Effects of gastrin and difluoromethylornithine on growth of human colon cancer

Gastrin activates autophagy and increases migration and survival of gastric adenocarcinoma cells

BACKGROUND

The peptide hormone gastrin exerts a growth-promoting effect in both normal and malignant gastrointestinal tissue. Gastrin mediates its effect via the cholecystokinin 2 receptor (CCKBR/CCK2R). Although a substantial part of the gastric adenocarcinomas express gastrin and CCKBR, the role of gastrin in tumor development is not completely understood. Autophagy has been implicated in mechanisms governing cytoprotection, tumor growth, and contributes to chemoresistance. This study explores the role of autophagy in response to gastrin in gastric adenocarcinoma cell lines.

METHODS

Immunoblotting, survival assays and the xCELLigence system were used to study gastrin induced autophagy. Chemical inhibitors of autophagy were utilized to assess the role of this process in the regulation of cellular responses induced by gastrin. Further, knockdown studies using siRNA and immunoblotting were performed to explore the signaling pathways that activate autophagy in response to gastrin treatment.

RESULTS

We demonstrate that gastrin increases the expression of the autophagy markers MAP1LC3B-II and SQSTM1 in gastric adenocarcinoma cells. Gastrin induces autophagy via activation of the STK11-PRKAA2-ULK1 and that this signaling pathway is involved in increased migration and cell survival. Furthermore, gastrin mediated increase in survival of cells treated with cisplatin is partially dependent on induced autophagy.

CONCLUSION

This study reveals a novel role of gastrin in the regulation of autophagy. It also opens up new avenues in the treatment of gastric cancer by targeting CCKBR mediated signaling and/or autophagy in combination with conventional cytostatic drugs.

Gastrin-induced proliferation involves MEK partner 1 (MP1)

The peptide hormone gastrin is an important factor for the maintenance and homeostasis of the gastric mucosa. We show that gastrin stimulates proliferation in a dose-dependent manner in the human gastric adenocarcinoma cell line AGS-GR. Furthermore, we demonstrate that the MAPK scaffold protein MEK partner 1 (MP1) is important for gastrin-induced phosphorylation of ERK1 and ERK2 and that MP1 promotes gastrin-induced proliferation of AGS-GR cells. Our results suggest a role of MP1 in gastrin-induced cellular responses involved in proliferation and homeostasis of the gastric mucosa.

Differentiation-inducing effect of retinoic acid, difluoromethylornithine, sodium butyrate and sodium suramin in human colon cancer cells

We investigated the relative effectiveness of four differentiation-inducing chemicals to induce a more normal or benign phenotype in the human colon cancer cell lines Moser and HT29. The differentiation-inducing capability of all-trans retinoic acid (ATRA), difluoromethylornithine (DFMO), sodium butyrate (NaB) and sodium suramin (NaS) was evaluated in terms of the efficacy of these chemicals in inhibiting cellular proliferation, growth in soft agarose, invasion of matrigel and induction of morphological alteration. The relative ability of these chemicals to induce production of the differentiation-related molecules fibronectin and carcinoembryonic antigen was also determined. Overall, ATRA was found to be the most effective chemical in inducing differentiation as measured by these parameters. The Moser cells were more susceptible to differentiation induction by comparison with the HT29 cells. Both similarities and differences in the cellular responses to DFMO, NaB and NaS were also observed for the Moser and HT29 cells. The differences in cellular responses to these chemicals may be due to different phenotypic properties of these two cell lines and different mechanisms of action of these chemicals.

CCK-B receptors produce similar signals but have opposite growth effects in CHO and Swiss 3T3 cells

Rat cholecystokinin-B (CCK-B) receptors were transfected into Chinese hamster ovary (CHO)-K1 (CHO-CCK-B) and Swiss 3T3 (Swiss 3T3-CCK-B) cells, and the effects of receptor activation on cell proliferation and intracellular signaling were investigated. CCK octapeptide (CCK-8) treatment had no effect on cell growth in quiescent CHO-CCK-B cells but inhibited DNA synthesis, proliferation, and colony formation when the cells were grown in fetal bovine serum (FBS). In contrast, CCK-8 stimulated DNA synthesis in quiescent Swiss 3T3-CCK-B cells and had no effect when the cells were grown in FBS. These differences in growth responses were not due to differences in the level of receptor expression, as similar numbers of receptors were present in both cell types. To determine whether the different growth effects were due to differences in receptor coupling to common second messenger pathways, we investigated the effects of CCK-8 on several known intracellular signals. In both cell types, CCK-8 stimulated increases in intracellular Ca2+ concentration and polyphosphoinositide hydrolysis with similar potencies and efficacies. CCK-8 also stimulated arachidonate release from both cell types, although the potency was higher in the CHO cells. Adenosine 3',5'-cyclic monophosphate generation was observed at high agonist concentrations in both cell types and was much greater in cells with higher receptor density. In summary, receptor activation had opposite effects on growth parameters in CHO and Swiss 3T3 cells, but only minor differences were observed in the characteristics of CCK-B receptor coupling to specific second messengers in the two cell types. Thus cellular context is a principal determinant of the biological effects of CCK-B receptor activation, and differences in biological responses may occur independently of major differences in receptor coupling.

