Tryptophan (Trp) has been proved that play a critical role in cell proliferation (50). The metabolism of tryptophan is regulated directly or indirectly by the gut microbiome (51). The metabolites of tryptophan have immune and neuroregulatory functions (52, 53), which may bring new opportunities for the application and transformation of gut microbiome-related research in drug therapy.

The level of free tryptophan in vivo is mainly determined by food intake and the activity of three tryptophan metabolic pathways. A very small part of free tryptophan is used in protein synthesis and the production of neurotransmitters (e.g., serotonin) and neuro-regulators (e.g., tryptamine). More than 95% of free tryptophan is metabolized through the tryptophan-canine pathway (53).

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor involved in regulating cell metabolism, proliferation, differentiation, cell death, and cell adhesion (34), which is widely and highly expressed in gliomas, especially in GBM (54). The gut microbiome is critically involved in dietary tryptophan metabolism and catalyzes tryptophan to produce AHR agonists. The latter binds to the AHR of astrocytes and gliomas to trigger related effects, including inducing T cell activation, regulating DC function, and recruiting tumor-associated macrophages (TAMs) through hypoxia-inducible factor 1 (HIF-1) (55). Currently, a study demonstrated a function of the neomorphic enzymatic product of mutant IDH, R-2-hydroxyglutarate (R-2-HG), in regulating amino acid metabolism in immune cells (56). Paracrine R-2-hydroxyglutarate not only further impaired monocyteto-DC differentiation in IDH-mutant glioma but also delays DC maturation and specifically suppresses MHC class I/II-mediated antigen (cross-)presentation and co-stimulation by IL-6, which translates to reduced T cell activating capacities (57). What’s more, R-2-HG is taken up by myeloid cells to enzymatically induce TDO2-dependent activation of the kynurenine pathway and the AHR (56), which excessively degraded tryptophan resulting in an amino acid starvation-like response that triggers the expression of LAT1–CD98, a key transporter for tryptophan in proliferating cells (58), which was previously linked to T cell activation and differentiation (59). At the same time, AHR in DC and TAM can also act on CD8⁺T cells to regulate the growth of glioma (60). What’s more, by consuming endogenous tryptophan, glioma cells activate AHR and to inhibit T cell function, induce T cell apoptosis, promote CD39 expression, and induce the differentiation of T cells mediated by interleukin 10 (IL-10) (61). AHR signaling pathway may modify metabolic pathways associated with amino acid, which inhibits the function of immune cells such as glioma-associated macrophages, T cells, antigen-presenting cells, astrocytes, and microglia, resulting in inhibitory changes in the immune microenvironment during the occurrence and development of gliomas, and promoting glioma invasion and migration (60, 62). The key factors for activation of the AHR pathway include IL4 inducible factor 1 (IL4I1), indoleamine-2, 3-dioxygenase 1/2 (IDO1/2) and Tryptophan 2, 3-dioxygenase (TDO). IL4I1 was positively correlated with AHR activity and negatively correlated with patient survival in both high-grade and low-grade gliomas (63). IDO1/TDO activated the Kynurenine-AHR signaling pathway, which was positively correlated with the pathological grade and Ki67 index of glioma as well as negatively correlated with overall survival (64). In conclusion, AHR is an important factor of the gut microbiome affecting the progression of glioma.

Arginine, a semi-essential amino acid in humans, is critical for cell division, healing of wounds, removing ammonia from the body, immune function, and the release of hormones (65, 66). Arginine-derived metabolites, including polyamines and nitric oxide, may affect tumor growth. However, the gut microbiota can absorb dietary arginine to produce polyamines and nitric oxide (67, 68), which are released into the blood circulation system and then transferred to the brain through the blood-brain barrier (BBB). The polyamine may induce tumor cell proliferation and metastasis by up-regulating the expression of ornithine decarboxylase, spermidine, and spermine acetyltransferase, and Akt1 (39). At present, there is still a lot of controversy about the effect of nitric oxide on glioma cells, but its effect may largely depend on the concentration of nitric oxide, exposure time, cell type, and microenvironment (69). In addition, the gut microbiota can produce nitric oxide after reacting with superoxide radicals (70). Nitric oxide can interfere with T cell function by inducing T cell apoptosis (35, 36). Nitric oxide is more likely to cause damage to DNA and mitochondria in tumor cells to enhance the sensitivity of drug-resistant tumor cells to apoptosis during chemotherapy and immunotherapy (71).

