Abstract
The death ligand TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) represents a promising strategy for melanoma due to significant expression of TRAIL receptor 1 in melanoma metastases and high TRAIL sensitivity through this receptor. However, prevalent and inducible resistance are limiting its clinical use. In previous work, we and others have described multiple strategies leading to TRAIL sensitization; however, the common principles of these strategies remained elusive. Here, we demonstrate in melanoma cell lines (TRAIL-sensitive, TRAIL-resistant, and TRAIL-selected cells with acquired resistance) that cell cycle arrest clearly correlates with enhanced TRAIL sensitivity. Cell cycle arrest was induced by high cell confluence, serum starvation, or cyclin-dependent kinase (CDK) 4/6 inhibition. Addressing the signaling pathways revealed disruption of mitochondrial membrane potential and production of reactive oxygen species (ROS) in response to antiproliferative conditions alone. Activation of the proapoptotic Bcl-2 protein Bax and inhibition of apoptosis by Bcl-2 overexpression or by the antioxidant N-acetyl cysteine underlined the critical involvement of mitochondrial apoptosis pathways and of ROS, respectively. Most pronounced was the upregulation of small proapoptotic Bcl-2 proteins (Puma and Bcl-xS). These data provide a general understanding on TRAIL sensitization as well as an alternative view on CDK inhibitors and may suggest selective targeting of melanoma cells by cell cycle inhibition and TRAIL.