title: A proteasome inhibitor-stimulated Nrf1 protein-dependent compensatory increase in proteasome subunit gene expression reduces polycomb group protein level.
abstract: The polycomb group (PcG) proteins, Bmi-1 and Ezh2, are important epigenetic regulators that enhance skin cancer cell survival. We recently showed that Bmi-1 and Ezh2 protein level is reduced by treatment with the dietary chemopreventive agents, sulforaphane and green tea polyphenol, and that this reduction involves ubiquitination of Bmi-1 and Ezh2, suggesting a key role of the proteasome. In the present study, we observe a surprising outcome that Bmi-1 and Ezh2 levels are reduced by treatment with the proteasome inhibitor, MG132. We show that this is associated with a compensatory increase in the level of mRNA encoding proteasome protein subunits in response to MG132 treatment and an increase in proteasome activity. The increase in proteasome subunit level is associated with increased Nrf1 and Nrf2 level. Moreover, knockdown of Nrf1 attenuates the MG132-dependent increase in proteasome subunit expression and restores Bmi-1 and Ezh2 expression. The MG132-dependent loss of Bmi-1 and Ezh2 is associated with reduced cell proliferation, accumulation of cells in G(2), and increased apoptosis. These effects are attenuated by forced expression of Bmi-1, suggesting that PcG proteins, consistent with a prosurvival action, may antagonize the action of MG132. These studies describe a compensatory Nrf1-dependent, and to a lesser extent Nrf2-dependent, increase in proteasome subunit level in proteasome inhibitor-treated cells and confirm that PcG protein levels are regulated by proteasome activity.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, Maryland 21201, USA.
authors.forename: Sivaprakasam
authors.initials: S
authors.lastname: Balasubramanian
authors.forename: Santosh
authors.initials: S
authors.lastname: Kanade
authors.forename: Bingshe
authors.initials: B
authors.lastname: Han
authors.forename: Richard L
authors.initials: RL
authors.lastname: Eckert
mesh_terms: D017209:Apoptosis / Q000187:drug effects / Q000235:genetics; D045744:Cell Line, Tumor; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D000071221:Enhancer of Zeste Homolog 2 Protein; D016195:G2 Phase / Q000187:drug effects / Q000235:genetics; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects* / Q000235:genetics; D006801:Humans; D007976:Leupeptins / Q000494:pharmacology*; D051266:Nuclear Respiratory Factor 1 / Q000235:genetics / Q000378:metabolism*; D063151:Polycomb Repressive Complex 2 / Q000235:genetics / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex / Q000096:biosynthesis* / Q000235:genetics; D061988:Proteasome Inhibitors / Q000494:pharmacology*; D054875:Ubiquitination / Q000187:drug effects / Q000235:genetics
chemical_list: D015853:Cysteine Proteinase Inhibitors; D007976:Leupeptins; C495633:NRF1 protein, human; D051266:Nuclear Respiratory Factor 1; D061988:Proteasome Inhibitors; C498409:EZH2 protein, human; D000071221:Enhancer of Zeste Homolog 2 Protein; D063151:Polycomb Repressive Complex 2; D046988:Proteasome Endopeptidase Complex; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde