Adiponectin and the metabolic syndrome: mechanisms mediating risk for metabolic and cardiovascular disease

Low serum adiponectin level as a predictor of impaired glucose regulation and type 2 diabetes mellitus in a middle-aged Finnish population

Low levels of adiponectin are associated with obesity and type 2 diabetes mellitus (DM2). However, only few studies on this topic have used the most recent World Health Organization 1999 criteria, which include a definition of impaired glucose regulation (IGR). Our objective was to find out if a baseline low adiponectin level in initially normoglycemic subjects predicted IGR or DM2 during a mean follow-up period of 5.1 years. A population-based cohort study was carried out in Oulu, Northern Finland. Subjects born in 1935 and living in Oulu in 1990 were invited to participate in a follow-up study. At baseline, oral glucose tolerance tests and measurements of adiponectin, lipids, blood pressure, and body mass index were performed; and oral glucose tolerance tests were repeated at follow-up. Analyses were performed for 201 subjects who were normoglycemic at baseline. Low adiponectin level was defined as the lowest quartile of adiponectin levels. During the follow-up, 47 (23%) of the 201 subjects developed IGR or DM2. Impaired glucose regulation or DM2 developed in 15 of 41 (37%) subjects with low adiponectin level at baseline, whereas the corresponding proportion was 32 of 160 (20%) subjects with higher adiponectin levels (P = .025). In logistic regression analysis, the adjusted odds ratio for IGR or DM2 was 2.1 (95% confidence interval, 1.0-4.5) when adjustment was made for sex and body mass index. Low concentrations of adiponectin predicted subsequent development of IGR and DM2 in initially normoglycemic middle-aged Finnish subjects. Our findings support the hypothesis that adiponectin may play a role in the pathogenesis of abnormal glucose metabolism.

Intravascular kinetics of C-reactive protein and their relationships with features of the metabolic syndrome

OBJECTIVE

The objective of the study was to describe, for the first time, the intravascular kinetics of C-reactive protein (CRP), using stable isotopes, and its relationship with features of the metabolic syndrome.

METHODS

Sixteen men and 16 women [aged 49 +/- 9 years, body mass index (BMI) 28.7 +/- 4.5 kg/m(2)] underwent a 12-h primed-constant infusion of 5,5,5-(2)H(3)-l-leucine. CRP was purified from the plasma fraction rho greater than 1.25 g/ml by affinity chromatography, and isotopic enrichment over time was determined by gas chromatography/mass spectrometry.

RESULTS

The CRP fractional catabolic rate was 60% higher in men than women (0.49 +/- 1.83 vs. 0.30 +/- 1.80 pool/d, P = 0.03), but this difference was no longer significant in a multivariate model that included several features associated with the metabolic syndrome. The CRP production rate (PR) and pool size were not statistically different between sexes. Plasma CRP concentrations were more strongly correlated with the PR (r = 0.80, P < 0.0001) than with the fractional catabolic rate of CRP (r = 0.39, P < 0.05). The PR of CRP was positively correlated with waist girth (r = 0.53, P < 0.01), plasma low-density lipoprotein apolipoprotein B-100 (r = 0.42, P = 0.07), triglyceride (r = 0.41, P = 0.06), and IL-6 concentrations (r = 0.61, P = 0.0008) and inversely correlated with high-density lipoprotein-cholesterol (r = -0.47, P = 0.03) and adiponectin (r = -0.63, P < 0.0005) after adjustment for sex. Blood pressure and low-density lipoprotein-cholesterol showed no association with CRP kinetics.

CONCLUSION

The PR of CRP appeared as the main determinant of CRP concentrations and showed significant associations with features of the metabolic syndrome as well as with adipose tissue-derived cytokines such as IL-6 and adiponectin.

Gender specificity of body adiposity and circulating adiponectin, visfatin, insulin, and insulin growth factor-I at term birth: relation to prenatal growth

CONTEXT

Fetal development is thought to be gender specific for adiposity and circulating insulin and IGF-I but not adipokinemia, as judged by serum visfatin and adiponectin at term birth. We studied the potential relationship between these gender specificities and fetal growth.

SETTING

The study was conducted at a university hospital.

STUDY POPULATION

Subjects included 96 strictly matched neonates born appropriate for gestational age (AGA; 24 girls, 24 boys) or small for gestational age (SGA; 24 girls, 24 boys).

