Apoptosis induced by cadmium in human lymphoma U937 cells through Ca²⁺-calpain and caspase-mitochondriadependent 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 understood. When the human histiocytic lymphoma cell line U937 was treated with cadmium for 12 h, evidence of apoptotic features, including change in nuclear morphology, DNA fragmentation, formation of DNA ladder in agarose gel electrophoresis, and phosphatidylserine externalization, were obtained. Moreover, loss of the mitochondrial membrane potential (δψm) was observed in the cadmium-treated cells and was inhibited by a broad caspase inhibitor (Z-VAD-FMK). Caspase inhibitors suppressed the DNA fragmentation in the order of Z-VAD-FMK > caspase-8 inhibitor > caspase-3 inhibitor. Expression 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. Tetrakis-(2-pyridylmethyl) ethylendiamine, a cell-permeable heavy metal chelator, partially reversed the increase of fluorescence of Fura-2 in the cadmium-treated cells. In addition, verapamil (70 μM), a voltage-dependent Ca²⁺ channel blocker, inhibited the DNA fragmentation induced by cadmium less than 100 μM and decreased the fluorescence of Fura-2. Cadmium up-regulated the expression of type 1 inositol 1,4,5-trisphosphate receptor (IP₃R) but not type 2 or type 3 IP₃3R. Calpain inhibitors I and II partially prevented DNA fragmentation. No effects of Z-VAD-FMK on the expression of type 1 IP₃R or of calpain inhibitors on the loss of δμ_m were observed. These results suggest that cadmium possibly induced apoptosis in U937 cells through two independent path-ways, the Ca²⁺-calpain-dependent pathway and the caspase-mitochondria-dependent pathway.