Interaction between CD133 and PI3K-p85 promotes chemoresistance in gastric cancer cells

Chemoresistance in gastric cancer is the leading cause of tumor recurrence and poses a substantial therapeutic challenge. The stem cell biomarker CD133 has been implicated in drug resistance of tumor-initiating cells in a number of cancers including gastric cancer. Therefore, we investigated the molecular mechanism of CD133-associated multidrug resistance in gastric cancer cells. Using CD133 overexpressing and knockdown gastric cancer cell lines, we demonstrated that loss of CD133 significantly increased the growth inhibition of chemotherapeutic agents; whereas, overexpression significantly reduced growth inhibition. Furthermore, CD133 knockdown significantly reduced the enzymatic activity of phosphatidylinositol-3 kinase (PI3K) and the expression of P-glycoprotein (P-gp), B-cell lymphoma 2 (BCL2), and phosphorylated-protein kinase B (p-AKT), but elevated the expression of BCL2 associated X (BAX). Conversely, overexpression of CD133 significantly increased PI3K enzymatic activity, expression of P-gp, BCL2, and p-AKT, and decreased BAX expression. The PI3K/AKT inhibitor LY294002 mirrored the effects of loss of CD133; whereas, the PI3K/AKT activator epidermal growth factor reproduced the effects of CD133 overexpression. To identify the interaction between CD133 and PI3K, we used site-directed mutagenesis to mutate individual tyrosine residues of CD133. We found that binding between CD133 and p85, the regulatory subunit of PI3K, was significantly reduced when tyrosine 852 was mutated. In summary, we have demonstrated that CD133 activates the PI3K/AKT signal transduction pathway through direct interaction with PI3K-p85, resulting in multidrug resistance of gastric cancer cells. These results suggest that the interaction between CD133 and PI3K-p85 may offer a novel therapeutic target in multidrug resistant gastric cancer.