GSK-3 promotes conditional association of CREB and its coactivators with MEIS1 to facilitate HOX-mediated transcription and oncogenesis

A cAMP and CREB-mediated feed-forward mechanism regulates GSK3β in polycystic kidney disease

Glycogen synthase kinase 3β (GSK3β), a serine/threonine protein kinase, is commonly known to be regulated at the level of its activity. However, in some diseases including polycystic kidney disease (PKD), GSK3β expression is increased and plays a pathophysiological role. The current studies aimed to determine the mechanism for the increased GSK3β expression in PKD and its significance to disease progression. In mouse models of PKD, increases in renal GSK3β corresponded with increases in renal cAMP levels and disease progression. In vivo and in vitro studies revealed that GSK3β is a cAMP-responsive gene, and elevated cAMP levels, as seen in PKD, can increase GSK3β expression. In normal mice, vasopressin signaling induced by water deprivation increased GSK3β expression, which decreased following rehydration. Examination of the GSK3β promoter revealed five potential binding sites for the transcription factor, cAMP response element binding protein (CREB). CREB was found to bind to GSK3β promoter and essential for cAMP-mediated regulation of GSK3β. Importantly, this regulation was demonstrated to be part of a feed-forward loop in which cAMP through CREB regulates GSK3β expression, and GSK3β in turn positively regulates cAMP generation. GSK3β or CREB inhibition reduced transepithelial fluid secretion and cyst expansion in vitro Thus, disruption at any point of this destructive cycle may be therapeutically useful to reduce cyst expansion and preserve renal function in PKD.

miR-22 has a potent anti-tumour role with therapeutic potential in acute myeloid leukaemia

MicroRNAs are subject to precise regulation and have key roles in tumorigenesis. In contrast to the oncogenic role of miR-22 reported in myelodysplastic syndrome (MDS) and breast cancer, here we show that miR-22 is an essential anti-tumour gatekeeper in de novo acute myeloid leukaemia (AML) where it is significantly downregulated. Forced expression of miR-22 significantly suppresses leukaemic cell viability and growth in vitro, and substantially inhibits leukaemia development and maintenance in vivo. Mechanistically, miR-22 targets multiple oncogenes, including CRTC1, FLT3 and MYCBP, and thus represses the CREB and MYC pathways. The downregulation of miR-22 in AML is caused by TET1/GFI1/EZH2/SIN3A-mediated epigenetic repression and/or DNA copy-number loss. Furthermore, nanoparticles carrying miR-22 oligos significantly inhibit leukaemia progression in vivo. Together, our study uncovers a TET1/GFI1/EZH2/SIN3A/miR-22/CREB-MYC signalling circuit and thereby provides insights into epigenetic/genetic mechanisms underlying the pathogenesis of AML, and also highlights the clinical potential of miR-22-based AML therapy.

Haploinsufficiency for NR3C1, the gene encoding the glucocorticoid receptor, in blastic plasmacytoid dendritic cell neoplasms

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare and highly aggressive leukemia for which knowledge on disease mechanisms and effective therapies are currently lacking. Only a handful of recurring genetic mutations have been identified and none is specific to BPDCN. In this study, through molecular cloning in an index case that presented a balanced t(3;5)(q21;q31) and molecular cytogenetic analyses in a further 46 cases, we identify monoallelic deletion of NR3C1 (5q31), encoding the glucocorticoid receptor (GCR), in 13 of 47 (28%) BPDCN patients. Targeted deep sequencing in 36 BPDCN cases, including 10 with NR3C1 deletion, did not reveal NR3C1 point mutations or indels. Haploinsufficiency for NR3C1 defined a subset of BPDCN with lowered GCR expression and extremely poor overall survival (P = .0006). Consistent with a role for GCR in tumor suppression, functional analyses coupled with gene expression profiling identified corticoresistance and loss-of-EZH2 function as major downstream consequences of NR3C1 deletion in BPDCN. Subsequently, more detailed analyses of the t(3;5)(q21;q31) revealed fusion of NR3C1 to a long noncoding RNA (lncRNA) gene (lincRNA-3q) that encodes a novel, nuclear, noncoding RNA involved in the regulation of leukemia stem cell programs and G1/S transition, via E2F. Overexpression of lincRNA-3q was a consistent feature of malignant cells and could be abrogated by bromodomain and extraterminal domain (BET) protein inhibition. Taken together, this work points to NR3C1 as a haploinsufficient tumor suppressor in a subset of BPDCN and identifies BET inhibition, acting at least partially via lncRNA blockade, as a novel treatment option in BPDCN.

GSK-3α Is a Novel Target of CREB and CREB-GSK-3α Signaling Participates in Cell Viability in Lung Cancer

Overexpression or activation of cyclic AMP-response element-binding protein (CREB) has been known to be involved in several human malignancies, including lung cancer. Genes regulated by CREB have been reported to suppress apoptosis, induce cell proliferation, inflammation, and tumor metastasis. However, the critical target genes of CREB in lung cancer have not been well understood. Here, we identified GSK-3α as one of the CREB target genes which is critical for the viability of lung cancer cells. The CREB knockdown significantly reduced the expression of GSK-3α and the direct binding of CREB on the promoter of GSK3A was identified. Kaplan-Meier analysis with a public database showed a prognostic significance of aberrant GSK-3α expression in lung cancer. Inhibition of GSK-3α suppressed cell viability, colony formation, and tumor growth. For the first time, we demonstrated that GSK-3α is regulated by CREB in lung cancer and is required for the cell viability. These findings implicate CREB-GSK-3α axis as a novel therapeutic target for lung cancer treatment.

MEIS1-mediated transactivation of synaptotagmin-like 1 promotes CXCL12/CXCR4 signaling and leukemogenesis

The TALE-class homeoprotein MEIS1 specifically collaborates with HOXA9 to drive myeloid leukemogenesis. Although MEIS1 alone has only a moderate effect on cell proliferation in vitro, it is essential for the development of HOXA9-induced leukemia in vivo. Here, using murine models of leukemogenesis, we have shown that MEIS1 promotes leukemic cell homing and engraftment in bone marrow and enhances cell-cell interactions and cytokine-mediated cell migration. We analyzed global DNA binding of MEIS1 in leukemic cells as well as gene expression alterations in MEIS1-deficent cells and identified synaptotagmin-like 1 (Sytl1, also known as Slp1) as the MEIS1 target gene that cooperates with Hoxa9 in leukemogenesis. Replacement of SYTL1 in MEIS1-deficent cells restored both cell migration and engraftment. Further analysis revealed that SYTL1 promotes cell migration via activation of the CXCL12/CXCR4 axis, as SYTL1 determines intracellular trafficking of CXCR4. Together, our results reveal that MEIS1, through induction of SYTL1, promotes leukemogenesis and supports leukemic cell homing and engraftment, facilitating interactions between leukemic cells and bone marrow stroma.

