Bilirubin has widespread inhibitory effects on protein phosphorylation

Molecular mechanisms and therapeutic strategies for ferroptosis and cuproptosis in ischemic stroke

Ischemic stroke, as one of the most severe and prevalent neurological disorders, poses a significant threat to the health and quality of life of affected individuals. Stemming from the obstruction of blood flow, ischemic stroke, leads to cerebral tissue hypoxia and ischemia, instigating a cascade of pathophysiological changes that markedly exacerbate neuronal damage and may even culminate in cell death. In recent years, emerging research has increasingly focused on novel cell death mechanisms such as ferroptosis and cuproptosis. Mounting evidence underscores the independent roles of ferroptosis and cuproptosis in ischemic stroke. This review aims to elucidate potential cross-regulatory mechanisms between ferroptosis and cuproptosis, exploring their regulatory roles in ischemic stroke. The objective is to provide targeted therapeutic intervention strategies.

Gamma-glutamyl transpeptidase and indirect bilirubin may participate in systemic inflammation of patients with psoriatic arthritis

BACKGROUND

Previous studies have suggested that systemic metabolic abnormalities are closely related to psoriatic arthritis (PsA). Gamma-glutamyl transpeptidase (GGT) and indirect bilirubin (IBIL), two essential active substances in hepatic metabolism that have been demonstrated as an oxidative and anti-oxidative factor respectively, have been proved to be involved in oxidative stress damage and inflammation in several human diseases. However, their role in PsA remains unclear.

METHODS

In this retrospective comparative cohort study, a case group of 68 PsA patients and a control group of 73 healthy volunteers from the Third Hospital of Hebei Medical University were enrolled. Serum GGT, IBIL, GGT/IBIL ratio and C-reactive protein (CRP), a well applied bio-marker of systemic inflammatory in PsA, were compared between the two groups. Furthermore, the relationship of GGT, IBIL and GGT/IBIL with CRP were explored in PsA patients. Finally, the patients were divided into high inflammation group and low inflammation group according to the median value of CRP. Multivariate logistic regression analyses were used for the association of systemic inflammation level with GGT, IBIL and GGT/IBIL.

RESULTS

Compared with healthy controls, PsA patients exhibited significantly higher serum GGT, GGT/IBIL, and CRP levels and lower IBIL levels. Serum GGT and GGT/IBIL were positively correlated with CRP, whereas IBIL were negatively correlated with CRP. Binary logistic regression analysis revealed that serum GGT was a risk factor for high CRP in PsA, whereas IBIL was a protective factor. Furthermore, GGT/IBIL was a better indicator of high CRP condition in PsA patients than either GGT or IBIL alone, as determined by the receiver operating characteristic curves.

CONCLUSION

GGT and IBIL may participate in the pathogenesis of PsA. Additionally, GGT, IBIL and the balance of the two may reflect systemic inflammation mediated by oxidative stress events related to metabolic abnormalities to a certain extent.

Gender Differences in Oxidative Stress in Relation to Cancer Susceptibility and Survival

Genetic, developmental, biochemical, and environmental variables interact intricately to produce sex differences. The significance of sex differences in cancer susceptibility is being clarified by numerous studies. Epidemiological research and cancer registries have revealed over the past few years that there are definite sex variations in cancer incidence, progression, and survival. However, oxidative stress and mitochondrial dysfunction also have a significant impact on the response to treatment of neoplastic diseases. Young women may be more protected from cancer than men because most of the proteins implicated in the regulation of redox state and mitochondrial function are under the control of sexual hormones. In this review, we describe how sexual hormones control the activity of antioxidant enzymes and mitochondria, as well as how they affect several neoplastic diseases. The molecular pathways that underlie the gender-related discrepancies in cancer that have been identified may be better understood, which may lead to more effective precision medicine and vital information on treatment options for both males and females with neoplastic illnesses.

Effects of bilirubin on the development and electrical activity of neural circuits

In the past several decades, bilirubin has attracted great attention for central nervous system (CNS) toxicity in some pathological conditions with severely elevated bilirubin levels. CNS function relies on the structural and functional integrity of neural circuits, which are large and complex electrochemical networks. Neural circuits develop from the proliferation and differentiation of neural stem cells, followed by dendritic and axonal arborization, myelination, and synapse formation. The circuits are immature, but robustly developing, during the neonatal period. It is at the same time that physiological or pathological jaundice occurs. The present review comprehensively discusses the effects of bilirubin on the development and electrical activity of neural circuits to provide a systematic understanding of the underlying mechanisms of bilirubin-induced acute neurotoxicity and chronic neurodevelopmental disorders.

The different facets of heme-oxygenase 1 in innate and adaptive immunity

Heme oxygenase (HO) enzymes are responsible for the main oxidative step in heme degradation, generating equimolar amounts of free iron, biliverdin and carbon monoxide. HO-1 is induced as a crucial stress response protein, playing protective roles in physiologic and pathological conditions, due to its antioxidant, anti-apoptotic and anti-inflammatory effects. The mechanisms behind HO-1-mediated protection are being explored by different studies, affecting cell fate through multiple ways, such as reduction in intracellular levels of heme and ROS, transcriptional regulation, and through its byproducts generation. In this review we focus on the interplay between HO-1 and immune-related signaling pathways, which culminate in the activation of transcription factors important in immune responses and inflammation. We also discuss the dual interaction of HO-1 and inflammatory mediators that govern resolution and tissue damage. We highlight the dichotomy of HO-1 in innate and adaptive immune cells development and activation in different disease contexts. Finally, we address different known anti-inflammatory pharmaceuticals that are now being described to modulate HO-1, and the possible contribution of HO-1 in their anti-inflammatory effects.

Bilirubin represents a negative regulator of ILC2 in allergic airway inflammation

Group 2 innate lymphoid cells (ILC2s) play an important role in allergic airway inflammation. Despite recent advances in defining molecular mechanisms that control ILC2 development and function, the role of endogenous metabolites in the regulation of ILC2s remains poorly understood. Herein, we demonstrated that bilirubin, an end product of heme catabolism, was a potent negative regulator of ILC2s. Bilirubin metabolism was found to be significantly induced during airway inflammation in mouse models. The administration of unconjugated bilirubin (UCB) dramatically suppressed ILC2 responses to interleukin (IL)-33 in mice, including cell proliferation and the production of effector cytokines. Furthermore, UCB significantly alleviated ILC2-driven airway inflammation, which was aggravated upon clearance of endogenous UCB. Mechanistic studies showed that the effects of bilirubin on ILC2s were associated with downregulation of ERK phosphorylation and GATA3 expression. Clinically, newborns with hyperbilirubinemia displayed significantly lower levels of ILC2 with impaired function and suppressed ERK signaling. Together, these findings indicate that bilirubin serves as an endogenous suppressor of ILC2s and might have potential therapeutic value in the treatment of allergic airway inflammation.

Bilirubin: The yellow hormone?

