In addition to its central role in the pathogenesis of autoinflammatory disorders, the NLRP3 inflammasome has emerged recently as an unexpected sensor for metabolic danger and stress [75,76]. Indeed, it has been implicated in the development of major diseases such as gout, type 2 diabetes and obesity-induced insulin resistance. Moreover, the NLRP3 inflammasome is increasingly suspected of playing a major role in other human pathologies such as cancer, asbestosis and Alzheimer's disease.
Gout is a sterile inflammatory disease caused by monosodium urate (MSU) crystal deposition in various tissues. The prototypical clinical manifestation is acute monoarthritis, where MSU crystals precipitate in the joint, triggering an acute local inflammatory response . MSU crystals were demonstrated to specifically activate the NLRP3 inflammasome, both in vitro and in vivo. Uric acid is normally produced as the end result of the metabolic pathway governing the degradation of purines, and hyperuricaemia is thus a main risk factor for the development of gout . Taken together, this suggests that the NLRP3 inflammasome has evolved as a sensor of metabolic endogenous danger, in addition to its pathogen-detecting functions.
Excitingly, preliminary clinical trials involving in vivo IL-1β blockade by anakinra or rilonacept in gout patients demonstrated high efficacy and the absence of adverse effects [79,80]. These findings require confirmation in large-scale controlled studies, and it will be interesting to see whether long-acting therapies such as canakinumab are able to tame chronic gout flares over time. Of special interest, anti-IL-1 therapy might be attractive to patients for which mainstream gout therapies are inefficient or contraindicated .
Type II diabetes
Another key metabolic danger signal resides in chronically elevated blood sugar levels and associated insulin resistance, which are hallmarks of type 2 diabetes. During recent years there has been a growing interest in the inflammatory component of the disease , and in particular in the role of IL-1β. Indeed, IL-1β has been proposed to play a critical role in the loss of β cell mass in the course of type 2 diabetes , and a current hypothesis suggests that the relative balance between IL-1β and endogenous IL-1Ra regulates pancreatic islet inflammation associated with the disease . Remarkably, a recent clinical trial supports the notion that IL-1β is indeed a key player in type 2 diabetes, as patients receiving IL-1β antagonists featured improved glycaemic control and β cell mass . Notably, diabetic markers such as increased levels of saturated fatty acids and islet-derived amyloid polypeptide have been reported as capable of activating the NLRP3 inflammasome [86,87], and NLRP3- and ASC-deficient mice fed a high-fat diet display improved insulin sensitivity when compared to control mice .
Obesity-induced insulin resistance
Further experimental data suggest that the NLRP3 inflammasome is an important regulator of adipocyte differentiation and insulin sensitivity . Adipocytes are rendered more metabolically active and insulin-sensitive upon NLRP3 inflammasome inhibition in murine models of obesity . Strikingly, calorie restriction and exercise-mediated weight loss in obese type 2 diabetes patients is associated with a decreased NLRP3 expression in adipose tissue, coupled to decreased inflammation and improved insulin sensitivity . Collectively, these findings suggest that the NLRP3 inflammasome is able to sense obesity-associated danger signals and contribute to the development of inflammation and insulin resistance .
The tumour microenvironment has been likened to a non-resolving wound response, with an inflammatory milieu capable of stimulating tumour survival, growth, angiogenesis, invasion and metastasis, immune suppression and genetic mutation [90,91]. Studies suggest that IL-1, like the other key proinflammatory cytokine TNF, is often associated with tumour promotion. An evaluation of several clinical trials using recombinant IL-1β or IL-1α showed that neither had any significant therapeutic benefit when used alone against ovarian cancer, renal cell carcinoma or melanoma, and the toxicity associated with IL-1 administration is likely to outweigh any potential benefits .
However, immune-mediated anti-tumour responses resulting from the production of IL-1β or IL-1α have been documented. Early reports showed that IL-1 treatment (alone or in combination with chemotherapeutic treatment) of cancer cell lines or murine syngenic tumours resulted in decreased tumour cell growth and often promoted tumour regression [92–94]. Similarly, when IL-1α transgenic mice expressing 17 kDa IL-1α under the keratin 14 promoter were treated with DMBA/TPA (7,12-dimethylbenzanthracene/12-O-tetradecanoylphorbol-13-acetate), or crossed to mutant Ha-Ras expressing mice, the IL-1α expressing mice were completely resistant to papilloma and carcinoma formation due to enhanced acute inflammatory responses . More recently it was also demonstrated that the NRLP3 inflammasome and subsequent IL-1β priming of T cells is critical for immune-mediated eradication of tumours following chemotherapy .
