β-Sitosterol inhibits cell cycle progression of rat aortic smooth muscle cells through increases of p21^cip1 protein

Abnormal proliferation of vascular smooth muscle cells (VSMCs) plays a central role in the pathogenesis of atherosclerosis. β-Sitosterol, an important phytosterol found in plant food, is known to exert antiatherosclerosis activity. However, the molecular mechanisms underlying β-sitosterol- induced antiproliferation of VSMCs were still not clear. This study demonstrated that β-sitosterol (1-20 μM) concentration-dependently inhibited proliferation of rat aortic smooth muscle cells (RASMCs) without cytotoxic effect. Flow cytometric analysis revealed that β-sitosterol arrested cell cycle progression through down-regulation of cyclin E and cyclin-dependent kinase (CDK)2 and up-regulation of p21^cip1. In the β-sitosterol-treated RASMCs, the formation of the CDK2-p21^cip1 complex was increased and the assayable CDK2 activity was decreased. Knockdown of the expression of p21^cip1 gene prevented β-sitosterol-induced cell cycle arrest in RASMCs. In conclusion, β-sitosterol inhibited VSMC proliferation by increasing the levels of p21^cip1 protein, which in turn inhibited the CDK2 activity, and finally interrupted the progress of the cell cycle.