Novel Anticancer Strategy by Targeting the Gut Microbial Neurotransmitter Signaling to Overcome Immunotherapy Resistance

Abstract

Significance:

Microbial neurotransmitters, as potential targets for cancer therapy, are expected to provide a new perspective on the interaction between the gut microbiome and cancer immunotherapy.

Recent Advances:

Mounting data reveal that most neurotransmitters can be derived from gut microbiota. Furthermore, modulation of neurotransmitter signaling can limit tumor growth and enhance antitumor immunity.

Critical Issues:

Here, we first present the relationships between microbial neurotransmitters and cancer cells mediated by immune cells. Then, we discuss the microbial neurotransmitters recently associated with cancer immunotherapy. Notably, the review emphasizes that neurotransmitter signaling plays a substantial role in cancer immunotherapy as an emerging cancer treatment target by regulating targeted receptors and interfering with the tumor microenvironment.

Future Directions:

Future studies are required to uncover the antitumor mechanisms of neurotransmitter signaling to develop novel treatment strategies to overcome cancer immunotherapy resistance. Antioxid. Redox Signal. 38, 298–315.

Introduction

Cancer immunotherapy, notably CTLA-4 and PD-1/PD-L1 immune checkpoint blockades, has made breakthroughs in various cancers, including melanoma, lung, stomach, colorectal, and liver cancer (Tontonoz and Gee, 2015). Despite these advances, immunotherapy has still failed in many patients; for example, in non-small cell lung cancer (NSCLC) patients, even up to 80% of patients do not respond (Gandhi et al, 2018; Herbst et al, 2020; Reck et al, 2016). A series of independent reports showed notable differences in the composition of gut microbes from responders and nonresponders to anti-PD-1 inhibitors.

By fecal microbiota transplantation (FMT) from patients to germ-free (GF) mice, many beneficial bacteria were found to have a favorable clinical response to anti-PD-1 immunotherapy, including Akkermansia and Alistipes (Routy et al, 2018), Clostridiales and Ruminococcaceae (Gopalakrishnan et al, 2018), and Bifidobacterium longum, Enterococcus faecium, and Collinsella aerofaciens (Matson et al, 2018).

Moreover, our previous study found that a higher relative abundance of Bacteroides vulgatus and Parabacteroides distasonis points to a better response to anti-PD-1 immunotherapy (Huang and Wu, 2021). In addition, two phase I clinical trials proved that modulation gut microbiome could augment the efficacy and overcome resistance to anti-PD-1 blockades in advanced melanoma (Baruch et al, 2021; Davar et al, 2021). Nonetheless, the mechanisms in which microbes, their products, and/or their metabolites influence tumor immunity were still poorly understood.

Multiple studies have suggested that microbial metabolites potentiated the efficacy of immune checkpoint inhibitors (ICIs) (He et al, 2021; Luu et al, 2021; Mager et al, 2020). Gut microbiota can produce and/or metabolize a wide range of mammalian neurotransmitters, including dopamine (DA), norepinephrine (NE), serotonin (5-HT), or γ-aminobutyric acid (GABA) (Roshchina, 2016; Strandwitz, 2018). Manipulation of these neurotransmitters can not only regulate physiological function but also has been shown to affect the initiation or progression of diverse cancers, including colon (Sarkar et al, 2008), pancreatic (Renz et al, 2018), prostate (Zahalka et al, 2017), gastric (Chakroborty et al, 2004), breast (Sarkar et al, 2008), and ovarian cancers (Basu et al, 2001).

Monoamine oxidase A (MAO-A) knockout mice could activate tumor-infiltrating CD8⁺ T cells by boosting the autocrine 5-HT to enhance the antitumor effect (Wang et al, 2021b). DA, glutamate, neuropeptide Y (NPY), and calcitonin gene-related peptide (CGRP) at low physiological concentrations (∼10 nM) alone or combined could decrease the percentage of PD-1⁺ T cells and the expression of PD-1 on T cells, and increase the cytotoxicity of hepatocellular carcinoma cells (Levite et al, 2021).

Nevertheless, these studies reveal several gaps in knowledge, including whether manipulation of these microbial neurotransmitters/receptors can potentiate cancer immunotherapy. In this study, we outline the pro/antitumor effect of neurotransmitters/receptors and the influence of microbial neurotransmitters on immune cells and cancer cells, and attempt to put forward a new potential strategy in combination with cancer immunotherapy.

Neurotransmitters Affect Immune Cells to Promote/Control Tumor Progression

Neurotransmitters can be divided into four categories; that is, biogenic amines, amino acids, peptides, and others (Fig. 1). Biogenic amine neurotransmitters were the first to be discovered, including Epi, NE, DA (Kumar, 2003), and 5-HT (Valenstein, 2002). Neurotransmitters are not only released from the brain, peripheral nerve plexus, ganglia, and adrenal medulla by the autonomic nervous system but also can be produced by gut microbes. Specific pathogen-free (SPF) mouse intestines are predominant with free-type NE and DA, while GF mice are mainly glucurono-conjugated type.

FIG. 1.

Neurotransmitters source and classification. Neurotransmitters not only originate from the nervous system but also synthesized by tumors and immune cells. Likewise, tumor angiogenesis and their microenvironments are affected by neurotransmitters.

When colonized either a mixture of Clostridia species or complete SPF microbiota rich in β-glucuronidase (GUS) activity, the concentration of free NE and DA significantly increased, indicating that gut microbiota can regulate the production of NE and DA via GUS (Asano et al, 2012). 5-HT can also be directly produced by deconjugation via bacterial GUS and sulfatase (Sudo, 2019) (Fig. 2A).

FIG. 2.

Biosynthesis of catecholamines and regulation of cancer cells. (A) Gut microbiota can convert glucuronide- or sulfate-conjugated catecholamines into free catecholamines in vivo. (B) Biosynthesis of catecholamines. The biosynthesis of catecholamine neurotransmitters is based on the conversion of phenylalanine into L-tyrosine as a biosynthetic precursor. Tyrosine is converted into DOPA by tyrosine hydroxylase. DOPA is then processed by aromatic amino acid decarboxylase into DA. Among them, NE formation is catalyzed by DA-β-hydroxylase. Finally, Epi is synthesized by adding methyl groups to norepinephrine by phenylethanolamine-N-methyltransferase. (C) Mutual regulation of cancer cells and catecholamine neurotransmitters. Catecholamine neurotransmitters and their receptors, target cells, and the tumor microenvironment are subject to reciprocal regulation. Neurotransmitters such as Epi and NE can activate their receptors expressed on cancer and immune cells to promote tumorigenesis and tumor progression. DA, dopamine; DOPA, dihydroxyphenylalanine; NE, norepinephrine.

In addition to the direct modulation via microbial enzymes, gut microbiota can indirectly control neurotransmitters production. For example, nucleotide-binding oligomerization domain 1 (Nod1) ligand from gut microbes can be sensed by adrenal chromaffin cells and directly modulates epinephrine secretion (Xiang et al, 2021). Enterochromaffin cells (ECs) can sense nutrients and gut microbiota to secrete 5-HT (De Vadder et al, 2018). It has been proven that gut microbiota can mediate the interplay between metabolites and antitumor immunity. Thus, we summarize the microbial-related neurotransmitter signaling on cancer development and progression, and provide insights and perspectives regarding the rationales and strategies to enhance the efficacy of cancer immunotherapy by targeting neurotransmitter signaling.

Cancer-inhibiting effect of biogenic amine neurotransmitters

In recent years, neurotransmitters have become important microenvironmental components that regulate tissue homeostasis and affect a variety of malignant phenotypes of human cancers (Boilly et al, 2017; Jiang et al, 2020). Growing evidence supports that concentrations of biogenic amines are associated with cancer cell proliferation.

Epinephrine and NE signaling exhibit tumor suppression or cancer progression depending on the immune state

Mounting evidence supports the bidirectional role of Epi and NE in antitumor and protumor in different cancers. Pheochromocytoma is a chromaffin cell tumor induced by excessive production of catecholamines. Catecholamines and their metabolites in urine or plasma could assist in the diagnosis of pheochromocytoma (Lenders et al, 2002). Psychosocial stress, exercise, and receptor agonists and antagonists such as β-adrenergic receptor (β-AR) blockers affect the concentrations of catecholamines. Psychological stress can cause immune dysfunction, thus inducing tumorigenesis and progression (Schoemaker et al, 2016).

Chronic stress-induced immunosuppressive microenvironment promoted the proliferation and activation of myeloid-derived suppressor cells (MDSCs) via adrenergic signaling in a mouse breast tumor model (Mohammadpour et al, 2019). Environmental eustress via an enriched environment activated the antitumor immunity of CD8⁺ T cells through the sympathetic nervous system (SNS)/β-AR/CCL2 pathway and exerted a better response to PD-L1 immunotherapy (Liu et al, 2021a). Moderate- to high-intensity exercise can upregulate peripheral NE and Epi concentrations to control breast cancer development via the Hippo signaling pathway (Dethlefsen et al, 2017).

In addition, voluntary running was shown to inhibit tumor growth by activating NK cells via Epi and IL-6. In further support of the antitumor effect of Epi, daily intraperitoneal injection of low-dose (0.5 mg/kg) Epi to B16 tumor-bearing mice could mobilize NK cells and β-blockade to blunt tumor growth (Pedersen et al, 2016). In addition, retrospective epidemiological and preclinical studies reported that β-AR blockers could improve the survival of melanoma patients, and enhance the antitumor efficacy of anti-PD-1 inhibitors in mice (Kokolus et al, 2018).

DA inhibits tumor growth and enhances antitumor immunity

DA is a precursor for synthesizing Epi and NE. It is also another catecholamine neurotransmitter, and exerts neuroimmune communication through its receptors in immune cells (Pinoli et al, 2017) (Fig. 2B). DA could improve the chemotherapeutic efficacy for pancreatic cancer in vitro and in vivo by inhibiting the PKA/p38 signal pathway to suppress the polarization of M2 of tumor-associated macrophages (TAMs) (Liu et al, 2021b). In glioma, DA induced caspase-dependent apoptosis and p50/p65 NF-κB signaling pathway to inhibit tumor growth in vivo (Lan et al, 2017).

DA was also reported to inhibit vascular permeability factor (VPF)/vascular endothelial growth factor (VEGF)-induced angiogenesis and delayed tumor growth through D2 receptors in sarcoma tumor-bearing mouse model (Chakroborty et al, 2008). In addition, human T cells express all DA receptors. Dopamine 4 receptor (D4R) agonists exerted the best to activate and rejuvenate T cells. Furthermore, the low concentration of 10 nM DA could decrease PD-1 expression in T cells and induce the proliferation of T cells in vitro (Levite, 2021).

5-HT acts as an immunomodulator in most cancers

5-HT synthesized from the amino acid tryptophan in the brain and the ECs of the intestine is thought initially to increase vascular tone (Walther et al, 2003). 5-HT is absorbed from the bloodstream by platelets and released after activation (Mohammad-Zadeh et al, 2008; Schoenichen et al, 2019).

