Abstract
FGF-23 has arisen as an early biomarker of renal dysfunction, but at the onset of chronic kidney disease (CKD), data suggest that FGF-23 may be produced independently of the parathyroid hormone (PTH), 1,25(OH)2 -vitamin D3 signaling axis. Iron status is inversely correlated to the level of circulating FGF-23, and improvement in iron bioavailability within patients correlates with a decrease in FGF-23. Alternately, recent evidence also supports a regulatory role of inflammatory cytokines in the modulation of FGF-23 expression. To determine the identity of the signal from the kidney-inducing upregulation of osteocytic FGF-23 at the onset of CKD, we utilized a mouse model of congenital CKD that fails to properly mature the glomerular capillary tuft. We profiled the sequential presentation of indicators of renal dysfunction, phosphate imbalance, and iron bioavailability and transport to identify the events that initiate osteocytic production of FGF-23 during the onset of CKD. We report here that elevations in circulating intact-FGF-23 coincide with the earliest indicators of renal dysfunction (P14), and precede changes in serum phosphate or iron homeostasis. Serum PTH was also not changed within the first month. Instead, production of the inflammatory protein IL-1β from the kidney and systemic elevation of it in the circulation matched the induction of FGF-23. IL-1β's ability to induce FGF-23 was confirmed on bone chips in culture and within mice in vivo. Furthermore, neutralizing antibody to IL-1β blocked FGF-23 expression in both our congenital model of CKD and a second nephrotoxic serum-mediated model. We conclude that early CKD resembles a situation of primary FGF-23 excess mediated by inflammation. These findings do not preclude that altered mineral availability or anemia can later modulate FGF-23 levels but find that in early CKD they are not the driving stimulus for the initial upregulation of FGF-23. © 2020 American Society for Bone and Mineral Research.