Pancreastatin-like immunoreactivity in human carcinoid disease

Significance of biochemical markers in the diagnosis of neuroendocrine tumours and in the follow-up of patients

Circulating markers of neuroendocrine tumours are useful tools in the diagnosis of these tumours. Laboratory tests for general biomarkers have acceptable sensitivity for the recognition of neuroendocrine tumours as these biologically active proteins are typically synthesized by all types of neuroendocrine cells. Measurement of chromogranin A is widely used not only in the diagnosis of neuroendocrine tumours but it may predict the prognosis of the diseases and the effect of the antitumor therapy. It is also a useful tool for the detection of residual tumours. Neurendocrine tumours represent a heterogeneous group of tumours with the ability to secrete several hormones and, therefore, measurement of these hormones can also serve as neuroendocrine cell type-specific markers in routine clinical practice. In this review the authors summarize the significance of tumour markers in the diagnosis of neuroendocrine tumours as well as in the management and follow-up of patients with this disease. Orv. Hetil., 2014, 155(45), 1775–1782.

The biochemical utility of chromogranin A, chromogranin B and cocaine- and amphetamine-regulated transcript for neuroendocrine neoplasia

Neuroendocrine neoplasia (NEN) is a heterogeneous group of tumours and often represents a therapeutic challenge to clinicians. The peptides chromogranin A (CgA), chromogranin B (CgB) and cocaine- and amphetamine-regulated transcript (CART) are widely distributed throughout the neuroendocrine system. CgA and CgB have been used as general NEN biomarkers for many years, while CART has only recently been identified. Of these biomarkers, CgA is the most commonly used. However, circulating CgA concentrations exhibit considerable intra-individual biological variation, are altered by proton pump inhibitors (PPIs) and somatostatin analogues and are elevated in non-NEN malignancies. Therefore, interpretation of CgA results must be in the context of these confounding factors. The effects of treatment and non-NEN conditions on circulating CgB and CART concentrations are less well understood. CgB is less affected by impaired renal function and PPIs than CgA; while, circulating CART concentrations lack a diurnal variation in humans and are more reliable markers of pancreatic NEN malignancy than CgA. The utility of circulating CgA measurements in NEN prognosis, surveillance and disease recurrence has been widely investigated. However, the utility of CgB and CART in NEN management is yet to be elucidated. Further studies are needed to establish whether CgB and CART are useful alternatives to CgA.

Circulating biomarkers in neuroendocrine tumors of the enteropancreatic tract: application to diagnosis, monitoring disease, and as prognostic indicators

Neuroendocrine tumors (NETs) are difficult to diagnose. Their symptoms may be vague or intermittent, and are frequently associated with much more common diseases; many of the tumors may be asymptomatic. Therefore, diagnosis can be delayed for some years. Because most NETs are secretory, the measurement of circulating biomarkers is helpful not only for diagnosis but also for assessing tumor response to treatment, monitoring disease progression, and use as prognostic indicators.

Granin-derived peptides as diagnostic and prognostic markers for endocrine tumors

Chromogranin A-like immunoreactivity (CgA-LI) has been, and remains, the most widely used diagnostic and prognostic marker for endocrine tumors. The availability of assay kits combined with moderately high sensitivity and specificity has meant that there has been no great incentive to develop alternative markers. However, circulating concentrations of CgA-LI are elevated in several non-neoplastic diseases and in patients receiving acid-suppression therapy which may lead to false positive diagnosis. Additionally, certain endocrine tumors, such as rectal carcinoids, do not express the CgA gene so that there is a need for additional markers to complement CgA measurements. Plasma concentrations of the CgA-derived peptide, pancreastatin, measured with antisera of defined regional specificity, have a prognostic value in patients with metastatic midgut carcinoid tumors receiving somatostatin analog therapy or hepatic artery chemoembolization. Other CgA-derived peptides with potential as tumor markers are vasostatin-1, WE-14, catestatin, GE-25, and EL-35 but their value has yet to be fully assessed. Circulating concentrations of chromogranin B-like immunoreactivity (CgB-LI) are not elevated in non-neoplastic diseases and measurements of CCB, the COOH-terminal fragment of CgB, may be useful as a biochemical marker for neuroendocrine differentiation in lung tumors. Antisera to the secretogranin II-derived peptide, secretoneurin detects carcinoid tumors of the appendix with greater frequency than antisera to CgA and are of value in identifying therapy-resistant carcinoma of the prostate (clinical stage D3). Measurement of concentrations of a second secretogranin II-derived peptide, EM-66 in tumor tissue has been used to differentiate between benign and malignant pheochromocytoma. These examples point to a limited although potentially valuable role for granin-derived peptides as tumor markers.

