• Discovery of adiponectin
• Atherosclerosis
• Diabetes mellitus
• Blood pressure and vasorelaxation
• Inflammation
• Organ fibrosis
• Cancer
• Insights into Preventive Medicine
• References

Adiponectin: a new "hot" marker of the Metabolic Syndrome

Tohru FUNAHASHI,Yuji MATSUZAWA Department of Internal Medicine and Molecular Science, Osaka University Graduate School of Medicine, and Sumitomo Hospital; Osaka; Japan Correspondence: tohru@imed2.med.osaka-u.ac.jp

The Metabolic Syndrome clusters glucose intolerance, dyslipidaemia, and hypertension in an individual and currently appears as a major target for preventing coronary heart disease (CHD). It is postulated that the multiple coronary risk factors associated with the Metabolic Syndrome do not gather by chance, but that a common upstream factor induces these risks. Insulin resistance is a common feature of the Metabolic Syndrome, but its aetiology remains unclear. Obesity is a common disorder accompanying glucose intolerance, dyslipidaemia, and hypertension. However, not all the obese individuals present with these disorders. In addition, patients with the Metabolic Syndrome are not extremely obese.

During the early 80’s, our group introduced computer-tomography (CT) scan method to evaluate the amount of adipose tissue associated with obesity. Although the assessment of the total amount of subcutaneous fat by this method did not provide any additional information on obesity-related diseases beyond body mass index (BMI), the CT scan method enabled us to evaluate the amount of adipose tissue found in the intra-abdominal cavity. Subjects with predominantly intra-abdominal visceral fat excess also frequently have glucose intolerance, dyslipidaemia and hypertension, as compared to those with predominantly subcutaneous fat accumulation. More importantly, approximately 40% of the patients with CHD have visceral fat associated with multiple coronary risk factors. We named this pathogenetic condition ‘visceral fat syndrome’, a concept which appears very close to the Metabolic Syndrome and associated CHD. But a major question remained unanswered: what are the molecular mechanisms linking visceral fat to CHD?

Discovery of adiponectin

To answer this question, our group investigated the gene expression profile in adipose depots during the early 90’s. Traditionally, the adipose tissue has been regarded only as a simple energy storage organ. Surprisingly, we found that adipose tissues, especially visceral fat, expressed a variety of genes for secretory proteins including growth factors, cytokines and vasoactive substances such as type 1 plasminogen activator inhibitor (PAI1) and heparin binding EGF-like growth factor (HB-EGF). Accumulated visceral fat overproduces and secretes PAI-1, which in turn raises the risk for thrombotic disorders. ¹ Thus, it appeared that a direct link exists between visceral fat and CHD. We named collectively these adipocyte-derived molecules as ‘adipocytokines’.² The ‘visceral fat syndrome’ accelerates atherosclerosis due to the dysregulation of adipocytokines and clustering of other coronary risk factors.

Approximately 50% of the genes analyzed were unknown, and had not been registered in the ‘Genbank’ database. Through the systemic analysis of adipose-expressed genes, we discovered a novel adipocyte-derived factor, named ‘adiponectin’. Adiponectin is specifically expressed in adipose tissue. The molecule has two distinct domains, namely a collagen-like fibrous domain and C1q-like globular domain. Single molecules combine together and form a high-ordered structure. Regarding its effects, adiponectin has two interesting characteristics:

• its plasma concentrations decrease upon accumulation of visceral fat, in contrast to PAI-1 (Figure 1).
• the protein bounds to collagens I, III, and V, which are present in the subendothelial intima and accumulate in injured vascular walls (Figure 2). ³ This is the reason why we named this protein ‘adiponectin’.

Atherosclerosis

We tested the effects of adiponectin on the vascular components. Administration of adiponectin suppressed:

• TNF alpha-induced expression of adhesion molecules in the vascular endothelial cells (by inhibiting nuclear translocation of NFkappaB),
• growth factor-induced proliferation of smooth muscle cells (by inhibiting classical mitogen activated protein (MAP) kinase pathway),
• foam cell transformation of the macrophages (by inhibiting the expression of scavenger receptor).

P⁴ Zoccali et al⁵ showed that people with high plasma adiponectin levels at baseline had lower incidence of cardiac death. These data suggested that circulating adiponectin accumulates in the places where the endothelial barrier is damaged, and has protective effects against almost the whole process of atherosclerosis. To test this hypothesis, we generated adiponectin knockout (KO) mice, which showed severe neointimal thickening in response to vascular injury. We also showed that the overexpression of adiponectin in apoE knockout mice prevented the progression of fatty streak lesion.⁶ Recently, it has been reported that adiponectin could be an independent negative risk factor for myocardial infarction in a case-control study of 18,000 US men.⁷

Diabetes mellitus

Pima Indians are prone to obesity and type 2 diabetes, but not all of them develop type 2 diabetes. Weyer, Tataranni et al⁸ showed that plasma adiponectin levels are lower in Pima Indians than in Caucasians. Interestingly, these authors showed that plasma adiponectin levels were correlated positively with insulin sensitivity, determined by glucose clamp test. We tested insulin sensitivity in adiponectin KO mice 9: they exhibited severer insulin resistance as compared with wild-type mice. Our and Tataranni’s groups 10 demonstrated that Pima Indians with high plasma adiponectin levels at baseline had low risk of later onset diabetes. The significance of adiponectin as a negative risk for diabetes has been confirmed by other groups. Therefore, adiponectin has a dual protecting effect both against diabetes and atherosclerosis.

