Vitamin D attenuates TGF-β1-induced lung fibroblast proliferation and migration through repression of RasGRP3

Background: Transforming growth factor-131 (TGF-131) is a pleiotropic cytokine that plays a central role in the pathogenesis of idiopathic pulmonary fibrosis (IPF). While previous studies have revealed a cross-talk between vitamin D and TGF-131 signaling, it is still unclear how they interact with each other to regulate the progression of IPF. Methods: In this work, we searched for a novel mediator of TGF-131 activity in lung fibroblasts and examined its regulation by vitamin D. In addition, we investigated the mechanism underlying the interaction between vitamin D and TGF-131 signaling in lung fibroblast activation. Bioinformatic analysis was performed to identify TGF-131 downstream target genes. Knockdown and overexpression expression experiments were conducted to determine gene function in the regulation of lung fibroblast proliferation and migration. Results: Analysis of publicly available datasets revealed that RAS guanyl releasing protein 3 (RasGRP3) was upregulated in TGF-131-treated lung fibroblasts and lung tissues from IPF patients relative to healthy controls. Our data confirmed the upregulation of RasGRP3 by TGF-131 in human MRC5 lung fibroblasts. Overexpression of RasGRP3 enhanced MRC5 cell proliferation and migration. Knockdown of RasGRP3 blocked TGF-131-induced MRC5 proliferation and migration. Vitamin D abolished TGF-131-induced RasGRP3 upregulation, which was reversed by inhibition of the vitamin D receptor (VDR). Mechanistically, vitamin D promoted VDR enrichment and prevented mothers against decapentaplegic homolog (SMAD) 2 and 3 occupancy at the promoter of RasGRP3. Additionally, overexpression of RasGRP3 reversed the suppressive effect of vitamin D on MRC5 cell proliferation and migration. Conclusion: In conclusion, vitamin D antagonizes TGF-131-induced lung fibroblast activation by repressing RasGRP3 transcription.