MAIN OUTCOMES

Outcomes included serum insulin, IGF-I, visfatin, total and high-molecular-weight (HMW) adiponectin, osteocalcin at term birth, and neonatal body composition by absorptiometry.

RESULTS

Cord insulin and IGF-I levels were higher in girls than boys (P < or = 0.01), in both the AGA and SGA subpopulation. In AGA newborns, fat and lean mass were each gender specific (P < 0.0001), whereas visfatin and total and HMW adiponectin were not. Conversely, in SGA newborns, visfatin and HMW adiponectin were gender specific (higher levels in girls), whereas body adiposity was not. In SGA fetuses, the distribution of adiponectin isoforms was in both genders shifted toward HMW (P < 0.005 vs. AGA). Cord osteocalcin did not differ by either gender or birth weight.

CONCLUSION

At term birth, the gender specificity of adiposity and circulating visfatin and HMW adiponectin appeared to depend on prenatal growth, whereas the gender specificity of insulin and IGF-I levels did not. The fetal shift in adiponectin isoforms may contribute to explain why SGA newborns tend to be hypersensitive to insulin.

Expression of long pentraxin PTX3 in human adipose tissue and its relation with cardiovascular risk factors

Pentraxin 3 (PTX3) is an acute phase protein strongly expressed by advanced atherosclerotic lesions. We investigated (a) PTX3 expression and secretion in subcutaneous adipose tissue (SAT) and omental visceral adipose tissue (VAT) obtained from 21 obese (37.4+/-8.15 yr) and 10 normal weight subjects (43.7+/-11.07 yr) and (b) the relationships of adipose PTX3 with tumour necrosis factor alpha (TNFalpha) and adiponectin expression and with cardiometabolic risk factors. Real-time PCR was used to quantify specific mRNA for PTX3, CD68 (macrophage marker), TNFalpha and adiponectin. Fresh adipose tissue was cultured and PTX3 measured in the medium. Serum insulin, glucose, HDL and LDL cholesterol, triglycerides, C-reactive protein (CRP), fibrinogen, adiponectin, TNFalpha and PTX3 were measured. PTX3 expression was similar in the two fat compartments and tended to be higher in obese than in normal weight subjects in VAT only (p=0.05). CD68 and PTX3 expressions were correlated with each other in SAT but not in VAT. After adjustment for age and sex, VAT-PTX3 expression and release were correlated with VAT-TNFalpha expression (p<0.01 for both) and with LDL/HDL ratio (p<0.01 and p<0.001). VAT-PTX3 expression was also correlated with BMI, triglycerides, CRP, fibrinogen and adiponectin (p<0.05 for all). In the multivariate analysis with VAT-PTX3 RNA levels as dependent variable, LDL/HDL ratio and fibrinogen remained independently associated with VAT-PTX3 expression (p<0.01 for both). These associations were not seen within SAT.

CONCLUSIONS

Human adipose tissue expresses and releases PTX3 likely under TNFalpha control. VAT production of PTX3 seems to contribute to the mechanisms underlying the development of atherosclerosis.

Reduced response to adiponectin and lower abundance of adiponectin receptor proteins in type 2 diabetic monocytes

The abundance of the adiponectin receptors, AdipoR1 and AdipoR2, and the effects of the antidiabetic adipokine adiponectin in monocytes of normal-weight and overweight controls and type 2 diabetic patients (T2D) were analyzed. AdipoR1 and AdipoR2 mRNAs were increased in monocytes of obese controls and T2D patients when compared to normal-weight controls, and AdipoR1 mRNA positively correlated to AdipoR2 mRNA, the waist to hip ratio and systemic adiponectin. However, AdipoR1 and AdipoR2 proteins were lower in monocytes of T2D compared to normal-weight donors. Induction of IL-6 and IL-8 by adiponectin, an effect involving p38 MAPK, was also reduced in T2D monocytes.

