Kristy M. Heppner

Unimolecular Dual Incretins Maximize Metabolic Benefits in Rodents, Monkeys, and Humans

Compared to best-in-class GLP-1 mono-agonists, unimolecular co-agonists of GLP-1 and GIP with optimized pharmacokinetics enhance glycemic and metabolic benefits in mammals. “Twincretins”: Two Is Better than One Despite obesity-linked diabetes approaching worldwide epidemic proportions and the growing recognition of it as a global health challenge, safe and effective medicines have remained largely elusive. Pharmacological options targeting multiple obesity and diabetes signaling pathways offer greater therapeutic potential compared to molecules targeting a single pathway. Finan et al. now report the discovery, characterization, and translational efficacy of a single molecule that acts equally on the receptors for the incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). In rodent models of obesity and diabetes, this dual incretin co-agonist more effectively lowered body fat and corrected hyperglycemia than selective mono-agonists for the GLP-1 and GIP receptors. An enhanced insulinotropic effect translated from rodents to monkeys and humans, with substantially improved levels of glycosylated hemoglobin A1c (HbA1c) in humans with type 2 diabetes. The dual incretin was engineered with selective chemical modifications to enhance pharmacokinetics. This, in combination with its inherent mixed agonism, lowered the drug dose and ameliorated the dose-limiting nausea that has plagued selective GLP-1 therapies. These dual incretin co-agonists signify a new direction for unimolecular combination therapy and represent a new class of drug candidates for the treatment of metabolic diseases. We report the discovery and translational therapeutic efficacy of a peptide with potent, balanced co-agonism at both of the receptors for the incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). This unimolecular dual incretin is derived from an intermixed sequence of GLP-1 and GIP, and demonstrated enhanced antihyperglycemic and insulinotropic efficacy relative to selective GLP-1 agonists. Notably, this superior efficacy translated across rodent models of obesity and diabetes, including db/db mice and ZDF rats, to primates (cynomolgus monkeys and humans). Furthermore, this co-agonist exhibited synergism in reducing fat mass in obese rodents, whereas a selective GIP agonist demonstrated negligible weight-lowering efficacy. The unimolecular dual incretins corrected two causal mechanisms of diabesity, adiposity-induced insulin resistance and pancreatic insulin deficiency, more effectively than did selective mono-agonists. The duration of action of the unimolecular dual incretins was refined through site-specific lipidation or PEGylation to support less frequent administration. These peptides provide comparable pharmacology to the native peptides and enhanced efficacy relative to similarly modified selective GLP-1 agonists. The pharmacokinetic enhancement lessened peak drug exposure and, in combination with less dependence on GLP-1–mediated pharmacology, avoided the adverse gastrointestinal effects that typify selective GLP-1–based agonists. This discovery and validation of a balanced and high-potency dual incretin agonist enables a more physiological approach to management of diseases associated with impaired glucose tolerance.

The metabolic actions of glucagon revisited

The initial identification of glucagon as a counter-regulatory hormone to insulin revealed this hormone to be of largely singular physiological and pharmacological purpose. Glucagon agonism, however, has also been shown to exert effects on lipid metabolism, energy balance, body adipose tissue mass and food intake. The ability of glucagon to stimulate energy expenditure, along with its hypolipidemic and satiating effects, in particular, make this hormone an attractive pharmaceutical agent for the treatment of dyslipidemia and obesity. Studies that describe novel preclinical applications of glucagon, alone and in concert with glucagon-like peptide 1 agonism, have revealed potential benefits of glucagon agonism in the treatment of the metabolic syndrome. Collectively, these observations challenge us to thoroughly investigate the physiology and therapeutic potential of insulins long-known opponent.

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.

