Astrocytes are an essential source of energy for neurons by providing the neurons lactate. Astrocytes also participate in the clearance of glutamate from the synaptic cleft, take up neuronal mitochondria and are an important source of antioxidant enzymes, such as GSH. In AD, these normal astrocytic functions are altered leading to increases in astrocytic intracellular calcium, reductions in the levels of GSH and mitochondrial dysfunction. The normal intracellular degradation pathway for non-functional mitochondria, mitophagy, is impaired, leading to accumulation of non-functional mitochondria. In summary, these events lead to impaired astrocytic bioenergetics and impaired glutamate uptake from the synaptic cleft, greatly influencing neuronal health and contributing to the pathology progression.

## **8. Therapeutic approaches targeted to modulation of mitochondrial function in Alzheimer's disease**

There are a huge number of therapeutic approaches that have been trialed in AD, yet the only approved treatments only delay the inevitable and no cure for this devastating disorder exists. A critical difficulty in neurodegenerative disorders such as AD is the relative late onset of the symptoms united to a progressive degeneration, and late disease diagnosis. At the time of diagnosis, neuronal impairment is often too far for effective intervention. Furthermore, the anatomical site affected in these disorders is often difficult to access by potential therapies.

improves overall oxidative stress resistance induced by caloric restriction and exercise, may

Mitochondrial Function in Alzheimer's Disease: Focus on Astrocytes

http://dx.doi.org/10.5772/intechopen.71825

151

The hypothesis that impaired mitophagy in both neurons and astrocytes may lead to AD neurodegeneration and the potential of the mitophagy process as a therapeutic target needs further clarification. Pre-clinical studies introduce novel therapeutic molecules such as p62-mediated mitophagy inducer (PMI) and Mitochondrial division inhibitor 1 (Midvi-1) for this purpose. PMI is a recently described compound developed to upregulate p62 via stabilization of the transcription factor Nrf2 and to promote mitophagy [132]. This molecule has not yet been tested against neurodegeneration. Midvi-1 is a small molecule non-competitive inhibitor of dynamin-related protein 1 (Drp1) GTPase activity, which attenuates Drp1 mediated mitochondrial-fission and enhances the mitochondrial rescue through inactivation of PINK1 [133]. Recently, Manczak et al. [134] proposed a Drp1 based therapy in the context of AD: the authors demonstrated that the interaction of Drp1 with Aβ increases as AD progresses and that a partial reduction of Drp1 reduces Aβ deposition, reduces mitochondrial

It is noteworthy that most of the therapies for AD applied target neurons as the main cell type involved in neurodegeneration. Given that several therapeutic approaches have been attempted that do not completely rescue AD progression suggests that we should once more consider the cellular target against which we focus. A step towards the right direction has been taken by the scientific community in beginning to consider microglia as a therapeutic target for neurodegeneration because of their involvement in neuroinflammation. For example, the synthetic compound Midvi-1 attenuates mitochondrial-induced apoptosis in primary microglial cells in an Aβ-induced model of AD, thereby counteracting the neuroinflammation [135], and the endogenous compound melatonin has a protective role against cognitive decline and restores mitochondrial respiratory rate in microglial cells of the APPsw mouse

The increasingly evident key role of astrocytes in supporting neurons against neurodegeneration suggests that probing the unexplored field of mitochondrial-targeted therapies in astrocytes is needed. Currently, there is no relevant literature about astrocyte-targeted therapies, possibly because they are thought to be more involved in the late phases of AD progression. However, modulating early changes in these cells might prove to be more beneficial than

Several questions remain unanswered to fully understand the mechanisms of mitochondrial function, transport, turnover and degradation in astrocytes. Given the complexity of astrocyte sub-populations and region-specific phenotypes [137] several experimental approaches and models are needed to study mitochondrial function and possible impair-

be inhibited by antioxidants [131].

model of AD [136].

**9. Future aspects**

ment in these cells.

targeting downstream pathways.

dysfunction and enhances mitochondrial biogenesis.

Pharmaceutical companies have invested heavily in a variety of potential therapies to modulate AD: the well-known memantine, reducing the glutamate excitotoxicity, is considered one of the best available therapeutics for AD but still the possible long-term side effects are unknown [108]. In order for new approaches for a future therapeutic to be effective it is believed that early and better diagnosis methods are needed in order to prevent AD progression [109]. Mitochondria-targeting therapies are a novel approach that have potential to be used in the early onset of cognitive impairment. Mitochondrial oxidative damage is considered an early event of the disease process, which becomes more pronounced as AD progresses [110]. Mitochondrial dysfunction precedes Aβ plaque deposition [111] and is accompanied by a progressive reduction of the cerebral metabolic rates of glucose. Thus, several new therapeutic approaches have tested the efficacy of mitochondria-targeted molecules in delaying AD progression. For example, around ten clinical trials demonstrate that modulation of mitochondrial function rescue neuronal death and synaptic toxicity caused by Aβ exposure [112].

Mitochondrial medicine includes both life style intervention and pharmacological approaches. The Mediterranean diet [113], exercise [114] and caloric restriction [115, 116] have been shown to modulate AD risk factors including the mitochondrial healthy homeostasis. In combination with these, several preventive approaches have been studied in AD patients, for example the antioxidant N-acetyl cysteine (NAC) [117], α-lipoic acid (LA) [118] and curcumin [119] have been tested. Some clinical studies in particular demonstrate that NAC reduces brain oxidative stress through increasing GSH-mediated protective activity against Aβ deposits and lipid peroxidation, and decreasing acetylcholine levels and choline acetyltransferase (ChAT) activity [120]. In addition, several clinical trials used well-known molecules to limit oxidative damage: vitamin E (α-tocopherol) rescues cognitive impairment and oxidative stress in early phase of AD in pre-clinical studies [121] even though there are controversial results in clinical trials in human AD subjects [122]; Donepezil enhances the mitochondrial resistance by inhibiting the mitochondrial permeability transition pore (mPTP) in a mouse model of AD [123]. Other pre-clinical studies demonstrating potent mitochondrial effects have not yet been assessed in clinical trials but show great promise. For example, conjugated Coenzyme Q with a lipophilic triphenylphosphonium (TPP+ ) form MitoQ that protect primary cortical neurons from Aβ toxicity, loss of mitochondrial membrane potential and ROS production [124]; Szeto-Shiller antioxidant peptides allow the localization of antioxidant molecules in the mitochondrial matrix, the major source of ROS, and in particular SS31 shows a neuroprotective effect [125, 126]. Furthermore, endogenous compounds such as peroxiredoxine (Prdx) [127] and the natural molecules such as alkaloid caffeine [128], polyphenol resveratrol [129] and gypenoside XVII (GP-17) [130] have been used to modulate the bioenergetic homeostasis at different levels in AD mouse models. The antioxidant approach may have wide applications; however, it could also present some controversial effects on mitochondrial adaptation. For example, mitohormesis, an adaptive response that improves overall oxidative stress resistance induced by caloric restriction and exercise, may be inhibited by antioxidants [131].

The hypothesis that impaired mitophagy in both neurons and astrocytes may lead to AD neurodegeneration and the potential of the mitophagy process as a therapeutic target needs further clarification. Pre-clinical studies introduce novel therapeutic molecules such as p62-mediated mitophagy inducer (PMI) and Mitochondrial division inhibitor 1 (Midvi-1) for this purpose. PMI is a recently described compound developed to upregulate p62 via stabilization of the transcription factor Nrf2 and to promote mitophagy [132]. This molecule has not yet been tested against neurodegeneration. Midvi-1 is a small molecule non-competitive inhibitor of dynamin-related protein 1 (Drp1) GTPase activity, which attenuates Drp1 mediated mitochondrial-fission and enhances the mitochondrial rescue through inactivation of PINK1 [133]. Recently, Manczak et al. [134] proposed a Drp1 based therapy in the context of AD: the authors demonstrated that the interaction of Drp1 with Aβ increases as AD progresses and that a partial reduction of Drp1 reduces Aβ deposition, reduces mitochondrial dysfunction and enhances mitochondrial biogenesis.

It is noteworthy that most of the therapies for AD applied target neurons as the main cell type involved in neurodegeneration. Given that several therapeutic approaches have been attempted that do not completely rescue AD progression suggests that we should once more consider the cellular target against which we focus. A step towards the right direction has been taken by the scientific community in beginning to consider microglia as a therapeutic target for neurodegeneration because of their involvement in neuroinflammation. For example, the synthetic compound Midvi-1 attenuates mitochondrial-induced apoptosis in primary microglial cells in an Aβ-induced model of AD, thereby counteracting the neuroinflammation [135], and the endogenous compound melatonin has a protective role against cognitive decline and restores mitochondrial respiratory rate in microglial cells of the APPsw mouse model of AD [136].

The increasingly evident key role of astrocytes in supporting neurons against neurodegeneration suggests that probing the unexplored field of mitochondrial-targeted therapies in astrocytes is needed. Currently, there is no relevant literature about astrocyte-targeted therapies, possibly because they are thought to be more involved in the late phases of AD progression. However, modulating early changes in these cells might prove to be more beneficial than targeting downstream pathways.

## **9. Future aspects**

At the time of diagnosis, neuronal impairment is often too far for effective intervention. Furthermore, the anatomical site affected in these disorders is often difficult to access by

Pharmaceutical companies have invested heavily in a variety of potential therapies to modulate AD: the well-known memantine, reducing the glutamate excitotoxicity, is considered one of the best available therapeutics for AD but still the possible long-term side effects are unknown [108]. In order for new approaches for a future therapeutic to be effective it is believed that early and better diagnosis methods are needed in order to prevent AD progression [109]. Mitochondria-targeting therapies are a novel approach that have potential to be used in the early onset of cognitive impairment. Mitochondrial oxidative damage is considered an early event of the disease process, which becomes more pronounced as AD progresses [110]. Mitochondrial dysfunction precedes Aβ plaque deposition [111] and is accompanied by a progressive reduction of the cerebral metabolic rates of glucose. Thus, several new therapeutic approaches have tested the efficacy of mitochondria-targeted molecules in delaying AD progression. For example, around ten clinical trials demonstrate that modulation of mitochondrial function rescue neuronal death and synaptic toxicity caused by

Mitochondrial medicine includes both life style intervention and pharmacological approaches. The Mediterranean diet [113], exercise [114] and caloric restriction [115, 116] have been shown to modulate AD risk factors including the mitochondrial healthy homeostasis. In combination with these, several preventive approaches have been studied in AD patients, for example the antioxidant N-acetyl cysteine (NAC) [117], α-lipoic acid (LA) [118] and curcumin [119] have been tested. Some clinical studies in particular demonstrate that NAC reduces brain oxidative stress through increasing GSH-mediated protective activity against Aβ deposits and lipid peroxidation, and decreasing acetylcholine levels and choline acetyltransferase (ChAT) activity [120]. In addition, several clinical trials used well-known molecules to limit oxidative damage: vitamin E (α-tocopherol) rescues cognitive impairment and oxidative stress in early phase of AD in pre-clinical studies [121] even though there are controversial results in clinical trials in human AD subjects [122]; Donepezil enhances the mitochondrial resistance by inhibiting the mitochondrial permeability transition pore (mPTP) in a mouse model of AD [123]. Other pre-clinical studies demonstrating potent mitochondrial effects have not yet been assessed in clinical trials but show great promise. For example,

protect primary cortical neurons from Aβ toxicity, loss of mitochondrial membrane potential and ROS production [124]; Szeto-Shiller antioxidant peptides allow the localization of antioxidant molecules in the mitochondrial matrix, the major source of ROS, and in particular SS31 shows a neuroprotective effect [125, 126]. Furthermore, endogenous compounds such as peroxiredoxine (Prdx) [127] and the natural molecules such as alkaloid caffeine [128], polyphenol resveratrol [129] and gypenoside XVII (GP-17) [130] have been used to modulate the bioenergetic homeostasis at different levels in AD mouse models. The antioxidant approach may have wide applications; however, it could also present some controversial effects on mitochondrial adaptation. For example, mitohormesis, an adaptive response that

) form MitoQ that

conjugated Coenzyme Q with a lipophilic triphenylphosphonium (TPP+

potential therapies.

150 Astrocyte - Physiology and Pathology

Aβ exposure [112].

Several questions remain unanswered to fully understand the mechanisms of mitochondrial function, transport, turnover and degradation in astrocytes. Given the complexity of astrocyte sub-populations and region-specific phenotypes [137] several experimental approaches and models are needed to study mitochondrial function and possible impairment in these cells.

It is important to note that very little literature exists about AD-associated mitochondrial dysfunction specifically in astrocytes, although recent reports have highlighted the importance of this cell type in neurodegeneration. In the future, it will be critically important to carry out more studies that focus on alterations of astrocytic mitochondria in AD because of the arising role of these cells in the early onset of this disease. Furthermore, the utilization of human models in AD research is expected to provide valuable tools and detailed mechanistic insight into the role of astrocytes that is central in understanding the features of this devastating human disease.

[4] van Hall G, Stømstad M, Rasmussen P, et al. Blood lactate is an important energy source for the human brain. Journal of Cerebral Blood Flow & Metabolism. 2009;**29**(6):

Mitochondrial Function in Alzheimer's Disease: Focus on Astrocytes

http://dx.doi.org/10.5772/intechopen.71825

153

[5] Magistretti PJ. Role of glutamate in neuron-glia metabolic coupling. The American

[6] Bélanger M, Allaman I, Magistretti P. Brain energy metabolism: Focus on astrocyte-neu-

[7] Fu W, Jhamandas JH. Role of astrocytic glycolytic metabolism in Alzheimer's disease

[8] Bolaños JP, Almeida A, Moncada S. Glycolysis: A bioenergetic or a survival pathway?

