In situ immunolocalization of lipoproteins in human arteriosclerotic tissue

LCAT, ApoD, and ApoA1 Expression and Review of Cholesterol Deposition in the Cornea

Lecithin:cholesterol acyltransferase (LCAT) is an enzyme secreted by the liver and circulates with high-density lipoprotein (HDL) in the blood. The enzyme esterifies plasma cholesterol and increases the capacity of HDL to carry and potentially remove cholesterol from tissues. Cholesterol accumulates within the extracellular connective tissue matrix of the cornea stroma in individuals with genetic deficiency of LCAT. LCAT can be activated by apolipoproteins (Apo) including ApoD and ApoA1. ApoA1 also mediates cellular synthesis of HDL. This study examined the expression of LCAT by epithelial cells, keratocytes, and endothelial cells, the cell types that comprise from anterior to posterior the three layers of the cornea. LCAT and ApoD were immunolocalized to all three cell types within the cornea, while ApoA1 was immunolocalized to keratocytes and endothelium but not epithelium. In situ hybridization was used to detect LCAT, ApoD, and ApoA1 mRNA to learn what cell types within the cornea synthesize these proteins. No corneal cells showed mRNA for ApoA1. Keratocytes and endothelium both showed ApoD mRNA, but epithelium did not. Epithelium and endothelium both showed LCAT mRNA, but despite the presence of LCAT protein in keratocytes, keratocytes did not show LCAT mRNA. RNA sequencing analysis of serum-cultured dedifferentiated keratocytes (commonly referred to as corneal stromal fibroblasts) revealed the presence of both LCAT and ApoD (but not ApoA1) mRNA, which was accompanied by their respective proteins detected by immunolabeling of the cultured keratocytes and Western blot analysis of keratocyte lysates. The results indicate that keratocytes in vivo show both ApoA1 and LCAT proteins, but do not synthesize these proteins. Rather, keratocytes in vivo must take up ApoA1 and LCAT from the corneal interstitial tissue fluid.

High levels of cathepsin D and cystatin B are associated with increased risk of coronary events

Objective

The majority of acute coronary syndromes are caused by plaque ruptures. Proteases secreted by macrophages play an important role in plaque ruptures by degrading extracellular matrix proteins in the fibrous cap. Matrix metalloproteinases have been shown to be markers for cardiovascular disease whereas the members of the cathepsin protease family are less studied.

Methods

Cathepsin D, cathepsin L and cystatin B were measured in plasma at baseline from 384 individuals who developed coronary events (CEs), and from 409 age-matched and sex-matched controls from the Malmö Diet and Cancer cardiovascular cohort.

Results

Cathepsin D (180 (142–238) vs 163 (128–210), p<0.001), cathepsin L (55 (44–73) vs 52 (43–67), p<0.05) and cystatin B levels (45 (36–57) vs 42 (33–52), p<0.001) were significantly increased in CE cases compared to controls. In addition, increased cathepsin D (220 (165–313) vs 167 (133–211), p<0.001), cathepsin L (61 (46–80) vs 53 (43–68), p<0.05) and cystatin B (46 (38–58) vs 43 (34–54), p<0.05) were associated with prevalent diabetes. Furthermore, cathepsin D and cystatin B were increased in smokers. The HRs for incident CE comparing the highest to the lowest tertile(s) of cathepsin D and cystatin B were 1.34 (95% CI 1.02 to 1.75) and 1.26 (95% CI 1.01 to 1.57), respectively, after adjusting for age, sex, low-density lipoprotein/high-density lipoprotein ratio, triglycerides, body mass index, hypertension and glucose, but these associations did not remain significant after further addition of smoking to the model. In addition, cathepsin D was increased in incident CE cases among smokers after adjusting for cardiovascular risk factors.

Conclusions

The associations of cathepsin D and cystatin B with future CE provide clinical support for a role of these factors in cardiovascular disease, which for cathepsin D may be of particular importance for smokers.

Atorvastatin modulates the profile of proteins released by human atherosclerotic plaques

