Association of a beta-2 adrenoceptor (ADRB2) gene variant with a blunted in vivo lipolysis and fat oxidation

BACKGROUND AND AIMS: Obesity is associated with a blunted beta-adrenoceptor-mediated lipolysis and fat oxidation. We investigated whether polymorphisms in codon 16, 27 and 164 of the beta (2)-adrenoceptor gene (ADRB2) and exon 10 of the G protein beta (3)-subunit gene (GNB3) are associated with alterations in in vivo lipolysis and fat oxidation. DESIGN AND METHODS: Sixty-five male and 43 female overweight and obese subjects (body mass index (BMI) range: 26.1-48.4 kg/m(2)) were included. Energy expenditure (EE), respiratory quotient (RQ), circulating free fatty acid (FFA) and glycerol levels were determined after stepwise infusion of increasing doses of the non-selective beta-agonist isoprenaline (ISO). RESULTS: In women, the Arg16 allele of the ADRB2 gene was associated with a blunted increase in circulating FFA, glycerol and a decreased fat oxidation during ISO stimulation. In men, the Arg16 allele was significantly associated with a blunted increase in FFA but not in glycerol or fat oxidation. CONCLUSION: These results suggest that genetic variation in the ADRB2 gene is associated with disturbances in in vivo beta-adrenoceptor-mediated lipolysis and fat oxidation during beta-adrenergic stimulation in overweight and obese subjects; these effects are influenced by gene-gender interactions.     

Angiotensin II: a hormone that affects lipid metabolism in adipose tissue

BACKGROUND: Alterations in adipose tissue lipolysis may contribute to the pathophysiology of obesity and insulin resistance. We examined the effects of angiotensin II (Ang II) on abdominal subcutaneous adipose tissue lipolysis in humans. METHODS AND RESULTS: First, adipocytes obtained from nine normal weight and seven obese subjects were stimulated with Ang II (10(-14)-10(-6) M). Glycerol concentration in the medium, used as an indicator of adipocyte lipolysis, was significantly reduced (approximately 20%) after Ang II stimulation in adipocytes from normal weight (P=0.04) and obese subjects (P<0.001). Based on these observations, adipocytes of seven additional obese subjects were stimulated with lower doses of Ang II (10(-17)-10(-6) M) in the presence and absence of Ang II type 1 (AT(1)) receptor blockade. Lipolysis was dose dependently inhibited by approximately 20 to 25% after Ang II stimulation (P=0.001). AT(1) receptor blockade completely abolished the Ang II-induced effects (P=0.35). CONCLUSION: Ang II directly inhibits abdominal subcutaneous adipocyte lipolysis in normal weight and obese subjects via the AT(1) receptor.     

Stimulation of adipose tissue lipolysis following insulin-induced hypoglycaemia: evidence of increased beta-adrenoceptor-mediated lipolytic response in IDDM

Summary The adrenergic regulation of adipose tissue lipolysis in response to insulin-induced hypoglycaemia (intravenous infusion of soluble insulin 0.10 IU · kg body weight⁻¹· h⁻¹ until the arterial plasma glucose fell below 2.8 mmol/l) was investigated directly in vivo in 11 insulin-dependent diabetic (IDDM) patients and 12 control subjects, using microdialysis of the extracellular space of abdominal subcutaneous adipose tissue. The tissue glycerol level (lipolysis index) and the escape of ethanol from the perfusion medium (blood flow index) were continuously monitored. During insulin infusion the arterial glucose level was reduced in parallel and the hypoglycaemic nadir was almost identical in the two groups (diabetic patients 2.2 ± 0.1 and control subjects 2.3 ± 0.1 mmol/l). While the maximum response of plasma epinephrine to hypoglycaemia was 30 % lower in diabetic patients than in the control subjects (p < 0.05), the glycerol levels in adipose tissue and in plasma, as well as in serum non-esterified fatty acids, increased twice as much in the former as in the latter group following hypoglycaemia (p < 0.01). Addition of the beta-adrenoceptor blocker propranolol (10⁻⁴ mol/l) to the tissue perfusate almost completely prevented the hypoglycaemia-induced increase in the adipose tissue glycerol level in both groups, whereas in situ perfusion with 10⁻⁴ mol/l of the alpha-adrenoceptor blocker phentolamine resulted in an additional increase in the tissue glycerol levels; during alpha-blockade, the glycerol response to hypoglycaemia remained enhanced by threefold in the diabetic patients (p < 0.01). In both groups local adipose tissue blood flow increased transiently in a similar way after hypoglycaemia; the increase being inhibited by in situ beta-adrenoceptor blockade. We conclude that both alpha- and beta-adrenergic mechanisms regulate adipose tissue lipolysis in response to hypoglycaemia. In IDDM, lipolysis is markedly enhanced following hypoglycaemia, despite a reduced catecholamine secretory response, because of increased beta-adrenoceptor action in adipose tissue. [Diabetologia (1996) 39: 845–853] Received: 5 July 1995 and in final revised form: 4 March 1996     

Stability over time of adrenergic sensitivity in isolated human fat cells.

