Evidence for increased collagenolysis by interstitial collagenases-1 and -3 in vulnerable human atheromatous plaques

Inhibition of CD40 signaling limits evolution of established atherosclerosis in mice

Interruption of inflammatory pathways may provide a novel approach to the therapy of atherosclerosis. Recently, we and others have implicated the immune mediator dyad CD40/CD40L (CD40 ligand), which is expressed on endothelial and smooth muscle cells, macrophages, and T lymphocytes within human atherosclerotic lesions, in aspects of atherogenesis and the acute coronary syndromes, including regulation of matrix metalloproteinases, procoagulant activity, cytokines, etc. In vivo, interruption of CD40 signaling reduced the initiation and early phases of atheroma formation in hypercholesterolemic mice. However, whether interruption of CD40 signaling can retard the progression or even regress established lesions remains unknown. We report here that anti-CD40L antibody treatment of randomly assigned low-density lipoprotein receptor-deficient mice during the second half of a 26-week regimen of high-cholesterol diet did not regress, but did significantly reduce further evolution of established atherosclerotic lesions within the aortic arch and particularly the thoracic and abdominal aorta, as compared with control treatment (application of rat-IgG or saline; 13 weeks, continued high-cholesterol diet). In addition to limiting lesion progression, anti-CD40L treatment changed the composition of atheroma in manners thought to favor plaque stability, e.g., reduced relative content of macrophages and lipid, as well as increased relative content of smooth muscle cells and collagen. These data implicate CD40/CD40L as crucial mediators not only in the initial events of atherogenesis but also during the evolution of established atheroma. This study lends further support to the importance of this specific inflammatory signaling pathway in atherosclerosis and its complications.

The extracellular matrix can regulate vascular cell migration, proliferation, and survival: relationships to vascular disease

The extracellular matrix (ECM) of the normal artery wall is a collection of fibrous proteins and associated glycoproteins embedded in a hydrated ground substance of glycosaminoglycans and proteoglycans. These distinct molecules are organized into a highly ordered network that are closely associated with the vascular cells that produce them. In addition to providing the architectural framework for the artery wall that imparts mechanical support and viscoelasticity, the ECM can regulate the behaviour of vascular cells, including their ability to migrate, proliferate and survive injury. The composition of the ECM is different within intimal lesions of atherosclerosis, which are composed of monocytes and lymphocytes from the circulation and smooth muscle cells (SMC) that migrate from the media to the intima (Ross 1993, 1999), and these differences may contribute to the altered phenotype of vascular cells within lesions. This review will briefly outline the ECM changes observed in atherosclerosis and restenosis and the potential relationship of these changes to altered vascular cell functions.

Collagen degradation and platelet-derived growth factor stimulate the migration of vascular smooth muscle cells

Cell migration is a key event in many biological processes and depends on signals from both extracellular matrix and soluble motogenic factors. During atherosclerotic plaque development, vascular smooth muscle cells migrate from the tunica media to the intima through a basement membrane and interstitial collagenous matrix and proliferate to form a neointima. Matrix metalloproteinases have previously been implicated in neointimal formation and in this study smooth muscle cell adhesion and migration on degraded collagen have been evaluated. Vascular smooth muscle cells adhered to native intact collagen type I and to its first degradation by-product, 3/4 fragment (generated by collagenase-3 cleavage), unwound at 35 degrees C to mimic physiological conditions. PDGF-BB pre-treatment induced a fourfold stimulation of smooth muscle cell motility on the collagen 3/4 fragment whereas no increase in smooth muscle cell motility on collagen type I was observed. Cell migration on collagen type I was mediated by alpha2 integrin, whereas PDGF-BB-stimulated migration on the 3/4 collagen fragment was dependent on alphavbeta3 integrin. alphavbeta3 integrin was organised in clusters concentrated at the leading and trailing edges of the cells and was only expressed when cells were exposed to the 3/4 collagen fragment. Tyrphostin A9, an inhibitor of PDGF receptor-beta tyrosine kinase activity, resulted in complete abolition of migration of PDGF-BB treated cells on collagen type I and 3/4 fragment. These results strongly support the hypothesis that the cellular migratory response to soluble motogens can be regulated by proteolytic modification of the extracellular matrix.

