AJAP1 Interactions with the β‐catenin‐E‐cadherin Complex in Epithelial Cells for Cardiovascular Development

Abstract

Coronary vessel anomalies affect up to 1% of the population. These blood vessels constantly supply the myocardium with oxygen and nutrients. The proepicardium (PE) is a source of progenitor cells for the epicardium and coronary vessels. It is a transitory cluster of epithelial‐like mesothelial cells that lies dorsal to the primitive heart tube. During cardiovascular development, epithelial cells of the PE migrate toward the heart tube and adhere to the myocardium to form the epicardium. Epicardial cells proliferate, undergo apoptosis and epithelial‐mesenchymal transition (EMT) to become epicardial‐derived cells (EPDCs). EPDCs delaminate and invade the myocardium to form a subset of coronary vascular cells. Conditional deletion of the Tbx5 transcription factor in the PE impairs epicardium and coronary vessel formation during mouse heart development and reduces cardiac expression of adherens junction‐associated protein 1 (AJAP1). AJAP1 is a cell adhesion molecule that regulates epithelial cell behavior in cancer cells by influencing migration, adhesion and epithelial‐mesenchymal transition (EMT). We observed AJAP1 expression in the PE, epicardium and coronary vessels of mouse embryos. However, its role in cardiovascular development is unknown. Study Objective Our goal was to determine how AJAP elicits changes in epithelial cell behaviors integral to cardiovascular development. Previous investigators indicated that AJAP1 repression in hepatocellular cancer cells increased β‐catenin nuclear translocation. This led to increased metastasis and tumor growth via β‐catenin‐induced transcriptional activation. We hypothesize that AJAP1 interacts with the β‐catenin‐E‐cadherin complex in normal epithelial cells to influence gene expression and regulate cell behaviors that are necessary for epicardium and coronary vessel formation. Methods We used primary human mammary epithelial cells (HMEpiCs) to examine in vitro contributions of AJAP1 to epithelial cell behaviors that are necessary for epicardium and coronary vessel formation. Small interfering RNAs (siRNA) silenced endogenous AJAP1 mRNA and reduced protein expression. We analyzed siRNA‐transfected HMEpiCs for associations between cell migration and cell expression and localization of β‐catenin‐E‐cadherin complexes. Also, we examined these cells for changes in gene expression that contribute to epicardium and coronary vessel formation. Results siRNA‐mediated silencing of AJAP1 led to a >85% decrease in AJAP1 mRNA expression in HMEpiCs. Epithelial cell migration was altered in AJAP1‐silenced HMEpiCs compared to control cells. β‐catenin expression was observed in AJAP1‐silenced cells. Also, we noted changes in gene expression that account for these observable differences. Conclusion Our data indicates that AJAP1 contributes to epithelial cell functions by altering cell migration. This behavior is potentially mediated through interactions with the β‐catenin‐E‐cadherin complex. These findings will enable us to understand the developmental role of AJAP1 during formation of the e...