FZD4 and rheumatoid arthritis: Saponin 23 inhibited proliferation and inflammatory response in FLSs from RA patients in vitro and ameliorated RA pathology in CIA mice by combining with frizzled class receptor 4 (FZD4) and blocking the circular pleiotrophin (circPTN)/miR-145-5p/FZD4 signal axis [108].