Trophic and neurotrophic factors in human pituitary adenomas (Review)
- Authors:
- Published online on: September 5, 2017 https://doi.org/10.3892/ijo.2017.4120
- Pages: 1014-1024
Abstract
1. Introduction
The pituitary gland is an organ that physiologically connects the hypothalamus with the peripheral organs. The pituitary gland is an important regulator of body homeostasis during development, stress, and other physiological processes. This small organ is localized in a tiny cavity called sella turcica. The pituitary fossa is a depression in the bone structure at the base of the brain.
The pituitary is functionally and anatomically connected to the hypothalamus by the median eminence. The gland receives blood through the hypophyseal portal circulation, which carries the hypothalamic hormones to the specialized adenohypophyseal cells.
The pituitary gland is composed by adenohypophysis (or anterior pituitary) and neurohypophysis (or posterior pituitary) that are two different lobes. The adenohypophysis contains three regions: the pars tuberalis (pars infundibularis), the pars intermedia (intermediate lobe) and pars distalis (also known as the anterior lobe). The pars intermedia is placed in the marginal area between the anterior pituitary and the posterior pituitary. The pars distalis represents the largest part of the adenohypophysis while the intermediate lobe is a small portion of the gland. The different cells of pituitary gland secrete many molecules: endocrine hormones, cytokines and growth factors.
The neurohypophysis, or posterior pituitary, is characterized by axonal terminals of the hypothalamic supraoptic and paraventricular (PVN) nuclei. The neurohypophysis contains the pituicytes: fusiform cells related with microglia. The pituicytes surround the axons influencing the secretion of neurohypophyseal hormones (1) as oxytocin and vasopressin.
Hypothalamic stimulatory and inhibitory factors and other molecules interact with the auto- and paracrine factors to induce transcriptional regulation, translation and secretion of the pituitary hormones. Hormones released by the anterior pituitary are produced by specialized cells. Growth hormone (GH) is produced by somatotroph, prolactin (PRL) by lactotrophs, adrenocorticotropic hormone (ACTH) by corticotrophs, thyroid-stimulating hormone (TSH) by thyreotrophs, luteinizing hormone (LH) and follicle-stimulating hormone (FSH) by gonadotrophs cells. The function of the specialized cells is influenced by the interaction between cells via systemic signals.
The lactotrophs are the cells located in the anterior pituitary and they interact with the gonadotrophs to improve the paracrine secretion. Gonadotrophs are in the pars distalis and tuberalis, while thyrotrophs, that represent a small section of the total pituitary cells, are found in the anterior-medial part of the pituitary gland and they secrete the subunits of PRL, LH, and FSH. In the central mucoid wedge of the pituitary gland, we can observe the presence of corticotrophs that constitute 15–20% of the adenophypophyseal cells (2). Folliculostellate cells (FS) represent a small rate of total population of pituitary cells. Some immunohistochemical studies described the presence of molecular connections between the FS cells in anterior gland (3,4). FS cells in the normal pituitary tissue secrete large amounts of VEGF (5,6) that improve blood vessel system of the gland (7).
The hypophysis may be affected by different pathologies causing endocrine and neurological disorders. Among these pathologies there are craniopharyngioma, pituicytoma and granular cell tumor, chordoma and the pituitary adenomas that are associated with endocrine dysfunctions such as Cushing's disease or acromegaly. Some pituitary neoplasms are simulated by pathological conditions as lymphocytic or granulomatous hypophysitis.
Craniopharyngiomas are relatively uncommon tumors and arise in the suprasellar region. They are believed to be congenital. Craniopharyngiomas involve the pituitary gland and they can modify the endocrine functions and damage optical nerves and chiasm, causing vision problems (8).
The pituicytoma is a rare neoplasm in the posterior pituitary gland that arises in the infundibulum or the neurohypophyseal cells. A possible development of this tumor was also noticed from the FS cells. In a recent study, a positivity for vascular endothelial growth factor (VEGF) was detected in patients with pituicytoma (9).
Granular cell tumors (GCTs) can arise in different regions of the body: skin, head and neck. GCTs occur rarely in the central nervous system developing in the posterior hypophysis and cerebral hemispheres (10,11).
