Proteolytic degradation of low density lipoproteins by arterial smooth muscle cells: the role of individual cathepsins

Lysosomal oxidation of LDL alters lysosomal pH, induces senescence, and increases secretion of pro-inflammatory cytokines in human macrophages

We have shown that aggregated LDL is internalized by macrophages and oxidized in lysosomes by redox-active iron. We have now investigated to determine whether the lysosomal oxidation of LDL impairs lysosomal function and whether a lysosomotropic antioxidant can prevent these alterations. LDL aggregated by SMase (SMase-LDL) caused increased lysosomal lipid peroxidation in human monocyte-derived macrophages or THP-1 macrophage-like cells, as shown by a fluorescent probe, Foam-LPO. The pH of the lysosomes was increased considerably by lysosomal LDL oxidation as shown by LysoSensor Yellow/Blue and LysoTracker Red. SMase-LDL induced senescence-like properties in the cells as shown by β-galactosidase staining and levels of p53 and p21. Inflammation plays a key role in atherosclerosis. SMase-LDL treatment increased the lipopolysaccharide-induced secretion of TNF-α, IL-6, and MCP-1. The lysosomotropic antioxidant, cysteamine, inhibited all of the above changes. Targeting lysosomes with antioxidants, such as cysteamine, to prevent the intralysosomal oxidation of LDL might be a novel therapy for atherosclerosis.

Soy extract has different effects compared with the isolated isoflavones on the proteome of homocysteine-stressed endothelial cells

Epidemiological studies suggest that soy consumption may provide a protection in the development and progression of atherosclerosis. It is under debate, however, whether the soy isoflavones or other compounds are the "active principle". As apoptosis is a driving force in the process of atherosclerosis, we tested whether a soy extract or a combination of the two predominant isoflavones genistein and daidzein, in concentrations as found in the extract, exert similar or different effects on apoptosis in EA.hy 926 endothelial cells after exposure to the endothelial stressor homocysteine. Plasma membrane disintegration and nuclear fragmentation served as relevant apoptosis markers. To assess whether the extract and the genistein/daidzein mixture differently affect cellular target proteins changed in amount by homocysteine treatment, proteome analysis was performed by two-dimensional gel-electrophoresis and peptide mass fingerprinting of regulated protein spots. Homocysteine induced apoptosis in the cells, and both extract and genistein/daidzein inhibited apoptosis to a comparable extent. Whereas the extract prevented for 10 proteins the changes in expression levels as caused by homocysteine, the genistein/daidzein mixture reversed the homocysteine effects on the proteome for 13 proteins. The cytoskeletal protein matrin 3 and a U5 snRNP-specific 40-kDa protein were the only protein entities where both extract and genistein/daidzein reversed the homocysteine-induced changes in a common way. In conclusion, our studies provide evidence that an isoflavone containing soy extract and isolated isoflavones, despite similar effects on inhibition of homocysteine-induced apoptosis in endothelial cells, affect a quite different spectrum of cellular target proteins.

Seeing Within

Molecular imaging is a rapidly evolving discipline with the goal of developing tools to display and quantify molecular and cellular targets in vivo. The heart of this field is based on the rational design and screening of targeted and activatable imaging reporter agents to sense fundamental processes of biology. Parallel advances in small animal imaging systems and in agent synthesis have allowed molecular imaging applications to extend into the in vivo arena. These advances have permitted, for example, in vivo sensing of inflammation, apoptosis, cell trafficking, and gene expression. In this review, we first review core principles of molecular imaging with an emphasis on smart, activatable agent technology. We then discuss applications of state-of-the-art molecular probes to interrogate important aspects of cardiovascular biology, with a focus on atherosclerosis, thrombosis, and heart failure. In the ensuing years, we anticipate that fundamental aspects of cardiovascular biology will be detectable in vivo, and that promising molecular imaging agents will be translated into the clinical arena to guide diagnosis and therapy of human cardiovascular illness.

