Carboxyl terminus of Hsp70-interacting protein (CHIP or STUB1) is an E3 ligase that regulates the stability of several proteins involved in tumor growth and metastasis. However, the role of CHIP in bone growth and bone remodeling in vivo has not been reported. This study was undertaken to investigate the role and mechanism of CHIP in regulation of bone mass and bone remodeling.
The bone phenotype of Chip(-/-) mice was assessed by histologic, histomorphometric, and micro-computed tomographic analyses. The mechanism by which CHIP regulates the degradation of tumor necrosis factor receptor-associated factor 6 (TRAF6) and the inhibition of NF-κB signaling was examined by immunoprecipitation, Western blot, and luciferase reporter assays.
Deletion of the Chip gene led to an osteopenic phenotype and increased osteoclast formation. TRAF6, an adaptor protein that is a key regulator of NF-κB signaling and is critical for RANKL-induced osteoclastogenesis, was up-regulated in osteoclasts from Chip(-/-) mice. CHIP interacted with TRAF6 to promote TRAF6 ubiquitination and proteasome degradation. Further, CHIP inhibited p65 nuclear translocation, leading to the repression of TRAF6-mediated NF-κB transcription.
CHIP inhibits NF-κB signaling by promoting TRAF6 degradation and plays an important role in osteoclastogenesis and bone remodeling. These findings suggest that CHIP may be a novel therapeutic target in bone loss-associated disorders.