research into tumor cell metabolism has recently entered a new age ( cairns et al . , 2011 ; kroemer and pouyssegur , 2008 ) . although this field was actively explored in the pre - genomics era , the discovery of tumor suppressor genes and oncogenes dampened interest in metabolism as a potential source of cancer cell specificities and related therapeutic targets .
however , although the study of tumor - associated genes has led to the identification of oncoproteins as new therapeutic targets , a cure for cancer is still far away .
scientists in the cancer field are therefore reconsidering earlier metabolic discoveries using the molecular tools that are now available .
the altered metabolism of cancer cells compared with normal cells confers a selective advantage for their survival and proliferation . as the primary tumor expands
, it outgrows the diffusion limits of its local blood supply , leading to hypoxia . among other effects
, hypoxia induces the expression of hypoxia - inducible factor ( hif ) , a transcription factor that initiates a range of responses , including angiogenesis and various pro - survival mechanisms ( denko , 2008 ) .
cellular metabolism is consecutively shifted towards the glycolytic pathway ( i.e. glucose to lactate ) through the increased expression of glycolytic enzymes and glucose transporters , together with a decreased dependence on the oxidative pathway ( i.e. pyruvate to lactate to acetyl - coa ) . in parallel ,
stimulation of angiogenesis leads to chaotic development of the tumor vasculature , which only alleviates hypoxia to a limited temporal and spatial extent , further selecting for tumors that constitutively upregulate glycolysis .
the warburg effect describes this capacity of tumor cells to exploit glycolysis ( i.e. without coupling to the krebs cycle and mitochondrial respiratory chain ) even in the presence of oxygen ( cairns et al . , 2011 ) .
the expression of oncogenes such as those encoding for myc and ras , and/or loss of tumor suppressor genes such as p53 , also promote the glycolytic pathway by acting on the same metabolic factors as those regulated by hif ( levine and puzio - kuter , 2010 ) .
lactate , the end product of glycolysis , is produced in large excess in tumors in response to the specific characteristics of the tumor microenvironment and the genetic features of tumor cells .
importantly , lactate also constitutes an alternative metabolic fuel for cancer cells ( sonveaux et al . , 2008 ;
feron , 2009 ; whitaker - menezes et al . , 2011 ) , which is a phenomenon that has been well characterized in non - tumor tissues , including in skeletal muscle , brain and liver ( brooks , 2009 ; gladden , 2004 ) .
oxidative tumor cells can use lactate instead of ( or in addition to ) glucose , thereby sparing available glucose , which can , in turn , diffuse deeper into the tumor to fuel hypoxic cells located farther away from tumor blood vessels ( sonveaux et al .
the use of lactate as an energy source requires the conversion of lactate into pyruvate ( and back ) as well as the transport of lactate into and out of tumor cells by way of specific transporters .
lactate dehydrogenase ( ldh ) isoforms and a family of monocarboxylate transporters ( mcts ) , respectively , regulate these processes . in this article and the accompanying poster , we describe lactate shuttles that involve these two lactate - related proteins in certain types of healthy tissues and tumor cells . given their importance as controllers of tumor cell metabolism , the components of lactate shuttles might actually be promising therapeutic targets for cancer .
ldhs catalyze the interconversion of pyruvate and lactate , with concomitant interconversion of nadh and nad .
these enzymes are homo- or hetero - tetramers composed of m and h protein subunits that are encoded by the ldha and ldhb genes , respectively .
five isoforms are therefore possible : ldh-1 [ four h subunits ( 4h ) ] , ldh-2 ( 3h1 m ) , ldh-3 ( 2h2 m ) , ldh-4 ( 1h3 m ) and ldh-5 ( 4 m ) ( see poster ) ; a third gene , ldhc ( also known as ldhx ) , is reportedly expressed in testes and sperm ( holmes and goldberg , 2009 )
. ldh enzymes with a high m - subunit content ( often referred to as ldha proteins ) are abundant in white skeletal muscle ( rapid twitch glycolytic fibers ) , in which they reduce pyruvate into lactate .