The mechanisms by which hydroxymethylglutaryl CoenzymeA reductase inhibitors (statins) reduce atherosclerotic cardiovascular morbidity and mortality remain poorly understood. Statins have been shown to modulate the levels of different inflammatory proteins both in carotid atherosclerotic plaques and in the blood of patients with atherosclerosis. In this work, we hypothesize that statins could also modulate the levels of the proteins secreted by cultured atherosclerotic plaques. Thus, the secretomes obtained from complicated atherosclerotic plaques incubated in the presence/absence of atorvastatin (10 micromol/l, 24 h) were analysed and compared by two-dimensional electrophoresis, considering the fibrous adjacent areas as controls. In total, 54 proteins (83 protein isoforms) were identified by Mass Spectrometry (MS): 24 proteins were increased and 20 proteins decreased in atheroma plaque supernatants compared to controls. Some of these proteins, like Cathepsin D, could play a significant role in plaque instability, becoming a potential target for therapeutical treatment. Interestingly, 66% of the proteins differentially released by atherosclerotic plaques reverted to control values after administration of atorvastatin, among them, Cathepsin D. Moreover, plaques obtained from patients who received atorvastatin treatment prior to carotid endarterectomy showed decreased Cathepsin D expression relative to plaques from non-treated patients. In conclusion, this proteomic approach has shown that statins are able to modulate the secretome of atherosclerotic plaques, and new therapeutical targets for statins have been characterised.

Myeloperoxidase-mediated oxidation of high-density lipoproteins: Fingerprints of newly recognized potential proatherogenic lipoproteins

Substantial evidence supports the notion that oxidative processes participate in the pathogenesis of atherosclerotic heart disease. Major evidence for myeloperoxidase (MPO) as enzymatic catalyst for oxidative modification of lipoproteins in the artery wall has been suggested in numerous studies performed with low-density lipoprotein. In contrast to low-density lipoprotein, plasma levels of high-density lipoprotein (HDL)-cholesterol and apoAI, the major apolipoprotein of HDL, inversely correlate with the risk of developing coronary artery disease. These antiatherosclerotic effects are attributed mainly to HDL's capacity to transport excess cholesterol from arterial wall cells to the liver during 'reverse cholesterol transport'. There is now strong evidence that HDL is a selective in vivo target for MPO-catalyzed oxidation impairing the cardioprotective and antiinflammatory capacity of this antiatherogenic lipoprotein. MPO is enzymatically active in human lesion material and was found to be associated with HDL extracted from human atheroma. MPO-catalyzed oxidation products are highly enriched in circulating HDL from individuals with cardiovascular disease where MPO concentrations are also increased. The oxidative potential of MPO involves an array of intermediate-generated reactive oxygen and reactive nitrogen species and the ability of MPO to generate chlorinating oxidants-in particular hypochlorous acid/hypochlorite-under physiological conditions is a unique and defining activity for this enzyme. All these MPO-generated reactive products may affect structure and function of HDL as well as the activity of HDL-associated enzymes involved in conversion and remodeling of the lipoprotein particle, and represent clinically useful markers for atherosclerosis.

Intracellular localization of oxidized low-density lipoproteins in atherosclerotic plaque cells revealed by electron microscopy combined with laser capture microdissection

Recently, laser capture microdissection (LCM) has become a powerful tool for investigating the genome and protein contents of cells populating normal and pathologically altered tissues. The present work reports a technique for the preparation of tissue specimens for further LCM combined with electron microscopy. In this work, atherosclerotic plaque cells containing oxidized low-density lipoproteins (LDL) were microdissected from human carotid arteries and the intracellular distribution of oxidized LDL was examined, providing novel information about the association of microvesicles containing oxidized LDL with "lipid droplets" in macrophage foam cells.

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.

Immunolocalization of apolipoproteins in aortic atherosclerosis in American youths and young adults: findings from the PDAY study

The immunohistochemical distribution of apolipoproteins in the abdominal aortas of 142 men, 15-34 years of age, collected in a cooperative multicenter study group (Pathobiological Determinants of Atherosclerosis in Youth) was examined in relationship to serum VLDL+LDL+HDL cholesterol levels. ApoB deposits were limited to the intima of specimens with intimal fibro cellular thickening or atherosclerotic lesions. Apo A-I, E and J were observed in both the intima and media of the aortas with intimal lesions. The pattern of apoJ distribution was similar to that of apoA-I and E. The distribution patterns of these apolipoproteins in these young adults were very similar to those in adults and old men seen in an earlier study. The extent of apolipoprotein distribution in the intima and media increased with age and the stage of atherosclerosis development, but was not correlated significantly with serum VLDL+LDL or HDL cholesterol levels. The infiltration of lipoprotein particles into the aortic wall seems to be more strongly associated with the progression of intimal lesions rather than with serum cholesterol levels.

Immunolocalization of high-density lipoproteins in arterial walls of rats

The inverse correlation between serum high-density lipoprotein (HDL) levels and coronary heart disease in humans suggests that HDL has a protective effect against the development of atherosclerosis. However, there is a lack of data concerning its distribution across the arterial wall. In order to detect this lipoprotein, we performed immunogold labeling on ultrathin sections of L.R. White embedded rat arterial tissue. Electron microscopic examination revealed that HDL was localized in the cytoplasm of the endothelial cells and the smooth muscle cells, but not in the nucleus or other organelles. The HDL was also present in the subendothelial space, the extracellular matrix as well as the intercellular clefts between the endothelial cells. Quantitative study revealed that rats on a high cholesterol diet for one month have more immunogold labeling (P < 0.05) in the subendothelial space, the smooth muscle cells and the extracellular matrix as compared to rats on a normal diet. After 12 months of normal diet, the intracellular labeling was significantly increased (P < 0.05) in the endothelial cells and the smooth muscle cells as compared to 1 month on the normal diet. The increase was greater (P < 0.05) for the high-cholesterol diet than for the normal diet treatment.

