Proliferation and bone marrow engraftment of AML blasts is dependent on β-catenin signalling

Cordycepin (3'dA) Induces Cell Death of AC133+ Leukemia Cells via Re-Expression of WIF1 and Down-Modulation of MYC

Wnt/β-catenin signaling is critically required for the development and maintenance of leukemia stem cells (LSCs) in acute myeloid leukemia (AML) by constitutive activation of myeloid regeneration-related pathways. Cell-intrinsic activation of canonical Wnt signaling propagates in the nucleus by β-catenin translocation, where it induces expression of target oncogenes such as JUN, MYC and CCND1. As the Wnt/β-catenin pathway is now well established to be a key oncogenic signaling pathway promoting leukemic myelopoiesis, targeting it would be an effective strategy to impair LSC functionality. Although the effects of the adenosine analogue cordycepin in repressing β-catenins and destabilizing the LSC niche have been highlighted, the cellular and molecular effects on AML-LSC have not been fully clarified. In the present study, we evaluated the potency and efficacy of cordycepin, a selective repressor of Wnt/β-catenin signaling with anti-leukemia properties, on the AC133+ LSC fraction. Cordycepin effectively reduces cell viability of the AC133+ LSCs in the MUTZ-2 cell model and patient-derived cells through the induction of apoptosis. By Wnt-targeted RNA sequencing panel, we highlighted the re-expression of WIF1 and DKK1 among others, and the consequent downregulation of MYC and PROM1 (CD133) following MUTZ-2 cell exposure to increasing doses of cordycepin. Our results provide new insights into the molecular circuits involved in pharmacological inhibition mediated by cordycepin reinforcing the potential of targeting the Wnt/β-catenin and co-regulatory complexes in AML.

Wnt regulation of hematopoietic stem cell development and disease

Hematopoietic stem cells (HSCs) are multipotent stem cells that give rise to all cells of the blood and most immune cells. Due to their capacity for unlimited self-renewal, long-term HSCs replenish the blood and immune cells of an organism throughout its life. HSC development, maintenance, and differentiation are all tightly regulated by cell signaling pathways, including the Wnt pathway. Wnt signaling is initiated extracellularly by secreted ligands which bind to cell surface receptors and give rise to several different downstream signaling cascades. These are classically categorized either β-catenin dependent (BCD) or β-catenin independent (BCI) signaling, depending on their reliance on the β-catenin transcriptional activator. HSC development, homeostasis, and differentiation is influenced by both BCD and BCI, with a high degree of sensitivity to the timing and dosage of Wnt signaling. Importantly, dysregulated Wnt signals can result in hematological malignancies such as leukemia, lymphoma, and myeloma. Here, we review how Wnt signaling impacts HSCs during development and in disease.

Recent progress on targeting leukemia stem cells

Leukemia is a type of malignant clonal disease of hematopoietic stem cells (HSCs). A small population of leukemic stem cells (LSCs) are responsible for the initiation, drug resistance, and relapse of leukemia. LSCs have the ability to form tumors after xenotransplantation in immunodeficient mice and appear to be common in most human leukemias. Therefore, the eradication of LSCs is an approach with the potential to improve survival or even to cure leukemia. Using recent research in the field of LSCs, we summarize the targeted therapy approaches for the removal of LSCs through surface markers including immune checkpoint molecules, pathways influencing LSC survival, or the survival microenvironment of LSCs. In addition, we introduce the survival microenvironment and survival regulation of LSCs.

Combination of a mitogen-activated protein kinase inhibitor with the tyrosine kinase inhibitor pacritinib combats cell adhesion-based residual disease and prevents re-expansion of FLT3-ITD acute myeloid leukaemia

Minimal residual disease (MRD) in acute myeloid leukaemia (AML) poses a major challenge due to drug insensitivity and high risk of relapse. Intensification of chemotherapy and stem cell transplantation are often pivoted on MRD status. Relapse rates are high even with the integration of first-generation FMS-like tyrosine kinase 3 (FLT3) inhibitors in pre- and post-transplant regimes and as maintenance in FLT3-mutated AML. Pre-clinical progress is hampered by the lack of suitable modelling of residual disease and post-therapy relapse. In the present study, we investigated the nature of pro-survival signalling in primary residual tyrosine kinase inhibitor (TKI)-treated AML cells adherent to stroma and further determined their drug sensitivity in order to inform rational future therapy combinations. Using a primary human leukaemia-human stroma model to mimic the cell-cell interactions occurring in patients, the ability of several TKIs in clinical use, to abrogate stroma-driven leukaemic signalling was determined, and a synergistic combination with a mitogen-activated protein kinase (MEK) inhibitor identified for potential therapeutic application in the MRD setting. The findings reveal a common mechanism of stroma-mediated resistance that may be independent of mutational status but can be targeted through rational drug design, to eradicate MRD and reduce treatment-related toxicity.

SOX30 confers a tumor suppressive effect in acute myeloid leukemia through inactivation of Wnt/β-catenin signaling

Recent studies suggested SRY-related high mobility group box 30 (SOX30) as a candidate tumor-promoter or tumor-inhibitor in multiple tumor types. Yet, the detailed role of SOX30 in acute myeloid leukemia (AML) has not been well studied. The present research was designed to investigate the detailed relevance of SOX30 in AML. The data of our study indicated that SOX30 expression was markedly downregulated in AML cells, a pattern associated with its hypermethylation. SOX30 overexpression caused a marked reduction in AML cell proliferation and colony formation, but it promoted AML cell apoptosis. By contrast, SOX30 depletion by small interfering RNA (siRNA)-mediated gene silencing had the opposite effect. Moreover, SOX30 overexpression markedly decreased β-catenin expression, a change that led to inactivation of Wnt/β-catenin pathway. Notably, restoration of β-catenin expression partially reversed SOX30-mediated tumor suppressive effect in AML cells. In an AML-derived mouse xenograft model, SOX30 overexpression remarkably retarded the tumor growth in vivo. Overall, these data of the study suggest a tumor-inhibition role of SOX30 in AML, and highlight a key role of SOX30/Wnt/β-catenin axis in the progression of AML.

