Hematopoietic cell fate and the initiation of leukemic properties in primitive primary human cells are influenced by Ras activity and farnesyltransferase inhibition

Disruption of dNTP homeostasis by ribonucleotide reductase hyperactivation overcomes AML differentiation blockade

Differentiation blockade is a hallmark of acute myeloid leukemia (AML). A strategy to overcome such a blockade is a promising approach against the disease. The lack of understanding of the underlying mechanisms hampers development of such strategies. Dysregulated ribonucleotide reductase (RNR) is considered a druggable target in proliferative cancers susceptible to deoxynucleoside triphosphate (dNTP) depletion. Herein, we report an unanticipated discovery that hyperactivating RNR enables differentiation and decreases leukemia cell growth. We integrate pharmacogenomics and metabolomics analyses to identify that pharmacologically (eg, nelarabine) or genetically upregulating RNR subunit M2 (RRM2) creates a dNTP pool imbalance and overcomes differentiation arrest. Moreover, R-loop-mediated DNA replication stress signaling is responsible for RRM2 activation by nelarabine treatment. Further aggravating dNTP imbalance by depleting the dNTP hydrolase SAM domain and HD domain-containing protein 1 (SAMHD1) enhances ablation of leukemia stem cells by RRM2 hyperactivation. Mechanistically, excessive activation of extracellular signal-regulated kinase (ERK) signaling downstream of the imbalance contributes to cellular outcomes of RNR hyperactivation. A CRISPR screen identifies a synthetic lethal interaction between loss of DUSP6, an ERK-negative regulator, and nelarabine treatment. These data demonstrate that dNTP homeostasis governs leukemia maintenance, and a combination of DUSP inhibition and nelarabine represents a therapeutic strategy.

In-situ proliferation contributes to the accumulation of myeloid cells in the spleen during progressive experimental visceral leishmaniasis

Visceral leishmaniasis (VL) is characterized by expansion of myeloid cells in the liver and spleen, which leads to a severe splenomegaly associated with higher risk of mortality. This increased cellularity is thought to be a consequence of recruitment of cells to the viscera. We studied whether the local proliferation of splenic myeloid cells contributes to increased splenic cellularity. We found that a monocyte-like population of adherent splenic cells from Leishmania donovani-infected hamsters had enhanced replicative capacity ex vivo and in vivo (BrdU incorporation, p<0.0001). In vitro assays demonstrated that proliferation was more pronounced in the proinflammatory M1 environment and that intracellular infection prevented proliferation. Secondary analysis of the published splenic transcriptome in the hamster model of progressive VL revealed a gene expression signature that included division of tumoral cells (Z = 2.0), cell cycle progression (Z = 2.3), hematopoiesis (Z = 2.8), proliferation of stem cells (Z = 2.5) and overexpression of proto-oncogenes. Regulators of myeloid cell proliferation were predicted in-silico (CSF2, TLR4, IFNG, IL-6, IL-4, RTK signaling, and STAT3). The in-silico prediction was confirmed with chemical inhibitors of PI3K/AKT, MAPK and STAT3 which decreased splenic myeloid cell division ex vivo. Hamsters infected with L. donovani treated with a STAT3 inhibitor had reduced in situ splenic myeloid proliferation (p = 0.03) and parasite burden. We conclude that monocyte-like myeloid cells have increased STAT3-dependent proliferation in the spleen of hamsters with visceral leishmaniasis and that inhibition of STAT3 reduces myeloid cell proliferation and parasite burden.

Wild-type Kras expands and exhausts hematopoietic stem cells

Oncogenic Kras expression specifically in hematopoietic stem cells (HSCs) induces a rapidly fatal myeloproliferative neoplasm in mice, suggesting that Kras signaling plays a dominant role in normal hematopoiesis. However, such a conclusion is based on expression of an oncogenic version of Kras. Hence, we sought to determine the effect of simply increasing the amount of endogenous wild-type Kras on HSC fate. To this end, we utilized a codon-optimized version of the murine Kras gene (Krasex3op) that we developed, in which silent mutations in exon 3 render the encoded mRNA more efficiently translated, leading to increased protein expression without disruption to the normal gene architecture. We found that Kras protein levels were significantly increased in bone marrow (BM) HSCs in Krasex3op/ex3op mice, demonstrating that the translation of Kras in HSCs is normally constrained by rare codons. Krasex3op/ex3op mice displayed expansion of BM HSCs, progenitor cells, and B lymphocytes, but no evidence of myeloproliferative disease or leukemia in mice followed for 12 months. BM HSCs from Krasex3op/ex3op mice demonstrated increased multilineage repopulating capacity in primary competitive transplantation assays, but secondary competitive transplants revealed exhaustion of long-term HSCs. Following total body irradiation, Krasex3op/ex3op mice displayed accelerated hematologic recovery and increased survival. Mechanistically, HSCs from Krasex3op/ex3op mice demonstrated increased proliferation at baseline, with a corresponding increase in Erk1/2 phosphorylation and cyclin-dependent kinase 4 and 6 (Cdk4/6) activation. Furthermore, both the enhanced colony-forming capacity and in vivo repopulating capacity of HSCs from Krasex3op/ex3op mice were dependent on Cdk4/6 activation. Finally, BM transplantation studies revealed that augmented Kras expression produced expansion of HSCs, progenitor cells, and B cells in a hematopoietic cell-autonomous manner, independent from effects on the BM microenvironment. This study provides fundamental demonstration of codon usage in a mammal having a biological consequence, which may speak to the importance of codon usage in mammalian biology.

