Toshifumi Morooka

Vascular Inflammation and Repair: Implications for Re-Endothelialization, Restenosis, and Stent Thrombosis

The cellular and molecular processes that control vascular injury responses after percutaneous coronary intervention involve a complex interplay among vascular cells and progenitor cells that control arterial remodeling, neointimal proliferation, and re-endothelialization. Drug-eluting stents (DES) improve the efficacy of percutaneous coronary intervention by modulating vascular inflammation and preventing neointimal proliferation and restenosis. Although positive effects of DES reduce inflammation and restenosis, negative effects delay re-endothelialization and impair endothelial function. Delayed re-endothelialization and impaired endothelial function are linked to stent thrombosis and adverse clinical outcomes after DES use. Compared with bare-metal stents, DES also differentially modulate mobilization, homing, and differentiation of vascular progenitor cells involved in re-endothelialization and neointimal proliferation. The effects of DES on vascular inflammation and repair directly impact clinical outcomes with these devices and dictate requirements for extended-duration dual antiplatelet therapy.

Platelet-derived S100 family member myeloid-related protein-14 regulates thrombosis.

Expression of the gene encoding the S100 calcium-modulated protein family member MRP-14 (also known as S100A9) is elevated in platelets from patients presenting with acute myocardial infarction (MI) compared with those from patients with stable coronary artery disease; however, a causal role for MRP-14 in acute coronary syndromes has not been established. Here, using multiple models of vascular injury, we found that time to arterial thrombotic occlusion was markedly prolonged in Mrp14⁻/⁻ mice. We observed that MRP-14 and MRP-8/MRP-14 heterodimers (S100A8/A9) are expressed in and secreted by platelets from WT mice and that thrombus formation was reduced in whole blood from Mrp14⁻/⁻ mice. Infusion of WT platelets, purified MRP-14, or purified MRP-8/MRP-14 heterodimers into Mrp14⁻/⁻ mice decreased the time to carotid artery occlusion after injury, indicating that platelet-derived MRP-14 directly regulates thrombosis. In contrast, infusion of purified MRP-14 into mice deficient for both MRP-14 and CD36 failed to reduce carotid occlusion times, indicating that CD36 is required for MRP-14-dependent thrombosis. Our data identify a molecular pathway of thrombosis that involves platelet MRP-14 and CD36 and suggest that targeting MRP-14 has potential for treating atherothrombotic disorders, including MI and stroke.

Critical role for Syk in responses to vascular injury

Although current antiplatelet therapies provide potent antithrombotic effects, their efficacy is limited by a heightened risk of bleeding and failure to affect vascular remodeling after injury. New lines of research suggest that thrombosis and hemorrhage may be uncoupled at the interface of pathways controlling thrombosis and inflammation. Here, as one remarkable example, studies using a novel and highly selective pharmacologic inhibitor of the spleen tyrosine kinase Syk [PRT060318; 2-((1R,2S)-2-aminocyclohexylamino)-4-(m-tolylamino)pyrimidine-5-carboxamide] coupled with genetic experiments, demonstrate that Syk inhibition ameliorates both the acute and chronic responses to vascular injury without affecting hemostasis. Specifically, lack of Syk (murine radiation chimeras) attenuated shear-induced thrombus formation ex vivo, and PRT060318 strongly inhibited arterial thrombosis in vivo in multiple animal species while having minimal impact on bleeding. Furthermore, leukocyte-platelet-dependent responses to vascular injury, including inflammatory cell recruitment and neointima formation, were markedly inhibited by PRT060318. Thus, Syk controls acute and long-term responses to arterial vascular injury. The therapeutic potential of Syk may be exemplary of a new class of antiatherothrombotic agents that target the interface between thrombosis and inflammation.

Stent-induced neutrophil activation is associated with an oxidative burst in the inflammatory process, leading to neointimal thickening.