ldha and pyruvate dehydrogenase kinase ( pdk ) are upregulated in solid tumors in response to hypoxia in a hif-1-dependent manner .
pdk inactivates pyruvate dehydrogenase ( pdh ) and prevents the import of pyruvate into the mitochondrial matrix , whereas ldha reduces the pyruvate into lactate and thereby regenerates the nad stock necessary to maintain the glycolytic flux . in tumors ,
the glucose - to - lactate glycolytic pathway can also occur in the presence of oxygen , an observation described as the warburg effect ( feron , 2009 ; cairns et al . , 2011 ) .
this is actually observed in most proliferating cells and occurs because dividing cells need to produce more biosynthetic intermediates to duplicate cell biomass and dna ( feron , 2009 ) .
as long as glucose is available , nad regeneration makes glycolysis self - sufficient and a crucial carbon source for protein , lipid and nucleotide biosynthesis .
ldha expression is also induced by a variety of oncogene products , including myc ( shim et al . , 1997 ) , and might therefore contribute to the rapid consumption of pyruvate , the accumulation of which is potentially damaging .
lactate that is produced from hypoxia - induced or oncogene - driven ldha expression also needs to be removed to avoid acidification of the intracellular compartment : this function is fulfilled by dedicated transporters called mcts ( see below ) .
ldh enzymes with high h - subunit content ( often referred to as ldhb proteins ) are mainly found in aerobic tissues ( such as heart and brain ) , where they convert lactate into pyruvate .
transcriptional silencing of ldhb expression caused by aberrant methylation of the gene promoter region has been reported in gastric and prostate cancers ( maekawa et al . , 2003 ; leiblich et al . ,
2006 ) , thereby reinforcing the idea that tumors preferentially express ldh isoenzymes with a high ldha gene product content ( ldh-5>ldh-4>ldh-3 ) . in colorectal cancer , suppression of ldhb transcript in a more invasive phenotype
was even proposed to account for a large part of the warburg metabolic switch ( i.e. independently of hypoxia - induced changes in ldha expression ) ( thorn et al . , 2009 ) .
mcts constitute a family of 14 transporters [ also known as solute carrier 16 ( slc16 ) proteins ] that carry single - carboxylate molecules across biological membranes ( halestrap and price , 1999 ; halestrap and meredith , 2004 ; kennedy and dewhirst , 2010 ) .
the mct proteins are predicted to have 12 transmembrane domains , with the n- and c - termini facing the intracellular side of the membrane and a large cytosolic loop between domains 6 and 7 ( see poster ) .
transmembrane domains are well conserved and mcts differ mainly in their n- and c - termini and intermediary loop sequences .
four members of the mct family ( mct1-mct4 ) have been described to be proton - linked mcts . at physiological ph ,
mct1 is the most widely expressed mct and plays an active role in the uptake of lactate in the heart , skeletal muscle and red blood cells , as well as in the liver ( for gluconeogenesis ) ( halestrap and meredith , 2004 ) .
mct2 is less ubiquitous and has been reported to play key roles in neurons at the postsynaptic density , and mct3 expression is limited to the retinal pigment epithelium and choroid plexus epithelia ( halestrap and meredith , 2004 ) .
mct4 is primarily expressed in highly glycolytic cells , such as white ( rapid twitch ) muscle fibers , and is upregulated in response to hypoxia ( ullah et al . , 2006 ) .
the affinity for lactate and pyruvate differs between mcts , as reflected by the michaelis constant ( km ) value for each ( i.e. the substrate concentration at which the transport rate is half of maximum ) ( see poster ) .
in particular , the low km value of mct1 for lactate indicates that , when expressed in a cell , this transporter is the predominant regulator of lactate fluxes . as discussed below , mct1 and mct4
are the two major mcts expressed in tumor cells and represent promising targets for therapy .
