Transcriptional Regulation of 25-Hydroxyvitamin D-24-Hydroxylase (CYP24A1) by Calcemic Factors in Keratinocytes.

CYP24A1 regulates serum vitamin D (VD) levels by inactivating 25(OH)2D3, which is the precursor of the active form of VD [1α,25(OH)2D3], and CYP24A1 expression is controlled by multiple calcemic factors such as 1α,25(OH)2D3, calcium, and phosphate. A major phosphaturic factor, FGF23, has also been identified as a regulator of serum VD levels by affecting renal CYP24A1 gene expression; however, its effect on CYP24A1 in extrarenal cells remains largely unstudied. Therefore, the direct effect of FGF23 on CYP24A1 was examined in a human keratinocyte cell line (HaCaT). In this cell line, significant induction of CYP24A1 gene expression by 1α,25(OH)2D3 was seen within 4 h by qRT-PCR, and this was mediated by the VD receptor, as shown in a mutant cell line genetically deficient in this receptor. However, FGF23 treatment up to 12 h did not induce CYP24A1 expression, although the expected activation of the downstream MAPK signaling pathway was seen. High calcium and phosphate treatments were also ineffective in inducing CYP24A1 gene expression. Furthermore, a luciferase assay showed no activation of a VD-sensitive proximal CYP24A1 promoter in response to the calcium and phosphate treatments, suggesting that the effect of FGF23 on dermal CYP24A1 gene expression is indirect. From these findings, we speculate that CYP24A1 gene regulation by FGF23 occurs mainly in renal cells, but not in extrarenal cells, at least not in keratinocytes.