Inactivation of p21WAF1/cip1 enhances intestinal tumor formation in Muc2-/- mice

In the Apc1638(+/-) mouse model of intestinal tumorigenesis, targeted inactivation of the cyclin-dependent kinase inhibitor p21(WAF1/cip1) is highly effective in enhancing Apc-initiated tumor formation in the intestine. Because p21(WAF1/cip1) plays a critical role in regulating intestinal cell proliferation, maturation, and tumorigenesis, we examined whether its inactivation would enhance tumor formation in a different mouse model of colon cancer. Therefore, we mated p21(-/-) mice with mice carrying a genetic deficiency of the Muc2 gene, which encodes the major gastrointestinal mucin. Muc2(-/-) mice develop tumors in the small and large intestine and the rectum, but in contrast to tumors in Apc1638(+/-) mice, this does not involve increased expression or nuclear localization of beta-catenin. We found that inactivation of p21(WAF1/cip1) significantly increased the frequency and size of intestinal tumors in Muc2 knockout mice and also led to development of more invasive adenocarcinomas. This enhanced tumorigenesis significantly decreased mouse life span. Further, inactivation of p21(WAF1/cip1) increased cell proliferation, decreased apoptosis, and decreased intestinal trefoil factor expression in the mucosa of both the small and large intestine. Surprisingly, reduced expression of p27(kip1) was also observed in the Muc2(-/-), p21(+/-), and p21(-/-) mice. In contrast, the expression of c-myc was significantly elevated. Thus, p21 modulates the formation of tumors whose initiation does (Apc) or does not (Muc2) involve altered beta-catenin-Tcf4 signaling, but which may converge on common elements downstream of this signaling pathway.

Targeted inactivation of p27kip1 is sufficient for large and small intestinal tumorigenesis in the mouse, which can be augmented by a Western-style high-risk diet

Mice with a targeted inactivation of both alleles of the cyclin-dependent kinase inhibitor p27(kip1) developed both small and large intestinal adenomas when fed a control AIN-76A diet. A Western-style diet that is high in fat and phosphate and low in calcium and vitamin D was also able to initiate adenoma formation in wild-type mice. The combination of p27(kip1) inactivation and the Western-style diet was additive in terms of tumor incidence, frequency and size, and in reducing the life span of the mice. The genetic and dietary combination also resulted in development of adenocarcinoma. Tumor formation was linked to a disruption in homeostasis of the intestinal mucosa, involving increased cell proliferation and decreased apoptosis. There was also decreased goblet cell differentiation as assessed by alcian blue staining and expression of the Muc2 gene, especially in mice fed the Western-style diet, although this differentiation lineage was still present as indicated by expression and staining for intestinal trefoil factor. The inactivation of p27(kip1) and the consequent disruption of normal colonic cell maturation in the mucosa were associated with modestly elevated c-myc, cdk4, and cyclin D1 expression. These data establish a fundamental role for p27(kip1) in maintenance of intestinal cell homeostasis and in suppressing tumor formation. The data also emphasize the critical role that dietary factors can have in both tumor initiation and progression through interaction with pathways that normally maintain intestinal homeostasis.