Synthetic lethal targeting of oncogenic transcription factors in acute leukemia by PARP inhibitors

Acute myeloid leukemia (AML) is mostly driven by oncogenic transcription factors, which have been classically viewed as intractable targets using small-molecule inhibitor approaches. Here we demonstrate that AML driven by repressive transcription factors, including AML1-ETO (encoded by the fusion oncogene RUNX1-RUNX1T1) and PML-RARα fusion oncoproteins (encoded by PML-RARA) are extremely sensitive to poly (ADP-ribose) polymerase (PARP) inhibition, in part owing to their suppressed expression of key homologous recombination (HR)-associated genes and their compromised DNA-damage response (DDR). In contrast, leukemia driven by mixed-lineage leukemia (MLL, encoded by KMT2A) fusions with dominant transactivation ability is proficient in DDR and insensitive to PARP inhibition. Intriguingly, genetic or pharmacological inhibition of an MLL downstream target, HOXA9, which activates expression of various HR-associated genes, impairs DDR and sensitizes MLL leukemia to PARP inhibitors (PARPis). Conversely, HOXA9 overexpression confers PARPi resistance to AML1-ETO and PML-RARα transformed cells. Together, these studies describe a potential utility of PARPi-induced synthetic lethality for leukemia treatment and reveal a novel molecular mechanism governing PARPi sensitivity in AML.

Phosphorylation of GSK3α/β correlates with activation of AKT and is prognostic for poor overall survival in acute myeloid leukemia patients

Background

Acute myeloid leukemia (AML) patients with highly active AKT tend to do poorly. Cell cycle arrest and apoptosis are tightly regulated by AKT via phosphorylation of GSK3α and β isoforms which inactivates these kinases. In the current study we examine the prognostic role of AKT mediated GSK3 phosphorylation in AML.

Methods

We analyzed GSK3α/β phosphorylation by reverse phase protein analysis (RPPA) in a cohort of 511 acute myeloid leukemia (AML) patients. Levels of phosphorylated GSK3 were correlated with patient characteristics including survival and with expression of other proteins important in AML cell survival.

Results

High levels of p-GSK3α/β correlated with adverse overall survival and a lower incidence of complete remission duration in patients with intermediate cytogenetics, but not in those with unfavorable cytogenetics. Intermediate cytogenetic patients with FLT3 mutation also fared better respectively when p-GSK3α/β levels were lower. Phosphorylated GSK3α/β expression was compared and contrasted with that of 229 related cell cycle arrest and/or apoptosis proteins. Consistent with p-GSK3α/β as an indicator of AKT activation, RPPA revealed that p-GSK3α/β positively correlated with phosphorylation of AKT, BAD, and P70S6K, and negatively correlated with β-catenin and FOXO3A. PKCδ also positively correlated with p-GSK3α/β expression, suggesting crosstalk between the AKT and PKC signaling pathways in AML cells.

Conclusions

These findings suggest that AKT-mediated phosphorylation of GSK3α/β may be beneficial to AML cell survival, and hence detrimental to the overall survival of AML patients. Intrinsically, p-GSK3α/β may serve as an important adverse prognostic factor for a subset of AML patients.

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Phospho-GSK3 is prognostic for poor survival in a subset of AML patients.

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Phospho-GSK3 is a biomarker for active AKT in AML.

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AKT is a PKCδ kinase in AML cells.

Lithium chloride antileukemic activity in acute promyelocytic leukemia is GSK-3 and MEK/ERK dependent

We recently identified that the MEK/ERK1/2 pathway synergized with retinoic acid (RA) to restore both transcriptional activity and RA-induced differentiation in RA-resistant acute promyelocytic leukemia (APL) cells. To target the MEK/ERK pathway, we identified glycogen synthase kinase-3β (GSK-3β) inhibitors including lithium chloride (LiCl) as activators of this pathway in APL cells. Using NB4 (RA-sensitive) and UF-1 (RA-resistant) APL cell lines, we observed that LiCl as well as synthetic GSK-3β inhibitors decreased proliferation, induced apoptosis and restored, in RA-resistant cells, the expression of RA target genes and the RA-induced differentiation. Inhibition of the MEK/ERK1/2 pathway abolished these effects. These results were corroborated in primary APL patient cells and translated in vivo using an APL preclinical mouse model in which LiCl given alone was as efficient as RA in increasing survival of leukemic mice compared with untreated mice. When LiCl was combined with RA, we observed a significant survival advantage compared with mice treated by RA alone. In this work, we demonstrate that LiCl, a well-tolerated agent in humans, has antileukemic activity in APL and that it has the potential to restore RA-induced transcriptional activation and differentiation in RA-resistant APL cells in an MEK/ERK-dependent manner.

Role of HOXA9 in leukemia: dysregulation, cofactors and essential targets

HOXA9 is a homeodomain-containing transcription factor that has an important role in hematopoietic stem cell expansion and is commonly deregulated in acute leukemias. A variety of upstream genetic alterations in acute myeloid leukemia lead to overexpression of HOXA9, which is a strong predictor of poor prognosis. In many cases, HOXA9 has been shown to be necessary for maintaining leukemic transformation; however, the molecular mechanisms through which it promotes leukemogenesis remain elusive. Recent work has established that HOXA9 regulates downstream gene expression through binding at promoter distal enhancers along with a subset of cell-specific cofactor and collaborator proteins. Increasing efforts are being made to identify both the critical cofactors and target genes required for maintaining transformation in HOXA9-overexpressing leukemias. With continued advances in understanding HOXA9-mediated transformation, there is a wealth of opportunity for developing novel therapeutics that would be applicable for greater than 50% of AML with overexpression of HOXA9.

Enriched transcription factor signatures in triple negative breast cancer indicates possible targeted therapies with existing drugs

PURPOSE

Triple negative (TN) breast cancers which lack expression of the estrogen (ER), progesterone (PR), and human epidermal growth factor 2 (HER2) receptors convey a poor prognosis due in part to a lack of targeted therapies.

METHODS

To identify viable targets for the treatment of TN disease, we have conducted a gene set enrichment analysis (GSEA) on seven different breast cancer whole genome gene expression cohorts comparing TN vs. ER + HER2 - to identify consistently enriched genes that share a common promoter motif. The seven cohorts were profiled on three different genome expression platforms (Affymetrix, Illumina and RNAseq) consisting in total of 2088 samples with IHC metadata.

RESULTS

GSEA identified enriched gene expression patterns in TN samples that share common promoter motifs associated with SOX9, E2F1, HIF1A, HMGA1, MYC BACH2, CEBPB, and GCNF/NR6A1. Unexpectedly, NR6A1 an orphan nuclear receptor normally expressed in germ cells of gonads is highly expressed in TN and ER + HER2 - samples making it an ideal drug target.

CONCLUSION

With the increasing number of large sample size breast cancer cohorts, an exploratory analysis of genes that are consistently enriched in TN sharing common promoter motifs allows for the identification of possible therapeutic targets with extensive validation in patient derived data sets.

Activating the Wnt/β-Catenin Pathway for the Treatment of Melanoma--Application of LY2090314, a Novel Selective Inhibitor of Glycogen Synthase Kinase-3

It has previously been observed that a loss of β-catenin expression occurs with melanoma progression and that nuclear β-catenin levels are inversely proportional to cellular proliferation, suggesting that activation of the Wnt/β-catenin pathway may provide benefit for melanoma patients. In order to further probe this concept we tested LY2090314, a potent and selective small-molecule inhibitor with activity against GSK3α and GSK3β isoforms. In a panel of melanoma cell lines, nM concentrations of LY2090314 stimulated TCF/LEF TOPFlash reporter activity, stabilized β-catenin and elevated the expression of Axin2, a Wnt responsive gene and marker of pathway activation. Cytotoxicity assays revealed that melanoma cell lines are very sensitive to LY2090314 in vitro (IC50 ~10 nM after 72hr of treatment) in contrast to other solid tumor cell lines (IC50 >10 uM) as evidenced by caspase activation and PARP cleavage. Cell lines harboring mutant B-RAF or N-RAS were equally sensitive to LY2090314 as were those with acquired resistance to the BRAF inhibitor Vemurafenib. shRNA studies demonstrated that β-catenin stabilization is required for apoptosis following treatment with the GSK3 inhibitor since the sensitivity of melanoma cell lines to LY290314 could be overcome by β-catenin knockdown. We further demonstrate that in vivo, LY2090314 elevates Axin2 gene expression after a single dose and produces tumor growth delay in A375 melanoma xenografts with repeat dosing. The activity of LY2090314 in preclinical models suggests that the role of Wnt activators for the treatment of melanoma should be further explored.

