Bilirubin

Bilirubin from heme oxygenase-1 attenuates vascular endothelial activation and dysfunction.[Pubmed: 15499042]

Arterioscler Thromb Vasc Biol. 2005 Jan;25(1):155-60.

Heme oxygenase-1 (HO-1), the rate-limiting enzyme of heme degradation, has recently been considered to have protective roles against various pathophysiological conditions. Since we demonstrated that HO-1 overexpression inhibits atherosclerotic formation in animal models, we examined the effect of HO modulation on proinflammatory cytokine production, endothelial NO synthase (eNOS) expression, and endothelium-dependent vascular relaxation responses.
METHODS AND RESULTS:
After HO-1 induction by heme arginate (HA), vascular endothelial cell cultures were exposed to oxidized low-density lipoprotein (oxLDL) or tumor necrosis factor-alpha (TNF-alpha). HA pretreatment significantly attenuated the production of vascular cell adhesion molecule-1, monocyte chemotactic protein-1, and macrophage colony-stimulating factor, suggesting that HO-1 induction attenuates proinflammatory responses. In addition, HO-1 overexpression also alleviated endothelial dysfunction as judged by restoration of attenuated eNOS expression after exposure to oxLDL and TNF-alpha. Importantly, impaired endothelium-dependent vascular relaxation responses in thoracic aortic rings from high-fat-fed LDL receptor knockout mice were also improved. These effects were observed by treatment with Bilirubin not by carbon monoxide.
CONCLUSIONS:
These results suggest that the antiatherogenic properties of HO-1 may be mediated predominantly through the action of Bilirubin by inhibition of vascular endothelial activation and dysfunction in response to proinflammatory stresses.

Bilirubin is an antioxidant of possible physiological importance.[Pubmed: 3029864]

Science. 1987 Feb 27;235(4792):1043-6.

METHODS AND RESULTS:
Bilirubin, the end product of heme catabolism in mammals, is generally regarded as a potentially cytotoxic, lipid-soluble waste product that needs to be excreted. However, it is here that Bilirubin, at micromolar concentrations in vitro, efficiently scavenges peroxyl radicals generated chemically in either homogeneous solution or multilamellar liposomes. The antioxidant activity of Bilirubin increases as the experimental concentration of oxygen is decreased from 20% (that of normal air) to 2% (physiologically relevant concentration). Furthermore, under 2% oxygen, in liposomes, Bilirubin suppresses the oxidation more than alpha-tocopherol, which is regarded as the best antioxidant of lipid peroxidation.
CONCLUSIONS:
The data support the idea of a "beneficial" role for Bilirubin as a physiological, chain-breaking antioxidant.

Bilirubin and glutathione have complementary antioxidant and cytoprotective roles.[Pubmed: 19286972]

Proc Natl Acad Sci U S A. 2009 Mar 31;106(13):5171-6.

METHODS AND RESULTS:
Glutathione (GSH) and Bilirubin are prominent endogenous antioxidant cytoprotectants. Despite tissue levels that are thousands of times lower than GSH, Bilirubin is effective because of the biosynthetic cycle wherein it is generated from biliverdin by biliverdin reductase (BVR). When Bilirubin acts as an antioxidant, it is oxidized to biliverdin, which is immediately reduced by BVR to Bilirubin. Why does the body employ both of these 2 distinct antioxidant systems?
CONCLUSIONS:
We show that the water-soluble GSH primarily protects water soluble proteins, whereas the lipophilic Bilirubin protects lipids from oxidation. Mice with deletion of heme oxygenase-2, which generates biliverdin, display greater lipid than protein oxidation, while the reverse holds for GSH depletion. RNA interference depletion of BVR increases oxidation of lipids more than protein. Depletion of BVR or GSH augments cell death in an oxidant-specific fashion.

Bilirubin decreases nos2 expression via inhibition of NAD(P)H oxidase: implications for protection against endotoxic shock in rats.[Pubmed: 16129699 ]

FASEB J. 2005 Nov;19(13):1890-2

We investigated a possible beneficial role for Bilirubin, one of the products of heme degradation by the cytoprotective enzyme heme oxygenase-1 in counteracting Escherichia coli endotoxin-mediated toxicity.
METHODS AND RESULTS:
Homozygous jaundice Gunn rats, which display high plasma Bilirubin levels due to deficiency of glucuronyl transferase activity, and Sprague-Dawley rats subjected to sustained exogenous Bilirubin administration were more resistant to endotoxin (LPS)-induced hypotension and death compared with nonhyperBilirubinemic rats. LPS-stimulated production of nitric oxide (NO) was significantly decreased in hyperBilirubinemic rats compared with normal animals; this effect was associated with reduction of inducible NO synthase (NOS2) expression in renal, myocardial, and aortic tissues. Furthermore, NOS2 protein expression and activity were reduced in murine macrophages stimulated with LPS and preincubated with Bilirubin at concentrations similar to that found in the serum of hyperBilirubinemic animals. This effect was secondary to inhibition of NAD(P)H oxidase since 1) inhibition of NAD(P)H oxidase attenuated NOS2 induction by LPS, 2) Bilirubin decreased NAD(P)H oxidase activity in vivo and in vitro, and 3) down-regulation of NOS2 by Bilirubin was reversed by addition of NAD(P)H.
CONCLUSIONS:
These findings indicate that Bilirubin can act as an effective agent to reduce mortality and counteract hypotension elicited by endotoxin through mechanisms involving a decreased NOS2 induction secondary to inhibition of NAD(P)H oxidase.

