Objectives: This study investigated the expression of proviral-integration site for Moloney murine leukaemia virus (PIM) -1 kinase in RA synovium and RA fibroblast-like synoviocytes (FLSs) along with its impact on RA-FLS aggressiveness.
Methods: The expression of PIM kinases was assessed in synovial tissues by immunohistochemistry and double IF. After PIM-1 inhibition using either small-interfering RNA or the chemical inhibitor AZD1208, we performed proliferation and migration assays and measured the levels of MMPs and IL-6 released from RA-FLSs under stimulation with proinflammatory cytokines (TNF-α, S100A4 and IL-6/soluble IL-6 receptor). Additionally, PIM-1-associated downstream signalling pathways were analysed by immunoblotting.
Results: Three isoforms of PIM kinases were immunodetected in the synovial tissues from patients with RA or OA. Specifically, PIM-1 and PIM-3 were upregulated in RA synovium and PIM-1 was expressed in T cells, macrophages and FLSs. Additionally, upon stimulation of RA-FLSs with TNF-α, S100A4 and IL-6/sIL-6R, PIM-1 and PIM-3, but not PIM-2, were significantly inducible. Moreover, PIM-1 knockdown or AZD1208 treatment significantly suppressed basal or cytokine-induced proliferation and migration of RA-FLS and the secretion of MMPs from stimulated RA-FLSs. PIM-1 knockdown significantly affected the phosphorylation levels of extracellular signal-regulated kinase and cAMP responsive element binding protein in RA-FLSs.
Conclusion: PIM-1 was upregulated in RA synovial tissues and RA-FLSs and its inhibition significantly reduced the proliferation, migration and MMP production of RA-FLSs in vitro. These findings suggest PIM-1 as a novel regulator of the aggressive and invasive behaviour of RA-FLSs and indicate its potential as a target for RA treatment.