Mechanisms of bilirubin neurotoxicity

Association of Breastfeeding and Neonatal Jaundice With Infant Neurodevelopment

INTRODUCTION

Exclusive breastfeeding is advantageous for infant neurodevelopment. Nevertheless, insufficient human milk supply in exclusively breastfed infants may elevate the risk of neonatal jaundice, which can potentially result in neurological harm. Whether mothers should adhere to exclusive breastfeeding in infants with neonatal jaundice remains unclear.

METHODS

Data comes from the Jiangsu Birth Cohort (JBC), a prospective and longitudinal birth cohort study in China. A total of 2,577 infants born from November 2017 to March 2021 were included in the analysis. Multivariate linear regression models were used to analyze the associations between breastfeeding status, neonatal jaundice, and their interaction with infant neurodevelopment. Analysis was performed in 2022.

RESULTS

Compared with "exclusive breastfeeding," fine motor scores of infants were lower for "mixed feeding" (βadj, -0.16; 95% CI, -0.29 to -0.03; p=0.016) and "no breastfeeding" (βadj, -0.41; 95% CI, -0.79 to -0.03; p=0.034). Compared with "no neonatal jaundice," infants with "severe neonatal jaundice" had lower scores for cognition (βadj, -0.44; 95% CI, -0.66 to -0.23; p<0.001) and fine motor (βadj, -0.19; 95% CI, -0.35 to -0.03; p=0.024). In infants with severe neonatal jaundice, the termination of exclusive breastfeeding before 6 months was associated with worse cognition (βadj, -0.28; 95% CI, -0.57 to 0.01), while this association was not observed in those without neonatal jaundice (βadj, 0.09; 95% CI, -0.26 to 0.43).

CONCLUSIONS

Exclusive breastfeeding for the first 6 months is beneficial to the neurodevelopment of infants, especially in those with severe neonatal jaundice.

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.

The Ontogeny of UDP-glucuronosyltransferase Enzymes, Recommendations for Future Profiling Studies and Application Through Physiologically Based Pharmacokinetic Modelling

Limited understanding of drug pharmacokinetics in children is one of the major challenges in paediatric drug development. This is most critical in neonates and infants owing to rapid changes in physiological functions, especially in the activity of drug-metabolising enzymes. Paediatric physiologically based pharmacokinetic models that integrate ontogeny functions for cytochrome P450 enzymes have aided our understanding of drug exposure in children, including those under the age of 2 years. Paediatric physiologically based pharmacokinetic models have consequently been recognised by the European Medicines Agency and the US Food and Drug Administration as innovative tools in paediatric drug development and regulatory decision making. However, little is currently known about age-related changes in UDP-glucuronosyltransferase-mediated metabolism, which represents the most important conjugation reaction for xenobiotics. Therefore, the objective of the review was to conduct a thorough literature survey to summarise our current understanding of age-related changes in UDP-glucuronosyltransferases as well as associated clinical and experimental sources of variance. Our findings indicate that there are distinct differences in UDP-glucuronosyltransferase expression and activity between isoforms for different age groups. In addition, there is substantial variability between individuals and laboratories reported for human liver microsomes, which results in part from a lack of standardised experimental conditions. Therefore, we provide a number of best practice recommendations for experimental conditions, which ultimately may help improve the quality of data used for quantitative clinical pharmacology approaches, and thus for safe and effective pharmacotherapy in children.

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.

Unconjugated Bilirubin exerts Pro-Apoptotic Effect on Platelets via p38-MAPK activation

Thrombocytopenia is one of the most frequently observed secondary complications in many pathological conditions including liver diseases, where hyperbilirubinemia is very common. The present study sought to find the cause of thrombocytopenia in unconjugated hyperbilirubinemic conditions. Unconjugated bilirubin (UCB), an end-product of heme catabolism, is known to have pro-oxidative and cytotoxic effects at high serum concentration. We investigated the molecular mechanism underlying the pro-apoptotic effect of UCB on human platelets in vitro, and followed it up with studies in phenylhydrazine-induced hyperbilirubinemic rat model and hyperbilirubinemic human subjects. UCB is indeed found to significantly induce platelet apoptotic events including elevated endogenous reactive oxygen species generation, mitochondrial membrane depolarization, increased intracellular calcium levels, cardiolipin peroxidation and phosphatidylserine externalization (p < 0.001) as evident by FACS analysis. The immunoblots show the elevated levels of cytosolic cytochrome c and caspase activation in UCB-treated platelets. Further, UCB is found to induce mitochondrial ROS generation leading to p38 activation, followed by downstream activation of p53, ultimately resulting in altered expression of Bcl-2 and Bax proteins as evident from immunoblotting. All these parameters conclude that elevated unconjugated bilirubin causes thrombocytopenia by stimulating platelet apoptosis via mitochondrial ROS-induced p38 and p53 activation.

