Gut microbiome and gastric cancer: microbial interactions and therapeutic potential

One of the main risk factors for intestinal GC is known to be Hp infection [30]. GC may develop as a result of microbial dysbiosis [31, 32]. Long and his associates discovered possible links between cancer and the gut microbiota. Using Mendelian Randomization study, they found a positive causal direction with cases of gastric cancer [33]. Chronic inflammation of the stomach mucosa, oxyntic cell death, elevated stomach pH, and an imbalance in the gastric microbiota caused by Hp infection lower Hp levels and allow non-Hp bacteria to colonize. Long-lasting inflammation causes DNA damage, gastric epithelial cell apoptosis, and autophagy, which in turn damages the gastric mucosa and causes GC and Hp-related gastropathy [34]. Hp is much less abundant in tumor tissues than in nearby non-tumor tissues, as it prefers to colonize healthy gastric mucosa [35, 36]. There are notable variations in the gastric microbiota between Hp-infected and non-infected patients, indicating that Hp might be involved in other microbial dysbiosis [37‐40]. Other microbes become more prevalent as Hp abundance declines [41]. There were no discernible differences in the gut microbiota composition of Hp-positive and Hp-negative individuals in patients with late-stage gastric cancer [42, 43]. While Pseudomonas was significantly more common in tumor tissues, Hp and Lysobacter were significantly more common in normal tissues [44]. Hp plays a role in oncogenesis at the gastric level through three mechanisms [45]. First, Hp injects two cytotoxins, VacA and CagA into the host cell, which activates oncogenic signal transduction pathways [46‐48]. A tiny RNA molecule; Hpnc 4160 in Hp has the capacity to suppress the expression of both outer membrane protein (OMP) and CagA causing autophagy inhibition and consequently malignant transformation [49, 50]. CagA-positive Hp mediates dysregulation of multiple signaling pathways including the Wnt/β-catenin signaling pathway, PI3K/Akt, NF-κB signaling pathway, Shh signaling pathway, JNK signaling pathway, JAK/STAT3 signaling pathway, and ERK/MAPK signaling pathway [51]. Second, it causes reactive oxygen species (ROS) to be produced, which in turn triggers inflammatory pathways. Finally, atrophic gastritis is characterized by the destruction of the parietal cells that produce acid so that there is a compensatory upregulation of gastrin that stimulates the cells to produce more acid, but also activates oncogenic signals. Following diminish of gastric acidity, the carcinogenic strength of a few bacterial lines might additionally increase. Additionally, the microbiota is likely populated with microorganisms that has the ability to form nitrites and carcinogenic N-nitroso compounds [52‐55]. The USF1 gene has an important shielding function in Hp carcinogenesis and can be used potentially as a marker for susceptibility to GC [56, 46]. Watanabe et al. [57, 47] found that Hp infection was associated with reduced richness and evenness of gastric bacteria, and eradication of Hp only partially restored microbial diversity.

Although other stomach microbes might have an impact on gastric carcinogenesis, their precise function is still unknown and needs more research [58]. The ‘Hp initiation–non-Hp acceleration’ cascade is increasingly recognized [59].

Gastric fluid samples from GC patients had larger amounts of Lactobacillus and Veillonella compared to lower levels of Verrucomicrobia and Deferribacteres [60]. There have been reports linking the phylum Verrucomicrobia, which includes Akkermansia, to the advancement of GC [61]. Fusobacterium was reported to be enriched in gastric adenocarcinoma [36, 62, 63].

Protumorigenic bacteria (e.g., E. coli and F. nucleatum) which are not predominant species in fecal microflora are enriched in the cancerous tissues, and may promote tumorigenesis by expression of genotoxins and virulence factors. E. coli, F. nucleatum, and B. fragilis by using experimental models are Potential tumorigenic pathobionts [64]. Mucosal colonization of Adherent-invasive E. coli (AIEC) through fimbriae-mediated adhesion was a crucial step for its colitogenic ability as a pathobiont [65]. Aside from causing genotoxicity, the induction of tumor-infiltrating macrophages and other unknown mechanisms may also play indispensable roles in E. coli-driven tumorigenesis. Enrichment of F. nucleatum was demonstrated in the stool and tissue samples of CRC patients [66]. The inconsistent data of F. nucleatum suggested that other unidentified mechanisms, such as interaction with other bacteria, may partly contribute to its protumoral characteristics. Entertoxigenic Bacteroides fragilis (ETBF) enterotoxin known as fragilysin acts as a metalloprotease that causes oxidative DNA damage, E-cadherin cleavage, epithelial barrier damage, and activation of STAT3/Th17 immune responses, and generation of protumoral monocytic myeloid suppressor cells [67]. Also, fragilysin stimulated the production of spermine oxidase in intestinal epithelial cell lines, suggesting a direct role of the enterotoxin on epithelial free radical production and DNA damage [68]. So, ETBF promote tumorigenesis through both direct and indirect mechanisms.

