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Heme oxygenase in liver transplantation: Heme catabolism and metabolites in the search of function
EDITORIALS Heme Oxygenase in Liver Transplantation: Heme Catabolism and Metabolites in the Search of Function See Article on Page 364
I
schemia and reperfusion (I/R), reflecting antigen-independent components of the “harvesting injury” deeply affect transplantation outcome. I/R injury may not only cause hyperacute primary nonfunction of the graft, but may also sustain acute as well as chronic rejection.1 Thus, control of factors responsible for harvesting injury might not only improve outcome in individual graft recipients but, due to higher susceptibility of marginal livers to I/R injury, may help to increase the number of organs available for successful transplantation.2 Generation of reactive oxygen species subsequent to reoxygenation not only is one of the critical factors inflicting tissue injury during organ procurement and transplantation,3 but also initiates an adaptive hepatocellular stress response.4 Transcriptional activation of the heme oxygenase-1 (HO-1) gene reflects an integral part of this “oxidative stress response” of the liver,5 although the functional implications of HO-1 induction as well as its modes of action remain somewhat enigmatic.6,7 In this issue of HEPATOLOGY, Kato et al. address in depth the role of bilirubin, a product of the HO pathway in mediating the beneficial effects of HO-1 gene expression during ischemia and reperfusion, suggesting that HO-1 is more than a simple “cellular waste management system.”8
ing proteins are abundantly present in the liver, and heme derived from these cellular sources is accumulating on hepatocellular injury and is highly cytotoxic. The concentration of “free” heme in parenchymal liver cells is critical for hepatocellular homeostasis and is tightly controlled by a meticulous balance of synthesis and degradation under physiologic as well as pathophysiologic conditions. Although regulation of heme biosynthesis is achieved through modulation of ␦-aminolevulinic acid synthase activity, heme catabolism is controlled by microsomal HO.9 Among the different isoenzymes cloned and characterized to date, only HO-1 can be induced by a wide variety of seemingly distinct stimuli, including accumulation of free heme as well as cold or warm ischemia, which are linked by their ability to provoke oxidative stress.10-12 Constitutive expression of HO-1 in the liver is restricted to Kupffer cells, but the gene is inducible in nonparenchymal as well as parenchymal liver cells upon I/R through oxygen-free radical triggered activation of activator protein 1, a redox-sensitive transcription factor.4 Blockade of the HO pathway on reperfusion from ischemia by false substrates, such as Sn- or Zn-protoporphyrin-IX, increases injury under most experimental conditions,13-16 whereas induction of the gene by pharmacologic preconditioning or gene transfer reflects a promising approach for preventing reperfusion injury even in marginal grafts.2,15,16
Accumulation of “Free” Heme, Heme Oxygenase, and Hepatic Oxidative Stress
By-products of Heme Degradation: The Good, the Bad, and the Ugly
Prosthetic heme groups serve by virtue of their active iron center, which carries a high affinity for molecular oxygen and can donate electrons, as catalytic sites tightly bound to proteins, such as respiratory chain or synthetic and degradative p450 cytochromes. Thus, heme-contain-
HO isozymes catalyze the oxidative cleavage of the ␣-mesocarbon of heme, yielding equimolar amounts of biliverdin-IXa, free divalent iron (Fe2⫹), and carbon monoxide (CO)9 (Fig. 1). Whereas removal of the prooxidant heme is intuitively cytoprotective, generation of the by-products biliverdin, CO, and Fe2⫹ is a risky business as they are all inherently toxic. Thus, the HO pathway has classically been considered as a sink to deal with excessive amounts of heme proteins accumulating on cellular damage. The products of this pathway received little attention because their biological functions were, at best, obscure. The seminal discovery that nitric oxide (NO), a gaseous compound, may act as a cellular messenger prompted a flurry of studies addressing the potential of CO as an alternative signaling system mediating protective effects of HO.17 In 1995, Suematsu et al. provided compelling evidence that CO rather than NO maintains
Abbreviations: I/R, ischemia reperfusion; HO-1, heme oxygenase-1; Fe2⫹, free divalent iron; CO, carbon monoxide; NO, nitric oxide; cGMP, cyclic guanosine monophosphate; sGC, soluble guanylyl cyclase. From the Department of Anesthesiology and Critical Care Medicine, University of Saarland, Homburg, Germany. Supported by DFG grants Ba1601/1-1, 1-2, and 1-3. Address reprint requests to: Michael Bauer, M.D., Associate Professor, Department of Anesthesiology and Critical Care Medicine, University of the Saarland, D-66421 Homburg, Germany. E-mail: [email protected]; fax: (49) 6841-162-2833. Copyright © 2003 by the American Association for the Study of Liver Diseases. 0270-9139/03/3802-0003$30.00/0 doi:10.1053/jhep.2003.50360 286
HEPATOLOGY, Vol. 38, No. 2, 2003
Fig. 1. Heme degradation and products of the HO pathway. HO catalyzes the oxidative cleavage of the ␣-mesocarbon yielding equimolar amounts of biliverdin, Fe2⫹, and CO.
