Control of HIF-1α Expression by eIF2α Phosphorylation-Mediated Translational Repression

Hypoxia inducible factor 1 alpha (HIF-1 alpha) plays a central role in regulating tumor angiogenesis via its effects on vascular endothelial growth factor (VEGF) transcription, and its expression is regulated through proteasome-mediated degradation. Paradoxically, previous studies have shown that proteasome inhibitors (PI) block tumor angiogensis by reducing VEGF expression, but the mechanisms have not been identified. Here, we report that Pis down-regulated HIF-1 alpha protein levels and blocked HIF-1 alpha transcriptional activity in human prostate cancer cells. Pis induced phosphorylation of the translation initiation factor 2 alpha (eIF2 alpha), which caused general translational repression to inhibit HIF-1 alpha expression. Furthermore, Pis induced HIF-1 alpha accumulation in LNCaP-Pro5 cells depleted of eIF2 alpha via siRNA transfection and in MEFs expressing a phosphorylation-deficient mutant form of eIF2 alpha. Finally, Pis failed to induce eIF2 alpha phosphorylation or translational attenuation in DU145 or 253JB-V cells, and, in these cells, Pis promoted HIF-1 alpha accumulation. Our data established that Pis down-regulated HIF-1 alpha expression in cells that display activation of the unfolded protein response by stimulating phosphorylation of eIF2 alpha and inhibiting HIF-1 alpha translation. [Cancer Res 2009;69(5):1836-43]