Reassessment of the unbound concentrations of unconjugated bilirubin in relation to neurotoxicity in vitro

Choline supplementation mitigates effects of bilirubin in cerebellar granule neurons in vitro

BACKGROUND

Premature infants may suffer from high levels of bilirubin that could lead to neurotoxicity. Bilirubin has been shown to decrease L1-mediated ERK1/2 signaling, L1 phosphorylation, and L1 tyrosine 1176 dephosphorylation. Furthermore, bilirubin redistributes L1 into lipid rafts (LR) and decreases L1-mediated neurite outgrowth. We demonstrate that choline supplementation improves L1 function and signaling in the presence of bilirubin.

METHODS

Cerebellar granule neurons (CGN) were cultured with and without supplemental choline, and the effects on L1 signaling and function were measured in the presence of bilirubin. L1 activation of ERK1/2, L1 phosphorylation and dephosphorylation were measured. L1 distribution in LR was quantified and neurite outgrowth of CGN was determined.

RESULTS

Forty µM choline significantly reduced the effect of bilirubin on L1 activation of ERK1/2 by 220% (p = 0.04), and increased L1 triggered changes in tyrosine phosphorylation /dephosphorylation of L1 by 34% (p = 0.026) and 35% (p = 0.02) respectively. Choline ameliorated the redistribution of L1 in lipid rafts by 38% (p = 0.02) and increased L1-mediated mean neurite length by 11% (p = 0.04).

CONCLUSION

Choline pretreatment of CGN significantly reduced the disruption of L1 function by bilirubin. The supplementation of pregnant women and preterm infants with choline may increase infant resilience to the effects of bilirubin.

IMPACT

This article establishes choline as an intervention for the neurotoxic effects of bilirubin on lipid rafts. This article provides clear evidence toward establishing one intervention for bilirubin neurotoxicity, where little is understood. This article paves the way for future investigation into the mechanism of the ameliorative effect of choline on bilirubin neurotoxicity.

Hyperbilirubinemia Among Infants Born Preterm: Peak Levels and Association with Neurodevelopmental Outcomes

OBJECTIVE

To describe the distribution of peak bilirubin levels among infants born before 29 weeks of gestation in the first 14 days of life and to study the association between quartiles of peak bilirubin levels at different gestational ages and neurodevelopmental outcomes.

STUDY DESIGN

Multicenter, retrospective, nationwide cohort study of neonatal intensive care units in the Canadian Neonatal Network and Canadian Neonatal Follow-Up Network, including neonates born preterm at 220/7 to 286/7 weeks of gestation born between 2010 and 2018. Peak bilirubin levels were recorded during the first 14 days of age. Main outcome was significant neurodevelopmental impairment, defined as cerebral palsy with Gross Motor Function Classification System ≥3, or Bayley III-IV scores of <70 in any domain, or visual impairment, or bilateral hearing loss requiring hearing aids.

RESULTS

Among 12 554 included newborns, median gestational age was 26 weeks (IQR 25-28) and birth weight was 920 g (IQR 750-1105 g). The median peak bilirubin values increased as gestational age increased (112 mmol/L [6.5 mg/dL] at 22 weeks and 156 mmol/L [9.1 mg/dL] at 28 weeks). Significant neurodevelopmental impairment was identified in 1116 of 6638 (16.8%) of children. Multivariable analyses identified an association between peak bilirubin in the highest quartile and neurodevelopmental impairment (aOR 1.27, 95% CI 1.01-1.60) and receipt of hearing aid/cochlear implant (aOR 3.97, 95%CI: 2.01-7.82) compared with the lowest quartile.

CONCLUSION

In this multicenter cohort study, peak bilirubin levels in neonates of <29 weeks of gestation increased with gestational age. Peak bilirubin values in the highest gestational age-specific quartile were associated with significant neurodevelopmental and hearing impairments.

Bilirubin Prevents the TH+ Dopaminergic Neuron Loss in a Parkinson's Disease Model by Acting on TNF-α

Parkinson's disease (PD), the fastest-growing movement disorder, is still challenged by the unavailability of disease-modifying therapy. Mildly elevated levels of unconjugated bilirubin (UCB, PubChem CID 5280352) have been shown to be protective against several extra-CNS diseases, and the effect is attributed to its well-known anti-oxidant and anti-inflammatory capability. We explored the neuroprotective effect of low concentrations of UCB (from 0.5 to 4 µM) in our PD model based on organotypic brain cultures of substantia nigra (OBCs-SN) challenged with a low dose of rotenone (Rot). UCB at 0.5 and 1 µM fully protects against the loss of TH⁺ (dopaminergic) neurons (DOPAn). The alteration in oxidative stress is involved in TH⁺ positive neuron demise induced by Rot, but is not the key player in UCB-conferred protection. On the contrary, inflammation, specifically tumor necrosis factor alpha (TNF-α), was found to be the key to UCB protection against DOPAn sufferance. Further work will be needed to introduce the use of UCB into clinical settings, but determining that TNF-α plays a key role in PD may be crucial in designing therapeutic options.

NAD+ attenuates bilirubin-induced augmentation of voltage-gated calcium currents in neurons of the ventral cochlear nucleus

Nicotinamide adenine dinucleotide (NAD+) is a ubiquitous molecule with wide-ranging roles in several cell processes, such as regulation of calcium homeostasis and protection against cell injuries. However, the roles of NAD+ in neuroprotection is poorly understood. The main neurons in ventral cochlear nucleus (VCN) are highly susceptible to bilirubin-associated excitotoxicity. We investigated the effects of NAD+ on VCN neurons by whole cell patch-clamp recordings. We found that NAD+ effectively reverses and inhibits bilirubin-mediated enhancement of voltage-gated calcium (VGCC) currents in VCN neurons. Moreover, NAD+ itself did not affect VGCC currents. These results collectively suggest that NAD+ may be neuroprotective by attenuating Ca²⁺ influx to suppress bilirubin-induced intracellular Ca²⁺ overloads. Our research provides a basis for evaluation of NAD+ as a promising therapeutic target for bilirubin encephalopathy and excitotoxicity associated with other neurological disorders.

Bilirubin inhibits lipid raft dependent functions of L1 cell adhesion molecule in rat pup cerebellar granule neurons

BACKGROUND

The mechanism of bilirubin neurotoxicity is poorly understood. We hypothesize that bilirubin inhibits the function of lipid rafts (LR), microdomains of the plasma membrane critical for signal transduction. To test this hypothesis, we measured the effect of free bilirubin (Bf) between 7.6 and 122.5 nM on LR-dependent functions of L1 cell adhesion molecule (L1).

METHODS

Cerebellar granule neurons (CGN) were plated on poly-L-lysine overnight, and neurite length was determined after 1 h treatment with L1 alone or L1 and bilirubin. L1 activation of ERK1/2 was measured in CGN in the presence or absence of bilirubin. The effect of bilirubin on L1 distribution in LR was quantitated, and the localization of bilirubin to LR was determined.

RESULTS

The addition of bilirubin to CGN treated with L1 significantly decreased neurite length compared to L1 alone. L1 activation of ERK1/2 was inhibited by bilirubin. Bilirubin redistributed L1 into LR. Bilirubin was associated only with LR-containing fractions of a sucrose density gradient.

CONCLUSION

Bf significantly inhibits LR-dependent functions of L1 and are found only associated with LR, suggesting one mechanism by which bilirubin may exert neurotoxicity is through the dysfunction of protein-LR interactions.

IMPACT

This article establishes lipid rafts as a target for the neurotoxic effects of bilirubin. This article provides clear evidence toward establishing one mechanism of bilirubin neurotoxicity, where little is understood. This article paves the way for future investigation into lipid raft dependent functions, and its role in neurodevelopmental outcome.

