Research focusing on how the gut microbiome influences distant organs, such as the lungs, is increasing. The proposed “gut-lung axis” concept holds that dysbiosis of the gut microbiome or gut microbiota metabolites can modulate the immune system in the lungs and cause various respiratory diseases [13, 16, 17]. Indeed, metabolism of dietary fiber by the gut microbiota was shown to affect allergic airway diseases in mice . Another study suggested that the gut microbiome affects the development of the microbial community of the respiratory tract in infants with cystic fibrosis . However, there are few reports of studies investigating the relationship between the gut microbiome and lung cancer. Considerable recent attention has focused on the use of probiotics, living microorganisms that can improve the composition or function of the gut microbiota, in the context of treatments for various neoplasms, including lung cancer [28, 29]. Probiotics are commonly found in yogurt, and an inverse association between yogurt intake and lung cancer risk was reported in a pooled analysis of 10 prospective cohorts involving 1,445,850 adults . Based on these data, we considered it important to evaluate the relationship between the gut microbiome and lung cancer, as the resulting data could help to establish new lung cancer treatments targeting the gut microbiome.
In this study, we found that T category and primary tumor size are significantly associated with the gut microbial community among female never-smokers with lung adenocarcinoma. To our knowledge, this is the first study to identify the gut microbiome as a promising biomarker for lung cancer progression. Our analyses also revealed a positive correlation between the relative abundance of Faecalibacterium and tumor progression, whereas a negative correlation was found between the relative abundances of Fusicatenibacter and Bacteroides and tumor progression. Previous studies have demonstrated that some specific gut bacteria can affect immune cells in the tumor microenvironment [14, 31]. This observation provides a potential clue for explaining the link between gut bacteria and lung cancer progression. For example, Faecalibacterium prausnitzii (sole known species within the genus Faecalibacterium), a common anaerobic bacterium usually representing more than 5% of the total gut bacterial population, upregulates regulatory T cells both in vivo and in vitro [32, 33]. Generally, regulatory T cells suppress the activity of cytotoxic T cells, and it was reported that the number of tumor-infiltrating regulatory T cells is associated with worse recurrence-free survival in non-small cell lung cancer . Based on these data, we speculate that Faecalibacterium plays a role in lung cancer progression by activating the function of regulatory T cells in the tumor microenvironment. Regarding Bacteroides, Vétizou et al reported that T-cell responses specific for Bacteroides thetaiotaomicron or Bacteroides fragilis were associated with the efficacy of cytotoxic T-lymphocyte-associated antigen 4 (a negative regulator of T-cell activation) blockade in mice and humans . This result suggests that Bacteroides can upregulate T cells in the tumor microenvironment and suppress tumor proliferation. It remains unclear whether Fusicatenibacter has an effect on immune cells because no detailed reports are available. Additional studies are needed to determine whether the specific bacteria identified in this study (Faecalibacterium, Fusicatenibacter, and Bacteroides) actually affect immune cells in the tumor microenvironment of lung cancer.
Our study has some clinical strengths, such as eliminating the confounding effect of smoking. Smoking is known to affect the gut microbiome and increase the numbers of Bacteroides and decrease those of Firmicutes . In a previous study comparing the gut microbiomes of lung cancer patients and healthy individuals, lung cancer patients had higher levels of Bacteroides and lower levels of Faecalibacterium (of the phylum Firmicutes) . However, the percentage of ever-smokers in that study was higher among lung cancer patients than healthy controls (63.4% vs 43.9%). Thus, it is likely that the result of the study was influenced by the effect of smoking. Although changes in the gut microbiome due to smoking may be concerned with carcinogenesis, that potential link should be examined in future prospective studies. By contrast, our study completely excluded the possible confounding effect of smoking, because only never-smokers were included. Second, this is the first study to examine the relationship between EGFR mutation status and the gut microbiome. Regarding colorectal cancer, Burns et al previously reported that the tumor microenvironment microbial community is correlated with mutations in tumor DNA, and that this correlation can be used to predict mutated genes based on the microbiome . Some bacteria identified in our study (Bifidobacterium, Faecalibacterium, and Blautia) were significantly correlated with EGFR mutation status. To the best of our knowledge, there are no reports describing the relationship between these bacteria and EGFR mutation status. With regard to the results of our study, we could not determine whether the changes in these bacteria occurred before or after the appearance of EGFR mutations. However, a previous study reported that Enterococcus faecalis can induce EGFR activation in human oral cancer cells via the production of a specific signaling molecule . Thus, it is possible to hypothesize that the bacteria identified in this study are also associated in development of EGFR mutations and that they may play a crucial role in developing new treatments for lung cancers with EGFR mutations.
The potential limitations of the present study are as follows. First, the number of included patients was small. Thus, larger studies are needed to confirm the results of our study. Second, although this study provides initial insights into the relationship between the gut microbiome and lung cancer progression, it was not possible to determine whether alterations in the gut microbiome contribute to lung cancer progression or whether lung cancer progression changes the gut microbiome. To elucidate this cause-and-effect relationship, animal experiments or future longitudinal studies that incorporate repeatedly collected fecal samples will be necessary. Third, we used primary tumor size as a marker for cancer progression, but this parameter does not necessarily reflect the growth rate of the tumor. To evaluate cancer progression more accurately, for example, tumor volume doubling time is preferable. However, we could not measure tumor volume doubling time because of the study design.