The stability over time for lipolysis in isolated human subcutaneous fat cells was investigated in 13 non-obese volunteers who underwent two gluteal fat biopsies under local anaesthesia at random intervals (mean 18 months). Glycerol release to the incubation medium was measured in the absence or the presence of lipolytic or antilipolytic adrenergic agents. The half-maximum effects (ED50) of norepinephrine, the non-selective beta-agonist isoprenaline and the selective alpha 2 agonist clonidine were determined and each had an intraindividual coefficient of variation of 7 per cent. The precision of the lipolysis assay, performed in duplicate, was 10 per cent. The coefficient of variation for lipolysis at the maximum effective concentrations of the agonists was between 20 per cent and 30 per cent. The addition of adenosine deaminase does not reduce the variability. In conclusion, the isolated human fat cell lipolysis assay shows some variability for maximum action of adrenergic agonists on lipolysis. However, there is a small variability in ED50 for such agents which indicates that the assay is well suited for determining drug sensitivity.     

Increased adipose tissue expression of tumor necrosis factor-alpha in human obesity and insulin resistance.

Obesity is frequently associated with insulin resistance and abnormal glucose homeostasis. Recent studies in animal models have indicated that TNF-alpha plays an important role in mediating the insulin resistance of obesity through its overexpression in fat tissue. However, the mechanisms linking obesity to insulin resistance and diabetes in humans remain largely unknown. In this study we examined the expression pattern of TNF-alpha mRNA in adipose tissues from 18 control and 19 obese premenopausal women by Northern blot analysis. TNF-alpha protein concentrations in plasma and in conditioned medium of explanted adipose tissue were measured by ELISA. Furthermore, the effects of weight reduction by dietary treatment of obesity on the adipose expression of TNF-alpha mRNA were also analyzed in nine premenopausal obese women, before and after a controlled weight-reduction program. These studies demonstrated that obese individuals express 2.5-fold more TNF-alpha mRNA in fat tissue relative to the lean controls (P < 0.001). Similar increases were also observed in adipose production of TNF-alpha protein but circulating TNF-alpha levels were extremely low or undetectable. A strong positive correlation was observed between TNF-alpha mRNA expression levels in fat tissue and the level of hyperinsulinemia (P < 0.001), an indirect measure of insulin resistance. Finally, body weight reduction in obese subjects which resulted in improved insulin sensitivity was also associated with a decrease in TNF-alpha mRNA expression (45%, P < 0.001) in fat tissue. These results suggest a role for the abnormal regulation of this cytokine in the pathogenesis of obesity-related insulin resistance.     

Studies of liver insulin receptors in non-obese and obese human subjects.

The insulin-binding isotherms and the structural composition of human liver insulin receptors were examined by using plasma membranes that were prepared from liver biopsies of nine non-obese and 10 obese subjects undergoing elective surgery. The insulin-binding characteristics of liver membranes from non-obese subjects were quite similar to those previously described in rat liver membranes. However, when the membranes from obese subjects were compared with the non-obese group, insulin-binding activity was reduced by 50% (P less than 0.01). The reduction in obesity resulted primarily from a decrease in total receptor number, although a small decrease in receptor affinity was also observed. Insulin binding was not correlated with sex or with the fasting plasma insulin level. The insulin-binding sites of liver membranes were affinity-labeled with 125I-insulin and the cross-linking reagent, disuccinimidyl suberate. The liver membranes from both the non-obese and the obese group had heterogenous (nonreduced) insulin-binding species of 300,000, 260,000, and 150,000 mol wt, which were again comparable to the findings reported in rat liver. Sulfhydryl reduction demonstrated a major sub-unit of 125,000 and a minor component of 40,000-45,000 in both groups. These results indicate a close similarity between the hepatic insulin receptor of man and the more intensely studied rat hepatic receptor. Obesity in human subjects is associated with a loss of hepatic insulin receptors. This alteration may contribute to the insulin resistance reported in this organ as well as to obesity-mediated glucose tolerance.     

Characterisation of insulin-like growth factor I receptor in skeletal muscles of normal and insulin resistant subjects

Summary The insulin-like growth factor I receptor and the activity of its associated tyrosine kinase activity were characterised in wheat germ agglutinin extracts from skeletal muscle biopsies from nine control and ten obese Type 2 (non-insulin-dependent) diabetic subjects, who had marked peripheral in vivo insulin resistance for glucose disposal and hyperinsulinaemia. In parallel studies, the concentration of the insulin receptor and its tyrosine kinase activity were examined in the biopsy extracts and compared to the findings for the insulin-like growth factor I receptor system. Specific binding sites for insulin-like growth factor I were detected. The receptor binding of insulin-like growth factor I was not changed in the obese diabetic subjects as compared to binding activity in the biopsies from the control subjects. The molecular weight of the insulin-like growth factor I receptor alpha subunit was similar in both groups (135 kDa). The insulin-like growth factor I stimulated tyrosine kinase activity was also similar for the two groups. In contrast, insulin binding activity was 30% less in the receptor extracts from the in vivo insulin resistant group when compared to the control group. Moreover, insulin-stimulated tyrosine kinase activity was reduced in the former group by 40% when the value was corrected for insulin binding. Thus, specific insulin-like growth factor I receptors are present in human skeletal muscle. These receptors are normal in insulin resistant obese Type 2 diabetic subjects. The findings argue that alterations in the insulin receptor number and tyrosine kinase activity of muscle, which may underlie the marked insulin resistance found in obese Type 2 diabetic patients for glucose disposal, are quite specific for the insulin receptor, since the closely related insulin-like growth factor I receptor was not affected in these patients.