Intestinal excretion of unconjugated bilirubin in man and rats with inherited unconjugated hyperbilirubinemia

Role of Heme Oxygenase in Gastrointestinal Epithelial Cells

The gastrointestinal tract is a unique organ containing both vascular and luminal routes lined by epithelial cells forming the mucosa, which play an important role in the entry of nutrients and act as a selective barrier, excluding potentially harmful agents. Mucosal surfaces establish a selective barrier between hostile external environments and the internal milieu. Heme is a major nutritional source of iron and is a pro-oxidant that causes oxidative stress. Heme oxygenases (HOs) catalyze the rate-limiting step in heme degradation, resulting in the formation of iron, carbon monoxide, and biliverdin, which are subsequently converted to bilirubin by biliverdin reductase. In gastrointestinal pathogenesis, HO-1, an inducible isoform of HO, is markedly induced in epithelial cells and plays an important role in protecting mucosal cells. Recent studies have focused on the biological effects of the products of this enzymatic reaction, which have antioxidant, anti-inflammatory, and cytoprotective functions. In this review, the essential roles of HO in the gastrointestinal tract are summarized, focusing on nutrient absorption, protection against cellular stresses, and the maintenance and regulation of tight junction proteins, emphasizing the potential therapeutic implications. The biochemical basis of the potential therapeutic implications of glutamine for HO-1 induction in gastrointestinal injury is also discussed.

Recent advances in the translation of drug metabolism and pharmacokinetics science for drug discovery and development

Drug metabolism and pharmacokinetics (DMPK) is an important branch of pharmaceutical sciences. The nature of ADME (absorption, distribution, metabolism, excretion) and PK (pharmacokinetics) inquiries during drug discovery and development has evolved in recent years from being largely descriptive to seeking a more quantitative and mechanistic understanding of the fate of drug candidates in biological systems. Tremendous progress has been made in the past decade, not only in the characterization of physiochemical properties of drugs that influence their ADME, target organ exposure, and toxicity, but also in the identification of design principles that can minimize drug-drug interaction (DDI) potentials and reduce the attritions. The importance of membrane transporters in drug disposition, efficacy, and safety, as well as the interplay with metabolic processes, has been increasingly recognized. Dramatic increases in investments on new modalities beyond traditional small and large molecule drugs, such as peptides, oligonucleotides, and antibody-drug conjugates, necessitated further innovations in bioanalytical and experimental tools for the characterization of their ADME properties. In this review, we highlight some of the most notable advances in the last decade, and provide future perspectives on potential major breakthroughs and innovations in the translation of DMPK science in various stages of drug discovery and development.

Intestinal Excretion, Intestinal Recirculation, and Renal Tubule Reabsorption Are Underappreciated Mechanisms That Drive the Distribution and Pharmacokinetic Behavior of Small Molecule Drugs

Drug reabsorption following biliary excretion is well-known as enterohepatic recirculation (EHR). Renal tubular reabsorption (RTR) following renal excretion is also common but not easily assessed. Intestinal excretion (IE) and enteroenteric recirculation (EER) have not been recognized as common disposition mechanisms for metabolically stable and permeable drugs. IE and intestinal reabsorption (IR:EHR/EER), as well as RTR, are governed by dug concentration gradients, passive diffusion, active transport, and metabolism, and together they markedly impact disposition and pharmacokinetics (PK) of small molecule drugs. Disruption of IE, IR, or RTR through applications of active charcoal (AC), transporter knockout (KO), and transporter inhibitors can lead to changes in PK parameters. The impacts of intestinal and renal reabsorption on PK are under-appreciated. Although IE and EER/RTR can be an intrinsic drug property, there is no apparent strategy to optimize compounds based on this property. This review seeks to improve understanding and applications of IE, IR, and RTR mechanisms.

Induction of fecal cholesterol excretion is not effective for the treatment of hyperbilirubinemia in Gunn rats

BACKGROUND

Unconjugated hyperbilirubinemia, a feature of neonatal jaundice or Crigler-Najjar syndrome, can lead to neurotoxicity and even death. We previously demonstrated that unconjugated bilirubin (UCB) can be eliminated via transintestinal excretion in Gunn rats, a model of unconjugated hyperbilirubinemia, and that this is stimulated by enhancing fecal fatty acid excretion. Since transintestinal excretion also occurs for cholesterol (TICE), we hypothesized that increasing fecal cholesterol excretion and/or TICE could also enhance fecal UCB disposal and subsequently lower plasma UCB concentrations.

