Kathryn B. Horwitz

MCF-7: A human breast cancer cell line with estrogen, androgen, progesterone, and glucocorticoid receptors

We have identified receptors for glucocorticoids, progestins, and androgens in a human breast tumor cell line (MCF-7) known to have estrogen receptor. Sucrose density gradients show that MCF-7 cytosol contains approximately 100 fm/mg protein estradiol (E2-3H) receptor, more than 300 fm/mg protein progesterone receptor (measured with R5020-3H), about 40 fm/mg protein 5alpha-dihydrotestosterone (5alpha-DHT-3H) receptor, and 800 fm/mg glucocorticoid receptor (measured with dexamethasone-3H). Dissociation constants obtained by Scatchard analyses were approximately 0.6 x 10(-10)M (E2), 1 x 10(-9)M (R5020), 2.8 x 10(-10)M (5alpha-DHT) and 8 x 10(-9)M (dexamethasone). No cross competition was found for estrogen receptor, but progestins competed for androgen and glucocorticoid binding. The androgen, but not the glucocorticoid, partially competed for R5020 binding to progesterone receptor. This first demonstration of 4 classes of steroid receptors in human breast cancer means that MCF-7 may be an excellent in vitro model for studying the mechanism of tumor response to endocrine therapy as well as the complex relationships between binding and biological actions of these hormones.

Specific progesterone receptors in human breast cancer

We have identified a specific progesterone receptor in 11 of 33 human breast cancer cytosols. Since progesterone itself binds to glucocorticoid receptor, to corticosteroid binding globulin (CBG), and to nonspecific components as well as to its own receptor, we have used a synthetic progestin, R5020 (17,21-dimethyl-19-nor-4,9-pregnadiene-3,20-dione), whose binding specificity is restricted to progesterone receptor. Bound R5020 sediments at 8 S in sucrose gradients; binding is competed by excess unlabeled R5020 or progesterone. The receptor is distinct from glucocorticoid receptor and CBG as determined by competition studies using dexamethasone and hydrocortisone. The dissociation constant for R5020 obtained by Scatchard analysis of dextran-coated charcoal assays is approximately 2 times 10- minus 9 M.

Hormones in breast cancer: Update 1978☆

The role of cytoplasmic estrogen receptor (ER) assays in determining therapeutic strategies for advanced breast cancer is certainly well established. The use of ER assays in the primary breast tumor specimen to predict for early recurrence and ultimate survival is a new finding, however, and will probably be employed in future trials of adjuvant therapy. The prevalence and significance of nuclear-bound ER still requires additional clarification. Our previous suggestion that progesterone receptor measurements might be a useful marker for hormone dependence in advanced breast cancer is gaining support and may soon have a place in routine therapeutic decision-making. The emphasis on early adjuvant therapy has hastened the search for a safe endocrine therapy that would have good patient compliance and achieve remission rates comparable to previous agents and procedures. Antiestrogens show promise of meeting these requirements. We are now beginning an era in which primary and secondary systemic therapies for breast cancer can be based on sound biologic principles. The empirical approach is outdated.

Antiestrogens: Mechanism of Action and Effects in Breast Cancer

Several of the nonsteroidal antiestrogens are in experimental use for the treatment of breast cancer. The treatment goal is to obtain, with minimal toxicity, specific control over cell growth by chemical means, thereby avoiding, on the one hand, pharmacologic doses of hormones and, on the other, major surgical ablative procedures.(1) One rationale for use of estrogen antagonists is an outgrowth of our current awareness of the role of estrogen receptors (ERs) and estrogen in breast cancer. Recent reports show that hormone dependence can be predicted by use of ER measurements. If estrogen antagonists block the action of estrogen at its receptor, it would be possible to obtain, by noninvasive means, the same therapeutic end as ablative hormonal procedures. Despite considerable research to that end, however, our knowledge of the mechanism of antiestrogen action remains unclear. The purpose of this review is to summarize this research. We will review first some of the biological activities of antiestrogens in normal tissues, and the mechanisms that have been proposed for their effects, then the role of antiestrogens in experimental animal breast cancer, and the data currently available on use of antiestrogens in humans and human breast cancer.

A physiological role for estrogen and progesterone in breast cancer

Abstract Breast cancer is often hormone responsive, since growth or regression of tumors can often be modulated by appropriate endocrine manipulations. Estrogen and progesterone are major hormones involved in regulation of breast cancer tumor growth. Considerable insight into the mechanism of action of these hormones on tumor growth stimulation has been provided by demonstration of specific receptors for each. The inference that each hormone acts independently through its receptor to control tumor growth is belied by current studies which show that certain hormones are capable of regulating the receptor sites, metabolism, or nuclear translocation of others. This may begin to explain the complex hormonal interactions and requirements of normal and neoplastic breast tissues. Considerable progress has thus been made in understanding the basis for success of various ablative therapies. The pharmacologie actions of estrogens and progestins in causing breast tumor regression is much less well understood. The role of hormone receptor sites has not been established in the mechanism of tumor regression caused by these pharmacological therapies. Nevertheless, when estrogen receptors are absent in a tumor, we can with accuracy pre.dict that endocrine therapies will fail, whereas when ER is present the likelihood of a successful response to pharmacological or ablative therapy is high. Receptor sites seem to be a common denominator and useful marker for hormone dependence or hormone responsiveness, irrespective of their actual role in the tumor regression process. Further investigations into the receptor functions should lead to new approaches in the endocrine management of patients with breast cancer.

