Effect of m-3m3FBS on Ca ²⁺ handling and viability in OC2 human oral cancer cells

The effect of 2,4,6-trimethyl-N-(meta-3-trifluoromethyl-phenyl)- benzenesulfonamide (m-3M3FBS), a presumed phospholipase C activator, on cytosolic free Ca ²⁺ concentrations ([Ca ²⁺] _i) in OC2 human oral cancer cells is unclear. This study explored whether m-3M3FBS changed basal [Ca ²⁺] _i levels in suspended OC2 cells by using fura-2 as a Ca ²⁺-sensitive fluorescent dye. M-3M3FBS at concentrations between 10-60 μM increased [Ca ²⁺] _i in a concentration-dependent manner. The Ca ²⁺ signal was reduced partly by removing extracellular Ca ²⁺. M-3M3FBS-induced Ca ²⁺ influx was inhibited by the store-operated Ca ²⁺ channel blockers nifedipine, econazole and SK&F96365, and by the phospholipase A2 inhibitor aristolochic acid. In Ca ²⁺-free medium, 30 μM m-3M3FBS pretreatment inhibited the [Ca ²⁺] _i rise induced by the endoplasmic reticulum Ca ²⁺ pump inhibitors thapsigargin and 2,5-di-tert-butylhydroquinone (BHQ). Conversely, pretreatment with thapsigargin, BHQ or cyclopiazonic acid partly reduced m-3M3FBS-induced [Ca ²⁺] _i rise. Inhibition of inositol 1,4,5-trisphosphate formation with U73122 did not alter m-3M3FBS-induced [Ca ²⁺] _i rise. At concentrations between 5 and 100 μM m-3M3FBS killed cells in a concentration-dependent manner. The cytotoxic effect of m-3M3FBS was not reversed by prechelating cytosolic Ca ²⁺ with 1,2-bis(2- aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA). Propidium iodide staining data suggest that m-3M3FBS (20 or 50 μM) induced apoptosis in a Ca ²⁺-independent manner. Collectively, in OC2 cells, m-3M3FBS induced [Ca ²⁺] _i rise by causing inositol 1,4,5-trisphosphate-independent Ca ²⁺ release from the endoplasmic reticulum and Ca ²⁺ influx via phospholipase A ₂-sensitive store-operated Ca ²⁺ channels. M-3M3FBS also induced Ca ²⁺-independent cell death and apoptosis.