Activation of leukocytes plays an essential role in the mechanism of restenosis. Prior research has focused on monocytes and little is known about the role of neutrophils in this process. Neutrophils are known to contribute to tissue injury through oxygen-derived free radicals that nitrate tyrosine. This study was designed to elucidate clinically the role of neutrophil-mediated oxidative burst in the regulation of the post-stent inflammatory process. In 36 patients undergoing coronary stenting, we serially measured serum levels of glycosyl-phosphatidil-inositol-anchored protein (GPI)-80,a modulator of Mac-1 on the surface of neutrophils, in samples of coronary sinus as well as peripheral blood. We also simultaneously measured the serum 3-nitrotyrosine/tyrosine ratio as an index of oxidative stress. The GPI-80 level and the 3-nitrotyrosine/tyrosine ratio increased in the coronary sinus after coronary stenting in a time-dependent manner; with the maximum increase of GPI-80 level (3.1 +/- 2.9 to 8.6 +/- 4.3 ng/ml, P < 0.01) at 48 hours, and 3-nitrotyrosine/tyrosine ratio at 24 hrs (5.2 +/- 4.8 to 28.4 +/- 13.2 x 10(-4), P < 0.01), more strikingly than in the peripheral blood. In the coronary sinus blood, the 3-nitrotyrosine/tyrosine ratio was correlated with GPI-80 levels at 24 hr (R = 0.58, P < 0.001) and at 48 hr (R = 0.41, P < 0.01). Multiple regressions analysis showed that the maximum responses of GPI-80 level and 3-nitrotyrosine/tyrosine ratio were independent predictors of angiographic late lumen loss. Our results may support a hypothesis that Mac-1-dependent activation of neutrophils causes oxidative burst in the post-stent inflammatory process, possibly leading to restenosis.

Intravascular Optical Coherence Tomography Detection of Atherosclerosis and Inflammation in Murine Aorta

Objective—The goal of this study was to evaluate the feasibility of imaging the aorta of apolipoprotein E–deficient (ApoE–/–) mice for the detection of atherosclerosis and macrophages using optical coherence tomography (OCT) compared with histology. Methods and Results—Atherosclerosis was induced by high-fat diet in 7-week-old ApoE–/– mice for 10 (n=7) and 22 (n=7) weeks. Nine-week-old ApoE–/– mice (n=7) fed a standard chow diet were used as controls. OCT images of a 10-mm descending aorta in situ were performed in 4 mice for each, and plaque and macrophages were determined at 0.5-mm intervals. Automated detection and quantification of macrophages were performed independently using a customized algorithm. Coregistered histological cross-sections were stained with hematoxylin-eosin, Mac-3, and von Kossa. Three mice in each group had en face OCT imaging to detect macrophages, which were compared with lipid-positive area with Sudan IV. OCT images were successfully acquired in all mice. OCT and histology were able to discriminate macrophages and plaque among the 3 groups and showed excellent correlation for (1) visual detection of plaque (r=0.98) and macrophages (r=0.93), (2) automated detection and quantification of macrophages by OCT versus Mac-3-positive area (r=0.92), and (3) en face OCT detection of macrophages versus Sudan IV–positive area (r=0.92). Conclusion—Murine intra-aortic OCT is feasible and shows excellent correlation with histology for detection of atherosclerotic plaque and macrophages.

Prolylcarboxypeptidase promotes angiogenesis and vascular repair

Prolylcarboxypeptidase (PRCP) is associated with leanness, hypertension, and thrombosis. PRCP-depleted mice have injured vessels with reduced Kruppel-like factor (KLF)2, KLF4, endothelial nitric oxide synthase (eNOS), and thrombomodulin. Does PRCP influence vessel growth, angiogenesis, and injury repair? PRCP depletion reduced endothelial cell growth, whereas transfection of hPRCP cDNA enhanced cell proliferation. Transfection of hPRCP cDNA, or an active site mutant (hPRCPmut) rescued reduced cell growth after PRCP siRNA knockdown. PRCP-depleted cells migrated less on scratch assay and murine PRCP(gt/gt) aortic segments had reduced sprouting. Matrigel plugs in PRCP(gt/gt) mice had reduced hemoglobin content and angiogenic capillaries by platelet endothelial cell adhesion molecule (PECAM) and NG2 immunohistochemistry. Skin wounds on PRCP(gt/gt) mice had delayed closure and reepithelialization with reduced PECAM staining, but increased macrophage infiltration. After limb ischemia, PRCP(gt/gt) mice also had reduced reperfusion of the femoral artery and angiogenesis. On femoral artery wire injury, PRCP(gt/gt) mice had increased neointimal formation, CD45 staining, and Ki-67 expression. Alternatively, combined PRCP(gt/gt) and MRP-14(-/-) mice were protected from wire injury with less neointimal thickening, leukocyte infiltration, and cellular proliferation. PRCP regulates cell growth, angiogenesis, and the response to vascular injury. Combined with its known roles in blood pressure and thrombosis control, PRCP is positioned as a key regulator of vascular homeostasis.

