Obesity represents an important public health challenge for the twenty-first century: globalised, highly prevalent and increasingly common with time, this condition is likely to reverse some of the hard-won gains in mortality accomplished in previous centuries. In the search for safe and effective therapies for obesity and its companion, type 2 diabetes mellitus (T2D), the gut hormone glucagon-like peptide-1 (GLP-1) has emerged as a forerunner and analogues thereof are now widely used in treatment of obesity and T2D, bringing proven benefits in improving glycaemia and weight loss and, notably, cardiovascular outcomes. However, GLP-1 alone is subject to limitations in terms of efficacy, and as a result, investigators are evaluating other gut hormones such as glucose-dependent insulinotropic peptide (GIP), glucagon and peptide YY (PYY) as possible partner hormones that may complement and enhance GLP-1’s therapeutic effects. Such combination gut hormone therapies are in pharmaceutical development at present and are likely to make it to market within the next few years. This review examines the physiological basis for combination gut hormone therapy and presents the latest clinical results that underpin the excitement around these treatments. We also pose, however, some hard questions for the field which need to be answered before the full benefit of such treatments can be realised.
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Bernard Khoo and Tricia Mei-Mei Tan
Edra London, Michelle Bloyd and Constantine A Stratakis
Both direct and indirect evidence demonstrate a central role for the cAMP-dependent protein kinase (PKA) signaling pathway in the regulation of energy balance and metabolism across multiple systems. However, the ubiquitous pattern of PKA expression across cell types poses a challenge in pinpointing its tissue-specific regulatory functions and further characterizing its many downstream effects in certain organs or cells. Mouse models of PKA deficiency and over-expression and studies in living cells have helped clarify PKA function in adipose tissue (AT), liver, adrenal, pancreas, and specific brain nuclei, as they pertain to energy balance and metabolic dysregulation. Limited studies in humans suggest differential regulation of PKA in AT of obese compared to lean individuals and an overall dysregulation of PKA signaling in obesity. Despite its complexity, under normal physiologic conditions, the PKA system is tightly regulated by changes in cAMP concentrations upstream via adenylate cyclase and downstream by phosphodiesterase-mediated cAMP degradation to AMP and by changes in PKA holoenzyme stability. Adjustments in the PKA system appear to be important to the development and maintenance of the obese state and its associated metabolic perturbations. In this review we discuss the important role of PKA in obesity and its involvement in resistance to obesity, through studies in humans and in mouse models, with a focus on the regulation of PKA in energy expenditure, intake behavior, and lipid and glucose metabolism.
Chaoyi Zhang, Qianli Zhang, Zhihong Huang and Quan Jiang
Adropin plays a role in the maintenance of energy homeostasis, insulin resistance prevention, and impaired glucose tolerance. However, the molecular mechanisms by which adropin affects hepatic glucose and lipid metabolism in vitro are not entirely understood. This study intended to examine the roles and underlying mechanisms of adropin in glucose and lipid metabolism in Nile tilapia. In primary cultured tilapia hepatocytes, adropin significantly attenuated oleic acid (OA)-induced glucose output and reduced the activities and mRNA expression of cytosolic phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase), which are involved in gluconeogenesis. In contrast, adropin facilitated glucose uptake activity via glucose transporter 1 (Glut1) upregulation in OA-treated hepatocytes. One-week of adropin treatment reduced the hepatic total lipid accumulation in OA-fed tilapia without changes in body weight. Subsequent studies revealed that adropin suppressed OA-induced intracellular triglyceride accumulation and decreased the expression of genes and proteins involved in lipid metabolisms such as sterol regulatory element-binding protein-1c (SREBP-1c), acetyl-CoA carboxylase α (ACCα) and CD36, but upregulated peroxisome proliferator-activated receptor α (PPARα) levels. In parallel studies, however, adropin had no detectable effects on fatty acid-binding protein 4 (Fabp4) and carnitine palmitoyltransferase 1α (Cpt1α) mRNA expression. Furthermore, adropin treatment dose-dependently increased the phosphorylation level of AMP-activated protein kinase (AMPK). Suppression of AMPK by compound C or AMPKα1 siRNA blocked adropin-induced decreases in the mature form of SREBP-1c expression, glucose output, and intracellular triglyceride content in OA-treated hepatocytes. These findings suggest that teleost adropin could suppress hepatic gluconeogenesis and triglyceride accumulation via a mechanism dependent on AMPK signalling.
Qinglei Yin, Liyun Shen, Yicheng Qi, Dalong Song, Lei Ye, Ying Peng, Yanqiu Wang, Zhou Jin, Guang Ning, Weiqing Wang, Dongping Lin and Shu Wang
SIRT1, a class III histone/protein deacetylase (HDAC), has been associated with autoimmune diseases. There is a paucity of data about the role of SIRT1 in Graves’ disease. The aim of this study was to investigate the role of SIRT1 in the pathogenesis of GD. Here, we showed that SIRT1 expression and activity were significantly decreased in GD patients compared with healthy controls. The NF-κB pathway was activated in the peripheral blood of GD patients. The reduced SIRT1 levels correlated strongly with clinical parameters. In euthyroid patients, SIRT1 expression was markedly upregulated and NF-κB downstream target gene expression was significantly reduced. SIRT1 inhibited the NF-κB pathway activity by deacetylating P65. These results demonstrate that reduced SIRT1 expression and activity contribute to the activation of the NF-κB pathway and may be involved in the pathogenesis of GD.
