The most frequent chromosomal translocations in pediatric acute myeloid leukemia affect the 11q23 locus and give rise to mixed lineage leukemia (MLL) fusion genes, MLL-AF9 being the most prevalent. The MLL-AF9 fusion gene has been shown to induce leukemia in both mouse and human models. In this study, we demonstrate that leukemogenic activity of MLL-AF9 requires RUVBL2 (RuvB-like 2), an AAA þ ATPase family member that functions in a wide range of cellular processes, including chromatin remodeling and transcriptional regulation. Expression of RUVBL2 was dependent on MLL-AF9, as it increased upon immortalization of human cord blood-derived hematopoietic progenitor cells with the fusion gene and decreased following loss of fusion gene expression in conditionally immortalized mouse cells. Short hairpin RNA-mediated silencing experiments demonstrated that both the immortalized human cells and the MLL-AF9-expressing human leukemia cell line THP-1 required RUVBL2 expression for proliferation and survival. Furthermore, inhibition of RUVBL2 expression in THP-1 cells led to reduced telomerase activity and clonogenic potential. These data were confirmed with a dominant-negative Walker B-mutated RUVBL2 construct. Taken together, these data suggest the possibility of targeting RUVBL2 as a potential therapeutic strategy for MLL-AF9-associated leukemia.

The Wnt signalling pathway, one of the core de-regulated pathways in chronic lymphocytic leukaemia (CLL), is activated in only a subset of patients through somatic mutations. Here we describe alternative, microenvironment-dependent mechanisms of Wnt activation in malignant B cells. We show that tumour cells specifically induce Notch2 activity in mesenchymal stromal cells (MSCs) required for the transcription of the complement factor C1q. MSC-derived C1q in turn inhibits Gsk3-β mediated degradation of β-catenin in CLL cells. Additionally, stromal Notch2 activity regulates N-cadherin expression in CLL cells, which interacts with and further stabilises β-catenin. Together, these stroma Notch2-dependent mechanisms induce strong activation of canonical Wnt signalling in CLL cells. Pharmacological inhibition of the Wnt pathway impairs microenvironment-mediated survival of tumour cells. Similarly, inhibition of Notch signalling diminishes survival of stroma-protected CLL cells in vitro and disease engraftment in vivo. Notch2 activation in the microenvironment is a pre-requisite for the activation of canonical Wnt signalling in tumour cells.

Key Points• STAT3 activity is necessary for TEL-AML1 leukemia maintenance.• TEL-AML1 induces STAT3 activation via RAC1 and leading to induction of MYC expression.The t(12;21)(p13;q22) translocation is the most common chromosomal abnormality in pediatric leukemia. Although this rearrangement involves 2 well-characterized transcription factors, TEL and AML1, the molecular pathways affected by the result of the translocation remain largely unknown. Also in light of recent studies showing genetic and functional heterogeneities in cells responsible for cancer clone maintenance and propagation, targeting a single common deregulated pathway may be critical for the success of novel therapies. Here we describe a novel signaling pathway that is essential for oncogenic addiction in TEL-AML1 leukemia. Our data indicate a direct role for TEL-AML1, via increasing the activity of RAC1, in regulating the phosphorylation of signal transducer and activator of transcription 3 (STAT3), which results in transcriptional induction of MYC. We demonstrate that human leukemic cell lines carrying this translocation are highly sensitive to treatment with S3I-201, a specific STAT3 inhibitor, and, more interestingly, that primary human leukemic samples are also responsive to the drug in the same concentration range. Thus, STAT3 inhibition represents a promising possible therapeutic strategy for the treatment of TEL-AML1 leukemia. (Blood. 2013;122(4):542-549)

Overcoming drug resistance remains a key challenge to cure patients with acute and chronic B cell malignancies. Here, we describe a stromal cell–autonomous signaling pathway, which contributes to drug resistance of malignant B cells. We show that protein kinase C (PKC)–β–dependent signals from bone marrow–derived stromal cells markedly decrease the efficacy of cytotoxic therapies. Conversely, small-molecule PKC-β inhibitors antagonize prosurvival signals from stromal cells and sensitize tumor cells to targeted and nontargeted chemotherapy, resulting in enhanced cytotoxicity and prolonged survival in vivo. Mechanistically, stromal PKC-β controls the expression of adhesion and matrix proteins, required for activation of phosphoinositide 3-kinases (PI3Ks) and the extracellular signal–regulated kinase (ERK)–mediated stabilization of B cell lymphoma–extra large (BCL-XL) in tumor cells. Central to the stroma-mediated drug resistance is the PKC-β–dependent activation of transcription factor EB, regulating lysosome biogenesis and plasma membrane integrity. Stroma-directed therapies, enabled by direct inhibition of PKC-β, enhance the effectiveness of many antileukemic therapies.