Effects of 1,25 and 24,25 Vitamin D on Corneal Fibroblast VDR and Vitamin D Metabolizing and Catabolizing Enzymes.

Purpose: To investigate the effects of 1,25-Vit D3 and 24,25-Vit D3 on corneal fibroblast expression of the vitamin D-associated enzymes CYP27B1 and CYP24A1 and the roles of the vitamin D receptor (VDR) and protein disulfide isomerase, family A, member 3 (Pdia3) in these cells.Methods: CYP24A1, CYP27B1, VDR, and Pdia3 expression in corneas was detected using immunohistochemistry. Western blotting was used to measure protein expression in human and mouse fibroblasts, including VDR KO mouse cells, treated with 1,25-Vit D3 (20 nM) and 24,25-Vit D3 (100 nM). The Pdia3 inhibitor LOC14 was used to explore the role of Pdia3 as a Vit D3 receptor in these cells.Results: CYP24A1, CYP27B1, VDR, and Pdia3 were all expressed in mouse and human corneal fibroblasts. 1,25-Vit D3 significantly increased VDR expression in human and mouse fibroblasts. 1,25-Vit D3 and 24,25-VitD3 significantly increased CYP24A1 and CYP27B1 expression level in human, VDR WT mouse, and VDR KO mouse corneal fibroblasts. CYP24A1 and CYP27B1 expression was unchanged in VDR KO mouse fibroblasts treated with 1,25-Vit D3 or 24,25-Vit D3 plus LOC14. Human fibroblast VDR, CYP24A1, and CYP27B1 expression were unaffected by LOC14.Conclusions: Vitamin D metabolic enzymes, VDR, and Pdia3 are all expressed in mouse and human corneal fibroblasts. 1,25-Vit D3 modulates fibroblast vitamin D enzymes through both the VDR and Pdia3 pathways in a species-dependent manner. 24,25-Vit D3 can increase expression of fibroblast CYP24A1 and CYP27B1 in the absence of VDR and is likely involved in fibroblast regulation independent of 1,25-Vit D3 or VDR.

[Current Topics on Vitamin D. Protein interaction and biological action of active vitamin D compounds].

To express its biological action, active vitamin D3 (1α,25-dihydroxyvitamin D3) interacts with three proteins : vitamin D binding protein (DBP), vitamin D receptor (VDR), and CYP24A1. This article explains how eldecalcitol interacts with these three proteins to achieve its mode of action. The main feature of eldecalcitol's molecular structure is the 3-hydroxypropyloxy group (3-HP group) at the 2ß-position. When interacting with DBP, eldecalcitol tightly binds to DBP via the 3-HP group and enhances systemic stability of eldecalcitol. By binding to VDR, the 3-HP group stabilizes the eldecalcitol-VDR complex, which leads to high efficacy. The 3-HP group also interferes with the binding to CYP24A1, which induces residence to CYP24A1 and liver metabolism. These protein interactions constitute the mode of action of eldecalcitol.