Abstract Background: The Notch effector Hes1 plays a critical role in stemness of cancer stem cells (CSCs). T-cell factor (TCF)-4 is a key transcription factor in Wnt signaling, which is suggested to be linked with Notch signaling. Among our TCF-4 isoforms cloned previously, the TCF-4J and K pair have been characterized based on the presence (K) or absence (J) of SxxSS motif (Exp Cell Res 2010). TCF-4J was highly expressed in poorly differentiated human hepatocellular carcinoma (HCC) tissues, and the TCF-4J-overexpressing HCC cells (J cells) exhibited high tumor-initiating potential, which is one of the features of CSCs (PLoS One 2013). Thus, the AIM of this study was to investigate whether the SxxSS motif of TCF-4 was involved in the regulation of Hes1 expression in HCC cells. Methods: TCF-4K mutants were prepared by site-directed mutagenesis. Sphere formation assay was used to condense CSC-like cells. Results: Hes1 was strikingly expressed in spheres of J cells and K-mutant cells in both protein and mRNA levels, while its expression level was 70% inhibited in K cells. Consistently, protein expression levels of Jagged1 and Notch1 were highest in J cells under both attached and floating conditions. The Notch inhibitor DAPT, a γ-secretase inhibitor, clearly decreased the expression levels of Hes1, suggesting that the Notch-Hes1 axis was functional. Conclusion: Lack of SxxSS motif in TCF-4 robustly upregulated Hes1 expression in HCC cell spheres. The finding strongly suggests that TCF-4 directly regulates Hes1 in HCC spheres, where CSCs abundantly exist. Citation Format: Hironori Koga, Yasuko Imamura, Yu Ikezono, Fumitaka Wada, Toru Nakamura, Hideki Iwamoto, Atsutaka Masuda, Takahiko Sakaue, Hirohisa Yano, Takuji Torimura. Regulation of Hes1 expression by the Wnt transcription factor T-cell factor-4. [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 4601.