WNT Signaling in Tumors: The Way to Evade Drugs and Immunity

WNT/β-catenin signaling is involved in many physiological processes. Its implication in embryonic development, cell migration, and polarization has been shown. Nevertheless, alterations in this signaling have also been related with pathological events such as sustaining and proliferating the cancer stem cell (CSC) subset present in the tumor bulk. Related with this, WNT signaling has been associated with the maintenance, expansion, and epithelial-mesenchymal transition of stem cells, and furthermore with two distinctive features of this tumor population: therapeutic resistance (MDR, multidrug resistance) and immune escape. These mechanisms are developed and maintained by WNT activation through the transcriptional control of the genes involved in such processes. This review focuses on the description of the best known WNT pathways and the molecules involved in them. Special attention is given to the WNT cascade proteins deregulated in tumors, which have a decisive role in tumor survival. Some of these proteins function as extrusion pumps that, in the course of chemotherapy, expel the drugs from the cells; others help the tumoral cells hide from the immune effector mechanisms. Among the WNT targets involved in drug resistance, the drug extrusion pump MDR-1 (P-GP, ABCB1) and the cell adhesion molecules from the CD44 family are highlighted. The chemokine CCL4 and the immune checkpoint proteins CD47 and PD-L1 are included in the list of WNT target molecules with a role in immunity escape. This pathway should be a main target in cancer therapy as WNT signaling activation is essential for tumor progression and survival, even in the presence of the anti-tumoral immune response and/or antineoplastic drugs. The appropriate design and combination of anti-tumoral strategies, based on the modulation of WNT mediators and/or protein targets, could negatively affect the growth of tumoral cells, improving the efficacy of these types of therapies.

Energy metabolism and drug response in myeloid leukaemic stem cells

Despite significant advances in the treatment of myeloid malignancies, many patients become resistant to therapy and ultimately succumb to their disease. Accumulating evidence over the past several years has suggested that the inadequacy of many leukaemia therapies results from their failure to target the leukaemic stem cell (LSC). For this reason, the LSC population currently represents the most critical target in the treatment of myeloid malignancies. However, while LSCs are ideal targets in the treatment of these diseases, they are also the most difficult population to target. This is due to both their heterogeneity within the LSC population, and also their phenotypic similarities with normal haematopoietic stem cells. This review will highlight the current landscape surrounding LSC biology in myeloid malignancies, with a focus on altered energy metabolism, and how that knowledge is being translated into clinical advances for the treatment of chronic and acute myeloid leukaemia and myelodysplastic syndromes.

Increased FLYWCH1 Expression is Negatively Correlated with Wnt/β-catenin Target Gene Expression in Acute Myeloid Leukemia Cells

Acute myeloid leukaemia (AML) is a heterogeneous clonal malignancy of hematopoietic progenitor cells. The Wnt pathway and its downstream targets are tightly regulated by β-catenin. We recently discovered a new protein, FLYWCH1, which can directly bind nuclear β-catenin. Herein, we studied the FLYWCH1/β-catenin pathway in AML cells using qRT-PCR, Western blot, and immunofluorescence assays. In addition, the stemness activity and cell cycle were analysed by the colony-forming unit (CFU) using methylcellulose-based and Propidium iodide/flow cytometry assays. We found that FLYWCH1 mRNA and protein were differentially expressed in the AML cell lines. C-Myc, cyclin D1, and c-Jun expression decreased in the presence of higher FLYWCH1 expression, and vice versa. There appeared to be the loss of FLYWCH1 expression in dividing cells. The sub-G0 phase was prolonged and shortened in the low and high FLYWCH1 expression cell lines, respectively. The G0/G1 arrest correlated with FLYWCH1-expression, and these cell lines also formed colonies, whereas the low FLYWCH1 expression cell lines could not. Thus, FLYWCH1 functions as a negative regulator of the Wnt/β-catenin pathway in AML.

Disruption of Wnt/β-Catenin Exerts Antileukemia Activity and Synergizes with FLT3 Inhibition in FLT3-Mutant Acute Myeloid Leukemia

Purpose: Wnt/β-catenin signaling is required for leukemic stem cell function. FLT3 mutations are frequently observed in acute myeloid leukemia (AML). Anomalous FLT3 signaling increases β-catenin nuclear localization and transcriptional activity. FLT3 tyrosine kinase inhibitors (TKI) are used clinically to treat FLT3-mutated AML patients, but with limited efficacy. We investigated the antileukemia activity of combined Wnt/β-catenin and FLT3 inhibition in FLT3-mutant AML.Experimental Design: Wnt/β-catenin signaling was inhibited by the β-catenin/CBP antagonist C-82/PRI-724 or siRNAs, and FLT3 signaling by sorafenib or quizartinib. Treatments on apoptosis, cell growth, and cell signaling were assessed in cell lines, patient samples, and in vivo in immunodeficient mice by flow cytometry, Western blot, RT-PCR, and CyTOF.Results: We found significantly higher β-catenin expression in cytogenetically unfavorable and relapsed AML patient samples and in the bone marrow-resident leukemic cells compared with circulating blasts. Disrupting Wnt/β-catenin signaling suppressed AML cell growth, induced apoptosis, abrogated stromal protection, and synergized with TKIs in FLT3-mutated AML cells and stem/progenitor cells in vitro The aforementioned combinatorial treatment improved survival of AML-xenografted mice in two in vivo models and impaired leukemia cell engraftment. Mechanistically, the combined inhibition of Wnt/β-catenin and FLT3 cooperatively decreased nuclear β-catenin and the levels of c-Myc and other Wnt/β-catenin and FLT3 signaling proteins. Importantly, β-catenin inhibition abrogated the microenvironmental protection afforded the leukemic stem/progenitor cells.Conclusions: Disrupting Wnt/β-catenin signaling exerts potent activities against AML stem/progenitor cells and synergizes with FLT3 inhibition in FLT3-mutant AML. These findings provide a rationale for clinical development of this strategy for treating FLT3-mutated AML patients. Clin Cancer Res; 24(10); 2417-29. ©2018 AACR.

