XII^th International Symposium on Atherosclerosis, Stockholm, Sweden. (June 25-29, 2000)
The degradation of extracellular matrix within the aortic wall by proteolytic enzymes has been implicated in several pathological conditions such as aneurysm formation and atherosclerotic plaque rupture, leading to atherosclerosis complications.

Stockholm, Sweden -- One of the workshops of the Symposium has been dedicated to the issue of plaque rupture. This is the most common pathology underlying acute myocardial infarction (AMI) and is defined as the condition in which the fibrous cap of an atherosclerotic plaque is ulcerated producing a fissure into the lipid core, with consequent intraplaque haemorrhage and adherent mural thrombosis. Dr. A.E. Becker (University of Amsterdam, The Netherlands) focused his presentation on the reasons why some plaques rupture and others do not. Plaques prone to rupture are characterised by a large lipid core, a thin fibrous cap, and a dense inflammatory infiltrate composed of macrophages, T lymphocytes, and mast cells. With the worsening of the situation, there is a relative reduction in the percentage of smooth muscle cells (SMC), and an increase in macrophages and T lymphocytes. "T lymphocytes are almost always in close association with macrophages," reported Dr. Becker, "and they express markers of activation." AMI is associated with an increase of the inflammatory cell-mediated (Th-1)-like response, responsible for the release of a variety of cytokines, growth factors, and enzymes. The Th-1 response leads to the release of IFN-g, IL-2, and TNF-b, which down-regulate extracellular matrix production, SMC proliferation and scavenger receptor expression. A wide range of extracellular proteolytic enzymes, such as matrix metalloproteinases (MMPs), is produced at focal sites in plaques. Inflammatory cells participate in this process by releasing MMPs that degrade collagen, predisposing the rupture of the fibrous cap. Atherectomy specimens from patients with AMI show recent onset activation of T lymphocytes, which correlates with clinical observation of increased inflammatory markers in peripheral blood. Dr Becker isolated T cells from atherosclerotic plaques, which respond to Chlamydia pneumoniae. This small gram-negative intracellular bacterium causes a humoral and a cell-mediated immune response that can be inhibited by antibodies against HLA-DR. Dr Becker concluded, "This could mean that C. pneumoniae might be the additional stimulus that serves as the last straw that breaks the camel's back," shifting the equilibrium toward the rupture of the plaque.

Dr. P.T. Kovanen (Wihuri Research Institute, Finland) reviewed the role of mast cells as storage site and rich source of proteolytic enzymes in advanced plaques. Mast cells originate from the bone marrow, and, when mature, are filled with secretory granules containing neutral proteases: tryptase, which alone makes 23% of total cellular protein, and chymase. Tryptase is a tetrameric serine protease, with trypsin-like activity. Chymase is a monomeric serine protease that produces several detrimental effects. Among these: apolipoprotein AI degradation, MMP-1 and -3 activation, and conversion of angiotensin I in angiotensin II and of big-endothelin in endothelin. Mast cells, like macrophages and T lymphocytes, are more present in areas of erosion than in adjacent areas. The opposite is true for SMC. The per cent of activated mast cells is 86% in erosion area, 63% in adjacent, and only 27% in unaffected sites. Dr. Kovanen said, "Mast cells are almost absent in normal intima, but their number increases in fatty streaks and in unstable plaques, in particular in the shoulder region, as it is the case for macrophages and T lymphocytes. How do mast cells contribute to AMI? They have to be activated and degranulate." Chymase is active upon secretion, moreover it binds to proteoglycans and in this way it is protected from physiologic inhibitors. Chymase activates MMP-1 and -3, thus activating the trigger for the onset of the proteolytic cascade.

Two of the following presentations focused on the important role played by MMPs in plaque rupture. Dr. P. Pöllänen (University of Tampere, Finland) studied the association between human stromelysin-1 (MMP-3) promoter 5A/6A polymorphism and autopsy-verified coronary atherosclerotic lesions in a cohort of 300 middle-aged men. The data showed that carriers (>53 years old) of the 5A/5A and 5A/6A polymorphisms have on average a 101% and a 79% increase, respectively, in the area of calcified lesions, when compared with the carriers of the 6A/6A genotype. Dr. Pöllänen concluded that the MMP-3 5A-allele is a significant genetic risk factor for calcified atherosclerotic lesion rupture at late middle age. Dr. S. Jormsjö (Karolinska Institute, Sweden) scanned the metalloelastase (MMP-12) and matrilysin (MMP-7) gene for polymorphisms, in order to elucidate the role that elastolytic enzymes may play in the degradation of extracellular matrix. She found two common polymorphisms in the MMP-12 gene. One located in exon 6 is silent, while the other is an A to G substitution in the promoter region and influences the binding of transcription factor AP-1. This affects the transcriptional activity of the MMP-12 promoter. Moreover, data obtained in a cohort of patients undergoing PTCA with stent implantation showed an allele-specific effect on arterial luminal dimension. Dr. Jormsjö has identified two promoter polymorphisms in the MMP-7 gene, but studies are still ongoing in order to analyze the functional significance of these mutations.

The two remaining presentations were dedicated to different factors involved in atherosclerotic plaque progression. Dr. B. van Vlijmen (TNO-PG/Gaubius Laboratory, The Netherlands) investigated the importance of the tumor suppressor gene p53 of macrophages in the progression of atherosclerotic lesions in apoE*3-Leiden mice. The deletion of macrophages p53 strongly exacerbated the progression of lesions, which contained more necrotic cells, less apoptotic cells, and more lipid-loaded macrophages. These data clearly indicate that p53 is an important player in the progression of atherosclerotic plaques due to its effect on macrophage death.

Finally, Dr. N. Ouchi (University of Osaka, Japan) described the role played by adiponectin, a novel adipocyte-derived plasma protein. Plasma adiponectin levels are negatively correlated with body mass index and are significantly low in patients with coronary artery disease. The addition of physiological concentrations of adiponectin reduced intracellular cholesteryl ester content in human macrophages in vitro. In addition adiponectin reduced the expression of class A macrophage scavenger receptor, without affecting the expression of CD36, and, consequently, decreased both binding and uptake of modified low-density lipoprotein. These data could give some hints on the link between atherosclerosis and obesity, one of the most common nutritional disorders and one of the major cardiovascular risk factors.