Hyperbilirubinemic Serum Is Cytotoxic and Induces Apoptosis in Murine Astrocytes

Bilirubin induces microglial NLRP3 inflammasome activation in vitro and in vivo

Despite current advancements in neonatal care, hyperbilirubinemia resulting in bilirubin-induced neurological dysfunction (BIND) continues to be one of the major reasons of mortality or lifelong disability. Although the exact mechanisms underlying brain injury upon bilirubin exposure remains unelucidated, inflammation is considered to be one of the major contributors to BIND. This study investigates the role of the NLRP3 inflammasome in bilirubin-induced injury using in vitro and in vivo models. We successfully demonstrated that the upregulation of NLRP3 expression is significantly associated with the release of active caspase-1 and IL-1β in N9 microglial cells exposed to bilirubin. Functional in vitro experiments with NLRP3 siRNA confirms that bilirubin-induced inflammasome activation and cell death are mediated by the NLRP3 inflammasome. Following injection of bilirubin into the cisterna magna of a neonatal mouse, activation of the NLRP3 inflammasome and microglia were determined by double staining with Iba1-NLRP3 and Iba1-Caspase-1. Upon injection of bilirubin into the cisterna magna, neuronal loss was significantly higher in the wild-type mouse compared to Nlrp3^-/- and Caspase-1^-/- strains. Collectively, these data indicate that NLRP3 inflammasome has a crucial role in microglial activation and bilirubin-induced neuronal damage.

Roles of Bilirubin in Hemorrhagic Transformation of Different Types and Severity

BACKGROUND

Hemorrhagic transformation (HT) is a severe complication in patients with acute ischemic stroke (AIS). This study was performed to explore and validate the relation between bilirubin levels and spontaneous HT (sHT) and HT after mechanical thrombectomy (tHT).

METHODS

The study population consisted of 408 consecutive AIS patients with HT and age- and sex-matched patients without HT. All patients were divided into quartiles according to total bilirubin (TBIL) level. HT was classified as hemorrhagic infarction (HI) and parenchymal hematoma (PH) based on radiographic data.

RESULTS

In this study, the baseline TBIL levels were significantly higher in the HT than non-HT patients in both cohorts (p < 0.001). Furthermore, the severity of HT increased with increasing TBIL levels (p < 0.001) in sHT and tHT cohorts. The highest quartile of TBIL was associated with HT in sHT and tHT cohorts (sHT cohort: OR = 3.924 (2.051-7.505), p < 0.001; tHT cohort: OR = 3.557 (1.662-7.611), p = 0.006).

CONCLUSIONS

Our results suggest that an increased TBIL is associated with a high risk of patients with sHT and tHT, and that TBIL is more suitable as a predictor for sHT than tHT. These findings may help to identify patients susceptible to different types and severity of HT.

Predictive value of different bilirubin subtypes for clinical outcomes in patients with acute ischemic stroke receiving thrombolysis therapy

Aims

To explore the association of total bilirubin (TBIL), direct bilirubin (DBIL), and indirect bilirubin (IBIL) levels with, as well as the incremental predictive value of different bilirubin subtypes for, poor outcomes in acute ischemic stroke patients after thrombolysis.

Methods

We analyzed 588 individuals out of 718 AIS participants, and all patients were followed up at 3 months after thrombolysis. The primary outcome was 3‐month death and major disability (modified Rankin Scale (mRS) score of 3–6). The secondary outcomes were 3‐month mortality (mRS score of 6), moderate‐severe cerebral edema, and symptomatic intracranial hemorrhage (sICH), respectively.

