Galanin and galanin receptor expression in neuroblastoma

The Galaninergic System: A Target for Cancer Treatment

Simple Summary

Peptidergic systems play an important role in cancer progression. The galaninergic system (the peptide galanin and its receptors: galanin 1, 2 and 3) is involved in tumorigenesis, the invasion and migration of tumor cells and angiogenesis and it has been correlated with tumor stage/subtypes, metastasis and recurrence rate in many types of cancer. Galanin exerts a dual action in tumor cells: a proliferative or an antiproliferative effect depending on the galanin receptor involved in these mechanisms. Galanin receptors could be used in certain tumors as therapeutic targets and diagnostic markers for treatment, prognosis and surgical outcome. This review shows the importance of the galaninergic system in the development of tumors and suggests future promising clinical antitumor applications using galanin agonists or antagonists.

Abstract

The aim of this review is to show the involvement of the galaninergic system in neuroendocrine (phaeochromocytomas, insulinomas, neuroblastic tumors, pituitary tumors, small-cell lung cancer) and non-neuroendocrine (gastric cancer, colorectal cancer, head and neck squamous cell carcinoma, glioma) tumors. The galaninergic system is involved in tumorigenesis, invasion/migration of tumor cells and angiogenesis, and this system has been correlated with tumor size/stage/subtypes, metastasis and recurrence rate. In the galaninergic system, epigenetic mechanisms have been related with carcinogenesis and recurrence rate. Galanin (GAL) exerts both proliferative and antiproliferative actions in tumor cells. GAL receptors (GALRs) mediate different signal transduction pathways and actions, depending on the particular G protein involved and the tumor cell type. In general, the activation of GAL₁R promoted an antiproliferative effect, whereas the activation of GAL₂R induced antiproliferative or proliferative actions. GALRs could be used in certain tumors as therapeutic targets and diagnostic markers for treatment, prognosis and surgical outcome. The current data show the importance of the galaninergic system in the development of certain tumors and suggest future potential clinical antitumor applications using GAL agonists or antagonists.

Galanin System in Human Glioma and Pituitary Adenoma

Expression of neuropeptides and their corresponding receptors has been demonstrated in different cancer types, where they can play a role in tumor cell growth, invasion, and migration. Human galanin (GAL) is a 30-amino-acid regulatory neuropeptide which acts through three G protein-coupled receptors, GAL₁-R, GAL₂-R, and GAL₃-R that differ in their signal transduction pathways. GAL and galanin receptors (GALRs) are expressed by different tumors, and direct involvement of GAL in tumorigenesis has been shown. Despite its strong expression in the central nervous system (CNS), the role of GAL in CNS tumors has not been extensively studied. To date, GAL peptide expression, GAL receptor binding and mRNA expression have been reported in glioma, meningioma, and pituitary adenoma. However, data on the cellular distribution of GALRs are sparse. The aim of the present study was to examine the expression of GAL and GALRs in different brain tumors by immunohistochemistry. Anterior pituitary gland (n = 7), pituitary adenoma (n = 9) and glioma of different WHO grades I-IV (n = 55) were analyzed for the expression of GAL and the three GALRs with antibodies recently extensively validated for specificity. While high focal GAL immunoreactivity was detected in up to 40% of cells in the anterior pituitary gland samples, only one pituitary adenoma showed focal GAL expression, at a low level. In the anterior pituitary, GAL₁-R and GAL₃-R protein expression was observed in up to 15% of cells, whereas receptor expression was not detected in pituitary adenoma. In glioma, diffuse and focal GAL staining was noticed in the majority of cases. GAL₁-R was observed in eight out of nine glioma subtypes. GAL₂-R immunoreactivity was not detected in glioma and pituitary adenoma, while GAL₃-R expression was significantly associated to high-grade glioma (WHO grade IV). Most interestingly, expression of GAL and GALRs was observed in tumor-infiltrating immune cells, including neutrophils and glioma-associated macrophages/microglia. The presence of GALRs on tumor-associated immune cells, especially macrophages, indicates that GAL signaling contributes to homeostasis of the tumor microenvironment. Thus, our data indicate that GAL signaling in tumor-supportive myeloid cells could be a novel therapeutic target.

