PPARβ/δ activation induces enteroendocrine L cell GLP-1 production

Cyanidin-3-O-glucoside enhances GLP-1 secretion via PPARβ/δ-β-catenin-TCF-4 pathway in type 2 diabetes mellitus

In late-stage type 2 diabetes mellitus (T2DM), impaired islet β cell function leads to absolute insulin deficiency, thereby disrupting blood glucose homeostasis. GLP-1, an incretin hormone, stimulates insulin secretion from islet β cells post-meals. This study investigated the effects of anthocyanin cyanidin-3-O-glucoside (C3G) on GLP-1 secretion using STC-1 (intestinal endocrine L cells) and NIT-1 (islet β cells). In a co-culture system, C3G treatment increased GLP-1 secretion in STC-1 cells, promoting insulin release in NIT-1 cells under high glucose. Mechanistically, C3G activated the PPARβ/δ-β-catenin-TCF-4 pathway in STC-1 cells, enhancing PG precursor transcription and GLP-1 synthesis.Inhibiting PPARβ/δ with GSK0660 blocked this C3G-induced upregulation. Overall, C3G stimulates GLP-1 secretion from intestinal L cells via this pathway, indirectly boosting insulin release from β cells. These findings enhance T2DM mechanism understanding and suggest the potential of C3G in GLP-1-based T2DM therapy.

High-fat diet promotes gestational diabetes mellitus through modulating gut microbiota and bile acid metabolism

Introduction

Gestational diabetes mellitus (GDM) is a condition characterized by glucose intolerance during pregnancy, estimated to affect approximately 20% of the whole pregnancies and is increasing in prevalence globally. However, there is still a big gap in knowledge about the association between gut microbiota associated metabolism alterations and GDM development.

Methods

All the participants accomplished the validated internet-based dietary questionnaire for Chinese and serum, fecal samples were collected. HFD, control diet or colesevelam intervention was fed to GDM mice models or Fxr-/- mice models, with or without antibiotics cocktail treatment. Fecal microbiota transplantation were used for further validation. Gut microbiota and metabolites were detected by metagenomic sequencing and high-performance liquid chromatography-mass spectrometry, respectively. Bile acids of serum, fecal samples from human and mice were analysised. Body weight, average feed intake, blood glucose, insulin levels and oral glucose tolerance test was performed among each groups. Expression levels of Fxr, Shp and Fgf15 mRNA and protein were detected by quantitative reverse transcription polymerase chain reaction and western blot, respectively.

Results

Our data indicated that high fat diet (HFD) was linked with higher prevalence of GDM, and HFD was positively associated with poor prognosis in GDM patients. Moreover, compared with normal diet (ND) group, GDM patients from HFD group performed a loss of gut microbiota diversity and enrichment of Alistipes onderdonkii, Lachnospiraceae bacterium 1_7_58FAA, and Clostridium aspaaragiforme while ruduction of Akkermansiaceae, Paraprevotell xylaniphila, and Prevotella copri. Additionally, HFD aggravated GDM in mice and gut microbiota depletion by antibiotics crippled the effect of excess fat intake. BAs profile altered in HFD GDM patients and mice models. Fecal microbiota transplantation (FMT) further confirmed that gut microbiota contributed to bile acids (BAs) metabolic dysfunction during HFD-associated GDM development. Mechanically, HFD-FMT administration activated Fxr, Shp, and Fgf15 activity, disturbed the glucose metabolism and aggravated insulin resistance but not in HFD-FMT Fxr-/- mice and ND-FMT Fxr-/- mice. Furthermore, colesevelam intervention alleviated HFD-associated GDM development, improved BAs metabolism, suppressed Fxr, Shp, and Fgf15 activity only in WT mice but not in the Fxr-/- HFD + Colesevelam group and Fxr-/- HFD group. HFD induced GDM and contributed to poor prognosis in GDM parturients through inducing gut microbial dysbiosis and metabolic alteration, especially appeared in BAs profile. Moreover, Fxr pathway participated in regulating HFD-associated gut microbiota disordered BAs metabolites and aggravating GDM in mice.

Discussion

Modulating gut microbiota and BAs metabolites could be a potential therapeutic strategy in the prevention and treatment of HFD-associated GDM.

The Versatile Role of Peroxisome Proliferator-Activated Receptors in Immune-Mediated Intestinal Diseases

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors that sense lipophilic molecules and act as transcription factors to regulate target genes. PPARs have been implicated in the regulation of innate immunity, glucose and lipid metabolism, cell proliferation, wound healing, and fibrotic processes. Some synthetic PPAR ligands are promising molecules for the treatment of inflammatory and fibrotic processes in immune-mediated intestinal diseases. Some of these are currently undergoing or have previously undergone clinical trials. Dietary PPAR ligands and changes in microbiota composition could modulate PPARs' activation to reduce inflammatory responses in these immune-mediated diseases, based on animal models and clinical trials. This narrative review aims to summarize the role of PPARs in immune-mediated bowel diseases and their potential therapeutic use.

Peroxisome proliferator-activated receptors: A key link between lipid metabolism and cancer progression

Lipids represent the essential components of membranes, serve as fuels for high-energy processes, and play crucial roles in signaling and cellular function. One of the key hallmarks of cancer is the reprogramming of metabolic pathways, especially abnormal lipid metabolism. Alterations in lipid uptake, lipid desaturation, de novo lipogenesis, lipid droplets, and fatty acid oxidation in cancer cells all contribute to cell survival in a changing microenvironment by regulating feedforward oncogenic signals, key oncogenic functions, oxidative and other stresses, immune responses, or intercellular communication. Peroxisome proliferator-activated receptors (PPARs) are transcription factors activated by fatty acids and act as core lipid sensors involved in the regulation of lipid homeostasis and cell fate. In addition to regulating whole-body energy homeostasis in physiological states, PPARs play a key role in lipid metabolism in cancer, which is receiving increasing research attention, especially the fundamental molecular mechanisms and cancer therapies targeting PPARs. In this review, we discuss how cancer cells alter metabolic patterns and regulate lipid metabolism to promote their own survival and progression through PPARs. Finally, we discuss potential therapeutic strategies for targeting PPARs in cancer based on recent studies from the last five years.

Low dietary carbohydrate induces structural alterations in enterocytes of the chicken ileum

We investigated the role of dietary carbohydrates in the maintenance of the enterocyte microvillar structure in the chicken ileum. Male chickens were divided into the control and three experimental groups, and the experimental groups were fed diets containing 50%, 25%, and 0% carbohydrates of the control diet. The structural alterations in enterocytes were examined using transmission electron microscopy and immunofluorescent techniques for β-actin and villin. Glucagon-like peptide (GLP)-2 and proglucagon mRNA were detected by immunohistochemistry and in situ hybridization, respectively. Fragmentation and wide gap spaces were frequently observed in the microvilli of the 25% and 0% groups. The length, width, and density of microvilli were also decreased in the experimental groups. The experimental groups had shorter terminal web extensions, and there were substantial changes in the mitochondrial density between the control and experimental groups. Intensities of β-actin and villin immunofluorescence observed on the apical surface of enterocytes were lower in the 0% group. The frequency of GLP-2-immunoreactive and proglucagon mRNA-expressing cells decreased with declining dietary carbohydrate levels. This study revealed that dietary carbohydrates contribute to the structural maintenance of enterocyte microvilli in the chicken ileum. The data from immunohistochemistry and in situ hybridization assays suggest the participation of GLP-2 in this maintenance system.

