Pigment epithelium derived factor suppresses expression of Sost/Sclerostin by osteocytes: implication for its role in bone matrix mineralization.

Mutations in Serpinf1 gene which encodes pigment epithelium derived factor (PEDF) lead to osteogenesis imperfecta type VI whose hallmark is defective mineralization. Mechanisms by which PEDF regulates matrix mineralization remain unknown. We examined effect of exogenous PEDF on expression of osteoblastic and osteocytic related genes and proteins in mineralizing osteoblast culture. Mineralizing human osteoblasts supplemented with exogenous PEDF for 14 days deposited 47% more mineral than cells cultured without PEDF. Analysis of selected gene expression by cells in mineralizing cultures supplemented with exogenous PEDF showed reduction in expression of Sclerostin (Sost) by 70%, matrix extracellular phosphoglycoprotein (MEPE) by 75% and dentin matrix protein (DMP-1) by 20% at day 14 of culture. Phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX) expression was not affected. Western blotting and immunoprecipitation showed that sclerostin and MEPE synthesis by osteocytes were reduced by 50% and 60% respectively in mineralizing osteoblasts containing exogenous PEDF. Primary osteocytes exposed to PEDF also reduced synthesis of Sost/sclerostin by 50% within 24 h. For osteoblastic genes, Bone sialoprotein (BSP) was expressed at 75% higher by day 7 in cultures containing exogenous PEDF while Col1A1 expression remained high at all-time points. Total beta-catenin was increased in mineralizing osteoblastic cells suggesting increased Wnt activity. Taken together, the data indicate that PEDF suppressed expression of factors that inhibit mineralization while enhancing those that promote mineralization. The findings also suggest that PEDF may regulate Sost expression by osteocytes leading to enhanced osteoblastic differentiation and increased matrix mineralization.