Serotonin synthesis protects the mouse colonic crypt from DNA damage and colorectal tumorigenesis

The dual roles of serotonin in antitumor immunity

Research has shown that a significant portion of cancer patients experience depressive symptoms, often accompanied by neuroendocrine hormone imbalances. Depression is frequently associated with decreased levels of serotonin with the alternate name 5-hydroxytryptamine (5-HT), leading to the common use of selective serotonin reuptake inhibitors (SSRIs) as antidepressants. However, the role of serotonin in tumor regulation remains unclear, with its expression levels displaying varied effects across different types of tumors. Tumor initiation and progression are closely intertwined with the immune function of the human body. Neuroimmunity, as an interdisciplinary subject, has played a unique role in the study of the relationship between psychosocial factors and tumors and their mechanisms in recent years. This article offers a comprehensive review of serotonin's regulatory roles in tumor onset and progression, as well as its impacts on immune cells in the tumor microenvironment. The aim is to stimulate further interdisciplinary research and discover novel targets for tumor treatment.

Serotonin signalling in cancer: Emerging mechanisms and therapeutic opportunities

BACKGROUND

Serotonin (5-hydroxytryptamine) is a multifunctional bioamine serving as a neurotransmitter, peripheral hormone and mitogen in the vertebrate system. It has pleiotropic activities in central nervous system and gastrointestinal function via an orchestrated action of serotonergic elements, particularly serotonin receptor-mediated signalling cascades. The mitogenic properties of serotonin have garnered recognition for years and have been exploited for repurposing serotonergic-targeted drugs in cancer therapy. However, emerging conflicting findings necessitate a more comprehensive elucidation of serotonin's role in cancer pathogenesis.

MAIN BODY AND CONCLUSION

Here, we provide an overview of the biosynthesis, metabolism and action modes of serotonin. We summarise our current knowledge regarding the effects of the peripheral serotonergic system on tumourigenesis, with a specific emphasis on its immunomodulatory activities in human cancers. We also discuss the dual roles of serotonin in tumour pathogenesis and elucidate the potential of serotonergic drugs, some of which display favourable safety profiles and impressive efficacy in clinical trials, as a promising avenue in cancer treatment.

KEY POINTS

Primary synthesis and metabolic routes of peripheral 5-hydroxytryptamine in the gastrointestinal tract. Advanced research has established a strong association between the serotonergic components and carcinogenic mechanisms. The interplay between serotonergic signalling and the immune system within the tumour microenvironment orchestrates antitumour immune responses. Serotonergic-targeted drugs offer valuable clinical options for cancer therapy.

The mast cell-T lymphocyte axis impacts cancer: Friend or foe?

Crosstalk between mast cells (MCs) and T lymphocytes (TLs) releases specific signals that create an environment conducive to tumor development. Conversely, they can protect against cancer by targeting tumor cells for destruction. Although their role in immunity and cancer is complex, their potential in anticancer strategies is often underestimated. When peripheral MCs are activated, they can affect cancer development. Tumor-infiltrating TLs may malfunction and contribute to aggressive cancer and poor prognoses. One promising approach for cancer patients is TL-based immunotherapies. Recent reports suggest that MCs modulate TL activity in solid tumors and may be a potential therapeutic layer in multitargeting anticancer strategies. Pharmacologically modulating MC activity can enhance the anticancer cytotoxic TL response in tumors. By identifying tumor-specific targets, it has been possible to genetically alter patients' cells into fully humanized anticancer cellular therapies for autologous transplantation, including the engineering of TLs and MCs to target and kill cancer cells. Hence, recent scientific evidence provides a broader understanding of MC-TL activity in cancer.

5-HT-treated mouse B cells alleviate ulcerative colitis via RIPK1: Insights from proteomic and phosphoproteomic analyses

5-hydroxytryptamine (5-HT) exerts various physiological effects on the intestine through different signaling pathways and molecular transmission mechanisms, including pro- and anti-inflammatory effects. Adoptive transfer of regulatory B cells (Bregs) into colitis mice has exhibited significant therapeutic benefits. We aimed to elucidate the mechanism through which 5-HT-treated B cells alleviate ulcerative colitis. To this end, we analyzed the proteomic and phosphoproteomic profiles of 5-HT-stimulated B cells from naïve mice. We identified 3124 phosphorylation sites in proteins via tandem mass tagging and found 110 differential peptides after protein phosphorylation. Furthermore, we obtained three differential proteins, RIPK1, ATXN2l, and Q8C5K5 through integration of both proteomic datasets. We discovered and validated that 5-HT binds to 5-HT7R and increases the expression of RIPK1 in B cells. We propose a theoretical and experimental basis for further research on the RIPK1 signaling pathway, kinase prediction, and phosphorylation sites in ulcerative colitis. SIGNIFICANCE: Some researchers demonstrated that 5-HT can effectively suppress colitis through a variety of molecular mechanisms. Our study discovered and consistently validated the 5-HT/5-HT7R/RIPK1 pathway, further clarifying the molecular mechanism through which 5-HT stimulates B cells to alleviate intestinal inflammation.

