Lisa L. Wei

Progestin action and progesterone receptor structure in human breast cancer: a review

Publisher Summary This chapter discusses the current information about progestin action, and progesterone receptors (PR) structure and function. It presents a recent study in which a progestin-responsive human breast cancer cell line has been used to study the biochemical mechanisms involved in progesterone action. Progesterone is essential for human breast alveolar gland development and growth. It is not only a mitogenic hormone but also acts in synergy with estradiol to prime the mammary gland to respond to mitogens like prolactin, glucocorticoids, and growth hormone. At puberty, estradiol induces mammary duct formation. As the hypothalamic-pituitary-ovarian axis matures, there is a brief phase of anovulatory cycles during which the mammary gland is influenced principally by estrogens. Once ovulation is established, corpus luteum produces progesterone, which stimulates growth of the lobuloalveolar structures. During the reproductive years, progesterone promotes differentiation of alveolar cells into secretory cells, and dilation of the duct system. In conjunction with prolactin and other metabolic hormones, progesterone stimulates lipid droplet formation, and secretory activity. While prolactin and growth hormone are the most important pituitary hormones affecting growth of the mammary gland during pregnancy, it is estradiol and progesterone that prevent the full expression of their secretory effects on the mammary epithelium. At parturition, the sudden withdrawal of these two hormones permits the breast to respond to the lactogenic hormones.

Human progesterone A-receptors can be synthesized intracellularly and are biologically functional

In order to investigate the origin and functional independence of the human progesterone receptor A binding protein, we have expressed a truncated human progesterone receptor cDNA in both gene transfer and in vitro translation assays. Proteins identical in size and antigenicity to the A-receptors found naturally in human progesterone target cells are synthesized from this cDNA that lacks the putative B receptor initiator methionine codon of the complete cDNA. The functional independence of A-receptors is suggested by their ability to bind hormone and to stimulate transcription from the progestin responsive mouse mammary tumor virus promoter.

Structural analyses of progesterone receptors

Progesterone receptors of T47Dco human breast cancer cells consist of two equimolar hormone-binding proteins of mol. wt approximately 85,000 (A protein) and 115,000 (B protein). Both proteins can be demonstrated in intact cells by in situ photoaffinity labeling; that is, in cells treated with the synthetic progestin [3H]R5020, irradiated 2 min with 300 nm u.v., solubilized directly in SDS and subjected to electrophoresis under denaturing conditions. These proteins are 6000-10,000 dalton heavier than the corresponding proteins of chick oviducts. This difference has been measured by direct comparison of photolabeled chick and human receptors on SDS-PAGE and by immunoblotting with the 9G10 antibody prepared against chick protein B. The antibody binds to a protein of mol. wt approximately 106,000 in human cells that is smaller than the hormone-bound B protein and larger than the hormone-bound A. In T47Dco cells, in situ photolabeled, untransformed receptors, as well as transformed nuclear-bound receptors, have equimolar amounts of A and B proteins. This ratio remains stable during a 1 h 37 degrees C in vitro incubation. Analysis of the in situ labeled receptors on gradient gels shows that the untransformed B protein exists as a doublet of mol. wt approximately 115,000 and 119,000 while the A protein is a singlet. After [3H]R5020 treatment, nuclear receptors change further: during the first 30 min in the nucleus the B protein shifts entirely to the heavier, mol. wt = 119,000 form. Between 30 and 60 min after nuclear binding, the A protein first becomes a doublet of 85,000 and 89,000 dalton then shifts entirely to the 4000 dalton heavier form. Later, nuclear processing leads to the simultaneous disappearance of both proteins without generation of smaller molecular weight fragments. Cleveland mapping studies show that the A and B proteins are closely related; despite the initial difference in the molecular weight of A and B, digestion with S. aureaus V8 protease yields identical fragmentation patterns for each, with sequential peptides of mol. wt approximately 49,000, 39,000, 26,000 and 14,000.(ABSTRACT TRUNCATED AT 400 WORDS)