A link between p53 tumor suppressor and KSHV K-cyclin

Kaposi's sarcoma associate herpesvirus (KSHV) is the etiological agent for Kaposi's sarcoma and primary effusion lymphoma. KSHV K-cyclin contributes to dysregulation of cell growth control by promoting abnormal cell cycle progression. It induces cell growth arrest under certain circumstances. Objectives: determine molecular link between K-cyclin induced growth arrest and p53 tumor suppressor pathway. Methods: With co-immunoprecipitation, we identified a new K-cyclin associated protein. We performed in vitro kinase assays and Western blot with phospho-specific antibodies to map the phosphorylation site on p53. Using recombinant adenoviruses, we expressed K-cyclin and Cdk9 in SaOS2 and U2OS cells, and examined the effect of K-cyclin on cell growth. Results: We identified cyclin dependent kinase 9 (Cdk9) as a new K-cyclin associated kinase. We found that K-cyclin greatly enhanced the kinase activity of Cdk9 toward p53. The phosphorylation site was mapped at Ser33 on p53. Using p53-positive U2OS and p53-null SaOS2 cells, we demonstrated that the K-cyclin induced growth arrest was associated with p53 expressed either endogenously or exogenously, and the growth inhibition was apparently mediated by Cdk9. Conclusions: KSHV K-cyclin interacts and activates cellular Cdk9 to phosphorylate p53 on Ser33. Cdk9 plays a role that links K-cyclin-induced cell growth arrest to p53 tumor suppressor pathway.

This study was supported by the Grant DE016644-02 from National Institute of Health.