Tryptophan metabolism in digestive system tumors: unraveling the pathways and implications

Tryptophan (Trp) metabolism plays a crucial role in influencing the development of digestive system tumors. Dysregulation of Trp and its metabolites has been identified in various digestive system cancers, including esophageal, gastric, liver, colorectal, and pancreatic cancers. Aberrantly expressed Trp metabolites are associated with diverse clinical features in digestive system tumors. Moreover, the levels of these metabolites can serve as prognostic indicators and predictors of recurrence risk in patients with digestive system tumors. Trp metabolites exert their influence on tumor growth and metastasis through multiple mechanisms, including immune evasion, angiogenesis promotion, and drug resistance enhancement. Suppressing the expression of key enzymes in Trp metabolism can reduce the accumulation of these metabolites, effectively impacting their role in the promotion of tumor progression and metastasis. Strategies targeting Trp metabolism through specific enzyme inhibitors or tailored drugs exhibit considerable promise in enhancing therapeutic outcomes for digestive system tumors. In addition, integrating these approaches with immunotherapy holds the potential to further enhance treatment efficacy.

Assessing Metabolic Differences Associated with Exposure to Polybrominated Biphenyl and Polychlorinated Biphenyls in the Michigan PBB Registry

BACKGROUND

Polybrominated biphenyls (PBB) and polychlorinated biphenyls (PCB) are persistent organic pollutants with potential endocrine-disrupting effects linked to adverse health outcomes.

OBJECTIVES

In this study, we utilize high-resolution metabolomics (HRM) to identify internal exposure and biological responses underlying PCB and multigenerational PBB exposure for participants enrolled in the Michigan PBB Registry.

METHODS

HRM profiling was conducted on plasma samples collected from 2013 to 2014 from a subset of participants enrolled in the Michigan PBB Registry, including 369 directly exposed individuals (F0) who were alive when PBB mixtures were accidentally introduced into the food chain and 129 participants exposed to PBB in utero or through breastfeeding, if applicable (F1). Metabolome-wide association studies were performed for PBB-153 separately for each generation and Σ PCB (PCB-118, PCB-138, PCB-153, and PCB-180) in the two generations combined, as both had direct PCB exposure. Metabolite and metabolic pathway alterations were evaluated following a well-established untargeted HRM workflow.

RESULTS

Mean levels were 1.75  ng / mL [standard deviation (SD): 13.9] for PBB-153 and 1.04  ng / mL (SD: 0.788) for Σ PCB . Sixty-two and 26 metabolic features were significantly associated with PBB-153 in F0 and F1 [false discovery rate (FDR) p < 0.2 ], respectively. There were 2,861 features associated with Σ PCB (FDR p < 0.2 ). Metabolic pathway enrichment analysis using a bioinformatics tool revealed perturbations associated with Σ PCB in numerous oxidative stress and inflammation pathways (e.g., carnitine shuttle, glycosphingolipid, and vitamin B9 metabolism). Metabolic perturbations associated with PBB-153 in F0 were related to oxidative stress (e.g., pentose phosphate and vitamin C metabolism) and in F1 were related to energy production (e.g., pyrimidine, amino sugars, and lysine metabolism). Using authentic chemical standards, we confirmed the chemical identity of 29 metabolites associated with Σ PCB levels (level 1 evidence).

CONCLUSIONS

Our results demonstrate that serum PBB-153 is associated with alterations in inflammation and oxidative stress-related pathways, which differed when stratified by generation. We also found that Σ PCB was associated with the downregulation of important neurotransmitters, serotonin, and 4-aminobutanoate. These findings provide novel insights for future investigations of molecular mechanisms underlying PBB and PCB exposure on health. https://doi.org/10.1289/EHP12657.

