Opposing roles for HIF-1α and HIF-2α in the regulation of angiogenesis by mononuclear phagocytes.

Macrophages contribute to tumor growth through the secretion of the proangiogenic molecule vascular endothelial growth factor (VEGF). We previously observed that monocytes treated with the cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) produce a soluble form of the VEGF receptor-1 (sVEGFR-1), which neutralizes VEGF biologic activity. The VEGF and VEGFR-1 promoters both contain a hypoxia regulatory element, which binds the hypoxia-inducible factor (HIF) transcription factors under hypoxic conditions. Based on this observation, we examined VEGF and sVEGFR-1 production from monocytes cultured at various O(2) concentrations. The amount of sVEGFR-1 production observed from GM-CSF-treated monocytes increased with decreasing levels of O(2). This sVEGFR-1 was biologically active and sequestered VEGF. To evaluate the role of the HIFs in sVEGFR-1 production, we used macrophages with a genetic deletion of HIF-1α. HIF-1α(-/-) macrophages cultured with GM-CSF at hypoxia secreted diminished amounts of VEGF compared with HIF-1α(+/+) macrophages, whereas sVEGFR-1 secretion was unaffected. In contrast, siRNA-mediated knockdown of HIF-2α inhibited the production of sVEGFR-1 in response to GM-CSF and low O(2), whereas VEGF production was unaffected. These studies suggest that hypoxia, generally thought to promote angiogenesis, can induce antiangiogenic behavior from macrophages within a GM-CSF-rich environment. Furthermore, these results suggest specific and independent roles for HIF-1α and HIF-2α in hypoxic macrophages.