Endogenous c-N-Ras provides a steady-state anti-apoptotic signal

We report that c-N-Ras possesses an isoform-specific, functional role in cell survival under steady-state conditions. This function includes protection from programmed cell death by serum deprivation or upon treatment with apoptosis-inducing agents. The data demonstrate that c-N-Ras may play a functional role in the regulation of steady-state phosphorylated Akt and serine 136-phosphorylated Bad (Ser(136)-pBad). Immortalized N-Ras knockout fibroblasts possess nearly undetectable levels of steady-state Ser(136)-pBad. In contrast, wild-type control cells and the N-Ras knockout cells ectopically expressing c-N-Ras at control levels maintained easily detectable levels of Ser(136)-pBad both at steady-state and following treatment with tumor necrosis factor alpha. Similar results were seen with Ser(112)-pBad. These differences did not arise from differences in total Bad protein levels. These data correlate with the observation that the N-Ras knockout cells exhibit a heightened susceptibility to the induction of apoptosis. Ectopic expression of c-N-Ras in the N-Ras knockout cells at endogenous levels, compared with control cells, significantly rescues the apoptotically sensitive phenotype. Elevated expression of either c-Kirsten A-Ras or c-Kirsten B-Ras did not reverse the apoptotic sensitivity of the N-Ras knockout cells or result in increased levels of either phospho-Akt or phospho-Bad. Our results indicate that, at steady state, c-N-Ras possesses an isoform-specific, functional role in cell survival.