Dichotomous colorectal cancer behaviour

Colorectal cancer (CRC) is the third most common malignant tumor and one of the deadliest cancers. At molecular level, CRC is a heterogeneous disease that could be divided in four Consensus Molecular Subtypes. Given the differences in the disease due to its anatomical location (proximal and distal colon), another classification should be considered. Here, we review the current knowledge on CRC dichotomic´s behaviour based on two different entities; right and left-sided tumors, their impact on clinical trial data, microbiota spatial composition and the interaction with the nervous system. We discuss recent advances in understanding how the spatial tumor heterogeneity influences the tumor growth, progression, and responses to current therapies.

Mangiferin inhibits chronic stress-induced tumor growth in colorectal liver metastases via WAVE2 signaling pathway

Background

Evidence indicates that chronic stress promotes progression of colorectal liver metastases (CLM). Mangiferin is the active chemical constituent of the rhizomes of Anemarrhena asphodeloides Bunge. Mangiferin (MGF) exerts anti-inflammatory, anti-proliferative, anti-angiogenic, anti-fibrotic and antioxidant effects in a variety of cancers. Its mechanism in chronic stress and tumor growth is still poorly understood.

Methods

To investigate the effects of MGF on the CLM and tumor-associated depression, activated hepatic stellate cells (a-HSCs), HT-29 CRC cells, were used in chronic unpredictable mild stress (CUMS) of tumor-bearing models. Potential antidepressant activity was determined by FST, TST, SIT and serum cytokine (IL-6, IL-18 and TNF-α) examination. Downstream signaling molecules were detected by Western blot, immunohistochemistry and fluorescence microscopy.

Results

CUMS induced depression behavior and depression-related cytokines and promoted tumor growth in CLM. MGF-treated mice significantly improved chronic stress behaviors by reducing depression-related cytokines. In addition, MGF treatment inhibits WAVE2 signaling pathway, leading to TGF-β1 induced HSC inhibition, thereby reducing depressive behavior and tumor growth in CLM.

Conclusion

MGF can alleviate CUMS induced tumor growth and the treatment of CLM patients with MGF may be beneficial.

Metabolic reprogramming in colorectal cancer: regulatory networks and therapy

With high prevalence and mortality, together with metabolic reprogramming, colorectal cancer is a leading cause of cancer-related death. Metabolic reprogramming gives tumors the capacity for long-term cell proliferation, making it a distinguishing feature of cancer. Energy and intermediate metabolites produced by metabolic reprogramming fuel the rapid growth of cancer cells. Aberrant metabolic enzyme-mediated tumor metabolism is regulated at multiple levels. Notably, tumor metabolism is affected by nutrient levels, cell interactions, and transcriptional and posttranscriptional regulation. Understanding the crosstalk between metabolic enzymes and colorectal carcinogenesis factors is particularly important to advance research for targeted cancer therapy strategies via the investigation into the aberrant regulation of metabolic pathways. Hence, the abnormal roles and regulation of metabolic enzymes in recent years are reviewed in this paper, which provides an overview of targeted inhibitors for targeting metabolic enzymes in colorectal cancer that have been identified through tumor research or clinical trials.

Tryptophan and its metabolites in normal physiology and cancer etiology

Within the growing field of amino acid metabolism, tryptophan (Trp) catabolism is an area of increasing interest. Trp is essential for protein synthesis, and its metabolism gives rise to biologically active catabolites including serotonin and numerous metabolites in the kynurenine (Kyn) pathway. In normal tissues, the production of Trp metabolites is directly regulated by the tissue-specific expression of Trp-metabolizing enzymes. Alterations of these enzymes in cancers can shift the balance and lead to an increased production of specific byproducts that can function as oncometabolites. For example, increased expression of the enzyme indoleamine 2,3-dioxygenase, which converts Trp into Kyn, leads to an increase in Kyn levels in numerous cancers. Kyn functions as an oncometabolite in cancer cells by promoting the activity of the transcription factor aryl hydrocarbon receptor, which regulates progrowth genes. Moreover, Kyn also inhibits T-cell activity and thus allows cancer cells to evade clearance by the immune system. Therefore, targeting the Kyn pathway has become a therapeutic focus as a novel means to abrogate tumor growth and immune resistance. This review summarizes the biological role and regulation of Trp metabolism and its catabolites with an emphasis on tumor cell growth and immune evasion and outlines areas for future research focus.

