Recent studies have suggested that Skp2, an SCF-type ubiquitin ligase, positively regulates cell cycle through degradation of p27, which is an inhibitor of cyclin-dependent kinase 2 (CDK2), which drives cells from the G1 to S phase of cell cycles. In the present study, we examined key regulatory proteins involved in serum starvation-induced cell cycle arrest in human ovarian cancer cells, SK-OV-3. Cell cycle analysis showed that cells were arrested at the G1 phase after serum starvation. Western blot analysis showed that the protein levels of CDK4 and CDK2 were significantly decreased in SK-OV-3 cells. Consistently, Roscovitine, an inhibitor of CDK2, induced cell cycle arrest in normally proliferating cells and a chemical inhibitor of CDK4, 3-ATA [3-Amino-9-thio(10H)-acridone], was found to induce growth arrest. We also found that the protein level of Skp2 was dramatically decreased in response to serum starvation. Moreover, CDK2 protein, which allows cell cycle transit from the G1 to the S phase, was decreased when the Skp2 expression was inhibited by specific siRNA of Skp2, but CDK4 was not decreased. Therefore, these results suggest that serum starvation induces G1 arrest through suppression of Skp2-dependent CDK2 activity and Skp2-independent CDK4 activity in human SK-OV-3 ovarian cancer cells.

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