Melanoma is an often fatal form of skin cancer which is remarkably resistant against radio- and chemotherapy.,Even new strategies that target RAS/RAF signaling and display unprecedented efficacy are characterized by resistance mechanisms.,The targeting of survival pathways would be an attractive alternative strategy, if tumor-specific cell death can be achieved.,Bcl-2 proteins play a central role in regulating survival of tumor cells.,In this study, we systematically investigated the relevance of antiapoptotic Bcl-2 proteins, i.e., Bcl-2, Bcl-xL, Bcl-w, Mcl-1, and A1, in melanoma cell lines and non-malignant cells using RNAi.,We found that melanoma cells required the presence of specific antiapoptotic Bcl-2 proteins: Inhibition of Mcl-1 and A1 strongly induced cell death in some melanoma cell lines, whereas non-malignant cells, i.e., primary human fibroblasts or keratinocytes were not affected.,This specific sensitivity of melanoma cells was further enhanced by the combined inhibition of Mcl-1 and A1 and resulted in 60% to 80% cell death in all melanoma cell lines tested.,This treatment was successfully combined with chemotherapy, which killed a substantial proportion of cells that survived Mcl-1 and A1 inhibition.,Together, these results identify antiapoptotic proteins on which specifically melanoma cells rely on and, thus, provide a basis for the development of new Bcl-2 protein-targeting therapies.
Past studies have shown that upregulation of the anti-apoptotic Bcl-2 family protein Mcl-1 is a major adaptive mechanism of melanoma cells to endoplasmic reticulum (ER) stress, and has an important role in resistance of the cells to apoptosis.,In this study, we show that the increase in transcription of Mcl-1 in melanoma cells triggered by pharmacological ER stress inducers is mediated by the transcription factor Ets-1.,By incremental deletion analysis of the Mcl-1 promoter, we identified a DNA fragment containing an Ets-1 binding site that is transcriptionally responsive to ER stress.,Mutations in the Ets-1 binding site or knockdown of Ets-1 inhibited the increase in Mcl-1, indicating that Ets-1 has a critical role in transcriptional upregulation of Mcl-1.,Similar to Mcl-1, Ets-1 was transcriptionally upregulated by ER stress.,This was mediated by the IRE1α/XBP-1 branch of the unfolded protein response, as upregulation of Ets-1 was inhibited in melanoma cell lines deficient in IRE1α or XBP-1 established by short hairpin RNA knockdown.,Activation of the PI3k/Akt pathway downstream of XBP-1 was also involved, in that inhibition of the pathway blocked upregulation of Ets-1.,Inhibition of Ets-1 enhanced ER stress-induced apoptosis in melanoma cell lines and in fresh melanoma isolates, recapitulating the effect of inhibition of Mcl-1.,These results reveal a key mechanism by which Mcl-1 is transcriptionally upregulated in melanoma cells by ER stress, and identify Ets-1 as a potential target for inhibition to sensitize melanoma cells to apoptosis.