Mutated K-rasAsp12 promotes tumourigenesis in ApcMin mice more in the large than the small intestines, with synergistic effects between K-ras and Wnt pathways

P>K-ras mutations are found in 40-50% of human colorectal adenomas and carcinomas, but their functional contribution remains incompletely understood. Here, we show that a conditional mutant K-ras mouse model (K-rasAsp12/Cre), with transient intestinal Cre activation by beta-Naphthoflavone (beta-NF) treatment, displayed transgene recombination and K-rasAsp12 expression in the murine intestines, but developed few intestinal adenomas over 2 years. However, when crossed with ApcMin/+ mice, the K-rasAsp12/Cre/ApcMin/+ offspring showed acceleration of intestinal tumourigenesis with significantly changed average lifespan (P < 0.05) decreased to 18.4 +/- 5.4 weeks from 20.9 +/- 4.7 weeks (control ApcMin/+ mice). The numbers of adenomas in the small intestine and large intestine were significantly (P < 0.01) increased by 1.5-fold and 5.7-fold, respectively, in K-rasAsp12/Cre/ApcMin/+ mice compared with ApcMin/+ mice, with the more marked increase in adenoma prevalence in the large intestine. To explore possible mechanisms for K-rasAsp12 and ApcMin co-operation, the Mitogen-activated protein kinase (Mapk), Akt and Wnt signalling pathways, including selected target gene expression levels, were evaluated in normal large intestine and large intestinal tumours. K-rasAsp12 increased activation of Mapk and Akt signalling pathway targets phospho-extracellular signal-regulated kinase (pErk) and pAkt, and increased relative expression levels of Wnt pathway targets vascular endothelial growth factor (VEGF), gastrin, cyclo-oxygenase 2 (Cox2) and T-cell lymphoma invasion and metastasis 1 (Tiam1) in K-rasAsp12/Cre/ApcMin/+ adenomas compared with that of ApcMin/+ adenomas, although other Wnt signalling pathway target genes such as Peroxisome proliferator-activated receptor delta (PPARd), matrix metalloproteinase 7 (MMP7), protein phosphatase 1 alpha (PP1A) and c-myc remained unchanged. In conclusion, intestinal expression of K-rasAsp12 promotes mutant Apc-initiated intestinal adenoma formation in vivo more in the large intestine than the small intestine, with evidence of synergistic co-operation between mutant K-ras and Apc involving increased expression of some Wnt-pathway target genes.