Ghrelin in the control of energy, lipid, and glucose metabolism

Ghrelin octanoylation by ghrelin O-acyltransferase: protein acylation impacting metabolic and neuroendocrine signalling

The acylated peptide hormone ghrelin impacts a wide range of physiological processes but is most well known for controlling hunger and metabolic regulation. Ghrelin requires a unique posttranslational modification, serine octanoylation, to bind and activate signalling through its cognate GHS-R1a receptor. Ghrelin acylation is catalysed by ghrelin O-acyltransferase (GOAT), a member of the membrane-bound O-acyltransferase (MBOAT) enzyme family. The ghrelin/GOAT/GHS-R1a system is defined by multiple unique aspects within both protein biochemistry and endocrinology. Ghrelin serves as the only substrate for GOAT within the human proteome and, among the multiple hormones involved in energy homeostasis and metabolism such as insulin and leptin, acts as the only known hormone in circulation that directly stimulates appetite and hunger signalling. Advances in GOAT enzymology, structural modelling and inhibitor development have revolutionized our understanding of this enzyme and offered new tools for investigating ghrelin signalling at the molecular and organismal levels. In this review, we briefly summarize the current state of knowledge regarding ghrelin signalling and ghrelin/GOAT enzymology, discuss the GOAT structural model in the context of recently reported MBOAT enzyme superfamily member structures, and highlight the growing complement of GOAT inhibitors that offer options for both ghrelin signalling studies and therapeutic applications.

Exogenous Ghrelin Increases Plasma Insulin Level in Diabetic Rats

Ghrelin, a 28-amino acid peptide, is a strong growth hormone secretagogue and a regulator of food intake. In addition, ghrelin is thought to play a role in insulin secretion and in glucose homeostasis. A lot of contradictory data have been reported in the literature regarding the co-localization of ghrelin with other hormones in the islet of Langerhans, its role in insulin secretion and attenuation of type 2 diabetes mellitus. In this study, we investigate the effect of chronic ghrelin treatment on glucose, body weight and insulin level in normal and streptozotocin-induced diabetic male Wistar rats. We have also examined the distribution pattern and co-localization of ghrelin with insulin in pancreatic islet cells using immunohistochemistry and immune-electron microscopy and the ability of ghrelin to stimulate insulin release from the CRL11065 beta cell line. Control, non-diabetic groups received intraperitoneal injection of normal saline, while treated groups received intraperitoneal injection of 5 µg/kg body weight of ghrelin (amino acid chain 24–51) on a daily basis for a duration of four weeks. Our results show that the administration of ghrelin increases the number of insulin-secreting beta cells and serum insulin level in both normal and diabetic rats. We also demonstrated that ghrelin co-localizes with insulin in pancreatic islet cells and that the pattern of ghrelin distribution is altered after the onset of diabetes. Moreover, ghrelin at a dose of 10⁻⁶ M and 10⁻¹² M increased insulin release from the CRL11065 beta cell line. In summary, ghrelin co-localizes with insulin in the secretory granules of pancreatic beta cells and enhances insulin production.

Ghrelin octanoylation by ghrelin O-acyltransferase: Unique protein biochemistry underlying metabolic signaling

Ghrelin is a small peptide hormone that requires a unique post-translational modification, serine octanoylation, to bind and activate the GHS-R1a receptor. Ghrelin signaling is implicated in a variety of neurological and physiological processes, but is most well known for its roles in controlling hunger and metabolic regulation. Ghrelin octanoylation is catalyzed by ghrelin O-acyltransferase (GOAT), a member of the membrane-bound O-acyltransferase (MBOAT) enzyme family. From the status of ghrelin as the only substrate for GOAT in the human genome to the source and requirement for the octanoyl acyl donor, the ghrelin-GOAT system is defined by multiple unique aspects within both protein biochemistry and endocrinology. In this review, we examine recent advances in our understanding of the interactions and mechanisms leading to ghrelin modification by GOAT, discuss the potential sources for the octanoyl acyl donor required for ghrelin's activation, and summarize the current landscape of molecules targeting ghrelin octanoylation through GOAT inhibition.

