Interleukin (IL) 18 was initially cloned in 1995 and identified as a proinflammatory cytokine that stimulates type 1 helper T cells to produce interferon (IFN)-γ [1]. The 23-kDa precursor form of IL18 is activated by cleaved caspase-1 and secreted as an active, 18-kDa mature form ^{[2][3][4][5][6]}[2,3,4,5,6]. IL18 is secreted by hematopoietic lineages, such as macrophage cells [1] and microglia [7], as well as non-immune cells such as neural cells [6]. IL18 plays multiple roles in immune function, energy metabolism, and psychiatric disorders ^{[1][8][9][10][11]}[1,8,9,10,11], and is also a therapeutic target for cancer immunotherapy, inhibition of body weight gain, and cognitive impairment ^[8][10][12][8,10,12]. 

2. IL18 and Cancer

An increasing number of studies have demonstrated a relationship between IL18 and cancer. In pancreatic ductal adenocarcinoma (PDA), serum and stromal IL18 is positively correlated with patient mortality ^[13][14][20,21]. High expression of IL18 in PDA was associated with worse disease progression and poor survival ^[15][22]. However, there is the other report that serum IL18 concentration was not correlated with patient survival of pancreatic adenocarcinoma ^[16][23]. In oral squamous cell carcinoma (OSCC), the serum levels of IL18 increase during tumor growth ^[17][18][24,25]. IL18 expression in peripheral blood mononuclear cells is also increased in OSCC patients compared with that in healthy individuals ^[18][25]. In OSCC patients with lymph node metastasis and a severe TNM stage, serum IL18 levels were significantly higher than those in patients without lymph node metastasis or a severe TNM stage. This trend has also been observed in patients with other cancers ^[18][25]. In clinical trials, systematic administration of IL18 significantly suppressed the growth of several kinds of carcinomas, such as melanoma and renal cell cancer, by stimulating the immune system ^[19][20][21][26,27,28]. Furthermore, the effectiveness of cancer immunotherapy using IL18 to augment immune checkpoint inhibitors [12]. Moreover, mutant IL18 engineered for resistance to inhibitory binding of the high-affinity IL-18 decoy receptor also promoted the activity of NK cells, resulting in the enhancement of anti-tumor effects in mouse tumor models ^[22][29]. These results suggest the possibility that IL18 may be an important cytokine in cancer treatment.

