Role of heat shock protein 90 dissociation in mediating agonist-induced activation of the aryl hydrocarbon receptor.

The aryl hydrocarbon receptor (AhR) is a cytosolic basic helix-loop-helix protein that associates with a chaperone complex that includes two molecules of heat shock protein 90 (HSP90). It has been hypothesized that after ligand binding, the AhR dissociates from its chaperone complex and translocates into the nucleus, where it heterodimerizes with its DNA binding partner, the AhR nuclear translocator (ARNT), and activates specific genes. However, it remains unclear whether nuclear translocation of the AhR occurs before or after dissociation of the HSP90/chaperone complex. Because sodium molybdate stabilizes the AhR-HSP90 interaction and inhibits the gene activation of a number of steroid receptors, we reasoned that molybdate would be a useful tool in delineating the role of HSP90 dissociation in AhR nuclear translocation. In this study, we demonstrate that molybdate inhibits AhR gene activation in both HepG2 and Hepa-1 cells in a concentration-dependent manner and protects the AhR against agonist-induced proteolysis. In addition, we demonstrate that AhR/ARNT dimerization, but not nuclear translocation of the AhR, is inhibited by molybdate. This indicates that 1) HSP90 dissociation is not required for nuclear translocation of the AhR, 2) HSP90 dissociation is essential for formation of the AhR/ARNT heterodimer, and 3) an additional undefined regulatory step is required for AhR/ARNT dimerization in the nucleus.