Metabolic activation of tachysterol to biologically active hydroxyderivatives that act on VDR, AhR, LXRs, and PPARγ receptors.

CYP11A1 and CYP27A1 hydroxylate tachysterol , a photoproduct of previtamin D , producing 20S-hydroxytachysterol [20S(OH)T ] and 25(OH)T , respectively. Both metabolites were detected in the human epidermis and serum. Tachysterol was also detected in human serum at a concentration of 7.3 ± 2.5 ng/ml. 20S(OH)T and 25(OH)T inhibited the proliferation of epidermal keratinocytes and dermal fibroblasts and stimulated the expression of differentiation and anti-oxidative genes in keratinocytes in a similar manner to 1,25-dihydroxyvitamin D [1,25(OH) D ]. They acted on the vitamin D receptor (VDR) as demonstrated by image flow cytometry and the translocation of VDR coupled GFP from the cytoplasm to the nucleus of melanoma cells, as well as by the stimulation of CYP24A1 expression. Functional studies using a human aryl hydrocarbon receptor (AhR) reporter assay system revealed marked activation of AhR by 20S(OH)T , a smaller effect by 25(OH)T , and a minimal effect for their precursor, tachysterol . Tachysterol hydroxyderivatives showed high-affinity binding to the ligan-binding domain (LBD) of the liver X receptor (LXR) α and β, and the peroxisome proliferator-activated receptor γ (PPARγ) in LanthaScreen TR-FRET coactivator assays. Molecular docking using crystal structures of the LBDs of VDR, AhR, LXRs, and PPARγ revealed high docking scores for 20S(OH)T and 25(OH)T , comparable to their natural ligands. The scores for the non-genomic-binding site of the VDR were very low indicating a lack of interaction with tachysterol ligands. Our identification of endogenous production of 20S(OH)T and 25(OH)T that are biologically active and interact with VDR, AhR, LXRs, and PPARγ, provides a new understanding of the biological function of tachysterol .