Obligatory role of cholesterol and apolipoprotein E in the formation of large cholesterol-enriched and receptor-active high density lipoproteins.
The formation of large cholesterol-enriched high density lipoproteins (HDL1/HDLc) from typical HDL3 requires lecithin:cholesterol acyltransferase activity, additional cholesterol, and a source of apolipoprotein (apo-) E. The present study explores the role of apo-E in promoting HDL1/HDLc formation and in imparting to these lipoprotein particles the ability to interact with the apo-B,E(low density lipoprotein (LDL] receptor. Incubation of normal canine serum with cholesterol-loaded mouse peritoneal macrophages resulted in the formation of HDL1/HDLc that competed with 125I-LDL for binding to the apo-B,E(LDL) receptors on cultured human fibroblasts. Cholesterol efflux from macrophages was necessary because incubation of normal canine serum with nonloaded macrophages did not cause HDL1/HDLc formation. However, cholesterol delivery to the serum was not sufficient to result in HDL1/HDLc formation. Apolipoprotein E had to be available. Incubation of apo-E-depleted canine serum with cholesterol-loaded J774 cells, a macrophage cell line that does not synthesize apo-E, demonstrated that no HDL1/HDLc formation was detected even in the presence of significant cholesterol efflux. However, addition of exogenous apo-E to the serum during the incubation with cholesterol-loaded J744 cells promoted the formation of large receptor-active HDL1/HDLc. The receptor binding activity of these particles produced in vitro correlated with the amount of apo-E incorporated into the HDL1/HDLc. Apolipoproteins A-I and C-III were ineffective in promoting HDL1/HDLc formation; thus, apo-E was unique in allowing HDL1/HDLc formation. These results demonstrate that when lecithin:cholesterol acyltransferase activity, cholesterol, and apo-E are present in serum, typical HDL can be transformed in vitro into large cholesterol-rich HDL1/HDLc that are capable of binding to lipoprotein receptors.
D000818:Animals; D001057:Apolipoproteins E / Q000378:metabolism*; D001667:Binding, Competitive; D002352:Carrier Proteins; D002460:Cell Line; D002478:Cells, Cultured; D002784:Cholesterol / Q000378:metabolism*; D005260:Female; D007700:Kinetics; D008075:Lipoproteins, HDL / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism*; D008264:Macrophages / Q000378:metabolism; D051379:Mice; D008970:Molecular Weight; D011487:Protein Conformation; D016601:RNA-Binding Proteins; D011956:Receptors, Cell Surface / Q000378:metabolism*; D011973:Receptors, LDL / Q000378:metabolism; D018110:Receptors, Lipoprotein
D001057:Apolipoproteins E; D002352:Carrier Proteins; D008075:Lipoproteins, HDL; D016601:RNA-Binding Proteins; D011956:Receptors, Cell Surface; D011973:Receptors, LDL; D018110:Receptors, Lipoprotein; C031616:high density lipoprotein receptors; C075197:high density lipoprotein binding protein; D002784:Cholesterol