Circulating markers for endocrine tumours of the gastroenteropancreatic tract

The diffuse endocrine system (DES) includes a wide range of secretory cells that may be the source of tumours. Gastroenteropancreatic endocrine (GEP) tumours arising within the DES secrete a variety of peptides and amines that are found in the circulation and are responsible for the syndromes associated with these tumours. In this review, the most common tumours of the GEP tract are outlined and the circulating products of these tumours identified. Where differential diagnosis is difficult these points are addressed. The peptides most commonly secreted by GEP neuroendocrine tumours are identified and described and their biological activities are discussed. Current methods available for measurement of these peptides are described. Attention is drawn towards molecular specificity where appropriate, as many pancreatic and gut peptides fall within families which show considerable homology, such as the tachykinin family or the glucagon family. Other peptides such as gastrin circulate in multiple molecular forms. This homology and diversity may cause difficulty in the interpretation of peptide measurements in the clinical situation if assays are not specific.

A rapid rise in circulating pancreastatin in response to somatostatin analogue therapy is associated with poor survival in patients with neuroendocrine tumours

Aim

To assess the value of pancreastatin as a predictive factor for identifying patients with neuroendocrine tumours (NETs) who respond poorly to somatostatin analogues.

Methods

A retrospective study of patients with NETs. Patient records from the Northern Ireland Neuroendocrine Tumour Register were interrogated. Those who had pancreastatin concentrations measured on two or more occasions, before and during somatostatin analogue therapy (within the set time-limits) were selected. Data relating to diagnosis, surgery, somatostatin analogue therapy and survival outcome were noted. Data were subjected to univariate and multivariate analysis using Cox proportional hazard model.

Results

Fifty-nine patients with gastroenteropancreatic NETs fulfilled the inclusion criteria. Factors associated with a poor survival outcome on univariate analysis were primary tumour site ( P = 0.006) and rapid rise in pancreastatin during somatostatin analogue treatment ( P < 0.001). In multivariate analysis, highly significant clinical prognostic indicators were: tumour location ( P < 0.001), pre-treatment pancreastatin ( P < 0.001) and pancreastatin change ( P < 0.001).

Conclusions

This study endorses the finding that pancreastatin is a useful prognostic indicator of neuroendocrine disease. On commencement of treatment, one-third of the subjects showed an immediate negative pancreastatin response to somatostatin analogues, which was associated with poor survival. This is the first study to document such an association. These findings have significant therapeutic consequences. In the presence of a rapidly rising pancreastatin alternative, treatment modalities should be sought.

Processing-independent quantitation of chromogranin a in plasma from patients with neuroendocrine tumors and small-cell lung carcinomas

BACKGROUND

Most neuroendocrine tumors express chromogranin A (CgA). The posttranslational processing of neuroendocrine proteins such as CgA is often specific for the individual tumor. To cope with this variability and improve tumor diagnosis, we developed a processing-independent analysis (PIA) method to measure the total CgA product.

METHODS

For PIA, samples underwent trypsin treatment followed by measurement of CgA by the "CgA(340-->)" assay, in which the antiserum binds an epitope starting at amino acid 340 of CgA and including amino acid residues located in the C-terminal direction. The diagnostic accuracy of the CgA PIA and 3 sequence-specific assays for CgA were evaluated on plasma samples from patients with neuroendocrine tumors and small-cell lung carcinomas. Furthermore, we investigated whether the CgA plasma concentrations correlated with the tumor burden.