Signaling pathways and intervention for therapy of type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) represents one of the fastest growing epidemic metabolic disorders worldwide and is a strong contributor for a broad range of comorbidities, including vascular, visual, neurological, kidney, and liver diseases. Moreover, recent data suggest a mutual interplay between T2DM and Corona Virus Disease 2019 (COVID-19). T2DM is characterized by insulin resistance (IR) and pancreatic β cell dysfunction. Pioneering discoveries throughout the past few decades have established notable links between signaling pathways and T2DM pathogenesis and therapy. Importantly, a number of signaling pathways substantially control the advancement of core pathological changes in T2DM, including IR and β cell dysfunction, as well as additional pathogenic disturbances. Accordingly, an improved understanding of these signaling pathways sheds light on tractable targets and strategies for developing and repurposing critical therapies to treat T2DM and its complications. In this review, we provide a brief overview of the history of T2DM and signaling pathways, and offer a systematic update on the role and mechanism of key signaling pathways underlying the onset, development, and progression of T2DM. In this content, we also summarize current therapeutic drugs/agents associated with signaling pathways for the treatment of T2DM and its complications, and discuss some implications and directions to the future of this field.

Interactions between Intestinal Homeostasis and NAD+ Biology in Regulating Incretin Production and Postprandial Glucose Metabolism

The intestine has garnered attention as a target organ for developing new therapies for impaired glucose tolerance. The intestine, which produces incretin hormones, is the central regulator of glucose metabolism. Glucagon-like peptide-1 (GLP-1) production, which determines postprandial glucose levels, is regulated by intestinal homeostasis. Nicotinamide phosphoribosyltransferase (NAMPT)-mediated nicotinamide adenine dinucleotide (NAD⁺) biosynthesis in major metabolic organs such as the liver, adipose tissue, and skeletal muscle plays a crucial role in obesity- and aging-associated organ derangements. Furthermore, NAMPT-mediated NAD⁺ biosynthesis in the intestines and its upstream and downstream mediators, adenosine monophosphate-activated protein kinase (AMPK) and NAD⁺-dependent deacetylase sirtuins (SIRTs), respectively, are critical for intestinal homeostasis, including gut microbiota composition and bile acid metabolism, and GLP-1 production. Thus, boosting the intestinal AMPK-NAMPT-NAD⁺-SIRT pathway to improve intestinal homeostasis, GLP-1 production, and postprandial glucose metabolism has gained significant attention as a novel strategy to improve impaired glucose tolerance. Herein, we aimed to review in detail the regulatory mechanisms and importance of intestinal NAMPT-mediated NAD⁺ biosynthesis in regulating intestinal homeostasis and GLP-1 secretion in obesity and aging. Furthermore, dietary and molecular factors regulating intestinal NAMPT-mediated NAD⁺ biosynthesis were critically explored to facilitate the development of new therapeutic strategies for postprandial glucose dysregulation.

Glucagon-like peptide-1 receptor regulates receptor of advanced glycation end products in high glucose-treated rat mesangial cells

BACKGROUND

Hyperglycemia-induced advanced glycation end products (AGEs) and receptor for AGEs (RAGEs) play major roles in diabetic nephropathy progression. In previous study, both glucagon-like peptide-1 (GLP-1) and peroxisome proliferator-activated receptors delta (PPARδ) agonists were shown to have anti-inflammatory effect on AGE-treated rat mesangial cells (RMCs). The interaction among PPARδ agonists, GLP-1, and AGE-RAGE axis is, however, still unclear.

METHODS

In this study, the individual and synergic effect of PPARδ agonist (L-165 041) and siRNA of GLP-1 receptor (GLP-1R) on the expression of GLP-1, GLP-1R, RAGE, and cell viability in AGE-treated RMCs were investigated.

RESULTS

L-165 041 enhanced GLP-1R mRNA and protein expression only in the presence of AGE. The expression of RAGE mRNA and protein was enhanced by AGE, attenuated by L-165 041, and siRNA of GLP-1R reversed L-165 041-induced inhibition. Cell viability was also inhibited by AGE. L-165 041 attenuated AGE-induced inhibition and siRNA GLP-1R diminished L-165 041 effect.

CONCLUSION

PPARδ agonists increase GLP-1R expression on RMC in the presence of AGE. PPARδ agonists also attenuate AGE-induced upregulated RAGE expression and downregulated cell viability. The effect of PPARδ agonists needs the cooperation of GLP-1R activation.

Circulating saturated fatty acids and risk of gestational diabetes mellitus: A cross-sectional study and meta-analysis

Background

Previous studies have analyzed the associations between the circulating saturated fatty acids (SFAs) and gestational diabetes mellitus (GDM), but no consistent conclusions have been reached. The aim of this study was to evaluate whether plasma SFAs were in correlation with GDM risks in our in-house women cross-sectional study and to better define their associations on the clinical evidence available to date by a dose-response meta-analysis.

Methods

We carried out a cross-sectional study of 807 pregnant women in 2018–2019 (Shanghai, China). GDM was defined according to the criteria of the International Association of Diabetes and Pregnancy Study Groups (IADPSG). Gas chromatography was used to determine the plasma fatty acids (FAs) in the 24–28 gestational weeks. The SFAs levels of non-GDM and GDM participants were compared by Mann–Whitney test, and the association between SFAs and GDM was explored by multivariate logistic models. Further, the potential diagnostic value of plasma SFAs was evaluated using the method of receiver operating characteristic (ROC) analysis. For meta-analysis, five databases were systematically searched from inception to March 2022, and we included 25 relevant studies for calculating pooled standard mean differences (SMDs) and 95% CI to describe the differences in SFAs profiles between non-GDM and GDM women. Study-specific, multivariable-adjusted ORs and 95% CI were also pooled using a fixed-effect model or random-effects model according to the heterogeneity to evaluate the associations between circulating SFAs and GDM prevalence.

Results

In our cross-sectional study, we found plasma proportion of palmitic acid (C16:0) was positively associated (aOR: 1.10 per 1% increase; 95% CI: 1.04, 1.17), while plasma stearic acid (C18:0) (aOR: 0.76 per 1% increase; 95% CI: 0.66, 0.89), arachidic acid (C20:0) (aOR: 0.92 per 0.1% increase; 95% CI: 0.87, 0.97), behenic acid (C22:0) (aOR: 0.94 per 0.1% increase; 95% CI: 0.92, 0.97), and lignoceric acid (C24:0) (aOR: 0.94 per 0.1% increase; 95% CI: 0.92, 0.97) were inversely associated with GDM. The area under the receiver operative characteristic curve increased from 0.7503 (the basic diagnostic model) to 0.8178 (p = 0.002) after adding total very-long-chain SFAs (VLcSFAs). A meta-analysis from 25 studies showed the circulating levels of three individual SFAs of GDM women were different from those of normal pregnant women. The summarized ORs for GDM was 1.593 (95% CI: 1.125, 2.255, p = 0.009), 0.652 (95% CI: 0.472, 0.901, p = 0.010) and 0.613 (95% CI: 0.449, 0.838, p = 0.002), respectively, comparing the highest vs. lowest quantile of the concentrations of C16:0, C22:0, and C24:0.

