Crosstalk Between the Gut and Brain: Importance of the Fecal Microbiota in Patient With Brain Tumors

Gut microbiota and their influence in brain cancer milieu

Microbial communities are not simply remnants of the past but dynamic entities that continuously evolve under the selective pressures of nature, reflecting the intricate and adaptive processes of evolution. The microbiota residing in the various regions of the human body has numerous roles in different physiological processes such as nutrition, metabolism, immune regulation, etc. In the zeal of achieving empirical insights into the ambit of the gut microbiome, the research over the years led to the revelation of reciprocal interaction between the gut microbiome and the cognitive functioning of the human body. Dysbiosis in the gut microbial composition disturbs the homeostatic cognitive functioning of the human body. This dysbiosis has been associated with various chronic diseases, including brain cancer, such as glioma, glioblastoma, etc. This review explores the mechanistic role of dysbiosis-mediated progression of brain cancers and their subtypes. Moreover, it demonstrates the regulatory role of microbial metabolites produced by the gut microbiota, such as short-chain fatty acids, amino acids, lipids, etc., in the tumour progression. Further, we also provide valuable insights into the microbiota mediating the efficiency of therapeutic regimens, thereby leveraging gut microbiota as potential biomarkers and targets for improved treatment outcomes.

Exploring the Microbiome's Impact on Glioma and Brain Metastases: Insights into Development, Progression, and Treatment Response-A Scoping Review

Background: The human microbiome plays a crucial role in health and disease. Dysbiosis, an imbalance of microorganisms, has been implicated in cancer development and treatment response, including in primary brain tumors and brain metastases, through interactions mediated by the gut-brain axis. This scoping review synthesizes current evidence on the relationship between the human microbiome and brain tumors. Methods: A systematic search of five electronic databases was conducted by an expert librarian, using controlled vocabulary and keywords. A targeted grey literature search in Google Scholar and clinical trial registries was also undertaken. Eligible studies included primary research involving human patients, or in vivo, or in vitro models of glioma or brain metastasis, with a focus on the microbiome's role in tumor development, treatment response, and outcomes. Results: Out of 584 citations screened, 40 studies met inclusion criteria, comprising 24 articles and 16 conference abstracts. These included 12 human studies, 16 using mouse models, 7 combining both, and 5 employing large datasets or next-generation sequencing of tumor samples. Thirty-one studies focused on primary brain tumors, six on brain metastases, and three on both. Of the 20 studies examining dysbiosis in tumor development, 95% (n = 19) found an association with tumor growth. Additionally, 71.4% (n = 5/7) of studies reported that microbiome alterations influenced treatment efficacy. Conclusions: Although the role of the gut-brain axis in brain tumors is still emerging and is characterized by heterogeneity across studies, existing evidence consistently supports a relationship between the gut microbiome and both brain tumor development and treatment outcomes.

Preliminary investigation of gut microbiota and associated metabolic pathways in the pathogenesis of primary central nervous system lymphoma

Background

Primary central nervous system lymphoma (PCNSL) is a rare and highly aggressive form of non-Hodgkin lymphoma, primarily confined to the central nervous system. In recent years, growing evidence has indicated that dysbiosis of the gut microbiota is closely associated with the development of various malignancies. This study aims to systematically explore the potential role of gut microbiota and their metabolic pathways in the pathogenesis of PCNSL by integrating metagenomic and metabolomic approaches.

Materials and methods

A total of 33 PCNSL patients and 32 healthy controls were enrolled in this study, and fecal samples were collected from each participant. The fecal samples were analyzed using metagenomic and metabolomic techniques, followed by KEGG pathway enrichment analysis to investigate the biological pathways enriched by the differential gut microbiota and metabolites.

Results

Significant differences were observed in the composition of gut microbiota and metabolites between PCNSL patients and healthy controls. In the gut microbiota of PCNSL patients, the abundance of the phylum Proteobacteria was markedly increased, while the Firmicutes/Bacteroidetes (F/B) ratio was significantly elevated. Metabolomic analysis revealed that the abundance of oleamide was significantly reduced in the PCNSL group, while the relative abundance of deoxycholic acid was significantly elevated. KEGG pathway analysis indicated that the differential gut microbiota and metabolites were primarily involved in key metabolic pathways such as nitrogen metabolism, phenylalanine metabolism, purine metabolism, and pyrimidine metabolism, with these pathways being more active in PCNSL patients.

