Adiponutrin: A new gene regulated by energy balance in human adipose tissue

A publication-wide association study (PWAS), historical language models to prioritise novel therapeutic drug targets

Most biomedical knowledge is published as text, making it challenging to analyse using traditional statistical methods. In contrast, machine-interpretable data primarily comes from structured property databases, which represent only a fraction of the knowledge present in the biomedical literature. Crucial insights and inferences can be drawn from these publications by the scientific community. We trained language models on literature from different time periods to evaluate their ranking of prospective gene-disease associations and protein-protein interactions. Using 28 distinct historical text corpora of abstracts published between 1995 and 2022, we trained independent Word2Vec models to prioritise associations that were likely to be reported in future years. This study demonstrates that biomedical knowledge can be encoded as word embeddings without the need for human labelling or supervision. Language models effectively capture drug discovery concepts such as clinical tractability, disease associations, and biochemical pathways. Additionally, these models can prioritise hypotheses years before their initial reporting. Our findings underscore the potential for extracting yet-to-be-discovered relationships through data-driven approaches, leading to generalised biomedical literature mining for potential therapeutic drug targets. The Publication-Wide Association Study (PWAS) enables the prioritisation of under-explored targets and provides a scalable system for accelerating early-stage target ranking, irrespective of the specific disease of interest.

Synergistic and Detrimental Effects of Alcohol Intake on Progression of Liver Steatosis

Nonalcoholic fatty liver disease (NAFLD) and alcoholic liver disease (ALD) are the most common liver disorders worldwide and the major causes of non-viral liver cirrhosis in the general population. In NAFLD, metabolic abnormalities, obesity, and metabolic syndrome are the driving factors for liver damage with no or minimal alcohol consumption. ALD refers to liver damage caused by excess alcohol intake in individuals drinking more than 5 to 10 daily units for years. Although NAFLD and ALD are nosologically considered two distinct entities, they show a continuum and exert synergistic effects on the progression toward liver cirrhosis. The current view is that low alcohol use might also increase the risk of advanced clinical liver disease in NAFLD, whereas metabolic factors increase the risk of cirrhosis among alcohol risk drinkers. Therefore, special interest is now addressed to individuals with metabolic abnormalities who consume small amounts of alcohol or who binge drink, for the role of light-to-moderate alcohol use in fibrosis progression and clinical severity of the liver disease. Evidence shows that in the presence of NAFLD, there is no liver-safe limit of alcohol intake. We discuss the epidemiological and clinical features of NAFLD/ALD, aspects of alcohol metabolism, and mechanisms of damage concerning steatosis, fibrosis, cumulative effects, and deleterious consequences which include hepatocellular carcinoma.

PNPLA3 I148M is involved in the variability in anti-NAFLD response to exenatide

PURPOSE

GLP-1 receptor agonists, such as exenatide, have been proven to attenuate nonalcoholic fatty liver disease (NAFLD) in vivo and in vitro. However, the efficiency of exenatide had interindividual differences. PNPLA3 is a major susceptibility gene for NAFLD and its I148M polymorphism increases the risk of all disorders of the NAFLD spectrum. Whether this variant contributes to variability in exenatide response is still unclear.

METHODS

PNPLA3 148I knockin HepG2 cells were constructed using the Cas9/sgRNA system. Oil Red O staining combined with TG quantification was used to evaluate lipid accumulation. Western blotting and qRT-qPCR were conducted, respectively, to measure the protein and mRNA expression of lipid metabolic and endoplasmic reticulum (ER) stress-related inflammatory markers. PNPLA3 I148M was genotyped in type 2 diabetics using Sanger sequencing. The exenatide-induced changes in liver fat content and other clinical parameters were compared between PNPLA3 I148M genotypes.

RESULTS

Lipid deposition increased in both PNPLA3 148I/I and 148M/M HepG2 cells treated with palmitoleic acid, while cells with 148M/M had a higher TG content than those with 148I/I. Exendin-4 treatment was showed to be more significant in 148I/I cells than in 148M/M cells in terms of reducing the intrahepatic fat content, inhibiting SREBP-1c and ER stress-related inflammation, and activating AMPK-ACC lipid oxidation pathway. In patients with type 2 diabetes, 24-week treatment with exenatide improved liver fat content in patients carrying PNPLA3 148I/I better than in patients with 148M/M.

CONCLUSIONS

PNPLA3 I148M might modify the anti-NAFLD response to exenatide.

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.

The effect of PNPLA3 polymorphism as gain in function mutation in the pathogenesis of non-alcoholic fatty liver disease

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is often associated with metabolic syndrome (type 2 diabetes, hypertension, hypertriglyceridemia, insulin resistance, and obesity). NAFLD is multi-factorial in pathogenesis with some genetic predisposition. The variant patatin-like phospholipase domain-containing protein 3 (PNPLA3) is known to be an independent risk factor for hepatocellular cancer (HCC). The aim of this study was to investigate the role of PNPLA3 polymorphism as the risk factor for NAFLD.

METHODOLOGY

Patients had histological, ultrasonographic, biopsy evidence of NAFLD (n=248) and 81 controls were studied for PNPLA3 polymorphism. PNPLA3 genotyping was done from peripheral blood DNA by real-time polymerase chain reaction (RT-PCR).

RESULTS

PNPLA3 genotyping of the groups NAFLD (CC [n = 76], CG [n = 83], GG [n = 89]) and control (CC [n= 42], CG [n = 22], GG [n = 17]) was determined. In the patient group, the G allele was 261 (52.63%) and the C allele was 235 (47.37%), whereas in the control group, the G allele was 56 (34.54%) and the C allele was 106 (65.43%). In our study, 53 out of 174 women had GG allele and 54 out of 155 men had GG allele.

CONCLUSION

The findings suggest that there is a predominant relationship between men with PNPLA3 I148M variant with NAFLD in women. Patients with NAFLD carrying PNPLA3 rs738409 G>C variant are at higher risk of NAFLD.

New Perspectives on Genetic Prediction for Pediatric Metabolic Associated Fatty Liver Disease

Non-alcoholic or recently re-defined metabolic associated fatty liver disease (MAFLD), a spectrum of progressive hepatic disease, has become a public health issue in obese children and adolescents. MAFLD is a complex metabolic disease strongly associated with obesity and insulin resistance. It is not known why not every obese subject will develop MAFLD. Different ethnic/racial groups display differences in MAFLD prevalence, indicating genetic factor plays a role. In the past two decades, sequence variations in genetic loci, including PNPLA3, TM6SF2, GCKR, MBOAT7, HSD17B13, etc. have been shown to confer susceptibility to MAFLD in children and adults. This review article provides an updated viewpoint of genetic predictors related to pediatric MAFLD. We discuss whether these susceptible genes can be clinically used for risk stratification and personalized care. Understanding human genetics and molecular mechanisms can give important information not only for prediction of risk but also on how to design drugs. In view of current epidemic of MAFLD worldwide, it is necessary to identify which children with MAFLD progress rapidly and need earlier intervention. In the future, a comprehensive analysis of individualized genetic and environmental factors may help assess the risk of children with MAFLD and personalize their treatment.

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.

Association of PNPLA3 rs738409 polymorphism with liver steatosis but not with cirrhosis in patients with HBV infection: Systematic review with meta-analysis

BACKGROUND

Hepatitis B virus (HBV) infection is a worldwide health issue and is well known for being the main cause of developing secondary liver complications such as cirrhosis and hepatocellular carcinoma (HCC). The PNPLA3 rs738409 polymorphism has been investigated conclusively with occurrence risk of steatosis and cirrhosis. Therefore, performing a meta-analysis of the available studies with the aim of clarifying the association between rs738409 and occurrence risk of steatosis and cirrhosis among HBV-infected patients would be helpful.

METHODS

Chronic HBV infection was defined as the persistence of HBsAg for more than 6 months. To gather sufficient data for this meta-analysis, reliable databases were conclusively searched using appropriate keywords. Only studies that satisfied the inclusion criteria were enrolled in the present study.

