Modification of low density lipoproteins by secretory granules of rat serosal mast cells

Open tubular CE forin vitro oxidation studies of human very-low-density lipoprotein particles

Human very-low-density lipoprotein (VLDL) particles were immobilised on the inner wall of electrochromatographic fused-silica capillaries, and the applicability of these capillary columns in oxidation studies was investigated. Capillaries coated with radiolabelled VLDL particles showed a coating efficiency of 97%, and allowed estimation of the amount of VLDL present in a capillary. Radioactivity measurements and atomic force microscopy with tapping mode confirmed the presence of VLDL particles as a monolayer. The pI determined for the VLDL was 4.7-4.8 varying with the human source. The effects of VLDL concentration, coating time and pH on the coating stability were clarified, and the stability was examined in terms of the repeatability of EOF and retention factors of selected steroids. The repeatability of run-to-run and the coating-to-coating reproducibility ranged from 2.6 to 4.9% and 3.2 to 6.6%, respectively. The lifetime of a coating was at least 7 days or 84 consecutive runs. The in situ copper-mediated VLDL oxidation carried out in the capillary with optimised VLDL coating showed that, during the oxidation of VLDL particles, the negative charges of the particles are increased, leading to enhanced EOF mobilities. Several oxidation parameters, including copper sulfate concentration, amount of EDTA needed to stop the reaction, pH and the oxidation procedure, were examined. Effect of the oxidation process on the stability of the coating in one capillary, and in five different capillaries ranged between 0.4-4.1% and 0.8-6.6%, respectively. The in situ oxidation of VLDL particles was compared with that of low-density lipoproteins.

Open Tubular Capillary Electrochromatography: Technique for Oxidation and Interaction Studies on Human Low-Density Lipoproteins

A novel, open tubular capillary electrochromatographic method was developed for the in vitro oxidation of low-density lipoprotein (LDL) particles. Low-density lipoprotein particles with molar mass of approximately 2.5 MDa yielded a stable stationary phase at temperatures 25 and 37 degrees C and at pH values from 3.2 to 7.4. The quality of the coatings was not influenced by variations in the LDL concentration in the coating solutions (within the range of 2-0.015 mg/mL) with the coating procedure used in the study. Radiolabeled LDL stationary phases and scanning electron microscopy, employed to shed light on the location and coating density of LDL particles on the inner surface of the capillary wall, confirmed the presence of an LDL monolayer and almost 100% coating efficiency (99 +/- 8%). In addition, the radioactivity measurements allowed estimation of the amount of LDL present in a single capillary coating. Capillaries coated with human LDL particles were submitted to different oxidative conditions by changing the concentration of the oxidant (CuSO4), oxidation time, pH value, and temperature. The oxidation procedure was followed with electroosmotic flow mobility, which served as an indicator of the increase in total negative charges of LDL coatings, and by asymmetrical field flow fractionation, which measured the changes in size of the lipoprotein particles. The results indicated that oxidation of LDL was progressing with increasing time, temperature, and concentration of the oxidant as expected. The oxidation process was faster around neutral pH values (pH 6.5-7.4) and inhibited at acidic pH values (pH 5.5 and lower).

Sphingomyelinase Induces Aggregation and Fusion of Small Very Low–Density Lipoprotein and Intermediate-Density Lipoprotein Particles and Increases Their Retention to Human Arterial Proteoglycans

OBJECTIVE

Infiltration of low-density lipoprotein (LDL) into subendothelial space is an early step in atherosclerosis. In addition to LDL particles, small very low-density lipoprotein (sVLDL) and intermediate-density lipoprotein (IDL) particles are also able to enter the arterial intima and be retained within the subendothelial extracellular matrix. Here we compared how proteolysis with alpha-chymotrypsin and phospholipid hydrolysis with phospholipase A2 or sphingomyelinase (SMase) of sVLDL, IDL, and LDL particles can influence their aggregation, fusion, and binding to human arterial proteoglycans in vitro.

METHODS AND RESULTS

In each of the 3 lipoprotein classes, the particles became only slightly aggregated with alpha-chymotrypsin or phospholipase A2. However, the particles strongly aggregated when treated with SMase. The aggregated/fused particles were found to bind to proteoglycans in proteoglycan affinity chromatography more tightly than the native-sized counterparts. In addition, in a microtiter well assay, the binding of SMase-treated lipoproteins was enhanced: the amounts of proteoglycan-bound SMase-treated LDL, IDL, and sVLDL were 4-, 5-, and 20-fold higher, respectively, than the amounts of proteoglycan-bound native lipoproteins.

