Unconjugated bilirubin exhibits spontaneous diffusion through model lipid bilayers and native hepatocyte membranes

Bile pigment pharmacokinetics and absorption in the rat: therapeutic potential for enteral administration

BACKGROUND AND PURPOSE

Bilirubin and biliverdin possess antioxidant and anti-inflammatory properties and their exogenous administration protects against the effects of inflammation and trauma in experimental models. Despite the therapeutic potential of bile pigments, little is known about their in vivo parenteral or enteral absorption after exogenous administration. This study investigated the absorption and pharmacokinetics of bile pigments after i.v., i.p. and intraduodenal (i.d.) administration in addition to their metabolism and routes of excretion.

EXPERIMENTAL APPROACH

Anaesthetized Wistar rats had their bile duct, jugular and portal veins cannulated. Bile pigments were infused and their circulating concentrations/biliary excretion were measured over 180 min. KEY RESULTS After i.v. administration of unconjugated bilirubin, biliverdin and bilirubin ditaurate, their plasma concentrations decreased exponentially over time. Subsequently, native and metabolized compounds appeared in the bile. When administered i.p., their absolute bioavailabilities equalled 14.0, 16.1 and 33.1%, respectively, and correspondingly 38, 28 and 34% of the same bile pigment doses were excreted in the bile. Administration of unconjugated bilirubin and bilirubin ditaurate i.d. increased their portal and systemic concentrations and their systemic bioavailability equalled 1.0 and 2.0%, respectively. Correspondingly, 2.7 and 4.6%, of the doses were excreted in the bile. Biliverdin was rapidly metabolized and these products were absorbed and excreted via the urine and bile.

CONCLUSIONS AND IMPLICATIONS

Bile pigment absorption from the peritoneal and duodenal cavities demonstrate new routes of administration for the treatment of inflammatory and traumatic pathology. Oral biliverdin administration may lead to the production of active metabolite that protect from inflammation/complement activation.

Complete OATP1B1 and OATP1B3 deficiency causes human Rotor syndrome by interrupting conjugated bilirubin reuptake into the liver

Bilirubin, a breakdown product of heme, is normally glucuronidated and excreted by the liver into bile. Failure of this system can lead to a buildup of conjugated bilirubin in the blood, resulting in jaundice. The mechanistic basis of bilirubin excretion and hyperbilirubinemia syndromes is largely understood, but that of Rotor syndrome, an autosomal recessive disorder characterized by conjugated hyperbilirubinemia, coproporphyrinuria, and near-absent hepatic uptake of anionic diagnostics, has remained enigmatic. Here, we analyzed 8 Rotor-syndrome families and found that Rotor syndrome was linked to mutations predicted to cause complete and simultaneous deficiencies of the organic anion transporting polypeptides OATP1B1 and OATP1B3. These important detoxification-limiting proteins mediate uptake and clearance of countless drugs and drug conjugates across the sinusoidal hepatocyte membrane. OATP1B1 polymorphisms have previously been linked to drug hypersensitivities. Using mice deficient in Oatp1a/1b and in the multispecific sinusoidal export pump Abcc3, we found that Abcc3 secretes bilirubin conjugates into the blood, while Oatp1a/1b transporters mediate their hepatic reuptake. Transgenic expression of human OATP1B1 or OATP1B3 restored the function of this detoxification-enhancing liver-blood shuttle in Oatp1a/1b-deficient mice. Within liver lobules, this shuttle may allow flexible transfer of bilirubin conjugates (and probably also drug conjugates) formed in upstream hepatocytes to downstream hepatocytes, thereby preventing local saturation of further detoxification processes and hepatocyte toxic injury. Thus, disruption of hepatic reuptake of bilirubin glucuronide due to coexisting OATP1B1 and OATP1B3 deficiencies explains Rotor-type hyperbilirubinemia. Moreover, OATP1B1 and OATP1B3 null mutations may confer substantial drug toxicity risks.

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.

Combined treatment strategies for unconjugated hyperbilirubinemia in Gunn rats

We recently demonstrated that acceleration of the gastrointestinal transit by polyethylene glycol (PEG) treats unconjugated hyperbilirubinemia in jaundiced Gunn rats. It is unclear whether acceleration of gastrointestinal transit also (partly) underlies the therapeutic effects of established hypobilirubinemic treatments or whether PEG cotreatment might enhance these effects. We treated Gunn rats with phototherapy (17 μW/cm2/nm), orlistat (200 mg/kg chow), ursodeoxycholate (5 g/kg chow), or calcium phosphate (CaP) (20 g/kg chow) either as single treatment or in combination with PEG. Three weeks of phototherapy, orlistat, ursodeoxycholic acid, or CaP treatment decreased plasma unconjugated bilirubin (UCB) levels by 47, 27, 28, and 45%, respectively (each p < 0.001), without a significant impact on gastrointestinal transit time. PEG cotreatment accelerated the gastrointestinal transit in all treatment groups, which resulted in an additive hypobilirubinemic effect of -20% and -26% (final plasma UCB -67 and -53%, respectively) in phototherapy- and orlistat-treated animals. PEG cotreatment did not enhance the hypobilirubinemic effect of ursodeoxycholic acid or CaP. We conclude that phototherapy, orlistat, ursodoxycholic acid, and CaP do not exert their hypobilirubinemic effect via acceleration of the gastrointestinal transit. PEG cotreatment enhanced the hypobilirubinemic effects of phototherapy and of orlistat treatment. Current results support a clinical trial to evaluate PEG cotreatment during phototherapy.

Unconjugated bilirubin contributes to early inflammation and edema after intracerebral hemorrhage

Intracerebral hemorrhage (ICH) is a stroke subtype with significant mortality and morbidity. The role of unconjugated bilirubin (UBR) in ICH brain injury is not well understood. Therefore, we studied the effects of UBR on brain injury markers and inflammation, as well as mechanisms involved therein. We induced ICH in mice by infusion of autologous whole blood with vehicle (dimethyl sulfoxide) or UBR. We found that UBR led to an increase in edema (P≤0.05), but a decrease in nitrate/nitrite formation (7.0±0.40  nmol/mg versus 5.2±0.70  nmol/mg protein, P≤0.05) and no change in protein carbonyls. Unconjugated bilirubin was also associated with an increase in neutrophil infiltration compared with ICH alone, as determined by both immunofluorescence and flow cytometry (36%±3.2% versus 53%±1.3% of CD45(+) cells, P≤0.05). In contrast, we observed reduced perihematomal microglia immunoreactivity in animals receiving UBR (P≤0.05). Using in vitro techniques, we show neutrophil activation by UBR and also show that protein kinase C participates in this signaling pathway. Finally, we found that UBR was associated with an increased expression of the leukocyte adhesion molecule intercellular adhesion molecule-1. Our results suggest that UBR possesses complex immune-modulatory and antioxidant effects.

