Signy Reynisdottir

Human beta-2 adrenoceptor gene polymorphisms are highly frequent in obesity and associate with altered adipocyte beta-2 adrenoceptor function.

Catecholamines play a central role in the regulation of energy expenditure, in part by stimulating lipid mobilization through lipolysis in fat cells. The beta-2 adrenoceptor (BAR-2) is a major lipolytic receptor in human fat cells. To determine whether known polymorphisms in codons 16, 27, and 164 of this receptor play a role in obesity and subcutaneous adipocyte BAR-2 lipolytic function, we investigated a group of 140 women with a large variation in body fat mass. Only the polymorphisms in codons 16 and 27 were common in the study population. The Gln27Glu polymorphism was markedly associated with obesity with a relative risk for obesity of approximately 7 and an odds ratio of approximately 10. Homozygotes for Glu27 had an average fat mass excess of 20 kg and approximately 50% larger fat cells than controls. However, no significant association with changes in BAR-2 function was observed. The Arg16Gly polymorphism was associated with altered BAR-2 function with Gly16 carriers showing a fivefold increased agonist sensitivity and without any change in BAR-2 expression. However, it was not significantly linked with obesity. These findings suggest that genetic variability in the human BAR-2 gene could be of major importance for obesity, energy expenditure, and lipolytic BAR-2 function in adipose tissue, at least in women.

Adipose tissue secretion of plasminogen activator inhibitor-1 in non-obese and obese individuals

Summary High plasma plasminogen activator inhibitor-1 (PAI-1) activity is a frequent finding in obesity, and both PAI-1 and obesity are risk factors for cardiovascular disease. To study the mechanisms underlying increased PAI-1 levels in obese individuals, gene expression and secretion of PAI-1 were measured in human abdominal subcutaneous adipose tissue. A total of 32 obese, otherwise healthy subjects and 10 never-obese healthy subjects with a body mass index (BMI) of 42.6 ± 1.2 and 24.3 ± 1.9 kg/m2 (mean ± SEM), respectively, were investigated. Plasma PAI-1 activity, adipose tissue PAI-1 secretion and adipocyte PAI-1 mRNA levels were increased sevenfold (p < 0.0001), sixfold (p < 0.0001) and twofold (p < 0.05), respectively, in the obese group. There were clear associations between adipose tissue secretion of PAI-1 and PAI-1 mRNA levels on the one hand and fat cell volume on the other (r = 0.68, p < 0.0001 and r = 0.51, p < 0.01, respectively, in the obese group). PAI-1 mRNA levels were also related to the amount of PAI-1 secreted among obese individuals (r = 0.31, p = 0.09). It is concluded that adipose tissue secretes significant amounts of PAI-1, that PAI-1 secretion from adipose tissue is increased in obesity, and that PAI-1 secretion is related to the lipid content and cell volume of fat cells. Plasma PAI-1 activity is elevated in obesity, at least in part due to increased gene expression in adipocytes, which, in turn, enhances PAI-1 secretion from adipose tissue. [Diabetologia (1998) 41: 65–71]

Regional difference in insulin inhibition of non-esterified fatty acid release from human adipocytes: relation to insulin receptor phosphorylation and intracellular signalling through the insulin receptor substrate-1 pathway