RESULTS

Size-exclusion chromatography of plasma showed that CgA immunoreactivity mainly consisted of high-molecular-weight forms, indicating that neuroendocrine tumors may secrete large amounts of poorly processed CgA. Accordingly, trypsination of plasma from 54 patients with neuroendocrine tumors or small-cell lung carcinomas increased the CgA(340-->) immunoreactivity up to 500-fold. Both the CgA(340-->) assay and the PIA measured significantly higher plasma concentrations in patients with very extensive disease than in patients with less widespread disease. The diagnostic sensitivity was 0.91 when using the CgA(340-->) assay and 0.82 using the CgA PIA.

CONCLUSION

The CgA(340-->) assay and CgA PIA are both useful for diagnosis of neuroendocrine tumors and small-cell lung carcinomas and both assays correlate with tumor burden.

A panel of 11 region-specific radioimmunoassays for measurements of human chromogranin A

INTRODUCTION

The primary structure of human chromogranin A (CgA) not only contains 10 pairs of basic amino acids, which are potential cleavage sites for specific endogenous proteases, but also other sites in the molecule can be subjected to cleavage. Several CgA-related peptides have been identified in tissue, and many of the biological effects attributed to CgA seem to be mediated by these peptides.

MATERIALS AND METHODS

Peptides homologous to defined parts of the human CgA molecule were selected and synthesised. Antibodies were raised, and 11 specific radioimmunoassays were developed. Plasma samples from 20 patients with neuroendocrine tumours were collected and measured in all assays.

RESULTS

All assays measured circulating levels of CgA-derived peptides. Only four of the assays measured concentrations that correlated with that of total CgA. However, concentrations of the individual CgA-related peptides were generally lower than the concentration of total CgA. Different neuroendocrine tumours seem to process CgA differently. The ratio between a given region-specific assay and total CgA is inversely correlated to tumour activity.

CONCLUSION

The assays presented allow measurements of defined regions of CgA and will thus become important tools for further studies of processing of CgA.

Serum peptide profiles in patients with carcinoid tumors

BACKGROUND

Patterns of elevated serum peptides may reveal additional markers and permit better classification of tumors based on (secondary) peptide secretion.

METHODS

Fasting peptide profiles were obtained from 31 carcinoid patients. vasoactive intestinal peptide (VIP), pancreatic polypeptide (PP), neurotensin, substance P, gastrin-releasing polypeptide (GRP), calcitonin, gastrin, and pancreastatin were measured. Peptide elevation patterns were correlated with disease sites, syndrome, and survival.

RESULTS

Elevations in patients were as follows: VIP 0%, PP 13%, neurotensin 10%, substance P 20%, GRP 3%, calcitonin 10%, and gastrin 3%. There were no consistent patterns of elevated peptides with regard to site or syndrome. Pancreastatin was elevated in 81% of profiles and was the only abnormal peptide in 57% of patients.

CONCLUSION

Peptide profile results do not permit improved classification, predict syndrome development, or correlate with survival. In contrast, pancreastatin is elevated in most cases and may be utilized to monitor disease progression and evaluate response to therapy.

Characterisation of WE-14 in porcine ocular tissue

WE-14 is derived from the cell-specific posttranslational processing of chromogranin A (CgA) in subpopulations of neuroendocrine cells and neurons. Region- and site-specific chromogranin A, pancreastatin and WE-14 antisera were employed to study the generation of WE-14 in porcine ocular tissues. No chromogranin A or pancreastatin immunostaining was detected in ocular tissue. Immunohistochemistry detected WE-14 immunostaining in a network of nerve fibre bundles and nerve fibres throughout the limbus, cornea, iris and ciliary body with sparse nerve fibres detected throughout the choroid and sclera. Retinal analysis detected intense WE-14 immunostaining in large ovoid cells in the ganglion cell layer with weak immunostaining in a population of small cells in the inner nuclear layer; weak immunostaining was detected within the fibre layers in the inner plexiform layer. Quantitatively, the highest WE-14 tissue concentration was recorded in aqueous retinal and corneal extracts with lower concentrations in the sclera, choroid and anterior uveal tissues. Chromatographic profiling resolved a minor chromogranin A-like immunoreactant and a predominant immunoreactant co-eluting with synthetic human WE-14. This is the first study to demonstrate that WE-14 is generated in neuronal fibres primarily innervating the anterior chamber and in select cell populations in the retina.

