Identification of Binding Regions of Bilirubin in the Ligand-Binding Pocket of the Peroxisome Proliferator-Activated Receptor-A (PPARalpha)

Recent work has shown that bilirubin has a hormonal function by binding to the peroxisome proliferator-activated receptor-α (PPARα), a nuclear receptor that drives the transcription of genes to control adiposity. Our previous in silico work predicted three potential amino acids that bilirubin may interact with by hydrogen bonding in the PPARα ligand-binding domain (LBD), which could be responsible for the ligand-induced function. To further reveal the amino acids that bilirubin interacts with in the PPARα LBD, we harnessed bilirubin’s known fluorescent properties when bound to proteins such as albumin. Our work here revealed that bilirubin interacts with threonine 283 (T283) and alanine 333 (A333) for ligand binding. Mutational analysis of T283 and A333 showed significantly reduced bilirubin binding, reductions of 11.4% and 17.0%, respectively. Fenofibrate competitive binding studies for the PPARα LBD showed that bilirubin and fenofibrate possibly interact with different amino acid residues. Furthermore, bilirubin showed no interaction with PPARγ. This is the first study to reveal the amino acids responsible for bilirubin binding in the ligand-binding pocket of PPARα. Our work offers new insight into the mechanistic actions of a well-known molecule, bilirubin, and new fronts into its mechanisms.

Bilirubin as a metabolic hormone: the physiological relevance of low levels

Recent research on bilirubin, a historically well-known waste product of heme catabolism, suggests an entirely new function as a metabolic hormone that drives gene transcription by nuclear receptors. Studies are now revealing that low plasma bilirubin levels, defined as "hypobilirubinemia," are a possible new pathology analogous to the other end of the spectrum of extreme hyperbilirubinemia seen in patients with jaundice and liver dysfunction. Hypobilirubinemia is most commonly seen in patients with metabolic dysfunction, which may lead to cardiovascular complications and possibly stroke. We address the clinical significance of low bilirubin levels. A better understanding of bilirubin's hormonal function may explain why hypobilirubinemia might be deleterious. We present mechanisms by which bilirubin may be protective at mildly elevated levels and research directions that could generate treatment possibilities for patients with hypobilirubinemia, such as targeting of pathways that regulate its production or turnover or the newly designed bilirubin nanoparticles. Our review here calls for a shift in the perspective of an old molecule that could benefit millions of patients with hypobilirubinemia.

Bilirubin in metabolic syndrome and associated inflammatory diseases: New perspectives

Diabetes mellitus is one of the major global health issues, which is closely related to metabolic dysfunction and the chronic inflammatory diseases. Multiple studies have demonstrated that serum bilirubin is negatively correlated with metabolic syndrome and associated inflammatory diseases, including atherosclerosis, hypertension, etc. However, the roles of bilirubin in metabolic syndrome and associated inflammatory diseases still remain unclear. Here, we explain the role of bilirubin in metabolic syndrome and chronic inflammatory diseases and its therapeutic potential. Understanding the role of bilirubin activities in diabetes may serve as a therapeutic target for the treatment of chronic inflammatory diseases in diabetic patients.

A link between bilirubin levels and atrial fibrillation recurrence after catheter ablation

BACKGROUND

Bilirubin is associated with different cardiovascular diseases. The relationship between bilirubin and atrial fibrillation (AF) remains unclear. The aim of this study is to determine the association between bilirubin and AF recurrence after catheter ablation.

METHODS

A total of 212 patients who received AF ablation were retrospectively studied. The total bilirubin level, clinical characteristics, and echocardiographic findings were analyzed to predict the outcome of AF ablations.

RESULTS

During a mean follow-up period of 12.2 ± 5.8 months, 61 (28.8%) patients had AF recurrence after catheter ablation. The patients with AF recurrence had a larger left atrial (LA) diameter (39.8 ± 6.3 versus 36.7 ± 5.8 mm; p = 0.001) and higher total bilirubin levels (0.82 ± 0.37 versus 0.63 ± 0.29 mg/dL; p < 0.001) than those without recurrence. The patients with recurrence had higher direct and indirect bilirubin levels than patients without recurrence. The total bilirubin level remained an independent predictor of AF recurrence after multivariate analysis (odds ratio, 4.95; 95% CI, 1.65-14.83; p = 0.004). We identified a cut point of the total bilirubin level for predicting AF recurrence by receiver operator characteristic curve (cut point, 0.7 mg/dL; area under the curve, 0.65; p < 0.001). The total bilirubin levels were positively correlated with the neutrophil counts. However, there were no associations among the total bilirubin level, left atrial (LA) diameter, and voltage.

CONCLUSION

Higher serum bilirubin levels were associated with AF recurrence in paroxysmal AF patients following catheter ablation.

Bilirubin: from an unimportant waste product to important myocardial infarction predictor

Bilirubin is the major catabolic product of heme degradation. It has long been regarded as an unimportant waste product. However, within the last twenty-five years, it has been demonstrated to play a very important role in maintaining the bodys redox equilibrium. Disturbances of this equilibrium - increased oxidative stress - are currently considered one of the major risk factors for the development of non-communicable diseases. Although the exact mechanism is not known, a number of human studies have proved a reduced incidence of a number of (especially cardiovascular but also cancer) diseases in individuals with mildly elevated bilirubin concentrations. Key words: bilirubin - cardiovascular disease - morbidity - mortality.

Hyperbilirubinemia, Hypertension, and CKD: the Links

PURPOSE OF REVIEW

This review aims to highlight recent advances on the role of hyperbilirubinemia in hypertension and chronic kidney disease, with a focus on the pathophysiological mechanisms explaining the protective effects of bilirubin. An overview of pharmacologic induction of hyperbilirubinemia will also be discussed.

