Effect of thimerosal on Ca(2+) movement and viability in human oral cancer cells

The effect of thimerosal on cytosolic free Ca(2+) concentrations ([Ca(2+)](i) ) in human oral cancer cells (OC2) is unclear. This study explored whether thimerosal changed basal [Ca(2+)](i) levels in suspended OC2 cells using fura-2. Thimerosal at concentrations between 1and 50 microM increased [Ca(2+)](i) in a concentration-dependent manner. The Ca(2+) signal was reduced partly by removing extracellular Ca( 2+). Thimerosal-induced Ca(2+) influx was not blocked by L-type Ca(2+) entry inhibitors and protein kinase C modulators (phorbol 12-myristate 13-acetate [PMA] and GF109203X). In Ca(2+)-free medium, 50 microM thimerosal failed to induce a [Ca(2+)](i) rise after pretreatment with thapsigargin (an endoplasmic reticulum Ca(2+) pump inhibitor). Inhibition of phospholipase C with U73122 did not change thimerosal-induced [Ca(2+)](i) rises. At concentrations between 5 and 10 microM, thimerosal killed cells in a concentration-dependent manner. The cytotoxic effect of 8 muM thimerosal was potentiated by prechelating cytosolic Ca(2+) with the Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate/acetomethyl (BAPTA/ AM). Flow cytometry data suggested that 1-7 microM thimerosal-induced apoptosis in a concentration-dependent manner. Collectively, in OC2 cells, thimerosal-induced [Ca(2+)](i) rises by causing phospholipase C-independent Ca(2+) release from the endoplasmic reticulum and Ca(2+) influx through non-L-type Ca(2+) channels. Thimerosal killed cells in a concentration-dependent manner through apoptosis.