Mutated K-ras(Asp12) promotes tumourigenesis in Apc(Min) mice more in the large than the small intestines, with synergistic effects between K-ras and Wnt pathways

K-ras exon 4A has a tumour suppressor effect on carcinogen-induced murine colonic adenoma formation

K-ras encodes two isoforms, K-ras 4A and 4B, that are jointly affected by K-ras activating mutations, which are prevalent in colorectal cancer (CRC). CRC shows alterations in the expressed K-ras 4A : 4B isoform ratio in favour of K-ras 4B, in tumours both with and without K-ras mutations. The present study evaluated whether K-ras 4A expression can suppress colonic adenoma development in the absence of its oncogenic allele. Mice with homozygous targeted deletions of K-ras exon 4A (K-ras(tmDelta4A/tmDelta4A)) that can express the K-ras 4B isoform only, along with heterozygous K-ras(tmDelta4A/+) and wild-type mice, were given ten weekly 1,2-dimethylhydrazine (DMH) treatments to induce colonic adenomas. There was a significant increase in both the number and the size of colonic adenomas in DMH-treated K-ras(tmDelta4A/tmDelta4A) mice, with reduced survival, compared with heterozygous and wild-type mice. No K-ras mutations were found in any of the 30 tumours tested from the three groups. Lack of expression of K-ras 4A transcripts was confirmed, whereas the relative expression levels of K-ras 4B transcripts were significantly increased in the adenomas of K-ras(tmDelta4A/tmDelta4A) mice compared with K-ras(tmDelta4A/+) and wild-type mice. Immunohistochemical studies showed that adenomas of K-ras(tmDelta4A/tmDelta4A) mice had significantly increased cell proliferation and significantly decreased apoptosis with evidence of activation of MapKinase and Akt pathways, with increased phospho-Erk1/2 and both phospho-Akt-Thr308 and phospho-Akt-Ser473 immunostaining, compared with adenomas from K-ras(tmDelta4A/+) and wild-type mice. In conclusion, following DMH treatment, K-ras exon 4A deletion promoted increased number and size of colonic adenomas showing increased K-ras 4B expression, increased proliferation, decreased apoptosis, and activation of MapKinase and Akt pathways, in the absence of K-ras mutations. Therefore, K-ras 4A expression had a tumour suppressor effect on carcinogen-induced murine colonic adenoma formation, explaining the selective advantage of the altered K-ras 4A : 4B isoform ratio found in human colorectal cancer.

Krüppel-like factor 5 is a crucial mediator of intestinal tumorigenesis in mice harboring combined ApcMin and KRASV12 mutations

Background

Both mutational inactivation of the adenomatous polyposis coli (APC) tumor suppressor gene and activation of the KRAS oncogene are implicated in the pathogenesis of colorectal cancer. Mice harboring a germline Apc^Min mutation or intestine-specific expression of the KRAS^V12 gene have been developed. Both mouse strains develop spontaneous intestinal tumors, including adenoma and carcinoma, though at a different age. The zinc finger transcription factor Krüppel-like factor 5 (KLF5) has previously been shown to promote proliferation of intestinal epithelial cells and modulate intestinal tumorigenesis. Here we investigated the in vivo effect of Klf5 heterozygosity on the propensity of Apc^Min/KRAS^V12 double transgenic mice to develop intestinal tumors.

Results

At 12 weeks of age, Apc^Min/KRAS^V12 mice had three times as many intestinal tumors as Apc^Min mice. This increase in tumor number was reduced by 92% in triple transgenic Apc^Min/KRAS^V12/Klf5^+/- mice. The reduction in tumor number in Apc^Min/KRAS^V12/Klf5^+/- mice was also statistically significant compared to Apc^Min mice alone, with a 75% decrease. Compared with Apc^Min/KRAS^V12, tumors from both Apc^Min/KRAS^V12/Klf5^+/- and Apc^Min mice were smaller. In addition, tumors from Apc^Min mice were more distally distributed in the intestine as contrasted by the more proximal distribution in Apc^Min/KRAS^V12 and Apc^Min/KRAS^V12/Klf5^+/- mice. Klf5 levels in the normal-appearing intestinal mucosa were higher in both Apc^Min and Apc^Min/KRAS^V12 mice but were attenuated in Apc^Min/KRAS^V12/Klf5^+/- mice. The levels of β-catenin, cyclin D1 and Ki-67 were also reduced in the normal-appearing intestinal mucosa of Apc^Min/KRAS^V12/Klf5^+/- mice when compared to Apc^Min/KRAS^V12 mice. Levels of pMek and pErk1/2 were elevated in the normal-appearing mucosa of Apc^Min/KRAS^V12 mice and modestly reduced in Apc^Min/KRAS^V12/Klf5^+/- mice. Tumor tissues displayed higher levels of both Klf5 and β-catenin, irrespective of the mouse genotype from which tumors were derived.

Conclusions

Results of the current study confirm the cumulative effect of Apc loss and oncogenic KRAS activation on intestinal tumorigenesis. The drastic reduction in tumor number and size due to Klf5 heterozygosity in Apc^Min/KRAS^V12 mice indicate a critical function of KLF5 in modulating intestinal tumor initiation and progression.