The classical neurotransmitters E and NE have been found to give promotion to PC progression through multiple mechanisms. E and NE are stress molecules produced by the sympathetic nervous system and linked to PC growth via β-adrenergic signaling in both in vitro and in vivo studies (Renz et al., 2018b). Specifically, E indirectly enhanced β-AR-dependent neurotrophin secretion, which in turn increased NE levels and promoted PC growth (Zhao et al., 2014). The activation of β-AR can promote tumor growth and angiogenesis via VEGF and metalloproteinase MMP2/MMP9 signaling pathways (Thaker et al., 2006). NE also tended to promote PC progression through β-AR/PKA/STAT3 signaling pathway (Coelho et al., 2017).

The neurotransmitter 5-HT, as well as its receptors, was found to be elevated in PC tissues (Jiang et al., 2017). Knockdown of 5-HT receptors inhibited the proliferation and invasion of human PC cells in vitro (Gurbuz et al., 2014). In contrast, the activation of 5-HT receptors enhanced glycolysis under metabolic stress, and thus promoted the growth of PC. Regarding to its molecular mechanism, 5-HT stimulation increased the Warburg effect through PI3K-Akt-mTOR signaling (Jiang et al., 2017). In addition, the increased levels of type 1 tryptophan hydroxylase (TPH1), which was a key enzyme for peripheral 5-HT synthesis, and the decreased level of MAOA, which is responsible for 5-HT degradation, in PC tissues were correlated with the poor survival of patients (Jiang et al., 2017). Of importance, the metaplasia of acinar-to-ductal metaplasia (ADM) is a key determinant in PC development (Liou et al., 2017). Serotonin uptake by acinar cells could promote the activation of the small GTPase Ras-related C3 botulinum toxin substrate 1 (Rac1), which is required for the transdifferentiation of acinar cells into ADM (Saponara et al., 2018).

The effect of DA on cancer cells is tumor type-specific. DA mainly reduced the proliferation and migration of endothelial cells in TME (Hoeppner et al., 2015). In PC, the dopamine receptor D2R is abnormally highly expressed and the antagonists of D2R (pimozide and haloperidol) were able to prevent the proliferation of PC cells, suggesting a PC-promoting effect of DA (Jandaghi et al., 2016).

Gamma-aminobutyric acid is a major inhibitory neurotransmitter in the central nervous system (CNS). Different GABA receptors play different roles in tumor growth. GABA was found to enhance prostate cancer cell proliferation through the GABA-A receptor pathway (Blanchart et al., 2017) and to inhibit cancer cell growth through the GABA-B receptor pathway in liver cancer (Wang et al., 2008; Hujber et al., 2018). GABRP, a subunit of the GABA-A receptor, was abnormally highly expressed in PC cells (Jiang et al., 2019). GABA treatment in GABRP-positive PC cells increased intracellular Ca²⁺ levels and activated the MAPK/Erk cascade, which led to a pro-tumor effect on PC (Takehara et al., 2007).

Neuropeptides, such as substance P, CGRP, and NPY, were also found to have a direct effect on PC cells. Substance P was a powerful regulator to PNI in PC during the early stage of primary tumor formation via the MMP1/PAR1/SP/NK-1R paracrine loop (Huang et al., 2018). Besides, substance P induced cancer cell proliferation and invasion as well as the expression of MMP-2 in PC cells, and sensory nerves in TME may help PC progression in part through up-regulation of its receptor (Sinha et al., 2017). Human PC cells possess distinct CGRP receptors. CGRP can stimulate the proliferation of human PC cells, suggesting a role of CGRP in the growth of PC cells (Zhou, 1993). What’s more, CGRP and substance P derived from pancreatic stellate cells mediated the PC pain via activation of sHH signaling pathway (Han et al., 2016), which provided a novel therapeutic option for PC pain. NPY could be detected in both human and murine pancreatic samples. Its receptor Y2 was significantly increased in PanIN lesions and PC samples both in murine and human. The enhanced Y2 receptor-mediated NPY signaling may modulate the angiogenesis of PC (Waldmann et al., 2018).

