Vascular collagens: spotlight on the role of type VIII collagen in atherogenesis

The influence of extracellular matrix composition on the pathogenesis of coronary atherosclerosis

The modern concept of the development of atherosclerosis implies that the underlying pathogenesis of this disease is vascular remodeling as a response of the vessel wall to hypertension associated with hyperlipidemia and subsequent inflammation. However, even though this disease has been investigated for decades, both from a basic and clinical research aspect, there are still many doubts as to what the initial phase of the disease is. In contemporary literature there are an increasing number of papers that stress the importance of the extracellular matrix (ECM) of the blood vessels connective tissue, particularly proteoglycans, in the formation of early atherosclerotic lesions of human coronary arteries.

Extracellular matrix molecules: potential targets in pharmacotherapy

The extracellular matrix (ECM) consists of numerous macromolecules classified traditionally into collagens, elastin, and microfibrillar proteins, proteoglycans including hyaluronan, and noncollagenous glycoproteins. In addition to being necessary structural components, ECM molecules exhibit important functional roles in the control of key cellular events such as adhesion, migration, proliferation, differentiation, and survival. Any structural inherited or acquired defect and/or metabolic disturbance in the ECM may cause cellular and tissue alterations that can lead to the development or progression of disease. Consequently, ECM molecules are important targets for pharmacotherapy. Specific agents that prevent the excess accumulation of ECM molecules in the vascular system, liver, kidney, skin, and lung; alternatively, agents that inhibit the degradation of the ECM in degenerative diseases such as osteoarthritis would be clinically beneficial. Unfortunately, until recently, the ECM in drug discovery has been largely ignored. However, several of today's drugs that act on various primary targets affect the ECM as a byproduct of the drugs' actions, and this activity may in part be beneficial to the drugs' disease-modifying properties. In the future, agents and compounds targeting directly the ECM will significantly advance the treatment of various human diseases, even those for which efficient therapies are not yet available.

Collagens

The collagens represent a family of trimeric extracellular matrix molecules used by cells for structural integrity and other functions. The three alpha chains that form the triple helical part of the molecule are composed of repeating peptide triplets of glycine-X-Y. X and Y can be any amino acid but are often proline and hydroxyproline, respectively. Flanking the triple helical regions (i.e., Col domains) are non-glycine-X-Y regions, termed non-collagenous domains. These frequently contain recognizable peptide modules found in other matrix molecules. Proper tissue function depends on correctly assembled molecular aggregates being incorporated into the matrix. This review highlights some of the structural characteristics of collagen types I-XXVIII.

Proteomics of acute coronary syndromes

Acute coronary syndromes (ACS), such as unstable angina, acute myocardial infarction, and sudden cardiac death, are commonly associated with the presence of vulnerable plaques in coronary arteries. Rupture or erosion of vulnerable plaques results in the formation of luminal thrombi due to the physical contact between platelets and thrombogenic elements within the atherosclerotic lesions. Considering the socioeconomic burden of ACS, it is imperative that the scientific community achieves a clear understanding of the multifaceted pathophysiology of vulnerable atheroma to identify accurate prognostic biomarkers and therapeutic targets. The analytical power of modern proteomic technologies could facilitate our understanding of vulnerable plaques and lead to the discovery of novel therapeutic targets and diagnostic biomarkers.

Collagens in the progression and complications of atherosclerosis

Collagens constitute a major portion of the extracellular matrix in the atherosclerotic plaque, where they contribute to the strength and integrity of the fibrous cap, and also modulate cellular responses via specific receptors and signaling pathways. This review focuses on the diverse roles that collagens play in atherosclerosis; regulating the infiltration and differentiation of smooth muscle cells and macrophages; controlling matrix remodeling through feedback signaling to proteinases; and influencing the development of atherosclerotic complications such as plaque rupture, aneurysm formation and calcification. Expanding our understanding of the pathways involved in cell-matrix interactions will provide new therapeutic targets and strategies for the diagnosis and treatment of atherosclerosis.

siRNA-targeted COL8A1 inhibits proliferation, reduces invasion and enhances sensitivity to D-limonence treatment in hepatocarcinoma cells