Neurotoxicity of Unconjugated Bilirubin in Neonatal Hypoxic-Ischemic Brain Injury in vitro

Background: The pathophysiology of bilirubin neurotoxicity in course of hypoxic-ischemic encephalopathy (HIE) in term and preterm infants is still poorly understood. We hypothesized that oxidative stress may be a common mechanism that link hyperbilirubinemia and HIE. Objectives: The objective of the present study was to evaluate whether unconjugated bilirubin (UCB) may enhance the HI brain injury by increasing oxidative stress and to test pioglitazone and allopurinol as new antioxidant therapeutic drugs in vitro. Methods: The effects of UCB were tested on organotypic hippocampal slices subjected to 30 min oxygen-glucose deprivation (OGD), used as in vitro model of HIE. The experiments were performed on mature (14 days in culture) and immature (7 days in culture) slices, to mimic the brains of term and preterm infants, respectively. Mature and immature slices were exposed to UCB, human serum albumin (HSA), pioglitazone, and/or allopurinol for 24 h, immediately after 30 min OGD. Neuronal injury was assessed using propidium iodide (PI) fluorescence. ROS formation was quantified by using the 2',7'-dichlorodihydrofluorescein diacetate (DCF-DA) method. Results: In mature slices, we found that the neurotoxicity, as well as oxidative stress, induced by OGD were enhanced by UCB. HSA significantly prevented UCB-increased neurotoxicity, but had a slight reduction on ROS production. Allopurinol, but not pioglitazone, significantly reduced UCB-increased neurotoxicity induced by OGD. In immature slices exposed to OGD, no increase of neuronal death was observed, whereas oxidative stress was detected after UCB exposure. HSA, pioglitazone and allopurinol have no protective effects on both OGD-induced neuronal death and on UCB-induced oxidative stress. For this reason, UCB, pioglitazone and allopurinol was also tested on ischemic preconditioning protocol. We found that UCB abolished the neuroprotection induced by preconditioning and increased oxidative stress. These effects were restored by allopurinol but not pioglitazone. Conclusions: UCB characterized a different path of neuronal damage and oxidative stress in mature and immature hippocampal slice model of HIE. Management of hyperbilirubinemia in a complex pathological condition, such as HIE and hyperbilirubinemia, should be very careful. Allopurinol could deserve attention as a novel pharmacological intervention for hyperbilirubinemia and HIE.

Microglia Susceptibility to Free Bilirubin Is Age-Dependent

Increased concentrations of unconjugated bilirubin (UCB), namely its free fraction (Bf), in neonatal life may cause transient or definitive injury to neurons and glial cells. We demonstrated that UCB damages neurons and glial cells by compromising oligodendrocyte maturation and myelination, and by activating astrocytes and microglia. Immature neurons and astrocytes showed to be especially vulnerable. However, whether microglia susceptibility to UCB is also age-related was never investigated. We developed a microglia culture model in which cells at 2 days in vitro (2DIV) revealed to behave as the neonatal microglia (amoeboid/reactive cells), in contrast with those at 16DIV microglia that performed as aged cells (irresponsive/dormant cells). Here, we aimed to unveil whether UCB-induced toxicity diverged from the young to the long-cultured microglia. Cells were isolated from the cortical brain of 1- to 2-day-old CD1 mice and incubated for 24 h with 50/100 nM Bf levels, which were associated to moderate and severe neonatal hyperbilirubinemia, respectively. These concentrations of Bf induced early apoptosis and amoeboid shape in 2DIV microglia, while caused late apoptosis in 16DIV cells, without altering their morphology. CD11b staining increased in both, but more markedly in 2DIV cells. Likewise, the gene expression of HMGB1, a well-known alarmin, as well as HMGB1 and GLT-1–positive cells, were enhanced as compared to long-maturated microglia. The CX3CR1 reduction in 2DIV microglia was opposed to the 16DIV cells and suggests a preferential Bf-induced sickness response in younger cells. In conformity, increased mitochondrial mass and NO were enhanced in 2DIV cells, but unchanged or reduced, respectively, in the 16DIV microglia. However, 100 nM Bf caused iNOS gene overexpression in 2DIV and 16DIV cells. While only arginase 1/IL-1β gene expression levels increased upon 50/100 nM Bf treatment in long-maturated microglia, MHCII/arginase 1/TNF-α/IL-1β/IL-6 (>10-fold) were upregulated in the 2DIV microglia. Remarkably, enhanced inflammatory-associated microRNAs (miR-155/miR-125b/miR-21/miR-146a) and reduced anti-inflammatory miR-124 were found in young microglia by both Bf concentrations, while remained unchanged (miR/21/miR-125b) or decreased (miR-155/miR-146a/miR-124) in aged cells. Altogether, these findings support the neurodevelopmental susceptibilities to UCB-induced neurotoxicity, the most severe disabilities in premature babies, and the involvement of immune-inflammation neonatal microglia processes in poorer outcomes.

Review of bilirubin neurotoxicity I: molecular biology and neuropathology of disease

Despite the availability of successful prevention strategies to prevent excessive hyperbilirubinemia, the neurological sequelae of bilirubin neurotoxicity (BNTx) still occur throughout the world. Kernicterus, encephalopathy due to BNTx, is now understood to be a spectrum of severity and phenotypes known as kernicterus spectrum disorder (KSD). A better understanding of the selective neuropathology and molecular biology of BNTx and using consistent clinical definitions of KSDs as outcome measure can lead to more accurately predicting the risk and causes of BNTx and KSDs. In Part I of our two-part review, we will summarize current and recent advances in the understanding of the selective neuropathology and molecular biology of the disease. Herein we emphasize the role of unbound, free unconjugated bilirubin as well as genetic contributions to the susceptibility BNTx and the development of KSDs. In Part II, we focus on current and possible novel methods to prevent BNTx and ABE and treat ABE and KSDs.

Review of bilirubin neurotoxicity II: preventing and treating acute bilirubin encephalopathy and kernicterus spectrum disorders

Previously in Part I of this two-part review, we discussed the current and recent advances in the understanding of the molecular biology and neuropathology of bilirubin neurotoxicity (BNTx). Here in Part II, we summarize current treatment options available to treat the severely jaundiced infants to prevent significant brain damage and improve clinical outcomes. In addition, we review potential novel therapies that are in various stages of research and development. We will emphasize treatments for both prevention and treatment of both acute bilirubin encephalopathy (ABE) and kernicterus spectrum disorders (KSDs), highlighting the treatment of the most disabling neurological sequelae of children with mild-to-severe KSDs whose "rare disease" status often means they are overlooked by the clinical research community at large. As with other secondary dystonias, treatment of the dystonic motor symptoms in kernicterus is the greatest clinical challenge.

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.

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.

Neurotoxicity of Unconjugated Bilirubin in Mature and Immature Rat Organotypic Hippocampal Slice Cultures

BACKGROUND

The physiopathology of bilirubin-induced neurological disorders is not completely understood.

OBJECTIVES

The aim of our study was to assess the effect on bilirubin neurotoxicity of the maturity or immaturity of exposed cells, the influence of different unconjugated bilirubin (UCB) and human serum albumin (HSA) concentrations, and time of UCB exposure.

METHODS

Organotypic hippocampal slices were exposed for 48 h to different UCB and HSA concentrations after 14 (mature) or 7 (immature) days of in vitro culture. Immature slices were also exposed to UCB and HSA for 72 h. The different effects of exposure time to UCB on neurons and astrocytes were evaluated.

RESULTS

We found that 48 h of UCB exposure was neurotoxic for mature rat organotypic hippocampal slices while 72 h of exposure was neurotoxic for immature slices. Forty-eight-hour UCB exposure was toxic for astrocytes but not for neurons, while 72-h exposure was toxic for both astrocytes and neurons. HSA prevented UCB toxicity when the UCB:HSA molar ratio was ≤1 in both mature and immature slices.

CONCLUSIONS

We confirmed UCB neurotoxicity in mature and immature rat hippocampal slices, although immature ones were more resistant. HSA was effective in preventing UCB neurotoxicity in both mature and immature rat hippocampal slices.

Depurative capacity of molecular adsorbent recycling system (MARS): A focus on bilirubin removal

The molecular adsorbent recycling system (MARS) is now widely used in the treatment of patients with hepatic failure (HF). A great deal of interest has been directed toward its effect on clinical outcome, whereas its depurative capacity also needs attention. Bilirubin, a tightly albumin-bound toxin accumulating in patients with HF, is regarded as a surrogate to evaluate the depurative capacity of albumin-bound toxins by blood purification modalities. The removal of bilirubin by MARS is difficult to predict, because both the clearance of bilirubin and the reduction ratio of bilirubin after a single session differ between patients and sessions. A reduction of depurative capacity over the course of a treatment is observed. Furthermore, the later sessions are likely less efficient than previous ones. It cannot be taken for granted that the reduction of depurative capacity is due to the saturation and reduced efficiency of the adsorbent columns used in MARS. The answer lies in the property of bilirubin/albumin binding. The removal of bilirubin by MARS is a diffusion process, dependent on the free bilirubin concentration. Bilirubin binds to albumin in 3 ways with different affinity. High-affinity binding bilirubin is difficult to dissociate from albumin and is accompanied by a smaller free fraction, which means it is also difficult for MARS to remove. The factors affecting the free fraction of bilirubin will impact on bilirubin removal by MARS. Among them, the molar ratio of bilirubin to albumin is the most important one. Other factors include the interaction of other agents with bilirubin/albumin binding, the albumin concentration, plasma ion strength, and pH.

