Disruption of the RP-MDM2-p53 pathway accelerates APC loss-induced colorectal tumorigenesis

Inactivation of the adenomatous polyposis coli (APC) tumor suppressor is frequently found in colorectal cancer. Loss of APC function results in deregulation of the Wnt/β-catenin signaling pathway causing overexpression of the c-MYC oncogene. In lymphoma, both p19ARF and ribosomal proteins RPL11 and RPL5 respond to c-MYC activation to induce p53. Their role in c-MYC-driven colorectal carcinogenesis is unclear, as p19ARF deletion does not accelerate APC loss-triggered intestinal tumorigenesis. To determine the contribution of the RP-MDM2-p53 pathway to APC loss-induced tumorigenesis, we crossed mice bearing MDM2(C305F) mutation, which disrupts RPL11- and RPL5-MDM2 binding, with Apc(min/+) mice, which are prone to intestinal tumor formation. Interestingly, loss of RP-MDM2 binding significantly accelerated colorectal tumor formation while having no discernable effect on small intestinal tumor formation. Mechanistically, APC loss leads to overexpression of c-MYC, RPL11 and RPL5 in mouse colonic tumor cells irrespective of MDM2(C305F) mutation. However, notable p53 stabilization and activation were observed only in Apc(min/+);Mdm2(+/+) but not Apc(min/+);Mdm2(C305F/C305F) colon tumors. These data establish that the RP-MDM2-p53 pathway, in contrast to the p19ARF-MDM2-p53 pathway, is a critical mediator of colorectal tumorigenesis following APC loss.