RECENT FINDINGS

The findings depict a protective role of bilirubin in the development of hypertension and cardiovascular diseases. Hyperbilirubinemia is also negatively correlated with the development and progression of chronic kidney disease. Commonly used drugs play a role in pharmacologic induction of hyperbilirubinemia. Bilirubin is therefore an exciting target for new therapeutic interventions for its antioxidant properties can be pivotal in the management of hypertension and in preventing and halting the progression of chronic kidney disease. Longitudinal studies are warranted to evaluate the prospective association between bilirubin levels and incident hypertension and chronic kidney disease in the general population. Interventions to induce hyperbilirubinemia need to be explored as a novel therapeutic approach in fighting disease burden.

Bilirubin scavenges chloramines and inhibits myeloperoxidase-induced protein/lipid oxidation in physiologically relevant hyperbilirubinemic serum

Hypochlorous acid (HOCl), an oxidant produced by myeloperoxidase (MPO), induces protein and lipid oxidation, which is implicated in the pathogenesis of atherosclerosis. Individuals with mildly elevated bilirubin concentrations (i.e., Gilbert syndrome; GS) are protected from atherosclerosis, cardiovascular disease, and related mortality. We aimed to investigate whether exogenous/endogenous unconjugated bilirubin (UCB), at physiological concentrations, can protect proteins/lipids from oxidation induced by reagent and enzymatically generated HOCl. Serum/plasma samples supplemented with exogenous UCB (≤250µM) were assessed for their susceptibility to HOCl and MPO/H2O2/Cl(-) oxidation, by measuring chloramine, protein carbonyl, and malondialdehyde (MDA) formation. Serum/plasma samples from hyperbilirubinemic Gunn rats and humans with GS were also exposed to MPO/H2O2/Cl(-) to: (1) validate in vitro data and (2) determine the relevance of endogenously elevated UCB in preventing protein and lipid oxidation. Exogenous UCB dose-dependently (P<0.05) inhibited HOCl and MPO/H2O2/Cl(-)-induced chloramine formation. Albumin-bound UCB efficiently and specifically (3.9-125µM; P<0.05) scavenged taurine, glycine, and N-α-acetyllysine chloramines. These results were translated into Gunn rat and GS serum/plasma, which showed significantly (P<0.01) reduced chloramine formation after MPO-induced oxidation. Protein carbonyl and MDA formation was also reduced after MPO oxidation in plasma supplemented with UCB (P<0.05; 25 and 50µM, respectively). Significant inhibition of protein and lipid oxidation was demonstrated within the physiological range of UCB, providing a hypothetical link to protection from atherosclerosis in hyperbilirubinemic individuals. These data demonstrate a novel and physiologically relevant mechanism whereby UCB could inhibit protein and lipid modification by quenching chloramines induced by MPO-induced HOCl.

Serum bilirubin and the risk of hypertension

BACKGROUND

Experimental studies suggest oxidative stress could lead to the development of hypertension. Serum bilirubin is a major contributor to the antioxidant capacity in blood plasma and has been identified as an independent cardiovascular risk factor in cohort studies. However, data on the relationship between bilirubin and blood pressure are scarce and inconclusive.

METHODS

We analysed data from the National Health and Nutrition Examination Surveys (NHANES) 1999-2012 (N=31069). Fifty multiple imputed data sets were generated and analysed to avoid selection/confounding bias due to excluding individuals/variables with missing values. A minimal sufficient adjustment set of variables (MSAS) needed to estimate the unconfounded effect of bilirubin on blood pressure and hypertension (systolic/diastolic blood pressure ≥ 140/90 mmHg or using antihypertensive medication) was identified using the back-door criterion and included in all regression models.

RESULTS

After adjustment for the MSAS variables, systolic blood pressure decreased progressively up to -2.5 mmHg (p<0.001) and the prevalence of hypertension was up to 25% lower (P<0.001) in those with bilirubin ≥ 1.0 mg/dl-the highest two deciles-compared with those with 0.1-0.4 mg/dl-the lowest decile. Sensitivity analyses showed these results were unlikely to be explained by residual confounding or selection bias.

CONCLUSIONS

High serum bilirubin may decrease the risk of hypertension by inactivating and inhibiting the synthesis of reactive oxygen species in vascular cells. Strategies to boost the bioavailability of circulating and tissue bilirubin or to mimic bilirubin's antioxidant properties could have a significant impact on prevention and control of hypertension as well as coronary heart disease.

Antioxidant activity of liver growth factor, a bilirubin covalently bound to albumin

We previously reported that treatment of spontaneously hypertensive rats (SHR) with liver growth factor (LGF), an albumin-bilirubin complex with a covalent bond, reduces blood pressure, improves nitric oxide (NO)-dependent vasodilatation, and exerts vascular antifibrotic actions. Because bilirubin, albumin, and albumin-bound bilirubins have antioxidant properties, we hypothesize that LGF might exert its cardiovascular actions through an antioxidant mechanism. We have tested in vitro the capacity of LGF to scavenge ABTS cation and peroxyl and hydroxyl radicals and to protect vascular NO from degradation by superoxide anion. We have also compared the antioxidant capacity of LGF with that of its molecular components albumin and bilirubin and the reference antioxidant trolox. LGF exhibited antioxidant capacity against all free radicals tested at lower concentrations than albumin, bilirubin, and trolox. LGF, bilirubin, and albumin were also able to protect endothelial NO from superoxide anion degradation in a fashion similar to that of superoxide dismutase or tiron, but at much lower concentrations. These data, together with our previous results in SHR, suggest that LGF might exert its cardiovascular regenerative actions, at least in part, through an antioxidant mechanism and that LGF could be a relevant circulating antioxidant in situations of oxidative stress.

Polyamine oxidase activity in peripheral blood of newborn infants with neonatal hyperbilirubinemia: is bilirubin an antioxidant?

