Pyroptosis,inflammasome,and gasdermins in tumor immunity

Abstract

The gasdermins (GSDM), a family of pore-forming proteins, consist of gasdermin A (GSDMA), gasdermin B (GSDMB), gasdermin C (GSDMC), gasdermin D (GSDMD), gasdermin E (GSDME) and DFNB59 (Pejvakin (PJVK)) in humans. These proteins play an important role in pyroptosis. Among them, GSDMD is the most extensively studied protein and is identified as the executioner of pyroptosis. Other family members have also been implicated in certain cancers. As a unique form of programmed cell death, pyroptosis is closely related to tumor progression, and the inflammasome, an innate immune mechanism that induces inflammation and pyroptosis. In this review, we explore the current developments of pyroptosis, the inflammasome, and especially we review the gasdermin family members and their role in inducing pyroptosis and the possible therapeutic values in antitumor effects.

Introduction

Under certain conditions, normal tissue cells undergo a series of genetic variations, transform into malignant cells, and ultimately develop into tumors. Although considerable progression has been made in tumor therapy, cancers are still the second leading cause of death worldwide. According to the report of the World Health Organization (WHO), tumor fatalities will increase by 80% till the year of 2030. ¹ One of the big challenges is how to effectively eliminate malignant cells without affecting normal cells. So far, the induction of apoptosis in tumor cells is considered an efficient therapeutic treatment for cancer patients. And yet, the intrinsic or acquired resistance of tumor cells to apoptosis occurs often. ² Hence, exploring alternative ways to kill tumors is urgently needed.

Pyroptosis, an unique form of programmed cell death (PCD), was first discovered in 1992. ³ It is usually initiated by microbial infection, accompanied by activation of the inflammasome and secretion of pro-inflammatory cytokines, such as interleukin (IL)-1β and IL-18. ⁴ In recent years, a series of studies of pyroptosis were carried out, yet the mechanism of how pyroptosis takes place within the cells remains unknown. Importantly, more reports showed that pyroptosis was involved in several human diseases, especially in malignant tumors. ⁵ Increasing evidence has shown that pyroptosis can be induced in tumor cells to prevent tumor growth.^6,7

Gasdermin (GSDM) is a recently identified protein from pore-forming protein family consisting of GSDMA, GSDMB, GSDMC, GSDMD, GSDME and DFNB59. GSDMD was discovered in 2015 and was a downstream effector of the inflammasome. After caspase cleavage, GSDMD relieves the auto-inhibition state and releases gasdermin-N and gasdermin-C domains. ⁴ The gasdermin-N domain combines with membrane phospholipids and perforates cell membrane with a pore size of 18 nm, thereby disrupting the membrane permeability and causing cell swelling with big bubbles. ⁴ Apart from GSDMD, other members of the gasdermin family including GSDMA, GSDMB, GSDMC, GSDME also possess the ability of perforating the cell membrane and of activating pyroptosis. ⁸

In this mini review, we will briefly introduce pyroptosis and the inflammasome. We will discuss how gasdermins are induced in tumor cells and their potential role in tumor therapy. This review will help to understand the antitumor activity of gasdermins and to develop new drugs for clinical application in cancer patients based on the modulation of gasdermins.

Pyroptosis: A double-edged sword in immune homeostasis

The pyroptosis phenomenon was first described in 1992, ³ but the term was coined in 2001 following the observation that bacteria-infected macrophages underwent a rapid lytic cell death dependent on caspase-1 activity. ⁹ Pyroptosis is characterized by the formation of pores in the plasma membrane, swelling and cell failure. It is often activated in response to various microbial ligands, such as bacterial flagellin, toxins, lipopolysaccharide (LPS) or DNA that gain access to the cell cytosol.^10–12 These ligands are usually strong danger signals that require the cells to react appropriately.