[9] Suzuki A, Stern S, Bozdagi O, et al. Astrocyte-neuron lactate transport is required for

[10] Rose J, Brian C, Woods J, et al. Mitochondrial dysfunction in glial cells: Implications for

[11] Walls AB, Heimbürger CM, Bouman SD, et al. Robust glycogen shunt activity in astrocytes: Effects of glutamatergic and adrenergic agents. Neuroscience. 2009;**158**(1):284-292

[12] Obel LF, Müller MS, Walls AB, et al. Brain glycogen-new perspectives on its metabolic function and regulation at the subcellular level. Frontiers in Neuroenergetics. 2012;**4**:

[13] Almeida A, Moncada S, Bolaños JP. Nitric oxide switches on glycolysis through the AMP protein kinase and 6-phosphofructo-2-kinase pathway. Nature Cell Biology. 2004;**6**(1):

[14] Voutsinos-Porche B, Bonvento G, Tanaka K, et al. Glial glutamate transporters mediate a functional metabolic crosstalk between neurons and astrocytes in the mouse developing

[15] Pellerin L, Magistretti PJ. Glutamate uptake into astrocytes stimulates aerobic glycolysis: A mechanism coupling neuronal activity to glucose utilization. Proceedings of the National Academy of Sciences of the United States of America. 1994;**91**(22):

[16] Sagan L. On the origin of mitosing cells. Journal of Theoretical Biology. 1967;**14**(3):255-274 [17] Held NM, Houtkooper RH. Mitochondrial quality control pathways as determinants of

[18] Duchen MR. Mitochondria and calcium: From cell signalling to cell death. The Journal

Journal of Clinical Nutrition. 2009;**90**(3):875-880

pathogenesis. Biogerontology. 2014;**15**(6):579-586

Trends in Biochemical Sciences. 2010;**35**(3):145-149

long-term memory formation. Cell. 2011;**144**(5):810-823

neuronal homeostasis and survival. Toxicology. 2017;**391**:109-115

ron metabolic cooperation. Cell Metabolism. 2011;**14**(6):724-738

1121-1129

1-15

45-51

10625-10629

cortex. Neuron. 2003;**37**(2):275-286

of Physiology. 2000;**529**:57-68

metabolic health. BioEssays. 2015;**37**(8):867-876

As research steers towards an in depth understanding of the molecular basis of mitochondria and the mitochondrial quality control system it is possible that this might in the future provide both a diagnostic and a therapeutic tool for neurodegeneration.

## **Acknowledgements**

We gratefully acknowledge the financial support from the Academy of Finland and The Sigrid Juselius Foundation.

## **Author details**

Riikka Lampinen, Irina Belaya, Isabella Boccuni, Tarja Malm and Katja M. Kanninen\*

\*Address all correspondence to<EMAIL_ADDRESS>
A. I. Virtanen Institute for Molecular Sciences, University of Eastern Finland, Kuopio, Finland

## **References**

[4] van Hall G, Stømstad M, Rasmussen P, et al. Blood lactate is an important energy source for the human brain. Journal of Cerebral Blood Flow & Metabolism. 2009;**29**(6): 1121-1129

It is important to note that very little literature exists about AD-associated mitochondrial dysfunction specifically in astrocytes, although recent reports have highlighted the importance of this cell type in neurodegeneration. In the future, it will be critically important to carry out more studies that focus on alterations of astrocytic mitochondria in AD because of the arising role of these cells in the early onset of this disease. Furthermore, the utilization of human models in AD research is expected to provide valuable tools and detailed mechanistic insight into the role of astrocytes that is central in understanding the features of this devastating

As research steers towards an in depth understanding of the molecular basis of mitochondria and the mitochondrial quality control system it is possible that this might in the future pro-

We gratefully acknowledge the financial support from the Academy of Finland and The Sigrid

Riikka Lampinen, Irina Belaya, Isabella Boccuni, Tarja Malm and Katja M. Kanninen\*

A. I. Virtanen Institute for Molecular Sciences, University of Eastern Finland, Kuopio,

[1] Wang Z, Ying Z, Bosy-Westphal A, et al. Specific metabolic rates of major organs and tissues across adulthood: Evaluation by mechanistic model of resting energy expenditure.

[2] Mergenthaler P, Lindauer U, Dienel GA, et al. Sugar for the brain: The role of glucose in physiological and pathological brain function. Trends in Neurosciences. 2013;**36**(10):

[3] Howarth C, Gleeson P, Attwell D. Updated energy budgets for neural computation in the neocortex and cerebellum. Journal of Cerebral Blood Flow & Metabolism. 2012;**32**(7):

The American Journal of Clinical Nutrition. 2010;**92**(6):1369-1377

vide both a diagnostic and a therapeutic tool for neurodegeneration.

\*Address all correspondence to<EMAIL_ADDRESS>
human disease.

152 Astrocyte - Physiology and Pathology

**Acknowledgements**

Juselius Foundation.

**Author details**

Finland

**References**

587-597

1222-1232

[19] Giorgi C, Baldassari F, Bononi A, et al. Mitochondrial Ca(2+) and apoptosis. Cell Calcium. 2012;**52**(1):36-43

[33] Jackson JG, Robinson MB. Reciprocal regulation of mitochondrial dynamics and calcium signaling in astrocyte processes. The Journal of Neuroscience. 2015;**35**(45):15199-15213

Mitochondrial Function in Alzheimer's Disease: Focus on Astrocytes

http://dx.doi.org/10.5772/intechopen.71825

155

[34] Jackson JG, O'Donnell JC, Takano H, et al. Neuronal activity and glutamate uptake decrease mitochondrial mobility in astrocytes and position mitochondria near gluta-

[35] Cagalinec M, Safiulina D, Liiv M, et al. Principles of the mitochondrial fusion and fission

[36] Jeong SY, Seol DW. The role of mitochondria in apoptosis. BMB Reports. 2008;**41**(1):11-22

[37] Lionaki E, Markaki M, Palikaras K, et al. Mitochondria, autophagy and age-associated neurodegenerative diseases: New insights into a complex interplay. Biochimica et

[38] Amadoro G, Corsetti V, Florenzano F, et al. Morphological and bioenergetic demands underlying the mitophagy in post-mitotic neurons: The pink-parkin pathway. Frontiers

[39] Lemasters JJ. Selective mitochondrial autophagy, or mitophagy, as a targeted defense against oxidative stress, mitochondrial dysfunction, and aging. Rejuvenation Research.

[40] Kissova I, Deffieu M, Manon S, et al. Uth1p is involved in the autophagic degradation of mitochondria. The Journal of Biological Chemistry. 2004;**279**(37):39068-39074

[41] Narendra DP, Youle RJ. Targeting mitochondrial dysfunction: Role for PINK1 and Parkin in mitochondrial quality control. Antioxidants & Redox Signaling. 2011;**14**(10):

[42] Cai Q, Lu L, Tian JH, et al. Snapin-regulated late endosomal transport is critical for effi-

[43] Davis CH, Kim KY, Bushong EA, et al. Transcellular degradation of axonal mitochondria. Proceedings of the National Academy of Sciences of the United States of America.

[44] Hayakawa K, Esposito E, Wang X, et al. Transfer of mitochondria from astrocytes to

[45] Berridge MV, Schneider RT, McConnell MJ. Mitochondrial transfer from astrocytes to neurons following ischemic insult: Guilt by association? Cell Metabolism. 2016;**24**(3):

[46] Derouiche A, Haseleu J, Korf HW. Fine astrocyte processes contain very small mitochondria: Glial oxidative capability may fuel transmitter metabolism. Neurochemical

cient autophagy-lysosomal function in neurons. Neuron. 2010;**68**(1):73-86

neurons after stroke. Nature. 2016;**535**(7613):551-555

mate transporters. The Journal of Neuroscience. 2014;**34**(5):1613-1624

cycle in neurons. Journal of Cell Science. 2013;**126**(10):2187-2197

Biophysica Acta. 2015;**1847**(11):1412-1423

in Aging Neuroscience. 2014;**6**:18

2005;**8**(1):3-5

1929-1938

376-378

2014;**111**(26):9633-9638

Research. 2015;**40**(12):2402-2413

[33] Jackson JG, Robinson MB. Reciprocal regulation of mitochondrial dynamics and calcium signaling in astrocyte processes. The Journal of Neuroscience. 2015;**35**(45):15199-15213

[19] Giorgi C, Baldassari F, Bononi A, et al. Mitochondrial Ca(2+) and apoptosis. Cell Calcium.

[20] Raps SP, Lai JC, Hertz L, et al. Glutathione is present in high concentrations in cultured

[21] Ziech D, Franco R, Georgakilas AG, et al. The role of reactive oxygen species and oxidative stress in environmental carcinogenesis and biomarker development. Chemico-

[22] Starkov AA. The role of mitochondria in reactive oxygen species metabolism and signal-

[23] Boveris A, Chance B. The mitochondrial generation of hydrogen peroxide. General properties and effect of hyperbaric oxygen. The Biochemical Journal. 1973;**134**(3):707-716

[24] Zorov DB, Filburn CR, Klotz LO, et al. Reactive oxygen species (ROS)-induced ROS release: A new phenomenon accompanying induction of the mitochondrial permeability transition in cardiac myocytes. The Journal of Experimental Medicine. 2000;**192**(7):

[25] Hara Y, Yuk F, Puri R, et al. Presynaptic mitochondrial morphology in monkey prefrontal cortex correlates with working memory and is improved with estrogen treatment. Proceedings of the National Academy of Sciences of the United States of America.

[26] von Bartheld CS, Bahney J, Herculano-Houzel S. The search for true numbers of neurons and glial cells in the human brain: A review of 150 years of cell counting. The Journal of

[27] Verkhratsky A, Olabarria M, Noristani HN, et al. Astrocytes in Alzheimer's disease.

[28] Sofroniew MV, Vinters HV. Astrocytes: Biology and pathology. Acta Neuropathologica.

[29] Ogata K, Kosaka T. Structural and quantitative analysis of astrocytes in the mouse hip-

[30] Nedergaard M, Ransom B, Goldman SA. New roles for astrocytes: Redefining the functional architecture of the brain. Trends in Neurosciences. 2003;**26**(10):523-530

[31] Lovatt D, Sonnewald U, Waagepetersen HS, et al. The transcriptome and metabolic gene signature of protoplasmic astrocytes in the adult murine cortex. The Journal of

[32] Acosta C, Anderson HD, Anderson C. Astrocyte dysfunction in Alzheimer disease.

astrocytes but not in cultured neurons. Brain Research. 1989;**493**(2):398-401

ing. Annals of the New York Academy of Sciences. 2008;**1147**:37-52

2012;**52**(1):36-43

154 Astrocyte - Physiology and Pathology

1001-1014

2014;**111**(1):486-491

2010;**119**(1):7-35

Biological Interactions. 2010;**188**(2):334-339

Comparative Neurology. 2016;**524**(18):3865-3895

pocampus. Neuroscience. 2002;**113**(1):221-233

Neuroscience. 2007;**27**(45):12255-12266

Journal of Neuroscience Research. 2017;**95**(12):2430-2447

Neurotherapeutics. 2010;**7**(4):399-412

[47] Mayeux R. Early Alzheimer's disease. The New England Journal of Medicine. 2010;**362**(23): 2194-2201

[62] Olabarria M, Noristani HN, Verkhratsky A, et al. Concomitant astroglial atrophy and astrogliosis in a triple transgenic animal model of Alzheimer's disease. Glia. 2010;**58**(7):

Mitochondrial Function in Alzheimer's Disease: Focus on Astrocytes

http://dx.doi.org/10.5772/intechopen.71825

157

[63] Orre M, Kamphuis W, Osborn LM, et al. Isolation of glia from Alzheimer's mice reveals inflammation and dysfunction. Neurobiology of Aging. 2014;**35**(12):2746-2760

[64] Pomilio C, Pavia P, Gorojod RM, et al. Glial alterations from early to late stages in a model of Alzheimer's disease: Evidence of autophagy involvement in Aβ internaliza-

[65] Hartlage-Rübsamen M, Zeitschel U, Apelt J, et al. Astrocytic expression of the Alzheimer's disease β-secretase (BACE1) is stimulus-dependent. Glia. 2003;**41**(2):169-179

[66] Oberheim NA, Takano T, Han X, et al. Uniquely hominid features of adult human astro-

[67] Oksanen M, Petersen AJ, Naumenko N, et al. PSEN1 mutant iPSC-derived model reveals severe astrocyte pathology in alzheimer's disease. Stem Cell Reports. 2017. pii:

[68] Pekny M, Wilhelmsson U, Pekna M. The dual role of astrocyte activation and reactive

[69] Zamanian JL, Xu L, Foo LC, et al. Genomic analysis of reactive astrogliosis. The Journal

[70] Liddelow SA, Guttenplan KA, Clarke LE, et al. Neurotoxic reactive astrocytes are

[71] Osborn LM, Kamphuis W, Wadman WJ, et al. Astrogliosis: An integral player in the pathogenesis of Alzheimer's disease. Progress in Neurobiology. 2016;**144**:121-141

[72] Dossi E, Vasile F, Rouach N. Human astrocytes in the diseased brain. Brain Research

[73] Cotrina ML, Nedergaard M. Astrocytes in the aging brain. Journal of Neuroscience

[74] Schipper H, Gupta A, Szarek W. Suppression of glial HO-1 activitiy as a potential neurotherapeutic intervention in AD. Current Alzheimer Research. 2009;**6**(5):424-430

[75] Schubert D, Soucek T, Blouw B. The induction of HIF-1 reduces astrocyte activation by amyloid beta peptide. The European Journal of Neuroscience. 2009;**29**(7):1323-1334 [76] Cai Z, Cheng-Qun W, Zhou L. Astrocyte and Alzheimer's disease. Journal of Neurology.

[77] Merlini M, Meyer E, Ulmann-Schuler A, et al. Vascular β-amyloid and early astrocyte alterations impair cerebrovascular function and cerebral metabolism in transgenic

induced by activated microglia. Nature. 2017;**541**(7638):481-487

Bulletin. 2017. pii: S0361-9230(17)30082-5 [Epub ahead of print]

arcAβ mice. Acta Neuropathologica. 2011;**122**(3):293-311

831-838

tion. Hippocampus. 2016;**26**(2):194-210

cytes. Journal of Neuroscience. 2009;**29**(10):3276-3287

S2213-6711(17)30471-X [Epub ahead of print]

gliosis. Neuroscience Letters. 2014;**565**:30

of Neuroscience. 2012;**32**(18):6391-6410

Research. 2002;**67**(1):1-10

2017;**264**(10):2068-2074

[62] Olabarria M, Noristani HN, Verkhratsky A, et al. Concomitant astroglial atrophy and astrogliosis in a triple transgenic animal model of Alzheimer's disease. Glia. 2010;**58**(7): 831-838

[47] Mayeux R. Early Alzheimer's disease. The New England Journal of Medicine. 2010;**362**(23):

[48] Nelson P, Braak H, Markesbery W. Neuropathology and cognitive impairment in Alzheimer disease: A complex but coherent relationship. Journal of Neuropathology

[49] Karch CM, Goate AM. Alzheimer's disease risk genes and mechanisms of disease patho-

[50] Tanzi RE. The genetics of Alzheimer disease. Cold Spring Harbor Perspectives in Medicine.

