Leptin actions on food intake and body temperature are mediated by IL-1

Cardiovascular and metabolic responses to fasting and thermoneutrality are conserved in obese Zucker rats

The primary purpose of the study was to test the hypothesis that reduced leptin signaling is necessary to elicit the cardiovascular and metabolic responses to fasting. Lean (Fa/?; normal leptin receptor; n = 7) and obese (fa/fa; mutated leptin receptor; n = 8) Zucker rats were instrumented with telemetry transmitters and housed in metabolic chambers at 23 degrees C (12:12-h light-dark cycle) for continuous (24 h) measurement of metabolic and cardiovascular variables. Before fasting, mean arterial pressure (MAP) was higher (MAP: obese = 103 +/- 3; lean = 94 +/- 1 mmHg), whereas oxygen consumption (VO(2): obese = 16.5 +/- 0.3; lean = 18.6 +/- 0.2 ml. min(-1). kg(-0.75)) was lower in obese Zucker rats compared with their lean controls. Two days of fasting had no effect on MAP in either lean or obese Zucker rats, whereas VO(2) (obese = -3.1 +/- 0.3; lean = -2.9 +/- 0.1 ml. min(-1). kg(-0.75)) and heart rate (HR: obese = -56 +/- 4; lean = -42 +/- 4 beats/min) were decreased markedly in both groups. Fasting increased HR variability both in lean (+1.8 +/- 0.4 ms) and obese (+2.6 +/- 0.3 ms) Zucker rats. After a 6-day period of ad libitum refeeding, when all parameters had returned to near baseline levels, the cardiovascular and metabolic responses to 2 days of thermoneutrality (ambient temperature 29 degrees C) were determined. Thermoneutrality reduced VO(2) (obese = -2.4 +/- 0.2; lean = -3.3 +/- 0.2 ml. min(-1). kg(-0.75)), HR (obese = -46 +/- 5; lean = -55 +/- 4 beats/min), and MAP (obese = -13 +/- 6; lean = -10 +/- 1 mmHg) similarly in lean and obese Zucker rats. The results indicate that the cardiovascular and metabolic responses to fasting and thermoneutrality are conserved in Zucker rats and suggest that intact leptin signaling may not be requisite for the metabolic and cardiovascular responses to reduced energy intake.

Modulation of body temperature, interleukin-6 and leptin by oral contraceptive use

OBJECTIVES

This study examined the hypothesis that oral contraceptives (OC) influence the production of thermoregulatory cytokines, i.e. interleukin-6 (IL-6), soluble IL-6 receptor (sIL-6R), soluble glycoprotein 130 (s-gp130) and leptin, and that OC-induced changes in oral temperature (T(oral)) are associated with changes in plasma concentrations of these cytokines. To determine if increases in T(oral) are part of a cytokine-driven inflammatory (acute-phase) response, circulating concentrations of the hepatic acute-phase protein C-reactive protein (CRP) were also measured.

METHODS

Morning T(oral) were measured and blood samples were collected from 18 women (19- to 22-years-old) on two occasions: Once during active pill usage (quasi-luteal (QL) phase) and once when no active pills were taken (quasi-follicular (QF) phase). Plasma cytokine and CRP concentrations were measured by immunoassay.

RESULTS

T(oral) and plasma leptin were higher during QL phase (36.4 +/- 0.1 degrees C, 9.3 +/- 1.0 ng/ml) than QF phase (36.1 +/- 0.1 degrees C, p < 0.01; 7.5 +/- 0.7 ng/ml, p < 0.01). Increases in T(oral) correlated with increases in plasma leptin (R = 0.55, p = 0.02) and with progestin dose (R = 0.47, p = 0.05) individually as well as with leptin and progestin combined in a multiple regression (R = 0.68, p = 0.01). Plasma IL-6 correlated with progestin dose (R = 0.62, p = 0.006). Although there were no phase-related differences in plasma IL-6, sIL-6R, s-gp130, or CRP, the variation in CRP between individuals correlated with the IL-6 agonist/antagonist ratio combined with progestin dose in a multiple regression (R = 0.71, p = 0.01).

CONCLUSIONS

These results (a) implicate leptin in basal thermoregulation; (b) indicate that progestins have a significant influence on circulating IL-6 concentrations, and (c) are consistent with the concept that plasma CRP concentrations depend upon combined influences of progestins and bioavailable IL-6.