E and NE are mainly secreted by the adrenal medulla and sympathetic nerves, respectively. Local sympathetic innervation provides the bulk of the catecholamine content within the tumor (Zahalka and Frenette, 2020). Excessive activation of sympathetic nerves could damage the anti-tumor immune response, increase the invasion ability of tumor cells, and accelerate the occurrence and development of tumors (Nissen et al., 2018).

E and NE mainly exert immunosuppressive effects. The receptor β-ARs is present in most immune cells such as T cells (including Treg), B cells, macrophages, NK and DC cells (Sarkar et al., 2013). E and NE can inhibit the activity of CD8⁺ T and NK cells via inducing the apoptosis of these lymphocytes (Zhao et al., 2014). They are known to directly suppress the production of cytokines and T cell proliferation, and potently inhibit TCR-mediated integrin activation on human antigen-specific CD8⁺ T cells (Leone et al., 2015; Dimitrov et al., 2019). β-AR signal suppressed the production of IL-2 and IFN-γ and proliferation of CD4⁺ T cells depending on their stage of differentiation (Muthu et al., 2007). The combined application of endogenous E and prostaglandin could reduce the anti-tumor activity of NK cells, thereby promoting the progression of leukemia (Inbar et al., 2011; Meron et al., 2013). Besides, several studies have demonstrated that chronic adrenergic signaling suppresses NK cell activity in solid tumors (Melamed et al., 2005; Tarr et al., 2012; Graff et al., 2018). In addition, β-AR-mediated hormonal signaling could reduce the deformability of macrophages, resulting in the acceleration of tumor metastasis (Kim et al., 2019). NE and E contribute to the macrophage polarization, recruitment, and cytokine production like IL-6 and TNF-α (Izeboud et al., 1999; Chiarella et al., 2014). Activation of β-AR signal could increase the infiltration of macrophages in the primary tumor parenchyma and induce the M2 polarization of macrophages (Dimitrov et al., 2019), which subsequently promoted tumor metastasis and stimulated tumor cells to produce chemokines like M-CSF (Van Overmeire et al., 2016). The use of β-blocker propranolol reduced the immunosuppression function of MDSC in breast cancer and enhances the effect of other cancer therapies like anti-PD-1 treatment and irradiation (Mohammadpour et al., 2019).

In pancreatic TME, it was shown that NE and E could decrease the expressions of MHC-I molecules and the costimulatory ligand B7-1 in PC cells, and increase the expressions of immunosuppressive IDO, PD-1, and PD-L1 (Arreola et al., 2015). Although these changes only last for a short time, these phenotypic changes of cancer cells do not only prevent the antigen recognition by T cells, but also damage T cell function by depleting essential nutrients, such as tryptophan, and inducing the exhaustion of activated lymphocytes (Zhao et al., 2014). β-adrenergic activation directly inhibits the generation of CTL and blocks the recruitment of protective T cells in the tumor microenvironment (Nakai et al., 2014). In PC, stress-induced neural activation is related to increased primary tumor growth and tumor cell dissemination to the normal adjacent pancreas. These effects were associated with increased expression of invasion genes by tumor cells and pancreatic stromal cells in the microenvironment (Kim-Fuchs et al., 2014). Enriching the housing environment for mice could enhance the cytotoxic activity of NK cells and promote tumor-infiltrating NK cells via sympathetic nerve-dependent mechanisms. Application of the β-blocker largely abolished the effects of the enriched environment on NK cells and attenuated its anti-tumor function (Song et al., 2017).

Taken together, adrenergic signaling mainly exhibits pro-tumorigenic properties. This effect is generated in part through the enhanced immune evasion induced by E and NE. The inhibition of adrenergic signaling increases the antitumor immune response via its impact on multiple immune cells, supporting the potential value of adrenergic antagonists in cancer prevention and treatment.

