Kinetics of adventitial repair in the rat carotid model

BACKGROUND

Discrepancies between success in experimental animals with a variety of pharmacologic strategies and failure with such agents in clinical trials have raised questions concerning the mechanism of restenosis. Recent observations suggest a potential implication for the adventitial (Adv) layer in neointimal formation.

METHODS

The purpose of this study was to examine the Adv changes in the rat carotid artery subjected to balloon injury. These changes were characterized by morphometric, immunohistochemical, and electron microscopy analyses, with special attention devoted to early time-points post-injury.

RESULTS

We report that the most important adventitial changes occurred in the first 48 h post-injury. Within 2 h there was extensive cell-loss by apoptosis and oncosis in the Adv and in the media; this was followed by the rapid onset of proliferation and a parallel slow increase in Adv thickening, reaching a maximum at 7 days. We further demonstrate an early migration of these Adv cells to the media and neointima. Moreover, we characterize the Adv cell phenotype with a panel of antibodies. Within 48 h after injury, a population of Adv cells expressed alpha-actin and vinculin with a maximum expression 7 days post-injury. At that time, these Adv cells started to express smooth muscle myosin heavy chain, a specific marker of smooth muscle cells. In parallel, we report an impaired production of elastic fibres in the Adv and medial layer.

CONCLUSIONS

We reported a detailed time-course of adventitial changes after rat carotid injury (cell death, proliferation, migration and differentiation) that supports an important role of adventitia in neointima formation.

Expression pattern of mouse sFRP-1 and mWnt-8 gene during heart morphogenesis

The Wnt genes encode a large family of secreted proteins that play a key role in embryonic development and tissue differentiation in many species (Rijsewijk et al., 1987; Nusse and Varmus, 1992). Genetic and biochemical studies have suggested that the frizzled proteins are cell surface receptors for Wnts (Vinson et al., 1989; Chan et al. , 1992; Bhanot et al., 1996; Wang et al., 1996). In parallel, a number of secreted frizzled-like proteins with a conserved N-terminal frizzled motif have been identified (Finch et al., 1997; Melkonyan et al., 1997; Rattner et al., 1997). One of these proteins, FrzA, the bovine counterpart of the murine sFRP-1 (93% identity) is involved in vascular cell growth control, binds Wg in vitro and antagonizes Xwnt-8 and hWnt-2 signaling in Xenopus embryos (Xu et al. , 1998; Duplàa et al., 1999). In this study, we report that sFRP-1 is expressed in the heart and in the visceral yolk sac during mouse development, and that sFRP-1 and mWnt-8 display overlapping expression patterns during heart morphogenesis. From 8.5 to 12.5 d.p. c., sFRP-1 is expressed in cardiomyocytes together with mWnt-8 but neither in the pericardium nor in the endocardium; at 17.5 d.p.c., they are no longer present in the heart. In mouse adult tissues, while sFRP-1 is highly detected in the aortic endothelium and media and in cardiomyocytes, mWnt-8 is not detected in these areas. Immunoprecipitation experiments demonstrates that FrzA binds to mWnt-8 in cell culture experiments.

Identification and cloning of a secreted protein related to the cysteine-rich domain of frizzled. Evidence for a role in endothelial cell growth control

We report the isolation of a cDNA, FrzA (frizzled in aorta; GenBank accession No. U85945), from bovine aortic endothelium. It is the bovine counterpart of the mouse sFRP1, which encodes for a secreted protein that is homologous to the cysteine-rich domain of frizzled. Members of the frizzled family of genes have been shown to be required for tissue polarity and to act as receptors for Wnt. The predicted protein product of this gene includes the cysteine-rich extracellular domain, but not the 7 putative transmembrane domains that are highly conserved among members of the frizzled family. Visualization of FrzA mRNA and protein revealed that it was widely distributed among adult tissues. FrzA is expressed by highly differentiated or polarized cells, eg, neurons, cardiocytes, or various epithelia. Analysis of its expression in endothelium revealed that FrzA mRNA levels were high in endothelial cells scraped from freshly obtained bovine aortas, decreased when cells were placed in culture and began to proliferate, but increased at confluence. Transient transfection assays and an assay using addition of purified protein indicate that FrzA reduces the proliferation of endothelial cells. These data demonstrate the existence of a secreted protein homologous to the extracellular domain of the fz receptor, which we speculate plays a role in controlling cell growth and differentiation, possibly by regulating accessibility to Wnt family members.