Mechanisms of bombesin on growth of gastrinoma (PT) in vivo

The growth of the human gastrinoma model (PT) in athymic nude mice is stimulated by bombesin (BBS), an amphibian peptide homologous to both human gastrin-releasing peptide (GRP) and neuromedin B (NMB). The mechanism is not known, and a potent and specific GRP-R antagonist BIM26226, which has low affinity for NMB-R, was used in vivo in athymic nude mice bearing gastrinoma subcutaneously. Both the BBS and BIM26226 stimulated the growth of PT, and the growth stimulation was even greater when given together. RT-PCR study of gastrinoma revealed the presence of both GRP-R and NMB-R mRNA, but much more abundant NMB-R mRNA. We conclude that BBS-stimulated growth of gastrinoma involves both GRP-R and NMB-R, and our findings suggest that GRP-R mediates negative and NMB-R produces positive growth effects on gastrinoma.

Antiproliferative effect of a novel cholecystokinin-B/gastrin receptor antagonist, YM022

Cholecystokinin (CCK)-B and gastrin receptors are expressed on a variety of human tumor cells. Recently, we have demonstrated that the human brain CCK-B receptors are identical to the gastrin receptors derived from the stomach mucosa, and that the brain-gut peptides, CCK-8 and gastrin I are mitogenic for mouse NIH 3T3 fibroblasts expressing human CCK-B/gastrin receptors (N-hCCKBR). In this report, we evaluated the antiproliferative potency of CCK-B/gastrin receptor antagonists by using N-hCCKBR cells. Among several antagonists, a benzodiazepine derivative, YM022 had the most potent activities in competing with [125I]CCK-8 or [125I]gastrin I binding, inhibition of CCK-8- or gastrin I-induced phosphoinositide hydrolysis and increasing cytoplasmic free calcium. Interestingly, a potent antagonist for rat CCK-B/gastrin receptors did not have such activities in N-hCCKBR cells. YM022 inhibited the CCK-8- or gastrin I-induced [methyl-3H]thymidine incorporation of N-hCCKBR cells in a dose-dependent manner. In the absence of exogenous peptide ligands, YM022 also inhibited the proliferation of several human cancer cell lines expressing the genes for both gastrin and its receptor. These results suggest that YM022 could intervene in the autocrine stimulation of human tumor cell lines through CCK-B/gastrin receptors. N-hCCKBR cells are an excellent tool to screen for novel human CCK-B/gastrin receptor antagonists possessing antiproliferative activity for human cancer cells.

The role of gastrin and cholecystokinin in normal and neoplastic gastrointestinal growth

Gastrin and cholecystokinin (CCK) act as growth factors for the gastric mucosa and the pancreas, respectively. CCK is also responsible, via the CCK-A receptor, for the pancreatic hyperplasia observed following the feeding of protease inhibitors or pancreaticobiliary diversion. Hypergastrinaemia does not increase the incidence of spontaneous gastrointestinal carcinoma, but does stimulate the proliferation of gastric enterochromaffin-like cells via the gastrin/CCK-B receptor, with a consequent increase in the incidence of gastric carcinoids. Whether gastrin influences mutagen-induced gastrointestinal carcinogenesis is still controversial, but CCK clearly enhances the induction by carcinogens of acinar tumours in the pancreas. While gastrin increases xenograft growth of 50% of gastrointestinal tumours tested, effects on the proliferation of gastrointestinal tumour cell lines in vitro have been more difficult to demonstrate, perhaps because many cell lines are already maximally stimulated by autocrine gastrin. Gastrin mRNA and progastrin, but not mature amidated gastrin, have been detected in all gastrointestinal cell lines tested. Although cell proliferation is inhibited by gastrin/CCK receptor antagonists, the spectrum of antagonist affinities is not consistent with binding to either CCK-A or gastrin/CCK-B receptors. Definition of the molecular structure of the receptor involved in the autocrine loop may lead to novel therapies for gastrointestinal cancer.