Identifying Potential Therapeutic Applications and Diagnostic Harms of Increased Bilirubin Concentrations: A Clinical and Genetic Approach

Bilirubin has antioxidant and anti-inflammatory properties in vitro and in animal studies and protects against inflammatory, cardiovascular, and other diseases in observational studies; therefore, bilirubin has potential as a therapeutic agent. However, observational studies could be confounded by many factors. We used a genetic (n = 61,281) and clinical (n = 234,670) approach to define the association between bilirubin and 19 conditions with a putative protective signal in observational studies. We also tested if individuals with genetically higher bilirubin levels underwent more diagnostic tests. We used a common variant in UGT1A1 (rs6742078) associated with an 26% increase in bilirubin levels in the genetic studies. Carriers of the variant had higher bilirubin levels (P = 2.2 × 10-16 ) but there was no significant association with any of the 19 conditions. In a phenome-wide association study (pheWAS) to seek undiscovered genetic associations, the only significant finding was increased risk of "jaundice-not of newborn." Carriers of the variant allele were more likely to undergo an abdominal ultrasound (odds ratio = 1.04, [1.00-1.08], P = 0.03). In contrast, clinically measured bilirubin levels were significantly associated with 15 of the 19 conditions (P < 0.003) and with 431 clinical diagnoses in the pheWAS (P < 1 × 10-5 adjusted for sex, age, and follow-up). With additional adjustment for smoking and body mass index, 7 of 19 conditions and 260 pheWAS diagnoses remained significantly associated with bilirubin levels. In conclusion, bilirubin does not protect against inflammatory or other diseases using a genetic approach; the many putative beneficial associations reported clinically are likely due to confounding.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

A novel newborn rat kernicterus model created by injecting a bilirubin solution into the cisterna magna

BACKGROUND

Kernicterus still occurs around the world; however, the mechanism of bilirubin neurotoxicity remains unclear, and effective treatment strategies are lacking. To solve these problems, several kernicterus (or acute bilirubin encephalopathy) animal models have been established, but these models are difficult and expensive. Therefore, the present study was performed to establish a novel kernicterus model that is simple and affordable by injecting unconjugated bilirubin solution into the cisterna magna (CM) of ordinary newborn Sprague-Dawley (SD) rats.

METHODS

On postnatal day 5, SD rat pups were randomly divided into bilirubin and control groups. Then, either bilirubin solution or ddH2O (pH = 8.5) was injected into the CM at 10 µg/g (bodyweight). For model characterization, neurobehavioral outcomes were observed, mortality was calculated, and bodyweight was recorded after bilirubin injection and weaning. Apoptosis in the hippocampus was detected by H&E staining, TUNEL, flow cytometry and Western blotting. When the rats were 28 days old, learning and memory ability were evaluated using the Morris water maze test.

RESULTS

The bilirubin-treated rats showed apparently abnormal neurological manifestations, such as clenched fists, opisthotonos and torsion spasms. Bodyweight gain in the bilirubin-treated rats was significantly lower than that in the controls (P<0.001). The early and late mortality of the bilirubin-treated rats were both dramatically higher than those of the controls (P = 0.004 and 0.017, respectively). Apoptosis and necrosis in the hippocampal nerve cells in the bilirubin-treated rats were observed. The bilirubin-treated rats performed worse than the controls on the Morris water maze test.

CONCLUSION

By injecting bilirubin into the CM, we successfully created a new kernicterus model using ordinary SD rats; the model mimics both the acute clinical manifestations and the chronic sequelae. In particular, CM injection is easy to perform; thus, more stable models for follow-up study are available.

Restoring HOmeostasis: is heme oxygenase-1 ready for the clinic?

Inflammation and immunity result in a wide range of disease processes, including atherosclerosis, vascular thrombosis and sepsis. Heme oxygenase-1 (HO-1) is a key enzyme that is integral to the temporal and spatial regulation of the host response and, together with its products carbon monoxide (CO) and bilirubin, is crucial for maintaining homeostasis and the preservation of function and life. An increasing number of reports demonstrates that HO-1, CO and bilirubin regulate the immune response. As CO and bilirubin enter clinical trials, there are obstacles to be addressed before their full therapeutic potential can be achieved. In this article, we delineate the challenges that lie ahead regarding toxicity, pharmacokinetics and mechanisms of action to be able to take full advantage of the powerful cytoprotective properties of these agents for clinical benefit.

