Reactive oxygen species produced by macrophage-derived foam cells regulate the activity of vascular matrix metalloproteinases in vitro. Implications for atherosclerotic plaque stability

Vascular expression of extracellular superoxide dismutase in atherosclerosis

We characterized a novel form of extracellular superoxide dismutase (ecSOD) in atherosclerotic vessels. Specific activity and protein expression of ecSOD was increased two- to threefold in apo E-deficient compared with control aortas. RNase protection assays demonstrated that the expected ecSOD transcript was not increased in either apo E-deficient mice or cholesterol-fed LDL receptor-deficient mice, but that a second, lower molecular weight transcript was present and became predominant as atherosclerosis progressed. Sequence analysis revealed that this novel ecSOD has a 10-bp deletion in the 3' untranslated region and an asparagine to aspartic acid mutation at amino acid 21. Studies of isolated macrophages and immunohistochemistry suggested that the truncated ecSOD transcript was expressed by lipid-laden but not control macrophages. Recombinant wild-type and novel ecSODs expressed in Sf9 cells exhibited similar SOD activities. These experiments show that ecSOD expression is increased in atherosclerotic vessels and that this is characterized by an alteration in mRNA and protein structure. Further, the source of this altered ecSOD is likely the lipid-laden macrophage. The enzymatic properties of this novel ecSOD may have important implications for the function of the lipid-laden macrophage and the atherosclerotic process.

17beta-oestradiol enhances release of matrix metalloproteinase-2 from human vascular smooth muscle cells

Vascular remodelling occurs during all stages of atherosclerotic progression. Anti-atherosclerotic drugs may function by restoring regulation of the processes involved in remodelling of the extracellular matrix. A key group of enzymes involved in these processes are the matrix metalloproteinases (MMPs). Oestrogens have been demonstrated to possess anti-atherosclerotic properties at low concentrations while being associated with lesion formation at high concentrations. We examined the effect of 17beta-oestradiol on MMP-2 expression in human coronary artery (CAVSMC) and umbilical artery vascular smooth muscle cells (UAVSMC). MMP-2 expression was measured by chemiluminescent immunoblotting and quantified by laser densitometry. pro-MMP-2 was secreted by VSMCs and increasing levels of 17beta-oestradiol, from physiological through supraphysiological, were associated with significant dose-dependent increases in MMP-2 levels in culture media. This effect was dependent on de novo protein synthesis and could be antagonised by the oestrogen receptor antagonist, tamoxifen, and the specific receptor antagonist ICI 182, 780. 17beta-Oestradiol appears to be a specific stimulator of MMP-2 release from human vascular cells. The concentration dependence of this effect suggests a basis for the differential effects of low and high oestrogen levels on vascular integrity.

Extracellular matrix modulates macrophage functions characteristic to atheroma: collagen type I enhances acquisition of resident macrophage traits by human peripheral blood monocytes in vitro

Activated resident macrophages sustain atheroma, and a high macrophage content is associated with plaque vulnerability. Factors leading to differentiation and activation of these blood-derived cells remain largely uncharacterized. We investigated the contribution of interaction with collagen type I, the predominant component of atherosclerotic matrix, to differentiation and modulation of characteristic macrophage functions, including intracellular lipid accumulation and production of the typical matrix-degrading enzyme matrix metalloproteinase (MMP)-9. When used as an adhesion substrate for human peripheral blood monocytes in vitro, collagen type I increased monocyte differentiation, assessed by analysis of CD71 expression and cell spreading. Culturing on collagen type I doubled the number of differentiated monocytes at 24 hours (44.9+/-1.4% versus 18.4+/-1.7% on uncoated dishes, P<.001, n=3 independent experiments) and was a stronger stimulus for differentiation than phorbol myristate acetate, a known inducer of monocyte differentiation. The effect of substrate on intracellular accumulation of modified lipoproteins was assessed by quantitative confocal microscopy of monocytes incubated with fluorescent acetylated LDL. The collagen type I substrate also doubled the number of macrophages containing intracellular lipid and significantly increased the individual intracellular loading. Monocytes cultured on collagen type I also released more MMP-9 than did cells plated directly on plastic. The role of monocyte spreading was further assessed by treatment with colchicine, an inhibitor of cytoskeletal function, or with genistein, a nonspecific inhibitor of tyrosine kinases, shown to participate in cell adhesion. Cell spreading was inhibited in 72.3+/-6.7% of colchicine-treated and in 62.4+/-6.4% of genistein-treated monocytes (n=3, P<.01 in both cases). The same conditions also decreased secretion of MMP-9, and genistein reduced the number of acetylated LDL-containing cells (from 286+/-7 to 184+/-8 cells/mm2 with genistein, n=3, P<.001). Data showed a strong correlation (r>.98) between monocyte spreading on collagen type I and intracellular lipid accumulation. Our results indicate that interaction with vascular matrix may play an important role in differentiation of peripheral blood monocytes into resident lipid-laden macrophages, which act as central stimulators throughout the natural history of atheroma.

Gene therapy for restenosis: are we ready?

