eif4e - binding protein 1 castration - resistant prostate cancer eukaryotic translation initiation factor 4e mammalian target of rapamycin complex 1 tumor suppression protein 53 positron emission tomography phosphatase and tensin homolog
accelerated glucose metabolism in cancer cells under aerobic conditions , a phenomenon known as the warburg effect , leads to high uptake of the labeled glucose analog fluorodeoxyglucose ( fdg ) , which can be clinically useful to detect tumors and monitor therapeutic responses of cancer patients by positron emission tomography ( pet ) . thus , identification of the enzyme(s ) that catalyze the elevated glucose metabolism in cancer cells could be exploited not only to distinguish cancer cells from normal cells , but also to preferentially target cancer cells while sparing healthy cells .
hexokinases ( hks ) catalyze the essentially irreversible first step of glucose metabolism in cells by phosphorylating glucose to glucose-6-phosphate ( g-6-p )
. there are 4 hk isoforms encoded by separate genes , hk1 , hk2 , hk3 , and hk4 ( also known as glucokinase ) .
hk1 is ubiquitously expressed in almost all mammalian tissues and hk2 is normally expressed in insulin - sensitive tissues such as adipose , skeletal , and cardiac muscles .
hk3 is usually expressed at low levels and hk4 expression is restricted to the pancreas and liver .
although elevated hk2 expression has been observed in certain types of cancer cell and in tumor tissues from mouse models and/or human patients , the molecular mechanisms underlying hk2 upregulation remain incompletely understood .
accumulating evidence suggests that co - deletion of tumor suppressor genes , such as phosphatase and tensin homolog ( pten ) and tumor suppressor protein p53 ( tp53 , best known as p53 ) , plays a crucial role in the development of castration - resistant prostate cancer ( crpc ) in vivo . through integrated analyses of mouse embryonic fibroblasts ( mefs ) deficient in pten and p53 , prostate cancer cell lines , xenografts , and genetically engineered mouse models ( gemms ) , as well as clinic prostate cancer samples
, we have found that pten / p53 deficiency selectively enhances expression of hk2 , but not hk1 , through post - transcriptional and translational regulation . regarding the underlying mechanism , we have demonstrated that activation of akt 
mtorc1 signaling as a result of pten deletion increases hk2 expression primarily at the translational level through phosphorylation of eif4e - binding protein 1 ( 4e - bp1 ) , whereas loss of p53 decreases the biogenesis of mir143 , which in turn causes degradation of hk2 mrna . as a result
, the combined deficiency of pten and p53 in prostate cancer cells synergistically leads to robustly elevated hk2 expression ( fig .  1 ; ref . 6 ) . 

figure 1.induction of the hk2-mediated warburg effect is required for pten / p53 deficiency - driven prostate tumorigenesis .
loss of pten in prostate epithelial cells activates akt  mtorc1 signaling to initiate phosphorylation of 4e - bp1 , which releases eif4e allowing formation of the translation initiation complex at the 5 end of hk2 mrna , and prompting cap - dependent translation .
loss of p53 in prostate epithelial cells decreases the biogenesis of mir143 , which in turn leads to the degradation of hk2 mrna .
the overexpressed hk2 protein initiates warburg effect - dependent prostate tumor growth ; therefore , targeting hk2 may inhibit prostate tumorigenesis that is driven by pten / p53 deficiency .
4e - bp1 , eif4e - binding protein 1 ; eif4e , eukaryotic translation initiation factor 4e ; mtorc1 , mammalian target of rapamycin complex 1 . 
 
induction of the hk2-mediated warburg effect is required for pten / p53 deficiency - driven prostate tumorigenesis .
loss of pten in prostate epithelial cells activates akt  mtorc1 signaling to initiate phosphorylation of 4e - bp1 , which releases eif4e allowing formation of the translation initiation complex at the 5 end of hk2 mrna , and prompting cap - dependent translation .
loss of p53 in prostate epithelial cells decreases the biogenesis of mir143 , which in turn leads to the degradation of hk2 mrna .
the overexpressed hk2 protein initiates warburg effect - dependent prostate tumor growth ; therefore , targeting hk2 may inhibit prostate tumorigenesis that is driven by pten / p53 deficiency . 4e - bp1 , eif4e - binding protein 1 ; eif4e , eukaryotic translation initiation factor 4e ; mtorc1 , mammalian target of rapamycin complex 1 .