Photo-isomerization and oxidation of bilirubin in mammals is dependent on albumin binding

The bilirubin (BR) photo-conversion in the human body is a protein-dependent process; an effective photo-isomerization of the potentially neurotoxic Z,Z-BR as well as its oxidation to biliverdin in the antioxidant redox cycle is possible only when BR is bound on serum albumin. We present a novel analytical concept in the study of linear tetrapyrroles metabolic processes based on an in-depth mapping of binding sites in the structure of human serum albumin (HSA). A combination of fluorescence spectroscopy, circular dichroism (CD) spectroscopy, and molecular modeling methods was used for recognition of the binding site for BR, its derivatives (mesobilirubin and bilirubin ditaurate), and the products of the photo-isomerization and oxidation (lumirubin, biliverdin, and xanthobilirubic acid) on HSA. The CD spectra and fluorescent quenching of the Trp-HSA were used to calculate the binding constants. The results of the CD displacement experiments performed with hemin were interpreted together with the findings of molecular docking performed on the pigment-HSA complexes. We estimated that Z,Z-BR and its metabolic products bind on two independent binding sites. Our findings support the existence of a reversible antioxidant redox cycle for BR and explain an additional pathway of the photo-isomerization process (increase of HSA binding capacity; the excess free [unbound] BR can be converted and also bound to HSA).

Cross-talk between neurons and astrocytes in response to bilirubin: adverse secondary impacts

Previous studies using monotypic nerve cell cultures have shown that bilirubin-induced neurological dysfunction (BIND) involves apoptosis and necrosis-like cell death, following neuritic atrophy and astrocyte activation,and that glycoursodeoxycholic acid (GUDCA) has therapeutic efficacy against BIND. Cross-talk between neurons and astrocytes may protect or aggravate neurotoxicity by unconjugated bilirubin (UCB). In a previous work we have shown that bidirectional signaling during astrocyte-neuron recognition attenuates neuronal damage by UCB. Here, we investigated whether the establishment of neuron-astrocyte homeostasis prior to cell exposure to UCB was instead associated with a lower resistance of neurons to UCB toxicity, and if the pro-survival properties of GUDCA were replicated in that experimental model. We have introduced a 24 h adaptation period for neuron-glia communication prior to the 48 h treatment with UCB. In such conditions, UCB induced glial activation, which aggravated neuronal damage, comprising increased apoptosis,cell demise and neuritic atrophy, which were completely prevented in the presence of GUDCA. Neuronal multidrug resistance-associated protein 1 expression and tumor necrosis factor-a secretion, although unchanged by UCB, increased in the presence of astrocytes. The rise in S100B and nitric oxide in the co-cultures medium may have contributed to UCB neurotoxicity. Since the levels of these diffusible molecules did not change by GUDCA we may assume that they are not directly involved in its beneficial effects. Data indicate that astrocytes, in an indirect neuron-astrocyte co-culture model and after homeostatic setting regulation of the system, are critically influencing neurodegeneration by UCB, and support GUDCA for the prevention of BIND.

Chiroptical properties of bilirubin-serum albumin binding sites

Although the interactions between bilirubin and serum albumin are among the most studied serum albumin-ligand interactions, the binding-site location and the participation of bilirubin-serum albumin complexes in pathological and physiological processes are under debate. In this article, we have benefited from the chiral structure of bilirubin and used CD spectroscopy to characterize the structure of bilirubin bound to human and bovine serum albumins. We determined that in a phosphate buffer at pH 7.8 there are three binding sites in both human and bovine serum albumins. While the primary binding sites in human and bovine serum albumins bind bilirubin with P- and M-helical conformations, respectively, the secondary binding sites in both albumins bind bilirubin in the P-helical conformation. We have shown that the bonding of bilirubin to the serum albumin matrix is a more favorable process than the self-association of bilirubin under the studied conditions, with a maximum of three bound bilirubins per serum albumin molecule. Although bilirubin bound to the primary binding site has attracted the most attention, the presented results have documented the impact of the secondary binding sites which are relevant in the displacement reactions between BR and drugs and in the phenomena where bilirubin plays antioxidant, antimutagenic, and anti-inflammatory roles.

