The role of hypothalamic estrogen receptors in metabolic regulation.

Estrogens regulate key features of metabolism, including food intake, body weight, energy expenditure, insulin sensitivity, leptin sensitivity, and body fat distribution. There are two 'classical' estrogen receptors (ERs): estrogen receptor alpha (ERS1) and estrogen receptor beta (ERS2). Human and murine data indicate ERS1 contributes to metabolic regulation more so than ESR2. For example, there are human inactivating mutations of ERS1 which recapitulate aspects of the metabolic syndrome in both men and women. Much of our understanding of the metabolic roles of ERS1 was initially uncovered in estrogen receptor α-null mice (ERS1(-/-)); these mice display aspects of the metabolic syndrome, including increased body weight, increased visceral fat deposition and dysregulated glucose intolerance. Recent data further implicate ERS1 in specific tissues and neuronal populations as being critical for regulating food intake, energy expenditure, body fat distribution and adipose tissue function. This review will focus predominantly on the role of hypothalamic ERs and their critical role in regulating all aspects of energy homeostasis and metabolism.

Estradiol-dependent decrease in the orexigenic potency of ghrelin in female rats.

Ghrelin, the only known orexigenic gut hormone, is secreted mainly from the stomach, increases with fasting and before meal initiation in humans and rats, and increases food intake after central or peripheral administration. To investigate sex differences in the action of ghrelin, we assessed the effects of exogenous ghrelin in intact male and female rats, the effects of exogenous ghrelin in ovariectomized (OVX) and estradiol (E2)-treated female rats, as well as the effects of OVX on plasma ghrelin and hypothalamic orexigneic neuropeptide expression in rats and on food intake and weight gain in transgenic mice lacking the ghrelin receptor (Ghsr(-/-) mice). Male and OVX female rats were significantly more sensitive than intact female rats to the orexigenic effects of both centrally (intra-third ventricular, i3vt, 0.01, 0.1, and 1.0 nmol) and systemically (ip, 3, 6, and 9 nmol) administered ghrelin. This difference is likely to be estradiol dependent because E2 attenuated the orexigenic action of ghrelin in OVX female and male rats. Furthermore, OVX increased food intake and body weight in wild-type mice, but not in Ghsr(-/-) mice, suggesting that OVX increases food intake by releasing ghrelin from a tonic inhibitory effect of estradiol. In addition, following OVX, there was an increase in plasma ghrelin that was temporally associated with increased food intake, body weight, and hypothalamic neuropeptide Y and Agouti-related protein mRNA expression. Collectively, these data suggest that estradiol inhibits the orexigenic action of ghrelin in females, that weight gain associated with OVX is ghrelin mediated, and that this endocrine interaction may account for an important sex differences in food intake and the regulation of body weight.

Gonadal hormones determine sensitivity to central leptin and insulin.

Males have proportionally more visceral fat and are more likely to develop complications associated with obesity than females, and the male brain is relatively more sensitive to the catabolic action of insulin and less sensitive to that of leptin than the female brain. To understand the underlying mechanism, we manipulated estrogen through ovariectomy (OVX) and estradiol administration. Rats with relatively high systemic estrogen (intact females and OVX females and males administered estrogen subcutaneously) were significantly more sensitive to leptin's anorexic action in the brain (i3vt), as well as significantly less sensitive to insulin's i3vt action, than intact males. Administering estradiol directly into the brain of our females increased i3vt leptin sensitivity while decreasing i3vt insulin sensitivity and changed the body fat distribution of our females to resemble that of intact females. These data indicate that estrogen acts within the brain to increase leptin sensitivity, decrease insulin sensitivity, and favor subcutaneous over visceral fat.

Intraventricular (i3vt) ghrelin increases food intake in fatty Zucker rats.

Ghrelin is an orexigenic peptide secreted from the stomach and also made in the brain. Ghrelin receptors are expressed on hypothalamic cells important in appetite and energy balance. We determined that intra-3rd-ventricular (i3vt) ghrelin dose-dependently increases acute (1 and 2 h) food intake in lean and fatty Zucker rats (0, 0.01, 0.1 and 1.0 nmol ghrelin). The percentage increase of food intake in fatty Zucker rats was significantly greater than that in lean rats. Fatty Zucker rats had 4.5 times more ghrelin receptor mRNA in the hypothalamus than lean Zucker rats, suggesting a possible mechanism for the increased sensitivity.

Intraventricular insulin and leptin reduce food intake and body weight in C57BL/6J mice.

As the incidence of obesity continues to increase, adequate animal models acquire increased importance for the investigation of energy homeostatic mechanisms. Understanding the central mechanism of action of the adiposity hormones, insulin and leptin, has become particularly important as researchers examine ways to treat or prevent obesity. Although the intra-3rd-ventricular (i3vt) administration of insulin reduces food intake in several species, its effects on food intake and body weight have not been previously been assessed in mice. Male C57BL/6J mice were administered insulin i3vt (0.05, 0.1 or 0.4 microU) or leptin i3vt (5 microg/1 microl) as a positive control. As it occurs in other species, i3vt insulin dose-dependently reduced 24-h food intake and body weight, and increased hypothalamic proopiomelanocortin (POMC) mRNA. Hence, genetic manipulations that influence brain insulin sensitivity in mice can now more easily be integrated with the broader literature on energy homeostasis.

