Received: April 05, 2016 Accepted: April 30, 2016 Published:May 06, 2016
Citation:Garcia SB, Minto SB, Kannen V, Oliveira EC (2016) Neuropeptides in the Development of Colon Cancer. Can Surg 1:104.
Copyright: © 2016 Garcia SB, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
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Neuropeptides modulate a broad range of physiologic processes as paracrine and endocrine factors in peripheral systems. These complex neuronal activities are only possible because neurons can release one or more neuropeptides together, which can signal as neuromodulators. Given that a large number of neuropeptides has been reported, it would be almost impossible to describe such complexity herein. Therefore, we will focus only on those molecules with more known pronounced activity throughout the development of colon cancer. Given that the intestinal neuro-immune axis plays a significant role in regulating the severity of inflammation, it possibly has a pivotal role in the colon cancer development. Nevertheless, the precise role and the real impact of neuropeptides in the development of colon cancer, remains to be fully elucidated.
Neuropeptides; Colon cancer; Growth factors; Peptide hormones; Neurotransmitters
A class of small protein-like molecules synthesized from short amino acid chains were coined ‘neuropeptides’ by David de Wied in the early 1970's. These hormones were originally thought to modulate brain-related events only [1,2]. Burbach et al.  has recently proposed that chemokines, growth factors, and hormones should be considered neuropeptides, although such idea remains debatable . Scientific research engaged in clarifying the functions of neuropeptides still is mostly carried out in the brain tissue under either physiological or pathological conditions. In the last 20 years, neuroscience has revealed that these molecules play a vital role in several physiological functions from virtually all peripheral tissues. Neuropeptides modulate a broad range of physiologic processes as paracrine and endocrine factors in peripheral systems. These complex neuronal activities are only possible because neurons can release one or more neuropeptides together, which can signal as neuromodulators. These molecules have been mainly reported to act through several G protein-coupled receptors . Given that a large number of neuropeptides has been reported, it would be almost impossible to describe such complexity herein. Therefore, we will focus only on those molecules with more known pronounced activity throughout the development of colon cancer (Table 1).
|Neuropeptide||*||PossibleMechanisms of Action|
|VIP||↑ (45)||- Pro-inflammatory effects (11)
- Ulcerative colitis associated with more VIP nerves (39)
- VIP receptors overexpressed in poorly differentiated colon cancer (44)
|GLP-2||↑ (17)||- Stimulation of colonic crypt cell proliferation and stem cell behavior (16)|
|SP||--||- Anti-apoptotic effects in the colonic mucosa by activating Akt (20)
- Increased serum levels in colon cancer patients (35)
|ENKEPHALIN||↑ (37)||- Enhanced levels in adenocarcinomas of the colon (36)|
|SOMATOSTATIN||--||- Anti-proliferative and pro-apoptotic effects in cancer cells (65)|
|CALCITONIN||--||- Enhancement of cancer cells survival (66)
- Promotion of colon cancer metastasis (67)
|GALANIN||↓ (49)||- Serum expression level significantly correlated with colon tumor size and tumor stage (screening tool for colon cancer?) (47)
- Contraditory effects in tumor cell lines (50)
|ADRENOMEDULLIN||--||- Upregulation of Bcl-2 / downregulation of pro-apoptotic factors (70)|
|GASTRIN||↓ (53)||- Controversical effects on human colon carcinogenesis (51,52)|
|MCH||↑ (72)||- Inhibition of tumors in APCmin mice lacking MCH (72)|
|NEUROTENSIN||↑ (58)||- Induces proliferation of colon cancer cells (56)
- MAP kinase phosphorylation and colon cancer cell growth (59)
|EGF||↑ (60)||- Increases cell proliferation (60)|
|BOMBESIN||↑ (61)||- Stimulates proliferation of human colon carcinoma cells (62)|
|ANGIOTENSIN||↑ (71)||- Higher concentration in colon cancer patients with diabetes presented and higher incidence of liver metastasis than in nondiabetic cases (72)|
|β- ENDORPHIN||--||- Cancer-preventive effects mediated through the suppression of sympathetic neuronal function, which results in increased peripheral natural killer cell, elevated levels of anti-inflammatory cytokines, and reduced levels of pro-inflammatory cytokines (75)|
Table 1: Reported effects of neuropeptides on experimental models of colonic cancer *↑ stimulatory effect; ↓ inhibitory effect.
