In many human cancers, the tumor suppressor, p27kip1 (p27), a cyclin-dependent kinase inhibitor critical to cell cycle arrest, undergoes perpetual ubiquitin-mediated proteasomal degradation by the E3 ligase complex SCF-Skp2/Cks1 and/or cytoplasmic mislocalization; lack of nuclear p27 causes aberrant cell cycle progression and cytoplasmic p27 can mediate cell migration/metastasis. We previously showed that estrogen (E2) induces Skp2-dependent degradation of p27 and cell proliferation in primary endometrial epithelial cells (EECs) and endometrial carcinoma (ECA) cell lines (e.g., ECC-1) suggesting a pathogenic mechanism for type I endometrial carcinoma (ECA), an estrogen (E2)-linked cancer. The current studies show that treatment of ECC-1 cells with small molecule inhibitors of Skp2/Cks1 E3 ligase activity (Skp2E3LIs) increase protein levels of p27, increase p27 half-life by 6 hours and inhibit cell proliferation (IC50 10μM). Two of five SKP2E3LIs, designated as C2 and C20 specifically increase p27 in the nucleus while decreasing p27 in the cytoplasm in both ECC-1 cells and primary ECA cells. In addition, C2 and C20 prevent both E2-induced proliferation and degradation of nuclear p27. These data suggest that Skp2E3LIs can function in the nucleus to prevent E2-induced degradation of p27 in primary ECA cells to regain growth control by directly blocking p27 ubiquitylation and subsequent degradation. Skp2E3LIs enable a chemical biology approach to understanding the functional significance of the ubiquitin pathway in p27-mediated cell cycle regulation and importantly, are the first specific proteasome inhibitors that pharmacologically target the binding interaction between the E3ligase, SCF-Skp2/Cks1 and p27 to prevent its ubiquitylation and subsequent degradation. These targeted inhibitors represent a major therapeutic advancement over general proteasome inhibitors for cancers characterized by SCF-Skp2/Cks1-mediated destruction of p27.

Citation Format: Savvas C. Pavlides, Kuang-Tzu Huang, Lily Wu, Bo R. Rueda, Stephanie V. Blank, Khushbakhat R. Mittal, Timothy Cardozo, Leslie I. Gold. Inhibitors of Skp2 E3ligase stabilize nuclear p27kip1 for regain of growth regulation in cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3410. doi:10.1158/1538-7445.AM2013-3410