Xenobiotic response element binding enriched in both nuclear and microsomal fractions of rat cerebellum

Abstract

Xenobiotic response element (XRE) is a core nucleotide sequence at the upstream of inducible target genes for the transcription factor aryl hydrocarbon receptor (AhR) that is responsible for signal transduction of exogenous environmental pollutants in eukaryotic cells. Immunoblotting analysis revealed the constitutive expression of AhR-related proteins in rat liver and brain, while specific binding of a radiolabelled 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-oblongata, 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 mitochondrial fractions. Limited proteolysis by V8 protease did not markedly affect XRE binding in cerebellar nuclear extracts, with concomitant diminution of that in hepatic and hippocampal nuclear extracts. In primary cultured cerebellar neurons, indigo was effective in significantly increasing XRE binding only when determined immediately after sustained exposure for 120 min in the presence of high potassium chloride. 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. In eukaryotic cells, de novo synthesis of functional proteins is mainly controlled at the level of transcription of genomic genes by RNA polymerase II responsible for the formation of mRNA from DNA in the nucleus. Transcription factors are nuclear proteins that specifically recognize particular core nucleotide sequences at promoter and/or enhancer regions on target genes and thereby elicit quantitative control of formation of mRNA as a result of modulation of the activity of RNA polymerase II in the nucleus. Transcription factors often have one of the most major protein motifs, leucinezipper/helix-loop-helix motif, through which homo and/or hetero dimeric protein complex is formed. Xenobiotic response element (XRE; TNGCGTG) is a core nucleotide sequence specifically recognized by the transcription factor of a nuclear receptor type, aryl hydrocarbon receptor (AhR) (Saatcioglu et al. 1990). AhR is shown as a receptor for particular lipophilic ligands that are highly toxic environmental pollutants including 3-methylcholanthrene, benzo[a]pyrene

Endocrine disrupting polyhalogenated organic pollutants interfere with thyroid hormone signalling in the developing brain

“…It was found that the binding of a radioactive DRE probe to nuclear extracts from adult rat cerebellum was clearly higher than for other brain regions (117) and AhR expression in mouse cerebellum was shown to peak between postnatal day 3-10, a critical period for granule neuroblast growth and maturation (116). In cerebellar granule cell cultures, TCDD treatment increased CYP1A1 and CYP1B1 expression and at the same time reduced granule neuroblast survival (116).…”

Independent actions on cyclin-dependent kinases and aryl hydrocarbon receptor mediate the antiproliferative effects of indirubins

“…Indirubins can then be evaluated for potential therapeutic applications according to their selectivity (GSK-3: type II diabetes; GSK-3 & CDK5: neurodegenerative disorders; CDKs: cancer; AhR: cancer) (Knockaert et al, 2002). In the case of applications in the nervous system, AhR activation should be given careful attention, as AhR, ARNT and indigo-responsive XRE binding are widely distributed in the brain (Petersen et al, 2000;Kuramoto et al, 2003).…”

AhR-mediated gene expression in the developing mouse telencephalon

“…Furthermore, several studies have shown activation of mammalian AhR does not need application of an exogenous ligand to be functional [25]. This has lead researchers to hypothesize about the presence of an as yet unknown endogenous ligand for AhR [15, 26-29]. Furthermore, recent studies in Caenorhabditis elegans have shown that the worm AhR protein does not respond to activation by classical xenobiotic ligands of the vertebrate AhR, but that this ancestral form functions to regulate neuronal differentiation, specifically by directing the fate and gene expression phenotype of two of the worm's 26 GABAergic neurons [30-32].…”