|Ahead of print publication
Nod-like receptor protein 3 inflammasome in head-and-neck cancer
K Sheeja, S Lakshmi
Division of Cancer Research, Regional Cancer Centre, Laboratory of Molecular Medicine, Medical College, Thiruvananthapuram, Kerala, India
|Date of Submission||13-Dec-2018|
|Date of Acceptance||13-May-2019|
|Date of Web Publication||31-Jan-2020|
Division of Cancer Research, Regional Cancer Centre, Laboratory of Molecular Medicine, Medical College, Thiruvananthapuram - 695 011, Kerala
Source of Support: None, Conflict of Interest: None
Activation of inflammasomes has a decisive role in host defense mechanism against pathogens and other intracellular risk factors, but recently, it has been revealed that they play a significant role in the pathogenesis of several diseases, including cancer. Nod-like receptor protein 3 (NLRP3) inflammasome, the best-studied inflammasome, has contrasting roles in cancer development and progressions. In head-and-neck cancers, the upregulated level of NLRP3 promotes tumor progression. The main objective of this review is to provide current knowledge on the involvement of NLRP3 inflammasome in head-and-neck cancers. Deeper understanding of the biology of this dynamic protein complex provides new scope for the development of more effective anticancer therapies.
Keywords: Head-and-neck cancer, inflammation, NRP3 inflammasome, pattern recognition receptors
| > Introduction|| |
Head-and-neck cancers are a heterogeneous group of diseases. They originate in the upper aerodigestive tract, including the oral cavity, larynx, pharynx, and nasopharynx, and are the sixth leading cancer worldwide. Head-and-neck cancers make up around 6% of all cancers. The major histological subtype within head-and-neck cancers is squamous cell carcinoma (HNSCC), arising from the epithelial membranes (mucus linings) of these regions. They are classified according to the part of the body in which they occur. The use of tobacco and alcohol is the most important risk factor for head-and-neck cancers. Infection with high-risk factor human papillomavirus and genetic predisposition may also contribute toward the development of this malignancy. Lung, mediastinal nodes, liver, and bone are the main sites of metastases. The main treatment options for head-and-neck cancers are surgery, radiotherapy, and chemotherapy. Recent advances in the use of chemotherapy plus radiation and hyperfractionated radiotherapy has led to longer survival times in clinical trials affecting various sites, with different prognosis. Treatment responses depend on many factors including stage of the disease.
The immune system plays a pivotal role in the development of malignancy. During the progression of tumor, an inflammatory microenvironment is created in the tumor vicinity that may accelerate the tumor growth into advanced stage. Tumor microenvironment is comprised of proliferating tumor cells, stromal cells, blood cells, and infiltrating inflammatory cells, and they provide signals to regulate various events of tumor progression. Many types of cancers are etiologically linked to inflammation. As the tumor grows and proliferates, it activates innate immune cells and recruits effector cells to exert antitumor activity while promoting tumor progression through the production of pro-inflammatory molecules., Inflammatory cells in the microenvironment play an important role in providing mitogenic growth factors for proliferating tumors. Chronic inflammation fuels the tumor growth by inducing genetic and epigenetic changes: DNA and tissue damage. Inflammation is also orchestrated by a cytoplasmic multimeric protein complex known as inflammasomes. They provide a platform for caspase-1 activation which catalyzes the proteolytic cleavage and maturation of inactive interleukin (IL) IL-1β and IL-18 to their respective active forms. This is also accompanied by inflammatory cell death known as pyroptosis. This might be involved in the suppression of tumors. Thus, inflammasomes play a crucial role in the progression or suppression of the developing tumor in a contextual manner based on cell type, tissue, and organ involved. Experimental and clinical data indicate that the upregulation of nod-like receptor protein 3 (NLRP3) inflammasome in head-and-neck tumor is associated with the self-renewal activation of cancer stem cells. In this review, we describe the recent research in the area of NLRP3 inflammasomes and head-and-neck malignancies.
| > Pattern Recognition Receptors|| |
Pattern recognition receptors (PRRs) are germline-encoded receptors present on innate immune cells including dendritic cells, monocytes, macrophages, neutrophils, natural killer (NK) cells, and epithelial cells, which play a central role in the immune response. PRRs can be expressed on cell surfaces, in the cytoplasm, or in endosomes. The host defense system of the body is activated when pathogen-associated molecular patterns and damage-associated molecular patterns (DAMPs) released from infected cells, tumors, and damaged tissues are recognized by cytosolic PRRs. The PRR family consists of various members, including toll-like receptors (TLRs), nucleotide-binding and oligomerization domain NLRs, retinoic acid-inducible gene I-like receptors (RLRs), C-type lectins (CTL), and absent in melanoma (AIM)-like receptors (ALRs)., Plasma membrane PRRs include TLRs and CTLs, whereas the NLRs, RLRs, and ALRs are intracellular in nature. Recently, the NLR family has gained a lot of attention in cancer research due to its regulatory effect on inflammation and immunity. Studies have revealed that certain members of NLR proteins participate in inflammatory process through the formation of inflammasomes.