Bilirubin augments radiation injury and leads to increased infection and mortality in mice: molecular mechanisms

Our earlier results demonstrated that clinically relevant concentrations of unconjugated bilirubin (UCB) possessed immunotoxic effects. Whole-body irradiation (WBI) with 1 to 6 Gy leads to acute radiation syndrome, immunosuppression, and makes the host susceptible to infection. Since hyperbilirubinemia has been shown to be associated with several types of cancer, the present studies were undertaken to evaluate the radiomodifying effects of UCB in radiation-exposed mice having elevated levels of UCB. Pretreatment of splenic lymphocytes with UCB (1-50 μM at UCB/BSA ratio <1) augmented radiation-induced DNA strand breaks, MMP loss, calcium release, and apoptosis. Combination treatment of mice with UCB (50mg/kg bw) followed by WBI (2 Gy) 0.5h later, resulted in significantly increased splenic atrophy, bone marrow aplasia, decreased counts of peritoneal exudate cells, and different splenocyte subsets such as CD3+ T, CD4+ T, CD8+ T, CD19+ B, and CD14+ macrophages as compared to either UCB or WBI treatment. Hematological studies showed that WBI-induced lymphopenia, thrombocytopenia, and neutropenia were further aggravated in the combination treatment group. UCB pretreatment of mice potentiated WBI-induced apoptosis and decreased WBI-induced loss of functional response of various immune cells leading to augmentation of immunosuppression and infection susceptibility caused by WBI. In an acute bacterial peritonitis model, UCB pretreatment of mice significantly increased WBI-induced proinflammatory cytokines, nitric oxide, and peritoneal bacterial load resulting in increased infection and death. Studies using the pharmacological inhibitor of p38MAPK demonstrated the involvement of p38MAPK activation in the inflammatory cascade of peritonitis. These findings should prove useful in understanding the potential risk to hyperbilirubinemic patients during radiotherapy and victims of acute radiation exposure in the course of radiation accidents.

Transport and metabolism at blood-brain interfaces and in neural cells: relevance to bilirubin-induced encephalopathy

Bilirubin, the end-product of heme catabolism, circulates in non-pathological plasma mostly as a protein-bound species. When bilirubin concentration builds up, the free fraction of the molecule increases. Unbound bilirubin then diffuses across blood-brain interfaces (BBIs) into the brain, where it accumulates and exerts neurotoxic effects. In this classical view of bilirubin neurotoxicity, BBIs act merely as structural barriers impeding the penetration of the pigment-bound carrier protein, and neural cells are considered as passive targets of its toxicity. Yet, the role of BBIs in the occurrence of bilirubin encephalopathy appears more complex than being simple barriers to the diffusion of bilirubin, and neural cells such as astrocytes and neurons can play an active role in controlling the balance between the neuroprotective and neurotoxic effects of bilirubin. This article reviews the emerging in vivo and in vitro data showing that transport and metabolic detoxification mechanisms at the blood-brain and blood-cerebrospinal fluid barriers may modulate bilirubin flux across both cellular interfaces, and that these protective functions can be affected in chronic unconjugated hyperbilirubinemia. Then the in vivo and in vitro arguments in favor of the physiological antioxidant function of intracerebral bilirubin are presented, as well as the potential role of transporters such as ABCC1 and metabolizing enzymes such as cytochromes P-450 in setting the cerebral cell- and structure-specific toxicity of bilirubin following hyperbilirubinemia. The relevance of these data to the pathophysiology of bilirubin-induced neurological diseases is discussed.

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.

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.

Effects of bilirubin ditaurate overload on canalicular membrane function and ultrastructure of the pig liver

AIM

Bilirubin overload caused by haemolysis of transfused blood and breakdown of extravasated blood constitutes an important causative factor of jaundice in trauma patients. Intravenous infusions of large amounts of unconjugated bilirubin (UCB) block biliary phospholipid secretion and produce canalicular membrane lesions in pigs, putatively because of enhanced cytotoxicity of bile. Severe intrahepatic cholestasis is the functional end result. The aim of the study was to investigate whether bilirubin ditaurate (BDT) overload, which also inhibits biliary phospholipid secretion, also induces intrahepatic cholestasis.

