5-amp - activated protein kinase ( ampk ) , a key regulator of cellular energy homeostasis , is activated in response to various metabolic stresses including starvation , hypoxia , and ischemia .
ampk is a heterotrimeric enzyme , which is activated by phosphorylation of thr172 on its  subunit .
several upstream kinases can modify thr172 , including liver kinase b1 ( lkb1 ) , calcium / calmodulin - dependent protein kinase kinase 2 ( camkk2 ) , and transforming growth factor  activated kinase-1 ( fig .
master switch  that regulates intracellular processes such as the cellular uptake of glucose , -oxidation of fatty acids , biogenesis of glucose transporter 4 , and mitochondrial biogenesis [ 1 - 4 ] . upon activation ,
ampk increases cellular energy levels by inhibiting anabolic energy - consuming pathways ( including fatty acid synthesis and protein synthesis ) and stimulating energy - producing catabolic pathways ( including fatty acid oxidation and glucose transport ) ( fig .
2 ) . because it is a major regulator of both lipid and glucose metabolism , ampk has been considered as a potential therapeutic target for the treatment of diabetes , obesity , and cancer .
recently , however , many reports show that ampk actually plays a critical role in tumor survival and growth [ 5 - 8 ] .
the metabolism reprogramming triggered by the activation of ampk is important for the survival of cells in the tumor microenvironment .
thus , it would not be surprising if , in some cases , ampk acts as a tumor promoter .
in contrast to normal cells , which produce energy through a relatively low rate of glycolysis and oxidation of pyruvate in the mitochondria , the energy production of most cancer cells relies on a high rate of glycolysis followed by lactic acid fermentation in the cytosol ; this phenomenon is known as the warburg effect [ 9 - 11 ] .
rapidly growing cancer cells achieve glycolytic rates that are up to 200 folds higher than those of normal cells , even when oxygen is plentiful
. the warburg effect may reflect adaptation to low - oxygen environments within tumors , mitochondrial damage , or oncogene - mediated shutdown of mitochondria , which would otherwise induce apoptosis in cancerous cells . on the other hand , the effect
might also be associated with rapid cell proliferation . because glycolysis provides most of the building blocks required for cell proliferation
, cancer cells may need to activate glycolysis despite the availability of oxygen   ( fig .
3 ) . in several types of cancer , genetic defects in the enzymes of the tricarboxylic acid cycle
are involved in tumorigenesis , whereas oncogenic h - ras transformation damages mitochondrial function . upon initiation of the warburg effect ,
cells depend on aerobic glycolysis with lactate production , rather than oxidative phosphorylation , to maintain adenosine triphosphate ( atp ) levels .
pfkfb3 is overexpressed and its phosphorylation is elevated in several types of cancer ( including colon and breast cancers ) as a consequence of ampk activation .
various stress signals , such as hypoxia and nutrient depletion , can drive pfkfb3 expression , and ampk contributes to the elevation of pfkfb3 levels and glycolytic flux in response to reduced ph .
in addition , expression of pfkfb2 , a phosphorylation target of both akt and ampk , is elevated in human cancers .
ampk can act as a negative regulator of the warburg effect under nutrient- and growth factor - rich conditions .
ampk depletion further increases mammalian target of rapamycin complex 1 ( mtorc1 ) activation , hypoxia - inducible factor 1 ( hif1 ) levels , and aerobic glycolysis in myc over - expressing cells .
depletion of ampk leads to increased lactate production , promoting the use of nonglucose carbon sources for anabolism , maintenance of bioenergetics , and lactate production .
the role of lactate production in proliferating tumor cells , the primary characteristic of the warburg effect , remains poorly understood .
the key glycolysis enzyme phosphofructokinase-1 is inhibited by o - glcnacylation , which is necessary for optimum tumor growth , suggesting that suppression of lactate - producing glycolysis can also confer survival and growth advantages on cancer cells , depending on the genetic background and cellular context .
although the ampk signaling pathway was originally characterized as a tumor - suppressive pathway , recent studies have demonstrated that many factors in the tumor microenvironment , as well as some oncogenic signals , are ampk activators .