The COL8A1 (collagen type VIII, alpha-1) gene, which encodes the alpha 1 chain of collagen, type VIII, may modulate migration, proliferation and adherence of various cells. Only very sparse information exists on COL8A1 expression in hepatocarcinoma. To investigate the possible role of COL8A1 in the mouse hepatocarcinoma cell line Hca-F with highly metastatic potential in the lymph nodes, we used an RNA interference (RNAi) approach to silence COL8A1 expression. The results showed that a small interfering RNA (siRNA) targeted against COL8A1 significantly impeded Hca-F cells proliferation and colony formation in soft agar. This reduction of COL8A1 expression also led to the decreased invasion of Hca-F cells dramatically in vitro. Furthermore, the downregulation of COL8A1 expression also sensitized cells to the action of D-limonene. These data together provide insights into the function of COL8A1 and suggest that COL8A1 might represent a new potential target for gene therapy in hepatocarcinoma.

Oxidized phospholipids induce type VIII collagen expression and vascular smooth muscle cell migration

Phenotypic switching of vascular smooth muscle cells (VSMCs) is known to play a critical role in the development of atherosclerosis. However, the factors present within lesions that mediate VSMC phenotypic switching are unclear. Oxidized phospholipids (OxPLs), including 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphorylcholine (POVPC), are active components of minimally modified low density lipoprotein and have been previously shown to induce multiple proatherogenic events in endothelial cells and macrophages, but their effects on VSMCs have been largely unexplored until recently. We previously showed that OxPLs induced phenotypic switching of VSMCs, including suppression of SMC differentiation marker genes. The goal of the present studies was to test the hypothesis that OxPLs alter extracellular matrix production and VSMC migration. Results showed that POVPC activated expression of several extracellular matrix proteins in VSMC. POVPC increased expression of type VIII collagen alpha1 chain (Col8a1) mRNA in cultured VSMCs and in vivo in rat carotid arteries by 9-fold and 4-fold, respectively. POVPC-induced activation of Col8a1 gene expression was reduced by small interfering RNA-mediated suppression of Krüppel-like factor 4 (Klf4) and Sp1, and was abolished in Klf4-knockout VSMCs. POVPC increased Klf4 binding to the Col8a1 gene promoter both in vivo in rat carotid arteries and in cultured VSMCs based on chromatin immunoprecipitation assays. Moreover, POVPC-induced VSMC migration was markedly reduced in Klf4- or type VIII collagen-knockout VSMCs. Given evidence that OxPLs are present within atherosclerotic lesions, it is interesting to suggest that OxPL-induced changes in VSMC phenotype may contribute to the pathogenesis of atherosclerosis at least in part through changes in extracellular matrix composition.

Role of NonO-histone interaction in TNFalpha-suppressed prolyl-4-hydroxylase alpha1

Inflammation is a key process in cardiovascular diseases. The extracellular matrix (ECM) of the vasculature is a major target of inflammatory cytokines, and TNFalpha regulates ECM metabolism by affecting collagen production. In this study, we have examined the pathways mediating TNFalpha-induced suppression of prolyl-4 hydroxylase alpha1 (P4Halpha1), the rate-limiting isoform of P4H responsible for procollagen hydroxylation, maturation, and organization. Using human aortic smooth muscle cells, we found that TNFalpha activated the MKK4-JNK1 pathway, which induced histone (H) 4 lysine 12 acetylation within the TNFalpha response element in the P4Halpha1 promoter. The acetylated-H4 then recruited a transcription factor, NonO, which, in turn, recruited HDACs and induced H3 lysine 9 deacetylation, thereby inhibiting transcription of the P4Halpha1 promoter. Furthermore, we found that TNFalpha oxidized DJ-1, which may be essential for the NonO-P4Halpha1 interaction because treatment with gene specific siRNA to knockout DJ-1 eliminated the TNFalpha-induced NonO-P4Halpha1 interaction and its suppression. Our findings may be relevant to aortic aneurysm and dissection and the stability of the fibrous cap of atherosclerotic plaque in which collagen metabolism is important in arterial remodeling. Defining this cytokine-mediated regulatory pathway may provide novel molecular targets for therapeutic intervention in preventing plaque rupture and acute coronary occlusion.

