Challenges to defining a role for progesterone in breast cancer

Progesterone is an ovarian steroid hormone that is essential for normal breast development during puberty and in preparation for lactation. The actions of progesterone are primarily mediated by its high affinity receptors, including the classical progesterone receptor (PR) -A and -B isoforms, located in diverse tissues such as the brain where progesterone controls reproductive behavior, and the breast and reproductive organs. Progestins are frequently prescribed as contraceptives or to alleviate menopausal symptoms, wherein progestin is combined with estrogen as a means to block estrogen-induced endometrial growth. Estrogen is undisputed as a potent breast mitogen, and inhibitors of the estrogen receptor (ER) and estrogen producing enzymes (aromatases) are effective first-line cancer therapies. However, PR action in breast cancer remains controversial. Herein, we review existing evidence from in vitro and in vivo models, and discuss the challenges to defining a role for progesterone in breast cancer.     

Potential interactions between estrogen receptor and thyroid receptors relevant for neuroendocrine systems

Environmental signals can profoundly affect reproductive behavior, physiology and responses to steroids. One consequence of nutritional or temperature stress is altered plasma concentrations of thyroid hormone. Recent in vivo and in vitro data indicate that manipulations of estrogen and thyroid hormone levels can alter each other's functions. One possible mechanism for interaction may be that thyroid and estrogen receptors bind to parts of the same hormone response elements of target genes and compete with each other, thus serving to integrate environmental signals with neuroendocrine responses.     

Progesterone as a neurosteroid: synthesis and actions in rat glial cellsProceedings of Xth International Congress on Hormonal Steroids, Quebec, Canada, 17–21 June 1998.

The central nervous system (CNS) and the peripheral nervous system (PNS) are targets for steroid hormones where they regulate important neuronal functions. Some steroid hormones are synthesized within the nervous system, either de novo from cholesterol, or by the metabolism of precursors originating from the circulation, and they were termed ‘neurosteroids'. The sex steroid progesterone can also be considered as a neurosteroid since its synthesis was demonstrated in rat glial cell cultures of the CNS (oligodendrocytes and astrocytes) and of the PNS (Schwann cells). Both types of glial cells express steroid hormone receptors, ER, GR and PR. As in target tissue, e.g. the uterus, PR is estrogen-inducible in brain glial cell cultures. In the PNS, similar PR-induction could not be seen in pure Schwann cells derived from sciatic nerves. However, a significant PR-induction by estradiol was demonstrated in Schwann cells cocultured with dorsal root ganglia (DRG), and we will present evidence that neuronal signal(s) are required for this estrogen-mediated PR-induction. Progesterone has multiple effects on glial cells, it influences growth, differentiation and increases the expression of myelin-specific proteins in oligodendrocytes, and potentiates the formation of new myelin sheaths by Schwann cells in vivo. Progesterone and progesterone analogues also promotes myelination of DRG-Neurites in tissue culture, strongly suggesting a role for this neurosteroid in myelinating processes in the CNS and in the PNS.     

Regulation by Gonadal Steroids of Estrogen and Progesterone Receptors Along the Reproductive Tract in Female Lambs

The regulation of estrogen and progesterone receptor (ER, PR) expression by estradiol (E2) and progesterone (P4) in the oviduct, uterus and cervix of female lambs was studied. The animals received three intramuscular injections of E2, P4 or vehicle with an interval of 24 h and they were slaugthered 24 h after the third injection. Determinations of ER and PR were performed by binding assays and mRNAs of ERα and PR by solution hybridization. High levels of ER and PR in both cervix and oviduct were found in the female lamb, differing from other mammalian species. No significant effects by either E2 or P4 treatment on ER and PR levels in the cervix and oviduct could be observed. E2 treatment increased the mRNA levels of ERa and PR more than 3-fold in the cervix, while P4 treatment increased the mRNA levels of ERa and PR in the uterus. The results show differential effects of gonadal steroids on sex steroid receptor expression along the reproductive tract in female lambs, suggesting that steroid target tissues can modulate responses to the same circulating levels of steroid hormones.     

