Effects of timolol on Ca 2+ handling and viability in human prostate cancer cells.

Timolol is a medication used widely to treat glaucoma. Regarding Ca2+ signaling, timolol was shown to modulate Ca2+ -related physiology in various cell types, however, the effect of timolol on Ca2+ homeostasis and cell viability has not been explored in human prostate cancer cells. The aim of this study was to explore the effect of timolol on intracellular Ca2+ concentrations ([Ca2+ ]i ) and viability in PC3 human prostate cancer cells. Timolol at concentrations of 100-1000 μM induced [Ca2+ ]i rises. The Ca2+ signal in Ca2+ -containing medium was reduced by removal of extracellular Ca2+ by approximately 75%. Timolol (1000 μM) induced Mn2+ influx suggesting of Ca2+ entry. Timolol-induced Ca2+ entry was partially inhibited by three inhibitors of store-operated Ca2+ channels: nifedipine, econoazole and SKF96365, and by a protein kinase C (PKC) activator (phorbol 12-myristate 13 acetate, PMA) or an inhibitor (GF109203X). In Ca2+ -free medium, treatment with the endoplasmic reticulum Ca2+ pump inhibitor thapsigargin abolished timolol-evoked [Ca2+ ]i rises. Conversely, treatment with timolol abolished thapsigargin-evoked [Ca2+ ]i rises. Inhibition of phospholipase C (PLC) with U73122 abolished timolol-induced [Ca2+ ]i rises. Timolol at concentrations between 200-600 μM killed cells in a concentration-dependent fashion. Chelation of cytosolic Ca2+ with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid/AM (BAPTA/AM) did not reverse cytotoxicity of timolol. Together, in PC3 cells, timolol induced [Ca2+ ]i rises by evoking Ca2+ release from the endoplasmic reticulum in a PLC-dependent manner, and Ca2+ influx via PKC-regulated store-operated Ca2+ entry. Timolol also caused cell death that was not linked to preceding [Ca2+ ]i rises.