Intracranial chordomas are characterized by soft and gelatinous lesions usually developing at the dorsum sellae. The cells are large and vacuolated. Chordomas may produce compression of the pituitary and destruction of the pituitary fossa. These lesions slowly reach neurovascular bundles and often cannot be treated surgically (12).
Pituitary adenomas constitute a common group of benign tumors arising in adenohypophyseal cells of the anterior lobe of the pituitary gland (13,14). Some adenomas are similar to malignant tumors, invading the cavernous sinus, sphenoid sinus and hypothalamus (15). The invasive adenomas are characterized by large size, rapid growth and scarce response to treatment. Pituitary adenomas may be subdivided to functional and non-functional, depending on their hormonal activity in vivo (14,16,17).
Tumors may hypersecrete pituitary hormones which can determine endocrine disturbances of physiological mechanisms that are controlled by the gland. The pituitary adenomas can produce great doses of GH causing gigantism or acromegaly, ACTH leading to Cushing's disease and PRL, which negatively influences reproduction. Some very rare pituitary adenomas secrete FSH and LH (which cause reproductive dysfunction) or TSH that leads to hyperthyroidism. However, some pituitary tumors do not produce hormones, but their growth and expansion may produce a reduced function of the gland (hypopituitarism). Non-functioning pituitary adenomas represent ~30% of all pituitary tumors (18). Non-functioning pituitary adenomas may be characterized by the feature of invasive macroadenomas that determine the onset of neurological symptoms (19).
Regarding the different therapeutic approaches, which include trans-sphenoidal resection, pharmacotherapy, and radiation therapy, the results remain inadequate in a significant number of patients (20,21). The invasiveness of pituitary adenomas seems to be an important determinant for the success rate of surgical treatment. Generally, this tumor has no capsule that could separate it from the adjacent tissue and, consequently, the growing adenoma can reach and invade the adjacent structures (22). Such adenomas may produce some symptoms through two mechanisms: i) hypersecretion or hyposecretion of hormones and i) compression exerted on the neighbouring structures (23). Functioning pituitary adenomas become symptomatic because they lead to hormone secretion whereas the non-functioning variety may grow slowly and compress the optic chiasm situated directly above the pituitary gland, producing progressive visual loss (24).
Actually, the causes responsible for the pathogenic processes of initiation, expansion and invasion of pituitary tumors are not clear, but different mechanisms are involved in the pituitary tumorigenesis. A small percentage of tumors is hereditary and so these tumors may be due to genetic mutations (20). Recent studies suggest that neurotrophins and other growth factors play a significant role in pituitary adenoma development (25–29) (Table I). In this review we summarize and discuss the data regarding the trophic and neurotrophic factors which seem to influence the proliferation and growth of pituitary adenomas.
2. Ciliary neurotrophic factor (CNTF)
The CNTF is a neurotrophin of the IL-6 family and has important neuroprotective effects on neurons. CNTF acts by binding to several receptors: CNTF receptor (CNTFR), gp130 and the leukemia inhibitory factor receptor (LIFR). The resultant CNTF-CNTFRα complex induces the formation of the LIFRα-gp130 heterodimer. When CNTF interacts with the receptors it activates the Janus kinases/signal transducer and activator of transcription (JAK/STAT), mitogen-activated protein kinase (MAPK), and phosphatidyl inositol 3-kinase/protein kinase B (PI3K/Akt) signalling pathways (30). Some studies found that the deletion of the CNTF gene in mice determines the degeneration of motor neurons and the increasing of an inflammatory demyelinating disease (31,32). The CNTF have been demonstrated to be important for autocrine and paracrine mechanisms that act in the pituitary gland (33). CNTF is expressed in folliculostellate cells and in lactosomatotropic cells and its secretion stimulates the production of GH and PRL (34,35).
