Induction of metallothionein synthesis in human peripheral blood leukocytes

Metallothionein (MT), a low molecular weight, metal-binding protein, has recently been shown to protect murine mononuclear phagocytic cells from the cytotoxic effects of bacterial lipopolysaccharides (LPS), the endotoxic component of Enterobacteriaceae. MT appears to function intracellularly as an antioxidant since autolysis results from lipid peroxidation initiated by free radicals of O 2. Since this activity is distinct from MT's capacity to specifically sequestrate heavy metals, we examined whether MT synthesis can be induced by direct membrane activation or through interaction with soluble leukocyte mediators. Normal human monocytes, polymorphonuclear neutrophils (PMN), and lymphocytes, isolated from heparinized whole blood, were incubated with and without LPS from Escherichia coli and Salmonella typhosa. MT in cell lysates was quantitated using a 203Hg-binding assay employing Sephadex G-10 “minicolumns.” When incubated with monocytes, PMN, or lymphocytes, neither preparation of LPS (10–100 μg/ml) was capable of enhancing 203Hg-binding activity after 24 or 72 hr incubation. CdCl 2 (2 μg/ml), however, increased binding activity in monocyte and lymphocyte cultures 4- and 15-fold, respectively. When monocytes and lymphocytes were cocultured with LPS, 203Hg-binding activity was not enhanced. Addition of human interleukin 1 (endogenous pyrogen) to these cultures had no significant effect. Leukocyte endogenous mediator (LEM), a product of LPS-activated PMN that possesses hypozincemic activity in vivo, did not induce MT synthesis. Collectively, these results demonstrate that leukocyte MT does not arise from direct LPS activation or from interaction with products secreted by LPS-activated cells. De novo synthesis of MT observed during endotoxemia and gram negative sepsis appears, therefore, to be induced by endogenously released corticosteroids.