Interaction of a recombinant form of apolipoprotein[a] with human fibroblasts and with the human hepatoma cell line HepG2.
We have studied the binding, uptake, and degradation of a recombinant form of apolipoprotein[a] (r-apo[a]) using a cultured cell model. In HepG2 cells and in human fibroblasts, r-apo[a] complexed with low density lipoprotein(LDL) is bound and internalized via high affinity (Kd = 10 nM) receptors; in both cell types, low affinity (Kd = 200-300 nM) sites also mediate free apo[a] uptake. Using competition studies, we found that the high affinity binding component corresponds to the LDL receptor. Involvement of the LDL receptor in r-apo[a] uptake by fibroblasts was confirmed using fibroblasts derived from an individual homozygous for familial hypercholesterolemia; in contrast to normal fibroblasts, these cells lacked the high affinity r-apo[a] binding component. Cell association of 125I-labeled r-apo[a] was increased and decreased concomitantly with the up- and down-regulation of the LDL receptor in response to a number of compounds. The addition of alpha 2-macroglobulin as well as treatment with heparinase, chondroitinase ABC, and sodium chlorate did not decrease total specific binding of r-apo[a], suggesting that neither the low density lipoprotein receptor-related protein nor cell surface proteoglycans are involved in r-apo[a] clearance. The low affinity binding component present in both fibroblasts and HepG2 cells likely corresponds to the plasminogen receptor, as binding of r-apo[a] to these sites was specifically decreased by the addition of plasminogen or the lysine analogue epsilon-aminocaproic acid, but not by the addition of tissue-type plasminogen activator. Heparin abolished uptake of r-apo[a] by the LDL receptor component only; this indicates that apo[a] must be associated with LDL to be cleared by this receptor. In contrast, free apo[a] can be effectively cleared by the plasminogen receptor which may represent a significant route of clearance for free apo[a] in vivo.
Tam
S P
SP
Department of Biochemistry, Queen's University, Kingston, Ontario, Canada.
Zhang
X
X
Koschinsky
M L
ML
D015119:Aminocaproic Acid / Q000494:pharmacology; D000911:Antibodies, Monoclonal / Q000276:immunology; D001054:Apolipoproteins A / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D001667:Binding, Competitive; D002704:Chlorates / Q000494:pharmacology; D002808:Chondroitin Lyases / Q000494:pharmacology; D015536:Down-Regulation; D004492:Edetic Acid / Q000494:pharmacology; D006493:Heparin / Q000494:pharmacology; D019763:Heparin Lyase; D006801:Humans; D006938:Hyperlipoproteinemia Type II / Q000378:metabolism; D008077:Lipoproteins, LDL / Q000378:metabolism; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D011133:Polysaccharide-Lyases / Q000494:pharmacology; D011956:Receptors, Cell Surface / Q000378:metabolism*; D011971:Receptors, Immunologic / Q000378:metabolism; D011973:Receptors, LDL / Q000037:antagonists & inhibitors / Q000276:immunology / Q000378:metabolism*; D055293:Receptors, Urokinase Plasminogen Activator; D011994:Recombinant Proteins / Q000378:metabolism; D014407:Tumor Cells, Cultured; D015854:Up-Regulation
D000911:Antibodies, Monoclonal; D001054:Apolipoproteins A; D002704:Chlorates; D008077:Lipoproteins, LDL; D026503:Low Density Lipoprotein Receptor-Related Protein-1; C524898:PLAUR protein, human; D011956:Receptors, Cell Surface; D011971:Receptors, Immunologic; D011973:Receptors, LDL; D055293:Receptors, Urokinase Plasminogen Activator; D011994:Recombinant Proteins; D006493:Heparin; D004492:Edetic Acid; D002808:Chondroitin Lyases; D011133:Polysaccharide-Lyases; D019763:Heparin Lyase; C032706:sodium chlorate; D015119:Aminocaproic Acid