Hormones and Their Receptors in Breast Cancer

Publisher Summary This chapter describes normal target tissues including mammary gland, which contain specific receptors for hormones: (1) cytoplasmic proteins for the steroid and (2) cell-surface receptors for polypeptides. These receptor sites are responsible for the initial interaction between the hormone and the cell and function to trigger the biochemical chain of events characteristic for the particular hormone. Estrogen acts directly on the normal mammary gland to promote growth and differentiation. However, estrogen also stimulates the release of pituitary prolactin, which likewise acts upon the mammary cell. At present, there are several procedures for measurement of estrogen (ER) in cytosols of target tissues. The receptor can be quantitated by demonstration of specific 8 S and 4 S binding of [3H] estradiol on sucrose density gradients (SDG). The dextran-coated-charcoal method (DCC) is equally quantitative and less expensive. Non-receptor-bound [3H] estradiol is removed from specific estradiol-bound receptor by charcoal. In castrated premenopausal or in postmenopausal breast cancer patients, estrogen precursors are secreted by the adrenal gland and converted to estrogens by peripheral tissues.

A role for progesterone in breast cancer.

Because of the cyclic changes of blood estrogen and progesterone levels that occur in females and the interrelationships among these hormones in regulating target tissue development and growth, it was inevitable that progesterone would be studied for its effect on breast cancer. Although progesterone itself is not a carcinogen, it may be a potent target-specific cocarcinogen for induction of mammary tumors by viral or chemical agents.l As such, the hormone has been implicated both in tumor enhancement and in tumor suppression. That progesterone plays a role in stimulating tumor growth is suggested by the pioneering studies of Huggins et al.2-4 They showed that pregnancy promoted the growth of dimethylbenzanthracene (DMBA)-induced rat mammary tumors. Administration of progesterone to intact rats accelerated the appearance of tumors, increased the numbers of tumors, and augmented the growth rate of established tumors. If DMBA-induced mammary tumor-bearing rats are ovariectornized and simultaneous lesions are placed in the median eminence to increase prolactin release, the tumor grows at an accelerated pace for only 10-12 days and then regresses, even though prolactin levels remain e l e ~ a t e d . ~ . ~ The ovarian factor responsible for maintaining tumor growth under these circumstances has not been identified, but the following experiments suggest the importance of progesterone. Pregnancy stimulates the growth of DMBA-induced mammary tumors, while parturition and weaning are followed by regression of a large number of these tum o r ~ . ~ , ~ , ~ The tumor growth-promoting factor of pregnancy is probably placental l a ~ t o g e n , ~ while prolactin has been implicated as being responsible for the maintenance of tumor size and growth during lactation, because tumors regress if suckling is prevented. I 0 * l I The true situation is more complex, however, because ovariectomy blocks the stimulatory effects of endogenous or exogenous prolactin on tumor growth, and injection of progesterone removes this block.I0 One interpretation would be that prolactin stimulation of tumors under these circumstances is dependent on progesterone or, alternatively, that the high levels of circulating progesterone found in the lactating rat, which are under prolactin control,I2 are responsible for the tumor growth. This does not necessarily mean that progesterone alone is responsible for maintaining rat mammary tumor growth, because in these experiments the animals had both high prolactin levels and intact adrenal glands. On the other hand, they do suggest that progesterone plays an important physiologic role in stimulating tumor growth. In contrast to the stimulatory effects of progesterone described above, progesterone can induce rat mammary tumor regression or prevent tumor appearance when combined with moderate to large doses of estrogen.2,13 In humans, the percentage of breast tumor regressions to a progesterone-estrogen combina-

Steroid Receptors in Breast Tumors—Current Status

Publisher Summary This chapter reviews the role of several hormones and their receptors in breast cancer tissues and examines mechanisms of control. Breast cancer is often hormone-responsive because growth or regression of tumors can often be modulated by appropriate endocrine manipulations. Estrogen and progesterone appear to be the major hormones involved in the regulation of breast tumor growth. The inference that each hormone acts independently through its receptor to control tumor growth is belied by the current studies showing that certain hormones are capable of regulating the receptor sites, metabolism, or nuclear translocation of others. This may begin to explain the complex hormonal interactions and the requirements of normal and neoplastic breast tissues. Considerable progress has, thus, been made in understanding the basis for success of various ablative therapies. Many of the studies on hormone-dependent breast carcinoma employ animal models, particularly carcinogen-induced rat mammary tumors, which regress after endocrine ablative surgery. The chapter discusses the relevance of these animal models to the clinical problem of human breast cancer. It illustrates the physiological principles of hormone action in breast cancer tissue so that an appreciation of the mechanism of endocrine-induced breast tumor regression will develop. Such an understanding should lead to a more rational approach for selecting or rejecting endocrine therapy for advanced breast cancer patients.

Receptors and breast cancer: Do we know it all?

Abstract The use of cytoplasmic estrogen receptor to predict endocrine responsiveness in breast cancer patients is now well established. It is our contention that this pertinent clinical application is only the first of many contributions from receptor studies to the clinic. Efforts are now underway to examine the distribution of receptors within tumor cells, and in particular the significance of receptors in tumor nuclei in the absence of hormone. Also, the demonstration of a direct stimulation of progesterone receptor synthesis by estradiol in cultured breast tumor cells now permits a careful dissection of the complete estrogen response system which should provide clues to the mechanism of antiestrogen action in causing tumor regression. Finally, it is now evident that pharmacologic effects of steroids in breast cancer may be mediated through different pathways than expected from studies of normal physiology.