Linked Deficiencies in Extracellular PP_i and Osteopontin Mediate Pathologic Calcification Associated With Defective PC-1 and ANK Expression

Osteopontin and PPi both suppress hydroxyapatite deposition. Extracellular PPi deficiency causes spontaneous hypercalcification, yet unchallenged osteopontin knockout mice have only subtle mineralization abnormalities. We report that extracellular PPi deficiency promotes osteopontin deficiency and correction of osteopontin deficiency prevents hypercalcification, suggesting synergistic inhibition of hydroxyapatite deposition. Nucleotide pyrophosphatase phosphodiesterase (NPP) isozymes including PC-1 (NPP1) function partly to generate PP_i, a physiologic calcification inhibitor. PP_i transport is modulated by the membrane channel protein ANK. Spontaneous articular cartilage calcification, increased vertebral cortical bone formation, and peripheral joint and intervertebral ossific ankylosis are associated with both PC-1 deficiency and expression of truncated ANK in ank/ank mice. To assess how PC-1, ANK, and PP_i regulate both calcification and cell differentiation, we studied cultured PC-1^-/- and ank/ank mouse calvarial osteoblasts. PC-1^-/- osteoblasts demonstrated ∼50% depressed NPP activity and markedly lowered extracellular PP_i associated with hypercalcification. These abnormalities were rescued by transfection of PC-1 but not of the NPP isozyme B10/NPP3. PC-1^-/- and ank/ank cultured osteoblasts demonstrated not only comparable extracellular PP_i depression and hypercalcification but also marked reduction in expression of osteopontin (OPN), another direct calcification inhibitor. Soluble PC-1 (which corrected extracellular PP_i and OPN), and OPN itself (≥15 pg/ml), corrected hypercalcification by PC-1^-/- and ank/ank osteoblasts. Thus, linked regulatory effects on extracellular PP _i and OPN expression mediate the ability of PC-1 and ANK to regulate calcification.