Abstract 1520: Regulation of stemness by perturbation of OCT4-PP1 axis reduces malignancy of embryonal carcinoma

OCT4 (encoded by pou5f1) is a master regulator of pluripotency and closely related to tumor malignancies. Its associations with cancer drug-resistance, recurrence and poor prognosis of the patients has been widely reported. As OCT4 transcriptional activity is directly and indirectly involved in the maintenance of cancer stemness, inhibition of OCT4 activity could be a strategy to reduced cancer malignancies by eliminating stemness. In this study, we investigated a way to regulate the stemness of embryonal carcinoma (EC) by perturbation of posttranslational modification of OCT4. First, to confirm the role of OCT4 in the maintenance of stemness in ECs, we generated the cells which are downregulated in OCT4 doxycycline-dependently. Down-regulation of OCT4 caused weaker alkaline phosphates staining, reduced tumor sphere formation, and reduced expression of stemness marker proteins. Also, the cells underwent retardation of proliferation, cell-cycle arrest in G1 phase, and eventual apoptosis by OCT4 downregulation. The most altered genes in ECs by OCT4 downregulation were networked to signaling pathways regulating pluripotent stem cells, Hippo signaling pathways and transcriptional misregulation in cancer. Second, to regulate OCT4 transcriptional activity, we used phosphorylation of serine 236th (S236) on OCT4. ECs cells, with which their endogenous OCT4 was substituted with an OCT4 phosphorylation-mimicking mutant (Serine 236 to aspartate; S236D), showed similar phenotype to those cells that were depleted in OCT4. Chemical inhibition or knockdown of PP1 (protein phosphatase 1), which remove the phosphorylation, induced accumulation of S236 phosphorylation, and also caused similar phenotype to those cells that were depleted in OCT4. Finally, to evaluate whether regulation of OCT4 can reduce tumor malignancy, we tested whether drug-tolerant persister cancer cells could be reduced by regulation of OCT4. Co-treatment of inhibits PP1 activity with cisplatin significantly reduced drug-resistant colonies compared with single treatment of cisplatin. These results suggested that stemness of ECs could be regulated by OCT4 posttranslational modifications and regulation of cancer stemness could reduce tumor malignancy. Citation Format: Bomin Song, Hyonchol Jang. Regulation of stemness by perturbation of OCT4-PP1 axis reduces malignancy of embryonal carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1520.