Oleic acid activates peroxisome proliferator-activated receptor δ to compensate insulin resistance in steatotic cells

Characterizing the metabolic divide: distinctive metabolites differentiating CAD-T2DM from CAD patients

OBJECTIVE

To delineate the metabolomic differences in plasma samples between patients with coronary artery disease (CAD) and those with concomitant CAD and type 2 diabetes mellitus (T2DM), and to pinpoint distinctive metabolites indicative of T2DM risk.

METHOD

Plasma samples from CAD and CAD-T2DM patients across three centers underwent comprehensive metabolomic and lipidomic analyses. Multivariate logistic regression was employed to discern the relationship between the identified metabolites and T2DM risk. Characteristic metabolites' metabolic impacts were further probed through hepatocyte cellular experiments. Subsequent transcriptomic analyses elucidated the potential target sites explaining the metabolic actions of these metabolites.

RESULTS

Metabolomic analysis revealed 192 and 95 significantly altered profiles in the discovery (FDR < 0.05) and validation (P < 0.05) cohorts, respectively, that were associated with T2DM risk in univariate logistic regression. Further multivariate regression analyses identified 22 characteristic metabolites consistently associated with T2DM risk in both cohorts. Notably, pipecolinic acid and L-pipecolic acid, lysine derivatives, exhibited negative association with CAD-T2DM and influenced cellular glucose metabolism in hepatocytes. Transcriptomic insights shed light on potential metabolic action sites of these metabolites.

CONCLUSIONS

This research underscores the metabolic disparities between CAD and CAD-T2DM patients, spotlighting the protective attributes of pipecolinic acid and L-pipecolic acid. The comprehensive metabolomic and transcriptomic findings provide novel insights into the mechanism research, prophylaxis and treatment of comorbidity of CAD and T2DM.

A novel PPARβ/FFA1 dual agonist Y8 promotes diabetic wound healing

BACKGROUND

Diabetes ulcer is one of the leading causes of disability and death in diabetics. Y8 [(2-(2-fluoro-4-((4-methyl-2-(4-(trifluoromethyl)phenyl)thiazol-5-yl)methoxy) phenoxy)acetic acid)], a dual agonist of peroxisome proliferation activated receptorβ (PPARβ) and free fatty acid receptor 1 (FFA1/FFAR1/GPR40), a new compound molecule with the potential for diabetes ulcer treatment.

OBJECTIVE

To research the effect of the dual target agonist Y8 and its mechanism of action in the treatment of diabetic ulcers.

METHODS

We have established a wound model in diabetic mice. After treatment with Y8, wound healing was evaluated by tissue pathology, reactive oxygen species (ROS) levels, and gene expression testing. Under high sugar conditions, the mechanism of Y8 affecting fibroblasts' proliferation and keratinocytes' migration is further studied.

RESULTS

We found that Y8 accelerated wound healing and shortened healing time in diabetic mice. Granulation tissue generation and extracellular matrix (ECM) deposition were significantly increased in Y8-treated mice. Mechanistically, Y8 promotes keratinocyte proliferation by activating PPARβ and migration of keratinocytes by triggering FFA1 in vitro. In addition, Y8 also decreased ROS levels in fibroblasts in vitro and in vivo by activating PPARβ, reducing their release of superoxide anions.

CONCLUSION

Our results suggest that PPARβ/FFA1 dual agonist Y8 has the effect of promoting the healing of diabetic ulcer wounds in vivo and in vitro, and its therapeutic effect is better than that of single-target agonists.

