Vascular endothelial growth factor signaling in hypoxia and inflammation

Abstract

Infection, cancer and cardiovascular diseases are the major causes for morbidity and mortality in the United States according to the Center for Disease Control. The underlying etiology that contributes to the severity of these diseases is either hypoxia induced inflammation or inflammation resulting in hypoxia. Therefore, molecular mechanisms that regulate hypoxia-induced adaptive responses in cells are important areas of investigation. Oxygen availability is sensed by molecular switches which regulate synthesis and secretion of growth factors and inflammatory mediators. As a consequence, tissue microenvironment is altered by re-programming metabolic pathways, angiogenesis, vascular permeability, pH homeostasis to facilitate tissue remodeling. Hypoxia inducible factor (HIF) is the central mediator of hypoxic response. HIF regulates several hundred genes and vascular endothelial growth factor (VEGF) is one of the primary target genes. Understanding the regulation of HIF and its influence on inflammatory response offers unique opportunities for drug development to modulate inflammation and ischemia in pathological conditions.

Figure 1

Hypoxia and inflammation associated diseases. A: Summary of major pathological conditions arising from hypoxia/ischemia-mediated inflammation. B: Shows how inflammation results in hypoxic conditions leading to pulmonary and gastrointestinal diseases.

Figure 2

A: HIF-1 regulation. Schematic diagram shows canonical regulation of HIF-1α. In normoxia, HIF-1α is modified by prolyl-4-hydroxylases (PHD). PHD enzymes (PHD1, PHD2, PHD3) are Fe²⁺ and oxoglutarate dependent dioxygenases that use oxygen as a cofactor. PHD2 is involved in the regulation of developmental angiogenesis. PHD2 Gene deletion increases angiogenesis and erythropoiesis. Hydroxylation of proline residues in HIF-1α is recognized by VHL, tumor suppressor protein. Subsequent assembly of ubiquitin ligases results in polyubiquitination of HIF-1α and proteasomal degradation. HIF-2α is also regulated by a similar mechanism. Hypoxia and iron chelation can inhibit PHD activity and thereby prevents proteasomal degradation of HIF-1/2α. In addition, HIF-1α - mediated transactivation is modulated by hydroxylation of an Asn residue and its ability to recruit co-activators. Noncanonical regulation of HIF-1α involves chaperon-mediated sequestration, NFκB-depdendent signaling and microRNA network. B: A representative confocal image showing HIF-2 nuclear translocation in hypoxia. A2780 ovarian cancer cells were exposed to either normoxia or hypoxia for 24 hours. Indirect immunofluorescence using an antibody to HIF-2α shows nuclear accumulation of HIF-2α (red). Nuclei are stained with DAPI (blue).

Figure 2

A: HIF-1 regulation. Schematic diagram shows canonical regulation of HIF-1α. In normoxia, HIF-1α is modified by prolyl-4-hydroxylases (PHD). PHD enzymes (PHD1, PHD2, PHD3) are Fe²⁺ and oxoglutarate dependent dioxygenases that use oxygen as a cofactor. PHD2 is involved in the regulation of developmental angiogenesis. PHD2 Gene deletion increases angiogenesis and erythropoiesis. Hydroxylation of proline residues in HIF-1α is recognized by VHL, tumor suppressor protein. Subsequent assembly of ubiquitin ligases results in polyubiquitination of HIF-1α and proteasomal degradation. HIF-2α is also regulated by a similar mechanism. Hypoxia and iron chelation can inhibit PHD activity and thereby prevents proteasomal degradation of HIF-1/2α. In addition, HIF-1α - mediated transactivation is modulated by hydroxylation of an Asn residue and its ability to recruit co-activators. Noncanonical regulation of HIF-1α involves chaperon-mediated sequestration, NFκB-depdendent signaling and microRNA network. B: A representative confocal image showing HIF-2 nuclear translocation in hypoxia. A2780 ovarian cancer cells were exposed to either normoxia or hypoxia for 24 hours. Indirect immunofluorescence using an antibody to HIF-2α shows nuclear accumulation of HIF-2α (red). Nuclei are stained with DAPI (blue).

Figure 3

Schematic diagram shows VEGF family of growth factors and their receptors. Alternate splicing results in multiple splice variants of VEGF (not shown). Larger variants of VEGF have a heparin binding domain and are sequestered by heparin sulfate proteoglycans (HSPG). Sufatases and proteases secreted into the tissue microenvironment can release HSPG-bound VEGF. Shorter forms of VEGF are secreted and act on distant targets. VEGF binds to multiple VEGFRs. Neuropilin-1 and neuropilin-2 function as co-receptors for VEGF. Protease-mediated release of soluble VEGFR1 (sVEGFR or sFlt-1) can regulate VEGF-mediated signaling by sequestering the ligand. Furthermore, sFlt can heterodimerize with VEGFR2 and inhibit signaling by blocking autophosphorylation.