An essential role for mitogen-activated protein kinases, ERKs, in preventing heat-induced cell death

Stimulation of mitogen-activated protein kinases (MAPKs) or extracellular s ignal regulated protein kinases (ERKs) after exposure of mammalian cells to ultraviolet (UV) and X-irradiation occurs through activation of receptor t yrosine kinases via Ras/Raf/Mek/ERKs cascade. This activation of MAPKs is p roposed to play a role in the replacement of damaged proteins during these stresses. Heat shock also activates MAPKs; however the signaling cascade an d the biochemical and physiological links between activation by heat and do wnstream effects are unknown. In this report we demonstrate that, unlike ir radiation, heat induces MAPKs through ceramide metabolism to sphingosine wi th stimulation of Raf-1 protein kinase. The activation of MAPKs by heat doe s not occur in all cell types, because the step(s) downstream of ceramide t o activation of Raf-1 protein kinase is missing in myeloid leukemic cells s uch as HL-60, U937, and K562, while it is present in NIH3T3 fibroblasts. He at-induced MAPK activation may enhance the ability of cells to survive a se vere heat shock. Blocking 60-70% of the activity of MAPK (ERK1) by stable o verexpression of the dominant negative allele ERK1-KR renders NIH3T3 and K5 62 cells up to 100-fold more sensitive to cytotoxic effects of heat. Conver sely, NIH3T3 and K562 cells stably overexpressing the wild-type ERK1 develo p resistance to killing by heat. These results suggest that increased therm al sensitivity of leukemic cells to thermal stress or other cancer therapy regimens could be attributable to lack of pertinent activation of the MAPK pathway by such stresses. (C) 1999 Wiley-Liss, Inc.