Role of Hypoxia-Inducible Factor (HIF)-1α versus HIF-2α in the Regulation of HIF Target Genes in Response to Hypoxia, Insulin-Like Growth Factor-I, or Loss of von Hippel-Lindau Function: Implications for Targeting the HIF Pathway

Jun 17, 2006Cancer research

Different roles of HIF-1alpha and HIF-2alpha in controlling HIF-related genes under low oxygen, growth signals, or loss of VHL function: insights for targeting the HIF pathway

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Abstract

Overexpression of hypoxia-inducible factors (HIF) is associated with tumor progression in several cancers.

HIF-1alpha primarily regulates VEGF expression in response to hypoxia and insulin-like growth factor (IGF)-I in MCF-7 cells.
A reciprocal relationship exists between HIF-1alpha and HIF-2alpha expression under hypoxic conditions.
In renal carcinoma cells with high basal levels of HIF, VEGF and other target gene expressions are predominantly dependent on HIF-2alpha.
A new small-molecule inhibitor of HIF-1, NSC-134754, significantly decreases both HIF-2alpha protein expression and HIF-2alpha-regulated VEGF levels.

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Overexpression of hypoxia-inducible factors (HIF), HIF-1alpha and HIF-2alpha, leads to the up-regulation of genes involved in proliferation, angiogenesis, and glucose metabolism and is associated with tumor progression in several cancers. However, the contribution of HIF-1alpha versus HIF-2alpha to vascular endothelial growth factor (VEGF) expression and other HIF-regulated target genes under different conditions is unclear. To address this, we used small interfering RNA (siRNA) techniques to knockdown HIF-1alpha and/or HIF-2alpha expression in response to hypoxia, insulin-like growth factor (IGF)-I, or renal carcinoma cells expressing constitutively high basal levels of HIF-1alpha and/or HIF-2alpha due to loss of von Hippel-Lindau (VHL) function. We found that HIF-1alpha primarily regulates transcriptional activation of VEGF in response to hypoxia and IGF-I compared with HIF-2alpha in MCF-7 cells. We also observed a reciprocal relationship between HIF-1alpha and HIF-2alpha expression in hypoxia in these cells: HIF-2alpha siRNA enhanced HIF-1alpha-mediated VEGF expression in MCF-7 cells in response to hypoxia, which could be completely blocked by cotransfection with HIF-1alpha siRNA. In contrast, in renal carcinoma cells that constitutively express HIF-1alpha and HIF-2alpha due to loss of VHL function, we found that high basal VEGF, glucose transporter-1, urokinase-type plasminogen activator receptor, and plasminogen activator inhibitor-1 expression was predominantly dependent on HIF-2alpha. Finally, we showed that a newly identified small-molecule inhibitor of HIF-1, NSC-134754, is also able to significantly decrease HIF-2alpha protein expression and HIF-2alpha-regulated VEGF levels in renal carcinoma cells. Our data have important implications for how we target the HIF pathway therapeutically.

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Role of Hypoxia-Inducible Factor (HIF)-1α versus HIF-2α in the Regulation of HIF Target Genes in Response to Hypoxia, Insulin-Like Growth Factor-I, or Loss of von Hippel-Lindau Function: Implications for Targeting the HIF Pathway

Abstract

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