Bilirubin is a tetrapyrrolic compound originating from heme catabolism. Although originally considered only a potentially dangerous waste product, it has become increasingly evident that this molecule represents an important modulator of various biological functions in the human body. Bilirubin appears to have versatile functions, from cell signaling (behaving almost like a "real" hormonal substance), modulation of metabolism, to immune regulation, affecting biological activities with apparent clinical and even therapeutic consequences. These activities may be the reason for the lower incidence of diseases of civilisation (cardiovascular diseases, arterial hypertension, diabetes, obesity, metabolic syndrome, certain cancers, autoimmune, and neurodegenerative diseases) observed in individuals with a chronic mild unconjugated hyperbilirubinemia, a typical sign of Gilbert's syndrome. While higher serum concentrations of unconjugated bilirubin may serve as an important protective factor against these diseases, low levels of bilirubin are associated with the opposite effect.

Associations between Prediagnostic Circulating Bilirubin Levels and Risk of Gastrointestinal Cancers in the UK Biobank

We investigated associations between serum levels of bilirubin, an endogenous antioxidant, and gastrointestinal cancer risk. In the UK Biobank, prediagnostic serum levels of total bilirubin were measured in blood samples collected from 440,948 participants. In multivariable-adjusted Cox proportional hazard regression, we estimated hazard ratios (HR) and 95% confidence intervals (CI) for associations between bilirubin levels and gastrointestinal cancer risk (colorectum, esophagus, stomach, mouth, pancreas, and liver). After a median follow-up of 7.1 years (interquartile range: 1.4), 5033 incident gastrointestinal cancer cases were recorded. In multivariable-adjusted models, bilirubin levels were negatively associated with risk of esophageal adenocarcinoma (EAC, HR per 1-SD increment in log-total bilirubin levels 0.72, 95%CI 0.56-0.92, p = 0.01). Weak and less robust negative associations were observed for colorectal cancer (CRC, HR per 1-SD increment in log-total bilirubin levels 0.95, 95%CI 0.88-1.02, p = 0.14). Bilirubin levels were positively associated with risk of hepatocellular carcinoma (HCC, HR per 1-SD increment in log-total bilirubin levels 2.07, 95%CI 1.15-3.73, p = 0.02) and intrahepatic bile duct (IBD) cancer (HR per 1-SD increment 1.67, 95%CI 1.07-2.62, p = 0.03). We found no associations with risks of stomach, oral, and pancreatic cancers. Prediagnostic serum levels of bilirubin were negatively associated with risk of EAC and positively associated with HCC and IBD cancer. Further studies are warranted to replicate our findings for specific GI cancers.

The Association between Serum Bilirubin Levels and Colorectal Cancer Risk: Results from the Prospective Cooperative Health Research in the Region of Augsburg (KORA) Study in Germany

Emerging studies have suggested that bilirubin, particularly unconjugated bilirubin (UCB), has substantial anti-inflammatory and antioxidant properties that protect against oxidative stress-associated diseases such as cancer. Few observational studies have investigated the etiological role of bilirubin in colorectal cancer (CRC) development. In this case-control study, nested in the population-based prospective cohort of the Cooperative Health Research in the Region of Augsburg (KORA) study in south Germany, pre-diagnostic circulating UCB concentrations were measured by high-performance liquid chromatography in 77 CRC cases and their individually matched controls. Multivariable unconditional logistic regression was used to estimate the odds ratios (OR) and 95% confidence intervals (CI) for associations between log-transformed UCB levels (log-UCB), standardized per one-standard-deviation (one-SD) increment, and CRC risk. The models were a priori stratified by sex based on previous evidence. In the fully adjusted models, each one-SD increment in log-UCB was indicative of a positive association with CRC risk (OR, 1.20; 95% CI, 0.52-2.79) among men, and of an inverse association (OR, 0.76; 95% CI, 0.34-1.84) among women (Pheterogeneity = 0.4 for differences between men and women). We found little evidence for sex-specific associations of circulating bilirubin with CRC risk, and further studies are needed to confirm or refute the potential associations.

Circulating bilirubin levels and risk of colorectal cancer: serological and Mendelian randomization analyses

BACKGROUND

Bilirubin, a byproduct of hemoglobin breakdown and purported anti-oxidant, is thought to be cancer preventive. We conducted complementary serological and Mendelian randomization (MR) analyses to investigate whether alterations in circulating levels of bilirubin are associated with risk of colorectal cancer (CRC). We decided a priori to perform analyses separately in men and women based on suggestive evidence that associations may differ by sex.

METHODS

In a case-control study nested in the European Prospective Investigation into Cancer and Nutrition (EPIC), pre-diagnostic unconjugated bilirubin (UCB, the main component of total bilirubin) concentrations were measured by high-performance liquid chromatography in plasma samples of 1386 CRC cases and their individually matched controls. Additionally, 115 single-nucleotide polymorphisms (SNPs) robustly associated (P < 5 × 10-8) with circulating total bilirubin were instrumented in a 2-sample MR to test for a potential causal effect of bilirubin on CRC risk in 52,775 CRC cases and 45,940 matched controls in the Genetics and Epidemiology of Colorectal Cancer Consortium (GECCO), the Colon Cancer Family Registry (CCFR), and the Colorectal Transdisciplinary (CORECT) study.

RESULTS

The associations between circulating UCB levels and CRC risk differed by sex (Pheterogeneity = 0.008). Among men, higher levels of UCB were positively associated with CRC risk (odds ratio [OR] = 1.19, 95% confidence interval [CI] = 1.04-1.36; per 1-SD increment of log-UCB). In women, an inverse association was observed (OR = 0.86 (0.76-0.97)). In the MR analysis of the main UGT1A1 SNP (rs6431625), genetically predicted higher levels of total bilirubin were associated with a 7% increase in CRC risk in men (OR = 1.07 (1.02-1.12); P = 0.006; per 1-SD increment of total bilirubin), while there was no association in women (OR = 1.01 (0.96-1.06); P = 0.73). Raised bilirubin levels, predicted by instrumental variables excluding rs6431625, were suggestive of an inverse association with CRC in men, but not in women. These differences by sex did not reach formal statistical significance (Pheterogeneity ≥ 0.2).

CONCLUSIONS

Additional insight into the relationship between circulating bilirubin and CRC is needed in order to conclude on a potential causal role of bilirubin in CRC development.

Molecular Physiology and Pathophysiology of Bilirubin Handling by the Blood, Liver, Intestine, and Brain in the Newborn

Bilirubin is the end product of heme catabolism formed during a process that involves oxidation-reduction reactions and conserves iron body stores. Unconjugated hyperbilirubinemia is common in newborn infants, but rare later in life. The basic physiology of bilirubin metabolism, such as production, transport, and excretion, has been well described. However, in the neonate, numerous variables related to nutrition, ethnicity, and genetic variants at several metabolic steps may be superimposed on the normal physiological hyperbilirubinemia that occurs in the first week of life and results in bilirubin levels that may be toxic to the brain. Bilirubin exists in several isomeric forms that differ in their polarities and is considered a physiologically important antioxidant. Here we review the chemistry of the bilirubin molecule and its metabolism in the body with a particular focus on the processes that impact the newborn infant, and how differences relative to older children and adults contribute to the risk of developing both acute and long-term neurological sequelae in the newborn infant. The final section deals with the interplay between the brain and bilirubin and its entry, clearance, and accumulation. We conclude with a discussion of the current state of knowledge regarding the mechanism(s) of bilirubin neurotoxicity.

