Mercuric chloride-induced apoptosis is dependent on protein synthesis.

Apoptosis is a mode of cell death with morphologic and biochemical features that distinguish it from necrosis. Recent studies demonstrating that mercury compounds initiate apoptosis in cultured cells did not elucidate if the biochemical mechanism of apoptosis involved a dependence on macromolecular synthesis post-insult, i.e. programmed cell death. The objectives of this in vitro study were (1) to determine if HgCl2 cytotoxicity includes an apoptotic component, and (2) to determine if apoptosis is dependent on protein synthesis, i.e. proceeds by an inducible mechanism. Suspensions of mouse lymphoma (L5178Y-R) cells were exposed to 0, 1, 5, or 10 microM HgCl2 and apoptosis was evaluated utilizing qualitative and quantitative methods. At 24 h after exposure, transmission electron microscopy revealed a concentration-related increase in morphologic changes typical of apoptosis: margination of condensed chromatin to the nuclear membrane, dilation of the rough endoplasmic reticulum, cytoplasmic condensation and vacuolation, nuclear dissolution, and plasma membrane blebbing. An increase in Hg-induced DNA fragmentation (DNA 'ladder') was observed using agarose gel electrophoresis. Time- and concentration-dependent increases in the percent of apoptotic cells were observed at 1, 6, 12, and 24 h after HgCl2 exposure using a flow cytometric method that discriminates between cells according to size and granularity. Pretreatment of cells with cycloheximide (CHX), an inhibitor of translation, prior to HgCl2 exposure resulted in a 25-50% reduction in apoptotic cells 24 h after exposure to 10 and 20 microM HgCl2, and concomitantly reduced the overall cytotoxicity compared to HgCl2 alone. These results, although limited to a single cell line, support the hypothesis that HgCl2 induces apoptosis that is dependent, at least in part, upon protein synthesis.