Association of thrombospondin-1 and cardiac allograft vasculopathy in human cardiac allografts

The molecular mechanism of thrombospondin family members in cardiovascular diseases

Cardiovascular diseases have been identified as vital factors in global morbidity and mortality in recent years. The available evidence suggests that various cytokines and pathological proteins participate in these complicated and changeable diseases. The thrombospondin (TSP) family is a series of conserved, multidomain calcium-binding glycoproteins that cause cell-matrix and cell-cell effects via interactions with other extracellular matrix components and cell surface receptors. The TSP family has five members that can be divided into two groups (Group A and Group B) based on their different structures. TSP-1, TSP-2, and TSP-4 are the most studied proteins. Among recent studies and findings, we investigated the functions of several family members, especially TSP-5. We review the basic concepts of TSPs and summarize the relevant molecular mechanisms and cell interactions in the cardiovascular system. Targeting TSPs in CVD and other diseases has a remarkable therapeutic benefit.

The Innate Cellular Immune Response in Xenotransplantation

Xenotransplantation is very attractive strategy for addressing the shortage of donors. While hyper acute rejection (HAR) caused by natural antibodies and complement has been well defined, this is not the case for innate cellular xenogeneic rejection. An increasing body of evidence suggests that innate cellular immune responses contribute to xenogeneic rejection. Various molecular incompatibilities between receptors and their ligands across different species typically have an impact on graft outcome. NK cells are activated by direct interaction as well as by antigen dependent cellular cytotoxicity (ADCC) mechanisms. Macrophages are activated through various mechanisms in xenogeneic conditions. Macrophages recognize CD47 as a "marker of self" through binding to SIRPα. A number of studies have shown that incompatibility of porcine CD47 against human SIRPα contributes to the rejection of xenogeneic target cells by macrophages. Neutrophils are an early responder cell that infiltrates xenogeneic grafts. It has also been reported that neutrophil extracellular traps (NETs) activate macrophages as damage-associated pattern molecules (DAMPs). In this review, we summarize recent insights into innate cellular xenogeneic rejection.

In the balance: how do thrombospondins contribute to the cellular pathophysiology of cardiovascular disease?

Thrombospondins (TSPs) are multidomain, secreted proteins that associate with cell surfaces and extracellular matrix. In mammals, there is a large body of data on functional roles of various TSP family members in cardiovascular disease (CVD), including stroke, cardiac remodeling and fibrosis, atherosclerosis, and aortic aneurysms. Coding single nucleotide polymorphisms (SNPs) of TSP1 or TSP4 are also associated with increased risk of several forms of CVD. Whereas interactions and functional effects of TSPs on a variety of cell types have been studied extensively, the molecular and cellular basis for the differential effects of the SNPs remains under investigation. Here, we provide an integrative review on TSPs, their roles in CVD and cardiovascular cell physiology, and known properties and mechanisms of TSP SNPs relevant to CVD. In considering recent expansions to knowledge of the fundamental cellular roles and mechanisms of TSPs, as well as the effects of wild-type and variant TSPs on cells of the cardiovascular system, we aim to highlight knowledge gaps and areas for future research or of translational potential.

Efficient Ex Vivo Screening of Agents Targeting Thrombospondin1-Induced Vascular Dysfunction Using a Digital Multiwire Myograph System

Homeostasis of vascular tone is intricately and delicately maintained systemically and locally, by autonomic nerves and hormones in the blood and by intimal vasoactive substances, respectively. The balance can be acutely or chronically interrupted secondary to many alterations, especially under pathological conditions. Excessive matricellular glycoprotein thrombospondin 1 (TSP1) levels in circulation have been found to play an important role in ischemia-reperfusion injuries of different organs, by acutely suppressing vasorelaxation and chronically remodeling vascular bed. Our laboratory has been interested in identifying new drug moieties, which can selectively and effectively counteract TSP1-induced vascular dysfunction, in order to address associated clinical complications. Preliminary studies using computational docking and molecular models revealed potential drug candidates for further evaluation via vascular functional bioassay to prove the antagonism using an ex vivo vascular model. Herein, we described an efficient screening method for the identification of active drug candidates, by adapting a multiwire myograph system to perform a protocol with different treatments, in the presence of pathological levels of TSP1. We discussed the promising pharmacological evaluation results and suggested suitable modification for versatile applications. We also described the necessity of pre-determination of optimal resting tension to obtain the maximal response, if the experimental test model is different from those with determined optimal resting tension.