Results

Elevated DBIL pre‐thrombolysis was associated with an increased risk of primary outcome (OR 3.228; 95% CI 1.595–6.535; p for trend = 0.014) after fully adjustment. Elevated TBIL pre‐thrombolysis showed the similar results (OR 2.185; 95% CI 1.111–4.298; p for trend = 0.047), while IBIL pre‐thrombolysis was not significantly associated with primary outcome (OR 1.895; 95% CI 0.974–3.687; p for trend = 0.090). Multivariable‐adjusted spline regression model showed a positive linear dose‐response relationship between DBIL pre‐thrombolysis and risk of primary outcome (p for linearity = 0.004). Adding DBIL pre‐thrombolysis into conventional model had greater incremental predictive value for primary outcome, with net reclassification improvement (NRI) 95% CI = 0.275 (0.084–0.466) and integrated discrimination improvement (IDI) 95% CI = 0.011 (0.001–0.024). Increased DBIL post‐thrombolysis had an association with primary outcome (OR 2.416; 95%CI 1.184–4.930; p for trend = 0.039), and it also elevated the incremental predictive value for primary outcome, with NRI (95% CI) = 0.259 (0.066–0.453) and IDI (95% CI) = 0.025 (0.008–0.043).

Conclusion

Increased DBIL pre‐thrombolysis had a stronger association with, as well as greater incremental predictive value for, poor outcomes than TBIL and IBIL did in AIS patients after thrombolysis, which should be understood in the context of retrospective design. The effect of DBIL on targeted populations should be investigated in further researches.

The Effects of Bilirubin and Lumirubin on the Differentiation of Human Pluripotent Cell-Derived Neural Stem Cells

The ‘gold standard’ treatment of severe neonatal jaundice is phototherapy with blue–green light, which produces more polar photo-oxidation products that are easily excreted via the bile or urine. The aim of this study was to compare the effects of bilirubin (BR) and its major photo-oxidation product lumirubin (LR) on the proliferation, differentiation, morphology, and specific gene and protein expressions of self-renewing human pluripotent stem cell-derived neural stem cells (NSC). Neither BR nor LR in biologically relevant concentrations (12.5 and 25 µmol/L) affected cell proliferation or the cell cycle phases of NSC. Although none of these pigments affected terminal differentiation to neurons and astrocytes, when compared to LR, BR exerted a dose-dependent cytotoxicity on self-renewing NSC. In contrast, LR had a substantial effect on the morphology of the NSC, inducing them to form highly polar rosette-like structures associated with the redistribution of specific cellular proteins (β-catenin/N-cadherin) responsible for membrane polarity. This observation was accompanied by lower expressions of NSC-specific proteins (such as SOX1, NR2F2, or PAX6) together with the upregulation of phospho-ERK. Collectively, the data indicated that both BR and LR affect early human neurodevelopment in vitro, which may have clinical relevance in phototherapy-treated hyperbilirubinemic neonates.

Minocycline protects neurons against glial cells-mediated bilirubin neurotoxicity

Unconjugated bilirubin, the end product of heme catabolism and antioxidant, induced brain damage in human neonates is a well-recognized clinical syndrome. However, the cellular and molecular mechanisms underlying bilirubin neurotoxicity remain unclear. To characterize the sequence of events leading to bilirubin-induced neurotoxicity, we investigated whether bilirubin-induced glial activation was involved in bilirubin neurotoxicity by exposing co-cultured rat glial cells and cerebellar granule neurons (CGN) to bilirubin. We found that bilirubin could markedly induce the expression of TNF-α and iNOS in glial cells, and even at low concentrations, the co-culture of glial cells with neurons significantly enhances neurotoxicity of bilirubin. Pretreatment of the co-cultured cells with minocycline protected CGN from glia-mediated bilirubin neurotoxicity and inhibited overexpression of TNF-α and iNOS in glia. Furthermore, we found that high doses of bilirubin were able to induce glial injury, and minocycline attenuated bilirubin-induced glial cell death. Our data suggest that glial cells play an important role in brain damage caused by bilirubin, and minocycline blocks bilirubin-induced encephalopathy possibly by directly and indirectly inhibiting neuronal death pathways.