RESULTS

This meta-analysis pooled four studies with 1135 cases of chronic hepatitis B (CHB) to evaluate the impact of PNPLA3 SNP on liver steatosis and also pooled five studies with 3713 cases of CHB to evaluate the impact of PNPLA3 SNP on cirrhosis. The association of rs738409 with each complication was investigated. The rs738409 was found to be associated with steatosis in recessive [p = 4.57 × 10^-6 , odds ratio (OR) = 2.85], dominant (p = 4.35 × 10^-6 , OR = 1.84), co-dominant (p = 6.18 × 10^-8 ; OR = 3.74) and allelic (p = 9.79 × 10^-9 ; OR = 1.78) models. No association was found between rs738409 and cirrhosis development in recessive (p = 0.99, OR = 1.00), dominant (p = 0.30, OR = 0.92), co-dominant (p = 0.74; OR = 0.96) and allelic (p = 0.45; OR = 0.96) models.

CONCLUSIONS

Although the PNPLA3 rs738409 G allele has been associated with the risk of steatosis in CHB patients, no association between this polymorphism and the risk of cirrhosis was seen.

Body condition of late pregnant gilts affects the expression of selected adipokines and their receptors in mammary fat and backfat tissues

The impact of body condition in late gestating gilts on gene expression of selected adipokines and their receptors in backfat and mammary fat tissues was studied. The presence of associations between mammary gland composition variables and the mRNA abundance of selected genes and serum concentrations of adiponectin and leptin was also investigated. A total of 45 gilts were selected at mating based on their backfat depth and were allocated to three groups: (1) low backfat (LBF; 12-15 mm; n = 14), (2) medium backfat (MBF; 17-19 mm; n = 15), and (3) high backfat (HBF; 22-26 mm; n = 16). Gilts were fed different amounts of a conventional diet to maintain differences in backfat depth throughout the gestation period. Blood samples were collected at day 109 of gestation to measure adiponectin and leptin serum concentrations. Gilts were slaughtered on day 110 of gestation, and mammary glands were collected to determine mammary composition. Mammary fat and backfat tissues were also sampled to measure the mRNA abundance of selected genes. In mammary fat tissue, there was an effect of body condition on the prolactin (PRL; P = 0.01), adiponutrin (PNPLA3; P < 0.10), and prolactin receptor long form (PRLR-LF; P < 0.10) genes. There was a greater PRL mRNA abundance in mammary fat tissue from HBF than LBF or MBF gilts (P < 0.05). The PNPLA3 mRNA abundance was lower in HBF than in MBF gilts (P < 0.05), and that of PRLR-LF was lower in LBF than in HBF gilts (P < 0.05). In backfat, body condition affected the mRNA abundance of leptin (P < 0.05) and PNPLA3 (P < 0.01), with the greatest expression levels being observed in HBF gilts for both genes. Association analyses suggest a detrimental effect of high circulating leptin concentrations on gilts mammary development, as reflected by the negative correlations between serum leptin and protein percent (r = -0.66, P < 0.01), and concentrations of DNA (r = -0.62, P < 0.01) and RNA (r = -0.60, P < 0.01) in mammary parenchyma. Current results show that body condition of gilts at the end of gestation can affect the expression of adipokines in mammary fat and backfat tissues, with a different regulation of transcript abundance being observed in these two fat depots. Results also suggest that circulating leptin is strongly associated with mammary gland composition of late pregnant gilts, whereas locally synthesized leptin from mammary fat tissue is not.

Joint Profiling of miRNAs and mRNAs Reveals miRNA Mediated Gene Regulation in the Göttingen Minipig Obesity Model

Obesity and its comorbidities are an increasing challenge for both affected individuals and health care systems, worldwide. In obese individuals, perturbation of expression of both protein-coding genes and microRNAs (miRNA) are seen in obesity-relevant tissues (i.e. adipose tissue, liver and skeletal muscle). miRNAs are small non-coding RNA molecules which have important regulatory roles in a wide range of biological processes, including obesity. Rodents are widely used animal models for human diseases including obesity. However, not all research is applicable for human health or diseases. In contrast, pigs are emerging as an excellent animal model for obesity studies, due to their similarities in their metabolism, their digestive tract and their genetics, when compared to humans. The Göttingen minipig is a small sized easy-to-handle pig breed which has been extensively used for modeling human obesity, due to its capacity to develop severe obesity when fed ad libitum. The aim of this study was to identify differentially expressed of protein-coding genes and miRNAs in a Göttingen minipig obesity model. Liver, skeletal muscle and abdominal adipose tissue were sampled from 7 lean and 7 obese minipigs. Differential gene expression was investigated using high-throughput quantitative real-time PCR (qPCR) on 90 mRNAs and 72 miRNAs. The results revealed de-regulation of several obesity and inflammation-relevant protein-coding genes and miRNAs in all tissues examined. Many genes that are known to be de-regulated in obese humans were confirmed in the obese minipigs and several of these genes have target sites for miRNAs expressed in the opposing direction of the gene, confirming miRNA-mediated regulation in obesity. These results confirm the translational value of the pig for human obesity studies.

Adiponutrin (PNPLA3) in liver fibrogenesis: Is unaltered HepG2 cell line a better model system compared to murine models?

Alcoholic liver disease is a major source of morbidity and mortality worldwide. Twin studies had demonstrated heritability of alcoholic liver disease. Although to date only Adiponutrin (PNPLA3) rs738409 polymorphism (I148M) had been unequivocally proved to be associated with increased risk of alcoholic liver disease across different ethnicities. This protein was previously thought to have a predominant lipolytic role. However, recent investigations have provided evidence of lipogenic activity of this protein. The current hypothesis paper is summarizing the recent evidences gleaned in biological role of Adiponutrin and bioinformatic pointers towards a role in lipid trafficking. A critical appraisal of the utility of murine models and cell based systems in investigating Adiponutrin is also presented. As the HepG2 cell line harbors the I148M mutation in homozygous state it is hypothesized that this should represent an ideal model system for PNPLA3 biology. Thus, as Adiponutrin is proposed as having both lipolytic and lipogenic/lipid trafficking roles it is termed as a Yin-Yang protein in analogy to ancient Chinese wisdom.

Tetra-primer ARMS-PCR identified four pivotal genetic variations in bovine PNPLA3 gene and its expression patterns

Patatin-like phospholipase domain-containing protein 3 (PNPLA3), a member of the patatin like phospholipase domain-containing (PNPLA) family, plays an important role in energy balance, fat metabolism regulation, glucose metabolism and fatty liver disease. Tetra-primer amplification refractory mutation system PCR (T-ARMS-PCR) is a new method offering fast detection and extreme simplicity at a negligible cost for SNP genotyping. In this paper, we investigated the genetic variations at different ages of 660 Chinese indigenous cattle belonging to three breeds (QC, NY, JX) and applied T-ARMS-PCR and PCR-RFLP methods to genotype four SNPs, SNP1: g.A2980G, SNP2: g.A2996T, SNP3: g.A36718G, SNP4: g.G36850A. The statistical analyses indicated that these 4 SNPs affected growth traits markedly (P<0.05) in QC population, whereas combined haplotypes were not (P>0.05). The qPCR (quantitative PCR) indicated that bovine PNPLA3 gene was exclusively expressed in fat tissues. Besides, the analysis between SNP and mRNA expression revealed that, in SNP1, the expression of AG was much higher than AA and GG (P<0.05), which was in accordance with the results of growth traits association analysis, while the results of SNP4 was not. These results supported high potential that SNPs of bovine PNPLA3 gene might be utilized as genetic markers in marker-assisted selection (MAS) for Chinese cattle breeding programs.

The SRE Motif in the Human PNPLA3 Promoter (-97 to -88 bp) Mediates Transactivational Effects of SREBP-1c

Patatin-like phospholipase domain containing 3 (PNPLA3) is a non-secreted protein primarily expressed in liver and adipose tissue. Recently, numerous genetic studies have shown that PNPLA3 is a major susceptibility gene for nonalcoholic fatty liver disease (NAFLD). However, the mechanism involved in transcriptional regulation of the PNPLA3 gene remains unknown. We performed a detailed analysis of the human PNPLA3 gene promoter and identified two novel cis-acting elements (SRE and NFY binding motifs) located at -97/-88 and -26/-22 bp, respectively. Overexpression of SREBP-1c in HepG2 cells significantly increased PNPLA3 promoter activity. Mutation of either of the putative SRE or NFY binding motifs blocked the transactivation effects of SREBP-1c on the promoter. Overexpression of SREBP-1c and NFY together increased PNPLA3 promoter activity twice as much as that of SREBP-1c or NFY expression alone. This result suggests that SREBP-1c and NFY synergistically transactivate the human PNPLA3 gene. The ability of SREBP-1c and NFY to bind these cis-elements was confirmed using gel shift analysis. Putative SRE and NFY motifs also mediated synergistic insulin-induced transactivation of the PNPLA3 promoter in HepG2 cells. Additionally, the ability of SREBP-1c to bind to the PNPLA3 promoter was increased by insulin in a dose-dependent manner. Moreover, the treatment of HepG2 cells with the PI3K inhibitor LY294002 led to reduced insulin promoter-activating ability accompanied by a decrease in PNPLA3 and SREBP-1c protein expression. These results demonstrate that SREBP-1c is a direct activator of the human PNPLA3 gene and insulin transactivates the PNPLA3 gene via the PI3K-SREBP-1c/NFY pathway in HepG2 cells.