the low vascular tone of liver sinusoids via cyclic guanosine monophosphate (cGMP)-mediated relaxation of hepatic stellate cells,18 an effect that is particularly emminent when the HO-1 gene is induced on oxidative stress.19 Under these conditions, the HO/CO system can set off an increased vasoconstrictor tone, e.g., resulting from release of endothelin-1 on reperfusion from ischemia.20,21 Although both gases can bind to the prosthetic heme moiety of soluble guanylyl cyclase (sGC), the impact of CO on sGC activity (approximately a 5-fold increase in activity of the ␣11 heterodimeric isoform in vitro) is rather moderate when compared with NO (inducing a 100- to 400-fold increase in activity) and the exact mechanisms of CO-mediated vasodilation is still a matter of controversy.22,23 However, there is increasing evidence that CO acts largely via sGC-cGMP–independent pathways and that its cytoprotective modes of action are not confined to vasodilation: For example, CO has been shown lately to confer protection via the mitogen kinase pathway against apoptotic and inflammatory stimuli,24 both of which can contribute to hepatic I/R injury. The release of Fe2⫹ during oxidative cleavage of the heme molecule is a particular matter of concern, because Fe2⫹, like other transition metal ions, catalyzes the formation of oxygen-free radicals in the Haber-Weiss or Fenton reaction. Therefore, the release of iron can mitigate any antioxidant actions of HO-1 gene expression and may explain the rather narrow threshold of overexpression of HO-1 conferring protection.25,26 Nevertheless, release of iron fosters gene expression of ferritin, a protein imparting additive cytoprotection against oxidative stress.27 The potential toxic actions of bile pigments range from itching in jaundiced patients to severe neuronal damage primarily of basal ganglia as observed in icterus neonatorum. Although neuronal tissue seems to be particularly susceptible to toxic actions of bile pigments, a more gen-
BAUER
287
eral toxic action of bile pigments is assumed to result from damage of lipid bilayers of biological membranes.28 Thus, cytoprotection by bilirubin is not an intuitively intriguing concept to most hepatologists, and removal of bile pigments is one potential beneficial factor of modern supportive treatment strategies in acute liver failure, such as albumin dialysis.29 On the other hand, cells primed by a wide variety of toxic compounds, such as heavy metals, develop tolerance to subsequent, otherwise lethal injury. Moreover, there is good evidence to suggest a significant role for bile pigments as a (hepato)cellular antioxidant sytem.30 Regarding the latter, Kato et al. provide evidence that a simple short-term rinse of the harvested liver with micromolar amounts of unconjugated bilirubin attenuates biliary dysfunction and cell injury both ex vivo in the isolated perfused rat liver as well as in an in vivo transplant model in the rat.8 This simple technique is as effective as pharmacologic preconditioning with hemin, which induces a cellular stress response and presumably acts cytoprotective via HO-1 gene expression. The investigators not only characterize a potential promising conditioning technique to attenuate harvesting injury, but they also try to shed light on the mechanisms by which HO induction by various preconditioning techniques confers protection: While short-term addition of bilirubin to the perfusate is able to ameliorate I/R injury even when the HO pathway is inhibited, supplementation of exogenous CO failed to confer protection in ex vivo asanguineously perfused livers. These results are at odds with previous reports that CO supplementation attenuates harvesting injury in an
Fig. 2. HO-1 immunoreactive protein in normal human liver tissue obtained (A) during liver resection surgery and (B) in a liver graft stored in UW-solution prior to liver transplantation. Whereas HO-1 immunoreactive protein is restricted to Kupffer cells under physiologic conditions, the gene is induced in the graft of a brain dead donor in parenchymal cells primarily of the pericentral region as well.
