Regulation of Diverse Endogenous Metabolic and Signaling Pathways by UDP-Glucuronosyltransferases (UGTs) Revealed by Metabolomics of Knockout Mice

Abstract

Uridine 5′-diphospho-glucuronosyltransferase (UGT) enzymes play a critical role in hepatic and other metabolism of endogenous and exogenous small molecules. The addition of glucuronic acid via UGT increases the polarity and subsequent solubility of small molecules, helping direct them for biliary excretion and renal excretion by organic anion transporter OAT3 (SLC22A8). Mutations in UGTs are associated with disorders in biliary metabolism (e.g., Gilbert’s syndrome, Crigler-Najjar syndrome). Here, we used comprehensive serum metabolomic profiling of pan Ugt1-knockout (Ugt1^–/–) mice to define the metabolic consequences of UGT deficiency. All 7 knockout mice exhibited markedly elevated bilirubin (mean = 5.96 mg/dL). Partial Least Squares Discriminant Analysis (PLS-DA) showed clear separation between Ugt1^–/– and wild type (WT), and volcano plots revealed significant endogenous metabolite alterations. These included upregulation of numerous bilirubin degradation products as well as major disturbances in lipid and amino acid metabolism. In particular, pathway enrichment analyses identified disruptions in fatty acid biosynthesis, glycine-serine metabolism, heme breakdown, porphyrin metabolism, and β-oxidation of fatty acids. Thus, the UGT1 proteins regulate different aspects of endogenous metabolism and signaling. Consistent with this notion, there were increases in liver expression of several nuclear receptors (e.g., HNF4a, PPARa, PXR) involved in metabolic signaling. These results support the remote sensing and signaling theory (RSST), which proposes that UGT1 proteins play a key role in organ crosstalk via routing small molecules between the liver and kidney, with glucuronidated molecules being mainly directed to the multispecific SLC kidney transporter OAT3. We propose that this glucuronidation-regulated RSST loop is essential to signaling along the gut-liver-kidney axis.