Upstream transcription factor 1 gene polymorphisms are associated with high antilipolytic insulin sensitivity and show gene–gene interactions

Single Nucleotide Polymorphisms in the G-Protein Coupled Receptor Kinase 5 (GRK5) Gene are associated with Plasma LDL-Cholesterol Levels in Humans

Genetically modified mice models suggest an important role for G-protein-coupled receptor kinase 5 (GRK5) in the pathophysiology of obesity and related disorders. We investigated whether single nucleotide polymorphisms (SNPs) in the gene encoding GRK5 affect cardiometabolic traits in humans. We genotyped 3 common SNPs in intron 1 (rs1980030, rs10466210, rs9325562) and one SNP in intron 3 (rs10886471) of GRK5 in 2332 subjects at risk for type 2 diabetes. Total- and visceral fat mass were measured by magnetic resonance (MR) tomography and liver fat content by ¹H-MR spectroscopy. Insulin secretion and sensitivity were estimated during an OGTT and measured during the euglycemic, hyperinsulinemic clamp (n = 498). Carriers of the minor allele of rs10466210 and rs1980030 had higher total- and LDL-cholesterol levels (p = 0.0018 and p = 0.0031, respectively, for rs10466210; p = 0.0035 and p = 0.0081, respectively, for rs1980030), independently of gender, age, BMI and lipid-lowering drugs. The effects of rs10466210 withstood Bonferroni correction. Similar associations were observed with apolipoprotein B levels (p = 0.0034 and p = 0.0122, respectively). Carriers of the minor allele of rs10466210 additionally displayed a trend for higher intima-media thickness of the carotid artery (p = 0.075). GRK5 may represent a novel target for strategies aiming at lowering LDL-cholesterol levels and at modifying cardiovascular risk.

Insulin-sensitive and insulin-resistant obese and non-obese phenotypes: role in prediction of incident pre-diabetes in a longitudinal biracial cohort

OBJECTIVE

We measured insulin sensitivity with euglycemic clamp (Si-clamp) in initially normoglycemic African Americans (AA) and European Americans (EA), to probe the existence of subphenotypes of obesity and leanness, and their impact on incident dysglycemia during longitudinal follow-up.

RESEARCH DESIGN AND METHODS

320 healthy subjects (176 AA, 144 EA; mean age 44.2±10.6 years) underwent baseline assessments, including Si-clamp and homeostasis model of insulin resistance (HOMA-IR) and were stratified into: insulin-resistant obese (IRO) (body mass index (BMI) >30 kg/m², Si-clamp <0.1, HOMA-IR >2.5); insulin-sensitive obesity (ISO) (BMI >30 kg/m², Si-clamp >0.1, HOMA-IR <2.5); insulin-resistant non-obese (IRN) (BMI <28 kg/m², Si-clamp <0.1, HOMA-IR >2.5); insulin-sensitive non-obese (ISN) (BMI <28 kg/m², Si-clamp >0.1, HOMA-IR <2.5). Outcome measures were cardiometabolic risks and incident pre-diabetes/type 2 diabetes (T2D) during 5.5 years.

RESULTS

Compared with IRO, subjects with ISO had lower abdominal fat, triglycerides and high-sensitivity C reactive protein and higher adiponectin (p=0.015 to <0.0001). IRN subjects had higher cardiometabolic risk markers than ISN (p=0.03 to <0.0001). During 5.5-year follow-up, incident pre-diabetes/T2D was lower in ISO (31.3% vs 48.7%) among obese subjects and higher in IRN (47.1% vs. 26.0%) among non-obese subjects (p=0.0024). Kaplan-Meier analysis showed significantly different pre-diabetes/T2D survival probabilities across insulin sensitivity/adiposity phenotypes (p=0.0001).

CONCLUSIONS

Insulin sensitivity predicts ~40% decrease in the relative risk of incident pre-diabetes/T2D among obese persons, whereas insulin resistance predicts ~80% increased risk among non-obese persons. This is the first documentation of healthy and unhealthy phenotypes of obesity and leanness in a prospective biracial cohort, using rigorous measurement of insulin sensitivity.

