Zinc Downregulates HIF-1α and Inhibits Its Activity in Tumor Cells In Vitro and In Vivo

Background: Hypoxia inducible factor-1 alpha (HIF-1 alpha) is responsible for the majority of HIF-1-induced gene expression changes under hypoxia and for the angiogenic switch'' during tumor progression. HIF-1 alpha is often upregulated in tumors leading to more aggressive tumor growth and chemoresistance, therefore representing an important target for antitumor intervention. We previously reported that zinc downregulated HIF-1 alpha levels. Here, we evaluated the molecular mechanisms of zinc-induced HIF-1 alpha downregulation and whether zinc affected HIF-1 alpha also in vivo. Methodology/Principal Findings: Here we report that zinc downregulated HIF-1 alpha protein levels in human prostate cancer and glioblastoma cells under hypoxia, whether induced or constitutive. Investigations into the molecular mechanisms showed that zinc induced HIF-1 alpha proteasomal degradation that was prevented by treatment with proteasomal inhibitor MG132. HIF-1 alpha downregulation induced by zinc was ineffective in human RCC4 VHL-null renal carcinoma cell line; likewise, the HIF-1 alpha P402/P564A mutant was resistant to zinc treatment. Similarly to HIF-1 alpha, zinc downregulated also hypoxia-induced HIF-2 alpha whereas the HIF-1 beta subunit remained unchanged. Zinc inhibited HIF-1 alpha recruitment onto VEGF promoter and the zinc-induced suppression of HIF-1-dependent activation of VEGF correlated with reduction of glioblastoma and prostate cancer cell invasiveness in vitro. Finally, zinc administration downregulated HIF-1 alpha levels in vivo, by bioluminescence imaging, and suppressed intratumoral VEGF expression. Conclusions/Significance: These findings, by demonstrating that zinc induces HIF-1 alpha proteasomal degradation, indicate that zinc could be useful as an inhibitor of HIF-1 alpha in human tumors to repress important pathways involved in tumor progression, such as those induced by VEGF, MDR1, and Bcl2 target genes, and hopefully potentiate the anticancer therapies.