Differential Effects of the Cyclin-Dependent Kinase Inhibitors p27, p21, and p16 on Vascular Smooth Muscle Cell Proliferation

Background—The cyclin-dependent kinase inhibitors (CKIs) have different patterns of expression in vascular diseases. The Kip/Cip CKIs, p27 and p21, are upregulated during arterial repair and negatively regulate the growth of vascular smooth muscle cells (VSMCs). In contrast, the Ink CKI, p16, is not expressed in vascular lesions. We hypothesized that a variation in the inactivation of cdk2 and cdk4 during the G1 phase of the cell cycle by p27 , p21, and p16 leads to different effects on VSMC growth in vitro and in vivo. Methods and Results—The expression of p27 and p21 in serum-stimulated VSMCs inactivated cdk2 and cdk4, leading to G1 growth arrest. p16 Ink4 inhibited cdk4, but not cdk2, kinase activity, producing partial inhibition of VSMC growth in vitro. In an in vivo model of vascular injury, overexpression of p27 reduced intimal VSMC proliferation by 52% (P,0.01) and the intima/media area ratio by 51% (P,0.005) after vascular injury and gene transfer to pig arteries, when compared with control arteries. p16 was a weak inhibitor of intimal VSMC proliferation in injured arteries (P5NS), and it did not significantly reduce intima/media area ratios (P5NS), which is consistent with its minor effects on VSMC growth in vitro. Conclusions—p27 and p21 are potent inhibitors of VSMC growth compared with p16 because of their different molecular mechanisms of cyclin-dependent kinase inhibition in the G1 phase of the cell cycle. These findings have important implications for our understanding of the pathophysiology of vascular proliferative diseases and for the development of molecular therapies. (Circulation. 2000;101:2022-2025.)