Regulation of 1 and 24 hydroxylation of vitamin D metabolites in the proximal tubule

Abstract

Calcium and phosphate are critical for numerous physiological processes. Consequently, the plasma concentration of these ions are tightly regulated. Calcitriol, the active form of vitamin D, is a positive modulator of mineralization as well as calcium and phosphate metabolism. The molecular and physiological effects of calcitriol are well documented. Calcitriol increases blood calcium and phosphate levels by increasing absorption from the intestine, and resorption of bone. Calcitriol synthesis is a multistep process. A precursor is first made via skin exposure to UV, it is then 25-hydroxylated in the liver to form 25-hydroxyitamin D. The next hydroxylation step occurs in the renal proximal tubule via the 1-αhydroxylase enzyme (encoded by CYP27B1) thereby generating 1,25-dihydroxyvitamin D, that is, calcitriol. At the same site, the 25-hydroxyvitamin D 24-hydroxlase enzyme encoded by CYP24A1 can hydroxylate 25-hydroxyvitamin D or calcitriol to deactivate the hormone. Plasma calcitriol levels are primarily determined by the regulated expression of CYP27B1 and CYP24A1. This occurs in response to parathyroid hormone (increases CYP27B1), calcitriol itself (decreases CYP27B1 and increases CYP24A1), calcitonin (increases or decreases CYP24A1 and increases CYP27B1), FGF23 (decreases CYP27B1 and increases CYP24A1) and potentially plasma calcium and phosphate levels themselves (mixed effects). Herein, we review the regulation of CYP27B1 and CYP24A1 transcription in response to the action of classic phophocalciotropic hormones and explore the possibility of direct regulation by plasma calcium.

Keywords: CYP24A1; CYP27B1; CaSR; PTH; Vitamin D; calcitriol; calcium; kidney.

Figure 1.

Regulation of CYP27B1 and CYP24A1 transcription by phosphocalciotropic hormones, calcium and phosphate. Calcitonin and PTH are secreted from the thyroid and parathyroid glands, respectively. Both act to upregulate the transcription of CYP27B1. In addition, calcitonin affects CYP24A1 by either upregulating or attenuating its transcription depending on the situation. High extracellular calcium has been associated with a decrease in CYP27B1 transcription, while low extracellular calcium and hypophosphatemia are associated with increased CYP27B1 transcription. FGF23 production in bone suppresses CYP27B1 expression while enhancing CYP24A1 production. Finally, calcitriol itself feedback inhibits its own production by inhibiting CYP27B1 transcription and promoting its own inactivation by increasing CYP24A1 transcription. (Created with BioRender.com, supported by previous studies^–). (A color version of this figure is available in the online journal.)

Figure 2.

Transcriptional regulation of CYP27B1 and CYP24A1 by PTH and calcitriol in proximal tubular epithelial cells. PTH-induced transcriptional regulation occurs primarily by a PKA-mediated pathway. Upon binding to the G-protein-coupled receptor in the apical or basolateral membrane, adenylate cyclase is activated producing cAMP, which in turn activates protein kinase A (PKA). PKA phosphorylates CREB, activating it and permitting binding to CRE sites in the promoter of CYP27B1. Calcitriol signaling occurs via binding to the intracellular vitamin D receptor (VDR), which heterodimerizes with the retinoid X receptor (RXR) prior to DNA binding. Together, liganded VDR-RXR enter the nucleus where it can bind to vitamin D response elements of target genes. This increases CYP24A1 transcription, resulting in decreased calcitriol levels, and decreases CYP27B1 transcription, reducing calcitriol production. (Created with BioRender.com, supported by previous studies^,,,,). (A color version of this figure is available in the online journal.)

Figure 3.

Transcriptional regulation of CYP27B1 and CYP24A1 by calcium, phosphate, FGF23, and calcitonin. Extracellular calcium activates the calcium sensing receptor, which in turn activates mitogen-activated protein kinase (MAPK) and protein kinase C pathways. These pathways are proposed to interact with the CYP27B1 promoter to increase its activity at low extracellular calcium concentrations and to decrease promoter activity at high extracellular calcium concentrations. Hypophosphatemia is linked to increased expression of CYP27B1. However, the mechanism behind this is unknown. Fibroblast growth factor 23 (FGF23) binds to the FGF receptor (FGFR) and its cofactor klotho to activate the MAPK pathway, phosphorylating ERK1/2, which inhibits CYP27B1 transcription. FGF23 signaling in the proximal tubule is also associated with increased CYP24A1 levels, although the mechanism behind this is unknown. Calcitonin binds to its G-protein-coupled receptor, activating both the PKA and PKC pathways. PKC increases the expression of the C/EBP $\beta$ transcription factor thereby promoting CYP27B1 transcription. Calcitonin can also signal through the Sp1 and NF-Y transcription factors to upregulate CYP24A1 transcription, although there may be an additional mechanism whereby CYP24A1 transcription is attenuated. (Created with BioRender.com, supported by references^,–,,.) (A color version of this figure is available in the online journal.)