Steatosis in the liver

Accumulation of triacylglycerols within the cytoplasm of hepatocytes to the degree that lipid droplets are visible microscopically is called liver steatosis. Most commonly, it occurs when there is an imbalance between the delivery or synthesis of fatty acids in the liver and their disposal through oxidative pathways or secretion into the blood as a component of triacylglycerols in very low density lipoprotein. This disorder is called nonalcoholic fatty liver disease (NAFLD) in the absence of alcoholic abuse and viral hepatitis, and it is often associated with insulin resistance, obesity and type 2 diabetes. Also, liver steatosis can be induced by many other causes including excessive alcohol consumption, infection with genotype 3 hepatitis C virus and certain medications. Whereas hepatic triacylglycerol accumulation was once considered the ultimate effector of hepatic lipotoxicity, triacylglycerols per se are quite inert and do not induce insulin resistance or cellular injury. Rather, lipotoxic injury in the liver appears to be mediated by the global ongoing fatty acid enrichment in the liver, paralleling the development of insulin resistance. A considerable number of fatty acid metabolites may be responsible for hepatic lipotoxicity and liver injury. Additional key contributors include hepatic cytosolic lipases and the "lipophagy" of lipid droplets, as sources of hepatic fatty acids. The specific origin of the lipids, mainly triacylglycerols, accumulating in liver has been unraveled by recent kinetic studies, and identifying the origin of the accumulated triacylglycerols in the liver of patients with NAFLD may direct the prevention and treatment of this condition.

Progress in understanding association of PNPLA3 I148M (rs738409) single nucleotide polymorphism with hepatocellular carcinoma and hepatic cirrhosis

Patatin-like phospholipase domain containing 3 (PNPLA3), also called adiponutrin, is mainly expressed in the hepatocellular membrane and involved in lipid metabolism. The rs738409 genetic variant causes an isoleucine-to-methionine substitution at amino acid position 148 (I148M). Recently, genome-wide association studies have described associations of PNPLA3 I148M with plasma liver enzyme levels, steatosis and fibrosis severity. Studies found that PNPLA3 I148M is associated with progression of alcoholic liver cirrhosis, clinical outcome and prognosis of alcohol related hepatocellular carcinoma (HCC), and clinical outcomes of chronic hepatitis C. PNPLA3 I148M plays an important role in liver disease progression, which can be an independent risk factor for HCC.

Correlation between patatin-like phospholipase domain-containing protein 3 gene polymorphisms and liver cirrhosis in a chinese han population with chronic hepatitis B

BACKGROUND

A single nucleotide polymorphism (SNP) of patatin-like phospholipase domain-containing 3 (PNPLA3) genes (rs738409) is associated with the severity of fibrosis and cirrhosis in patients with fatty liver disease. However, in a small group of Italian patients, there was no significant correlation between the rs738409 SNP and hepatitis B virus (HBV) infection-associated liver cirrhosis.

OBJECTIVES

This study aimed to investigate whether PNPLA3 polymorphisms are a risk factor for liver cirrhosis in a Chinese Han population with chronic hepatitis B (CHB).

PATIENTS AND METHODS

The study population consisted of 344 Chinese Han patients with CHB, among which 203 presented with liver cirrhosis (LC group) and 141 had no sign of liver cirrhosis (CHB group). TaqMan genotyping assay was used to investigate the association of two PNPLA3 SNPs (rs738409 and rs2281135) with the risk of liver cirrhosis.

RESULTS

The allele and genotype distributions of PNPLA3 rs738409 and rs2281135 were not significantly different between the CHB and LC groups. After segregation on the basis of sex, no significant correlation between PNPLA3 (rs738409 and rs2281135) genotypes/alleles and liver cirrhosis was detected. Moreover, none of the haplotypes in PNPLA3 (rs738409 and rs2281135) was found to be statistically different between the two groups.

CONCLUSIONS

Our results showed no association between PNPLA3 polymorphisms (rs738409 and rs2281135) and the susceptibility to HBV-related liver cirrhosis in a Chinese Han population.

Race and the insulin resistance syndrome

Type 2 diabetes remains an important cause of morbidity and mortality. The metabolic syndrome affects 25% of the adult US population based on the Third Report of the Expert Panel on Detection, Evaluation and Treatment of High Blood Cholesterol in Adults from the National Cholesterol Education Program. Knowledge on the impact of obesity on metabolic health parameters has increased greatly over the past decade. This review discusses the limitations of the National Cholesterol Education Program metabolic syndrome definition and the racial disparities in the clinical presentation of the insulin resistance syndrome. We also examine the current literature with particular emphasis on albuminuria, nonalcoholic fatty liver disease, and intramyocellular lipid content. This review explores potential environmental and genetic reasons for differences in the manifestation of insulin resistance across racial/ethnic groups and highlights several promising areas for further study.

Hepatic patatin-like phospholipase domain-containing protein 3 sequence, single nucleotide polymorphism presence, protein confirmation, and responsiveness to energy balance in dairy cows

Patatin-like phospholipase domain-containing protein 3 (PNPLA3), commonly known as adiponutrin, is part of a novel subfamily of triglyceride lipase enzymes with potential effects on triglyceride metabolism in adipose and hepatic tissues. The predicted bovine PNPLA3 sequence has been identified, but expression of the gene had not been examined. The objectives of this study were to confirm the predicted bovine PNPLA3 gene sequence, determine expression of the bovine PNPLA3 gene in response to whole-animal energy balance, identify single nucleotide polymorphisms present in dairy cows, and verify the presence of the protein in the liver. Using liver biopsy samples collected from cows at +28d relative to calving (DRTC), RNA was isolated and used to generate a cDNA template for amplification of the entire predicted coding sequence of PNPLA3 via PCR. To determine if energy balance alters the expression of PNPLA3, RNA was isolated and mRNA expression quantified in liver samples from mid-lactation cows after a 5-d ad libitum period (n=5) and after a subsequent 5-d 50% feed restriction period (n=5), and in samples collected from cows at -14, +1, +14, and +28 DRTC (n=16). The presence of PNPLA3 protein was detected by Western blot in liver protein samples collected at +28 DRTC. Expression of hepatic PNPLA3 was decreased after a period of feed restriction (8.14 vs. 1.08±2.17 arbitrary units, ad libitum vs. fasted). Expression of PNPLA3 mRNA was decreased at +1 and +14 DRTC compared with -14 DRTC (23.35, 7.28, 10.17, and 14.5±4.9 arbitrary units, -14, +1, +14, and +28 DRTC, respectively). The presence of PNPLA3 protein was detected as a 55-kDa band in hepatic protein isolations from liver tissue collected at +28 DRTC. These data confirm the presence and sequence of the bovine hepatic PNPLA3 gene and single nucleotide polymorphisms. Furthermore, these data indicate responsiveness of bovine hepatic PNPLA3 to energy balance.

