Molecular insights into mineralotropic hormone inter-regulation

Abstract

The regulation of mineral homeostasis involves the three mineralotropic hormones PTH, FGF23 and 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃). Early research efforts focused on PTH and 1,25(OH)₂D₃ and more recently on FGF23 have revealed that each of these hormones regulates the expression of the other two. Despite early suggestions of transcriptional processes, it has been only recently that research effort have begun to delineate the genomic mechanisms underpinning this regulation for 1,25(OH)₂D₃ and FGF23; the regulation of PTH by 1,25(OH)₂D₃, however, remains obscure. We review here our molecular understanding of how PTH induces Cyp27b1 expression, the gene encoding the enzyme responsible for the synthesis of 1,25(OH)₂D₃. FGF23 and 1,25(OH)₂D₃, on the other hand, function by suppressing production of 1,25(OH)₂D₃. PTH stimulates the PKA-induced recruitment of CREB and its coactivator CBP at CREB occupied sites within the kidney-specific regulatory regions of Cyp27b1. PKA activation also promotes the nuclear translocation of SIK bound coactivators such as CRTC2, where it similarly interacts with CREB occupied Cyp27b1 sites. The negative actions of both FGF23 and 1,25(OH)₂D₃ appear to suppress Cyp27b1 expression by opposing the recruitment of CREB coactivators at this gene. Reciprocal gene actions are seen at Cyp24a1, the gene encoding the enzyme that degrades 1,25(OH)₂D₃, thereby contributing to the overall regulation of blood levels of 1,25(OH)₂D₃. Relative to PTH regulation, we summarize what is known of how 1,25(OH)₂D₃ regulates PTH suppression. These studies suggest that it is not 1,25(OH)₂D₃ that controls PTH levels in healthy subjects, but rather calcium itself. Finally, we describe current progress using an in vivo approach that furthers our understanding of the regulation of Fgf23 expression by PTH and 1,25(OH)₂D₃ and provide the first evidence that P may act to induce Fgf23 expression via a complex transcriptional mechanism in bone. It is clear, however, that additional advances will need to be made to further our understanding of the inter-regulation of each of these hormonal genes.

Keywords: CRISPR/Cas9; ChIP-seq analysis; Cyp27b1/Cyp24a1 genes; FGF23 gene; PTH gene; mineral regulating hormones; mutant mice; transcription.

Figure 1

Diagram of the inter-regulation of PTH, FGF23 and 1,25(OH)₂D₃ and the target tissues relevant to each hormone. Arrows together with (+) or (-) signs indicate the direction and influence of each hormone.

Figure 2

Schematic diagram of the mouse Cyp2b1 gene. The Cyp27b1, Mettl1 and Mettl21b genes are indicated. PTH (blue) sites of action at CREB are noted. FGF23 (green) and 1,25D₃ (yellow) sites of action at the VDR are noted. Arrow indicates direction of transcription.

Figure 3

Schematic diagram of the mouse Cyp24a1 gene. The Cyp24a1 gene and downstream direction of transcription is noted. CREB sites (purple squares) and VDR sites (red ovals) are indicated. PTH (blue) sites of action negative action at CREB are noted. FGF23 (green) and 1,25D₃ (yellow) sites of positive action at the VDR are noted.

Figure 4

Summary of the of PTH, FGF23 and 1,25(OH)₂D₃ are diagrammed.

Figure 5

Schematic diagram of the mouse Pth gene located in blue. Arrow indicates direction of transcription. Location of a single VDR binding site (red oval) indicated at the 3’ end of the Pth gene, as assessed by ChIP-seq analysis of thyroparathyroid tissue.

Figure 6

Schematic diagram of the mouse fgf23 gene located in green with arrow indicting direction of transcription. Brown squares indicate regions of increased epigenetic acetylation. Positive regulation by PTH occurs from the CREB site in the DE16K enhancer (DE16K) and by 1,25D₃ and P at the promoter-proximal enhancer (PPE).