Gastrin is a target of the beta-catenin/TCF-4 growth-signaling pathway in a model of intestinal polyposis

Abstract

Mutations in the adenomatous polyposis coli (APC) tumor suppressor gene occur in most colorectal cancers and lead to activation of beta-catenin. Whereas several downstream targets of beta-catenin have been identified (c-myc, cyclin D1, PPARdelta), the precise functional significance of many of these targets has not been examined directly using genetic approaches. Previous studies have shown that the gene encoding the hormone gastrin is activated during colon cancer progression and the less-processed forms of gastrin are important colonic trophic factors. We show here that the gastrin gene is a downstream target of the beta-catenin/TCF-4 signaling pathway and that cotransfection of a constitutively active beta-catenin expression construct causes a threefold increase in gastrin promoter activity. APC(min-/+) mice overexpressing one of the alternatively processed forms of gastrin, glycine-extended gastrin, show a significant increase in polyp number. Gastrin-deficient APC(min-/+) mice, conversely, showed a marked decrease in polyp number and a significantly decreased polyp proliferation rate. Activation of gastrin by beta-catenin may therefore represent an early event in colorectal tumorigenesis and may contribute significantly toward neoplastic progression. The identification of gastrin as a functionally relevant downstream target of the beta-catenin signaling pathway provides a new target for therapeutic modalities in the treatment of colorectal cancer.

Figure 1

Gastrin is a downstream target of the APC/β-catenin pathway. (a) Schematic representation of a 1,300-bp human gastrin promoter construct (19) containing several potential TCF-4–binding sites. TCF-4–binding sites with high homology to the known TCF/Lef-HMG box transcription factor consensus motifs [CC(A/T)TTG(A/T)(A/T)(T/C); ref. 25, 27] were identified in the gastrin promoter using Lasergene99 DNA-analysis software. Homologous bases are printed as capital letters: –1166 tCTTTGgca-1156, –1000 GATGAAACC-990, –822 aCTTTGTcT-812, –516 CCATTGcTC –506, –103 CCATTccTC-93. (b) Northern blot analysis of RNA taken from HT-29-APC and HT-29-GAL cells that were induced with zinc to express either wild-type APC or β-galactosidase. The blot was probed with either a riboprobe to human gastrin exon 2 or GAPDH. (c) HeLa cells were transfected with a series of gastrin promoter-luciferase reporter deletion constructs (number reflects length of promoter from the transcriptional start site) and then cotransfected with a constitutively active β-catenin expression construct alone (filled bars) or with both the constitutively active β-catenin expression construct with a dominant-negative TCF-4 construct (open bars). The basal promoter construct Pxp2 was used as a control. The results are taken from three independent experiments. (d) Sequences containing either the wild-type TCF-4–binding site from –103 to –90 on the gastrin promoter, a mutated TCF-4–binding site, or a corresponding perfect TCF-4 consensus sequence (Con) are shown. These sequences were used to generate heterologous promoter constructs by cloning them upstream of the thymidine kinase promoter in the luciferase reporter construct pT81. (e) HeLa cells were then transfected with the wild-type TCF-4-pT81 construct, the mutant TCF-4-pT81 construct, the consensus TCF-4-pT81 construct, or an empty pT81 construct and a constitutively active β-catenin expression construct (filled bars) or both the constitutively active β-catenin expression construct and a dominant-negative TCF-4 construct (open bars).

Figure 2

Effect of overexpression of glycine-extended gastrin on APC^min–/+ mice. Total polyp number and the number of large polyps (>3 mm) in the small intestine from twenty 6-month-old APC^min–/+ mice that overexpress glycine-extended gastrin (MTI/G-Gly) (shaded bars) compared with twenty 6-month-old control APC^min–/+ mice (filled bars) in the same genetic background (^AP < 0.05).

Figure 3

Effect of gastrin deficiency on APC^min–/+ mice. (a) Total polyp number in the small intestine and hematocrit from fifteen 6-month-old gastrin-deficient APC^min–/+ mice compared (shaded bars) with fifteen 6-month-old control APC^min–/+ mice (wild-type for the gastrin allele in the same genetic background; filled bars) (^AP < 0.05). (b) The number of small intestinal polyps from the gastrin-deficient (shaded bars) and control APC^min–/+ mice (filled bars) as stratified by size (^AP < 0.05).

Figure 4

Effect of gastrin deficiency on polyp proliferation rates. (a) BrdU staining in 6-month old gastrin-deficient APC^min–/+ mice and (b) control APC^min–/+ mice. (c) Labeling indices were calculated by counting the number of immunopositive-staining cells as a percentage of the total number of cells in each polyp (^AP < 0.05) from both wild-type (filled bars) and gastrin-deficient mice (shaded bars).

Figure 5

Effect of gastrin deficiency on life expectancy. Survival curves from 15 APC^min–/+ gastrin-deficient mice and 15 control APC^min–/+ mice (^AP < 0.05).

Figure 6

A role for gastrin in the molecular pathogenesis of colorectal cancer.