Isoliquiritigenin Suppresses E2-Induced Uterine Leiomyoma Growth through the Modulation of Cell Death Program and the Repression of ECM Accumulation

Uterine leiomyomas, also known as fibroids, are common and prevalent in women of reproductive age. In this study, the effect of Isoliquiritigenin (ISL), a licorice flavonoid, on the anti-proliferation of uterine leiomyoma was investigated. We found that the survival of uterine leiomyoma ELT3 cells and primary uterine smooth muscle (UtSMC) cells was reduced by treatment with ISL alone or with ISL plus estradiol (E2). Cell cycles were arrested through the reduction of G2/M- and S-phase populations in ELT3 and UtSMC cells, respectively. Furthermore, increased sub-G1 phase and nucleus condensation were observed in ELT3 cells but not in UtSMC cells. Co-treatment of ELT3 cells with E2 and ISL inhibited ERK1/2 activation, whereas p38 and c-Jun N-terminal kinase (JNK) activation was enhanced. Moreover, ISL-induced apoptosis and autophagy cell death in ELT3 cells were observed. Serum E2 and P4 levels were reduced in a E2-enhanced uterine myometrium hyperplasia mouse model by ISL treatment, which contributed to the downregulation of the expression of extracellular matrix (ECM) associated proteins and matrix metalloproteinase (MMPs). Taken together, these results showed that ISL exerted a higher effect on the inhibition of estrogen-induced uterine leiomyoma growth for both in vitro and in vivo ECM accumulation, demonstrating its potential as a new option for treatment of uterine leiomyoma.

Ovarian steroids, stem cells and uterine leiomyoma: therapeutic implications

BACKGROUND

Uterine leiomyoma is the most common benign tumor in women and is thought to arise from the clonal expansion of a single myometrial smooth muscle cell transformed by a cellular insult. Leiomyomas cause a variety of symptoms, including abnormal uterine bleeding, pelvic pain, bladder or bowel dysfunction, and recurrent pregnancy loss, and are the most common indication for hysterectomy in the USA. A slow rate of cell proliferation, combined with the production of copious amounts of extracellular matrix, accounts for tumor expansion. A common salient feature of leiomyomas is their responsiveness to steroid hormones, thus providing an opportunity for intervention.

METHODS

A comprehensive search of PUBMED was conducted to identify peer-reviewed literature published since 1980 pertinent to the roles of steroid hormones and somatic stem cells in leiomyoma, including literature on therapeutics that target steroid hormone action in leiomyoma. Reviewed articles were restricted to English language only. Studies in both animals and humans were reviewed for the manuscript.

RESULTS

Estrogen stimulates the growth of leiomyomas, which are exposed to this hormone not only through ovarian steroidogenesis, but also through local conversion of androgens by aromatase within the tumors themselves. The primary action of estrogen, together with its receptor estrogen receptor α (ERα), is likely mediated via induction of progesterone receptor (PR) expression, thereby allowing leiomyoma responsiveness to progesterone. Progesterone has been shown to stimulate the growth of leiomyoma through a set of key genes that regulate both apoptosis and proliferation. Given these findings, aromatase inhibitors and antiprogestins have been developed for the treatment of leiomyoma, but neither treatment results in complete regression of leiomyoma, and tumors recur after treatment is stopped. Recently, distinct cell populations were discovered in leiomyomas; a small population showed stem-progenitor cell properties, and was found to be essential for ovarian steroid-dependent growth of leiomyomas. Interestingly, these stem-progenitor cells were deficient in ERα and PR and instead relied on the strikingly higher levels of these receptors in surrounding differentiated cells to mediate estrogen and progesterone action via paracrine signaling.

CONCLUSIONS

It has been well established that estrogen and progesterone are involved in the proliferation and maintenance of uterine leiomyoma, and the majority of medical treatments currently available for leiomyoma work by inhibiting steroid hormone production or action. A pitfall of these therapeutics is that they decrease leiomyoma size, but do not completely eradicate them, and tumors tend to regrow once treatment is stopped. The recent discovery of stem cells and their paracrine interactions with more differentiated cell populations within leiomyoma has the potential to provide the missing link between developing therapeutics that temper leiomyoma growth and those that eradicate them.

