beta-VLDL and acetylated-LDL binding to pigeon monocyte macrophages

The pathogenesis of foam cell formation: modified LDL stimulates uptake of co-incubated LDL via macropinocytosis

Previously, modified LDLs were shown to stimulate macropinocytosis in pigeon macrophages. Simultaneous intracellular trafficking of LDL and AcLDL, differentially labeled with colloidal gold, was done to determine whether uptake of LDL, which does not cause foam cell formation, was internalized via a separate route from AcLDL, which stimulates foam cell formation. AcLDL and LDL were followed at either low (12 microg/mL) concentrations near the saturation of high affinity binding sites or high (50 to 150 microg/mL) lipoprotein concentrations used to induce foam cell formation. The colloidal gold distribution and percentage of co-labeling as observed by transmission electron microscopy were determined for organelles involved with coated-pit endocytosis or macropinocytosis. LDL simultaneously incubated with AcLDL on macrophages at the low concentration was predominately internalized via coated-pit endocytosis. AcLDL was internalized via both coated-pit endocytosis and macropinocytosis at low concentration. At higher lipoprotein concentrations (50 to 150 microg/mL), AcLDL continued to be internalized via macropinocytosis. Interestingly, a significant portion of the co-incubated LDL, at high concentrations, also trafficked via macropinocytosis. LDL internalized by macropinosomes at high lipoprotein concentrations suggests that AcLDL-stimulated macropinocytosis might increase uptake of co-incubated lipoproteins. When (125)I-LDL was incubated with cold AcLDL, LDL degradation at 37 degrees C doubled, without a corresponding increase in cell association or total binding of LDL at 4 degrees C. These studies suggest that modified LDL-stimulated macropinocytosis is a mechanism for increased degradation of co-incubated LDL potentially leading to foam cell formation.

Modified LDLs induce and bind to membrane ruffles on macrophages

Macrophage foam cell formation in vitro requires uptake of modified low density lipoproteins (LDL) such as acetylated LDL (AcLDL) and moderately oxidized LDL (OxLDL), or beta-migrating very low density lipoprotein (betaVLDL), a naturally occurring lipoprotein. Incubation ofmacrophages with AcLDL and OxLDL resulted in stimulation of membrane ruffle formation, while betaVLDL primarily resulted in increased numbers of microvilli. Time-lapse Allen video enhanced contrast differential interference contrast (AVEC-DIC) light microscopy and correlative whole mount intermediate-voltage transmission electron microscopy (IVEM) was used to examine the dynamics ofAcLDL stimulated membrane ruffling and membrane ruffle ultrastructure. Stereo 3D surface replicas confirmed that AcLDL bound to these AcLDL-induced membrane ruffles. Quantification of the plasma membrane surface area after incubation with AcLDL, betaVLDL or LDL confirmed that AcLDL stimulated membrane ruffling, while betaVLDL and LDL stimulated microvilli formation. These studies suggest that modified LDLs induce circular membrane ruffles and modified LDLs bind to these ligand-induced membrane ruffles.

Modified LDLs are internalized by macrophages in part via macropinocytosis

Macrophage foam cell formation in vitro requires uptake of modified low density lipoproteins (LDL) such as acetylated LDL (AcLDL) and moderately oxidized LDL (OxLDL). Macrophages incubated with AcLDL and OxLDL, but not LDL, showed increased membrane ruffling as seen with time-lapse phase contrast video light microscopy. Modified LDLs stimulated circular membrane ruffles between 2 and 10 min after incubation. These membrane ruffles were readsorbed into the plasma membrane between 5 and 15 min later. Phase-bright macropinosomes formed at the base of the stimulated membrane ruffles. The fluid-phase marker lucifer yellow labeled the modified LDL stimulated macropinosomes. Modified LDLs stimulate fluid-phase uptake by 1.5-fold to threefold as measured with 14C-sucrose uptake. Transmission electron microscopy showed that gold conjugated AcLDL and OxLDL bound preferentially to membrane ruffles and were present in macropinosomes (diameter >0.2 pm) underneath these membrane ruffles. AcLDL and OxLDL were also present in clathrin-coated pits and endosomes. These studies suggest that modified lipoproteins stimulate macropinocytosis. AcLDL and OxLDL are partially internalized by macropinocytosis and partially internalized via clathrin-coated pit endocytosis.

