Fas ligand-deficient mice display enhanced leukocyte infiltration and intima hyperplasia in flow-restricted vessels

Fas ligand and nitric oxide combination to control smooth muscle growth while sparing endothelium

Metallic stents cause vascular wall damage with subsequent smooth muscle cell (SMC) proliferation, neointimal hyperplasia, and treatment failure. To combat in-stent restenosis, drug-eluting stents (DES) delivering mTOR inhibitors such as sirolimus or everolimus have become standard for coronary stenting. However, the relatively non-specific action of mTOR inhibitors prevents efficient endothelium recovery and mandates dual antiplatelet therapy to prevent thrombosis. Unfortunately, long-term dual antiplatelet therapy leads to increased risk of bleeding/stroke and, paradoxically, myocardial infarction. Here, we took advantage of the fact that nitric oxide (NO) increases Fas receptors on the SMC surface. Fas forms a death-inducing complex upon binding to Fas ligand (FasL), while endothelial cells (ECs) are relatively resistant to this pathway. Selected doses of FasL and NO donor synergistically increased SMC apoptosis and inhibited SMC growth more potently than did everolimus or sirolimus, while having no significant effect on EC viability and proliferation. This differential effect was corroborated in ex vivo pig coronaries, where the neointimal formation was inhibited by the drug combination, but endothelial viability was retained. We also deployed FasL-NO donor-releasing ethylene-vinyl acetate copolymer (EVAc)-coated stents into pig coronary arteries, and cultured them in perfusion bioreactors for one week. FasL and NO donor, released from the stent coating, killed SMCs close to the stent struts, even in the presence of flow rates mimicking those of native arteries. Thus, the FasL-NO donor-combination has a potential to prevent intimal hyperplasia and in-stent restenosis, without harming endothelial restoration, and hence may be a superior drug delivery strategy for DES.

Human mesenchymal stromal cells (MSCs) reduce neointimal hyperplasia in a mouse model of flow-restriction by transient suppression of anti-inflammatory cytokines

AIM

Mesenchymal stromal cells from human bone marrow (hMSCs) were observed to produce therapeutic benefits in some models for cardiac and vascular injuries but their mode of action was not defined. We tested the effects of hMSCs in models for restricted vascular flow.

METHODS

We made model for restricted vascular flow produced by permanent ligation of a carotid artery and injected hMSCs to clarify the effects of hMSCs to vascular lesions.

RESULTS

Seven, 14, and 28 days after infusion of hMSCs into the cardiac left ventricle of the mice, there was a significant reduction in neointimal hyperplasia (p<0.05). Seven days after administration of the hMSCs, macrophages infiltration into the ligated artery and serum levels of monocyte chemoattractive protein-1 (MCP-1/CCL-2) (p<0.05) were reduced. However, no hMSCs were detected in the lesions by sensitive PCR assays. We then observed that the serum level of MCP-1 was a potential biomarker for the therapeutic effects of hMSCs in a mouse model for high-fat-diet.

CONCLUSIONS

These results indicated the administration of hMSCs decreased the initial and excess inflammatory responses to carotid artery ligation. The decrease in inflammatory response apparently decreased the subsequent neointimal hyperplasia.

Angiotensin II-inducible smooth muscle cell apoptosis involves the angiotensin II type 2 receptor, GATA-6 activation, and FasL-Fas engagement

RATIONALE

Fas ligand (FasL)-mediated smooth muscle cell (SMC) apoptosis within the vulnerable plaque may lead to plaque instability and rupture, events that underlie myocardial infarction and stroke.

OBJECTIVE

The molecular mechanisms underlying FasL transcription and FasL-dependent SMC apoptosis were investigated in this study in vitro and in vivo.

METHODS AND RESULTS

We demonstrate that GATA-6, the predominant GATA family member expressed in SMCs, stimulates SMC apoptosis in an extracellular FasL-dependent manner. Both GATA-6 and FasL were inducibly and transiently expressed following balloon injury to rat carotid arteries. We identified two potential GATA binding in the FasL promoter and demonstrated using DNA binding and chromatin immunoprecipitation assays that GATA-6 regulates FasL through one ((-298)TTATCA(-303)) but not both these elements. Angiotensin II (Ang II) stimulated expression of both GATA-6 and FasL. Ang II increased SMC apoptosis in an Ang II type 2 receptor-, caspase 8-, and FasL-dependent fashion. GATA-6 activation was MEK-ERK1/2- and JNK-dependent, and GATA-6 small interfering RNA blocked Ang II-inducible FasL expression and SMC apoptosis. Administration of Ang II to rats increased FasL expression and apoptosis in carotid artery SMCs in an Ang II type 2 receptor- and GATA-6-dependent manner.

