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Query: UNIPROT:P15088 (mast cell)
 14,925 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

When low density lipoprotein (LDL) is incubated with granules isolated from rat serosal mast cells, a fraction of LDL is bound to the granule heparin proteoglycan. If incubation is continued at 37 degrees C, the bound LDL, but not the unbound LDL, is degraded by granule neutral proteases. In the early stage of incubation, all the granule-bound LDL can be released by 0.3 M NaCl (the "salt-sensitive" fraction of LDL). With time, an increasing proportion of the granule-bound LDL requires 0.5 M NaCl for release (the "salt-resistant" fraction of LDL). Chemical analysis showed that, on average, 20% of the apolipoprotein B LDL was lost from the salt-sensitive fraction and 60% from the salt-resistant fraction, without any change in the composition of the lipid portion. Electron microscopic analysis disclosed large fused particles of LDL (diameters up to 100 nm) in the highly proteolyzed salt-resistant fraction, but no fused particles could be found in the less proteolyzed salt-sensitive fraction. We conclude that both binding and extensive degradation of LDL by mast cell granules is required for fusion of LDL particles on the granule surface. As compared with native LDL, the mast cell granule-modified LDL particles exhibit (i) increased particle size, (ii) selective loss of protein (apoB), (iii) a decrease in hydrated density, and (iv) stronger ionic interaction between apoB and heparin proteoglycan. The particles resemble the extracellular lipid droplets found in atherosclerotic lesions of both man and animals. Modification of LDL by mast cells may therefore provide a model of how these lipid structures are formed.
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PMID:Modification of low density lipoproteins by secretory granules of rat serosal mast cells. 199 27

Secretory granules exocytosed from rat serosal mast cells bind low density lipoprotein (LDL), and on being phagocytosed by macrophages, carry the bound LDL into these cells (Kokkonen, J. O., and Kovanen, P. T. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 2287-2291). The binding of LDL to the granules is mediated through interactions between the apolipoprotein B (apoB) component of LDL and the heparin proteoglycan component of the granules. Here we report how degradation of apoB by the neutral proteases of the granules affects the granule-mediated uptake of LDL by cultured mouse macrophages. During incubation of LDL with proteolytically inactive granules, the rate of uptake of LDL by macrophages increased by 10-fold; whereas during incubation with proteolytically active granules, it increased by 50-fold, the increase in the rate of uptake during proteolysis correlating with the degree of apoB degradation. The 5-fold greater capacity of the proteolytically active granules to enhance the uptake of LDL resulted from their greater capacity to bind LDL, and consequently, to carry it into the macrophages. Electron microscopic analysis of LDL bound to the proteolytically active granules disclosed large spherical particles of fused LDL. The diameters of the granule-bound particles ranged up to 90 nm compared with an average diameter of 22 nm for both native LDL and the LDL bound to proteolytically inactive granules. The results show that granule proteases, by inducing fusion of granule-bound LDL, increase the amount of LDL bound per unit weight of granule heparin proteoglycan. Hence, the two components of mast cell granules, the proteases and the heparin proteoglycan, act in concert to promote the uptake of LDL by macrophages in vitro.
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PMID:Proteolytic enzymes of mast cell granules degrade low density lipoproteins and promote their granule-mediated uptake by macrophages in vitro. 265 92

