Xanthotoxin, a linear furanocoumarin in the hostplants of the caterpillar Papilio polyxenes, activates transcription of the cytochrome P450 gene, CYP6B1, via a xenobiotic response element (XRE-Xan) in its promoter. XRE-AhR, a response element first identified in the mammalian xenobiotic response cascade, also resides in the promoter of CYP6B1 but its function is unknown. In mammals, XRE-AhR is activated by the heterodimer-forming proteins, AhR and ARNT. In Drosophila melanogaster, homologous proteins, spineless (Ss) with tango (Tgo) and single-minded (Sim) with Tgo, activate genes in the developmental regulatory cascade via two similar elements, XRE-AhR and CME, respectively. To determine if XRE-AhR acts in the regulation of CYP6B1, combinations of ss/tgo and sim/tgo expressible plasmids were cotransfected with a CYP6B1 promoter:CAT reporter plasmid into insect Sf9 cells. Treatment of the transfected cells with xanthotoxin in methanol (1 ug/ul) and methanol alone indicated that coexpression of the Ss and Tgo proteins significantly increased both constitutive and xanthotoxin-induced CAT activity above that detected in cells transfected with the CYP6B1:CAT construct alone. Coexpression of Sim and Tgo proteins demonstrated a more limited ability to increase CAT activity. These results parallel previous experiments in Drosophila indicating that the Ss/Tgo heterodimer binds XRE-AhR with greater efficiency than the Sim/Tgo heterodimer. Activation of detoxification genes by binding proteins involved in regulation of insect developmental genes suggests a common ancestor for both pathways co-opted for multiple functions.
Species 1: Lepidoptera Papilionidae Papilio polyxenes (black swallowtail)
Species 2: Diptera Drosophilidae Drosophila melanogaster (fruit fly)
Keywords: transcriptional regulation, aryl hydrocarbon
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