Elevation of heme oxygenase-1 by proteasome inhibition affords dopaminergic neuroprotection

Postmortem studies have shown that heme oxygenase-1 (HO-1) immunoreactivity is increased in patients with Parkinson disease. HO-1 expression is highly upregulated by a variety of stress. Since the proteasome activity is decreased in patients with Parkinson disease, we investigated whether proteasome activity regulates HO-1 content. MG-132, a proteasome inhibitor, increased the amount of HO-1 protein mainly in astrocytes of primary mesencephalic cultures. Quantitative RT-PCR analysis revealed that lactacystin upregulated HO-1 mRNA expression. Proteasome inhibition with MG132 also increased the cytomegalovirus promoter-driven expression of Flag-HO-1 protein and resulted in an accumulation of ubiquitinated Flag-HO-1 in Flag-HO-1-overexpressing PC12 cells. In addition, a cycloheximide chase assay demonstrated that the degradation of Flag-HO-1 protein was slowed by MG-132. Next, the function of HO-1 which was upregulated by proteasome inhibitors was examined. Proteasome inhibitors protected dopaminergic neurons from 6-hydroxydopamine (6-OHDA)-induced toxicity and this neuroprotection was abrogated by co-treatment with zinc protoporphyrin IX, a HO-1 inhibitor. Furthermore, 6-OHDA-induced toxicity was blocked by bilirubin and carbon monoxide, products of the HO-1-catalyzed degradation of heme. These results suggest that mesencephalic HO-1 protein level is regulated by proteasome activity and the elevation by proteasome inhibition affords neuroprotection.

Controversies in bilirubin biochemistry and their clinical relevance

Despite a century of research, several clinically relevant areas of bilirubin biochemistry remain controversial, poorly understood, or unrecognized. These include: (i) The structure and molecularity of bilirubin under physiological environments such as membranes, brain tissue and when bound to proteins. Related to this is the large number of structurally different bilirubin species that may occur in blood under pathological conditions and their potential effects on measurements of bilirubin and free bilirubin. (ii) The mechanism of phototherapy, the neurotoxicity of the photoisomers produced and their influence on measurements of bilirubin and free bilirubin. (iii) The role of membrane transporters in the passage of unconjugated bilirubin across the placenta, intestine, vascular epithelium, blood-brain barrier, and into the liver. (iv) Biochemical mechanisms of bilirubin toxicity, pharmacologic prevention of kernicterus, the contribution of bilirubin to antioxidant defenses, and the practical value of free bilirubin measurements for identifying infants at most risk of kernicterus.

Contributions of the Ah receptor to bilirubin homeostasis and its antioxidative and atheroprotective functions

The homeostasis and atheroprotective function of bilirubin could be an appealing model to investigate one of the many physiologic functions of the human aryl hydrocarbon receptor (AhR). Several clinical and epidemiological studies have been carried out on key enzymes generating and eliminating bilirubin (heme oxygenase-1 and UDP-glucuronosyltransferase UGT1A1, respectively) and their regulation by the AhR. Studies with AhR-deficient mice strongly suggest a role of the AhR in vascular biology. Atherosclerosis, a major cause of premature death, is initiated by pro-oxidative insults of the vascular endothelium. The strong antioxidant and activator of AhR bilirubin is generated in vascular endothelial cells, smooth muscles and macrophages. It acts mostly in the lipid environment, thereby complementing other antioxidants such as glutathione which act mostly on water-soluble proteins. In conclusion, the atheroprotective functions of bilirubin might not only provide models to study physiologic functions of the human AhR but also provide opportunities to improve prevention and treatment of a major life-threatening disease.

Review: Bilirubin pKa studies: new models and theories indicate high pKa values in water, dimethylformamide and DMSO

Background

Correct aqueous pKa values of unconjugated bilirubin (UCB), a poorly-soluble, unstable substance, are essential for understanding its functions. Our prior solvent partition studies, of unlabeled and [¹⁴C] UCB, indicated pKa values above 8.0. These high values were attributed to effects of internal H-bonding in UCB. Many earlier and subsequent studies have reported lower pKa values, some even below 5.0, which are often used to describe the behavior of UCB. We here review 18 published studies that assessed aqueous pKa values of UCB, critically evaluating their methodologies in relation to essential preconditions for valid pKa measurements (short-duration experiments with purified UCB below saturation and accounting for self-association of UCB).

Results

These re-assessments identified major deficiencies that invalidate the results of all but our partition studies. New theoretical modeling of UCB titrations shows remarkable, unexpected effects of self-association, yielding falsely low pKa estimates, and provides some rationalization of the titration anomalies. The titration behavior reported for a soluble thioether conjugate of UCB at high aqueous concentrations is shown to be highly anomalous. Theoretical re-interpretations of data in DMSO and dimethylformamide show that those indirectly-derived aqueous pKa values are unacceptable, and indicate new, high average pKa values for UCB in non-aqueous media (>11 in DMSO and, probably, >10 in dimethylformamide).

Conclusions

No reliable aqueous pKa values of UCB are available for comparison with our partition-derived results. A companion paper shows that only the high pKa values can explain the pH-dependence of UCB binding to phospholipids, cyclodextrins, and alkyl-glycoside and bile salt micelles.

A transcriptome analysis identifies molecular effectors of unconjugated bilirubin in human neuroblastoma SH-SY5Y cells

BACKGROUND

The deposition of unconjugated bilirubin (UCB) in selected regions of the brain results in irreversible neuronal damage, or Bilirubin Encephalopathy (BE). Although UCB impairs a large number of cellular functions in other tissues, the basic mechanisms of neurotoxicity have not yet been fully clarified. While cells can accumulate UCB by passive diffusion, cell protection may involve multiple mechanisms including the extrusion of the pigment as well as pro-survival homeostatic responses that are still unknown.