| > Nucleotide-Binding and Oligomerization Domain-Like Receptors|| |
NLRs are located in the cytoplasm of immune-competent cells that recognize microbial products and DAMPs and activate the nuclear factor kappa B (NF-κB) complex, leading to the expression of pro-inflammatory and chemotactic cytokines. NLRs are classified into 22 isoforms in humans. In general, NLRs display three domains, namely the C-terminal domain which contains a leucine-rich repeat (LRR), which is a sensing unit; the N-terminal which participates in protein–protein interaction; and the central region called NOD domain/NACHAT domain, which mediates NLR oligomerization. NLRs have been subdivided into five types based on the domain structure and evolutionary analysis. They are NLRA (acidic transactivation domain), NLRB (the baculoviral inhibitory repeat-like domain), NLRC (the caspase activation and recruitment domain), and NLRP (the pyrin domain), and NLRX (N-terminal mitochondria-targeting sequence). NLR family members take part in innate immune signaling through activation or inhibition of the inflammasome. Among different NLRs, NLRP1, NLRP2, NLRP3, NLRC4, NLRP6, NLRP7, and NLRP 12, as well as the PYHIN family member AIM2, have been shown to form inflammasomes.,
| > Nod-Like Receptor Protein 3 Inflammasome|| |
The NLRP3 inflammasome was first identified by Tschopp in 2002 as a multiprotein complex containing CASP1, CASP5, Pycard/Asc, and NLR protein 1 (NLRP1). They are multicomplex proteins formed by the oligomerization of certain NLRs and serve as a central molecular platform for inflammatory reaction. Recently, more than twenty inflammasomes have been identified, and NLRP3 inflammasome is composed of NLRP3; ASC (apoptosis-associated speck-like protein containing a caspase-recruitment domain) which is an adaptor protein encoded by PYCARD; and the precursor pro-caspase-1. NLRP3 has three domains, namely amino-terminal death-fold domain, carboxy-terminal LRRs, and a central NACHT nucleotide-binding domain. The ASC consists of the following two death-fold domains: a pyrin domain and a caspase activation and recruitment domain (CARD). Procaspase-1 connects to NLRP3 via ASC. The association of NLRP3 with ASC is required for the recruitment of procaspase-1., Active caspase-1 converts pro-IL-1β and pro-IL-18 cytokines to active form. The active IL-1β recruits the native immune cells, whereas the active IL-18 induces interferon-γ and enhances the activity of T cells and NK cells. In addition, IL-18 stimulates the expression of programmed death 1 (PD1) on NK cell, causes immunosuppression and promote metastases.
| > Nod-Like Receptor Protein 3 Inflammasome Assembly And Activation|| |
NLRP3 inflammasome uniquely requires a two-step mechanism for activation. First, a priming signal that involves the upregulation of NLRP3 expression induced by TLRs-NF-κB pathway and production of pro-IL-1 beta and pro-IL-18. Second signals include Reactive Oxygen Species (ROS), membrane perturbations, lysosomal destabilization, K+ efflux and extracellular ATP that promotes the oligomerization of NLRP3, ASC and procaspase-1 and leads to the formation of functional NLRP3 inflammasome complex. During this assembly, NLRP3 and ASC interact through their PYD domains while association of ASC with caspase I via CARD/ CARD interactions. The bipartite nature of ASC represents the core structure of the inflammasome. Finally, clustering of caspase 1 induces processing of pro-IL-1β and pro-IL-18 into their active form. Activation of caspase results not only in the release of active pro-inflammatory cytokines but also induces inflammatory cell death, called pyroptosis., Pyroptosis is characterized by cell membrane rupture brought about by cell swelling and lysis. Hence, the event of pyroptosis and NLRP3 inflammasome may have important implications for the clinical development of anticancer therapeutics.
| > Dual Role Of Nod-Like Receptor Protein 3 Inflammasome In Cancer|| |
NLRP3 inflammasome is probably the most versatile inflammasome subtype with diverse biologic and chemical agents. It is predominantly expressed in neutrophils, macrophages, monocytes, and conventional dendritic cells, and its expression is inducible in response to the inflammatory stimuli. The NLRP3 inflammasome regulates the initiation and progression and therapeutic responses of cancer. The interplay between malignant cells and inflammasome complex has a very contrasting role. It exerts procarcinogenic effect by suppressing NK and T cell–mediated anticancer activities. In addition, it stimulates the production of trophic factors that favours the development of secondary tumors. On other hand this protein complex produce anticancer effects through immunostimulation and inflammatory cell death.