METHODS

To determine whether increasing fecal cholesterol excretion could ameliorate unconjugated hyperbilirubinemia, we treated hyperbilirubinemic Gunn rats with ezetimibe (EZE), an intestinal cholesterol absorption inhibitor, and/or a liver X receptor (LXR) and farnesoid X receptor (FXR) agonist (T0901317 (T09) and obeticholic acid (OCA), respectively), known to stimulate TICE.

RESULTS

We found that EZE treatment alone or in combination with T09 or OCA increased fecal cholesterol disposal but did not lower plasma UCB levels.

CONCLUSIONS

These findings do not support a link between the regulation of transintestinal excretion of cholesterol and bilirubin. Furthermore, induction of fecal cholesterol excretion is not a potential therapy for unconjugated hyperbilirubinemia.

IMPACT

Increasing fecal cholesterol excretion is not effective to treat unconjugated hyperbilirubinemia. This is the first time a potential relation between transintestinal excretion of cholesterol and unconjugated bilirubin is investigated. Transintestinal excretion of cholesterol and unconjugated bilirubin do not seem to be quantitatively linked. Unlike intestinal fatty acids, cholesterol cannot "capture" unconjugated bilirubin to increase its excretion. These results add to our understanding of ways to improve and factors regulating unconjugated bilirubin disposal in hyperbilirubinemic conditions.

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.

Stable gastric pentadecapeptide BPC 157 in the therapy of the rats with bile duct ligation

Recently, stable gastric pentadecapeptide BPC 157 reversed the high MDA- and NO-tissue values to the healthy levels. Thereby, BPC 157 therapy cured rats with bile duct ligation (BDL) (sacrifice at 2, 4, 6, 8 week). BPC 157-medication (10 μg/kg, 10 ng/kg) was continuously in drinking water (0.16 μg/ml, 0.16 ng/ml, 12 ml/rat/day) since awakening from surgery, or since week 4. Intraperitoneal administration was first at 30 min post-ligation, last at 24 h before sacrifice. Local bath BPC 157 (10 µg/kg) with assessed immediate normalization of portal hypertension was given immediately after establishing portal hypertension values at 4, 6, 8 week. BPC 157 therapy markedly abated jaundice, snout, ears, paws, and yellow abdominal tegmentum in controls since 4th week, ascites, nodular, steatotic liver with large dilatation of main bile duct, increased liver and/or cyst weight, decreased body weight. BPC 157 counteracts the piecemeal necrosis, focal lytic necrosis, apoptosis and focal inflammation, disturbed cell proliferation (Ki-67-staining), cytoskeletal structure in the hepatic stellate cell (α-SMA staining), collagen presentation (Mallory staining). Likewise, counteraction includes increased AST, ALT, GGT, ALP, total bilirubin, direct and indirect and decreased albumin serum levels. As the end-result appear normalized MDA- and NO-tissue values, next to Western blot of NOS2 and NOS3 in the liver tissue, and decreased IL-6, TNF-α, IL-1β levels in liver tissue. Finally, although portal hypertension is sustained in BDL-rats, with BPC 157 therapy, portal hypertension in BDL-rats is either not even developed or rapidly abated, depending on the given BPC 157's regimen. Thus, BPC 157 may counteract liver fibrosis and portal hypertension.

Systems pharmacology modeling of drug-induced hyperbilirubinemia: Differentiating hepatotoxicity and inhibition of enzymes/transporters

Elevations in serum bilirubin during drug treatment may indicate global liver dysfunction and a high risk of liver failure. However, drugs also can increase serum bilirubin in the absence of hepatic injury by inhibiting specific enzymes/transporters. We constructed a mechanistic model of bilirubin disposition based on known functional polymorphisms in bilirubin metabolism/transport. Using physiologically based pharmacokinetic (PBPK) model-predicted drug exposure and enzyme/transporter inhibition constants determined in vitro, our model correctly predicted indinavir-mediated hyperbilirubinemia in humans and rats. Nelfinavir was predicted not to cause hyperbilirubinemia, consistent with clinical observations. We next examined a new drug candidate that caused both elevations in serum bilirubin and biochemical evidence of liver injury in rats. Simulations suggest that bilirubin elevation primarily resulted from inhibition of transporters rather than global liver dysfunction. We conclude that mechanistic modeling of bilirubin can help elucidate underlying mechanisms of drug-induced hyperbilirubinemia, and thereby distinguish benign from clinically important elevations in serum bilirubin.