Bilirubin as a signaling molecule

For long time bilirubin was only considered as a potentially dangerous sign of liver diseases, but it now appears clear that it is also a powerful signaling molecule. Together with potent antioxidant activities that were only reported in the last few decades, many other biological effects have now been clearly described. These include especially profound inhibitory effects on almost all effectors of the immune system, with their clinical consequences in the bilirubin-mediated protection against autoimmune and inflammatory diseases. Separate from these, bilirubin activates various nuclear and cytoplasmic receptors, resembling the endocrine activities of actual hormonal substances. This is true for the "classical" hepatic nuclear receptors, including the aryl hydrocarbon receptor, or the constitutive androstane receptor; and also for some lesser-explored receptors such as peroxisome proliferator-activated receptors α and γ; Mas-related G protein-coupled receptor; or other signaling molecules including fatty acid binding protein 1, apolipoprotein D, or reactive oxygen species. All of these targets have broad metabolic effects, which in turn may offer protection against obesity, diabetes mellitus, and other metabolic diseases. The (mostly experimental) data are also supported by clinical evidence. In fact, data from the last three decades have convincingly demonstrated the protective effects of mildly elevated serum bilirubin concentrations against various "diseases of civilization." Additionally, even tiny, micromolar changes of serum bilirubin concentrations have been associated with substantial alteration in the risks of these diseases. It is highly likely that all of the biological activities of bilirubin have yet to be exhaustively explored, and thus we can expect further clinical discoveries about this evolutionarily old molecule into the future.

Ca2+-dependent recruitment of voltage-gated sodium channels underlies bilirubin-induced overexcitation and neurotoxicity

Neonatal jaundice is prevalent among newborns and can lead to severe neurological deficits, particularly sensorimotor dysfunction. Previous studies have shown that bilirubin (BIL) enhances the intrinsic excitability of central neurons and this can potentially contribute to their overexcitation, Ca²⁺ overload, and neurotoxicity. However, the cellular mechanisms underlying elevated neuronal excitability remain unknown. By performing patch-clamp recordings from neonatal neurons in the rat medial vestibular nucleus (MVN), a crucial relay station for locomotor and balance control, we found that BIL (3 μM) drastically increases the spontaneous firing rates by upregulating the current-mediated voltage-gated sodium channels (VGSCs), while shifting their voltage-dependent activation toward more hyperpolarized potentials. Immunofluorescence labeling and western immunoblotting with an anti-NaV1.1 antibody, revealed that BIL elevates the expression of VGSCs by promoting their recruitment to the membrane. Furthermore, we found that this VGSC-trafficking process is Ca²⁺ dependent because preloading MVN neurons with the Ca²⁺ buffer BAPTA-AM, or exocytosis inhibitor TAT-NSF700, prevents the effects of BIL, indicating the upregulated activity and density of functional VGSCs as the core mechanism accountable for the BIL-induced overexcitation of neonatal neurons. Most importantly, rectification of such overexcitation with a low dose of VGSC blocker lidocaine significantly attenuates BIL-induced cell death. We suggest that this enhancement of VGSC currents directly contributes to the vulnerability of neonatal brain to hyperbilirubinemia, implicating the activity and trafficking of NaV1.1 channels as a potential target for neuroprotection in cases of severe jaundice.

Hyperbilirubinemia in Gunn Rats is Associated with Decreased Inflammatory Response in LPS-Mediated Systemic Inflammation

Decreased inflammatory status has been reported in subjects with mild unconjugated hyperbilirubinemia. However, mechanisms of the anti-inflammatory actions of bilirubin (BR) are not fully understood. The aim of this study is to assess the role of BR in systemic inflammation using hyperbilirubinemic Gunn rats as well as their normobilirubinemic littermates and further in primary hepatocytes. The rats were treated with lipopolysaccharide (LPS, 6 mg/kg intraperitoneally) for 12 h, their blood and liver were collected for analyses of inflammatory and hepatic injury markers. Primary hepatocytes were treated with BR and TNF-α. LPS-treated Gunn rats had a significantly decreased inflammatory response, as evidenced by the anti-inflammatory profile of white blood cell subsets, and lower hepatic and systemic expressions of IL-6, TNF-α, IL-1β, and IL-10. Hepatic mRNA expression of LPS-binding protein was upregulated in Gunn rats before and after LPS treatment. In addition, liver injury markers were lower in Gunn rats as compared to in LPS-treated controls. The exposure of primary hepatocytes to TNF-α with BR led to a milder decrease in phosphorylation of the NF-κB p65 subunit compared to in cells without BR. In conclusion, hyperbilirubinemia in Gunn rats is associated with an attenuated systemic inflammatory response and decreased liver damage upon exposure to LPS.

Modulation of Cav2.3 channels by unconjugated bilirubin (UCB) - Candidate mechanism for UCB-induced neuromodulation and neurotoxicity

Elevated levels of unbound unconjugated bilirubin (UCB) can lead to bilirubin encephalopathy and kernicterus. In spite of a large number of studies demonstrating UCB-induced changes in central neurotransmission, it is still unclear whether these effects involve alterations in the function of specific ion channels. To assess how different UCB concentrations and UCB:albumin (U/A) molar ratios affect neuronal R-type voltage-gated Ca²⁺ channels, we evaluated their effects on whole-cell currents through recombinant Ca_v2.3 + β₃ channel complexes and ex-vivo electroretinograms (ERGs) from wildtype and Ca_v2.3-deficient mice. Our findings show that modestly elevated levels of unbound UCB (U/A = 0.5) produce subtle but significant changes in the voltage-dependence of activation and prepulse inactivation, resulting in a stimulation of currents activated by weak depolarization and inhibition at potentials on the plateau of the activation curve. Saturation of the albumin binding capacity (U/A = 1) produced additional suppression that became significant when albumin was omitted completely and might involve a complete loss of channel function. Acutely administered UCB (U/A = 0.5) has recently been shown to affect transsynaptic signaling in the isolated vertebrate retina. The present report reveals that sustained exposure of the murine retina to UCB significantly suppresses also late responses of the inner retina (b-wave) from wildtype compared to Ca_v2.3-deficient mice. In addition, recovery during washout was significantly more complete and faster in retinae lacking Ca_v2.3 channels. Together, these findings show that UCB affects cloned and native Ca_v2.3 channels at clinically relevant U/A molar ratios and indicate that supersaturation of albumin is not required for modulation but associated with a loss of channel functional that could contribute to chronic neuronal dysfunction.

High unbound bilirubin for age: a neurotoxin with major effects on the developing brain

Neonatal hyperbilirubinemia is one of the most frequent diagnoses made in neonates. A high level of unconjugated bilirubin that is unbound to albumin is neurotoxic when the level exceeds age-specific thresholds or at lower levels in neonates with neurotoxic risk factors. Lower range of unbound bilirubin results in apoptosis, while moderate-to-high levels result in neuronal necrosis. Basal ganglia and various brain stem nuclei are more susceptible to bilirubin toxicity. Proposed mechanisms of bilirubin-induced neurotoxicity include excessive release of glutamate, mitochondrial energy failure, release of proinflammatory cytokines, and increased intracellular calcium concentration. These mechanisms are similar to the events that occur following hypoxic-ischemic insult in neonates. Severe hyperbilirubinemia in term neonates has been shown to be associated with increased risk for autism spectrum disorders. The neuropathological finding of bilirubin-induced neurotoxicity also includes cerebellar injury with a decreased number of Purkinje cells, and disruption of multisensory feedback loop between cerebellum and cortical neurons which may explain the clinical characteristics of autism spectrum disorders. Severe hyperbilirubinemia occurs more frequently in infants from low- and middle-income countries (LMIC). Simple devices to measure bilirubin, and timely treatment are essential to reduce neurotoxicity, and improve outcomes for thousands of neonates around the world.

