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amp - activated protein kinase birt - hogg - dub syndrome folliculin interacting protein hypoxia - inducible factor mouse embryonic fibroblast peroxisome proliferator - activated receptor  coactivator 1  reactive oxygen species
sustaining rapid and persistent proliferation represents a bioenergetic and biosynthetic challenge for cancer cells , which must efficiently manage their energetic resources to survive and grow in unfavorable environments .
this is achieved through metabolic reprogramming to stimulate aerobic glycolysis , a process known as the warburg effect , which is now a well - appreciated hallmark of cancer .
however , the signaling pathways and key regulators of this metabolic transformation are still poorly defined .
amp - activated protein kinase ( ampk ) , a major physiological regulator of cellular energy homeostasis , is located at the center of a metabolic network . indeed , ampk functions as a sensor of cellular energy fluctuation and a driver of pathways that minimize energy consumption and maximize energy production upon energetic stress .
despite its critical function in cell metabolism , the role of ampk in cancer is controversial as both pro- and anti - tumorigenic effects have been described .
in fact , while ampk activation might prevent tumor initiation through its ability to restrict cell proliferation , several reports have shown that ampk gain of function acts as a driver of tumorigenesis by enhancing glycolytic energy production and cell survival under metabolic stress conditions .
our recent work sheds light on how ampk activation could drive metabolic transformation and tumorigenesis .
germline inactivating mutations in the folliculin ( flcn ) tumor suppressor gene predispose to birt - hogg - dub ( bhd ) syndrome , an inherited cancer disorder associated with lung cysts , pneumothorax susceptibility , renal cell carcinoma , and skin tumors .
although flcn and its uncharacterized binding partners folliculin interacting protein 1 ( fnip1 ) and 2 ( fnip2 ) were identified as ampk binding partners and phosphorylation tar binding partner and phosphorylation target , its physiological role and tumor suppressor mechanism and its impact on ampk - dependent functions are still poorly characterized .
we previously reported that loss of flcn enhances transcriptional activity of hypoxia - inducible factor ( hif ) and increases the glycolytic rate in human kidney cancer cells .
moreover , a recent publication demonstrated that conditional deletion of flcn in mouse kidney and muscle results in increased mitochondrial oxidative phosphorylation , suggesting that loss of flcn enhances cellular metabolism . to maintain cellular homeostasis under hypoxic conditions
, hif drives the transcription of genes that stimulate glycolysis , angiogenesis , and energy supply , which in turn promotes solid tumor growth , invasion , and metastasis .
however , how flcn and ampk regulate hif activity and metabolic adaptation in normoxic conditions , and whether this effect is linked to tumorigenesis , is not defined . using untransformed flcn mouse embryonic fibroblasts ( mefs ) and cancer cell lines naturally deficient for flcn expression , we demonstrated that flcn depletion constitutively activates ampk independent of the cellular energy state . using a non - phosphorylatable form of flcn that is mutated at a previously identified ampk phosphorylation and binding site we established that loss of flcn binding to ampk
chronic ampk activation leads to upregulation of the peroxisome proliferator - activated receptor  coactivator 1  ( pgc-1 ) , a known target of ampk , which in turn increases mitochondrial content and the oxidative phosphorylation rate , leading to enhanced production of reactive oxygen species ( ros ) .
surprisingly , elevated mitochondrial ros production is not associated with increased oxidative protein and dna damage , but rather acts as a signaling molecule to activate hif transcriptional activity without affecting hif-1 protein stability . upon flcn depletion ,
the active hif complex mediates transcription of glycolytic effectors , thus enhancing glucose uptake and the glycolytic rate .
strikingly , knockout of ampk , knockdown of pgc-1 , or reduction of ros levels by antioxidants abolish the hif - dependent glycolytic increase ( fig .
1 ) . by concomitantly upregulating mitochondrial oxidative phosphorylation and glycolysis , the chronic ampk activation induced by
flcn deficiency increases cellular atp levels and biosynthetic precursors , mimicking the metabolic signature of highly proliferative cells .
interestingly , this metabolic transformation is not associated with a direct effect on cell proliferation or spontaneous transformation of primary cells per se .
however , loss of flcn significantly enhances the hif-1-dependent anchorage - independent and in vivo tumor growth of human cancer cell lines .
finally , hif-1 nuclear translocation and upregulation of glycolytic effectors controlled by hif were observed in a chromophobe tumor from a bhd patient , further suggesting that flcn deficiency induces ampk- and hif - dependent metabolic reprogramming that confers a tumorigenic advantage in vivo .
taken together , our recently reported data suggest a tumor suppressor mechanism for flcn . we identify flcn as a new physiological negative regulator of ampk - dependent metabolic reprogramming , which might have wider implications for other cancers .
although ampk was previously reported to increase the transcriptional activity of hif without affecting its stability , the mechanism involved was not identified .
strikingly , our findings confirm a link between ampk and hif transcriptional activation in normoxic conditions and identify pgc-1 as an intermediate effector .
in addition , our data highlight a non - canonical role for ros in hif activation that differs from the classical ros - dependent inhibition of prolyl - hydroxylases .
moreover , our work demonstrated that an increase in glycolysis is not necessarily associated with reduced mitochondrial oxidative phosphorylation and that both atp generation processes could work simultaneously to provide metabolic flexibility , thus conferring bioenergetic and biosynthetic advantages on cancer cells .
our data are compatible with recent reports showing that ampk and pgc-1 gain of function mutations are drivers of tumorigenesis by promoting the metabolic plasticity required to support malignant growth and survival under energetic stress conditions , such as hypoxia and nutrient limitation
. this ampk - dependent metabolic adaptation process might be particularly crucial during cancer progression and the development of metastasis . 

folliculin ( flcn ) binds and inhibits phosphorylation ( p ) of amp - activated protein kinase ( ampk ) via serine 62 ( s62 ) , a previously described ampk phospho - site . upon loss of flcn ,
ampk is activated by phosphorylation by an unidentified kinase ( kinase x ) and stimulates transcription and expression of peroxisome proliferator - activated receptor  coactivator 1 ( pgc-1 ) , leading to enhanced mitochondrial biogenesis and reactive oxygen species ( ros ) production .
this drives hypoxia - inducible factor ( hif ) transcriptional activation and enhances warburg metabolic reprogramming . 
 
folliculin ( flcn ) binds and inhibits phosphorylation ( p ) of amp - activated protein kinase ( ampk ) via serine 62 ( s62 ) , a previously described ampk phospho - site . upon loss of flcn , ampk is activated by phosphorylation by an unidentified kinase ( kinase x ) and stimulates transcription and expression of peroxisome proliferator - activated receptor  coactivator 1 ( pgc-1 ) , leading to enhanced mitochondrial biogenesis and reactive oxygen species ( ros ) production .
this drives hypoxia - inducible factor ( hif ) transcriptional activation and enhances warburg metabolic reprogramming .
tumor cells manage their energy to support aberrant proliferation by reprogramming their cellular metabolism , for example through the warburg effect . 
 although ampk is a major regulator of energy homeostasis , its role in cancer metabolic adaptation is unclear . 
 we recently identified the tumor suppressor folliculin as a new regulator of ampk - dependent metabolic transformation .