Colorectal cancer patients exhibit increased levels of galanin in serum and colon tissues

Galanin (GAL) is a 30-amino acid neuropeptide that is expressed in both the central and peripheral nervous system, including the enteric nervous system (ENS). Increased GAL concentrations have been identified in the blood of colorectal cancer (CRC) patients. The aim of the present study was to assess whether sections of the colon wall containing ENS plexuses or CRC tumor are associated with increased GAL concentrations. Blood samples were collected from 68 CRC patients and 39 healthy volunteers. In addition, samples of CRC tumors and colon wall tissue in close proximity to and distant from the neoplastic tissue were obtained from 22 CRC patients. The GAL concentration of sera and tissue homogenates obtained from three sections of the colon wall (mucosa with submucosa, muscularis externa and CRC tumor) was analyzed by ELISA. The localization of GAL was evaluated using immunohistochemistry and morphometry was used to measure the distribution of GAL-immunoreactive (GAL-Ir) myenteric plexuses in the vicinity of cancer invasion and in sections of the colon wall distant from the tumor. The GAL serum concentration of CRC patients was 2.4 times higher than that of the control group. The GAL concentration was highest in the homogenates of neoplastic tissue and mucosa obtained from the control (distant) section of the intestinal wall, followed by that in the mucosa and muscularis externa proximal to the tumor. The lowest GAL concentrations were identified within the muscular layer of the colon wall located distant from the tumor. Strong GAL immunoreactivity was identified in the cancer cells, intestinal epithelium and the submucosal and myenteric plexuses. Morphometric analysis revealed that the GAL-Ir myenteric plexuses in the vicinity of tumor infiltration were significantly smaller in size than those in the intact section of the large intestine. Furthermore, no associations were identified between the clinicopathological characteristics of CRC patients and GAL serum and tissue concentration. The increased GAL serum concentrations observed in CRC patients in comparison to healthy controls may result from GAL secretion by CRC tumors, however, other sources of GAL cannot be excluded. The atrophy of myenteric plexuses within close proximity to the tumor may affect the colon function of CRC patients. In conclusion, investigation into the presence of GAL in the colon wall and serum of CRC patients revealed that serum and tissue GAL levels may present useful potential biomarkers in CRC patients.

Physiology, signaling, and pharmacology of galanin peptides and receptors: three decades of emerging diversity

Galanin was first identified 30 years ago as a "classic neuropeptide," with actions primarily as a modulator of neurotransmission in the brain and peripheral nervous system. Other structurally-related peptides-galanin-like peptide and alarin-with diverse biologic actions in brain and other tissues have since been identified, although, unlike galanin, their cognate receptors are currently unknown. Over the last two decades, in addition to many neuronal actions, a number of nonneuronal actions of galanin and other galanin family peptides have been described. These include actions associated with neural stem cells, nonneuronal cells in the brain such as glia, endocrine functions, effects on metabolism, energy homeostasis, and paracrine effects in bone. Substantial new data also indicate an emerging role for galanin in innate immunity, inflammation, and cancer. Galanin has been shown to regulate its numerous physiologic and pathophysiological processes through interactions with three G protein-coupled receptors, GAL1, GAL2, and GAL3, and signaling via multiple transduction pathways, including inhibition of cAMP/PKA (GAL1, GAL3) and stimulation of phospholipase C (GAL2). In this review, we emphasize the importance of novel galanin receptor-specific agonists and antagonists. Also, other approaches, including new transgenic mouse lines (such as a recently characterized GAL3 knockout mouse) represent, in combination with viral-based techniques, critical tools required to better evaluate galanin system physiology. These in turn will help identify potential targets of the galanin/galanin-receptor systems in a diverse range of human diseases, including pain, mood disorders, epilepsy, neurodegenerative conditions, diabetes, and cancer.

