No abstract

Th2 cytokine IL-4 has been previously shown to suppress the production of proinflammatory cytokines in monocytes. However, the underlying molecular mechanism by which IL-4 signaling antagonizes proinflammatory responses is poorly characterized. In particular, whether IL-4 can modulate inflammasome signaling is unknown. Here, we provide evidence that IL-4 suppresses NLRP3-dependent caspase-1 activation and the subsequent IL-1β secretion but does not inhibit AIM2- or NLRC4-dependent caspase-1 activation in THP-1 and mouse bone marrow-derived macrophages. Upon LPS or LPS/ATP stimulation, IL-4 markedly inhibited the assembly of NLRP3 inflammasome, including NLRP3-dependent ASC oligomerization, NLRP3-ASC interaction, and NLRP3 speck-like oligomeric structure formation. The negative regulation of NLRP3 inflammasome by IL-4 was not due to the impaired mRNA or protein production of NLRP3 and proinflammatory cytokines. Supporting this observation, IL-4 attenuated NLRP3 inflammasome activation even in reconstituted NLRP3-expressing macrophages in which NLRP3 expression is not transcriptionally regulated by TLR-NF-κB signaling. Furthermore, the IL-4-mediated suppression of NLRP3 inflammasome was independent of STAT6-dependent transcription and mitochondrial ROS. Instead, IL-4 inhibited subcellular redistribution of NLRP3 into mitochondria and microtubule polymerization upon NLRP3-activating stimulation. Our results collectively suggest that IL-4 could suppress NLRP3 inflammasome activation in a transcription-independent manner, thus providing an endogenous regulatory machinery to prevent excessive inflammasome activation.

BackgroundInflammation may play a significant role in the pathogenesis of depression, although the molecular target for the treatment of inflammation-mediated depressive symptoms remains to be elucidated. Recent studies have implicated the NLRP3 inflammasome in various psychiatric disorders, including depression. However, the underlying mechanism by which NLRP3 inflammasome activation mediates the progression of depressive-like behaviors remains poorly understood.MethodsWe examined whether NLRP3 deficiency influenced depressive-like behaviors and cerebral inflammation following systemic administration of lipopolysaccharide in mice. To further assess the contribution of the NLRP3 inflammasome to the progression of depression, we evaluated the effects of NLRP3 signaling on levels of indoleamine 2,3-dioxygenase.Results Nlrp3-deficient mice exhibited significant attenuation of depressive-like behaviors and cerebral caspase-1 activation in a lipopolysaccharide-induced model of depression. Treatment with the antidepressant amitriptyline failed to block NLRP3-dependent activation of caspase-1, but inhibited lipopolysaccharide-promoted production of interleukin-1β mRNA via suppressing NF-κB signaling in mouse mixed glial cultures. Interestingly, lipopolysaccharide administration produced NLRP3-dependent increases in indoleamine 2,3-dioxygenase expression and activity of mouse brain. Furthermore, inflammasome-activating stimulations, but not treatment with the inflammasome product interleukin-1β, triggered indoleamine 2,3-dioxygenase mRNA induction in mixed glial cells.ConclusionsOur data indicate that the NLRP3 inflammasome is significantly implicated in the progression of systemic inflammation-induced depression. NLRP3-dependent caspase-1 activation produced significant increases in indoleamine 2,3-dioxygenase levels, which may play a significant role in lipopolysaccharide-induced depression. Collectively, our findings suggest that indoleamine 2,3-dioxygenase is a potential downstream mediator of the NLRP3 inflammasome in inflammation-mediated depressive-like behaviors.

Endoplasmic reticulum (ER)‐Golgi vesicle trafficking plays a pivotal role in the conventional secretory pathway of many cytokines; however, the precise release mechanism of a major inflammasome mediator, IL‐1β, is not thought to follow the conventional ER‐Golgi route and remains elusive. Here, we found that perturbation of ER‐Golgi trafficking by brefeldin A (BFA) treatment attenuated nucleotide‐binding oligomerization domain‐like receptor family, pyrin–domain–containing 3 (NLRP3) inflammasome activation in mouse bone marrow–derived macrophages (BMDMs). BFA treatment inhibited NLRP3‐mediated inflammasome assembly and caspase‐1 activation but did not block IL‐1β secretion from BMDMs following BFA administration after NLRP3 inflammasome activation. Consistently, short‐hairpin RNA–dependent knockdown of BFA‐inhibited guanine nucleotide‐exchange protein 1 (BIG1), a molecular target of BFA and an initiator of Golgi‐specific vesicle trafficking, abolished NLRP3‐dependent apoptosis‐associated speck‐like protein containing a caspase‐recruitment domain oligomerization and caspase‐1 activation in BMDMs. Similarly, knockdown of Golgi‐specific BFA‐resistance guanine nucleotide exchange factor 1, another target of BFA, clearly attenuated NLRP3‐mediated caspase‐1 activation in BMDMs. Mechanistically, inhibition of BIGl‐mediated vesicle trafficking did not impair NLRP3‐activating signal 2‐promoted events, such as potassium efflux and mitochondrial rearrangement, but caused significant impairment of signal 1‐triggered priming steps, including NF‐κB–mediated pathways. These data suggest that BFA‐targeted vesicle trafficking at the Golgi contributes to activation of the NLRP3 inflammasome signaling.—Hong, S., Hwang, I., Gim, E., Yang, J., Park, S., Yoon, S.‐H., Lee, W.‐W., Yu, J.‐W. Brefeldin A–sensitive ER‐Golgi vesicle trafficking contributes to NLRP3‐dependent caspase‐1 activation. FASEB J. 33, 4547–4558 (2019). http://www.fasebj.org

Inflammasome signaling can contribute to host innate immune defense against bacterial pathogens such as Pseudomonas aeruginosa. However, bacterial evasion of host inflammasome activation is still poorly elucidated. Quorum sensing (QS) is a bacterial communication mechanism that promotes coordinated adaptation by triggering expression of a wide range of genes. QS is thought to strongly contribute to the virulence of P. aeruginosa, but the molecular impact of bacterial QS on host inflammasome defense is completely unknown. Here, we present evidence that QS-related factors of the bacterial secretant (BS) from P. aeruginosa can dampen host inflammasome signaling in mouse bone marrow-derived macrophages. We found that BS from QS-defective ΔlasR/rhlR mutant, but not from wild-type (WT) P. aeruginosa, induces robust activation of the NLRC4 inflammasome. P. aeruginosa-released flagellin mediates this inflammasome activation by ΔlasR/rhlR secretant, but QS-regulated bacterial proteases in the WT BS impair extracellular flagellin to attenuate NLRC4 inflammasome activation. P. aeruginosa-secreted proteases also degrade inflammasome components in the extracellular space to inhibit the propagation of inflammasome-mediated responses. Furthermore, QS-regulated virulence factor pyocyanin and QS autoinducer 3-oxo-C12-homoserine lactone directly suppressed NLRC4- and even NLRP3-mediated inflammasome assembly and activation. Taken together, our data indicate that QS system of P. aeruginosa facilitates bacteria to evade host inflammasome-dependent sensing machinery.