mct1 , mct3 and mct4 associate with the chaperone protein basigin ( also known as cd147 , emmprin or ox-47 ) , whereas embigin ( also known as gp70 ) is the preferred binding partner of mct2 ( wilson et al . , 2005 ;
the current consensus on the mechanism of mct1-mct4 transporters is that monocarboxylate - proton symport occurs via a rapid equilibrium - ordered mechanism , with proton binding followed by monocarboxylate binding ( halestrap and meredith , 2004 ; halestrap and price , 1999 ) .
lactate export in response to high glycolytic flux ( see above ) is not the only function of mcts : lactate can also be taken up by various specialized cells in a variety of tissues .
circulating lactate can be used by the liver for gluconeogenesis ( cori cycle ) ( miller et al . , 2002b ; roef et al . , 2003 ) or directly consumed by oxidative cells located near to glycolytic lactate - producing cells .
this symbiosis system  referred to as the cell - cell lactate shuttle ( brooks , 1985 ; brooks , 1998 )  has been extensively studied in skeletal muscle and brain , and was more recently documented in tumors ( sonveaux et al . , 2008 ; vegran et al . , 2011 ) .
in addition to lactate exchange between cells , an intracellular lactate shuttle paradigm ( brooks , 1998 ; brooks et al . , 1999 ) was proposed to account for the apparently paradoxical conversion of exogenous lactate into pyruvate in glucose - fuelled cells .
although lactate shuttles were identified only recently in tumors ( see below ) , this process has been described in skeletal muscle for several years .
accumulation of lactate in contracting skeletal muscle was originally thought to be the consequence of anaerobic glycolysis ( wasserman , 1984 ) .
however , lately , numerous studies have documented that lactate can be produced and used continuously under fully aerobic conditions [ for reviews and historical perspectives , see gladden and brooks ( gladden , 2004 ; brooks , 2009 ) ] . during exercise , white glycolytic
( rapid twitch ) muscle fibers produce lactate and release it into the blood ( skelton et al . , 1995 ) , mainly via the mct4 transporter ; a large part ( one third ) of plasma lactate is then transported into red blood cells via mct1 ( garcia et al . , 1994 ) . when exercise is prolonged , but also during recovery from short - term exercise , lactate is taken up from the muscle interstitial fluid or back from the blood to red oxidative ( slow twitch ) muscle fibers ( brooks , 2000 ; gladden , 2000 ; miller et al . , 2002a )
similarly , the heart and brain are active consumers of lactate , which , under conditions of increased circulating lactate concentrations , can represent up to 60% and 25% of the metabolic fuel for these organs , respectively ( stanley , 1991 ; ide and secher , 2000 ; gertz et al . , 1988 ) .
tracer studies indicate that essentially all of the lactate taken up by these organs is oxidized ( stanley , 1991 ; van hall et al . , 2009 ) , suggesting that there is little carbon exchange from lactate to other metabolites . of note , whereas cardiac muscle mainly takes up lactate from the blood , in the brain , lactate exchange between neighboring cells has been documented ( bouzier - sore et al . , 2003 ) .
pellerin and magistretti originally reported that neurons can metabolize lactate originating from astrocytes in vitro ( see poster ) ( pellerin and magistretti , 1994 ) .
the symbiosis in this case is further reinforced by the production and release of glutamate from neurons : glutamate is consecutively taken up by astrocytes together with na , thus requiring ( na , k)-atpase activation to restore ionic homeostasis .
glycolysis is in turn stimulated in response to reduced atp levels and glutamine synthesis from glutamate . as a consequence ,
lactate concentration rises in astrocytes and then lactate moves outward through mct4 into the extracellular space , driving lactate influx into the surrounding neurons via mct2 .
however , stoichiometric analyses do not support a strict lactate exchange between the two cell types , and it is generally accepted that glycolysis is also directly coupled to oxidative metabolism in astrocytes ( mangia et al . , 2003 ) .
a common requirement of the capacity of muscle and brain cells to use lactate is the presence of oxygen . in tumors
, the temporal and/or local deficit in oxygen imposes an additional layer of regulation to the cell - cell lactate shuttle ( see below ) .
the concept of the intracellular lactate shuttle originates from the need to understand how oxidation of lactate to pyruvate can occur in well - oxygenated tissues , including in skeletal muscle , cardiac muscle and liver .
