Human prostate carcinoma cell line DU-145 was used to examine the relationship between the intracellular levels of cysteine-rich metallothionein (MT) and the sensitivity or resistance of cells to Adriamycin (ADR). The basis for the poor response of human prostate carcinomas to ADR was studied. Cadmium-resistant (Cdr) cells, capable of growth in 10-5 m cadmium, were derived from DU-145 cadmium-sensitive (Cds) cells, by exposure to increasing concentrations of cadmium. The relative rates of MT synthesis were measured by l-[35S]cysteine incorporation and MT separation by high-performance liquid chromatography. Cdr cells, continuously exposed to cadmium, show a steady-state rate of MT synthesis (designated as control = 100%) which is 3.5 times the basal rate in Cds cells (29%). Dose-response curves, using clonal and cell count assays, show that the dose levels required to produce inhibition of growth to 50% and 90% of control, respectively, of ADR for Cdr cells (19.00 and 132.0 ng/ml) are 1.5 to 1.7 times those for Cds cells (12.5 and 77.5 ng/ml). In the absence of cadmium, deinduction of MT occurs with MT synthesis declining, after 70 and 118 h, to 29% and 19% of control. Correspondingly, in such deinduced cells (Cdr minus cadmium), the 50% inhibitory doses of ADR in clonal and growth assays are 3.5 and 4.8 ng/ml, respectively. Thus, deinduced cells are 3 and 4 times more sensitive to ADR than Cds and Cdr cells. This increased sensitivity is explained by the rapid and marked inhibition of MT synthesis upon exposure to ADR, even in the presence of cadmium, so that after 6 and 10 h in the presence of 10 ng/ml of ADR, the rates drop to 62% and 19% of control. On the basis of these results, we propose that: (a) the increased levels of MT increase the resistance of Cdr cells to ADR and that this may be partly responsible for the poor response of prostate carcinomas to ADR; (b) MT deinduction results in increased sensitivity to ADR; and (c) ADR inhibits MT synthesis. Thus, it is suggested that a treatment regimen consisting of ADR exposure followed by a second exposure, during increased ADR sensitivity, may be effective for growth inhibition of slow-growing prostatic carcinomas.
Supported in part by USPHS Grants CA-33169 and CA-34308 from the National Cancer Institute, Department of Health and Human Services, and by a grant from the Center for Alternatives to Animal Testing, The Johns Hopkins School of Hygiene and Public Health.