PPARδ activation attenuates hepatic steatosis in Ldlr-/- mice by enhanced fat oxidation, reduced lipogenesis, and improved insulin sensitivity

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.

Transcriptome and Metabolome Analyses Provide Insight into the Glucose-Induced Adipogenesis in Porcine Adipocytes

Glucose is a major energy substrate for porcine adipocytes and also serves as a regulatory signal for adipogenesis and lipid metabolism. In this study, we combined transcriptome and metabolome analyses to reveal the underlying regulatory mechanisms of high glucose (HG) on adipogenesis by comparing differentially expressed genes (DEGs) and differentially accumulated metabolites (DAMs) identified in porcine adipocytes. Results showed that HG (20 mmol/L) significantly increased fat accumulation in porcine adipocytes compared to low glucose (LG, 5 mmol/L). A total of 843 DEGs and 365 DAMs were identified. Functional enrichment analyses of DEGs found that multiple pathways were related to adipogenesis, lipid metabolism, and immune-inflammatory responses. PPARγ, C/EBPα, ChREBP, and FOS were identified as the key hub genes through module 3 analysis, and PPARγ acted as a central regulator by linking genes involved in lipid metabolism and immune-inflammatory responses. Gene-metabolite networks found that PPARγ-13-HODE was the most important interaction relationship. These results revealed that PPARγ could mediate the cross-talk between adipogenesis and the immune-inflammatory response during adipocyte maturation. This work provides a comprehensive view of the regulatory mechanisms of glucose on adipogenesis in porcine adipocytes.

Seladelpar combined with complementary therapies improves fibrosis, inflammation, and liver injury in a mouse model of nonalcoholic steatohepatitis

Seladelpar, a selective peroxisome proliferator-activated receptor δ (PPARδ) agonist, improves markers of hepatic injury in human liver diseases, but histological improvement of nonalcoholic steatohepatitis (NASH) and liver fibrosis has been challenging with any single agent. To discover how complementary agents could work with seladelpar to achieve optimal outcomes, this study evaluated a variety of therapeutics (alone and in combination) in a mouse model of NASH. Mice on a high-fat amylin liver NASH (AMLN) diet were treated for 12 wk with seladelpar, GLP-1-R (glucagon-like peptide-1 receptor) agonist liraglutide, apoptosis signal-regulating kinase 1 (ASK1) inhibitor selonsertib, farnesoid X receptor (FXR) agonist obeticholic acid, and with seladelpar in combination with liraglutide or selonsertib. Seladelpar treatment markedly improved plasma markers of liver function. Seladelpar alone or in combination resulted in stark reductions in liver fibrosis (hydroxyproline, new collagen synthesis rate, mRNA indices of fibrosis, and fibrosis staining) compared with vehicle and the other single agents. Robust reductions in liver steatosis were also observed. Seladelpar produced a reorganization of metabolic gene expression, particularly for those genes promoting peroxisomal and mitochondrial lipid oxidation. In summary, substantial improvements in NASH and NASH-induced fibrosis were observed with seladelpar alone and in combination with liraglutide in this model. Broad gene expression analysis suggests seladelpar should be effective in concert with diverse mechanisms of action.NEW & NOTEWORTHY NASH is a chronic, progressive, and increasingly problematic liver disease that has been resistant to treatment with individual therapeutics. In this study using a diet-induced mouse model of NASH, we found that the PPARδ agonist seladelpar reduced fibrosis and NASH pathology alone and in combinations with a GLP-1-R agonist (liraglutide) or an ASK1 inhibitor (selonsertib). Liver transcriptome analysis comparing each agent and coadministration suggests seladelpar should be effective in combination with a variety of therapeutics.

PPARβ/δ activation protects against hepatic ischaemia-reperfusion injury

BACKGROUND AND AIMS

Hepatic ischaemia/reperfusion injury (HIRI) is a pathophysiological process that occurs during the liver resection and transplantation. Reportedly, peroxisome proliferator-activated receptor β/δ (PPARβ/δ) can ameliorate kidney and myocardial ischaemia/reperfusion injury. However, the effect of PPARβ/δ in HIRI remains unclear.

METHODS

Mouse hepatic ischaemia/reperfusion (I/R) models were constructed for in vivo study. Primary hepatocytes and Kupffer cells (KCs) isolated from mice and cell anoxia/reoxygenation (A/R) injury model were constructed for in vitro study. Liver injury and inflammation were investigated. Small molecular compounds (GW0742 and GSK0660) and adenoviruses were used to interfere with PPARβ/δ.

RESULTS

We found that PPARβ/δ expression was increased in the I/R and A/R models. Overexpression of PPARβ/δ in hepatocytes alleviated A/R-induced cell apoptosis, while knockdown of PPARβ/δ in hepatocytes aggravated A/R injury. Activation of PPARβ/δ by GW0742 protected against I/R-induced liver damage, inflammation and cell death, whereas inhibition of PPARβ/δ by GSK0660 had the opposite effects. Consistent results were obtained in mouse I/R models through the tail vein injection of adenovirus-mediated PPARβ/δ overexpression or knockdown vectors. Furthermore, knockdown and overexpression of PPARβ/δ in KCs aggravated and ameliorated A/R-induced hepatocyte injury, respectively. Gene ontology and gene set enrichment analysis showed that PPARβ/δ deletion was significantly enriched in the NF-κB pathway. PPARβ/δ inhibited the expression of p-IKBα and p-P65 and decreased NF-κB activity.

CONCLUSIONS

PPARβ/δ exerts anti-inflammatory and anti-apoptotic effects on HIRI by inhibiting the NF-κB pathway, and hepatocytes and KCs may play a synergistic role in this phenomenon. Thus, PPARβ/δ is a potential therapeutic target for HIRI.

Treating NASH by targeting peroxisome proliferator-activated receptors

The pathophysiology of non-alcoholic steatohepatitis (NASH) encompasses a complex set of intra- and extrahepatic driving mechanisms, involving numerous metabolic, inflammatory, vascular and fibrogenic pathways. The peroxisome proliferator-activated receptors (PPARs) α, β/δ and γ belong to the nuclear receptor family of ligand-activated transcription factors. Activated PPARs modulate target tissue transcriptomic profiles, enabling the body's adaptation to changing nutritional, metabolic and inflammatory environments. PPARs hence regulate several pathways involved in NASH pathogenesis. Whereas single PPAR agonists exert robust anti-NASH activity in several preclinical models, their clinical effects on histological endpoints of NASH resolution and fibrosis regression appear more modest. Simultaneous activation of several PPAR isotypes across different organs and within-organ cell types, resulting in pleiotropic actions, enhances the therapeutic potential of PPAR agonists as pharmacological agents for NASH and NASH-related hepatic and extrahepatic morbidity, with some compounds having already shown clinical efficacy on histological endpoints.