The PNPLA3 Ile148Met interacts with overweight and dietary intakes on fasting triglyceride levels

The Ile148Met (rs738409, G-allele) in the patatin-like phospholipase domain-containing protein 3 gene (PNPLA3) associates with liver fat content and may lead to loss-of-function (hydrolysis) or gain-of-function (CoA-dependent lysophosphatidic acid acyltransferase) defects. PNPLA3 is up-regulated by dietary carbohydrates, and interactions between rs738409 and carbohydrates, and sugar and ω6:ω3-polyunsaturated fatty acid (PUFA) ratio on hepatic fat accumulation have been reported. We examined interaction between rs738409 and overweight, and between rs738409 and dietary intakes (carbohydrates, sucrose and ω6:ω3-PUFA ratio), on fasting triglyceride levels. From the Malmo Diet and Cancer Study-Cardiovascular Cohort, 4,827 individuals without diabetes aged 58 ± 6 years, 2,346 with BMI ≤ 25 kg/m² and 2,478 with BMI > 25 kg/m², were included in cross-sectional analyses. Dietary data were collected by a modified diet history method. Overweight modified the association between rs738409 and fasting triglyceride levels (P_interaction = 0.003). G-allele associated with lower triglycerides only among overweight individuals (P = 0.01). Nominally, significant interaction on triglyceride levels was observed between rs738409 and sucrose among normal-weight individuals (P_interaction = 0.03). G-allele associated with lower triglycerides among overweight individuals in the lowest tertiles of carbohydrate and ω6:ω3-PUFA ratio (P = 0.04 and P = 0.001) and with higher triglycerides among normal-weight individuals in the highest tertile of sucrose (P = 0.001). We conclude that overweight and dietary sucrose may modify the association between rs738409 and fasting triglyceride levels.

Sexually dimorphic genome-wide binding of retinoid X receptor alpha (RXRα) determines male-female differences in the expression of hepatic lipid processing genes in mice

Many hepatic functions including lipid metabolism, drug metabolism, and inflammatory responses are regulated in a sex-specific manner due to distinct patterns of hepatic gene expression between males and females. Regulation for the majority of these genes is under control of Nuclear Receptors (NRs). Retinoid X Receptor alpha (RXRα) is an obligate partner for multiple NRs and considered a master regulator of hepatic gene expression, yet the full extent of RXRα chromatin binding in male and female livers is unclear. ChIP-Seq analysis of RXRα and RNA Polymerase2 (Pol2) binding was performed livers of both genders and combined with microarray analysis. Mice were gavage-fed with the RXR ligand LG268 for 5 days (30 mg/kg/day) and RXRα-binding and RNA levels were determined by ChIP-qPCR and qPCR, respectively. ChIP-Seq revealed 47,845 (male) and 46,877 (female) RXRα binding sites (BS), associated with ∼12,700 unique genes in livers of both genders, with 91% shared between sexes. RXRα-binding showed significant enrichment for 2227 and 1498 unique genes in male and female livers, respectively. Correlating RXRα binding strength with Pol2-binding revealed 44 genes being male-dominant and 43 female-dominant, many previously unknown to be sexually-dimorphic. Surprisingly, genes fundamental to lipid metabolism, including Scd1, Fasn, Elovl6, and Pnpla3-implicated in Fatty Liver Disease pathogenesis, were predominant in females. RXRα activation using LG268 confirmed RXRα-binding was 2-3 fold increased in female livers at multiple newly identified RXRα BS including for Pnpla3 and Elovl6, with corresponding ∼10-fold and ∼2-fold increases in Pnpla3 and Elovl6 RNA respectively in LG268-treated female livers, supporting a role for RXRα regulation of sexually-dimorphic responses for these genes. RXRα appears to be one of the most widely distributed transcriptional regulators in mouse liver and is engaged in determining sexually-dimorphic expression of key lipid-processing genes, suggesting novel gender- and gene-specific responses to NR-based treatments for lipid-related liver diseases.

PNPLA3 I148M variant and hepatocellular carcinoma: a common genetic variant for a rare disease

Hepatocellular carcinoma (HCC) is highly associated with chronic liver disease. The rs738409 genetic variant in the patatin-like phospholipase domain-containing 3 (PNPLA3, adiponutrin) gene has been implicated as a genetic determinant of the entire spectrum of liver diseases, ranging from steatosis, chronic hepatitis, cirrhosis and ultimately to HCC. In this review, first we will examine the current genetic theories of disease susceptibility. Next, we will analyze the evidences for the association between PNPLA3 I148M variant and HCC. Moreover, we will exploit this association to propose a new paradigm in human genetics: a common genetic variant contributing to a rare disease. Finally, we will examine the molecular genetics of PNPLA3 and, specifically, the theories that have been proposed to explain the function of PNPLA3 in health and disease.

A frequent PNPLA3 variant is a sex specific disease modifier in PSC patients with bile duct stenosis

Background & Aims

Primary sclerosing cholangitis predominantly affects males and is an important indication for liver transplantation. The rs738409 variant (I148M) of the PNPLA3 gene is associated with alcoholic and non-alcoholic liver disease and we evaluated its impact on the disease course of PSC.

Methods

The I148M polymorphism was genotyped in 121 German PSC patients of a long-term prospective cohort and 347 Norwegian PSC patients.

Results

In the prospective German cohort, actuarial survival free of liver transplantation was significantly reduced for I148M carriers (p = 0.011) compared to wildtype patients. This effect was restricted to patients with severe disease, as defined by development of dominant stenosis (DS) requiring endoscopic intervention. DS patients showed markedly decreased survival (p = 0.004) when carrying the I148M variant (I148M: mean 13.8 years; 95% confidence interval: 11.6–16.0 vs. wildtype: mean 18.6 years; 95% confidence interval: 16.3–20.9) while there was no impact on survival in patients without a DS (p = 0.87). In line with previous observations of sex specific effects of the I148M polymorphism, the effect on survival was further restricted to male patients (mean survival 11.9 years; 95% confidence interval: 10.0–14.0 in I148M carriers vs. 18.8 years; 95% confidence interval: 16.2–21.5 in wildtype; p<0.001) while female patients were unaffected by the polymorphism (p = 0.65). These sex specific findings were validated in the Norwegian cohort (p = 0.013).

Conclusions

In male PSC patients with severe disease with bile duct stenosis requiring intervention, the common I148M variant of the PNPLA3 gene is a risk factor for reduced survival.

Decreased transcription of ChREBP-α/β isoforms in abdominal subcutaneous adipose tissue of obese adolescents with prediabetes or early type 2 diabetes: associations with insulin resistance and hyperglycemia

Insulin resistance associated with altered fat partitioning in liver and adipose tissues is a prediabetic condition in obese adolescents. We investigated interactions between glucose tolerance, insulin sensitivity, and the expression of lipogenic genes in abdominal subcutaneous adipose and liver tissue in 53 obese adolescents. Based on their 2-h glucose tests they were stratified in the following groups: group 1, 2-h glucose level <120 mg/dL; group 2, 2-h glucose level between 120 and 140 mg/dL; and group 3, 2-h glucose level >140 mg/dL. Liver and adipose tissue insulin sensitivity were greater in group 1 than in group 2 and group 3, and muscle insulin sensitivity progressively decreased from group 1 to group 3. The expression of the carbohydrate-responsive element-binding protein (ChREBP) was decreased in adipose tissue but increased in the liver (eight subjects) in adolescents with impaired glucose tolerance or type 2 diabetes. The expression of adipose ChREBPα and ChREBPβ was inversely related to 2-h glucose level and positively correlated to insulin sensitivity. Improvement of glucose tolerance in four subjects was associated with an increase of ChREBP/GLUT4 expression in the adipose tissue. In conclusion, early in the development of prediabetes/type 2 diabetes in youth, ChREBPβ expression in adipose tissue predicts insulin resistance and, therefore, might play a role in the regulation of glucose tolerance.

PNPLA3, a genetic marker of progressive liver disease, still hiding its metabolic function?

Non-alcoholic fatty liver disease (NAFLD) is an emerging epidemic disease. It represents a large spectrum of liver diseases, and affects both adults and children. The etiology of NAFLD is multifactorial. Indeed, several events such as caloric imbalance including sedentary lifestyle, obesity and/or a predisposing genetic background are key players in the increasing risk for NAFLD development and its progression. Recently, a sequence variation within the gene encoding for patatin-like phospholipase containing 3 (PNPLA3, rs738409) was found to modulate steatosis, inflammation and fibrosis in NAFLD. It was also demonstrated as a novel genetic marker associated with progressive ALD (alcoholic liver disease). In this mini-review, we summarize the current knowledge on (i) PNPLA3 variant(s) in the pathogenesis of liver diseases, and (ii) PNPLA3 gene regulation and potential function in liver.