the oncogene src ( proto - oncogene tyrosine - protein kinase ) , which is overexpressed and activated in many cancers , activates ampk through the activation of the protein kinase c -lkb1 pathway in ovcar3 and a431 cells .
similarly , the expression of myc and h - ras activates ampk in some cancers .
moreover , androgen receptor ( ar ) signaling activates ampk through the transactivation of camkk2 in prostate cancer [ 22 - 24 ] .
in addition to the activation of oncogenes , the loss of tumor suppressor gene folliculin ( flcn ) , which is associated with birt - hogg - dub syndrome , also activates ampk .
initially , two groups reported that lkb1 and ampk play protumorigenic roles in the context of oncogene - induced transformation of mouse embryonic fibroblasts ( mefs ) .
they showed that both lkb1-null mefs and ampk1/2-null ( ampk knockout ) mefs are resistant to oncogenic effects such as h - ras / sv40t - induced anchorage - independent growth and solid tumor growth in vivo .
remarkably , several studies demonstrated that ampk is also essential for other types of oncogene - induced tumorigenesis , such as mycinduced development of hepatocellular carcinoma ( hcc ) , h - raspten deletioninduced astrocytic tumor cell proliferation , and kinase suppressor of ras 2-induced anoikis resistance in mefs .
depletion of ampk activity in breast and pancreatic cancer cells hampers anchorage - independent growth and tumor formation in vivo [ 30 - 32 ] .
the lkb1-ampk signaling pathway is also essential for glioma cell survival under low - glucose conditions .
two recent approaches have identified the ampk1 and ampk1 subunits as essential protumorigenic genes in melanoma   and prostate cancer , respectively .
activation of ampk following loss of the tumor suppressor flcn increases mitochondrial biogenesis by inducing peroxisome proliferator - activated receptor  ( ppar ) coactivator 1 ( pgc1 ) expression , which in turn results in the production of reactive oxygen species ( ros ) .
.   also demonstrated that high levels of ros activate hif1 transcriptional activity , resulting in elevated expression of glycolytic enzymes , which induce atp production .
the ar - camkk2-ampk axis in prostate cancer increases aerobic glycolysis and anabolic metabolism partly through pgc1-mediated mitochondrial biogenesis , which can provide both the energy and the building blocks required for rapid cell growth .
recently , mechanisms associated with the metabolic functions of ampk , other than those involved in the production of energy or building blocks , have attracted increasing attention .
jeon et al .   showed that in the absence of ampk activity , oxidative stress is the major cause of cancer cell death during glucose deprivation and matrix detachment . under such stressful metabolic conditions ,
maintenance of nadph levels rather than atp levels is crucial for ampk - induced solid tumor formation in vivo , tumor cell survival , and anchorage - independent growth .
acetyl - coa carboxylase ( acc ) 1 and acc2 decrease nadph consumption by inhibiting fatty acid synthesis and increase nadph production by activation of fatty acid oxidation , respectively .
they both need to be inhibited to maintain the glutathione / oxidized glutathione ratio and to reduce h2o2 levels , thereby protecting against oxidative stress - induced cell death . in osteosarcoma
loss of flcn results in the constitutive activation of ampk , which in turn induces autophagy , inhibits apoptosis , improves cellular bioenergetics , and confers resistance to energy - depleting stresses such as oxidative stress , heat , anoxia , and serum deprivation .
despite accumulating evidence highlighting the oncogenic role of ampk , this enzyme was originally identified as a tumor suppressor following the discovery of lkb1 and mtorc1 [ 38 - 41 ] .
germline mutations in the gene encoding ampk were detected in peutz - jeghers syndrome . additionally , somatic mutations in the tumor suppressor lkb1 were observed in lung and cervical cancers .
mtorc1 , which is inactivated by the lkb1-ampk axis , is an important regulator of growth factor and nutrient signaling that controls protein synthesis and cell growth , and is over - activated in most human cancers . since lkb1 activates ampk and ampk inhibits mtorc1 , it is reasonable to classify ampk as a tumor suppressor .