Some authors observed that folliculostellate and lactosomatotrophic cells express the mRNA of CNTF (34) and the mRNA of the α-chain specific for the CNTFR. This mRNA for the CNTFR was detected in tumors secreting PRL, GH and in non-functioning tumors (35). CNTFR are expressed on lactotropic, somatotropic and non-functioning pituitary adenomas demonstrating that these receptors are also present on human pituitary cells. CNTFR was seen to be involved in pituitary pathophysiology (35). In particular, CNTF did not influence the secretion of either GH or PRL and on GH mRNA in monolayer cell cultures obtained from normal rat anterior pituitary. However, CTNF significantly stimulated both PRL and GH secretion when the cells were amassed in cultures, restoring the three-dimensional structure of the cells. These results underline that the three-dimensional structure of the pituitary cells represents a key role for the regulatory action of CTNF in anterior pituitary cells (35). The three-dimensional organization of the cells constitutes their physiological conformation that can also result in a better expression of receptors. It was also observed that the interaction of hormone-secreting cells with the extracellular matrix is determining the role of CNTF in regulation of GH and PRL in producing the pituitary cells.
3. Transforming growth factors β (TGF-β)
TGF-β is a suppressor or a promoter of tumor development in relation to the tumor stage and type (25,36). TGF-β signalling starts through the binding of some ligands (TGF-β1, TGF-β2 and TGF-β3) with type II TGF-β receptors (TGF-β RII): subsequently the recruitment of type I TGF-β receptor (TGF-β RI) forms a complex (37–39). Moreover, TGF-β RII phosphorylates TGF-β RI to activate it (37–39). TGF-β signal transducer proteins are Smads and the activated TGF-β receptor complexes can phosphorylate Smad2 and Smad3. These proteins bind Smad4 to form a complex that regulates the transcriptional activity. On the other hand, Smad7 is an inhibitory protein that suppresses the phosphorylation of Smad2 and Smad3 (40–42). Some authors reported that Smad3 and phospho-Smad3 are potential markers of invasive non-functioning pituitary adenomas. In particular, it was observed that the invasion of non-functioning pituitary adenoma is associated with low level of expression of Smad3 and phospho-Smad3 and that proliferative activity was higher in invasive non-functioning pituitary adenomas when compared to non-invasive non-functioning pituitary adenomas (43).
The clinical importance of TGF-β ligands and downstream signalling mediators has been analyzed in some studies performed in different types of tumors although the results obtained are discordant (37–39). In non-functioning pituitary adenomas, the expression of mRNA TGF-β1 levels was significantly lower than in invasive non-functioning pituitary adenomas and in non-invasive non-functioning pituitary adenomas, in comparison to normal anterior pituitaries (43). It has been suggested that the invasiveness of pituitary adenomas could be predicted by TGF-β1 blood serum concentration (44). Some studies widely evaluated the role of the TGF-β1 gene in some tumors. The expression of this gene was studied in subjects with breast cancer and a relationship was found between TGF-β1 gene and a poorer patient clinical outcome (45). The evaluation of a tumor in lung cell lines, established that this gene plays an important role in cell proliferation (46). TGF-β1 is commonly evaluated in tumors: in fact, the in vitro studies were performed on TGF-β1 function in cancer cell lines other than lung cancer and pituitary adenoma cells (47).
A study described the development of prolactinomas in transgenic female mice with pituitary TGF-α transgene expression (48). The levels of pituitary TGF-α mRNA become high before initiation of lactotroph hyperplasia. TGF effects are increased in vivo conditions by estrogen, and TGFs do not seem to improve other pituitary cell type cancers.
4. Glial cell line-derived neurotrophic factor (GDNF)
GDNF is a component of a large family of neurotrophic factors that include GDNF, Neurturin, Artemin, and Persephin (49,50). GDNF plays a crucial role in the development and survival of various neuronal populations (51,52). However, the intracellular trafficking mechanisms of GDNF are not fully understood. Some studies demonstrated that although GDNF is not essential for the development of dopaminergic neurons (49,50), the presence of GDNF is important for the maintenance of these cells in A9 dopaminergic neurons of tyrosine kinase receptor (RET) knockout mice (51). In fact, degeneration of A9 neurons is evident in Parkinson's disease (PD) and GDNF was retained to be a possible therapeutic drug for PD. The signalling of GDNF is mediated through a system including GFRa1 which binds to GDNF. Subsequently, this complex binds and triggers the RET (52,53). Recently, GDNF and RET gene expression have been found in anterior pituitary glands from male rats (54). In an interesting study, a positive immunostaining for GDNF was observed in all of the GH-secreting pituitary adenomas and in 10% of the corticotropinomas (55). Some experimental evaluations showed that both RET and GDNF are normally secreted in the human pituitary gland. These results were confirmed through other experimental techniques, demonstrating that the pituitary gland produces GDNF and that the gland itself is also a target tissue of neurotrophins (55). GDNF is mostly present in somatotrophs and, to a lesser extent, in corticotrophs, but it is not present in gonadotrophs of the human pituitary gland (55). In particular, a positive immunostaining for GDNF was observed in all of the GH secreting adenomas and in 10% of the corticotropinomas, but it was negative in all other pituitary tumors.
