The plasma level of apolipoprotein B100 (apoB100) is among the strongest risk factors for coronary artery disease, making the regulation of its production by the liver a significant pursuit. We and others have shown that apoB100 production by the liver is regulated primarily by post-translational degradation, with 2 major intracellular proteolytic systems involved, namely the ubiquitin-proteasome pathway (UPP) when lipid ligands in the endoplasmic reticulum (ER) for apoB100 are limiting, and autophagy, when hepatic cells are incubated with dietary n-3 fatty acids (FAs). In this proposal, we plan to establish the mechanisms and sites of action for chaperone-like holdases and for regulators of the Sec61 translocation channel that regulate apoB degradation by the UPP (Aim 1).
Under Aim 2, we propose to pursue promising preliminary data to test in vitro and in vivo the hypotheses that autophagy, in addition to its role in n-3 FA-stimulated degradation, also is a regulator of apoB100 degradation under basal metabolic conditions, as well as under conditions relevant to obesity and insulin resistance (e.g., chronic exposure to high levels of common dietary FAs, and insulin- stimulated apoB100 degradation). Because apoB-lipid bodies form in hepatic cells in some of the metabolic settings under study, including, as we show, in human primary heptocytes, we will monitor their formation and turnover and seek their relationship to apoB100 degradation by the proteasome and autophagy.
Under Aim 3, we plan to focus on the role of the over-expression of sortilin 1 (sort 1) in directing apoB100 to degradation in hepatic cells, based not only on its establishment in GWAS studies that hepatic over-expression of sort1 is asssociated with decreased plasma levels of LDL and apoB, but also on strong supporting data showing that its effects are dependent on autophagy. These studies will be conducted in vitro (including in primary human hepatocytes as warranted) and in vivo, using novel mouse models, and will examine the role and cell biological mechanisms by which sort1 participates in the autophagic degradation of apoB100 autophagy.
Given the strong positive association of plasma apoB100 levels and the risk of coronary artery disease, knowledge about its regulated production by the liver is vital to the understanding of hyperlipidemia and to the identification of factors that canbe targeted to reduce high levels of atherogenic apoB-lipoproteins. In addition, people with obesity (insulin resistance), diabetes, and lower expression of sortilin 1 relatively overproduce VLDL-apoB100, which may represent the loss of the insulin-related, sortilin, and autophagy pathways we are investigating.
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|Amengual, Jaume; Guo, Liang; Strong, Alanna et al. (2018) Autophagy Is Required for Sortilin-Mediated Degradation of Apolipoprotein B100. Circ Res 122:568-582|