Differential molecular mechanisms of bortezomib sensitization to rhTRAIL in non-small cell lung cancer cell lines.

Aim: TNF-related apoptosis-inducing ligand (TRAIL) is a promising targeted anti-cancer agent for several types of cancer, including non-small cell lung cancer (NSCLC). The proteasome inhibitor bortezomib can further potentiate rhTRAIL-induced apoptosis in NSCLC cells. Here, the mechanisms underlying this sensitization were examined in TRAIL-sensitive H460 and TRAIL-resistant A549 and SW1573 NSCLC cells. Methods: NSCLC cell lines were treated with rhTRAIL and bortezomib, and apoptosis was assessed through caspase activation assays, western blotting, and gene silencing of key apoptotic regulators, including Bid, XIAP, and cFLIP. Clonogenic assays were performed to evaluate long-term tumor growth suppression. Results: Bortezomib sensitization mechanisms varied across NSCLC cell lines. Combined rhTRAIL/bortezomib treatment enhanced apoptosis across all cell lines. In TRAIL-sensitive H460 cells, rapid caspase activation was observed, with both extrinsic and intrinsic apoptotic pathways contributing to cell death. Sensitization in H460 cells was predominantly mediated via the caspase-8/Bid amplification loop. In A549 cells, the bortezomib sensitizing effect also relied on the caspase-8/Bid amplification loop. Additionally, the inhibition of Bid and XIAP emphasized the critical role of mitochondrial pathways in apoptosis. In SW1573 cells, limited caspase cleavage was detected, with distinct cleavage patterns suggesting cell-specific apoptotic mechanisms. In this cell line, bortezomib primarily enhanced the extrinsic apoptotic pathway, with XIAP depression further increasing apoptosis. Silencing cFLIP, a caspase-8 inhibitor, significantly improved rhTRAIL sensitivity, emphasizing the critical role of caspase-8 activation in overcoming resistance in SW1573. The clonogenic assay demonstrated that bortezomib combined with rhTRAIL significantly suppressed tumor growth, especially in resistant cell lines. Conclusions: This study underscores bortezomib's ability to differentially enhance rhTRAIL-induced apoptosis by targeting multiple apoptotic regulators. The variety of effects that bortezomib can exert to enhance rhTRAIL-induced apoptosis makes it a very powerful combination for the treatment of NSCLC and various other types of cancer cells.