NLRP3 inflammasome activation in response to metals

Implant surgery is followed by a series of inflammatory reactions that directly affect its postoperative results. The inflammasome plays a vital role in the inflammatory response by inducing pyroptosis and producing interleukin-1β, which plays a critical role in inflammation and tissue damage. Therefore, it is essential to study the activation of the inflammasome in the bone healing process after implant surgery. As metals are the primary implant materials, metal-induced local inflammatory reactions have received significant attention, and there has been more and more research on the activation of the NLRP3 (NOD-like receptor protein-3) inflammasome caused by these metals. In this review, we consolidate the basic knowledge on the NLRP3 inflammasome structures, the present knowledge on the mechanisms of NLRP3 inflammasome activation, and the studies of metal-induced NLRP3 inflammasome activation.


Introduction
Implantation has been widely used in orthopedic and dental treatments to replace the non-regenerative part of the human body (1,2). Surgical implantation is accompanied by hemorrhage, which causes various cells and proteins between the bone and surrounding soft tissue to participate in a series of biological processes, including protein deposition, coagulation, inflammation, and tissue formation (3). Leukocytes migrate into the periimplant site, and the activation of leukocytes results in the release of inflammatory mediators, including interleukin-1 beta (IL-1b), IL-6, tumor necrosis factor alpha (TNF-a), and macrophage colony-stimulating factor (3). The initial reaction of inflammatory cells to foreign materials influences the bone healing process. Thus, it is essential to study biomaterial-induced inflammation in the osteogenesis field (4). The inflammasome is an essential part of the innate immune system that is involved in metal-induced hypersensitivity, bacterial infection-induced peri-implantitis, and other possible side effects of implantation (5,6). The activation and assembly of the inflammasome are complicated programmed processes that involve the upstream sensors, the adaptors, and the downstream effectors (7). Therefore, studies on implantation-induced inflammasome activation have constantly increased over the years.
Broadly, biomaterials are grouped into natural precursors or synthetic materials (8,9). Suitable implant materials must satisfy the biochemical, physiological, and antibacterial property requirements of implantations. The implant surface chemistry and topography influence the process of osteogenesis (10). There are two main methods to improve osseointegration: 1) sintering of the metallic beads or fibers over the implant surface and 2) plasma spray deposition of the metallics or ceramics onto the implant surface (3). Metal implants apply load-bearing sections, such as in long bone and dental implants. Metal implantation can constantly precipitate metal ions, which are components of cellular proteins, bone structures, and intracellular or extracellular matrices (11). In addition, the release of metal ions into the surrounding bone tissues participates in osseointegration, which has been used to describe the successful healing of an implant within a host bone (12).
Peri-implant tissue healing starts with an inflammatory response after the implant is inserted into the bone cavity (13); however, inflammation at the peri-implantation site is also the leading cause of implantation failure (14). Therefore, it is essential to study the immune response to implant biomaterials. However, the role of implant-released metal ions in inflammasome activation, a critical component of the host immune system, is still unclear. Recent studies on metal-activated inflammasomes have mainly focused on the NODlike receptor protein-3 (NLRP3) inflammasome. In this review, we first present an overview of the series of NLRP3 inflammasome activation mechanisms and then summarize recently published research on various metal ions, metal particles, and metalloproteins that activate the NLRP3 inflammasome signals.

