Agonistic and antagonistic effects of alpha-naphthoflavone on dioxin receptor function. Role of the basic region helix-loop-helix dioxin receptor partner factor Arnt

Area of Science:

• Molecular Toxicology
• Gene Regulation
• Transcription Factors

Background:

• The dioxin receptor and Arnt (aryl hydrocarbon receptor nuclear translocator) form a complex that regulates gene expression upon ligand binding.
• Alpha-naphthoflavone (ANF) is a known ligand for the dioxin receptor, exhibiting both agonistic and antagonistic effects.
• Understanding the role of Arnt in ANF's dual action is crucial for deciphering dioxin receptor signaling pathways.

Purpose of the Study:

• To investigate the role of Arnt in the agonistic and antagonistic effects of alpha-naphthoflavone (ANF) on the dioxin receptor.
• To elucidate the molecular mechanisms underlying ANF's dual activity.
• To determine if Arnt dimerization is essential for ANF's antagonistic effects.

Main Methods:

• In vitro experiments using specific antisera to detect dioxin receptor and Arnt interactions.
• Transient transfection assays with reporter genes to assess transcriptional activity.
• Construction and utilization of chimeric receptors to functionally uncouple dioxin receptor from Arnt.

Main Results:

• ANF induced Arnt recruitment to the dioxin receptor in vitro, enhancing heterodimer binding to xenobiotic responsive elements (XREs).
• High ANF concentrations stimulated XRE-driven reporter gene expression, indicating an agonistic effect mediated by dioxin receptor-Arnt heterodimerization.
• ANF antagonized dioxin receptor agonist activity, and this effect was observed even in Arnt-deficient cells using chimeric receptors, demonstrating Arnt-independent antagonism.

Conclusions:

• ANF's agonistic effect on the dioxin receptor is mediated by promoting Arnt heterodimerization and subsequent DNA binding.
• ANF's antagonistic effect is independent of Arnt dimerization, suggesting direct interaction with dioxin receptor sequences that regulate transcription.
• The dioxin receptor possesses Arnt-independent mechanisms for mediating ANF's antagonistic actions.