it provides structural support in bone , cartilage , and the basement membrane ; specific association of cells with the ecm also provides contextual information that controls cellular phenotype , including differentiation , proliferation , or apoptosis .
ecm also regulates availability and activity of many signaling molecules , including tgf- , through controlled sequestration , presentation , and release .
the primary cell surface receptors for the ecm are the integrins , a family of 24 heterodimeric proteins composed of one of 18 -subunits and 8 -subunits .
integrins bind to motifs present in the ecm though an interaction domain located between the - and -subunit ; while many integrin - binding motifs have been identified , the best studied is the arginine - glycine - aspartate ( rgd ) sequence that is present in fibronectin and many other extracellular molecules .
integrins become activated in a process that is regulated both by availability of ecm substrate ( outside - in activation ) and signals from within the cell ( inside - out activation ) .
activated integrins can bind to the actin cytoskeleton and recruit a variety of cytosolic components into adhesion complexes . through those interactions integrins
can transduce biochemical signaling dependent on ecm composition , as well as directly link physical forces acting on the ecm to the cellular cytoskeleton ( fig .
stimulation of cellular signaling pathways can lead to increased affinity for binding sites in the ecm .
integrin ligation to the ecm triggers assembly of cytoplasmic molecules that can lead to focal adhesion complex formation and connection to the actin cytoskeleton 
  integrin activation .
stimulation of cellular signaling pathways can lead to increased affinity for binding sites in the ecm .
integrin ligation to the ecm triggers assembly of cytoplasmic molecules that can lead to focal adhesion complex formation and connection to the actin cytoskeleton
three tgf- isoforms are present in mammals , tgf-1 , -2 , and -3 , each encoded by a separate gene , and each playing distinct physiological roles during development .
exposure of cells to an active tgf- isoform leads to assembly of a tgf--ligated tetrameric receptor complex , composed of 2 type i and 2 type ii tgf- receptor subunits ( tgfbri and tgfbrii ) .
tgfbrii then phosphorylates tgfbri , enabling it to activate downstream signaling responses , which are regulated through canonical and noncanonical signaling pathways ( fig .
tgf- signaling can be initiated by binding of tgf- to tgf- receptor type iii ( tgfbriii ) , which can then assemble with tgf- receptors types ii and i ( tgfbrii , tgfbri ) to form an active signaling complex . in the canonical signaling pathway , the active signaling complex phosphorylates the receptor smads ( smad2 and smad3 ) , which then associate with smad4 , translocate to the nucleus and affect gene transcription . in the noncanonical pathway , phosphorylation of the tgf- receptor complex leads to activation of cytosolic signaling pathways , including mapk , pi3k / akt , and rho gtpases
tgf- signaling can be initiated by binding of tgf- to tgf- receptor type iii ( tgfbriii ) , which can then assemble with tgf- receptors types ii and i ( tgfbrii , tgfbri ) to form an active signaling complex . in the canonical signaling pathway , the active signaling complex phosphorylates the receptor smads ( smad2 and smad3 ) , which then associate with smad4 , translocate to the nucleus and affect gene transcription . in the noncanonical pathway , phosphorylation of the tgf- receptor complex leads to activation of cytosolic signaling pathways , including mapk , pi3k / akt , and rho gtpases . in the canonical signaling pathway , tgfbri phosphorylates the receptor smad proteins , smad2 or smad3 .
phosphorylated receptor smads associate with smad4 and the resultant oligomeric complex becomes translocated to the nucleus , where it can bind to a variety of other transcription factors and cofactors .
transcriptional alterations induced by the canonical pathway vary according to the strength and sustained maintenance of the tgf- receptor signaling , the composition and availability of existing transcriptional cofactors , and the epigenetic landscape of the chromatin at the time of tgf- pathway activation .
the gene expression effects are highly cell type and context dependent ; for example , the canonical signaling pathway inhibits expression of the inhibitor of differentiation ( id1 ) gene in normal mammary epithelial cells , but activates its expression in breast cancer cells . in the noncanonical signaling pathway , the activated tgf- receptor complex directly regulates non - smad - dependent pathways to activate , sustain , or modulate cellular responses .
non - canonical pathways include activation of erk / mapk signaling through tyrosine phosphorylation of tgfbr1 and recruitment of grb / shc , and subsequent activation of ras , which may contribute to tgf--dependent induction of senescence and prevention of transformation in normal human mammary epithelial cells .