NLRP3 inflammasome
The inflammasome, identified by Tschopp and co-workers in 2002, is described as a high-molecular-weight complex present in the cytosol of stimulated immune cells that mediates the activation of inflammatory caspases (15). The activated caspases convert IL-1b and IL-18 from their inactive to their active forms. At the same time, fulllength gasdermin D (FL-GSDMD) is cleaved to the N-terminal GSDMD (GSDMD-NT) and forms membrane pores, resulting in cytokine release and/or programmed cell death called pyroptosis (16,17). The inflammasomes are named after the pattern recognition receptors (PRRs) that sense the pathogen-associated molecular patterns (PAMPs) and the damage-associated molecular patterns (DAMPs) in the cytosol and initiate downstream responses. In general, inflammasomes are classified into canonical and noncanonical inflammasomes.
Canonical inflammasomes comprise a sensor molecule, the adaptor ASC (apoptosis-associated speck-like protein containing a C-terminal caspase recruitment domain (CARD), and the effector caspase-1. Several sensors have been identified. There are sensors that consist of the nucleotide-binding oligomerization domain (NOD) and leucine-rich repeat (LRR)-containing protein (NLR) family members, including NLRP1, NLRP3, NLRC4, NLRP6, NLRP7, and NLRP12. These sensors, which activate inflammasomes, are classified as NLR N-terminal domains. Another class of inflammasomes assembling sensors is represented by the PYHIN protein family members, such as melanoma 2 (AIM2) and pyrin (7). The ASC adaptor contains two death-fold domains: one pyrin domain (PYD) and one caspase recruitment domain (CARD). On the one hand, PYD forms PYD-PYD interactions with the PYD domain in activated sensors. On the other hand, the CARD domain interacts with the CARD domain in pro-caspase-1. Therefore, ASC serves as a bridge between the sensors and pro-caspase-1, forming inflammasomes as a result of recognition of the PAMPs and DAMPs in canonical inflammasomes (6).
The NLRP3 inflammasome plays critical roles in various inflammatory disorders, including Alzheimer's disease, diabetes, and other inflammation-related diseases (18)(19)(20). It consists of an N-terminal PYD, a central NOD (also called the NACHT domain), and a C-terminal LRR domain (21). The ATPase activity of NOD is essential for NLRP3 oligomerization and is targeted by MCC950, a commonly used NLRP3 inhibitor (22). In addition, it has been determined that the NIMA-related kinase 7 (NEK7) interacts with the LRR domain to promote the activation of NLRP3 (23). The structure of the NLRP3 inflammasome is shown in Figure 1.
The NLRP3 inflammasome has been extensively studied for its broad spectrum of stimuli and complicated activation signaling. The stimulators of NLRP3 include crystalline materials, extracellular ATP, pore-forming toxins, RNA-DNA hybrids, peptide aggregates, and viral, fungal, and bacterial pathogens (6,24). It is generally accepted that the activation of the NLRP3 inflammasome involves a two-step process. Signal 1, also known as the priming signal, is triggered by PRR signaling, such as Toll-like receptors (TLRs) or cytokine receptors, e.g., TNF and IL-1 receptors. The activation of signal 1 leads to the transcriptional activation of nuclear factor kappa B (NF-kB), which upregulates the gene expression of NLRP3, IL-1b, and IL-18. Signal 2, also known as the activation signal, is induced by various PAMPs and DAMPs. Multiple molecular and cellular signaling events have been proposed to activate the NLRP3 inflammasome, as follows: 1) ionic flux, including K + efflux, Ca 2+ mobilization, Cl − efflux, and Na + influx. As a DAMP, ATP activates the P2X7 receptor on the cell membrane, inducing K + efflux that triggers the activation signal; 2) mitochondrial dysfunction and the production of reactive oxygen species (ROS); and 3) lysosome damage. However, none of these is recognized as a common event induced by all the NLRP3 inflammasome stimuli (21,25). To date, studies on metal-activated inflammasomes have mainly focused on the NLRP3 inflammasome (26,27). Details of these studies are discussed in the following sections and are summarized in Figure 2.

Metals involved in NLRPinflammasome activation
As metals are the primary implant materials, metal-induced local inflammatory reactions have received significant attention, and there has been more and more research on the activation of inflammasomes caused by metals (28,29). One of the present trends of biomaterials is the development of strategies and solutions that modulate the immune cell biology to avoid or minimize the undesired side effects in regenerative medicine (30). In the host response to biomaterials, the inflammasome is the bridge between inflammation and tissue regeneration. As the widest studied inflammasome, the NLRP3 inflammasome has become a major topic in the development of immunomodulatory biomaterials. Modulation of NLRP3 inflammasome activation in response to biomaterials in order to minimize the tissue inflammatory response to implants can promote tissue regeneration around the implant and determine the outcomes post-implantation (31). The reaction and mechanisms of metals activating the NLRP3 inflammasome depend on the different metal elements and metal states. We summarize recent studies on the roles of metals in NLRP3 inflammasome activation in Table 1. Details of specific metals are discussed as follows.

