THE LIVER RESPONSE TO IN-VIVO HEAT-SHOCK INVOLVES THE ACTIVATION OF MAP KINASES AND RAF AND THE TYROSINE PHOSPHORYLATION OF SHC PROTEINS

We have investigated the mechanisms of signal transduction in the resp onse of liver to heat shock in vivo. By immunoblot experiments we have shown that heat shock decreases the electrophoretic mobility of the 4 0 and 43 kDa mitogen activated protein kinases (MAPKs) and we have fou nd a significant increase of MAPK activity measured as phosphotransfer ase capacity of both cytosolic extracts and MAPK immunoprecipitates. T o elucidate the signalling pathway which accounts for MAPK activation, we focused our attention on its upstream factors, Raf and Res. We hav e shown that, heat shock activates Raf-1 kinase and causes an increase in phosphotyrosine content of the 52 kDa She protein accompanied by a n increment in the amount of coimmunoprecipitated Grb2. These findings provide the first evidence that the Ras-Raf-MAPK pathway is activated in liver during heat shock in vivo. (C) 1995 Academic Press, Inc.