Endothelial overexpression of LOX-1 increases plaque formation and promotes atherosclerosis in vivo

AIMS

Lectin-like oxLDL receptor-1 (LOX-1) mediates the uptake of oxidized low-density lipoprotein (oxLDL) in endothelial cells and macrophages. However, the different atherogenic potential of LOX-1-mediated endothelial and macrophage oxLDL uptake remains unclear. The present study was designed to investigate the in vivo role of endothelial LOX-1 in atherogenesis.

METHODS AND RESULTS

Endothelial-specific LOX-1 transgenic mice were generated using the Tie2 promoter (LOX-1TG). Oxidized low-density lipoprotein uptake was enhanced in cultured endothelial cells, but not in macrophages of LOX-1TG mice. Six-week-old male LOX-1TG and wild-type (WT) mice were fed a high-cholesterol diet (HCD) for 30 weeks. Increased reactive oxygen species production, impaired endothelial nitric oxide synthase activity and endothelial dysfunction were observed in LOX-1TG mice as compared with WT littermates. LOX-1 overexpression led to p38 phosphorylation, increased nuclear factor κB activity and subsequent up-regulation of vascular cell adhesion molecule-1, thereby favouring macrophage accumulation and aortic fatty streaks. Consistently, HCD-fed double-mutant LOX-1TG/ApoE(-/-) displayed oxidative stress and vascular inflammation with higher aortic plaques than ApoE(-/-) controls. Finally, bone marrow transplantation experiments showed that endothelial LOX-1 was sufficient for atherosclerosis development in vivo.

CONCLUSIONS

Endothelial-specific LOX-1 overexpression enhanced aortic oxLDL levels, thereby favouring endothelial dysfunction, vascular inflammation and plaque formation. Thus, LOX-1 may serve as a novel therapeutic target for atherosclerosis.

Atherosclerotic mice exhibit systemic inflammation in periadventitial and visceral adipose tissue, liver, and pancreatic islets

OBJECTIVE

Atherosclerosis is a chronic inflammatory disease of major conduit arteries. Similarly, obesity and type 2 diabetes mellitus are associated with accumulation of macrophages in visceral white adipose tissue and pancreatic islets. Our goal was to characterize systemic inflammation in atherosclerosis with hypercholesterolemia, but without obesity.

METHODS AND RESULTS

We compared 22-week-old apolipoprotein E knockout (ApoE(-/-)) with wild-type mice kept for 14 weeks on a high cholesterol (1.25%) diet (CD, n=8) and 8-week-old ApoE(-/-) with wild-type mice kept on a normal diet (ND, n=8). Hypercholesterolemic, atherosclerotic ApoE(-/-) mice on CD exhibited increased macrophages and T-cells in plaques and periadventitial adipose tissue that revealed elevated expression of MIP-1alpha, IL-1beta, IL-1 receptor, and IL-6. Mesenteric adipose tissue and pancreatic islets in ApoE(-/-) mice showed increased macrophages. Expression of IL-1beta was enhanced in mesenteric adipose tissue of ApoE(-/-) mice on CD. Furthermore, these mice exhibited steatohepatitis with macrophage and T-cell infiltrations as well as increased MIP-1alpha and IL-1 receptor expression. Blood glucose, insulin and total body weight did not differ between the groups.

CONCLUSIONS

In hypercholesterolemic lean ApoE(-/-) mice, inflammation extends beyond atherosclerotic plaques to the periadventitial and visceral adipose tissue, liver, and pancreatic islets without affecting glucose homeostasis.

PARP1 is required for adhesion molecule expression in atherogenesis

AIMS

Atherosclerosis is the leading cause of death in Western societies and a chronic inflammatory disease. However, the key mediators linking recruitment of inflammatory cells to atherogenesis remain poorly defined. Poly(ADP-ribose) polymerase 1 (PARP1) is a nuclear enzyme, which plays a role in acute inflammatory diseases.

METHODS AND RESULTS

In order to test the role of PARP in atherogenesis, we applied chronic pharmacological PARP inhibition or genetic PARP1 deletion in atherosclerosis-prone apolipoprotein E-deficient mice and measured plaque formation, adhesion molecules, and features of plaque vulnerability. After 12 weeks of high-cholesterol diet, plaque formation in male apolipoprotein E-deficient mice was decreased by chronic inhibition of enzymatic PARP activity or genetic deletion of PARP1 by 46 or 51%, respectively (P < 0.05, n >or= 9). PARP inhibition or PARP1 deletion reduced PARP activity and diminished expression of inducible nitric oxide synthase, vascular cell adhesion molecule-1, and P- and E-selectin. Furthermore, chronic PARP inhibition reduced plaque macrophage (CD68) and T-cell infiltration (CD3), increased fibrous cap thickness, and decreased necrotic core size and cell death (P < 0.05, n >or= 6).