The galanin system in cancer

Many tumours of neuroendocrine origin, and also an increasing number of non-neuroendocrine cancers, have been shown to express neuropeptides and/or their corresponding receptors. These peptides and receptors represent the molecular basis for in vivo diagnostic or therapeutic targeting of cancer with radiolabelled or cytotoxic peptide analogues. Galanin is a classical neuropeptide that functions in diverse physiological processes such as food intake, nociception, and blood pressure regulation, and it can also act as a growth factor for neurons. Expression of galanin peptide has been detected in pheochromocytoma, pituitary adenoma, neuroblastic tumours, gastrointestinal cancer, squamous cell carcinoma, brain tumours, melanoma, breast cancer and embryonal carcinoma. In several cancers and tumour cell lines expression of galanin receptors--three are known (GalR1, 2 and 3)--has been shown as well. Expression of peptide or receptors has been correlated with tumour stage or subtypes of pituitary adenoma, neuroblastic tumours, colon carcinoma and squamous cell carcinoma. Galanin treatment has tumour-reducing effects in murine models of gastrointestinal cancer, whereas in animal experiments on adenoma formation, galanin seems to act as a growth factor, promoting both proliferation and tumour formation. In cell culture experiments on tumour cell lines, galanin has shown growth promoting or inhibiting effects. Activation of GalR1 is generally anti-proliferative, whereas activation of GalR2 can have pro- or anti-proliferative effects. Therefore, galanin and its receptors are promising targets for diagnosis and treatment of several types of tumours.

Galanin decreases proliferation of PC12 cells and induces apoptosis via its subtype 2 receptor (GalR2)

Galanin is a neuropeptide with a wide range of effects in the nervous and endocrine systems, mediated through three G protein-coupled receptor subtypes (GalR1-3). Interestingly, galanin and its receptors are also expressed in certain tumors. Here we studied the effects of galanin in rat pheochromocytoma (PC12) cells stably transfected with GFP-tagged GalR2. Galanin at 100 nM inhibited cell proliferation in both nontransfected and transfected cells. Conversly, both galanin and the GalR2(R3)-agonist AR-M1896 induced caspase-dependent apoptotic cell death only in GalR2-transfected cells. Western-blot analyses of downstream mediators of the G(q/11)-type G protein showed down-regulation of pAkt and pBad in galanin-exposed transfected cells. Also, the specific PI3 kinase inhibitor LY-294002 increased the level of pBad and decreased activation of caspases. In addition, p21(cip1) levels were up-regulated in galanin-exposed PC12 cells and down-regulated in galanin-exposed GalR2-transfected cells. In agreement, FACS analyses of galanin exposed cells showed occurrence of cell cycle arrest in PC12 cells and cell death in transfected cells. Finally, as shown with real-time PCR, galanin and its receptors were expressed at very high levels in human pheochromocytoma tissues as compared with normal adrenal medulla. These findings point to GalR2 as a possible target for therapeuthic interventions in pheochromocytoma.

Galanin and galanin receptors in human cancers

The increasing interest in peptides and peptide receptors in cancer is based on the possibility of receptor targeting, because peptide receptors are often expressed in different human tumors. The neuropeptide galanin has also been suggested to be involved in the development of neuroendocrine tumors based on the development of estrogen-induced tumors in estrogen-sensitive rats. This study summarizes our current knowledge on the expression of galanin peptide and galanin receptors in different human neuroendocrine tumors. The expression of both, peptide and corresponding receptor, seems to be a common feature of human gliomas, pheochromocytomas, pituitary and neuroblastic tumors. The co-expression of galanin and its receptors supports a role for galanin in tumor cell pathology via autocrine/paracrine mechanisms.