3. Cancer-Related Genes in Il18^−/− Mice

Among the DEGs identified in our microarray analysis of Il18^−/− mice, those with involvement in various cancers are listed in Table 1. In human tongue squamous cell carcinoma cells, overexpression of IL18 led to apoptosis of tumor cells and decreased Ccnd1 expression ^[23][30]. Though Jihong et al. reported that Ccnd1 expression in the liver was unaffected by the administration of rIL18, they used BRL-3A rat liver cells to show that cell viability was increased with IL18 treatment ^[24][31]. In contrast, IL18 administration increased the expression of Ccnd1 in the liver of wild-type and Il18^−/− mice [8]. These results suggest that the IL18 receptor is expressed in the liver; however, the influence of IL18 on Ccnd1 may occur through an indirect mechanism. Erdr1 is related to cancer and IL18 ^[25][26][32,33]. Increased expression of Erdr1 in mouse melanoma inhibited tumor growth and metastasis to the lung ^[25][32]. In another study, treatment with recombinant Erdr1 prevented invasion and migration of gastric cancer ^[27][34]. Overexpression of Erdr1 has also been shown to suppress the expression of Bcl2 and promote apoptosis ^[26][28][29][33,35,36]. Thus, Erdr1 plays a role in cell homeostasis. One study has shown that Erdr1 is negatively regulated by IL18 in melanoma cells ^[25][32]. Erdr1 and Nfkb regulate the activation of STAT3, which increases tumor progression ^[30][31][37,38]. STAT3 phosphorylation was impaired in the liver of Il18^−/− mice, and was restored by r-IL18 treatment ^[32][15]. Erdr1 also functions as an immune activator that specifically activates NK cells ^[33][39]. Additionally, administration of recombinant ERDR1 augmented the cytotoxicity of primary human NK cells against leukemia cancer cells ^[33][39]. IL18 in combination with IL2 induces NK cells and has a cytotoxic role against cancers ^[34][13].  The potential association between Tmem267 with IL18 has also not been reported. One study showed a poor prognosis of liver and colon cancer in patients with elevated Tmem267 levels ^[35][55]. Increased expression of Tmem267 was also observed in Il18^−/− mice. Therefore, Il18^−/− mice may have a high risk of liver cancer. The expression of Axin2 in Il18^−/− mice was significantly decreased. Axin2 is a critical modulator of the Wnt/β-catenin signaling pathway ^[36][56]. The Axin2-Wnt pathway involves negative feedback regulation, and Axin2 is also a direct target of Wnt/β-catenin ^[36][56]. Axin2 is a known tumor suppressor gene in some cancers ^[37][57]. In contrast, several reports identified Axin2 as an oncogene in colorectal, liver, and gastric cancers ^[38][58]. Axin2 is a β-catenin target that is highly expressed in human colorectal cancer ^[39][59]. Another study showed that the Axin2 axis suppressed tumor growth and metastasis in colorectal cancer ^[40][60]. Caspase-4 (Casp4) is related to various malignancies and metastasis ^[41][61]. In non-small cell lung cancer (NSCLC) patients, the circulating Casp4 level was much higher than that in healthy individuals. Furthermore, increased levels of Casp4 in NSCLC patients led to higher mortality compared with those in NSCLC patients with low Casp4 gene levels ^[42][62]. In gastric cancer patients, high expression of Casp4 was associated with a better survival rate ^[43][63]. In esophageal squamous cell carcinoma, Casp4 may be a tumor suppressor gene ^[44][64]. Casp4 expression is decreased in Il18^−/− mice. Therefore, tumor growth might be increased in Il18^−/− mice compared with Il18^+/+ mice. Several reports have indicated the involvement of Chrm1 in both the promotion and inhibition of cancer growth. Chrm1 activates cholinergic signals and the hedgehog signaling pathway, resulting in the promotion of prostate cancer invasion ^[45][46][65,66]. Activation of Chrm1 also induced the migration and invasion of two cancer cell lines, HepG2 and SMMC-7721, via the PI3K/Akt pathway ^[47][67]. Signaling through Chrm1 inhibited primary pancreatic tumor growth via downregulation of the growth factor pathway ^[48][68]. In Il18^−/− mice, Chrm1 expression was increased, indicating that tumor growth might be increased. Ifi16 reportedly functions as both a tumor suppressor and a promoter. High expression of Ifi16 was observed in colorectal cancer ^[49][50][69,70]. Another study reported that Ifi16 promoted cancer development in vitro and in vivo ^[51][71]. Ifi16 protein also activates the STING-TBK1 pathway for IFN-β production ^[52][72]. Ifi16 functions as an activator of the inflammasome, resulting in the production of cleaved IL1β and IL18 ^[53][73]. One report showed that Ifi16 suppresses cell viability and increases apoptosis in hepatocellular carcinoma (HCC) cell lines ^[54][74]. In Il18^−/− mice, the expression of Ifi16 is significantly decreased. Further study is required to determine whether IL18 suppresses or promotes cancer development through Ifi16. Klf13 exhibits important functions in cell proliferation, migration, and differentiation ^[55][56][75,76]. Klf13 inhibits cell proliferation and accelerates apoptosis in pancreatic cancer cells ^[57][77], and functions as a tumor suppressor protein in prostate cancer and colorectal cancer ^[58][59][78,79]. Klf13 is also necessary for Ccnd1 expression, which is an oncogene in oral squamous cell carcinoma ^[60][80]. Klf13 and Fgfr3 are highly expressed in oral cancer cells ^[61][81]. In Il18^−/− mice, the expression of Klf13 is significantly increased. Therefore, tumor proliferation might be promoted in Il18^−/− mice. Upregulated Lrrc8e led to cervical cancer cell proliferation and metastasis of breast cancer ^[62][63][82,83]. In Il18^−/− mice, the expression of Lrrc8e is decreased. Lrrc8e and Il18 may be positively correlated; however, further study is warranted to determine whether IL18 can promote or suppress the growth of these tumor types. In mouse models of cancer, LY6A has been identified as an important regulator of tumor progression ^[64][65][66][84,85,86]. LY6A exhibits marked influences on cellular activity and tumorigenicity, both in vitro and in vivo ^[67][87]. In Il18^−/− mice, the expression of Ly6a is decreased. Further study is needed to determine whether IL18 suppresses or promotes cancer progression through Ly6a. Nnt is overexpressed in gastric cancer. Nnt accelerates tumor growth, lung metastasis, and peritoneal dispersion of cancer ^[68][88]. Furthermore, Nnt expression is upregulated in adrenocortical carcinoma and triggers anti-apoptosis pathways in cancer cells ^[69][89]. In mouse models of lung tumor initiation and progression, the expression of Nnt significantly enhances tumor growth, invasion, and aggressiveness ^[70][90]. Expression of Nnt is increased in Il18^−/− mice, indicating that tumor growth might be promoted. Several previous studies have linked Samsn1 and cancer. The human SAMSN1 gene is located on chromosome 21q11-21, a region associated with heterozygous deletions frequently found in lung cancer cells, suggesting that SAMSN1 may be a tumor suppressor ^[71][72][91,92]. Additionally, SAMSN1 is a suppressive factor of multiple myeloma migration, both in vitro and in vivo ^[73][93]. Decreased expression of SAMSN1 may promote the progression and recurrence of gastric cancer ^[74][94]. High expression of Samsn1 was associated with high mortality in glioblastoma multiforme ^[75][95], but Samsn1 was found to be expressed at significantly low levels in HCC ^[76][96]. In Il18^−/− mice, the expression of Samsn1 is significantly decreased, suggesting that tumor growth might be promoted. There are no reports on the involvement of Npas1, Or10ad1, Ppcdc, or Wscd1 in cancer.

