FGF23-Driven Kidney Adaptations: Unveiling Chromatin Remodeling and Transcriptional Dynamics with Implications for Disease

Abstract

The hormone 1,25(OH)2-vitamin D3-(‘1,25D’) is required for maintaining musculoskeletal structure and function. The synthesis of 1,25D from liver-produced 25(OH)D3 is directed in final metabolizing steps in the kidney proximal tubule (PT) by the catabolic enzyme vitamin D 24-hydroxylase (CYP24A1) and anabolic vitamin D 1α- hydroxylase (CYP27B1). FGF23 and 1,25D lower 1,25D by increasing CYP24A1 and suppressing CYP27B1, in contrast to the actions of PTH. Using scRNAseq after FGF23 injections into mice, we found that the E-twenty-six family member ETV1 was stimulated in PT S1-S2 cells in parallel with increased Cyp24a1, and a 10-fold induction of ETS occupancy across the genome. In vivo, FGF23 induced localization of ETV1 to megalin+ PT nuclei. ETV1 was rapidly recruited to genomic enhancers within the Cyp24a1 promoter and a kidney-specific distal enhancer (DS1) responsible for both FGF23 and PTH activities that overlapped with VDR binding. PTH suppressed nuclear ETV1 protein in vivo, and opposed FGF23 increases by completely removing ETV1 from the Cyp24a1 enhancers. In vitro, ETV1 drove CYP24A1 mRNA in HEK-mKL cells, and ETV1 protein was upregulated by FGF23, but not by 1,25D. However, 1,25D strongly enhanced coprecipitated ETV1-VDR complexes. Conditional targeting of kidney epithelial Etv1 in mice resulted in ‘FGF23 resistance’ with elevated FGF23 and almost complete blockade of Cyp24a1 mRNA responses, which altered systemic 1,25D metabolism. FGF23 was shown to inhibit COP1 nuclear localization, enhancing ETV1 protein stability. Further, a ‘PTH mimetic’ SIK inhibitor abolished ETV1 production by enhancing COP1-mediated degradation, whereas COP1 knockdown and proteasome inhibition, rescuing SIKimediated ETV1 suppression. In sum, our findings demonstrate that ETV1 acts as a novel ‘unifying’ TF positively regulating FGF23/KL bioactivity with VDR on Cyp24a1 and opposing PTH actions, thus opening novel pathways needed to address severe diseases involving FGF23 and vitamin D.