RESULTS

Transcriptome changes induced by UCB exposure in SH-SY5Y neuroblastoma cell line were examined by high density oligonucleotide microarrays. Two-hundred and thirty genes were induced after 24 hours. A Gene Ontology (GO) analysis showed that at least 50 genes were directly involved in the endoplasmic reticulum (ER) stress response. Validation of selected ER stress genes is shown by quantitative RT-PCR. Analysis of XBP1 splicing and DDIT3/CHOP subcellular localization is presented.

CONCLUSION

These results show for the first time that UCB exposure induces ER stress response as major intracellular homeostasis in surviving neuroblastoma cells in vitro.

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.

Laminar-flow immediate-overlay hepatocyte sandwich perfusion system for drug hepatotoxicity testing

Drug hepatotoxicity testing requires in vitro hepatocyte culture to maintain the long-term and stable liver specific functions. We developed a drug testing platform based on laminar-flow immediate-overlay hepatocyte sandwich perfusion culture. The immediate-overlay sandwich (collagen-coated porous polymeric membrane as top overlay) protects the cells and integrity of the top collagen matrix from the impact of flow. A bioreactor was designed that allowed proper control of shear stress and mass transfer. The culture parameters such as the optimal perfusion initiation time and flow rate were systematically and mechanistically determined. The optimized system could re-establish hepatocyte polarity to support biliary excretion and to maintain other liver specific functions, such as the biotransformation enzyme activities, for two weeks that extended the usable in vitro hepatocyte-based drug testing window. When the perfusion cultured hepatocytes from days 7 or 14 were used for drug testing, the APAP-induced hepatotoxicity measurements were more sensitive and consistent over time than the static culture control, enabling further exploitations in large-scale drug testing applications.

Bilirubin impairs intestinal regrowth following massive small bowel resection in a rat model

OBJECTIVES

The purpose of the present study was to evaluate the effects of exogenous bilirubin on structural intestinal adaptation, cell proliferation, and apoptosis in a rat model of short bowel syndrome (SBS).

MATERIALS AND METHODS

Male Sprague-Dawley rats were divided into 5 experimental groups: Sham rats underwent bowel transection and reanastomosis, sham multiple doses of bilirubin (MDB) rats underwent bowel transection and were treated with bilirubin, SBS rats underwent a 75% small bowel resection, SBS-SDB (single dose bilirubin) rats underwent a bowel resection and were treated with a single dose of bilirubin, and SBS-MDB underwent a bowel resection and were treated with 3 doses of bilirubin. Bilirubin was administered intraperitoneally from the 7th day through the 14th day postoperatively. Serum total bilirubin concentration over time was evaluated in 5 SBS-SDB rats following a single intraperitoneal dose. Total bilirubin, alanine aminotransferase, and aspartate aminotransferase in serum and parameters of intestinal adaptation, enterocyte proliferation, and enterocyte apoptosis were determined on day 15.

RESULTS

SBS-SDB and SBS-MDB animals demonstrated lower ileal bowel and mucosal weights, jejunal mucosal DNA and ileal mucosal protein, and jejunal and ileal villus height and crypt depth (vs SBS animals). Bilirubin-treated rats showed a lower apoptotic index in jejunum and ileum and a trend toward an increase in cell proliferation in jejunum and ileum (vs SBS group).

CONCLUSIONS

In a rat model of SBS, exogenous bilirubin inhibits structural intestinal adaptation. Increased cell proliferation and decreased apoptosis may be considered adaptive mechanisms that maintain cell mass.

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.

Neonatal jaundice and stool production in breast- or formula-fed term infants

It has remained unclear whether the amount of fecal fat excreted in the stool and stool production influences the severity of neonatal jaundice. We determined the relationship between stool production, fecal fat excretion and jaundice in healthy breast-fed (BF) or formula-fed (FF) (near-)term neonates. From postnatal day 1-4, we quantitatively collected stools from 27 FF and 33 BF infants in daily fractions. Stool production and fecal fat contents were related to unconjugated bilirubin (UCB) levels, as determined by transcutaneous bilirubinometry (TcB). Bilirubin concentrations and stool production did not differ between FF and BF neonates during the study period. Neonatal bilirubin levels were not inversely correlated with stool production. FF and BF infants had similar fecal fat excretion rates. The stool production of FF infants was profoundly lower in the present study than in a 1985 study by De Carvalho et al. [J Pediatr (1985) 107:786-790]. We conclude that increased jaundice during the first postnatal days in healthy term neonates can no longer be attributed to breast-feeding and speculate that improved absorbability of formulas since 1985 has contributed to similar fat excretion and stool production in FF and BF neonates in 2007.

Bilirubin in clinical practice: a review

Bilirubin is an endogenous compound that can be toxic under certain conditions but, on the other hand, mild unconjugated hyperbilirubinaemia might protect against cardiovascular diseases and tumour development. Serum bilirubin levels are often enhanced under a variety of clinical conditions. These are discussed and the mechanisms are outlined.

Highly sensitive method for quantitative determination of bilirubin in biological fluids and tissues

Unconjugated bilirubin (UCB) exhibits potent antioxidant and cytoprotective properties, but causes apoptosis and cytotoxicity at pathologically elevated concentrations. Accurate measurement of UCB concentrations in cells, fluids and tissues is needed to evaluate its role in redox regulation, prevention of atherosclerotic and malignant diseases, and bilirubin encephalopathy. In the present study, we developed and validated a highly sensitive method for tissue UCB determinations. UCB was extracted from rat organs with chloroform/methanol/hexane at pH 6.2 and then partitioned into a minute volume of alkaline buffer that was subjected to HPLC using an octyl reverse phase (RP) column. Addition of mesobilirubin as an internal standard corrected for losses of UCB during extraction. Recoveries averaged 75+/-5%. The detection limit was 10pmol UCB/g wet tissue. Variance was +/-2.5%. When used to measure UCB concentrations in tissues of jaundiced Gunn rats, this procedure yielded UCB levels directly comparable to published methods, and accurately determined very low tissue bilirubin concentrations (</=40pmol UCB/g tissue) in non-jaundiced rats.

