Targeting FoxM1 by thiostrepton inhibits growth and induces apoptosis of laryngeal squamous cell carcinoma

We have previously reported that forkhead box M1 (FoxM1) transcription factor was overexpressed in laryngeal squamous cell carcinoma (LSCC) and was associated with development of LSCC. However, there are limited studies regarding the functional significance of FoxM1 and FoxM1 inhibitor thiostrepton in LSCC. Therefore, the aim of this study was to examine both in vitro and in vivo activity of FoxM1 inhibitor thiostrepton against LSCC cell line and nude mice. Cell viability was studied by CCK-8 assay. Cell growth was evaluated by CFSE staining and cell cycle analysis. Apoptosis was measured by flow cytometry. The mRNA and protein expression were detected by quantitative real-time RT-PCR, Western blot and immunohistochemical staining. Xenograft model of tumor formation was used to investigate how thiostrepton influences tumorigenesis in vivo. Overexpression of FoxM1 in LSCC cells was down-regulated by thiostrepton in a dose-dependent manner. Thiostrepton caused dose- and time-dependent suppression of cell viability of LSCC. Moreover, thiostrepton induced cell cycle arrest at S phase at early time and inhibited DNA synthesis in LSCC cells in a dose- and time-dependent manner by down-regulation of cyclin D1 and cyclin E1. Thiostrepton also induced dose- and time-dependent apoptosis of LSCC cells by down-regulation of Bcl-2, up-regulation of Bax and p53, and inducing release of cytochrome c accompanied by activation of cleaved caspase-9, cleaved caspase-3 and cleaved PARP. In addition, z-VAD-fmk, a universal inhibitor of caspases, prevented activation of cleavage caspase-3 and abrogates cell death induced by thiostrepton treatment. Furthermore, FADD and cleaved caspase-8 were activated, and expression of cIAP1, XIAP and survivin were inhibited by thiostrepton. Finally, treatment of LSCC cell line xenografts with thiostrepton resulted in tumorigenesis inhibition of tumors in nude mice by reducing proliferation and inducing apoptosis of LSCC cells. Collectively, our finding suggest that targeting FoxM1 by thiostrepton inhibit growth and induce apoptosis of LSCC through mitochondrial- and caspase-dependent intrinsic pathway and Fas-dependent extrinsic pathway as well as IAP family. Thiostrepton may represent a novel lead compound for targeted therapy of LSCC.