Titanium
Titanium (Ti) and its alloys have recently attracted significant interest (75). They are the most widely used metals for implantation due to their advantageous characteristics, including their excellent corrosion resistance and bone-bonding ability (76). The interface between metals and the surrounding tissues is also critical as insufficient bonding provides a bacterial invasion route, resulting in peri-implantitis. Titanium ions are frequently detected in the implant region, especially in peri-implantitis tissue (75). It has been reported The structure of NLRP3 inflammasome. Canonical inflammasomes comprise a sensor molecule, the adaptor ASC, and the effector Caspase-1. NLRP3 comprises a N-terminal PYD, a central NOD, and a C-terminal LRR domain. NEK7 interacts with LRR domain to promote NLRP3 activation. ASC comprises a PYD and a CARD domain. Caspase-1 comprises a CARD and two subunits, p10 and p20. Sensors interact with ASC by PYD domains and ASC interacts with caspases-1 by CARD domains. Roles of metals in NLRP3 inflammasome activation. NLRP3 inflammasome activation requires a two-step process. In signal 1, NF-kB transcriptional activation can be accelerated by Mn 2+ binding prion fibrils, Ni 2+ ions, ZrO 2 and inhibited by magnesium isoglycyrrhizinate (MGL). In signal 2, ionic flux activating NLRP3 inflammasome is controlled by the P2X7 and pannexin-1 receptors. The pannexin-1 activity can be regulated by Zn 2+ . Mg 2+ can inhibit the ATP-gated Ca 2+ channel P2X7 and limits the oligomerization and membrane localization of GSDMD-NT, which form the GSDMD-N pore. In addition, caspase-11 cleave pannexin-1, resulting in K + efflux and NLRP3 inflammasome activation. Mitochondrion dysfunction and ROS production can induce NLRP3 inflammasome activation. Ti ions, Fe 2+ , K + , Nano-Co and Ni particles promote this process, while MnTBAP inhibits it. Zinc can attenuate ROS production while Mn 2+ , Cr (VI) and Ni 2+ induce the release of ROS, triggering NLRP3 inflammasome. Ti particles, Zn 2+ depletion, ZnONPs, CuONPs, Cr pariticles and Ni particles can induce lysosome damage, which induced cathepsins release and result in NLRP3 inflammasome activation.  that Ti ions can promote the production of ROS, and N-acetyl-Lcysteine (NAC), a ROS scavenger, decreased the Ti ion-induced NLRP3 gene expression and IL-1b release, suggesting that Ti ions activated the NLRP3 inflammasome in an ROS-dependent manner (32). However, research has shown that Ti ions alone cannot stimulate the transcription of the inflammasome components, but they form particles that stimulate inflammasome activation and, consequently, IL-1b release (77). It is widely accepted that Ti particles can activate the NLRP3 inflammasome (78,79). However, the mechanisms of Tiinduced inflammasome activation in inflammatory diseases are still controversial. Several studies have suggested that Ti particle-induced activation of the NLRP3 inflammasome is dependent on lysosomes and lysosomal cathepsins. St. Pierre et al. found that the macrophage uptake of Ti particles was cathepsin B-dependent and induced acute inflammation by activating the NLRP3 inflammasome, resulting in IL-1b release and neutrophil recruitment (33). In a rat peri-implant osteolysis model, Ti particles induced NLRP3 inflammasome activation depending on mitochondrial function. Sirtuin 3, an NAD + -dependent deacetylase of the mitochondria that regulates its function, suppressed the Ti particle-induced NLRP3 inflammasome activation via the GSK-3b/b-catenin pathway (35). In addition, Ti particles can also induce cell death. However, NLRP3 and gasdermin D did not participate in the cell death process, suggesting that the Ti particle-induced cell death was not pyroptosis (80). An in vitro study also showed that Ti particles alone were insufficient at inducing the IL-1b release in macrophages; an additional priming signal, such as bacterial lipopolysaccharide (LPS), was required to enable inflammasome activation (63). Therefore, reducing the amount of particles produced in the process of implant surgery and application is critical to suppressing the activation of the NLRP3 inflammasome.

