Melanotan II, a melanocortin agonist, partially rescues the impaired thermogenic capacity of pituitary adenylate cyclase-activating polypeptide deficient mice.

NEW FINDINGS: What is the central question of this study? Can chronic treatment of pituitary adenylate cyclase-activating polypeptide (PACAP) deficient mice with the melanocortin agonist melanotan II during cold acclimation rescue the impaired thermogenic capacity previously observed in PACAP deficient mice? What is the main finding and its importance? Using a genetic model of PACAP deficiency, this study provides evidence that PACAP acts upstream of the melanocortin system in regulating sympathetic nerve activity to brown adipose tissue in mice. ABSTRACT: Impaired adipose tissue function in obesity, including reduced thermogenic potential, has detrimental consequences for metabolic health. Hormonal regulation of adaptive thermogenesis is being explored as a potential therapeutic target for human obesity. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide expressed in nuclei of the hypothalamus known to regulate energy expenditure, and functional studies reveal a role for PACAP in the central regulation of thermogenesis, although mechanisms are not well understood. We hypothesized that PACAP acts upstream of the melanocortin system to regulate sympathetic nerve activity to stimulate thermogenesis. To assess this, female PACAP-/- and PACAP+/+ mice were given daily peripheral injections of a melanocortin receptor agonist, melanotan II (MTII), for 3 weeks during cold acclimation, and the effect of MTII on thermogenic capacity and adipose tissue remodelling was examined by physiological and histological analyses. MTII partially rescued the impaired thermogenic capacity in PACAP-/- mice as compared to PACAP+/+ mice as determined by measuring noradrenaline-induced metabolic rate. In addition, MTII treatment during cold acclimation corrected the previously identified deficit in lipid utilization in response to adrenergic stimulation in PACAP-/- null mice, suggesting impaired lipid mobilization may contribute to the impaired thermogenic capacity of PACAP-/- mice. Results presented here provide physiological evidence to suggest that PACAP acts upstream of melanocortin receptors to facilitate sympathetically induced mechanisms of adaptive thermogenesis in response to cold acclimation.

PACAP in the Defense of Energy Homeostasis.

The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) mediates diverse physiology from neuroprotection to thermoregulation. PACAP is well established as a master regulator of the stress response, regulating psychological and physiological equilibrium via the autonomic nervous system. Neuroanatomical and functional evidence support a role for PACAP in energy metabolism, including thermogenesis, activity, mobilization of energy stores, and appetite. Through integration of this evidence we suggest PACAP be included in the growing list of neuropeptides that mediate energy homeostasis. Future work to uncover the intricacies of PACAP expression and the molecular pathways responsible for PACAP signaling may show potential for this neuropeptide as a therapeutic target as well as further elucidate the complex neuroanatomical networks involved in defending energy balance.

PACAP is essential for the adaptive thermogenic response of brown adipose tissue to cold exposure.

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a widely distributed neuropeptide that acts as a neurotransmitter, neuromodulator, neurotropic factor, neuroprotectant, secretagogue, and neurohormone. Owing to its pleiotropic biological actions, knockout of Pacap (Adcyap1) has been shown to induce several abnormalities in mice such as impaired thermoregulation. However, the underlying physiological and molecular mechanisms remain unclear. A previous report has shown that cold-exposed Pacap null mice cannot supply appropriate levels of norepinephrine (NE) to brown adipocytes. Therefore, we hypothesized that exogenous NE would rescue the impaired thermogenic response of Pacap null mice during cold exposure. We compared the adaptive thermogenic capacity of Pacap(-/-) to Pacap(+/+) mice in response to NE when housed at room temperature (24 °C) and after a 3.5-week cold exposure (4 °C). Biochemical parameters, expression of thermogenic genes, and morphological properties of brown adipose tissue (BAT) and white adipose tissue (WAT) were also characterized. Results showed that there was a significant effect of temperature, but no effect of genotype, on the resting metabolic rate in conscious, unrestrained mice. However, the normal cold-induced increase in the basal metabolic rate and NE-induced increase in thermogenesis were severely blunted in cold-exposed Pacap(-/-) mice. These changes were associated with altered substrate utilization, reduced ß3-adrenergic receptor (ß3-Ar (Adrb3)) and hormone-sensitive lipase (Hsl (Lipe)) gene expression, and increased fibroblast growth factor 2 (Fgf2) gene expression in BAT. Interestingly, Pacap(-/-) mice had depleted WAT depots, associated with upregulated uncoupling protein 1 expression in inguinal WATs. These results suggest that the impairment of adaptive thermogenesis in Pacap null mice cannot be rescued by exogenous NE perhaps in part due to decreased ß3-Ar-mediated BAT activation.

Overexpression of PPARγ specifically in pancreatic ß-cells exacerbates obesity-induced glucose intolerance, reduces ß-cell mass, and alters islet lipid metabolism in male mice.

The contribution of peroxisomal proliferator-activated receptor (PPAR)-γ agonism in pancreatic ß-cells to the antidiabetic actions of thiazolidinediones has not been clearly elucidated. Genetic models of pancreatic ß-cell PPARγ ablation have revealed a potential role for PPARγ in ß-cell expansion in obesity but a limited role in normal ß-cell physiology. Here we overexpressed PPARγ1 or PPARγ2 specifically in pancreatic ß-cells of mice subjected to high-fat feeding using an associated adenovirus (ß-PPARγ1-HFD and ß-PPARγ2-HFD mice). We show ß-cell-specific PPARγ1 or PPARγ2 overexpression in diet-induced obese mice exacerbated obesity-induced glucose intolerance with decreased ß-cell mass, increased islet cell apoptosis, and decreased plasma insulin compared with obese control mice (ß-eGFP-HFD mice). Analysis of islet lipid composition in ß-PPARγ2-HFD mice revealed no significant changes in islet triglyceride content and an increase in only one of eight ceramide species measured. Interestingly ß-PPARγ2-HFD islets had significantly lower levels of lysophosphatidylcholines, lipid species shown to enhance insulin secretion in ß-cells. Gene expression profiling revealed increased expression of uncoupling protein 2 and genes involved in fatty acid transport and ß-oxidation. In summary, transgenic overexpression of PPARγ in ß-cells in diet-induced obesity negatively impacts whole-animal carbohydrate metabolism associated with altered islet lipid content, increased expression of ß-oxidative genes, and reduced ß-cell mass.