β-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.