On the one hand, pyroptosis functions as a protective process for the host since it controls the release of inflammatory cytokines and danger signals, and removes the replicative niche of a pathogen. The importance of pyroptosis in controlling multiple viral and bacterial infections has been clearly demonstrated. For example, pyroptosis restricts replication of the intracellular bacterium Legionella pneumophila by controlling the NAIP5/NLRC4 inflammasome.^13,14 In addition, the pyroptosome traps intracellular bacteria into the cellular debris of the pyroptotic cells. ¹⁵ These pore-induced intracellular traps further recruit neutrophils and provoke other immunologic reactions to eliminate the trapped bacteria. ¹⁶

However, pyroptosis was also found to cause adverse consequences to the host. For instance, in an endotoxic or septic shock, excessive pyroptosis results in an overwhelming inflammatory reaction, causing tissue damage.^17,18 Furthermore, pyroptosis plays a critical role in promoting disease progression, such as cardiovascular, central nervous system, and liver diseases.^19–21 Although pyroptosis is traditionally defined as a caspase-1-mediated cell death, other caspases, such as caspase-11 and its orthologs caspase-4 and -5,^22,23 as well as the apoptotic effector caspase-3^6,24 are capable of triggering pyroptosis. The execution of pyroptotic cell death via these caspases requires the activation and cleavage of specific members of the gasdermin family including GSDMA, GSDMB, GSDMC, GSDMD and GSDME. The cleavage of the link between N- and C- terminal domains of gasdermin results in the release of an activated N-terminal region from an inhibitory C-terminal fragment, thereby forming oligomeric death-inducing pores.^25,26 It is widely accepted that the pyroptosis of mammalian cells is dependent on gasdermins (Figure 1).

Figure 1.

Pyroptosis. Pyroptotic stimulation evokes inflammasome assembly and subsequent caspase activation. Activated caspases cleave gasdermins, generating the N-terminal fragments which oligomerize and translocate to the plasma membrane, finally causing plasma membrane rupture via non-selective pore formation.

Inflammasome: Old dog new tricks in pyroptosis

The inflammasome, a cytosolic multimeric innate immune signaling complex, has emerged as a central pillar of mammalian innate immunity since its discovery in 2002. ²⁷ Inflammasome pathways are traditionally divided into the canonical and noncanonical, which differ mainly in the initiation step. The formation of pores in the cell membrane and the release of endogenous molecules and inflammatory cytokines, such as IL-1β and IL-18, are the final steps for both pathways (Figure 2). ²⁸ In terms of canonical pathways, formation of a functional inflammasome is initiated by germline-encoded pattern recognition receptors (PRRs) which are capable of sensing pathogen-associated molecular patterns (PAMPs), danger-associated molecular patterns (DAMPs), and homeostasis-altering molecular processes (HAMPs).^29,30 Upon sensing a specific molecular pattern, certain PRRs recruit the adaptor protein, apoptosis-associated speck-like protein and further activate downstream events.

Figure 2.

Canonical and noncanonical inflammasome pathways. In the canonical model of pyroptosis, pattern recognition receptors (PRRs) can recognize cellular stressors, including pathogen-associated molecular patterns (PAMPs), danger-associated molecular patterns (DAMPs), and homeostasis-altering molecular processes (HAMPs). These PRRs subsequently activate caspase-1 directly or via the adaptor protein ASC (apoptosis-associated speck-like protein containing a CARD). Caspase-1 further activates IL-1β and IL-18, and also cleaves GSDMD to release the GSDMD-N domain, which next forms oligomeric death-inducing pores. GSDMD-N pores promote the release of biologically active IL-1β and IL-18, thereby resulting in a series of subsequent inflammatory responses. In terms of noncanonical pathways, cytosolic LPS directly binds Caspase-4/5/11 to trigger the cleavage of GSDMD, but not IL-1β and IL-18.

