H. pylori infection is related to mitochondrial microsatellite instability in gastric carcinogenesis

Although a causal relationship exists between H. pylori infection and the development of gastric carcinoma, the molecular mechanisms underlying
this relationship have remained elusive. In this study, we show that H. pylori infection may correlate with up-regulated IL-8 expression and mtMSI in the gastric
mucosa of patients with various gastric pathologies. Up-regulation of IL-8 is correlated
with mtMSI. These findings are consistent with IL-8 and mtMSI playing important roles
in H. pylori-associated gastric cancer. Taking into consideration the increase in the mtMSI frequency
observed in H. pylori-associated lesions, our results suggest early involvement and continuous accumulation
of mtMSI in gastric cells with H. pylori infection that have entered the multistep gastric carcinogenesis pathway.

Mutations in the mitochondrial genome have been detected in nearly every type of cancer
investigated to date 25], including gastric cancer and the corresponding preneoplastic lesions 26]. H. pylori infection has been reported as an important risk factor for gastric carcinogenesis
5], 27]. Previous studies have identified a significant association of H. pylori infection and gastric cancer with nMSI 24], 28]. Although some studies addressing mitochondrial mutations in gastric cancer have
attempted to identify the role of mitochondrial genetic instability 22], 24], 29], reports on the role of H. pylori in mtMSI in gastric carcinogenesis have been rare. mtDNA mutations are associated
with H. pylori infections responsible for chronic gastritis and peptic ulcer tissues, indicating
that the consequences of H. pylori infection include the aggregation of mutations in mtDNA in early phases of gastric
cancer development. Machado et al. 30] showed that gastric epithelial cells are closely related to the genetic instability
of mitochondrial DNA due to H. pylori infection. To study the role of mtMSI in H. pylori-associated gastric carcinogenesis, we analyzed mtMSI in H. pylori-positive and H. pylori-negative gastric mucosa using seven microsatellite markers known to be altered in
gastrointestinal carcinomas. It was found that as carcinogenesis progresses, the level
of mtMSI increases, and the rate of mtMSI was found to be significantly higher in
the H. pylori-positive groups than the H. pylori-negative groups, implying that mtMSI might play a role in the occurrence of those
gastric cancers that are H. pylori positive.

The mechanisms underlying mtMSI induced by H. pylori in the gastric mucosa remain unclear. Reactive oxygen species (ROS) are commonly
released in gastric mucosae that are inflamed as a result of infection with H. pylori, especially with CagA+ strains, and could be responsible for mtMSI-positive gastric
cancer 31], 32]. The mitochondrial genome is particularly susceptible to oxidative damage and mutation
because of the high rate of ROS generation in this organelle and its inefficient DNA
repair system 33], 34]. Increased damage caused by ROS and defective DNA repair are the two causes that
have been proposed to explain mtMSI in H. pylori-associated gastric cancer 31], 33]. A significant correlation between high IL-8 expression in the gastric mucosa and
gastric cancer risk has been reported 35]. In IL-8 transgenic mice, the expression of IL-8 increases tumorigenesis, suggesting
that IL-8 might play a crucial role in gastrointestinal cancers 36]. The induction of higher levels IL-8 expression by H. pylori has been observed in gastric carcinoma and premalignant lesions 12]. In the present study, we found that the levels of IL-8 were significantly higher
in patients with mtMSI than in those without mtMSI, suggesting that IL-8 may play
a role in the development of mtMSI induced by H. pylori. However, the mechanisms through which IL-8 leads to the development of mtMSI in
H. pylori-positive gastric mucosae need to be studied further.

Cancers arising from different mutational pathways are thought to have different clinical
features. nMSI+ gastric cancer is characterized by an older age, antral location,
intestinal type, lower prevalence of lymph node metastasis, and a lower pTNM stage
37], 38]. However, the clinicopathological characteristics of mtMSI+ gastric cancers remain
unclear. In the current study, we did not find an obvious relationship between mtMSI
and tumor size, the depth of invasion, node metastasis or clinical stage, indicating
a limited role of mtMSI in predicting the prognosis of gastric carcinoma. However,
a marked difference in mtMSI was noted in gastric cancers distinguished by histological
type. mtMSI was significantly more frequent in intestinal-type gastric cancers than
diffuse-type gastric cancers, suggesting that mtMSI is a predisposing event in intestinal
gastric cancer.

IL-8 can regulate neovascularization, thereby promoting the growth and spread of human
gastric carcinoma 39]. Yamaoka et al. found that the expression of IL-8 was 10 times higher in gastric
cancer tissue than in normal tissue and it was twice as high in advanced gastric cancer
tissue compared with early cancer tissue 15]. Macrì et al. 40] reported that serum levels of IL-8 serve as a marker of gastric cancer. Increased
expression of IL-8 mRNA in tissue extracts from gastric cancer patients has been associated
with certain clinicopathological aspects of the disease, including a poor prognosis
41]. In a previous study, we showed that co-culture with H. pylori stimulates AGS cell motility and invasion, upregulates ezrin expression at the protein
level and induces a Hummingbird phenotype 42]. In the present study, IL-8 levels were found to be associated with invasion, lymph
node spreading and clinical stage. These observations indicate that high levels of
IL-8 may be associated with a poor prognosis and that IL-8 may be indicative of more
aggressive gastric cancer.