G protein-coupled receptors: Key molecules in metabolic associated fatty liver disease development

Endothelial dysfunction, platelet hyperactivity, hypertension, and the metabolic syndrome: molecular insights and combating strategies

Metabolic syndrome (MetS) is a multifaceted condition that increases the possibility of developing atherosclerotic cardiovascular disease. MetS includes obesity, hypertension, dyslipidemia, hyperglycemia, endothelial dysfunction, and platelet hyperactivity. There is a concerning rise in the occurrence and frequency of MetS globally. The rising incidence and severity of MetS need a proactive, multipronged strategy for identifying and treating those affected. For many MetS patients, achieving recommended goals for healthy fat intake, blood pressure control, and blood glucose management may require a combination of medicine therapy, lifestyles, nutraceuticals, and others. However, it is essential to note that lifestyle modification should be the first-line therapy for MetS. In addition, MetS requires pharmacological, nutraceutical, or other interventions. This review aimed to bring together the etiology, molecular mechanisms, and dietary strategies to combat hypertension, endothelial dysfunction, and platelet dysfunction in individuals with MetS.

The short-chain fatty acid butyrate exerts a specific effect on VE-cadherin phosphorylation and alters the integrity of aortic endothelial cells

Short-chain fatty acids (SCFAs) like butyrate (BUT) largely influence vascular integrity and are closely associated with the onset and progression of cardiovascular diseases. However, their impact on vascular endothelial cadherin (VEC), a major vascular adhesion and signaling molecule, is largely unknown. Here, we explored the effect of the SCFA BUT on the phosphorylation of specific tyrosine residues of VEC (Y731, Y685, and Y658), which are reported to be critical for VEC regulation and vascular integrity. Moreover, we shed light on the signaling pathway engaged by BUT to affect the phosphorylation of VEC. Thereby, we used phospho-specific antibodies to evaluate the phosphorylation of VEC in response to the SCFA sodium butyrate in human aortic endothelial cells (HAOECs) and performed dextran assays to analyze the permeability of the EC monolayer. The role of c-Src and SCFA receptors FFAR2 and FFAR3 in the induction of VEC phosphorylation was analyzed using inhibitors and antagonists for c-Src family kinases and FFAR2/3, respectively, as well as by RNAi-mediated knockdown. Localization of VEC in response to BUT was assessed by fluorescence microscopy. BUT treatment of HAOEC resulted in the specific phosphorylation of Y731 at VEC with minor effects on Y685 and Y658. Thereby, BUT engages FFAR3, FFAR2, and c-Src kinase to induce phosphorylation of VEC. VEC phosphorylation correlated with enhanced endothelial permeability and c-Src-dependent remodeling of junctional VEC. Our data suggest that BUT, an SCFA and gut microbiota-derived metabolite, impacts vascular integrity by targeting VEC phosphorylation with potential impact on the pathophysiology and therapy of vascular diseases.

Astragalus mongholicus polysaccharides ameliorate hepatic lipid accumulation and inflammation as well as modulate gut microbiota in NAFLD rats

