Microphthalmia-associated transcription factor (Mitf) is critically involved in melanin synthesis as well as differentiation of cells of the melanocytic lineage. Some earlier studies suggested that Mitf is also essential in the survival of melanoma cells, but this notion remains controversial. We synthesized short interfering RNA (siRNA) duplexes corresponding to the mitf sequence and transfected them into B16 melanoma. Lipid-mediated transfection in vitro of Mitf-specific siRNA resulted in specific downregulation of Mitf and of the tyrosinase that is a transcriptional target of Mitf. This treatment also remarkably reduced the viability of melanoma cells by inducing apoptosis. To examine the potential feasibility of RNAi therapy against melanoma, B16 cells were subcutaneously injected into syngenic mice and siRNA was transfected into the pre-established tumor by means of electroporation. The Mitf-specific siRNA drastically reduced outgrowth of subcutaneous melanoma, while nonspecific siRNA failed to affect tumor progression. Terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling-based analysis of tumor specimens demonstrated that the tumor cells transfected with Mitf-siRNA effectively underwent apoptosis in vivo. The present results indicate that Mitf plays important roles in melanoma survival. Intratumor electrotransfer of Mitf-specific siRNA may provide a powerful strategy for therapeutic intervention of malignant melanoma.
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