The anti-mutagenic properties of bile pigments

Bile pigments, including bilirubin and biliverdin, are endogenous compounds belonging to the porphyrin family of molecules. In the past, bile pigments and bilirubin in particular were thought of as useless by-products of heme catabolism that can be toxic if they accumulate. However, in the past 20 years, research probing the physiological relevance of bile pigments has been mounting, with evidence to suggest bile pigments possess significant antioxidant and anti-mutagenic properties. More specifically, bile pigments are potent peroxyl radical scavengers and inhibit the mutagenic effects of a number of classes of mutagens (polycyclic aromatic hydrocarbons, heterocyclic amines, oxidants). Coincidentally, persons with elevated circulating bilirubin concentrations have a reduced prevalence of cancer and cardio-vascular disease. Despite the encouraging in vitro anti-mutagenic effects of bile pigments, relatively little research has been conducted on their inhibitory capacity in bacterial and cultured cell assays of mutation, which might link the existing in vitro and in vivo observations. This is the first review to summarise the published data and it is our hope it will stimulate further research on these potentially preventative compounds.

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.

''Iatrogenic Gilbert syndrome''--a strategy for reducing vascular and cancer risk by increasing plasma unconjugated bilirubin

The catabolism of heme, generating biliverdin, carbon monoxide, and free iron, is mediated by heme oxygenase (HO). One form of this of this enzyme, heme oxygenase-1, is inducible by numerous agents which promote oxidative stress, and is now known to provide important antioxidant protection, as demonstrated in many rodent models of free radical-mediated pathogenesis, and suggested by epidemiology observing favorable health outcomes in individuals carrying high-expression alleles of the HO-1 gene. The antioxidant impact of HO-1 appears to be mediated by bilirubin, generated rapidly from biliverdin by ubiquitously expressed biliverdin reductase. Bilirubin efficiently scavenges a wide range of physiological oxidants by electron donation. In the process, it is often reconverted to biliverdin, but biliverdin reductase quickly regenerates bilirubin, thereby greatly boosting its antioxidant potential. There is also suggestive evidence that bilirubin inhibits the activity or activation of NADPH oxidase. Increased serum bilirubin is associated with reduced risk for atherogenic disease in epidemiological studies, and more limited data show an inverse correlation between serum bilirubin and cancer risk. Gilbert syndrome, a genetic variant characterized by moderate hyperbilirubinemia attributable to reduced hepatic expression of the UDP-glucuronosyltransferase which conjugates bilirubin, has been associated with a greatly reduced risk for ischemic heart disease and hypertension in a recent study. Feasible strategies for boosting serum bilirubin levels may include administration of HO-1 inducers, supplementation with bilirubin or biliverdin, and administration of drugs which decrease the efficiency of hepatic bilirubin conjugation. The well-tolerated uricosuric drug probenecid achieves non-competitive inhibition of hepatic glucuronidation reactions by inhibiting the transport of UDP-glucuronic acid into endoplasmic reticulum; probenecid therapy is included in the differential diagnosis of hyperbilirubinemia, and presumably could be used to induce an ''iatrogenic Gilbert syndrome''. Other drugs, such as rifampin, can raise serum bilirubin through competitive inhibition of hepatocyte bilirubin uptake--although unfortunately rifampin is not as safe as probenecid. Measures which can safely achieve moderate serum elevations of bilirubin may prove to have value in the prevention and/or treatment of a wide range of disorders in which oxidants play a prominent pathogenic role, including many vascular diseases, cancer, and inflammatory syndromes. Phycobilins, algal biliverdin metabolites that are good substrates for biliverdin reductase, may prove to have clinical antioxidant potential comparable to that of bilirubin.

Bilirubin as an antioxidant in micelles and lipid bilayers: its contribution to the total antioxidant capacity of human blood plasma

The antioxidant capacities, antioxidant activities, k(inh), and stoichiometric factors, n, of water-soluble derivatives of bilirubin (BR), BR-human serum albumin (BR-HSA), and BR-ditaurate disodium conjugate (BRC) were determined in aqueous/lipid dispersions of sodium dodecyl sulfate (SDS) micelles/methyl linoleate and in bilayers of dilinoleoylphosphatidylcholine (DLPC) during initiation by water-soluble azo-bis-amidinopropane dihydrochloride (ABAP). The inhibition rate constants for BRC and BR-HSA were similar in micelles (k(inh) approximately 1.3 x 10(4) M(-1) s(-1)), where n approximately 2, whereas the k(inh) for BR-HSA dropped by (1/2) in bilayers. The dimethyl ester of bilirubin (BRDE) gave a k(inh) only one-tenth that of the vitamin E analog, pentamethylhydroxychroman (PMHC) in SDS micelles/methyl linoleate when initiated by lipid-soluble azo-bis-2,4-dimethylvaleronitrile (DMVN). Biliverdin hydrochloride (BVHCl) was NOT an effective peroxyl radical-trapping agent in the micellar phase during initiation by ABAP or DMVN containing methyl linoleate but it inhibited oxygen uptake in the aqueous phase. Both BRC and BR-HSA extended the total radical antioxidant parameter (TRAP) of human blood plasma and their contribution to TRAP was in the range of 5-10% of the natural TRAP of blood plasma, depending on the BR content determined in the blood plasma.

Revalidation and rationale for high pKa values of unconjugated bilirubin

BACKGROUND

Our prior solvent partition analysis, published in 1992, yielded pKa values for unconjugated bilirubin of about 8.1 and 8.4, but these results have been challenged and studies by other methods have suggested pKa values below 5.0.

METHODS

We repeated our published solvent partition studies, using 14C-unconjugated bilirubin highly purified by extraction of residual labeled impurities from CHCl3 into an aqueous buffer, pH 7.0. Partition ratios at six pH values from 5.0 to 9.0 were determined by radioassay and compared with our prior values obtained by diazo assay.

RESULTS

At pH values ranging from 4.8 to 9.2, stable aqueous/chloroform 14C-partition ratios did not differ significantly from our published partition ratios based on diazo assay.

