Dual effect of tamoxifen, an anti-breast-cancer drug, on intracellular Ca2+ and cytotoxicity in intact cells

The effect of tamoxifen on Ca2+ signaling and viability in Madin Darby canine kidney (MDCK) cells was investigated by using fura-2 as a Ca2+ probe. Tamoxifen evoked a rise in cytosolic free Ca2+ levels ([Ca2+](i)) concentration-dependently between 1 and 50 mu M with an EC50 of 10 mu M. The response was decreased by extracellular Ca2+ removal. In Ca2+-free medium, pretreatment with 5 mu M tamoxifen abolished the [Ca2+](i) increase induced by the endoplasmic reticulum Ca2+ pump inhibitor thapsigargin (1 mu M), but pretreatment with brefeldin A (50 mu M; a Ca2+ mobilizer of the Golgi complex), thapsigargin (an inhibitor of the endoplasmic reticulum Ca2+ pump), and carbonylcyanide m-chlorophenylhydrazone (CCCP; a mitochondrial uncoupler), only partly inhibited tamoxifen-induced [Ca2+](i) increases. This suggests that tamoxifen released Ca2+ from multiple pools. Addition of 3 mM Ca2+ induced a [Ca2+](i) rise after pretreatment with 5 mu M tamoxifen in Ca2+-free medium. Inhibiting inositol 1,4,5-trisphosphate formation with the phospholipase C inhibitor U73122 (2 mu M) did not alter 5 mu M tamoxifen-induced Ca2+ release. The [Ca2+](i) increase induced by 5 mu M tamoxifen was not altered by La3+, nifedipine, verapamil, or diltiazem. Tamoxifen (1-10 mu M) decreased cell viability in a concentration- and time-dependent manner. Tamoxifen (5 mu M) also increased [Ca2+](i) in neutrophils, bladder cancer cells, and prostate cancer cells from humans and glioma cells from rats. Collectively, it was found that tamoxifen increased [Ca2+](i) in MDCK cells by releasing Ca2+ from multiple Ca2+ stores in a manner independent of the production of inositol 1,4,5-trisphosphate and also by triggering Ca2+ influx from extracellular space. The [Ca2+](i) increase was accompanied by cytotoxicity. (C) 2000 Academic Press.