tgf- can also induce jnk / p38 through a smad - independent pathway , which can then reinforce smad - dependent transcriptional alterations through a reactive oxygen species ( ros)-mediated mechanism .
rho family gtpases can be regulated through smad - independent tgf- signaling : rhoa , rac1 , and cdc42 can be activated in epithelial cells , or rhoa can be targeted for degradation via a pathway initiated by direct phosphorylation of the polarity protein par6 by tgfbrii .
tgf- can also activate akt through smad - independent induction of pi3k , which can in turn act as a regulator of the canonical pathway through phosphorylation of erk and consequent activation of smads .
the principal determinant and mediator of whether tgf- will signal through the canonical or noncanonical pathway in normal cells is smad7 , which is able to inhibit phosphorylation of receptor smads through multiple mechanisms .
smad7 is a transcriptional target of the canonical pathway , which constitutes a mechanism for channeling signaling from canonical to noncanonical pathways .
`` tgf- expression has been studied in many tumor types , where it has been found to function both as a tumor suppressor and a tumor promoter . in normal cells and in early stage tumors , tgf- acts to block cell proliferation through the canonical pathways , including via smad - dependent inhibition of myc , as well as activation of cyclin - dependent kinase inhibitors . additionally , in premalignant cells which have acquired oncogenic mutations , tgf- can induce apoptosis . in more advanced tumors ,
the tgf--dependent cytostatic effects are suppressed , and emt - associated cell invasion and metastasis become dominant . the transition from tumor - inhibitory to tumor - promoting behaviors has been described as the tgf- paradox , and while multiple components of this transition have been discovered , many aspects remain unknown . additionally , tgf- can induce a variety of different anti- and pro - tumorigenic effects indirectly by acting on stromal cells in the cancer microenvironment .
one of the most highly investigated roles of tgf- in pathology is in the context of development of tissue fibrosis . under normal circumstances
, tissue damage triggers a wound healing response characterized by deposition of transitional ecm , followed by activation and invasion of fibroblasts that remodel and contract the wound ecm ; once tissue homeostasis is restored , the fibroblasts undergo apoptosis . under fibrotic conditions ,
however , a feedback loop is activated in which excessive ecm deposition leads to increased proliferation and activation of ecm - producing fibroblasts . maintained for extended periods of time
, fibrosis can become a significant problem in its own right , and can also stimulate malignant transformation and promote tumor progression .
tgf- has been implicated as a critical player in chronic fibrosis of many organs , including lung , kidney , liver , and skin .
tgf- directly stimulates expression of ecm proteins , including collagen , fibronectin , and proteoglycans .
tgf- induces conversion of fibroblasts into myofibroblasts which can further contract and distort the ecm .
tgf- can also directly stimulate myofibroblast formation from epithelial and endothelial cells through emt - related processes , and this function is essential for development of fibrosis in several organs .
the relative amount of epithelial- vs fibroblast - derived myofibroblasts is a current point of debate and is likely to be highly tissue specific .
in addition to inducing integrin signaling through increased production of ecm , tgf- can also regulate integrin function directly .
tgf- can control expression of v- , 3- , and 1-integrin subunits through both canonical and noncanonical pathways , according to cell type .
tgf- signaling can also directly phosphorylate and activate 1-integrin , stimulating cell invasion and facilitating tissue regeneration .
tgf- can also induce cross - talk between integrins and growth factor receptors , including via activation of focal adhesion kinase ( fak)-dependent clustering of erbb2 ( her2 ) and integrins 6 , 1 , and 4 through a pathway initiated by egfr - dependent phosphorylation and activation of src ; the overall effect of this pathway is increased cell migration and survival .
tgf- isoforms are translated as preproproteins that contain a 2530  kda latency associated peptide ( lap ) and the 13  kda tgf- molecule .
lap - tgf- homodimers linked by disulfide bonds are formed in the endoplasmic reticulum ( er ) , followed by protoelytic cleavage of lap from tgf- in the golgi ; the resultant homodimers of tgf- and lap remain noncovalently associated following secretion as an inactive protein complex called the small latency complex ( slc ) .