Acute exposure to high-fat diets increases hepatic expression of genes related to cell repair and remodeling in female rats.

High-fat diets (HFD) promote the development of both obesity and fatty liver disease through the up-regulation of hepatic lipogenesis. Insulin resistance, a hallmark of both conditions, causes dysfunctional fuel partitioning and increases in lipogenesis. Recent work has demonstrated that systemic insulin resistance occurs in as little as the first 72 hours of an HFD, suggesting the potential for hepatic disruption with HFD at this time point. The current study sought to determine differences in expression of lipogenic genes between sexes in 3-month-old male and female Long-Evans rats after 72 hours of a 40% HFD or a 17% fat (chow) diet. Owing to the response of estrogen on hepatic signaling, we hypothesized that a sexual dimorphic response would occur in the expression of lipogenic enzymes, inflammatory cytokines, apoptotic, and cell repair and remodeling genes. Both sexes consumed more energy when fed an HFD compared with their low fat-fed controls. However, only the males fed the HFD had a significant increase in body fat. Regardless of sex, HFD caused down-regulation of lipogenic and inflammatory genes. Interestingly, females fed an HFD had up-regulated expression of apoptotic and cell repair-related genes compared with the males. This may suggest that females are more responsive to the acute hepatic injury effects caused by HFDs. In summary, neither male nor female rats displayed disrupted hepatic metabolic pathways after 72 hours of the HFD treatment. In addition, female rats appear to have protection from increases in fat deposition, possibly due to increased caloric expenditure; male rats fed an HFD were less active, as demonstrated by distance traveled in their home cage.

Traits of the metabolic syndrome alter corpulent obesity in LAN, SHR and DSS rats: behavioral and metabolic interactions with adrenalectomy.

Obesity results from a complex interaction of genes with environmental factors. Our experimental design compared obesity in three rat strains with the corpulent (cp) mutation. The three strains included Lister and Albany NIH (LAN) rats, Spontaneously Hypertensive Rats (SHR) and Dahl Salt Sensitive (DSS) rats that were congenically bred. The strains were selected because of different reported metabolic complications generally clustered with obesity, and defined as the metabolic syndrome. Body weight, food intake, carcass composition, plasma hormones and hypothalamic expression of Y5 receptors were assessed in obese (cp) and lean (wt) rats after adrenalectomy (ADX) or sham surgeries. Plasma corticosterone in sham-operated wtDSS and cpDSS were significantly higher (approx. 165ng/ml) than that in cpLAN and cpSHR (~77 and 68ng/ml respectively). All cp groups had a higher % carcass fat than wt groups. The % carcass fat was greater in cpDSS>cpLAN>cpSHR but plasma leptin was greatest in cpLAN>cpSHR>cpDSS. Hypothalamic expression of the Y5R after ADX resulted in a phenotype×surgery interaction since Y5R expression was slightly increased in cp rats and slightly decreased in wt rats. The strain with greatest number of metabolic syndrome traits, SHR, was not the fattest of the strains and had little response to ADX. The strains with fewer metabolic syndrome traits LAN and DSS had more extreme obesities which were attenuated after ADX. The results of the current experiment provide evidence that the corpulent mutation is not fully characterized in one strain.

The effect of TNFα on food intake and central insulin sensitivity in rats.

Circulating and tissue levels of the proinflammatory cytokine tumor necrosis factor α (TNFα) are elevated in obesity. TNFα interferes with insulin signaling in many tissues and also plays a causal role in the anorexia that accompanies severe challenges to the immune system. The interactions between TNFα and insulin in the control of eating are less well known. The present study evaluated the role of TNFα in the central nervous system control of food intake by insulin in adult male Long Evans rats. We first determined the ability of several doses of TNFα injected into the 3rd cerebral ventricle (i3vt) to reduce food intake in male rats. Subsequently, we assessed the ability of a subthreshold dose of TNFα to modulate the effect of i3vt insulin on food intake in male rats fed a low-fat chow or a high-fat (HF) diet. TNFα administered i3vt dose-dependently reduced food intake in rats fed a standard low-fat chow diet. Moreover, a low, sub-threshold dose of TNFα diminished the reduction in food intake by insulin in rats maintained on a chow diet, but enhanced insulin action in rats maintained on a HF diet. These data suggest that the interaction of TNFα with central insulin varies with nutritional and/or dietary conditions.

Comparison of DEXA and QMR for assessing fat and lean body mass in adult rats.

