Sortilin restricts secretion of apolipoprotein B-100 by hepatocytes under stressed but not basal conditions
Donna M. Conlon, Carolin V. Schneider, Yi-An Ko, Amrith Rodrigues, Kathy Guo, Nicholas J. Hand, Daniel J. Rader
Donna M. Conlon, Carolin V. Schneider, Yi-An Ko, Amrith Rodrigues, Kathy Guo, Nicholas J. Hand, Daniel J. Rader
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Research Article Genetics Vascular biology

Sortilin restricts secretion of apolipoprotein B-100 by hepatocytes under stressed but not basal conditions

Abstract

Genetic variants at the SORT1 locus in humans, which cause increased SORT1 expression in the liver, are significantly associated with reduced plasma levels of LDL cholesterol and apolipoprotein B (apoB). However, the role of hepatic sortilin remains controversial, as genetic deletion of sortilin in mice has resulted in variable and conflicting effects on apoB secretion. Here, we found that Sort1-KO mice on a chow diet and several Sort1-deficient hepatocyte lines displayed no difference in apoB secretion. When these models were challenged with high-fat diet or ER stress, the loss of Sort1 expression resulted in a significant increase in apoB-100 secretion. Sort1-overexpression studies yielded reciprocal results. Importantly, carriers of SORT1 variant with diabetes had larger decreases in plasma apoB, TG, and VLDL and LDL particle number as compared with people without diabetes with the same variants. We conclude that, under basal nonstressed conditions, loss of sortilin has little effect on hepatocyte apoB secretion, whereas, in the setting of lipid loading or ER stress, sortilin deficiency leads to increased apoB secretion. These results are consistent with the directionality of effect in human genetics studies and suggest that, under stress conditions, hepatic sortilin directs apoB toward lysosomal degradation rather than secretion, potentially serving as a quality control step in the apoB secretion pathway in hepatocytes.

Authors

Donna M. Conlon, Carolin V. Schneider, Yi-An Ko, Amrith Rodrigues, Kathy Guo, Nicholas J. Hand, Daniel J. Rader

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Figure 4

In cells overexpressing human apoB or treated with fatty acids loss of sortilin results in a significant increase in apoB-100 secretion.

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In cells overexpressing human apoB or treated with fatty acids loss of s...
(A) McA and McA-HAPOB cells were transfected with rat Sort1 (rSort1) siRNA or control siRNA. After 36 hours, cells were incubated for 1 hour without Met/Cys and then labeled in the same media with 35S Met/Cys for 2 hours. Media was collected and cells were lysed and subjected to autoradiography, and 35S-labeled apoB-100 was quantified by liquid scintillation counting counted. n = 3 wells/group. Representative of 3 independent experiments. (B) McA and McA-HAPOB cells were transfected with mSort1 or empty control vector (pcDNA), and apoB-100 secretion was measured as described in A. n = 3 wells/group. Representative of 3 independent experiments. (C) siRNA-transfected McA cells were incubated without Met/Cys with 0(NT), 0.4, 0.8, or 1.2 mM oleic acid (OA) for 2 hours and then labeled in the same media with 35S Met/Cys for 2 hours, after which apoB-100 secretion was measured. n = 9 wells/treatment/group. (D) apoB secretion from McA cells treated with 0, 0.4 mM, or 0.8 mM palmitic acid (PA). n = 9 wells/treatment/group. (E) apoB secretion in siRNA-transfected HepG2 cells treated with 0 or 1.2 mM OA. n = 9 well/treatment/group. (F) apoB secretion in primary hepatocytes isolated from Sort1-KO mice and WT littermate controls treated with 0.4 mM OA or 0.4 mM PA or nontreated (NT) for 4 hours. n = 3 wells/mouse. Representative of 3 independent isolations. *P < 0.05 versus control siRNA or pcDNA by ANOVA.