Stress fibers in endothelial cells overlying atherosclerotic lesions in rabbit aorta

In Development-A New Paradigm for Understanding Vascular Disease

Under physiological conditions, the arterial endothelium exerts a powerful protective influence to maintain vascular homeostasis. However, during the development of vascular disease, these protective activities are lost, and dysfunctional endothelial cells actually promote disease pathogenesis. Numerous investigations have analyzed the characteristics of dysfunctional endothelium with a view to understanding the processes responsible for the dysfunction and to determining their role in vascular pathology. This review adopts an alternate approach: reviewing the mechanisms that contribute to the initial formation of a healthy protective endothelium and on how those mechanisms may be disrupted, precipitating the appearance of dysfunctional endothelial cells and the progression of vascular disease. This approach, which highlights the role of endothelial adherens junctions and vascular endothelial-cadherin in endothelial maturation and endothelial dysfunction, provides new insight into the remarkable biology of this important cell layer and its role in vascular protection and vascular disease.

The atypical structure and function of newborn arterial endothelium is mediated by Rho/Rho kinase signaling

Endothelium of fetal or newborn arteries is atypical, displaying actin stress fibers and reduced nitric oxide (NO)-mediated dilatation. This study tested the hypothesis that Rho/Rho kinase signaling, which promotes endothelial stress fibers and inhibits endothelial dilatation, contributed to this phenotype. Carotid arteries were isolated from newborn [postnatal day 1 (P1)], P7, and P21 mice. Endothelial dilatation to acetylcholine (pressure myograph) was minimal at P1, increased at P7, and further increased at P21. Inhibition of Rho (C3 transferase) or Rho kinase (Y27632, fasudil) significantly increased dilatation to acetylcholine in P1 arteries but had no effect in P7 or P21 arteries. After inhibition of NO synthase (N(G)-nitro-l-arginine methyl ester), Rho kinase inhibition no longer increased acetylcholine responses in P1 arteries. Rho kinase inhibition did not affect dilatation to the NO donor DEA-NONOate. The endothelial actin cytoskeleton was labeled with phalloidin and visualized by laser-scanning microscopy. In P1 arteries, the endothelium had prominent transcytoplasmic stress fibers, whereas in P7 and P21 arteries, the actin fibers had a significantly reduced intensity and were restricted to cell borders. Phosphorylation of myosin light chains, a Rho kinase substrate, was highest in P1 endothelium and significantly reduced in P7 and P21 endothelium (laser-scanning microscopy). In P1 arteries, inhibition of Rho (C3 transferase) or Rho kinase (Y27632) significantly reduced the intensity of actin fibers, which were restricted to cell borders. Similarly, in P1 arteries, Rho inhibition significantly reduced endothelial levels of phosphorylated myosin light chains. These results indicate that the atypical function and morphology of newborn endothelium is mediated by Rho/Rho kinase signaling.

Atherogenecity of lipoprotein(a) and oxidized low density lipoprotein: insight from in vivo studies of arterial wall influx, degradation and efflux

The accumulation of atherogenic lipoproteins in the arterial intima is pathognomonic of atherosclerosis. Modification of LDL by covalent linkage of apo(a) (resulting in the formation of Lp(a)) or oxidation probably enhances its atherogenecity. Although the metabolism of LDL in arterial intima has been rather extensively characterized, little has been known about the interaction of Lp(a) and oxidized LDL (ox-LDL) with the arterial wall. The present paper reviews a series of recent in vivo studies of the interaction of Lp(a) and ox-LDL with the arterial wall. The results have identified several factors that affect the accumulation of Lp(a) and ox-LDL in the arterial intima and have provided fresh insight into unique metabolic characteristics of Lp(a) and ox-LDL that may explain the large atherogenic potential of these modified LDL species.

