Regulation Of Caspase-1 Activation Research Articles

Thymic stromal lymphopoietin protects in a model of airway damage and inflammation via regulation of caspase-1 activity and apoptosis inhibition

Thymic stromal lymphopoietin (TSLP), an epithelial cell-derived cytokine, exhibits both pro-inflammatory and pro-homeostatic properties depending on the context and tissues in which it is expressed. It remains unknown whether TSLP has a similar dual role in the airways, where TSLP is known to promote allergic inflammation. Here we show that TSLP receptor (TSLPR)-deficient mice (Tslpr−/−) and mice treated with anti-TSLP antibodies exhibited increased airway inflammation and morbidity rates after bleomycin-induced tissue damage. We found that signaling through TSLPR on non-hematopoietic cells was sufficient for TSLP’s protective function. Consistent with this finding, we showed that TSLP reduces caspase-1 and caspase-3 activity levels in primary human bronchial epithelial cells treated with bleomycin via Bcl-xL up-regulation. These observations were recapitulated in vivo by observing that Tslpr−/− mice showed reduced Bcl-xL expression that paralleled increased lung caspase-1 and caspase-3 activity levels and IL-1β concentrations in the bronchial-alveolar lavage fluid. Our studies reveal a novel contribution for TSLP in preventing damage-induced airway inflammation.

Involvement of caspase-4 in IL-1 beta production and pyroptosis in human macrophages during dengue virus infection

Caspase-4 physically interacts with caspase-1 and is believed to be a proinflammatory caspase that can induce the inflammatory form of programmed cell death (pyroptosis) and the release of mature interleukin (IL)-1β. However, the function of caspase-4 in dengue virus infection is not yet fully understood. We examined the function of caspase-4 in IL-1β production and pyroptosis during dengue virus serotype-2 (DENV-2) infection in human macrophages. In this study, DENV-2 infection increased IL-1β protein level with activated caspase-4 activity. Using primary macrophages, we observed that caspase-4 induces activation of caspase-1 and secretion of IL-1β in response to DENV-2 infection, without the need for secondary signals to stimulate the assembly of the inflammasome. These findings indicate that the regulation of caspase-1 activity by capsase-4 could represent a unique mechanism. Our data suggest that caspase-4 is upstream of caspase-1 in the pathway that regulates pyroptosis and IL-1β synthesis in macrophages during DENV-2 infection.

A critical role for human caspase-4 in endotoxin sensitivity.

Response to endotoxins is an important part of the organismal reaction to Gram-negative bacteria and plays a critical role in sepsis and septic shock, as well as other conditions such as metabolic endotoxemia. Humans are generally more sensitive to endotoxins when compared with experimental animals such as mice. Inflammatory caspases mediate endotoxin-induced IL-1β secretion and lethality in mice, and caspase-4 is an inflammatory caspase that is found in the human, and not mouse, genome. To test whether caspase-4 is involved in endotoxin sensitivity, we developed a transgenic mouse expressing human caspase-4 in its genomic context. Caspase-4 transgenic mice exhibited significantly higher endotoxin sensitivity, as measured by enhanced cytokine secretion and lethality following LPS challenge. Using bone marrow-derived macrophages, we then observed that caspase-4 can support activation of caspase-1 and secretion of IL-1β and IL-18 in response to priming signals (LPS or Pam3CSK4) alone, without the need for second signals to stimulate the assembly of the inflammasome. These findings indicate that the regulation of caspase-1 activity by human caspase-4 could represent a unique mechanism in humans, as compared with laboratory rodents, and may partially explain the higher sensitivity to endotoxins observed in humans. Regulation of the expression, activation, or activity of caspase-4 therefore represents targets for systemic inflammatory response syndrome, sepsis, septic shock, and related disorders.