Nerves in gastrointestinal cancer: from mechanism to modulations

Maintenance of gastrointestinal health is challenging as it requires balancing multifaceted processes within the highly complex and dynamic ecosystem of the gastrointestinal tract. Disturbances within this vibrant environment can have detrimental consequences, including the onset of gastrointestinal cancers. Globally, gastrointestinal cancers account for ~19% of all cancer cases and ~22.5% of all cancer-related deaths. Developing new ways to more readily detect and more efficiently target these malignancies are urgently needed. Whereas members of the tumour microenvironment, such as immune cells and fibroblasts, have already been in the spotlight as key players of cancer initiation and progression, the importance of the nervous system in gastrointestinal cancers has only been highlighted in the past few years. Although extrinsic innervations modulate gastrointestinal cancers, cells and signals from the gut's intrinsic innervation also have the ability to do so. Here, we shed light on this thriving field and discuss neural influences during gastrointestinal carcinogenesis. We focus on the interactions between neurons and components of the gastrointestinal tract and tumour microenvironment, on the neural signalling pathways involved, and how these factors affect the cancer hallmarks, and discuss the neural signatures in gastrointestinal cancers. Finally, we highlight neural-related therapies that have potential for the management of gastrointestinal cancers.

Mast cell-T cell axis alters development of colitis-dependent and colitis-independent colorectal tumours: potential for therapeutically targeting via mast cell inhibition

Background

Colorectal cancer (CRC) has a high mortality rate and can develop in either colitis-dependent (colitis-associated (CA)-CRC) or colitis-independent (sporadic (s)CRC) manner. There has been a significant debate about whether mast cells (MCs) promote or inhibit the development of CRC. Herein we investigated MC activity throughout the multistepped development of CRC in both human patients and animal models.

Methods

We analyzed human patient matched samples of healthy colon vs CRC tissue alongside conducting a The Cancer Genome Atlas-based immunogenomic analysis and multiple experiments employing genetically engineered mouse (GEM) models.

Results

Analyzing human CRC samples revealed that MCs can be active or inactive in this disease. An activated MC population decreased the number of tumor-residing CD8 T cells. In mice, MC deficiency decreased the development of CA-CRC lesions, while it increased the density of tumor-based CD8 infiltration. Furthermore, co-culture experiments revealed that tumor-primed MCs promote apoptosis in CRC cells. In MC-deficient mice, we found that MCs inhibited the development of sCRC lesions. Further exploration of this with several GEM models confirmed that different immune responses alter and are altered by MC activity, which directly alters colon tumorigenesis. Since rescuing MC activity with bone marrow transplantation in MC-deficient mice or pharmacologically inhibiting MC effects impacts the development of sCRC lesions, we explored its therapeutic potential against CRC. MC activity promoted CRC cell engraftment by inhibiting CD8+ cell infiltration in tumors, pharmacologically blocking it inhibits the ability of allograft tumors to develop. This therapeutic strategy potentiated the cytotoxic activity of fluorouracil chemotherapy.

Conclusion

Therefore, we suggest that MCs have a dual role throughout CRC development and are potential druggable targets against this disease.

Integrated Analysis of Colorectal Cancer Reveals Cross-Cohort Gut Microbial Signatures and Associated Serum Metabolites

BACKGROUND & AIMS

Studies have reported abnormal gut microbiota or circulating metabolome associated with colorectal cancer (CRC), but it remains a challenge to capture the CRC-relevant features consistent across geographic regions. This is particularly the problem for metabolic traits of CRC because the analyses generally use different platforms and laboratory methods, which poses a barrier to cross-dataset examination. In light of this, we sought to elucidate the microbial and metabolic signatures of CRC with broad population relevance.

METHODS

In this integrated metagenomic (healthy controls [HC], n = 91; colorectal adenoma [CRA], n = 63; CRC, n = 71) and metabolomic (HC, n = 34; CRA, n = 31; CRC, n = 35) analysis, CRC-associated features and microbe-metabolite correlations were first identified from a Shanghai cohort. A gut microbial panel was trained in the in-house cohort and cross-validated in 7 published metagenomic datasets of CRC. The in-house metabolic connections to the cross-cohort microbial signatures were used as evidence to infer serum metabolites with potentially external relevance. In addition, a combined microbe-metabolite panel was produced for diagnosing CRC or adenoma.

RESULTS

CRC-associated alterations were identified in the gut microbiome and serum metabolome. A composite microbe-metabolite diagnostic panel was developed and yielded an area under the curve of 0.912 for adenoma and 0.994 for CRC. We showed that many CRC-associated metabolites were linked to cross-cohort gut microbiome signatures of the disease, including CRC-enriched leucylalanine, serotonin, and imidazole propionate; and CRC-depleted perfluorooctane sulfonate, 2-linoleoylglycerol (18:2), and sphingadienine.

