Molecular Mechanisms of IL18 in Disease

Interleukin 18 (IL18) was originally identified as an inflammation-induced cytokine that is secreted by immune cells. An increasing number of studies have focused on its non-immunological functions, with demonstrated functions for IL18 in energy homeostasis and neural stability. IL18 is reportedly required for lipid metabolism in the liver and brown adipose tissue. Furthermore, IL18 (Il18) deficiency in mice leads to mitochondrial dysfunction in hippocampal cells, resulting in depressive-like symptoms and cognitive impairment. Microarray analyses of Il18−/− mice have revealed a set of genes with differential expression in liver, brown adipose tissue, and brain; however, the impact of IL18 deficiency in these tissues remains uncertain. In this review article, we discuss these genes, with a focus on their relationships with the phenotypic disease traits of Il18−/− mice.


Introduction
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]. 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], and is also a therapeutic target for cancer immunotherapy, inhibition of body weight gain, and cognitive impairment [8, 10,12].Furthermore, we previously reported the effectiveness of the combination of IL18 and immune checkpoint inhibitors in suppressing tumor metastasis [12].IL18 may exert anti-metastatic effects by increasing the numbers of effector-like natural killer (NK) cells or decreasing immunosuppressive cells, such as regulatory T cells.While IL18 alone can prime lymphocytes, IL18 combined with IL2 can promote the proliferation of NK cells, resulting in increased cytotoxicity against cancer [13].Moreover, the expansion and function of NK cells stimulated by IL15 and IL18 are controlled by IL12 [14].Together, this leads to a mechanism whereby antigen-mediated activation of dendritic cells leads to the production of cleaved caspase-1 and release of active IL18 and IL2, induced proliferation of NK cells, and secretion of IFN-γ.Thus, IL18 is an essential cytokine in inflammatory responses and cancer immunotherapy.
Research has shown that IL18 is also closely associated with energy metabolism.In our studies on IL18-knockout (Il18 −/− ) mice, we observed a remarkable body weight gain over time in Il18 −/− mice compared with wild-type littermate mice [8,15].Furthermore, Il18 −/− mice exhibited higher blood glucose and lipid levels, insulin resistance, non-alcoholic fatty liver disease (NAFLD), and non-alcoholic steatohepatitis (NASH) with aging.Regarding the mechanism of higher blood glucose levels and insulin resistance in Il18 −/− mice, the phosphorylation of signal transducer and activator of transcription 3 (STAT3) was impaired in the liver and was recovered by the administration of recombinant-IL18 (rIL18) [15].Additionally, Il18 −/− mice with dyslipidemia, NAFLD, and NASH showed inhibition of the Wnt signaling pathway, reduced expression of cyclin D1 (Ccnd1), and disturbances in the circadian rhythm [8].The dyslipidemia, NAFLD, and NASH in Il18 −/− mice were improved by administration of rIL18.We performed an additional study on brown adipose tissue (BAT) in these mice and confirmed that IL18 deficiency had similar effects to those observed in the liver [9].Nevertheless, the direct molecular role of IL18 in dyslipidemia, NAFLD, and NASH remains to be clarified.
IL18 also plays a role in psychiatric and neurologic conditions [10,16].We reported that Il18 −/− mice showed cognitive impairment and depressive-like behavioral changes compared with wild-type mice [10].Another study on Il18 −/− mice revealed degenerated mitochondria in presynaptic axon terminals of the molecular and polymorphic layer of hippocampal dentate gyrus, suggesting the possible dysfunction of neurotransmitter release [10].These disruptions may be related to the mechanism underlying the observed cognitive impairment and behavioral changes in Il18 −/− mice.Additionally, regulation of mitochondrial function may be involved in the role of IL18 in the brain.Furthermore, reports have identified several other genes, including transthyretin (Ttr), as having potential involvement in depression and cognitive impairment [10,17].Therefore, the precise role and mechanisms of IL18 in brain dysfunction have not yet been clarified.
In this review, we summarize the current literature on genes showing differential expression (DEGs) between Il18 −/− and wild-type mice, focusing on those categorized as being related to energy metabolism, psychiatric conditions, and the brain, and discuss the potential relationships of these DEGs with IL18-deficient phenotypes in mice.Considering the role of IL18 in cancer, we also discuss the current literature and potential mechanisms of differentially expressed cancer-related genes.

