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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.

Despite recent developments for new targeted therapies in melanoma, as BRAF inhibitors and immune-stimulating antibodies, tumor relapse frequently follows within less than a year. Therapy resistance is explained by defects in proapoptotic signalling. Thus, efficient induction of apoptosis in tumor cells appears as predominant therapeutic goal. In apoptosis control of melanoma, the balance between pro- and antiapoptotic Bcl-2 proteins plays a decisive role. In particular, members of the subfamily of BH3-only proteins function as proapoptotic triggers, and mimetics of these proteins are already in clinical trials in other cancers. Recent experimental work has revealed that the effects of different treatments in melanoma are related to the activation of BH3-only proteins, and also the proapoptotic effects of BRAF inhibitors are prevented by knockdown of the BH3-only protein Bim. Thus, melanoma therapy might be critically improved by the combination of survival pathway antagonists as BRAF inhibitors with BH3 mimetics., (© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Mutated BRAF represents a critical oncogene in melanoma, and selective inhibitors have been approved for melanoma therapy. However, the molecular consequences of RAF inhibition in melanoma cells remained largely elusive. Here, we investigated the effects of the pan-RAF inhibitor L-779,450, which inhibited cell proliferation both in BRAF-mutated and wild-type melanoma cell lines. It furthermore enhanced apoptosis in combination with the death ligand tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and overcame TRAIL resistance in melanoma cells. Enhanced apoptosis coincided with activation of mitochondrial pathways, seen by loss of mitochondrial membrane potential and release of cytochrome c, Smac (second mitochondria-derived activator of caspases), and apoptosis-inducing factor (AIF). Subsequently, caspase-9 and -3 were activated. Apoptosis induction by L-779,450/TRAIL was prevented by Bcl-2 overexpression and was dependent on Bax. Thus, activation of Bax by L-779,450 alone was demonstrated by Bax conformational changes, whereas Bak was not activated. Furthermore, the BH3-only protein Bim was upregulated in response to L-779,450. The significant roles of Smac, Bax, and Bim in this setting were proven by small interfering RNA (siRNA)-mediated knockdown experiments. L-779,450 also resulted in morphological changes indicating autophagy confirmed by the autophagy marker light chain 3-II (LC3-II). The pro-apoptotic effects of L-779,450 may explain the antitumor effects of RAF inhibition and may be considered when evaluating RAF inhibitors for melanoma therapy.

The death ligand TRAIL (TNF-related apoptosis-inducing ligand) represents a promising therapeutic strategy for metastatic melanoma, however prevalent and inducible resistance limits its applicability and therapeutic use. Recent work has revealed that combinations with survival pathway inhibitors could efficiently sensitize melanoma cells for TRAIL. Here, a particular role was attributed to the activation of Bax, which is regulated by phosphorylation. Thus, TRAIL resistance in melanoma is explained by three major steps, namely high levels of antiapoptotic Bcl-2 proteins, high levels of inhibitor of apoptosis proteins (cIAPs) and suppressed Bax activity. Importantly, Bid was activated in response to TRAIL alone also in resistant cells to antagonize Bcl-2, and Bax was activated in response to pathway inhibitors. However, only in combinations, mitochondrial apoptosis pathways were opened to result in release of Smac/DIABLO, which functions as antagonist of cIAPs. Opening the caspase cascade by Smac then allowed efficient induction of apoptosis. Thus, direct or indirect targeting of Bax represents a suitable strategy to overcome TRAIL resistance in melanoma and may allow the establishment of TRAIL-based therapeutic approaches., (Copyright © 2013 Elsevier GmbH. All rights reserved.)

