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]. High expression of IL18 in PDA was associated with worse disease progression and poor survival
[15]. However, there is the other report that serum IL18 concentration was not correlated with patient survival of pancreatic adenocarcinoma
[16]. In oral squamous cell carcinoma (OSCC), the serum levels of IL18 increase during tumor growth
[17][18]. IL18 expression in peripheral blood mononuclear cells is also increased in OSCC patients compared with that in healthy individuals
[18]. 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].
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]. 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]. 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.
Table 1. Cancers and related genes with differential expression in liver, brown adipose tissue, and brain under IL18 deficiency.
In human tongue squamous cell carcinoma cells, overexpression of IL18 led to apoptosis of tumor cells and decreased
Ccnd1 expression
[23]. 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]. 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]. Increased expression of
Erdr1 in mouse melanoma inhibited tumor growth and metastasis to the lung
[25]. In another study, treatment with recombinant Erdr1 prevented invasion and migration of gastric cancer
[27]. Overexpression of
Erdr1 has also been shown to suppress the expression of
Bcl2 and promote apoptosis
[26][28][29]. Thus,
Erdr1 plays a role in cell homeostasis. One study has shown that Erdr1 is negatively regulated by IL18 in melanoma cells
[25]. Erdr1 and Nfkb regulate the activation of STAT3, which increases tumor progression
[30][31]. STAT3 phosphorylation was impaired in the liver of
Il18−/− mice, and was restored by r-IL18 treatment
[32]. Erdr1 also functions as an immune activator that specifically activates NK cells
[33]. Additionally, administration of recombinant ERDR1 augmented the cytotoxicity of primary human NK cells against leukemia cancer cells
[33]. IL18 in combination with IL2 induces NK cells and has a cytotoxic role against cancers
[34].
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]. 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]. The
Axin2-Wnt pathway involves negative feedback regulation, and
Axin2 is also a direct target of Wnt/β-catenin
[36].
Axin2 is a known tumor suppressor gene in some cancers
[37]. In contrast, several reports identified
Axin2 as an oncogene in colorectal, liver, and gastric cancers
[38].
Axin2 is a β-catenin target that is highly expressed in human colorectal cancer
[39]. Another study showed that the
Axin2 axis suppressed tumor growth and metastasis in colorectal cancer
[40].
Caspase-4 (
Casp4) is related to various malignancies and metastasis
[41]. 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]. In gastric cancer patients, high expression of
Casp4 was associated with a better survival rate
[43]. In esophageal squamous cell carcinoma,
Casp4 may be a tumor suppressor gene
[44].
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]. Activation of
Chrm1 also induced the migration and invasion of two cancer cell lines, HepG2 and SMMC-7721, via the PI3K/Akt pathway
[47]. Signaling through
Chrm1 inhibited primary pancreatic tumor growth via downregulation of the growth factor pathway
[48]. 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]. Another study reported that
Ifi16 promoted cancer development in vitro and in vivo
[51]. Ifi16 protein also activates the STING-TBK1 pathway for IFN-β production
[52].
Ifi16 functions as an activator of the inflammasome, resulting in the production of cleaved IL1β and IL18
[53]. One report showed that
Ifi16 suppresses cell viability and increases apoptosis in hepatocellular carcinoma (HCC) cell lines
[54]. 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].
Klf13 inhibits cell proliferation and accelerates apoptosis in pancreatic cancer cells
[57], and functions as a tumor suppressor protein in prostate cancer and colorectal cancer
[58][59].
Klf13 is also necessary for
Ccnd1 expression, which is an oncogene in oral squamous cell carcinoma
[60].
Klf13 and
Fgfr3 are highly expressed in oral cancer cells
[61]. 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]. 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]. LY6A exhibits marked influences on cellular activity and tumorigenicity, both in vitro and in vivo
[67]. 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]. Furthermore,
Nnt expression is upregulated in adrenocortical carcinoma and triggers anti-apoptosis pathways in cancer cells
[69]. In mouse models of lung tumor initiation and progression, the expression of
Nnt significantly enhances tumor growth, invasion, and aggressiveness
[70]. 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]. Additionally,
SAMSN1 is a suppressive factor of multiple myeloma migration, both in vitro and in vivo
[73]. Decreased expression of
SAMSN1 may promote the progression and recurrence of gastric cancer
[74]. High expression of
Samsn1 was associated with high mortality in glioblastoma multiforme
[75], but
Samsn1 was found to be expressed at significantly low levels in HCC
[76]. 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]. Furthermore, IL18 levels in serum or plasma are negatively correlated with carbohydrate tolerance and positively related to insulin resistance
[80][81][82]. 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]. In women with obesity, weight loss was found to reduce the levels of IL18
[85]. 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]. Previous studies in
Il18−/− mice have indicated that IL18 is involved in glucose metabolism, lipid metabolism, and mitochondrial function
[8][9][32]. 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.
Table 2. Glucose and lipid metabolism-related genes with differential expression in liver, brown adipose tissue, and brain under IL18 deficiency.
Atm is required to maintain mitochondrial homeostasis
[87]. Regulation of the DNA damage response by
Atm involves inflammatory cytokines such as tumor necrosis factor-α and nuclear factor-κB
[88]. Atm is implicated in intermediary metabolism through signaling pathways such as insulin and AMPK
[89][90]. 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].
Atm−/− mice also display disturbances in carbohydrate metabolism, such as glucose intolerance, insulin resistance, and insufficient insulin secretion
[91][93]. Furthermore, diet-induced hepatic steatosis is reduced in
Atm−/− mice compared with that in wild-type mice
[94]. The
Atm pathway is associated with oncogenesis
[95]. In NASH,
Atm mRNA expression accelerates signaling of oncogenic pathways
[96][97]. Activation of
Atm increases the accumulation of cholesterol
[98]. 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]. The ER is a crucial site of lipid metabolism, and a number of enzymes related to lipid metabolism exist there
[100].
Casp4 has been implicated in inflammasome activation through ER stress
[101]. 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]. One study showed that increased
Ifi16 expression stimulates adipogenesis in mice and humans
[103]. 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]. 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]. Another report showed that overexpression of
Nnmt in mice led to fatty liver disease and fibrosis
[106].
Nnmt expression in adipose tissue was also inversely correlated with insulin sensitivity
[107]. 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]. 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]. 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]. First-episode psychosis patients display increased plasma levels of IL18 that correlate with its severity
[113]. 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]. 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].