Inflammation and matrix degrading proteases in coronary artery disease

Abstract

Coronary artery disease (CAD) is a major cause of morbidity and death in the world today. The underlying process, atherosclerosis, is caused by lipid accumulation and inflammatory processes within the intimal layer of the vessel wall. Stable CAD is characterized by gradually expanding atherosclerotic plaques that narrow the lumen and, by restricting the blood flow, give rise to ischemic symptoms. Acute coronary syndromes (ACS) are usually caused by vulnerable plaques, which rupture or erode, thus initiating thrombus formation that suddenly impairs the blood flow and may cause acute myocardial infarction (MI).

Inflammation is now considered to be an important feature of both atherosclerosis and atherothrombotic complications. Inflammatory cells, such as macrophages, T-cells and mast cells, are present in the atherosclerotic tissue and interact with the cells present in the vascular wall.

Matrix metalloproteinases (MMPs), a family of more than 20 zinc- and calcium-dependent proteases, are capable of degrading extracellular matrix components. MMPs are involved in both the expansion of atherosclerotic plaques and the weakening of the fibrous cap, thus contributing to the development of vulnerable plaques. Inflammatory cells are suggested to be the major source of MMPs.

The aim of this thesis was to evaluate the roles of MMP-3 and MMP-1 in CAD and MI, and how the expression of these MMPs may be modulated by inflammation. This was addressed by genetic analyses of known and novel polymorphisms in the MMP-3 and MMP-1 genes, biochemical analyses of circulating MMP-3, MMP-1, and various cytokines, and angiographic analyses of the extent and severity of CAD.

The individual studies were conducted in three different cohorts: i) the Stockholm Coronary Artery Risk Factor (SCARF) study, a case-control study of 387 post-infarction patients and 387 healthy control persons, ii) 164 patients from the Thrombolysis and REOCclusion (TREOC) study, a longitudinal cohort study of patients with ST-elevation MI, and iii) 1177 patients from the Southampton Atherosclerosis Study (SAS) with significant CAD as assessed by coronary angiography.

Scrum MMP-3 concentrations were lower in postinfarction patients compared with control subjects. Also, serum concentrations of MMP-3 were lower in the acute stage of MI (within 48 hours) than at the three months follow-up visit. Plasma MMP-1 concentrations did not differ between postinfarction patients and controls.

The MMP-3 -1612 5A/6A promoter polymorphism markedly influenced the serum MMP-3 concentrations in two different cohorts of postinfarction patients and in healthy controls. Serum MMP-3 concentrations increased with the number of 6A-alleles. Furthermore, the serum MMP-3 concentration increased with the number of diseased major coronary arteries, as evaluated by coronary angiography. Women had substantially lower serum MMP-3 concentrations compared with men in the SCARF and the TREOC study cohorts. In addition, plasma MMP-1 concentrations were lower in women in the SCARF study.

Studies of the MMP-1 gene revealed a haplotype effect of the MMP-1 -519 A/G and -340 C/T promoter polymorphisms on risk of MI. Compared with the A-519 -T-340 haplotype, both the A-519 -C-340 and G-519 -T-340 haplotypes showed a protective effect against MI whereas the G-519-C-340 haplotype was associated with an increased risk of MI. Functional studies of gene expression demonstrated a lower promoter activity of the A-519 -C340 and G-519 -T-340 haplotypes than of the A-519 -T-340 haplotype. Also, mRNA levels in atherosclerotic plaques were significantly lower in plaques retrieved from carriers of the A-519 -C-340 and G-519 -T-340 haplotypes than from carriers of the A-519 -T-340 haplotype.

Plasma concentrations of interleukin-2 (IL-2), IL-6, IL-8, and tumour necrosis factor-alpha were higher in postinfarction patients than in controls. Furthermore, there was a difference between MMP-3 and MMP-1 concentrations as regards their respective correlations with several circulating inflammatory cytokines. Correlations were found for MMP-1, but not for MMP-3.

In conclusion, the results presented in this thesis reinforce the novel alternative hypothesis of divergent effects of different MMPs in atherosclerosis and CAD. Inflammation is suggested to exert a stronger influence on MMP-1 than on MMP-3. Reduced MMP-3 concentration seems to be associated with MI, whereas increased concentration may promote progression of stable CAD.