Human cytomegalovirus increases HUVEC sensitivity to thrombin and modulates expression of thrombin receptors

Endothelial protease-activated receptor 4: impotent or important?

The protease thrombin, which increases its levels with various pathologies, can signal through the G protein-coupled receptors protease-activated receptors 1 and 4 (PAR1/PAR4). PAR1 is a high-affinity receptor for thrombin, whereas PAR4 is a low-affinity receptor. Finding functions for PAR4 in endothelial cells (ECs) has been an elusive goal over the last two decades. Several studies have demonstrated a lack of functionality for PAR4 in ECs, with many claiming that PAR4 function is confined mostly to platelets. A recent study from our lab identified low expressing but functional PAR4 in hepatic ECs in vivo. We also found that PAR4 likely has a higher signaling potency than PAR1. Given this potency, ECs seem to limit PAR4 signaling except for extreme cases. As a result, we claim PAR4 is not an impotent receptor because it is low expressing, but rather PAR4 is low expressing because it is a very potent receptor. Since we have finally shown PAR4 to be present and functional on ECs in vivo, it is important to outline why such controversy arose over the last two decades and, more importantly, why the receptor was undervalued on ECs. This timely review aims to inspire investigators in the field of vascular biology to study the regulatory aspect of endothelial PAR4 and its relationship with the more highly expressed PAR1.

The role of human cytomegalovirus in atherosclerosis: a systematic review

Atherosclerosis is a progressive vascular disease with increasing morbidity and mortality year by year in modern society. Human cytomegalovirus (HCMV) infection is closely associated with the development of atherosclerosis. HCMV infection may accelerate graft atherosclerosis and the development of transplant vasculopathy in organ transplantation. However, our current understanding of HCMV-associated atherosclerosis remains limited and is mainly based on clinical observations. The underlying mechanism of the involvement of HCMV infection in atherogenesis remains unclear. Here, we summarized current knowledge regarding the multiple influences of HCMV on a diverse range of infected cells, including vascular endothelial cells, vascular smooth muscle cells, monocytes, macrophages, and T cells. In addition, we described potential HCMV-induced molecular mechanisms, such as oxidative stress, endoplasmic reticulum stress, autophagy, lipid metabolism, and miRNA regulation, which are involved in the development of HCMV-associated atherogenesis. Gaining an improved understanding of these mechanisms will facilitate the development of novel and effective therapeutic strategies for the treatment of HCMV-related cardiovascular disease.

Infectious myocarditis: the role of the cardiac vasculature

Infectious myocarditis is the result of an immune response to a microbial infection of the heart. The blood vessels of the heart, both the intramyocardial microvasculature and the large epicardial coronary arteries, play an important role in the pathogenesis of infectious myocarditis. First of all, in addition to cardiomyocytes, endothelial cells of the cardiac (micro)vasculature are direct targets for infection. Moreover, through the expression of adhesion molecules and antigen presenting Major Histocompatibility Complex molecules, the blood vessels assist in shaping the cellular immune response in infectious myocarditis. In addition, damage and dysfunction of the cardiac (micro)vasculature are associated with thrombus formation as well as aberrant regulation of vascular tone including coronary vasospasm. These in turn can cause cardiac perfusion abnormalities and even myocardial infarction. In this review, we will discuss the role of the cardiac (micro)vasculature in the pathogenesis of infectious myocarditis.

Evaluation on potential contributions of protease activated receptors related mediators in allergic inflammation

Protease activated receptors (PARs) have been recognized as a distinctive four-member family of seven transmembrane G protein-coupled receptors (GPCRs) that can be cleaved by certain serine proteases. In recent years, there has been considerable interest in the role of PARs in allergic inflammation, the fundamental pathologic changes of allergy, but the potential roles of PARs in allergy remain obscure. Since many of these proteases are produced and actively involved in the pathologic process of inflammation including exudation of plasma components, inflammatory cell infiltration, and tissue damage and repair, PARs appear to make important contribution to allergy. The aim of the present review is to summarize the expression of PARs in inflammatory and structural cells, the influence of agonists or antagonists of PARs on cell behavior, and the involvement of PARs in allergic disorders, which will help us to better understand the roles of serine proteases and PARs in allergy.