BACKGROUND

Neonatal hyperbilirubinemia can be physiological and pathological and most frequently is a consequence of faster erythrocytes (RBC) hemolysis. Free unconjugated bilirubin is a highly toxic compound, especially for the central nervous system. The most abundant polyamines circulating in blood are spermidine (Spd) and spermine (Sp), which are mainly localized in RBC, where they control membrane permeability. Polyamine oxidase (PAO) exerts an important activity in the plasma and erythrocytes of newborn infants with hyperbilirubinemia, catalyzing the oxidative deamination of Sp and Spd, producing potentially toxic agents that induce apoptosis of mammalian cells. The present study investigated polyamine metabolism by measuring PAO activity in the blood of newborn infants with hyperbilirubinemia and explored the possible antioxidant function of bilirubin through monitoring malondialdehyde (MDA) levels.

METHODS

The study included 43 newborns, 10 in the control and 33 in the diseased group. Blood PAO activity and bilirubin and MDA levels were measured using spectrophotometric methods.

RESULTS/DISCUSSION

Our results indicate that bilirubin, at physiologic concentrations, protects neonatal erythrocytes against oxidative stress. The positive correlation between PAO activity and MDA levels with high bilirubin concentrations (> 200 micromol/L) in newborn infants indicates that in pathological conditions, bilirubin cannot exert its antioxidant function.

CONCLUSION

Investigating the function of polyamines in erythrocytes and the importance of PAO related to hemolysis and bilirubin synthesis is necessary to shed light on the functions of PAO and its metabolites on the permeability of the erythrocyte membrane.

A device for adsorbing unbound, unconjugated bilirubin and its effects in vitro and in a hyperbilirubinemic newborn piglet

AIMS

A novel blood purification material that we previously reported as a superantigen- and cytokine-adsorbing device (SCAD) was evaluated for its ability to adsorb unbound, unconjugated bilirubin (UUBil) in vitro and in vivo.

METHODS

In albumin-containing buffer, UUBil was dissolved and circulated through the SCAD column. Also, bilirubin was infused into low-body weight newborn piglets and hemoperfused for 3 h over SCAD columns.

RESULTS

In albumin-containing buffer, concentration of bilirubin decreased from 34 to 0.6 mg/dL within 5 h and the SCAD fiber turned brown, indicating that bilirubin was adsorbed onto the surface of the adsorbent and was not degraded during the circulation. Using the hyperbilirubinemia swine, clearances of total bilirubin (TBil), direct bilirubin (DBil), and indirect bilirubin (IdBil) were significantly higher (P<0.01) in the SCAD group compared with the control group. The clearances of TBil, DBil, and IdBil at 3 h after the initiation of the bilirubin infusion were 0.47, 0.53, and 0.45 mL/min, respectively, at a blood flow rate of 2.5 mL/min, and this result indicates that almost 20% of bilirubins were adsorbed to the SCAD column in a single passage.

CONCLUSION

These results provide initial evidence that SCAD treatment is effective in the removal of UUBil and can be performed safely in newborn animals.

Interdiction of the diabetic state in NOD mice by sustained induction of heme oxygenase: possible role of carbon monoxide and bilirubin

The aims of the present study were to assess whether sustained HO-1 expression could moderate or prevent diabetes in an animal model of the disease and, if so, to examine the possible mechanisms involved. Our results showed that HO-1 expression and HO activity were upregulated in the pancreas of non-obese diabetic (NOD) mice by the weekly administration of cobalt protoporphyrin (CoPP). Blood glucose levels in CoPPtreated mice decreased to normal, but continuously increased in untreated controls. Beta-cell numbers were preserved in the islets of CoPP-treated mice, whereas no beta cells were found in untreated diabetic mice. The number of CD11c(+) dendritic cells was significantly decreased in the pancreas of CoPP-treated NOD mice, but this effect was reversed by the inhibition of HO activity. Increased levels of HO-1 produced a new pancreatic phenotype, as reflected by increases in phosphorylated AKT, BcL-xL and RSK levels, and decreases in O(2)- and 3-NT levels. These novel findings provide a link between the increase in HO-1 activity, with its concurrent enhanced production of carbon monoxide (CO) and bilirubin, a decrease in infiltrated CD11c(+) dendritic cells and an increase in anti-apoptotic proteins, including RSK and BcL-xL, in the interdiction of the diabetic state.

Can pigment gallstones be induced by biliary stricture and prevented by medicine in Guinea pigs?

AIM: To determine the relationship between biliary stricture and pigment gallstone formation, and the prevention of pigment gallstones with medicine.

METHODS: One hundred and eighteen male guinea pigs were randomly divided into four groups: stricture group (S, n = 30) underwent partial ligation of common bile duct, and fed on regular chow; S plus medicine group (S+M, n = 27) underwent the same operation but fed on medicinal chow (0.3 g chenodeoxycholic acid, 0.5 g glucurolactone, and 0.5 g aspirin were mixed up in 1.2 kg regular chow); medicinal control group (C+M, n = 30) was free of operation, and fed on medicinal chow; and control group (C, n = 31) was free of operation and fed on regular chow. One week later, laparotomy was performed, and the bile of gallbladder was collected, measured, and cultured.

RESULTS: Gallstones were identified. Pigment gallstones were induced by biliary stricture in 95% (22/23) of S group. In the S+M group, the incidence of gallstone was reduced to 55% (11/20, vs S group, P < 0.01). The changes of indirect bilirubin and ionized calcium in the bile were consistent with gallstone incidences.

CONCLUSION: Biliary stricture can cause pigment gallstone formation in guinea pigs, and the medicines used can lower the incidence of gallstones. The bilirubin and ionized calcium play important roles in pigment gallstone formation.

[Schizophrenia and idiopathic unconjugated hyperbilirubinemia (Gilbert's syndrome)]

Idiopathic unconjugated hyperbilirubinemia (Gilbert's syndrome, GS) is a relatively common congenital hyperbilirubinemia occurring in 3-7% of the world's population. It has been recognized as a benign familial condition in which hyperbilirubinemia occurs in the absence of structural liver disease or hemolysis, and the plasma concentration of conjugated bilirubin is normal. Recently, it was reported that unconjugated bilirubin exhibited neurotoxicity in the developing nervous system. The 'neurodevelopmental hypothesis' of schizophrenia proposes that an as yet unidentified event occurs in utero or during early postnatal life. We have observed that patients suffering from schizophrenia frequently present an increased unconjugated bilirubin plasma concentration when admitted to the hospital. Therefore, we noticed a relation between unconjugated bilirubin and the etiology of and vulnerability to schizophrenia. Our reported findings suggest that there are significant biological and clinical character differences between schizophrenic patients with and without GS. From the viewpoint of the heterogeneity of schizophrenia, there may be a poor outcome for the subtype of schizophrenia with GS.

