Safrole-induced Ca²⁺ mobilization and cytotoxicity in human PC3 prostate cancer cells

The effect of the carcinogen safrole on intracellular Ca²⁺ mobilization and on viability of human PC3 prostate cancer cells was examined. Cytosolic free Ca²⁺ levels ([Ca²⁺]_i) were measured by using fura-2 as a probe. Safrole at concentrations above 10 μM increased [Ca²⁺]_i in a concentration-dependent manner with an EC₅₀ value of 350 μM. The Ca²⁺ signal was reduced by more than half after removing extracellular Ca²⁺ but was unaffected by nifedipine, nicardipine, nimodipine, diltiazem, or verapamil. In Ca²⁺-free medium, after treatment with 650 μM safrole, 1 μM thapsigargin (an endoplasmic reticulum Ca²⁺ pump inhibitor) failed to release Ca²⁺. Neither inhibition of phospholipase C with U73122 nor modulation of protein kinase C activity affected safrole-induced Ca²⁺ release. Overnight incubation with 0.65-65 μM safrole did not affect cell viability, but incubation with 325-625 μM safrole decreased viability. Collectively, the data suggest that in PC3 cells, safrole induced a [Ca ²⁺]_i increase by causing Ca²⁺ release from the endoplasmic reticulum in a phospholipase C- and protein kinase C-independent fashion, and by inducing Ca²⁺ influx. Safrole can decrease cell viability in a concentration-dependent manner.