Hypoxic Regulation of Vascular Endothelial Growth Factor through the Induction of Phosphatidylinositol 3-Kinase/Rho/ROCK and c-Myc

Abstract

The induction of vascular endothelial growth factor (VEGF) is an essential feature of tumor angiogenesis. Hypoxia is a potent stimulator of VEGF expression, and hypoxia-inducible factor-1 (HIF-1) is considered to be critical for this induction. However, we have previously demonstrated that induction of VEGF by hypoxia was preserved when HIF-1␣ was silenced. We sought to better define the molecular basis of this HIF-1-independent regulation. In colon cancer cells, hypoxia stimulated multiple K-ras effector pathways including phosphatidylinositol 3-kinase. VEGF promoter deletion studies identified a novel promoter region between ؊418 and ؊223 bp that was responsive to hypoxia in a PI3K/Rho/ROCK-dependent manner. Electrophoretic mobility shift assays identified a fragment between ؊300 and ؊251 bp that demonstrated a unique shift only in hypoxic conditions. Inhibition of PI3K or ROCK blocked the formation of this complex. A binding site for c-Myc, a target of ROCK, was identified at ؊271 bp. A role for c-Myc in the hypoxic induction of VEGF was demonstrated by site-directed mutagenesis of the VEGF promoter and silencing of c-Myc by small interfering RNA. Collectively, these findings suggest an alternative mechanism for the hypoxic induction of VEGF in colon cancer that does not depend upon HIF-1␣ but instead requires the activation of PI3K/Rho/ROCK and c-Myc.

Fasudil-induced hypoxia-inducible factor-1α degradation disrupts a hypoxia-driven vascular endothelial growth factor autocrine mechanism in endothelial cells

“…However, in colon cancer cells, Mizukami et al clarified a role for the activation of c-Myc by Rho/ROCK signaling as an alternative HIF-1 -independent mechanism for the induction of VEGF in hypoxia (29). HIF-1 -independent mechanism also might be available in hypoxia-induced VEGF autocrine loop in ECs.…”

“…Recent reports showed that the Rho protein is up-regulated in hypoxia and involved in HIF-1a induction in renal cell carcinoma (27) and trophoblast cells (28). On the other hand, Mizukami et al reported an alternative mechanism for the hypoxic induction of VEGF in colon cancer that does not depend on HIF-1a but instead requires the activation of phosphatidylinositol 3-kinase/Rho/Rhoassociated kinase (ROCK) and c-Myc (29). Then, we characterized Rho GTPase expression and activity under hypoxic conditions and investigated its potential roles in hypoxic responses in ECs.…”

The progress of angiogenic factors in the development of leukemias

“…In recent years, it have been discovered that c-myc also acts as an important tumor angiogenesis factor (TAF) and participates in tumor angiogenesis, and c-myc was related to the expression of VEGF (44). There is a c-myc-binding region located at the 271bp of the VEGF promoter.…”

Mitogen-Activated Protein 3 Kinase 6 Mediates Angiogenic and Tumorigenic Effects via Vascular Endothelial Growth Factor Expression

“…16 However, the induction of VEGF expression in hypoxic conditions is not fully HIF-dependent. [17][18][19] In evidence of this, the tyrosine kinase inhibitor gefitinib decreased VEGF expression in SQ20B squamous cell carcinoma by both HIF-dependent and -independent mechanisms, 18 whereas in colon cancer cells, the hypoxic induction of VEGF was only partially blocked by HIF-1␣ knock-down. 19 RNA interference (RNAi) was originally described as a powerful tool for the inhibition of gene expression in Caenorhabditis elegans and Drosophila.…”