The gut microbiome of patients with esophageal cancer (EC), GC, and CRC differs from those of healthy people. The abundance alteration of R. faecis in patients with GI cancer might be a predictor of chemotherapy efficacy. Bifidobacterium, Ruminococcus and Roseburia; a member of the Clostridium coccodis cluster of the phylum Firmicutes, are considered a protective taxa in patients with EC, GC, and/or CRC and might be useful in identifying novel diagnostic biomarkers [69, 70]. The clinical chemotherapy response was not significantly associated with baseline microbiota. Moreover, no bacterial differences between responders and non-responders were observed in the patients with EC, GC, and CRC [70].

Results on microbial diversity were inconsistent. After analyzing nearly 200 gastric mucosal samples, Olabisi and his colleagues reported variations in the microbial composition without a discernible difference in microbial diversity [62]. Francisco et al., on the other hand, reported a decline in bacterial diversity from intestinal metaplasia (IM) to intestinal-type GC after non-atrophic gastritis (NAG) [58, 60, 61, 71]. This could be caused by a variety of factors, such as diet, ethnicity, sequencing methods, and the fact that the study populations included both Hp-positive and Hp-negative people. The analysis of different microbial groups in variable gastric regions can explain the heterogeneous results. There are differences between the proximal and distal GC in terms of microbial composition and metabolic products, despite the fact that there is no significant difference in species variety and abundance [72]. Clinically, EBV-associated GC (EBVaGC) is more common in men than women, tends to be found in the proximal region [73], has a better prognosis [74] and a comparatively low rate of lymph node metastasis [75].

The mechanism by which microbes and their metabolites affect gastric carcinogenesis is generally unclear. S. anginosus is consistently observed in mucosa biopsies of patients with GC and produces proinflammatory cytokines. The surface protein TMPC on S. anginosus mediates its attachment and colonization of gastric tissues. Activation of the oncogenic pathway can take place by the interaction of TMPC with the annexin A2 receptor on gastric epithelial cells. Compared with Hp which is mostly depleted in GC, S. anginosus is consistently involved in different stages of gastric carcinogenesis from precancerous lesion, mucosal atrophy, intestinal metaplasia, gastric dysplasia cascade, and finally to malignancy [76].

Fungus sequencing showed that each tumor cell had a single fungus microbe. These microorganisms are less common in the esophagus and somewhat more common in the head and neck, colorectal, and stomach tissues [77]. The organization of fungal communities is drastically changed during gastric carcinogenesis, and the species richness, variety, and evenness of fungal components decline. Ascomycetes was the most enriched phylum in GC tissues, in contrast to the normally lower enrichment [78]. It is unknown if GC is caused by or follows from fungal dysbiosis [79]. Pu and his colleagues have recently emphasized the significance of age in determining the differences in the gut mycobiome. They further emphasize that the gut mycobiome of long-lived people has particular signatures that set them apart from other seniors. These signatures include an increase in core taxa and an overrepresentation of the Candida enterotype. Crucially, gut bacterial compositions are also strongly connected with these longevity-associated traits, which may be used as biomarkers to distinguish long-lived individuals from others [80].

Epstein-Barr virus (EBV) is one of the pathogenic viruses that has the ability to infect eukaryotic cells and produce cancer [81]. Other viruses such as human papillomavirus, human herpesvirus, and hepatitis virus showed no causal relationship and GC [82]. Figure 1.

Through its impact on the tumor microenvironment (TME), the intestinal microbiome has a significant impact on the course and outcome of GC. The TME is primarily composed of tumor cells, stromal cells, immune cells, endothelial cells, and various secreted factors [83]. Because it interacts with tumor cells, promotes tumor growth and metastasis, and offers immune escape features, the TME’s immune cell population is extremely vital [84, 85]. Specific elements of the gut microbiota, such as distinct bacterial communities or distinct secretory factors, have a variety of functions in controlling immune cells in the TME, which affects the prognosis and results of cancer therapies [83].

Bacterial extracellular vesicles (BEVs) are small molecule active substances formed from bacteria, characterized by longer circulation times and structural stability [99]. They play a crucial role in mediating interactions between bacteria and the host, impacting a range of physiological and pathological processes in both organisms [100] including the transport of virulence factors, biofilm formation, and antibiotic resistance [101].

