Bone morphogenetic protein-2 activity is regulated by secreted phosphoprotein-24 kd, an extracellular pseudoreceptor, the gene for which maps to a region of the human genome important for bone quality.

The material properties of bone are the sum of the complex and interrelated anabolic and catabolic processes that modulate formation and turnover. The 2q33-37 region of the human genome contains quantitative trait loci important in determining the broadband ultrasound attenuation (an index of trabecular microarchitecture, bone elasticity, and susceptibility to fracture) of the calcaneus, but no genes of significance to bone metabolism have been identified in this domain. Secreted phosphoprotein-24 kd (SPP24 or SPP2) is a novel and relatively poorly characterized growth hormone-regulated gene that maps to 2q37. The purpose of this review is to summarize the status of research related to spp24 and how it regulates bone morphogenetic protein (BMP) bioactivity in bone. SPP24 codes for an extracellular matrix protein that contains a high-affinity BMP-2-binding transforming growth factor-beta receptor II homology 1 loop similar to those identified in fetuin and the receptor itself. SPP24 is transcribed primarily in the liver and bone. High levels of spp24 (a hydroxyapatite-binding protein) are found in bone, and small amounts are found in fetuin-mineral complexes. Full-length secretory spp24 inhibits ectopic bone formation, and overexpression of spp24 reduces murine bone mass and density. Spp24 is extremely labile to proteolysis, a process that regulates its bioactivity in vivo. For example, an 18.5-kd degradation product of spp24, designated spp18.5, is pro-osteogenic. A synthetic cyclized Cys(1)-to-Cys(19) disulfide-bonded peptide (BMP binding peptide) corresponding to the transforming growth factor-beta receptor II homology 1 domain of spp24 and spp18.5 binds BMP-2 and increases the rate and magnitude of BMP-2-mediated ectopic bone formation. Thus, the mechanism of action of spp18.5 and spp24 may be to regulate the local bioavailability of BMP cytokines. SPP24 is regulated by growth hormone and 3 major families of transcription factors (nuclear factor of activated T cells, CCAAT/enhancer-binding protein, Cut/Cux/CCAAT displacement protein) that regulate mesenchymal cell proliferation, embryonic patterning, and terminal differentiation. The gene contains at least 2 single nucleotide polymorphisms. Given its mechanism of action and sequence variability, SPP24 may be an interesting candidate for future studies of the genetic regulation of bone mass, particularly during periods of BMP-mediated endochondral bone growth, development, and fracture healing.