Diminished gastric prokinetic response to ghrelin in a rat model of spinal cord injury

BACKGROUND

Patients with cervical or high-thoracic spinal cord injury (SCI) often present reduced gastric emptying and early satiety. Ghrelin provokes motility via gastric vagal neurocircuitry and ghrelin receptor agonists offer a therapeutic option for gastroparesis. We have previously shown that experimental high-thoracic injury (T3-SCI) diminishes sensitivity to another gastrointestinal peptide, cholecystokinin. This study tests the hypothesis that T3-SCI impairs the vagally mediated response to ghrelin.

METHODS

We investigated ghrelin sensitivity in control and T3-SCI rats at 3-days or 3-weeks after injury utilizing: (i) acute (3-day post-injury) fasting and post-prandial serum levels of ghrelin; (ii) in vivo gastric reflex recording following intravenous or central brainstem ghrelin; and (iii) in vitro whole cell recording of neurons within the dorsal motor nucleus of the vagus (DMV).

KEY RESULTS

The 2-day food intake of T3-SCI rats was reduced while fasting serum ghrelin levels were higher than in controls. Intravenous and fourth ventricle ghrelin increased in vivo gastric motility in fasted 3-day control rats but not fasted T3-SCI rats. In vitro recording of DMV neurons from 3-day T3-SCI rats were insensitive to exogenous ghrelin. For each measure, vagal responses returned after 3-weeks.

CONCLUSIONS AND INFERENCES

Hypophagia accompanying T3-SCI produces a significant and physiologically appropriate elevation in serum ghrelin levels. However, higher ghrelin levels did not translate into increased gastric motility in the acute stage of T3-SCI. We propose that this may reflect diminished sensitivity of peripheral vagal afferents to ghrelin or a reduction in the responsiveness of medullary gastric vagal neurocircuitry following T3-SCI.

Ghrelin protects adult rat hippocampal neural stem cells from excessive autophagy during oxygen-glucose deprivation

Ghrelin functions as a neuroprotective agent and saves neurons from various insults include ischemic injury. However, it remains to be elucidated whether ghrelin protects neuronal cells against ischemic injury-induced excessive autophagy. Autophagy is required for the maintenance of neural stem cell homeostasis. However, regarding autophagic cell death, it is commonly assumed that excessive autophagy leads to self-elimination of mammalian cells. The purpose of this study was to investigate the potential neuroprotection effects of ghrelin from excessive autophagy in adult rat hippocampal neural stem cells (NSCs). Oxygen-Glucose Deprivation (OGD) strongly induces autophagy in adult rat hippocampal NSCs. Ghrelin treatment inhibited OGD-induced cell death of adult rat hippocampal NSCs assessed by cell-counting-kit-8 assay. Ghrelin also suppressed OGD-induced excessive autophagy activity. The protective effect of ghrelin was accompanied by an increased expression levels of Bcl-2, p-62 and decreased expression level of LC3-II, Beclin-1 by Western blot. Furthermore, ghrelin reduced autophagosome formation and number of GFP-LC3 transfected puncta. In conclusion, our data suggest that ghrelin protects adult rat hippocampal NSCs from excessive autophagy in experimental stroke (oxygen-glucose deprivation) model. Regulating autophagic activity may be a potential optimizing target for promoting adult rat hippocampal NSCs based therapy for stroke.

The octanoylated energy regulating hormone ghrelin: An expanded view of ghrelin's biological interactions and avenues for controlling ghrelin signaling

Ghrelin is a small peptide hormone that requires a unique post-translational modification, serine octanoylation, to bind and activate the GHS-R1a receptor. Initially demonstrated to stimulate hunger and appetite, ghrelin-dependent signaling is implicated in a variety of neurological and physiological processes influencing diseases such as diabetes, obesity, and Prader-Willi syndrome. In addition to its cognate receptor, recent studies have revealed ghrelin interacts with a range of binding partners within the bloodstream. Defining the scope of ghrelin's interactions within the body, understanding how these interactions work in concert to modulate ghrelin signaling, and developing molecular tools for controlling ghrelin signaling are essential for exploiting ghrelin for therapeutic effect. In this review, we discuss recent findings regarding the biological effects of ghrelin signaling, outline binding partners that control ghrelin trafficking and stability in circulation, and summarize the current landscape of inhibitors targeting ghrelin octanoylation.

