Suppressing thrombin generation is compatible with the development of atherosclerosis in mice

Noncanonical Matrix Metalloprotease 1-Protease-Activated Receptor 1 Signaling Drives Progression of Atherosclerosis

OBJECTIVE

Protease-activated receptor-1 (PAR1) is classically activated by thrombin and is critical in controlling the balance of hemostasis and thrombosis. More recently, it has been shown that noncanonical activation of PAR1 by matrix metalloprotease-1 (MMP1) contributes to arterial thrombosis. However, the role of PAR1 in long-term development of atherosclerosis is unknown, regardless of the protease agonist.

APPROACH AND RESULTS

We found that plasma MMP1 was significantly correlated (R=0.33; P=0.0015) with coronary atherosclerotic burden as determined by angiography in 91 patients with coronary artery disease and acute coronary syndrome undergoing cardiac catheterization or percutaneous coronary intervention. A cell-penetrating PAR1 pepducin, PZ-128, currently being tested as an antithrombotic agent in the acute setting in the TRIP-PCI study (Thrombin Receptor Inhibitory Pepducin-Percutaneous Coronary Intervention), caused a significant decrease in total atherosclerotic burden by 58% to 70% (P<0.05) and reduced plaque macrophage content by 54% (P<0.05) in apolipoprotein E-deficient mice. An MMP1 inhibitor gave similar beneficial effects, in contrast to the thrombin inhibitor bivalirudin that gave no improvement on atherosclerosis end points. Mechanistic studies revealed that inflammatory signaling mediated by MMP1-PAR1 plays a critical role in amplifying tumor necrosis factor α signaling in endothelial cells.

CONCLUSIONS

These data suggest that targeting the MMP1-PAR1 system may be effective in tamping down chronic inflammatory signaling in plaques and halting the progression of atherosclerosis.

Atherosclerosis in Mice Is Not Affected by a Reduction in Tissue Factor Expression

Objective— To determine whether tissue factor (TF) contributes to the progression of atherosclerotic lesions in mice.

Methods and Results— We determined the effect of a 50% reduction of TF levels in all cells on atherosclerosis in apolipoprotein E-deficient (apoE −/− ) mice. No differences were observed in the extent of atherosclerosis in apoE −/− /TF +/+ and apoE −/− /TF +/− mice fed regular chow for 34 weeks. Atherosclerosis could not be analyzed in apoE −/− mice expressing low levels of TF because of premature death of these mice. Macrophages are a major source of TF in atherosclerotic plaques. Therefore, in a second series of experiments, we investigated the effect on atherosclerosis of selectively reducing hematopoietic cell-derived TF by transplanting bone marrow from mice expressing low levels of TF into low-density lipoprotein receptor deficient (LDLR −/− ) mice. Atherosclerosis within the arterial tree and aortic root were similar in LDLR −/− mice with low-TF bone marrow compared with control bone marrow (TF +/+ or TF +/− ) after 4 and 16 weeks on an atherogenic diet. Furthermore, the cellular composition of the aortic root lesions was similar between the 2 groups.

Conclusions— Our data indicate that either a 50% reduction of TF in all cells or a selective reduction in hematopoietic cell-derived TF does not affect the development of atherosclerotic lesions in mice.

Apolipoprotein E Modulates Clearance of Apoptotic Bodies In Vitro and In Vivo, Resulting in a Systemic Proinflammatory State in Apolipoprotein E-Deficient Mice

Apolipoprotein E (apoE) is a 34-kDa glycoprotein involved in lipoprotein transport through interaction with the low-density lipoprotein receptor and related receptors. Recently, it has become clear that apoE binding to its receptors plays a role both in development and in control of the immune system. In this study, we show that apoE modulates the rate of uptake of apoptotic cells by macrophages. In vitro, apoE-deficient macrophages ingest less apoptotic thymocytes (but not latex beads) than wild-type macrophages, and this defect can be corrected by addition of exogenous apoE protein. In vivo, the number of dying macrophages is increased in a range of tissues, including lung and brain. Possibly in response to the larger numbers of persistent apoptotic bodies, the number of live macrophages in these tissues are also increased compared with those of wild-type control mice. In addition to the significant changes in macrophage population dynamics we observed, levels of the proinflammatory cytokine TNF-alpha and the positive acute phase reactant fibrinogen are also elevated in the livers from apoE-deficient mice. In contrast, neither deletion of the gene encoding the LDL receptor nor cholesterol feeding of wild-type mice affected either the number of apoptotic bodies or the number of live macrophages. We conclude that apoE deficiency results in impaired clearance of apoptotic cell remnants and a functionally relevant systemic proinflammatory condition in mice, independent of its role in lipoprotein metabolism. Any similar reduction of apoE activity in humans may contribute to the pathogenesis of a wide range of chronic diseases including atherosclerosis, dementia, and osteoporosis.

Atherosclerotic plaque rupture: local or systemic process?

It is generally established that the unstable plaque is the major cause of acute clinical sequelae of atherosclerosis. Unfortunately, terms indicating lesions prone to plaque instability, such as "vulnerable plaque," and the different phenotypes of unstable plaques, such as plaque rupture, plaque fissuring, intraplaque hemorrhage, and erosion, are often used interchangeably. Moreover, the different phenotypes of the unstable plaque are mostly referred to as plaque rupture. In the first part of this review, we will focus on the definition of true plaque rupture and the definitions of other phenotypes of plaque instability, especially on intraplaque hemorrhage, and discuss the phenotypes of available animal models of plaque instability. The second part of this review will address the pathogenesis of plaque rupture from a local and a systemic perspective. Plaque rupture is thought to occur because of changes in the plaque itself or systemic changes in the patient. Interestingly, contributing factors seem to overlap to a great extent and might even be interrelated. Finally, we will propose an integrative view on the pathogenesis of plaque rupture.