Role of cytosolic copper, metallothionein and glutathione in copper toxicity in rat hepatoma tissue culture cells.

Effects of metallothionein (MT) synthesis inhibiting compounds (actinomycin D, cycloheximide), MT synthesis stimulating compounds (dexamethasone, dibu-cAMP) and interfering metals (Cd, Zn) on copper accumulation were investigated in rat hepatoma tissue culture cells. Copper-metallothionein (Cu-MT) and MT-associated copper levels were determined to find a possible correlation between cytosolic copper concentrations and MT as a Cu-detoxifying protein. Further, intracellular non-MT associated copper levels and levels of GSH and SOD were determined. Cell viability was tested under all experimental conditions by measuring LDH-release, K+ uptake and total cell protein. Administration of dexamethasone and dibu-cAMP showed no effect on MT levels (compared with controls), and only a marginal effect on 64Cu and total Cu accumulation. Administration of actinomycin D resulted in increased copper accumulation in the particulate fraction, possibly due to inhibition of copper secretion processes and/or protein synthesis. Presence of zinc had no effect on MT levels nor on total Cu and 64Cu levels, in contrast with cadmium which drastically enhanced copper accumulation and MT levels in the cells. Cu/MT ratios varied from 1.0 +/- 0.3 to 3.3 +/- 1.2, which is far below the assumed maximum molar ratio of 8-12 mol Cu per mol MT. SOD levels appeared to be enhanced up to 2- or 3-fold in the presence of Cd2+, relative to control values. The role of GSH as Cu-intermediate in intracellular Cu distribution plus its role in copper defence mechanism(s) was tested by application of BSO, an inhibitor of GSH synthesis. It was found that BSO had no effect on intracellular MT level; it was found however that MT-bound copper levels were markedly decreased. The results presented support a model for copper metabolism in hepatoma tissue culture (HTC) cells, where Cu(I) is complexed by GSH immediately after entering the cell. GSH is capable of transferring copper to MT where it is stored. Depletion of GSH (by administration of Cd2+, actinomycin D, cycloheximide) almost instantaneously results in enhanced cellular toxicity. When also MT is depleted (by actinomycin D) non-MT associated, 'free' cytosolic Cu2+ is elevated, and HTC cells rapidly loose their resistance to copper toxicity, as also reflected in loss of cell viability (LDH, K+ and total cell protein).