Introduction
The majority of acute coronary syndromes are caused by plaque ruptures resulting in embolisation or thrombotic occlusion of vessels. Vulnerable plaques, which are more prone to rupture, are characterised by a large lipid-rich necrotic core in combination with a thin fibrous cap infiltrated by macrophages and T lymphocytes, but with fewer smooth muscle cells. The underlying processes causing plaque rupture involve accumulation of pro-inflammatory and toxic lipids, changes in shear stress, degradation of extracellular matrix proteins by macrophage proteases and impaired tissue repair due to smooth muscle cell death.1 One major class of proteases in the vessel wall are the matrix metalloproteinases, which are inhibited by the naturally occurring tissue inhibitors of matrix metalloproteinases.2 While a number of studies have identified different matrix metalloproteinases as possible plasma markers of cardiovascular events, less is known about the members of the cathepsin protease family.
Cathepsins are normally present intracellularly in lysosomes but can in some cases be secreted and found in human serum. While normal arteries contain no or small amounts of cathepsins, increased expression is seen in abdominal aneurysms, as well as in macrophages and smooth muscle cells present within atherosclerotic lesions.3–6 Pro-inflammatory mediators induce cathepsin expression in aortic macrophages and smooth muscle cells, and increase the matrix degrading activity, thus implicating a role for cathepsins in plaque destabilisation.7 Furthermore, cathepsins influence macrophage apoptosis, which may further increase plaque vulnerability.6
Studies on human atherosclerotic lesions have shown that cathepsin L is associated with plaque destabilisation and apoptosis.6 ,8 ,9 Cathepsin L is also expressed in endothelial cells and is suggested to play a role in flow-induced vascular remodelling and atherogenesis.10 Clinical studies have shown that plasma/serum levels of cathepsin L are increased in patients with coronary artery stenosis, as well as in patients with abdominal aortic aneurysms.11 ,12 However, the association between plasma levels of cathepsin L and risk for future coronary events (CEs) has not been studied previously.
Less is known about cystatin B, an inhibitor of cathepsin L, and cardiovascular disease (CVD). Cystatin B levels in tissue and body fluids have been suggested as biomarkers in some cancers.13–15 Mutations in the cystatin B gene are linked to progressive myoclonus epilepsy 1, a neurodegenerative disorder.16 Recently, cystatin B in plasma has been proposed as a biomarker for cardiovascular disease based on data obtained from a small patient cohort.17
Cathepsin D has been suggested as an independent prognostic factor in a variety of cancers.18 Under pathological conditions, cathepsin D is present extracellularly in tissues, such as in the synovia of patients with rheumatoid arthritis,19 in the brain of patients with Alzheimer's disease,20 in invasive carcinomas and in macrophage-rich regions of human atherosclerotic lesions.3 In addition, cathepsin D has been implicated in apoptosis of macrophages.6 Interestingly, proteomic studies have identified cathepsin D as a potential novel biomarker of CVD. In these studies, cathepsin D was shown to be increased in cell supernatants of oxidised low-density lipoprotein (LDL)-stimulated macrophages, as well as in supernatants of ex vivo plaques compared to control samples.21–23
Although cathepsin D and cystatin B have both been proposed as biomarkers for cardiovascular disease in proteomic studies or small patient cohorts, so far, they have not been investigated in a prospective cardiovascular cohort. In the present study, we investigated if plasma levels of cathepsin D, as well as cathepsin L, and its inhibitor cystatin B, are associated with future CEs.