Progastrin Explained

Progastrin is an 80-amino acid intracellular protein and the precursor of gastrin, a gastrointestinal hormone produced by G cells in the gastric antrum.[1] The main function of gastrin is to regulate acid secretion.[2] During digestion, only gastrin is released into the bloodstream and stimulates the secretion of hydrochloric acid in the stomach as well as pancreatic digestive enzymes. In humans, progastrin is encoded by the GAST[3] gene. Progastrin is expressed primarily in stomach tissue.

Discovery

In 1905, John Sydney Edkins demonstrated the existence of a hormone responsible for the secretion of gastric acid.[4] This hormone was named gastric secretin or gastrin. But it was not until 1979 and later in 1987 and 1988 that progastrin was identified as the precursor to gastrin.[5] [6] [7] His protein sequence and mRNA were revealed.

Not to be confused with progastrin, Pro-Gastrin-Releasing-Peptide is the precursor of Gastrin-releasing peptide (GRP), a neuropeptide which belongs to the bombesin/neuromedin B family and whose expression is important in the intestine and brain. GRP is involved in many physiological and pathophysiological processes.[8] Gastrin-Releasing Peptide stimulates the release of gastrin and other gastrointestinal hormones. It helps regulate food intake. There are also two types of progastrin, the intracellular progastrin discussed in this article and the extracellular progastrin, mainly called hPG80.[9]

Structure and production

The GAST gene is located on chromosome 17 (17q21). It consists of three exons containing the sequence that encodes preprogastrin. Progastrin is made up of several alpha helices. The primary structure of human preprogastrin consists of a 21 amino acid signal sequence at the N-terminus followed by a spacer peptide, a bioactive domain and finally a C-terminal flanking peptide (CTFP).[10]

Action

The production of gastrin allows the production of gastric acid during food intake to be promoted. Progastrin has also been shown to be expressed in other healthy tissues (cerebellum, pituitary gland, pancreas, testicles), but to a much lesser extent than in the stomach and in different forms.[11] [12] [13] [14] For example, carboxyamide gastrin is found in the testicles.[15] However, the role of progastrin in these organs is not clearly established.

Biosynthesis

In humans, the GAST gene encodes a 101-amino acid precursor peptide, preprogastrin.[16] The latter is synthesized and matured in the endoplasmic reticulum. Upon initiation of translation, the signal sequence facilitating the translocation of the polypeptide is eliminated by a membrane-bound signal peptidase. This enzyme cleaves the born polypeptide chain between alanine residue 21 and serine 22 to generate the 80-amino acid peptide progastrin. The progastrin is then cleaved by an enzyme to give the main circulating biologically active forms of gastrin: gastrin-34 and gastrin-17, in sulfated and unsulfated forms. Both sulfation and phosphorylation play a role in the maturation process: they increase the maturation of progastrin. While phosphorylation can also affect the conversion of intermediate products with carboxy-terminal glycine (G34-Gly and G17-Gly) to mature gastrins.[17] Small amounts of gastrin-52 (also called component 1), gastrin-14 (mini-gastrin) and even smaller fragments were detected in the serum.[18] At this stage, two pathways of post-translational modifications exist within the antral G cells. In the dominant pathway, progastrin is cleaved at three sites, resulting in two major bioactive gastrins, gastrin-34 and gastrin-17. In the putative alternative pathway, progastrin can be cleaved only at the dibasic site closest to the C-terminus, resulting in the synthesis of gastrin-71.[19] The other maturation products, in particular G34-Gly, G17-Gly and CTFP have various functions. CTFP has been described as capable of inducing or inhibiting apoptosis, depending on the tissue or cell type involved. After the progastrin conversion step, there is a passage through the secretory pathway.

Under pathological conditions

As early as 1996, it was demonstrated that the expression of the GAST gene is required for the cellular tumorigenicity of human colorectal cancer. The GAST gene has been shown to be a downstream target of the β-catenin/TCF-4 signalling pathway. Transfection of a construct expressing a constitutively active form of β-catenin triples the activity of the GAST gene promoter.[20] This study establishes a link between progastrin and cancer across many cellular functions involving the Wnt pathway in a cancer cell, starting with its importance for cancer stem cell survival.[21] In pathological situations, progastrin is secreted by tumor cells into the bloodstream and is referred to as hPG80.[22]

The expression of progastrin in many cancers has been demonstrated.[23] It has been noted that in colorectal cancers, progastrin is more than 700 times more abundant than amidated gastrin.[24] Another study showed that colorectal carcinomas have progastrin-derived peptides that are not converted to gastrin.[25] Finally, it has been shown that half of the tumor cells express progastrin. Ovarian, liver and pancreatic cancers present the same scenario: unmatured progastrin is more abundant than amidated gastrin.[26] [27] [28] This is because progastrin is not fully matured and is secreted into the bloodstream for many cancers.

