Studies on the binding of plasma low density lipoproteins to arterial elastin

Preferential localization of varying forms of photoactive 1, 8-naphthalimide compounds within the atheromatous arterial wall

BACKGROUND AND OBJECTIVE

We are currently working with a novel class of photoactivated 4-amino substituted 1,8-naphthalimide compounds for tissue bonding. With promising results in other tissues, we are pursuing potential vascular applications. This study focused on determining the appropriate compound formulation(s), concentration, and exposure times to optimize penetration of the heterogeneous arterial wall.

STUDY DESIGN/MATERIALS AND METHODS

Segments of atheromatous rabbit carotid artery were immersed in hydrophilic or lipophilic forms of the compound, then frozen, cryosectioned, and examined by confocal microscopy.

RESULTS

The hydrophilic compound exhibited preferential localization within the intima and media and limited presence in the adventitia. Conversely, the lipophilic compound concentrated in the intima and adventitia with virtual exclusion from the media. Exposure to both forms resulted in complete penetration of the arterial wall.

CONCLUSION

These results extend our knowledge and permit a more practical approach to potential vascular applications using these photoactivated compounds for tissue bonding.

Accumulation of co-localised unesterified cholesterol and neutral lipids within vacuolised elastin fibres in athero-prone areas of the human aorta

To investigate whether there are alterations of elastin fibres in the arterial intima at the pre-atherosclerotic stage, grossly normal areas of human thoracic aorta were taken soon after death from 13 healthy trauma victims whose ages ranged from 16 to 40 years. Two areas were compared: atherosclerosis-prone (AP) areas localised to the dorsal aspect of the aorta along the rows of intercostal branch origins, and atherosclerosis-resistant (AR) areas from the ventral aorta. Electron microscopic analysis combined with cytochemical staining was applied. Unesterified cholesterol was identified using the filipin-staining technique while neutral lipids were visualised by the OTO-technique. Intimal features were studied by combining the filipin-staining and the OTO-technique. Electron microscopical examination showed that in both AR and AP areas, some elastin fibres in the intima were vacuolised. Unesterified cholesterol was found to be predominantly localised in the musculoelastic layer, in particular, inside the vacuolised elastin fibres. This localisation was seen in all 13 AP areas studied in contrast to the AR areas where it was observed in only four of 13 aortas studied (P < 0.0005, chi2-test). Accumulation of neutral lipids inside vacuolised elastin fibres was found in five out of 13 AP areas but was not observed in any of the AR areas (P=0.01, chi2). A combination of the filipin-staining and OTO-techniques showed that some deposits of neutral lipids and unesterified cholesterol within vacuolised elastin fibres were independently located from each other, but more frequently, neutral lipids were co-located with unesterified cholesterol. The present observations indicate a difference between AP and AR intimal areas which, in particular, relates to the structure of elastin fibres in the musculoelastic layer. The observations suggest that alterations of the extracellular matrix are involved in the trapping and retention of cholesterol and neutral lipids within the intima at an early stage in the development of atherosclerotic lesions.

In vitro interactions of oxidatively modified LDL with type I, II, III, IV, and V collagen, laminin, fibronectin, and poly-D-lysine

The accumulation of LDL in the arterial intima is considered a key event in atherogenesis. We investigated the binding of oxidized LDL (ox-LDL) to microtiter plates coated with type I or II collagen, laminin, fibronectin, or poly-D-lysine. Oxidation of LDL, 125I-LDL, or Eu(3+)-LDL was performed with CuCl2, varying the time of oxidation. Bound lipoprotein was assessed by counting radioactivity or fluorescence in the wells. Binding of highly ox-LDL in PBS followed the order: type I collagen > poly-D-lysine > type II collagen > laminin > fibronectin. Comparing various collagen types, the binding of ox-LDL followed the order: type I > type V and, type III > type IV > type II collagen. Binding of ox-LDL in PBS was dependent on an increase in negative charge of ox-LDL. Testing certain amino acids as competitors for binding of highly ox-LDL to type I collagen put lysine first, followed by arginine and histidine. On laminin, histidine competed most, followed by lysine and arginine. When studying the influence of Na+, K+, Ca2+, Mg2+ (equivalent to their concentrations in the interstitial fluid), native LDL, moderately ox-LDL, and highly ox-LDL showed the same affinity to type I collagen. However, a fivefold dilution of the buffer increased the affinity of moderately and highly ox-LDL 3.9- and 10-fold compared with native LDL. Application of the F(ab')2 from a monoclonal antibody to ox-LDL revealed a strong competition of the binding of highly ox-LDL to type II collagen (60%), laminin (35%), type I collagen (20%), and poly-D-lysine (15%), whereas the binding to fibronectin was not affected.

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.

