Robert C. Frederich

Activation of β3 Adrenergic Receptors Suppresses Leptin Expression and Mediates a Leptin-Independent Inhibition of Food Intake in Mice

To examine potential interactions between leptin and the β3 adrenergic system in the regulation of food intake, we determined the effects of treatment with a selective β3 adrenergic receptor (AR) agonist (CL 316,243 [1 mgfleg]) on body weight, food intake, and leptin expression. Studies were carried out in C57B1/6J and FVB male control mice as well as in mice with targeted disruption of the β3 AR gene. These findings were correlated with measurement of the expression in hypothalamus of neuropeptide Y (NPY) and melanin concentrating hormone (MCH), two neuropeptides that may be involved in the central regulation of food intake. Treatment with CL 316,243 (1 mg/kg) for 12 or 24 h decreased leptin mRNA abundance and circulating levels to 20% of baseline in normal animals. No effect of the CL 316,243 compound was seen in mice with targeted disruption of the β3 AR gene. Despite the falling leptin levels, β3 agonist administration acutely suppressed food intake. Finally, the induced suppression of food intake and leptin levels occurred despite unchanged or increased hypothalamic expression of the orexigenic neuropeptides NPY and MCH. Thus, β3 AR agonists via β3 ARs suppress leptin levels acutely and simultaneously suppress food intake via a mechanism that operates downstream of leptin and two of its putative central targets.

Tissue-specific nutritional regulation of angiotensinogen in adipose tissue.

Recent studies have found that angiotensinogen is expressed in white and brown fat pads, and adipocytes have been implicated as a primary source of angiotensinogen in several other tissues. The functional significance of this unexpected expression is unknown. To address this, we studied angiotensinogen messenger RNA (mRNA) expression and angiotensinogen secretion in adipose tissue and isolated adipocytes comparing fasted and refed rodents and those with genetic obesity with normal controls. Control 2-month-old Sprague-Dawley rats, those fasted for 3 days, or those fasted for 2 days and refed for 6 days were killed, and adipocytes were isolated from epididymal fat pads using collagenase digestion. Angiotensinogen mRNA was reduced to 14.6 +/- 2.3% of control levels under fasted conditions and increased to 228 +/- 53% of control levels after refeeding. Angiotensinogen release from adipocytes was reduced to 33% of control levels by fasting and increased to 183% by refeeding. These effects of fasting and refeeding on angiotensinogen regulation were tissue specific since liver angiotensinogen mRNA and serum angiotensinogen concentrations were unaffected. Systolic blood pressure, however, was modulated by fasting and refeeding in a manner parallel to adipocyte angiotensinogen expression. In related experiments, angiotensinogen secretion per epididymal fat pad of the ob/ob mouse model of obesity was increased an average of 3.4-fold compared with control. We conclude angiotensinogen expression in white adipocytes is regulated nutritionally in a tissue-specific manner. We propose that adipocyte angiotensinogen could play a previously unrecognized role in regulating adipose tissue blood supply and thereby fatty acid efflux from fat.(ABSTRACT TRUNCATED AT 250 WORDS)

Melanin-concentrating hormone: a functional melanocortin antagonist in the hypothalamus

Melanin-concentrating hormone (MCH) and alpha-melanocyte-stimulating hormone (alpha-MSH) demonstrate opposite actions on skin coloration in teleost fish. Both peptides are present in the mammalian brain, although their specific physiological roles remain largely unknown. In this study, we examined the interactions between MCH and alpha-MSH after intracerebroventricular administration in rats. MCH increased food intake in a dose-dependent manner and lowered plasma glucocorticoid levels through a mechanism involving ACTH. In contrast, alpha-MSH decreased food intake and increased glucocorticoid levels. MCH, at a twofold molar excess, antagonized both actions of alpha-MSH. alpha-MSH, at a threefold molar excess, blocked the orexigenic properties of MCH. MCH did not block alpha-MSH binding or the ability of alpha-MSH to induce cAMP in cells expressing either the MC3 or MC4 receptor, the principal brain alpha-MSH receptor subtypes. These data suggest that MCH and alpha-MSH exert opposing and antagonistic influences on feeding behavior and the stress response and may function in a coordinate manner to regulate metabolism through a novel mechanism mediated in part by an MCH receptor.Melanin-concentrating hormone (MCH) and α-melanocyte-stimulating hormone (α-MSH) demonstrate opposite actions on skin coloration in teleost fish. Both peptides are present in the mammalian brain, although their specific physiological roles remain largely unknown. In this study, we examined the interactions between MCH and α-MSH after intracerebroventricular administration in rats. MCH increased food intake in a dose-dependent manner and lowered plasma glucocorticoid levels through a mechanism involving ACTH. In contrast, α-MSH decreased food intake and increased glucocorticoid levels. MCH, at a twofold molar excess, antagonized both actions of α-MSH. α-MSH, at a threefold molar excess, blocked the orexigenic properties of MCH. MCH did not block α-MSH binding or the ability of α-MSH to induce cAMP in cells expressing either the MC3 or MC4 receptor, the principal brain α-MSH receptor subtypes. These data suggest that MCH and α-MSH exert opposing and antagonistic influences on feeding behavior and the stress response and may function in a coordinate manner to regulate metabolism through a novel mechanism mediated in part by an MCH receptor.

