title: Dioxin mediates downregulation of the reduced folate carrier transport activity via the arylhydrocarbon receptor signalling pathway.
abstract: Dioxins such as 2,3,7,8-tetrachlordibenzo-p-dioxin (TCDD) are common environmental contaminants known to regulate several genes via activation of the transcription factor aryl hydrocarbon receptor (AhR) associated with the development of numerous adverse biological effects. However, comparatively little is known about the molecular mechanisms by which dioxins display their toxic effects in vertebrates. The 5' untranslated region of the hepatocellular Reduced folate carrier (Rfc1; Slc19a1) exhibits AhR binding sites termed dioxin responsive elements (DRE) that have as yet only been found in the promoter region of prototypical TCDD target genes. Rfc1 mediated transport of reduced folates and antifolate drugs such as methotrexate (MTX) plays an essential role in physiological folate homeostasis and MTX cancer chemotherapy. In order to determine whether this carrier represents a target gene of dioxins we have investigated the influence of TCDD on functional Rfc1 activity in rat liver. Pre-treatment of rats with TCDD significantly diminished hepatocellular Rfc1 uptake activity in a time- and dose-dependent manner. In further mechanistic studies we demonstrated that this reduction was due to TCDD-dependent activation of the AhR signalling pathway. We additionally showed that binding of the activated receptor to DRE motifs in the Rfc1 promoter resulted in downregulation of Rfc1 gene expression and reduced carrier protein levels. As downregulation of pivotal Rfc1 activity results in functional folate deficiency associated with an elevated risk of cardiovascular diseases or carcinogenesis, our results indicate that deregulation of this essential transport pathway represents a novel regulatory mechanism how dioxins display their toxic effects through the Ah receptor.
authors.affiliation: Institute of Pharmacology, Pharmacy and Toxicology, Faculty of Veterinary Medicine, University of Leipzig, Leipzig, Germany. halwachs@vetmed.uni-leipzig.de
authors.forename: Sandra
authors.initials: S
authors.lastname: Halwachs
authors.forename: Cathleen
authors.initials: C
authors.lastname: Lakoma
authors.forename: Rolf
authors.initials: R
authors.lastname: Gebhardt
authors.forename: Ingo
authors.initials: I
authors.lastname: Schäfer
authors.forename: Peter
authors.initials: P
authors.lastname: Seibel
authors.forename: Walther
authors.initials: W
authors.lastname: Honscha
mesh_terms: D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects / Q000502:physiology; D045744:Cell Line, Tumor; D004147:Dioxins / Q000494:pharmacology*; D015536:Down-Regulation / Q000187:drug effects; D015870:Gene Expression / Q000187:drug effects; D017930:Genes, Reporter / Q000187:drug effects; D008099:Liver / Q000187:drug effects / Q000378:metabolism; D008297:Male; D026901:Membrane Transport Proteins / Q000187:drug effects* / Q000502:physiology; D051379:Mice; D000072317:Polychlorinated Dibenzodioxins / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D018336:Receptors, Aryl Hydrocarbon / Q000502:physiology*; D058978:Reduced Folate Carrier Protein; D051818:Replication Protein C / Q000096:biosynthesis; D012097:Repressor Proteins / Q000096:biosynthesis; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction / Q000187:drug effects* / Q000502:physiology
chemical_list: D004147:Dioxins; D026901:Membrane Transport Proteins; D000072317:Polychlorinated Dibenzodioxins; D018336:Receptors, Aryl Hydrocarbon; D058978:Reduced Folate Carrier Protein; D012097:Repressor Proteins; C113967:Rfc1 protein, rat; C548700:Slc19a1 protein, mouse; D051818:Replication Protein C