Direct Control of Brown Adipose Tissue Thermogenesis by Central Nervous System Glucagon-Like Peptide-1 Receptor Signaling

We studied interscapular brown adipose tissue (iBAT) activity in wild-type (WT) and glucagon-like peptide 1 receptor (GLP-1R)–deficient mice after the administration of the proglucagon-derived peptides (PGDPs) glucagon-like peptide (GLP-1), glucagon (GCG), and oxyntomodulin (OXM) directly into the brain. Intracerebroventricular injection of PGDPs reduces body weight and increases iBAT thermogenesis. This was independent of changes in feeding and insulin responsiveness but correlated with increased activity of sympathetic fibers innervating brown adipose tissue (BAT). Despite being a GCG receptor agonist, OXM requires GLP-1R activation to induce iBAT thermogenesis. The increase in thermogenesis in WT mice correlates with increased expression of genes upregulated by adrenergic signaling and required for iBAT thermogenesis, including PGC1a and UCP-1. In spite of the increase in iBAT thermogenesis induced by GLP-1R activation in WT mice, Glp1r−/− mice exhibit a normal response to cold exposure, demonstrating that endogenous GLP-1R signaling is not essential for appropriate thermogenic response after cold exposure. Our data suggest that the increase in BAT thermogenesis may be an additional mechanism whereby pharmacological GLP-1R activation controls energy balance.

Ghrelin-induced adiposity is independent of orexigenic effects

Ghrelin is a hormone produced predominantly by the stomach that targets a number of specific areas in the central nervous system to promote a positive energy balance by increasing food intake and energy storage. In that respect, similarities exist with the effects of consuming a high‐fat diet (HFD), which also increases caloric intake and the amount of stored calories. We determined whether the effects of ghrelin on feeding and adiposity are influenced by the exposure to an HFD. Chronic intracerebroventricular ghrelin (2.5 nmol/d) increased feeding in lean rats fed a low‐fat control diet (CD) [192±5 g (ghrelin+CD) vs. 152±5 g (control i.c.v. saline+CD), P<0.001], but the combination of ghrelin plus HFD did not result in significantly greater hyperphagia [150±7 g (ghrelin+HFD) vs. 136±4 g (saline+HFD)]. Despite failing to increase food intake in rats fed the HFD, ghrelin nonetheless increased adiposity [fat mass increase of 14±2 g (ghrelin+HFD) vs. 1±1 g (saline+HFD), P<0.001] up‐regulating the gene expression of lipogenic enzymes in white adipose tissue. Our findings demonstrate that factors associated with high‐fat feeding functionally interact with pathways regulating the effect of ghrelin on food intake. We conclude that ghrelins central effects on nutrient intake and nutrient partitioning can be separated and suggest an opportunity to identify respective independent neuronal pathways.— Perez‐Tilve, D., Heppner, K., Kirchner, H., Lockie, S. H., Woods, S. C., Smiley, D. L., Tschöp, M., and Pfluger, P. Ghrelin‐induced adiposity is independent of orexigenic effects. FASEB J. 25, 2814‐2822 (2011). www.fasebj.org

Glucagon regulation of energy metabolism.

Glucagon has long been known as a counter-regulatory hormone to insulin of fundamental importance to glucose homeostasis. Its prominent ability to stimulate glycogenolysis and gluconeogenesis, has historically cast this peptide as one hormone where the metabolic consequences of increasing blood glucose levels, especially in obesity, are viewed largely as being deleterious. This perspective may be changing in light of emerging data and reconsideration of historic studies, which suggest that glucagon has beneficial effects on body fat mass, food intake, and energy expenditure. In this review, we discuss the mechanisms of glucagon-mediated body weight regulation as well as possible novel therapeutic approaches in the treatment of obesity and glucose intolerance that may arise from these findings. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

Goat: the master switch for the ghrelin system?