METHODS

Six pigs were infused with 2.8 g of BDT for 150 min. Six control pigs were infused with albumin sham solution for 150 min. Bile samples were analysed for bile acid- and phospholipid-secretion rates. Bile and serum samples were analysed for bilirubin concentration. Liver biopsies were obtained for scanning electron microscopic studies (SEM).

RESULTS

Biliary bile acid secretion fell by 7.7% and biliary phospholipid secretion rate fell by 88% after BDT infusion. Thus, infusion of BDT for 150 min did not cause intrahepatic cholestasis. SEM showed some variability in the size of canalicular membrane microvilli, but no evidence of gross destruction.

CONCLUSION

BDT overload markedly lowers biliary phospholipid secretion. In contrast to UCB, BDT does not induce canalicular membrane damage nor cause intrahepatic cholestasis. Sustained, marked inhibition of phospholipid secretion does therefore not adequately explain UCB-induced cholestasis. Accumulation of UCB in the canalicular membrane may be the important factor in the pathogenesis of canalicular membrane lesions and intrahepatic cholestasis during UCB overload.

Adeno-associated Virus Vector Serotypes Mediate Sustained Correction of Bilirubin UDP Glucuronosyltransferase Deficiency in Rats

Crigler-Najjar (CN) patients have no bilirubin UDP glucuronosyltransferase (UGT1A1) activity and suffer brain damage because of bilirubin toxicity. Vectors based on adeno-associated virus (AAV) serotype 2 transduce liver cells with relatively low efficiency. Recently, AAV serotypes 1, 6, and 8 have been shown to be more efficient for liver cell transduction. We compared AAV serotypes 1, 2, 6, and 8 for correction of UGT1A1 deficiency in the Gunn rat model of CN disease. Adult Gunn rats were injected with CMV-UGT1A1 AAV vectors. Serum bilirubin was decreased over the first year by 64% for AAV1, 16% for AAV2, 25% for AAV6, and 35% for AAV8. Antibodies to UGT1A1 were detected after injection of all AAV serotypes. An AAV1 UGT1A1 vector with the liver-specific albumin promoter corrected serum bilirubin levels but did not induce UGT1A1 antibodies. Two years after injection of AAV vectors all animals had large lipid deposits in the liver. These lipid deposits were not seen in age-matched control animals. AAV1 vectors are promising candidates for CN gene therapy because they can mediate a reduction in serum bilirubin levels in Gunn rats that would be therapeutic in humans.

Immune response to lentiviral bilirubin UDP-glucuronosyltransferase gene transfer in fetal and neonatal rats

Gene therapy for inherited disorders might cause an immune response to the therapeutic protein. A solution would be to introduce the gene in the fetal or neonatal period, which should lead to tolerization. Lentiviral vectors mediate long-term gene expression, and are well suited for gene therapy early in development. A model for fetal or neonatal gene therapy is the inherited disorder of bilirubin metabolism, Crigler-Najjar disease (CN). The absence of bilirubin UDP-glucoronyltransferase (UGT1A1) activity in CN patients causes high serum levels of unconjugated bilirubin and brain damage in infancy. CN is attractive for the development of gene therapy because the mutant Gunn rat closely mimics the human disease. Injection of UGT1A1 lentiviral vectors corrected the hyperbilirubinemia for more than a year in rats injected as fetuses and for up to 18 weeks in rats injected the day of birth. UGT1A1 gene transfer was confirmed by the presence of bilirubin glucuronides in bile. All animals injected with UGT1A1 lentiviral vectors developed antibodies to UGT1A1. Animals injected with green fluorescent protein (GFP) lentiviral vectors did not develop antibodies to GFP. Our results indicate that fetal and neonatal gene therapy with immunogenic proteins such as UGT1A1 does not necessarily lead to tolerization.

