Abstract
The levels of proteins that control the cell cycle are regulated by ubiquitin-mediated degradation via the ubiquitin-proteasome system (UPS) by substrate-specific E3 ubiquitin ligases. The cyclin-dependent kinase inhibitor, p27kip1 (p27), that blocks the cell cycle in G1, is ubiquitylated by the E3 ligase SCF-Skp2/Cks1 for degradation by the UPS. In turn, Skp2 and Cks1 are ubiquitylated by the E3 ligase complex APC/Cdh1 for destruction thereby maintaining abundant levels of nuclear p27. We previously showed that perpetual proteasomal degradation of p27 is an early event in Type I endometrial carcinogenesis (ECA), an estrogen (E2)-induced cancer. The present studies demonstrate that E2 stimulates growth of ECA cell lines and normal primary endometrial epithelial cells (EECs) and induces MAPK-ERK1/2-dependent phosphorylation of p27 on Thr187, a prerequisite for p27 ubiquitylation by nuclear SCF-Skp2/Cks1 and subsequent degradation. In addition, E2 decreases the E3 ligase [APC]Cdh1 leaving Skp2 and Cks1 intact to cause p27 degradation. Furthermore, knocking-down Skp2 prevents E2-induced p27 degradation and growth stimulation suggesting that the pathogenesis of E2-induced ECA is dependent on Skp2-mediated degradation of p27. Conversely, progesterone (Pg) as an inhibitor of endometrial proliferation increases nuclear p27 and Cdh1 in primary EECs and ECA cells. Pg, also increases Cdh1 binding to APC to form the active E3ligase. Knocking-down Cdh1 obviates Pg-induced stabilization of p27 and growth inhibition. Notably, neither E2 nor Pg affected transcription of Cdh1, Skp2, Cks1 nor p27. These studies provide new insights into hormone regulation of cell proliferation through the UPS. The data implicates that preventing nuclear p27 degradation by blocking Skp2/Cks1-mediated degradation of p27 or increasing Cdh1 to mediate degradation of Skp2-Cks1 are potential strategies for the prevention and treatment of ECA.
Highlights
Estrogen (E2) stimulates proliferation of the endometrium and progesterone (Pg) suppresses E2-driven proliferation
Consistent with hormonal growth regulatory effects on the endometrium, we previously showed that the ovarian hormones, estrogen (E2) and progesterone (Pg; in the presence of E2 to increase Pg receptors [PR] for a physiological response that mimics the menstrual cycle [6,28,29]) had opposite effects on the levels of p27 in primary epithelial cells (EECs) such that E2 decreased the levels of p27 via ubiquitin-mediated degradation and Pg induced a marked increase in p27 levels in both EECs and primary endometrial carcinoma (ECA) cells [6]
P27 is not at the level of transcription; 3) the ECA cell lines used are appropriate paradigms for this study since their responses to E2 and Pg were identical to primary EECs
Summary
Estrogen (E2) stimulates proliferation of the endometrium and progesterone (Pg) suppresses E2-driven proliferation. A molecular level understanding of normal and malignant growth regulation of the endometrium by E2 and Pg is important to advance the field in terms of defining novel preventative and therapeutic molecular targets for this disease. Aligned with the opposing effects of E2 and Pg on proliferation, we further showed that E2 caused proteasomal degradation of p27 in primary EECs whereas Pg markedly increased p27 in both primary endometrial epithelial cells (EECs) and ECA cells. These data suggest that p27 is a significant molecular target involved in both the pathogenesis and treatment of ECA
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have