Pituitary adenylate-cyclase-activating polypeptide (PACAP) binding sites and PACAP/vasoactive intestinal polypeptide receptor expression in human pituitary adenomas

Diagnostic and Prognostic Value of PACAP in Multiple Myeloma

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a multifunctional neuropeptide with well-known anti-inflammatory, antioxidant, antitumor, and immunomodulatory effects. PACAP regulates the production of various proinflammatory factors and may influence the complex cytokine network of the bone marrow microenvironment altered by plasma cells, affecting the progression of multiple myeloma (MM) and the development of end-organ damage. The aim of our study was to investigate the changes in PACAP-38 levels in patients with MM to explore its value as a potential biomarker in this disease. We compared the plasma PACAP-38 levels of MM patients with healthy individuals by ELISA method and examined its relationship with various MM-related clinical and laboratory parameters. Lower PACAP-38 levels were measured in MM patients compared with the healthy controls, however, this difference vanished if the patient achieved any response better than partial response. In addition, lower peptide levels were found in elderly patients. Significantly higher PACAP-38 levels were seen in patients with lower stage, lower plasma cell infiltration in bone marrow, lower markers of tumor burden in serum, lower total urinary and Bence-Jones protein levels, and in patients after lenalidomide therapy. Higher PACAP-38 levels in newly diagnosed MM patients predicted longer survival and a higher probability of complete response to treatment. Our findings confirm the hypothesis that PACAP plays an important role in the pathomechanism of MM. Furthermore, our results suggest that PACAP might be used as a valuable, non-invasive, complementary biomarker in diagnosis, and may be utilized for prognosis prediction and response monitoring.

Colorectal Cancer Early Detection in Stool Samples Tracing CpG Islands Methylation Alterations Affecting Gene Expression

Colorectal cancer (CRC) is a major cause of cancer mortality. Early diagnosis is relevant for its prevention and treatment. Since DNA methylation alterations are early events in tumourigenesis and can be detected in cell-free DNA, they represent promising biomarkers for early CRC diagnosis through non-invasive methods. In our previous work, we identified 74 early altered CpG islands (CGIs) associated with genes involved in cell cross-talking and cell signalling pathways. The aim of this work was to test whether methylation-based biomarkers could be detected in non-invasive matrices. Our results confirmed methylation alterations of GRIA4 and VIPR2 in CRC tissues, using MethyLight, as well as in stool samples, using a much more sensitive technique as droplet digital PCR. Furthermore, we analysed expression levels of selected genes whose promoter CGIs were hypermethylated in CRC, detecting downregulation at mRNA and protein levels in CRC tissue for GRIA4, VIPR2, SPOCK1 and SLC6A3. Most of these genes were already lowly expressed in colon normal tissues supporting the idea that cancer DNA methylation targets genes already barely expressed in the matched normal tissues. Our study suggests GRIA4 and VIPR2 as biomarkers for early CRC diagnosis using stool samples and confirms downregulation of genes hypermethylated in CRC.

Expression of cell cycle regulators and biomarkers of proliferation and regrowth in human pituitary adenomas

PURPOSE

The pathogenesis of pituitary adenomas (PA) is complex. Ki-67, pituitary tumour transforming gene (PTTG), vascular endothelial growth factor (VEGF), cyclin D1, c-MYC and pituitary adenylate cyclase-activating peptide (PACAP) protein expression were analysed and correlated with tumour and patient characteristics.

METHODS

74 pituitary tumour samples (48 non-functional PA, 26 functional PAs); Immunohistochemical analysis of protein expression, retrospective analysis of MR images and in vitro analysis of octreotide treatment was carried out on GH3 cells.

RESULTS

PTTG expression was negatively associated with age and positively with PA size, regrowth and Ki-67 index. Cyclin D1 correlated with Ki-67 and tumour size. c-MYC negatively correlated with size of tumour and age; and correlated with PTTG expression. Somatostatin analogue treatment was associated with lower Ki-67, PTTG and Cyclin D1 expression while T2 hypointense PAs were associated with lower PTTG, cyclin D1, c-MYC and Ki-67. In vitro analyses confirmed the effect of somatostatin analogue treatment on Pttg and Cyclin D1 expression.