Several canonical inflammasomes have been newly identified to mediate pyroptosis in vivo, denoted by different cytoplasmic PRRs, including nucleotide-binding domain, NLRP1, NLRP3 and NLRP4, but not the melanoma2 (AIM2) or pyrin inflammasomes.^31–35 The activated canonical inflammasomes further process the zymogen procaspase-1 into the active noncovalently linked subunits p10 and p20 (active caspase-1). The latter facilitate the maturation of proinflammatory cytokines (pro-IL-18 and pro-IL-1β) and induce pore formation on plasma membranes. With the human caspase-1-mediated canonical inflammasome pathway of pyroptosis, subsequent studies identified murine caspase-11 and human caspase-4/5 as cytosolic sensors that mediate the noncanonical inflammasome pathway of pyroptosis in response to LPS (Figure 2).^22,36

It was recently shown for the canonical and noncanonical pathways that inflammasome-associated caspases (i.e., human caspase-1, -4, and -5, and murine caspase-1 and -11) have the capacity to mediate cleavage and activation of the pore-forming protein GSDMD. This, in turn liberates the N-terminal domain to oligomerize in the host plasma membrane, and drives pyroptosis in macrophages or in other immune cells such as neutrophils (Figure 2).^4,37,38 Of note, oligomerization of the N-terminal domains leads to pore formation, cell swelling, and release of cytoplasmic contents including biologically active IL-1b and IL-18, which causes an inflammasome-associated inflammatory response in the host.^25,26,39

The gasdermins modulate the host immune responses and tumor progression

GSDM is a family of functionally diversified proteins that are expressed in a variety of cells and tissue types, such as the small intestine, colon, esophagus, bladder and skin epithelial tissues.^40,41 Currently, six GSDMs are found in humans.^42–48 All these GSDMs except Pejvakin adopt a similar structure consisting of a pore-forming N-terminal domain and a regulatory C-terminal domain. Cleavage and separation of these two domains are thought to be essential for activation.

Gasdermin A (GSDMA)

The human genome encodes one form of GSDMA, while the mouse genome encodes three paralogs known as GSDMA1, GSDMA2 and GSDMA3.^44,45,49 Human GSDMA and mouse GSDMA3 adopt an auto-inhibited two-domain structure, in which the N-terminal domain is inhibited by the C-terminal domain.^25,50,51 Gasdermin A is involved in tumor apoptosis by regulating mitochondrial homeostasis^50,52 and can generate a lytic effector when experimentally truncated, but has no known activator. ⁵³ Recent studies have demonstrated that human GSDMA and murine GSDMA can be cleaved by SpeB, a streptococcal protease, which initiates pyroptosis.^54,55

A previous study showed that upon incubation with phenylalanine trifluoroborate (Phe-BF3), administration of nanocomplex GSDMA3 in a tumor mouse model bearing 4T1 or EMT6 cells led to the formation of pores on the liposome membranes and ultimately the induction of pyroptosis (Figure 3, Table 1).^25,53 Afterwards, the host immune system underwent significant changes. ⁵³ Further, pyroptosis-induced inflammation within the tumor microenvironment can synergize with checkpoint blockade to induce anti-tumor immunity.^53,56,57

Figure 3.

Anti-tumor effects of gasdermins by pyroptosis in tumor cells. (a) GSDMA3 is combined with nanoparticles, generating NP–GSDMA3. Upon incubation with phenylalanine trifluoroborate (Phe-BF3) in the tumor, GSDMA3 is released from NP-GSDMA3 and induces pyroptosis. (b) Under trastuzumab stimulus, the increased antibody-GSDMB–sulfatide binding might promote pyroptosis. On the other hand, in the presence of IFN-γ, cytotoxic T lymphocytes(CTLs) and natural killer(NK) cells secrete perforin granzyme A into tumor cells, which mediates the inter-domain cleavage of GSDMB. After that, GSDMB-N could trigger extensive pyroptosis in tumor cells. (c) Under hypoxic stress, Stat3-Y705 is phosphorylated to p-Y705-Stat3. The latter interacts physically with programmed death ligand 1 (PD-L1) and facilitates its nuclear import via the importin α and β pathways. After that, the nuclear PD-L1(nPD-L1)/p-Y705-Stat3 complex transcriptionally activates GSDMC. In the presence of TNF-α, GSDMC is cleaved by caspase-8, and the GSDMC N-terminal domain forms pores on the cell membrane to induce pyroptosis. Interestingly, boosts in ROS levels induce the oxidation of death receptor DR6, which then internalizes into the cytosol, and recruits both pro-caspase-8 and GSDMC to the DR6 receptosome. After that, GSDMC is cleaved and pyroptosis generated. (d) Some drugs or molecules can activate caspase-1 and cleave GSDMD to promote pyroptosis. (e) NK cells and CD8⁺T cells release granzyme B into the tumor cells via a perforin-dependent manner. Granzyme B activates caspase-3 to cleave GSDME or directly processes GSDME, leading to pyroptosis.