The protumorigenic effect of 5-HT has been previously shown in lung, bladder, and colon cancer (Cattaneo et al, 1993; Nocito et al, 2008; Siddiqui et al, 2006). For example, 5-HT promotes cell proliferation in multiple tumors through different 5-HT receptor (5-HTR) types (Jiang et al, 2020; Liu et al, 2019) (Fig. 2C). In addition, dysregulated 5-HT signaling has been experimentally observed in epithelial tumors (Sarrouilhe et al, 2015). 5-HT /HTR1E signaling also plays a vital role in preventing ovarian cancer progression promoted by chronic psychological stress (Qin et al, 2021) (Fig. 2C).

Most immune cells express 5-HTR on their cell membranes, as well as other serotonergic components, including tryptophan hydroxylase 1 (TPH1) serotonin transporter (SERT), monoamine oxidase (MAO) (Millan et al, 2008). Most of these 5-HTR affect the activation or inhibition of immune response (Ahern, 2011; Grimaldi et al, 1998). In the intestinal mucosa, 5-HT can act as a proinflammatory or anti-inflammatory tissue protection signal molecule by activating 5-HT₇ or 5-HT₄ receptors, respectively (Spohn and Mawe, 2017).

It was reported that depleting peripheral 5-HT could downregulate PD-L1 expression on cancer cells, elevate tumor-infiltrating CD8⁺ T cell accumulation, and delay tumor growth in pancreatic and colorectal cancer mouse models. In addition, the 5-HT inhibitors could augment the effects of anti-PD-L1 inhibitors and induce long-term tumor control (Schneider et al, 2021). Taken together, 5-HT is a key immunomodulation factor in most cancers.

Epi, NE, DA, and/or 5-HT are implicated in tumor growth and progression of multiple cancer types (Fig. 2C). Nevertheless, how the fluctuation of these neurotransmitters in the tumor microenvironment affects cancer progression and how to translate this knowledge into the clinical environment are worthy of further exploration. Further preclinical and clinical data may reveal the molecular mechanisms by which autonomic neurotransmitters regulate cancer at the system and microenvironment level.

Amino acid neurotransmitters in tumor immunity

Role of GABA in tumors

GABA promotes or inhibits tumor growth

GABA mainly acts as an inhibitory neurotransmitter in the mature central nervous system (Li and Xu, 2008). Studies have shown that GABA and its receptors not only exist in nerve tissues (Watanabe et al, 2002) but they also play an essential role in the development of tumors. GABA can promote tumor growth by inducing the polarization of M2 macrophages and limiting the antitumor effect of CD8⁺ T cells. In contrast, GABA can also exhibit antitumor activity via GABAB receptors (Fig. 3). In humans and animals, GABA receptors were identified on macrophages, dendritic cells (DCs), T cells, and B cells (Alam et al, 2006; Dionisio et al, 2011; Mendu et al, 2012; Tian et al, 2004). Modulation of GABA_A receptors has been found to regulate the polarization of macrophages (Januzi et al, 2018), the migration of DCs (Fuks et al, 2012), the secretion of various cytokines (Bhandage et al, 2018), and also the suppression of T cell proliferation (Sparrow et al, 2021; Tian et al, 2019; Tian et al, 1999).

FIG. 3.

Role of GABA neurotransmitters in tumors. GABA can promote tumor growth by inducing the polarization of M2 macrophages and limiting the antitumor effect of CD8⁺ T cells. In contrast, GABA can also exhibit antitumor activity via GABA_B receptors. GABA, γ-aminobutyric acid.

In pancreatic cancer, GABA can promote the proliferation of tumor cells by promoting the upregulation of GABA receptor π subunit (GABRP) expression (Takehara et al, 2007). B-cell–derived GABA limited the antitumor immunity of CD8⁺ T cells by stimulating the differentiation of IL10⁺ M2 macrophages through GABA_A receptors (Zhang et al, 2021). Moreover, increased GABA in tumor tissues points to a worse prognosis in NSCLC and colon cancer patients.

Further study in the mouse model showed that cancer-cell–derived GABA promoted tumor growth and suppressed the infiltration of CD8⁺ T cells through β-catenin signaling (Huang et al, 2022a). However, preclinical studies also found that exogenous supplements of GABA exerted an inhibitory effect on tumor growth and metastasis in colorectal, gastric, liver, and NSCLC cells (Al-Wadei et al, 2012; Joseph et al, 2002; Sun et al, 2003). So, GABA modulation may be an effective strategy to improve the response to cancer immunotherapy.

Glutamate stimulates proliferation and migration of tumor cells

Glutamate is the primary excitatory neurotransmitter, which mediates most of the excitatory interaction between different parts of the central nervous system. In addition to its crucial role in the nervous system, glutamate is involved in tumors' occurrence, development, invasion, and migration.

As one of the essential amino acids for protein biosynthesis and the precursor of several other amino acids, glutamate is required for normal and malignant cell growth. Because cancer cells have a higher metabolism than normal tissues due to uncontrolled growth, some genes related to the metabolic chain have been mutated somehow, making cancer cells more dependent on glutamate in various cancers (Yi et al, 2019). Abnormal glutamate signaling can lead to tumorigenesis and may be the mechanism that causes many cancers.

As cancer cells release excess glutamate, nearby cells either die or are transformed to participate in increased glutamatergic signaling by increasing glutamate receptor expression. In this way, cancer cells invade normal tissues and transform normal cell signals (Seidlitz et al, 2009). In glioblastoma, glutamate was increased in the tumor microenvironment. Downregulation of glutamate synergized the response to anti-PD-1 immunotherapy in the GL261 glioma tumor mouse model (Medikonda et al, 2021; Medikonda et al, 2020).

Neuropeptides promote tumor growth by modulating cancer immune microenvironment

Peptide neurotransmitters include but are not limited to substance P (SP), NPY, and CGRP. Apart from traditional roles, the tumor-promoting effects of neuropeptides, especially those of SP and NPY, have been widely reported (Ebrahimi et al, 2020; Garcia-Recio et al, 2013; Moody et al, 2015; Tilan and Kitlinska, 2016). For example, SP mediated by neurokinin-1 (NK-1) receptors is involved in the proliferation, angiogenesis, migration, and invasion of breast cancer (Ebrahimi et al, 2020). NPY is considered to be the growth-promoting and vascularization factor in various cancers, including prostate, breast, and neuroblastoma (Li et al, 2015; Tilan and Kitlinska, 2016). NPY was also reported to modulate immune cell trafficking and alter macrophage function (De la Fuente et al, 1993; Phan and Taylor, 2013; Zhou et al, 2013).

NE-treated Myc-CaP cells promoted the release of NPY, which resulted in the infiltration of myeloid cells and increased IL-6 of TAMs in the prostate tumor microenvironment (Cheng et al, 2019). In addition, the NPY5 receptor (NPY5R) in liver cancer was correlated with tumor growth and survival. NPY promoted the progression of hepatocellular carcinoma by NPY5R activation via TGF-β/NPY/NPY5R axis (Dietrich et al, 2020). Although the oncogenic mechanism of neuropeptides in regulating tumor progression still needs further investigation, the existing evidence strongly supports that neuropeptides inhibition and their receptor blockers may benefit cancer patients.

Gut Microbiota Release Functional Neurotransmitters via Microbial Metabolism

Accumulating evidence supports the notion that gut microbiota potentiates the effectiveness of cancer immunotherapy, especially ICI therapy, mainly through the activation of tumor-infiltrating CD8⁺ T cells (Gopalakrishnan et al, 2019; Matson et al, 2018; Routy et al, 2018). Microbiota-derived metabolites (induced or produced by the microbiota) play a crucial role in the interactions between gut microbiota and the immune system by modulating antitumor immunity and facilitating the efficacy of cancer immunotherapy. Microbial butyric acid was demonstrated to boost the antitumor effect by enhancing CD8⁺ T cell immunity in an ID2-dependent manner by IL-12 signaling. Bifidobacterium pseudolongum-derived inosine enhanced the efficacy of immune checkpoint blockade therapy by activating cDC1-dependent Th1 effector function.

In mice monocolonized with B. pseudolongum, treatment with ICIs dramatically elevated inosine concentration in the serum by eight- to nine fold compared with GF mice or mice colonized with bacteria that did not produce inosine (26.16 ± 3.32 μM and 37.5 ± 10.2 μM vs. 3.26 ± 1.01 μM, and 4.8 ± 1.3 μM) (Mager et al, 2020). These findings support the notion that microbial metabolism influences cancer development and response to cancer immunotherapy. Certain gut microbes can produce enzymes to synthesize neurotransmitters and their precursors.

These microbial neurotransmitters can modulate host homeostasis extending beyond the gastrointestinal tract to the entire body, such as the brain, via complex neuronal, immunological, and humoral signaling cascades (Cryan et al, 2019; Fischbach and Segre, 2016). Gut microbiota can also synthesize gaseous neurotransmitters nitric oxide (NO) and hydrogen sulfide (H₂S) (Luhachack and Nudler, 2014). Although mechanistic relationships between microbial metabolites and tumor development remain largely unexplored, it may be envisioned that neurotransmitters have the potential to be a new target for cancer immunotherapy (Table 1 and Fig. 4).

FIG. 4.

Communications between microbial neurotransmitters, immune cells, and cancer cells. Neurotransmitter-associated bacteria, including Faecalibacterium prausnitzii, Roseburia hominis, Akkermansia muciniphila, Enterococcus faecium, Bacteroides thetaiotaomicron, Bacteroides fragilis, Bacteroides vulgatus, Parabacteroides distasonis, Lactobacillus rhamnosus GG, and Lactobacillus reuteri, mainly elicit their antitumor efficacy by mediating the augmentation of T cells, the polarization of TAMs from M2 to M1, inhibition of tumor-infiltrating MDSCs, and augmentation of NK cells. MDSCs, myeloid-derived suppressor cells; TAMs, tumor-associated macrophages.

Table 1.

Interactions Between Neurotransmitters and Immune Checkpoint Inhibitors Mediated by Gut Microbiota

Neurotransmitter	Gut microbes	Immune checkpoint	References
Epinephrine and norepinephrine	Faecalibacterium prausnitzii; Akkermansia muciniphila	PD-1	(Gopalakrishnan et al, 2018, Routy et al, 2018)
DA	Enterococcus faecalis; Enterococcus faecium	PD-1; PD-L1	(Matson et al, 2018, Wang et al, 2021)
Serotonin	F. prausnitzii	PD-1	(Gopalakrishnan et al, 2018)
GABA	Bacteroides thetaiotaomicron; Bacteroides fragilis	CTLA-4	(Vetizou et al, 2015, Strandwitz et al, 2019)
	Lactobacillus rhamnosus GG
	A. muciniphila	PD-1	(Dooling and Costa-Mattioli, 2018)
	Pacrabacteroides merdae

DA, dopamine; GABA, γ-aminobutyric acid.

Microbial monoamine neurotransmitters in cancer immunotherapy

Epinephrine and NE

GUS-encoded bacteria can produce biologically active, free forms of NE, Epi, and DA in the gut lumen of mice by comparing the findings among GF mice, SPF mice, and gnotobiotic mice (Asano et al, 2012). In addition, exercise can increase the production of Epi and NE. Exercise could mobilize NK cells via increasing Epi to suppress tumor growth (Pedersen et al, 2016). Other studies reported that >3 h of exercise per week could enrich the abundance of Faecalibacterium prausnitzii, Roseburia hominis, and Akkermansia muciniphila (Bressa et al, 2017).