Pauline Campos, Jamie J Walker and Patrice Mollard
In most species, survival relies on the hypothalamic control of endocrine axes that regulate critical functions such as reproduction, growth, and metabolism. For decades, the complexity and inaccessibility of the hypothalamic–pituitary axis has prevented researchers from elucidating the relationship between the activity of endocrine hypothalamic neurons and pituitary hormone secretion. Indeed, the study of central control of endocrine function has been largely dominated by ‘traditional’ techniques that consist of studying in vitro or ex vivo isolated cell types without taking into account the complexity of regulatory mechanisms at the level of the brain, pituitary and periphery. Nowadays, by exploiting modern neuronal transfection and imaging techniques, it is possible to study hypothalamic neuron activity in situ, in real time, and in conscious animals. Deep-brain imaging of calcium activity can be performed through gradient-index lenses that are chronically implanted and offer a ‘window into the brain’ to image multiple neurons at single-cell resolution. With this review, we aim to highlight deep-brain imaging techniques that enable the study of neuroendocrine neurons in awake animals whilst maintaining the integrity of regulatory loops between the brain, pituitary and peripheral glands. Furthermore, to assist researchers in setting up these techniques, we discuss the equipment required and include a practical step-by-step guide to performing these deep-brain imaging studies.
Marianna Minnetti, Valeria Hasenmajer, Riccardo Pofi, Mary Anna Venneri, Krystallenia I Alexandraki and Andrea M Isidori
The circadian rhythm derives from the integration of many signals that shape the expression of clock-related genes in a 24-h cycle. Biological tasks, including cell proliferation, differentiation, energy storage, and immune regulation, are preferentially confined to specific periods. A gating system, supervised by the central and peripheral clocks, coordinates the endogenous and exogenous signals and prepares for transition to activities confined to periods of light or darkness. The fluctuations of cortisol and its receptor are crucial in modulating these signals. Glucocorticoids and the autonomous nervous system act as a bridge between the suprachiasmatic master clock and almost all peripheral clocks. Additional peripheral synchronizing mechanisms including metabolic fluxes and cytokines stabilize the network. The pacemaker is amplified by peaks and troughs in cortisol and their response to food, activity, and inflammation. However, when the glucocorticoid exposure pattern becomes chronically flattened at high- (as in Cushing’s syndrome) or low (as in adrenal insufficiency) levels, the system fails. While endocrinologists are well aware of cortisol rhythm, too little attention has been given to interventions aimed at restoring physiological cortisol fluctuations in adrenal disorders. However, acting on glucocorticoid levels may not be the only way to restore clock-related activities. First, a counterregulatory mechanism on the glucocorticoid receptor itself controls signal transduction, and second, melatonin and/or metabolically active drugs and nutrients could also be used to modulate the clock. All these aspects are described herein, providing some insights into the emerging role of chronopharmacology, focusing on glucocorticoid excess and deficiency disorders.
Gregory S Y Ong, Timothy J Cole, Gregory H Tesch, James Morgan, Jennifer K Dowling, Ashley Mansell, Peter J Fuller and Morag J Young
MR activation in macrophages is critical for the development of cardiac inflammation and fibrosis. We previously showed that MR activation modifies macrophage pro-inflammatory signalling, changing the cardiac tissue response to injury via both direct gene transcription and JNK/AP-1 second messenger pathways. In contrast, MR-mediated renal electrolyte homeostasis is critically determined by DNA-binding-dependent processes. Hence, ascertaining the relative contribution of MR actions via DNA binding or alternative pathways on macrophage behaviour and cardiac inflammation may provide therapeutic opportunities which separate the cardioprotective effects of MR antagonists from their undesirable renal potassium-conserving effects. We developed new macrophage cell lines either lacking MR or harbouring a mutant MR incapable of DNA binding. Western blot analysis demonstrated that MR DNA binding is required for lipopolysaccharide (LPS), but not phorbol 12-myristate-13-acetate (PMA), induction of the MAPK/pJNK pathway in macrophages. Quantitative RTPCR for pro-inflammatory and pro-fibrotic targets revealed subsets of LPS- and PMA-induced genes that were either enhanced or repressed by the MR via actions that do not always require direct MR-DNA binding. Analysis of the MR target gene and profibrotic factor MMP12 identified promoter elements that are regulated by combined MR/MAPK/JNK signalling. Evaluation of cardiac tissue responses to an 8-day DOC/salt challenge in mice selectively lacking MR DNA-binding in macrophages demonstrated levels of inflammatory markers equivalent to WT, indicating non-DNA binding-dependent MR signalling in macrophages is sufficient for DOC/salt-induced tissue inflammation. Our data demonstrate that the MR regulates a macrophage pro-inflammatory phenotype and cardiac tissue inflammation, partially via pathways that do not require DNA binding.