Low anticoagulant heparin blocks thrombin-induced endothelial permeability in a PAR-dependent manner

Acute lung injury and acute respiratory distress syndrome are accompanied by thrombin activation and fibrin deposition that enhance lung inflammation, activate endothelial cells and disrupt lung paracellular permeability. Heparin possesses anti-inflammatory properties but its clinical use is limited by hemorrhage and heparin induced thrombocytopenia. We studied the effects of heparin and low anticoagulant 2-O, 3-O desulfated heparin (ODSH) on thrombin-induced increases in paracellular permeability of cultured human pulmonary endothelial cells (ECs). Pretreatment with heparin or ODSH blocked thrombin-induced decrease in the EC transendothelial electrical resistance (TER), attenuated thrombin-stimulated paracellular gap formation and actin cytoskeletal rearrangement. Our data demonstrated that heparin and ODSH had inhibitory effects on thrombin-induced RhoA activation and intracellular calcium elevation. Thrombin-stimulated phosphorylation of the cytoskeletal regulatory proteins, myosin light chain and ezrin/radixin/moesin was also reduced. In these effects, low anticoagulant ODSH was more potent than heparin. Heparin or ODSH alone produced decreases in the EC TER that were abolished by siRNA-mediated depletion of the thrombin receptor, PAR-1. We also demonstrated that, in contrast to heparin, ODSH did not possess thrombin-binding activity. Results suggest that heparin and low anticoagulant ODSH can interfere with thrombin-activated signaling.

PAR-1 contributes to the innate immune response during viral infection

Coagulation is a host defense system that limits the spread of pathogens. Coagulation proteases, such as thrombin, also activate cells by cleaving PARs. In this study, we analyzed the role of PAR-1 in coxsackievirus B3-induced (CVB3-induced) myocarditis and influenza A infection. CVB3-infected Par1(-/-) mice expressed reduced levels of IFN-β and CXCL10 during the early phase of infection compared with Par1(+/+) mice that resulted in higher viral loads and cardiac injury at day 8 after infection. Inhibition of either tissue factor or thrombin in WT mice also significantly increased CVB3 levels in the heart and cardiac injury compared with controls. BM transplantation experiments demonstrated that PAR-1 in nonhematopoietic cells protected mice from CVB3 infection. Transgenic mice overexpressing PAR-1 in cardiomyocytes had reduced CVB3-induced myocarditis. We found that cooperative signaling between PAR-1 and TLR3 in mouse cardiac fibroblasts enhanced activation of p38 and induction of IFN-β and CXCL10 expression. Par1(-/-) mice also had decreased CXCL10 expression and increased viral levels in the lung after influenza A infection compared with Par1(+/+) mice. Our results indicate that the tissue factor/thrombin/PAR-1 pathway enhances IFN-β expression and contributes to the innate immune response during single-stranded RNA viral infection.

Increased expression of LDL receptor-related protein 1 during human cytomegalovirus infection reduces virion cholesterol and infectivity

In response to virus infection, cells can alter protein expression to modify cellular functions and limit viral replication. To examine host protein expression during infection with human cytomegalovirus (HCMV), an enveloped DNA virus, we performed a semiquantitative, temporal analysis of the cell surface proteome in infected fibroblasts. We determined that resident low density lipoprotein related receptor 1 (LRP1), a plasma membrane receptor that regulates lipid metabolism, is elevated early after HCMV infection, resulting in decreased intracellular cholesterol. siRNA knockdown or antibody-mediated inhibition of LRP1 increased intracellular cholesterol and concomitantly increased the infectious virus yield. Virions produced under these conditions contained elevated cholesterol, resulting in increased infectivity. Depleting cholesterol from virions reduced their infectivity by blocking fusion of the virion envelope with the cell membrane. Thus, LRP1 restricts HCMV infectivity by controlling the availability of cholesterol for the virion envelope, and increased LRP1 expression is likely a defense response to infection.

Humoral innate immune response and disease

The humoral innate immune response consists of multiple components, including the naturally occurring antibodies (NAb), pentraxins and the complement and contact cascades. As soluble, plasma components, these innate proteins provide key elements in the prevention and control of disease. However, pathogens and cells with altered self proteins utilize multiple humoral components to evade destruction and promote pathogy. Many studies have examined the relationship between humoral immunity and autoimmune disorders. This review focuses on the interactions between the humoral components and their role in promoting the pathogenesis of bacterial and viral infections and chronic diseases such as atherosclerosis and cancer. Understanding the beneficial and detrimental aspects of the individual components and the interactions between proteins which regulate the innate and adaptive response will provide therapeutic targets for subsequent studies.

The roles of thrombin and protease-activated receptors in inflammation

Inflammation and coagulation constitute two host defence systems with complementary physiological roles in limiting tissue damage, restoring homeostasis and eliminating invading pathogens, functions reliant on effective regulation of both processes at a variety of levels. Dysfunctional activation or regulation of either pathway may lead to pathology and contribute to human diseases as diverse as myocardial infarction and septic shock. The serine protease thrombin, a key protein in the coagulation pathway, can activate cellular signalling directly via proteolytic cleavage of the N-terminal domain of a family of G protein-coupled receptors or indirectly through the generation of molecules such as activated protein C. These events transmit signals to many cell types and can elicit the production of various pro-inflammatory mediators such as cytokines, chemokines and growth factors, thereby influencing cell activation, differentiation, survival and migration. This review discusses recent progress in understanding how thrombin and protease-activated receptors influence biological processes, highlighting the detrimental and protective cellular effects of thrombin and its signalling pathways.