Beta VLDL uptake by pigeon monocyte-derived macrophages: correlation of binding dynamics with three-dimensional ultrastructure

Lysosomes, cholesterol and atherosclerosis

Cholesterol-engorged macrophage foam cells are a critical component of the atherosclerotic lesion. Reducing the sterol deposits in lesions reduces clinical events. Sterol accumulations within lysosomes have proven to be particularly hard to mobilize out of foam cells. Moreover, excess sterol accumulation in lysosomes has untoward effects, including a complete disruption of lysosome function. Recently, we demonstrated that treatment of sterol-engorged macrophages in culture with triglyceride-containing particles can reverse many of the effects of cholesterol on lysosomes and dramatically reduce the sterol burden in these cells. This article describes what is known about lysosomal sterol engorgement, discusses the possible mechanisms by which triglyceride could produce its effects, and evaluates the possible positive and negative effects of reducing the lysosomal cholesterol levels in foam cells.

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.

Evidence for prolonged cell-surface contact of acetyl-LDL before entry into macrophages

Acetyl-LDL stimulates acyl-CoA:cholesterol acyltransferase (ACAT) much more effectively than LDL in mouse peritoneal macrophages. Previous work with another potent ACAT stimulator, beta-VLDL, suggested that atherogenic lipoproteins may use internalization pathways distinct from that of LDL. Brief incubation of fluorescently labeled acetyl-LDL and LDL followed by a short chase period without lipoproteins was used to compare endocytic pathways. LDL was delivered rapidly to perinuclear vesicles, corresponding to late endosomes and lysosomes. A substantial fraction (> 40%) of acetyl-LDL was initially retained in the cell periphery, while the rest was rapidly delivered to late endosomes that also contained LDL. Fluorescence of peripheral 1,1'-dioctadecyl-3,3,3', 3'-tetramethylindocarbocyanine perchlorate (DiI)-acetyl-LDL could be quenched by TNBS, indicating accessibility of the peripheral acetyl-LDL to the extracellular space. Quantification of fluorescence intensities demonstrated that > 40% of the cell-associated DiI-acetyl-LDL but only about 10% of DiI-LDL fluorescence was quenchable by TNBS after a 3-minute chase. Fucoidin can efficiently displace DiI-acetyl-LDL bound to cells at 0 degree C. DiI-acetyl-LDL in the TNBS-quenchable peripheral compartments, however, was resistant to fucoidin. Electron microscopy of colloidal gold-acetyl-LDL showed that acetyl-LDL on the cell surface was often associated with microvilli or ruffles. After clearance from the surface, the peripheral acetyl-LDL was also delivered to the late endosomes and lysosomes. These results indicate that a substantial portion of acetyl-LDL enters macrophages through a pathway that initially differs from that of LDL. This pathway involves a prolonged retention of acetyl-LDL on the plasma membrane. This surface retention may affect ACAT activation in macrophages.

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.

Beta-very low density lipoprotein is sequestered in surface-connected tubules in mouse peritoneal macrophages

beta-very low density lipoprotein (VLDL) is a large lipoprotein with multiple apoprotein E (apoE) molecules that bind to the LDL receptors on mouse macrophages. Even though they bind to the same receptor, the endocytic processing of beta-VLDL differs from low density lipoprotein (LDL). LDL is rapidly delivered to perinuclear lysosomes and degraded, but much of the beta-VLDL is retained in peripheral compartments for several minutes. We have investigated the properties of these peripheral compartments. Measurement of the pH was made using FITC-phosphatidylethanolamine incorporated into the beta-VLDL, and we found that the peripheral compartments were near neutral in pH. These peripheral, beta-VLDL containing compartments were poorly accessible to antibodies, but a low molecular weight fluorescence quencher (trypan blue) entered the compartments within a few seconds. Intermediate voltage EM of cells labeled with colloidal-gold-beta-VLDL revealed that the peripheral compartments are tubular, surface-connected invaginations. Kinetic studies with fluorescent beta-VLDL showed that the compartments become fully sealed with a half-time of 6 min, and the beta-VLDL is then delivered rapidly to perinuclear lysosomes. By monitoring fluorescence energy transfer between lipid analogs incorporated into the beta-VLDL, some processing of the lipoprotein in the peripheral tubular compartments is demonstrated. The novel mode of uptake of beta-VLDL may account for the high cholesterol ester accumulation induced by this lipoprotein.