Caffeine prevents bilirubin-induced cytotoxicity in cultured newborn rat astrocytes

OBJECTIVE

Unconjugated bilirubin (UCB) may cause neurotoxicity in preterm neonates due to immaturity of UGT1A1 leading to bilirubin accumulation in the brain. Caffeine used in the treatment of apnea of prematurity was reported to decrease mechanical ventilation requirement, the frequencies of bronchopulmonary dysplasia, patent ductus arteriosus, cerebral palsy and neurodevelopmental disorders in very low birth weight infants. However, the effect of caffeine on hyperbilirubinemia was not yet clarified.

METHODS

We used astrocyte cell cultures obtained from 2-day-old Wistar albino rats via modified Cole and de Vellis method. UCB concentration toxic to 50% of astrocytes, and caffeine concentration increasing cell viability 100% were used in experiments. While no medication was applied to the control group, UCB (50 μM) and caffeine (100 μM) were applied to the bilirubin and caffeine groups for 24 h. Prophylactic and therapeutic caffeine groups were treated with caffeine 4 h before and after UCB exposure. The effects of caffeine were investigated in rat astrocytes exposed to UCB in terms of cell viability, apoptosis, antioxidant defense, proinflammatory cytokines, and Toll-like receptor (TLR)s.

RESULTS

Compared to the control group, UCB increased apoptosis, malondialdehyde (MDA), tumor necrosis factor-α (TNF-α), interleukin (IL)-1β, IL-6, total nitrate/nitrite, and TLR4 levels, and decreased cell viability, catalase (CAT), glutathione peroxidase (GPx), superoxide dismutase (SOD) activities, glutathione, and TLR9 levels (for all p < .001). Conversely, prophylactic and therapeutic caffeine improved the detrimental effects of UCB.

CONCLUSIONS

Caffeine seems encouraging for the prevention and treatment of bilirubin neurotoxicity in rats by means of its antiapoptotic, antioxidant, anti-inflammatory, anti-nitrosative, and anti-TLR-4 properties.

The Neuroprotective Effects of Hypothermia on Bilirubin-Induced Neurotoxicity in vitro

BACKGROUND

In high-risk newborns indirect hyperbilirubinemia can lead to acute bilirubin encephalopathy and kernicterus. Despite the current therapeutic modalities, preventing or reversing the neurotoxicity cannot be achieved in all infants.

OBJECTIVE

To investigate the neuroprotective effects of hypothermia on bilirubin-induced toxicity in primary mouse neuronal cell cultures.

METHODS

Hippocampal cell cultures, isolated from newborn mouse brains, were incubated with unconjugated bilirubin (UCB) at 3 days in vitro (DIV) and immediately exposed to varying degrees of hypothermia. Neuronal viability and mitochondrial health were compared between the normothermia (37°C), mild (34°C), moderate (32°C) and severe (29°C) hypothermia groups. Confocal microscopy and fluorescent dyes (propidium iodide and JC-1) were used for cell evaluation. To determine the late effects of hypothermia, the cultures were also examined at 7 DIV after returning to normothermic conditions.

RESULTS

Induction of any degree of hypothermia increased the neuronal survival after 24 h of UCB treatment. Neuronal death rate and mitochondrial membrane potential loss were lowest in the neurons exposed to moderate hypothermia. We also observed that mild to moderate hypothermia had late protective effects on neuronal cell viability, whereas deep hypothermia did not improve neuronal survival.

CONCLUSIONS

We conclude that hypothermia reduces the cell death induced by bilirubin toxicity in neuronal cells. Although moderate hypothermia has a better outcome than mild hypothermia, deep hypothermia as low as 29°C has adverse effects on neuronal cell viability.

The activation of autophagy protects neurons and astrocytes against bilirubin-induced cytotoxicity

Unconjugated bilirubin (UCB) neurotoxicity involves oxidative stress, calcium signaling and ER-stress. The same insults can also induce autophagy, a process of "self-eating", with both a pro-survival or a pro-apoptotic role. Our aim was to study the outcome of autophagy activation by UCB in the highly sensitive neuronal SH-SY5Y cells and in the resistant astrocytoma U87 cells. Upon treatment with a toxic dose of UCB, the conversion of LC3-I to LC3-II was detected in both cell lines. Inhibition of autophagy by E64d before UCB treatment increased SH-SY5Y cell mortality and made U87 cells sensitive to UCB. In SH-SY5Y autophagy related genes ATG8 (5 folds), ATG18 (5 folds), p62 (3 folds) and FAM 129A (4.5 folds) were induced 8h after UCB treatment while DDIT4 upregulation (13 folds) started at 4h. mTORC1 inactivation by UCB was confirmed by phosphorylation of 4EBP1. UCB induced LC3-II conversion was completely prevented by pretreating cells with the calcium chelator BAPTA and reduced by 65% using the ER-stress inhibitor 4-PBA. Pretreatment with the PKC inhibitor reduced LC3 mRNA by 70% as compared to cells exposed to UCB alone. Finally, autophagy induction by Trifluoroperazine (TFP) increased the cell viability of rat hippocampal primary neurons upon UCB treatment from 60% to 80%. In SH-SY5Y cells, TFP pretreatment blocked the UCB-induced cleaved caspase-3 protein expression, decreased LDH release from 50% to 23%, reduced the UCB-induction of HO1, CHOP and IL-8 mRNAs by 85%, 70% and 97%. Collectively these data indicate that the activation of autophagy protects neuronal cells from UCB cytotoxicity. The mechanisms of autophagy activation by UCB involves mTOR/ER-stress/PKC/calcium signaling.

Genetic and epigenetic changes in host ABCB1 influences malaria susceptibility to Plasmodium falciparum

Multiple mechanisms such as genetic and epigenetic variations within a key gene may play a role in malarial susceptibility and response to anti-malarial drugs in the population. ABCB1 is one of the well-studied membrane transporter genes that code for the P-glycoprotein (an efflux protein) and whose effect on malaria disease predisposition and susceptibility to drugs remains to be understood. We studied the association of single nucleotide variations in human ABCB1 that influences its function in subjects with uncomplicated and complicated malaria caused by Plasmodium falciparum (Pf). Global DNA methylation and ABCB1 DNA promoter methylation levels were performed along with transcriptional response and protein expression in subjects with malaria and healthy controls. The rs2032582 locus was significantly associated with complicated and combined malaria groups when compared to controls (p < 0.05). Significant DNA methylation difference was noticed between case and control (p < 0.05). In addition, global DNA methylation levels of the host DNA were inversely proportional to parasitemia in individuals with Pf infection. Our study also revealed the correlation between ABCB1 DNA promoter methylation with rs1128503 and rs2032582 polymorphisms in malaria and was related to increased expression of ABCB1 protein levels in complicated malaria group (p < 0.05) when compared to uncomplicated malaria and control groups. The study provides evidence for multiple mechanisms that may regulate the role of host ABCB1 function to mediate aetiology of malaria susceptibility, prognosis and drug response. These may have clinical implications and therapeutic application for various malarial conditions.

Anemia, bilirubin, and cardiovascular autonomic neuropathy in patients with type 2 diabetes

To investigate the relationship among anemia, physiological serum bilirubin levels, and cardiovascular autonomic neuropathy (CAN) in subjects with type 2 diabetes. In total, 2230 subjects with type 2 diabetes were evaluated in this cross-sectional study. CAN was diagnosed with a cardiovascular reflex test. The prevalence of anemia was greater in subjects with CAN. In multivariable analysis, the relationship between anemia and CAN remained statistically significant after adjusting for the risk factors (odds ratio [OR] 1.39; 95% confidence interval [CI] 1.07-1.80, P = .015). Additional adjustment for serum bilirubin concentrations abolished this relationship (OR 1.20, 95% CI 0.91-1.58, P = .189). Anemia is positively associated with the prevalence of CAN in subjects with type 2 diabetes. In addition, our results suggest that the putative increased CAN risk associated with anemia might be mediated by a correlated decrease in serum bilirubin levels.