288
BAUER
isolated perfused liver preparation with red cells added to the buffer.31 Although the exact mechanisms by which manipulation of the HO system confers protection still remain unclear and may depend on experimental conditions and model systems studied, it is obvious that treatment of the brain dead donor in an intensive care unit for hours or even days prior to organ preservation is likely to induce a stress response involving HO-1 gene expression (Fig. 2) even in the absence of any further “pharmacologic preconditioning.” Thus, studies using unprimed healthy animals as organ donors may overestimate any protective effect of induction of a stress response in graft donors. Nevertheless, bilirubin rinse may reflect a simple strategem against post-transplantation reperfusion injury, although its superiority over other antioxidants or additional modes of action besides scavenging of oxygenfree radicals remain to be delineated. Acknowledgment: The author is indebted to Martin K. Schilling, Professor and Chair, Department of Surgery, University of Saarland for providing tissue samples and critical discussion of the manuscript. MICHAEL BAUER, M.D.
Department of Anesthesiology and Critical Care Medicine University of Saarland Homburg, Germany
References 1. Howard TK, Klintmalm GB, Cofer JB, Husberg BS, Goldstein RM, Gonwa TA. The influence of preservation injury on rejection in the hepatic transplant recipient. Transplantation 1990;49:103-107. 2. Amersi F, Buelow R, Kato H, Ke B, Coito AJ, Shen XD, Zhao D, et al. Upregulation of heme oxygenase-1 protects genetically fat Zucker rat livers from ischemia/reperfusion injury. J Clin Invest 1999;104:1631-1639. 3. Jaeschke H. Preservation injury: mechanisms, prevention and consequences. J Hepatol 1996;25:774-780. 4. Rensing H, Jaeschke H, Bauer I, Patau C, Datene V, Pannen BH, Bauer M. Differential activation pattern of redox-sensitive transcription factors and stress-inducible dilator systems heme oxygenase-1 and inducible nitric oxide synthase in hemorrhagic and endotoxic shock. Crit Care Med 2001; 29:1962-1971. 5. Bauer M, Bauer I. Heme oxygenase-1: redox regulation and role in the hepatic response to oxidative stress. Antioxid Redox Signal 2002;4:749-758. 6. Maines MD. Heme oxygenase: function, multiplicity, regulatory mechanisms, and clinical applications. FASEB J 1988;2:2557-2568. 7. Choi AMK, Alam J. Heme oxygenase-1: function, regulation, and implication of a novel stress-inducible protein in oxidant-induced lung injury. Am J Respir Cell Mol Biol 1996;15:9-19. 8. Kato Y, Shimazu M, Kondo M, Uchida K, Kumamoto Y, Wakabayashi G, Kitajima M, et al. Bilirubin rinse: a simple protectant against the rat liver graft injury mimicking heme oxygenase-1 preconditioning. HEPATOLOGY 2003;38:364-373. 9. Tenhunen R, Marver HS, Schmid R. The enzymatic conversion of heme to bilirubin by microsomal heme oxygenase. Proc Natl Acad Sci U S A 1968; 61:748-755. 10. Keyse SM, Tyrrell RM. Heme oxygenase is the major 32-kDa stress protein induced in human skin fibroblasts by UVA radiation, hydrogen peroxide, and sodium arsenite. Proc Natl Acad Sci U S A 1989;86:99-103.