Interestingly, in patients treated with anti-PD-1 immunotherapy, responders have a relatively high abundance of these bacteria, enhancing the antitumor effect of cytotoxic T cells (Davar et al, 2021; Gopalakrishnan et al, 2018; Routy et al, 2018). Although the direct interactions between these beneficial bacteria and Epi are still unclear, it is interesting to explore whether they can modulate Epi to enhance the antitumor efficacy of anti-PD-1 immunotherapy. Whether the antitumor effect of these bacteria is dependent on Epi/NE remains to be investigated.

Dopamine

DA serves as a precursor of Epi and NE. It was found to remodel tumor blood vessels and improve the delivery of 5-fluorouracil, doubling the amount of the drug in HT29 tumors and increasing its efficacy in chemotherapy (Chakroborty et al, 2011). Peripheral DA exerts antitumor effects via the interactions of DA and immunity, including the activation of CD8⁺ T cells (Levite, 2016), altering the polarization state of TAMs from M2 to M1 (Qin et al, 2015), inhibiting tumor-infiltrating MDSCs (Hoeppner et al, 2015), and augmenting cytotoxicity of NK cells (Zhao et al, 2013).

Recent evidence suggests that >50% of the peripheral DA has an intestinal source. Tyrosine is the biosynthetic precursor of all three catecholamines. The first step of the reaction pathway is catalyzed by tyrosine hydroxylase (TH) and is rate limiting. This reaction requires oxygen as a cosubstrate and tetrahydrobiopterin as a cofactor to synthesize dihydroxyphenylalanine (DOPA) (Molinoff and Axelrod, 1971). Then, DOPA can be converted into DA by the enzyme dopa decarboxylase (DDC).

Interestingly, gut microbes, such as Enterococcus faecalis (E. faecalis) and Enterococcus faecium (E. faecium), also exhibit TH and DDC activity, indicating the capability to synthesize DA (Wang et al, 2021a). Oral E. faecalis or E. faecium was demonstrated to elevate brain DA levels in mice by converting L-dopa in the gut and blood (Wang et al, 2021a). DA at 10⁻⁵ and 10⁻⁷ M could downregulate regulatory T (Treg) cell activity, and increase the proliferation of effector T (T_eff) cells through DA receptors (D1-R and D5-R) via the ERK signaling pathway (Kipnis et al, 2004).

In a melanoma mouse model, mice monocolonized with E. faecium exhibited the capability to decrease Treg cells and augment Th1 responses to improve tumor control of anti-PD-L1 therapy (Routy et al, 2018). In a pioneering phase I trial of anti-PD-1 immunotherapy by fecal microbiota transplant from responder patients to nonresponders, gut microbiota composition in responders had a higher representation of Enterococcus spp. (Baruch et al, 2021). However, the detailed mechanism is still elusive, and DA may be a potential target for E. faecium to overcome the anti-PD-L1 immunotherapy by activating antitumor immunity.

Elevated 5-HT promotes tumor immune escape

The various effects of 5-HT signaling on immune cells make it a target for studies of tumor immunity. For example, macrophages express 5-HTR_2B, and 5-HTR₇ signaling stimulates the polarization of M2 macrophages, which promotes tumor development (Nieto et al, 2020; Nocito et al, 2008). 5-HT signaling also promotes activation of Treg cells via suppression of antitumor immunity (Karmakar and Lal, 2021; O'Connell et al, 2006). In the intestinal tract, ∼90% of 5-HT is synthesized by ECs, 50% of which is regulated by gut microbiota. Microbial metabolites, such as SCFAs or secondary bile acids, can act on ECs to induce the transcription of the rate-limiting biosynthetic enzyme, TPH1 of 5-HT.

The effect of gut microbiota on 5-HT production was investigated by measuring peripheral 5-HT levels in conventional mice with normal gut microbiota and GF mice. GF mice have lower 5-HT levels in serum and colon than conventional controls (Yano et al, 2015). 5-HT can act directly or indirectly on the immune system, shaping the microbiota composition and localization. 5-HT can also control gut motility, secretory reflexes, platelet aggregation, bone development, and cardiac function to regulate immune responses.

In mouse models, F. prausnitzii and its supernatant can also modulate intestinal 5-HT levels to protect intestinal integrity (Martin et al, 2015). F. prausnitzii was reported as beneficial bacteria, which was observed to be highly enriched in responders to anti-PD-1 immunotherapy and correlated with higher infiltration of CD8⁺ T cells in tumors. When performed FMT from responders to the melanoma mouse model, mice with a higher abundance of F. prausnitzii exhibited an improved response to anti-PD-1 immunotherapy (Gopalakrishnan et al, 2018).

Besides, another 5-HT-producing bacterium, Escherichia coli (K-12), was reported to upregulate the expression of PD-L1 in IFN-γ–sensitized HT-29 cells via the NF-κB pathway, which may lead to immune escape (Lee et al, 2020). Collectively, all the evidence indicates that regulating the level of 5-HT by gut microbiota may have an essential role in regulating PD-L1 expression in tumor cells, thus sensitizing response to ICIs.

Microbial GABA signaling augments antitumor efficacy

L-Glutamate and GABA are not only known for their roles as the primary excitatory and inhibitory neurotransmitters in the mammalian central nervous system, respectively. A growing body of evidence suggests that glutamate signaling promotes cancer cell initiation, proliferation, and metastasis through their receptors (Luksch et al, 2011; Stepulak et al, 2014). The GABAergic system may develop an antitumor activity by suppressing cancer cells in the inflammatory response (Jiang et al, 2020). For example, GABA receptor GABRP interacts with KCNN4 inducing an influx of Ca²⁺ and activating NF-κB signaling. This facilitates macrophage infiltration and tumor progression in pancreatic cancer via the recruitment of CXCL5 and CCL20 in pancreatic cancer (Jiang et al, 2019).

Lower glutamate correlates with a better response to anti-PD-1 immunotherapy in glioblastoma in vivo (Medikonda et al, 2020). As a matter of particular interest, some Bacteroides species can convert glutamate into GABA, which points to a better response to anti-PD-1/CTLA-4 immunotherapy. For instance, Bacteroides thetaiotaomicron and Bacteroides fragilis, the GABA-producing commensals, facilitated the antitumor efficacy of CTLA-4 blockade via the IL-12–dependent Th1 immune response (Vetizou et al, 2015). B. vulgatus and P. distasonis can produce GABA in vitro culture (Strandwitz et al, 2019). Both strains were associated with a better response to anti-PD-1 immunotherapy in NSCLC patients (Huang et al, 2022b).

In addition, Lactobacillus could convert glutamate into GABA (Li and Cao, 2010). Oral administration of live Lactobacillus rhamnosus GG augmented the antitumor activity of anti-PD-1 immunotherapy by increasing tumor-infiltrating DCs and T cells (Si et al, 2022). Lactobacillus reuteri has been reported to alleviate colitis associated with immune checkpoint blockade by inhibiting group 3 innate lymphoid cells (ILC3s) (Wang et al, 2019). A. muciniphila and Pacrabacteroides merdae were found to increase the ratio of GABA/glutamate (Dooling and Costa-Mattioli, 2018).

Notably, previous studies suggested that A. muciniphila could potentiate the response to PD-1 immunotherapy with melanoma, NSCLC, and renal cell carcinoma in an IL-12–dependent manner by recruiting CCR9⁺CXCR3⁺CD4⁺ T cells into tumor sites (Routy et al, 2018). Altogether, these results hint that A. muciniphila may elicit a synergistic antitumor effect via GABA signaling.

However, there is still no study to examine the direct interaction mechanisms between microbial neurotransmitters and cancer immunotherapy. Further research is needed to identify the mode of action of microbial neurotransmitters.

Neurotransmitters Signaling or Receptors as Emerging Targets for Cancer Therapy

A variety of neurotransmitter receptors, expressed on nerve fibers or tumor-infiltrating immune cells, are critical for tumorigenesis. Thus, antagonists or agonists of their receptors can be scrutinized for their use in cancer therapy (Jiang et al, 2020) (Fig. 5).

FIG. 5.

Microbial neurotransmitters as emerging targets for cancer therapy. (A) Neurotransmitters exert pro-tumor or tumor suppressing effect depending on the activation or blocking of the corresponding receptor. Epi and NE can promote tumor growth by inducing MDSCs and TAMs via β2 adrenergic receptors. Blocking β2 adrenergic receptors can inhibit the tumor growth and augment the antitumor immunity of CD8⁺ T cells. 5-HTR_2B and 5-HTR₇ signaling can promote tumor development through the polarization of M2 macrophages. DA can activate CD8⁺ T cells and suppress Tregs to control tumor growth via the D2 receptor. (B) Small molecule microbial neurotransmitters and their analogs are implicated at all stages of cancer. With the participation of their targeted receptors, their rational utilization may be an effective strategy for cancer treatment. As examples, small molecules that have shown efficacy in cancer treatment are presented. Tregs, regulatory T cells.

Early studies have linked gut microbes with neurobehavioral phenotypes and proposed the concept of the “microbe–gut–brain” axis (Cryan et al, 2019). Mounting evidence supports that certain microorganisms can synthesize and/or regulate various neurochemical substances to influence neurotransmission. They can also synthesize and/or regulate other environmental metabolites to directly or indirectly affect neuronal activity (Jameson et al, 2020; Schledwitz et al, 2021) (Figs. 4 and 5). Thus, the role of microorganisms in regulating neuronal communication through the gut–brain axis is of great significance and has been actively elucidated as a novel therapeutic target for cancer treatment.

Neurotransmitters and their analogs for cancer therapy

The discovery of microbial synthesis of neurotransmitters recapitulates the first discovery of chemical transmitters in the early 20th century (Jameson et al, 2020; Valenstein, 2002). GABA, NE, DA, and 5-HT are also produced by cultured bacteria (Huang and Wu, 2021; Strandwitz, 2018). Recent studies have identified GABA as a potential tumor suppressor for small airway-derived lung adenocarcinoma (Schuller et al, 2008b).

Likewise, DA treatment controls tumor growth by inhibiting angiogenesis (Chakroborty et al, 2004) or regulating diverse immunocompetent cells within the tumor microenvironment (Nasi et al, 2019) (Table 2). The role of small molecule neurotransmitters in tumors is detailed above. Given a potential link between neurotransmitters and antitumor immune response, we next summarized the current development of neurotransmitter receptors in cancer treatment.

Table 2.