Pyrin Domain (PYD)-containing Inflammasome in Innate Immunity

Inflammasome is a cytosolic multiprotein complex to activate caspase-1 leading to the subsequent processing of inactive pro-interleukin-1-beta (Pro-IL-1β) into its active interleukin-1 beta (IL-1β) in response to pathogen- or dangerassociated molecular pattern. In recent years, a huge progress has been made to identify inflammasome component as a molecular platform to recruit and activate caspase-1. Nucleotide-binding oligomerization domain-like receptor (NLR) family proteins such as NLRP1, NLRP3 or interleukin-1β-converting enzyme (ICE)-protease activating factor (IPAF) have been first characterized to form inflammasome complex to induce caspase-1 activation. More recently, non-NLR type, pyrin-domain (PYD)-containing proteins such as pyrin or absent in melanoma2 (AIM2) were also proposed to form caspase-1-activating inflammasome machinery with apoptosis-associated speck-like protein containing a CARD (ASC), an essential adaptor molecule. Inflammasome pathways were shown to be crucial for protecting host organisms against diverse pathogen infections, but accumulating evidences also suggest that excessive activation of inflammasome/ caspase-1 might be related to the pathogenesis of inflammation-related diseases. Indeed, mutations in NLRP3 or pyrin are closely associated with autoinflammatory diseases such as familial Mediterranean fever (FMF) syndrome or Muckle-Wells syndrome (MWS), indicating that the regulation of caspase-1 activity by inflammasome is a central process in these hereditary inflammatory disorders. Here, recent advances on the molecular mechanism of caspase-1 activation by PYD-containing inflammasomes are summarized and discussed.

12.2 Mevalonate kinase deficiency: Impaired isoprenoid synthesis induces IL-1beta production via activation of Rac1

Mevalonate kinase deficiency is an autosomal recessive disorder characterized by recurring episodes of fever and inflammation. Peripheral blood mononuclear cells from mevalonate kinase deficiency patients secrete high levels of IL-1β when stimulated with lipopolysaccharide (LPS) due to the presence of hyperactive caspase-1. The molecular mechanism of mevalonate kinase deficiency-induced caspase-1 activation remains unclear. We artificially impaired isoprenoid biosynthesis in the monocytic cells (THP-1) with simvastatin, after which cells were stimulated with LPS. Simvastatin-treated THP-1 cells stimulated with LPS demonstrated enhanced release of IL-1β. LPS enhanced transcription of IL-1β., whereas simvastatin enhanced proteolytic activation of IL-1β. This effect was mediated by phosphatidylinositol 3 kinase (PI3K) and protein kinase B (PKB/c-Akt). In addition, simvastatin-induced IL-1β secretion required the small GTPase Rac1. Simvastatin treatment increased the levels of biologically active GTP-bound Rac1 and inhibition of Rac1 reduced simvastatin-mediated IL-1β secretion. Rac1 functioned upstream of PKB, since Rac1 inhibition abolished the effect of simvastatin on PKB. Simvastatin-mediated activation of the Rac1/PI3K/PKB pathway enhanced IL-1β secretion through activation of caspase-1, since inhibition of both Rac1 and PI3K blocked the release of active caspase-1 subunits. The importance of Rac1 in mevalonate kinase deficiency was confirmed when a specific Rac1 inhibitor was shown to inhibit spontaneous IL-1β release by mononuclear cells from mevalonate kinase deficiency patients. Together, these results demonstrate that Rac1, PI3K and PKB are involved in simvastatin-induced secretion of IL-1β through regulation of caspase-1 activity and that Rac1 is a potential new therapeutic target in mevalonate kinase deficiency.