The role of ghrelin in metabolic regulation

PURPOSE OF REVIEW

To discuss recent research on the role of ghrelin in the regulation of carbohydrate and lipid metabolism in the context of its wider role in regulating energy balance.

RECENT FINDINGS

Ghrelin possesses a range of centrally and peripherally mediated metabolic actions influencing insulin glucose homeostasis and fatty acid metabolism and appetite. Although acyl ghrelin was previously thought to be the active hormone, recent evidence suggests that des-acyl ghrelin also possesses activity, and the enzyme ghrelin-O-acyl transferase regulates their interconversion. In partnership with insulin and leptin, ghrelin defends against energy deficit by enhancing hunger, conserving carbohydrate and promoting fat oxidation. In the postprandial state, it contributes to satiety, energy storage and favours glucose oxidation. New research suggests a range of new roles including addictive behaviours, cardiovascular protection, neuroprotection and regeneration and perhaps the ageing process.

SUMMARY

Ghrelin functions primarily as a short-term metabolic switch at the onset of fasting, gearing the fuel economy away from glucose uptake, conserving glucose for vital functions, favouring fatty acid oxidation and triggering food-seeking behaviour. The ghrelin system is a potential target for a range of pharmacological interventions, but its pleiotropic nature makes selective treatments challenging.

Neonatal birth waist is positively predicted by second trimester maternal active ghrelin, a pro-appetite hormone, and negatively associated with third trimester maternal leptin, a pro-satiety hormone

INTRODUCTION

In pregnancy physiological mechanisms activated by maternal appetite contribute to adequate energy intake for the mother and for the fetus. The role of maternal appetite-related peptides and their possible association with neonatal energy stores and glucose metabolism have not been investigated as yet. The aim was to investigate, during pregnancy, the association of fasting maternal appetite-related hormones levels [ghrelin (active), GLP1 (active), total PYY and leptin] with neonatal waist, percent total body fat and insulin levels at birth.

METHODS

Forty-two normal and thirty eight overweight women (mean±SD; age: 26.9±2.5years; pre-pregnancy BMI 26±2.2kg/m(2)) were seen during each of the three trimesters, had blood sampling and a 75g oral glucose tolerance test. At birth, neonates underwent anthropometry and cord blood sampling for c-peptide, glucose, insulin.

RESULTS

During all three trimesters maternal weight correlated positively with percent total neonatal body fat while during the second and third trimesters it correlated positively with birth weight. The second trimester maternal active ghrelin levels correlated positively with neonatal waist and were its best positive predictor. The third trimester maternal active ghrelin levels correlated positively with neonatal waist and negatively with percent total neonatal body fat, fetal cord blood insulin levels and were the best negative predictor of the latter. The third trimester maternal leptin levels correlated negatively with neonatal waist.

CONCLUSIONS

During pregnancy circulating maternal active ghrelin, a pro-appetite hormone, is associated with neonatal visceral energy storage (as expressed by neonatal waist). By inhibiting glucose-driven maternal insulin secretion, ghrelin might ensure adequate fasting glucose and nutrient supplies to the fetus while limiting overall fetal adipose tissue deposition.

