Progestins both stimulate and inhibit breast cancer cell cycle progression while increasing expression of transforming growth factor alpha, epidermal growth factor receptor, c-fos, and c-myc genes.

This study documents a biphasic change in the rate of cell cycle progression and proliferation of T-47D human breast cancer cells treated with synthetic progestins, consisting of an initial transient acceleration in transit through G1, followed by cell cycle arrest and growth inhibition. Both components of the response were mediated via the progesterone receptor. The data are consistent with a model in which the action of progestins is to accelerate cells already progressing through G1, which are then arrested early in G1 after completing a round of replication, as are cells initially in other phases of the cell cycle. Such acceleration implies that progestins act on genes or gene products which are rate limiting for cell cycle progression. Increased production of epidermal growth factor and transforming growth factor alpha, putative autocrine growth factors in breast cancer cells, does not appear to account for the initial response to progestins, since although the mRNA abundance for these growth factors is rapidly induced by progestins, cells treated with epidermal growth factor or transforming growth factor alpha did not enter S phase until 5 to 6 h later than those stimulated by progestin. The proto-oncogenes c-fos and c-myc were rapidly but transiently induced by progestin treatment, paralleling the well-known response of these genes to mitogenic signals in other cell types. The progestin antagonist RU 486 inhibited progestin regulation of both cell cycle progression and c-myc expression, suggesting that this proto-oncogene may participate in growth modulation by progestins.     

Inhibition of AKT with the Orally Active Allosteric AKT Inhibitor, MK-2206, Sensitizes Endometrial Cancer Cells to Progestin

Progestin resistance is a major obstacle to treating early stage, well-differentiated endometrial cancer as well as recurrent endometrial cancer. The mechanism behind the suboptimal response to progestin is not well understood. The PTEN tumor suppressor gene is frequently mutated in type I endometrial cancers and this mutation results in hyperactivation of the PI3K/AKT pathway. We hypothesized that increased activation of AKT promotes an inadequate response to progestins in endometrial cancer cells. Ishikawa cells stably transfected with progesterone receptor B (PRB23 cells) were treated with the AKT inhibitor, MK-2206, which effectively decreased levels of p(Ser473)-AKT in a dose-dependent (10 nM to 1 uM) and time-dependent manner (0.5 h to 24 h). MK-2206 inhibited levels of p(Thr308)-AKT and a downstream target, p(Thr246)-PRAS40, but did not change levels of p(Thr202/Tyr204)ERK or p(Thr13/Tyr185)SAPK/JNK, demonstrating specificity of MK-2206 for AKT. Additionally, MK-2206 treatment of PRB23 cells resulted in a significant increase in levels of progesterone receptor B (PRB) protein. Microarray analysis of PRB23 cells identified PDK4 as the most highly upregulated gene among 70 upregulated genes in response to R5020. Inhibition of AKT further upregulated progestin-mediated expression of PDK4 but did not affect another progestin-responsive gene, SGK1. Treatment of PRB23 cells with R5020 and MK-2206 independently decreased viability of cells while the combination of R5020 and MK-2206 caused the greatest decrease in cell viability. Furthermore, mice with xenografted tumors treated with MK-2206 alone or with progesterone alone exhibited modest reductions in their tumor volume. The largest decrease in tumor size was observed in the mice treated with both MK-2206 and progesterone; these tumors exhibited the least proliferation (Ki67) and the most apoptosis (cleaved caspase-3) of all the treatment groups. In summary, inhibition of AKT stabilizes the Progesterone Receptor B and augments progesterone response in endometrial cancer cells that have hyperactivated AKT.     

Estrogens in the causation of breast, endometrial and ovarian cancers - evidence and hypotheses from epidemiological findings

Estrogens along with progesterone/progestins, and other hormones, are important determinants of cancer in the breast, endometrium and ovary. Estrogens may increase the risk of breast cancer through various mechanisms and at various phases of life, with a possible synergistic effect of progesterone/progestins. Exposure to high doses of placental hormones, such as estrogens and/or progesterone, during pregnancy may play a pivotal role in reducing subsequent breast cancer susceptibility. Estrogens cause endometrial cancer, an effect that can be reduced, prevented or reversed by progesterone/progestin — if allowed to act for a sufficiently long period of each cycle. The role of sex hormones seems important for ovarian carcinogenesis. Intake of combined oral contraceptives has a substantial and well-documented protective effect on endometrial and ovarian cancer risks. Epidemiological observations and experimental data from an animal model indicate that estrogens may have an adverse effect, while progesterone/progestins have a risk reducing effect directly on the ovarian epithelium. Thus, estrogens and other sex hormones have potential effects on the three most important female cancers. Research has yet to define how some of the risk factors can be modified or treatment regimens can be improved to reduce these cancer risks.     