CONCLUSIONS

This study provides new insights into the transcriptional events underpinning FasL-dependent SMC apoptosis after exposure to Ang II.

Smooth muscle apoptosis and vascular remodeling

PURPOSE OF REVIEW

The present review is to summarize recent advances in molecular mechanisms that regulate vascular smooth muscle cell apoptosis during vascular remodeling. In normal blood vessels apoptosis counteracts cell division, whereas apoptosis is especially crucial for regulating vascular remodeling during cardiovascular diseases.

RECENT FINDINGS

Recent results have expanded our knowledge regarding the signaling pathways and molecules that regulate vascular smooth muscle cell death in postnatal vascular remodeling. Compelling data from genetic disorders associated with vascular smooth muscle cell loss (e.g., Hutchinson-Gilford progeria syndrome) and experimental studies suggest that changes in hemodynamic and mechanical forces are major modulators for vascular smooth muscle cell apoptosis. Furthermore, understanding the therapeutic effects of antihypertensive drugs related to apoptosis may identify pathways that can improve outcomes independent of the blood pressure fall.

SUMMARY

Regulation of vascular smooth muscle cell apoptosis is a potential target to modify pathological vascular remodeling and new drugs development.

Genetic determinants of vascular remodelling

Vascular remodelling is an important physiological mechanism that occurs as a result of changes in hemodynamics, and is a pathological process that plays a major role in the clinical manifestations of cardiovascular diseases. Using a mouse model, it was recently established that vascular remodelling is partially based on ligation of the carotid. In this model, low flow was associated with intima media thickening (IMT). IMT is a major manifestation of atherosclerosis of the carotid artery, and it is an important predictor of cardiovascular events. Carotid IMT has a strong genetic component. It was hypothesized that there would be genetically determined differences in outward remodelling and IMT induced by carotid flow alterations. Vascular remodelling among five inbred strains of mice were compared. Despite similar changes in flow in the left carotid among the strains, dramatic differences in remodelling of the partially ligated left carotid relative to control were observed. IMT correlated significantly with heart rate, outward remodelling and changes in plasminogen activator expression, cell proliferation and apoptosis. There were significant strain-dependent differences in the remodelling index (measured as the ratio of vessel area to IMT), which suggest fundamental alterations in sensing or transducing hemodynamic signals among strains. This model should be useful to identify and characterize the role of genes that mediate vascular remodelling.

Axl, a receptor tyrosine kinase, mediates flow-induced vascular remodeling

Intima-media thickening (IMT) in response to hemodynamic stress is a physiological process that requires coordinated signaling among endothelial, inflammatory, and vascular smooth muscle cells (VSMC). Axl, a receptor tyrosine kinase, whose ligand is Gas6, is highly induced in VSMC after carotid injury. Because Axl regulates cell migration, phagocytosis and apoptosis, we hypothesized that Axl would play a role in IMT. Vascular remodeling in mice deficient in Axl (Axl(-/-)) and wild-type littermates (Axl(+/+)) was induced by ligation of the left carotid artery (LCA) branches maintaining flow via the left occipital artery. Both genotypes had similar baseline hemodynamic parameters and carotid artery structure. Partial ligation altered blood flow equally in both genotypes: increased by 60% in the right carotid artery (RCA) and decreased by 80% in the LCA. There were no significant differences in RCA remodeling between genotypes. However, in the LCA Axl(-/-) developed significantly smaller intima+media compared with Axl(+/+) (31+/-4 versus 42+/-6x10(-6) microm3, respectively). Quantitative immunohistochemistry of Axl(-/-) LCA showed increased apoptosis compared with Axl(+/+) (5-fold). As expected, p-Akt was decreased in Axl(-/-), whereas there was no difference in Gas6 expression. Cell composition also changed significantly, with increases in CD45+ cells and decreases in VSMC, macrophages, and neutrophils in Axl(-/-) compared with Axl(+/+). These data demonstrate an important role for Axl in flow-dependent remodeling by regulating vascular apoptosis and vascular inflammation.