The American neuro-gastroenterologist Gershon  has precisely nicknamed the intestines as ‘The second brain’, taking into account the size and complexity of the enteric nervous system (ENS). Enterochromaffin cells (EC) are functionally connected with the ENS and can also synthesize neuropeptides to control intestinal functions . Novel evidence has shown that EC indeed differentiates from the intestinal cryptal stem cell niche [5,6]. Besides, mammalian bombesinlike peptides, cholecystokinin (CCK), gastrin, and neurotensin are gastrointestinal neuropeptides that impact on the body development, inflammation, tissue regeneration, and neoplastic transformation .
These molecules signal through their receptors that are highly expressed in the human colon but have, however, a differentiated pattern of expression for each one of them, which might illustrate their specific physiological roles. Hence, GRP receptors, Y2 receptors, PACAP type1-receptors, cholecystokinin-A receptors, neurotensin1 and somatostatin st2 receptors were abundantly expressed in the myenteric plexus, whereas VIP/PACAP and sst2 receptors were found in lymphoid follicles, and Y2, VIP type1 and sst2 receptors were present in the colonic mucosa . Specific binding sites for VIP, have been characterized in isolated human colonic epithelial cells in the early 80s [9,10]. Given that the intestinal neuro-immune axis plays a significant role in regulating the severity of inflammation, it possibly has a pivotal role in the colon cancer development. For instance, neuropeptides might have either anti-inflammatory (VIP and galanin) or pro-inflammatory effects (NPY, substance P, serotonin, and neurotensin). The activation of specific immune signaling pathways might be the key to understanding the complex and controversial effects of these neuropeptides, from which inflammatory signals increase in a cascade of events . Corticotropin-releasing factor (CRF), which is a stress peptide, has been shown to control the colonic IL-6 release and neuronal activation, a fact that helps to clarify the effects of stressful conditions on the development of colonic inflammation .
Whether colitis increases the expression of neuropeptide Y (NPY) in the enteric nervous system, NPY knockout mice showed reduced intestinal inflammation . Pharmacological inhibition of vasoactive intestinal peptide (VIP), which is a neurotransmitter and vasodilator, protected experimental models from colitis . VIP-deficient mice, authors have also found reduced inflammation in trinitrobenzene sulfonic acid-induced colitis . Then, glucagon-like peptide 2 (GLP-2), which is an enteroendocrine hormone trophic for intestinal mucosa, has been shown to increase enteric neuronal expression of VIP . Sigalet et al.  suggested that GLP-2 acts on enteric neurons and glial cells via a PI3Kγ/Akt pathway, stimulating neuronal differentiation via mTOR and ERK pathways, as well as the expression of receptors and ligands for IGF-I and ErbB pathways. Conversely, the anti-inflammatory and blood flow effects of GLP-2 are related to VIP and nitric oxide synthesizes and release from submucosal enteric neurons .
Although GLP-2 is a hormone with multiple beneficial intestinal effects, as regeneration of the mucosal surface area through increased crypt cell proliferation, and promotion of nutrient digestion and absorption, we have discussed its potential pro-malignant effects in colon cancer through the PI3Kγ/Akt pathway . Another study has shown that GLP-2 not only modulated intestinal stem cell behaviour but also promoted colon carcinogenesis in experimental models .
Substance P (SP) also seems to modulate intestinal inflammation, and recovery process of chemically-induced colitis [18,19]. This peptide inhibits apoptosis activating the Akt signaling through NK-1R, which promotes the tissue recovery during colitis . SP is yet recognized by the neurokinin receptor NK-1R in stromal colonic cells enhancing angiogenesis in the chronic phase of colitis . In intestinal bowel diseases (IBD), expression of SP and vanilloid-type ion channel receptor transient receptor potential vanilloid type 1 (TRPv1) was observed to be increased in enteric nervous fibers, which grown in density near to mesenteric blood vessels instead of those submucosal units . Yet, TRPV-1 has been suggested to inhibit the neuroinflammation in IBD, because its activation induces the release of somatostatin and SP . Such control on the neuroinflammatory processes by TRPV-1 has been hypothesized to decrease the initiation and progression of colon cancer .
Tutton et al.  developed an elegant study that revealed that colonic cell proliferation, but not the tumour proliferative activity, was controlled by the autonomic nervous system. We have also explored the relationship between the myenteric neuronal activity and colon carcinogenesis for two decades now and found that reduced myenteric neuronal density impairs the development chemical-induced colonic pre-neoplastic lesions [26,27]. It helped to clarify the old empirical idea that Brazilian physicians had that ‘carcinoma of the colon does not occur in cases of acquired chagasic megacolon’. Indeed, our latest report has shown that DNA damage in epithelial cells is decreased in patients and experimental murine models with reduced enteric neuronal density .