Zinc
Nearly 90% of zinc (Zn) is found in muscles and bones. Zn has a stimulatory effect on bone metabolism and the ability to promote bone formation and mineralization (81). It is accepted that the immune system is regulated by Zn homeostasis, and Zn 2+ functions as a second passenger in innate immunity (27). Zn homeostasis is maintained by the Zn 2+ transporter proteins, including SLC30A (ZnT) and SLC39A (ZIP, Zir/Irt-like proteins) (82). The effects of Zn in NLRP3 inflammasome activation have been proven in series studies. Brough et al. explored pretreating the macrophage with N,N, N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), the Zn chelator, and found significant inhibition of the activity of pannexin-1, thus suppressing the activation of the NLRP3 inflammasome and decreasing the production of IL-1b (36). Pretreatment of primary mouse macrophages with TPEN also damaged the integrity of the lysosome, thus suppressing the NLRP3  inflammasome activation and IL-1b secretion (37). In addition, Zn can inhibit high glucose-induced NLRP3 inflammasome activation in peritoneal mesothelial cells by attenuating ROS production (38). Zn also played a neuroprotective role by suppressing NLRP3 inflammasome activation through autophagy and ubiquitination in an experimental spinal cord injury model (39,83). Another study also showed that Zn suppressed NLRP3 activation by inducing microglia autophagy and played a neuroprotective role in spinal cord injury (40). Moreover, Zn participated in LPS and hypoxia inducing NLRP3 inflammasome activation in the microglia (41). Zinc oxide nanoparticles (ZnONPs) can suppress the NLRP3 inflammasome using the NLRP3 inflammasome-autophagy-exosomal pathway (42).

Magnesium
Magnesium (Mg) is a degradable and absorbable biomaterial. It is widely used in the clinic as its structural and mechanical characteristics are similar to those of the trabecular bone, which is beneficial to obtaining early fixation (84). Mg homeostasis in cells is maintained by Mg channels and transporters, including Mrs2, SLC41A1, SLC41A2, and TRPM6 (82).
It has been reported that Mg ions can suppress both the canonical and non-canonical pyroptotic pathways in macrophages by inhibiting the ATP-gated Ca 2+ channel P2X7 and limiting the oligomerization and membrane localization of GSDMD-NT, thus blocking the GSDMD-NT-induced pyroptosis (43). In addition, magnesium isoglycyrrhizinate (MGL), a new stereoisomer of glycyrrhizic acid, performs immune modulation through its anti-inflammatory effect and is clinically used as a hepatoprotective agent for the treatment of liver diseases. The anti-inflammatory effects of MGL may involve suppressing the activation of inflammasomes (44,45,85). MGL inhibited both NF-kB activation and NLRP3 inflammasome formation, thus alleviating liver inflammation in fructose-fed rats with metabolic syndrome (44). MGL was also used to treat chronic obstructive pulmonary disease by suppressing NLRP3 and cleaving caspase-1 expression (85). In addition, MGL administration can decrease the expression of NLRP3, NLRP6, and caspase-3 in mice, suggesting its downregulatory inflammasome expression effect in liver tissue (45).

Copper
Copper (Cu), an indispensable trace element in organisms, plays a crucial role in a lot of physiological activities, including respiration, iron metabolism, antioxidant activity, and tissue integrity (86). It is well known that the addition of Cu can endow biomaterials with antibacterial properties, osteogenesis, and angiogenesis (87). In organisms, Cu exists in two states: Cu(I)/Cu + (cuprous ion) and Cu (II)/Cu 2+ (cupric ion). Cu 2+ is the predominant redox state in blood, whereas Cu + is the form found in the reducing environment of the cell cytosol (88). Cu 2+ can be reduced to Cu + through the plasma membrane reductase of the STEAP (six-transmembrane epithelial antigen of prostate) family or DCYTB (duodenal cytochrome B) and then transported into cells by CTR1 (copper transporter 1) or DMT1 (divalent metal ion transporter 1), which are located in the membrane (89). In addition, proteins called metallochaperones also distribute Cu to specific sites in cells (90).
The dyshomeostasis of Cu has been reported to trigger inflammasome activation. Metal chelators remove metal ions from the body, reducing the metal concentration. Tetrathiomolybdate (TTM) is a highly specific, clinically approved Cu chelator (91) that can be used as an anti-inflammatory agent to prevent LPS-induced inflammatory reactions in vivo. Deigendesch et al. showed that TTM could prevent the activation of NLRP3 by removing Cu from the active site of superoxide dismutase 1 (SOD1) in macrophages. This regulation targets macrophages, not monocytes, in both mice and humans (46). In addition, TTM did not block the NF-kB and mitogen-activated protein kinase (MAPK) pathways or the other major antimicrobial inflammasomes such as NLRC4, NLRP1, and AIM2 (44). In vivo, depletion of bioavailable Cu led to a decreased caspase-1-dependent inflammation and reduced the susceptibility to LPS-induced endotoxic shock (44). Exposure to copper oxide nanoparticles (CuONPs) also resulted in NLRP3 activation by inducing lysosomal damage and the release of cathepsin B. Moreover, after lysosomal deposition, CuONPs released Cu 2+ due to the acidic environment of lysosomes. Cu 2+ then induced cellular oxidative stress and further mediated the activation of the NLRP3 inflammasome (47).

