CLONING AND IDENTIFICATION OF HUMAN SEA AS A NOVEL INHIBITOR OF OSTEOCLAST FORMATION AND BONE-RESORPTION

Increased osteoclast activity is responsible for the enhanced bone des truction in postmenopausal osteoporosis, Paget's disease, bone metasta sis, and hypercalcemia of malignancy. However, the number of known inh ibitory factors that block osteoclast formation and bone resorption ar e limited. Therefore, we used an expression-cloning approach to identi fy novel factors produced by osteoclasts that inhibit osteoclast activ ity. A candidate clone was identified and isolated from a human osteoc last-like multinucleated cell (MNC) cDNA library, named osteoclast inh ibitory peptide-1 (OIP-1), and the cDNA sequence was determined. This sequence matched that of the recently identified human stem cell antig en, was structurally similar to the mouse Ly-6 gene family, and the se quence predicted it was a glycosyl phosphatidyl inositol (GPI)-anchore d protein that had a cleavable COOH-terminal peptide. Western blot ana lysis of conditioned media from 293 cells transfected with the OIP-1 c DNA clone confirmed that OIP-1 was released into the media as a membra ne-bound GPI-linked protein. Interestingly, both recombinant OIP-1 exp ressed in Escherichia coli (which does not have GPI linker) and OIP-1 expressed by mammalian cells significantly reduced osteoclast-like MNC formation induced by 1,25-dihydroxyvitamin D-3 or PTH-related protein in mouse and human bone marrow cultures, and inhibited Ca-45 release from prelabeled bone in fetal rat organ cultures. In contrast, recombi nant OIP-1 did not inhibit the growth of a variety of other cell types . These data indicate that OIP-1 is a novel, specific inhibitor of ost eoclast formation and bone resorption.