Knockdown of the Wnt receptor Frizzled-1 (FZD1) reduces MDR1/P-glycoprotein expression in multidrug resistant leukemic cells and inhibits leukemic cell proliferation

Multidrug resistance (MDR) is a major obstacle to leukemia treatment. The Frizzled-1 (FZD1) Wnt receptor is involved in MDR in some solid cancers, but has rarely been reported to act in acute myeloid leukemia (AML). We investigated whether the knockdown of FZD1 affects MDR1 expression and P-glycoprotein (P-gp) function in multidrug resistant leukemic cell lines, as well as FZD1 and MDR1/P-gp expression in leukemic cells taken from patients with AML (n = 112). FZD1 knockdown significantly reduced MDR1 expression through the Wnt/β-catenin pathway, disrupted the P-gp efflux function, induced the recovery of sensitivity to chemotherapeutic agents, and hindered cell proliferation in cell lines. FZD1 expression in leukemic cells was significantly higher in patients experiencing relapse (n = 34) than in those with no relapse (n = 44, P = .003). Leukemic cells unable to achieve complete response (CR) showed an increased expression of MDR1 and P-gp, compared to patients who achieved CR. Obtaining CR in patients with higher FZD1 expression at diagnosis is difficult. Moreover, they tend to present instances of relapse, suggesting that AML cells with increased FZD1 expression are resistant to chemotherapy. We conclude that the activated FZD1 observed in leukemic cells likely confers acquired drug resistance, whereas FZD1 silencing may be more effective in reversing MDR.

Wnt Signaling in Hematological Malignancies

Leukemia and lymphoma are a wide encompassing term for a diverse set of blood malignancies that affect people of all ages and result in approximately 23,000 deaths in the United States per year (Siegel RL, Miller KD, Jemal A. Cancer statistics, 2016. CA Cancer J Clin. 2016;66(1):7-30.). Hematopoietic stem cells (HSCs) are tissue-specific stem cells at the apex of the hierarchy that gives rise to all of the terminally differentiated blood cells, through progressively restricted progenitor populations, a process that is known to be Wnt-responsive. In particular, the progenitor populations are subject to uncontrolled expansion during oncogenic processes, namely the common myeloid progenitor and common lymphoid progenitor, as well as the myeloblast and lymphoblast. Unregulated growth of these cell-types leads to mainly three types of blood cancers (i.e., leukemia, lymphoma, and myeloma), which frequently exhibit deregulation of the Wnt signaling pathway. Generally, leukemia is caused by the expansion of myeloid progenitors, leading to an overproduction of white blood cells; as such, patients are unable to make sufficient numbers of red blood cells and platelets. Likewise, an overproduction of lymphocytes leads to clogging of the lymph system and impairment of the immune system in lymphomas. Finally, cancer of the plasma cells in the blood is called myeloma, which also leads to immune system failure. Within each of these three types of blood cancers, there are multiple subtypes, usually characterized by their timeline of onset and their cell type of origin. Of these, 85% of leukemias are encompassed by the four most common diseases, that is, acute myeloid leukemia (AML), chronic myeloid leukemia (CML), acute lymphoblastic leukemia (ALL), and chronic lymphocytic leukemia (CLL); AML accounts for the majority of leukemia-related deaths (Siegel RL, Miller KD, Jemal A. Cancer statistics, 2016. CA Cancer J Clin. 2016;66(1):7-30.). Through understanding how HSCs are normally developed and maintained, we can understand how the normal functions of these pathways are disrupted during blood cancer progression; the Wnt pathway is important in regulation of both normal and malignant hematopoiesis. In this chapter, we will discuss the role of Wnt signaling in normal and aberrant hematopoiesis. Our understanding the relationship between Wnt and HSCs will provide novel insights into therapeutic targets.

SALL1 expression in acute myeloid leukemia

Similar signaling pathways could operate in both normal hematopoietic stem and progenitor cells (HSPCs) and leukemia stem cells (LSCs). Thus, targeting LSCs signaling without substantial toxicities to normal HSPCs remains challenging. SALL1, is a member of the transcriptional network that regulates stem cell pluripotency, and lacks significant expression in most adult tissues, including normal bone marrow (NBM). We examined the expression and functional characterization of SALL1 in NBM and in acute myeloid leukemia (AML) using in vitro and in vivo assays. We showed that SALL1 is expressed preferentially in LSCs- enriched CD34+CD38- cell subpopulation but not in NBM. SALL1 inhibition resulted in decreased cellular proliferation and in inferior AML engraftment in NSG mice and it was also associated with upregulation of PTEN and downregulation of m-TOR, β-catenin, and NF-қB expression. These findings suggest that SALL1 inhibition interrupts leukemogenesis. Further studies to validate SALL1 as a potential biomarker for minimal residual disease (MRD) and to determine SALL1's role in prognostication are ongoing. Additionally, pre-clinical evaluation of SALL1 as a therapeutic target in AML is warranted.

