HIF-1α: A key survival factor for serum-deprived prostate cancer cells

BACKGROUND Hypoxia‐inducible factor‐1α (HIF‐1α) is commonly overexpressed in prostate cancer (PCa) cells. As PCa cells are known to survive serum deprivation, we investigated the effect of prolonged serum deprivation on HIF‐1α expression, and the function of HIF‐1α in regulating the survival of normoxic serum‐deprived PCa cells. METHODS HIF‐1α protein was assessed by immunoblots. Cell viability and proliferation were assessed by trypan blue assay and flow cytometric analysis. Transcriptional activity was assessed by luciferase reporter assay and RT‐PCR. HIF‐1α expression was suppressed with siRNA. Activities of HIF‐1α‐target genes were inhibited with neutralizing antibody. RESULTS Prolonged serum deprivation is a potent inducer of HIF‐1α in PC‐3 and LNCaP PCa cells, despite normal oxygen conditions. In contrast, cells grown in the presence of serum did not show HIF‐1α protein accumulation. Moreover, HIF‐1α protein increase during serum deprivation correlated with increased cell survival, while suppression of HIF‐1α expression significantly decreased PCa cell viability. Our results further demonstrate that HIF‐1α protein increase is due to increased HIF‐1α protein synthesis. First, there was no significant increase in HIF‐1α mRNA. Secondly, cycloheximide, a protein synthesis inhibitor, prevented HIF‐1α protein increase in serum‐deprived PCa cells. Moreover, the expression of HIF‐1α‐target genes, VEGF and IGF‐2, was concomitantly increased in serum‐deprived PCa cells, while suppression of HIF‐1α expression significantly inhibited their induction. Furthermore, inhibition of IGF‐2 activity resulted in a significant decline in PCa cell survival. CONCLUSION PCa cells counteract the stress of prolonged serum deprivation by upregulating HIF‐1α protein which increases IGF‐2 expression to promote cell survival. Prostate 68: 1405–1415, 2008. © 2008 Wiley‐Liss, Inc.