Microbial-Derived Neurotransmitters and Small Molecules Targeting Neurotransmitter Receptors for Cancer Therapy

Small molecules	Functions	Models	References
GABA	A potential tumor suppressor	Lung adenocarcinoma	(Schuller et al, 2008b)
DA	Suppressing the growth of cancer cells by inhibiting angiogenesis	Gastric cancer	(Chakroborty et al, 2004)
	Inhibiting human CD8⁺ Treg function through D1-like DAR	Immune-competent cells	(Nasi et al, 2019)
Propranolol	Reducing the incidence of cancer	Prostate cancer patients with high-risk or metastatic disease	(Grytli et al, 2014, Santala et al, 2019) (Li et al, 2015)
GABA or baclofen	Repressing isoproterenol-induced cAMP signaling below base levels	Pancreatic cancer	(Schuller et al, 2008a)
Paliperidone, risperidone, and pimozide	Curbing the serotonin receptor-7	Glioblastoma	(Kast, 2010)
Cyproheptadine	Inhibiting the mTOR and β-catenin signaling pathways		(Hsieh et al, 2016)
SB204741	Antagonizing HTR-2B and inhibiting the Warburg effect	Pancreatic cancer	(Jiang et al, 2017)

Treg, regulatory T cell.

Targeting neurotransmitter receptors for cancer treatment

Targeting β-AR

Adrenergic receptors α or β (AR-α or β) are widely expressed in most mammalian tissues and cells. β-ARs are expressed in T suppressor lymphocytes, NK cells, B lymphocytes, DCs, and macrophages (Sarkar et al, 2013), which mediate the suppression of antitumor immune response and regulate immune cells within the tumor microenvironment.

At present, epidemiological studies generally support the notion that cardiovascular patients treated with β-AR antagonists, such as propranolol, have a reduced cancer incidence (Grytli et al, 2014; Santala et al, 2019) (Table 2). Preclinical studies revealed that cold-induced stress is sufficient to elevate NE. Propranolol could reduce tumor growth by increasing the secretion of functional cytokines of tumor-infiltrating CD8⁺ T cells and downregulating the expression of PD-1 on CD8⁺ T cells in B16-OVA and 4T1 tumor-bearing mouse models.

Furthermore, combination treatment of propranolol and anti-PD-1 inhibitor significantly delayed 4T1 tumor growth by increasing the level of IFN-γ⁺ CD8⁺ T cells (Bucsek et al, 2017). In addition, a retrospective analysis revealed that metastatic melanoma patients who received β-AR blockers prolonged overall survival after at least one immunotherapy (IL-2, anti-CTLA-4, and anti-PD-1).

Moreover, the in vivo results further supported the notion that combining β-AR blockers with anti-PD-1 or anti-PD-1/IL-2 could improve tumor control and prolonged survival in the melanoma mouse model (Kokolus et al, 2018). The prospective phase I clinical trial of propranolol and anti-PD-1 ICIs in metastatic melanoma patients provided preliminary evidence of antitumor activity based on safety and tolerability (Gandhi et al, 2021).

Collectively, these data implicate that targeting NE/β-AR signaling is an attractive strategy to improve the efficacy of anti-PD-1 immunotherapy. Still, further data are required experimentally and clinically to classify the potential mechanisms.

Targeting DA receptor

DA exerts its function by binding to D1-like (D1 and D5) DA receptors and D2-like (D2, D3, and D4) receptors. Most immune cells, including monocytes, NK cells, macrophages, DCs, neutrophils, B cells, effector and Treg cells, express DARs (Zhang et al, 2017). DA could directly activate T_eff cells at the physiological concentration of ∼10 nM by binding to D2-like receptors or suppress Treg cells via D1-like receptors.

In addition, DA can selectively induce the adhesion, migration, and homing of CD8⁺ T cells (Levite, 2016). DAR agonists seem to exert inhibitory effects on the growth of various tumors, including gastric cancer, breast cancer, and sarcoma (Roney and Park, 2018). However, whether this anticancer effect is associated with the enhancement of immunity is unclear, and it may provide broad implications for the modulation of DA signaling for cancer immunotherapy.

Targeting 5-HTR

5-HT performs multiple functions by interacting with its receptors. To date, 15 different receptor subtypes have been identified to modulate a wide range of functions on stromal, cancer, and immune cells. The importance of the 5-HT signaling in tumors and the immune system has been well studied by different agonists and antagonists of 5-HTR.

A recent study found that pancreatic cancer cells increase the expression of receptor 5-HT_2B to allow glycolysis in tumors under metabolic pressure, thereby promoting tumor growth (Jiang et al, 2017). Likewise, selective inhibition of the 5-HT_2B receptor can inhibit tumor angiogenesis by inhibiting endothelial NO synthase and p-ERK1/2 (Asada et al, 2009). Selective inhibition of 5-HT reuptake has been shown to induce anticancer effects by affecting cytokine secretion in mouse models of melanoma (Grygier et al, 2013) (Table 2). The 5-HTR_5A agonist, valerenic acid, could attenuate glioblastoma cell growth in vitro and in vivo by increasing innate immune signals, including elevating intracellular ROS and activating AMPK signaling (Lu et al, 2020).

The 5-HTR₇ inhibitors paliperidone, risperidone, and pimozide are also being studied to treat glioblastoma (Kast, 2010). Cyproheptadine, a 5-HT antagonist, can inhibit the proliferation of urothelial cancer cells by inhibiting mTOR and β-catenin signaling pathways (Hsieh et al, 2016). In addition, SB204741, a recent specific antagonist of 5-HTR_2B, has shown a significant inhibitory effect on tumor growth of pancreatic cancer by inhibiting the Warburg effect (Jiang et al, 2017).

In addition, 5-HTR_2B and 5-HTR₇ signaling have been reported to promote tumor development through the polarization of M2 macrophages (de las Casas-Engel et al, 2013). 5-HT could boost the expression of PD-L1 in the cancer cell to help the immune escape. Using 5-HT synthesis inhibitors efficiently slowed tumor growth in colon and pancreatic cancers in vivo. And when combined with anti-PD-L1, the tumors were even eliminated (Schneider et al, 2021). Hence, 5-HT signaling is a promising target for cancer immunotherapy.

Targeting GABA receptor

Many tumor tissues have identified ionotropic GABA_A, GABA_C receptors, and metabotropic GABA_B receptors (Kanbara et al, 2018; Sung et al, 2017). These GABA receptors play an essential role in cancer cell proliferation and migration. It is worth noting that GABA_A receptors are upregulated in breast, prostate, and pancreatic cancer (Gumireddy et al, 2016; Jiang et al, 2020), while GABA_B receptors are downregulated in pancreatic and liver cancers (Schuller, 2018).

Interestingly, in most cases, GABA stimulates the proliferation of cancer cells through the GABA_A receptor pathway, whereas it inhibits the growth of cancer cells through the GABA_B receptor (Jiang et al, 2020; Zhang et al, 2014). Studies have found that GABA_A receptor agonists increase the proliferation of gastric and pancreatic cancer cells by activating the MAPK signaling pathway (Takehara et al, 2007).

In contrast, activation of GABA_B receptors inhibits isoproterenol-induced phosphorylation of ERK1/2 to prevent DNA synthesis and cell migration effectively (Schuller et al, 2008b). Moreover, the activation of GABA_B receptors by GABA or baclofen can serve as potential targets for cancer treatment (Nieto et al, 2022; Schuller et al, 2008a) (Table 2). Thus, the specific type of cancer or GABA receptor type may lead to different effects of GABA activation on cancer growth/migration.

In addition, since GABA exists in the tumor microenvironment, it may increase the possibility that GABA can regulate the inflammatory response by targeting infiltrating immune cells (Jiang et al, 2020). It may also affect various functional properties of immune cells. The π subunit of the GABA_A receptor promotes the progression of pancreatic cancer by regulating KCNN4-mediated Ca²⁺ in a GABA-independent manner (Jiang et al, 2019). GABRP, a subunit of GABA_A, can upregulate the expression of CXCL5 and CCL20 to regulate the recruitment of macrophages in pancreatic cancer (Jiang et al, 2019).

A recent study revealed that B cell-secreted GABA limited the antitumor immune response and controlled the polarization of tumor-infiltration macrophages via IL-10 in the MC38 CRC mouse model. When blocked GABA_A receptor or reduced GABA, the tumor growth was controlled with enhanced cytotoxic CD8⁺ T cells in the tumor microenvironment (Zhang et al, 2021). Altogether, this evidence provides a rationale for manipulating GABA signaling to enhance the efficacy of cancer immunotherapy.

Glutamate receptors play a vital role in the biological effects of glutamate. Extracellular glutamate binds to α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPAR) and activates signaling pathways, such as KRAS-MAPK and PI3K, to promote cell proliferation, survival, and migration (Yi et al, 2019). In this way, glutamate can increase the invasion and migration of pancreatic cancer cells (Herner et al, 2011).

Recently, another type of glutamate receptor, N-methyl-D-aspartate (NMDA), has also been shown to activate neural signaling pathways and affect breast cancer brain metastasis (Zeng et al, 2019). The breast-to-brain metastasis (B2BM) activated by NMDA plays a crucial role in spreading metastasis via neuronal signaling pathways (Arvanitis et al, 2020; Zeng et al, 2019). The effects of glutamate on cancer cells are collectively mediated through glutamate metabolism and glutamate receptors.

Current Challenges and Future Directions

Cancer results from a combination of genetic and environmental factors (Hanahan and Weinberg, 2011); it is not surprising that intestinal microbial neurotransmitters play essential roles in cancer occurrence, development, and treatment. While the evidence for interaction between these regulatory elements is overwhelming, the question of how the signals conveyed by neurotransmitters in the gut reach their targets in tumors is still a mystery. Exceptions are colon and other intestine cancers, for which direct interaction between neurotransmitters and cancer cells is possible.

The enteric nervous system (ENS) is a complex network involving ∼500 million neurons (Furness, 2012). It is often referred to as the “second brain.” The ENS can function autonomously (Travagli et al, 2003), but it interacts with the brain and the autonomic nervous system via the vagus nerve (Cryan et al, 2019). Some neurons are well positioned to directly sense neurotransmitters and neuromodulators of microbial origin, such as 5-HT (Li et al, 2012; Watanabe et al, 2002).

Besides, bacterial ion channels have functional roles in information signaling and processing, which needs membrane potential (V_mem) to provide free energy across their membranes. For example, potassium ion channel YugO mediates electrical signaling within a Bacillus subtilis biofilm (Prindle et al, 2015). This ion channel mediates bacterial communication through glutamate metabolism within a biofilm (Liu et al, 2015). These studies suggest that membrane potential could be dynamic and mediate signaling like neurons (Benarroch and Asally, 2020).

Also, it is well established that the brain and the peripheral nervous system strongly influence the immune system and its role in the tumor microenvironment (Bahrambeigi et al, 2019; Chen et al, 2021; He et al, 2021; Karmakar and Lal, 2021; Levite, 2016; Liu et al, 2019; Nasi et al, 2019; Pinoli et al, 2017; Sarkar et al, 2013). It closes the link between signals originating in the gut and those acting on cancers and their microenvironment.

A robust multidisciplinary approach will be required to better understand the communications among gut microbiota, neurotransmitters, and antitumor immunity (Fig. 6). For example, functional metagenomics can help reveal potential mechanisms of host–microbial interactions by identifying effector molecules and functionally essential genes in the human microbiome (Cohen et al, 2015). To further define the role of microbiota-encoded effectors in host–microbial interactions, synthetic biology (such as CRISPR-Cas9 genome editing or in situ microbiome engineering) and bioinformatic analysis of gut microbiome sequencing data were of great significance (Cohen et al, 2017).

