Introduction

Biliverdin is a tetrapyrrolic, water-soluble compound formed by the breakdown of heme.[1] Heme is broken down into biliverdin and carbon monoxide and iron by heme oxidase. Biliverdin is then quickly broken down to bilirubin by biliverdin reductase.[2] The buildup of biliverdin and bilirubin, due to increased hemolysis or liver damage resulting in impaired glucuronidation, causes jaundice.[3] While the buildup of bilirubin and biliverdin have been known to have negative effects, recently, numerous beneficial effects of these compounds have been discovered. Biliverdin is anti-mutagenic, an antioxidant, anti-inflammatory, and immunosuppressant. The buildup of biliverdin has been shown to upregulate the activity of biliverdin reductase, an enzyme that is a regulator of the innate immune system.[4]

Fundamentals

Elevated levels of biliverdin and bilirubin have been shown to decrease the risk of vascular injury due to cytoprotective activity, which lowers the incidence of atherosclerosis in these patients. Biliverdin and bilirubin downregulate the proliferation of vascular smooth muscle cells and demonstrate an antioxidant effect in the damaged blood vessels. This decreases the formation of the tunica intima of blood vessels and blocks neovascularization and angiogenesis. Due to this effect of biliverdin and bilirubin, the hypothesis is that biliverdin may be useful in the prevention of atherosclerosis.[5] One study found a significant decrease in intimal hyperplasia following balloon angioplasty and vascular grafting in rats treated with biliverdin before the procedure.[6]

In addition to the cytoprotective activity, biliverdin and bilirubin have demonstrated an anti-inflammatory effect. Biliverdin decreases inflammation by downregulating pro-inflammatory cytokines such as TLR-4 and by stimulating the production of anti-inflammatory cytokines such as IL-10.[7][8] The thought is that biliverdin decreases the expression of TLR-4 due to nitrosylation of biliverdin reductase in response to biliverdin. The nitrosylated biliverdin reductase is then thought to translocate to the nucleus and bind to TLR-4, decreasing the expression of this pro-inflammatory cytokine. Researchers surmise that biliverdin increases the production of IL-10 due to the phosphorylation of biliverdin reductase when biliverdin binds to this enzyme on the surface of macrophages. When biliverdin reductase is phosphorylated, it can bind to PI3K, which then activates Akt. This activation of Akt leads to the increased production of the pro-inflammatory cytokine, IL-10.[8] The anti-inflammatory effects of biliverdin are under investigation in asthmatic patients. The hypothesis is that with elevated levels of biliverdin and bilirubin, patients may experiencing a decrease in their asthma symptoms.[9] These anti-inflammatory effects of biliverdin and bilirubin have been shown to have beneficial effects in patients with rheumatic arthritis due to inhibition of cartilage breakdown.[2]

In addition to the anti-inflammatory effect of biliverdin, it has also been shown to be protective against insulin resistance — elevated levels of biliverdin cause dysregulation of adipogenesis. Biliverdin downregulates the expression of Cd11c and Tnfa, two genes that are responsible for the production of an M1 macrophage and tissue necrosis factor-alpha, a proinflammatory cytokine, respectively. This downregulation due to elevated biliverdin results in decreased fat production and reduced inflammation, thus protecting against insulin resistance in patients indulging in high-fat diets.[10]

Bilirubin and biliverdin have also been shown to be scavengers of hydroxyl peroxide groups showing the antioxidant effects of biliverdin.[2] Researchers have hypothesized that bilirubin is recycled back to biliverdin, which is then metabolized to bilirubin again. This recycling process may play a role in the antioxidant effects of biliverdin. The thought is that bilirubin donates hydrogen to free radicals during this recycling process. Biliverdin has antioxidant effects by forming resonance-stabilized radicals. Further studies are still needed to determine fully how bilirubin and biliverdin demonstrate these antioxidant effects.[6][1] Patients with elevated levels of biliverdin have had a decreased incidence of various cancers.[1] Biliverdin and bilirubin also have immunosuppressive effects. Biliverdin reductase is a direct regulator of the innate immune system.[2] Because biliverdin increases the activity of biliverdin reductase, elevated levels of biliverdin have been shown to have direct effects on the immune syste[4] Also, biliverdin and bilirubin have both been shown to inhibit the activity of the classical complement cascade.[2][4][9] These immunosuppressive effects of biliverdin have demonstrated that biliverdin may be an effective treatment option for patients receiving allograft transplants.[11]

Issues of Concern

Biliverdin and bilirubin may elevate for several reasons. Liver failure results in elevated bilirubin and biliverdin due to impaired metabolism and excretion of bilirubin. Hemolytic diseases cause increased production of biliverdin and bilirubin due to hemoglobin breakdown.[3] Genetic disorders, such as Gilbert Syndrome and Crigler-Najar Syndrome, cause decreased metabolism of bilirubin due to deficiencies of UDP-glucuronosyl transferase, the enzyme responsible for the metabolism of unconjugated bilirubin to conjugated bilirubin. As a result, there are excess levels of biliverdin and bilirubin in the bloodstream.[12][13] All of these conditions lead to jaundice, the clinical symptom of elevated biliverdin and bilirubin in the bloodstream. Increased levels of biliverdin and bilirubin are not dangerous in adults due to a fully formed blood-brain barrier. In neonates, the elevated biliverdin and bilirubin can be harmful because they have a poorly developed blood-brain barrier. Due to the underdeveloped blood-brain barrier, the unconjugated bilirubin can diffuse across the blood-brain barrier to the brain and causes kernicterus, which is permanent brain damage due to the neurotoxic effects of biliverdin and bilirubin.[4][3]

Cellular

Biliverdin is considered a waste product of heme degradation. When red blood cells lyse, either due to hemolytic diseases or by natural apoptosis, heme get released. The heme is metabolized by heme oxygenase using three molecules of oxygen and seven electrons and producing biliverdin IX-alpha, carbon monoxide, and ferrous iron. The biliverdin IX-alpha undergoes metabolism to bilirubin IX-alpha via NADPH-dependent biliverdin reductase. The bilirubin IX-alpha is initially in the unconjugated, fat-soluble, indirect form. It gets conjugated in the liver via UDP-glucuronosyl transferase.[14] The conjugated bilirubin is then further metabolized and excreted in the urine and feces.

Clinical Significance

When red blood cells are injured, they burst open and release their contents into the skin and surrounding tissue. Injury to tissue may result in a bruise due to these contents from the blood. Blood breakdown first releases heme, which is responsible for the purple discoloration of a bruise.[15] Later, as the heme becomes metabolized into biliverdin, the bruise will turn green because biliverdin is a green pigmented chemical.[4] As the biliverdin metabolizes into bilirubin, the bruise will turn yellow.[3] The bruise will finally resolve once the bilirubin fully clears from the affected tissue.