Altered S-phase kinase-associated protein-2 levels are a major mediator of cyclic nucleotide-induced inhibition of vascular smooth muscle cell proliferation

Cyclic nucleotides inhibit vascular smooth muscle cell ( VSMC) proliferation but the underlying molecular mechanisms are incompletely understood. We studied the role of S- phase kinase- associated protein- 2 ( Skp2), an F- box protein of SCFSkp2 ubiquitin ligase responsible for polyubiquitylation of and subsequent proteolysis of p27(Kip1), a key step leading to cell cycle progression. Skp2 mRNA and protein were upregulated in mitogen- stimulated VSMCs and after balloon injury in rat carotid arteries, where the time course and location of Skp2 expression closely paralleled that of proliferating cell nuclear antigen. Skp2 small interference RNA ( siRNA) reduced Skp2 expression, increased p27(Kip1) levels, and inhibited VSMC proliferation in vitro. cAMP- elevating agents prominently inhibited VSMC proliferation and Skp2 expression through inhibiting Skp2 transcription as well as decreasing Skp2 protein stability. Consistent with this, activation of protein kinase A, a downstream target of cAMP, was shown to negatively regulate focal adhesion kinase ( FAK) phosphorylation and Skp2 expression. Adenovirus- mediated Skp2 expression reversed cAMP- induced p27(Kip1) upregulation and rescued cAMP- related S- phase entry inhibition up to 50%. 8- Bromo- cGMP also moderately reduced Skp2 and cell proliferation when VSMCs were incubated with low serum concentration. Interestingly, we showed that 8- bromo- cGMP inhibited Skp2 expression also through activation of protein kinase A, not protein kinase G, which conversely enhanced FAK(Y397) phosphorylation and Skp2 expression. After balloon injury of rat carotid arteries, local forskolin treatment significantly reduced FAK(Y397) phosphorylation, Skp2 expression, VSMC proliferation, and subsequent neointimal thickening. These data demonstrate for the first time that Skp2 is an important factor in VSMC proliferation and its inhibition by cyclic nucleotides.