CONCLUSIONS

We generated cross-cohort metagenomic signatures of CRC, some of which linked to in-house CRC-associated serum metabolites. The microbial and metabolic shifts may have wide population relevance.

Neurotransmitter signaling: a new frontier in colorectal cancer biology and treatment

The brain-gut axis, a bidirectional network between the central and enteric nervous system, plays a critical role in modulating the gastrointestinal tract function and homeostasis. Recently, increasing evidence suggests that neuronal signaling molecules can promote gastrointestinal cancers, however, the mechanisms remain unclear. Aberrant expression of neurotransmitter signaling genes in colorectal cancer supports the role of neurotransmitters to stimulate tumor growth and metastatic spread by promoting cell proliferation, migration, invasion, and angiogenesis. In addition, neurotransmitters can interact with immune and endothelial cells in the tumor microenvironment to promote inflammation and tumor progression. As such, pharmacological targeting of neurotransmitter signaling represent a promising novel anticancer approach. Here, we present an overview of the current evidence supporting the role of neurotransmitters in colorectal cancer biology and treatment.

Akkermansia muciniphila and its outer protein Amuc_1100 regulates tryptophan metabolism in colitis

Dietary interventions, including dietary ingredients, nutrients and probiotics, exert anti-inflammatory effects in ulcerative colitis (UC). Our previous study showed that Akkermansia muciniphila (Akk), a promising probiotic, could protect against colitis via the regulation of the immune response. However, whether it can restore aberrant tryptophan (Trp) metabolism during colitis remains unclear. In this study, untargeted serum metabolomics of patients with UC and colitis mice showed that Trp metabolism was activated, which was confirmed by quantification of Trp metabolites from a validation cohort and animal study. Integrative analysis of faecal metagenomes and serum metabolomes revealed significant associations between Akk and three Trp metabolites. Live Akk, pasteurised Akk and Amuc_1100 failed to restore the reduction in Trp metabolites involved in the serotonin pathway in colitis mice. However, live Akk, pasteurised Akk and Amuc_1100 increased kynurenine (Kyn) but decreased 2-picolinic acid (PIC) levels and the PIC/Kyn ratio without regulating any of the genes involved in Trp metabolism, suggesting that they could suppress the Kyn pathway (KP) independent of colon tissue. In addition, they could significantly restore the enrichment of Trp metabolism mediated by faecal microbiota. Specifically, live Akk, pasteurised Akk and Amuc_1100 could significantly offset the reduction in indoleacetic acid (IAA) levels. Pasteurised Akk significantly elevated the serum levels of indole acrylic acid (IA). In addition, live Akk, pasteurised Akk and Amuc_1100 could upregulate aryl hydrocarbon receptor (AhR) targeted genes, including CYP1A1, IL-10 and IL-22, suggesting that Akk could activate AhR signaling by regulating Trp metabolism, thereby attenuating colonic inflammation.

Microbiomics in Collusion with the Nervous System in Carcinogenesis: Diagnosis, Pathogenesis and Treatment

The influence of the naturally occurring population of microbes on various human diseases has been a topic of much recent interest. Not surprisingly, continuously growing attention is devoted to the existence of a gut brain axis, where the microbiota present in the gut can affect the nervous system through the release of metabolites, stimulation of the immune system, changing the permeability of the blood–brain barrier or activating the vagus nerves. Many of the methods that stimulate the nervous system can also lead to the development of cancer by manipulating pathways associated with the hallmarks of cancer. Moreover, neurogenesis or the creation of new nervous tissue, is associated with the development and progression of cancer in a similar manner as the blood and lymphatic systems. Finally, microbes can secrete neurotransmitters, which can stimulate cancer growth and development. In this review we discuss the latest evidence that support the importance of microbiota and peripheral nerves in cancer development and dissemination.

The role of 5-HT metabolism in cancer

Serotonin (5-hydroxytryptamine, 5-HT) metabolism has long been linked to tumorigenesis and tumor progression. Numerous studies have shown the functions of 5-HT and its metabolites in the regulation of tumor biological processes like cell proliferation, invasion, metastasis, tumor angiogenesis and immunomodulatory through multi-step complex mechanisms. Reprogramming of 5-HT metabolism has been revealed in various tumors paving way for development of drugs that target enzymes, metabolites or receptors involved in 5-HT metabolic pathway. However, information on the role of 5-HT metabolism in cancer is scanty. This review briefly describes the main metabolic routes of 5-HT, the role of 5-HT metabolism in cancer and systematically summarizes the most recent advances in 5-HT metabolism-targeted cancer therapy.