NMDA channel antagonist MK-801 does not protect against bilirubin neurotoxicity

BACKGROUND

Bilirubin encephalopathy or kernicterus is a potentially serious complication of neonatal hyperbilirubinemia. The mechanism of bilirubin-induced neurotoxicity is not known. Many neurological insults are mediated through NMDA receptor activation.

OBJECTIVE

We assessed the effect of the NMDA channel antagonist, MK-801 on bilirubin neurotoxicity in vivo and in vitro.

METHODS

Bilirubin toxicity in vitro was assessed using trypan blue staining. Sulfadimethoxine injected (i.p.) jaundiced Gunn rat pups exhibit many neurological sequelae observed in human hyperbilirubinemia. Brainstem auditory-evoked potentials (BAEPs), a noninvasive sensitive tool to assess auditory dysfunction due to bilirubin neurotoxicity, were used to assess neuroprotection with MK-801 (i.p.) in vivo.

RESULTS

In primary cultures of hippocampal neurons, 20 min exposure to 64:32 microM bilirubin:human serum albumin reduced the cell viability by approximately 50% ten hours later. MK-801 treatment did not protect the cells. MK-801 pretreatment doses ranging from 0.1-4.0 mg/kg did not protect against BAEP abnormalities in Gunn rat pups 6 h after sulfadimethoxine injection.

CONCLUSION

Our findings suggest that bilirubin neurotoxicity is not mediated through NMDA receptor activation.

Minocycline blocks acute bilirubin-induced neurological dysfunction in jaundiced Gunn rats

BACKGROUND

Extreme hyperbilirubinemia is treated with double volume exchange transfusion, which may take hours to commence. A neuroprotective agent that could be administered immediately might be clinically useful. Minocycline, an anti-inflammatory and anti-apoptotic semisynthetic tetracycline, prevents hyperbilirubinemia-induced cerebellar hypoplasia in Gunn rats. Acute brainstem auditory evoked potential (BAEP) abnormalities occur after giving sulfadimethoxine to 16-day-old jaundiced Gunn rats to displace bilirubin into tissue including brain.

OBJECTIVE

To assess whether minocycline is neuroprotective in this model of acute bilirubin encephalopathy.

METHODS

We recorded BAEPs at baseline and 6 h after injecting sulfadimethoxine. Minocycline 0.5 mg/kg (n = 4), 5 mg/kg (n = 9), 50 mg/kg (n = 9) or 500 mg/kg (n = 3, all died) was administered 15 min before sulfadimethoxine (0 h). Controls received saline followed by either sulfadimethoxine (n = 13) or saline (n = 7).

RESULTS

At 6 h total plasma bilirubin decreased from 10.84 +/- 0.88 mg/dl (mean +/- SD) to 0.70 +/- 0.35 mg/dl (p <10(-9)) in all sulfadimethoxine-injected groups. At 6 h, there was complete protection against decreased amplitudes of BAEP waves II and III and increased I-II and I-III interwave intervals (brainstem conduction times corresponding to I-III and I-V in humans) with 50 mg/kg minocycline, and partial protection with lower doses.

CONCLUSIONS

Minocycline 50 mg/kg 15 min prior to an intervention that normally produces acute bilirubin neurotoxicity is neuroprotective in jaundiced Gunn rat pups. Further studies are needed to investigate the temporal course and mechanism of neuroprotection. Minocycline, administered immediately, may be clinically useful in treating extreme neonatal hyperbilirubinemia and preventing kernicterus. We believe our model provides an efficient in vivo model to screen and evaluate new agents that are neuroprotective against bilirubin toxicity and kernicterus.

Biological properties and therapeutic potential of bilirubin

Bilirubin was long considered a useless metabolite of heme catabolism, responsible for the clinical manifestation of jaundice, and potentially toxic in high doses, particularly in neonates. In the past two decades the potent biological properties of bilirubin, particularly as an antioxidant, have been recognised, and this has prompted a number of investigations into this molecule concerning its in vitro and in vivo properties. This review summarises that work, as well as more recent investigations into the potential therapeutic uses of bilirubin.