Conclusion

Our results, combined with the findings from meta-analysis, showed that women with GDM had a particular circulating SFA profile, characterized by higher levels of palmitic acid, and lower levels of VLcSFAs. Alterations in the chain lengths of blood SFA profile were shown to be associated with the occurrence of GDM.

Dietary carbohydrate influences the colocalization pattern of Glucagon-like Peptide-1 with neurotensin in the chicken ileum

Glucagon-like peptide (GLP)-1 colocalizes with neurotensin (NT) in the same enteroendocrine cells (EECs) of the chicken ileum. The present study was designed to clarify the influence of dietary carbohydrate (CHO) on the colocalization pattern of GLP-1 with NT in the chicken distal ileum. Male White Leghorn chickens at 6 weeks of age (n = 15) were divided into three groups, a control and two experimental (low-CHO and CHO-free), with five chickens in each, and fed control or experimental diets for 7 d. Distal ileum was collected from each bird as a tissue sample and subjected to double immunofluorescence staining to detect GLP-1 and NT. Three types of EEC, GLP-1+/NT+, GLP-1+/NT- and GLP-1-/NT+, were demonstrated in the chicken ileum. GLP-1+/NT+ cells in the control group had a spindle-like shape with a long cytoplasmic process, but those in the experimental groups were round and lacked a cytoplasmic process. The ratio of GLP-1+/NT+ cells was significantly decreased in the two experimental groups compared with that in the control group. The ratio of GLP-1+/NT+ cells was significantly lower than those of GLP-1+/NT- and GLP-1-/NT+ cells in the two experimental groups. Most cells that were immunoreactive for GLP-1 and NT antisera lacked signals of proglucagon (PG) and NT precursor (NTP) mRNA in the experimental groups. The number of EECs expressing PG and NTP mRNA signals showed tendencies for decreases with a reduction of dietary CHO level. These findings suggest that dietary CHO could be a significant regulator of the pattern of colocalization pattern of GLP-1 with NT in the chicken ileum.

Dietary carbohydrate modifies the density of L cells in the chicken ileum

Glucagon-like peptides (GLPs) are secreted from intestinal L cells and stimulate various physiological functions in the gastrointestinal tract. The secretion of GLPs is influenced by macronutrient ingestion. This study aims to clarify the effects of dietary carbohydrate (CHO) on L cells in the chicken ileum. Six-week-old, male White Leghorn chickens were divided into three groups: control, low-CHO and CHO-free, with five chickens in each group. Paraffin sections were made from the proximal and distal ileum of each animal and subjected to immunohistochemistry for GLP-1 and GLP-2 peptides and in situ hybridization for proglucagon (PG) mRNA. A significant reduction of GLP-1- and GLP-2-immunoreactive cells was observed in the two experimental groups compared with that in the control. A reduction of cells expressing PG mRNA was observed in the proximal and distal ileum of the CHO-free group compared with that in the control. The ratio of GLP-1-immunoreactive cells showing Ki-67 immunoreactivity was significantly lower in the distal ileum of the CHO-free group than that in the control group. These data suggest that dietary CHO is an effective stimulator for modifying L cell density in the chicken ileum.

Design, synthesis, and biological evaluation of novel dual FFA1 and PPARδ agonists possessing phenoxyacetic acid scaffold

The free fatty acid receptor 1 (FFA1/GPR40) and peroxisome proliferator-activated receptor δ (PPARδ) have been widely considered as promising targets for type 2 diabetes mellitus (T2DM) due to their respective roles in promoting insulin secretion and improving insulin sensitivity. Hence, the dual FFA1/PPARδ agonists may exert synergistic effects by simultaneously activating FFA1 and PPARδ. The present study performed systematic exploration around previously reported FFA1 agonist 2-(2-fluoro-4-((2'-methyl-4'-(3-(methylsulfonyl)propoxy)-[1,1'-biphenyl]-3-yl)methoxy)phenoxy)acetic acid (lead compound), leading to the identification of a novel dual FFA1/PPARδ agonist 2-(2-fluoro-4-((3-(6-methoxynaphthalen-2-yl)benzyl)oxy)phenoxy)acetic acid (the optimal compound), which displayed high selectivity over PPARα and PPARγ. In addition, the docking study provided us with detailed binding modes of the optimal compound in FFA1 and PPARδ. Furthermore, the optimal compound exhibited greater glucose-lowering effects than lead compound, which might attribute to its synergistic effects by simultaneously modulating insulin secretion and resistance. Moreover, the optimal compound has an acceptable safety profile in the acute toxicity study at a high dose of 500 mg/kg Therefore, our results provided a novel dual FFA1/PPARδ agonist with excellent glucose-lowering effects in vivo.

Discovery of the first-in-class dual PPARδ/γ partial agonist for the treatment of metabolic syndrome

The peroxisome proliferator-activated receptors (PPARs) exert vital function in the regulation of energy metabolism, which were considered as promising targets of metabolic syndrome. Until now, PPARδ/γ dual agonist is rarely reported, and thereby the pharmacologic action of PPARδ/γ dual agonist is still unclear. In this study, we identified a dual PPARδ/γ partial agonist 6 (ZLY06) based on the cyclization strategy of PPARα/δ dual agonist GFT505. ZLY06 revealed excellent pharmacokinetic profiles suitable for oral medication. Moreover, ZLY06 markedly improved glucolipid metabolism without weight gain, and alleviated fatty liver by promoting the β-oxidation of fatty acid and inhibiting hepatic lipogenesis. In contrast, weight gain and hepatic steatosis were observed in Rosiglitazone, a widely used PPARγ full agonist. All of these results indicated that ZLY06 exhibits potential benefits on metabolic syndrome, while no adverse effects related to PPARγ full agonist.

PPAR control of metabolism and cardiovascular functions

Peroxisome proliferator-activated receptor-α (PPARα), PPARδ and PPARγ are transcription factors that regulate gene expression following ligand activation. PPARα increases cellular fatty acid uptake, esterification and trafficking, and regulates lipoprotein metabolism genes. PPARδ stimulates lipid and glucose utilization by increasing mitochondrial function and fatty acid desaturation pathways. By contrast, PPARγ promotes fatty acid uptake, triglyceride formation and storage in lipid droplets, thereby increasing insulin sensitivity and glucose metabolism. PPARs also exert antiatherogenic and anti-inflammatory effects on the vascular wall and immune cells. Clinically, PPARγ activation by glitazones and PPARα activation by fibrates reduce insulin resistance and dyslipidaemia, respectively. PPARs are also physiological master switches in the heart, steering cardiac energy metabolism in cardiomyocytes, thereby affecting pathological heart failure and diabetic cardiomyopathy. Novel PPAR agonists in clinical development are providing new opportunities in the management of metabolic and cardiovascular diseases.

