1,25-Dihydroxyvitamin D₃ and extracellular inorganic phosphate activate mitogen-activated protein kinase pathway through fibroblast growth factor 23 contributing to hypertrophy and mineralization in osteoarthritic chondrocytes

Abstract

Hypertrophy and impaired mineralization are two processes closely associated with osteoarthritis (OA). 1,25-dihydroxyvitamin D(3) (1a,25(OH)(2)D(3)) and inorganic phosphate (Pi) are two important factors that are implicated in calcium and phosphate homeostasis of bone metabolism and both can be regulated by the circulating phosphaturic factor fibroblast growth factor 23 (FGF23). The objective of this study was to investigate the role of 1a,25(OH)(2)D(3) and Pi and the molecular mechanism through which they contribute to hypertrophy and mineralization in human osteoarthritic chondrocytes. For this purpose, primary human chondrocytes were obtained from articular cartilage which was collected after total knee replacement surgery in OA patients. FGF23, fibroblast growth factor receptor 1c (FGFR1c), vitamin D(3) receptor (VDR), and phosphate inorganic transporter-1 and -2 (PiT-1 and PiT-2) expression levels were evaluated and found to be significantly higher in OA chondrocytes compared with normal. In addition, we observed that the binding of FGF23 to FGFR1c was stronger in OA chondrocytes compared with normal. Chromatin immunoprecipitation (ChIP) assay revealed, for the first time, the presence of two vitamin D response elements (VDREs) in the FGF23 promoter. Treatment of normal chondrocytes with 1a,25(OH)(2)D(3) or Pi resulted in significant up-regulation of VDR, FGF23, PiT-1, PiT-2 mRNA and protein levels, extracellular signal-regulated kinases 1/2 (ERK1/2) phosphorylation and induction of hypertrophy markers collagen type X (COL10A1), osteopontin (OPN), osteocalcin (OC), catabolic markers metalloproteinase-13 (MMP-13) and the apoptotic marker caspase-9. Furthermore, VDR silencing in OA chondrocytes negatively regulated FGF23, COL10A1, OPN, OC, MMP-13 and caspase-9 expressions and ERK1/2 phosphorylation. Finally, combined VDR silencing and PiT-1, PiT-2 inhibition in OA chondrocytes resulted in additive down-regulation of FGF23 expression, ERK1/2 activation and COL10A1, OPN, OC, MMP-13 and caspase-9 expression levels. We propose that 1a,25(OH)(2)D(3) and Pi act synergistically through FGF23 signaling and ERK1/2 phosphorylation contributing to late hypertrophic events and impaired mineralization in osteoarthritic chondrocytes.

1,25-Dihydroxyvitamin D3 Regulation of Fibroblast Growth Factor-23 Expression in Bone Cells: Evidence for Primary and Secondary Mechanisms Modulated by Leptin and Interleukin-6

“…However, the identity of a phosphate “sensor” in osteocytes that might detect high blood phosphate levels and signal FGF23 induction has remained elusive, but recently phosphate signaling of FGF23 has been shown to be dominant over that of 1,25(OH) 2 D 3 /VDR [47]. Additionally, high phosphate [48], along with PHEX and DMP-1 [49], has been demonstrated to require the FGF23 receptor for regulation of FGF23 gene expression. This implies the existence of a crucial autoregulatory circuit that involves extracellular FGF23 and its cognate receptor on osteocytes.…”

Fibroblast Growth Factor 23 drives MMP13 expression in human osteoarthritic chondrocytes in a Klotho-independent manner

“…As a consequence, the cells start to express several proteins involved in endochondral ossification such as type X collagen, Indian hedgehog, annexins, VEGF and collagenase 3 16,17 . Interestingly, FGF23, a critical factor of phosphate homeostasis shown to be involved in endochondral ossification 18 is also produced by both growth plate hypertrophic 19 and OA 20,21 chondrocytes suggesting that it may play a role in the switch of OA chondrocytes towards hypertrophy. Indeed, it has been demonstrated that FGF23 triggers normal human chondrocyte toward a hypertrophic phenotype mainly by the upregulation of RUNX2 20 .…”

Inferring Genes and Biological Functions That Are Sensitive to the Severity of Toxicity Symptoms

“…The authors of some previous studies have reported the relatedness between hypertrophy and the extracellular region of the heart, chondrocytes, and astrocytes [24,25,26,27,28,29,30,31]. In particular, Yang et al [29] concluded that SCUBE3, which is an extracellular protein, may account for the accelerated onset and progression of cardiac hypertrophy.…”