Abstract 1817: C/EBPβ antagonist peptide, ST101, attenuates oncogenic gene transactivation in cancer cells to drive antitumor activity

Abstract

Abstract Transcription factor dysregulation is common in cancer, resulting in aberrant gene expression that drives oncogenesis. Antagonism of oncogenic transcription factor activity, by disrupting essential protein-protein interactions needed for activation of downstream effector molecules or association with DNA, represents a powerful approach to target this previously ‘undruggable' class of proteins. CCAAT/Enhancer Binding Protein Beta (C/EBPβ) is a transcription factor overexpressed in many cancers that regulates expression of factors that promote tumor survival, proliferation and inhibit differentiation. Here, we decribe the anti-tumor activity of ST101, a cell-penetrating peptide antagonist of C/EBPβ. To demonstrate ST101 disruption of the interaction of C/EBPβ with co-factor activating transcription factor 5 (ATF5), a competition ELISA assay was performed. Recombinant ATF5 associated with plate-bound C/EBPβ with a binding affinity of ∼1 nM. Addition of ST101 inhibited this interaction in dose-dependent manner, resulting in an IC50 of 25 nM. To demonstrate ST101 disruption of C/EBPβ phosphorylation and gene transactivation in cancer cells, western blot analysis and quantitative polymerase chain reaction (qPCR) were performed on U251 glioblastoma, MCF7 breast adenocarcinoma and A549 lung adenocarcinoma cells. Administration of ST101 resulted in a dose-dependent decrease in C/EBPβ activation, as evidenced by a decrease in Thr189 phosphorylation. Anatgonism of C/EBPβ activity resulted in a dose-dependent decrease in mRNA expression of genes involved in survival (BCL2 and the baculoviral inhibitor of apoptosis factors BIRC3, BIRC5), inhibition of differentiation (Inhibitor of DNA binding proteins ID1, ID2 and ID3) and proliferation (cyclins CCNB1 and CCNA2 and cyclin-dependent kinases CDK1 and CDK2). Finally, in a mouse xenograft model, 25mg/kg ST101 administered three times per week for three weeks resulted in significant and sustained tumor growth inhibition in U251 subcutaneous tumors, both when ST101 administration was initiated early (day 2, 200 mm3 tumors, p<0.05) or late (day 16, >500mm3 tumors, p<0.05). These data demonstrate the therapeutic potential of systemic administration of ST101 and support clinical development of ST101 as a potent peptide therapeutic for a variety of solid tumor malignancies. Citation Format: Jim A. Rotolo, Rick Ramirez, Mark Koester, Erin Gallagher, Siok Leong, Lila Ghamsari, Gene Merutka, Barry J. Kappel. C/EBPβ antagonist peptide, ST101, attenuates oncogenic gene transactivation in cancer cells to drive antitumor activity [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 1817.