Novel Pharmaceutical Strategy for Selective Abrogation of TSP1-Induced Vascular Dysfunction by Decoy Recombinant CD47 Soluble Receptor in Prophylaxis and Treatment Models

Elevated thrombospondin 1 (TSP1) is a prevalent factor, via cognate receptor CD47, in the pathogenesis of cardiovascular conditions, including ischemia-reperfusion injury (IRI) and pulmonary arterial hypertension (PAH). Moreover, TSP1/CD47 interaction has been found to be associated with platelet hyperaggregability and impaired nitric oxide response, exacerbating progression in IRI and PAH. Pathological TSP1 in circulation arises as a target of our novel therapeutic approach. Our “proof-of-concept” pharmacological strategy relies on recombinant human CD47 peptide (rh-CD47p) as a decoy receptor protein (DRP) to specifically bind TSP1 and neutralize TSP1-impaired vasorelaxation, strongly implicated in IRI and PAH. The binding of rh-CD47p and TSP1 was first verified as the primary mechanism via Western blotting and further quantified with modified ELISA, which also revealed a linear molar dose-dependent interaction. Ex vivo, pretreatment protocol with rh-CD47p (rh-CD47p added prior to TSP1 incubation) demonstrated a prophylactic effect against TSP1-impairment of endothelium-dependent vasodilation. Post-treatment set-up (TSP1 incubation prior to rh-CD47p addition), mimicking pre-existing excessive TSP1 in PAH, reversed TSP1-inhibited vasodilation back to control level. Dose titration identified an effective molar dose range (approx. ≥1:3 of tTSP1:rh-CD47p) for prevention of/recovery from TSP1-induced vascular dysfunction. Our results indicate the great potential for proposed novel decoy rh-CD47p-therapy to abrogate TSP1-associated cardiovascular complications, such as PAH.

Thrombospondin-1 in maladaptive aging responses: a concept whose time has come

Numerous age-dependent alterations at the molecular, cellular, tissue and organ systems levels underlie the pathophysiology of aging. Herein, the focus is upon the secreted protein thrombospondin-1 (TSP1) as a promoter of aging and age-related diseases. TSP1 has several physiological functions in youth, including promoting neural synapse formation, mediating responses to ischemic and genotoxic stress, minimizing hemorrhage, limiting angiogenesis, and supporting wound healing. These acute functions of TSP1 generally require only transient expression of the protein. However, accumulating basic and clinical data reinforce the view that chronic diseases of aging are associated with accumulation of TSP1 in the extracellular matrix, which is a significant maladaptive contributor to the aging process. Identification of the relevant cell types that chronically produce and respond to TSP1 and the molecular mechanisms that mediate the resulting maladaptive responses could direct the development of therapeutic agents to delay or revert age-associated maladies.

Elimination of donor CD47 protects against vascularized allograft rejection in mice

CD47 is a ubiquitously expressed transmembrane glycoprotein that plays a complex role in regulation of cell survival and function. We have previously shown that the interspecies incompatibility of CD47 plays an important role in triggering rejection of cellular xenografts by macrophages. However, the role of CD47 in solid organ transplantation remains undetermined. Here, we explored this question in mouse models of heart allotransplantation. We observed that the lack of CD47 in donor hearts had no deleterious effect on graft survival in syngeneic or single MHC class I-mismatched recipients, in which both wild-type (WT) and CD47 knockout (CD47 KO) mouse hearts survived long term with no sign of rejection. Paradoxically, elimination of donor CD47 was beneficial for graft survival in signal MHC class II- and class I- plus class II-mismatched combinations, in which CD47 KO donor hearts showed significantly improved survival compared to WT donor hearts. Similarly, CD47 KO donor hearts were more resistant than WT hearts to humoral rejection in α1,3-galactosyltransferase-deficient mice. Moreover, a significant prolongation of WT allografts was observed in recipient mice treated with antibodies against a CD47 ligand thrombospondin-1 (TSP1) or with TSP1 deficiency, indicating that TSP1-CD47 signaling may stimulate vascularized allograft rejection. Thus, unlike cellular transplantation, donor CD47 expression may accelerate the rejection of vascularized allografts.

Matricellular protein thrombospondin-1 in pulmonary hypertension: multiple pathways to disease

Matricellular proteins are secreted molecules that have affinities for both extracellular matrix and cell surface receptors. Through interaction with structural proteins and the cells that maintain the matrix these proteins can alter matrix strength. Matricellular proteins exert control on cell activity primarily through engagement of membrane receptors that mediate outside-in signaling. An example of this group is thrombospondin-1 (TSP1), first identified as a component of the secreted product of activated platelets. As a result, TSP1 was initially studied in relation to coagulation, growth factor signaling and angiogenesis. More recently, TSP1 has been found to alter the effects of the gaseous transmitter nitric oxide (NO). This latter capacity has provided motivation to study TSP1 in diseases associated with loss of NO signaling as observed in cardiovascular disease and pulmonary hypertension (PH). PH is characterized by progressive changes in the pulmonary vasculature leading to increased resistance to blood flow and subsequent right heart failure. Studies have linked TSP1 to pre-clinical animal models of PH and more recently to clinical PH. This review will provide analysis of the vascular and non-vascular effects of TSP1 that contribute to PH, the experimental and translational studies that support a role for TSP1 in disease promotion and frame the relevance of these findings to therapeutic strategies.

