Lp(a) lipoprotein enters cultured fibroblasts independently of the plasma membrane low density lipoprotein receptor

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Abstract
Lp(a) lipoprotein shares the apoB antigen with low density lipoprotein (LDL). The Lp(a) antigen is unique for Lp(a) lipoprotein. Fibroblast association (i.e., plasma membrane binding plus intracellular accumulation), plasma membrane binding, intracellular accumulation and degradation of 125I-Lp(a) lipoprotein were studied in strains from subjects with or without autosomal dominant hypercholesterolemia (HC). Subjects without HC (non-HC) have cell surface receptors for low density lipoprotein (LDL receptors). On the average, HC heterozygotes have half-normal LDL receptor activity and receptor-negative HC homozygous cell strains lack functional receptors. Fibroblast processing of 125I-Lp(a) lipoprotein was compared to fibroblast processing of 125I-L. LDL receptor-dependent processing of 125I-LDL was saturated at about 50 .mu.g apo 125I-LDL.cntdot.ml-1 in non-HC fibroblasts. 125I-Lp(a) lipoprotein was largely processed independently of receptor mechanisms by non-HC cells (highest concentration examined 150 .mu.g apo 125I-Lp(a) lipoprotein.cntdot.ml-1). Lp(a) lipoprotein did not interfere with 125I-L for fibroblast association, but inhibited 125I-LDL degradation. The interference with 125I-LDL degradation was time dependent. Only slightly higher 125I-Lp(a) lipoprotein processing values were found in non-HC and HC heterozygous strains than in receptor-negative HC homozygous strains. Non-HC cells had more than 10-fold higher 125I-LDL processing values than receptor-negative HC homozygous cells.