Importin β1 Protein-mediated Nuclear Localization of Death Receptor 5 (DR5) Limits DR5/Tumor Necrosis Factor (TNF)-related Apoptosis-inducing Ligand (TRAIL)-induced Cell Death of Human Tumor Cells

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)/death receptor 5 (DR5)-mediated cell death plays an important role in the elimination of tumor cells and transformed cells. Recently, recombinant TRAIL and agonistic anti-DR5 monoclonal antibodies have been developed and applied to cancer therapy. However, depending on the type of cancer, the sensitivity to TRAIL has been reportedly different, and some tumor cells are resistant to TRAIL-mediated apoptosis. Using confocal microscopy, we found that large amounts of DR5 were localized in the nucleus in HeLa and HepG2 cells. Moreover, these tumor cells were resistant to TRAIL, whereas DU145 cells, which do not have nuclear DR5, were highly sensitive to TRAIL. By means of immunoprecipitation and Western blot analysis, we found that DR5 and importin beta 1 were physically associated, suggesting that the nuclear DR5 was transported through the nuclear import pathway mediated by importin beta 1. Two functional nuclear localization signals were identified in DR5, the mutation of which abrogated the nuclear localization of DR5 in HeLa cells. Moreover, the nuclear transport of DR5 was also prevented by the knockdown of importin beta 1 using siRNA, resulting in the up-regulation of DR5 expression on the cell surface and an increased sensitivity of HeLa and HepG2 cells to TRAIL. Taken together, our findings suggest that the importin beta 1-mediated nuclear localization of DR5 limits the DR5/TRAIL-induced cell death of human tumor cells and thus can be a novel target to improve cancer therapy with recombinant TRAIL and anti-DR5 antibodies.