Dexamethasone inhibits the phosphorylation of retinoblastoma protein in the suppression of human vascular smooth muscle cell proliferation

We have previously demonstrated that dexamethasone (DEX) suppresses neointi mal hyperplasia and proliferation of rat aortic smooth muscle cells (SMC) b y inducing a late G1 phase cell cycle arrest. Phosphorylation of retinoblas toma protein (Rb) regulates cell proliferation by controlling progression f rom G1 to S phase of the cell cycle. We hypothesized that DEX inhibits huma n vascular SMC proliferation and causes cell cycle arrest through inhibitio n of Rb phosphorylation. Human aortic SMC were cultured and treated with in cremental doses of DEX. Cell counts and [H-3]thymidine uptake were determin ed after 72 h. To examine the effects of DEX on the cell cycle, cells were synchronized by serum deprivation, restimulated to enter G1 phase, and trea ted with 10(-5) M DEX, and protein was extracted at sequential time points. Flow cytometry was performed to track cell cycle progression. Western blot s were performed to examine Rb phosphorylation. DEX inhibited smooth muscle cell proliferation and DNA synthesis in a concentration-dependent fashion. Flow cytometry indicated that DEX induces a G1 phase cell cycle arrest, DE X inhibited the phosphorylation of Rb protein compared to control. DEX inhi bits the proliferation of human vascular SMC by inducing G1 phase cell cycl e arrest, DEX inhibited the phosphorylation of Rb, a key step in the progre ssion of the cell from G1 to S phase, Elucidation of the mechanism of DEX m ay be helpful in treatment strategies for preventing neointimal hyperplasia as well as other disorders of cell proliferation. (C) 2000 Academic Press.