Differential expression of cysteine and aspartic proteases during progression of atherosclerosis in apolipoprotein E-deficient mice

Several groups of proteolytic enzymes are able to degrade components of the extracellular matrix. During atherosclerosis, matrix remodeling is believed to influence the migration and proliferation of cells within the plaque. In the present study, gene expression of several proteases and their inhibitors was analyzed during the development of atherosclerosis in apolipoprotein E-deficient (ApoE-/-) mice. Quantitative real-time polymerase chain reaction was used to study gene expression of proteases after 10 and 20 weeks in ApoE-/- and C57BL/6 mice and in atherosclerotic lesions and nonaffected regions of the same ApoE-/- mouse. Some of the differentially expressed proteolytic enzymes were studied by immunohistochemistry. The matrix metalloproteinase (MMP)-9 and its inhibitor TIMP-1 were differentially expressed and the expression increased with time. Urokinase-type plasminogen activator showed no major expression. In contrast, cathepsins B, D, L, and S all showed strong and increased expression in ApoE-/- mice compared to C57BL/6 mice whereas the expression of their inhibitor, cystatin C, did not differ between the two mouse strains. The expression of cathepsins was mainly localized to the lesions and not to nonaffected regions of the aorta of ApoE-/- mice. Furthermore, cathepsin expression was similar to the expression of the macrophage marker macrosialin (CD68) although expression of cathepsins B, D, and L could be demonstrated in healthy C57BL/6 mice and in nonaffected vessel segments of atherosclerotic ApoE-/- mice. Cathepsin S mRNA expression was restricted to lesions of ApoE-/- mice. Furthermore, cathepsin S was the only cathepsin that was expressed in the media and absent in lipid-rich regions. All cathepsins studied showed intimal expression, the degree and localization of which differed between individual cathepsins. In conclusion, increased expression of several cathepsins in atherosclerotic lesions suggests that these proteases may participate in the remodeling of extracellular matrix associated with the atherosclerotic process.

Vascular inflammation and the renin-angiotensin system

It is now well established that vascular inflammation is an independent risk factor for the development of atherosclerosis. In otherwise healthy patients, chronic elevations of circulating interleukin-6 or its biomarkers are predictors for increased risk in the development and progression of ischemic heart disease. Although multifactorial in etiology, vascular inflammation produces atherosclerosis by the continuous recruitment of circulating monocytes into the vessel wall and by contributing to an oxidant-rich inflammatory milieu that induces phenotypic changes in resident (noninflammatory) cells. In addition, the renin-angiotensin system (RAS) has important modulatory activities in the atherogenic process. Recent work has shown that angiotensin II (Ang II) has significant proinflammatory actions in the vascular wall, inducing the production of reactive oxygen species, inflammatory cytokines, and adhesion molecules. These latter effects on gene expression are mediated, at least in part, through the cytoplasmic nuclear factor-kappaB transcription factor. Through these actions, Ang II augments vascular inflammation, induces endothelial dysfunction, and, in so doing, enhances the atherogenic process. Our recent studies have defined a molecular mechanism for a biological positive-feedback loop that explains how vascular inflammation can be self-sustaining through upregulation of the vessel wall Ang II tone. Ang II produced locally by the inflamed vessel induces the synthesis and secretion of interleukin-6, a cytokine that induces synthesis of angiotensinogen in the liver through a janus kinase (JAK)/signal transducer and activator of transcription (STAT)-3 pathway. Enhanced angiotensinogen production, in turn, supplies more substrate to the activated vascular RAS, where locally produced Ang II synergizes with oxidized lipid to perpetuate atherosclerotic vascular inflammation. These observations suggest that one mechanism by which RAS antagonists prevent atherosclerosis is by reducing vascular inflammation. Moreover, antagonizing the vascular nuclear factor-kappaB and/or hepatic JAK/STAT pathways may modulate the atherosclerotic process.