High-fat diet-activated fatty acid oxidation mediates intestinal stemness and tumorigenicity

Obesity is an established risk factor for cancer in many tissues. In the mammalian intestine, a pro-obesity high-fat diet (HFD) promotes regeneration and tumorigenesis by enhancing intestinal stem cell (ISC) numbers, proliferation, and function. Although PPAR (peroxisome proliferator-activated receptor) nuclear receptor activity has been proposed to facilitate these effects, their exact role is unclear. Here we find that, in loss-of-function in vivo models, PPARα and PPARδ contribute to the HFD response in ISCs. Mechanistically, both PPARs do so by robustly inducing a downstream fatty acid oxidation (FAO) metabolic program. Pharmacologic and genetic disruption of CPT1A (the rate-controlling enzyme of mitochondrial FAO) blunts the HFD phenotype in ISCs. Furthermore, inhibition of CPT1A dampens the pro-tumorigenic consequences of a HFD on early tumor incidence and progression. These findings demonstrate that inhibition of a HFD-activated FAO program creates a therapeutic opportunity to counter the effects of a HFD on ISCs and intestinal tumorigenesis.

Peroxisome Proliferator-Activated Receptors as Molecular Links between Caloric Restriction and Circadian Rhythm

The circadian rhythm plays a chief role in the adaptation of all bodily processes to internal and environmental changes on the daily basis. Next to light/dark phases, feeding patterns constitute the most essential element entraining daily oscillations, and therefore, timely and appropriate restrictive diets have a great capacity to restore the circadian rhythm. One of the restrictive nutritional approaches, caloric restriction (CR) achieves stunning results in extending health span and life span via coordinated changes in multiple biological functions from the molecular, cellular, to the whole-body levels. The main molecular pathways affected by CR include mTOR, insulin signaling, AMPK, and sirtuins. Members of the family of nuclear receptors, the three peroxisome proliferator-activated receptors (PPARs), PPARα, PPARβ/δ, and PPARγ take part in the modulation of these pathways. In this non-systematic review, we describe the molecular interconnection between circadian rhythm, CR-associated pathways, and PPARs. Further, we identify a link between circadian rhythm and the outcomes of CR on the whole-body level including oxidative stress, inflammation, and aging. Since PPARs contribute to many changes triggered by CR, we discuss the potential involvement of PPARs in bridging CR and circadian rhythm.

PPARs and Microbiota in Skeletal Muscle Health and Wasting

Skeletal muscle is a major metabolic organ that uses mostly glucose and lipids for energy production and has the capacity to remodel itself in response to exercise and fasting. Skeletal muscle wasting occurs in many diseases and during aging. Muscle wasting is often accompanied by chronic low-grade inflammation associated to inter- and intra-muscular fat deposition. During aging, muscle wasting is advanced due to increased movement disorders, as a result of restricted physical exercise, frailty, and the pain associated with arthritis. Muscle atrophy is characterized by increased protein degradation, where the ubiquitin-proteasomal and autophagy-lysosomal pathways, atrogenes, and growth factor signaling all play an important role. Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor family of transcription factors, which are activated by fatty acids and their derivatives. PPARs regulate genes that are involved in development, metabolism, inflammation, and many cellular processes in different organs. PPARs are also expressed in muscle and exert pleiotropic specialized responses upon activation by their ligands. There are three PPAR isotypes, viz., PPARα, -β/δ, and -γ. The expression of PPARα is high in tissues with effective fatty acid catabolism, including skeletal muscle. PPARβ/δ is expressed more ubiquitously and is the predominant isotype in skeletal muscle. It is involved in energy metabolism, mitochondrial biogenesis, and fiber-type switching. The expression of PPARγ is high in adipocytes, but it is also implicated in lipid deposition in muscle and other organs. Collectively, all three PPAR isotypes have a major impact on muscle homeostasis either directly or indirectly. Furthermore, reciprocal interactions have been found between PPARs and the gut microbiota along the gut–muscle axis in both health and disease. Herein, we review functions of PPARs in skeletal muscle and their interaction with the gut microbiota in the context of muscle wasting.

Peroxisome Proliferator-Activated Receptors and Caloric Restriction-Common Pathways Affecting Metabolism, Health, and Longevity

Caloric restriction (CR) is a traditional but scientifically verified approach to promoting health and increasing lifespan. CR exerts its effects through multiple molecular pathways that trigger major metabolic adaptations. It influences key nutrient and energy-sensing pathways including mammalian target of rapamycin, Sirtuin 1, AMP-activated protein kinase, and insulin signaling, ultimately resulting in reductions in basic metabolic rate, inflammation, and oxidative stress, as well as increased autophagy and mitochondrial efficiency. CR shares multiple overlapping pathways with peroxisome proliferator-activated receptors (PPARs), particularly in energy metabolism and inflammation. Consequently, several lines of evidence suggest that PPARs might be indispensable for beneficial outcomes related to CR. In this review, we present the available evidence for the interconnection between CR and PPARs, highlighting their shared pathways and analyzing their interaction. We also discuss the possible contributions of PPARs to the effects of CR on whole organism outcomes.

Antidiabetic effects of selenium-enriched Bifidobacterium longum DD98 in type 2 diabetes model of mice

Both selenium and probiotics have shown antidiabetic effects in a type 2 diabetes model. The objective of this study is to investigate the alleviating effects of selenium-enriched Bifidobacterium longum DD98 (Se-B. longum DD98) on diabetes in mice and explore the possible underlying mechanism. A type 2 diabetes model was established using a high-fat diet and streptozotocin (STZ) injection in mice. To investigate the beneficial effects of Se-B. longum DD98, diabetic mice were then treated with B. longum DD98, Se-B. longum DD98, or sodium selenite (Na₂SeO₃) for three weeks. The results suggested that all three treatments could reduce the levels of fasting blood glucose (FBG), glycated hemoglobin (HbA1c), insulin and leptin, improve glucose tolerance, regulate lipid metabolism, and protect against the impairment of the liver and pancreas, while Se-B. longum DD98 showed a greater effect on relieving the above mentioned symptoms of type 2 diabetes in mice. Furthermore, this effect was associated with butyrate production and inflammatory response. Se-B. longum DD98 better increased the level of butyrate in feces and decreased the levels of proinflammatory cytokines in the pancreas compared with B. longum DD98 and Na₂SeO₃, leading to ameliorative insulin resistance. Se-B. longum DD98 also improved the glucagon like peptide-1 (GLP-1) level in serum and intestinal cells, which protected the pancreatic β-islet cells from damage induced by type 2 diabetes. These results demonstrated that Na₂SeO₃, B. longum DD98, or Se-B. longum DD98 could alleviate the progression of type 2 diabetes in mice. Se-B. longum DD98 showed greater antidiabetic effects than the other two treatments, and could be considered as a promising candidate for treating type 2 diabetes.

