Low Estrogen Exposure and/or Defective Estrogen Signaling Induces Disturbances in Glucose Uptake and Energy Expenditure

Correlations between estrogen signaling and human morbidity seems to be very unclear and difficult topic. Nowadays, the opinions of scientists have been partially changed regarding “estrogen induced diseases”, but the overwhelming literary data are still contradictory. Physiological estrogen levels in healthy premenopausal women supply protection against insulin resistance, hypertension, cardiovascular diseases and malignancies as compared with men of the same age. However, after menopause a decreased ovarian estrogen synthesis will rapidly deteriorate the glucose sensitivity and increases the prevalence of life threatening diseases. New findings on the functions of tissular estrogen receptors (ERs) indicate that estradiol plays important role in the maintenance of glucose homeostasis and energy expenditure. Diverse disorders associated with insulin resistance are usually well treatable by estradiol substitution both in preand postmenopausal women as well as in animal experiments. ERs seem to have balanced interplay in the maintenance of adaptation to the momentarily changing intra and extracellular stimuli. This equilibrium may be shattered in case of a defective estrogen supply or by the derangement of ER signaling pathways. Understanding the crosstalk and interplay between ERs illuminates the fact that there is no good or bad ER isoform, but they construe a complex system so as to achieve an ideal internal milieu. Considering the regulatory effects of ERs on food intake, insulin secretion, glucose uptake and metabolic processes, estrogen administration may be a therapeutic avenue to repair insulin sensitivity in patients with dysmetabolism and diabetes mellitus. Low Estrogen Exposure and/or Defective Estrogen Signaling Induces Disturbances in Glucose Uptake and Energy Expenditure


Introduction
A famous hypothesis of Reaven established a causal association between cellular insulin resistance and compensatory hyperinsulinemia, which were than regarded as basic disorders for several human diseases [1]. Earlier, cardiovascular lesions, hypertension, dyslipidemia, obesity and elevated fasting glucose level were attributed to be the complications of type 2 diabetes. Nevertheless, the sequence of these alterations was frequently inconsequent and contradictory. The new theory revealed that many divergent symptoms and findings all have a common soil; the defective glucose uptake of mammalian cells [2].
Actions of female sexual steroids are much wider than having crucial roles in female physiology and reproduction [3]. Physiological estrogen levels in healthy premenopausal women supply protection against insulin resistance, hypertension, cardiovascular diseases and malignancies as compared with men of the same age [4,5]. However, after menopause a decreased ovarian estrogen synthesis will rapidly deteriorate the glucose uptake and increases the prevalence of life threatening diseases.
New findings on functions of tissular estrogen receptors indicate that estradiol plays important role in the maintenance of glucose homeostasis and energy expenditure [6]. It beneficially modulates the expression of genes that are involved in insulin secretion, cellular insulin sensitivity and glucose uptake at different sites [6,7]. Nevertheless, there are conflicting data concerning estradiol action in women and its relation to glucose homeostasis and insulin sensitivity in pre-and postmenopausal cases.
Recent studies suppose that a functional imbalance between the signals of estrogen receptor (ER) isoforms and gene polymorphism of ERs may have important implications for the development of dysmetabolism, such as metabolic syndrome and type 2 diabetes [8]. Moreover, hyperestrogenism is erroneously supposed to disrupt the glucose uptake in the adipose tissue mass in obese women and is presumed to contribute to insulin resistance and the associated comorbidities [9,10].
Results of animal experiments support that estradiol administration improves the metabolic functions in insulin resistance and obesity [11,12]. Elevated estrogen level is advantageous against metabolic disorders and decreases the risk for associated diseases. In experimental animals, pregnancy analogue estradiol administration counteracts to the growth of chemically induced or transplanted malignant tumors [13][14][15]. Moreover, high dose estrogen administration as ovulation provocation and good hormonal equilibrium in multiparous women prove to be metabolically advantageous and exhibit anticancer capacities [16].
In human metabolic diseases the beneficial effects of estradiol treatment justify that ERs conduct advantageous intracellular signals if the estrogen supply is sufficient [17]. Conversely, in estrogen deficient states or in case of defective ER signals an increased prevalence of metabolic syndrome, type 2 diabetes and their comorbidities is characteristic [7].