The ghrelin-ghrelin receptor system is one of the most important mechanisms regulating energy balance and metabolism. Among other actions, central and peripheral administration of ghrelin increases food intake and adiposity. During the last years, many efforts have been made in the investigation of the cellular and molecular mechanisms modulating the effects of ghrelin. One particularity of this peptide hormone is its acylation at serine-3 with an eight-carbon fatty acid (octanoate), which confers its biological activity. Recent reports have demonstrated that the ghrelin O-acyltransferase (GOAT) is the enzyme that catalyzes ghrelin octanoylation. Therefore, all questions concerning the posttranslational acylation of ghrelin are of great interest for the complete understanding of this system. In this review, we summarize the discovery and characterization of GOAT, and remark the importance of GOAT as a novel and potential target that regulates the biological actions of ghrelin, revealing several therapeutical possibilities for the treatment of the metabolic syndrome.

Both Acyl and Des-Acyl Ghrelin Regulate Adiposity and Glucose Metabolism via Central Nervous System Ghrelin Receptors

Growth hormone secretagogue receptors (GHSRs) in the central nervous system (CNS) mediate hyperphagia and adiposity induced by acyl ghrelin (AG). Evidence suggests that des-AG (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 (100 nmol/L) and dAG (100 nmol/L) significantly increased inositol triphosphate formation in human embryonic kidney-293 cells transfected with human GHSR. As expected, intracerebroventricular infusion of AG in mice increased fat mass (FM), in comparison with the saline-infused controls. Intracerebroventricular dAG also increased FM at the highest dose tested (5 nmol/day). Chronic intracerebroventricular infusion of AG or dAG increased glucose-stimulated insulin secretion (GSIS). Subcutaneously infused AG regulated FM and GSIS in comparison with saline-infused control mice, whereas dAG failed to regulate these parameters even with doses that were efficacious when delivered intracerebroventricularly. Furthermore, intracerebroventricular dAG failed to regulate FM and induce hyperinsulinemia in GHSR-deficient (Ghsr−/−) mice. In addition, a hyperinsulinemic-euglycemic clamp suggests that intracerebroventricular dAG impairs glucose clearance without affecting endogenous glucose production. 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.

p62 Links β-adrenergic input to mitochondrial function and thermogenesis

The scaffold protein p62 (sequestosome 1; SQSTM1) is an emerging key molecular link among the metabolic, immune, and proliferative processes of the cell. Here, we report that adipocyte-specific, but not CNS-, liver-, muscle-, or myeloid-specific p62-deficient mice are obese and exhibit a decreased metabolic rate caused by impaired nonshivering thermogenesis. Our results show that p62 regulates energy metabolism via control of mitochondrial function in brown adipose tissue (BAT). Accordingly, adipocyte-specific p62 deficiency led to impaired mitochondrial function, causing BAT to become unresponsive to β-adrenergic stimuli. Ablation of p62 leads to decreased activation of p38 targets, affecting signaling molecules that control mitochondrial function, such as ATF2, CREB, PGC1α, DIO2, NRF1, CYTC, COX2, ATP5β, and UCP1. p62 ablation in HIB1B and BAT primary cells demonstrated that p62 controls thermogenesis in a cell-autonomous manner, independently of brown adipocyte development or differentiation. Together, our data identify p62 as a novel regulator of mitochondrial function and brown fat thermogenesis.

The Role of Ghrelin in the Control of Energy Balance

Ghrelin is the only potent orexigenic peptide in circulation. It stimulates food intake and leads to positive energy balance, adipogenesis, and body weight gain. However, the physiological significance of ghrelin in the regulation of energy homeostasis is controversial, since loss of ghrelin function in rodents does not necessarily lead to anorexia and weight loss. In this chapter, we discuss the metabolic function of ghrelin and are highlighting recent findings including the discovery and function of ghrelin-acylating enzyme ghrelin O-acyltransferase (GOAT). Based on available published data, we conclude that ghrelin is a principally important endogenous regulator of energy balance, which however may affect both food intake and systemic metabolism via independent mechanisms. Importantly, ghrelin, when acylated by GOAT, might represent a key molecular link between the sensing of consumed calories and the neuroendocrine control of energy homeostasis. Thus, agents antagonizing the action of ghrelin may have therapeutic potential in the therapy of obesity.