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.

[Genetic aspects of type 1 glucagon peptide agonists clinical efficacy: A review]

A review of publications devoted to the analysis of genetic polymorphisms of the gene encoding the glucagon-like peptide type 1 receptor and some other genes directly and indirectly involved in the implementation of its physiological action is presented. The aim of the study: to search for information on genes polymorphism that can affect the effectiveness of glucagon-like peptide type 1 agonists. The review was carried out in accordance with the PRISMA 2020 recommendations, the search for publications was based on PubMed databases (including Medline), Web of Science, as well as Russian scientific electronic source eLIBRARY.RU from 1993 to 2022. The several genes polymorphisms (GLP1R, TCF7L2, CNR1, SORCS1, WFS1, PPARD, CTRB1/2) that may affect the course and therapy of type 2 diabetes mellitus, metabolic syndrome and obesity, was described. Single nucleotide substitutions in some regions of these genes can both decrease and increase the clinical efficacy of the treatment of diabetes mellitus and metabolic syndrome with the help of type 1 glucagon-like peptide agonists: exenatide, liraglutide. Data on the role of genetic variations in the structure of the products of these genes in the effectiveness of other type 1 glucacone-like peptide agonists have not been found.

Recent updates on targeting the molecular mediators of NAFLD

Nonalcoholic fatty liver disease (NAFLD) is rapidly becoming the most common disease worldwide in an era of rapid economic growth. NAFLD is a multifactorial disease, involving multiple genetic, metabolic, and environmental factors, and is closely associated with metabolic syndrome, obesity, and cardiovascular disease. NAFLD can be classified into nonalcoholic fatty liver disease (NAFL) and nonalcoholic steatohepatitis (NASH), which can both progress to cirrhosis and even hepatocellular carcinoma (HCC). Due to the enormous burden of NAFLD and its complications, no FDA-approved drugs for the treatment of NAFLD are on the market, and therapeutic targets and drug therapies are being actively investigated. In view of the various pathological mechanisms of NAFLD, numbers of preclinical studies and clinical trials have made rapid progress. This review mainly summarizes the most recently characterized mechanisms and therapeutic targets in each mechanism of NAFLD, focusing on the mechanism and application potential.

(Pro)Renin Receptor Antagonism Attenuates High-Fat-Diet-Induced Hepatic Steatosis

Non-alcoholic fatty liver disease (NAFLD) comprises a spectrum of liver damage directly related to diabetes, obesity, and metabolic syndrome. The (pro)renin receptor (PRR) has recently been demonstrated to play a role in glucose and lipid metabolism. Here, we test the hypothesis that the PRR regulates the development of diet-induced hepatic steatosis and fibrosis. C57Bl/6J mice were fed a high-fat diet (HFD) or normal-fat diet (NFD) with matching calories for 6 weeks. An 8-week methionine choline-deficient (MCD) diet was used to induce fibrosis. Two weeks following diet treatment, mice were implanted with a subcutaneous osmotic pump delivering either the peptide PRR antagonist, PRO20, or scrambled peptide for 4 or 6 weeks. Mice fed a 6-week HFD exhibited increased liver lipid accumulation and liver triglyceride content compared with NFD-fed mice. Importantly, PRO20 treatment reduced hepatic lipid accumulation in HFD-fed mice without affecting body weight or blood glucose. Furthermore, PRR antagonism attenuated HFD-induced steatosis, particularly microvesicular steatosis. In the MCD diet model, the percentage of collagen area was reduced in PRO20-treated compared with control mice. PRO20 treatment also significantly decreased levels of liver alanine aminotransferase, an indicator of liver damage, in MCD-fed mice compared with controls. Mechanistically, we found that PRR antagonism prevented HFD-induced increases in PPARγ and glycerol-3-phosphate acyltransferase 3 expression in the liver. Taken together, our findings establish the involvement of the PRR in liver triglyceride synthesis and suggest the therapeutic potential of PRR antagonism for the treatment of liver steatosis and fibrosis in NAFLD.

Therapeutic effects of icariin and icariside II on diabetes mellitus and its complications

Diabetes mellitus (DM) is a global health issue in the twenty-first century, and there are numerous challenges in preventing and alleviating its chronic complications. The herb Epimedium has beneficial therapeutic effects on various human diseases, including DM. Its major flavonoid component, icariin, has significant anti-DM activity and may help improve pancreatic β-cell dysfunction and insulin resistance. Furthermore, preclinical evidence has shown that icariin and its in vivo bioactive form, icariside II, have preventive and therapeutic effects on several diabetic complications, including diabetic cardiomyopathy, diabetic vascular endothelial disorder, diabetic nephropathy, and diabetic erectile dysfunction. In this review, we present the general and toxicological information concerning icariin and icariside II and review the anti-DM effects of icariin from a molecular perspective. Additionally, we discuss the potential benefits of icariin and icariside II on the important pathological mechanisms of various diabetic complications. Despite positive preclinical evidence, additional investigations are needed before relevant clinical studies can be conducted. Therefore, we conclude with suggestions for future research. Hopefully, this review will provide a comprehensive molecular perspective for future research and product development related to icariin and icariside II in treating DM and diabetic complications.

Effects of PPARD gene variants on the therapeutic responses to exenatide in chinese patients with type 2 diabetes mellitus

BACKGROUND

Exenatide is a GLP-1R agonist that often exhibits considerable interindividual variability in therapeutic efficacy. However, there is no evidence about the impact of genetic variants in the PPARD on the therapeutic efficacy of exenatide. This research was aimed to explore the influence of PPARD gene polymorphism on the therapeutic effect of exenatide, and to identify the potential mechanism futher.

METHODS

A total of 300 patients with T2DM and 200 control subjects were enrolled to identify PPARD rs2016520 and rs3777744 genotypes. A prospective clinical study was used to collect clinical indicators and peripheral blood of T2DM patients treated with exenatide monotherapy for 6 months. The SNaPshot method was used to identify PPARD rs2016520 and rs3777744 genotypes, and then we performed correlation analysis between PPARD gene variants and the efficacy of exenatide, and conducted multiple linear regression analysis of factors affecting the therapeutic effect of exenatide. HepG2 cells were incubated with exenatide in the absence or presence of a PPARδ agonist or the siPPARδ plasmid, after which the levels of GLP-1R and the ratio of glucose uptake were determined.