Notes and References

  1. Book: Fiona M.. Gribble. Gastrointestinal Hormones. Frank. Reimann. Geoffrey. P. Roberts. Physiology of the Gastrointestinal Tract, Elsevier. 2018. 31–70.
  2. Book: Rehfeld. Jens F.. 2 The Post-Translational Phase of Gene Expression in Tumor Diagnosis. 2005. Handbook of Immunohistochemistry and in Situ Hybridization of Human Carcinomas. 4. 23–32. Elsevier. en. 10.1016/s1874-5784(05)80057-1. 978-0-12-369402-7. Goetze. Jens Peter.
  3. Web site: GAST - Gastrin precursor - Homo sapiens (Human) - GAST gene & protein. www.uniprot.org. 2020-06-26.
  4. en. World Journal of Surgery. 10.1007/s002689900221. 0364-2313. February 1997. 2. 21. The pivotal role of John S. Edkins in the discovery of gastrin. Irvin M.. Tang. Laura H.. Marks. I. N.. Kidd. Mark. Modlin. 226–34. 8995084. 28243696.
  5. en. 76. 10.1073/pnas.76.4.1770. 383472. 88048. 0027-8424. 1979-04-01. 4. Proceedings of the National Academy of Sciences. B. E.. Detection and partial sequence analysis of gastrin mRNA by using an oligodeoxynucleotide probe.. Agarwal. K. L.. Stein. R.. Mevarech. M.. Noyes. 1770–4. 1979PNAS...76.1770N. free .
  6. en. FEBS Letters. 10.1016/0014-5793(87)81334-0. 1987-01-05. 2. 210. Isolation and characterization of the intact gastrin precursor from a gastrinoma. H.. Gregory. H.. Varro. A.. Pauwels. S.. Desmond. 185–8. 3792562. 20561440. free.
  7. Announcements. Regulatory Peptides. 34. 1. 1991. 0167-0115. 10.1016/0167-0115(91)90226-7. 71–72. 208789664.
  8. You. Benoit. Mercier. Frédéric. Assenat. Eric. Langlois-Jacques. Carole. Glehen. Olivier. Soulé. Julien. Payen. Léa. Kepenekian. Vahan. Dupuy. Marie. Belouin. Fanny. Morency. Eric. January 2020. The oncogenic and druggable hPG80 (Progastrin) is overexpressed in multiple cancers and detected in the blood of patients. EBioMedicine. en. 51. 102574. 10.1016/j.ebiom.2019.11.035. 6938867. 31877416.
  9. en. Benoit You, Frédéric Mercier, Eric Assenat. Alexandre Prieur. Léa Payen, Vahan Kepenekian, Marie Dupuy. Fanny Belouin, Eric Morency. The oncogenic and druggable hPG80 (Progastrin) is overexpressed in multiple cancers and detected in the blood of patients.. EBioMedicine. 51. January 2020. 10.1016/j.ebiom.2019.11.035. 102574. 31877416. 6938867.
  10. Web site: GAST (gastrin). atlasgeneticsoncology.org. 2020-06-26.
  11. en. Journal of Clinical Investigation. 10.1172/JCI114758. 0021-9738. 1990-08-01. 2. 86. Expression and localization of gastrin messenger RNA and peptide in spermatogenic cells.. M. Odum. L. Pelto-Huikko. M. Persson. H. Schalling. 660–9. 2117026. 296774. free.
  12. en. Linda. Bardram. Progastrin in serum from Zollinger-Ellison patients. Gastroenterology. 98. 6. 1990. 1420–6. 10.1016/0016-5085(90)91071-D. 2338186.
  13. en. J.F.. Rehfeld. Progastrin and its products in the cerebellum. Neuropeptides. 20. 4. 1991. 239–45. 10.1016/0143-4179(91)90014-A. 1812406. 21211891.
  14. L.. Larsson. J.. Rehfeld. Pituitary gastrins occur in corticotrophs and melanotrophs. Science. 213. 4509. 1981-08-14. 768–70. 0036-8075. 10.1126/science.6266012. 6266012. 1981Sci...213..768L.
  15. en. Cancer. 10.1002/1097-0142(19930901)72:5<1726::aid-cncr2820720536>3.0.co;2-s. 1097-0142. 1993. 5. 72. Primary carcinoid tumor of testis. Immunohistochemical, ultrastructural, and DNA flow cytometric study of three cases with a review of the literature. Angel. OrdóNtez. Nelson G.. El‐Naggar. Adel. Ro. Jae Y.. Zavala‐Pompa. 1726–32. 7688660.
