The large number of microbiome in the body affects the susceptibility to cancer, partly because its metabolites or co-metabolites have important effects on the function of immune cells. Studies report that 15 to 20 percent of cancer cases are associated with microbial infections [18], but the role of microorganisms in tumor genesis and development is still controversial today. There were some highlighted questions about whether oral microbiome changes are an important risk factor for oral cancer development [19]. According to the results, patients with oral leukoplakia (OLK), a lesion with malignant potential, were more enriched with Fusobacteria compared to normal tissue from the same patients [20]. In addition, the opinion that microbiome changes preceded the malignant transformation process was confirmed, supporting the possible role of microbiome changes in the pathogenesis of disease [9].
Porphyromonas gingivalis is one of the most studied oral microorganisms in vivo and in vitro. Studies have found that continuous stimulation of normal epithelial cells by this kind of bacteria can lead to tumour-like changes in cells [21]. Another reported that the bacteria had an inhibitory effect on tongue squamous cell carcinoma cells [22]. Some researchers have summarized the role of oral microbiota in cancer development. The results suggested that among streptococci, S.anginosus seems to be an especially relevant marker of head, neck, and esophageal cancers and is more common in oral squamous cell carcinoma. They also pointed out three mechanisms of oral microbiota in cancer pathogenesis. The first is bacterial stimulation of chronic inflammation.;The second is the activation of NF-κB and inhibition of cellular apoptosis༛And the third is the carcinogens produced by bacteria [23].
S.anginosus, as a human symbiotic bacterium, was first proposed in 1983 that it may be involved in the occurrence and development of oral infectious diseases [14]. In recent years, many relevant studies have claimed that the detection rate of this bacterium in OSCC tissues has increased [24], but its actual effect and mechanism on tumor cells have not been reported. SASAKI et al. isolated and purified a new bioactive antigen SAA (S.anginosus antigen) from S.anginosus supernatant, and they was found that SAA can stimulate macrophages to produce high concentrations of nitric oxide (NO) and various inflammatory factors. NO can interact with O2 or O2- to form reactive nitrogen species (RNS), which leads to a DNA damage response by DNA base oxidation and nitration. In addition, the β-hemolysin of S.anginosus and the streptolysin S(SLS) of Streptococcus pyogenes are homologous series encoded by a similar gene cluster; SLS is a highly toxic cytolysin, which can help bacteria to cross the epithelial barrier with a tissue damage and also can against the immune clearance of hosts [25].Based on the reported toxic effects of metabolites of this bacteria, we selected S.anginosus supernatant of stable period for treatment and retained the metabolites in the supernatant for related experiments.
Autophagy is a physiological compensation process which cell removes damaged proteins and organelles to maintain its homeostasis [26]. Stress reactions such as starvation, hypoxia and microbial infection can stimulate autophagy. Many literatures have stated the relationship between tumor, microorganism and autophagy [27, 28, 29]. However, it is not clear whether the decrease in autophagy activity observed in malignant cells is mechanically significant or merely incidental to the progression of malignancy. Invasion and metastasis are the critical markers in the development of cancer, and active cell migration plays an essential role in the invasion and metastasis cascade of cancers. Our study confirmed that S.anginosus can reduce the proliferation, migration and invasion of SCC15 cells and promote cell apoptosis; Furthermore, it has been found that Beclin1 located in the cytoplasmic endoplasmic reticulum is involved in the formation of autophagic vesicles and also an important factor in inducing autophagic death of tumor cells. Structurally, Beclin1 has the BH3 region that constitutes the apoptotic bcl-2 protein family, and it is a specific receptor of apoptosis, so it plays an important role in promoting apoptosis [30]. In this study, MDC staining, qPCR and western blot results also confirmed increased autophagy activity and up-regulated expression of Beclin1 during this process, which is consistent with our results of increased apoptosis rate.
Picardo SL et al. suggested that microbial regulation may affect the course of the disease [31]. In patients with advanced cancer treated with immunotherapy, resistance is associated with microbiome abnormalities and antibiotic treatment. There is certainly evidence that the gut flora plays a key role in the response of cancer patients to chemoradiotherapy and immunotherapy. Microbiota transplantation (MT), including fecal microbiota transplantation(FMT) and selective microbiota transplantation (SMT), may improve the effect of anti-cancer treatment and/or reduce the related side effects [32]. Viaud et al. demonstrated that cyclophosphamide causes discontinuity of the intestinal barrier and subsequently promotes selective transfer of specific Gram-positive bacteria to secondary lymphoid organs. These transplanted bacteria can enhance the anti-cancer adaptive immune response of T cells [33].
A new study indicates that the composition of the patient's intestinal flora is an important factor in regulating the host's response to anti-PD-1 / PD-L1 or anti-CTLA-4 immunotherapy [34]. In addition, the view that regulation of the microbiome can be used in cancer treatment was proposed [35]. The study in a mouse model of colonic carcinogenesis showed that oral intake of probiotics containing lactobacilli can reduce IL-17-producing T cells and inhibit proliferation and tumor formation, which may be achieved by changing the gut microbiome. National Institute of Health reconstructed the laboratory mice with natural “wild-type” microbiota, and increased resistance to mutagen and inflammation-induced colorectal tumorigenesis were found [36]. Wang et al. observed that S. anginosus stimulated peripheral blood of OSCC patients and healthy people, and found that CD8 T cells were significantly higher in OSCC patients, proving that Streptococcus-reactive CD8 T cell responses might contribute to antitumor immunity in OSCC patients [37]. Above all, microbiome profoundly affects immune development and carcinogenesis. microbiome-modulating agents are poised to become bona fide anti-cancer strategies, complementing the currently available arsenal of surgery, chemotherapy, radiotherapy, and immunotherapy.