Evaluation of region selective bilirubin-induced brain damage as a basis for a pharmacological treatment

The neurologic manifestations of neonatal hyperbilirubinemia in the central nervous system (CNS) exhibit high variations in the severity and appearance of motor, auditory and cognitive symptoms, which is suggestive of a still unexplained selective topography of bilirubin-induced damage. By applying the organotypic brain culture (OBC: preserving in vitro the cellular complexity, connection and architecture of the in vivo brain) technique to study hyperbilirubinemia, we mapped the regional target of bilirubin-induced damage, demonstrated a multifactorial toxic action of bilirubin, and used this information to evaluate the efficacy of drugs applicable to newborns to protect the brain. OBCs from 8-day-old rat pups showed a 2–13 fold higher sensitivity to bilirubin damage than 2-day-old preparations. The hippocampus, inferior colliculus and cerebral cortex were the only brain regions affected, presenting a mixed inflammatory-oxidative mechanism. Glutamate excitotoxicity was appreciable in only the hippocampus and inferior colliculus. Single drug treatment (indomethacin, curcumin, MgCl₂) significantly improved cell viability in all regions, while the combined (cocktail) administration of the three drugs almost completely prevented damage in the most affected area (hippocampus). Our data may supports an innovative (complementary to phototherapy) approach for directly protecting the newborn brain from bilirubin neurotoxicity.

Bilirubin-induced ER stress contributes to the inflammatory response and apoptosis in neuronal cells

Unconjugated bilirubin (UCB) in newborns may lead to bilirubin neurotoxicity. Few studies investigated the activation of endoplasmic reticulum stress (ER stress) by UCB. We performed an in vitro comparative study using undifferentiated SH-SY5Y, differentiated GI-ME-N neuronal cells and human U87 astrocytoma cells. ER stress and its contribution to inflammation and apoptosis induced by UCB were analyzed. Cytotoxicity, ER stress and inflammation were observed only in neuronal cells, despite intracellular UCB accumulation in all three cell types. UCB toxicity was enhanced in undifferentiated SH-SY5Y cells and correlated with a higher mRNA expression of pro-apoptotic CHOP. Mouse embryonic fibroblast knockout for CHOP and CHOP siRNA-silenced SH-SY5Y increased cells viability upon UCB exposure. In SH-SY5Y, ER stress inhibition by 4-phenylbutyric acid reduced UCB-induced apoptosis and decreased the cleaved forms of caspase-3 and PARP proteins. Reporter gene assay and PERK siRNA showed that IL-8 induction by UCB is transcriptionally regulated by NFкB and PERK signaling. These data suggest that ER stress has an important role in the UCB-induced inflammation and apoptosis, and that targeting ER stress may represent a potential therapeutic approach to decrease UCB-induced neurotoxicity.

17α-Ethinylestradiol can disrupt hemoglobin catabolism in amphibians

Different chemical substances, which enter the environment due to anthropogenic influences, can affect the endocrine system and influence development and physiology of aquatic animals. One of these endocrine disrupting chemicals is the synthetic estrogen, 17α-ethinylestradiol (EE2), which is a main component of various oral contraceptives and demonstrably affects many different aquatic vertebrates at extremely low concentrations by feminization phenomena. The aim of the present study was to investigate whether a four week exposure to three different concentrations of EE2 (0.3 ng/L, 29.6 ng/L and 2960 ng/L) affects the catabolism of hemoglobin of the amphibian Xenopus laevis. The results of this study demonstrate for the first time that beside an increase of the hepatic vitellogenin gene expression, exposure to EE2 also decreases the gene expression of the hepatic heme oxygenase 1 and 2 (HO1, HO2), degrading heme of different heme proteins to biliverdin, as well as of the biliverdin reductase A (BLVRA), which converts biliverdin to bilirubin. The results further suggest that EE2 already at the environmentally relevant concentration of (29.6 ng/L) can disrupt hemoglobin catabolism, indicated by decreased gene expression of HO2, which becomes evident at the highest EE2 concentration that led to a severe increase of biliverdin in plasma.

Bilirubin-albumin binding, bilirubin/albumin ratios, and free bilirubin levels: where do we stand?

Treatment for unconjugated hyperbilirubinemia is predominantly based on one parameter, i.e., total serum bilirubin (TSB) levels. Yet, overt kernicterus has been reported in preterm infants at relatively low TSB levels, and it has been repeatedly shown that free unconjugated bilirubin (freeUCB) levels, or bilirubin/albumin (B/A) ratios for that matter, are more closely associated with bilirubin neurotoxicity. In this article, we review bilirubin-albumin binding, UCBfree levels, and B/A ratios in addition to TSB levels to individualize and optimize treatment especially in preterm infants. Methods to measure bilirubin-albumin binding or UCBfree are neither routinely performed in Western clinical laboratories nor incorporated in current management guidelines on unconjugated hyperbilirubinemia. For bilirubin-albumin binding, this seems justified because several of these methods have been challenged, and sufficiently powered prospective trials on the clinical benefits are lacking. Technological advances in the measurement of UCBfree may provide a convenient means for integrating UCBfree measurements into routine clinical management of jaundiced infants. A point-of-care method, as well as determination of UCBfree levels in various newborn populations, is desirable to learn more about variations in time and how various clinical pathophysiological conditions affect UCBfree levels. This will improve the estimation of approximate UCBfree levels associated with neurotoxicity. To delineate the role of UCBfree in the management of jaundiced (preterm) infants, trials are needed using UCBfree as treatment parameter. The additional use of the B/A ratio in jaundiced preterms has been evaluated in the Bilirubin Albumin Ratio Trial (BARTrial; Clinical Trials: ISRCTN74465643) but failed to demonstrate better neurodevelopmental outcome in preterm infants <32 weeks assigned to the study group. Awaiting a study in which infants are assigned to be managed solely on the basis of their B/A ratio (with TSB excluded ) versus TSB levels alone-and determining which group does better-the additional use of the B/A ratio in the management of hyperbilirubinemia in preterms is not advised. In conjunction with TSB levels, other parameters possibly allow for more accurate prediction of bilirubin toxicity. Yet, different methodologies for estimating these parameters exist, and sufficiently powered, prospective clinical trials supporting their clinical benefit, i.e., reduced bilirubin neurotoxicity when using these parameters, are lacking. Their use in addition to TSB needs to be prospectively evaluated, especially in preterm neonates, and preferentially in randomized clinical trials, which include specific risk factors and assessment of clinical relevant outcome measures for detecting those infants at risk of bilirubin toxicity.

Difference in cerebral blood flow velocity in neonates with and without hyperbilirubinemia

PURPOSE

To evaluate the difference in cerebral blood flow velocity (CBFV) in neonates with and without hyperbilirubinemia.

METHODS

CBFV of 70 healthy late-preterm and term newborns with unconjugated hyperbilirubinemia (UCH) reaching the threshold of phototherapy requirement was compared with 70 gestational- and postnatal age-matched controls without hyperbilirubinemia. Resistance index (RI), pulsatility index (PI), peak systolic velocity (PSV) and vascular diameter were measured in internal carotid, vertebral and middle cerebral arteries by transcranial color Doppler ultrasound at the beginning of phototherapy, after 48-72h of starting phototherapy and at 5-7days after its stoppage. In controls CBFV was assessed once at inclusion.

RESULTS

Both the groups were comparable. An increase in CBFV (decreased RI and PI, increased PSV and vasodilation) was observed in the UCH group. A further increase in CBFV was noticed after 48h of phototherapy. After 5-7days of stoppage of phototherapy, though there was a significant reduction in CBFV in mild-to-moderate UCH (serum bilirubin ⩽25mg/dL), in severe UCH (serum bilirubin >25mg/dL), CBFV remained increased. Four neonates developed features of acute bilirubin encephalopathy and had significantly higher CBFV compared to those with normal outcome.

CONCLUSIONS

An increase in CBFV was observed in neonates with UCH compared to those without hyperbilirubinemia.

