Abstract 4180: Therapeutic targeting of STAT3 in nasopharyngeal carcinoma
Authors: Hui Shinyee , Lo Kwok Wai , Liu Alyssa Ming-Ting , Law Marcus Hung-Nam , Ng Linda Wing-Chi , Li Peter Tin-Chung , Cheung Siu Tim 
Published: September 2020

Abstract
Cancer stem cells (CSCs) have been reported to associate with resistance, metastasis, recurrence, and related to treatment failure in Nasopharyngeal Carcinoma (NPC). Signal transducer and activator of transcription 3 (STAT3) was associated with CSCs. When activated at tyrosine-705 (Y705) and serine-727 (S727) by phosphorylation, STAT3 promotes tumour inflammation. In CSCs, STAT3 was activated independently of upstream modulator, further increases resistance towards chemotherapy. Hence, we aim to investigate the potential of STAT3 as a therapeutic target in NPC. Immunohistochemistry revealed STAT3 activation in 100% primary NPC tissues (P-STAT3 Y705, n=27). Comparable STAT3 status was observed in patient derived xenografts and cell line established xenografts. Western blot analysis of NPC cell lines (C17, NPC43 and C666-1) substantiated P-STAT3 (Y705 and S727) activation status. STAT3 targeting were examined by specific small interfering RNA (siRNA) and cancer stemness inhibitor Napabucasin, both were shown to knockdown STAT3 activation. Napabucasin reduced cell viability, induced cell death, decelerated NPC cell migration, and inhibited STAT3 phosphorylation. Notably, chemoresistant NPC cell line (C666-1 FR) despite showing 300% higher resistance towards standard chemodrug 5-fluorouracil (5-FU), showed comparable sensitivity towards Napabucasin with reference to its parental cells C666-1. This demonstrates feasibility of STAT3 targeted therapy as alternative treatment strategy in overcoming chemoresistance of NPC. Interestingly, despite significant STAT3 knockdown by siRNA approach, minimal effect on cell proliferation and cell death was observed. In addition, Napabucasin was showed to impair NPC stemness properties including colony and spheroid formation. NPC spheroids were observed to have elevated STAT3 and classical cancer stemness markers (ALDH1A1, CD133) comparing the three-dimensional culture to their corresponding cells in two-dimensional culture. ALDH1A1 and CD133 were downregulated when treated with Napabucasin and STAT3 siRNA. RNA-sequencing and real-time qPCR (RT-qPCR) analysis showed significant downregulation of cancer stemness gene, Euchromatic histone-lysine N-methyltransferase 2 (EHMT2), upon Napabucasin treatment, but not with STAT3 siRNA. This work first reports that Napabucasin downregulates EHMT2 in NPC. qPCR analysis also demonstrated overexpressed EHMT2 in NPC spheroids, suggesting EHMT2 role in regulating NPC cancer stemness. Indeed, when treated with Napabucasin, EHMT2 was downregulated in spheroids. These observations indicate specificity of Napabucasin in controlling NPC cancer stemness. Congruent to in vitro data, Napabucasin suppressed tumor growth in vivo, diminished P-STAT3 (Y705) activation, decreased proliferation (Ki67), and elevated apoptosis (cleaved caspase-3). In summary, this work demonstrated promising therapeutic potential of Napabucasin targeting STAT3 and cancer stemness in NPC.
Citation Format: Shinyee Hui, Kwok Wai Lo, Alyssa Ming-Ting Liu, Marcus Hung-Nam Law, Linda Wing-Chi Ng, Peter Tin-Chung Li, Siu Tim Cheung. Therapeutic targeting of STAT3 in nasopharyngeal carcinoma [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4180.