Serum amyloid A triggers the mosodium urate -mediated mature interleukin-1β production from human synovial fibroblasts

Human synovial fibroblasts were stimulated with MSU in the presence or absence of serum amyloid A (SAA). The cellular supernatants were analyzed by immunoblotting using anti-IL-1β or anti-caspase-1 antibodies. IL-1β or NLRP3 mRNA expressions were analyzed by real-time PCR or reverse transcription-PCR (RT-PCR) method.Neither SAA nor MSU stimulation resulted in IL-1β or interleukin-1α (IL-1α) secretions and pro-IL-1β processing in synovial fibroblasts. However, in SAA-primed synovial fibroblasts, MSU stimulation resulted in the activation of caspase-1 and production of active IL-1β and IL-1α. The effect of SAA on IL-1β induction was impaired in cells by silencing NLRP3 using siRNA or treating with caspase-1 inhibitor. In addition, SAA induced the secretion of cathepsin B and NLRP3 mRNA expression in synovial fibroblasts.Our data demonstrate that exposure of human synovial fibroblasts to SAA promotes MSU-mediated caspase-1 activation and IL-1β secretion in the absence of microbial stimulation. These findings provide insight into the molecular processes underlying the synovial inflammatory condition of gout.Gout is a paradigm for acute sterile inflammation that is triggered by interactions between monosodium urate (MSU) crystals and inflammatory cells in the joint connective tissues [1]. Interleukin-1β (IL-1β) has been identified as a pivotal cytokine in gout and MSU crystal-induced inflammation [2]. IL-1β is induced as an inactive pro-molecule by immune cells, such as macrophages and monocytes, and then cleaved into the active p17 form of IL-1 by caspase-1 [3,4]. Tschopp et al. demonstrated that MSU is capable of activating the NLRP3 inflammasome to process and secrete active IL-1β [5]. These findings suggest that macrophages can recognize MSU as danger-associated molecular patterns (DAMPs) in the damaged tissues and release proinflammatory IL-1β [6]. Upon activation, NLRP3 binds to the ASC, which in turn recruits procaspase-1 for activation. Activated caspase-1 cleave