Physiological serum bilirubin concentrations are inversely associated with the prevalence of cardiovascular autonomic neuropathy in patients with Type 2 diabetes

AIMS

Although severe hyperbilirubinaemia causes kernicterus in neonates, normal to modestly elevated bilirubin concentrations have been reported to be neuroprotective. However, the relationship between serum bilirubin concentrations and cardiovascular autonomic neuropathy in patients with Type 2 diabetes is currently unknown. This study assessed the relationships between physiological serum total bilirubin concentrations and cardiovascular autonomic neuropathy in patients with Type 2 diabetes.

METHODS

A total of 2991 patients with Type 2 diabetes were investigated in this cross-sectional study. Cardiovascular autonomic neuropathy was diagnosed by a cardiovascular reflex test. According to the American Diabetes Association criteria, the degree of cardiovascular autonomic dysfunction was graded into normal, early, definite and severe. Cardiovascular autonomic neuropathy was either definite or severe in the present study. An analysis of covariance after adjustment for other covariates was performed. A logistic regression model was used to assess an association of cardiovascular autonomic neuropathy with serum total bilirubin tertiles.

RESULTS

Serum total bilirubin concentrations were significantly lower in subjects with cardiovascular autonomic neuropathy. The mean total bilirubin values differed significantly according to the severity of cardiovascular autonomic dysfunction (normal 13.0 μmol/l; early 12.3 μmol/l; definite 11.8 μmol/l; severe 10.1 μmol/l; P for trend < 0.001) after adjustment for other covariates. In multivariate analysis, serum total bilirubin levels were significantly associated with cardiovascular autonomic neuropathy (odds ratio 0.36; 95% CI 0.21-0.63 for the highest vs. the lowest bilirubin tertile, P < 0.001).

CONCLUSIONS

Serum total bilirubin concentrations within the physiologic range are inversely associated with the prevalence of cardiovascular autonomic neuropathy in patients with Type 2 diabetes.

Albumin administration protects against bilirubin-induced auditory brainstem dysfunction in Gunn rat pups

BACKGROUND

Free bilirubin (Bf), the unbound fraction of unconjugated bilirubin (UCB), can induce neurotoxicity, including impairment of the auditory system, which can be assessed by brainstem auditory evoked potentials (BAEPs). We hypothesized that albumin might reduce the risk of neurotoxicity by decreasing Bf and its translocation into the brain.

AIM

To determine the effects of albumin on BAEPs and brain bilirubin content in two Gunn rat pup models of acute hyperbilirubinemia.

METHODS

We used Gunn rat pups, which have a deficiency of the bilirubin-conjugating enzyme UGT1A1. We induced haemolysis by injection of phenylhydrazine (phz) into 14-days old pups. Subsequently, pups were treated with either i.p. human serum albumin (HSA; 2.5 g/kg; n = 8) or saline (control, n = 8). We induced acute neurotoxicity by injecting 16-days old pups with sulphadimethoxine (sulpha) and treated them with either HSA (n = 9) or saline (control, n = 10). To assess bilirubin neurotoxicity, we used the validated BAEP method and compared relevant parameters; i.e. peak latency values and interwave interval (IWI) between peak I and peak II, a marker of acute neurotoxicity.

RESULTS

Phz and sulpha significantly increased IWI I-II by 26% and 29% (P < 0.05) in the haemolysis and the displacement model, respectively. Albumin completely prevented the increase of IWI I-II in either model. The beneficial effect of albumin in the displacement-model by means of normal BAEPs was in line with less bilirubin in the brain (NS). Interestingly, in the haemolysis model the accumulation of total bilirubin in the brain was unaltered, and BAEPs still appeared normal. This might advocate for a role of brain Bf which was calculated and showed that albumin treatment non-significantly reduces Bf concentrations in brain, compared with saline treatment.

CONCLUSIONS

Albumin treatment is neuroprotective in acute hyperbilirubinemia in Gunn rat pups. Our present results underline the importance of functional diagnostic test of neurotoxicity above biochemical concentrations.

Alterations in the cell cycle in the cerebellum of hyperbilirubinemic Gunn rat: a possible link with apoptosis?

Severe hyperbilirubinemia causes neurological damage both in humans and rodents. The hyperbilirubinemic Gunn rat shows a marked cerebellar hypoplasia. More recently bilirubin ability to arrest the cell cycle progression in vascular smooth muscle, tumour cells, and, more importantly, cultured neurons has been demonstrated. However, the involvement of cell cycle perturbation in the development of cerebellar hypoplasia was never investigated before. We explored the effect of sustained spontaneous hyperbilirubinemia on cell cycle progression and apoptosis in whole cerebella dissected from 9 day old Gunn rat by Real Time PCR, Western blot and FACS analysis. The cerebellum of the hyperbilirubinemic Gunn rats exhibits an increased cell cycle arrest in the late G0/G1 phase (p < 0.001), characterized by a decrease in the protein expression of cyclin D1 (15%, p < 0.05), cyclin A/A1 (20 and 30%, p < 0.05 and 0.01, respectively) and cyclin dependent kinases2 (25%, p < 0.001). This was associated with a marked increase in the 18 kDa fragment of cyclin E (67%, p < 0.001) which amplifies the apoptotic pathway. In line with this was the increase of the cleaved form of Poly (ADP-ribose) polymerase (54%, p < 0.01) and active Caspase3 (two fold, p < 0.01). These data indicate that the characteristic cerebellar alteration in this developing brain structure of the hyperbilirubinemic Gunn rat may be partly due to cell cycle perturbation and apoptosis related to the high bilirubin concentration in cerebellar tissue mainly affecting granular cells. These two phenomena might be intimately connected.

The evolving landscape of neurotoxicity by unconjugated bilirubin: role of glial cells and inflammation

Unconjugated hyperbilirubinemia is a common condition in the first week of postnatal life. Although generally harmless, some neonates may develop very high levels of unconjugated bilirubin (UCB), which may surpass the protective mechanisms of the brain in preventing UCB accumulation. In this case, both short-term and long-term neurodevelopmental disabilities, such as acute and chronic UCB encephalopathy, known as kernicterus, or more subtle alterations defined as bilirubin-induced neurological dysfunction (BIND) may be produced. There is a tremendous variability in babies' vulnerability toward UCB for reasons not yet explained, but preterm birth, sepsis, hypoxia, and hemolytic disease are comprised as risk factors. Therefore, UCB levels and neurological abnormalities are not strictly correlated. Even nowadays, the mechanisms of UCB neurotoxicity are still unclear, as are specific biomarkers, and little is known about lasting sequelae attributable to hyperbilirubinemia. On autopsy, UCB was shown to be within neurons, neuronal processes, and microglia, and to produce loss of neurons, demyelination, and gliosis. In isolated cell cultures, UCB was shown to impair neuronal arborization and to induce the release of pro-inflammatory cytokines from microglia and astrocytes. However, cell dependent sensitivity to UCB toxicity and the role of each nerve cell type remains not fully understood. This review provides a comprehensive insight into cell susceptibilities and molecular targets of UCB in neurons, astrocytes, and oligodendrocytes, and on phenotypic and functional responses of microglia to UCB. Interplay among glia elements and cross-talk with neurons, with a special emphasis in the UCB-induced immunostimulation, and the role of sepsis in BIND pathogenesis are highlighted. New and interesting data on the anti-inflammatory and antioxidant activities of different pharmacological agents are also presented, as novel and promising additional therapeutic approaches to BIND.

Functional induction of the cystine-glutamate exchanger system Xc(-) activity in SH-SY5Y cells by unconjugated bilirubin

We have previously reported that exposure of SH-SY5Y neuroblastoma cells to unconjugated bilirubin (UCB) resulted in a marked up-regulation of the mRNA encoding for the Na⁺ -independent cystine∶glutamate exchanger System X_c⁻ (SLC7A11 and SLC3A2 genes). In this study we demonstrate that SH-SY5Y cells treated with UCB showed a higher cystine uptake due to a significant and specific increase in the activity of System X_c⁻, without the contribution of the others two cystine transporters (X_AG⁻ and GGT) reported in neurons. The total intracellular glutathione content was 2 folds higher in the cells exposed to bilirubin as compared to controls, suggesting that the internalized cystine is used for gluthathione synthesis. Interestingly, these cells were significantly less sensitive to an oxidative insult induced by hydrogen peroxide. If System X_c⁻ is silenced the protection is lost. In conclusion, these results suggest that bilirubin can modulate the gluthathione levels in neuroblastoma cells through the induction of the System X_c⁻, and this renders the cell less prone to oxidative damage.