although lkb1 deletion alone is not sufficient to cause lung cancer , it elevates mtorc1 and src activity , which enhances lung cancer induced by k - ras activation or pten deletion [ 44 - 46 ] .
conversely , in the endometrium , lkb1 deletion alone is sufficient for the development of invasive endometrial cancer , which can be effectively treated by rapamycin , an mtorc1 inhibitor , suggesting that mtorc1 activation is the major downstream target of lkb1 inhibition [ 48 - 50 ] .
notably , lkb1 also plays an ampk / mtorc1-independent role in tumor suppression that depends on microtubule affinity - regulating kinase - mediated regulation of snail or hippo signaling .
deletion of prkaa1 , the gene that encodes ampk1 , accelerates development of myc - induced lymphoma .
this observation contradicts the findings of liu et al . , who showed that ampk plays an important role in the development of myc - induced hcc .
.   also demonstrated that deletion of either lkb1 or prkaa1 induces mtorc1/hif1-dependent metabolic reprogramming that helps cancer cells meet their energetic and anabolic demands .
this observation also conflicts with the findings of yan et al . , who suggested that ampk activates aerobic glycolysis through the pgc1-ros - hif1 axis in the absence of flcn .
although this discrepancy could be due to tissue - specific and context - dependent functions of ampk , further research is needed to obtain an unequivocal interpretation of these seemingly contradictory findings .
several additional studies have reported conflicting pharmacological evidence regarding the role of ampk in cancer .
the antidiabetic drug metformin , which indirectly activates ampk by inhibiting mitochondrial metabolism , decreases the incidence and mortality rate of breast cancer .
moreover , several lines of evidence support the idea that ampk activators , such as 5-aminoimidazole-4-carboxamide-1-b - d - ribofuranoside ( aicar ) , metformin , and phenformin , suppress cancer cell growth and proliferation [ 56 - 59 ] .
however , in contradiction to these findings , liu et al .   showed that aicar and metformin reduce glioblastoma proliferation and viability .
furthermore , the direct ampk activator a769662 has no effect on proliferation , suggesting that the antitumor effects of aicar and metformin are ampk - independent .
shackelford et al .   also demonstrated that phenformin selectively kills lkb1-deficient lung tumor cells in vivo , suggesting that the antitumor effect of phenformin is not dependent on activation of ampk .
notably , vincent et al .   showed that all these nonspecific and indirect ampk activators strongly inhibit cell proliferation and viability in the absence of ampk , whereas such cytotoxic effects are reduced in the presence of ampk , suggesting that the activation of ampk actually opposes the cytotoxic effect of the indirect ampk activators .
along these lines , vincent et al .   demonstrated that the direct ampk agonist a769662 promoted cell proliferation under metabolically stressful conditions , supporting the idea that ampk plays an oncogenic role in the tumor microenvironment .
therefore , we must consider the effects of organ , cancer type , and acute vs. long - term ampk activation and inactivation when investigating the mechanistic contribution of ampk to cancer [ 63 - 65 ] .
non  muscle - invasive bladder cancer ( nmibc ) and muscle - invasive bladder cancer ( mibc ) are associated with distinct molecular pathways .
nmibc is characterized by activation of the ras pathway through mutations in h - ras , fgfr-3 , and pi3k , while mibc is associated with a loss of tumor suppressor genes , including p53 , rb , and pten [ 66 - 72 ] .
activation of the ras pathway occurs in approximately 80% of all nmibcs , whereas more than 50% of mibcs exhibit loss of p53 function [ 70 - 72 ] .
importantly , there is crosstalk between these pathways and the mtor pathway , which means that alterations in either pathway are predicted to influence mtor activity .
reported that intravesical rapamycin instillation exerts striking inhibitory effects on tumor progression in a bladder - specific pten and p53 double knockout transgenic mouse model , which develops carcinoma in situ lesions that progress to mibc .
in addition to the h - ras , p53 , and pten pathways , diverse upstream signals such as ampk and insulin can affect the mtor pathway .
although no direct evidence about the role of the ampk pathway in bladder cancer is currently available , several studies support the idea that the ampk pathway might influence both bladder cancer development and progression .
liu et al .   reported that rhodiola rosea extract and salidroside inhibit the mtor pathway and translational initiation via activation of ampk in umuc-3 bladder cancer cells .
metformin inhibits the growth of bladder cancer cells via indirect activation of ampk , which in turn suppresses the mtor / p70 s6 kinase-1 ( s6k1 ) pathway in 253j and rt4 bladder cancer cell lines .
