The spectrum of endogenous human chromogranin A-derived peptides identified using a modified proteomic strategy

Abstract

The hypothesis that chromogranin A (CgA), a protein of neuroendocrine cell secretory granules, may be a precursor of biologically active peptides, rests on observed activities of peptide fragments largely produced by exogenous protease digestion of the bovine protein. Here we have adopted a modified proteomic strategy to isolate and characterise human CgA-derived peptides produced by endogenous prohormone convertases. Initial focus was on an insulinoma as previous studies have shown that CgA is rapidly processed in pancreatic beta cells and that tumours arising from these express appropriate prohormone convertases. Eleven novel peptides were identified arising from processing at both monobasic and dibasic sites and processing was most evident in the C-terminal domain of the protein. Some of these peptides were identified in endocrine tumours, such as mid-gut carcinoid and phaeochromocytoma, which arise from endocrine cells of different phenotype and in different anatomical sites. Two of the most interesting peptides, GR-44 and ER-37, representing the C-terminal region of CgA, were found to be amidated. These data would imply that the intact protein is C-terminally amidated and that these peptides are probably biologically active. The spectrum of novel CgA-derived peptides, described in the present study, should provide a basis for biological evaluation of authentic entities.

Chromogranin A as a marker of neuroendocrine neoplasia: an Italian Multicenter Study

“…However, further studies are needed to address the specific CgA cleavage in different tumors. Initial proteomic studies have identified 11 novel CgA-derived peptides in endocrine tumors, supporting the hypothesis that different tumors may process differently the entire molecule, representing a possible specific signature (Orr et al 2002).…”

Evaluation of chromogranin A determined by three different procedures in patients with benign diseases, neuroendocrine tumors and other malignancies

“…These differences among the three commercial available assay kits that use three different methodologies may be important if we consider the hypothesis that in pathological conditions, such as NETs, different proteolytic processes may take place, generating a variable number of fragments [30,35]. One study [37] identified 11 novel CgA-derived peptides in endocrine tumors by proteomic studies, supporting the hypothesis that different tumors may process the entire molecule differently, representing a possible specific signature. In summary, it is important to know whether the configuration of the different assays allows the detection of the same fragmentation pattern since this may significantly influence the clinical accuracy of the tests [5,16].…”

“…This aspect has been rarely studied by other authors and in only a small subset of patients [5,10,37]. Abnormal CgA concentrations have been indicated mainly in patients with gastrointestinal tumors and prostatic cancer suggesting that the presence of this tumor marker indicates small areas with neuroendocrine differentiation in the primary tumor.…”

“…These discrepancies between the results of the two assays do not seem to be due to the use of different blood derivates [45]. Most authors have suggested that these differences may be due to the different abilities of the antibodies to detect CgA-derived peptides or the standardization method [16,[36][37][38]48]. However, discrepancies among the different results may also be related to patient characteristics, tumor characteristics (stage, tumor origin), or to the cut-off used [13,16,38].…”

“…Different methods have been described using different antibodies to detect CgA, different standardizations and the threshold considered as pathologic [14,16,17,[33][34][35] The method to determine CgA is particularly important since, as reported in pathological conditions such as NETs, different proteolytic processes may take place generating a variable number of fragments that may be detected by all these techniques [5,16,30,35]. Most studies that have compared different methods to determine chromogranin normally compared only two of the three most common commercially available assay kits that use three different methodologies to determine CgA [32,36,37]. Likewise, these studies evaluated the performance of these commercial kits, but only comparing NETs with some control groups, usually healthy people.…”