Role of patatin-like phospholipase domain-containing 3 on lipid-induced hepatic steatosis and insulin resistance in rats

The Role of PNPLA3_rs738409 Gene Variant, Lifestyle Factors, and Bioactive Compounds in Nonalcoholic Fatty Liver Disease: A Population-Based and Molecular Approach towards Healthy Nutrition

This study aimed to investigate the impact of a common non-synonymous gene variant (C>G, rs738409) in patatin-like phospholipase domain-containing 3 (PNPLA3), leading to the substitution of isoleucine with methionine at position 148 (PNPLA3-I148M), on susceptibility to nonalcoholic fatty liver disease (NAFLD) and explore potential therapeutic nutritional strategies targeting PNPLA3. It contributed to understanding sustainable dietary practices for managing NAFLD, recently referred to as metabolic-dysfunction-associated fatty liver. NAFLD had been diagnosed by ultrasound in a metropolitan hospital-based cohort comprising 58,701 middle-aged and older Korean individuals, identifying 2089 NAFLD patients. The interaction between PNPLA3 and lifestyle factors was investigated. In silico analyses, including virtual screening, molecular docking, and molecular dynamics simulations, were conducted to identify bioactive compounds from foods targeting PNPLA3(I148M). Subsequent cellular experiments involved treating oleic acid (OA)-exposed HepG2 cells with selected bioactive compounds, both in the absence and presence of compound C (AMPK inhibitor), targeting PNPLA3 expression. Carriers of the risk allele PNPLA3_rs738409G showed an increased association with NAFLD risk, particularly with adherence to a plant-based diet, avoidance of a Western-style diet, and smoking. Delphinidin 3-caffeoyl-glucoside, pyranocyanin A, delta-viniferin, kaempferol-7-glucoside, and petunidin 3-rutinoside emerged as potential binders to the active site residues of PNPLA3, exhibiting a reduction in binding energy. These compounds demonstrated a dose-dependent reduction in intracellular triglyceride and lipid peroxide levels in HepG2 cells, while pretreatment with compound C showed the opposite trend. Kaempferol-7-glucoside and petunidin-3-rutinoside showed potential as inhibitors of PNPLA3 expression by enhancing AMPK activity, ultimately reducing intrahepatic lipogenesis. In conclusion, there is potential for plant-based diets and specific bioactive compounds to promote sustainable dietary practices to mitigate NAFLD risk, especially in individuals with genetic predispositions.

The Intersection of Genetic Factors, Aberrant Nutrient Metabolism and Oxidative Stress in the Progression of Cardiometabolic Disease

Cardiometabolic disease (CMD), which encompasses metabolic-associated fatty liver disease (MAFLD), chronic kidney disease (CKD) and cardiovascular disease (CVD), has been increasing considerably in the past 50 years. CMD is a complex disease that can be influenced by genetics and environmental factors such as diet. With the increased reliance on processed foods containing saturated fats, fructose and cholesterol, a mechanistic understanding of how these molecules cause metabolic disease is required. A major pathway by which excessive nutrients contribute to CMD is through oxidative stress. In this review, we discuss how oxidative stress can drive CMD and the role of aberrant nutrient metabolism and genetic risk factors and how they potentially interact to promote progression of MAFLD, CVD and CKD. This review will focus on genetic mutations that are known to alter nutrient metabolism. We discuss the major genetic risk factors for MAFLD, which include Patatin-like phospholipase domain-containing protein 3 (PNPLA3), Membrane Bound O-Acyltransferase Domain Containing 7 (MBOAT7) and Transmembrane 6 Superfamily Member 2 (TM6SF2). In addition, mutations that prevent nutrient uptake cause hypercholesterolemia that contributes to CVD. We also discuss the mechanisms by which MAFLD, CKD and CVD are mutually associated with one another. In addition, some of the genetic risk factors which are associated with MAFLD and CVD are also associated with CKD, while some genetic risk factors seem to dissociate one disease from the other. Through a better understanding of the causative effect of genetic mutations in CMD and how aberrant nutrient metabolism intersects with our genetics, novel therapies and precision approaches can be developed for treating CMD.

Mechanism of Action and Related Natural Regulators of Nrf2 in Nonalcoholic Fatty Liver Disease

With the acceleration of people's pace of life, non-alcoholic fatty liver disease (NAFLD) has become the most common chronic liver disease in the world, which greatly threatens people's health and safety. Therefore, there is still an urgent need for higher-quality research and treatment in this area. Nuclear factor Red-2-related factor 2 (Nrf2), as a key transcription factor in the regulation of oxidative stress, plays an important role in inducing the body's antioxidant response. Although there are no approved drugs targeting Nrf2 to treat NAFLD so far, it is still of great significance to target Nrf2 to alleviate NAFLD. In recent years, studies have reported that many natural products treat NAFLD by acting on Nrf2 or Nrf2 pathways. This article reviews the role of Nrf2 in the pathogenesis of NAFLD and summarizes the currently reported natural products targeting Nrf2 or Nrf2 pathway for the treatment of NAFLD, which provides new ideas for the development of new NAFLD-related drugs.

Unraveling the pathogenesis of non-alcoholic fatty liver diseases through genome-wide association studies

Non-alcoholic fatty liver disease (NAFLD) is a significant health burden around the world, affecting approximately 25% of the population. Recent advances in human genetic databases have allowed for the identification of various single nucleotide polymorphisms associated with NAFLD-related traits. Investigating the functions of these genetic factors provides insight into the pathogenesis of NAFLD and potentially identifies novel therapeutic targets for NAFLD. In this review, we summarized current research on genes with NAFLD-associated mutations, highlighting phospholipid remodeling and spatially clustered loci as common pathological and genetic features of these mutations. These features suggest a complex yet intriguing mechanism of dissociated steatosis and insulin resistance, which is observed in a subset of patients and may lead to more precise therapy against NAFLD in the future.

Porphyra haitanensis Polysaccharides Attenuates Blood Lipid via Gut-Liver Axis in Diet-Induced High-Fat Mesocricetus auratus through Multiple Integrated Omics

SCOPE

Hyperlipidemia is currently a global public health problem severely affecting people's physical and mental health, as well as their quality of life.

METHODS AND RESULTS

The present study is aimed at revealing the mechanism of Porphyra haitanensis polysaccharide (PHP) in decreasing blood lipids by acting through gut-liver axis in Mesocricetus auratus fed a high-fat diet. PHP significantly prevented increases in serum total cholesterol, triglycerides and low-density lipoprotein cholesterol, and alleviated damage to liver cells induced by a high-fat diet M. auratus, in a dose-dependent manner. PHP promotes proliferation of Muribaculaceae and Faecalibaculum, thereby enhancing the production of butyric acid both in the colon and liver, particularly high-dose PHP (HPHP). Low-dose PHP (LPHP) promotes the expression of phosphatidylcholine metabolites and fatty acid transport genes, and inhibits the expression of genes involved in fat degradation (Abhd5), adipogenesis (Me1), fatty acid synthesis (Fasn and Pnpla3), and fatty acid chain elongation (Elovl6) in the liver. However, HPHP inhibits the expression of triglyceride metabolites and promotes the expression of fatty acid transporter (CD36), fatty acid oxidation (Acacb), and peroxisome proliferator-activated receptor gamma (PPARg) genes in the liver.

CONCLUSION

PHP regulates lipid metabolism through the gut microbiota, and the gut-liver axis plays an important role in its hypolipidemic effects.

Antisense oligonucleotide is a promising intervention for liver diseases

As the body's critical metabolic organ, the liver plays an essential role in maintaining proper body homeostasis. However, as people's living standards have improved and the number of unhealthy lifestyles has increased, the liver has become overburdened. These have made liver disease one of the leading causes of death worldwide. Under the influence of adverse factors, liver disease progresses from simple steatosis to hepatitis, to liver fibrosis, and finally to cirrhosis and cancer, followed by increased mortality. Until now, there has been a lack of accepted effective treatments for liver disease. Based on current research, antisense oligonucleotide (ASO), as an alternative intervention for liver diseases, is expected to be an effective treatment due to its high efficiency, low toxicity, low dosage, strong specificity, and additional positive characteristics. In this review, we will first introduce the design, modification, delivery, and the mechanisms of ASO, and then summarize the application of ASO in liver disease treatment, including in non-alcoholic fatty liver disease (NAFLD), hepatitis, liver fibrosis, and liver cancer. Finally, we discuss challenges and perspectives on the transfer of ASO drugs into clinical use. This review provides a current and comprehensive understanding of the integrative and systematic functions of ASO for its use in liver disease.

