Cell cycle protein expression in vascular smooth muscle cells in vitro andin vivo is regulated through phosphatidylinositol 3-kinase and mammalian target of rapamycin

Cell cycle progression represents a key event in vascular proliferative dis eases, one that depends on an increased rate of protein synthesis. An incre ase in phosphatidylinositol 3-kinase (PI 3-kinase) activity is associated w ith vascular smooth muscle cell proliferation, and rapamycin, which blocks the activity of the mammalian target of rapamycin, inhibits this proliferat ion in vitro and in vivo. We hypothesized that these 2 molecules converge o n a critical pathway of translational regulation that is essential for succ essful upregulation of cell cycle-regulatory proteins in activated smooth m uscle cells, p70(S6) kinase, a target of PI 3-kinase and the mammalian targ et of rapamycin, was rapidly activated on growth factor stimulation of quie scent coronary artery smooth muscle cells and after balloon injury of rat c arotid arteries. The translational repressor protein 4E-binding protein 1 w as similarly hyperphosphorylated under these conditions. These events were associated with increases in the protein levels of cyclin B1, cyclin DI, cy clin E, cyclin-dependent kinase 1, cyclin-dependent kinase 2, proliferating cell nuclear antigen, and p21(Cip1) in vivo and in vitro, whereas inhibiti on of the PI 3-kinase signaling pathway with either rapamycin or wortmannin blocked the upregulation of these cell cycle proteins, but not mRNA, and a rrested the cells in vitro before S phase. In contrast to findings in other cell types, growth factor- or balloon injury-induced downregulation of the cell cycle inhibitor p27(Kip1) was not affected by rapamycin treatment. Th ese data suggest that cell cycle progression in vascular cells in vitro and in vivo depends on the integrity of the PI 3-kinase signaling pathway in a llowing posttranscriptional accumulation of cell cycle proteins.