Silibinin sensitizes TRAIL-mediated apoptosis by upregulating DR5 through ROS-induced endoplasmic reticulum stress-Ca 2+ -CaMKII-Sp1 pathway

// Matharage Gayani Dilshara 1 , Rajapaksha Gedara Prasad Tharanga Jayasooriya 1 , Ilandarage Menu Neelaka Molagoda 1 , Jin-Woo Jeong 2 , Seungheon Lee 1 , Sang Rul Park 1 , Gi-Young Kim 1 and Yung Hyun Choi 2 1 Department of Marine Life Sciences, Jeju National University, Jeju 63243, Republic of Korea 2 Department of Biochemistry, College of Oriental Medicine, Dong-Eui University, Busan 47227, Republic of Korea Correspondence to: Gi-Young Kim, email: immunkim@jejunu.ac.kr Yung Hyun Choi, email: choiyh@deu.ac.kr Keywords: silibinin; TRAIL; DR5; ROS; ER stress Received: April 24, 2016      Accepted: November 14, 2017      Published: December 07, 2017 ABSTRACT In this study, we addressed how silibinin enhances tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis in various cancer cells. Combined treatment with silibinin and TRAIL (silibinin/TRAIL) induced apoptosis accompanied by the activation of caspase-3, caspase-8, caspase-9, and Bax, and cytosolic accumulation of cytochrome c. Anti-apoptotic proteins such as Bcl-2, IAP-1, and IAP-2 were inhibited as well. Silibinin also triggered TRAIL-induced apoptosis in A549 cells through upregulation of death receptor 5 (DR5). Pretreatment with DR5/Fc chimeric protein and DR5-targeted small interfering RNA (siRNA) significantly blocked silibinin/TRAIL-mediated apoptosis in A549 cells. Furthermore, silibinin increased the production of reactive oxygen species (ROS), which led to the induction of TRAIL-mediated apoptosis through DR5 upregulation. Antioxidants such as N -acetyl-L-cysteine and glutathione reversed the apoptosis-inducing effects of TRAIL. Silibinin further induced endoplasmic reticulum (ER) stress as was indicated by the increase in ER marker proteins such as PERK, eIF2α, and ATF-4, which stimulate the expression of CCAAT/enhancer binding protein homologous protein (CHOP). CHOP-targeted siRNA eliminated the induction of DR5 and resulted in a significant decrease in silibinin/TRAIL-mediated apoptosis. We also found that silibinin/TRAIL-induced apoptosis was accompanied with intracellular influx of Ca 2+ , which was stimulated by ER stress and the Ca 2+ chelator, ethylene glycol tetraacetic acid (EGTA). Ca 2+ /calmodulin-dependent protein kinase (CaMKII) inhibitor, K252a, blocked silibinin/TRAIL-induced DR5 expression along with TRAIL-mediated apoptosis. Accordingly, we showed that ROS/ER stress-induced CaMKII activated Sp1, which is an important transcription factor for DR5 expression. Our results showed that silibinin enhanced TRAIL-induced apoptosis by upregulating DR5 expression through the ROS-ER stress-CaMKII-Sp1 axis.