The phosphatase Cdc25A was shown to be a target of the transcription factor c-Myc. Myc-induced apoptosis appeared dependent on Cdc25A expression and Cdc25A over-expression could substitute for Myc-triggered apoptosis. These findings suggested that an important downstream component of Myc-mediated apoptosis was identified. However and in contrast, we recently reported that during TNFalpha-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 refractory to apoptosis, which was induced by serum deprivation and in absence of detectable c-myc levels. The survival promoting activity of cdc25A was abolished upon infection of cells with a full-length cdc25A antisense construct. To identify the signaling proteins mediating the survival function of the 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 Cdc25A 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 control 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 were found, which co-precipitated with Cdc25A and Raf1. Since activation of proteins requires dephosphorylation of particular residues in addition to site-specific phosphorylation, the anti-apoptotic effect of Cdc25A might derive from its participation in a multimeric protein complex with phosphoAkt and Raf1, two prominent components of survival pathways.
