Role of peroxisome proliferator-activated receptor {delta}/{beta} in hepatic metabolic regulation

Effects of elafibranor on liver fibrosis and gut barrier function in a mouse model of alcohol-associated liver disease

BACKGROUND

Alcohol-associated liver disease (ALD) is a leading cause of liver-related morbidity and mortality, but there are no therapeutic targets and modalities to prevent ALD-related liver fibrosis. Peroxisome proliferator activated receptor (PPAR) α and δ play a key role in lipid metabolism and intestinal barrier homeostasis, which are major contributors to the pathological progression of ALD. Meanwhile, elafibranor (EFN), which is a dual PPARα and PPARδ agonist, has reached a phase III clinical trial for the treatment of metabolic dysfunction-associated steatotic liver disease and primary biliary cholangitis. However, the benefits of EFN for ALD treatment is unknown.

AIM

To evaluate the inhibitory effects of EFN on liver fibrosis and gut-intestinal barrier dysfunction in an ALD mouse model.

METHODS

ALD-related liver fibrosis was induced in female C57BL/6J mice by feeding a 2.5% ethanol (EtOH)-containing Lieber-DeCarli liquid diet and intraperitoneally injecting carbon tetrachloride thrice weekly (1 mL/kg) for 8 weeks. EFN (3 and 10 mg/kg/day) was orally administered during the experimental period. Histological and molecular analyses were performed to assess the effect of EFN on steatohepatitis, fibrosis, and intestinal barrier integrity. The EFN effects on HepG2 lipotoxicity and Caco-2 barrier function were evaluated by cell-based assays.

RESULTS

The hepatic steatosis, apoptosis, and fibrosis in the ALD mice model were significantly attenuated by EFN treatment. EFN promoted lipolysis and β-oxidation and enhanced autophagic and antioxidant capacities in EtOH-stimulated HepG2 cells, primarily through PPARα activation. Moreover, EFN inhibited the Kupffer cell-mediated inflammatory response, with blunted hepatic exposure to lipopolysaccharide (LPS) and toll like receptor 4 (TLR4)/nuclear factor kappa B (NF-κB) signaling. EFN improved intestinal hyperpermeability by restoring tight junction proteins and autophagy and by inhibiting apoptosis and proinflammatory responses. The protective effect on intestinal barrier function in the EtOH-stimulated Caco-2 cells was predominantly mediated by PPARδ activation.

CONCLUSION

EFN reduced ALD-related fibrosis by inhibiting lipid accumulation and apoptosis, enhancing hepatocyte autophagic and antioxidant capacities, and suppressing LPS/TLR4/NF-κB-mediated inflammatory responses by restoring intestinal barrier function.

Probiotics Alleviate Microcystin-LR-Induced Developmental Toxicity in Zebrafish Larvae

Microcystin-LR (MCLR) poses a significant threat to aquatic ecosystems and public health. This study investigated the protective effects of the probiotic Lactobacillus rhamnosus against MCLR-induced developmental toxicity in zebrafish larvae. Zebrafish larvae were exposed to various concentrations of MCLR (0, 0.9, 1.8, and 3.6 mg/L) with or without L. rhamnosus from 72 to 168 h post-fertilization (hpf). Probiotic supplementation significantly improved survival, hatching, and growth rates and reduced malformation rates in MCLR-exposed larvae. L. rhamnosus alleviated MCLR-induced oxidative stress by reducing reactive oxygen species (ROS) levels and enhancing glutathione (GSH) content and catalase (CAT) activity. Probiotics also mitigated MCLR-induced lipid metabolism disorders by regulating key metabolites (triglycerides, cholesterol, bile acids, and free fatty acids) and gene expression (ppara, pparb, srebp1, and nr1h4). Moreover, 16S rRNA sequencing revealed that L. rhamnosus modulated the gut microbiome structure and diversity in MCLR-exposed larvae, promoting beneficial genera like Shewanella and Enterobacter and inhibiting potential pathogens like Vibrio. Significant correlations were found between gut microbiota composition and host antioxidant and lipid metabolism parameters. These findings suggest that L. rhamnosus exerts protective effects against MCLR toxicity in zebrafish larvae by alleviating oxidative stress, regulating lipid metabolism, and modulating the gut microbiome, providing insights into probiotic-based strategies for mitigating MCLR toxicity in aquatic organisms.

Metabolomics analyses reveal the liver-protective mechanism of Wang's metabolic formula on metabolic-associated fatty liver disease

Wang's metabolic formula (WMF) is a traditional Chinese medicine formula developed under the guidance of Professor Kungen Wang. WMF has been clinically utilized for several years. However, the therapeutic mechanism of WMF in treating metabolic-associated fatty liver disease (MAFLD) remains unclear. In this study, we performed phytochemical analysis on WMF using LC-MS. To study the role of WMF in MAFLD, we orally administered WMF (20.6 g/kg) to male MAFLD mice induced by a high-cholesterol high-fat diet (HCHFD). Then pathological, biochemical, and metabolomic analyses were performed. The main components of WMF are chlorogenic acid, geniposide, albiflorin, paeoniflorin, and calycosin-7-O-glucoside. MAFLD mice treated with WMF exhibited significant improvements in obesity, abnormal lipid metabolism, inflammation, and liver pathology. WMF decreased aspartate aminotransferase (AST), alanine aminotransferase (ALT), and triglyceride (TG) levels in the serum of MAFLD mice while increasing high-density lipoprotein cholesterol (HDL-c) levels. WMF lowered liver TG levels and inflammatory factors (IL-1β, IL-6, TNF-α, and NF-κB). Metabolomic analysis of the liver annotated 78 differentially regulated metabolites enriched in four pathways: glycerophospholipid metabolism, retinol metabolism, PPAR signaling pathway, and choline metabolism. Western blot experiments showed that WMF increased the expression of PPAR-α, PPAR-β, and RXR in the liver while decreasing the expression of RAR. The study demonstrates that WMF has a solid preventive and therapeutic effect on MAFLD. The anti-inflammatory and regulation of abnormal liver metabolism activities of WMF involve retinol metabolism and the PPAR signaling pathway.

PPARβ/δ as a promising molecular drug target for liver diseases: A focused review

Various liver diseases pose great threats to humans. Although the etiologies of these liver diseases are quite diverse, they share similar pathologic phenotypes and molecular mechanisms such as oxidative stress, lipid and glucose metabolism disturbance, hepatic Kupffer cell (KC) proinflammatory polarization and inflammation, insulin resistance, and hepatic stellate cell (HSC) activation and proliferation. Peroxisome proliferator-activated receptor β/δ (PPARβ/δ) is expressed in various types of liver cells with relatively higher expression in KCs and HSCs. Accumulating evidence has revealed the versatile functions of PPARβ/δ such as controlling lipid homeostasis, inhibiting inflammation, regulating glucose metabolism, and restoring insulin sensitivity, suggesting that PPARβ/δ may serve as a potential molecular drug target for various liver diseases. This article aims to provide a concise review of the structure, expression pattern and biological functions of PPARβ/δ in the liver and its roles in various liver diseases, and to discuss potential future research perspectives.

MASLD/MASH and type 2 diabetes: Two sides of the same coin? From single PPAR to pan-PPAR agonists

Type 2 diabetes (T2D) and metabolic dysfunction-associated steatotic liver disease (MASLD), mainly related to nutrition and lack of physical activity, are both very common conditions, share several disease pathways and clinical manifestations, and increasingly co-occur with disease progression. Insulin resistance is an upstream node in the biology of both conditions and triggers liver parenchymal injury, inflammation and fibrosis. Peroxisome proliferator-activated receptor (PPAR) nuclear transcription factors are master regulators of energy homeostasis - insulin signaling in liver, adipose and skeletal muscle tissue - and affect immune and fibrogenesis pathways. Among distinct yet overlapping effects, PPARα regulates lipid metabolism and energy expenditure, PPARβ/δ has anti-inflammatory effects and increases glucose uptake by skeletal muscle, while PPARγ improves insulin sensitivity and exerts direct antifibrotic effects on hepatic stellate cells. Together PPARs thus represent pharmacological targets across the entire biology of MASH. Single PPAR agonists are approved for hypertriglyceridemia (PPARα) and T2D (PPARγ), but these, as well as dual PPAR agonists, have shown mixed results as anti-MASH treatments in clinical trials. Agonists of all three PPAR isoforms have the potential to improve the full disease spectrum from insulin resistance to fibrosis, and correspondingly to improve cardiometabolic and hepatic health, as has been shown (phase II data) with the pan-PPAR agonist lanifibranor.