Highly selective inhibition of estrogen biosynthesis by CGS 20267, a new non-steroidal aromatase inhibitor

CGS 20267 is a new non-steroidal compound which potently inhibits aromatase in vitro (IC₅₀ of 11.5 nM) and in vivo (ED₅₀ of 1–3 μg/kg p.o.). CGS 20267 maximally inhibits estradiol production in vitro in LH-stimulated hamster ovarian tissue at 0.1 μM with an IC₅₀ of 0.02 μM and does not significantly affect progesterone production up to 350 μM. In ACTH-stimulated rat adrenal tissue in vitro, aldosterone production was inhibited with an IC₅₀ of 210 μM (10,000 times higher than the IC₅₀ for estradiol production); no significant effect on corticosterone production was seen at 350 μM. In vivo, in ACTH-treated rats, CGS 20267 does not affect plasma levels of corticosterone or aldosterone at a dose of 4 mg/kg p.o. (1000 times higher than the ED₅₀ for aromatase inhibition in vivo). In adult female rats, a 14-day treatment with 1 mg/kg p.o. daily, completely interrupts ovarian cyclicity and suppresses uterine weight to that seen 14 days after ovariectomy. In adult female rats bearing estrogen-dependent DMBA-induced mammary tumors, 0.1 mg/kg p.o. given daily for 42 days caused almost complete regression of tumors present at the start of treatment. Thus compared to each other, CGS 16949A and CGS 20267 are both highly potent in inhibiting estrogen biosynthesis in vitro and in vivo. The striking difference between them is that unlike CGS 16949A, CGS 20267 does not affect adrenal steroidogenesis in vitro or in vivo, at concentrations and doses several orders of magnitude higher than those required to inhibit estrogen biosynthesis.     

孕酮受体介导哺乳动物雌性生殖活动的分子机理

孕酮作为一种甾体激素,在哺乳动物雌性生殖活动的调控中起着关键作用。孕酮的生理功能依赖于核孕酮受体介导的基因组效应和膜孕酮受体介导的非基因组效应,这两种效应共同介导了孕酮在各种雌性生殖活动中的不同作用,包括排卵、胚胎植入、妊娠维持、分娩启动和乳腺发育等。近年来,通过基因芯片技术筛选出大量的孕酮下游靶基因,但至今未能在这些基因的启动子区域上找到传统意义上的孕酮响应元件,故推测核孕酮受体调节下游靶基因转录活动的方式可能不同于传统的类固醇核受体。基于目前最新的研究成果,文章综述了在哺乳动物雌性生殖活动中,孕酮受体介导各种生理效应的分子机理。     

Androgen receptor mediated growth control of breast cancer and endometrial cancer modulated by antiandrogen- and androgen-like steroids

Androgens are involved in many regulatory processes in mammary and endometrial epithelium, but their role in the development and progression of breast and endometrial carcinoma is poorly understood. Androgen receptors (AR) are found in normal epithelium as well as in more than 50% of specimen from both tumor types. The occurrence of AR is correlated with estrogen and progesterone receptors. Androgen receptor positive cell lines were established during the last few years in our laboratory from malignant mammary (MFM-223) and endometrial (MFE-296) tumors supplementing the small number of androgen-responsive cell lines published so far. In this paper some aspects of the role of androgens in these two types of hormone responsive female cancer are presented. The proliferation of ZR-75-1, MFM-223 and MFE-296 cells is inhibited by androgens. The progestin medroxyprogesterone acetate inhibits the proliferation of estrogen- and progesterone receptor negative MFM-223 cells via the androgen receptor. Some steroid metabolites with distinct estrogenic properties like androst-5-ene-3β,17β-diol possess androgenic properties in this model system. Androgens stimulate the in vitro secretion of gross cystic disease fluid proteins by human mammary cancer cells. These proteins are normally found in benign breast cysts in vivo. The occurrence of gross cystic disease is correlated with an increased risk of breast cancer. The AR is autoregulated in MFM-223 mammary cancer cells on the protein and mRNA level. In MFE-296 cells with endometrial origin AR protein was increased after incubation with androgens.     

Protection by α-tocopherol but not ascorbic acid from hydrogen peroxide induced cell death in normal human breast epithelial cells in culture

-Tocopherol and ascorbic acid have been suggested to play a role in breast cancer prevention due to their antioxidative capacity. Increased exposure to endogenous and exogenous sex steroids is a known risk factor for breast cancer. We have studied the effects of -tocopherol and ascorbic acid on hydrogen peroxide induced cell death in sex hormone treated normal breast epithelial cells in culture. We found that -tocopherol but not ascorbic acid alone protected the cells. The effect of -tocopherol increased when ascorbic acid was added to the cultures. The hydrogen peroxide degradation rate decreased in cultures treated with -tocopherol alone and in combination with ascorbic acid compared to cells grown in medium or with ascorbic acid only. Oestradiol and progesterone treatment did not influence the results. Possible beneficial effects of combining various antioxidants, endogenous as well as exogenous, on human breast tissue need to be investigated further both in vivo and in vitro.     

