Regulation of transcription factors by XAP-2, a member of the Hsp90 chaperone complex

The AhR belongs to the bHLH/PAS family of transcription factors, which&#13;\nmediates biological effects of environmental pollutants such as&#13;\n2,3,7,8-tetrachlorodibenzo-rho-dioxin (TCDD). In the absence of ligand&#13;\nthe AhR is located in the cytoplasmatic compartment of the cell&#13;\nassociated with the molecular chaperone Hsp90, p23 and a member of the&#13;\nimmunophilin family of proteins, XAP-2. Upon ligand binding the receptor&#13;\ntranslocates to the nuclear compartment of the cell where it dimerizes&#13;\nwith its partner factor ARNT. This dimerization event induces the release&#13;\nof the Hsp90 complex and subsequent binding of the AhR/ARNT complex to&#13;\npromoter regions of TCDD regulated genes such as Cyp1A1 and Cyp1A2.&#13;\n&#13;\n&#13;\n&#13;\nFollowing transcriptional activation AhR is exported out of the nucleus&#13;\nby the export protein CRM1. CRM1 binds to nuclear export sequences (NES)&#13;\nwithin transcription factors and in turn mediates export of the AhR to&#13;\nthe cytoplasm. In this study we show that nuclear export of the AhR is&#13;\nmediated by two different NES depending on ligand status. In paper I we&#13;\nshow that in the absence of ligand, export of the AhR is mediated by a&#13;\nNES located in the PAS A domain of the AhR. In contrast, nuclear export&#13;\nof the ligand activated AhR is mediated by a NES located in the bHLH&#13;\ndomain of the receptor. In paper II we show that cytoplasmatic&#13;\nlocalization of the AhR is mediated by two non-overlapping mechanisms. One is mediated through CRM1 and the second mechanism to maintain the AhR&#13;\nin the cytoplasm is dependent on the Hsp90 associated co-chaperone XAP-2.&#13;\nHere we show that XAP-2 mediates anchoring of the non-activated AhR to&#13;\nactin filaments in the cytoplasm. XAP-2 is a 37kDa Hsp90 associated&#13;\nprotein that belongs to the immunophilin family of proteins such as&#13;\nFKBP51 and 52. It is mainly located in the cytoplasmatic compartment of&#13;\nthe cell where it protects the AhR against proteosomal degradation by&#13;\ninhibit ubiquitinylation of the AhR in the cytosol. XAP-2 has previously&#13;\nbeen identified as an Hsp90 associated protein that specifically&#13;\ninteracts with the AhR. This notion, however have been challenged upon&#13;\nfindings that XAP-2 interacts with other proteins such as PPARalpha. In&#13;\nthis study we show that XAP-2 interacts with both ERalpha and TRbeta1.&#13;\nInterestingly, this interaction is isoform specific. Furthermore, in&#13;\npaper III we show that XAP-2 regulates the transcription of hypothalamic&#13;\nthyrotropin releasing hormone (TRH) in vivo demonstrating an important&#13;\nregulatory and physiological role for an immunophilin like factor in&#13;\nvivo.&#13;\n&#13;\n&#13;\n&#13;\nIn paper IV we observe that XAP-2 exerts a repressive effect on ERalpha&#13;\nbut not ERbeta transcriptional activation. In addition, this effect is&#13;\nmediated by the co-activator TIF-2. Taken together our results show that&#13;\nXAP-2 function as a general co-factor that modulates numerous cellular&#13;\nregulatory pathways including the AhR, TR and ER. These studies provide&#13;\nnew perspectives in the understanding of the regulation of transcription&#13;\nfactors by members of the Hsp90 complex.

Topic: Heat shock proteins research
Subfield: Molecular Biology
Field: Biochemistry, Genetics and Molecular Biology
Domain: Life Sciences