Immunomodulatory and immunotoxic effects of bilirubin: molecular mechanisms

The immunomodulatory and immunotoxic effects of purified UCB have not been evaluated previously at clinically relevant UCB concentrations and UCB:BSA ratios. To delineate the molecular mechanism of UCB-induced immunomodulation, immune cells were exposed to clinically relevant concentrations of UCB. It inhibited LPS-induced B cell proliferation and cytokine production from splenic macrophages. UCB (≥25 μM) was toxic to unfractionated splenocytes, splenic T cells, B cells, macrophages, LPS-stimulated CD19(+) B cells, human PBMCs, and RBCs. Purified UCB also was found to be toxic to splenocytes and human PBMCs. UCB induced necrosis and apoptosis in splenocytes. UCB activated the extrinsic and intrinsic pathways of apoptosis, as reflected by the markers, such as CD95, caspase-8, Bax, MMP, cytoplasmic Ca(+2), caspase-3, and DNA fragmentation. UCB depleted GSH and activated p38MAPK. NAC, caspase inhibitors, and p38MAPK inhibitor attenuated the UCB-induced apoptosis. In vivo administration of ≥25 mg/kbw UCB induced atrophy of spleen, depletion of bone marrow cells, and leukopenia and decreased lymphocyte count and the T and B cell response to mitogens. UCB administration to mice led to induction of oxidative stress, activation of p38MAPK, and cell death in splenocytes. These parameters were attenuated by the injection of NAC and the p38MAPK inhibitor. Our results demonstrate for the first time that clinically relevant concentrations of UCB induce apoptosis and necrosis in immune cells by depleting cellular GSH. These findings should prove useful in understanding the immunosuppression associated with hyperbilirubinemia.

The mitochondrial connection in auditory neuropathy

'Auditory neuropathy' (AN), the term used to codify a primary degeneration of the auditory nerve, can be linked directly or indirectly to mitochondrial dysfunction. These observations are based on the expression of AN in known mitochondrial-based neurological diseases (Friedreich's ataxia, Mohr-Tranebjærg syndrome), in conditions where defects in axonal transport, protein trafficking, and fusion processes perturb and/or disrupt mitochondrial dynamics (Charcot-Marie-Tooth disease, autosomal dominant optic atrophy), in a common neonatal condition known to be toxic to mitochondria (hyperbilirubinemia), and where respiratory chain deficiencies produce reductions in oxidative phosphorylation that adversely affect peripheral auditory mechanisms. This body of evidence is solidified by data derived from temporal bone and genetic studies, biochemical, molecular biologic, behavioral, electroacoustic, and electrophysiological investigations.

Chronic bilirubin encephalopathy: diagnosis and outcome

Chronic bilirubin encephalopathy (kernicterus) can be diagnosed using semi-objective criteria based on history, physical and neurological examination and laboratory findings including auditory brainstem responses and magnetic resonance imaging. Classical kernicterus is a well-described clinical tetrad of (i) abnormal motor control, movements and muscle tone, (ii) an auditory processing disturbance with or without hearing loss, (iii) oculomotor impairments, especially impairment of upward vertical gaze, and (iv) dysplasia of the enamel of deciduous teeth. Subtle kernicterus or bilirubin-induced neurologic dysfunction (BIND) refers to individuals with subtle neurodevelopmental disabilities without classical findings of kernicterus that, after careful evaluation and consideration, appear to be due to bilirubin neurotoxicity. Kernicterus can be further classified as auditory predominant or motor predominant and characterized based on the severity of clinical sequelae. Proposed research definitions for kernicterus diagnosis in infants from 3 to 18 months are reviewed, as are treatments of auditory and motor deficits and other complications of bilirubin encephalopathy.

Unconjugated bilirubin exposure impairs hippocampal long-term synaptic plasticity

BACKGROUND

Jaundice is one of the most common problems encountered in newborn infants, due to immaturity of hepatic conjugation and transport processes for bilirubin. Although the majority of neonatal jaundice is benign, some neonates with severe hyperbilirubinemia develop bilirubin encephalopathy or kernicterus. Accumulation of unconjugated bilirubin (UCB) in selected brain regions may result in temporary or permanent impairments of auditory, motor, or cognitive function; however, the molecular mechanisms by which UCB elicits such neurotoxicity are still poorly understood. The present study is undertaken to investigate whether prolonged exposure of rat organotypic hippocampal slice cultures to UCB alters the induction of long-term synaptic plasticity.

METHODOLOGY/PRINCIPAL FINDINGS

Using electrophysiological recording techniques, we find that exposure of hippocampal slice cultures to clinically relevant concentrations of UCB for 24 or 48 h results in an impairment of CA1 long-term potentiation (LTP) and long-term depression (LTD) induction in a time- and concentration-dependent manner. Hippocampal slice cultures stimulated with UCB show no changes in the secretion profiles of the pro-inflammatory cytokines, interleukin-1beta and tumor necrosis factor-alpha, or the propidium ioide uptake. UCB treatment produced a significant decrease in the levels of NR1, NR2A and NR2B subunits of N-methyl-D-aspartate (NMDA) receptors through a calpain-mediated proteolytic cleavage mechanism. Pretreatment of the hippocampal slice cultures with NMDA receptor antagonist or calpain inhibitors effectively prevented the UCB-induced impairment of LTP and LTD.

CONCLUSION/SIGNIFICANCE

Our results indicate that the proteolytic cleavage of NMDA receptor subunits by calpain may play a critical role in mediating the UCB-induced impairment of long-term synaptic plasticity in the hippocampus. These observations provide new insights into the molecular mechanisms underlying UCB-induced impairment of hippocampal synaptic plasticity which, in turn, might provide opportunities for the development of novel therapeutic strategies that targets these pathways for treatment.

Unbound (free) bilirubin: improving the paradigm for evaluating neonatal jaundice

BACKGROUND

The serum or plasma total bilirubin concentration (B(T)) has long been the standard clinical laboratory test for evaluating neonatal jaundice, despite studies showing that B(T) correlates poorly with acute bilirubin encephalopathy (ABE) and its sequelae including death, classical kernicterus, or bilirubin-induced neurological dysfunction (BIND). The poor correlation between B(T) and ABE is commonly attributed to the confounding effects of comorbidities such as hemolytic diseases, prematurity, asphyxia, or infection. Mounting evidence suggests, however, that B(T) inherently performs poorly because it is the plasma non-protein-bound (unbound or free) bilirubin concentration (B(f)), rather than B(T), that is more closely associated with central nervous system bilirubin concentrations and therefore ABE and its sequelae.

CONTENT

This article reviews (a) the complex relationship between serum or plasma bilirubin measurements and ABE, (b) the history underlying the limited use of B(f) in the clinical setting, (c) the peroxidase method for measuring B(f) and technical and other issues involved in adapting the measurement to routine clinical use, (d) clinical experience using B(f) in the management of newborn jaundice, and (e) the value of B(f) measurements in research investigating bilirubin pathochemistry.

SUMMARY

Increasing evidence from clinical studies, clinical experience, and basic research investigating bilirubin neurotoxicity supports efforts to incorporate B(f) expeditiously into the routine evaluation of newborn jaundice.

Ibuprofen augments bilirubin toxicity in rat cortical neuronal culture

Premature infants are at risk for bilirubin-associated brain damage. In cell cultures bilirubin causes neuronal apoptosis and necrosis. Ibuprofen is used to close the ductus arteriosus, and is often given when hyperbilirubinemia is at its maximum. Ibuprofen is known to interfere with bilirubin-albumin binding. We hypothesized that bilirubin toxicity to cultured rat embryonic cortical neurons is augmented by coincubation with ibuprofen. Incubation with ibuprofen above a concentration of 125 microg/mL reduced cell viability, measured by methylthiazole tetrazolium reduction, to 68% of controls (p < 0.05). Lactate dehydrogenase (LDH) release increased from 29 to 38% (p < 0.01). The vehicle solution did not affect cell viability. Coincubation with 10 microM unconjugated bilirubin (UCB)/human serum albumin in a molar ratio of 3:1 and 250 microg/mL ibuprofen caused additional loss of cell viability and increased LDH release (p < 0.01), DNA fragmentation, and activated caspase-3. Preincubation with the pan-caspase inhibitor z-val-ala-asp-fluoromethyl ketone abolished ibuprofen- and UCB-induced DNA fragmentation. The study demonstrates that bilirubin in low concentration of 10 microM reduces neuron viability and ibuprofen increases this effect. Apoptosis is the underlying cell death mechanism.