Hepatic deficiency of selenoprotein S exacerbates hepatic steatosis and insulin resistance

Nonalcoholic fatty liver disease (NAFLD) is closely associated with insulin resistance (IR) and type 2 diabetes mellitus (T2DM), which are all complex metabolic disorders. Selenoprotein S (SelS) is an endoplasmic reticulum (ER) resident selenoprotein involved in regulating ER stress and has been found to participate in the occurrence and development of IR and T2DM. However, the potential role and mechanism of SelS in NAFLD remains unclear. Here, we analyzed SelS expression in the liver of high-fat diet (HFD)-fed mice and obese T2DM model (db/db) mice and generated hepatocyte-specific SelS knockout (SelS^H-KO) mice using the Cre-loxP system. We showed that hepatic SelS expression levels were significantly downregulated in HFD-fed mice and db/db mice. Hepatic SelS deficiency markedly increased ER stress markers in the liver and caused hepatic steatosis via increased fatty acid uptake and reduced fatty acid oxidation. Impaired insulin signaling was detected in the liver of SelS^H-KO mice with decreased phosphorylation levels of insulin receptor substrate 1 (IRS1) and protein kinase B (PKB/Akt), which ultimately led to disturbed glucose homeostasis. Meanwhile, our results showed hepatic protein kinase Cɛ (PKCɛ) activation participated in the negative regulation of insulin signaling in SelS^H-KO mice. Moreover, the inhibitory effect of SelS on hepatic steatosis and IR was confirmed by SelS overexpression in primary hepatocytes in vitro. Thus, we conclude that hepatic SelS plays a key role in regulating hepatic lipid accumulation and insulin action, suggesting that SelS may be a potential intervention target for the prevention and treatment of NAFLD and T2DM.

Metabolic disorders and the risk of cholangiocarcinoma

Introduction: Cholangiocarcinoma (CCA) is a malignancy which arises from the biliary epithelium. Carcinogenesis of CCA is mainly linked to aberrant glucose metabolism and creation of an immunosuppressive environment around normal biliary epithelium. The incidence of CCA is higher in the East due to Opisthorchis viverrini, an endemic liver fluke. CCA has also be attributed to genetic, metabolic, and lifestyle risk factors.Areas covered: Differences in epidemiological risk factors are associated with varying phenotypes of CCA. Metabolic risk factors include diabetes, obesity, nonalcoholic fatty liver disease (NAFLD)/nonalcoholic steatohepatitis (NASH), dyslipidemia, and metabolic syndrome. Inherited metabolic risk factors include Wilson's disease and hemochromatosis. Metabolic disease is associated with a higher risk of CCA, with higher risk for the intrahepatic form. In this review, the authors provide an overview of available evidence regarding metabolic conditions associated with the development of CCA.Expert opinion: Metabolic disease is associated with a higher risk of intrahepatic CCA compared to its extrahepatic or hilar counterpart. As rates of obesity and metabolic syndrome increase, particularly in the West, it is conceivable that the incidence of CCA will also rise in the next years.

The Role of Fatty Acids in Non-Alcoholic Fatty Liver Disease Progression: An Update

Non-alcoholic fatty liver disease (NAFLD) is a major public health problem worldwide. NAFLD (both simple steatosis and steatohepatitis) is characterized by alterations in hepatic lipid metabolism, which may lead to the development of severe liver complications including cirrhosis and hepatocellular carcinoma. Thus, an exhaustive examination of lipid disorders in the liver of NAFLD patients is much needed. Mass spectrometry-based lipidomics platforms allow for in-depth analysis of lipid alterations in a number of human diseases, including NAFLD. This review summarizes the current research on lipid alterations associated with NAFLD and related complications, with special emphasis on the changes in long-chain and short-chain fatty acids levels in both serum and liver tissue, as well as in the hepatic expression of genes encoding the enzymes catalyzing lipid interconversions.

Mechanisms and disease consequences of nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is the leading chronic liver disease worldwide. Its more advanced subtype, nonalcoholic steatohepatitis (NASH), connotes progressive liver injury that can lead to cirrhosis and hepatocellular carcinoma. Here we provide an in-depth discussion of the underlying pathogenetic mechanisms that lead to progressive liver injury, including the metabolic origins of NAFLD, the effect of NAFLD on hepatic glucose and lipid metabolism, bile acid toxicity, macrophage dysfunction, and hepatic stellate cell activation, and consider the role of genetic, epigenetic, and environmental factors that promote fibrosis progression and risk of hepatocellular carcinoma in NASH.

Natural Progression of Non-Alcoholic Steatohepatitis to Hepatocellular Carcinoma

Non-alcoholic steatohepatitis (NASH) is a chronic and progressive form of non-alcoholic fatty liver disease (NAFLD). Its global incidence is increasing which makes NASH an epidemic and a public health threat. Due to repeated insults to the liver, patients are at risk for developing hepatocellular carcinoma (HCC). The progression of NASH to HCC was initially defined according to a two-hit model which involved the development of steatosis, followed by lipid peroxidation and inflammation. However, current research defines a “multi-hit” or “multi-parallel hit” model which synthesizes several contributing pathways involved in progressive fibrosis and oncogenesis. This perspective considers the effects of cellular, genetic, immunologic, metabolic, and endocrine pathways leading up to HCC which underscores the complexity of this condition. This article will provide an updated review of the pathogenic mechanisms leading from NASH to HCC as well as an exploration of the role of biomarkers and screening.

The PNPLA3-I148M Variant Confers an Antiatherogenic Lipid Profile in Insulin-resistant Patients

CONTEXT

The I148M (rs738409-G) variant in PNPLA3 increases liver fat content but may be protective against cardiovascular disease. Insulin resistance (IR) amplifies the effect of PNPLA3-I148M on liver fat.

OBJECTIVE

To study whether PNPLA3-I148M confers an antihyperlipidemic effect in insulin-resistant patients.

DESIGN

Cross-sectional study comparing the impact of PNPLA3-I148M on plasma lipids and lipoproteins in 2 cohorts, both divided into groups based on rs738409-G allele carrier status and median HOMA-IR.

SETTING

Tertiary referral center.

PATIENTS

A total of 298 obese patients who underwent a liver biopsy during bariatric surgery (bariatric cohort: age 49 ± 9 years, body mass index [BMI] 43.2 ± 6.8 kg/m2), and 345 less obese volunteers in whom liver fat was measured by proton magnetic resonance spectroscopy (nonbariatric cohort: age 45 ± 14 years, BMI 29.7 ± 5.7 kg/m2).

MAIN OUTCOME MEASURES

Nuclear magnetic resonance profiling of plasma lipids, lipoprotein particle subclasses and their composition.

RESULTS

In both cohorts, individuals carrying the PNPLA3-I148M variant had significantly higher liver fat content than noncarriers. In insulin-resistant and homozygous carriers, PNPLA3-I148M exerted a distinct antihyperlipidemic effect with decreased very-low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) particles and their constituents, and increased high-density lipoprotein particles and their constituents, compared with noncarriers. VLDL particles were smaller and LDL particles larger in PNPLA3-I148M carriers. These changes were geometrically opposite to those due to IR. PNPLA3-I148M did not have a measurable effect in patients with lower IR, and its effect was smaller albeit still significant in the less obese than in the obese cohort.

CONCLUSIONS

PNPLA3-I148M confers an antiatherogenic plasma lipid profile particularly in insulin-resistant individuals.

Non-Alcoholic Steatohepatitis: A Review of Its Mechanism, Models and Medical Treatments

Non-alcoholic steatohepatitis (NASH) develops from non-alcoholic fatty liver disease (NAFLD). Currently, around 25% of the population is estimated to have NAFLD, and 25% of NAFLD patients are estimated to have NASH. NASH is typically characterized by liver steatosis inflammation, and fibrosis driven by metabolic disruptions such as obesity, diabetes, and dyslipidemia. NASH patients with significant fibrosis have increased risk of developing cirrhosis and liver failure. Currently, NASH is the second leading cause for liver transplant in the United States. More importantly, the risk of developing hepatocellular carcinoma from NASH has also been highlighted in recent studies. Patients may have NAFLD for years before progressing into NASH. Although the pathogenesis of NASH is not completely understood, the current “multiple-hits” hypothesis suggests that in addition to fat accumulation, elevated oxidative and ER stress may also drive liver inflammation and fibrosis. The development of clinically relevant animal models and pharmacological treatments for NASH have been hampered by the limited understanding of the disease mechanism and a lack of sensitive, non-invasive diagnostic tools. Currently, most pre-clinical animal models are divided into three main groups which includes: genetic models, diet-induced, and toxin + diet-induced animal models. Although dietary models mimic the natural course of NASH in humans, the models often only induce mild liver injury. Many genetic and toxin + diet-induced models rapidly induce the development of metabolic disruption and serious liver injury, but not without their own shortcomings. This review provides an overview of the “multiple-hits” hypothesis and an evaluation of the currently existing animal models of NASH. This review also provides an update on the available interventions for managing NASH as well as pharmacological agents that are currently undergoing clinical trials for the treatment of NASH.