Genetic variants in PNPLA3 and risk of non-alcoholic fatty liver disease in a Han Chinese population

We investigated the possible association between genetic variants in the Patatin like phospholipase-3 (PNPLA3) gene and nonalcoholic fatty liver disease (NAFLD) in a Han Chinese population. We evaluated twelve tagging single-nucleotide polymorphisms (tSNPs) of the PNPLA3 gene in a frequency matched case–control study from Fuzhou city of China (553 cases, 553 controls). In the multivariate logistic regression analysis, the rs738409 GG or GC, and rs139051 TT genotypes were found to be associated with increased risk of NAFLD, and a significant trend of increased risk with increasing numbers of risk genotype was observed in the cumulative effect analysis of these single nucleotide polymorphisms. Furthermore, haplotype association analysis showed that, compared with the most common haplotype, the CAAGAATGCGTG and CGAAGGTGTCCG haplotypes conferred a statistically significant increased risk for NAFLD, while the CGGGAACCCGCG haplotype decreased the risk of NAFLD. Moreover, rs738409 C>G appeared to have a multiplicative joint effect with tea drinking (P<0.005) and an additive joint effect with obesity (Interaction contrast ratio (ICR) = 2.31, 95% CI: 0.7–8.86), hypertriglyceridemia (ICR = 3.07, 95% CI: 0.98–5.09) or hypertension (ICR = 1.74, 95% CI: 0.52–3.12). Our data suggests that PNPLA3 genetic polymorphisms might influence the susceptibility to NAFLD development independently or jointly in Han Chinese.

PNPLA3 polymorphisms and liver aminotransferase levels in a Mexican American population

PURPOSE

This study examined genetic associations of patatin-like phospholipase domain containing 3 gene (PNPLA3) polymorphisms and liver aminotransferases in an extensively documented, randomly recruited Mexican American population at high risk of liver disease.

METHODS

Two single nucleotide polymorphisms (SNP) in the PNPLA3 gene (i.e., rs738409 and rs2281135) were genotyped in 1532 individuals. Population stratification was corrected by the genotyping of 103 ancestry informative markers (AIMs) for Mexican Americans.

RESULTS

Both PNPLA3 SNPs showed highly significant association with alanine aminotransferase (ALT) levels, but was also, in males, associated with aspartate aminotransferase (AST) levels. Haplotypic association test of the two SNPs suggested stronger genetic association with rs738409 than rs2281135. Obvious sex effects were observed: rs738409-sex interaction in ALT levels P = 8.37 x 10(-4); rs738409-sex interaction in AST levels P = 5.03 x 10(-3).

CONCLUSIONS

This population study highlights a sex-specific association of PNPLA3 polymorphisms and elevated liver enzymes in a population-based study, independent of common pathological factors of the metabolic syndrome. The strong genetic association found in women ≤ 50 years old, but not in women > 50 years old, suggests that sex hormones may mediate the sex effect.

Adipose tissue gene expression analysis reveals changes in inflammatory, mitochondrial respiratory and lipid metabolic pathways in obese insulin-resistant subjects

Background

To get insight into molecular mechanisms underlying insulin resistance, we compared acute in vivo effects of insulin on adipose tissue transcriptional profiles between obese insulin-resistant and lean insulin-sensitive women.

Methods

Subcutaneous adipose tissue biopsies were obtained before and after 3 and 6 hours of intravenously maintained euglycemic hyperinsulinemia from 9 insulin-resistant and 11 insulin-sensitive females. Gene expression was measured using Affymetrix HG U133 Plus 2 microarrays and qRT-PCR. Microarray data and pathway analyses were performed with Chipster v1.4.2 and by using in-house developed nonparametric pathway analysis software.

Results

The most prominent difference in gene expression of the insulin-resistant group during hyperinsulinemia was reduced transcription of nuclear genes involved in mitochondrial respiration (mitochondrial respiratory chain, GO:0001934). Inflammatory pathways with complement components (inflammatory response, GO:0006954) and cytokines (chemotaxis, GO:0042330) were strongly up-regulated in insulin-resistant as compared to insulin-sensitive subjects both before and during hyperinsulinemia. Furthermore, differences were observed in genes contributing to fatty acid, cholesterol and triglyceride metabolism (FATP2, ELOVL6, PNPLA3, SREBF1) and in genes involved in regulating lipolysis (ANGPTL4) between the insulin-resistant and -sensitive subjects especially during hyperinsulinemia.

Conclusions

The major finding of this study was lower expression of mitochondrial respiratory pathway and defective induction of lipid metabolism pathways by insulin in insulin-resistant subjects. Moreover, the study reveals several novel genes whose aberrant regulation is associated with the obese insulin-resistant phenotype.

Cold exposure down-regulates adiponutrin/PNPLA3 mRNA expression and affects its nutritional regulation in adipose tissues of lean and obese Zucker rats

Adiponutrin/PNPLA3 is a protein highly produced in adipose tissue whose expression is under tight nutritional regulation. It possesses lipogenic/lipolytic capacity and, although adiponutrin polymorphisms are related to obesity, its physiological role is not clear. To help clarify its role, we studied the effect of acute cold exposure on adiponutrin mRNA expression in different adipose tissues of lean/obese Zucker rats subjected to feeding/fasting/refeeding. The effect of cold on the expression of key lipogenic enzymes and on uncoupling protein-1 (UCP1) was evaluated in selected adipose depots. Adiponutrin mRNA levels were also determined in the adipose tissue of isoprenaline-treated rats and in cultured adipocytes treated with noradrenaline, isoprenaline and a selective β3-adrenoceptor (AR) agonist. Adiponutrin expression was strongly down-regulated by cold in the different adipose depots in lean animals, while this down-regulation was impaired in obese rats. Adiponutrin pattern of expression in response to cold correlated positively with that of the lipogenic enzymes and negatively with UCP1 expression. Acute intraperitoneal administration of isoprenaline also produced a decrease in adiponutrin expression in adipose tissue. In vitro data suggest that adiponutrin's inhibitory effect could be mediated, at least in part, by the sympathetic system via β1/β2-AR. In addition, improvement in metabolic parameters related to obesity in cold-exposed animals was related to an improvement in adiponutrin nutritional regulation. Thus, cold inhibition of adiponutrin expression in adipose tissue (which correlates with the response of lipogenic enzymes) supports a physiological role for this protein in lipogenesis. Moreover, alterations in adiponutrin expression and regulation in adipose tissue are related to obesity.

Advances in understanding the role of PNPLA3 in the pathogenesis of non-alcoholic fatty liver disease

The incidence of non-alcoholic fatty liver disease (NAFLD) is rising worldwide. Investigation of genes involved in the pathogenesis of NAFLD is significant for replenishing treatment scheme and improving prognosis. Multiple studies have established a correlation between patatin-like phospholipase domain-containing 3 (PNPLA3) gene mutation and the pathogenesis of NAFLD, suggesting that PNPLA3 may affect lipid metabolism. However, the precise mechanism remains to be elucidated. Some researchers believed that PNPLA3 as a patatin-like protein might have triglyceride hydrolysis activity and therefore affect fat metabolism in the liver, while some others thought that PNPLA3 mutation might interfere with the lipid transfer process. In this paper, we give an overview of the PNPLA3 gene and its expression, and explore the correlation between PNPLA3 gene mutation and the pathogenesis of NAFLD.

A common variant in the PNPLA3 gene is a risk factor for non-alcoholic fatty liver disease in obese Taiwanese children

OBJECTIVE

To test the hypothesis that the presence of the PNPLA3 rs738409 G allele would increase the susceptibility of non-alcoholic fatty liver disease (NAFLD) in obese Taiwanese children.

STUDY DESIGN

A total of 520 obese children aged 6-18 years were recruited. Their PNPLA3 rs738409 genotypes-CC, CG, or GG-were detected by the 5'-nuclease assay. The effects of the PNPLA3 rs738409 G allele on pediatric NAFLD were evaluated based on liver ultrasonography findings and mean serum alanine aminotransferase levels in these children.

RESULTS

NAFLD was present in 19.6% of the obese children. In comparison to the subjects with CC alleles, the risk of NAFLD was increased by 2.96-fold (95% CI, 1.57 to 5.59, P = .0008) in the subjects with CG alleles and by 5.84-fold (95% CI, 2.59 to 13.16; P < .0001) in those with GG alleles. Variant PNPLA3 rs738409 genotypes were associated with increases in mean serum alanine aminotransferase level of 4.77 IU/L (P = .0435) in subjects with CG alleles and of 10.86 IU/L (P < .0001) in those with GG alleles compared with subjects with CC alleles.