Targeting nuclear receptors for NASH/MASH: From bench to bedside

The onset of metabolic dysfunction-associated steatohepatitis (MASH) or non-alcoholic steatohepatitis (NASH) represents a tipping point leading to liver injury and subsequent hepatic complications in the natural progression of what is now termed metabolic dysfunction-associated steatotic liver diseases (MASLD), formerly known as non-alcoholic fatty liver disease (NAFLD). With no pharmacological treatment currently available for MASH/NASH, the race is on to develop drugs targeting multiple facets of hepatic metabolism, inflammation, and pro-fibrotic events, which are major drivers of MASH. Nuclear receptors (NRs) regulate genomic transcription upon binding to lipophilic ligands and govern multiple aspects of liver metabolism and inflammation. Ligands of NRs may include hormones, lipids, bile acids, and synthetic ligands, which upon binding to NRs regulate the transcriptional activities of target genes. NR ligands are presently the most promising drug candidates expected to receive approval from the United States Food and Drug Administration as a pharmacological treatment for MASH. This review aims to cover the current understanding of NRs, including nuclear hormone receptors, non-steroid hormone receptors, circadian NRs, and orphan NRs, which are currently undergoing clinical trials for MASH treatment, along with NRs that have shown promising results in preclinical studies.

The Role of Cannabidiol in Liver Disease: A Systemic Review

Cannabidiol (CBD), a non-psychoactive phytocannabinoid abundant in Cannabis sativa, has gained considerable attention for its anti-inflammatory, antioxidant, analgesic, and neuroprotective properties. It exhibits the potential to prevent or slow the progression of various diseases, ranging from malignant tumors and viral infections to neurodegenerative disorders and ischemic diseases. Metabolic dysfunction-associated steatotic liver disease (MASLD), formerly known as non-alcoholic fatty liver disease (NAFLD), alcoholic liver disease, and viral hepatitis stand as prominent causes of morbidity and mortality in chronic liver diseases globally. The literature has substantiated CBD's potential therapeutic effects across diverse liver diseases in in vivo and in vitro models. However, the precise mechanism of action remains elusive, and an absence of evidence hinders its translation into clinical practice. This comprehensive review emphasizes the wealth of data linking CBD to liver diseases. Importantly, we delve into a detailed discussion of the receptors through which CBD might exert its effects, including cannabinoid receptors, CB1 and CB2, peroxisome proliferator-activated receptors (PPARs), G protein-coupled receptor 55 (GPR55), transient receptor potential channels (TRPs), and their intricate connections with liver diseases. In conclusion, we address new questions that warrant further investigation in this evolving field.

The Role of Nuclear Receptors in the Pathogenesis and Treatment of Non-alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD) is a global health burden closely linked to insulin resistance, obesity, and type 2 diabetes. The complex pathophysiology of NAFLD involves multiple cellular pathways and molecular factors. Nuclear receptors (NRs) have emerged as crucial regulators of lipid metabolism and inflammation in NAFLD, offering potential therapeutic targets for NAFLD. Targeting PPARs and FXRs has shown promise in ameliorating NAFLD symptoms and halting disease progression. However, further investigation is needed to address side effects and personalize therapy approaches. This review summarizes the current understanding of the involvement of NRs in the pathogenesis of NAFLD and explores their therapeutic potential. We discuss the role of several NRs in modulating lipid homeostasis in the liver, including peroxisome proliferator-activated receptors (PPARs), liver X receptors (LXRs), farnesoid X receptors (FXRs), REV-ERB, hepatocyte nuclear factor 4α (HNF4α), constitutive androstane receptor (CAR) and pregnane X receptor (PXR).The expanding knowledge of NRs in NAFLD offers new avenues for targeted therapies, necessitating exploration of novel treatment strategies and optimization of existing approaches to combat this increasingly prevalent disease.

Metabolic Biomarkers in Adults with Type 2 Diabetes: The Role of PPAR-γ2 and PPAR-β/δ Polymorphisms

Glucose and lipid metabolism regulation by the peroxisome proliferator-activated receptors (PPARs) has been extensively reported. However, the role of their polymorphisms remains unclear.

OBJECTIVE

To determine the relation between PPAR-γ2 rs1801282 (Pro12Ala) and PPAR-β/δ rs2016520 (+294T/C) polymorphisms and metabolic biomarkers in adults with type 2 diabetes (T2D).

MATERIALS AND METHODS

We included 314 patients with T2D. Information on anthropometric, fasting plasma glucose (FPG), HbA1c and lipid profile measurements was taken from clinical records. Genomic DNA was obtained from peripheral blood. End-point PCR was used for PPAR-γ2 rs1801282, while for PPAR-β/δ rs2016520 the PCR product was digested with Bsl-I enzyme. Data were compared with parametric or non-parametric tests. Multivariate models were used to adjust for covariates and interaction effects.

RESULTS

minor allele frequency was 12.42% for PPAR-γ2 rs1801282-G and 13.85% for PPAR-β/δ rs2016520-C. Both polymorphisms were related to waist circumference; they showed independent effects on HbA1c, while they interacted for FPG; carriers of both PPAR minor alleles had the highest values. Interactions between FPG and polymorphisms were identified in their relation to triglyceride level.

CONCLUSIONS

PPAR-γ2 rs1801282 and PPAR-β/δ rs2016520 polymorphisms are associated with anthropometric, glucose, and lipid metabolism biomarkers in T2D patients. Further research is required on the molecular mechanisms involved.

Transcriptomic and lipid profiling analysis reveals a functional interplay between testosterone and growth hormone in hypothyroid liver

Preclinical and clinical studies suggest that hypothyroidism might cause hepatic endocrine and metabolic disturbances with features that mimic deficiencies of testosterone and/or GH. The absence of physiological interactions between testosterone and GH can be linked to male differentiated liver diseases. Testosterone plays relevant physiological effects on somatotropic-liver axis and liver composition and the liver is a primary organ of interactions between testosterone and GH. However, testosterone exerts many effects on liver through complex and poorly understood mechanisms. Testosterone impacts liver functions by binding to the Androgen Receptor, and, indirectly, through its conversion to estradiol, and cooperation with GH. However, the role of testosterone, and its interaction with GH, in the hypothyroid liver, remains unclear. In the present work, the effects of testosterone, and how they impact on GH-regulated whole transcriptome and lipid composition in the liver, were studied in the context of adult hypothyroid-orchiectomized rats. Testosterone replacement positively modulated somatotropic-liver axis and impacted liver transcriptome involved in lipid and glucose metabolism. In addition, testosterone enhanced the effects of GH on the transcriptome linked to lipid biosynthesis, oxidation-reduction, and metabolism of unsaturated and long-chain fatty acids (FA). However, testosterone decreased the hepatic content of cholesterol esters and triacylglycerols and increased fatty acids whereas GH increased neutral lipids and decreased polar lipids. Biological network analysis of the effects of testosterone on GH-regulated transcriptome confirmed a close connection with crucial proteins involved in steroid and fatty acid metabolism. Taken together, this comprehensive analysis of gene expression and lipid profiling in hypothyroid male liver reveals a functional interplay between testosterone and pulsed GH administration.

Nonalcoholic fatty liver disease: Current therapies and future perspectives in drug delivery

Nonalcoholic fatty liver disease (NAFLD) affects approximately 25% of the adult population worldwide. This pathology can progress into end-stage liver disease with life-threatening complications, and yet no pharmacologic therapy has been approved. NAFLD is commonly characterized by excessive fat accumulation in the liver and is in closely associated with insulin resistance and metabolic disorders, which suggests that NAFLD is the hepatic manifestation of metabolic syndrome. Regarding treatment options, the current validated strategy relies on lifestyle modifications (exercise and diet restrictions). Although there are no approved drug-based treatments, several clinical trials are ongoing. Novel targets are being discovered, and the repurposing of drugs that show promising effects in NAFLD is starting to gain more interest. The field of nanotechnology has been growing at an increasing rate, with new and more efficient drug delivery strategies being developed for NAFLD treatment. Nanocarriers can easily encapsulate drugs that need to be better protected from the organism to exert their effect or that need help at reaching their target, thereby helping achieve a better bioavailability. Drug delivery systems can also be designed to target the site of the disease, in this case, the liver. In this review, we focus on the current knowledge of NAFLD pathology, the targets being considered for clinical trials, and the current guidelines and ongoing clinical trials, with a specific focus on potential oral treatments for NAFLD using promising drug delivery strategies.