Combined effects of bilirubin and hypoxia on cultured neurons from the developing rat forebrain

Hyperbilirubinemia and hypoxia are common causes of brain injury in the newborn. To determine the effects of free bilirubin associated with transient hypoxia on developing rat neurons, the cells were exposed to bilirubin (0.25 to 5 micromol/L) and/or to hypoxia for 3 or 6 hours (95% N2-5% CO2). Glutamate receptor antagonists were added to some cultures. Cell death characteristics, energy metabolism, and protein synthesis were analyzed for 96 hours. Bilirubin increased apoptotic cell death. When associated with hypoxia, the neuronal loss was worsened. Bilirubin reduced energy metabolism, whereas a 6-hour exposure to hypoxia increased it for at least 24 hours, with no influence of additional bilirubin. Bilirubin with or without hypoxia induced 2 increases in protein synthesis, at 1 and 72 hours. In this model, bilirubin may promote programmed neuronal death. When bilirubin is associated with hypoxia, the deleterious effects are enhanced. The suppression of bilirubin induced neuronal damage by the NMDA (N-methyl-D-aspartate) receptor antagonist MK801 suggests the involvement of glutamate.

[Combined neuronal toxicity of bilirubin and hypoxia. Study of cultured rat neurons]

Clinical observations suggest that bilirubin encephalopathy is often seen in newborn infants presenting not only with hyperbilirubinemia but also with alterations in oxygen transport like in severe anaemia. Since bilirubin and hypoxia have been shown to be detrimental to the central nervous system, the present study was designed to test the additional effects of the two insults on the outcome of cultured neurons from the forebrain of 14 day-old embryos. After 6 days in vitro, neurons were exposed either to bilirubin (0.5 microM) or to bilirubin and hypoxia for 6 hours. Thereafter, cells were reoxygenated for 96 hours in standard conditions. Control cells were kept in normoxia. Cell viability was assessed by the methyltetrazolium method. Cell death (apoptosis or necrosis) was characterized by fluorescent nuclear staining with DAPI. Rates of protein synthesis and energy metabolism in neurons were measured by [3H]leucine and [3H]2-deoxyglucose incorporation, respectively. Data are reported as percentages of change as compared to controls. Each experiment involved 5 to 10 dishes per time point and was repeated 2 to 4 times.

RESULTS

Bilirubin reduced cell viability by 24.5% vs controls (p < 0.001) at 96 h while 16% of the neurons exhibited morphological features of apoptosis (p < 0.001). The combination of hypoxia with bilirubin induced a 34% decrease in cell viability (p < 0.001) and the percentage of apoptotic cells was higher than after the exposure to hypoxia or to bilirubin alone. The rate of protein synthesis increased significantly in all experimental conditions as early as 1 h after the onset of the insult and at 48 h post reoxygenation. It increased again at 72 h in the cells exposed to bilirubin or to bilirubin and hypoxia. These sequential changes in synthesis of specific proteins seem to be involved in delayed neuronal death. Bilirubin decreased significantly [3H]2-deoxyglucose incorporation at 24 h while it increased when the neurons were exposed to both bilirubin and hypoxia (+60%, p < 0.001) and decreased thereafter. These data confirm the deleterious effects of bilirubin on neuronal viability, on protein synthesis and metabolic rates. The combination of bilirubin with hypoxia resulted in stronger detrimental effects on neurons than bilirubin alone.

Brain edema after experimental intracerebral hemorrhage: role of hemoglobin degradation products

OBJECT

The mechanisms involved in brain edema formation following intracerebral hemorrhage (ICH) have not been fully elucidated. The authors have found that red blood cell lysis plays an important role in edema development after ICH. In the present study, they sought to determine whether degradation products of hemoglobin cause brain edema.

METHODS

Hemoglobin, hemin, bilirubin, or FeCl2 were infused with stereotactic guidance into the right basal ganglia of Sprague-Dawley rats. The animals were killed 24 hours later to determine brain water and ion contents. Western blot analysis and immunohistochemistry were applied for heme oxygenase-1 (HO-1) measurement. The effects of an HO inhibitor, tin-protoporphyrin (SnPP), and the iron chelator deferoxamine, on hemoglobin-induced brain edema were also examined. Intracerebral infusion of hemoglobin, hemin, bilirubin, or FeCl2 caused an increase in brain water content at 24 hours. The HO-1 was upregulated after hemoglobin infusion and HO inhibition by SnPP-attenuated hemoglobin-induced edema. Brain edema induced by hemoglobin was also attenuated by the intraperitoneal injection of 500 mg/kg deferoxamine.

CONCLUSIONS

Hemoglobin causes brain edema, at least in part, through its degradation products. Limiting hemoglobin degradation coupled with the use of iron chelators may be a novel therapeutic approach to limit brain edema after ICH.