Summary Increased mobilization of non-esterified fatty acids (NEFA) from visceral as opposed to peripheral fat depots can lead to metabolic disturbances because of the direct portal link between visceral fat and the liver. Compared with peripheral fat, visceral fat shows a decreased response to insulin. The mechanisms behind these site variations were investigated by comparing insulin action on NEFA metabolism with insulin receptor signal transduction through the insulin receptor substrate-1 (IRS-1) pathway in omental (visceral) and subcutaneous human fat obtained during elective surgery. Insulin inhibited lipolysis and stimulated NEFA re-esterification. This was counteracted by wortmannin, an inhibitor of phosphaditylinositol (PI) 3-kinase. The effects of insulin on antilipolysis and NEFA re-esterification were greatly reduced in omental fat cells. Insulin receptor binding capacity, mRNA and protein expression did not differ between the cell types. Insulin was four times more effective in stimulating tyrosine phosporylation of the insulin receptor in subcutaneous fat cells (p < 0.001). Similarly, insulin was two to three times more effective in stimulating tyrosine phosphorylation of IRS-1 in subcutaneous fat cells (p < 0.01). This finding could be explained by finding that IRS-1 protein expression was reduced by 50 ± 8 % in omental fat cells (p < 0.01). In omental fat cells, maximum insulin-stimulated association of the p85 kDa subunit of PI 3-kinase to phosphotyrosine proteins and phosphotyrosine associated PI 3-kinase activity were both reduced by 50 % (p < 0.05 or better). Thus, the ability of insulin to induce antilipolysis and stimulate NEFA re-esterification is reduced in visceral adipocytes. This reduction can be explained by reduced insulin receptor autophosphorylation and signal transduction through an IRS-1 associated PI 3-kinase pathway in visceral adipocytes. [Diabetologia (1998) 41: 1343–1354]

Mechanisms Involved in the Regulation of Free Fatty Acid Release from Isolated Human Fat Cells by Acylation-stimulating Protein and Insulin

The effects of acylation-stimulating protein (ASP) and insulin on free fatty acid (FFA) release from isolated human fat cells and the signal transduction pathways to induce these effects were studied. ASP and insulin inhibited basal and norepinephrine-induced FFA release by stimulating fractional FFA re-esterification (both to the same extent) and by inhibiting FFA produced during lipolysis (ASP to a lesser extent than insulin). Protein kinase C inhibition influenced none of the effects of ASP or insulin. Phosphatidylinositol 3-kinase inhibition counteracted the effects of insulin but not of ASP. Phosphodiesterase 3 (PDE3) activity was stimulated by ASP and insulin, whereas PDE4 activity was slightly increased by ASP only. Selective PDE3 inhibition reversed the effects of both ASP and insulin on fractional FFA re-esterification and lipolysis. Selective PDE4 inhibition slightly counteracted the ASP but not the effect of insulin on fractional FFA re-esterification and did not prevent the action of ASP or insulin on lipolysis. Thus, ASP and insulin play a major role in regulating FFA release from fat cells as follows: insulin by stimulating fractional FFA re-esterification and inhibiting lipolysis and ASP mainly by stimulating fractional FFA re-esterification. For both ASP and insulin these effects on FFA release are mediated by PDE3, and for ASP PDE4 might also be involved. The signaling pathway preceding PDE is not known for ASP but involves phosphatidylinositol 3-kinase for insulin.

Catecholamine resistance in fat cells of women with upper-body obesity due to decreased expression of beta2-adrenoceptors

SummaryUpper-body obesity is an important risk factor for developing non-insulin dependent diabetes. To investigate the possibility that a lipolysis defect is present in this form of obesity, we examined the adrenergic regulation of lipolysis in abdominal subcutaneous fat cells from 25 women with upper-body obesity and 24 non-obese women. Lipolytic noradrenaline sensitivity (but not the maximum rate of lipolysis) was reduced by 10-fold in obese women (p<0.01). The noradrenaline resistance could be ascribed to a 10-fold decrease in lipolytic beta2-adrenoceptor sensitivity (p<0.01). The lipolytic sensitivity of beta1- and alpha2-adrenergic receptors was normal in the obese women. A 70 % reduction in the cell surface density of beta2-adrenoceptors was observed compared to the control subjects (p<0.01). However, beta1-receptor density as well as steady-state mRNA levels for beta1- and beta2-receptors were normal in obese women. Lipolytic noradrenaline sensitivity correlated inversely with BMI (adjusted r2=0.76 together with fat cell volume in stepwise regression analysis). The fasting plasma level of free cortisol was 30 % lower in obese compared to non-obese women (p<0.05) but obesity did not influence resting plasma catecholamine levels. Thus, lipolytic catecholamine resistance is present in abdominal obesity, due to low density of beta2-adrenoceptors, which in its turn may be caused by a post-transcriptional defect in beta2-receptor expression. Whether abnormalities in circulating free cortisol levels have caused the impaired lipolytic function of these receptors in upper-body obesity remains to be established.