.   demonstrated that troglitazone ( a synthetic ligand of ppar ) activates autophagy concurrent with the activation of ampk and suppression of the mtor signaling pathway in t24 cells .
activation of mtor occurs via a multistep process including upstream pi3k and ras activation , or inactivation of ampk , leading to phosphorylation and inactivation of the tuberous sclerosis complex 1 and 2 ( tsc1/tsc2 ) heterodimer .
inactivation of tsc1/tsc2 results in release of rheb inhibition and subsequent mtor activation through rheb - gtpase activity .
finally , mtor activity regulates the effects of a number of downstream molecules , including s6k and elongation - initiation factor 4e binding protein-1 . in mibc
, pten loss appears to correlate with an increased mtor activity , suggesting that it exerts a direct effect on downstream signaling components in bladder cancer .
thirty - nine percent of mibc cases exhibit either loss - of - heterozygosity ( loh ) or homozygous deletion of the pten locus , and occasionally , mutations in the pten coding region occur .
additionally , loss of p53 function appears to synergize with pten loss to promote activation of the mtor signaling pathway .
finally , loh of tsc1 has been reported in approximately 50% of bladder cancer cases . taken together
, the deregulation of components upstream of the mtor pathway may occur in bladder tumors , suggesting that mtor signaling may be elevated in this type of cancer .
the results of our analysis revealed that patients with bladder cancer have elevated levels of urinary acetyl - coa and carnitine . because carnitine is crucial for the entry of fatty acids into the mitochondria for oxidation , and acetyl - coa is the final product of this oxidation event , we speculate that -oxidation of fatty acids might be an important factor in the development of bladder cancer .
when we examined the gene expression levels of the enzymes involved in fatty acid oxidation using our published microarray data , we found that bladder cancer specimens expressed significantly higher levels of carnitine palmitoyl transferase 1a ( cpt1a ) than normal bladder mucosae .
cpt is a key enzyme that uses carnitine to transfer fatty acids to the mitochondria for oxidation . in particular ,
our microarray data also showed that bladder cancers express carnitine acylcarnitine translocase - like protein ( cacl ) , another enzyme involved in fatty acid transport to mitochondria , at higher levels than normal bladder mucosae .
although cacl has not been studied as extensively as cpt1a , strategies that target it may also have therapeutic potential in bladder cancer .
thus , in line with other studies implicating fatty acid oxidation in various types of tumorigenesis , our metabolomics study   and microarray analysis   indicated that -oxidation of fatty acid plays an important role in bladder tumorigenesis and aggressiveness .
the level of acetyl - coa , another molecule associated with -oxidation , can be influenced by input from pyruvate through the pyruvate dehydrogenase complex .
we also found that the level of a third component of the complex , dihydrolipoyl dehydrogenase , is significantly reduced in bladder cancer .
this observation suggests that the higher acetyl - coa levels in bladder cancer are largely due to elevated -oxidation , rather than conversion from pyruvate .
our speculation is concordant with the warburg effect in cancer cells , in which pyruvate is converted to lactate rather than acetyl - coa .
although the ampk - fatty acid -oxidation pathway has not been completely characterized in the context of bladder cancer , fatty acid -oxidation is mainly controlled by the ampk pathway , suggesting that ampk acts as an oncogenic regulator of bladder cancer .
the pathways related to ampk - fatty acid -oxidation that are putatively affected by bladder cancer , based on data from our two studies , are summarized in fig .
4 . several lines of evidence suggest that activation of the mtor pathway and stimulation of the fatty acid -oxidation pathway might be important in the development of bladder cancer .