Impact of Phytochemicals on PPAR Receptors: Implications for Disease Treatments

Peroxisome proliferator-activated receptors (PPARs) are members of the ligand-dependent nuclear receptor family. PPARs have attracted wide attention as pharmacologic mediators to manage multiple diseases and their underlying signaling targets. They mediate a broad range of specific biological activities and multiple organ toxicity, including cellular differentiation, metabolic syndrome, cancer, atherosclerosis, neurodegeneration, cardiovascular diseases, and inflammation related to their up/downstream signaling pathways. Consequently, several types of selective PPAR ligands, such as fibrates and thiazolidinediones (TZDs), have been approved as their pharmacological agonists. Despite these advances, the use of PPAR agonists is known to cause adverse effects in various systems. Conversely, some naturally occurring PPAR agonists, including polyunsaturated fatty acids and natural endogenous PPAR agonists curcumin and resveratrol, have been introduced as safe agonists as a result of their clinical evidence or preclinical experiments. This review focuses on research on plant-derived active ingredients (natural phytochemicals) as potential safe and promising PPAR agonists. Moreover, it provides a comprehensive review and critique of the role of phytochemicals in PPARs-related diseases and provides an understanding of phytochemical-mediated PPAR-dependent and -independent cascades. The findings of this research will help to define the functions of phytochemicals as potent PPAR pharmacological agonists in underlying disease mechanisms and their related complications.

Differential effects of single fatty acids and fatty acid mixtures on the phosphoinositide 3-kinase/Akt/eNOS pathway in endothelial cells

SCOPE

Dietary fat composition is an important modulator of vascular function. Non-esterified fatty acids (NEFA) enriched in saturated fatty acids (SFA) are thought to reduce vascular reactivity by attenuating insulin signalling via vasodilator pathways (phosphoinositide 3-kinase (PI3K)/Akt/endothelial nitric oxide synthase (eNOS)) and enhancing signalling via pro-inflammatory pathways.

METHODS

To examine the effects of fatty acids on these pathways, human aortic endothelial cells were incubated with single fatty acids, and mixtures of these fatty acids to mimic typical NEFA composition and concentrations achieved in our previous human study. RNA was extracted to determine gene expression using real-time RT-PCR and cell lysates prepared to assess protein phosphorylation by Western blotting.

RESULTS

Oleic acid (OA, 100 µM) was shown to down regulate expression of the insulin receptor, PTEN and a PI3K catalytic (p110β) and regulatory (p85α) subunit compared to palmitic, linoleic and stearic acids (P < 0.04), and promote greater eNOS phosphorylation at Ser¹¹⁷⁷. Both concentration and composition of the SFA and SFA plus n-3 polyunsaturated fatty acids (PUFA) mixtures had significant effects on genes involved in the PI3K/Akt pathway. Greater up-regulation was found with 800 than 400 µM concentration (respective of concentrations in insulin resistant and normal individuals), whereas greater down-regulation was evident with SFA plus n-3 PUFA than SFA mixture alone.

CONCLUSION

Our findings provide novel insights into the modulation of the PI3K/Akt/eNOS pathway by single fatty acids and fatty acid mixtures. In particular, OA appears to promote signalling via this pathway, with further work required to determine the primary molecular site(s) of action.

Discovery of new and highly effective quadruple FFA1 and PPARα/γ/δ agonists as potential anti-fatty liver agents

Non-alcoholic fatty liver disease (NAFLD) has become the most common hepatic disease, while no drug was approved until now. The previous study reported that the quadruple FFA1/PPAR-α/γ/δ agonist RLA8 provided better efficacy than obeticholic acid on NASH. In the present study, two design strategies were introduced to explore better quadruple FFA1/PPAR-α/γ/δ agonists with improved metabolic stability. These efforts ultimately resulted in the identification of ZLY18, a quadruple FFA1/PPAR-α/γ/δ agonist with twice higher metabolic half-life than RLA8 in the liver microsome. In the triton-1339W-induced hyperlipidemic model, ZLY18 reversed hyperlipidemia to an almost normal level, which exhibited far stronger lipid-lowering effects than that of RLA8. Moreover, ZLY18 significantly decreased steatosis, hepatocellular ballooning, inflammation and liver fibrosis in NASH model even better than RLA8. Further mechanism studies suggested that ZLY18 exerts stronger effects than RLA8 on the regulation of the gene related to lipid synthesis, oxidative stress, inflammation and fibrosis. In addition, ZLY18 is more effective than pirfenidone in the prevention of CCl4-induced liver fibrosis. Besides, ZLY18 has an acceptable safety profile in the acute toxicity study at a high dose of 500 mg/kg. Therefore, ZLY18 represents a novel and highly promising quadruple FFA1/PPAR-α/γ/δ agonist worth of further investigation and development.

