ABSTRACT: Objectives: HAPLN1 stabilize the macromolecular structure of the extracellular matrix (ECM) and its expression is increased in a few types of musculoskeletal diseases including rheumatoid arthritis (RA). However, its functions are obscure. Methods: Here, how HAPLN1 contributes to the viability, proliferation, mobility and pro-inflammatory phenotype of RA-FLSs was investigated using small interfering RNA (siHAPLN1), over-expression vector (HAPLN1OE) and a recombinant HAPLN1 (rHAPLN1) protein. Results: Interestingly, HAPLN1 promoted proliferation but inhibited migration of RA-FLSs. Earlier we reported the function of metformin, an AMPK activator, in the inhibition of FLSs activation but promotion of HAPLN1 secretion. In this study, we confirmed the findings of up-regulated HAPLN1 in RA patients, and it is positively correlated with AMPK. Both si-HAPLN1 and HAPLN1OE treatment has down-regulated the expression of AMPK-ɑ gene but the levels of phosphorylated AMPK-ɑ were found to be up-regulated in RA-FLSs. Furthermore, si-HAPLN1 had down-regulated the gene and protein expression of proinflammatory factors like TNF-ɑ, MMPs and IL-6, while HAPLN1OE up-regulated their levels. qPCR assay also indicated down regulated TGF-β, ACAN, Fibronectin, collagen Ⅱ, and Ki-67 by si-HAPLN1 treatment, while up-regulated ACAN, Ki-67 and down regulated cyclin-D1 by HAPLN1OE treatment. Proteomics of si-HAPLN1, rHAPLN1 and mRNA-Seq analysis of rHAPLN1 confirmed the functions of HAPLN1 in the activation of inflammation, proliferation, increased cell adhesion, and strengthening of ECM function. Conclusions: Collectively, these findings demonstrate HAPLN1 function in promoting proliferation and pro-inflammatory phenotype of RA-FLSs, which in turn could contribute to RA pathogenesis.
