Abstract
The interactions of derivatives of lumisterol (L3) and vitamin D3 (D3) with liver X receptors (LXRs) were investigated. Molecular docking using crystal structures of the ligand binding domains (LBDs) of LXRα and β revealed high docking scores for L3 and D3 hydroxymetabolites, similar to those of the natural ligands, predicting good binding to the receptor. RNA sequencing of murine dermal fibroblasts stimulated with D3-hydroxyderivatives revealed LXR as the second nuclear receptor pathway for several D3-hydroxyderivatives, including 1,25(OH)2D3. This was validated by their induction of genes downstream of LXR. L3 and D3-derivatives activated an LXR-response element (LXRE)-driven reporter in CHO cells and human keratinocytes, and by enhanced expression of LXR target genes. L3 and D3 derivatives showed high affinity binding to the LBD of the LXRα and β in LanthaScreen TR-FRET LXRα and β coactivator assays. The majority of metabolites functioned as LXRα/β agonists; however, 1,20,25(OH)3D3, 1,25(OH)2D3, 1,20(OH)2D3 and 25(OH)D3 acted as inverse agonists of LXRα, but as agonists of LXRβ. Molecular dynamics simulations for the selected compounds, including 1,25(OH)2D3, 1,20(OH)2D3, 25(OH)D3, 20(OH)D3, 20(OH)L3 and 20,22(OH)2L3, showed different but overlapping interactions with LXRs. Identification of D3 and L3 derivatives as ligands for LXRs suggests a new mechanism of action for these compounds.
Highlights
Vitamin D3 is a product of ultraviolet B (UVB)-induced photochemical transformation of 7-dehydrocholestrol (7DHC), of which the intermediate, pre-vitamin D3, can isomerize to lumisterol and tachysterol[1,2,3,4]
Retrospective analysis of the microarray data deposited at the NCBI GEO (GSE117351)[44], identified liver X receptors (LXRs)/retinoid X receptor (RXR) as the second nuclear receptor complexes after vitamin D receptor (VDR)/RXR, based on ranking in the canonical (p = 0.0039) and toxicity-related (p = 0.0041) pathways activated in primary human keratinocytes treated with 1,25(OH)2D3 for 24 h
The regulation of LXR targets by these secosteroids was supported by chromatin immunoprecipitation (ChIP) analysis with chromatin isolated from HaCaT keratinocytes treated with 1,25(OH)2D3 and antibodies against LXRα and β, which showed significant stimulation of LXRα/β binding to the LXR-response element (LXRE) within the ABCA1 promoter region (Fig. 1C,D)
Summary
Vitamin D3 is a product of ultraviolet B (UVB)-induced photochemical transformation of 7-dehydrocholestrol (7DHC), of which the intermediate, pre-vitamin D3, can isomerize to lumisterol and tachysterol[1,2,3,4]. Neither affects calcium metabolism nor has any significant biological activity, except that UVB-led phototransformation of pre-D3 into lumisterol explained the lack of systemic intoxication by vitamin D 32 We have challenged these dogmas by questioning whether these diverse and sometime opposite effects are regulated by only one receptor (VDR) and one molecule (1,25(OH)2D3). Recent evidence from our laboratory has shown that CYP11A1-derived (OH)nD3, in addition of acting as biased agonists on VDR25–27, can, together with lumisterol hydroxyderivatives, act as inverse agonists of retinoic acid-related orphan receptors (ROR) α and γ26,28 and as agonists on the arylhydrocarbon receptor (AhR)[29] This breaks the dogma that VDR serves as the only NR for active forms of vitamin D3. Functional LXRs are expressed in the s kin[40,41,42,43], the site of lumisterol and vitamin D3 production and a site of their metabolism[5,10,24]
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