Hepatic lipid accumulation, inflammation and gut microbiota dysbiosis are hallmarks of non-alcoholic fatty liver disease (NAFLD), which is the leading cause of chronic liver disease with no therapeutic consensus. The aim of the present study was to elucidate the mechanism of the effects of Astragalus mongholicus polysaccharides (mAPS) on lipid metabolism, inflammation and gut microbiota in a rat model of NAFLD induced by a high-fat diet (HFD). Our results showed that mAPS and Berberine supplementation reduced HFD-induced increases in body weight, alanine aminotransferase (ALT), aspartate aminotransferase (AST) and homeostasis model assessment of insulin resistance (HOMA-IR), and these changes were accompanied by improved histological changes in the liver. Moreover, administration of mAPS and Berberine resulted in lower levels of serum triglycerides, total cholesterol and low-density lipoprotein cholesterol (LDL-c) but higher levels of high-density lipoprotein cholesterol (HDL-c) in HFD-fed rats. mAPS and Berberine treatment markedly reduced HFD-induced hepatic lipid accumulation, which was associated with increased expression of phosphorylated- adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK) and peroxisome proliferator-activated receptor-α (PPAR-α) but decreased expression of sterol-regulatory element binding proteins (SREBP-1). Pretreatment with mAPS or Berberine reduced HFD-induced expression of proinflammatory cytokines, such as tumor necrosis factor-α (TNF-α). In addition, mAPS downregulated the expression of colonic and hepatic Toll-like receptor 4 (TLR4) as well as phosphorylated- nuclear factor-κB (NF-κB) and nucleotide-binding domain, leucine-rich repeat-containing receptor, pyrin domain-containing-3 (NLRP3) but upregulated the expression of zonula occludens-1 (ZO-1) and occludin in HFD-fed rats. Notably, mAPS treatment reshaped the intestinal microbiome by lowering the Firmicutes to Bacteroidetes (F/B) ratio and increasing the abundance of Proteobacteria and Epsilonbacteria. mAPS supplementation had little effect on the profile of fecal short-chain fatty acids (SCFAs), but it significantly decreased the expression of colonic and hepatic G-protein coupled receptor (GPR) 41 and 43. Therefore, mAPS supplementation ameliorates hepatic inflammation and lipid accumulation in NAFLD by modulating the gut microbiota and SCFA-GPR signaling pathways. The present study provides new evidence for mAPS as a natural active substance in the treatment of NAFLD.

G Protein-Coupled Receptor GPR35 Suppresses Lipid Accumulation in Hepatocytes

Although prevalent, nonalcoholic fatty liver disease is not currently treated effectively with medicines. Initially, using wild-type and genome-edited clones of the human hepatocyte cell line HepG2, we show that activation of the orphan G protein-coupled receptor GPR35 is both able and sufficient to block liver X-receptor-mediated lipid accumulation. Studies on hepatocytes isolated from both wild-type and GPR35 knock-out mice were consistent with a similar effect of GPR35 agonists in these cells, but because of marked differences in the pharmacology of GPR35 agonists and antagonists at the mouse and human orthologues, as well as elevated basal lipid levels in hepatocytes from the GPR35 knock-out mice, no definitive conclusion could be reached. To overcome this, we generated and characterized a transgenic knock-in mouse line in which the corresponding human GPR35 splice variant replaced the mouse orthologue. In hepatocytes from these humanized GPR35 mice, activation of this receptor was shown conclusively to prevent, and also reverse, lipid accumulation induced by liver X-receptor stimulation. These studies highlight the potential to target GPR35 in the context of fatty liver diseases.

Functional Characterization of the Obesity-Linked Variant of the β3-Adrenergic Receptor

Adrenergic receptor β₃ (ADRβ₃) is a member of the rhodopsin-like G protein-coupled receptor family. The binding of the ligand to ADRβ₃ activates adenylate cyclase and increases cAMP in the cells. ADRβ₃ is highly expressed in white and brown adipocytes and controls key regulatory pathways of lipid metabolism. Trp64Arg (W64R) polymorphism in the ADRβ₃ is associated with the early development of type 2 diabetes mellitus, lower resting metabolic rate, abdominal obesity, and insulin resistance. It is unclear how the substitution of W64R affects the functioning of ADRβ₃. This study was initiated to functionally characterize this obesity-linked variant of ADRβ₃. We evaluated in detail the expression, subcellular distribution, and post-activation behavior of the WT and W64R ADRβ₃ using single cell quantitative fluorescence microscopy. When expressed in HEK 293 cells, ADRβ₃ shows a typical distribution displayed by other GPCRs with a predominant localization at the cell surface. Unlike adrenergic receptor β₂ (ADRβ₂), agonist-induced desensitization of ADRβ₃ does not involve loss of cell surface expression. WT and W64R variant of ADRβ₃ displayed comparable biochemical properties, and there was no significant impact of the substitution of tryptophan with arginine on the expression, cellular distribution, signaling, and post-activation behavior of ADRβ₃. The obesity-linked W64R variant of ADRβ₃ is indistinguishable from the WT ADRβ₃ in terms of expression, cellular distribution, signaling, and post-activation behavior.