The purpose of this study is to examine the real significance of estradiol supply in cellular metabolic functions. Experimental studies on disabled ER knockout mice help to evaluate the unique and apparently controversial functions of each ER isoform. Nevertheless, understanding the crosstalk and interplay between them illuminates the fact that there is no good or bad ER isoform, but they construe a complex system so as to achieve an ideal internal milieu.

Role of Estradiol and its Receptors in the Regulation of Cellular Glucose Uptake and Energy Homeostasis
The metabolic state of the body is controlled by a central regulation of the brain through signals arriving from the pancreas, liver, adipose tissue, skeletal muscle, and gut [8]. A wide variety of these signals includes hormones (insulin, leptin, adiponectin, etc.), cytokines (TNF-α, IL-6) and nutrients (glucose, free fatty acids, lipids).
Estrogens have pivotal effects on energy metabolism and glucose homeostasis [6]. Two receptor isoforms were identified; estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ), which belong to the steroid-thyroid hormone nuclear receptor supergene family [17,18].
The classic genomic mechanism of estrogen binding activates ERs in the nucleus and they act as transcriptional modulators in the promoter region of target genes. ERs can also regulate gene expression without direct binding to DNA through interaction with transcription factor proteins in the nucleus [3]. Estrogen action has also non-genomic signaling cascades through cell membrane associated ERs [19]. Finally, genomic and non-genomic pathways of estrogen receptor signaling converge on the target genes.
ERα and ERβ regulates in thorough interplay the metabolic processes, the cell growth and the cell proliferation of mammalian cells. They may oppose the activities of each-other eliciting sometimes quite opposite reactions in the presence of estradiol [20], which may be crucial for the dynamism of regulatory mechanisms.
Discovery of selective agonists for ERα and ERβ yielded new possibilities to research the specificity of estrogen receptors [21]. PPT, an ERα selective agonist provoked an increase in cell number nearly 50%, while DPN, an ERβ selective ligand caused a decrease of 30%. Selective activation of each receptor alone increased cell metabolism independently as either mitotic activity or cell growth require enhanced cellular metabolism. However, the simultaneous activation either by estradiol or by PPT-DPN combination produced equilibrium in metabolic activity.
Development of the three knockout mouse lines: estrogen receptor αand β knockout (ERαKO, ERβKO) and aromatase enzyme knockout (ARKO) mice provided new opportunities for understanding the individual actions of estrogen receptor isoforms. ERα knockout mice exhibit insulin resistance, impaired glucose tolerance and obesity affecting both males and females [22]. Ovariectomy of insulin resistant ERα knockout mice (i.e. removal of the action of estradiol on ERβ) improved glucose and insulin metabolism [23], suggesting that ERβ might have a diabetogenic, opposing action against ERα. In the maintenance of normal glucose homeostasis ERα mainly enhances the capacity for glucose uptake, whereas ERβ exerts contraregulatory activity [6].

Role of Estrogen Signaling in the Central Regulation of Metabolism and Energy Homeostasis
The central nervous system may induce metabolic and behavioral changes by anorexigenic and orexigenic stimuli so as to maintain the serum glucose level and energy homeostasis [8]. The hypothalamic nuclei are pivotal regulators of food intake and energy expenditure by means of their estrogen receptors [24].
The main hypothalamic areas involved in feeding behavior are the arcuate nucleus (AN), the lateral hypothalamus (LH), the paraventricular nucleus (PVN), the ventromedial nucleus (VMN) and the dorsomedial nucleus (DMN) [25] (Figure 1) In rats and mice the ERα-selective agonist, PPT, rapidly results in a decrease in food intake [33], whereas silencing of ERα leads to hyperphagia, obesity, decreased glucose tolerance and reduced energy expenditure [34]. In ovariectomized rodents treatment with an ERαspecific ligand (PPT) resulted in an inhibition of eating and reduced body weight.