Potential Role of Upstream Stimulatory Factor 1 Gene Variant in Familial Combined Hyperlipidemia and Related Disorders

Objective—

Genetic studies implicated upstream stimulatory factor 1 (USF1) in familial combined hyperlipidemia because the rs2073658 minor allele was associated with reduced risk of familial combined hyperlipidemia and related disorders. The molecular mechanisms whereby rs2073658 influences trait expression have remained elusive.

Methods and Results—

Plasma lipids, rs2073658 genotypes (N=372), and hepatic transcript levels (N=96) of USF1 and genes involved in hepatic lipoprotein production were determined in obese subjects. The rs2073658 minor allele was associated with reduced plasma triglycerides (TGs) ( P <0.001), hepatic USF1 ( P <0.01), and microsomal TG transfer protein transcript levels ( P <0.05). Functional studies in human hepatocellular carcinoma cells showed that rs2073658 is located in a forkhead box A2 (FOXA2) binding site and that major allele constructs displayed higher transcriptional activity than minor allele constructs. Knockdown of FOXA2 reduced the activity of major, but not minor allele constructs. Furthermore, an interaction between hepatic FOXA2 transcript levels and rs2073658 minor allele carrier status on hepatic USF1 transcript levels was observed in vivo ( P <0.05). USF1 activated the transcription of FOXA2 and FOXA2 strongly activated the transcription of microsomal TG transfer protein.

Conclusion—

A feed-forward loop comprising activation of USF1 transcription by FOXA2 and activation of FOXA2 transcription by USF1 , driving microsomal TG transfer protein expression, is modulated by rs2073658. Hence, rs2073658 likely influences hepatic TG secretion.

Genetics of adipose tissue biology

Adipose tissue morphology and release of free fatty acids, as well as peptide hormones, are believed to contribute to obesity and related metabolic disorders. These adipose tissue phenotypes are influenced by adiposity, but there is also a strong hereditary impact. Polymorphisms in numerous adipose-expressed genes have been evaluated for association with adipocyte and clinical phenotypes. In our opinion, some results are convincing. Thus ADRB2 and GPR74 genes are associated with adipocyte lipolysis, GPR74 also with BMI; PPARG and SREBP1, which promote adipogenesis and lipid storage, are associated with T2D and possible adiposity; ADIPOQ and ARL15 are associated with circulating levels of adiponectin, ARL15 also with coronary heart disease. We anticipate that the use of complementary approaches such as expression profiling and RNAi screening, and studies of additional levels of gene regulation, that is, miRNA and epigenetics, will be important to unravel the genetics of adipose tissue function.

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.

Non-invasive assessment and quantification of liver steatosis by ultrasound, computed tomography and magnetic resonance

Hepatic steatosis is the most prevalent liver disorder in the developed world. It is closely associated with features of metabolic syndrome, especially insulin resistance and obesity. The two most common conditions associated with fatty liver are alcoholic liver disease (ALD) and non-alcoholic fatty liver disease (NAFLD). Liver biopsy is considered the gold standard for the assessment of liver fat, but there is a need for less invasive diagnostic techniques. New imaging modalities are emerging, which could provide more detailed information about hepatic tissue or even replace biopsy. In the present review, available imaging modalities (ultrasound, computed tomography, magnetic resonance imaging and proton magnetic resonance spectroscopy) are presented which are employed to detect or even quantify the fat content of the liver. The advantages and disadvantages of the above-mentioned imaging modalities are discussed. Although none of these techniques is able to differentiate between microvesicular and macrovesicular steatosis and to reveal all features visible using histology, the proposed diagnostic modalities offer a wide range of additional information such as anatomical and morphological information non-invasively. In particular, magnetic resonance imaging and proton magnetic resonance spectroscopy are able to quantify the hepatic fat content hence avoiding exposure to radiation. Except for proton magnetic resonance spectroscopy, all modalities offer additional information about regional fat distribution within the liver. MR elastography, which can estimate the amount of fibrosis, also appears promising in the differentiation between simple steatosis and steatohepatitis.

Functional variant disrupts insulin induction of USF1: mechanism for USF1-associated dyslipidemias

BACKGROUND

The upstream transcription factor 1 (USF1) gene is associated with familial combined hyperlipidemia, the most common genetic dyslipidemia in humans, as well as with various dyslipidemic changes in numerous other studies. Typical of complex disease-associated genes, neither the explicit mutations have been described nor the functional consequences for risk allele carriers been reported at the cellular or tissue level.

