The novel untranslated exon "exon 0T" encoded between the exon 0 and exon 1 of the rat estrogen receptor alpha (ER alpha) gene

Uterine ERα epigenetic modifications are induced by the endocrine disruptor endosulfan in female rats with impaired fertility

High ERα activity may disrupt the window of uterine receptivity, causing defective implantation. We investigated whether implantation failures prompted by endosulfan are associated with aberrant ERα uterine expression and DNA methylation status during the pre-implantation period. ERα-dependent target genes that play a crucial role in the uterine receptivity for embryo attachment and implantation were also investigated. Newborn female rats received corn oil (vehicle, Control), 6 μg/kg/d of endosulfan (Endo6) or 600 μg/kg/d of endosulfan (Endo600) on postnatal days (PND) 1, 3, 5, and 7. On PND90, females were made pregnant and on gestational day 5 (GD5, pre-implantation period) uterine samples were collected. ERα expression was assessed at protein and mRNA levels by immunohistochemistry and real time RT-PCR, respectively. ERα transcript variants mRNA containing alternative 5'-untranslated regions (5'UTRs) were also evaluated. We searched for predicted transcription factors binding sites in ERα regulatory regions and assessed their methylation status by Methylation-Sensitive Restriction Enzymes-PCR technique (MSRE-PCR). The expression of the ERα-dependent uterine target genes, i.e. mucin-1 (MUC-1), insulin-like growth factor-1 (IGF-1), and leukemia inhibitory factor (LIF), was assessed by real time RT-PCR. Both doses of endosulfan increased the expression of ERα and its transcript variants ERα-OS, ERα-O, ERα-OT and ERα-E1. Moreover, a decreased DNA methylation levels were detected in some ERα regulatory regions, suggesting an epigenetic up-regulation of it transcription. ERα overexpression was associated with an induction of its downstream genes, MUC-1 and IGF-1, suggesting that endosulfan might alter the uterine estrogenic pathway compromising uterine receptivity. These alterations could account, at least in part, for the endosulfan-induced implantation failures.

Alteration of Mammary Gland Development and Gene Expression by In Utero Exposure to Cadmium

Environmental exposure to estrogens and estrogen like contaminants during early development is thought to contribute to the risk of developing breast cancer primarily due to an early onset of puberty; however, exposure during key developing windows may also influence the risk of developing the disease. The goal of this study was to ask whether in utero exposure to the metalloestrogen cadmium alters mammary gland development due to acceleration of puberty onset or to an effect on early development of the mammary gland. The results show that, in addition to advancing the onset of puberty, in utero exposure to the metalloestrogen cadmium altered mammary gland development prior to its effect on puberty onset. In utero exposure resulted in an expansion of the number of mammosphere-forming cells in the neonatal mammary gland and an increase in branching, epithelial cells, and density in the prepubertal mammary gland. In the postpubertal mammary gland, there was a further expansion of the mammary stem/progenitor cell population and overexpression of estrogen receptor-alpha (ERα) that was due to the overexpression and altered regulation of the ERα transcripts derived from exons O and OT in response to estradiol. These results suggest that in utero exposure to cadmium increases stem/progenitor cells, cell density, and expression of estrogen receptor-alpha that may contribute to the risk of developing breast cancer.

Alteration of mammary gland development and gene expression by in utero exposure to arsenic

Early life exposure to estrogens and estrogen like contaminants in the environment is thought to contribute to the early onset of puberty and consequently increases the risk of developing breast cancer in the exposed female. The results of this study show that in utero exposure to the metalloestrogen arsenite altered mammary gland development prior to its effect on puberty onset. In the prepubertal gland, in utero exposure resulted in an increase in the number of mammosphere-forming cells and an increase in branching, epithelial cells, and density. In the postpubertal gland, in utero exposure resulted in the overexpression of estrogen receptor-alpha (ERα) that was due to the increased and altered response of the ERα transcripts derived from exons O and OT to estradiol. These results suggest that, in addition to advancing puberty onset, in utero exposure to arsenite alters the pre- and postpubertal development of the mammary gland and possibly, the risk of developing breast cancer.

