Role of the Raf/MEK/ERK and the PI3K/Akt(PKB) pathways in fibroblast senescence.

Replicative senescence is characterized by numerous phenotypic alterations including loss of proliferative capacity and numerous changes in gene expression such as impaired serum inducibility of the immediate early gene c-fos and increased expression of collagenase. Transcription of c-fos in response to mitogens depends on the activation of a multiprotein complex formed on the c-fos serum response element (SRE), which includes the transcription factors serum response factor (SRF) and ternary complex factor (TCF). TCF is activated after phosphorylation by the Extracellular signals Regulated Kinase 1 and 2 (ERK1/2), two kinases of the Raf/MEK/ERK signaling pathway. We have previously demonstrated that collagenase expression is under positive regulation by the transcription factor FKHRL1 and that this transcription factor is under negative regulation by the phosphatidylinositol 3-kinase(PI3K)/Akt(PKB) pathway. Although total activity of ERK and Akt was similar in total cell lysates from early and late passage fibroblasts our data indicate that in senescent cells neither ERK nor Akt are able to phosphorylate efficiently their nuclear targets. Our findings suggest that although they can be fully activated in the cytosol of both early and late passage cells, the Raf/MEK/ERK and the PI3K/Akt pathways, which are essential for cellular proliferation, are down regulated in the nuclei of senescent cells.