The regulation of the aryl hydrocarbon receptor (AhR) protein levels has been an 
area of keen interest, given its important role in mediating the cellular 
adaptation and toxic response to several environmental pollutants. The carboxyl 
terminus of hsc70-interacting protein (CHIP) ubiquitin ligase was previously 
associated with the regulation of the aryl hydrocarbon receptor, although the 
mechanisms were not directly demonstrated. In this study, we established that 
CHIP could associate with the AhR at cellular levels of these two proteins, 
suggesting a potential role for CHIP in the regulation of the AhR complex. The 
analysis of the sucrose-gradient-fractionated in vitro translated AhR complexes 
revealed that CHIP can mediate hsp90 ubiquitination while cooperating with 
unidentified factors to promote the ubiquitination of mature unliganded AhR 
complexes. In addition, the immunophilin-like protein XAP2 was able to partially 
protect the AhR from CHIP-mediated ubiquitination in vitro. This protection 
required the direct interaction of the XAP2 with the AhR complex. Surprisingly, 
CHIP silencing in Hepa-1c1c7 cells by siRNA methods did not reveal the function 
of CHIP in the AhR complex, because it did not affect well-characterized 
activities of the AhR nor affect its steady-state protein levels. However, the 
presence of potential compensatory mechanisms may be confounding this particular 
observation. Our results suggest a model where the E3 ubiquitin ligase CHIP 
cooperates with other ubiquitination factors to remodel native AhR-hsp90 
complexes and where co-chaperones such as the XAP2 may affect the ability of 
CHIP to target AhR complexes for ubiquitination.