Real-time imaging of the dynamics of death receptors and therapeutics that overcome TRAIL resistance in tumors.

Tumor necrosis factor related apoptosis-inducing ligand (TRAIL) induces apoptosis specifically in tumor cells and its efficacy has been tested in pre-clinical models by delivering it systemically as a purified ligand or via engineered stem cells (SC). However, about 50% of tumor lines are resistant to TRAIL and overcoming TRAIL resistance in aggressive tumors, such as glioblastoma-multiforme (GBM), and understanding the molecular dynamics of TRAIL-based combination therapies are critical to broadly use TRAIL as a therapeutic agent. In this study, we developed death receptor (DR)4/5-reporters that offer an imaging-based platform to identify agents that act in concert with a potent, secretable variant of TRAIL (S-TRAIL) by monitoring changes in DR4/5 expression. Utilizing these reporters, we show a differential regulation of DR4/5 when exposed to a panel of clinically relevant agents. A histone deacetylase inhibitor, MS-275, resulted in upregulation of DR4/5 in all GBM cell lines, and these changes could be followed in real time both in vitro and in vivo in mice bearing tumors and they correlated with increased TRAIL sensitivity. To further assess the dynamics of combinatorial strategies that overcome resistance of tumors to SC released S-TRAIL, we also engineered tumor cells to express live-cell caspase-reporters and SCs to express S-TRAIL. Utilizing DR4/5 and caspase reporters in parallel, we show that MS-275 sensitizes TRAIL-resistant GBM cells to stem cell (SC) delivered S-TRAIL by changing the time-to-death in vitro and in vivo. This study demonstrates the effectiveness of a combination of real-time reporters of TRAIL-induced apoptosis pathway in evaluating the efficacy of SC-TRAIL-based therapeutics and may have implications in targeting a broad range of cancers.