Echistatin inhibits pp125FAK autophosphorylation, paxillin phosphorylation and pp125FAK-paxillin interaction in fibronectin-adherent melanoma cells

Echistatin, a snake-venom RGD-containing protein, was previously shown to disrupt cell-matrix adhesion by a mechanism that involves the reduction of pp125(FAK) tyrosine phosphorylation levels. The aim of this study was to establish the sequence of events downstream pp125(FAK) dephosphorylation that could be responsible for echistatin-induced disassembly of actin cytoskeleton and focal adhesions in fibronectin-adherent B16-BL6 melanoma cells. The results obtained show that echistatin induces a decrease of both autophosphorylation and kinase activity of pp125(FAK). One hour of cell exposure to echistatin caused a 39% decrease of pp125(FAK) Tyr397 phosphorylation and a 31% reduction of pp125(FAK) autophosphorylation activity as measured by immune-complex kinase assay. Furthermore, 1 h of cell treatment by echistatin produced a 63% decrease of paxillin phosphorylation, as well as a reduction in the amount of paxillin bound to pp125(FAK). Immunofluorescence analysis of echistatin treated cells showed the concomitant disappearance of both paxillin and pp125(FAK) from focal adhesions. The reduction of paxillin phosphorylation may represent a critical step in the pathway by which disintegrins exert their biological activity, including the inhibition of experimental metastasis in vivo.