Hyperbilirubinemia exaggerates endotoxin-induced hypothermia

Systemic inflammation is accompanied by an increased production of reactive oxygen species (ROS) and by either fever or hypothermia (or both). To study aseptic systemic inflammation, it is often induced in rats by the intravenous administration of bacterial lipopolysaccharide (LPS). Knowing that bilirubin is a potent ROS scavenger, we compared responses to LPS between normobilirubinemic Gunn rats (heterozygous, asymptomatic; J/+) and hyperbilirubinemic Gunn rats (homozygous, jaundiced; J/J) to establish whether ROS mediate fever and hypothermia in aseptic systemic inflammation. These two genotypes correspond to undisturbed versus drastically suppressed (by bilirubin) tissue accumulation of ROS, respectively. A low dose of LPS (10 μg/kg) caused a typical triphasic fever in both genotypes, without any intergenotype differences. A high dose of LPS (1,000 μg/kg) caused a complex response consisting of early hypothermia followed by late fever. The hypothermic response was markedly exaggerated, whereas the subsequent fever response was strongly attenuated in J/J rats, as compared to J/+ rats. J/J rats also tended to respond to 1,000 μg/kg with blunted surges in plasma levels of all hepatic enzymes studied (alanine aminotransferase, aspartate aminotransferase, gamma-glutamyl transferase), thus suggesting an attenuation of hepatic damage. We propose that the reported exaggeration of LPS-induced hypothermia in J/J rats occurs via direct inhibition of nonshivering thermogenesis by bilirubin and possibly via a direct vasodilatatory action of bilirubin in the skin. This hypothermia-exaggerating effect might be responsible, at least in part, for the observed tendency of J/J rats to be protected from LPS-induced hepatic damage. The attenuation of the fever response to 1,000 μg/kg could be due to either direct actions of bilirubin on thermoeffectors or the ROS-scavenging action of bilirubin. However, the experiments with 10 μg/kg strongly suggest that ROS signaling is not involved in the fever response to low doses of LPS.

Quantitative assessment of the multiple processes responsible for bilirubin homeostasis in health and disease

Serum bilirubin measurements are commonly obtained for the evaluation of ill patients and to screen for liver disease in routine physical exams. An enormous research effort has identified the multiple mechanisms involved in the production and metabolism of conjugated (CB) and unconjugated bilirubin (UB). While the qualitative effects of these mechanisms are well understood, their expected quantitative influence on serum bilirubin homeostasis has received less attention. In this review, each of the steps involved in bilirubin production, metabolism, hepatic cell uptake, and excretion is quantitatively examined. We then attempt to predict the expected effect of normal and defective function on serum UB and CB levels in health and disease states including hemolysis, extra- and intrahepatic cholestasis, hepatocellular diseases (eg, cirrhosis, hepatitis), and various congenital defects in bilirubin conjugation and secretion (eg, Gilbert's, Dubin-Johnson, Crigler-Najjar, Rotor syndromes). Novel aspects of this review include: 1) quantitative estimates of the free and total UB and CB in the plasma, hepatocyte, and bile; 2) detailed discussion of the important implications of the recently recognized role of the hepatic OATP transporters in the maintenance of CB homeostasis; 3) discussion of the differences between the standard diazo assay versus chromatographic measurement of CB and UB; 4) pharmacokinetic implications of the extremely high-affinity albumin binding of UB; 5) role of the enterohepatic circulation in physiologic jaundice of newborn and fasting hyperbilirubinemia; and 6) insights concerning the clinical interpretation of bilirubin measurements.

A product of heme catabolism modulates bacterial function and survival

Bilirubin is the terminal metabolite in heme catabolism in mammals. After deposition into bile, bilirubin is released in large quantities into the mammalian gastrointestinal (GI) tract. We hypothesized that intestinal bilirubin may modulate the function of enteric bacteria. To test this hypothesis, we investigated the effect of bilirubin on two enteric pathogens; enterohemorrhagic E. coli (EHEC), a Gram-negative that causes life-threatening intestinal infections, and E. faecalis, a Gram-positive human commensal bacterium known to be an opportunistic pathogen with broad-spectrum antibiotic resistance. We demonstrate that bilirubin can protect EHEC from exogenous and host-generated reactive oxygen species (ROS) through the absorption of free radicals. In contrast, E. faecalis was highly susceptible to bilirubin, which causes significant membrane disruption and uncoupling of respiratory metabolism in this bacterium. Interestingly, similar results were observed for other Gram-positive bacteria, including B. cereus and S. aureus. A model is proposed whereby bilirubin places distinct selective pressure on enteric bacteria, with Gram-negative bacteria being protected from ROS (positive outcome) and Gram-positive bacteria being susceptible to membrane disruption (negative outcome). This work suggests bilirubin has differential but biologically relevant effects on bacteria and justifies additional efforts to determine the role of this neglected waste catabolite in disease processes, including animal models.