Evaluation of collagen in atherosclerotic plaques: the use of two coherent laser-based imaging methods

Acute coronary events such as myocardial infarction are frequently caused by the rupture of unstable atherosclerotic plaque. Collagen plays a key role in determining plaque stability. Methods to measure plaque collagen content are invaluable in detecting unstable atherosclerotic plaques. Recently, novel coherent laser-based imaging techniques, such as polarization-sensitive optical coherence tomography (PSOCT) and laser speckle imaging (LSI) have been investigated, and they provide a wealth of information related to collagen content and plaque stability. Additionally, given their potential for intravascular use, these technologies will be invaluable for improving our understanding of the natural history of plaque development and rupture and, hence, enable the detection of unstable plaques. In this article we review recent developments in these techniques and potential challenges in translating these methods into intra-arterial use in patients.

Analysis of structural changes in normal and aneurismal human aortic tissues using FTIR microscopy

Aortic aneurisms are frequently asymptomatic but can induce dramatic complications. The diagnosis is only based on the aortic diameter and not on a structural and compositional basis. In this preliminary study, we propose infrared microspectroscopy to nondestructively probe normal and aneurismal human aortas. Spectra from 19 human ascending aortic biopsies (10 normal and 9 aneurismal) were acquired using infrared microspectroscopy. A 1500 x 150 microm(2) area of each 7-microm thick cryosection was investigated using a 30-microm spatial resolution with a total of about 200 spectra per sample. Spectral differences between normal and aneurismal tissues were mainly located in spectral regions related to proteins, such as elastin and collagen, and proteoglycans (1750-1000 cm(-1)). Tissue heterogeneity and sample classification have been evaluated using hierarchical cluster analysis of individual or mean spectra and their second derivative. Using spectral range related to proteins, 100% of good classification was obtained whereas the proteoglycan spectral range was less discriminant. This in vitro study demonstrates the potential of such technique to differentiate between normal and aneurismal aortas using selected spectral ranges. Future investigations will be focused on these specific spectral regions to determine the role of elastin and collagen in the discrimination of normal and pathological aortas.

Collagen type VIII expression in human diabetic nephropathy

BACKGROUND

Collagen type VIII is a non-fibrillar short-chain collagen that may modulate migration, proliferation and adherence of various cells. Only very sparse information exists on collagen type VIII expression in human diabetic nephropathy.

MATERIAL AND METHODS

We retrospectively studied mRNA expression for the two collagen type VIII chains (COL8A1 and COL8A2) in 20 biopsies with histologically confirmed diabetic nephropathy by real-time PCR, and compared glomerular and tubular expression with normal kidney [pre-transplant biopsies (n = 10)]. Expression of collagen type VIII was also studied in biopsies from patients with benign nephrosclerosis (BNS; n = 16) and focal-segmental glomerulosclerosis (FSGS; n = 9).

RESULTS

A strong specific induction of COL8A1 mRNA was found in diabetic nephropathy in both glomerular and tubular compartments. There was also a robust induction of COL8A2 in diabetic nephropathy, but overall expression was lower than that of COL8A1 transcripts. No significant increase in COL8A1 and COL8A2 mRNAs expression was found in biopsies from patients with BNS and FSGS compared with normal kidneys. The cross-reactivity of the used anti-alpha1(VIII) antibody with human tissue was confirmed by Western blots. Immunohistological analysis revealed only little staining for collagen type VIII in the normal kidney, localized to vessels. There was an up-regulation of collagen type VIII protein expression as shown by immunohistochemistry in the diabetic nephropathy biopsies mainly localized to mesangial cells, tubules and the interstitium. Proteinuria and serum creatinine did not correlate with glomerular or tubular COL8A1 and COL8A2 mRNA expression in diabetic patients.

CONCLUSION

Our study systemically investigates collagen type VIII expression in human biopsies. Induction of collagen type VIII was specific for diabetic nephropathy and did not occur in the other renal diseases studied. More specific factors of the diabetic environment are likely involved in the stimulated expression because there was no correlation of collagen type VIII mRNA expression with proteinuria. Since collagen type VIII may influence proliferation and migration of cells, it is possible that an increase in renal expression of collagen type VIII initiates other pathophysiological processes (e.g. proliferation of renal fibroblasts) involved in diabetic nephropathy.