CONCLUSIONS

These results imply a specific role for SMase as an sVLDL- and IDL-modifying enzyme and also suggest a novel mechanism of lipid accumulation in atherogenesis, namely enhanced retention of atherogenic triglyceride-rich lipoprotein particles in intimal areas expressing extracellular SMase activity.

Cysteine protease cathepsin F is expressed in human atherosclerotic lesions, is secreted by cultured macrophages, and modifies low density lipoprotein particles in vitro

During atherogenesis, low density lipoprotein (LDL) particles in the arterial intima become modified and fuse to form extracellular lipid droplets. Proteolytic modification of apolipoprotein (apo) B-100 may be one mechanism of droplet formation from LDL. Here we studied whether the newly described acid protease cathepsin F can generate LDL-derived lipid droplets in vitro. Treatment of LDL particles with human recombinant cathepsin F led to extensive degradation of apoB-100, which, as determined by rate zonal flotation, electron microscopy, and NMR spectroscopy, triggered both aggregation and fusion of the LDL particles. Two other acid cysteine proteases, cathepsins S and K, which have been shown to be present in the arterial intima, were also capable of degrading apoB-100, albeit less efficiently. Cathepsin F treatment resulted also in enhanced retention of LDL to human arterial proteoglycans in vitro. Cultured monocyte-derived macrophages were found to secrete active cathepsin F. In addition, similarly with cathepsins S and K, cathepsin F was found to be localized mainly within the macrophage-rich areas of the human coronary atherosclerotic plaques. These results suggest that proteolytic modification of LDL by cathepsin F may be one mechanism leading to the extracellular accumulation of LDL-derived lipid droplets within the proteoglycan-rich extracellular matrix of the arterial intima during atherogenesis.

Effect of fluid mechanical stresses and plasma constituents on aggregation of LDL

LDL aggregates when exposed to even moderate fluid mechanical stresses in the laboratory, yet its half-life in the circulation is 2-3 days, implying that little aggregation occurs. LDL may be protected from aggregation in vivo by components of plasma, or by a qualitative difference in flows. Previous studies have shown that HDL and albumin inhibit the aggregation induced by vortexing. Using a more reproducible method of inducing aggregation and assessing aggregation both spectrophotometrically and by sedimentation techniques, we showed that at physiological concentrations, albumin is the more effective inhibitor, and that aggregation is substantially but not completely inhibited in plasma. Heat denatured and fatty-acid-stripped albumin were more effective inhibitors than normal albumin, supporting the idea that hydrophobic interactions are involved. Aggregation of LDL in a model reproducing several aspects of flow in the circulation was 200-fold slower, but was still inhibited by HDL and albumin, suggesting similar mechanisms are involved. Within the sensitivity of our technique, LDL aggregation did not occur in plasma exposed to these flows. Thus, as a result of the characteristics of blood flow and the inhibitory effects of plasma components, particularly albumin, LDL aggregation is unlikely to occur within the circulation.

Lipolysis of LDL by human secretory phospholipase A(2) induces particle fusion and enhances the retention of LDL to human aortic proteoglycans

The first morphological sign of atherogenesis is the accumulation of extracellular lipid droplets in the proteoglycan-rich subendothelial layer of the arterial intima. Secretory nonpancreatic phospholipase A(2) (snpPLA(2)), an enzyme capable of lipolyzing LDL particles, is found in the arterial extracellular matrix and in contact with the extracellular lipid droplets. We have recently shown that in the presence of heparin, lipolysis of LDL with bee venom PLA(2) induces aggregation and fusion of the particles. Here, we studied the effect of human snpPLA(2) on the integrity of LDL particles and on their interaction with human aortic proteoglycans. In addition, the capacity of the proteoglycans to retain PLA(2)-lipolyzed LDL particles was tested in a microtiter well assay. We found that lipolysis of LDL induced fusion of proteoglycan-bound LDL particles, which increased their binding strength to the proteoglycans. Moreover, lipolysis of LDL with snpPLA(2) under physiological salt and albumin concentrations induced a 3-fold increase in the amount of LDL bound to proteoglycans. The results imply a role for PLA(2) in the retention and accumulation of LDL to the proteoglycan matrix in atherosclerosis.

