Possible involvement of protein kinase C in mediating gastrin-induced response in rat colonic epithelium

The role of Sonic Hedgehog as a regulator of gastric function and differentiation

The Hedgehog (Hh) genes play a key role in the regulation of embryonic development and govern processes such as cell differentiation, cell proliferation, and tissue patterning. In vertebrate embryos, Hh gene expression regulates correct formation of limbs, skeleton, muscles, and organs including stomach. In the adult, the Hh pathway functions in tissue repair and regeneration, along with maintenance of stem cells. Sonic Hedgehog (Shh) signaling has been extensively studied for its role in developmental and cancer biology. Recent advances in the field of gastroenterology show that in the stomach, Shh is responsible for proper differentiation of the gastric glands. The aberrant activity of the Shh signaling pathway leads to an altered gastric differentiation program and loss of gastric acid secretion that is the predominant function of the stomach. In this chapter, we review the most recent findings that reveal the role of Shh as a regulator of gastric function and differentiation and how this signaling is dysregulated during the development of gastric cancer in response bacterial infection.

The production and role of gastrin-17 and gastrin-17-gly in gastrointestinal cancers

The gastrointestinal peptide hormone gastrin is responsible for initiating the release of gastric acid in the stomach in response to the presence of food and/or humoral factors such as gastrin releasing peptide. However, it has a role in the growth and maintenance of the gastric epithelium, and has been implicated in the formation and growth of gastric cancers. Hypergastrinemia resulting from atrophic gastritis and pernicious anemia leads to hyperplasia and carcinoid formation in rats, and contributes to tumor formation in humans. Additionally, gastrin has been suspected to play a role in the formation and growth of cancers of the colon, but recent studies have instead implicated gastrin processing intermediates, such as gastrin-17-Gly, acting upon a putative, non-cholecystokinin receptor. This review summarizes the production and chemical structures of gastrin and of the processing intermediate gastrin-17-Gly, as well as their activities in the gastrointestinal tract, particularly the promotion of colon cancers.

Effects of gastrin and its antagonist proglumide on proliferation of human hepatoma cell lines

AIM: To determine the effects of gastrin and its receptor antagonist proglumide on the proliferation of hepatoma cell lines, and to evaluate the possibility of non-cytotoxic therapy for liver cancers.

METHODS: The vitality and cycle distribution of 4 liver cancer cell lines treated with gastrin and proglumide were analyzed using MTT colorimetric assay and flow cytometry.

RESULTS: Gastrin significantly stimulated the proliferation of QGY-7701 and Bel-7402 at a concentration approximate to physiological levels, and shifted G₀/G₁ phase to S, G₂/M phase of Bel-7402 ,which could be blocked by gastrin receptor antagonist proglumide. And excessive gastrin exerted inhibitory effect on QGY-7701. However, Gastrin had no effect on HepG2 and SMMC-7721 cell lines. Proglumide at a high concentration significantly inhibited all cell lines.

CONCLUSION: Gastrin can promote proliferation of liver cancer cells and DNA synthesis while Proglumide inhibit gastrin. Gastrin receptor antagonist may become a new endocrine approach to some liver cancers.

Regulatory effect and mechanism of gastrin and its antagonists on colorectal carcinoma

AIM: To explore the effect and mechanism of gastrin and its an tagonists proglumide and somatostatin on colorectal carcinoma and their clinical significance.

METHODS: A model of transplanted human colonic carcinoma was established from SW480 cell line in gymnomouse body. The volume and weight of transplanted carcinoma was observed under the effect of pentagatrin (PG), proglumide (PGL) and octapeptide somotostatin (SMS201-995, SMS). The cAMP content of carcinoma cell was determined by radioimmunoassay and the DNA, protein content and cell cycle were determined by flow-cytometry. The amount of viable cells was determined by MTT colorimetric analysis, IP₃ content was determined by radioimmuno assay, Ca²⁺ concentration in cell by fluorometry and PKC activity by isotopic enzymolysis. The expression of gastrin, c-myc, c-fos and rasP21 in 48 case s of colorectal carcinoma tissue was detected by the immuno-cytochemistry SP method. Argyrophilia nucleolar organizer regions was determined with argyrophilia stain.