Discovery and Targeting of the Signaling Controls of PNPLA3 to Effectively Reduce Transcription, Expression, and Function in Pre-Clinical NAFLD/NASH Settings

Non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) are emerging worldwide epidemics, projected to become the leading cause of liver transplants. The strongest genetic risk factor for NAFLD/NASH susceptibility and progression is a single-nucleotide polymorphism (SNP) in the patatin-like phospholipase domain-containing 3 gene (PNPLA3), rs738409, encoding the missense mutation I148M. This aminoacidic substitution interferes with the normal remodeling of lipid droplets in hepatocytes. It is also thought to play a key role in promoting liver fibrosis by inhibiting the release of retinol from hepatic stellate cells. Reducing PNPLA3 levels in individuals homozygous for 148M may be an effective treatment for the entire spectrum of NAFLD, based on gene dosage analysis in the human population, as well as the protective effect of another naturally occurring SNP (rs2294918) in PNPLA3 which, when co-inherited, reduces PNPLA3 mRNA levels to 50% and counteracts disease risk. By screening a clinical compound library targeting specific signaling pathways active in primary human hepatocytes, we identified momelotinib, a drug evaluated in clinical trials to treat myelofibrosis, as a potent down-regulator of PNPLA3 expression, across all genotypes. We found that momelotinib treatment yielded >80% reduction in PNPLA3 mRNA in human primary hepatocytes and stellate cells, as well as in vivo via acute and chronic treatment of WT mice. Using a human multilineage 3D spheroid model of NASH homozygous for the PNPLA3 mutant protein, we additionally show that it decreases PNPLA3 mRNA as well as intracellular lipid content. Furthermore, we show that the effects on PNPLA3 coincide with changes in chromatin accessibility within regulatory regions of the PNPLA3 locus, consistent with inhibition occurring at the level of transcription. In addition to its primary reported targets, the JAK kinases, momelotinib inhibits several non-JAK kinases, including ACVR1. Using a combination of targeted siRNA knockdowns and signaling pathway perturbations, we show that momelotinib reduces the expression of the PNPLA3 gene largely through the inhibition of BMP signaling rather than the JAK/STAT pathway. Overall, our work identified momelotinib as a potential NASH therapeutic and uncovered previously unrecognized connections between signaling pathways and PNPLA3. These pathways may be exploited by drug modalities to “tune down” the level of gene expression, and therefore offer a potential therapeutic benefit to a high at-risk subset of NAFLD/NASH patients.

The PNPLA3-I148M variant increases polyunsaturated triglycerides in human adipose tissue

BACKGROUND & AIMS

The I148M variant in PNPLA3 is the major genetic risk factor for non-alcoholic fatty liver disease (NAFLD). The liver is enriched with polyunsaturated triglycerides (PUFA-TGs) in PNPLA3-I148M carriers. Gene expression data indicate that PNPLA3 is liver-specific in humans, but whether it functions in adipose tissue (AT) is unknown. We investigated whether PNPLA3-I148M modifies AT metabolism in human NAFLD.

METHODS

Profiling of the AT lipidome and fasting serum non-esterified fatty acid (NEFA) composition was conducted in 125 volunteers (PNPLA3^148MM/MI , n = 63; PNPLA3^148II , n = 62). AT fatty acid composition was determined in 50 volunteers homozygous for the variant (PNPLA3^148MM , n = 25) or lacking the variant (PNPLA3^148II , n = 25). Whole-body insulin sensitivity of lipolysis was determined using [² H₅ ]glycerol, and PNPLA3 mRNA and protein levels were measured in subcutaneous AT and liver biopsies in a subset of the volunteers.

RESULTS

PUFA-TGs were significantly increased in AT in carriers versus non-carriers of PNPLA3-I148M. The variant did not alter the rate of lipolysis or the composition of fasting serum NEFAs. PNPLA3 mRNA was 33-fold higher in the liver than in AT (P < .0001). In contrast, PNPLA3 protein levels per tissue protein were three-fold higher in AT than the liver (P < .0001) and nine-fold higher when related to whole-body AT and liver tissue masses (P < .0001).

CONCLUSIONS

Contrary to previous assumptions, PNPLA3 is highly abundant in AT. PNPLA3-I148M locally remodels AT TGs to become polyunsaturated as it does in the liver, without affecting lipolysis or composition of serum NEFAs. Changes in AT metabolism do not contribute to NAFLD in PNPLA3-I148M carriers.

Role of Patatin-Like Phospholipase Domain-Containing 3 Gene for Hepatic Lipid Content and Insulin Resistance in Diabetes

OBJECTIVE

The rs738409(G) single nucleotide polymorphism (SNP) in the patatin-like phospholipase domain-containing 3 (PNPLA3) gene associates with increased risk and progression of nonalcoholic fatty liver disease (NAFLD). As the recently described severe insulin-resistant diabetes (SIRD) cluster specifically relates to NAFLD, this study examined whether this SNP differently associates with hepatic lipid content (hepatocellular lipids [HCL]) and insulin sensitivity in recent-onset diabetes.

RESEARCH DESIGN AND METHODS

A total of 917 participants in the German Diabetes Study (GDS) underwent genotyping, hyperinsulinemic-euglycemic clamps with stable isotopic tracer dilution, and MRS.

RESULTS

The G allele associated positively with HCL (β = 0.36, P < 0.01), independent of age, sex, and BMI across the whole cohort, but not in the individual clusters. Those with SIRD exhibited lowest whole-body insulin sensitivity compared with those with severe insulin-deficient (SIDD), moderate obesity-related (MOD), moderate age-related (MARD), and severe autoimmune diabetes (SAID) clusters (all P < 0.001). Interestingly, the SIRD group presented with higher prevalence of the rs738409(G) SNP compared with other clusters and the glucose-tolerant control group (P < 0.05). HCL was higher in the SIRD group (median 13.6% [1st quartile 5.8; 3rd quartile 19.1] compared with the MOD (6.4 % [2.1; 12.4], P < 0.05), MARD (3.0% [1.0; 7.9], P < 0.001), SAID (0.4% [0.0; 1.5], P < 0.001), and glucose-tolerant (0.9% [0.4; 4.9), P < 0.001) group. Although the PNPLA3 polymorphism did not directly associate with whole-body insulin sensitivity in SIRD, the G-allele carriers had higher circulating free fatty acid concentrations and greater adipose tissue insulin resistance compared with noncarriers (both P < 0.001).

CONCLUSIONS

Members of the SIRD cluster are more frequently carriers of the rs738409(G) variant. The SNP-associated adipose tissue insulin resistance and excessive lipolysis may contribute to their NAFLD.

Genetics and epigenetics purpose in nonalcoholic fatty liver disease

INTRODUCTION

nonalcoholic fatty liver disease (NAFLD) comprises a broad spectrum of diseases, which can progress from benign steatosis to nonalcoholic steatohepatitis, liver cirrhosis and hepatocellular carcinoma. NAFLD is the most common chronic liver disease in developed countries, affecting approximately 25% of the general population. Insulin resistance, adipose tissue dysfunction, mitochondrial and endoplasmic reticulum stress, chronic inflammation, genetic and epigenetic factors are NAFLD triggers that control the disease susceptibility and progression.

AREAS COVERED

In recent years a large number of investigations have been carried out to elucidate genetic and epigenetic factors in the disease pathogenesis, as well as the search for diagnostic markers and therapeutic targets. This paper objective is to report the most studied genetic and epigenetic variants around NAFLD.

EXPERT OPINION

NAFLD lead to various comorbidities, which have a considerable impact on the patient wellness and life quality, as well as on the costs they generate for the country's health services. It is essential to continue with molecular research, since it could be used as a clinical tool for prognosis and disease severity. Specifically, in the field of hepatology, plasma miRNAs could provide a novel tool in liver diseases diagnosis and monitoring, representing an alternative to invasive diagnostic procedures.

Review article: the emerging role of genetics in precision medicine for patients with non-alcoholic steatohepatitis

BACKGROUND

Non-alcoholic steatohepatitis (NASH) is a severe form of non-alcoholic fatty liver disease (NAFLD) characterised by liver fat accumulation, inflammation and progressive fibrosis. Emerging data indicate that genetic susceptibility increases risks of NAFLD, NASH and NASH-related cirrhosis.

