Divergent Roles of Hepatitis B Virus X-Associated Protein 2 (XAP2) in Human versus Mouse Ah Receptor Complexes

The aryl hydrocarbon receptor (AhR) mediates the toxicologic and carcinogenic properties of 2,3,7,8-tetrachlorodibenzo-p-dioxin. In the cytoplasm, the AhR is complexed with a dimer of hsp90, and the hepatitis B virus X-associated protein 2 (XAP2). Most studies that have examined the ability of XAP2 to modulate the AhR have characterized the mouse receptor (mAhR). However, the amino acid sequence of mAhR is significantly different from human AhR (hAhR) in the carboxy terminal half of the protein, and this could lead to differences in the behavior of the two receptors. mAhR-yellow fluorescent protein (YFP) and hAhR-YFP were used to compare nucleocytoplasmic shuttling properties and the ability of XAP2 to modulate their activity. As reported previously, mAhR localized predominantly in the nucleus and was redistributed to the cytoplasm by coexpression of XAP2 in COS-1 cells. Leptomycin B treatment revealed that XAP2 blocked mAhR-YFP translocation to the nucleus in the absence of ligand. In contrast, hAhR-YFP localized predominantly in the cytoplasm, and coexpression of XAP2 did not affect this localization, and did not block nuclear accumulation in the presence of leptomycin B. An XAP2 fusion protein with a nuclear localization signal fused to the carboxy terminus (XAP2-NLS) was utilized to test whether this protein could drag the AhR into the nucleus. Coexpression of mAhR-YFP and XAP2-NLS caused cytoplasmic localization of the mAhR, while hAhR-YFP was partially localized in the nucleus, suggesting that XAP2 remains bound to the hAhR during nucleocytoplasmic shuttling. The presence of XAP2 in the ligand-bound hAhR complex enhanced the rate of nuclear translocation but repressed transcriptional activity. Together, these results suggest that the hAhR differs biochemically from the mAhR.