Key Points• Inhibition of Myb activity by a small molecule blocks proliferation of AML cells and prolongs survival of mice in an in vivo AML model.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. (Blood. 2016;127(9):1173-1182 Introduction Myb, the protein encoded by the MYB proto-oncogene, is now recognized as an attractive therapeutic target for the treatment of leukemia and potentially for other human tumors.1 Myb was originally discovered as the cellular progenitor of the transforming v-Myb transduced by avian myeloblastosis virus. 2,3 Myb is expressed in the hematopoietic progenitor cells, where it acts as a transcription factor to control genes important for lineage determination, cell proliferation, and differentiation. 4,5 The analysis of Myb null and conditional knockout mice and of mice bearing hypomorphic Myb alleles has demonstrated that Myb is essential for most hematopoietic lineages. [6][7][8][9][10][11] Myb is also expressed in several nonhematopoietic tissues, 12 such as the colonic crypts, where it controls the proliferation and differentiation of the intestinal stem cells. 13 Recent work has shown that deregulated Myb plays critical roles in leukemias and other types of cancer. Recurrent translocations and duplications of the Myb locus occur in acute lymphoblastic leukemia of young children. [14][15][16] In addition, genomic rearrangements of Myb have been reported in acute myelomonocytic and basophilic leukemia. [17][18][19] Although such rearrangements are relatively rare, they indicate that aberrant Myb expression contributes to the development of leukemia. Importantly, it has now been realized that Myb also plays essential roles in leukemias caused by genetic lesions of other genes, such as leukemias driven by human acute myeloid leukemia (AML) oncogenes. [20][21][22][23][24][25][26] High expression of Myb is a common characteristic of these leukemias and is essential for maintenance of the leukemic cells. This was initially observed in studies with Myb ant...

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Myb is a key regulator of hematopoietic progenitor cell proliferation and differentiation and has emerged as a potential target for the treatment of acute leukemia. Using a myeloid cell line with a stably integrated Myb-inducible reporter gene as a screening tool we have previously identified Celastrol, a natural compound with anti-tumor activity, as a potent Myb inhib-itor that disrupts the interaction of Myb with the co-activator p300. We showed that Celastrol inhibits the proliferation of acute myeloid leukemia (AML) cells and prolongs the survival of mice in an in vivo model of AML, demonstrating that targeting Myb with a small-molecule inhibitor is feasible and might have potential as a therapeutic approach against AML. Recently we became aware that the reporter system used for Myb inhibitor screening also responds to inhibition of C/EBPβ, a transcription factor known to cooperate with Myb in mye-loid cells. By re-investigating the inhibitory potential of Celastrol we have found that Celastrol also strongly inhibits the activity of C/EBPβ by disrupting its interaction with the Taz2 domain of p300. Together with previous studies our work reveals that Celastrol independently targets Myb and C/EBPβ by disrupting the interaction of both transcription factors with p300.

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