Abstract 16: Tribbles1 Regulates Hepatic Lipogenesis in Mice through Post-Translational Regulation of C/EBPα

Abstract

Tribbles1 (TRIB1) was recently identified in genome-wide association studies as being associated with plasma levels of VLDL, HDL, LDL, and TG as well as coronary artery disease in humans. Here we report studies in a Trib1 liver-specific knockout mouse (Trib1_LSKO) created via AAV-mediated delivery of Cre recombinase into mice with a floxed allele of Trib1. Four weeks after infection, Trib1_LSKO mice exhibit a 25% increase in liver weight (p<0.01) as compared to controls given null virus, and examination of hepatic lipids reveals a 78% increase in TG content (p<0.001) of Trib1_LSKO livers. Hepatic transcription of lipogenic genes (i.e. Scd1, Fasn, and Acaca) was increased greater than 2-fold in Trib1_LSKO mice, and when in vivo lipogenesis was measured, Trib1_LSKO animals exhibited significantly increased hepatic production of fatty acids (2.2-fold, p=0.02), diacylglycerol (1.8-fold, p<0.01), and TG (3.6-fold, p<0.001). Microarray and subsequent pathway analysis of Trib1_LSKO livers revealed altered transcription of genes downstream of the transcription factor C/EBPα, and western blot analysis of liver extracts showed increased protein levels of C/EBPα in Trib1_LSKO mice compared to controls. C57Bl/6 mice overexpressing C/EBPα via AAV (AAV_Cebpa) exhibit increases in hepatic production of TG (3.6-fold, p<0.001), fatty acids (2.5-fold, p<0.001), and diacylglycerol (2.1-fold, p<0.005) that mirror those observed in Trib1_LSKO mice. Interestingly, while Trib1_LSKO mice have altered plasma lipids, AAV_Cebpa animals do not, suggesting that Trib1 modulates plasma lipids via a mechanism separate from C/EBPα-mediated lipogenesis. Finally, ChIP-Seq analysis of C/EBPα DNA occupancy in Trib1_LSKO and AAV_Cebpa mice showed increased binding near lipogenic genes as well as upstream of Trib1 itself. Trib1 does show increased gene expression in response to C/EBPα overexpression, suggesting that Trib1 and C/EBPα participate in a feedback loop. In summary, we demonstrate here that Trib1 regulates de novo lipogenesis through modulation of hepatic protein levels of C/EBPα, which in turn regulates Trib1 expression. These data assign novel roles for both Trib1 and C/EBPα in the regulation of hepatic lipid metabolism.