Microarray and Ingenuity Pathway Analysis (IPA)
In our previous studies, we obtained microarray data from the liver, BAT, and brain of Il18 −/− mice at 12 weeks of age [8][9][10]17].Genes with significantly increased or decreased expression compared with wild-type controls were extracted, and our analysis focused on DEGs that were commonly expressed in all three tissues (liver, BAT, and brain).
IPA was applied to analyze the functions of the DEGs, as previously described [18,19].Of the tissue-specific DEGs, those associated with cancer, energy metabolism, and psychiatric and brain disorders (depression, dementia, Alzheimer's disease [AD], and cognitive impairment) were selected on the basis of behaviors observed in Il18 −/− mice [10].The IPA network explorer was run with default settings to reveal molecule-molecule interactions and to detect pathways between the molecules.
The correlation between microarray and quantitative real-time PCR (RT-qPCR) results was analyzed using Spearman's rank correlation tests.The rs and two-tailed p values for the microarray and RT-qPCR results of the liver, BAT, and brain were previously reported [8,9,17].
A diagram illustrating the interaction of the selected DEGs is provided in Figure 1.

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 [20,21].High expression of IL18 in PDA was associated with worse disease progression and poor survival [22].However, there is the other report that serum IL18 concentration was not correlated with patient survival of pancreatic adenocarcinoma [23].In oral squamous cell carcinoma (OSCC), the serum levels of IL18 increase during tumor growth [24,25].IL18 expression in peripheral blood mononuclear cells is also increased in OSCC patients compared with that in healthy individuals [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 [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 [26][27][28].Furthermore, we previously demonstrated the effectiveness of cancer immunotherapy using IL18 to augment immune checkpoint inhibitors [12].Moreover, mutant IL18 engineered for resistance to inhibitory binding of the highaffinity IL-18 decoy receptor also promoted the activity of NK cells, resulting in the enhancement of anti-tumor effects in mouse tumor models [29].These results suggest the possibility that IL18 may be an important cytokine in cancer treatment.

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 [20,21].High expression of IL18 in PDA was associated with worse disease progression and poor survival [22].However, there is the other report that serum IL18 concentration was not correlated with patient survival of pancreatic adenocarcinoma [23].In oral squamous cell carcinoma (OSCC), the serum levels of IL18 increase during tumor growth [24,25].IL18 expression in peripheral blood mononuclear cells is also increased in OSCC patients compared with that in healthy individuals [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 [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 [26][27][28].Furthermore, we previously demonstrated 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 [29].These results suggest the possibility that IL18 may be an important cytokine in cancer treatment.