Role of endogenous Fas (CD95/Apo-1) ligand in balloon-induced apoptosis, inflammation, and neointima formation

BACKGROUND

Fas (CD95/Apo-1) ligand (FasL)-induced apoptosis in Fas-bearing cells is critically involved in modulating immune reactions and tissue repair. Apoptosis has also been described after mechanical vascular injury such as percutaneous coronary intervention. However, the relevance of cell death in this context of vascular repair remains unknown.

METHODS AND RESULTS

To determine whether FasL-induced apoptosis is causally related to neointimal lesion formation, we subjected FasL-deficient (generalized lymphoproliferative disorder [gld], C57BL/6J) and corresponding wild-type (WT) mice to carotid balloon distension injury, which induces marked endothelial denudation and medial cell death. FasL expression in WT mice was induced in injured vessels compared with untreated arteries (P<0.05; n=5). Conversely, absence of functional FasL in gld mice decreased medial and intimal apoptosis (terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling [TUNEL] index) at 1 hour and 7 days after balloon injury (P<0.05; n=6). In addition, peritoneal macrophages isolated from gld mice showed no apoptosis and enhanced migration (P<0.05; n=4). In parallel, we observed increased balloon-induced macrophage infiltrations (anti-CD68) in injured arteries of FasL-deficient animals (P<0.05; n=6). Together with enhanced proliferation (bromodeoxyuridine index; P<0.05), these events resulted in a further increase in medial and neointimal cells (P<0.01; n=8) with thickened neointima in gld mice (intima/media ratio, x3.8 of WT; P<0.01).

CONCLUSIONS

Our data identify proapoptotic and antiinflammatory effects of endogenous FasL as important factors in the process of neointimal lesion formation after balloon injury. Moreover, they suggest that activation of FasL may decrease neointimal thickening after percutaneous coronary intervention.

Impaired clearance of apoptotic cells promotes synergy between atherogenesis and autoimmune disease

To clarify the link between autoimmune disease and hypercholesterolemia, we created the gld.apoE^−/− mouse as a model of accelerated atherosclerosis. Atherosclerotic lesion area was significantly increased in gld.apoE^−/− mice compared with apoE^−/− mice. gld.apoE^−/− mice also displayed increases in lymphadenopathy, splenomegaly, and autoantibodies compared with gld mice, and these effects were exacerbated by high cholesterol diet. gld.apoE^−/− mice exhibited higher levels of apoptotic cells, yet a reduced frequency of engulfed apoptotic nuclei within macrophages. Infusion of lysophosphatidylcholine, a component of oxidized low density lipoprotein, markedly decreased apoptotic cell clearance in gld mice, indicating that hypercholesterolemia promotes autoimmune disease in this background. These data suggest that defects in apoptotic cell clearance promote synergy between atherosclerotic and autoimmune diseases.

Decreased circulating Fas ligand in patients with familial combined hyperlipidemia or carotid atherosclerosis: normalization by atorvastatin

OBJECTIVES

We sought to study whether patients with familial combined hyperlipidemia (FCH) or carotid atherosclerosis have modified circulating solubilized Fas ligand (sFasL) levels, as well as the potential modifications by atorvastatin. We also examined the effect of atorvastatin on FasL expression and sFasL release in cytokine-stimulated cultured human endothelial cells (ECs).

BACKGROUND

In normal situations, FasL is expressed in most cells, including ECs. Proinflammatory stimuli can downregulate its expression in ECs and facilitate the vascular infiltration of inflammatory cells.

METHODS

We have measured sFasL plasma levels (by ELISA) in 58 patients with FCH, 14 normocholesterolemic patients with carotid atherosclerosis, and 15 healthy volunteers. We analyzed FasL expression (by Western blot analysis) and sFasL release in cultured ECs stimulated with tumor necrosis factor (TNF)-alpha.

