Proprotein convertase subtilisin/kexin type 6 promotes in vitro proliferation, migration and inflammatory cytokine secretion of synovial fibroblast-like cells from rheumatoid arthritis via nuclear-κB, signal transducer and activator of transcription 3 and extracellular signal regulated 1/2 pathways

Our previous studies demonstrated that expression of proprotein convertase subtilisin/kexin type 6 (PCSK6) is greatly enhanced in rheumatoid arthritis fibroblast‑like synoviocytes (RASFs), and that PCSK6 inhibition decreases cell proliferation, migration and invasion. The present study aimed to investigate the functional role of PCSK6 in the hyperplasia of RASFs. Cultured RASFs from RA patients were stimulated with recombinant human (rh)PCSK6. Subsequent changes in proliferation, invasion, migration and the secretion of inflammatory cytokines were measured in vitro using MTT, wound healing and Transwell assays, and ELISA. Cell cycle and apoptosis were analyzed by flow cytometry. Influence on downstream gene expression levels were analyzed using reverse transcription‑quantitative polymerase chain reaction. Specific signaling pathways responsible for these effects were analyzed using western blotting and confirmed with pathway‑specific inhibitors. It was demonstrated that rhPCSK6 significantly increased RASF cell invasion, migration and proliferation, which was influenced through both reduced cell cycle arrest and reduced apoptosis. Furthermore, rhPCSK6 stimulated RASFs to secrete the inflammatory cytokines interleukin (IL)‑1α, IL‑1β and IL‑6, and exhibit altered expression of genes involved in angiogenesis, hypoxia, proliferation and inflammation. These cellular effects were mediated via the nuclear factor (NF)‑κB, signal transducer and activator of transcription 3 (STAT3) and extracellular signal regulated (ERK)1/2 signaling pathways. The results demonstrated that signaling via NF‑κB and STAT3 mediated cell cycle arrest, and signaling through NF‑κB mediated apoptosis in RASF cells stimulated with PCSK6. PCSK6 can activate NF‑κB, STAT3 and ERK1/2 signaling pathways in vitro to enhance cell proliferation, migration, invasion and inflammation in RASF cells. These findings suggest that PCSK6 may be an important therapeutic target in RA.