Severe leptin resistance in brown fat-deficient uncoupling protein promoter-driven diphtheria toxin A mice despite suppression of hypothalamic neuropeptide Y and circulating corticosterone concentrations

Brown adipose tissue (BAT) has the capacity for uncoupled mitochondrial respiration and is proposed to be a key site for regulating energy expenditure in rodents. To better define the role of BAT in energy homeostasis, we previously created a line of transgenic mice with deficiency of BAT (UCP promoter-driven diphtheria toxin A transgenic mice [UCP-DTA]) mice. These mice develop obesity that initially is due to decreased energy expenditure and later accompanied by hyperphagia despite increased levels of circulating leptin. In addition, the obesity of these mice is accompanied by severe insulin-resistant diabetes and hyperlipidemia. To better define the basis for leptin resistance in this model, we treated UCP-DTA mice with leptin (300 ug i.p., b.i.d.) and compared their response with that of leptin-treated ob/ob and FVB control mice (30 μg i.p., b.i.d.). Leptin treatment of FVB and ob/ob mice decreased their body weight and food intake and improved their glucose homeostasis. In contrast, tenfold higher dosages of leptin had no effect on body weight, food intake, or circulating insulin or glucose concentrations of UCP-DTA mice. Hypothalamic neuropeptide Y (NPY) mRNA expression was lower in UCP-DTA mice than in littermate control FVB mice in the fed state, and increased progressively in response to food restriction as leptin levels fell. In parallel to the levels of hypothalamic NPY, corticosterone levels were initially suppressed and rose with food restriction. Thus food intake, body weight, and insulin and glucose homeostasis of UCP-DTA mice are all extraordinarily resistant to leptin, whereas hypothalamic NPY and the hypothalamopituitary adrenal (HPA) axis may remain under leptin control. Further elucidation of the mechanisms underlying leptin resistance in UCP-DTA mice may provide valuable insights into the basis for leptin resistance in human obesity.

Metabolic effects of insulin and insulin-like growth factor-I in endotoxemic rats during total parenteral nutrition feeding

The effects of insulin and insulin-like growth factor-I (IGF-I) on protein, energy, and glucose metabolism were examined in endotoxemic rats receiving total parenteral nutrition (TPN) for 3 days. The endotoxemic model was induced by constant infusion of lipopolysaccharide (1 mg/kg x d) for 3 days. The TPN regimen provided 200 kcal/kg x d and 1.5 g protein/kg x d. The dosage of insulin (5 mU/kg x h) and IGF-I (20 microg/kg x h), either alone or in combination, was chosen to maintain normal levels of leucine and glucose in plasma during feeding. One normal control and 4 endotoxemic groups with different treatments (saline, IGF-I, insulin, or IGF-I and insulin) were included. The effects of endotoxin were compared between the group receiving endotoxin alone and normal controls, and the effects of insulin and IGF-I were compared within the endotoxemic groups. The results show that endotoxin significantly increased the mortality and induced a hypermetabolic state, and nutrition alone could not overcome the catabolism induced by endotoxin. However, administration of insulin and IGF-I enhanced protein preservation in muscle tissue in endotoxemic rats during TPN. This effect was greater for insulin either alone or in combination with IGF-I. Insulin also significantly reduced the mortality. There were no additive effects of these two anabolic hormones on any measured parameter in these experimental conditions.