Iron
Iron (Fe) is involved in many critical biological processes, such as oxygen transport, ATP generation, and DNA biosynthesis. With regard to tissue engineering, Fe has excellent mechanical properties, making it a good candidate for implants requiring high structural strength, such as bone defect repair and vascular stents (92). Fe has redox activity, and a high Fe level can induce ROS, leading to oxidative stress and signal pathways critical to cell survival and death (93). The homeostasis of Fe metabolism in the human body is strictly regulated by a variety of proteins, including, among others, ferritin (FTH1 and FTL), a protein complex that stores Fe in cells for future use; transferrin (TF), an Fe-binding serum protein; transferrin receptor 1 (TfR1, TFRC), a plasma membrane protein that allows cells to ingest transferrin; divalent metal transporter 1 (DMT1, SLC11A2), a critical metal transporter for TfR1-mediated Fe uptake and dietary Fe absorption; and ferroportin (Fpn, SLC40A1), the only known cell Fe efflux pump (94).
The multi-target Fe chelator M30 has an Fe chelating/free radical scavenging effect. It inhibits lipid peroxidation, which has been proven to inhibit oxidative stress and inflammation in many diseases, such as type 2 diabetes and Alzheimer's disease. In an in vitro ethanol-induced hepatocyte injury model, M30 inhibited the activation of the AC/cAMP/PKA/HIF-1/NLRP3 inflammasome pathway, ameliorated oxidative cell stress, and reduced cell damage (48). Fe 2+ -specific chelators can also rescue peripheral blood mononuclear cells from an LPS stimulation-induced Fe 2+ increase following an Fe 2+ dose-dependent IL-1b production, which results from NLRP3 inflammasome activation (49). Gelfand et al. found that an Fe overload caused retinal degeneration by enhancing the stability of Alu RNAs, thereby promoting retinal pigmented epithelium (RPE) degeneration, thus inducing NLRP3 inflammasome activation (95).
Moreover, Liu et al. explored how morphology affects the NLRP3 inflammasome-activating property of iron oxide nanoparticles (IONPs). Research indicates that morphology is a critical determinant of IONP-induced IL-1b release and pyroptosis, and this process is not all mediated by NLRP3 (96).

