Tamoxifen-induced Ca²⁺ mobilization in bladder female transitional carcinoma cells

This study examined the effect of tamoxifen, an anti-breast cancer drug, on Ca²⁺ handling in bladder female transitional cancer cells. Changes in cytosolic free Ca²⁺ levels were recorded by using the Ca²⁺-sensitive dye fura-2. In a dose-dependent manner, tamoxifen induced intracellular free Ca²⁺ concentrations ([Ca²⁺]_i) increases between 5 and 20 μM with an EC₅₀ of 10 μM. External Ca²⁺ removal reduced the response by 60±6%. Addition of 3 mM Ca²⁺ caused a [Ca²+]_i increase after pretreatment with 10 μM tamoxifen in Ca²⁺-free medium. In Ca²⁺-free medium, pretreatment with 10 μM tamoxifen abolished the [Ca²⁺]_i increase induced by 1 μM thapsigargin, an endoplasmic reticulum Ca²⁺ pump inhibitor. Conversely, pretreatment with 1 μM thapsigargin prevented tamoxifen from releasing more Ca²⁺. Inhibition of phospholipase C-dependent inositol 1,4,5-tris-phosphate formation with 2 μM U73122 did not alter 10 μM tamoxifen-induced Ca²⁺ release. The [Ca²⁺]_i increase induced by 5 μM tamoxifen was not altered by 10 μM La³⁺, nifedipine, verapamil, and diltiazem. Collectively, it was found that tamoxifen increased [Ca²⁺]_i in bladder cancer cells by releasing Ca²⁺ from the endoplasmic reticulum Ca²⁺ stores in a manner independent of phospholipase C activity, and by inducing Ca²⁺ entry from external medium.