[51] Demetrius LA, Driver J. Alzheimer's as a metabolic disease. Biogerontology. 2013;**14**(6):

[52] Lee J, Han P. An update of animal models of Alzheimer disease with a reevaluation of

[53] Arber C, Lovejoy C, Wray S. Stem cell models of Alzheimers disease: Progress and chal-

[54] Jones VC, Atkinson-dell R, Verkhratsky A, et al. Aberrant iPSC-derived human astro-

[55] Liao M, Muratore CR, Gierahn TM, et al. Single-cell detection of secreted Aβ and sAPPα from human IPSC-derived neurons and astrocytes. The Journal of Neuroscience.

[56] Alzheimer A. Contribution about the knowledge of the pathologic neuroglia, and its relationships with the course of the decomposition of the nervous system. Histologic

[57] Rodríguez-Arellano JJ, Parpura V, Zorec R, et al. Astrocytes in physiological aging and

[58] Oakley H, Cole SL, Logan S, et al. Intraneuronal beta-amyloid aggregates, neurodegeneration, and neuron loss in transgenic mice with five familial Alzheimer's disease mutations: Potential factors in amyloid plaque formation. Journal of Neuroscience.

[59] van Gijsel-Bonnello M, Baranger K, Benech P, et al. Metabolic changes and inflammation in cultured astrocytes from the 5xFAD mouse model of Alzheimer's disease: Alleviation

[60] Iram T, Trudler D, Kain D, et al. Astrocytes from old Alzheimer's disease mice are impaired in Aβ uptake and in neuroprotection. Neurobiology of Disease. 2016;**96**:84-94

[61] Kulijewicz-Nawrot M, Verkhratsky A, Chvátal A, et al. Astrocytic cytoskeletal atrophy in the medial prefrontal cortex of a triple transgenic mouse model of Alzheimer's dis-

plaque depositions. Experimental Neurobiology. 2013;**22**(2):84-95

cytes in Alzheimer's disease. Cell Death and Disease. 2017;**8**(3):e2696

and Histopathologic Studies on Cerebral Cortex. 1910:401-562

Alzheimer's disease. Neuroscience. 2016;**323**:170-182

lenges. Alzheimer's Research & Therapy. 2017;**9**(1):42

2194-2201

156 Astrocyte - Physiology and Pathology

2012;**2**(10):1-10

2016;**36**(5):1730-1746

2006;**26**(40):10129-10140

by pantethine. PLoS One. 2017;**12**(4):1-22

ease. Journal of Anatomy. 2012;**221**(3):252-262

641

and Experimental Neurology. 2009;**68**(1):1-14

genesis. Biological Psychiatry. 2015;**77**(1):43-51

[78] Lu W, Huang J, Sun S, et al. Changes in lactate content and monocarboxylate transporter 2 expression in Aβ₂₅₋₃₅-treated rat model of Alzheimer's disease. Neurological Sciences. 2015;**36**(6):871

[93] Sheng B, Wang X, Su B, et al. Impaired mitochondrial biogenesis contributes to mitochondrial dysfunction in Alzheimer's disease. Journal of Neurochemistry. 2012;**120**(3):

Mitochondrial Function in Alzheimer's Disease: Focus on Astrocytes

http://dx.doi.org/10.5772/intechopen.71825

159

[94] Baloyannis SJ. Mitochondrial alterations in Alzheimer's disease. Journal of Alzheimers

[95] Rice AC, Keeney PM, Algarzae NK, et al. Mitochondrial DNA copy numbers in pyramidal neurons are decreased and mitochondrial biogenesis transcriptome signaling is disrupted in Alzheimer's disease hippocampi. Journal of Alzheimer's Disease. 2014;**40**(2):

[96] Brown AM, Sheu RK, Mohs R, et al. Correlation of the clinical severity of alzheimer's disease with an aberration in mitochondrial DNA (mtDNA). Journal of Molecular Neuro-

[97] Hirai K, Aliev G, Nunomura A, et al. Mitochondrial abnormalities in Alzheimer's dis-

[98] Fukui H, Moraes CT. The mitochondrial impairment, oxidative stress and neurodegeneration connection: Reality or just an attractive hypothesis? Trends in Neurosciences.

[99] Fu W, Shi D, Westaway D, et al. Bioenergetic mechanisms in astrocytes may contribute to amyloid plaque deposition and toxicity. The Journal of Biological Chemistry.

[100] Misko A, Jiang S, Wegorzewska I, et al. Mitofusin 2 is necessary for transport of axonal mitochondria and interacts with the Miro/Milton complex. Journal of Neuroscience.

[101] Bak LK, Schousboe A, Waagepetersen HS. The glutamate/GABA-glutamine cycle: Aspects of transport, neurotransmitter homeostasis and ammonia transfer. Journal of

[102] Voloboueva LA, Suh SW, Swanson RA, et al. Inhibition of mitochondrial function in astrocytes: Implications for neuroprotection. Journal of Neurochemistry. 2007;**102**(4):

[103] Genda EN, Jackson JG, Sheldon AL, et al. Co-compartmentalization of the astroglial glutamate transporter, GLT-1, with glycolytic enzymes and mitochondria. The Journal

[104] Sekar S, McDonald J, Cuyugan L, et al. Alzheimer's disease is associated with altered expression of genes involved in immune response and mitochondrial processes in

[105] Sarkar P, Zaja I, Bienengraeber M, et al. Epoxyeicosatrienoic acids pretreatment improves amyloid -induced mitochondrial dysfunction in cultured rat hippocampal

astrocytes. AJP - Heart and Circulatory Physiology. 2014;**306**(4):475-484

419-429

319-330

Disease. 2006;**9**(9):119-126

science. 2001;**16**(1):41-48

2008;**31**(5):251-256

2015;**290**(20):12504-12513

2010;**30**(12):4232-4240

1383-1394

Neurochemistry. 2006;**98**(3):641-653

of Neuroscience. 2011;**31**(50):18275-18288

astrocytes. Neurobiology of Aging. 2015;**36**(2):583-591

ease. Journal of Neuroscience. 2001;**21**(9):3017-3023

[93] Sheng B, Wang X, Su B, et al. Impaired mitochondrial biogenesis contributes to mitochondrial dysfunction in Alzheimer's disease. Journal of Neurochemistry. 2012;**120**(3): 419-429

[78] Lu W, Huang J, Sun S, et al. Changes in lactate content and monocarboxylate transporter 2 expression in Aβ₂₅₋₃₅-treated rat model of Alzheimer's disease. Neurological Sciences.

[79] Kubik LL, Philbert MA. The role of astrocyte mitochondria in differential regional susceptibility to environmental neurotoxicants: Tools for understanding neurodegenera-

[80] Stephen T, Gupta-Agarwal S, Kittler JT. Mitochondrial dynamics in astrocytes.

[81] Demetrius LA, Simon DK.  Journal of the Royal Society Interface. 2013;**10**(82):20130006

[82] Slemmer JE, Shacka JJ, Sweeney MI, et al. Antioxidants and free radical scavengers for the treatment of stroke, traumatic brain injury and aging. Current Medicinal Chemistry.

[83] Kaminsky YG, Kosenko EA. Effects of amyloid-beta peptides on hydrogen peroxidemetabolizing enzymes in rat brain in vivo. Free Radical Research. 2008;**42**(6):564-573 [84] Abramov AY, Canevari L, Duchen MR. Beta-amyloid peptides induce mitochondrial dysfunction and oxidative stress in astrocytes and death of neurons through activation

[85] Kuchibhotla KV, Lattarulo CR, Hyman BT, et al. Synchronous hyperactivity and intercellular calcium waves in astrocytes in Alzheimer mice. Science. 2009;**323**(5918):1211-1215

[86] Begley JG, Duan W, Chan S, et al. Altered calcium homeostasis and mitochondrial dysfunction in cortical synaptic compartments of presenilin-1 mutant mice. Journal of

[87] Swerdlow RH, Burns JM, Khan SM. The Alzheimer's disease mitochondrial cascade hypothesis: Progress and perspectives. BBA - Molecular Basis of Disease. 2014;**1842**(8):

[88] Pagani L, Eckert A. Amyloid-Beta interaction with mitochondria. International Journal

[89] Anandatheerthavarada HK, Biswas G, Robin M, et al. Mitochondrial targeting and a novel transmembrane arrest of Alzheimer's amyloid precursor protein impairs mitochondrial function in neuronal cells. The Journal of Cell Biology. 2003;**161**(1):41-54 [90] Swerdlow RH. The neurodegenerative mitochondriopathies. Journal of Alzheimer's

[91] Halliwell B. Reactive oxygen species and the central nervous system. Journal of Neuro-

[92] Sagara JI, Miura K, Bannai S. Maintenance of neuronal glutathione by glial cells. Journal

of NADPH oxidase. The Journal of Neuroscience. 2004;**24**(2):565-575

2015;**36**(6):871

158 Astrocyte - Physiology and Pathology

2008;**15**(4):404-414

1219-1231

tion. Toxicological Sciences. 2015;**144**(1):7-16

Neurochemistry. 1999;**72**(3):1030-1039

of Alzheimer's Disease. 2011;**2011**:925050

Disease. 2009;**17**(4):737-751

chemistry. 1992;**59**(5):1609-1623

of Neurochemistry. 1993;**61**(5):1672-1676

Biochemical Society Transactions. 2014;**42**(5):1302-1310

[106] Lee H, Pancholi N, Esposito L, et al. Early induction of oxidative stress in mouse model of Alzheimer disease with reduced mitochondrial superoxide dismutase activity. PLoS One. 2012;**7**(1):e28033

[120] Pocernich CB, Butterfield DA. Elevation of glutathione as a therapeutic strategy in Alzheimer disease. BBA - Molecular Basis of Disease. 2012;**1822**(5):625-630

Mitochondrial Function in Alzheimer's Disease: Focus on Astrocytes

http://dx.doi.org/10.5772/intechopen.71825

161

[121] Sung S, Yao Y, Uryu K, et al. Early vitamin E supplementation in young but not aged mice reduces Abeta levels and amyloid deposition in a transgenic model of Alzheimer's

[122] Usoro OB, Mousa SA. Vitamin E forms in Alzheimer's disease: A review of controversial and clinical experiences. Critical Reviews in Food Science and Nutrition.

[123] Ye CY, Lei Y, Tang XC, et al. Donepezil attenuates Aβ-associated mitochondrial dysfunction and reduces mitochondrial Aβ accumulation in vivo and in vitro. Neuro-

[124] Ng LF, Gruber J, Cheah IK, et al. The mitochondria-targeted antioxidant MitoQ extends lifespan and improves healthspan of a transgenic *Caenorhabditis elegans* model of

[125] Reddy PH, Manczak M, Kandimalla R. Mitochondria-targeted small molecule SS31, a potential candidate for the treatment of Alzheimer's disease. Human Molecular

[126] Szeto H. Mitochondria-targeted peptide antioxidants: Novel neuroprotective agents.

[127] Kim IK, Lee KJ, Rhee S, et al. Protective effects of Peroxiredoxin 6 overexpression on amyloid β-induced apoptosis in PC12 cells. Free Radical Research. 2013;**47**(10):

[128] Dragicevic N, Delic V, Cao C, et al. Caffeine increases mitochondrial function and blocks melatonin signaling to mitochondria in Alzheimer's mice and cells. Neuro-

[129] Rege SD, Geetha T, Griffin GD, et al. Neuroprotective effects of resveratrol in Alzheimer

[130] Meng X, Wang M, Sun G, et al. Attenuation of Aβ25-35-induced parallel autophagic and apoptotic cell death by gypenoside XVII through the estrogen receptor-dependent activation of Nrf2/ARE pathways. Toxicology and Applied Pharmacology. 2014;**279**(1):

[131] Onyango IG, Dennis J, Khan SM. Mitochondrial dysfunction in Alzheimer's disease and the rationale for bioenergetics based therapies. Aging and Disease. 2016;**7**(2):201-214

[132] East DA, Fagiani F, Crosby J, et al. PMI: A ΔΨm independent pharmacological regula-

[133] Cui M, Tang X, Christian WV, et al. Perturbations in mitochondrial dynamics induced by human mutant PINK1 can be rescued by the mitochondrial division inhibitor mdivi-1.

disease pathology. Frontiers in Aging Neuroscience. 2014;**6**:218

tor of mitophagy. Chemistry & Biology. 2014;**21**(11):1585-1596

The Journal of Biological Chemistry. 2010;**285**(15):11740-11752

Alzheimer disease. Free Radical Biology & Medicine. 2014;**71**:390-401

disease. FASEB Journal. 2004;**18**(2):323-325

2010;**50**(5):414-419

pharmacology. 2015;**95**:29-36

Genetics. 2017;**26**(8):1483-1496

836-846

63-75

The AAPS Journal. 2006;**8**(3):521-531

pharmacology. 2012;**63**(8):1368-1379

[120] Pocernich CB, Butterfield DA. Elevation of glutathione as a therapeutic strategy in Alzheimer disease. BBA - Molecular Basis of Disease. 2012;**1822**(5):625-630

[106] Lee H, Pancholi N, Esposito L, et al. Early induction of oxidative stress in mouse model of Alzheimer disease with reduced mitochondrial superoxide dismutase activity. PLoS

[107] Tamagno E, Guglielmotto M, Aragno M, et al. Oxidative stress activates a positive feedback between the γ- and β-secretase cleavages of the β-amyloid precursor protein.

[108] Matsunaga S, Kishi T, Iwata N. Memantine monotherapy for Alzheimer's disease: A

[109] Amanatkar HR, Papagiannopoulos B, Grossberg GT. Analysis of recent failures of disease modifying therapies in Alzheimer's disease suggesting a new methodology for

[110] Reddy AP, Reddy PH. Mitochondria-targeted molecules as potential drugs to treat patients with Alzheimer's disease. Progress in Molecular Biology and Translational

[111] Yao J, Irwin RW, Zhao L, et al. Mitochondrial bioenergetic deficit precedes Alzheimer's pathology in female mouse model of Alzheimer's disease. Proceedings of the National

Academy of Sciences of the United States of America. 2009;**106**(34):14670-14675

[112] Wilkins HM, Morris HM. New therapeutics to modulate mitochondrial function in neurodegenerative disorders. Current Pharmaceutical Design. 2017;**23**(5):731-752

[113] Morris MC, Tangney CC, Wang Y, et al. MIND diet associated with reduced incidence

[114] Chirles TJ, Reiter K, Weiss LR, et al. Exercise training and functional connectivity changes in mild cognitive impairment and healthy elders. Journal of Alzheimer's Disease. 2017;

[115] Van Cauwenberghe C, Vandendriessche C, Libert C, et al. Caloric restriction: Beneficial effects on brain aging and Alzheimer's disease. Mammalian Genome. 2016;**27**(7):300-319

[116] Martin B, Mattson MP, Maudsley S. Caloric restriction and intermittent fasting: Two potential diets for successful brain aging. Ageing Research Reviews. 2006;**5**(3):332-353

[117] Bavarsad Shahripour R, Harrigan MR, Alexandrov AV. N-acetylcysteine (NAC) in neurological disorders: Mechanisms of action and therapeutic opportunities. Brain and

[118] Maczurek A, Hager K, Kenklies M, et al. Lipoic acid as an anti-inflammatory and neuroprotective treatment for Alzheimer's disease. Advanced Drug Delivery Reviews.

