Periodontitis and oesophageal cancer

It has been confirmed that chronic periodontitis can cause changes in systemic inflammation levels and is a risk factor for a variety of diseases, such as cardiovascular disease, diabetes, premature birth, and low birth weight. In recent years, periodontitis, tooth loss, and oesophageal cancer have been found to be closely related. For instance, Lo et al. [Reference Lo, Kwon, Wang, Polychronidis, Knudsen, Zhong, Cao, Wu, Ogino, Giovannucci, Chan and Song20] found in a large cohort study that individuals with a history of periodontal disease have an increased risk of EAC (hazard ratio (HR) = 1.43). In another cohort study, Nwizu et al. [Reference Nwizu, Marshall, Moysich, Genco, Hovey, Mai, LaMonte, Freudenheim and Wactawski-Wende13] showed that periodontal disease in postmenopausal women increases the risk of oesophageal cancer (HR = 3.28). A retrospective study by Lee et al. [Reference Lee, Hu, Yang, Chou and Chu21] determined that the risk of oesophageal cancer decreases after periodontal prophylaxis in men (HR = 0.54, 95% confidence interval (CI) 0.44–0.66). In 2001, Abnet et al. [Reference Abnet, Qiao, Mark, Dong, Taylor and Dawsey22] conducted a 5.25-year prospective study using 620 ESCC samples from Linzhou city, an area with a high incidence of oesophageal cancer in China, and concluded that tooth loss is significantly positively correlated with the occurrence of ESCC. Additionally, Chen et al. [Reference Chen, Nie, Zhu and Lu23] found that tooth loss increases the risk of ESCC (risk ratio (RR) = 1.3). A study on the Chinese population verified that poor oral hygiene is closely related to the occurrence of ESCC [Reference Chen, Yuan, Lu, Zhang, Jin and Ye24] and that poor oral habits and changes in the oral microecological environment may play an important role.

P. gingivalis in the oral and oesophageal microbiome in ESCC

The human oral microbiome includes as many as 700 different species of bacteria [Reference Verma, Garg and Dubey25], an increasing number of which can be isolated and cultured in vitro and have been named. These microorganisms constitute a complex oral microecology. Under normal physiological conditions, oral microbes maintain a dynamic balance; however, oral microecological imbalance can directly damage adjacent tissues or indirectly damage distant tissues through abnormal immune responses, leading to the occurrence and development of a variety of human diseases [Reference Hajishengallis and Lamont26, Reference Jenkinson27]. Chronic periodontitis is an inflammatory disease caused by oral bacterial infection, and oral microorganisms play an important role in the tissue inflammation and destruction caused by periodontitis. Therefore, the correlation between periodontitis and oesophageal cancer may be associated with periodontal pathogens, and P. gingivalis infection and oral microecological imbalance are the main pathogenic bacteria that cause chronic periodontitis and tooth loss [Reference Cugini, Klepac-Ceraj, Rackaityte, Riggs and Davey28]. Accordingly, the influence of P. gingivalis, an important periodontitis pathogen, on oesophageal cancer has attracted the attention of researchers.

In a large case–control study, Chen et al. [Reference Chen, Winckler, Lu, Cheng, Yuan, Yang, Jin and Ye29] explored the correlation between changes in oral microbiota and ESCC risk and observed differences in the relative abundance of oral bacteria in 87 ESCC patients, 63 atypical hyperplasia patients, and 85 healthy controls through 16S rRNA gene sequencing. The diversity of the oral microbiome in ESCC patients was significantly lower than that in the normal healthy controls and patients with atypical hyperplasia, and the bacteria that were significantly enriched included Prevotella, Streptococcus, and Porphyromonas. Similarly, Meng et al. [Reference Meng, Li, Ma, Liu, Lai, Yang, Wei, Ma and Li30] evaluated saliva samples from 30 ESCC patients in addition to 22 healthy controls. 16S rRNA sequencing results showed that Porphyromonas, Streptococcus, and Leptotrichia were the most enriched in ESCC saliva. Wang et al. [Reference Wang, Rao, Guo, Liu, Liu, Wen, Li and Li31] showed that Actinomyces and Antopobium were related to an increased risk of ESCC, whereas the presence of Fusobacterium and Porphyromonas was highly associated with the healthy group. In this study, since the ESCC patients had periodontitis or gingivitis, the authors selected healthy controls with periodontitis or gingivitis to control the effects of confounding factors. Therefore, the difference may not have been significant because patients with periodontitis or gingivitis themselves have a higher prevalence of ESCC than the normal population. A recent study showed that Leptotrichia, Porphyromonas, Streptococcus, Rothia, Lactobacillus, and Peptostreptococcus were more abundant in ESCC patient saliva than in healthy control saliva [Reference Chen, Xian, Wei, Chen, Yang, Lai, Liu, Wu, Lin, Deng, Zhang, Liu, Qiao and Li32]. These findings suggest that oral microecological abnormalities can increase the risk of ESCC.