Serum amyloid P component associates with high density lipoprotein as well as very low density lipoprotein but not with low density lipoprotein

Serum amyloid P component (SAP) is a glycoprotein in human plasma. We previously showed that SAP is specifically localized in human atherosclerotic lesions, suggesting that SAP may play a role in atherogenesis. In this study, the interactions between human SAP and high density lipoprotein (HDL), low density lipoprotein (LDL) and very low density lipoprotein (VLDL) were investigated by using a solid phase plate assay. Biotinylated SAP bound to immobilized HDL and VLDL in a calcium-dependent, saturable manner. The SAP-HDL and SAP-VLDL bindings reached saturation at 4 nM and 16 nM of SAP, respectively. The bindings were inhibited by native SAP in a dose-dependent manner. No binding between SAP and LDL was found in the presence of calcium or EDTA, which indicates the specificity of SAP-lipoproteins interactions. These results suggest that the function of SAP is related to its capability to interact with lipoproteins and this may have important implications in atherosclerosis and in amyloidosis.

Effects of reagent and enzymatically generated hypochlorite on physicochemical and metabolic properties of high density lipoproteins

Myeloperoxidase (MPO), a protein secreted by activated phagocytes, may be a potential candidate for the generation of modified/oxidized lipoproteins in vivo via intermediate formation of HOCl, a powerful oxidant. During the present study, the effects of reagent NaOCl and OCl- generated by the MPO/H2O2/Cl- system on physicochemical and metabolic properties of high density lipoprotein (HDL) subclass 3 (HDL3) were investigated. Up to a molar oxidant:lipoprotein ratio of approximately 30:1, apolipoprotein A-I (apoA-I), the major HDL3 apolipoprotein component, represented the preferential target for OCl- attack (consuming 35-76% of the oxidant), thereby protecting HDL3 fatty acids (consuming between 17 and 30% of the oxidant) against OCl--mediated modification. At molar oxidant:HDL3 ratios >/= 60:1, we have observed pronounced consumption of HDL3 unsaturated fatty acids with concomitant formation of fatty acid chlorohydrins. Modification of HDL3 in the presence of the MPO/H2O2/Cl- system resulted in amino acid oxidation in a manner comparable with that found with reagent NaOCl only. Treatment of HDL3 with reagent NaOCl as well as modification by the MPO/H2O2/Cl- system resulted in significantly enhanced turnover rates of HDL3 by mouse peritoneal macrophages, an effect that was not a result of HDL3 aggregation as judged by dynamic and static light-scattering experiments. In comparison with native HDL3, the degradation by macrophages was enhanced by 4- and 15-fold when HDL3 was modified with reagent NaOCl or the MPO/H2O2/Cl- system. Finally, the ability of HDL3 to promote cellular cholesterol efflux from macrophages was significantly diminished after modification with reagent NaOCl. Collectively, these results demonstrate that the modification of HDL3 by hypochlorite (added as reagent or generated by the MPO/H2O2/Cl- system) transformed an antiatherogenic lipoprotein particle into a modified lipoprotein with characteristics similar to lipoproteins commonly thought to initiate foam cell formation in vivo.

The toxicity to macrophages of oxidized low-density lipoprotein is mediated through lysosomal damage

Oxidized low-density lipoprotein (ox-LDL) has been shown to degrade poorly within the secondary lysosomes of macrophages but its possible effect on lysosomal integrity has received less attention. The effect of ultraviolet-C oxidized LDL (UVox-LDL) on cellular viability, and lysosomal membrane stability, was examined on cultured murine J-774 cells and human monocyte-derived macrophages (HMDMs). The acridine orange (AO) relocalization test was applied to study the lysosomal integrity of living cells. UVox-LDL dramatically reduced J-774 cell proliferation at a concentration of 25 microg/ml. Incubation with 5 microM copper alone, normally used to induce LDL oxidation, was also toxic. In contrast to the effects of ox-LDL, in concentrations up to 75 microg/ml, native LDL (nLDL) rather stimulated J-774 cell replication. Incubation with UVox-LDL (25-75 microg/ml) also altered cellular AO uptake, depending on time and dose: its lysosomal accumulation decreased and its cytosolic accumulation increased. This shift indicates damaged lysosomal membranes with decreased intralysosomal, and increased cytosolic, H+ concentration. Many J-774 cells exposed to UVox-LDL initially transformed into foam cells and then assumed an apoptotic-type morphology with TUNEL-positive nuclei. We conclude that ox-LDL is cytotoxic to macrophages due to oxidative damage of lysosomal membranes, with ensuing destabilization and leakage to the cytosol of lysosomal contents, such as hydrolytic enzymes, causing degeneration of apoptotic type.

