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Iron primes hepatic macrophages for NF-kappaB activation in alcoholic liver injury
1Department of Medicine and Pathology, University of Southern California School of Medicine, Los Angeles 90033, California. htsukamoto@hsc.usc.edu
Abstract:
NF-kappaB activation induced by lipopolysaccharide (LPS) in cultured hepatic macrophages (HM) may be abrogated by pretreatment of cells with a lipophilic iron chelator, 1,2-dimethyl-3-hydroxypyrid-4-one (L1, deferiprone), suggesting a role for iron in this molecular event [M. Lin, M., R. A. Rippe, O. Niemelä, G. Brittenham, and H. Tsukamoto, Am. J. Physiol. 272 (Gastrointest. Liver Physiol. 35): G1355-G1364, 1997]. To ascertain the relevance in vivo of this hypothesis, HM from an experimental model of alcoholic liver injury were examined for the relationship between nuclear factor (NF)-kappaB activation and iron storage. HM showed a significant increase in nonheme iron concentration (+70%), accompanied by enhanced generation of electron paramagnetic resonance-detected radicals (+200%), NF-kappaB activation (+100%), and tumor necrosis factor-alpha (+150%) and macrophage inflammatory protein-1 (+280%) mRNA induction. Treatment of the cells ex vivo with L1 normalized all these parameters. HM content of ferritin protein, ferritin L chain mRNA, and hemeoxygenase-1 mRNA and splenic content of nonheme iron were increased, suggesting enhanced heme turnover as a cause of the increased iron storage and NF-kappaB activation. To test this possibility, increased iron content in HM was reproduced in vitro by phagocytosis of heat-treated red blood cells. Treatment caused a 40% increase in nonheme iron concentration and accentuated LPS-induced NF-kappaB activation twofold. Both effects could be abolished by pretreatment of cells with zinc protoporphyrin, a hemeoxygenase inhibitor. To extend this observation, animals were splenectomized before 9-wk alcohol feeding. Splenectomy resulted in further increments in HM nonheme iron storage (+60%) and NF-kappaB activation (+90%) and mononuclear cell infiltration (+450%), particularly around the iron-loaded HM in alcohol-fed animals. These results support the pivotal role of heme-derived iron in priming HM for NF-kappaB activation and expression of proinflammatory genes in alcoholic liver injury.
Insights
Iron overload in liver macrophages primes them for inflammation in alcoholic liver injury. Blocking iron metabolism with deferiprone or hemeoxygenase inhibitors reduces this inflammatory response, highlighting iron
Area of Science:
- Hepatology
- Immunology
- Biochemistry
Background:
- Lipopolysaccharide (LPS)-induced NF-kappaB activation in hepatic macrophages (HM) may be inhibited by iron chelators.
- Alcoholic liver injury is associated with inflammation and altered iron metabolism.
Purpose of the Study:
- To investigate the in vivo relationship between nuclear factor (NF)-kappaB activation and iron storage in HM during alcoholic liver injury.
- To determine the role of heme-derived iron in priming HM for NF-kappaB activation and pro-inflammatory gene expression.
Main Methods:
- Analysis of nonheme iron concentration, radical generation, NF-kappaB activation, and inflammatory cytokine mRNA in HM from an experimental model of alcoholic liver injury.
- Ex vivo treatment with deferiprone (L1), an iron chelator, and zinc protoporphyrin, a hemeoxygenase inhibitor.
- In vitro experiments involving phagocytosis of heat-treated red blood cells to increase HM iron content.
- Splenectomy in alcohol-fed animals to assess its impact on HM iron storage and NF-kappaB activation.
Main Results:
- Alcoholic liver injury model showed increased nonheme iron, radical generation, NF-kappaB activation, and pro-inflammatory cytokine mRNA in HM.
- Deferiprone treatment normalized these parameters, while zinc protoporphyrin abolished the effects of increased iron content in vitro.
- Splenectomy exacerbated HM iron storage and NF-kappaB activation in alcohol-fed animals.
Conclusions:
- Heme-derived iron plays a critical role in activating NF-kappaB and promoting pro-inflammatory gene expression in HM during alcoholic liver injury.
- Targeting iron metabolism presents a potential therapeutic strategy for alcoholic liver disease.