Ectopic Expression of Snail and Twist in Ph+ Leukemia Cells Upregulates CD44 Expression and Alters Their Differentiation Potential

Philadelphia chromosome-positive (Ph+) leukemia is characterized by reciprocal translocation between chromosomes 9 and 22. The resultant BCR/ABL fusion protein displays constitutive tyrosine kinase activity, leading to the induction of aberrant proliferation and neoplastic transformation. The bone marrow (BM) microenvironment is tumor-promoting, and contributes to disease recurrence in Ph+ leukemia. Activity in the BM microenvironment is mediated by several cellular compartments, extracellular matrix, various soluble factors including transforming growth factor beta 1 (TGF-β1), and the hypoxic conditions in the BM niche. TGF-β1 is released during bone remodeling and plays a role in maintaining leukemic stem cells, as well as being implicated in the epithelial-mesenchymal transition (EMT) process in most solid tumors. Although EMT is largely implicated in epithelial tumors, recent findings argue for an EMT-like process in leukemia as well. The surface receptor CD44 is involved in cell adhesion, cell migration, and homing of normal and malignant hematopoietic stem cells. Elevation of CD44 expression is considered a marker for a worse prognosis in most hematological malignancies. We explored the functions of Snail and Twist1 in Ph+ leukemia. We showed that ectopic expression of Snail and, to a lesser extent, Twist1, upregulates CD44 expression that is β-catenin-dependent. Moreover, the presence of Snail or Twist1 partially blocked phorbol 12-myristate 13-acetate-induced megakaryocyte differentiation, while that of Twist significantly altered imatinib-induced erythroid differentiation. Thus EMT modulators affected proliferation, CD44 gene expression and differentiation ability of Ph+ leukemia cells.

β-catenin induces expression of prohibitin gene in acute leukemic cells

Prohibitin (PHB) is a multifunctional protein conserved in eukaryotic systems and shows various expression levels in tumor cells. However, regulation of PHB is not clearly understood. Here, we focused on the regulation of PHB expression by Wnt signaling, one of dominant regulatory signals in various leukemic cells. High mRNA levels of PHB were found in half of clinical leukemia samples. PHB expression was increased by inhibition of the MAPK pathway and decreased by activation of EGF signal. Although cell proliferating signals downregulated the transcription of PHB, treatment with lithium chloride, an analog of the Wnt signal, induced PHB level in various cell types. We identified the TCF-4/LEF-1 binding motif, CATCTG, in the promoter region of PHB by site-directed mutagenesis and ChIP assay. This β-catenin-mediated activation of PHB expression was independent of c‑MYC activation, a product of Wnt signaling. These data indicate that PHB is a direct target of β-catenin and the increased level of PHB in leukemia can be regulated by Wnt signaling.

Understanding of leukemic stem cells and their clinical implications

Since leukemic stem cells (LSCs) or cancer stem cells (CSCs) were found in acute myeloid leukemia (AML) in 1997, extensive studies have been contributed to identification and characterization of such cell populations in various tissues. LSCs are now generally recognized as a heterogeneous cell population that possesses the capacities of self-renewal, proliferation and differentiation. It has been shown that LSCs are regulated by critical surface antigens, microenvironment, intrinsic signaling pathways, and novel molecules such as some ncRNAs. To date, significant progress has been made in understanding of LSCs, leading to the development of numerous LSCs-targeted therapies. Moreover, various novel therapeutic agents targeting LSCs are undergoing clinical trials. Here, we review current knowledge of LSCs, and discuss the potential therapies and their challenges that are being tested in clinical trials for evaluation of their effects on leukemias.

Hypoxia regulates CD44 expression via hypoxia-inducible factor-1α in human gastric cancer cells

Hypoxia induces proliferation and invasion in cancer cells via hypoxia-inducible factor (HIF)-1α. The cell adhesion molecule cluster of differentiation (CD) 44 has been associated with increased cell invasion and metastasis. Whether hypoxia regulates the expression of CD44 in gastric cancer cells remains to be established. In the current study, the effects of hypoxia on HIF-1α and CD44 expression levels in human gastric cell lines SGC-7901 and BGC-823 were evaluated. The cells were cultured in 1% O₂ for 1 week and then treated with 20 nM rapamycin for 72 h. Cell viability was evaluated using the Cell Counting kit-8 assay, and cell invasion was detected by the Transwell invasion assay. The protein and messenger (m) RNA expression levels of HIF-1α and CD44 were detected using western blotting and reverse transcription-quantitative polymerase chain reaction, respectively. The results revealed that cell viability and invasion increased under hypoxic conditions, but decreased following rapamycin treatment in SGC-7901 and BGC-823 cells. Hypoxia also increased the protein and mRNA expression levels of HIF-1α and CD44 in these two cell lines. However, this hypoxia-induced increase in HIF-1α and CD44 protein and mRNA expression levels was inhibited by rapamycin. These findings suggest that hypoxia induced the proliferation and invasion of SGC-7901 and BGC-823 cells. Furthermore, CD44 expression levels were potentially associated with HIF-1α expression levels. Therefore, in gastric cancer cells, hypoxia may regulate CD44 expression via HIF-1α in order to promote cell proliferation and invasion.

Role of Signal Regulatory Protein α in Arsenic Trioxide-induced Promyelocytic Leukemia Cell Apoptosis

Signal regulatory protein α (SIRPα) has been shown to operate as a negative regulator in cancer cell survival. The mechanism underneath such function, however, remains poorly defined. In the present study, we demonstrate that overexpression of SIRPα in acute promyelocytic leukemia (APL) cells results in apoptosis possibly via inhibiting the β-catenin signaling pathway and upregulating Foxo3a. Pharmacological activation of β-catenin signal pathway attenuates apoptosis caused by SIRPα. Interestingly, we also find that the pro-apoptotic effect of SIRPα plays an important role in arsenic trioxide (ATO)-induced apoptosis in APL cells. ATO treatment induces the SIRPα protein expression in APL cells and abrogation of SIRPα induction by lentivirus-mediated SIRPα shRNA significantly reduces the ATO-induced apoptosis. Mechanistic study further shows that induction of SIRPα protein in APL cells by ATO is mediated through suppression of c-Myc, resulting in reduction of three SIRPα-targeting microRNAs: miR-17, miR-20a and miR-106a. In summary, our results demonstrate that SIRPα inhibits tumor cell survival and significantly contributes to ATO-induced APL cell apoptosis.

