Inhibition of mitogenic signaling and induction of apoptosis in human bladder smooth muscle cells treated with doxazosin

Purpose: The alpha1 antagonist doxazosin is used to treat lower urinary tract symptoms and is believed to function primarily as a smooth muscle relaxant. However, doxazosin has been shown to inhibit proliferation and induce apoptosis in nonbladder smooth muscle. Consequently, we examined the effects of doxazosin on human bladder smooth muscle cell (SMC) proliferation and apoptosis. Materials and Methods: Primary human bladder SMCs were cultured in M199 with 10% fetal bovine serum (FBS) until they reached 65% confluency and then they were made quiescent by serum starvation in M199 with 0.4% FBS for 24 hours. The quiescent bladder SMCs were pretreated for 30 minutes with doxazosin or vehicle (dimethyl sulfoxide) and then stimulated with 10% FBS for 24 hours. Measurement of 5'-bromo-2'-deoxyuridine (BrdU) uptake by flow cytometry was used to determine the effect of doxazosin on cell cycle progression. Western immunoblot was used to examine cell cycle protein expression and phosphorylation of the retinoblastoma protein (Rb) and cyclin A, both of which regulate cell cycle progression. Results: Cellular proliferation was inhibited in a dose dependent manner by doxazosin. There was nearly a 50% decrease in BrdU uptake at 10 muM doxazosin and an approximately 90% decrease in BrdU at 25 muM doxazosin. Notably, doxazosin inhibited phosphorylation of Rb and expression of cyclin A, both of which are necessary for cell cycle progression. At concentrations of 25 muM doxazosin or greater apoptosis was induced in the bladder SMCs, as indicated by an increase in subG1 DNA content. Conclusions: Our study demonstrates that doxazosin inhibits mitogen induced proliferation of human bladder SMC by blocking cell cycle progression at the of G1/S border. Doxazosin induced cell cycle inhibition appears to be at least in part due to an inhibition of mitogen induced Rb phosphorylation and cyclin A expression. At higher concentrations doxazosin induces apoptosis in human bladder SMCs.