C/EBPß is a MYB- and p300-cooperating pro-leukemogenic factor and promising drug target in acute myeloid leukemia.

Transcription factor MYB has recently emerged as a promising drug target for the treatment of acute myeloid leukemia (AML). Here, we have characterized a group of natural sesquiterpene lactones (STLs), previously shown to suppress MYB activity, for their potential to decrease AML cell proliferation. Unlike what was initially thought, these compounds inhibit MYB indirectly via its cooperation partner C/EBPß. C/EBPß-inhibitory STLs affect the expression of a large number of MYB-regulated genes, suggesting that the cooperation of MYB and C/EBPß broadly shapes the transcriptional program of AML cells. We show that expression of GFI1, a direct MYB target gene, is controlled cooperatively by MYB, C/EBPß, and co-activator p300, and is down-regulated by C/EBPß-inhibitory STLs, exemplifying that they target the activity of composite MYB-C/EBPß-p300 transcriptional modules. Ectopic expression of GFI1, a zinc-finger protein that is required for the maintenance of hematopoietic stem and progenitor cells, partially abrogated STL-induced myelomonocytic differentiation, implicating GFI1 as a relevant target of C/EBPß-inhibitory STLs. Overall, our data identify C/EBPß as a pro-leukemogenic factor in AML and suggest that targeting of C/EBPß may have therapeutic potential against AML.

Monensin, a novel potent MYB inhibitor, suppresses proliferation of acute myeloid leukemia and adenoid cystic carcinoma cells.

The master transcriptional regulator MYB is a key oncogenic driver in several human neoplasms, particularly in acute myeloid leukemia (AML) and adenoid cystic carcinoma (ACC). MYB is therefore an attractive target for drug development in MYB-activated malignancies. Here, we employed a MYB-reporter cell line and identified the polyether ionophores monensin, salinomycin, and nigericin as novel inhibitors of MYB activity. As a proof of principle, we show that monensin affects the expression of a significant number of MYB-regulated genes in AML cells and causes down-regulation of MYB expression, loss of cell viability, and induction of differentiation and apoptosis. Furthermore, monensin significantly inhibits proliferation of primary murine AML cells but not of normal hematopoietic progenitors, reflecting a high MYB-dependence of leukemic cells and underscoring the efficacy of monensin in MYB-activated malignancies. Importantly, monensin also suppressed the viability and non-adherent growth of adenoid cystic carcinoma (ACC) cells expressing MYB-NFIB fusion oncoproteins. Our data show that a single compound with significant MYB-inhibitory activity is effective against malignant cells from two distinct MYB-driven human neoplasms. Hence, monensin and related compounds are promising molecular scaffolds for development of novel MYB inhibitors.

Withaferin A, a natural compound with anti-tumor activity, is a potent inhibitor of transcription factor C/EBPß.

Recent work has shown that deregulation of the transcription factor Myb contributes to the development of leukemia and several other human cancers, making Myb and its cooperation partners attractive targets for drug development. By employing a myeloid Myb-reporter cell line we have identified Withaferin A (WFA), a natural compound that exhibits anti-tumor activities, as an inhibitor of Myb-dependent transcription. Analysis of the inhibitory mechanism of WFA showed that WFA is a significantly more potent inhibitor of C/EBPß, a transcription factor cooperating with Myb in myeloid cells, than of Myb itself. We show that WFA covalently modifies specific cysteine residues of C/EBPß, resulting in the disruption of the interaction of C/EBPß with the co-activator p300. Our work identifies C/EBPß as a novel direct target of WFA and highlights the role of p300 as a crucial co-activator of C/EBPß. The finding that WFA is a potent inhibitor of C/EBPß suggests that inhibition of C/EBPß might contribute to the biological activities of WFA.

Targeting acute myeloid leukemia with a small molecule inhibitor of the Myb/p300 interaction.

The transcription factor Myb plays a key role in the hematopoietic system and has been implicated in the development of leukemia and other human cancers. Inhibition of Myb is therefore emerging as a potential therapeutic strategy for these diseases. However, because of a lack of suitable inhibitors, the feasibility of therapeutic approaches based on Myb inhibition has not been explored. We have identified the triterpenoid Celastrol as a potent low-molecular-weight inhibitor of the interaction of Myb with its cooperation partner p300. We demonstrate that Celastrol suppresses the proliferative potential of acute myeloid leukemia (AML) cells while not affecting normal hematopoietic progenitor cells. Furthermore, Celastrol prolongs the survival of mice in a model of an aggressive AML. Overall, our work demonstrates the therapeutic potential of a small molecule inhibitor of the Myb/p300 interaction for the treatment of AML and provides a starting point for the further development of Myb-inhibitory compounds for the treatment of leukemia and, possibly, other tumors driven by deregulated Myb.