TNF-alpha suppresses prolyl-4-hydroxylase alpha1 expression via the ASK1-JNK-NonO pathway

BACKGROUND

Inflammation is known to contribute to the pathogenesis of vascular diseases in which arterial wall extracellular matrix (ECM) homeostasis is disrupted. Tumor necrosis factor-alpha (TNF-alpha), a pivotal cytokine that regulates ECM metabolism by increasing degradation and decreasing production of arterial collagens, is associated with vulnerable plaques and aortic aneurysms.

METHODS AND RESULTS

In the current study, we showed that, when administered in doses of 1 to 100 ng/mL, TNF-alpha dose-dependently downregulated the expression of prolyl-4-hydroxylase alphaI [P4H alpha(I)]-the rate-limiting subunit for the P4H enzyme essential for procollagen hydroxylation, secretion, and deposition in primary human aortic smooth muscle cells (HASMCs). Using a progressive deletion cloning approach, we characterized the TNF-alpha-responsive element (TaRE) in the human P4H alpha(I) promoter and found that a negative regulatory region at the position of -32 to +18 bp is responsible for approximately 80% of TNF-alpha-mediated suppression. Using oligonucleotide-based transcription factor pull-down method in which proteins were resolved in 1-D gel electrophoresis and identified using LC-MS/MS, we identified the NonO protein binds this region. When NonO expression silenced with specific siRNA, we found that 70% of the TNF-alpha-mediated P4H alpha suppression was abolished, which appeared to be mediated by the ASK1-JNK pathway.

CONCLUSIONS

Our findings define a novel molecular pathway for inflammation associated extracellular matrix dysregulation, which may account for atherosclerotic plaque rupture and aortic aneurysm formation. Further understanding of this pathway may facilitate development of novel therapeutics for vascular diseases.

Mechanism of gender-specific differences in aortic stiffness with aging in nonhuman primates

BACKGROUND

Our hypothesis was that the changes in vascular properties responsible for aortic stiffness with aging would be greater in old male monkeys than old female monkeys.

METHODS AND RESULTS

We analyzed the effects of gender differences in aging on in vivo measurements of aortic pressure and diameter and on extracellular matrix of the thoracic aorta in young adult (age, 6.6+/-0.5 years) versus old adult (age, 21.2+/-0.2 years) monkeys (Macaca fascicularis). Aortic stiffness, as represented by the pressure strain elastic modulus (Ep), increased more in old male monkeys (5.08+/-0.81; P<0.01) than in old females (3.06+/-0.52). In both genders, collagen density was maintained, collagen-bound glycation end products increased, and collagen type 1 decreased. However, elastin density decreased significantly (from 22+/-1.5% to 15+/-1.2%) with aging (P<0.05) only in males. Furthermore, only old males were characterized by a decrease (P<0.05) in collagen type 3 (an isoform that promotes elasticity) and an increase in collagen type 8 (an isoform that promotes the neointimal migration of vascular smooth muscle cells). In contrast to the data in monkeys, collagen types 1 and 3 both increased significantly in aging rats.

CONCLUSIONS

There are major species differences in the effects of aging on aortic collagen types 1 and 3. Furthermore, because alterations in collagen density, collagen content, hydroxyproline, and collagen advanced glycation end products were similar in both old male and female monkeys, these factors cannot be responsible for the greater increase in stiffness in old males. However, changes in collagen isoforms and the decrease in elastin observed only in old males likely account for the greater increase in aortic stiffness.

