Role of the carboxy-terminal transactivation domain and active transcription in the ligand-induced and ligand-independent degradation of the mouse Ahb-1 receptor.

To assess the importance of transactivation domains (TAD), DNA binding and transcription on the degradation of the AH receptor (AHR), Hepa-1 cells were pre-treated with actinomycin D (AD) or cycloheximide (CHX) and exposed to 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD). AD or CHX did not affect nuclear localization or DNA binding of the AHR but inhibited ligand-induced degradation. In contrast, AD or CHX did not inhibit geldanamycin (GA) induced degradation of the AHR. To assess the role of the COOH-terminal TAD in AHR degradation, stop codons were placed at nucleotide 1501 and 1921 of the Ah(b-1) AHR coding region to generate AHR(500) and AHR(640). Stable cell lines were generated and exposed to TCDD. Cells expressing AHR(500) did not induce CYP1A1 protein, but exhibited significant degradation of AHR(500). Cells expressing AHR(640) induced CYP1A1 protein to 50% of the level of cells expressing wild type AHR and exhibited significant degradation of AHR(640). Importantly, AD and CHX did not inhibit the TCDD-induced degradation of either AHR(500) and AHR(640) and these receptors showed a more rapid profile of ligand-induced degradation compared to cells expressing wild type AHR. TCDD exposure to Hepa-1 cells with reduced aryl hydrocarbon receptor nuclear translocator (ARNT), showed ligand-induced degradation of the AHR that was not blocked by AD. However, AD inhibited TCDD-induced degradation when ARNT expression was restored. These results show that multiple mechanisms exist for the ligand and GA-induced degradation of the AHR and suggest that ligand-induced degradation can switch between two mechanisms depending on the presence of a functional TAD and the binding to DNA.