CONCLUSIONS

The variant PNPLA3 rs738409 genotypes increased the risk of NAFLD in our population of obese Taiwanese children. The effect of the G allele on pediatric NAFLD followed a dominant genetic model.

Pnpla3/Adiponutrin deficiency in mice does not contribute to fatty liver disease or metabolic syndrome

PNPLA3 (adiponutrin, calcium-independent phospholipase A(2) epsilon [iPLA(2)ε]) is an adipose-enriched, nutritionally regulated protein that belongs to the patatin-like phospholipase domain containing (PNPLA) family of lipid metabolizing proteins. Genetic variations in the human PNPLA3 gene (i.e., the rs738409 I148M allele) has been strongly and repeatedly associated with fatty liver disease. Although human PNPLA3 has triacylglycerol (TAG) hydrolase and transacylase activities in vitro, its in vivo function and physiological relevance remain controversial. The objective of this study was to determine the metabolic consequences of global targeted deletion of the Pnpla3 gene in mice. We found that Pnpla3 mRNA expression is altered in adipose tissue and liver in response to acute and chronic nutritional challenges. However, global targeted deletion of the Pnpla3 gene in mice did not affect TAG hydrolysis, nor did it influence energy/glucose/lipid homoeostasis or hepatic steatosis/injury. Experimental interventions designed to increase Pnpla3 expression (refeeding, high-sucrose diet, diet-induced obesity, and liver X receptor agonism) likewise failed to reveal differences in the above-mentioned metabolic phenotypes. Expression of the Pnpla3 paralog, Pnpla5, was increased in adipose tissue but not in liver of Pnpla3-deficient mice, but compensatory regulation of genes involved in TAG metabolism was not identified. Together these data argue against a role for Pnpla3 loss-of-function in fatty liver disease or metabolic syndrome in mice.

Genetic determinants of hepatic steatosis in man

Hepatic steatosis is one of the most common liver disorders in the general population. The main cause of hepatic steatosis is nonalcoholic fatty liver disease (NAFLD), representing the hepatic component of the metabolic syndrome, which is characterized by type 2 diabetes, obesity, and dyslipidemia. Insulin resistance and excess adiposity are considered to play key roles in the pathogenesis of NAFLD. Although the risk factors for NAFLD are well established, the genetic basis of hepatic steatosis is largely unknown. Here we review recent progress on genomic variants and their association with hepatic steatosis and discuss the potential impact of these genetic studies on clinical practice. Identifying the genetic determinants of hepatic steatosis will lead to a better understanding of the pathogenesis and progression of NAFLD.

Recent progress in phospholipase A₂ research: from cells to animals to humans

Mammalian genomes encode genes for more than 30 phospholipase A₂s (PLA₂s) or related enzymes, which are subdivided into several classes including low-molecular-weight secreted PLA₂s (sPLA₂s), Ca²+-dependent cytosolic PLA₂s (cPLA₂s), Ca²+-independent PLA₂s (iPLA₂s), platelet-activating factor acetylhydrolases (PAF-AHs), lysosomal PLA₂s, and a recently identified adipose-specific PLA. Of these, the intracellular cPLA₂ and iPLA₂ families and the extracellular sPLA₂ family are recognized as the "big three". From a general viewpoint, cPLA₂α (the prototypic cPLA₂ plays a major role in the initiation of arachidonic acid metabolism, the iPLA₂ family contributes to membrane homeostasis and energy metabolism, and the sPLA₂ family affects various biological events by modulating the extracellular phospholipid milieus. The cPLA₂ family evolved along with eicosanoid receptors when vertebrates first appeared, whereas the diverse branching of the iPLA₂ and sPLA₂ families during earlier eukaryote development suggests that they play fundamental roles in life-related processes. During the past decade, data concerning the unexplored roles of various PLA₂ enzymes in pathophysiology have emerged on the basis of studies using knockout and transgenic mice, the use of specific inhibitors, and information obtained from analysis of human diseases caused by mutations in PLA₂ genes. This review focuses on current understanding of the emerging biological functions of PLA₂s and related enzymes.

Increased hepatic fat in overweight Hispanic youth influenced by interaction between genetic variation in PNPLA3 and high dietary carbohydrate and sugar consumption

BACKGROUND

Recently, a genetic variant (rs738409; C→G) of the PNPLA3 gene was identified to be associated with increased hepatic fat deposition, and the effect was more pronounced in Hispanics. Animal models have also shown that PNPLA3 expression can be regulated by dietary carbohydrate.

OBJECTIVE

The aim of this study was to examine whether the influence of PNPLA3 genotype on hepatic fat is modulated by dietary factors in Hispanic children.

DESIGN

PNPLA3 was genotyped in 153 Hispanic children (75% female, ages 8-18 y) by using the TaqMan method. Dietary intake was assessed by using three 24-h dietary recalls or diet records. Visceral adipose tissue (VAT), subcutaneous abdominal adipose tissue (SAAT), and hepatic fat fraction (HFF) were assessed in multiple abdominal slices by magnetic resonance imaging. Analysis of covariance was used to assess the diet × genotype interaction in liver fat, with the following a priori covariates: sex, age, energy, VAT, and SAAT.

RESULTS

HFF was influenced by a significant interaction between genotype and diet (genotype × carbohydrate, P = 0.04; genotype × total sugar, P = 0.01). HFF was positively related to carbohydrate (r = 0.31, P = 0.04) and total sugar (r = 0.34, P = 0.02) intakes but only in the GG group, independent of covariates. Dietary variables were not related to HFF in the CC or CG group or to other fat depots in all genotype groups.

CONCLUSIONS

These findings suggest that Hispanic children carrying the GG genotype are susceptible to increased hepatic fat when dietary carbohydrate intake, specifically sugar, is high. Specific dietary interventions based on genetic predisposition in this population may lead to more effective therapeutic outcomes for fatty liver. This trial was registered at clinicaltrials.gov as NCT00697580, 195-1642394A1, and NCT00693511.

The latest idea in NAFLD/NASH pathogenesis

Nonalcoholic fatty liver disease (NAFLD) is a metabolic disorder characterized by fatty accumulation in the liver without alcohol consumption. NAFLD ranges from simple steatosis to nonalcoholic steatohepatitis (NASH), which may progress to end-stage liver disease. The prevalence of NAFLD is rising because of an increasing prevalence of obesity and metabolic syndrome. The progression of these diseases is considered to be related to metabolic syndrome, which is characterized by obesity, glucose impairment, dyslipidemia, hypertension, and adipocytokine impairment. In addition, the pathogenesis of NAFLD/NASH is considered to be multifactorial and complex and is influenced by lifestyle habits, nutritional factors, and genetics. In particular, the PNPLA3 gene has been recently recognized as the most important functional gene polymorphism in the progression of NASH. Disruption in hepatic lipid metabolism is closely related to the development of fatty liver. Accumulation of excess triglycerides (TGs) induces hepatic steatosis. However, TG accumulation itself is not harmful to hepatocytes and may instead act as a protective mechanism against free fatty acid (FFA)-induced lipotoxicity. Excess FFAs also contribute to hepatotoxicity in NAFLD/NASH because oxidation of FFAs in hepatic microsomes generates excessive oxidative stress. Oxidative stress is considered one of the most important pathogenic factors in the development of NASH. Mitochondrial abnormalities, which are frequently observed in NASH-affected livers, are associated with impaired electron transport and result in further oxidative stress formation. The aims of this review are to assess the mechanisms of lipid metabolism and hepatic steatosis, the background of the disease, and the potential molecular mechanisms involved.

Pnpla3/Adiponutrin deficiency in mice does not contribute to fatty liver disease or metabolic syndrome

PNPLA3 (adiponutrin, calcium-independent phospholipase A(2) epsilon [iPLA(2)ε]) is an adipose-enriched, nutritionally regulated protein that belongs to the patatin-like phospholipase domain containing (PNPLA) family of lipid metabolizing proteins. Genetic variations in the human PNPLA3 gene (i.e., the rs738409 I148M allele) has been strongly and repeatedly associated with fatty liver disease. Although human PNPLA3 has triacylglycerol (TAG) hydrolase and transacylase activities in vitro, its in vivo function and physiological relevance remain controversial. The objective of this study was to determine the metabolic consequences of global targeted deletion of the Pnpla3 gene in mice. We found that Pnpla3 mRNA expression is altered in adipose tissue and liver in response to acute and chronic nutritional challenges. However, global targeted deletion of the Pnpla3 gene in mice did not affect TAG hydrolysis, nor did it influence energy/glucose/lipid homoeostasis or hepatic steatosis/injury. Experimental interventions designed to increase Pnpla3 expression (refeeding, high-sucrose diet, diet-induced obesity, and liver X receptor agonism) likewise failed to reveal differences in the above-mentioned metabolic phenotypes. Expression of the Pnpla3 paralog, Pnpla5, was increased in adipose tissue but not in liver of Pnpla3-deficient mice, but compensatory regulation of genes involved in TAG metabolism was not identified. Together these data argue against a role for Pnpla3 loss-of-function in fatty liver disease or metabolic syndrome in mice.

