Bortezomib represses HIF-1α protein expression and nuclear accumulation by inhibiting both PI3K/Akt/TOR and MAPK pathways in prostate cancer cells

Bortezomib represents the first proteasome inhibitor (PI) with demonstrated antitumor activity in the clinical setting, particularly for treatment of hematological malignancies. At the preclinical level, its action is shown to be mediated by induction of growth arrest and apoptosis in many tumor types, including androgen-dependent (AD) and androgen-independent (AI) prostate cancer (PCa) cells. Hypoxiainducible factor-1 alpha (HIF-1 alpha), which is directly involved in tumor growth, is one of the most studied and promising molecular targets for anti-cancer therapy and is often overexpressed in PCa. Bortezomib has been reported to impair tumor growth by also inhibiting HIF-1 alpha. In this study, we investigated the effect of bortezomib on the expression, activity and localization of HIF-1 alpha in LNCaP (AD) and PC3 (AI) PCa cells. First, we show that hypoxic upregulation of HIF-1 alpha protein levels and activity involves both the PI3K/Akt/mTOR and p44/42 MAPK pathways. Second, bortezomib inhibits expression of HIF-1 alpha protein under both normoxic and hypoxic conditions, represses HIF-1 transcriptional activity and attenuates the release of vascular endothelial growth factor. These effects correlate with the ability of bortezomib to cause dephosphorylation of phospho-Akt, phospho-p70S6K, and phospho-S6RP, thus inactivating a pathway known to be required for HIF-1 alpha protein expression at the translational level. Furthermore, bortezomib also abrogates p44/42 MAPK phosphorylation, which results to reduced nuclear translocation of HIF-1 alpha. Taken together, these results suggest that bortezomib inhibits HIF-1 alpha protein synthesis and its nuclear targeting through suppression of PI3K/Akt/mTOR and MAPK pathways, respectively, in both AD and AI PCa cells.