Progressive Changes in Adherens Junction Structure during Intestinal Adenoma Formation in Apc Mutant Mice

Abstract

The C57BL/6J-Min/+ (Min/+) mouse bears a mutant Apc gene and therefore is an important in vivo model of intestinal tumorigenesis. Min/+ mice develop adenomas that exhibit loss of the wild-type Apc allele (ApcMin/-). Previously, we found that histologically normal enterocytes bearing a truncated Apc protein (ApcMin/+) migrated more slowly in vivo than enterocytes with either wild-type Apc (Apc+/+) or with heterozygous loss of Apc protein (Apc1638N). To study this phenotype further, we determined the effect of the ApcMin mutation upon cell. cell adhesion by examining the components of the adherens junction (AJ). We observed a reduced association between E-cadherin and β-catenin in ApcMin/+ enterocytes. Subcellular fractionation of proteins from Apc+/+, ApcMin/+, and ApcMin/- intestinal tissues revealed a cytoplasmic localization of intact E-cadherin only in ApcMin/+, suggesting E-cadherin internalization in these enterocytes. β-Catenin tyrosine phosphorylation was also increased in ApcMin/+ enterocytes, consistent with its dissociation from E-cadherin. Furthermore, Apc Min/+ enterocytes showed a decreased association between β-catenin and receptor protein-tyrosine phosphatase β/ζ (RPTPβ/ζ), and ApcMin/- cells demonstrated an association between β-catenin and receptor protein-tyrosine phosphatase γ. In contrast to the ApcMin/+ enterocytes, ApcMin/- adenomas displayed increased expression and association of E-cadherin, β-catenin, and α-catenin relative to Apc+/+ controls. These data show that Apc plays a role in regulating adherens junction structure and function in the intestine. In addition, discovery of these effects in initiated but histologically normal tissue (ApcMin/+) defines a pre-adenoma stage of tumorigenesis in the intestinal mucosa.