Supraphysiologic levels of the AML1-ETO isoform AE9a are essential for transformation.

Chromosomal translocation 8;21 is found in 40% of the FAB M2 subtype of acute myeloid leukemia (AML). The resultant in-frame fusion protein AML1-ETO (AE) acts as an initiating oncogene for leukemia development. AE immortalizes human CD34(+) cord blood cells in long-term culture. We assessed the transforming properties of the alternatively spliced AE isoform AE9a (or alternative splicing at exon 9), which is fully transforming in a murine retroviral model, in human cord blood cells. Full activity was realized only upon increased fusion protein expression. This effect was recapitulated in the AE9a murine AML model. Cotransduction of AE and AE9a resulted in a strong selective pressure for AE-expressing cells. In the context of AE, AE9a did not show selection for increased expression, affirming observations of human t(8;21) patient samples where full-length AE is the dominant protein detected. Mechanistically, AE9a showed defective transcriptional regulation of AE target genes that was partially corrected at high expression. Together, these results bring an additional perspective to our understanding of AE function and highlight the contribution of oncogene expression level in t(8;21) experimental models.

MEIS1 regulates an HLF-oxidative stress axis in MLL-fusion gene leukemia.

Leukemias with MLL translocations are often found in infants and are associated with poor outcomes. The pathogenesis of MLL-fusion leukemias has been linked to upregulation of HOX/MEIS1 genes. The functions of the Hox/Meis1 complex in leukemia, however, remain elusive. Here, we used inducible Meis1-knockout mice coupled with MLL-AF9 knockin mice to decipher the mechanistic role of Meis1 in established MLL leukemia. We demonstrate that Meis1 is essential for maintenance of established leukemia. In addition, in both the murine model and human leukemia cells, we found that Meis1 loss led to increased oxidative stress, oxygen flux, and apoptosis. Gene expression and chromatin immunoprecipitation studies revealed hepatic leukemia factor (HLF) as a target gene of Meis1. Hypoxia or HLF expression reversed the oxidative stress, rescuing leukemia development in Meis1-deficient cells. Thus, the leukemia-promoting properties of Meis1 are at least partly mediated by a low-oxidative state, aided by HLF. These results suggest that stimulants of oxidative metabolism could have therapeutic potential in leukemia treatment.

Transcription factor RUNX1 promotes survival of acute myeloid leukemia cells.

RUNX1 is generally considered a tumor suppressor in myeloid neoplasms. Inactivating RUNX1 mutations have frequently been found in patients with myelodysplastic syndrome (MDS) and cytogenetically normal acute myeloid leukemia (AML). However, no somatic RUNX1 alteration was found in AMLs with leukemogenic fusion proteins, such as core-binding factor (CBF) leukemia and MLL fusion leukemia, raising the possibility that RUNX1 could actually promote the growth of these leukemia cells. Using normal human cord blood cells and those expressing leukemogenic fusion proteins, we discovered a dual role of RUNX1 in myeloid leukemogenesis. RUNX1 overexpression inhibited the growth of normal cord blood cells by inducing myeloid differentiation, whereas a certain level of RUNX1 activity was required for the growth of AML1-ETO and MLL-AF9 cells. Using a mouse genetic model, we also showed that the combined loss of Runx1/Cbfb inhibited leukemia development induced by MLL-AF9. RUNX2 could compensate for the loss of RUNX1. The survival effect of RUNX1 was mediated by BCL2 in MLL fusion leukemia. Our study unveiled an unexpected prosurvival role for RUNX1 in myeloid leukemogenesis. Inhibiting RUNX1 activity rather than enhancing it could be a promising therapeutic strategy for AMLs with leukemogenic fusion proteins.

The thrombopoietin/MPL/Bcl-xL pathway is essential for survival and self-renewal in human preleukemia induced by AML1-ETO.

AML1-ETO (AE) is a fusion product of translocation (8;21) that accounts for 40% of M2 type acute myeloid leukemia (AML). In addition to its role in promoting preleukemic hematopoietic cell self-renewal, AE represses DNA repair genes, which leads to DNA damage and increased mutation frequency. Although this latter function may promote leukemogenesis, concurrent p53 activation also leads to an increased baseline apoptotic rate. It is unclear how AE expression is able to counterbalance this intrinsic apoptotic conditioning by p53 to promote survival and self-renewal. In this report, we show that Bcl-xL is up-regulated in AE cells and plays an essential role in their survival and self-renewal. Further investigation revealed that Bcl-xL expression is regulated by thrombopoietin (THPO)/MPL-signaling induced by AE expression. THPO/MPL-signaling also controls cell cycle reentry and mediates AE-induced self-renewal. Analysis of primary AML patient samples revealed a correlation between MPL and Bcl-xL expression specifically in t(8;21) blasts. Taken together, we propose that survival signaling through Bcl-xL is a critical and intrinsic component of a broader self-renewal signaling pathway downstream of AML1-ETO-induced MPL.

Core binding factor at the crossroads: determining the fate of the HSC.

