Regarding the regulation of HAPLN1 expression, Chen et al. showed that Metformin, an AMP-activated protein kinase (AMPK) activator, promotes HAPLN1 secretion in rheumatoid arthritis (RA)-fibroblast-like synoviocytes (FLSs), but it inhibits FLSs activation [19]. This evidence concerns the gene HAPLN1 and rheumatoid arthritis.