Inhibition of mast cell-dependent conversion of cultured macrophages into foam cells with antiallergic drugs

Degranulation of isolated, rat peritoneal mast cells in the presence of low density lipoprotein (LDL) induces cholesteryl ester accumulation in cocultured macrophages with ensuing foam cell formation. This event occurs when the macrophages phagocytose LDL particles that have been bound to the heparin proteoglycans of exocytosed granules. In an attempt to inhibit such foam cell formation pharmacologically, rat peritoneal mast cells that had been passively sensitized with anti-ovalbumin-IgE were treated with 2 mast cell-stabilizing antianaphylactic drugs, MY-1250 or disodium cromoglycate (DSCG). Both drugs were found to inhibit antigen (ovalbumin)-triggered release of histamine from the mast cells, revealing mast cell stabilization. In cocultures of rat peritoneal macrophages and passively sensitized mast cells, addition of MY-1250 before addition of the antigen resulted in parallel reductions in histamine release from mast cells, uptake of [(14)C]sucrose-LDL, and accumulation of LDL-derived cholesteryl esters in the cocultured macrophages. Similarly, when passively sensitized mast cells were stimulated with antigen in the presence of DSCG and the preconditioned media containing all substances released from the drug-treated mast cells were collected and added to macrophages cultured in LDL-containing medium, uptake and esterification of LDL cholesterol by the macrophages were inhibited. The inhibitory effects of both drugs were mast cell-specific because neither drug inhibited the ability of macrophages to take up and esterify LDL cholesterol. Analysis of heparin proteoglycan contents of the incubation media revealed that both drugs had inhibited mast cells from expelling their granule remnants. Thus, both MY-1250 and DSCG prevent mast cells from releasing the heparin proteoglycan-containing vehicles that bind LDL and carry it into macrophages. This study suggests that antiallergic pharmacological agents could be used in animal models to prevent mast cell-dependent formation of foam cells in vivo.

Modified LDL - trigger of atherosclerosis and inflammation in the arterial intima

Atherosclerosis is characterized by chronic inflammation of an injured intima. The pathological processes are initiated by accumulation of morphologically distinct, modified forms of LDL, and followed by cellular infiltration and foam cell formation. Activated intimal cells secrete enzymes and agents capable of modifying LDL, and the modified lipids of LDL, in turn, are able to activate intimal cells and to trigger various inflammatory signals. These processes can initiate and maintain a vicious circle in the intima and lead to lesion progression. In this review, we focus on the LDL modifications relevant to the initial lipid accumulation and discuss their pro-inflammatory effects.

Lipolytic modification of LDL by phospholipase A2 induces particle aggregation in the absence and fusion in the presence of heparin

One of the first events in atherogenesis is modification of low density lipoprotein (LDL) particles in the arterial wall with ensuing formation of aggregated and fused lipid droplets. The accumulating particles are relatively depleted in phosphatidylcholine (PC). Recently, secretory phospholipase A2 (PLA2), an enzyme capable of hydrolyzing LDL PC into fatty acid and lysoPC molecules, has been found in atherosclerotic arteries. There is also evidence that both LDL and PLA2 bind to the glycosaminoglycan (GAG) chains of extracellular proteoglycans in the arterial wall. Here we studied the effect of heparin GAG on the lipolytic modification of LDL by PLA2. Untreated LDL, heparin-treated LDL, and heparin-bound LDL were lipolyzed with bee venom PLA2. In the presence of albumin, lipolysis resulted in aggregation in all 3 preparations of the LDL particles. Lipolysis of untreated LDL did not result in aggregation if albumin was absent from the reaction medium, and the lipolytic products accumulated in the particles rendering them negatively charged. However, heparin-treated and heparin-bound lipolyzed LDL particles aggregated even in the absence of albumin. Importantly, in the presence of albumin, some of the heparin-treated and heparin-bound lipolyzed LDL particles fused, the proportion of fused particles being substantially greater when LDL was bound to heparin during lipolysis. In summary, lipolysis of LDL PC by PLA2 under physiological conditions, which allow transfer of the lipolytic degradation products to albumin, leads to fusion of LDL particles in the presence, but not in the absence, of heparin. Thus, it is possible that within the GAG meshwork of the arterial intima, PLA2-induced modification of LDL is one source of the lipid droplets during atherogenesis.