FIG. 6.

Mechanism of action of modified gut bacteria as anticancer drugs. Through genetic editing, microbiome engineering, synthetic biology, and other methods, gut bacteria are modified to increase or decrease neurotransmitter production. Further modifications may be necessary for such agents to reach the tumor microenvironment for the purpose of tumor treatment.

Recently, CRISPR-Cas9 genome editing has been applied to Lactobacillus plantarum, Clostridium beijerinckii, and Clostridium difficile (Leenay et al, 2018; Li et al, 2016; Wang et al, 2015). An engineered bacterium was designed to specifically lyse within the tumor microenvironment and activate tumor-infiltrating T cells to reduce tumor growth in a syngeneic tumor-bearing mouse model (Chowdhury et al, 2019), providing proof of concept for engineered bacterial immunotherapy.

In addition, in situ microbiome engineering can be used to manipulate the gut microbiota to generate specific neurotransmitters instead of culturing a specific strain and modifying it in the laboratory (Sheth et al, 2016). Collectively, synthetic biology tools combined with modulation of bacteria's electrical response by neurotransmitters may provide a new approach to modulate bacterial functions such as metabolic reactions, signaling, and gene expression, regarding its capacity to construct new signaling and metabolic pathways or redesign pathways in biological systems (Selberg et al, 2018).

Moreover, the antitumor effect of the modified microbes can be further elucidated by FMT in vivo (Sheth et al, 2016). Besides, culture-independent metagenomic approaches empower to reveal the potential implications for human health. However, <17% of species of human gut microbiota have been cultured (Hugon et al, 2015). Thus, culturomics was developed to culture, identify, and extend our understanding of unknown bacteria.

Although we have taken significant steps in gut microbiota genetic modification, biological and ecological knowledge gaps still need to be filled, which prevent its immediate application. However, it is challenging to evaluate these neurotransmitter-regulatory strategies to modulate native ion-channel circuits and different signaling pathways. The current lack of datasets for physiomic profiling is one of the significant barriers. Nevertheless, such innovative research should be promoted by encouraging crossdisciplinary collaborations between biophysicists, biochemists, and clinicians.

Footnotes

Authors' Contributions

E.L.-H.L. and J.H. designed and conceived the review. E.L.-H.L., J.H., Junmin Zhang, Juanhong Zhang, L.M., and H.Y. wrote the article. M.W., Y.Z., Z.M., E.N., X.Y., and E.L.-H.L. revised, extended, and provided final proof of the article. All authors reviewed the article.

Author Disclosure Statement

No financial or commercial conflict of interest was disclosed.

Funding Information

This work was funded by Dr. Neher's Biophysics Laboratory for Innovative Drug Discovery (Grant No. 001/2020/ALC) and regular grants (File No. 0096/2018/A3 & 0111/2020/A3, 0015/2019/AMJ, and 0114/2020/A3) supported by Macao Science and Technology Development Fund, Macau (SAR) China, 2020 Young Qihuang Scholar funded by National Administration of Traditional Chinese Medicine, the National Natural Science Foundation of China (82003779 and 82001995), the Natural Science Foundation of Gansu Province (20JR10RA010), the Macao Young Scholars Program (AM201926 and AM2020018), FDCT Funding Scheme for Postdoctoral Researchers of Higher Education Institutions (0017/2021/APD), and the Science, Technology and Innovation Commission of Shenzhen Municipality (ZDSYS20190902093601675). This work is also financially supported by the Start-up Research Grant of University of Macau (SRG2022-00020-FHS) and the Faculty of Health Science, University of Macau.

Abbreviations Used

References

Ahern

. 5-HT and the immune system. Curr Opin Pharmacol, 2011; 11(1):29–33; doi: 10.1016/j.coph.2011.02.004

Al-Wadei

, Plummer

, 3rd, Ullah

, et al. Social stress promotes and gamma-aminobutyric acid inhibits tumor growth in mouse models of non-small cell lung cancer. Cancer Prev Res (Phila), 2012; 5(2):189–196; doi: 10.1158/1940-6207.CAPR-11-0177

Alam

, Laughton

, Walding

, et al. Human peripheral blood mononuclear cells express GABAA receptor subunits. Mol Immunol, 2006; 43(9):1432–1442; doi: 10.1016/j.molimm.2005.07.025

Arvanitis

, Ferraro

, Jain

. The blood-brain barrier and blood-tumour barrier in brain tumours and metastases. Nat Rev Cancer, 2020; 20(1):26–41; doi: 10.1038/s41568-019-0205-x

Asada

, Ebihara

, Yamanda

, et al. Depletion of serotonin and selective inhibition of 2B receptor suppressed tumor angiogenesis by inhibiting endothelial nitric oxide synthase and extracellular signal-regulated kinase 1/2 phosphorylation. Neoplasia (New York, NY), 2009; 11(4):408–417; doi: 10.1593/neo.81630

Asano

, Hiramoto

, Nishino

, et al. Critical role of gut microbiota in the production of biologically active, free catecholamines in the gut lumen of mice. Am J Physiol Gastrointest Liver Physiol, 2012; 303(11):G1288–G1295; doi: 10.1152/ajpgi.00341.2012

Bahrambeigi

, Sanajou

, Shafiei-Irannejad

. Major fundamental factors hindering immune system in defense against tumor cells: The link between insufficiency of innate immune responses, metabolism, and neurotransmitters with effector immune cells disability. Immunol Lett, 2019; 212:81–87; doi: 10.1016/j.imlet.2019.06.008

Baruch

, Youngster

, Ben-Betzalel

, et al. Fecal microbiota transplant promotes response in immunotherapy-refractory melanoma patients. Science, 2021; 371(6529):602–609; doi: 10.1126/science.abb5920

Basu

, Nagy

, Pal

, et al. The neurotransmitter dopamine inhibits angiogenesis induced by vascular permeability factor/vascular endothelial growth factor. Nat Med, 2001; 7(5):569–574; doi: 10.1038/87895

10.

Benarroch

, Asally

. The microbiologist's guide to membrane potential dynamics. Trends Microbiol, 2020; 28(4):304–314; doi: 10.1016/j.tim.2019.12.008

11.

Bhandage

, Jin

, Korol

, et al. GABA regulates release of inflammatory cytokines from peripheral blood mononuclear cells and CD4(+) T cells and is immunosuppressive in type 1 diabetes. EBioMedicine, 2018; 30:283–294; doi: 10.1016/j.ebiom.2018.03.019

12.

Boilly

, Faulkner

, Jobling

, et al. Nerve dependence: From regeneration to cancer. Cancer Cell, 2017; 31(3):342–354; doi: 10.1016/j.ccell.2017.02.005

13.

Bressa

, Bailen-Andrino

, Perez-Santiago

, et al. Differences in gut microbiota profile between women with active lifestyle and sedentary women. PLoS One, 2017; 12(2):e0171352; doi: 10.1371/journal.pone.0171352

14.

Bucsek

, Qiao

, MacDonald

, et al. beta-Adrenergic signaling in mice housed at standard temperatures suppresses an effector phenotype in CD8(+) T cells and undermines checkpoint inhibitor therapy. Cancer Res, 2017; 77(20):5639–5651; doi: 10.1158/0008-5472.CAN-17-0546

15.

Cattaneo

, Codignola

, Vicentini

, et al. Nicotine stimulates a serotonergic autocrine loop in human small-cell lung carcinoma. Cancer Res, 1993; 53(22):5566–5568.

16.

Chakroborty

, Chowdhury

, Sarkar

, et al. Dopamine regulates endothelial progenitor cell mobilization from mouse bone marrow in tumor vascularization. J Clin Invest, 2008; 118(4):1380–1389; doi: 10.1172/JCI33125

17.

Chakroborty

, Sarkar

, Mitra

, et al. Depleted dopamine in gastric cancer tissues: Dopamine treatment retards growth of gastric cancer by inhibiting angiogenesis. Clin Cancer Res, 2004; 10(13):4349–4356; doi: 10.1158/1078-0432.CCR-04-0059

18.

Chakroborty

, Sarkar

, Yu

, et al. Dopamine stabilizes tumor blood vessels by up-regulating angiopoietin 1 expression in pericytes and Kruppel-like factor-2 expression in tumor endothelial cells. Proc Natl Acad Sci U S A, 2011; 108(51):20730–20735; doi: 10.1073/pnas.1108696108

19.

Chen

C-S

, Barnoud

, Scheiermann

. Peripheral neurotransmitters in the immune system. Curr Opin Physiol, 2021; 19:73–79; doi: https://doi.org/10.1016/j.cophys.2020.09.009

20.

Cheng

, Tang

, Li

, et al. Depression-induced neuropeptide Y secretion promotes prostate cancer growth by recruiting myeloid cells. Clin Cancer Res, 2019; 25(8):2621–2632; doi: 10.1158/1078-0432.CCR-18-2912

21.

Chowdhury

, Castro

, Coker

, et al. Programmable bacteria induce durable tumor regression and systemic antitumor immunity. Nat Med, 2019; 25(7):1057–1063; doi: 10.1038/s41591-019-0498-z

22.

Cohen

, Esterhazy

, Kim

, et al. Commensal bacteria make GPCR ligands that mimic human signalling molecules. Nature, 2017; 549(7670):48–53; doi: 10.1038/nature23874

23.

Cohen

, Kang

, Chu

, et al. Functional metagenomic discovery of bacterial effectors in the human microbiome and isolation of commendamide, a GPCR G2A/132 agonist. Proc Natl Acad Sci U S A, 2015; 112(35):E4825–E4834; doi: 10.1073/pnas.1508737112

24.

Cryan

, O'Riordan

, Cowan

CSM

, et al. The microbiota-gut-brain axis. Physiol Rev, 2019; 99(4):1877–2013; doi: 10.1152/physrev.00018.2018

25.

Davar

, Dzutsev

, McCulloch

, et al. Fecal microbiota transplant overcomes resistance to anti-PD-1 therapy in melanoma patients. Science, 2021; 371(6529):595–602; doi: 10.1126/science.abf3363

26.

De la Fuente

, Bernaez

, Del Rio

, et al. Stimulation of murine peritoneal macrophage functions by neuropeptide Y and peptide YY. Involvement of protein kinase C. Immunology, 1993; 80(2):259–265.

27.

de las Casas-Engel

, Dominguez-Soto

, Sierra-Filardi

, et al. Serotonin skews human macrophage polarization through HTR2B and HTR7. J Immunol, 2013; 190(5):2301–2310; doi: 10.4049/jimmunol.1201133

28.

De Vadder

, Grasset

, Manneras Holm

, et al. Gut microbiota regulates maturation of the adult enteric nervous system via enteric serotonin networks. Proc Natl Acad Sci U S A, 2018; 115(25):6458–6463; doi: 10.1073/pnas.1720017115

29.

Dethlefsen

, Hansen

, Lillelund

, et al. Exercise-induced catecholamines activate the hippo tumor suppressor pathway to reduce risks of breast cancer development. Cancer Res, 2017; 77(18):4894–4904; doi: 10.1158/0008-5472.CAN-16-3125

30.

