Cadmium induces Ca2+ mediated, calpain-1/caspase-3-dependent apoptosis in primary cultured rat proximal tubular cells

Calcium, as a ubiquitous second messenger, governs a large array of cellular processes and is necessary for cell survival. More recently, it was observed that the cytosolic Ca2+ concentration ([Ca2+](c)) elevation could induce apoptosis in primary cultured rat proximal tubular (rPT) cells exposed to cadmium (Cd), but the concrete mechanism is still unclear. This study was designed to investigate the signal pathway involved in [Ca2+](c) elevation-mediated apoptosis. The results confirmed the elevation of [Ca2+](c) by confocal microscopy and enhancement of the apoptosis by Hoechst 33258 staining and flow cytometer when rPT cells were exposed to Cd for 12 h. Then we demonstrated that Cd enhanced the protein levels of active calpain-1 and caspase-3 in rPT cells. Pretreatment with a cytosolic Ca2+ chelator, 1,2-Bis (2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM), markedly blocked the up-regulation of active calpain-1 and caspase-3 and inhibited the apoptosis induced by Cd. Further, rPT cells were pretreated with a cell-permeable selective calpain-1 inhibitor, 3-(4-iodophenyl)-2-mercapto-(Z)-2-propenoic acid (PD150606) and caspase-3 inhibitor, N-Acetyl-Asp-Glu-Val-Asp-CHO (Ac-DEVD-CHO), respectively. PD150606 significantly attenuated the up-regulation of active caspase-3 and the apoptosis induced by Cd. As expected, inhibition of active caspase-3 by Ac-DEVD-CHO decreased the apoptosis induced by Cd. Taken together, it could be concluded that [Ca2+](c) elevation did act as a pro-apoptotic signal in Cd-induced cytotoxicity of rPT cells, triggered calpain-1 and caspase-3 activation in turn, and induced apoptosis of rPT cells.