Cot protooncoprotein activates the dual specificity kinases MEK-1 and SEK-1 and induces differentiation of PC12 cells

Mitogenic signals initiated at the plasma membrane are transmitted to the n ucleus through an intricate signalling network. We identified the protoonco protein Cot as a new component of mitogenic signalling cascades, which acti vates both the classic cytoplasmic cascade and the SAFE stress pathway. Wil dtype and activated Cot phosphorylate and activate MEK-1 and SEK-1 in vitro . These findings are consistent with the sequence homology between Cot and the rat gene Tpl-2. Expression of oncogenic Cot in 293, NIH3T3 and PC12 cel ls leads to in vivo phosphorylation of endogenous c-Jun and Erk-1/2 suggest ing that the serine/threonine kinase Cot functions beside c-Raf-l and Mos a sa direct activator of MEK-1. Furthermore, we have examined the biological effects of Cot on the phenotype of fibroblastic and neuronal cells. In orde r to test a potential c-Raf-l dependency of Cot transformation, the effect of oncogenic Cot on Raf revertant CHP25 cells was determined. Cot could res tore the transformed phenotype indicating that Cot transformation is not de pendent on active c-Raf-l and that Cot is not a target for the putative Raf inhibitor, which is presumably active in the revertant cell line. Expressi on of oncogenic versions of Raf as well as v-Mos leads to differentiation o f PC12 cells. Cot also induces neurite outgrowth of PC12 cells. These data are consistent with the role of Cot in the classic mitogenic cascade and su ggest that the simultaneously activated JNK/SAPK stress pathway has no anta gonistic effects in this context.