Pterostilbene Enhances TRAIL-Induced Apoptosis through the Induction of Death Receptors and Downregulation of Cell Survival Proteins in TRAIL-Resistance Triple Negative Breast Cancer Cells

Tumor necrosis factor-related apoptosis-induced ligand (TRAIL) is nontoxic to normal cells and preferentially cytotoxic to cancer cells. Recent data suggest that malignant breast cancer cells often become resistant to TRAIL. Pterostilbene (PTER), a naturally occurring analogue of resveratrol found in blueberries, is known to induce cancer cells to undergo apoptosis. In the present study, we examined whether PTER affects TRAIL-induced apoptosis and its mechanism in TRAIL-resistant triple negative breast cancer (TNBC) cells. Our data indicated that PTER induced apoptosis (14.68 +/- 3.78% for 40 mu M PTER vs 1.98 +/- 0.25% for control, p < 0.01) in TNBC cells and enhanced TRAIL-induced apoptosis in TRAIL-resistant TNBC cells (18.45 +/- 4.65% for 40 mu M PTER vs 29.38 +/- 6.35% for combination of 40 mu M PTER and 100 ng/mL TRAIL, p < 0.01). We demonstrated that PTER induced death receptors DRS and DR4 as well as decreased decoy receptor DcR-1 and DcR-2 expression. PTER also decreased the antiapoptotic proteins c-FLIPS/L, Bc1-Xl, Bcl-2, survivin, and XIAP. PTER induced the cleavage of bid protein and caused proapoptotic Bax accumulation. Moreover, we found that PTER induced the expression of DR4 and DRS through the reactive oxygen species (ROS)/endoplasmic reticulum (ER) stress/ERK 1/2 and p38/C/EBP-homologous protein (CHOP) signaling pathways. Overall, our results showed that PTER potentiated TRAIL-induced apoptosis via ROS-mediated CHOP activation leading to the expression of DR4 and DRS.