Dual induction of apoptotic and autophagic cell death by targeting survivin in head neck squamous cell carcinoma

Survivin is ubiquitously expressed in patients with head neck squamous cell carcinoma (HNSCC) and is associated with poor survival and chemotherapy resistance. Sepantronium bromide (YM155) is a selective survivin suppressant that exhibits potent antitumor activities by inducing apoptosis and autophagy in various types of cancer. However, the curative effects and underlying mechanisms of YM155 in HNSCC remain unclear. This study showed that survivin overexpression positively correlated with p-S6, p-Rb and LAMP2 but negatively correlated with the autophagic marker LC3 in human HNSCC tissues. In vitro studies revealed that YM155 triggered apoptosis of HNSCC cells in mitochondria and death receptor-dependent manner. The treatment also significantly enhanced autophagy by upregulating Beclin1, which led to cell death. YM155 not only downregulated the expression of survivin but also remarkably suppressed the activation of the mTOR signaling pathway in vitro and in vivo. YM155 displayed potent antitumor activities in both CAL27 xenograft and transgenic HNSCC mice models by delaying tumor onset and suppressing tumor growth. Furthermore, YM155 combined with docetaxel promoted tumor regression better than either treatment alone without causing considerable body weight loss in the HNSCC xenograft models. Overall, targeting survivin by YM155 can benefit HNSCC therapy by increasing apoptotic and autophagic cell death, and suppressing prosurvival pathways.Head and neck squamous cell carcinoma (HNSCC), which occurs in the oral cavity, oropharynx, larynx and hypopharynx, is the sixth most common malignancy worldwide.¹ It affects 600 000 new patients each year, which accounts for over 90% of head and neck cancers.^{2, 3} The current preferred therapy for HNSCC is combined surgery, radiotherapy, chemotherapy and biotherapy; however, the 5-year survival rate is still <50%, and the long-term survival rate has only marginally improved.^{4, 5, 6} As an important hallmark of head and neck cancer, apoptosis resistance restricts the efficacy of traditional therapies.⁷Survivin (also called BIRC5) inhibits apoptosis-related proteins, regulates cell division, relates to stress response and promotes tumor-associated angiogenesis in HNSCC.⁸ Survivin is also associated with high-grade and advanced HNSCC, poor survival, high recurrence rate and chemotherapy and radiation resistance. Therefore, targeting survivin is promisingly beneficial for head and neck cancer therapies.Sepantronium bromide (YM155) is a small imidazolium-based compound (1-(2-methoxyethyl)-2-methyl-4,9-dioxo-3-(pyrazin-2-ylmethyl)-4,9-dihydro-1H-naphtho[2,3-d]imidazolium bromide) that selectively suppresses survivin expression⁹ and displays potent anticancer activities against various types of cancer.^{10, 11, 12} Previous researches have focused on that YM155 induced the apoptosis by downregulating survivin in cancer cells.^{10, 13, 14} Recent studies including ours have demonstrated YM155 also triggered autophagy in cancer cells.^{15, 16, 17} Macroautophagy or autophagy is considered to be another type of programmed cell death wherein proteins are degraded by autophagosomes and lysosomes.¹⁸ Autophagy also has an important role in tumorigenesis.¹⁹ Autophagy shares several regulatory systems and common pathways with apoptosis; thus, autophagy is closely linked with apoptosis. Beclin1 (ATG6), an autophagy-specific gene that is essential for autophagosome induction and elongation, interacts with several apoptosis-related genes, such as bcl-2, bcl-xl and survivin.²⁰ Therefore, YM155 may not only induce the apoptosis but also affect the autophagy in HNSCC.The present study investigated the antitumor effects of YM155 on HNSCC in vitro and in vivo through dual induction of apoptotic and autophagic cell death. Although it specifically suppressed the expression of survivin, we here proved YM155 also targeted the mTOR signaling pathway, which was the principal regulator of cancer cell survival and autophagy. Most importantly, in an inducible tissue-specific spontaneous HNSCC mouse model with 100% penetrance by the combined deletion of Tgfbr1 and Pten (Tgfbr1/Pten 2cKO) in the oral mucosa²¹ with ubiquitous activation of the Akt/mTOR/survivin pathway,²² YM155 exerted significant therapeutic effects by delaying tumor onset and suppressing tumor growth. This finding coincided with the xenograft results. Finally, the effects of YM155 when combined with traditional chemotherapeutic agent were also determined.