A multidrug-resistant MCF-7 human breast cancer cell line which exhibits cross-resistance to antiestrogens and hormone-independent tumor growth in vivo

MCF-7 human breast cancer cells provide a useful in vitro model system to study hormone-responsive breast cancer as they contain receptors for estrogen and progesterone, and estrogen both induces the synthesis of specific proteins in these cells and increases their rate of proliferation. An MCF-7 cell line which was selected for resistance to adriamycin (MCF-7/Adr^R) exhibits the phenotype of multidrug resistance (MDR), and displays multiple biochemical changes. MDR in MCF-7/Adr^R is also associated with a loss of mitogenic response to estrogen and the development of cross-resistance to the antiestrogen 4-hydroxytamoxifen. In addition, while the parental MCF-7 cell line responds to estrogen with increased levels of progesterone receptors and the secretion of specific proteins, these estrogen responses are lost in MCF-7/Adr^R. Furthermore, while the formation of tumors in nude mice by wild-type MCF-7 cells is dependent upon the presence of estrogen, MCF-7/Adr^R cells form tumors in the absence of exogenous estrogen administration. These changes in hormonal sensitivity and estrogen-independent tumorigenicity of the multidrug-resistant MCF-7 cell line are associated with a loss of the estrogen receptor and a concomitant increase in the level of receptors for epidermal growth factor. Thus, in MCF-7/Adr^R cells, the development of MDR is associated with alterations in the expression of both cytosolic and membrane receptors, resulting in resistance to hormonal agents and the expression of hormone-independent tumor formation.