J BIOCHEM MOLECULAR TOXICOLOGY Volume 15, Number 4, 2001 Activation of the Ah Receptor Signaling Pathway by Prostaglandins Shawn D. Seidel, Greg M. Winters, William J. Rogers, Michael H. Ziccardi, Violet Li, Bart Keser, and Michael S. Denison Department of Environmental Toxicology, University of California, Davis, CA 95616-8588, US; E-mail: msdenison@ucdavis.edu Received 14 March 2001; revised 6 May 2001; accepted 12 May 2001 ABSTRACT: The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that medi- ates many of the biological and toxicological actions of a diverse range of chemicals, including the en- vironmental contaminant 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD, dioxin). Although no endogenous physiological ligand for the AhR has yet been de- scribed, numerous studies support the existence of such a ligand(s). Here we have examined the ability of prostaglandins and related chemicals to activate the AhR signaling system. Using two AhR-based bioas- say systems we report that relatively high concentra- tions of several prostaglandins (namely, PGB 3 , PGD 3 , PGF 3 , PGG 2 , PGH 1 , and PGH 2 ) can not only stim- ulate AhR transformation and DNA binding in vitro, but also induce AhR-dependent reporter gene expres- sion in mouse hepatoma cells in culture. PGG 2 also induced AhR-dependent reporter gene expression to a level three-to four fold greater than that observed with a maximal inducing dose of TCDD. Sucrose gradient ligand binding analysis revealed that PGG 2 could com- petitively displace [ 3 H]TCDD from the AhR. Overall, our results demonstrate that selected prostaglandins are weak agonists for the AhR and they represent a structurally distinct and novel class of activator of the AhR signal transduction pathway. C  2001 John Wiley & Sons, Inc. J Biochem Mol Toxicol 15:187–196, 2001 Correspondence to: Dr. Michael S. Denison. Present Address of Shawn D. Seidel: Department of Neuro- science, 4447 Boyce Hall, University of California, Riverside, CA 92521, US. Contract Grant Sponsor: National Institutes of Environmental Health Sciences. Contract Grant Number: ES07685, ES05707 Contract Grant Sponsor: Environmental Toxicology Training. Contract Grant Number: ES07059 Contract Grant Sponsor: Superfund Basic Research. Contract Grant Number: ES04699 Contract Grant Sponsor: California Agriculture Experiment Station. c  2001 John Wiley & Sons, Inc. KEYWORDS: Ah Receptor; Prostaglandins; TCDD; Prostaglandin G2 INTRODUCTION The Ah receptor (AhR) is a ligand-dependent tran- scription factor which regulates many of the toxic and biological effects of a variety of hazardous chemicals, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin). Exposure to TCDD, the prototypical and most potent AhR ligand, results in a wide variety of species- and tissue-specific toxic and biological responses, many of which have been shown to be AhR-dependent [1–3]. The induction of gene expression, most notably that of CYP1A1, is one such response that has been used as the model to define the mechanism of TCDD- and AhR-dependent action [2–4]. Following ligand (TCDD) binding, the cytosolic TCDD:AhR complex undergoes transformation † , during which it translo- cates into the nucleus and dissociates from two molecules of hsp90 (a heat shock protein of 90 kDa) and at least one immunophilin-like protein [5–7]. Once in the nucleus, the dimerization of the lig- anded AhR with the ARNT (AhR nuclear transloca- tor) protein, converts the AhR complex into its high affinity DNA binding form [2,3,8–11]. Binding of the transformed heteromeric AhR complex to its spe- cific DNA recognition site, the dioxin responsive el- ement (DRE), leads to alterations in chromatin struc- ture, increased promoter accessibility and increased rates of CYP1A1 gene transcription [3,4]. DREs have also been identified in the upstream region of other TCDD-inducible genes, including CYP1A2/1B1, UDP- glucuronosyltransferase (UGT ∗ 01) and glutathione S- transferase Ya [reviewed in 3], and they are responsible † In this report, we have defined transformation as the process by which the TCDD:AhR complex is converted into a form which can bind to DNA with high affinity. 187