RESULTS

Solubilized FasL levels were decreased in hyperlipidemic patients (49 pg/ml), as compared with healthy volunteers (123 pg/ml, p < 0.0001). Patients were randomized to atorvastatin (n = 28) or bezafibrate (n = 30) during 12 months. Atorvastatin treatment increased sFasL concentrations (111 pg/ml, p < 0.0001), reaching normal values. However, treatment with bezafibrate only marginally affected sFasL (85 pg/ml, p < 0.05). Solubilized FasL was also diminished in patients with carotid atherosclerosis (39 pg/ml), and intensive treatment with atorvastatin normalized sFasL levels (90 pg/ml, p = 0.02). Finally, atorvastatin prevented the diminution of FasL expression and sFasL release elicited by TNF-alpha in cultured ECs.

CONCLUSIONS

Patients with FCH or carotid atherosclerosis have decreased circulating sFasL levels, probably indicating endothelial dysfunction, but treatment with atorvastatin restored normal blood levels. These data provide a novel effect of atorvastatin and add support for the well-known anti-inflammatory properties of statins.

Human Fas-ligand expression on porcine endothelial cells does not protect against xenogeneic natural killer cytotoxicity*

Several human leukocyte subsets including natural killer (NK) cells, cytotoxic T lymphocytes (CTL), and polymorphonuclear neutrophils (PMN) participate in cellular immune responses directed against vascularized pig-to-human xenografts. As these leukocytes express the death receptor Fas either constitutively (PMN) or upon activation (NK, CTL), we explored in vitro whether the transgenic expression of Fas ligand (FasL) on porcine endothelial cells (EC) is a valuable strategy to protect porcine xenografts. The porcine EC line 2A2 was stably transfected with human FasL (2A2-FasL) and interactions of 2A2-FasL with human leukocytes were analyzed using functional assays for apoptosis, cytotoxicity, chemotaxis, adhesion under shear stress, and transmigration. FasL expressed on porcine EC induced apoptosis in human NK and T cells, but did not protect porcine EC against killing mediated by human NK cells. 2A2-FasL released soluble FasL, which induced strong chemotaxis in human PMN. Adhesion under shear stress of PMN on 2A2-FasL cells was increased whereas transendothelial migration was decreased. In contrast, FasL had no effect on the adhesion of NK cells but increased their transmigration through porcine EC. Although FasL expression on porcine EC is able to induce apoptosis in human effector cells, it did not provide protection against xenogeneic cytotoxicity. The observed impact of FasL on adhesion and transendothelial migration provides evidence for novel biological functions of FasL.

Diverse Contribution of Bone Marrow Cells to Neointimal Hyperplasia After Mechanical Vascular Injuries

We and others have suggested that bone marrow-derived progenitor cells may contribute to the pathogenesis of vascular diseases. On the other hand, it was reported that bone marrow cells do not participate substantially in vascular remodeling in other experimental systems. In this study, three distinct types of mechanical vascular injuries were induced in the same mouse whose bone marrow had been reconstituted with that of GFP or LacZ mice. All injuries are known to cause smooth muscle cell (SMC) hyperplasia. At 4 weeks after wire-mediated endovascular injury, a significant number of the neointimal and medial cells derived from bone marrow. In contrast, marker-positive cells were seldom detected in the lesion induced by perivascular cuff replacement. There were only a few bone marrow-derived cells in the neointima after ligation of the common carotid artery. These results indicate that the origin of intimal cells is diverse and that contribution of bone marrow-derived cells to neointimal hyperplasia depends on the type of model.

Intratumoral T cell subset ratios and Fas ligand expression on brain tumor endothelium

INTRODUCTION

T cell presence in TIL, and the ratio of CD8+ and CD4+ T cell subsets in particular, can correlate with tumor prognosis in some tumors, although the significance of such infiltration into glioma is controversial. However, gliomas represent a lower extreme in their extent of T cell infiltration, and are thus useful in assessing factors that can decrease T cell presence within tumor tissue. Fas ligand, a pro-apoptotic cell surface protein, may play a key role in reduction of T cells in tumor tissue.

OBJECTIVE

To assess the level of FasL expression on brain tumor endothelium and to correlate this with relative levels of CD4+ and CD8+ T cell subsets in TIL from brain tumors.

METHODS

CD3+, CD4+, and CD8+ cells were quantified in fresh TIL by flow cytometry. Paraffin embedded sections of tumors, including meningiomas and gliomas as well as extracranial malignancies, underwent immunohistochemical staining for FasL and Von-Willebrand's factor (Factor VIII) to determine expression levels of endothelial FasL.