Familial adenomatous polyposis predisposes to pathologic exposure of the stomach to bilirubin

BACKGROUND

The role of duodenogastric reflux in the genesis of gastric polyps in familial adenomatous polyposis (FAP), although suggested by scintigraphy scanning studies, remains unclear.

METHODS

Twenty-four hour intragastric bilirubin monitoring with the Bilitec optoelectronic device was carried out in 25 FAP patients, of whom 19 had gastric polyps (fundic gland in 13, adenomatous in 2, and both histologic types in 4) on endoscopic examination. Gastric exposure to bilirubin was expressed as the percentage of total recording time that absorbance exceeded the threshold of 0.25 and was calculated in reference to values obtained from 25 healthy volunteers. Helicobacter pylori status of the stomach was checked as well.

RESULTS

Gastric exposure to bilirubin was pathologic in 14 (56%) patients. Gastric exposure to bilirubin was of longer duration in FAP patients than in healthy volunteers (mean+/-SEM: 19%+/-4% vs 6%+/-2%) (P<.005). It increased from healthy volunteers (6%+/-2%) to FAP patients without gastric polyps (10%+/-3%), and to FAP patients with gastric polyps (22%+/-5%) (P<.004). Bilirubin exposure times were similar in FAP patients with fundic gland polyps only and in those having either adenomatous polyps only or both types of polyps (24%+/-7% vs 17%+/-4%). No patient with pathologic gastric exposure to bilirubin as well as none having gastric polyps, had H. pylori in the antrum.

CONCLUSIONS

This study shows that gastric exposure to bilirubin is of longer duration in FAP patients than in healthy volunteers, and in FAP patients with gastric polyps than in those without polyps. This study supports the existence of a direct correlation between pathologic duodenogastric reflux (DGR), the absence of H. pylori in the antrum, and the presence of gastric polyps in FAP patients.

Role of carbon monoxide and biliverdin in renal ischemia/reperfusion injury

Heme oxygenase (HO) isoforms catalyze the conversion of heme to carbon monoxide (CO) and biliverdin/bilirubin with a concurrent release of iron. There is strong evidence that HO activity and products play a major role in renoprotection, however the exact molecular mechanisms underlying the beneficial effects exerted by this pathway are not fully understood. This review is aimed at illustrating the possible mechanism/s by which HO is renoprotective in the context of ischemia/reperfusion. We will first analyze the effects of exogenous administration of bilirubin/biliverdin and CO and then describe their biological activities once generated endogenously following stimulation of the HO pathway by either pharmacological means or gene targeting-mediated approaches.

Antioxidant role of heme oxygenase-1 in prehepatic portal hypertensive rats

AIM: To study the effect of bilirubin on the oxidative liver status and the activity and expression of heme oxygenase-1 (HO-1) in rat liver injury induced by prehepatic portal hypertension.

METHODS: Wistar male rats, weighing 200-250 g, were divided at random into two groups: one group with prehepatic portal hypertension (PH) induced by regulated prehepatic portal vein ligation (PPVL) and the other group corresponded to sham operated rats. Portal pressure, oxidative stress parameters, antioxidant enzymes, HO-1 activity and expression and hepatic sinusoidal vasodilatation were measured.

RESULTS: In PPVL rats oxidative stress was evidenced by a marked increase in thiobarbituric acid reactive substances (TBARS) content and a decrease in reduced glutathione (GSH) levels. The activities of liver antioxidant enzymes, superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) were also diminished while activity and expression of HO-1 were enhanced. Administration of bilirubin (5 μmol/kg body weight) 24 h before the end of the experiment entirely prevented all these effects. Pretreatment with Sn-protoporphyrin IX (Sn-PPIX) (100 μg/kg body weight, i.p.), a potent inhibitor of HO, completely abolished the oxidative stress and provoked a slight decrease in liver GSH levels as well as an increase in lipid peroxidation. Besides, carbon monoxide, another heme catabolic product, induced a significant increase in sinusoidal hepatic areas in PPVL group. Pretreatment of PPVL rats with Sn-PPIX totally prevented this effect.

CONCLUSION: These results suggest a beneficial role of HO-1 overexpression in prehepatic portal hypertensive rats.

Unconjugated bilirubin modulates the intestinal epithelial barrier function in a human-derived in vitro model

Unconjugated bilirubin promotes intestinal secretion without affecting nutrient digestion or absorption. In the current study, the effects of unconjugated bilirubin (UCB) on the barrier function of the intestinal epithelium were investigated. The apical side of human intestinal cell line Caco-2 monolayers was challenged with purified UCB. Transepithelial electrical resistance and paracellular fluxes of 10 kD Cascade blue conjugate dextran were measured. Cell monolayer viability was studied using LDH release and trypan blue exclusion tests. Redistribution of enterocyte tight junction occludin was studied by confocal microscopy. Bilirubin induced a dose-dependent decrease of transepithelial electrical resistance (TEER). This effect was maximal at 6 h and tended to be reversed at 48 h. Oxidated bilirubin was ineffective. Bilirubin significantly increased fluorescent dextran paracellular passage. Cell viability was not affected by UCB over the 5-200 nmol/L concentration range. Finally, bilirubin triggered a reversible redistribution of tight junctional occludin. UCB increases the permeability of intestinal epithelium. This effect is reversible, dependent on the redox status of the molecule and the rearrangement of the tight junction. These data attribute to bilirubin a novel role of functional modulator of intestinal paracellular permeability in vitro.