A novel graph mining approach to predict and evaluate food-drug interactions

Food-drug interactions (FDIs) arise when nutritional dietary consumption regulates biochemical mechanisms involved in drug metabolism. This study proposes FDMine, a novel systematic framework that models the FDI problem as a homogenous graph. Our dataset consists of 788 unique approved small molecule drugs with metabolism-related drug-drug interactions and 320 unique food items, composed of 563 unique compounds. The potential number of interactions is 87,192 and 92,143 for disjoint and joint versions of the graph. We defined several similarity subnetworks comprising food-drug similarity, drug-drug similarity, and food-food similarity networks. A unique part of the graph involves encoding the food composition as a set of nodes and calculating a content contribution score. To predict new FDIs, we considered several link prediction algorithms and various performance metrics, including the precision@top (top 1%, 2%, and 5%) of the newly predicted links. The shortest path-based method has achieved a precision of 84%, 60% and 40% for the top 1%, 2% and 5% of FDIs identified, respectively. We validated the top FDIs predicted using FDMine to demonstrate its applicability, and we relate therapeutic anti-inflammatory effects of food items informed by FDIs. FDMine is publicly available to support clinicians and researchers.

PA and OA induce abnormal glucose metabolism by inhibiting KLF15 in adipocytes

Background

Obesity-induced elevated serum free fatty acids (FFAs) levels result in the occurrence of type 2 diabetes mellitus (T2DM). However, the molecular mechanism remains largely enigmatic. This study was to explore the effect and mechanism of KLF15 on FFAs-induced abnormal glucose metabolism.

Methods

Levels of TG, TC, HDL-C, LDL-C, and glucose were measured by different assay kits. qRT-PCR and Western Blot were used to detect the levels of GPR120, GPR40, phosphorylation of p38 MAPK, KLF15, and downstream factors.

Results

KLF15 was decreased in visceral adipose tissue of obesity subjects and high-fat diet (HFD) mice. In HFD mice, GPR120 antagonist significantly promoted KLF15 protein expression level and phosphorylation of p38 MAPK, meanwhile reduced the blood glucose levels. While, blocking GPR40 inhibited the KLF15 expression. In 3T3-L1 adipocytes, 1500 μM PA inhibited KLF15 through a GPR120/P-p38 MAPK signal pathway, and 750 μM OA inhibited KLF15 mainly through GPR120 while not dependent on P-p38 MAPK, ultimately resulting in abnormal glucose metabolism. Unfortunately, GPR40 didn’t contribute to PA or OA-induced KLF15 reduction.

Conclusions

Both PA and OA inhibit KLF15 expression through GPR120, leading to abnormal glucose metabolism in adipocytes. Notably, the inhibition of KLF15 expression by PA depends on phosphorylation of p38 MAPK.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12986-021-00628-2.

Phosphatase and Tensin Homolog in Non-neoplastic Digestive Disease: More Than Just Tumor Suppressor

The Phosphatase and tensin homolog (PTEN) gene is one of the most important tumor suppressor genes, which acts through its unique protein phosphatase and lipid phosphatase activity. PTEN protein is widely distributed and exhibits complex biological functions and regulatory modes. It is involved in the regulation of cell morphology, proliferation, differentiation, adhesion, and migration through a variety of signaling pathways. The role of PTEN in malignant tumors of the digestive system is well documented. Recent studies have indicated that PTEN may be closely related to many other benign processes in digestive organs. Emerging evidence suggests that PTEN is a potential therapeutic target in the context of several non-neoplastic diseases of the digestive tract. The recent discovery of PTEN isoforms is expected to help unravel more biological effects of PTEN in non-neoplastic digestive diseases.

Exploratory Data Analysis of Cell and Mitochondrial High-Fat, High-Sugar Toxicity on Human HepG2 Cells