Smad3 Mediates Diabetic Dyslipidemia and Fatty Liver in db/db Mice by Targeting PPARδ

Transforming growth factor-β (TGF-β)/Smad3 signaling has been shown to play important roles in fibrotic and inflammatory diseases. However, the role of Smad3 in dyslipidemia and non-alcoholic fatty liver disease (NAFLD) in type 2 diabetes remains unclear, and whether targeting Smad3 has a therapeutic effect on these metabolic abnormalities remains unexplored. These topics were investigated in this study in Smad3 knockout (KO)-db/db mice and by treating db/db mice with a Smad3-specific inhibitor SIS3. Compared to Smad3 wild-type (WT)-db/db mice, Smad3 KO-db/db mice were protected against dyslipidemia and NAFLD. Similarly, treatment of db/db mice with SIS3 at week 4 before the onset of type 2 diabetes until week 12 was capable of lowering blood glucose levels and improving diabetic dyslipidemia and NAFLD. In addition, using RNA-sequencing, the potential Smad3-target genes related to lipid metabolism was identified in the liver tissues of Smad3 KO/WT mice, and the regulatory mechanisms were investigated. Mechanistically, we uncovered that Smad3 targeted peroxisome proliferator-activated receptor delta (PPARδ) to induce dyslipidemia and NAFLD in db/db mice, which was improved by genetically deleting and pharmacologically inhibiting Smad3.

Targeting PPARs for therapy of atherosclerosis: A review

Atherosclerosis, a chief pathogenic factor of cardiovascular disease, is associated with many factors including inflammation, dyslipidemia, and oxidative stress. Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors and are widely expressed with tissue- and cell-specificity. They control multiple genes that are involved in lipid metabolism, inflammatory response, and redox homeostasis. Given the diverse biological functions of PPARs, they have been extensively studied since their discovery in 1990s. Although controversies exist, accumulating evidence have demonstrated that PPAR activation attenuates atherosclerosis. Recent advances are valuable for understanding the mechanisms of action of PPAR activation. This article reviews the recent findings, mainly from the year of 2018 to present, including endogenous molecules in regulation of PPARs, roles of PPARs in atherosclerosis by focusing on lipid metabolism, inflammation, and oxidative stress, and synthesized PPAR modulators. This article provides information valuable for researchers in the field of basic cardiovascular research, for pharmacologists that are interested in developing novel PPAR agonists and antagonists with lower side effects as well as for clinicians.

Metabolism-Disrupting Chemicals Affecting the Liver: Screening, Testing, and Molecular Pathway Identification

The liver is the central metabolic organ of the body. The plethora of anabolic and catabolic pathways in the liver is tightly regulated by physiological signaling but may become imbalanced as a consequence of malnutrition or exposure to certain chemicals, so-called metabolic endocrine disrupters, or metabolism-disrupting chemicals (MDCs). Among different metabolism-related diseases, obesity and non-alcoholic fatty liver disease (NAFLD) constitute a growing health problem, which has been associated with a western lifestyle combining excessive caloric intake and reduced physical activity. In the past years, awareness of chemical exposure as an underlying cause of metabolic endocrine effects has continuously increased. Within this review, we have collected and summarized evidence that certain environmental MDCs are capable of contributing to metabolic diseases such as liver steatosis and cholestasis by different molecular mechanisms, thereby contributing to the metabolic syndrome. Despite the high relevance of metabolism-related diseases, standardized mechanistic assays for the identification and characterization of MDCs are missing. Therefore, the current state of candidate test systems to identify MDCs is presented, and their possible implementation into a testing strategy for MDCs is discussed.

The Role of PPARs in Breast Cancer

Breast cancer is a malignant tumor with high morbidity and lethality. Its pathogenesis is related to the abnormal expression of many genes. The peroxisome proliferator-activated receptors (PPARs) are a class of ligand-dependent transcription factors in the nuclear receptor superfamily. They can regulate the transcription of a large number of target genes, which are involved in life activities such as cell proliferation, differentiation, metabolism, and apoptosis, and regulate physiological processes such as glucose metabolism, lipid metabolism, inflammation, and wound healing. Further, the changes in its expression are associated with various diseases, including breast cancer. The experimental reports related to "PPAR" and "breast cancer" were retrieved from PubMed since the discovery of PPARs and summarized in this paper. This review (1) analyzed the roles and potential molecular mechanisms of non-coordinated and ligand-activated subtypes of PPARs in breast cancer progression; (2) discussed the correlations between PPARs and estrogen receptors (ERs) as the nuclear receptor superfamily; and (3) investigated the interaction between PPARs and key regulators in several signaling pathways. As a result, this paper identifies PPARs as targets for breast cancer prevention and treatment in order to provide more evidence for the synthesis of new drugs targeting PPARs or the search for new drug combination treatments.

Review article: vascular effects of PPARs in the context of NASH

BACKGROUND

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors known to regulate glucose and fatty acid metabolism, inflammation, endothelial function and fibrosis. PPAR isoforms have been extensively studied in metabolic diseases, including type 2 diabetes and cardiovascular diseases. Recent data extend the key role of PPARs to liver diseases coursing with vascular dysfunction, including nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH).

AIM

This review summarises and discusses the pathobiological role of PPARs in cardiovascular diseases with a special focus on their impact and therapeutic potential in NAFLD and NASH.

RESULTS AND CONCLUSIONS

PPARs may be attractive for the treatment of NASH due to their liver-specific effects but also because of their efficacy in improving cardiovascular outcomes, which may later impact liver disease. Assessment of cardiovascular disease in the context of NASH trials is, therefore, of the utmost importance, both from a safety and efficacy perspective.

Effects of active, inactive, and derivatives of Akkermansia muciniphila on the expression of the endocannabinoid system and PPARs genes

This study aimed to investigate the effects of active and heat-inactivated forms of Akkermansia muciniphila, bacterium-derived outer membrane vesicles (OMVs), and cell-free supernatant on the transcription of endocannabinoid system (ECS) members, including cannabinoid receptors 1 and 2 (CB1 and CB2), fatty acid amide hydrolase (FAAH), and peroxisome proliferator-activated receptors (PPARs) genes (i.e., α, β/δ, and δ) in Caco-2 and HepG-2 cell lines. After the inoculation of A. muciniphila in brain heart infusion enriched medium, OMVs and cell-free supernatant were extracted. For the investigation of the effects of bacteria and its derivatives on the expression of ECS and PPARs genes, the aforementioned cells were treated by active and heat-inactivated bacteria, OMVs, and cell-free supernatant. Quantitative real-time polymerase chain reaction analysis revealed that both forms of the bacterium, bacterial-derived OMVs, and cell-free supernatant could affect the expression of CB1, CB2, FAAH, and PPARs genes (i.e., α, β/δ, and δ) significantly (P < 0.05). Considering the engagement of the aforementioned genes in metabolic pathways, it might be suggested that both forms of the bacterium, OMVs, and cell-free supernatant might have the potential to serve as a probiotic, paraprobiotic, and postbiotic candidate to prevent obesity, metabolic disorders, and liver diseases.

PPAR-Targeted Therapies in the Treatment of Non-Alcoholic Fatty Liver Disease in Diabetic Patients

Peroxisome proliferator-activated receptors (PPAR), ligand-activated transcription factors of the nuclear hormone receptor superfamily, have been identified as key metabolic regulators in the liver, skeletal muscle, and adipose tissue, among others. As a leading cause of liver disease worldwide, non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) cause a significant burden worldwide and therapeutic strategies are needed. This review provides an overview of the evidence on PPAR-targeted treatment of NAFLD and NASH in individuals with type 2 diabetes mellitus. We considered current evidence from clinical trials and observational studies as well as the impact of treatment on comorbid metabolic conditions such as obesity, dyslipidemia, and cardiovascular disease. Future areas of research, such as possible sexually dimorphic effects of PPAR-targeted therapies, are briefly reviewed.

