Abstract-C-reactive protein, a hepatic acute phase protein largely regulated by circulating levels of interleukin-6, predicts coronary heart disease incidence in healthy subjects. We have shown that subcutaneous adipose tissue secretes interleukin-6 in vivo. In this study we have sought associations of levels of C-reactive protein and interleukin-6 with measures of obesity and of chronic infection as their putative determinants. We have also related levels of C-reactive protein and interleukin-6 to markers of the insulin resistance syndrome and of endothelial dysfunction. We performed a cross-sectional study in 107 nondiabetic subjects: (1) Levels of C-reactive protein, and concentrations of the proinflammatory cytokines interleukin-6 and tumor necrosis factor-␣, were related to all measures of obesity, but titers of antibodies to Helicobacter pylori were only weakly and those of Chlamydia pneumoniae and cytomegalovirus were not significantly correlated with levels of these molecules. Levels of C-reactive protein were significantly related to those of interleukin-6 (rϭ0.37, PϽ0.0005) and tumor necrosis factor-␣ (rϭ0.46, PϽ0.0001).(2) Concentrations of C-reactive protein were related to insulin resistance as calculated from the homoeostasis model assessment model, blood pressure, HDL, and triglyceride, and to markers of endothelial dysfunction (plasma levels of von Willebrand factor, tissue plasminogen activator, and cellular fibronectin). A mean standard deviation score of levels of acute phase markers correlated closely with a similar score of insulin resistance syndrome variables (rϭ0.59, PϽ0.00005), this relationship being weakened only marginally by removing measures of obesity from the insulin resistance score (rϭ0.53, PϽ0.00005). These data suggest that adipose tissue is an important determinant of a low level, chronic inflammatory state as reflected by levels of interleukin-6, tumor necrosis factor-␣, and C-reactive protein, and that infection with H pylori, C pneumoniae, and cytomegalovirus is not. Moreover, our data support the concept that such a low-level, chronic inflammatory state may induce insulin resistance and endothelial dysfunction and thus link the latter phenomena with obesity and cardiovascular disease. (Arterioscler Thromb Vasc Biol. 1999;19:972-978.)

Cytokines appear to be major regulators of adipose tissue metabolism. Therapeutic modulation of cytokine systems offers the possibility of major changes in adipose tissue behaviour. Cytokines within adipose tissue originate from adipocyte, preadipocyte and other cell types. mRNA expression studies show that adipocytes can synthesise both tumour necrosis factor a (TNF-a) and several interleukins (IL), notably IL-1b and IL-6. Other adipocyte products with ‘immunological’ actions include complement system products and macrophage colony-stimulating factor. Cytokine secretion within adipocytes appears similar to that of other cells. There is general agreement that circulating TNF-a and IL-6 concentrations are mildly elevated in obesity. Most studies suggest increased TNF-a mRNA expression or secretion in vitro in adipose tissue from obese subjects. The factors regulating cytokine release within adipose tissue appear to include usual ‘inflammatory‘ stimuli such as lipopolysaccaride, but also the size of the fat cells per se and catecholamines. There is conflicting data about whether insulin and cortisol regulate TNF-a. The effects of cytokines within adipose tissue include some actions that might be characterised as metabolic. TNF-a and IL-6 inhibit lipoprotein lipase, and TNF-a additionally stimulates hormone-sensitive lipase and induces uncoupling protein expression. TNF-a also down regulates insulin-stimulated glucose uptake via effects on glucose transporter 4, insulin receptor autophosphorylation and insulin receptor substrate-1. All these effects will tend to reduce lipid accumulation within adipose tissue. Other effects appear more ‘trophic’, and include the induction of apoptosis, regulation of cell size and induction of de-differentiation (the latter involving reduced peroxisome proliferator-activated receptor g). Cytokines are important stimulators and repressors of other cytokines. In addition, cytokines appear to modulate other regulatory systems. Examples of the latter include effects on leptin secretion (probably stimulation followed by inhibition) and reduction of b3-adrenoceptor expression. There seems to be no clear agreement as to which cytokines derived from adipose tissue act as remote regulators, i.e. hormones. Leptin, which is structurally a cytokine, is also a hormone. IL-6 appears to be released systemically by adipose tissue, but TNF-a is probably not. Both leptin and IL-6 appear to act on the hypothalamus, IL-6 acts on the liver, while leptin may have actions on the pancreas. The importance of the immune system in whole-body energy balance provides a rationale for the links between cytokines and adipose tissue. It seems clear that TNF-a is a powerful autocrine and paracrine regulator of adipose tissue. Other cytokines, notably leptin, and possibly IL-6, have lesser actions on adipose tissue. These cytokines act as hormones, reporting the state of adipose tissue stores throughout the body.

Insulin resistance has been proposed as the metabolic basis of atherogenesis. This hypothesis is based on the concept of the "insulin resistance syndrome," according to which insulin resistance is viewed as the primary abnormality that gives rise to dyslipidemia, essential hypertension, impaired glucose tolerance, and NIDDM. However, this hypothesis takes no account of the well-established and central role of vascular endothelium in the atherogenic process. Although endothelial injury is an early and prominent feature of atherogenesis, relatively little attention has been given to its metabolic consequences. In subjects with NIDDM, we have shown that endothelial dysfunction is associated with insulin resistance, raising the question of whether this relationship could be causal. In this article, we review the factors that are considered to be responsible for the development of endothelial dysfunction during atherogenesis, together with the metabolic consequences of endothelial dysfunction. While dysfunction of the endothelium in large and medium-sized arteries plays a central role in atherogenesis, we argue that dysfunction of peripheral vascular endothelium, at arteriolar and capillary level, plays the primary role in the pathogenesis of both insulin resistance and the associated features of the insulin resistance syndrome. We propose that the insulin resistance syndrome, together with many aspects of atherogenesis, can be viewed as the diverse consequences of endothelial dysfunction in different vascular beds. This new and testable hypothesis accounts for both the endothelial and metabolic abnormalities associated with atherogenesis.

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