PPP1CA contributes to the senescence program induced by oncogenic Ras

Ectopic expression of conditional murine p53 (p53val135) and oncogenic ras is enough to induce a senescent-like growth arrest at the restrictive temperature. We took advantage of this cellular system to identify new key players in the ras/p53-induced senescence. Applying a retroviral-based genetic screen, we obtained an antisense RNA fragment against PPP1CA, the catalytic subunit of protein phosphatase 1 alpha, whose loss of function bypasses ras/p53-induced growth arrest and senescence. Expression of a specific short hairpin (sh)RNA against PPP1CA impairs the p53-dependent induction of p21 after DNA damage and blocks the subsequent pRb dephosphorylation, thus bypassing p53-induced arrest. We found that oncogenic ras promotes an increase in the intracellular level of ceramides together with an increase in the PPP1CA protein levels. Addition of soluble ceramide to the cells induced a senescence phenotype that is blocked through PPP1CA downregulation by specific shRNA. Analysis of human tumors suggests that one of the PPP1CA alleles might be lost in a high percentage of carcinomas such as kidney and colorectal. The overexpression of two out of five PPP1CA alternative spliced variants reduced tumor cell growth and the downregulation of the protein to hemizygosity increased the anchorage-independent growth. We propose that oncogenic stress induced by ras causes ceramide accumulation, therefore, increasing PPP1CA activity, pRb dephosphorylation and onset of the p53-induced arrest, contributing to tumor suppression.