The exact role of ERβ in the central regulation of feeding is less known [8]. In ovariectomized rats, estradiol and ERβ anti-sense oligodeoxynucleotides (ODN) administered into the third ventricle in the brain reduced the inhibitory effect of estrogen on food intake [35]. This result suggests that in some hypothalamic areas ERβ mediates the central anorexic effect of estradiol.  The hypothalamus has a wide control over insulin, glucocorticoid and gonadal hormone functions, which have important role in defining insulin sensitivity and consequently in peripheral carbohydrate and lipid metabolism [36]. The ratio of male to female sexual hormone levels affects peripheral fat distribution attributed to a modulation of insulin sensitivity. Estrogen deficiency or defective estrogen signaling results in insulin resistance and an increase in the adipose tissue mass, preferentially in visceral location [37].

Estrogen Receptor Actions on Pancreatic Islets and Insulin Production
Estradiol and its receptors are key players in the physiology and insulin production capacity of the β cells of pancreatic islets [38]. Estradiol administration is associated with pancreatic islet hypertrophy and increased insulin release from the β cells in rats. Islet cells isolated from ovariectomized mice respond to glucose with a smaller insulin release than islet cells from intact mice. Estradiol replacement in ovariectomized mice normalizes the insulin response to glucose ingestion [6]. ERα activation promotes β-cell mass proliferation and insulin biosynthesis in diabetic and obese cases, whereas ERβ activation improves glucose stimulated insulin secretion [39].
After menopause, estrogen loss decreases the insulin secretion, which is transitorily compensated by its reduced elimination [40]. During postmenopausal hormone replacement therapy or contraceptive use estradiol improves insulin secretion, sensitivity and elimination.
Estrogen signaling seems to be essential for structural and functional β cell adaptation, especially during high metabolic demand and insulin resistant periods [8]. Recognition of these correlations leads to novel therapies for β-cell related diseases as estrogen administration preserves functional β-cell mass in patients with diabetes mellitus.

Estrogen Receptors and Energy Homeostasis in the Liver
Liver function disturbances are in close correlation with insulin resistance, hyperglycemia and dyslipidemia. Estrogen plays a pivotal role in the regulation of hepatic glucose homeostasis. ERα is the predominant receptor isoform in human hepatocytes and ERα signal is essential for the glucose tolerance of the liver [41,42].
Insulin modulates hepatic glucose uptake by the activation of glycogen synthase and glycogen phosphorylase leading to glucose storage as glycogen, whereas it may stimulate glycolysis through the activation of several hepatic enzymes [43].
PPT, a selective ERα agonist, improved glucose tolerance and insulin sensitivity in genetically obese mice suggesting that estradiol has antidiabetogenic impact via ERα [44]. In the liver of ERα knock out (ERαKO) mice, hepatic insulin resistance, increased glucose production and lipid synthesis as well as decreased lipid transport were observed [42]. By contrast ERβ might be diabetogenic as ERβ knock out (ERβKO) mice with increased body weight exhibited improved hepatic and muscular insulin sensitivity due to reduced accumulation of triglycerides [45].
Estradiol have pivotal interactions with growth hormone (GH)regulated endocrine (e.g., IGF-I), metabolic (e.g., lipid and glucose metabolism) and sex-differentiated (e.g., endo-and xenobiotic metabolism) functions in liver [45]. Estrogens modulate GH action at the level of pituitary GH secretion and have pivotal role in the regulation of GH activity, receptor expression and signaling as well [46]. Estrogen induced suppression of cytokine signaling negatively regulates GHR-Janus kinase (JAK)-2-signal transducer and activator of transcription (STAT)-5 signaling pathway [46,47]. Estrogen induced disruption of GHR-JAK2-STAT5 signaling may be associated with hepatic metabolic changes that include fatty liver, fibrosis, and hepatocellular carcinoma [48]. Complex interplay between GH and estrogen seemed to be disquieting because of the physiological roles of these hormones in mammals, and the widespread use of estrogenrelated compounds [45].
Conversely, 17β-estradiol administration could suppress tumor growth in mice with hepatocellular carcinoma [49]. Estradiol suppressed the alternative macrophage activation and tumor progression by keeping ERβ away from interacting with ATPase-coupling factor 6, a part of ATPase, thus inhibiting the JAK1-STAT6 signaling pathway. This study introduced a novel mechanism for the estrogen suppression of male-predominant hepatocellular carcinoma.