this phenomenon is indeed paradoxical , considering that the vmax of ldh is the highest of any enzyme in the glycolytic pathway and that the reaction equilibrium constant for the pyruvate - to - lactate conversion is more favorable than for the reverse reaction ( brooks , 2000 ; brooks , 1998 ; brooks et al . , 1999 ) . because of thermodynamic issues related to the conversion of lactate into pyruvate inside mitochondria , and the lack of undisputable evidence of significant mitochondrial ldh activity ( yoshida et al . , 2007 ) , a consensus is emerging that suggests that differential concentrations of lactate and pyruvate are present in different subcellular regions .
if one considers that mitochondria can act as a  sink for pyruvate ( into which it is taken up and used for the krebs cycle ) , it can be proposed that there exists a steep pyruvate gradient between the plasma membrane and mitochondria .
in other words , pyruvate concentrations are the lowest close to mitochondria , making lactate concentrations relatively high in this remote cytosolic location away from the plasma membrane . as a consequence , in the immediate vicinity of mitochondria , lactate and nad can be converted back to pyruvate and nadh before they are taken up into mitochondria ( stainsby and brooks , 1990 ; gladden , 2004 ) .
the nature of the transporter that brings pyruvate into mitochondria is unclear : mcts [ in particular mct2 , which has a high affinity for pyruvate ( yoshida et al . , 2007 ) ] and a six transmembrane helix structure known as mitochondrial pyruvate carrier ( mpc ) ( kuan and saier , jr , 1993 ; sugden and holness , 2003 ) have been described .
the intracellular lactate shuttle model provides a framework to understand how ( millimolar ) lactate exchange and conversion into its more oxidized analog pyruvate can be used to maintain the redox balance in the cytosol and mitochondria .
the presence of cell - cell and intracellular lactate shuttles gives rise to the notion that glycolytic ( i.e. glucose to lactate ) and oxidative ( i.e. lactate to pyruvate to acetyl - coa ) pathways can simultaneously co - exist and even be linked in a given cell . in the following section , we comment on recent findings documenting that these different modes of lactate exchange , which are physiological in essence , can be usurped by tumors .
we recently reported that both oxidative cancer cells and endothelial cells lining tumor blood vessels can take up lactate released by glycolytic tumor cells ( sonveaux et al . , 2008 ; vegran et al . ,
the cell - cell and intracellular lactate shuttle concepts identified in non - cancer tissues can be applied in this context and open up exciting avenues , with concrete therapeutic perspectives . using cervix cancer cells as an in vitro model of oxidative cancer cells
, we documented that lactate derived from glycolytic tumor cells could be used as a main source of metabolic fuel ( sonveaux et al . , 2008 ) .
as a consequence , aerobic tumor cells ( i.e. those located near to blood vessels ) spare glucose , which can then diffuse a greater distance into the tumor to nourish glycolytic cells .
exogenous lactate uptake by oxidative tumor cells occurs through the high - affinity lactate transporter mct1 , whereas glycolysis - derived lactate is released through the low - affinity lactate transporter mct4 ( sonveaux et al . ,
2008 ) , recapitulating the cell - cell lactate shuttling processes observed in muscle and brain ( see above ) .
interestingly , alteration in this symbion can have dramatic consequences ( sonveaux et al . , 2008 ) .
inhibition of mct1 can shift the preference of oxidative tumor cells towards using glucose , thereby reducing the amount of glucose that can reach hypoxic tumor cells and altering their survival ( see poster ) .
inhibition of mct4 has instead the potential to directly target hypoxic tumor cells and promote their death by intracellular lactic acid accumulation .
chatham and colleagues previously reported that , in rat heart , the production of lactate via glycolysis and the oxidation of exogenous lactate are functionally separate metabolic pathways ( chatham et al . , 2001 ) .
this observation , which is in line with the intracellular lactate shuttle hypothesis , further supports the paradigm of preferential ( or at least co - existing ) fuelling of mitochondrial respiration in tumors by exogenous lactate ( sonveaux et al . , 2008 ) .