Sphingomyelinase induces aggregation and fusion, but phospholipase A2 only aggregation, of low density lipoprotein (LDL) particles. Two distinct mechanisms leading to increased binding strength of LDL to human aortic proteoglycans

During atherogenesis, low density lipoprotein (LDL) particles bind to extracellular matrix proteoglycans in the arterial wall, become modified, and appear as aggregated and fused particles. Sphingomyelinase (SMase) and phospholipase A2 (PLA2) have been found in the arterial wall, and, moreover, lesional LDL shows signs of hydrolysis of both sphingomyelin and phosphatidylcholine. We have now studied the effects of these two lipolytic modifications on the aggregation and fusion of LDL particles by hydrolyzing the particles with Bacillus cereus SMase or bee venom PLA2. In addition, the binding strengths of the modified LDL to human aortic proteoglycans (PG) were analyzed on an affinity column. We found that SMase induced aggregation and fusion of LDL, but PLA2 induced only aggregation of the particles. In addition, the SMase-induced aggregation and fusion of LDL was promoted by pretreatment of LDL with PLA2. Determination of the binding strengths of the hydrolyzed LDL revealed that mere lipolysis of LDL without aggregation or fusion, either by SMase or PLA2, did not affect the binding of the particles to PG. Aggregation and fusion of lipolyzed LDL particles, however, increased their strength of binding to PG. Active lysine residues in apolipoprotein B-100 (apoB-100) appear to be involved in the binding of LDL to PG, and, in fact, quantitative 13C NMR analysis revealed that, in the fused LDL particles, the number of active lysine residues per apoB-100 moiety was increased. Moreover, aggregation and fusion of LDL increased the number of apoB-100 copies and, consequently, the number of active lysine residues per aggregate or fused particle. Our present findings therefore (i) show that treatment of LDL with SMase and PLA2 generates modified LDL particles, which then bind to human aortic PG with increased strength, and (ii) suggest that SMase- and PLA2-induced aggregation and fusion of LDL are potential mechanisms leading to focal retention of extracellular lipid in the arterial wall.

Development of the lipid-rich core in human atherosclerosis

In recent years the role of the atherosclerotic core in promoting plaque rupture has become well recognized. A new insight into core development is its origination early in atherogenesis, before formation of the fibrous plaque. The early core is associated with accumulation of vesicular lipid rich in free cholesterol. Later in core development, lipid deposits become more diverse. The weight of evidence points toward a direct extracellular process, probably lipoprotein aggregation and fusion, as the chief pathway of cholesteryl ester accumulation, although foam cell death may also contribute cholesteryl ester. The mechanism or mechanisms of formation of vesicular, cholesterol-rich deposits are unknown. Since the increase in free cholesterol is likely to have deleterious effects on cells bordering the core, the further elucidation of cellular and biochemical pathways leading to and responding to free cholesterol accumulation is of great importance. Complement activation and cellular stress responses are prominent in the vicinity of core lipids, but their pathogenetic roles remain to be established. Since the core appears so early in atherogenesis, these as well as other, yet to be determined cellular responses to core lipids, oxidized and unoxidized, could have a considerable effect on overall lesion development. Much remains to be learned about macrophage and smooth muscle responses, calcification, capillarization, and matrix protein alterations in the evolution of the core and surrounding arterial intima.

Proteolysis and fusion of low density lipoprotein particles strengthen their binding to human aortic proteoglycans

Lipid droplets resembling those seen in the extracellular space of the arterial intima were generated in vitro when granule proteases of rat serosal mast cells degraded the apolipoprotein B-100 (apoB-100) component of granule-bound low density lipoprotein (LDL), and the particles fused on the granule surface (Paanenen, K., and Kovanen, P. T. (1994) J. Biol. Chem. 269, 2023-2031). Moreover, the binding of the fused particles to the heparin proteoglycan component of the granules was found to be strengthened. We have now treated LDL particles with alpha-chymotrypsin and examined the strength with which the proteolytically modified LDL binds to human aortic proteoglycans on an affinity column. We found that chymotryptic degradation of the LDL particles triggered particle fusion. The higher the degree of proteolytic degradation, the higher were the degree of fusion and the strength of binding to the aortic proteoglycans. Separation of the proteolyzed particles by size exclusion chromatography into two fractions, unfused and fused particles, and analysis of their binding strengths revealed that not only the fused but also the unfused proteolyzed particles bound more tightly to the proteoglycans than did the native LDL particles. To investigate the mechanism underlying this increase in binding strength, we attached [13C]dimethyl groups to the lysines and used NMR spectroscopy to quantify the active lysine residues of apoB-100, which are thought to be located in basic areas of apoB-100 and involved in binding of LDL to proteoglycans. Analysis of the 13C-labeled particles showed that, despite loss of apoB-100 fragments from the particles, the number of active lysine residues in the unfused proteolyzed particles had not decreased. In the fused proteolyzed particles, the number of active lysine residues was markedly increased. Thus, proteolytic fusion appears to increase the number of basic domains of apoB-100, which would explain the observed increase in the strength of binding of the modified LDL particles to arterial proteoglycans. Since the fused particles resemble the small lipid droplets found in the atherosclerotic arterial intima, this LDL modification offers a plausible mechanism for the focal accumulation of lipid droplets in the extracellular proteoglycan matrix during atherogenesis.