The integrin very late antigen-4 is expressed in human smooth muscle cell. Involvement of alpha 4 and vascular cell adhesion molecule-1 during smooth muscle cell differentiation

Vascular cell adhesion molecule-1 (VCAM-1) and its counterreceptor, the integrin very late antigen-4 (VLA-4), have recently been identified in smooth muscle cells during intimal thickening in humans and in newly forming vessels during ontogeny in mice, respectively. We examined the coexpression of VCAM-1 and the alpha 4 integrin subunit in human smooth muscle cells. The expression of VCAM-1 and alpha 4 subunit were studied during development of the aorta. In the 10-week-old human fetal aorta, VCAM-1 and alpha 4 were strongly expressed in smooth muscle cells. Their expression was dramatically reduced within the 24th week of gestation and disappeared in the adult aortic media. However, smooth muscle cells from intimal atherosclerotic thickening of adult aorta reexpressed both VCAM-1 and alpha 4. In a culture model mimicking smooth muscle differentiation, VCAM-1 mRNA and protein and alpha 4 integrin protein were coexpressed with smooth muscle-specific variants of cytoskeletal and contractile proteins, smooth muscle myosin heavy chain, caldesmon heavy chain, and desmin. Treatment with antibodies against VCAM-1 or alpha 4 integrin subunit interfered with the mRNA induction of smooth muscle-specific markers of differentiation. These results in vitro, associated with the transitory expression of VCAM-1 and VLA-4 during vascular ontogeny and the atherosclerosis process, point to a possible role of VCAM-1 and VLA-4 in the induction of smooth muscle differentiation.

Monocyte/macrophage recruitment and expression of endothelial adhesion proteins in human atherosclerotic lesions

Since mononuclear cells are recruited in atherosclerotic lesions, the expression of adhesion proteins by the arterial endothelium may play a major role in atherogenesis. The relationships between ICAM-1, E-selectin, and VCAM-1 expression on the arterial endothelium and the presence and degree of maturation of intimal macrophages in human atherosclerotic lesions was investigated. By quantitative double immunostaining with a pan-macrophage-specific monoclonal antibody, HAM-56, and a recently developed monoclonal antibody that is specific for mature macrophages, 3MA-B38, arterial sections were classified as (I) normal, (II) thickened without macrophage infiltration, (III) atherosclerotic with recent macrophage infiltration or (IV) atherosclerotic with infiltration of mature differentiated macrophages. A marked increase in the expression of ICAM-1, E-selectin, and VCAM-1 was observed on endothelial cells adjacent to recently recruited macrophages. Endothelial cells overlying differentiated macrophages exhibited a lower but significant increase in VCAM-1 expression, with no difference in ICAM-1 and E-selectin expression with respect to that observed in endothelium of normal arteries. These findings indicate that the endothelium covering the human arterial wall exhibits different states of activation as reflected by the expression of adhesion proteins, and that intimal monocyte/macrophage recruitment appears to depend on the level of expression of adhesion proteins.

Progression of coronary artery disease in non-dilated sites in the months following balloon angioplasty: time-dependent relation with restenosis

There is scant information on the progression of coronary artery disease in non-dilated sites in the months following percutaneous transluminal coronary angioplasty (PTCA) or on its relationship with restenosis. To assess the incidence of this progression and its relationship with restenosis at various times after PTCA, the authors selected 371 consecutive patients who had undergone a first successful PTCA for angina on native coronaries followed by a repeat angiographic study. The angiograms were analysed by a computer-assisted method; progression was defined as a 20% decrease in diameter and restenosis as a 30% decrease in diameter or a return to > 50% stenosis. The relationship between progression and restenosis was analysed in the whole population and then, using the Mantel-Haenszel chi-square test, in two subgroups: patients with a stable clinical state, who were restudied routinely and those whose worsened state had prompted repeat angiography. The relationship was assessed at different times between angioplasty and the repeat angiography. Progression was observed in 80 patients (22%) and restenosis in 155 patients (42%). There was a highly significant relationship between progression and restenosis in the total population (chi 2 = 26.4, odds ratio = 3.9 and P < 0.0003) and in the group of patients that were routinely restudied (chi 2 = 31.6, odds ratio = 5.3 and P < 0.0001), but not in the group of patients in whom restudy was performed because of clinical worsening (chi 2 = 0.13, odds ratio = 1.5 and P = NS). With respect to the length of follow-up, in the total population the relationship was significant only at 6 and 7 months (P < 0.0001), and in the group receiving a routine restudy only at 4-5 and 6-7 months (P < 0.001). Progression in non-dilated sites appeared to be strongly and transiently linked with restenosis, suggesting that PTCA may enhance both restenosis and progression over a short period.