this model might , in particular , apply in the context of cyclic hypoxia , in which tumor cells must switch from using glucose to an alternative source of energy to survive during periods of ischemia ( denko , 2008 ) .
tumor - associated fibroblasts have also been documented to participate in lactate homeostasis in tumors ( see poster ) .
koukourakis and colleagues reported that preferential expression of mct1 and ldh-1 together with elevated pdh activity in tumor fibroblasts supports the metabolic use of lactate produced by tumor cells , and thereby prevents the development of a hostile acidic environment ( koukourakis et al .
by contrast , lisanti and colleagues documented that tumor - associated fibroblasts can instead undergo aerobic glycolysis and  feed adjacent oxidative cancer cells with the released lactate ( bonuccelli et al . , 2010 ; whitaker - menezes et al . ,
more recently , we reported that lactate can also enter tumor endothelial cells through mct1 ( vegran et al . , 2011 ) .
this study not only showed that lactate can favor the survival of serum - starved endothelial cells , but , more interestingly , that lactate can directly initiate pro - angiogenic signaling .
we found that lactate stimulates an autocrine pathway involving nuclear factor-b ( nfb ) and interleukin-8 [ il-8 ; also known as c - x - c chemokine 8 ( cxcl8 ) ] , which drives endothelial cell migration and tube formation .
il-8-specific blocking antibodies and il-8-targeted sirna both prevented lactate - induced angiogenesis . using human colorectal and breast cancer models in mice , we further documented that lactate that is released from tumor cells through mct4 ( but not mct1 ) is sufficient to stimulate il-8-dependent angiogenesis and tumor growth ( vegran et al . , 2011 ) .
importantly , we also documented the anti - angiogenic potential of mct1 and mct4 inhibition through the blockade of endothelial cell lactate uptake and the reduction in extracellular lactate availability , respectively ( see poster ) .
these findings establish a dual role for lactate in tumors : it acts as both a metabolic fuel and a signaling molecule , positioning lactate at the intersection of key processes in cancer progression , namely tumor metabolism and angiogenesis .
therefore , although lactate shuttles are physiologically active in some types of healthy tissue , the specific characteristics of some tumors make the regulators of cellular lactate handling , namely mcts and ldhs , potential anti - cancer targets .
metabolic symbions that we and others have described between oxidative and glycolytic tumor cells  but also between tumor cells and stromal cells , including endothelial cells and fibroblasts  offer diverse rationales to support the development of mct and ldh inhibitory strategies .
however , as for all new anti - cancer therapies , the extent to which tumors are addicted ( feron , 2010 ) to these pathways ( i.e. to lactate transport and conversion ) , and the capacity to therapeutically target these pathways without altering healthy tissues ( bouzin and feron , 2007 ) , will determine the success of these new therapeutic avenues in the anti - cancer drug armamentarium .
 hypoxia and oncogene expression both stimulate glycolytic metabolism in tumors , thereby leading to lactate production . 
 however , lactate is more than merely a by - product of glycolysis : it can be used as a metabolic fuel by oxidative cancer cells . 
 this phenomenon resembles processes that have been described for skeletal muscle and brain that involve what are known as cell - cell and intracellular lactate shuttles . 
 two control points regulate lactate shuttles : the lactate dehydrogenase ( ldh)-dependent conversion of lactate into pyruvate ( and back ) , and the transport of lactate into and out of cells through specific monocarboxylate transporters ( mcts ) . in tumors , 
 mct4 is largely involved in hypoxia - driven lactate release , whereas the uptake of lactate into both tumor cells and tumor endothelial cells occurs via mct1 . 
 translating knowledge of lactate shuttles to the cancer field offers new perspectives to therapeutically target the hypoxic tumor microenvironment and to tackle tumor angiogenesis . 
Introduction
Lactate dehydrogenase: one enzyme, two functions
Monocarboxylate transporters
Cell-cell lactate shuttle
Intracellular lactate shuttle
Lactate shuttles in tumors