Coexpression of Ah receptor and CYP1A1 in hepatocytes of C57BL/6J and DBA/2J mice.

The distribution of the hepatic aryl hydrocarbon receptor (AhR) and CYP1A1 was investigated in control and 3-methylcholanthrene (3-MC)-treated C57BL/6J (B6) and DBA/2J (D2) mice. The AhR was identified as protein bands of 95 and 104 kDa in cytosol of B6 and D2 mice, respectively, while a protein of 93 kDa was detected in nuclear extracts from both murine strains. CYP1A1 was recognized as a protein of 56 kDa in microsomes from B6 and D2 mice. Lower amounts of immunodetectable AhR and CYP1A1 were observed in D2 mice, compared to B6 mice. Immunohistochemical studies were used to colocalize the AhR and CYP1A1 in adjacent liver sections. Staining for the AhR was localized most prominently in the cytoplasm of centrilobular hepatocytes and was minimal in the nuclei. Periportal hepatocytes were also reactive for the AhR, but the staining was usually localized in the nuclei. Other hepatocytes contained moderate amounts of cytoplasmic staining, whereas nuclear staining was present at low levels. Immunodetectable CYP1A1 also predominated in centrilobular hepatocytes and was negligible in periportal hepatocytes. The staining was exclusively cytoplasmic and was not seen in the nuclei of hepatocytes. Observations of adjacent liver sections revealed that the AhR and CYP1A1 were coexpressed in individual hepatocytes. A similar regional distribution for the AhR and CYP1A1 was manifested in B6 and D2 mice, with the exception that staining was more pronounced in the former under both control and induced conditions. These results showed heterogeneity in the distribution of the AhR and CYP1A1, with comparable levels of both proteins residing in the same hepatocytes. They further demonstrated that the AhR is localized in both the cytoplasm and nuclei of hepatocytes.