Role of iron and iron metabolism in hypoxia inducible factor (HIF)-mediated events

Hypoxia initiates an important mechanism for the survival and progression of cancer. Tumor cells that adapt to the selective pressure of hypoxic stress can further advance to more aggressive phenotypes. Hypoxia inducible factor (HIF) regulates the expression of genes that help cells to adapt to low oxygen conditions. The goal of this research was to examine how cellular iron contributes to hypoxic signaling pathways that lead to angiogenesis.; We have tested the hypothesis that decrease in cellular iron during hypoxia contribute to HIF stabilization and downstream biological processes. In the present study, human breast cancer MDA-MB-231 cells exposed to hypoxia (1% O2) showed significant decreases in both low molecular-weight (LMW) iron (p < 0.02) and ferritin (Ft) (p < 0.05) as compared to normoxic controls. Concurrently, iron regulatory protein binding to iron response elements increased, indicating that cells under hypoxia were in an iron-depleted state. The observed hypoxia-mediated iron decreases lead to HIF-1alpha stabilization and increased vascular endothelial growth factor (VEGF) as well as membrane type 1-matrix metalloprotease MT1-MMP.; Our data also showed that cells treated with iron under hypoxia elevate LMW iron levels, cause HIF-1alpha destabilization, and reduce VEGF production. In vitro angiogenesis assays also showed that hypoxia-treated tumor cells activate endothelial cells in co-culture, which could be prevented by iron treatment prior to hypoxic exposure.; To further confirm the role of iron in hypoxia-mediated biological events such as angiogenesis, MDA-MB-231 cells were transfected with transferrin receptor (TfR) small interfering RNA (siRNA) to constitutively impair iron uptake. Significant decrease in Ft with concomitant increase in VEGF was observed in TfR siRNA-transfected cells as compared to parental MDA-MB-231 cells. TfR siRNA transfectants also evoked a stronger angiogenic response than did parental MDA-MB-231 cells after the cells were injected subcutaneously into nude mice.; Thus, our results show that hypoxia decreases LMW iron in cells, which contributes to HIF stabilization and angiogenic signaling in cancer cells. This project demonstrated a new important role for iron in angiogenesis of the tumor microenvironment, which contributes to tumor progression.