| > Nod-Like Receptor Protein 3 in Head -And -Neck Cancer|| |
Head-and-neck squamous cell carcinoma (HNSCC) is an aggressive disease, and mortality from this disease remains high because of the development of distant metastases and poor prognosis. Surgery, chemotherapy, and radiotherapy are the most important treatment strategies and are used either alone or in combination to obtain complete remission and cure. NLR is found to be involved in the progression of head-and-neck cancers. Oral cancer is the sixth-most prevalent malignancy in the world, and oral squamous cell carcinoma (OSCC) accounts for approximately 90% of all oral malignancies. Studies have suggested that IL-1β can be induced by tobacco and betel quid-related carcinogens, and it participates in the early and late stages of oral carcinogenesis by increasing the proliferation of dysplasia oral cells, stimulating oncogenic cytokines, and promoting the aggressiveness of OSCC. Despite advances in therapy, which have improved the quality of life, survival rates of head-and-neck cancers have remained static for many years. The efficacy of immunotherapies is hampered in head-and-neck cancer due to immunosuppressive tumor microenvironment., HNSCC-related inflammation is characterized by increased pro-inflammatory cytokines and acute-phase reactant proteins (C-reactive protein, serum amyloid A protein) that enhance the recruitment of circulating neutrophils, monocytes, and myeloid-derived suppressor cells (MDSCs) while inhibiting the recruitment of lymphocytes to the circulation. Clinical studies indicate that the NF-κB play a pivotal role in the carcinogenesis of HNSCC. Owing to their ability to induce pro-inflammatory cytokines, TLR and other innate immune receptor signaling pathways, in the context of tumor initiation, progression, and metastasis, have attracted close attention in recent years. In oral squamous cell carcinoma, lower expression of NLRP3 is related to less aggressiveness of the disease. Wang et al. in 2018 demonstrated that increased expression of NLRP3 in OSCC was associated with tumor growth and metastasis and the knockdown of NLRP3 inhibited the proliferation, migration, and invasion of OSCC cells in in vivo. Recently, it has been identified that blocking NLRP3 inflammasome by MCC950 reduced IL1β, MDSCs, Tregs (regulatory T), and TAMs (tumor-associated macrophages), which directly correlated with the reduction of tumors. NLRP3 inflammasome was elevated after receiving 5-fluorouracil (FU)-based chemotherapy both in patient sample and cell lines. It has been found that NLRP3 silencing significantly enhanced the proapoptosis effect of 5-FU and inhibited the proliferation in oral cancer cells. Detailed investigation revealed that 5-FU-mediated chemoresistance is by generating intracellular ROS and by the activation of NLRP3 inflammasome.
The purinergic receptor P2X7 (P2X7R), a member of the P2XR subfamily, facilitates the metastasis of cancer cells. P2X7 is mainly expressed by monocytes, macrophages, as well as dendritic cells. During innate immune responses, activated PRRs induce ATP release, which in turn can activate P2X7 receptor. The NLRP3 inflammasome pathway is considered to be one of the most important P2X7R-induced downstream pathways., Prolonged activation of P2X7 creates nonselective pores on cell membranes, leading to cell death. Bae et al. demonstrated that HNSCC can produce active IL-1β via P2X7/NLRP3 inflammasome pathways and reduce the survival and invasiveness of HNSCC by blocking P2X7R/NLRP3 inflammasome. Altogether, the downregulation of NLRP3 inflammasome may contribute to the development of novel therapeutic approaches or diagnostic markers predicting the prognosis and the degree of HNSCC malignancy. Downregulation of NLRP3 inflammasome has the potential to be a new therapeutic approach in head-and-neck cancer therapy.
| > Nod-Like Receptor Mutations In Head -And -Neck Cancer|| |
A series of mutations are necessary for the malignant change which leads to an increased cell proliferation in potentially malignant disorders. Rapid advancement of genomic sequencing and high-throughput techniques has identified novel, targetable genes and genetic mutations in a variety of cancers., Mutational events reflect a high degree of genome instability, which is one of the hallmarks of cancer. In humans, there are 22 known NLRs, and the association of mutations and single-nucleotide polymorphisms (SNPs) in their genes with human diseases reflects their vital role in host defense. Mutations in NLR genes are found to be associated with different types of cancers and are closely associated with higher degree of cancer genome instability. NLRP3 inflammasome polymorphism also exists in different types of cancers. An SNP in NLRP3 (rs35929419), in combination with CARD-8 polymorphism (rs2043211), has been associated with chronic inflammatory conditions., Lei et al. identified twenty novel NLRP mutations in HNSCC, and mutations in this group of genes were correlated with increased cancer cell genome mutation rates. These mutations were clustered at the LRR region of NLRP proteins, and the affected NLRP genes were mostly localized at chromosomes 11p15.4 and 19q13.42-19q13.43. This documented that NLRP3 inflammasome can be a potential therapeutic target and molecular biomarker of HNSCC genome instability.
| > Conclusion|| |
NLRP3 has emerged as a central regulator in the inflammatory process, and its activation directly correlates with the progression of head-and-neck tumors. However, very few studies have been done to explore the molecular mechanism behind cross talk between NLRP3 inflammasome and head-and-neck cancers. Further studies are required to unravel the molecular mechanisms behind the modulation of inflammasome and to determine their potential therapeutic role in head-and-neck cancers. The involvement of the NLRP3 inflammasome in head-and-neck cancers makes it a highly attractive drug target.
The authors would like to acknowledge Department of Biotechnology, Govt, of India for providing financial support.
Financial support and sponsorship
This study was financially supported by the Department of Biotechnology under BioCARe.
Conflicts of interest
There are no conflicts of interest.
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