ERα confers estradiol mediated protection of the liver from inflammatory injuries [50]. Ovariectomized mice were treated by IL-18 to induce hepatic inflammation and estradiol hampered the expression of cytokines in ERβKO but not in ERαKO mice. Estrogen may suppress the production of hepatocyte growth factors and interleukin-6, which modulates the inflammatory environment of hepatocellular carcinoma in rats and inhibits metastatic spread [51].
Estradiol replacement in postmenopausal women increased HDL and decreased LDL, total cholesterol, lipoprotein, fasting insulin and glucose levels, exhibiting antidiabetogenic and antiatherogenic impacts [52]. Conversely, estrogen deficiency, such as antiestrogen (tamoxifen) therapy or ovariectomy resulted in atherogenic lipid profile and hepatic steatosis increasing the risk of metabolic syndrome and cardiovascular diseases [53].

Estradiol Actions on the Energy Homeostasis of Skeletal Muscles
Skeletal muscle mass is responsible for 75% of the insulin-mediated glucose uptake in the body and consequently, physical activity is in close correlation with insulin sensitivity [8].
Insulin receptors (IRs) help the active glucose transport through the double lipid layer of the cell membrane ( Figure 2). IRs has outer α-subunits with binding sites for insulin and two transmembrane β-subunits. Insulin interaction with the external α-subunits induces auto-phosphorylation of the β subunits at multiple tyrosines resulting in an activation of signal transduction [54]. The phosphorylation cascade provokes translocation of glucose transporter (GLUT4) containing cytoplasmic vesicles to the cell membrane. GLUT4 anchored and incorporated into the cell membrane enables the facilitated diffusion of glucose from the extracellular space into the cell [55]. Alterations in this mechanism, such as insulin deficiency, disturbance in insulin signal transduction, GLUT4 expression and/or translocation result in insulin resistance.
Estradiol stimulates the phosphorylation of Akt, AMPK and the Akt substrate in soleus muscle [56]. Estradiol administration to insulin resistant rats or mice increases the insulin receptor substrate content and the concentration of the phosphorylated form of Akt in muscles, restoring the action of insulin [57].
ERs advantageously modulate insulin stimulated glucose uptake through the regulation of the tyrosine phosphorylation of insulin receptor protein [58]. ERs promote GLUT4 expression and intracellular translocation as well. Moreover, estradiol improves glucose homeostasis through the facilitation of GLUT4 incorporation into the cell membrane ( Figure 2). By this way, estradiol treatment increases the GLUT4 content of the cell membrane [59]. In ovariectomized rats, the decreased amount of GLUT1 protein in the blood-brain barrier was increased after estradiol substitution [60].
Aromatase knockout (ARKO) mice with inactivation of the enzyme for estrogen synthesis, exhibit reduced glucose oxidation, increased adiposity and hyperinsulinemia both in males and females [61,62]. Glucose intolerance and insulin resistance can be reversed by estradiol administration even in male ARKO mice [63].
Skeletal muscle expresses both ERs, and in mice ERβ is the predominant isoform [8]. Treatment with the ERα selective agonist, PPT increased GLUT4 translocation to the cell membrane of L6 myoblasts, and when ERα was silenced the translocation decreased [64]. ERα knockout (ERαKO) mice are glucose intolerant and insulin resistant [22] as the absence of ERα involves a reduced glucose uptake in muscles [42]. ERβ is a repressor of GLUT4 expression and translocation. In ERβKO mice both glucose tolerance and insulin release remains normal or better than in wild type mice [42,44]. These data exhibit that a steadily balanced activation of both receptors may ensure the ideal glucose tolerance and energy expenditure.
During estrogen loss in the postmenopausal period, muscle strength exhibits a striking decline that can be reversed by hormone replacement therapy (HRT), suggesting that estrogens are important modulators of muscle physiology [65,66].
Some authors have established that hyperestrogenism is also related to insulin resistance similarly, to estrogen deficiency [67]. In women with irregular menstrual cycles and gestational diabetes, hyperestrogenism was presumed to be a contributor to insulin resistance [9,10]. Nevertheless, today these pathologic insulin resistant states are attributed to hyperandrogenism and deficient estrogen synthesis rather than to estrogen excess [7,16].
Taken together, estrogens increase insulin sensitivity, whereas an estrogen deficient milieu endangers the balanced glucose uptake and energy expenditure of skeletal muscles leading to insulin resistance [7].

