Preferential Uptake of Core Lipids of HDL3 by Hep G2 Cells

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The uptake and metabolism of human LDL and HDL3 by Hep G2 (human hepatoma) cells were assessed using lipoproteins labeled either in the protein moiety with l25I or in the core lipid with [3H]cholesteryl linoleyl ether ([3H]CLE). The uptake of LDL involved the high-affinity LDL apo-B,E receptor. Relatively low-affinity binding and uptake of LDL were also observed. The uptake of LDL via both low-and high-affinity processes involved whole-particle uptake, since the [3H]CLE/[l25I]protein uptake ratio, standardized to LDL particle number, remaine constant at about 1 at all LDL concentrations offered to the cells (2.5-200 μg protein/ml). On the other hand, the uptake of HDL3 components involved both whole-particle uptake, possibly via receptor-mediated endocytosis (distinguishable from the LDL-receptor-mediated process), and preferential core lipid delivery. The [3H]CLE/[125I]protein uptake ratio, standardized to HDL3 particle number, increased from 1.5 ± 0.1 (n = 3) when HDL3 at 10 μg protein/ml was offered to the cells to 3.9 ± 0.2 (n = 3) at 200 μg HDL protein/ml. The uptake of whole particles thus appeared to occur via a high-affinity process, whereas core lipids were preferentially delivered to the cells via a relatively low-affinity event. High-affinity uptake of [125I]HDL3 led to apolipoprotein degradation via a choloroquine-inhibitable step. The uptake of HDL3 components via both processes was not increased by a preincubation of the cells with ethanolic cholesterol, but under the same conditions, high-affinity binding of [125I]HDL3 was increased by 80%, and LDL-receptor-mediated binding and uptake of [125I]LDL was reduced by 70%. Overall, HDL3 binding to Hep G2 cells probably led to a net loss of cholesterol from the cells, since HDL3 treatment resulted in upregulation of the LDL receptor. Alternatively, HDL3 whole-particle uptake and preferential core lipid uptake may have led to the delivery of cholesterol to intracellular cholesterol pools that were not involved in LDL receptor regulation.