Glycogen synthase kinase-3 (Gsk-3) plays a fundamental role in maintaining DNA methylation at imprinted loci in mouse embryonic stem cells

A genome-wide analysis is given of DNA methylation in mouse embryonic stem cells in which both Gsk-3α and Gsk-3β have been genetically deleted. DNA methylation patterns are compared to those of wild-type cells. More than 75% of known imprinted loci have reduced DNA methylation in the Gsk-3–knockout cells.

Glycogen synthase kinase-3 (Gsk-3) is a key regulator of multiple signal transduction pathways. Recently we described a novel role for Gsk-3 in the regulation of DNA methylation at imprinted loci in mouse embryonic stem cells (ESCs), suggesting that epigenetic changes regulated by Gsk-3 are likely an unrecognized facet of Gsk-3 signaling. Here we extend our initial observation to the entire mouse genome by enriching for methylated DNA with the MethylMiner kit and performing next-generation sequencing (MBD-Seq) in wild-type and Gsk-3α^−/−;Gsk-3β^−/− ESCs. Consistent with our previous data, we found that 77% of known imprinted loci have reduced DNA methylation in Gsk-3-deficient ESCs. More specifically, we unambiguously identified changes in DNA methylation within regions that have been confirmed to function as imprinting control regions. In many cases, the reduced DNA methylation at imprinted loci in Gsk-3α^−/−;Gsk-3β^−/− ESCs was accompanied by changes in gene expression as well. Furthermore, many of the Gsk-3–dependent, differentially methylated regions (DMRs) are identical to the DMRs recently identified in uniparental ESCs. Our data demonstrate the importance of Gsk-3 activity in the maintenance of DNA methylation at a majority of the imprinted loci in ESCs and emphasize the importance of Gsk-3–mediated signal transduction in the epigenome.

Two decades of leukemia oncoprotein epistasis: the MLL1 paradigm for epigenetic deregulation in leukemia

MLL1, located on human chromosome 11, is disrupted in distinct recurrent chromosomal translocations in several leukemia subsets. Studying the MLL1 gene and its oncogenic variants has provided a paradigm for understanding cancer initiation and maintenance through aberrant epigenetic gene regulation. Here we review the historical development of model systems to recapitulate oncogenic MLL1-rearrangement (MLL-r) alleles encoding mixed-lineage leukemia fusion proteins (MLL-FPs) or internal gene rearrangement products. These largely mouse and human cell/xenograft systems have been generated and used to understand how MLL-r alleles affect diverse pathways to result in a highly penetrant, drug-resistant leukemia. The particular features of the animal models influenced the conclusions of mechanisms of transformation. We discuss significant downstream enablers, inhibitors, effectors, and collaborators of MLL-r leukemia, including molecules that directly interact with MLL-FPs and endogenous mixed-lineage leukemia protein, direct target genes of MLL-FPs, and other pathways that have proven to be influential in supporting or suppressing the leukemogenic activity of MLL-FPs. The use of animal models has been complemented with patient sample, genome-wide analyses to delineate the important genomic and epigenomic changes that occur in distinct subsets of MLL-r leukemia. Collectively, these studies have resulted in rapid progress toward developing new strategies for targeting MLL-r leukemia and general cell-biological principles that may broadly inform targeting aberrant epigenetic regulators in other cancers.

Glycogen synthase kinase-3 (GSK3): regulation, actions, and diseases

Glycogen synthase kinase-3 (GSK3) may be the busiest kinase in most cells, with over 100 known substrates to deal with. How does GSK3 maintain control to selectively phosphorylate each substrate, and why was it evolutionarily favorable for GSK3 to assume such a large responsibility? GSK3 must be particularly adaptable for incorporating new substrates into its repertoire, and we discuss the distinct properties of GSK3 that may contribute to its capacity to fulfill its roles in multiple signaling pathways. The mechanisms regulating GSK3 (predominantly post-translational modifications, substrate priming, cellular trafficking, protein complexes) have been reviewed previously, so here we focus on newly identified complexities in these mechanisms, how each of these regulatory mechanism contributes to the ability of GSK3 to select which substrates to phosphorylate, and how these mechanisms may have contributed to its adaptability as new substrates evolved. The current understanding of the mechanisms regulating GSK3 is reviewed, as are emerging topics in the actions of GSK3, particularly its interactions with receptors and receptor-coupled signal transduction events, and differential actions and regulation of the two GSK3 isoforms, GSK3α and GSK3β. Another remarkable characteristic of GSK3 is its involvement in many prevalent disorders, including psychiatric and neurological diseases, inflammatory diseases, cancer, and others. We address the feasibility of targeting GSK3 therapeutically, and provide an update of its involvement in the etiology and treatment of several disorders.

TALE factors poise promoters for activation by Hox proteins

Hox proteins form complexes with TALE cofactors from the Pbx and Prep/Meis families to control transcription, but it remains unclear how Hox:TALE complexes function. Examining a Hoxb1b:TALE complex that regulates zebrafish hoxb1a transcription, we find maternally deposited TALE proteins at the hoxb1a promoter already during blastula stages. These TALE factors recruit histone-modifying enzymes to promote an active chromatin profile at the hoxb1a promoter and also recruit RNA polymerase II (RNAPII) and P-TEFb. However, in the presence of TALE factors, RNAPII remains phosphorylated on serine 5 and hoxb1a transcription is inefficient. By gastrula stages, Hoxb1b binds together with TALE factors to the hoxb1a promoter. This triggers P-TEFb-mediated transitioning of RNAPII to the serine 2-phosphorylated form and efficient hoxb1a transcription. We conclude that TALE factors access promoters during early embryogenesis to poise them for activation but that Hox proteins are required to trigger efficient transcription.

GSK3-mediated MAF phosphorylation in multiple myeloma as a potential therapeutic target

Multiple myeloma (MM) is an incurable haematological malignancy characterised by the proliferation of mature antibody-secreting plasma B cells in the bone marrow. MM can arise from initiating translocations, of which the musculoaponeurotic fibrosarcoma (MAF) family is implicated in ∼5%. MMs bearing Maf translocations are of poor prognosis. These translocations are associated with elevated Maf expression, including c-MAF, MAFB and MAFA, and with t(14;16) and t(14;20) translocations, involving c-MAF and MAFB, respectively. c-MAF is also overexpressed in MM through MEK/ERK activation, bringing the number of MMs driven by the deregulation of a Maf gene close to 50%. Here we demonstrate that MAFB and c-MAF are phosphorylated by the Ser/Thr kinase GSK3 in human MM cell lines. We show that LiCl-induced GSK3 inhibition targets these phosphorylations and specifically decreases proliferation and colony formation of Maf-expressing MM cell lines. Interestingly, bortezomib induced stabilisation of Maf phosphorylation, an observation that could explain, at least partially, the low efficacy of bortezomib for patients carrying Maf translocations. Thus, GSK3 inhibition could represent a new therapeutic approach for these patients.