In the brain, 5-HT is synthesized by neurons located in the raphe nucleus of the brainstem (Migliarini et al., 2013). Externally, less than 1% of free 5-HT exists in the blood, and the rest is stored in platelets, presynaptic neurons, and intestinal enterochromaffin cells (Kim et al., 2019). 5-HT is a multifunctional molecule that regulates immune function (Arreola et al., 2015). It is now known that there are seven different 5-HT receptor subtypes: 5-HT1 to 5-HT7. Except 5-HT3, which is a ligand-gated ion channel, all the other 5-HT receptors belong to the G protein coupled receptor family. Most immune cells express 5-HT receptors. 5-HT1 is mainly expressed in innate immune cells such as the mast cells (Kushnir-Sukhov et al., 2006), macrophages (Nakamura et al., 2008), DCs (Durk et al., 2005), and monocytes (Soga et al., 2007); 5HT-2 is expressed in eosinophils and macrophages (Mikulski et al., 2010). In the adaptive immune system, proliferated T cells mainly express 5-HT1B, 5-HT2A, and 5-HT7, and B cells mainly express 5-HT1A and 5-HT3 (Yin et al., 2006; Inoue et al., 2011).

5-HT is a neuromodulator with neurotransmitter and neuroendocrine functions in cancer (Balakrishna et al., 2021). Meanwhile, it also regulates a variety of immune processes, such as immune cell chemotaxis, activation, proliferation, and cytokine secretion (Herr et al., 2017). SERT blockers including the serotonin selective reuptake inhibitors (SSRI) which increase extracellular 5-HT concentration have immunoinhibitory effects (Gobin et al., 2014). In T cells, 5-HT induces T cell differentiation into Treg cells and promotes the shift of Th17 cells to Tregs. 5-HT acts on Th17 to induce the secretion of IFN-γ and IL-17 and elevates the release of IL-10 from Tregs, which indirectly promote the development of tumors (Sacramento et al., 2018). In DC cells, 5-HT elevates the Ca²⁺ concentration in immature cells, and contributes to the differentiation and maturation of DCs by activating 5-HT4 and 5-HT7. This activation could up-regulate cAMP levels in DCs (Carhart-Harris and Nutt, 2017) and the secretion of related cytokines such as IL-1β, IL-6, IL-8, and IL-10 (Idzko et al., 2004; Katoh et al., 2006). In macrophages, 5-HT regulates the polarization of macrophages through both activated and inhibitory signals. 5-HT2A activation increases the production of M2-type cytokines and migration while 5-HT1A activation enhances the capability of phagocytosis in macrophages (Mikulski et al., 2010; de las Casas-Engel et al., 2013). Activation of 5-HT1A on lung cancer cells could induce immune evasion via autophagy. After the activation, the ratio of TH1/TH2 cells decreased and the number of Tregs increased in TME, which suggesting a resistance to CTL attack (Liu et al., 2019).

In the pancreas, 5-HT is profoundly implicated in acute pancreatitis, pancreas regeneration after pancreatitis and PC. In the pancreatic TME, the SSRI fluoxetine reduced the stromal reaction that surrounds pancreatic lesions, evidenced by decreased fibrosis, inflammation and angiogenesis (Saponara et al., 2018). The intratumoral MAOA expression was also associated with T cell dysfunction and decreased patient survival in a broad range of cancer patients including PC (Wang et al., 2021).

In summary, in most cases, serotonin signaling influences immune cells and facilitates tumor development via suppression of anti-tumor immunity. Therefore, the use of anti-anxiety or anti-depressant drugs targeting serotoninergic system may have potential implications in cancer therapy.

Dopamine contributes to neuroimmune communication and acts on immune cells in an autocrine/paracrine manner through its receptors (Roy et al., 2000). Dopamine receptors are functionally classified into the D1-like subtypes consisting of D1R and D5R and D2-like subtypes (D2R, D3R, and D4R receptors), based on their ability to stimulate the formation or inhibition of cAMP (Kebabian, 1978).

