Evidence for two modes of development of acquired TRAIL resistance: Involvement of Bcl-xL Free

5135

Previous studies have shown that repeated application of TRAIL induces acquired resistance to TRAIL, which limits the use of TRAIL clinically. However, it is still unclear how cancer cells have acquired TRAIL resistance. Using human prostate adenocarcinoma DU-145 and human pancreatic carcinoma Mia PaCa-2 cells as a model, we now demonstrate, for the first time, that two states of acquired TRAIL resistance can develope after TRAIL treatment. Data from survival assay and western blot analysis show that acquired TRAIL resistance developed within 1 day and gradually decayed within 6 days after TRAIL treatment in both cell lines. After TRAIL treatment, the level of Bcl-xL increased and reached maximal within 2 days and gradually decreased in both cell lines. Bcl-xL-mediated development of acquired TRAIL resistance was suppressed by knockdown of Bcl-xL expression. Protein interaction assay revealed that during the development of TRAIL resistance, Bcl-xL dissociated from Bad and then associated with Bax. We also observed that TRAIL promoted the Akt signal transduction pathway by activating (phosphorylating) Akt in attached cells (TRAIL-resistant cells) but not in detached cells (TRAIL-sensitive cells). We believe that inactivation of Bad and increase in Bcl-xL, which play an important role in acquired TRAIL resistance, occur by activating Akt and its signal transduction pathway in attached cells. Thus, our results suggest that (a) dissociation of Bad from Bcl-xL and (b) an increase in the intracellular level of Bcl-xL, which are both induced by Akt, are responsible for development of acquired TRAIL resistance.