The mechanism of heat shock activation of ERK mitogen-activated protein kinases in the interleukin 3-dependent ProB cell line BaF3

We have investigated heat shock stimulation of MAPK cascades in an interleu kin 3-dependent cell line, BaF3. Following exposure to 42 degreesC, the str ess-activated JNK MAPKs were phosphorylated and activated, but p38 MAPKs re mained unaffected. Surprisingly, heat shock also activated ERK MAPKs in a p otent (>60-fold), delayed (>30 min), and sustained (greater than or equal t o 120 min) manner. These characteristics suggested a novel mechanism of ERK MAPK activation and became the focus of this study. A MEK-specific inhibit or, PD98059, inhibited heat shock ERK MAPK activation by >75%. Surprisingly , a role for Ras in the heat shock response was eliminated by the failure o f a dominant-negative Ras(Asn-17) mutant to inhibit ERK MAPK activation and the failure to observe increases in Ras GTP. Heat shock also failed to sti mulate activation of A-, B-, and c-Raf. Instead, a serine/threonine phospha tase inhibitor, okadaic acid, activated ERK MAPK in a similar manner to hea t shock. Furthermore, pretreatment with suramin, generally recognized as ab road range inhibitor of growth factor receptors, inhibited both okadaic aci d-stimulated and heat shock-stimulated ERK MAPK activity by >40%. Inhibitin g ERK MAPK activation during heat shock with PD98059 enhanced losses in cel l viability. These results demonstrate Ras- and Raf-independent ERK MAPK ac tivation maintains cell viability following heat shock.