Inactivation of protein-tyrosine phosphatases as mechanism of UV-induced signal transduction

Abstract

UV irradiation of cells causes ligand-independent activation of receptor tyrosine kinases. On the basis of dephosphorylation kinetics, UV-induced inactivation of receptor-directed tyrosine phosphatases (PTP) has been proposed as the mechanism of receptor activation (Knebel, A., Rahmsdorf, H. J., Ullrich, A., and Herrlich, P. (1996) EMBO J. 15, 5314-5325). Here we show that four defined protein-tyrosine phosphatases (PTPs), SHP-1, RPTPα, RPTPα, and DEP-1, are partially inactivated upon UV irradiation of PTP- overexpressing cells. The dephosphorylation of coexpressed platelet-derived growth factor β (PDGFβ) receptor by RPTPα is inhibited upon UV irradiation. UV converts RPTPα into a substrate-trapping enzyme which can coprecipitate PDGFβ receptor, similarly to the PTP mutant at the active- center cysteine: C433S. In agreement with the proposed mechanism that inactivation of PTPs accounts for receptor tyrosine kinase activation, no evidence for a UV-induced receptor cross-linking could be obtained in PDGFβ receptor-enriched membrane micelle preparations and in PDGFβ receptor overexpressing 293 cells. The intrinsic activity of PDGFβ receptor kinase was required for the UV-induced enhancement of receptor phosphorylation, but was not changed upon UV irradiation. The data support a mechanism of UV- induced signal transduction involving inactivation of PTPs through an unknown reactive intermediate that oxidizes the conserved cysteine in the active sites of PTPs.