The oesophagus is close to the mouth, and the epithelium is squamous; thus, this tissue inevitably serves as a colonisation site for oral bacteria. Similar to the microbiome of the mouth, stomach, colon, vagina, and skin, the composition of the oesophageal microbiome is extremely complex [Reference Hajishengallis and Lamont26]. Bacteria belonging to 41 genera and 95 species in 6 phyla have been identified in the normal oesophageal microbiome, and at least 100 symbiotic bacteria of different species colonise the lower oesophagus [Reference Pei, Bini, Yang, Zhou, Francois and Blaser33]. These bacteria mainly comprise gram-positive Streptococcus, whereas the microbiome of individuals with oesophagitis and Barrett’s oesophagus mainly comprises gram-negative bacteria [Reference Yang, Francois and Pei34]. Yu et al. [Reference Yu, Gail, Shi, Klepac-Ceraj, Paster, Dye, Wang, Wei, Fan, Qiao, Dawsey, Freedman and Abnet35] indicated that the relative abundance of oesophageal microbes correlates negatively with oesophageal squamous epithelial dysplasia. The authors found that the decreased diversity and change in the relative abundance of oesophageal microbes can promote oesophageal squamous cell atypical hyperplasia and may play an important role in the occurrence and development of ESCC. Furthermore, the oesophageal microbiota was prospectively investigated in 18 patients with ESCC and 11 patients with physiologically normal oesophagus by 16S rRNA gene profiling. The results indicated that Aggregatibacter, Veillonella, Parvimonas, Catonella, Streptococcus, Selenomonas, Porphyromonas, and non-Fusobacterium. Fus, Lautropia, Peptococcus, Fusobacterium spp., Peptostreptococcus, Campylobacter, Dialister, Prevotella, Treponema, and Granulicatella represent significantly enriched genera in ESCC [Reference Yang, Chen, Jia, Xing, Gao, Tsai, Zhang, Liu, Zeng, Yeung, Lee and Cheng36]. These results suggest that bacterial infection or oesophageal microecological changes may play an important role in the progression of multistage oesophageal cancer.

Peter et al. [Reference Peters, Wu, Pei, Yang, Purdue, Freedman, Jacobs, Gapstur, Hayes and Ahn37] reported that microorganisms may play a role in the aetiology of oesophageal cancer. These authors conducted a prospective study involving 81 patients with ECA, 25 patients with ESCC, and a healthy control group to identify oral microbes by 16S rRNA gene sequencing. The diversity of the oral microbiota in ESCC patients was significantly reduced and the abundance of Fustanella correlated highly with the occurrence of EAC (95% CI: 1.01–1.46). The increased abundance of P. gingivalis increased the risk of ESCC, and P. gingivalis was associated with lymph node metastasis and short survival in ESCC patients. Kageyama et al. [Reference Kageyama, Takeshita, Takeuchi, Asakawa, Matsumi, Furuta, Shibata, Nagai, Ikebe, Morita, Masuda, Toh, Kiyohara, Ninomiya and Yamashita38] conducted a case–control study examining the salivary microbiota in patients with different gastrointestinal tract cancers, including 12 with unspecified oesophageal cancer and their age- and sex-matched controls. P. gingivalis and Corynebacterium species were more abundant in the saliva of patients with oesophageal cancer than in healthy controls. In addition, Chen et al. [Reference Chen, Lu, Hsieh and Chen39] reported that the oral biofilms in ESCC patients contained more Streptococcus species, Veillonella parvula, and P. gingivalis than in healthy volunteers. Although there have been only a few studies, all of which had small sample sizes, these studies indicate that P. gingivalis may promote the occurrence of oesophageal cancer, and P. gingivalis may represent a risk factor for ESCC.