Progesterone action in endometrial cancer, endometriosis, uterine fibroids, and breast cancer

Progesterone receptor (PR) mediates the actions of the ovarian steroid progesterone, which together with estradiol regulates gonadotropin secretion, prepares the endometrium for implantation, maintains pregnancy, and differentiates breast tissue. Separation of estrogen and progesterone actions in hormone-responsive tissues remains a challenge. Pathologies of the uterus and breast, including endometrial cancer, endometriosis, uterine fibroids, and breast cancer, are highly associated with estrogen, considered to be the mitogenic factor. Emerging evidence supports distinct roles of progesterone and its influence on the pathogenesis of these diseases. Progesterone antagonizes estrogen-driven growth in the endometrium, and insufficient progesterone action strikingly increases the risk of endometrial cancer. In endometriosis, eutopic and ectopic tissues do not respond sufficiently to progesterone and are considered to be progesterone-resistant, which contributes to proliferation and survival. In uterine fibroids, progesterone promotes growth by increasing proliferation, cellular hypertrophy, and deposition of extracellular matrix. In normal mammary tissue and breast cancer, progesterone is pro-proliferative and carcinogenic. A key difference between these tissues that could explain the diverse effects of progesterone is the paracrine interactions of PR-expressing stroma and epithelium. Normal endometrium is a mucosa containing large quantities of distinct stromal cells with abundant PR, which influences epithelial cell proliferation and differentiation and protects against carcinogenic transformation. In contrast, the primary target cells of progesterone in the breast and fibroids are the mammary epithelial cells and the leiomyoma cells, which lack specifically organized stromal components with significant PR expression. This review provides a unifying perspective for the diverse effects of progesterone across human tissues and diseases.

Identifying the molecular signature of the interstitial deletion 7q subgroup of uterine leiomyomata using a paired analysis

Uterine leiomyomata (UL), the most common neoplasm in reproductive-age women, have recurrent cytogenetic abnormalities including interstitial deletion of 7q. To develop a molecular signature, matched del(7q) and non-del(7q) tumors identified by FISH or karyotyping from 11 women were profiled with expression arrays. Our analysis using paired t tests demonstrates this matched design is critical to eliminate the confounding effects of genotype and environment that underlie patient variation. A gene list ordered by genome-wide significance showed enrichment for the 7q22 target region. Modification of the gene list by weighting each sample for percent of del(7q) cells to account for the mosaic nature of these tumors further enhanced the frequency of 7q22 genes. Pathway analysis revealed two of the 19 significant functional networks were associated with development and the most represented pathway was protein ubiquitination, which can influence tumor development by stabilizing oncoproteins and destabilizing tumor suppressor proteins. Array CGH (aCGH) studies determined the only consistent genomic imbalance was deletion of 9.5 megabases from 7q22-7q31.1. Combining the aCGH data with the del(7q) UL mosaicism-weighted expression analysis resulted in a list of genes that are commonly deleted and whose copy number is correlated with significantly decreased expression. These genes include the proliferation inhibitor HPB1, the loss of expression of which has been associated with invasive breast cancer, as well as the mitosis integrity-maintenance tumor suppressor RINT1. This study provides a molecular signature of the del(7q) UL subgroup and will serve as a platform for future studies of tumor pathogenesis.

Murine xenograft model for human uterine fibroids: an in vivo imaging approach

Uterine fibroids are the most prevalent benign tumors in women of reproductive age. The current knowledge on the fibroid disease mechanism has derived from studies of the Eker rat model where a unique germ line defect in the tuberous sclerosis 2 (Tsc2) tumor suppressor gene leads to the development of leiomyosarcoma, leiomyoma, and renal cancer. To study fibroids of human origin, we sought to establish fibroid xenografts in immune-compromised mice. We determined that lentiviral-mediated transduction of a green fluorescence protein (GFP)-luciferase (LUC) fusion gene and bioluminescence-based whole animal imaging allowed for the monitoring of transplanted fibroid cells in mice. We used this in vivo imaging approach to test a series of transplantation protocols and found that only freshly dissociated fibroid cells, but not the fibroid-derived smooth muscle cells grown in ex vivo cultures, can generate stable xenografts in subcutaneous Matrigel implants. Formation of the fibroid-xenografts requires the implantation of 17betaestradiol-releasing pellets in the recipient mice. Furthermore, freshly dissociated myometrial cells do not form xenografts under the experimental conditions. The xenograft protocol developed from this study provides an avenue for investigating the pathogenesis and drug responses of human fibroids.