Altered lipid and salt taste responsivity in ghrelin and GOAT null mice

Taste perception plays an important role in regulating food preference, eating behavior and energy homeostasis. Taste perception is modulated by a variety of factors, including gastric hormones such as ghrelin. Ghrelin can regulate growth hormone release, food intake, adiposity, and energy metabolism. Octanoylation of ghrelin by ghrelin O-acyltransferase (GOAT) is a specific post-translational modification which is essential for many biological activities of ghrelin. Ghrelin and GOAT are both widely expressed in many organs including the gustatory system. In the current study, overall metabolic profiles were assessed in wild-type (WT), ghrelin knockout (ghrelin^−/−), and GOAT knockout (GOAT^−/−) mice. Ghrelin^−/− mice exhibited decreased food intake, increased plasma triglycerides and increased ketone bodies compared to WT mice while demonstrating WT-like body weight, fat composition and glucose control. In contrast GOAT^−/− mice exhibited reduced body weight, adiposity, resting glucose and insulin levels compared to WT mice. Brief access taste behavioral tests were performed to determine taste responsivity in WT, ghrelin^−/− and GOAT^−/− mice. Ghrelin and GOAT null mice possessed reduced lipid taste responsivity. Furthermore, we found that salty taste responsivity was attenuated in ghrelin^−/− mice, yet potentiated in GOAT^−/− mice compared to WT mice. Expression of the potential lipid taste regulators Cd36 and Gpr120 were reduced in the taste buds of ghrelin and GOAT null mice, while the salt-sensitive ENaC subunit was increased in GOAT^−/− mice compared with WT mice. The altered expression of Cd36, Gpr120 and ENaC may be responsible for the altered lipid and salt taste perception in ghrelin^−/− and GOAT^−/− mice. The data presented in the current study potentially implicates ghrelin signaling activity in the modulation of both lipid and salt taste modalities.

Differential effects of central ghrelin on fatty acid metabolism in hypothalamic ventral medial and arcuate nuclei

Fatty acid metabolism is an important pathway involved in the hypothalamus-mediated control of food intake. Previous studies using whole hypothalamic tissue lysates have shown that fatty acid metabolism plays a key role in ghrelin's effect on feeding. Here, we report site-specific effects of central ghrelin on fatty acid metabolism in two critical hypothalamic nuclei, the ventral medial nucleus (VMN) and the arcuate nucleus (Arc). Intracerebroventricular administration of ghrelin to rats activates AMP-activated protein kinase in both the VMN and the Arc, while ghrelin treatment has a site-specific effect on fatty acid metabolic pathways in these two nuclei. In the VMN, central ghrelin increases the phosphorylation level of ACC, indicating the decrease in activity, and decreases the level of malonyl-CoA (the product of ACC). Malonyl-CoA is an inhibitor of carnitine palmitoyltransferase-1 (CPT-1) that is a key enzyme in mitochondrial fatty acid oxidation. Consistent with this action of malonyl-CoA on CPT-1, central ghrelin treatment increases the activity of CPT-1 in the VMN. In contrast, in the Arc, neither malonyl-CoA level nor CPT-1 activity is affected following central ghrelin. Taken together, our data suggest ghrelin exerts differential effects on fatty acid metabolic pathways in the VMN and the Arc.

Action of Neurotransmitter: A Key to Unlock the AgRP Neuron Feeding Circuit

The current obesity epidemic and lack of efficient therapeutics demand a clear understanding of the mechanism underlying body weight regulation. Despite intensive research focus on obesity pathogenesis, an effective therapeutic strategy to treat and cure obesity is still lacking. Exciting studies in last decades have established the importance of hypothalamic agouti-related protein-expressing neurons (AgRP neurons) in the regulation of body weight homeostasis. AgRP neurons are both required and sufficient for feeding regulation. The activity of AgRP neurons is intricately regulated by nutritional hormones as well as synaptic inputs from upstream neurons. Changes in AgRP neuron activity lead to alterations in the release of mediators, including neuropeptides Neuropeptide Y (NPY) and AgRP, and fast-acting neurotransmitter GABA. Recent studies based on mouse genetics, novel optogenetics, and designer receptor exclusively activated by designer drugs have identified a critical role for GABA release from AgRP neurons in the parabrachial nucleus and paraventricular hypothalamus in feeding control. This review will summarize recent findings about AgRP neuron-mediated control of feeding circuits with a focus on the role of neurotransmitters. Given the limited knowledge on feeding regulation, understanding the action of neurotransmitters may be a key to unlock neurocircuitry that governs feeding.