Free fatty acids activate a high-affinity saturable pathway for degradation of low-density lipoproteins in fibroblasts from a subject homozygous for familial hypercholesterolemia.
This paper describes a mechanism for degradation of low-density lipoprotein (LDL) in fibroblasts unable to synthesize the LDL receptor. In this cell line, long-chain free fatty acids (FFA) activated 125I-LDL uptake; unsaturated FFA were the most efficient. The first step of this pathway was the binding of LDL apoB to a single class of sites on the plasma membrane and was reversible in the presence of greater than or equal to 10 mM suramin. Binding equilibrium was achieved after a 60-90-min incubation at 37 degrees C with 1 mM oleate; under these conditions, the apparent Kd for 125I-LDL binding was 12.3 micrograms/mL. Both cholesterol-rich (LDL and beta-VLDL) and triglyceride-rich (VLDL) lipoproteins, but not apoE-free HDL, efficiently competed with 125I-LDL for this FFA-induced binding site. After LDL bound to the cell surface, they were internalized and delivered to lysosomes; chloroquine inhibited subsequent proteolysis of LDL and thereby increased the cellular content of the particles. A physiological oleate to albumin molar ratio, i.e., 1:1 (25 microM oleate and 2 mg/mL albumin), was sufficient to significantly (p less than 0.01) activate all three steps of this alternate pathway: for example, 644 +/- 217 (25 microM oleate) versus 33 +/- 57 (no oleate) ng of LDL/mg of cell protein was degraded after incubation (2 h, 37 degrees C) with 50 micrograms/mL 125I-LDL. We speculate that this pathway could contribute to the clearance of both chylomicron remnants and LDL.
Bihain
B E
BE
Department of Physiology, Louisiana State University Medical Center, New Orleans 70112-2822.
Yen
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FT
D000418:Albumins / Q000494:pharmacology; D000818:Animals; D001055:Apolipoproteins B / Q000737:chemistry; D001667:Binding, Competitive; D002460:Cell Line; D003080:Cold Temperature; D005230:Fatty Acids, Nonesterified / Q000494:pharmacology*; D005347:Fibroblasts / Q000187:drug effects / Q000378:metabolism*; D006720:Homozygote; D006801:Humans; D006938:Hyperlipoproteinemia Type II / Q000235:genetics / Q000378:metabolism*; D007457:Iodine Radioisotopes; D007700:Kinetics; D008077:Lipoproteins, LDL / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism*; D019301:Oleic Acid; D009829:Oleic Acids / Q000494:pharmacology; D011485:Protein Binding; D011817:Rabbits; D011973:Receptors, LDL / Q000187:drug effects; D013498:Suramin / Q000378:metabolism
D000418:Albumins; D001055:Apolipoproteins B; D005230:Fatty Acids, Nonesterified; D007457:Iodine Radioisotopes; D008077:Lipoproteins, LDL; D009829:Oleic Acids; D011973:Receptors, LDL; D019301:Oleic Acid; D013498:Suramin