Insights in dynamic kinome reprogramming as a consequence of MEK inhibition in MLL-rearranged AML

Single kinase-targeted cancer therapies often failed prolonged responses because cancer cells bypass through alternative routes. In this study, high-throughput kinomic and proteomic approaches enabled to identify aberrant activity profiles in mixed lineage leukemia (MLL)-rearranged acute myeloid leukemia (AML) that defined druggable targets. This approach revealed impaired activity of proteins belonging to the mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) pathway. Pharmacological druggable MAPK pathway targets tested in primary MLL-rearranged AML included MAPKK1/2 (MEK), cyclic AMP-responsive element-binding protein (CREB) and MAPK8/9 (JNK). MEK inhibition showed to severely decrease MLL-rearranged AML cell survival without showing cytotoxicity in normal controls, whereas inhibition of CREB and JNK failed to exhibit MLL selectivity. Exploring the working mechanism of MEK inhibition, we assessed proteome activity in response to MEK inhibition in THP-1. MAPK1/3 (Erk) phosphorylation was instantly decreased in concurrence with a sustained Akt/mammalian target of rapamycin (mTOR) phosphorylation that enabled a subpopulation of cells to survive MEK inhibition. After exhaustion of MEK inhibition the AML cells recovered via increased activity of vascular endothelial growth factor receptor-2 (VEGFR-2) and Erk proteins to resume their proliferative state. Combined MEK and VEGFR-2 inhibition strengthened the reduction in MLL-rearranged AML cell survival by blocking the Akt/mTOR and MAPK pathways simultaneously. The generation of insights in cancerous altered activity profiles and alternative escape mechanisms upon targeted therapy allows the rational design of novel combination strategies.

Epigenetic roles of MLL oncoproteins are dependent on NF-κB

MLL fusion proteins in leukemia induce aberrant transcriptional elongation and associated chromatin perturbations; however, the upstream signaling pathways and activators that recruit or retain MLL oncoproteins at initiated promoters are unknown. Through functional and comparative genomic studies, we identified an essential role for NF-κB signaling in MLL leukemia. Suppression of NF-κB led to robust antileukemia effects that phenocopied loss of functional MLL oncoprotein or associated epigenetic cofactors. The NF-κB subunit RELA occupies promoter regions of crucial MLL target genes and sustains the MLL-dependent leukemia stem cell program. IKK/NF-κB signaling is required for wild-type and fusion MLL protein retention and maintenance of associated histone modifications, providing a molecular rationale for enhanced efficacy in therapeutic targeting of this pathway in MLL leukemias.

Hox regulation of transcription: more complex(es)

Hox genes encode transcription factors with important roles during embryogenesis and tissue differentiation. Genetic analyses initially demonstrated that interfering with Hox genes has profound effects on the specification of cell identity, suggesting that Hox proteins regulate very specific sets of target genes. However, subsequent biochemical analyses revealed that Hox proteins bind DNA with relatively low affinity and specificity. Furthermore, it became clear that a given Hox protein could activate or repress transcription, depending on the context. A resolution to these paradoxes presented itself with the discovery that Hox proteins do not function in isolation, but interact with other factors in complexes. The first such "cofactors" were members of the Extradenticle/Pbx and Homothorax/Meis/Prep families. However, the list of Hox-interacting proteins has continued to grow, suggesting that Hox complexes contain many more components than initially thought. Additionally, the activities of the various components and the exact mechanisms whereby they modulate the activity of the complex remain puzzling. Here, we review the various proteins known to participate in Hox complexes and discuss their likely functions. We also consider that Hox complexes of different compositions may have different activities and discuss mechanisms whereby Hox complexes may be switched between active and inactive states.

Mechanisms of mixed-lineage leukemia

Advances in our understanding of the genetic determinants of leukemia have translated to better treatment options and improved survival of patients with acute myeloid and acute lymphoid leukemia. However, some leukemias, such as those bearing 11q23 (MLL) translocations, result in aggressive diseases with a relatively poor prognosis, despite improved treatments such as allogeneic hematopoietic stem cell transplantation. This article will briefly review the functions and regulation of wild-type MLL during normal hematopoiesis, while focusing on recent advances in our understanding of the molecular mechanisms governing MLL leukemias. The transcriptional targets, cooperating signaling pathways and molecular machinery involved in MLL-associated leukemias will be discussed, as well as how these may be harnessed for more personalized treatment of this disease.

Targeting self-renewal pathways in myeloid malignancies

A fundamental property of hematopoietic stem cells (HSCs) is the ability to self-renew. This is a complex process involving multiple signal transduction cascades which control the fine balance between self-renewal and differentiation through transcriptional networks. Key activators/regulators of self-renewal include chemokines, cytokines and morphogens which are expressed in the bone marrow niche, either in a paracrine or autocrine fashion, and modulate stem cell behaviour. Increasing evidence suggests that the downstream signaling pathways induced by these ligands converge at multiple levels providing a degree of redundancy in steady state hematopoiesis. Here we will focus on how these pathways cross-talk to regulate HSC self-renewal highlighting potential therapeutic windows which could be targeted to prevent leukemic stem cell self-renewal in myeloid malignancies.

Recent progress toward epigenetic therapies: the example of mixed lineage leukemia

The importance of epigenetic gene regulatory mechanisms in normal and cancer development is increasingly evident. Genome-wide analyses have revealed the mutation, deletion, and dysregulated expression of chromatin-modifying enzymes in a number of cancers, including hematologic malignancies. Genome-wide studies of DNA methylation and histone modifications are beginning to reveal the landscape of cancer-specific chromatin patterns. In parallel, recent genetic loss-of-function studies in murine models are demonstrating functional involvement of chromatin-modifying enzymes in malignant cell proliferation and self-renewal. Paradoxically, the same chromatin modifiers can, depending on cancer type, be either hyperactive or inactivated. Increasingly, cross talk between epigenetic pathways is being identified. Leukemias carrying MLL rearrangements are quintessential cancers driven by dysregulated epigenetic mechanisms in which fusion proteins containing N-terminal sequences of MLL require few or perhaps no additional mutations to cause human leukemia. Here, we review how recent progress in the field of epigenetics opens potential mechanism-based therapeutic avenues.

Zebrafish hoxd4a acts upstream of meis1.1 to direct vasculogenesis, angiogenesis and hematopoiesis

Mice lacking the 4^th-group paralog Hoxd4 display malformations of the anterior vertebral column, but are viable and fertile. Here, we report that zebrafish embryos having decreased function of the orthologous hoxd4a gene manifest striking perturbations in vasculogenesis, angiogenesis and primitive and definitive hematopoiesis. These defects are preceded by reduced expression of the hemangioblast markers scl1, lmo2 and fli1 within the posterior lateral plate mesoderm (PLM) at 13 hours post fertilization (hpf). Epistasis analysis revealed that hoxd4a acts upstream of meis1.1 but downstream of cdx4 as early as the shield stage in ventral-most mesoderm fated to give rise to hemangioblasts, leading us to propose that loss of hoxd4a function disrupts hemangioblast specification. These findings place hoxd4a high in a genetic hierarchy directing hemangioblast formation downstream of cdx1/cdx4 and upstream of meis1.1. An additional consequence of impaired hoxd4a and meis1.1 expression is the deregulation of multiple Hox genes implicated in vasculogenesis and hematopoiesis which may further contribute to the defects described here. Our results add to evidence implicating key roles for Hox genes in their initial phase of expression early in gastrulation.