Infectious mononucleosis and hepatic function

Abnormal hepatic function is common in infectious mononucleosis (IM). However, it remains unknown why increased transferase levels are more common than bilirubin abnormalities in IM. The current study aimed to investigate these associations in the Chinese population. A total of 95 patients with IM (47 males and 48 females) were enrolled in the current study, as well as 95 healthy controls. Patients were sorted by sex. A receiver operating characteristic (ROC) curve was used to determine cut-off values for IM diagnosis and prediction. Crude and adjusted odds ratios (OR) for IM were analyzed using binary logistic regression. It was determined that alanine aminotransferase (ALT), aspartate aminotransferase (AST) and γ-glutamyl transferase (GGT) levels were significantly higher in patients with IM compared with controls; however, total bilirubin (TB) levels were significantly lower in patients with IM. ROCs demonstrated that, if ALT, AST and GGT concentrations were higher than, or if TB was lower than, cut-off values, they were predictive of IM. Binary logistic regression identified that the risk of IM in patients exhibiting high levels of transferases was significantly increased, particularly in males. Crude ORs in ALT quartile 4 were 21.667 and 10.111 for males and females, respectively and adjusted ORs were 38.054 and 9.882, respectively. A significant IM risk of IM was evident in patients with low bilirubin levels and females appeared to be particularly susceptible. For example, crude ORs in quartile 1 were 8.229 and 8.257 for males and females, respectively and adjusted ORs were 8.883 and 10.048, respectively. Therefore, the current study identified a positive association between transferase levels and IM and a negative association between TB and IM. Therefore, the results of the current study indicate that high transferases are suggestive of IM, particularly in males, whereas low TB is suggestive for IM, particularly in females.

Bilirubin and atherosclerotic diseases

Bilirubin is the final product of heme catabolism in the systemic circulation. For decades, increased serum/plasma bilirubin levels were considered an ominous sign of an underlying liver disease. However, data from recent years convincingly suggest that mildly elevated bilirubin concentrations are associated with protection against various oxidative stress-mediated diseases, atherosclerotic conditions being the most clinically relevant. Although scarce data on beneficial effects of bilirubin had been published also in the past, it took until 1994 when the first clinical study demonstrated an increased risk of coronary heart disease in subjects with low serum bilirubin levels, and bilirubin was found to be a risk factor for atherosclerotic diseases independent of standard risk factors. Consistent with these results, we proved in our own studies, that subjects with mild elevation of serum levels of unconjugated bilirubin (benign hyperbilirubinemia, Gilbert syndrome) have much lower prevalence/incidence of coronary heart as well as peripheral vascular disease. We have also demonstrated that this association is even more general, with serum bilirubin being a biomarker of numerous other diseases, often associated with increased risk of atherosclerosis. In addition, very recent data have demonstrated biological pathways modulated by bilirubin, which are responsible for observed strong clinical associations.

Conjugated Bilirubin Differentially Regulates CD4+ T Effector Cells and T Regulatory Cell Function through Outside-In and Inside-Out Mechanisms: The Effects of HAV Cell Surface Receptor and Intracellular Signaling

We recently reported an immune-modulatory role of conjugated bilirubin (CB) in hepatitis A virus (HAV) infection. During this infection the immune response relies on CD4+ T lymphocytes (TLs) and it may be affected by the interaction of HAV with its cellular receptor (HAVCR1/TIM-1) on T cell surface. How CB might affect T cell function during HAV infection remains to be elucidated. Herein, in vitro stimulation of CD4+ TLs from healthy donors with CB resulted in a decrease in the degree of intracellular tyrosine phosphorylation and an increase in the activity of T regulatory cells (Tregs) expressing HAVCR1/TIM-1. A comparison between CD4+ TLs from healthy donors and HAV-infected patients revealed changes in the TCR signaling pathway relative to changes in CB levels. The proportion of CD4+CD25+ TLs increased in patients with low CB serum levels and an increase in the percentage of Tregs expressing HAVCR1/TIM-1 was found in HAV-infected patients relative to controls. A low frequency of 157insMTTTVP insertion in the viral receptor gene HAVCR1/TIM-1 was found in patients and controls. Our data revealed that, during HAV infection, CB differentially regulates CD4+ TLs and Tregs functions by modulating intracellular pathways and by inducing changes in the proportion of Tregs expressing HAVCR1/TIM-1.

The molecular basis for the immunomodulatory activities of unconjugated bilirubin

Nearly a century ago, jaundiced patients were observed to have surprising and spontaneous remissions from incurable immunologic diseases including rheumatoid arthritis, allergy, and asthma. The mystery of why this phenomenon occurred remains unresolved to this day. Bilirubin has traditionally been considered an excretory product resulting from heme metabolism with little benefit to human physiology. In the past few decades, however, the salutary role of this byproduct as a potent antioxidant has been repeatedly noted. Most recently, the molecule has been found to possess immunomodulatory properties that rival its redox capacity, possibly explaining its ability to suppress inflammation. In this review, we specifically examine unconjugated bilirubin (UCB) as an immunomodulator and explore the molecular basis for its immunosuppressive effects.

Regulation of vulnerable plaque development by the heme oxygenase/carbon monoxide system

Plaque rupture and luminal thrombosis is the most common cause of coronary occlusion that leads to acute coronary syndromes. High-risk plaques, or vulnerable plaques, are defined as lesions that are prone to rupture, also known as thin cap fibroatheroma (TCFA), or lesions prone to erosion or with calcified cores. This review will focus mainly on the vulnerable plaque, which is thought to be the precursor of the thrombogenic or ruptured plaque. Heme oxygenase 1 (HO-1) protein expression is specifically increased in lesions with a vulnerable plaque phenotype resembling TCFAs and correlates with a rise in expression levels of intimal proinflammatory markers. Data from several human and animal studies imply an important function for HO-1 in the genetic regulation of early, as well as late atherogenesis, and plaque destabilization toward a vulnerable phenotype. Although a direct association between HO-1, vulnerable plaque development, and clinical outcome is for now missing, the correlations that have been reported for HO-1 and coronary artery disease point to a possible link.

Circular dichroism spectroscopy is a sensitive tool for investigation of bilirubin-enzyme interactions

Noncovalent complex formation of unconjugated bilirubin with various enzymes has been demonstrated by measuring induced circular dichroism (ICD) peaks associated with the pigment VIS absorption band. Preferential binding of the P- or M-helical conformer of bilirubin to dehydrogenases, catalase, alkaline phosphatase, and α-chymotrypsin is responsible for the characteristic exciton CD couplet that undergoes remarkable changes upon the addition of enzymatic cofactors (NADH, AMP) and an inhibitor (acridine). Alterations of the ICD spectra refer to a direct binding competition between bilirubin and NADH for a common binding site on alcohol dehydrogenase and catalase, suggesting a potential mechanism for the inhibitory effect of BR reported on NAD(P)H dependent enzymes. NADH and bilirubin form a ternary complex with glutamate dehydrogenase indicated by peculiar CD spectral changes that are proposed to be generated by allosteric mechanism. α-chymotrypsin binds bilirubin in its catalytic site, as indicated by CD displacement experiments performed with the competitive inhibitor acridine. Surprisingly, the closely related trypsin does not induce any CD signal with bilirubin. Taking into consideration the clinically relevant but controversial and poorly understood areas of bilirubin biochemistry, the fast and simple CD spectroscopic approach presented here may help to unfold diverse physiological and pathophysiological roles of BR on a molecular level.

