| Research Abstract |
Hypergastrinemia is frequently observed in patients with Zollinger-Ellison (ZE) tumor and with atrophic gastritis. ZE tumor is known to produce a high level of gastrin, which stimulates parietal cells and enterochromaffin-like (ECL) cells, both of which are known to express a gastrin receptor. Upon gastrin stimulation parietal cells produce gastric acid, and ECL cells produce histamine, a potent acid secretagogue on parietal cells. With a synergistic effect of gastrin and histamine, parietal cells produce a plentiful of gastric acid, and gastric mucosa becomes hypertrophic by yet unknown mechanisms. Another cause of hypergastrinemia is atrophic gastritis that is known as a precancerous state. In atrophic gastritis, gastric fluid becomes neutral due to deficiency of gastric acid because parietal cells are often impaired by autoimmune mechanisms or toxins from Helicobacter pylori. Resultantly, gastrin production is up-regulated from antral G cells by the lack of an inhibitory message, ac
… More
idity in gastric fluid.
To elucidate the mechanism of hypertrophic growth of gastric mucosa by gastrin and TGFalpha, We attempted to produce transgenic mice with overexpression of gastrin or TGFalpha in gastric mucosa. Fortunately, co-investigator, H.Takagi, has already made transgenic mice with overexpressing TGFalpha in the gastric mucosa before. Thus, we focused on the production of gastrin-overexpressing transgenic mice. Gastrin is synthesized as a precursor preprogastrin. This precursor requires a series of post-translational processing reactions to become bioactive gastrin, which include dibasic cleavage at paired basic residues, their subsequent removal by carboxypeptidase H,and formation of a carboxyl (C-) terminal amide moiety via the action of the amidation enzyme, peptidyl-glycine alpha-amidating mono-oxygenase (PAM). The amidated gastrin (G17-NH_2) thus formed, exhibits gastric acid-secreting activity three orders of magnitude higher than does glycine-extended a gastrin (G17-Gly). To attain hypergastrinemia in tansgenic mice, we expressed a gastrin cDNA under the control of a beta-actin promoter, which exhibits strong expression in a variety of tissues. Another devise we used is the cleavage site for proprotein-processing enzyme furin, which distributes in virtually all tissues. By creating a furin creavable site before gastrin-17 and deleting the C-terminal extension peptide to become glycine as a last amino acid for efficient amidation reaction, we succeeded in producing bioactive gastrin from two conventional culture cell lines CHO and COS-7. Then, we injected the gastrin expression uint DNA into a rat ovum for producing transgenic mice. We obtained five hypergastrinemic mice with 400-500 pg/ml serum gastrin. By cross-mating these heterozygous mice, we obtained homozygous mice with a 600-1500 pg/ml serum gastrin level. As expected, hypergastrinemic mice had hypertrophic gastric mucosa with an extended hight of a surface mucous cell layr along a gastric pit. Although a gastric gland unit is highly extended, its structure was normally arranged and surface mucous cells appear to be normal. In contrast, TGFalpha-overexpressing gastric mucosa exhibited enlarged mucous epithelia with hyperplastic, dysplastic and cystic changes. Gastric glandular structure was similar to Menetrier's disease in humans, which is a precancerous state for gastric cancer. Since gastrin receptors are localized in parietal cells and ECL cells, the growth of mucosa by gastrin is thought to be mediated by unknown factors from parietal cells. One of the candidates of these factors was TGFalpha. However, considering the difference in histological changes between gastrin and TGFalpha, gastrin appears to exert its asction directly onto gastric surface mucous cells. Our preliminary data suggests that gastric surface mucous cells may express gastrin receptors on their plasma membrane.
As expected, hypergastrinemic mice had hypertrophic gastric mucosa with an extended hight of a surface mucous cell layr along a gastric pit. Although a gastric gland unit is highly extended, its structure was normally arranged and surface mucous cells appear to be normal. In contrast, TGFalpha-overexpressing gastric mucosa exhibited enlarged mucous epithelia with hyperplastic, dysplastic and cystic changes. Gastric glandular structure was similar to Menetrier's disease in humans, which is a precancerous state for gastric cancer. Since gastrin receptors are localized in parietal cells and ECL cells, the growth of mucosa by gastrin is thought to be mediated by unknown factors from parietal cells. One of the candidates of these factors was TGFalpha. However, considering the difference in histological changes between gastrin and TGFalpha, gastrin appears to exert its asction directly onto gastric surface mucous cells. Our preliminary data suggests that gastric surface mucous cells may express gastrin receptors on their plasma membrane.
Gastrin has long been thought to act as a tumor promoter for gastic cancer. But, our gastrin-overexpressing model dose not support this possibility although TGFalpha appears to be involved in the formation of precancerous changes in the gastric mucosa. Less
|
