The protective effects of pomelo extract (Citrus grandis L. Osbeck) against fructose-mediated protein oxidation and glycation

Chronic hyperglycemia induces non-enzymatic protein glycation, which plays an important role in the development of diabetic complications. Immense efforts have been made to determine effective antiglycation compounds from natural products. Pomelo has shown beneficial effects for human health. The objective of this study was to determine the antiglycation effect of pomelo extract against fructose-mediated protein oxidation and glycation. Our results showed that the pomelo extract (0.25 - 2.00 mg/mL) significantly inhibited the overall formation of advanced glycation end products (AGEs) in a concentration-dependent manner. The pomelo extract markedly decreased the level of fructosamine, which is directly associated with reduction in formation of AGEs and Nε -(carboxymethyl)lysine (CML). In addition, the pomelo extract inhibited protein oxidation through its ability to prevent the loss of thiol groups and reduced protein carbonyl formation. We characterized the active components in the pomelo extract by using high-performance liquid chromatography (HPLC), which showed that the pomelo extract contained naringin (11.90 ± 0.21 mg/g dried extract), hesperidin (12.04 ± 0.12 mg/g dried extract), neohesperidin (25.4 ± 0.12 mg/g dried extract), and naringenin (9.20 ± 0.19 mg/g dried extract). Our findings could provide a new insight into the antiglycation properties of the extract of the naturally occurring fruit pomelo for preventing AGE-mediated diabetic complications

The GPR120 agonist TUG‐891 promotes metabolic health by stimulating mitochondrial respiration in brown fat

Brown adipose tissue (BAT) activation stimulates energy expenditure in human adults, which makes it an attractive target to combat obesity and related disorders. Recent studies demonstrated a role for G protein‐coupled receptor 120 (GPR120) in BAT thermogenesis. Here, we investigated the therapeutic potential of GPR120 agonism and addressed GPR120‐mediated signaling in BAT. We found that activation of GPR120 by the selective agonist TUG‐891 acutely increases fat oxidation and reduces body weight and fat mass in C57Bl/6J mice. These effects coincided with decreased brown adipocyte lipid content and increased nutrient uptake by BAT, confirming increased BAT activity. Consistent with these observations, GPR120 deficiency reduced expression of genes involved in nutrient handling in BAT. Stimulation of brown adipocytes in vitro with TUG‐891 acutely induced O2 consumption, through GPR120‐dependent and GPR120‐independent mechanisms. TUG‐891 not only stimulated GPR120 signaling resulting in intracellular calcium release, mitochondrial depolarization, and mitochondrial fission, but also activated UCP1. Collectively, these data suggest that activation of brown adipocytes with the GPR120 agonist TUG‐891 is a promising strategy to increase lipid combustion and reduce obesity

Understanding the mechanisms regulating SCFA mediated release of anorectic gut hormones

Obesity is a fast-growing epidemic that poses a major challenge to the public health. There is a current lack of safe and effective anti-obesity treatment options, therefore an improved treatment option is critical. Short chain fatty acids (SCFAs), produced in the colon via fermentation of non-digestible carbohydrates by gut microbiota, activates FFA2 and FFA3, G-protein coupled receptors (GPCRs) that stimulate the release of anorectic gut hormones GLP-1 and PYY. However, the underlying molecular mechanisms stimulating their release are poorly understood. A fundamental mechanism controlling the signalling capacity of GPCRs is via receptor trafficking to diverse cellular compartments such as early endosomes (EE), or very early endosomes (VEE). A subpopulation of VEEs contains the adaptor protein APPL1, essential for driving receptor recycling from the VEE and regulating endosomal G protein signalling. I therefore characterised the signalling pathways exerted by SCFAs in intestinal enteroendocrine cells and colonic organoids and elucidated the trafficking properties of FFA2 that regulate anorectic gut hormone release. In enteroendocrine cells, SCFAs are unable to elicit Gαq/11-signalling but robustly activates Gαi/o signalling which is important for propionate induced GLP-1 secretion. FFA2 undergoes both constitutive and ligand induced internalisation. Following ligand-induced internalisation, FFA2 traffics to the VEE to activate Gαi endosomal signalling that is regulated by APPL1. In addition, by employing high resolution single vesicle imaging, I unveiled propionate-induced FFA2 recycling is APPL1 dependent. I also examined the dependence of receptor internalisation and found that receptor internalisation is critical only for propionate induced- Gαi/o signalling, p38 activation and GLP-1 release, while Gαq/11 signalling occurs from the plasma membrane. Together these findings suggest an important spatial requirement for propionate-mediated activity and uncovers novel mechanisms regulating the release of anorectic gut hormone, GLP-1.Open Acces

Distinct phosphorylation sites in a prototypical GPCR differently orchestrate β-arrestin interaction, trafficking, and signaling

Agonist-induced phosphorylation of G protein-coupled receptors (GPCRs) is a key determinant for their interaction with β-arrestins (βarrs) and subsequent functional responses. Therefore, it is important to decipher the contribution and interplay of different receptor phosphorylation sites in governing βarr interaction and functional outcomes. Here, we find that several phosphorylation sites in the human vasopressin receptor (V2R), positioned either individually or in clusters, differentially contribute to βarr recruitment, trafficking, and ERK1/2 activation. Even a single phosphorylation site in V2R, suitably positioned to cross-talk with a key residue in βarrs, has a decisive contribution in βarr recruitment, and its mutation results in strong G-protein bias. Molecular dynamics simulation provides mechanistic insights into the pivotal role of this key phosphorylation site in governing the stability of βarr interaction and regulating the interdomain rotation in βarrs. Our findings uncover important structural aspects to better understand the framework of GPCR-βarr interaction and biased signaling