Localization of Xenobiotic-responsive Element Binding Protein in Rat Hepatocyte Nuclei After Methylcholanthrene Administration as Revealed by In Situ Southwestern HybridizationYouko Asakaa, Jun Watanabea, and Shinsuke Kanamuraaa Department of Anatomy, Kansai Medical University, Moriguchi, Osaka, Japan Correspondence to: Shinsuke Kanamura, Dept. of Anatomy, Kansai Medical Univ., 10-15 Fumizono-cho, Moriguchi, Osaka 570, Japan. Xenobiotic-responsive element binding protein (XRE-BP), a heterodimer of aryl hydrocarbon receptor (AhR) and its nuclear translocator (Arnt), regulates the transcription of cytochrome P-450 1A1 gene (CYP1A1) through XRE in response to xenobiotic inducers. For a better understanding of the regulatory mechanism of CYP1A1 through XRE, localization of XRE-BP was examined in liver sections or isolated hepatocyte nuclei from control and 3-methylcholanthrene (MC)-treated rats by in situ Southwestern hybridization, using synthetic XRE as a probe, and was observed by confocal laser scanning microscopy and electron microscopy. Gel mobility shift assay and competitive binding assay showed specificity of the synthetic XRE probe. XRE-BP was exclusively localized in hepatocyte nuclei in liver sections from animals 3 hr after MC injection, whereas the protein was absent in hepatocyte cytoplasm in MC-treated animals and in hepatocyte nuclei and cytoplasm in control animals. In isolated hepatocyte nuclei, XRE-BP began to accumulate in the central region between 0.5 and 3 hr, showed a peak between 3 and 6 hr, decreased gradually between 6 and 72 hr, and disappeared at 72 hr after MC injection. The protein was scarce in peripheral and nucleolar regions of the nucleus. Therefore, XRE-BP is formed in the nuclei of hepatocytes after MC stimulation. In addition, XRE-BP was found in isolated hepatocyte nuclei from control animals after preincubation with cytoplasmic lysate from MC-treated animals, although the protein was absent in the nuclei before the preincubation. These findings strongly suggest that AhR translocates from hepatocyte cytoplasm to the nucleus and forms XRE-BP in the nucleus after MC stimulation. (J Histochem Cytochem 46:825832, 1998) Key Words: xenobiotic-responsive element, DNA binding protein, in situ Southwestern, hybridization, cytochrome P-450, methylcholanthrene, liver, rat
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