Dietrich

, Wormser

, Fritz

, et al. Molecular crosstalk between Y5 receptor and neuropeptide Y drives liver cancer. J Clin Invest, 2020; 130(5):2509–2526; doi: 10.1172/JCI131919

31.

Dionisio

, Jose De Rosa

, Bouzat

, et al. An intrinsic GABAergic system in human lymphocytes. Neuropharmacology, 2011; 60(2–3):513–519; doi: 10.1016/j.neuropharm.2010.11.007

32.

Dooling

, Costa-Mattioli

. Gut bacteria seize control of the brain to prevent epilepsy. Cell Host Microbe, 2018; 24(1):3–5; doi: 10.1016/j.chom.2018.06.014

33.

Ebrahimi

, Javid

, Alaei

, et al. New insight into the role of substance P/neurokinin-1 receptor system in breast cancer progression and its crosstalk with microRNAs. Clin Genet, 2020; 98(4):322–330; doi: 10.1111/cge.13750

34.

Fischbach

, Segre

. Signaling in host-associated microbial communities. Cell, 2016; 164(6):1288–1300; doi: 10.1016/j.cell.2016.02.037

35.

Fuks

, Arrighi

, Weidner

, et al. GABAergic signaling is linked to a hypermigratory phenotype in dendritic cells infected by Toxoplasma gondii . PLoS Pathog, 2012; 8(12):e1003051; doi: 10.1371/journal.ppat.1003051

36.

Furness

. The enteric nervous system and neurogastroenterology. Nat Rev Gastroenterol Hepatol, 2012; 9(5):286–294; doi: 10.1038/nrgastro.2012.32

37.

Gandhi

, Rodriguez-Abreu

, Gadgeel

, et al. Pembrolizumab plus chemotherapy in metastatic non-small-cell lung cancer. N Engl J Med, 2018; 378(22):2078–2092; doi: 10.1056/NEJMoa1801005

38.

Gandhi

, Pandey

, Attwood

, et al. Phase I clinical trial of combination propranolol and pembrolizumab in locally advanced and metastatic melanoma: Safety, tolerability, and preliminary evidence of antitumor activity. Clin Cancer Res, 2021; 27(1):87–95; doi: 10.1158/1078-0432.CCR-20-2381

39.

Garcia-Recio

, Fuster

, Fernandez-Nogueira

, et al. Substance P autocrine signaling contributes to persistent HER2 activation that drives malignant progression and drug resistance in breast cancer. Cancer Res, 2013; 73(21):6424–6434; doi: 10.1158/0008-5472.CAN-12-4573

40.

Gopalakrishnan

, Andrews

, Chen

W-S

, et al. Abstract 1493: Therapeutic efficacy and tolerability of combined immune checkpoint blockade in metastatic melanoma patients is influenced by the gut microbiome. Cancer Res, 2019; 79:1493; doi: 10.1158/1538-7445.AM2019-1493

41.

Gopalakrishnan

, Spencer

, Nezi

, et al. Gut microbiome modulates response to anti-PD-1 immunotherapy in melanoma patients. Science, 2018; 359(6371):97–103; doi: 10.1126/science.aan4236

42.

Grimaldi

, Sibella-Arguelles

, Bonnin

, et al. Functional properties of 5-HT-moduline in the immune system: A model for central nervous system investigation. Ann N Y Acad Sci, 1998; 861:249–250; doi: 10.1111/j.1749-6632.1998.tb10206.x

43.

Grygier

, Arteta

, Kubera

, et al. Inhibitory effect of antidepressants on B16F10 melanoma tumor growth. Pharmacol Rep, 2013; 65(3):672–681; doi: 10.1016/s1734-1140(13)71045-4

44.

Grytli

, Fagerland

, Fosså

, et al. Association between use of β-blockers and prostate cancer-specific survival: A cohort study of 3561 prostate cancer patients with high-risk or metastatic disease. Eur Urol, 2014; 65(3):635–641; doi: 10.1016/j.eururo.2013.01.007

45.

Gumireddy

, Li

, Kossenkov

, et al. The mRNA-edited form of GABRA3 suppresses GABRA3-mediated Akt activation and breast cancer metastasis. Nat Commun, 2016; 7:10715; doi: 10.1038/ncomms10715

46.

Hanahan

, Weinberg

. Hallmarks of cancer: The next generation. Cell, 2011; 144(5):646–674; doi: 10.1016/j.cell.2011.02.013

47.

, Fu

, Li

, et al. Gut microbial metabolites facilitate anticancer therapy efficacy by modulating cytotoxic CD8(+) T cell immunity. Cell Metab, 2021;33(5):988.e7–1000.e7; doi: 10.1016/j.cmet.2021.03.002

48.

Herbst

, Giaccone

, de Marinis

, et al. Atezolizumab for first-line treatment of PD-L1-selected patients with NSCLC. N Engl J Med, 2020; 383(14):1328–1339; doi: 10.1056/NEJMoa1917346

49.

Herner

, Sauliunaite

, Michalski

, et al. Glutamate increases pancreatic cancer cell invasion and migration via AMPA receptor activation and Kras-MAPK signaling. Int J Cancer, 2011; 129(10):2349–2359; doi: 10.1002/ijc.25898

50.

Hoeppner

, Wang

, Sharma

, et al. Dopamine D2 receptor agonists inhibit lung cancer progression by reducing angiogenesis and tumor infiltrating myeloid derived suppressor cells. Mol Oncol, 2015; 9(1):270–281; doi: 10.1016/j.molonc.2014.08.008

51.

Hsieh

, Shen

, Lin

, et al. Cyproheptadine exhibits antitumor activity in urothelial carcinoma cells by targeting GSK3beta to suppress mTOR and beta-catenin signaling pathways. Cancer Lett, 2016; 370(1):56–65; doi: 10.1016/j.canlet.2015.09.018

52.

Huang

, Wang

, Thompson

, et al. Cancer-cell-derived GABA promotes beta-catenin-mediated tumour growth and immunosuppression. Nat Cell Biol, 2022a;24(2):230–241; doi: 10.1038/s41556-021-00820-9

53.

Huang

, Wu

. Brain neurotransmitter modulation by gut microbiota in anxiety and depression. Front Cell Dev Biol, 2021; 9:649103; doi: 10.3389/fcell.2021.649103

54.

Huang

, Liu

, Wang

, et al. Ginseng polysaccharides alter the gut microbiota and kynurenine/tryptophan ratio, potentiating the antitumour effect of antiprogrammed cell death 1/programmed cell death ligand 1 (anti-PD-1/PD-L1) immunotherapy. Gut, 2022b;71(4):734–745; doi: 10.1136/gutjnl-2020-321031

55.

Hugon

, Dufour

, Colson

, et al. A comprehensive repertoire of prokaryotic species identified in human beings. Lancet Infect Dis, 2015; 15(10):1211–1219; doi: 10.1016/S1473-3099(15)00293-5

56.

Jameson

, Olson

, Kazmi

, et al. Toward understanding microbiome-neuronal signaling. Mol Cell, 2020; 78(4):577–583; doi: 10.1016/j.molcel.2020.03.006

57.

Januzi

, Poirier

, Maksoud

MJE

, et al. Autocrine GABA signaling distinctively regulates phenotypic activation of mouse pulmonary macrophages. Cell Immunol, 2018; 332:7–23; doi: 10.1016/j.cellimm.2018.07.001

58.

Jiang

, Hu

, Wang

, et al. Neurotransmitters: Emerging targets in cancer. Oncogene, 2020; 39(3):503–515; doi: 10.1038/s41388-019-1006-0

59.

Jiang

, Li

, Dong

, et al. Increased serotonin signaling contributes to the Warburg effect in pancreatic tumor cells under metabolic stress and promotes growth of pancreatic tumors in mice. Gastroenterology, 2017;153(1):277.e19–291.e19; doi: 10.1053/j.gastro.2017.03.008

60.

Jiang

, Zhu

, Zhang

, et al. GABRP regulates chemokine signalling, macrophage recruitment and tumour progression in pancreatic cancer through tuning KCNN4-mediated Ca(2+) signalling in a GABA-independent manner. Gut, 2019; 68(11):1994–2006; doi: 10.1136/gutjnl-2018-317479

61.

Joseph

, Niggemann

, Zaenker

, et al. The neurotransmitter γ-aminobutyric acid is an inhibitory regulator for the migration of SW 480 colon carcinoma cells. Cancer Res, 2002; 62(22):6467–6469.

62.

Kanbara

, Otsuki

, Watanabe

, et al. GABA(B) receptor regulates proliferation in the high-grade chondrosarcoma cell line OUMS-27 via apoptotic pathways. BMC Cancer, 2018; 18(1):263; doi: 10.1186/s12885-018-4149-4

63.

Karmakar

, Lal

. Role of serotonin receptor signaling in cancer cells and anti-tumor immunity. Theranostics, 2021; 11(11):5296–5312; doi: 10.7150/thno.55986

64.

Kast

. Glioblastoma chemotherapy adjunct via potent serotonin receptor-7 inhibition using currently marketed high-affinity antipsychotic medicines. Br J Pharmacol, 2010; 161(3):481–487; doi: 10.1111/j.1476-5381.2010.00923.x

65.

Kipnis

, Cardon

, Avidan

, et al. Dopamine, through the extracellular signal-regulated kinase pathway, downregulates CD4⁺CD25⁺ regulatory T-cell activity: Implications for neurodegeneration. J Neurosci, 2004; 24(27):6133–6143; doi: 10.1523/JNEUROSCI.0600-04.2004

66.

Kokolus

, Zhang

, Sivik

, et al. Beta blocker use correlates with better overall survival in metastatic melanoma patients and improves the efficacy of immunotherapies in mice. Oncoimmunology, 2018; 7(3):e1405205; doi: 10.1080/2162402X.2017.1405205

67.

Kumar

. Biogenic amine neurotransmitters: Their importance and measurement in human tissue and body fluids. Sep Tech Clin Chem, 2003; 10:289; doi: 10.1201/9780203911105.ch7

68.

Lan

, Wang

, Xing

, et al. Anti-cancer effects of dopamine in human glioma: Involvement of mitochondrial apoptotic and anti-inflammatory pathways. Oncotarget, 2017; 8(51):88488–88500; doi: 10.18632/oncotarget.19691

69.

Lee

, Wang

, Liu

, et al. Escherichia coli K12 upregulates programmed cell death ligand 1 (PD-L1) expression in gamma interferon-sensitized intestinal epithelial cells via the NF-κB pathway. Infect Immun, 2020; 89(1):e00618–e00620; doi: 10.1128/IAI.00618-20

70.

Leenay

, Vento

, Shah

, et al. Streamlined, recombinase-free genome editing with CRISPR-Cas9 in Lactobacillus plantarum reveals barriers to efficient editing. bioRxiv 2018; doi: 10.1101/352039

71.

Lenders

, Pacak

, Walther

, et al. Biochemical diagnosis of pheochromocytoma: Which test is best?. JAMA, 2002; 287(11):1427–1434; doi: 10.1001/jama.287.11.1427

72.