Macrophages possess both neutral and acidic protease activities toward low density lipoproteins

Low density lipoproteins (LDL) have been strongly implicated in the pathogenesis of atherosclerosis. We have studied the proteolytic degradation of these lipoproteins by macrophages, which are a major cellular constituent of atherosclerotic lesions. Mouse peritoneal macrophages contained both an acidic and a less active but distinct neutral/alkaline protease activity toward human 125I-labelled LDL. The acidic activity had a pH optimum of 4.5 and the neutral/alkaline activity one of 8-8.5. The acidic activity started to plateau with increasing lipoprotein concentrations whereas the neutral activity was directly proportional to the lipoprotein concentration up to at least 150 micrograms of protein/ml. The acidic protease activity had a complex time course whereas the neutral activity was directly proportional to the time of incubation up to at least 48 h. Leupeptin (35 microM) and pepstatin (5 microM) inhibited the acidic activity by about 70% individually and almost entirely in combination, indicating that cathepsins B and D are important in the degradation of LDL by lysosomal cathepsins. In contrast, there was little, if any, inhibition of the neutral protease activity by leupeptin or pepstatin. The acidic protease activity was increased by both DL-dithiothreitol (5 mM) and disodium EDTA (1 mM) whereas the neutral protease activity was increased by dithiothreitol but inhibited partially by EDTA. The possible significance of macrophage neutral and acidic protease activities toward LDL in atherosclerosis needs to be assessed.

The effects of acetylated low-density lipoproteins on fluid-phase pinocytosis by macrophages

A simple method has been set up to measure the rate of fluid-phase pinocytosis in resident mouse peritoneal macrophages in culture. The method uses 125I-labelled polyvinylpyrrolidone as a nondegradable marker of fluid-phase pinocytosis. The accumulation of 125I-labelled polyvinylpyrrolidone by the cells was directly proportional to its concentration in the culture medium up to at least 200 micrograms/ml. The estimates of the rate of fluid-phase pinocytosis were reproducible within each experiment (coefficient of variation 8.5%) but varied between individual experiments. Fluid-phase pinocytosis was undetectable at 4 degrees C and reduced greatly at 37 degrees C by metabolic inhibitors and 1 mM ZnSO4. High concentrations of human acetylated low-density lipoproteins, which are taken up rapidly by macrophages, decreased the rate of fluid-phase pinocytosis by up to about 70%. The inhibition was seen after only 2 h of incubation. Unmodified low-density lipoproteins, which are taken up only slowly by macrophages, did not usually inhibit fluid-phase pinocytosis (in fact, they sometimes increased it). Modified low-density lipoprotein uptake, leading to massive lipid accumulation in macrophages in the arterial wall, has been postulated to be involved in the pathogenesis of atherosclerosis. This study raises the possibility that the rate of fluid-phase pinocytosis in these lipid-laden arterial macrophages may be reduced.

[Lysosomal enzyme activity of monocytes/macrophages following incubation with postprandial hyperlipemic serum and its significance for the development of atherosclerosis]

Lipid accumulation in macrophages is a prominent feature of the atherosclerotic lesion. Decreased lysosomal function of these cells might play an important role in the pathogenesis of the atherosclerotic foam cell. In this investigation six normal volunteers were fed a meal with a high fat content (68.9% energy, P/S ratio 0.13). The hyperlipidemic postprandial serum was incubated with monocyte derived macrophages. The enzyme activity of cathepsin B, acid cholesterylester-hydrolase and N-acetyl-beta-hydrolase decreased significantly in these cells. Thus, inadequate response in enzyme activity of lysosomal enzymes in case of fat overload might contribute to the development of the atherosclerotic foam cell.

A new method for the reconstitution of the anion transport system of the human erythrocyte membrane