[119] Baum L, Lam CWK, Cheung SK, et al. Six-month randomized, placebo-controlled, double-blind, pilot clinical trial of curcumin in patients with Alzheimer disease. Journal of

of Alzheimer's disease. Alzheimer's & Dementia. 2015;**11**(9):1007-1014

systematic review and meta-analysis. PLoS One. 2015;**10**(4):e0123289

future studies. Expert Review of Neurotherapeutics. 2017;**17**(1):7-16

One. 2012;**7**(1):e28033

160 Astrocyte - Physiology and Pathology

Science. 2017;**146**:173-201

**57**:845-856

Behavior. 2014;**4**(2):108-122

Clinical Psychopharmacology. 2008;**28**(1):110-113

2008;**60**(13):1463-1470

Journal of Neurochemistry. 2008;**104**(3):683-695

[134] Manczak M, Kandimalla R, Fry D, et al. Protective effects of reduced dynamin-related protein 1 against amyloid beta-induced mitochondrial dysfunction and synaptic damage in Alzheimer's disease. Human Molecular Genetics. 2016;**25**(23):5148-5166

**Chapter 8**

Provisional chapter

**The Role of Astrocytes in Tumor Growth and**

DOI: 10.5772/intechopen.72720

Current research is continually implicating the importance of astrocytes as active participants in neurological injury, disease, and tumor progression. This chapter will discuss some of these emerging concepts, especially as they relate to tumor biology. Astrocytes themselves can become tumorigenic, such as the case in gliomas, which often have aberrant signaling in key regulating genes of astrocyte development. Astrocytes secrete factors that maintain the tight junctions of the blood brain barrier (BBB), which in turn regulates the success or failure of metastatic cells extravasating into the brain. This astrocytic association with the brain vasculature also promotes brain tumor stem cell characteristics, which are known to be necessary for tumor initiation. Tumor cells within the brain make direct contacts with astrocytes through gap junctions, which subsequently lead to increased chemoresistance of the tumor cells. Astrocytes have also been shown to effect tumors cells via secretion of degradative enzymes, cytokines, chemokines, and growth factors, all of which have been shown to promote tumor cell proliferation, survival, and invasion. Thus, research in astrocyte biology and the role of astrocytes in the tumor microenvironment has and will likely continue to reveal novel targets for cancer

Keywords: astrocytes, metastasis, blood-brain barrier, reactive astrogliosis, cancer

The tumor microenvironment plays a critical role in tumor progression. Tumors within the central nervous system (CNS) include primary brain tumors originating from a CNS resident cell, or secondary tumors that came from extraneural origins. The brain microenvironment consists of multiple cell types including the most abundant glial cell, astrocytes. Astrocytes have very diverse and microenvironment-dependent morphologies; for a long time, this

> © The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The Role of Astrocytes in Tumor Growth

Emily Gronseth, Ling Wang, David R. Harder and

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Emily Gronseth, Ling Wang, David R. Harder

http://dx.doi.org/10.5772/intechopen.72720

**Progression**

Ramani Ramchandran

Abstract

intervention.

1. Introduction

and Ramani Ramchandran

and Progression

## **The Role of Astrocytes in Tumor Growth and Progression** The Role of Astrocytes in Tumor Growth and Progression

DOI: 10.5772/intechopen.72720

Emily Gronseth, Ling Wang, David R. Harder and Ramani Ramchandran Emily Gronseth, Ling Wang, David R. Harder

Additional information is available at the end of the chapter and Ramani Ramchandran

http://dx.doi.org/10.5772/intechopen.72720 Additional information is available at the end of the chapter

#### Abstract

[134] Manczak M, Kandimalla R, Fry D, et al. Protective effects of reduced dynamin-related protein 1 against amyloid beta-induced mitochondrial dysfunction and synaptic dam-

[135] Xie N, Wang C, Lian Y, et al. Inhibition of mitochondrial fission attenuates Aβ-induced

[136] Jang M, Jung S, Lee M, et al. Melatonin attenuates amyloid beta25-35-induced apoptosis

[137] Schitine C, Nogaroli L, Costa MR, et al. Astrocyte heterogeneity in the brain: From

in mouse microglial BV2 cells. Neuroscience Letters. 2005;**380**(1-2):26-31

development to disease. Frontiers in Cellular Neuroscience. 2015;**9**:76

age in Alzheimer's disease. Human Molecular Genetics. 2016;**25**(23):5148-5166

microglia apoptosis. Neuroscience. 2014;**256**:36-42

162 Astrocyte - Physiology and Pathology

Current research is continually implicating the importance of astrocytes as active participants in neurological injury, disease, and tumor progression. This chapter will discuss some of these emerging concepts, especially as they relate to tumor biology. Astrocytes themselves can become tumorigenic, such as the case in gliomas, which often have aberrant signaling in key regulating genes of astrocyte development. Astrocytes secrete factors that maintain the tight junctions of the blood brain barrier (BBB), which in turn regulates the success or failure of metastatic cells extravasating into the brain. This astrocytic association with the brain vasculature also promotes brain tumor stem cell characteristics, which are known to be necessary for tumor initiation. Tumor cells within the brain make direct contacts with astrocytes through gap junctions, which subsequently lead to increased chemoresistance of the tumor cells. Astrocytes have also been shown to effect tumors cells via secretion of degradative enzymes, cytokines, chemokines, and growth factors, all of which have been shown to promote tumor cell proliferation, survival, and invasion. Thus, research in astrocyte biology and the role of astrocytes in the tumor microenvironment has and will likely continue to reveal novel targets for cancer intervention.

Keywords: astrocytes, metastasis, blood-brain barrier, reactive astrogliosis, cancer

## 1. Introduction

The tumor microenvironment plays a critical role in tumor progression. Tumors within the central nervous system (CNS) include primary brain tumors originating from a CNS resident cell, or secondary tumors that came from extraneural origins. The brain microenvironment consists of multiple cell types including the most abundant glial cell, astrocytes. Astrocytes have very diverse and microenvironment-dependent morphologies; for a long time, this

© 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

structural contribution was considered their main purpose. Present in gray matter, protoplasmic astrocytes are the most common types of astrocytes and are stellate in nature with branching processes or "endfeet" [1]. These endfeet make important contacts with neurons and other cells within the brain microenvironment. Importantly for this chapter, one of these interfaces, which we will discuss further, is the astrocyte endfeet connections made with endothelial cells and pericytes, commonly referred to as the blood brain barrier (BBB). This barrier allows for select metabolites to enter and toxic waste to exit the brain.

The term "glial cells" describes a broader group of cells including astrocytes, ependymal cells and oligodendrocytes, not all gliomas are specifically astrocytic in nature. Gliomas which are thought to originate or histologically resemble astrocytes include astrocytomas, mixed gliomas or oligoastrocytomas, diffuse intrinsic pontine gliomas [10], and high grade astrocytomas called glioblastoma multiforme (GBM). There are also several types of mixed neuronal-glial tumors. These tumors are extremely heterogeneous, differing in histology, location in the brain, molecular biology, karyotype, age of onset, and survival prognosis of the patient. Gliomas share many characteristics with astrocytes, particularly activated astrocytes, which will be discussed in a later section of this chapter. Some of these include migration capabilities, growth factor expression pattern, stem cell-like characteristics, and the ability for anchorage-independent

The Role of Astrocytes in Tumor Growth and Progression

http://dx.doi.org/10.5772/intechopen.72720

165

In general, cellular origins of the previously mentioned gliomas are astrocyte precursor cells. However, the diversity in the distinct molecular/genetic alterations of the tumors suggests that different stages or types of precursor cells have different sensitivities to specific genetic mutations. One of the most notable genetic signatures of GBM is EGFR amplification and overexpression which, as previously mentioned, is also involved in regulating astrocyte differentiation [14–17]. There have been several mechanisms associating EGFR overexpression with astrocyte tumor malignancy. Several known ligands of EGFR, including EGF and transforming growth factor-α (TGF-α), promote proliferation of astrocytes and astrocyte precursor cells, thus contributing to the malignancy of the tumors [14, 18, 19]. Additionally, cell cycle regulators such as Rb, p53 and CDKN2A are commonly mutated and inactivated in low grade gliomas and GBM, [15, 20–23]. Mutations in isocitrate dehydrogenase (IDH)-1 and -2 are also extremely common, but only in certain gliomas; they are present in 70% of grade II and III astrocytomas and oligodendrogliomas, as well as secondary GBMs, but are rare in primary GBMs [15, 24].

As will be discussed in more detail throughout this chapter, astrocytes are very heterogeneous in regards to function and influence on tumors within the CNS. This fact, combined with the heterogeneity that encompasses the transformation of cells results in unique tumor genotypes and phenotypes, plus many other contextual factors, equates to interactions that are both tumor promoting and tumor suppressive (Figure 1). Arguably, there is more evidence suggesting how astrocytes can be tumor promoting, which will be covered in this section. Some functions of astrocytes that are known within the literature to be tumor promoting to both primary and

One of the critical steps in the life time of tumor progression is tumor metastasis, especially brain metastasis. This step results in catastrophic consequences from a patient perspective. The metastases from extraneural tumors in the brain are actually the most common sources of

growth which is correlated with invasiveness of a tumor [11–13].

3. The role of astrocytes in tumor growth and progression

metastatic brain tumors are summarized and illustrated (Figure 2).

tumors in the CNS, as shown in Table 1 [25, 26].

3.1. Metastatic tumors: Interactions with astrocytes at the blood-brain barrier

Homeostasis in the brain is of utmost importance to maintain neural function and prevent potentially detrimental immune responses from occurring. An invading tumor cell normally encounters enormous barriers before it can colonize the brain.  These and other mechanisms are in place to thwart tumor cell entrance, however, in some cases these mechanisms are either not adequate to prevent tumor cell invasion, or exploited to aid in tumor cell extravasation into the brain. In addition to regulating brain metastases, astrocytes, which develop from neural stem cells (NSCs), can become transformed and undergo developmental dysregulation due to aberrant gene activation, resulting in various types of brain tumors, including gliomas.

In this chapter, we will further discuss autocrine, paracrine, and juxtacrine mechanisms in which astrocytes influence surrounding cells in the brain microenvironment and tumor progression within the CNS. We will discuss the underlying mechanisms that regulate these processes, and provide examples of possible interventions that could eventually be translated into successful clinical treatment for patients.

## 2. Primary tumors of astrocytic origin

We begin this chapter by understanding how astrocytes themselves may become transformed and discuss the key features of these types of tumors. The cellular origin of many brain tumors can be traced back to multipotent NSCs, which are able to self-renew and differentiate into all subtypes of mature neurons and glial cells. However, many tumors with more distinct cellular origins exist along the glial cell differentiation axis, and are traced back to more restricted and differentiated astrocyte progeny [2]. During development of mature astrocytes, NSCs first partially differentiate into neuronal precursor cells where, in the presence of specific growth factors and their cognate receptors, they differentiate into various cellular lineages [3]. Early astrocyte precursors are characterized by their expression of fibroblast growth factor receptor (FGFR), nestin, and epidermal growth factor receptor (EGFR), while mature astrocytes express markers such as glutamate aspartate transporter (GLAST), FGFR3, S100β and glial fibrillary acidic protein (GFAP) [4–7]. These astrocyte precursor cells are perhaps most vulnerable to transformation, and depending on the stage of these cells, fatal adult primary brain tumors (gliomas) may arise. Because mature astrocytes maintain their ability to proliferate throughout adulthood (an uncommon characteristic of many CNS cells), it is hypothesized that this is a contributing reason for why astrocytic tumors are so common overall and most common in adults [8, 9].

The term "glial cells" describes a broader group of cells including astrocytes, ependymal cells and oligodendrocytes, not all gliomas are specifically astrocytic in nature. Gliomas which are thought to originate or histologically resemble astrocytes include astrocytomas, mixed gliomas or oligoastrocytomas, diffuse intrinsic pontine gliomas [10], and high grade astrocytomas called glioblastoma multiforme (GBM). There are also several types of mixed neuronal-glial tumors. These tumors are extremely heterogeneous, differing in histology, location in the brain, molecular biology, karyotype, age of onset, and survival prognosis of the patient. Gliomas share many characteristics with astrocytes, particularly activated astrocytes, which will be discussed in a later section of this chapter. Some of these include migration capabilities, growth factor expression pattern, stem cell-like characteristics, and the ability for anchorage-independent growth which is correlated with invasiveness of a tumor [11–13].

structural contribution was considered their main purpose. Present in gray matter, protoplasmic astrocytes are the most common types of astrocytes and are stellate in nature with branching processes or "endfeet" [1]. These endfeet make important contacts with neurons and other cells within the brain microenvironment. Importantly for this chapter, one of these interfaces, which we will discuss further, is the astrocyte endfeet connections made with endothelial cells and pericytes, commonly referred to as the blood brain barrier (BBB). This

Homeostasis in the brain is of utmost importance to maintain neural function and prevent potentially detrimental immune responses from occurring. An invading tumor cell normally encounters enormous barriers before it can colonize the brain.  These and other mechanisms are in place to thwart tumor cell entrance, however, in some cases these mechanisms are either not adequate to prevent tumor cell invasion, or exploited to aid in tumor cell extravasation into the brain. In addition to regulating brain metastases, astrocytes, which develop from neural stem cells (NSCs), can become transformed and undergo developmental dysregulation due to aberrant

In this chapter, we will further discuss autocrine, paracrine, and juxtacrine mechanisms in which astrocytes influence surrounding cells in the brain microenvironment and tumor progression within the CNS. We will discuss the underlying mechanisms that regulate these processes, and provide examples of possible interventions that could eventually be translated

We begin this chapter by understanding how astrocytes themselves may become transformed and discuss the key features of these types of tumors. The cellular origin of many brain tumors can be traced back to multipotent NSCs, which are able to self-renew and differentiate into all subtypes of mature neurons and glial cells. However, many tumors with more distinct cellular origins exist along the glial cell differentiation axis, and are traced back to more restricted and differentiated astrocyte progeny [2]. During development of mature astrocytes, NSCs first partially differentiate into neuronal precursor cells where, in the presence of specific growth factors and their cognate receptors, they differentiate into various cellular lineages [3]. Early astrocyte precursors are characterized by their expression of fibroblast growth factor receptor (FGFR), nestin, and epidermal growth factor receptor (EGFR), while mature astrocytes express markers such as glutamate aspartate transporter (GLAST), FGFR3, S100β and glial fibrillary acidic protein (GFAP) [4–7]. These astrocyte precursor cells are perhaps most vulnerable to transformation, and depending on the stage of these cells, fatal adult primary brain tumors (gliomas) may arise. Because mature astrocytes maintain their ability to proliferate throughout adulthood (an uncommon characteristic of many CNS cells), it is hypothesized that this is a contributing reason for why astrocytic tumors are so common overall and most common in

barrier allows for select metabolites to enter and toxic waste to exit the brain.

gene activation, resulting in various types of brain tumors, including gliomas.

into successful clinical treatment for patients.

