OVERCOMING TNF-ALPHA AND DRUG-RESISTANCE OF HUMAN RENAL-CELL CARCINOMA-CELLS BY TREATMENT WITH PENTOXIFYLLINE IN COMBINATION WITH TNF-ALPHAOR DRUGS - THE ROLE OF TNF-ALPHA MESSENGER-RNA DOWN-REGULATION IN TUMOR-CELL SENSITIZATION

Previous studies have demonstrated that one of the possible mechanisms responsible for the resistance of tumor cells to tumor necrosis facto r-alpha (TNF-alpha) is the expression of TNF-alpha mRNA and/or protein . Pentoxifylline (PTX) suppressed TNF-alpha gene transcription and dow nregulates the expression of TNF-alpha mRNA and the secretion of TNF-a lpha protein in macrophages and monocytes. This study investigates whe ther PTX downregulates the expression of TNF-alpha mRNA and/or protein in renal cell carcinoma (RCC) cells and whether PTX enhances the sens itivity of TNF-alpha-resistant RCC cells to TNF-alpha. Further, we exp lored whether PTX enhances the sensitivity of RCC cells to agents othe r than TNF-alpha by downregulation of the expression of TNF-alpha mRNA and protein. The R4 human RCC cell line constitutively expressed TNF- alpha mRNA and protein and was resistant to TNF-alpha. When R4 cells w ere incubated with PTX, the level of TNF-alpha mRNA and protein was ma rkedly reduced. Pentoxifylline and TNF-alpha together overcame the res istance of R4 cells to TNF-alpha. The R11 human RCC cell line did not constitutively express TNF-alpha mRNA or protein, and was resistant to TNF-alpha. The expression of TNF-alpha mRNA in R11 cells, but not the production of TNF-alpha protein, was induced by TNF-alpha. When PTX w as used in combination with TNF-alpha, the level of TNF-alpha mRNA ind uced by TNF-alpha was markedly reduced. The combination of PTX and TNF -alpha overcame the resistance of R11 cells to TNF-alpha. Pentoxifylli ne also enhanced the sensitivity of R4 cells to interferon-alpha. Pent oxifylline and anti-TNF-alpha monoclonal antibody augmented the sensit ivity of R4 cells to cis-diamminedichloroplatinum (II) (CDDP). This st udy demonstrated that PTX, in combination with TNF-alpha, IFN-alpha or CDDP, overcame the drug resistance to RCC cells and that downregulati on of TNF-alpha mRNA by PTX may be related to the cytotoxicity enhance d by the combination. The implications of these findings for clinical therapy are discussed.