**3.3. Tumor originates from dedifferentiation of epithelial cells**

E-cadherin expression is lost in most diffuse-type gastric carcinomas, but E-cadherin loss alone is not sufficient to initiate diffuse-type gastric cancer in mice [40]. Loss of the tight junction protein IQGAP1 is also insufficient to induce diffuse-type gastric carcinoma in transgenic mice after challenge with *Helicobacter* infection, as it indeed promotes intestinal-type carcinogenesis [41]. To try to reproduce the diffuse-type gastric cancer, some authors infected *Mist1*;CreERT2;*Cdh1*flox/flox mice with *H. felis* to induce chronic inflammation. In this context,

[13]. The double inactivation of E-cadherin and p53 in a conditional mouse model also suc-

Some data have shown that bone marrow-derived cells (BMDCs) can migrate to peripheral tissues in case of injury or inflammation where they are engrafted and participate in tissue repair, giving rise to all cell lineages. Houghton et al. showed that BMDCs are recruited into the gastric mucosa of C57BL/6 mice chronically infected by *H. felis* and contribute overtime to metaplasia, dysplasia, and finally cancer [34]. In fact, to the contrary, they found that it was a rare event in the context of a normal homeostasis, without injury. Results from this study did not exclude the possibility that BMDCs participate in the development of lesions via fusion with epithelial cells. Our group confirmed these observations in the same mouse genetic background but with different strains of the human pathogen *H. pylori* and found that nearly a quarter of high-grade dysplastic lesions are composed of BMDCs [35]. BMDC epithelial gland repopulation was significantly associated with pseudointestinal metaplasia, suggesting that BMDC recruitment may play a role in preneoplastic lesion progression. These BMDCs are recruited only in response to chronic *H. felis* and *H. pylori* infection but not in response to acute injury [34]. BMDC recruitment occurs in response to the secretion of several chemokines such as SDF1 and TNFα by infected epithelial cells in a NF-κB-dependent manner [34, 43]. Once recruited, the BMDCs can differentiate into local gastric epithelial cells via transdifferentiation or cell/cell fusion with local gastric epithelial cells [35, 44]. Interestingly, BMDC recruitment into the gastric mucosa was a late event in the cascade of gastric carcinogenesis, occurring only in infected animals of more than 1 year of age. In these chimera mice, metaplastic lesions were comprised, inside the same gland, of a mosaic of tagged-BMDCs and native gastric epithelial cells, revealing a multiclonal composition [35]. These metaplastic lesions are now considered as a "point of no return," after which, most of the time, eradication of *H. pylori* cannot lead to a regression of the metaplastic and associated dysplastic lesions, because mutations deregulating stem cell properties and proliferation are already present. A monoclonal conversion will occur during the evolution of intestinal metaplasia toward dysplasia and finally carcinoma. As the mice never develop real metastatic gastric adenocarcinoma in contrast to the human situation, there is no evidence in the literature to date of the role of BMDCs composing metaplastic/dysplastic lesions as the tumor-initiating cells in invasive gastric adenocarcinoma. However, it is very interesting to note that BMDCs were also detected in gastric carcinoma of the esophagus in mice models and in humans, which also develop on a background of chronic inflammation and intestinal metaplasia cascade [45, 46]. Very few studies have been described in humans to strengthen the results obtained in animal models, because there is a limited possibility to trace BMDCs in an individual developing a carcinoma of the GI tract. The only technical approach

cells resulted in the development of diffuse-type gastric carcinomas

E-cadherin loss in Mist1<sup>+</sup>

70 Gastric Cancer

cessfully led to metastatic diffuse-type gastric cancer [42].

**3.2. Tumors originating from bone marrow-derived cells**

In an inflammatory setting, differentiated cells can reacquire the ability to divide and to give rise to all cell lineages. In vitro, *H. pylori* infection induces a destabilization of cell/cell junctions and an elongated phenotype associated with motility [52]. We reported that *H. pylori* infection leads to the generation of a CD44<sup>+</sup> cancer stem-like cell population with mesenchymal phenotype and tumorigenic properties, through complex signaling pathways involving activation of the mitogen-activated protein kinase ERK, c-Jun N-terminal kinase, miR200, and NF-κB signaling pathways, leading to the activation of ZEB1 and Snail1 transcription factors, the main drivers of the epithelial-mesenchymal transition (EMT) [36]. We showed that this effect was associated with the bacterial oncoprotein CagA produced by *H. pylori* and with secreted factors such as hepatocyte growth factor (HGF) as described by others [36, 53]. In vivo, the expansion of the compartment of CD44<sup>+</sup> stem cells at the isthmus in the corpus and at the base of the glands in the antro-pyloric region is associated with the expression of mesenchymal markers in the context of *H. pylori-*associated gastritis, metaplasia, and dysplasia, in humans and in wild-type mice [23, 36, 41]. We recently showed that invalidation of *iqgap1*, a partner of E-cadherin at the cell/cell junctions, increased EMT both in vitro and in vivo, promoted *H. felis*- and *H. pylori*-induced regenerative hyperplasia expressing CD44 and mesenchymal markers, and accelerated and worsened metaplasia and dysplasia development, reinforcing the causal link between EMT and emergence of CSC-like cells [41].