Table 1.

Gasdermins and examined relevant tumor models.

Gasdermin	Relevant tumor models	Reference
GSDMA	4T1, EMT6	PMID32188939
GSDMB	HER2 breast cancer SW837, OE19, SKCO1	PMID: 31064780 PMID: 32299851
GSDMC	MDA-MB-231 HeLa cervical carcinoma SGC-7901 BGC-823 HCT116 Huh7 B16	PMID: 32929201 PMID: 34012073
GSDMD	esophageal squamous cell carcinoma MDA-MB-231 endometrial cancer	PMID: 30771436 PMID: 29386662 PMID: 31924176
GSDME	MCF-7 HEPG2 CEM-C7 B16 human B leukemic cells	PMID: 28045099 PMID: 30976076 PMID: 32188940 PMID: 31953257

Gasdermin B (GSDMB)

GSDMB belongs to the cytoplasmic proteins of the gasdermin family, which displays either pro-tumor or anti-tumor effects. ⁵⁸ Humans carry a gene encoding GSDMB, whereas mice do not.^44,49 GSDMB has at least four splice variants. ⁴³ To date, the biological function of GSDMB in normal physiology and human disease has been unknown.

Indeed, high expression of GSDMB is associated with tumor progression in different types of cancers, such as gastric, hepatic, colon and cervical cancers.^59,60 In 2019, GSDMB was first confirmed as a therapeutic target in tumor treatment. Intracellular delivery of an antibody targeting GSDMB reduced HER2 breast cancer growth and metastasis. ⁶¹ Under trastuzumab stimulus, the increased GSDMB–sulfatide binding might promote pyroptosis and subsequently amplify cell death signals. ⁶¹ Recently, GSDMB-mediated anti-tumor effects have been further demonstrated. ⁶² Cytotoxic T lymphocytes (CTLs) and natural killer cells (NK) can utilize perforin to deliver granzyme A into tumor cells where granzyme A-mediated cleavage of GSDMB further causing pyroptosis. GSDMB was shown to be a key executioner of inflammatory lysis of cancer cells targeted by innate lymphocytes (Figure 3, Table 1). ⁶²

The tumor microenvironment (TME) plays a crucial role in tumor initiation, progression, metastasis and response to therapies. Investigation of TME will help to further understand the mechanism how gasdermin B induces pyroptosis. The function of immune cells continues to change during the tumor progression. Natural killer cells and cytotoxic T lymphocytes can secrete IFN-γ and granzyme A. IFN-γ binds to its receptor on the surface of tumor cells and further up-regulates GSDMB expression, while granzyme A cleaves GSDMB. ⁶² However, how GSDMB-N terminal interacts with phospholipids commonly found in the mammalian cell plasma membrane still remains unknown. GSDMB binds and forms pores in membranes with enriched cardiolipin. ⁶³ The surface exposure of cardiolipin could trigger GSDMB pore formation and a mitochondrial-dependent cell death pathway. One possible hypothesis is that cardiolipin may be mis-localized to the plasma membrane in TME.^63,64 Lastly, cytosolic pathogens prevent GSDMB-mediated lysis by pathogenic ubiquitination of GSDMB. ⁶³ However, whether similar mechanisms exist in tumor cells may be an area of further inquiry.

