Inflammasomes are critical regulators of inflammation by stimulating the secretion of proinflammatory cytokines after sensing harmful pathogens and endogenous danger signals by innate immune system receptors (pattern-recognition receptor (PRR)) [1,2]. The tight regulation of inflammasomes is essential, as their deficient activation can lead to exacerbating and persistent pathogenic infections while upregulation can cause autoinflammatory disorders [3,4]. Therefore, understanding the structure of the inflammasome complex and activation/regulation mechanisms could help to modulate and manage different disorders.

Inflammasomes are multiprotein complexes that activate inflammatory caspases in response to pathogens or danger signals. The activation of caspases lead to the production of inflammatory cytokines (specifically, interleukin 1β (IL-1β) and interleukin 18 (IL-18)) and the induction of pyroptotic cell death [5]. IL-1β and IL-18 have a central role in inflammation and immunity, and it has been established that IL-1β/IL-18 activation by inflammasomes are considered the main factors in various inflammatory disorders [6,7]. Assembling this cytoplasmic complex is triggered by the activation of the PRR by pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs). Receptors that are able to assemble inflammasomes include leucine-rich repeat containing proteins (NLR) family members (like NLRC4, NLRP1 and NLRP3); proteins absent in melanoma 2 (AIM2) and pyrin (MEFV) [5]. NLRs have a nucleotide-binding and oligomerization domain (NACHT), which is located in the center and takes part in the oligomerization and dNTPase activity. NLRs also have either a caspase recruitment domain (CARD) or pyrin domain (PYD) and, in a few cases, a baculovirus IAP repeat (BIR) or a leucine-rich repeat (LRR). An apoptosis-associated speck-like protein containing a CARD (ASC) is an adaptor protein that acts as a bridge between upwards inflammasome components and caspase-1. The PYD and CARD domains are the main components of ASC. The interaction between the PYD domain of ASC and the PYD of the NLR leads to the aggregation of ASC molecules and formation of ASC filaments. On the other hand, the CARD domain of ASC interacts with the CARD of the Zymogen form of caspase-1. The Zymogen form of caspase-1 matures into caspase 1 through the proteolytic reaction. The interaction of PYD with ASC leads to the recruitment of the Zymogen form of caspase-1 and the activation of caspase-1 [8]. Upon activation, caspase-1, through its proteolytic cleavage properties, stimulates the maturation of the dominant proinflammatory precursor cytokines (IL-1β and IL-18) and releases active forms of these cytokines. Additionally, caspase-1 cleaves the substrate gasdermin D into an N-terminal fragment of gasdermin D that induces pyroptosis [9,10].

ASC has a dual role associated with cancer. It has been demonstrated that ASC expression is silenced via methylation, which inhibits tumor cell apoptosis. On the other hand, as mentioned earlier, ASC is also recognized as an inflammasome complex adaptor molecule, which mediates inflammatory cytokines production (such as IL-1β and IL-18), mediating tumor-promoting functions. Therefore, ASC may perform opposing functions, promoting tumor progression by increasing inflammatory cytokines production or tumor-suppressing by provoking tumor cell apoptosis [11].

In addition to the activation of caspases and proinflammatory cytokines, the activation of inflammasomes leads to the programmed cell death pyroptosis as a gasdermin-dependent form of cell death [12]. This is the term used to describe the release of cytoplasmic components in the extracellular space by the creation of membrane pores [13]. Pyroptosis, as an inflammatory type of programmed cell death, can protect against intracellular pathogens via removing intracellular replication niches and concurrently triggering an inflammatory response [14]. The phagocytes (dendritic cells, macrophages and neutrophils); CD4+ T cells; epithelial cells; endothelial cells; keratinocytes and neurons also undergo pyroptosis [15]. The PRRs that these cells express can recognize a broad spectrum of PAMPs and DAMPs upon microbial infection. The common PRRs include Toll-like receptors (TLRs) and NOD-like receptors (NLRs) [16]. PAMPs and DAMPs as the main stimuli trigger the formation of multiprotein complex inflammasomes, which later activate the caspases to induce pyroptosis. The inflammasome-mediated pyroptosis pathway may be canonical or noncanonical, with the prior applying caspase-1-activating inflammasomes and the end utilizing other caspases [17].

