NRF2 Blockade Suppresses Colon Tumor Angiogenesis by Inhibiting Hypoxia-Induced Activation of HIF-1α

Abstract

Transcription factor NRF2 is an important modifier of cellular responses to oxidative stress. Although its cytoprotective effects are firmly established, recent evidence suggesting important roles in cancer pathobiology has yet to be mechanistically developed. In the current study, we investigated the role of NRF2 in colon tumor angiogenesis. Stable RNAi-mediated knockdown of NRF2 in human colon cancer cells suppressed tumor growth in mouse xenograft settings with a concomitant reduction in blood vessel formation and VEGF expression. Similar antiangiogenic effects of NRF2 knockdown were documented in chick chorioallantoic membrane assays and endothelial tube formation assays. Notably, NRF2-inhibited cancer cells failed to accumulate HIF-1α protein under hypoxic conditions, limiting expression of VEGF and other HIF-1α target genes. In these cells, HIF-1α was hydroxylated but pharmacological inhibition of PHD domain-containing prolyl hydroxylases was sufficient to restore hypoxia-induced accumulation of HIF-1α. Mechanistic investigations demonstrated that reduced mitochondrial O(2) consumption in NRF2-inhibited cells was probably responsible for HIF-1α degradation during hypoxia; cellular O(2) consumption and ATP production were lower in NRF2 knockdown cells than in control cells. Our findings offer novel insights into how cellular responses to O(2) and oxidative stress are integrated in cancer cells, and they highlight NRF2 as a candidate molecular target to control tumor angiogenesis by imposing a blockade to HIF-1α signaling.