Vascular endothelial growth factor and vascular homeostasis.

Vascular endothelial growth factor (VEGF) is the angiogenic factor promoting and orchestrating most, if not all, processes of neovascularization taking place in the embryo and the adult. VEGF is also required to sustain newly formed vessels and plays additional multiple roles in the maintenance and function of certain mature vascular beds. Correspondingly, perturbations in VEGF signaling may impact organ homeostasis in multiple ways. Here we briefly review potential consequences of VEGF loss of function in adult organs. Different vascular beds display highly variable dependencies on VEGF for survival, and its loss of function may trigger the regression of many VEGF-dependent vasculatures. Normal turnover of blood vessels, in conjunction with the fact that VEGF is indispensable for compensatory angiogenesis to restore adequate perfusion, accounts for progressive vascular rarefaction under conditions of chronic VEGF inhibition of even vasculatures that are not intrinsically dependent on VEGF. Because blood vessels may have paracrine functions other than their traditional role in tissue perfusion, vascular regression resulting from VEGF withdrawal may cause substantial collateral tissue damage. VEGF may also impact tissue homeostasis via acting directly on nonvascular cells expressing cognate receptors. In the particular case of the lung, constitutive abundant expression of VEGF together with the fact that its receptors are distributed on both endothelial and epithelial cells is compatible with multiple homeostatic VEGF functions in the adult lung. Indeed, experimental inhibition of VEGF in the mature lung produces lesions resembling common lung pathologies, including emphysema and respiratory distress syndrome.