Non-alcoholic steatohepatitis (NASH), one of the deleterious stages of non-alcoholic fatty liver disease, remains a significant cause of liver-related morbidity and mortality worldwide. In the current work, we used an exploratory data analysis to investigate time-dependent cellular and mitochondrial effects of different supra-physiological fatty acids (FA) overload strategies, in the presence or absence of fructose (F), on human hepatoma-derived HepG2 cells. We measured intracellular neutral lipid content and reactive oxygen species (ROS) levels, mitochondrial respiration and morphology, and caspases activity and cell death. FA-treatments induced a time-dependent increase in neutral lipid content, which was paralleled by an increase in ROS. Fructose, by itself, did not increase intracellular lipid content nor aggravated the effects of palmitic acid (PA) or free fatty acids mixture (FFA), although it led to an up-expression of hepatic fructokinase. Instead, F decreased mitochondrial phospholipid content, as well as OXPHOS subunits levels. Increased lipid accumulation and ROS in FA-treatments preceded mitochondrial dysfunction, comprising altered mitochondrial membrane potential (ΔΨm) and morphology, and decreased oxygen consumption rates, especially with PA. Consequently, supra-physiological PA alone or combined with F prompted the activation of caspase pathways leading to a time-dependent decrease in cell viability. Exploratory data analysis methods support this conclusion by clearly identifying the effects of FA treatments. In fact, unsupervised learning algorithms created homogeneous and cohesive clusters, with a clear separation between PA and FFA treated samples to identify a minimal subset of critical mitochondrial markers in order to attain a feasible model to predict cell death in NAFLD or for high throughput screening of possible therapeutic agents, with particular focus in measuring mitochondrial function.

Fatty acids and their role in type-2 diabetes (Review)

Age, lifestyle and diet are major risk factors for the onset of type 2 diabetes mellitus (T2DM). Insulin resistance (IR) and β-cell dysfunction underlie the pathophysiology of T2DM. Diabetic populations are also prone to lipid and lipoprotein abnormalities as an indirect effect of IR on key metabolic enzymes. However, recent studies suggested that lipid changes may not only be a consequence of impaired glucose metabolism but also a causative factor. Fatty acids (FAs) influence translocation of glucose transporters and insulin receptor binding and signalling, in addition to cell membrane fluidity and permeability. It is thus suggested that FAs may have an essential role in the development of IR and T2DM. Specific combinations of FAs within phospholipids and triglycerides were indicated to exhibit the strongest associations with the risk of T2DM. The aim of the present review was to investigate the role of FAs in the pathogenesis of T2DM, as it has yet to be fully elucidated.

GPCRs get fatty: the role of G protein-coupled receptor signaling in the development and progression of nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD), characterized by the abnormal deposition of lipids within the liver not due to alcohol consumption, is a growing epidemic affecting over 30% of the United States population. Both simple fatty liver and its more severe counterpart, nonalcoholic steatohepatitis, represent one of the most common forms of liver disease. Recently, several G protein-coupled receptors have emerged as targets for therapeutic intervention for these disorders. These include those with known hepatic function as well as those involved in global metabolic regulation. In this review, we highlight these emerging therapeutic targets, focusing on several common themes including their activation by microbial metabolites, stimulatory effect on insulin and incretin secretion, and contribution to glucose tolerance. The overlap in ligands, localization, and downstream effects of activation indicate the interdependent nature of these receptors and highlight the importance of this signaling family in the development and prevention of NAFLD.

ω-6/ω-3 fatty acid ratio as an essential predictive biomarker in the management of type 2 diabetes mellitus

OBJECTIVES

Intake of dietary fatty acid may play a major role in the prevention and management of lifestyle-related diseases such as type 2 diabetes mellitus (T2DM). Therefore, the aim of this study was to find an association between ω-6 to ω-3 fatty acid ratio and T2DM.

METHODS

Fasting plasma glucose, glycated hemoglobin, and insulin were measured using commercially available kits. Fatty acid methyl esters were prepared using standard protocols. Delta-5 desaturase (D5D) and delta-6 desaturase (D6D) activities were determined from product-to-precursor ratios of individual fatty acids in plasma. Statistical analysis was performed using SPSS version 20.

RESULTS

The ratio of ω-6 to ω-3 was higher in the group with diabetes (13:1) when compared with the group without diabetes (4:1) and was statistically significant (P < 0.0001). Further association studies showed that univariate model with the ω-6 to ω-3 ratio and a multivariate model with D5D, D6D, and ω-6 to ω-3 ratio could serve as predictive polyunsaturated fatty acid pathway models for T2DM.

CONCLUSIONS

From the study results, it is evident that ω-6 to ω-3 fatty acid ratios can serve as essential predictive biomarkers in the management of patients with T2DM. This would not only help in management but would also aid in prevention of increased T2DM incidence in India. These results potentiate the need to maintain an ideal balance of ω-6 to ω-3, as prevention is always better than cure.