Choroid plexus: target for polypeptides and site of their synthesis

Choroid plexus (CP) is an important target organ for polypeptides. The fenestrated phenotype of choroidal endothelium facilitates the penetration of blood-borne polypeptides across the capillary walls. Thus, both circulating and cerebrospinal fluid (CSF)-borne polypeptides can reach their receptors on choroidal epithelium. Several polypeptides have been demonstrated to regulate CSF formation by controlling blood flow to choroid plexus and/or the activity of ion transport in choroidal epithelium. However, many ligand-receptor interactions occurring in the CP are not involved in the regulation of fluid secretion. Increasing evidence suggests that the choroidal epithelium plays an important role in hormonal signaling via a receptor-mediated transport into the brain (e.g., leptin) and helps to clear certain CSF-borne polypeptides (e.g., soluble amyloid beta-protein). Thus, impaired choroidal transport or insufficient clearance of polypeptides may contribute to pathogenesis of systemic or central nervous system (CNS) disorders, such as obesity or Alzheimer's disease. CP epithelium is not only a target but is also a source of neuropeptides, growth factors, and cytokines in the CNS. These polypeptides following their release into the CSF may exert distal, endocrine-like effects on target cells in the brain due to bulk flow of this fluid. Distinct temporal patterns of choroidal expression of several polypeptides are observed during brain development and in various CNS disorders, including traumatic brain injury and ischemia. Therefore, it is proposed that the CP plays an integral role not only in normal brain functioning, but also in the recovery from the injury. This review attempts to critically analyze the available data to support the above hypothesis.

Hot flashes in the late reproductive years: risk factors for Africa American and Caucasian women

Hot flashes are a primary reason that midlife women seek medical care, but there is little information about the onset or the predictors of hot flashes in the years before the menopause. This study examines women's experience of hot flashes in the late reproductive years, comparing African American and Caucasian women, and identifies hormonal, behavioral, and environmental risk factors for hot flashes associated with ovarian aging. Data are from a population-based prospective cohort study of ovarian aging in women who were ages 35--47, in general good health, and had regular menstrual cycles at study enrollment. Hot flashes were assessed by subject report in a structured interview at the first follow-up period and correlated highly with previous prospective daily ratings of hot flashes (p = 0.0001). Blood samples were obtained in the first 6 days of the menstrual cycle in two consecutive cycles at enrollment and two consecutive cycles at follow-up. Predictor variables include hormone measures, structured interview, and standard questionnaire data. Thirty-one percent of the sample (n = 375) reported hot flashes (mean age 41 years). In bivariate analysis, more African American than Caucasian women reported hot flashes (38% vs. 25%, p = 0.01). Significant predictors of hot flashes in the final multivariable logistic regression model were higher follicle-stimulating hormone (FSH) levels (odds ratio [OR] 3.19), anxiety (OR 1.06), baseline menopausal symptoms (OR 4.91), alcohol use (OR 1.09), body mass index (BMI) (OR 1.04), and parity (OR 1.20). Race did not predict hot flashes after adjusting for these variables. Hot flashes commonly occur before observable menstrual irregularities in the perimenopause and are associated with both hormonal and behavioral factors. The association of hot flashes with increased body mass (BMI) challenges the current "thin" hypothesis and raises important questions about the role of BMI in hormone dynamics in the late reproductive years.

Leptin acts on metabolism in a photoperiod-dependent manner, but has no effect on reproductive function in the seasonally breeding Siberian hamster (Phodopus sungorus)

Leptin may play a role in appetite regulation and metabolism, but its reproductive role is less clear. In photoperiodic Siberian hamsters, seasonal changes in fatness, leptin gene expression, and metabolism occur synchronously with activation or suppression of reproduction, analogous to puberty. Here, we test the hypothesis that seasonal changes in leptin secretion mediate the photoperiodic regulation of reproduction. Mature male and ovariectomized estrogen-treated female Siberian hamsters were kept in long (LD; 16 h of light, 8 h of darkness) or short days (SD; 8 h of light, 16 h of darkness) for 8 weeks, and recombinant murine leptin (15 microg/day) was infused for 2 weeks via osmotic minipumps. SD hamsters exhibited significant weight and fat losses, reduced serum leptin and food intake, and suppressed pituitary LH concentration. Leptin did not suppress food intake over the 2-week treatment on either photoperiod, but significantly reduced fat reserves in SD hamsters. Leptin had no significant effect on pituitary LH concentrations in either sex or photoperiod or on testicular size and testosterone concentrations in males. These results suggest hamsters are more responsive to leptin on SD than on LD and that effects on food intake and fat loss can be dissociated in this species. Our data suggest that leptin does not mediate photoperiodic reproductive changes.

Expression of leptin receptors and induction of IL-1beta transcript in glial cells

To examine the role of leptin in the immune function of the brain, we examined the effect of leptin on interleukin-1beta (IL-1beta) expression in mouse primary cultured glial cells. The expression of leptin receptor isoforms Ob-Ra and Ob-Rb mRNA was detected by RT-PCR analysis of total RNA from primary cultured glial cells. Protein of leptin receptor was also expressed in mouse primary cultured glial cells as evaluated by Western blotting analysis. Leptin increased the expression of IL-1beta mRNA evaluated by RT-PCR. The expression of IL-1beta transcript peaked 2 to 6 h after leptin application. These results indicate that leptin could induce IL-1beta transcript in the brain and that one of the target cells of the leptin-induced IL-1beta transcript may be a glial cell.

Leptin facilitates histamine release from the hypothalamus in rats

We studied the effect of leptin on the release of histamine from the anterior hypothalamus using an in vivo microdialysis in anesthetized rats. Histamine release was significantly increased by leptin administration (1.3 mg/kg, i.p.) and kept at a high level for 4 h after the injection. The same dose of leptin significantly reduced food intake in another group of rats. This finding suggests that leptin activates the histaminergic system in the hypothalamus, which may contribute the expression of leptin-induced anorectic effect.