Metals and breast cancer

Metalloestrogens are metals that activate the estrogen receptor in the absence of estradiol. The metalloestrogens fall into two subclasses: metal/metalloid anions and bivalent cationic metals. The metal/metalloid anions include compounds such as arsenite, nitrite, selenite, and vanadate while the bivalent cations include metals such as cadmium, calcium, cobalt, copper, nickel, chromium, lead, mercury, and tin. The best studied metalloestrogen is cadmium. It is a heavy metal and a prevalent environmental contaminant with no known physiological function. This review addresses our current understanding of the mechanism by which cadmium and the bivalent cationic metals activate estrogen receptor-α. The review also summarizes the in vitro and in vivo evidence that cadmium functions as an estrogen and the potential role of cadmium in breast cancer.

Novel splicing events and post-transcriptional regulation of human estrogen receptor α E isoforms

Expression of the estrogen receptor α (ERα) gene is subject to complex regulation. To elucidate the mechanisms of this regulation, the genomic organization and the physiological role of the multiple 5'-untranslated regions (5'-UTRs) must be determined. Here, we investigated the expression and splicing patterns of the human ERα E isoforms. We identified two novel untranslated exons, N1 and N2, in the 5'-region of the human ERα gene and multiple E isoform mRNA variants generated by alternative usage of non-coding internal exons. Expression of the N1-containing variants was observed only in the human breast adenocarcinoma cell line, MCF7, while the N2-containing variants were expressed in the adult liver and MCF7 cells. We examined post-transcriptional regulation of the variant mRNAs using luciferase reporter assays and quantitative PCR. The insertion of untranslated internal exons into the 5'-UTRs of the E isoforms reduced their translation efficiency, but barely influenced mRNA turnover. Our results indicate that the genomic organization of the human ERα gene and the splicing profiles of the human ERα E isoforms are more complicated than previously reported. Furthermore, the 5'-UTRs of the E isoforms post-transcriptionally control human ERα expression mainly through translational repression.

Sexually dimorphic expression of receptor-alpha in the cerebral cortex of neonatal Caiman latirostris (Crocodylia: Alligatoridae)

In mammals, estrogens have been described as endocrine and paracrine modulators of neuronal differentiation and synapse formation. However, the functional role of circulating estrogens and the distribution of estrogen receptors (ERs) in the cerebral cortex of reptiles have not been clearly established. Caiman latirostris (C. latirostris) is a South American species that presents temperature-dependent sex determination (TSD). By using immunohistochemistry, we have studied the distribution of ERα in the cerebral cortex of neonatal caimans. We studied brain samples from ten-day-old TSD-females and TSD-males and from female caimans that were administered estradiol during embryonic development (hormone-dependent sex determination, HSD-females). ERα was detected in the medial (MC), dorsal (DC) and lateral (LC) cortices. ERα expression in the MC showed sex-associated differences, being significantly greater in TSD-females compared to TSD-males. Interestingly, the highest ERα expression in the MC was exhibited by HSD-females. In addition, the circulating levels of estradiol were significantly higher in females (both TSD and HSD) than in TSD-males. Double immunostaining showed that ERα is expressed by neural precursor cells (as detected by ERα/doublecortin or ERα/glial fibrillary acidic protein) and mature neurons (ERα/neuron-specific nuclear protein). Our results demonstrate that the expression of ERα in the neonatal caiman cortex is sexually dimorphic and is present in the early stages of neuronal differentiation.

Complex organization of the 5'-untranslated region of the mouse estrogen receptor α gene: identification of numerous mRNA transcripts with distinct 5'-ends

The 5'-untranslated region (5'-UTR) of the estrogen receptor α (ERα) gene plays an important role in determining its tissue-specific expression. We examined the 5'-UTRs of the mouse ERα mRNA variants in depth using the Basic Local Alignment Search Tool (BLAST), rapid amplification of 5'-cDNA ends (5'-RACE) and RT-PCR. We demonstrated the presence of multiple variants containing unique 5'-UTRs. We mapped the cDNA sequences onto the mouse genome, and found that both alternative splicing from four different leader exons (A, C, F1, and H) to exon 1, and combinations of 12 internal exons (X1, X2, X3, X4, F2/X5, X6, X7, X8, X9, X10, X11, and B) generate multiple ERα transcripts. Mouse exon B, that has homologies with human exon B and rat exon 0T, was used as an internal exon, not as a leader exon. RT-PCR analysis revealed distinct expression patterns of the variants, suggesting that the alternative promoter usage and alternative splicing are regulated in a tissue-specific manner. Our results indicate that the genomic organization of the mouse ERα gene is complicated as previously shown in the rat ERα gene. In addition, both alternative promoter usage and alternative splicing contribute to the remarkable mRNA diversity of the mouse ERα gene.

