The combination of genetic, epigenetic, and environmental elements participate in the carcinogenesis and development of GC. The epigenetic alterations in GC is influenced by various factors such as infection with H pylori, EBV, tumor microenvironment and cytokines (30) ( Figure 1A ).

Infection with H. pylori causes a prolonged inflammatory reaction of the immune response, H. pylori is not only one of the main risk factors influencing the development of inflammation in GC but can also cause epigenetic deregulations, such as histone protein deacetylation (31) and phosphorylation (32). Although its primary impact is on DNA methylation, affecting tumor suppressor genes related to autophagy, whose modification silences them and favors GC (25).

Additionally, H. pylori modifies the methylation of genes associated with signaling pathways activated by G protein-coupled receptors, such as guanine nucleotide-binding protein subunit beta-4 (33). Treatments that eradicate the bacteria show a reversal in methylation in patients without intestinal metaplasia (34) In GC, EBV-induced host genome hypermethylation directly targets key tumor suppressor genes, including APC, PTEN, and p14ARF, among others. EBV-induced hypermethylation silences genes that regulate the cell cycle and cell differentiation, leading to increased proliferation and dedifferentiation (35).

The environment around a tumor, known as the tumor microenvironment (TME), is composed of different cell types such as immune cells, fibroblasts, and endothelial cells. It is a key player in tumor formation, tumor growth, and metastasis. Additionally, it influences the treatment response (36).

It has been described that cytokines may be aberrantly regulated by different epigenetic mechanisms in tumor tissues, contributing to carcinogenesis in multiple ways. Some of these cytokines also function as regulators of other genes crucial to tumor biology, so a direct or indirect relationship can occur (3) (37). ( Figure 1B , Table 1 ).

Angiogenesis (AG) (18) is a process by which new blood vessels are formed, where endothelial cells form tubular structures that bind together to form stable blood vessels (57), which is critical to tumor growth, invasion, and metastasis. GC tumor and stromal cells produce various pro-angiogenic growth factors, such as vascular endothelial and platelet-derived endothelial cells, as well as IL-8 and angiopoietin (8, 18) ( Figure 1B , Table 1 ).

Epigenetic regulation, like DNA hypermethylation in gene promoter regions, is generally associated with transcriptional silencing, whereas hypomethylation facilitates gene expression (3). Post-transcriptional modifications of mRNA, including N6-methyladenosine (m6A) and 5-methylcytosine (m5C), are involved in mRNA stability, translation, and translocation (58). Wang et al. report that methyltransferase 3-mediated m6A promotes the activation and maturation of dendritic cells involved in the immune response (58). This methylation mechanism is also associated with the activation of several biological pathways, such as transforming growth factor beta (TGF-β) signaling, epithelial-mesenchymal transition (EMT), and chemokine signaling (37).

TGF-β is a cytokine that has a dual function; when the tumor is in early stages it acts as a proliferation suppressor, arresting the cell cycle and promoting apoptosis. In advanced stages of the tumor, it favors the epithelial-mesenchymal transition (EMT). This last process favors the change in cellular characteristics such as intercellular adhesion molecules, which favors invasion and metastasis to other tissues. TGF-β acts through its membrane receptors, allowing its phosphorylation and downstream activation of the SMAD 2/3/4 system that targets the nucleus to regulate proliferation, immune mediators, EMT and metastasis. This pathway becomes central in the interrelation between epigenetic modifications and cytokines in GC (59) and will be developed in subsequent sections.

Another cross-signaling pathway for the interrelation of epigenetics and cytokines in GC is Chemokine receptor signaling. Chemokines are part of the cytokine family, but they are characterized by their function in promoting chemotaxis, that is, they allow the trafficking of leukocytes to be directed towards the tumor and its microenvironment. Chemokines activate receptors found in membranes and are coupled to G proteins, which allows the activation of downstream signaling of MAPK and PI3K in tumor cells, which favors cell survival and proliferation. Additionally, through a pathway independent of Protein G, chemokines activate the JAK/STAT signaling pathway. This pathway is relevant since it allows the regulation of transcription of genes related to inflammation, allowing the cell to regulate itself through autocrine signaling and promote their survival and resistance to drugs (60).