Immunohistochemical evaluation of the post-translational processing of chromogranin A in human pituitary adenomas

Chromogranin A (CgA), pancreastatin (PST), intervening-peptide (IP) and WE-14 antisera were employed to investigate the proteolysis of CgA in 50 pituitary adenomas. All non-functioning (NF) pituitary tumours (n = 28) exhibited CgA immunoreactivity. PST, IP and WE-14 immunostaining was observed in 85%, 89% and 67%, respectively. CgA, PST and IP immunostaining were comparable in the majority of NF tumours, while less intense WE-14 immunoreactivity was detected in a subpopulation of NF tumour cells. Approximately half of the functioning pituitary tumours expressed CgA immunoreactivity. Six of nine ACTH-secreting tumours displayed CgA and IP immunostaining; four of these tumours displayed PST immunoreactivity. WE-14 immunoreactivity was detected in one corticotroph tumour. Three of six growth hormone (GH) secreting tumours displayed CgA immunostaining, two exhibited PST and IP, and one exhibited WE-14 immunoreactivity. Clusters of WE-14 immunopositive cells were detected in one GH tumour. One of seven prolactinomas exhibited weak CgA immunostaining, while weak IP and WE-14 immunostaining was detected in an additional tumour. No PST immunostaining was detected in prolactinomas. Therefore CgA is a valuable marker of NF pituitary tumours, however it is a more sporadic marker of functioning adenomas. In general, the cellular pattern and intensities of CgA, PST and IP immunoreactivity were comparable in the majority of pituitary adenomas. In contrast, WE-14 immunostaining was observed in a subpopulation of tumour cells. The pathophysiological significance of the proteolysis of CgA to generate bioactive peptides in both NF and functioning pituitary adenomas remains to be established.

Library of Sequence-specific Radioimmunoassays for Human Chromogranin A

Background: Human chromogranin A (CgA) is an acidic protein widely expressed in neuroendocrine tissue and tumors. The extensive tissue- and tumor-specific cleavages of CgA at basic cleavage sites produce multiple peptides.

Methods: We have developed a library of RIAs specific for different epitopes, including the NH2 and COOH termini and three sequences adjacent to dibasic sites in the remaining part of CgA.

Results: The antisera raised against CgA(210–222) and CgA(340–348) required a free NH2 terminus for binding. All antisera displayed high titers, high indexes of heterogeneity (∼1.0), and high binding affinities (Keff0 ∼ 0.1 × 1012 to 1.0 × 1012 L/mol), implying that the RIAs were monospecific and sensitive. The concentration of CgA in different tissues varied with the assay used. Hence, in a carcinoid tumor the concentration varied from 0.5 to 34.0 nmol/g tissue depending on the specificity of the CgA assay. The lowest concentration in all tumors was measured with the assay specific for the NH2 terminus of CgA. This is consistent with the relatively low concentrations measured in plasma from carcinoid tumor patients by the N-terminal assay, whereas the assays using antisera raised against CgA(210–222) and CgA(340–348) measured increased concentrations.

Conclusion: Only some CgA assays appear useful for diagnosis of neuroendocrine tumors, but the entire library is valuable for studies of the expression and processing of human CgA.

Evidence that rat stomach ECL cells represent the main source of circulating pancreastatin

Recently, we showed that the ECL cells in the oxyntic mucosa of the rat stomach are an important source of circulating pancreastatin, a fragment of chromogranin A. The present study examined how much the ECL cells contribute to the circulating levels of pancreastatin during omeprazole-evoked hypergastrinemia. Rats received omeprazole (400 mumol kg-1 day-1) by the oral route for 3 weeks. Two weeks after the start of the treatment, the rats were subjected to a sham operation or fundectomy. The concentrations of gastrin and pancreastatin in serum were monitored before and after the operations. The ECL cells were visualized by pancreastatin immunostaining and their number was determined. The activity of oxyntic mucosal histidine decarboxylase (HDC) was measured before and after 2 weeks of omeprazole treatment. Omeprazole-induced hypergastrinemia resulted in elevated serum pancreastatin and increased oxyntic mucosal HDC activity. Pancreastatin-immunoreactive cells were equally numerous before and after 2 weeks of omeprazole treatment. After surgical removal of the ECL cells by fundectomy, the serum gastrin concentration remained high whereas the serum pancreastatin concentration decreased by 90%. We conclude that the ECL cells in omeprazole-treated rats are responsible for 90% of circulating pancreastatin.