5. Nerve growth factor (NGF)
NGF is a growth factor expressed by peripheral tissues that are innervated by sensory and sympathetic neuronal projections and it belongs to the nerve growth factor family of neurotrophins. NGF controls neuronal survival, differentiation and growth binding to two receptors: the p75 neurotrophin receptor (p75NTR) and tropomyosin-related kinase A (TrkA) (56–58). Some studies have demonstrated that NGF is involved in tumor progression, increasing cancer cell survival and proliferation (59–61). NGF is not only described in the nervous system, but also in some normal and neoplastic human tissues (62,63). In the submandibular gland of the mouse NGF is composed of 2α subunits, 1β subunit, 2γ subunits (α2βγ2) and also one or two zinc ions (64). The β subunit of NGF represents a biologically active region, the 2γ subunits of NGF have proteolytic activity and the 2α subunits do not possess enzymatic activity (65–67).
NGF acts as a regulator of neuronal survival, proliferation, and differentiation in the peripheral and central nervous systems by binding to its receptors: TrkA and p75NTR. The binding between p75NTR and NGF controls the activation of c-Jun N-terminal kinase (JNK) signalling pathways to promote apoptosis, and the activation of nuclear factor κB (NF-κB) pathways to promote cell survival. Moreover, NGF increases cell proliferation and metastasis binding to TrkA (68,69). Some researchers have also found that p75NTR may help to improve the binding of NGF with TrkA, TrkA activation and the number of binding sites (70). TrkA regulates growth and differentiation of neurons in peripheral and central nervous systems (71). NGF facilitates the development of perivascular nerves to regulate the blood flow in tumors. NGF may promote angiogenesis by interacting with α9β1 integrin (72). In one study it was observed that p75NTR in tumor cells may negatively regulate cell growth and proliferation (73). An increased apoptosis (pro-apoptotic effect) was demonstrated in cells with a high expression of p75NTR from patients with medulloblastoma (74). In prolactinoma cells, NGF binds p75NTR and activates NF-κB in a TrkA-independent way (75). It was observed that TrkA triggers proliferation in some tumor cells (76,77), but inhibits cell growth in other tumors (78,79). In some patients with pituitary adenomas, a moderate expression for NGF, and its relative receptor TrKA and p75NTR were observed on epithelial glands (29) (Fig. 1). It has also been found that p75NTR suppresses some tumors (80,81), but has a mitogenic effect in others (82). The cell cycle controls the cell proliferation and the G1 phase of the cycle is regulated by the protein p53 that is a tumor suppressor (83). Inactivation of this pathway influences tumorigenesis. Some mutations that inactivate the p53 gene are observed in more than 50% of all human cancers (84). However, p53 is rarely mutated in human pituitary adenomas (85). Mammosomatotroph pituitary cells express NGF and its receptors. Mice, in which transgenic NGF is driven by the prolactin promoter, develop lactotroph hyperplasia without adenomas, despite having markedly enlarged pituitary glands (86).
6. Vascular endothelial growth factor (VEGF)
VEGF is a central regulatory protein of angiogenesis with a hydrophobic leader sequence. It has a homodimeric structure with three intramolecular and two intermolecular disulfide (S-S) bonds. It is a member of a family that exerts important functions in vasculogenesis, angiogenesis and lymphangiogenesis. It was found that VEGF is a tumor-secreted protein that can increase microvascular permeability to plasma proteins. In particular, it improves vascular permeability to plasma and plasma proteins, a typical characteristic of the tumor microvasculature and a critical early step in cancer stroma generation. VEGF is overexpressed in the cells of several human vascular tumors of the brain, colon, gastrointestinal tract, ovary, and breast (87). Some studies found that VEGF may not directly participate to tumoral invasion, but it may regulate pathways that increase tumor volume or invasiveness (88–91). Results obtained in a study, described that VEGF operates as a neurotrophic factor and plays an important role during the regeneration of peripheral nerves (92).