HLA Class II-Triggered Signaling Cascades Cause Endothelial Cell Proliferation and Migration: Relevance to Antibody-Mediated Transplant Rejection

Transplant recipients developing donor-specific HLA class II (HLA-II) Abs are at higher risk for Ab-mediated rejection (AMR) and transplant vasculopathy. To understand how HLA-II Abs cause AMR and transplant vasculopathy, we determined the signaling events triggered in vascular endothelial cells (EC) following Ab ligation of HLA-II molecules. HLA-II expression in EC was induced by adenoviral vector expression of CIITA or by pretreatment with TNF-α/IFN-γ. Ab ligation of class II stimulated EC proliferation and migration. Class II Ab also induced activation of key signaling nodes Src, focal adhesion kinase, PI3K, and ERK that regulated downstream targets of the mammalian target of rapamycin (mTOR) pathway Akt, p70 ribosomal S6 kinase, and S6 ribosomal protein. Pharmacological inhibitors and small interfering RNA showed the protein kinases Src, focal adhesion kinase, PI3K/Akt, and MEK/ERK regulate class II Ab-stimulated cell proliferation and migration. Treatment with rapalogs for 2 h did not affect HLA-II Ab-induced phosphorylation of ERK; instead, mTOR complex (mTORC)1 targets were dependent on activation of ERK. Importantly, suppression of mTORC2 for 24 h with rapamycin or everolimus or treatment with mTOR active-site inhibitors enhanced HLA-II Ab-stimulated phosphorylation of ERK. Furthermore, knockdown of Rictor with small interfering RNA caused overactivation of ERK while abolishing phosphorylation of Akt Ser⁴⁷³ induced by class II Ab. These data are different from HLA class I Ab-induced activation of ERK, which is mTORC2-dependent. Our results identify a complex signaling network triggered by HLA-II Ab in EC and indicate that combined ERK and mTORC2 inhibitors may be required to achieve optimal efficacy in controlling HLA-II Ab-mediated AMR.

Thrombospondins: A Role in Cardiovascular Disease

Thrombospondins (TSPs) represent extracellular matrix (ECM) proteins belonging to the TSP family that comprises five members. All TSPs have a complex multidomain structure that permits the interaction with various partners including other ECM proteins, cytokines, receptors, growth factors, etc. Among TSPs, TSP1, TSP2, and TSP4 are the most studied and functionally tested. TSP1 possesses anti-angiogenic activity and is able to activate transforming growth factor (TGF)-β, a potent profibrotic and anti-inflammatory factor. Both TSP2 and TSP4 are implicated in the control of ECM composition in hypertrophic hearts. TSP1, TSP2, and TSP4 also influence cardiac remodeling by affecting collagen production, activity of matrix metalloproteinases and TGF-β signaling, myofibroblast differentiation, cardiomyocyte apoptosis, and stretch-mediated enhancement of myocardial contraction. The development and evaluation of TSP-deficient animal models provided an option to assess the contribution of TSPs to cardiovascular pathology such as (myocardial infarction) MI, cardiac hypertrophy, heart failure, atherosclerosis, and aortic valve stenosis. Targeting of TSPs has a significant therapeutic value for treatment of cardiovascular disease. The activation of cardiac TSP signaling in stress and pressure overload may be therefore beneficial.

Antibodies to HLA Molecules Mimic Agonistic Stimulation to Trigger Vascular Cell Changes and Induce Allograft Injury

Human leukocyte antigen (HLA)-induced signaling in endothelial and smooth muscle cells causes dramatic cytoskeletal rearrangement, increased survival, motility, proliferation, adhesion molecule and chemokine expression, and adhesion of leukocytes. These mechanisms are directly related to endothelial activation, neointimal proliferation, and intragraft accumulation of leukocytes during antibody-mediated rejection (AMR) and chronic rejection. Clustering of HLA by ligands in trans, such as in antigen-presenting cells at the immune synapse, triggers physiological functions analogous to HLA antibody-induced signaling in vascular cells. Emerging evidence has revealed previously unknown functions for HLA beyond antigen presentation, including association with coreceptors in cis to permit signal transduction, and modulation of intracellular signaling downstream of other receptors that may be relevant to HLA signaling in the graft vasculature. We discuss the literature regarding HLA-induced signaling in vascular endothelial and smooth muscle cells, as well as under endogenous biological conditions, and how such signaling relates to functional changes and pathological mechanisms during graft injury.

Invoking the power of thrombospondins: regulation of thrombospondins expression

Increasing evidence suggests critical functions of thrombospondins (TSPs) in a variety of physiological and pathological processes. With the growing understanding of the importance of these matricellular proteins, the need to understand the mechanisms of regulation of their expression and potential approaches to modulate their levels is also increasing. The regulation of TSP expression is multi-leveled, cell- and tissue-specific, and very precise. However, the knowledge of mechanisms modulating the levels of TSPs is fragmented and incomplete. This review discusses the known mechanisms of regulation of TSP levels and the gaps in our knowledge that prevent us from developing strategies to modulate the expression of these physiologically important proteins.