Palmitic and Oleic Acid: The Yin and Yang of Fatty Acids in Type 2 Diabetes Mellitus

Increased plasma non-esterified fatty acids (NEFAs) link obesity with insulin resistance and type 2 diabetes mellitus (T2DM). However, in contrast to the saturated FA (SFA) palmitic acid, the monounsaturated FA (MUFA) oleic acid elicits beneficial effects on insulin sensitivity, and the dietary palmitic acid:oleic acid ratio impacts diabetes risk in humans. Here we review recent mechanistic insights into the beneficial effects of oleic acid compared with palmitic acid on insulin resistance and T2DM, including its anti-inflammatory actions, and its capacity to inhibit endoplasmic reticulum (ER) stress, prevent attenuation of the insulin signaling pathway, and improve β cell survival. Understanding the molecular mechanisms of the antidiabetic effects of oleic acid may contribute to understanding the benefits of this FA in the prevention or delay of T2DM.

Free fatty acids sensitize dendritic cells to amplify TH1/TH17-immune responses

Obesity is associated with body fat gain and impaired glucose metabolism. Here, we identified both body fat gain in obesity and impaired glucose metabolism as two independent risk factors for increased serum levels of free fatty acids (FFAs). Since obesity is associated with increased and/or delayed resolution of inflammation observed in various chronic inflammatory diseases such as psoriasis, we investigated the impact of FFAs on human monocyte-derived and mouse bone marrow-derived dendritic cell (DCs) functions relevant for the pathogenesis of chronic inflammation. FFAs such as palmitic acid (PA) and oleic acid (OA) did not affect the pro-inflammatory immune response of DCs. In contrast, PA and OA sensitize DCs resulting in augmented secretion of TH1/TH17-instructive cytokines upon pro-inflammatory stimulation. Interestingly, obesity in mice worsened a TH1/TH17-driven psoriasis-like skin inflammation. Strong correlation of the amount of total FFA, PA, and OA in serum with the severity of skin inflammation points to a critical role of FFA in obesity-mediated exacerbation of skin inflammation. Our data suggest that increased levels of FFAs might be a predisposing factor promoting a TH1/TH17-mediated inflammation such as psoriasis in response to an inflammatory danger signal.

Dihydromyricetin ameliorates oleic acid-induced lipid accumulation in L02 and HepG2 cells by inhibiting lipogenesis and oxidative stress

AIMS

Dihydromyricetin (DMY), a flavonoid component isolated from Ampelopsis grossedentata, was recently reported to ameliorate nonalcoholic fatty liver disease (NAFLD) in patients. However, the underlying mechanisms of this action remain unknown. Here, we evaluate the effect of DMY on an in vitro model of NAFLD and investigate the signal transduction pathways underlying DMY treatment.

MAIN METHODS

Oleic acid (OA) induced hepatic steatosis was established in L02 and HepG2 cells as in vitro model of NAFLD. Cell apoptosis, lipid accumulation and oxide stress were evaluated by flow cytometry, oil red O staining, and cellular biochemical assays, respectively. Signaling pathways involved in lipid metabolism including PPARγ, AMPK, and AKT were investigated by Western blot and RT-qPCR.

KEY FINDINGS

DMY protected cells against apoptosis and lipid accumulation induced by oleic acid. DMY decreased the levels of cellular triglycerides (TG), cholesterol (TC) and malondialdehyde (MDA), while at the same time increasing the level of superoxide dismutase (SOD). DMY suppressed the expression of PPARγ and the phosphorylation of AKT, and promoted the phosphorylation of AMPK.

SIGNIFICANCE

Our study suggests that DMY ameliorates OA-induced hepatic steatosis by inhibiting cell apoptosis, lipid accumulation and oxide stress. Furthermore, the effect of DMY is likely associated with its role in the regulating of PPARγ, AMPK and AKT signaling pathways.