In general, arginine starvation may have both beneficial and adverse effects on glioma. On the one hand, the gut microbiome depletes arginine in the tumor microenvironment, which inhibit T cell cycle regulators, thus inhibiting T cell proliferation (72). On the other hand, the consumption of nutritional arginine by the gut microbiome may be beneficial for the eradication of arginine-deficient tumors, which lack the argininosuccinate synthetase converting citrulline to arginine, and therefore can’t meet the high-energy demand in rapid proliferation (37, 38, 73). Arginine depletion in GBM can induce excessive autophagy, which will be toxic to tumor cells and may induce apoptosis (37, 38, 40).

Glutamate(Glu) is the most abundant excitatory neurotransmitter in the brain and plays a crucial role in brain structure and function including learning, memory, emotion, and cognition (74). Alpha-ketoglutarate (αKG), a product of Glu metabolism, is required in DNA demethylation. The gut microbiome can regulate the dynamic balance between αKG and Glu (41), and thus affect DNA methylation. At present, isocitrate dehydrogenase (IDH) gene mutation has been identified as an important biomarker of glioma. IDH1/IDH2 are NADP+-dependent enzymes that catalyze the oxidative decarboxylation of isocitrate to 2-oxoglutarate in cytosol and mitochondria (75). The mutation of the IDH1 and IDH2 protein leads to enzymes with neomorphic enzyme activity that results in the conversion of αKG to the metabolite D‐2‐hydroxyglutarate (D2HG) inhibiting αKG-dependent dioxygenase (76–78). Thus, the mutation of the IDH1 and IDH2 causes abnormal DNA and histone methylation, ultimately leading to widespread hypermethylation of cytosine-phosphate-guanine (CpG) islands (79, 80). The IDH mutations highlight the interaction between metabolism and epigenetics. Thus, the dynamic interaction between the gut microbiome and epigenetic modifications can contribute to regulating glioma growth and development (81).

Glutamine (Gln) is a crucial energy source for glioma cells In addition, Glu and Gln are both involved in energy metabolism and neurotransmission in CNS (82). Starvation therapy has also been shown to reduce the proliferation activity of GBM cells (42–45). However, more than half of the Glnwas synthesized in situ in CNS (83), as such the influence of Gln in the peripheral blood on the energy metabolism of glioma seems to be very limited. At present, most of the studies concerning energy metabolism of Gln in gliomas are confined to the CNS. Studies have shown that the Gln level in the peripheral blood circulation of glioma patients is lower than that of normal people (84). Gln may be used to compensate for the excessive energy consumption of glioma cells. The majority of Gln in the gut comes from the diet, and the rest is produced by various types of bacteria in the gut (85). In condition of intestinal lesions such as inflammatory bowel disease or irritable bowel syndrome where the permeability of the intestinal barrier and BBB was increased (86), the glutaminergic receptor in CNS is affected by the microbiome-gut-brain axis and in turn alters the energy metabolism of CNS. Moreover, changes in the gut microbiome can directly or indirectly alter the level of glutamine in the brain (41), ultimately affecting the energy supply of gliomas. Restricting calories and the ketogenic diet, which limit energy metabolism of glioma cells and induce metabolic oxidative stress and apoptosis, may be potential metabolic therapies for glioma (87, 88).

Short-chain fatty acids (SCFAs), formed by the fermentation of undigested carbohydrates by gut probiotics such as Lactobacillus and Bifidobacterium, can influence the glucose and energy metabolism (89, 90). The most common SCFAs included acetate, propionate, and butyrate (91). Studies have demonstrated that SCFAs can enter the circulation through the intestinal mucosa and regulate the maturation process and function of microglia (92). Microglia, known as brain macrophages, are essential for brain development and physiological functions (92). The deficiency of the gut microbiome can lead to defects in the morphology and function of microglia, which can be partly restored by replanting the complex gut microbiome (92). The disturbance of the abundance and composition of the gut microbiome that decreases circulatory SCFAs not only influences microglial maturation and function but also leads to a state of chronic stress, which has a profound effect on the development and prognosis of tumors through stress-related pathways (48).