Hematopoietic development requires coordinated actions from a variety of transcription factors. The core binding factor (CBF), consisting of a Runx protein and the CBFbeta protein, is a transcription factor complex that is essential for emergence of the hematopoietic stem cell (HSC) from an endothelial cell stage. The hematopoietic defects observed in either Runx1 or CBFbeta knockout mice underscore the necessity of this complex for definitive hematopoiesis. Despite the requirement for CBF in establishing definitive hematopoiesis, Runx1 loss has minimal impact on maintaining the HSC state postnatally, while CBFbeta may continue to be essential. Lineage commitment, on the other hand, is significantly affected upon CBF loss in the adult, indicating a primary role for this complex in modulating differentiation. Given the impact of normal CBF function in the hematopoietic system, the severe consequences of disrupting CBF activity, either through point mutations or generation of fusion genes, are obvious. The physiologic role of CBF in differentiation is subverted to an active process of self-renewal maintenance by the genetic aberrations, through several possible mechanisms, contributing to the development of hematopoietic malignancies including myelodysplastic syndrome and leukemia. The major impact of CBF on the hematopoietic system in both development and disease highlights the need for understanding the intricate functions of this complex and reiterate the necessity of continued efforts to identify potential points of therapeutic intervention for CBF-related diseases.

Targeting the BAF57 SWI/SNF subunit in prostate cancer: a novel platform to control androgen receptor activity.

The androgen receptor (AR) is critical for disseminated prostate cancer proliferation and survival. AR activity is targeted either through prevention of ligand synthesis or through the use of antagonists that bind the COOH-terminal ligand-binding domain. Although initially effective, treatment fails due to restored AR activity in the presence of therapeutics. Thus, new means must be developed to target AR activity. The SWI/SNF chromatin remodeling complex is critical for AR transcriptional activity, and the BAF57 SWI/SNF subunit facilitates direct interaction with the receptor. Although selected SWI/SNF subunit expression is reduced in prostate cancer, we show that BAF57 is retained in human disease and is elevated in a subset of tumors. Functional analyses showed that BAF57 contributes uniquely to androgen-mediated stimulation of transcription without compromising the effectiveness of AR antagonists. Subsequent studies revealed that BAF57 is recruited to the AR DNA-binding domain/hinge region, which occurs concomitant with receptor activation. These data provided the basis for a novel inhibitor derived from BAF57 [BAF57 inhibitory peptide (BIPep)], which blocked AR residence on chromatin and resultant AR-dependent gene activation. Importantly, BIPep expression was sufficient to inhibit androgen-dependent prostate cancer cell proliferation in AR-positive cells. In summary, these data identify blockade of AR-BAF57 interaction as a novel means to target agonist-induced AR function in prostate cancer, and provide the first evidence that abrogation of SWI/SNF function can be developed as a point of therapeutic intervention in prostate cancer.

BAF57 governs androgen receptor action and androgen-dependent proliferation through SWI/SNF.

Androgen receptor (AR) activity is required for prostate cancer development and progression. Thus, there is a major impetus to understand the regulation of AR action. We and others have previously shown that AR transactivation potential is dependent on the presence of an active SWI/SNF chromatin remodeling complex. However, the mechanisms underlying SWI/SNF regulation of the AR remained unsolved. We show here that the BAF57 subunit, an accessory component of the remodeling complex, is a critical regulator of AR function. We show that BAF57 is expressed in the luminal epithelia of the prostate and is required for AR-dependent transactivation in prostatic adenocarcinoma cells. Our data reveal that BAF57 can directly bind to the AR and is recruited to endogenous AR targets upon ligand activation. Loss of BAF57 or inhibition of BAF57 function severely compromised AR activity, as observed with both exogenous and endogenous AR targets. Rescue of BAF57 function restored AR activity, thus demonstrating a specific requirement of BAF57 for AR activity. This action of BAF57 proved to be dependent on SWI/SNF ATPase function. BAF57 has previously been implicated in nuclear receptor coactivator function, and we show that, although BAF57 facilitated coactivator activity, only a selected subset required BAF57 for coactivator function. Lastly, we demonstrate that both BAF57 and BRM are required for the proliferation of AR-dependent prostatic adenocarcinoma cells. In summary, these findings identify BAF57 as a critical modulator of the AR that is capable of altering AR activity, coactivator function, and AR-dependent proliferation.

Differential requirement of SWI/SNF for androgen receptor activity.

The androgen receptor (AR) is a ligand-dependent transcription factor whose activity is required for prostate cancer proliferation. Because ablation of AR activity is a critical goal of prostate cancer therapy, much emphasis has been placed on understanding the accessory proteins that regulate AR function in the prostate. Several co-activators have been shown to be required for full AR activity, including histone acetyl-transferases and TRAP/mediator complexes. SWI/SNF comprises a family of large, multisubunit complexes present in the cell, which contain one of two core ATPases required for nucleosome re-positioning, BRG1 or hBRM. We investigated the specific requirement of the SWI/SNF core ATPases for AR function. Using cells deficient in both BRG1 and hBRM, we show that activation of one AR target promoter, prostate-specific antigen (PSA), requires SWI/SNF chromatin remodeling for activity. A second AR target promoter, probasin, maintained a low level of activation in the absence of SWI/SNF. AR stimulation on the probasin core promoter could be partially induced with BRG1, but hBRM strongly stimulated AR activity. The PSA promoter was only induced by the restoration of hBRM. In contrast, ligand-dependent activation of the estrogen receptor was equally stimulated by BRG1 or hBRM. We demonstrate that the addition of a known enhancer region to the core PSA promoter bypasses the requirement for SWI/SNF on the PSA promoter, indicating that elements upstream of specific proximal promoters can impact the influence of the SWI/SNF complex on target gene activation. Addition of the enhancer to the probasin core promoter failed to impact the SWI/SNF requirement. In summary, SWI/SNF function potently regulates core AR target gene promoter activation, with a preference for hBRM-containing complexes. These studies highlight a role for the enhancer in altering the impact of SWI/SNF action and suggest a disparity in AR target genes for SWI/SNF requirement.