| Research Abstract |
Vascular smooth muscle cells (VSMCs) exhibit enhanced proliferation under mitogenic stimulation and accumulate in neointima after arterial injury. The cell cycle progression is positively regulated by cyclin/cyclin-dependent kinase (CDK) complex, and negatively controlled by p21 family. P57kip2 is one of the members of p21 family and its distribution is relatively limited to terminally differentiated tissues, including muscle. However, p57kip2 contribution to VSMC proliferation remains scarcely unknown. In this study, we investigated the role of p57kip2 in VSMC proliferation, in combination with other cell cycle related proteins. VSMCs were G0 arrested by serum deprivation, followed by mitogenic stimulation with growth medium, and harvested at every four hour. The phenotype of immature VSMCs didn't differentiate even after mitogenic deprivation. The protein levels of p57kip2 and p27kip1 diminished by mitogenic stimulation, in correlation with the upregulation of cyclin-D, -E and CDK2 protein amount and their kinase activities, resulting in the phosphorylation of retinoblastoma (Rb) protein family members, p107 and p130. Immunoprecipitation revealed that p27kip1 could associate with cyclin-D1, but did not inhibit its activity. On the contrary, p57kip2 could bind to cyclin-D1, and overexpressed p57kip2 prevented the phosphorylation of Rb family, probably due to canceling the cyclin-D1 (and/or cyclin-E) kinase activation. In conclusion, p57 decline is likely to be the key event for initiation of immature VSMCs proliferation. These results suggested that spplemental overexpression of p57kip2 could be a powerful strategy to prevent the development of intimal hyperplasia after arterial injury.
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