The cytotoxic effect of unconjugated bilirubin in human neuroblastoma SH-SY5Y cells is modulated by the expression level of MRP1 but not MDR1

In vitro and in vivo studies have demonstrated that UCB (unconjugated bilirubin) is neurotoxic. Although previous studies suggested that both MRP1 (multidrug resistance-associated protein 1) and MDR1 (multidrug resistance protein 1) may protect cells against accumulation of UCB, direct comparison of their role in UCB transport was never performed. To this end, we used an inducible siRNA (small interfering RNA) expression system to silence the expression of MRP1 and MDR1 in human neuroblastoma SH-SY5Y cells. The effects of in vitro exposure to clinically-relevant levels of unbound UCB were compared between unsilenced (control) cells and cells with similar reductions in the expression of MRP1 or MDR1, documented by RT–PCR (reverse transcription–PCR) (mRNA), immunoblotting (protein), and for MDR1, the enhanced net uptake of a specific fluorescent substrate. Cytotoxicity was assessed by the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide] test. MRP1-deficient cells accumulated significantly more UCB and suffered greater cytotoxicity than controls. By contrast, MDR1-deficient cells exhibited UCB uptake and cytotoxicity comparable with controls. At intermediate levels of silencing, the increased susceptibility to UCB toxicity closely correlated with the decrease in the expression of MRP1, but not of MDR1. These data support the concept that limitation of cellular UCB accumulation, due to UCB export mediated by MRP1, but not MDR1, plays an important role in preventing bilirubin encephalopathy in the newborn.

Tau and S100B proteins as biochemical markers of bilirubin-induced neurotoxicity in term neonates

We investigated the relationship between total serum bilirubin and serum Tau and S100B protein levels, and predicted a cutoff level of bilirubin-induced neurotoxicity in term newborns. Total serum bilirubin, serum Tau, and S100B levels were measured in 92 jaundiced term newborns. A neurologic examination, electroencephalogram, brainstem auditory-evoked response, and otoacoustic emission were performed in the infants on admission and at age 3 months. Serum Tau (r = 0.921, P < 0.001) and S100B (r = 0.927, P < 0.001) levels were correlated with total serum bilirubin levels in all infants. Serum Tau and S100B protein levels remained at a steady level up to a total serum bilirubin level of 19.1 mg/dL, and then demonstrated a significant increase. Mean total serum bilirubin, serum Tau, and S100B levels of infants who manifested auditory neuropathy, neurologic abnormalities, or electroencephalogram abnormalities were significantly higher than in infants without these abnormalities (P < 0.05). Clinical and laboratory findings of bilirubin-induced neurotoxicity developed after a total serum bilirubin level of 22 mg/dL was reached. Serum levels of Tau and S100B proteins in jaundiced term newborns were strongly correlated with early-phase bilirubin encephalopathy.

Bilirubin inhibits Plasmodium falciparum growth through the generation of reactive oxygen species

Free heme is very toxic because it generates highly reactive hydroxyl radicals ((.)OH) to cause oxidative damage. Detoxification of free heme by the heme oxygenase (HO) system is a very common phenomenon by which free heme is catabolized to form bilirubin as an end product. Interestingly, the malaria parasite, Plasmodium falciparum, lacks an HO system, but it forms hemozoin, mainly to detoxify free heme. Here, we report that bilirubin significantly induces oxidative stress in the parasite as evident from the increased formation of lipid peroxide, decrease in glutathione content, and increased formation of H(2)O(2) and (.)OH. Bilirubin can effectively inhibit hemozoin formation also. Furthermore, results indicate that bilirubin inhibits parasite growth and induces caspase-like protease activity, up-regulates the expression of apoptosis-related protein (Gene ID PFI0450c), and reduces the mitochondrial membrane potential. (.)OH scavengers such as mannitol, as well as the spin trap alpha-phenyl-n-tert-butylnitrone, effectively protect the parasite from bilirubin-induced oxidative stress and growth inhibition. These findings suggest that bilirubin, through the development of oxidative stress, induces P. falciparum cell death and that the malaria parasite lacks an HO system probably to protect itself from bilirubin-induced cell death as a second line of defense.

Cytotoxicity is predicted by unbound and not total bilirubin concentration

Although it has been suggested that the unbound, free, (B(f)) rather than total (B(T)) bilirubin level correlates with cell toxicity, direct experimental evidence supporting this conclusion is limited. In addition, previous studies never included a direct measurement of B(f), using newer, accurate methods. To test "the free bilirubin hypothesis", in vitro cytotoxicity was assessed in four cell lines exposed to different B(f) concentrations obtained by varying B(T)/Albumin ratio, using serum albumins with different binding affinities, and/or displacing unconjugated bilirubin (UCB) from albumin with a sulphonamide. B(f) was assessed by the modified, minimally diluted peroxidase method. Cytotoxicity varied among cell lines but was invariably related to B(f) and not B(T). Light exposure decreased toxicity parallel to a decrease in B(f). In the absence of albumin, no cytotoxicity was found at a B(f) of 150 nM whereas in the presence of albumin a similar B(f) resulted in a 40% reduction of viability indicating the importance of total cellular uptake of UCB in eliciting toxic effect. In the presence of albumin-bound UCB, bilirubin-induced cytotoxicity in a given cell line is accurately predicted by B(f) irrespective of the source and concentration of albumin, or total bilirubin level.

Unconjugated bilirubin efflux by bovine brain microvascular endothelial cells in vitro

OBJECTIVES

The passage of unconjugated bilirubin (UCB) across the blood-brain barrier into the central nervous system is a crucial first step in the development of kernicterus. The objective of the current study was to characterize the passage of UCB across primary bovine brain microvascular endothelial cell (BBMVEC) monolayers in vitro.

DESIGN

Experimental study.

SETTING

Research institute.

SUBJECTS

BBMVECs.

INTERVENTIONS

Tritiated UCB (H-UCB) transport at 60, 80, 100, 200, 300, and 400 nM concentrations was tested in both the apical to basolateral (A--> B) and basolateral to apical (B-->A) directions in BBMVEC monolayers in vitro with or without preincubation with pharmacologic active transport inhibitors cyclosporine A, indomethacin, or MK571.

MEASUREMENTS AND MAIN RESULTS

The rate of H-UCB transport in the B-->A direction was 6.2- to 7.3-fold higher than in the A-->B direction, suggesting active efflux of UCB. Cyclosporine A (5 microM), a model inhibitor of P-glycoprotein, enhanced A-->B while decreasing B-->A UCB transport, resulting in an overall decrease in BBMVEC UCB efflux of between 46% and 54%. Indomethacin (10 microM) and MK-571 (50 microM), respectively a substrate and potent inhibitor of multidrug resistance-associated protein-1, had no effect.

CONCLUSIONS

We conclude that 1) UCB is transported by BBMVEC monolayers in vitro in a net B-->A direction (i.e., active efflux); and 2) cyclosporine A partially inhibits such transport. We speculate that the blood-brain barrier limits the passage and central nervous system retention of UCB by active transport and that this may be accounted in part by P-glycoprotein.

Factors affecting the binding of bilirubin to serum albumins: validation and application of the peroxidase method

The unbound "free" bilirubin concentration (Bf), not the total bilirubin concentration, is the critical determinant of cellular uptake and toxicity of bilirubin. We compared Bf measured by a modified peroxidase method with published data obtained with ultrafiltration and examined conditions that affect the affinity (KF) of human (HSA) and bovine (BSA) serum albumin for bilirubin. The peroxidase and ultrafiltration methods yielded similar KF values that decreased with increasing HSA concentration and the presence of 50 mM chloride. When related to ionic strength, inhibition of BSA-bilirubin binding by chloride, bromide, and sulfate were similar, whereas phosphate buffer had a smaller effect. KF was lower at 37 degrees C than at 25 degrees C for HSA but not for BSA. KF for BSA was similar at pH 7.4 and 8.0. BSA and FCS had similar binding properties. The close agreement of Bf and KF values determined by the peroxidase method with published results obtained by ultrafiltration validates both methods and supports the use of the peroxidase method as a practical technique for measuring Bf under steady state conditions in minimally diluted serum or culture medium.