164 Astrocyte - Physiology and Pathology

2. Primary tumors of astrocytic origin

adults [8, 9].

In general, cellular origins of the previously mentioned gliomas are astrocyte precursor cells. However, the diversity in the distinct molecular/genetic alterations of the tumors suggests that different stages or types of precursor cells have different sensitivities to specific genetic mutations. One of the most notable genetic signatures of GBM is EGFR amplification and overexpression which, as previously mentioned, is also involved in regulating astrocyte differentiation [14–17]. There have been several mechanisms associating EGFR overexpression with astrocyte tumor malignancy. Several known ligands of EGFR, including EGF and transforming growth factor-α (TGF-α), promote proliferation of astrocytes and astrocyte precursor cells, thus contributing to the malignancy of the tumors [14, 18, 19]. Additionally, cell cycle regulators such as Rb, p53 and CDKN2A are commonly mutated and inactivated in low grade gliomas and GBM, [15, 20–23]. Mutations in isocitrate dehydrogenase (IDH)-1 and -2 are also extremely common, but only in certain gliomas; they are present in 70% of grade II and III astrocytomas and oligodendrogliomas, as well as secondary GBMs, but are rare in primary GBMs [15, 24].

## 3. The role of astrocytes in tumor growth and progression

As will be discussed in more detail throughout this chapter, astrocytes are very heterogeneous in regards to function and influence on tumors within the CNS. This fact, combined with the heterogeneity that encompasses the transformation of cells results in unique tumor genotypes and phenotypes, plus many other contextual factors, equates to interactions that are both tumor promoting and tumor suppressive (Figure 1). Arguably, there is more evidence suggesting how astrocytes can be tumor promoting, which will be covered in this section. Some functions of astrocytes that are known within the literature to be tumor promoting to both primary and metastatic brain tumors are summarized and illustrated (Figure 2).

#### 3.1. Metastatic tumors: Interactions with astrocytes at the blood-brain barrier

One of the critical steps in the life time of tumor progression is tumor metastasis, especially brain metastasis. This step results in catastrophic consequences from a patient perspective. The metastases from extraneural tumors in the brain are actually the most common sources of tumors in the CNS, as shown in Table 1 [25, 26].

Figure 1. Research shows astrocytes have functions that result in tumor supporting and tumor suppressing mechanisms, and sometimes both. Like many physiological responses, this demonstrates the context dependent balancing act that occurs when homeostasis is breached, and the many factors that play a role in tipping the balance one way or another.

The process of metastasis, in brief, involves invasion of a tumor cell away from the tumor to a blood vessel, entry into and survival in the blood circulation, extravasation from the blood vessel into the secondary organ, and survival, engraftment, and proliferation into a secondary tumor. Extravasating into the brain provides an added challenge: that of getting past the BBB. The most functionally important component of the BBB are the tight junctions held between brain microvascular endothelial cells. Thus, substances that get into the brain parenchyma are tightly controlled. Both para-cellular and trans-cellular diffusion are low; most solutes that get in and out through the BBB, such as glucose and other nutrients, do so through transporters expressed on endothelial cells [27–29]. Despite this added barrier, many extraneural tumors have a strong tendency to metastasize to the brain. To note, there are regions within the brain that lack BBB, and could also be a potential avenue of metastasis [30]. The recent discovery of brain lymphatics is also suggestive of an alternative route given the already known function of lymphatics to carry tumor cells [31].

Breast cancer, melanoma, and lung cancer are three tumor types that show proclivity to go to the brain.  Interestingly, specific subtypes of these tumors have a much higher frequency of brain metastases, including non-small cell lung cancer (NSLC), triple negative breast cancer cells that are estrogenic receptor (ER), progesterone receptor (PR) and epidermal growth factor receptor-2 (HER2) (ER, PR, HER2), and HER2-enriched (HER2+ ) breast cancer cells [32–35]. One theory behind a specific tumors' proclivity to the brain is explained by Paget's seed and soil hypothesis, which suggests that for a seed (tumor cell) to take up in a soil (brain), it must adapt itself and make changes that will favor the soil [26, 36]. In support of this idea, genes associated with breast cancer metastasis to brain have been discovered and efforts continue to identify new targets for lung cancer cell metastasis to the brain and for other cancers as well [37–40]. However, the question raised by the

seed-soil hypothesis is whether the soil influences the seed, and if so, how it is accomplished. One could argue that in the first place, the soil is influenced by the seed. Therefore, in this "circular logic," both seed and soil appear to contribute together, ultimately, for the growth of the tumor cell. Astrocytes are vital to the development and maintenance of the BBB, therefore understanding their role in this process is necessary to understand how they also may influence brain metastatic tumor cells attempting to breach the BBB. The tight junctions between the endothelial cells of the BBB are comprised of many junctional proteins, notably claudin-5 and occludin [41, 42]. Vascular endothelial (VE)-cadherin is also of importance within adherens junctions which associate with tight junctions, as well as cytoplasmic scaffolding proteins such as zonula occludens (ZO)-1 and -2 [27, 43–45]. In normal conditions, the BBB homeostasis and junctional complexes are partially supported at the structural and physiological functional levels by astrocytes. Astrocytes contact

been shown to regulate CNS immune suppression, weakening the innate tumor killing response of the body.

Figure 2. Many signaling mechanisms have been identified that implicate astrocytes in tumor progression. Some of these functions include secretion of factors that have been shown to increase invasiveness and malignancy of established brain tumors, in addition to factors that enable brain metastasizing cells to enter the brain. Astrocytes have also been shown to maintain the vascular niche in the brain which can promote stem-like characteristics in BTSCs. Astrocytes directly interact with tumor cells and communicate via gap junctions, leading to increased intracellular calcium and resistance to treatments. Indirect or paracrine communication with surrounding cells often occurs via secretion of exosomes known to carry miRNAs to target key tumor suppressor genes in tumor cells and surrounding microenvironment. Lastly, astrocytes have

The Role of Astrocytes in Tumor Growth and Progression

http://dx.doi.org/10.5772/intechopen.72720

167

The process of metastasis, in brief, involves invasion of a tumor cell away from the tumor to a blood vessel, entry into and survival in the blood circulation, extravasation from the blood vessel into the secondary organ, and survival, engraftment, and proliferation into a secondary tumor. Extravasating into the brain provides an added challenge: that of getting past the BBB. The most functionally important component of the BBB are the tight junctions held between brain microvascular endothelial cells. Thus, substances that get into the brain parenchyma are tightly controlled. Both para-cellular and trans-cellular diffusion are low; most solutes that get in and out through the BBB, such as glucose and other nutrients, do so through transporters expressed on endothelial cells [27–29]. Despite this added barrier, many extraneural tumors have a strong tendency to metastasize to the brain. To note, there are regions within the brain that lack BBB, and could also be a potential avenue of metastasis [30]. The recent discovery of brain lymphatics is also suggestive of an alternative route given the already known function of

Figure 1. Research shows astrocytes have functions that result in tumor supporting and tumor suppressing mechanisms, and sometimes both. Like many physiological responses, this demonstrates the context dependent balancing act that occurs when homeostasis is breached, and the many factors that play a role in tipping the balance one way or another.

Breast cancer, melanoma, and lung cancer are three tumor types that show proclivity to go to the brain.  Interestingly, specific subtypes of these tumors have a much higher frequency of brain metastases, including non-small cell lung cancer (NSLC), triple negative breast cancer cells that are estrogenic receptor (ER), progesterone receptor (PR) and epidermal growth factor receptor-2 (HER2) (ER, PR,

tumors' proclivity to the brain is explained by Paget's seed and soil hypothesis, which suggests that for a seed (tumor cell) to take up in a soil (brain), it must adapt itself and make changes that will favor the soil [26, 36]. In support of this idea, genes associated with breast cancer metastasis to brain have been discovered and efforts continue to identify new targets for lung cancer cell metastasis to the brain and for other cancers as well [37–40]. However, the question raised by the

) breast cancer cells [32–35]. One theory behind a specific

lymphatics to carry tumor cells [31].

166 Astrocyte - Physiology and Pathology

HER2), and HER2-enriched (HER2+

Figure 2. Many signaling mechanisms have been identified that implicate astrocytes in tumor progression. Some of these functions include secretion of factors that have been shown to increase invasiveness and malignancy of established brain tumors, in addition to factors that enable brain metastasizing cells to enter the brain. Astrocytes have also been shown to maintain the vascular niche in the brain which can promote stem-like characteristics in BTSCs. Astrocytes directly interact with tumor cells and communicate via gap junctions, leading to increased intracellular calcium and resistance to treatments. Indirect or paracrine communication with surrounding cells often occurs via secretion of exosomes known to carry miRNAs to target key tumor suppressor genes in tumor cells and surrounding microenvironment. Lastly, astrocytes have been shown to regulate CNS immune suppression, weakening the innate tumor killing response of the body.

seed-soil hypothesis is whether the soil influences the seed, and if so, how it is accomplished. One could argue that in the first place, the soil is influenced by the seed. Therefore, in this "circular logic," both seed and soil appear to contribute together, ultimately, for the growth of the tumor cell.

Astrocytes are vital to the development and maintenance of the BBB, therefore understanding their role in this process is necessary to understand how they also may influence brain metastatic tumor cells attempting to breach the BBB. The tight junctions between the endothelial cells of the BBB are comprised of many junctional proteins, notably claudin-5 and occludin [41, 42]. Vascular endothelial (VE)-cadherin is also of importance within adherens junctions which associate with tight junctions, as well as cytoplasmic scaffolding proteins such as zonula occludens (ZO)-1 and -2 [27, 43–45]. In normal conditions, the BBB homeostasis and junctional complexes are partially supported at the structural and physiological functional levels by astrocytes. Astrocytes contact

3.2. Astrocytes' direct cell-cell interactions with tumor cells

Astrocytes have multiple primary and branching endfeet which expand and contract, allowing them to dynamically contact both synapses and the microvasculature. Also, astrocytes regulate communication between neuronal networks and glial-vascular coupling by forming independent contact network [58–61]. Therefore, it has been widely accepted that astrocytes directly contact and communicate with neurons to regulate neuronal function at the synaptic and network levels, which provides a significant impact on physiological and pathological state of the CNS. Subsequently, direct interactions with astrocytes and tumor cells, often in the form of gap junctions, has

The Role of Astrocytes in Tumor Growth and Progression

http://dx.doi.org/10.5772/intechopen.72720

169

As discussed, gliomas are the most lethal primary intracranial tumors. The proliferative dysfunction and invasion of gliomas are associated with changes in gap junction communication [62, 63]. In metastatic brain tumors, reactive astrocytes protect melanoma cells from chemotherapy induced cell death by sequestering intracellular calcium through gap junctions [64]. In the brain, metastases from breast and lung cancer show upregulation of many survival genes which is dependent on the direct contact through gap junctions between the astrocytes and tumor cells, which was found to be causal for developing resistance [65]. These data suggest that reactive astrocytes participate in tumor progression and chemo-resistance by their direct

Gap junctions are efficient tools for intercellular communication. In astrocytes, they are composed of connexins 30 and 43 (Cx30, Cx43) [66]. Cx43 is widely expressed in adult astrocytes and exhibits increased expression in reactive astrocytes induced by various brain pathologies and intercellular calcium signaling [67–73]. Also, Cx43-mediated intercellular communication between astrocytes plays an important role in the invasion of glioma cells in the brain [63]. A recent study has also revealed that breast and lung cancer cells express proto-cadherin 7 (PCDH7) to promote tumor-astrocyte gap junction formation by recruiting Cx43, which allows the transfer of cGAMP from tumor cells to astrocytes to trigger the secretion of inflammatory

3.3. Astrocytes' secretome and paracrine signaling mechanisms that influence tumor cells

Astrocytes can synthesize a host of biologically interesting growth factors and cytokines. Previous studies have shown that sphingosine-1-phosphate (S1P), which shows the highest expression in the brain and is only expressed by astrocytes, induces cell motility in GBM cell lines that express S1P receptor-1 and S1P receptor-3 [75, 76]. Other neurotrophic factors secreted by astrocytes, such as TGF-α, C-X-C motif chemokine 12 (CXCL12), and GDNF, have also revealed the potential to increase the invasive capacity of GBM cells [77, 78]. In brain metastatic tumors, an early study found that metastatic MDA-MB-435 breast cancer cells, when cultured with astrocyte conditioned media, exhibit better growth in response to the conditioned medium. However, the growth-stimulatory effect was partially reversed by anti-IL-6, anti-TGF-β, and anti-insulin like growth factor-1 (IGF-I) antibodies [79]. Another study showed that reactive astrocytes expressed phosphorylated platelet-derived growth factor receptor β at

also been discovered to be significant for tumor progression and resistance to therapy.

physical contacts and gap junctional communication with tumor cells in the brain.

cytokines, which further promote tumor growth and chemo-resistance [74].

