Pmepa1 induced by RANKL-p38 MAPK pathway has a novel role in osteoclastogenesis

Osteoclasts are multinucleated cells formed by fusion of preosteoclasts (POCs) derived from cells of the monocyte/macrophage lineage. We have reported a culture system that supports the formation of POCs from stroma-depleted rat bone marrow cells. Global gene expression analysis of this culture system identified genes highly expressed in POCs. Here, we have analyzed the expression and function of one of these highly expressed genes, prostate transmembrane protein androgen induced 1 (Pmepa1), a target of TGF-β and binds Nedd4 ubiquitin ligase, which plays a role in intracellular trafficking. We show here that the expression of Pmepa1 was strongly induced by RANKL in mouse bone marrow macrophage and in the osteoclast precursor cell line RAW-D. The expression of Pmepa1 was increased at 24 hr of culture, but was decreased at 72 hr. Pmepa1 protein was localized to intracellular vesicle membrnane of mononuclear cells, some of which were cathepsin-K positive. RANKL-induced expression of Pmepa1 was significantly reduced by inhibitors of p38 MAPK signaling. Pmepa1 siRNA suppressed the formation of osteoclasts in RAW-D cells, and inhibited the expression of cathepsin K and c-fos but not RANK. In addition, inhibition of Pmepa1 expression reduced the surface expression of RANK in RAW-D cells induced by RANKL. These results demonstrate that Pmepa1 is induced by RANK-p38 MAPK pathway signaling, and upregulates cell surface expression of RANK, suggesting that Pmepa1 plays a role in osteoclastogenesis and osteoclast signaling.