PC12 cells show immunoreactivity to a number of proteins and peptides, including vasostatin

N-terminal chromogranin A (CGA) contains peptides with vasoinhibitory properties, called vasostatin I (VST) and II [CGA (1-76) and (1-113) in human and bovine; (1-128) in rat]. Three fragments of VST were synthesized and antisera raised: human CGA (68-76) (VST I) rat CGA (121-128) (VST II fragment 2), and bovine/human CGA (83-91) (VST II, fragment 3). Strong immunoreactivity was observed in PC12 cells with antisera to VST II, fragment 3, VST I, and neuron-specific enolase. Little or no immunoreactivity was observed using antisera to synaptophysin, whole molecule CGA, pancreastatin, protein gene product 9.5, somatostatin, pancreatic polypeptide, or with antibodies 875 and 876 to VST II, fragment 2. Most of the VST antisera cross-reacted, with a species of molecular weight, 61 kDa but one, 874, cross-reacted with two species of molecular weights, 7.2 and 12 kDa. Our results show the presence of N-terminally processed CGA in PC12 cells.

Circulating pancreastatin is a marker for the enterochromaffin-like cells of the rat stomach

BACKGROUND/AIMS

Peptides of the chromogranin family occur in peptide hormone-producing cells throughout the body. One source of such peptides is the enterochromaffin-like (ECL) cells, which constitute the predominant population of endocrine cells in the fundus (the acid-producing part) of the rat stomach. The purpose of this study was to examine whether ECL cells, which are controlled by gastrin, represent a major source of circulating pancreastatin, a fragment of chromogranin A.

METHODS

Rats underwent surgical procedures and treatments in which the ECL cells could be manipulated. The procedures included antrectomy, fundectomy, and gastrectomy (and adrenalectomy), and the treatments included fasting or feeding, gastrin-17 infusion, and administration of omeprazole or ranitidine. The concentrations of pancreastatin-like immunoreactivity (LI) and gastrin in the serum were determined by radioimmunoassay.

RESULTS

The serum pancreastatin-LI concentration was lowered by about 80% by fundectomy and gastrectomy; both of these procedures eliminated the ECL cell population. Adrenalectomy had no effect on the serum pancreastatin-LI concentration. Gastrin infusion, which activates the ECL cells, promptly increased serum pancreastatin-LI concentration. Refeeding after fasting and administration of omeprazole or ranitidine increased the serum pancreastatin-LI concentrations; these responses were prevented by antrectomy.

CONCLUSIONS

The concentration of circulating pancreastatin-LI reflects the activity of the ECL cells and the size of the ECL cell population in the rat stomach.

Immunoassays for measurement of chromogranin A and pancreastatin-like immunoreactivity in humans: correspondence in patients with neuroendocrine neoplasia

Chromogranin A (CgA) is a useful marker of neuroendocrine tumors in humans. Here we describe and compare two immunoassay methods for determination of CgA, a radioimmunoassay (RIA) and an enzyme linked immunoassay (ELISA). The detection limit of the ELISA was lower than that of the RIA method (2 ng/ml versus 10 ng/ml, respectively), though the CgA RIA method covered a wider range than the CgA ELISA (10-920 ng/ml versus 2-500 ng/ml, respectively). There was no cross-reactivity with synthetic human and porcine pancreastatin (PST) in the two assays. There was a significant positive correlation between levels of CgA in sera from patients with carcinoid disease, measured by the two methods (r = 0.9, p < 0.0001), and the values were in the same range. Similarly, serum CgA levels in normal controls were also in the same range when assayed by the two methods. A commercially available porcine PST RIA method was evaluated, especially with respect to the influence of Sep-Pak extraction of serum on the levels of pancreastatin-like immunoreactivity (PST-LI). Ten sera from carcinoid patients were treated with Sep-Pak extraction, and levels of PST-LI were determined in non-extracted and extracted sera. There was a significant positive correlation between the concentrations of PST-LI measured in extracted and non-extracted carcinoid sera (r = 0.9, p < 0.002), and the levels were in the same range. There was also a significant positive correlation between levels of CgA and PST-LI in 49 carcinoid sera (r = 0.8, p < 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Chromogranin A(210-301) is the major form of pancreastatin-like material in human gut extracts and endocrine tumors