Estrogen receptor {alpha} gene promoter 0/B usage in the rat sexually dimorphic nucleus of the preoptic area

The volume of the sexually dimorphic nucleus of the preoptic area (SDN-POA) is two to four times larger in male rats than in females; however, the mechanism for the establishment of sexual dimorphism and the function of this nucleus is almost unknown. Perinatal estrogen can cause sexual dimorphism via the estrogen receptor alpha (ERalpha). Recently, transgenic rats were generated that express enhanced green fluorescent protein (EGFP) under the control of the ERalpha gene promoter 0/B to tag ERalpha-positive neurons in the brain. In the present study, we examined whether this EGFP expression could be a marker for the SDN-POA in adults. EGFP-labeled cells were distributed in the core of the SDN-POA (0/B-SDN) of male and female transgenic rats, in accordance with the Nissl staining and immunoreactivity for the SDN marker, calbindin. They were also immunoreactive for ERalpha. The core was bigger in volume and contained more 0/B-SDN neurons in males than in females. The EGFP-tagged cells were packed more densely in the female core than that in males. Subcutaneous injection of 100 mug 17beta-estradiol to females on the day of birth, or orchidectomy of male neonates, reversed the sexually dimorphic phenotype of the volume of the 0/B-SDN, despite not affecting the cell number. We suggest that this EGFP expression in the SDN-POA could be a useful marker to clarify the sexual differentiation and function of the SDN-POA. Moreover, the ERalpha gene promoter 0/B plays a key role in the organization of the sexual differentiation of the SDN-POA.

Alternative promoter usage and alternative splicing of the rat estrogen receptor alpha gene generate numerous mRNA variants with distinct 5'-ends

The 5'-untranslated region (UTR) of the estrogen receptor alpha (ERalpha) gene plays an important role in determining tissue-specific expression. To elucidate the regulatory mechanisms of rat ERalpha gene expression, the genomic organization must be investigated. We therefore analyzed the structure of the rat ERalpha mRNA 5'-UTR using rapid amplification of 5'-cDNA ends (5'-RACE) and RT-PCR. The analysis showed the presence of multiple variants containing unique 5'-UTRs. We mapped the cDNA sequences on the rat genome, and newly identified one leader exon (exon 0U) and ten untranslated internal exons (exons I1-10). Both splicing from four different leader exons (exons 0S, 0N, 0U, and 0/B) onto exon 1 and alternative splicing in combination with eleven internal exons (exons I1-10, and 0T) produce multiple transcripts. RT-PCR analysis revealed that each variant had preferred expression sites, suggesting that promoter usage and splicing are regulated in tissue-specific manners. Moreover, we determined a splicing event to yield Deltaexon 1 variants (0S-2-3-4-5-6-7-8), which are translated into rat 46 kDa ERalpha proteins. Our results indicate that the rat ERalpha gene is more complex than previously thought in terms of genomic organization and that both alternative promoter usage and alternative splicing contribute to the remarkable diversity of ERalpha mRNAs.

Dynamic regulation of estrogen receptor-alpha gene expression in the brain: a role for promoter methylation?

Estrogen has long been known to play an important role in coordinating the neuroendocrine events that control sexual development, sexual behavior and reproduction. Estrogen actions in other, non-reproductive areas of the brain have also been described. It is now known that estrogen can also influence learning, memory, and emotion and has neurotrophic and neuroprotective properties. The actions of estrogen are largely mediated through at least two intracellular estrogen receptors. Both estrogen receptor-alpha and estrogen receptor-beta are expressed in a wide variety of brain regions. Estrogen receptor-alpha (ERalpha), however, undergoes developmental and brain region-specific changes in expression. The precise molecular mechanisms that regulate its expression at the level of gene transcription are not well understood. Adding to the complexity of its regulation, the estrogen receptor gene contains multiple promoters that drive its expression. In the cortex in particular, the ERalpha mRNA expression is dynamically regulated during postnatal development and again following neuronal injury. Epigenetic modification of chromatin is increasingly being understood as a mechanism of neuronal gene regulation. This review examines the potential regulation of the ERalpha gene by such epigenetic mechanisms.