DECIPHERING THE SIGNALING MECHANISM OF ENHANCED TRAIL SENSITIZATION BY LY303511

Abstract

Resistance to apoptosis induced by the proapoptotic ligand, TRAIL has dimmed its potential as a possible anticancer modality in a clinical setting. Thus, understanding mechanisms that contribute to this resistance has been of considerable interest. This has also led to the discovery of compounds that can overcome resistance or enhance the efficacy of TRAIL in tumor cells. This project has been an effort to understand the signaling mechanism of enhanced TRAIL sensitization by one such compound, LY303511 in a neuroblastoma cell line, SHEP-1. Early experimental evidence indicated the role of intracellular H2O2 production in activating MAPKs in SHEP-1 cells. This activated MAPK¿s was seen to be an important signal for upregulating expression of TRAIL receptors (DR4 and DR5). The resulting increase in availability of receptors for ligand (TRAIL) binding at the cell surface allows significant amplification of TRAIL-mediated caspase 8 processing, activity and apoptotic cell death. The dominance of the extrinsic pathway of apoptosis was indicated by the ability of blocking antibodies against DR4 and/or DR5 to completely inhibit LY30- induced TRAIL sensitization. Pharmacological inhibition and siRNA- mediated gene silencing of c-jun N-terminal-kinase (JNK) and extracellular signal-regulated kinase (ERK) inhibited LY30-induced increase in surface expression of DR4 and DR5, respectively. In addition, the role played by the phosphoprotein, PEA-15 in regulating intracellular localization of activated ERK could be important in directing the signaling cascade and biological outcome. Elucidating the possible mechanism of action of LY30 holds implications for the use of LY30 and other similar compounds for enhancing the apoptotic sensitivity of neuroblastoma cells that often become resistant to TRAIL and refractory to chemotherapy.