HEPATOLOGY, August 2003
11. Shibahara S, Muller RM, Taguchi H. Transcriptional control of rat heme oxygenase by heat shock. J Biol Chem 1985;262:12889-12892. 12. Bauer I, Wanner GA, Rensing H, Alte C, Miescher EA, Wolf B, Pannen BH, et al. Expression pattern of heme oxygenase isoenzymes 1 and 2 in normal and stress-exposed rat liver. HEPATOLOGY 1998;27:829-838. 13. Rensing H, Bauer I, Datene V, Patau C, Pannen BH, Bauer M. Differential expression pattern of heme oxygenase-1/heat shock protein 32 and nitric oxide synthase-II and their impact on liver injury in a rat model of hemorrhage and resuscitation. Crit Care Med 1999;27:2766-2775. 14. Pannen BH, Kohler N, Hole B, Bauer M, Clemens MG, Geiger KK. Protective role of endogenous carbon monoxide in hepatic microcirculatory dysfunction after hemorrhagic shock in rats. J Clin Invest 1998;102: 1220-1228. 15. Kato H, Amersi F, Buelow R, Melinek J, Coito AJ, Ke B, Busuttil RW, et al. Heme oxygenase-1 overexpression protects rat livers from ischemia/ reperfusion injury with extended cold preservation. Am J Transplant 2001; 1:121-128. 16. Redaelli CA, Tian YH, Schaffner T, Ledermann M, Baer HU, Dufour JF. Extended preservation of rat liver graft by induction of heme oxygenase-1. HEPATOLOGY 2002;35:1082-1092. 17. Furchgott RF, Jothianandan D. Endothelium-dependent and -independent vasodilation involving cyclic GMP: relaxation induced by nitric oxide, carbon monoxide and light. Blood Vessels 1991;28:52-61. 18. Suematsu M, Goda N, Sano T, Kashiwagi S, Egawa T, Shinoda Y, Ishimura Y. Carbon monoxide: an endogenous modulator of sinusoidal tone in the perfused rat liver. J Clin Invest 1995;96:2431-2437. 19. Bauer M, Pannen BHJ, Bauer I, Herzog C, Wanner GA, Hanselmann R, Zhang JX, et al. Evidence for a functional link between stress response and vascular control in hepatic portal circulation. Am J Physiol (Gastrointest Liver Physiol) 1996;271:G929-G935. 20. Wakabayashi Y, Takamiya R, Mizuki A, Kyokane T, Goda N, Yamaguchi T, et al. Carbon monoxide overproduced by heme oxygenase-1 causes a reduction of vascular resistance in perfused rat liver. Am J Physiol (Gastrointest Liver Physiol) 1999;277:G1088-G1096. 21. Rensing H, Bauer I, Zhang JX, Paxian M, Pannen BH, Yokoyama Y, Clemens MG, et al. Endothelin-1 and heme oxygenase-1 as modulators of sinusoidal tone in the stress-exposed rat liver. HEPATOLOGY 2002;36:1453-1465. 22. Stone JR, Marletta MA. Soluble guanylate cyclase from bovine lung: activation with nitric oxide and carbon monoxide and spectral characterization of the ferrous and ferric states. Biochemistry 1994;33:5636-5640. 23. Naik JS, Walker BR. Heme oxygenase-mediated vasodilation involves vascular smooth muscle cell hyperpolarization. Am J Physiol (Heart Circ Physiol) 2003;285:H220-H228. 24. Otterbein LE, Bach FH, Alam J, Soares M, Tao LH, Wysk M, Davis RJ, et al. Carbon monoxide has anti-inflammatory effects involving the mitogenactivated protein kinase pathway. Nat Med 2000;6:422-428. 25. Ryter SW, Tyrrell RM. The heme synthesis and degradation pathways: role in oxidant sensitivity. Heme oxygenase has both pro-and antioxidant properties. Free Radic Biol Med 2000;28:289-309. 26. Suttner DM, Dennery PA. Reversal of HO-1 related cytoprotection with increased expression is due to reactive iron. FASEB J 1999;13:1800-1809. 27. Regan RF, Kumar N, Gao F, Guo Y. Ferritin induction protects cortical astrocytes from heme-mediated oxidative injury. Neuroscience 2002;113: 985-994. 28. Wennberg RP. Cellular basis of bilirubin toxicity. N Y State J Med 1991; 91:493-496. 29. Mitzner SR, Stange J, Klammt S, Risler T, Erley CM, Bader BD, et al. Improvement of hepatorenal syndrome with extracorporeal albumin dialysis MARS: results of a prospective, randomized, controlled clinical trial. Liver Transplant 2000;6:277-286. 30. Stocker R, Yamamoto Y, McDonagh AF, Glazer AN, Ames BN. Bilirubin is an antioxidant of possible physiological importance. Science 1987;235: 1043-1046. 31. Amersi F, Shen XD, Anselmo D, Melinek J, Iyer S, Southard DJ, Katori M, et al. Ex vivo exposure to carbon monoxide prevents hepatic ischemia/ reperfusion injury through p38 MAP kinase pathway. HEPATOLOGY 2002; 35:815-823.
