SIX1 attenuates inflammation and rheumatoid arthritis by silencing MyD88-dependent TLR1/2 signaling

Objectives: Rheumatoid arthritis (RA) is a chronic autoimmune disease that severely affects the patients' quality of life. Sine oculis homeobox 1 (SIX1) has been reported as a key regulator of organogenesis and inflammation. This study aimed to explore the effects of SIX1 on RA.

Methods: Wistar rats were immunized with type II collagen to induce an animal model of RA. RA synovial fibroblasts (RASFs) were isolated from the rats. SIX1 expression in RA rats and RASFs was detected by qRT-PCR and western blot. CCK-8, EdU, transwell, flow cytometer, and ELISA were conducted to assay the effects of SIX1 on RASFs. The effects of SIX1 on RA rats were studied by Safranin O staining, H&E staining, and ELISA. Besides, GSEA and KEGG analysis were used to predict the underlying signaling pathways.

Results: SIX1 was low expressed in synovial tissue of RA rats and RASFs. SIX1 overexpression inhibited the proliferation, invasion, and levels of TNF-α, IL-6, and IL-8 in RASFs. However, SIX1 overexpression promoted the Apoptosis of RASFs. SIX1 overexpression enhanced body weight, and attenuated the cartilage damage, pathological injury, and pro-inflammatory cytokine release of RA rat model. MyD88-dependent TLR1/2 might be a downstream signaling of SIX1. RelA acted as a transcription factor of TLR1/2, and SIX1 inhibited TLR1/2 signaling possibly via interaction with RelA. Adding with Pam₃CSK₄, a specific agonist of TLR1/2 signaling, attenuated the effects of SIX1 on RASFs.

Conclusion: SIX1 attenuated inflammation and RA by silencing MyD88-dependent TLR1/2 signaling. SIX1 may be a promising target for RA treatment.