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

Distinct regulation of adiponutrin/PNPLA3 gene expression by the transcription factors ChREBP and SREBP1c in mouse and human hepatocytes

BACKGROUND & AIMS

The adiponutrin/PNPLA3 (patatin-like phospholipase domain-containing protein 3) variant I148M has recently emerged as an important marker of human fatty liver disease. In order to understand the role of the adiponutrin/PNPLA3 protein, we investigated the regulation of its expression in both human and mouse hepatocytes.

METHODS

Adiponutrin/PNPLA3 and lipogenic enzyme expression was determined by real-time PCR analysis in a wide panel of analysis in vivo in the mouse liver and in vitro in murine hepatocytes and human hepatocyte cell lines infected with ChREBP or SREBP1c-expressing adenoviruses.

RESULTS

We show that in the mouse liver, adiponutrin/PNPLA3 gene expression is under the direct transcriptional control of ChREBP (carbohydrate-response element-binding protein) and SREBP1c (sterol regulatory element binding protein1c) in response to glucose and insulin, respectively. In silico analysis revealed the presence of a ChoRE (carbohydrate response element) and of a SRE (sterol response element) binding site on the mouse adiponutrin/PNPLA3 gene promoter. Point mutation analysis in reporter gene assays identified the functional response of these two binding sites in the mouse adiponutrin/PNPLA3 promoter. In contrast, in human immortalized hepatocytes and in HepG2 hepatoma cells, only SREBP1c was able to induce adiponutrin/PNPLA3 expression, whereas ChREBP was unable to modulate its expression.

CONCLUSIONS

All together, our results suggest that adiponutrin/PNPLA3 is regulated by two key factors of the glycolytic and lipogenic pathways, raising the question of its implication in the metabolism of carbohydrates and lipids.

NASH: A global health problem

Non-alcoholic fatty liver disease (NAFLD) is a major public health problem both in the Western world and in the East. This is mainly due to the high prevalence of the disease and its effects on the individual with NAFLD. In the USA, it is estimated that approximately a third of the general population has NAFLD. Increasing age, obesity and the presence of multiple features of metabolic syndrome, especially diabetes, are associated with a higher probability of having non-alcoholic steatohepatitis (NASH). In the individual with NAFLD, excess hepatic fat is associated with an increased risk of developing diabetes, hypertension, cardiovascular events, abnormal resting electrocardiography and endothelial dysfunction. These findings have been corroborated in studies in teenagers as well as adults. There is also an increase in cardiovascular mortality, especially in those with NASH. In addition, there is an increased risk of death from a variety of non-hepatocellular cancers. From a liver perspective, NAFLD is associated with a 15-20% risk of progression to cirrhosis. The disease progresses more rapidly in those with diabetes, increasing age and obesity. The PNPLA3 gene mutation at position 148 is associated with not only steatosis, but with the likelihood of having steatohepatitis and increased inflammation and fibrosis. Once cirrhosis develops, the liver disease decompensates at the rate of 3-4% per year. NASH-related cirrhosis is a risk factor for hepatocellular cancer. All of these factors indicate that NAFLD is a common condition that has significant adverse health consequences for those who are afflicted. It is therefore a major public health hazard throughout the world.

Human fatty liver disease: old questions and new insights

Nonalcoholic fatty liver disease (NAFLD) is a burgeoning health problem that affects one-third of adults and an increasing number of children in developed countries. The disease begins with the aberrant accumulation of triglyceride in the liver, which in some individuals elicits an inflammatory response that can progress to cirrhosis and liver cancer. Although NAFLD is strongly associated with obesity and insulin resistance, its pathogenesis remains poorly understood, and therapeutic options are limited. Here, we discuss recent mechanistic insights into NAFLD, focusing primarily on those that have emerged from human genetic and metabolic studies.

Association of PNPLA3 SNP rs738409 with liver density in African Americans with type 2 diabetes mellitus

AIM

Non-alcoholic fatty liver disease (NAFLD) is commonly diagnosed in patients with obesity and type 2 diabetes mellitus (T2DM), and has been associated with the single nucleotide polymorphism (SNP) rs738409 in the PNPLA3 gene. This association remains to be investigated in African Americans with T2DM, a group at lower risk for hepatic steatosis relative to European Americans with T2DM.