Transfer of low density lipoprotein into the arterial wall and risk of atherosclerosis

The aim of the review is to summarize the present knowledge on determinants of transfer of low density lipoprotein (LDL) into the arterial wall, particularly in relation to the risk of development of atherosclerosis. The flux of LDL into the arterial wall (in moles of LDL per surface area per unit of time) has two major determinants, i.e. the LDL concentration in plasma and the arterial wall permeability. LDL enters the arterial wall as intact particles by vesicular ferrying through endothelial cells and/or by passive sieving through pores in or between endothelial cells. Estimates in vivo of the LDL permeability of a normal arterial wall vary between 5 and 100 nl/cm2/h. In laboratory animals, the regional variation in the arterial wall permeability predicts the pattern of subsequent dietary induced atherosclerosis. Moreover, mechanical or immunological injury of the arterial wall increases the LDL permeability and is accompanied by accelerated development of experimental atherosclerosis. This supports the idea that an increased permeability to LDL, like an increased plasma LDL concentration, increases the risk of atherosclerosis. Hypertension, smoking, genetic predisposition, atherosclerosis, and a small size of LDL may all increase the arterial wall permeability to LDL and in this way increase the risk of accelerated development of atherosclerosis. The hypothesis that atherosclerosis risk can be reduced by improving the barrier function of the arterial wall towards the entry of LDL remains to be investigated; agents which directly modulate the LDL permeability of the arterial wall in vivo await identification.

Atherogenic levels of low-density lipoprotein increase hydrogen peroxide generation in cultured human endothelial cells: possible mechanism of heightened endocytosis

Cultured human umbilical vein endothelial cells(EC) exposed to atherogenic low-density lipoprotein (LDL) levels have augmented reactive oxygen species generation. Confluent EC were incubated with 30-330 mg/dl LDL cholesterol and cellular hydrogen peroxide (H2O2) generation measured. EC incubated with 30 and 90 mg/dl LDL cholesterol showed similar low level H2O2 production. In contrast, EC exposed to 180 and 330 mg/dl LDL cholesterol have a marked, dose-related elevation in H2O2 generation. Subsequent studies have explored if direct EC exposure to H2O2 promotes cellular functional changes similar to those induced by high LDL levels (> 160 mg/dl cholesterol). Confluent EC were incubated with 0.1-10 mM H2O2 for 30 minutes and endocytosis measured and cytoskeletal structure examined. H2O2 exposure (0.5 and 1 mM) promoted heightened EC endocytosis, which similarly occurs with high LDL exposure. Likewise, cytoskeletal examination of EC perturbed with 1 mM H2O2 reveals structural remodeling with a marked increase in stress fibers, which similarly happens with high LDL levels. The above observations that high LDL levels cause increased EC H2O2 production, and direct H2O2 exposure promotes cellular functional changes similar to those induced by high LDL concentrations, suggest a modulatory role for reactive oxygen species. Thus LDL-induced reactive oxygen species generation may contribute mechanistically to endothelial perturbation, which has been hypothesized to be a major contributing factor in the pathogenesis of atherosclerosis.

A definition of initial, fatty streak, and intermediate lesions of atherosclerosis. A report from the Committee on Vascular Lesions of the Council on Arteriosclerosis, American Heart Association

The compositions of lesion types that precede and that may initiate the development of advanced atherosclerotic lesions are described and the possible mechanisms of their development are reviewed. While advanced lesions involve disorganization of the intima and deformity of the artery, such changes are absent or minimal in their precursors. Advanced lesions are either overtly clinical or they predispose to the complications that cause ischemic episodes; precursors are silent and do not lead directly to complications. The precursors are arranged in a temporal sequence of three characteristic lesion types. Types I and II are generally the only lesion types found in children, although they may also occur in adults. Type I lesions represent the very initial changes and are recognized as an increase in the number of intimal macrophages and the appearance of macrophages filled with lipid droplets (foam cells). Type II lesions include the fatty streak lesion, the first grossly visible lesion, and are characterized by layers of macrophage foam cells and lipid droplets within intimal smooth muscle cells and minimal coarse-grained particles and heterogeneous droplets of extracellular lipid. Type III (intermediate) lesions are the morphological and chemical bridge between type II and advanced lesions. Type III lesions appear in some adaptive intimal thickenings (progression-prone locations) in young adults and are characterized by pools of extracellular lipid in addition to all the components of type II lesions.

