RESISTANCE TO TNF-ALPHA AND ADRIAMYCIN IN THE HUMAN BREAST-CANCER MCF-7 CELL-LINE - RELATIONSHIP TO MDR1, MNSOD, AND TNF GENE-EXPRESSION

The molecular basis of cross-resistance to tumor necrosis factor (TNF) and Adriamycin has been investigated using the breast adenocarcinoma cell line MCF7/p, its Adriamycin-resistant counterpart MCF7Adr(R), and the MDR1 gene-transduced MCF-7 cells (MCF7/MDR1). While the parental cell line MCF7/p was TNF-sensitive, MCF7Adr(R) was TNF-resistant. The TNF resistance exhibited by MCF7Adr(R) cells was not due to a lack of TNF receptor expression because both cell lines express comparable lev els of p75 and p55 receptors as revealed by immunofluorescence analysi s. NF-kappaB translocation, which is an essential transducing signal o f the TNF-induced lysis pathway, does not appear to be involved in thi s resistance as assessed by gel shift experiments. In order to determi ne the role of MDR1 gene expression in the development of this cross-r esistance, MCF7/p cells transfected by the MDR1 gene were examined. Ou r data showed that the expression of the MDR1 gene in these cells resu lted in a relative resistance of these cells to Adriamycin without aff ecting their susceptibility to TNF killing. The implication of the man ganese superoxide dismutase and endogenous TNF expression in the cross -resistance by MCF7Adr(R) cells to Adriamycin and TNF has also been in vestigated. Northern blot analysis indicated that following TNF stimul ation, the expression of 4-kilobase and 1-kilobase manganese superoxid e dismutase mRNAs were 9- to 10-fold induced in MCF7Adr(R) cells as co mpared to MCF7/p and MCF7/MDR1 cells. This suggests a possible involve ment of this enzyme in the Adriamycin-induced resistance to TNF. Altho ugh TNF-treatment of MCF7/p and MDR-cells induced endogenous TNF expre ssion in these cells. the level of mRNA induction was selectively enha nced in MCF7Adr(R) cells (7- to 8-fold greater in MCF7Adr(R) cells as compared to MCF7/p and MCF7/MDR1 cells). Collectively, these data indi cate that the expression of the MDR1 gene in MCF7/p cells following ge ne transfection is not sufficient for the acquisition of TNF resistanc e by MCF7/MDR1 cells. Furthermore, our data provide the first evidence that Adriamycin-induced resistance to TNF in MCF7Adr(R) cells may, in part, involve an overexpression of endogenous TNF and manganese super oxide dismutase genes.