Basically, hypoadiponectinaemia is caused by accumulation of visceral fat. However, analysis of the clinical phenotype associated with hypoadiponectinaemia caused by genetic mutations can provide further information on the role of this protein in the Metabolic Syndrome. We screened the mutations in the adiponectin gene and focused on a missense mutation (substitution of isoleucine 164 to threonine in the globular domain). A disturbed secretion of this mutant protein was noted, together with markedly low plasma adiponectin levels. Subjects carrying this mutation were frequently hypertensive, hyperlipidaemic, diabetic, and had atherosclerotic vascular diseases (i.e., they had the clinical phenotype of Metabolic Syndrome).¹¹ Thus, both environmental and genetic causes of hypoadiponectinaemia are associated with this condition. Adiponectin may be a key player in the Metabolic Syndrome.

Blood pressure and vasorelaxation

Wiecek et al¹² reported that plasma adiponectin levels are negatively correlated with mean blood pressure (BP) in patients with essential hypertension. We investigated the relation between adiponectin and vasoreactivity in hypertensive patients, and analyzed also BP levels and vasoreactivity in adiponectin-knockout (KO) and wild-type mice.¹³ The plasma adiponectin levels were significantly correlated with the forearm vasodilator response but not with nitroglycerin-induced hyperemia in patients with essential hypertension. Blood pressure was slightly higher in adiponectin KO mice. The acetylcholine-induced but not sodium nitroprusside induced vasorelaxation was impaired in adiponectin KO animals, suggesting that the endothelium-dependent vasodilation is disturbed in adiponectin deficiency. Administration of adiponectin activated AMP kinase, Akt and endogenous nitric oxide synthase enhanced nitric oxide production in vascular endothelial cells.¹⁴

Inflammation

Adiponectin suppresses the TNF alpha-signaling pathway in endothelial cells and expression of TNF alpha in macrophages, suggesting anti-inflammatory effect of this protein. In contrast, TNF alpha suppresses the expression of adiponectin in adipocytes. Thus, these adipocytokines suppress the expression of each other. Plasma levels of an inflammatory marker, the high-sensitive C-reactive protein (CRP), were negatively correlated with plasma adiponectin levels. We found that the CRP mRNA was expressed and its levels were increased in the adipose tissue of the KO mice.¹⁵ The reduction of adiponectin enhances the production of TNF alpha and CRP in adipocytes. Reduced adiponectin and increased TNF alpha will lead to insulin resistance in the muscle. Decreased plasma adiponectin also promotes the local production of TNF alpha in macrophages, which accelerates the inflammatory process in vascular wall.

Organ fibrosis

Recently, non-alcoholic steatohepatitis (NASH) and subsequent hepatic fibrosis were noticed as obesity-related disorders. Atherosclerosis is a sort of wound healing process. Adiponectin may have some protective roles on the organ fibrosis. Adiponectin KO mice showed more severe carbon tetrachloride-induced hepatic fibrosis as compared with wildtype mice.¹⁶ The stellate cells, which synthesize collagen fibres, were remarkably activated in the KO mice. The mRNA levels of fibrosis-inducing cytokines, TGF-beta1 and the connective tissue growth factor, were increased in the KO mice. Smad2 controls the expression of these genes. Adiponectin suppressed the nuclear translocation of Smad2 in cultured stellate cells.

Cancer

Association of obesity with increased risk for breast and endometrial cancers has been well established. Aromatizing conversion of the androgens to estrone in the adipose tissue may contribute to the development of these cancers. However, dysregulation of adipocyte-derived hormones may also play a role in increasing the risk for these cancers. We have recently reported that low serum adiponectin levels are associated with an increased risk for breast cancer.¹⁷ It has been reported also that serum adiponectin levels are inversely related to the risk of endometrial cancer, independently of BMI, in women younger than 65. Recently, Cao et al¹⁸ showed that adiponectin inhibits primary tumour growth in a mouse tumour model. This inhibition was associated with increased tumour cell apoptosis. Although further investigations on the anti-tumourigenic effects of adiponectin are needed, hypoadiponectinaemia may be involved in the development of obesity-associated cancers.

Insights into Preventive Medicine

Visceral fat accumulation is associated with disturbance of multiple adipocytokines. Increased PAI-1 enhances thrombotic tendency. Adiponectin suppresses both the atherosclerotic process and the production of an inflammatory cytokine, TNF alpha, from macrophages. Recently, we found that adiponectin increases the production of tissue inhibitor of metalloproteinase-1 from macrophages, suggesting that the reduction of adiponectin may facilitate coronary plaque rupture.¹⁹ Adipocytokines may play the role of molecular links between two syndromes, the Metabolic Syndrome and the acute coronary syndrome (Figure 3). Direct infusion of adiponectin for the treatment of atherosclerotic vascular disease or diabetes will not be practical because plasma concentrations of adiponectin are very high. However, many reports show that body weight reduction, physical exercise and lifestyle changes can raise plasma adiponectin levels. In addition, agents such as thiazolidinediones, renin-angiotensin system blockers and glimepiride have been reported to increase plasma adiponectin concentrations. Population-based studies are needed to assess the predictive power of plasma adiponectin levels on atherosclerotic vascular diseases during these interventions. In the current era of overnutrition, plasma adiponectin may become a novel biomarker for atherosclerotic vascular diseases as well as plasma cholesterol and glucose levels.

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