Abstract 387: Regulation of Apolipoprotein B Secretion by Hepatic Sortilin is Dependent on Secretory Stress

Abstract

Sortilin is a multi-ligand sorting receptor involved in trafficking of proteins from the Golgi apparatus to the lysosome and has been widely shown to be associated with plasma lipid traits and coronary artery disease. While over expression of sortilin in the liver reduces VLDL production, the reported effects of the genetic loss of sortilin on apolipoprotein B100 (apoB) and VLDL secretion have been contradictory and perplexing; loss of sortilin has been shown in different studies to result in both increased and decreased apoB/VLDL secretion. These conflicting studies were carried out in a variety of different models and used different methods of knocking down sortilin expression. To attempt to further clarify the role of sortilin, we explored the role of sortilin deficiency on apoB secretion in the hepatocyte by utilizing 2 in vitro models of sortilin knockdown; primary hepatocytes from Sort1-/- mice and siRNA -treated McA-RH7777 (McA) cells. In both primary hepatocytes and McA cells, loss of sortilin alone was not associated with any change in apoB secretion. The previously reported increases in VLDL secretion occurred on either the background of apoB over expression or in livers of mice on a high fat diet, suggesting the requirement for a metabolic stress. We found that apoB secretion was increased with Sort1 knockdown as compared to control in isolated primary hepatocytes from Apobec1-/-; hAPOB Tg mice and McA cells stably over expressing apoB. We then sought to increase apoB secretion by lipid loading with oleic acid (OA). While OA increased apoB secretion in all cells, there was no effect of Sort1 knockdown in this context. However, when the cells were further treated with either palmitic acid, proteasomal inhibitors, or tunicamycin (an ER stress inducer), there was an observed increase in apoB secretion with Sort1 knockdown, suggesting that sortilin regulates apoB secretion only when both apoB secretion is increased and the cell is stressed. Based on this data, we propose that hepatic sortilin regulates the post-ER fate of apoB for degradation and export and acts to coordinate 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.