The Role of Peroxisomal CoA Degradation in the Regulation of Bile Acid Metabolism

Abstract

Coenzyme A (CoA) is an essential metabolic cofactor that activates fatty acids and other organic acids as CoA thioesters for a multitude of metabolic pathways in different subcellular compartments. For example, CoA is utilized in the bile acid synthesis pathway and for shortening of very long chain fatty acids in the peroxisomes. In the liver, CoA levels are tightly regulated and dynamically adjusted in response to changes in nutritional status and energy demands. Balance between CoA synthesis and degradation is responsible for the regulation of CoA. Three intracellular CoA diphosphohydrolases, Nudt7, Nudt8, and Nudt19, catalyze the degradation of CoA by catalyzing the hydrolysis of its pyrophosphate bond. Nudt7 is the major CoA-degrading enzyme in the liver and resides in the peroxisomes. This enzyme displays a broad substrate specificity; it is active against free CoA, short- and medium-chain acyl-CoAs, including acetyl-CoA, and the acyl-CoA derivatives of bile acids.Nudt7 expression and activity are regulated by nutritional state, as they both decrease upon fasting. Overexpression of Nudt7 in the liver was recently found to decrease the levels of a variety of short-chain acyl-CoAs and choloyl-CoA, and to decrease both the rate of peroxisomal fatty acid oxidation and the concentration of bile acids in the liver. To gain further insight into the role of Nudt7 in vivo, we generated Nudt7-/- mice and analyzed their lipid metabolism in response to a dietary challenge with a high fat, high cholesterol (western) diet. Deletion of Nudt7 caused a significant increase in total hepatic CoA levels in Nudt7-/- males compared to control mice.Targeted and untargeted metabolomics analysis of whole liver extracts revealed an increase in glutaryl- and adipoyl-CoA in the Nudt7-/- males, associated with an accumulation of glutaryl-carnitine and other short-chain dicarboxylic acylcarnitines and with a decrease in long-chain acyl carnitines, suggesting an increase in the rate of fatty acid oxidation. Pathway enrichment analysis revealed changes in bile acid metabolism in both male and female Nudt7-/- mice compared to gender-matched controls.Specifically, deletion of Nudt7 led to an accumulation of bile acids in the liver, gallbladder, and small intestine, which also correlated with higher serum levels of Fgf15. This increase in bile acids occurred without changes in the expression of genes involved in bile acid synthesis or transport and without a correspondent increase in the concentration of bile acid precursors 7α-hydroxycholesterol and 7α-hydroxycholestenone. These results suggest that higher CoA levels in the peroxisomes may mediate an increase in flux through the bile acid biosynthetic pathway by activating the last, peroxisomal steps in the pathway. Overall, analysis of lipid metabolism following manipulation of Nudt7 expression in mouse liver indicates that this enzyme contributes to the regulation of peroxisomal fatty acids and bile acid metabolism.