In this sense, activation of EMT and TGF-β in GC leads to decreased T cell trafficking in tumors, modulation of angiogenesis, and activation of fibroblasts (61). In patients with GC positive for H. pylori, efficient transcriptional activation of IL-8, CXCL1, and CCL5 has been observed in a histone protein demethylase-dependent manner. Hence, it is implicated as an inducible epigenetic factor under stress conditions and as a contributor to the dysregulation of gastric TME and angiogenesis (38).

In addition, dysregulation of miRNA, such as miR-149 and miR-204, also contributes to angiogenesis in GC. Hypermethylation of the miR-149 promoter region in cancer-associated fibroblasts (CAF) is related to H. pylori infection and the activation of Cyclooxygenase-2/Prostaglandin E2 (COX-2/PGE2) signaling, regulating chronic inflammation. PGE2 binds to prostaglandin E receptors and activates downstream signaling pathways such as the β-catenin pathway, the PI3K/AKT pathway, and the NF-κB pathway, which may promote proliferation, survival, and regulation of the immune system. The COX-2/PGE2 pathway favors immune evasion of the tumor, which stimulate not only cell survival, but also chemoresistance (62), resulting in an increased secretion of IL-6 (39). Conversely, miR-204, when overexpressed, inhibits NF-κB pathway-associated genes like IL-6 and IL-8 (40). NF-kB is a transcription-factor involved in cellular immunity, inflammation, and stress. inflammatory responses, is one of the most important molecules linking chronic inflammation with cancer, and its activity is tightly regulated by several mechanisms. NF-κB activation induces several target genes, such as proproliferative and antiapoptotic genes, and NF-κB signaling crosstalk affects many signaling pathways, including those involving STAT3, AP1, interferon regulatory factors, NRF2, Notch, WNT–β-catenin and p53 (63, 64).

Angiogenesis in GC is influenced by epigenetic mechanisms, such as DNA methylation, mRNA post-transcriptional modifications, and miR regulation, underscoring the complexity of the molecular events involved in this process.

GC progression and metastasis are critical aspects that arise mainly due to the late diagnosis of the disease. The potential of genetically unstable tumor cells to invade adjacent tissues and migrate to distant organs is known as metastasis (65). This process is associated with altered cell adhesion, cell motility, invasion, resistance to cell death signals, basement membrane disruption, and extracellular matrix (65).

In the context of epigenetics and cytokine regulation in GC progression and metastasis, it has been noted that epigenetic silencing can influence the loss and imbalance in expression levels of the chemokine CXCL12 and its receptor, CXCR4. Hypermethylation of the CXCL12 promoter has been associated with inhibition of tumor metastasis, and restoration of expression by methyltransferase inhibitors has been shown to reverse this effect (41, 66). Using the same methodology, Hu et al. proved that the chemokine CXCL14 could be involved in the development and progression of GC. CXCL14 was reduced in GC tissues compared to normal tissues. Abnormal hypermethylation of the promoter region in tumor tissue is one of the mechanisms causing the reduction. Promoter demethylation has been shown to restore CXCL14 expression, correlating positively with the prognosis in stages III/IV (42).

TGF- β1 (anti-inflammatory cytokine) has biphasic effects on tumorigenesis (67), acting as a tumor suppressor in the early stages and promoting tumor progression in the late stages. Methylation of the TGF-β1 promoter has been linked to the development of various solid tumors, including GC. Wang et al. showed that high methylation levels in patients with GC who test positive for H. pylori are associated with the increase and production of proinflammatory cytokines such as IL-1β (46).

Expression of suppressor of cytokine signaling-1 (SOCS1) prevents the activation of the JAK/STAT signaling pathway (a pathway that promotes tumor development) (68). Loss of SOCS1 expression through promoter hypermethylation is strongly associated with the overproduction of inflammatory cytokines such as TNF-α and IL-6 (69). This loss of expression contributes to the activation of the JAK/STAT pathway and tumor progression; its demethylation restores SOCS1 expression in GC and suppresses constitutive STAT3 phosphorylation (45), like what occurs with SOCS3 (44). In addition, studies have shown that downregulation of SOCS1 by promoter hypermethylation is related to infection by H. pylori and the generation of inflammatory cytokines during gastric carcinogenesis (47).