In pyroptosis, unlike apoptosis, a different set of caspases, such as caspase-1/4/5 in humans and caspase-11 in mice, are activated by the inflammasome [18]. These caspases will lead to the activation of several proinflammatory cytokines and the pore-forming protein gasdermins. The pores that are formed during pyroptosis will result in cell membrane rupture and cytokine release, as well as the release of various DAMPs such as DNA, HMGB-1 and ATP outside of the cell. DAMPs recruit immune cells and sustain the inflammatory cascade in the tissue [19,20].

The canonical inflammasome pathway is a two-step process, involving the priming and activation steps (signals 1 and 2). At first, signal 1 is provided by TNF-α and IL-1 or via the sensing of PAMPs and DAMPs by TLRs or NOD1/2, which activate NF-κB. NF-κB activation will lead to the induction of pro-IL-1β and pro-IL-18 expression. Additionally, priming prepares the inflammasomes for activation through another undiscovered mechanism. The second step is followed by PAMP and DAMP sensing by NLRs (NLRP3, NLRC4, etc.) or AIM2 and pyrin via mechanisms that are not fully understood [21]. The activation of these receptors increases the assembly of the NLRP3 inflammasome, caspase-1-mediated IL-1β and IL-18 release and, subsequently, pyroptosis. A sensor protein (PRRs), an adaptor (ASC) and an effector (caspase-1) are the main components of the canonical pathway [17].

Some NLRs, like NLRP3 and NLRC4, using ASC, will interact with the Zymogen form of caspase-1, while others like NLRP1 can directly interact with caspase-1. The Zymogen form of caspase-1 activation, and its cleavage, can catalyze the proteolytic cleavage and activation of IL-1β and IL-18. Furthermore, caspase-1 activation is likely to have a direct effect on the induction of pyroptotic cell death [21].

In addition, non-inflammasome-forming PRRs like TLRs and NOD1/NOD2 are also important in pyroptosis. These receptors, via the activation of NF-κB and MAPK-signaling pathways, will upregulate inflammatory cytokine expressions (IFN α/β,TNF, IL-12 and IL-6) [22,23]. These active inflammatory cytokines will be released from the host cells. Caspase-1 also cleaves the cytosolic gasdermin D (GSDMD) to generate an N-terminal domain (GSDMD-N) and a C-terminal domain (GSDMD-C) [24]. In normal conditions, the GSDMD-C auto-represses GSDMD-N cleavage to suppress cell lysis [25]. GSDMD-N via oligomerization forms pores in the plasma membrane with an inner diameter of 10-14 nm, which facilitates potassium efflux and the release of IL-1β, IL-18 and other cytosolic components into the extracellular area. This interrupts the cellular ionic gradient, leading to an increase in osmotic pressure and, subsequently, pyroptosis [24,25]. GSDMD-N restricts the damage to normal neighboring cells by inserting itself into the inner membrane [26].

The activation of caspase-4/5 in humans and caspase-11 in mice responds to the noncanonical inflammasome pathway, which is initiated by the binding of a lipopolysaccharide (LPS) of Gram-negative bacteria directly onto these caspases. The oligomerization and activation of these caspases cleaves GSDMD to release GSDMD-N, triggering pyroptosis. This effect is seen to increase with LPS binding to the caspases [27]. Furthermore, the influx of potassium ions upon membrane permeabilization also stimulates NLRP3 activation, resulting in NLRP3 inflammasome formation and the activation of caspase-1. These processes promote GSDMD cleaving and improve the maturation and release of the proinflammatory cytokines [17].

Previous studies have revealed that the high production of IL-1β and IL-18, through the dysregulation of inflammasome activity during the microbial invasion or intestinal inflammation, plays a critical role in the pathogenesis of inflammatory bowel diseases (IBD) and colorectal cancer. However, the exact role of the inflammasome in the progression of these disorders requires more investigation [28].

IBD (ulcerative colitis and Crohn’s disease) is at an enhanced risk of colorectal cancer (CRC) development. The disease duration, stage of disease, degree of mucosal inflammation and portion of the bowel, family history, the related primary sclerosing cholangitis and age at diagnosis are the main CRC risk factors [29,30]. The development of CRC is two to six times greater in IBD patients compared to the general population. The major factor of CRC development in IBD is dysplasia, which is a neoplastic intraepithelial transformation. The inflammation induces intestinal epithelial cell (IEC) apoptosis through tumor suppressor p53 pathways; impaired signaling by p53 may be an initial step of the dysplasia progression to cancer [30].

2. Inflammasomes and Inflammatory Bowel Disease

3. Inflammasomes Regulate Intestinal Inflammation