Although these reports seem controversial at first, we should consider that pre-neoplastic cells are not malignant cells yet, from which we might hypothesize that enteric neurons and intestinal neuropeptides possibly have different activities according to each phase of colon carcinogenesis. In a chemically-induced colon cancer model, the relative density of substance P- and VIP-IR nerve fibres was increased in the muscularis propria . In colon cancer samples, the density of VIP, PACAP, and NPY-positive neurons and neural fibres have been, however, found decreased in sub-mucousal and enteric plexuses . Neuropeptides and their receptors have been also found expressed by either neuroendocrine or non-neuroendocrine tumour cells . Indeed, colon carcinomas containing NE populations, even if small, have been reported to behave worse than their counterparts without these NE cells . In adenomas, argyrophilic cells were detected in such large number, and so intricately blended with other cell types, that they were regarded as an intrinsic part of the tumor. An immunocytochemistry analysis then revealed that these tumor cells expressed glucagon, pancreatic polypeptide, somatostatin, and serotonin in close resemblance to their expression pattern in the normal colorectal mucosa [33,34]. However, novel evidence has shown that serum levels of SP and TNF receptor 1 were higher in colon cancer patients than in healthy subjects .
Understanding the Effects of Main Neuropeptides in the Development of Colon Carcinogenesis
Davis et al.  were the first to report that enkephalin levels in colon adenocarcinomas were significantly higher than in normal controls. Then, Taniguchi and colleagues observed that enkephalin also enhanced experimental and chemically induced colon carcinogenesis . A recent report has discussed that tachykinins, neuropeptide Y, and β-adrenergic receptors might enhance the progression of gastrointestinal tumours through the immune system .
In ulcerative colitis patients, who have increased risk of colon cancer, VIP neural fibres were found in higher density in comparison with normal colon samples, although those positive fibres for SP and neurotensin were reduced . VIP is known to act through specific receptors in the plasma membrane of human colonic adenocarcinomas [40-43]. Recently, VIP has been reported to bind to some receptors, named as VIP/pituitary adenylate cyclase-activating peptide receptors (VPACs), which are overexpressed in poor differentiated colon adenocarcinomas . Experimentally, VIP also enhanced the development of azoxymethane-induced colonic tumours .
Galanin (GAL) is a neuropeptide expressed by neurons, and with complex physiological functions. In colon cancer patients, GAL expression by myenteric neurons was increased in comparison with healthy subjects . According to the colon cancer size and stage, GAL serum levels were significantly increased . A worsen prognosis was related to high-GAL serum levels in colon cancer patients at the stage II . Iishi et al.  had, however, reported that this neuropeptide might suppress the development of colon tumors. Notably, cell culture experiments have clarified that GAL receptor type I (GALr1) has anti-proliferative effects, whereas its receptor type II could induce either anti- or pro-proliferative according to the stimuli intensity. Hence, GAL and its receptors might be powerful biomarkers in colon cancer diagnosis .
The physiological functions of gastrin and cholecystokinin (CCK) are well-known in the nervous and gastrointestinal systems. However, pro-gastrin, a gastrin precursor has been reported to promote colonic epithelial proliferation and cancer, as well as to inhibit the acquisition of an M2-phenotype by tumour-associated macrophages [51,52]. Notably, long-term treatment with the proton pump inhibitor (PPI), which increases gastrin levels, did not increase the risk of colon cancer. Authors reported that PPI-induced anti-inflammatory, anti-oxidative, and anti-mutagenic effects, reducing the risk of colon cancer . Furthermore, pro-gastrin inhibited the acquisition of a M2-phenotype by tumour-associated macrophages . These facts illustrate the complex activity of neuropeptides in colon carcinogenesis.
Neurotensin, which is another intestinal neuropeptide, has been reported to modulate pro-inflammatory and tissue repair signalling pathways by activating the insulin-like growth factor-1 receptor (IGF-1R) that induces Akt phosphorylation and NF-κB activation after this neuropeptide is coupled to its receptor (NTr1) in colonic epithelial cells . Notably, NTr1 expression was found increased in colon cancer but not in normal epithelial cells. Authors suggested that that cancer outgrowth induced by mutations in the Wnt/APC signalling pathway further enhanced the NTr1 expression . Another recent report has shown that neurotensin induces proliferation and differentiation of colon cancer cells through calcium signalling , which has its intracellular levels modulated by this neuropeptide in an inositol triphosphate-dependent manner . In azoxymethaneexposed rats, the neurotensin treatment increased colon tumours in size and numbers, as well as their development into adenocarcinomas . In the HT29 cells, NT stimulated MAP kinase phosphorylation through its receptors promoting cell growth aside from the epidermal growth factor (EGF) signalling .