Bilirubin is the terminal breakdown product of heme, which is deposited at high concentrations in the human intestine, where it can come into contact with host cells, the gastrointestinal (GI) microflora, and invading pathogens. Here, we report that bilirubin can act as a protectant for the Gram-negative bacterial pathogen E. coli O157:H7, which causes severe hemorrhagic diarrhea and life-threatening kidney damage. Paradoxically, bilirubin is highly toxic towards another enteric opportunistic pathogen, the Gram-positive bacterium E. faecalis. Whereas the protection of E. coli stems from the neutralization of host reactive oxygen species, bilirubin's toxicity toward E. faecalis is rooted in its lipophilic properties, which drives the rapid association of bilirubin with bacteria, leading to disrupted cell membranes and concomitant death. These results suggest small molecule metabolites can modulate bacterial communities in the intestine, a finding that may have important implications for diseases caused by enteric bacteria and disrupted flora.

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.

Assessment of UGT polymorphisms and neonatal jaundice

Elevation of the serum bilirubin level is a common, if not universal, finding during the first week of life. This can be a transient phenomenon that resolves spontaneously or can signify a serious or even life-threatening condition. There are many causes of hyperbilirubinemia and related therapeutic and prognostic implications. The diseases in which there is a primary disorder of the metabolism of bilirubin will be reviewed regarding their clinical presentation, pathophysiology, diagnosis, and treatment. These disorders-Gilbert's syndrome and Crigler-Najjar Syndrome-both involve abnormalities in bilirubin conjugation secondary to deficiency of bilirubin uridine diphosphate glucuronosyltransferase. The purpose of this article is to review the current understanding of the genetic polymorphisms that result in these diseases and discuss recent advances in diagnosis and treatment.

Bile acids decrease intracellular bilirubin levels in the cholestatic liver: implications for bile acid-mediated oxidative stress

High plasma concentrations of bile acids (BA) and bilirubin are hallmarks of cholestasis. BA are implicated in the pathogenesis of cholestatic liver damage through mechanisms involving oxidative stress, whereas bilirubin is a strong antioxidant. We evaluated the roles of bilirubin and BA on mediating oxidative stress in rats following bile duct ligation (BDL). Adult female Wistar and Gunn rats intraperitoneally anaesthetized with ketamine and xylazine underwent BDL or sham operation. Cholestatic markers, antioxidant capacity, lipid peroxidation and heme oxygenase (HO) activity were determined in plasma and/or liver tissue 5 days after surgery. HepG2-rNtcp cells were used for in vitro experiments. Plasma bilirubin levels in control and BDL animals positively correlated with plasma antioxidant capacity. Peroxyl radical scavenging capacity was significantly higher in the plasma of BDL Wistar rats (210 ± 12%, P < 0.0001) compared to controls, but not in the liver tissues. Furthermore after BDL, lipid peroxidation in the livers increased (179 ± 37%, P < 0.01), whereas liver HO activity significantly decreased to 61% of control levels (P < 0.001). Addition of taurocholic acid (TCA, ≥ 50 μmol/l) to liver homogenates increased lipid peroxidation (P < 0.01) in Wistar, but not in Gunn rats or after the addition of bilirubin. In HepG2-rNtcp cells, TCA decreased both HO activity and intracellular bilirubin levels. We conclude that even though plasma bilirubin is a marker of cholestasis and hepatocyte dysfunction, it is also an endogenous antioxidant, which may counteract the pro-oxidative effects of BA in circulation. However, in an animal model of obstructive cholestasis, we found that BA compromise intracellular bilirubin levels making hepatocytes more susceptible to oxidative damage.

Effective treatment of unconjugated hyperbilirubinemia with oral bile salts in Gunn rats

BACKGROUND & AIMS

We tested the hypothesis that oral administration of bile salts, which are known to increase the biliary excretion of unconjugated bilirubin (UCB), decreases unconjugated hyperbilirubinemia in the Gunn rat model.

METHODS

Adult Gunn rats were fed a standard diet or the same diet supplemented with 0.5 weight % ursodeoxycholic acid (UDCA) or cholic acid (CA) for 1 or 6 weeks. UCB and urobilinoids, a family of intestinal UCB breakdown products, were determined in plasma, feces, or both. After 6 weeks of treatment, tracer 3H-UCB was administered intravenously to determine steady-state UCB kinetics over the next 60 hours.

