Introduction: Apolipoprotein A-IV (apoA-IV), an intestinally and cerebrally synthesized satiety factor and anti-atherogenic plasma apolipoprotein, was recently identified as an anti-inflammatory protein. In order to elucidate whether intestinal apoA-IV exerts similar repair function as its hepatic homologue apolipoprotein A-V (apoA-V), apoA-IV-interactive proteins were searched and in vitro functional studies were performed with apoA-IV overexpressing cells. ApoA-IV was also analyzed in the intestinal mucosa of patients with inflammatory bowel diseases (IBD), together with other genes involved in epithelial junctional integrity.
Methods: A yeast-two-hybrid screening was used to identify apoA-IV-interactors. ApoA-IV was overexpressed in Caco-2 and HT-29 mucosal cells for colocalization and in vitro epithelial permeability studies. Mucosal biopsies from quiescent regions of colon transversum and terminal ileum were subjected to DNA-microarray analysis and pathway-related data mining.
Results: Four proteins interacting with apoA-IV were identified, including apolipoprotein B-100, alpha1-antichymotrypsin, cyclin C, and the cytosolic adaptor alpha-catenin, thus linking apoA-IV to adherens junctions. Overexpression of apoA-IV was paralleled with a differentiated phenotype of intestinal epithelial cells, upregulation of junctional proteins, and decreased paracellular permeability. Colocalization between alpha-catenin and apoA-IV occurred exclusively in junctional complexes. ApoA-IV was downregulated in quiescent mucosal tissues from patients suffering from IBD. In parallel, only a distinct set of junctional genes was dysregulated in non-inflamed regions of IBD gut.
Conclusions: ApoA-IV may act as a stabilizer of adherens junctions interacting with alpha-catenin, and is likely involved in the maintenance of junctional integrity. ApoA-IV expression is significantly impaired in IBD mucosa, even in non-inflamed regions.