No abstract

No abstract

No abstract

Thrombosis is integral to the development and progression and clinical sequelae of atherosclerosis. Acute thrombosis can occur spontaneously, leading to catastrophic arterial occlusion and resulting in myocardial infarction, unstable angina, stroke, and sudden death (1,2). Acute thrombosis also occurs as a complication of arterial bypass surgery, balloon angioplasty, atherectomy, or coronary artery stenting (3-5). Non-occlusive thrombus is an important component of the atherosclerotic plaque. Thrombus is comprised principally of platelets and fibrin. White blood cells and circulating proteins are also trapped within the platelet-fibrin thrombus. Activated platelets release a variety of growth factors and cytokines that have been implicated in vessel inflammation and in vascular smooth muscle cell (SMC) proliferation and migration (6,7). Thrombin (8) and factor XlXa (9,10) also have direct effects on SMC and may play a role in the development of intimal hyperplasia. In addition, products secreted by leukocytes trapped within the thrombus may have direct effects on the vessel wall. Tissue factor (TF) is a membrane-bound glycoprotein that initiates coagulation (11,12).Human TF consists of three domains: a short cytoplasmic domain of 19 residues, a single transmembrane domain of 23 residues, and a large extracellular domain of 219 residues. In addition, there is a 32 residue amino-terminal leader sequence which is cleaved to produce the mature molecule. TF binds to factor VIIA/IIa, and the resulting complex acts as a catalyst for the conversion of factors IX and X to IXa and Xa respectively, triggering the clotting cascade. This ultimately leads to the generation of thrombin, which in turn cleaves fibrinogen to fibrin, the major ingredient of the thrombus. Recent studies suggest that the accumulation of TF in atherosclerotic plaques plays a major role in determining plaque thrombogenicity. TF is also rapidly induced in the vessel wall as a consequence of acute arterial injury. Both phenomena may be important in the thrombotic complications of atherosclerotic heart disease.

Tissue factor (TF), the initiator of coagulation, is thought to function predominantly at the cell surface. Recent data have suggested that active TF is present extracellularly in atherosclerotic plaques, the arterial wall, and the blood. This study was conducted to determine whether smooth muscle cells (SMCs), a major source of arterial TF, could generate extracellular TF. Active TF accumulated in the medium of cultured human SMCs, representing approximately 10% of that measured in the underlying cells at 24 hours. Platelet-derived growth factor, phorbol ester, and tumor necrosis factor-alpha caused approximately 3-fold increases in TF activity in the medium. Release of TF into the medium was dependent on the presence of the TF transmembrane domain but not the cytoplasmic domain. Antibodies to TF precipitated most of the activity from the culture medium, whereas antibodies to the beta(1)-integrin subunit precipitated approximately 33% of the activity. Treatment with detergent or phosphatidylserine:phosphatidylcholine did not increase activity, suggesting that all TF released by SMCs was in the appropriate lipid milieu and not encrypted. Western blotting showed that the medium contained full-length TF protein. Fluorescent cytometry showed that extracellular TF was present largely in particles < or =200 nm, which had a density of 1.10 g/mL. We hypothesize that active extracellular TF found in the injured arterial wall and atherosclerotic plaques derives, in part, from SMC microparticles.

No abstract

CC chemokine receptors are important modulators of inflammation. Although CC chemokine receptors have been found predominantly on leukocytes, recent studies have suggested that vascular smooth muscle cells respond to CC chemokines. We now report that human smooth muscle cells express CCR5, a co-receptor for human immunodeficiency virus. CCR5 mRNA was detectable by RNA blot hybridization in human aortic and coronary artery smooth muscle cells. The cDNA generated by reverse transcription-polymerase chain reaction from aortic smooth muscle cells had 100% identity throughout the entire coding region with the CCR5 cloned from THP-1 cells. By immunohistochemistry, CCR5 and the CCR5 ligand, macrophage inflammatory protein-1␤ (MIP-1␤), were detected in smooth muscle cells and macrophages of the atherosclerotic plaque. In smooth muscle cell culture, MIP-1␤ induced a significant increase in intracellular calcium concentrations, which was blocked by an antibody to CCR5. In addition, MIP-1␤ caused a calcium-dependent increase in tissue factor activity. Tissue factor is the initiator of coagulation and is thought to play a key role in arterial thrombosis. These data suggest that human arterial smooth muscle cells express functional CCR5 receptors and MIP-1␤ is an agonist for these cells.

Human immunodeficiency virus (HIV) infection is associated with accelerated atherosclerosis and vasculopathy, although the mechanisms underlying these findings have not been determined. Hypotheses for these observations include: 1) an increase in the prevalence of established cardiac risk factors observed in HIV-infected individuals who are currently experiencing longer life expectancies; 2) the dyslipidemia reported with certain HIV anti-retroviral therapies; and/or 3) the proinflammatory effects of infiltrating HIV-infected monocytes/macrophages. An unexplored possibility is whether HIV itself can infect vascular smooth muscle cells (SMCs) and, by doing so, whether SMCs can accelerate vascular disease. Our studies demonstrate that human SMCs can be infected with HIV both in vivo and in vitro. The HIV protein p24 was detected by fluorescence confocal microscopy in SMCs from tissue sections of human atherosclerotic plaques obtained from HIV-infected individuals. Human SMCs could also be infected in vitro with HIV by a mechanism dependent on CD4, the chemokine receptors CXCR4 or CCR5, and endocytosis, resulting in a marked increase in SMC secretion of the chemokine CCL2/MCP-1, which has been previously shown to be a critical mediator of atherosclerosis. In addition, SMC proliferation appeared concentric to the vessel lumen, and minimal inflammation was detected, unlike typical atherosclerosis. Our data suggest that direct infection of human arterial SMCs by HIV represents a potential mechanism in a multifactorial paradigm to explain the exacerbated atherosclerosis and vasculopathy reported in individuals infected with HIV.