Messenger molecules and cell death: therapeutic implications

Programmed cell death, also called apoptosis, participates not only in normal physiologic processes such as development of the immune system, but also in many diseases. A loss of normal cell death may occur in cancer, and excessive cell death is found in a variety of neurodegenerative conditions. We describe 3 distinct pathways that regulate cell death. First, bilirubin, often thought to be a toxic end product of heme metabolism, serves as a physiologic cytoprotectant that may attenuate multiple forms of morbidity. In a second pathway, the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mediates a novel cell death cascade. Cytotoxic stimuli, via nitric oxide generation, lead to the binding of GAPDH to the protein Siah1, translocation of GAPDH-Siah1 to the nucleus, and ultimately cell death. Third, cytochrome c, released from mitochondria early in apoptosis, synergizes with inositol-1,4,5-triphosphate (IP3) to elicit massive cellular calcium release, resulting in cell death. These pathways may regulate cell survival in a variety of pathologic states and represent fertile targets for novel therapies.

Kernicterus and the Molecular Mechanisms of Bilirubin-Induced CNS Injury in Newborns

Kernicterus is a devastating, chronic disabling neurological disorder whose central nervous system (CNS) sequelae reflect both a predilection of bilirubin toxicity for neurons (rather than glial cells) and the regional topography of bilirubin-induced neuronal injury that is characterized by prominent basal ganglia, cochlear, and oculomotor nuclei involvement. The molecular pathogenesis of bilirubin-induced neuronal cell injury, although incompletely understood, likely reflects the untoward effects of hazardous unconjugated bilirubin concentrations on plasma, mitochondrial, and/or endoplasmic reticulum (ER) membranes. These membrane perturbations, in turn, might lead to the genesis of neuronal excitotoxicity, mitochondrial energy failure, or increased intracellular calcium concentration [Ca2+]i. These three phenomena are likely to be linked spatially and temporally in the pathogenesis of bilirubin-induced neuronal injury. Downstream events triggered by increased [Ca2+]i may include, among others, the activation of proteolytic enzymes, apoptotic pathways, and/or necrosis, the individual occurrence of which is likely a function of the degree and duration of bilirubin exposure. A recent study demonstrates the activation of mitogen-activated protein kinase signal transduction pathways by bilirubin heralding a degree of complexity regarding the molecular mechanism(s) of bilirubin-induced neurotoxicity not previously appreciated. There remains, however, a paucity of data regarding specific effects of bilirubin on intracellular signaling and cell death pathways, particularly in vivo. An enhanced understanding of the molecular pathogenesis of bilirubin-induced neuronal injury will lead to the identification of potential novel interventional strategies to protect the CNS against kernicterus.

Hyperbilirubinemia is not a major contributing factor to altered bone mineral density in patients with chronic liver disease

Reduced bone density is commonly encountered in patients with chronic liver disease. Prior studies have shown that unconjugated bilirubin inhibits osteoblast activity and function in vitro and in animal models of bone mineralization. To determine whether hyperbilirubinemia promotes the development of hepatic osteodystrophy, bone mineral density (BMD) was measured by dual energy X-ray absorptiometry in a cohort of 86 consecutive patients with chronic liver disease referred for liver transplant evaluation. The mean age of the study population was 52 years (range, 22-73), in which 52% were female and 90% were white. Average bone density values were significantly lower than expected for age, race, and sex, with Z-scores for the femoral neck and spine of -0.50 (95% confidence interval [CI] -0.63 to -0.37; p=0.0003) and -0.69 (95% CI -0.85 to -0.52; p=0.0001), respectively. Sixty-one subjects (71%) exhibited reduced BMD (T-score of femoral neck or spine<or=-1 standard deviation [SD] below the young-adult mean), and 18 subjects (21%) met criteria for osteoporosis (T-score<-2.5 SD). Stepwise logistic regression analyses identified significant associations between BMD and serum creatinine, alkaline phosphatase, age, and gender. On the other hand, neither unconjugated, nor conjugated, nor total serum bilirubin levels were found to predict diminished BMD. The lack of association between serum unconjugated bilirubin levels and bone mineralization was validated in hyperbilirubinemic Gunn rats, in which BMD and serum osteocalcin levels were no different than in wild-type rodents. In conclusion, the finding that serum bilirubin levels do not correlate with reduced BMD in patients with end-stage liver disease, and that chronic unconjugated hyperbilirubinemia does not lead to alterations in bone mineralization in Gunn rats, suggests that bilirubin is not a major contributing factor to hepatic osteodystrophy.

Heme oxygenase-2 protects against glutathione depletion-induced neuronal apoptosis mediated by bilirubin and cyclic GMP

Heme oxygenase (HO) enzymes catalyze the breakdown of heme to iron, carbon monoxide (CO), and biliverdin, which is rapidly converted to bilirubin. HO-2 has been implicated in protection against oxidative stress, ischemia, and traumatic brain injury. The neuroprotective effects of HO-2 have been attributed to the generation of bilirubin, which is an important radical scavenger. However, the mechanism by which HO-2 provides protection is unclear. We utilized the olfactory system as a model to define the roles of HO-2 in glutathione depletion-induced oxidative injury, since olfactory receptor neurons (ORNs) express high levels of HO isoforms. We demonstrated that L-buthionine-[S, R]-sulfoximine (BSO), an inhibitor of glutathione biosynthesis, lowered glutathione levels and induced apoptosis of ORNs. Despite the presence of HO-1 in ORNs, HO-2 null animals displayed increased levels of neuronal death after BSO treatment compared to wild type mice. Levels of bilirubin and cGMP were also reduced in HO-2 null mice. Primary cultures of ORNs confirmed that the neuroprotective role of HO-2 was mediated by bilirubin and cGMP. Taken together, these results suggest that HO-2 plays a major role in neuroprotection from oxidative stress, an effect that is mediated by cGMP and bilirubin.