CONCLUSIONS

Interesting and novel observations on the differences in expression of tumour markers studied are reported. Correlation between Ki-67 expression, PTTG nuclear expression and recurrence/regrowth of PAs, emphasizes the role that Ki-67 and PTTG expression have as markers of increased proliferation. c-MYC and PTTG nuclear expression levels were correlated providing evidence that PTTG induces c-MYC expression in PAs and we propose that c-MYC might principally have a role in early pituitary tumorigenesis. Evidence is shown that the anti-proliferative effect of somatostatin analogue treatment in vivo occurs through regulation of the cell cycle.

PACAP and VIP Inhibit the Invasiveness of Glioblastoma Cells Exposed to Hypoxia through the Regulation of HIFs and EGFR Expression

Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) through the binding of vasoactive intestinal peptide receptors (VIPRs), perform a wide variety of effects in human cancers, including glioblastoma multiforme (GBM). This tumor is characterized by extensive areas of hypoxia, which triggers the expression of hypoxia-inducible factors (HIFs). HIFs not only mediate angiogenesis but also tumor cell migration and invasion. Furthermore, HIFs activation is linked to epidermal growth factor receptor (EGFR) overexpression. Previous studies have shown that VIP interferes with the invasive nature of gliomas by regulating cell migration. However, the role of VIP family members in GBM infiltration under low oxygen tension has not been clarified yet. Therefore, in the present study we have investigated, for the first time, the molecular mechanisms involved in the anti-invasive effect of PACAP or VIP in U87MG glioblastoma cells exposed to hypoxia induced by treatment with desferrioxamine (DFX). The results suggest that either PACAP or VIP exert an anti-infiltrative effect under low oxygen tension by modulating HIFs and EGFR expression, key elements involved in cell migration and angiogenesis. These peptides act through the inhibition of PI3K/Akt and MAPK/ERK signaling pathways, which are known to have a crucial role in HIFs regulation.

VPAC2 receptor expression in human normal and neoplastic tissues: evaluation of the novel MAB SP235

The vasoactive intestinal peptide receptor 2 (VPAC2) is widely distributed throughout the body and is also overexpressed in a variety of human neoplastic tissues. However, little is known about its precise tissue distribution, regulation and function, which is in part be due to the lack of specific monoclonal anti-VPAC2 antibodies. In this study, we extensively characterised the novel rabbit monoclonal anti-VPAC2 antibody (clone SP235) using transfected cells and mouse, rat and human tissues. SP235 was then subjected to a comparative immunohistochemical study on a series of 167 histological specimens from formalin-fixed, paraffin-embedded human tumours and adjacent normal tissues. SP235 detected a broad band migrating at a molecular weight of 50-70 kDa in western blotting analyses of various mouse tissues as well as VPAC2- but not VPAC1-transfected human embryonic kidney 293 cells. SP235 yielded an efficient immunostaining of distinct cell populations in human tissue samples with a predominance of plasma membrane staining, which was completely abolished by preadsorption with its immunising peptide. SP235 immunohistochemistry detected VPAC2 receptors in lymphocytes present in spleen, tonsils, lymph nodes and Peyer's patches, chief cells of gastric mucosa, exocrine and endocrine pancreas, kidney tubules and blood vessels. In addition, VPAC2 was observed in thyroid, gastric and lung carcinomas, pancreatic adenocarcinomas, sarcomas and neuroendocrine tumours. SP235 may prove of great value in the identification of VPAC2 receptors during routine histopathological examination. VPAC2 visualisation with this simple and rapid immunohistochemical method will facilitate identification of candidate tumours for vasoactive intestinal peptide (VIP)-based diagnostics or therapeutic interventions.

cAMP signalling in the normal and tumorigenic pituitary gland

cAMP signalling plays a key role in the normal physiology of the pituitary gland, regulating cellular growth and proliferation, hormone production and release. Deregulation of the cAMP signalling pathway has been reported to be a common occurrence in pituitary tumorigenesis. Several mechanisms have been implicated including somatic mutations, gene-gene interactions and gene-environmental interactions. Somatic mutations in G-proteins and protein kinases directly alter cAMP signalling, while malfunctioning of other signalling pathways such as the Raf/MAPK/ERK, PI3K/Akt/mTOR and Wnt pathways which normally interact with the cAMP pathway may mediate indirect effects on cAMP and varying downstream effectors. The aryl hydrocarbon receptor signalling pathway has been implicated in pituitary tumorigenesis and we review its role in general and specifically in relation to cAMP de-regulation.

