Hyperbilirubinemic Serum Is Cytotoxic and Induces Apoptosis in Murine Astrocytes

Silva

et al. 2007

Eur J of Neuroscience

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.

Section: Resultsmentioning

confidence: 99%

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

Silva

et al. 2007

Eur J of Neuroscience

“…2001b, 2002; Fernandes et al . 2004; Kumral et al . 2005), we next examined the time course of UCB‐induced astroglial cytotoxicity.…”

Section: Resultsmentioning

confidence: 99%

Inflammatory signalling pathways involved in astroglial activation by unconjugated bilirubin

Journal of Neurochemistry

Silva

et al. 2006

108

During neonatal hyperbilirubinaemia, astrocytes activated by unconjugated bilirubin (UCB) may contibute 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)-a receptor 1 (TNFR1), mitogen-activated protein kinase (MAPK) and nuclear factor jB (NF-jB) 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-jB. Interestingly, the induction of these signal effectors preceded the early up-regulation of TNF-a and interleukin (IL)-1b mRNAs, and later secretion of TNF-a, IL-1b 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-a and IL-1b. Moreover, loss of cell viability and cytokine secretion were reduced when the NF-jB signal transduction pathway was inhibited, suggesting a key role for NF-jB 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.

“…Several toxic effects of UCB have been reported on neuronal and glial cells affecting cell morphology and inducing cell death by necrosis and apoptosis (Amit and Brenner, 1993; Silva et al, 2001, 2002; Genc et al, 2003; Kumral et al, 2005). Here, we found for the first time that UCB induces microglial cell death by apoptosis and necrosis in a concentration‐dependent manner.…”

Section: Discussionmentioning

confidence: 99%

Unconjugated bilirubin activates and damages microglia

Gordo

J of Neuroscience Research

et al. 2006

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.