Janin Berndt

Dysregulation of the Peripheral and Adipose Tissue Endocannabinoid System in Human Abdominal Obesity

The endocannabinoid system has been suspected to contribute to the association of visceral fat accumulation with metabolic diseases. We determined whether circulating endocannabinoids are related to visceral adipose tissue mass in lean, subcutaneous obese, and visceral obese subjects (10 men and 10 women in each group). We further measured expression of the cannabinoid type 1 (CB1) receptor and fatty acid amide hydrolase (FAAH) genes in paired samples of subcutaneous and visceral adipose tissue in all 60 subjects. Circulating 2-arachidonoyl glycerol (2-AG) was significantly correlated with body fat (r = 0.45, P = 0.03), visceral fat mass (r = 0.44, P = 0.003), and fasting plasma insulin concentrations (r = 0.41, P = 0.001) but negatively correlated to glucose infusion rate during clamp (r = 0.39, P = 0.009). In visceral adipose tissue, CB1 mRNA expression was negatively correlated with visceral fat mass (r = 0.32, P = 0.01), fasting insulin (r = 0.48, P < 0.001), and circulating 2-AG (r = 0.5, P < 0.001), whereas FAAH gene expression was negatively correlated with visceral fat mass (r = 0.39, P = 0.01) and circulating 2-AG (r = 0.77, P < 0.001). Our findings suggest that abdominal fat accumulation is a critical correlate of the dysregulation of the peripheral endocannabinoid system in human obesity. Thus, the endocannabinoid system may represent a primary target for the treatment of abdominal obesity and associated metabolic changes.

Fatty acid synthase gene expression in human adipose tissue: association with obesity and type 2 diabetes

Aims/hypothesisIncreased expression and activity of the lipogenic pathways in adipose tissue may contribute to the development of obesity. As a central enzyme in lipogenesis, the gene encoding fatty acid synthase (FASN) was identified as a candidate gene for determining body fat. In the present study we tested the hypothesis that increased FASN expression links metabolic alterations of excess energy intake, including hyperinsulinaemia, dyslipidaemia and altered adipokine profile to increased body fat mass.Subjects and methodsIn paired samples of visceral and subcutaneous adipose tissue from 196 participants (lean or obese), we investigated whether FASN mRNA expression (assessed by PCR) in adipose tissue is increased in obesity and related to visceral fat accumulation, measures of insulin sensitivity (euglycaemic–hyperinsulinaemic clamp) and glucose metabolism.ResultsFASN mRNA expression was increased by 1.7-fold in visceral vs subcutaneous fat. Visceral adipose tissue FASN expression was correlated with FASN protein levels, subcutaneous FASN expression, visceral fat area, fasting plasma insulin, serum concentrations of IL-6, leptin and retinol-binding protein 4 (RBP4), and inversely with measures of insulin sensitivity, independently of age, sex and BMI. Moreover, we found significant correlations between FASN expression and markers of renal function, including serum creatinine and urinary albumin excretion.Conclusions/interpretationIncreased FASN gene expression in adipose tissue is linked to visceral fat accumulation, impaired insulin sensitivity, increased circulating fasting insulin, IL-6, leptin and RBP4, suggesting an important role of lipogenic pathways in the causal relationship between consequences of excess energy intake and the development of obesity and type 2 diabetes.

Effects of Genetic Variation in the Human Retinol Binding Protein-4 Gene (RBP4) on Insulin Resistance and Fat Depot–Specific mRNA Expression

OBJECTIVE— Serum retinol binding protein 4 (RBP4) is a new liver- and adipocyte-derived signal that may contribute to insulin resistance. Therefore, the RBP4 gene represents a plausible candidate gene involved in susceptibility to type 2 diabetes. RESEARCH DESIGN AND METHODS— In this study, the RBP4 gene was sequenced in DNA samples from 48 nonrelated Caucasian subjects. Five novel and three known single nucleotide polymorphisms (SNPs) were identified. Furthermore, five recently reported SNPs were genotyped in 90 subjects. Six SNPs, representative of their linkage disequilibrium groups, were then genotyped in 934 diabetic and 716 nondiabetic subjects. RESULTS— A haplotype of six common SNPs (A-G-G-T-G-C) was significantly increased in 934 case subjects with type 2 diabetes compared with 537 healthy control subjects with normal glucose tolerance (P = 0.02; odds ratio 1.37 [95% CI 1.05–1.79]). Furthermore, in the cohort of 716 nondiabetic Caucasian subjects, carriers of the A-G-G-T-G-C haplotype had significantly higher mean fasting plasma insulin and 2-h plasma glucose than subjects without the haplotype. Two single SNPs (rs10882283 and rs10882273) were also associated with BMI, waist-to-hip ratio, and fasting plasma insulin, and several SNPs were associated with circulating free fatty acids (all adjusted P < 0.05). In addition, subjects carrying a previously reported diabetes-associated haplotype had significantly higher mRNA levels in visceral adipose tissue (adjusted P < 0.05) in a subgroup of nondiabetic subjects (n = 170) with measurements of RBP4 mRNA expression in visceral and subcutaneous fat depots. CONCLUSIONS— Our data indicate a role of RBP4 genetic variation in susceptibility to type 2 diabetes and insulin resistance, possibly through an effect on RBP4 expression.

Inverse relationship between obesity and FTO gene expression in visceral adipose tissue in humans.