4. IL18 and Energy Metabolism

Previous studies have linked IL18 to energy metabolism, with potential roles in glucose and lipid homeostasis. High plasma levels of IL18 lead to a significant increase in the risk of type 2 diabetes (T2D), and serum levels of IL18 are significantly increased in patients with T2D compared with healthy controls ^[77][78][79][97,98,99]. Furthermore, IL18 levels in serum or plasma are negatively correlated with carbohydrate tolerance and positively related to insulin resistance ^[80][81][82][100,101,102]. High serum levels of IL18 increase the risk of metabolic syndrome characteristics such as hypertriglyceridemia, and are also linked to serum triglyceride levels ^[83][84][103,104]. In women with obesity, weight loss was found to reduce the levels of IL18 ^[85][105]. Plasma levels of IL18 were increased, but mRNA expression of Il18 in adipose tissue was significantly decreased in obese mice compared with control mice ^[86][106]. Previous studies in Il18^−/− mice have indicated that IL18 is involved in glucose metabolism, lipid metabolism, and mitochondrial function ^[8][9][32][8,9,15]. IL18 deficiency was also shown to inhibit the phosphorylation of STAT3 in the liver, which may play a role in the mechanism of impaired energy metabolism, indicating the possible involvement of the Wnt signaling system [8].

