We have previously reported the isolation of a human breast cancer cell line resistant to doxorubicin (adriamycin; AdrR MCF-7 cells) that has also developed the phenotype of multidrug resistance (MDR). MDR in this cell line is associated with increased expression of mdr (P glycoprotein) gene sequences. The development ofMDR in AdrR MCF-7 cells is also associated with changes in the expression of several phase I and phase II drug-detoxifying enzymes. These changes are remarkably similar to those associated with development of xenobiotic resistance in rat hyperplastic liver nodules, a wellstudied model system of chemical carcinogenesis. Using an mdr-encoded cDNA sequence isolated from AdrR MCF-7 cells, we have examined the expression of mdr sequences in rat livers under a variety of experimental conditions. The expression of mdr increased 3-fold in regenerating liver. It was also elevated (3-to 12-fold) in several different samples of rat hyperplastic nodules and in four of five hepatomas that developed in this system. This suggests that overexpression of mdr, a gene previously associated with resistance to antineoplastic agents, may also be involved in the development of resistance to xenobiotics in rat hyperplastic nodules. In addition, although the acute administration of 2-acetylaminofluorene induced an 8-fold increase in hepatic mdr-encoded RNA, performance of a partial hepatectomy either before or after administration of 2-acetylaminofluorene resulted in a >80-fold increase in mdr gene expression over that in normal untreated livers. This represents an important in vivo model system in which to study the acute regulation of this drug resistance gene.

The adsorption (minor coat) protein of the bacteriophage fd has been implicated to function in several steps of viral morphogenesis. The protein has been purified by sodium dodecyl sulfate gel filtration after dissociation of the virus. The adsorption protein preparation was estimated to have less than 5% contamination by analysis on sodium dodecyl sulfate-polyacrylamide gels and by the results of semiquantitative dansyl-Edman degradation. The amino-terminal sequence of the adsorption protein is H2N-Ala-Glx-Thr-Val-Glx-Ser-Pro-Leu-Pro-. Carboxypeptidase A plus B digestion of the protein under a variety of denaturing conditions did not release any amino acids. There are 3-4 adsorption proteins per virion as estimated by the distribution of E114C]leucine between the major and minor coat protein peaks on sodium dodecyl sulfate-polyacrylamide gels. Adsorption protein-specific antibodies were induced in the rabbit and used as electronmicroscopic markers to determine the position of the adsorption proteins in the viral particle. The adsorption proteins were found at only one end of the filamentous viral particles.

The development of multdrug iresisance in MCF7 human breast cancer cells is acted with ovexpression of P-glycoprotein, changes in activities of several deo don enzymes, and loss of hornmoe senivity and etg receptors (ERs). We In vitro exposure of cancer cells to certain cytotoxins not only leads to resistance of the cells to the selecting agent but can also lead to broad crossresistance to structurally dissimilar cytotoxins (1-7). This phenomenon of multidrug resistance has been associated with decreased intracellular drug accumulation (2-4) and with overexpression of a 130-to 180-kDa membrane-associated glycoprotein (P-glycoprotein) (3,(5)(6)(7) (14,(23)(24)(25). Among these phase II enzymes is the anionic isozyme of glutathione S-transferase (GST; RX:glutathione R-transferase, EC 2.5.1.18), an isozyme found in placenta and usually present only in low levels in human liver. Another phenotypic change associated with the development of multidrug resistance in MCF7 cells is the loss of hormonal sensitivity and the development of resstae to antiestrogens (26). This transformation from estrogenresponsive, wild-type (WI) MCF7 to estrogn-unresponsive Adr' MCF7 is associated with the loss of estren receptors (ERs).Since the increase in anionic GST is one of the marked changes in both AdrR MCF7 cells and rat hyperplastic nodules, we have cloned the cDNA encoding GSThr, the human anionic GST, and used it as a probe to examine the expression of this enzyme in human tumors. In this study we sought to determine whether the regulation of this drug-metabolizing enzyme was related to hormone-receptor regulation.MATERALS AND METHODS Cell Cuture. WT MCF7 and AdrR MCF7 cells were grown in improved minimum essential medium (IMEM) containing 5% (vol/vol) fetal bovine serum (GIBCO) as described (20). AdrR MCF7 cells were grown in medium containing 10 jm doxorubicin. When used in specific studies, Adr' MCF7 cells were grown in drug-free medium for 2-4 weeks.Abbreviations: WT MCF7, wild-type, drug-sensitive MCF7 human breast cancer cell line; AdrR MCF7, multidrug-resistant MCF7 cells initially selected for resistance to doxorubicin (former generic name, adriamycin); GST, glutathione S-transferase; GSTvr, human anionic isozyme ofGST; ER, estrogen receptor; PR, progesterone receptor.

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