Evidence That the Co-chaperone p23 Regulates Ligand Responsiveness of the Dioxin (Aryl Hydrocarbon) Receptor

Abstract

The dioxin (aryl hydrocarbon) receptor is a ligand-dependent transcription factor that induces expression of a number of genes encoding drug metabolizing enzymes. In the absence of ligand the dioxin receptor is present in the cytoplasmic compartment of the cell associated with the molecular chaperone hsp90, which has been implicated in regulating the correct folding of the ligand binding domain of the receptor. In this study we have examined a potential role of the hsp90-associated p23 protein in the activation process of the dioxin receptor to a DNA binding form. In an in vitro model we show that addition of ligand alone to the dioxin receptor fails to induce release of hsp90 from the dioxin receptor. In the presence of ligand, this release was, however, induced upon addition of purified preparations of Arnt. Interestingly, p23 was also found to be associated with the nonactivated form of the dioxin receptor. Following fractionation on sucrose gradients p23 was dissociated from the receptor-hsp90 complex generating a receptor form, which showed ligand-independent release of hsp90 by Arnt and, consequently, ligand-independent activation of the DNA binding activity of the dioxin receptor. Ligand dependence was reconstituted in the presence of molybdate, a transition metal ion known to stabilize the interaction between the molecular chaperone hsp90 and p23. Taken together these experiments suggest a role of p23 in modulating ligand responsiveness in the activation process of the dioxin receptor.The bHLH-PAS dioxin (aryl hydrocarbon) 1 receptor (1, 2) is a member of a growing family of transcription factors that includes, among others, a number of circadian rhythmicity regulatory proteins (3, 4) and proteins such as the hypoxia-inducible factor HIF-1␣ (5), its endothelial cell-specific homologue E-PAS/HLF (6, 7), and Arnt (8), which is a partner factor both of the dioxin receptor and of hypoxia-inducible factors. These factors, with the possible exception of Arnt, appear to be conditionally regulated and respond to different stimuli. The dioxin receptor is activated by dioxin (9) and related environmental pollutants, whereas HIF 1␣ is activated under conditions of oxygen deprivation to induce the expression of genes encoding, e.g. erythropoietin and vascular endothelial growth factor (10).In the absence of ligand the dioxin receptor is present in a latent conformation in the cytoplasmic compartment of the cell (11) associated with the molecular chaperone hsp90 (12). Hsp90 is required both for maintaining the dioxin receptor in a latent non-DNA binding state and a ligand binding conformation (13). Expression of the dioxin receptor in mutant yeast cells containing reduced levels of hsp90 abolishes ligand responsiveness demonstrating the critical importance of hsp90 for dioxin receptor function (14, 15). The nuclear form of the dioxin receptor interacts with Arnt (16 -18) and does no longer posses the ability to bind ligand and does not interact with the molecular chaperone hsp90 (12,13). This form of the receptor spe...

Signaling pathway for 2,3,7,8-tetrachlorodibenzo- p-dioxin-induced TNF-α production in differentiated THP-1 human macrophages

“…AhR is a ligand-activated transcription factor belonging to a basic-helix-loop-helix (bHLH)/Per-ARNT-Sim (PAS) family (Schmidt and Bradfield, 1996). At a resting state, AhR exists in the cytoplasm as an inactive complex with a chaperone, heat shock protein 90, X-associated protein 2, and cochaperone protein, p23 (Whitelaw et al, 1993;Ma and Whitlock, 1997;Kazlauskas et al, 1999). When TCDD binds to the cytoplasmic AhR, AhR is released from the complex and translocates to the nucleus.…”

Two Parallel Pathways Mediate Cytoplasmic Localization of the Dioxin (Aryl Hydrocarbon) Receptor

“…In the presence of ligand the receptor accumulates in the nucleus (1,11) where it interacts with the general dimerization partner factor ARNT (12). This event induces release of the hsp90 complex (4,13). Nuclear accumulation of the receptor is regulated by a nuclear localization signal present in the bHLH domain (14) and by the hsp90 complex which regulates the interaction between the import machinery and the activated form of the receptor (1).…”

“…For instance, the dioxin receptor is a ligand-dependent transcription factor which, in the absence of ligand, is found in the cytoplasmic compartment of the cell or, depending on cell type, evenly distributed between the cytoplasm and the nucleus (1,2). This non-activated form of the receptor interacts with the hsp90 1 -dependent molecular chaperone complex (3) and its associated proteins, such as the hsp90-associated factor p23 (4,5) and the dioxin receptor specific immunophilin XAP-2 (6 -8).…”

Aryl hydrocarbon receptor regulates Stat1 activation and participates in the development of Th17 cells

“…Ahr, also known as dioxin receptor, is a ligand-activated transcription factor that belongs to the basic-helix-loop-helix-PER-ARNT-SIM family (12,13). Ahr is present in the cytoplasm, where it forms a complex with heat shock protein (HSP) 90, Ahr-interacting protein (AIP), and p23 (14)(15)(16). Upon binding with a ligand, Ahr undergoes a conformation change, translocates to the nucleus, and dimerizes with Ahr nuclear translocator (Arnt).…”