Role of Estrogen Receptors in the Energy Homeostasis of Adipose Tissue
Fatty tissue participates in a variety of metabolic, endocrine and immunologic processes and interacts with CNS and peripheral organs by means of adipokine secretion. The size of the fatty tissue compartment clearly reflects the balance between whole-body energy intake and expenditure [68]. Increased adipose tissue in visceral location has been linked to a self-generating process of insulin resistance [16].
In rats, ovariectomy increased body weight, intra-abdominal fat mass, fasting glucose, insulin levels and insulin resistance. Estradiol substitution restored normoglycemia, increased the expression of adiponectin and decreased resistin expression resulting in improvement of insulin resistance [69].
In healthy premenopausal women, estradiol counteracts to the accumulation of visceral fat, and decreases the lipogenic activity of lipoprotein lipase in adipose tissue [34]. By contrast, irregular or long menstrual cycles in young women are associated with insulin resistance and predict occurrence of type 2 diabetes attributed to the defective estrogen synthesis [70,71]. Postmenopausal decreasing estradiol production, deepening insulin resistance and decreased lipid utilization result in visceral fat mass accumulation [72].
The presence of ERα and ERβ isoforms was confirmed in human adipocytes from both subcutaneous and intra-abdominal fat, with clear predominance of ERα [73]. Although the separated functions of the two ERs in adipose tissue may be studied on disabled ERαKO and ERβKO mice, these results may hardly be extrapolated to human practice. The only clinical case was a 28-year-old man with defective ERα who presented with glucose intolerance, hyperinsulinemia and obesity [74]. Hyperestrogenism in this patient seemed to be a contra-regulatory effect. Failure of his ERα signal leaded to premature coronary artery disease and decreased HDL cholesterol level [75].
In male mice deletion of ERα induces insulin resistance and progressive increase in adipose tissue with advancing age. In female ERαKO mice insulin resistance, increased adiposity, higher leptin and cholesterol levels and smaller LDL particles are characteristic [22].
Estradiol substitution in ovariectomized mice kept on highfat diet preserved glucose tolerance and insulin sensitivity [12]. Studies on ERα and ERβ knockout mice suggest that ERα is the main regulator of GLUT4 expression in adipose tissue [8] and the two ER isoforms seem to have opposite functions on fat metabolism [23]. In human adipocytes GLUT4 abundance is highly correlated with insulin responsiveness. In polycystic ovarian syndrome (PCOS) cases with ovarian overproduction of testosterone and defective estrogen synthesis, insulin stimulated glucose uptake was reduced due to decreased amounts of GLUT4 on adipocyte membrane [76].

Presumed Role of Gene Polymorphism in the Alterations of Estrogen Signaling and in the Development of Metabolic Syndrome
Epidemiological and prospective studies suggest controversial associations between estrogen signaling and several aspects of the metabolic syndrome [8]. Rapid development of new methods for genetic investigations enabled scientists to analyze the polymorphism of the estrogen receptor α gene (ESR1) coding for ERα. Gene polymorphism may induce either enhancing or silencing of the signal transduction of ERs, yielding possibilities for the rational explanation of puzzling links among estrogen, metabolic disorders and the associated morbidities.
Of the many hypotheses trying to explain the health advantage of women over men, none has received more attention than the estrogen hypothesis [77,78]. Nevertheless, some of the large studies have indicated a lack of metabolic benefit and an early increase in CVD and breast cancer risk from the use of hormone replacement therapy in postmenopausal cases [79]. As a plausible possibility, genetic variations in estrogen mediated pathways emerged as contributors to inter-individual and sex-specific variation in health risk [80]. Gene polymorphisms of ESR1 and their correlations with each component of metabolic syndrome and insulin resistance associated comorbidities were thoroughly investigated but the results proved to be contradictory.
Apparently controversial results were published in relation to ESR1 polymorphism and the prevalence of myocardial infarct. In the Framingham Heart study, male carriers of the common ESR1 IVS1-397T>C C/C genotype, were found to have a substantial increase in the risk of myocardial infarction [81]. Conversely, in the Rotterdam study, postmenopausal female carriers of the ESR1 haplotype 1 (IVS1-397T>C*T allele and IVS1-351A>G*A allele), were shown to have an increased risk of myocardial infarct and ischemic heart disease, whereas in men an absence of this association was reported [82].