Binding and activity of bisphenol analogues to human peroxisome proliferator-activated receptor β/δ

Several studies have indicated metabolic function disruption effects of bisphenol analogues through peroxisome proliferator-activated receptor (PPAR) alpha and gamma pathways. In the present study, we found for the first time that PPARβ/δ might be a novel cellular target of bisphenol analogues. By using the fluorescence competitive binding assay, we found seven bisphenol analogues could bind to PPARβ/δ directly, among which tetrabromobisphenol A (TBBPA, 18.38-fold) and tetrachlorobisphenol A (TCBPA, 12.06-fold) exhibited stronger binding affinity than bisphenol A (BPA). In PPARβ/δ-mediated luciferase reporter gene assay, the seven bisphenol analogues showed transcriptional activity toward PPARβ/δ. Bisphenol AF (BPAF), bisphenol F (BPF) and bisphenol B (BPB) even showed higher transcriptional activity than BPA, while TBBPA and TCBPA showed comparable activity with BPA. Moreover, in human liver HL-7702 cells, the bisphenol analogues promoted the expression of two PPARβ/δ target genes PDK4 and ANGPTL4. Molecular docking simulation indicated the binding potency of bisphenol analogues to PPARβ/δ might depend on halogenation and hydrophobicity and the transcriptional activity might depend on their binding affinity and hydrogen bond interactions. Overall, the PPARβ/δ pathway may provide a new mechanism for the metabolic function disruption of bisphenol analogues, and TBBPA and TCBPA might exert higher metabolic disruption effects than BPA via PPARβ/δ pathway.

The Role and Mechanism of Oxidative Stress and Nuclear Receptors in the Development of NAFLD

The overproduction of reactive oxygen species (ROS) and consequent oxidative stress contribute to the pathogenesis of acute and chronic liver diseases. It is now acknowledged that nonalcoholic fatty liver disease (NAFLD) is characterized as a redox-centered disease due to the role of ROS in hepatic metabolism. However, the underlying mechanisms accounting for these alternations are not completely understood. Several nuclear receptors (NRs) are dysregulated in NAFLD, and have a direct influence on the expression of a set of genes relating to the progress of hepatic lipid homeostasis and ROS generation. Meanwhile, the NRs act as redox sensors in response to metabolic stress. Therefore, targeting NRs may represent a promising strategy for improving oxidation damage and treating NAFLD. This review summarizes the link between impaired lipid metabolism and oxidative stress and highlights some NRs involved in regulating oxidant/antioxidant turnover in the context of NAFLD, shedding light on potential therapies based on NR-mediated modulation of ROS generation and lipid accumulation.

Signal Transduction and Molecular Regulation in Fatty Liver Disease

Significance: Fatty liver disease is a major liver disorder in the modern societies. Comprehensive understanding of the pathophysiology and molecular mechanisms is essential for the prevention and treatment of the disease. Recent Advances: Remarkable progress has been made in the recent years in basic and translational research in the field of fatty liver disease. Multiple signaling pathways have been implicated in the development of fatty liver disease, including AMP-activated protein kinase, mechanistic target of rapamycin kinase, endoplasmic reticulum stress, oxidative stress, inflammation, transforming growth factor β, and yes1-associated transcriptional regulator/transcriptional coactivator with PDZ-binding motif (YAP/TAZ). In addition, critical molecular regulations at the transcriptional and epigenetic levels have been linked to the pathogenesis of fatty liver disease. Critical Issues: Some critical issues remain to be solved so that research findings can be translated into clinical applications. Robust and reliable biomarkers are needed for diagnosis of different stages of the fatty liver disease. Effective and safe molecular targets remain to be identified and validated. Prevention strategies require solid scientific evidence and population-wide feasibility. Future Directions: As more data are generated with time, integrative approaches are needed to comprehensively understand the disease pathophysiology and mechanisms at multiple levels from population, organismal system, organ/tissue, to cell. The interactions between genes and environmental factors require deeper investigation for the purposes of prevention and personalized treatment of fatty liver disease. Antioxid. Redox Signal. 35, 689-717.

GDF15 mediates the metabolic effects of PPARβ/δ by activating AMPK

Peroxisome proliferator-activated receptor β/δ (PPARβ/δ) activates AMP-activated protein kinase (AMPK) and plays a crucial role in glucose and lipid metabolism. Here, we examine whether PPARβ/δ activation effects depend on growth differentiation factor 15 (GDF15), a stress response cytokine that regulates energy metabolism. Pharmacological PPARβ/δ activation increases GDF15 levels and ameliorates glucose intolerance, fatty acid oxidation, endoplasmic reticulum stress, and inflammation, and activates AMPK in HFD-fed mice, whereas these effects are abrogated by the injection of a GDF15 neutralizing antibody and in Gdf15-/- mice. The AMPK-p53 pathway is involved in the PPARβ/δ-mediated increase in GDF15, which in turn activates again AMPK. Consistently, Gdf15-/- mice show reduced AMPK activation in skeletal muscle, whereas GDF15 administration results in AMPK activation in this organ. Collectively, these data reveal a mechanism by which PPARβ/δ activation increases GDF15 levels via AMPK and p53, which in turn mediates the metabolic effects of PPARβ/δ by sustaining AMPK activation.

PPARs as Metabolic Sensors and Therapeutic Targets in Liver Diseases

Carbohydrates and lipids are two components of the diet that provide the necessary energy to carry out various physiological processes to help maintain homeostasis in the body. However, when the metabolism of both biomolecules is altered, development of various liver diseases takes place; such as metabolic-associated fatty liver diseases (MAFLD), hepatitis B and C virus infections, alcoholic liver disease (ALD), and in more severe cases, hepatocelular carcinoma (HCC). On the other hand, PPARs are a family of ligand-dependent transcription factors with an important role in the regulation of metabolic processes to hepatic level as well as in other organs. After interaction with specific ligands, PPARs are translocated to the nucleus, undergoing structural changes to regulate gene transcription involved in lipid metabolism, adipogenesis, inflammation and metabolic homeostasis. This review aims to provide updated data about PPARs’ critical role in liver metabolic regulation, and their involvement triggering the genesis of several liver diseases. Information is provided about their molecular characteristics, cell signal pathways, and the main pharmacological therapies that modulate their function, currently engaged in the clinic scenario, or in pharmacological development.

The Emerging Factors and Treatment Options for NAFLD-Related Hepatocellular Carcinoma

Simple Summary

Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disease, and it is an increasing factor in the cause of hepatocellular carcinoma (HCC). The incidence of NAFLD has increased in recent decades, accompanied by an increase in the prevalence of other metabolic diseases, such as obesity and type 2 diabetes. However, current treatment options are limited. Both genetic factors and non-genetic factors impact the initiation and progression of NAFLD-related HCC. The early diagnosis of liver cancer predicts curative treatment and longer survival. Some key molecules play pivotal roles in the initiation and progression of NAFLD-related HCC, which can be targeted to impede HCC development. In this review, we summarize some key factors and important molecules in NAFLD-related HCC development, the latest progress in HCC diagnosis and treatment options, and some current clinical trials for NAFLD treatment.

Abstract

Hepatocellular carcinoma (HCC) is the most common type of primary liver cancer, followed by cholangiocarcinoma (CCA). HCC is the third most common cause of cancer death worldwide, and its incidence is rising, associated with an increased prevalence of obesity and nonalcoholic fatty liver disease (NAFLD). However, current treatment options are limited. Genetic factors and epigenetic factors, influenced by age and environment, significantly impact the initiation and progression of NAFLD-related HCC. In addition, both transcriptional factors and post-transcriptional modification are critically important for the development of HCC in the fatty liver under inflammatory and fibrotic conditions. The early diagnosis of liver cancer predicts curative treatment and longer survival. However, clinical HCC cases are commonly found in a very late stage due to the asymptomatic nature of the early stage of NAFLD-related HCC. The development of diagnostic methods and novel biomarkers, as well as the combined evaluation algorithm and artificial intelligence, support the early and precise diagnosis of NAFLD-related HCC, and timely monitoring during its progression. Treatment options for HCC and NAFLD-related HCC include immunotherapy, CAR T cell therapy, peptide treatment, bariatric surgery, anti-fibrotic treatment, and so on. Overall, the incidence of NAFLD-related HCC is increasing, and a better understanding of the underlying mechanism implicated in the progression of NAFLD-related HCC is essential for improving treatment and prognosis.