RESULTS

One-week treatment with UDCA or CA decreased plasma UCB concentrations by 21% and 30%, respectively (each P < .01). During the first 4 days of treatment, both UDCA and CA increased the combined fecal excretion of UCB and urobilinoids (+52% and +32%, respectively; each P < .01). Prolongation of treatment to 6 weeks caused a persistent decrease in plasma UCB concentrations to approximately 40% below baseline (each bile salt P < .001). (3)H-UCB kinetic studies showed that UDCA and CA administration decreased UCB pool size (-33% and -32%, respectively; each P < .05) and increased UCB fractional turnover (+33% and +25%, respectively; each P < .05).

CONCLUSIONS

Dietary bile salt administration induces a large, persistent decrease in plasma UCB concentrations in Gunn rats. Both UDCA and CA enhance UCB turnover by increasing its fecal disposal. These results support the application of oral bile salt treatment in patients with unconjugated hyperbilirubinemia.

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.

Species-dependent transport and modulation properties of human and mouse multidrug resistance protein 2 (MRP2/Mrp2, ABCC2/Abcc2)

Multidrug resistance protein 2 (MRP2/Mrp2) is a transporter that can influence the absorption, distribution, and elimination of many drugs. Mrp2 knockout mice are being used to study Mrp2 functions in vivo, including pharmacokinetics of drugs. To assess possible species-specific differences between human MRP2 and mouse Mrp2, we generated polarized cell lines expressing mouse Mrp2 and used these to investigate transport of clinically important agents. We also tested the ability of other drugs to modulate MRP2/Mrp2-mediated transport, a phenomenon that can lead to drug-drug interactions. In MDCK cells stably expressing human MRP2 or mouse Mrp2, saquinavir and docetaxel were more efficiently transported by mouse Mrp2, whereas vinblastine was transported better by human MRP2. MRP2/Mrp2-mediated transepithelial transport of several drugs could be stimulated by probenecid and sulfanitran, but stimulation was often more pronounced for human MRP2 than for mouse Mrp2. Interestingly, for some drugs the MRP2 modulator sulfinpyrazone had opposite effects on both transporters, stimulating human MRP2 and inhibiting mouse Mrp2 activity. In vesicular transport studies, transport of estradiol-17beta-glucuronide by mouse Mrp2 showed homotropic cooperativity, as previously described for human MRP2. The MRP2 modulators again showed differential effects on estradiol-17beta-glucuronide transport, most notably with sulfinpyrazone stimulating human MRP2 and profoundly inhibiting mouse Mrp2 activity. In conclusion, although human and mouse MRP2/Mrp2 have largely overlapping substrate specificities, there are important species differences in the transport efficiency of MRP2 substrates and in the modulation of transport by other compounds. These differences should be taken into account when results obtained in mice are extrapolated to humans.

Orlistat treatment of unconjugated hyperbilirubinemia in Crigler-Najjar disease: a randomized controlled trial

Unconjugated hyperbilirubinemia in Crigler-Najjar (CN) disease is conventionally treated with phototherapy and phenobarbital. Orlistat treatment increases fecal fat excretion and decreases plasma unconjugated bilirubin (UCB) concentrations in Gunn rats, the animal model for CN disease. We determined in CN patients the effects of orlistat treatment on plasma UCB concentrations, and on fecal excretion of fat and UCB. A randomized, placebo-controlled, double-blind, cross-over trial was conducted in 16 patients, simultaneous with their regular treatment (phototherapy, n = 11, and/or phenobarbital, n = 6). Patients received orlistat or placebo, each for 4-6 wk. Compared with placebo, orlistat increased fecal fat excretion (+333%) and fecal UCB excretion (+43%). Orlistat treatment significantly decreased plasma UCB concentration (-9%). In 7 of 16 patients, the decrease in plasma UCB levels was clinically relevant (>10%, mean 21%). In patients with a clinically relevant response, plasma UCB concentrations during orlistat were strongly, negatively correlated with fecal fat excretion (r = -0.93). Clinically relevant response to orlistat treatment was not correlated with age, sex, CN type, BMI, or co-treatment with phototherapy or phenobarbital, but appeared correlated with a relatively lower dietary fat intake. In conclusion, orlistat treatment decreases plasma UCB concentrations, particularly in a subgroup of CN patients. Dietary fat intake may determine the responsiveness to orlistat treatment.

