Polycystic ovary syndrome (PCOS) pathophysiology is poorly understood, due partly to lack of PCOS animal models fully recapitulating this complex disorder. Recently, a PCOS rat model using letrozole (LET), a nonsteroidal aromatase inhibitor, mimicked multiple PCOS phenotypes, including metabolic features absent in other models. Given the advantages of using genetic and transgenic mouse models, we investigated whether LET produces a similar PCOS phenotype in mice. Pubertal female C57BL/6N mice were treated for 5 wk with LET, which resulted in increased serum testosterone and normal diestrus levels of estradiol, similar to the hyperandrogenemia and follicular phase estrogen levels of PCOS women. As in PCOS, ovaries from LET mice were larger, polycystic, and lacked corpora lutea versus controls. Most LET females were acyclic, and all were infertile. LET females displayed elevated serum LH levels and higher Lhb mRNA in the pituitary. In contrast, serum FSH and Fshb were significantly reduced in LET females, demonstrating differential effects on gonadotropins, as in PCOS. Within the ovary, LET females had higher Cyp17, Cyp19, and Fsh receptor mRNA expression. In the hypothalamus, LET females had higher kisspeptin receptor mRNA expression but lower progesterone receptor mRNA levels. LET females also gained more weight than controls, had increased abdominal adiposity and adipocyte size, elevated adipose inflammatory mRNA levels, and impaired glucose tolerance, mirroring the metabolic phenotype in PCOS women. This is the first report of a LET paradigm in mice that recapitulates both reproductive and metabolic PCOS phenotypes and will be useful to genetically probe the PCOS condition.
Kisspeptin is a product of the Kiss1 gene and is expressed in the forebrain. Neurons that express Kiss1 play a crucial role in the regulation of pituitary luteinizing hormone secretion and reproduction. These neurons are the direct targets for the action of estradiol-17 (E 2 ), which acts via the estrogen receptor ␣ isoform (ER␣) to regulate Kiss1 expression. In the arcuate nucleus (Arc), where the dynorphin gene (Dyn) is expressed in Kiss1 neurons, E 2 inhibits the expression of Kiss1 mRNA. However, E 2 induces the expression of Kiss1 in the anteroventral periventricular nucleus (AVPV). The mechanism for differential regulation of Kiss1 in the Arc and AVPV by E 2 is unknown. ER␣ signals through multiple pathways, which can be categorized as either classical, involving the estrogen response element (ERE), or nonclassical, involving ERE-independent mechanisms. To elucidate the molecular basis for the action of E 2 on Kiss1 and Dyn expression, we studied the effects of E 2 on Kiss1 and Dyn mRNAs in the brains of mice bearing targeted alterations in the ER␣ signaling pathways. We found that stimulation of Kiss1 expression by E 2 in the AVPV and inhibition of Dyn in the Arc required an ERE-dependent pathway, whereas the inhibition of Kiss1 expression by E 2 in the Arc involved ERE-independent mechanisms. Thus, distinct ER␣ signaling pathways can differentially regulate the expression of identical genes across different brain regions, and E 2 can act within the same neuron through divergent ER␣ signaling pathways to regulate different neurotransmitter genes.
In addition to its role in reproduction, estradiol-17β is critical to the regulation of energy balance and body weight. Estrogen receptor α-null (Erα -/-) mutant mice develop an obese state characterized by decreased energy expenditure, decreased locomotion, increased adiposity, altered glucose homeostasis, and hyperleptinemia. Such features are reminiscent of the propensity of postmenopausal women to develop obesity and type 2 diabetes. The mechanisms by which ERα signaling maintains normal energy balance, however, have remained unclear. Here we used knockin mice that express mutant ERα that can only signal through the noncanonical pathway to assess the role of nonclassical ERα signaling in energy homeostasis. In these mice, we found that nonclassical ERα signaling restored metabolic parameters dysregulated in Erα -/-mutant mice to normal or near-normal values. The rescue of body weight and metabolic function by nonclassical ERα signaling was mediated by normalization of energy expenditure, including voluntary locomotor activity. These findings indicate that nonclassical ERα signaling mediates major effects of estradiol-17β on energy balance, raising the possibility that selective ERα agonists may be developed to reduce the risks of obesity and metabolic disturbances in postmenopausal women. IntroductionIn addition to its critical functions as a reproductive hormone, estradiol-17β (E 2 ) plays a vital role in the regulation of energy balance and body weight (1). Estrogen deficiency at menopause is associated with an increased probability of obesity as well as increased risk for the development of type 2 diabetes (2). In experimental animals, reduction of circulating estrogen levels by ovariectomy leads to the development of obesity, which can be reversed or prevented by E 2 treatment (1). The effects of E 2 on energy balance bear many similarities to the actions of leptin and insulin, key molecules involved in energy homeostasis (3, 4). Genetic and pharmacological studies have demonstrated that leptin and insulin act directly on neural networks to modulate energy homeostasis, where the net effect is to decrease food intake and increase energy expenditure (5-9). Both can activate STAT3 in various tissues, and hypothalamic leptin and insulin signaling are known to converge on the PI3K pathway (10-13). Similarly, E 2 is now also known to activate STAT3 as well as PI3K signaling cascades, suggestive of possible cross-talk among these molecules and possibly representing a common neuronal signaling mechanism that may help explain the similarities in their central effects on energy homeostasis (14-17).That these metabolic actions of E 2 are mediated by estrogen receptor α (ERα) has been demonstrated in Erα-null (Erα -/-) mutant mice, in which the ablation of ERα signaling results in a metabolic syndrome characterized by increased body weight, adiposity, altered glucose homeostasis, decreased energy expenditure, hyperinsulinemia, and hyperleptinemia (18)(19)(20). However, these metabolic character-
Ovarian estrogen exerts both positive and negative feedback control over luteinizing hormone (LH) secretion during the ovulatory cycle. Estrogen receptor (ER) ␣ but not ER knockout mice lack estrogen feedback. Thus, estrogen feedback appears to be primarily mediated by ER␣. However, it is now recognized that, in addition to binding to estrogen response elements (EREs) in DNA to alter target gene transcription, ER␣ signals through ERE-independent or nonclassical pathways, and the relative contributions of these pathways in conveying estrogen feedback remain unknown. Previously we created a knockin mouse model expressing a mutant form of ER␣ (AA) with ablated ERE-dependent but intact ERE-independent activity. Breeding this allele onto the ER␣-null (؊/؊) background, we examine the ability of ERE-independent ER␣ signaling pathways to convey estrogen feedback regulation of the female hypothalamic-pituitary axis in vivo. ER␣ ؊/AA exhibited 69.9% lower serum LH levels compared with ER␣ ؊/؊ mice. Additionally, like wild type, ER␣ ؊/AA mice exhibited elevated LH after ovariectomy (OVX). Furthermore, the post-OVX rise in serum LH was significantly suppressed by estrogen treatment in OVX ER␣ ؊/AA mice. However, unlike wild type, both ER␣ ؊/AA and ER␣ ؊/؊ mice failed to exhibit estrous cyclicity, spontaneous ovulation, or an afternoon LH surge response to estrogen. These results indicate that ERE-independent ER␣ signaling is sufficient to convey a major portion of estrogen's negative feedback actions, whereas positive feedback and spontaneous ovulatory cyclicity require ERE-dependent ER␣ signaling.neuroendocrine ͉ reproduction
Estrogen receptor alpha (ERα) mediates estrogen (E 2 ) actions in the brain and is critical for normal reproductive function and behavior. In the classical pathway, ERα binds to estrogen response elements (EREs) to regulate gene transcription. ERα can also participate in several non-classical pathways, including ERE-independent gene transcription via protein-protein interactions with transcription factors and rapid, non-genotropic pathways. To distinguish between ERE-dependent and ERE-independent mechanisms of E 2 action in vivo, we have created ERα null mice that possess an ER knock-in mutation (E207A/G208A; "AA"), in which the mutant ERα cannot bind to DNA but retains activity in ERE-independent pathways (ERα −/AA mice). Understanding the molecular mechanisms of ERα action will be helpful in developing pharmacological therapies that differentiate between ERE-dependent and -independent processes. This review focuses on how the ERα −/AA model has contributed to our knowledge of ERα signaling mechanisms in estrogen regulation of the reproductive axis and sexual behavior. Keywordsestrogen receptor alpha; estrogen response element; non-classical signaling; negative feedback; sexual behaviorThe biological effects of estrogens are mediated through at least two distinct nuclear receptors, ERα and ERβ, which belong to the nuclear hormone receptor superfamily (Mangelsdorf et al., 1995). In the classical pathway of estrogen action, E 2 binds to ER, inducing conformational changes within the receptor that promote dimerization and interaction with coactivator and corepressor molecules. The ligand-receptor complex binds with high affinity to specific estrogen response elements (EREs) in the regulatory regions of target genes to either activate or repress gene expression (Glass, 1994;McKenna et al
During the female reproductive cycle, the neuroendocrine action of estradiol switches from negative feedback to positive feedback to initiate the preovulatory GnRH and subsequent LH surges. Estrogen receptor-alpha (ERalpha) is required for both estradiol negative and positive feedback regulation of LH. ERalpha may signal through estrogen response elements (EREs) in DNA and/or via ERE-independent pathways. Previously, a knock-in mutant allele (ERalpha-/AA) that selectively restores ERE-independent signaling onto the ERalpha-/- background was shown to confer partial negative but not positive estradiol feedback on serum LH. The current study investigated the roles of the ERE-dependent and ERE-independent ERalpha pathways for estradiol feedback at the level of GnRH neuron firing activity. The above ERalpha genetic models were crossed with GnRH-green fluorescent protein mice to enable identification of GnRH neurons in brain slices. Targeted extracellular recordings were used to monitor GnRH neuron firing activity using an ovariectomized, estradiol-treated mouse model that exhibits diurnal switches between negative and positive feedback. In wild-type mice, GnRH neuron firing decreased in response to estradiol during negative feedback and increased during positive feedback. In contrast, both positive and negative responses to estradiol were absent in GnRH neurons from ERalpha-/- and ERalpha-/AA mice. ERE-dependent signaling is thus required to increase GnRH neuron firing to generate a GnRH/LH surge. Furthermore, ERE-dependent and -independent ERalpha signaling pathways both appear necessary to mediate estradiol negative feedback on serum LH levels, suggesting central and pituitary estradiol feedback may use different combinations of ERalpha signaling pathways.
The estrogen receptor-alpha (ERalpha) acts through multiple pathways, including estrogen response element (ERE)-dependent (classical) and ERE-independent (nonclassical) mechanisms. We previously created a mouse model harboring a two-amino-acid mutation of the DNA-binding domain (E207A, G208A) that precludes direct binding of ERalpha to an ERE. After crossing heterozygous mutant mice with an ERalpha knockout (ERKO) line, it was possible to assess the degree of physiological rescue by the isolated ERalpha nonclassical allele (-/AA; AA) when compared with ERKO mice (-/-) and to wild type (+/+; WT). In male ERKO mice up to 8 months of age, testosterone levels were high, although LH levels were similar to WT. Testosterone was normal in the AA mice, indicating that the AA allele rescues the enhanced testosterone biosynthesis in ERKO mice. Male ERKO mice exhibited distention of the seminiferous tubules as early as 2-3 months of age as a consequence of decreased water resorption in the efferent ducts. By 3-4 months of age, ERKO mice had impaired spermatogenesis in approximately 40% of their tubules, and sperm counts and motility declined in association with the histological changes. In the AA mice, histological defects were greatly reduced or absent, and sperm counts and motility were rescued. Levels of aquaporins 1 and 9, which contribute to water uptake in the efferent ducts, were reduced in ERKO mice and partially or fully rescued in AA mice, whereas another water transporter, sodium-hydrogen exchanger-3, was decreased in both ERKO and AA mice. We conclude that non-ERE-dependent estrogen pathways are sufficient to rescue the defective spermatogenesis observed in ERKO mice and play a prominent role in ERalpha action in the testis, including pathways that regulate water resorption and androgen biosynthesis.
Estrogen receptor (ER)-alpha mediates estradiol (E(2)) actions in the male gonads and brain and is critical for normal male reproductive function. In the classical pathway, ERalpha binds to estrogen response elements (EREs) to regulate gene transcription. ERalpha can also regulate gene transcription independently of EREs via protein-protein interactions with transcription factors and additionally signal via rapid, nongenomic pathways originating at the cell membrane. This study assessed the degree to which ERE-independent ERalpha signaling can rescue the disrupted masculine sexual behaviors and elevated serum testosterone (T) levels that have been shown to result from ERalpha gene deletion. We utilized male ERalpha null mice that possess a ER knock-in mutation (E207A/G208A; AA), in which the mutant ERalpha is incapable of binding to DNA and can signal only through ERE-independent pathways (ERalpha(-/AA) mice). We found that sexual behavior, including mounting, is virtually absent in ERalpha(-/-) and ERalpha(-/AA) males, suggesting that ERE-independent signaling is insufficient to maintain any degree of normal sexual behavior in the absence of ERE binding. By contrast, ERE-independent signaling in the ERalpha(-/AA) mouse is sufficient to restore serum T levels to values observed in wild-type males. These data indicate that binding of ERs to EREs mediates most if not all of E(2)'s effects on male sexual behavior, whereas ERE-independent ERalpha signaling may mediate E(2)'s inhibitory effects on T production.
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