Macrophages possess a number of surface receptors that are capable of mediating the internalization of lipoproteins. The low-density lipoprotein (LDL) receptor of human monocyte macrophages recognizes apolipoprotein B-100 and apolipoprotein E and is rapidly regulated in response to changes in intracellular cholesterol levels. In contrast, in J774 macrophages LDL receptor regulation is defective and LDL can cause massive cholesterol accumulation. The beta migrating very low density lipoprotein (beta-VLDL) receptor is poorly regulated by cellular cholesterol concentrations, readily recognizes apolipoprotein E, poorly recognizes apolipoprotein B-100, and is immunologically related to the LDL receptor. The scavenger receptor (acetyl-LDL receptor) appears to have a molecular weight of 250,000 and is not regulated by cellular cholesterol levels. This receptor recognizes LDL that has been chemically or biologically altered. LDL complexes can also enter macrophages and cause cholesterol accumulation. Examples of such complexes are LDL-dextran sulphate complexes, LDL-proteoglycan aggregates, LDL-mast cell granule complexes, LDL-heparin-fibronectin-denatured collagen complexes, and LDL-antibody complexes. The entry of lipoprotein into macrophages by a pathway that is poorly regulated or is not regulated by cellular cholesterol concentrations appears to be a prerequisite for the formation of arterial foam cells.
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PMID:Macrophage lipoprotein receptors. 285 2

The secretory granules of rat serosal mast cells are able efficiently to degrade the apolipoprotein B component of low density lipoproteins (LDL) Kokkonen, J. O., and Kovanen, P. T. (1985) J. Biol. Chem. 260, 14756-14763). The granules are known to contain two neutral proteases with complementary specificities: a chymotrypsin-like endopeptidase called chymase, and an exopeptidase, the granule carboxypeptidase A. The role of this enzyme pair in the proteolytic degradation of LDL was studied with the aid of specific enzyme inhibitors. Incubation of LDL with intact granules (both enzymes active) led to the formation of numerous low molecular weight peptides and the liberation of free amino acids, most of which (95%) were aromatic (Phe, Tyr, Trp) or branched-chain aliphatic (Leu, Ile, Val). Selective inhibition of granule carboxypeptidase A (leaving chymase active) blocked the liberation of free amino acids, but left the formation of peptides uninhibited. On the other hand, selective inhibition of granule chymase (leaving carboxypeptidase A active) totally abolished the proteolytic degradation of LDL. The results are consistent with a model according to which the proteolytic degradation of LDL by mast cell granules results from coordinated action of the two granule-bound enzymes, whereby the chymase first cleaves peptides from the apolipoprotein B of LDL, and thereafter the carboxypeptidase A cleaves amino acids from the peptides formed.
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PMID:Low density lipoprotein degradation by secretory granules of rat mast cells. Sequential degradation of apolipoprotein B by granule chymase and carboxypeptidase A. 353 21

The interaction between rat serosal mast cells and low density lipoproteins (LDL) was studied in vitro. When rat 125I-LDL was incubated with mast cells, it was bound to a binding site on the mast cell surface but was not internalized by the cells. Even though 125I-LDL was not internalized, its protein component, apolipoprotein B, was rapidly degraded. The proteolytic activity responsible for the degradation of apolipoprotein B was present in the extracellular fluid of mast cells. It could be shown that the degradation was caused entirely by specific cell organelles of mast cells, the granules, which were spontaneously released into the extracellular fluid during preparation and incubation of the cells. In contrast to uncontrolled spontaneous degranulation, a controlled specific degranulation of mast cells can be induced by treating the cells with the compound 48/80. When increasing amounts of 48/80 were added to mast cell suspensions, a dose-dependent release of granules was observed and an increase in the rate of 125I-LDL degradation resulted. The increase in 125I-LDL degradation closely followed the increase in granule release. Thus, a quantitative relationship between the amount of granules present in the extracellular fluid and the amount of degradation of 125I-LDL could be established. The apolipoprotein part of LDL was extensively degraded by isolated mast cell granules. Analysis by polyacrylamide gel electrophoresis showed that upon incubation of LDL with isolated granules, the apolipoprotein B band rapidly disappeared with simultaneous appearance of several low molecular weight bands. The degradation of 125I-LDL by mast cell granules proceeded optimally at neutral pH and at physiological ionic strength. The results show that mast cell granules are able to efficiently degrade LDL in vitro, once released from mast cells into the extracellular fluid.
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PMID:Low density lipoprotein degradation by rat mast cells. Demonstration of extracellular proteolysis caused by mast cell granules. 390 39