I
schemia and reperfusion (I/R), reflecting antigen-independent components of the “harvesting injury” deeply affect transplantation outcome. I/R injury may not only cause hyperacute primary nonfunction of the graft, but may also sustain acute as well as chronic rejection.1 Thus, control of factors responsible for harvesting injury might not only improve outcome in individual graft recipients but, due to higher susceptibility of marginal livers to I/R injury, may help to increase the number of organs available for successful transplantation.2 Generation of reactive oxygen species subsequent to reoxygenation not only is one of the critical factors inflicting tissue injury during organ procurement and transplantation,3 but also initiates an adaptive hepatocellular stress response.4 Transcriptional activation of the heme oxygenase-1 (HO-1) gene reflects an integral part of this “oxidative stress response” of the liver,5 although the functional implications of HO-1 induction as well as its modes of action remain somewhat enigmatic.6,7 In this issue of HEPATOLOGY, Kato et al. address in depth the role of bilirubin, a product of the HO pathway in mediating the beneficial effects of HO-1 gene expression during ischemia and reperfusion, suggesting that HO-1 is more than a simple “cellular waste management system.”8
ing proteins are abundantly present in the liver, and heme derived from these cellular sources is accumulating on hepatocellular injury and is highly cytotoxic. The concentration of “free” heme in parenchymal liver cells is critical for hepatocellular homeostasis and is tightly controlled by a meticulous balance of synthesis and degradation under physiologic as well as pathophysiologic conditions. Although regulation of heme biosynthesis is achieved through modulation of ␦-aminolevulinic acid synthase activity, heme catabolism is controlled by microsomal HO.9 Among the different isoenzymes cloned and characterized to date, only HO-1 can be induced by a wide variety of seemingly distinct stimuli, including accumulation of free heme as well as cold or warm ischemia, which are linked by their ability to provoke oxidative stress.10-12 Constitutive expression of HO-1 in the liver is restricted to Kupffer cells, but the gene is inducible in nonparenchymal as well as parenchymal liver cells upon I/R through oxygen-free radical triggered activation of activator protein 1, a redox-sensitive transcription factor.4 Blockade of the HO pathway on reperfusion from ischemia by false substrates, such as Sn- or Zn-protoporphyrin-IX, increases injury under most experimental conditions,13-16 whereas induction of the gene by pharmacologic preconditioning or gene transfer reflects a promising approach for preventing reperfusion injury even in marginal grafts.2,15,16
Accumulation of “Free” Heme, Heme Oxygenase, and Hepatic Oxidative Stress
By-products of Heme Degradation: The Good, the Bad, and the Ugly
Prosthetic heme groups serve by virtue of their active iron center, which carries a high affinity for molecular oxygen and can donate electrons, as catalytic sites tightly bound to proteins, such as respiratory chain or synthetic and degradative p450 cytochromes. Thus, heme-contain-
HO isozymes catalyze the oxidative cleavage of the ␣-mesocarbon of heme, yielding equimolar amounts of biliverdin-IXa, free divalent iron (Fe2⫹), and carbon monoxide (CO)9 (Fig. 1). Whereas removal of the prooxidant heme is intuitively cytoprotective, generation of the by-products biliverdin, CO, and Fe2⫹ is a risky business as they are all inherently toxic. Thus, the HO pathway has classically been considered as a sink to deal with excessive amounts of heme proteins accumulating on cellular damage. The products of this pathway received little attention because their biological functions were, at best, obscure. The seminal discovery that nitric oxide (NO), a gaseous compound, may act as a cellular messenger prompted a flurry of studies addressing the potential of CO as an alternative signaling system mediating protective effects of HO.17 In 1995, Suematsu et al. provided compelling evidence that CO rather than NO maintains
Abbreviations: I/R, ischemia reperfusion; HO-1, heme oxygenase-1; Fe2⫹, free divalent iron; CO, carbon monoxide; NO, nitric oxide; cGMP, cyclic guanosine monophosphate; sGC, soluble guanylyl cyclase. From the Department of Anesthesiology and Critical Care Medicine, University of Saarland, Homburg, Germany. Supported by DFG grants Ba1601/1-1, 1-2, and 1-3. Address reprint requests to: Michael Bauer, M.D., Associate Professor, Department of Anesthesiology and Critical Care Medicine, University of the Saarland, D-66421 Homburg, Germany. E-mail: [email protected]; fax: (49) 6841-162-2833. Copyright © 2003 by the American Association for the Study of Liver Diseases. 0270-9139/03/3802-0003$30.00/0 doi:10.1053/jhep.2003.50360 286