Conclusion

This study is the first to systematically investigate the differences in gut microbiota and their metabolites between PCNSL patients and healthy individuals, highlighting the potential role of gut microbiota alterations in the pathogenesis of PCNSL.

Influence of gut microbial metabolites on tumor immunotherapy: mechanisms and potential natural products

In recent years, tumor immunotherapy has made significant breakthroughs in the treatment of malignant tumors. However, individual differences in efficacy have been observed in clinical practice. There is increasing evidence that gut microbial metabolites influence the efficacy of distal tumor immunotherapy via the gut-liver axis, the gut-brain axis and the gut-breast axis, a process that may involve modulating the expression of immune cells and cytokines in the tumor microenvironment (TME). In this review, we systematically explore the relationship between gut microbial metabolites and tumor immunotherapy, and examine the corresponding natural products and their mechanisms of action. The in-depth exploration of this research area will provide new ideas and strategies to enhance the efficacy of tumor immunotherapy and mitigate adverse effects.

The bacterial microbiome modulates the initiation of brain metastasis by impacting the gut-to-brain axis

Brain metastases (BrMs) are the most common brain tumors in patients and are associated with poor prognosis. Investigating the systemic and environmental factors regulating BrM biology represents an important strategy to develop effective treatments. Toward this goal, we explored the contribution of the gut microbiome to BrM development by using in vivo breast-BrM models under germ-free conditions or antibiotic treatment. This revealed a detrimental role of gut microbiota in fostering BrM initiation. We thus evaluated the impact of antibiotics and BrM outgrowth on the gut-brain axis. We found the bacterial genus Alistipes was differentially present under antibiotic treatment and BrM progression. In parallel, we quantified circulating metabolites, revealing kynurenic acid as a differentially abundant molecule that impaired the interaction between cancer cells and the brain vasculature in ex vivo functional assays. Together, these results illuminate the potential role of gut microbiota in modulating breast-BrM via the gut-to-brain axis.

The Role of the Gut Microbiota in Modulating Signaling Pathways and Oxidative Stress in Glioma Therapies

Tumors of the central nervous system (CNS), especially gliomas, pose a significant clinical challenge due to their aggressive nature and limited therapeutic options. Emerging research highlights the critical role of the gut microbiota in regulating CNS health and disease. The composition of the gut microbiota is essential for maintaining CNS homeostasis, as it modulates immune responses, oxidative status, and neuroinflammation. The microbiota-gut-brain axis, a bidirectional communication network, plays a pivotal role in cancer and CNS disease treatment, exerting its influence through neural, endocrine, immunological, and metabolic pathways. Recent studies suggest that the gut microbiota influences the solidification of the tumor microenvironment and that dysbiosis may promote glioma development by modulating systemic inflammation and oxidative stress, which contributes to tumorigenesis and CNS tumor progression. This review interrogates the impact of the gut microbiota on glioma, focusing on critical pathways such as NF-κB, MAPK, PI3K/Akt/mTOR, and Kynurenine/AhR that drive tumor proliferation, immune evasion, and therapy resistance. Furthermore, we explore emerging therapeutic strategies, including probiotics and microbiota-based interventions, which show potential in modulating these pathways and enhancing immunotherapies such as checkpoint inhibitors. By focusing on the multifaceted interactions between the gut microbiota, oxidative stress, and CNS tumors, this review highlights the potential of microbiota-targeted therapies and their manipulation to complement and enhance current treatments.

Exploring the associations of gut microbiota with inflammatory and the early hematoma expansion in intracerebral hemorrhage: from change to potential therapeutic objectives

Background

Although a great deal of research has explored the possibility of a systemic inflammatory response and dysbiosis of the gut microbiota after an intracerebral hemorrhage (ICH), the relationships between gut microbiota and blood inflammatory indicators as well as their role in the hematoma expansion following an early-stage mild-to-moderate ICH (emICH) remain unknown. This study analyzes these changes and associations in order to predict and prevent hematoma expansion after emICH.

Methods

The study included 100 participants, with 70 individuals diagnosed with emICH (30 with hematoma expansion and 40 without hematoma expansion, referred to as the HE and NE groups) and 30 healthy controls matched in terms of age and gender (HC). We used 16S rRNA gene sequencing to explore the gut microbial structure and its underlying associations with blood inflammatory parameters in the HE group.

