Abstract 20583: Sortilin Promotes Secretion of Apolipoprotein(a) by Hepatic Cells
Introduction: Lp(a) is a unique lipoprotein species consisting of an LDL-like moiety that is covalently linked to the hallmark glycoprotein apo(a). Plasma concentrations of Lp(a) vary enormously in the population, and are largely governed by the rate of biosynthesis in hepatocytes. Sortilin is a member of the VSP10 family of multi-ligand sorting receptors, and regulates Golgi to lysosomal transport. Sortilin has been shown to act as a clearance receptor for LDL in hepatocytes; its role in VLDL secretion is controversial, however, as sortilin has been shown in different settings to have either an increasing or decreasing effect. Our own work has shown that sortilin can promote Lp(a) catabolism.
Hypothesis: Sortilin regulates secretion of apo(a) from hepatocytes.
Methods: HepG2 cells were transfected with expression plasmids encoding apo(a) and sortilin. Apo(a) secretion was monitored using a pulse-chase protocol employing 35S-Cys/Met followed by pulldown from cell lysates or conditioned media. Binding of apo(a) to sortilin was assessed using non-denaturing agarose gel electrophoresis or by pulldown assay. Deep sequencing of the SORT1 locus was performed in a cohort of subjects with high Lp(a).
Results: Overexpression of sortilin increased the rate of apo(a) secretion from HepG2 cells, without affecting the rate of intracellular maturation of apo(a). The effect of sortilin was absolutely dependent on its carboxyl-terminal cytosolic domain, and was diminished by mutation of the dileucine- or tyrosine-containing sortin motifs in this domain. Interestingly, while the effect of sortilin was similar when a mutant form of apo(a) lacking the strong lysine binding site (LBS) in KIV10 was used, mutation of the weak LBS in KIV7-8, necessary for interaction with apoB, eliminated the effect of sortilin. No direct binding between apo(a) and sortilin was observed. We have found 19 occurrences of rare, novel heterozygous missense or splicing variants in SORT1. Interestingly, 9 of these patients have Lp(a) levels in excess of the top 5th percentile for the general population.
Conclusions: Sortilin promotes apo(a) secretion, possibly in a manner dependent on an intracellular interaction between apo(a) and apoB, but not through direct binding of sortilin to apo(a).
Author Disclosures: M. Gemin: None. R. Romagnuolo: None. A.D. McIntyre: None. C.A. Scipione: None. N.G. Seidah: None. M.B. Boffa: None. R.A. Hegele: None. M.L. Koschinsky: Research Grant; Modest; Pfizer. Other Research Support; Modest; Regeneron. Consultant/Advisory Board; Modest; Regeneron.
- © 2016 by American Heart Association, Inc.