Low density lipoprotein binding, internalization, and degradation in human adipose cells

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Abstract
Human adipose tissue derives its cholesterol primarily from circulating lipoproteins. To study fat cell-lipoprotein interactions, low density lipoprotein (LDL) uptake and metabolism were examined using isolated human adipocytes. The 125I-labeled LDL (d [density] = 1.025-1.045) was bound and incorporated by human fat cells in a dose dependent manner with an apparent Km of 6.9 .+-. 0.9 .mu.g LDL protein/mL and a Vmax of 15-80 .mu.g LDL protein/mg lipid per 2 h. In time course studies LDL uptake was characterized by rapid initial binding followed by a linear accumulation for at least 4 h. The 125I-labeled LDL degradation products (trichloroacetic acid soluble iodopeptides) accumulated in the incubation medium in a progressive manner with time. Azide and F- inhibited LDL internalization and degradation, suggesting that these processes are energy dependent. Binding and cellular internalization of 125I-labeled LDL lacked lipoprotein class specificity in that excess (25.times.) unlabeled very low density lipoprotein (VLDL) (d < 1.006) and high density lipoprotein (HDL) (d = 1.075-1.21) inhibited binding and internalization of 125I-labeled LDL. On an equivalent protein basis HDL was the most potent. The 125I-labeled LDL binding to an adipocyte plasma membrane preparation was a saturable process and almost completely abolished by a 3-4.times. greater concentration of HDL. The binding, internalization and degradation of LDL by human adipocytes resembled that of other mesenchymal cells and could account for a significant proportion of in vivo LDL catabolism. Apparently adipose tissue is an important site of LDL and HDL interactions.

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