RESULTS

After 6 months exenatide monotherapy, we observed that homeostasis model assessment for insulin resistance (HOMA-IR) levels of the subjects with at least one C allele of the PPARD rs2016520 were significantly lower than those with the TT genotype, which suggested that the PPARD rs2016520 TT genotype conferred the poor exenatide response through a reduction of insulin resistance, as measured by HOMA-IR. The carriers of G alleles at rs3777744 exhibited higher levels of in waist to hip ratio (WHR), fasting plasma glucose (FPG), hemoglobin A1c (HbA1c) and HOMA-IR compared to individuals with the AA genotype following 6 months of exenatide treatment, potentially accounting for the lower failure rate of exenatide therapy among the AA homozygotes. In an insulin resistant HepG2 cell model, the PPARδ agonists enhanced exenatide efficacy on insulin resistance, with the expression of GLP-1R being up-regulated markedly.

CONCLUSION

These data suggest that the PPARD rs2016520 and rs3777744 polymorphisms are associated with exenatide monotherapy efficacy, due to the pivotal role of PPARδ in regulating insulin resistance through affecting the expression of GLP-1R. This study was registered in the Chinese Clinical Trial Register (No. ChiCTR-CCC13003536).

Grape seed extract supplementation modulates hepatic lipid metabolism in rats. Implication of PPARβ/δ

Grape seed extract supplementationat low doses (25 mg per kg BW per day) modulates the transcriptional programs that controls the hepatic lipid metabolism in lean normolipidemic Wistar rats through PPARβ/δ activation.

PPARD rs2016520 (T/C) and NOS1AP rs12742393 (A/C) polymorphisms affect therapeutic efficacy of nateglinide in Chinese patients with type 2 diabetes mellitus

Background

Genetic polymorphisms in the PPARD and NOS1AP is associated with type 2 diabetes mellitus (T2DM); however, there is no evidence about its impact on the therapeutic efficacy of nateglinide. This study was designed to investigate a potential association of PPARD rs2016520 (T/C) and NOS1AP rs12742393 (A/C) polymorphisms with efficacy of nateglinide in newly diagnosed Chinese patients with type 2 diabetes mellitus (T2DM).

Methods

Sixty patients with newly diagnosed T2DM were enrolled to identify PPARD rs2016520 and NOS1AP rs12742393 genotypes using the polymerase chain reaction-restriction fragment length polymorphism assay (PCR–RFLP). All subjects were treated with nateglinide (360 mg/day) for 8 weeks. Anthropometric measurements, clinical laboratory tests were obtained at baseline and after 8 weeks of nateglinide treatment.

Results

After nateglinide treatment for 8 consecutive weeks, patients with at least one C allele of PPARD rs2016520 showed a smaller decrease in post plasma glucose (PPG), homeostasis model assessment for beta cell function (HOMA-B) than those with the TT genotype did (P < 0.05). In patients with the AA genotype of NOS1AP rs12742393, the drug showed better efficacy with respect to levels of fasting plasma glucose (FPG), fasting serum insulin (FINS), HOMA-B and homeostasis model assessment for insulin resistance (HOMA-IR) than in patients with the AC + CC genotype (P < 0.05). NOS1AP rs12742393 genotype distribution and allele frequency were associated with responsiveness of nateglinide treatment (P < 0.05).

Conclusions

The PPARD rs2016520 and NOS1AP rs12742393 polymorphisms were associated with nateglinide monotherapy efficacy in Chinese patients with newly diagnosed T2DM.

Trial registration

Chinese Clinical Trial Register ChiCTR13003536, date of registration: May 14, 2013.

Targeting Peroxisome Proliferator-Activated Receptor-β/δ (PPARβ/δ) for the Treatment or Prevention of Alcoholic Liver Disease

Excessive, chronic alcohol consumption can lead to alcoholic liver disease. The etiology of alcoholic liver disease is multifactorial and is influenced by alterations in gene expression and changes in fatty acid metabolism, oxidative stress, and insulin resistance. These events can lead to steatosis, fibrosis, and eventually to cirrhosis and liver cancer. Many of these functions are regulated by peroxisome proliferator-activated receptors (PPARs). Thus, it is not surprising that PPARs can modulate the mechanisms that cause alcoholic liver disease. While the roles of PPARα and PPARγ are clearer, the role of PPARβ/δ in alcoholic liver disease requires further clarification. This review summarizes the current understanding based on recent studies that indicate that PPARβ/δ can likely be targeted for the treatment and/or the prevention of alcoholic liver disease.

AMPK phosphorylates PPARδ to mediate its stabilization, inhibit glucose and glutamine uptake and colon tumor growth

Peroxisome proliferator-activated receptor δ (PPARδ) is a nuclear receptor transcription factor that plays an important role in the regulation of metabolism, inflammation, and cancer. In addition, the nutrient-sensing kinase 5'AMP-activated protein kinase (AMPK) is a critical regulator of cellular energy in coordination with PPARδ. However, the molecular mechanism of the AMPK/PPARδ pathway on cancer progression is still unclear. Here, we found that activated AMPK induced PPARδ-S50 phosphorylation in cancer cells, whereas the PPARδ/S50A (nonphosphorylation mimic) mutant reversed this event. Further analysis showed that the PPARδ/S50E (phosphorylation mimic) but not the PPARδ/S50A mutant increased PPARδ protein stability, which led to reduced p62/SQSTM1-mediated degradation of misfolded PPARδ. Furthermore, PPARδ-S50 phosphorylation decreased PPARδ transcription activity and alleviated PPARδ-mediated uptake of glucose and glutamine in cancer cells. Soft agar and xenograft tumor model analysis showed that the PPARδ/S50E mutant but not the PPARδ/S50A mutant inhibited colon cancer cell proliferation and tumor growth, which was associated with inhibition of Glut1 and SLC1A5 transporter protein expression. These findings reveal a new mechanism of AMPK-induced PPARδ-S50 phosphorylation, accumulation of misfolded PPARδ protein, and inhibition of PPARδ transcription activity contributing to the suppression of colon tumor formation.

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.

