Structure and expression of principal proteins involved in dioxin signal transduction and potentially in dioxin sensitivity
Year: 2005
Type: dissertation
Abstract: Dioxins are a group of environmental contaminants that raise concern because of their potency, widespread presence and persistence within the food chain.2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and other dioxins bring about a wide variety of biochemical and toxic effects, most of which are mediated by the AH receptor (AHR).The AHR functions as a ligand-activated transcription factor and binds to DNA as a heterodimeric complex with the AHR nuclear translocator (ARNT) or its homologue in the brain, ARNT2.The AHR repressor (AHRR) negatively regulates the dioxin signalling by competing with AHR for dimerizing with ARNT.All these principal proteins in dioxin signal transduction, AHR, ARNT and AHRR, belong to the bHLH/PAS superfamily of transcriptional regulators.A characteristic feature of TCDD toxicity is its wide variation in sensitivity among animal species and strains, which complicates dioxin risk assessment.For the guinea pig, TCDD is the most toxic synthetic compound known, while the resistant hamster tolerates over 1000fold higher doses.A similar difference exists between two rat strains, the sensitive Long-Evans (Turku A/B) (L-E) and the resistant Han/Wistar (Kuopio) (H/W).Cloning of H/W rat AHR revealed changes in the structure of C-terminal transactivation domain, which appeared to be the principal reason for TCDD resistance in H/W rats.Because the reason for sensitivity difference between hamsters and guinea pigs was unknown, their AHRs were cloned.Sequencing of hamster AHR revealed a restructured transactivation domain as compared with that in guinea pigs.The results imply that AHR structure, especially in the C-terminal transactivation region, may be an important or even crucial factor to the sensitivity differences in TCDD toxicity.Although the AHR appears to be the major reason for TCDD resistance in H/W rats, there is also another, currently unknown factor involved.Therefore, the primary structures of the ARNT, ARNT2 and AHRR were compared between L-E and H/W rats to determine whether some of these proteins could be this auxiliary factor participating in the strainspecific differences in TCDD toxicity.The AHRR gene had not been cloned earlier in rats, and thus its time-, dose-and tissue-dependent expression was also determined using quantitative RT-PCR.However, no marked differences were found between H/W and L-E rats in the structure or expression of ARNT, ARNT2 or AHRR, suggesting that these bHLH/PAS proteins do not contribute to strain differences in TCDD sensitivity.Moreover, several novel splice variants were discovered in the structures of ARNT and ARNT2, but none of these variants appeared to be related to TCDD resistance in this rat model.The effects of TCDD on the hypothalamic expression of several genes encoding bHLH/PAS proteins participating in dioxin signal transduction and possibly in wasting syndrome were also determined using quantitative RT-PCR.The wasting syndrome is a TCDD-induced response that shows a clear sensitivity difference between L-E and H/W rats.However, although some minor changes were detected, they may not account for the wasting syndrome.Thus, among all the studied proteins involved in dioxin signal transduction, only the structure of the AHR stood out as an essential determinant of dioxin sensitivity in the rat model.The AHR structure also appeared to be a critical factor in the species model.
Source: Työväentutkimus Vuosikirja
Author: Merja Korkalainen
Language: English
Subfield: Health, Toxicology and Mutagenesis
Field: Environmental Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W599342691