ZLY032, the first-in-class dual FFA1/PPARδ agonist, improves glucolipid metabolism and alleviates hepatic fibrosis

The free fatty acid receptor 1 (FFA1) and peroxisome proliferator-activated receptor δ (PPARδ) are considered as anti-diabetic targets based on their role in improving insulin secretion and resistance. Based on their synergetic mechanisms, we have previously identified the first-in-class dual FFA1/PPARδ agonist ZLY032. After long-term treatment, ZLY032 significantly improved glucolipid metabolism and alleviated fatty liver in ob/ob mice and methionine choline-deficient diet-fed db/db mice, mainly by regulating triglyceride metabolism, fatty acid β-oxidation, lipid synthesis, inflammation, oxidative stress and mitochondrial function. Notably, ZLY032 exhibited greater advantages on lipid metabolism, insulin sensitivity and pancreatic β-cell function than TAK-875, the most advanced candidate of FFA1 agonists. Moreover, ZLY032 prevented CCl₄-induced liver fibrosis by reducing the expressions of genes involved in inflammation and fibrosis development. These results suggest that the dual FFA1/PPARδ agonists such as ZLY032 may be useful for the treatment of metabolic disorders.

ID1-Mediated BMP Signaling Pathway Potentiates Glucagon-Like Peptide-1 Secretion in Response to Nutrient Replenishment

Glucagon-like peptide-1 (GLP-1) is a well-known incretin hormone secreted from enteroendocrinal L cells in response to nutrients, such as glucose and dietary fat, and controls glycemic homeostasis. However, the detailed intracellular mechanisms of how L cells control GLP-1 secretion in response to nutrients still remain unclear. Here, we report that bone morphogenetic protein (BMP) signaling pathway plays a pivotal role to control GLP-1 secretion in response to nutrient replenishment in well-established mouse enteroendocrinal L cells (GLUTag cells). Nutrient starvation dramatically reduced cellular respiration and GLP-1 secretion in GLUTag cells. Transcriptome analysis revealed that nutrient starvation remarkably reduced gene expressions involved in BMP signaling pathway, whereas nutrient replenishment rescued BMP signaling to potentiate GLP-1 secretion. Transient knockdown of inhibitor of DNA binding (ID)1, a well-known target gene of BMP signaling, remarkably reduced GLP-1 secretion. Consistently, LDN193189, an inhibitor of BMP signaling, markedly reduced GLP-1 secretion in L cells. In contrast, BMP4 treatment activated BMP signaling pathway and potentiated GLP-1 secretion in response to nutrient replenishment. Altogether, we demonstrated that BMP signaling pathway is a novel molecular mechanism to control GLP-1 secretion in response to cellular nutrient status. Selective activation of BMP signaling would be a potent therapeutic strategy to stimulate GLP-1 secretion in order to restore glycemic homeostasis.

HWL-088, a new potent free fatty acid receptor 1 (FFAR1) agonist, improves glucolipid metabolism and acts additively with metformin in ob/ob diabetic mice

BACKGROUND AND PURPOSE

The free fatty acid receptor 1 (FFAR1) plays an important role in glucose-stimulated insulin secretion making it an attractive anti-diabetic target. This study characterizes the pharmacological profile of HWL-088 (2-(2-fluoro-4-((2'-methyl-[1,1'- biphenyl]-3-yl)methoxy)phenoxy)acetic acid), a novel highly potent FFAR1 agonist in vitro and in vivo. Moreover, we investigated the long-term effects of HWL-088 alone and in combination with metformin in diabetic mice.

EXPERIMENTAL APPROACH

In vitro effects of HWL-088 on FFAR1 and PPARα/γ/δ were studied in cell-based assays. Glucose-dependent insulinotropic effects were evaluated in MIN6 cell line and in rats. Long-term effects on glucose and lipid metabolism were investigated in ob/ob mice.

KEY RESULTS

HWL-088 is a highly potent FFAR1 agonist (EC50 = 18.9 nM) with moderate PPARδ activity (EC50 = 570.9 nM) and promotes glucose-dependent insulin secretion in vitro and in vivo. Long-term administration of HWL-088 exhibited better glucose control and plasma lipid profiles than those of another FFAR1 agonist, TAK-875, and synergistic improvements were observed when combined with metformin. Moreover, HWL-088 and combination therapy improved β-cell function by up-regulation of pancreas duodenum homeobox-1, reduced fat accumulation in adipose tissue and alleviated fatty liver in ob/ob mice. The effect of HWL-088 involves a reduction in hepatic lipogenesis and oxidative stress, increased lipoprotein lipolysis, glucose uptake, mitochondrial function and fatty acid β-oxidation.

CONCLUSION AND IMPLICATIONS

These data indicate that long-term treatment with HWL-088, a highly potent FFAR1 agonist, improves glucose and lipid metabolism and may be useful for the treatment of diabetes mellitus by mono-therapy or combination with metformin.

Myeloid-Derived Growth Factor Promotes Intestinal Glucagon-Like Peptide-1 Production in Male Mice With Type 2 Diabetes

Myeloid-derived growth factor (MYDGF), which is produced by bone marrow-derived cells, mediates cardiac repair following myocardial infarction by inhibiting cardiac myocyte apoptosis to subsequently reduce the infarct size. However, the function of MYDGF in the incretin system of diabetes is still unknown. Here, loss-of-function and gain-of-function experiments in mice revealed that MYDGF maintains glucose homeostasis by inducing glucagon-like peptide-1 (GLP-1) production and secretion and that it improves glucose tolerance and lipid metabolism. Treatment with recombinant MYDGF increased the secretion and production of GLP-1 in STC-1 cells in vitro. Mechanistically, the positive effects of MYDGF are potentially attributable to the activation of protein kinase A/glycogen synthase kinase 3β/β-catenin (PKA/GSK-3β/β-catenin) and mitogen-activated protein kinase (MAPK) kinases/extracellular regulated protein kinase (MEK/ERK) pathways. Based on these findings, MYDGF promotes the secretion and production of GLP-1 in intestinal L-cells and potentially represents a potential therapeutic medication target for type 2 diabetes.