Serotonin as a Mitogen in the Gastrointestinal Tract: Revisiting a Familiar Molecule in a New Role

Serotonin signaling is ubiquitous in the gastrointestinal (GI) system, where it acts as a neurotransmitter in the enteric nervous system (ENS) and influences intestinal motility and inflammation. Since its discovery, serotonin has been linked to cellular proliferation in several types of tissues, including vascular smooth muscle, neurons, and hepatocytes. Activation of serotonin receptors on distinct cell types has been shown to induce well-known intracellular proliferation pathways. In the GI tract, potentiation of serotonin signaling results in enhanced intestinal epithelial proliferation, and decreased injury from intestinal inflammation. Furthermore, activation of the type 4 serotonin receptor on enteric neurons leads to neurogenesis and neuroprotection in the setting of intestinal injury. It is not surprising that the mitogenic properties of serotonin are pronounced within the GI tract, where enterochromaffin cells in the intestinal epithelium produce 90% of the body's serotonin; however, these proliferative effects are attributed to increased serotonin signaling within the ENS compartment as opposed to the intestinal mucosa, which are functionally and chemically separate by virtue of the distinct tryptophan hydroxylase enzyme isoforms involved in serotonin synthesis. The exact mechanism by which serotonergic neurons in the ENS lead to intestinal proliferation are not known, but the activation of muscarinic receptors on intestinal crypt cells indicate that cholinergic signaling is essential to this signaling pathway. Further understanding of serotonin's role in mucosal and enteric nervous system mitogenesis may aid in harnessing serotonin signaling for therapeutic benefit in many GI diseases, including inflammatory bowel disease, malabsorptive conditions, and cancer.

Targeting SERT promotes tryptophan metabolism: mechanisms and implications in colon cancer treatment

Background

Serotonin signaling has been associated with tumorigenesis and tumor progression. Targeting the serotonin transporter to block serotonin cellular uptake confers antineoplastic effects in various tumors, including colon cancer. However, the antineoplastic mechanism of serotonin transporter inhibition and serotonin metabolism alterations in the absence of serotonin transporter have not been elucidated, especially in colon cancer, which might limit anti-tumor effects associating with targeting serotonin transporter.

Methods

The promotion in the uptake and catabolism of extracellular tryptophan and targeting serotonin transporter was detected by using quantitative reverse-transcription polymerase chain reaction, western blotting and liquid chromatography tandem mass spectrometry. Western blotting Immunoprecipitation and immunofluorescence was utilized to research the serotonylation of mTOR by serotonin and serotonin transporter inhibition. The primary mouse model, homograft model and tissue microarry was used to explore the tryptophan pathway in colon cancer. The cell viability assay, western blotting, xenograft and primary colon cancer mouse model were used to identify whether the combination of sertraline and tryptophan restriction had a synergistic effect.

Results

Targeting serotonin transporter through genetic ablation or pharmacological inhibition in vitro and in vivo induced a compensatory effect by promoting the uptake and catabolism of extracellular tryptophan in colon cancer. Mechanistically, targeting serotonin transporter suppressed mTOR serotonylation, leading to mTOR inactivation and increased tryptophan uptake. In turn, this process promoted serotonin biosynthesis and oncogenic metabolite kynurenine production through enhanced tryptophan catabolism. Tryptophan deprivation, or blocking its uptake by using trametinib, a MEK inhibitor, can sensitize colon cancer to selective serotonin reuptake inhibitors.

Conclusions

The present study elucidated a novel feedback mechanism involved in the regulation of serotonin homeostasis and suggested innovative strategies for selective serotonin reuptake inhibitors-based treatment of colon cancer.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-021-01971-1.

