Abstract 2910: Overcoming therapy resistance in SOX2-expressing head and neck squamous cell carcinoma cells by targeting the downstream BCL2-pathway

SOX2 (SRY (sex determining region Y)-box 2) is a transcriptional master regulator that confers “stemness” in early embryogenesis and acts as an oncogene in various cancers. Recently, SOX2 expression has been shown in several solid tumor types. However, the SOX2 expression pattern and the correlation with histopathologic status and clinical outcome are highly variable among tumors, suggesting distinct roles of SOX2 in individual tumors. In head and neck squamous cell carcinoma (HNSCC), we reported that expression of SOX2 is detectable and associates with high-risk disease in non-HPV human HNSCC. Functionally, SOX2 overexpression enhanced apoptosis resistance in response to treatments with cisplatin and other apoptosis-inducing agents, indicating SOX2 as a mediator of therapy resistance in human HNSCC. On the molecular level, induction of the anti-apoptotic protein BCL2 was observed following SOX2-induction. Here we explore if the anti-apoptotic effects of SOX2 are functionally mediated through up-regulation of BCL2 expression and explore BCL2 as a potential direct downstream target of SOX2. We tested SOX2 and BCL2 expression in different human HNSCC cell-lines by qPCR and respectively immunoblot. To analyse the functional effect of SOX2, SOX2 overexpressing and control SCC25 cells generated via transduction with lentiviral particles were analysed in in vitro apoptosis assays after treatments with staurosporine, TRAIL or cisplatin. Furthermore, to analyse the influence of BCL2 in SOX2-overexpressing cells, treatment with the BCL2 inhibitor ABT-199 was used. We could confirm the strong inductive effect of SOX2 overexpression on BCL2 gene and protein levels in the human SCC25 cell line. In line, cells treated with SOX2 overexpressing lentiviruses showed enhanced apoptotic resistance as compared to control-transduced cells. Notably, HNSCC cells were less sensitive to ABT-199 as previously reported for lymphatic leukemic cells (5-10 μM versus 0.1-1 μM for EC 50 ). However, ABT-199 treatment in low dosage (1 μM) was indeed able to sensitize SOX2-overexpressing human HNSCC cells to apoptosis, indicating up-regulation of BCL2 as an important mechanism by which SOX2 confers treatment resistance in these cells. Moreover, analysis of the BCL2 promoter revealed a potential SOX2-binding site, and a direct association of SOX2 protein with this site could be confirmed by chromatin immunoprecipitation assays (CHIP). We are currently further exploring the effect of ABT-199 on the therapy resistance of SOX2-overexpressing cells using in vivo models. Taken together, our findings indicate that, in human HNSCC cells, SOX2 plays an important role in drug resistance by elevating BCL2 expression. In patients with SOX2-expressing HNSCC, combinatorial treatment with ABT-199 may enhance response to conventional chemotherapies by re-sensitizing cells to apoptosis induction. Citation Format: Hui Wang, Angela Quiesser, Andreas Schrock, Sven Perner, Claudia Lengerke. Overcoming therapy resistance in SOX2-expressing head and neck squamous cell carcinoma cells by targeting the downstream BCL2-pathway. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2910.