l-779-450 and Skin-Neoplasms

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.

Topics: Apoptosis; Apoptosis Regulatory Proteins; Autophagy; bcl-2-Associated X Protein; Bcl-2-Like Protein 11; Cell Proliferation; Drug Resistance, Neoplasm; HCT116 Cells; Humans; Melanoma; Membrane Proteins; Mitochondria; Proto-Oncogene Proteins; Proto-Oncogene Proteins B-raf; Proto-Oncogene Proteins c-bcl-2; Pyridines; Skin Neoplasms; TNF-Related Apoptosis-Inducing Ligand

BRAF inhibition has been an instant, although short-lasting, success in BRAF-mutated melanoma treatment. Novel data by Berger et al. now suggest that BRAF-inhibitor-mediated "priming to death" facilitates tumor necrosis factor-related apoptosis-inducing ligand-mediated apoptosis. We give an overview about the importance of the crosstalk of extrinsic and mitochondrial apoptotic signaling and propose other combination therapies that may prevent or overcome secondary resistance in melanoma.

Topics: Apoptosis; Humans; Melanoma; Proto-Oncogene Proteins B-raf; Pyridines; Skin Neoplasms; TNF-Related Apoptosis-Inducing Ligand