Imaging Collagen in Intact Viable Healthy and Atherosclerotic Arteries Using Fluorescently Labeled CNA35 and Two-Photon Laser Scanning Microscopy

We evaluated CNA35 as a collagen marker in healthy and atherosclerotic arteries of mice after both ex vivo and in vivo administration and as a molecular imaging agent for the detection of atherosclerosis. CNA35 conjugated with fluorescent Oregon Green 488 (CNA35/OG488) was administered ex vivo to mounted viable muscular (uterine), elastic (carotid), and atherosclerotic (carotid) arteries and fresh arterial rings. Two-photon microscopy was used for imaging. CNA35/OG488 labeling in healthy elastic arteries was compared with collagen type I, III, and IV antibody labeling in histologic sections. For in vivo labeling experiments, CNA35/OG488 was injected intravenously in C57BL6/J and apolipoprotein E−/− mice. Ex vivo CNA35/OG488 strongly labeled collagen in the tunica adventitia, media, and intima of muscular arteries. In healthy elastic arteries, tunica adventitia was strongly labeled, but labeling in tunica media and intima was prevented by endothelium and elastic laminae. Histology confirmed the affinity of CNA35 for type I, III, and IV collagen in arteries. Strong CNA35/OG488 labeling was found in atherosclerotic plaques. In vivo applied CNA35/OG488 minimally labeled the tunica intima of healthy carotid arteries. Atherosclerotic plaques in apolipoprotein E−/− mice exhibited large uptake. CNA35/OG488 imaging in organs revealed endothelium as a limiting barrier for in vivo uptake. CNA35/OG488 is a good molecular imaging agent for atherosclerosis.

C1q and its growing family

C1q is the target recognition protein of the classical complement pathway and a major connecting link between innate and acquired immunity. As a charge pattern recognition molecule of innate immunity, C1q can engage a broad range of self and non-self ligands via its heterotrimeric globular (gC1q) domain and thus trigger the classical pathway. The trimeric gC1q signature domain has been identified in a variety of non-complement proteins that can be grouped together as a C1q family. The X-ray crystal structures of the gC1q domain of a few members of the C1q family reveal a compact jelly-roll beta-sandwich fold similar to that of the multifunctional tumor necrosis factor (TNF) ligand family, hence the C1q and TNF superfamily. This review is an update on the structural and functional aspects of the gC1q domain of human C1q. We also mention the diverse range of proteins that utilize a gC1q domain in order to reflect on its importance as a versatile scaffold to support a variety of functions.

A tissue-specific variant of the human lysyl oxidase-like protein 3 (LOXL3) functions as an amine oxidase with substrate specificity

The human lysyl oxidase-like 3 (LOXL3) encodes a member of the emerging family of lysyl oxidase (LOX) that functions as a copper-dependent amine oxidase. The LOXL3 protein contains four scavenger receptor cysteine-rich domains in the N terminus in addition to the C-terminal characteristic domains of the LOX family, such as a copper binding domain, a cytokine receptor-like domain and residues for the lysyl-tyrosyl quinone cofactor. Using BLASTN searches, we identified a LOXL3 variant LOXL3-sv1 that lacked the sequences corresponding to exons 1, 2, 3, and 5 of LOXL3. LOXL3-sv1 showed an exon-intron structure distinct from LOXL3, additionally containing an 80-bp sequence corresponding to intron 3 of LOXL3 in the 5'-UTR and a 561-bp sequence corresponding to the 3'-flanking genomic region of exon 14 in the 3'-UTR. LOXL3-sv1 was predicted to encode a polypeptide of 392 amino acids that contains the C-terminal domains required for amine oxidase activity but lacks the N-terminal SRCR domains 1, 2, and 3. The recombinant LOXL3-sv1 protein showed a beta-aminopropionitrile-inhibitable amine oxidase activity toward elastin and collagen with substrate specificity. In RT-PCR assays with various human tissues, LOXL3-sv1 and LOXL3 showed distinct expression patterns. Further, luciferase reporter assays revealed a strong promoter element in intron 3 that probably functions as a regulatory region for the expression of LOXL3-sv1. These findings strongly indicate that LOXL3 encodes two variants, LOXL3 and LOXL3-sv1, both of which function as amine oxidases with distinct tissue and substrate specificities from one another.

