Increased resistance to chlorambucil in cultured cells with a high concentration of cytoplasmic metallothionein.

Cultured cells with elevated levels of cytoplasmic metallothionein (MT), a small-molecular-weight protein extremely rich in cysteine, showed resistance against the alkylating drug chlorambucil. One human epithelial line (HE) and one mouse fibroblast line (Cl 1D) were used, as well as substrains of both cell lines (HE100 and Cl 1D100). The substrains contain large amounts of MT. Dose-survival curves obtained from cloning studies with the murine cells after exposure to chlorambucil for 1 hr revealed that Cl 1D100 was approximately 3-fold more resistant than were Cl 1D cells (D0 = 32.9 and 11.6 micrograms/ml, respectively); this difference was highly significant (p less than 0.001; t test). In growth rate studies, both MT-containing strains showed resistance against chlorambucil over the total dose range of 0.5 to 100 micrograms/ml compared to the parent strains. After 4 days treatment with 25 micrograms/ml, the growth inhibition of Cl 1D was 67%, but only 13% in Cl 1D100. For HE and HE100, 50 micrograms/ml gave growth inhibitions of 83 and 65%, respectively. Following 1- and 24-hr incubations of cells with [14C]chlorambucil and subsequent ultracentrifugation and gel filtration of the cytosols, about 20 to 40% of the 14C-derived radioactivity coeluted with the MT from the resistant strains. The results indicate that intracellular MT sequesters chlorambucil or its toxic metabolites and that the protein thus contributes to the resistance against this drug.