Cdc2 and Cdk2 kinase activated by transforming growth factor-β1 trigger apoptosis through the phosphorylation of retinoblastoma protein in FaO hepatoma cells

The signaling pathway leading to TGF-beta 1-induced apoptosis was investigated using a TGF-beta 1-sensitive hepatoma cell line, FaO. Cell cycle analysis demonstrated that the accumulation of apoptotic cells was pre ceded by a progressive decrease of the cell, population in the G(1) phase concomitant with a slight increase of the cell population in the G(2)/M phase in response to TGB-beta 1. TGF-beta 1 induced a transient increase in the expression of Cdc2, cyclin A, cyclin B, and cyclin D1 at an early phase of apoptosis. During TGF-beta 1-induced apoptosis, the transient increase in cyclin-dependent kinase (Cdk) activities coincides with a dramatic increase in the hyperphosphorylated forms of RE. Treatment with roscovitine or olomoucine, inhibitors of Cdc2 and Cdk2, blocked TGF-beta 1-induced apoptosis by inhibiting RE phosphorylation. Overexpression of Bcl-2 or adenovirus E1B 19K suppressed TGF-beta 1-induced apoptosis by blocking the induction of Cdc2 mRNA and the subsequent activation of Cdc2 kinase, whereas activation of Cdk2 was not affected, suggesting that Cdc2 plays a more critical role in TGF-beta 1-induced apoptosis. in conclusion, we present the evidence that Cdc2 and Cdk2 kinase activity transiently induced by TGF-beta 1 phosphorylates RE as a physiological target in FaO cells and that RE hyperphosphorylation may trigger abrupt cell cycle progression, leading to irreversible cell death.