Cytoguardin: A Tryptophan Metabolite against Cancer Growth and Metastasis

Cytoguardin was identified in the conditioned medium of fibroblasts as a tryptophan metabolite, 5-methoxytryptophan (5-MTP). It is synthesized via two enzymatic steps: tryptophan hydroxylase (TPH) and hydroxyindole O-methyltransferase (HIOMT). A truncated HIOMT isoform, HIOMT298, catalyzes 5-MTP synthesis. Cancer cells produce scarce 5-MTP due to defective HIOMT298 expression. 5-MTP inhibits cancer cell COX-2 expression and thereby reduces COX-2-mediated cell proliferation and migration. 5-MTP also inhibits MMP-9 expression and thereby reduces cancer cell invasion. 5-MTP exerts its anti-cancer effect by blocking p38 MAPK and p38-mediated NF-κB and p300 HAT activation. The stable transfection of A549 cells with HIOMT298 restores 5-MTP production which renders cancer cells less aggressive. The implantation of HIOMT-transfected A549 into subcutaneous tissues of a murine xenograft tumor model shows that HIOMT-transduced A549 cells form smaller tumors and generate fewer metastatic lung nodules than control A549 cells. HIOMT298 transfection suppresses aromatic amino acid decarboxylase (AADC) expression and serotonin production. Serotonin is a cancer-promoting factor. By restoring 5-MTP and suppressing serotonin production, HIOMT298 overexpression converts cancer cells into less malignant phenotypes. The analysis of HIOMT expression in a human cancer tissue array showed reduced HIOMT levels in a majority of colorectal, pancreatic, and breast cancer. HIOMT298 may be a biomarker of human cancer progression. Furthermore, 5-MTP has the potential to be a lead compound in the development of new therapy for the chemoprevention of certain cancers such as hepatocellular cancer.

Targeting Dietary and Microbial Tryptophan-Indole Metabolism as Therapeutic Approaches to Colon Cancer

Tryptophan metabolism, via the kynurenine (Kyn) pathway, and microbial transformation of tryptophan to indolic compounds are fundamental for host health; both of which are altered in colon carcinogenesis. Alterations in tryptophan metabolism begin early in colon carcinogenesis as an adaptive mechanism for the tumor to escape immune surveillance and metastasize. The microbial community is a key part of the tumor microenvironment and influences cancer initiation, promotion and treatment response. A growing awareness of the impact of the microbiome on tryptophan (Trp) metabolism in the context of carcinogenesis has prompted this review. We first compare the different metabolic pathways of Trp under normal cellular physiology to colon carcinogenesis, in both the host cells and the microbiome. Second, we review how the microbiome, specifically indoles, influence host tryptophan pathways under normal and oncogenic metabolism. We conclude by proposing several dietary, microbial and drug therapeutic modalities that can be utilized in combination to abrogate tumorigenesis.

Paradoxical interaction between cancer and long-term postsepsis disorder: impairment of de novo carcinogenesis versus favoring the growth of established tumors

Background

Previous data have reported that the growth of established tumors may be facilitated by postsepsis disorder through changes in the microenvironment and immune dysfunction. However, the influence of postsepsis disorder in initial carcinogenesis remains elusive.

Methods

In the present work, the effect of postsepsis on inflammation-induced early carcinogenesis was evaluated in an experimental model of colitis-associated colorectal cancer (CAC). We also analyzed the frequency and role of intestinal T regulatory cells (Treg) in CAC carcinogenesis.

Results

The colitis grade and the tumor development rate were evaluated postmortem or in vivo through serial colonoscopies. Sepsis-surviving mice (SSM) presented with a lower colonic DNA damage, polyp incidence, reduced tumor load, and milder colitis than their sham-operated counterparts. Ablating Treg led to restoration of the ability to develop colitis and tumor polyps in the SSM, in a similar fashion to that in the sham-operated mice. On the other hand, the growth of subcutaneously inoculated MC38luc colorectal cancer cells or previously established chemical CAC tumors was increased in SSM.

Conclusion

Our results provide evidence that postsepsis disorder has a dual effect in cancer development, inhibiting inflammation-induced early carcinogenesis in a Treg-dependent manner, while increasing the growth of previously established tumors.