CONCLUSION

Our data provide pharmacological and genetic evidence that endogenous PARP1 is required for atherogenesis in vivo by increasing adhesion molecules with endothelial activation, enhancing inflammation, and inducing features of plaque vulnerability. Thus, inhibition of PARP1 may represent a promising therapeutic target in atherosclerosis.

Human antibody against C domain of tenascin-C visualizes murine atherosclerotic plaques ex vivo

Targeting proteins that are overexpressed in atherosclerotic plaques may open novel diagnostic applications. The C domain of tenascin-C is absent from normal adult tissues but can be inserted during tumor progression or tissue repair into the molecule by alternative splicing. We tested the ability of the human antibody G11, specific to this antigen, to reveal murine atherosclerotic plaques ex vivo. The antibody directed against the extra domain B of fibronectin (L19) was used as a reference.

METHODS

We intravenously injected (125)I-labeled G11 or L19 antibodies into apolipoprotein E-deficient (ApoE(-/-)) mice and harvested the aortae 4 or 24 h later. En face analyses of distal aortae and longitudinal sections of the aortic arch were performed to compare antibody uptake using autoradiography with plaque staining using oil red O. Plaque macrophages were detected by immunohistochemistry (anti-CD68 staining). Biodistribution of injected antibodies was investigated in aortae and blood at 4 and 24 h.

RESULTS

En face analyses revealed a significant correlation between radiolabeled G11 and fat-stained areas, increasing from 4 to 24 h, with a correlation coefficient of 0.92 (P < 0.0001) and an average signal-to-noise ratio of 104:1 at 24 h. Plaque imaging using L19 showed similar results (r = 0.86; P < 0.0001; signal-to-noise ratio, 72:1 at 24 h). Uptake of radiolabeled antibodies in histologic sections colocalized with fat staining and activated macrophages in aortic plaques. Biodistribution analyses confirmed specific accumulation in aortic plaques as well as rapid blood pool clearance of the antibodies 24 h after injection. Immunofluorescence analyses revealed increased expression of tenascin and fibronectin isoforms in macrophage-rich plaques.

CONCLUSION

The antibody G11, specific to the C domain of tenascin-C, visualizes murine atherosclerotic plaques ex vivo. In conjunction with the increased expression of the C domain of tenascin-C in macrophage-rich plaques, the colocalization of G11 uptake with activated macrophages, and the favorable target-to-blood ratio at 24 h, this antibody may be useful for molecular imaging of advanced atherosclerotic plaques in the intact organism.

Increased Balloon-Induced Inflammation, Proliferation, and Neointima Formation in Apolipoprotein E (ApoE) Knockout Mice

BACKGROUND AND PURPOSE

The pathophysiology of vascular lesions after balloon angioplasty remains poorly understood. A major limitation of most experimental studies in this regard is that injury was assessed in healthy arteries. Our aim was to study the effects of hypercholesterolemia in a mouse vascular injury model that mimics human balloon angioplasty.

METHODS

Carotid balloon distension was performed in wild-type (WT) mice on a normal diet (ND), in apolipoprotein E-deficient (ApoE-/-) mice on ND and in ApoE-/- mice fed a high cholesterol diet (CD).

RESULTS

Medial cell death (TUNEL) was elevated in all mice at 1 hour and 1 day after angioplasty without differences between the groups. We found enhanced intimal inflammation (%CD45-positive cells) and vascular cell adhesion molecule-1 expression at 7 days (P < 0.05; n > or = 4) as well as increased proliferation rates (BrdU-index) in ApoE-/- CD at 7 and 28 days postinjury (P < 0.05; n > or = 5). Four weeks after injury, these events led to enhanced neointima in ApoE-/- CD compared with WT ND mice (intima/media, P < 0.001; n > or = 8). The amount of lesion formation paralleled the incremental increase in total plasma cholesterol in WT ND, ApoE-/- ND and ApoE-/- CD (P < 0.01).

CONCLUSIONS

Carotid balloon distension injury in ApoE-/- mice on CD induced enhanced inflammation and proliferation leading to increased neointima. Further applications of this microballoon catheter in genetically modified mice will provide opportunities to elucidate molecular mechanisms of vascular lesion formation in a model that reflects clinical balloon angioplasty. This know-how may pave the way to catheter-based interventions of human microvessels in the peripheral or cerebral circulation.

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.

