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

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