THE MOS MAP KINASE PATHWAY STABILIZES C-FOS BY PHOSPHORYLATION AND AUGMENTS ITS TRANSFORMING ACTIVITY IN NIH 3T3 CELLS/

The c-mos proto-oncogene product, Mos, is a serine/threonine kinase th at can activate ERK1 and 2 mitogen-activated protein (MAP) kinases by direct phosphorylation of MAP/ERK kinase (MEK). ERK activation is esse ntial for oncogenic transformation of NIH 3T3 cells by Mos. In this st udy, we examined how mitogenic and oncogenic signalling from the Mos/M EK/ERK pathway reaches the nucleus to activate downstream target genes . We show that c-Fos (the c-fos protooncogene product), which is an in trinsically unstable nuclear protein, is metabolically highly stabiliz ed, and greatly enhances the transforming efficiency of NIH 3T3 cells, by Mos. This stabilization of c-Fos required Mos-induced phosphorylat ion of its C-terminal region on Ser362 and Ser374, and double replacem ents of these serines with acidic (Asp) residues markedly increased th e stability and transforming efficiency of c-Fos even in the absence o f Mos. Moreover, activation of the ERK pathway was necessary and suffi cient for the c-Fos phosphorylation and stabilization by Mos. These re sults indicate that c-Fos undergoes stabilization, and mediates at lea st partly the oncogenic signalling, by the Mos/MEK/ERK pathway. The pr esent findings also suggest that, in general, the ERK pathway may regu late the cell fate and function by affecting the metabolic stability o f c-Fos.