CONCLUSION

These results support the high pKa values of unconjugated bilirubin, above 8.0, derived from our earlier solvent partition study. In both studies, our measurements were based on the rapid analysis of clearly under-saturated solutions of highly-purified bilirubin over a wide pH range, using properly purified and preserved solvents. No previous direct estimate of the aqueous pKa values of unconjugated bilirubin meets all these preconditions. Three theoretical factors acting in combination, each related to the unique, extensive internal H-bonding of the -COOH groups, are proposed to support high pKa values of unconjugated bilirubin in water: a) donation of an H-bond from the -OH moiety of the -COOH group, which is broken on ionization; b) hindered solvation of the -COO- group after ionization; and c) restricted rotation of the -COO- and -COOH groups. Our findings and rationale rebut methodological and theoretical criticisms leveled against our prior work. High pKa values for unconjugated bilirubin dictate that: a) bilirubin diacid, which readily diffuses across membranes and can cause neurotoxicity, is the dominant unbound bilirubin species of unconjugated bilirubin in plasma at physiological pH; b) at the near-neutral pH range of gallbladder bile, the monoanion is the major unconjugated bilirubin anion present, concordant with the finding that the calcium bilirubinate precipitated in gallstones is the monoanion salt. Our conclusions are thus relevant to understanding bilirubin-induced neurological disease in severely jaundiced neonates and the precipitation of calcium bilirubinate salts in gallstones.

The anti-mutagenic and antioxidant effects of bile pigments in the Ames Salmonella test

The aim of this study was to explore the potential pro- and anti-mutagenic effects of endogenous bile pigments unconjugated bilirubin (BR), biliverdin (BV) and a synthetic, water soluble conjugate, bilirubin ditaurate (BRT) in the Ames Salmonella test. The bile pigments were tested over a wide concentration range (0.01-2 micromol/plate) in the presence of three bacterial strains (TA98, TA100, TA102). A variety of mutagens including benzo[alpha]pyrene (B[alpha]P), 2,4,7 trinitrofluorenone (TNFone), 2-aminofluorene (2-AF), sodium azide (NaN(3)) and tertiary-butyl hydroperoxide (t-BuOOH), were used to promote the formation of mutant revertants. Tests were conducted with (B[alpha]P, 2-AF, t-BuOOH) and without (TNFone, NaN(3), t-BuOOH) metabolic activation incorporating the addition of the microsomal liver preparation, S9. The bile pigments alone did not induce mutagenicity in any of the strains tested (p>0.05). Anti-mutagenic effects of the bile pigments were observed in the presence of all mutagens except for NaN(3) and the anti-mutagenic effects appeared independent of the strain tested. For TNFone induced genotoxicity, the order of effectiveness was BR> or =BRT>BV. However, the order was BV> or =BRT> or =BR for 2-AF. Antioxidant testing in the TA102 strain revealed bile pigments could effectively inhibit the genotoxic effect of t-BuOOH induced oxidative stress. The apparent antioxidant and anti-mutagenic behaviour of bile pigments further suggests their presence in biological systems is of possible physiological importance.

Transport and transporters in the endoplasmic reticulum

Enzyme activities localized in the luminal compartment of the endoplasmic reticulum are integrated into the cellular metabolism by transmembrane fluxes of their substrates, products and/or cofactors. Most compounds involved are bulky, polar or even charged; hence, they cannot be expected to diffuse through lipid bilayers. Accordingly, transport processes investigated so far have been found protein-mediated. The selective and often rate-limiting transport processes greatly influence the activity, kinetic features and substrate specificity of the corresponding luminal enzymes. Therefore, the phenomenological characterization of endoplasmic reticulum transport contributes largely to the understanding of the metabolic functions of this organelle. Attempts to identify the transporter proteins have only been successful in a few cases, but recent development in molecular biology promises a better progress in this field.

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.

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.

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.

Hyperbilirubinemia in the setting of antiviral therapy

Hyperbilirubinemia is a common side effect of antiviral medications. The mechanisms underlying its development are multiple and unique to each therapy. During administration of antiviral medications, the hyperbilirubinemia observed in the absence of liver injury is most frequently manifested by isolated increases in the indirect-reacting fraction. Relevant mechanisms leading to indirect hyperbilirubinemia in this setting include hemolysis, decreased hepatic bilirubin clearance as a result of impairment of bilirubin conjugation, or circumstances in which both processes occur simultaneously. Underlying genetic susceptibilities may potentiate these side effects of antiviral therapy. Conjugated (direct-reacting) hyperbilirubinemia can be a consequence of generalized hepatocellular injury, selective cholestatic defects, biliary obstruction, or, rarely, genetic disorders of bilirubin transport. In the specific setting of antiviral therapy, preexisting liver disease or antiviral hepatotoxicity, such as is encountered with the use of the nucleoside and non-nucleoside human immunodeficiency virus reverse transcriptase inhibitors, are the most frequent causes of direct-reacting or mixed direct- and indirect-reacting hyperbilirubinemia. Modification in antiviral drug choice or dose may be required in cases of liver injury or of brisk hemolysis leading to significant anemia. The mild indirect hyperbilirubinemia associated with impairment in conjugation tends to be well tolerated and of little consequence. The decision to continue or discontinue antiviral therapy in the face of hyperbilirubinemia should be made after an assessment of the cause of the elevated bilirubin level and a thorough assessment of the risks and benefits of antiviral therapy.

Unconjugated bilirubin induces apoptosis in colon cancer cells by triggering mitochondrial depolarization

Bilirubin is the principal end product of heme degradation. Prompted by epidemiologic analyses demonstrating an inverse correlation between serum bilirubin levels and cancer mortality, we examined the effect(s) of bilirubin on the growth and survival of colon adenocarcinoma cells. Adenocarcinoma cell monolayers were treated with bilirubin over a range of bilirubin:BSA molar ratios (0-0.6), and viability was assessed colorimetrically. Apoptosis was characterized by TUNEL assay, annexin V staining and caspase-3 activation. The mechanism(s) by which bilirubin induces apoptosis was investigated by Western blotting for cytochrome c release, assaying for caspase-8 and caspase-9 activation and for mitochondrial depolarization by JC-1 staining. The direct effect of bilirubin on the membrane potential of isolated mitochondria was evaluated using light-scattering and fluorescence techniques. Bilirubin decreased the viability of all colon cancer cell lines tested in a dose-dependent manner. Cells exhibited substantial apoptosis when exposed to bilirubin concentrations ranging 0-50 microM, as demonstrated by an 8- to 10-fold increase in TUNEL and annexin V staining and in caspase-3 activity. Bilirubin treatment evokes specific activation of caspase-9, enhances cytochrome c release into the cytoplasm and triggers the mitochondrial permeability transition in colon cancer monolayers. Additionally, bilirubin directly induces the depolarization of isolated rat liver mitochondria, an effect that is not inhibited by cyclosporin A. Bilirubin stimulates apoptosis of colon adenocarcinoma cells in vitro through activation of the mitochondrial pathway, apparently by directly dissipating mitochondrial membrane potential. As this effect is triggered at concentrations normally present in the intestinal lumen, we postulate a physiologic role for bilirubin in modulating colon tumorigenesis.