P. gingivalis promotes the occurrence and development of ESCC

As the most important pathogenic bacterium in periodontal disease, P. gingivalis has been found to be associated with the development of oesophageal cancer in recent years. Through immunohistochemistry experiments and real-time quantitative polymerase chain reaction (PCR) detection, our team’s previous research reported in 2016 that the detection rate of P. gingivalis (61%) and 16S rDNA (71%) in ESCC tissues was much higher than that in adjacent tissue, whereas P. gingivalis and 16S rDNA were not detected in normal oesophageal mucosa tissue. In addition, P. gingivalis levels correlated positively with clinicopathological features, including tumour differentiation, lymph node metastasis, and TNM stage, and negatively with overall survival in ESCC patients [Reference Gao, Li, Ma, Liang, Shan, Zhang, Zhu, Zhang, Liu, Zhou, Yuan, Jia, Potempa, Scott, Lamont, Wang and Feng40]. Very similar results were obtained by Chen et al. [Reference Chen, Lu, Hsieh and Chen39], who in 2021 reported that 57% of patients enrolled in their study were infected with P. gingivalis. Moreover, the authors highlighted that P. gingivalis was associated with advanced clinical stages of ESCC and poor prognosis. In 2017, further research by our team revealed that large levels of P. gingivalis can be detected in ESCC tissues, but the low abundance or absence of this bacterium in cardia cancer or gastric cancer may be related to the low tolerance value of P. gingivalis to acidic environments. In addition, the positive rate of P. gingivalis detection was the highest at 48.3%, followed by the rates of F. nucleatum (35%) and Streptococcus anginosus (17.5%) detection, demonstrating that P. gingivalis potentially represents the predominant anaerobe in late aggressive ESCC [Reference Yuan, Liu, Kong, Gu, Qi, Wang, Sun, Chen, Sun, Wang, Zhou and Gao41]. In summary, P. gingivalis colonisation of the oesophagus is not only related to ESCC but also closely related to disease severity and prognosis.

In 2012, Ahn et al. [Reference Ahn, Segers and Hayes42] conducted a prospective cohort study using 7852 serum P. gingivalis IgG antibody-positive samples from the third National Health and Nutrition Survey of the United States from 1988 to 1994. After controlling for other variables, such as sex, age, smoking, education level, and body mass index (BMI), the researchers found that the P. gingivalis serum IgG antibody level correlated positively with mortality due to digestive malignancies, including ESCC, pancreatic cancer, and colorectal cancer. Thus, P. gingivalis is independent of periodontal disease and other factors that could cause cancer and is closely associated with digestive system malignant tumours, and the presence of P. gingivalis may increase mortality in cancer patients. Our team’s previous research measured and evaluated serum P. gingivalis-specific IgG and IgA antibody levels in 96 ESCC patients, 50 oesophagitis patients, and 80 healthy people using enzyme-linked immunosorbent assays (ELISA) in 2018 [Reference Gao, Yang, Ma, Yuan, Zhao, Wang, Wei, Feng and Qi43]. P. gingivalis-specific IgG and IgA antibody titres in ESCC patients were significantly higher than those in oesophagitis patients and healthy controls, and the expression levels of the two antibodies were significantly negatively correlated with the survival of oesophageal cancer patients, especially those with stage 0-II or negative lymph node metastasis [Reference Gao, Yang, Ma, Yuan, Zhao, Wang, Wei, Feng and Qi43]. Overall, P. gingivalis may be involved in the pathogenesis of ESCC, and the combined analysis of several P. gingivalis serum biomarkers is more sensitive and specific than the analysis of any single biomarker for diagnosis. Due to the continuity of the physiological structure between the oesophagus and the mouth, ESCC is more likely to be in contact with the oral flora than other parts of the digestive system. If the exact relationship between ESCC and P. gingivalis can be demonstrated, monitoring the development of ESCC by detecting P. gingivalis will have important clinical value.

Possible mechanisms by which P. gingivalis promotes oesophageal cancer

Tumours may affect the abundance and diversity of microorganisms. In patients with cancer exhibiting low immunity, harmful microbes may take advantage of this condition and disrupt the normal distribution of human microbes. Conversely, alterations in microorganisms may also serve as a risk or protective factor for the tumourigenesis of various types of cancer [Reference Li, Zhu, Zhang and Chen44]. Currently, the possible carcinogenic mechanisms of P. gingivalis include the production of virulence factors or active metabolites, the activation of carcinogenic signalling pathways, the promotion of abnormal immune responses, and the inhibition of apoptosis in host cells.