Induction of intercellular adhesion molecule-1 by monocyte adhesion to endothelial cells in human culture system

Increased monocyte adhesion to the endothelial lining of blood vessels by cytokine-inducible adhesion proteins is a crucial event in inflammatory processes. Moreover, adherence is known to induce cytokine gene expression, suggesting a possible positive feedback mechanism. Therefore, we determined whether monocyte adhesion to endothelial cells (ECs) amplifies their adhesion by inducing intercellular adhesion molecule 1 (ICAM-1), and whether such positive feedback mechanism could be mediated by secretion of interleukin-1 (IL-1). Using monocyte-EC couples obtained after monocyte adhesion to ECs, and methods of quantitative polymerase chain reaction and immunofluorescence flow cytometry, we showed a biphasic increase of ICAM-1 mRNA content (2 and 16 hours) and a time-dependent increase of cell surface expression of ICAM-1, mainly on ECs, and couple adhesiveness, after monocyte adhesion to ECs. Anti-ICAM-1 monoclonal antibody inhibited 63% of the enhancement of adhesiveness induced on monocyte-EC couples by previous monocyte adhesion, suggesting that monocyte adhesion to ECs induces an increase of couple adhesiveness which is partially dependent on the ICAM-1 pathway. The early ICAM-1 mRNA induction was associated with a fast induction of IL-1 beta mRNA and a 7.7-fold increase in IL-1 beta protein in supernatant. However, 30% of this 2-hour ICAM-1 mRNA peak was abolished by recombinant soluble human IL-1 receptor, suggesting that the early ICAM-1 over-expression was partially mediated by IL-1 beta, and could be induced directly by adherence. The second ICAM-1 mRNA peak was accompanied by a marked increase in IL-1 beta mRNA and protein secretion (2.6 ng/ml). The binding to ICAM-1 did not appear to directly stimulate IL-1 beta synthesis. These results indicate that monocyte adhesion to endothelial cells appears to stimulate their own recruitment via induction of ICAM-1 thereby constituting a self-perpetuating positive feedback system.

Regulation of vascular cell adhesion molecule-1 and intercellular adhesion molecule-1 in human vascular smooth muscle cells

Vascular cell adhesion molecule-1 (VCAM-1), intercellular adhesion molecule-1 (ICAM-1), and E-selectin are inducible proteins involved in cell-cell adhesion. Immunohistochemical studies have indicated that human atherosclerotic plaques contain smooth muscle cells (SMCs) that express ICAM-1 and VCAM-1. Recently, we demonstrated that SMCs in culture express a functionally active cytokine-inducible ICAM-1. SMCs and mononuclear cells participate in the local accumulation of cytokines and related growth factors in atherosclerotic lesions. Therefore, we determined the effects of different cytokines and growth factors on mRNA content and cell surface expression of VCAM-1, ICAM-1, and E-selectin in cultured human aortic SMCs by Northern blotting, quantitative polymerase chain reaction amplification, and immunofluorescence flow cytometry. Under basal conditions of cultivation, both VCAM-1 mRNA and membrane expression of VCAM-1 were low and were induced very little by interleukin-1 beta (100 U/mL). Platelet-derived growth factor or transforming growth factor-beta decreased VCAM-1 mRNA basal expression. Treatment of SMCs with tumor necrosis factor-alpha (TNF-alpha) led to an increase in both VCAM-1 mRNA and cell surface expression for VCAM-1 in a dose- and time-dependent manner. Interferon-gamma induced a weak increase in VCAM-1 mRNA expression, with no synergistic effect on the stimulation by TNF-alpha. Various differences were noted between the expression of ICAM-1 and VCAM-1 genes, because interleukin-1 beta induced substantial amounts of ICAM-1 but not VCAM-1. The addition of interferon-gamma delays the time at which peak expression of ICAM-1 in response to TNF-alpha stimulation occurs. Under our conditions, we did not detect any expression of E-selectin by SMCs. These results suggest that cytokines regulate VCAM-1 and ICAM-1 expression on arterial SMCs and could play an important role in the pathophysiology of inflammatory and immune processes in atherosclerosis.