Dopamine mainly acts on T cells and trigger DA receptor-dependent activation of ERK, LCK, FYN, and NF-κB pathways (Ghosh et al., 2003). In T cells, the activation of D2R and D5R can induce IL-10 (Besser et al., 2005; Cosentino et al., 2007), while D3R activation induces TNF-α and IFN-γ secretion (Ilani et al., 2004). Besides, the stimulation of D3R in naive CD8⁺ T cells also contributed to the regulation of chemotaxis and related cellular function like extravasation and adhesion (Huang et al., 2010; Figueroa et al., 2017). DA affects Th1/Th2/Th17 differentiation. Specifically, the activation of D1R and D4R enhanced the TH2 differentiation by up-regulating the activity of cAMP, STAT5, and GATA3. Besides, DA mediated the chemotactic migration of naive CD8⁺ T cells by inducing chemokines like CCL19, CCL21, and CXCL12 (Watanabe et al., 2006). It was reported that DA inhibits the proliferation of T cell and the secretion of IL-2, IL-6, and IFN-γ when exposed to a high level of DA (Bergquist et al., 1997; Saha et al., 2001). DA could activate D1-like receptors in Treg, which in turn indirectly activates effect T cell (Cosentino et al., 2007). Except for T cells, inhibition of the DR3-mediated signal in DC cells increased the cross-presentation of antigen to CD8⁺ T cells (Figueroa et al., 2017).

Dopamine functions as a regulatory component on immune cells in the TME (Yan et al., 2015; Petrilli, 2017). Increased level of plasma DA (40–80 pg/ml) was reported to evidently impair physiological proliferation and cytotoxicity of T cells in cancer patients (Saha et al., 2001). In PC, DA enhanced the chemotherapeutic efficacy of gemcitabine both in vitro and in immunocompetent murine models, and changed TME by suppressing the M2 characters of TAMs. Specifically, the activation of D4R in macrophages by DA reduced the production of cAMP, and then inhibited the activation of PKA/p38 signal pathway, which suppressed the transcription of tumor-promoting cytokines of TAMs such as IL-1β and TNF-α (Liu Q. et al., 2021). What’s more, DA was found to hinder the function of tumor-induced monocytic MDSCs on the proliferation and IFN-γ production of T cells in lung and melanoma cancer (Hoeppner et al., 2015; Wu J. et al., 2015). DA attenuated NO production by MDSCs directly, mediated by decreased iNOS expression and the downregulation of ERK and JNK signaling pathways. DA-induced activation of resting Teffs and suppression of Tregs seem beneficial for the immunotherapy of cancer. As for the receptors, D2R was identified as an upregulated protein in PC, and D2R antagonists (pimozide and haloperidol) reduced PC growth and particularly metastasis (Jandaghi et al., 2016). The anti-tumor efficacy of ONC201 and ONC212, two small molecule antagonists of D2R, were observed in vivo either administrated as a single agent or in combination with 5-fluorouracil, irinotecan, and oxaliplatin. When treated with ONC201, a broad induction of immune cytokines and effector molecules was observed among PC patients with longer progression-free survival (Lev et al., 2017; Stein et al., 2019). Besides, ONC201 has been reported to induce the proliferation of NK cells and activation via TRAIL and granzyme B in preclinical studies (Stein et al., 2019).

Together, the current findings suggest an important immunomodulatory effect of DA in cancer microenvironment. DA may confer immunopromoting or immunosuppressive effect dependent on the types of immune cells and the specific DA receptor they express.

Gamma-aminobutyric acid is the main inhibitory neurotransmitter in the central nervous system. In the periphery, GABA is produced by pancreatic β cells, T cells, and macrophages. These cells also express other components of the GABAergic system, including receptors, transporters, and metabolic enzymes (Wu et al., 2017). The GABAergic signaling system affects various functional characteristics of immune cells, such as antigen-induced T cell proliferation (Bjurstom et al., 2008) and LPS-induced cytokine release and effector T cell activity (Lang et al., 2003).

Both in vivo and in vitro studies have proved that GABA suppresses the immune effect by inhibiting the activation of NF-κB and reducing the production of inflammatory cytokines (Bhat et al., 2010). As a negative regulator in the TME, GABA inhibits the activation of macrophages and T cells by blocking calcium signals and inhibiting NF-κB pathway (Prud’homme et al., 2015). In addition, GABA was shown to regulate the expression of GABA-A receptor subunits on immune cells (Bergeret et al., 1998; Feske et al., 2012). In the pancreatic TME, GABRP expression was remarkably increased in PC tissues among other neurotransmitters’ receptors. GABRP expression correlated with macrophages density closely, and the deletion of macrophages largely abrogated the oncogenic functions of GABRP in PC. The GABRP on cancer cells promoted macrophages recruitment by inducing CXCL5 and CCL20 expression. Specifically, GABRP might act as a chaperone protein and regulate the activity of KCNN4 channel to induce Ca²⁺ signaling in PC cells. The GABRP-KCNN4 complex led to the activation of NF-κB, which further facilitated CXCL5 and CCL20 transcription to induce macrophage infiltration in PC (Jiang et al., 2019).