The PPARβ/δ-AMPK Connection in the Treatment of Insulin Resistance

The current treatment options for type 2 diabetes mellitus do not adequately control the disease in many patients. Consequently, there is a need for new drugs to prevent and treat type 2 diabetes mellitus. Among the new potential pharmacological strategies, activators of peroxisome proliferator-activated receptor (PPAR)β/δ show promise. Remarkably, most of the antidiabetic effects of PPARβ/δ agonists involve AMP-activated protein kinase (AMPK) activation. This review summarizes the recent mechanistic insights into the antidiabetic effects of the PPARβ/δ-AMPK pathway, including the upregulation of glucose uptake, muscle remodeling, enhanced fatty acid oxidation, and autophagy, as well as the inhibition of endoplasmic reticulum stress and inflammation. A better understanding of the mechanisms underlying the effects resulting from the PPARβ/δ-AMPK pathway may provide the basis for the development of new therapies in the prevention and treatment of insulin resistance and type 2 diabetes mellitus.

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.

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.

From Food to Genes: Transcriptional Regulation of Metabolism by Lipids and Carbohydrates

Lipids and carbohydrates regulate gene expression by means of molecules that sense these macronutrients and act as transcription factors. The peroxisome proliferator-activated receptor (PPAR), activated by some fatty acids or their derivatives, and the carbohydrate response element binding protein (ChREBP), activated by glucose-derived metabolites, play a key role in metabolic homeostasis, especially in glucose and lipid metabolism. Furthermore, the action of both factors in obesity, diabetes and fatty liver, as well as the pharmacological development in the treatment of these pathologies are indeed of high relevance. In this review we present an overview of the discovery, mechanism of activation and metabolic functions of these nutrient-dependent transcription factors in different tissues contexts, from the nutritional genomics perspective. The possibility of targeting these factors in pharmacological approaches is also discussed. Lipid and carbohydrate-dependent transcription factors are key players in the complex metabolic homeostasis, but these factors also drive an adaptive response to non-physiological situations, such as overeating. Possibly the decisive role of ChREBP and PPAR in metabolic regulation points to them as ideal therapeutic targets, but their pleiotropic functions in different tissues makes it difficult to "hit the mark".

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.

Valine Supplementation Does Not Reduce Lipid Accumulation and Improve Insulin Sensitivity in Mice Fed High-Fat Diet

Branched-chain amino acids (BCAAs), particularly leucine, were reported to decrease obesity and relevant metabolic syndrome. However, whether valine has a similar effect has rarely been investigated. In the present study, mice were assigned into four treatments (n = 10): chow diet supplemented with water (CW) or valine (CV) and high-fat diet supplemented with water (HW) or valine (HV). Valine (3%, w/v) was supplied in the drinking water. The results showed that valine treatment markedly increased serum triglyceride and insulin levels of chow diet-fed mice. The body weight, serum triglyceride level, white adipose tissue weight, and glucose and insulin intolerance were significantly elevated by valine supplementation in high-fat diet-fed mice. Metabolomics and transcriptomics showed that several genes related to fat oxidation were downregulated, and arachidonic acid and linoleic acid metabolism were altered in the HV group compared to the HW group. In conclusion, valine supplementation did not suppress lipid deposition and metabolic disorders in mice, which provides a new understanding for BCAAs in the modulation of lipid metabolism.

Cyclin-like proteins tip regenerative balance in the liver to favour cancer formation

Hepatocellular carcinoma (HCC) is one of the leading causes of cancer-related deaths worldwide. A variety of factors can contribute to the onset of this disease, including viral infection, obesity, alcohol abuse and non-alcoholic fatty liver disease (NAFLD). These stressors predominantly introduce chronic inflammation leading to liver cirrhosis and finally the onset of HCC; however, approximately 20% of HCC cases arise in the absence of cirrhosis via a poorly defined mechanism. The atypical cyclin-like protein Spy1 is capable of overriding cell cycle checkpoints, promoting proliferation and has been implicated in HCC. We hypothesize that Spy1 promotes sustained proliferation making the liver more susceptible to accumulation of deleterious mutations, leading to the development of non-cirrhotic HCC. We report for the first time that elevation of Spy1 within the liver of a transgenic mouse model leads to enhanced spontaneous liver tumourigenesis. We show that the abundance of Spy1 enhanced fat deposition within the liver and decreased the inflammatory response. Interestingly, Spy1 transgenic mice have a significant reduction in fibrosis and sustained rates of hepatocyte proliferation, and endogenous levels of Spy1 are downregulated during the normal fibrotic response. Our results provide support that abnormal regulation of Spy1 protein drives liver tumorigenesis in the absence of elevated fibrosis and, hence, may represent a potential mechanism behind non-cirrhotic HCC. This work may implicate Spy1 as a prognostic indicator and/or potential target in the treatment of diseases of the liver, such as HCC. The cyclin-like protein Spy1 enhances lipid deposition and reduces fibrosis in the liver. Spy1 also promotes increased hepatocyte proliferation and onset of non-cirrhotic hepatocellular carcinoma (HCC). Thus, Spy1 may be used as a potential target in the treatment of HCC.

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.

Discovery and SAR of Natural-Product-Inspired RXR Agonists with Heterodimer Selectivity to PPARδ-RXR

A known natural product, magnaldehyde B, was identified as an agonist of retinoid X receptor (RXR) α. Magnaldehyde B was isolated from Magnolia obovata (Magnoliaceae) and synthesized along with more potent analogs for screening of their RXRα agonistic activities. Structural optimization of magnaldehyde B resulted in the development of a candidate molecule that displayed a 440-fold increase in potency. Receptor-ligand docking simulations indicated that this molecule has the highest affinity with the ligand binding domain of RXRα among the analogs synthesized in this study. Furthermore, the selective activation of the peroxisome proliferator-activated receptor (PPAR) δ-RXR heterodimer with a stronger efficacy compared to those of PPARα-RXR and PPARγ-RXR was achieved in luciferase reporter assays using the PPAR response element driven reporter (PPRE-Luc). The PPARδ activity of the molecule was significantly inhibited by the antagonists of both RXR and PPARδ, whereas the activity of GW501516 was not affected by the RXR antagonist. Furthermore, the molecule exhibited a particularly weak PPARδ agonistic activity in reporter gene assays using the Gal4 hybrid system. The obtained data therefore suggest that the weak PPARδ agonistic activity of the optimized molecule is synergistically enhanced by its own RXR agonistic activity, indicating the potent agonistic activity of the PPARδ-RXR heterodimer.