Further studies also demonstrated conflicting results. The presence of the IVS1-397T>C*C allele was protective against the risk of in-stent restenosis [83]. In postmenopausal women with HRT use, homozygous for the C allele at the IVS1-397T>C site, the severity of atherosclerosis was milder and less progressive [84]. By contrast, negative effects of the same allele have also been described; it was associated with risk of aortic valve sclerosis and coronary atherosclerosis at angiography in women [85]. Nevertheless, age distribution, body weight, HRT use and other factors of the examined population may thoroughly influence the CVD risk besides the ER genotype.
Associations of polymorphism in the estrogen receptor β gene (ESR2) with recurrent spontaneous abortion were examined in the Chinese population. There were no significant differences in gene polymorphisms between women with full term pregnancy and with recurrent spontaneous abortion [86].
Correlations between genetic polymorphisms in the ESR1 gene and breast cancer risk are highly inconclusive. Pvull polymorphism was associated with a moderately increased risk, whereas, Xbal polymorphism was related to non-significantly elevated risk confined to older postmenopausal women in Shanghai [87]. Conversely, in a Korean study Pvull genotype distribution did not show any differences between breast cancer cases and controls, while the adjusted odds ratio for the Xbal allele containing genotypes was advantageously decreased (OR=0.4), particularly in postmenopausal cases [88]. These results suggest that even in genetic investigations, mainly postmenopausal state and ageing associated estrogen deficiency define the altered estrogen signaling leading to breast cancer [89]. Moreover, these studies cannot clarify the ligand binding strength and the negative or positive changes in the signaling pathways of estrogen attributed to gene polymorphism.
Although endometrial physiology is closely dependent on estrogen exposure, there were no significant associations between ERS1 gene polymorphisms and endometriosis in Japanese women [90]. Correlations between ESR1 polymorphism and endometrial cancer risk are also controversial. Some studies indicate that common genetic variants within ER cofactor genes are unlikely to play significant role in endometrial cancer risk in the European population [91]. In a further study the c.454-397C→T CC and c.454-351A→G GG genotypes were associated with a decreased risk for endometrial cancers [92].
In PCOS cases, Pvull and Xbal polymorphisms of ESR1, as well as Alul and Rsal polymorphisms of ESR2 were genotyped, and no difference was found in the distribution of these gene variants between patients and healthy controls [93]. Nevertheless, in PCOS women, carriers of TC and TT genotypes of Pvull polymorphism had lower fasting glucose to insulin ratio compared with carriers of CC genotype. These results suggest some associations of ERα polymorphisms with insulin resistance in PCOS.
Correlations between hormone replacement therapy (HRT) and mortality of women were examined to determine whether the risk of HRT associated mortality varies depending on the genetic variability of ERs [94]. These studies suggested that some women are genetically more vulnerable to the effects of HRT in terms of their estrogen receptor genotype.

Lifelong Changes in the Sex Hormone Levels of Women and their Correlation with Insulin Resistance, Type-2 Diabetes and Obesity
In premenopausal women, the equilibrium of sexual steroid synthesis defines somatic health and reproductive capacity, whereas symptom-free adaptation to the estrogen deficient environment is a prerequisite of postmenopausal health [16,89]. Changes in the sexual hormone equilibrium during women's lives strongly influence insulin sensitivity and the associated risk for type 2 diabetes and obesity (Table  1).

Changes in sexual hormone levels and insulin sensitivity in puberty
In puberty, the extreme somatic growth and explosion-like sexual development mean a great challenge for the metabolic and hormonal systems. In this period there is a higher risk for development of insulin resistance, particularly in overweight cases [16]. Recent results support that poor natural light exposure in puberty also mediates insulin resistance and hormonal alterations by excessive melatonin secretion [95].
In adolescent girls, the developing insulin resistance leads to abnormal ovarian sexual steroidgenesis as well, resulting in excessive androgen and defective estrogen production associated with menstrual irregularity and anovulatory cycles [96][97][98]. Hyperandrogenism and insulin resistance in adolescents are preserved into adulthood and result in defective fertility patterns and dysmetabolism at least until 30 years of age [99][100][101]. This prolonged hormonal and metabolic imbalance might be a defining, dangerous factor for adult metabolic syndrome, type 2 diabetes and their comorbidities.

