Activated human T lymphocytes exhibit reduced susceptibility to methylmercury chloride-induced apoptosis

Mercurials have been shown to cause apoptosis in human T cells. The objective of this study was to evaluate and compare the relative susceptibility of resting versus activated T cells to methyl mercury chloride (MeHgCl)-induced cell death. Apoptosis was assessed by Hoechst 33258 and 7-AAD staining and annexin V binding. Our results show that activation of T cells by PHA, PMA, and ionomycin, or IL-2, reduces mercury-induced apoptosis by approximately 50%. We have previously shown that the underlying basis for these toxic effects involves perturbation of mitochondrial function leading to oxidative stress and the release of cytochrome c to the cytosol. Therefore, the ability of MeHgCl to alter the mitochondrial transmembrane potential (Delta Psi(m)) and to induce the generation of reactive oxygen species (ROS) was evaluated in activated T-cells. Both resting and activated cells treated with MeHgCl exhibited a decrease in Delta Psi(m), when compared to respective control cells. ROS production was elevated in resting cells following treatment with mercury; in contrast, fewer activated T cells exhibit increased levels of ROS in the presence of MeHgCl. Similarly, MeHgCl treatment resulted in the release of cytochrome c to the cytoplasm in non-activated T cells but failed to do so in the activated population. These results lead us to examine intracellular levels of bcl-2, a protein that has been shown to regulate apoptosis, presumably via its ability to associate with the mitochondrial membrane. Bcl-2 levels were found, in resting cells, to be low in the presence or absence of mercury. In comparison, activated T cells expressed elevated levels of bcl-2. The relationship between mercury-induced apoptosis in human T cells, mitochondrial dysfunction, and intracellular levels of bcl-2 are discussed.