Elevated plasma levels of bilirubin in patients with neurotrauma reflect its pathophysiological role in free radical scavenging

Bilirubin, a powerful endogenous antioxidant, is one of the catabolites of heme oxygenases (HOs). In this study, the plasma bilirubin concentration was measured to establish bilirubin kinesis after traumatic brain injury (TBI). Furthermore, in in vitro studies, the free radical scavenging activity and antioxidant potency of bilirubin was also investigated at various concentrations, including physiological ones. Indirect plasma bilirubin was measured in 25 patients on days 1, 2, 3 and 4 after presentation with TBI. The ability of bilirubin to scavenge the hydroxyl (OH) and 1,1-diphenyl-2-picrylhyrazyl (DPPH) radicals, and its antioxidant potency, were also analyzed using electron spin resonance (ESR) and the bioantioxidant power (BAP) methods, respectively. Plasma bilirubin levels were significantly higher on days 2, 3 and 4 than on patient admission (day 1; p < 0.05). ESR and BAP results revealed that bilirubin has direct OH and DPPH radical scavenging activities and potent antioxidant effects in vitro at physiological concentrations. These data indicate that physiological concentrations of bilirubin have antioxidant properties and that it constitutes one of the biological defense mechanisms in neurotrauma patients.

Bilirubin and the risk of common non-hepatic diseases

Bilirubin is a potent antioxidant but can be toxic at high concentrations. This article critically reviews the reported relationships of plasma bilirubin levels to the severity and/or incidence of various common non-hepatic diseases. Plasma bilirubin levels are reportedly negatively related to the risk of atherosclerotic diseases, cancers, demyelinating neuropathies and seasonal affective disorder. By contrast, the incidence and severity of schizophrenia are increased by elevated bilirubin levels. The data strongly suggest that the level of plasma bilirubin should be considered as a risk factor for several common non-hepatic diseases. Additional studies are needed to clarify the mechanisms of this influence, which are thought to be related to unconjugated bilirubin counteracting the oxidative stress underlying these disorders.

Barrett's esophagus in females: a comparative analysis of risk factors in females and males

OBJECTIVES

Gastroesophageal reflux symptoms occur with similar frequency in males and females, yet Barrett's esophagus is less common in females. The reason for this disparity is unknown. The aim of this study was to determine the factors related to Barrett's in females.

METHODS

The records of 796 patients (462 male, 334 female) evaluated from 1990 to 2000 for symptoms of reflux were retrospectively reviewed. Physiologic abnormalities based on results of endoscopic, motility, pH, and Bilitec testing were identified, and factors related to the presence of Barrett's were determined using univariate and multivariate analysis.

RESULTS

Females with reflux symptoms were significantly less likely to have a positive 24-h pH test, a defective lower esophageal sphincter, or a hiatal hernia than males with reflux symptoms. Further, females with reflux on the basis of an abnormal 24-h pH test had significantly less esophageal acid exposure than males with reflux. In contrast, esophageal exposure to refluxed acid and bilirubin was similar in females (n = 50) and males (n = 136) with Barrett's. On multivariable analysis increased esophageal bilirubin exposure was the only significant factor associated with the presence of Barrett's in male and female patients with reflux disease.

CONCLUSIONS

Females with reflux symptoms have less esophageal acid exposure on average than males. However, females and males with Barrett's have a similar severity of reflux, and the female gender does not protect against the development of Barrett's in the setting of advanced reflux disease. Esophageal bilirubin exposure is the major risk factor for the presence of Barrett's in patients with reflux disease.

[Oxidative stress related diseases and biopyrrins]

Reactive oxygen species (ROS) cause damage to the structure and function of tissues. Therefore tissues have systems that eliminate ROS. Bilirubin is one antioxidant that reacts with ROS to produce oxidative metabolites. Biopyrrins are one of the metabolites, the level of which in urine reflects oxidative stress. They are measured by non-competitive inhibition ELISA that employs anti-bilirubin antibody (24G7) and the results are corrected for the urinary concentration of cereatinine. Some reports suggested that psychological stress increased oxidative stress markers. Urinary biopyrrins were also elevated by speech stress, and the subjective stress score recorded by the speakers correlated with the level. The result suggests that bilirubin might eliminate ROS generated by psychological stress. From the beginning of the study of biopyrrins, their urinary level has been known to be increased by surgical stress. Furthermore, it was significantly higher in a major operation patient group than in a minor one, and correlated with operation duration. Sepsis increased the level in surgical patients. Ischemia-reperfusion elevates ROS and, as a result, biopyrrin production. An increase in urinary biopyrrins was observed in a coronary spastic angina group after a spasm provocation test, and the level in myocardial infarction patients with NYHA (New York Heart Association) classification became higher. Correlation between urinary biopyrrins and plasma B-type natriuretic peptide (BNP) was also reported. Research that determines the structures of biopyrrins and their clinical application are in progress.

[Cytoprotective effects of bilirubin]

Bilirubin, a major product of heme catabolism, belongs to compounds with pleiotropic biologic effects. For a long time bilirubin was considered as a metabolite dangerous for human health, neonatologists know well serious clinical complication of neonatal jaundice called bilirubin encephalopathy. Nevertheless, recent data has demonstrated that bilirubin exhibits potent antioxidant and even anti-inflammatory effects with substantial clinical impacts. The aim of the present study was to summarize present knowledge in this rapidly evolving field and suggest further possible clinical consequences.