VEGF binds and activates the type 1 and 2 vascular endothelial growth factor receptors (VEGFR1 and VEGFR2) on the vascular endothelium (93). In the pituitary VEGF and VEGFR2 are expressed (94,95). Various experimental publications reported that VEGF expression is not necessarily related to endothelium and vessels, but it is also found in adenoma cells (28,96). Distribution and location of VEGF receptors in pituitary adenomas has also been studied. A relevant expression of fetal liver kinase-1, a type of VEGF receptor that controls mitogenesis and influences endothelial cell characteristics, may have a potential role in the pituitary tumorigenesis (97). According to this observation fetal liver kinase-1 expression was very marked in non-functioning adenomas in comparison to functioning tumors (97). Other reports concerning the expression and the distribution of VEGF and its receptors in pituitary adenomas are not conclusive. In another study, VEGF expression was recognized as different in the subtypes, thus suggesting possible different modalities of VEGF expression and/or effects (28). The tyrosine kinase receptors of VEGF are mainly expressed on vascular endothelium, and it behaves as a selective mitogen for vascular endothelium.
VEGF contributes to the formation of the vascular network of a new pituitary tumor (98,99) and is also involved in the proliferative action of estrogen on lactotrophs (100), VEGF might contribute to adequate temporal vascular supply. It was observed that VEGF is expressed in all cell types of the pituitary, but mainly in somatotrophic and follicle-stellate cells. In the normal human pituitary gland, VEGF expression was higher than adenomas (28). One study found no differences in VEGF expression among tumors of different histotypes (101). Also, comparing VEGF in different types, the highest expression was observed in non-functioning adenomas and GH producing adenomas (97). Elevated serum VEGF levels have been determined in patients with pituitary tumors and VEGF secretion was measured in human pituitary tumors cultured in vitro. In a group of patients with pituitary adenomas it was observed that VEGF protein expression was higher in dopamine agonist resistant prolactinomas compared to non-functioning GH and ACTH secreting adenomas (102). Different VEGF levels in ACTH-secreting adenomas may be produced by glucocorticoids which are efficient inhibitors of VEGF secretion (103). VEGF targeting in pituitary adenomas may be useful as shown in a study performed on mice (104).
The VEGF is characterized from several variants of the growth factor that can bind one of the three VEGF-specific receptor tyrosine kinases to influence angiogenesis or related processes (105). The VEGF secretion is increased by hypoxia of the tumor. The deficiency of oxygen increases the expression of hypoxia-inducible factor 1 (HIF1), which enhances the VEGF expression.
7. Vascular endothelial growth inhibitor (VEGI)
VEGI is a protein member of the TNF superfamily that can inhibit the proliferation of endothelial cells and exerts an anti-angiogenic effect on the endothelial cells (106). VEGI is mainly produced by vessel endothelial cells and also be expressed on antigen-presenting cells and lymphocytes such as T cells and dendritic cells. VEGI always acts as a co-stimulator to induce T cell proliferation and cytokine secretion (107,108). VEGI acts by interacting with two receptors: death receptor-3 (DR-3) and the decoy receptor-3 (DcR-3). DR-3 is also known as TNFRSF25: this receptor is a protein of approximate 47 kDa in size and is a member of the TNF receptor superfamily. So far DR3 is the only known functional receptor for VEGI and its role is to induce apoptosis after activation.