Spontaneously Occurring Fibroid Tumors of the Laying Hen Oviduct

Spontaneously occurring benign uterine leiomyomas (fibroids) are the most common tumors of reproductive-age women. It is estimated that more than 70% of all women will develop uterine fibroids, and the presence of these tumors is a primary cause of hysterectomies. Research into the causes and treatment of uterine fibroids is hampered by a lack of reliable animal models for the disease. Leiomyomas that appear to be outwardly similar to human uterine fibroid tumors are known to occur on the oviducts of laying hens over 2 yr of age. The objective of this study was to characterize these tumors and compare them to human uterine fibroids to determine the suitability of the aging hen as a model system for the study of the disease. In this study, hens at 5 yr of age were examined for the presence of oviduct-associated fibroid tumors. Tumors were found attached to the internal surface of the oviduct, embedded in the oviduct wall, or attached to the exterior of the magnum and isthmus. Tumor and normal oviduct samples were frozen or fixed in formalin for histological analyses or immunohistochemistry for estrogen and progesterone receptors, proliferating cell nuclear antigen and Bcl-2 protein expression. Human uterine fibroid samples were acquired and evaluated compared with hen oviduct fibroids. The results indicate that laying hen fibroid tumors are similar to human fibroid tumors with respect to estrogen and progesterone receptors, localized cellular proliferation, and expression of the Bcl-2 protein.

Catechol-O-methyltransferase polymorphism is associated with increased uterine leiomyoma risk in different ethnic groups

OBJECTIVES

Uterine leiomyomas (ULMs) are estrogen-dependent tumors that are more common in African American women. The etiology for such ethnic disparity is currently unknown. Catechol-O-methyltransferase (COMT) is an essential enzyme in estrogen metabolism. In the current study, we investigated the association of the functional COMT Val158Met polymorphism with ULM in different ethnic groups. We also studied the biologic role of COMT in tumor formation in human and rat leiomyoma cell lines and the potential therapeutic utility of COMT inhibitors.

METHODS

The genotype frequencies of the functional COMT Val158Met polymorphism among participants with (186 women) or without (142 women) ULMs were compared, as was the differential ethnic distribution of that polymorphism using polymerase chain reaction (PCR) and restriction-fragment linkage polymorphism. Proliferation, Western blot, and reporter transactivation analyses were applied to myometrial and leiomyoma cells representative of different COMT genotypes.

RESULTS

Women with the high-activity COMT Val/Val genotype are 2.5 times more likely to develop ULMs than women with other genotypes (confidence interval, 1.017 to 6.151; P <.001). The prevalence of this genotype was significantly higher in African American women (47%) compared with white (19%) or Hispanic (30%) women (P = .003). Myometrial cell lines expressing the Val/Val genotype exhibited significantly enhanced responses to estrogen in proliferation and in estrogen-responsive element reporter assays. COMT-specific inhibitors reversed such a response and induced apoptosis. Myometrial specimens from Val/Val women demonstrated distinct estrogen-regulated gene expression that was consistent with enhanced proliferation and decreased apoptosis.

CONCLUSIONS

The high-activity COMT Val/Val genotype is associated with increased risk of ULM. Our results provide a possible explanation for the higher prevalence of ULMs among African American women and offer a potential new target for nonsurgical treatment using COMT inhibitors.

Expression of steroid receptor coactivators in cultured cells from paired myometrial and fibroid tissues

OBJECTIVES

Fibroid tumor growth in the myometrium appears to be regulated by estrogens but the role of estrogen receptor (ER) coregulators, such as the steroid receptor coactivator (SRC) family members, in fibroid growth is currently unknown. The aims of this study were to compare the expression of the SRC-1, SRC-2, and SRC-3 coactivators between fibroids and normal myometrium in pure populations of cultured smooth muscle cells (SMC) and microvascular endothelial cells (MEC), and also between both cell types, and to identify any relationship between the SRC expression profiles and the known ER status of the SMC and MEC samples examined in this study.

METHODS

Reverse transcriptase-polymerase chain reaction (RT-PCR) coupled with Southern blot analysis was used to derive a semiquantitative estimate of the relative levels of SRC-1, SRC-2, and SRC-3 expression in pure populations of SMC (>98% alpha-smooth muscle actin [SMA](+)) and MEC (>99% CD31(+)) isolated and cultured from eight samples of paired human myometrial and fibroid tissue.

RESULTS

The mean levels of SRC-1, SRC-2, and SRC-3 were each similar in normal myometrium compared to fibroids for SMC and also for MEC. However, SRC-1, SRC-2, and SRC-3 levels were each significantly higher in SMC compared to MEC from both myometrial and fibroid samples, although for SRC-3 there was a trend for higher levels in myometrial samples that did not reach significance. While all SMC samples expressed ERalpha and high coactivator levels, there does not appear to be a relationship between coactivator expression levels and the presence or absence of ERalpha in MEC samples.