Lactobacillus plantarum inhibits intestinal epithelial barrier dysfunction induced by unconjugated bilirubin

Although a large number of in vitro and in vivo tests have confirmed that taking probiotics can improve the intestinal barrier, few studies have focused on the relationship between probiotics and the intestinal epithelial barrier in hyperbilirubinaemia. To investigate the effects of and mechanisms associated with probiotic bacteria (Lactobacillus plantarum; LP) and unconjugated bilirubin (UCB) on the intestinal epithelial barrier, we measured the viability, apoptotic ratio and protein kinase C (PKC) activity of Caco-2 cells. We also determined the distribution and expression of tight junction proteins such as occludin, zonula occludens (ZO)-1, claudin-1, claudin-4, junctional adhesion molecule (JAM)-1 and F-actin using confocal laser scanning microscopy, immunohistochemistry, Western blotting and real-time quantitative PCR. The present study demonstrated that high concentrations of UCB caused obvious cytotoxicity and decreased the transepithelial electrical resistance (TER) of the Caco-2 cell monolayer. Low concentrations of UCB inhibited the expression of tight junction proteins and PKC but could induce UDP-glucuronosyltransferases 1 family-polypeptide A1 (UGT1A1) expression. UCB alone caused decreased PKC activity, serine phosphorylated occludin and ZO-1 levels. After treatment with LP, the effects of UCB on TER and apoptosis were mitigated; LP also prevented aberrant expression and rearrangement of tight junction proteins. Moreover, PKC activity and serine phosphorylated tight junction protein levels were partially restored after treatment with LP, LP exerted a protective effect against UCB damage to Caco-2 monolayer cells, and it restored the structure and distribution of tight junction proteins by activating the PKC pathway. In addition, UGT1A1 expression induced by UCB in Caco-2 cells could ameliorate the cytotoxicity of UCB.

Dantrolene exerts protective activity in double and triple combination with nimodipine, ruthenium red and basilene blue in bilirubin-induced neurotoxicity in cell culture of rats

In the present study, dantrolene, nimodipine, basilen blue, and ruthenium red were tested in experimental bilirubin toxicity in cortical cell culture of rats. Neurotoxicity was induced by 10(-4) M bilirubin. Basilen blue in the highest concentration of 10(-4) M was determined as the most protective agent when applied alone. Dantrolene alone was found surprisingly ineffective in all doses tested. But it was found very protective both in double and triple combinations. Nimodipine, basilen blue, and ruthenim red neuroprotective potentials were enhanced by adding dantrolene into the media. Best double combination was determined as dantrolene plus ruthenium red. On the other hand, most useful triple combination was found as dantrolene plus nimodipine plus basilen blue. As a result, dantrolene wasn't found to be effective alone, while it seems most potential compound in combined application in bilirubin-induced neurotoxicity. The importance of calcium intrusion was confirmed in bilirubin-induced neurotoxicity.

Hemoxygenase-1 in cardiovascular disease

Hemoxygenase (HO)-1 is an inducible isoform of the first and rate-controlling enzyme of the degradation of heme into iron, carbon monoxide, and biliverdin, the latter being subsequently converted into bilirubin. Several positive biological effects exerted by this enzyme have gained attention, as anti-inflammatory, antiapoptotic, angiogenic, and cytoprotective functions are attributable to carbon monoxide and/or bilirubin. Thus, the physiological induction of HO-1 may be an adaptive and beneficial response to several possibly noxious stimuli, including heme itself, suggesting a potentially autoprotective and autodefensive role in several pathophysiological states including acute coronary syndromes and stroke. This review article provides a comprehensive overview of the biochemistry, physiology, and pathophysiology of HO-1 in relation to cardiovascular disease (CVD). Furthermore, we present some of the emerging evidence in support of the view that the induction of the HO-1 gene may be a new opportunity to target the pathophysiology of CVD, with therapeutic implications for management.

Association of serum bilirubin with pulsatile arterial function in asymptomatic young adults: the Bogalusa Heart Study

The inverse association between serum bilirubin, a potent antioxidant, and oxidative stress-mediated diseases like cardiovascular disease is known. However, information is scant regarding the influence of bilirubin in relation to traditional cardiovascular risk factors on pulsatile arterial function in asymptomatic younger adults. The present study examines this aspect in 777 black and white subjects (71% white, 42% male) aged 18 to 44 years. Pulsatile arterial function was assessed in terms of large-artery (capacitive) and small-artery (oscillatory) compliances by radial artery pressure pulse contour analysis. In bivariate analysis adjusted for race and sex, bilirubin related significantly and positively to large- and small-artery compliances and high-density lipoprotein cholesterol, and inversely to age, body mass index, blood pressure variables, non-high-density lipoprotein cholesterol, triglycerides, and insulin resistance index. In multivariable analysis including race, sex, body surface area, and risk factor variables mentioned above, bilirubin did not relate to large-artery compliance, without or with smoking status in the model, whereas bilirubin associated beneficially with small-artery compliance (P=.01) in a model that excluded smoking status. When smoking status was included in the model, this association became less strong (P=.04); and smoking entered the model as an adverse predictor (P=.003). The observed beneficial association of serum bilirubin on pulsatile arterial function, albeit the attenuating effect of smoking on this relationship, in asymptomatic younger adults supports the antioxidant function of bilirubin in providing protection against oxidative stress-mediated vascular dysfunction.

Regulation of TNF-alpha-activated PKC-zeta signaling by the human biliverdin reductase: identification of activating and inhibitory domains of the reductase

Human biliverdin reductase (hBVR) is a dual function enzyme: a catalyst for bilirubin formation and a S/T/Y kinase that shares activators with protein kinase C (PKC) -zeta, including cytokines, insulin, and reactive oxygen species (ROS). Presently, we show that hBVR increases PKC-zeta autophosphorylation, stimulation by TNF-alpha, as well as cytokine stimulation of NF-kappaB DNA binding and promoter activity. S149 in hBVR S/T kinase domain and S230 in YLS230F in hBVR's docking site for the SH2 domain of signaling proteins are phosphorylation targets of PKC-zeta. Two hBVR-based peptides, KRNRYLS230F (#1) and KKRILHC281 (#2), but not their S-->A or C-->A derivatives, respectively, blocked PKC-zeta stimulation by TNF-alpha and its membrane translocation. The C-terminal-based peptide KYCCSRK296 (#3), enhanced PKC-zeta stimulation by TNF-alpha; for this, Lys296 was essential. In metabolically 32P-labeled HEK293 cells transfected with hBVR or PKC-zeta, TNF-alpha increased hBVR phosphorylation. TNF-alpha did not stimulate PKC-zeta in cells infected with small interfering RNA for hBVR or transfected with hBVR with a point mutation in the nucleotide-binding loop (G17), S149, or S230; this was similar to the response of "kinase-dead" PKC-zeta(K281R). We suggest peptide #1 blocks PKC-zeta-docking site interaction, peptide #2 disrupts function of the PKC-zeta C1 domain, and peptide #3 alters ATP presentation to the kinase. The findings are of potential significance for development of modulators of PKC-zeta activity and cellular response to cytokines.