Bilirubin inhibits iNOS expression and NO production in response to endotoxin in rats

The inducible isoform of heme oxygenase (HO), HO-1, has been shown to play an important role in attenuating tissue injury. Because HO-1 catalyzes the rate-limiting step in bilirubin synthesis, we examined the hypothesis that bilirubin is a key mediator of HO-1 cytoprotection, employing a rat model of endotoxemia. Bilirubin treatment resulted in improved survival and attenuated liver injury in response to lipopolysaccharide infusion. Serum levels of NO and tumor necrosis factor alpha, key mediators of endotoxemia, and hepatic inducible nitric oxide synthase (iNOS) expression were significantly lower in bilirubin-treated rodents versus control animals. Both intraperitoneal and local administration of bilirubin also was found to ameliorate hindpaw inflammation induced by the injection of lambda-carrageenan. Consistent with in vivo results, bilirubin significantly inhibited iNOS expression and suppressed NO production in lipopolysaccharide (LPS)-stimulated RAW 264.7 murine macrophages. In contrast, bilirubin treatment induced a threefold increase in LPS-mediated prostaglandin synthesis in the absence of significant changes in cyclooxygenase expression or activity, suggesting that bilirubin enhances substrate availability for eicosanoid synthesis. Bilirubin had no effect on LPS-mediated activation of nuclear factor kappaB or p38 mitogen-activated protein kinase, consistent with a nuclear factor kappaB-independent mechanism of action. Taken together, these data support a cytoprotective role for bilirubin that is mediated, at least in part, through the inhibition of iNOS expression and, potentially, through stimulation of local prostaglandin E2 production. In conclusion, our findings suggest a role for bilirubin in mollifying tissue injury in response to inflammatory stimuli and support the possibility that the phenomenon of "jaundice of sepsis" represents an adaptive physiological response to endotoxemia. Supplementary material for this article can be found on the HEPATOLOGY website (http://interscience.wiley.com/jpages/0270-9139/suppmat/index.html).

Heme oxygenase-1 inhibits angiotensin II-induced cardiac hypertrophy in vitro and in vivo

BACKGROUND

Heme oxygenase-1 (HO-1) is a stress-response enzyme implicated in cardioprotection. To explore whether HO-1 has a role in cardiac remodeling response, the effect of its overexpression on angiotensin II (Ang II)-induced cardiac hypertrophy was examined.

METHODS AND RESULTS

HO-1 was induced in cultured rat neonatal cardiomyocytes by treatment with cobalt protoporphyrin IX (CoPPIX) or a recombinant adenovirus carrying the human HO-1 gene. Ang II-induced myocyte hypertrophy assessed by increments in cell size, [3H]leucine uptake, and protein content was suppressed by HO-1 overexpression. Cotreatment of cells with tin protoporphyrin IX, a HO inhibitor, significantly reversed the suppressive effect of HO-1. Bilirubin, one of the byproducts of heme degradation by HO-1, mediated the suppressive effect through the inhibition of Ang II-induced production of reactive oxygen species, as detected by a 2',7'-dichlorofluorescein probe. The antihypertrophic effect of HO-1 was also demonstrated in rats receiving chronic Ang II infusions. Cotreatment of animals with CoPPIX significantly attenuated Ang II-induced left ventricular hypertrophy and hyperdynamic contractions, whereas concomitant treatment with tin protoporphyrin IX abolished CoPPIX-mediated cardioprotection in vivo.

CONCLUSIONS

HO-1 attenuates Ang II-induced cardiac hypertrophy both in vitro and in vivo, and bilirubin mediates, at least in part, the antihypertrophic effect of HO-1 via inhibition of reactive oxygen species production after Ang II stimulation.

Neonatal jaundice: physiologic variation or pathologic process

Neonatal hyperbilirubinemia and jaundice affect approximately 60% of the 4 million newborns in the United States each year. Jaundice results from bilirubin deposition in the skin and mucous membranes, becoming clinically visible at a serum bilirubin level of 5 to 7 mg/dL. At a higher but undefined level, bilirubin may deposit in the brain where it can cause transient dysfunction or permanent neurologic impairment.

Bilirubin protects astrocytes from its own toxicity by inducing up-regulation and translocation of multidrug resistance-associated protein 1 (Mrp1)

Unconjugated bilirubin (UCB) causes encephalopathy in severely jaundiced neonates by damaging astrocytes and neurons. Astrocytes, which help defend the brain against cytotoxic insults, express the ATP-dependent transporter, multidrug resistance-associated protein 1 (Mrp1), which mediates export of organic anions, probably including UCB. We therefore studied whether exposure to UCB affects the expression and intracellular localization of Mrp1 in cultured mouse astroglial cells (>95% astrocytes). Mrp1 was localized and quantitated by confocal laser scanning microscopy and double immunofluorescence labeling by using specific antibodies against Mrp1 and the astrocyte marker glial fibrillary acidic protein, plus the Golgi marker wheat germ agglutinin (WGA). In unexposed astrocytes, Mrp1 colocalized with WGA in the Golgi apparatus. Exposure to UCB at a low unbound concentration (Bf) of 40 nM caused rapid redistribution of Mrp1 from the Golgi throughout the cytoplasm to the plasma membrane, with a peak 5-fold increase in Mrp1 immunofluorescence intensity from 30 to 120 min. Bf above aqueous saturation produced a similar but aborted response. Exposure to this higher Bf for 16 h markedly decreased Trypan blue exclusion and methylthiazoletetrazoilum activity and increased apoptosis 5-fold by terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling assay. These toxic effects were modestly increased by inhibition of Mrp1 activity with 3-([3-(2-[7-chloro-2-quinolinyl]ethenyl)phenyl-(3-dimethylamino-3-oxopropyl)-thio-methyl]thio)propanoic acid (MK571). By contrast, Bf=40 nM caused injury only if Mrp1 activity was inhibited by MK571, which also blocked translocation of Mrp1. Our conclusion is that in astrocytes, UCB up-regulates expression of Mrp1 and promotes its trafficking from the Golgi to the plasma membrane, thus moderating cytotoxicity from UCB, presumably by limiting its intracellular accumulation.

Vascular heme oxygenase-1 induction suppresses microvascular thrombus formation in vivo

OBJECTIVE

By heme degradation, heme oxygenase-1 (HO-1) provides endogenous carbon monoxide and bilirubin, both of which play major roles in vascular biology. The current study aimed to examine whether induction of HO-1 and its byproducts modulate the process of microvascular thrombus formation in vivo.