HEPATOLOGY, Vol. 38, No. 2, 2003
Fig. 1. Heme degradation and products of the HO pathway. HO catalyzes the oxidative cleavage of the ␣-mesocarbon yielding equimolar amounts of biliverdin, Fe2⫹, and CO.
the low vascular tone of liver sinusoids via cyclic guanosine monophosphate (cGMP)-mediated relaxation of hepatic stellate cells,18 an effect that is particularly emminent when the HO-1 gene is induced on oxidative stress.19 Under these conditions, the HO/CO system can set off an increased vasoconstrictor tone, e.g., resulting from release of endothelin-1 on reperfusion from ischemia.20,21 Although both gases can bind to the prosthetic heme moiety of soluble guanylyl cyclase (sGC), the impact of CO on sGC activity (approximately a 5-fold increase in activity of the ␣11 heterodimeric isoform in vitro) is rather moderate when compared with NO (inducing a 100- to 400-fold increase in activity) and the exact mechanisms of CO-mediated vasodilation is still a matter of controversy.22,23 However, there is increasing evidence that CO acts largely via sGC-cGMP–independent pathways and that its cytoprotective modes of action are not confined to vasodilation: For example, CO has been shown lately to confer protection via the mitogen kinase pathway against apoptotic and inflammatory stimuli,24 both of which can contribute to hepatic I/R injury. The release of Fe2⫹ during oxidative cleavage of the heme molecule is a particular matter of concern, because Fe2⫹, like other transition metal ions, catalyzes the formation of oxygen-free radicals in the Haber-Weiss or Fenton reaction. Therefore, the release of iron can mitigate any antioxidant actions of HO-1 gene expression and may explain the rather narrow threshold of overexpression of HO-1 conferring protection.25,26 Nevertheless, release of iron fosters gene expression of ferritin, a protein imparting additive cytoprotection against oxidative stress.27 The potential toxic actions of bile pigments range from itching in jaundiced patients to severe neuronal damage primarily of basal ganglia as observed in icterus neonatorum. Although neuronal tissue seems to be particularly susceptible to toxic actions of bile pigments, a more gen-
BAUER
287
eral toxic action of bile pigments is assumed to result from damage of lipid bilayers of biological membranes.28 Thus, cytoprotection by bilirubin is not an intuitively intriguing concept to most hepatologists, and removal of bile pigments is one potential beneficial factor of modern supportive treatment strategies in acute liver failure, such as albumin dialysis.29 On the other hand, cells primed by a wide variety of toxic compounds, such as heavy metals, develop tolerance to subsequent, otherwise lethal injury. Moreover, there is good evidence to suggest a significant role for bile pigments as a (hepato)cellular antioxidant sytem.30 Regarding the latter, Kato et al. provide evidence that a simple short-term rinse of the harvested liver with micromolar amounts of unconjugated bilirubin attenuates biliary dysfunction and cell injury both ex vivo in the isolated perfused rat liver as well as in an in vivo transplant model in the rat.8 This simple technique is as effective as pharmacologic preconditioning with hemin, which induces a cellular stress response and presumably acts cytoprotective via HO-1 gene expression. The investigators not only characterize a potential promising conditioning technique to attenuate harvesting injury, but they also try to shed light on the mechanisms by which HO induction by various preconditioning techniques confers protection: While short-term addition of bilirubin to the perfusate is able to ameliorate I/R injury even when the HO pathway is inhibited, supplementation of exogenous CO failed to confer protection in ex vivo asanguineously perfused livers. These results are at odds with previous reports that CO supplementation attenuates harvesting injury in an
Fig. 2. HO-1 immunoreactive protein in normal human liver tissue obtained (A) during liver resection surgery and (B) in a liver graft stored in UW-solution prior to liver transplantation. Whereas HO-1 immunoreactive protein is restricted to Kupffer cells under physiologic conditions, the gene is induced in the graft of a brain dead donor in parenchymal cells primarily of the pericentral region as well.
