Abstract 504: p27 transcriptionally coregulates STAT3 to drive cancer stem cells

Abstract

Abstract p27 is a cell cycle inhibitor and a tumor suppressor. It can also regulate cellular processes including migration and transcription through mechanisms independent of its CDK-inhibitory role. In cancers, p27 C-terminal phosphorylation by PI3K-activated kinases, AKT, RSK1, and SGK1 at T157 and T198 alters p27 protein-protein interactions and shifts p27 from CDK-inhibitor to an oncogene. Previously, our group showed that CDK-binding defective p27pT157pT198 phosphomimetic (p27CK-DD) upregulates epithelial-mesenchymal transition (EMT) and the metastatic potential of cancer cell lines. In addition to its action to promote EMT, p27 appears to promote CSC expansion and or maintenance. Here we demonstrated that C-terminally phosphorylated p27 increases CSC properties, including tumor sphere formation and CSC markers. p27CK-DD increases the expression of several embryonic stem cell transcription factors (ES-TFs), including SOX2, NANOG and cMYC, which are known to drive embryonic stem cell self-renewal and to promote CSC expansion. A human phospho-kinase array showed Pyk2 is activated by p27CK-DD. We demonstrated that Pyk2 activation and its binding to p27 are dependent on phosphorylation of p27 at T198 and T157. Treatment with a Pyk2 inhibitor, and PYK2-knockdown by siRNA or shRNA revealed that Pyk2 is a key mediator of the increase in tumor spheres, ALDH1 activity and ES-TFs in cancer cells expressing abundant C-terminally phosphorylated p27. Pyk2 enhances STAT3 phosphorylation and activation. We identified that C-terminally phosphorylated p27 recruits STAT3 to form p27-Pyk2-STAT3 complex leading to STAT3 activation. p27/STAT3 complexes co-localize to the nucleus and co-occupy chromatin. We confirmed by ChIP-qPCR that p27/STAT3 bind and activate the cMYC promotor. Altogether, in breast cancer cells with high p27pT157pT198 or expressing p27CK−DD, STAT3 is partly activated by Pyk2 and interacts with p27. p27 is a STAT3 coregulator, whose assembly and chromatin association is governed by p27 phosphorylation. These data reveal a novel mechanism whereby p27-driven Pyk2 activation promotes CSC expansion and tumor progression via transcriptionally activation of the STAT3 and its target genes. Citation Format: Seyedehfatemeh Razavipour, Kibeom Jang, Hyunho Yoon, Minsoon Kim, Miyoung Shin, Dekuang Zhao, Joyce Slingerland. p27 transcriptionally coregulates STAT3 to drive cancer stem cells [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 504.