Levite

. Dopamine and T cells: Dopamine receptors and potent effects on T cells, dopamine production in T cells, and abnormalities in the dopaminergic system in T cells in autoimmune, neurological and psychiatric diseases. Acta Physiol (Oxf), 2016; 216(1):42–89; doi: 10.1111/apha.12476

73.

Levite

. T cells plead for rejuvenation and amplification; with the brain's neurotransmitters and neuropeptides we can make it happen. Front Immunol, 2021; 12:617658; doi: 10.3389/fimmu.2021.617658

74.

Levite

, Safadi

, Milgrom

, et al. Neurotransmitters and neuropeptides decrease PD-1 in T cells of healthy subjects and patients with hepatocellular carcinoma (HCC), and increase their proliferation and eradication of HCC cells. Neuropeptides, 2021; 89:102159; doi: 10.1016/j.npep.2021.102159

75.

, Cao

. Lactic acid bacterial cell factories for gamma-aminobutyric acid. Amino Acids, 2010; 39(5):1107–1116; doi: 10.1007/s00726-010-0582-7

76.

, Tian

, Wu

. Neuropeptide Y receptors: A promising target for cancer imaging and therapy. Regen Biomater, 2015; 2(3):215–219; doi: 10.1093/rb/rbv013

77.

, Xu

. The role and the mechanism of γ-aminobutyric acid during central nervous system development. Neurosci Bull, 2008; 24(3):195; doi: 10.1007/s12264-008-0109-3

78.

, Chen

, Minton

, et al. CRISPR-based genome editing and expression control systems in Clostridium acetobutylicum and Clostridium beijerinckii . Biotechnol J, 2016; 11(7):961–972; doi: 10.1002/biot.201600053

79.

, Xiang

, Lu

, et al. A novel role of intestine epithelial GABAergic signaling in regulating intestinal fluid secretion. Am J Physiol Gastrointest Liver Physiol, 2012; 303(4):G453–G460; doi: 10.1152/ajpgi.00497.2011

80.

Liu

, Yang

, Chen

, et al. Environmental eustress modulates beta-ARs/CCL2 axis to induce anti-tumor immunity and sensitize immunotherapy against liver cancer in mice. Nat Commun, 2021a;12(1):5725; doi: 10.1038/s41467-021-25967-9

81.

Liu

, Prindle

, Humphries

, et al. Metabolic co-dependence gives rise to collective oscillations within biofilms. Nature, 2015; 523(7562):550–554; doi: 10.1038/nature14660

82.

Liu

, Zhang

, et al. Dopamine improves chemotherapeutic efficacy for pancreatic cancer by regulating macrophage-derived inflammations. Cancer Immunol Immunother, 2021b;70(8):2165–2177; doi: 10.1007/s00262-020-02816-0

83.

Liu

, Zhang

, Wang

, et al. 5-Hydroxytryptamine1a receptors on tumour cells induce immune evasion in lung adenocarcinoma patients with depression via autophagy/pSTAT3. Eur J Cancer, 2019; 114:8–24; doi: 10.1016/j.ejca.2019.03.017

84.

, Ding

, Li

, et al. 5-HT receptor agonist valerenic acid enhances the innate immunity signal and suppresses glioblastoma cell growth and invasion. Int J Biol Sci, 2020; 16(12):2104–2115; doi: 10.7150/ijbs.44906

85.

Luhachack

, Nudler

. Bacterial gasotransmitters: An innate defense against antibiotics. Curr Opin Microbiol, 2014; 21:13–17; doi: 10.1016/j.mib.2014.06.017

86.

Luksch

, Uckermann

, Stepulak

, et al. Silencing of selected glutamate receptor subunits modulates cancer growth. Anticancer Res, 2011; 31(10):3181–3192.

87.

Luu

, Riester

, Baldrich

, et al. Microbial short-chain fatty acids modulate CD8(+) T cell responses and improve adoptive immunotherapy for cancer. Nat Commun, 2021; 12(1):4077; doi: 10.1038/s41467-021-24331-1

88.

Mager

, Burkhard

, Pett

, et al. Microbiome-derived inosine modulates response to checkpoint inhibitor immunotherapy. Science, 2020; 369(6510):1481–1489; doi: 10.1126/science.abc3421

89.

Martin

, Miquel

, Chain

, et al. Faecalibacterium prausnitzii prevents physiological damages in a chronic low-grade inflammation murine model. BMC Microbiol, 2015; 15:67; doi: 10.1186/s12866-015-0400-1

90.

Matson

, Fessler

, Bao

, et al. The commensal microbiome is associated with anti-PD-1 efficacy in metastatic melanoma patients. Science, 2018; 359(6371):104–108; doi: 10.1126/science.aao3290

91.

Medikonda

, Choi

, Pant

, et al. Glutamate modulation synergizes with anti-PD-1 immunotherapy in glioblastoma. Neurosurgery, 2020; 67(Suppl 1):294; doi: 10.1093/neuros/nyaa447_845

92.

Medikonda

, Choi

, Pant

, et al. Synergy between glutamate modulation and anti-programmed cell death protein 1 immunotherapy for glioblastoma. J Neurosurg, 2021; 136(2):379–388; doi: 10.3171/2021.1.JNS202482

93.

Mendu

, Bhandage

, Jin

, et al. Different subtypes of GABA-A receptors are expressed in human, mouse and rat T lymphocytes. PLoS One, 2012; 7(8):e42959; doi: 10.1371/journal.pone.0042959

94.

Millan

, Marin

, Bockaert

, et al. Signaling at G-protein-coupled serotonin receptors: Recent advances and future research directions. Trends Pharmacol Sci, 2008; 29(9):454–464.

95.

Mohammad-Zadeh

, Moses

, Gwaltney-Brant

. Serotonin: A review. J Vet Pharmacol Ther, 2008; 31(3):187–199; doi: 10.1111/j.1365-2885.2008.00944.x

96.

Mohammadpour

, MacDonald

, Qiao

, et al. beta2 Adrenergic receptor-mediated signaling regulates the immunosuppressive potential of myeloid-derived suppressor cells. J Clin Invest, 2019; 129(12):5537–5552; doi: 10.1172/JCI129502

97.

Molinoff

, Axelrod

. Biochemistry of catecholamines. Annu Rev Biochem, 1971; 40:465–500; doi: 10.1146/annurev.bi.40.070171.002341

98.

Moody

, Moreno

, Jensen

. Neuropeptides as lung cancer growth factors. Peptides, 2015; 72:106–111; doi: 10.1016/j.peptides.2015.03.018

99.

Nasi

, Ahmed

, Rasini

, et al. Dopamine inhibits human CD8⁺ Treg function through D1-like dopaminergic receptors. J Neuroimmunol, 2019; 332:233–241; doi: 10.1016/j.jneuroim.2019.02.007

100.

Nieto

, Bailey

, Kaczanowska

, et al. GABA B receptor chemistry and pharmacology: agonists, antagonists, and allosteric modulators. Curr Top Behav Neurosci, 2022; 52:81–118; doi: 10.1007/7854_2021_232

101.

Nieto

, Rayo

, de Las Casas-Engel

, et al. Serotonin (5-HT) shapes the macrophage gene profile through the 5-HT2B-dependent activation of the aryl hydrocarbon receptor. J Immunol, 2020; 204(10):2808–2817; doi: 10.4049/jimmunol.1901531

102.

Nocito

, Dahm

, Jochum

, et al. Serotonin regulates macrophage-mediated angiogenesis in a mouse model of colon cancer allografts. Cancer Res, 2008; 68(13):5152–5158; doi: 10.1158/0008-5472.CAN-08-0202

103.

O'Connell

, Wang

, Leon-Ponte

, et al. A novel form of immune signaling revealed by transmission of the inflammatory mediator serotonin between dendritic cells and T cells. Blood, 2006; 107(3):1010–1017; doi: 10.1182/blood-2005-07-2903

104.

Pedersen

, Idorn

, Olofsson

, et al. Voluntary running suppresses tumor growth through epinephrine- and IL-6-dependent NK cell mobilization and redistribution. Cell Metab, 2016; 23(3):554–562; doi: 10.1016/j.cmet.2016.01.011

105.

Phan

, Taylor

. The neuropeptides alpha-MSH and NPY modulate phagocytosis and phagolysosome activation in RAW 264.7 cells. J Neuroimmunol, 2013; 260(1–2):9–16; doi: 10.1016/j.jneuroim.2013.04.019

106.

Pinoli

, Marino

, Cosentino

. Dopaminergic regulation of innate immunity: A review. J Neuroimmune Pharmacol, 2017; 12(4):602–623; doi: 10.1007/s11481-017-9749-2

107.

Prindle

, Liu

, Asally

, et al. Ion channels enable electrical communication in bacterial communities. Nature, 2015; 527(7576):59–63; doi: 10.1038/nature15709

108.

Qin

, Wang

, Chen

, et al. Dopamine induces growth inhibition and vascular normalization through reprogramming M2-polarized macrophages in rat C6 glioma. Toxicol Appl Pharmacol, 2015; 286(2):112–123; doi: 10.1016/j.taap.2015.03.021

109.

Qin

, Li

, Wang

, et al. Serotonin/HTR1E signaling blocks chronic stress-promoted progression of ovarian cancer. Theranostics, 2021; 11(14):6950–6965; doi: 10.7150/thno.58956

110.

Reck

, Rodriguez-Abreu

, Robinson

, et al. Pembrolizumab versus chemotherapy for PD-L1-positive non-small-cell lung cancer. N Engl J Med, 2016; 375(19):1823–1833; doi: 10.1056/NEJMoa1606774

111.

Renz

, Tanaka

, Sunagawa

, et al. Cholinergic signaling via muscarinic receptors directly and indirectly suppresses pancreatic tumorigenesis and cancer stemness. Cancer Discov, 2018; 8(11):1458–1473; doi: 10.1158/2159-8290.CD-18-0046

112.

Roney

MSI

, Park

. Antipsychotic dopamine receptor antagonists, cancer, and cancer stem cells. Arch Pharm Res, 2018; 41(4):384–408; doi: 10.1007/s12272-018-1017-3

113.

Roshchina

. New trends and perspectives in the evolution of neurotransmitters in microbial, plant, and animal cells. Adv Exp Med Biol, 2016; 874:25–77; doi: 10.1007/978-3-319-20215-0_2

114.

Routy

, Le Chatelier

, Derosa

, et al. Gut microbiome influences efficacy of PD-1-based immunotherapy against epithelial tumors. Science, 2018; 359(6371):91–97; doi: 10.1126/science.aan3706

115.

Santala

, Rannikko

, Murtola

. Antihypertensive drugs and prostate cancer survival after radical prostatectomy in Finland-A nationwide cohort study. Int J Cancer, 2019; 144(3):440–447; doi: 10.1002/ijc.31802

116.

Sarkar

, Chakroborty

, Basu

. Neurotransmitters as regulators of tumor angiogenesis and immunity: The role of catecholamines. J Neuroimmune Pharmacol, 2013; 8(1):7–14; doi: 10.1007/s11481-012-9395-7

117.

