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An Estrogen-Independent MCF-7 Breast Cancer Cell Line Which Contains a Novel 80-Kilodalton Estrogen Receptor-related Protein¹

Department of Human Oncology, University of Wisconsin Comprehensive Cancer Center, Madison, Wisconsin 53792 [J. J. P., S. Y. J., M. F., V. C. J.], and Robert H. Lurie Cancer Center, Northwestern University Medical School, Chicago, Illinois 60611 [V. C. J.]

Long-term growth of estrogen-responsive human breast cancer cell lines in estrogen-free media leads inevitably to the development of estrogen-independent growth. We have identified and characterized a unique subclone of the MCF-7 human breast cancer cell line, named MCF-7:2A, which grows maximally in the absence of endogenous estrogens but whose growth is inhibited by the antiestrogens 4-hydroxytamoxifen and ICI 164,384. The MCF-7:2A cells express high levels of estrogen receptor (ER; 477 fmol/mg protein), which can be reduced by growth in 10 nM 17ß-estradiol (201 fmol/mg protein). Basal progesterone receptor synthesis is very low in the 2A cells (<1 fmol/mg protein) but can be dramatically increased by 10 nM 17ß-estradiol (384 fmol/mg protein). Clearly, the pathways that control growth and estrogen-regulated genes such as the progesterone receptor are now dissociated in these cells. MCF-7:2A cells also possess two unique characteristics. First, the MCF-7:2A cells constitutively activate an ER-responsive luciferase reporter construct in the absence of any estrogens, and this activation can be blocked by either 4-hydroxytamoxifen or ICI 164,384. This constitutive activity is not observed in the parental MCF-7 cells. Second, they express an 80-kDa protein that cross-reacts with three distinct antibodies to the ER.

The MCF-7:2A cells were subjected to an additional round of limiting dilution subcloning, and 10 independent clones were all shown to express both the 66- and 80-kDa ERs as observed in the MCF-7:2A line. This confirms that both ERs are being expressed in each cell and are not the result of a mixed population of cells. While numerous ER variants have been reported previously, no ER has until now been described that is larger than the wild-type 66-kDa ER. The MCF-7:2A cells provide a unique model to use in the study of ER action and the development of estrogen-independent growth in human breast cancer cells.

¹ This work was supported by NIH Grant CA32713. J. J. P. was supported in part by NIH Training Grant 5T32-CA0947. S. Y. J. was supported by a scholarship from the National Science Council and National Defense Medical Center (Taiwan, Republic of China).

² To whom correspondence should be addressed, at Robert H. Lurie Cancer Center, Northwestern University Medical School, 303 East Chicago Avenue, Olsen Pavilion 8258, Chicago, IL 60611.

Received 1/13/95. Accepted 4/18/95.

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