Combined effects of levonorgestrel and quinestrol on reproductive hormone levels and receptor expression in females of the Mongolian gerbil (Meriones unguiculatus)

The effects of treatment with a combination of levonorgestrel and quinestrol (EP-1; ratio of 2:1) on reproductive hormone levels and the expression of their receptors in female Mongolian gerbils were examined. We show that serum follicle-stimulating hormone (FSH) and luteinizing hormone (LH) decreased, whereas serum estradiol (E2) and progesterone (P4) increased after EP-1 treatment. EP1 down-regulated mRNA expression of the follicle-stimulating hormone receptor (FSHR) and the estrogen receptor (ER) βin the ovary. EP-1 up-regulated the mRNA expression of the luteinizing hormone receptor (LHR) and the progesterone receptor (PR) in the ovary as well as ERα and PR in the uterus of Mongolian gerbils. The effects were time-dependent and dose-dependent. EP-1 had no obvious effects on ERα mRNA expression in the ovary. The current study demonstrates that the effect of EP-1 on the expression of ER subtypes is tissue-specific in Mongolian gerbils. EP-1 disrupted the reproductive endocrinology of the Mongolian gerbil. These findings suggest that the effects of EP-1 on reproductive hormone levels and their receptor expression in Mongolian gerbils may be the result of synergistic actions of levonorgestrel and quinestrol, with quinestrol playing the major role.     

Expression of sex steroid receptors and IGF-1 mRNA in breast tissue--effects of hormonal treatment

The mechanisms behind increased breast tissue proliferation and a possibly increased breast cancer risk in women using hormonal contraception (HC) and hormonal replacement therapy (HRT) are incompletely understood. We analyzed breast tissue from 20 premenopausal and seven postmenopausal women undergoing reduction mammoplasties for estrogen receptor (ER) and progesterone receptor (PR) content as well as mRNA levels for ER, PR and insulin-like growth factor-1 (IGF-1). The receptor values were correlated to IGF-1 mRNA concentrations and levels of steroid and peptide hormones and SHBG. In women using HC, we found significantly lower ER values (p=0.02) but non-significantly lower ER mRNA levels compared to those in naturally cycling women. PR and PR mRNA were no different. Women on HC displayed a higher breast tissue proliferation (p=0.05) expressed as Ki-67, MIB-1 positivity, which was correlated with IGF-1 mRNA (r_s=0.82, p=0.04). Since the concentration of sex steroid receptors in breast tissue is comparatively low and steroid receptors are down-regulated during hormonal treatment, mechanisms other than direct sex steroid receptor action are likely to be present. Our results suggest a role for IGF-1 in the proliferative response of breast tissue during exogenous hormonal treatment.     

Role of sex steroids in development of anxiety in female mice

Level of sex steroid hormones being changed throughout an estrous cycle influences physiological and behavioral features of female subjects. To test how estrogen and progesterone affect the anxiety level in mice the ovariectomy (OVX) followed by hormone treatment was carried out. After 1 week of recovery period estradiol benzonate (20 micrograms, s/c) was injected once a day during 7 consequent days. By the 7th day in addition to EB injection progesterone (500 micrograms, s/c) was also injected. Four hours later the mice were tested in elevated plus-maze to measure the anxiety level. Control animals were treated with sesame oil only. Behavioral data obtained demonstrate that the hormonal treatment altered anxiety state in experimental animals. In plusmaze paradigm, it has been demonstrated that progesterone-treated mice revealed the lowest level of open arm activity. In contrast, these mice showed the highest grooming activity as compared to other experimental groups. Immunohistochemical data on progesterone receptor (PR), immunoreactivity in brain have shown that the manipulation with different hormonal treatments modified the number of PR-ir cells in many brain areas. Our data suggest that sex steroid hormones play an important role in induction of anxiety level, as measured by elevated plus-maze, and this action might be partially mediated through the classical steroid receptors.