The nuclear receptor FXR inhibits Glucagon-Like Peptide-1 secretion in response to microbiota-derived Short-Chain Fatty Acids

The gut microbiota participates in the control of energy homeostasis partly through fermentation of dietary fibers hence producing short-chain fatty acids (SCFAs), which in turn promote the secretion of the incretin Glucagon-Like Peptide-1 (GLP-1) by binding to the SCFA receptors FFAR2 and FFAR3 on enteroendocrine L-cells. We have previously shown that activation of the nuclear Farnesoid X Receptor (FXR) decreases the L-cell response to glucose. Here, we investigated whether FXR also regulates the SCFA-induced GLP-1 secretion. GLP-1 secretion in response to SCFAs was evaluated ex vivo in murine colonic biopsies and in colonoids of wild-type (WT) and FXR knock-out (KO) mice, in vitro in GLUTag and NCI-H716 L-cells activated with the synthetic FXR agonist GW4064 and in vivo in WT and FXR KO mice after prebiotic supplementation. SCFA-induced GLP-1 secretion was blunted in colonic biopsies from GW4064-treated mice and enhanced in FXR KO colonoids. In vitro FXR activation inhibited GLP-1 secretion in response to SCFAs and FFAR2 synthetic ligands, mainly by decreasing FFAR2 expression and downstream Gαq-signaling. FXR KO mice displayed elevated colonic FFAR2 mRNA levels and increased plasma GLP-1 levels upon local supply of SCFAs with prebiotic supplementation. Our results demonstrate that FXR activation decreases L-cell GLP-1 secretion in response to inulin-derived SCFA by reducing FFAR2 expression and signaling. Inactivation of intestinal FXR using bile acid sequestrants or synthetic antagonists in combination with prebiotic supplementation may be a promising therapeutic approach to boost the incretin axis in type 2 diabetes.

Enteric Microbiota⁻Gut⁻Brain Axis from the Perspective of Nuclear Receptors

Nuclear receptors (NRs) play a key role in regulating virtually all body functions, thus maintaining a healthy operating body with all its complex systems. Recently, gut microbiota emerged as major factor contributing to the health of the whole organism. Enteric bacteria have multiple ways to influence their host and several of them involve communication with the brain. Mounting evidence of cooperation between gut flora and NRs is already available. However, the full potential of the microbiota interconnection with NRs remains to be uncovered. Herewith, we present the current state of knowledge on the multifaceted roles of NRs in the enteric microbiota–gut–brain axis.

Hypothalamic peroxisome proliferator-activated receptor gamma regulates ghrelin production and food intake

Peroxisome proliferator-activated receptor-γ (PPARγ) regulates fatty acid storage, glucose metabolism, and food intake. Ghrelin, a gastric hormone, provides a hunger signal to the central nervous system to stimulate appetite. However, the effects of PPARγ on ghrelin production are still unclear. In the present study, the effects of PPARγ on ghrelin production were examined in lean- or high-fat diet-induced obese (DIO) C57BL/6J mice and mHypoE-42 cells, a hypothalamic cell line. 3rd intracerebroventricular injection of adenoviral-directed overexpression of PPARγ (Ad-PPARγ) reduced hypothalamic and plasma ghrelin, food intake in both lean C57BL/6J mice and diet-induced obese mice. These changes were associated with a significant increase in mechanistic target of rapamycin complex 1 (mTORC1) activity. Overexpression of PPARγ enhanced mTORC1 signaling and suppressed ghrelin production in cultured mHypoE-42 cells. Our results suggest that hypothalamic PPARγ plays a vital role in ghrelin production and food intake in mice.

PPARβ/δ: A Key Therapeutic Target in Metabolic Disorders

Research in recent years on peroxisome proliferator-activated receptor (PPAR)β/δ indicates that it plays a key role in the maintenance of energy homeostasis, both at the cellular level and within the organism as a whole. PPARβ/δ activation might help prevent the development of metabolic disorders, including obesity, dyslipidaemia, type 2 diabetes mellitus and non-alcoholic fatty liver disease. This review highlights research findings on the PPARβ/δ regulation of energy metabolism and the development of diseases related to altered cellular and body metabolism. It also describes the potential of the pharmacological activation of PPARβ/δ as a treatment for human metabolic disorders.

Lactobacillus casei CCFM419 attenuates type 2 diabetes via a gut microbiota dependent mechanism

Probiotics, as dietary supplements, transmit their major effects through the regulation of gut microbiota. According to a previous study, one possible mechanism of Lactobacillus casei CCFM419 protection against diabetes may involve gut flora. To test this hypothesis, high fat and streptozotocin-induced C57BL/6J mice were fed L. casei CCFM419 at 10⁸, 10⁹, and 10¹⁰ colony forming units (CFU). Compared to untreated mice, 10⁹ CFU of L. casei CCFM419 attenuated several symptoms of diabetes, including fasting blood glucose, postprandial blood glucose, glucose intolerance, and insulin resistance. In addition, this CFU level also decreased the levels of the inflammatory markers tumor necrosis factor-α and interleukin-6 and increased intestinal glucagon-like peptide-1 (GLP-1) levels, which are associated with the production of short chain fatty acids (SCFAs). The 16S rRNA gene sequencing of fecal samples demonstrated that 10⁹ CFU of L. casei CCFM419 dramatically increased the abundance of Bacteroidetes and decreased the proportion of Firmicutes at the phylum level, and enriched Bifidobacterium, Lactobacillus, and SCFA-producing bacteria, including Allobaculum and Bacteroides. These findings suggested that L. casei CCFM419 modified the gut flora-SCFA-inflammation/GLP-1 mechanism to ameliorate type 2 diabetes.

Glucagon-like peptide receptor agonists attenuate advanced glycation end products-induced inflammation in rat mesangial cells

BACKGROUND

Hyperglycemia-induced advanced glycation end products (AGEs) and receptor for AGEs (RAGE) production play major roles in progression of diabetic nephropathy. Anti-RAGE effect of peroxisome proliferator-activated receptor-delta (PPARδ) agonists was shown in previous studies. PPARδ agonists also stimulate glucagon-like peptide-1 (GLP-1) secretion from human intestinal cells.

METHODS

In this study, the individual and synergic anti-inflammatory effects of GLP-1 receptor (exendin-4) and PPARδ (L-165,041) agonists in AGE-treated rat mesangial cells (RMC) were investigated.

RESULTS

The results showed both exendin-4 and L-165,041 significantly attenuated AGE-induced IL-6 and TNF-α production, RAGE expression, and cell death in RMC. Similar anti-inflammatory potency was seen between 0.3 nM exendin-4 and 1 μM L-165,041. Synergic effect of exendin-4 and L-165,041 was shown in inhibiting cytokines production, but not in inhibiting RAGE expression or cell death.

CONCLUSIONS

These results suggest that both GLP-1 receptor and PPARδ agonists have anti-inflammatory effect on AGE-treated rat mesangial cells.

Intestinal PPARδ protects against diet-induced obesity, insulin resistance and dyslipidemia

Peroxisome proliferator-activated receptor δ (PPARδ) is a ligand-activated transcription factor that has an important role in lipid metabolism. Activation of PPARδ stimulates fatty acid oxidation in adipose tissue and skeletal muscle and improves dyslipidemia in mice and humans. PPARδ is highly expressed in the intestinal tract but its physiological function in this organ is not known. Using mice with an intestinal epithelial cell-specific deletion of PPARδ, we show that intestinal PPARδ protects against diet-induced obesity, insulin resistance and dyslipidemia. Furthermore, absence of intestinal PPARδ abolished the ability of PPARδ agonist GW501516 to increase plasma levels of HDL-cholesterol. Together, our findings show that intestinal PPARδ is important in maintaining metabolic homeostasis and suggest that intestinal-specific activation of PPARδ could be a therapeutic approach for treatment of the metabolic syndrome and dyslipidemia, while avoiding systemic toxicity.

