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ToxSci Advance Access originally published online on March 23, 2005
Toxicological Sciences 2005 85(2):935-943; doi:10.1093/toxsci/kfi154
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Toxicological Sciences vol. 85 no. 2 © The Author 2005. Published by Oxford University Press on behalf of the Society of Toxicology. All rights reserved. For Permissions, please email: journals.permissions@oupjournals.org

Identification of the Tryptophan Photoproduct 6-Formylindolo[3,2-b]carbazole, in Cell Culture Medium, as a Factor That Controls the Background Aryl Hydrocarbon Receptor Activity

Mattias Öberg1,2,*, Linda Bergander1,{dagger}, Helen Håkansson*, Ulf Rannug{dagger} and Agneta Rannug*

* Institute of Environmental Medicine, Karolinska Institutet, SE-171 77 Stockholm, Sweden, and {dagger} Division of Cellular and Genetic Toxicology, Stockholm University, Stockholm, Sweden

Received October 22, 2004; accepted March 18, 2005

The presence of high affinity ligands for the aryl hydrocarbon receptor (AhR) in cell culture medium has generally been overlooked. Such compounds may confound mechanistic studies of the important AhR regulatory network. Numerous reports have described that light exposed cell culture medium induces AhR-dependent activity. In this study, we aimed at identifying the causative substance(s). A three-dimensional factorial design was used to study how the background activity of CYP1A1 in a rat hepatoma cell line (MH1C1) was controlled by photoproducts formed in the medium exposed to normal laboratory light. The light induced activity was found to be tryptophan dependent, but independent of riboflavin and other components in the medium. The light exposed medium showed the same transient enzyme inducing activity in vitro as the AhR ligand 6-formylindolo[3,2-b]carbazole (FICZ). This substance, which we have previously identified as being formed in UV-exposed tryptophan solutions, is a substrate for CYP1A1 and it has a higher AhR binding affinity than TCDD. Several tryptophan related photoproducts were detected in the light-exposed medium. For the first time one of the formed photoproducts was identified as FICZ with bioassay driven fractionation coupled with HPLC/MS. These results clearly show that tryptophan derived AhR ligands, which have been suggested to be endogenous AhR ligands, influence the background levels of CYP1A1 activity in cells in culture.

Key Words: tryptophan; 6-formylindolo[3,2-b]carbazole; light; aryl hydrocarbon receptor.


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