VEGF in Signaling and Disease: Beyond Discovery and Development

Abstract

The discovery of vascular endothelial-derived growth factor (VEGF) has revolutionized our understanding of vasculogenesis and angiogenesis during development and physiological homeostasis. Over a short span of two decades, our understanding of the molecular mechanisms by which VEGF coordinates neurovascular homeostasis has become more sophisticated. The central role of VEGF in the pathogenesis of diverse cancers and blinding eye diseases has also become evident. Elucidation of the molecular regulation of VEGF and the transformative development of multiple therapeutic pathways targeting VEGF directly or indirectly is a powerful case study of how fundamental research can guide innovation and translation. It is also an elegant example of how agnostic discovery and can transform our understanding of human disease. This review will highlight critical nodal points in VEGF biology, including recent developments in immunotherapy for cancer and multitarget approaches in neovascular eye disease.

Figure 1.. A historical timeline of VEGF discovery:

In diabetic retinopathy, it was proposed that diffuse angiogenic factor(s) were involved in neovascularization and the term ‘factor X’ was coined to describe such molecule(s) (Algire et al., 1945; Ashton, 1952; Carrel, 1913; Ide et al., 1939; Michaelson, 1948; Wise, 1956). In the early 1970s, it was suggested that an anti-angiogenic approach might be a unique and novel strategy to inhibit growth and proliferation of tumors (Folkman, 1971). Senger et al. reported the identification and initial biochemical characterization of VPF, a permeability-enhancing protein in the supernatant of a guinea pig tumor cell line (Senger et al., 1983). (Senger et al., 1990). The Ferrara laboratory reported the isolation and cloning of a heparin-binding endothelial cell mitogen from medium conditioned by bovine pituitary follicular cells (Ferrara and Henzel, 1989; Leung et al., 1989) and the term VEGF was coined to describe this novel 45-kDa heparin binding endothelial cell mitogen protein. (Keck et al., 1989; Senger et al., 1990; Senger et al., 1983). Inactivation of a single allele of the VEGF gene in mice resulted in defective vascular development and early embryonic lethality (Carmeliet et al., 1996; Ferrara et al., 1996), highlighting the importance of VEGF during embryonic development. Neutralizing anti-VEGF antibodies dramatically reduced angiogenesis and growth of tumor cells implanted in immune deficient mice (Kim et al., 1993) opening up novel therapeutic opportunities.

Figure 2.. VEGF activation and signaling pathways:

A) VEGF-A is a member of a family of proteins including VEGF-A, VEGF-B, VEGF-C, VEGF-D, virally encoded VEGF-E, and placental growth factor (PlGF). Hypoxia inducible factor (HIF), Epidermal growth factor (EGF), platelet derived growth factor (PDGF) are among many hypoxia/ischemia induced genes that regulate VEGF expression. Canonical VEGF signaling through VEGF-R1/R2 (with R2 being the dominant signaling receptor) regulates the activities of several kinases and ultimately guides cell proliferation, migration, survival and vascular permeability during vasculogenesis and angiogenesis. B) VEGF-A binds to both R1 and R2, VEGF-B and PlGF bind to VEGF-R1, and VEGF C and D bind to VEGF-R3 that may regulate lymphangiogenesis. VEGF-A or PlGF can also bind to neuropilin 1 (NRP-1) to increase their binding affinity to VEGF-R2 or independent of this function. Neuropilin 2 (NRP-2) performs a similar role in regulating lymphangiogenesis through its interactions with VEGF-R3 (adapted from (Ferrara and Adamis, 2016))

Figure 2.. VEGF activation and signaling pathways:

A) VEGF-A is a member of a family of proteins including VEGF-A, VEGF-B, VEGF-C, VEGF-D, virally encoded VEGF-E, and placental growth factor (PlGF). Hypoxia inducible factor (HIF), Epidermal growth factor (EGF), platelet derived growth factor (PDGF) are among many hypoxia/ischemia induced genes that regulate VEGF expression. Canonical VEGF signaling through VEGF-R1/R2 (with R2 being the dominant signaling receptor) regulates the activities of several kinases and ultimately guides cell proliferation, migration, survival and vascular permeability during vasculogenesis and angiogenesis. B) VEGF-A binds to both R1 and R2, VEGF-B and PlGF bind to VEGF-R1, and VEGF C and D bind to VEGF-R3 that may regulate lymphangiogenesis. VEGF-A or PlGF can also bind to neuropilin 1 (NRP-1) to increase their binding affinity to VEGF-R2 or independent of this function. Neuropilin 2 (NRP-2) performs a similar role in regulating lymphangiogenesis through its interactions with VEGF-R3 (adapted from (Ferrara and Adamis, 2016))

Figure 3.. VEGF and tumor angiogenesis.

VEGF secreted by cancer and stromal cells stimulates the proliferation and survival of endothelial cells leading to the formation of new blood vessels, often with impaired tight junctions and increased permeability. Combining VEGF inhibitors with cytotoxic agents is synergistic and improves therapeutic outcomes. Although the mechanisms of synergy are debated, it has been suggested that the anti-vascular effects of the cytotoxic agents complement the proapoptotic effects of anti-VEGF agents on the vascular endothelium. Normalization of the vasculature by anti-VEGF agents that allows enhanced uptake of chemotherapeutic agents may also play a role. VEGF may skew maturation of myeloid progenitor away from differentiation into dendritic cells and towards endothelial cells, impacting T cell activation. VEGF can also decrease expression of VCAM-1, important for anti-cancer T cell adhesion and infiltration into tumors, and increase expression of FASL, leading to apoptosis of anti-cancer T cells at the vascular border to cancer. Clinical studies have recently supported a role for anti-VEGF agents in combination with in PD-L1/PD-1 inhibitors in anti-cancer immunity.