The tumor suppressor protein p53 responds to genotoxic stress to induce cell growth arrest and to promote apoptosis. Posttranslational modifications of p53 and interactions with its co-factors such as Mdm2 and CBP/p300 play an important role in regulating p53 stability and activity. p300, a ubiquitous transcriptional co-activator, binds to the p53 activation domain, and functions as an acetyltransferase for p53 that increases its stability and transcriptional activity.

S-phase kinase associated protein 2 (Skp2), a member of the F-box protein family, promotes the ubiquitin-mediated degradation of several key regulators involved in cell cycle progression, including the CDK inhibitor p27. Skp2 is oncogenic and its amplification and over-expression correlates with the grade of malignancy of many tumors. Conversely, inactivation of Skp2 decreases cell growth and increases apoptosis in many cancers.

Here we show that inactivation of Skp2 by small interfering (si) RNA together with adriamycin, a DNA-damage agent, or with nutlin-3, a selective small-molecule antagonist of Mdm2, synergistically induces apoptosis in several types of tumor cells in a p53-dependent manner. Conversely, we demonstrate that over-expression of Skp2 suppresses apoptosis induced by adriamycin or nutlin-3, by inhibiting the trans-activation ability of p53. We show that Skp2 forms a complex with p300 and promotes its proteasome-mediated degradation. Furthermore, we demonstrate that Skp2 functions to suppress p300-mediated acetylation to p53 by antagonizing p300-binding to p53, whereas ectopic expression of p300 rescues the trans-activation ability of p53 from Skp2-mediated inhibition. Taken together, our results implicate p300 as a new target for Skp2 to promote ubiquitin-mediated protein degradation. Furthermore, our results implicate Skp2 as an important factor that controls p53-dependent signaling pathways through inhibiting p300 in cancer cells, and we propose Skp2 as a potential molecular target for cancer therapy.

M.K. and S-H.L. contributed equally to this work.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA