Aging of Brain: Role of Estrogen

The brain undergoes many structural and functional changes during aging. Some of these changes are regulated by estrogens which act mainly through their intracellular receptors, estrogen receptor ERα and ERβ. The expression of these receptors is regulated by several factors including their own ligand estrogen, and others such as growth hormone and thyroid hormone. The levels of these factors decrease during aging which in turn influence estrogen signaling leading to alterations in brain functions. In the present paper, we review the effects of aging on brain structure and function, and estrogen action and signaling during brain aging. The findings suggest key role of estrogen in the maintenance of brain functions during aging.     

Uterine epithelial and eosinophil estrogen receptors in rats during the estrous cycle

Summary In the uterus of the adult female rats, the luminal epithelial cells and the eosinophil leukocytes are rich in cytoplasmic estrogen receptors. During the estrous cycle, the epithelial estrogen receptor concentration reaches its peak level, in proestrus, drops precipitously in estrus, and hits the trough, at metestrus. Repopulation of the cytoplasm with estrogen binding sites occurs during diestrus. This pattern of cyclic change is indicative of a rapid turnover of estrogen receptors in the epithelial cells and its regulation by endogenous estrogens. The concentration of estrogen receptors in the cytoplasm of the eosinophils does not appear to fluctuate during the cycle. But the intrauterine, distribution of these leukocytes is clearly cyclic in pattern, ostensibly influenced by estrogens. While progesterone binding activity is consistently demonstrated in tandem with estrogen receptors in the cytoplasm of the epithelial cells, it has not been observed in the eosinophil leukocytes. These findings support the claim that there are two estrogen receptor systems in the rat uterus, one mediating the intracellular events of the genomic response to estrogens, and the other being concerned with non-genomic responses.Abbreviations BSA Bovine serum albumin - FITC fluorescein isothiocyanate - TMRITC tetramethylrhodamine isothiocyanate - CMO O-carboxymethyl oxime     

Assessing the mammary gland of nonhuman primates: effects of endogenous hormones and exogenous hormonal agents and growth factors

This review provides a summary of the normal biology, development, and morphology of the breast in nonhuman primates (macaques), and of the major published work addressing hormonally-induced changes in the breast of these animals. The mammary glands of macaques are anatomically, developmentally, and physiologically similar to the human breast, with similar expression of sex steroid receptors (estrogen receptors alpha and beta, progesterone receptor A and B, androgen receptors), estrogen dependent markers, and steroid metabolizing enzymes. Genetic similarity between human beings and macaques is high, varying from 95-99% depending on the sequence evaluated. Macaques develop hyperplastic and cancerous lesions of the breast spontaneously, which are similar in type and prevalence to those of human beings. They have a reproductive physiology typical of anthropoid primates, including a distinct menarche and menopause, and a 28-day menstrual cycle. These similarities give unique value to the macaque model for evaluation of the effectiveness and safety of hormonal agents. Such agents considered in this review include estrogens and progestogens, combined therapies such as oral contraceptives and post-menopausal hormone therapies, androgens, selective estrogen receptor modulators, phytoestrogens, prolactin, somatotropin, epidermal growth factor, and other novel agents with hormonal or growth factor-like activity. This review also includes a consideration of selected background changes and typical strategies and markers used for evaluation of experimentally-induced changes, including biopsy-based strategies designed to control for inter-individual variability and minimize numbers of animals used.     

Estrogen resistance and aromatase deficiency in humans

The primordial role of estrogens in female reproductive function is well known. The recent production of transgenic mice deficient in estrogen receptors (ERKO) or in aromatase (ArKO) and the discovery in man of inactivating mutations of the corresponding genes (ER) have contributed to the understanding of the role of estrogens in metabolic processes in female as well as male. To date 8 well documented cases (5 women and 3 men) of congenital deficiencies in estrogens have been reported. As mice deficient in ERa had been previously described, these cases definitely proved that estrogen absence was compatible with survival and disproved the "lethality concept" previously held because the role of estrogens in implantation and gestation maintenance. ERKO mice are phenotypically normal though sterile, but their bone density is lower (20-25%) than that of controls. Similarly, men with no aromatase or no ER display continuous growth, osteoporosis and also (but not necessarily) alterations in testicular functions. How much do primordial functions such as bone development, control of gonadotrophin secretion and lipid metabolism depend on estrogens? These interrogations, elegantly clarified following testosterone and estradiol treatment in an aromatase deficient man are considered in this present synthesis.     

Challenges to defining a role for progesterone in breast cancer

Progesterone is an ovarian steroid hormone that is essential for normal breast development during puberty and in preparation for lactation. The actions of progesterone are primarily mediated by its high affinity receptors, including the classical progesterone receptor (PR) -A and -B isoforms, located in diverse tissues such as the brain where progesterone controls reproductive behavior, and the breast and reproductive organs. Progestins are frequently prescribed as contraceptives or to alleviate menopausal symptoms, wherein progestin is combined with estrogen as a means to block estrogen-induced endometrial growth. Estrogen is undisputed as a potent breast mitogen, and inhibitors of the estrogen receptor (ER) and estrogen producing enzymes (aromatases) are effective first-line cancer therapies. However, PR action in breast cancer remains controversial. Herein, we review existing evidence from in vitro and in vivo models, and discuss the challenges to defining a role for progesterone in breast cancer.     

Pregnenolone as a potential candidate for hormone therapy for female reproductive disorders targeting ERβ

Many steroid hormones such as estrogen (E2) bind to their receptors for the regulation of biological processes. Pregnenolone (P5) is the precursor form of almost all steroid hormones and is often used to treat skin disorders and neurological complications. However, the mechanism and physiological function of P5 in reproductive organs are not well established. In this study, we investigated the effects of P5 on activation and expression of E2 receptor (ER) in the uteri and ovaries. To study the mechanism of P5 directly, Ishikawa cells were transfected with E2 response element (ERE)‐luciferase plasmid and isoforms of ER. ERE‐luciferase activity induced by P5 was similar to that induced by E2, and P5 showed high activity for ERβ without any relevance to P5‐metabolizing hormones such as progesterone (P4) and E2. In an animal study, immature female rats treated with P5 showed upregulation of ERα and downregulation of ERβ in the uteri, which is the main organ expressing ERα. In ERβ‐expressing organ ovaries, estrogen receptor 1, estrogen receptor 2, and P4 receptor were all downregulated by P5 and E2. Also, a decrease of ovarian cell proliferation and viability was observed in response to P5 relative to the control, suggesting that P5 may be a candidate for antiproliferative hormone of ovarian cancer. These findings suggest that P5 stimulates ERE promoter by ERβ‐mediated signaling in the uteri and ovaries. Activation of ERβ by P5 may help in understanding the mechanism of ER‐related female reproductive diseases such as endometriosis and ovarian cancer.     

Role of endocytosis in cellular uptake of sex steroids

Androgens and estrogens are transported bound to the sex hormone binding globulin (SHBG). SHBG is believed to keep sex steroids inactive and to control the amount of free hormones that enter cells by passive diffusion. Contrary to the free hormone hypothesis, we demonstrate that megalin, an endocytic receptor in reproductive tissues, acts as a pathway for cellular uptake of biologically active androgens and estrogens bound to SHBG. In line with this function, lack of receptor expression in megalin knockout mice results in impaired descent of the testes into the scrotum in males and blockade of vagina opening in females. Both processes are critically dependent on sex-steroid signaling, and similar defects are seen in animals treated with androgen- or estrogen-receptor antagonists. Thus, our findings uncover the existence of endocytic pathways for protein bound androgens and estrogens and their crucial role in development of the reproductive organs.     

Dehydroepiandrosterone stimulates the estrogen response element

Studies suggest that the steroid, dehydroepiandrosterone (DHEA) can exert effects directly, in addition to its indirect role serving as a precursor for other steroids such as androgens and estrogens. Because DHEA is one of the most abundant adrenal steroids secreted in man, we investigated the functional activity of DHEA on the classic estrogen response element (ERE) in the presence of the estrogen receptor (ER) in transiently transfected cells. GT1-7 hypothalamic neuronal cells, devoid of the estrogen receptor, were transiently transfected with the estrogen receptor expression plasmid (HEGO) and the estrogen response element luciferase (ERELUC) reporter vector. As expected, a dose-response stimulation of luciferase activity was observed in cells treated with estradiol. Concentrations of estradiol from 10⁻¹⁰–10⁻⁶ M resulted in a 136–195 percent increase in luciferase activity compared with control. A dose-response stimulation was also observed in the cells treated with DHEA. A maximum stimulation of 177 percent increase in luciferase activity compared with control was observed with DHEA at a concentration of 10⁻⁵ M. Both the estradiol and DHEA stimulation of ERE luciferase activity was inhibited by the estrogen receptor antagonist, ICI 182,780. The aromatase inhibitor, formestane in combination with estradiol or DHEA had no effect on luciferase activity, suggesting that the effect of DHEA is independent of its conversion to estadiol. Estradiol levels, as measured by ELISA, were appropriately elevated in the estradiol-treated cells but were not significantly different from the control cells in the DHEA-treated cells. These studies suggest a functional in vitro role of DHEA in activating the ERE in the presence of the classic ER.     

Identification of estrogen target genes in human neural cells

In mammals, estrogens have a multiplicity of effects ranging from control of differentiation of selected brain nuclei, reproductive functions, sexual behavior. In addition, these hormones influence the manifestation of disorders like depression and Alzheimer's. Study of the cells target for the hormone has shown that estrogen receptors (ERs) are expressed in all known neural cells, including microglia. In view of the potential interest in the use of estrogens in the therapy of several pathologies of the nervous system, it would be of interest to fully understand the mechanism of estrogen activity in the various neural target cells and get an insight on the molecular means allowing the hormone to display such a variety of effects. We have proposed the use of a reductionist approach for the systematic understanding of the estrogen activities in each specific type of target cell. Thus, we have generated a model system in which to study the activation of one of the known (ERs), estrogen receptor alpha. This system allowed us to identify a number of novel genes which expression may be influenced following the activation of this receptor subtype by estradiol (E₂). We here report on data recently obtained by the study of one of these target genes, nip2, which encodes a proapoptotic protein product. We hypothesize that nip2 might be an important molecular determinant for estrogen anti-apoptotic activity in cells of neural origin and represents a potential target for drugs aimed at mimicking the E₂ beneficial effects in neural cells.     

Regulation of uterine epidermal growth factor (EGF) receptors by estrogen in the mature rat and during the estrous cycle

Previous work has shown that the immature rat uterus contains epidermal growth factor (EGF) receptors and that tissue levels of this receptor are increased by the administration of exogenous estrogens. This study was undertaken to determine if estrogen administration also elevated EGF receptor levels in the mature animal and if the growth factor receptor levels varied in concert with endogenous estrogens throughout the estrous cycle. In the mature, castrate rat administration of estradiol, but not non-estrogenic steroids, causes a 2-3-fold elevation of uterine EGF receptors as judged by ligand binding. This increase is maximum in 18 h and is due to an increase in the number of binding sites. In cycling animals EGF receptor levels are low at metestrus, rise at diestrus, reach a maximum (approximately twice metestrus values) at proestrus, and then return at estrus to metestrus levels. These changes in EGF receptor levels parallel changes in plasma estrogens and occupied nuclear estrogen receptor reported by other workers. These results indicate that uterine EGF receptors are increased by exogenous estrogens in both mature and immature animals, and support a physiological role for estrogens in the regulation of this growth factor receptor.     

Estrogen-mediated regulation of CYP7B1: a possible role for controlling DHEA levels in human tissues

The current study examines regulation of CYP7B1, a DHEA 7alpha-hydroxylase, by sex hormones. Transfection with estrogen receptor alpha and treatment with 17beta-estradiol in human embryonic kidney 293 cells significantly increased CYP7B1 catalytic activity and mRNA, and stimulated a human CYP7B1 reporter gene. Transfection with estrogen receptor beta showed similar but less significant effects. In the absence of receptors, 17beta-estradiol suppressed CYP7B1 activity, suggesting that estrogenic effects may be different in cells not expressing receptors. Quantitation of CYP7B1 mRNA in adult and fetal human tissues showed markedly higher CYP7B1 mRNA levels in fetal tissues compared with the corresponding adult ones, except in the liver. This indicates a tissue-specific, developmental regulation of CYP7B1 and suggests an important function for this enzyme in fetal life. DHEA secreted by fetal adrenals is an essential precursor for placental estrogen formation. Since CYP7B1 diverts DHEA from the sex hormone biosynthetic pathway, estrogen receptor-mediated up-regulation of CYP7B1 should lead to less DHEA available for sex hormone synthesis and may help to maintain normal levels of estrogens and androgens in human tissues, especially during fetal development. Regulation by estrogens may also be of importance in other processes where CYP7B1 is involved, including cholesterol homeostasis, cellular proliferation, and CNS function.