The LDL receptor and LRP are receptors for beta VLDL on pigeon monocyte-derived macrophages

Receptors for the lipoprotein, beta very low density lipoprotein (beta VLDL), have been identified through the binding of beta VLDL-gold conjugates on two ligand-induced regions of pigeon monocyte-derived macrophages. These regions were microvilli/retraction fibers and membrane ruffles. The present study investigated the location and identity of beta VLDL receptors using an antiserum directed against the epidermal growth factor (EGF) precursor region of the human low density lipoprotein (LDL) receptor. The anti-receptor serum recognized two membrane proteins from pigeon monocyte-derived macrophages, a 116 kDa (LDL receptor) protein and a 600 kDa (low density lipoprotein receptor-related protein; LRP) protein. Ligand blot analysis demonstrated that pigeon beta VLDL bound to both the LDL receptor and LRP. Immuno-gold electron microscopy using the anti-receptor serum resulted in immunoglobulin localization on the same two ligand-induced regions, microvilli/retraction fibers and membrane ruffles, to which the ligand had bound. Furthermore, simultaneous immunogold localization of the lipoprotein receptor antigens and beta VLDL-gold (ligand) binding substantiated co-localization of the receptor antigens and beta VLDL on the ligand-induced regions. Cross-competition studies with the anti-receptor serum and beta VLDL-gold conjugate documented that increasing concentration of the anti-receptor serum resulted in 70% inhibition of beta VLDL-gold conjugate binding. These data suggest that pigeon monocyte-derived macrophages utilize both the LDL receptor and LRP as receptors for pigeon beta VLDL.

Localization of lipoprotein in pre- and post-transition atherosclerotic lesions following short-term incubation with [125I]LDL

Ultrastructural autoradiography has been used to test the hypothesis that atherosclerotic regions of vessels differ with respect to lipoprotein uptake and localization. White Carneau pigeons, in which the prevalence and localization of aortic lesions are highly predictable, were fed a 0.25% cholesterol-supplemented diet to accelerate atherosclerosis. One hour prior to necropsy the birds were given a single intravenous injection of homologous [125I]LDL (low-density lipoprotein). Plasma die-away and tissue distribution of label were determined, and after the birds had been killed, the aortas, spleen and liver were processed for electron microscope autoradiography. Initial [125I]LDL uptake was rapid, with 35% of the label removed within 30 min. Predominant accumulation was in the liver, followed by the lung, kidney, the spleen and the aorta, in which the [125I]LDL level was approximately 4% that of the liver. Autoradiographic analysis documented hepatocyte (33%) and Kupffer cell (19.9%) localization in the liver and reticuloendothelial cell (57.4%) localization in the spleen. The aortic analysis involved serially sectioned lesions for direct comparison of non-lesion, lesion/non-lesion interface (edge) and deep lesion regions. Analysis of 2275 silver grains documented a ten-fold increase in LDL accumulation at the lesion edge (as compared to adjacent non-lesion) where macrophage foam cells contained more than 70% of the label. The other 30% was distributed equally among endothelium, the intimal matrix and smooth muscle cells. This distribution changed with more complex (deeper) lesions, although grain density in the complex lesions was comparable to the edge. In the complex regions, macrophage foam cell grains were reduced to 37%, whereas smooth muscle cell (22%) and the extracellular matrix (24%) label were both increased. These studies substantiate enhanced accumulation of lipoprotein specifically at lesion sites in the aorta and demonstrate a shift from macrophage localization at the developing edge to smooth muscle cell and the extracellular matrix in more complex deeper lesions.

