Amylin/leptin synergy is absent in extreme obesity and not restored by calorie restriction-induced weight loss in rats

Amylin, Another Important Neuroendocrine Hormone for the Treatment of Diabesity

Diabetes mellitus is a devastating chronic metabolic disease. Since the majority of type 2 diabetes mellitus patients are overweight or obese, a novel term-diabesity-has emerged. The gut-brain axis plays a critical function in maintaining glucose and energy homeostasis and involves a variety of peptides. Amylin is a neuroendocrine anorexigenic polypeptide hormone, which is co-secreted with insulin from β-cells of the pancreas in response to food consumption. Aside from its effect on glucose homeostasis, amylin inhibits homeostatic and hedonic feeding, induces satiety, and decreases body weight. In this narrative review, we summarized the current evidence and ongoing studies on the mechanism of action, clinical pharmacology, and applications of amylin and its analogs, pramlintide and cagrilintide, in the field of diabetology, endocrinology, and metabolism disorders, such as obesity.

Factors of Obesity and Metabolically Healthy Obesity in Asia

The East Asian region (China, Japan, and South Korea) is comprised of almost 1.5 billion people and recent industrialization has brought with it a pandemic of rising obesity, even in children. As these countries are rapidly aging and functioning at sub-replacement birthrates, the burgeoning costs of obesity-related care may threaten socialized healthcare systems and quality of life. However, a condition called metabolically healthy obesity (MHO) has been found to be without immediate cardiopulmonary or diabetic risk. Thus, maintenance of the MHO condition for the obese in East Asia could buffer the burden of long-term obesity care on medical systems and knowledge of the biochemical, genetic, and physiological milieu associated with it could also provide new targets for intervention. Diverse physiological, psychological, environmental, and social factors play a role in obesogenesis and the transition of MHO to a metabolically unhealthy obesity. This review will give a broad survey of the various causes of obesity and MHO, with special emphasis on the East Asian population and studies from that region.

Mediators of Amylin Action in Metabolic Control

Amylin (also called islet amyloid polypeptide (IAPP)) is a pancreatic beta-cell hormone that is co-secreted with insulin in response to nutrient stimuli. The last 35 years of intensive research have shown that amylin exerts important physiological effects on metabolic control. Most importantly, amylin is a physiological control of meal-ending satiation, and it limits the rate of gastric emptying and reduces the secretion of pancreatic glucagon, in particular in postprandial states. The physiological effects of amylin and its analogs are mediated by direct brain activation, with the caudal hindbrain playing the most prominent role. The clarification of the structure of amylin receptors, consisting of the calcitonin core receptor plus receptor-activity modifying proteins, aided in the development of amylin analogs with a broad pharmacological profile. The general interest in amylin physiology and pharmacology was boosted by the finding that amylin is a sensitizer to the catabolic actions of leptin. Today, amylin derived analogs are considered to be among the most promising approaches for the pharmacotherapy against obesity. At least in conjunction with insulin, amylin analogs are also considered important treatment options in diabetic patients, so that new drugs may soon be added to the only currently approved compound pramlintide (Symlin^®). This review provides a brief summary of the physiology of amylin’s mode of actions and its role in the control of the metabolism, in particular energy intake and glucose metabolism.

Systemic and Central Amylin, Amylin Receptor Signaling, and Their Physiological and Pathophysiological Roles in Metabolism

This article in the Neural and Endocrine Section of Comprehensive Physiology discusses the physiology and pathophysiology of the pancreatic hormone amylin. Shortly after its discovery in 1986, amylin has been shown to reduce food intake as a satiation signal to limit meal size. Amylin also affects food reward, sensitizes the brain to the catabolic actions of leptin, and may also play a prominent role in the development of certain brain areas that are involved in metabolic control. Amylin may act at different sites in the brain in addition to the area postrema (AP) in the caudal hindbrain. In particular, the sensitizing effect of amylin on leptin action may depend on a direct interaction in the hypothalamus. The concept of central pathways mediating amylin action became more complex after the discovery that amylin is also synthesized in certain hypothalamic areas but the interaction between central and peripheral amylin signaling remains currently unexplored. Amylin may also play a dominant pathophysiological role that is associated with the aggregation of monomeric amylin into larger, cytotoxic molecular entities. This aggregation in certain species may contribute to the development of type 2 diabetes mellitus but also cardiovascular disease. Amylin receptor pharmacology is complex because several distinct amylin receptor subtypes have been described, because other neuropeptides [e.g., calcitonin gene-related peptide (CGRP)] can also bind to amylin receptors, and because some components of the functional amylin receptor are also used for other G-protein coupled receptor (GPCR) systems. © 2020 American Physiological Society. Compr Physiol 10:811-837, 2020.