Thrombospondin-1 and CD47 regulation of cardiac, pulmonary and vascular responses in health and disease

Cardiovascular homeostasis and health is maintained through the balanced interactions of cardiac generated blood flow and cross-talk between the cellular components that comprise blood vessels. Central to this cross-talk is endothelial generated nitric oxide (NO) that stimulates relaxation of the contractile vascular smooth muscle (VSMC) layer of blood vessels. In cardiovascular disease this balanced interaction is disrupted and NO signaling is lost. Work over the last several years indicates that regulation of NO is much more complex than previously believed. It is now apparent that the secreted protein thrombospondin-1 (TSP1), that is upregulated in cardiovascular disease and animal models of the same, on activating cell surface receptor CD47, redundantly inhibits NO production and NO signaling. This inhibitory event has implications for baseline and disease-related responses mediated by NO. Further work has identified that TSP1-CD47 signaling stimulates enzymatic reactive oxygen species (ROS) production to further limit blood flow and promote vascular disease. Herein consideration is given to the most recent discoveries in this regard which identify the TSP1-CD47 axis as a major proximate governor of cardiovascular health.

Thrombospondins: old players, new games

PURPOSE OF REVIEW

Thrombospondins (TSPs) are secreted extracellular matrix (ECM) proteins from TSP family, which consists of five homologous members. They share a complex domain structure and have numerous binding partners in ECM and multiple cell surface receptors. Information that has emerged over the past decade identifies TSPs as important mediators of cellular homeostasis, assigning new important roles in cardiovascular pathology to these proteins.

RECENT FINDINGS

Recent studies of the functions of TSP in the cardiovascular system, diabetes and aging, which placed several TSPs in a position of critical regulators, demonstrated the involvement of these proteins in practically every aspect of cardiovascular pathophysiology related to atherosclerosis: inflammation, immunity, leukocyte recruitment and function, function of vascular cells, angiogenesis, and responses to hypoxia, ischemia and hyperglycemia. TSPs are also critically important in the development and ultimate outcome of the complications associated with atherosclerosis--myocardial infarction, and heart hypertrophy and failure. Their expression and significance increase with age and with the progression of diabetes, two major contributors to the development of atherosclerosis and its complications.

SUMMARY

This overview of recent literature examines the latest information on the newfound functions of TSPs that emphasize the importance of ECM in cardiovascular homeostasis and pathology. The functions of TSPs in myocardium, vasculature, vascular complications of diabetes, aging and immunity are discussed.

Thrombospondin-4, tumour necrosis factor-like weak inducer of apoptosis (TWEAK) and its receptor Fn14: novel extracellular matrix modulating factors in cardiac remodelling

Cardiac remodelling is defined as changes in the size, shape, and function of the heart, which are most commonly caused by hypertension-induced left ventricular hypertrophy and myocardial infarction. Both neurohumoral and inflammatory factors have critical roles in the regulation of cardiac remodelling. A characteristic feature of cardiac remodelling is modification of the extracellular matrix (ECM), often manifested by fibrosis, a process that has vital consequences for the structure and function of the myocardium. In addition to established modulators of the ECM, the matricellular protein thrombospondin-4 (TSP-4) as well as the tumour necrosis factor-like weak inducer of apoptosis (TWEAK) and its receptor Fn14 has been recently shown to modulate cardiac ECM. TSP-4 null mice develop pronounced cardiac hypertrophy and fibrosis with defects in collagen maturation in response to pressure overload. TWEAK and Fn14 belong to the tumour necrosis factor superfamily of proinflammatory cytokines. Recently it was shown that elevated levels of circulating TWEAK via Fn14 critically affect the cardiac ECM, characterized by increasing fibrosis and cardiomyocyte hypertrophy in mice. Here we review the literature concerning the role of matricellular proteins and inflammation in cardiac ECM remodelling, with a special focus on TSP-4, TWEAK, and its receptor Fn14.

Matricellular proteins and matrix metalloproteinases mark the inflammatory and fibrotic response in human cardiac allograft rejection

Aims

The cardiac extracellular matrix is highly involved in regulating inflammation, remodelling, and function of the heart. Whether matrix alterations relate to the degree of inflammation, fibrosis, and overall rejection in the human transplanted heart remained, until now, unknown.

Methods and results

Expression of matricellular proteins, proteoglycans, and metalloproteinases (MMPs) and their inhibitors (TIMPs) were investigated in serial endomyocardial biopsies (n = 102), in a cohort of 39 patients within the first year after cardiac transplantation. Out of 15 matrix-related proteins, intragraft transcript and protein levels of syndecan-1 and MMP-9 showed a strong association with the degree of cardiac allograft rejection (CAR), the expression of pro-inflammatory cytokines tumour necrosis factor (TNF)-α, interleukin (IL)-6 and transforming growth factor (TGF)-β, and with infiltrating CD3⁺T-cells and CD68⁺monocytes. In addition, SPARC, CTGF, TSP-2, MMP-14, TIMP-1, Testican-1, TSP-1, Syndecan-1, MMP-2, -9, and -14, as well as IL-6 and TGF-β transcript levels and inflammatory infiltrates all strongly relate to collagen expression in the transplanted heart. More importantly, receiver operating characteristic curve analysis demonstrated that syndecan-1 and MMP-9 transcript levels had the highest area under the curve (0.969 and 0.981, respectively), thereby identifying both as a potential decision-making tool to discriminate rejecting from non-rejecting hearts.