Kernicterus in term and near-term infants--the specter walks again

Diagnostic and therapeutic intervention is common in newborns with neonatal jaundice, motivated by the fear of bilirubin-associated brain damage, kernicterus. In recent years, a resurgence of kernicterus has been noted in countries in which this complication had essentially disappeared. Both early postnatal discharge and relaxation of attitudes to neonatal jaundice have been implicated. Guidelines for the management of neonatal jaundice show significant disparity, attesting to our inadequate understanding of the underlying biology. Aggressive guidelines expose infants to unnecessary risks, risks that are significant when it comes to exchange transfusion, and may also involve improper use of limited resources. Relaxed guidelines, on the other hand, may expose infants to increased risk of brain toxicity.

CONCLUSION

At present we have no tools for ensuring certain identification of individuals with increased vulnerability to bilirubin toxicity. Relaxation of guidelines which have been proven safe through prolonged use should therefore be undertaken only in an atmosphere of increased vigilance. Guidelines that allow for a range of therapeutic and diagnostic options underline the need for careful assessment of each case on its individual merits.

Effect of bilirubin on the activity and thermokinetic characteristics of brain (synaptosomal) membrane NO synthase

NO synthase activity was found in the plasma (synaptosomal) membrane particles isolated from the homogenate of adult rat brain (without cerebellum) under conditions preventing the protease attack and formation of reactive oxygen species. The NO synthase discovered exhibited some properties of a neuronal constitutive integral membrane enzyme and was inhibited by N-nitro-L-arginine. NO synthase activity decreased when bilirubin entered the synaptosomal membrane in vitro. Bilirubin caused the shift of the transition temperature in the temperature dependence of NO synthase activity in Arrhenius plots. The incorporation of bilirubin into synaptosomal membranes resulted in an increase in the apparent activation energy for NO synthase within a temperature range of 10-30 degrees C. The membrane NO synthase was susceptible to the photodynamic effect of membrane-bound bilirubin molecules. Monomeric human serum albumin without organophilic ligands exerted a protective effect on NO synthase in bilirubin-containing membrane particles.

Difference in susceptibilities of different cell lines to bilirubin damage

OBJECTIVE

To investigate if there are differences in susceptibilities to bilirubin toxicity of different cell lines.

METHODOLOGY

A modified 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) method was adopted to study the cytotoxic effect of bilirubin on several commercially available cell lines including human glioblastoma (ATCC CRL 1690, T98G), human neuroblastoma (ATCC HTB-10, SK-N-MC), human liver (ATCC CCL 13, Chang Liver, HeLa markers) and a mouse fibroblast (ATCC CCL-1, NCTC Colon 929).

RESULTS

Cytotoxicity was observed when certain bilirubin:albumin molar ratios were exceeded in the medium of a cell line in culture. Different cells exhibited different susceptibilities to the cytotoxic effects of bilirubin; neuroblastoma and glioblastoma were most susceptible, fibroblasts were the least vulnerable.

CONCLUSIONS

Our findings have confirmed the clinical impression that different cells sustain different degrees of cytotoxicities caused by bilirubin.

Red meat and colon cancer: the cytotoxic and hyperproliferative effects of dietary heme

The intake of a Western diet with a high amount of red meat is associated with a high risk for colon cancer. We hypothesize that heme, the iron carrier of red meat, is involved in diet-induced colonic epithelial damage, resulting in increased epithelial proliferation. Rats were fed purified control diets, or purified diets supplemented with 1.3 micromol/g of hemin (ferriheme), protoporphyrin IX, ferric citrate, or bilirubin (n = 8/group) for 14 days. Feces were collected for biochemical analyses. Fecal cytotoxicity was determined from the degree of lysis of erythrocytes by fecal water. Colonic epithelial proliferation was measured in vivo using [3H]thymidine incorporation into colonic mucosa. The colonic epithelial proliferation in heme-fed rats was significantly increased compared to control rats [55.2 +/- 5.8 versus 32.6 +/- 6.3 dpm/microg DNA (mean +/- SE); P < 0.05]. The fecal water of the heme group was highly cytotoxic compared to the controls (90 +/- 2% versus 2 +/- 1%; P < 0.001), although the concentrations of cytotoxic bile acids and fatty acids were significantly lower. Organic iron was significantly increased compared to the controls (257 +/- 26 versus 80 +/- 21, microM; P < 0.001). Spectrophotometric analyses suggest that this organic iron is heme-associated. Thiobarbituric acid-reactive substances were greatly increased in the fecal water of heme-fed rats compared to the controls (177 +/- 12 versus 59 +/- 7 microM; P < 0.05). Heme itself could not account for the increased cytotoxicity because the addition of heme to the fecal water of the control group, which was equimolar to the organic iron content of the fecal water of the heme group, did not influence the cytotoxicity. Hence, an additional heme-induced cytotoxic factor is involved, which may be modulated by the generation of luminal-reactive oxygen species. Protoporphyrin IX, ferric citrate, and bilirubin did not increase proliferation and cytotoxicity. In conclusion, dietary heme leads to the formation of an unknown, highly cytotoxic factor in the colonic lumen. This suggests that, in heme-fed rats, colonic mucosa is damaged by the intestinal contents. This results in a compensatory hyperproliferation of the epithelium, which supposedly increases the risk for colon cancer.