Leptin and psychiatric illnesses: does leptin play a role in antipsychotic-induced weight gain?

Antipsychotic-induced weight gain is the most prevalent somatic adverse event occurring in patients treated by antipsychotics, especially atypical antipsychotics. It is of particular interest because of its repercussion on cardiovascular morbidity and mortality especially now that the use of second-generation antipsychotics has been extended to other mental health illnesses such as bipolar disorders and major depressive disorder. The mechanism underlying antipsychotics-induced weight gain is still poorly understood despite a significant amount of work on the topic. Recently, there has been an on-going debate of tremendous research interest on the relationship between antipsychotic-induced weight gain and body weight regulatory hormones such as leptin. Given that, researchers have brought to light the question of leptin's role in antipsychotic-induced weight gain. Here we summarize and discuss the existing evidence on the link between leptin and weight gain related to antipsychotic drugs, especially atypical antipsychotics.

Is "Leptin Resistance" Another Key Resistance to Manage Type 2 Diabetes?

Although novel pharmacological options for the treatment of type 2 diabetes mellitus (DM2) have been observed to modulate the functionality of several key organs in glucose homeostasis, successful regulation of insulin resistance (IR), body weight management, and pharmacological treatment of obesity remain notable problems in endocrinology. Leptin may be a pivotal player in this scenario, as an adipokine which centrally regulates appetite and energy balance. In obesity, excessive caloric intake promotes a low-grade inflammatory response, which leads to dysregulations in lipid storage and adipokine secretion. In turn, these entail alterations in leptin sensitivity, leptin transport across the blood-brain barrier and defects in post-receptor signaling. Furthermore, hypothalamic inflammation and endoplasmic reticulum stress may increase the expression of molecules which may disrupt leptin signaling. Abundant evidence has linked obesity and leptin resistance, which may precede or occur simultaneously to IR and DM2. Thus, leptin sensitivity may be a potential early therapeutic target that demands further preclinical and clinical research. Modulators of insulin sensitivity have been tested in animal models and small clinical trials with promising results, especially in combination with agents such as amylin and GLP-1 analogs, in particular, due to their central activity in the hypothalamus.

Amylin and Leptin interaction: Role During Pregnancy, Lactation and Neonatal Development

Amylin is co-secreted with insulin by pancreatic β-cells in response to a meal and produced by neurons in discrete hypothalamic brain areas. Leptin is proportionally secreted by the adipose tissue. Both hormones control food intake and energy homeostasis post-weaning in rodents. While amylin's main site of action is located in the area postrema (AP) and leptin's is located in the mediobasal hypothalamus, both hormones can also influence the other's signaling pathway; amylin has been shown enhance hypothalamic leptin signaling, and amylin signaling in the AP may rely on functional leptin receptors to modulate its effects. These two hormones also play major roles during other life periods. During pregnancy, leptin levels rise as a result of an increase in fat depot resulting in gestational leptin-resistance to prepare the maternal body for the metabolic needs during fetal development. The role of amylin is far less studied during pregnancy and lactation, though amylin levels seem to be elevated during pregnancy relative to insulin. Whether amylin and leptin interact during pregnancy and lactation remains to be assessed. Lastly, during brain development, amylin and leptin are major regulators of cell birth during embryogenesis and act as neurotrophic factors in the neonatal period. This review will highlight the role of amylin and leptin, and their possible interaction, during these dynamic time periods of pregnancy, lactation, and early development.

Emerging hormonal-based combination pharmacotherapies for the treatment of metabolic diseases

Obesity and its comorbidities, such as type 2 diabetes mellitus and cardiovascular disease, constitute growing challenges for public health and economies globally. The available treatment options for these metabolic disorders cannot reverse the disease in most individuals and have not substantially reduced disease prevalence, which underscores the unmet need for more efficacious interventions. Neurobiological resilience to energy homeostatic perturbations, combined with the heterogeneous pathophysiology of human metabolic disorders, has limited the sustainability and efficacy of current pharmacological options. Emerging insights into the molecular origins of eating behaviour, energy expenditure, dyslipidaemia and insulin resistance suggest that coordinated targeting of multiple signalling pathways is probably necessary for sizeable improvements to reverse the progression of these diseases. Accordingly, a broad set of combinatorial approaches targeting feeding circuits, energy expenditure and glucose metabolism in concert are currently being explored and developed. Notably, several classes of peptide-based multi-agonists and peptide-small molecule conjugates with superior preclinical efficacy have emerged and are currently undergoing clinical evaluation. Here, we summarize advances over the past decade in combination pharmacotherapy for the management of obesity and type 2 diabetes mellitus, exclusively focusing on large-molecule formats (notably enteroendocrine peptides and proteins) and discuss the associated therapeutic opportunities and challenges.