Hyperelastic modelling of arterial layers with distributed collagen fibre orientations

Constitutive relations are fundamental to the solution of problems in continuum mechanics, and are required in the study of, for example, mechanically dominated clinical interventions involving soft biological tissues. Structural continuum constitutive models of arterial layers integrate information about the tissue morphology and therefore allow investigation of the interrelation between structure and function in response to mechanical loading. Collagen fibres are key ingredients in the structure of arteries. In the media (the middle layer of the artery wall) they are arranged in two helically distributed families with a small pitch and very little dispersion in their orientation (i.e. they are aligned quite close to the circumferential direction). By contrast, in the adventitial and intimal layers, the orientation of the collagen fibres is dispersed, as shown by polarized light microscopy of stained arterial tissue. As a result, continuum models that do not account for the dispersion are not able to capture accurately the stress-strain response of these layers. The purpose of this paper, therefore, is to develop a structural continuum framework that is able to represent the dispersion of the collagen fibre orientation. This then allows the development of a new hyperelastic free-energy function that is particularly suited for representing the anisotropic elastic properties of adventitial and intimal layers of arterial walls, and is a generalization of the fibre-reinforced structural model introduced by Holzapfel & Gasser (Holzapfel & Gasser 2001 Comput. Meth. Appl. Mech. Eng. 190, 4379-4403) and Holzapfel et al. (Holzapfel et al. 2000 J. Elast. 61, 1-48). The model incorporates an additional scalar structure parameter that characterizes the dispersed collagen orientation. An efficient finite element implementation of the model is then presented and numerical examples show that the dispersion of the orientation of collagen fibres in the adventitia of human iliac arteries has a significant effect on their mechanical response.

Endovascular Cryotherapy Accentuates the Accumulation of the Fibrillar Collagen Types I and III After Percutaneous Transluminal Angioplasty in Pigs

PURPOSE

To investigate the effect of endovascular cryotherapy (Cryo) on the density of collagen types I (CI) and III (CIII), which are involved in the dynamic modulation of extracellular matrix (ECM) after percutaneous transluminal angioplasty (PTA).

METHODS

Twenty-one juvenile farm swine and 10 miniswine underwent PTA of the femoral arteries with and without Cryo (-50 degrees C for 2 minutes). Quantitative angiography, histomorphometry, and quantification of CI and CIII were performed at 1 week (n=7), 1 month (n=7), 3 months (n=7), and 6 months (n=10).

RESULTS

PTA decreased the minimal luminal diameter (MLD) (range 3.48+/-0.18 to 4.2+/-0.39 mm) compared to baseline values (range 3.67+/-0.15 to 4.59+/-0.23 mm), but the application of Cryo maintained the MLDs at preprocedural levels (range 3.88+/-0.31 to 4.58+/-0.21 mm). At the time of sacrifice, the MLDs were similar in PTA and Cryo-treated arteries, but the external elastic lamina was significantly greater after Cryo application (range 10.17+/-0.54 to 14.34+/-0.76 mm2) than after PTA (range 8.69+/-0.70 to 11.77+/-0.73 mm2, p<0.05). Cryo did not alter the luminal area or prevent neointimal growth. A time-dependent increase of both CI and CIII was observed as early as 1 week after PTA, peaking at 3 months, and declining thereafter. Cryo accentuated this increase at all time points.

CONCLUSION

The application of Cryo accentuates the accumulation of CI and CIII in PTA-treated femoral arteries. This effect may be of clinical relevance in the stabilization of peripheral atherosclerotic plaque.

Migration and growth are attenuated in vascular smooth muscle cells with type VIII collagen-null alleles

OBJECTIVE

Type VIII collagen is upregulated after vascular injury and in atherosclerosis. However, the role of type VIII collagen endogenously expressed by smooth muscle cells (SMCs) and in the context of the vascular matrix microenvironment, which is rich in type I collagen, is not known. To address this, we have compared aortic SMCs from wild-type (WT) mice to SMCs from type VIII collagen-deficient (KO) mice when plated on type I collagen.