METHODS AND RESULTS

In this study, we aimed at describing the molecular mechanism through which the strongest associating intronic single-nucleotide polymorphism variant in USF1 is involved in the development of dyslipidemia. The effects of the risk variant on gene expression were studied in 2 relevant human tissues, fat and muscle. Global transcript profiles of 47 fat biopsies ascertained for carriership of the risk allele were tested for differential expression of known USF1 target genes as well as for broader effects on the transcript profile. Allelic imbalance of USF1 in fat was assessed using a quantitative sequencing approach. The possible allele-specific effect of insulin on the expression of USF1 was studied in 118 muscle biopsies before and after a euglycemic hyperinsulinemic clamp. The risk allele of single-nucleotide polymorphism rs2073658 seems to eradicate the inductive effect of insulin on the expression of USF1 in muscle and fat. The expression of numerous target genes is in turn perturbed in adipose tissue.

CONCLUSIONS

In risk allele carriers, a defective response of USF1 to insulin results in the suboptimal response of relevant target genes that contributes to the enhanced risk of developing dyslipidemia and coronary heart disease.

Genetic factors contribute to variation in serum alanine aminotransferase activity independent of obesity and alcohol: a study in monozygotic and dizygotic twins

BACKGROUND/AIMS

This study aimed to determine the heritability of serum alanine aminotransferase (S-ALT) and fasting serum insulin (fS-insulin) concentration as well as determine the association of these measures with liver fat content in young adult monozygotic (MZ) and dizygotic (DZ) twins.

METHODS

Three hundred and thirteen individual twins were recruited from a population-based cohort (n = 4929). The study subjects represented a wide range of body mass indexes (BMI), were free of any diseases or regular medications and had an intake of less than two drinks of alcohol/day. To verify that S-ALT is a marker of liver fat, it was measured by proton magnetic resonance spectroscopy ((1)H MRS) in 66 subjects. Heritability estimations were performed using BMI- and gender-adjusted values.

RESULTS

Intra-pair correlations were significantly higher in the MZ twins than the DZ twins for both S-ALT (0.65 for MZ and 0.04 for DZ) and fS-insulin (0.58 and 0.34, respectively). Heritability of S-ALT was 55% and that of fS-insulin 61%. In the 66 subjects S-ALT (r = 0.70 for women and r = 0.50 for men, p < or = 0.01 for both) and fS-insulin (r = 0.58 and r = 0.59, respectively, p < or = 0.01 for both) concentrations correlated significantly with liver fat content.

CONCLUSIONS

These twin data suggest that approximately 60% of the variation in S-ALT, a marker of liver fat content, is genetically determined.

Genetic variation in the ADIPOR2 gene is associated with liver fat content and its surrogate markers in three independent cohorts

AIMS

We investigated whether polymorphisms in candidate genes involved in lipid metabolism and type 2 diabetes are related to liver fat content.

METHODS

Liver fat content was measured using proton magnetic resonance spectroscopy ((1)H-MRS) in 302 Finns, in whom single nucleotide polymorphisms (SNPs) in acyl-CoA synthetase long-chain family member 4 (ACSL4), adiponectin receptors 1 and 2 (ADIPOR1 and ADIPOR2), and the three peroxisome proliferator-activated receptors (PPARA, PPARD, and PPARG) were analyzed. To validate our findings, SNPs significantly associated with liver fat content were studied in two independent cohorts and related to surrogate markers of liver fat content.

RESULTS

In the Finnish subjects, polymorphisms in ACSL4 (rs7887981), ADIPOR2 (rs767870), and PPARG (rs3856806) were significantly associated with liver fat content measured with (1)H-MRS after adjusting for age, gender, and BMI. Anthropometric and circulating parameters were comparable between genotypes. In the first validation cohort of approximately 600 Swedish men, ACSL4 rs7887981 was related to fasting insulin and triglyceride concentrations, and ADIPOR2 rs767870 to serum gamma glutamyltransferase concentrations after adjusting for BMI. The SNP in PPARG (rs3856806) was not significantly associated with any relevant metabolic parameter in this cohort. In the second validation cohort of approximately 3000 subjects from Western Finland, ADIPOR2 rs767870, but not ACSL4 rs7887981 was related to fasting triglyceride concentrations.