Ultrastructural localization of tissue factor on monocyte-derived macrophages and macrophage foam cells associated with atherosclerotic lesions

The expression of tissue factor (TF) antigen by circulating monocytes, cultured macrophages, and macrophages associated with atherosclerotic lesions was ultrastructurally analysed using immunogold labeling. A subpopulation of macrophages associated with the intimal surface overlying lesions had a significant TF expression. Macrophages and macrophage foam cells that projected from the intima into the arterial lumen also expressed a high level of TF (14-fold increase over control). In contrast, circulating monocytes and macrophages in culture did not express TF above background control levels. This TF expression by macrophages in vivo but not by macrophages cultured from either normal or hypercholesterolemic animals suggests that monocyte activation and macrophage transition, as measured by TF expression, is lesion-dependent and not stimulated solely by intimal attachment, surface migration, or hypercholesterolemia. These results further suggest that macrophages and foam cells associated with early lesions of atherosclerosis can initiate fibrin formation, which could contribute to lesion complications and transition to a fibromuscular stage.

Eicosapentaenoic acid inhibits cholesteryl ester accumulation in rat peritoneal macrophages by decreasing the number of specific binding sites of acetyl LDL

We found that rat peritoneal macrophages bind acetyl low density lipoprotein (AcLDL) by a saturable and specific manner and accumulate a substantial amount of cholesteryl ester (CE) when incubated with AcLDL. In macrophages enriched with eicosapentaenoic acid (EPA) by the ingestion of highly purified eicosapentaenoic acid-ethyl ester (EPA-E), the accumulation of CE was significantly decreased in a dose-dependent manner. In addition, the contents of EPA and docosapentaenoic acid (DPA) in macrophage phospholipids were dose dependently and significantly increased by EPA-E feeding. In contrast, the contents of arachidonic acid (AA) and docosahexaenoic acid (DHA) were unchanged. Furthermore EPA-E ingestion significantly decreased the Bmax of the AcLDL receptor without affecting the Kd in rat peritoneal macrophages. In addition, specific proteolytic degradation of AcLDL was also dose dependently inhibited by EPA-E feeding, indicating that the number of AcLDL receptor was significantly decreased after EPA-E ingestion. These findings indicate that EPA-E feeding inhibited CE accumulation mainly by decreasing the AcLDL receptors in macrophages. We speculate that EPA inhibits foam cell formation and this inhibitory effect may partly account for its anti-atherogenic action.

Characterization of dietary-induced hypercholesterolemia in the chicken

The effect of 2% dietary cholesterol on the distribution of cholesterol among the plasma lipoproteins was studied in 2-week old male chickens. Very-low-, intermediate-, low- and high-density lipoproteins (VLDL, IDL, LDL and HDL) were separated from plasma by density gradient ultracentrifugation in order to determine their concentration and chemical composition. VLDL were furthermore characterized as concerned their size, mobility and protein content. The lipoprotein profile was quantitatively and qualitatively normal in the control group (n = 6) fed the diet without cholesterol, HDL representing the major lipoprotein class (5.06 +/- 0.36 g/l) and the main carrier of cholesterol. Birds fed the cholesterol containing diets for 5 weeks (n = 6) exhibited a dramatic hypercholesterolemia (1.60 +/- 0.89 g/l free cholesterol and 6.70 +/- 3.22 g/l cholesteryl esters) and a shift in their lipoprotein pattern, with an accumulation of beta-VLDL (6.08 +/- 4.21 g/l) and a marked decrease in HDL level (3.53 +/- 0.91 g/l). The decrease or absence of LDL was balanced by a considerable amount of beta-VLDL remnants (namely IDL), so that the concentration of IDL + LDL considered as a whole was not modified significantly (2.10 +/- 0.95 g/l compared to 1.66 +/- 1.13 g/l in controls). Chicken beta-VLDL, smaller in size (31.0 nm) than control VLDL (33.5 nm), were typically enriched in cholesterol (67%) but they lacked apoE. About 60% of plasma cholesterol was associated with beta-VLDL which therefore represented the main atherogenic lipoprotein class and were probably responsible for the greater amount of cholesterol found in the aorta in these chickens (2.44 +/- 0.99 mg/g aorta vs. 1.32 +/- 0.57 in controls). Since LDL were very reduced or absent, the cholesterol-fed chicken provides a suitable model in which to study the role of beta-VLDL in atherogenesis.