  16. Web site: GAST (gastrin). atlasgeneticsoncology.org. 2020-06-26.
  17. en. Gastroenterology. 10.1016/S0016-5085(98)70086-1. 1998. 5. 115. Modulation of the cleavage of the gastrin precursor by prohormone phosphorylation. L. Deavall. D. Blackmore. C. Dimaline. R. Bishop. 1154–62. 9797370.
  18. Book: Rehfeld. Jens F.. 2 The Post-Translational Phase of Gene Expression in Tumor Diagnosis. 2005. Handbook of Immunohistochemistry and in Situ Hybridization of Human Carcinomas. 4. 23–32. Elsevier. en. 10.1016/s1874-5784(05)80057-1. 978-0-12-369402-7. Goetze. Jens Peter.
  19. Web site: GAST - Gastrin precursor - Homo sapiens (Human) - GAST gene & protein. www.uniprot.org. 2020-06-26.
  20. en. 106. 10.1172/JCI9476. 380254. 10953028. 0021-9738. 2000-08-15. 4. Journal of Clinical Investigation. Theodore J.. Gastrin is a target of the β-catenin/TCF-4 growth-signaling pathway in a model of intestinal polyposis. Dockray. Graham J.. Fleming. John V.. Bulitta. Clemens J.. Koh. 533–9.
  21. en. 23. 2020-06-23. Targeting the Wnt Pathway and Cancer Stem Cells with Anti-progastrin Humanized Antibodies as a Potential Treatment for K-RAS-Mutated Colorectal Cancer. 10.1158/1078-0432.CCR-17-0533. 28600477. 1078-0432. 2017. 17. Clinical Cancer Research. Alexandre. Flaceliere. Maud. Habif. Guillaume. Dominique Joubert. Monica. Prieur. 5267–5280. 22085894. free.
  22. en. EBioMedicine. 10.1016/j.ebiom.2019.11.035. 6938867. 31877416. 2020. 51. The oncogenic and druggable hPG80 (Progastrin) is overexpressed in multiple cancers and detected in the blood of patients. Benoit. Langlois-Jacques. Carole. Assenat. Eric. Mercier. Frédéric. You. 102574.
  23. en. Biochemical and Biophysical Research Communications. 10.1016/0006-291X(92)92326-S. 1992. 2. 189. Post-translational processing of gastrin in neoplastic human colonic tissues. Michael Lee. Boland. C.Richard. Dickinson. Chris John. DelValle. John. Kochman. 1165–9. 1472026. 2027.42/29682. free.
  24. en. 34. 10.1136/gut.34.1.90. 1374107. 8432459. 0017-5749. 1993-01-01. 1. Gut. J. Identification of progastrin derived peptides in colorectal carcinoma extracts.. Varro. A. Pauwels. S. Taylor. B. Nemeth. 90–5.
  25. Wouter W.. 104. 10.1016/0016-5085(93)90279-l. 0016-5085. 1993. 4. Gastroenterology. Van Solinge. Expression but incomplete maturation of progastrin in colorectal carcinomas. Falkmer. Ursula G.. Friis-Hansen. Lennart. Nielsen. Finn C.. 1099–107. 8462798. free.
  26. en. Journal of Hepatology. 10.1016/S0168-8278(99)80114-7. 1999. 3. 30. Expression and processing of gastrin in hepatocellular carcinoma, fibrolamellar carcinoma and cholangiocarcinoma. Martyn. Rode. Jurgen. Savage. Kay. Khan. Kosser. Caplin. 519–26. 10190738.
  27. en. British Journal of Surgery. 10.1046/j.1365-2168.2000.01488.x. 2000-08-01. 8. 87. Expression and processing of gastrin in pancreatic adenocarcinoma: Gastrin and pancreatic cancer. M.. Brett. B.. Khan. K.. Savage. K.. Caplin. 1035–40. 10931047. 24292877.
  28. en. American Journal of Physiology. Gastrointestinal and Liver Physiology. 10.1152/ajpgi.1994.266.3.G459. 0193-1857. 1994-03-01. 3. 266. Incomplete processing of progastrin expressed by human colon cancer cells: role of noncarboxyamidated gastrins. P.. Rajaraman. S.. Dai. B.. Xu. Z.. Singh. G459-68. 8166285.

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