Manganese
Manganese (Mn) is an essential metal required for proper immune function, regulation of blood sugar and cellular energy, reproduction, digestion, bone growth, blood coagulation and hemostasis, and defense against ROS (27). Mn is used as an additive of biomaterials because of its advantages in stabilizing and promoting osteoblast differentiation and bone metabolism (97). In biological systems, Mn exists in two oxidation states, Mn 2+ and Mn 3+ , which mediate the redox cycling of Mn and are involved in the biological effects of metals, including the Fenton reaction, transferrinmediated transport, and interference, as well as interference with other divalent metals (98). In addition, Mn forms various Mndependent enzymes, including oxidoreductases, isomerases, transferases, ligases, lyases, and hydrolases (27). Mn 2+ is also an essential component of some metalloenzymes, such as SOD, glutamate synthetase, pyruvate carboxylase, arginase, hydrolases, phosphatases, transferases, dehydrogenases, kinases, peptidases, and decarboxylases (99). However, excessive levels of Mn can cause cellular toxicity, which leads to oxidative stress, genotoxicity, membrane perturbation, and protein dysfunction by catalyzing the conversion of hydrogen peroxide (H 2 O 2 ) into oxygen radical species via the Fenton reaction (99). There are several Mn 2+ importers in the plasma membrane, including SLC39A14, SLC39A8, SLC30A10, and DMT1. In addition, NRAMP1 transports Mn 2+ from the phagosome to the cytoplasm (99, 100).
Excessive Mn exposure can activate the NLRP3 inflammasome in the microglia, the principal central nervous system (CNS) immune cells, and may result in neurodegenerative disorders. Mn 2+ activated the NLRP3 inflammasome in the striatum of adult rats and induced the release of ROS, triggering the NLRP3 inflammasome (50). In addition, Mn exposure also resulted in mitochondrial defects that drove the NLRP3 inflammasome signal amplification and propagation and the exosomal release of ASC in microglial cells (51). Another study also showed that Mn 2+ binding on prion fibrils was critical to inducing the priming signal in NLRP3 inflammasome activation in the microglia (52). Sodium para-aminosalicylic acid inhibited Mn-induced NLRP3 inflammasome by inhibiting NF-kB activation and oxidative stress in the microglia (101,102). On the other hand, the SIRT3 activating enzyme manganese superoxide dismutase (MnSOD) in the mitochondria significantly enhanced the ability to scavenge ROS and suppressed the activation of the NLRP3 inflammasome to protect the heart against oxidative stress (53). Anakinra, the recombinant form of the IL-1 receptor antagonist, dampened the NLRP3 activity by increasing the MnSOD protein longevity (103). Manganese tetrakis porphyrin chloride (MnTBAP), a mitochondrial SOD2 mimic, suppressed NLRP3 inflammasome activation by blocking the albumin-induced mitochondrial dysfunction in renal tubular injury (54). MnTE-2-PyP, another artificial mitochondrial SOD2 mimic, was also proven to suppress the pulmonary hypertension-induced NLRP3 inflammasome activation in macrophages (55). Mn-TAT PTD-Ngb, an artificial metalloprotein containing a TAT protein transduction domain (TAT PTD), which was recombined from apo-neuroglobin (apo-Ngb) with Mn porphyrin, suppressed both NF-kB and ROS-NLRP3 inflammasome activation in the microglia (27,57).
To date, the role of Mn in NLRP3 inflammasome activation has been examined mainly in relation to the brain neuroscience field. The effect of Mn and the Mn-dependent enzymes on NLRP3 inflammasome activation in the bone healing process needs further exploration, which is beneficial to the application of Mn in bone tissue engineering.

Potassium
Potassium (K) is an essential element in the human body. As a monovalent cation, the potassium ion directly controls other ionic signaling pathways by regulating the membrane potential. In addition, the balance between the intracellular and extracellular fluids is maintained by Na + /K + ATPase, which pumps sodium ion (Na + ) to extra cells while it uptakes K + into the cell in reverse concentration gradients (27,104).
Activating the NLRP3 inflammasome inevitably leads to potassium efflux, which can also induce NLRP3 inflammasome activation. The molecular mechanisms underlying NLRP3 inflammasome activation through potassium efflux have been extensively investigated. In this review, we only briefly summarized the main mechanisms. For details of the studies, please refer to an elegant review published previously (59). Two potential mechanisms are acknowledged. Potassium efflux may be related to the interaction of NLRP3-NEK7, which is essential to the activation of the NLRP3 inflammasome (58). Another theory is that potassium efflux might promote NLRP3 inflammasome activation by inducing mitochondrial dysfunction and mitochondrial ROS (mtROS) production (105). P2X7 has been reported to be expressed in immune cells such as macrophages, lymphocytes, mast cells, and neutrophils. P2X7 appears to play a critical role in inflammation and autoimmune diseases (82). Extracellular ATP can activate the P2X7 receptor and lead to the activation of the NLRP3 inflammasome. Furthermore, it has also been reported that the non-canonical inflammasome caspase-11 can cleave pannexin-1, followed by ATP release, P2X7 receptor activation, potassium efflux, and NLRP3 inflammasome activation (60). Another non-canonical inflammasome activating the NLRP3 inflammasome mechanism is potassium efflux through the GSDMD-NT-forming pores induced by non-canonical inflammasomes, which further activates the NLRP3 inflammasome (61). The two-pore domain potassium (K2P) is responsible for maintenance of the resting membrane potential in almost all cells. It has been suggested to cooperate with the P2X7 receptor mechanistically (59).