Many studies have shown that VEGI is related to various diseases including bowel diseases (109), lung cancer (110), prostate cancer (111), breast cancer (112) and pituitary adenomas (27). The authors of these studies described how VEGI expression is decreased in late stage tumors and is associated with survival of patients. VEGI and other factors may influence the development of diseases through the involvement of some pathways (Fig. 2). It was recently found that VEGI has a direct effect on both epithelial and cancer cells, also by exerting an inhibitory effect on the migration and growth of neoplastic cells. Some results indicated that VEGI plays an essential role in activating the transcription factor κB and caspase-3 (113). Other studies suggested that VEGI may also be involved in the immune response by inducing the secretion of granulocyte-macrofage colony-stimulating factor (GM-CSF) and interferon γ IFN-γ (114). VEGI gene expression also decreases inflammation and angiogenesis in cancers or wounds (114). Studies that involved the cell cycle suggested that VEGI maintained early G1 arrest in the G0/G1 cells and induced programmed death in the endothelial cell cycle (115).
Pituitary tumors with cystic lesions tended to express low levels of VEGI and, in particular, pituitary tumors that invade the floor of the sella turcica showed reduced levels of VEGI. VEGI has an anti-angiogenesis function and some studies demonstrated that it acts as a powerful angiogenesis inhibitor. Lack of VEGI was negatively correlated with angiogenesis in solid tumors, due to the removal of the inhibitory mechanisms in the tissues (116,117). In pituitary adenomas, VEGI inhibits the growth and the migration of tumor cells through DR3 (113). In particular, high levels of VEGI and DR3 were found to be associated with intratumoral haemorrhage. It is thus argued that, in a similar pattern, the lack of VEGI in pituitary tumors may trigger an increase in angiogenesis within the tumor tissues.
8. Fibroblast growth factors (FGFs)
FGFs are a family of molecules that control the differentiation, migration, and angiogenesis of the cells (118,119). FGFs includes 23 ligands and FGF2 is the basic FGF, which regulates the production of pituitary hormones and the proliferation and differentiation of parenchyma and vessels (120). FGF2 isoform of 18 kDa is highly secreted in the normal human pituitary gland, whereas the 24 kDa isoform is largely secreted by pituitary adenomas (121). It was observed that high levels of FGF2 were expressed in patients with pituitary neoplasms and the secretion decreased after surgical adenomectomy (122). In a recent study, it was found that FGF2 is secreted in different types of pituitary tumors, including GH secreting adenomas (123). FGF2 expression was significantly higher in patients without postoperative remission observed at the third month and later, if compared to subjects with remission (123). Also, the levels of FGF2 were higher in patients who presented a sphenoid bone invasion if compared to patients without bony lesions (123).
The interaction between FGF2 and some transmembrane receptors (FGFRs) with tyrosine kinase activity, determines the biological effects of FGF2 (124). The FGFRs are encoded by different genes to obtain multiple isoforms, also the FGFRs are expressed in the membrane surfaces of different type of cells, including endothelial cells in which FGF2 influences the angiogenic development. Each FGFR is characterized by three Ig-like extracellular domains, a transmembrane domain, a tyrosine kinase cytoplasmic domain and a -COOH domain that comprehends a group of tyrosines phosphorylated by the binding with the ligand (125,126). A great percentage of FGFs shows a specific affinity for the receptor isoforms. It was observed that FGFRs are expressed in different types of tumors, including pituitary adenomas (127–132). In particular, FGFR1 was highly expressed in pituitary adenomas compared to healthy gland (132) and the cytoplasmic immunoreactivity of the receptor was inversely correlated with pituitary lesion size (133).
A study has demonstrated the lack of FGFR2 in the pituitary adenomas that causes the upregulation of the melanoma-associated antigen 3 (MAGEA3) (134). The presence of arginine at codon 388 of the FGFR4 gene, which encodes the receptor, was associated with the phosphorylation process of mitochondrial STAT3 serine that improves the GH secretion in pituitary cells promoting tumorigenesis development (135). Also, this arginine allele was associated with other forms of tumors resistant to the pharmacological treatment (136–139). The expression of FGFR isoforms was really modified in pituitary adenomas (140). It was observed that the modified FGFR4 expression in pituitary tumors is caused by the pituitary tumor-derived FGFR4 (ptd-FGFR4) (141). This is an isoform with a deletion at the N-terminal produced by substitute transcription initiation (141,142). The FGFR4 isoforms have a different capability to link the cell adhesion molecules and, in particular, ptd-FGFR4 blocks cell adhesion interacting with N-cadherin signaling (143). The complex constituted by FGFR4 and N-cadherin may be considered an important therapeutic target to reduce the growth and the invasiveness of tumor cells (143).
