Non-hypoxic transcriptional activation of the aryl hydrocarbon receptor nuclear translocator in concert with a novel hypoxia-inducible factor-1alpha isoform.

Aryl hydrocarbon receptor nuclear translocator (ARNT) belongs to the basic helix-loop-helix Per-Arnt-Sim (bHLH PAS) protein which dimerizes with other PAS proteins. Although it has a transactivation domain (TAD), ARNT functions as an assistant partner of main factors, such as aryl hydrocarbon receptor and hypoxia-inducible factors, rather than acting as a straightforward transcription factor. However, ARNT may function as an active transcription factor using its TAD either in association with itself, single-minded protein 1, or trachealess protein. In the present study, we identified a novel ARNT partner, a HIF-1alpha variant, which is ubiquitously expressed in human tissues and cancer cell lines. The HIF-1alpha variant, designated HIF-1alpha417, bound to ARNT and, moreover, stimulated the transcription of the erythropoietin enhancer reporter gene. This stimulation was markedly augmented by ARNT but not by the ARNT603 mutant lacking the TAD. Thus, augmentation by ARNT suggests that ARNT determined the transcriptional activity. HIF-1alpha417 was found to be associated with ARNT and to bind to the hypoxia response element containing the E-box core. Moreover, HIF-1alpha417 promoted the nuclear translocation of ARNT, and conversely ARNT stabilized HIF-1alpha417. Taken together, our results suggest that HIF-1alpha417 is a novel partner that is required for transcription activity of ARNT.