Multiple lipolysis defects in the insulin resistance (metabolic) syndrome.

Bearing in mind the importance of upper-body obesity for the insulin resistance (or metabolic) syndrome and the abnormalities in free fatty acid metabolism associated with this disorder, the regulation of lipolysis in isolated subcutaneous adipocytes was investigated in 13 72-yr old upper-body obese men with insulin resistance and glucose intolerance and in 10 healthy 72-yr-old men. There was a marked resistance to the lipolytic effect of noradrenaline in the metabolic syndrome due to defects at two different levels in the lipolytic cascade. First, an 80-fold decrease in sensitivity to the beta 2-selective agonist terbutaline (P < 0.001) which could be ascribed to a 50% reduced number of beta 2-receptors (P < 0.005) as determined with radioligand binding. The groups did not differ as regards dobutamine (beta 1) or clonidine (alpha-2) sensitivity, nor beta 1-receptor number. The mRNA levels for beta 1- and beta 2-receptors were similar in the two groups. Second, the maximum stimulated lipolytic rate was markedly reduced in the metabolic syndrome. This was true for isoprenaline (nonselective beta-agonist), forskolin (activating adenylyl cyclase), and dibutyryl cAMP (activating protein kinase). In regression analysis, the observed abnormalities in lipolysis regulation correlated in an independent way with the degree of glucose intolerance (r = -0.67) and beta 2-receptor number with insulin resistance (r = 0.67). In conclusion, the results of this study indicate the existence of lipolytic resistance to catecholamines in the adipose tissue of elderly men with the metabolic syndrome, which may be of importance for impaired insulin action and glucose intolerance. The resistance is located at a posttranscriptional level of beta 2-receptor expression and at the protein kinase-hormone sensitive lipase level.

The association of human adipose angiotensinogen gene expression with abdominal fat distribution in obesity.

OBJECTIVE: To investigate in obese subjects the relationship between angiotensinogen gene expression in the abdominal omental and subcutaneous adipose tissue on the one hand and body fat distribution as measured by waist-to-hip ratio (WHR) on the other hand and to compare angiotensinogen gene expression between the two adipose tissue regions.SUBJECTS: Twenty obese subjects undergoing weight reduction surgery with adjustable gastric banding (12 men, eight women; WHR 0.89–1.09; body mass index (BMI) 29–51 kg/m2, age 26–54 y).MEASUREMENTS: Omental and subcutaneous adipose angiotensinogen mRNA and 18S ribosomal RNA (reference gene) levels were measured by competitive quantitative reverse transcriptase–polymerase chain reaction.RESULTS: Angiotensinogen mRNA levels were one-third higher in the omental than in the subcutaneous adipose tissue region (P=0.02). The 18S rRNA levels did not differ significantly between the two adipose tissue regions. WHR correlated positively and significantly with angiotensinogen mRNA in both the subcutaneous and the omental adipose tissue (r=0.5). This relationship was independent of age and BMI. However, WHR did not correlate with 18S rRNA in any of the adipose tissue regions.CONCLUSION: The angiotensinogen gene in adipose tissue might be involved in the development of upper-body obesity.

Impaired activation of adipocyte lipolysis in familial combined hyperlipidemia.