9. Epidermal growth factor (EGF)
EGF is a small protein of 6 kDa containing 53 amino acids which comprises three disulfide bridges (144). EGF displays a homology of function and sequence with human type-α transforming growth factor (hTGF-α), which can link the EGF receptor sites. EGF is characterized by mitogenic activity and so it is involved in the process of cell growth and tumorigenesis. In fact, EGF is a protein contained in a network of growth factors and receptors that controls the growth and the division of cells. EGF is produced by pituitary cells and it acts as a growth factor, stimulating prolactin synthesis (145). In human pituitary gland EGF is localized mainly in thyrotrophic and gonadotrophic cells (145). An immunohistochemical study described the EGF expression in functional and non-functional pituitary adenomas (145).
It was observed that EGF can promote the transphosphorylation of the related oncogene neu through the binding with epidermal growth factor receptor (EGF-R) (146). EGF-R is a 170-kDa transmembrane glycoprotein constituted by: a tyrosine kinase domain, a transmembrane domain and an extracellular domain of binding. EFG-R is a component of a family which includes four known members and it is considered the homolog of the v-erbB oncogene protein (147). Some studies evidenced EGF-R overexpression in different tumors: lung (148), breast (149), ovarian (150) and gastric (151) cancers. Different studies observed a higher expression in non-functional pituitary tumors (152–154). Interestingly, Onguru et al described a moderate or a strong positivity of EGFR in more than 50% of adenomas (154). In each group of adenomas the percentage of EGFR overexpression was variable and it was higher in non-functional compared to functional adenomas.
Also, in ACTH secreting adenomas the lowest number of tumors expressing EGFR was detected (154). However, high expression of EGFR is frequently found in ACTH secreting adenomas, in which EGF signaling is deleted and consequently there is inhibition of ACTH secretion (155,156). The binding between intracellular EGFR domain and its specific antibody showed an immunoreactivity in corticotroph adenomas and many cells of ACTH secreting tumors were EGFR immunoreactive (157). Also, a low or absent immunoreactivity was observed for p27 protein that blocks the cell cycle (157). Probably, EGFR promotes the signaling activation that induces the downregulation of p27, which promotes tumorigenesis by the stimulation of ACTH secretion (158).
10. Conclusion
Pituitary adenomas are common tumors that are classified on the basis of certain characteristics: size, invasion of adjacent structures, sporadic or familial cases, biochemical activity, clinical manifestations, morphological characteristics, response to treatment and recurrence. Although they are considered benign tumors, some of them are difficult to treat because they can recur after standardized treatment. Some studies indicate that trophic and neurotrophic factors play a significant role in neuroendocrine systems and in the biological effects of molecules involved in the development and maintenance of the nervous system. There is some evidence suggesting that trophic and neurotrophic factors may be involved in pituitary endocrine cell function, suggesting a possible role of trophic and neurotrophic factors in the normal development of the pituitary gland and in the progression of some pituitary adenomas.
The pituitary cells are regulated by endocrine and paracrine systems through the action of some growth factors and their receptors. A different expression and action of these factors and their receptors may determine the development and progression of pituitary tumors. The vascularization in the normal pituitary regulates the growth of pituitary cells and the hormone secretion by some molecules: hormones, hypothalamic and pituitary growth factors. However, the high secretion of EGF, FGF and VEGF in pituitary is involved in gland tumorigenesis (159). Generally, the pituitary adenomas have a low grade of carcinogenicity. In fact, these tumors are rarely aggressive and the angiogenesis is not predominantly involved in the improvement of the release of nutrients to the tumor. Basing upon these reasons, it may be interesting to study the possible role of VEGF involved in pituitary angiogenesis. The development of the vascularization in pituitary tumors is less present than in the normal anterior pituitary tissue (160,161).
The biological role of these factors in the development and progression of this type of tumor should be further investigated to ameliorate the knowledge of the pathogenesis of pituitary adenomas.
Acknowledgments
This study was supported by Nobile S.p.A. Grant from REGIONE LAZIO Prot. FILAS-RU-2014-1020 is gratefully acknowledged (EA).
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