Even though propionate has also been reported to interact with GPR41 and GPR43 and activate GPCR-induced signaling,57,58 we detected no expression of GPR41 and GPR43 in RA-FLSs, suggesting that propionate worked on RA-FLSs in GPR41 and GPR4-independent manners. This evidence concerns the gene GPR4 and rheumatoid arthritis.