XIAP inhibitor Embelin potentiates TRAIL-induced apoptosis in human lung cancer cells intrinsically resistant to TRAIL therapy alone

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BACKGROUND: Recombinant soluble TRAIL have been found to induce apoptosis in select cancers but not in others. Down-stream prevention of pro-apoptotic signaling may be mediated by robust expression of the inhibitors of apoptosis (IAPs). Embelin is a recently discovered small molecule which exhibits XIAP inhibitor properties as shown by others. We hypothesize that co-administration of embelin (therefore inhibiting XIAP function) and soluble TRAIL will result in enhanced apoptosis in previously TRAIL-resistant cancer cells.
 OBJECTIVE: Our goal is to determine whether co-treatment with embelin (an XIAP inhibitor) can render a TRAIL-resistant cancer cell susceptible to the pro-apoptotic effects of TRAIL.
 METHODS: Human lung cancer cells, consisting of A549 (TRAIL-resistant) and H460 (TRAIL-sensitive), were cultured under standard conditions. Embelin and recombinant human soluble TRAIL were procured commercially. Each lung cancer cell line was treated with either TRAIL-alone; embelin-alone; or concurrent administration of embelin and TRAIL. IAP and caspase alterations resulting from treatment were measured semi-quantitatively by Western immunoblot and densitometric analyses. Cancer cell apoptosis was identified using terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling assay and quantified by flow cytometry. The effects of TRAIL and embelin co-administration were also analyzed using computer software for dose-effect analysis.
 RESULTS: A549 lung cancer cells were resistant to TRAIL administration despite escalating TRAIL doses. Embelin-alone was able to induce significant G1 cell-cycle arrest and apoptosis in a dose-dependent relationship. However, the co-administration of low-dose embelin with TRAIL resulted in caspase-mediated apoptosis at rates higher than those compared to embelin-alone.
 CONCLUSIONS: The anti-apoptotic effects of XIAP may be mitigated by embelin treatment. Concurrent treatment with embelin appears to effectively inhibit XIAP function and converts a TRAIL-resistant cancer cell into a TRAIL-susceptible one. Moreover, the respective effects of embelin and TRAIL appear to work cooperatively in the induction of caspase-mediated apoptosis.