VPAC1 overexpression is associated with poor differentiation in colon cancer

Vasoactive intestinal peptide (VIP) is a neurotransmitter that primarily functions as a vasodilator. VIP plays its role through binding to its receptors known as VIP/pituitary adenylate cyclase-activating peptide receptors (VPACs). In this study, we examined the expression of VPAC1 in human colon cancer tissues, analyzed the relationship between VPAC1 expression and cancer malignancy, and explored the possible mechanisms using immunohistochemistry and immunofluorescence double staining. The results showed that (1) poorly differentiated colon cancers have significantly higher VPAC1 expression than well-differentiated colon cancers do (p < 0.01); (2) phospho-epithelial growth factor receptor (EGFR) overexpression/activation in the cytoplasm of cancer cells is related to VPAC1 overexpression; (3) blood vessels surrounding colon cancer have significantly more VPAC1-positive than normal colon mucosa does; (4) tumor-associated macrophages (TAMs) of colon cancer have a higher level of VPAC1 expression than macrophages in normal colon mucosa do. These data suggest that VPAC1 overexpression is associated with poorer differentiation of colon cancer, which is likely caused by subsequent EGFR activation in cancer cells. In addition, VPAC1 overexpression in both blood vessels and macrophages in tumors may also play an important role in the development of aggressive cancer.

Neuropeptide Y receptors in primary human brain tumors: overexpression in high-grade tumors

Peptide receptors are often overexpressed in tumors, and they may be targeted in vivo. We evaluated neuropeptide Y (NPY) receptor expression in 131 primary human brain tumors, including gliomas, embryonal tumors, meningiomas, and pituitary adenomas, by in vitro receptor autoradiography using the 125I-labeled NPY receptor ligand peptide YY in competition with NPY receptor subtype-selective analogs. Receptor functionality was investigated in selected cases using [35S]GTPgammaS-binding autoradiography. World Health Organization Grade IV glioblastomas showed a remarkably high expression of the NPY receptor subtype Y2 with respect to both incidence (83%) and density (mean, 4,886 dpm/mg tissue); astrocytomas World Health Organization Grades I to III and oligodendrogliomas also exhibited high Y2 incidences but low Y2 densities. In glioblastomas, Y2 agonists specifically stimulated [35S]GTPgammaS binding, suggesting that tumoral Y2 receptors were functional. Furthermore, nonneoplastic nerve fibers containing NPY peptide were identified in glioblastomas by immunohistochemistry. Medulloblastomas, primitive neuroectodermal tumors of the CNS, and meningiomas expressed Y1 and Y2 receptor subtypes in moderate incidence and density. In conclusion, Y2 receptors in glioblastomas that are activated by NPY originating from intratumoral nerve fibers might mediate functional effects on the tumor cells. Moreover, identification of the high expression of NPY receptors in high-grade gliomas and embryonal brain tumors provides the basis for in vivo targeting.

Pituitary adenylate cyclase-activating polypeptide (PACAP) as a growth hormone (GH)-releasing factor in grass carp. I. Functional coupling of cyclic adenosine 3',5'-monophosphate and Ca2+/calmodulin-dependent signaling pathways in PACAP-induced GH secretion and GH gene expression in grass carp pituitary cells