Essential Roles of PPARs in Lipid Metabolism during Mycobacterial Infection

The mycobacterial cell wall is composed of large amounts of lipids with varying moieties. Some mycobacteria species hijack host cells and promote lipid droplet accumulation to build the cellular environment essential for their intracellular survival. Thus, lipids are thought to be important for mycobacteria survival as well as for the invasion, parasitization, and proliferation within host cells. However, their physiological roles have not been fully elucidated. Recent studies have revealed that mycobacteria modulate the peroxisome proliferator-activated receptor (PPAR) signaling and utilize host-derived triacylglycerol (TAG) and cholesterol as both nutrient sources and evasion from the host immune system. In this review, we discuss recent findings that describe the activation of PPARs by mycobacterial infections and their role in determining the fate of bacilli by inducing lipid metabolism, anti-inflammatory function, and autophagy.

Liver macrophages and inflammation in physiology and physiopathology of non-alcoholic fatty liver disease

Non‐alcoholic fatty liver disease (NAFLD) is the hepatic manifestation of metabolic syndrome, being a common comorbidity of type 2 diabetes and with important links to inflammation and insulin resistance. NAFLD represents a spectrum of liver conditions ranging from steatosis in the form of ectopic lipid storage, to inflammation and fibrosis in nonalcoholic steatohepatitis (NASH). Macrophages that populate the liver play important roles in maintaining liver homeostasis under normal physiology and in promoting inflammation and mediating fibrosis in the progression of NAFLD toward to NASH. Liver macrophages are a heterogenous group of innate immune cells, originating from the yolk sac or from circulating monocytes, that are required to maintain immune tolerance while being exposed portal and pancreatic blood flow rich in nutrients and hormones. Yet, liver macrophages retain a limited capacity to raise the alarm in response to danger signals. We now know that macrophages in the liver play both inflammatory and noninflammatory roles throughout the progression of NAFLD. Macrophage responses are mediated first at the level of cell surface receptors that integrate environmental stimuli, signals are transduced through multiple levels of regulation in the cell, and specific transcriptional programmes dictate effector functions. These effector functions play paramount roles in determining the course of disease in NAFLD and even more so in the progression towards NASH. The current review covers recent reports in the physiological and pathophysiological roles of liver macrophages in NAFLD. We emphasise the responses of liver macrophages to insulin resistance and the transcriptional machinery that dictates liver macrophage function.

Transcriptional Regulation of Metabolic Pathways via Lipid-Sensing Nuclear Receptors PPARs, FXR, and LXR in NASH

Nonalcoholic fatty liver disease comprises a wide spectrum of liver injuries from simple steatosis to steatohepatitis and cirrhosis. Nonalcoholic steatohepatitis (NASH) is defined when liver steatosis is associated with inflammation, hepatocyte damage, and fibrosis. A genetic predisposition and environmental insults (ie, dietary habits, obesity) are putatively responsible for NASH progression. Here, we present the impact of the lipid-sensing nuclear receptors in the pathogenesis and treatment of NASH. In detail, we discuss the pros and cons of the putative transcriptional action of the fatty acid sensors (peroxisome proliferator-activated receptors), the bile acid sensor (farnesoid X receptor), and the oxysterol sensor (liver X receptors) in the pathogenesis and bona fide treatment of NASH.

PPARs in liver physiology

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors and transcriptional modulators with crucial functions in hepatic and whole-body energy homeostasis. Besides their well-documented roles in lipid and glucose metabolism, emerging evidence also implicate PPARs in the control of other processes such as inflammatory responses. Recent technological advances, such as single-cell RNA sequencing, have allowed to unravel an unexpected complexity in the regulation of PPAR expression, activity and downstream signaling. Here we provide an overview of the latest advances in the study of PPARs in liver physiology, with a specific focus on formerly neglected aspects of PPAR regulation, such as tissular zonation, cellular heterogeneity, circadian rhythms, sexual dimorphism and species-specific features.

Revealing the role of peroxisome proliferator-activated receptor β/δ in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD), a form of chronic liver disease that occurs in individuals with no significant alcohol abuse, has become an increasing concern for global health. NAFLD is defined as the presence of lipid deposits in hepatocytes and it ranges from hepatic steatosis (fatty liver) to steatohepatitis. Emerging data from both preclinical studies and clinical trials suggest that the peroxisome proliferator-activated receptor (PPAR)β/δ plays an important role in the control of carbohydrate and lipid metabolism in liver, and its activation might hinder the progression of NAFLD. Here, we review the latest information on the effects of PPARβ/δ on NAFLD, including its capacity to reduce lipogenesis, to alleviate inflammation and endoplasmic reticulum stress, to ameliorate insulin resistance, and to attenuate liver injury. Because of these effects, activation of hepatic PPARβ/δ through synthetic or natural ligands provides a promising therapeutic option for the management of NAFLD.

Nonalcoholic steatohepatitis: the role of peroxisome proliferator-activated receptors

The increasing epidemic of obesity worldwide is linked to serious health effects, including increased prevalence of type 2 diabetes mellitus, cardiovascular disease and nonalcoholic fatty liver disease (NAFLD). NAFLD is the liver manifestation of the metabolic syndrome and includes the spectrum of liver steatosis (known as nonalcoholic fatty liver) and steatohepatitis (known as nonalcoholic steatohepatitis), which can evolve into progressive liver fibrosis and eventually cause cirrhosis. Although NAFLD is becoming the number one cause of chronic liver diseases, it is part of a systemic disease that affects many other parts of the body, including adipose tissue, pancreatic β-cells and the cardiovascular system. The pathomechanism of NAFLD is multifactorial across a spectrum of metabolic derangements and changes in the host microbiome that trigger low-grade inflammation in the liver and other organs. Peroxisome proliferator-activated receptors (PPARs) are a group of nuclear regulatory factors that provide fine tuning for key elements of glucose and fat metabolism and regulate inflammatory cell activation and fibrotic processes. This Review summarizes and discusses the current literature on NAFLD as the liver manifestation of the systemic metabolic syndrome and focuses on the role of PPARs in the pathomechanisms as well as in the potential targeting of disease.

Peroxisome proliferator-activated receptors in the pathogenesis and therapies of liver fibrosis

Liver fibrosis is a dynamic wound-healing process associated with the deposition of extracellular matrix produced by myofibroblasts. HSCs activation, inflammation, oxidative stress, steatosis and aging play critical roles in the progression of liver fibrosis, which is correlated with the regulation of the peroxisome proliferator-activated receptor (PPAR) pathway. As nuclear receptors, PPARs reduce inflammatory response, regulate lipid metabolism, and inhibit fibrogenesis in the liver associated with aging. Thus, PPAR ligands have been investigated as possible therapeutic agents. Mounting evidence indicated that some PPAR agonists could reverse steatohepatitis and liver fibrosis. Consequently, targeting PPARs might be a promising and novel therapeutic option against liver fibrosis. This review summarizes recent studies on the role of PPARs on the pathogenesis and treatment of liver fibrosis.

Nuclear receptors and transcriptional regulation in non-alcoholic fatty liver disease

BACKGROUND

As a result of a sedentary lifestyle and excess food consumption in modern society, non-alcoholic fatty liver disease (NAFLD) characterized by fat accumulation in the liver is becoming a major disease burden. Non-alcoholic steatohepatitis (NASH) is an advanced form of NAFLD characterized by inflammation and fibrosis that can lead to hepatocellular carcinoma and liver failure. Nuclear receptors (NRs) are a family of ligand-regulated transcription factors that closely control multiple aspects of metabolism. Their transcriptional activity is modulated by various ligands, including hormones and lipids. NRs serve as potential pharmacological targets for NAFLD/NASH and other metabolic diseases.

SCOPE OF REVIEW

In this review, we provide a comprehensive overview of NRs that have been studied in the context of NAFLD/NASH with a focus on their transcriptional regulation, function in preclinical models, and studies of their clinical utility.