The association of genetic variability in patatin-like phospholipase domain-containing protein 3 (PNPLA3) with histological severity of nonalcoholic fatty liver disease

Genome-wide association studies identified single-nucleotide polymorphisms (SNPs) that are associated with increased hepatic fat or elevated liver enzymes, presumably reflecting nonalcoholic fatty liver disease (NAFLD). To investigate whether these SNPs are associated with histological severity of NAFLD, 1117 (894 adults/223 children) individuals enrolled in the Nonalcoholic Steatohepatitis (NASH) Clinical Research Network and National Institutes of Health Clinical Center studies with histologically confirmed NAFLD were genotyped for six SNPs that are associated with hepatic fat or liver enzymes in genome-wide association studies. In adults, three SNPs on chromosome 22 showed associations with histological parameters of NASH. After adjustment for age, sex, diabetes, and alcohol consumption, the minor allele of rs738409 C/G, a nonsynonymous coding SNP in the patatin-like phospholipase domain-containing protein 3 (PNPLA3) (adiponutrin) gene encoding an Ile148Met change, was associated with steatosis (P = 0.03), portal inflammation (P = 2.5 x 10(-4)), lobular inflammation (P = 0.005), Mallory-Denk bodies (P = 0.015), NAFLD activity score (NAS, P = 0.004), and fibrosis (P = 7.7 x 10(-6)). Two other SNPs in the same region demonstrated similar associations. Three SNPs on chromosome 10 near the CHUK (conserved helix-loop-helix ubiquitous kinase) gene were independently associated with fibrosis (P = 0.010). In children, no SNP was associated with histological severity. However, the rs738409 G allele was associated with younger age at the time of biopsy in multivariate analysis (P = 0.045).

CONCLUSION

In this large cohort of histologically proven NAFLD, we confirm the association of the rs738409 G allele with steatosis and describe its association with histological severity. In pediatric patients, the high-risk rs738409 G allele is associated with an earlier presentation of disease. We also describe a hitherto unknown association between SNPs at a chromosome 10 locus and the severity of NASH fibrosis.

A common variant in the patatin-like phospholipase 3 gene (PNPLA3) is associated with fatty liver disease in obese children and adolescents

The genetic factors associated with susceptibility to nonalcoholic fatty liver disease (NAFLD) in pediatric obesity remain largely unknown. Recently, a nonsynonymous single-nucleotide polymorphism (rs738409), in the patatin-like phospholipase 3 gene (PNPLA3) has been associated with hepatic steatosis in adults. In a multiethnic group of 85 obese youths, we genotyped the PNLPA3 single-nucleotide polymorphism, measured hepatic fat content by magnetic resonance imaging and insulin sensitivity by the insulin clamp. Because PNPLA3 might affect adipogenesis/lipogenesis, we explored the putative association with the distribution of adipose cell size and the expression of some adipogenic/lipogenic genes in a subset of subjects who underwent a subcutaneous fat biopsy. Steatosis was present in 41% of Caucasians, 23% of African Americans, and 66% of Hispanics. The frequency of PNPLA3(rs738409) G allele was 0.324 in Caucasians, 0.183 in African Americans, and 0.483 in Hispanics. The prevalence of the G allele was higher in subjects showing hepatic steatosis. Surprisingly, subjects carrying the G allele showed comparable hepatic glucose production rates, peripheral glucose disposal rate, and glycerol turnover as the CC homozygotes. Carriers of the G allele showed smaller adipocytes than those with CC genotype (P = 0.005). Although the expression of PNPLA3, PNPLA2, PPARγ2(peroxisome proliferator-activated receptor gamma 2), SREBP1c(sterol regulatory element binding protein 1c), and ACACA(acetyl coenzyme A carboxylase) was not different between genotypes, carriers of the G allele showed lower leptin (LEP)(P = 0.03) and sirtuin 1 (SIRT1) expression (P = 0.04).

CONCLUSION

A common variant of the PNPLA3 gene confers susceptibility to hepatic steatosis in obese youths without increasing the level of hepatic and peripheral insulin resistance. The rs738409 PNPLA3 G allele is associated with morphological changes in adipocyte cell size.

The association of genetic variability in patatin-like phospholipase domain-containing protein 3 (PNPLA3) with histological severity of nonalcoholic fatty liver disease

Genome-wide association studies identified single-nucleotide polymorphisms (SNPs) that are associated with increased hepatic fat or elevated liver enzymes, presumably reflecting nonalcoholic fatty liver disease (NAFLD). To investigate whether these SNPs are associated with histological severity of NAFLD, 1117 (894 adults/223 children) individuals enrolled in the Nonalcoholic Steatohepatitis (NASH) Clinical Research Network and National Institutes of Health Clinical Center studies with histologically confirmed NAFLD were genotyped for six SNPs that are associated with hepatic fat or liver enzymes in genome-wide association studies. In adults, three SNPs on chromosome 22 showed associations with histological parameters of NASH. After adjustment for age, sex, diabetes, and alcohol consumption, the minor allele of rs738409 C/G, a nonsynonymous coding SNP in the patatin-like phospholipase domain-containing protein 3 (PNPLA3) (adiponutrin) gene encoding an Ile148Met change, was associated with steatosis (P = 0.03), portal inflammation (P = 2.5 x 10(-4)), lobular inflammation (P = 0.005), Mallory-Denk bodies (P = 0.015), NAFLD activity score (NAS, P = 0.004), and fibrosis (P = 7.7 x 10(-6)). Two other SNPs in the same region demonstrated similar associations. Three SNPs on chromosome 10 near the CHUK (conserved helix-loop-helix ubiquitous kinase) gene were independently associated with fibrosis (P = 0.010). In children, no SNP was associated with histological severity. However, the rs738409 G allele was associated with younger age at the time of biopsy in multivariate analysis (P = 0.045).

CONCLUSION

In this large cohort of histologically proven NAFLD, we confirm the association of the rs738409 G allele with steatosis and describe its association with histological severity. In pediatric patients, the high-risk rs738409 G allele is associated with an earlier presentation of disease. We also describe a hitherto unknown association between SNPs at a chromosome 10 locus and the severity of NASH fibrosis.

Patatin-like phospholipase domain-containing 3/adiponutrin deficiency in mice is not associated with fatty liver disease

PNPLA3 (adiponutrin), a novel patatin-like phospholipase domain-containing enzyme, is expressed at high level in fat, but also in other tissues including liver. Polymorphisms in PNPLA3 have been linked to obesity and insulin sensitivity. Notably, a nonsynonymous variant rs738409(G) allele of the PNPLA3 gene was found to be strongly associated with both nonalcoholic and alcoholic fatty liver disease. We have generated Pnpla3(-/-) mice by gene targeting. Loss of Pnpla3 has no effect on body weight or composition, adipose mass, or development, whether the mice were fed regular chow or high-fat diet or bred into the genetic obese Lep(ob/ob) background. Plasma and liver triglyceride content and plasma aspartate aminotransferase and alanine aminotransferase levels were not different between Pnpla3(+/+) and Pnpla3(-/-) mice while they were on regular chow, fed three different fatty liver-inducing diets, or after they were bred into Lep(ob/ob) background. Hepatic Pnpla5 messenger RNA (mRNA) levels were similar in wild-type and Pnpla3(-/-) mice, although adipose Pnpla5 mRNA level was increased in Pnpla3(-/-) mice. A high-sucrose lipogenic diet stimulated hepatic Pnpla3 and Pnpla5 mRNA levels to a similar degree, but it did not affect adipose or liver triglyceride lipase (ATGL, known also as Pnpla2) mRNA in Pnpla3(+/+) and Pnpla3(-/-) mice. Finally, Pnpla3(+/+) and Pnpla3(-/-) mice displayed similar glucose tolerance and insulin tolerance tests while on regular chow or three different fatty liver-inducing diets.