Heme regulation in traumatic brain injury: relevance to the adult and developing brain

Intracranial bleeding is one of the most prominent aspects in the clinical diagnosis and prognosis of traumatic brain injury (TBI). Substantial amounts of blood products, such as heme, are released because of traumatic subarachnoid hemorrhages, intraparenchymal contusions, and hematomas. Despite this, surprisingly few studies have directly addressed the role of blood products, in particular heme, in the setting of TBI. Heme is degraded by heme oxygenase (HO) into three highly bioactive products: iron, bilirubin, and carbon monoxide. The HO isozymes, in particular HO-1 and HO-2, exhibit significantly different expression patterns and appear to have specific roles after injury. Developmentally, differences between the adult and immature brain have implications for endogenous protection from oxidative stress. The aim of this paper is to review recent advances in the understanding of heme regulation and metabolism after brain injury and its specific relevance to the developing brain. These findings suggest novel clinical therapeutic options for further translational study.

Unconjugated bilirubin inhibits VCAM-1-mediated transendothelial leukocyte migration

During lymphocyte migration, engagement of VCAM-1 stimulates the generation of endothelial cell-derived reactive oxygen species (ROS) and activation of matrix metalloproteinases, facilitating endothelial retraction. Because bilirubin is a potent antioxidant, we examined the hypothesis that this bile pigment inhibits VCAM-1-dependent cellular events. The migration of isolated murine splenic lymphocytes across monolayers of murine endothelial cell lines (which constitutively express VCAM-1) is significantly inhibited by physiological concentrations of bilirubin, in the absence of an effect on lymphocyte adhesion. Bilirubin administration also suppresses VCAM-1-stimulated ROS generation and reduces endothelial cell matrix metalloproteinase activity. In a murine asthma model characterized by VCAM-1-dependent airway inflammation, treatment of C57BL6/J mice with i.p. bilirubin decreases the total leukocyte count in the lung parenchyma and lavage fluid, through specific inhibition of eosinophil and lymphocyte infiltration. Blood eosinophil counts were increased in bilirubin-treated animals, while VCAM-1 expression in the capillary endothelium and cytokine levels in both lung lavage and supernatants from cultured lymph node lymphocytes were unchanged, suggesting that bilirubin inhibits leukocyte migration.

CONCLUSION

bilirubin blocks VCAM-1-dependent lymphocyte migration in vitro and ameliorates VCAM-1-mediated airway inflammation in vivo, apparently through the suppression of cellular ROS production. These findings support a potential role for bilirubin as an endogenous immunomodulatory agent.

The human multidrug-resistance-associated protein MRP1 mediates ATP-dependent transport of unconjugated bilirubin

Results of previous studies have suggested that UCB (unconjugated bilirubin) may be transported by MRP1/Mrp1 (multidrug-resistance-associated protein 1). To test this hypothesis directly, [3H]UCB transport was assessed in plasma-membrane vesicles from MDCKII cells (Madin-Darby canine kidney II cells) stably transfected with human MRP1 or MRP2; wild-type MDCKII cells served as controls. As revealed by Western blotting, transfection achieved abundant expression of MRP1 and MRP2. [3H]UCB uptake was measured in the presence of 60 microM human serum albumin at a free (unbound) concentration of UCB (B(F)) ranging from 5 to 72 nM and in the presence of 3 mM ATP or 3 mM AMP-PCP (adenosine 5'-[beta,gamma-methylene]triphosphate). MRP1-transfected vesicles showed transport activity three and five times higher respectively compared with MRP2 or wild-type vesicles, whose transport did not differ significantly. [3H]UCB transport was stimulated 4-fold by 1.5 mM GSH, occurred into an osmotically sensitive space, was inhibited by 3 microM MK571 and followed saturative kinetics with K(m)=10+/-3 nM (B(F)) and V(max)=100+/-13 pmol x min(-1) x (mg of protein)(-1). UCB significantly inhibited the transport of LTC4 (leukotriene C4), a leukotriene substrate known to have high affinity for MRP1. Collectively, these results prove directly that MRP1 mediates ATP-dependent cellular export of UCB and supports its role in protecting cells from bilirubin toxicity.

