Activity of the epidermal-growth-factor receptor and phospholipase C-gamma 1 in heat-stressed fibroblasts and A-431 cells.
A variety of changes in the functions of specific plasma-membrane components have been reported in cells exposed to a heat shock. In this study, we examined the consequences of heat stress on epidermal-growth-factor (EGF)-induced receptor autophosphorylation and receptor-mediated tyrosine phosphorylation of phospholipase C-gamma 1 (PLC-gamma 1), a cellular substrate. Although the tyrosine kinase activity of the EGF receptor is rapidly inactivated at 45 degrees C in vitro [Carpenter, King & Cohen (1979) J. Biol. Chem. 254, 4884-4891], EGF stimulates autophosphorylation of its receptor in both A-431 cells and human fibroblasts after a prolonged heat shock. Phosphoamino acid analysis of the receptor reveals an EGF-induced increase in phosphotyrosine and phosphoserine at 46 degrees C. EGF also stimulates the phosphorylation of phospholipase C-gamma 1 and induces the formation of inositol phosphates under heat-shock conditions. 125I-EGF binding and internalization in A-431 cells is not decreased during incubations at 46 degrees C for up to 90 min. EGF-induced dimerization of EGF receptors on the cell surface is preserved during heat shock. Though EGF-receptor-mediated endocytosis is not inhibited by elevated temperature, the degradation of internalized 125I-EGF is dramatically decreased. These results indicate that, aside from ligand degradation, the EGF-mediated pathway of signal transduction through phospholipase C-gamma 1 remains remarkably intact during conditions of extreme cellular stress.