In vitro effect of liposome-incorporated valinomycin on growth and macromolecular synthesis of normal and ras-transformed 3T3 cells.
Valinomycin is a depsipeptide antibiotic that selectively translocates potassium ion across biologic membranes. This drug has been reported to display antitumor effects, but its use has been limited by its extreme toxicity. However, its incorporation into lipid vesicles (liposomes) has resulted in a reduction in toxicity and in the enhancement of the drug's therapeutic index. As a preliminary investigation of the mechanistic basis for this enhancement, the in vitro response of normal 3T3 and ras-transformed cells to free (VM) and liposomal valinomycin (VM-MLV) was examined. The incorporation of [3H]-leucine and [methyl-3H]-thymidine was used to assess macromolecular synthesis, and the MTT vital dye assay was used to measure cell survival and growth. Pretreatment of exponentially growing NIH/3T3 cells with 20 nM VM for 1 h decreased [3H]-leucine and [methyl-3H]-thymidine incorporation by 90% and 80%, respectively. However, Ha-ras 3T3 cells showed resistance to VM treatment with inhibitory doses in the range of 200 nM. At equimolar VM concentrations, VM-MLV was found to be less inhibitory than VM for protein and DNA synthesis. Specifically, marked protective activity was apparent with normal 3T3 cells. In this report we also demonstrate that VM selectively killed normal cells compared with ras-transformed cells grown in vitro. However, VM-MLV displayed a modest cytotoxic selectivity (3- to 4-fold) to ras-transformed cells. Our data suggests that first, there is good correlation between growth inhibition and inhibition of DNA and protein synthesis by VM, and second, VM-MLV exhibits a modest, selective toxicity to the ras-transformed 3T3 cell line as compared with nontransformed 3T3 cells, whereas free VM has the opposite selectivity.