The anion transport protein of the human erythrocyte membrane, band 3, was solubilized and purified in solutions of the non-ionic detergent Triton X-100. It was incorporated into spherical lipid bilayers by the following procedure: Dry phosphatidylcholine was suspended in the protein solution. Octylglucopyranoside was added until the milky suspension became clear. The sample was dialyzed overnight against detergent-free buffer. Residual Triton X-100 was removed from the opalescent vesicle suspension by sucrose density gradient centrifugation and subsequent dialysis. Sulfate efflux from the vesicles was studied, under exchange conditions, using a filtration method. Three vesicle subpopulations could be distinguished by analyzing the time course of the efflux. One was nearly impermeable to sulfate, and efflux from another was due to leaks. The largest subpopulation, however, showed transport characteristics very similar to those of the anion transport system of the intact erythrocyte membrane: transport numbers (at 30 degrees C) close to 20 sulfate molecules per band 3 and min, an activation energy of approx. 140 kJ/mol, a pH maximum at pH 6.2, saturation of the sulfate flux at sulfate concentrations around 100 mM, inhibition of the flux by H2DIDS and flufenamate (approx. KI-values at 30 degrees C: 0.1 and 0.7 microM, respectively), and "right-side-out" orientation of the transport protein (as judged from the inhibition of sulfate efflux by up to 98% by externally added H2DIDS). Thus, the system represents, for the first time, a reconstitution of all the major properties of the sulfate transport across the erythrocyte membrane.

Reversible high affinity uptake of apo E-free high density lipoproteins in cultured pig hepatocytes

We examined the high affinity binding, uptake, and degradation of apo E-free 125I-high density lipoprotein (HDL) in cultured pig hepatocytes. At steady state, the cells degraded 9.4% of cell-associated 125I-HDL/hour, compared with 41.7%/hour for 125I-LDL. Pulse-chase experiments at 4 degrees C revealed that high affinity 125I-HDL binding was reversible. Similar experiments at 37 degrees C revealed that about 70% of the cell-associated 125I-HDL was released as a macromolecule; the remainder was degraded to acid-soluble products. In contrast, over 75% of the 125I-LDL that was released had been degraded to acid soluble products. The amount of macromolecular 125I-HDL released at 37 degrees C was similar to the amount that was bound to the cell surface, as estimated from measurements of trypsin-releasable radioactivity. Density gradient ultracentrifugation and SDS-polyacrylamide gel electrophoretic analysis of macromolecular 125I-HDL released to the medium revealed an increase in density, and the apparent partial proteolysis of apo A-I (Mr 25,000) to products of apparent Mr 12,000-14,000. The findings suggest that high affinity 125I-HDL uptake had a reversible component in which HDL was concentrated temporarily at the cell surface, modified, and then released as a somewhat denser lipoprotein particle. Measurement of 125I-HDL and 125I-LDL degradation in cell homogenates revealed no difference in the inherent susceptibility of the two lipoproteins to proteolysis by lysosomal enzymes. The overall slower rate of degradation of 125I-HDL compared to 125I-LDL was therefore due in part to the smaller fraction of HDL that was committed to irreversible catabolism. The rate of catabolism of this fraction, however, was considerable. Cells pulsed at 4 degrees C and subsequently warmed to 37 degrees C released one-half the acid-soluble products from 125I-HDL within about 4 hours, compared with 2 hours for cells pulsed with 125I-LDL. These findings indicate that HDL was internalized, transported to lysosomes, and degraded at about one-half the rate of LDL.

Properties and subcellular localization of elastase-like activities of arterial smooth muscle cells in culture

The properties and subcellular localization of the elastase-like activities of smooth muscle cells cultured from pig aortas have been investigated. Homogenates of the cells hydrolysed N-succinyl-L-alanyl-L-alanyl-L-alanine-p-nitroanilide, a synthetic substrate for elastases, with a distinct pH optimum of 8.2 and hydrolysed insoluble elastin with a distinct pH optimum of 8.5. Both enzyme activities were directly proportional to the concentration of homogenate in the assay mixture. The activities toward both substrates were inhibited by phenylmethylsulphonyl fluoride and were therefore probably due to a serine peptidase(s). The activities were also inhibited by EDTA and, in a dose-related manner, by alpha 1-antiprotease. Pepstatin, which inhibits cathepsin D, and leupeptin, which inhibits cathepsin B, did not significantly inhibit the elastase-like activities in these cells. The cells were homogenized and a post-nuclear supernatant subjected to sucrose density gradient centrifugation. The distribution of elastase-like activity toward both substrates was similar to that of the plasma membrane marker 5'-nucleotidase, and distinct from those of marker enzymes for the other organelles. Cells were also homogenized with digitonin, which selectively increases the equilibrium density of the plasma membrane. The equilibrium densities of both 5'-nucleotidase and of the elastase-like activities were increased considerably, confirming the plasma membrane localization of the elastase-like activities. The subcellular localization of the elastase-like activities of arterial smooth muscle cells is therefore consistent with a role for them in the degradation of elastin in the normal arterial wall and in atherosclerotic lesions.

