Angiogenesis activators and inhibitors differentially regulate caveolin-1 expression and caveolae formation in vascular endothelial cells - Angiogenesis inhibitors block vascular endothelial growth factor-induced down-regulation of caveolin-1

Angiogenesis is the process by which new blood vessels are formed via proli feration of vascular endothelial cells. A variety of angiogenesis inhibitor s that antagonize the effects of vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) have recently been identified. Ho wever, the mechanism by which these diverse angiogenesis inhibitors exert t heir common effects remains largely unknown. Caveolin-1 and -2 are known to be highly expressed in vascular endothelial cells both in vitro and in viv o, Here, we examine the potential role of caveolins in the angiogenic respo nse, For this purpose, we used the well established human umbilical vein en dothelial cell line, ECV 304, Treatment of ECV 304 cells with known angioge nic growth factors (VEGF, bFGF, or hepatocyte growth factor/scatter factor) , resulted in a dramatic reduction in the expression of caveolin-1, This do wn-regulation event was selective for caveolin-1, as caveolin-2 levels rema ined constant under these conditions of growth factor stimulation. VEGF-ind uced down-regulation of caveolin-1 expression also resulted in the morpholo gical loss of cell surface caveolae organelles as seen by trans mission ele ctron microscopy, A variety of well characterized angiogenesis inhibitors ( including angiostatin, fumagillin, 2-methoxy estradiol, transforming growth factor-A and thalidomide) effectively blocked VEGF-induced down-regulation of caveolin-1 as seen by immunoblotting and immunofluorescence microscopy, However, treatment with angiogenesis inhibitors alone did not significantl y affect the expression of caveolin-1. PD98059, a specific inhibitor of mit ogen-activated protein kinase and a known angiogenesis inhibitor, also bloc ked the observed VEGF-induced down-regulation of caveolin-1. Furthermore, w e show that caveoliln-1 can function as a negative regulator of VEGF-R (KDR ) signal transduction in vivo, Thus, down-regulation of caveolin-1 may be a n important step along the pathway toward endothelial cell proliferation.