SCFAs exert both local and systemic effects through comprehensive signal networks, and the main mechanisms involving binding to G-protein coupled receptors and inhibition of histone deacetylase activity (93, 94). These intracellular mechanisms have been found in the gut, gut-related immune tissues, as well as in the nervous system (94, 95). Butyrate affects the immune system by inducing Treg differentiation and regulating inflammation (49). What’s more, SCFAs can traverse the BBB and enter the brain, subsequently modulate the microglia through epigenetic modifications (96). Acetyl-CoA, an optional products of lipids, functions in regulating protein acetylation (97). Acetate is one of the most abundant nutrients in the brain, which can be absorbed by tumor cells, and affect the production of acetyl-CoA in GBM (46). Acetylation of Rictor by acetyl-CoA actives mTORC2, which drives the proliferation and survival of GBM (47).

In general, the role of gut microbiota-derived metabolites in the glioma microenvironment can be demonstrated in Figure 2 . However, it remains to be seen that whether these metabolites produced by the gut microbiome also disrupt the BBB and induce immunosuppression in the brain.

Previously, due to the existence of the BBB and the lack of a classical lymphatic drainage system, the brain has been considered an immune-privileged organ (98). The resultant disruption of BBB during the process of tumorigenesis may permit the entry of peripheral immune cells into the brain microenvironment, such as T cells, macrophages, and B cells (99). Specifically, a major character of the GBM immune microenvironment is the TAMs (100). In the process of gliomagenesis, due to the destruction of the BBB, the resulting bidirectional communication between immune cells and glioma cells creates an immunosuppressive microenvironment that promotes the survival and growth of tumor (101). Then, Glioma-associated microglia and macrophages are recruited to glioma tissues and can be polarized into M2-like cells, which become tumor-supportive and immunosuppressive (102, 103). The increase of M2-like cells in the brain was associated with GBM and negatively correlated with the survival time of glioma patients (104). Besides, previous studies have demonstrated that functional lymphatic vessels link CNS with lymphatic drainage, and that its dysfunction may related with brain cancer (105), further, showing the uniqueness of the brain as an immune-privileged organ (106). The lymphatic outflow of cerebrospinal fluid is reduced in GBM, thus more pro-inflammatory and chemokines can be captured in the glioma microenvironment (107).

Present studies have demonstrated that gut commensal bacteria of newborns can affect the development and function of the immune system (108). Moreover, the gut microbiome plays an indispensable role in the induction, training, regulation, and function of the host immune system (109, 110). In the presence of some gut commensal bacteria, human DCs can induce the differentiation of T helper (Th) cells into Th1 and Th17, promoting the secretion of pro-inflammatory factors (111). Studies suggested that B. fragilis can induce the differentiation of IL10-secreting Tregs, which can impair the anticancer immunity of Th1 and are related to the progression and invasiveness of gliomas (112). Moreover, interestingly, the certain gut microbiome may lead to immunosuppression, which may result in severe local immunosuppression in patients with GBM (113, 114).

Recent studies have shown that the gut microbiome can affect the morphology and function of microglia. For example, in the absence of particular gut microbiome, the microglia of germ-free (GF) mice altered in their morphological characteristics and gene expression profiles, and their maturation process were inhibited (85, 115). The increase in the number of immature microglia is considered to functionally impair the immune activation and stress response in GF mice, which is involved in the down-regulation of inflammatory factors and the inhibition of the innate immune signal pathway (49, 115). In addition, the entry of the gut microbiome and their metabolites into the systemic circulation can also affect the morphology and function of CNS microglia (116). As aforementioned, SCFAs, fermented by bacteria, can control the maturation process of microglia, whose lack may lead to the disruption of the structure and immune function of CNS microglia (115, 117).