P. gingivalis enhances the malignant abilities of ESCC cells

To date, studies on the mechanism by which microorganisms influence oesophageal cancer have been limited, but scholars have found that the long-term colonisation of P. gingivalis may be involved in the occurrence and development of ESCC. Rousseau et al. [Reference Rousseau, Hsu, Spicer, McDonald, Chan, Perera, Giannias, Chow, Rousseau, Law and Ferri45] reported that LPS can increase the migratory ability of human oesophageal cancer cells by increasing their adhesive properties through Toll-like receptor 4(TLR4) signalling and selectin ligands. Zhang et al. [Reference Zhang, Lin, Wen, Li and Tong46] identified P. gingivalis lipopolysaccharide (LPS) as a potential carcinogenic factor for ESCC that affects cell proliferation and migration. The mechanism may involve influencing the biological behaviour of ESCC cells by increasing the expression of ARTN. Meng et al. [Reference Meng, Li, Ma, Liu, Lai, Yang, Wei, Ma and Li30] revealed that P. gingivalis upregulates the expression of key molecules involved in the NF-κB signalling pathway in vitro, such as cyclin D1, matrix metalloproteinase 2 (MMP2), and C-MYC, promoting the proliferation, migration, and invasion of ESCC cells. Liang et al. [Reference Liang, Wang, Shi, Zhu, An, Qi, Du, Li and Gao47] used high-throughput sequencing to show that miR-194 was significantly upregulated and its direct target GRHL3 was decreased after P. gingivalis infection. Since tumour suppressor gene PTEN is a direct regulatory target of GRHL3, GRHL3 has a positive regulatory effect on it in vivo and can regulate the occurrence and development of tumours by activating the PI3K/Akt pathway. Moreover, Chen et al. [Reference Chen, Lu, Hsieh and Chen39] found that P. gingivalis-infected ESCC cells exhibited enhanced EMT-associated characteristics, including elevated β-catenin and matrix metalloproteinase-9 (MMP9) expression levels, which was accompanied by reduced E-cadherin expression levels. Importantly, P. gingivalis infection increased the incidence of developing invasive carcinoma in a mouse oesophageal tumour model induced by 4-nitroquinoline 1-oxide. A recent article by Jia et al. [Reference Jia, Liu, He and Huang48] revealed that in the presence of P. gingivalis, normal human oesophageal epithelial cell proliferation and migration increased, and aneuploid cells appeared. This study indicated that P. gingivalis can induce the malignant transformation of normal oesophageal epithelium by activating HHIP and GLI1, which are key genes in Sonic hedgehog pathways. This study provides direct causal evidence for the carcinogenic effect of a periodontal pathogen on the normal oesophagus. The above results confirmed that P. gingivalis and its virulence factors could enhance the malignant abilities of ESCC cells by activating multiple signal transduction pathways (Fig. 1).

Figure 1. The mechanism by which Porphyromonas gingivalis enhances the malignant abilities of ESCC cells.

P. gingivalis induces tumour cell evasion of the host immune response

To date, there have been few studies on the immune escape of tumour cells induced by P. gingivalis, and those that have been conducted have mainly focused on the expression of immune coregulatory receptors and immune checkpoint molecules (Fig. 2). P. gingivalis can induce the expression of B7-H1 and B7-DC receptors on the surface of gingival epithelial cells and squamous cell carcinoma cells. B7-H1 enhances the production of regulatory T cells, which can inhibit the function of effector T cells and promote immune avoidance in tumour cells [Reference Groeger, Jarzina, Mamat and Meyle49]. Gingipains produced and secreted by P. gingivalis induces the expression of pro-inflammatory mediator, as matrix metalloproteinases (MMPs), degrades extracellular matrix (ECM), destroys immunoglobulin and complement components C3 and C5, and enables ESCC to escape killing by macrophages and neutrophils, leading to ESCC occurrence and development [Reference Malinowski, Węsierska, Zalewska, Sokołowska, Bursiewicz, Socha, Ozorowski, Pawlak-Osińska and Wiciński50]. From the perspective of the tumour immune microenvironment, our previous study found that P. gingivalis colonisation in oesophageal cancer cells induces strong expression of the immune checkpoint molecule B7-H4 and lysine demethylation 5B (KDM5B), thereby promoting the evasion or inhibition of the host immune response by tumour cells, with sustained colonisation [Reference Yuan, Liu, Li, Lan, Ma, Li, Kong, Sun, Hou, Hou, Ma, Ren, Zhou and Gao51]. The dual factors of infection and tumours aggravate the suppression of host immunity and the weakening of immunogenicity. The study further explored the novel mechanisms of immune escape by tumour cells promoted by P. gingivalis, providing a new target for immune checkpoint inhibitors in the treatment of oesophageal cancer and a new strategy for immunotherapy of oesophageal cancer based on microecology. Chen et al. [Reference Chen, Lu, Hsieh and Chen39] reported that P. gingivalis infection induces interleukin (IL)-6 expression in ESCC cells and significantly elevates IL-6 levels in cell culture supernatants. In addition, P. gingivalis infection regulates the local ESCC microenvironment by promoting the recruitment of myeloid-derived suppressor cells (MDSCs), thus enabling cancer cells to escape the host immune response.

