Sensitization of immunoresistant prostate carcinoma cell lines to Fas/Fas ligand-mediated killing by cytotoxic lymphocytes: Independence of de novo protein synthesis

BACKGROUND. We recently reported that drug-resistant prostate tumor cells ( DU145, PC3) are resistant to Fas-mediated killing by cytotoxic lymphocytes, and that this resistance can be overcome by treatment with subtoxic concen trations of chemotherapeutic drugs. Fas belongs to the tumor necrosis facto r (TNF) family of receptors. Since resistance to TNF-alpha-mediated killing has been shown to be due, in part, to the presence of protective factors a nd that inhibitors of protein synthesis can sensitize cells to TNF-alpha ki lling, we hypothesized that resistance to Fas-mediated killing may be due t o similar mechanisms. Since sensitization is achieved with chemotherapeutic drugs, and some chemotherapeutic drugs can also inhibit protein synthesis, we tested whether sensitization of prostate tumor cells to Fas ligand (Fas -L) occurred through inhibition of protein synthesis in a manner analogous to that of TNF-alpha. METHODS. The effect of chemotherapeutic drugs on protein synthesis in DU145 and PC-3 cells was characterized by H-3-leucine incorporation assays. We a lso determined the ability of inhibitors of protein synthesis and chemother apeutic drugs to sensitize Fas and TNF-resistant DU145 cells to killing. Th e ability of RNA (actinomycin-D, Act-D) and protein synthesis inhibitors (c yclohexamide (CHX), emetine) to block drug-mediated sensitization to Fas-L killing was analyzed. Sensitivity to Fas-L killing was determined by the Cr -51-release assay using human lymphokine-activated killer cells (LAK) and t umor-infiltrating lymphocyte (TIL) effector cells and the murine Fas-L-expr essing PMMI cells. RESULTS. The drugs cis-diamminedichloroplatinum (II) (CDDP), adriamycin (AD R), and etoposide (VP-16) sensitized DU145 and PC-3 cells to Fas killing. C DDP and ADR, which sensitized DU145 and PC-3 cells to Fas-L- and TNF-mediat ed killing, inhibited de novo protein synthesis in both cell lines, while V P-16 only inhibited protein synthesis in DU145 cells. Further, neither CHX nor emetine sensitized DU145 or PC-3 cells to Fas-L-mediated killing, despi te blocking >90% de novo protein synthesis. In contrast, CDDP, VP-16, and t he protein synthesis inhibitors, Act-D and CHX sensitized DU145 cells to TN F-alpha killing. Finally, pretreating cells with protein synthesis inhibito rs (CHX, emetine) did not abrogate drug-mediated sensitization to Pas-media ted killing. CONCLUSIONS. These findings demonstrate that downregulation of protective f actors by protein synthesis inhibition may not be the primary mechanism of drug-mediated sensitization to Fas-L killing in prostate cell lines. These findings also suggest that drug-mediated sensitization to Fas-L killing may be due to modifications of preexisting gene products that participate in F as-L-mediated apoptosis. Prostate 41:20-30, 1999. (C) 1999 Wiley-Liss, Inc.