Renal hemodynamic, inflammatory, and apoptotic responses to lipopolysaccharide in HO-1-/- mice

Lipopolysaccharide (LPS) induces the stress-responsive gene heme oxygenase-1 (HO-1). The present study examined the significance of HO-1 in response to LPS. In HO-1(-/-) mice, as compared with HO-1(+/+) mice, LPS provoked a greater reduction in glomerular filtration rate and renal blood flow, increased renal cytokine expression, and increased activation of nuclear factor (NF)-kappaB. Conversely, HO-1-overexpressing renal epithelial cells, exposed to LPS, exhibited a blunted activation of NF-kappaB and less phosphorylation of its inhibitor, IkappaB. In HO-1(-/-) mice, as compared with HO-1(+/+) mice, LPS provoked markedly greater elevations in serum levels of Th1 cytokines, Th2 cytokines, chemokines, and cytokines that stimulate bone marrow progenitors. The liver, a major source of serum cytokines, showed an increased activation of NF-kappaB in LPS-treated HO-1(-/-) mice. In addition, LPS provoked widespread apoptosis of immune cells in the spleen and thymus in HO-1(-/-) mice but not in HO-1(+/+) mice. We conclude that HO-1 deficiency exhibits a heightened and dysregulated inflammatory response to LPS accompanied by greater impairment in renal hemodynamic response and widespread apoptosis of immune cells. Because polymorphisms in the HO-1 gene with diminished HO activity predispose to human disease, we speculate that our findings may be relevant to the clinical outcome in patients with sepsis syndromes.

MAPKs are key players in mediating cytokine release and cell death induced by unconjugated bilirubin in cultured rat cortical astrocytes

When activated by unconjugated bilirubin (UCB), astrocytes are important sources of inflammatory mediators such as TNF-alpha, IL-1beta and IL-6, which may contribute for the neurotoxicity observed during severe neonatal hyperbilirubinemia. In the present study, we have addressed the role of the mitogen-activated protein kinases (MAPKs) p38, Jun N-terminal kinase (JNK)1/2 and extracellular signal-regulated kinase (ERK)1/2 pathways and their relation with the nuclear factor kappaB (NF-kappaB) cascade in the signalling events involved in cytokine release and cell death caused by UCB in primary cultures of rat astrocytes. Stimulation of astrocytes with UCB in the presence of all the MAPK inhibitors prevented UCB-induced release of TNF-alpha and IL-6, while IL-1beta secretion was only reduced by JNK1/2 and ERK1/2 inhibitors. In addition, activation of the NF-kappaB transcription factor, needed for cytokine release by UCB-stimulated astrocytes, was shown to be dependent on JNK1/2 and ERK1/2 phosphorylation. Moreover, all MAPK inhibitors prevented astroglial apoptosis triggered by UCB. Interestingly, UCB-induced lactate dehydrogenase release was prevented by blockade of JNK1/2, ERK1/2 and NF-kappaB cascades but enhanced by p38 inhibition. Taken together, our data demonstrate for the first time that MAPK transduction pathways are key players in the UCB-induced inflammatory response and cell death in astrocytes, probably also involving NF-kappaB modulation. These findings contribute to unraveling the complex mechanisms of astrocyte reactivity to UCB and may ultimately prove useful in the development of new therapeutic strategies to prevent nerve cell damage during acute bilirubin encephalopathy.

Reading dynamic kinase activity in living cells for high-throughput screening

Protein kinases, as crucial signaling molecules, represent an emerging class of drug targets, and the ability to assay their activities in living cells with high-throughput screening should provide exciting opportunities for drug discovery and chemical and functional genomics. Here, we describe a general method for high-throughput reading of dynamic kinase activities using ratiometric fluorescent sensors, and showcase an example of reading intracellular activities of protein kinase A (PKA) and the cyclic adenosine monophosphate (cAMP)/PKA pathway downstream of many G-protein coupled receptors (GPCRs). We further demonstrate the first compound screen based on the ability of compounds to modulate dynamic kinase activities in living cells and show that such screening of a collection of clinical compounds has successfully identified modulators of the GPCR/cAMP/PKA pathway.

New insights into biliverdin reductase functions: linking heme metabolism to cell signaling

Biliverdin reductase (BVR) functions in cell signaling through three distinct tracks: a dual-specificity kinase that functions in the insulin receptor/MAPK pathways (25, 29, 51); a bzip-type transcription factor for ATF-2/CREB and HO-1 regulation (1, 25); and a reductase that catalyzes the conversion of biliverdin to bilirubin (27). These, together with the protein's primary and secondary features, intimately link BVR to the entire spectrum of cell-signaling cascades.

Bilirubin-induced cell death during continuous and intermittent phototherapy and in the dark

AIM

To compare continuous and intermittent light exposure in the presence of bilirubin with respect to cellular damage. Furthermore, it was of interest to characterize the nature of cellular toxicity of bilirubin in the dark.

METHOD

A murine lymphoma cell line, L5178Y-R (LY-R), was exposed to solutions of bilirubin (160 microM) supplemented with human serum albumin (200 microM) and irradiated with phototherapy light (Philips 20W/52) at a constant total dose of approximately 500 kJ/m2. The irradiation was given either as intermittent or continuous treatment with light of variable irradiance. The three lower irradiance levels were clinically relevant. Cells treated with bilirubin were also kept in the dark for various periods of time. Cell toxicity was determined by measuring apoptosis and necrosis. Apoptosis was measured by terminal deoxynucleotide transferase and propidium iodide staining assay, while trypan blue assay was used for detection of necrosis.

RESULTS

There was no difference (n = 6, p > 0.05) between continuous and intermittent irradiation in the induction of early and late apoptotic cell death. Necrosis was more pronounced after intermittent treatment. Bilirubin dark toxicity was observed and classified as both apoptotic and necrotic.

CONCLUSION

Continuous and intermittent light exposure caused the same degree of apoptotic cell death, while the cells underwent more necrotic death after intermittent exposure. Bilirubin was cytotoxic in the dark by both cell death mechanisms.

Phototherapy: old questions, new answers

Despite its popularity and widespread use, there remains much more to be learned about phototherapy.

CONCLUSION

There is continued debate on whether intermittent phototherapy is more effective than continuous phototherapy; if light of a longer wavelength is more effective than blue light phototherapy; and if phototherapy in the presence of bilirubin is toxic to normal or abnormal red blood cells. Three in vitro studies in this issue of the journal provide answers to some of these questions.