Caffeine prevents bilirubin-induced cytotoxicity in cultured newborn rat astrocytes

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Bilirubin and its oxidation products damage brain white matter

Brain injury after intracerebral hemorrhage (ICH) occurs in cortex and white matter and may be mediated by blood breakdown products, including hemoglobin and heme. Effects of blood breakdown products, bilirubin and bilirubin oxidation products, have not been widely investigated in adult brain. Here, we first determined the effect of bilirubin and its oxidation products on the structure and function of white matter in vitro using brain slices. Subsequently, we determined whether these compounds have an effect on the structure and function of white matter in vivo. In all, 0.5 mmol/L bilirubin treatment significantly damaged both the function and the structure of myelinated axons but not the unmyelinated axons in brain slices. Toxicity of bilirubin in vitro was prevented by dimethyl sulfoxide. Bilirubin oxidation products (BOXes) may be responsible for the toxicity of bilirubin. In in vivo experiments, unmyelinated axons were found more susceptible to damage from bilirubin injection. These results suggest that unmyelinated axons may have a major role in white-matter damage in vivo. Since bilirubin and BOXes appear in a delayed manner after ICH, preventing their toxic effects may be worth investigating therapeutically. Dimethyl sulfoxide or its structurally related derivatives may have a potential therapeutic value at antagonizing axonal damage after hemorrhagic stroke.

Bilirubin injury to neurons and glial cells: new players, novel targets, and newer insights

Encephalopathy by hyperbilirubinemia in infants has been described for decades, but neither the underlying cellular and molecular mechanisms nor the selective pattern of bilirubin deposition in the brain is well understood. The brain is composed of highly specialized and diverse populations of cells, represented by neurons and glia that comprise astrocytes, oligodendrocytes, and microglia. Although microscopic evaluation of icteric brain sections revealed bilirubin within neurons, neuronal processes, and microglia, cell dependent-sensitivity to bilirubin toxicity and the role of each nerve cell type are poorly understood. Even less considered are glial and neuronal pathologic alterations as integrated phenomena. The available knowledge on reactivity of glial cells to bilirubin and on the impairment to neuronal network dynamics that it causes, here summarized, suggests that a better comprehension of the interplay between neurons and glia is essential to understand bilirubin neurotoxicity and highlight potential molecular targets that may lead to disease-modifying therapeutic approaches.

Glycoursodeoxycholic Acid and Interleukin-10 Modulate the Reactivity of Rat Cortical Astrocytes to Unconjugated Bilirubin

The pathogenesis of bilirubin encephalopathy seems to result from accumulation of unconjugated bilirubin (UCB) within the brain. We have recently demonstrated that UCB causes astroglial release of proinflammatory cytokines and glutamate, as well as cell death. The bile acid glycoursodeoxycholic acid (GUDCA) and the anti-inflammatory cytokine interleukin (IL)-10 have been reported to modulate inflammation and cell survival. In this study we investigated the effect of these therapeutic agents on the astroglial response to UCB. Only GUDCA prevented UCB-induced astroglial death. The secretion of tumor necrosis factor-alpha (TNF-alpha) and IL-1beta elicited by UCB in astrocytes was reduced in the presence of GUDCA and IL-10, whereas the suppression of IL-6 was only counteracted by GUDCA. Neither GUDCA nor IL-10 modulated the accumulation of extracellular glutamate. Additionally, IL-10 markedly inhibited UCB-induced nuclear factor-kappaB nuclear translocation and cytokine mRNA expression, whereas GUDCA only prevented TNF-alpha mRNA expression. Moreover, GUDCA inhibited TNF-alpha- and IL-1beta-converting enzymes, preventing the maturation of these cytokines and their consequent release. Collectively, this study shows that IL-10 action is restricted to UCB-induced release of TNF-alpha and IL-1beta from the astrocytes, whereas GUDCA presents a more ubiquitous action on the astroglial reactivity to UCB. Hence, GUDCA may have potential benefits over an IL-10 therapeutic approach in reducing UCB-induced astrocyte immunostimulation and death.

