Abstract
Cyclin A-Cdk2, a cell cycle regulated Ser/Thr kinase, plays important roles in a variety of apoptoticprocesses. However, the mechanism of cyclin A-Cdk2 regulated apoptosis remains unclear. Here, we demonstrated that Rad9, a member of the BH3-only subfamily of Bcl-2 proteins, could be phosphorylated by cyclin A-Cdk2 in vitro and in vivo. Cyclin A-Cdk2 catalyzed the phosphorylation of Rad9 at serine 328 in HeLa cells during apoptosis induced by etoposide, an inhibitor of topoisomeraseII. The phosphorylation of Rad9 resulted in its translocation from the nucleus to the mitochondria and its interaction with Bcl-xL. The forced activation of cyclin A-Cdk2 in these cells by the overexpression of cyclin A,triggered Rad9 phosphorylation at serine 328 and thereby promoted the interaction of Rad9 with Bcl-xL and the subsequent initiation of the apoptotic program. The pro-apoptotic effects regulated by the cyclin A-Cdk2 complex were significantly lower in cells transfected with Rad9S328A, an expression vector that encodes a Rad9 mutant that is resistant to cyclin A-Cdk2 phosphorylation. These findings suggest that cyclin A-Cdk2 regulates apoptosis through a mechanism that involves Rad9phosphorylation.
Highlights
Cyclin A-Cdk2, a Ser/Thr kinase, regulates mammalian cell cycle progression by phosphorylating specific substrates during S phase [1,2]
Because cyclin A-Cdk2 is a protein kinase, we considered the possibility that it mightpromote apoptosis by directly phosphorylating and thereby activating a component of the cell death regulatory machinery
Four major observations were made in this study: (i) Rad9, a member of the BH3-only protein subfamily, can be phosphorylated by cyclin A-Cdk2 in vitro and in vivo; (ii) cyclin A-Cdk2 phosphorylates Rad9 at serine 328 during etoposide-induced apoptosis in HeLa cells; (iii) the up-regulation of cyclin A-Cdk2 activity enhances Rad9-induced apoptosis by phosphorylating Rad9 at serine 328; and (iv) the phosphorylation of Rad9 at serine 328 is required for the interaction of Rad9 with Bcl-xL
Summary
Cyclin A-Cdk, a Ser/Thr kinase, regulates mammalian cell cycle progression by phosphorylating specific substrates during S phase [1,2]. We showed previously that Cdk activity, but notCdc activity, was selectively up-regulated and that this up-regulation wasrequired for the induction of apoptosis by G-Rh2 [7], panaxadiol [8], or etoposide [4] in several cancer celllines, including SK-HEP-1 cells and HeLa cells. Cyclin A-Cdk may act upstream of mitochondrial cytochrome c release in the apoptosis pathway [4,9]. These studies suggest an essential role of cyclin A-Cdk in apoptosis, the precise molecular mechanism by which cyclin A-Cdk regulates apoptotic pathways is largelyunknown
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
