Differential Regulation of bFGF- and VEGF-Induced Angiogenesis in Mice Lacking Junctional Adhesion Molecule-A.

Angiogenesis, the formation of new blood vessels from preexisting vessels, is a complex process involving vascular endothelial cell activation, proliferation, migration, and tube formation. Several growth factors have been shown to induce this process. Among them vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) are most widely studied. These two growth factors are believed to induce angiogenesis through distinct signaling pathways. Recently, we have shown that signaling through junctional adhesion molecule-A (JAM-A) is required for bFGF-induced angiogenesis. Here, we present evidence that JAM-A differentially regulates both bFGF-induced and VEGF-induced angiogenic responses. Using human umbilical cord vein endothelial cells (HUVEC) we found that bFGF-induced but not VEGF-induced migration was inhibited by function blocking anti-JAM-A antibody. We also found that mutant JAM-A blocked bFGF-induced but not VEGF-induced MAP kinase activation. To conclusively determine the role of JAM-A in angiogenic signaling, we generated JAM-A null mice. Endothelial cells isolated from JAM-A null mice failed to migrate in response to bFGF stimulation, but showed enhanced cell migration in response to VEGF. Further, using aortic ring and Matrigel® plug assays, we found that bFGF failed to induce blood vessel formation in JAM-A null mice. Interestingly, an increase in the number of blood vessels was observed in response to VEGF in these mice suggesting that in the absence of JAM-A bFGF-induced angiogenic pathway is impaired, but VEGF-induced pathway is upregulated to compensate. We next determined the effect of increased angiogenesis on tumor growth. We found that B16F0 melanoma growth was enhanced at least by two-fold in JAM-A null mice compared to wild-type mice. To understand the mechanism of hypersensitivity of JAM-A null mice to VEGF, we determined the expression of VEGF receptors on endothelial cells. We found that surface expression of VEGFR-2 is upregulated in endothelial cells isolated from JAM-A null mice. These results suggest that signaling through JAM-A is essential for the bFGF-induced pathway and when this pathway is impaired, the VEGF-induced pathway is upregulated which leads to enhanced angiogenesis and tumor growth.