Cdc25A phosphatase suppresses apoptosis induced by serum deprivation

The phosphatase Cdc25A was shown to be a target of the transcription factor c-Myc. Myc-induced apoptosis appeared dependent on Cdc25A expression and C dc25A over-expression could substitute for Myc-triggered apoptosis. These f indings suggested that an important downstream component of Myc-mediated ap optosis was identified. However and in contrast, we recently reported that during TNF alpha -induced apoptosis, which required c-Myc function, Cdc25A was down-regulated in a human carcinoma cell line. We now provide evidence that Cdc25A rendered the non-transformed rat embryonic cell line 423 refrac tory to apoptosis, which was induced by serum deprivation and in absence of detectable c-myc levels. The survival promoting activity of cdc25A was abo lished upon infection of cells with a full-length cdc25A antisense construc t. To identify the signaling proteins mediating the survival function of th e phosphatase, cdc25A- and akt- over-expressing pooled clones were exposed to selected chemicals, which inhibit or activate key proteins in signaling pathways. Inhibition of apoptosis by SU4984, NF023 and Rapamycin placed Cdc 25A and Akt function downstream of FGF.R, PDGF.R, and compensated G-protein - and PP2A-activity. Interestingly, upon treatment with LY-294002, cdc25A- and akt- over-expressing clones exhibited similar apoptotic patterns as con trol cells, which indicates that neither Akt- nor Cdc25A-mediated survival functions are dependent on PI.3 kinase activity in rat 423 cells. In cdc25A -overexpressing cells increased levels of serine 473 phosphorylated Akt wer e found, which coprecipitated with Cdc25A and Raf1. Since activation of pro teins requires dephosphorylation of particular residues in addition to site -specific phosphorylation, the antiapoptotic effect of Cdc25A might derive from its participation in a multimeric protein complex with phosphoAkt and Raf1, two prominent components of survival pathways.