The death ligand TRAIL represents a promising therapeutic strategy for metastatic melanoma, however prevalent and inducible resistance limit its applicability. A new approach is presented here for sensitization to TRAIL. It is based on inhibition of the membrane potassium channel KCa3.1 (IK1), which serves fundamental cellular functions related to membrane potential. The selective inhibitor TRAM-34 did not induce apoptosis by itself but synergistically enhanced TRAIL sensitivity and overrode TRAIL resistance in a large panel of melanoma cell lines. Expression of IK1 was also found in mitochondria, and its inhibition resulted in mitochondrial membrane hyperpolarization and an early activation of Bax. The combination of TRAM-34 and TRAIL resulted in massive release of mitochondrial factors, cytochrome c, AIF and SMAC/DIABLO. Bax knockdown and Bcl-2 overexpression abolished apoptosis. Overexpression of XIAP diminished apoptosis by two-fold, and SMAC knockdown almost completely abolished apoptosis. These data uncover the existence of a rheostat in melanoma cells, consisting of inhibitor of apoptosis proteins and SMAC, which regulates TRAIL sensitivity. Thus, a new strategy is described based on mitochondrial membrane channels, which correspond to Bax activation. As both TRAIL and IK1 inhibitors had shown only minor side effects in clinical trials, a clinical application of this combination is conceivable.

Rheumatoid arthritis synovial fibroblasts (RA-SFs) show an aggressive phenotype and support joint inflammation and tissue destruction. New druggable targets in RA-SFs would therefore be of high therapeutic interest. The present study shows that the intermediate-conductance, calcium-activated potassium channel KCa3.1 (KCNN4) is expressed at the mRNA and protein level in RA-SFs, is functionally active, and has a regulatory impact on cell proliferation and secretion of pro-inflammatory and pro-destructive mediators. Whole-cell patch-clamp recordings identified KCa3.1 as the dominant potassium channel in the physiologically relevant membrane voltage range below 0 mV. Stimulation with transforming growth factor β1 (TGF-β1) significantly increased transcription, translation, and channel function of KCa3.1. Inhibition of KCa3.1 by the selective, pore-blocking inhibitor TRAM-34, (and, in part, by siRNA) significantly reduced cell proliferation, as well as expression and secretion of pro-inflammatory factors (IL-6, IL-8, and MCP1) and the tissue-destructive protease MMP3. These effects were observed in non-stimulated and/or TGF-β1-stimulated RA-SFs. Since small molecule-based interference with KCa3.1 is principally well tolerated in clinical settings, further evaluation of channel blockers in models of rheumatoid arthritis may be a promising approach to identify new pharmacological targets and develop new therapeutic strategies for this debilitating disease.

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