many cell types also produce latent tgf--binding protein ( ltbp ) , which can covalently bind to the slc , producing the large latency complex ( llc ) , which can become associated with fibrillar ecm molecules . outside of the cell
, tgf- can be activated following release from the slc , which can occur through selective proteolytic digestion , exposure to ros , or through direct interaction with ecm molecules .
recent studies have identified a process by which tgf-1 and tgf-3 can be released from their lap - tgf- complex through a force - dependent conformational shift induced by association of rgd motifs in their respective lap proteins with integrins ; all v - containing integrins ( v1 , v3 , v5 , v6 , and v8 ) as well as 81 integrin have been shown to bind lap , although whether v1 and 81 can activate latent tgf- has not been determined .
the relevance of v integrin - mediated activation of latent tgf- was demonstrated by studies showing that transgenic mice with mutations in the rgd motif of the tgf-1-associated lap protein recapitulate the phenotype of the tgf-1 knockout mouse ; similar developmental alterations are seen in mice lacking a functional v - integrin gene . of v - containing integrins that can activate latent tgf- , v6- and v8-integrins have been shown to play a critical role in tgf- activation during development and in immune homeostasis , while v3 and v5 may play a more important role in tgf- activation during fibrosis . 

( a ) integrin binding to latent tgf- complexes can lead to release of active tgf-. ( b ) signaling from integrins or tgf- receptors can stimulate expression of receptors or effectors of the other signaling pathway .
( c ) activation of integrins or tgf- receptors can lead to activation or inhibition of the other signaling receptor .
( d ) cooperative signaling from integrins and tgf- receptor may be necessary to stimulate phenotypic outcomes , including emt . 
 
interaction of integrins and tgf- signaling pathways . ( a ) integrin binding to latent tgf- complexes can lead to release of active tgf-. ( b ) signaling from integrins or tgf- receptors can stimulate expression of receptors or effectors of the other signaling pathway .
( c ) activation of integrins or tgf- receptors can lead to activation or inhibition of the other signaling receptor .
( d ) cooperative signaling from integrins and tgf- receptor may be necessary to stimulate phenotypic outcomes , including emt .
in addition to controlling tgf- activation , integrins can also affect signaling downstream of the tgf- receptor .
this can occur through integrin - mediated activation of the tgf- receptor , through stimulation of canonical and noncanonical signaling pathways , and through increased transcription of genes encoding tgf- and tgf- receptor isoforms .
recent studies have shown that the pathways regulating these effects vary in different cell and tissue types .
activity of 1-integrins and the integrin signaling mediator integrin - linked kinase ( ilk ) are required for tgf--induced emt in mammary epithelial cells .
integrin 31 is necessary for tgf--induced emt of alveolar epithelial cells and development of pulmonary fibrosis , and integrin 11 is required for tgf--induced kidney fibrosis .
furthermore , the response of the tgf receptor can be potentiated through binding of specific integrins to tgfbrii , leading to phosphorylation of the receptor in a fak- and src- dependent manner .
these effects can be quite cell - type specific : in mammary epithelial cells , the activation of tgfbrii by interaction with integrin 3 is inhibited by signaling from integrin 1 , while in chondrocytes , integrin 1 preferentially associates with and activates tgfbrii . as an additional layer of control
, the strength of the signal from the tgf- receptor complex depends on which tgf- ligand is bound , as the ligands vary in their ability to bind to tgfbri and to assemble the active tetrameric receptor complex with tgfbrii ; the co - receptor , tgfbriii , can further modulate the ligand - dependent assembly and activation of the complex .
while a role for integrin - mediated regulation of tgf- and receptor isoform expression is relatively unexplored , it is known that activation of integrin 51 stimulates tgfbrii expression , while expression of both tgfbri and tgfbrii can be repressed by integrin v3 .
as tgf- has been found to play a central role in promotion of tumor cell invasion and metastasis , stimulation of pathological emt , and induction of cancer - promoting microenvironmental changes , it is unsurprising that there has been considerable effort to develop inhibitors targeting the tgf- pathway as potential cancer therapeutics . to date , multiple inhibitors of the tgf- signaling have been developed and tested in animal models and clinical trials .
these include monoclonal antibody - based inhibitors of tgf- signaling , for example fresolimumab ( gc1008 ) , a fully human monoclonal antibody against tgf--1 , -2 and -3 , currently tested in clinical trials for combinatorial treatment with radiation therapy in metastatic breast cancer ( clinicaltrials.gov identifier : nct01401062 ) and in glioma ( clinicaltrials.gov identifier : nct01472731 ) .