MicroRNA-34b promoter hypermethylation induces CREB overexpression and contributes to myeloid transformation

MicroRNA-34b down-regulation in acute myeloid leukemia was previously shown to induce CREB overexpression, thereby causing leukemia proliferation in vitro and in vivo. The role of microRNA-34b and CREB in patients with myeloid malignancies has never been evaluated. We examined microRNA-34b expression and the methylation status of its promoter in cells from patients diagnosed with myeloid malignancies. We used gene expression profiling to identify signatures of myeloid transformation. We established that microRNA-34b has suppressor ability and that CREB has oncogenic potential in primary bone marrow cell cultures and in vivo. MicroRNA-34b was found to be up-regulated in pediatric patients with juvenile myelomonocytic leukemia (n=17) and myelodysplastic syndromes (n=28), but was down-regulated in acute myeloid leukemia patients at diagnosis (n=112). Our results showed that hypermethylation of the microRNA-34b promoter occurred in 66% of cases of acute myeloid leukemia explaining the low microRNA-34b levels and CREB overexpression, whereas preleukemic myelodysplastic syndromes and juvenile myelomonocytic leukemia were not associated with hypermethylation or CREB overexpression. In paired samples taken from the same patients when they had myelodysplastic syndrome and again during the subsequent acute myeloid leukemia, we confirmed microRNA-34b promoter hypermethylation at leukemia onset, with 103 CREB target genes differentially expressed between the two disease stages. This subset of CREB targets was confirmed to associate with high-risk myelodysplastic syndromes in a separate cohort of patients (n=20). Seventy-eight of these 103 CREB targets were also differentially expressed between healthy samples (n=11) and de novo acute myeloid leukemia (n=72). Further, low microRNA-34b and high CREB expression levels induced aberrant myelopoiesis through CREB-dependent pathways in vitro and in vivo. In conclusion, we suggest that microRNA-34b controls CREB expression and contributes to myeloid transformation from both healthy bone marrow and myelodysplastic syndromes. We identified a subset of CREB target genes that represents a novel transcriptional network that may control myeloid transformation.

Frat2 mediates the oncogenic activation of Rac by MLL fusions

Mixed lineage leukemia (MLL) fusion genes arise from chromosomal translocations and induce acute myeloid leukemia through a mechanism involving transcriptional deregulation of differentiation and self-renewal programs. Progression of MLL-rearranged acute myeloid leukemia is associated with increased activation of Rac GTPases. Here, we demonstrate that MLL fusion oncogenes maintain leukemia-associated Rac activity by regulating Frat gene expression, specifically Frat2. Modulation of FRAT2 leads to concomitant changes in Rac activity, and transformation of Frat knockout hematopoietic progenitor cells by MLL fusions results in leukemias displaying reduced Rac activation and increased sensitivity to chemotherapeutic drugs. FRAT2 activates Rac through a signaling mechanism that requires glycogen synthase kinase 3 and DVL. Disruption of this pathway abrogates the leukemogenic activity of MLL fusions. This suggests a rationale for the paradoxical requirement of canonical Wnt signaling and glycogen synthase kinase 3 activity for MLL fusion oncogenicity and identifies novel therapeutic targets for this disease.

GSK3β and CREB3 gene expression profiling in benign and malignant salivary gland tumors

BACKGROUND

Salivary gland tumors (SGT) are rare lesions with uncertain histopathology. One of the major signaling pathways that participate in the development of several tumors is protein kinase A. In this pathway, glycogen synthase kinase β (GSK3β) and cAMP responsive element binding protein (CREB3) are two genes which are supposed to be down regulated in most human tumors. The expression level of the genes was evaluated in SGT to scrutinize their possible under expression in these tumors.

METHODS

Forty eight fresh tissue samples were obtained from patients with benign and malignant SGT, including pleomorphic adenoma, warthin's tumor, mucoepidermoid carcinoma (MEC), salivary duct carcinoma and carcinoma ex pleomorphic adenoma. Eight normal samples were used as controls. Quantitative real-time PCR was used to analyze the expression level of interest genes.

RESULTS

Data was analyzed by statistical methods. GSK3β was downregulate in all samples and all results were statistically significant (P<0.05). CREB3 did not show a significant decrease or increase in its mRNA expression, but the results were significant in MEC and salivary duct carcinoma.

CONCLUSION

GSK3β down regulation has been reported in many human tumors. This gene stimulates CREB3, inducing cell proliferation and oncogenesis. Our findings showed GSK β down regulation; however, CREB3 expression level was close to normal group. No association between CREB3 expression and inactivated GSK3β could be postulated in SGT.

Insulin detemir enhances proglucagon gene expression in the intestinal L cells via stimulating β-catenin and CREB activities

Insulin therapy using insulin detemir (d-INS) has demonstrated weight-sparing effects compared with other insulin formulations. Mechanisms underlying these effects, however, remain largely unknown. Here we postulate that the intestinal tissues' selective preference allows d-INS to exert enhanced action on proglucagon (Gcg) expression and the production of glucagon-like peptide (GLP)-1, an incretin hormone possessing both glycemia-lowering and weight loss effects. To test this hypothesis, we used obese type 2 diabetic db/db mice and conducted a 14-day intervention with daily injection of a therapeutic dose of d-INS or human insulin (h-INS) in these mice. The body weight of the mice after 14-day daily injection of d-INS (5 IU/kg) was decreased significantly compared with those injected with the same dose of h-INS or saline. The weight-sparing effect of d-INS was associated with significantly elevated circulating levels of total GLP-1 and reduced food intake. Histochemistry analysis demonstrated that d-INS induced rapid phosphorylation of protein kinase B (Akt) in the gut L cells of normal mice. Western blotting showed that d-INS stimulated Akt activation in a more rapid and enhanced fashion in the mouse distal ileum compared with those by h-INS. In vitro investigation in primary fetal rat intestinal cell (FRIC) cultures showed that d-INS increased Gcg mRNA expression as determined by Northern blotting and real-time RT-PCR. Consistent with these in vivo investigations, d-INS significantly increased GLP-1 secretion in FRIC cultures. Consistently, d-INS was also shown to induce rapid phosphorylation of Akt in the clonal gut cell line GLUTag. Furthermore, d-INS increased β-catenin phosphorylation, its nuclear translocation, and enhanced cAMP response element-binding protein (CREB) phosphorylation in a phosphatidylinositol 3-kinase and/or mitogen-activated protein kinase kinase/extracellular signal-regulated kinase-sensitive manner. We suggest that the weight-sparing benefit of d-INS in mice is related to its intestinal tissues preference that leads to profound stimulation of Gcg expression and enhanced GLP-1 secretion in intestinal L cells, potentially involving the activation of insulin/β-catenin/CREB signaling pathways.

Menin as a hub controlling mixed lineage leukemia

Mixed lineage leukemia (MLL) fusion protein (FP)-induced acute leukemia is highly aggressive and often refractory to therapy. Recent progress in the field has unraveled novel mechanisms and targets to combat this disease. Menin, a nuclear protein, interacts with wild-type (WT) MLL, MLL-FPs, and other partners such as the chromatin-associated protein LEDGF and the transcription factor C-Myb to promote leukemogenesis. The newly solved co-crystal structure illustrating the menin-MLL interaction, coupled with the role of menin in recruiting both WT MLL and MLL-FPs to target genes, highlights menin as a scaffold protein and a central hub controlling this type of leukemia. The menin/WT MLL/MLL-FP hub may also cooperate with several signaling pathways, including Wnt, GSK3, and bromodomain-containing Brd4-related pathways to sustain MLL-FP-induced leukemogenesis, revealing new therapeutic targets to improve the treatment of MLL-FP leukemias.