PPARs as Metabolic Regulators in the Liver: Lessons from Liver-Specific PPAR-Null Mice

Peroxisome proliferator-activated receptor (PPAR) α, β/δ, and γ modulate lipid homeostasis. PPARα regulates lipid metabolism in the liver, the organ that largely controls whole-body nutrient/energy homeostasis, and its abnormalities may lead to hepatic steatosis, steatohepatitis, steatofibrosis, and liver cancer. PPARβ/δ promotes fatty acid β-oxidation largely in extrahepatic organs, and PPARγ stores triacylglycerol in adipocytes. Investigations using liver-specific PPAR-disrupted mice have revealed major but distinct contributions of the three PPARs in the liver. This review summarizes the findings of liver-specific PPAR-null mice and discusses the role of PPARs in the liver.

The citrus flavonoid nobiletin confers protection from metabolic dysregulation in high-fat-fed mice independent of AMPK

Obesity, dyslipidemia, and insulin resistance, the increasingly common metabolic syndrome, are risk factors for CVD and type 2 diabetes that warrant novel therapeutic interventions. The flavonoid nobiletin displays potent lipid-lowering and insulin-sensitizing properties in mice with metabolic dysfunction. However, the mechanisms by which nobiletin mediates metabolic protection are not clearly established. The central role of AMP-activated protein kinase (AMPK) as an energy sensor suggests that AMPK is a target of nobiletin. We tested the hypothesis that metabolic protection by nobiletin required phosphorylation of AMPK and acetyl-CoA carboxylase (ACC) in mouse hepatocytes, in mice deficient in hepatic AMPK (Ampkβ1 ^-/-), in mice incapable of inhibitory phosphorylation of ACC (AccDKI), and in mice with adipocyte-specific AMPK deficiency (iβ1β2AKO). We fed mice a high-fat/high-cholesterol diet with or without nobiletin. Nobiletin increased phosphorylation of AMPK and ACC in primary mouse hepatocytes, which was associated with increased FA oxidation and attenuated FA synthesis. Despite loss of ACC phosphorylation in Ampkβ1 ^-/- hepatocytes, nobiletin suppressed FA synthesis and enhanced FA oxidation. Acute injection of nobiletin into mice did not increase phosphorylation of either AMPK or ACC in liver. In mice fed a high-fat diet, nobiletin robustly prevented obesity, hepatic steatosis, dyslipidemia, and insulin resistance, and it improved energy expenditure in Ampkβ1 ^-/-, AccDKI, and iβ1β2AKO mice to the same extent as in WT controls. Thus, the beneficial metabolic effects of nobiletin in vivo are conferred independently of hepatic or adipocyte AMPK activation. These studies further underscore the therapeutic potential of nobiletin and begin to clarify possible mechanisms.

Identification of hub genes and key pathways of dietary advanced glycation end products‑induced non‑alcoholic fatty liver disease by bioinformatics analysis and animal experiments

Non-alcoholic fatty liver disease (NAFLD) is a common chronic liver disease. Advanced glycation end products (AGEs) negatively affect the liver and accelerate NAFLD progression; however, the underlying mechanisms remain unclear. The present study aimed to examine the effect and mechanism of dietary AGEs on the mouse liver using bioinformatics and in vivo experimental approaches. Gene expression datasets associated with NAFLD were obtained from the Gene Expression Omnibus and differentially expressed genes (DEGs) were identified using GEO2R. Functional enrichment analyses were performed using the Database for Annotation, Visualization and Integrated Discovery and a protein-protein interaction network for the DEGs was constructed using the Search Tool for the Retrieval of Interacting Genes database. MCODE, a Cytoscape plugin, was subsequently used to identify the most significant module. The key genes involved were verified in a dietary AGE-induced non-alcoholic steatohepatitis (NASH) mouse model using reverse transcription-quantitative PCR (RT-qPCR). The 462 DEGs associated with NAFLD in the two datasets, of which 34 overlapping genes were found in two microarray datasets. Functional analysis demonstrated that the 34 DEGs were enriched in the ‘PPAR signaling pathway’, ‘central carbon metabolism in cancer’, and ‘cell adhesion molecules (CAMs)’. Moreover, four hub genes (cell death-inducing DFFA-like effector a, cell death-inducing DFFA-like effector c, fatty acid-binding protein 4 and perilipin 4) were identified from a protein-protein interaction network and were verified using RT-qPCR in a mouse model of NASH. The results suggested that AGEs and their receptor axis may be involved in NAFLD onset and/or progression. This integrative analysis identified candidate genes and pathways in NAFLD, as well as DEGs and hub genes related to NAFLD progression in silico and in vivo.

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.

Comparative Evaluation of Gemcabene and Peroxisome Proliferator-Activated Receptor Ligands in Transcriptional Assays of Peroxisome Proliferator-Activated Receptors: Implication for the Treatment of Hyperlipidemia and Cardiovascular Disease

Gemcabene, a late-stage clinical candidate, has shown efficacy for LDL-C, non-HDL cholesterol, apoB, triglycerides, and hsCRP reduction, all risk factors for cardiovascular disease. In rodents, gemcabene showed changes in targets, including apoC-III, apoA-I, peroxisomal enzymes, considered regulated through peroxisome proliferator-activated receptor (PPAR) gene activation, suggesting a PPAR-mediated mechanism of action for the observed hypolipidemic effects observed in rodents and humans. In the current study, the gemcabene agonist activity against PPAR subtypes of human, rat, and mouse were compared with known lipid lowering PPAR activators. Surprisingly, gemcabene showed no or little PPAR-α transactivation compared with reference agonists, which showed concentration-dependent transactivation against human PPAR-α of 2.4- to 30-fold (fenofibric acid), 17-fold (GW590735), and 2.3- to 25-fold (WY-14643). These agents also showed robust transactivation of mouse and rat PPAR-α in a concentration-dependent manner. The known PPAR-δ agonists, GW1516, L165041, and GW0742, showed potent agonist activity against human, mouse, and rat receptors (ranging from 165- to 396-fold). By contrast, gemcabene at the highest concentration tested (300 μM) showed no response in mouse and rat and a marginal response against human PPAR-δ receptors (3.2-fold). For PPAR-γ, gemcabene showed no agonist activity against all 3 species at 100 μM and marginal activity (3.6- to 5-fold) at 300 μM. By contrast, the known agonists, rosiglitazone, indomethacin, and muraglitazar showed strong activation against the mouse, rat, and human PPAR-γ receptors. No clear antagonist activity was observed with gemcabene against any PPAR subtypes for all 3 species over a wide range of concentrations. In summary, the transactivation studies rule out gemcabene as a direct agonist or antagonist of PPAR-α, PPAR-γ, and PPAR-δ receptors of these 3 species. These data suggest that the peroxisomal effects observed in rodents and the lipid regulating effects observed in rodents and humans are not related to a direct activation of PPAR receptors by gemcabene.

LRG ameliorates steatohepatitis by activating the AMPK/mTOR/SREBP1 signaling pathway in C57BL/6J mice fed a high‑fat diet

The pathogenesis of nonalcoholic fatty liver disease non‑alcoholic steatohepatitis (NASH) has not been fully elucidated, and there are currently no effective treatments for NASH. The aim of the present study was to explore the therapeutic effects of the glucagon‑like peptide‑1 (GLP‑1) receptor agonist liraglutide (LRG) on NASH and the underlying mechanisms. C57BL6J mice were fed a high‑fat diet (HFD) for 8 weeks to induce hepatic steatosis, and then LRG was injected subcutaneously for 4 weeks. The expression of sterol regulatory element‑binding protein 1 (SREBP1) and adenosine monophosphate‑activated protein kinase (AMPK) as well as the phosphorylation of mechanistic target of rapamycin (mTOR) and p70 ribosomal S6 kinase (p70S6K) were determined by western blot analysis. The intracellular distribution of SREBP1 was assessed by immunofluorescence staining. The results revealed that LRG treatment ameliorated HFD‑induced hepatic lipid accumulation and inhibited body weight gain. In addition, LRG treatment significantly suppressed the expression of hepatic SREBP1 as well as the phosphorylation of mTOR and p70S6K; it also increased the phosphorylation of AMPK and acetyl coenzyme A carboxylase. Furthermore, LRG treatment inhibited the hepatic nuclear translocation of SREBP1. It was suggested that the GLP‑1 receptor agonist LRG may have ameliorated hepatic steatosis by activating the AMPK/mTOR/SREBP1 signaling pathway as opposed to inhibiting body weight gain.

PPARδ attenuates hepatic steatosis through autophagy-mediated fatty acid oxidation

Peroxisome proliferator-activated receptor δ (PPARδ) belongs to the nuclear receptor family and is involved in metabolic diseases. Although PPARδ is known to attenuate hepatic lipid deposition, its mechanism remains unclear. Here, we show that PPARδ is a potent stimulator of hepatic autophagic flux. The expression levels of PPARδ and autophagy-related proteins were decreased in liver tissues from obese and ageing mice. Pharmacological and adenovirus-mediated increases in PPARδ expression and activity were achieved in obese transgenic db/db and high fat diet-fed mice. Using genetic, pharmacological and metabolic approaches, we demonstrate that PPARδ reduces intrahepatic lipid content and stimulates β-oxidation in liver and hepatic cells by an autophagy-lysosomal pathway involving AMPK/mTOR signalling. These results provide novel insight into the lipolytic actions of PPARδ through autophagy in the liver and highlight its potential beneficial effects in NAFLD.

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.

PPARβ/δ: A Key Therapeutic Target in Metabolic Disorders

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

Hepatic regulation of VLDL receptor by PPARβ/δ and FGF21 modulates non-alcoholic fatty liver disease

Objective

The very low-density lipoprotein receptor (VLDLR) plays an important role in the development of hepatic steatosis. In this study, we investigated the role of Peroxisome Proliferator-Activated Receptor (PPAR)β/δ and fibroblast growth factor 21 (FGF21) in hepatic VLDLR regulation.

Methods

Studies were conducted in wild-type and Pparβ/δ-null mice, primary mouse hepatocytes, human Huh-7 hepatocytes, and liver biopsies from control subjects and patients with moderate and severe hepatic steatosis.

Results

Increased VLDLR levels were observed in liver of Pparβ/δ-null mice and in Pparβ/δ-knocked down mouse primary hepatocytes through mechanisms involving the heme-regulated eukaryotic translation initiation factor 2α (eIF2α) kinase (HRI), activating transcription factor (ATF) 4 and the oxidative stress-induced nuclear factor (erythroid-derived 2)-like 2 (Nrf2) pathways. Moreover, by using a neutralizing antibody against FGF21, Fgf21-null mice and by treating mice with recombinant FGF21, we show that FGF21 may protect against hepatic steatosis by attenuating endoplasmic reticulum (ER) stress-induced VLDLR upregulation. Finally, in liver biopsies from patients with moderate and severe hepatic steatosis, we observed an increase in VLDLR levels that was accompanied by a reduction in PPARβ/δ mRNA abundance and DNA-binding activity compared with control subjects.

Conclusions

Overall, these findings provide new mechanisms by which PPARβ/δ and FGF21 regulate VLDLR levels and influence hepatic steatosis development.

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PPARβ/δ deficiency leads to increased levels of hepatic VLDLR levels.

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FGF21 protects against hepatic steatosis by attenuating ER stress-induced VLDLR upregulation.

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Human hepatic steatosis is accompanied by increased levels of VLDLR and reduced expression of PPARβ/δ.

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PPARβ/δ and FGF21 may influence NAFLD development by regulating VLDLR levels.

Erythropoietin Increases Myelination in Oligodendrocytes: Gene Expression Profiling Reveals Early Induction of Genes Involved in Lipid Transport and Metabolism

Several studies have shown that erythropoietin (EPO) has neuroprotective or neuroreparative actions on diseases of the nervous system and that improves oligodendrocyte (OL) differentiation and myelination in vivo and in vitro. This study aims at investigating the early molecular mechanisms for the pro-myelinating action of EPO at the gene expression level. For this purpose, we used a differentiating OL precursor cell line, rat central glia-4 cells. Cells were differentiated or not, and then treated with EPO for 1 or 20 h. RNA was extracted and changes in the gene expression profile were assessed using microarray analysis. Experiments were performed in biological replicates of n = 4. Differentiation alone changed the expression of 11% of transcripts (2,663 out of 24,272), representing 2,436 genes, half of which were upregulated and half downregulated. At 20 h of treatment, EPO significantly affected the expression of 99 genes that were already regulated by differentiation and of 150 genes that were not influenced by differentiation alone. Analysis of the transcripts most upregulated by EPO identified several genes involved in lipid transport (e.g., Cd36) and lipid metabolism (Ppargc1a/Pgc1alpha, Lpin1, Pnlip, Lpin2, Ppard, Plin2) along with Igf1 and Igf2, growth factors known for their pro-myelinating action. All these genes were only induced by EPO and not by differentiation alone, except for Pnlip which was highly induced by differentiation and augmented by EPO. Results were validated by quantitative PCR. These findings suggest that EPO might increase remyelination by inducing insulin-like growth factors and increasing lipid metabolism.

Oral Exposure to Silver Nanoparticles or Silver Ions May Aggravate Fatty Liver Disease in Overweight Mice

As the applications and environmental release of silver ions and nanoparticles are increasing, increasing human exposure to these pollutants has become an emerging health concern. The impeding effects of such pollutants on susceptible populations are severely under-studied. Here, we demonstrate that silver nanoparticles (Ag NPs), at a dose that causes no general toxicity in normal mice, promotes the progression of fatty liver disease from steatosis to steatohepatitis only in overweight mice. Exposure to Ag⁺ ions induces the same effects in overweight mice. Ag NPs rather than Ag⁺ ions cause this disease progression based on our findings that Ag⁺ ions are partly reduced to Ag NPs in fatty livers, and the toxic effect is correlated with the liver dose of Ag NPs, not Ag⁺ ions. Furthermore, the Ag NP-induced pro-inflammatory activation of Kupffer cells in the liver, enhancement of hepatic inflammation, and suppression of fatty acid oxidation are identified as key factors in the underlying mechanisms.

The endotoxemia cardiac dysfunction is attenuated by AMPK/mTOR signaling pathway regulating autophagy

AMP-activated protein kinase (AMPK), an enzyme that plays a role in cellular energy homeostasis, modulates myocardial signaling in the heart. Myocardial dysfunction is a common complication of sepsis. Autophagy is involved in the aging related cardiac dysfunction. However, the role of AMPK in sepsis-induced cardiotoxicity has yet to be clarified, especially in aging. In this study, we explored the role of AMPK in lipopolysaccharide (LPS)-induced myocardial dysfunction and elucidated the potential mechanisms of AMPK/mTOR pathway regulating autophagy in young and aged mice. We harvested cardiac tissues by intraperitoneal injection of LPS treatment. The results by echocardiography, pathology, contractile and intracellular Ca²⁺ property as well as western blot analysis revealed that LPS induced remarkable cardiac dysfunction and cardiotoxicity in mice hearts and cardiomyocytes, which were more seriously in the aged mice. Western blot analysis indicated that the underlying mechanisms included inhibition autophagy mediated by AMPK/mTOR activation. LPS overtly promoted the expression of AMPK upstream regulator PP2A and PP2Cα. Pharmacological activation of AMPK improved cardiac function and upregulated cardiac autophagy induced by LPS in the aged mice. Collectively, our findings suggest that upregulation of autophagy by administration of AMPK could attenuate LPS-induced cardiotoxicity, which enhances our knowledge to explore new drugs and strategies for combating cardiac dysfunction induced by sepsis.

Distinct but complementary contributions of PPAR isotypes to energy homeostasis

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

Prevention of Diet-Induced Metabolic Dysregulation, Inflammation, and Atherosclerosis in Ldlr −/− Mice by Treatment With the ATP-Citrate Lyase Inhibitor Bempedoic Acid

Objective—

Bempedoic acid (ETC-1002, 8-hydroxy-2,2,14,14-tetramethylpentadecanedioic acid) is a novel low-density lipoprotein cholesterol–lowering compound. In animals, bempedoic acid targets the liver where it inhibits cholesterol and fatty acid synthesis through inhibition of ATP-citrate lyase and through activation of AMP-activated protein kinase. In this study, we tested the hypothesis that bempedoic acid would prevent diet-induced metabolic dysregulation, inflammation, and atherosclerosis.

Approach and Results—

Ldlr −/− mice were fed a high-fat, high-cholesterol diet (42% kcal fat, 0.2% cholesterol) supplemented with bempedoic acid at 0, 3, 10 and 30 mg/kg body weight/day. Treatment for 12 weeks dose-dependently attenuated diet-induced hypercholesterolemia, hypertriglyceridemia, hyperglycemia, hyperinsulinemia, fatty liver and obesity. Compared to high-fat, high-cholesterol alone, the addition of bempedoic acid decreased plasma triglyceride (up to 64%) and cholesterol (up to 50%) concentrations, and improved glucose tolerance. Adiposity was significantly reduced with treatment. In liver, bempedoic acid prevented cholesterol and triglyceride accumulation, which was associated with increased fatty acid oxidation and reduced fatty acid synthesis. Hepatic gene expression analysis revealed that treatment significantly increased expression of genes involved in fatty acid oxidation while suppressing inflammatory gene expression. In full-length aorta, bempedoic acid markedly suppressed cholesteryl ester accumulation, attenuated the expression of proinflammatory M1 genes and attenuated the iNos / Arg1 ratio. Treatment robustly attenuated atherosclerotic lesion development in the aortic sinus by 44%, with beneficial changes in morphology, characteristic of earlier-stage lesions.

Conclusions—

Bempedoic acid effectively prevents plasma and tissue lipid elevations and attenuates the onset of inflammation, leading to the prevention of atherosclerotic lesion development in a mouse model of metabolic dysregulation.

PPARs in obesity-induced T2DM, dyslipidaemia and NAFLD

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

PPARs and Mitochondrial Metabolism: From NAFLD to HCC

Metabolic related diseases, such as type 2 diabetes, metabolic syndrome, and nonalcoholic fatty liver disease (NAFLD), are widespread threats which bring about a significant burden of deaths worldwide, mainly due to cardiovascular events and cancer. The pathogenesis of these diseases is extremely complex, multifactorial, and only partially understood. As the main metabolic organ, the liver is central to maintain whole body energetic homeostasis. At the cellular level, mitochondria are the metabolic hub connecting and integrating all the main biochemical, hormonal, and inflammatory signaling pathways to fulfill the energetic and biosynthetic demand of the cell. In the liver, mitochondria metabolism needs to cope with the energetic regulation of the whole body. The nuclear receptors PPARs orchestrate lipid and glucose metabolism and are involved in a variety of diseases, from metabolic disorders to cancer. In this review, focus is placed on the roles of PPARs in the regulation of liver mitochondrial metabolism in physiology and pathology, from NAFLD to HCC.

Coordinated Activation of VEGF/VEGFR-2 and PPARδ Pathways by a Multi-Component Chinese Medicine DHI Accelerated Recovery from Peripheral Arterial Disease in Type 2 Diabetic Mice

Diabetic mellitus (DM) patients are at an increased risk of developing peripheral arterial disease (PAD). Danhong injection (DHI) is a Chinese patent medicine widely used for several cardiovascular indications but the mechanism of action is not well-understood. We investigated the therapeutic potential of DHI on experimental PAD in mice with chemically induced as well as genetic (KKAy) type 2 DM and the overlapping signaling pathways regulating both therapeutic angiogenesis and glucose homeostasis. Compared with normal genetic background wild type (WT) mice, both DM mice showed impaired perfusion recovery in hind-limb ischemia (HLI) model. DHI treatment significantly accelerated perfusion recovery, lowered blood glucose and improved glucose tolerance in both DM models. Bioluminescent imaging demonstrated a continuous ischemia-induced vascular endothelial growth factor receptor 2 (VEGFR-2) gene expressions with a peak time coincident with the maximal DHI stimulation. Flow cytometry analysis showed a DHI-mediated increase in endothelial progenitor cell (EPC) mobilization from bone marrow to circulating peripheral blood. DHI administration upregulated the expression of vascular endothelial growth factor A (VEGF-A) and VEGF receptor-2 (VEGFR-2) in ischemic muscle. A cross talk between ischemia-induced angiogenesis and glucose tolerance pathways was analyzed by Ingenuity Pathway Analysis (IPA) which suggested an interaction of VEGF-A/VEGFR-2 and peroxisome proliferator-activated receptor δ (PPARδ)/peroxisome proliferator-activated receptor γ (PPARγ) genes. We confirmed that upregulation of VEGF-A/VEGFR-2 by DHI promoted PPARδ gene expression in both type 2 diabetic mice. Our findings demonstrated that a multi-component Chinese medicine DHI effectively increased blood flow recovery after tissue ischemia in diabetic mice by promoting angiogenesis and improving glucose tolerance through a concomitant activation of VEGF-A/VEGFR-2 and PPARδ signaling pathways.

Identification of Hepatic Lysophosphatidylcholine Acyltransferase 3 as a Novel Target Gene Regulated by Peroxisome Proliferator-activated Receptor δ

Peroxisome proliferator-activated receptor δ (PPARδ) regulates many genes involved in lipid metabolism. Hepatic lysophosphatidylcholine acyltransferase 3 (LPCAT3) has critical functions in triglycerides transport and endoplasmic reticulum stress response due to its unique ability to catalyze the incorporation of polyunsaturated fatty acids into phospholipids. Previous studies identified liver X receptor as the transcription factor controlling LPCAT3 expression in mouse liver tissue. Here we show that the hepatic LPCAT3 gene is transcriptionally regulated by PPARδ. Adenovirus-mediated knockdown of PPARδ in cultured hepatic cells and liver tissue reduced LPCAT3 mRNA levels, and exogenous overexpression of PPARδ increased LPCAT3 mRNA expression. Activation of PPARδ in HepG2, Huh7, and Hepa 1-6 cells with its specific agonists increased LPCAT3 mRNA levels in all three hepatic cell lines. Through conducting sequence analysis, LPCAT3 promoter assays, and direct DNA binding assays, we have mapped the functional PPAR-responsive element to a proximal region from -135 to -123 of the LPCAT3 promoter that plays an essential role in mediating PPARδ-induced transactivation of the LPCAT3 gene. Finally, we have provided in vivo evidence showing that activation of PPARδ by agonist L165041 in mice increased hepatic LPCAT3 mRNA abundance and LPCAT enzymatic activity, which is associated with increased incorporations of arachidonate into liver phosphatidylcholine and phosphatidylethanolamine. Furthermore, transient liver-specific knockdown of LPCAT3 in mice affected PPARδ-mediated activation of several hepatic genes involving in FA metabolism. Altogether, our new findings identify LPCAT3 as a direct PPARδ target gene and suggest a novel function of PPARδ in regulation of phospholipid metabolism through LPCAT3.

Regulation of Cytochrome P450 2B10 (CYP2B10) Expression in Liver by Peroxisome Proliferator-activated Receptor-β/δ Modulation of SP1 Promoter Occupancy

Alcoholic liver disease is a pathological condition caused by overconsumption of alcohol. Because of the high morbidity and mortality associated with this disease, there remains a need to elucidate the molecular mechanisms underlying its etiology and to develop new treatments. Because peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) modulates ethanol-induced hepatic effects, the present study examined alterations in gene expression that may contribute to this disease. Chronic ethanol treatment causes increased hepatic CYP2B10 expression inPparβ/δ^+/+ mice but not in Pparβ/δ^-/- mice. Nuclear and cytosolic localization of the constitutive androstane receptor (CAR), a transcription factor known to regulate Cyp2b10 expression, was not different between genotypes. PPARγ co-activator 1α, a co-activator of both CAR and PPARβ/δ, was up-regulated in Pparβ/δ^+/+ liver following ethanol exposure, but not in Pparβ/δ^-/- liver. Functional mapping of the Cyp2b10 promoter and ChIP assays revealed that PPARβ/δ-dependent modulation of SP1 promoter occupancy up-regulated Cyp2b10 expression in response to ethanol. These results suggest that PPARβ/δ regulates Cyp2b10 expression indirectly by modulating SP1 and PPARγ co-activator 1α expression and/or activity independent of CAR activity. Ligand activation of PPARβ/δ attenuates ethanol-induced Cyp2b10 expression in Pparβ/δ^+/+ liver but not in Pparβ/δ^-/- liver. Strikingly, Cyp2b10 suppression by ligand activation of PPARβ/δ following ethanol treatment occurred in hepatocytes and was mediated by paracrine signaling from Kupffer cells. Combined, results from the present study demonstrate a novel regulatory role of PPARβ/δ in modulating CYP2B10 that may contribute to the etiology of alcoholic liver disease.

Treatment of nonalcoholic fatty liver disease: role of AMPK

Nonalcoholic fatty liver disease (NAFLD) is a growing worldwide epidemic and an important risk factor for the development of insulin resistance, type 2 diabetes, nonalcoholic steatohepatitis (NASH), and hepatic cellular carcinoma (HCC). Despite the prevalence of NAFLD, lifestyle interventions involving exercise and weight loss are the only accepted treatments for this disease. Over the last decade, numerous experimental compounds have been shown to improve NAFLD in preclinical animal models, and many of these therapeutics have been shown to increase the activity of the cellular energy sensor AMP-activated protein kinase (AMPK). Because AMPK activity is reduced by inflammation, obesity, and diabetes, increasing AMPK activity has been viewed as a viable therapeutic strategy to improve NAFLD. In this review, we propose three primary mechanisms by which AMPK activation may improve NAFLD. In addition, we examine the mechanisms by which AMPK is activated. Finally, we identify 27 studies that have used AMPK activators to reduce NAFLD. Future considerations for studies examining the relationship between AMPK and NAFLD are highlighted.