PRT 4165

PRT4165 is a potent inhibitor of PRC1-mediated H2A ubiquitylation.

In Vitro:PRT4165 is a potent inhibitor of PRC1-mediated H2A ubiquitylation. In vitro E3 ubiquitin ligase activity assays reveal that PRT4165 inhibits both RNF2 and RING 1A, but not RNF8 nor RNF168. In the presence of PRT4165, H2A ubiquitylation could be completely inhibited regardless of whether RING1 or RNF2 contributes the E3 ubiquitin ligase activity. Treatment of cells for 60 min with 50 μM PRT4165 results in a dramatic reduction in total ubiquitylated histone H2A. It is also found that longer exposure of the cells with the PRT4165 (30 and 60 min) leads to increased levels of γ-H2AX in unirradiated cells. PRT4165 inhibits double-strand break (DSB) repair at the 8-h time point compare with mock treated cells. Cells treated with increasing concentrations of PRT4165 show increasing numbers of cells in G2/M[1].

References:
[1]. Ismail IH, et al. A small molecule inhibitor of polycomb repressive complex 1 inhibits ubiquitin signaling at DNA double-strand breaks. J Biol Chem. 2013 Sep 13;288(37):26944-54.

A small molecule inhibitor of polycomb repressive complex 1 inhibits ubiquitin signaling at DNA double-strand breaks.[Pubmed:23902761]

J Biol Chem. 2013 Sep 13;288(37):26944-54.

Polycomb-repressive complex 1 (PRC1)-mediated histone ubiquitylation plays an important role in aberrant gene silencing in human cancers and is a potential target for cancer therapy. Here we show that 2-pyridine-3-yl-methylene-indan-1,3-dione (PRT4165) is a potent inhibitor of PRC1-mediated H2A ubiquitylation in vivo and in vitro. The drug also inhibits the accumulation of all detectable ubiquitin at sites of DNA double-strand breaks (DSBs), the retention of several DNA damage response proteins in foci that form around DSBs, and the repair of the DSBs. In vitro E3 ubiquitin ligase activity assays revealed that PRT4165 inhibits both RNF2 and RING 1A, which are partially redundant paralogues that together account for the E3 ubiquitin ligase activity found in PRC1 complexes, but not RNF8 nor RNF168. Because ubiquitylation is completely inhibited despite the efficient recruitment of RNF8 to DSBs, our results suggest that PRC1-mediated monoubiquitylation is required for subsequent RNF8- and/or RNF168-mediated polyubiquitylation. Our results demonstrate the unique feature of PRT4165 as a novel chromatin-remodeling compound and provide a new tool for the inhibition of ubiquitylation signaling at DNA double-strand breaks.

The E3 ubiquitin-ligase Bmi1/Ring1A controls the proteasomal degradation of Top2alpha cleavage complex - a potentially new drug target.[Pubmed:19956605]

PLoS One. 2009 Dec 1;4(12):e8104.

BACKGROUND: The topoisomerases Top1, Top2alpha and Top2beta are important molecular targets for antitumor drugs, which specifically poison Top1 or Top2 isomers. While it was previously demonstrated that poisoned Top1 and Top2beta are subject to proteasomal degradation, this phenomena was not demonstrated for Top2alpha. METHODOLOGY/PRINCIPAL FINDINGS: We show here that Top2alpha is subject to drug induced proteasomal degradation as well, although at a lower rate than Top2beta. Using an siRNA screen we identified Bmi1 and Ring1A as subunits of an E3 ubiquitin ligase involved in this process. We show that silencing of Bmi1 inhibits drug-induced Top2alpha degradation, increases the persistence of Top2alpha-DNA cleavage complex, and increases Top2 drug efficacy. The Bmi1/Ring1A ligase ubiquitinates Top2alpha in-vitro and cellular overexpression of Bmi1 increases drug induced Top2alpha ubiquitination. A small-molecular weight compound, identified in a screen for inhibitors of Bmi1/Ring1A ubiquitination activity, also prevents Top2alpha ubiquitination and drug-induced Top2alpha degradation. This ubiquitination inhibitor increases the efficacy of topoisomerase 2 poisons in a synergistic manner. CONCLUSIONS/SIGNIFICANCE: The discovery that poisoned Top2alpha is undergoing proteasomal degradation combined with the involvement of Bmi1/Ring1A, allowed us to identify a small molecule that inhibits the degradation process. The Bmi1/Ring1A inhibitor sensitizes cells to Top2 drugs, suggesting that this type of drug combination will have a beneficial therapeutic outcome. As Bmi1 is also a known oncogene, elevated in numerous types of cancer, the identified Bmi1/Ring1A ubiquitin ligase inhibitors can also be potentially used to directly target the oncogenic properties of Bmi1.