AIMS

To review NASH genetics and discuss the potential for precision medicine approaches to treatment.

METHOD

PubMed search and inclusion of relevant literature.

RESULTS

Single-nucleotide polymorphisms in PNPLA3, TM6SF2, GCKR, MBOAT7 and HSD17B13 are clearly associated with NASH development or progression. These genetic variants are common and have moderate-to-large effect sizes for development of NAFLD, NASH and hepatocellular carcinoma (HCC). The genes play roles in lipid remodelling in lipid droplets, hepatic very low-density lipoprotein (VLDL) secretion and de novo lipogenesis. The PNPLA3 I148M variant (rs738409) has large effects, with approximately twofold increased odds of NAFLD and threefold increased odds of NASH and HCC per allele. Obesity interacts with PNPLA3 I148M to elevate liver fat content and increase rates of NASH. Although the isoleucine-to-methionine substitution at amino acid position 148 of the PNPLA3 enzyme inactivates its lipid remodelling activity, the effect of PNPLA3 I148M results from trans-repression of another lipase (ATGL/PNPLA2) by sequestration of a shared cofactor (CGI-58/ABHD5), leading to decreased hepatic lipolysis and VLDL secretion. In homozygous Pnpla3 I148M knock-in rodent models of NAFLD, targeted PNPLA3 mRNA knockdown reduces hepatic steatosis, inflammation and fibrosis.

CONCLUSION

The emerging genetic and molecular understanding of NASH paves the way for novel interventions, including precision medicines that can modulate the activity of specific genes associated with NASH.

Emerging Pharmacological Targets for the Treatment of Nonalcoholic Fatty Liver Disease, Insulin Resistance, and Type 2 Diabetes

Type 2 diabetes (T2D) is characterized by persistent hyperglycemia despite hyperinsulinemia, affects more than 400 million people worldwide, and is a major cause of morbidity and mortality. Insulin resistance, of which ectopic lipid accumulation in the liver [nonalcoholic fatty liver disease (NAFLD)] and skeletal muscle is the root cause, plays a major role in the development of T2D. Although lifestyle interventions and weight loss are highly effective at reversing NAFLD and T2D, weight loss is difficult to sustain, and newer approaches aimed at treating the root cause of T2D are urgently needed. In this review, we highlight emerging pharmacological strategies aimed at improving insulin sensitivity and T2D by altering hepatic energy balance or inhibiting key enzymes involved in hepatic lipid synthesis. We also summarize recent research suggesting that liver-targeted mitochondrial uncoupling may be an attractive therapeutic approach to treat NAFLD, nonalcoholic steatohepatitis, and T2D.

The Impacts of Herbal Medicines and Natural Products on Regulating the Hepatic Lipid Metabolism

The dysregulation of hepatic lipid metabolism is one of the hallmarks in many liver diseases including alcoholic liver diseases (ALD) and non-alcoholic fatty liver diseases (NAFLD). Hepatic inflammation, lipoperoxidative stress as well as the imbalance between lipid availability and lipid disposal, are direct causes of liver steatosis. The application of herbal medicines with anti-oxidative stress and lipid-balancing properties has been extensively attempted as pharmaceutical intervention for liver disorders in experimental and clinical studies. Although the molecular mechanisms underlying their hepatoprotective effects warrant further exploration, increasing evidence demonstrated that many herbal medicines are involved in regulating lipid accumulation processes including hepatic lipolytic and lipogenic pathways, such as mitochondrial and peroxisomal β-oxidation, the secretion of very low density lipoprotein (VLDL), the non-esterified fatty acid (NEFA) uptake, and some vital hepatic lipogenic enzymes. Therefore, in this review, the pathways or crucial mediators participated in the dysregulation of hepatic lipid metabolism are systematically summarized, followed by the current evidences and advances in the positive impacts of herbal medicines and natural products on the lipid metabolism pathways are detailed. Furthermore, several herbal formulas, herbs or herbal derivatives, such as Erchen Dection, Danshen, resveratrol, and berberine, which have been extensively studied for their promising potential in mediating lipid metabolism, are particularly highlighted in this review.

Leveraging Human Genetics to Identify Potential New Treatments for Fatty Liver Disease

Fatty liver disease (FLD), including its more severe pathologies, namely steatohepatitis, hepatocarcinoma, and cirrhosis, is the most common cause of chronic liver disease worldwide and is projected to become the leading cause of hepatocellular carcinoma and end-stage liver disease. FLD is heterogeneous with multiple etiologies and diverse histological phenotypes, so therapies will ultimately need to be individualized for relevant targets. Inherited factors contribute to FLD, and most of the genetic variation influencing liver disease development and progression is derived from genes involved in lipid biology, including PNPLA3, TM6SF2, GCKR, MBOAT7, and HSD17B13. From this point of view, we focus in this perspective on how human molecular genetics of FLD have highlighted defects in hepatic lipid handling as a major common mechanism of its pathology and how this insight could be leveraged to treat and prevent its more serious complications.

PNPLA3-I148M: a problem of plenty in non-alcoholic fatty liver disease

Fatty liver disease (FLD) affects more than one-third of the population in the western world and an increasing number of children in the United States. It is a leading cause of obesity and liver transplantation. Mechanistic insights into the causes of FLD are urgently needed since no therapeutic intervention has proven to be effective. A sequence variation in patatin like phospholipase domain-containing protein 3 (PNPLA3), rs 738409, is strongly associated with the progression of fatty liver disease. The resulting mutant causes a substitution of isoleucine to methionine at position 148. The underlying mechanism of this disease remains unsolved although several studies have illuminated key insights into its pathogenesis. This review highlights the progress in our understanding of PNPLA3 function in lipid droplet dynamics and explores possible therapeutic interventions to ameliorate this human health hazard.

Decreased liver triglyceride content in adult rats exposed to protein restriction during gestation and lactation: roles of hepatic lipogenesis and lipid utilization in muscle and adipose tissue

Protein restriction throughout pregnancy and lactation reduces liver triglyceride (TG) content in adult male rat offspring. The study determined the contribution of hepatic lipogenesis to the reduction in liver TG content. Rats received either control or protein-restricted diets throughout pregnancy and lactation. Offspring were sacrificed on day 65. Hepatic fatty acid uptake and de novo fatty acid and TG biosynthesis were similar between control and low-protein (LP) offspring. These results indicate that hepatic lipogenesis cannot mediate the decrease in liver TG content in LP offspring. We then determined whether increased lipid utilization in adipose tissue and muscle was responsible for the decrease in liver TG content. There was suggestive evidence of increased sympathetic nervous system tone in epididymal adipose tissue of LP offspring that increased fatty acid uptake, TG lipolysis, and utilization of fatty acids in mitochondrial thermogenesis. Measurement of similar parameters demonstrated that such alterations do not occur in gastrocnemius muscle, another major lipid-utilizing tissue. Our results suggest that the decrease in liver TG content in LP offspring is likely due to increased diversion of fatty acids to white and brown adipose tissue depots and their enhanced utilization to fuel mitochondrial thermogenesis.

Dynamic interactions of ABHD5 with PNPLA3 regulate triacylglycerol metabolism in brown adipocytes

Patatin-Like Phospholipase Domain Containing 2 (PNPLA2)/Adipose Triglyceride Lipase (ATGL) and PNPLA3/Adiponutrin are close paralogs that appear to have opposite functions on triacylglycerol (TAG) mobilization and storage. PNPLA2/ATGL is a major triglyceride lipase in adipose tissue and liver, whereas a common human variant of PNPLA3, I148M, greatly increases risk of hepatosteatosis. Nonetheless, the function of PNPLA3 and the mechanism by which the I148M variant promotes TAG accumulation are poorly understood. Here we demonstrate that PNPLA3 strongly interacts with α/β hydrolase domain-containing 5 (ABHD5/CGI-58), an essential co-activator of PNPLA2/ATGL. Molecular imaging experiments demonstrate that PNPLA3 effectively competes with PNPLA2/ATGL for ABHD5, and that PNPLA3 I148M is more effective in this regard. Inducible overexpression of PNPLA3 I148M greatly suppressed PNPLA2/ATGL-dependent lipolysis and triggered massive TAG accumulation in brown adipocytes, and these effects were dependent on ABHD5. The interaction of PNPLA3 and ABHD5 can be regulated by fatty acid supplementation and synthetic ABHD5 ligands, raising the possibility that this interaction might be targeted for treatment of fatty liver disease.

Human Multilineage 3D Spheroids as a Model of Liver Steatosis and Fibrosis

Non-alcoholic fatty liver disease (NAFLD) is the most common liver disorder in western countries. Despite the high prevalence of NAFLD, the underlying biology of the disease progression is not clear, and there are no approved drugs to treat non-alcoholic steatohepatitis (NASH), the most advanced form of the disease. Thus, there is an urgent need for developing advanced in vitro human cellular systems to study disease mechanisms and drug responses. We attempted to create an organoid system genetically predisposed to NAFLD and to induce steatosis and fibrosis in it by adding free fatty acids. We used multilineage 3D spheroids composed by hepatocytes (HepG2) and hepatic stellate cells (LX-2) with a physiological ratio (24:1). HepG2 and LX-2 cells are homozygotes for the PNPLA3 I148M sequence variant, the strongest genetic determinant of NAFLD. We demonstrate that hepatic stellate cells facilitate the compactness of 3D spheroids. Then, we show that the spheroids develop accumulations of fat and collagen upon exposure to free fatty acids. Finally, this accumulation was rescued by incubating spheroids with liraglutide or elafibranor, drugs that are in clinical trials for the treatment of NASH. In conclusion, we have established a simple, easy to handle, in vitro model of genetically induced NAFLD consisting of multilineage 3D spheroids. This tool may be used to understand molecular mechanisms involved in the early stages of fibrogenesis induced by lipid accumulation. Moreover, it may be used to identify new compounds to treat NASH using high-throughput drug screening.

Crosstalk between nonalcoholic fatty liver disease and cardiometabolic syndrome

Nonalcoholic fatty liver disease (NAFLD) is a chronic condition characterized by fat accumulation combined with low-grade inflammation in the liver. A large body of clinical and experimental data shows that increased flux of free fatty acids from increased visceral adipose tissue and de novo lipogenesis can lead to NAFLD and insulin resistance. Thus, individuals with obesity, insulin resistance, and dyslipidaemia are at the greatest risk of developing NAFLD. Conversely, NAFLD is a phenotype of cardiometabolic syndrome. Notably, researchers have discovered a close association between NAFLD and impaired glucose metabolism and focused on the role of NAFLD in the development of type 2 diabetes. Moreover, recent studies provide substantial evidence for an association between NAFLD and atherosclerosis and cardiometabolic disorders. Even if NAFLD can progress into severe liver disorders including nonalcoholic steatohepatitis (NASH) and cirrhosis, the majority of subjects with NAFLD die from cardiovascular disease eventually. In this review, we propose a potential pathological link between NAFLD/NASH and cardiometabolic syndrome. The potential factors that can play a pivotal role in this link, such as inflammation, insulin resistance, alteration in lipid metabolism, oxidative stress, genetic predisposition, and gut microbiota are discussed.

Pnpla3 silencing with antisense oligonucleotides ameliorates nonalcoholic steatohepatitis and fibrosis in Pnpla3 I148M knock-in mice

OBJECTIVE

Nonalcoholic fatty liver disease (NAFLD) is becoming a leading cause of advanced chronic liver disease. The progression of NAFLD, including nonalcoholic steatohepatitis (NASH), has a strong genetic component, and the most robust contributor is the patatin-like phospholipase domain-containing 3 (PNPLA3) rs738409 encoding the 148M protein sequence variant. We hypothesized that suppressing the expression of the PNPLA3 148M mutant protein would exert a beneficial effect on the entire spectrum of NAFLD.

METHODS

We examined the effects of liver-targeted GalNAc3-conjugated antisense oligonucleotide (ASO)-mediated silencing of Pnpla3 in a knock-in mouse model in which we introduced the human PNPLA3 I148M mutation.

RESULTS

ASO-mediated silencing of Pnpla3 reduced liver steatosis (p = 0.038) in homozygous Pnpla3 148M/M knock-in mutant mice but not in wild-type littermates fed a steatogenic high-sucrose diet. In mice fed a NASH-inducing diet, ASO-mediated silencing of Pnpla3 reduced liver steatosis score and NAFLD activity score independent of the Pnpla3 genotype, while reductions in liver inflammation score (p = 0.018) and fibrosis stage (p = 0.031) were observed only in the Pnpla3 knock-in 148M/M mutant mice. These responses were accompanied by reduced liver levels of Mcp1 (p = 0.026) and Timp2 (p = 0.007) specifically in the mutant knock-in mice. This may reduce levels of chemokine attracting inflammatory cells and increase the collagenolytic activity during tissue regeneration.

CONCLUSION

This study provides the first evidence that a Pnpla3 ASO therapy can improve all features of NAFLD, including liver fibrosis, and suppress the expression of a strong innate genetic risk factor, Pnpla3 148M, which may open up a precision medicine approach in NASH.

Current Progress in Pharmacogenetics of Second-Line Antidiabetic Medications: Towards Precision Medicine for Type 2 Diabetes

Precision medicine is a scientific and medical practice for personalized therapy based on patients’ individual genetic, environmental, and lifestyle characteristics. Pharmacogenetics and pharmacogenomics are also rapidly developing and expanding as a key element of precision medicine, in which the association between individual genetic variabilities and drug disposition and therapeutic responses are investigated. Type 2 diabetes (T2D) is a chronic metabolic disorder characterized by hyperglycemia mainly associated with insulin resistance, with the risk of clinically important cardiovascular, neurological, and renal complications. The latest consensus report from the American Diabetes Association and European Association for the Study of Diabetes (ADA-EASD) on the management of T2D recommends preferential use of glucagon-like peptide-1 (GLP-1) receptor agonists, sodium-glucose cotransporter-2 (SGLT2) inhibitors, and some dipeptidyl peptidase-4 (DPP-4) inhibitors after initial metformin monotherapy for diabetic patients with established atherosclerotic cardiovascular or chronic kidney disease, and with risk of hypoglycemia or body weight-related problems. In this review article, we summarized current progress on pharmacogenetics of newer second-line antidiabetic medications in clinical practices and discussed their therapeutic implications for precision medicine in T2D management. Several biomarkers associated with drug responses have been identified from extensive clinical pharmacogenetic studies, and functional variations in these genes have been shown to significantly affect drug-related glycemic control, adverse reactions, and risk of diabetic complications. More comprehensive pharmacogenetic research in various clinical settings will clarify the therapeutic implications of these genes, which may be useful tools for precision medicine in the treatment and prevention of T2D and its complications.

Distinguishing Lipid Subtypes by Amplifying Contrast from J-Coupling

Previous work has highlighted the complicated and distinctive dynamics that set signal evolution during a train of spin echoes, especially with nonuniform echo spacing applied to complex molecules like fats. The work presented here regards those signal patterns as codes that can be used as a contrast mechanism, capable of distinguishing mixtures of molecules with an imaging sequence, sidestepping many challenges of spectroscopy. For particular arrays of echo spacings, non-monotonic and distinctive signal evolution can be enhanced to improve contrast between target species. This work presents simulations that show how contrast between two molecules: (a) depends on the specific sequence of echo spacing, (b) is directly linked to the presence of J-coupling, and (c) can be relatively insensitive to variations in B0, T2 and B1. Imaging studies with oils demonstrate this phenomenon experimentally and also show that spin echo codes can be used for quantification. Finally, preliminary experiments apply the method to human liver in vivo, verifying that the presence of fat can lead to nonmonotonic codes like those seen in vitro. In summary, nonuniformly spaced echo trains introduce a new approach to molecular imaging of J-coupled species, such as lipids, which may have implications diagnosing metabolic diseases.

PEPCK1 Antisense Oligonucleotide Prevents Adiposity and Impairs Hepatic Glycogen Synthesis in High-Fat Male Fed Rats

The increased hepatic gluconeogenesis in type 2 diabetes mellitus has often been ascribed to increased transcription of phosphoenolpyruvate carboxykinase 1, cystolic form (PEPCK1), although recent evidence has questioned this attribution. To assess the metabolic role of PEPCK1, we treated regular chow fed and high-fat fed (HFF) male Sprague-Dawley rats with a 2'-O-methoxyethyl chimeric antisense oligonucleotide (ASO) against PEPCK1 and compared them with control ASO-treated rats. PEPCK1 ASO effectively decreased PEPCK1 expression in the liver and white adipose tissue. In chow fed rats, PEPCK1 ASO did not alter adiposity, plasma glucose, or insulin. In contrast, PEPCK1 ASO decreased the white adipose tissue mass in HFF rats but without altering basal rates of lipolysis, de novo lipogenesis, or glyceroneogenesis in vivo. Despite the protection from adiposity, hepatic insulin sensitivity was impaired in HFF PEPCK1 ASO-treated rats. PEPCK1 ASO worsened hepatic steatosis, although without additional impairments in hepatic insulin signaling or activation of inflammatory signals in the liver. Instead, the development of hepatic insulin resistance and the decrease in hepatic glycogen synthesis during a hyperglycemic clamp was attributed to a decrease in hepatic glucokinase (GCK) expression and decreased synthesis of glycogen via the direct pathway. The decrease in GCK expression was associated with increased expression of activating transcription factor 3, a negative regulator of GCK transcription. These studies have demonstrated that PEPCK1 is integral to coordinating cellular metabolism in the liver and adipose tissue, although it does not directly effect hepatic glucose production or adipose glyceroneogenesis.

The role of lysophosphatidic acid in the physiology and pathology of the skin

Lysophosphatidic acid (LPA) is the simplest phospholipid found in nature. LPA is mainly biosynthesized in tissues and cells by autotoxin and PA-PLA1α/PA-PLA1β and is degraded by lipid phosphate phosphatases (LPPs). It is an important component of biofilm, an extracellular signal transmitter and intracellular second messenger. After targeting to endothelial differentiation gene (Edg) family LPA receptors (LPA1, LPA2, LPA3) and non-Edg family LPA receptors (LPA4, LPA5, LPA6), LPA mediates physiological and pathological processes such as embryonic development, angiogenesis, tumor progression, fibrogenesis, wound healing, ischemia/reperfusion injury, and inflammatory reactions. These processes are induced through signaling pathways including mitogen-activated protein kinase (MAPK), phosphatidylinositol-3-kinase (PI3K)/Akt, protein kinase C (PKC)-GSK3β-β-catenin, Rho, Stat, and hypoxia-inducible factor 1-alpha (HIF-1α). LPA is involved in multiple physiological and pathological processes in the skin. It not only regulates skin function but also plays an important role in hair follicle development, skin wound healing, pruritus, skin tumors, and scleroderma. Pharmacological inhibition of LPA synthesis or antagonization of LPA receptors is a new strategy for the treatment of various skin disorders. This review focuses on the current understanding of the pathophysiologic role of LPA in the skin.

Mechanisms of sleep deprivation-induced hepatic steatosis and insulin resistance in mice

Sleep deprivation is associated with increased risk for type 2 diabetes mellitus. However, the underlying mechanisms of sleep deprivation-induced glucose intolerance remain elusive. The aim of this study was to investigate the mechanisms of sleep deprivation-induced glucose intolerance in mice with a special focus on the liver. We established a mouse model of sleep deprivation-induced glucose intolerance using C57BL/6J male mice. A single 6-h sleep deprivation by the gentle handling method under fasting condition induced glucose intolerance. Hepatic glucose production assessed by a pyruvate challenge test was significantly increased, as was hepatic triglyceride content (by 67.9%) in the sleep deprivation group, compared with freely sleeping control mice. Metabolome and microarray analyses were used to evaluate hepatic metabolites and gene expression levels and to determine the molecular mechanisms of sleep deprivation-induced hepatic steatosis. Hepatic metabolites, such as acetyl coenzyme A, 3β-hydroxybutyric acid, and certain acylcarnitines, were significantly increased in the sleep deprivation group, suggesting increased lipid oxidation in the liver. In contrast, fasted sleep-deprived mice showed that hepatic gene expression levels of elongation of very long chain fatty acids-like 3, lipin 1, perilipin 4, perilipin 5, and acyl-CoA thioesterase 1, which are known to play lipogenic roles, were 2.7, 4.5, 3.7, 2.9, and 2.8 times, respectively, those of the fasted sleeping control group, as assessed by quantitative RT-PCR. Sleep deprivation-induced hepatic steatosis and hepatic insulin resistance seem to be mediated through upregulation of hepatic lipogenic enzymes.

Dissociation of Fatty Liver and Insulin Resistance in I148M PNPLA3 Carriers: Differences in Diacylglycerol (DAG) FA18:1 Lipid Species as a Possible Explanation

Fatty liver is tightly associated with insulin resistance and the development of type 2 diabetes. I148M variant in patatin-like phospholipase domain-containing protein 3 (PNPLA3) gene is associated with high liver fat but normal insulin sensitivity. The underlying mechanism of the disassociation between high liver fat but normal insulin sensitivity remains obscure. We investigated the effect of I148M variant on hepatic lipidome of subjects with or without fatty liver, using the Lipidyzer method. Liver samples of four groups of subjects consisting of normal liver fat with wild-type PNPLA3 allele (group 1); normal liver fat with variant PNPLA3 allele (group 2); high liver fat with wild-type PNPLA3 allele (group 3); high liver fat with variant PNPLA3 allele (group 4); were analyzed. When high liver fat to normal liver fat groups were compared, wild-type carriers (group 3 vs. group 1) showed similar lipid changes compared to I148M PNPLA3 carriers (group 4 vs. group 2). On the other hand, in wild-type carriers, increased liver fat significantly elevated the proportion of specific DAGs (diacylglycerols), mostly DAG (FA18:1) which, however, remained unchanged in I148M PNPLA3 carriers. Since DAG (FA18:1) has been implicated in hepatic insulin resistance, the unaltered proportion of DAG (FA18:1) in I148M PNPLA3 carriers with fatty liver may explain the normal insulin sensitivity in these subjects.

The role of PNPLA3 in health and disease

The human patatin-like phospholipase domain-containing 3 (PNPLA3) gene encodes for a protein of 481 amino-acids. The variant rs738409 is a cytosine to guanine substitution, encoding for the isoleucine to methionine substitution at position 148 (I148M) of the protein. This variant is strongly associated with the entire spectrum of liver disease. Although this variant is one of the best characterized and deeply studied, the mechanism behind the PNPLA3 and the liver disease is still not well defined. Functionally, it has become clear that the PNPLA3 protein is an enzyme with lipase activity towards triglycerides and retinyl esters, and acyltransferase activity on phospholipids. The aim of this review is to collect the latest data, obtained by in vitro and in vivo experiments, on the functional aspects of the PNPLA3 protein. Defining the precise role of PNPLA3 in the liver lipid metabolism, in order to develop novel therapies for the treatment of liver disease, will be the key of future research.

Genetic and Epigenetic Profile of Patients With Alcoholic Liver Disease

Alcoholic liver disease (ALD) is a definition encompassing a spectrum of disorders ranging from simple steatosis to cirrhosis and hepatocellular carcinoma. Excessive alcohol consumption triggers a series of metabolic reactions that affect the liver by inducing lipogenesis, increasing oxidative stress, and causing abnormal inflammatory responses. The metabolic pathways regulating lipids, reactive oxygen species (ROS), and immune system are closely related and in some cases cross-regulate each other. Therefore, it must be taken into account that major genetic and epigenetic abnormalities affecting enzymes involved in one of such pathways can play a pivotal role in ALD pathogenesis. However, recent studies have pointed out how a significant predisposition can also be determined by minor variants, such as relatively common polymorphisms, epigenetic modifications, and microRNA abnormalities. Genetic and epigenetic factors can also affect the progression of liver diseases, promoting fibrogenesis, cirrhosis, and ultimately hepatocellular carcinoma. It is noteworthy that some of these factors, such as some of the cytokines involved in the abnormal inflammatory responses, are shared with non-alcoholic liver disease, while other factors are unique to ALD. The study of the genetic and epigenetic components involved in the liver damages caused by alcohol is crucial to identify individuals with high risk of developing ALD, design personalized protocols for prevention and/or treatment, and select the best molecular targets for new therapies.

Risk Factors for the Development of Nonalcoholic Fatty Liver Disease/Nonalcoholic Steatohepatitis, Including Genetics

Nonalcoholic fatty liver disease is emerging as the most common cause of chronic liver disease worldwide. This trend is, in part, secondary, to the growing incidence of obesity, type 2 diabetes, and metabolic syndrome. Other risk factors include age, gender, race/ethnicity, genetic predisposition, and polycystic ovarian disease. With the introduction of genome-wide association studies, genetic mutations contributing to inherited susceptibility to steatosis have been identified, which hold keys to future improvement in diagnosis and management. This article expands on the aforementioned risk factors and summarizes the current available data on genetic and environmental factors associated with this common entity.

Nonalcoholic Fatty Liver Disease as a Nexus of Metabolic and Hepatic Diseases

NAFLD is closely linked with hepatic insulin resistance. Accumulation of hepatic diacylglycerol activates PKC-ε, impairing insulin receptor activation and insulin-stimulated glycogen synthesis. Peripheral insulin resistance indirectly influences hepatic glucose and lipid metabolism by increasing flux of substrates that promote lipogenesis (glucose and fatty acids) and gluconeogenesis (glycerol and fatty acid-derived acetyl-CoA, an allosteric activator of pyruvate carboxylase). Weight loss with diet or bariatric surgery effectively treats NAFLD, but drugs specifically approved for NAFLD are not available. Some new pharmacological strategies act broadly to alter energy balance or influence pathways that contribute to NAFLD (e.g., agonists for PPAR γ, PPAR α/δ, FXR and analogs for FGF-21, and GLP-1). Others specifically inhibit key enzymes involved in lipid synthesis (e.g., mitochondrial pyruvate carrier, acetyl-CoA carboxylase, stearoyl-CoA desaturase, and monoacyl- and diacyl-glycerol transferases). Finally, a novel class of liver-targeted mitochondrial uncoupling agents increases hepatocellular energy expenditure, reversing the metabolic and hepatic complications of NAFLD.

Suppressed Fat Mobilization Due to PNPLA3 rs738409 -Associated Liver Damage in Heavy Drinkers: The Liver Damage Feedback Hypothesis

PNPLA3 variant rs738409 has been identified as important progression factor in patients with ALD and NAFLD, the most common liver diseases worldwide. These findings point towards similarities between metabolism of alcohol and fat with regard to the PNPLA3 gene. However, despite many efforts, neither the mechanisms of PNPLA3-related liver damage nor the physiological role of PNPLA3 are fully understood. Based on a large monocentric cohort of Caucasian heavy drinkers we could recently provide evidence that PNPLA3 GG primarily correlated with signs of liver damage (steatohepatitis, ballooning) but less with steatosis. Moreover, upon alcohol withdrawal, PNPLA3 GG carriers showed a delayed inflammation-associated resolution of liver stiffness. In line with the histological findings, hepatic fat content as quantified by CAP (controlled attenuation parameter) did not depend on PNPLA3 status and decreased equally in all genotypes by ca. 30 dB/m during alcohol withdrawal. Preliminary additional analysis from this large cohort indicates that PNPLA3 GG carriers (8.2%) drink significantly less high percentage beverages (23% vs 55%, p < 0.001) but show no metabolic phenotype such as increased weight, BMI or diabetes. On the molecular level, key molecules, important for lipolysis and flow of free fatty acids to the liver were drastically reduced in G carriers. These included the liver-synthesized serum ApoA1, the LD-associated protein perilipin5 and the recently identified hepato-protective transcriptional cofactor transducin beta-like-related 1 (TBLR1). Based on these findings, we here introduce the liver damage feedback hypothesis. Accordingly, PNPLA3-mediated liver damage (e.g. by enhanced metabolic activity) suppresses the mobilization of fat towards the liver at various levels (reduced serum lipid flux to the liver and fat mobilization from peripheric adipose tissues, suppressed hepatocyte fat release and avoidance of high percentage alcohol beverages). Finally, the liver damage feedback hypothesis identifies a novel and central role of liver damage on systemic fat homeostasis, which has not been appreciated so far.

Roles of Diacylglycerols and Ceramides in Hepatic Insulin Resistance

Although ample evidence links hepatic lipid accumulation with hepatic insulin resistance, the mechanistic basis of this association is incompletely understood and controversial. Diacylglycerols (DAGs) and ceramides have emerged as the two best-studied putative mediators of lipid-induced hepatic insulin resistance. Both lipids were first associated with insulin resistance in skeletal muscle and were subsequently hypothesized to mediate insulin resistance in the liver. However, the putative roles for DAGs and ceramides in hepatic insulin resistance have proved more complex than originally imagined, with various genetic and pharmacologic manipulations yielding a vast and occasionally contradictory trove of data to sort. In this review we examine the state of this field, turning a critical eye toward both DAGs and ceramides as putative mediators of lipid-induced hepatic insulin resistance.

Binge alcohol alters PNPLA3 levels in liver through epigenetic mechanism involving histone H3 acetylation

The human PNPLA3 (patatin-like phospholipase domain-containing 3) gene codes for a protein which is highly expressed in adipose tissue and liver, and is implicated in lipid homeostasis. While PNPLA3 protein contains regions homologous to functional lipolytic proteins, the regulation of its tissue expression is reflective of lipogenic genes. A naturally occurring genetic variant of PNPLA3 in humans has been linked to increased susceptibility to alcoholic liver disease. We have examined the modulatory effect of alcohol on PNPLA3 protein and mRNA expression as well as the association of its gene promoter with acetylated histone H3K9 by chromatin immunoprecipitation (ChIP) assay in rat hepatocytes in vitro, and in vivo in mouse and rat models of acute binge, chronic, and chronic followed by acute binge ethanol administration. Protein expression of PNPLA3 was significantly increased by alcohol in all three models used. PNPLA3 mRNA also increased, albeit to a varying degree. ChIP assay using H3AcK9 antibody showed increased association with the promoter of PNPLA3 in hepatocytes and in mouse liver. This was less evident in rat livers in vivo except under chronic treatment. It is concluded for the first time that histone acetylation plays a role in the modulation of PNPLA3 levels in the liver exposed to binge ethanol both in vitro and in vivo.

The association of nonalcoholic fatty liver disease with genetic polymorphisms: a multicenter study

INTRODUCTION

Growing evidence suggests that multiple factors, such as insulin resistance, nutritional factors, gut microbiota, and hormones released from the adipose tissue, act together on genetically predisposed individuals. We aimed to investigate whether various single-nucleotide polymorphisms (SNPs) play a role in the development of nonalcoholic fatty liver disease (NAFLD) and severity of liver damage in the Anatolian population.

METHODS

Two hundred and sixteen patients with biopsy-proven NAFLD and 150 control participants, aged 18-70 years, were consecutively enrolled in this multicenter study. Blood samples were genotyped for the PNPLA3 (rs738409), IL28B (rs12979860, rs12980275, rs8099917), PPAR-α 227 ALA, PPAR-γ pro 12 ALA, SOD2 C47T, and LOX-1 IVS4-14 polymorphisms using the custom-made LightSNiP assays on a LightCycler 480 II instrument.

RESULTS

Genotypic distributions of PNPLA3 rs738409 SNPs were different between NAFLD and control participants, but not for other SNPs. The PNPLA3 rs738409 GG polymorphism was associated with a 27-fold increased risk of development of NAFLD (odds ratio=27.8, 95% confidence interval: 3.5-218.4; P=0.002). Patients with the PNPLA3 GG genotype had higher nonalcoholic fatty liver disease activity score levels compared with patients with the PNPLA3 CC genotype (P<0.005). NAFLD patients without fibrosis had a higher frequency of IL28B rs12979860 TT and rs12980275 GG genotypes compared with NAFLD patients with fibrosis (P<0.005).

CONCLUSION

The present study proposes that polymorphisms in the PNPLA3 gene have highly predictive value in the development of NAFLD and are independently associated with the severity of liver histology in patients with NAFLD. The results of this study suggest that IL28B rs12979860 TT or rs12980275 GG may play an important protective role against the development of advanced fibrosis and even cirrhosis.

Weight loss induced by bariatric surgery restores adipose tissue PNPLA3 expression

BACKGROUND & AIMS

Obesity and its related co-morbidities such as non-alcoholic fatty liver disease (NAFLD) are increasing dramatically worldwide. The genetic variation in Patatin-like phospholipase domain-containing protein 3 (PNPLA3), which is also called adiponutrin (ADPN), in residue 148 (I148M, rs738409) has been associated with NAFLD. However, the regulation and function of PNPLA3 in metabolic diseases remains unclear. Laparoscopic gastric banding (LAGB) of severely obese patients reduces body weight, liver and adipose tissue inflammation. In this study, we investigated whether weight loss induced by LAGB affected PNPLA3 expression in hepatic and adipose tissue.

METHODS

Liver and subcutaneous adipose tissue samples were collected from 28 severely obese patients before and 6 months after LAGB. PNPLA3 expression was assessed by quantitative real-time PCR. To understand whether inflammatory stimuli regulated PNPLA3 expression, we studied the effect of tumour necrosis factor alpha (TNFα) and lipopolysaccharide (LPS) on PNPLA3 expression in human adipocytes and hepatocytes.

RESULTS

PNPLA3 was strongly expressed in the liver and clearly detectable in subcutaneous adipose tissue of obese patients. Weight loss induced by LAGB of severely obese patients led to significantly increased adipose, but not hepatic, tissue expression of PNPLA3. Subcutaneous PNPLA3 expression negatively correlated with body-mass-index, fasting glucose and fasting insulin. TNFα potently suppressed PNPLA3 expression in adipocytes but not hepatocytes.