Figure 2. The mechanism by which Porphyromonas gingivalis induces tumour cell evasion of the host immune response.

P. gingivalis inhibits apoptosis in host cells and accelerates the cell cycle

P. gingivalis inhibits apoptosis through various mechanisms in primary epithelial cells (Fig. 3). (1) P. gingivalis secretes nucleoside diphosphate kinase (NDK) after entering human cells, which has a similar function to adenosine triphosphate (ATP). NDK from P. gingivalis antagonises ATP activation of P2X7 receptors, and thus reduces IL-1β production from epithelial cells, which inhibits ATP-dependent apoptosis and increases the incidence of cancer [Reference Ramos-Junior, Morandini, Almeida-da-Silva, Franco, Potempa, Nguyen, Oliveira, Zamboni, Ojcius, Scharfstein and Coutinho-Silva52, Reference Morandini, Ramos-Junior, Potempa, Nguyen, Oliveira, Bellio, Ojcius, Scharfstein and Coutinho-Silva53]. Additionally, NDK phosphorylation of heat shock protein-27 (HSP-27) curtails cytochrome C release and caspase-9 activation, thus stalling apoptosis [Reference Lee, Roberts, Atanasova, Chowdhury and Yilmaz54]. (2) At the mitochondrial membrane, P. gingivalis inhibits apoptosis by activating the Janus kinase 1(Jak1)/threonine kinase (Akt)/signal transducer and activator of transcription 3 (Stat3) signalling pathways, reducing the content of the proapoptotic protein Bad, increasing the ratio of the antiapoptotic factor Bcl2 protein to the proapoptotic factor Bax, decreasing the release of the apoptosis effector cytochrome oxidase C, and inhibiting downstream caspases-3 activation [Reference Yao, Jermanus, Barbetta, Choi, Verbeke, Ojcius and Yilmaz55, Reference Yilmaz, Jungas, Verbeke and Ojcius56]. Our previous study verified that P. gingivalis infection reduces the sensitivity of ESCC cells to chemotherapy drugs by activating the Stat3 signalling pathway in vivo and in vitro [Reference Gao, Liu, Duan, Liu, Mohammed, Gu, Ren, Yakoumatos, Yuan, Lu, Liang, Li, Scott, Lamont, Zhou and Wang57]. Compared with the control group, P. gingivalis infection significantly enhanced Stat3 expression and inhibited caspase-3 expression in paclitaxel-treated KYSE-30 cells. These results suggest that differential regulation of Stat3 and caspase-3 is the key pathway involved in the resistance of oesophageal cancer cells to apoptosis induced by P. gingivalis infection. (3) P. gingivalis upregulates the expression level of miRNA-203, which results in the decreasing levels of suppressor of cytokine signalling 3 (SOCS3) and subsequently suppresses apoptosis of primary gingival epithelial cells [Reference Moffatt and Lamont58]. Since SOCS3 can bind to phosphorylated JAK receptors, it consequently inhibits JAK/STAT3 signalling [Reference Sasaki, Yasukawa, Suzuki, Kamizono, Syoda, Kinjyo, Sasaki, Johnston and Yoshimura59]. In addition, P. gingivalis regulates the activity of cyclin/cyclin-dependent kinase (CDK), accelerating the progression of primary gingival epithelial cells through the cell cycle. Besides, P. gingivalis also reduces the level of the tumour suppressor p53, leading to either progression of cell cycle, or to inhibition of apoptosis [Reference Kuboniwa, Hasegawa, Mao, Shizukuishi, Amano, Lamont and Yilmaz60] (Fig. 3).

Figure 3. The mechanism by which Porphyromonas gingivalis inhibits apoptosis in host cells and accelerates the cell cycle.

Summary and outlook

There is a high incidence of oesophageal cancer in China, and the morbidity and mortality of this disease are much greater in China than the world’s average. Overall, oesophageal cancer is a major public health problem that urgently needs to be addressed. Although previous studies have shown that P. gingivalis may be closely related to the occurrence and development of oesophageal cancer, tumour occurrence is the result of the synergistic effects of multiple factors. Whether P. gingivalis can be exclusively pathogenic or has related synergistic pathogenic factors and how these factors function need to be further explored. Most studies to date have only focused on the correlation between P. gingivalis and oesophageal cancer, and there are still many gaps in our knowledge of the aetiology, pathology, and immunology. Therefore, further research is needed to prevent and treat oesophageal cancer by monitoring oral bacteria and to identify therapeutic targets through relevant mechanisms.