ToxSci Advance Access originally published online on October 4, 2006
Toxicological Sciences 2007 95(1):172-181; doi:10.1093/toxsci/kfl126
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Effects of Tryptophan Photoproducts in the Circadian Timing System: Searching for a Physiological Role for Aryl Hydrocarbon Receptor
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* Department of Veterinary Biosciences, College of Veterinary Medicine, University of Illinois at Urbana-Champaign, Urbana, IL 61802
Neuroscience Program, University of Illinois at Urbana-Champaign, Urbana, Illinois 61802
1 To whom correspondence should be addressed at Department of Veterinary Biosciences, College of Veterinary Medicine, University of Illinois at Urbana-Champaign, 2001 S. Lincoln Avenue, Veterinary Medicine Basic Sciences Building, Urbana, IL 61802. Fax: +1-217-244-1652. E-mail: tischkau{at}uiuc.edu.
Received September 1, 2006; accepted September 30, 2006
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Abstract |
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The aryl hydrocarbon receptor (AhR) mediates adverse effects of dioxins, but its physiological role remains ambiguous. The similarity between AhR and canonical circadian clock genes suggests potential involvement of AhR in regulation of circadian timing. Photoproducts of tryptophan (TRP), including 6-formylindolo[3,2-b]carbazole (FICZ), have high affinity for AhR and are postulated as endogenous ligands. Although TRP photoproducts activate AhR signaling in vitro, their effects in vivo have not been investigated in mammals. Because TRP photoproducts may act as transducers of light, we examined their effects on the circadian clock. Intraperitoneal injection of TRP photoproducts or FICZ to C57BL/6J mice dose dependently induced AhR downstream targets, cytochrome P4501A1 (CYP1A1) and cytochrome P4501B1 mRNA expression, in liver. c-fos mRNA, a commonly used marker for light responses, was also induced with FICZ, and all responses were AhR dependent. A rat-immortalized suprachiasmatic nucleus (SCN) cell line, SCN 2.2, was used to examine the direct effect of TRP photoproducts on the molecular clock. Both TRP photoproducts and FICZ-increased CYP1A1 expression and prolonged FICZ incubation altered the circadian expression of clock genes (Per1, Cry1, and Cry2) in SCN 2.2 cells. Furthermore, FICZ inhibited glutamate-induced phase shifting of the mouse SCN electrical activity rhythm. Circadian light entrainment is critical for adjustment of the endogenous rhythm to environmental light cycle. Our results reveal a potential for TRP photoproducts to modulate light-dependent regulation of circadian rhythm through triggering of AhR signaling. This may lead to further understanding of toxicity of dioxins and the role of AhR in circadian rhythmicity.
Key Words: dioxin; suprachiasmatic nucleus; circadian rhythm; light entrainment; phase shift.
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