Vitamin D receptor suppresses pulmonary fibroblast activation by downregulating the TGF-β1/Smad signaling pathway

The vitamin D receptor (VDR) has been implicated in anti-inflammatory and antifibrotic effects, but its role in regulating TGF-β1/Smad signaling and fibroblast activation in pulmonary fibrosis remains unclear. This study investigates the regulatory effects of VDR on TGF-β1/Smad signaling and its impact on fibrogenic responses in lung fibroblasts. MRC-5 cells were treated with L-lactate sodium to generate a fibrotic model, and VDR and TGF-β1 expression were manipulated using plasmids and siRNA. Fibroblast activation, TGF-β1/Smad signaling, and ECM remodeling were assessed using qRT-PCR, western blot, and immunofluorescence, while cell proliferation, migration, invasion, oxidative stress, and inflammation were also evaluated. Lactate stimulation increased α-SMA and collagen I/III expression, confirming fibroblast activation. VDR overexpression reduced fibrotic markers, downregulated ECM-degrading enzymes (MMP2, MMP9), and upregulated TIMP-1, while inhibiting migration, invasion, and reducing ROS and inflammatory cytokines (IL-6, IL-1β). In contrast, VDR knockdown enhanced fibrotic marker expression and fibroblast activity. Phosphorylation of Smad2/3 decreased with VDR overexpression and increased with knockdown. TGF-β1 overexpression elevated fibrotic markers and Smad signaling, while TGF-β1 knockdown reduced these markers and alleviated the activated phenotype. Exogenous TGF-β1 treatment reversed the antifibrotic effects of VDR overexpression, linking VDR to TGF-β1/Smad signaling. VDR suppresses fibroblast activation and fibrotic responses in lung fibroblasts by downregulating the TGF-β1/Smad signaling pathway, highlighting its potential as a therapeutic target for pulmonary fibrosis.