Regulation of gene expression by PRA-910, a novel progesterone receptor modulator, in T47D cells

Progestins play an important role in women's health and are used in oral contraception, hormone therapy, and treatment of reproductive disorders. The effects of progestins upon gene expression in breast epithelium are poorly understood. In an attempt to characterize the molecular mechanism of progestin action, we used a gene expression profiling approach to examine the action of a novel progestin in the T47D cell model, a human breast cancer cell line. PRA-910 is a novel, nonsteroidal progesterone receptor modulator (PRM) with species-specific activities identified in a screen for selective PRMs. To understand the mechanism of action for PRA-910 in T47D cells, we compared its gene regulation to progesterone (P4) and RU486 through Affymetrix U95A GeneChip analysis and TaqMan RT-PCR. PRA-910, P4, and RU486 regulated 50, 108, and 16 genes by threefold or greater versus vehicle, respectively, with 18 genes having similar regulation for P4 and PRA-910. These data confirm and extend previous findings for T47D cells. We also obtained time course, concentration-response, cyclohexamide sensitivity, and PR-specificity data for two progestin-regulated genes, ATP1A1 and CLDN8. Our data demonstrate that PRA-910 has a unique gene regulation profile distinct from both P4 and RU486. Further investigation of the underlying mechanism for these differences is ongoing.     

Regulation of growth hormone and epidermal growth factor receptors by progestins in breast cancer cells

A 24 hr incubation of T-47D human breast cancer cells with R5020, a synthetic progestin, resulted in a 200-250% increase in the specific binding of human growth hormone (hGH) and epidermal growth factor (EGF) by these cells. This effect was specific for progestins in that similar responses were observed with progesterone, medroxyprogesterone acetate and ORG 2058 but no significant increases in hGH or EGF binding were observed in cells incubated with testosterone, estradiol or hydrocortisone. Increased binding was due to an increase in the concentration of receptors (hGH, control = 6,490 +/- 500, progestin treated = 13,180 +/- 3,270 sites/cell; EGF, control = 33,380 +/- 7,410, progestin treated = 67,460 +/- 20,330 sites/cell) while the affinity constants for the hormone-receptor interactions were unchanged by progestin treatment. The specific binding of insulin, calcitonin, transferrin and concanavalin A was unaffected by these treatments. It is concluded that expression of hGH and EGF receptors in this breast cancer cell line is regulated by progestins.     

Prolonged progestin treatment induces the promoter of CDK inhibitor p21 Cip1,Waf1 through activation of p53 in human breast and endometrial tumor cells

Progestins are frequently used in the treatment of advanced breast and endometrial cancer. The human breast carcinoma cell line T47D shows a biphasic response to progestins. Short-term progestin treatment leads to enhanced DNA synthesis, while this line is growth inhibited upon prolonged exposure. An important protein involved in growth regulation by progestins in this cell is the CDK inhibitor p21(Cip1,Waf1). We show that after 1 day of progestin treatment in T47D cells, the p21 promoter-proximal region containing Sp1 binding sites is crucial in the induction by progestins. However, after 3 days the activity of the promoter-distal region becomes predominant in T47D cells or the endometrial carcinoma cell line ECC1. This is dependent upon two domains within this region that contain p53 response elements. In ECC1 and T47D cells 3-day progestin treatment induces a reporter containing a p53 response element, but not a mutated version. This induction is due to activation of p53 by progestin, which may be caused by nuclear translocation of p53. These data indicate that upon prolonged exposure, progestins activate p53, in human breast and endometrial tumor cells, which up-regulates the p21(Cip1,Waf1) promoter. This may be an important mechanism involved in progestin-inhibited cellular proliferation in these cells.     

Molecular mechanisms of steroid receptor-mediated actions by synthetic progestins used in HRT and contraception

Synthetic progestins are used by millions of women as contraceptives and in hormone replacement therapy (HRT), although their molecular mechanisms of action are not well understood. The importance of investigating these mechanisms, as compared to those of progesterone, has been highlighted by clinical evidence showing that medroxyprogesterone acetate (MPA), a first generation progestin, increases the risk of breast cancer and coronary heart disease in HRT users. A diverse range of later generation progestins with varying structures and pharmacological properties is available for therapeutic use and it is becoming clear that different progestins elicit beneficial and adverse effects to different extents. These differences in biological activity are likely to be due to many factors including variations in dose, metabolism, pharmacokinetics, bioavailability, and regulation of, and/or binding, to serum-binding proteins and steroidogenic enzymes. Since the intracellular effects on gene expression and cell signaling of steroids are mediated via intracellular steroid receptors, differential actions via the progesterone and other steroid receptors and their isoforms, are likely to be the major cause of differential intracellular actions of progestins. Since many progestins bind not only to the progesterone receptor, but also to the glucocorticoid, androgen, mineralocorticoid, and possibly the estrogen receptors, it is plausible that synthetic progestins exert therapeutic actions as well as side-effects via some of these receptors. Here we review the molecular mechanisms of intracellular actions of old (MPA, norethisterone, levonorgestrel, gestodene) vs. new (drospirenone, dienogest, trimegestone) generation progestins, via steroid receptors.     