In experimental studies in vitro, mast cells have induced uptake of apolipoprotein B-100 (apoB-100)-containing low-density lipoproteins by macrophages, with the subsequent formation of foam cells, the hallmarks of atherosclerosis. Recently, increased numbers of activated, ie, degranulated, mast cells were found to be present in human coronary fatty streaks and atheromas. We therefore sought evidence of a connection between mast cells and foam cell formation in vivo. In electron microscopic studies of human aortic and coronary fatty streaks and atheromas, exocytosed cytoplasmic secretory granules of mast cells were detected in the vicinity of their parent cells. These exocytosed granules had bound apoB-100-containing lipoproteins, as indicated by their positive staining with MB 47, a monoclonal antibody against apoB-100. A smooth muscle cell was observed to be in the process of phagocytosing one such exocytosed granule, and in the vicinity of a degranulated mast cell a foam cell contained an ingested mast cell granule. Therefore, the micrographs show that exocytosed granules of intimal mast cells may contribute to intimal foam cell formation and suggest a role for mast cells in human atherogenesis. More generally, the findings provide evidence that phagocytosis of apoB-100-carrying particles is one mechanism by which lipoproteins enter human arterial intimal cells.
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PMID:Extracellular mast cell granules carry apolipoprotein B-100-containing lipoproteins into phagocytes in human arterial intima. Functional coupling of exocytosis and phagodytosis in neighboring cells. 758 88

During atherogenesis, lipid droplets appear in the extracellular space of the arterial intima. We previously observed generation of lipid droplets on the surface of exocytosed mast cell granules when granule neutral proteases degraded the granule-bound LDL particles and the particles became unstable and fused [Kovanen, P.T., & Kokkonen, J.O. (1991) J. Biol. Chem. 266, 4430-4436]. We have now extended our studies to the fluid phase and examined the effects of several proteases (trypsin, alpha-chymotrypsin, Pronase, plasmin, kallikrein, and thrombin) all known for their ability to cleave the apolipoprotein B-100 component (apoB-100) of LDL. The fused LDL particles were separated from unfused particles by gel filtration or by density gradient ultracentrifugation. Proteolytic degradation of LDL with trypsin, alpha-chymotrypsin, or Pronase led to fragmentation of apoB-100 and release of the fragments from the LDL particles and triggered particle fusion. In contrast, proteolytic degradation of LDL with plasmin, kallikrein, or thrombin, which also led to fragmentation of apoB-100 but not to release of fragments, did not trigger particle fusion. With advancing degradation of apoB-100, particles having progressively lower densities and larger sizes were generated. Thus, after incubation for 24 h with alpha-chymotrypsin (apoB-100:alpha-chymotrypsin mass ratio 10:1) 40% of the apoB-100 was degraded and about 30% of the LDL particles had fused and reached diameters of up to 70 nm and densities ranging from 1.020 to < 1.005 g/mL. When the proteolyzed LDL particles, both unfused and fused, were incubated with macrophages, only those particles that had undergone fusion were ingested and converted into intracellular cholesteryl ester droplets. Thus proteolysis of LDL with release of apoB-100 fragments renders the particles sufficiently unstable to fuse and thus to become liable to ingestion by macrophages. Since the fused LDL particles resemble the extracellular lipid droplets in the atherosclerotic arterial intima and generate foam cells in vitro, these findings support the idea that proteolytic fusion of LDL is an atherogenic process.
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PMID:Fusion of proteolyzed low-density lipoprotein in the fluid phase: a novel mechanism generating atherogenic lipoprotein particles. 764 Feb 66