Molecular basis of bilirubin-induced neurotoxicity

Unconjugated bilirubin (UCB), at slightly elevated unbound concentrations, is toxic to astrocytes and neurons, damaging mitochondria (causing impaired energy metabolism and apoptosis) and plasma membranes (causing oxidative damage and disrupting transport of neurotransmitters). Accumulation of UCB in the CSF and CNS is limited by its active export, probably mediated by MRP1/Mrp1 present in choroid plexus epithelia, capillary endothelia, astrocytes and neurons. Upregulation of MRP1/Mrp1 protein levels by UCB might represent an important adaptive mechanism that protects the CNS from UCB toxicity. These concepts could explain the varied susceptibility of newborns to bilirubin neurotoxicity and the occurrence of neurological damage at plasma UCB concentrations well below therapeutic guidelines, and are relevant to the increasing prevalence of bilirubin encephalopathy in newborns.

Serum bilirubin levels in the U.S. population: gender effect and inverse correlation with colorectal cancer

Bilirubin, the primary end product of heme catabolism, is a key marker of liver and hematological disorders, and important cytoprotective properties have been ascribed to this bile pigment. The Third National Health and Nutrition Examination Survey, a comprehensive assessment of health and nutrition in the United States, was analyzed to determine the demographics and correlates of serum bilirubin levels in the general population. Men and women aged 17 and older were included in the weighted analysis, representing a total of 176,748,462 subjects. The mean serum total bilirubin in the adult population is 0.62 +/- 0.003 mg/dL (SEM), with a 97.5% cut-off of 1.4 mg/dL. Serum bilirubin levels are significantly higher in men (0.72 +/- 0.004) than in women (0.52 +/- 0.003 mg/dL) and are lower in non-Hispanic blacks (0.55 +/- 0.005 mg/dL) compared with non-Hispanic whites (0.63 +/- 0.004 mg/dL) and Mexican Americans (0.61 +/- 0.005 mg/dL). Bilirubin concentrations are unrelated to body weight but are reduced in active smokers. Individuals with a history of nondermatological malignancy exhibit significantly lower serum bilirubin concentrations compared with those who do not have a history of nondermatological cancer. In particular, each 1-mg/dL increase in serum bilirubin is associated with a markedly decreased prevalence of colorectal cancer (OR = 0.257; 95% CI 0.254-0.260). In conclusion, serum bilirubin levels vary significantly with gender, race, and smoking status. The observed inverse correlation between serum bilirubin concentrations and a history of nondermatological malignancy, particularly colorectal cancer, warrants further investigation of a potentially important chemopreventive function of bilirubin.

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

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

Bilirubin as an Antioxidant: Kinetic Studies of the Reaction of Bilirubin with Peroxyl Radicals in Solution, Micelles, and Lipid Bilayers

Bilirubin (BR) showed very weak antioxidant activity in a nonpolar medium of styrene or cumene in chlorobenzene. In contrast, BR exhibited strong antioxidant activity in polar media such as aqueous lipid bilayers or SDS micelles/methyl linoleate (pH 7.4), where the rate with peroxyl radicals, k(inh) = 5.0 x 10(4) M(-)(1) s(-)(1), was comparable to that with vitamin E analogues, Trolox, or PMHC. An electron-transfer mechanism accounts for the effect of the medium on the antioxidant properties of BR.

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.

Growth hormone modulation of the rat hepatic bile transporter system in endotoxin-induced cholestasis

Treatment with high dose human GH, although an effective anabolic agent, has been associated with increased incidence of sepsis, inflammation, multiple organ failure, and death in critically ill patients. We hypothesized that GH might increase mortality by exacerbating cholestasis through modulation of bile acid transporter expression. High dose GH was continuously infused over 4 d into rats, and on the final day lipopolysaccharides were injected. Hepatic bile acid transporter expression was measured by Northern analysis and immunoblotting and compared with serum markers of cholestasis and endotoxinemia. Compared with non-GH-treated controls, GH increased endotoxin-induced markers of cholestasis and liver damage as well as augmented IL-6 induction. In endotoxinemia, GH treatment significantly induced multidrug resistance-associated protein 1 mRNA and protein and suppressed organic anion transporting polypeptides, Oatp1 and Oatp4, mRNA, suggesting impaired uptake of bilirubin and bile acids at the basolateral surface of the hepatocyte, which could contribute to the observed worsening of cholestasis by GH. This study of endotoxinemia may thus provide a mechanistic link between GH treatment and exacerbation of cholestasis through modulation of basolateral bile acid transporter expression in the rat hepatocyte.