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

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

Novel kinetic insights into treatment of unconjugated hyperbilirubinemia: phototherapy and orlistat treatment in Gunn rats

Treatment with phototherapy or with the lipase inhibitor orlistat decreases plasma unconjugated bilirubin (UCB) concentrations in hyperbilirubinemic Gunn rats. We investigated the mechanism(s) underlying the effects of orlistat, phototherapy, and combined treatment, using steady-state 3H-UCB kinetics. After three weeks of treatment with orlistat (200 mg/kg chow), phototherapy (19 microW/cm2/nm) or combined treatment, tracer 3H-UCB was administered IV to treated and untreated (control) Gunn rats. Plasma samples and feces were collected every 12h for 60h, and bile for 30 min at 60h. The following results were obtained: 1) each treatment decreased plasma bilirubin levels compared with controls: orlistat- 19%, phototherapy-32%, combined treatment-53%; 2) plasma bilirubin concentrations were strongly, negatively correlated with fractional bilirubin turnover; 3) orlistat treatment induced net transmucosal excretion of UCB into the intestinal lumen, whereas phototherapy increased biliary UCB excretion rate; 4) all treatments profoundly increased the enterohepatic circulation of UCB derivatives, indicating enhanced metabolism by intestinal bacteria. In conclusion, orlistat and phototherapy lower plasma bilirubin concentrations in Gunn rats by increasing (net) intestinal influx of UCB, either by transmucosal excretion (orlistat), or increased biliary secretion (phototherapy). The mechanism of phototherapy and orlistat treatment involves increasing the availability of UCB in the intestinal lumen for fecal excretion and for metabolism by intestinal bacteria.

Transcutaneous bilirubinometry: a noninvasive tool for studying newborn jaundiced rats before and after exposure to light

The homozygous Gunn rat is the most frequently used animal model for the study of neonatal jaundice. We evaluated the applicability of noninvasive transcutaneous bilirubin (TcB) measurements as an index of serum total bilirubin (STB) levels in neonatal rats by comparison to invasive STB measurements. TcB measurements were made during the first 96 h of life with the Model 101 Minolta/Air-Shields Jaundice Meter (JM) and SpectRx BiliCheck System (BC). Measurements with both devices displayed parallel TcB profiles, rapidly rising within 24 h, increasing during the next 6 h, then leveling off after 30 h. Linear regressions for the JM (n = 60) were as follows: STB (mg/dL) = 0.79 (JM) - 0.01 (units, r = 0.95, head); STB (mg/dL) = 0.82 (JM) + 1.51 (units, r = 0.95, upper back); and STB (mg/dL) = 0.74 (JM) + 1.60 (units, r = 0.91, lower back). Mean bias +/- imprecision were as follows: -0.02 +/- 3.99 mg/dL, -0.01 +/- 3.90, and 0.01 +/- 4.28 at the head, upper back, and lower back, respectively. For the BC, only lower back measurements were taken, and the regression was as follows: STB (mg/dL) = 0.77 (BC) + 1.65 mg/dL, (r = 0.93, n = 29) with a mean bias +/- imprecision of -1.08 +/- 3.08 mg/dL. When pups were exposed to light, correlations remained strong but intercepts increased. These results demonstrate that noninvasive TcB measurements correlate highly with STB in the Gunn rat during the first 96 h of life and after exposure to light. We conclude that JM measurements at the head and BC at the lower back reflect STB most reliably and consistently. Thus, in addition to being a useful tool for evaluating jaundice in human neonates, TcB methodology can be used successfully for the noninvasive monitoring of jaundice in neonatal Gunn rats pre- and postlight exposure.

