Glucocorticoids inhibit proliferation, cyclin D1 expression, and retinoblastoma protein phosphorylation, but not activity of the extracellular-regulated kinases in human cultured airway smooth muscle

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 neointimal hyperplasia and proliferation of rat aortic smooth muscle cells (SMC) by inducing a late G1 phase cell cycle arrest. Phosphorylation of retinoblastoma protein (Rb) regulates cell proliferation by controlling progression from G1 to S phase of the cell cycle. We hypothesized that DEX inhibits human vascular SMC proliferation and causes cell cycle arrest through inhibition of Rb phosphorylation. Human aortic SMC were cultured and treated with incremental doses of DEX. Cell counts and [(3)H]thymidine uptake were determined 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 treated with 10(-5) M DEX, and protein was extracted at sequential time points. Flow cytometry was performed to track cell cycle progression. Western blots 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. DEX inhibited the phosphorylation of Rb protein compared to control. DEX inhibits the proliferation of human vascular SMC by inducing G1 phase cell cycle arrest. DEX inhibited the phosphorylation of Rb, a key step in the progression of the cell from G1 to S phase. Elucidation of the mechanism of DEX may be helpful in treatment strategies for preventing neointimal hyperplasia as well as other disorders of cell proliferation.

Beta2-adrenergic receptor agonists and cAMP arrest human cultured airway smooth muscle cells in the G(1) phase of the cell cycle: role of proteasome degradation of cyclin D1

Hyperplasia of airway smooth muscle (ASM) contributes to the airway hyperresponsiveness that characterizes asthma. We have investigated the relationship between cAMP-induced growth arrest of ASM cells and thrombin-stimulated, extracellular-regulated protein kinase (ERK) activity, cyclin D1, and the restriction protein retinoblastoma. The beta(2)-adrenergic receptor agonist albuterol (100 nM) inhibited DNA synthesis after incubation with ASM for periods as brief as 1 h when these coincided with the timing of the restriction point. Inhibition of thrombin-stimulated DNA synthesis by albuterol (1-100 nM), 8-bromo-cAMP (300 microM), or prostaglandin E(2) (1 microM) was accompanied by a reduction in cyclin D1 protein levels. The ERK kinase inhibitor PD98059 (3-30 microM) attenuated thrombin-stimulated ERK phosphorylation and activity and the increase in cyclin D1 protein levels, as did albuterol (1-100 nM) or 8-bromo-cAMP (300 microM). In contrast, neither albuterol (100 nM) nor PD98059 (30 microM) reduced cyclin D1 mRNA levels between 4 and 20 h after thrombin addition, which suggests that elevation of cAMP regulates cyclin D1 by a post transcriptional mechanism. The proteasome inhibitor MG132 (30 and 100 nM) and the calpain I inhibitor N-acetyl-Leu-Leu-leucinal (10 microM) attenuated the reduction in thrombin-stimulated cyclin D1 levels in ASM exposed to albuterol (100 nM), 8-bromo-cAMP (300 microM), or the phosphodiesterase inhibitor isobutylmethylxanthine (100 microM). Thus, the cAMP-induced arrest of ASM in the G(1) phase of the cell cycle is associated with a proteasomal degradation of cyclin D1 protein and a reduced protein retinoblastoma phosphorylation that prevents passage through the restriction point.