Lipid lowering reduces proteolytic and prothrombotic potential in rabbit atheroma

Thrombus formation at sites of atheromatous plaque disruption cause most acute coronary events such as myocardial infarction and unstable angina. Lesional macrophages and smooth muscle cells produce matrix metallo-proteinases (MMPs) and tissue factor (TF), the molecules likely contribute to plaque rupture and thrombus formation. Recent clinical studies have suggested that lipid lowering can reduce the incidence of acute coronary events. We have recently determined the effects of long-term dietary lipid lowering on atheroma of high-cholesterol-fed rabbits. Lipid lowering diminished macrophage accumulation, reduced expression and activity of MMPs, and increased interstitial collagen accumulation in rabbit atheroma. Expression and activity of TF in atheroma also substantially decreased during lipid lowering. Dietary lipid lowering also promoted accumulation of mature smooth muscle cells expressing less MMPs and TF in the plaque's fibrous cap. These results suggest potential mechanisms by which lipid lowering reduces acute coronary events in patients by decreasing proteolytic and prothrombotic activity within the atheroma.

The serpin proteinase inhibitor 9 is an endogenous inhibitor of interleukin 1beta-converting enzyme (caspase-1) activity in human vascular smooth muscle cells

Interleukin-1beta-converting enzyme (ICE, caspase-1) regulates key steps in inflammation and immunity, by activating the proinflammatory cytokines interleukin (IL-)1beta and IL-18, or mediating apoptotic processes. We recently provided evidence for the regulation of caspase-1 activity via an endogenous inhibitor expressed by human vascular smooth muscle cells (SMCs) (Schönbeck, U., M. Herzberg, A. Petersen, C. Wohlenberg, J. Gerdes, H.-D. Flad, and H. Loppnow. 1997. J. Exp. Med. 185:1287-1294). However, the molecular identity of this endogenous inhibitor remained undefined. We report here that the serine proteinase inhibitor (serpin) PI-9 accounts for the endogenous caspase-1 inhibitory activity in human SMCs and prevents processing of the enzyme's natural substrates, IL-1beta and IL-18 precursor. Treatment of SMC lysates with anti-PI-9 antibody abrogated the caspase-1 inhibitory activity and coprecipitated the enzyme, demonstrating protein-protein interaction. Furthermore, PI-9 antisense oligonucleotides coordinately reduced PI-9 expression and promoted IL-1beta release. Since SMCs comprise the majority of cells in the vascular wall, and because IL-1 is implicated in atherogenesis, we tested the biological validity of our in vitro findings within human atheroma in situ. The unaffected arterial wall contains abundant and homogeneously distributed PI-9. In human atherosclerotic lesions, however, PI-9 expression correlated inversely with immunoreactive IL-1beta, supporting a potential role of the endogenous caspase-1 inhibitor in this chronic inflammatory disease. Thus, our results provide new insights into the regulation of this enzyme involved in immune and inflammatory processes of chronic inflammatory diseases, and point to an endogenous antiinflammatory action of PI-9, dysregulated in a prevalent human disease.

Therapeutic potential of matrix metalloproteinase inhibitors in atherosclerosis

The activity of matrix-degrading metalloproteinases (MMPs) is essential for many of the processes involved in atherosclerotic plaque formation, for example, infiltration of inflammatory cells, smooth muscle cell migration and proliferation and angiogenesis. Furthermore, matrix degradation by MMPs may cause the plaque instability and rupture that leads to the clinical symptoms of atherosclerosis; unstable angina, myocardial infarction and stroke. Together, the family of MMPs can degrade all of the components of the blood vessel extracellular matrix and their activity therefore, is tightly regulated in normal blood vessels. The increased MMP activity during atherosclerotic plaque development and instability must therefore be caused by increased cytokine and growth factor-stimulated gene transcription, elevated zymogen activation and an imbalance in the MMP:TIMP ratio. It is therefore conceivable that inhibition of MMPs or re-establishing the MMP:TIMP balance may be useful in treating the symptoms of atherosclerosis. Recent studies using synthetic MMP inhibitors and gene therapy have highlighted the potential of such an approach.

The extracellular matrix dynamically regulates smooth muscle cell responsiveness to PDGF

Focal accumulation of smooth muscle (SMC) within the arterial intima contributes to the formation of lesions of atherosclerosis. Platelet-derived growth factor (PDGF) is a potent stimulant of SMC migration and proliferation in culture that may play a role in the accumulation of SMC in atherogenesis. SMCs normally reside in the media of the artery wall surrounded by extracellular matrix (ECM), including type I collagen. In atherogenesis, the ECM is degraded, new ECM components, such as fibronectin, are synthesized and assembled, and these alterations in ECM components are associated with changes in SMC phenotype. To model the changes in ECM in normal and diseased arteries, we have analyzed SMCs cultured on different forms of type I collagen. Our studies demonstrate that integrin-mediated signals from various forms of type I collagen lead to specific and rapid modulation of the integrin signaling complex, including cytoskeletal connections, and of the responsiveness of SMC to PDGF stimulation.