Effective oral treatment of unconjugated hyperbilirubinemia in Gunn rats

We sought to develop an oral treatment for unconjugated hyperbilirubinemia. In the Gunn rat model of unconjugated hyperbilirubinemia, dietary supplementation with the lipase inhibitor orlistat (Orl) or with calcium phosphate (CaP) decreases plasma unconjugated bilirubin (UCB) levels. We determined whether Orl, CaP, or their combination is superior to phototherapy, the conventional treatment, and whether the effects of Orl and CaP are influenced by dietary fat content. Gunn rats were treated with Orl (200 mg/kg chow), CaP (20 g/kg chow), Orl + CaP, or continuous phototherapy (19 muW/cm(2)/nm) during a low-fat (LF) diet (13 energy%) or high-fat (HF) diet (35 energy%). Plasma UCB and fecal fat excretion were measured before, during, and/or at the end of treatment. Orl treatment for 2 weeks (HF diet) reduced plasma UCB concentrations similar to phototherapy (-34% and -28%, respectively); the combination of both was more effective than either treatment alone (-48%; P < .001). After 3 weeks of a HF diet, plasma UCB was 46% lower compared with the LF diet (P < .001). Plasma UCB concentrations were negatively correlated with fecal fat excretion (r = -0.96; P < .001). Irrespective of dietary fat content, 3 weeks of combined treatment (Orl + CaP) decreased plasma UCB by approximately 50% (P < .01) and was more effective than phototherapy (P < .05) at the intensity provided. In conclusion, plasma UCB concentrations in Gunn rats are negatively related to fecal fat excretion and dietary fat content. Orlistat is equally effective as phototherapy for the treatment of unconjugated hyperbilirubinemia in Gunn rats, and combined oral treatment with Orl + CaP is more effective than phototherapy. The present results support the feasibility of an efficient oral treatment of unconjugated hyperbilirubinemia.

The effect of zinc salts on serum bilirubin levels in hyperbilirubinemic rats

OBJECTIVES

Intestinal metabolism of bilirubin is implicated in the pathogenesis of neonatal jaundice and Crigler-Najjar syndrome. In the present study the authors investigated the effect of oral administration of zinc salts on serum bilirubin levels in hyperbilirubinemic rats.

METHODS

Bilirubin-binding activities of zinc sulfate and water-insoluble zinc methacrylate were determined in vitro. Congenitally hyperbilirubinemic Gunn rats and artificially hyperbilirubinemic Wistar rats were used in in vivo studies. Animals were fed a normal diet for 1 week and then a treatment diet of either zinc sulfate or zinc methacrylate for additional 2 weeks. Serum and fecal bile pigments were determined at the end of each phase. Biliary bilirubin secretion rates were determined in hyperbilirubinemic Wistar rats fed zinc methacrylate.

RESULTS

Substantial bilirubin-binding activities of zinc salts were demonstrated in in vitro experiments. Treatment with oral zinc salts significantly decreased serum bilirubin levels in Gunn rats (166 +/- 53 versus 123 +/- 38 and 206 +/- 34 versus 131 +/- 31 micromol/L, P < 0.05 for zinc methacrylate and zinc sulfate, respectively). A similar effect of zinc methacrylate was also observed in hyperbilirubinemic Wistar rats (102 +/- 10 versus 14 +/- 4 micromol/L, P < 0.0001). In accord, biliary bilirubin secretion decreased significantly in these animals (45 +/- 11 versus 28 +/- 4 nmol/h 100g body weight, P < 0.02). In contrast to zinc sulfate, treatment with zinc methacrylate did not lead to the elevation of serum zinc levels.

CONCLUSIONS

Oral administration of zinc salts efficiently decreased serum bilirubin levels in hyperbilirubinemic rats, presumably as a result of inhibition of enterohepatic circulation of bilirubin. This approach might be useful in the treatment of severe unconjugated hyperbilirubinemias.

The impact of intestinal microflora on serum bilirubin levels

BACKGROUND/AIMS

Intestinal microflora plays an important role in the pathogenesis of neonatal jaundice by inhibiting enterosystemic circulation of bilirubin. The present study aimed to investigate the influence of intestinal microflora on serum bilirubin levels in hyperbilirubinemic Gunn rats.

METHODS

After a baseline phase Gunn rats received oral antibiotics (either clindamycin/neomycine or co-trimethoxazole for four days, phase II). Intestinal colonization was carried out either with a bilirubin-reducing strain of C. perfringens or C. pasteurianum incapable of reducing bilirubin (phase III). Serum bilirubin and fecal bile pigments were determined at the end of each phase.

RESULTS

Oral administration of clindamycin/neomycine resulted in the disappearance of fecal urobilinoids. Simultaneously, serum bilirubin increased dramatically (186+/-31 vs. 289+/-35 micromol/l, P=0.004). Intestinal colonization with C. perfringens led to reappearance of fecal urobilinoid production accompanied with a partial decrease of serum bilirubin (289+/-35 vs. 239+/-17 micromol/l, P=0.013), whereas the effect of C. pasteurianum on bile pigment metabolism was negligible. Co-trimethoxazole therapy had no effect on serum and intestinal metabolism of bilirubin.

CONCLUSIONS

Intestinal microflora greatly affects intravascular metabolism of bilirubin. Prolonged use of certain antibiotics in man may lead to an increase in serum bilirubin levels, while the enhancement of intestinal catabolism may have an opposite effect.

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.

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.