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 [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 [31].In contrast, we found that 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 [32,33].Increased expression of Erdr1 in mouse melanoma inhibited tumor growth and metastasis to the lung [32].In another study, treatment with recombinant Erdr1 prevented invasion and migration of gastric cancer [34].Overexpression of Erdr1 has also been shown to suppress the expression of Bcl2 and pro-mote apoptosis [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 [32].Erdr1 and Nfkb regulate the activation of STAT3, which increases tumor progression [37,38].We observed that STAT3 phosphorylation was impaired in the liver of Il18 −/− mice, and was restored by r-IL18 treatment [15].Erdr1 also functions as an immune activator that specifically activates NK cells [39].Additionally, administration of recombinant ERDR1 augmented the cytotoxicity of primary human NK cells against leukemia cancer cells [39].IL18 in combination with IL2 induces NK cells and has a cytotoxic role against cancers [13].Consistent with other research [32], our microarray analysis showed that Erdr1 expression was increased in liver, BAT, and brain of Il18 −/− mice, implying that Erdr1 and Il18 negatively regulate each other.We also reported decreased tumor sizes in lung metastasis models of melanoma treated with the combination of immune checkpoint inhibitors and IL18 [12].Although Erdr1expression was not analyzed in our previous study, it is possible that the balance between Il18 and Erdr1 expression is important for the treatment of melanoma.
To the best of our knowledge, there are no previous reports on the relationship between Nxpe4 and IL18.In our microarray analysis, the expression of Nxpe4 was significantly decreased in all analyzed tissues.Colon cancer patients with decreased Nxpe4 expression were found to have high mortality [40], and Cygb-deficient mice with increased colon tumors also exhibited decreased Nxpe4 expression [41].Therefore, future studies should examine whether Il18 -/-mice with decreased Nxpe4 expression develop tumors over time.
Some reports have linked Nnmt and IL18 with cancer progression through the STAT3/IL1β pathway [42].Our microarray analysis showed that the expression of Nnmt is significantly decreased in Il18 −/− mice.In PC-3 cells, a prostate cancer cell line, Nnmt promoted cell viability, whereas knockdown of Nnmt in PC-3 cells decreased cell viability [43].Further studies are required to understand the functional relationship between IL18 and Nnmt.
No studies have examined the relationship between Tmem25 and IL18.Only two studies have examined Tmem25 expression in colon and breast cancer [44,45].One clinical study showed that Tmem25 expression was decreased in colorectal cancer; therefore, Tmem25 has been considered as a therapeutic target [46].Reduced Tmem25 expression in breast cancer has been correlated with a better response to chemotherapy [44,45].
One study reported an association of Atm with IL18 [47].In the colonic epithelium, the suppression of Atm reduces the activity of inflammasomes, including IL18, resulting in suppressed inflammation.In our microarray analysis of Il18 −/− mice, the expression of Atm was also reduced.Taken together, these results indicate a positive correlation between the expression of Atm and that of Il18 [47].Atm is reportedly a cancer risk factor [48,49].One report showed that Atm variants and mutations are associated with the risk of pancreatic cancer [50,51].Therefore, the relevance of the relationship between Atm and IL18 should be determined in further analyses.
No studies have examined the relationship between IL18 and Rps25.One clinical research study revealed that Rps25 expression was associated with the disease-specific survival rate in stage II mucinous colorectal cancer [52].Furthermore, Rps25 is considered a potential biomarker in lung adenocarcinoma and T-cell leukemia [53,54].In Il18 −/− mice, the expression of Rps25 is decreased.Thus, further study is required to clarify the interaction between these two factors.
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 [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 [56].The Axin2-Wnt pathway involves negative feedback regulation, and Axin2 is also a direct target of Wnt/β-catenin [56].Axin2 is a known tumor suppressor gene in some cancers [57].In contrast, several reports identified Axin2 as an oncogene in colorectal, liver, and gastric cancers [58].Axin2 is a β-catenin target that is highly expressed in human colorectal cancer [59].Another study showed that the Axin2 axis suppressed tumor growth and metastasis in colorectal cancer [60].
Caspase-4 (Casp4) is related to various malignancies and metastasis [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 [62].In gastric cancer patients, high expression of Casp4 was associated with a better survival rate [63].In esophageal squamous cell carcinoma, Casp4 may be a tumor suppressor gene [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 [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 [67].Signaling through Chrm1 inhibited primary pancreatic tumor growth via downregulation of the growth factor pathway [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 [69,70].Another study reported that Ifi16 promoted cancer development in vitro and in vivo [71].Ifi16 protein also activates the STING-TBK1 pathway for IFN-β production [72].Ifi16 functions as an activator of the inflammasome, resulting in the production of cleaved IL1β and IL18 [73].One report showed that Ifi16 suppresses cell viability and increases apoptosis in hepatocellular carcinoma (HCC) cell lines [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 [75,76].Klf13 inhibits cell proliferation and accelerates apoptosis in pancreatic cancer cells [77], and functions as a tumor suppressor protein in prostate cancer and colorectal cancer [78,79].Klf13 is also necessary for Ccnd1 expression, which is an oncogene in oral squamous cell carcinoma [80].Klf13 and Fgfr3 are highly expressed in oral cancer cells [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 [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 [84][85][86].LY6A exhibits marked influences on cellular activity and tumorigenicity, both in vitro and in vivo [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 [88].Furthermore, Nnt expression is upregulated in adrenocortical carcinoma and triggers anti-apoptosis pathways in cancer cells [89].In mouse models of lung tumor initiation and progression, the expression of Nnt significantly enhances tumor growth, invasion, and aggressiveness [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 [91,92].Additionally, SAMSN1 is a suppressive factor of multiple myeloma migration, both in vitro and in vivo [93].Decreased expression of SAMSN1 may promote the progression and recurrence of gastric cancer [94].High expression of Samsn1 was associated with high mortality in glioblastoma multiforme [95], but Samsn1 was found to be expressed at significantly low levels in HCC [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.