Monocyte adherence to endothelial cells in patients with atherosclerosis: relationships with risk factors

Monocyte adhesion to endothelium appearing determinant in atherosclerosis, the possibility that circulating monocytes have an increase of their adherence on endothelial cells were investigated in patients with atherosclerosis. The adherence of circulating monocytes on endothelial cell monolayers was determined in 26 patients with atherosclerosis (age 59 +/- 4), and 25 healthy individuals (age 55 +/- 4). No difference of monocyte adherence was observed between the two groups (18.8 +/- 13.8% vs 19.2 +/- 13.4%), or following atherosclerosis severity. However, monocyte adherence appeared positively correlated to smoking habits (r = 0.34, P < 0.02) and fibrinogen level (r = 0.31, P < 0.03), and negatively to the degree of plasmatic LDL oxidation (r = -0.28, P < 0.05). These results suggest that the adherence of monocytes to endothelial cells is not increased in atherosclerosis, but enhanced by risk factors. A weak plasmatic LDL oxidation could inhibit monocyte adhesion.

Quantitative analysis of polymerase chain reaction products using biotinylated dUTP incorporation

A method for relative quantitation of specific mRNA species by polymerase chain reaction (PCR) has been developed by using the incorporation of biotinylated dUTP. Transferred biotinylated PCR products gave a sensitive colorimetric signal which could be quantitated by video analysis. In the exponential phase of amplification, the linearity and reproducibility of reverse transcription and PCR demonstrated the same efficiency of cDNA synthesis and PCR for the two target genes, ICAM-1 and beta-actin, and allowed the normalization of ICAM-1 expression. These results suggested that in the exponential phase of amplification a relative quantitation of mRNA could be determined. We used this approach to analyze the different effects of TNF-alpha, IL-1 beta, and purified porcine platelet-derived growth factor stimulations on ICAM-1 expression in smooth muscle cells.

Tumor necrosis factor-alpha stimulates ICAM-1 expression in human vascular smooth muscle cells

Human atherosclerotic plaques contain numerous smooth muscle cells (SMCs) that express intercellular adhesion molecule-1 (ICAM-1). Expression of ICAM-1 in different cells is known to be regulated by tumor necrosis factor-alpha (TNF-alpha), which has recently been found to be present in the intimal thickening of human arteries. Therefore, we studied the effect of TNF-alpha on ICAM-1 mRNA content and surface expression in cultured human aortic SMCs by using the methods of Northern blotting and immunofluorescence flow cytometry. Under basal conditions of cultivation, ICAM-1 mRNA was not revealed in SMCs. However, treatment of the cells with recombinant human TNF-alpha induced substantial levels of ICAM-1 mRNA. The content of ICAM-1 on the surface of SMCs also increased in a dose- and time-dependent manner after incubation with TNF-alpha. Twenty-four hours of treatment with 10 ng/mL TNF-alpha led to an approximately 10-fold increase in ICAM-1 surface expression in the SMCs. Under the same conditions, pretreatment of SMCs with TNF-alpha resulted in a twofold increase of their adhesiveness for monocytes. In the presence of anti-ICAM-1 monoclonal antibody 10F3, monocyte adhesion to TNF-alpha-pretreated SMCs was significantly inhibited, suggesting that the observed monocyte-SMC interaction involved the ICAM-1 expressed on SMC surfaces as a result of TNF-alpha stimulation. These results led us to propose that TNF-alpha may act a regulator of functional ICAM-1 expression on the SMC surface and thus can increase the possibility of interactions between mononuclear cells and SMCs in atherosclerotic plaques.

Effect of low density lipoprotein on monocyte adhesiveness to endothelial cells in vitro

Adhesion of monocytes to the endothelium is an early event in the development of atherosclerosis. The possibility that low density lipoproteins enhance this process by activating monocytes was investigated using an in vitro adhesion test on endothelial cell monolayer cultures. Preincubation of monocytes with low density lipoprotein (LDL) (100 micrograms LDL protein/l x 10(6) cells/ml) for 15 min induced a 70% increase in adhesion to endothelial cells with a maximal effect at 100 micrograms LDL protein/ml and a short latency of effect (2 min). Anti-LDL receptor antibody, which inhibited LDL binding, blocked this activation. The LDL effect appeared to depend on receptor binding of LDL rather than on receptor-mediated endocytosis, since preincubation of monocytes with LDL at either 4 degrees C or 37 degrees C resulted in the same stimulation of adhesion. A cytofluorimetric study using integrin monoclonal antibodies (MAbs) against CD18 and CD11b did not reveal any increase in expression of the integrins on the surface of LDL-activated monocytes. However, a 30-min preincubation of monocytes with anti-CD18 abolished the LDL-activated adhesion. These results indicate that LDL induces a rapid activation of monocyte adhesiveness to endothelial cells. This effect appears to be mediated by interaction of LDL with its receptor rather than LDL-receptor complex internalization or integrin membrane mobilization from intracellular pools. The integrin system nevertheless appears to be involved.