Uptake by J774 macrophages of very-low-density lipoproteins isolated from apoE-deficient mice is mediated by a distinct receptor and stimulated by lipoprotein lipase

Apolipoprotein (apo) E-deficient mice display marked accumulation in the plasma of VLDL deficient in both apoE and apoB100 but containing apoB48, apoA-I apoCs, and apoA-IV. Since apoE-deficient mice develop severe atherosclerotic lesions with lipid-laden macrophages, we reasoned that the uptake of lipoproteins by intimal macrophages can take place in the absence of both apoE and apoB100. To get more insight into the mechanism of foam cell formation in apoE-deficient mice, we measured the interaction of VLDL from apoE-deficient mice (apoE(null) VLDL) with the murine macrophage cell line J774. Scatchard analysis revealed that apoE(null) VLDL is bound to J774 cells with a K-d value comparable to that of control VLDL (8.1 versus 4.7 mu g/mL) and with a B-max value about half that of control VLDL (40 versus 70 ng/mg cell protein, respectively). ApoE(null) VLDL is also taken up and degraded by J774 macrophages via a high-affinity process less efficiently than control mouse VLDL (6-fold and 50-fold less efficiently, respectively). In line with this observation, incubation of J774 cells with 50 mu g/mL apoE(null) VLDL for 24 hours resulted in an increase in intracellular cholesteryl ester (CE) content, although 5-fold less pronounced than after incubation with 50 mu g/mL control mouse VLDL. Under the conditions applied, simultaneous addition of 5 mu g/mL lipoprotein lipase (LPL) stimulated the cellular uptake and degradation of apoE(null) VLDL about 10-fold and resulted in a 5-fold stimulation of the intracellular CE accumulation, from 9+/-2 to 46+/-5 mu g CE per milligram cell protein. In contrast to control mouse VLDL, apoE(null) VLDL could not compete with I-125-labeled LDL for binding to the LDL receptor of J774 cells. Furthermore, neither LDL nor acetylated LDL could compete with I-125-labeled apoE(null) VLDL for binding to these cells, whereas control mouse VLDL, VLDL from a hypertriglyceridemic patient, and apoE(null) VLDL itself were efficient competitors. Thus, VLDL from apoE-deficient mice is taken up by J774 macrophages through recognition by a distinct receptor, which could be the triglyceride-rich lipoprotein receptor. We conclude that in apoE-deficient mice, foam cell formation occurs via a receptor-mediated uptake of apoE(null) VLDL, which can be stimulated by the presence of LPL.