MAJOR CONCLUSIONS

The transcriptional regulation of NRs is context-dependent. During the dynamic progression of NAFLD/NASH, NRs play diverse roles in multiple organs and different cell types in the liver, which highlights the necessity of targeting NRs in a stage-specific and cell-type-specific manner to enhance the efficacy and safety of treatment methods.

In utero exposure to phenanthrene induces hepatic steatosis in F1 adult female mice

Environmental pollutants are thought to be a risk factor for the prevalence of hepatic steatosis. Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous, and human exposure is inevitable. In the present study, phenanthrene (Phe) was used as a representative PAH to investigate the effects of in utero exposure to PAH on hepatic lipid metabolism and the toxicological mechanism involved. Pregnant mice (C57BL/6J) were orally administered Phe (0, 60, 600 and 6000 μg kg-1 body weight) once every 3 days with 6 doses in total. F1 female mice aged 125 days showed significantly elevated hepatic lipid levels in the liver. The protein expression of hepatic peroxisome proliferator-activated receptors (PPARβ and PPARγ) and retinoid X receptors (RXRs) was upregulated; the transcription of genes related to lipogenesis, such as srebp1 (encoding sterol regulatory element binding proteins), acca (acetyl-CoA carboxylase), fasn (fatty acid synthase) and pcsk9 (proprotein convertase subtilisin/kexin type 9), showed an upregulation, while the mRNA levels of the lipolysis gene lcat (encoding lecithin cholesterol acyl transferase) were downregulated. These results could be responsible for lipid accumulation. The promoter methylation levels of pparγ were reduced and were the lowest in the 600 μg kg-1 group, and the promoter methylation levels of lcat were significantly increased in all the Phe treatments. These changes were matched with the alterations in their mRNA levels, suggesting that prenatal Phe exposure could induce abnormal lipid metabolism in later life via epigenetic modification.

Diet-Induced Physiological Responses in the Liver of Atlantic Salmon (Salmo salar) Inferred Using Multiplex PCR Platforms

The simultaneous quantification of several transcripts via multiplex PCR can accelerate research in fish physiological responses to diet and enable the development of superior aquafeeds for farmed fish. We designed two multiplex PCR panels that included assays for 40 biomarker genes representing key aspects of fish physiology (growth, metabolism, oxidative stress, and inflammation) and 3 normalizer genes. We used both panels to assess the physiological effects of replacing fish meal and fish oil by terrestrial alternatives on Atlantic salmon smolts. In a 14-week trial, we tested three diets based on marine ingredients (MAR), animal by-products and vegetable oil (ABP), and plant protein and vegetable oil (VEG). Dietary treatments affected the expression of genes involved in hepatic glucose and lipid metabolism (e.g., srebp1, elovl2), cell redox status (e.g., txna, prdx1b), and inflammation (e.g., pgds, 5loxa). At the multivariate level, gene expression profiles were more divergent between fish fed the marine and terrestrial diets (MAR vs. ABP/VEG) than between the two terrestrial diets (ABP vs. VEG). Liver ARA was inversely related to glucose metabolism (gck)- and growth (igfbp-5b1, htra1b)-related biomarkers and hepatosomatic index. Liver DHA and EPA levels correlated negatively with elovl2, whereas ARA levels correlated positively with fadsd5. Lower hepatic EPA/ARA in ABP-fed fish correlated with the increased expression of biomarkers related to mitochondrial function (fabp3a), oxidative stress (txna, prdx1b), and inflammation (pgds, 5loxa). The analysis of hepatic biomarker gene expression via multiplex PCR revealed potential physiological impacts and nutrient-gene interactions in Atlantic salmon fed lower levels of marine-sourced nutrients.

Peroxisome Proliferator-Activated Receptors and Their Novel Ligands as Candidates for the Treatment of Non-Alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD) is a major health issue worldwide, frequently associated with obesity and type 2 diabetes. Steatosis is the initial stage of the disease, which is characterized by lipid accumulation in hepatocytes, which can progress to non-alcoholic steatohepatitis (NASH) with inflammation and various levels of fibrosis that further increase the risk of developing cirrhosis and hepatocellular carcinoma. The pathogenesis of NAFLD is influenced by interactions between genetic and environmental factors and involves several biological processes in multiple organs. No effective therapy is currently available for the treatment of NAFLD. Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors that regulate many functions that are disturbed in NAFLD, including glucose and lipid metabolism, as well as inflammation. Thus, they represent relevant clinical targets for NAFLD. In this review, we describe the determinants and mechanisms underlying the pathogenesis of NAFLD, its progression and complications, as well as the current therapeutic strategies that are employed. We also focus on the complementary and distinct roles of PPAR isotypes in many biological processes and on the effects of first-generation PPAR agonists. Finally, we review novel and safe PPAR agonists with improved efficacy and their potential use in the treatment of NAFLD.

The Journey of Thiazolidinediones as Modulators of PPARs for the Management of Diabetes: A Current Perspective

Peroxisome Proliferator-Activated Receptors (PPARs) also known as glitazone receptors are a family of receptors that regulate the expression of genes and have an essential role in carbohydrate, lipid and protein metabolism apart from other functions. PPARs come in 3 sub-types: PPAR-α, PPAR-β/δ and PPAR-γ - with PPAR-γ having 2 isoforms - γ1 and γ2. Upon activation, the PPARs regulate the transcription of various genes involved in lipid and glucose metabolism, adipocyte differentiation, increasing insulin sensitivity, prevention of oxidative stress and to a certain extent, modulation of immune responses via macrophages that have been implicated in the pathogenesis of insulin resistance. Hence, PPARs are an attractive molecular target for designing new anti-diabetic drugs. This has led to a boost in the research efforts directed towards designing of PPAR ligands - particularly ones that can selectively and specifically activate one or more of the PPAR subtypes. Though, PPAR- γ full agonists such as Thiazolidinediones (TZDs) are well established agents for dyslipidemia and type 2 diabetes mellitus (T2D), the side effect profile associated with TZDs has potentiated an imminent need to come up with newer agents that act through this pathway. Several newer derivatives having TZD scaffold have been designed using structure based drug designing technique and computational tools and tested for their PPAR binding affinity and efficacy in combating T2D and some have shown promising activities. This review would focus on the role of PPARs in the management of T2D; recently reported TZD derivatives which acted as agonists of PPAR- γ and its subtypes and are potentially useful in the new drug discovery for the disease.

Lifestyle and Food Habits Impact on Chronic Diseases: Roles of PPARs

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors that exert important functions in mediating the pleiotropic effects of diverse exogenous factors such as physical exercise and food components. Particularly, PPARs act as transcription factors that control the expression of genes implicated in lipid and glucose metabolism, and cellular proliferation and differentiation. In this review, we aim to summarize the recent advancements reported on the effects of lifestyle and food habits on PPAR transcriptional activity in chronic disease.

PPAR δ inhibition protects against palmitic acid-LPS induced lipidosis and injury in cultured hepatocyte L02 cell

Background: Non-alcoholic fatty liver disease (NAFLD) is the most common chronic liver disease, and its pathogenesis and mechanism are intricate. In the present study, we aimed to evaluate the role of PPAR δ in LPS associated NAFLD and to investigate the signal transduction pathways underlying PPAR δ treatment in vitro. Material and Methods: L02 cells were exposed to palmitic acid (PA) and/or LPS in the absence or presence of PPAR δ inhibition and/or activation. Results: LPS treatment markedly increased lipid deposition, FFA contents, IL-6 and TNF-α levels, and cell apoptosis in PA treatment (NAFLD model). PPAR δ inhibition protects L02 cells against LPS-induced lipidosis and injury. Conversely, the result of PPAR δ activation showed the reverse trend. LPS+PA treatment group significantly decreases the relative expression level of IRS-1, PI3K, AKT, phosphorylation of AKT, TLR-4, MyD88, phosphorylation of IKKα, NF-κB, Bcl-2 and increases the relative expression level of Bax, cleaved caspase 3 and cleaved caspase 8, compared with the cells treated with NAFLD model. PPAR δ inhibition upregulated the related proteins' expression level in insulin resistance and inflammation pathway and downregulated apoptotic relevant proteins. Instead, PPAR δ agonist showed the reverse trend. Conclusion: Our data show that PPAR δ inhibition reduces steatosis, inflammation and apoptosis in LPS-related NAFLD damage, in vitro. PPAR δ may be a potential therapeutic implication for NAFLD.

