Investigation of ARNT isoform-specific regulation of AHR signaling

The aryl hydrocarbon receptor nuclear translocator (ARNT) is alternatively spliced into two distinct isoforms, isoform 1 and 3. Although ARNT is found to be critical in immunity, xenobiotic, and hypoxic response, ARNT isoform-specific function has yet to be investigated. We previously demonstrated that primary lymphocytes express both of these isoforms, however malignant T cells overexpress ARNT isoform 1 to promote cell viability. In this study, we find that the ARNT isoforms have opposing roles in aryl hydrocarbon receptor (AHR) signaling, as ARNT isoform 1 suppresses AHR activity, whereas ARNT isoform 3 is needed for AHR target-gene transcription. Furthermore, to explore this suppressive role, we investigated a unique modification specific only to ARNT isoform 1 – phosphorylation of serine 77 (S77). We determined that phosphorylation at S77 is initiated following AHR activation and is critical for the augmentation of AHR-target gene transcription. These results further highlight the importance of investigating ARNT isoform-specific function and reveal an essential role of ARNT isoform 1 phosphorylation in AHR signaling. Collectively, these findings increase our understanding of a complex regulatory mechanism by which ARNT regulates AHR signaling, further aiding in the comprehension of their roles in immunity and supporting the potential of targeting ARNT alternative splicing as a means of therapeutic intervention in hematological diseases and malignancies.