Kruppel-like Factor 15 Regulates Smooth Muscle Response to Vascular Injury—Brief Report

Abstract—To determine the role of Kruppel-like factor (KLF) 15, a zinc finger transcriptional factor that is expressed in vascular smooth muscle cells (VSMCs) in vascular biology. VSMCs respond to mechanical injury via a tightly orchestrated series of gene regulatory events. KLF15 is broadly expressed in both arterial and venous vascular beds in a VSMC restricted fashion. KLF15 expression is markedly reduced by both pharmacological and mechanical stimuli. To examine the specific role of KLF15 in the vascular response to injury, we performed femoral artery wire injury in KLF15−/− and wild-type mice. KLF15−/− mice develop exaggerated neointimal growth, with evidence of increased SMC proliferation and migration within the neointima. In concordance, gain and loss of function studies in isolated VSMCs demonstrate that KLF15 can directly inhibit SMC proliferation and migration. To our knowledge, these data are the first to identify KLF15 as a novel inhibitor of VSMC proliferation and migration and to implicate this factor as a critical regulator of the vascular response to injury.

Kruppellike Factor 15 Regulates Smooth Muscle Response to Vascular Injury

Abstract—To determine the role of Kruppel-like factor (KLF) 15, a zinc finger transcriptional factor that is expressed in vascular smooth muscle cells (VSMCs) in vascular biology. VSMCs respond to mechanical injury via a tightly orchestrated series of gene regulatory events. KLF15 is broadly expressed in both arterial and venous vascular beds in a VSMC restricted fashion. KLF15 expression is markedly reduced by both pharmacological and mechanical stimuli. To examine the specific role of KLF15 in the vascular response to injury, we performed femoral artery wire injury in KLF15−/− and wild-type mice. KLF15−/− mice develop exaggerated neointimal growth, with evidence of increased SMC proliferation and migration within the neointima. In concordance, gain and loss of function studies in isolated VSMCs demonstrate that KLF15 can directly inhibit SMC proliferation and migration. To our knowledge, these data are the first to identify KLF15 as a novel inhibitor of VSMC proliferation and migration and to implicate this factor as a critical regulator of the vascular response to injury.

The Intrinsic Complement Regulator Decay-Accelerating Factor Modulates the Biological Response to Vascular Injury

Objective—To investigate whether the presence of decay-accelerating factor (or CD55), an intrinsic complement regulator, protects against the development of vascular disease, given that complement activation can affect leukocytes and platelets. Methods and Results—Leukocyte-platelet complexes are critical for the initiation and progression of atherosclerosis and restenosis; however, the mechanism by which these processes promote vascular injury is incompletely defined. We performed femoral artery wire injury in Daf1−/− mice and their wild-type controls. Leukocyte accumulation, cellular proliferation, and neointimal thickening were enhanced in Daf1−/− mice versus wild-type mice. Deficiency of either the C3a or the C5a receptor, respectively, reversed the increased vascular inflammation, cellular proliferation, and neointimal formation in Daf1−/− mice. Conclusion—Decay-accelerating factor control of C3a and C5a generation and prevention of the binding of these activation fragments to the C3a and C5a receptors are critical for the biological response to vascular injury. Targeting the C3a and C5a receptors may be useful for the prevention of neointimal hyperplasia.

Low-density lipoprotein subfractions and the prevalence of silent lacunar infarction in subjects with essential hypertension.

Recent lipid research has focused on low-density lipoprotein (LDL) subfractions as new markers for cardiovascular risk. However, the clinical significance of measurement of LDL subfractions in subjects with essential hypertension is yet to be established. We studied the association between the prevalence of silent lacunar infarction (SLI) and LDL subfractions in patients with essential hypertension. We performed brain MRI to detect SLI and measured LDL subfractions in 100 asymptomatic non-diabetic middle-aged subjects with essential hypertension (mean age, 62 years). We fractionated LDL into three parts, LDL-1, LDL-2, and LDL-3, with LDL-3 being the oxidized subfraction. Of the 100 study subjects, 24 (24%) had one or more SLIs, while the remaining 76 (76%) were considered as a non-SLI group. The LDL-3 levels were significantly higher in the SLI group than in the non-SLI group (8.3±4.4 mg/dl vs. 6.3±2.0 mg/dl, p=0.006). Multiple logistic regression analysis showed that LDL-3 levels alone were an independent predictor of SLI (odds ratio [OR]: 1.380; 95% confidence interval [CI]: 1.113–1.663; p=0.003). When subjects were divided into quartiles based on LDL-3 levels, the prevalence of SLI was significantly higher in the highest LDL-3 level group than in the lowest LDL-3 level group (p=0.0036). The present study suggests that LDL-3 levels are associated with the prevalence of SLI in subjects with essential hypertension.