Molecular and cellular analysis of a neoplastic pancreatic A cell tumor

Endocrine tumors of the pancreas may produce characteristic syndromes attributable to the increased secretion of one or more hormones. These tumors provide valuable opportunities for the analysis of hormone biosynthesis and secretion in the neoplastic human endocrine cell. The authors studied a pancreatic endocrine tumor obtained from a patient with classical glucagonoma syndrome. Characterization of plasma and tumor glucagon-like immunoreactivity (GLI) by high-performance liquid chromatography and radioimmunoassay for GLI showed different chromatographic profiles, with glucagon the major molecular form in the tumor, and glicentin and oxyntomodulin predominating in plasma. Although immunocytochemical staining of the tumor showed only focal weak positivity for glucagon, tumor extracts contained large amounts of immunoreactive GLI peptide. Northern blot analysis of tumor RNA demonstrated that abundant glucagon mRNA transcripts were present, just slightly larger in size than those detected in normal pancreas and intestine. Electron microscopic analysis of the tumor cellular ultrastructure revealed only occasional small electron dense secretory granules. A large number of complex lysosome-like structures of variable size and electron density were detected throughout the cytoplasm and ringing the nucleus of most cells, a feature atypical of endocrine tumors of the pancreas. Primary cultures of dispersed tumor cells were established and, in contrast to previous results, were obtained using normal or neoplastic islet cell models, GLI secretion was found to be stimulated eightfold by incubation with 5 mM dibutyryl cyclic adenosine monophosphate. Phorbol myristate acetate, the calcium ionophore A23187, and sodium butyrate had no effect on GLI secretion in vitro. These observations indicate that neoplastic human A cells may have abnormalities at different points in the biosynthesis and secretion of glucagon.