Bilirubin deficiency renders mice susceptible to hepatic steatosis in the absence of insulin resistance

Abstract

Background & aims Plasma concentrations of bilirubin, a product of heme catabolism formed by biliverdin reductase A (BVRA), inversely associate with the risk of metabolic diseases including hepatic steatosis and diabetes mellitus in humans. Bilirubin has antioxidant and anti-inflammatory activities and may also regulate insulin signaling and peroxisome proliferator-activated receptor alpha (PPARα) activity. However, a causal link between bilirubin and metabolic diseases remains to be established. Here, we used the global Bvra gene knockout ( Bvra –/– ) mouse as a model of deficiency in bilirubin to assess its role in metabolic diseases. Approach & results We fed mice fat-rich diets to induce hepatic steatosis and insulin resistance. Bile pigments were measured by LC-MS/MS, and hepatic lipids by LC-MS/MS (non-targeted lipidomics), HPLC-UV and Oil-Red-O staining. Oxidative stress was evaluated measuring F 2 -isoprostanes by GC-MS. Glucose metabolism and insulin sensitivity were verified by glucose and insulin tolerance tests, ex vivo and in vivo glucose uptake, and Western blotting for insulin signaling. Compared with wild type littermates, Bvra –/– mice contained negligible bilirubin in plasma and liver, and they had comparable glucose metabolism and insulin sensitivity. However, Bvra –/– mice exhibited an inflamed and fatty liver phenotype, accompanied by hepatic accumulation of oxidized triacylglycerols and F 2 -isoprostanes, in association with depletion of α-tocopherol. α-Tocopherol supplementation reversed the hepatic phenotype and observed biochemical changes in Bvra –/– mice. Conclusions Our data suggests that BVRA deficiency renders mice susceptible to oxidative stress-induced hepatic steatosis in the absence of insulin resistance.

Reactive Oxygen Species (ROS) and Antioxidants as Immunomodulators in Exercise: Implications for Heme Oxygenase and Bilirubin

“…In this same study, low HO-1 mRNA levels in human liver transplants correlated with increased expression of M1 pro-inflammatory markers [ 48 , 49 ]. Liver-specific biliverdin reductase A (BVRA) knockout animals with reduced hepatic bilirubin had worsened fatty liver on a high-fat diet compared to littermate controls [ 50 ], which was confirmed in global BVRA knockout animals [ 51 ]. Bilirubin reduces lipid content and inflammatory markers in mouse models of obesity-induced NAFLD [ 46 , 47 ].…”

“…While there are limited studies showing that BVRA is regulated by exercise [ 70 ], there have been supporting studies showing a role for the enzyme in metabolism [ 50 , 51 , 124 , 125 , 127 ]. Adipocyte-specific deletion of BVRA results in adipocyte hypertrophy and increased inflammation while decreasing mitochondrial number and markers of adipocyte browning such as PPARα and β3 adrenergic receptor ( Adrb3 ) [ 127 ].…”

Association of Serum Bilirubin With Metabolic Syndrome and Non-Alcoholic Fatty Liver Disease: A Systematic Review and Meta-Analysis

“…There is a basic research showed that bilirubin deficiency renders mice susceptible to hepatic steatosis in the absence of insulin resistance. It adopted a kind of contrarian strategy to prove that the pathogenesis of MetS and NAFLD is not exactly identically ( 57 ). In addition, the variety of definitions of MetS and the variety of diagnostic methods for NAFLD in different studies is also make it challenging that assess the consistent-association of serum bilirubin with MetS and NAFLD.…”

Long-Term Effects of Biliverdin Reductase a Deficiency in Ugt1−/− Mice: Impact on Redox Status and Metabolism

“…The observed discrepancy suggests that insulin signaling seems less affected in global Bvra -deficient mice, fed a chow diet, than in liver-specific Bvra -deficient animals. Bvra −/− mice, fed with a high-fat diet, have comparable fasted plasma glucose and insulin, glucose and insulin tolerance, glucose uptake, as well as insulin signaling, but developed a fatty and inflamed liver as a result of enhanced oxidative stress reflected in increased non-enzymatic lipid peroxidation [ 74 ]. Thus, in the context of Bvra −/− Ugt1 −/− and Ugt1 −/− mice, further studies with more specific glucose and diet challenges may be needed to reach more definitive conclusions.…”