Genome-wide linkage analysis for circulating levels of adipokines and C-reactive protein in the Quebec family study (QFS)

Adipose tissue synthesizes and secretes a wide range of biologically active molecules considered as inflammatory markers whose dysregulation in obesity plays a role in the development of insulin resistance and vascular disorders. Thus, finding genes that influence circulating levels of inflammatory biomarkers may provide insights into the genetic determinants of obesity-related metabolic diseases. We performed linkage analyses for fasting plasma levels of adiponectin, C-reactive protein (CRP), interleukin-6 (IL-6) and tumor-necrosis factor-alpha (TNF-alpha) in 764 subjects enrolled in the Quebec family study (QFS). A maximum of 393 pairs of siblings from 211 nuclear families were available for analyses. A total of 443 markers spanning the 22 autosomal chromosomes with an average inter-marker distance of 6.24 Mb were genotyped. Linkage was tested using both allele-sharing (SIBPAL) and variance component linkage methods (MERLIN). We showed suggestive evidence of linkage for plasma adiponectin levels on chromosome 15q21.1 [D15S659; logarithm of the odds (LOD) score = 2.23], 3q13.33 (D3S3023; LOD = 2.09), 20q13.2 (D20S197; LOD = 1.96) and 14q32.2 (D14S1426; LOD = 1.79). Evidence of linkage (SIBPAL) was also found for CRP on 12p11.23 (P = 0.001) and 12q15 (P = 0.0005) and for IL-6 on 14q12 (P = 0.002). None of these linkages remained significant after adjustment for body mass index. No evidence of linkage was found for TNF-alpha plasma levels. These results suggest that several QTLs can influence plasma levels of adiponectin and CRP, partly via their effects on adiposity.

Vascular effects of adiponectin: molecular mechanisms and potential therapeutic intervention

Adiponectin is a major adipocyte-secreted adipokine abundantly present in the circulation as three distinct oligomeric complexes. In addition to its role as an insulin sensitizer, mounting evidence suggests that adiponectin is an important player in maintaining vascular homoeostasis. Numerous epidemiological studies based on different ethnic groups have identified adiponectin deficiency (hypoadiponectinaemia) as an independent risk factor for endothelial dysfunction, hypertension, coronary heart disease, myocardial infarction and other cardiovascular complications. Conversely, elevation of circulating adiponectin concentrations by either genetic or pharmacological approaches can alleviate various vascular dysfunctions in animal models. Adiponectin exerts its vasculoprotective effects through its direct actions in the vascular system, such as increasing endothelial NO production, inhibiting endothelial cell activation and endothelium-leucocyte interaction, enhancing phagocytosis, and suppressing macrophage activation, macrophage-to-foam cell transformation and platelet aggregation. In addition, adiponectin reduces neointima formation through an oligomerization-dependent inhibition of smooth muscle proliferation. The present review highlights recent research advances in unveiling the molecular mechanisms that underpin the vascular actions of adiponectin and discusses the potential strategies of using adiponectin or its signalling pathways as therapeutic targets to combat obesity-related metabolic and vascular diseases.

Insulin resistance: a proinflammatory state mediated by lipid-induced signaling dysfunction and involved in atherosclerotic plaque instability

The dysregulation of the insulin-glucose axis represents the crucial event in insulin resistance syndrome. Insulin resistance increases atherogenesis and atherosclerotic plaque instability by inducing proinflammatory activities on vascular and immune cells. This condition characterizes several diseases, such as type 2 diabetes, impaired glucose tolerance (IGT), impaired fasting glucose (IFG), obesity, hypertension, dyslipidemia, and other endocrinopathies, but also cancer. Recent studies suggest that the pathophysiology of insulin resistance is closely related to interferences with insulin-mediated intracellular signaling on skeletal muscle cells, hepatocytes, and adipocytes. Strong evidence supports the role of free fatty acids (FFAs) in promoting insulin resistance. The FFA-induced activation of protein kinase C (PKC) delta, inhibitor kappaB kinase (IKK), or c-Jun N-terminal kinase (JNK) modulates insulin-triggered intracellular pathway (classically known as PI3-K-dependent). Therefore, reduction of FFA levels represents a selective target for modulating insulin resistance.

Inflammation in atherosclerosis: from vascular biology to biomarker discovery and risk prediction

Recent investigations of atherosclerosis have focused on inflammation, providing new insight into mechanisms of disease. Inflammatory cytokines involved in vascular inflammation stimulate the generation of endothelial adhesion molecules, proteases, and other mediators, which may enter the circulation in soluble form. These primary cytokines also induce production of the messenger cytokine interleukin-6, which stimulates the liver to increase production of acute-phase reactants such as C-reactive protein. In addition, platelets and adipose tissue can generate inflammatory mediators relevant to atherothrombosis. Despite the irreplaceable utility of plasma lipid profiles in assessment of atherosclerotic risk, these profiles provide an incomplete picture. Indeed, many cardiovascular events occur in individuals with plasma cholesterol concentrations below the National Cholesterol Education Program thresholds of 200 mg/dL for total cholesterol and 130 mg/dL for low-density lipoprotein (LDL) cholesterol. The concept of the involvement of inflammation in atherosclerosis has spurred the discovery and adoption of inflammatory biomarkers for cardiovascular risk prediction. C-reactive protein is currently the best validated inflammatory biomarker; in addition, soluble CD40 ligand, adiponectin, interleukin 18, and matrix metalloproteinase 9 may provide additional information for cardiovascular risk stratification and prediction. This review retraces the biology of atherothrombosis and the evidence supporting the role of inflammatory biomarkers in predicting primary cardiovascular events in this biologic context.