Perifosine enhances mTORC1-targeted cancer therapy by activation of GSK3β in NSCLC cells

mTORC1 inhibitors, including rapamycin and its analogs, have been actively studied both pre-clinically and clinically. However, the single treatment of mTORC1 inhibitors has been modest in most cancer types. We have previously demonstrated that the activation of PI3K/Akt and MEK/ERK signaling pathways attenuates the anticancer efficacy of mTORC1 inhibitors. In this study, we report that mTORC1 inhibition also phosphorylates and inactivates GSK3β, which is a tumor suppressor in lung cancer. Moreover, we show that perifosine, as an Akt inhibitor, decreases rapamycin-induced phosphorylation of GSK3β and elevated p-GSK3β levels in rapamycin-resistant cell lines. Combination of perifosine with mTORC1 inhibitors showed enhanced anticancer efficacy both in cell cultures and in a xenograft mouse model. In addition, perifosine inhibits the growth of both rapamycin sensitive and resistant A549 cells. However, inhibition of GSK3β by a selective inhibitor- LiCl, or downregulation of GSK3β expression by siRNA, reverses the growth inhibitory effects of perifosine on rapamycin resistant cells, suggesting the important role of GSK3β activation in enhancing mTORC1 inhibitors efficacy by perifosine. Thus, our results provide a potential therapeutic strategy to enhance mTORC1-targeted cancer therapy by using perifosine or targeting GSK3β.

Selective treatment of mixed-lineage leukemia leukemic stem cells through targeting glycogen synthase kinase 3 and the canonical Wnt/β-catenin pathway

PURPOSE OF REVIEW

Leukemia carrying mutation of the mixed-lineage leukemia (MLL) gene is particularly refractory to current treatment, and is associated with frequent relapse. We will review the biology of MLL leukemia, and explore the potential of targeting multiple signaling pathways deregulated in MLL leukemic stem cells (LSCs).

RECENT FINDINGS

Glycogen synthase kinase 3 (GSK3) plays a critical role in mediating Hox/MEIS1 transcriptional program and its inhibition shows promise in suppressing leukemia carrying MLL fusions or aberrant Hox expression. However, recent evidence indicates that GSK3 inhibition can be overcome by hyperactivation of the canonical Wnt signaling pathway in MLL LSCs, whereas suppression of β-catenin resensitizes MLL LSCs to the GSK3 inhibitor treatment. These results suggest a differential GSK3 dependence in different subsets of leukemic populations during disease development.

SUMMARY

On the basis of the results from preclinical model studies, a combination treatment targeting both GSK3 and the canonical Wnt signaling pathway emerges as a promising avenue to eradicate MLL LSCs. Future effort in identifying the key tractable components along these signaling pathways will be critical for the development of effective inhibitors to target this aggressive disease.

State of chromosome 11q23 in T-ALL/LBL and their relation to prognosis

The purpose of this study is to investigate the mixed lineage leukemia gene (MLL, located on chromosome 11q23) expression in T acute lymphoblastic leukemia (T-ALL)/lymphoblastic lymphoma (LBL) and its relationship to prognosis. Fifty cases of T-ALL/LBL with clinical data were selected from the Shanxi Cancer Hospital in China. The immunohistochemical EnVision method was used for the expression of CD3, CD7, CD10, CD20, CD23, CD43, CD45RO, CD99, terminal deoxynucleotidyl transferase, myeloperoxidase and ki67. Fluorescent in situ hybridization for MLL gene expression was performed on paraffin-embedded tissue. Among the 50 cases of T-ALL/LBL, the percentages of tumor cells expressing terminal deoxynucleotidyl transferase, CD99, CD3, CD7, CD10, CD43, and CD45RO were 92.0%, 96.0%, 72.0%, 92.0%, 34%, 60.0%, and 40.0%, respectively, whereas myeloperoxidase, CD20, and CD23 were all negative. A level of Ki67 expression >80% was found in 18 cases and ≤80% in 32 cases. The period of follow-up ranged from 1 to 108 months. The overall survival rate was 35.8%, with a median survival time of 330 days. Breakage of 11q23 was detected in 8 (16.00%) and amplification in 14 (28.00%) of the 50 cases. The rate of amplification in stage III-IV was higher than that in stage I-II (P<0.05). The prognosis in the 11q23 breakage group was worse than that in the nonbreakage group (P<0.05). The prognosis in the 11q23 amplification group was also worse than that in the nonamplification group (P<0.05). MLL gene rearrangement is a new subgroup concerned with prognosis in T-ALL/LBL. Both breakage and amplification of 11q23 in T-ALL/LBL might play important roles in the development and progression of T-ALL/LBL.

Retinoid differentiation therapy for common types of acute myeloid leukemia

Many cancers arise in a tissue stem cell, and cell differentiation is impaired resulting in an accumulation of immature cells. The introduction of all-trans retinoic acid (ATRA) in 1987 to treat acute promyelocytic leukemia (APL), a rare subtype of acute myeloid leukemia (AML), pioneered a new approach to obtain remission in malignancies by restoring the terminal maturation of leukemia cells resulting in these cells having a limited lifespan. Differentiation therapy also offers the prospect of a less aggressive treatment by virtue of attenuated growth of leukemia cells coupled to limited damage to normal cells. The success of ATRA in differentiation therapy of APL is well known. However, ATRA does not work in non-APL AML. Here we examine some of the molecular pathways towards new retinoid-based differentiation therapy of non-APL AML. Prospects include modulation of the epigenetic status of ATRA-insensitive AML cells, agents that influence intracellular signalling events that are provoked by ATRA, and the use of novel synthetic retinoids.

CREB and leukemogenesis

Acute myeloid leukemia (AML) is one of the most common leukemias with a 20% 5-year event-free survival in adults and 50% overall survival in children, despite aggressive chemotherapy treatment and bone marrow transplantation. The incidence and mortality rates for acute leukemia have only slightly decreased over the last 20 years, and therefore greater understanding of the molecular mechanisms associated with leukemic progression is needed. To this end, a number of transcription factors that appear to play a central role in leukemogenesis are being investigated; among them is the cAMP response element binding protein (CREB). CREB is a transcription factor that can regulate downstream targets involving in various cellular functions including cell proliferation, survival, and differentiation. In several studies, the majority of bone marrow samples from patients with acute lymphoid and myeloid leukemia demonstrate CREB overexpression. Moreover, CREB overexpression is associated with a poor outcome in AML patients. This review summarizes the role of CREB in leukemogenesis.

The intersection of genetic and chemical genomic screens identifies GSK-3α as a target in human acute myeloid leukemia

Acute myeloid leukemia (AML) is the most common form of acute leukemia in adults. Long-term survival of patients with AML has changed little over the past decade, necessitating the identification and validation of new AML targets. Integration of genomic approaches with small-molecule and genetically based high-throughput screening holds the promise of improved discovery of candidate targets for cancer therapy. Here, we identified a role for glycogen synthase kinase 3α (GSK-3α) in AML by performing 2 independent small-molecule library screens and an shRNA screen for perturbations that induced a differentiation expression signature in AML cells. GSK-3 is a serine-threonine kinase involved in diverse cellular processes, including differentiation, signal transduction, cell cycle regulation, and proliferation. We demonstrated that specific loss of GSK-3α induced differentiation in AML by multiple measurements, including induction of gene expression signatures, morphological changes, and cell surface markers consistent with myeloid maturation. GSK-3α-specific suppression also led to impaired growth and proliferation in vitro, induction of apoptosis, loss of colony formation in methylcellulose, and anti-AML activity in vivo. Although the role of GSK-3β has been well studied in cancer development, these studies support a role for GSK-3α in AML.