288
BAUER
isolated perfused liver preparation with red cells added to the buffer.31 Although the exact mechanisms by which manipulation of the HO system confers protection still remain unclear and may depend on experimental conditions and model systems studied, it is obvious that treatment of the brain dead donor in an intensive care unit for hours or even days prior to organ preservation is likely to induce a stress response involving HO-1 gene expression (Fig. 2) even in the absence of any further “pharmacologic preconditioning.” Thus, studies using unprimed healthy animals as organ donors may overestimate any protective effect of induction of a stress response in graft donors. Nevertheless, bilirubin rinse may reflect a simple strategem against post-transplantation reperfusion injury, although its superiority over other antioxidants or additional modes of action besides scavenging of oxygenfree radicals remain to be delineated. Acknowledgment: The author is indebted to Martin K. Schilling, Professor and Chair, Department of Surgery, University of Saarland for providing tissue samples and critical discussion of the manuscript. MICHAEL BAUER, M.D.
Department of Anesthesiology and Critical Care Medicine University of Saarland Homburg, Germany
References 1. Howard TK, Klintmalm GB, Cofer JB, Husberg BS, Goldstein RM, Gonwa TA. The influence of preservation injury on rejection in the hepatic transplant recipient. Transplantation 1990;49:103-107. 2. Amersi F, Buelow R, Kato H, Ke B, Coito AJ, Shen XD, Zhao D, et al. Upregulation of heme oxygenase-1 protects genetically fat Zucker rat livers from ischemia/reperfusion injury. J Clin Invest 1999;104:1631-1639. 3. Jaeschke H. Preservation injury: mechanisms, prevention and consequences. J Hepatol 1996;25:774-780. 4. Rensing H, Jaeschke H, Bauer I, Patau C, Datene V, Pannen BH, Bauer M. Differential activation pattern of redox-sensitive transcription factors and stress-inducible dilator systems heme oxygenase-1 and inducible nitric oxide synthase in hemorrhagic and endotoxic shock. Crit Care Med 2001; 29:1962-1971. 5. Bauer M, Bauer I. Heme oxygenase-1: redox regulation and role in the hepatic response to oxidative stress. Antioxid Redox Signal 2002;4:749-758. 6. Maines MD. Heme oxygenase: function, multiplicity, regulatory mechanisms, and clinical applications. FASEB J 1988;2:2557-2568. 7. Choi AMK, Alam J. Heme oxygenase-1: function, regulation, and implication of a novel stress-inducible protein in oxidant-induced lung injury. Am J Respir Cell Mol Biol 1996;15:9-19. 8. Kato Y, Shimazu M, Kondo M, Uchida K, Kumamoto Y, Wakabayashi G, Kitajima M, et al. Bilirubin rinse: a simple protectant against the rat liver graft injury mimicking heme oxygenase-1 preconditioning. HEPATOLOGY 2003;38:364-373. 9. Tenhunen R, Marver HS, Schmid R. The enzymatic conversion of heme to bilirubin by microsomal heme oxygenase. Proc Natl Acad Sci U S A 1968; 61:748-755. 10. Keyse SM, Tyrrell RM. Heme oxygenase is the major 32-kDa stress protein induced in human skin fibroblasts by UVA radiation, hydrogen peroxide, and sodium arsenite. Proc Natl Acad Sci U S A 1989;86:99-103.
HEPATOLOGY, August 2003
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