METHODS

We examined 422 African Americans with T2DM (40.3% male; age: 56.4±9.6 years; Body Mass Index: 35.2±8.2 kg/m(2)), all with measures of liver density reflecting hepatic fat content on abdominal computed tomography, and blood glucose and lipid profiles. Associations between rs738409 and phenotypes of interest were determined using SOLAR, assuming an additive model of inheritance with covariates age, sex, BMI and use of lipid-lowering medications.

RESULTS

Mean±SD liver density was 55.4±10.2 Hounsfield Units. SNP rs738409 in PNPLA3 was significantly associated with liver density (P=0.0075) and hepatic steatosis (P=0.0350), but not with blood glucose, HbA(1c), total cholesterol, triglycerides, high-density or low-density lipoprotein levels or liver function tests (P=0.15-0.96).

CONCLUSION

These findings provide evidence that the PNPLA3 SNP rs738409 contributes to risk for increased liver fat content in African Americans with T2DM, an effect that appears to be independent from serum lipids. Although African Americans are less susceptible to fatty liver than European Americans, PNPLA3 appears to be a risk locus for hepatic steatosis in diabetic African Americans.

Nonalcoholic fatty liver disease and diabetes mellitus: pathogenesis and treatment

Nonalcoholic fatty liver disease (NAFLD) and type 2 diabetes mellitus (T2DM) frequently coexist as they share the pathogenic abnormalities of excess adiposity and insulin resistance. Although type 1 diabetes mellitus (T1DM) is due to a relative lack of insulin, an increased prevalence of obesity and insulin resistance in this population means that NAFLD also commonly coexists with this condition. Both T2DM and NAFLD are associated with adverse outcomes of the other; T2DM is a risk factor for progressive liver disease and liver-related death in patients with NAFLD, whereas NAFLD may be a marker of cardiovascular risk and mortality in individuals with T2DM. Nonalcoholic steatohepatitis-a histological subtype of NAFLD characterized by hepatocyte injury and inflammation-is present in approximately 10% of patients with T2DM and is associated with an increased risk for the development of cirrhosis and liver-related death. Current treatment strategies aim to improve insulin resistance via weight loss and exercise, improve insulin sensitivity by the use of insulin-sensitizing agents (for example, pioglitazone) and reduce oxidative stress by the use of antioxidants, such as vitamin E. Pioglitazone and vitamin E supplementation show the most promise in improving hepatic steatosis and inflammation but have not yet been demonstrated to improve fibrosis, and concern remains regarding the toxicity of long-term use of both of these agents.

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.

Lipolysis - a highly regulated multi-enzyme complex mediates the catabolism of cellular fat stores

Lipolysis is the biochemical pathway responsible for the catabolism of triacylglycerol (TAG) stored in cellular lipid droplets. The hydrolytic cleavage of TAG generates non-esterified fatty acids, which are subsequently used as energy substrates, essential precursors for lipid and membrane synthesis, or mediators in cell signaling processes. Consistent with its central importance in lipid and energy homeostasis, lipolysis occurs in essentially all tissues and cell types, it is most abundant, however, in white and brown adipose tissue. Over the last 5years, important enzymes and regulatory protein factors involved in lipolysis have been identified. These include an essential TAG hydrolase named adipose triglyceride lipase (ATGL) [annotated as patatin-like phospholipase domain-containing protein A2], the ATGL activator comparative gene identification-58 [annotated as α/β hydrolase containing protein 5], and the ATGL inhibitor G0/G1 switch gene 2. Together with the established hormone-sensitive lipase [annotated as lipase E] and monoglyceride lipase, these proteins constitute the basic "lipolytic machinery". Additionally, a large number of hormonal signaling pathways and lipid droplet-associated protein factors regulate substrate access and the activity of the "lipolysome". This review summarizes the current knowledge concerning the enzymes and regulatory processes governing lipolysis of fat stores in adipose and non-adipose tissues. Special emphasis will be given to ATGL, its regulation, and physiological function.