1. Introduction
Hepatocellular carcinoma (HCC) evolves from chronic liver disease and accounts for 85% of all primary liver cancers [
1]. Viral hepatitis was the primary cause, whereas advances in treatment have gradually lowered its incidence [
2]. Meanwhile, the incidence of HCC (MASH-HCC) related to metabolic dysfunction-associated steatohepatitis (MASH, formerly known as NASH) [
3] has increased over the years owing to an increase in the incidence of obesity and metabolic syndrome [
4]. However, the pathogenesis of MASH-HCC remains unclear.
To date, the mechanism by which periodontal bacteria affect the entire body is hypothesized to be as follows: bacteria enter the bloodstream directly from periodontal pocket ulceration and affect organs outside the oral cavity [
5,
6]. In recent studies, another mechanism has been identified: oral bacteria in saliva are transferred enterally to the intestine by swallowing and affect the intestinal bacterial flora and metabolism [
7,
8]. The liver is an organ of the digestive system that is anatomically and physiologically connected to the enterohepatic circulation via the portal vein. Hence, periodontal bacteria and lipopolysaccharides (LPS) derived from periodontal bacteria in the saliva may be implicated in the pathogenic mechanism of MASH-HCC by affecting the intestinal flora.
In recent years, epidemiological reports have shown that periodontal pathogen is a risk factor in onset of various cancers and cancer-associated mortality [
9].
Fusobacterium nucleatum was specifically detected in organs of the digestive system that are anatomically close to the oral cavity [
10,
11,
12], and the presence of
Porphyromonas gingivalis was correlated with the malignancy of esophageal cancer [
13]. Only a few reports have shown an association between HCC and periodontal pathogen, such as high circulating reactive oxygen species levels in patients with HCC and periodontitis [
14]. As far as we are aware, our recent report showing an association between MASH-HCC and salivary
P. gingivalis,
F. nucleatum, and IgA is the only report to date highlighting a relationship between MASH-HCC and periodontopathic bacteria [
15].
Based on these reports, we hypothesized that MASH-HCC is associated with periodontopathic bacteria in the oral cavity. This study aimed to analyze the clinical parameters and oral and intestinal bacterial flora in patients with MASH and MASH-HCC to determine the relationship between MASH-HCC and periodontal bacteria.
2. Materials and Methods
2.1. Participants
Participants in this study included patients with MASH and MASH-HCC aged 20 years or older who attended or were admitted to the Department of Gastroenterology at Yokohama City University (YCU) Hospital between November 2020 and April 2022. Those who were taking antimicrobials within one month prior to periodontal examination and those with edentulous jaws were excluded from the study. This study was approved by the research ethics committee of Kanagawa Dental University (KDU) and YCU and was conducted at YCU Hospital in compliance with the Declaration of Helsinki. All participants were informed of the purpose, outline, safety, and protection of personal information of this study, and their written consent to participate in the study based on their free will was obtained. Initially, sixty-nine participants were enrolled, and data from sixty participants (forty-one with MASH and nineteen with MASH-HCC) for whom all testing and sample collection data were available were used for the analysis.
2.2. Background Information
The participants' gender, age and smoking status were interviewed using a questionnaire form. The dentist filled out the response form based on the participants’ responses. Body mass index (BMI) was calculated from the medical records by obtaining the height and weight values closest to the date of periodontal examination.
2.3. Periodontal Examination
Periodontal examinations were performed by two dentists from the Department of Periodontology at KDU. The probing depth and bleeding on probing were measured at the six probing points per tooth. The plaque index was recorded at the four points per tooth, and tooth mobility was evaluated. Probing was performed at constant pressure using a plastic probe (Contact Probe, Nihon Dental Laboratory Co., Ltd., Tokyo, Japan) with a probing pressure of 0.2 N. The dentists calibrated their probing tools in advance. A periodontal jaw model (P15FE-500HPRO-S2A1-GSF, Nissin, Kyoto, Japan) was used for calibration.
