Transcription Factor Hematopoietically Expressed Homeobox Protein (Hhex) Negatively Regulates Osteoclast Differentiation by Controlling Cyclin-Dependent Kinase Inhibitors

We investigated the role of hematopoietically expressed homeobox protein (Hhex) in osteoclast development. Trimethylation of lysine 27 of histone H3 at the cis-regulatory element of Hhex was maintained and that of lysine 4 was reduced during receptor activator of nuclear factor κB ligand (RANKL)-induced osteoclastogenesis, which was associated with a reduction of Hhex expression. Overexpression of Hhex in bone marrow–derived macrophages inhibited, whereas Hhex suppression promoted, RANKL-induced osteoclastogenesis in vitro. Conditional deletion of Hhex in osteoclast-lineage cells promoted osteoclastogenesis and reduced cancellous bone volume in mice, confirming the negative regulatory role of Hhex in osteoclast differentiation. Expression of cyclin-dependent kinase inhibitors such as Cdkn2a and Cdkn1b in osteoclast precursors was negatively regulated by Hhex, and Hhex deletion increased the ratio of cells at the G1 phase of the cell cycle. In conclusion, Hhex is an inhibitor of osteoclast differentiation that is regulated in an epigenetic manner and regulates the cell cycle of osteoclast precursors and the skeletal homeostasis.