Sarkar

, Chakroborty

, Chowdhury

, et al. Dopamine increases the efficacy of anticancer drugs in breast and colon cancer preclinical models. Clin Cancer Res, 2008; 14(8):2502–2510; doi: 10.1158/1078-0432.CCR-07-1778

118.

Sarrouilhe

, Clarhaut

, Defamie

, et al. Serotonin and cancer: What is the link?. Curr Mol Med, 2015; 15(1):62–77; doi: 10.2174/1566524015666150114113411

119.

Schledwitz

, Xie

, Raufman

. Exploiting unique features of the gut-brain interface to combat gastrointestinal cancer. J Clin Invest, 2021; 131(10):e143776; doi: 10.1172/jci143776

120.

Schneider

, Heeb

, Beffinger

, et al. Attenuation of peripheral serotonin inhibits tumor growth and enhances immune checkpoint blockade therapy in murine tumor models. Sci Transl Med, 2021; 13(611):eabc8188; doi: 10.1126/scitranslmed.abc8188

121.

Schoemaker

, Jones

, Wright

, et al. Psychological stress, adverse life events and breast cancer incidence: A cohort investigation in 106,000 women in the United Kingdom. Breast Cancer Res, 2016; 18(1):72; doi: 10.1186/s13058-016-0733-1

122.

Schoenichen

, Bode

, Duerschmied

. Role of platelet serotonin in innate immune cell recruitment. Front Biosci (Landmark Ed), 2019; 24:514–526; doi: 10.2741/4732

123.

Schuller

. Regulatory role of G protein-coupled receptors in pancreatic cancer development and progression. Curr Med Chem, 2018; 25(22):2566–2575; doi: 10.2174/0929867324666170303121708

124.

Schuller

, Al-Wadei

, Majidi

. GABA B receptor is a novel drug target for pancreatic cancer. Cancer, 2008a;112(4):767–778; doi: 10.1002/cncr.23231

125.

Schuller

, Al-Wadei

, Majidi

. Gamma-aminobutyric acid, a potential tumor suppressor for small airway-derived lung adenocarcinoma. Carcinogenesis, 2008b;29(10):1979–1985; doi: 10.1093/carcin/bgn041

126.

Seidlitz

, Sharma

, Saikali

, et al. Cancer cell lines release glutamate into the extracellular environment. Clin Exp Metastasis, 2009; 26(7):781; doi: 10.1007/s10585-009-9277-4

127.

Selberg

, Gomez

, Rolandi

. The potential for convergence between synthetic biology and bioelectronics. Cell Syst, 2018; 7(3):231–244; doi: 10.1016/j.cels.2018.08.007

128.

Sheth

, Cabral

, Chen

, et al. Manipulating bacterial communities by in situ microbiome engineering. Trends Genet, 2016; 32(4):189–200; doi: 10.1016/j.tig.2016.01.005

129.

, Liang

, Bugno

, et al. Lactobacillus rhamnosus GG induces cGAS/STING-dependent type I interferon and improves response to immune checkpoint blockade. Gut, 2022; 71(3):521–533; doi: 10.1136/gutjnl-2020-323426

130.

Siddiqui

, Shabbir

, Mikhailidis

, et al. The effect of serotonin and serotonin antagonists on bladder cancer cell proliferation. BJU Int, 2006; 97(3):634–639; doi: 10.1111/j.1464-410X.2006.06056.x

131.

Sparrow

, James

, Hussain

, et al. Activation of GABA(A) receptors inhibits T cell proliferation. PLoS One, 2021; 16(5):e0251632; doi: 10.1371/journal.pone.0251632

132.

Spohn

, Mawe

. Non-conventional features of peripheral serotonin signalling - the gut and beyond. Nat Rev Gastroenterol Hepatol, 2017; 14(7):412–420; doi: 10.1038/nrgastro.2017.51

133.

Stepulak

, Rola

, Polberg

, et al. Glutamate and its receptors in cancer. J Neural Transm (Vienna), 2014; 121(8):933–944; doi: 10.1007/s00702-014-1182-6

134.

Strandwitz

. Neurotransmitter modulation by the gut microbiota. Brain Res, 2018; 1693(Pt B):128–133; doi: 10.1016/j.brainres.2018.03.015

135.

Strandwitz

, Kim

, Terekhova

, et al. GABA-modulating bacteria of the human gut microbiota. Nat Microbiol, 2019; 4(3):396–403; doi: 10.1038/s41564-018-0307-3

136.

Sudo

. Biogenic amines: Signals between commensal microbiota and gut physiology. Front Endocrinol (Lausanne), 2019; 10:504; doi: 10.3389/fendo.2019.00504

137.

Sun

, Gong

, Kojima

, et al. Increasing cell membrane potential and GABAergic activity inhibits malignant hepatocyte growth. Am J Physiol Gastrointest Liver Physiol, 2003; 285(1):G12–G19; doi: 10.1152/ajpgi.00513.2002

138.

Sung

, Yang

, Ju

, et al. Aberrant epigenetic regulation of GABRP associates with aggressive phenotype of ovarian cancer. Exp Mol Med, 2017; 49(5):e335; doi: 10.1038/emm.2017.62

139.

Takehara

, Hosokawa

, Eguchi

, et al. Gamma-aminobutyric acid (GABA) stimulates pancreatic cancer growth through overexpressing GABAA receptor pi subunit. Cancer Res, 2007; 67(20):9704–9712; doi: 10.1158/0008-5472.Can-07-2099

140.

Tian

, Chau

, Hales

, et al. GABA(A) receptors mediate inhibition of T cell responses. J Neuroimmunol, 1999; 96(1):21–28; doi: 10.1016/s0165-5728(98)00264-1

141.

Tian

, Dang

, O'Laco

, et al. Homotaurine treatment enhances CD4(+) and CD8(+) regulatory T cell responses and synergizes with low-dose anti-CD3 to enhance diabetes remission in type 1 diabetic mice. Immunohorizons, 2019; 3(10):498–510; doi: 10.4049/immunohorizons.1900019

142.

Tian

, Lu

, Zhang

, et al. Gamma-aminobutyric acid inhibits T cell autoimmunity and the development of inflammatory responses in a mouse type 1 diabetes model. J Immunol, 2004; 173(8):5298–5304; doi: 10.4049/jimmunol.173.8.5298

143.

Tilan

, Kitlinska

. Neuropeptide Y (NPY) in tumor growth and progression: Lessons learned from pediatric oncology. Neuropeptides, 2016; 55:55–66; doi: 10.1016/j.npep.2015.10.005

144.

Tontonoz

, Gee

. Cancer immunotherapy out of the gate: The 22nd annual Cancer Research Institute International Immunotherapy Symposium. Cancer Immunol Res, 2015; 3(5):444–448; doi: 10.1158/2326-6066.CIR-15-0072

145.

Travagli

, Hermann

, Browning

, et al. Musings on the wanderer: What's new in our understanding of vago-vagal reflexes? III. Activity-dependent plasticity in vago-vagal reflexes controlling the stomach. Am J Physiol Gastrointest Liver Physiol, 2003; 284(2):G180–G187; doi: 10.1152/ajpgi.00413.2002

146.

Valenstein

. The discovery of chemical neurotransmitters. Brain Cogn, 2002; 49(1):73–95; doi: 10.1006/brcg.2001.1487

147.

Vetizou

, Pitt

, Daillere

, et al. Anticancer immunotherapy by CTLA-4 blockade relies on the gut microbiota. Science, 2015; 350(6264):1079–1084; doi: 10.1126/science.aad1329

148.

Walther

, Peter

, Bashammakh

, et al. Synthesis of serotonin by a second tryptophan hydroxylase isoform. Science, 2003; 299(5603):76; doi: 10.1126/science.1078197

149.

Wang

, Zheng

, Luo

, et al. Probiotics Lactobacillus reuteri abrogates immune checkpoint blockade-associated colitis by inhibiting group 3 innate lymphoid cells. Front Immunol, 2019; 10:1235; doi: 10.3389/fimmu.2019.01235

150.

Wang

, Tong

, Ma

, et al. Oral berberine improves brain dopa/dopamine levels to ameliorate Parkinson's disease by regulating gut microbiota. Signal Transduct Target Ther, 2021a;6(1):77; doi: 10.1038/s41392-020-00456-5

151.

Wang

, Zhang

Z-T

, Seo

S-O

, et al. Markerless chromosomal gene deletion in Clostridium beijerinckii using CRISPR/Cas9 system. J Biotechnol, 2015; 200:1–5; doi: 10.1016/j.jbiotec.2015.02.005

152.

Wang

, Wang

, Yu

, et al. Targeting monoamine oxidase A-regulated tumor-associated macrophage polarization for cancer immunotherapy. Nat Commun, 2021b;12(1):3530; doi: 10.1038/s41467-021-23164-2

153.

Watanabe

, Maemura

, Kanbara

, et al. GABA and GABA receptors in the central nervous system and other organs. Int Rev Cytol, 2002; 213:1–47; doi: 10.1016/s0074-7696(02)13011-13017

154.

Xiang

, Chen

, Zhang

, et al. Intestinal microbiota modulates adrenomedullary response through Nod1 sensing in chromaffin cells. iScience, 2021; 24(8):102849; doi: 10.1016/j.isci.2021.102849

155.

Yano

, Yu

, Donaldson

, et al. Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell, 2015; 161(2):264–276; doi: 10.1016/j.cell.2015.02.047

156.

, Talmon

, Wang

. Glutamate in cancers: From metabolism to signaling. J Biomed Res, 2019; 34(4):260–270; doi: 10.7555/JBR.34.20190037

157.

Zahalka

, Arnal-Estape

, Maryanovich

, et al. Adrenergic nerves activate an angio-metabolic switch in prostate cancer. Science, 2017; 358(6361):321–326; doi: 10.1126/science.aah5072

158.

Zeng

, Michael

, Zhang

, et al. Synaptic proximity enables NMDAR signalling to promote brain metastasis. Nature, 2019; 573(7775):526–531; doi: 10.1038/s41586-019-1576-6

159.

Zhang

, Vogelzang

, Miyajima

, et al. B cell-derived GABA elicits IL-10(+) macrophages to limit anti-tumour immunity. Nature, 2021; 599(7885):471–476; doi: 10.1038/s41586-021-04082-1

160.

Zhang

, Du

, Liu

, et al. Γ-Aminobutyric acid receptors affect the progression and migration of tumor cells. J Recept Signal Transduct Res, 2014; 34(6):431–439; doi: 10.3109/10799893.2013.856918

161.

Zhang

, Liu

, Liao

, et al. Potential roles of peripheral dopamine in tumor immunity. J Cancer, 2017; 8(15):2966–2973; doi: 10.7150/jca.20850

162.

Zhao

, Huang

, Liu

, et al. Dopamine receptors modulate cytotoxicity of natural killer cells via cAMP-PKA-CREB signaling pathway. PLoS One, 2013; 8(6):e65860; doi: 10.1371/journal.pone.0065860

163.

Zhou

, Zhang

, Wei

, et al. Neuropeptide Y induces secretion of high-mobility group box 1 protein in mouse macrophage via PKC/ERK dependent pathway. J Neuroimmunol, 2013; 260(1–2):55–59; doi: 10.1016/j.jneuroim.2013.04.005