Tamoxifen-Induced [Ca2+]i Rises 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 μM 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 μM 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 μM U73122 did not change tamoxifen-induced Ca²⁺]_i rises. At concentrations between 10 and 50 μM tamoxifen killed cells in a concentration-dependent manner. The cytotoxic effect of 23 μM 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.