Mast cells of two types differing in neutral protease composition in the human aortic intima. Demonstration of tryptase- and tryptase/chymase-containing mast cells in normal intimas, fatty streaks, and the shoulder region of atheromas

Biochemical studies in vitro have demonstrated that stimulated mast cells induce macrophage foam cell formation through the synergistic action of mast cell granule neutral proteases and proteoglycans. To determine the presence and number of mast cells in human arterial intima, the site of atherogenesis, specimens of normal and atherosclerotic human aortic intima from 35 autopsies of persons ranging from 13 to 67 years old were stained with monoclonal antibodies against the two major proteases of mast cells, tryptase and chymase. All mast cells present were found to contain tryptase, and an average of 40% contained chymase as well. In sections of normal intimas, fatty streaks, and atheromas, the mast cells had average densities of 15/mm2, 15/mm2, and 3/mm2, respectively. In contrast to the normal intimas and fatty streaks, however, the atheromas had mast cells distributed unevenly in a typical pattern: 8/mm2 in the shoulder region, 1/mm2 in the fibrous cap, and none in the core region. In normal intimas, fatty streaks, and the shoulder region of atheromas, the mast cells amounted to 3% of all nucleated cells. The ratios of mast cells to T lymphocytes and to macrophages, respectively, were 2:1 and 1:4 in normal intimas, 1:3 and 1:10 in fatty streaks, and 1:5 and 1:20 in the shoulder region of atheromas. Thus, among the blood-borne cells in the human aortic intima, mast cells compose a significant cell population, and in terms of their protease content, these intimal mast cells are heterogeneous.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for loss of apo B from LDL in human atherosclerotic lesions: extracellular cholesteryl ester lipid particles lacking apo B

Previous studies have demonstrated the accumulation of low density lipoprotein (LDL) in the extracellular spaces of the intima of normal and atherosclerotic human vessels. In this study we have assessed the degree of colocalization in vessels of apolipoprotein B (apo B), the major protein of LDL, with cholesteryl ester, the predominant lipid of LDL. Apo B was detected immunohistochemically and cholesteryl ester was detected after its enzymatic hydrolysis and staining with the fluorescent probe, filipin. Most normal intima showed apo B staining without associated cholesteryl ester staining. This result would be expected with LDL having intact apo B; intact apo B interferes with hydrolysis and filipin staining of LDL cholesteryl ester. Fatty streaks and fibrous plagues showed regions of congruent apo B and cholesteryl ester staining in the extracellular space, suggesting fragmentation of apo B without loss of its immunoreactivity. Still other areas of lesions showed cholesteryl ester staining in the extracellular space without apo B staining. This staining pattern suggests loss of apo B from LDL leaving only the cholesteryl ester-rich core of LDL. Progressive loss of apo B from LDL can explain the patterns of apo B and cholesteryl ester colocalization that occur in vessel wall intima. The distribution of these patterns in normal and atherosclerotic lesions suggests that loss of apo B from the cholesteryl ester core of LDL is associated with lesion development.

Proteolytic degradation of low-density lipoprotein by lipoprotein(a) and by recombinant apo(a)

The plasma concentration of lipoprotein(a) (Lp(a)) is correlated with the risk of atherosclerosis, and both Lp(a) and LDL are present in atherosclerotic lesions. Lp(a) is similar in structure to LDL, its distinguishing feature from LDL being the presence of one additional glycoprotein, apo(a), that is linked to apoB-100. Upon incubation of 125I-LDL with isolated Lp(a), we found a dose and time-dependent increase in the proportion of TCA-soluble radioactive material, demonstrating degradation of LDL. The addition of unlabelled LDL decreased the degradation of 125I-LDL, while HDL or albumin had no such effect. Recombinant DNA-derived apo(a), R-apo(a), which itself expressed no amidolytic activity, displayed an increase in amidolytic activity after pre-incubation with LDL. Furthermore, activated R-apo(a) caused degradation of 125I-LDL. Treatment of R-apo(a) with phenylmethanesulfonyl fluoride inhibited LDL apoB-100 degradation, indicating that R-apo(a) has serine esterase type proteolytic activity. The results show that apo(a) is activated in the presence of LDL, and that this activation leads to proteolytic modification of LDL. The induction of apo(a) proteolytic activity by LDL suggests a novel mechanism whereby Lp(a) may be atherogenic.