Gasdermin C (GSDMC)

GSDMC is expressed in the esophagus, stomach, large and small intestines, ⁶⁵ and in certain cancers such as colorectal cancer, ⁶⁶ metastatic melanoma, ⁴⁶ and esophageal cancer. ⁶⁷ GSDMC expression is drived by STAT6 O-GlcNAcylation in epithelial cells. ⁶⁸ Similar to other GSDM family members, the N-terminus of GSDMC has intrinsic cytotoxicity and can induce pyroptosis-like features in 293T cells. ²⁵

According to recent research, GSDMC was proposed to have an anti-tumor effect. Under hypoxic conditions, p-Stat3 physically interacts with PD-L1 and facilitates its nuclear translocation, hence enhancing the transcription of the GSDMC. GSDMC is then specifically cleaved by caspase-8 upon TNF-α treatment, generating a GSDMC N-terminal domain that forms pores on the cell membrane and further induces pyroptosis (Figure 3, Table 1). ⁷ Enhanced reactive oxygen species (ROS) induces the oxidation of the plasma membrane-localized death receptor 6 (DR6), which is then internalized into the cytosol, and further recruits both pro-caspase-8 and GSDMC to the DR6 receptosome, in which activated caspase-8 cleaves GSDMC, thereby inducing pyroptosis (Figure 3, Table 1). ⁶⁹ Furthermore, it's noted that GSDMC can also be cleaved by caspase-6, which raises the interesting question whether the anti-tumor effects of GSDMC may also occur under a caspase-6-activated microenvironment, such as occurs with ROS and DNA damage.^7,70

Gasdermin D (GSDMD)

GSDMD, the first extensively studied member of the GSDM family, was discovered in the context of inflammasome biology and linked to pyroptosis. ⁷¹ GSDMD consists of a N-terminal pore-forming domain, and a C-terminal inhibitory domain. ⁷¹ The cleavage site is targeted by caspase-1-activated inflammasome complexes, such as AIM2 sensing of intracellular double-stranded DNA or viral DNA, NLRC4 sensing of bacterial flagellin, or NLRP3 sensing of cellular disturbances and plasma membrane disruption. ¹⁰ Additionally, LPS-triggered caspase-11 activation leads to robust GSDMD cleavage, which is upregulated by IFN-γ and IFN-β.^72,73 The caspase-4-GSDMD pathway has been suggested to be transcriptionally regulated by interferon regulatory factor 2 (IRF2).^74,75 Furthermore, in response to Yersinia spp. infection, where the activity of TAK1 and IKK kinases is blocked, a caspase-8 pathway is activated, which leads to the cleavage and activation of GSDMD at the same site. Among these three caspases, caspase-1 appears to be the strongest driver for GSDMD cleavage, while caspase-8 is the weakest, but perhaps acts more as a backup measure in situations when other caspases are impaired or absent. ⁷⁶ Finally, in addition to the caspases mentioned above, both the neutrophil specific elastase and cathepsin G have been shown to be able to cleave the upstream cleavage site of the canonical caspase within GSDMD.^77,78

Once GSDMD is cleaved, the N-terminal domain is liberated and free to oligomerize and insert into the plasma membrane, which in turn results in pore formation. ⁷¹ Expression of the N-terminal domain itself is also sufficient to trigger pyroptosis as it can bind to phospholipids such as cardiolipin, phosphatidylinositol-4-phosphate, phosphatidylinositol-4, -5- bisphosphate, phosphatidic acid and phosphatidyl serine with weaker affinity. ²⁶ GSDMD was also found to be cleaved by caspase-3 within the N-terminal domain.^8,79 However, this caspase-3 cleavage was shown to decrease pyroptosis by inhibiting oligomerization and pore formation by the N-terminal GSDMD domain. Finally, it is believed that pore formation causes the loss of osmotic homeostasis, cell swelling, and cell death. ⁴

As early as 2004, the GSDMD gene was reported to be involved in tumor formation. ⁴⁷ Later, GSDMD was found to be expressed in most tumor cells and connected to tumor progression and prognosis. Although the direct anti-tumor effect mediated by GSDMD has yet to be discovered, certain drugs or molecules could trigger GSDMD-mediated pyroptosis in various types of cancers which indirectly control tumor progression. The metformin targeted miR-497/proline, glutamic acid and leucine-rich protein-1(PELP1) axis induced GSDMD-mediated pyroptosis of esophageal squamous cell carcinoma. ⁸⁰ Docosahexaenoic acid (DHA) inhibits the growth of breast cancer cells through caspase-1-mediated cleavage of GSDMD, which promotes the secretion of IL-1β and membrane pore formation, thereby leading to pyroptosis. ⁸¹ Endometrial cancer growth could be inhibited by hydrogen via a ROS/NLRP3/caspase-1/GSDMD- mediated pyroptosis pathway (Figure 3, Table 1). ⁸² It has been demonstrated that GSDMD is required for an optimal CTL response to cancer cells. ⁸³ Therefore, GSDMD is indeed closely involved in cancer progression and prognosis, and could serve as a new and promising target for cancer therapies.