Palmitic acid-rich diet suppresses glucose-stimulated insulin secretion (GSIS) and induces endoplasmic reticulum (ER) stress in pancreatic islets in mice

The objective was to clarify whether dietary palmitic acid supplementation affects glucose-stimulated insulin secretion (GSIS) and the endoplasmic reticulum (ER) stress pathway in pancreatic islets in mice. Eight-week-old male C57BL/6J mice were randomly divided into three treatment diet groups: control diet, palmitic acid-supplemented diet (PAL) and oleic acid-supplemented diet (OLE). After 2 weeks of treatment, intraperitoneal glucose tolerance test and intraperitoneal insulin tolerance test were performed. GSIS was assessed by pancreatic perfusion in situ with basal (100 mg/dL) glucose followed by a high (300 mg/dL) glucose concentration. We measured mRNA levels of ER stress markers such as C/EBP homologous protein (CHOP), immunoglobulin heavy-chain binding protein (BIP) and X-box binding protein (XBP)-1 using real-time polymerase chain reaction (PCR) analyses in isolated islets. Immunohistochemical staining was also performed. Mice fed PAL showed significantly decreased glucose tolerance (p < 0.05). In the perfusion study, GSIS was significantly suppressed in the PAL group (p < 0.05). Semi-quantitative RT-PCR revealed that islet CHOP, BIP, and XBP-1 mRNA expression were significantly increased in the PAL group (p < 0.05). TUNEL-positive β-cells were not detected in all groups. Dietary palmitic acid-supplementation for 2 weeks might suppress GSIS and induce ER stress in pancreatic islets in mice, in the early stage of lipotoxicity.

Protective effects of dietary avocado oil on impaired electron transport chain function and exacerbated oxidative stress in liver mitochondria from diabetic rats

Electron transport chain (ETC) dysfunction, excessive ROS generation and lipid peroxidation are hallmarks of mitochondrial injury in the diabetic liver, with these alterations also playing a role in the development of non-alcoholic fatty liver disease (NAFLD). Enhanced mitochondrial sensitivity to lipid peroxidation during diabetes has been also associated to augmented content of C22:6 in membrane phospholipids. Thus, we aimed to test whether avocado oil, a rich source of C18:1 and antioxidants, attenuates the deleterious effects of diabetes on oxidative status of liver mitochondria by decreasing unsaturation of acyl chains of membrane lipids and/or by improving ETC functionality and decreasing ROS generation. Streptozocin-induced diabetes elicited a noticeable increase in the content of C22:6, leading to augmented mitochondrial peroxidizability index and higher levels of lipid peroxidation. Mitochondrial respiration and complex I activity were impaired in diabetic rats with a concomitant increase in ROS generation using a complex I substrate. This was associated to a more oxidized state of glutathione, All these alterations were prevented by avocado oil except by the changes in mitochondrial fatty acid composition. Avocado oil did not prevented hyperglycemia and polyphagia although did normalized hyperlipidemia. Neither diabetes nor avocado oil induced steatosis. These results suggest that avocado oil improves mitochondrial ETC function by attenuating the deleterious effects of oxidative stress in the liver of diabetic rats independently of a hypoglycemic effect or by modifying the fatty acid composition of mitochondrial membranes. These findings might have also significant implications in the progression of NAFLD in experimental models of steatosis.

Targeting Peroxisome Proliferator-Activated Receptor-β/δ (PPARβ/δ) for Cancer Chemoprevention

The role of peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) in cancer remains contentious due in large part to divergent publications indicating opposing effects in different rodent and human cell culture models. During the past 10 years, some facts regarding PPARβ/δ in cancer have become clearer, while others remain uncertain. For example, it is now well accepted that (1) expression of PPARβ/δ is relatively lower in most human tumors as compared to the corresponding non-transformed tissue, (2) PPARβ/δ promotes terminal differentiation, and (3) PPARβ/δ inhibits pro-inflammatory signaling in multiple in vivo models. However, whether PPARβ/δ is suitable to target with natural and/or synthetic agonists or antagonists for cancer chemoprevention is hindered because of the uncertainty in the mechanism of action and role in carcinogenesis. Recent findings that shed new insight into the possibility of targeting this nuclear receptor to improve human health will be discussed.