CONCLUSIONS

Weight loss induced by LAGB restored adipose tissue PNPLA3 expression which is suppressed by TNFα. Further studies will be required to determine the functional impact of PNPLA3 and its related genetic variation on adipose tissue inflammation and NAFLD.

Clinical and Metabolic Characterization of Lean Caucasian Subjects With Non-alcoholic Fatty Liver

OBJECTIVES

Non-alcoholic fatty liver disease (NAFLD) is closely linked to obesity; however, 5-8% of lean subjects also have evidence of NAFLD. We aimed to investigate clinical, genetic, metabolic and lifestyle characteristics in lean Caucasian subjects with NAFLD.

METHODS

Data from 187 subjects allocated to one of the three groups according to body mass index (BMI) and hepatic steatosis on ultrasound were obtained: lean healthy (BMI≤25 kg/m², no steatosis, N=71), lean NAFLD (BMI≤25 kg/m², steatosis, N=55), obese NAFLD (BMI≥30 kg/m², steatosis; N=61). All subjects received a detailed clinical and laboratory examination including oral glucose tolerance test. The serum metabolome was assessed using the Metabolomics AbsoluteIDQ p180 kit (BIOCRATES Life Sciences). Genotyping for single-nucleotide polymorphisms (SNPs) associated with NAFLD was performed.

RESULTS

Lean NAFLD subjects had fasting insulin concentrations similar to lean healthy subjects but had markedly impaired glucose tolerance. Lean NAFLD subjects had a higher rate of the mutant PNPLA3 CG/GG variant compared to lean controls (P=0.007). Serum adiponectin concentrations were decreased in both NAFLD groups compared to controls (P<0.001 for both groups) The metabolomics study revealed a potential role for various lysophosphatidylcholines (lyso-PC C18:0, lyso-PC C17:0) and phosphatidylcholines (PCaa C36:3; false discovery rate (FDR)-corrected P-value<0.001) as well as lysine, tyrosine, and valine (FDR<0.001).

CONCLUSIONS

Lean subjects with evidence of NAFLD have clinically relevant impaired glucose tolerance, low adiponectin concentrations and a distinct metabolite profile with an increased rate of PNPLA3 risk allele carriage.

The role of mitochondrial genomics in patients with non-alcoholic steatohepatitis (NASH)

Background

Visceral obesity and metabolic syndrome are commonly associated with non-alcoholic fatty liver disease (NAFLD). The progression of steatosis to NASH depends on a number of metabolic and patient-related factors. The mechanisms of genetic predisposition towards the development of NASH and related fibrosis remain unclear. In this study, our aim was to utilize mitotyping and identify mitochondrial haplotypes that may be associated with NAFLD.

Methods

We examined mitochondrial haplotypes along with patatin-like phospholipase domain containing 3 (PNPLA3) rs738409 genotype to determine their association with NAFLD phenotypes. Whole blood samples were obtained from 341 patients (BMI > 35) undergoing weight reduction surgery after written consent. Liver biopsies were centrally reviewed by a single pathologist based on predetermined pathologic protocol (41.9 % Non-NASH NAFLD, 30.4 % NASH, 27.5 % controls). A 1,122 bp of the mitochondrial control loop was sequenced for each sample and classified into haplogroups.

Results

The presence of haplogroup L exhibits protection against the development of NASH and pericellular fibrosis. The alleles of PNPLA3 locus showed differential distribution in cohorts with NAFLD, NASH and pericellular fibrosis. Heterozygosity at this locus is independently associated with higher risk of having NASH and pericellular fibrosis.

Conclusion

Mitochondrial genetics play an important role in NASH probably by modulation of oxidative stress and the efficiency of oxidative phosphorylation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12881-016-0324-0) contains supplementary material, which is available to authorized users.

Untargeted Metabolomics Reveals Intervention Effects of Total Turmeric Extract in a Rat Model of Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is one of the most common forms of chronic liver disease. Currently, there are no recognized medical therapies effective for NAFLD. Previous studies have demonstrated the effects of total turmeric extract on rats with NAFLD induced by high-fat diet. In this study, serum metabolomics was employed using UHPLC-Q-TOF-MS to elucidate the underlying mechanisms of HFD-induced NAFLD and the therapeutic effects of TE. Supervised orthogonal partial least-squares-discriminant analysis was used to discover differentiating metabolites, and pathway enrichment analysis suggested that TE had powerful combined effects of regulating lipid metabolism by affecting glycerophospholipid metabolism, glycerolipid metabolism, and steroid hormone biosynthesis signaling pathways. In addition, the significant changes in glycerophospholipid metabolism proteins also indicated that glycerophospholipid metabolism might be involved in the therapeutic effect of TE on NAFLD. Our findings not only supply systematic insight into the mechanisms of NAFLD but also provide a theoretical basis for the prevention or treatment of NAFLD.

Influence of the rs738409 polymorphism in patatin-like phospholipase 3 on the treatment efficacy of non-alcoholic fatty liver disease with type 2 diabetes mellitus

AIM

A genome-wide association study revealed that the single nucleotide polymorphism (SNP) rs738409 in the patatin-like phospholipase 3 gene (PNPLA3) was strongly associated with non-alcoholic fatty liver disease (NAFLD). Recent pilot studies investigated the effects of dipeptidyl peptidase-4 inhibitors on liver function and glucose metabolism in NAFLD with type 2 diabetes mellitus (DM). We herein evaluated the efficacy of alogliptin in NAFLD patients with type 2 DM as well as the relationship between genotypes at rs738409 in PNPLA3 and treatment efficacy.

METHODS

Forty-one biopsy-proven NAFLD patients with type 2 DM treated with 25 mg/day alogliptin were retrospectively enrolled. SNP rs738409 in PNPLA3 was present in all patients. Clinical data were measured before and after the treatment.

RESULTS

Average hemoglobin A1c (HbA1c) levels mostly remained unchanged. Moreover, significant changes were not noted in the levels of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) during the follow-up period. A positive correlation was observed between improvements in HbA1c (ΔHbA1c) levels and changes in AST (ΔAST) and ALT (ΔALT) levels (r = 0.325 and 0.439, respectively). Patients with the risk allele (G-allele) showed more positive correlation between ΔHbA1c and changes in transaminase. Furthermore, improvements in the levels of total cholesterol, triglycerides and hyaluronic acid were significantly greater in G-allele patients in the weight loss group.

CONCLUSION

The treatment of NAFLD with type 2 DM with alogliptin contributed to the amelioration of NAFLD. Our results suggested that differences in the PNPLA3 risk allele affected the therapeutic effects of this treatment.

The rs2294918 E434K variant modulates patatin-like phospholipase domain-containing 3 expression and liver damage

The patatin-like phosholipase domain-containing 3 (PNPLA3) rs738409 polymorphism (I148M) is a major determinant of hepatic fat and predisposes to the full spectrum of liver damage in nonalcoholic fatty liver disease (NAFLD). The aim of this study was to evaluate whether additional PNPLA3 coding variants contribute to NAFLD susceptibility, first in individuals with contrasting phenotypes (with early-onset NAFLD vs. very low aminotransferases) and then in a large validation cohort. Rare PNPLA3 variants were not detected by sequencing coding regions and intron-exon boundaries either in 142 patients with early-onset NAFLD nor in 100 healthy individuals with alanine aminotransferase <22/20 IU/mL. Besides rs738409 I148M, the rs2294918 G>A polymorphism (E434K sequence variant) was over-represented in NAFLD (adjusted P = 0.01). In 1,447 subjects with and without NAFLD, the 148M-434E (P < 0.0001), but not the 148M-434K, haplotype (P > 0.9), was associated with histological NAFLD and steatohepatitis. Both the I148M (P = 0.0002) and E434K variants (P = 0.044) were associated with serum ALT levels, by interacting with each other, in that the 434K hampered the association with liver damage of the 148M allele (P = 0.006). The E434K variant did not affect PNPLA3 enzymatic activity, but carriers of the rs2294918 A allele (434K) displayed lower hepatic PNPLA3 messenger RNA and protein levels (P < 0.05).

CONCLUSIONS

Rare loss-of-function PNPLA3 variants were not detected in early-onset NAFLD. However, PNPLA3 rs2294918 E434K decreased PNPLA3 expression, lessening the effect of the I148M variant on the predisposition to steatosis and liver damage. This suggests that the PNPLA3 I148M variant has a codominant negative effect on triglycerides mobilization from lipid droplets, mediated by inhibition of other lipases.