Galanin immunoreactivity in paragangliomas but not in carcinoid tumors

Galanin is a 29 to 30-amino acid neuropeptide involved in diverse regulatory effects in the central and peripheral nervous systems. Although galanin has been found in paragangliomas and other tumors, its expression has not been studied in carcinoid tumors arising in various locations. Galanin was detected in 62% (15/24) of adrenal pheochromocytomas, 40% (2/5) of jugulotympanic paragangliomas, and 18% (2/11) of carotid body paragangliomas, but was absent in 4 extra-adrenal pheochromocytomas and 3 metastatic or recurrent paragangliomas. All 26 carcinoid tumors were negative. Galanin may have limited utility as a diagnostic marker alone but would be most useful in conjunction with other immunohistochemical markers for the differential diagnosis of neuroendocrine tumors.

Galanin and galanin receptor expression in neuroblastic tumours: correlation with their differentiation status

Neuroblastoma and its benign differentiated counterpart, ganglioneuroma, are paediatric neuroblastic tumours arising in the sympathetic nervous system. Their broad spectrum of clinical virulence is mainly related to heterogeneous biologic background and tumour differentiation. Neuroblastic tumours synthesize various neuropeptides acting as neuromodulators. Previous studies suggested that galanin plays a role in sympathetic tissue where it could be involved in differentiation and development. We investigated the expression and distribution of galanin and its three known receptors (Gal-R1, Gal-R2, Gal-R3) in 19 samples of neuroblastic tumours tissue by immunohistochemistry, in situ hybridization and fluorescent-ligand binding. This study provides clear evidence for galanin and galanin receptor expression in human neuroblastic tumours. The messengers coding for galanin, Gal-R1 and -R3 were highly expressed in neuroblastoma and their amount dramatically decreased in ganglioneuroma. In contrast, Gal-R2 levels remained unchanged. Double labelling studies showed that galanin was mainly co-expressed with its receptors whatever the differentiation stage. In neuroblastic tumours, galanin might promote cell-survival or counteract neuronal differentiation through the different signalling pathways mediated by galanin receptors. Finally, our results suggest that galanin influences neuroblastoma growth and development as an autocrine/paracrine modulator. These findings suggest potential critical implications for galanin in neuroblastic tumours development.

Galanin and galanin receptors

The development of a strain of galanin knockout mice has provided confirmation of a neuroendocrine role for galanin, as well as supporting results of previous physiological investigations indicating a role for galanin in analgesia and neuropathic pain, and potentially in neuronal growth and regeneration processes. Whether elevation of galanin expression in neurodegenerative disorders such as Alzheimer's disease represents a survival response or exacerbates functional deficit in afflicted individuals remains to be determined. More detailed analysis of the phenotype of the galanin knockout mouse should provide insights into the physiological role of galanin in memory and learning processes, as well as in hypothalamic function and other aspects of neuroendocrine regulation. Biochemical and molecular cloning efforts have demonstrated that the multiplicity of actions of galanin is matched by complexity in the distribution and regulation of galanin and its receptors. A focus on characterisation of galanin receptors has resulted in the molecular cloning of three receptor subtypes to date. The distribution and functional properties of these receptors have not yet been fully elucidated, currently precluding assignment of discrete functions of galanin to any one receptor subtype. It is not currently possible to reconcile available pharmacological data using analogs of galanin and chimeric peptides in functional assay systems with the pharmacological properties of cloned receptor subtypes. This highlights the value of further knockout approaches targeting galanin receptor subtypes, but also raises the possibility of the existence of additional receptor subtypes that have yet to be cloned, or that receptor activity may be modulated by regulatory molecules that remain to be identified. The development of receptor subtype-specific compounds remains a high priority to advance work in this area. The ability to selectively modulate the many different actions of galanin, through a clearer understanding of receptor structure-function relationships and neuronal distribution, promises to provide important insights into the molecular and cellular basis of galanin action in normal physiology, and may provide lead compounds with therapeutic application in the prevention and treatment of a range of disorders.