Abstract 022: Hepatic Sortilin Regulates Apolipoprotein B Secretion only Under Conditions of Secretory StressDependent of the Presence of Secretory Stress

Abstract

SORT1 is strongly associated with plasma lipid traits and coronary artery disease. The protein it encodes, sortilin is a multi-ligand receptor involved in trafficking of proteins from the Golgi to the lysosome. Hepatic sortilin overexpression reduces VLDL secretion, but the effects of loss of sortilin function on apolipoprotein B100 (apoB) and VLDL secretion have been contradictory and perplexing. Conflicting studies have variously reported increased and decreased apoB/VLDL secretion in response to loss of sortilin, but used a variety of different models and methods of ablating sortilin function. To resolve this, we first measured VLDL secretion in Sort1 knockout (KO) mice on a chow diet and observed no differences in either apoB or TG secretion. ApoB secretion was similarly unaffected in Sort1 KO primary hepatocytes or in McA-RH7777 cells treated with siRNA to knockdown sortilin. We concluded that in a basal secretory state, loss of sortilin alone does not impact apoB secretion, leading us to test if differences in secretory state might explain the discrepancies in published results. We found that when hAPOB was overexpressed, Sort1 knockdown further increased its secretion. The reports of increased VLDL secretion with Sort1 knockout occurred with apoB overexpression, or on high fat diet (HFD); consistent with our hypothesis that secretory overload or metabolic stress are required to uncover the deficiency phenotype. We placed Sort1 KO mice on a 45% HFD for 12 weeks and observed a significant increase in VLDL secretion compared to controls. In addition, when we lipid-loaded hepatocytes with oleic acid or palmitic acid, apoB secretion was increased, and Sort1 knockdown further increased this compared to control. We treated cells with Tunicamycin to induce ER stress, and found that apoB secretion was decreased in the control group was observed, but not in the absence of Sort1, suggesting that sortilin regulates apoB secretion the cell is stressed. Based on these data, we propose that hepatic sortilin regulates the post-ER fate of apoB for either degradation or export; thereby coordinating intracellular apoB metabolism in response to the number and quality of apoB particles that reach the Golgi, and the level of post-ER pre-secretory proteolysis activity.