**5. The tumor microenvironment and gastric carcinogenesis**

More and more evidence supported the idea that not only malignant GC cells, but also those nonmalignant cells involved in the tumor microenvironment play indispensable roles throughout GC pathogenesis. Generally, nonmalignant cells participate in various mechanisms related to GC development, such as stromal interactions, angiogenesis, and some immune responses.

Stromal components, including fibroblasts and extracellular matrix (ECM) adjacent to cancer cells, create a suitable environment for GC development. Stromal fibroblasts are known to play a central role in tumor microenvironment by interacting with cancer cells [59, 60]. However, other than ordinary fibroblasts, cancer-associated fibroblasts (CAFs) undergo a phenotypic change into myofibroblasts. Also, CAFs exhibit distinct gene expression patents that pertain to aberrant cell growth, focal adhesion, and ECM remodeling [61]. The remodeling ECM promotes the survival ability and distant colonization cancer cells by synthesizing components, such as type I and type III collagens, fibronectins, tenascin, and versican [62–64], as well as proteases like urokinase, plasminogen activator, fibroblast activation protein (FAP), and matrix metalloproteinases (MMPs) [65–67]. These characteristics are usually associated with poor prognostics. Moreover, tumor-related stromal fibroblasts secrete small molecules, such as IL-6 to stimulate cell growth, as well as factors associated with TGF-β signaling, triggering epithelial-mesenchymal transition (EMT) [68–70].

Angiogenesis is also a pivotal process contributes to tumor progression. Cell population in GC tumor microenvironment regulates the density and architecture of new blood vessels by stimulating the proliferation and differentiation of myofibroblasts and vascular endothelial cells [71, 72]. The growing number of new vessels was reported to facilitate GC metastasis [73, 74]. To promote angiogenesis, GC cells provide numerous angiogenic factors, including VEGF, FGF-2, CXCL1, and Ang-2, to microenvironment [75–78]. It has been noted that high level of VEGF-A contributed to endothelium-dependent angiogenesis. VEGF-A signaling increased both the new blood vessel number and permeability in GC [79]. VEGF-A strongly promotes the angiogenesis and aggressive phenotype of human intestinal-type GC [80]. Bevacizumab is a specific antibody against VEGF-A, and dominantly regulates normal and pathological angiogenesis. Clinical trials of bevacizumab in phase II advanced GC suggested a satisfied curative effect [81]. Unfortunately, in the randomized phase III Avastin in Gastric Cancer (AVAGAST) study, the combination of chemotherapy and bevacizumab did not show a better overall survival extension in the first-line treatment when compared to advanced GC patients, who only subjected chemotherapy [82]. Intriguingly, REGARD and RAINBOW trials using VEGFR2 targeting antibody ramucirumab have also shown a significant increase in the overall survival of patients with advanced GC [83]. It can be partially explained by the geographical differences of GC patients.

Immune reactivity in GC development is based on various types of immune cells in different stages [84]. First, in eliminating stage, macrophages and dendritic cells recruit to secrete chemokines and cytokines, such as IL-12 and IFN-γ, to phagocytize and remove apoptotic cancer cells [85]. Cancer cells survived from elimination subsequently under equilibrium stage and adapted to immune-suppression. The number of tumor-infiltrating lymphocytes (TILs) was noted as independent predictors to evaluate lymph node metastasis and GC patient survival in this stage. Interestingly, TILs exert either oncogenic or tumor-suppressive influence in GC cells, attributing to their unique functions. In a recent report, tumor-associated macrophages with high levels of CD163 expression exhibited aggressive characteristics and expression of cancer stem cell markers in recurrent GC cases. CD163<sup>+</sup> macrophages might, therefore, related to independent worse prognosis [86]. T-lymphocyte subsets are also significant cell types associated with both early onset of tumor and tumor progression. It has been reported that the high ratio of CD8<sup>+</sup> cytotoxic TILs/FOXP3<sup>+</sup> regulatory T cells (Treg) correlated with high overall survival rate and good prognosis, especially in MSI gastric cancer [87, 88]. Eventually, cancer cells escape from immune responses.

Immunogenicity could be blunted by releasing cytokines including TGF-β, TNF-α, and IL-10. Therefore, cancer cells can evade the detection by immune effectors in the tumor microenvironment [89]. PD-1 is one of the most important receptors expressed by T cells and monocytes. PD-1 negatively regulates the effectors by binding to its ligands PD-L1 and PD-L2. Blocking PD-1/PD-L1 has been suggested as a promising immune-therapeutic option [90, 91]. The relationship between PD-L1 level and prognosis remains under debate, however, a recent meta-analysis pooled all present data and suggested PD-L1 to be a valuable prognostic indicator of GC patients by dividing patients into different groups [92].

