(a) After treatment with different concentrations of arsenic trioxide (ATO) (0.1, 0.5, 1.0, 2.0, or 4.0 μM) or Vitamin D (Vit D) (50 or 100 nM) for 48 h (n=3), CCK-8 was added to rheumatoid arthritis (RA) fibroblast-like synoviocytes (FLS) and incubated for 1 h. Absorbance (optical density value) at 450 nm was detected by a microplate reader. The results showed no significant influence on cell proliferation after the administration of different concentrations of ATO and Vit D. The results are presented as the mean ± SEM. (b) Real-time PCR and western blot analysis showed significantly increased expression of the Vitamin D receptor (VDR), PPAR-γ, LC-3, and p62 in RA FLS (n=3) compared with normal human (NH) FLS (n=3; ∗ p<0.05, ∗∗ p<0.01, ∗∗∗ p<0.001). The results are presented as means ± SEM. (c) Silencing of VDR in RA FLS significantly reduced the expression of VDR, PPAR-γ, and LC-3 but induced the expression of p62. Addition of ATO could reverse the effect of small interfering RNA (siRNA) (n=3, #p<0.05 ##p<0.01 ###p<0.001 versus scrambled siRNA-treated cells (SCR-siRNA); ∗p<0.05; ∗∗p<0.01 ∗∗∗p<0.001 versus vehicle). Vehicle = RA FLS treated with the medium alone after silencing by VDR siRNA.