another avenue for tgf- pathway inhibition is through synthetic antisense oligonucleotides , such as trabedersen ( ap 12009 ) , targeting tgf-2 , currently being tested for treatment of glioma , pancreatic cancer , melanoma and colorectal cancer .
other types of tgf- pathway inhibitors include small molecule tgf- receptor kinase inhibitors which bind to and directly block receptor signaling , as well as peptide aptamers which bind to and inhibit downstream mediators of the tgf- pathway , such as smads . an alternative way to interfere with tgf- signaling would be to target v integrin subunit integrins required for activation of latent tgf-. multiple anti - integrin v small molecule synthetic inhibitors and monoclonal anitbodies have been developed and are being tested in preclinical and clinical trials . among these , the monoclonal antibody intentumumab is currently being tested in patients with melanoma ( clinicaltrials.gov identifier : nct00246012 ) and prostate cancer ( clinicaltrials.gov identifier : nct00537381 ) .
it should be noted that , given the pleiotropic effects of tgf- and potential development of resistance , the current paradigms for targeting tgf- signaling in cancer treatment have focused on short term dosing in combination with other therapies .
it can act as a tumor suppressor , blocking proliferation and inhibiting stromal mitogens , as well as a tumor promoter and an inducer of cancer - associated emt , allowing evasion of immune surveillance and stimulating invasion and metastatic spread .
those opposing consequences of tgf- signaling are highly cell type dependent and regulated by the cellular context .
ecm is a key regulator of both initiation of tgf- signaling and a determinant of its outcomes .
integrins , acting as bidirectional signal transducers between the cellular microenvironment and the cell itself , play crucial roles in this process . often , the presence of certain integrins is required for activation of latent tgf- and thus induction of downstream signaling pathways ; integrins can also lead to ligand - independent signaling via activation of tgf- receptor .
the complexity and dual nature of tgf- signaling effects reinforce the need to specifically study the context and determinants of those different responses .
moreover , while it is the paradigmatic view that in early stages of tumor development tgf- acts to inhibit proliferation , whereas in later stages that aspect of signaling is lost in favor of promotion of emt and invasiveness , we should be mindful that this phenotypic transition from cytostasis to motility is unlikely to occur in every cell within a tumor , and that competition between pathways responsible for these different phenotypes likely leads to complex and sometimes unexpected outcomes .
most importantly , if tgf- pathway is to be targeted in cancer therapy , the challenge remains to predict the types and stages of tumors in which tgf- signaling inhibition would prevent metastasis without facilitating growth of the primary tumor .
this work was supported by grants from nih ( ca116201 ) , susan b. komen foundation ( kg110542 ) , and jimmy v. foundation .
 extracellular matrix ( ecm ) provides both structural support and contextual information to cells within tissues and organs . 
 the combination of biochemical and biomechanical signals from the ecm modulates responses to extracellular signals toward differentiation , proliferation , or apoptosis ; alterations in the ecm are necessary for development and remodeling processes , but aberrations in the composition and organization of ecm are associated with disease pathology and can predispose to development of cancer . 
 the primary cell surface sensors of the ecm are the integrins , which provide the physical connection between the ecm and the cytoskeleton and also convey biochemical information about the composition of the ecm . 
 transforming growth factor- ( tgf- ) is an extracellular signaling molecule that is a powerful controller of a variety of cellular functions , and that has been found to induce very different outcomes according to cell type and cellular context . 
 it is becoming clear that ecm - mediated signaling through integrins is reciprocally influenced by tgf- : integrin expression , activation , and responses are affected by cellular exposure to tgf- , and tgf- activation and cellular responses are in turn controlled by signaling from the ecm through integrins . 
 epithelial - mesenchymal transition ( emt ) , a physiological process that is activated by tgf- in normal development and in cancer , is also affected by the composition and structure of the ecm . here 
 , we will outline how signaling from the ecm controls the contextual response to tgf- , and how this response is selectively modulated during disease , with an emphasis on recent findings , current challenges , and future opportunities . 
Basics of ECM and Integrin Signaling
Basics of TGF- Signaling
TGF- Regulation of ECM Secretion and Integrin Function
Integrin Regulation of TGF- Activation and Signaling
Approaches to Therapeutic Intervention
Conclusions
Funding