Epigenetic regulation of endometrium during the menstrual cycle

Coordinate regulation of heterogeneous nuclear ribonucleoprotein dynamics by steroid hormones in the human fallopian tube and endometrium in vivo and in vitro

Heterogeneous nuclear ribonucleoproteins (hnRNPs), which are chromatin-associated RNA-binding proteins, participate in mRNA stability, transport, intracellular localization, and translation by acting as transacting factors. Several studies have shown that steroid hormones can regulate hnRNP expression. However, to date, the regulation of hnRNPs and their interactions with steroid hormone signaling in fallopian tubes and endometrium are not fully elucidated. In the present study, we determined whether hnRNP expression is regulated during the menstrual cycle and correlates with estrogen receptor (ER) and progesterone receptor (PR) levels in human fallopian tubes in vivo. Because of the limited availability of human tubal tissues for the research, we also explored the mechanisms of hnRNP regulation in human endometrium in vitro. Fallopian tissue was obtained from patients in the early, late, and postovulatory phases and the midsecretory phase and endometrial tissue from premenopausal and postmenopausal women undergoing hysterectomy. We measured expression of hnRNPs and assessed their intracellular localization and interactions with ERs and PRs. We also determined the effects of human chorionic gonadotropin, 17β-estradiol (E(2)), and progesterone (P(4)) on hnRNP expression. In fallopian tubes, mRNA and protein levels of hnRNP A1, AB, D, G, H, and U changed dynamically during ovulation and in the midsecretory phase. In coimmunolocation and coimmunoprecipitation experiments, hnRNPs interacted with each other and with ERs and PRs in fallopian tubes. After treatment with E(2) and/or P(4) to activate ERs and PRs, hnRNP A1, AB, D, G, and U proteins displayed overlapping but distinct patterns of regulation in the endometrium in vitro. Our findings expand the physiological repertoire of hnRNPs in human fallopian tubes and endometrium and suggest that steroid hormones regulate different hnRNPs directly by interacting with ERs and/or PRs or indirectly by binding other hnRNPs. Both actions may contribute to regulation of gene transcription.

Cell cycle regulation of human endometrial stromal cells during decidualization

OBJECTIVE

Differentiation of endometrial stromal cells into decidual cells is crucial for optimal endometrial receptivity. Data from our previous microarray study implied that expression of many cell cycle regulators are changed during decidualization and inhibition of DNA methylation in vitro. In this study, we hypothesized that both the classic progestin treatment and DNA methylation inhibition would inhibit stromal cell proliferation and cell cycle transition.

METHODS

The human endometrial stromal cell line (HESC) was treated from 2 days to 18 days with the DNA methylation inhibitor, 5-aza-2'-deoxycytidine (AZA), a mixture of estradiol/progestin/cyclic adenosine monophosphate ([cAMP]; medroxy-progesterone acetate [MPA mix]) or both. Cell growth was measured by cell counting, cell cycle transition and apoptosis were analyzed by flow cytometry, expression of cell cycle regulators were analyzed by quantitative polymerase chain reaction (qPCR) and Western blotting, and change in DNA methylation profiles were detected by methylation-specific PCR.

RESULTS

Both AZA and MPA mix inhibited the proliferation of HESC for at least 7 days. Treatment with MPA mix resulted in an early G0/G1 inhibition followed by G2/M phase inhibition at 18 days. In contrast, AZA treatment inhibited cell cycle progression at the G2/M phase throughout. The protein levels of p21(Cip1)and 14-3-3σ were increased with both AZA and MPA mix treatments without any change in the DNA methylation profiles of the genes.

CONCLUSIONS

Our data imply that the decidualization of HESC is associated with cell cycle arrest at G0/G1 phase initially and G2/M phase at later stages. Our results also suggest that p53 pathway members play a role in the cell cycle regulation of endometrial stromal cells.

