Receptor protein tyrosine phosphatase α activates Src-family kinases and controls integrin-mediated responses in fibroblasts

Background: Fyn and c-Src are two of the most widely expressed Src-family kinases, Both are strongly implicated in the control of cytoskeletal organization and in the generation of integrin-dependent signalling responses in fibroblasts. These proteins are representative of a large family of tyrosine kinases, the activity of which is tightly controlled by inhibitory phosphorylation of a carboxyterminal tyrosine residue (Tyr527 in chicken c-Src); this phosphorylation induces the kinases to form an inactive conformation. Whereas the identity of such inhibitory Tyr527 kinases has been well established, no corresponding phosphatases have been identified that, under physiological conditions, function as positive regulators of c-Src and Fyn in fibroblasts. Results: Receptor protein tyrosine phosphatase alpha (RPTP alpha) was inactivated by homologous recombination. Fibroblasts derived from these RPTP alpha(-/-) mice had impaired tyrosine kinase activity of both c-Src and Fyn, and this was accompanied by a concomitant increase in c-Src Tyr527 phosphorylation, RPTP alpha(-/-) fibroblasts also showed a reduction in the rate of spreading on fibronectin substrates, a trait that is a phenocopy of the effect of inactivation of the c-src gene. In response to adhesion on a fibronectin substrate, RPTP alpha(-/-) fibroblasts also exhibited characteristic deficiencies in integrin-mediated signalling responses, such as decreased tyrosine phosphorylation of the c-Src substrates Fak and p130(cas), and reduced activation of extracellular signal regulated (Erk) MAP kinases. Conclusions: These observations demonstrate that RPTPa functions as a physiological upstream activator of Src-family kinases in fibroblasts and establish this tyrosine phosphatase as a newly identified regulator of integrin signalling. (C) Elsevier Science Ltd ISSN 0960-9822.