5. Metabolism-Related Genes in Il18^−/− Mice

DEGs identified in the microarray analysis of Il18^−/− mice that are involved in lipid and glucose metabolism are shown in Table 2. Atm is required to maintain mitochondrial homeostasis ^[87][107]. Regulation of the DNA damage response by Atm involves inflammatory cytokines such as tumor necrosis factor-α and nuclear factor-κB ^[88][108]. Atm is implicated in intermediary metabolism through signaling pathways such as insulin and AMPK ^[89][90][109,110]. Aged Atm^−/− mice show an increase in blood glucose levels with lower insulin and C-peptide levels, whereas young Atm^−/− mice exhibit temporal hyperglycemia during oral glucose challenge comparing to age-matched wild-type controls ^[91][92][111,112]. Atm^−/− mice also display disturbances in carbohydrate metabolism, such as glucose intolerance, insulin resistance, and insufficient insulin secretion ^[91][93][111,113]. Furthermore, diet-induced hepatic steatosis is reduced in Atm^−/− mice compared with that in wild-type mice ^[94][114]. The Atm pathway is associated with oncogenesis ^[95][115]. In NASH, Atm mRNA expression accelerates signaling of oncogenic pathways ^[96][97][116,117]. Activation of Atm increases the accumulation of cholesterol ^[98][118]. In Il18^−/− mice, the expression of Atm is significantly decreased, raising the possibility that IL18 might regulate Atm, resulting in an imbalance of glucose and lipid metabolism. Casp4 is related to energy metabolism and responds to endoplasmic reticulum (ER) stress ^[99][119]. The ER is a crucial site of lipid metabolism, and a number of enzymes related to lipid metabolism exist there ^[100][120]. Casp4 has been implicated in inflammasome activation through ER stress ^[101][121]. IL18 is an important component of the inflammasome. Decreased expression of Casp4 was observed in Il18^−/− mice, which is consistent with these previous studies. Ifi16 is related to energy metabolism, and both lipid and glucose metabolism are affected by Ifi16 expression ^[102][122]. One study showed that increased Ifi16 expression stimulates adipogenesis in mice and humans ^[103][123]. Furthermore, the authors found that overexpression of Ifi16 in mice led to obesity. Expression of Ifi16 is significantly decreased in Il18^−/− mice, which leads us to speculate that Ifi16 might not be related to obesity in Il18^−/− mice. Nnmt is closely related to energy metabolism, and the expression of Nnmt in liver improves lipid parameters ^[104][124]. In humans and mice, the expression of Nnmt is negatively correlated with the levels of lipids, such as total cholesterol, low-density lipoprotein cholesterol, and triglycerides ^[105][125]. Another report showed that overexpression of Nnmt in mice led to fatty liver disease and fibrosis ^[106][126]. Nnmt expression in adipose tissue was also inversely correlated with insulin sensitivity ^[107][127]. In Il18^−/− mice, the expression of Nnmt is decreased. The phenotypes of Il18^−/− mice are partially consistent with some results of previous these papers. No studies have examined the relationship between Chrm1 and Hmbs expression and energy metabolism.

6. IL18 and Psychiatric Disorders

Previous studies have revealed that IL18 is closely related to several psychiatric disorders, including depressive disorders and schizophrenia ^[108][128]. Serum or plasma levels of IL18 in patients with depression, AD, or mild cognitive impairment were found to be higher than in healthy individuals ^[109][110][129,130]. Although IL18 is abnormally upregulated in neurons and glial cells in AD patients, IL18 levels are not associated with the severity of AD ^[111][112][131,132]. First-episode psychosis patients display increased plasma levels of IL18 that correlate with its severity ^[113][133]. An in vitro study using a human neuroblastoma cell line, Sh-sy5y, showed that IL18 promotes amyloid beta (Aβ) production and kinase activity, which is important for tau phosphorylation ^[114][115][134,135]. These findings indicate that IL18 is closely associated with various psychiatric disorders and cognitive impairment. IL18-deficient mice show depressive-like behavioral changes and impairments in learning and memory [10]. In another study, IL18 might have an adjustive function against stress ^[116][136].