Caspase-1 inflammasomes: choosing between death and taxis

Among the arsenal of the innate immune system are the Nodlike receptors (NLRs), which detect intracellular pathogens or other ‘alarms’ and participate in the formation of the inflammasome, an oligomeric platform that dimerizes and thereby activates caspase-1. The caspase, in turn, processes pro-IL1b and pro-IL18, key cytokines for the recruitment and engagement of inflammatory cells (hence, ‘taxis’). But the activation of caspase-1 can also trigger a form of cell death with characteristics of necrosis and apoptosis that has been called ‘pyroptosis’. Two papers in this issue (FernandesAlnemri et al. and Petrilli et al.) address the role of Kþ efflux in the control of caspase-1 activation through the regulation of oligomerization of the ASC adapter molecule. In mammals, there are approximately 20 NLRs, including NALP1b, NALP3/cryopyrin, and IPAF. Upon activation, these can oligomerize and recruit ASC, which in turn binds to caspase-1 to trigger its protease activity. Such complexes were proposed as the inflammasomes, the platforms on which caspase-1 is activated. It has been known for over a decade that the activation of caspase-1 is inhibited by Kþ ions, which efflux from the cell when the cell surface receptor P2X7 is engaged by extracellular ATP, or when microbial toxins such as nigericin, gramacidin, maitotoxin, or a-toxin form potassium channels in the plasma membrane. Several studies had suggested that the Kþ efflux acts by inducing a calcium-independent phospholipase, iPLA2, required for caspase-1 processing of cytokines. Now, the laboratories of Tschopp and Alnemri show that the efflux of Kþ directly promotes inflammasome formation by promoting the oligomerization of ASC to associate with and activate caspase-1. Petrilli et al. show that ASC oligomerization and recruitment of caspase-1 by NALP3 is inhibited by Kþ , and provide evidence that the same may also be true for NALP1, while Fernandes-Alnemri et al. show that ASC can oligomerize and activate caspase-1 independent of NLRs or other proteins at sufficiently low Kþ concentrations. The idea that ASC may be the target of Kþ regulation of caspase-1 activation is further supported by the observation that the IPAF inflammasome is not regulated by Kþ . While ASC is required for activation of caspase-1 downstream of various inflammatory triggers, it is dispensable for inflammasome activity in response to Legionella, a response that requires IPAF. Thus, under some circumstances, a requirement for Kþ efflux may be overcome by engaging IPAF and bypassing ASC. Stimuli that induce the activation of caspase-1 to process cytokines do not necessarily lead to caspase-1-dependent cell death. What determines this choice (which we have facetiously labeled as between death and taxis)? FernandesAlnemri et al. suggest that the large ASC oligomer (the ‘speck’ that earned ASC its name: apoptosis-associated speck-like protein containing a CARD), which they have dubbed the ‘ASC pyroptosome’, triggers pyroptosis via caspase-1. Simplistically, a low level of active caspase-1 may process cytokines without killing, as inhibitory mechanisms have sufficient time to counteract the potentially deadly effects of the caspase unless the latter exceeds some threshold. Since levels of activation do not affect the rates or specificities of proteases, different outcomes are likely to be a consequence of the thresholds set by endogenous inhibitors and counteracting mechanisms. While some inhibitors of the inflammasome are known (e.g., caspase-12, Iceberg), little is known about how these are engaged by different inflammasomes. It is tempting to propose that Kþ levels regulate the levels of caspase-1 activation in the following way (Figure 1). When Kþ levels are normal, the inflammasome is inhibited, unless IPAF is engaged. As Kþ levels fall, NALPs can bind to ASC and promote inflammasome formation; however, as NALPs are expressed at relatively low levels as compared to ASC and caspase-1, they limit the extent of inflammasome activity. Therefore, cytokine processing rather than death prevails. Should Kþ levels fall further, ASC can oligomerize independently of the NALPs, and therefore much higher levels of caspase-1 manifest to produce deadly results. It is intriguing that the activation of caspase-1 can trigger the repair of the plasma membrane in response to microbial toxins that cause Kþ efflux, enhancing cell survival. Such feedback effects might suggest that at low levels of Kþ efflux, cell survival strategies persist, while at higher levels the cells trigger a caspase-1 suicide pathway. One problem with this simple scenario, however attractive, is that in the cell-free systems employed, similar levels of Kþ were inhibitory to both the NALP3 inflammasome and the ASC

The Inflammasome: First Line of the Immune Response to Cell Stress

The NALP3-inflammasome is a protein complex that stimulates caspase-1 activation to promote the processing and secretion of proinflammatory cytokines. Recent work indicates that the NALP3-inflammasome can be activated by endogenous "danger signals" as well as compounds associated with pathogens (Kanneganti et al., 2006; Mariathasan et al., 2006, Martinon et al., 2006; Sutterwala et al., 2006). Here, we discuss new insights into the regulation of caspase-1 activity in the inflammatory response.