HDAC1 and HDAC2 are differentially expressed in endometriosis

Epigenetic mechanisms have been ascribed important roles in endometriosis. Covalent histone modifications at lysine residues have been shown to regulate gene expression and thus contribute to pathological states in many diseases. In endometriosis, histone deacetylase inhibition (HDACi) resulted in reactivation of E-cadherin, attenuation of invasion, decreased proliferation of endometriotic cells, and caused lesion regression in an animal model. This study was conducted to assess basal and hormone-regulated gene expression levels of HDAC1 and HDAC2 (HDAC1/2) in cell lines and protein expression levels in tissues. Basal and steroid hormone-regulated HDAC1/2 gene expression levels were determined by quantitative polymerase chain reaction in cell lines and tissues. Protein levels were measured by immunohistochemistry (IHC) in tissues on an endometriosis tissue microarray (TMA). Basal HDAC1/2 gene expression levels were significantly higher in endometriotic versus endometrial stromal cells, which was confirmed by Western blot analysis. Estradiol (E2) and progesterone (P4) significantly downregulated HDAC1 expression in endometrial epithelial cells. Levels of HDAC2 were upregulated by E2 and downregulated by E2 + P4 in endometrial stromal cells. Hormone modulation of HDAC1/2 gene expression was lost in the endometriotic cell line. Immunohistochemistry showed that HDAC1/2 proteins were expressed in a substantial proportion of lesions and endometrium from patients, and their expression levels varied according to lesion localization. The highest proportion of strong HDAC1 immunostaining was seen in ovarian, skin, and gastrointestinal lesions, and of HDAC2 in skin lesions and endometrium from patients with endometriosis. These studies suggest that endometriosis etiology may be partially explained by epigenetic regulation of gene expression due to dysregulations in the expression of HDACs.

Inhibition of histone deacetylase activity in human endometrial stromal cells promotes extracellular matrix remodelling and limits embryo invasion

Invasion of the trophoblast into the maternal decidua is regulated by both the trophoectoderm and the endometrial stroma, and entails the action of tissue remodeling enzymes. Trophoblast invasion requires the action of metalloproteinases (MMPs) to degrade extracellular matrix (ECM) proteins and in turn, decidual cells express tissue inhibitors of MMPs (TIMPs). The balance between these promoting and restraining factors is a key event for the successful outcome of pregnancy. Gene expression is post-transcriptionally regulated by histone deacetylases (HDACs) that unpacks condensed chromatin activating gene expression. In this study we analyze the effect of histone acetylation on the expression of tissue remodeling enzymes and activity of human endometrial stromal cells (hESCs) related to trophoblast invasion control. Treatment of hESCs with the HDAC inhibitor trichostatin A (TSA) increased the expression of TIMP-1 and TIMP-3 while decreased MMP-2, MMP-9 and uPA and have an inhibitory effect on trophoblast invasion. Moreover, histone acetylation is detected at the promoters of TIMP-1 and TIMP-3 genes in TSA-treated. In addition, in an in vitro decidualized hESCs model, the increase of TIMP-1 and TIMP-3 expression is associated with histone acetylation at the promoters of these genes. Our results demonstrate that histone acetylation disrupt the balance of ECM modulators provoking a restrain of trophoblast invasion. These findings are important as an epigenetic mechanism that can be used to control trophoblast invasion.

Is DNA methylation an epigenetic contribution to transcriptional regulation of the bovine endometrium during the estrous cycle and early pregnancy?

Epigenetic events controlling the transcriptional regulation of genes involved in endometrial function during the estrous cycle and early pregnancy have only sparsely been investigated. We analyzed the gene expression of DNA methyltransferases and the most prominent endocrine transcriptional mediator estrogen receptor alpha (ESR1) in the bovine endometrium of heifers at 0, 12 and 18 days following estrous and at day 18 after insemination. The luminometric methylation assay for the investigation of global DNA methylation and an elegant combination of methylation-sensitive high resolution melting and pyrosequencing for local methylation levels of ESR1 were deployed. In spite of differential gene expression of ESR1 among groups, no differences in endometrial ESR1 DNA methylation during neither estrous cycle nor early pregnancy were determined. Global DNA methylation prevailed at similar low levels in endometrium, likely controlled by the observed moderate DNMT3b expression. Thus, the epigenetic contribution of DNA methylation influencing endometrial function seems rather limited. However, because a control tissue expressing only minute amounts of ESR1 transcripts was locally significantly higher methylated, DNA methylation might contribute to an appropriate tissue-specific expression status underlying further specific control mechanisms of gene transcription.

