Contortrostatin, a snake venom disintegrin, induces alphavbeta3-mediated tyrosine phosphorylation of CAS and FAK in tumor cells

Contortrostatin is a homodimeric disintegrin that inhibits platelet aggregation and cell adhesion to extracellular matrix proteins by blocking integrins. The effect of contortrostatin on integrin-mediated signaling in tumor cells was investigated by studying tyrosine phosphorylation events and activation of specific signaling molecules. We found that at concentrations as low as 1 nM, soluble contortrostatin activates integrin signals leading to increased tyrosine phosphorylation of FAK and CAS, and that these signals are abolished by inhibiting Src family kinases. Using transfected 293 cells expressing specific integrins, it was determined that contortrostatin-generated signals are mediated exclusively by the alphavbeta3 integrin. This observation was extended by showing that cells lacking alphavbeta3, but expressing alphavbeta5 and alpha5beta1, do not respond in this way to contortrostatin treatment. In cells expressing alphavbeta3, blocking contortrostatin binding with antibodies against alphavbeta3 completely abrogates contortrostatin signals. Monovalent disintegrins echistatin and flavoridin were incapable of affecting tyrosine phosphorylation alone, but when added simultaneously with contortrostatin, completely inhibited contortrostatin-initiated signals. We propose that the homodimeric nature of contortrostatin imparts the ability to crosslink alphavbeta3 integrins, causing Src activation and hyperphosphorylation of FAK and CAS. This activity may represent a novel mechanism by which tumor cell motility can be inhibited.