Properties and subcellular localization of adenosine diphosphatase in arterial smooth muscle cells in culture

The properties and subcellular localization of adenosine diphosphatase (ADPase) activity in smooth muscle cells cultured from pig aortas have been investigated. The pH optimum of ADPase activity was 7.3 and the apparent Km for ADP was 10.3 microM. ADPase activity was inhibited completely by EDTA and was restored by the addition of divalent cations. The enzyme activity was not inhibited by 2-glycerophosphate, a substrate for non-specific phosphatases, nor by levamisole, a specific inhibitor of alkaline phosphatase. Smooth muscle cells were homogenized and a post-nuclear supernatant was applied to a sucrose density gradient in a Beaufay automatic zonal rotor. The distribution of ADPase activity in the density gradient was similar to that of 5'-nucleotidase activity, a marker enzyme for the plasma membrane, and distinct from the distributions of the marker enzymes for the other organelles. When the cells were homogenized in the presence of digitonin, an agent which binds to cholesterol and increases the equilibrium density of the plasma membrane, the modal equilibrium densities of ADPase activity and of 5'-nucleotidase activity were increased to similar extents, thus confirming the plasma membrane localization of ADPase activity.

Lipid accumulation in arterial smooth muscle cells in culture. Morphological and biochemical changes caused by low density lipoproteins and chloroquine

Cultured smooth muscle cells from pig aortas were incubated with low density lipoproteins (LDL) and chloroquine for up to 5 days, as an in vitro model for lipid accumulation in atherosclerosis. Cells incubated with LDL alone had a normal morphology, except that some cells contained large lipid droplets. The activities of acid phosphatase, catalase and malate dehydrogenase were increased in homogenates prepared from these cells. Cells incubated with chloroquine alone developed large autophagic vacuoles. The activities of the three acid hydrolases, acid phosphatase, N-acetyl-beta-glucosaminidase and beta-glucuronidase, were decreased, as was the proteolytic activity of the cell homogenates at acid pH toward 125I-labelled LDL. There was, however, a transient increase in the activity of malate dehydrogenase. Chloroquine by itself was toxic to the cells, but LDL protected against this toxic effect. Cells incubated with LDL and chloroquine together developed both autophagic vacuoles and large lipid droplets. The cholesteryl ester content of the cells was increased many-fold and the non-esterified cholesterol content was increased to a lesser extent. The above four acid hydrolase activities were decreased, as was the activity of catalase, whereas the activities of lactate dehydrogenase and malate dehydrogenase were increased.

Analytical subcellular fractionation of rat pituitary homogenates, with special reference to prolactin proteolysis by lysosomes

Prolactin proteolysis by rat pituitary homogenates was assayed by measuring the release of trichloroacetic acid-soluble peptides from 125I-labelled rat prolactin. There was a distinct optimum at pH 4.3, with only trace amounts of activity at neutral and alkaline pH. Rat pituitary homogenates were subjected to analytical subcellular fractionation by sucrose density gradient centrifugation in a Beaufay automatic zonal rotor. The principal organelles were characterized by their respective marker enzymes, including: cytosol (lactate dehydrogenase); plasma membrane (5'-nucleotidase); lysosomes (N-acetyl-beta-glucosaminidase, beta-glucuronidase); mitochondria (particulate malate dehydrogenase); endoplasmic reticulum (neutral alpha-glucosidase); prolactin granules (radioimmunoassayable prolactin). Acid prolactin protease had a similar distribution to the lysosomal marker enzymes. A localisation of the activity to lysosomes was confirmed by subcellular fractionation experiments in which the lysosomes were selectively disrupted with low concentrations of the membrane perturbant, digitonin. Experiments with specific inhibitors of the lysosomal cathepsins indicate that both cathepsins B and D are implicated in pituitary prolactin proteolysis.