Event Abstract

Back to Event

Regulation of bilirubin entrance into the brain and its effects on glucose uptake in endothelial cells of the blood-brain barrier

Jaime Kapitulnik1*, Clara Benaim1, Guy Cohen1 and Shlomo Sasson1
  • 1 The Hebrew University of Jerusalem, Department of Pharmacology, Institute of Drug Research, School of Pharmacy, Faculty of Medicine, Israel

Bilirubin (BR) is a lipophilic product of heme degradation formed by the concerted action of heme oxygenase, which converts heme into biliverdin (BV), and BV reductase, which reduces BV into BR [1,2]. Under normal circumstances, BR is conjugated in the liver with one or two glucuronic acid residues by the UDP-glucuronosyltransferase isoform UGT1A1. The resulting glucuronides are excreted into bile by the transporters MRP2 and MRP3.

Plasma BR levels are usually elevated in normal newborn infants during the first two weeks of postnatal life due to a marked breakdown of fetal erythrocytes at birth, coupled with a transient inability of the newborn to form BR glucuronides in the liver and excrete them into bile. Once the UGT1A1 enzyme and the biliary excretory system reach maturity, at about 1 month of age, plasma BR levels decrease and reach the adult levels of <20 μM (reviewed in [3]). Neonatal hyperbilirubinemia (jaundice) may worsen because of: 1) increased BR production due to severe hemolysis (e.g., in Rh or ABO incompatibility or in G6PD deficiency); 2) delayed maturation of the hepatic conjugation system (e.g., in prematurely-born neonates); 3) increased enterohepatic circulation of BR; 4) genetic abnormalities (e.g., mutations in the ugt1a1 gene, such as in patients with the Crigler-Najjar syndrome type 1). When the plasma BR levels are excessively elevated and surpass the capacity of albumin for high-affinity binding of BR, the free (unbound) fraction of the pigment increases. This fraction may also be elevated in the plasma of newborns with 'physiologic jaundice' in association with a reduced capacity of plasma albumin for highaffinity binding of BR, resulting from a low blood pH (acidosis) or use of drugs that compete with BR for binding to plasma albumin (e.g., sulfonamides). The highly lipophilic free (unbound) pigment may cross the blood–brain barrier (BBB), precipitate in discrete brain areas such as the basal ganglia (kernicterus), and produce a wide array of neurological deficits collectively known as 'bilirubin encephalopathy'. These include irreversible abnormalities in motor, sensory (auditory and ocular), and cognitive functions (reviewed in [4]). Newborn infants display an increased susceptibility for brain damage because of the temporal 'immaturity' of their BBB.

A host of adverse effects of BR on neuronal and astroglial cells have been reported, including changes in cell morphology and viability, apoptotic cell death, impairment of mitochondrial function leading to uncoupling of oxidative phosphorylation, inhibition of DNA and protein synthesis, changes in carbohydrate metabolism, and modulation of the synthesis, release and uptake of neurotransmitters. Although BR-induced neurotoxicity has been extensively studied for more than half a century, and bilirubin encephalopathy is a subject of great clinical importance, its pathogenesis is still not fully understood (for reviews on UCB neurotoxicity see [5-7]).

BR is a 'double-edged sword'; it displays cytoprotective effects at low concentrations while at higher concentrations it is cytotoxic (reviewed in [6-8]). The final outcome of its cellular effects - toxicity or protection - will depend on the target cell/tissue involved, plasma BR concentration and extent of protein binding, local BR concentration at the target, cellular redox state, and the developmental stage.

Although the highly lipophilic free BR can easily diffuse into the brain, it has become increasingly evident that both the BBB (located at the endothelial cells of brain microvessels) as well as the blood-CSF barrier (located at the epithelial cells of the choroid plexuses (CPs)) play an important role in protecting the brain from the toxic effects of BR. Transport proteins present in the above barriers are involved in preventing BR diffusion into brain cells and stimulating its elimination from the CNS. BR is a substrate for both the MRP1/ABCC1 transporter and P-glycoprotein (P-gp; MDR1/ABCB1), although its apparent affinity for P-gp is lower than that for MRP1 (reviewed in [9]). The localization of P-gp and MRP1 differs markedly between the BBB and the blood-CSF barrier; P-gp is found mainly in brain microvessels while MRP1 is much more abundant in the CP [10]. Although earlier reports suggested that BR elimination from brain is regulated by P-gp [11], a recent study using siRNAs showed that MRP1 plays a major role in BR elimination from a cell line co-expressing both functional MRP1 and P-gp [12]. These findings were not confirmed yet in in vivo studies.