CONCLUSION

Coactivators may be more important in ERalpha-mediated growth of SMC than for MEC. Although the SRC family members are likely to play a role in the response of fibroid SMC to estrogen, via ERalpha, changes in their levels do not appear to contribute to the increased sensitivity of fibroid SMC to estrogen.

In vitro culture significantly alters gene expression profiles and reduces differences between myometrial and fibroid smooth muscle cells

Cultured myometrial (M) and fibroid (F) smooth muscle cells (SMCs) have been widely used as a model for the study of F growth. The aim of this study was to compare gene expression profiles using microarrays between six paired M and F tissues from hysterectomy specimens, as well as cells isolated from the same tissues and cultured for up to three passages. A total of 2055 genes were differentially expressed by ANOVA between all experimental groups. Among them, 128 genes were found to be statistically different between M and F tissues. More than 1100 genes were significantly changed between tissues and cultured cells, with 648 genes common between both M and F cells at P0 and P3. Expression profiles of six genes including estrogen receptor-alpha (ERalpha) and progesterone receptor (PR) were also validated using real-time PCR. These data demonstrate that large changes occur in SMC gene expression in culture, reducing differences between M and F cells. They also show that ERalpha and PR levels are reduced in cells compared with whole tissue. These results indicate that although M and F cell cultures provide an important tool to study these tumours, in vitro studies must be carefully planned and evaluated to provide meaningful results.

Etiology and pathogenesis of uterine leiomyomas: a review

Uterine leiomyomas, or fibroids, represent a major public health problem. It is believed that these tumors develop in the majority of American women and become symptomatic in one-third of these women. They are the most frequent indication for hysterectomy in the United States. Although the initiator or initiators of fibroids are unknown, several predisposing factors have been identified, including age (late reproductive years), African-American ethnicity, nulliparity, and obesity. Nonrandom cytogenetic abnormalities have been found in about 40% of tumors examined. Estrogen and progesterone are recognized as promoters of tumor growth, and the potential role of environmental estrogens has only recently been explored. Growth factors with mitogenic activity, such as transforming growth factor- (subscript)3(/subscript), basic fibroblast growth factor, epidermal growth factor, and insulin-like growth factor-I, are elevated in fibroids and may be the effectors of estrogen and progesterone promotion. These data offer clues to the etiology and pathogenesis of this common condition, which we have analyzed and summarized in this review.

An in vivo/in vitro model to assess endocrine disrupting activity of xenoestrogens in uterine leiomyoma

Xenoestrogens with endocrine disrupting activity have been associated with the dysregulation of reproductive function and promotion of malignancies in experimental animals and human populations. The high incidence of uterine leiomyomas, a benign estrogen-responsive neoplasm of the uterine myometrium, calls into question the potential influence of xenoestrogens in the pathogenesis of these tumors. An in vivo/in vitro animal model, the Eker rat, that can be used to assess the estrogen-like agonist activity of potential endocrine disruptors in the uterine myometrium is discussed. Using this model, several in vitro assays are developed that demonstrate that compounds from three major classes of xenoestrogens can mimic the effect of estrogen on leiomyoma cells and act as estrogen receptor (ER) agonists: phytoestrogens, organochlorine pesticides and pharmacologic agents. These compounds can stimulate transcription via the ER and upregulate the expression of an estrogen-responsive gene in uterine leiomyoma cells. The use of these in vitro assays has also advanced our ability to predict the agonist activity of potential therapeutic agents in the uterine myometrium. Selective estrogen receptor modulators (SERMs), while able to act as agonists in some tissues such as the bone and uterine endometrium, act as antagonists in vivo in the uterine myometrium and in our in vitro assays. This antagonist activity in the uterine myometrium suggests that SERMs may be useful in the treatment of uterine leiomyoma.

17-beta estradiol elicits an autocrine leiomyoma cell proliferation: evidence for a stimulation of protein kinase-dependent pathway

