Distinct Linkage between Post-translational Processing and Differential Secretion of Progastrin Derivatives in Endocrine Cells

Abstract

Prohormones often undergo extensive cellular processing prior to secretion. These post-translational processing events occur in organelles of the constitutive or regulated secretory pathway. The aim of this study was to examine the relationship between post-translational modifications and the secretory pathways taken by peptides derived from progastrin, the prohormone of gastrin, which in vivo is secreted by cells of the pyloric glands and stimulates the release of gastric acid. Targeting progastrin to compartments of the early secretory pathway shows that endoproteolytic processing is initiated in a pretrans-Golgi network compartment of endocrine but not nonendocrine cells. The resulting N-terminal fragments of progastrin are secreted via the constitutive pathway, whereas endoproteolytically processed C-terminal fragments are secreted via the regulated or constitutive-like pathways. C-terminal fragments derived from progastrin differ in characteristic manners in levels and patterns of carboxyamidation and tyrosine sulfation in accordance with the secretory pathway taken. Point mutations introduced into a sorting motif disrupt these patterns, suggesting that differences in post-translational modifications are attributable to differential intracellular sorting of precursors. The results suggest a two-step sorting mechanism for progastrin leading to differential secretion of processed fragments via different secretory pathways.

Ferric ions inhibit proteolytic processing of progastrin

“…Bundgaard and coworkers have begun to unravel by mutational studies the interconnections between tyrosine sulfation, serine phosphorylation and the acidic metal ion binding domain of progastrin as a potential regulatory mechanism for differential progastrin secretion. The acidic penta-glutamate domain and the dibasic cleavage sites of progastrin have been proposed to synergise in the sorting of the maturing peptides to regulated, constitutive or constitutive-like pathways [7,28]. In the present study we provide direct evidence that metal ion binding to progastrin is likely to have a significant role in determining the cleavage pattern of progastrin as it traverses the secretory pathway.…”

Cell-Specific Precursor Processing

“…G-cells are equipped with an extensive machinery of processing enzymes located along the secretory pathway (Bundgaard et al 1995Bundgaard and Rehfeld 2008). Matured by the enzymes, the original progastrin molecule ends up in secretory granules almost completely processed to bioactive alfa-amidated gastrins, of which nearly 90% are gastrin-17 and 5-10% are gastrin-34 (Håkanson et al 1982;Hilsted and Rehfeld 1987).…”

“…Generally, the posttranslational modifications are never complete, leaving varying amounts of intact precursors and partly processed intermediates along the secretory pathway. Some of the prohormones and processing intermediates are subsequently released by constitutive or constitutive-like secretion, but portions of the precursors may also be secreted in a regulated manner together with the bioactive endproducts from secretory granules (Bundgaard and Rehfeld 2008;Rehfeld et al 2004).…”

Do Glycine-Extended Hormone Precursors have Clinical Significance?

“…The granular content of glycine-extended hormone precursors is released together with the mature amidated peptide hormones after the intracellular travel along the regulated secretory pathway. But a minor fraction of the glycine-extended hormones may also be released from the cells via the constitutive-like secretory pathway together with other processing intermediates [25,28].…”

“…The variable equipment with these enzymes explains the cell-specific processing pattern seen for many hormones [7-10, 23,24]. The processing of prohormones begins in the Golgi apparatus and is then followed up in the secretory pathway of neuroendocrine cells, that is, from the Golgi apparatus to small-and intermediate-sized vesicles and then to the larger, mature secretory granules that eventually fuse in a regulated manner with the cell membranes and empty their content into the local capillary network [25][26][27][28]. The granular content of glycine-extended hormone precursors is released together with the mature amidated peptide hormones after the intracellular travel along the regulated secretory pathway.…”