The application of gene therapy techniques to the clinical problem of coronary restenosis has generated tremendous attention and enthusiasm. Use of gene transfer technology to prevent a common intractable illness would represent a watershed event for human gene therapy. However, the time is not yet right to initiate gene therapy trials for restenosis. The biology of restenosis is incompletely understood, catheter-based gene delivery is poorly adapted to the coronary circulation, and current gene transfer vectors are ill-suited for safe and effective gene delivery to the coronary artery wall. Basic research designed to overcome these obstacles is currently more appropriate than the initiation of clinical trials.

Overexpression of the matrix metalloproteinase matrilysin results in premature mammary gland differentiation and male infertility

To examine the role of matrilysin (MAT), an epithelial cell-specific matrix metalloproteinase, in the normal development and function of reproductive tissues, we generated transgenic animals that overexpress MAT in several reproductive organs. Three distinct forms of human MAT (wild-type, active, and inactive) were placed under the control of the murine mammary tumor virus promoter/enhancer. Although wild-type, active, and inactive forms of the human MAT protein could be produced in an in vitro culture system, mutations of the MAT cDNA significantly decreased the efficiency with which the MAT protein was produced in vivo. Therefore, animals carrying the wild-type MAT transgene that expressed high levels of human MAT in vivo were further examined. Mammary glands from female transgenic animals were morphologically normal throughout mammary development, but displayed an increased ability to produce beta-casein protein in virgin animals. In addition, beginning at approximately 8 mo of age, the testes of male transgenic animals became disorganized with apparent disintegration of interstitial tissue that normally surrounds the seminiferous tubules. The disruption of testis morphology was concurrent with the onset of infertility. These results suggest that overexpression of the matrix-degrading enzyme MAT alters the integrity of the extracellular matrix and thereby induces cellular differentiation and cellular destruction in a tissue-specific manner.

Exposure to chronic hypoxia induces qualitative changes of collagen in the walls of peripheral pulmonary arteries

Qualitative changes of vascular wall matrix collagens in chronic hypoxic pulmonary hypertension were studied by gel electrophoresis. Male adult rats (n = 12) were exposed to hypoxia (FiO2 = 0.1, 3 wks). Control rats (n = 13) were kept in air. Samples of peripheral pulmonary arteries (PPA, diam. 100-400 microm), main branches of pulmonary artery, and aorta were dissected. Arterial samples were treated with 4M guanidine-HCl to remove noncollagenous moieties and the collagenous stroma was dissolved by limited pepsin digestion at low pH. Low molecular mass peptides (M. W. approx. 76 and 66 kD) were detected in the gel electrophoretic profile of collagen peptides of PPA of the chronically hypoxic animals and in aorta of both hypoxic and normoxic groups. These peptides were absent in the PPA of normoxic rats. Since the 76 kD peptide bound anticollagen type I antibodies, it appears to be of collagenous nature and it may be the result of collagenolytic activity in PPA isolated from hypoxic lungs. This was confirmed by zymography. We conclude that exposure of rats to chronic hypoxia results in the presence of low molecular mass peptides in the wall matrix of PPA which resemble those found in aorta of normoxic animals. Collagenolytic activity in the walls of peripheral pulmonary arteries may participate in the mechanism of lung vascular remodelling in chronic hypoxia.

Cardioprotective mechanisms of ACE inhibition. The angiotensin II-nitric oxide balance

The current challenge facing clinicians is to develop pharmacotherapies that move beyond the treatment of symptoms towards a new agenda in cardiovascular therapeutics that includes interventions to actually prevent the development of end-stage coronary heart disease. The development of new strategies to alter the natural history of cardiovascular disease will be fostered by gaining insights into the fundamental pathobiological mechanisms that promote the morbidity and mortality associated with these disorders. An emerging body of evidence indicates that locally generated vasoactive substances such as angiotensin II and nitric oxide are important determinants of the natural history of vascular disease. It is anticipated that ongoing clinical trials will extend the concept that modulating the activity of vasoactive substances generated by the endothelium has important implications for altering the course of coronary heart disease.

Endothelial function as a determinant of vascular function and structure: a new therapeutic target

Recent insights into the pathogenesis of vascular disease have opened up a new frontier that has implications for future therapies. The vasculature has been redefined as a vital organ that can regulate its own tone and structure via numerous cellular mechanisms. The endothelium plays the role of gatekeeper in this process, sensing and responding to stimuli and activating various vasoactive systems that function as mediators. Locally generated vasoactive substances such as angiotensin II and nitric oxide appear to be important determinants of vessel function and structure. Vasoactive substances generated within the endothelium influence cell proliferation and cell death in a complex interplay that, when disturbed, can result in structural alteration known as vascular remodeling. Normal vascular homeostasis is maintained by a balance between vasoconstrictors such as angiotensin II and vasodilators such as nitric oxide. Endothelial dysfunction involves an imbalance between vasoactive substances such that perturbations in the regulation of tone, hemostasis, and vessel structure result in the development of cardiovascular diseases, such as hypertension, atherosclerosis, and heart failure.