TRIB1 downregulates hepatic lipogenesis and glycogenesis via multiple molecular interactions

Mammalian tribbles homolog 1 (TRIB1) regulates hepatic lipogenesis and is genetically associated with plasma triglyceride (TG) levels and cholesterol, but the molecular mechanisms remain obscure. We explored these mechanisms in mouse livers transfected with a TRIB1 overexpression, a shRNA template or a control (LacZ) adenovirus vector. The overexpression of TRIB1 reduced, whereas induction of the shRNA template increased, plasma glucose, TG, and cholesterolandsimultaneouslyhepaticTGandglycogenlevels.TheinvolvementofTRIB1inhepatic lipid accumulation was supported by the findings of a human SNP association study. ATRIB1 SNP, rs6982502, was identified in an enhancer sequence, modulated enhancer activity in reporter gene assays, and was significantly (PZ9.39!10 K7 ) associated with ultrasonographically diagnosed nonalcoholic fatty liver disease in a population of 5570 individuals. Transcriptome analyses of mouse livers revealed significant modulation of the gene sets involved in glycogenolysis and lipogenesis. Enforced TRIB1 expression abolished CCAAT/enhancer binding protein A (CEBPA), CEBPB, and MLXIPL proteins, whereas knockdown increased the protein level. Levels of TRIB1 expression simultaneously affected MKK4 (MAP2K4), MEK1 (MAP2K1), and ERK1/2 (MAPK1/3) protein levels andthephosphorylationofJNK,butnotofERK1/2.Pull-downandmammaliantwo-hybridanalyses revealed novel molecular interaction between TRIB1 and a hepatic lipogenic master regulator, MLXIPL. Co-expressionof TRIB1andCEBPAor MLXIPLreduced their protein levels and proteasome inhibitorsattenuatedthereduction.ThesedatasuggestedthatthemodulationofTRIB1expression