There are several techniques used to measure body composition in experimental models including dual energy X-ray absorptiometry (DEXA) and quantitative magnetic resonance (QMR). DEXA/QMR data have been compared in mice, but have not been compared previously in rats. The goal of this study was to compare DEXA and QMR data in rats. We used rats that varied by sex, diet, and age, in addition we compared dissected samples containing subcutaneous (pelt) or visceral fat (carcass). The data means were compared by focusing on the differences between DEXA/QMR data using a series of scatter plots without assuming that either method is more accurate as suggested by Bland and Altman. DEXA/QMR data did not agree sufficiently in carcass or pelt FM or in pelt LBM. The variation observed within these groups suggests that DEXA and QMR measurements are not comparable. Carcass LBM in young rats did yield comparable data once the data for middle-aged rats was removed. The variation in our data may be a result of different direct and indirect measures that DEXA and QMR technologies use to quantify FM and LBM. DEXA measures FM and estimates fat-free mass. In contrast, QMR uses separate equations of magnetic resonance to measure FM, LBM, total body water and free water. We found that QMR overestimated body mass in our middle-aged rats, and this increased the variation between methods. Our goal was to evaluate the precision of DEXA/QMR data in rats to determine if they agree sufficiently to allow direct comparison of data between methods. However DEXA and QMR did not yield the same estimates of FM or LBM for the majority of our samples.

Consumption of a high-fat diet induces central insulin resistance independent of adiposity.

Plasma insulin enters the CNS where it interacts with insulin receptors in areas that are related to energy homeostasis and elicits a decrease of food intake and body weight. Here, we demonstrate that consumption of a high-fat (HF) diet impairs the central actions of insulin. Male Long-Evans rats were given chronic (70-day) or acute (3-day) ad libitum access to HF, low-fat (LF), or chow diets. Insulin administered into the 3rd-cerebral ventricle (i3vt) decreased food intake and body weight of LF and chow rats but had no effect on HF rats in either the chronic or the acute experiment. Rats chronically pair-fed the HF diet to match the caloric intake of LF rats, and with body weights and adiposity levels comparable to those of LF rats, were also unresponsive to i3vt insulin when returned to ad libitum food whereas rats pair-fed the LF diet had reduced food intake and body weight when administered i3vt insulin. Insulin's inability to reduce food intake in the presence of the high-fat diet was associated with a reduced ability of insulin to activate its signaling cascade, as measured by pAKT. Finally, i3vt administration of insulin increased hypothalamic expression of POMC mRNA in the LF- but not the HF-fed rats. We conclude that consumption of a HF diet leads to central insulin resistance following short exposure to the diet, and as demonstrated by reductions in insulin signaling and insulin-induced hypothalamic expression of POMC mRNA.

Metabolic impact of sex hormones on obesity.

Obesity and its associated health disorders and costs are increasing. Men and post-menopausal women have greater risk of developing complications of obesity than younger women. Within the brain, the hypothalamus is an important regulator of energy homeostasis. Two of its sub-areas, the ventrolateral portion of the ventral medial nucleus (VL VMN) and the arcuate (ARC) respond to hormones and other signals to control energy intake and expenditure. When large lesions are made in the hypothalamus which includes both the VL VMN and the ARC, animals eat more, have reduced energy expenditure, and become obese. The ARC and the VL VMN, in addition to other regions in the hypothalamus, have been demonstrated to contain estrogen receptors. There are two estrogen receptors, estrogen receptor alpha (ERalpha) and estrogen receptor beta (ERbeta). We and others have previously demonstrated that activation of ERalpha by estrogens reduces food intake and increases body weight. This review focuses on the relative contribution of activation of ERalpha by estrogens in the ARC and the VL VMN in the regulation of food intake and body weight. Additionally, estrogen receptors have been found in many peripheral tissues including adipose tissue. Estrogens are thought to have direct effects on adipose tissue and estrogens may provide anti-inflammatory properties both in the periphery and the in the central nervous system (CNS) which may protect women from diseases associated with inflammation. Understanding the mechanisms by which estrogens regulate body weight and inflammation will assist in determining potential therapeutic agents for menopausal women to decrease the propensity of diseases associated with obesity.

Computer access to research on dietary supplements: a database of federally funded dietary supplement research.

Dietary supplement use is prevalent in the United States, but support for supplement research has been relatively modest and only recently emphasized at the NIH. The Dietary Supplement Health and Education Act of 1994 led to the creation of the Office of Dietary Supplements (ODS) at the NIH to promote research on dietary supplements. In order to track federally funded dietary supplement research, the ODS developed a database known as Computer Access to Research on Dietary Supplements (CARDS). This article provides an overview of the development and potential uses of the CARDS database. In addition, we report that NIH-funded dietary supplement research steadily increased from fiscal year (FY) 1999 through 2002. The majority of NIH institutes or centers (ICs) funded research relevant to dietary supplements during this time, led by the National Cancer Institute and one of the newest NIH ICs, the National Center for Complementary and Alternative Medicine. CARDS data indicate that NIH-funded dietary supplement research from FY 1999 through 2002 involved primarily vitamins, minerals, botanicals and phytochemicals. Cancer and cardiovascular disease, two of the leading causes of morbidity and mortality in the United States, collectively accounted for almost 45% of the research related to dietary supplements. A variety of types of research studies were funded, with the majority consisting of human intervention studies. This information is useful to evaluate trends in federally funded dietary supplement research, identify research gaps, and help research scientists identify potential sources of NIH funding.