Heme oxygenase protects hippocampal neurons from ethanol-induced neurotoxicity

Ethanol has deleterious effects on neuronal cells both in vivo and in vitro, but the mechanisms are unknown. Here, treatment with increasing doses of ethanol (from 20 up to 600mM) decreased the viability of a mouse hippocampal neuroblastoma cell line, HT22. The glutathione concentration decreased and intracellular reactive oxygen species (ROS) increased in a dose-and time-dependent manner, suggesting that the neurotoxicity was due to oxidative stress. Expression of heme oxygenase (HO)-1, a redox regulator and heat shock protein, increased with time after ethanol treatment, but HO-2 was expressed constitutively. The addition of 5microM zinc protoporphyrin IX (ZnPP IX), a competitive HO inhibitor, with the ethanol further reduced cell viability and increased intracellular ROS, but these effects were reversed by co-treatment with 50nM bilirubin, a well-known antioxidant and a product of HO catalysis. These results suggest that HO has a protective role in hippocampal neurons as an intrinsic factor against ethanol-induced oxidative stress and the protection depends on the degree of oxidative stress.

Protective effect of L-carnitine against bilirubin-induced neuronal cell death

There is growing evidence that glutamate receptor-mediated injury plays a crucial role in bilirubin neurotoxicity. L-carnitine (LC) has been shown to prevent glutamate-induced toxicity in neuronal cell culture. The purpose of this study is to assess whether LC is able to prevent bilirubin neurotoxicity. Unconjugated bilirubin at different concentrations was administered to cerebellar granular cell cultures prepared from 1-day-old Sprague-Dawley rats. The neuroprotective effect of LC was examined. LC at doses of 10(-6), 10(-5), 10(-4) and 10(-3) M was applied to culture flasks. LC at a dose of 10(-4) M significantly blocked bilirubin neurotoxicity. On the other hand, LC significantly increased bilirubin toxicity at a higher dose (10(-3) M). LC at the doses of 10(-5) and 10(-6) M was found to be ineffective. 10(-4) M LC decreased bilirubin-induced neuronal cell death from 47.72+/-3.68 to 27.23+/-5.14%, (P=0.003). The present study demonstrates, for the first time, that LC protects against bilirubin neurotoxicity in a dose-dependent manner in cerebellar granular cell culture of rats. Further research is needed to confirm our findings and to clarify the mechanisms responsible for the protective effect of LC.

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.

Calculated in vivo free bilirubin levels in the central nervous system of Gunn rat pups

In vitro studies suggest a free bilirubin (B(F)) concentration in the range of 71-770 nmol/L can induce neurotoxicity. In vivo data regarding central nervous system (CNS) B(F) levels have not been determined. We calculated in vivo CNS B(F) levels in Gunn rat pups (15-19 d old; heterozygous nonjaundiced Gunn rats (J/j) and homozygous jaundiced Gunn rats (j/j); saline or sulfadimethoxine treated) based on 1) total brain bilirubin (TBB) content, 2) brain albumin level, 3) CNS bilirubin binding capacity attributable to brain albumin determined using an ultrafiltration technique, and 4) published Gunn rat albumin-bilirubin binding constants (k). Gunn rat brain bilirubin binding capacity was approximately 22 x 10(-3) micromol/g, of which two thirds was accounted for by brain albumin. Using a Gunn rat pup in vivo, k of 9.2 L/micromol, calculated CNS B(F) levels ranged from 72 to 112 nmol/L [95% confidence interval (CI)] in saline and from 59 to 156 nmol/L (95% CI) in sulfadimethoxine-treated J/j pups. These animals demonstrated no neurobehavioral abnormalities and normal cerebellar weight. Calculated CNS B(F) levels were severalfold higher (p < 0.001) in saline (95% CI: 556-1110 nmol/L) and sulfadimethoxine-treated (95% CI: 3461-8985 nmol/L) j/j pups; the former evidenced reduced cerebellar weight; the latter both reduced cerebellar weight and acute neurobehavioral abnormalities. We conclude that calculated CNS B(F) values in j/j pups are substantially higher than those in J/j animals. Given the absence of CNS abnormalities in J/j pups, the presence of such in j/j animals, and the CNS B(F) levels in these groups, we speculate that the CNS B(F) neurotoxicity threshold in vivo is subsumed within the range (71-770 nmol/L) reported in vitro.

Inflammatory signalling pathways involved in astroglial activation by unconjugated bilirubin

During neonatal hyperbilirubinaemia, astrocytes activated by unconjugated bilirubin (UCB) may contribute to brain toxicity through the production of cytokines. As a first step in addressing the signal transduction cascades involved in the UCB-induced astroglial immunological response, we tested whether tumour necrosis factor (TNF)-alpha receptor 1 (TNFR1), mitogen-activated protein kinase (MAPK) and nuclear factor kappaB (NF-kappaB) would be activated in astrocytes exposed to UCB, and examined the profile of cytokine production. Astrocyte cultures stimulated with UCB showed a rapid rise in TNFR1 protein levels, followed by activation of the MAPKs p38, Jun N-terminal kinase1/2 and extracellular signal-regulated kinase1/2, and NF-kappaB. Interestingly, the induction of these signal effectors preceded the early up-regulation of TNF-alpha and interleukin (IL)-1beta mRNAs, and later secretion of TNF-alpha, IL-1beta and IL-6. Treatment of astrocytes with UCB also induced cell death, with levels comparable to those obtained after exposure of astrocytes to recombinant TNF-alpha and IL-1beta. Moreover, loss of cell viability and cytokine secretion were reduced when the NF-kappaB signal transduction pathway was inhibited, suggesting a key role for NF-kappaB in the astroglial response to UCB. These results demonstrate the complexity of the molecular mechanisms involved in cell injury by UCB during hyperbilirubinaemia and provide a basis for the development of novel therapeutic strategies.

Toward understanding kernicterus: a challenge to improve the management of jaundiced newborns

PURPOSE

We sought to evaluate the sensitivity and specificity of total serum bilirubin concentration (TSB) and free (unbound) bilirubin concentration (Bf) as predictors of risk for bilirubin toxicity and kernicterus and to examine consistency between these findings and proposed mechanisms of bilirubin transport and brain uptake.

METHODS

A review of literature was undertaken to define basic principles of bilirubin transport and brain uptake leading to neurotoxicity. We then reviewed experimental and clinical evidence that relate TSB or Bf to risk for bilirubin toxicity and kernicterus.

RESULTS

There are insufficient published data to precisely define sensitivity and specificity of either TSB or Bf in determining risk for acute bilirubin neurotoxicity or chronic sequelae (kernicterus). However, available laboratory and clinical evidence indicate that Bf is better than TSB in discriminating risk for bilirubin toxicity in patients with severe hyperbilirubinemia. These findings are consistent with basic pharmacokinetic principles involved in bilirubin transport and tissue uptake.

CONCLUSIONS

Experimental and clinical data strongly suggest that measurement of Bf in newborns with hyperbilirubinemia will improve risk assessment for neurotoxicity, which emphasizes the need for additional clinical evaluation relating Bf and TSB to acute bilirubin toxicity and long-term outcome. We speculate that establishing risk thresholds for neurotoxicity by using newer methods for measuring Bf in minimally diluted serum samples will improve the sensitivity and specificity of serum indicators for treating hyperbilirubinemia, thus reducing unnecessary aggressive intervention and associated cost and morbidity.

Multidrug resistance associated protein 1 protects against bilirubin-induced cytotoxicity

We have shown that multidrug resistance associated protein 1 (MRP1) mediates ATP-dependent extrusion of bilirubin, possibly limiting its potentially toxic accumulation in cells. To determine directly if Mrp1 protects cells against unconjugated bilirubin (UCB) toxicity, mouse embryo fibroblasts (MEF) were isolated from Mrp1 knockout (-/-) mice and their wild type (WT) (+/+) littermates. Compared to WT cells, cultured MEF (-/-) cells exposed to 40-140 nM unbound [H3]-bilirubin accumulated twice as much [H3]-bilirubin (P<0.01). This was associated with greater, dose-related cytotoxicity, assessed by the methylthiazoletetrazolium test, lactate dehydrogenase release and cellular ATP content. The data confirm that Mrp1 limits intracellular accumulation of UCB and thus decreases its cytotoxicity.

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.