Chromogranin A Peptide-Specific Antisera and High-Performance Size Exclusion Chromatography Demonstrate Amino-Terminal and Carboxy-Terminal Fragments of the Native Molecule in Human Cell Lines

We have studied the pattern of amino-terminal and carboxy-terminal protein processing of chromogranin A (CgA) in five human cell lines, four derived from lung cancers: NCI-H478, NCI-H1011, NCI-H727, and BEN; and one derived from human medullary thyroid carcinoma: TT. This was accomplished by fractionation of cell extracts by high-performance size exclusion chromatography (HPSEC) and measurement of CgA in the fractions by radioimmunoassay (RIA). Three RIA's were used, one specific for the amino-terminus of CgA, one specific for the carboxy-terminal region, and a monoclonal antibody-based assay that recognizes only the native CgA molecule. We demonstrated the presence of different amino- and carboxy-terminal immunoreactive species of CgA in the different cell lines. The amino-terminal assay demonstrated distinct low-molecular-size species in the NCI-H478 and NCI-H1011 cell lines, and a similar peak in the TT cells. The amino-terminal assay did not recognize any distinct species in BEN and NCI-H727 cell lines. The carboxy-regional assay demonstrated distinct low-molecular-size species in the NCI-H478 and NCI-H101 cell lines and high-molecular-size species in the NCI-H727 and BEN cells. Our studies demonstrate with region-specific RIA's the presence of both amino- and carboxy- forms of CgA in human cells that secrete this protein. These results provide direct evidence that CgA-producing cells produce, probably through endoproteolytic processing of the native molecule, amino- and carboxy-terminal forms of the protein.