Chromogranin A (CGA) is a secretory glycoprotein identified in nearly all endocrine-neuroendocrine cells. CGA is stored typically with co-resident peptides in secretory granules whose contents are released in response to external stimulation. Although the exact physiological significance of CGA remains an enigma, several ideas have been put forth. One suggests that CGA is a precursor (ie, prohormone) of smaller, biologically active peptides. For instance, pancreastatin, a product of CGA processing, is found in enteroendocrine cells of the pancreas and GI tract. Pancreastatin is a potent inhibitor of insulin secretion. In endocrine- neuroendocrine cells, the conversion of CGA to pancreastatin most likely involves cleavage of CGA at single or paired basic amino acids by endoproteolytic enzymes. Only recently have some of the endoproteases (also called prohormone convertases [PCs]) been identified. We intend to examine the relevance of two specific PCs, PC-1 and PC-2, in the processing of CGA to pancreastatin. Another hypothesis regarding CGA function suggests that it is linked to the storage and processing of peptides destined for the regulated secretory pathway; therefore, we will examine the regulation of homeostasis of CGA and its co-resident peptide. Gene expression of a stomach peptide, gastrin, accompanies gene expression for CGA in a human ovarian adenocarcinoma cell line; and, a calcium regulatory peptide, parathyroid hormone-related peptide PTHrp), is synthesized and released by a human hepatic cell line. We intend to examine whether CGA and its co-resident peptide show characteristics of regulated secretion, and if CGA is processed to pancreastatin in these cell lines. The working hypotheses for this research proposal include: chromogranin A (CGA) serves as precursor for biologically active peptides in enteroendocrine cells of the gastrointestinal (GI) tract and pancreases; prohormone convertases (PCs) participate in the processing of CG to its product, pancreastatin; cellular homeostasis of CGA and its co-resident peptide are linked; and, production and secretion of gastrin- related peptides in pancreatic, colon and ovarian cancer cell lines, and of parathyroid hormone-related peptide (PTHrp in a hepatic cell line are accompanied by CGA.
The Specific Aims of this proposal are: 1) to characterize pancreastatin-like peptides in the pancreas and gastrointestinal tract, 2) to examine the involvement of the prohormone convertases, PC-1 and PC-2, in the formation of pancreastatin variants from CGA, 3) to investigate the regulation of CGA and its co-resident peptide (ie, insulin) homeostasis, and the control of CGA processing to pancreastatin in the endocrine pancreas, 4) to examine the hypothesis that CGA co-resides with gastrin peptides in pancreatic, colon and ovarian cancer cells and with PTHrp in liver cells (HEP-G2); and, that CGA and its co-resident peptide show properties of regulated secretion.
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