TZD have been recently shown to activate AMPK, a cellular sensor of energy CDK inhibitor status.37 AMPK may suppress tumorigenesis regulating cell growth via inhibition of mammalian target of rapamycin (mTOR) signaling and p53 activation, and it is indicated as a beneficial target for cancer treatment.38 We showed that TZD induced AMPK activation in PPARγ-deficient hepatocytes and that inhibition of AMPK activity completely prevented the TZD-induced growth arrest and NPM expression. These results are in agreement with the observation that TZD specifically inhibit IGF-I tumor-promoting activity

in mouse skin through activation of AMPK and subsequent inhibition of mTOR pathway.20 In addition, AMPK activation was demonstrated to induce p53 phosphorylation and p53-dependent apoptotic cell death in response to energetic stress.39 Although there is no evidence for a direct involvement of NPM in the regulation of the apoptotic machinery, NPM might function as an antiapoptotic protein through indirect mechanisms. Interaction with p53 might be an important step by which NPM inhibits programmed cell death. In fact, NPM overexpression protects mouse embryonic fibroblast against hypoxic cell death, but this effect is not observed in cell that lacks p53.40 We showed that in cultured PPARγ-deficient hepatocytes, ectopic expression of wild-type NPM significantly blocked TZD inhibition whereas

a mutant click here variant lacking the p53-interacting domain did not prevent TZD antiproliferative and proapoptotic actions. Similarly, in malignant haematopoietic cells, the same NPM mutant does not prevent apoptosis in response to stress stimuli, unlike the overexpression of wild-type NPM.41 NPM-p53 interaction inhibits p53 phosphorylation at the serine 15, and subsequently represses p53 target genes expression such as the cell cycle inhibitor p21. However, in hepatic cells TZD may promote p53 phosphorylation by inhibiting NPM gene expression. Interestingly, NPM has also been shown to interact with p53 in hypoxic cells and to inhibit hypoxia-induced p53 phosphorylation on the same residue.42 Besides, regulation N-acetylglucosamine-1-phosphate transferase of p53 expression and activity by TZD has been

also demonstrated in human cholangiocarcinoma cells.43 In consideration that the ability of AMPK to induce cell cycle arrest is dependent on p53 phosphorylation at Ser15,39 it might be conceivable that TZD modify p53 phosphorylation status and activity by an AMPK-mediated down-regulation of NPM. In conclusion, we have shown that chronic administration of TZD inhibits hepatic tumor formation in mice with a PPARγ-independent mechanism. Furthermore, we found that the anticancer activity of these drugs in the liver was mediated, at least in part, by inhibition of NPM expression and p53 activation. Collectively, these observations provide new insight into the molecular mechanisms of hepatic carcinogenesis and emphasize relevant clinical implication.