CONCLUSION

Loss of Pnpla3 does not cause fatty liver, liver enzyme elevation, or insulin resistance in mice.

Unravelling the pathogenesis of fatty liver disease: patatin-like phospholipase domain-containing 3 protein

PURPOSE OF REVIEW

Hepatic steatosis is a leading cause of adult and paediatric liver disease and is inextricably linked to obesity, insulin resistance and cardiovascular disease. Here we summarize our current understanding of the role of the patatin-like phospholipase domain-containing 3 gene (PNPLA3) in hepatic steatosis.

RECENT FINDINGS

Multiple studies have revealed an association between the common I148M variant in PNPLA3 and increased hepatic fat. In the presence of obesity and chronic alcohol intake, the variant is associated with even more striking phenotypes such as hepatitis and cirrhosis, respectively. These findings suggest that genetic variants in PNPLA3 predispose towards hepatic steatosis and, in the context of other environmental stressors, its progression to irreversible liver failure. PNPLA3 is predominantly expressed in human liver and adipose tissue, possesses both lipolytic and lipogenic activity in vitro and localizes to the surface of lipid droplets in heptocytes. The 148M mutant protein has reduced lipolytic activity, with attendant increased cellular triglyceride accumulation. However, the precise physiological role of PNPLA3 remains mysterious.

SUMMARY

Recent studies have implicated PNPLA3 in the pathogenesis of hepatic steatosis. Attempts to describe its function in vivo may provide us with both an opportunity to understand and a strategy to overcome this leading cause of human morbidity.

The expression level of non-alcoholic fatty liver disease-related gene PNPLA3 in hepatocytes is highly influenced by hepatic lipid status

BACKGROUND & AIMS

Recent studies have suggested that variations in PNPLA3 are associated with non-alcoholic fatty liver disease (NAFLD). To gain insight into the potential function of PNPLA3 in liver, we have determined the effect of metabolic shifts on the hepatic expression profile of PNPLA3 in mice.

METHODS

PNPLA3 expression in wild-type C57BL/6 and NAFLD-susceptible LDL receptor knockout (LDLR-/-) mice was determined using microarray and real-time PCR analysis.

RESULTS

PNPLA3 expression in livers is 50- to 100-fold lower as compared to (cardiac) muscle and adipose tissue in regular chow diet-fed mice. Feeding a Western-type diet stimulated hepatic relative PNPLA3 expression level 23-fold (p<0.001) both in C57BL/6 mice and LDLR-/- mice, suggesting that PNPLA3 does become an important player in hepatic lipid metabolism under conditions of lipid excess. Subjecting mice to fasting fully reversed the effect of the Western-type diet on hepatic PNPLA3 expression. Under these conditions, the expression level of PNPLA3 in adipose tissue is also decreased 90% (p<0.001). Cellular distribution analysis revealed that PNPLA3 is expressed in hepatocytes but not in liver endothelial and Kupffer cells. Microarray-based gene profiling showed that the expression level of PNPLA3 in hepatocytes is correlated with that of genes associated with the lipogenic pathway such as ME1, SPOT14, and SCD1.

CONCLUSIONS

It appears that the NAFLD-related gene PNPLA3 is highly responsive to metabolic changes in hepatocytes within the liver and its relative change in expression level suggests an essential function in lipogenesis.

Tri-iodothyronine upregulates adiponutrin mRNA expression in rat and human adipocytes

Adiponutrin (PNPLA3) is expressed in adipose tissue. Although its precise function is unknown, some data suggest a dual role in lipid homeostasis. We have investigated the influence of thyroid hormone (TH) on PNPLA3 mRNA, in rat and human cultured white adipocytes and in rat white adipose tissue (WAT). Pnpla3 mRNA increased during differentiation of rat adipocytes in an insulin-dependent manner. Tri-iodothyronine further increased Pnpla3 expression at any day during differentiation and its effects were time and dose-dependent. The Pnpla3 mRNA half-life was stabilized by tri-iodothyronine, but a transcriptional component was also observed. Pnpla3 mRNA decreased in WAT of hypothyroid rats and was partially restored by treatment with TH. Taqman analysis showed that tri-iodothyronine also increased human PNPLA3 expression in cultured subcutaneous adipocytes from obese patients. In conclusion, PNPLA3 mRNA expression is upregulated by tri-iodothyronine in adipocytes in vitro, in humans and rats, and in vivo in rat WAT.

Genetic evidence for a role of adiponutrin in the metabolism of apolipoprotein B-containing lipoproteins

Adiponutrin (PNPLA3) is a predominantly liver-expressed transmembrane protein with phospholipase activity that is regulated by fasting and feeding. Recent genome-wide association studies identified PNPLA3 to be associated with hepatic fat content and liver function, thus pointing to a possible involvement in the hepatic lipoprotein metabolism. The aim of this study was to examine the association between two common variants in the adiponutrin gene and parameters of lipoprotein metabolism in 23,274 participants from eight independent West-Eurasian study populations including six population-based studies [Bruneck (n = 800), KORA S3/F3 (n = 1644), KORA S4/F4 (n = 1814), CoLaus (n = 5435), SHIP (n = 4012), Rotterdam (n = 5967)], the SAPHIR Study as a healthy working population (n = 1738) and the Utah Obesity Case-Control Study including a group of 1037 severely obese individuals (average BMI 46 kg/m2) and 827 controls from the same geographical region of Utah. We observed a strong additive association of a common non-synonymous variant within adiponutrin (rs738409) with age-, gender-, and alanine-aminotransferase-adjusted lipoprotein concentrations: each copy of the minor allele decreased levels of total cholesterol on average by 2.43 mg/dl (P = 8.87 x 10(-7)), non-HDL cholesterol levels by 2.35 mg/dl (P = 2.27 x 10(-6)) and LDL cholesterol levels by 1.48 mg/dl (P = 7.99 x 10(-4)). These associations remained significant after correction for multiple testing. We did not observe clear evidence for associations with HDL cholesterol or triglyceride concentrations. In conclusion, our study suggests that adiponutrin is involved in the metabolism of apoB-containing lipoproteins.

A single nucleotide polymorphism (SNP) rs2072907 in the adiponutrin gene (ADPN) was not associated with obesity and type 2 diabetes in Chinese population

We aimed to assess the association between adiponutrin gene (ADPN) SNPrs2072907 and obesity or type 2 diabetes mellitus (T2DM) in Chinese population. 500 T2DM and 331 nondiabetic subjects were recruited. No association was found between SNPrs2072907 and obesity or T2DM. ADPN was not an obesity/T2DM susceptibility gene in Chinese population.

A common variant in PNPLA3, which encodes adiponutrin, is associated with liver fat content in humans

AIMS/HYPOTHESIS

It has recently been suggested that the rs738409 G allele in PNPLA3, which encodes adiponutrin, is strongly associated with increased liver fat content in three different ethnic groups. The aims of the present study were as follows: (1) to try to replicate these findings in European individuals with quantitative measures of hepatic fat content; (2) to study whether the polymorphism influences hepatic and adipose tissue insulin sensitivity; and (3) to investigate whether PNPLA3 expression is altered in the human fatty liver.

METHODS

We genotyped 291 Finnish individuals in whom liver fat had been measured using proton magnetic resonance spectroscopy. Hepatic PNPLA3 expression was measured in 32 participants. Hepatic and adipose tissue insulin sensitivities were measured using a euglycaemic-hyperinsulinaemic (insulin infusion 0.3 mU kg(-1) min(-1)) clamp technique combined with infusion of [3-(3)H]glucose in 109 participants.

RESULTS

The rs738409 G allele in PNPLA3 was associated with increased quantitative measures of liver fat content (p = 0.011) and serum aspartate aminotransferase concentrations (p = 0.002) independently of age, sex and BMI. Fasting serum insulin and hepatic and adipose tissue insulin sensitivity were related to liver fat content independently of genotype status. PNPLA3 mRNA expression in the liver was positively related to obesity (r = 0.62, p < 0.0001) and to liver fat content (r = 0.58, p = 0.025) in participants who were not morbidly obese (BMI < 40 kg/m(2)).

CONCLUSIONS/INTERPRETATION

A common variant in PNPLA3 increases the risk of hepatic steatosis in humans.