MMP1 and rheumatoid arthritis: And that FSTL1 protects against the joint erosion seen in RA by down‐regulating the production of matrix metalloproteinase (MMP)‐1, MMP‐3 and prostaglandin E2.13 Yury et al found a significant positive correlation between serum FSTL‐1 levels and the arthritis index in mice with collagen‐induced arthritis.