Metabolism of LDL in mast cells recovering from degranulation. Description of a novel intracellular pathway leading to proteolytic modification of the lipoprotein

Rat serosal mast cells contain cytoplasmic secretory granules composed of a proteoglycan matrix in which histamine and neutral proteases are embedded. On stimulation, these granules are exocytosed, but some of them remain in the degranulation channels where on exposure to the extracellular fluid, they lose their histamine and a fraction of their proteoglycans. In vitro, such granule remnants efficiently bind low density lipoprotein (LDL) present in the incubation medium. After a lag period of about 10 minutes, the granule remnants, still within the channels and coated with LDL particles, are internalized by the parent mast cells. During subsequent recovery from degranulation, the apolipoprotein B of the intracellularly located remnant-bound LDL becomes efficiently (up to 70%) degraded by the proteolytic enzymes of the granule remnants. Since the granule remnants lack cholesteryl esterase activity, no LDL cholesterol is made available for cellular nutrition. Instead, selective proteolytic degradation of the bound LDL leads to formation of LDL particles enlarged by fusion on the granule remnant surface. In response to restimulation of the mast cells, about 50% of the fused LDL particles are exocytosed with the granule remnants. Of these, about one in five are expelled into the incubation medium. The granule remnants that again remain in the degranulation channels bind and internalize more LDL. This "round trip" of LDL in mast cells exposed to repeated stimulation constitutes a hitherto-unknown intracellular pathway for modification of LDL.

Inhibition of copper-mediated oxidation of LDL by rat serosal mast cells. A novel cellular protective mechanism involving proteolysis of the substrate under oxidative stress

Rat serosal mast cells, when stimulated to exocytose their cytoplasmic granules, effectively blocked the copper-mediated oxidation of low density lipoproteins (LDLs) in vitro. This effect depended on the proteolytic activity of the formed extracellular granule remnants, since specific inhibition of chymase, the neutral protease that they contain, blocked the protective effect of the mast cells. The mechanism of this chymase-mediated inhibition of LDL oxidation was found to be binding of the copper ions present in the incubation medium by peptides released from LDL on proteolytic degradation of their apolipoprotein B (apoB) component. This was verified by demonstrating that addition of such peptides to LDL--copper ion mixtures completely prevented oxidation of LDL and that this protective effect could be overcome by adding copper ions in excess. Furthermore, proteolytic degradation of the apoB of LDL, with concomitant release of copper-containing peptides, left the partially degraded apoB without the copper ions necessary for propagation of LDL oxidation. These observations provide the first evidence for cell-mediated inhibition of LDL oxidation.

Mast cell granule-mediated uptake of low density lipoproteins by macrophages: a novel carrier mechanism leading to the formation of foam cells

Mast cells are present in the arterial intima, the site of atherogenesis. To gain insight into the possible role of mast cells in the formation of the cholesterol-loaded macrophage foam cells typical of both early and late atherosclerotic lesions, a model system was developed in which isolated rat serosal mast cells were incubated with mouse peritoneal macrophages in medium to which low-density lipoproteins (LDL) had been added. Stimulation of the mast cells was found to induce a 50-fold enhancement of LDL uptake by the macrophages, which concomitantly accumulated LDL-derived cholesterol. This process, called the "granule-mediated uptake of LDL", involves the following steps: (i) exocytosis of the cytoplasmic granules of the mast cells, (ii) escape of soluble granule components, such as histamine and a fraction of the granule heparin proteoglycans into the medium, leaving granule remnants consisting of neutral proteases embedded in a heparin proteoglycan matrix, (ii) binding of LDL to binding sites on the glycosaminoglycan side chains of the heparin proteoglycan component of the granule remnants, (iv) proteolytic degradation of the bound LDL by the neutral proteases of the granule remnants, (v) fusion of degraded LDL particles on the surfaces of the granule remnants, and (vi) phagocytosis of the LDL-laden granule remnants by the macrophages. Simultaneously, the soluble heparin proteoglycans, to which no proteolytic enzymes are bound, interact with LDL with formation of insoluble complexes which are also phagocytosed by the macrophages.(ABSTRACT TRUNCATED AT 250 WORDS)