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


RESULTS
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
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.
Alchanati
Iris
I
Proteologics Ltd, Rehovot, Israel.
Teicher
Carmit
C
Cohen
Galit
G
Shemesh
Vivian
V
Barr
Haim M
HM
Nakache
Philippe
P
Ben-Avraham
Danny
D
Idelevich
Anna
A
Angel
Itzchak
I
Livnah
Nurit
N
Tuvia
Shmuel
S
Reiss
Yuval
Y
Taglicht
Daniel
D
Erez
Omri
O
D000951:Antigens, Neoplasm / Q000378:metabolism*; D016923:Cell Death / Q000187:drug effects; D002460:Cell Line; D002474:Cell-Free System; D004250:DNA Topoisomerases, Type II / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000737:chemistry / Q000494:pharmacology; D020868:Gene Silencing / Q000187:drug effects; D005947:Glucose / Q000172:deficiency / Q000494:pharmacology; D006801:Humans; D009687:Nuclear Proteins / Q000378:metabolism*; D000075223:Poly-ADP-Ribose Binding Proteins; D063150:Polycomb Repressive Complex 1; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D011485:Protein Binding / Q000187:drug effects; D011499:Protein Processing, Post-Translational / Q000187:drug effects; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D034741:RNA, Small Interfering / Q000378:metabolism; D012097:Repressor Proteins / Q000378:metabolism*; D013713:Teniposide / Q000494:pharmacology; D059005:Topoisomerase II Inhibitors; D044767:Ubiquitin-Protein Ligases / Q000378:metabolism*; D054875:Ubiquitination / Q000187:drug effects
D000951:Antigens, Neoplasm; C069077:BMI1 protein, human; D004268:DNA-Binding Proteins; D004791:Enzyme Inhibitors; D009687:Nuclear Proteins; D000075223:Poly-ADP-Ribose Binding Proteins; D011518:Proto-Oncogene Proteins; D034741:RNA, Small Interfering; D012097:Repressor Proteins; D059005:Topoisomerase II Inhibitors; D013713:Teniposide; D063150:Polycomb Repressive Complex 1; D044767:Ubiquitin-Protein Ligases; D046988:Proteasome Endopeptidase Complex; D004250:DNA Topoisomerases, Type II; C000618811:TOP2A protein, human; D005947:Glucose