3.3.1. Cytokines and growth factors

Table 1. Primary and metastatic CNS tumors with their respective incidence rates per 100,000 persons. All tumors are accounting for all ages, except embryonal tumors which only includes persons' age 0–19 in the population study.

brain endothelial cells via their end feet processes. This contact was shown to maintain BBB permeability characteristics. However, it was later found that secreted factors alone in astrocyteconditioned media also upheld the tight junction characteristics in endothelial cells, demonstrating the importance of both astrocyte contact and paracrine actions in BBB function [46–48]. For example, Sonic hedgehog (Shh), an important developmental signaling protein, is known to be secreted by astrocytes and bind its cognate receptor, Patched-1, expressed at the cell membrane of brain endothelial cells. This induces a signaling cascade mediated by β-catenin that bolsters tight junctions by increasing expression of occludin [49, 50]. Other proteins secreted by astrocytes that regulate and maintain tight junctions of brain ECs (most often by increased gene expression of junctional proteins) include angiotensin-1, FGF, TGF-β, glia derived neurotropic factor (GDNF), and retinoic acid (RA) [51–55].

Just as astrocytes are important for maintaining the BBB in homeostatic conditions, astrocytes also play key roles when the BBB is disrupted, which can occur during extravasation of tumor cells metastasizing to the brain. Several groups utilized mouse models of melanoma, lung, and breast cancer combined with histological and fluorescent imaging modalities to visualize very early interactions with tumor cells and the BBB. This work demonstrates that tumor cells first arrest in the brain capillaries, often at branch points [56, 57]. Lorger et al. (2010) show that very early on, astrocytes become activated and associate around vasculature in the brain where breast tumor cells are present, but in some cases have not yet extravasated or visibly altered the BBB [56]. This suggests that signals secreted by tumor cells are reaching astrocytes either directly or indirectly through the endothelial cells; a topic that requires further investigation. This association of tumor cells with reactive astrocytes persists throughout metastases formation, characterized by increased astrocyte expression of GFAP, nestin, and matrix metalloproteinase 9 (MMP-9), all of which aid in tumor extravasation mechanisms that will be discussed in later sections.

#### 3.2. Astrocytes' direct cell-cell interactions with tumor cells

Astrocytes have multiple primary and branching endfeet which expand and contract, allowing them to dynamically contact both synapses and the microvasculature. Also, astrocytes regulate communication between neuronal networks and glial-vascular coupling by forming independent contact network [58–61]. Therefore, it has been widely accepted that astrocytes directly contact and communicate with neurons to regulate neuronal function at the synaptic and network levels, which provides a significant impact on physiological and pathological state of the CNS. Subsequently, direct interactions with astrocytes and tumor cells, often in the form of gap junctions, has also been discovered to be significant for tumor progression and resistance to therapy.

As discussed, gliomas are the most lethal primary intracranial tumors. The proliferative dysfunction and invasion of gliomas are associated with changes in gap junction communication [62, 63]. In metastatic brain tumors, reactive astrocytes protect melanoma cells from chemotherapy induced cell death by sequestering intracellular calcium through gap junctions [64]. In the brain, metastases from breast and lung cancer show upregulation of many survival genes which is dependent on the direct contact through gap junctions between the astrocytes and tumor cells, which was found to be causal for developing resistance [65]. These data suggest that reactive astrocytes participate in tumor progression and chemo-resistance by their direct physical contacts and gap junctional communication with tumor cells in the brain.

Gap junctions are efficient tools for intercellular communication. In astrocytes, they are composed of connexins 30 and 43 (Cx30, Cx43) [66]. Cx43 is widely expressed in adult astrocytes and exhibits increased expression in reactive astrocytes induced by various brain pathologies and intercellular calcium signaling [67–73]. Also, Cx43-mediated intercellular communication between astrocytes plays an important role in the invasion of glioma cells in the brain [63]. A recent study has also revealed that breast and lung cancer cells express proto-cadherin 7 (PCDH7) to promote tumor-astrocyte gap junction formation by recruiting Cx43, which allows the transfer of cGAMP from tumor cells to astrocytes to trigger the secretion of inflammatory cytokines, which further promote tumor growth and chemo-resistance [74].

#### 3.3. Astrocytes' secretome and paracrine signaling mechanisms that influence tumor cells

#### 3.3.1. Cytokines and growth factors

brain endothelial cells via their end feet processes. This contact was shown to maintain BBB permeability characteristics. However, it was later found that secreted factors alone in astrocyteconditioned media also upheld the tight junction characteristics in endothelial cells, demonstrating the importance of both astrocyte contact and paracrine actions in BBB function [46–48]. For example, Sonic hedgehog (Shh), an important developmental signaling protein, is known to be secreted by astrocytes and bind its cognate receptor, Patched-1, expressed at the cell membrane of brain endothelial cells. This induces a signaling cascade mediated by β-catenin that bolsters tight junctions by increasing expression of occludin [49, 50]. Other proteins secreted by astrocytes that regulate and maintain tight junctions of brain ECs (most often by increased gene expression of junctional proteins) include angiotensin-1, FGF, TGF-β, glia derived neurotropic factor

Table 1. Primary and metastatic CNS tumors with their respective incidence rates per 100,000 persons. All tumors are accounting for all ages, except embryonal tumors which only includes persons' age 0–19 in the population study.

0.62 (ages 0–19 only) [198]

CNS tumor type Incidence rate (per 100,000 persons) (all ages) References All brain metastases 8.3/11.1/14.3 [195–197]

[32]

[32]

[32]

metastases)

metastases)

metastases)

Primary malignant CNS tumors 7.2 [198] GBM 3.2 [198] Nerve sheath tumors 1.82 [198] Other astrocytomas 1.2 [198] CNS lymphoma 0.43 [198]

Lung cancer brain metastases ~ 3.2–8(estimation based on 39–56% of all brain

Breast cancer brain metastases ~ 1.1–4.3 (estimation based on 13–30% of all brain

Melanoma brain metastases ~ 0.5–1.6 (estimation based on 6–11% of all brain

Just as astrocytes are important for maintaining the BBB in homeostatic conditions, astrocytes also play key roles when the BBB is disrupted, which can occur during extravasation of tumor cells metastasizing to the brain. Several groups utilized mouse models of melanoma, lung, and breast cancer combined with histological and fluorescent imaging modalities to visualize very early interactions with tumor cells and the BBB. This work demonstrates that tumor cells first arrest in the brain capillaries, often at branch points [56, 57]. Lorger et al. (2010) show that very early on, astrocytes become activated and associate around vasculature in the brain where breast tumor cells are present, but in some cases have not yet extravasated or visibly altered the BBB [56]. This suggests that signals secreted by tumor cells are reaching astrocytes either directly or indirectly through the endothelial cells; a topic that requires further investigation. This association of tumor cells with reactive astrocytes persists throughout metastases formation, characterized by increased astrocyte expression of GFAP, nestin, and matrix metalloproteinase 9 (MMP-9), all of which aid in tumor extravasation mechanisms that will be discussed in later sections.

(GDNF), and retinoic acid (RA) [51–55].

Embryonal tumors (medulloblastoma, ATRT, and

168 Astrocyte - Physiology and Pathology

PNET)

Astrocytes can synthesize a host of biologically interesting growth factors and cytokines. Previous studies have shown that sphingosine-1-phosphate (S1P), which shows the highest expression in the brain and is only expressed by astrocytes, induces cell motility in GBM cell lines that express S1P receptor-1 and S1P receptor-3 [75, 76]. Other neurotrophic factors secreted by astrocytes, such as TGF-α, C-X-C motif chemokine 12 (CXCL12), and GDNF, have also revealed the potential to increase the invasive capacity of GBM cells [77, 78]. In brain metastatic tumors, an early study found that metastatic MDA-MB-435 breast cancer cells, when cultured with astrocyte conditioned media, exhibit better growth in response to the conditioned medium. However, the growth-stimulatory effect was partially reversed by anti-IL-6, anti-TGF-β, and anti-insulin like growth factor-1 (IGF-I) antibodies [79]. Another study showed that reactive astrocytes expressed phosphorylated platelet-derived growth factor receptor β at tyrosine 751 (p751-PDGFRβ). Pazopanib, an inhibitor of PDGFRs, inhibited the activation of p-PDGFR expressing astrocytes, and thus prevented brain metastasis formation in the HER2 transfected MDA-MB-231 breast cancer cells [80]. Taken together, this work demonstrates that paracrine signaling by astrocyte secreted cytokines and growth factors facilitates tumor metastasis formation in the brain.

tumor biology. Exosomes are released from cells by fusion of multi-vesicular structures with the plasma membrane through the process of exocytosis [97]. Exosomes are a general mode of intercellular communication and can interact with neighboring cells, thus mediating signals

The Role of Astrocytes in Tumor Growth and Progression

http://dx.doi.org/10.5772/intechopen.72720

171

Among RNA cargo, microRNA (miRNA) transcripts, specifically miR-26a, is highly expressed in astrocytes and is present in astrocyte-derived exosomes [100, 101]. MiR-26a targets mRNAs that impact neuronal function and morphology, and was first implicated in many neuronal disorders [102–104]. Moreover, miR-26a can be sorted to exosomes and transported by these vesicles in the plasma, serum, whole blood, urine, or secreted in vitro by human umbilical vein endothelial cells [105–109]. In addition, miR-26a autonomously regulates primary gliomas by increasing de novo tumor formation and radiosensitivity through targeting of suppressor phosphatase and tensin homolog (PTEN) and ataxia-telangiectasia mutated (ATM), respectively [110, 111]. Therefore, it is plausible that miR-26a in astrocyte-derived exosomes may function to regulate the surrounding tumor environment. A recent study found that primary breast tumor cells express tumor suppressor PTEN, however this expression of PTEN was lost reversibly after tumor cells metastasized into the brain [112]. Astrocyte-derived exosomal miR-19a reversibly mediated the downregulation of PTEN expression in cancer cells, thus providing one mechanism for loss of PTEN in tumor cells that enter the brain. Further, miR-19a also increased C-C motif chemokine ligand 2 (CCL2) secretion and recruitment of myeloid cells,

between astrocytes and other cells in the brain microenvironment [98, 99].

thus facilitating changes in the brain microenvironment to promote metastasis [112].

An important attribute of brain tumor biology regarding tumor initiation and propagation is the existence of brain tumor stem cells (BTSCs). These cells have been found to, in many ways, resemble adult NSCs that exist in distinct regions of the brain, including the subventricular zone (SVZ) and the subgranular zone (SGZ) [113, 114]. Many groups identified CD133, Nestin, and sex determining region Y-box 2 (SOX2) as markers to isolate NSCs which maintain the essential properties of stem cells (self-renewal and ability to differentiate into multiple progeny) [115–118]. Using the NSC neurosphere culturing method, CD133 and/or CD15 have also been found to be expressed on BTSCs from GBM, medulloblastoma, ependymoma, and astrocytoma tumors [118– 120]. Interestingly, Singh et al. (2003) found CD133+ cells to be tumor initiating, whereas CD133 cells could not initiate a tumor or self-renew in a mouse xenograft model [118, 121]. The levels of CD133+ BTSCs has since been correlated to negative prognoses in gliomas, and have been found to be particularly enriched in recurrent tumors after radiation and chemotherapy [122–124]. These findings highlight the importance of stem cells in the overall initiation, malignancy, and

It is clear that BTSCs play an important role in the progression of all brain tumors. Therefore, cells in the microenvironment that influence BTSCs are of interest from a clinical therapy perspective. Interestingly, astrocytes seem to affect normal NSCs and BTSCs quite differently.

4. The role of astrocytes in brain tumor stem cell biology

recurrence of brain tumors.

4.1. Astrocytes' direct influence on cancer stem cells

#### 3.3.2. Extracellular matrix (ECM) proteins and degradative enzymes

ECM proteins are important participants in the tumorigenic process, as they are involved in not only the physical adhesion and migration of tumors cells, but also the regulation of intracellular signaling. The brain parenchyma is high in proteoglycans, glycoproteins, and matricellular proteins, all of which astrocytes express and secrete [4]. Specifically, some astrocyte secreted matricellular proteins have been studied in regulation of various brain tumors, including secreted protein acidic and rich in cysteine (SPARC) and CYR61/CTGF/NOV (CCN). Both SPARC and CCN2 have been shown to be secreted by activated astrocytes proximal to brain tumors or injuries [81, 82]. While increases in CCN2 secretion have been correlated with negative glioblastoma outcomes, expression of SPARC and its effect on tumor cells is tumor dependent. In gliomas and astrocytomas, tumor secretion of SPARC promotes invasion, angiogenesis, and a negative prognosis [83, 84]; however medulloblastoma tumor cells have increased loss of SPARC, which when rescued induces cell cycle arrest, neuronal differentiation, and limits radioresistant DNA damage response [85–87].

Previously, we discussed MMP-related mechanisms in which astrocytes assist tumor cells in extravasating the BBB. The secretion of these matrix degrading enzymes also supports brain tumor progression by breaking down the barriers induced by the ECM. Heparanase degrades the glycosaminoglycan side chains of heparan sulfate proteoglycans, which are essential and ubiquitous macromolecules associated with the cell surface [88–90]. Reactive astrocytes have been frequently found in areas surrounding melanoma-related lesions and produce nerve growth factor (NGF), the prototypic neurotrophin [91]. Neurotrophins can stimulate heparanase production in astrocytes and thus contribute to the brain colonization of melanoma cells [88].

MMP-2 and -9 have been observed in secretory vesicles in astrocytes [92]. Stimulation of astrocytes with lipopolysaccharide, IL1-α, IL1-β, or TNF-α induces MMP-2 and -9 secretion [93]. MMP-9 also promotes the growth of primary brain tumors by releasing vascular endothelial growth factor (VEGF) sequestered in the surrounding matrix [94]. The expression of MMP-9 was up-regulated in reactive astrocytes, which was involved in the brain metastases of MDA-MB-435 cells [56]. Moreover, both MMP-2 and -9 secreted by astrocytes contribute to breast cancer MDA-MB-231 cell invasion and brain metastases [95]. In addition to secreting MMPs themselves, astrocytes can also induce tumor cells to secrete MMPs, as shown by Mendes et al. (2007), where they found breast cancer cells to secrete significantly more MMP-2 in the presence of astrocyte conditioned media, aiding metastasis to the brain [96].

