Coronary artery disease (CAD) is the leading cause of mortality worldwide, for which the strongest causal risk factors are elevated plasma levels of low-density lipoprotein cholesterol (LDL) and triglyceride (TG)-rich lipoproteins (TRL). One of the strongest novel loci identified by genome wide association studies for plasma lipid traits is at chromosomal locus 8q24, which contains the TRIB1 gene. TRIB1 encodes the pseudokinase, Tribbles homolog 1 (TRIB1) which is catalytically inactive, but has been proposed to act as an adaptor and scaffolding protein, targeting the transcription factor CEBP? for ubiquitination and proteosomal degradation. Variants at this locus in humans are associated with lower CAD risk, lower levels of LDL, total cholesterol and TG, and beneficially, higher levels of high-density lipoproteins (HDL). Hepatic overexpression of Trib1 in mice results in decreased plasma lipid levels and hepatic fat, while hepatic-specific Trib1 deficiency (Trib1 LSKO) increases plasma lipids, lipogenesis and hepatic steatosis. A tissue-specific approach was necessary because mice homozygous for Trib1 whole body deletion (Trib1 KO) on a pure C57BL/6 background have a highly penetrant neonatal lethality. My first goal is to elucidate how Trib1 hepatic deficiency leads to the plasma lipid phenotype in mice. Plasma lipids are regulated by the balance between their rates of secretion and clearance from circulation; both processes heavily influenced by the liver. Preliminary data shows decreased LDL clearance rate when comparing Trib1 LSKO with control mice, suggesting that a defect in clearance could explain the phenotype. We previously demonstrated that increased hepatic expression of Cebpa phenocopies Trib1 LSKO with respect to increased lipogenesis and the development of hepatic steatosis. However, it is still unknown if this is also the causal mechanism for increased plasma lipid in Trib1 LSKO mice. My preliminary data shows that deleting Cebpa in a liver specific manner (Cebpa LSKO) induces a decrease in plasma lipids. I hypothesize that the elevated plasma lipid phenotype in Trib1 LSKO mice is due to increase in Cebpa expression and by impaired LDL clearance. My second goal is to determine the roles of Trib1 in regulating glucose metabolism and to define the physiological mechanism by which Trib1 whole body deletion leads to neonatal death. My preliminary data from neonates (less than 12 hours post-partum) shows a significant reduction in blood glucose levels in mice homozygous and heterozygous for the Trib1 deletion compared to control mice. Consistent with this, adult Trib1 LSKO mice exhibit rapid glucose clearance, which correlates with increased plasma insulin levels. I propose to test the hypothesis that neonatal lethality in Trib1 KO mice is results from severe hypoglycemia due to increased plasma insulin levels (hyperinsulinemia). Taken together, the proposed studies will provide substantial new insight into the molecular and cellular mechanisms by which Trib1 modulates plasma lipid levels and glucose metabolism, as well as CAD.
Despite advances in lipid lowering drugs, Coronary Artery Disease (CAD) remains the leading cause of death worldwide, and a better understanding of the genetics underlying CAD risk may offer new avenues for improving human health. The TRIB1 gene (at chromosomal locus 8q24) has been implicated by genome wide association with plasma lipid traits (LDL, HDL, total cholesterol, triglycerides), hepatic steatosis, and with CAD risk. The proposed studies will provide substantial new insights into the molecular and cellular mechanisms by which Trib1 modulates lipid and glucose metabolism in mice, and are likely to lead to a greater understanding its role in CAD risk and progression. !
