LOW-LEVEL METHYLMERCURY EXPOSURE CAUSES HUMAN T-CELLS TO UNDERGO APOPTOSIS - EVIDENCE OF MITOCHONDRIAL DYSFUNCTION

There is growing evidence that heavy metals, in general, and mercurial compounds, in particular, are immunotoxic to the human immune system. The major focus of our study is to demonstrate that methylmercuric ch loride (MeHgCl) kills human lymphocytes by inducing apoptosis. T-cells exposed to 0.6-5 mu M MeHgCl for 24 h were analyzed by flow cytometry . Methylmercury-treated cells exhibited increased Hoechst 33258 fluore scence while maintaining their ability to exclude the vital stain 7-am inoactinomycin. Furthermore, T-cells exposed to methylmercury exhibite d changes in light scatter patterns that included decreased forward li ght scatter and increased side light scatter. The light scatter and fl uorescent changes were consistent with morpholoscal alterations displa yed by cells during apoptosis. Cell death was further evaluated by ass essing annexin V binding to the plasma membrane. Methylmercury-treated cells exhibited increased annexin V binding indicative of phosphatidy lserine translocation to the outer leaflet of the plasma membrane. Usi ng the fluorescent probe DiOC(6)(3), we noted that methylmercury expos ure resulted in a decrease in mitochondrial transmembrane potential (P si(m)). Since a low Psi(m), is associated with altered mitochondrial f unction, we also determined if exposure to methylmercury potentiated r eactive oxygen species (ROS) generation. We noted that treated cells g enerated ROS, as evidenced by oxidation of hydroethidine and the gener ation of the fluorescent product, ethidium. Finally, we evaluated the effect of methylmercury on T-cell GSH content utilizing the fluorescen t probe monochlorobimane; in the presence of MeHgCl, there is a marked loss in reduced cell thiols. The results of the study indicate that a key event in the induction of T-cell apoptosis by mercuric compounds is depletion in the thiol reserve which predisposes cells to ROS damag e and at the same time activates death signaling pathways. (C) 1998 Ac ademic Press.