Experimental bilirubin pigmentation of rat dentine and its detection by a qualitative analytical method

An animal model of bilirubinemia was used to determine whether bilirubin present in pigmented teeth can be extracted and qualitatively analysed. The bile ducts of 10 Long-Evans Agouti rats were ligated and bilirubin (14 mg/kg per day) was injected intraperitoneally for 4 days. When the animals were killed 2 weeks later, pigmented lower incisors were observed in three animals. These teeth were dried, powdered and bilirubin was extracted with chloroform/methanol/acetic acid, 30:10:0.5, v/v for 10 min under sonication. After centrifugation, the supernatant was collected and evaporated. The residue was dissolved in chloroform and its absorption spectrum measured before and after diazo reaction. This resulted in a shift of the absorption maximum from 450 to 540 nm and indicated the presence of bilirubin in pigmented teeth. No bilirubin was found in the lower incisors of untreated control rats. This technique may be useful in distinguishing bilirubin staning from other intrinsic discolorations of teeth.

Does hyperbilirubinemia damage the brain of healthy full-term infants?

In the 1950s, exchange transfusion to keep the total serum bilirubin below 20 mg/dl was shown to be an effective way of preventing kernicterus in babies with erythroblastosis fetalis. For the last 15 to 20 years this level has also been used to determine the need for intervention in healthy full-term infants who do not have hemolytic disease. A critical review of all the available data including six studies from the collaborative perinatal project (more than 30,000 infants) and several smaller studies of term infants without hemolysis reveals essentially no evidence of adverse effects of bilirubin on IQ, neurologic examination, or hearing. The investigation and treatment of normal infants with jaundice is expensive and potentially harmful. We need to reassess our approach to hyperbilirubinemia in healthy full-term infants.

[The toxicity of bilirubin (author's transl)]

This report is a short review about the cytopathogenic effect of bilirubin on the different cells of the human organism. Own investigations deal with the excretion of enzymes in urine in relation to the bilirubin in serum. The second part concerns bilirubin and teeth. Unconjugated bilirubin is only toxic, when it is not coppled to albumin. Therapeutic efforts must consider this aspect.

Bilirubin as a cholestatic agent. III. Prevention of bilirubin-related cholestasis by sulfobromophthalein

The administration of sulfobromophthalein sodium (BSP) to manganese-loaded animals had been found to prevent effectively the cholestasis normally created by bilirubin infusion following manganese loading. Measurement of bilirubin levels in blood, liver, and bile in manganese-bilirubin animals with and without BSP fails to provide convincing evidence for a BSP-induced shift in site or magnitude of total bilirubin concentration as an explanation for this anticholestatic action of BSP. Nevertheless, increasing the dose of bilirubin partially reinstitutes the cholestasis. The results are important in that they demonstrate (1) that under certain circumstances intrahepatic cholestasis can be immediately and directly prevented by a pharmacologic agent (by a means not as yet defined) and (2) that the cholestatic activity of bilirubin is, to some extent or under some circumstances, separable from its concentration in blood, liver, and bile.

Bilirubin as a cholestatic agent. IV. Effect of bilirubin and sulfobromophthalein (BSP) on biliary manganese excretion

These experiments demonstrate that the adminstration of bilirubin does not significantly alter biliary manganese excretion in manganese-loaded animals. Sulfobromophthalein sodium (BSP), which has been shown to afford prophylaxis against manganese-bilirubin cholestasis, significantly increases the biliary excretion of manganese. These results are meaningful in that they indicate that the amount of manganese per se in the bile is not critical in this cholestasis, and suggest that intracanalicular reactions are probably not etiologically critical in this model. They point to the conclusion that the critical cholestatic events are occurring within the hepatocyte. The results also indicate that the biliary excretion of manganese is obligate, and to some extent independent of bile flow. The critical cholestatic events are presumably mediated through the biochemical effects of either a manganese-bilirubin complex or of manganese and bilirubin acting separately but synergistically.