Heme Degradation by Heme Oxygenase Protects Mitochondria but Induces ER Stress via Formed Bilirubin.[Pubmed: 25942605]

Biomolecules. 2015 Apr 30;5(2):679-701.

Heme oxygenase (HO), in conjunction with biliverdin reductase, degrades heme to carbon monoxide, ferrous iron and Bilirubin (BR); the latter is a potent antioxidant. The induced isoform HO-1 has evoked intense research interest, especially because it manifests anti-inflammatory and anti-apoptotic effects relieving acute cell stress. The mechanisms by which HO mediates the described effects are not completely clear. However, the degradation of heme, a strong pro-oxidant, and the generation of BR are considered to play key roles. The aim of this study was to determine the effects of BR on vital functions of hepatocytes focusing on mitochondria and the endoplasmic reticulum (ER).
METHODS AND RESULTS:
The affinity of BR to proteins is a known challenge for its exact quantification. We consider two major consequences of this affinity, namely possible analytical errors in the determination of HO activity, and biological effects of BR due to direct interaction with protein function. In order to overcome analytical bias we applied a polynomial correction accounting for the loss of BR due to its adsorption to proteins. To identify potential intracellular targets of BR we used an in vitro approach involving hepatocytes and isolated mitochondria. After verification that the hepatocytes possess HO activity at a similar level as liver tissue by using our improved post-extraction spectroscopic assay, we elucidated the effects of increased HO activity and the formed BR on mitochondrial function and the ER stress response.
CONCLUSIONS:
Our data show that BR may compromise cellular metabolism and proliferation via induction of ER stress. ER and mitochondria respond differently to elevated levels of BR and HO-activity. Mitochondria are susceptible to hemin, but active HO protects them against hemin-induced toxicity. BR at slightly elevated levels induces a stress response at the ER, resulting in a decreased proliferative and metabolic activity of hepatocytes. However, the proteins that are targeted by BR still have to be identified.

Bilirubin, formed by activation of heme oxygenase-2, protects neurons against oxidative stress injury.[Pubmed: 10051662 ]

Proc Natl Acad Sci U S A. 1999 Mar 2;96(5):2445-50.

Heme oxygenase (HO) catalyzes the conversion of heme to carbon monoxide, iron, and biliverdin, which is immediately reduced to Bilirubin (BR). Two HO active isozymes exist: HO1, an inducible heat shock protein, and HO2, which is constitutive and highly concentrated in neurons.
METHODS AND RESULTS:
We demonstrate a neuroprotective role for BR formed from HO2. Neurotoxicity elicited by hydrogen peroxide in hippocampal and cortical neuronal cultures is prevented by the phorbol ester, phorbol 12-myristate 13-acetate (PMA) via stimulation of protein kinase C. We observe phosphorylation of HO2 through the protein kinase C pathway with enhancement of HO2 catalytic activity and accumulation of BR in neuronal cultures.
CONCLUSIONS:
The neuroprotective effects of PMA are prevented by the HO inhibitor tin protoporphyrin IX and in cultures from mice with deletion of HO2 gene. Moreover, BR, an antioxidant, is neuroprotective at nanomolar concentrations.

Bilirubin: a natural inhibitor of vascular smooth muscle cell proliferation.[Pubmed: 16087796 ]

Circulation. 2005 Aug 16;112(7):1030-9.

Bilirubin, a natural product of heme catabolism by heme oxygenases, was considered a toxic waste product until 1987, when its antioxidant potential was recognized. On the basis of observations that oxidative stress is a potent trigger in vascular proliferative responses, that heme oxygenase-1 is antiatherogenic, and that several studies now show that individuals with high-normal or supranormal levels of plasma Bilirubin have a lesser incidence of atherosclerosis-related diseases, we hypothesized that Bilirubin would have salutary effects on preventing intimal hyperplasia after balloon injury.
METHODS AND RESULTS:
We found less balloon injury-induced neointima formation in hyperBilirubinemic Gunn rats and in wild-type rats treated with biliverdin, the precursor of Bilirubin, than in controls. In vitro, Bilirubin and biliverdin inhibited serum-driven smooth muscle cell cycle progression at the G1 phase via inhibition of the mitogen-activated protein kinase signal transduction pathways and inhibition of phosphorylation of the retinoblastoma tumor suppressor protein.
CONCLUSIONS:
Bilirubin and biliverdin might be potential therapeutics in vascular proliferative disorders.