PPARs as Nuclear Receptors for Nutrient and Energy Metabolism

It has been more than 36 years since peroxisome proliferator-activated receptors (PPARs) were first recognized as enhancers of peroxisome proliferation. Consequently, many studies in different fields have illustrated that PPARs are nuclear receptors that participate in nutrient and energy metabolism and regulate cellular and whole-body energy homeostasis during lipid and carbohydrate metabolism, cell growth, cancer development, and so on. With increasing challenges to human health, PPARs have attracted much attention for their ability to ameliorate metabolic syndromes. In our previous studies, we found that the complex functions of PPARs may be used as future targets in obesity and atherosclerosis treatments. Here, we review three types of PPARs that play overlapping but distinct roles in nutrient and energy metabolism during different metabolic states and in different organs. Furthermore, research has emerged showing that PPARs also play many other roles in inflammation, central nervous system-related diseases, and cancer. Increasingly, drug development has been based on the use of several selective PPARs as modulators to diminish the adverse effects of the PPAR agonists previously used in clinical practice. In conclusion, the complex roles of PPARs in metabolic networks keep these factors in the forefront of research because it is hoped that they will have potential therapeutic effects in future applications.

Endocannabinoid System in Hepatic Glucose Metabolism, Fatty Liver Disease, and Cirrhosis

There is growing evidence that glucose metabolism in the liver is in part under the control of the endocannabinoid system (ECS) which is also supported by its presence in this organ. The ECS consists of its cannabinoid receptors (CBRs) and enzymes that are responsible for endocannabinoid production and metabolism. ECS is known to be differentially influenced by the hepatic glucose metabolism and insulin resistance, e.g., cannabinoid receptor type 1(CB₁) antagonist can improve the glucose tolerance and insulin resistance. Interestingly, our own study shows that expression patterns of CBRs are influenced by the light/dark cycle, which is of significant physiological and clinical interest. The ECS system is highly upregulated during chronic liver disease and a growing number of studies suggest a mechanistic and therapeutic impact of ECS on the development of liver fibrosis, especially putting its receptors into focus. An opposing effect of the CBRs was exerted via the CB₁ or CB₂ receptor stimulation. An activation of CB₁ promoted fibrogenesis, while CB₂ activation improved antifibrogenic responses. However, underlying mechanisms are not yet clear. In the context of liver diseases, the ECS is considered as a possible mediator, which seems to be involved in the synthesis of fibrotic tissue, increase of intrahepatic vascular resistance and subsequently development of portal hypertension. Portal hypertension is the main event that leads to complications of the disease. The main complication is the development of variceal bleeding and ascites, which have prognostic relevance for the patients. The present review summarizes the current understanding and impact of the ECS on glucose metabolism in the liver, in association with the development of liver cirrhosis and hemodynamics in cirrhosis and its complication, to give perspectives for development of new therapeutic strategies.

Mito-Nuclear Communication in Hepatocellular Carcinoma Metabolic Rewiring

As the main metabolic and detoxification organ, the liver constantly adapts its activity to fulfill the energy requirements of the whole body. Despite the remarkable adaptive capacity of the liver, prolonged exposure to noxious stimuli such as alcohol, viruses and metabolic disorders results in the development of chronic liver disease that can progress to hepatocellular carcinoma (HCC), which is currently the second leading cause of cancer-related death worldwide. Metabolic rewiring is a common feature of cancers, including HCC. Altered mito-nuclear communication is emerging as a driving force in the metabolic reprogramming of cancer cells, affecting all aspects of cancer biology from neoplastic transformation to acquired drug resistance. Here, we explore relevant aspects (and discuss recent findings) of mito-nuclear crosstalk in the metabolic reprogramming of hepatocellular carcinoma.

Elafibranor restricts lipogenic and inflammatory responses in a human skin stem cell-derived model of NASH

Non-alcoholic steatohepatitis (NASH) is characterized by hepatocellular steatosis with concomitant hepatic inflammation. Despite its pandemic proportions, no anti-NASH drugs have been approved yet. This is partially because drug development is decelerated due to the lack of adequate tools to assess the efficacy of potential new drug candidates. The present study describes the development and application of a new preclinical model for NASH using hepatic cells generated from human skin-derived precursors. Exposure of these cells to lipogenic (insulin, glucose, fatty acids) and pro-inflammatory factors (IL-1β, TNF-α, TGF-β) resulted in a characteristic NASH response, as indicated by intracellular lipid accumulation, modulation of NASH-specific gene expression, increased caspase-3/7 activity and the expression and/or secretion of inflammatory markers, including CCL2, CCL5, CCL7, CCL8, CXCL5, CXCL8, IL1a, IL6 and IL11. The human relevance of the proposed NASH model was verified by transcriptomics analyses that revealed commonly modulated genes and the identification of the same gene classes between the in vitro system and patients suffering from NASH. The application potential of this in vitro model was demonstrated by testing elafibranor, a promising anti-NASH compound currently under clinical phase III trial evaluation. Elafibranor attenuated in vitro key features of NASH, and dramatically lowered lipid load as well as the expression and secretion of inflammatory chemokines, which in vivo are responsible for the recruitment of immune cells. This reduction in inflammatory response was NFκB-mediated. In summary, this human-relevant, in vitro system proved to be a sensitive testing tool for the investigation of novel anti-NASH compounds.

PPARδ: A Potential Therapeutic Target for the Treatment of Metabolic Hypertension

High blood pressure and its associated cardiovascular diseases have been major risks for public health. Multiple metabolic risk factors can cause the vascular dysfunction and vascular lesion, and the hypertension due to metabolic disturbances was defined as metabolic hypertension. The members of a subfamily of the nuclear receptors, peroxisome proliferator-activated receptors (PPARs), were found to be key regulators of metabolism and vascular function. We provide up-to-date knowledge on the role of subtype PPARδ in the regulation of metabolism and vascular function and the effect of its intervention on the metabolic hypertension management. We hope to give some insights into the development of more effective treatments of metabolic hypertension and its main complications.

Nutritional Regulation of Gene Expression: Carbohydrate-, Fat- and Amino Acid-Dependent Modulation of Transcriptional Activity

The ability to detect changes in nutrient levels and generate an adequate response to these changes is essential for the proper functioning of living organisms. Adaptation to the high degree of variability in nutrient intake requires precise control of metabolic pathways. Mammals have developed different mechanisms to detect the abundance of nutrients such as sugars, lipids and amino acids and provide an integrated response. These mechanisms include the control of gene expression (from transcription to translation). This review reports the main molecular mechanisms that connect nutrients’ levels, gene expression and metabolism in health. The manuscript is focused on sugars’ signaling through the carbohydrate-responsive element binding protein (ChREBP), the role of peroxisome proliferator-activated receptors (PPARs) in the response to fat and GCN2/activating transcription factor 4 (ATF4) and mTORC1 pathways that sense amino acid concentrations. Frequently, alterations in these pathways underlie the onset of several metabolic pathologies such as obesity, insulin resistance, type 2 diabetes, cardiovascular diseases or cancer. In this context, the complete understanding of these mechanisms may improve our knowledge of metabolic diseases and may offer new therapeutic approaches based on nutritional interventions and individual genetic makeup.

