Partial sensitization of human bladder cancer cells to a gene-therapeutic adenovirus carrying REIC/Dkk-3 by downregulation of BRPK/PINK1.

REIC/Dkk-3 is a tumor suppressor gene that was first identified as a gene downregulated in association with immortalization of normal human fibroblasts. We have demonstrated that an adenovirus carrying REIC/Dkk-3 (Ad-REIC) showed a tumor-specific killing effect on a wide range of cancers. However, some human cancers, bladder cancers in particular, are resistant to Ad-REIC. In this study, we investigated the combination effect of downregulation of BRPK/PINK1 (PINK1) and Ad-REIC on bladder cancer cells. Five bladder cancer cell lines among six cell lines examined were resistant to Ad-REIC. Among the cell lines, the resistance of two cell lines was probably due to low infection efficiency of the adenovirus. PINK1-specific siRNA remarkably downregulated Bcl-xL and TRAP1 proteins and upregulated BAX protein expression. Finally, downregulation of PINK1 partially sensitized the other three cell lines that were resistant to Ad-REIC. This sensitization was associated with increasing production of reactive oxygen species (ROS). These results indicate that PINK1 is one of the key molecules for the mitochondrial protection system and that PINK1 can be a new target molecule to sensitize bladder cancer cells that are resistant to Ad-REIC.