Whereas the exact mechanism of the gut microbiome affecting human brain microglia remains unclear,. the structure and function of microglia are closely associated with the diversity and specificity of the gut microbiome. The smallest colonies of three kinds of bacteria (i.e., Bacteroides distasonis, Lactobacillus salivarius, and Clostridium cluster XIV) can maintain the activation and growth of microglia (115). Recently, a study, for the first time, found that two antibiotics that change the distribution of the gut microbiome can promote the growth of gliomas by inducing early damage of NK cells and phenotypic changes of microglia. In conclusion, the regulation of the gut microbiome can induce alterations of microglia and change the immune microenvironment of glioma.

It is reported that the gut microbiome plays a therapeutic role in several types of cancer. Antibiotics may impair the efficacy of chemotherapy and radiation since they upset the balance of the gut microbiome (118). Some gut microbiome can metabolize chemotherapeutic drugs, which leads to drug resistance (119, 120). Currently, temozolomide (TMZ) is a first-choice alkylating agent considered as a gold standard chemotherapeutic drug for newly diagnosed and recurrent GBM (121). Although the new alkylating agent TMZ can improve the outcome of GBM patients and has an impact on the treatment of malignant gliomas, GBM is still an incurable disease. Orally administered TMZ is converted to 5-(3-methyltriazen-l-yl) imidazole-4-carboximide (MTIC) in the blood (122, 123). MTIC is broken down to methyldiazonium cation and 5-aminoimidazole-4-carboxamide (AIC) (122). Subsequently, methyldiazonium cation transfers its methyl group to DNA, RNA, and cellular proteins (123). These methyl groups are transferred to the 6th position oxygen atoms of guanine and O6-methylguanines are formed. Methylation on the O6 position of guanine is a cytotoxic lesion, which stimulates the mismatch of nucleotide bases during DNA replication (124). If O6-methylguanine-DNA methyltransferase (MGMT) mediated repair does not occur, mismatch repair(MMR) proteins identify mispairing in the newly synthesized strand and thymine excision or DNA damage, followed by cell cycle arrest, leading to programmed cell death (125). Besides, MGMT promoter methylation can predict responsiveness to alkylating chemotherapies in glioblastoma. MGMT testing to select patients with glioblastoma for clinical trials is feasible, and withholding TMZ from patients without MGMT promoter methylation is justified in this context.

Clinically, a majority of patients do not respond to TMZ during the course of their treatment and the efficacy of TMZ is limited by antibiotics due to the lack of related immune response (126). Activation of DNA repair pathways is the principal mechanism for this phenomenon that detaches TMZ-induced O-6-methylguanine adducts and restores genomic integrity (125). Consequently, much remains to be clarified; the mechanism of chemoresistance and the roles of related molecules including MGMT, mismatch repair enzymes, DNA excision repair enzymes, PARP, p53, ABC superfamily, and apoptosis-related factors. Not only approaches to increase sensitivity to TMZ but also understanding the cellular biology underlying chemoresistance and the stem cell phenotype will be helpful to prolong the survival time of patients with GBM.

Compared with surgical resection and radiotherapy (RT) alone, the combination of RT and TMZ can improve the 2-year survival rate from 10% to 27% (127). Previous studies have demonstrated that the destruction of the gut microbiome affects the anti-tumor effect of chemotherapy by changing the tumor microenvironment (128, 129), and shown that the gut microbiome can regulate the sensitivity of patients to chemotherapy. Cyclophosphamide is another major alkylating anticancer drug, and its anti-tumor efficacy is also affected by the gut microbiome. What’s more, cyclophosphamide is capable of altering the composition of the gut microbiome in healthy mice (130). However, the changes in microbiome induced by TMZ in the treatment of gliomas are rarely studied.

Studies in the glioma mice model have reported that the abundance of Akkermansia, Bifidobacterium, and Verrucomicrobia increased at 7 days after TMZ treatment (126), which the abundance of Anaerotruncus increased at 21 days after TMZ treatment (126). These results suggest that the gut microbiome may play a crucial role in anti-tumor response to chemotherapy and immunotherapy.

