Binding to the extracellular matrix and proteolytic processing: two key mechanisms regulating vascular endothelial growth factor action

Abstract

Vascular endothelial growth factor (VEGF, VEGF-A) is a major regulator of physiological and pathological angiogenesis. One feature of VEGF is the existence of multiple isoforms arising from alternative exon splicing. Our initial biochemical and biological studies indicated that such isoforms are uniquely suited to generate angiogenic gradients by virtue of their differential ability to interact with the extracellular matrix (ECM). Although ECM-bound VEGF was bioactive, processing by physiologically relevant proteases such as plasmin was identified as a key mechanism to convert ECM-bound VEGF into freely diffusible forms. This retrospective article examines the early studies and also emphasizes the subsequent progress in our understanding of these processes in health and disease.

Figure 1.

(A) Organization of the human VEGF-A gene. The gene comprises of eight exons. Alternative splicing results in the generation of multiple isoforms (Houck et al., 1991; Tischer et al., 1991). The monomeric precursor of each isoform is represented here. The most frequently detected isoforms are VEGF₁₂₁, VEGF₁₆₅, and VEGF₁₈₉. Less abundant splice variants have been described, including VEGF₁₄₅, VEGF₁₆₂, VEGF₁₈₃, and VEGF_165b. VEGF₁₆₅ interacts with HSPG through exon 7–encoded sequences. VEGF₁₈₉ and VEGF₂₀₆ have one additional heparin-binding domain, encoded by exon 6, explaining the particularly strong binding of these isoforms to HSPG in the ECM. (B) Plasmin-mediated cleavage of VEGF₁₆₅. The mature VEGF homodimer is represented here. Note the sequential cleavage, generating first the heterodimer VEGF_165/110 (Keyt et al., 1996). The final product is VEGF₁₁₀, lacking the sequences encoded by exons 7 and 8 and part of exon 5. This protein is biologically and biochemically similar to alternatively spliced VEGF₁₂₁. The diagram also illustrates the site-specific binding of two inhibitors, ranibizumab and pegaptanib. VEGF₁₁₀ activity is blocked by ranibizumab but not by pegaptanib.