Cdc2 and Cdk2 kinase activated by transforming growth factor-beta 1 trigger apoptosis through the phosphorylation of retinoblastoma protein in FaO hepatoma cells

The signaling pathway leading to TGF-beta 1-induced apoptosis was investiga ted using a TGF-beta 1-sensitive hepatoma cell line, FaO. Cell cycle analys is 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 respo nse to TGB-beta 1. TGF-beta 1 induced a transient increase in the expressio n 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-dep endent kinase (Cdk) activities coincides with a dramatic increase in the hy perphosphorylated forms of RE. Treatment with roscovitine or olomoucine, in hibitors of Cdc2 and Cdk2, blocked TGF-beta 1-induced apoptosis by inhibiti ng RE phosphorylation. Overexpression of Bcl-2 or adenovirus E1B 19K suppre ssed TGF-beta 1-induced apoptosis by blocking the induction of Cdc2 mRNA an d 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 Cd k2 kinase activity transiently induced by TGF-beta 1 phosphorylates RE as a physiological target in FaO cells and that RE hyperphosphorylation may tri gger abrupt cell cycle progression, leading to irreversible cell death.