Survival regulation of leukemia stem cells

Leukemia stem cells (LSCs) are a subpopulation cells at the apex of hierarchies in leukemia cells and responsible for disease continuous propagation. In this article, we discuss some cellular and molecular components, which are critical for LSC survival. These components include intrinsic signaling pathways and extrinsic microenvironments. The intrinsic signaling pathways to be discussed include Wnt/β-catenin signaling, Hox genes, Hh pathway, Alox5, and some miRNAs, which have been shown to play important roles in regulating LSC survival and proliferation. The extrinsic components to be discussed include selectins, CXCL12/CXCR4, and CD44, which involve in LSC homing, survival, and proliferation by affecting bone marrow microenvironment. Potential strategies for eradicating LSCs will also discuss.

CD44, Hyaluronan, the Hematopoietic Stem Cell, and Leukemia-Initiating Cells

CD44 is an adhesion molecule that varies in size due to glycosylation and insertion of so-called variant exon products. The CD44 standard isoform (CD44s) is highly expressed in many cells and most abundantly in cells of the hematopoietic system, whereas expression of CD44 variant isoforms (CD44v) is more restricted. CD44s and CD44v are known as stem cell markers, first described for hematopoietic stem cells and later on confirmed for cancer- and leukemia-initiating cells. Importantly, both abundantly expressed CD44s as well as CD44v actively contribute to the maintenance of stem cell features, like generating and embedding in a niche, homing into the niche, maintenance of quiescence, and relative apoptosis resistance. This is surprising, as CD44 is not a master stem cell gene. I here will discuss that the functional contribution of CD44 relies on its particular communication skills with neighboring molecules, adjacent cells and, last not least, the surrounding matrix. In fact, it is the interaction of the hyaluronan receptor CD44 with its prime ligand, which strongly assists stem cells to fulfill their special and demanding tasks. Recent fundamental progress in support of this “old” hypothesis, which may soon pave the way for most promising new therapeutics, is presented for both hematopoietic stem cell and leukemia-initiating cell. The contribution of CD44 to the generation of a stem cell niche, to homing of stem cells in their niche, to stem cell quiescence and apoptosis resistance will be in focus.

DNMT3B7 expression promotes tumor progression to a more aggressive phenotype in breast cancer cells

Epigenetic changes, such as DNA methylation, have been shown to promote breast cancer progression. However, the mechanism by which cancer cells acquire and maintain abnormal DNA methylation is not well understood. We have previously identified an aberrant splice form of a DNA methyltransferase, DNMT3B7, expressed in virtually all cancer cell lines but at very low levels in normal cells. Furthermore, aggressive MDA-MB-231 breast cancer cells have been shown to express increased levels of DNMT3B7 compared to poorly invasive MCF-7 cells, indicating that DNMT3B7 may have a role in promoting a more invasive phenotype. Using data gathered from The Cancer Genome Atlas, we show that DNMT3B7 expression is increased in breast cancer patient tissues compared to normal tissue. To determine the mechanism by which DNMT3B7 was functioning in breast cancer cells, two poorly invasive breast cancer cell lines, MCF-7 and T-47D, were stably transfected with a DNMT3B7 expression construct. Expression of DNMT3B7 led to hypermethylation and down-regulation of E-cadherin, altered localization of β-catenin, as well as increased adhesion turnover, cell proliferation, and anchorage-independent growth. The novel results presented in this study suggest a role for DNMT3B7 in the progression of breast cancer to a more aggressive state and the potential for future development of novel therapeutics.

Leukemia and ABC transporters

Acute myeloid leukemia (AML) is a heterogeneous disease caused by aberrant proliferation and/or differentiation of myeloid progenitors. However, only ~65% of AML patients respond to induction chemotherapy and the overall survival rate for AML remains low (~24% for 5-year survival). The conventional view suggests that ATP-binding cassette (ABC) transporters contribute to treatment failure due to their drug-effluxing capabilities. This might be overly simplistic. Some ABC transporters export endogenous substrates that have defined roles in normal hematopoietic progenitors. It is conceivable that these substances also provide an advantage to leukemic progenitors. This review will highlight how certain endogenous substrates impact normal hematopoietic cells and suggest that ABC transporters facilitate export of these substances to affect both normal hematopoietic and leukemic progenitors. For example, the ability to export certain endogenous ligands may facilitate leukemogenesis by modifying leukemic progenitor cell proliferation or survival. If so, the addition of ABC transporter inhibitors to traditional chemotherapy might improve therapeutic efficacy by not just increasing intracellular drug accumulation but also blocking the beneficial effects ABC transporter ligands have on cell survival.

Pre-clinical efficacy of combined therapy with novel β-catenin antagonist BC2059 and histone deacetylase inhibitor against AML cells

The canonical WNT-β-catenin pathway is essential for self-renewal, growth and survival of AML stem/blast progenitor cells (BPCs). Deregulated WNT signaling inhibits degradation of β-catenin, causing increased nuclear translocation and co-factor activity of β-catenin with the transcriptional regulator TCF4/LEF1 in AML BPCs. Here, we determined the pre-clinical anti-AML activity of the anthraquinone oxime-analog BC2059 (BC), known to attenuate β-catenin levels. BC treatment disrupted the binding of β-catenin with the scaffold protein TBL1 (transducin β-like 1) and proteasomal degradation and decline in the nuclear levels of β-catenin. This was associated with reduced transcriptional activity of TCF4 and expression of its target genes, cyclin D1, c-MYC and survivin. BC treatment dose-dependently induced apoptosis of cultured and primary AML BPCs. Treatment with BC also significantly improved the median survival of immune-depleted mice engrafted with either cultured or primary AML BPCs exhibiting nuclear expression of β-catenin. Co-treatment with the pan-histone deacetylase inhibitor panobinostat and BC synergistically induced apoptosis of cultured and primary AML BPCs, including those expressing FLT3-ITD, as well as further significantly improved the survival of immune-depleted mice engrafted with primary AML BPCs. These findings underscore the promising pre-clinical activity and warrant further testing of BC against human AML, especially those expressing FLT3-ITD.