Collectively, tumor microenvironment composed of multiple subjects, such as tumor stromal fibroblasts, lymphocytes, and angiogenic factors. All these components cooperate together, configuring a context for GC development.

## **6. Conclusion**

and miR-16-1, which target a member of Hippo pathway called YAP1, are downregulated in GC) [50]. Thus, identifying the target genes of these miRNAs is crucial and it may lead us to

For example, miR-21 was the first miRNA which was influenced by *H. pylori*. In tissues of both GC and *H. Pylori* infection, it was highly detected [51, 52]. Several data have reported that this miRNA can be used as a biomarker in GC diagnosis in the clinic [53–55]. And most recently, a research conducted in China suggested this miR-21 to be a GC biomarker in both diagnosis and prognosis, for the reason that, besides the high levels found in tumor tissues compared with the normal ones, miR-21 was revealed to be associated with poor survivals in clinical patients [56]. Behind the phenomena, the molecular mechanism is still unclear in GC. In other types of cancer, however, such as colorectal cancer, miR-21 decreased the tumor suppressor protein programmed cell death 4 (PDCD4) and exhibited an oncogenic function [57]. Additionally, in nonsmall cell lung carcinoma, miR-21 was found to deregulate PTEN, a

More and more evidence supported the idea that not only malignant GC cells, but also those nonmalignant cells involved in the tumor microenvironment play indispensable roles throughout GC pathogenesis. Generally, nonmalignant cells participate in various mechanisms related to GC development, such as stromal interactions, angiogenesis, and some immune responses. Stromal components, including fibroblasts and extracellular matrix (ECM) adjacent to cancer cells, create a suitable environment for GC development. Stromal fibroblasts are known to play a central role in tumor microenvironment by interacting with cancer cells [59, 60]. However, other than ordinary fibroblasts, cancer-associated fibroblasts (CAFs) undergo a phenotypic change into myofibroblasts. Also, CAFs exhibit distinct gene expression patents that pertain to aberrant cell growth, focal adhesion, and ECM remodeling [61]. The remodeling ECM promotes the survival ability and distant colonization cancer cells by synthesizing components, such as type I and type III collagens, fibronectins, tenascin, and versican [62–64], as well as proteases like urokinase, plasminogen activator, fibroblast activation protein (FAP), and matrix metalloproteinases (MMPs) [65–67]. These characteristics are usually associated with poor prognostics. Moreover, tumor-related stromal fibroblasts secrete small molecules, such as IL-6 to stimulate cell growth, as well as factors associated with TGF-β signaling, trig-

Angiogenesis is also a pivotal process contributes to tumor progression. Cell population in GC tumor microenvironment regulates the density and architecture of new blood vessels by stimulating the proliferation and differentiation of myofibroblasts and vascular endothelial cells [71, 72]. The growing number of new vessels was reported to facilitate GC metastasis [73, 74]. To promote angiogenesis, GC cells provide numerous angiogenic factors, including VEGF, FGF-2, CXCL1, and Ang-2, to microenvironment [75–78]. It has been noted that high level of VEGF-A contributed to endothelium-dependent angiogenesis. VEGF-A signaling increased both the new blood vessel number and permeability in GC [79]. VEGF-A strongly

a better understanding of the miRNAs themselves.

12 Stomach Disorders

tumor suppressor, to promote carcinogenesis [58].

gering epithelial-mesenchymal transition (EMT) [68–70].

**5. The tumor microenvironment and gastric carcinogenesis**

There is still a large realm of the unknown area about GC pathogenies even though the related research studies have been going deeper and our horizon has been broadened in the past few years. Epigenetically, we are seeking to unravel the mechanisms of gastric tumorigenesis affected by chromatin remodeling. Besides, there are miRNAs, lncRNAs, and some other classic molecules that are involved in GC development, which requires a more detailed investigation regarding these aspects. The most crucially and urgently, we are struggling to explore specific and precise therapies for patients who are suffering or likely to suffer from GC, especially genomically stable GC (a type of GC with a high mortality).

When it comes to the novel small entities, there are even more issues waiting to be addressed, such as how lncRNAs interact with chromatin alterations during gastric carcinogenesis. Moreover, in addition to *H. pylori* and EBV, scientists have identified a wild variety of bacteria in the stomach through DNA sequencing. These uncultivatable bacteria have formed a complicated community and there are few clues about how they interact with *H. pylori* and host immunity during GC initiation and progression. In the early detection field, molecular findings may facilitate new approaches for diagnosing GC early, by identifying high-risk potential sufferers via the molecular features of precancerous lesions. Finally, therapeutic strategies need to be designed precisely for each GC subtypes, according to their somatic driver change of genome or tumor-associated cell compartments, such as stromal cells and tumor-infiltrating immune cells.