Katherine N Makowski, Michael J Kreisman, Richard B McCosh, Ali A Raad and Kellie M Breen
Peripheral immune/inflammatory challenges rapidly disrupt reproductive neuroendocrine function. This inhibition is considered to be centrally mediated via suppression of gonadotropin-releasing hormone secretion, yet the neural pathway(s) for this effect remains unclear. We tested the hypothesis that interleukin-1β inhibits pulsatile luteinizing hormone secretion in female mice via inhibition of arcuate kisspeptin cell activation, a population of neurons considered to be the gonadotropin-releasing hormone pulse generator. In the first experiment, we determined that the inhibitory effect of peripheral interleukin-1β on luteinizing hormone secretion was enhanced by estradiol. We next utilized serial sampling and showed that interleukin-1β reduced the frequency of luteinizing hormone pulses in ovariectomized female mice treated with estradiol. The interleukin-1β-induced suppression of pulse frequency was associated with reduced kisspeptin cell activation, as determined by c-Fos coexpression, but not as a result of impaired responsiveness to kisspeptin challenge. Together, these data suggest an inhibitory action of interleukin-1β upstream of kisspeptin receptor activation. We next tested the hypothesis that estradiol enhances the activation of brainstem nuclei responding to interleukin-1β. We determined that the expression of interleukin-1 receptor was elevated within the brainstem following estradiol. Interleukin-1β induced c-Fos in the area postrema, ventrolateral medulla, and nucleus of the solitary tract; however, the response was not increased by estradiol. Collectively, these data support a neural mechanism whereby peripheral immune/inflammatory stress impairs reproductive neuroendocrine function via inhibition of kisspeptin cell activation and reduced pulsatile luteinizing hormone secretion. Furthermore, these findings implicate the influence of estradiol on peripherally mediated neural pathways such as those activated by peripheral cytokines.
Hans-Peter Holthoff, Kerstin Uhland, Gabor Laszlo Kovacs, Andreas Reimann, Kristin Adler, Clara Wenhart and Martin Ungerer
Graves’ disease is an autoimmune disorder, which is characterized by stimulatory antibodies targeting the human thyrotropin receptor (TSHR), resulting in hyperthyroidism and multiple organ damage. We systematically investigated monomeric and dimeric fusion proteins of the A subunit of TSHR for efficacy to bind to the monoclonal patient antibody M22, to interact with Graves’ patient serum samples, and to impact on anti-TSHR antibody titers, hyperthyroidism, tachycardia and other in vivo read-outs in a long-term mouse model of Graves’ disease induced by immunization with a recombinant adenovirus encoding TSHR A. Binding assays and functional measurements of TSHR-dependent cAMP formation showed binding of monomeric TSHR-His and dimeric TSHR-Fc to the anti-TSHR antibody M22 at low-effective concentrations (EC50 of 5.7 nmol/L and 8.6 nmol/L) and inhibition of the effects of this antibody at high efficiencies (IC50 values of 16–20 nmol/L). Both proteins also block the effects of polyclonal anti-TSHR antibodies occurring in Graves’ patient sera with somewhat lower average efficiencies (mean IC50 values of 29 nmol/L and 68 nmol/L). However, in vivo characterization of epicutaneous patch administrations of TSHR-Fc at doses of 0.3 and 0.6 mg/kg body weight in a murine Graves’ disease model did not result in any improvement of disease parameters. In conclusion, high affinity binding of TSHR-Fc to pathological anti-TSHR antibodies was not matched by efficacy to improve Graves’ disease parameter in a long-term mouse model.
Xiaoyi Ma, Fei Gao, Qi Chen, Xiuping Xuan, Ying Wang, Hongjun Deng, Fengying Yang and Li Yuan
The angiotensin-converting enzyme 2 (ACE2)/angiotensin 1-7 (A1-7)/MAS axis and glutamate decarboxylase 67 (GAD67)/gamma-aminobutyric acid (GABA) signal both exist in the islet and play important roles in regulating blood glucose metabolism. It has been reported that the activation of ACE2 in the brain increases GABA expression to improve biological effects; however, it is unclear whether there is functional correlation between the ACE2/A1-7/MAS axis and GAD67/GABA signal in the islet. In this study, we showed that the ACE2/A1-7/MAS and GABA signaling systems decreased in the islet of different metabolic stress models. In ACE2-knockout mice, we found that GAD67 and GABA expression decreased significantly, which was reversed by exogenous administration of A1-7. Furthermore, A1-7 mediated PDX1 and AKT activation was inhibited by allylglycine (a specific GAD67 inhibitor) in MIN6 cells. Moreover, giving A1-7 and GABA could significantly reduce beta-cell dedifferentiation and improved glucose metabolism during metabolic stress in vivo and in vitro. In conclusion, our study reveals that the ACE2/A1-7/MAS axis improves beta-cell function through regulating GAD67/GABA signal in beta cells, and up-regulating the ACE2/A1-7/MAS axis and GABA signals delays the development of obesity-induced diabetes.