Fibroblast growth factor 23 (FGF23) production is regulated by both calciotropic hormones and inflammation. Consistent with this, elevated FGF23 levels are associated with inflammatory markers as well as parathyroid hormone (PTH) in various disease states, including chronic kidney disease (CKD). However, the molecular mechanisms underpinning Fgf23 transcription in response to these regulators are largely unknown. We therefore utilized chromatin immunoprecipitation followed by DNA sequencing (ChIP-seq) data from an osteocyte cell line to identify potential regulatory regions of the Fgf23 gene. Based on ChIP-seq analysis of enhancer-associated histone modifications, including H3K4 methylation and H3K9 acetylation, we discovered several potential enhancers for Fgf23, one of which was located 16kb upstream of the gene's transcriptional start site. Deletion of this putative enhancer from the mouse genome using CRISPR-Cas9 technology led to lower bone, thymus, and spleen expression of Fgf23 mRNA without altering circulating levels of the intact hormone, although as previously reported, only bone displayed significant basal expression. Nevertheless, lack of the -16kb enhancer blunted FGF23 upregulation in a tissue-specific manner by the acute inflammatory inducers lipopolysaccharide (LPS), interleukin-1-beta (IL-1β), and tumor necrosis factor-alpha (TNFα) in bone, non-osseous tissues, and in circulation. Lack of the -16kb enhancer also inhibited PTH-induced bone Fgf23 mRNA. Moreover, the absence of this Fgf23 enhancer in an oxalate diet-induced murine CKD model prevented the early onset induction of osseous, renal, and thymic Fgf23 mRNA levels and led to a significant blunting of elevated circulating intact FGF23 levels. These results suggest that -16kb enhancer mediates the induction of Fgf23 by inflammation and PTH and facilitates the increase in FGF23 expression in a murine model of CKD. As exemplified herein, these Fgf23 enhancer-deleted mice will provide a unique model in which to study the role of FGF23 expression in inflammatory diseases.
Keywords: CYTOKINES; DISORDERS OF CALCIUM/PHOSPHATE METABOLISM; GENETIC ANIMAL MODELS; OSTEOCYTES; PTH/VIT D/FGF23; TRANSCRIPTIONAL REGULATION.