Background and aims: Self-renewal and differentiation of intestinal epithelium is a tightly regulated process, whose perturbations are implicated in human colorectal tumorigenesis. The insulin/IGF signaling pathway may play important roles in intestinal epithelium homeostasis. Insulin receptor substrate 2 (IRS2) is a poorly characterized component in these pathways.
Methods: Using complementary in vitro and in vivo human and murine models, we investigated expression (mRNA and protein levels), localization (immunohistochemistry) and regulation of IRS2 in normal intestine and colorectal tumors. In silico analysis of human IRS2 promoter was performed together with reporter and ChIP assays.
Results: Significant IRS2 expression was detected in the intestine with specific protein localization in the villus region of the ileum and in the surface epithelium of the colon. In human HT29 and Caco2 cells, IRS2 mRNA levels increased with spontaneous and induced differentiation, together with CDX2, p21 and KLF4. Adenoviral infection with human CDX2 induced IRS2 expression in APC and b-catenin mutated cells. On the other hand, IRS2 down-regulation was observed in differentiated enterocytes after adenoviral infection with shCDX2, in the intestine of CDX2 heterozygous mice and in colorectal tumors of ApcMin/+ and FAP patients. The human IRS2 promoter region presents several CDX2 binding sites, where CDX2 immunoprecipitated in vivo. IRS2 reporters were functionally activated via CDX2 and blocked via a dominant-negative CDX2 protein.
Conclusions: Combining gain- and loss-of-function approaches, we present an intriguing scenario whereby IRS2 is significantly expressed in the apical intestinal compartment and is directly controlled by CDX2 in normal intestine and tumors.