An overview of collagenase-3 expression in malignant tumors and analysis of its potential value as a target in antitumor therapies

Collagenase-3 (MMP-13) is a member of the matrix metalloproteinase family of endopeptidases that is characterized by a potent degrading activity against a wide spectrum of substrates. This enzyme was first detected in breast carcinomas but it is also overexpressed in a variety of malignant tumors including head and neck carcinomas, chondrosarcomas, skin carcinomas, and carcinomas of the female genital tract. Clinical studies have revealed that in all these tumors collagenase-3 expression is associated with invasive and metastatic tumors. Analysis of the molecular mechanisms underlying its marked overexpression in malignant tumors has allowed to identify different cytokines, growth factors and tumor promoters with ability to up-regulate collagenase-3 expression in tumor cells, or in stromal fibroblasts surrounding epithelial tumor cells. The first strategies designed to target this enzyme are being developed, and are mainly directed to prepare synthetic inhibitors with ability to selectively block the collagenase-3 proteolytic activity. Alternatively, inhibitors of the signal transduction pathways mediating the expression of this enzyme by tumor cells may also be useful for collagenase-3 targeting. These studies together with those performed on other enzymes associated with tumor processes may lead to the development of novel therapeutic strategies to control the progression and metastatic capacity of neoplastic cells.

Regulation of matrix proteins and impact on vascular structure

The vascular extracellular matrix is responsible for the mechanical properties of the vessel wall and is also involved in biologic processes such as cellular adhesion, regulation, and proliferation. Thus, an adequate balance of its components is necessary for the normal functioning of the vasculature. Vascular disorders affect this balance, and this plays a key role in their pathophysiology. Atherogenesis is accompanied by an increase in matrix deposition in response to low-density lipoprotein accumulation. However, this matrix, mainly collagen, also has a protective role by forming a fibrous cap around the lipid core, avoiding contact with blood. A decrease in the amount of collagen will weaken the cap and make it prone to rupture, leading to thrombosis and acute coronary syndromes. In hypertension, the increase in matrix deposition results in vascular stiffness and cardiac dysfunction. In this paper, we discuss the relevance of matrix regulation in these conditions.

Selective adhesion of macrophages to denatured forms of type I collagen is mediated by scavenger receptors

Macrophages (Mφs) are multifunctional immune cells which are involved in the regulation of immune and inflammatory responses, as well as in tissue repair and remodeling. In tissues, Mφs reside in areas which are rich in extracellular matrix (ECM), the structural component which also plays an essential role in regulating a variety of cellular functions. A major ECM protein encountered by Mφs is type I collagen, the most abundant of the fibril-forming collagens. In this study, the adhesion of RAW 264.7 murine Mphis to native fibrillar, monomeric, and denatured type I collagen was investigated. Using atomic force microscopy, structural differences between fibrillar and monomeric type I collagen were clearly resolved. When cultured on fibrillar type I collagen, Mphis adhered poorly. In contrast, they adhered significantly to monomeric, heat-denatured, or collagenase-modified type I collagen. Studies utilizing anti-beta1 and -beta2 integrin adhesion-blocking antibodies, RGD-containing peptides, or divalent cation-free conditions did not inhibit Mphi; adhesion to monomeric or denatured type I collagen. However, macrophage scavenger receptor (MSR) ligands and anti-MSR antibodies significantly blocked Mphi; adhesion to denatured and monomeric type I collagen strongly suggesting the involvement of the MSR as an adhesion molecule for denatured type I collagen. Further analysis by Western blot identified the MSR as the primary receptor for denatured type I collagen among Mphi; proteins purified from a heat-denatured type I collagen affinity column. These findings indicate that Mphis adhere selectively to denatured forms of type I collagen, but not the native fibrillar conformation, via their scavenger receptors.

Modification of coronary artery disease progression by cholesterol-lowering therapy: the angiographic studies

Large randomized placebo-controlled trials have demonstrated that cholesterol lowering with statin therapy reduces the incidence of adverse cardiac events. Smaller angiographic studies have shown that coronary artery disease progression can be slowed and, in some cases, reversed by cholesterol-lowering interventions. These anatomical changes, however, are small and occur too slowly to account for the early clinical benefit. Current evidence suggests that plaque stabilization is the most important mechanism, by which cholesterol-lowering therapy reduces both the incidence of adverse cardiac events and coronary artery disease progression.