CONCLUSIONS

Genetic variation, particularly in the ADIPOR2 gene, contributes to variation in hepatic fat accumulation in humans.

Causes and metabolic consequences of Fatty liver

Type 2 diabetes and cardiovascular disease represent a serious threat to the health of the population worldwide. Although overall adiposity and particularly visceral adiposity are established risk factors for these diseases, in the recent years fatty liver emerged as an additional and independent factor. However, the pathophysiology of fat accumulation in the liver and the cross-talk of fatty liver with other tissues involved in metabolism in humans are not fully understood. Here we discuss the mechanisms involved in the pathogenesis of hepatic fat accumulation, particularly the roles of body fat distribution, nutrition, exercise, genetics, and gene-environment interaction. Furthermore, the effects of fatty liver on glucose and lipid metabolism, specifically via induction of subclinical inflammation and secretion of humoral factors, are highlighted. Finally, new aspects regarding the dissociation of fatty liver and insulin resistance are addressed.

Upstream transcription factor 1 (USF1) in risk of type 2 diabetes: association study in 2000 Dutch Caucasians

Type 2 diabetes shares substantial genetic and phenotypic overlap with familial combined hyperlipidemia. Upstream stimulatory factor 1 (USF1), a well-established susceptibility gene for familial combined hyperlipidemia, is postulated to be such a shared genetic determinant. We evaluated two established variants in familial combined hyperlipidemia (rs2073658 and rs3737787) for association with type 2 diabetes in two Dutch case-control samples (N=2011). The first case-control sample comprised 501 subjects with type 2 diabetes from the Breda cohort and 920 healthy blood bank donors of Dutch Caucasian origin. The second case-control sample included 211 subjects with type 2 diabetes, and 379 normoglycemic controls. SNP rs2073658 and SNP rs3737787 were in perfect linkage disequilibrium. In the first case-control sample, prevalence of the major allele was higher in patients than in controls (75% versus 71%, OR=1.25, p=0.018). A similar effect-size and -direction was observed in the second case-control sample (76% versus 72%, OR=1.22, p=0.16). A combined analysis strengthened the evidence for association (OR=1.23, p=0.006). Notably, the increased risk for type 2 diabetes could be ascribed to the major allele, and its high frequency translated to a substantial population attributable risk of 14.5%. In conclusion, the major allele of rs2073658 in the USF1 gene is associated with a modestly increased risk to develop type 2 diabetes in Dutch Caucasians, with considerable impact at the population level.

Low hepatic stearoyl-CoA desaturase 1 activity is associated with fatty liver and insulin resistance in obese humans

AIMS/HYPOTHESIS

Stearoyl-CoA desaturase 1 (SCD1) is the rate-limiting enzyme in monounsaturated fatty acid synthesis. It is imperative for the assembly of VLDL particles, which transport triacylglycerol (TG) from liver to adipose tissue and other sites. We aimed to determine the role of hepatic SCD1 activity in human glucose and lipid metabolism.

METHODS

We studied 54 people participating in a lifestyle intervention programme with diet modification and increased physical activity. Insulin sensitivity was determined during a euglycaemic-hyperinsulinaemic clamp and estimated from an OGTT. Liver fat was quantified by (1)H-magnetic resonance spectroscopy at baseline and after 9 months of intervention. The pattern of fatty acids in serum VLDL-TGs was determined by ultracentrifugation followed by thin layer and gas chromatography, with the 18:1 n-9: 18:0 ratio providing an index of hepatic SCD1 activity.

RESULTS

The hepatic SCD1 activity index correlated negatively with liver fat (r= -0.29, p=0.04) and positively with insulin sensitivity, both OGTT-derived (r=0.42, p=0.003) and clamp-derived (r=0.27, p=0.07). These correlations depended on overall adiposity. They were absent in leaner participants (n=27, liver fat: p=0.34, insulin sensitivity [OGTT]: p=0.75, insulin sensitivity [clamp]: p=0.24), but were strong in obese individuals (n=27, p=0.004, p=0.0002 and p=0.006, respectively). Furthermore, during intervention a high SCD1 activity index at baseline predicted a decrease in liver fat only in obese participants (r= -0.46, p=0.02).