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 [97][98][99].Furthermore, IL18 levels in serum or plasma are negatively correlated with carbohydrate tolerance and positively related to insulin resistance [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 [103,104].In women with obesity, weight loss was found to reduce the levels of IL18 [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 [106].Previous studies in Il18 −/− mice have indicated that IL18 is involved in glucose metabolism, lipid metabolism, and mitochondrial function [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].

Metabolism-Related Genes in
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 [107].Regulation of the DNA damage response by Atm involves inflammatory cytokines such as tumor necrosis factorα and nuclear factor-κB [108].Atm is implicated in intermediary metabolism through signaling pathways such as insulin and AMPK [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 [111,112].Atm −/− mice also display disturbances in carbohydrate metabolism, such as glucose intolerance, insulin resistance, and insufficient insulin secretion [111,113].Furthermore, diet-induced hepatic steatosis is reduced in Atm −/− mice compared with that in wild-type mice [114].The Atm pathway is associated with oncogenesis [115].In NASH, Atm mRNA expression accelerates signaling of oncogenic pathways [116,117].Activation of Atm increases the accumulation of cholesterol [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 [119].The ER is a crucial site of lipid metabolism, and a number of enzymes related to lipid metabolism exist there [120].Casp4 has been implicated in inflammasome activation through ER stress [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.
Ccnd1 expression is decreased in Il18 −/− mice.We discussed the relationship between IL18, cyclin D1, and lipid metabolism in a previous study [8].
Ifi16 is related to energy metabolism, and both lipid and glucose metabolism are affected by Ifi16 expression [122].One study showed that increased Ifi16 expression stimulates adipogenesis in mice and humans [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 [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 [125].Another report showed that overexpression of Nnmt in mice led to fatty liver disease and fibrosis [126].Nnmt expression in adipose tissue was also inversely correlated with insulin sensitivity [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.

IL18 and Psychiatric Disorders
Previous studies have revealed that IL18 is closely related to several psychiatric disorders, including depressive disorders and schizophrenia [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 [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 [131,132].First-episode psychosis patients display increased plasma levels of IL18 that correlate with its severity [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 [134,135].These findings indicate that IL18 is closely associated with various psychiatric disorders and cognitive impairment.Our study reported that IL18-deficient mice show depressive-like behavioral changes and impairments in learning and memory [10].In another study, we reported that IL18 might have an adjustive function against stress [136].