2.4. Sample Collection
Saliva samples used for IgA concentration assay were collected using the SALIVETⓇ (SARSTEDT, Nümbrecht, Germany). A polypropylene-polyethylene polymer sponge was held under the tongue for 2 min, and the saliva-containing sponge was returned to the tube. The tubes were quickly ice-cooled, centrifuged (1,200 × g, 20 min, 4 °C), and stored at –80 °C until analysis. Mouth-rinsed water was used to analyze the oral microbiota. Participants rinsed with saline for 10 s and then collected into tubes. The rinsed water was stored at –80 °C until analysis. Fecal samples were collected from the participants using a Mykinso fecal collection kitⓇ (Cykinso, Inc., Tokyo, Japan) containing a guanidine thiocyanate solution. Fecal samples were collected by the participants themselves according to the manufacturer's manual.
2.5. Medical Examination
Peripheral blood samples were collected on the same day as periodontal examination. Endotoxin, high-sensitivity C-reactive protein (CRP), aspartate transaminase (AST), alanine aminotransferase (ALT), and total bilirubin (T-Bil) levels were analyzed.
2.6. Assay of IgA Concentration in Saliva
IgA concentration in saliva were determined by enzyme-linked immunosorbent assay (ELISA) using a Human IgA ELISA Kit (Bethyl Laboratories, Inc., Montgomery, TX, USA). The ELISA was performed according to the manufacturer’s instructions.
2.7. DNA Preparation and Microbiota Analysis
DNA extraction and bacterial flora analysis of the mouth-rinsed water and fecal samples were performed at the Medical Laboratory (Cykinso, Inc.) [
16,
17]. DNA was extracted using an automated DNA extraction machine (GENE PREP STAR PI-1200A; Kurabo Industries Ltd., Osaka, Japan) according to the manufacturer’s protocol. Detailed sequencing methods are described in previous report [
18]. Data processing and assignment were performed using the QIIME2 pipeline (version 2020.8), and based on the work of Fujihara et al. [
19].
2.8. Bayesian Network Analysis and Classification Trees
A Bayesian network is a directed acyclic graph composed of a set of variables {X1, X2,...,XN} and a set of directed edges between them [
20]. The details of the analytical methods are described in our previous report [
21]. Because the Bayesian network could not be analyzed with missing values, we excluded one participant from the MASH group who had missing T-Bil values, and data from fifty-nine participants (forty patients with MASH and nineteen patients with MASH-HCC) were used for the analysis. Based on the results of Bayesian network analysis, a classification tree analysis was performed using rpart.
2.9. Statistical Analysis
Statistical analyses were performed using SPSS Statistics (version 27.0; IBM, Tokyo, Japan) and R (version 3.5.1 (The R Project for Statistical Computing, Vienna, Austria, 2018). The Mann–Whitney’s U test was used for comparisons between two groups, except for gender and smoking status, which were verified by χ2 test. The Spearman's rank correlation coefficient was used for the correlation analysis. Statistical significance was set at p < 0.05.
4. Discussion
This is the first study to analyze the relationship between oral periodontal pathogenic bacteria and intestinal bacteria in patients with MASH and MASH-HCC. In this study, the presence of HCC directly affected several periodontopathogenic bacteria in the saliva. In addition, a higher abundance of
F. nucleatum in saliva was observed in the MASH-HCC group than in the MASH group. It has been reported that oral bacterial flora changes in patients with pancreatic cancer [
22], and the presence of
F. nucleatum in the oral cavity is elevated in patients with lung and colorectal cancers [
23,
24]. Cancer occurs when the systemic immune response decreases [
25], and immune function clearly decreases in patients with cancer [
26]. In addition, since the type and number of oral bacteria are related to systemic immune status [
27], in this study, the decline in systemic immune function caused by MASH-HCC may have affected periodontal bacteria in the oral cavity, which is a remote organ.