CONCLUSIONS/INTERPRETATION

Our data suggest that high hepatic SCD1 activity may regulate fat accumulation in the liver and possibly protects from insulin resistance in obesity.

Association analysis of allelic variants of USF1 in coronary atherosclerosis

OBJECTIVE

USF1 regulates the transcription of more than 40 cardiovascular related genes and is well established as a gene associated with familial combined hyperlipidemia, a condition increasing the risk for coronary heart disease. No detailed data, however, exists on the impact of this gene to the critical outcome at the tissue level: different types of atherosclerotic lesions.

METHODS AND RESULTS

We analyzed the USF1 in 2 autopsy series of altogether 700 middle-aged men (the Helsinki Sudden Death Study) with quantitative morphometric measurements of coronary atherosclerosis. SNP rs2516839, tagging common USF1 haplotypes, associated with the presence of several types of atherosclerotic lesions, particularly with the proportion of advanced atherosclerotic plaques (P=0.02) and area of calcified lesions (P<0.001) of the coronary arteries. Importantly, carriers of risk alleles of rs2516839 also showed a 2-fold risk for sudden cardiac death (genotype TT versus CC; OR 2.10, 95% CI 1.17 to 3.75, P=0.04). The risk effect of rs2516839 was present also in aorta samples of the men.

CONCLUSIONS

Our findings in this unique study sample suggest that USF1 contributes to atherosclerosis, the pathological arterial wall phenotype resulting in coronary heart disease and in its most dramatic consequence-sudden cardiac death.

Fatty Liver

Although the epidemic of obesity has been accompanied by an increase in the prevalence of the metabolic syndrome, not all obese develop the syndrome and even lean individuals can be insulin resistant. Both lean and obese insulin resistant individuals have an excess of fat in the liver which is not attributable to alcohol or other known causes of liver disease, a condition defined as nonalcoholic fatty liver disease (NAFLD) by gastroenterologists. The fatty liver is insulin resistant. Liver fat is highly significantly and linearly correlated with all components of the metabolic syndrome independent of obesity. Overproduction of glucose, VLDL, CRP, and coagulation factors by the fatty liver could contribute to the excess risk of cardiovascular disease associated with the metabolic syndrome and NAFLD. Both of the latter conditions also increase the risk of type 2 diabetes and advanced liver disease. The reason why some deposit fat in the liver whereas others do not is poorly understood. Individuals with a fatty liver are more likely to have excess intraabdominal fat and inflammatory changes in adipose tissue. Intervention studies have shown that liver fat can be decreased by weight loss, PPARγ agonists, and insulin therapy.

Characterization of the Basal promoter element of human organic cation transporter 2 gene

Human organic cation transporter 2 (hOCT2; SLC22A2) is abundantly expressed in the kidney, and it plays important roles in the renal tubular secretion of cationic drugs. Although the transport characteristics of hOCT2 have been studied extensively, there is no information available for the transcriptional regulation of hOCT2. The present study was undertaken to identify the cis-element and trans-factor for basal expression of hOCT2. The transcription start site was located 385 nucleotides above the translation start site by using 5'-rapid amplification of cDNA ends. An approximately 4-kilobase fragment of the hOCT2 promoter region was isolated and the promoter activities were measured in the renal epithelial cell line LLC-PK1. A deletion analysis suggested that the region spanning -91 to -58 base pairs was essential for basal transcriptional activity. This region lacked a TATA-box but contained a CCAAT box and an E-box. Electrophoretic mobility shift assays showed that specific DNA/protein complexes were present in the E-box but not in the CCAAT box, and supershift assays revealed that upstream stimulatory factor 1 (USF-1), which belongs to the basic helix-loop-helix-leucine zipper family of transcription factors, bound to the E-box. Mutation of the E-box resulted in a decrease in hOCT2 promoter activity, and overexpression of USF-1 enhanced the hOCT2 promoter activity in a dose-dependent manner. This article reports the first characterization of the hOCT2 promoter and shows that USF-1 functions as a basal transcriptional regulator of the hOCT2 gene via the E-box.