Contact between low density lipoproteins (LDL) and exocytosed mast cell granules, the "granule remnants," leads to binding of LDL to the granule remnants via ionic interactions between the apolipoprotein B-100 (apoB-100) component of LDL and the heparin proteoglycan component of the granule remnants. Upon incubation at 37 degrees C, the heparin proteoglycan-bound apoB-100 is progressively proteolyzed by remnant chymase and carboxypeptidase A, which are also bound to the heparin proteoglycans. Thereupon, the LDL particles fuse, and their binding to the granule remnants strengthens, as defined by the decreased ability of NaCl to release LDL from the remnants. We now have examined separately the effects of proteolysis and fusion on LDL binding. Proteolysis without fusion was induced by lowering the incubation temperature to 15 degrees C, and proteolysis-independent fusion was induced by treating granule remnant-bound LDL with sphingomyelinase in the presence of protease inhibitors. It was found that degradation of the heparin proteoglycan-bound apoB-100, even without accompanying particle fusion, increased the strength of LDL binding to the granule remnants, suggesting exposure of buried heparin binding regions of apoB-100. When such proteolyzed LDL particles were allowed to fuse, the strength of their binding to the granule remnants increased still further, probably because of an increase in the number of apoB-100 fragments in the enlarged particles. Proteolysis-independent fusion, induced by sphingomyelinase treatment of granule remnant-bound LDL, also increased the strength of binding. The results show that proteolytic degradation and fusion, the two modifications of granule remnant-bound LDL subsequent to action by chymase and carboxypeptidase A of the granule remnants, represent two separate mechanisms by which LDL particles become tightly bound to the heparin proteoglycans of exocytosed mast cell granules. Since the formation of an atheroma, the hallmark of atherosclerosis, is characterized by accumulation in the proteoglycan matrix of the arterial intima of extracellular lipid droplets resembling the fused LDL particles on the granule remnant surfaces, the modifications of LDL described in this study may provide a clue to the actual processes by which the lipid droplets are anchored to the arterial intima.
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PMID:Proteolysis and fusion of low density lipoprotein particles independently strengthen their binding to exocytosed mast cell granules. 829 53

Mast cells and macrophages coexist in the human arterial intima where oxidation of low density lipoproteins (LDL) also takes place during atherosclerosis. To investigate whether mast cells play a role in macrophage-mediated oxidation of LDL, a model system was designed in which mast cells and macrophages were cocultured in incubation medium containing LDL. Stimulation of rat serosal mast cells to induce exocytosis of their cytoplasmic granules was found to inhibit macrophage-mediated oxidation of LDL. The inhibitory effect depended on the ability of mast cell-derived histamine, released from the exocytosed granules into the medium, to bind the copper ions necessary for propagation of the macrophage-initiated oxidation of LDL. In addition to binding free copper ions, the mast cell-derived histamine was also capable of inhibiting oxidation of LDL propagated by copper ions bound to the apolipoprotein B component of the LDL particle. The results indicate that mast cells may prevent cell-mediated oxidation of LDL and imply a potentially preventive role for the mast cell in atherosclerosis.
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PMID:Inhibition of macrophage-mediated low density lipoprotein oxidation by stimulated rat serosal mast cells. 846 3

Recent findings have helped to explain the fate of cholesterol entering the arterial wall. LDL can undergo both fusion and aggregation. These changes may cause increased retention of LDL in lesion connective tissue matrix and LDL uptake by macrophages. In the cornea, apparent fusion of LDL occurs in the absence of macrophages. Mast cells may be important in LDL fusion, as mast cell-derived proteases can induce fusion of LDL through proteolysis of apolipoprotein B. LDL in arterial wall atherosclerotic lesions was found to be sialic acid-poor and ceramide-enriched. These chemical changes promote LDL aggregation. Processes that may function to remove cholesterol from the arterial wall have been reported. Macrophage-produced apolipoprotein E can mediate macrophage cholesterol efflux and macrophages can convert cholesterol to 27-oxygenated products that macrophages excrete. Alternately, another oxygenated sterol, 7-ketocholesterol, impairs macrophage cholesterol efflux. In addition, mast-cell derived chymase proteolyses HDL and reduces its capacity to stimulate cholesterol efflux.
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PMID:The fate of lipoprotein cholesterol entering the arterial wall. 933 47


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