Cobalt
Cobalt (Co) is essential for humans in the form of cobalamin (coenzyme B12), which is tightly bound to a corrin ring and serves as a methyl group carrier with Co oxidation states (106). Co-based alloys are considered one of the most successful materials used for implants as they have satisfactory corrosion, wear, and mechanical properties (107). The homeostasis of Co is maintained by Co 2+ transporter proteins, including CbiMNQO, NiCoT, HupE/UreJ, CorA, and TBDT. Human serum albumin is considered the primary transporter of Co 2+ in the blood (108). Although humans are exposed to Co 2+ , the most stable form under ambient conditions, in the course of normal nutrition (109), a high level of Co 2+ is toxic to cells (104). Co 2+ has also been reported to induce an immune response by stimulating TLR4 (110).
Exposure to high levels of CoCl 2 significantly increased the NLRP3 expression, caspase-1 activity, and IL-1b secretion (64). Although a high CoCl 2 level can induce apoptosis in T lymphocytes, CoCl 2 -treated monocytes did not undergo apoptosis as the effect of p53 was counteracted by the anti-apoptotic activity of the activation of NF-kB and the inflammasome danger signaling pathway leading to the production of pro-inflammatory cytokines (111). However, a study demonstrated that CoCl 2 -induced hypoxia may negatively regulate the NLRP3 inflammasome signaling in brain glial cells (65). Feng et al. found that exposure to Co nanoparticles (Nano-Co) promoted intracellular oxidative stress damage and mtROS, which activated the NLRP3 inflammasome in hepatocytes, suggesting an essential role of the ROS/NLRP3 pathway in Nano-Coinduced hepatotoxicity (66). A study also showed that hemin and cobalt protoporphyrin (CoPP) inhibited NLRP3 inflammasome assembly by reducing the amount of intracellular ASC in cultured macrophages (67).

Chromium
Chromium (Cr), which belongs to the group of trace elements, exists in many different oxidation states in the environment and is essential in numerous functions of the human body. Cr deficiency can cause various physical dysfunctions, while exposure to Cr at higher concentrations is also toxic and may lead to neoplastic diseases. The Co-Cr-Mo alloy is the most widely used combination of Co-based alloys due to its unique combination of strength and ductility (107). Cr(VI) and Cr(III) are the most stable forms of Cr. Cr(VI), the most cancer-related among all Cr oxidation states, enters cells through the sulfate anion transporter system and is reduced to the intermediate oxidation states, e.g., Cr(V) and Cr(IV), in the process of forming stable Cr(III) forms (112).
Adam et al. found the indirect effects of Cr(VI) compounds in pro-inflammation activation. Cr(VI) compounds can induce NLRP3 inflammasome activation and IL-1b production, amplifying the innate immune activation inflammatory response. The authors also confirmed the production of mtROS upstream of the NLRP3 inflammasome assembly by treatment with NAC, suggesting that Cr(VI) induces the production of mtROS and thus activates the NLRP3 inflammasome. In addition, the Cr(III) compounds were also examined. However, the Cr(III) compounds failed to induce these reactions in cells, suggesting that oxidation state-specific differences in mitochondrial reactivity may determine the activation of the inflammasome (62). Jämsen et al. observed that Cr particles alone were insufficient to induce NLRP3 inflammasome activation. Priming human primary macrophages with LPS and exposing the cells to Cr particles were discovered to induce the production of IL-1b, which was significantly reduced by the NLRP3 inflammasome or cathepsin B inhibitor, suggesting that Cr-induced NLRP3 activation is lysosome/cathepsin B-dependent (63). In addition, Cr 3+ can activate both priming signaling and activation signaling of the NLRP3 inflammasome by inducing ROS accumulation (28).

