Transcription factor C/EBPβ isoform ratio regulates osteoclastogenesis through MafB

Disequilibrium between bone-forming osteoblasts and bone-resorbing osteoclasts is central to many bone diseases. Here, we show that dysregulated expression of translationally controlled isoforms of CCAAT/enhancer-binding protein beta (C/EBP beta) differentially affect bone mass. Alternative translation initiation that is controlled by the mammalian target of rapamycin (mTOR) pathway generates long transactivating (LAP*, LAP) and a short repressive (LIP) isoforms from a single C/EBPb transcript. Rapamycin, an inhibitor of mTOR signalling increases the ratio of LAP over LIP and inhibits osteoclastogenesis in wild type (WT) but not in C/EBP beta null (c/ebp beta(-/-)) or in LIP knock-in (L/L) osteoclast precursors. C/EBP beta mutant mouse strains exhibit increased bone resorption and attenuated expression of MafB, a negative regulator of osteoclastogenesis. Ectopic expression of LAP and LIP in monocytes differentially affect the MafB promoter activity, MafB gene expression and dramatically affect osteoclastogenesis. These data show that mTOR regulates osteoclast formation by modulating the C/EBP beta isoform ratio, which in turn affects osteoclastogenesis by regulating MafB expression. The EMBO Journal (2009) 28, 1769-1781. doi:10.1038/emboj.2009.127; Published online 14 May 2009