Effects of Human Cathelicidin on Hyaluronan Metabolism and Mechanisms Related to Rheumatoid Arthritis

Abstract

LL-37 is the only human cathelicidin-family host defense peptide and has been reported to interact with invading pathogens causing inflammation at various body sites. Recent studies showed high levels of LL-37 in the synovial-lining membrane of patients with rheumatoid arthritis, a common type of inflammatory arthritis. The present study aims to investigate the role of LL-37 on mechanisms associated with pathogenesis of rheumatoid arthritis and how the product of LL-37 induction, HA fragment, affected the disease mechanisms. The effects of LL-37 on the expression of proinflammatory cytokines, hyaluronan (HA) metabolism-related genes, cell death-related pathways, and cell invasion were investigated in SW982, a human synovial sarcoma cell line. Time-course measurements of proinflammatory cytokines and mediators showed that LL-37 significantly induced IL6 and IL17A mRNA levels at early time points (3–6 hr). HA-metabolism-related genes (i.e., HA synthase 2 (HAS2), HAS3, hyaluronidase 1 (HYAL1), HYAL2, and CD44) were co-expressed in parallel. In combination, LL-37 and IL-17A significantly enhanced COX2, TNF, and HAS3 gene expression concomitantly with the elevation of their respective products, PGE2, TNF, and HA. Cell invasion rates and FN1 gene expression were also significantly enhanced. However, LL-37 alone or combined with IL-17A did not affect cell mortality or cell cycle. Treatment of SW982 cells with both LL-37 and IL-17A significantly enhanced IKK and p65 phosphorylation. These findings suggest that the chronic production of a high level of LL-37 may synchronize with its downstream proinflammatory cytokines, especially IL-17A, contributing to the co-operative enhancement of pathogenesis mechanisms of rheumatoid arthritis, such as high production of proinflammatory cytokines and mediators together with the activation of HA-metabolism-associated genes and cell invasion. Furthermore, the by-product of the induced inflammation, the accumulated HA fragment possessed the pro-inflammatory properties as well. Partly through TLR4 receptor, HA fragment could upregulate gene expression of TNF, IL1 and IL6. It also showed a positive feedback loop induction on the production of LMW-HA by HAS3 and induced TLR4-independently cell migration in SW982 cell. These phenomena demonstrate how LL-37 could be a potent inducer of RA at an early stage, induced by exogenous invasion or the hypersensitized innate immune system.