Role of the Wilms' tumor 1 gene in the aberrant biological behavior of leukemic cells and the related mechanisms

The Wilms' tumor 1 (WT1) gene is one of the regulating factors in cell proliferation and development. It is a double-functional gene: an oncogene and a tumor suppressor. This gene was found to be highly expressed in many leukemic cell lines and in patients with acute myeloid leukemia. In the present study, we demonstrated that the WT1 gene was commonly expressed in leukemic cell lines apart from U937 cells. The K562 cell line which expresses WT1 at a high level (mRNA and protein) was used in the entire experiment. By MTT and colony formation assays, we found that curcumin, an inhibitor of the WT1 protein, inhibited cell proliferation and clonogenicity in a time- and dose-dependent manner. It also caused cell cycle arrest at the G2/M phase. We then designed specific short hairpin RNAs (shRNAs) which could downregulate WT1 by 70-80% at the mRNA and protein levels. Reduction in the WT1 levels attenuated the proliferative ability and clonogenicity. Cell cycle progression analysis indicated that the proportion of cells in the G0/G1 phase increased while the proportion in the S phase decreased distinctively. ChIP-DNA selection and ligation (DSL) experiment identified a cohort of genes whose promoters are targeted by WT1. These genes were classified into different cellular signaling pathways using MAS software and included the Wnt/β-catenin pathway, MAPK signaling pathway, apoptosis pathway, and the cell cycle. We focused on the Wnt/β-catenin signaling pathway, and compared expression of several genes in the K562 cells transfected with the control shRNA and WT1-specific shRNA. β-catenin, an important gene in the Wnt canonical pathway, was downregulated after WT1 RNAi. Target genes of β-catenin which participate in cell proliferation and cell cycle regulation, such as CCND1 and MYC, were also significantly downregulated. Collectively, these data suggest that WT1 functions as an oncogene in leukemia cells, and one important mechanism is regulation of the Wnt/β-catenin pathway.

Inhibiting CCN1 blocks AML cell growth by disrupting the MEK/ERK pathway

Background

CCN1 plays distinct roles in various tumor types, but little is known regarding the role of CCN1 in leukemia.

Methods

We analyzed CCN1 protein expression in leukemia cell lines and in AML bone marrow samples. We also evaluated the effects of antibody- or siRNA-mediated inhibition of CCN1 on the growth of two AML cell lines (U937 and Kasumi-1 cells) and on the MEK/ERK pathway, β-catenin and other related genes.

Results

U937 and Kasumi-1 cells had markedly higher CCN1 expression than the 5 other leukemia cell lines, and CCN1 protein expression was higher in the AML bone marrow samples than in the normal bone marrow samples. Blocking CCN1 with an antibody in U937 and Kasumi-1 cells suppressed proliferation, increased apoptosis, down-regulated Bcl-xL and c-Myc expression, up-regulated Bax expression, and had no effect on Survivin. siRNA-mediated down-regulation of CCN1 inhibited the proliferation and colony formation of U937 and Kasumi-1 cells and increased cytarabine-induced apoptosis. Furthermore, CCN1 siRNA reduced MEK and ERK phosphorylation without affecting β-catenin; the CCN1 antibody similarly affected MEK and ERK phosphorylation. These changes in phosphorylation could influence the expression of Bcl-xL, c-Myc and Bax in AML cells.

Conclusions

The data suggested that CCN1 is a tumor promoter in AML that acts through the MEK/ERK pathway to up-regulate c-Myc and Bcl-xL and to down-regulate Bax.

CD44 standard and CD44v10 isoform expression on leukemia cells distinctly influences niche embedding of hematopoietic stem cells

Background

A blockade of CD44 is considered a therapeutic option for the elimination of leukemia initiating cells. However, anti-panCD44 can interfere with hematopoiesis. Therefore we explored, whether a CD44 variant isoform (CD44v)-specific antibody can inhibit leukemia growth without attacking hematopoiesis. As a model we used CD44v10 transfected EL4 thymoma cells (EL4-v10).

Methods

The therapeutic efficacy of anti-panCD44 and anti-CD44v10 was evaluated after intravenous application of EL4/EL4-v10. Ex vivo and in vitro studies evaluated the impact of anti-panCD44 and anti-CD44v10 as well as of EL4 and EL4-v10 on hematopoietic stem cells (HSC) in cocultures with bone marrow stroma cells with a focus on adhesion, migration, cell cycle progression and apoptosis resistance.

Results

Intravenously injected EL4-v10 grow in bone marrow and spleen. Anti-panCD44 and, more pronounced anti-CD44v10 prolong the survival time. The higher efficacy of anti-CD44v10 compared to anti-panCD44 does not rely on stronger antibody-dependent cellular cytotoxicity or on promoting EL4-v10 apoptosis. Instead, EL4 compete with HSC niche embedding. This has consequences on quiescence and apoptosis-protecting signals provided by the stroma. Anti-panCD44, too, more efficiently affected embedding of HSC than of EL4 in the bone marrow stroma. EL4-v10, by catching osteopontin, migrated on bone marrow stroma and did not or weakly interfere with HSC adhesion. Anti-CD44v10, too, did not affect the HSC – bone marrow stroma crosstalk.