#### 3.3.3. Exosomes

The topic of exosomes, which are endosome-derived microvesicles between 50 and 100 nm in size that carry specific protein and RNA cargo, has become a subject of intense interest in tumor biology. Exosomes are released from cells by fusion of multi-vesicular structures with the plasma membrane through the process of exocytosis [97]. Exosomes are a general mode of intercellular communication and can interact with neighboring cells, thus mediating signals between astrocytes and other cells in the brain microenvironment [98, 99].

tyrosine 751 (p751-PDGFRβ). Pazopanib, an inhibitor of PDGFRs, inhibited the activation of p-PDGFR expressing astrocytes, and thus prevented brain metastasis formation in the HER2 transfected MDA-MB-231 breast cancer cells [80]. Taken together, this work demonstrates that paracrine signaling by astrocyte secreted cytokines and growth factors facilitates tumor metas-

ECM proteins are important participants in the tumorigenic process, as they are involved in not only the physical adhesion and migration of tumors cells, but also the regulation of intracellular signaling. The brain parenchyma is high in proteoglycans, glycoproteins, and matricellular proteins, all of which astrocytes express and secrete [4]. Specifically, some astrocyte secreted matricellular proteins have been studied in regulation of various brain tumors, including secreted protein acidic and rich in cysteine (SPARC) and CYR61/CTGF/NOV (CCN). Both SPARC and CCN2 have been shown to be secreted by activated astrocytes proximal to brain tumors or injuries [81, 82]. While increases in CCN2 secretion have been correlated with negative glioblastoma outcomes, expression of SPARC and its effect on tumor cells is tumor dependent. In gliomas and astrocytomas, tumor secretion of SPARC promotes invasion, angiogenesis, and a negative prognosis [83, 84]; however medulloblastoma tumor cells have increased loss of SPARC, which when rescued induces cell cycle arrest, neuronal differentia-

Previously, we discussed MMP-related mechanisms in which astrocytes assist tumor cells in extravasating the BBB. The secretion of these matrix degrading enzymes also supports brain tumor progression by breaking down the barriers induced by the ECM. Heparanase degrades the glycosaminoglycan side chains of heparan sulfate proteoglycans, which are essential and ubiquitous macromolecules associated with the cell surface [88–90]. Reactive astrocytes have been frequently found in areas surrounding melanoma-related lesions and produce nerve growth factor (NGF), the prototypic neurotrophin [91]. Neurotrophins can stimulate heparanase production in astrocytes and thus contribute to the brain colonization of melanoma cells [88].

MMP-2 and -9 have been observed in secretory vesicles in astrocytes [92]. Stimulation of astrocytes with lipopolysaccharide, IL1-α, IL1-β, or TNF-α induces MMP-2 and -9 secretion [93]. MMP-9 also promotes the growth of primary brain tumors by releasing vascular endothelial growth factor (VEGF) sequestered in the surrounding matrix [94]. The expression of MMP-9 was up-regulated in reactive astrocytes, which was involved in the brain metastases of MDA-MB-435 cells [56]. Moreover, both MMP-2 and -9 secreted by astrocytes contribute to breast cancer MDA-MB-231 cell invasion and brain metastases [95]. In addition to secreting MMPs themselves, astrocytes can also induce tumor cells to secrete MMPs, as shown by Mendes et al. (2007), where they found breast cancer cells to secrete significantly more MMP-2 in the presence of astrocyte

The topic of exosomes, which are endosome-derived microvesicles between 50 and 100 nm in size that carry specific protein and RNA cargo, has become a subject of intense interest in

tasis formation in the brain.

170 Astrocyte - Physiology and Pathology

3.3.2. Extracellular matrix (ECM) proteins and degradative enzymes

tion, and limits radioresistant DNA damage response [85–87].

conditioned media, aiding metastasis to the brain [96].

3.3.3. Exosomes

Among RNA cargo, microRNA (miRNA) transcripts, specifically miR-26a, is highly expressed in astrocytes and is present in astrocyte-derived exosomes [100, 101]. MiR-26a targets mRNAs that impact neuronal function and morphology, and was first implicated in many neuronal disorders [102–104]. Moreover, miR-26a can be sorted to exosomes and transported by these vesicles in the plasma, serum, whole blood, urine, or secreted in vitro by human umbilical vein endothelial cells [105–109]. In addition, miR-26a autonomously regulates primary gliomas by increasing de novo tumor formation and radiosensitivity through targeting of suppressor phosphatase and tensin homolog (PTEN) and ataxia-telangiectasia mutated (ATM), respectively [110, 111]. Therefore, it is plausible that miR-26a in astrocyte-derived exosomes may function to regulate the surrounding tumor environment. A recent study found that primary breast tumor cells express tumor suppressor PTEN, however this expression of PTEN was lost reversibly after tumor cells metastasized into the brain [112]. Astrocyte-derived exosomal miR-19a reversibly mediated the downregulation of PTEN expression in cancer cells, thus providing one mechanism for loss of PTEN in tumor cells that enter the brain. Further, miR-19a also increased C-C motif chemokine ligand 2 (CCL2) secretion and recruitment of myeloid cells, thus facilitating changes in the brain microenvironment to promote metastasis [112].

## 4. The role of astrocytes in brain tumor stem cell biology

An important attribute of brain tumor biology regarding tumor initiation and propagation is the existence of brain tumor stem cells (BTSCs). These cells have been found to, in many ways, resemble adult NSCs that exist in distinct regions of the brain, including the subventricular zone (SVZ) and the subgranular zone (SGZ) [113, 114]. Many groups identified CD133, Nestin, and sex determining region Y-box 2 (SOX2) as markers to isolate NSCs which maintain the essential properties of stem cells (self-renewal and ability to differentiate into multiple progeny) [115–118]. Using the NSC neurosphere culturing method, CD133 and/or CD15 have also been found to be expressed on BTSCs from GBM, medulloblastoma, ependymoma, and astrocytoma tumors [118– 120]. Interestingly, Singh et al. (2003) found CD133+ cells to be tumor initiating, whereas CD133 cells could not initiate a tumor or self-renew in a mouse xenograft model [118, 121]. The levels of CD133+ BTSCs has since been correlated to negative prognoses in gliomas, and have been found to be particularly enriched in recurrent tumors after radiation and chemotherapy [122–124]. These findings highlight the importance of stem cells in the overall initiation, malignancy, and recurrence of brain tumors.

#### 4.1. Astrocytes' direct influence on cancer stem cells

It is clear that BTSCs play an important role in the progression of all brain tumors. Therefore, cells in the microenvironment that influence BTSCs are of interest from a clinical therapy perspective. Interestingly, astrocytes seem to affect normal NSCs and BTSCs quite differently. While astrocyte secreted factors have been shown to promote neurogenesis of normal adult NSCs, astrocytes within the microenvironment of brain tumors have also been shown to promote stem-like characteristics in BTSCs and enrich the stem cell population, thus worsening the malignancy of such brain tumors [125–128]. GBM CD133<sup>+</sup> stem cells co-cultured both directly and indirectly with astrocytes show gene expression signatures known to be involved in GBM invasion and metastasis, such as a disintegrin and metalloproteinase domaincontaining protein 10 (ADAM10), hyaluronan synthase 2 (HAS2), and vascular cell adhesion molecule-1 (VCAM1). Interestingly, although there were many overlapping genes in conditions where astrocytes and tumor cells were and were not in direct contact, even the distinct gene expression changes in each condition were still related to tumor cell invasion. This emphasizes the role of astrocytes in GBM invasion, which is one of the most challenging traits of GBM [127]. Indeed, CD133<sup>+</sup> GBM cells were found to be more invasive, whereas CD133 GBM cells did not have the same gene expression and invasion changes [127]. Later, it was shown that indirect co-culture with CD133<sup>+</sup> GBM cells and astrocytes resulted in cytokine release from astrocytes that reduced radiosensitivity of the GBM cells; again, this same phenotype and crosstalk with astrocytes was absent in CD133 GBM cells [128]. Some of the astrocyte secreted cytokines that induced radioresistance include CXCL1, IL-4, IL-6, and CCL7 [128]. These differences suggest that cancer stem (or stem-like) cells signal differently with astrocytes compared to tumor cells lacking stem characteristics. The reverse effect, which is tumor stem cells influencing astrocytes has also been observed in GBM. GBM stem cells provide signals that block the expression of p53 in surrounding astrocytes [129]. P53 is a tumor suppressor often found mutated in many tumors [130], and is classically known for its function in DNA damage response. However, recently p53 has been shown to have non-autonomous cellular functions, particularly in the tumor microenvironment, by influencing secretion of proteins, including ECM proteins [129, 131, 132]. Thus, the interaction between astrocytes and BTSCs are bi-directional and influence each other's development.

and regulating NSC/BTSCs' survival and differentiation status [136–139]. Astrocytes also play a vital role within the PVN. First and foremost, they play an indirect but obvious role in the structural and chemical maintenance of endothelial cell-BBB phenotypes, as discussed earlier. Studies have shown that BTSCs maintain close proximity to angiogenic regions of the tumor microenvironment, providing evidence that these regions phenocopy the PVN within the SVZ/

The Role of Astrocytes in Tumor Growth and Progression

http://dx.doi.org/10.5772/intechopen.72720

173

5. Astrocytes as an immune regulator in the tumor microenvironment

The immune system within the CNS is tightly controlled. In addition to the BBB, there are other barriers that maintain the CNS as an immune-privileged system, including the blood-meningeal barriers and the blood-cerebrospinal fluid (CSF) barriers [141, 142]. During homeostasis, these barriers do not allow entry of pathogens or blood-borne immune cells. Only upon CNS injury do some of these cellular barriers become fenestrated to allow for immune cell entrance. Although microglia are thought to be the main regulator of immune responses within the brain, astrocytes (and other cells) also play key roles in this function [143]. A vast amount of work investigating the astrocyte function in either normal or activated state is often related to the regulation of the immune environment in the CNS, as shown in functional studies and astrocyte secretome studies, summarized well by Sofroniew et al. [144–147]. As suggested by Yang et al. (2013), the presence of classical immunological surface molecules, such as major histocompatibility (MHC) antigen and intercellular adhesion molecule-1 (ICAM-1) on astrocytes underlines their importance in CNS immune function [11, 148, 149]. We will discuss next how astrocytes control immune responses to invading tumor cells, and the immune-related concepts associated with

As mentioned in the discussion of astrocytes and tumor cell interactions at the BBB, there have been a few key studies observing the cellular events that take place when metastatic cells extravasate into the brain parenchyma [56, 57]. From these studies, it is known that astrocytes are the first to respond to extravasating metastatic tumor cells entering the brain, followed by microglia [56]. Regardless of whether a CNS tumor is primary or metastatic, microglia and astrocytes control the immune response; therefore, it is upon their activation that other immune cells, such as macrophages or lymphocytes, may infiltrate [144–146, 150]. Activated astrocytes secrete pro-inflammatory molecules such as CXCL12, CCI2, Il15, CCL8, and CXCL1, all of which are known to regulate recruitment, activation and proliferation of T-cells, B-cells,

As it is often seen with any local or systemic inflammation, CNS immune responses can often persist or be dysregulated by tumor cells to become pathogenic. Many of these signaling responses are mediated by astrocytes. For example, Valiente et al. [151] reported that astrocytes produce FasL and plasmin ligands as defense mechanisms to kill brain-invading tumor cells. In response, tumor cells secrete serpins, which thwart the lethal action of plasmin [151]. Thus, Fas-mediated tumor cell apoptosis is blocked, leading to tumor survival. Other cells, such as endothelial cells, in the brain

SGZ to provide enrichment signals to BTSCs [140].

5.1. Immune responses to tumor cell presence

or natural killer (NK) cells [144–146].

this process.

In addition to primary brain tumors, cancer stem cells of brain metastatic tumors are also influenced by astrocytes.  While MMP-1 allows for BBB tight junction and basement membrane degradation to aid brain metastasis, prostaglandins are able to activate astrocytes and subsequently increase astrocyte expression of CCL7, which was shown to significantly increase self-renewal and survival of breast cancer stem cells through increased expression of Nanog, a key stem cell regulator [133, 134]. This study provides evidence that astrocytes enrich breast cancer stem cells in brain metastases and aid in their ability to extravasate the BBB.

#### 4.2. Astrocytes as a part of the perivascular niche

Normal NSCs in the SVZ and SGZ are maintained by specialized vascular regions called the perivascular niche (PVN) [135]. The PVN consists of the endothelial cells lining the vasculature, as well as astrocytes, pericytes, macrophages, microglia, fibroblasts, and vascular smooth muscle cells. These cells function and signal together to maintain structure and provide signals to NSCs. Evidence exists which demonstrates the vital role endothelial cells play in maintaining and regulating NSC/BTSCs' survival and differentiation status [136–139]. Astrocytes also play a vital role within the PVN. First and foremost, they play an indirect but obvious role in the structural and chemical maintenance of endothelial cell-BBB phenotypes, as discussed earlier. Studies have shown that BTSCs maintain close proximity to angiogenic regions of the tumor microenvironment, providing evidence that these regions phenocopy the PVN within the SVZ/ SGZ to provide enrichment signals to BTSCs [140].

## 5. Astrocytes as an immune regulator in the tumor microenvironment

The immune system within the CNS is tightly controlled. In addition to the BBB, there are other barriers that maintain the CNS as an immune-privileged system, including the blood-meningeal barriers and the blood-cerebrospinal fluid (CSF) barriers [141, 142]. During homeostasis, these barriers do not allow entry of pathogens or blood-borne immune cells. Only upon CNS injury do some of these cellular barriers become fenestrated to allow for immune cell entrance. Although microglia are thought to be the main regulator of immune responses within the brain, astrocytes (and other cells) also play key roles in this function [143]. A vast amount of work investigating the astrocyte function in either normal or activated state is often related to the regulation of the immune environment in the CNS, as shown in functional studies and astrocyte secretome studies, summarized well by Sofroniew et al. [144–147]. As suggested by Yang et al. (2013), the presence of classical immunological surface molecules, such as major histocompatibility (MHC) antigen and intercellular adhesion molecule-1 (ICAM-1) on astrocytes underlines their importance in CNS immune function [11, 148, 149]. We will discuss next how astrocytes control immune responses to invading tumor cells, and the immune-related concepts associated with this process.