Activation of free fatty acid receptor 1 improves hepatic steatosis through a p38-dependent pathway

Hepatic steatosis is highly correlated with insulin resistance and diabetes. Although, it has been demonstrated that activation of free fatty acid receptor 1 (FFAR1) by agonists showed benefits for the improvement of diabetes, the effects of FFAR1 agonists on hepatic steatosis were unknown. In this study, a high fat diet (HFD)-induced hepatic steatosis animal model was utilized to evaluate the effects of an FFAR1 agonist, GW9508, on hepatic lipid accumulation, and HepG2 hepatoma cells were also used to clarify the possible mechanisms. Administration of GW9508 significantly decreased the hepatic lipid accumulation with decreased expressions of lipogenesis-related proteins in HFD mice. Knockdown of hepatic Ffar1 by lentiviral vectors containing short hairpin RNA targeted to Ffar1 diminished the effect of GW9508 in HFD mice. In addition, GW9508 decreased oleic acid-induced lipid accumulation in HepG2 cells by decreases in the expression of lipogenesis-related proteins. Moreover, GW9508 downregulated the expression of sterol regulatory element-binding protein 1 (SREBP1) through a p38-dependent pathway, whereas knockdown of Ffar1 in HepG2 cells diminished the effect of GW9508 on the decrease in SREBP1. Considering all these results together, GW9508 exerts a therapeutic effect to improve hepatic steatosis through a p38-dependent pathway. Thus, investigation of chemicals that act on FFAR1 might be a new strategy for the treatment of hepatic steatosis.

The effect of FFAR1 on pioglitazone-mediated attenuation of palmitic acid-induced oxidative stress and apoptosis in βTC6 cells

OBJECTIVE

We sought to determine whether free fatty acid receptor 1 (FFAR1), a receptor for free fatty acids on the β-cell membrane, can mediate the pioglitazone (PIO)-attenuating effect on lipoapoptosis in β cells and its relationship to oxidative stress.

METHODS

The glucose-sensitive mouse beta pancreatic cell line βTC6 was used to investigate the effect of FFAR1 on PIO-attenuating palmitic acid (PA)-induced oxidative stress and apoptosis.

RESULTS

(1) PIO reduced PA-induced lipoapoptosis in β cells and upregulated the expression of FFAR1 at the mRNA and protein levels in a dose- and time-dependent manner. Silencing of FFAR1 expression was shown to weaken the protective effect of PIO on PA-induced lipoapoptosis in βTC6 cells; while lentiviral-mediated overexpression of FFAR1 was shown to enhance the protective effect of PIO against lipoapoptosis in β cells. (2) Downregulation of FFAR1 expression reduced the attenuating effect of PIO on the expression of NAPDH oxidase subunit p47(phox), Bax, cleaved caspase 3, and the production of reactive oxygen specific (ROS) induced by lipotoxicity, thereby preventing the upregulation of the expression of bcl-2. Inducing the overexpression of FFAR1 enhanced the anti-oxidative stress effect of PIO. Similarly, these effects of FFAR1 on PIO were reproduced under conditions of oxidative stress and apoptosis in βTC6 cells that were induced by H2O2. (3) PIO was found to increase the expression of PLCγ, ERK1/2, and PPARγ in lipotoxic β cells. Silencing FFAR1 expression reduced the PIO-mediated increases in the expression of above proteins; while inducing FFAR1 overexpression showed the opposite effect. Use of an inhibitor of PLCγ, ERK1/2, PPARγ was shown to restrict the protective effect of PIO on oxidative stress and lipoapoptosis of β cells.

CONCLUSIONS

FFAR1 can mediate PIO suppression of β-cell lipoapoptosis through anti-oxidative stress, which may be related to the activation of the PLCγ-ERK1/2-PPARγ pathway.

The role of hepassocin in the development of non-alcoholic fatty liver disease

BACKGROUND & AIMS

While non-alcoholic fatty liver disease (NAFLD) is the most common risk factor of chronic liver disease, the mechanisms that initiate its development are obscure. Hepassocin (HPS) is a hepatokine that has been reported to be involved in liver regeneration. In addition to the mitogenic activity of HPS, HPS expression is decreased in patients with hepatoma. However, the role of HPS in NAFLD is still unknown.