Gasdermin E (GSDME)

GSDME is reported to perform tumor-suppression function through caspase-3 pathway. Caspase-3 is an apoptotic caspase which can be activated by intrinsic and extrinsic apoptotic pathways. The former involves permeabilization of the mitochondrial membrane and the assembly of the apoptosome, which then activates caspase-9, and the extrinsic apoptotic pathway requires the activation of death receptors and caspase-8. ⁸⁴ Both apoptotic pathways can cause activation of caspase-3 and induce cleavage of GSDME at the site of residue Asp270, converting non-inflammatory apoptosis to pyroptosis in human or mouse cancer cells.^24,85 Induction of inflammatory cell death in GSDME-expressing cancer cells will lead to intrinsic stresses or extrinsic challenges to activate caspase 3, and ultimately have a marked effect on the tumor microenvironment, immune-cell recruitment, and tumor growth. ⁸⁶ Therefore, GSDME has been proposed as a tumor suppressor by inducing pyroptosis in tumor cells.

Interestingly, GSDME can be activated in a cell-extrinsic manner by induction of anti-tumor immunity. A recent report demonstrated that a tumor suppression environment could be achieved by tumor-infiltrating killer lymphocytes such as NK cells and CD8⁺ T cells without any extrinsic treatment. These killer cells released granzyme B that targeting tumor cells in a perforin-dependent manner, then granzyme B directly cleaved GSDME at D270 in addition to activation of caspase-3, ⁸⁶ eventually leading to pore formation and pyroptosis in GSDME-positive tumors.^86,87 In addition, it showed that chimeric antigen receptor (CAR) T cells, rather than existing CD8⁺ T cells, by virtue of their release of a large amount of perforin and granzyme B, activated the caspase 3–GSDME pathway in tumors, leading to pyroptosis. ⁸⁸ Importantly, the tumor suppressive effect of GSDME is abrogated in mice lacking killer cytotoxic lymphocytes, indicating that the GSDME-mediated tumor-suppressive function requires pyroptosis-dependent induction of anti-tumor immunity (Figure 3, Table 1). ⁸⁶ Occasionally, pyroptosis-released factors activate caspase-1 for GSDME cleavage in macrophages, resulting in the release of cytokines and the subsequent cytokine release syndrome (CRS). Therefore, the safety of activating GSDME in tumor immunotherapy or the administration of GSDME as a tumor-killing agent requires further evaluation.

Transcriptional and epigenetic regulation of gasdermins

Many cancers demonstrate positive or negative modulation of GSDM expression correlating with distinct tumor prognosis. GSDMA expression was silenced in gastric cancer due to methylation of its promoter.^52,89 TGF-β or 5-aza-2′-deoxyxytidine may reverse GSDMA silencing and result in increased GSDMA expression. ⁵² GSDMB expression is lower in esophageal and gastric cancers than in the adjacent normal tissues. ⁶² Silenced GSDMB in tumors may be due to DNA methylation. ⁸⁹ GSDMC expression may respond to external cues. For example, hypoxic stress, frequently encountered within solid tumors, also induces GSDMC expression through a STAT3-dependent mechanism. Hypomethylation of the GSDMC promoter leads to GSDMC overexpression in lung adenocarcinoma. ⁹⁰ GSDMD expression was either associated with an unfavorable prognosis in lung adenocarcinoma ⁹¹ or a favorable prognosis in skin cutaneous melanoma. ⁶² Transcription factor Foxo1 binds to the GSDMD promoter and controls GSDMD expression. ⁹² GSDME is absent from most cancer cell lines and is silenced in numerous cancers. This epigenetic silencing is caused by methylation of a CpG island around the transcription start site. Accordingly, hypermethylation of the GSDME promoter is found in 50% of gastric cancers. ⁹³ Various epigenetic mechanisms control GSDME expression. For example, p53 binds to its consensus sequence in the intron 1 of GSDME and activates transcription at the GSDME promoter. ⁹⁴ In summary, promoter methylation is closely related to the inactivation of GSDMs. Demethylation of promoters may be an area worth exploring for tumor therapy.