METHODS AND RESULTS

Type VIII collagen was upregulated after wounding of WT SMCs. KO SMCs exhibited greater adhesion to type I collagen than WT SMCs (optical density [OD595]=0.458+/-0.044 versus 0.193+/-0.071). By contrast, the WT SMCs spread more (389+/-75% versus 108+/-14% increase in cell area), migrated further (total distance 80.6+/-6.2 microm versus 64.2+/-4.4 microm), and exhibited increased [3H]-thymidine uptake (160,000+/-22,300 versus 63,100+/-12,100 counts per minute) when compared with KO SMCs. Gelatin zymograms showed that WT SMCs expressed latent matrix metalloproteinase 2, whereas KO SMCs did not. Addition of exogenous type VIII collagen returned levels of KO SMC adhesion (OD595=0.316+/-0.038), migration (79.5+/-5.8 microm), and latent matrix metalloproteinase 2 expression to levels comparable to WT SMCs.

CONCLUSIONS

This study suggests that SMCs can modify the matrix microenvironment by producing type VIII collagen, using it to overlay type I collagen, and generating a substrate favorable for migration.

Connexin43 deficiency causes dysregulation of coronary vasculogenesis

The connexin43 knockout (Cx43alpha1 KO) mouse dies at birth from outflow obstruction associated with infundibular pouches. To elucidate the origin of the infundibular pouches, we used microarray analysis to investigate gene expression changes in the pouch tissue. We found elevated expression of many genes encoding markers for vascular smooth muscle (VSM), endothelial cells, and fibroblasts, cell types that are epicardially derived and essential for coronary vasculogenesis. This was accompanied by increased expression of VEGF and genes in the TGFbeta and VEGF/Notch/Eph cell-signaling pathways known to regulate vasculogenesis/angiogenesis. Using immunohistochemistry and a VSM lacZ reporter gene, we confirmed an abundance of ectopic VSM and endothelial cells in the infundibular pouch and in some regions of the right ventricle forming secondary pouches. This was associated with distinct thinning of the compact myocardium. TUNEL labeling showed increased apoptosis in the pouch tissue, in agreement with the finding of altered expression of many apoptotic genes. Defects in vascular remodeling were indicated by a marked reduction in the branching complexity of the distal coronary arteries. In the near term KO mouse, we also observed a profusion of large coronary vascular plexuses subepicardially. This was associated with elevated epicardial expression of VEGF and abnormal epicardial cell morphology. Together, these observations indicate that dysregulated coronary vasculogenesis plays a pivotal role in formation of the infundibular pouches and suggests an essential role for Cx43alpha1 gap junctions in coronary vasculogenesis and vascular remodeling.

Prolonged culture of endothelial cells and deposition of basement membrane modify the recruitment of neutrophils

We tested whether endothelial cell conditioning during prolonged culture and deposition of basement membrane (BM) could modify neutrophil recruitment induced by the inflammatory cytokine, tumour necrosis factor-alpha (TNF). Confluent endothelial cells (EC) from human umbilical veins were cultured for 1 to 20 days and then stimulated with 1, 10 or 100 U/ml of TNF for 4 h. When isolated neutrophils were settled on EC stimulated with the lower doses of TNF, the levels of adhesion and the proportion of adherent cells that transmigrated increased markedly with time of culture. At 100 U/ml TNF, time of culture had little effect on recruitment, but the transmigrated neutrophils moved more slowly under the monolayer in longer-term cultures. The inhibitory effects of function-blocking antibodies against E-selectin and beta2-integrin, and studies in which neutrophils were perfused over short- or long-term cultures, suggested that increased adhesion and migration arose from increased efficiency of neutrophil activation by the EC. Prolonged culture was also associated with deposition of a distinct BM. When fresh EC were seeded on day 20 BM, transmigrated neutrophils moved more slowly under the EC than under control monolayers. Thus, EC change their pro-inflammatory phenotype during prolonged culture, and the deposited basement membrane influences neutrophil migration.

Regulation of collagen type I in vascular smooth muscle cells by competition between Nkx2.5 and deltaEF1/ZEB1

A major component of the vessel wall of large arteries and veins is the extracellular matrix (ECM), which consists of collagens, elastin, and proteoglycans. Collagen type I is one of the most abundant of the ECM proteins. We have previously shown that the pro-collagen type I alpha 2 gene contains an enhancer which confers tissue-specific expression in the majority of collagen-producing cells, including blood vessels. In this paper, we delineate a specific vascular smooth muscle cell (vSMC) element: a 100-bp sequence around -16.6 kb upstream of the transcription start site that regulates collagen expression exclusively in vSMCs. Furthermore, we show that the expression is activated through the binding of the homeodomain protein Nkx2.5, which is further potentiated in the presence of GATA6. In contrast, this element was repressed by the binding of the zinc-finger protein deltaEF1/ZEB1. We propose a model of regulation where the activating transcription factor Nkx2.5 and the repressor deltaEF1/ZEB1 compete for an overlapping DNA binding site. This element is important in understanding the molecular mechanisms of vessel remodeling and is a potential target for intervention in vascular diseases where there is excessive deposition of collagen in the vessel wall.