Distinct but complementary contributions of PPAR isotypes to energy homeostasis

Peroxisome proliferator-activated receptors (PPARs) regulate energy metabolism and hence are therapeutic targets in metabolic diseases such as type 2 diabetes and non-alcoholic fatty liver disease. While they share anti-inflammatory activities, the PPAR isotypes distinguish themselves by differential actions on lipid and glucose homeostasis. In this Review we discuss the complementary and distinct metabolic effects of the PPAR isotypes together with the underlying cellular and molecular mechanisms, as well as the synthetic PPAR ligands that are used in the clinic or under development. We highlight the potential of new PPAR ligands with improved efficacy and safety profiles in the treatment of complex metabolic disorders.

PPARs in obesity-induced T2DM, dyslipidaemia and NAFLD

Obesity is a worldwide epidemic that predisposes individuals to cardiometabolic complications, such as type 2 diabetes mellitus (T2DM) and nonalcoholic fatty liver disease (NAFLD), which are all related to inappropriate ectopic lipid deposition. Identification of the pathogenic molecular mechanisms and effective therapeutic approaches are highly needed. The peroxisome proliferator-activated receptors (PPARs) modulate several biological processes that are perturbed in obesity, including inflammation, lipid and glucose metabolism and overall energy homeostasis. Here, we review how PPARs regulate the functions of adipose tissues, such as adipogenesis, lipid storage and adaptive thermogenesis, under healthy and pathological conditions. We also discuss the clinical use and mechanism of PPAR agonists in the treatment of obesity comorbidities such as dyslipidaemia, T2DM and NAFLD. First generation PPAR agonists, primarily those acting on PPARγ, are associated with adverse effects that outweigh their clinical benefits, which led to the discontinuation of their development. An improved understanding of the physiological roles of PPARs might, therefore, enable the development of safe, new PPAR agonists with improved therapeutic potential.

Intestinal bile acid receptors are key regulators of glucose homeostasis

In addition to their well-known function as dietary lipid detergents, bile acids have emerged as important signalling molecules that regulate energy homeostasis. Recent studies have highlighted that disrupted bile acid metabolism is associated with metabolism disorders such as dyslipidaemia, intestinal chronic inflammatory diseases and obesity. In particular, type 2 diabetes (T2D) is associated with quantitative and qualitative modifications in bile acid metabolism. Bile acids bind and modulate the activity of transmembrane and nuclear receptors (NR). Among these receptors, the G-protein-coupled bile acid receptor 1 (TGR5) and the NR farnesoid X receptor (FXR) are implicated in the regulation of bile acid, lipid, glucose and energy homeostasis. The role of these receptors in the intestine in energy metabolism regulation has been recently highlighted. More precisely, recent studies have shown that FXR is important for glucose homeostasis in particular in metabolic disorders such as T2D and obesity. This review highlights the growing importance of the bile acid receptors TGR5 and FXR in the intestine as key regulators of glucose metabolism and their potential as therapeutic targets.

Molecular Pathways: Dietary Regulation of Stemness and Tumor Initiation by the PPAR-δ Pathway

Peroxisome proliferator-activated receptor delta (PPAR-δ) is a nuclear receptor transcription factor that regulates gene expression during development and disease states, such as cancer. However, the precise role of PPAR-δ during tumorigenesis is not well understood. Recent data suggest that PPAR-δ may have context-specific oncogenic and tumor-suppressive roles depending on the tissue, cell-type, or diet-induced physiology in question. For example, in the intestine, pro-obesity diets, such as a high-fat diet (HFD), are associated with increased colorectal cancer incidence. Interestingly, many of the effects of an HFD in the stem and progenitor cell compartment are driven by a robust PPAR-δ program and contribute to the early steps of intestinal tumorigenesis. Importantly, the PPAR-δ pathway or its downstream mediators may serve as therapeutic intervention points or biomarkers in colon cancer that arise in patients who are obese. Although potent PPAR-δ agonists and antagonists exist, their clinical utility may be enhanced by uncovering how PPAR-δ mediates tumorigenesis in diverse tissues and cell types as well as in response to diet. Clin Cancer Res; 22(23); 5636-41. ©2016 AACR.

Unraveling the Effects of PPARβ/δ on Insulin Resistance and Cardiovascular Disease

Insulin resistance precedes dyslipidemia and type 2 diabetes mellitus (T2DM) development. Preclinical evidence suggests that peroxisome proliferator-activated receptor (PPAR) β/δ activators may prevent and treat obesity-induced insulin resistance and T2DM, while clinical trials highlight their potential utility in dyslipidemia. This review summarizes recent mechanistic insights into the antidiabetic effects of PPARβ/δ activators, including their anti-inflammatory actions, their ability to inhibit endoplasmic reticulum (ER) stress and hepatic lipogenesis, and to improve atherogenesis and insulin sensitivity, as well as their capacity to activate pathways that are also stimulated by exercise. Findings from clinical trials are also examined. Dissecting the effects of PPARβ/δ ligands on insulin sensitivity and atherogenesis may provide a basis for the development of therapies for the prevention and treatment of T2DM and cardiovascular disease (CVD).

The regulation of function, growth and survival of GLP-1-producing L-cells

Glucagon-like peptide-1 (GLP-1) is a peptide hormone, released from intestinal L-cells in response to hormonal, neural and nutrient stimuli. In addition to potentiation of meal-stimulated insulin secretion, GLP-1 signalling exerts numerous pleiotropic effects on various tissues, regulating energy absorption and disposal, as well as cell proliferation and survival. In Type 2 Diabetes (T2D) reduced plasma levels of GLP-1 have been observed, and plasma levels of GLP-1, as well as reduced numbers of GLP-1 producing cells, have been correlated to obesity and insulin resistance. Increasing endogenous secretion of GLP-1 by selective targeting of the molecular mechanisms regulating secretion from the L-cell has been the focus of much recent research. An additional and promising strategy for enhancing endogenous secretion may be to increase the L-cell mass in the intestinal epithelium, but the mechanisms that regulate the growth, survival and function of these cells are largely unknown. We recently showed that prolonged exposure to high concentrations of the fatty acid palmitate induced lipotoxic effects, similar to those operative in insulin-producing cells, in an in vitro model of GLP-1-producing cells. The mechanisms inducing this lipototoxicity involved increased production of reactive oxygen species (ROS). In this review, regulation of GLP-1-secreting cells is discussed, with a focus on the mechanisms underlying GLP-1 secretion, long-term regulation of growth, differentiation and survival under normal as well as diabetic conditions of hypernutrition.

A comparative study of the antidiabetic effects exerted by live and dead multi-strain probiotics in the type 2 diabetes model of mice

A comparative study of the anti-diabetic effects exerted by live and dead multi-strain probiotics in the type 2 diabetes model of mice.