Serotonin Pathway in Cancer

Serotonin (5-hydroxytryptamine, 5-HT) is a biogenic monoamine produced from the essential amino acid tryptophan. Serotonin's role as a neurotransmitter in the central nervous system and a motility mediator in the gastrointestinal tract has been well defined, and its function in tumorigenesis in various cancers (gliomas, carcinoids, and carcinomas) is being studied. Many studies have shown a potential stimulatory effect of serotonin on cancer cell proliferation, invasion, dissemination, and tumor angiogenesis. Although the underlying mechanism is complex, it is proposed that serotonin levels in the tumor and its interaction with specific receptor subtypes are associated with disease progression. This review article describes serotonin's role in cancer pathogenesis and the utility of the serotonin pathway as a potential therapeutic target in cancer treatment. Octreotide, an inhibitor of serotonin release, is used in well-differentiated neuroendocrine cancers, and the tryptophan hydroxylase (TPH) inhibitor, telotristat, is currently being investigated in clinical trials to treat patients with metastatic neuroendocrine tumors and advanced cholangiocarcinoma. Several in vitro studies have shown the anticancer effect of 5-HT receptor antagonists in various cancers such as prostate cancer, breast cancer, urinary bladder, colorectal cancer, carcinoid, and small-cell lung cancer. More in vivo studies are needed to assess serotonin's role in cancer and its potential use as an anticancer therapeutic target. Serotonin is also being evaluated for its immunoregulatory properties, and studies have shown its potential anti-inflammatory effect. Therefore, it would be of interest to explore the combination of serotonin antagonists with immunotherapy in the future.

The Emerging Role of Nerves and Glia in Colorectal Cancer

Simple Summary

The influence of nerves on different types of cancers, including colorectal cancer, is increasingly recognized. The intestines are highly innervated, both from outside the intestines (extrinsic innervation) and by a nervous system of their own; the enteric nervous system (intrinsic innervation). Nerves and cancer cells have been described to communicate with each other, although the exact mechanism in colorectal cancer is not yet explored. Nerves can enhance cancer progression by secreting signaling molecules, and cancer cells are capable of stimulating nerve growth. This review summarizes the innervation of the intestines and current knowledge on the role of the nervous system in colorectal cancer. Additionally, the therapeutic potential of these new insights is discussed.

Abstract

The role of the nervous system as a contributor in the tumor microenvironment has been recognized in different cancer types, including colorectal cancer (CRC). The gastrointestinal tract is a highly innervated organ system, which is not only innervated by the autonomic nervous system, but also contains an extensive nervous system of its own; the enteric nervous system (ENS). The ENS is important for gut function and homeostasis by regulating processes such as fluid absorption, blood flow, and gut motility. Dysfunction of the ENS has been linked with multiple gastrointestinal diseases, such as Hirschsprung disease and inflammatory bowel disease, and even with neurodegenerative disorders. How the extrinsic and intrinsic innervation of the gut contributes to CRC is not fully understood, although a mutual relationship between cancer cells and nerves has been described. Nerves enhance cancer progression through the secretion of neurotransmitters and neuropeptides, and cancer cells are capable of stimulating nerve growth. This review summarizes and discusses the nervous system innervation of the gastrointestinal tract and how it can influence carcinogenesis, and vice versa. Lastly, the therapeutic potential of these novel insights is discussed.

The Dual Role of Serotonin in Colorectal Cancer

Serotonin (5-HT) has complex effects on the central nervous system (CNS), neuroendocrine mechanisms, immunological reactions, intestinal microbiome, and cancer. It has been associated with more severe signs and symptoms of colitis, as well as promoting colorectal cancer (CRC) cells toward expansion. However, recent findings revealed that impairments in 5-HT synthesis lead to high levels of DNA damage in colonocytes, which is linked with inflammatory reactions promoting the development of CRC. Here, we review the diverse roles of 5-HT in intestinal homeostasis and in CRC and discuss how improved understanding of the modulation of the 5-HT pathway could be helpful for the design of novel anticancer therapies.

Inhibition of serotonin synthesis: A novel therapeutic paradigm

The rate-limiting enzyme in serotonin synthesis is tryptophan hydroxylase (TPH). There are two independent serotonin systems in the body characterized by two isoforms of TPH, TPH1 and TPH2. While TPH2 synthesizes serotonin in the brain, TPH1 is expressed in the gut and in other peripheral tissues and supplies platelets in the circulation with serotonin. This duality of the serotonin system is enforced by the blood-brain barrier which is impermeable for serotonin. In the brain serotonin acts as neurotransmitter and is a main target for the treatment of psychiatric disorders. In the periphery it is released by platelets at the site of activation and elicits numerous physiological effects. TPH1 deficient mice were shown to be protected from diverse diseases including hemostatic, inflammatory, fibrotic, gastrointestinal, and metabolic disorders and therefore serotonin synthesis inhibition emerged as a reasonable therapeutic paradigm. Recently the first TPH inhibitor, telotristat ethyl, came on the market for the treatment of carcinoid syndrome. This review summarizes the state of development and the therapeutic opportunities of such compounds.