M. Wan et al., Transcriptional mechanisms of bone morphogenetic protein-induced osteoprotegrin gene expression, J BIOL CHEM, 276(13), 2001, pp. 10119-10125
Osteoprotegerin (OPG), an osteoblast-secreted decoy receptor, specifically
binds to osteoclast differentiation factor and inhibits osteoclast maturati
on. Members of the transforming growth factor-beta superfamily including bo
ne morphogenetic proteins (SMPs) stimulate OPG mRNA expression. In this stu
dy, we have characterized the transcription mechanism of BMP-induced OPG ge
ne expression. Transfection of Smadl and a constitutively active BMP type I
A receptor ALK3 (Q233) stimulated the OPG promoter. Deletion analysis of th
e OPG promoter identified two Hoxc-8 binding sites that respond to BMP stim
ulation. Glutathione S-transferase-Hoxc-8 protein binds to these two Hox si
tes specifically. Consistent with the transfection results of the native pr
omoter, ALK3 or Smadl linker region, which interacts with Hoxc-8, stimulate
d the activation of the reporter construct with the two Hox sites. Overexpr
ession of Hoxc-8 inhibited the induced promoter activity. When the two Hox
binding sites were mutated, ALK3 or Smadl linker region no longer activated
the transcription. Importantly, Smadl linker region induced both OPG promo
ter activity and endogenous OPG protein expression in 2T3 osteoblastic cell
s. The medium from cells transfected with Smadl linker region expression pl
asmid effectively inhibited osteoclastogenesis. Collectively, our data indi
cate that Hox sites mediate both OPG promoter construct activity and endoge
nous OPG gene expression in response to BMP stimulation.