Psychiatric and Brain Disorder-Related Genes in Il18
DEGs identified in the microarray analysis of Il18 −/− mice that are related to psychiatric disorders or psychiatric symptoms are shown in Table 3.
The Atm gene has multiple roles in central neurons.Previous studies have shown that Atm is required for apoptosis of the developing nervous system in response to DNA damage [137][138][139].Deficiency of Atm in dentate gyrus led to decreased survival of proliferating neurons, suggesting that Atm may have a role in neural progenitor survival or differentiation in the hippocampus [140].In Il18 −/− mice, the expression of Atm is significantly decreased, and neurogenesis in the hippocampus was suppressed in Il18 −/− mice compared with that in Il18 +/+ mice [10].Therefore, the histological phenotypes as suppressed neurogenesis in Il18 −/− mice may be caused by decreased expression of Atm.Casp4 is associated with the risk genes for AD [141].Casp4 expression is increased in the hippocampus and prefrontal cortex of CASP4/APP/PS1 mice, and increased expression of Casp4 leads to hippocampal synaptic plasticity in APP/PS1 mice.Casp4 is also expressed in microglia, and the presence of Casp4 led to more microglia clustered around amyloid plaques [141].In Il18 −/− mice, the expression of Casp4 is significantly decreased.In a previous study, less mature neural cells were observed in dentate gyrus of Il18 −/− mice (10); therefore, Casp4 might be one of the genes responsible for the behavioral phenotypes of Il18 −/− mice.
Chrm1 is related to various psychiatric disorders, including schizophrenia and mood disorders [142,143].In patients with schizophrenia, the expression of Chrm1 is decreased in the cortex [144].Chrm1 encodes a receptor that is highly expressed in glutamatergic neurons [145] and in postsynaptic regions of the hippocampus [146], and is a potential target molecule for schizophrenia treatment [142].Chrm1 expression is decreased in Il18 −/− mice, leading us to speculate that Chrm1 may not be responsible for the behavioral phenotypes of Il18 −/− mice.
Npas1 is expressed in neurons in the brain.Npas1 protein is a transcriptional suppressor of neuronal differentiation, development, and maturity functions [147,148].Furthermore, Npas1-positive neural cells in the ventral pallidum modulate the susceptibility to stressors and anxiety-like behaviors [149].In our previous study, Il18 −/− mice showed a depression-like phenotype, in which Npas1 was upregulated and suppressed neurogenesis in the hippocampus [10].While Npas1 may be one mediator of the depression-like phenotype in Il18 −/− mice [10], further analysis of the brain in these mice, particularly Npas1-positive neural cells, is warranted.
Nnmt is expressed in cholinergic neurons of the hippocampus [150].In AD patients, Nnmt expression is increased in this area.In a post-mortem study, the expression of Nnmt in the prefrontal cortex was decreased in patients with schizophrenia compared with that in healthy individuals [151].Patients with bipolar disorders also exhibit decreased serum levels of Nnmt compared with healthy individuals [152].Nnmt expression is decreased in Il18 −/− mice.While Nnmt might be associated with several psychiatric disorders, further analysis of Nnmt in other brain regions, such as the hippocampus or prefrontal cortex, may improve our understanding of these relationships.

Conclusions and Future Directions
In this review, we discussed the microarray analysis of DEGs identified in the liver, BAT, and brain of Il18 −/− mice.Building on a previous report linking IL18 deficiency

Figure 1 .
Figure 1.Ingenuity pathway analysis (IPA) diagram showing direct and indirect networks among interleukin (IL)18, its receptor (Il18r), and other differentially expressed genes common to liver, brown adipose tissue, and brain in Il18 −/− mice.Only Casp4 was predicted to have a direct interaction with IL18.

Figure 1 .
Figure 1.Ingenuity pathway analysis (IPA) diagram showing direct and indirect networks among interleukin (IL)18, its receptor (Il18r), and other differentially expressed genes common to liver, brown adipose tissue, and brain in Il18 −/− mice.Only Casp4 was predicted to have a direct interaction with IL18.

Table 1 .
Cancers and related genes with differential expression in liver, brown adipose tissue, and brain under IL18 deficiency.

Table 2 .
Glucose and lipid metabolism-related genes with differential expression in liver, brown adipose tissue, and brain under IL18 deficiency.

Table 3 .
Functions and diseases of psychiatric disorder-related genes with differential expression in liver, brown adipose tissue, and brain under IL18 deficiency.