RESULTS: The volume, weight, cAMP, DNA and protein content in carcinoma cell, cell amount and proliferation index of S and G₂M phase in PG group were all significantly higher than those of control group. When PG was at the concentration of 25 mg/L, the amount of viable cells, IP₃ content and Ca²⁺ concentration in cell and membrane PKC activity in PG group were significantly higher than those in control group; when PGL was at a concentration of 32 mg/L, they dropped to the lowest level in PG (25 mg/L) + PGL group, but without significant difference from the control group. The positive expression rate of gastrin, c-myc, c-fos and rasP21 in carcinoma tissue was 39.6%, 54.2%, 47.9% and 54.2% respectively and significantly higher than that in mucosa 3 cm and 6 cm adjacent to carcinoma tissue and normal colorectal mucosa. The positive expression rate of gastrin of highly-differentiated adenocarcinoma group was significantly higher than that of poorly-differentiated and mucinous adenoc arcinoma groups. The AgNORs count of carcinoma tissue was significantly higher than that in mucosa 3 cm and 6 cm adjacent to carcinoma tissue and norm al colorectal mucosa; and the positive expression of c-myc and c-fos and the A gNORs count in gastrin-positive group was significantly higher than those in gastrin-negative group.

CONCLUSION: Pentagastrin has a promoting effect on the growth of transplanted human colonic carcinoma from SW480 cell line. PGL has no obvious effect on the growth of human colonic carcinoma SW480 cell line, but could inhibit the growth promoting effect of PG on transplanted carcinoma. Somatostatin can not only inhibit the growth of transplanted human colonic carcinoma from SW480 cell line directly but also depress the growth-promoting effect of gastrin on the transplanted carcinoma. Some colorectal carcinoma cells can produce and secrete gastrin through autocrine, highly-differentiated adenocarcinoma express the highest level gastrin. Endogenous gastrin can stimulate the cell division and proliferation of carcinoma cell and promote the growth of colorectal carcinoma regulating the expression of oncogene c-myc, c-fos. Our study has provided experimental basis for the adjuvant treatment using gastrin antagonist such as PGL, so matostatin of patients with colorectal carcinoma.

Signaling pathways mediating gastrin's growth-promoting effects

In addition to its fundamental role in stimulating gastric acid secretion, the peptide hormone gastrin induces growth-promoting effects on diversity of target cells. Various mechanisms, including endocrine, paracrine, and autocrine, have been proposed for gastrin's growth-promoting actions. The mitogenic effects of gastrin are mediated by specific cell surface receptors activated after gastrin binding. The functionally defined receptors for gastrin include cholecystokinin A (CCKA) receptor, which is discriminating for sulfated CCK8; cholecystokinin B (CCKB)/gastrin receptor, which binds gastrin17 sulfated, and nonsulfated CCK8 with nearly equal affinities; cholecystokinin C (CCKC), which is a low-affinity gastrin binding protein; and novel, high-affinity receptors selective for amidated gastrin, processing intermediates of gastrin, or both. The signaling pathways mediating gastrin's stimulation of the CCKB/gastrin receptor have been progressively outlined, and the pathways mediating other receptors have been slowly emerging. Engagement of the gastrin receptor initiates various biochemical and molecular events, including recruitment and activation of tyrosine kinases, activation of the phospholipase C signaling pathway leading to phosphoinositide breakdown, intracellular calcium mobilization and protein kinase C stimulation, activation of the mitogen-activated protein kinase pathway, and induction of early response genes. Current emphasis is on understanding the functional significance of processing intermediate forms of gastrin, and the receptor subtypes and pathways that promote the trophic/mitogenic effects of the different molecular forms of gastrin.

Gastrin induces IP3 formation through phospholipase C gamma 1 and pp60c-src kinase

We have previously reported that gastrin induces a rapid and transient tyrosine phosphorylation of phospholipase C gamma 1 (PLC gamma 1) in association with inositol 1,4,5-trisphosphate (IP3) formation in rat colonic epithelial cells (34). In this study, we demonstrate that gastrin regulates IP3 formation mainly through PLC gamma 1 isozyme. Immunoblotting analysis revealed the expression of PLC beta 3 and -gamma 1, but not PLC beta 1, -beta 2, or -beta 4 in the rat colonic epitheliums. To explore what PLC isozyme(s) modulates gastrin effect on IP3, immunoneutralizing antibody to PLC beta 1, -beta 3, or -gamma 1 was introduced into the colonic cells using a lipid carrier. The gastrin-stimulated increase in IP3 concentration was specifically prevented by anti-PLC gamma 1 but not by anti-PLC beta 1 or -beta 3 antibody. Immunoprecipitation assays have also revealed that gastrin promoted an increase in tyrosine phosphorylation and co-precipitation of a 60 kDa src kinase with PLC gamma 1. Administration of antibody specific to pp60c-src into the colonic cells prevented the gastrin-stimulated increases in IP3. Tyrosine phosphorylation of PLC gamma 1 may be a major mechanism through which gastrin regulates IP3 level in the colonic cells. Pretreatment of cells with the tyrosine kinase inhibitor genistein abrogated gastrin's effect on IP3, while extended pretreatment with pertussis toxin, a G-protein inhibitor, did not affect the ability of gastrin to stimulate IP3 formation. Colonic cells expressed the G alpha i subunits1-3; however, immunoblotting analysis did not reveal any difference in G alpha i proteins' expression between control and gastrin treated cells. The results provide direct evidence that gastrin regulates IP3 level by a signaling mechanism that involves PLC gamma 1 and pp60c-src kinase.

