The cell cycle-regulatory CDC25A phosphatase inhibits apoptosis signal-regulating kinase 1

CDC25A phosphatase promotes cell cycle progression by activating G(1) cycli n-dependent kinases and has been postulated to be an oncogene because of it s ability to cooperate with RAS to transform rodent fibroblasts. In this st udy, we have identified apoptosis signal-regulating kinase 1 (ASK1) as a CD C25A-interacting protein by yeast two-hybrid screening. ASK1 activates the p38 mitogen-activated protein kinase (MAPK) and c-Jun NH2-terminal protein kinase-stress-activated protein kinase (JNK/SAPK) pathways upon various cel lular stresses. Coimmunoprecipitation studies demonstrated that CDC25A phys ically associates with ASK1 in mammalian cells, and immunocytochemistry wit h confocal laser-scanning microscopy showed that these two proteins colocal ize in the cytoplasm. The carboxyl terminus of CDC25A binds to a domain of ASK1 adjacent to its kinase domain and inhibits the kinase activity of ASK1 , independent of and without effect on the phosphatase activity of CDC25A. This inhibitory action of CDC25A on ASK1 activity involves diminished homo- oligomerization of ASK1. Increased cellular expression of wild-type or phos phatase-inactive CDC25A from inducible transgenes suppresses oxidant-depend ent activation of ASK1, p38, and JNK1 and reduces specific sensitivity to c ell death triggered by oxidative stress, but not other apoptotic stimuli. T hus, increased expression of CDC25A, frequently observed in human cancers, could contribute to reduced cellular responsiveness to oxidative stress und er mitogenic or oncogenic conditions, while it promotes cell cycle progress ion. These observations propose a mechanism of oncogenic transformation by the dual function of CDC25A on cell cycle progression and stress responses.