MAPKs are key players in mediating cytokine release and cell death induced by unconjugated bilirubin in cultured rat cortical astrocytes

When activated by unconjugated bilirubin (UCB), astrocytes are important sources of inflammatory mediators such as TNF-alpha, IL-1beta and IL-6, which may contribute for the neurotoxicity observed during severe neonatal hyperbilirubinemia. In the present study, we have addressed the role of the mitogen-activated protein kinases (MAPKs) p38, Jun N-terminal kinase (JNK)1/2 and extracellular signal-regulated kinase (ERK)1/2 pathways and their relation with the nuclear factor kappaB (NF-kappaB) cascade in the signalling events involved in cytokine release and cell death caused by UCB in primary cultures of rat astrocytes. Stimulation of astrocytes with UCB in the presence of all the MAPK inhibitors prevented UCB-induced release of TNF-alpha and IL-6, while IL-1beta secretion was only reduced by JNK1/2 and ERK1/2 inhibitors. In addition, activation of the NF-kappaB transcription factor, needed for cytokine release by UCB-stimulated astrocytes, was shown to be dependent on JNK1/2 and ERK1/2 phosphorylation. Moreover, all MAPK inhibitors prevented astroglial apoptosis triggered by UCB. Interestingly, UCB-induced lactate dehydrogenase release was prevented by blockade of JNK1/2, ERK1/2 and NF-kappaB cascades but enhanced by p38 inhibition. Taken together, our data demonstrate for the first time that MAPK transduction pathways are key players in the UCB-induced inflammatory response and cell death in astrocytes, probably also involving NF-kappaB modulation. These findings contribute to unraveling the complex mechanisms of astrocyte reactivity to UCB and may ultimately prove useful in the development of new therapeutic strategies to prevent nerve cell damage during acute bilirubin encephalopathy.

Influence of hypoxia and ischemia preconditioning on bilirubin damage to astrocytes

Hypoxia-ischemia in the perinatal period is a common cause of neurologic disability in children and is often associated with neonatal morbidity and mortality. Another frequent condition of the newborn is hyperbilirubinemia and it is well known that deposition of unconjugated bilirubin (UCB) in the central nervous system can damage nerve cells and cause encephalopathy. Interestingly, some studies report the onset of cerebral hypoxia-ischemia as a risk factor for UCB encephalopathy, since that condition often precedes neonatal hyperbilirubinemia. However, the cellular mechanisms triggered by hypoxia-ischemia that may enforce UCB deleterious effects are not well elucidated. Therefore, we designed this study to investigate whether hypoxia (HP) or combined oxygen-glucose deprivation (OGD) followed by reoxygenation, modifies glial cell susceptibility to UCB injury. Thus, cultured astrocytes were exposed to HP or OGD for 4 h and returned to normoxic conditions for another 12 h prior to incubation with UCB for 4 h. HP and OGD effects in UCB toxicity were compared to normoxic conditions. Our results demonstrate that HP and OGD preconditioning increase the vulnerability of glial cells to UCB damage by enhancing some of the deleterious effects of UCB, namely cell death by both apoptosis and necrosis. This preconditioning also augments the UCB-induced stimulation of an inflammatory response by an effect that involves the activation of the nuclear factor kappaB activation. These findings provide a novel basis for the increased risk of brain damage in jaundiced newborns that were previously exposed to hypoxia or ischemia during the perinatal period, namely during delivery.

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.

Unconjugated bilirubin activates and damages microglia

Microglia are the resident immune cells of the brain and are the principal source of cytokines produced during central nervous system inflammation. We have previously shown that increased levels of unconjugated bilirubin (UCB), which can be detrimental to the central nervous system during neonatal life, induce the secretion of inflammatory cytokines and glutamate by astrocytes. Nevertheless, the effect of UCB on microglia has never been investigated. Hence, the main goal of the present study was to evaluate whether UCB leads to microglial activation and to the release of the cytokines tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta, and IL-6. Additionally, we investigated the effects of UCB on glutamate efflux and cell death. The results showed that UCB induces morphological changes characteristic of activated microglia and the release of high levels of TNF-alpha, IL-1beta, and IL-6 in a concentration-dependent manner. In addition, UCB triggered extracellular accumulation of glutamate and an increased cell death by apoptosis and necrosis. These results demonstrate, for the first time, that UCB is toxic to microglial cells and point to microglia as an important target of UCB in the central nervous system. Moreover, they suggest that UCB-induced cytokine production, by mediating cell injury, can further contribute to exacerbate neurototoxicity. Interestingly, microglia cells are much more responsive to UCB than astrocytes. Collectively, these data indicate that microglia may play an important role in the pathogenesis of encephalopathy during severe hyperbilirubinemia.