Conclusion

The therapeutic effect of anti-panCD44 and anti-CD44v10 is based on stimulation of antibody-dependent cellular cytotoxicity. The superiority of anti-CD44v10 is partly due to blocking CD44v10-stimulated osteopontin expression that could drive HSC out of the niche. However, the main reason for the superiority of anti-CD44v10 relies on neither EL4-v10 nor anti-CD44v10 severely interfering with HSC – stroma cell interactions that, on the other hand, are affected by EL4 and anti-panCD44. Anti-panCD44 disturbing HSC embedding in the osteogenic niche weakens its therapeutic effect towards EL4. Thus, as far as leukemic cells express CD44v isoforms, the therapeutic use of anti-panCD44 should be avoided in favor of CD44v-specific antibodies.

SPARC promotes leukemic cell growth and predicts acute myeloid leukemia outcome

Aberrant expression of the secreted protein, acidic, cysteine-rich (osteonectin) (SPARC) gene, which encodes a matricellular protein that participates in normal tissue remodeling, is associated with a variety of diseases including cancer, but the contribution of SPARC to malignant growth remains controversial. We previously reported that SPARC was among the most upregulated genes in cytogenetically normal acute myeloid leukemia (CN-AML) patients with gene-expression profiles predictive of unfavorable outcome, such as mutations in isocitrate dehydrogenase 2 (IDH2-R172) and overexpression of the oncogenes brain and acute leukemia, cytoplasmic (BAALC) and v-ets erythroblastosis virus E26 oncogene homolog (ERG). In contrast, SPARC was downregulated in CN-AML patients harboring mutations in nucleophosmin (NPM1) that are associated with favorable prognosis. Based on these observations, we hypothesized that SPARC expression is clinically relevant in AML. Here, we found that SPARC overexpression is associated with adverse outcome in CN-AML patients and promotes aggressive leukemia growth in murine models of AML. In leukemia cells, SPARC expression was mediated by the SP1/NF-κB transactivation complex. Furthermore, secreted SPARC activated the integrin-linked kinase/AKT (ILK/AKT) pathway, likely via integrin interaction, and subsequent β-catenin signaling, which is involved in leukemia cell self-renewal. Pharmacologic inhibition of the SP1/NF-κB complex resulted in SPARC downregulation and leukemia growth inhibition. Together, our data indicate that evaluation of SPARC expression has prognosticative value and SPARC is a potential therapeutic target for AML.

β-Catenin activation synergizes with Pten loss and Myc overexpression in Notch-independent T-ALL

Wnt activation, Pten loss, and Myc translocation synergize to define a novel subset of murine Notch-independent T-ALL.

Regeneration-associated WNT signaling is activated in long-term reconstituting AC133bright acute myeloid leukemia cells

Acute myeloid leukemia (AML) is a genetically heterogeneous clonal disorder characterized by two molecularly distinct self-renewing leukemic stem cell (LSC) populations most closely related to normal progenitors and organized as a hierarchy. A requirement for WNT/β-catenin signaling in the pathogenesis of AML has recently been suggested by a mouse model. However, its relationship to a specific molecular function promoting retention of self-renewing leukemia-initiating cells (LICs) in human remains elusive. To identify transcriptional programs involved in the maintenance of a self-renewing state in LICs, we performed the expression profiling in normal (n = 10) and leukemic (n = 33) human long-term reconstituting AC133(+) cells, which represent an expanded cell population in most AML patients. This study reveals the ligand-dependent WNT pathway activation in AC133(bright) AML cells and shows a diffuse expression and release of WNT10B, a hematopoietic stem cell regenerative-associated molecule. The establishment of a primary AC133(+) AML cell culture (A46) demonstrated that leukemia cells synthesize and secrete WNT ligands, increasing the levels of dephosphorylated β-catenin in vivo. We tested the LSC functional activity in AC133(+) cells and found significant levels of engraftment upon transplantation of A46 cells into irradiated Rag2(-/-)γc(-/-) mice. Owing to the link between hematopoietic regeneration and developmental signaling, we transplanted A46 cells into developing zebrafish. This system revealed the formation of ectopic structures by activating dorsal organizer markers that act downstream of the WNT pathway. In conclusion, our findings suggest that AC133(bright) LSCs are promoted by misappropriating homeostatic WNT programs that control hematopoietic regeneration.

Normal hematopoiesis and hematologic malignancies: role of canonical Wnt signaling pathway and stromal microenvironment

Wnts are a family of evolutionary-conserved secreted signaling molecules critically involved in a variety of developmental processes and in cell fate determination. A growing body of evidence suggests that Wnt signaling plays a crucial role in the influence of bone marrow stromal microenvironment on the balance between hematopoietic stem cell self-renewal and differentiation. Emerging clinical and experimental evidence also indicates Wnt signaling involvement in the disruption of the latter balance in hematologic malignancies, where the stromal microenvironment favors the homing of cancer cells to the bone marrow, as well as leukemia stem cell development and chemoresistance. In the present review, we summarize and discuss the role of the canonical Wnt signaling pathway in normal hematopoiesis and hematologic malignancies, with regard to recent findings on the stromal microenvironment involvement in these process and diseases.

γ-Catenin is overexpressed in acute myeloid leukemia and promotes the stabilization and nuclear localization of β-catenin

Canonical Wnt signaling regulates the transcription of T-cell factor (TCF)-responsive genes through the stabilization and nuclear translocation of the transcriptional co-activator, β-catenin. Overexpression of β-catenin features prominently in acute myeloid leukemia (AML) and has previously been associated with poor clinical outcome. Overexpression of γ-catenin mRNA (a close homologue of β-catenin) has also been reported in AML and has been linked to the pathogenesis of this disease, however, the relative roles of these catenins in leukemia remains unclear. Here we report that overexpression and aberrant nuclear localization of γ-catenin is frequent in AML. Significantly, γ-catenin expression was associated with β-catenin stabilization and nuclear localization. Consistent with this, we found that ectopic γ-catenin expression promoted the stabilization and nuclear translocation of β-catenin in leukemia cells. β-Catenin knockdown demonstrated that both γ- and β-catenin contribute to TCF-dependent transcription in leukemia cells. These data indicate that γ-catenin expression is a significant factor in the stabilization of β-catenin in AML. We also show that although normal cells exclude nuclear translocation of both γ- and β-catenin, this level of regulation is lost in the majority of AML patients and cell lines, which allow nuclear accumulation of these catenins and inappropriate TCF-dependent transcription.

