Bilirubin bioconversion to urobilin in the gut-liver-kidney axis: A biomarker for insulin resistance in the Cardiovascular-Kidney-Metabolic (CKM) Syndrome

Abstract

The rising rates of obesity worldwide have increased the incidence of cardiovascular disease (CVD), making it the number one cause of death. Higher plasma bilirubin levels have been shown to prevent metabolic dysfunction and CVD. However, reducing levels leads to deleterious outcomes, possibly due to reduced bilirubin half-life that escalates the production of its catabolized product, urobilinogen, produced by gut bacteria and naturally oxidized to urobilin. Recent findings suggest that the involvement of the microbiome catabolism of bilirubin to urobilin and its absorption via the hepatic portal vein contributes to CVD, suggesting a liver-gut axis involvement. We discuss the studies that demonstrate that urobilin is frequently raised in the urine of persons with CVD and its probable role in acquiring the disease. Urobilin is excreted from the kidneys into the urine and may serve as a biomarker for Cardiovascular-Kidney-Metabolic (CKM) Syndrome. We deliberate on the newly discovered bilirubin reductase (BilR) bacterial enzyme that produces urobilin. We discuss the bacterial species expressing BilR, how they impact CVD, and whether suppressing urobilin production and increasing bilirubin may provide new therapeutic strategies for CKM. Possible therapeutic mechanisms for achieving this goal are discussed.

Keywords: Antioxidant; BLVRA; BilR; Bilirubin reductase; Biliverdin reductase; HMOX1; Heme oxygenase; MASLD; Microbiome; Obesity.

Fig. 1 |. Bilirubin and Urobilin Have Inverse Effects on Cardiovascular and Metabolic Diseases.

a | Plasma bilirubin is regulated by the UDP glucuronosyltransferase family 1 member A1 (UGT1A1) and bilirubin reductase (BilR) enzymes that control bilirubin catabolism to urobilin. Hyperbilirubinemia is inversely associated with the cardiovascular and metabolic effects of urobilinuria, atherosclerosis, hypertension, adiposity, insulin resistance, and oxidized low-density lipoprotein (LDL) levels. b | Hyperurobilinemia positively correlates with these cardiovascular and metabolic effects.

Fig. 2 |. Bilirubin Functions as a Hormone that Reduces Lipid Accumulation.

Biliverdin released from heme is reduced to bilirubin by biliverdin reductase A (BVRA), which also suppresses glycogen synthase kinase 3β (GSK3β). Bilirubin binds directly to the PPARα nuclear receptor transcription factor but not biliverdin. BVRA inhibition of GSK3 protects PPARα from protein degradation targeting when GSK3β phosphorylates the serine 73 (S73) amino acid. The bilirubin-PPARα interaction regulates genes that increase hepatic β-oxidation via mitochondria, reducing hepatocyte fat content.

Fig. 3 |. The Distribution of Plasma Bilirubin Levels Across Body Mass Index in the Normal and Fatty Liver Patient Populations.

A retrospective chart review of patient data was collected from the University of Kentucky Center for Clinical and Translational Science (CCTS) Biomedical Informatics team. (A) The distribution of bilirubin compared to body mass index (BMI) in 164,151 University of Kentucky Healthcare System patients. (B) The distribution of bilirubin compared to BMI in 6,257 patients diagnosed with fatty liver – metabolic dysfunction-associated steatotic liver disease (MASLD). The green and red arrows denote a BMI of 25. The images below represent healthy and MASLD liver phenotypes, the impact of fat accumulation (shown inside the circle), and changes in plasma bilirubin levels observed in the general population of these patients.

Fig. 4 |. Urinary Urobilinogen/Urobilin is a Biomarker for Cardiovascular Disease.

During a heart attack and in people with cardiovascular diseases, urinary urobilinogen/urobilin levels have been shown to be significantly increased (known as urobilinuria).

Fig. 5 |. Raising Plasma Urobilin Increases the Risk Factors for Cardiovascular Disease.

Bilirubin is conjugated in the liver by the UDP glucuronosyltransferase family 1 member A1 (UGT1A1) enzyme to remove it from blood and deliver it to the intestine through the biliary system. Within the intestine, gut microbiota containing the bilirubin reductase (BilR) enzyme removes the conjugation to form urobilinogen, which can be reabsorbed into the circulation and oxidized to urobilin. Plasma urobilin levels have been positively correlated to heart failure, acute myocardial ischemia, adiposity, visceral fat area, and insulin resistance.

Fig. 6 |. Bilirubin and Urobilin Have Inverse Effects on Cardiometabolic Diseases.

Bilirubin and urobilin have opposing effects on metabolic and cardiovascular diseases. Through its antioxidant and hormonal functions, bilirubin is protective against atherosclerosis, obesity, oxidized low-density lipoprotein (LDL), type 2 diabetes, and hypertension. However, urobilin is positively associated with obesity, oxidized LDL, insulin resistance, heart failure, and acute myocardial infarction. Abbreviations: PPARα, peroxisome proliferator-activated receptor alpha; FGF21, fibroblast growth factor 21; CPT1a, carnitine palmitoyltransferase 1A.

Fig. 7 |. Hypothetical Methods to Regulate Bilirubin Catabolism to Urobilin.

Future therapeutics for treating cardiovascular and metabolic diseases and their related comorbidities might target the catabolism of bilirubin to urobilin. This could be achieved by targeting a | UDP glucuronosyltransferase family 1 member A1 (UGT1A1) enzyme, b | gut microbiota, or c | the bilirubin reductase (BilR) enzyme.

Box 1 |. Structural Changes to Bilirubin During Production and Catabolism to Urobilin.

Bilirubin is produced from the breakdown of red blood cells when heme is released and converted to biliverdin by heme oxygenase (HO). Biliverdin is reduced to bilirubin by biliverdin reductase (BVR) to produce bilirubin. For excretion through the biliary system, bilirubin is glucuronidated by the UDP-glucuronosyltransferase-1 (UGT1A1) enzyme forming conjugated bilirubin (diglucuronidated bilirubin). Within the intestine, the microbiome bacteria containing bilirubin reductase (BilR) reduces the conjugation of bilirubin to urobilinogen. The urobilinogen is oxidized to urobilin, which can be reabsorbed into circulation via the hepatic portal system.