PROLONGATION OF HUMAN NEUTROPHIL SURVIVAL BY LOW-LEVEL MERCURY VIA INHIBITION OF SPONTANEOUS APOPTOSIS

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Abstract
Low levels of organic and inorganic mercury compounds have been reported previously to induce cell death by apoptosis in human peripheral blood mononuclear cells (MNC), but little is known about their potential effects on the viability and death of polymorphonuclear neutrophils (PMN). In contrast to MNC, PMN are known to undergo readily spontaneous apoptosis both in vivo and in vitro. Therefore, it was hypothesized that PMN may differ from MNC in their reactions to low mercury levels. The effects of methylmercuric chloride (MeHgCl) and mercuric chloride (HgCl 2 ) were evaluated in concentration- response and time-course studies on human PMN viability and on their modes of cell death after in vitro incubation at 37°C. Cell death by apoptosis or necrosis was assessed by annexin V-fluorescein isothiocyanate binding to externalized phosphatidylserine in conjunction with propidium iodide, and flow cytometry analysis. Morphologic counting of pyknotic nuclei and the fluorescence properties of the DNA-binding dye Hoechst 33342 in combination with propidium iodide were used to further confirm apoptotic cell death and to characterize the sequence of Hg-induced cell death. Results show that low concentrations of MeHgCl (1-7.5 µ M ) that were cytotoxic to MNC actually inhibited PMN spontaneous apoptosis. Low-level HgCl 2 reproduced the anti-apoptotic effects of MeHgCl on PMN, but to a lower extent. Higher concentrations of MeHgCl and HgCl 2 were necro-genic to PMN, but MeHgCl was about an order of magnitude more toxic, and discrete differences were observed in the modalities of cell death induced by both species. These data reveal for the first time that (1) low levels of organic and inorganic mercury species protect human PMN from cell death via inhibition of spontaneous apoptosis, and (2) PMN are more resistant than MNC to mercury-induced cytotoxicity. Since delayed apoptosis and increased resistance to toxicant-induced cell death may lead to excessive accumulation of senescent PMN, evidence indicates that findings of this study may have implications for mercury-induced autoimmunity and inflammation.