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We recently isolated a cardiac glycoside (CG), α<span style="font-variant: small-caps;">l</span>diginoside, from an indigenous plant in Taiwan, which exhibits potent tumor-suppressive efficacy in oral squamous cell carcinoma (OSCC) cell lines (SCC2095 and SCC4, IC₅₀ < 0.2 µM; 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays). Here, we report that α<span style="font-variant: small-caps;">l</span>diginoside caused Sphase arrest and apoptosis, through the inhibition of a series of signaling pathways, including those mediated by cyclin E, phospho-CDC25C (p-CDC25C), and janus kinase/signal transducer and activator of transcription (JAK/STAT)3. α<span style="font-variant: small-caps;">l</span>diginoside induced apoptosis, as indicated by caspase activation and poly (ADP-ribose) polymerase (PARP) cleavage. Equally important, α<span style="font-variant: small-caps;">l</span>diginoside reduced Mcl-1 expression through protein degradation, and overexpression of Mcl-1 partially protected SCC2095 cells from α<span style="font-variant: small-caps;">l</span>diginoside’s cytotoxicity. Taken together, these data suggest the translational potential of α<span style="font-variant: small-caps;">l</span>diginoside to foster new therapeutic strategies for OSCC treatment.