Tamoxifen-Induced [Ca²⁺]_iRises and Ca²⁺-Independent Cell Death in Human Oral Cancer Cells

The purpose of this study was to explore the effect of tamoxifen on cytosolic free Ca2+ concentrations ([Ca2+](i)) and cell viability in OC2 human oral cancer cells. [Ca2+](i) and cell viability were measured by using the fluorescent dyes fura-2 and WST-1, respectively. Tamoxifen at concentrations above 2 mu M increased [Ca2+](i) in a concentration-dependent manner. The Ca2+ signal was reduced partly by removing extracellular Ca2+. The tamoxifen-induced Ca2+ influx was sensitive to blockade of L-type Ca2+ channel blockers but insensitive to the estrogen receptor antagonist ICI 182,780 and protein kinase C modulators. In Ca2+-free medium, after pretreatment with 1 mu M thapsigargin (an endoplasmic reticulum Ca2+ pump inhibitor), tamoxifen-induced [Ca2+](i) rises were substantially inhibited; and conversely, tamoxifen pretreatment inhibited a part of thapsigargin-induced [Ca2+](i) rises. Inhibition of phospholipase C with 2 mu M U73122 did not change tamoxifen-induced [Ca2+](i) rises. At concentrations between 10 and 50 mu M tamoxifen killed cells in a concentration-dependent manner. The cytotoxic effect of 23 mu M tamoxifen was not reversed by prechelating cytosolic Ca2+ with BAPTA. Collectively, in OC2 cells, tamoxifen induced [Ca2+](i) rises, in a nongenomic manner, by causing Ca2+ release from the endoplasmic reticulum, and Ca2+ influx from L-type Ca2+ channels. Furthermore, tamoxifen-caused cytotoxicity was not via a preceding [Ca2+](i) rise.