Effect of Methoxsalen on Ca²⁺ Homeostasis and Viability in Human Osteosarcoma Cells

Methoxsalen is a natural compound found in many seed plants. The effect of methoxsalen on Ca²⁺- related physiology in human osteosarcoma is unclear. This study investigated the effect of methoxsalen on cytosolic free Ca²⁺ concentrations ([Ca²⁺]i) in MG63 human osteosarcoma cells. Methoxsalen induced [Ca²⁺]i rises concentration-dependently. Methoxsalen-induced Ca²⁺ entry was confirmed by Mn²⁺-induced quench of fura-2 fluorescence. This Ca²⁺ entry was suppressed by nifedipine, econazole, and SKF96365. In Ca²⁺-free medium, incubation with the endoplasmic reticulum Ca²⁺ pump inhibitor 2,5-di-tert-butylhydroquinone (BHQ) inhibited methoxsalen-evoked [Ca²⁺]i rises by 96%. In contrast, incubation with methoxsalen abolished BHQ-evoked [Ca²⁺]i rises. Inhibition of phospholipase C (PLC) with U73122 abolished methoxsalen-induced [Ca²⁺]i rises. Methoxsalen was cytotoxic at 300-700 μM in a concentration-dependent fashion. Chelating cytosolic Ca²⁺ with 1,2-bis(2-aminophenoxy)ethane-N,N,N’,N’-tetraacetic acid/acetoxymethyl ester (BAPTA/AM) did not prevent methoxsalen-induced cytotoxicity. Collectively, our data suggest that in MG63 cells, methoxsalen induced [Ca²⁺]i rises by evoking PLC-dependent Ca²⁺ release from the endoplasmic reticulum, and Ca²⁺ entry via store-operated Ca²⁺ entry. Methoxsalen also induced Ca²⁺- disassociated cell death.