Dougherty et al.  have recently suggested that EGF acts through its receptor (EGFr) promoting tumorigenesis in azoxymethane/dextran sulphate sodium-exposed mice. In another carcinogen-induced colon cancer murine model has been revealed that bombesin stimulates cell proliferation, a fact later confirmed in human colon carcinoma cells [61,62]. In Caco-2 colon cancer cells, this neuropeptide was also reported to induce gene and protein expression of cyclooxygenase-2 expression, which promoted cell growth, invasiveness, and tumour progression .
Given that somatostatin (SST) has anti proliferative and proapoptotic effects in cancer cells, and is usually decreased in tumours, its reduced expression was suggested to release cancer cells toward proliferation. Such reduced expression of SST was revealed to related to a gene silencing via hyper methylation of its promoter region, which is a central event in the pathogenesis of several types of cancer [64,65]. Another example of the neuropeptide complex activity in cancer is illustrated by comparing the calcitonin to its precursor pro-calcitonin. Whereas hypermethylating the promoter gene region of calcitonin enhance cancer cell survival , high-pro-calcitonin serum levels were observed to promote distant metastases in colon cancer patients . Increased expression levels of adrenomedullin (ADM) have been also observed in colon cancer samples, which was then hypothesized to determine the cancer stage and the clinical survival rate [68,69]. Notably, tumour hypoxia was suggested to upregulate this neuropeptide expression that increased Bcl-2 activity, but down regulated those Bax and Bid proapototic factors .
Angiotensin is a peptide with stimulatory effects on colon carcinogenesis, since it has been observed that angiotensin-converting enzyme inhibitor strongly suppress chemically-induced colon carcinogenesis by attenuating chronic inflammation and reducing oxidative stress in obese mice . Diabetic patients with primary colon cancer patients have been reported to have high serum levels of angiotensin-II (A-II), which seemed to increase the number of liver metastasis in comparison with nondiabetic subjects. In cell culture experiments these findings were confirmed, since A-II treatment promoted growth, invasion, and anti-apoptotic signals . Another neuropeptide close related to metabolic events is the melaninconcentrating hormone (MCH), which modulates appetite and energy balance in the hypothalamus. In APCmin mice, knocking out strategies against MCH has been reported to reduce the number of intestinal tumours, which were smaller and less dysplastic tumours than those from the wild-type group . Interestingly, reduced expression of corticotrophin-Releasing-Hormone receptors (CRHr) enhanced the risk of distant metastases and poor clinical prognosis .
Finally, it must be mentioned the hypothesis of existence of links between psychosocial factors and colon cancer development. If such relationship eventually will be demonstrated, certainly neuropeptides will play a major role on it. Studying the β-endorphin activity, which is an opioid peptide enrolled in controlling the central nervous stress axis, has been applied to understand how nervous, endocrine, and immune systems can impact on cancer growth and metastasis. For instance, high-β-endorphin hypothalamic levels have been found to suppress sympathetic neuronal functions, improving the killing activity of cancer cells by the immune system that reduced tumour incidence in prostate and breast cancer models . On the other hand, the tachykinins family of neuropeptides, such as substance P, neurokinin A and B, hemokinin-1 and endokinins, can act in different steps of carcinogenesis [76,77]. Indeed, they seem to promote proliferation and survival of cancer cells and facilitate tumor metastases by infiltrating in the bone marrow . Regardless of tumour histology, tachykinins might favour cancer incidence and metastases promoting immunosuppression together with improved malignant neovascularization in tumours .
We have mentioned here various reports on effects of isolated given neuropeptides in colon cancer development, as summarized on Table 1. Nevertheless, it must be taken into account that: (1) concomitant effects of various neuropeptides in cancer have not been evaluated so far and (2) the huge complexity and the vast number of variables involved in the modulation of colonic carcinogenesis by neuropeptides defy our current capacity of analysis. Thus, the precise role and the real impact of neuropeptides in the development of colon cancer, remains a major scientific challenge.