Long-term correction of bilirubin UDPglucuronyltransferase deficiency in rats by in utero lentiviral gene transfer

Bilirubin is glucuronidated by bilirubin UDP-glucuronyltransferase (UGT1A1) before biliary excretion. Because bilirubin is toxic, patients with Crigler-Najjar type I (CN), who have no UGT1A1 activity, suffer severe brain damage early in childhood. The Gunn rat is the model for CN type 1. Gunn rat fetuses were injected with 10(7) transducing units of UGT1A1 lentiviral vector at the end of the third trimester on embryonic day 19. Serum bilirubin of injected Gunn rats was lowered by 45% compared to untreated controls. This decrease was highly significant (P < 10(6)) and was sustained for more than a year. In treated Gunn rats, bilirubin glucuronides were present in bile and UGT1A1 protein was detected in tissue. Liver, intestine, stomach, pancreas, and other organs were transduced and mostly contained 1% or less vector copies per genome. Tissue distribution was variable among experimental animals but high transduction levels were seen in pancreas and intestine in most animals. Immunohistochemistry of these organs revealed transduction of pancreatic acinar cells and intestinal epithelium. Injection of a lentiviral UGT1A1 vector into third-trimester Gunn rat fetuses corrects the metabolic deficiency and mediates a reduction of serum bilirubin levels that would be therapeutic in humans.

New concepts in bilirubin encephalopathy

Revised concepts of bilirubin encephalopathy have been revealed by studies of bilirubin toxicity in cultured CNS cells and in congenitally jaundiced Gunn rats. Bilirubin neurotoxicity is related to the unbound (free) fraction of unconjugated bilirubin (Bf), of which the dominant species at physiological pH is the protonated diacid, which can passively diffuse across cell membranes. As the binding affinity of plasma albumin for bilirubin decreases strikingly as albumin concentration increases, previously reported Bf values were underestimated. Newer diagnostic tests can detect reversible neurotoxicity before permanent damage occurs from precipitation of bilirubin (kernicterus). Early toxicity can occur at Bf only modestly above aqueous saturation and affects astrocytes and neurons, causing mitochondrial damage, resulting in impaired energy metabolism and apoptosis, plus cell-membrane perturbation, which causes enzyme leakage and hampers transport of neurotransmitters. The concentrations of unbound bilirubin in the cerebro-spinal fluid and CNS cells are probably limited mainly by active export of bilirubin back into plasma, mediated by ABC transporters present in the brain capillary endothelium and choroid plexus epithelium. Intracellular bilirubin levels may be diminished also by oxidation, conjugation and binding to cytosolic proteins. These new concepts may explain the varied susceptibility of neonates to develop encephalopathy at any given plasma bilirubin level and the selective distribution of CNS lesions in bilirubin encephalopathy. They also can suggest better strategies for predicting, preventing and treating this syndrome.

Bilirubin toxicity in the developing nervous system

Bilirubin toxicity remains a significant problem despite recent advances in the care of jaundiced (hyperbilirubinemic) neonates. A recent surge in reported cases of classical kernicterus, due in part to earlier hospital discharge and relaxation of treatment criteria for hyperbilirubinemia, and new reports of hyperbilirubinemia-induced auditory dysfunction using evoked potential based infant testing and hearing screening, underscore the need to better understand how hyperbilirubinemia causes brain damage in some infants, especially because the damage is preventable. Recent progress in understanding bilirubin binding and neurotoxicity resulting from unbound or "free" unconjugated bilirubin, how bilirubin affects the central nervous system in vivo and in vitro, and the use of new clinical tools in neonates, for example magnetic resonance imaging revealing bilateral lesions in globus pallidus and subthalamus, and abnormal brainstem auditory evoked potentials with normal inner ear function, may lead to improved detection and prevention of neurologic dysfunction and damage from bilirubin. Finally, the concern is raised that partial or isolated neurologic sequelae, for example auditory neuropathy and other central auditory processing disorders, may result from excessive amount and duration of exposure to free, unconjugated bilirubin at different stages of neurodevelopment.

Upregulation in the expression of multidrug resistance protein Mrp1 mRNA and protein by increased bilirubin production in rat

Earlier studies suggest that Mrp1 may mediate ATP-dependent cellular extrusion of unconjugated bilirubin (UCB). We studied the serial responses of expression of Mrp1 mRNA and protein in rats with increased bilirubin production due to hemolysis induced by phenylhydrazine (PHZ) treatment. Mrp1 mRNA was analyzed by quantitative PCR and protein by Western blot. Hepatic expression of Mrp1 mRNA and protein peaked at day 3 of PHZ treatment. Splenic expression of Mrp1 mRNA peaked within 24h and returned to baseline at day 5 whereas Mrp1 protein expression peaked at day 3. Pretreatment with heme-oxygenase inhibitor, tin mesoporphyrin, blunted the increase in serum UCB and erased the overexpression of Mrp1 both in liver and spleen. Thus, the upregulation of Mrp1 in hemolysis is mediated by UCB and/or other products of heme oxygenase, further supporting a role of Mrp1 in UCB transport and protection from its cellular toxicity.