Results

Our findings showed a significant decrease in the diversity and even distribution of microorganisms in the HE group when compared to the HC and NE groups. The composition of the gut microbiota experienced notable alterations in the emICH group, especially in HE. These changes included a rise in the number of gram-negative pro-inflammatory bacteria and a decline in the level of probiotics. Furthermore, we observed strong positive connections between bacteria enriched in the HE group and levels of systemic inflammation. Several microbial biomarkers (e.g. Escherichia_Shigella, Enterobacter, and Porphyromonas) were revealed in disparateiating HE from HC and NE. Analysis of the Kyoto Encyclopedia of Genes and Genomes (KEGG) exposed disturbances in essential physiological pathways, especially those related to inflammation (such as the Toll-like receptor signaling pathway), in the HE group.

Conclusions

Our exploration indicated that individuals with emICH, especially those with HE, demonstrate notably different host-microbe interactions when compared to healthy individuals. We deduced that emICH could rapidly trigger the dysbiosis of intestinal flora, and the disturbed microbiota could, in turn, exacerbate inflammatory response and increase the risk of hematoma expansion. Our comprehensive research revealed the potential of intestinal flora as a potent diagnostic tool, emphasizing its significance as a preventive target for HE.

Brain Tumors and Beyond: Multi-Compartment Microbiome and Mycobiome Analysis

Brain tumors are frequently diagnosed diseases in which etiology and progression largely depend on mutations and genetic factors. Additionally, recent reports document that the microbiome may influence tumor growth, tumor microenvironment, and response to therapy. Our goal was to examine the extent to which the bacterial composition-microbiota-and fungal composition-mycobiota-characteristic of the tumor and its microenvironment correlate with the composition of the gut and blood microbiota and mycobiota in five randomly selected brain tumor patients. The bacterial composition of the tumor, tumor-adjacent tissue (TAT), blood, and gut samples of the five patients were analyzed by 16S rRNA and ITS-based sequencing in order to determine the bacterial and fungal composition. The gut microbiome and mycobiome composition showed individual and tissue-specific signatures in each patient. The microbiome composition of the blood, TAT, and tumor tissue was very similar in each patient, dominated by Klebsiella, Enterococcus, Blautia, and Lactobacillus spp. In contrast, the mycobiome composition of the blood, TAT, and tumor showed a diverse, individual picture. The most common fungal species in the blood and TAT were Tomentella, Didymosphaeria, Alternaria, Penicillium, Mycosphaerella, and Discosia. The blood and TAT mycobiome were similar to each other but unique and characteristic of the patients. In contrast, in the tumor tissues, Alternaria, Malassezia, Schizophyllum, and Tomentella genus were the most common fungi genus. Our results showed that the presence of fungi in tumors shows a unique pattern that is independent of the pattern observed in the gut, blood, and tumor environment and that the effects of the mycobiome are distinct and cannot be associated with those of the microbiome. Elucidating the role of fungi in tumors and exploring the relationship between fungi and brain tumor types may open up further therapeutic options.

From Gut to Brain: The Impact of Short-Chain Fatty Acids on Brain Cancer

The primary source of short-chain fatty acids (SCFAs), now recognized as critical mediators of host health, particularly in the context of neurobiology and cancer development, is the gut microbiota's fermentation of dietary fibers. Recent research highlights the complex influence of SCFAs, such as acetate, propionate, and butyrate, on brain cancer progression. These SCFAs impact immune modulation and the tumor microenvironment, particularly in brain tumors like glioma. They play a critical role in regulating cellular processes, including apoptosis, cell differentiation, and inflammation. Moreover, studies have linked SCFAs to maintaining the integrity of the blood-brain barrier (BBB), suggesting a protective role in preventing tumor infiltration and enhancing anti-tumor immunity. As our understanding of the gut-brain axis deepens, it becomes increasingly important to investigate SCFAs' therapeutic potential in brain cancer management. Looking into how SCFAs affect brain tumor cells and the environment around them could lead to new ways to prevent and treat these diseases, which could lead to better outcomes for people who are dealing with these challenging cancers.