Elastase-type enzymes and their relation to blood lipids in atherosclerotic patients

Serum elastase-type activity, elastase inhibitory capacity and their relation to lipids were examined in 140 male patients with ischemic vascular disease (coronary, cerebral, peripheral) and in 60 control subjects. In a further 24 patients with acute myocardial infarction elastase activity, inhibitory capacity and lipids during the course of the illness have also been investigated. Serum elastase-type activity was found to be significantly lower and inhibitory capacity significantly higher in the groups of patients than in the controls. HDL- and HDL2-cholesterol as well as apo A concentration showed significant negative correlation with elastase inhibitory capacity both in atherosclerotic and in control subjects. During the course of myocardial infarction a significant elevation of serum elastase-type activity could be observed at the end of the first week; serum triglyceride levels increased, HDL- and HDL2-concentrations decreased significantly in the first 3 weeks, than gradually approached the initial values. In the patients with an elevation of serum elastase-like activity by more than 30% in the first week, there was a significantly higher elevation of serum GOT and LDH1 and a greater occurrence of transmural (Q) infarction than in those with a smaller variation of elastase-like activity.

Interaction of low density lipoproteins with human aortic elastin

Interaction between lipoproteins and elastin in the arterial wall may play an important role in atherosclerotic lipid deposition, but binding affinities and other characteristics of the interaction have not been determined previously. Elastin was isolated by hot alkali treatment of human aortic tissue. At 4 degrees C, radioiodinated human low density lipoprotein (LDL) bound to more than one class of binding sites on elastin. Sites of highest affinity had an apparent dissociation constant of 3.6 x 10(-8) M. Total binding at an LDL concentration of 50 micrograms/ml ranged from 4 to 50 ng LDL protein/mg elastin. The binding was relatively specific, since binding was competitively inhibited by LDL and apo E-containing high density lipoprotein (HDL) but only modestly by HDL3. Atherosclerotic elastin exhibited a twofold to fourfold higher capacity for binding LDL, but a reduced affinity. At 37 degrees C, normal elastin exhibited an initial rapid binding of LDL, with a slower linear phase of binding over a 15-hour period, indicating an additional complex process at this temperature. Consideration of the expected LDL concentrations in the arterial intima, in comparison with binding affinities, suggests that LDL binding to elastin probably occurs in the intima and may foster atherosclerotic lipid deposition.

Human aortic fibrolipid lesions. Immunochemical localization of apolipoprotein B and apolipoprotein A

Comparative localization of apolipoproteins (apo) B and A in small, raised fibrolipid lesions was performed to determine whether low density lipoproteins (LDL) and high density lipoproteins (HDL) may be involved in the formation of the earliest identifiable lipid-rich core regions found in atherosclerotic fibrous plaques. Apo B was observed associated with the superficial layer of foam cells in collagenous areas of the lesion cap and within the lipid-rich core region. The lipid-rich core region was well-circumscribed by an intense band of apo B staining along both luminal and medial aspects. Apo A staining was confined to the noncellular elements of the fibrolipid lesion. Collagenous areas were speckled with fine, punctate granules of immunoreactive apo A. A homogeneous, granular apo A staining pattern was characteristic throughout the core region. With these data from a relatively early lesion in the development of the atherosclerotic fibrous plaque, we conclude that: 1) both LDL and HDL can accumulate in the lipid-rich core region since no preferential staining for apo B over apo A was observed in this region, and 2) the paucity of cell-associated apo A staining in small fibrolipid lesions suggests that HDL interacts with the arterial wall differently than does HDL.

Interaction of native and cell-modified low density lipoprotein with collagen gel

We have examined the binding of native and cell-modified low density lipoprotein (LDL) to gels of Type I collagen. Diffusion of native 125I-LDL into the collagen gel was slow, reaching equilibrium after 24 to 48 hours, while L-3H-glucose, a low molecular weight marker, equilibrated in 6 hours. Binding of 125I-LDL was measured at 48 hours as the amount associated with the collagen after extensive washing. Binding was saturable with an increasing concentration of LDL. Prior incubation with cell-free culture medium resulted in modest, but progressive, increases in electrophoretic mobility and binding to collagen. Incubation with cells produced a marked increase in electrophoretic mobility and a 5- to 10-fold increase in collagen binding; the presence of butylated hydroxytoluene during incubation prevented both effects. These changes in LDL were induced by porcine aortic endothelial cells, smooth muscle cells, human skin fibroblasts, and a variety of cell lines, as well as by acetylation. There was a curvilinear relationship between the amount of LDL protein bound and the net negative charge of the LDL; increasing net charge was associated with progressively greater increases in binding. These results suggest a potential role for collagen in trapping lipid in the extracellular matrix of arterial intima by slowing the diffusion of and by binding LDL. The data also demonstrate that binding of LDL to collagen is enhanced by modifications that increase its net negative charge.