METHODS AND RESULTS

In individual microvessels of mouse cremaster muscle preparations, ferric chloride-induced thrombus formation was analyzed using intravital fluorescence microscopy. When mice were pretreated with an intraperitoneal injection of hemin, a HO-1 inducer, immunohistochemistry and Western blot protein analysis of cremaster muscle tissue displayed a marked induction of HO-1. In these animals, superfusion with ferric chloride solution induced arteriolar and venular thrombus formation, which, however, was significantly delayed when compared with thrombus formation in animals without HO-1 induction. The delay in thrombus formation in hemin-treated mice was completely blunted by tin protoporphyrin-IX, a HO-1 inhibitor, but not by copper protoporphyrin-IX, which does not inhibit the enzyme. Coadministration of the vitamin E analogue Trolox in HO-1-blocked animals almost completely restored the delay in thrombus formation, implying that, besides CO, the antioxidant HO pathway metabolite bilirubin mainly contributes to the antithrombotic property of HO-1. This was further supported by the fact that bilirubin was found as effective as hemin in delay of ferric chloride-induced thrombus formation. Animals with HO-1 induction revealed reduced P-selectin protein expression in cremaster muscle tissue, which most probably presented the molecular basis for delayed thrombus growth.

CONCLUSIONS

Local induction of HO-1 activity may be of preventive and therapeutic value for clinical disorders with increased risk of thrombotic events.

Bilirubin inhibits bile acid induced apoptosis in rat hepatocytes

BACKGROUND AND AIMS

Hydrophobic bile acids contribute to hepatocellular injury in cholestasis and rapidly induce apoptosis in vitro; however, unlike Fas agonists, cholestasis does not cause extensive hepatocyte apoptosis. As antioxidants provide protection against bile acid induced liver injury, our premise was that bilirubin, a free radical scavenger with increased plasma levels in the presence of liver disease, could protect hepatocytes against bile acid induced apoptosis.

METHODS

Freshly isolated rat hepatocytes were incubated for four hours with 100 micromol/l glycochenodeoxycholate (GCDC) alone or with increasing concentrations of unconjugated (UCB) or conjugated (CB) bilirubin.

RESULTS

Both UCB and CB inhibited GCDC induced apoptosis in a dose dependent fashion and suppressed the generation of reactive oxygen species by hepatocytes.

CONCLUSIONS

The antiapoptotic effect of bilirubin associated with its antioxidant properties indicates that hyperbilirubinaemia may have a protective role in liver disease.

Role of heme oxygenase-1 protein in the neuroprotective effects of cyclopentenone prostaglandin derivatives under oxidative stress

Previously we found that some cyclopenteone prostaglandin derivatives (PGs), referred to as neurite outgrowth-promoting PGs (NEPPs), have dual biological activities of promoting neurite outgrowth and preventing neuronal death [Satoh et al. (2000) J. Neurochem., 75, 1092-1102; Satoh et al. (2001) J. Neurochem., 77, 50-62; Satoh et al. (2002) In Kikuchi, II. (ed.), Strategenic Medical Science Against Brain Attack. Springer-Verlag, Tokyo, pp. 78-93]. To investigate possible cellular mechanisms of the neuroprotective effects, we performed oligo hybridization-based DNA array analysis with mRNA isolated from HT22, a cell line that originated from a mouse hippocampal neuron. Several transcripts up-regulated by NEPP11 were identified. Because heme oxygenase 1 (HO-1) mRNA was the most prominently induced and was earlier reported to protect neuronal and non-neuronal cells against oxidative stress, we focused on it as a possible candidate responsible for the neuroprotective effects. We found NEPP11 to induce HO-1 protein (32 kDa) in HT22 cells in both the presence and the absence of glutamate, whereas non-neuroprotective prostaglandins (PGs) Delta12-PGJ2 or PGA2 did not. Overexpression of HO-1-green fluorescence protein (GFP) fusion protein significantly protected HT22 cells against oxidative glutamate toxicity, whereas that of GFP alone did not. Furthermore, biliverdin and bilirubin, products of HO-1 enzymatic activity on heme, protected HT22 cells from oxidative glutamate toxicity. These results, together with our previous results, suggest that NEPP11 activates the expression of HO-1 and that HO-1 produces biliverdin and bilirubin, which result in the inhibition of neuronal death induced by oxidative stress. NEPP11 is the first molecular probe reported to have a neuroprotective action through induction of HO-1 in neuronal cells.

Transient relief of asthma symptoms during jaundice: a possible beneficial role of bilirubin

Bilirubin arises from enzymatic reduction by biliverdin reductase of biliverdin, a product of heme oxygenase activity. Recent literature describes that bilirubin is a major physiologic antioxidant that can protect cells from chemical oxidants such as hydrogen peroxide. Recently, it has been reported that oxidative stress may play a crucial role in the pathogenesis of asthma. We report a case of complete resolution of persistent difficult-to-control asthma in accordance with increased levels of serum bilirubin due to acute hepatitis B. The present case suggests that anti-oxidative agents might be effective for the treatment of asthma.

Biliverdin reductase: a major physiologic cytoprotectant

Bilirubin, an abundant pigment that causes jaundice, has long lacked any clear physiologic role. It arises from enzymatic reduction by biliverdin reductase of biliverdin, a product of heme oxygenase activity. Bilirubin is a potent antioxidant that we show can protect cells from a 10,000-fold excess of H2O2. We report that bilirubin is a major physiologic antioxidant cytoprotectant. Thus, cellular depletion of bilirubin by RNA interference markedly augments tissue levels of reactive oxygen species and causes apoptotic cell death. Depletion of glutathione, generally regarded as a physiologic antioxidant cytoprotectant, elicits lesser increases in reactive oxygen species and cell death. The potent physiologic antioxidant actions of bilirubin reflect an amplification cycle whereby bilirubin, acting as an antioxidant, is itself oxidized to biliverdin and then recycled by biliverdin reductase back to bilirubin. This redox cycle may constitute the principal physiologic function of bilirubin.