Pituitary adenylate cyclase-activating polypeptide (PACAP), a member of the glucagon/secretin peptide family, has been recently proposed to be the ancestral GH-releasing factor. Using grass carp as a model for bony fish, we examined the mechanisms for PACAP regulation of GH synthesis and secretion at the pituitary level. Nerve fibers with PACAP immunoreactivity were identified in the grass carp pituitary overlapping with the distribution of somatotrophs. At the somatotroph level, PACAP was shown to induce cAMP synthesis and Ca(2+) entry through voltage-sensitive Ca(2+) channels (VSCC). In carp pituitary cells, PACAP but not vasoactive intestinal polypeptide increased GH release, GH content, total GH production, and steady-state GH mRNA levels. PACAP also enhanced GH mRNA stability, GH promoter activity, and nuclear expression of GH primary transcripts. Increasing cAMP levels, induction of Ca(2+) entry, and activation of VSCC were all effective in elevating GH secretion and GH mRNA levels. PACAP-induced GH secretion and GH mRNA expression, however, were abolished by inhibiting adenylate cyclase and protein kinase A, removing extracellular Ca(2+) or VSCC blockade, or inactivating calmodulin (CaM)-dependent protein kinase II (CaM kinase II). Similar sensitivity to VSCC and CaM kinase II blockade was also observed by activating cAMP production as a trigger for GH release and GH gene expression. These results suggest that PACAP stimulates GH synthesis and secretion in grass carp pituitary cells through PAC(1) receptors. These stimulatory actions probably are mediated by the adenylate cyclase/cAMP/protein kinase A pathway coupled to Ca(2+) entry via VSCC and subsequent activation of CaM/CaM kinase II cascades.

Immunocytochemical identification of VPAC1, VPAC2, and PAC1 receptors in normal and neoplastic human tissues with subtype-specific antibodies

Human tumors frequently overexpress receptors for vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating peptide (PACAP). However, none of the VIP/PACAP receptor proteins has been visualized individually in human tumors. Here, we developed and characterized a panel of antipeptide antibodies to the carboxyl-terminal regions of the VIP/PACAP receptor subtypes vasoactive intestinal peptide receptor (VPAC)1, VPAC2, and pituitary adenylate cyclase-activating peptide receptor (PAC)1. Specificity of the antisera was shown by the following: (1) detection of broad bands migrating at Mr 50,000 to 70,000 in Western blots of membranes from receptor-expressing tumors and receptor-transfected cells; (2) cell surface staining of VIP/PACAP receptor-transfected cells; (3) translocation of VIP/PACAP receptor immunostaining in transfected cells after agonist exposure; and (4) abolition of tissue immunostaining by preadsorbtion of the antibodies with their immunizing peptides. The distribution of VIP/PACAP receptors was investigated in 98 human tumors and their tissues of origin. VPAC1, VPAC2, and PAC1 receptors were clearly located at the plasma membrane of the tumor cells in a variety of human neoplasms. In the gastrointestinal tract, VPAC1 receptor immunoreactivity was abundant in the mucosa and myenteric neurons; VPAC2 receptor immunoreactivity was detected in neuroendocrine cells, blood vessels, and smooth muscle; and PAC1 receptor immunoreactivity was found in myenteric neurons. This is the first localization of all of the VIP/PACAP receptor subtypes in human formalin-fixed, paraffin-embedded tissues. VIP/PACAP receptor visualization with this simple and rapid immunohistochemical method will facilitate identification of tumors with a sufficient receptor overexpression for diagnostic or therapeutic intervention.

No relationship between vasoactive intestinal polypeptide expression and headache in pituitary tumours

BACKGROUND

Clinical studies have noted the common presentation of pituitary tumours with significant headache. This has been considered to be one, or a combination of, increased cranial pressure, tumour size with dural stretch, or cavernous sinus invasion. Newer hypotheses suggest an association between the presence of pituitary tumour-associated headaches and the expression and release of nociceptive substances. Vasoactive intestinal polypeptide (VIP), a marker of the cranial parasympathetic system, is increased during acute attacks of some primary headaches, and with its expression in the pituitary may link some pituitary tumours to their headache presentations.

METHODS

Using immunohistochemical techniques, VIP expression in pituitary tumour specimens was examined to determine if there was a relationship between the presence or absence of pituitary-associated headache and the expression of VIP immunoreactivity (VIP-IR). A qualitative analysis of the VIP-IR in pituitary cells was performed by observers blinded to the headache status of each patient. The presence of VIP-IR and headache were treated as binary variables and associations tested with chi-square tests.