Early signalling mechanism in colonic epithelial cell response to gastrin

The hormone gastrin exerts a growth-promoting effect on gastrointestinal cells. The molecular mechanisms by which colonic epithelial cells respond to gastrin are still poorly understood. In this study, we demonstrate a novel feature of the action of gastrin on normal colonic cells, namely the rapid phosphorylation on tyrosine of phospholipase C gamma 1 (PLC gamma 1). Tyrosine phosphorylation of PLC gamma 1, elicited by gastrin, was transient, concentration-dependent, and was abrogated by pretreating the colonic cells with the gastrin-receptor antagonist proglumide, the tyrosine kinase inhibitor genistein, and by removal of the tyrosine phosphatase inhibitor orthovanadate from the isolation buffer. Tyrosine phosphorylation of PLC gamma 1 correlated with the time- and concentration-dependent decrease in the mass of membrane phosphatidylinositol 4,5-bisphosphate (PIP2) and the increase in the epithelial concentration of inositol 1,4,5-trisphosphate (IP3). Likewise, the stimulated increase in IP3 was also prevented by proglumide and genistein. Gastrin induced a definite but transient increase in the intracellular concentration of free Ca2+ [Ca2+]i, and increased membrane-translocation of immunoreactive alpha- and beta-protein kinase C. The data thus indicate that gastrin elicits at least one signalling cascade, through rapid tyrosine phosphorylation of PLC gamma 1, leading to the activation of a PIP2-specific PLC pathway.

Protein kinase C alpha-, beta- and gamma-subspecies in basal granulated cells of rat duodenal mucosa

Protein kinase C [cPKC: alpha, beta (beta I, beta II), gamma], a Ca(2+)- and phospholipid-dependent enzyme, has been thought to play a critical role in the synthesis and secretion of gut hormones in gastrointestinal mucosa. However, the localization of PKC has not yet been clarified at the cellular level in the gastrointestinal epithelium. The present study was made to identify cPKC-containing cells immunohistochemically in the rat duodenal epithelium by light and electron microscopy and by confocal laser scanning microscopy. Special attention was paid to the demonstration of cPKC in basal granulated cells. By light microscopy, some duodenal epithelial cells were demonstrated to be immunopositive for PKC alpha-, beta- and gamma-subspecies. Their distribution and incidence were almost similar to those of cells stained by the silver impregnation method of Grimelius. By electron microscopy, profiles of secretory granules were found at the basal region of the PKC-immunopositive epithelial cells. When the cells were double-immunostained for gastrin, serotonin or somatostatin and for PKC alpha-, beta- or gamma-subspecies, these gut hormones and PKC subspecies were shown to colocalize as examined by confocal laser scanning microscopy. These findings show that cPKC (alpha, beta, gamma) is present in basal granulated cells such as G-, EC- and D-cells, presumably playing some important role in regulation of gut hormones, including their synthesis and/or secretion.

Gastrin's trophic effect in the colon: identification of a signaling pathway mediated by protein kinase C

In previous studies we have reported that gastrin exerts a trophic effect on rat colonic epithelial cells in vitro. The effect of gastrin appeared to be mediated through a protein kinase C mechanism. In this study, we have characterized the role of protein kinase C in the gastrin-induced stimulation. Gastrin, in a time- and dose-dependent manner, increased protein kinase C translocation from the cytosol to the membrane, an index of enzyme activation. Maximum translocation occurred in 1 to 2 min following exposure to gastrin (10(-8) M), before declining back to baseline level within 5 min. Gastrin did not change total protein kinase C activity in the colonic cells. Staurosporine, an inhibitor of protein kinase C, totally abolished the basal as well as the gastrin-stimulated activity of protein kinase C. The tumor promoter phorbol 12-myristate 13-acetate also stimulated colonic epithelial protein kinase C. However, prolonged treatment of cells with phorbol inhibited their subsequent response to gastrin stimulation. The response to gastrin was also prevented by the gastrin receptor antagonist proglumide. These observations suggest that protein kinase C mediates the stimulatory effect of gastrin on colonic epithelial cells, possibly through a receptor mechanism.