Anti-Obesity Therapy: from Rainbow Pills to Polyagonists

With their ever-growing prevalence, obesity and diabetes represent major health threats of our society. Based on estimations by the World Health Organization, approximately 300 million people will be obese in 2035. In 2015 alone there were more than 1.6 million fatalities attributable to hyperglycemia and diabetes. In addition, treatment of these diseases places an enormous burden on our health care system. As a result, the development of pharmacotherapies to tackle this life-threatening pandemic is of utmost importance. Since the beginning of the 19th century, a variety of drugs have been evaluated for their ability to decrease body weight and/or to improve deranged glycemic control. The list of evaluated drugs includes, among many others, sheep-derived thyroid extracts, mitochondrial uncouplers, amphetamines, serotonergics, lipase inhibitors, and a variety of hormones produced and secreted by the gastrointestinal tract or adipose tissue. Unfortunately, when used as a single hormone therapy, most of these drugs are underwhelming in their efficacy or safety, and placebo-subtracted weight loss attributed to such therapy is typically not more than 10%. In 2009, the generation of a single molecule with agonism at the receptors for glucagon and the glucagon-like peptide 1 broke new ground in obesity pharmacology. This molecule combined the beneficial anorectic and glycemic effects of glucagon-like peptide 1 with the thermogenic effect of glucagon into a single molecule with enhanced potency and sustained action. Several other unimolecular dual agonists have subsequently been developed, and, based on their preclinical success, these molecules illuminate the path to a new and more fruitful era in obesity pharmacology. In this review, we focus on the historical pharmacological approaches to treat obesity and glucose intolerance and describe how the knowledge obtained by these studies led to the discovery of unimolecular polypharmacology.

Serum Preptin and Amylin Levels with Respect to Body Mass Index in Polycystic Ovary Syndrome Patients

Background

Preptin and amylin are pancreatic hormones which participate in glucose homeostasis. This study aimed to evaluate how serum preptin and amylin levels are altered in polycystic ovary syndrome (PCOS) patients and healthy women based on BMI groups (<25 kg/m² and ≥25 kg/m²).

Material/Methods

This was a prospective randomized control study of 40 PCOS patients and 40 healthy women who were matched with respect to BMI (<25 kg/m² and ≥25 kg/m²).

Results

When compared to the healthy women, PCOS patients had significantly higher ovarian volumes, Ferriman-Gallwey scores, and free and total testosterone levels, but significantly lower amylin concentrations (p=0.001, p=0.001, p=0.049, p=0.021, and p<0.001, respectively). Both the normal-weight and overweight PCOS patients had significantly lower amylin levels than the normal-weight and overweight controls (p<0.001, p=0.009, p=0.001, and p=0.001, respectively). Amylin levels were negatively and significantly correlated with the Ferriman-Gallwey scores (r=−0.272, p=0.001) and ovarian volume (r=−0.206, p=0.007). Serum preptin levels were not elevated in either group.

Conclusions

Serum preptin levels are statistically similar in PCOS patients and BMI-matched healthy controls. Serum amylin levels are significantly higher in healthy controls than PCOS patients whether they are slim or overweight. These findings suggest the presence of mechanisms that can prevent the elevation in serum amylin concentrations that can occur in response to the impaired glucose metabolism in PCOS patients.

Rodent models of leptin receptor deficiency are less sensitive to amylin

The pancreatic hormone amylin is released from beta cells following nutrient ingestion and contributes to the control of body weight and glucose homeostasis. Amylin reduces food intake by activating neurons in the area postrema (AP). Amylin was also shown to synergize with the adipokine leptin, with combination therapy producing greater weight loss and food intake reduction than either hormone alone. Although amylin and leptin were initially thought to interact downstream of the AP in the hypothalamus, recent findings show that the two hormones can act on the same AP neurons, suggesting a more direct relationship. The objective of this study was to determine whether amylin action depends on functional leptin signaling. We tested the ability of amylin to induce satiation and to activate its primary target neurons in the AP in two rodent models of LepR deficiency, the db/db mouse and the Zucker diabetic fatty (ZDF) rat. When compared with wild-type (WT) mice, db/db mice exhibited reduced amylin-induced satiation, reduced amylin-induced Fos in the AP, and a lower expression of calcitonin receptor (CTR) protein, the core component of all amylin receptors. ZDF rats also showed no reduction in food intake following amylin treatment; however, unlike the db/db mice, levels of amylin-induced Fos and CTR in the AP were no different than WT rats. Our results suggest that LepR expression is required for the full anorexic effect of amylin; however, the neuronal activation in the AP seems to depend on the type of LepR mutation.

