The number of identical kringle IV repeats in apolipoprotein(a) affects its processing and secretion by HepG2 cells

A variable number of 5.6-kilobase kringle IV repeats in the human apolipoprotein(a) (apo(a)) gene results in a size polymorphism of the protein and correlates inversely with the plasma levels of the atherogenic lipoprotein(a) (Lp(a)). In order to analyze whether this association reflects a direct effect of kringle IV repeat number on Lp(a) plasma concentration, we have studied the expression of recombinant apo(a) (r-apo(a)) isoforms in the human hepatocarcinoma cell line HepG2. Following transient transfection of apo(a) cDNA expression plasmids that differed only in the number of kringle IV repeats, we observed a gradual decrease of Lp(a) in the medium of the cells with an increasing number of kringle IV repeats, mimicking the relationship present in humans in vivo. The analysis of apo(a) protein in the lysate and in the medium of cells that were transfected with a plasmid encoding an apo(a) isoform with 22 kringles revealed a predominant intracellular precursor with little secretion of the mature apo(a) protein. In contrast, transfection of a plasmid encoding an isoform with 11 kringles led to effective secretion of the mature peptide into the medium, indicating differential processing rates of apo(a) isoforms in the secretory path way. The intracellular accumulation of an apo(a) precursor in the endoplasmic reticulum was demonstrated by cell fractionation and [35S]Met metabolic labeling/temperature block experiments using HepG2 cells stably transfected with recombinant apo(a). The direct and causal effect of kringle IV repeat number on the expression of recombinant apo(a) in HepG2 cells, and presumably liver cells, provides a novel mechanism for the genetic regulation of the concentration of a protein.