The Role of Bacteria in Central Nervous System Tumors: Opportunities and Challenges

Tumors of the central nervous system (CNS) are severe and refractory diseases with poor prognosis, especially for patients with malignant glioblastoma and brain metastases. Currently, numerous studies have explored the potential role of bacteria and intestinal flora in tumor development and treatment. Bacteria can penetrate the blood-brain barrier (BBB), targeting the hypoxic microenvironment at the core of tumors, thereby eliminating tumors and activating both the innate and adaptive immune responses, rendering them promising therapeutic agents for CNS tumors. In addition, engineered bacteria and derivatives, such as bacterial membrane proteins and bacterial spores, can also be used as good candidate carriers for targeted drug delivery. Moreover, the intestinal flora can regulate CNS tumor metabolism and influence the immune microenvironment through the "gut-brain axis". Therefore, bacterial anti-tumor therapy, engineered bacterial targeted drug delivery, and intervention of the intestinal flora provide therapeutic modalities for the treatment of CNS tumors. In this paper, we performed a comprehensive review of the mechanisms and therapeutic practices of bacterial therapy for CNS tumors and discussed potential future research directions in this field.

Unlocking the secrets: exploring the influence of the aryl hydrocarbon receptor and microbiome on cancer development

Gut microbiota regulates various aspects of human physiology by producing metabolites, metabolizing enzymes, and toxins. Many studies have linked microbiota with human health and altered microbiome configurations with the occurrence of several diseases, including cancer. Accumulating evidence suggests that the microbiome can influence the initiation and progression of several cancers. Moreover, some microbiotas of the gut and oral cavity have been reported to infect tumors, initiate metastasis, and promote the spread of cancer to distant organs, thereby influencing the clinical outcome of cancer patients. The gut microbiome has recently been reported to interact with environmental factors such as diet and exposure to environmental toxicants. Exposure to environmental pollutants such as polycyclic aromatic hydrocarbons (PAHs) induces a shift in the gut microbiome metabolic pathways, favoring a proinflammatory microenvironment. In addition, other studies have also correlated cancer incidence with exposure to PAHs. PAHs are known to induce organ carcinogenesis through activating a ligand-activated transcriptional factor termed the aryl hydrocarbon receptor (AhR), which metabolizes PAHs to highly reactive carcinogenic intermediates. However, the crosstalk between AhR and the microbiome in mediating carcinogenesis is poorly reviewed. This review aims to discuss the role of exposure to environmental pollutants and activation of AhR on microbiome-associated cancer progression and explore the underlying molecular mechanisms involved in cancer development.

Role of Probiotics against Human Cancers, Inflammatory Diseases, and Other Complex Malignancies

Probiotics have growing medical importance as a result of their potential in the prevention and therapeutic support of several complex diseases, including different types of cancers. The anticarcinogenic properties of probiotics are attributed to various mechanisms, including alterations in the composition of the intestinal microbiota, suppression of cell proliferation, stimulation of apoptosis, inhibition of NF-kB, reduction in levels of H2AX, 8-hydroxy-deoxyguanosine, RIG-I, downregulation of IL-17, and TNF signaling pathway. Furthermore, probiotics have demonstrated significant advantages in the prevention and management of other complex diseases, including diabetes, obesity, and cardiovascular diseases. Probiotics had a considerable effect in reducing inflammatory infiltration and the occurrence of precancerous lesions. Additionally, the administration of probiotics led to a decrease in the appearance level of genes related to proinflammatory pathways, including NF-κB, IL-17, and TNF signaling pathways. However, further research studies are required to comprehend the processes via which probiotics exert their effects and to authenticate their potential as alternative therapeutic interventions.

Gastrointestinal Dysbiosis in Neuro-Critically Ill Patients: A Systematic Review of Case-Control Studies

The human gastrointestinal tract (GIT) has a rich and pre-programmed microbiome. This microbiome is essential for physiological functions such as digestion, immunity, metabolism, and structural integrity, and of prime concern to us in conducting this study is the nervous system communication. This two-way communication between the GIT and central nervous system (CNS) is known as the gut-brain axis (GBA) and has implications for neurocritical disease. A change in any factor relating to this microbiome is known as gut dysbiosis; this can lead to aberrant communication through the GBA and in turn, can contribute to disease states. The primary objective of this study is to determine the cause-specific dysbiotic organisms in neuro-critically ill patients and their effects. We performed this study by searching published literature as per Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Studies that defined gastrointestinal dysbiosis in neuro-critically ill patients were retrieved using Boolean search from 2000 to 2023 via PubMed and Google Scholar and narrowed the results down to five prospective case-control studies. We performed their quality assessment. The results concluded that in neurocritical illnesses such as encephalitis, brain tumors, intracerebral hemorrhage, and ischemic stroke, fluctuations in specific microbiota correlated with disease severity and prognosis. Moreover, the inhabiting population of dysbiotic organisms in neuro-critically ill patients were different in different diseases and there were no similarities in the composition of gut microbiota in these diseases. Taking stroke patients as an example; increased Enterobacteriaceae and lower Lachnospiraceae microbiome levels were found in patients with a higher stroke dysbiosis index (SDI). Those patients who developed stroke-associated pneumonia (SAP) displayed higher levels of Enterococcus species. In conclusion, dysbiosis has a major effect on neuro-critically ill patients' disease states and dysbiotic organisms can be used as a biomarker for disease. Further prospective studies on this topic are warranted for potential neurological and prognostic correlations.