The pathophysiology of familial combined hyperlipidemia (FCHL) is unknown, but altered lipid turnover in peripheral tissues as well as hepatic overproduction of apolipoprotein B have been suggested as possible causes. In the present study, we explored whether a change in triglyceride breakdown by lipolysis in fat cells is present in FCHL. Lipolysis activation by catecholamines was examined in isolated subcutaneous adipocytes from 10 patients with FCHL and 22 healthy control subjects. Lipolysis rate was linear for at least 3 h in both groups. However, a marked (approximately 65%) decrease in the lipolytic response to noradrenaline was found in FCHL. This was also true when lipolysis was maximally stimulated at the receptor level with isoprenaline (nonselective beta-adrenergic agonist), at the adenylyl cyclase level with forskolin, or at the level of the protein kinase hormone-sensitive lipase complex with dibutyryl cAMP. The maximum enzymatic activity of hormone-sensitive lipase was decreased by approximately 40% in FCHL. On the other hand, the lipolytic sensitivity of alpha 2-, beta 1-, and beta 2-adrenoceptors was normal in this condition, as was the number and affinity of beta 1- and beta 2-adrenoceptors. Variations in the maximum lipolysis rate correlated significantly with the variations in hormone-sensitive lipase activity in the whole material, and with the serum values for triglycerides, HDL cholesterol and apoB lipoprotein within the control group, but the serum triglyceride values in FCHL were higher than this correlation predicted. In conclusion, the data demonstrate a marked resistance to the lipolytic effect of catecholamines in fat cells from patients with FCHL, in spite of normal adrenoceptor function. The lipolytic defect appears predominantly to be due to a defect in hormone-sensitive lipase, and may be of importance in the pathophysiology of FCHL.

The different effects of a Gln27Glu β2-adrenoceptor gene polymorphism on obesity in males and in females

Objectives. To investigate the role of a polymorphism in codon 27 (Gln27Glu) of the β2‐adrenoceptor gene for obesity in males compared to previously investigated females with an association of this polymorphism to obesity.

Noradrenaline-induced lipolysis in isolated mesenteric, omental and subcutaneous adipocytes from obese subjects.

OBJECTIVE: The action of noradrenaline on human mesenteric, omental and subcutaneous adipocytes was compared. We also determined whether regional differences in the noradrenaline-effect were linked to variations in adrenoceptor subtype function. DESIGN: The lipolytic effects of different concentrations of noradrenaline (β1-, β2-, β3- and α2-adrenoceptor agonist), isoprenaline (β1-, β2- and β3-adrenoceptor agonist) and selective β1-, β2- and β3-adrenoceptor agonists (dobutamine, terbutaline and CGP 12177, respectively) were studied in adipocytes isolated from the three adipose tissue regions in the same subject. In addition, the effect of the α2-adrenoceptor antagonist, yohimbine, was studied on noradrenaline-induced glycerol release. SUBJECTS: Thirteen otherwise healthy obese subjects (nine females, four males). RESULTS: The noradrenaline-induced lipolytic response did not differ between omental and mesenteric adipocytes but was 50% higher than in subcutaneous adipocytes (P<0.05). Furthermore, noradrenaline sensitivity and intrinsic activity (in relation to isoprenaline) were higher in the two visceral fat cells than in the subcutaneous fat cells. The intrinsic activity of noradrenaline increased close to that of isoprenaline when yohimbine was added to the incubation system. Isoprenaline sensitivity was five times higher in the two visceral fat cells than in the subcutaneous fat cells. For CGP 12177, sensitivity and intrinsic activity did not differ between mesenteric and omental adipocytes, but was higher in these two regions when compared to subcutaneous adipocytes. For dobutamine and terbutaline no significant regional differences were found. CONCLUSION: β3-adrenoceptor action is enhanced and α2-adrenoceptor action is decreased in both mesenteric and omental adipocytes as compared to subcutaneous adipocytes. However, the two visceral fat depots show no difference in adrenoceptor function. The difference in β3- and α2-adrenoceptor function might explain why noradrenaline induced lipolysis is increased in the two visceral fat depots, as compared to the subcutaneous fat depot.