Abstract
The mitochondrial enzyme 25-hydroxyvitamin D 1α-hydroxylase, which is encoded by the CYP27B1 gene, converts 25OHD to the biological active form of vitamin D, 1,25-dihydroxyvitamin D (1,25(OH)2D). Renal 1α-hydroxylase activity is the principal determinant of the circulating 1,25(OH)2D concentration and enzyme activity is tightly regulated by several factors. Fibroblast growth factor-23 (FGF-23) decreases serum 1,25(OH)2D concentrations by suppressing CYP27B1 mRNA abundance in mice. In extra-renal tissues, 1α-hydroxylase is responsible for local 1,25(OH)2D synthesis, which has important paracrine actions, but whether FGF-23 regulates CYP27B1 gene expression in extra-renal tissues is unknown. We sought to determine whether FGF-23 regulates CYP27B1 transcription in the kidney and whether extra-renal tissues are target sites for FGF-23-induced suppression of CYP27B1. In HEK293 cells transfected with the human CYP27B1 promoter, FGF-23 suppressed promoter activity by 70%, and the suppressive effect was blocked by CI-1040, a specific inhibitor of extracellular signal regulated kinase 1/2. To examine CYP27B1 transcriptional activity in vivo, we crossed fgf-23 null mice with mice bearing the CYP27B1 promoter-driven luciferase transgene (1α-Luc). In the kidney of FGF-23 null/1α-Luc mice, CYP27B1 promoter activity was increased by 3-fold compared to that in wild-type/1α-Luc mice. Intraperitoneal injection of FGF-23 suppressed renal CYP27B1 promoter activity and protein expression by 26% and 60% respectively, and the suppressive effect was blocked by PD0325901, an ERK1/2 inhibitor. These findings provide evidence that FGF-23 suppresses CYP27B1 transcription in the kidney. Furthermore, we demonstrate that in FGF-23 null/1α-Luc mice, CYP27B1 promoter activity and mRNA abundance are increased in several extra-renal sites. In the heart of FGF-23 null/1α-Luc mice, CYP27B1 promoter activity and mRNA were 2- and 5-fold higher, respectively, than in control mice. We also observed a 3- to 10-fold increase in CYP27B1 mRNA abundance in the lung, spleen, aorta and testis of FGF-23 null/1α-Luc mice. Thus, we have identified novel extra-renal target sites for FGF-23-mediated regulation of CYP27B1.
Highlights
Vitamin D is activated by sequential hydroxylations in the liver and kidney to produce the active form of vitamin D; 1,25 dihydroxyvitamin D (1,25(OH)2D), which plays a central role in calcium and phosphorus homeostasis, skeletal growth and mineralization, and tissue differentiation [1]
We previously demonstrated that the suppression of CYP27B1 mRNA expression by Fibroblast growth factor-23 (FGF-23) depends upon mitogen activated protein kinase signaling pathway (MAPK) signaling via extracellular signal regulated kinase 1/2 (ERK1/2) [35,50,53]
We demonstrate that MAPK signaling via MEK/ERK1/2 plays a critical role in the transcriptional regulation of CYP27B1 in the kidney
Summary
Vitamin D is activated by sequential hydroxylations in the liver and kidney to produce the active form of vitamin D; 1,25 dihydroxyvitamin D (1,25(OH)2D), which plays a central role in calcium and phosphorus homeostasis, skeletal growth and mineralization, and tissue differentiation [1]. Hepatic synthesis of 25 hydroxyvitamin D (25OHD) is dependent on substrate concentration, which is determined by dietary intake of vitamin D and exposure to sunlight. Vitamin D 24-hydroxylase (24-hydroxylase) catalyzes the 24-hydroxylation of 25OHD to 24,25(OH)2D and of 1,25(OH)2D to 1,24,25(OH)3D; both reactions initiate the metabolic inactivation of vitamin D via the C24-oxidation pathway [19,20]. 24-hydroxylase activity is found in kidney, intestine, skin, macrophages and other tissues [21]. Circulating concentrations of 1,25(OH)2D primarily reflect its synthesis in the kidney; 1a-hydroxylase activity is found in keratinocytes, macrophages, osteoblasts and other tissues [22,23,24]. 1a-hydroxylase activity in extra-renal tissues is an important source of 1,25(OH)2D for its autocrine and paracrine functions in bone, skin, macrophages, prostate, parathyroid and several other tissues [26,27,28,29,30]
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have