Several groups have also examined the role of NLRP3 and caspase-1 in inflammatory bowel diseases using the dextran sulphate sodium (DSS) mouse model of colitis. Ulcerative colitis and Crohn's disease predispose to colorectal cancer, where an inappropriate inflammatory response to commensal bacteria is believed to play a major role in the neoplastic transformation of the intestinal epithelium. Two studies have suggested that caspase-1 or NLRP3 deficiency leads to reduced colitis severity in DSS-treated mice when compared to wild-type mice [97,98]. However, opposing results reported by several groups showed that NLRP3, ASC and caspase-1 knock-out mice are all more susceptible to DSS-induced colitis and death [99–101]. In inflammasome-deficient mice, it was reported that a lack of IL-18 activation prevented the repair of the mucosal barrier following DSS-induced damage, resulting in systemic commensal bacterial spread . It was also demonstrated that colitis-associated cancer, induced by DSS and azoxymethane, is enhanced significantly upon genetic deletion of either NLRP3, caspase-1 or ASC, while the role of NLRC4 remains controversial [102–104]. It has been observed previously that other TLR/IL-1R family signalling members, such as myeloid differentiation factor 88 (MydD88), also protect from DSS-associated colitis and intestinal tumorigenesis [105–107]. Therefore, the accumulative evidence suggests that appropriate innate immune signalling responses to commensal bacteria, mediated at least in part by the NLRP3 inflammasome, are a general requirement for intestinal homeostasis. Consistent with this notion, single nucleotide polymorphisms (SNPs) within the NLRP3 region that result in decreased NLRP3 expression have been identified as contributing to Crohn's disease susceptibility, suggesting that the NRLP3 inflammasome may also play a protective role in inflammatory bowel disease in humans .
Ulcerative colitis results from hyper-responsive inflammation, and in this context it has been demonstrated that excessive IL-1β and IL-18 production can also contribute to DSS-induced colitis and possibly cancer, as was observed when the autophagy gene ATG16L1, or the caspase-1 negative regulator, caspase-12, were deleted [43,101]. Therefore, the NLRP3 inflammasome may play an important role in cellular repair and regeneration following acute tissue damage, but if tissues are exposed to chronic or excessive inflammasome activity, NLRP3 stimulation is likely to enhance neoplastic processes.
Despite its ability to promote an immune cell-mediated anti-tumour response, high levels of IL-1 in the tumour microenvironment often correlate with a poor prognosis (reviewed in ). Tumour-associated macrophages and dendritic cells are likely to contribute to IL-1β levels within the tumour infiltrate, while some cancer cell lines, such as those derived from myeloma, melanoma and acute myeoblastic leukaemia, can produce active IL-1β constitutively which can contribute towards tumour cell growth and invasiveness [110–112]. It is notable that some common oncogenes, such as Ras, can induce IL-1β expression  and IL-1β is a known target for the transcription factor NF-κB, which is activated in many neoplastic malignancies.
IL-1 receptor signalling can induce either directly or indirectly the production genes that stimulate tumour growth, angiogenesis and metastasis [i.e. IL-6, IL-8, TNF, matrix metalloproteinases (MMPs), basic fibroblast growth factor (bFGF), vascular endothelial growth factor (VEGF), vascular cell adhesion molecule 1 (VCAM-1), intercellular adhesion molecule 1 (ICAM-1), monocyte chemotactic protein-1 (MCP-1), CXCL-2]. Melanoma cells expressing high levels of IL-1β show reduced tumour growth and metastases when treated with IL-1Ra in murine xenograft experiments . Similarly, Lewis lung cell carcinoma cells engineered to produce IL-1β showed increased tumour growth and increased expression of angiogenic factors when implanted into mice . In other mouse models, murine B16 melanoma growth, invasiveness, lung metastasis and stimulation of angiogenesis is severely attenuated in IL-1β, and to a lesser extent IL-1α, knock-out mice , an affect which anakinra treatment appears to recapitulate somewhat in B16 melanoma injected wild-type mice . Similarly, chemically induced skin carcinogenesis is severely compromised in IL-1β knock-out mice and, conversely, tumour growth accelerated upon genetic deletion of IL-1Ra . The mechanisms and potential contribution of different inflammasome(s) in IL-1β activation in murine cancer models has yet to be examined in detail, although caspase-1 function does contribute to B16 melanoma hepatic metastasis .
