Elevated plasma levels of lipoproteins containing apolipoprotein B (apoB), including low density lipoproteins (LDL) and triglyceride-rich lipoproteins (TRL), are among the most important causal risk factors for atherosclerotic cardiovascular disease (CVD). Despite major advances, the factors that regulate plasma levels of apoB-containing lipoproteins remain incompletely understood. Furthermore, elevated levels of apoB- containing lipoproteins, particularly TRL, are often associated with non-alcoholic fatty liver disease (NAFLD), which is a major cause of end-stage liver disease but of which the pathophysiology remains poorly understood. Unbiased `genome-wide' human genetics studies have identified a genomic 8q24 locus near the gene TRIB1 significantly associated with all major lipid traits (total cholesterol, LDL-C, HDL-C, TG), CVD, and liver enzymes, making TRIB1 of high interest for functional evaluation. In vivo overexpression and loss-of-function mouse studies have confirmed this association. Studies in mice with liver-specific deletion of Trib1 (Trib1_LSKO) have shown that hepatic Trib1 not only regulates plasma lipids in mice, but also hepatic de novo lipogenesis, the latter of which is accomplished via turnover of the transcription factor C/EBP?. Interestingly though, while Trib1_LSKO mice exhibit increased plasma lipids, this appears to be due to a C/EBP?-independent mechanism. Thus the underlying mechanism for hepatic Trib1 regulation of plasma lipids remains unclear. We present evidence suggesting that increased expression of Angptl8 due to the absence of Trib1 may lead to decreased hepatic clearance of apoB-containing lipoprotein particles. We propose to formally test this hypothesis and to identify the downstream effector of Trib1 responsible for the increased Angptl8 expression. Variants in the TRIB1 genomic locus are all significantly associated with CAD in humans, and while the effect of Trib1 on plasma lipid levels is clear, no study to date has tested its implications for atherosclerosis. Additionally, no role for extra-hepatic TRIB1 in atherogenesis has been investigated. Trib1 has been shown to regulate the polarization of macrophages, and thus, the combination of its roles in liver and extrahepatic tissues likely interplay in Trib1's overall contribution to risk for CAD. We will perform a deep phenotypic analysis of atherosclerosis in mice that lack extrahepatic Trib1 (with and without rescue of liver Trib1 expression). Finally, while our mouse ChIP-Seq data indicates a negative feedback loop between Trib1 and C/EBP?, (C/EBP? binds to downstream non-coding regions corresponding to the human GWAS signal), the functional sequence variation linking TRIB1 and lipoprotein metabolism has yet to be identified. We will identify the functional non-coding SNP underlying the TRIB1 GWAS signal though chromatin conformation capture, and we will validate these functional SNPs with reporter assays and in iPS-derived hepatocytes. In addition, through resequencing efforts, we have found multiple coding sequence variants in TRIB1 and have begun functional testing of them. The studies proposed here will increase our understanding of the novel roles TRIB1 plays in lipid metabolism, shed light on another novel regulator of lipid metabolism (ANGPTL8), and create genomics tools allowing for interrogation of human variation in the TRIB1 locus.
Unbiased `genome-wide' human genetics studies have identified the gene TRIB1, which encodes a protein Tribbles-1, as significantly associated with all major lipid traits (total cholesterol, LDL-C, HDL-C, TG), coronary artery disease, and liver enzymes suggestive of fatty liver disease. This proposal will investigate how TRIB1 alters plasma lipid levels and contributes to the progression of atherosclerosis, while also identifying the regulatory network that controls it, identifying new biological pathways relevant to cardiometabolic disease.
| Khetarpal, Sumeet A; Zeng, Xuemei; Millar, John S et al. (2017) A human APOC3 missense variant and monoclonal antibody accelerate apoC-III clearance and lower triglyceride-rich lipoprotein levels. Nat Med 23:1086-1094 |