Heme Oxygenase and Atherosclerosis

Overproduction of reactive oxygen species under pathophysiological conditions, including dyslipidemia, hypertension, diabetes, and smoking, is integral in the development of cardiovascular diseases (CVD). The reactive oxygen species released from all types of vascular cells regulate various signaling pathways that mediate not only vascular inflammation in atherogenesis but also antioxidative and antiinflammatory responses. One such protective and stress-induced protein is heme oxygenase (HO). HO is the first rate-limiting enzyme in heme breakdown to generate equimolar quantities of carbon monoxide, biliverdin, and free ferrous iron. Accumulating evidence has shown that inducible HO (HO-1) and its products function as adaptive molecules against oxidative insults. The proposed mechanisms by which HO-1 exerts its cytoprotective effects include its abilities to degrade the pro-oxidative heme, to release biliverdin and subsequently convert it bilirubin, both of which have antioxidant properties, and to generate carbon monoxide, which has antiproliferative and antiinflammatory as well as vasodilatory properties. Herein, I highlight the relationship of HO and cardiovascular disease, especially atherosclerosis, gene-targeting approaches in animal models, and the potential for and concern about HO-1 as a novel therapeutic target for cardiovascular diseases.

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

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. 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. 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.

Small interference RNA-mediated gene silencing of human biliverdin reductase, but not that of heme oxygenase-1, attenuates arsenite-mediated induction of the oxygenase and increases apoptosis in 293A kidney cells

BVR reduces biliverdin, the HO-1 and HO-2 product, to bilirubin. Human biliverdin (BVR) is a serine/threonine kinase activated by free radicals. It is a leucine zipper (bZip) DNA-binding protein and a regulatory factor for 8/7-bp AP-1-regulated genes, including HO-1 and ATF-2/CREB. Presently, small interference (si) RNA constructs were used to investigate the role of human BVR in sodium arsenite (As)-mediated induction of HO-1 and in cytoprotection against apoptosis. Activation of BVR involved increased serine/threonine phosphorylation but not its protein or transcript levels. The peak activity at 1 h (4-5-fold) after treatment of 293A cells with 5 mum As preceded induction of HO-1 expression by 3 h. The following suggests BVR involvement in regulating oxidative stress response of HO-1: siBVR attenuated As-mediated increase in HO-1 expression; siBVR, but not siHO-1, inhibited As-dependent increased c-jun promoter activity; treatment of cells with As increased AP-1 binding of nuclear proteins; BVR was identified in the DNA-protein complex; and AP-1 binding of the in vitro translated BVR was phosphorylation-dependent and was attenuated by biliverdin. Most unexpectedly, cells transfected with siBVR, but not siHO-1, displayed a 4-fold increase in apoptotic cells when treated with 10 mum As as detected by flow cytometry. The presence of BVR small interference RNA augmented the effect of As on levels of cytochrome c, TRAIL, and DR-5 mRNA and cleavage of poly(ADP-ribose) polymerase. The findings describe the function of BVR in HO-1 oxidative response and, demonstrate, for the first time, not only that BVR advances the role of HO-1 in cytoprotection but also affords cytoprotection independent of heme degradation.

Protective role of heme oxygenase in the blood vessel wall during atherogenesis

Several lines of evidence suggest that antioxidant processes and (or) endogenous antioxidants inhibit proatherogenic events in the blood vessel wall. Heme oxygenase (HO), which catabolizes heme to biliverdin, carbon monoxide, and catalytic iron, has been shown to have such antioxidative properties. The HO-1 isoform of heme oxygenase is ubiquitous and can be increased several fold by stimuli that induce cellular oxidative stress. Products of the HO reaction have important effects: carbon monoxide is a potent vasodilator, which is thought to play a role in modulation of vascular tone; biliverdin and its by-product bilirubin are potent antioxidants. Although HO induction results in an increase in catalytic free iron release, the enhancement of intracellular ferritin protein through HO-1 has been reported to decrease the cytotoxic effects of iron. Oxidized LDL has been shown to increase HO-1 expression in endothelial and smooth muscle cell cultures, and during atherogenesis. Further evidence of HO-1 expression associated with atherogenesis has been demonstrated in human, murine and rabbit atherosclerotic lesions. Moreover, genetic models of HO deficiency suggest that the actions of HO-1 are important in modulating the severity of atherosclerosis. Recent experiments in gene therapy using the HO gene suggest that interventions aimed at HO in the vessel wall could provide a novel therapeutic approach for the treatment or prevention of atherosclerotic disease.Key words: heme oxygenase, atherosclerosis, antioxidant enzymes, oxidized LDL, gene therapy.

Biliverdin, a natural product of heme catabolism, induces tolerance to cardiac allografts

Biliverdin, a product of heme oxygenase-1 (HO-1) enzymatic action, is converted into bilirubin, which has been considered a waste product in the past. We now show that administration of biliverdin has a salutary effect in organ transplantation. A brief course of treatment with biliverdin leads to long-term survival of H-2 incompatible heart allografts. Furthermore, those recipients harboring long-surviving (>100 days) allografts were tolerant to donor antigens indicated by the acceptance of second donor strain hearts but not third-party grafts. Treatment with biliverdin decreased intragraft leukocyte infiltration and inhibited T cell proliferation. Likely related to tolerance induction, biliverdin interferes with T cell signaling by inhibiting activation of nuclear factor of activated T cells (NFAT) and nuclear factor kappaB (NF-kappaB), two transcription factors involved in interleukin-2 (IL-2) transcription and T cell proliferation, as well as suppressing Th1 interferon-gamma (IFN-gamma) production in vitro. These findings support the potential use of biliverdin, a natural product, in transplantation and other T cell mediated immune disorders.

Heme oxygenase inhibits human airway smooth muscle proliferation via a bilirubin-dependent modulation of ERK1/2 phosphorylation

The aim of this study was to investigate whether the heme oxygenase (HO) pathway could modulate proliferation of airway smooth muscle (ASM) and the mechanism(s) involved in this phenomenon. In cultured human ASM cells, 10% fetal calf serum or 50 ng/ml platelet-derived growth factor AB induced cell proliferation, extracellular and intracellular reactive oxygen species (ROS) production and ERK1/2 phosphorylation. Pharmacological HO-1 induction (by 10 microm hemin or by 20 microm cobalt-protoporphyrin) and HO inhibition (by 25 microm tin-protoporphyrin or by an antisense oligonucleotide), respectively, reduced and enhanced significantly both cell proliferation and ROS production. Neither the carbon monoxide scavenger myoglobin (5-20 microm) nor the guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one could reverse ASM proliferation induced by tin-protoporphyrin, making a role of the CO-cGMP pathway in HO-modulated proliferation unlikely. By contrast, bilirubin (1 microm) and the antioxidant N-acetyl-cysteine (1 mm) significantly reduced mitogen-induced cell proliferation, ROS production, and ERK1/2 phosphorylation. Furthermore, both bilirubin and N-acetyl-cysteine and the ERK1/2 inhibitor PD98059 significantly reversed the effects of HO inhibition on ASM proliferation. These results could be relevant to ASM alterations observed in asthma because activation of the HO pathway prevented the increase in bronchial smooth muscle area induced by repeated ovalbumin challenge in immunized guinea pigs, whereas inhibition of HO had the opposite effect. In conclusion, this study provides evidence for an antiproliferative effect of the HO pathway in ASM in vitro and in vivo through a bilirubin-mediated redox modulation of phosphorylation of ERK1/2.