Nuclear receptor, coregulator signaling, and chromatin remodeling pathways suggest involvement of the epigenome in the steroid hormone response of endometrium and abnormalities in endometriosis

Human endometrium, a steroid hormone-dependent tissue, displays complex cellular regulation mediated by nuclear receptors (NRs). The NRs interact with histone-modifying and DNA-methylating/-demethylating enzymes in the transcriptional complex. We investigated NRs, their coregulators, and associated signaling pathways in endometrium across the normal menstrual cycle and in endometriosis, an estrogen-dependent, progesterone-resistant disorder. Endometrial tissue was processed for analysis of 84 genes using NR and coregulator polymerase chain reaction (PCR) arrays. Select genes were validated by immunohistochemistry. Ingenuity pathway analysis identified DNA methylation and transcriptional repression signaling as the most affected pathway in endometrium in women with versus without endometriosis, regardless of cycle phase. Thyroid hormone receptor (THR) and vitamin D receptor (VDR) pathways were also regulated in normal and disease endometrium by activation of TH or vitamin D regulated genes. These data support the involvement of the epigenome in steroid hormone response of normal endometrium throughout the cycle and abnormalities in endometrium in women with endometriosis.

Assessment of endometrial receptivity

OBJECTIVE

To provide a focused review of the scientific literature pertaining to endometrial receptivity.

DESIGN

Review of the literature and appraisal of relevant articles.

SETTING

Academic teaching hospital.

PATIENT(S)

Women with infertility.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Critical review of the literature.

RESULT(S)

Although a consensus has been achieved regarding the existence of a temporally defined period during which embryo attachment and invasion can occur (called the "window of implantation"), reliable methods to assess "receptivity" have not been established or adequately validated. In women with certain gynecologic disorders, including endometriosis, tubal disease, and polycystic ovary syndrome, endometrial receptivity seems to be compromised, leading to infertility and pregnancy loss. The establishment of reliable biomarkers for the detection of defects in endometrial receptivity has been a long-sought goal that remains an elusive target. The validation of endometrial biomarkers will require properly designed and implemented studies based on the recognition that endometrial receptivity defects are not equally distributed in women with endometriosis or these other conditions.

CONCLUSION(S)

Rapidly advancing technologies are bringing new biomarkers to the clinical arena that promise to further reveal the complexities of the implantation process.

The molecular basis of recurrent pregnancy loss: impaired natural embryo selection

Recurrent pregnancy loss (RPL) is a common and distressing disorder. Chromosomal errors in the embryo are the single most common cause, whereas uterine factors are invariably invoked to explain non-chromosomal miscarriages. These uterine factors are, however, poorly defined. The ability of a conceptus to implant in the endometrium is normally restricted to a few days in the menstrual cycle. A limited 'window of implantation' ensures coordinated embryonic and endometrial development, thereby minimizing the risk of late implantation of compromised embryos. In this paper, we review emerging evidence, indicating that RPL is associated with impaired differentiation of endometrial stromal cells into specialized decidual cells. From a functional perspective, this differentiation process, termed decidualization, is not only critical for placental development but also signals the end of the implantation window and bestows on the endometrium the ability to recognize, respond to and eliminate implanting compromised embryos. Thus, we propose that spontaneous decidualization of the human endometrium, which inevitably causes menstrual shedding in the absence of a viable conceptus, serves as functional 'window for natural embryo selection'. Conversely, impaired decidualization predisposes to late implantation, negates embryo quality control and causes early placental failure, regardless of the embryonic karyotype. This pathological pathway also explains the common observation that many RPL patients seem exceptionally fertile, often conceiving within one or two cycles. Thus, as the clinical correlate of inappropriate uterine receptivity, 'superfertility' should be considered as a genuine reproductive disorder that requires targeted intervention.