The abundance of salivary
F. nucleatum in the MASH-HCC group was higher than that of other periodontal pathogenic bacteria, except for
P. gingivalis.
F. nucleatum is an opportunistic bacterium present in the oral cavity of individuals without periodontal disease [
28]. In addition, Leigh et al. [
29] reported that opportunistic bacteria in the oral cavity increase owing to a decline in immune function. Hence, although there was no difference in periodontal conditions between the two groups in this study, the MASH-HCC group had decreased systemic immune function; therefore, the occupancy rate of
F. nucleatum in the oral cavity may have been high.
Bayesian network analysis revealed that HCC directly affected several fecal bacteria; however, none of the fecal bacteria directly affected HCC. Among the intestinal bacteria directly affected by HCC, the genus
Roseburia was found to have a lower abundance in the MASH-HCC group than in the MASH group, whereas the genus
Fusobacterium had a higher abundance. Various studies have reported that gastrointestinal cancer is associated with the intestinal microbiome. The genus
Roseburia was decreased in the intestinal microbiota of patients with colorectal and pancreatic cancer [
30,
31]. Although there is a known case of MASH that developed into MASH-HCC through liver cirrhosis [
32],
Roseburia occupancy was decreased in the intestinal microbiota of patients with liver cirrhosis [
33]. The abundance of the genus
Fusobacterium increases in the feces of patients with colorectal cancer [
34,
35]. The abundance of
Fusobacterium increases, and that of
Roseburia decreases when dysbiosis occurs in the intestinal microbiota [
36]. The liver and intestinal microbiomes have a close bidirectional relationship [
37], and dysbiosis occurs in patients with liver cancer [
38]. Moreover, the Shannon Index, which shows the diversity of the intestinal microflora, decrease owing to dysbiosis [
39]. The Shannon index was lower in the MASH-HCC group than in the MASH group in this study. Hence, it is likely that the MASH-HCC group in the present study had more advanced dysbiosis.
High-fat, high-glucose, and low-fiber Western diets are known to accelerate progression from MASH to MASH-HCC [
40]. In addition, the Western diet increases the abundance of the genus
Fusobacterium and decreases the abundance of the genus
Roseburia in the intestine [
41]. Thus, high-fat, high-glucose, and low-fiber diets that cause HCC may also affect the intestinal bacteria.
Our results showed that the genus
Fusobacterium in feces did not affect HCC, and
F. nucleatum in the saliva did not affect the genus
Fusobacterium in feces. Guo et al. reported that
F. nucleatum is increased in hepatocellular carcinoma tissues and that hepatocellular carcinoma is affected by
F. nucleatum because methyltransferase-like protein 3 expression during
F. nucleatum infection is involved in tumor progression [
42]. Although not revealed in the present study, intestinal bacteria may affect HCC.
Primary bile acids increase in MASH-HCC, and
Lactobacillus, which metabolizes them, has been reported to increase in the intestine [
43]. Therefore, Bayesian network analysis revealed a direct effect of MASH-HCC on
Lactobacillus spp in feces. Interestingly, not only HCC but also
P. gingivalis in the saliva directly affected the genus
Lactobacillus in feces.
P. gingivalis is a typical periodontal pathogenic bacterium in the oral cavity that can affect intestinal bacteria and cause dysbiosis [
44]. Park et al. [
45] found that mice infected with
P. gingivalis in the oral cavity showed increased levels of the intestinal phyla
Actinobacteria and
Deferribacteres. In addition, Nakajima et al. [
46] reported that a single oral dose of
P. gingivalis administered to mice increased
Bacteroidetes and decreased
Firmicutes in the intestine. Oral administration of
P. gingivalis causes changes in the intestinal microbiota, impairs intestinal barrier function, and damages the liver [
46]. Although the difference was not significant, the occupancy rate of salivary
P. gingivalis was higher in the MASH-HCC group. Hence, in addition to HCC, the high abundance of
P. gingivalis in saliva could have damaged the liver and altered the amount of primary bile acids, which could have affected the genus
Lactobacillus.
We found that
P. gingivalis in the saliva had a direct effect on the genus
Streptococcus in the feces. In patients with atrophic gastritis in the gastric corpus who are at a high risk of gastric cancer, an increase in
Streptococcus spp. in the stomach [
47] and in the feces of patients with CRC [
48] has been reported, and the genus
Streptococcus is associated with digestive disorders. Thus, in the MASH-HCC group, an increase in
P. gingivalis in the oral cavity can cause dysbiosis in the intestine, which may have affected
Streptococcus spp.
