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

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.

Chromogranin 'A' in normal subjects, essential hypertensives and adrenalectomized patients

OBJECTIVE

Chromogranin A (CgA) is an acidic glycoprotein co-stored in vesicles and co-released with catecholamines. Although currently used as a humoral marker of endocrine tumours, several aspects of CgA secretion still need to be clarified in humans.

PATIENTS

Fifty-four controls, 83 essential hypertensive and six adrenalectomized patients were studied.

DESIGN

In the controls and hypertensive patients, CgA and catecholamines were measured before (supine position) and after changes in posture (2' upright position), insulin-induced hypoglycaemia (0.15 IU/kg i.v.) and glucagon injection (1 mg i.v.). In addition, blood samples were taken in the morning (0800 h) and in the afternoon (1800 h), and every 5 h for 24 h. In the adrenalectomized patients, blood samples were obtained in the morning and in the afternoon.

MEASUREMENTS

CgA was measured by an immunoradiometric assay, and noradrenaline and adrenaline by high-performance liquid chromatography.

RESULTS

In controls, posture slightly increased plasma catecholamines without affecting CgA levels. Hypoglycaemia evoked a rise in noradrenaline (P < 0.04), adrenaline (P < 0.01) and CgA (79.6 +/- 11.8 vs. 46.1 +/- 10.1 microg/l, P < 0.03). Glucagon injection increased plasma adrenaline (P < 0.01) but not noradrenaline or CgA levels. At variance with blood pressure and catecholamines, CgA increased significantly in the afternoon (51.1 +/- 4.0 vs. 45.0 +/- 3.9 microg/l, P < 0.05); it also had a circadian rhythm, with peak values during the night (at 2300 h, 65.4 +/- 9.0 microg/l) and a nadir in the morning (at 0800 h, 43.1 +/- 6.6 microg/l). In hypertensives, basal and stimulated CgA levels as well as diurnal/circadian variations of this peptide were similar to those in normal subjects. In adrenalectomized patients plasma CgA in the morning (34.3 +/- 6.5 microg/l) was lower (P < 0.03) than in all controls and hypertensives studied, but also showed an afternoon increment (46.4 +/- 6.6 microg/l, P < 0.003). No correlation was found between CgA and catecholamines or blood pressure in all subjects or in the subgroups.

CONCLUSIONS

In normal humans, chromogranin A and catecholamines are not always co-secreted, and co-secretion occurs only for marked exocytotic adrenergic stimuli, such as hypoglycaemic stress. In addition, chromogranin A has a circadian rhythm unrelated to plasma catecholamines. Basal plasma concentrations and the secretory pattern of chromogranin A in hypertensives do not differ from the findings in controls. Finally, the adrenal glands contribute partially to circulating chromogranin A and are not involved in the circadian rhythm of this peptide in humans.

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.

Application of region-specific immunoassay for human chromogranin A: substantial clue for detection and measurement of chromogranin A in human plasma

Chromogranin A (CgA), a secretory protein, is co-released with catecholamines from storage vesicles. It is known to be elevated in the circulation of patients with neuroendocrine and endocrine tumors. For further investigation of the protein, especially in humans, it is essential to facilitate quantitative analysis of the protein in human biological materials. In order to introduce novel immunological methodology for this purpose, we purposely selected human CgA(344-374) for the synthetic immunogen to produce region-specific CgA antibodies. The anti-synthetic peptide antibody thus obtained made it possible to develop an immunological method for measurement and characterization of CgA in human plasma. The plasma CgA-immunoreactivity (LI) level measured by the method was 0.31+/-0.01 pmol/ml (mean+/-SEM) in normal subjects and 1.55+/-0.29 pmol/ml in pheochromocytoma. On gel chromatography and HPLC analysis of the plasma of patients with pheochromocytoma, the region-specific assay system enabled us to show the presence of N-terminal truncated CgA, besides CgA itself. By following up changes of plasma CgA-LI in a pheochromocytoma patient using samples that were collected consecutively over a two-year period, the present assay system using the region-specific antibody, anti-human CgA (344-374) serum, was confirmed to be extremely valuable for the measurement of CgA-LI in human plasma. The characteristic features and high sensitivity of the present assay system will give us a substantial clue to the detection and measurement of CgA to develop further investigation of the protein in humans.