Farnesoid X receptor inhibits glucagon-like peptide-1 production by enteroendocrine L cells

Bile acids are signalling molecules, which activate the transmembrane receptor TGR5 and the nuclear receptor FXR. BA sequestrants (BAS) complex bile acids in the intestinal lumen and decrease intestinal FXR activity. The BAS-BA complex also induces glucagon-like peptide-1 (GLP-1) production by L cells which potentiates β-cell glucose-induced insulin secretion. Whether FXR is expressed in L cells and controls GLP-1 production is unknown. Here, we show that FXR activation in L cells decreases proglucagon expression by interfering with the glucose-responsive factor Carbohydrate-Responsive Element Binding Protein (ChREBP) and GLP-1 secretion by inhibiting glycolysis. In vivo, FXR deficiency increases GLP-1 gene expression and secretion in response to glucose hence improving glucose metabolism. Moreover, treatment of ob/ob mice with the BAS colesevelam increases intestinal proglucagon gene expression and improves glycaemia in a FXR-dependent manner. These findings identify the FXR/GLP-1 pathway as a new mechanism of BA control of glucose metabolism and a pharmacological target for type 2 diabetes.

Lipid-rich diet enhances L-cell density in obese subjects and in mice through improved L-cell differentiation

The enterohormone glucagon-like peptide-1 (GLP-1) is required to amplify glucose-induced insulin secretion that facilitates peripheral glucose utilisation. Alteration in GLP-1 secretion during obesity has been reported but is still controversial. Due to the high adaptability of intestinal cells to environmental changes, we hypothesised that the density of GLP-1-producing cells could be modified by nutritional factors to prevent the deterioration of metabolic condition in obesity. We quantified L-cell density in jejunum samples collected during Roux-en-Y gastric bypass in forty-nine severely obese subjects analysed according to their fat consumption. In mice, we deciphered the mechanisms by which a high-fat diet (HFD) makes an impact on enteroendocrine cell density and function. L-cell density in the jejunum was higher in obese subjects consuming >30 % fat compared with low fat eaters. Mice fed a HFD for 8 weeks displayed an increase in GLP-1-positive cells in the jejunum and colon accordingly to GLP-1 secretion. The regulation by the HFD appears specific to GLP-1-producing cells, as the number of PYY (peptide YY)-positive cells remained unchanged. Moreover, genetically obese ob/ob mice did not show alteration of GLP-1-positive cell density in the jejunum or colon, suggesting that obesity per se is not sufficient to trigger the mechanism. The higher L-cell density in HFD-fed mice involved a rise in L-cell terminal differentiation as witnessed by the increased expression of transcription factors downstream of neurogenin3 (Ngn3). We suggest that the observed increase in GLP-1-positive cell density triggered by high fat consumption in humans and mice might favour insulin secretion and therefore constitute an adaptive response of the intestine to balance diet-induced insulin resistance.

Integrative and systemic approaches for evaluating PPARβ/δ (PPARD) function

The peroxisome proliferator-activated receptors (PPARs) are a group of nuclear receptors that function as transcription factors regulating the expression of genes involved in cellular differentiation, development, metabolism and also tumorigenesis. Three PPAR isotypes (α, β/δ and γ) have been identified, among which PPARβ/δ is the most difficult to functionally examine due to its tissue-specific diversity in cell fate determination, energy metabolism and housekeeping activities. PPARβ/δ acts both in a ligand-dependent and -independent manner. The specific type of regulation, activation or repression, is determined by many factors, among which the type of ligand, the presence/absence of PPARβ/δ-interacting corepressor or coactivator complexes and PPARβ/δ protein post-translational modifications play major roles. Recently, new global approaches to the study of nuclear receptors have made it possible to evaluate their molecular activity in a more systemic fashion, rather than deeply digging into a single pathway/function. This systemic approach is ideally suited for studying PPARβ/δ, due to its ubiquitous expression in various organs and its overlapping and tissue-specific transcriptomic signatures. The aim of the present review is to present in detail the diversity of PPARβ/δ function, focusing on the different information gained at the systemic level, and describing the global and unbiased approaches that combine a systems view with molecular understanding.

Physiological functions of peroxisome proliferator-activated receptor β

The peroxisome proliferator-activated receptors, PPARα, PPARβ, and PPARγ, are a family of transcription factors activated by a diversity of molecules including fatty acids and fatty acid metabolites. PPARs regulate the transcription of a large variety of genes implicated in metabolism, inflammation, proliferation, and differentiation in different cell types. These transcriptional regulations involve both direct transactivation and interaction with other transcriptional regulatory pathways. The functions of PPARα and PPARγ have been extensively documented mainly because these isoforms are activated by molecules clinically used as hypolipidemic and antidiabetic compounds. The physiological functions of PPARβ remained for a while less investigated, but the finding that specific synthetic agonists exert beneficial actions in obese subjects uplifted the studies aimed to elucidate the roles of this PPAR isoform. Intensive work based on pharmacological and genetic approaches and on the use of both in vitro and in vivo models has considerably improved our knowledge on the physiological roles of PPARβ in various cell types. This review will summarize the accumulated evidence for the implication of PPARβ in the regulation of development, metabolism, and inflammation in several tissues, including skeletal muscle, heart, skin, and intestine. Some of these findings indicate that pharmacological activation of PPARβ could be envisioned as a therapeutic option for the correction of metabolic disorders and a variety of inflammatory conditions. However, other experimental data suggesting that activation of PPARβ could result in serious adverse effects, such as carcinogenesis and psoriasis, raise concerns about the clinical use of potent PPARβ agonists.

Activation of toll-like receptors and inflammasome complexes in the diabetic cardiomyopathy-associated inflammation

Diabetic cardiomyopathy is defined as a ventricular dysfunction initiated by alterations in cardiac energy substrates in the absence of coronary artery disease and hypertension. Hyperglycemia, hyperlipidemia, and insulin resistance are major inducers of the chronic low-grade inflammatory state that characterizes the diabetic heart. Cardiac Toll-like receptors and inflammasome complexes may be key inducers for inflammation probably through NF-κB activation and ROS overproduction. However, metabolic dysregulated factors such as peroxisome proliferator-activated receptors and sirtuins may serve as therapeutic targets to control this response by mitigating both Toll-like receptors and inflammasome signaling.

Glucagon-like peptide-1: glucose homeostasis and beyond

Glucagon-like peptide-1 (GLP-1), an incretin hormone secreted primarily from the intestinal L-cells in response to meals, modulates nutrient homeostasis via actions exerted in multiple tissues and cell types. GLP-1 and its analogs, as well as compounds that inhibit endogenous GLP-1 breakdown, have become an effective therapeutic strategy for many subjects with type 2 diabetes. Here we review the discovery of GLP-1; its synthesis, secretion, and elimination from the circulation; and its multiple pancreatic and extrapancreatic effects. Finally, we review current options for GLP-1-based diabetes therapy, including GLP-1 receptor agonism and inhibition of GLP-1 breakdown, as well as the benefits and drawbacks of different modes of therapy and the potential for new therapeutic avenues.