Presentation
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Presentation |
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6th Internet World Congress for Biomedical Sciences |
S. Dhakshinamoorthy(1), D. Bloom(2), Anil Jaiswal(3)
(1)(2)Department of Pharmacology. Baylor College of Medicine. - Houston. United States
(3)Baylor College of Medicine - Houston. United States
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Contact address: |
S. Dhakshinamoorthy Department of Pharmacology Baylor College of Medicine. One Baylor Plaza Houston Texas United States ajaiswal@bcm.tmc.edu |
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Detoxifying enzymes including quinone oxidoreductases (NQO1 and NQO2) and glutathione S-transferases (GSTs) catalyze metabolic detoxification of xenobiotics, drugs and carcinogens and, thus, protect the cells against redox cycling and oxidative stress. Genes encoding the various detoxifying enzymes are ubiquitiosly expressed and coordinately induced in response to antioxidants and xenobiotics. Deletion mutagenesis and transfection assays have identified antioxidant response element (ARE) in the promoters of the various detoxifying enzyme genes, which regulate the expression and coordinated induction of detoxifying enzyme genes. ARE is a unique cis-element that contains two AP1/AP1 like elements followed by a ´GC´ box. Band and supershift assays were used to demonstrate that nuclear transcription factors Nrf1, Nrf2, Jun, Fos, and Fra bind to the NQO1 and GST Ya genes ARE. Overexpression of Nrf1 and Nrf2 individually in human hepatoblastoma (Hep-G2) cells significantly increased the ARE-mediated genes expression and induction by b-naphthoflavone (b-NF) and tert-butyl hydroquinone (t-BHQ). Nrf2 containing one mutated leucine in its leucine zipper region was more efficient in upregulation of ARE-mediated gene expression, as compared to Nrf1 with two mutated leucines. Immunoprecipitation assays demonstrated that Nrf1 and Nrf2 heterodimerize with Jun (c-Jun, Jun-B and Jun-D) proteins that bind to the ARE. This heterodimerization required unknown cytosolic factor(s). Overexpression of Nrf and Jun combinations led to superactivation of ARE-mediated gene expression in transfected Hep-G2 cells. Recently, an inhibitor of Nrf2, designated as INrf2/Keap1 that retains the Nrf2 in the cytoplasm was identified and cloned. A hypothetical model will be presented based on the available information on ARE-mediated regulation of detoxifying enzyme genes. Briefly, the Nrf2 is retained in the cytosplasm by a repressor protein Inrf2 (Keap1) in untreated normal cells. The treatment of cells with xenobiotics and antioxidants leads to the activation of unknown cytosolic factor(s) that catalyze modification of Nrf2 and/or INrf2/Keap1. The modification follows dissociation of Nrf2 and INrf2/Keap1. The free Nrf2 translocates in the nucleus. Nrf2 in the nucleus heterodimerizes with c-Jun and binds to the ARE resulting in the induction of NQO1 and other ARE-regulated genes expression. The identity of cytosolic factor(s) remains unknown.
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