Aims/hypothesisRecently, FTO was identified as a candidate gene contributing to both childhood and severe adult obesity. We tested the hypothesis that mRNA expression of FTO and/or of the neighbouring RPGRIP1L in adipose tissue correlates with measures of obesity and fat distribution. We also investigated whether the FTO obesity risk alleles might explain variability in FTO and RPGRIP1L mRNA expression.MethodsIn paired samples of visceral and subcutaneous adipose tissue from 55 lean and obese participants, we investigated whether FTO and RPGRIP1L mRNA expression is fat depot-specific, altered in obesity and related to measures of fat accumulation, insulin sensitivity and glucose metabolism. All participants were genotyped for the obesity-associated rs8050136 FTO variant.ResultsFTO mRNA expression was threefold higher in subcutaneous than in visceral adipose tissue. Subcutaneous FTO expression correlated with visceral FTO expression. FTO gene expression in both depots correlated with age and was negatively correlated to BMI and per cent body fat. FTO mRNA levels were not related to measures of insulin sensitivity and glucose metabolism. RPGRIP1L mRNA expression was 1.6-fold higher in visceral than in subcutaneous adipose tissue, but did not correlate with anthropometric and metabolic characteristics. There was no association between rs8050136 and FTO or RPGRIP1L mRNA expression in adipose tissue.Conclusions/interpretationExpression of adipose tissue FTO mRNA is fat depot-specific and negatively correlates with measures of obesity. However, the direction of this relationship still needs to be elucidated.

TCF7L2 gene expression in human visceral and subcutaneous adipose tissue is differentially regulated but not associated with type 2 diabetes mellitus.

Variants in the TCF7L2 gene have been associated with type 2 diabetes mellitus (T2DM), but the causal variant(s) is still unknown. We studied the TCF7L2 messenger RNA (mRNA) expression in paired samples of visceral and subcutaneous adipose tissue from 49 subjects using quantitative real-time polymerase chain reaction and its relation to obesity and T2DM. All subjects were genotyped for the previously described TCF7L2 diabetes risk variants. Independent of age, sex, obesity, and diabetes status, we found >3-fold higher TCF7L2 mRNA expression in subcutaneous compared with visceral adipose tissue. There was no correlation between visceral and subcutaneous TCF7L2 expression. No differences in adipose tissue TCF7L2 mRNA expression levels were found between diabetic and nondiabetic subjects, or between lean and obese subjects (all Ps > .05). In addition, there was no association between TCF7L2 genetic variants and mRNA expression. Based on our data, TCF7L2 mRNA expression is fat-depot specific but does not seem to provide the mechanistic link explaining genetic association with T2DM.

Role of genetic variation in the cannabinoid type 1 receptor gene (CNR1) in the pathophysiology of human obesity

AIMS The endocannabinoid system may contribute to the association of visceral fat accumulation with metabolic diseases. Here we investigated the effects of genetic variation in the cannabinoid type 1 receptor gene (CNR1) on its mRNA expression in adipose tissue from visceral and subcutaneous depots and on the development of obesity. MATERIALS & METHODS CNR1 was sequenced in 48 nonrelated German Caucasians to detect genetic variation. Five representative variants including HapMap tagging SNPs (rs12720071, rs806368, rs806370, rs1049353 and rs806369) were genotyped for subsequent association studies in two independent cohorts (total n = 2774) with detailed metabolic testing: subjects from the Leipzig Study (n = 1857) and a self-contained population of Sorbs from Germany (n = 917). RESULTS In a case-control study of lean (BMI <25 kg/m(2)) versus obese (BMI >30 kg/m(2)) subjects, rs806368 was found to be nominally associated with obesity in the Sorbian cohort (adjusted p < 0.05), but not in the Leipzig cohort. Also, several SNPs (rs806368, rs806370 and rs12720071) were nominally associated with serum leptin levels (p < 0.05 after adjusting for age, sex and BMI). However, none of these associations remained significant after accounting for multiple testing. Furthermore, none of the SNPs were related to CNR1 mRNA expression in visceral and subcutaneous fat. CONCLUSION The data suggest that common genetic variation in the CNR1 gene does not influence mRNA expression in adipose tissue nor does it play a significant role in the pathophysiology of obesity in German and Sorbian populations.

Effect of genetic variation in the human fatty acid synthase gene (FASN) on obesity and fat depot-specific mRNA expression.

Inhibition of fatty acid synthase (FASN) induces a rapid decline in fat stores in mice, suggesting a role for this enzyme in energy homeostasis. To investigate the potential role of FASN in the pathophysiology of human obesity, the FASN gene was sequenced in 48 German whites. Thirty‐five single‐nucleotide polymorphisms (SNPs) were identified. Eight SNPs representative for their linkage disequilibrium groups and the Val1483Ile (rs2228305) substitution were genotyped for subsequent association analyses in 1,311 adults from Germany. Further, the tagging SNPs were genotyped also in German childhood cohorts (738 schoolchildren, 205 obese children). Effects of genetic variation on FASN mRNA expression in visceral and subcutaneous adipose tissue from a subgroup of 172 subjects were analyzed. Several polymorphisms in the FASN (rs62078748, rs2229422, rs2229425, and rs17848939) were nominally associated with obesity in case–control studies including 446 obese subjects (BMI ≥30 kg/m2) and 389 lean controls (BMI ≤25 kg/m2) (adjusted P < 0.05). The strongest significant effect was found for rs2229422 (P = 1.3 × 10−5 adjusted for age, sex, type 2 diabetes status), which was supported by associations with BMI, waist‐to‐hip ratio (WHR), fasting plasma insulin and glucose infusion rate (adjusted P < 0.05). Subjects with the Val1483Ile substitution appeared to be protected against obesity. In addition, rs17848939 was nominally significantly associated with the ratio of visceral/subcutaneous FASN mRNA expression (adjusted P = 0.04). No effect of genetic variation in FASN on obesity was found in children. In conclusion, our data indicate a role of FASN genetic variation in susceptibility to obesity in adults.