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

Abstract

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/EBPβ. C/EBPβ-inhibitory STLs affect the expression of a large number of MYB-regulated genes, suggesting that the cooperation of MYB and C/EBPβ 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/EBPβ, and co-activator p300, and is down-regulated by C/EBPβ-inhibitory STLs, exemplifying that they target the activity of composite MYB-C/EBPβ-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/EBPβ-inhibitory STLs. Overall, our data identify C/EBPβ as a pro-leukemogenic factor in AML and suggest that targeting of C/EBPβ may have therapeutic potential against AML.

Bcr-TMP, a Novel Nanomolar-Active Compound That Exhibits Both MYB- and Microtubule-Inhibitory Activity

“…We have previously observed that small-molecule mediated inhibition of transcription factor C/EBPβ exerted similar effects on MYB-regulated genes and caused comparable biological effects in AML cells as inhibition of MYB itself [ 37 , 42 , 45 ]. This led us to propose that MYB acts in concert with C/EBPβ and p300 to form a “transcriptional module” that controls the expression of genes involved in maintaining AML cells in an undifferentiated state [ 45 ]. Support for this idea came from the analysis of the GFI1 gene, whose expression is required for the maintenance of hematopoietic stem and progenitor cells [ 54 , 55 , 56 , 57 ].…”

“…Support for this idea came from the analysis of the GFI1 gene, whose expression is required for the maintenance of hematopoietic stem and progenitor cells [ 54 , 55 , 56 , 57 ]. We showed that GFI1 expression is regulated in a direct manner via MYB and C/EBP binding sites in the GFI1 promoter through the cooperation of MYB, C/EBPβ, and p300 [ 45 ]. To examine if Bcr-TMP disrupts the function of this module in AML cells we compared the effect of Bcr-TMP on HL60-control cells and HL60 cells expressing ectopic C/EBPβ.…”

“…We have previously observed that compounds that inhibit the activity of C/EBPβ, but not of MYB, also down-regulate the expression of many MYB target genes and exert similar biological effects in AML cells as MYB inhibitors [ 45 ]. Therefore, we have proposed that MYB, C/EBPβ, and p300 constitute a transcriptional module as an essential part of the oncogenic transcriptional program that is responsible for the maintenance of AML cells in an undifferentiated state [ 45 , 64 ].…”

“…We have previously observed that compounds that inhibit the activity of C/EBPβ, but not of MYB, also down-regulate the expression of many MYB target genes and exert similar biological effects in AML cells as MYB inhibitors [ 45 ]. Therefore, we have proposed that MYB, C/EBPβ, and p300 constitute a transcriptional module as an essential part of the oncogenic transcriptional program that is responsible for the maintenance of AML cells in an undifferentiated state [ 45 , 64 ]. This is strongly supported by genome-wide ChIP studies showing that MYB, C/EBPβ, and p300 (together with other myeloid transcription factors) co-localize at many genomic sites in AML cells which correspond to cis-acting transcriptional control regions of genes, such as MYC and GFI1 , which are expressed in these cells [ 65 ].…”

“…All cell lines were originally obtained from ATCC and are free of mycoplasma contamination. HL60 cells expressing a C-terminally truncated MYB (MYB-CT3) and control HL60 cells were generated by lentiviral infection as described previously [ 43 , 45 ]. AML cells were grown in RPMI1640 medium supplemented with 10% FCS.…”