Conclusion

Out of 15 matrix-related proteins, we identified synd-1 and MMP-9 intragraft transcript levels of as strong predictors of human CAR. In addition, a multitude of non-structural matrix-related proteins closely associate with collagen expression in the transplanted heart. Therefore, we are convinced that these findings deserve further investigation and are likely to be of clinical value to prevent human CAR.

Matricellular proteins in cardiac adaptation and disease

The term matricellular proteins describes a family of structurally unrelated extracellular macromolecules that, unlike structural matrix proteins, do not play a primary role in tissue architecture, but are induced following injury and modulate cell-cell and cell-matrix interactions. When released to the matrix, matricellular proteins associate with growth factors, cytokines, and other bioactive effectors and bind to cell surface receptors transducing signaling cascades. Matricellular proteins are upregulated in the injured and remodeling heart and play an important role in regulation of inflammatory, reparative, fibrotic and angiogenic pathways. Thrombospondin (TSP)-1, -2, and -4 as well as tenascin-C and -X secreted protein acidic and rich in cysteine (SPARC), osteopontin, periostin, and members of the CCN family (including CCN1 and CCN2/connective tissue growth factor) are involved in a variety of cardiac pathophysiological conditions, including myocardial infarction, cardiac hypertrophy and fibrosis, aging-associated myocardial remodeling, myocarditis, diabetic cardiomyopathy, and valvular disease. This review discusses the properties and characteristics of the matricellular proteins and presents our current knowledge on their role in cardiac adaptation and disease. Understanding the role of matricellular proteins in myocardial pathophysiology and identification of the functional domains responsible for their actions may lead to design of peptides with therapeutic potential for patients with heart disease.

Reperfusion injury intensifies the adaptive human T cell alloresponse in a human-mouse chimeric artery model

OBJECTIVE

Perioperative nonimmune injuries to an allograft can decrease graft survival. We have developed a model for studying this process using human materials.

METHODS AND RESULTS

Human artery segments were transplanted as infrarenal aortic interposition grafts into an immunodeficient mouse host, allowed to "heal in" for 30 days, and then retransplanted into a second mouse host. To induce a reperfusion injury, the healed-in artery segments were incubated for 3 hours under hypoxic conditions ex vivo before retransplantation. To induce immunologic rejection, the animals receiving the retransplanted artery segment were adoptively transferred with human peripheral blood mononuclear cells or purified T cells from a donor allogeneic to the artery 1 week before surgery. To compare rejection of injured versus healthy tissues, these manipulations were combined. Results were analyzed ex vivo by histology, morphometry, immunohistochemistry, and mRNA quantitation or in vivo by ultrasound. Our results showed that reperfusion injury, which otherwise heals with minimal sequelae, intensifies the degree of allogeneic T cell-mediated injury to human artery segments.

CONCLUSIONS

We developed a new human-mouse chimeric model demonstrating interactions of reperfusion injury and alloimmunity using human cells and tissues that may be adapted to study other forms of nonimmune injury and other types of adaptive immune responses.

Thrombospondin-4 expression is rapidly upregulated by cardiac overload

The precise mechanisms regulating gene expression of thrombospondins (TSPs) in the heart remain incompletely understood. Here we characterized cardiac TSP-4 expression in response to pressure overload and myocardial infarction in vivo. Arginine(8)-vasopressin (AVP) infusion increased left ventricular (LV) TSP-4 mRNA levels within 30 min. Also angiotensin II infusion rapidly activated LV TSP-4 expression, TSP-4 mRNA levels being highest at 6h and protein at 72 h and 2 weeks. During remodeling process following myocardial infarction, LV TSP-4 mRNA levels increased at day one, as studied by quantitative RT-PCR. TSP-4 immunostaining was localized to endothelial cells in hypertrophied hearts of spontaneously hypertensive rats. AVP-infusion increased LV TSP-1 mRNA levels similarly to TSP-4 within 30 min showing that rapid induction of gene expression, well before the development of cardiac hypertrophy, is typical for the thrombospondin family. These results further suggest that TSP-4 may be an endothelial specific marker of cardiac overload.

Cell type-specific post-transcriptional regulation of production of the potent antiangiogenic and proatherogenic protein thrombospondin-1 by high glucose