RESULTS

FasL expression was high in aggressive intracranial malignancies compared to more indolent neoplasms, and correlated inversely with CD8+/CD4+ TIL ratios in all tumor classes combined (ANOVA, p < 0.05).

CONCLUSION

Low levels of T cells within TIL, as well as low CD8+/CD4+ TIL ratios appear to be a property of parenchymal tumor presence. Together with the inverse correlation seen between FasL expression and CD8+/CD4+ TIL ratios, the high levels of endothelial FasL expression in gliomas suggests that FasL decreases T cell presence in brain tumors in a subset-selective manner, thus contributing to glioma immune privilege.

Alterations of arterial physiology in osteopontin-null mice

OBJECTIVE

In this study, we characterized the effects of an osteopontin (OPN)-null mutation in normal arterial function and remodeling in a murine model.

METHODS AND RESULTS

OPN-null mutant mice were compared with wild-type mice before and after carotid artery ligation. Before ligation, OPN-null mice had increased heart rate, lower blood pressure, and increased circulating lymphocytes compared with wild-type mice. OPN-null vessels also demonstrated greater compliance accompanied by a loosely organized collagen network. After carotid artery ligation, significant differences were also found in the remodeling response of OPN-null animals. At 4 days after ligation, leukocyte adhesion/invasion was diminished by 10-fold in OPN-null mice compared with wild-type mice. At 14 days after ligation, the ligated arteries of OPN-null mice had smaller neointimal lesions but greater constrictive remodeling compared with wild-type mice, resulting in similar lumen areas. Continued remodeling resulted in a similar morphological phenotype in both groups at 28 days.

CONCLUSIONS

These data show that endogenous OPN regulates normal vascular physiology and contributes to the vascular remodeling response by regulating vascular compliance and the inflammatory response.

Signaling and transcriptional control of Fas ligand gene expression

Fas ligand (FasL), a member of the tumor necrosis factor family, initiates apoptosis by binding to its surface receptor Fas. As a consequence, there is sequential activation of caspases and the release of cytochrome c from the mitochondria, with additional caspase activation followed by cellular degradation and death. Recent studies have shed important insight into the molecular mechanisms controlling FasL gene expression at the level of transcription. Nuclear factors such as nuclear factor in activated T cells, nuclear factor-kappa B, specificity protein-1, early growth response factor, interferon regulatory factor, c-Myc and the forkhead transcriptional regulator, alone or cooperatively, activate FasL expression. These factors are often coexpressed with FasL in pathophysiologic settings including human atherosclerotic lesions. Here, we review these important advances in our understanding of the signaling and transcriptional mechanisms controlling FasL gene expression.

Reduced apoptosis and increased lesion development in the flow-restricted carotid artery of p75(NTR)-null mutant mice

Apoptosis of neointimal smooth muscle cells is a well-recognized component of the pathogenesis of vascular lesions. In recent studies, we have identified the neurotrophin receptor, p75(NTR), as a mediator of apoptosis of neointimal smooth muscle cells. Neurotrophin ligands and p75(NTR) are selectively expressed in areas of atherosclerotic lesions with increased smooth muscle cell apoptosis and the neurotrophins are potent apoptotic agents for p75(NTR)-expressing smooth muscle cells in vitro. In the present study, we directly assess the role of p75(NTR) in lesion development in the flow-restricted carotid artery, a model of murine vascular injury. Ligation of the left carotid artery resulted in a 3- to 4-fold increase in lesion development in p75(NTR)-null mutant mice as compared with wild-type mice. The increase in lesion size was associated with a 70% decrease in apoptosis of neointimal smooth muscle cells, as assessed by in situ TUNEL analysis. These data suggest that under conditions of flow restriction, p75(NTR) activation impairs lesion formation by promoting smooth muscle cell apoptosis. These results further implicate p75(NTR) as an important regulator of smooth muscle cell apoptosis and lesion development after vascular injury.