RESULTS

Forty-five per cent of specimens positive for VIP were from patients presenting with headache. There was no statistically significant association between the presence of VIP-IR and headache (chi(2) = 0.077, P = 0.781).

CONCLUSION

Although the significance of VIP positivity in pituitary tumour-associated headache is unknown it seems unrelated to headache. It remains possible that the mechanism of these headaches relates to the production of either an as yet unidentified peptide, or a combination of nociceptive peptides.

Peptide receptors as molecular targets for cancer diagnosis and therapy

During the past decade, proof of the principle that peptide receptors can be used successfully for in vivo targeting of human cancers has been provided. The molecular basis for targeting rests on the in vitro observation that peptide receptors can be expressed in large quantities in certain tumors. The clinical impact is at the diagnostic level: in vivo receptor scintigraphy uses radiolabeled peptides for the localization of tumors and their metastases. It is also at the therapeutic level: peptide receptor radiotherapy of tumors emerges as a serious treatment option. Peptides linked to cytotoxic agents are also considered for therapeutic applications. The use of nonradiolabeled, noncytotoxic peptide analogs for long-term antiproliferative treatment of tumors appears promising for only a few tumor types, whereas the symptomatic treatment of neuroendocrine tumors by somatostatin analogs is clearly successful. The present review summarizes and critically evaluates the in vitro data on peptide and peptide receptor expression in human cancers. These data are considered to be the molecular basis for peptide receptor targeting of tumors. The paradigmatic peptide somatostatin and its receptors are extensively reviewed in the light of in vivo targeting of neuroendocrine tumors. The role of the more recently described targeting peptides vasoactive intestinal peptide, gastrin-releasing peptide, and cholecystokinin/gastrin is discussed. Other emerging and promising peptides and their respective receptors, including neurotensin, substance P, and neuropeptide Y, are introduced. This information relates to established and potential clinical applications in oncology.

Expression and function of vasoactive intestinal peptide, pituitary adenylate cyclase-activating polypeptide, and their receptors in the human adrenal gland

VIP and pituitary adenylate cyclase-activating polypeptide (PACAP) are two regulatory peptides that possess remarkable amino acid sequence homology and act through common receptors, named PAC(1), VPAC(1), and VPAC(2). PAC(1) receptor is selective for PACAP, whereas VPAC(1) and VPAC(2) receptors bind both VIP and PACAP. We have investigated the expression and function of VIP, PACAP, and their receptors in the zona glomerulosa (ZG), zonae fasciculata and reticularis, and adrenal medulla (AM) of the human adrenal cortex. RT-PCR and RIA detected VIP and PACAP expression exclusively in AM cells. RT-PCR demonstrated the presence of PAC(1) mRNA only in AM and of VPAC(1) and VPAC(2) mRNAs in both ZG and AM cells. VIP and PACAP concentration-dependently increased aldosterone and catecholamine secretion from cultured ZG and AM cells. The catecholamine response to both peptides was higher than the aldosterone response, and the secretagogue action of PACAP was more intense than that of VIP. The aldosterone response of cultured ZG cells to VIP or PACAP was unaffected by the PAC(1) receptor antagonist PACAP-(6-38) (PAC(1)-A), but was significantly decreased by the VPAC(1) receptor antagonist [Ac-His(1),D-Phe(2),Lys(15),Arg(16)]VIP-(3-7),GH-releasing factor-(8-27)-NH(2) (VPAC(1)-A). The catecholamine response of cultured AM cells to VIP was lowered by VPAC(1)-A and unaffected by PAC(1)-A; conversely, the catecholamine response to PACAP was reduced by both PAC(1)-A and VPAC(1)-A. Simultaneous exposure to both antagonists did not abolish the catecholamine response to PACAP. Collectively, our findings allow us to conclude that in human adrenals 1) VIP and PACAP biosynthesis exclusively occurs in AM cells; 2) ZG cells are provided with functional VPAC(1) and VPAC(2) receptors, whose activation by VIP or PACAP elicits a moderate aldosterone response; 3) AM cells possess PAC(1), VPAC(1), and VPAC(2) receptors, whose activation evokes a marked catecholamine response; and 4) the catecholamine response to PACAP is more intense than that to VIP, because it is mediated by all subtypes of VIP/PACAP receptors.