The role of pyroptosis in modulating the tumor immune microenvironment

Generally speaking, the tumor microenvironment is formed in a chronic inflammatory process in which stromal components and polarized macrophages promote the progression of tumors. Solid tumors contain hypoxic areas, often surrounding sites of necrosis. ⁹⁵ Macrophages tend to accumulate in hypoxic areas of solid tumors and secrete TNF-α, which is essential for tumor necrosis.^96,97 The hypoxic environment enhances the expression of GSDMC, which is cleaved by TNFα-activated caspase-8, inducing pyroptosis and subsequent chronic tumor necrosis, and promoting tumor progression while suppressing anti-tumor immune responses. ⁷ Nevertheless, pyroptosis exerts a dual role in modulating the tumor environment. Chronic tumor pyroptosis suppresses anti-tumor immunity and accelerates tumor growth, while acute inflammation in the tumor microenvironment enhances the immune response to tumors and inhibits tumor development.^7,86 Pyroptosis enhances phagocytosis of tumor cells by tumor-associated macrophages, as well as the number and function of tumor-infiltrating natural killer lymphocytes and CD8⁺ T lymphocytes. ⁸⁶ Pyroptosis of cancer cells promotes the infiltration of dendritic cells and T cells, and boosts the anti-tumor immune response by releasing high mobility protein B1. ⁸⁶ Granzymes derived from cytotoxic lymphocytes are able to cleave GSDMB to induce tumor cell pyroptosis, which in turn enhances anti-cancer immunity. ⁶² Caspase-independent pyroptosis is more likely to induce a rapid anti-tumor response and create a tumor-suppressive microenvironment. Certainly, the relationship between pyroptosis and the tumor microenvironment deserves further investigation.

Discussion and perspectives

Recently, attention has been paid to the GSDMs which play a vital role in pyroptosis, as well as in the inflammasome. ⁶ It has been proved that all GSDM members display anti-tumor function through pyroptosis, which would be an effective measure in correcting the apoptosis-resistant cancers. ⁹⁸

Therapeutic regimens including chemotherapy, radiotherapy, targeted therapy and immune therapy induce pyroptosis in tumor cells and further potentiate local and systemic anti-tumor immunity. As a highly-immunogenic form of cell death, pyroptosis causes local inflammation, attracts inflammatory cell infiltration and amplifies inflammation, thereby providing a great opportunity to relieve the immunosuppression of TME and induces systemic immune responses in the treatment of solid tumors. ⁹⁹ Microenvironment homeostasis is prerequisite for initiating anti-tumor immunity. ¹⁰⁰ As a major form of programmed cell death, tumor cells undergoing pyroptosis generate large amounts of neo-antigens that stimulate the systemic immune to significantly limit tumor progression. ¹⁰¹ Tumor cell pyroptosis causes the release of a large amount of proinflammatory cytokines, causing the cytokine release syndrome and other side effects. ⁸⁸

Until now, our comprehension of the roles of pyroptosis in tumors has been limited and the mechanisms of how these molecules affect tumor cells in detail is also not fully understood. ⁷ The new theories about tumor cell pyroptosis may lead to the discovery of new drugs for tumor treatment in the future, however, further studies are needed to further understand the specific mechanisms by which GSDMs regulate tumor progression.

Given the unknown functions of GSDMs and their mechanisms of activation, we are only beginning to understand their biological and pathological functions. Future studies towards elucidating the new biological roles of GSDMs will deepen our comprehension of GSDMs in tumor growth and proliferation, and allow us to develop more potent anti-tumor targeting molecules.