Tamoxifen has been shown to increase cytoplasmic free Ca2+ levels [Ca2+]i in renal tubular cells and bladder cancer cells, and to after Ca2+ signaling in MCF-7 breast cancer cells. The present study examined the effect of tamoxifen on [Ca2+], in ZR-75-1 human breast cancer cells using fura-2 as an indicator. Tamoxifen increased [Ca2+]i at a concentration above 2 microM with an EC50 of 5 microM. Removing extracellular Ca2+ reduced the response by 48+/-2%. In Ca2+-free medium, after tamoxifen-induced [Ca2+]i increased had returned to baseline, adding 3 mM Ca2+ increased [Ca2+]i in a concentration-dependent manner. Further, pretreatment with 10 microM tamoxifen abolished the [Ca2+]i increase induced by 1 microM thapsigargin (an endoplasmic reticulum Ca2+ pump inhibitor); and conversely, pretreatment with thapsigargin prevented tamoxifen from releasing more Ca2+. Tamoxifen (10 microM)-induced Ca2+ release was not changed by inhibiting phospholipase C activity with 2 microM U73122. Trypan blue exclusion assay revealed that tamoxifen (1-10 microM) did not alter viability after 1 min of incubation, but killed 10% of cells after 3-10 min of incubation. Together, this study shows that tamoxifen (>2 microM) induced a significant, immediate increase in [Ca2+]i in ZR-75-1 breast cancer cells. Tamoxifen acted by releasing Ca2+ from the endoplasmic reticulum Ca2+ stores in a manner independent of phospholipase C activity, and by inducing Ca2+ entry from extracellular medium. Tamoxifen may be of mild cytotoxicity after acute exposure.

The effect of the phospholipase A2 inhibitor palmitoyl trifluoromethyl ketone (PACOCF3) on Ca2+ signaling in Madin Darby canine kidney (MDCK) cells was examined using fura-2 as the fluorescent Ca2+ indicator. At a concentration of 20 microM, PACOCF3 did not change basal cytosolic free calcium concentrations ([Ca2+]i), but at concentrations of 50-250 microM PACOCF3 induced an increase in [Ca2+]i by activating extracellular Ca2+ entry which was partly suppressed by 50 microM La3+. The effect of PACOCF3 was abolished by removal of extracellular Ca2+. PACOCF3 (10 microM) enhanced both the peak value and the area under the curve of the [Ca2+]i increase induced by 10 microM ATP and 1 microM bradykinin by potentiating extracellular Ca2+ influx without affecting internal Ca2+ release. Several other phospholipase A2 inhibitors had no effect on basal [Ca2+]i or agonist-induced [Ca2+]i increases. Collectively, the results suggest that PACOCF3 alters Ca2+ signaling in renal tubular cells in a manner independent of phospholipase A2 inhibition.

The effect of 17β-estradiol on intracellular Ca²⁺ concentrations ([Ca²⁺]_i) in Madin Darby canine kidney cells was investigated by using the fluorescent dye fura-2. 17β-Estradiol (5–100 µmol/l) induced instantaneous increases in [Ca²⁺]_i in a concentration-dependent manner. Ca²⁺ removal inhibited 45 ± 15% of the Ca²⁺ signal. In Ca²⁺-free medium, pretreatment with 50 µmol/l 17β-estradiol abolished the [Ca²⁺]_i increases induced by 2 µmol/l carbonylcyanide m-chlorophenylhydrazone (CCCP; a mitochondrial uncoupler), 1 µmol/l thapsigargin (an endoplasmic reticulum Ca²⁺ pump inhibitor) and 50 µmol/l brefeldin A (an antibiotic which disperses the Golgi complex), but pretreatment with brefeldin A, CCCP and thapsigargin only partly inhibited the 17β-estradiol-induced [Ca²⁺]_i signal. Adding 3 mmol/lCa²⁺ increased [Ca²⁺]_i in cells pretreated with 5–100 µmol/l 17β-estradiol in Ca²⁺-free medium. Pretreatment with 1 µmol/l U73122 to abolish the formation of inositol-1,4,5-trisphosphate inhibited 50% of the Ca²⁺ release induced by 50 µmol/l 17β-estradiol. 17β-Estradiol (20 µmol/l) also increased [Ca²⁺]_i in human bladder cancer cells and prostate cancer cells. Collectively, this study shows that 17β-estradiol evoked a significant internal Ca²⁺ release and external Ca²⁺ entry possibly in a nongenomic manner.

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