Abstract
Background Vitamin D receptor (VDR) is a member of the nuclear receptor superfamily. Its ligand, 1,25-(OH)(2)D, is a metabolically active hormone derived from vitamin D-3.
The levels of vitamin D-3 are decreased in patients with systemic sclerosis (SSc). Here, we aimed to analyse the role of VDR signalling in fibrosis.
Methods VDR expression was analysed in SSc skin, experimental fibrosis and human fibroblasts. VDR signalling was modulated by siRNA and with the selective agonist paricalcitol.
The effects of VDR on Smad signalling were analysed by reporter assays, target gene analyses and coimmunoprecipitation. The effects of paricalcitol were evaluated in the models of bleomycin-induced fibrosis and fibrosis induced by overexpression of a constitutively active transforming growth factor-beta (TGF-beta) receptor I (TBRICA).
Results VDR expression was decreased in fibroblasts of SSc patients and murine models of SSc in a TGF-beta-dependent manner. Knockdown of VDR enhanced the sensitivity of fibroblasts towards TGF-beta.
In contrast, activation of VDR by paricalcitol reduced the stimulatory effects of TGF-beta on fibroblasts and inhibited collagen release and myofibroblast differentiation. Paricalcitol stimulated the formation of complexes between VDR and phosphorylated Smad3 in fibroblasts to inhibit Smad-dependent transcription.
Preventive and therapeutic treatment with paricalcitol exerted potent antifibrotic effects and ameliorated bleomycin-as well as TBRICA-induced fibrosis. Conclusions We characterise VDR as a negative regulator of TGF-beta/Smad signalling.
Impaired VDR signalling with reduced expression of VDR and decreased levels of its ligand may thus contribute to hyperactive TGF-beta signalling and aberrant fibroblast activation in SSc.