The mechanism by which estradiol (E2) acts on cell proliferation is still unclear. In this paper, we report the results of a series of experiments in an attempt to elucidate the effector pathway(s) involved in coupling the E2 receptors binding to cellular growth response in leiomyoma cells (LSMC). Under conditions of E2-dependent growth, E2 treatment of LSMC triggers rapid and transient activation of the MAP-kinase pathway. Interestingly, we demonstrate that the early downstream signal transduction events determined by E2-stimulation in quiescent LSMC, including the rapid protein tyrosine phosphorylation of a subset of intracellular proteins, such GAP, PI-3-K, and PLCgamma, and the concomitant activation of ancillary protein kinases, are related to E2-induced PDGF secretion. Moreover, we identify the PDGF, alone or in association with other growth factors, as the main growth factor involved in the proliferation response of LSMC to E2 stimulation. The addition of neutralizing antibodies anti-PDGF was able to inhibit the mitogenic activity present in LSMC conditioned media samples. On the other hand, E2 did not affect the constitutive expression as well as the ligand affinity of PDGF receptors on LSMC plasmamembrane. Cell treatment with the antiestrogen ICI 182780 correlate both with a perturbation of E2-induced transductional circuit and with the disappearance of the mitogenic factor, PDGF, in LSMC conditioned media; the latter therefore, represents the main autocrine mediator of cell growth modulation, upregulated by E2 and down-regulated by antiestrogenic compound. Our experiments suggest that growth factor secretion is an initial and integral part of the signaling events mediated by the estradiol receptors, not related, at least in part, to E2 transcriptional modulation.

Localization and expression of the human estrogen receptor beta gene in uterine leiomyomata

Estrogens have an important function in the natural history of uterine leiomyomata. The human estrogen receptor beta gene (ESR2) has been identified recently and mapped to 14q22-24, a region frequently rearranged in uterine leiomyomata and other benign tumors, including pulmonary chondroid hamartomas and endometrial polyps. Using fluorescence in situ hybridization and radiation hybrid mapping, we map ESR2 within 14q23-24.1, to a region approximately 2 Mb centromeric to the t(12;14) breakpoint in uterine leiomyomata, between markers D14S63 and WI-7536. Two YAC clones, 948B6 and 741H4, contain ESR2. Using RT-PCR, we show that ESR2 is expressed in uterine leiomyomata and pulmonary chondroid hamartomas as well as in normal myometrium. Lack of a direct relationship between rearrangement of 14q23-24 and ESR2 expression suggests that ESR2 is not involved with HMGIC or HMGIY in t(12;14) or t(6;14). However, because of its relatively close physical distance from the characteristic site of rearrangements in 14q23-24, a role for ESR2 in the pathobiology of these tumors warrants future consideration.

Elevated connexin-43 expression in term human myometrium correlates with elevated c-Jun expression and is independent of myometrial estrogen receptors

Just previous to the onset of parturition, a number of genes such as the one that codes for connexin-43 (Cx43) gap junction protein are induced in the myometrium. We have shown previously that activation of protein kinase C in human myometrial cultured cells leads to an up-regulation of cx43 transcription through an activating protein-1 element in the 5'-flanking promoter. Analyses were now performed on extracts of term myometrial tissue to test for an association between the up-regulation of cx43 expression and the expression of transcription factors and steroid hormone receptors that might regulate cx43 expression at term. Immunoblot analyses were performed on extracts of term myometrial tissue from women receiving elective or indicated cesarean sections to test for an association between the up-regulation of cx43 expression and the up-regulation of expression of the transcription factors c-Jun, c-Fos, and Sp1, which have cognate binding elements in the cx43 5'-flanking promoter. Immunoblot analysis, immunohistochemistry, and receptor binding assays were also performed to analyze the levels of progesterone receptors (PR) and estrogen receptors (ER) in the same term myometrial tissue, and these were compared to the levels in nonpregnancy myometrial tissue. The levels of PR were consistently 2- to 3-fold higher in term myometrial tissue than in nonpregnancy values and did not fluctuate during the menstrual cycle as did ER levels. Surprisingly, in term myometrium, ER was barely detectable by immunoblot and had whole cell diffuse staining by immunohistochemistry. In addition, very low levels of estrogen binding were observed in the term myometrial tissue. Treatment of primary myometrial cultures containing ER with estrogen for 3 or 48 h did not result in up-regulation of c-Jun or c-Fos proteins or in trans-activation from the proximal cx43 promoter with the activating protein-1 element. In contrast, an activated form of c-Jun protein was 10- to 18-fold higher in term myometrial tissue that also had elevated cx43 expression compared to c-Jun levels in term myometrial tissue with low cx43 expression. Likewise, c-Fos and Sp1 levels were 2-4 fold higher in term myometrial tissue with elevated cx43 expression. Although c-Fos and Sp1 proteins could be detected by immunoblot in myometrial tissue from nonpregnant women, c-Jun and Cx43 proteins could not. In summary, these results suggest that up-regulation of human myometrial cx43 gene expression at term involves induction of primarily c-jun expression through a mechanism that does not directly involve myometrial ER or the loss of PR. Peptide hormones that activate protein kinase cascades, such as the protein kinase C cascade, may be important to signal the onset of labor in humans.