Oncogenic Ras-mediated cell growth arrest and apoptosis are associated with increased ubiquitin-dependent cyclin D1 degradation

The cellular responses to activated Ras vary depending on cell type. Normal cells are often induced into pathways that lead to cell growth arrest, sen escence, and/or apoptosis in response to activated Ras expression. These ar e important protective anti-tumorigenic responses that restrict the propaga tion of cells bearing activated oncogenes. Here we show that induction of H a-Ras(Val-12) in Rat-1 fibroblasts resulted in G(1) growth arrest and apopt osis with loss of viable cells that is accompanied by a marked decrease in cyclin D1 levels via increased ubiquitin-proteasome-dependent cyclin D1 tur nover. This is in contrast with a rat intestinal epithelial cell line in wh ich induction of Ha-Ras(Val-12) results in transformation associated with s ustained proliferation and increased levels of cyclin D1, that is not accom panied by anoikis or apoptosis. Expression of the cyclin D1 mutant (T286A) that contains an alanine for threonine 286 substitution and is resistant to ubiquitin-proteasome degradation in the Ha.Ras(Val-12) expressing Rat-1 ce lls resulted in a sustained transformed phenotype with no accumulation of c ells in G(1). Inhibition of mitogen-activated protein kinase (MEK1/2) pathw ay partially reversed the Ras-mediated decrease in cyclin D1. Induction of Ha-Ras(Val-12) resulted in activation of Akt kinase and inactivation of gly cogen-synthase-3 beta kinase that are associated with reduction of cyclin D 1 protein. These results suggest that has-mediated cyclin D1 degradation in Rat-1 cells appears to be partially dependent on activation of mitogen-act ivated protein kinase pathway and independent of glycogen-synthase-3 beta k inase pathway.