Cells from acute myeloid leukaemia (AML) patients can produce and secrete IL-1β and show substantially reduced proliferation and decreased growth factor levels [i.e. granulocyte–macrophage colony-stimulating factor (GM-CSF)] when treated with IL-1Ra, although in a subpopulation of patients AML cells may also proliferate when exposed to IL-1Ra [112,120–123]. A Phase I safety trial reported no responses in patients with refractory or relapsed AML when treated with soluble decoy human IL-1R . However, it was noted that the decoy IL-1R serum levels were below those that completely blocked AML cell growth in vitro and were likely to be even lower within marrow. It may therefore be worth revisiting the effects of IL-1 blockade on AML in the clinic using more efficacious IL-1 inhibitors.
Evidence for the tumorigenic role of IL-1β also comes from its association with gastric cancer, the second deadliest form of cancer worldwide after lung cancer . IL-1β is induced by Helicobacter pylori within the gastric mucosa and is a potent inhibitor of gastric acid secretion, which may lead to gastric atrophy, a precursor of gastric cancer. In 2000, Rabkin et al. described IL-1 gene cluster polymorphisms that correlated with a predisposition to hypochlorhydria, gastric atrophy and gastric cancer in humans infected with H. pylori. Several studies in different human populations have since confirmed these observations (reviewed in ), and mice engineered to express IL-1β in the stomach develop gastric inflammation and cancer . However, it is still unclear how the human polymorphisms affect IL-1β production and which, if any, inflammasomes are involved.
The expression of IL-1β by either myeloma cells or innate immune cells has been associated for some time with the induction of IL-6, a key growth factor that promotes myeloma cell survival and proliferation. Recent clinical trials using IL-1Ra (combined with low-dose dexamethasone) demonstrated that IL-1 inhibition induced a chronic disease state in smouldering or indolent multiple myeloma patients, and substantially improved progression-free survival by preventing the transition to active multiple myeloma . This represents the first demonstration of the therapeutic benefit of IL-1 inhibition in a human cancer.
Given the general safety of inhibiting IL-1 in vivo, and its probable role in cancer metastasis, future clinical trials examining IL-1 inhibition in cancer are deemed warranted . It will also be important to determine the mechanisms and inflammasomes by which cancer cells directly or indirectly modulate IL-1β activity.
Numerous NLRP3 inflammasome activators have been identified and characterized in vitro. In some cases, they have pointed to the unexpected implication of the NLRP3 inflammasome in the pathogenesis of various inflammatory diseases. For example, asbestosis and silicosis have been shown to activate the NLRP3 inflammasome in murine models of chronic pulmonary fibrotic disorders [131,132], raising the intriguing possibility that IL-1β may contribute to inflammation-induced lung cancer, and that anti-IL-1β therapy might be beneficial for patients suffering from these diseases. The fibrillar peptide amyloid-β, which plays a key function in the development of Alzheimer's disease, was also shown to activate the NLRP3 inflammasome . Moreover, the NLRP3 inflammasome was suggested to be instrumental in the inflammatory component of the disease and its associated brain tissue damage . In the skin, NLRP3 inflammasome activation has been linked to UVB-induced damage [133,134] and contact hypersensitivity [133,135]. Recent studies have shown that haemozoin, a crystal produced by plasmodium species in the course of malarial infection, activates the NLRP3 inflammasome [30,136]. More surprising still, non-coding NLRP3 mutations were linked to essential hypertension susceptibility, possibly due to increased expression of the protein .
In most of theses cases, including cancer, the evidence pointing to an involvement of the NLRP3 inflammasome in disease development in vivo remains preliminary and awaits further confirmation. Collectively, however, they stand as a testimony to the impressive versatility of the NLRP3 inflammasome as a danger-detection system with potentially far-reaching implications for human health.