notably , hk2 is almost exclusively expressed in human prostate cancer tissue compared with normal prostate tissue , and its expression is particularly elevated in human prostate cancer harboring pten / p53 mutations . in line with our findings that hk2 expression level positively correlates with the gleason score ,
the sensitivity and positive predictive value of fdg - pet based on hk2-mediated phosphorylation of fdg for detecting patients with advanced prostate cancers was as high as 87% .
these results imply that hk2 expression distinguishes cancer cells from normal cells and could serve as a potential diagnostic and prognostic biomarker for advanced human prostate cancer , especially for patients harboring defects or mutations in pten and p53 .
our genetic studies demonstrated that the hk2-mediated warburg effect is required for the growth of pten / p53-deficient prostate cancer cells in vitro and in xenograft models carrying mouse or human pten / p53-deficient prostate cancer cell lines in vivo .
these findings are consistent with a previous study of glioblastoma showing that hk2 depletion by shrna inhibits tumor growth in a xenograft model .
more recently , a study using hk2 conditional knockout mice found that hk2 is required for tumor initiation and maintenance in mouse models of kras - driven lung cancer and erbb2-driven breast cancer .
our study extends the biological significance of hk2 to prostate cancers carrying pten / p53 mutations , which drive the genesis of currently incurable castration - resistant prostate cancer ( crpc ) .
systemic deletion of hk2 in genetic mouse models inhibits tumor progression but does not impair normal glucose homeostasis or elicit any notable phenotypes in vivo , indicating that hk2 could be a selective therapeutic target for cancer without any adverse physiological consequences .
given that our genetic findings support the crucial role of increased hk2 expression in driving the warburg effect and prostate tumor progression in the presence of physiological levels of hk1 , selectively targeting hk2 in these prostate cancer cells could be exploited as a promising personalized therapeutic strategy for patients with crpc carrying pten / p53 mutations ( fig .  1 ) .
however , it would be very challenging to design an isoform - specific pharmacological inhibitor because hk2 has overlapping enzymatic activities with the ubiquitously expressed hk1 , which is required for glucose metabolism of normal cells .
considering that only the c - terminal half of hk1 retains catalytic activity , whereas both n- and c - terminal halves of hk2 are catalytically active with the n - terminal half showing higher enzyme activity , it is possible to use computational bioinformatics to identify small molecular compounds that might specifically block hk2 activity by targeting the n - terminal half of hk2 in cancer cells .
in addition , the enzymatic activities of hk1 and hk2 are inhibited to the same extent by their own product g-6-p , yet inorganic phosphate prevents the inhibition of hk1 by g-6-p while enhancing the inhibition of hk2 .
does genetic deficiency of hk2 ( conditional deletion of hk2 in prostate epithelial cells ) effectively inhibit pten / p53 deficiency - driven crpc in genetically engineered 
triple - deficient  mouse models ? can currently available hk2 enzymatic inhibitors such as 2-deoxyglucoe ( 2-dg ) and 3-bromopyruvate ( 3-brpa ) inhibit pten / p53 deficiency - driven crpc in vivo ? does akt  mtorc14ebp1-mediated translation signaling contribute to hk2 overexpression in prostate cancer cells carrying alterations in genes other than pten and p53 ? addressing these questions will provide deeper insights into the regulation and crucial role of hk2 in prostate tumorigenesis and potentially open up new avenues to treat currently incurable crpc .
this work was supported , in part , by grants from the us national cancer institute ( r01 ca160333 , yd ; r21 ca155522 , yd and jl ; r01 ca172169 , jl and yd ) , the university of minnesota grant - in aid ( yd ) and start - up funds from the hormel foundation ( yd ) .
 aerobic glycolysis , known as the warburg effect , is one of the hallmarks of cancer cells . 
 we recently reported that the hexokinase 2 ( hk2)-mediated warburg effect is required for castration - resistant prostate cancer that is driven by pten / p53 deficiency , suggesting that hk2 might be a therapeutic target for prostate cancer patients carrying pten and p53 mutations . 
Abbreviations
None
Disclosure of Potential Conflicts of Interest
Funding