Preferential association of apolipoprotein E Leiden with very low density lipoproteins of human plasma.

Abstract

Apolipoprotein (apo) E Leiden is a rare variant of human apoE characterized by defective receptor binding and associated with dominant transmission of type III hyperlipoproteinemia. In heterozygotes, apoE Leiden is present in higher concentrations in both total plasma and very low density lipoproteins (VLDL) than the other apoE allele product. In the present study we analyzed cell expression and plasma lipoprotein association of apoE Leiden to determine whether the unequal concentration of the two apoE allele products could be explained by differences in secretion rate from the hepatocyte or by preferential association with VLDL. We transfected the rat hepatoma cell line McA-RH7777 with apoE Leiden or normal human apoE3, and studied their secretion and media distribution. In pulse-chase experiments, the secretion of apoE Leiden was comparable to that of both human apoE3 and rat endogenous apoE, approaching 100% in 90 min. In similar transfection experiments, secreted apoE Leiden was significantly less glycosylated than normal apoE3 (21.7% vs. 36.6%, P < 0.005, n = 4), a finding also noted for apoE Leiden in human plasma. In in vitro incubation experiments, apoE Leiden showed a markedly higher preference for VLDL of normolipidemic human plasma when compared to both apoE3 (2.6-fold, P < 0.001) and apoE4 (1.6-fold, P < 0.001). These results suggest that the accumulation of apoE Leiden in VLDL derives from a high affinity of the mutant protein for the VLDL. This enrichment in defective apoE probably exacerbates impairment of VLDL removal from the circulation, thus contributing to the dominant transmission of type III hyperlipoproteinemia.