Autocrine/Paracrine Action of Vitamin D on FGF23 Expression in Cultured Rat Osteoblasts

To explore the local mechanisms of fibroblast growth factor (FGF) 23 regulations, we examined the FGF23 expression patterns in an osteoblast culture model. The characteristics of cultured rat calvaria osteoblasts in half-confluence, confluence, osteoid deposition, and osteoid mineralization stages might reflect the proliferation, differentiation, maturation, and matrix mineralization stages, respectively. Compared with proliferating cells in half-confluence, FGF23 expression was upregulated by 7.5-fold at the mRNA level and 126% at the protein level in confluent differentiated cells as determined by real-time RT-PCR and Western blot analysis. Interestingly, mRNA levels of CYP27B1 (the gene coding for 1α-hydroxylase enzyme which catalyses the conversion of 1α,25-dihydroxyvitamin D, 1α,25[OH]₂D, from its inactive form, 25-hydroxycholecalciferol, 25[OH]D) and CYP24A (the gene coding for 24-hydroxylase, a target gene of 1α,25[OH]₂D) were significantly increased by twofold and 34-fold, respectively, in differentiated osteoblasts compared with proliferating cells. We next examined if the local production of 1α,25(OH)₂D might contribute to the FGF23 upregulation. We cultured osteoblasts in serum-free medium with or without 25-(OH)D (the substrate of 1α-hydroxylase). FGF23 mRNA levels were increased in proliferating cells (16-fold) and in differentiated cells (28-fold) by the physiological dose of 25-(OH)D₃ treatment. CYP27B1 was slightly but significantly upregulated and CYP24A was increased by 1,700-fold and 800-fold, respectively, in transcriptional levels. Because FGF23 was upregulated in confluent osteoblasts regardless of the presence or absence of 25-(OH)D in serum-free medium, we further examined the possible impact of cell communication on FGF23 expression. We treated osteoblasts with carbenoxolone, a gap junction Cx43 blocker in serum-free medium. The FGF23 mRNA level was reduced by 50% in confluent differentiated cells and slightly but not significantly reduced in half-confluent cells by carbenoxolone treatments. The results suggested that upregulation of FGF23 in differentiated osteoblast appeared to be due to increased autocrine/paracrine action of osteoblast-derived 1α,25(OH)₂D and increased cell communication, which were identified in cultured rat calvaria osteoblasts. These results indicate that FGF23 expression was stimulated not only by circulating calcitriol but also by locally produced 1α,25(OH)₂D. The local mechanisms of FGF23 expression remain to be characterized.