NLRP3 is the most crucial member of the NLR family, as it detects the existence of pathogen invasion and self-derived molecules associated with cellular damage. Several studies have reported that excessive NLRP3 inflammasome-mediated caspase-1 activation is a key factor in the development of diseases. Recent studies have reported that Syk is involved in pathogen-induced NLRP3 inflammasome activation; however, the detailed mechanism linking Syk to NLRP3 inflammasome remains unclear. In this study, we showed that Syk mediates NLRP3 stimuli-induced processing of procaspase-1 and the consequent activation of caspase-1. Moreover, the kinase activity of Syk is required to potentiate caspase-1 activation in a reconstituted NLRP3 inflammasome system in HEK293T cells. The adaptor protein ASC bridges NLRP3 with the effector protein caspase-1. Herein, we find that Syk can associate directly with ASC and NLRP3 by its kinase domain but interact indirectly with procaspase-1. Syk can phosphorylate ASC at Y146 and Y187 residues, and the phosphorylation of both residues is critical to enhance ASC oligomerization and the recruitment of procaspase-1. Together, our results reveal a new molecular pathway through which Syk promotes NLRP3 inflammasome formation, resulting from the phosphorylation of ASC. Thus, the control of Syk activity might be effective to modulate NLRP3 inflammasome activation and treat NLRP3-related immune diseases.

Toll-like receptors (TLRs) are a major family of pattern recognition receptors, and they play a crucial role in innate immune responses. Activation of TLR4 signaling at the plasma membrane by its ligand lipopolysaccharide (LPS) stimulates a proinflammatory pathway dependent on the E3 ubiquitin ligase TRAF6 (tumor necrosis factor receptor-associated factor 6) and the kinase TAK1 (transforming growth factor β-activated kinase 1), whereas TLR4 signaling at endosomes stimulates the production of type I interferons (IFNs) through a pathway that depends on TRAF3 and the kinase TBK1 (TANK-binding kinase-1). We found that the nonreceptor tyrosine kinase Syk partially mediated the endocytosis of TLR4, but it also played a dual role in TLR4-mediated signaling. LPS-dependent stimulation of TLR4 in Syk-deficient macrophages led to enhanced activation of TAK1 and increased production of proinflammatory cytokines compared to that in wild-type macrophages. In contrast, Syk-deficient macrophages exhibited decreased TLR4-dependent activation of TBK1 signaling and production of type I IFNs. We found that Syk was present in both TRAF6- and TRAF3-containing signaling complexes; however, the LPS-dependent, lysine 63-linked ubiquitination of TRAF6 and TRAF3 was oppositely regulated by Syk. We identified the domains of Syk that interacted with TRAF3, TRAF6, TAK1, and TBK1, factors activated by multiple TLRs, which suggests that Syk may act as a common regulator of various TLR responses. Together, our results demonstrate the opposing regulatory roles of Syk in TLR-mediated TRAF6 and TRAF3 signaling pathways, which suggests that Syk may fine-tune the innate immune response to lessen inflammation.

IL-17 has an important role in the immunopathogenesis of autoimmune diseases, and spleen tyrosine kinase (Syk) has been implicated as a critical molecule in the signaling pathways of various immunoreceptors. Chemokine (C-C motif) ligand 20 (CCL20) interacts with chemokine (C-C motif) receptor 6 to recruit IL-17-producing cells into the skin to promote progression of psoriasis. Herein we investigate how Syk regulates IL-17 signaling to affect CCL20 expression in primary human epidermal keratinocytes. We found that IL-17 can induce CCL20 expression and activate TAK, IKK, NF-κB, c-Jun N-terminal kinase, and Syk. Data of TAK inhibitor and Syk small interfering RNA (siRNA) indicate Syk being an upstream molecule of TAK in IL-17-elicited signaling. The promoter activity assay combined with site-directed mutagenesis showed that IL-17-elicited CCL20 upregulation is depending on the Syk-mediated NF-κB pathway. Immunoprecipitation also indicated the interaction of Syk with signal molecules of IL-17R, such as TRAF6 and Act1, under IL-17A stimulation. However, the essential signaling events including TRAF6 interaction with Act1 and TRAF6 polyubiquitination under IL-17A stimulation were diminished by Syk siRNA and pharmacologically inhibiting Syk. Taken together, we identify Syk as an upstream signaling molecule in IL-17A-induced Act1-TRAF6 interaction in keratinocytes, and inhibition of Syk can attenuate CCL20 production, which highlights Syk as a potential therapeutic target for inflammatory skin diseases such as psoriasis.

Recent studies have demonstrated the stimulatory effects of HMG-CoA reductase inhibitors, statins, on IL-1β secretion in monocytes and suggest a crucial role for isoprenoids in the inhibition of caspase-1 activity. In this study, we further elucidated the molecular mechanisms underlying the stimulatory effects of statins on caspase-1. Three commonly recognized mechanistic models for NLRP3 inflammasome activation (i.e., ATP/P2X7/K(+) efflux, ROS production, and lysosomal rupture) were investigated in statin-stimulated human THP-1 monocytes. We found that fluvastatin and lovastatin can synergize with LPS to trigger inflammasome activation. Moreover, statin-induced caspase-1 activation and IL-1β production in LPS-primed THP-1 cells are dependent on GGPP deficiency and P2X7 activation. In particular, increased ATP release accounts for the action of statins in P2X7 activation. We also provide evidence that statin-induced moderate ROS elevation is involved in this event. Moreover, the cathepsin B inhibitor was shown to reduce statin-induced IL-1β secretion. Consistently statins can induce cathepsin B activation and lysosomal rupture, as evidenced by LysoTracker staining. Statins also increase intracellular ATP secretion and IL-1β release in primary human monocytes and murine macrophages. Notably, exogenous ATP-elicited P2X7 activation and consequent IL-1β release, an index of direct NLRP3 inflammasome activation, were not altered by statins. Taken together, statin-induced enhancement of inflammasome activation in monocytes and macrophages covers multiple mechanisms, including increases in ATP release, ROS production, and lysosomal rupture. These data not only shed new insight into isoprenylation-dependent regulation of caspase-1 but also unmask mechanisms for statin-elicited inflammasome activation.

A marked increase was noted since 2002. One of the causes is the performance of chest CT. Chest CT is useful to discover mediastinal lymphadenopathy and other lesions suggestive of sarcoidosis, as well as to help guide tissue confirmation in patients with peculiar uveitis features indicative of sarcoidosis. Female predominance and peak incidence of uveitis onset in the 6th decade of life were found. The posterior segment was the most common localization of uveitis in biopsy-proven sarcoidosis in Chinese.