In vitro suppression of thymocyte apoptosis by metal-rich complex environmental mixtures: potential role of zinc and cadmium excess

Excessive amounts of heavy metals Že.g. Zn, Cu, Mn, Cr. are accumulated in river bottom sediments ŽRBS., being available to humans and animals along food chains. Increased exposure of mammals to certain metals ŽCr, Cu. induces immunosuppresion, due to DNA damage and decreased survival of lymphoid cells. By contrast, excess of Zn and Cd causes inhibition of apoptosis thus suggesting increased survival of genetically mutated cells and higher cancer risks in exposed populations. Rat thymic lymphocytes represent a well-established model for apoptosis testing. The primary goal of our study was to assess the degree of apoptosis modulation with a number of RBS extracts differing in their metal contents. A series of freshly deposited RBS was collected at nine sampling stations along the Elbe River. All sediments were rich in Fe, Mn and Zn. The contents of Cu, Cr, Ni, Cd, Hg, Pb and As were much lower and interrelated. The short-term cytotoxicity of aqueous sediment extracts was assessed, using the following criteria: total cell counts; incidence of apoptosis and necrosis Žmorphological detection by fluorescent microscopy.; and nuclear chromatin decay Žby DNA flow cytometry.. RBS extracts produced both apoptosis and necrosis of thymocytes. High contents of zinc and other heavy metals in the samples correlated with decreased thymocyte apoptosis Žr 0.543 to 0.608, P 0.01.. The rates of thymocyte damage showed a distinct dependence on the time and region of sampling. Apoptosis modulation was also tested with pure salts of MnŽII., ZnŽII., CuŽII., CrŽIII. and CdŽII., at the test concentrations of 1, 10 and 100 M. CuŽII. and CrŽIII. proved to induce marked dose-related apoptosis, whereas ZnŽII. ions caused significant suppression of apoptosis. These effects were similar to those trends observed with metal-rich sediments. In the present study, DNA flow cytometry proved to be a less sensitive index of cell death than morphological assay of apoptosis and or necrosis. In summary, inhibition of lymphocyte apoptosis by RBS extracts and pure metals is associated with excess of zinc and, probably, cadmium. The proposed model of lymphoid cell apoptosis is a promising tool for screening cytotoxic effects of complex environmental samples