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 that can activate ERK1 and 2 mitogen-activated protein (MAP) kinases by direct phosphorylation of MAP/ERK kinase (MEK). ERK activation is essential for oncogenic transformation of NIH 3T3 cells by Mos. In this study, we examined how mitogenic and oncogenic signalling from the Mos/MEK/ERK pathway reaches the nucleus to activate downstream target genes. We show that c-Fos (the c-fos protooncogene product), which is an intrinsically unstable nuclear protein, is metabolically highly stabilized, and greatly enhances the transforming efficiency of NIH 3T3 cells, by Mos. This stabilization of c-Fos required Mos-induced phosphorylation of its C-terminal region on Ser362 and Ser374, and double replacements of these serines with acidic (Asp) residues markedly increased the stability and transforming efficiency of c-Fos even in the absence of Mos. Moreover, activation of the ERK pathway was necessary and sufficient for the c-Fos phosphorylation and stabilization by Mos. These results indicate that c-Fos undergoes stabilization, and mediates at least partly the oncogenic signalling, by the Mos/MEK/ERK pathway. The present findings also suggest that, in general, the ERK pathway may regulate the cell fate and function by affecting the metabolic stability of c-Fos.