Association between medically diagnosed postnatal infection and childhood cancers: A matched case-control study in Denmark, 1978 to 2016

Although the association between infection and childhood cancer has been long investigated, there is limited information on rarer cancers. This article aimed to explore the association between postnatal infection and childhood cancers in the Danish population. A matched case-control study was conducted using Danish nationwide registries from 1978 to 2016. Each childhood cancer case was matched 1:25 with controls by birth date within a week and sex. Postnatal infections were identified from the Danish National Patient Registry, which lists diagnoses seen in hospital, specialist or emergency care services. Multivariable conditional logistic regression was used to estimate adjusted odds ratios (adj.OR) and 95% confidence intervals (CI). Specific types of infections and the number of infection episodes were also considered. The study included 4125 childhood cancer cases and 103 526 matched controls with ages ranging from 0 to 19 years. Medically diagnosed postnatal infections were positively associated with many types of childhood cancer including acute lymphoblastic leukemia (adj.OR = 1.42; 95% CI: 1.23-1.63), acute myeloid leukemia (adj.OR = 1.80; 95% CI: 1.28-2.52), non-Hodgkin lymphoma (adj.OR = 1.53; 95% CI: 1.19-1.97) and central nervous system tumors (adj.OR = 1.57; 95% CI: 1.39-1.77). A higher number of infection episodes were also associated with an increased risk of these cancers. Specific infections such as viral, enteric and urinary tract infections were also strongly associated with specific types of cancer. In conclusion, children who later develop cancer appear to have adverse reactions to infections necessitating referral to specialized health care services, perhaps indicating dysregulated immune function.

Brain-gut-liver axis: Chronic psychological stress promotes liver injury and fibrosis via gut in rats

Background

The effect of chronic psychological stress on hepatitis and liver fibrosis is concerned. However, its mechanism remains unclear. We investigated the effect and mechanism of chronic psychological stress in promoting liver injury and fibrosis through gut.

Methods

Sixty male SD rats were randomly assigned to 6 groups. Rat models of chronic psychological stress (4 weeks) and liver fibrosis (8 weeks) were established. The diversity of gut microbiota in intestinal feces, permeability of intestinal mucosa, pathologies of intestinal and liver tissues, collagen fibers, protein expressions of toll-like receptor 4 (TLR4), myeloid differentiation factor 88 (MyD88), nuclear factor kappa β (NF-κβ), tumor necrosis factor α (TNF-α) and interleukin 1 (IL-1) in liver tissue, liver function and coagulation function in blood and lipopolysaccharide (LPS) in portal vein blood were detected and analyzed.

Results

The diversities and abundances of gut microbiota were significant differences in rats among each group. The pathological lesions of intestinal and liver tissues, decreased expression of occludin protein in intestinal mucosa, deposition of collagen fibers and increased protein expression of TLR4, MyD88, NF-κβ, TNF-α and IL-1 in liver tissue, increased LPS level in portal vein blood, and abnormalities of liver function and coagulation function, were observed in rats exposed to chronic psychological stress or liver fibrosis. There were significant differences with normal rats. When the dual intervention factors of chronic psychological stress and liver fibrosis were superimposed, the above indicators were further aggravated.

Conclusion

Chronic psychological stress promotes liver injury and fibrosis, depending on changes in the diversity of gut microbiota and increased intestinal permeability caused by psychological stress, LPS that enters liver and acts on TLR4, and active LPS-TLR4 pathway depend on MyD88. It demonstrates the possibility of existence of brain-gut-liver axis.