Adiponectin and inflammation: consensus and controversy

Circulating levels of adiponectin decrease with increasing visceral obesity and are lower in patients with type 2 diabetes, the metabolic syndrome, and cardiovascular disease compared with controls matched by body mass index. Several reports demonstrated anti-inflammatory effects of adiponectin. Because increased adipose tissue is associated with low-grade chronic inflammation and proinflammatory factors inhibit adiponectin production, the current hypothesis states that chronic inflammation associated with visceral obesity inhibits production of adiponectin, perpetuating inflammation. The negative correlation between adiponectin and markers of inflammation in the aforementioned conditions supports this hypothesis. In contrast with disorders typically associated with excess adiposity and positive energy balance, adiponectin levels are elevated--rather than decreased--in classic chronic inflammatory/autoimmune diseases that are unrelated to increased adipose tissue, such as rheumatoid arthritis, SLE, inflammatory bowel disease, type 1 diabetes, and cystic fibrosis. In these patients, adiponectin levels positively--rather than negatively--correlate with inflammatory markers. Furthermore, proinflammatory effects of adiponectin have been reported in tissues such as joint synovium and colonic epithelium. Thus, adiponectin is regulated in the opposite direction and may exert differential functions in classic versus obesity-associated inflammatory conditions. This article discusses this apparent paradox and presents possible alternative and/or complementary explanations.

Recent advances in the measurement of adiponectin isoform distribution

PURPOSE OF REVIEW

Adiponectin has potent antidiabetic and antiatherosclerotic actions. Recent studies in animals and humans suggest that the high-molecular weight adiponectin complex, consisting of many adiponectin monomers, is the biologically active form of the peptide. This article will present recent methodological approaches for analyzing adiponectin isoform distribution.

RECENT FINDINGS

A handful of methods have been used for the isolation and measurement of high-molecular weight adiponectin, based on velocity gradient centrifugation, gel filtration chromatography and polyacrylamide gel electrophoresis. Recently, two novel sandwich enzyme-linked immuno-sorbent assays have been developed. The first makes use of antibodies raised against human high-molecular weight adiponectin and thus allows for the specific determination of high-molecular weight adiponectin in plasma. The second and more versatile enzyme-linked immuno-sorbent assay system enables the measurement of all adiponectin isoforms (i.e. low, middle and high-molecular weight) by means of selective digestion by proteases.

SUMMARY

The development and commercial availability of novel enzyme-linked immuno-sorbent assay kits enables the easy and rapid measurement of high-molecular weight adiponectin in both research and clinical practice and will undoubtedly advance further our understanding of the role of adiponectin in health and disease.

Low molecular weight adiponectin negatively correlates with the waist circumference and monocytic IL-6 release

Adiponectin circulates as trimer (LMW), hexamer (MMW) and high molecular weight multimer (HMW) but the distribution and effects of these isoforms have not been studied in detail. Monocytes were isolated from normal weight and overweight controls and patients with type 2 diabetes mellitus (T2D) and monocytic release of IL-6 positively correlated with the body mass index (BMI). HMW-adiponectin further enhanced and LMW-adiponectin reduced IL-6 release in monocytes. Systemic total adiponectin, and the HMW isoform were not different in these groups but MMW-adiponectin was lower in T2D, and LMW-adiponectin was reduced in the obese and T2D. Circulating LMW-adiponectin negatively correlated to monocytic IL-6 release. Systemic IL-6 was higher in the obese control group and T2D, respectively, but did not correlate with monocytic IL-6 secretion. Therefore, the current study indicates that HMW-adiponectin exerts pro- and LMW-adiponectin antiinflammatory effects and reduced LMW-adiponectin in obesity may partly contribute to elevated monocytic IL-6 release.