Dual PI3K/mTOR inhibitor NVP-BEZ235 suppresses hypoxia-inducible factor (HIF)-1α expression by blocking protein translation and increases cell death under hypoxia.

The PI3K/Akt pathway is activated in many cancers; therefore, we investigated NVP-BEZ235, a dual PI3K/mTOR inhibitor. BEZ235 was more potent than either the mTOR inhibitor rapamycin or the PI3K inhibitor LY294002 in blocking HIF-1α induction. BEZ235 decreases protein translation, and 7-methyl GTP chromatography showed that the drug induced robust recruitment of 4E-BP1 to eIF4E and a near absence of binding of eIF4G. BEZ235 also decreased expression of other proteins known to be regulated by eIF4E including cyclin B1 and D1 and vascular endothelial growth factor (VEGF). BEZ235 also decreased the level of eIF4G but not eIF4E. As HIF-1α has been associated with adaptation to hypoxic stress, we examined the effect of the drug on cell survival in low pO 2. BEZ235 increased killing of cells under hypoxia, measured by short-term (MTT) and long-term (clonogenic) assays. To understand the underlying mechanism, we examined BEZ235's effect on the expression of factors associated with cell survival. Under normoxia, Akt Ser473 phosphorylation decreased within an hour of BEZ235 treatment, but then increased by 24 h. In contrast, under hypoxia, BEZ235 caused prolonged suppression of Akt Ser473 phosphorylation. Furthermore, there was greater PARP cleavage in hypoxic cells than in normoxic cells, consistent with increased apoptosis. BEZ235 increased autophagy as measured by LC3-I to LC3-II conversion under both normoxic and hypoxic conditions, but our data indicate that this is actually a pro-survival mechanism. In conclusion, we have found that BEZ235 blocks HIF-1α induction by decreasing protein translation and increases cell killing under hypoxia, likely by increasing apoptosis.