Discovery of Embelin as a Cell-Permeable, Small-Molecular Weight Inhibitor of XIAP through Structure-Based Computational Screening of a Traditional Herbal Medicine Three-Dimensional Structure Database

Abstract

The X-linked inhibitor of apoptosis (XIAP) is a promising new molecular target for the design of novel anticancer drugs aiming at overcoming apoptosis-resistance of cancer cells to chemotherapeutic agents and radiation therapy. Recent studies demonstrated that the BIR3 domain of XIAP where caspase-9 and Smac proteins bind is an attractive site for designing small-molecule inhibitors of XIAP. Through computational structure-based screening of an in-house traditional herbal medicine three-dimensional structure database of 8221 individual natural products, followed by biochemical testing of selected candidate compounds, we discovered embelin from the Japanese Ardisia herb as a small-molecular weight inhibitor that binds to the XIAP BIR3 domain. We showed that embelin binds to the XIAP BIR3 protein with an affinity similar to that of the natural Smac peptide using a fluorescence polarization-based binding assay. Our NMR analysis further conclusively confirmed that embelin interacts with several crucial residues in the XIAP BIR3 domain with which Smac and caspsase-9 bind. Embelin inhibits cell growth, induces apoptosis, and activates caspase-9 in prostate cancer cells with high levels of XIAP, but has a minimal effect on normal prostate epithelial and fibroblast cells with low levels of XIAP. In stably XIAP-transfected Jurkat cells, embelin effectively overcomes the protective effect of XIAP to apoptosis and enhances the etoposide-induced apoptosis and has a minimal effect in Jurkat cells transfected with vector control. Taken together, our results showed that embelin is a fairly potent, nonpeptidic, cell-permeable, small-molecule inhibitor of XIAP and represents a promising lead compound for designing an entirely new class of anticancer agents that target the BIR3 domain of XIAP.

An anthraquinone derivative, emodin sensitizes hepatocellular carcinoma cells to TRAIL induced apoptosis through the induction of death receptors and downregulation of cell survival proteins

“…Additionally, we found dose-dependent down-regulation in XIAP expression upon emodin treatment and the knockdown of XIAP by siRNA abrogated the sensitization effect of emodin on TRAILinduced apoptosis in HCC cells. Similarly embelin, a natural XIAP inhibitor [48] has been found to enhance TRAILinduced apoptosis in pancreatic cancer cells [49].…”

Elevated cell proliferation and VEGF production by high-glucose conditions in Müller cells involve XIAP

“…Embelin, a major constituent of embelia ribes, is a cellpermeable, small molecular inhibitor of XIAP. 17 Embelin has been shown to have anti-tumor, anti-inflammatory, and analgesic properties. 30 Our study showed that embelin counteracted the glucose-related stimulatory effect on the proliferation and production of VEGF in Mü ller cells, supporting the use of embelin in the treatment of diabetic retinopathy.…”

“…Embelin is a small molecule that specifically inhibits XIAP. 17 Mü ller cells were incubated with various concentrations of embelin in the presence of high-glucose conditions (50 mM), and the viable cells were determined by the MTT assay. The results showed that embelin reversed the pro-proliferative effect of highglucose conditions in a dose-dependent manner (Figure 1c).…”

A small molecule inhibitor of XIAP induces apoptosis and synergises with vinorelbine and cisplatin in NSCLC

“…However, it is not clear whether blocking BIR2 to mediate release of effector caspases, or selectively targeting the BIR3 domain, which binds to inhibit the upstream initiator protease caspase-9, is the more effective strategy (Srinivasula et al, 2001a;Nikolovska-Coleska et al, 2004;Sun et al, 2006). It has been shown in H460 cells that disruption of XIAP -caspase-9 binding with SMAC mimetics restored apoptosis (Yang et al, 2003).…”