A definition of initial, fatty streak, and intermediate lesions of atherosclerosis. A report from the Committee on Vascular Lesions of the Council on Arteriosclerosis, American Heart Association

The compositions of lesion types that precede and that may initiate the development of advanced atherosclerotic lesions are described and the possible mechanisms of their development are reviewed. While advanced lesions involve disorganization of the intima and deformity of the artery, such changes are absent or minimal in their precursors. Advanced lesions are either overtly clinical or they predispose to the complications that cause ischemic episodes; precursors are silent and do not lead directly to complications. The precursors are arranged in a temporal sequence of three characteristic lesion types. Types I and II are generally the only lesion types found in children, although they may also occur in adults. Type I lesions represent the very initial changes and are recognized as an increase in the number of intimal macrophages and the appearance of macrophages filled with lipid droplets (foam cells). Type II lesions include the fatty streak lesion, the first grossly visible lesion, and are characterized by layers of macrophage foam cells and lipid droplets within intimal smooth muscle cells and minimal coarse-grained particles and heterogeneous droplets of extracellular lipid. Type III (intermediate) lesions are the morphological and chemical bridge between type II and advanced lesions. Type III lesions appear in some adaptive intimal thickenings (progression-prone locations) in young adults and are characterized by pools of extracellular lipid in addition to all the components of type II lesions.

Oleic acid supplementation reduces oxidant-mediated dysfunction of cultured porcine pulmonary artery endothelial cells

We have previously shown that supplementing cultured porcine pulmonary artery endothelial cells (PAEC) with exogenous oleic acid (18:1 omega 9) alters the fatty acid composition of the cells and reduces oxidant-mediated cytotoxicity. Because the mechanisms by which lipid alterations modulate oxidant susceptibility have not been defined, the ability of 18:1 to reduce hydrogen peroxide (H2O2)-mediated PAEC dysfunction was evaluated. PAEC monolayers on polycarbonate filters were incubated for 3 h in maintenance medium supplemented with either 0.1 mM 18.1 in ethanol vehicle (ETOH) or with an equivalent volume of vehicle alone. Twenty-four hours later monolayers were treated for 30 min with 50 or 100 microM H2O2 in Hanks' balanced salt solution (HBSS) or with HBSS alone (nonoxidant control). As a functional index of PAEC monolayer integrity, the permeability of monolayers to albumin was then measured for 3 h. Treatment with 100 microM H2O2 caused cytotoxicity and progressive increases in PAEC monolayer permeability that were attenuated by 18:1 supplementation, whereas 50 microM H2O2 caused only a transient increase in permeability without cytotoxicity. Supplementation with 18:1 also attenuated H2O2-induced reductions in PAEC adenosine triphosphate (ATP) content and disruption of PAEC microfilament architecture. The ATP content of PAEC monolayers was reversibly reduced in the absence of oxidant stress by incubation with glucose-depleted medium containing deoxyglucose and antimycin A. Metabolic inhibitor-induced ATP depletion increased monolayer permeability and altered cytoskeletal architecture, alterations that resolved during recovery of PAEC ATP content. These results demonstrate that ATP depletion plays a critical role in barrier dysfunction and suggests that the ability of 18:1 to reduce oxidant-mediated PAEC dysfunction and injury may relate directly to its ability to preserve PAEC ATP content.

Actin cytoskeletal network in aging and cancer

The cytoskeleton is being recognized as an important modulator of metabolic functions of the cell. The actin cytoskeletal network, in particular, is involved in events regulating cell proliferation and differentiation. The state of actin in a variety of cell types is regulated by signals arising from the cell surface through a wide spectrum of interactions. In this review, we explore the role of actin cytoskeletal network in a series of events which are known to influence cell proliferation and differentiation. These include interaction of actin network with extracellular matrix proteins, cell surface membranes, second messengers, cytoplasmic enzymes and the nucleus. Because of the involvement of the actin network in such diverse interactions, we propose that alterations in the actin cytoskeletal function may be an important aspect of generalized decrease in cellular functions associated with aging. Preliminary data indicate that alterations in the cytoskeletal network do occur in cells obtained from older individuals. Alterations in actin state are also reported during malignant transformation of cells in culture, and in naturally occurring tumors. Taken together, the existing data seem to suggest that changes in the actin cytoskeletal network may be a part of the aging process as well as malignant transformation. Therefore, the study of the actin cytoskeletal network and its regulation has the potential to yield important information regarding cellular senescence and neoplastic transformation.