Hyperglycemia is an independent risk factor for development of vascular diabetic complications. Vascular dysfunction in diabetics manifests in a tissue-specific manner; macrovasculature is affected by atherosclerotic lesions, and microvascular complications are described as "aberrant angiogenesis": in the same patient angiogenesis is increased in some tissues (e.g. retinal neovascularization) and decreased in others (e.g. in skin). Molecular cell- and tissue-specific mechanisms regulating the response of vasculature to hyperglycemia remain unclear. Thrombospondin-1 (TSP-1), a potent antiangiogenic and proatherogenic protein, has been implicated in the development of several vascular diabetic complications (atherosclerosis, nephropathy, and cardiomyopathy). This study examines cell type-specific regulation of production of thrombospondin-1 by high glucose. We previously reported the increased expression of TSP-1 in the large arteries of diabetic animals. mRNA and protein levels were up-regulated in response to high glucose. Unlike in macrovascular cells, TSP-1 protein levels are dramatically decreased in response to high glucose in microvascular endothelial cells and retinal pigment epithelial cells (RPE). This down-regulation is post-transcriptional; mRNA levels are increased. In situ mRNA hybridization and immunohistochemistry revealed that the level of mRNA is up-regulated in RPE of diabetic rats, whereas the protein level is decreased. This cell type-specific posttranscriptional suppression of TSP-1 production in response to high glucose in microvascular endothelial cells and RPE is controlled by untranslated regions of TSP-1 mRNA that regulate coupling of TSP-1 mRNA to polysomes and its translation. The cell-specific regulation of TSP-1 suggests a potential mechanism for the aberrant angiogenesis in diabetics and TSP-1 involvement in development of various vascular diabetic complications.

Glycosylation mediates up-regulation of a potent antiangiogenic and proatherogenic protein, thrombospondin-1, by glucose in vascular smooth muscle cells

Accelerated development of atherosclerotic lesions remains the most frequent and dangerous complication of diabetes, accounting for 80% of deaths among diabetics. However, our understanding of the pathways mediating glucose-induced gene expression in vascular cells remains controversial and incomplete. We have identified an intracellular metabolic pathway activated by high glucose in human aortic smooth muscle cells that mediates up-regulation of thrombospondin-1 (TSP-1). TSP-1 is a potent antiangiogenic and proatherogenic protein that may represent an important link between diabetes and vascular complications. Using different glucose analogs and metabolites sharing distinct, limited metabolic steps with glucose, we demonstrated that activation of TSP-1 transcription is mediated by the hexosamine pathway of glucose catabolism, possibly resulting in modulation of the activity of nuclear proteins activity through their glycosylation. Specific inhibitors of glutamine: fructose 6-phosphate amidotransferase (GFAT), an enzyme controlling the hexosamine pathway, as well as direct inhibitors of protein glycosylation efficiently inhibited TSP-1 transcription and the activity of a TSP-1 promoter-reporter construct stimulated by high glucose. Overexpression of recombinant GFAT resulted in increased TSP-1 levels. Pharmacological inhibition of GFAT or protein glycosylation inhibited increased proliferation of human aortic smooth muscle cells caused by glucose. We have demonstrated that the hexosamine metabolic pathway mediates up-regulation of TSP-1 by high glucose. Our results suggest that the hexosamine pathway and intracellular glycosylation may control important steps in initiation and development of atherosclerotic lesions.

Thrombospondin-1 controls vascular platelet recruitment and thrombus adherence in mice by protecting (sub)endothelial VWF from cleavage by ADAMTS13

The function of thrombospondin-1 (TSP-1) in hemostasis was investigated in wild-type (WT) and Tsp1-/- mice, via dynamic platelet interaction studies with A23187-stimulated mesenteric endothelium and with photochemically injured cecum subendothelium. Injected calcein-labeled WT platelets tethered or firmly adhered to almost all A23187-stimulated blood vessels of WT mice, but Tsp1-/- platelets tethered to 45% and adhered to 25.8% of stimulated Tsp1-/- vessels only. Stimulation generated temporary endothelium-associated ultralarge von Willebrand factor (VWF) multimers, triggering platelet string formation in 48% of WT versus 20% of Tsp1-/- vessels. Injection of human TSP-1 or thrombotic thrombocytopenic purpura (TTP) patient-derived neutralizing anti-ADAMTS13 antibodies corrected the defective platelet recruitment in Tsp1-/- mice, while having a moderate effect in WT mice. Photochemical injury of intestinal blood vessels induced thrombotic occlusions with longer occlusion times in Tsp1-/- venules (1027 +/- 377 seconds) and arterioles (858 +/- 289 seconds) than in WT vessels (559 +/- 241 seconds, P < .001; 443 +/- 413 seconds, P < .003) due to defective thrombus adherence, resulting in embolization of complete thrombi, a defect restored by both human TSP-1 and anti-ADAMTS13 antibodies. We conclude that in a shear field, soluble or local platelet-released TSP-1 can protect unfolded endothelium-bound and subendothelial VWF from degradation by plasma ADAMTS13, thus securing platelet tethering and thrombus adherence to inflamed and injured endothelium, respectively.

Angiogenesis occurs within the intimal proliferation that characterizes transplant coronary artery vasculopathy

BACKGROUND

Vascular remodeling is central to the development of transplant coronary artery vasculopathy (CAV). For remodeling to occur, a sustained blood and nutrient supply is essential. Here we report on the presence of angiogenesis within the neointima of coronary arteries from cardiac transplant recipients.

METHODS

Coronary arteries from 57 cardiac transplant recipients with CAV were analyzed. Immunocytochemistry with antibodies raised against endothelial cells (CD31, CD34, and vWF), vascular smooth muscle cells (SmA), and activated endothelial cells (MHC 2, P-SEL, E-SEL, and VCAM-1) was performed.