Orlistat treatment increases fecal bilirubin excretion and decreases plasma bilirubin concentrations in hyperbilirubinemic Gunn rats

OBJECTIVE

To determine whether serum levels of unconjugated bilirubin (UCB) can be decreased by enhancing fecal fat excretion.

STUDY DESIGN

Gunn rats were fed a high-fat diet (control) or the same diet mixed with the lipase inhibitor orlistat. At regular intervals, plasma UCB concentrations were determined and 72-hour fat balances were performed.

RESULTS

Orlistat treatment decreased plasma UCB concentrations (at 3 weeks; 100 mg/kg, -33%+/-8%, P<.05; 200 mg/kg, -46%+/-10%, P<.01). Within days of treatment, orlistat treatment increased fecal excretion of UCB (at day 3; +220%, P<.05). During 24 weeks of orlistat treatment (200 mg/kg diet), the plasma bilirubin concentrations were continuously approximately 35% lower than in control rats. Plasma UCB concentrations were inversely correlated with the amount of fecal fat excretion (n=12, r=-0.87, P<.001).

CONCLUSIONS

In Gunn rats, orlistat treatment increases the fecal excretion of fat and enhances the disposal of UCB. This approach could lead to novel strategies for prevention and treatment of unconjugated hyperbilirubinemia in patients.

Mechanism of indinavir-induced hyperbilirubinemia

Indinavir is a viral protease inhibitor used for the treatment of HIV infection. Unconjugated hyperbilirubinemia develops in up to 25% of patients receiving indinavir, prompting drug discontinuation and further clinical evaluation in some instances. We postulated that this side-effect is due to indinavir-mediated impairment of bilirubin UDP-glucuronosyltransferase (UGT) activity and would be most pronounced in individuals with reduced hepatic enzyme levels, as occurs in approximately 10% of the population manifesting Gilbert's syndrome. This hypothesis was tested in vitro, in the Gunn rat model of UGT deficiency, and in HIV-infected patients with and without the Gilbert's polymorphism. Indinavir was found to competitively inhibit UGT enzymatic activity (K(I) = 183 microM) while concomitantly inducing hepatic bilirubin UGT mRNA and protein expression. Although oral indinavir increased plasma bilirubin levels in wild-type and heterozygous Gunn rats, the mean rise was significantly greater in the latter group of animals. Similarly, serum bilirubin increased by a mean of 0.34 mg/dl in indinavir-treated HIV patients lacking the Gilbert's polymorphism versus 1.45 mg/dl in those who were either heterozygous or homozygous for the mutant allele. Whereas saquinavir also competitively inhibits UGT activity, this drug has not been associated with hyperbilirubinemia, most likely because of the higher K(I) (360 microM) and substantially lower therapeutic levels as compared with indinavir. Taken together, these findings indicate that elevations in serum-unconjugated bilirubin associated with indinavir treatment result from direct inhibition of bilirubin-conjugating activity.

Bilirubin-IXbeta is a marker of meconium, like zinc coproporphyrin

BACKGROUND

Because meconium accumulates continuously in the fetal intestine, analysis of the postnatally excreted material could yield important information of intrauterine metabolism and maturation. Therefore, a study of the bilirubin pigments in meconium and in the first neonatal stools was carried out.

METHODS

Meconium and stools from 37 neonates of various gestational ages were collected carefully, and stored at -20 degrees C, protected by aluminium foil. Samples were defrosted, vortex mixed with an equal amount of dimethyl sulfoxide, centrifuged, and submitted to analysis by high-pressure liquid chromatography using newly developed methods to identify and to quantitate the bilirubin-IXalpha, -IXbeta, -IXgamma, and -IXdelta isomers. In addition, samples were also submitted to diazo coupling with ethyl anthranilate. Total coproporphyrins and zinc coproporphyrin were assayed for comparison.

RESULTS

Unconjugated bilirubin-IXalpha and -IXbeta were detected in meconium but not the -IXgamma or the -IXbeta isomer. Bilirubin-IXbeta was the predominant pigment and comprised 63% to 96% of the unconjugated bilirubins in the first sample of meconium excreted. Its amount decreased rapidly during the first 5 days in full-term newborns, but this occurred more slowly in preterm neonates, especially in those with a gestational age less than 30 weeks. The decrease of bilirubin-IXbeta over time correlated with that of coproporphyrin.

CONCLUSIONS

Bilirubin-IXbeta is the prevailing bile pigment in the first excreted sample of meconium. It gradually decreases after birth and can be considered a biochemical marker of meconium, like zinc coproporphyrin.