Estrogen receptor-mediated, nitric oxide-dependent modulation of the immunologic barrier function of the endothelium: regulation of fas ligand expression by estradiol

BACKGROUND

Premenopausal women have a lower incidence of coronary artery disease than postmenopausal women or same-age men. Although the mechanisms of this apparent relative protection against atherosclerosis remain ill defined, estradiol, which is present in higher concentrations before menopause, is considered to play a central role. Recently, Fas ligand (FasL) expression by the vascular endothelium has been shown to inhibit the migration of inflammatory cells into the vessel wall, an event that is considered crucial for the development of atherosclerosis.

METHODS AND RESULTS

The regulation of endothelial FasL expression by estradiol was investigated in vivo and in vitro. In an ovariectomized, cholesterol-clamped rabbit model, FasL expression was shown to be downregulated by elevations in serum cholesterol, which also resulted in invasion of the arterial wall by macrophages. Estradiol replacement resulted in restoration of FasL expression, with resultant inhibition of leukocyte traffic across the endothelium. Inhibition of NO production by addition of L-NAME to the drinking water of the estradiol-treated rabbits abrogated these effects. In vitro, estradiol is shown to regulate FasL expression at the transcriptional level via an estrogen receptor-mediated, NO-dependent mechanism.

CONCLUSIONS

Estradiol transcriptionally regulates endothelial FasL expression by a mechanism involving at least one of the estrogen receptors. In an animal model of atherosclerosis, estradiol restores FasL expression, which is suppressed by atherogenic levels of serum cholesterol. The maintenance of endothelial FasL expression by estradiol may represent a mechanism of estrogen's apparent antiatherogenic effect.

Acute and chronic smooth muscle cell apoptosis after mechanical vascular injury can occur independently of the Fas-death pathway

Vascular smooth muscle cell (VSMC) apoptosis has been demonstrated in vascular lesions, such as atherosclerotic and postangioplasty restenotic lesions. Balloon injury also induces VSMC apoptosis. Fas is a death factor that mediates apoptosis when it is activated by its ligand, FasL. Fas-mediated apoptosis was found to be implicated in the pathogenesis of vascular diseases in which Fas/FasL expression was detected. We investigated whether the Fas/FasL interaction mediated acute and chronic VSMC apoptosis and lesion formation in a vascular injury model that may resemble balloon angioplasty. A large spring wire was inserted into the femoral artery of C3H/HeJ (wild-type), C3H-gld (Fas ligand-/-), and C3H-lpr (Fas-/-) mice. The wire was left in place for 1 minute to denude and expand the artery. Massive apoptosis was observed in medial VSMCs from 1 to 7 hours later. There was no difference in the number of apoptotic cells among the 3 groups of mice 4 hours after injury. At 4 weeks, the injured arteries presented signs of concentric neointimal hyperplasia composed exclusively of VSMCs. There was no difference in the degree of neointima hyperplasia (intima/media ratios were as follows: wild type 1.4+/-0.3, gld 1.0+/-0.2, and lpr 1.3+/-0.2) or in the number of apoptotic nuclei among the 3 groups. These findings suggest the existence of other signaling pathways for acute and chronic VSMC apoptosis, at least that induced by mechanical vascular injury.

Anti-inflammatory mechanisms in the vascular wall

The role of vascular cells during inflammation is critical and is of particular importance in inflammatory diseases, including atherosclerosis, ischemia/reperfusion, and septic shock. Research in vascular biology has progressed remarkably in the last decade, resulting in a better understanding of the vascular cell responses to inflammatory stimuli. Most of the vascular inflammatory responses are mediated through the IkappaB/nuclear factor-kappaB system. Much recent work shows that vascular inflammation can be limited by anti-inflammatory counteregulatory mechanisms that maintain the integrity and homeostasis of the vascular wall. The anti-inflammatory mechanisms in the vascular wall involve anti-inflammatory external signals and intracellular mediators. The anti-inflammatory external signals include the anti-inflammatory cytokines, transforming growth factor-beta, interleukin-10 and interleukin-1 receptor antagonist, HDL, as well as some angiogenic and growth factors. Physiological laminar shear stress is of particular importance in protecting endothelial cells against inflammatory activation. Its effects are partly mediated through NO production. Finally, endogenous cytoprotective genes or nuclear receptors, such as the peroxisome proliferator-activated receptors, can be expressed by vascular cells in response to proinflammatory stimuli to limit the inflammatory process and the injury.