title: The Bmi-1 helix-turn and ring finger domains are required for Bmi-1 antagonism of (-) epigallocatechin-3-gallate suppression of skin cancer cell survival.
abstract: The Bmi-1 Polycomb group (PcG) protein is an important epigenetic regulator of chromatin status. Elevated Bmi-1 expression is observed in skin cancer and contributes to cancer cell survival. (-) Epigallocatechin-3-gallate (EGCG), an important green tea-derived cancer prevention agent, reduces Bmi-1 level resulting in reduced skin cancer cell survival. This is associated with increased p21(Cip1) and p27(Kip1) expression, reduced cyclin, and cyclin dependent kinase expression, and increased cleavage of apoptotic markers. These EGCG-dependent changes are attenuated by vector-mediated maintenance of Bmi-1 expression. In the present study, we identify Bmi-1 functional domains that are required for this response. Bmi-1 expression reverses the EGCG-dependent reduction in SCC-13 cell survival, but Bmi-1 mutants lacking the helix-turn-helix-turn-helix-turn (Bmi-1ΔHT) or ring finger (Bmi-1ΔRF) domains do not reverse the EGCG impact. The reduction in Ring1B ubiquitin ligase activity, observed in the presence of mutant Bmi-1, is associated with reduced ability of these mutants to interact with and activate Ring1B ubiquitin ligase, the major ligase responsible for the ubiquitination of histone H2A during chromatin condensation. This results in less chromatin condensation leading to increased tumor suppressor gene expression and reduced cell survival; thereby making the cells more susceptible to the anti-survival action of EGCG. We further show that these mutants act in a dominant-negative manner to inhibit the action of endogenous Bmi-1. Our results suggest that the HT and RF domains are required for Bmi-1 ability to maintain skin cancer cell survival in response to cancer preventive agents.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, MD, USA.
authors.forename: Sivaprakasam
authors.initials: S
authors.lastname: Balasubramanian
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, MD, USA.
authors.forename: Tiffany M
authors.initials: TM
authors.lastname: Scharadin
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, MD, USA.
authors.forename: Bingshe
authors.initials: B
authors.lastname: Han
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, MD, USA.
authors.forename: Wen
authors.initials: W
authors.lastname: Xu
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, MD, USA; Department of Dermatology, University of Maryland School of Medicine, Baltimore, MD, USA; Department of Obstetrics and Gynecology, University of Maryland School of Medicine, Baltimore, MD, USA; Department of Greenebaum Cancer Center, University of Maryland School of Medicine, Baltimore, MD, USA. Electronic address: reckert@umaryland.edu.
authors.forename: Richard L
authors.initials: RL
authors.lastname: Eckert
mesh_terms: D016588:Anticarcinogenic Agents / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects*; D002392:Catechin / Q000031:analogs & derivatives* / Q000494:pharmacology; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000187:drug effects; D005786:Gene Expression Regulation / Q000187:drug effects; D006801:Humans; D009154:Mutation; D063150:Polycomb Repressive Complex 1 / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D063146:Polycomb-Group Proteins / Q000378:metabolism; D054829:RING Finger Domains; D012878:Skin Neoplasms / Q000378:metabolism / Q000473:pathology
chemical_list: D016588:Anticarcinogenic Agents; C069077:BMI1 protein, human; D063146:Polycomb-Group Proteins; D002392:Catechin; C045651:epigallocatechin gallate; D063150:Polycomb Repressive Complex 1; C489841:RNF2 protein, human
keywords: Chemoprevention; EGCG; Ezh2; Histone methylation; Polycomb group proteins; Skin cancer
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