No abstract

Oncogenic mutations in the K-ras gene occur in Ϸ50% of human colorectal cancers. However, the precise role that K-ras oncogenes play in tumor formation is still unclear. To address this issue, we have conditionally expressed an oncogenic K-ras V12 allele in the small intestine of adult mice either alone or in the context of Apc deficiency. We found that expression of K-ras V12 does not affect normal intestinal homeostasis or the immediate phenotypes associated with Apc deficiency. Mechanistically we failed to find activation of the Raf͞MEK͞ERK pathway, which may be a consequence of the up-regulation of a number of negative feedback loops. However, K-ras V12 expression accelerates intestinal tumorigenesis and confers invasive properties after Apc loss over the long term. In renal epithelium, expression of the oncogenic Kras V12 allele in the absence of Apc induces the rapid development of renal carcinoma. These tumors, unlike those of intestinal origin, display activation of the Raf͞MEK͞ERK and Akt signaling pathways. Taken together, these data indicate that normal intestinal and kidney epithelium are resistant to malignant transformation by an endogenous K-ras oncogene. However, activation of K-ras V12 after Apc loss results in increased tumorigenesis with distinct kinetics. Whereas the effect of K-ras oncogenes in the intestine can been observed only after long latencies, they result in rapid carcinogenesis in the kidney epithelium. These data imply a window of opportunity for anti-K-ras therapies after tumor initiation in preventing tumor growth and invasion.colorectal cancer ͉ renal carcinoma ͉ Wnt signaling

Mutations in the adenomatous polyposis coli (APC) tumor suppressor gene initiate a majority of colorectal cancers. Acquisition of chromosomal instability is an early event in these tumors. We provide evidence that the loss of APC leads to a partial loss of interkinetochore tension at metaphase and alters mitotic progression. Furthermore, we show that inhibition of APC in U2OS cells compromises the mitotic spindle checkpoint. This is accompanied by a decrease in the association of the checkpoint proteins Bub1 and BubR1 with kinetochores. Additionally, APC depletion reduced apoptosis. As expected from this combination of defects, tetraploidy and polyploidy are consequences of APC inhibition in vitro and in vivo. The removal of APC produced the same defects in HCT116 cells that have constitutively active β-catenin. These data show that the loss of APC immediately induces chromosomal instability as a result of a combination of mitotic and apoptotic defects. We suggest that these defects amplify each other to increase the incidence of tetra- and polyploidy in early stages of tumorigenesis.

A subset of patients with gastric cancer have mutations in genes that participate in or regulate Wnt signaling at the level of ligand (Wnt) receptor (Fzd) binding. Moreover, increased Fzd expression is associated with poor clinical outcome. Despite these findings, there are no in vivo studies investigating the potential of targeting Wnt receptors for treating gastric cancer, and the specific Wnt receptor transmitting oncogenic Wnt signaling in gastric cancer is unknown. Here, we use inhibitors of Wnt/Fzd (OMP-18R5/vantictumab) and conditional gene deletion to test the therapeutic potential of targeting Wnt signaling in preclinical models of intestinal-type gastric cancer and ex vivo organoid cultures. Pharmacologic targeting of Fzd inhibited the growth of gastric adenomas in vivo. We identified Fzd7 to be the predominant Wnt receptor responsible for transmitting Wnt signaling in human gastric cancer cells and mouse models of gastric cancer, whereby Fzd7-deficient cells were retained in gastric adenomas but were unable to respond to Wnt signals and consequently failed to proliferate. Genetic deletion of Fzd7 or treatment with vantictumab was sufficient to inhibit the growth of gastric adenomas with or without mutations to Apc. Vantictumab is currently in phase Ib clinical trials for advanced pancreatic, lung, and breast cancer. Our data extend the scope of patients that may benefit from this therapeutic approach as we demonstrate that this drug will be effective in treating patients with gastric cancer regardless of APC mutation status. Significance: The Wnt receptor Fzd7 plays an essential role in gastric tumorigenesis irrespective of Apc mutation status, therefore targeting Wnt/Fzd7 may be of therapeutic benefit to patients with gastric cancer.

Contributions of null and hypomorphic alleles of Apc in mice produce both developmental and pathophysiological phenotypes. To ascribe the resulting genotype-to-phenotype relationship unambiguously to the Wnt/β-catenin pathway, we challenged the allele combinations by genetically restricting intracellular β-catenin expression in the corresponding compound mutant mice. Subsequent evaluation of the extent of resulting Tcf4-reporter activity in mouse embryo fibroblasts enabled genetic measurement of Wnt/β-catenin signaling in the form of an allelic series of mouse mutants. Different permissive Wnt signaling thresholds appear to be required for the embryonic development of head structures, adult intestinal polyposis, hepatocellular carcinomas, liver zonation, and the development of natural killer cells. Furthermore, we identify a homozygous Apc allele combination with Wnt/β-catenin signaling capacity similar to that in the germline of the Apcmin mice, where somatic Apc loss-of-heterozygosity triggers intestinal polyposis, to distinguish whether co-morbidities in Apcmin mice arise independently of intestinal tumorigenesis. Together, the present genotype–phenotype analysis suggests tissue-specific response levels for the Wnt/β-catenin pathway that regulate both physiological and pathophysiological conditions.