Molecular mechanisms involved in the adaptive response to cadmium-induced apoptosis in human myelomonocytic lymphoma U937 cells

We examined the molecular mechanisms involved in the adaptive response to cadmium (Cd)-induced apoptosis in human myelomonocytic lymphoma U937 cells. When U937 cells were treated with 50μM cadmium chloride (CdCl ₂) for 12h, significant apoptosis occurred. This was associated with an increase in intracellular reactive oxygen species (ROS), sustained phosphorylation of JNK, activation of caspase-3, a decrease in Mcl-1 (anti-apoptotic Bcl-2 proteins), and increases in Bim, Noxa and tBid (a pro-apoptotic protein under the Bcl-2 family). No apoptosis occurred when the cells were treated with 1μM CdCl ₂ for 72h. However, pretreatment with low-dose CdCl ₂ dramatically altered the sensitivity of the cells to 50μM CdCl ₂ with inhibition of apoptosis. Concomitantly, there were significant decreases in the generation of intracellular ROS and the activation of JNK. Pretreatment with 1μM CdCl ₂ also attenuated the decrease in Mcl-1 and the increases in Bim, Noxa and tBid induced by 50μM CdCl ₂. In conclusion, pretreatment with low-dose Cd inhibited apoptosis induced by high-dose Cd. The mechanism involves inhibition of intracellular ROS generation and JNK activation, and modulating the balance between the expression of Mcl-1 and its binding partners, Bim, Noxa and tBid.