The effects of bilirubin on evoked potentials and long-term potentiation in rat hippocampus in vivo

Neonatal jaundice is a common condition that could potentially lead to severe neurotoxicity. In this electrophysiological study we observed the effects of a short-term bilirubin injection on evoked potentials (population spike, PS) and long-term potentiation (LTP) in the hippocampal CA3 region of Sprague Dawley rats in vivo. The animal received a bolus i.v. injection of either 60 mg/kg, or 30 mg/kg of bilirubin, or an equivalent volume of bilirubin-free vehicle in 5 min. The results showed that both bilirubin-treated groups had a dose-independent prolongation of peak latencies and decrease of slopes of the PS at all measured time points following injection (1, 3, 5, 10, 15, 30, 45, 60, 90, 120 min), while the amplitudes of the PS did not change significantly. The peak latency, slope, and the amplitude of PS stayed unchanged in the control group. Furthermore, while LTP could be induced by high-frequency stimulation in control animals, this phenomenon was absent in both bilirubin-treated groups. The amplitudes of the PS in the two treated groups after stimulation were also smaller than those of the control animals at every time points. These findings are in accordance with previous observations showing significant depressive effects of bilirubin on the nervous system. Our novel finding that short-term exposure to bilirubin can inhibit the induction of LTPs in the hippocampus, is compatible with the suggestion that neonatal hyperbilirubinemia can impact on learning and memory.

Cognitive and behavioral deficits in premature graduates of intensive care

A substantial number of VLBW graduates of intensive care develop cognitive and behavioral problems, even in the absence of neuroimaging abnormalities. Although this article has highlighted the potential, important, contributing role of medical and stressful, neonatal, environmental conditions to the development of these deficits, it is not all-encompassing, and there are additional prenatal (ie, in utero stress, drug exposure) and neonatal (ie, infectious) contributing factors. The long-term, outcome data presented in this article are pertinent to the more mature, VLBW infant, and it remains unclear and critically important to delineate the long-term, neurobehavioral outcome of those extremely low birth-weight survivors born at the cutting limit of viability.

Mechanisms of bilirubin toxicity: clinical implications

The basic mechanism of kernicterus and bilirubin encephalopathy has not been unequivocally determined. Much knowledge has been gained about phenomena that contribute to bilirubin neurotoxicity, and this knowledge has implications for clinical practice. Conditions that impact on blood-brain barrier function, increase brain blood flow, or impact on bilirubin metabolism, including its transport in serum, should be avoided, if possible. Such conditions include drugs and drug stabilizers that compete with bilirubin binding to albumin, or that inhibit P-glycoprotein in the blood-brain barrier, prematurity/immaturity, and clinically significant illness in the infant that involves hemolysis, respiratory and metabolic acidosis, infection, asphyxia, hypoxia and (perhaps) hyperoxia, and hyperosmolality. If these conditions are not avoidable then there should be a more aggressive approach to the treatment of hyperbilirubinemia. The limits of tolerance for hyperbilirubinemia varies among neonates and there are no tools to determine with certainty when a particular infant is approaching the danger zone. Neurological symptoms in a jaundiced infant require extreme vigilance, and, in most cases, immediate intervention.

Heme oxygenase modulates oxidant-signaled airway smooth muscle contractility: role of bilirubin

Reactive oxygen species (ROS) increase the contractile response of airway smooth muscle (ASM). Heme oxygenase (HO) catabolizes heme to the powerful antioxidant bilirubin. Because HO is expressed in the airways, we investigated its effects on ASM contractility and ROS production in guinea pig trachea. HO expression was higher in the epithelium than in tracheal smooth muscle. Incubation of tracheal rings (TR) with the HO inhibitor tin protoporphyrin (SnPP IX) or the HO substrate hemin increased and decreased, respectively, ASM contractile response to carbamylcholine. The effect of hemin was reversed by SnPP and mimicked by the antioxidants superoxide dismutase (SOD) and catalase. Hemin significantly reduced the effect of carbamylcholine in rings treated with the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ), compared with ODQ-treated rings without hemin incubation, suggesting that the CO-guanosine 3',5'-cyclic monophosphate pathway was not involved in the control of tracheal reactivity. SnPP and hemin increased and decreased ROS production by TR by 18 and 38%, respectively. Bilirubin (100 pM) significantly decreased TR contractility and ROS production. Hemin, bilirubin, and SOD/catalase decreased phosphorylation of the contractile protein myosin light chain, whereas SnPP significantly augmented it. These data suggest that modulation of the redox status by HO and, moreover, by bilirubin modulates ASM contractility by modulating levels of phosphorylated myosin light chain.

Apoptosis in murine hepatoma hepa 1c1c7 wild-type, C12, and C4 cells mediated by bilirubin

Elevated serum and tissue bilirubin concentrations that occur in pathological conditions such as cholestasis, jaundice, and other liver diseases are known to stimulate cytotoxic responses. In preliminary studies, we noted that bilirubin seemed to cause apoptosis in murine hepatoma Hepa 1c1c7 wild-type (WT) cells. Consequently, we investigated apoptosis caused by bilirubin in WT, mutant C12 [aryl hydrocarbon receptor (AHR)-deficient], and C4 (AHR nuclear translocator-deficient) Hepa 1c1c7 cells. Three independent measures of apoptosis were used to quantify the effects of exogenous bilirubin (0, 1, 10, 25, 50, or 100 microM). Caspase-3 activity and cytochrome c release from mitochondria increased at 3 h post-treatment, before increased caspase-8 activity at 6 h, and nuclear condensation by 24 h after treatment with bilirubin. No differences in whole-cell lipid peroxidation were observed between the cell types; however, intracellular reactive oxygen species (ROS) production was greater in WT cells than C12 or C4 cells 3 h after bilirubin exposure. Pretreatment of cells for 1 h with 1 or 10 microM alpha-naphthoflavone, an AHR antagonist, before bilirubin exposure resulted in decreased caspase-3 activity at 6 h and nuclear condensation at 24 h in WT cells. These results indicate that bilirubin, a potential AHR ligand, causes apoptosis in murine Hepa 1c1c7 WT cells by a mechanism(s) partially involving the AHR, disruption of membrane integrity, and increased intracellular ROS production.

Bilirubin induces apoptosis via the mitochondrial pathway in developing rat brain neurons

Increased levels of unconjugated bilirubin, the end-product of heme catabolism, are detrimental to the central nervous system. To examine the role of apoptosis in bilirubin-induced toxicity and to characterize the biochemical pathway of cell death, we exposed developing rat brain neurons to purified unconjugated bilirubin at concentrations below and above saturation of human serum albumin. Isolated neurons treated with bilirubin showed increased levels of apoptosis. Mitochondrial cytochrome c was extensively released and accumulated in cytosol. Consistent with this observation, caspase-3 was activated and the full-length substrate poly(ADP)ribose polymerase (PARP) degraded, even in the presence of very modestly elevated concentrations of bilirubin. In parallel, all events were prevented in cells preincubated with ursodeoxycholate. Further experiments showed that bilirubin diminished mitochondrial transmembrane potential (DeltaPsi(m)) and increased mitochondrial-associated Bax protein levels, while directly disrupting membrane lipid and protein structure. In conclusion, bilirubin induces mitochondrial depolarization and Bax translocation via physical interaction with membranes, mediating the mitochondrial pathway of apoptosis in neurons exposed to bilirubin. These results provide a novel insight into the mechanism of bilirubin-induced toxicity.