Inducible and constitutive modes of gene regulation by the dioxin receptor and Arnt

Heterodimeric complexes between the basic helix-loop-helix (bHLH)/PAS (Per-Arnt-Sim) transcription factors, dioxin receptor and Arnt, are involved in mediating cellular responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin). The unliganded dioxin receptor is found in the cytoplasm complexed with the 90-kDa heat shock protein hsp90, in a latent inactive (non-DNA binding) configuration. Upon ligand binding, the dioxin receptor translocates to the nucleus and dimerizes with its DNA binding partner Arnt. During this process, association of the dioxin receptor with hsp90 is disrupted. The dioxin receptor Arnt heterodimer interacts with specific DNA sequences called xenobiotic response elements (XREs) in control regions within regulated genes. Both ligand binding and hsp90 binding domains are co-localized in the C-terminal part of the PAS domain. In the present work, we describe that two distinct domains of the dioxin receptor mediated interaction with hsp90: the ligand binding domain and the bHLH domain. Whereas ligand-binding activity correlated with association with hsp90, bHLH-hsp90 interaction appeared to be important for DNA-binding activity but not for dimerization of the receptor. The bHLH domain of the dioxin receptor exhibited a broad dimerization potential that was narrowly restricted by the PAS domain, which determined very strict specificity for Arnt. In the absence of dioxin, Arnt does not bind to the XRE sequence. However, we revealed that full-length Arnt constitutively activated CACGTG E box-driven reporter genes in vivo, most likely as a homodimeric complex. This E box motif is also recognized by dimeric complexes of distinct bHLH proteins including Myc, Max and USF These results indicate that homodimeric complexes of Arnt may regulate E box regulated genes involved in control of cell growth, differentiation and circadian rhythmicity.