Reporting temporal fluctuations of hepatic C16 and C18 fatty acids during late gestation and early lactation in dromedary camel

Based on current knowledge, C16 and C18 fatty acids (FA) are considered the most functional FA in hepatic metabolism. Although these FAs have been satisfyingly investigated in cattle, other species such as camel have been neglected. For this reason, the current study was designed to scrutinize changing patterns of C16 and C18 FAs in 10 dromedary camels from the last 2 months of gestation to the first months of lactation. Camels were grazed on natural pasture and supplemented with a balanced ration. Liver biopsies were obtained through blind biopsy technique at about 60, 45, 30, and 15-day antepartum (AP), and at 3, 15, 30, 45, and 60 post-partum (PP). Data were analyzed by the ANOVA procedure of SPSS with repeated measurements. From 15-day AP, saturated FA content of the liver declined (P < 0.01) and 15-day PP reached its peak (P = 0.02). At 30-day PP it went down (P < 0.01), and re-elevated at 45-day PP (P < 0.01) but remained at a steady state for the duration of the study. Mono-unsaturated and polyunsaturated FA content of hepatic tissue were constant throughout AP, albeit observed to peak at 15-day AP compared with 45 (P = 0.04) and 30-day AP (P < 0.01) for mono-unsaturated FAs, and with 60-, 45-, and 30-day AP (P ≤ 0.01) for polyunsaturated FAs. The palmitic acid content of the liver reached a nadir at 30-day AP (P < 0.01), increased sharply (P < 0.01) at the next sampling time-point, and had a trend to escalate until 3-day PP. Palmitoleic acid levels were unchanged from 60- to 30-day AP, decreased at 15 AP and 3-day PP, increased at 15-day PP, then remained constant until the end of the study period (P ≤ 0.04). Stearic acid content started to grow at 15-day AP and reached its peak at 15-day PP (P < 0.01). At 30-day PP, stearic level in liver dropped abruptly (P < 0.01), then intensified at 45-day PP and did not change after; hepatic content of stearic acid was lower during AP compared with PP time-points. Other C18 FAs changed significantly during the study period. These results suggest that parturition could have a profound effect on FA composition and other metabolites in camel liver. Further research is required to establish the metabolic mechanism behind these changes.

Peroxisome Proliferator-Activated Receptor-δ Supports the Metabolic Requirements of Cell Growth in TCRβ-Selected Thymocytes and Peripheral CD4+ T Cells

During T cell development, progenitor thymocytes undergo a large proliferative burst immediately following successful TCRβ rearrangement, and defects in genes that regulate this proliferation have a profound effect on thymus cellularity and output. Although the signaling pathways that initiate cell cycling and nutrient uptake after TCRβ selection are understood, less is known about the transcriptional programs that regulate the metabolic machinery to promote biomass accumulation during this process. In this article, we report that mice with whole body deficiency in the nuclear receptor peroxisome proliferator-activated receptor-δ (PPARδ^mut) exhibit a reduction in spleen and thymus cellularity, with a decrease in thymocyte cell number starting at the double-negative 4 stage of thymocyte development. Although in vivo DNA synthesis was normal in PPARδ^mut thymocytes, studies in the OP9-delta-like 4 in vitro system of differentiation revealed that PPARδ^mut double-negative 3 cells underwent fewer cell divisions. Naive CD4⁺ T cells from PPARδ^mut mice also exhibited reduced proliferation upon TCR and CD28 stimulation in vitro. Growth defects in PPAR-δ-deficient thymocytes and peripheral CD4⁺ T cells correlated with decreases in extracellular acidification rate, mitochondrial reserve, and expression of a host of genes involved in glycolysis, oxidative phosphorylation, and lipogenesis. By contrast, mice with T cell-restricted deficiency of Ppard starting at the double-positive stage of thymocyte development, although exhibiting defective CD4⁺ T cell growth, possessed a normal T cell compartment, pointing to developmental defects as a cause of peripheral T cell lymphopenia in PPARδ^mut mice. These findings implicate PPAR-δ as a regulator of the metabolic program during thymocyte and T cell growth.

Grifola frondosa (Maitake) extract activates PPARδ and improves glucose intolerance in high-fat diet-induced obese mice

Grifola frondosa is a mushroom that has anti-obesity effects, but the detailed mechanism is poorly understood. In this study, we found that lipid soluble extracts derived from G. frondosa (GFE) had peroxisome proliferator-activated receptor δ (PPARδ) agonist activity, inducing the expression of PPARδ-target genes in vitro. Furthermore, administration of GFE to high-fat diet-induced obese mice lowered the total blood cholesterol levels, upregulated the expression of PPARδ-target genes in skeletal muscles and improved glucose intolerance. Additionally, analyses of C2C12 myotubes revealed that GFE restored glucose uptake, which was inhibited by sodium palmitate, to normal levels. Unexpectedly, such acceleration was not abolished by a PPARδ antagonist. These results suggest that G. frondosa is a novel functional food that may prevent life-style related diseases like obesity and diabetes, and that these beneficial effects are likely to be mediated through the activation of PPARδ and a PPARδ-independent insulin signaling pathway.

PPARβ/δ: Linking Metabolism to Regeneration

In contrast to the general belief that regeneration is a rare event, mainly occurring in simple organisms, the ability of regeneration is widely distributed in the animal kingdom. Yet, the efficiency and extent of regeneration varies greatly. Humans can recover from blood loss as well as damage to tissues like bone and liver. Yet damage to the heart and brain cannot be reversed, resulting in scaring. Thus, there is a great interest in understanding the molecular mechanisms of naturally occurring regeneration and to apply this knowledge to repair human organs. During regeneration, injury-activated immune cells induce wound healing, extracellular matrix remodeling, migration, dedifferentiation and/or proliferation with subsequent differentiation of somatic or stem cells. An anti-inflammatory response stops the regenerative process, which ends with tissue remodeling to achieve the original functional state. Notably, many of these processes are associated with enhanced glycolysis. Therefore, peroxisome proliferator-activated receptor (PPAR) β/δ—which is known to be involved for example in lipid catabolism, glucose homeostasis, inflammation, survival, proliferation, differentiation, as well as mammalian regeneration of the skin, bone and liver—appears to be a promising target to promote mammalian regeneration. This review summarizes our current knowledge of PPARβ/δ in processes associated with wound healing and regeneration.

Insights into the Role of PPARβ/δ in NAFLD

Non-alcoholic fatty liver disease (NAFLD) is a major health issue in developed countries. Although usually associated with obesity, NAFLD is also diagnosed in individuals with low body mass index (BMI) values, especially in Asia. NAFLD can progress from steatosis to non-alcoholic steatohepatitis (NASH), which is characterized by liver damage and inflammation, leading to cirrhosis and hepatocellular carcinoma (HCC). NAFLD development can be induced by lipid metabolism alterations; imbalances of pro- and anti-inflammatory molecules; and changes in various other factors, such as gut nutrient-derived signals and adipokines. Obesity-related metabolic disorders may be improved by activation of the nuclear receptor peroxisome proliferator-activated receptor (PPAR)β/δ, which is involved in metabolic processes and other functions. This review is focused on research findings related to PPARβ/δ-mediated regulation of hepatic lipid and glucose metabolism and NAFLD development. It also discusses the potential use of pharmacological PPARβ/δ activation for NAFLD treatment.

Dual PPARα/γ agonist saroglitazar improves liver histopathology and biochemistry in experimental NASH models

BACKGROUND & AIMS

Non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) are common clinico-pathological conditions that affect millions of patients worldwide. In this study, the efficacy of saroglitazar, a novel PPARα/γ agonist, was assessed in models of NAFLD/NASH.

METHODS & RESULTS

HepG2 cells treated with palmitic acid (PA;0.75 mM) showed decreased expression of various antioxidant biomarkers (SOD1, SOD2, glutathione peroxidase and catalase) and increased expression of inflammatory markers (TNFα, IL1β and IL6). These effects were blocked by saroglitazar, pioglitazone and fenofibrate (all tested at 10μM concentration). Furthermore, these agents reversed PA-mediated changes in mitochondrial dysfunction, ATP production, NFkB phosphorylation and stellate cell activation in HepG2 and HepG2-LX2 Coculture studies. In mice with choline-deficient high-fat diet-induced NASH, saroglitazar reduced hepatic steatosis, inflammation, ballooning and prevented development of fibrosis. It also reduced serum alanine aminotransferase, aspartate aminotransferase and expression of inflammatory and fibrosis biomarkers. In this model, the reduction in the overall NAFLD activity score by saroglitazar (3 mg/kg) was significantly more prominent than pioglitazone (25 mg/kg) and fenofibrate (100 mg/kg). Pioglitazone and fenofibrate did not show any improvement in steatosis, but partially improved inflammation and liver function. Antifibrotic effect of saroglitazar (4 mg/kg) was also observed in carbon tetrachloride-induced fibrosis model.

CONCLUSIONS

Saroglitazar, a dual PPARα/γ agonist with predominant PPARα activity, shows an overall improvement in NASH. The effects of saroglitazar appear better than pure PPARα agonist, fenofibrate and PPARγ agonist pioglitazone.