In general, GABA exerts immunosuppressive effects on diverse immune cells and blockade of GABA signaling in the microenvironment may improve the anti-tumor immune response against cancer cells.

Most studies related to the immunomodulatory effect of neuropeptides focused on the substance P. In the central system, substance P is released from the brain regions and regulates emotions and specific sensory nerve endings (Cunin et al., 2011; Munoz and Covenas, 2015). In the periphery, substance P is mainly secreted by immune cells such as macrophages and DC cells (Janelsins et al., 2013). Substance P exerts its biological activity through G protein-coupled neurokinin receptors (NKRs), namely NK-1R, NK-2R, and NK-3R (Suvas, 2017). NK-1R mainly exists in the immune system and mediates the effect of substance P on immune cells (Shahzad et al., 2018). Substance P acts on NK-1R to induce a local inflammatory environment in a concentration-dependent manner. Specifically, substance P mediates the migration, proliferation, and activation of immune cells. Substance P activated NF-κB signaling in macrophages, increased the production of pro-inflammatory cytokines such as CCL2, CXCL2, and IL-8 (Levite, 2008). Therefore substance P amplified the inflammatory response mediated by TH1 or TH17 (Cunin et al., 2011; Mashaghi et al., 2016). Recent studies illustrated that administration of substance P during the primary immune response amplifies the secondary immune response by activating CD8⁺ T cells (Ikeda et al., 2007). Substance P increased the migration of immune cells, including T cells and neutrophils, through a β-arrestin-dependent mechanism (Nichols et al., 2012). Substance P also stimulated human PBMC to produce pro-inflammatory cytokines including IL-1, IL-6, IL-12, and TNF-α (Cunin et al., 2011). As for innate immune cells, substance P activated NK cells and neutrophils by up-regulating their production of cytotoxic-associated molecules, such as perforin and granzyme (Fu et al., 2011; Mashaghi et al., 2016). However, the up-regulation of the NK-1R can be seen in both chronic pancreatitis and PC and enhanced NK-1R expressions were related to advanced tumor stage and a poorer prognosis (Li et al., 2013). In addition, CD10⁺ fibroblasts can inhibit squamous cancer cells invasion ability by diminishing substance P (Xie et al., 2010).

The peripheral sensory nerves, which mediate pain reflexes, may influence immune responses through the release of neuropeptides CGRP (Holzmann, 2013). CGRP directly acts on macrophages and dendritic cells and inhibits the capacity of these cells to produce inflammatory cytokines and to present antigens to T cells. The molecular mechanisms, by which CGRP acts on innate immune cells, include the upregulation of IL-10, IL-10-independent induction of the inducible cAMP early repressor (ICER) and inhibition of NF-κB activity (Fox et al., 1997). In addition, Liu et al. demonstrated that increased CGRP and neuronal p75 immune-reactivities in tumor-bearing mice, promoting chronic pain in bone metastasis (Liu et al., 2020).

Neuropeptide Y was found to induce a dose-dependent migration of immature DCs through the engagement of NPY Y1 receptor and the activation of ERK and p38. Meanwhile, NPY promoted the polarization of TH2 polarizing by upregulating the production of IL-6 and IL-10 (Wheway et al., 2007a,b). Thus, NPY may exert proinflammatory effects through the recruitment of immature DCs, but it may exert anti-inflammatory effects by promoting a TH2 polarization. In prostate cancer, depression-induced NPY secretion might promote the tumor infiltration of myeloid cells and therefore contribute to cancer progression (Cheng et al., 2019).

Taken together, the neuropeptides CGRP and NPY were suggested to impair the anti-tumor immunity and their inhibition may be a potential strategy for cancer treatment. On the contrary, substance P may facilitate the anti-tumor immune response. However, enhancing the substance P signaling in cancer cells could promote the progression of pancreatic cancer. The distinction between signaling mechanisms of substance P in immune cells and cancer cells warrants further studies.