RESULTS

A total of 89% of patients had significant angiogenesis. These vessels appeared as endothelial lined channels and were present in a concentric circumferential pattern within the mid portion of the neointima. These new vessels were present at an interface between an area of intimal hyperplasia and below an area of fibrous regeneration. These 2 distinct zones were present in 64% of the cases, and were clearly demonstrated with an elastic van Gieson (EVG) stain and are distinctly different from that seen in native atherosclerosis. Endothelial activation markers were strongly expressed by the endothelial cells lining new vessels, suggesting that they are functional and may aid in the recruitment of inflammatory cells.

CONCLUSIONS

These data suggest that angiogenesis is present within the intima of CAV lesions and may contribute to the continued obliteration of the vessel lumen. The vessels appear to originate in the intima and may represent the location of the donor endothelium before transplantation. Inhibition of endothelial damage may provide therapeutic options to prevent the progression of CAV.

Endogenous thrombospondin-1 is not necessary for proliferation but is permissive for vascular smooth muscle cell responses to platelet-derived growth factor

We have reexamined the role of endogenous thrombospondin-1 (TSP1) in growth and motility of vascular smooth muscle cells (SMCs). Based on the ability of aortic-derived SMCs isolated from TSP1 null mice and grown in the absence of exogenous TSP1 to grow at comparable rates and to a slightly higher density than equivalent cells from wild-type mice, TSP1 is not necessary for their growth. Low concentrations of exogenous TSP1 stimulate growth of TSP1 null SMCs, but higher doses of TSP1 or its C-terminal domain are inhibitory. However, SMCs from TSP1 null mice are selectively deficient in chemotactic and proliferative responses to platelet-derived growth factor and in outgrowth in three-dimensional cultures. Recombinant portions of the N- and C-terminal domains of TSP1 stimulate SMC chemotaxis through different integrin receptors. Based on these data, the relative deficiency in SMC outgrowth during an ex vivo angiogenic response of muscle tissue from TSP1 null mice is probably due to restriction of platelet-derived growth factor dependent SMC migration and/or proliferation.

Coronary artery disease and the thrombospondin single nucleotide polymorphisms

GeneQuest was a high throughput, large-scale analysis of single nucleotide polymorphisms (SNPs) to identify gene associated with familial, premature coronary artery disease and myocardial infarction. The three SNPs showing the highest and most significant associations with disease were all members of the thrombospondin gene family, thrombospondin-1, thrombospondin-2 and thrombospondin-4. These unanticipated associations have kindled efforts to understand how the three SNPs influence the structures and functions of the thrombospondins. The SNP in thrombospondin-1 and thrombospondin-4 reside in their coding regions and result in single amino acid changes: in thrombospondin-1, the predominant asparagine at position 700 is changed to a serine while, in thrombospondin-4, it is a change of an alanine to a proline at position 387. The SNP in thrombospondin-2 is a base change in the 3'-untranslated region of the mRNA. At this early stage of investigation, predictive analyses suggest that the substitutions in thrombospondin-2 and thrombospondin-4 should alter structure, and there is direct evidence to indicate that the thrombospondin-1 SNP alters conformational stability. In addition, profound differences in the function of the thrombospondin-4 SNP variants have been identified with respect to their capacity to support endothelial cell adhesion and proliferation. While substantial additional information is needed to understand if and how the polymorphic forms of the thrombospondins affect coronary artery disease, the data assembled to date suggest marked effects of these SNPs on the structures and functions of the thrombospondins, which are consistent with induction of a proatherogenic and prothrombotic phenotype.

The role of nitric oxide in renal transplantation

This review discusses the concept that nitric oxide synthase (NOS) may orchestrate both the inflammatory response to the renal allograft and anti-inflammatory defense in the graft itself. NO is produced by endothelial, epithelial, as well as inflammatory cells. In the setting of transplantation, the endothelium is the first lining to be subjected to the early response to injury. In turn, activated endothelial cells facilitate leukocyte recruitment, immune-mediated injury, and angiogenesis. On activation by inflammatory stimuli, endothelial cells up-regulate multiple vasoactive substances, oxygen radicals, cytokines, chemokines, and growth factors. Therefore, endothelial integrity, especially the expression of protecting vasoactive agents, such as NO, may be a key factor in resistance or sensitivity to transplantation-mediated injury. Thus, evaluating the mechanisms by which NO is involved in either protecting or injuring the transplanted allogeneic kidney is important for our understanding of renal allograft rejection. This review focuses on the role of NO in the inflammatory endothelial-leukocyte interactions, which are implicated in acute and chronic rejection of the transplanted kidney.

Early growth responsive gene (Egr)-1 expression correlates with cardiac allograft rejection

BACKGROUND

Early growth response factor (Egr)-1 is a transcription factor induced by inflammatory cytokines that regulates the expression of cytokines, adhesion molecules, other genes pertinent to inflammatory, and proliferative pathologies. Its expression in allografted tissue and role in the pathogenesis of graft rejection has not been explored. The goal of this work is to determine whether Egr-1 expression could be used as a surrogate marker of cardiac allograft rejection.