In the case of EBV-positive GC, cells expressed the interleukin IL-15Rα receptor binds to IL 15 and allows the increase of natural killer cells, in addition to the regulation of the JAK/STAT pathway and thyrosine kinase pathways such as MAPK and PI3K (70), at a lower level than EBV-negative cells due to promoter hypermethylation (43); therefore, administration of IL-15 to stimulate immune responses against cancer could be a promising strategy in GC.

In the context of epigenetic mechanisms associated with miRNA in GC progression and metastasis, an interaction has been found between human epidermal growth factor receptor 2 (HER2) is a central tyrosine kinase receptor in cancer development, has epidermal growth factor (EGF) among its ligands and allows activation of a series of downstream signaling pathways such as MAPK, PI3K and PKC, influencing at the level of nuclear transcription and modulating cell-cycle progression, proliferation, and survival (71) and CD44 that contributes to metastasis by deacetylating histones, which suppresses the transcription of miR-139. This miR represses the chemokine receptor CXCR4, thereby decreasing tumor invasion and growth (48). Treatment with trastuzumab, a drug that inhibits HER2, has been shown to restore CXCR4-associated miR-139 and reduce invasiveness (48).

Other miRs, such as miR-17-5p (49) and miR-370 (50), have also been implicated in GC progression and metastasis by regulating genes such as SOCS6 and TGF-β-RII, respectively. Reduced patient survival has been linked to overexpression of miR-370, indicating the potential significance of this biomarker (50).

miR-922, which has been found to be overexpressed in GC, targets the SOCS1 gene and negatively regulates its expression by activating the JAK and AKT pathways, promoting tumor cell proliferation and motility. Conversely, downregulation of miR-922 increases SOCS1 expression and promotes the apoptosis of GC cells (52).

GC-associated EBV-encoded miRs, such as miR-BART6-3p, have also been linked to the carcinogenesis and progression of malignant neoplasms by suppressing IFN-β production and targeting genes like retinoic acid-inducible gene I (RIG1) (51). This activates a signaling cascade downstream and leads to the production of type I interferons, proinflammatory cytokines (72), and IL-6R (73), causing the immune system to deteriorate.

Chemoresistance in GC can arise from inherent mechanisms inside the tumor cell or can be acquired during treatment. Classic mechanisms include abnormalities in cell membrane transporters, increased DNA repair, reduced apoptosis, presence of tumor stem cells, changes in detoxification enzymes, disorders in miR regulation, development of EMT, and hypoxic conditions (74). Components of the tumor microenvironment and cytokine secretion also generate chemoresistance (75) ( Table 1 ).

TGF-β acts as a tumor suppressor, but tumor cells can develop resistance to its inhibitory effects. Hypermethylation of the TGF-βRI receptor is associated with resistance to TGF-β function in GC (76). Demethylation can increase TGF-βRI expression, suggesting a synergistic role with anticancer drugs. TGF-β is associated with sensitivity to chemotherapy (55).

Tumor-associated macrophage (TAM) infiltration in GC tissues is associated with high expression of DNMT1 (77). M2 macrophages upregulate DNMT1, and suppression of DNMT1 has antitumor effects related to the inhibition of CCR5 involvement stimulated by CCL5 (8). DNMT1 inhibition with 5-AZA or the C-C chemokine receptor (CCR) antagonist (Maraviroc) could be a therapy option with anti-inflammatory effect.

In resistance to cisplatin in GC, an H. pylori infection produces high levels of inflammatory cytokines such as TNF-α (78) and induces miR-135b-5p production by suppressing KLF4 (Kruppel-like factor 4) is a transcription factor that regulates proliferation, apoptosis, inflammation, and tumorigenesis, acting as a tumor suppressor in gastrointestinal tumors (79). The repression of which decreases apoptosis and increases drug resistance (53). In contrast, miR-204 acts as a tumor suppressor, sensitizing GC cells to 5-FU by suppressing the TGF-β-mediated EMT signaling pathway (54).

lncRNA also play a role in chemoresistance in GC. The overexpression of the lncRNA MACC1-AS1 in GC tissues is caused by TGF-β secreted by mesenchymal stem cells (MSC) through immune response-associated SMAD 2/3 activation, which promotes proliferation and chemoresistance by inhibiting miR-145-5p (56).