The causal analysis indicated that salivary
P. gingivalis caused a decrease in
Blautia and
Bacteroides via the genus
Lactobacillus and
Butyricicoccus via the genus
Streptococcus. Studies have shown an association between these three intestinal bacteria and gastrointestinal cancer.
Blautia spp. are decreased in the feces of liver cancer patients [
49],
Bacteroides spp. are decreased in the feces of mice that developed liver cancer due to a high-fat, high-cholesterol diet [
50], and
Butyricicoccus spp. are decreased in the feces of patients with esophageal cancer [
51]. All of these bacterial genera are short-chain fatty acid (SCFA) producers [
52,
53,
54]. McBrearty et al. [
55] reported that since SCFAs have strong anti-inflammatory and antitumor effects, the administration of SCFA to mice delayed the development of hepatocellular carcinoma. These bacterial genera did not directly affect HCC but may have affected HCC via SCFA. Hence, reducing
P. gingivalis in the oral cavity, which indirectly affects these three intestinal bacteria, may help prevent the development of MASH-HCC.
Our data demonstrated that salivary
F. nucleatum affected the fecal
Serratia spp., which is an opportunistic bacterium like
F. nucleatum [
27,
56]. Lin et al. [
57] reported increased levels of both oral
F. nucleatum and gut opportunistic bacteria of a mouse model of ulcerative colitis. In the present study, patients with MASH-HCC would have had a generalized state of weakened immune system that made them susceptible to an increase in both the opportunistic bacteria
F. nucleatum and the genus
Serratia. Therefore, our results show that
F. nucleatum in the oral cavity directly affects
Serratia.
Interestingly, salivary IgA concentrations only affected
P. intermedia in the saliva. Salivary IgA levels increase with the number of periodontal pathogenic bacteria and control them [
58,
59,
60]. Despite this, the fact that salivary IgA concentration only affected
P. intermedia in this study suggests that the effect of HCC on periodontal pathogenic bacteria in the oral cavity was greater than that of the salivary IgA concentration.
Of the two factors directly affecting HCC, one was blood T-Bil level, which was directly affected by fecal
Oscillospira spp. T-Bil levels increase as liver function declines in patients with liver cancer [
61]. Increased T-Bil levels have also been reported in rats with liver cancer [
62]. Therefore, it is likely that the MASH-HCC group in this study showed a decline in liver function, resulting in high T-Bil levels. Furthermore, an increase in secondary bile acids produced by intestinal bacteria decreases liver function [
63], and the level of secondary bile acids in feces is positively correlated with genus
Oscillospira in feces [
64]. The genus
Oscillospira may have affected the increase in T-Bil levels by reducing liver function through secondary bile acids.
Age is another factor that directly affects HCC development. The median age of the patients in the MASH-HCC group was higher than that in the MASH group. Recently, Shimomura et al. [
65] reported that patients with MASH-HCC were older and had lower antioxidant function than patients with MASH and that oxidative stress correlated with MASH activation markers, both of which were increased. However, young patients had lower levels of MASH activation markers because their antioxidant functions were preserved [
65]. Hence, old age may be a major risk factor for MASH development.
The results of the classification tree analysis suggested that a T-Bil of 1.35 mg/ dL or higher in the MASH group was related to the occurrence of HCC. HCC occurrences at the age of 77 years or older, even when T-Bil is less than 1.35 mg/dL in MASH. Therefore, MASH patients with T-Bil of 1.35 mg/dL or higher, or older MASH patients with T-Bil less than 1.35 mg/dL, may require more medical assistance to prevent them from developing HCC. High T-Bil levels are the major factors affecting HCC. The genus
Oscillospira, which elevates T-Bil levels, increases when dysbiosis occurs in the intestines [
66]. Because
P. gingivalis in the oral cavity causes intestinal dysbiosis [
46], patients with MASH may require periodontal management to suppress the abundance of
P. gingivalis in the oral cavity to prevent dysbiosis.
Limitations
The current study has several limitations. First, the number of participants was low. This is because it was difficult to recruit participants who met the inclusion criteria. Therefore, the number of participants in the two groups could not be matched. Second, differences were observed in the ages of participants in the MASH and MASH-HCC groups. Future studies will need to set the age of the participants higher in order to keep the age of both groups the same.