METHODS

A total of 393 subjects with (n=194) or without (n=199) NAFLD were enrolled to evaluate the serum HPS concentration. In order to clarify the causal inference between HPS and NAFLD, we used experimental animal and cell models. Hepatic overexpression or silencing of HPS was achieved by lentiviral vector delivery in mice and lipofectamine transfection in HepG2 cells. Lipogenesis related proteins were detected by Western blots. The expression of inflammatory factors was determined by real-time polymerase chain reaction.

RESULTS

Subjects with NAFLD had a higher serum HPS concentration than those without it. Overexpression of HPS increased hepatic lipid accumulation and NAFLD activity scores (NAS), whereas deletion of HPS improved high fat diet-induced hepatic steatosis and decreased NAS in mice. Additionally, oleic acid, a steatogenic reagent, increased HPS expression in hepatocytes. Furthermore, overexpression of HPS in HepG2 cells induced lipid accumulation through an extracellular signal-regulated kinase 1/2 (ERK1/2)-dependent pathway, whereas deletion of HPS decreased oleic acid-induced lipid accumulation.

CONCLUSIONS

The present study provides evidence that HPS plays an important role in NAFLD and induces hepatic lipid accumulation through an ERK1/2-dependent pathway.

Multiple mechanisms of GW-9508, a selective G protein-coupled receptor 40 agonist, in the regulation of glucose homeostasis and insulin sensitivity

Activation of G protein-coupled receptor 40 (GPR40) by agonists increases insulin release in isolated islets, whereas it is inconclusive whether GPR40 antagonists decrease blood glucose and increase insulin sensitivity. Although some clinical trials indicated that administration of a GPR40 agonist shows benefits in the regulation of blood glucose homeostasis, the pharmacological mechanisms of this receptor in the improvement of glycemic control remain unclear. Therefore, we used a selective GPR40 agonist, GW-9508, to clarify the role of GPR40 in the regulation of blood glucose. Bolus intraperitoneal injection of GW-9508 in mice showed a slight decrease in blood glucose, with an increase in plasma insulin levels under glucose stimuli. However, long-term treatment with low doses of GW-9508 in high-fat diet-induced (HFD) diabetic mice decreased blood glucose with decreased plasma insulin significantly and improved glucose intolerance and insulin resistance. Using small interfering ribonucleic acid to delete GPR40 in HepG2 cells, we demonstrated that GW-9508 reversed palmitate-induced insulin signaling impairment through a GPR40-dependent pathway. We also found that GW-9508 activates the Akt/GSK-3β pathway to increase glycogen levels in HepG2 cells. Furthermore, administration of GW-9508 decreased the hepatic expression of fetuin-A in HFD mice significantly and regulated high-glucose- or palmitate-induced fetuin-A expression to increase insulin sensitivity through a GPR40/PLC/PKC pathway in HepG2 cells. Taken together, GW-9508 exerts a partial agonist effect to regulate blood glucose through multiple mechanisms. Investigation of chemicals that act on GPR40 might be a new strategy for the treatment of diabetes.

PPARs in Liver Diseases and Cancer: Epigenetic Regulation by MicroRNAs

Peroxisome-proliferator-activated receptors (PPARs) are ligand-activated nuclear receptors that exert in the liver a transcriptional activity regulating a whole spectrum of physiological functions, including cholesterol and bile acid homeostasis, lipid/glucose metabolism, inflammatory responses, regenerative mechanisms, and cell differentiation/proliferation. Dysregulations of the expression, or activity, of specific PPAR isoforms in the liver are therefore believed to represent critical mechanisms contributing to the development of hepatic metabolic diseases, disorders induced by hepatic viral infections, and hepatocellular adenoma and carcinoma. In this regard, specific PPAR agonists have proven to be useful to treat these metabolic diseases, but for cancer therapies, the use of PPAR agonists is still debated. Interestingly, in addition to previously described mechanisms regulating PPARs expression and activity, microRNAs are emerging as new important regulators of PPAR expression and activity in pathophysiological conditions and therefore may represent future therapeutic targets to treat hepatic metabolic disorders and cancers. Here, we reviewed the current knowledge about the general roles of the different PPAR isoforms in common chronic metabolic and infectious liver diseases, as well as in the development of hepatic cancers. Recent works highlighting the regulation of PPARs by microRNAs in both physiological and pathological situations with a focus on the liver are also discussed.