Amiloride augments TRAIL-induced apoptotic death by inhibiting phosphorylation of kinases and phosphatases associated with the PI3K-Akt pathway

We have previously shown that low extracellular pH (pHe) promotes cell killing by the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). In this study, we examined whether amiloride, an inhibitor of the Na+/H+ antiporter capable of lowering the intracellular pH (pHi), can potentiate TRAIL-induced apoptotic death. Human prostate adenocarcinoma DU-145 cells were treated with various concentrations of TRAIL (10-200 ng/ml) and/or amiloride (0.1-1 mM) for 4 h. Amiloride, which caused little or no cytotoxicity by itself, enhanced TRAIL-induced apoptosis. The TRAIL-mediated activation of caspase, and PARP (poly (ADP-ribose) polymerase) cleavage were both promoted by amiloride. Western blot analysis showed that combined treatment with TRAIL and amiloride did not change the levels of TRAIL receptors (death receptor (DR)4, DR5, and DcR2 (decoy recepter 2) or antiapoptotic proteins (FLICE-inhibitory protein ( FLIP), inhibitor of apoptosis (IAP), and Bcl-2). However, unlike pHe, amiloride promoted the dephosphorylation of Akt. Interestingly, amiloride also induced the dephosphorylation of PI3K (phosphatidylinositol 3-kinase) and PDK-1 (phosphoinositide-dependent kinase-1) kinases along with PTEN (phosphatase and tensin homolog deleted on chromosome 10) and PP1alpha phosphatases. In vitro kinase assays revealed that amiloride inhibited phosphorylation of kinases and phosphatases by competing with ATP. Taken together, the present studies suggest that amiloride enhances TRAIL-induced cytotoxicity by inhibiting phosphorylation of the PI3K-Akt pathway-associated kinases and phosphatases.