A radioimmunoassay of human pancreastatin was developed using a rabbit antiserum that selectively recognized the C-terminal amidated end of the peptide, and it was used for the identification of the molecular forms of pancreastatin in human gut (stomach, duodenum, small intestine, colon) and endocrine tumor extracts (liver metastasis of a gastrinoma and a medullary carcinoma of thyroid, one nonsecreting pancreatic tumor, one recurrence of a gut carcinoid, one vipoma and one insulinoma). In all gut extracts, a gel filtration chromatography revealed the presence of three peaks of pancreastatin-like immunoreactivity. The predominant form eluted with an apparent molecular weight higher than that of pancreastatin. This form was also predominant in the endocrine tumors analyzed, except in the insulinoma, where a lower molecular weight form predominated. The high molecular form was further purified from a liver metastasis of a gastrinoma. The pancreastatin-like immunoreactivity eluted in all the chromatographical systems (reverse-phase, ion exchange) as a single peak that was finally purified to homogeneity and sequenced. The sequence of the first 29 N-terminal amino acids was obtained unambiguously and corresponded to the sequence 210-238 of chromogranin A. Considering the selectivity of the assay used for peptide identification, this major form was identified as the fragment 210-301 of chromogranin A. It is likely that the predominant form of pancreastatin in human gut extracts and noninsular tumors is a 92 amino acid peptide.

Chromogranin A and pancreastatin-like immunoreactivity in serum of gastrinoma patients

Gastrin and pancreastatin-like immunoreactivity were determined by radioimmunoassay methods and chromogranin A was determined by enzyme-linked immunoassay in sera from 18 patients with gastrinomas (Zollinger-Ellison syndrome) and in 20 age and sex matched controls. Gastrin serum levels in the gastrinoma patients were in the range 26-80,000 pmol/l, and in the controls 5-31 pmol/l. Chromogranin A serum levels in the gastrinoma group were in the range 6-2,700 ng/ml (mean +/- SEM: 400 +/- 147 ng/ml). The mean value of chromogranin A was significantly higher than in the control group (8 +/- 2 ng/ml, p = 0.008). The serum levels of pancreastatin-like immunoreactivity in the gastrinoma patients were in the range 23-1,994 pg/ml (597 +/- 123 pg/ml). The mean value of pancreastatin-like immunoreactivity in the gastrinoma group was significantly higher than in the control group (104 +/- 25 pg/ml, p = 0.0002). The levels of chromogranin A and pancreastatin-like immunoreactivity were significantly higher in patients with verified metastatic disease (p = 0.04, p = 0.01 respectively). There was a significantly positive correlation between levels of gastrin and pancreastatin-like immunoreactivity (r = 0.7, p = 0.002), while no correlation was found between gastrin and chromogranin A levels or between levels of chromogranin A and pancreastatin-like immunoreactivity. The study demonstrates an elevation of both chromogranin A and pancreastatin-like immunoreactivity in serum of gastrinoma patients. The lack of correlation between gastrin and chromogranin A, however, gives an indication that the gastrinoma cells are not the main source of serum chromogranin A elevation.

Immunoreactivity to two specific regions of chromogranin A in the nervous system of Ascaris suum: an immunocytochemical study

Antisera to a highly conserved region of chromogranin A (sequence KELTAE) and to a hexapeptide (sequence KGQELE) adjacent to the putative C-terminus of pancreastatin, a peptide whose sequence is found within the chromogranin A molecule, have been used to examine the localisation of immunoreactivity (IR) to these peptides in Ascaris suum. IR to both peptides was found in the nerve rings and nerve cords. In addition, KGQELE-IR was also observed in the pharyngeal neurones and in a network of fibres on the surface of the female gonoduct. The staining was specific in that it could be abolished by preincubation of the antisera with the appropriate antigen. The two antisera appeared to be staining different subsets of neurones, suggesting that (at least) two peptides were being recognised. The wide-spread distribution of IR to both peptides throughout the nervous system of the parasite suggests that the peptides carrying the epitopes recognised by the antisera are of fundamental importance to the functioning of the parasite's nervous system.