The anti-breast cancer drug tamoxifen alters Ca²⁺ movement in Chinese hamster ovary (CHO-K1) cells

The anti-breast cancer drug tamoxifen has recently been shown to cause an increase in intracellular free-Ca²⁺ concentrations ([Ca²⁺]_i) in renal tubular cells, breast cells and bladder cells. Because tamoxifen is known to alter ovary function in human patients and in rats, the present study was aimed at exploring whether tamoxifen could alter Ca²⁺ movement in Chinese hamster ovary (CHO-K1) cells. Cytosolic free-Ca²⁺ levels in populations of cells have been explored by using fura-2 as a fluorescent Ca²⁺ indicator. Tamoxifen at concentrations above 1 μM increased [Ca²⁺]_i in a concentration-dependent manner with an EC₅₀ value of 8 μM. The Ca²⁺ signal was reduced by removing extracellular Ca²⁺, but was not affected by nifedipine, verapamil, diltiazem or ICI 182,780 (an estrogen receptor antagonist). Pretreatment with 1 μM thapsigargin (an endoplasmic reticulum Ca²⁺ pump inhibitor) to deplete the endoplasmic reticulum Ca²⁺ abolished 10 μM tamoxifen-induced Ca²⁺ release. Neither inhibition of phospholipase C with 2 μM U73122 nor depletion of ryanodine-sensitive Ca²⁺ stores with 50 μM ryanodine affected tamoxifen-induced Ca²⁺ release. Cell proliferation assays using ELISA revealed that overnight incubation with 5-10 μM tamoxifen inhibited cell proliferation by 20%, and 20 μM tamoxifen killed all cells. Together, the results suggest that, in CHO-K1 cells, tamoxifen induced a [Ca²⁺]_i increase by causing store-Ca²⁺ release from the endoplasmic reticulum in an phospholipase C-independent manner, and by inducing Ca²⁺ influx. The action of tamoxifen appears to be dissociated from estrogen receptor activation. Longer incubation with tamoxifen (> 5 μM) was cytotoxic.