Leptin: a multifunctional hormone

Leptin is the protein product encoded by the obese (ob) gene. It is a circulating hormone produced primarily by the adipose tissue. ob/ob mice with mutations of the gene encoding leptin become morbidly obese, infertile, hyperphagic, hypothermic, and diabetic. Since the cloning of leptin in 1994, our knowledge in body weight regulation and the role played by leptin has increased substantially. We now know that leptin signals through its receptor, OB-R, which is a member of the cytokine receptor superfamily. Leptin serves as an adiposity signal to inform the brain the adipose tissue mass in a negative feedback loop regulating food intake and energy expenditure. Leptin also plays important roles in angiogenesis, immune function, fertility, and bone formation. Humans with mutations in the gene encoding leptin are also morbidly obese and respond to leptin treatment, demonstrating that enhancing or inhibiting leptin's activities in vivo may have potential therapeutic benefits.

Reduced leptin levels in starvation increase susceptibility to endotoxic shock

Malnutrition compromises immune function, reducing resistance to infection. We examine whether the decrease in leptin induced by starvation increases susceptibility to lipopolysaccharide (LPS)- and tumor necrosis factor (TNF)-induced lethality. In mice, fasting for 48 hours enhances sensitivity to LPS. Decreasing the fasting-induced fall in leptin by leptin administration markedly reduced sensitivity to LPS. Although fasting decreases basal leptin levels, LPS treatment increased leptin to the same extent as in fed animals. Fasting increased basal serum corticosterone; leptin treatment blunted this increase. Fasting decreased the ability of LPS to increase corticosterone; leptin restored the corticosterone response to LPS. Serum glucose levels were decreased in fasted mice and LPS induced a further decrease. Leptin treatment affected neither basal glucose nor that after LPS. LPS induced a fivefold greater increase in serum TNF in fasted mice, which was blunted by leptin replacement. In contrast, LPS induced lower levels of interferon-gamma and no differences in interleukin-1beta in fasted compared to fed animals; leptin had no effect on those cytokines. Furthermore, fasting increased sensitivity to the lethal effect of TNF itself, which was also reversed by leptin treatment. Thus, leptin seems to be protective by both inhibiting TNF induction by LPS and by reducing TNF toxicity.

Central nervous system control of food intake

New information regarding neuronal circuits that control food intake and their hormonal regulation has extended our understanding of energy homeostasis, the process whereby energy intake is matched to energy expenditure over time. The profound obesity that results in rodents (and in the rare human case as well) from mutation of key signalling molecules involved in this regulatory system highlights its importance to human health. Although each new signalling pathway discovered in the hypothalamus is a potential target for drug development in the treatment of obesity, the growing number of such signalling molecules indicates that food intake is controlled by a highly complex process. To better understand how energy homeostasis can be achieved, we describe a model that delineates the roles of individual hormonal and neuropeptide signalling pathways in the control of food intake and the means by which obesity can arise from inherited or acquired defects in their function.

Cytokines and the anorexia of infection: potential mechanisms and treatments

Anorexia during infection is thought to be mediated by immunoregulatory cytokines such as interleukins 1 and 6 and tumor necrosis factor. This article reviews the potential mechanisms of action by which these cytokines are thought to suppress food intake during infection and examines the proposition that blocking of cytokine activity might be one approach to improving food intake of the infected host.

Leptin: an essential regulator of lipid metabolism

This paper reviews the general mechanisms by which leptin acts as a regulator of lipid reserves through changes in food intake, energy expenditure and fuel selection, with an emphasis on its direct effects on cellular lipid metabolism. Briefly, when leptin levels increase, food consumption decreases via modulation of hypothalamic neuropeptides. As well, normal decreases in energy expenditures (e.g. with diurnal cycles or reduced caloric intake) do not occur. This is probably caused by an increase in mitochondrial proton leak mediated by leptin via increases in sympathetic nervous system stimulation and thyroid hormone release. The decrease in caloric input coupled with relatively higher energy expenditure, therefore, leads to negative energy balance. Leptin also changes the fuel source from which ATP is generated. Fuel preference switches from carbohydrate (glucose) to lipid (fatty acids). This effect arises through stimulation of triacylglycerol catabolism by leptin. In vitro studies show that leptin is a potent stimulator of lipolysis and fatty acid oxidation in adipocytes and other cell types. Consequently, leptin is also a regulator of cellular triacylglycerol content. Hormonal regulation of leptin, as well as its role in fasting and seasonal weight gain and energy expenditure are also briefly discussed.

Hypothalamic control of feeding

Our understanding of the hypothalamic control of energy homeostasis has increased greatly since the discovery of leptin, the adipose cell derived protein. Recent studies have identified several new hypothalamic neuropeptides that affect food intake and energy balance. By studying these molecules and their neuronal systems, receptors and interactions, we are beginning to unravel the circuitry between peripheral adipogenic signals and hypothalamic effector pathways.