Association of low-density lipoprotein with particulate connective tissue matrix components enhances cholesterol accumulation in cultured subendothelial cells of human aorta

Low-density lipoproteins (LDL) were incubated with elastin particles, collagenase-resistant debris isolated from human aorta, and latex beads of 1.13 microns in diameter. As a result of incubation, insoluble LDL-associates were formed. These associates, as well as LDL-heparin-fibronectin-gelatin complexes described by other workers, were added to a 7-day primary culture of enzyme-isolated cells of human aortic subendothelial intima. The culture contained a mixed cell population made up mostly of typical and modified smooth muscle cells. 24 h later, total cholesterol, phospholipid, triacylglycerol, free cholesterol and cholesteryl ester levels were measured. Addition of insoluble LDL-complexes as well as LDL-associates to culture brought about a substantial accumulation of intracellular lipids; primarily, cholesteryl esters. The total cholesterol level in cultured cells was raised 3- to 8-fold. Addition of free LDL or LDL-free particles had no effect on the content of intracellular lipids. The results obtained allow the assumption that the occurrence of the LDL-mediated accumulation of intracellular lipids is due mainly to the LDL penetration inside the cell via 'nonspecific' phagocytosis and not through a regulated receptor-dependent pathway.

Quantitative ultrastructural analysis of perifibrous lipid and its association with elastin in nonatherosclerotic human aorta

Nonatherosclerotic areas in human arteries display an age-related accumulation of cholesteryl ester in the form of small, perifibrous lipid droplets in the deeper intimal layers. We treated human aortic specimens with an osmium-thiocarbohydrazide-osmium sequence en bloc after glutaraldehyde fixation in order to provide electron dense staining of neutral lipid for ultrastructural study. Neutral lipid was quantified in terms of area fractions on thin sections. Extracellular lipid, primarily in the form of small (less than 300 nm) droplets, accounted for 91% of the lipid found in the deep intimal region. Seventy-four percent of extracellular lipid appeared in droplets or aggregates that were demonstrated as adjacent to or within elastic fibers in the plane of section. The fraction of lipid adjacent to elastin in three dimensions is likely to be considerably higher than 74%. The results support the concept that an interaction between elastin or its associated components and lipids or lipoproteins may be important in extracellular lipid deposition in human arteries.

The effect of procyanidolic oligomers on the composition of normal and hypercholesterolemic rabbit aortas

Rabbits were fed with normal (group 1 and 2) and cholesterol rich diets (group 3 and 4) concomitantly to a daily peroral administration of 50 mg/kg procyanidolic oligomers (PCO) to groups 2 and 4. After 10 weeks, the cholesterol content of the blood serum and the excised aortic intima-media were significantly higher in groups 3 and 4 than in groups 1 and 2. The DNA, hydroxyproline, uronic acid contents were similar in aortic dry weight basis in all four groups. The intima-media samples were extracted successively with 0.15 M NaCl, 0.02 M sodium phosphate pH 7.4 (NaCl extract) and with 4 M guanidinium chloride, 0.05 M sodium acetate pH 5.8 prior (G1 extract) and following (G2 extract) hydrolysis of the collagen with collagenase. The cholesterol contents of G1 extracts were higher in groups 2 and 4 than in groups 1 and 3. The cholesterol content of aortic elastin increased with cholesterol feeding (group 3). With simultaneous administration of cholesterol and PCO the cholesterol content of aortic elastin in group 4 was significantly lower than in group 3. The uronic acid contents increased in G1 extracts and in the collagenase digest with PCO treatment of both normal and hypercholesterolemic rabbits. The ratio of dermatan-sulphate to chondroitin-sulphate decreased with hypercholesterolemia (group 3) and with PCO (group 2 and 4). The parallelism between increased cholesterol and uronic acid contents and modified glycosaminoglycan composition in G1 extract, indicate that the interaction of cholesterol with macromolecules of the aorta can be modulated by PCO. This drug modifies the extractibility of aortic cholesterol and glycosaminoglycans and reduces the association of cholesterol to elastin.

Inhibitory effect of high density lipoprotein subfractions on the in vitro binding of low density lipoproteins to arterial elastin

The inhibitory effects of high density lipoprotein (HDL) subfractions on the in vitro complex formation between plasma low density lipoprotein (LDL) and arterial elastin were studied. The inhibitory effects were significantly higher with HDL3 than HDL2, and with HDL-without E than HDL-with E. The inhibitory effect of a phospholipid complex with apoHDL3 was higher than that with apoHDL2. In contrast with the inhibitory effects, the binding abilities of HDL2 and HDL-with E to elastin were significantly higher than those of HDL3 and HDL-without E. These results suggest that the inhibitory effects of HDL subfractions are not due to competitive binding with arterial elastin.