Targeting of GSK3β promotes imatinib-mediated apoptosis in quiescent CD34+ chronic myeloid leukemia progenitors, preserving normal stem cells

The targeting of BCR-ABL, a hybrid oncogenic tyrosine (Y) kinase, does not eradicate chronic myeloid leukemia (CML)-initiating cells. Activation of β-catenin was linked to CML leukemogenesis and drug resistance through its BCR-ABL-dependent Y phosphorylation and impaired binding to GSK3β (glycogen synthase kinase 3β). Herein, we show that GSK3β is constitutively Y(216) phospho-activated and predominantly relocated to the cytoplasm in primary CML stem/progenitor cells compared with its balanced active/inactive levels and cytosolic/nuclear distribution in normal cells. Under cytokine support, persistent GSK3β activity and its altered subcellular localization were correlated with BCR-ABL-dependent and -independent activation of MAPK and p60-SRC/GSK3β complex formation. Specifically, GSK3β activity and nuclear import were increased by imatinib mesylate (IM), a selective ABL inhibitor, but prevented by dasatinib that targets both BCR-ABL- and cytokine-dependent MAPK/p60-SRC activity. SB216763, a specific GSK3 inhibitor, promoted an almost complete suppression of primary CML stem/progenitor cells when combined with IM, but not dasatinib, while sparing bcr-abl-negative cells. Our data indicate that GSK3 inhibition acts to prime a pro-differentiative/apoptotic transcription program in the nucleus of IM-treated CML cells by affecting the β-catenin, cyclinD1, C-EBPα, ATF5, mTOR, and p27 levels. In conclusion, our data gain new insight in CML biology, indicating that GSK3 inhibitors may be of therapeutic value in selectively targeting leukemia-initiating cells in combination with IM but not dasatinib.

Identification and characterization of Hoxa9 binding sites in hematopoietic cells

The clustered homeobox proteins play crucial roles in development, hematopoiesis, and leukemia, yet the targets they regulate and their mechanisms of action are poorly understood. Here, we identified the binding sites for Hoxa9 and the Hox cofactor Meis1 on a genome-wide level and profiled their associated epigenetic modifications and transcriptional targets. Hoxa9 and the Hox cofactor Meis1 cobind at hundreds of highly evolutionarily conserved sites, most of which are distant from transcription start sites. These sites show high levels of histone H3K4 monomethylation and CBP/P300 binding characteristic of enhancers. Furthermore, a subset of these sites shows enhancer activity in transient transfection assays. Many Hoxa9 and Meis1 binding sites are also bound by PU.1 and other lineage-restricted transcription factors previously implicated in establishment of myeloid enhancers. Conditional Hoxa9 activation is associated with CBP/P300 recruitment, histone acetylation, and transcriptional activation of a network of proto-oncogenes, including Erg, Flt3, Lmo2, Myb, and Sox4. Collectively, this work suggests that Hoxa9 regulates transcription by interacting with enhancers of genes important for hematopoiesis and leukemia.

Glycogen synthase kinase (GSK)-3 promotes p70 ribosomal protein S6 kinase (p70S6K) activity and cell proliferation

The p70 ribosomal protein S6 kinase 1 (S6K1) plays a key role in cell growth and proliferation by regulating insulin sensitivity, metabolism, protein synthesis, and cell cycle. Thus, deregulation of S6K contributes to the progression of type 2 diabetes, obesity, aging, and cancer. Considering the biological and clinical importance of S6K1, a complete understanding of its regulation is critical. One of the key motifs in the activation of S6K1 is a turn motif, but its regulation is not well understood. Here we provide evidence for two mechanisms of modulating turn motif phosphorylation and S6K1 activity. First, mammalian target of rapamycin regulates turn motif phosphorylation by inhibiting its dephosphorylation. Second, we unexpectedly found that glycogen synthase kinase (GSK)-3 promotes turn motif phosphorylation. Our studies show that mammalian target of rapamycin and GSK-3 cooperate to control the activity of S6K1, an important regulator of cell proliferation and growth. Our unexpected results provide a clear rationale for the development and use of drugs targeting GSK-3 to treat diseases such as diabetes, cancer, and age-related diseases that are linked to improper regulation of S6K1.

The pathogenesis of mixed-lineage leukemia

Aggressive leukemias arise in both children and adults as a result of rearrangements to the mixed-lineage leukemia gene (MLL) located on chromosome 11q23. MLL encodes a large histone methyltransferase that directly binds DNA and positively regulates gene transcription, including homeobox (HOX) genes. MLL is involved in chromosomal translocations, partial tandem duplications, and amplifications, all of which result in hematopoietic malignancies due to sustained HOX expression and stalled differentiation. MLL lesions are associated with both acute myeloid leukemia and acute lymphoid leukemia and are usually associated with a relatively poor prognosis despite improved treatment options such as allogeneic hematopoietic stem cell transplantation, which underscores the need for new treatment regimens. Recent advances have begun to reveal the molecular mechanisms that drive MLL-associated leukemias, which, in turn, have provided opportunities for therapeutic development. Here, we discuss the etiology of MLL leukemias and potential directions for future therapy.

Sex differences in the GSK3β-mediated survival of adherent leukemic progenitors

Therapeutic resistance of acute myeloid leukemia stem cells, enriched in the CD34(+)38(-)123(+) progenitor population, is supported by extrinsic factors such as the bone marrow niche. Here, we report that when adherent onto fibronectin or osteoblast components, CD34(+)38(-)123(+) progenitors survive through an integrin-dependent activation of glycogen synthase kinase 3β (GSK3β) by serine 9-dephosphorylation. Strikingly, GSK3β-mediated survival was restricted to leukemic progenitors from female patients. GSK3β inhibition restored sensitivity to etoposide, and impaired the clonogenic capacities of adherent leukemic progenitors from female patients. In leukemic progenitors from female but not male patients, the scaffolding protein RACK1, activated downstream of α(5)β(1)-integrin engagement, was specifically upregulated and controlled GSK3β activation through the phosphatase protein phosphatase 2A (PP2A). In a mirrored manner, survival of adherent progenitors (CD34(+)38(-)) from male but not female healthy donors was partially dependent on this pathway. We conclude that the GSK3β-dependent survival pathway might be sex-specific in normal immature population and flip-flopped upon leukemogenesis. Taken together, our results strengthen GSK3β as a promising target for leukemic stem cell therapy and reveal gender differences as a new parameter in anti-leukemia therapy.