Peritoneal macrophage cholesteryl ester content as a function of plasma cholesterol in rats

Cholesterol accumulation in macrophages that have migrated in the subintimal space leads to foam cell formation, which is believed to be one of the initiating events in atherosclerosis. In this study we investigated the effect of cholesterol feeding on peritoneal monocyte/macrophage cholesterol content and peritoneal cavity lipoprotein composition in rats. A cholesterol (2%) and cholic acid (1%) diet caused significant hypercholesterolemia in plasma, and at the same time the cholesterol content of peritoneal monocytes/macrophages was increased. At day 7, the cellular cholesteryl ester content had risen to 30.1 micrograms/mg cellular protein from a baseline value of 9.2 micrograms/mg. The unesterified cholesterol content also increased by 56%. At this time, acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity was doubled, whereas neutral and acidic cholesteryl ester hydrolase activities were unchanged. Reversal to the regular chow diet after 7 days of the cholesterol-enriched diet normalized plasma cholesterol levels as well as peritoneal monocyte/macrophage cholesteryl ester content. ACAT activity also decreased toward normal levels. Analysis of the d less than 1.21 g/ml peritoneal lipoproteins isolated by ultracentrifugation revealed the presence, in both normal and hypercholesterolemic rats, of apolipoprotein A-I-rich lipid complexes with pre-beta mobility on agarose gel electrophoresis. The size of the peritoneal lipoproteins was smaller than that of plasmatic high density lipoproteins, and their chemical composition was also different from that of the major plasma lipoproteins. The cholesteryl ester content of peritoneal lipoproteins increased after feeding of the cholesterol-enriched diet. In conclusion, our results show that cholesterol feeding leads to rapid accumulation of cholesteryl esters in monocytes/macrophages. As soon as plasma cholesterol levels are returned to normal, cellular cholesterol content is also normalized.(ABSTRACT TRUNCATED AT 250 WORDS)

Lysosomal lipid accumulation in vascular smooth muscle cells

Using an inverted culture technique, the accumulation of lipid within vascular smooth muscle cells incubated with lipid droplets was studied. Initially, lipid was found exclusively within cytoplasmic inclusions but, as accumulation continued, lysosomes became the predominant site of lipid storage. After 3 hr of incubation, 84% of lipid was within lysosomes. This lysosomal lipid accumulation produced a tripling of the average size of lysosomes and resulted in lysosomes with complex, multilobed shapes. In contrast, although the number of cytoplasmic inclusions increased with lipid loading, individual inclusions maintained a spherical shape and a consistent diameter of 1-1.3 microns. Concomitant with changes in cellular lipid storage, incubation with lipid droplets induced development of an anastomosing network of acid phosphatase-containing tubules which were spatially related to sites of lysosomal lipid accumulation. Thus lipid accumulation produced ultrastructural alterations in a number of metabolic compartments. Similar alterations in the intracellular compartmentalization of acquired lipid have been demonstrated in foam cells during atherogenesis and have been hypothesized to have profound effects on lipid metabolism and disease progression.

