Background—The cyclin-dependent kinase inhibitors (CKIs) have different patterns of expression in vascular diseases. The Kip/Cip CKIs, p27^Kip1 and p21^Cip1, are upregulated during arterial repair and negatively regulate the growth of vascular smooth muscle cells (VSMCs). In contrast, the Ink CKI, p16^Ink4, 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^Kip1, p21^Cip1, and p16^Ink4 leads to different effects on VSMC growth in vitro and in vivo.

Methods and Results—The expression of p27^Kip1 and p21^Cip1 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^Kip1 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^Ink4 was a weak inhibitor of intimal VSMC proliferation in injured arteries (P=NS), and it did not significantly reduce intima/media area ratios (P=NS), which is consistent with its minor effects on VSMC growth in vitro.

Conclusions—p27^Kip1 and p21^Cip1 are potent inhibitors of VSMC growth compared with p16^Ink4 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.