Targeting levels or oligomerization of nucleophosmin 1 induces differentiation and loss of survival of human AML cells with mutant NPM1

Nucleophosmin 1 (NPM1) is an oligomeric, nucleolar phosphoprotein that functions as a molecular chaperone for both proteins and nucleic acids. NPM1 is mutated in approximately one-third of patients with AML. The mutant NPM1c+ contains a 4-base insert that results in extra C-terminal residues encoding a nuclear export signal, which causes NPM1c+ to be localized in the cytoplasm. Here, we determined the effects of targeting NPM1 in cultured and primary AML cells. Treatment with siRNA to NPM1 induced p53 and p21, decreased the percentage of cells in S-phase of the cell cycle, as well as induced differentiation of the AML OCI-AML3 cells that express both NPMc+ and unmutated NPM1. Notably, knockdown of NPM1 by shRNA abolished lethal AML phenotype induced by OCI-AML3 cells in NOD/SCID mice. Knockdown of NPM1 also sensitized OCI-AML3 to all-trans retinoic acid (ATRA) and cytarabine. Inhibition of NPM1 oligomerization by NSC348884 induced apoptosis and sensitized OCI-AML3 and primary AML cells expressing NPM1c+ to ATRA. This effect was significantly less in AML cells coexpressing FLT3-ITD, or in AML or normal CD34+ progenitor cells expressing wild-type NPM1. Thus, attenuating levels or oligomerization of NPM1 selectively induces apoptosis and sensitizes NPM1c+ expressing AML cells to treatment with ATRA and cytarabine.

Glycogen synthase kinase 3 activation is important for anthrax edema toxin-induced dendritic cell maturation and anthrax toxin receptor 2 expression in macrophages

Anthrax edema toxin (ET) is one of two binary toxins produced by Bacillus anthracis that contributes to the virulence of this pathogen. ET is an adenylate cyclase that generates high levels of cyclic AMP (cAMP), causing alterations in multiple host cell signaling pathways. We previously demonstrated that ET increases cell surface expression of the anthrax toxin receptors (ANTXR) in monocyte-derived cells and promotes dendritic cell (DC) migration toward the lymph node-homing chemokine MIP-3β. In this work, we sought to determine if glycogen synthase kinase 3 (GSK-3) is important for ET-induced modulation of macrophage and DC function. We demonstrate that inhibition of GSK-3 dampens ET-induced maturation and migration processes of monocyte-derived dendritic cells (MDDCs). Additional studies reveal that the ET-induced expression of ANTXR in macrophages was decreased when GSK-3 activity was disrupted with chemical inhibitors or with small interfering RNA (siRNA) targeting GSK-3. Further examination of the ET induction of ANTXR revealed that a dominant negative form of CREB could block the ET induction of ANTXR, suggesting that CREB or a related family member was involved in the upregulation of ANTXR. Because CREB and GSK-3 activity appeared to be important for ET-induced ANTXR expression, the impact of GSK-3 on ET-induced CREB activity was examined in RAW 264.7 cells possessing a CRE-luciferase reporter. As with ANTXR expression, the ET induction of the CRE reporter was decreased by reducing GSK-3 activity. These studies not only provide insight into host pathways targeted by ET but also shed light on interactions between GSK-3 and CREB pathways in host immune cells.

Adenomatous polyposis coli protein associates with C/EBP beta and increases Bacillus anthracis edema toxin-stimulated gene expression in macrophages

The production of cAMP from Bacillus anthracis edema toxin (ET) activates gene expression in macrophages through a complex array of signaling pathways, most of which remain poorly defined. In this study, the tumor suppressor protein adenomatous polyposis coli (APC) was found to be important for the up-regulation of previously defined ET-stimulated genes (Vegfa, Ptgs2, Arg2, Cxcl2, Sdc1, and Cebpb). A reduction in the expression of these genes after ET exposure was observed when APC was disrupted in macrophages using siRNA or in bone marrow-derived macrophages obtained from C57BL/6J-Apc(Min) mice, which are heterozygous for a truncated form of APC. In line with this observation, ET increased the expression of APC at the transcriptional level, leading to increased amounts of APC in the nucleus. The mechanism utilized by APC to increase ET-induced gene expression was determined to depend on the ability of APC to interact with C/EBP β, which is a transcription factor activated by cAMP. Coimmunoprecipitation experiments found that APC associated with C/EBP β and that levels of this complex increase after ET exposure. A further connection was uncovered when silencing APC was determined to reduce the ET-induced phosphorylation of C/EBP β at Thr-188. This ET-mediated phosphorylation of C/EBP β was blocked by glycogen synthase kinase 3 (GSK-3) inhibitors, suggesting that GSK-3 is involved in the activation of C/EBP β and supporting the idea of APC helping direct interactions between GSK-3 and C/EBP β. These results indicate that ET stimulates gene expression by promoting the formation of an inducible protein complex consisting of APC and C/EBP β.

The multitargeted receptor tyrosine kinase inhibitor linifanib (ABT-869) induces apoptosis through an Akt and glycogen synthase kinase 3β-dependent pathway

The FMS-like receptor tyrosine kinase 3 (FLT3) plays an important role in controlling differentiation and proliferation of hematopoietic cells. Activating mutations in FLT3 occur in patients with acute myeloid leukemia (AML; 15%-35%), resulting in abnormal cell proliferation. Furthermore, both adult and pediatric patients with AML harboring the FLT3 internal tandem duplication (ITD) mutation have a poor prognosis. Several inhibitors have been developed to target mutant FLT3 for the treatment of AML, yet the molecular pathways affected by drug inhibition of the mutated FLT3 receptor alone have not been characterized as yet. Linifanib (ABT-869) is a multitargeted tyrosine kinase receptor inhibitor that suppresses FLT3 signaling. In this article, we show that treatment with linifanib inhibits proliferation and induces apoptosis in ITD mutant cells in vitro and in vivo. We show that treatment with linifanib reduces phosphorylation of Akt and glycogen synthase kinase 3β (GSK3β). In addition, we show that inhibition of GSK3β decreases linifanib-induced apoptosis. This study shows the importance of GSK3 as a potential target for AML therapy, particularly in patients with FLT3 ITD mutations.

Pbx homeodomain proteins: TALEnted regulators of limb patterning and outgrowth

Limb development has long provided an excellent model for understanding the genetic principles driving embryogenesis. Studies utilizing chick and mouse have led to new insights into limb patterning and morphogenesis. Recent research has centered on the regulatory networks underlying limb development. Here, we discuss the hierarchical, overlapping, and iterative roles of Pbx family members in appendicular development that have emerged from genetic analyses in the mouse. Pbx genes are essential in determining limb bud positioning, early bud formation, limb axes establishment and coordination, and patterning and morphogenesis of most elements of the limb and girdle. Pbx proteins directly regulate critical effectors of limb and girdle development, including morphogen-encoding genes like Shh in limb posterior mesoderm, and transcription factor-encoding genes like Alx1 in pre-scapular domains. Interestingly, at least in limb buds, Pbx appear to act not only as Hox cofactors, but also in the upstream control of 5' HoxA/D gene expression.

β-Catenin mediates the establishment and drug resistance of MLL leukemic stem cells

Identification of molecular pathways essential for cancer stem cells is critical for understanding the underlying biology and designing effective cancer therapeutics. Here, we demonstrated that β-catenin was activated during development of MLL leukemic stem cells (LSCs). Suppression of β-catenin reversed LSCs to a pre-LSC-like stage and significantly reduced the growth of human MLL leukemic cells. Conditional deletion of β-catenin completely abolished the oncogenic potential of MLL-transformed cells. In addition, established MLL LSCs that have acquired resistance against GSK3 inhibitors could be resensitized by suppression of β-catenin expression. These results unveil previously unrecognized multifaceted functions of β-catenin in the establishment and drug-resistant properties of MLL stem cells, highlighting it as a potential therapeutic target for an important subset of AMLs.