Ciliary neurotrophic factor improves diabetic parameters and hepatic steatosis and increases basal metabolic rate in db/db mice

Obesity plays a central role in the development of insulin resistance and type 2 diabetes. We therefore examined the effects of a modified form of ciliary neurotrophic factor [Axokine, which is hereafter referred to as ciliary neurotrophic factor (CNTF)(Ax15)], which uses a leptin-like mechanism to reduce body weight, in the db/db murine model of type 2 diabetes. In previous studies, weight loss produced by CNTF treatment could largely be attributed to its effects on food intake. In contrast, CNTF(Ax15) treatment of db/db mice caused significantly greater weight loss and marked improvements in diabetic parameters (e.g., levels of glucose, insulin, triglyceride, cholesterol, and nonesterified free fatty acids) than could be accounted for by reduced caloric intake alone. These beneficial effects, above and beyond those seen in animals controlled for either food restriction or body weight, correlated with the ability of CNTF(Ax15) to increase metabolic rate and energy expenditure and reduce hepatic steatosis while enhancing hepatic responsiveness to insulin. The hepatic effects were linked to rapid alterations in hepatic gene expression, most notably reduced expression of stearoyl-CoA desaturase 1, a rate-limiting enzyme in the synthesis of complex lipids that is also markedly suppressed by leptin in ob/ob mice. These observations further link the mechanisms of CNTF and leptin action, and they suggest important, beneficial effects for CNTF in diabetes that may be distinct from its ability to decrease food intake; instead, these effects may be more related to its influence on energy expenditure and hepatic gene expression

N-acetylation of hypothalamic α-melanocyte-stimulating hormone and regulation by leptin

The central melanocortin system is critical in the regulation of appetite and body weight, and leptin exerts its anorexigenic actions partly by increasing hypothalamic proopiomelanocortin (POMC) expression. The POMC-derived peptide α-melanocyte-stimulating hormone (αMSH) is a melanocortin 4 receptor agonist, and its potency in reducing energy intake is strongly increased by N-acetylation. The reason for the higher biological activity of N-acetylated αMSH (Act-αMSH) compared with that of N-desacetylated αMSH (Des-αMSH) is unclear, and regulation of acetylation by leptin has not been investigated. We show here that total hypothalamic αMSH levels are decreased in leptin-deficient ob/ob mice and increased in leptin-treated ob/ob and C57BL/6J mice. The increase in total αMSH occurred as soon as 3 h after leptin injection and was entirely due to an increase in Act-αMSH. Consistent with this observation, leptin rapidly induced the enzymatic activity of a N-acetyltransferase in the hypothalamus of mice. In 293T cells expressing the melanocortin 4 receptor, Act-αMSH is far more potent than Des-αMSH in stimulating cAMP accumulation, an effect caused by a dramatically increased stability of Act-αMSH. Moreover, Des-αMSH is rapidly degraded in the hypothalamus after intracerebroventricular injection in rats and was less potent in inhibiting energy intake. The results suggest that leptin activates a N-acetyltransferase in POMC neurons, leading to increased hypothalamic levels of Act-αMSH. Due to its increased stability, this posttranslational modification of αMSH may play a critical role in leptin action via the central melanocortin pathway

Dissociation of the neuronal regulation of bone mass and energy metabolism by leptin in vivo

The leptin regulation of bone remodeling, which has been documented through studies of loss-of-function mutations of this hormone or of its receptor in mice and humans, still raised several unanswered questions. For instance, it has been assumed but not formally demonstrated that this regulation occurs through neuronal means. Likewise, it has not been possible until now to dissociate the influence leptin exerts on appetite and energy expenditure from this function. We show here through mouse genetic studies that a deletion of the leptin receptor in neurons results in an increase in bone formation and bone resorption, resulting in a high bone mass as seen in leptin-deficient mice. In contrast, the same deletion in osteoblasts only does not influence bone remodeling. Furthermore, through the use of l/l mice, a model of gain of function in leptin signaling harboring a Y985L substitution in the leptin receptor, we show that leptin signaling inhibits bone mass accrual by up-regulating sympathetic activity independently of any change in appetite or energy expenditure. This work establishes that in vivo leptin regulates bone mass accrual by acting through neuronal means and provides a direct demonstration that this function of leptin can occur independently of its regulation of energy metabolism