In vitro evaluation of VIP/PACAP receptors in healthy and diseased human tissues. Clinical implications

The evaluation of peptide receptors in man is relevant to identifying the physiological target tissues of a given peptide and to selecting diseases with a sufficient receptor overexpression for diagnostic or therapeutic intervention. VIP/PACAP receptors have been evaluated in normal and diseased human non-neuronal tissues by using in vitro receptor autoradiography with 125I-VIP or 125I-PACAP in tissue sections. As assessed by subtype-selective VIP analogs, VIP receptors of the VPAC1 subtype are found in a wide variety of tissues including liver, breast, kidney, prostate, ureter, bladder, pancreatic ducts, gastrointestinal mucosa, lung, thyroid, adipose, and lymphoid tissues. VPAC2 receptors are predominantly found in vessels and smooth muscles, whereas PAC1 receptors are present in the adrenal medulla. VIP/PACAP receptors are expressed in the majority of the most frequently occurring human tumors, including breast, prostate, pancreas, lung, colon, stomach, liver, and bladder carcinomas, as well as lymphomas and meningiomas, predominantly as VPAC1 receptors, as do their tissues of origin. Although leiomyomas predominantly express VPAC2 receptors, glial tumors, pituitary adenomas, neuroblastomas, paragangliomas, pheochromocytomas, and endometrial carcinomas preferentially express PAC1 receptors. The very wide distribution of VIP/PACAP receptors in the normal human body is indicative of the key role of these peptides in human physiology and pathophysiology. Moreover, the receptor expression in tumors is the molecular basis for clinical applications of VIP/PACAP such as in vivo scintigraphy and radiotherapy of tumors as well as VIP/PACAP analog treatment for tumor growth inhibition.

Pituitary adenylate cyclase-activating polypeptide inhibits transforming growth factor-beta1-induced apoptosis in a human pituitary adenoma cell line

Pituitary adenylate cyclase-activating polypeptide (PACAP) was originally isolated from hypothalamic tissues based on its ability to stimulate cAMP production in cultured anterior pituitary cells. Recent studies have suggested a functional role for PACAP in the apoptosis of brain cells. However, the role of PACAP in regulating apoptosis in human pituitary adenomas has not previously been examined. Analysis of the cultured human pituitary adenoma cell line HP75, which expresses all three major PACAP receptors, showed that both PACAP-38 and PACAP-27 inhibited TGF-beta1-induced apoptosis. Treatment with the PACAP receptor antagonists PACAP 6-38 (PACAP type I receptor antagonist) and (p-chloro-D-Phe(6), Leu(17))-VIP (PACAP type II receptor antagonist) blocked the effects of PACAP-38 on the inhibition of transforming growth factor-beta1 (TGF-beta1)-induced apoptosis, confirming the specificity of the role of PACAP. Treatment with forskolin but not phorbol 12-myristate 13-acetate (PMA) also inhibited TGF-beta1-induced apoptosis. TGF-beta1 treatment was associated with an increase in mitogen-activated protein kinase (MAP kinase) when analyzed by Western blotting, but PACAP inhibition of TGF-beta1-induced apoptosis was not associated with activation of MAP kinase. Immunocytochemical analysis of the cell cycle cyclin-dependent kinase inhibitor p27 showed that treatment with TGF-beta1, forskolin, PMA, and PACAP increased p27 expression in cultured HP75 cells. These results indicate that PACAP is a highly specific inhibitor of TGF-beta1-induced apoptosis in the HP75 human pituitary adenoma cell line and that PACAP, TGF-beta1, forskolin, and PMA all stimulate expression of the TGF-beta-regulated cell cycle protein p27 in the HP75 human pituitary adenoma cell line. The HP75 cell line can be used as a model to study the regulation of apoptosis in human pituitary cells.