, no study to date has described the connection between the ampk , mtor , and -oxidation pathways .
additional research into the missing link between mtor and -oxidation of fatty acids following ampk activation may reveal novel mechanisms underlying the roles of ampk in bladder cancer .
although ampk activation ( resulting in torc1 inhibition ) is considered to be a reasonable anticancer tactic , ampk may in fact exert very complex effects in tumor cells .
the antidiabetic drug metformin , which inhibits mitochondrial metabolism , inhibits mtorc1 in both an ampk - dependent and ampk - independent manner .
compound c , a selective ampk inhibitor , binds to the ampk subunit and acts as an atp - competitive inhibitor .
however , several studies have demonstrated that compound c can also inhibit many other kinases as well as the bone morphogenetic protein receptor , and this promiscuity raises doubts about whether its effects are mediated by ampk inhibition .
sunitinib , a multiple tyrosine kinase inhibitor that is used clinically against advanced clear - cell renal cell carcinoma , can directly inhibit ampk by binding the ampk subunit .
table 1 summarizes how ampk activity can be affected not only in vitro , but also in growing cells , organs , or whole organisms .
these data were collected from several publications describing the molecular mechanisms and tissue - specific effects of ampk activity .
currently , several lines of evidence suggest that ampk could act as a tumor promoter rather than tumor suppressor .
notably , secondary activation of ampk by many metabolic inhibitors actually induces resistance to apoptosis , suggesting that inhibition of ampk with concurrent induction of metabolic stress represents a more reasonable anticancer therapeutic strategy than activating ampk .
such an approach might provide an effective way to investigate the synergistic anticancer effects of combination therapy using current treatment modalities in conjunction with the relevant ampk inhibitors .
although ampk was originally considered a tumor suppressor that acted via mtorc1 inhibition , today the consensus is that ampk might in fact be a tumor promoter .
this suggests that inhibition of ampk is a more reasonable strategy for treating cancers than activating the enzyme .
although activation of the mtor pathway and elevated fatty acid -oxidation are closely associated with the ampk pathway , no publication has described the connection between the ampk , mtor , and -oxidation pathways . therefore , further research aimed at identifying the missing link between mtor and -oxidation of fatty acid following ampk activation will provide better insight into the role of ampk in bladder cancer .
 the 5-amp - activated protein kinase ( ampk ) is a key regulator of cellular metabolism and energy homeostasis in mammalian tissues . 
 metabolic adaptation is a critical step in ensuring cell survival during metabolic stress . because of its critical role in the regulation of glucose homeostasis and carbohydrate , lipid , and protein metabolism , 
 ampk is involved in many human diseases , including cancers . 
 although ampk signaling was originally characterized as a tumor - suppressive signaling pathway , several lines of evidence suggest that ampk plays a much broader role and can not simply be defined as either an oncogenic regulator or tumor suppressor . 
 notably , several recent studies demonstrated that the antitumorigenic effects of many indirect ampk activators , such as metformin , do not depend on ampk . 
 conversely , activation of ampk induces the progression of cancers , emphasizing its oncogenic effect . 
 bladder cancer can be divided into two groups : non  muscle - invasive bladder cancer ( nmibc ) and muscle - invasive bladder cancer ( mibc ) . 
 the molecular mechanisms underlying these two types of cancer are distinct : nmibc is associated with activation of the ras pathway , whereas mibc is characterized by loss of major tumor suppressors . 
 importantly , both pathways are connected to the mammalian target of rapamycin ( mtor ) pathway . 
 in addition , our recent metabolomic findings suggest that -oxidation of fatty acids is an important factor in the development of bladder cancer . 
 both mtor and -oxidation are tightly associated with the ampk pathway . 
 here , i summarize and discuss the recent findings on the two distinct roles of ampk in cancer , as well as the relationship between bladder cancer and ampk . 
INTRODUCTION
AMPK AND THE WARBURG EFFECT
AMPK: AN ONCOGENIC REGULATOR?
AMPK: A TUMOR SUPPRESSOR?
REGULATION OF AMPK IN BLADDER CANCER
AMPK AS A THERAPEUTIC TARGET FOR CANCER
CONCLUSIONS