Studies in mice glioma models have shown that Akkermansia is involved in glucose and lipid metabolism, thus improving metabolic disorders (131). Bifidobacterium plays an anti-inflammatory and immunomodulatory role by inducing regulatory T cells and regulating the release of inflammatory cytokines (132, 133). Besides, Bifidobacterium is also capable of producing folic acid, which is intimately linked to the DNA methylation of MGMT (134). The status of MGMT promoter methylation not only is involved in the inhibition of tumor proliferation but also is related to the anti-tumor effect of TMZ. As a result, it may improve the therapeutic efficacy of TMZ by increasing Bifidobacterium, which induced methylation of MGMT promoter by producing folic acid. It was also found that glutathione and lipid metabolism pathways were up-regulated after TMZ treatment, suggesting that there was a connection between TMZ, oxidative stress, and fatty acid levels (126). The increase of Akkermansia and Bifidobacterium and the decrease of Anaerotruncus may be one of the mechanisms of anti-tumor response to TMZ therapy. Further studies are required to confirm the role of the specific gut microbiota in the anti-tumor response of TMZ.

The immunosuppressive microenvironment limits the efficacy of GBM immunotherapy. Balancing the abundance and composition of the gut microbiome can reduce immunosuppression in the GBM tumor microenvironment (32). What’s more, GBM can attract CTLA-4-expressing T cells and PD-L1-expressing T cells, resulting in inhibiting the activation and continuation of the immune response of cytotoxic T cells. PD-1 and CTLA-4 cells can produce a strong synergistic inhibitory effect in T cells (135). Currently, immunotherapy strategies of GBMs include monoclonal antibodies (PD-1/PD-L1) that block suppressor T cell pathways (136). Unexpectedly, the anti-tumor effects of Bifidobacterium cooperating with the innate immune system and PD-L1 blocking were observed. These studies suggested that oral administration of Bifidobacterium in mice can affect the immune microenvironment of glioma, including induction of DC maturation, stimulation of tumor-specific CD8⁺T cells, recruitment of other immune cells, and activation of type I interferon signal, which hinder tumor growth (137). PD-1 blockade therapy combined with antibiotic therapy can damage the therapeutic effect and reduce overall survival (138). On the other hand, the anti-CTLA-4 antibodies therapy induces intestinal mucosal injury, gut microbiome imbalance, and translocation of specific Burkholderia and Bacteroides fragilis, which can induce IL-12 activation, DC proliferation, activate fecal specific Th1 cells, and work together to create an ideal immune microenvironment for CTLA-4 antibody to stimulate a protective anti-tumor immune response (139). Therefore, the potential effect of the gut microbiome on the treatment and intervention of glioma should be further studied.

Other immunotherapies, such as chimeric antigen receptor T cell (CAR-T) therapy, oncolytic virotherapy, and tumor vaccine therapy, have also been widely studied in the treatment of GBM (140, 141). The first CAR-T cell therapy in GBM, interleukin-13 receptor α 2(IL13Rα2), is the target (142). With the deepening ofresearch, more and more targets have been excavated. Three Phase I trials of CAR T cells targeting IL13Ralpha2, Her2/CMV, and EGFRvIII in the treatment of recurrent GBM have shown promising results (143, 144). However, as there is a lack of clinical efficacy in the application, CAR-T therapy has not been used in the clinical treatment of GBM. Moreover, oncolytic virotherapy and tumor vaccine for GBM treatment is still in the clinical trial stage. None of these treatments have been proved clinically. Currently, few studies are showing that the gut microbiome has an effect on these treatments. Interestingly, studies have demonstrated that the regulation of the gut microbiome can enhance PD-L1 therapy (145). What’s more, there are common immunological characteristics between immune checkpoint therapy and these immunotherapies. Consequently, the regulation of the gut microbiome may have the potential to improve the efficacy of these immunotherapies (146). We speculate that the gut microbiome can be used to maximize the effectiveness of existing anti-tumor approaches, and could even be used as a biomarker to predict the prognosis and therapy response of glioma patients (147). However, further studies are needed to determine the detailed functions of certain gut microbiome components in the treatment of gliomas.