Progestin regulation of progesterone biosynthetic enzymes in cultured rat granulosa cells

Progestins have recently been shown to augment gonadotropin-stimulated progesterone and 20 alpha-hydroxypregn-4-en-3-one (20 alpha-OH-P) biosynthesis in cultured rat granulosa cells. The mechanism by which progestins autoregulate ovarian progestin biosynthesis was investigated by studying the modulation of pregnenolone biosynthesis as well as the activities of the enzymes 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) and 20 alpha-hydroxysteroid dehydrogenase (20 alpha-HSD). Granulosa cells obtained from immature hypophysectomized, estrogen-treated rats were cultured with FSH and/or progestins. Pregnenolone production was measured in the presence of cyanoketone (10(-6) M) to inhibit 3 beta-HSD activity. Enzymatic activities of 3 beta-HSD and 20 alpha-HSD were determined in cell homogenates by direct enzyme assays. FSH stimulated pregnenolone production, while treatment with progesterone or R5020 alone was ineffective. Concomitant treatment with the progestins further enhanced FSH-stimulated pregnenolone production in a dose-dependent manner with minimal effective doses of 10(-8) and 10(-7) M for R5020 and progesterone, respectively. In FSH-primed cells, LH increased pregnenolone accumulation, and concomitant treatment with R5020 also enhanced the LH action. Furthermore, the gonadotropins stimulated the activity of 3 beta-HSD, and this effect was further enhanced by concomitant treatment with either R5020 or progesterone in a dose-dependent manner. In addition, the 20 alpha-HSD activities were enhanced by progestins in cells treated with FSH but not with LH. Thus, both natural and synthetic progestins stimulate the gonadotropin-induced progesterone production in cultured granulosa cells via enhancing the 3 beta-HSD enzyme as well as pregnenolone biosynthesis.     

Honey, we need to talk about the membrane progestin receptors

The recent discovery of three closely related cell surface receptors that bind to progesterone and mediate its actions on various cytoplasmic signalling cascades has been heralded as a major break-through. The reason for this is all too obvious. Progesterone is an essential regulator of all major reproductive events and progestins and antiprogestins are widely used in the treatment of many different gynaecological and obstetrical disorders. The novel membrane progestin receptors (mPRalpha, beta, gamma) reportedly resemble and function as G-protein-coupled receptors and therefore are promising pharmaceutical targets. However, our studies failed to corroborate that mPRs are expressed on the cell surface, that they mediate rapid progesterone signalling events, and even that they are bona fide progestin binding moieties. While the reason for these startling opposing results remains unclear, a critical review of existing data may help to shed some light onto the controversial mPRs. Time has come to talk.     

Progestins induce down-regulation of insulin-like growth factor-I (IGF-I) receptors in human breast cancer cells: potential autocrine role of IGF-II.

Insulin-like growth factor-I (IGF-I) receptors are present in breast cancer cells and may play a role in breast cancer cell growth. We have studied the effect of progestins on IGF-I receptors in T47D human breast cancer cells. T47D cells constitutively express high levels of progesterone receptors and are a model for studying the regulation of cellular functions by progestins. Treatment of T47D cells with either progesterone or the synthetic progestin promegestone (R5020) decreased IGF-I receptor content by approximately 50%, as measured by Scatchard analysis and receptor biosynthesis studies. In contrast to progestins, estradiol, dexamethasone, and dihydrotestosterone did not influence IGF-I receptor content. No effect of R5020 was seen after 12 h of incubation, a near-maximal effect was seen after 24 h, and greatest effects were seen after 72 h. R5020 decreased IGF-I receptor mRNA abundance, indicating that progestins acted at the level of gene expression. However, progestins also increased the secretion of IGF-II, a ligand for the IGF-I receptor. In contrast to IGF-II, T47D cells did not express IGF-I. The addition of exogenous IGF-II to T47D cells down-regulated both IGF-I receptor binding and IGF-I receptor mRNA abundance. This study indicates, therefore, that progestins regulate IGF-I receptors in breast cancer cells and suggests that this regulation occurs via an autocrine pathway involving enhanced IGF-II secretion.