Atherosclerosis and extracellular matrix

Atherosclerosis is primarily a lesion that progresses due to a series of reactions that are induced by repair of injured intima. The intercellular networking that occurs among smooth muscle cells, macrophages, T lymphocytes and endothelial cells leads to a fibroproliferative response, in which the extracellular matrix (ECM) plays an important role. The ECM, composed of a mixture of vastly different macromolecules including collagen, elastin, glycoproteins and proteoglycans, confers tensile strength and viscoelasticity to the arterial wall. Each component of the ECM possesses unique structural properties that determine its own roles during the development of atherosclerotic plaques. Not only does the ECM provide the structural integrity of the plaques, but it also participates in several key events such as cell migration and proliferation, lipoprotein retention and thrombosis. The various matrix metalloproteinases (MMPs), major enzymes in ECM degradation, and their inhibitors (tissue inhibitors of MMPs) are demonstrated in plaque. An excess of MMPs over inhibitors contributes significantly to ECM destruction rendering the plaque more prone to rupture. Accumulating information on the molecular regulation of ECM synthesis and degradation will help investigators attain a more thorough understanding of the mechanisms of plaque formation and plaque instability and rupture.

Different expression of MMPs/TIMP-1 in human atherosclerotic lesions. Relation to plaque features and vascular bed

BACKGROUND

Proteolytic imbalance might determine arterial remodeling and plaque destabilization in atherosclerotic vessels. The aim of this study was to examine differences in the patterns of metalloproteinases (MMPs) and MMP inhibitor (TIMP-1) expression in advanced human atheromas, both in relation to the plaque features and the vascular bed involved.

METHODS AND RESULTS

Immunohistochemistry for MMP-1, -3, -9 and TIMP-1 as well as the collagen content were measured in vascular sections from patients undergoing peripheral revascularization (carotid n=11, femoral n=23) and aorto-coronary bypass surgery (mammary arteries n=20, as controls). Increased expression of all MMPs was detected in atherosclerotic as compared with control sections (P<0.01). Aneurysmal plaques showed a significant increase of MMP-1 and-3 and a reduction in total collagen (P<0.05) in relation to occlusive lesions. Calcification areas in atherosclerotic plaques were consistently associated with increased TIMP-1 expression (P<0.01). Finally, MMP-9 expression was higher in occlusive lesions from carotid than femoral arteries (P<0.01).

CONCLUSIONS

Aneurysm lesions expressed higher MMP-1 and-3 expression than occlusive plaques, and MMP-9 was mainly detected in carotid as compared with femoral arteries. TIMP-1 was associated with arterial calcification. These differences in the MMPs/TIMP-1 expression might determine the evolution of advanced atherosclerotic plaques and contribute to its vulnerability.

LRP: role in vascular wall integrity and protection from atherosclerosis

Vascular smooth muscle cell (SMC) proliferation and migration are important events in the development of atherosclerosis. The low-density lipoprotein receptor-related protein (LRP1) mediates suppression of SMC migration induced by platelet-derived growth factor (PDGF). Here we show that LRP1 forms a complex with the PDGF receptor (PDGFR). Inactivation of LRP1 in vascular SMCs of mice causes PDGFR overexpression and abnormal activation of PDGFR signaling, resulting in disruption of the elastic layer, SMC proliferation, aneurysm formation, and marked susceptibility to cholesterol-induced atherosclerosis. The development of these abnormalities was reduced by treatment with Gleevec, an inhibitor of PDGF signaling. Thus, LRP1 has a pivotal role in protecting vascular wall integrity and preventing atherosclerosis by controlling PDGFR activation.