Sensitization of tumor target cells to cytolysis by murine macrophage cytolytic factor by drugs inhibiting DNA, RNA, and protein synthesis.

ABSTRACT

The mechanism(s) with which tumor target cells actively resist the lethal lesion induced by murine macrophage cytolytic factory (CF) has been probed by drug-induced sensitization of these cells. We have examined whether drug-induced sensitization is attributable to the action of these drugs on cellular DNA, RNA, or protein synthetic rates. The murine mammary adenocarcinoma cell line EMT-6 pretreated with a dose range of actinomycin-D (.03-3.0 micrograms/ml) for 4 hr was inhibited from 66 to 98% in DNA synthesis rate, from 81 to 93% in RNA synthesis rate, and from 38 to 52% in protein synthesis rate. The maximum sensitization toward lysis by CF was achieved with a drug dose of 1.0 micrograms/ml. The lack of correlation between the drug-induced effects on sensitization and effects on these metabolic rates was evident from the correlation coefficients for the percentage of maximum sensitization of the target toward CF-induced lysis, and the percent of inhibition of DNA, RNA, and protein synthesis. These were 0.11, -0.11, -0.44, respectively. Similarly, target cells treated with a dose range of cycloheximide (0.3-30 micrograms/ml) were inhibited from 81 to 95% in DNA synthesis rate, 68 to 81% in RNA synthesis rate, and 82 to 93% in protein synthesis rate. However, at none of the drug levels employed was significant sensitization of the target cell to CF-induced lysis observed. This was reflected in the correlation coefficients of -0.55, -0.28, and -0.34 for DNA, RNA, and protein synthetic rates, respectively. The essential role of cellular microtubule-dependent events early in the lytic mechanism was demonstrated by exposing colchicone-treated targets to CF. While colchicone could markedly inhibit lysis if introduced before CF, this inhibitory effect was not detected if the drug were withheld for 4 hr after CF exposure. The importance of the repair mechanism(s) early in the cellular response to lesion formation was demonstrated by altering the schedule for CF and actinomycin D administration to the target. If actinomycin D treatment was withheld from the targets for as little as 3 hr following introduction of CF, the lytic mechanism had already bypassed the drug-sensitive steps, manifest in markedly decreased susceptibility to lysis. Collectively, the data show that the ability of the EMT-6 target cells to resist CF-induced lysis does not depend solely on the ability of the cells to synthesize DNA, RNA, or protein.