Mechanisms underlying maintenance of smooth muscle cell quiescence in rat aorta: role of the cyclin dependent kinases and their inhibitors

OBJECTIVE: We sought to understand why smooth muscle cell proliferation is effectively repressed in intact rat aortic tissue. METHODS: Quiescent isolated rat aortic smooth muscle cells and segments of intact rat aorta were stimulated with 10% serum and the time course of expression and activity of proteins involved in cell cycle control were determined. RESULTS: After serum stimulation, smooth muscle cells in intact aortic tissue exhibit no proliferation, whereas isolated cells entered S phase 14-16 h later. Activation of ERKs 1 and 2, and induction of cyclin D1 occurred both in isolated cells and aortic tissue. Regulation of Cdk4, cyclin E and Cdk2 protein levels was also not different. Levels of the cyclin-dependent kinase inhibitors (CKIs), p16 and p27, were initially high in quiescent isolated cells and tissue; levels were downregulated by serum in isolated cells but not in aortic tissue. Cyclin D1/Cdk4, and cyclin E/Cdk2 kinases were active before S phase entry in isolated cells, but remained inactive in aortic tissue. CONCLUSIONS: Cell cycle entry is prevented in aortic tissue, and this is associated with an inability to downregulate p16 and p27 CKIs, and therefore to activate cyclin D1 and cyclin E associated kinase activities.