Abstract 1461: C-terminally phosphorylated p27 modulates chromatin remodeling in breast cancer

Abstract Breast Cancer (BC) is the leading cause of cancer-related death in women. Transcriptional deregulation is a frequent characteristic of cancers. Chromatin remodeling determines the accessibility of the chromatin through dynamic modification of chromatin architecture, and thereby regulates gene expression. Alteration in chromatin remodeling is associated with cancer. Therefore, a greater understanding of chromatin remodeling and its role in tumorigenesis could provide a significant cornerstone to develop more effective therapeutic strategies. p27 is an integral regulator of the cell cycle. It plays a dual role in tumorigenesis. p27 acts as a tumor suppressor to restrain the cell cycle, but upon C-terminal phosphorylation at T157 and T198 by PI3K activated kinases, it acts as an oncogene to promote cancer metastasis. We previously showed that p27pT157pT198 (p27pTpT) acts as a transcriptional co-regulator of cJun. Our new data now indicate that p27pTpT also binds to and transcriptionally co-regulates STAT3 to drive cancer stem cell gene programs. We also found that p27 is widely recruited to consensus motifs shared by many other transcription factors, suggesting a broader role in transcriptional regulation. Here we investigated if p27pTpT regulates transcriptional programs in BC through modulating chromatin accessibility. ATACseq was carried out in 231 cells, 231 with overexpression of a cell cycle-defective phosphomimetic p27 (231p27CK-DD), and in 1883 cells with high phospho-activated p27 levels, and 1833 with p27 knockdown and analyzed together with our prior p27 ChIPseq. p27pTpT increased ATAC-seq signals at the promoter of a sub-set of STAT3 target genes, where p27 ChIPseq and ATACseq signals overlapped. These genes are upregulated in 231p27CK-DD and in 1833 compared to 231. Thus, p27 appears to potentiate a more open chromatin state at these binding sites to modulate gene expression. p27 and STAT3 co-occupy at the promoters of the STAT3 target genes including MYC, IL6, and VEGFA, in which p27 knockdown decreased STAT3 recruitment to these sites. Using ENCODE p300 and CBP ChIP-seq data, we found potential co-localization of histone acetyltransferase (HAT), CBP and p300, with p27 and STAT3 on MYC, IL6, and VEGFA promoters. Thus, p27 might recruit HATs to the chromatin to increase chromatin accessibility through H3K27 acetylation (H3K27ac). ChIP-qPCR assays validated that p27, STAT3 and CBP recruitment to the MYC promoter was greater in p27-activated 231p27CK-DD and 1833 lines than in 231, and decreased upon p27 depletion in 1833. Notably, ChIP assays also showed CBP recruitment increased H3K27ac at this site. These data support a model in which p27 enrichment to the promoter of a subset of STAT3 target genes increases both CBP and STAT3 recruitment, to increase chromatin accessibility through H3K27ac and upregulate their expression. This work revealed a novel genome-wide action of p27 in chromatin remodeling. Citation Format: Seyedeh Fatemeh Razavipour, Hyunho Yoon, Miyoung Shin, Alec McIntosh, Dekuang Zhao, Amir Bagheri, Derek Van Booven, Chunling Yi, Joyce Slingerland. C-terminally phosphorylated p27 modulates chromatin remodeling in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1461.