Xenobiotic response element binding proteins expressed in rat brain

The transcription factor aryl hydrocarbon receptor (AhR) is responsible for signal transduction of exogenous environmental pollutants through recognition of the core nucleotide sequence xenobiotic response element (XRE) located at the upstream of inducible target genes in eukaryotic cells. Immunoblotting analysis revealed the constitutive expression of AhR-related proteins in rat liver and brain, while specific binding of a radiolabeled probe containing XRE was detected in nuclear preparations of both liver and brain on gel retardation electrophoresis. Among discrete rat brain structures examined, cerebellum exhibited the highest. XRE binding with less potent binding in hypothalamus, midbrain, medulla-pons, hippocampus, cerebral cortex and striatum. In contrast to liver and hippocampus, cerebellum also contained unusually higher XRE binding in microsomal fractions than that in either nuclear or synaptosomal fractions. The addition of magnesium ions did not affect XRE binding in nuclear extracts of 3 different regions tested. Limited proteolysis by V8 protease did not markedly affect XRE binding in hippocampal and cerebellar nuclear extracts, with concomitant diminution of that in hepatic nuclear extracts. In primary cultured rat cortical neurons benzo[a]pyrene was not effective in affecting XRE binding, while in primary cultured rat cerebellar neurons indigo was effective in significantly increasing XRE binding only when determined immediately after sustained exposure for 120 min in the presence of potassium chloride at a high concentration. These results suggest the abundance of as-yet unidentified proteins with high affinity for XRE and responsiveness to indigo in both nuclear and microsomal fractions of rat cerebellum.