BEVs have the potential to play a significant role in gastrointestinal cancer by infiltrating gastric mucosa and epithelial cells [102]. BEVs produced from host cells infected with Hp affect inflammatory signaling pathways, which in turn affect immune cell modification, cytokine release, cell proliferation, apoptosis, and endothelial dysfunction, cause cytoskeletal reorganization, damage cellular junctional structures, and significantly influence the course of subsequent immune-pathological reactions. These elements impact the course of GC and impede its pathogenesis [103]. Additionally, Hp liberates vesicles, referred to as outer membrane vesicles (Hp-OMVs), which contribute to atrophic and cell transformation in the gastric epithelium [104].

In order to promote antitumor immune responses in vivo for the treatment of cancer, BEVs can also act as strong immune stimulators. It has been demonstrated that OMVs released by bacteria cause anti-BFGF autoantibodies in tumor-bearing mice. These autoantibodies enhance tumor cell apoptosis, increase CTL responses, reverse tumor immunosuppressive microenvironments, inhibit tumor angiogenesis, and ultimately impede tumor growth [105]. By producing inflammatory mediators from gastric epithelial cells following their selective uptake by the cells, Hp EVs can cause inflammation and potentially cancer in the stomach [106]. Furthermore, studies by Li et al. demonstrate that BEV-derived HSP60 is essential for the emergence of Hp-related GC [107].

Via a biomimetic mineralization technique and utilizing a calcium phosphate coating on OMVs that dissolute in the acidic media upon arrival at the tumor location exposing the OMVs, this exposure successfully enhanced the tumor immune suppression microenvironment [108]. Guo et al. used BEVs as carriers to create a co-delivery system for chemical drugs [109].

With strong anti-tumor effects, E. Coli OMV may be a promising cancer immunotherapy agent [110]. Engineered OMV promotes the accumulation of effector T cells in tumors by a dual mechanism of checkpoint inhibition and immune activation [111]. Three intestinal bacterial strains were used to create novel nanovesicles (HNVs), which were linked to favorable immune checkpoint therapy outcomes. They also improve TME, encourage dendritic cell maturation and antigen presentation, and trigger innate immune activation [112]. OMV immunotherapy and HP DA-mediated phototherapy (PTT) were combined to increase the antitumor efficacy against melanoma. Combining PTT with the anti-tumor immune response greatly enhances treatment and results in the total eradication of melanoma [113]. Through extracellular vesicles, bacteria release bioactive metabolites that can change TME and selectively accumulate around tumor cells [114, 115]. TME-resident microbiota interactions are usually responsible for the presence of inflammatory carcinogenic metabolites in cancer cells [102]. Toxin-infected EV, which is released by certain intestinal bacteria, exacerbates inflammatory conditions and contributes to the development of CRC. They postulate that BEVs are crucial in the progression from inflammation to cancer [116]. BEVs are highly immunogenic and can be used as adjuvants, vaccines, and disease-treating vectors, particularly when delivering tumor antigens or small molecule drug-targeted treatments. BEVs are an emerging biomarker that can be found using liquid biopsy, providing new avenues for disease diagnosis [117]. In comparison to healthy controls, Kim et al. discovered that BEVs isolated from stool samples of CRC patients had a noticeably higher abundance of Firmicutes [118].

Non-coding RNAs (ncRNAs) have a pivotal role in gene expression, cancer progression, and cell-cell communication through the involvement of extracellular vesicles (EV). The gut microbiota controls the expression of microRNAs and abnormalities in their expression can result in pathogenic processes linked to the initiation and spread of cancer [119]. Crosstalk among microRNA-microbiota within the intestine performs a pivotal function in intestine homeostasis [120]. NcRNAs can act as tumor suppressors and oncogenes in cancers and they may be dealt with as promising diagnostic and healing markers. The microbiota can affect the occasion and development of most cancers by influencing the expression of ncRNA. MiRNAs play a vital role in the relationship in-between the host and the microbiota in cancer. Chang et al. in 2015 [121] showed that Hp-positive gastric cancer patients had considerably greater levels of miR99b-3p, miR-564, and miR-638 compared to Hp-negative patients, despite exhibiting significantly less miR-204-5p, miR-338-5p, miR-375, and miR-548c-3p. MiR-18a-3p and miR-4286 levels had been drastically large in gastric cancers related to Hp, following the analysis of Tsai et al. [79]. The gut microbiome and ncRNAs’ relation and pathway mechanisms are no longer known.