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Characterization of Estrogen-binding Proteins in Variants of a Rat Mammary Tumor¹

Grace Cancer Drug Center, Roswell Park Memorial Institute, Buffalo, New York 14263

Estrogen-binding proteins have been characterized in variants of the MTW-9B rat mammary tumor in an attempt to determine the functional significance of the low affinity cytosolic estrogen binder. In vivo selection of tumor variants was carried out by transplant of the tumor into intact and castrated male and female Wistar-Furth rats for four or five successive transplant generations. Tumors that developed in each of the four lines were taken for retransplant into intact and castrated male and female rats and all recipient tumor groups were compared for high and low affinity estrogen-binding proteins using isoelectric focusing analysis. No alterations in the isoelectric focusing profiles were observed in tumors that developed after repeated passage in ovariectomized female or castrated male rats when compared to the profile of the estrogen-binding proteins of the parent tumor carried routinely in intact female rats. However, a tumor variant containing only the low affinity, more basic estrogen-binding protein resulted from repeated passage of the MTW-9B mammary tumor in male rats. The high affinity estrogen receptor was absent in this variant and could not be induced by retransplant of the tumor into intact female hosts. Growth of the parent tumor and each of the variants was shown to be ovarian independent, suggesting that the presence of the low affinity estrogen-binding protein is not predictive of estrogen responsiveness. This suggestion is further supported by the observation that estrogen-stimulated progesterone receptor synthesis could be demonstrated in the parent tumor which demonstrated both binders, but not in the tumor variant developed in male rats which contained only the low affinity estrogen-binding protein. Progesterone receptor synthesis in this latter tumor appeared to be constitutive. Studies are continuing in an attempt to identify a role for the cytosolic low affinity estrogen-binding protein.

¹ This work was supported by American Cancer Society Grant PDT-76 and by USPHS Grant CA-22761 and core Grant CA-24538 awarded by the National Cancer Institute, Department of Health and Human Services. A preliminary report of this work was presented at the 67th annual meeting of the Endocrine Society, Baltimore, MD, 1985.

² Parts of this manuscript are from a thesis submitted to the Faculty of the Roswell Park Division of the Graduate School of the State University of New York at Buffalo, in partial fulfillment of the requirements of the PhD degree. Present address: Fels Research Institute, Temple University School of Medicine, 3420 North Broad Street, Philadelphia, PA 19140.

³ To whom requests for reprints should be addressed, at Grace Cancer Drug Center, Roswell Park Memorial Institute, 666 Elm Street, Buffalo, NY 14263.

Received 1/20/87. Revised 5/12/87. Accepted 5/20/87.