Impact of physiological levels of chenodeoxycholic acid supplementation on intestinal and hepatic bile acid and cholesterol metabolism in Cyp7a1-deficient mice.

Mice deficient in cholesterol 7α-hydroxylase (Cyp7a1) have a diminished bile acid pool (BAP) and therefore represent a useful model for investigating the metabolic effects of restoring the pool with a specific BA. Previously we carried out such studies in Cyp7a1(-/-) mice fed physiological levels of cholic acid (CA) and achieved BAP restoration, along with an increased CA enrichment, at a dietary level of just 0.03% (w/w). Here we demonstrate that in Cyp7a1(-/-) mice fed chenodeoxycholic acid (CDCA) at a level of 0.06% (w/w), the BAP was restored to normal size and became substantially enriched with muricholic acid (MCA) (>70%), leaving the combined contribution of CA and CDCA to be <15%. This resulted in a partial to complete reversal of the main changes in cholesterol and BA metabolism associated with Cyp7a1 deficiency such as an elevated rate of intestinal sterol synthesis, an enhanced level of mRNA for Cyp8b1 in the liver, and depressed mRNA levels for Ibabp, Shp and Fgf15 in the distal small intestine. When Cyp7a1(-/-) and matching Cyp7a1(+/+) mice were fed a diet with added cholesterol (0.2%) (w/w), either alone, or also containing CDCA (0.06%) (w/w) or CA (0.03%) (w/w) for 18days, the hepatic total cholesterol concentrations (mg/g) in the Cyp7a1(-/-) mice were 26.9±3.7, 16.4±0.9 and 47.6±1.9, respectively, vs. 4.9±0.4, 5.0±0.7 and 6.4±1.9, respectively in the corresponding Cyp7a1(+/+) controls. These data affirm the importance of using moderate levels of dietary BA supplementation to elicit changes in hepatic cholesterol metabolism through shifts in BAP size and composition.

Direct leptin action on POMC neurons regulates glucose homeostasis and hepatic insulin sensitivity in mice.

Leptin action on its receptor (LEPR) stimulates energy expenditure and reduces food intake, thereby lowering body weight. One leptin-sensitive target cell mediating these effects on energy balance is the proopiomelano-cortin (POMC) neuron. Recent evidence suggests that the action of leptin on POMC neurons regulates glucose homeostasis independently of its effects on energy balance. Here, we have dissected the physiological impact of direct leptin action on POMC neurons using a mouse model in which endogenous LEPR expression was prevented by a LoxP-flanked transcription blocker (loxTB), but could be reactivated by Cre recombinase. Mice homozygous for the Lepr(loxTB) allele were obese and exhibited defects characteristic of LEPR deficiency. Reexpression of LEPR only in POMC neurons in the arcuate nucleus of the hypothalamus did not reduce food intake, but partially normalized energy expenditure and modestly reduced body weight. Despite the moderate effects on energy balance and independent of changes in body weight, restoring LEPR in POMC neurons normalized blood glucose and ameliorated hepatic insulin resistance, hyperglucagonemia, and dyslipidemia. Collectively, these results demonstrate that direct leptin action on POMC neurons does not reduce food intake, but is sufficient to normalize glucose and glucagon levels in mice otherwise lacking LEPR.

5-HT2CRs expressed by pro-opiomelanocortin neurons regulate insulin sensitivity in liver.

Mice lacking 5-HT 2C receptors (5-HT(2C)Rs) displayed hepatic insulin resistance, a phenotype normalized by re-expression of 5-HT(2C)Rs only in pro-opiomelanocortin (POMC) neurons. 5-HT(2C)R deficiency also abolished the anti-diabetic effects of meta-chlorophenylpiperazine (a 5-HT(2C)R agonist); these effects were restored when 5-HT(2C)Rs were re-expressed in POMC neurons. Our findings indicate that 5-HT(2C)Rs expressed by POMC neurons are physiologically relevant regulators of insulin sensitivity and glucose homeostasis in the liver.

Direct insulin and leptin action on pro-opiomelanocortin neurons is required for normal glucose homeostasis and fertility.

Circulating leptin and insulin convey information regarding energy stores to the central nervous system, particularly the hypothalamus. Hypothalamic pro-opiomelanocortin (POMC) neurons regulate energy balance and glucose homeostasis and express leptin and insulin receptors. However, the physiological significance of concomitant leptin and insulin action on POMC neurons remains to be established. Here, we show that mice lacking both leptin and insulin receptors in POMC neurons (Pomc-Cre, Lepr(flox/flox) IR(flox/flox) mice) display systemic insulin resistance, which is distinct from the single deletion of either receptor. In addition, Pomc-Cre, Lepr(flox/flox) IR(flox/flox) female mice display elevated serum testosterone levels and ovarian abnormalities, resulting in reduced fertility. We conclude that direct action of insulin and leptin on POMC neurons is required to maintain normal glucose homeostasis and reproductive function.

A beta3-adrenergic-leptin-melanocortin circuit regulates behavioral and metabolic changes induced by chronic stress.

BACKGROUND: Obesity has been associated with an increased risk of developing several psychiatric illnesses, including major depression and posttraumatic stress disorder. Likewise, these stress-related disturbances are associated with a higher rate of obesity; yet, the neurobiological mechanisms linking obesity and stress remain incompletely understood. METHODS: Following exposure to chronic social defeat stress (CSDS), mice were given free access to either regular chow or a Western-style diet high in triglycerides and cholesterol. Comprehensive metabolic and behavioral testing was then conducted. RESULTS: Mice subjected to CSDS and then fed a high-fat diet for 30 days display severe behavioral deficits accompanied by redistribution of body fat. Stressed mice have decreased adipose tissue as well as decreased serum leptin levels compared with control mice. Pharmacological inhibition of beta(3)-adrenergic signaling during CSDS normalizes these metabolic abnormalities but worsens behavioral symptoms. Furthermore, mice subjected to CSDS display central leptin resistance including reduced expression of pro-opiomelanocortin in hypothalamus. Administration of a central melanocortin agonist worsens stress-induced behavioral deficits, while mice lacking the melanocortin-4 receptor display attenuated symptoms. CONCLUSIONS: These results indicate that chronic signaling through beta(3)-adrenergic receptors during social stress is an adaptive response that improves behavioral function. However, these responses come at the expense of central leptin resistance and melanocortin signaling alterations that contribute to significant and long-lasting metabolic abnormalities.