The human organic anion transport protein SLC21A6 is not sufficient for bilirubin transport

A recent study (Cui, Y., Konig, J., Leier, I., Buchholz, U., and Keppler, D. (2001) J. Biol. Chem. 276, 9626-9630) suggests that human OATP2 (SLC21A6), also known as OATP-C and LST1, mediates hepatic bilirubin transport. Because of methodologic concerns, this study was designed to examine this issue using a bilirubin transport assay that was validated in overnight cultured rat hepatocytes. These studies showed that cultured rat hepatocytes transported bilirubin with kinetics virtually identical to the transport of sulfobromophthalein. This assay was then used to quantify bilirubin transport by HeLa cells that had been stably transfected with OATP2 under regulation of a metallothionein promoter. Immunoblot analysis revealed abundant expression of OATP2 after incubation of cells for 48 h in zinc, whereas uninduced cells had no expression of this protein. In OATP2-expressing (zinc-induced) HeLa cells at 37 degrees C, the uptake of [35S]sulfobromophthalein was substantial (51.6 +/- 16.5 pmol/15 min/mg protein, n = 5) with little cell-associated ligand in non-expressing (uninduced) cells (0.54 +/- 0.16 pmol/15 min/mg protein, n = 5, p < 0.002). In contrast, there was no difference (p > 0.2) in cell-associated [3H]bilirubin in induced (OATP2-expressing) as compared with uninduced cells (11.25 +/- 3.02 pmol/15 min/mg protein versus 9.15 +/- 1.68 pmol/min/mg protein, respectively, n = 5) We obtained similar results in OATP2-transfected HEK293 cells that were used in the original report. The existence of a bilirubin transporter has been an important field of investigation for many years. Although the current study indicates that a role for OATP2 in hepatocyte bilirubin transport is unlikely, it provides new and sensitive tools that can be adapted to examine the function of putative bilirubin transporters in the future.

Probing red cell membrane cholesterol movement with cyclodextrin

We probed the kinetics with which cholesterol moves across the human red cell bilayer and exits the membrane using methyl-beta-cyclodextrin as an acceptor. The fractional rate of cholesterol transfer (% s(-1)) was unprecedented, the half-time at 37 degrees C being ~1 s. The kinetics observed under typical conditions were independent of donor concentration and directly proportional to acceptor concentration. The rate of exit of membrane cholesterol fell hyperbolically to zero with increasing dilution. The energy of activation for cholesterol transfer was the same at high and low dilution; namely, 27-28 Kcal/mol. This behavior is not consistent with an exit pathway involving desorption followed by aqueous diffusion to acceptors nor with a simple one-step collision mechanism. Rather, it is that predicted for an activation-collision mechanism in which the reversible partial projection of cholesterol molecules out of the bilayer precedes their collisional capture by cyclodextrin. Because the entire membrane pool was transferred in a single first-order process under all conditions, we infer that the transbilayer diffusion (flip-flop) of cholesterol must have proceeded faster than its exit, i.e., with a half-time of <1 s at 37 degrees C.

Mdr P-glycoproteins are not essential for biliary excretion of the hydrophobic heme precursor protoporphyrin in a griseofulvin-induced mouse model of erythropoietic protoporphyria

Hepatic complications in erythropoietic protoporphyria (EPP) have been attributed to toxic actions of accumulated protoporphyrin (PP). PP can only be removed via the bile but transport systems involved have not been defined. The aim of this study was to gain insight in the mode of biliary PP excretion, with emphasis on the potential contribution of the Mdr1 P-glycoprotein export pump and biliary lipids as PP carriers. Control mice and mice homozygous for Mdr1a/b (Abcb1) or Mdr2 (Abcb4) gene disruption, the latter unable to secrete phospholipids and cholesterol into bile, were treated with griseofulvin to chemically induce protoporphyria. All groups showed dramatically increased PP levels in erythrocytes and liver after griseofulvin treatment. Histologically, massive PP deposits were found in livers of control and Mdr1a/b(-/-) mice but not in those of Mdr2(-/-) mice. Serum unesterified cholesterol and phospholipids were increased by griseofulvin because of formation of lipoprotein-X in control and Mdr1a/b(-/-) mice only. Yet, bile flow was not impaired in griseofulvin-treated mice, and biliary bile salt, phospholipid, and cholesterol secretion rates were significantly increased. Surprisingly, biliary PP excretion was similar in all 3 groups of griseofulvin-treated mice: the observed linear relationship between hepatic and biliary PP concentrations and identical liver-to-bile concentration ratios in treated and untreated mice suggest a passive mode of excretion. In conclusion, the data show that Mdr P-glycoproteins are not critically involved in biliary removal of excess PP and indicate that the presence of biliary lipids is required for formation of intrahepatic PP deposits.

A unique multifunctional transporter translocates estradiol-17beta -glucuronide in rat liver microsomal vesicles

A wide array of drugs, xenobiotics, and endogenous compounds undergo detoxification by conjugation with glucuronic acid in the liver via the action of UDP-glucuronosyltransferases. The mechanism whereby glucuronides, generated by this enzyme system in the lumen of the endoplasmic reticulum (ER), are exported to the cytosol prior to excretion is unknown. We examined this process in purified rat liver microsomes using a rapid filtration technique and [(3)H]estradiol-17beta-d-glucuronide ([(3)H]E(2)17betaG) as model substrate. Time-dependent uptake of intact [(3)H]E(2)17betaG was observed and shrinkage of ER vesicles by raffinose lowered the steady-state level of [(3)H]E(2)17betaG accumulation. In addition, rapid efflux of [(3)H]E(2)17betaG from rat liver microsomal vesicles suggested that the transport process is bidirectional. Microsomal uptake was saturable with an apparent K(m) and V(max) of 3.29 +/- 0.58 microm and 0.19 +/- 0.02 nmol.min(-1).mg protein(-1), respectively. Transport of [(3)H]E(2)17betaG was inhibited by the anion transport inhibitors 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and probenecid. Specificity of the transport process was investigated by studying the cis-inhibitory effect of anionic metabolites, as well as substrates of the plasma membrane multidrug resistance-associated proteins on the uptake of [(3)H]E(2)17betaG. Collectively, these data are indicative of a novel multifunctional and bidirectional protein carrier for E(2)17betaG and other anionic compounds in the hepatic ER. This intracellular membrane transporter may contribute to the phenomenon of multidrug resistance.

Bilirubin-induced apoptosis in cultured rat neural cells is aggravated by chenodeoxycholic acid but prevented by ursodeoxycholic acid

BACKGROUND/AIMS

Unconjugated bilirubin (UCB) can be neurotoxic in jaundiced neonates and in patients with Crigler-Najjar syndrome. UCB toxicity may culminate in cell death, however, the occurrence of apoptosis has never been investigated. Ursodeoxycholic acid (UDCA) is a strong modulator of the apoptotic threshold in both hepatic and nonhepatic cells. The aims of this study were to determine whether apoptosis plays a role in neural cell death induced by UCB, and to investigate the ability of UDCA to prevent cell death.

