Apoptosis induced by cadmium in human lymphoma U937 cells through Ca2+-calpain and caspase-mitochondria dependent pathways

Apoptosis induced by cadmium has been shown in many tissues in vivo and in cultured cells in vitro. However, its molecular mechanism is not fully unde rstood. When the human histiocytic lymphoma cell line U937 was treated with cadmium for 12 h, evidence of apoptotic features, including change in nucl ear morphology, DNA fragmentation, formation of DNA ladder in agarose gel e lectrophoresis, and phosphatidylserine externalization, were obtained. More over, loss of the mitochondrial membrane potential (Delta psi (m)) was obse rved in the cadmium-treated cells and was inhibited by a broad caspase inhi bitor (Z-VAD-FMK). Caspase inhibitors suppressed the DNA fragmentation in t he order of Z-VAD-FMK > caspase-8 inhibitor > caspase-3 inhibitor. Expressi on of Bcl-x(L) and Bid decreased significantly in the cadmium-treated cells , although no apparent change in Bcl-2 and Bax expression was found. Tetrak is-(2-pyridylmethyl) ethylendiamine, a cell-permeable heavy metal chelator, partially reversed the increase of fluorescence of Fura-a in the cadmium-t reated cells. In addition, verapamil (70 muM), a voltage-dependent Ca2+ cha nnel blocker, inhibited the DNA fragmentation induced by cadmium less than 100 muM and decreased the fluorescence of Fura-8. Cadmium up-regulated the expression of type 1 inositol 1,4,5-trisphosphate receptor (IP3R) but not t ype 2 or type 3 IP3R Calpain inhibitors I and II partially prevented DNA fr agmentation. No effects of Z-VAD-FMK on the expression of type 1 IP3R or of calpain inhibitors on the loss of Delta psi (m) were observed. These resul ts suggest that cadmium possibly induced apoptosis in U937 cells through tw o independent pathways, the Ca2+-calpain-dependent pathway and the caspase- mitochondria-dependent pathway.