Until today effective therapies are lacking for metastatic melanoma. The death ligand TRAIL appears as promising in cancer treatment; however, melanoma cells reveal both preexisting and inducible TRAIL resistance. Here, we present evidence that the recently described indirubin derivative 8-Rha-β enhances melanoma cell sensitivity for death ligands and overcomes resistance to TRAIL and CD95 agonists. Indirubin is known from traditional Chinese medicine and is a potent kinase inhibitor. Unraveling of apoptotic signaling pathways revealed that TRAIL resulted in a quick (within 8h) downregulation of both agonistic TRAIL receptors DR4 and DR5, in a kind of negative feed-back loop. Treatment with indirubin, however, mediated upregulation of both receptors, thus compensating this negative feed-back loop by TRAIL. Furthermore, indirubin activated intrinsic apoptosis pathways, seen in loss of mitochondrial membrane potential and release of cytochrome c. The mitochondrial response appeared as related to upregulation of Bax and Bad and to downregulation of Mcl-1. Remarkably, indirubin in combination with TRAIL was also able to overcome apoptosis resistance due to ectopic Bcl-2 overexpression. The tumor suppressor p53 appeared as master regulator of these propapoptotic changes and is the transactivator of proapoptotic proteins which was upregulated by indirubin. Taking into account the physiological role of death ligands in immune surveillance, sensitization of melanoma cells for death ligands may be supportive for an anti-tumor immune response. Furthermore, combinations with kinase inhibitors, such as indirubin 8-Rha-β may help for a breakthrough of TRAIL-mediated strategies in melanoma., (Copyright © 2010 Elsevier Inc. All rights reserved.)

Chemotherapy resistance and related defects in apoptotic signaling are crucial for the high mortality of melanoma. Effective drugs are lacking, also due to the fact that apoptosis regulation in this tumor is essentially not understood. The cytosine analogue ferropoptoside (N69), which contains an iron carbonyl complex, resulted in strong induction of apoptosis in melanoma cells starting already after 2h, whereas cytotoxicity remained at a low level. Surprisingly, there was no indication for any caspase activation at early times, although cytochrome c was released from mitochondria. Indicative for new proapoptotic pathways was the production of reactive oxygen species (ROS) as an early effect, and the inhibition of apoptosis by the antioxidant vitamin E. Apoptosis was also blocked by exogenous Bcl-2 overexpression and by the pan-protease inhibitor zVAD. However, only zVAD also prevented ROS production, for which Bcl-2 remained without an effect. Thus, new proapoptotic pathways are described here for melanoma cells clearly related to ROS production. A cascade enclosing enhanced levels of intracellular iron, which lead to enhanced ROS production in a Fenton reaction, appears as suggestive. Whereas off-target effects of zVAD appear as upstream, Bcl-2 may exert its inhibitory activity downstream of ROS. New proapoptotic pathways are of particular interest for melanoma as they may open new options for targeting this highly therapy-refractory tumor.

The proapoptotic B-cell lymphoma (Bcl)-2 protein Bcl-xS encloses the Bcl-2 homology (BH) domains BH3 and BH4 and triggers apoptosis via the multidomain protein Bak, however, the mechanism remained elusive. For investigating Bcl-xS efficacy and pathways, an adenoviral vector was constructed with its cDNA under tetracycline-off control. Bcl-xS overexpression resulted in efficient apoptosis induction and caspase activation in melanoma cells. Indicative of mitochondrial apoptosis pathways, Bcl-xS translocated to the mitochondria, disrupted the mitochondrial membrane potential and induced release of cytochrome c, apoptosis-inducing factor and second mitochondria-derived activator of caspases. In melanoma cells, Bcl-xS resulted in significant Bak activation, and Bak knockdown as well as Bcl-xL overexpression abrogated Bcl-xS-induced apoptosis, whereas Mcl-1 (myeloid cell leukemia-1) knockdown resulted in a sensitization. With regard to the particular role of voltage-dependent anion channel 2 (VDAC2) for inhibition of Bak, we identified here a notable interaction between Bcl-xS and VDAC2 in melanoma cells, which was proven in reciprocal coimmunoprecipitation analyses. On the other hand, Bcl-xS showed no direct interaction with Bak, and its binding to VDAC2 appeared as also independent of Bak expression. Suggesting a new proapoptotic mechanism, Bcl-xS overexpression resulted in disruption of the VDAC2-Bak interaction leading to release of Bak. Further supporting this pathway, overexpression of VDAC2 strongly decreased apoptosis by Bcl-xS. New proapoptotic pathways are of principle interest for overcoming apoptosis deficiency of melanoma cells. [ABSTRACT FROM AUTHOR]