METHODS

Egr-1 protein expression was analyzed in endomyocardial biopsies of different rejection grades by immunohistochemistry. Egr-1 mRNA expression was analyzed in 106 biopsies from 11 transplant patients by semiquantitative reverse-transcriptase polymerase chain reaction. Egr-1 was also analyzed in coronary arteries from patients with coronary artery vasculopathy (CAV) by Western blot and immunohistochemistry.

RESULTS

No expression of Egr-1 protein was observed in grade 0 biopsies by immunohistochemistry. Strong nuclear Egr-1 was noted in leukocytes and cardiac myocytes in grade 3 biopsies. A clear pattern emerged where 20% (6/30), 34% (20/58), 22% (2/9), and 89%(8/9) of International Society For Heart and Lung Transplantation grade 0, 1, 2, and 3 biopsies were positive for Egr-1 mRNA. There was a significant (P<0.005) relationship between Egr-1 mRNA expression and rejection grade in endomyocardial biopsies. The calculated odds ratio indicates that a biopsy has a 2.18% greater probability of Egr-1 expression per increasing grade of rejection. Egr-1 was also up-regulated in vascular cells in coronary arteries from patients with CAV.

CONCLUSIONS

In consideration of its role as a transcription factor for genes involved in pathologic processes, Egr-1 expression in endomyocardial biopsies may act as a surrogate marker of cardiac allograft rejection.

Cardiac allograft vasculopathy: advances in understanding its pathophysiology, prevention, and treatment

PURPOSE OF REVIEW

To discuss the current understanding of the pathogenesis, natural history, and diagnosis of cardiac allograft vasculopathy, and to outline new preventive and treatment strategies.

RECENT FINDINGS

The central event in the development of allograft vasculopathy is the inflammatory response to immune or nonimmune-mediated endothelial damage. This response is characterized by the release of inflammatory cytokines, upregulation of cell-surface adhesion molecules, and the subsequent binding of leukocytes. Once induced, vascular smooth muscle cells proliferate and migrate from the media to form a neointima. Circulating progenitor cells are recruited to sites of arterial injury where they may then differentiate into smooth muscle cells. Because of its diffuse nature, allograft vasculopathy is best detected by intravascular ultrasound. Noninvasive tests, such as dobutamine echocardiography, are gaining in favor. Although the only definitive treatment is retransplantation, the immunosuppressant rapamycin can limit disease progression. Its synthetic derivative, everolimus, effectively prevented intimal hyperplasia in de novo transplant recipients.

SUMMARY

These advances have provided hope that allograft vasculopathy may finally be manageable.

Differential expression of the IFN-gamma-inducible CXCR3-binding chemokines, IFN-inducible protein 10, monokine induced by IFN, and IFN-inducible T cell alpha chemoattractant in human cardiac allografts: association with cardiac allograft vasculopathy and acute rejection

CXCR3 chemokines exert potent biological effects on both immune and vascular cells. The dual targets suggest their important roles in cardiac allograft vasculopathy (CAV) and rejection. Therefore, we investigated expression of IFN-inducible protein 10 (IP-10), IFN-inducible T cell alpha chemoattractant (I-TAC), monokine induced by IFN (Mig), and their receptor CXCR3 in consecutive endomyocardial biopsies (n = 133) from human cardiac allografts and corresponding normal donor hearts (n = 11) before transplantation. Allografts, but not normal hearts, contained IP-10, Mig, and I-TAC mRNA. Persistent elevation of IP-10 and I-TAC was associated with CAV. Allografts with CAV had an IP-10-GAPDH ratio 3.7 +/- 0.8 compared with 0.8 +/- 0.2 in those without CAV (p = 0.004). Similarly, I-TAC mRNA levels were persistently elevated in allografts with CAV (6.7 +/- 1.9 in allografts with vs 1.5 +/- 0.3 in those without CAV, p = 0.01). In contrast, Mig mRNA was induced only during rejection (2.4 +/- 0.9 with vs 0.6 +/- 0.2 without rejection, p = 0.015). In addition, IP-10 mRNA increased above baseline during rejection (4.1 +/- 2.3 in rejecting vs 1.8 +/- 1.2 in nonrejecting biopsies, p = 0.038). I-TAC did not defer significantly with rejection. CXCR3 mRNA persistently elevated after cardiac transplantation. Double immunohistochemistry revealed differential cellular distribution of CXCR3 chemokines. Intragraft vascular cells expressed high levels of IP-10 and I-TAC, while Mig localized predominantly in infiltrating macrophages. CXCR3 was localized in vascular and infiltrating cells. CXCR3 chemokines are induced in cardiac allografts and differentially associated with CAV and rejection. Differential cellular distribution of these chemokines in allografts indicates their central roles in multiple pathways involving CAV and rejection. This chemokine pathway may serve as a monitor and target for novel therapies to prevent CAV and rejection.