BIOCHEMICAL AND PHARMACOLOGICAL CHARACTERIZATION OF MCF-7 DRUG-SENSITIVE AND ADRR MULTIDRUG-RESISTANT HUMAN BREAST-TUMOR XENOGRAFTS IN ATHYMIC NUDE-MICE

The phenotypic expression of multidrug resistance by the doxorubicin-selected Adr(R) human breast tumor cell line is associated with overexpression of plasma membrane P-170 glycoprotein and increased cytosolic selenium-dependent GSH-peroxidase activity relative to the parental MCF-7 wild-type line (WT). To determine whether doxorubicin resistance by Adr(R) cells persists in vivo, and to further investigate the possibility of biochemical differences between WT and Adr(R) solid tumors, both tumor cell lines were grown as subcutaneous xenografts in athymic nude mice. Tumorigenicity depended upon cell inoculation burden, and tumor incidence was similar for both cell lines (> 80% tumor takes at 10(7) cells/mouse) at 14 days, provided 17-beta-estradiol was supplied to the animals bearing the WT tumors. However, the growth rate for the Adr(R) xenografts was only about half that of WT xenografts. Doxorubicin (2-8 mg/kg, i.p., injected weekly) significantly diminished the growth of the WT tumors, but Adr(R) solid tumors failed to respond to doxorubicin. The accumulation of C-14-labeled doxorubicin was 2-fold greater in WT xenografts that in Adr(R), although there were no differences in host organ drug levels in mice bearing either type of tumors. Membrane P-170 glycoprotein mRNA was detected by slot-blot analysis in the Adr(R) tumors, but not in WT. Electron spin resonance 5,5-dimethylpyrroline-N-oxide-spin-trapping experiments with microsomes and mitochondria from WT and Adr(R) xenographs demonstrated a 2-fold greater oxygen radical (superoxide and hydroxyl) formation from activated doxorubicin with WT xenographs compared to Adr(R). Selenium-dependent glutathione (GSH)-peroxidase, superoxide dismutase and GSH-S-aryltransferase activities in Adr(R) xenografts were elevated relative to WT. Although the activities of the latter two enzymes were similar to those measured in both tumor cell lines, GSH-peroxidase activities were elevated 70-fold (WT) and 10-fold (Adr(R)) in xenografts compared to tumor cells. In contrast, in both WT and Adr(R) solid tumors in vivo, catalase, NAD(P)H-oxidoreductases, and glutathione disulfide (GSSG)-reductase activities, and GSH and GSSG levels were not markedly different, and were essentially the same as in cells in vitro. Like the MDR cells in culture, Adr(R) tumor xenografts were extremely resistant to doxorubicin and retained most of the characteristics of the altered phenotype. These results suggest that WT and Adr(R) breast tumor xenografts provide a useful model for the study of biochemical and pharmacological mechanisms of drug resistance by solid tumors in vivo.