The BBB also regulates the supply of nutrients to the brain. Drugs or endogenous compounds may affect the transport of nutrients, such as glucose, at the BBB. In a previous study [13], primary cultures of bovine aortic endothelial cells (VEC) were exposed to increasing BR concentrations (Fig. 1). BR (1-40 μM) significantly increased the rate of glucose uptake in these cells when exposed to either low or high glucose concentrations (5.5 and 23 mM, respectively). Exposure to BR increased the expression of the glucose transporter 1 (GLUT1) and its plasma membrane localization. Interestingly, BR at the lower concentrations (<20 μM) abolished the high glucose-induced downregulation of the glucose transport system in the VEC, but failed to do so at BR concentrations which exceed the 'normal' values of <20 μM found in human plasma. It was therefore of interest to determine the effect of BR on uptake of glucose in the brain microvascular endothelial cell line bEnd.3. BR, at concentrations up to 20 μM, had no effect on glucose uptake and high glucose-induced downregulation of glucose transport in bEnd.3 cells, in contrast with the results obtained in VEC (Fig. 2), suggesting that brain microvascular endothelial cells may behave differently than their macrovascular counterparts by being resistant to BR-induced changes in nutrient uptake and thus protecting the brain against disturbances in glucose homeostasis.

Acknowledgements

J. Kapitulnik and S. Sasson are members of the David R. Bloom Center for Pharmacy and the Dr. Adolph and Klara Brettler Center for Research in Molecular Pharmacology and Therapeutics at the School of Pharmacy of the Hebrew University of Jerusalem. G. Cohen received a fellowship from the Hebrew University Center for Diabetes Research. This work was supported by grants from the Yedidut Foundation (Mexico), the David R. Bloom Center for Pharmacy and the Dr. Adolph and Klara Brettler Center for Research in Molecular Pharmacology and Therapeutics.

pic 1
Image
pic 2
Image

References

1. Maines, MD (2007) Biliverdin reductase: PKC interaction at the cross-talk of MAPK and PI3K signaling pathways. Antioxid. Redox Signaling 9: 2187-2195

2. Abraham, NG and Kappas, A (2008) Pharmacological and clinical aspects of heme oxygenases. Pharmacol. Rev. 60: 79-127

3. Gourley, GR (1997) Bilirubin metabolism and kernicterus. Adv. Pediatr. 44: 173–229

4. Shapiro SM (2003) Bilirubin toxicity in the developing nervous system. Pediatr. Neurol. 29: 410-421

5. Ostrow, JD, Pascolo, L, Shapiro, SM and Tiribelli, C (2003) New concepts in bilirubin encephalopathy. Eur. J. Clin. Invest. 33: 988-997

6. Kapitulnik, J (2004) Bilirubin: an endogenous product of heme degradation with both cytotoxic and cytoprotective properties. Mol. Pharmacol. 66: 773-779

7. Kapitulnik, J and Maines, MD (2009) Pleiotropic functions of biliverdin reductase: cellular signaling and generation of cytoprotective and cytotoxic bilirubin. Trends Pharmacol. Sci. 30: 129-137

8. Basiglio, CL, Arriaga, SM, Pelusa, F, Almara, AM, Kapitulnik, J and Mottino AD (2010) Complement activation and disease: protective effects of hyperbilirubinaemia. Clin. Sci. 118: 99-113

9. Ghersi-Egea, JF, Gazzin, S and Strazielle, N (2009) Blood-brain interfaces and bilirubininduced neurological diseases. Curr. Pharm. Des. 15: 2893-2907

10. Gazzin, S, Strazielle, N, Schmitt, C, Fevre-Montange, M, Ostrow, JD, Tiribelli, C, et al. (2008) Differential expression of the multidrug resistance-related proteins ABCb1 and ABCc1 between blood-brain interfaces. J. Comp. Neurol. 510: 497-507

11. Watchko, JF, Daood, MJ and Hansen, TW (1998) Brain bilirubin content is increased in Pglycoprotein- deficient transgenic null mutant mice. Pediatr. Res. 44: 763-766

12. Corich, L, Aranda, A, Carrassa, L, Bellarosa, C, Ostrow JD and Tiribelli, C (2009) The cytotoxic effect of unconjugated bilirubin in human neuroblastoma SH-SY5Y cells is modulated by the expression level of MRP1 but not MDR1. Biochem. J. 417: 305-312

13. Cohen, G, Livovsky, DM, Kapitulnik, J and Sasson, S (2006) Bilirubin increases the expression of glucose transporter-1 and the rate of glucose uptake in vascular endothelial cells. Rev. Diabet. Stud. 3: 127-133

Conference: Pharmacology and Toxicology of the Blood-Brain Barrier: State of the Art, Needs for Future Research and Expected Benefits for the EU, Brussels, Belgium, 11 Feb - 12 Feb, 2010.

Presentation Type: Oral Presentation

Topic: Presentations

Citation: Kapitulnik J, Benaim C, Cohen G and Sasson S (2010). Regulation of bilirubin entrance into the brain and its effects on glucose uptake in endothelial cells of the blood-brain barrier. Front. Pharmacol. Conference Abstract: Pharmacology and Toxicology of the Blood-Brain Barrier: State of the Art, Needs for Future Research and Expected Benefits for the EU. doi: 10.3389/conf.fphar.2010.02.00007

Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters.

The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated.

Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed.

For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions.

Received: 23 Feb 2010; Published Online: 23 Feb 2010.

* Correspondence: Jaime Kapitulnik, The Hebrew University of Jerusalem, Department of Pharmacology, Institute of Drug Research, School of Pharmacy, Faculty of Medicine, Jerusalem, Israel, jaimek@savion.huji.ac.il