Ghrelin-induced hypothermia: A physiological basis but no clinical risk

Ghrelin increases food intake and decreases energy expenditure, promoting a positive energy balance. We observed a single case of serious hypothermia during sustained ghrelin treatment in a male subject, suggesting that ghrelin may play a role in the regulation of body temperature. We therefore investigated the effect of ghrelin treatment on body temperature in rodents and humans under controlled conditions. Intriguingly, we could demonstrate ghrelin binding in axon terminals of the medial preoptic area of the hypothalamus located in the vicinity of cold-sensitive neurons. This localization of ghrelin receptors provides a potential anatomical basis for the regulation of body temperature by ghrelin. However, our follow-up studies also indicated that neither a chronic i.c.v. application of ghrelin in rats, nor a single s.c. injection under cold exposure in mice resulted in a relevant decrease in body core temperature. In addition, a four-hour intravenous ghrelin infusion did not decrease body surface temperature in healthy humans. We concluded that while there is a theoretical molecular basis for ghrelin to modify body temperature in mammals, its magnitude is irrelevant under physiologic circumstances. Hypothermia is not likely to represent a serious risk associated with this agent and pathway

Fibroblast growth factor 21 mediates specific glucagon actions

Glucagon, an essential regulator of glucose homeostasis, also modulates lipid metabolism and promotes weight loss, as reflected by the wasting observed in glucagonoma patients. Recently, coagonist peptides that include glucagon agonism have emerged as promising therapeutic candidates for the treatment of obesity and diabetes. We developed a novel stable and soluble glucagon receptor (GcgR) agonist, which allowed for in vivo dissection of glucagon action. As expected, chronic GcgR agonism in mice resulted in hyperglycemia and lower body fat and plasma cholesterol. Notably, GcgR activation also raised hepatic expression and circulating levels of fibroblast growth factor 21 (FGF21). This effect was retained in isolated primary hepatocytes from wild-type (WT) mice, but not GcgR knockout mice. We confirmed this link in healthy human volunteers, where injection of natural glucagon increased plasma FGF21 within hours. Functional relevance was evidenced in mice with genetic deletion of FGF21, where GcgR activation failed to induce the body weight loss and lipid metabolism changes observed in WT mice. Taken together, these data reveal for the first time that glucagon controls glucose, energy, and lipid metabolism at least in part via FGF21-dependent pathways

Discovery of High Potency, Single-Chain Insulin Analogs with a Shortened B‑Chain and Nonpeptide Linker

A series of novel, single chain insulin analogs containing polyethylene glycol based connecting segments were synthesized by native chemical ligation and tested for biological activity. While the full length single chain insulin analogs exhibited low potency, deletion of amino acids B26–B30 unexpectedly generated markedly higher activity. This observation is unprecedented in all previous studies of single chain insulin analogs and is consistent with the presumption that in the native hormone this sequence must translocate to achieve high potency insulin receptor interaction. Optimization of the sequence yielded an insulin analog with potency and selectivity comparable to that of native insulin. These results establish a basis for discovery of novel higher potency, single chain insulin analogs of shortened length

Both acyl and des-acyl ghrelin regulate adiposity and glucose metabolism via CNS ghrelin receptors.

Ghrelin receptors (GHSRs) in the central nervous system (CNS) mediate hyperphagia and adiposity induced by acyl ghrelin (AG). Evidence suggests that des-acyl ghrelin (dAG) has biological activity through GHSR independent mechanisms. We combined in vitro and in vivo approaches to test possible GHSR-mediated biological activity of dAG. Both AG (100nM) and dAG (100nM) significantly increased IP3 formation in HEK-293 cells transfected with human GHSR. As expected, intracerebroventricular (icv) infusion of AG in mice increased fat mass (FM), in comparison with the saline-infused controls. Icv-dAG also increased FM at the highest dose tested (5 nmol/day). Chronic icv infusion of AG or dAG increased glucose-stimulated insulin secretion (GSIS). Subcutaneously infused AG regulated FM and GSIS in comparison to saline-infused control mice, whereas dAG failed to regulate these parameters even with doses that were efficacious when delivered icv. Furthermore, icv-dAG failed to regulate FM and induce hyperinsulinemia in GHSR deficient (Ghsr-/-) mice. In addition, a hyperinsulinemic-euglycemic clamp suggests that icv-dAG impairs glucose clearance without affecting endogenous glucose production. Taken together, these data demonstrate that dAG is an agonist of GHSR and regulates body adiposity and peripheral glucose metabolism through a CNS GHSR-dependent mechanism