Correlations between changes in estrogen levels and insulin sensitivity in premenopausal women
In premenopausal young women, defective estrogen synthesis is frequently associated with anovulation and infertility. Clinical signs of ovarian insufficiency are the long and/or irregular menstrual cycles [102,103]. These reproductive dysfunctions are related to hyperinsulinism and excessive androgen production.
Among premenopausal hormonal disorders with insulin resistance and fertility failure, the polycystic ovarian syndrome (PCOS) is the most prevalent, presumably caused by a large number of different genetic abnormalities [104]. PCOS may usually be manifested by menstrual disorders, anovulatory infertility, hirsutism and obesity or overweight. Nevertheless, polycystic ovaries are common findings in symptom-free cases with normal menstrual cycles as well; only the laboratory findings PCOS is not only an infertility disease but also represents a high systemic health risk for the affected women. Increased levels of insulin and insulin like growth factors (IGFs) are directly related to the high risk of cardiovascular lesions in women with PCOS [107]. Moreover, the excessive ovarian androgen production and defective estrogen synthesis are strong risks for cardiovascular diseases as the gender dependent equilibrium of male to female sexual hormone level is the prerequisite of vascular health [4]. Atherosclerotic complications were directly related to hyperandrogenism in PCOS cases [107].
Increased prevalence of type 2 diabetes, hypertension and cardiovascular complications were observed in a follow up study of a Dutch population of women with PCOS [70]. Close associations between PCOS and premature coronary and aortic atherosclerosis were revealed in middle-aged women [108,109]. A retrospective Swedish study found a 7.4-fold risk of myocardial infarction among women suffering of PCOS [110].
In young infertile, nulliparous women with or without PCOS, an elevated risk for endometrial cancer was observed [111]. The high prevalence of endometrial cancer is frequently associated with synchronous primary cancers of the ovary or breast [111,112]. This female organ triad has the highest estrogen demand showing peculiar cancer risk even in a slightly estrogen deficient environment [7,16,113].
Earlier, some authors presumed that elevated estrogen levels unopposed by progestin continuously stimulate estrogen receptors in women with PCOS. This postulation explained the high risk of endometrial and breast cancers observed in these cases based on the concept of the carcinogenic capacity of estrogen [114]. Recently, insulin resistance and hyperinsulinemia in PCOS patients are regarded as concomitants of high ovarian and adrenal androgen synthesis at the expense of defective estrogen production [16,115].
Hyperprolactinemia is also associated with insulin resistance, obesity, cycle disorders, reproductive dysfunction and hyperandrogenism in women. Glucose intolerance and obesity are characteristic in hyperprolactinemia, suggesting that the associated hormonal disturbances might also be modulators of insulin sensitivity and body weight [116,117]. In a population based cohort study the overall cancer risk was elevated in patients with hyperprolactinemia [118].
Oral contraceptive (OC) use replaces the natural menstrual cycle with relatively steady levels and fluctuations of artificial sex hormones. In PCOS cases, hormone treatment by oral contraceptives reduces the volume of cystic ovaries, decreases testosterone secretion and improves the carbohydrate and lipid metabolism as well [119]. Conversely, administration of the insulin sensitizing metformin in PCOS cases primarily lowers the high insulin level and at the same time improves menstrual abnormalities, ovulatory dysfunction and hirsutism [120].
Epidemiologic studies have confirmed that combined oral contraceptives provide substantial protection against endometrial and ovarian cancers in endangered anovulatory women [120]. A recent patent disclosed a method for treating hyperandrogenic states, including PCOS by an estrogen derivative compound, which is more advantageous against the dangerous dysmetabolism of PCOS cases than oral contraceptive administration [121].
Tamoxifen is a nonsteroidal anti-estrogenic drug used for adjuvant therapy of breast cancer and recently as a chemopreventive agent for breast cancer and for other cancers as well [122]. Nevertheless, worldwide administration of antiestrogen compounds yielded thorough disappointment [123].
