There is mounting evidence that
inflammation plays a role in the development of
coronary heart disease (CHD). Observations have been made linking the presence of
infections in the vessel wall with
atherosclerosis, and epidemiological data also implicate
infection in remote sites in the aetiology of CHD. In this article we propose a key role for the proinflammatory
cytokine interleukin-6 (IL-6) in several mechanisms that contribute to the development of CHD.
IL-6 is a powerful inducer of the hepatic
acute phase response. Elevated concentrations of
acute phase reactants, such as
C-reactive protein (CRP), are found in patients with
acute coronary syndromes, and predict future risk in apparently healthy subjects. The
acute phase reaction is associated with elevated levels of
fibrinogen, a strong risk factor for CHD, with autocrine and paracrine activation of monocytes by
IL-6 in the vessel wall contributing to the deposition of
fibrinogen. The
acute phase response is associated with increased blood viscosity, platelet number and activity. Furthermore, raised
serum amyloid A lowers
HDL-cholesterol levels.
IL-6 decreases
lipoprotein lipase (LPL) activity and monomeric LPL levels in plasma, which increases macrophage uptake of
lipids. In fatty streaks and in the atheromatous 'cap' and 'shoulder' regions, macrophage foam cells and smooth muscle cells (SMC) express
IL-6, suggesting a role for this
cytokine along with
interleukin-1 (IL-1) and tumour
necrosis factor-alpha (
TNF-alpha), in the progression of
atherosclerosis. Both these
cytokines induce the release of
IL-6 from several cell types, including SMC. During
vascular injury SMC are exposed to platelets or their products, and
cytokine production by SMC further contributes to vascular damage. Furthermore, circulating
IL-6 stimulates the hypothalamic-pituitary-adrenal (HPA) axis, activation of which is associated with
central obesity,
hypertension and
insulin resistance. Thus we propose a role for
IL-6 in the pathogenesis of CHD through a combination of autocrine, paracrine and endocrine mechanisms. This hypothesis lends itself to testing using interventions to influence
IL-6 secretion and actions.