METHODS

Cultured rat astrocytes were incubated with UCB (17 and 86 microM) plus albumin (5.7 and 28.7 microM) for 4-22 h. In addition, astrocytes and neurones were treated with either UCB, 50 microM UDCA, or their combination for 4 h. Cultures were scored for nonviable cells by trypan blue dye exclusion. Apoptosis was assessed by Hoechst staining and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labelling assay.

RESULTS

UCB induced a concentration- and time-dependent decrease in astrocyte viability. Apoptosis was 4- and 7-fold increased after 4 h exposure to 17 and 86 microM UCB, respectively (P < 0.01). UDCA reduced apoptosis to <7%, which represents a appoximately 60% protection (P < 0.01). Cholic acid was not protective, and chenodeoxyholic acid aggravated UCB toxicity (P < 0.05). Finally, neurones showed a 1.5-fold greater sensitivity than astrocytes to UCB, while UDCA was still protective.

CONCLUSIONS

UCB is toxic to both astrocytes and neurones, causing cell death through an apoptotic process. Moreover, UDCA inhibits UCB-induced apoptosis in neural cells and this could not be mimicked by other bile acids.

Role of apolipoprotein D in the transport of bilirubin in plasma

Apolipoprotein D (apo D) is a 30-kDa glycoprotein of unknown function that is associated with high-density lipoproteins (HDL). Because unconjugated bilirubin has been shown to bind apo D with a 0. 8:1 stoichiometry, we examined the contribution of this protein to transport of bilirubin in human plasma. Density gradient centrifugation analysis using physiological concentrations of [(14)C]bilirubin reveals that 9% of unconjugated bilirubin is associated with HDL, with the remaining pigment bound primarily to serum proteins (i.e., albumin). The percentage of total plasma bilirubin bound to HDL was found to increase proportionally with bilirubin concentration. Affinity of human apo D for bilirubin was determined by steady-state fluorescence quenching, with Scatchard analysis demonstrating a single binding site for unconjugated bilirubin with an affinity constant (K(a)) of approximately 3 x 10(7) M(-1). Incorporation of apo D into phosphatidylcholine vesicles had no effect on K(a), suggesting that a lipid environment does not alter the affinity of the protein for bilirubin. Using stopped-flow techniques, the first-order rate constant for bilirubin dissociation from apo D was measured at 5.4 s(-1) (half-time = 129 ms). Our findings indicate that HDL is the principal nonalbumin carrier of bilirubin in human plasma and further support the proposition that the affinity of HDL for bilirubin is primarily the result of binding to apo D.

Assessment of bilirubin toxicity to erythrocytes. Implication in neonatal jaundice management

BACKGROUND

Neonatal hyperbilirubinaemia remains one of the most common clinical conditions requiring therapeutic intervention. Nevertheless, reliable indicators of bilirubin toxicity are still missing. This prompted us to investigate (a) the progression of cytotoxic events produced by increasing concentrations of bilirubin; (b) the relevance of the membrane lipid package on bilirubin binding to erythrocytes; and (c) the reliability of chloroform extraction compared with albumin extraction to evaluate erythrocyte-bound bilirubin and cytotoxicity.

MATERIALS AND METHODS

Morphological alterations, free bilirubin, erythrocyte-bound bilirubin (albumin- and chloroform-extractable), haemolysis and membrane-released lipids, were determined in human erythrocytes at 4 degrees C or 37 degrees C, after 4 h incubation at pH 7.4, with increasing molar ratios of bilirubin to albumin (0.5-5). The reversibility of cytotoxicity by albumin washing was assessed by morphological analysis.

RESULTS

Decreased free bilirubin, lower erythrocyte-bound bilirubin concentration by albumin extraction (superficial/non-aggregated bilirubin) and higher values by chloroform extraction (deep/aggregated bilirubin) were observed for 37 degrees C vs. 4 degrees C, at molar ratios > 1. Echinocytosis increased with bilirubin concentration and temperature and was not fully reversed by albumin washing. Haemolysis was already significant at a molar ratio of 1, and was enhanced by temperature at molar ratios 3 and 5 (P < 0.01). The loss of membrane lipids was remarkable at molar ratios > or = 0.5, both at 4 degrees C and 37 degrees C (P < 0.01), although correlation with bilirubin concentration was only significant at 37 degrees C (r = 0.971; P < 0.01).

CONCLUSIONS

These results suggest that increased lipid fluidity and high bilirubin concentrations promote membrane bilirubin translocation and toxicity. They also show that albumin is not able to displace the bilirubin located deeply or aggregated within the membrane, which in turn is removed by chloroform. Accordingly, chloroform-extractable rather than albumin-extractable bilirubin is a more accurate parameter to assess erythrocyte-bound bilirubin during severe hyperbilirubinaemia.

Mechanism of hepatocellular uptake of albumin-bound bilirubin

We previously demonstrated that unconjugated bilirubin spontaneously diffuses through phospholipid bilayers at a rate which exceeds albumin dissociation, suggesting that solvation from albumin represents the rate-limiting step in hepatic bilirubin clearance. To further examine this hypothesis, we studied the uptake of bovine serum albumin (BSA)-bound bilirubin by cultured hepatoblastoma (HepG2) cells. Uptake of bilirubin was saturable, with a K(m) and V(max) of 4.2+/-0.5 microM (+/-S.E.M.) and 469+/-41 pmol min(-1) mg(-1) at 25 degrees C. Substantial bilirubin uptake also was observed at 4 degrees C (K(m)=7.0+/-0.8 microM, V(max)=282+/-26 pmol min(-1) mg(-1)), supporting a diffusional transport mechanism. Consistent with reported solvation rates, the cellular uptake of bilirubin bound to human serum albumin was more rapid than for BSA-bound bilirubin, indicative of dissociation-limited uptake. Counterintuitively, an inverse correlation between pH and the rate of bilirubin flip-flop was observed, due to pH effects on the rate of dissociation of bilirubin from albumin and from the membrane bilayer. The identification of an inflection point at pH 8.1 is indicative of a pK(a) value for bilirubin in this range. Taken together, our data suggest that hepatocellular uptake of bilirubin is dissociation-limited and occurs principally by a mechanism involving spontaneous transmembrane diffusion.