18F-choline images murine atherosclerotic plaques ex vivo

OBJECTIVE

Current imaging modalities of atherosclerosis mainly visualize plaque morphology. Valuable insight into plaque biology was achieved by visualizing enhanced metabolism in plaque-derived macrophages using 18F-fluorodeoxyglucose (18F-FDG). Similarly, enhanced uptake of 18F-fluorocholine (18F-FCH) was associated with macrophages surrounding an abscess. As macrophages are important determinants of plaque vulnerability, we tested 18F-FCH for plaque imaging.

METHODS AND RESULTS

We injected 18F-FCH (n=5) or 18F-FDG (n=5) intravenously into atherosclerotic apolipoprotein E-deficient mice. En face measurements of aortae isolated 20 minutes after 18F-FCH injections demonstrated an excellent correlation between fat stainings and autoradiographies (r=0.842, P<0.0001), achieving a sensitivity of 84% to detect plaques by 18F-FCH. In contrast, radiotracer uptake 20 minutes after 18F-FDG injections correlated less with en face fat stainings (r=0.261, P<0.05), reaching a sensitivity of 64%. Histological analyses of cross-sections 20 minutes after coinjections of 18F-FCH and 14C-FDG (n=3) showed that 18F-FCH uptake correlated better with fat staining (r=0.740, P<0.0001) and macrophage-positive areas (r=0.740, P<0.0001) than 14C-FDG (fat: r=0.236, P=0.29 and CD68 staining: r=0.352, P=0.11), respectively.

CONCLUSIONS

18F-FCH identifies murine plaques better than 18F-FDG using ex vivo imaging. Enhanced 18F-FCH uptake into macrophages may render this tracer a promising candidate for imaging plaques in patients.

BNP as a marker of diastolic dysfunction in the general population: Importance of left ventricular hypertrophy

BNP is a marker of systolic left ventricular dysfunction (LVSD) and heart failure. To assess BNP for the detection of diastolic dysfunction in the general population, we examined 1678 subjects within an age- and sex-stratified survey (MONICA Augsburg). BNP was measured using a commercially available RIA (Shionogi). BNP increased in subjects with diastolic dysfunction (mean 20.3+/-4.7 pg/ml vs. control 9.6+/-0.5 pg/ml, p<0.001), but to a lesser extent than in subjects with LV hypertrophy (LVH, mean 37.3+/-49.1 pg/ml, p<0.001 vs. control) or LVSD (mean 76.2+/-23.2 pg/ml, p<0.001 vs. control). Individuals with sole diastolic abnormality displayed BNP concentrations at the control level (mean 9.7+/-1.7 pg/ml). In univariate analysis, age, BMI, systolic blood pressure, left atrial size, LV mass index, diastolic dysfunction and EF displayed a significant correlation with BNP (p<0.001). However, LV mass index displaced diastolic dysfunction as a significant predictor of BNP in multivariate analysis. Upon ROC analysis, sensitivity and specificity for the detection of diastolic dysfunction by BNP were only 61% and 55%, respectively. Nevertheless, a normal BNP test virtually excluded the presence of diastolic dysfunction and concomitant LVH (NPV 99.9%). Increased BNP concentrations in subjects with diastolic dysfunction are strongly related to LVH. Population-wide screening for diastolic dysfunction with BNP cannot be recommended although a normal BNP test usually excludes diastolic dysfunction and LV hypertrophy.

Optimization of radiation controlled gene expression by adenoviral vectors in vitro

The radiation-inducible EGR-1-promoter has been used in different gene therapy approaches in order to enhance and locally restrict therapeutic efficacy. The aim of this study was to reduce nonspecific gene expression in the absence of irradiation (IR) in an adenoviral vector. Rat rhabdomyosarcoma R1H tumor cells were infected with adenoviral vectors expressing either EGFP or HSV-TK under control of the murine EGR-1 promoter/enhancer. Cells were irradiated at 0-6 Gy. Gene expression was determined by FACS-analysis (EGFP), or crystal violet staining (HSV-TK). The bovine growth hormone polyadenylation signal (BGH pA) was used as insulating sequence and was introduced upstream or upstream and downstream of the expression cassette. Infected R1H cells displayed IR dose-dependent EGFP expression. Cells treated with IR, AdEGR.TK and ganciclovir displayed a survival of 17.3% (6 Gy). However, significant gene expression was observed in the absence of IR with EGR.TK and EGR.EGFP constructs. Introduction of BGHpA upstream or upstream and downstream of expression cassette resulted in decreased nonspecific cytotoxicity by a factor of 1.6-2.3 with minor influence on the induced level of cytotoxicity. Introduction of insulating sequences in adenoviral vectors might allow tighter temporospatial control of gene expression by the radiation-inducible EGR-1 promoter.