INTEGRIN-MEDIATED SIGNALING EVENTS IN HUMAN ENDOTHELIAL-CELLS

Vascular endothelial cells are important in a variety of physiological and pathophysiological processes. The growth and functions of vascula r endothelial cells are regulated both by soluble mitogenic and differ entiation factors and by interactions with the extracellular matrix; h owever, relatively little is known about the role of the matrix. In th e present study, we investigate whether integrin-mediated anchorage to a substratum coated with the extracellular matrix protein fibronectin regulates growth factor signaling events in human endothelial cells. We show that cell adhesion to fibronectin and growth factor stimulatio n trigger distinct initial tyrosine phosphorylation events in endothel ial cells. Thus, integrin-dependent adhesion of endothelial cells lead s to tyrosine phosphorylation of both focal adhesion kinase and paxill in, but not of several growth factor receptors. Conversely, EGF stimul ation causes receptor autophosphorylation, with no effect on focal adh esion kinase or paxillin tyrosine phosphorylation. Adhesion to fibrone ctin, in the absence of growth factors, leads to activation of MAPK. L n addition, adhesion to fibronectin also potentiates growth factor sig naling to MAPK. Thus, polypeptide growth factor activation of MAPK in anchored cells is far more effective than in cells maintained in suspe nsion. Other agonists known to activate MAPK were also examined for th eir ability to activate MAPK in an anchorage-dependent manner. The neu ropeptide bombesin, the bioactive lipid lysophosphatidic acid (LPA), a nd the cytokine tumor necrosis factor alpha, which signal through dive rse mechanisms, were all able to activate MAPK to a much greater degre e in fibronectin-adherent cells than in suspended cells. In addition, tumor necrosis factor ct activation of c-Jun kinase (JNK) was also muc h more robust in anchored cells. Together, these data suggest a cooper ation between integrins and soluble mitogens in efficient propagation of signals to downstream kinases. This cooperation may contribute to a nchorage dependence of mitogenic cell cycle progression.