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

Most studies of the cellular toxicity of unconjugated bilirubin (UCB) have been performed at concentrations of unbound UCB (BF) that exceed those in the plasma of neonates with bilirubin encephalopathy. We assessed whether UCB could be toxic to neurons and astrocytes at clinically relevant BF values (<or=1.0 microM), a range in which spontaneous precipitation of UCB would be unlikely to occur, even though BF exceeded the aqueous saturation limit of 70 nM. A meta-analysis yielded twelve published studies that had determined the in vitro effects of UCB on the function of cultured neurons or astrocytes at calculable BF values <or= 1.0 microM. BF values were recalculated from the stated UCB, albumin, and chloride concentrations by applying affinity constants derived from ultrafiltration of comparable solutions containing 14C-UCB and delipidated human serum albumin. At BF slightly above aqueous solubility, UCB impaired mitochondrial function and viability of astrocytes. Exposure of neuroblastoma and embryonic neuronal cell lines to BF above 250 nM impaired cellular proliferation and mitochondrial function and increased apoptosis. Purified UCB inhibited the uptake of glutamate into astrocytes at BF as low as 309 nM and induced apoptosis in brain neurons at BF as low as 85 nM. UCB can impair various cellular functions of astrocytes and neurons exposed to BF near or modestly above its aqueous solubility limit, at which UCB exists as soluble oligomers and metastable microaggregates. The results render doubtful the long-held concept that precipitation of UCB in or on cells is required to produce neurotoxicity.

Cholestatic syndromes

Further insights into the cellular and molecular mechanisms underlying hepatobiliary transport function and its regulation now permit a better understanding of the pathogenesis and treatment options of cholestatic liver diseases. Identification of the molecular basis of hereditary cholestatic syndromes will result in an improved diagnosis and management of these conditions. New insights into the pathogenesis of extrahepatic manifestations of cholestasis (eg, pruritus) have facilitated new treatment strategies. Important new studies have been published about the pathogenesis, clinical features, diagnosis, and treatment of primary biliary cirrhosis, primary sclerosing cholangitis, cholestasis of pregnancy, total parenteral nutrition-induced cholestasis, drug-induced cholestasis, and viral cholestatic syndromes.

Low solubility of unconjugated bilirubin in dimethylsulfoxide--water systems: implications for pKa determinations

BACKGROUND

Aqueous pKa values of unconjugated bilirubin are important determinants of its solubility and transport. Published pKa data on an analog, mesobilirubin-XIIIalpha, studied by 13C-NMR in buffered solutions containing 27 and 64 vol% (C2H3)2SO because of low aqueous solubility of mesobilirubin, were extrapolated to obtain pKa values in water of 4.2 and 4.9. Previous chloroform-water partition data on bilirubin diacid led to higher estimates of its pKa, 8.12 and 8.44, and its aqueous solubility. A thermodynamic analysis, using this solubility and a published solubility in DMSO, suggested that the systems used to measure 13C-NMR shifts were highly supersaturated. This expectation was assessed by measuring the residual concentrations of bilirubin in the supernatants of comparable DMSO-buffer systems, after mild centrifugation to remove microprecipitates.

RESULTS

Extensive sedimentation was observed from numerous systems, many of which appeared optically clear. The very low supernatant concentrations at the lowest pH values (4.1-5.9) were compatible with the above thermodynamic analysis. Extensive sedimentation and low supernatant concentrations occurred also at pH as high as 7.2.

CONCLUSIONS

The present study strongly supports the validity of the aqueous solubility of bilirubin diacid derived from partition data, and, therefore, the corresponding high pKa values. Many of the mesobilirubin systems in the 13C-NMR studies were probably supersaturated, contained microsuspensions, and were not true solutions. This, and previously documented errors in pH determinations that caused serious errors in pKa values of the many soluble reference acids and mesobilirubin, raise doubts regarding the low pKa estimates for mesobilirubin from the 13C-NMR studies.