THE VERY LOW-DENSITY INTERMEDIATE-DENSITY LIPOPROTEIN FRACTION ISOLATED FROM APOLIPOPROTEIN E-KNOCKOUT MICE TRANSFORMS MACROPHAGES TO FOAM CELLS THROUGH AN APOLIPOPROTEIN E-INDEPENDENT PATHWAY

Apolipoprotein E (apoE)-knockout mice develop severe atherosclerosis a ssociated with high levels of very low-density Lipoprotein (VLDL) and intermediate-density lipoprotein (IDL) in plasma. To investigate the a therogenic role of VLDL and IDL, the lipoprotein fraction containing b oth VLDL and IDL (apoEko-VLDL/IDL) was isolated from plasma of apoE-kn ockout mice by ultracentrifugation, and its interaction with macrophag es was studied. When peritoneal macrophages obtained from apoE-knockou t mice were incubated with apoEko-VLDL/IDL, the level of cellular chol esteryl esters (CE) increased with the concentration of apoEko-VLDL/ID L. The level of cellular cholesteryl [H-3]oleale formed reached 15.1 n mol/mg of cell protein upon incubation with 50 mu g/mL apoEko-VLDL/IDL for 18 h, which was an 8.4-fold increase over the corresponding level induced by low-density lipoprotein (LDL). The cellular CE mass was al so significantly increased by apoEko-VLDL/IDL. Morphologically, after exposure to apoEko-VLDL/IDL, macrophages became strongly stained with Sudan black B, The total binding of [I-125]apoEko-VLDL/IDL to macropha ges was effectively replaced by more than 80% by an excess of the unla beled Ligand, Specific binding, calculated by subtracting the nonspeci fic binding from the total binding, exhibited a saturation pattern. Si milar results were obtained with cell association and degradation expe riments. In addition, the endotytic degradation of [I-125]apoEko-VLDL/ IDL was partially inhibited by LDL, whereas acetyl-LDL did not show an y effect. These results indicated that apoEko-VLDL/IDL in its unmodifi ed form produced significant CE accumulation in macrophages through a specific and apoE-independent pathway. This pathway may explain, in pa rt, the mechanisms of foam cell formation in arterial walls and the su bsequent development of atherosclerosis in apoE-knockout mice.