Antiestrogens are cytostatic agents blocking the most important regulatory mechanisms associated with estrogen signaling. They have ambiguous impacts on mammary tumor development, however, the estrogen deprivation induces several life-threatening sideeffects and exhibits strong carcinogenic capacity, particularly in the highly estrogen dependent endometrium [124]. Results of casecontrol studies demonstrated an increased prevalence of fatty liver, intraabdominal fat accumulation and type 2 diabetes in breast cancer cases receiving tamoxifen [122,125,126]. Artificial blocking of estrogen signaling pathways seems to confer serious insulin resistance leading to metabolic syndrome and type 2 diabetes. These disorders are well-known promoters of cancer initiation and progression affecting preferentially the endometrium [127,128].

Correlations between estrogen loss and the risk of increasing insulin resistance in postmenopausal women
Menopause at 50-52 years of age means a sudden loss of ovarian estrogen synthesis and confers further decline in the circulating hormone level.
Postmenopausal women never using HRT are obviously insulin resistant and exhibit increasing inclination to the associated comorbidities. With ageing, every year after menopause is associated with continuous estrogen loss and parallel advancing insulin resistance [129]. For women aged 55-65 years, weight gain and obesity are their major health risks [130]. In postmenopausal women, deepening dysmetabolism, obesity and disturbance of male to female sexual steroid levels are associated with increased prevalence of metabolic syndrome, type 2 diabetes, cardiovascular disease and malignancies.
In HRT user postmenopausal cases, the protective effect of estrogen substitution may counteract to the developing insulin resistance and their metabolic and hormonal equilibrium becomes reminiscent to that of young women with preserved circulatory estrogen levels [16]. Estradiol administration increases insulin sensitivity [6], yields favorable changes in plasma lipid levels [131] and its anti-obesity effect decreases fat-accumulation, particularly in visceral location [132,133]. All these impacts justify that HRT use is beneficially protective against insulin resistance and its comorbidities in postmenopausal women.
Hysterectomy and bilateral oophorectomy mean an abrupt, shocking hormone deprivation as compared with natural menopause. These patients are highly endangered as they have no possibility for gradual adaptation to estrogen loss by a compensatory hormone synthesis of the peripheral tissues [89,113]. Bilateral oophorectomy is used as a risk reduction strategy in BRCA1/2 mutation carriers, although data on long-term side effects are not yet available. In the US population, oophorectomy, particularly at a young age, has been associated with highly increased overall and cardiovascular disease mortality [134].
In 2011, the WHI Randomized Controlled Trial substantiated that estrogen treatment in women with prior hysterectomy resulted in a significantly lower breast cancer risk than in untreated controls [135][136][137]. Hysterectomy seems to be a near uniformly high breast cancer risk for women, thus HRT studies on these homogenously selected cases proved to be methodologically strong and yielded unexpectedly correct results [7].
In postmenopausal women, antiestrogen administration is a worldwide practice as either therapeutic or cancer preventive agent. Both ER-blocker and aromatase inhibitor types of antiestrogens further aggravate the estrogen deficiency and insulin resistance of aged female patients [123].

Conclusion
Correlations between estrogen signaling and human morbidity seems to be very unclear and difficult topic. Nowadays, the opinions of scientists have been partially changed regarding "estrogen induced diseases" however; the overwhelming literary data are still contradictory.
Individual and population specific ER associated gene polymorphisms were presumed to illuminate the highly controversial clinical and experimental results on the role of estrogens in health and disease. Nevertheless, the puzzling biology behind estrogen action was not reassuringly clarified by studies on either disabled ERKO animals or patients with genotyping of their ERs.
Diverse disorders associated with insulin resistance are usually well treatable by estradiol substitution both in pre-and postmenopausal women as well as in animal experiments. ERs seem to have balanced interplay in the maintenance of ideal glucose uptake and energy expenditure with continuous adaptation to the momentarily changing intra and extracellular stimuli. This equilibrium may be shattered in case of a defective estrogen supply or by the derangement of ER signaling pathways.
Recently, new therapeutic options are planned based on the development of tissue specific selective estrogen receptor modulators (SERMs) by the pharmaceutical industry. Nevertheless, considering the beneficial results of estradiol substitution in diverse diseases, we should understand the marvelous network of ER signal transduction pathways instead of trying to manipulate it.