Revised Manuscript MS C2900 Regulation of mRNA Expression of Matrix Extracellular Phosphoglycoprotein (MEPE)/ Osteoblast/Osteocyte Factor 45 (OF45) by Fibroblast Growth Factor 2 in Cultures of Rat Bone Marrow-derived Osteoblastic Cells

Matrix extracellular phosphoglycoprotein (MEPE)/ osteoblast/osteocyte factor 45 (OF45) is a recently isolated RGD-containing matrix protein that acts as the tumor-derived phosphaturic factor in oncogenic hypophosphatemic osteomalacia. It is also highly expressed by osteoblasts and osteocytes. We examined the regulation of MEPE/OF45 mRNA expression in osteoblastic cells derived from high-density cultures of primary rat bone marrow stromal cells incubated with dexamethasone, β-glycerophosphate and ascorbic acid. The level of MEPE/OF45 mRNA in these cells was down-regulated by the addition of fibroblast growth factor 2 (FGF2) for 48 h. These effects were observed in a dose-dependent manner between 2 and 10 ng/ml. FGF2 also reduced the expression of osteocalcin mRNA in these cells. In contrast, bone sialoprotein mRNA expression was increased by FGF2, while α1(I) procollagen mRNA expression was not altered. Additionally, neither Runx-2 and osterix mRNA expression nor cell proliferation were affected by the addition of FGF2 in these high-density cultures, indicating that regulation by FGF2 may not be dependent on these transcription factors or on the proliferation of cells. Experiments using actinomycin D indicated that FGF2 decreased the stability of the MEPE/OF45 mRNA. Moreover, inhibition of a specific