Peripheral T-lymphocytes express WNT7A and its restoration in leukemia-derived lymphoblasts inhibits cell proliferation

Background

WNT7a, a member of the Wnt ligand family implicated in several developmental processes, has also been reported to be dysregulated in some types of tumors; however, its function and implication in oncogenesis is poorly understood. Moreover, the expression of this gene and the role that it plays in the biology of blood cells remains unclear. In addition to determining the expression of the WNT7A gene in blood cells, in leukemia-derived cell lines, and in samples of patients with leukemia, the aim of this study was to seek the effect of this gene in proliferation.

Methods

We analyzed peripheral blood mononuclear cells, sorted CD3 and CD19 cells, four leukemia-derived cell lines, and blood samples from 14 patients with Acute lymphoblastic leukemia (ALL), and 19 clinically healthy subjects. Reverse transcription followed by quantitative Real-time Polymerase chain reaction (qRT-PCR) analysis were performed to determine relative WNT7A expression. Restoration of WNT7a was done employing a lentiviral system and by using a recombinant human protein. Cell proliferation was measured by addition of WST-1 to cell cultures.

Results

WNT7a is mainly produced by CD3 T-lymphocytes, its expression decreases upon activation, and it is severely reduced in leukemia-derived cell lines, as well as in the blood samples of patients with ALL when compared with healthy controls (p ≤0.001). By restoring WNT7A expression in leukemia-derived cells, we were able to demonstrate that WNT7a inhibits cell growth. A similar effect was observed when a recombinant human WNT7a protein was used. Interestingly, restoration of WNT7A expression in Jurkat cells did not activate the canonical Wnt/β-catenin pathway.

Conclusions

To our knowledge, this is the first report evidencing quantitatively decreased WNT7A levels in leukemia-derived cells and that WNT7A restoration in T-lymphocytes inhibits cell proliferation. In addition, our results also support the possible function of WNT7A as a tumor suppressor gene as well as a therapeutic tool.

Wnt/ß-catenin: a new therapeutic approach to acute myeloid leukemia

Recent studies have shown genetic and epigenetic aberrations resulting in aberrant activation of the Wingless-Int (Wnt) pathway, thus influencing the initiation and progression of acute myeloid leukemia (AML). Of major importance, these findings may lead to novel treatment strategies exploiting targeted modulation of Wnt signaling. This paper comprises the latest status of knowledge concerning the role of Wnt pathway alteration in AML and outlines future lines of research and their clinical perspectives.

Stromal interaction essential for vascular endothelial growth factor A-induced tumour growth via transforming growth factor-β signalling

BACKGROUND

High vascular endothelial growth factor (VEGFA) levels at the time of diagnosis confer a worse prognosis to multiple malignancies. Our aim was to investigate the role of VEGFA in promoting tumour growth through interaction with its environment.

METHODS

HL-60 cells were transduced with VEGFA165 or control vector using retroviral constructs. Control cells (n=7) or VEGFA165 cells (n=7) were subcutaneously injected into NOD/SCID mice. Immunohistochemistry of markers for angiogenesis (CD31) and cell proliferation (Ki67) and gene expression profiling of tumours were performed. Paracrine effects were investigated by mouse-specific cytokine arrays.

RESULTS

In vivo we observed a twofold increase in tumour weight when VEGFA165 was overexpressed (P=0.001), combined with increased angiogenesis (P=0.002) and enhanced tumour cell proliferation (P=0.001). Gene expression profiling revealed human genes involved in TGF-β signalling differentially expressed between both tumour groups, that is, TGFBR2 and SMAD5 were lower expressed whereas the inhibitory SMAD7 was higher expressed with VEGFA165. An increased expression of mouse-derived cytokines IFNG and interleukin 7 was found in VEGFA165 tumours, both described to induce SMAD7 expression.

CONCLUSION

These results suggest a role for VEGFA-driven tumour growth by TGF-β signalling inhibition via paracrine mechanisms in vivo, and underscore the importance of stromal interaction in the VEGFA-induced phenotype.

Heterogeneous sensitivity of human acute myeloid leukemia to β-catenin down-modulation

Dysregulation of the Wnt/ß-catenin pathway has been observed in various malignancies, including acute myeloid leukemia (AML), where the over-expression of ß-catenin is an independent adverse prognostic factor. ß-catenin was found up-regulated in the vast majority of AML samples and more frequently localized in the nucleus of leukemic stem cells compared to normal bone marrow CD34⁺ cells. The knockdown of ß-catenin, using a short hairpin RNA (shRNA) lentiviral approach, accelerates ATRA-induced differentiation and impairs the proliferation of HL60 leukemic cell line. Using in vivo quantitative tracking of these cells, we observed a reduced engraftment potential after xenotransplantation when ß-catenin was silenced. However when studying primary AML cells, despite effective down-regulation of ß-catenin we did not observe any impairment of their in vitro long-term maintenance on MS-5 stroma nor of their engraftment potential in vivo. Altogether, these results demonstrate that despite a frequent ß-catenin up-regulation in AML, leukemia initiating cells might not be ‘addicted’ to this pathway and thus targeted therapy against ß-catenin might not be successful in all patients.