In vivo clearance of low-density lipoproteins and beta-very-low-density lipoproteins in normal and hypercholesterolemic White Carneau pigeons

Low-density lipoproteins (hLDL) and beta-migrating-very-low-density lipoproteins (beta-VLDL) were isolated from the plasma of cholesterol-fed White Carneau (WC) pigeons and low-density lipoproteins (nLDL) were isolated from the plasma of grain-fed WC pigeons. The lipoproteins were radiolabeled with 125I or 131I and injected into normocholesterolemic or hypercholesterolemic WC pigeons to determine their rate of clearance from the plasma. The fractional catabolic rate (FCR) of nLDL and hLDL in normocholesterolemic pigeons averaged 0.202 and 0.206 pools/h.respectively. beta-VLDL was cleared at a significantly slower rate of 0.155 pools/h. The FCR of the same lipoproteins injected into hypercholesterolemic pigeons was reduced by 17% for nLDL, 50% for hLDL and 57% for beta-VLDL, indicating that the effect of hypercholesterolemia on clearance in vivo was different for the three lipoproteins. The FCR of reductively methylated pigeon LDL (MeLDL), which gives a measure of receptor-independent clearance of LDL, was shown previously to be 0.037 pools/h. These studies suggest therefore that LDL and beta-VLDL are cleared from the plasma of normocholesterolemic and hypercholesterolemic pigeons at a rate substantially greater than that predicted for non-specific processes. Despite the reduction in the clearance rate of hLDL and beta-VLDL due to cholesterol feeding, the absolute amount of cholesterol that was cleared from the plasma by these lipoproteins was increased from approx. 200 mg/kg body weight per day in the normocholesterolemic pigeons to greater than 1000 mg/kg body weight per day in the hypercholesterolemic pigeons. This is due principally to the enrichment in cholesterol relative to protein of the lipoproteins isolated from cholesterol-fed pigeons and the failure of hypercholesterolemia to completely inhibit receptor-dependent clearance of LDL and beta-VLDL. The lower rate of clearance of beta-VLDL relative to LDL is in marked contrast to mammalian beta-VLDL, which is cleared much faster than LDL, but is consistent with the lack of apo E on pigeon lipoproteins. Apo E is the apoprotein that is thought to be responsible for the rapid clearance of beta-VLDL in normocholesterolemic mammals. The low rate of beta-VLDL clearance in pigeons also suggests that pigeons lack an apolipoprotein that function like mammalian apo E.

Morphological characterization of beta-VLDL and acetylated-LDL binding and internalization by cultured pigeon monocytes

The ultrastructure of binding, internalization, and translocation of beta-migrating very low density lipoprotein (beta-VLDL) and acetylated low density lipoprotein (Ac-LDL) by cultured pigeon monocytes was examined using lipoprotein-gold conjugates. Through morphometry, differences in the binding and uptake of beta-VLDL-gold and Ac-LDL-gold were documented. Cells exposed to either beta-VLDL-gold or Ac-LDL-gold for 2 hr at 4 degrees C had the label over noncoated regions of the plasma membrane. Upon warming the cells to 37 degrees C for 2 min, 35% of the surface-bound beta-VLDL-gold was within coated pits on the cell surface. Although coated pits occupied less than 2% of the surface, binding of beta-VLDL-gold was 53 times more concentrated in coated pits as compared to noncoated membrane regions. In contrast, Ac-LDL-gold neither bound to coated pits nor relocated into coated regions of the membrane upon warming to 37 degrees C. Both the beta-VLDL-gold and the Ac-LDL-gold were internalized when the cells were rewarmed at 37 degrees C. Most of the internalized gold particles for both lipoproteins were located in electron-lucent vesicles; however, 9% of the intracellular beta-VLDL-gold was observed within coated vesicles at early times. Upon prolonged rewarming (30-90 min), both lipoprotein-gold conjugates were within acid phosphatase-positive lysosomes. Ultimately 83% of the Ac-LDL-gold and 90% of the beta-VLDL-gold were within electron-dense and electron-lucent lysosomes. These results suggested that the receptor-mediated binding and internalization of beta-VLDL and Ac-LDL by pigeon monocyte macrophages proceeded by separate, distinct routes; beta-VLDL by both coated and noncoated pathways while Ac-LDL was internalized exclusively by noncoated mechanisms. Regardless of these internalization differences, both lipoproteins were delivered to lysosomes for degradation.