Nickel
Nickel (Ni) is an abundant element in the earth's core and is a commonly used implant material as it grants necessary strength and durability to the implant. However, it is also associated with metal hypersensitivity reactions and can be found in trace amounts in "commercially pure" Ti materials used in surgical implants (113,114). The uptake of Ni has a toxic effect on cell metabolism and physiology in humans. The toxicity of Ni is dependent on the solubility of the Ni compounds. Insoluble Ni compounds are phagocytosed by cells, while Ni ions are delivered into cells and induce the production of free radicals (68). Ni 2+ triggers an inflammatory response by activating human TLR4 (115).
Xin et al. found that Ni-refining fume particles can induce the decrease of the mitochondrial membrane potential (MMP) and the increase of the opening rate of the mitochondrial permeability transition pore (MPTP) in human lung epithelial BEAS-2B cells, and activation of the NLRP3 inflammasome induced by Ni-refining fume particles can be significantly suppressed by NAC, an effective ROS remover, suggesting that Ni-refining fume particles activate the NLRP3 inflammasome by causing mitochondrial dysfunction and ROS production (68). Another study also showed that Ni-refining fumes promoted the expression of the NLRP3 inflammasome by inducing the Warburg effect in BEAS-2B cells (69). In addition, it has also been confirmed that activation of the NLRP3 inflammasome by Ni ions is independent of the phagolysosome-cathepsin B pathway (70). However, in lung pathology, Ni-contaminated particles activated the NLRP3 inflammasome by disrupting macrophage phagolysosomes, which resulted in prolonged inflammation (71). In a study of Ni ions, NiCl 2 induced the accumulation of ROS and mitochondrial DNA, resulting in the activation of the NLRP3 inflammasome. It was also found that NiCl 2 -induced apoptosis is dependent on ROS generation, suggesting that NiCl 2 can induce both apoptosis and pyroptosis (72).

Metal element-bearing ceramic materials
Ceramic materials are widely used in orthopedic implantations because of their low osteolytic potential and friction coefficients and high biocompatibility. The particles of ceramic materials can be taken up by immune cells and can induce an immune response, including inflammasome activation (31, 116). Titanium dioxide (TiO 2 ), zirconium oxide (ZrO 2 ), and aluminum oxide (Al 2 O 3 ) are widely applied in bioceramic implantation due to their satisfactory properties of wear, tear, hardness, biocompatibility, and corrosion resistance (107). Cytotoxicity significantly increases when macrophages are exposed to high concentrations of ZrO 2 particles (≥10 7 particles/ ml). However, compared to TiO 2 , ZrO 2 particles produce less inflammatory cytokines, suggesting that they are less toxic than TiO 2 (117). Jamieson et al. found that Al 2 O 3 and ZrO 2 treatment can enhance the gene expression of IL-1b, which is TLR4-dependent. Priming cells with LPS following Al 2 O 3 or ZrO 2 treatment can induce cell death. In addition, LPS and ZrO 2 treatment can also induce IL-1b protein secretion, while treatment with LPS and Al 2 O 3 was insufficient to induce it. These results suggest that ZrO 2 , but not Al 2 O 3 , may activate the inflammasome (73). In another in vivo study, intraperitoneal injection of a water-soluble supernatant with Al 2 O 3 in mice revealed that the mRNA expression of NLRP3 decreased while that of caspase-1 did not change (74).

Conclusion and perspective
Metals are widely used in the fabrication of implants due to their advantage of having good mechanical properties and ductility compared to other biomaterials. However, metal corrosion inducing ion release and metal debris production is inevitable. At the same time, the amount of metal released is highest after the operation, which is also the acute inflammatory phase initiated by injury to the tissue and is one of the factors that define the outcome of the implant (118). A growing number of studies has provided new insights into how these metals affect the early inflammatory response of bone regeneration after metal implantation. However, whether the specific ion activates inflammasomes synergistically or singly is still unclear. The detailed mechanisms of metal ions activating inflammasomes are still under investigation. Additional studies are also needed to further understand the roles of metal implantation debris and the metal ions released from implantation in mediating the process of inflammasome activation. Immunomodulatory alloy biomaterials based on the NLRP3 inflammasome activation mechanism could be developed, but the immune response of tissues to these biomaterials needs to be further confirmed.

Author contributions
WH, ZZ, and YQ wrote the manuscript and created the figures and the table. QZ and QW reviewed and edited the manuscript and provided guidance. YG and YF provided important perspective of the manuscript. All authors contributed to the article and approved the submitted version.

Funding
This work is supported by the Natural Science Foundation Project of Liaoning Province (2019-ZD-0775 and 2020-MS-150).

Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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