Leptin and Leptin Resistance in the Pathogenesis of Obstructive Sleep Apnea: A Possible Link to Oxidative Stress and Cardiovascular Complications

Obesity-related sleep breathing disorders such as obstructive sleep apnea (OSA) and obesity hypoventilation syndrome (OHS) cause intermittent hypoxia (IH) during sleep, a powerful trigger of oxidative stress. Obesity also leads to dramatic increases in circulating levels of leptin, a hormone produced in adipose tissue. Leptin acts in the hypothalamus to suppress food intake and increase metabolic rate. However, obese individuals are resistant to metabolic effects of leptin. Leptin also activates the sympathetic nervous system without any evidence of resistance, possibly because these effects occur peripherally without a need to penetrate the blood-brain barrier. IH is a potent stimulator of leptin expression and release from adipose tissue. Hyperleptinemia and leptin resistance may upregulate generation of reactive oxygen species, increasing oxidative stress and promoting inflammation. The current review summarizes recent data on a possible link between leptin and oxidative stress in the pathogenesis of sleep breathing disorders.

Amylin and Leptin: Co-Regulators of Energy Homeostasis and Neuronal Development

While the regulation of energy homeostasis by amylin is already well-characterized, emerging data suggest that amylin is also crucial for the development of neural pathways in the hypothalamus and caudal hindbrain (area postrema, AP; nucleus tractus solitarius, NTS). Exciting new findings demonstrate crucial amylin-leptin interactions in altering the activity of specific hypothalamic and AP neurons, and a role for amylin as a novel class of 'leptin sensitizers' which enhance leptin signaling in both leptin-sensitive and -resistant individuals, in part by stimulating IL-6 production by hypothalamic microglia. This review summarizes these findings and provides a hypothetical framework for future studies to elucidate the mechanisms by which amylin and leptin act individually and as co-conspirators to alter energy homeostasis and neuronal development.

Amylin/leptin synergy is absent in extreme obesity and not restored by calorie restriction-induced weight loss in rats

Objective

Co‐administration of amylin and leptin induces synergistic and clinically meaningful (>10%) weight loss that is attenuated as the degree of obesity increases. We explored whether calorie restriction (CR) could restore amylin/leptin synergy in very obese rats.

Methods

Sprague Dawley rats on high‐fat diet (696 ± 8 g, n = 72) were randomized to three cohorts (C1–C3). Rats in C1 were administered vehicle, rat amylin (50 µg kg⁻¹ d⁻¹), murine leptin (125 µg kg⁻¹ d⁻¹) or amylin and leptin for 28 days (n = 6 per group) via subcutaneous minipump. Simultaneously, C2 and C3 rats initiated CR. After moderate (12.4 ± 0.3%, 86.7 ± 2.8 g; C2) or severe (24.9 ± 0.3%, 172.7 ± 4.7 g; C3) weight loss, amylin and/or leptin was administered as described.

Results

In C1, leptin did not alter weight, and amylin induced 40.2 ± 6.1 g weight loss (−6.0 ± 0.9%), which was not enhanced by leptin (44.4 ± 4.9 g, −6.1 ± 0.8%). In C2, vehicle‐treated (75.1 ± 7.8 g weight change from start of treatment, 1.1 ± 0.8% difference from start of pre‐CR phase) and leptin‐treated rats (68.6 ± 9.2 g, −1.3 ± 1.0%) rebounded to pre‐restriction weight that was attenuated by amylin (29.2 ± 11.4 g, −6.2 ± 0.7%). Leptin did not enhance the effect of amylin (22.8 ± 11.7 g, −8.3 ± 1.5%). In C3, vehicle‐treated and leptin‐treated rats regained most of their weight (161.9 ± 11.8, −2.3 ± 0.8% and 144.6 ± 9.5 g, −2.3 ± 0.9%, respectively), which was attenuated by amylin (91.1 ± 16.8 g, −11.2 ± 0.7%), but not enhanced by leptin (83.0 ± 7.6 g, −10.7 ± 0.8%).

Conclusions

Extreme obesity associated with leptin resistance perturbs amylin/leptin weight loss synergy in rats, which cannot be restored by pre‐treatment weight loss.