Hypercholesterolemia increases manganese superoxide dismutase immunoreactive macrophages in myocardium

The effect of hypercholesterolemia on manganese superoxide dismutase (MnSOD)-containing macrophages was investigated in male New Zealand white rabbits. Macrophages from control animals, which were marked with the RAM-11 antibody, demonstrated co-localization with MnSOD immunoreactivity, e.g. in the peri- and paravascular space within the myocardium, but not in the bone marrow. In rabbits fed a 0.5% cholesterol-enriched diet for 42 days, a significant increase (P < 0.01) of MnSOD-immunoreactive macrophages within the myocardium was found concomitant to the drastic elevation of serum cholesterol level. In the bone marrow, MnSOD immunoreactivity did not change after cholesterol feeding. Thus in cholesterol-fed rabbits, the increase of MnSOD-containing macrophages seems to parallel that of lipoproteins. MnSOD is considered as being protective against the cytotoxic effects of those superoxide anions, possibly generated in macrophages, which are involved in the metabolism of modified lipoproteins.

Growth-stimulating effect of lipoproteins on human arterial smooth-muscle cells and lung fibroblasts is due to apo B-containing lipoproteins, type LDL and VLDL, and requires LDL receptors

Excessive growth of the arterial smooth muscle is essential for the development of atherosclerosis and leads to arterial insufficiency in several other conditions. It is therefore important to elucidate the mechanisms that regulate the growth of the human arterial smooth-muscle cell, SMC. Like other untransformed cells, SMC require plasma for sustained growth in vitro. As found in an earlier study most of the material in plasma which stimulates SMC growth is related to the lipoproteins (LP), and is widespread among LP of different density classes. In the present study we investigated whether the growth-stimulating activity might be more specifically related to certain lipoproteins defined by criteria other than density or particle size. Activity was assayed using human SMC and human lung fibroblasts as both a change of culture size and DNA synthesis. The growth-stimulating activity was confined to apo B-containing LP, as defined by their strong affinity to heparin-Sepharose, electrophoretic beta-mobility, the presence of apo B and the absolute requirement of low density lipoprotein (LDL) receptors for the growth-stimulating effect to appear. It was strongly potentiated by PDGF-BB. A much higher level of LDL was required to initiate synthesis of DNA in SMC than in fibroblasts but at optimal LDL concentration the degree of activation was similar for both cell types. Apo B-containing LP are very powerfully related to atherosclerosis. As intimal thickening is a primary change in atherogenesis, the growth-stimulating effect of them may be of direct pathogenetic importance.

Low density lipoproteins bind more to type I and III collagens by negative charge-dependent mechanisms than to type IV and V collagens

The accumulation of low density lipoprotein (LDL) in the arterial intima is an important characteristic of atherosclerosis. We investigated the mechanisms by which LDL binds to different types of collagen. The binding activities of 125I-labeled human native LDL (nLDL) and copper-oxidized LDL (oxLDL) with different collagen gels prepared in type I collagen-based mixtures with types I, III, IV and V (I+I, I+III, I+IV and I+V, respectively) were examined. A concentration of 20 micrograms LDL protein/150 micrograms collagen/well was used. The diffusion of both nLDL and oxLDL into the collagen gels reached an equilibrium after 48 h. All of the collagen gels showed the same rates of diffusion with both LDLs. The binding activities of oxLDL were significantly greater than those of nLDL (P < 0.001%), while the binding activities for both LDLs followed the order I+I and I+III > I+V > I+IV. However, the increased binding rate of oxLDL compared to nLDL was 1.66 for I+IV, 1.50 for I+V, 1.33 for I+I and 1.19 for I+III. When a 10-fold higher dose of NaCl (1 M) was added to the oxLDL medium, the binding rate of oxLDL was reduced (rate of reduction: 52% (I+I), 48% (I+III), 35% (I+IV), 13% (I+V)). These results suggest that oxLDL binds more to type I and III collagens by negative charge-dependent mechanisms than to type IV and V collagens. Therefore, types I and III collagens may play an important role in trapping LDL, especially oxLDL. Therefore, oxidatively modified LDL may contribute to atherogenesis due to its longer retention in the arterial wall.