#### 5.1. Immune responses to tumor cell presence

While astrocyte secreted factors have been shown to promote neurogenesis of normal adult NSCs, astrocytes within the microenvironment of brain tumors have also been shown to promote stem-like characteristics in BTSCs and enrich the stem cell population, thus worsening the malignancy of such brain tumors [125–128]. GBM CD133<sup>+</sup> stem cells co-cultured both directly and indirectly with astrocytes show gene expression signatures known to be involved in GBM invasion and metastasis, such as a disintegrin and metalloproteinase domaincontaining protein 10 (ADAM10), hyaluronan synthase 2 (HAS2), and vascular cell adhesion molecule-1 (VCAM1). Interestingly, although there were many overlapping genes in conditions where astrocytes and tumor cells were and were not in direct contact, even the distinct gene expression changes in each condition were still related to tumor cell invasion. This emphasizes the role of astrocytes in GBM invasion, which is one of the most challenging traits of GBM [127]. Indeed, CD133<sup>+</sup> GBM cells were found to be more invasive, whereas CD133 GBM cells did not have the same gene expression and invasion changes [127]. Later, it was shown that indirect co-culture with CD133<sup>+</sup> GBM cells and astrocytes resulted in cytokine release from astrocytes that reduced radiosensitivity of the GBM cells; again, this same phenotype and crosstalk with astrocytes was absent in CD133 GBM cells [128]. Some of the astrocyte secreted cytokines that induced radioresistance include CXCL1, IL-4, IL-6, and CCL7 [128]. These differences suggest that cancer stem (or stem-like) cells signal differently with astrocytes compared to tumor cells lacking stem characteristics. The reverse effect, which is tumor stem cells influencing astrocytes has also been observed in GBM. GBM stem cells provide signals that block the expression of p53 in surrounding astrocytes [129]. P53 is a tumor suppressor often found mutated in many tumors [130], and is classically known for its function in DNA damage response. However, recently p53 has been shown to have non-autonomous cellular functions, particularly in the tumor microenvironment, by influencing secretion of proteins, including ECM proteins [129, 131, 132]. Thus, the interaction between astrocytes and BTSCs

In addition to primary brain tumors, cancer stem cells of brain metastatic tumors are also influenced by astrocytes.  While MMP-1 allows for BBB tight junction and basement membrane degradation to aid brain metastasis, prostaglandins are able to activate astrocytes and subsequently increase astrocyte expression of CCL7, which was shown to significantly increase self-renewal and survival of breast cancer stem cells through increased expression of Nanog, a key stem cell regulator [133, 134]. This study provides evidence that astrocytes enrich breast cancer stem cells in brain metastases and aid in their ability to

Normal NSCs in the SVZ and SGZ are maintained by specialized vascular regions called the perivascular niche (PVN) [135]. The PVN consists of the endothelial cells lining the vasculature, as well as astrocytes, pericytes, macrophages, microglia, fibroblasts, and vascular smooth muscle cells. These cells function and signal together to maintain structure and provide signals to NSCs. Evidence exists which demonstrates the vital role endothelial cells play in maintaining

are bi-directional and influence each other's development.

4.2. Astrocytes as a part of the perivascular niche

extravasate the BBB.

172 Astrocyte - Physiology and Pathology

As mentioned in the discussion of astrocytes and tumor cell interactions at the BBB, there have been a few key studies observing the cellular events that take place when metastatic cells extravasate into the brain parenchyma [56, 57]. From these studies, it is known that astrocytes are the first to respond to extravasating metastatic tumor cells entering the brain, followed by microglia [56]. Regardless of whether a CNS tumor is primary or metastatic, microglia and astrocytes control the immune response; therefore, it is upon their activation that other immune cells, such as macrophages or lymphocytes, may infiltrate [144–146, 150]. Activated astrocytes secrete pro-inflammatory molecules such as CXCL12, CCI2, Il15, CCL8, and CXCL1, all of which are known to regulate recruitment, activation and proliferation of T-cells, B-cells, or natural killer (NK) cells [144–146].

As it is often seen with any local or systemic inflammation, CNS immune responses can often persist or be dysregulated by tumor cells to become pathogenic. Many of these signaling responses are mediated by astrocytes. For example, Valiente et al. [151] reported that astrocytes produce FasL and plasmin ligands as defense mechanisms to kill brain-invading tumor cells. In response, tumor cells secrete serpins, which thwart the lethal action of plasmin [151]. Thus, Fas-mediated tumor cell apoptosis is blocked, leading to tumor survival. Other cells, such as endothelial cells, in the brain microenvironment are also co-opted, which is facilitated by up-regulation of L1 cell adhesion molecules (L1CAM). All these mechanisms work together to initiate brain metastasis [151].

6. Therapeutic opportunities for cancer emerging from astrocyte-tumor

be considered when trying to block tumor cell entrance into the brain [30].

As stated previously, homeostasis in brain environment is key for the functionality of the brain, and therefore key checkpoints, such as the BBB, are responsible for maintaining homeostasis [30]. The BBB also prevents access of key drugs into the brain for targeting tumor cells. Any surgical intervention in the brain clearly has quality of life considerations, and does not offer complete disease-free state. Therefore, it is of importance to prevent tumor cells from entering the brain or block the target routes and underlying mechanisms used by tumors to circumvent checkpoints. It is worth noting that the regions of the brain which are free from the protections of the junctional characteristics of the BBB, such as the stroma of the choroid plexus and area postrema have increased vascular permeability which can be problematic, and therefore must

The Role of Astrocytes in Tumor Growth and Progression

http://dx.doi.org/10.5772/intechopen.72720

175

As we know, astrocytes are capable of signaling to trigger tumor cell (breast, lung, skin, and brain) migration, invasion and metastasis in vivo [88, 95, 127, 160]. There are many targets in the brain microenvironment that provide effective intervention strategies for metastasis, and is reviewed elsewhere [164]. Here, we will discuss targets and mechanisms at the signaling interface of tumor cells and astrocytes that offer fresh perspective on intervention strategies.

As discussed earlier, we and others have identified astrocyte secreted MMP-2, MMP-9, and MMP-1 to promote tumor progression, and blocking them with broad spectrum MMP inhibitors does influence tumor metastasis in pre-clinical models [95, 96, 165–167]. Interestingly, MMP-1 was one of 21 MMPs that showed clinical significance in regards to breast cancer brain metastasis, and expression analysis of brain-seeking triple negative breast cancer clonal cells confirm MMP-1 and MMP-9 as potential targets [133, 168]. Therefore, these studies suggest either MMPs or the underlying pathways that regulate their expression as pharmaceutical targets. Given that targeting MMPs in the past using first generation MMP inhibitors resulted in disappointing results in the clinic, we also suggest that next generation, highly-specific MMP inhibitors, applied locally, could be effective new strategies to consider in preventing

further growth and movement of tumor cells to a second location in the brain [169].

Astrocytes are co-opted to up-regulate survival genes in tumor cells and induce protection from chemotherapy [65]. Downregulation of the astrocyte-initiated survival gene expression in tumor cells will render tumor cells sensitive to chemotherapy [65]. This chemoprevention role, however, appears to be contact dependent, utilizing gap junctions to mediate the changes in tumor cells. Previously, gap junction proteins Cx43 and Cx26 were utilized by breast cancer and melanoma cells to initiate brain metastatic lesion formation in cohort with the vasculature [170]. Indeed, patient data analysis revealed increased cancer recurrence and metastasis with increased expression of Cx26 and Cx43 in primary melanoma and breast tumor cells. The recent work done by Chen et al. shows that brain metastatic breast and lung cancer cells initiate contact with astrocytes through gap junctions, which produces a signaling

cross talk

6.1. Enzyme targets

6.2. Gap junction protein targets

Astrocytes are also known to function in immunosuppression. This is accomplished by downregulating the pro-inflammatory cytokine TNF-α in surrounding microglia, and suppressing the antigen presenting abilities of various immune cells by downregulating their expression of MHCII and CD80 [152, 153]. Additionally, activated astrocytes can co-localize and induce apoptosis in T-cells attempting to infiltrate the brain parenchyma by expression of the "death ligand," CD95L, which binds the receptor on T-cells [152, 154].

#### 5.2. Reactive Astrogliosis

Arguably, the most important feature of astrocytes in relation to their immune function is their ability to activate, a process called reactive astrogliosis. What determines whether an astrocyte is "activated" or not has not been clearly defined, however Sofroniew summarized the existing research into four key features. First, reactive astrogliosis is a spectrum of molecular, cellular, and functional changes among astrocytes in response to CNS injury of many kinds [147]. Second, the changes can vary in severity and the response can be sequential and/or progressive. Third, the changes are regulated by intra- and inter-cellular signals and lastly, signaling events can be both gain and loss of function in nature, resulting in both beneficial and detrimental outcomes [147, 155]. In other words, reactive astrogliosis is spectral in nature; the triggers can vary and therefore the "activation" or response can vary and is context dependent, which is also true in regards to how reactive astrocytes affect tumor progression and/or tumor death.

The activation responses can be as small as a transient upregulation of GFAP, to permanent structural changes in the brain from a process called glial scar formation. Scar formation occurs when astrocytes proliferate and overlap to a point that causes dense, compact barriers around necrotic tissue [147, 156]. In between these two extremes, other phenotypic changes that occur include hypertrophy of the cell body and processes, a vast array of gene expression changes, and varying degrees of proliferation up to the point of scar formation. Some of the chemical activators of astrocytes known to be secreted by or induced by tumor cells include EGF (glioblastoma and medulloblastoma), TGF-α (medulloblastoma), receptor activator of nuclear factor kappa-B (NFκB) ligand (RANKL) (glioma), macrophage migration inhibitory factor (MIF), interleukin-8 (IL-8), and plasminogen activator inhibitor-1 (PAI-1) (lung cancer metastases) [11, 157–160].

In addition to chemical activation, astrocytes can also be activated by tumor cells mechanically. Although extremely abundant, astrocytes hold a highly regulated, non-overlapping distribution that plays an important role in morphology and contact-dependent inhibition of proliferation [11, 61]. This distribution and homeostasis is mediated by contact inhibition and adherens junctions. Therefore, mechanical disruption occurs when processes such as migration and/or proliferation of surrounding cells is initiated. Such mechanical signals could come potentially emerge from tumor cells, subsequently triggering astrocyte activation via disruption of these cell surface complexes such as cadherins and β-catenin [161, 162]. The genes activated by β-catenin signaling are regulatory and often lead to proliferation and migration [11, 163]. Interestingly, Yang et al. (2012) found this contact initiated activation of astrocytes to parallel what occurs in the transformation of astrocytomas, further coupling the process of astrocyte activation and tumor progression [162].

## 6. Therapeutic opportunities for cancer emerging from astrocyte-tumor cross talk

As stated previously, homeostasis in brain environment is key for the functionality of the brain, and therefore key checkpoints, such as the BBB, are responsible for maintaining homeostasis [30]. The BBB also prevents access of key drugs into the brain for targeting tumor cells. Any surgical intervention in the brain clearly has quality of life considerations, and does not offer complete disease-free state. Therefore, it is of importance to prevent tumor cells from entering the brain or block the target routes and underlying mechanisms used by tumors to circumvent checkpoints. It is worth noting that the regions of the brain which are free from the protections of the junctional characteristics of the BBB, such as the stroma of the choroid plexus and area postrema have increased vascular permeability which can be problematic, and therefore must be considered when trying to block tumor cell entrance into the brain [30].

As we know, astrocytes are capable of signaling to trigger tumor cell (breast, lung, skin, and brain) migration, invasion and metastasis in vivo [88, 95, 127, 160]. There are many targets in the brain microenvironment that provide effective intervention strategies for metastasis, and is reviewed elsewhere [164]. Here, we will discuss targets and mechanisms at the signaling interface of tumor cells and astrocytes that offer fresh perspective on intervention strategies.

#### 6.1. Enzyme targets

microenvironment are also co-opted, which is facilitated by up-regulation of L1 cell adhesion molecules (L1CAM). All these mechanisms work together to initiate brain metastasis [151].

Astrocytes are also known to function in immunosuppression. This is accomplished by downregulating the pro-inflammatory cytokine TNF-α in surrounding microglia, and suppressing the antigen presenting abilities of various immune cells by downregulating their expression of MHCII and CD80 [152, 153]. Additionally, activated astrocytes can co-localize and induce apoptosis in T-cells attempting to infiltrate the brain parenchyma by expression of the "death ligand,"

Arguably, the most important feature of astrocytes in relation to their immune function is their ability to activate, a process called reactive astrogliosis. What determines whether an astrocyte is "activated" or not has not been clearly defined, however Sofroniew summarized the existing research into four key features. First, reactive astrogliosis is a spectrum of molecular, cellular, and functional changes among astrocytes in response to CNS injury of many kinds [147]. Second, the changes can vary in severity and the response can be sequential and/or progressive. Third, the changes are regulated by intra- and inter-cellular signals and lastly, signaling events can be both gain and loss of function in nature, resulting in both beneficial and detrimental outcomes [147, 155]. In other words, reactive astrogliosis is spectral in nature; the triggers can vary and therefore the "activation" or response can vary and is context dependent, which is also true in regards to how reactive astrocytes affect tumor progression and/or tumor death. The activation responses can be as small as a transient upregulation of GFAP, to permanent structural changes in the brain from a process called glial scar formation. Scar formation occurs when astrocytes proliferate and overlap to a point that causes dense, compact barriers around necrotic tissue [147, 156]. In between these two extremes, other phenotypic changes that occur include hypertrophy of the cell body and processes, a vast array of gene expression changes, and varying degrees of proliferation up to the point of scar formation. Some of the chemical activators of astrocytes known to be secreted by or induced by tumor cells include EGF (glioblastoma and medulloblastoma), TGF-α (medulloblastoma), receptor activator of nuclear factor kappa-B (NFκB) ligand (RANKL) (glioma), macrophage migration inhibitory factor (MIF), interleukin-8 (IL-8), and plasminogen activator inhibitor-1 (PAI-1) (lung cancer metastases) [11, 157–160].

In addition to chemical activation, astrocytes can also be activated by tumor cells mechanically. Although extremely abundant, astrocytes hold a highly regulated, non-overlapping distribution that plays an important role in morphology and contact-dependent inhibition of proliferation [11, 61]. This distribution and homeostasis is mediated by contact inhibition and adherens junctions. Therefore, mechanical disruption occurs when processes such as migration and/or proliferation of surrounding cells is initiated. Such mechanical signals could come potentially emerge from tumor cells, subsequently triggering astrocyte activation via disruption of these cell surface complexes such as cadherins and β-catenin [161, 162]. The genes activated by β-catenin signaling are regulatory and often lead to proliferation and migration [11, 163]. Interestingly, Yang et al. (2012) found this contact initiated activation of astrocytes to parallel what occurs in the transformation of astrocytomas, further coupling the process of astrocyte

CD95L, which binds the receptor on T-cells [152, 154].

5.2. Reactive Astrogliosis

174 Astrocyte - Physiology and Pathology