From top to bottom: midkine and pleiotrophin as emerging players in immune regulation

Cytokines are pivotal in the generation and resolution of the inflammatory response. The midkine/pleiotrophin (MK/PTN) family of cytokines, composed of just two members, was discovered as heparin-binding neurite outgrowth-promoting factors. Since their discovery, expression of this cytokine family has been reported in a wide array of inflammatory diseases and cancer. In this minireview, we will discuss the emerging appreciation of the functions of the MK/PTN family in the immune system, which include promoting lymphocyte survival, sculpting myeloid cell phenotype, driving immune cell chemotaxis, and maintaining hematopoiesis.

Pleiotrophin mediates hematopoietic regeneration via activation of RAS

Hematopoietic stem cells (HSCs) are highly susceptible to ionizing radiation-mediated death via induction of ROS, DNA double-strand breaks, and apoptotic pathways. The development of therapeutics capable of mitigating ionizing radiation-induced hematopoietic toxicity could benefit both victims of acute radiation sickness and patients undergoing hematopoietic cell transplantation. Unfortunately, therapies capable of accelerating hematopoietic reconstitution following lethal radiation exposure have remained elusive. Here, we found that systemic administration of pleiotrophin (PTN), a protein that is secreted by BM-derived endothelial cells, substantially increased the survival of mice following radiation exposure and after myeloablative BM transplantation. In both models, PTN increased survival by accelerating the recovery of BM hematopoietic stem and progenitor cells in vivo. PTN treatment promoted HSC regeneration via activation of the RAS pathway in mice that expressed protein tyrosine phosphatase receptor-zeta (PTPRZ), whereas PTN treatment did not induce RAS signaling in PTPRZ-deficient mice, suggesting that PTN-mediated activation of RAS was dependent upon signaling through PTPRZ. PTN strongly inhibited HSC cycling following irradiation, whereas RAS inhibition abrogated PTN-mediated induction of HSC quiescence, blocked PTN-mediated recovery of hematopoietic stem and progenitor cells, and abolished PTN-mediated survival of irradiated mice. These studies demonstrate the therapeutic potential of PTN to improve survival after myeloablation and suggest that PTN-mediated hematopoietic regeneration occurs in a RAS-dependent manner.

Eicosapentaenoic acid activates RAS/ERK/C/EBPβ pathway through H-Ras intron 1 CpG island demethylation in U937 leukemia cells

Epigenetic alterations, including aberrant DNA methylation, contribute to tumor development and progression. Silencing of tumor suppressor genes may be ascribed to promoter DNA hypermethylation, a reversible phenomenon intensely investigated as potential therapeutic target. Previously, we demonstrated that eicosapentaenoic acid (EPA) exhibits a DNA demethylating action that promotes the re-expression of the tumor suppressor gene CCAAT/enhancer-binding protein δ (C/EBPδ). The C/EBPβ/C/EBPδ heterodimer formed appears essential for the monocyte differentiation commitment. The present study aims to evaluate the effect of EPA on RAS/extracellular signal regulated kinases (ERK1/2)/C/EBPβ pathway, known to be induced during the monocyte differentiation program. We found that EPA conditioning of U937 leukemia cells activated RAS/ERK/C/EBPβ pathway, increasing the C/EBPβ and ERK1/2 active phosphorylated forms. Transcriptional induction of the upstream activator H-Ras gene resulted in increased expression of H-Ras protein in the active pool of non raft membrane fraction. H-Ras gene analysis identified an hypermethylated CpG island in intron 1 that can affect the DNA-protein interaction modifying RNA polymerase II (RNAPII) activity. EPA treatment demethylated almost completely this CpG island, which was associated with an enrichment of active RNAPII. The increased binding of the H-Ras transcriptional regulator p53 to its consensus sequence within the intronic CpG island further confirmed the effect of EPA as demethylating agent. Our results provide the first evidence that an endogenous polyunsaturated fatty acid (PUFA) promotes a DNA demethylation process responsible for the activation of RAS/ERK/C/EBPβ pathway during the monocyte differentiation commitment. The new role of EPA as demethylating agent paves the way for studying PUFA action when aberrant DNA methylation is involved.

RET fusion genes are associated with chronic myelomonocytic leukemia and enhance monocytic differentiation

Myeloproliferative neoplasms are frequently associated with aberrant constitutive tyrosine kinase (TK) activity resulting from chimaeric fusion genes or point mutations such as BCR-ABL1 or JAK2 V617F. We report here the cloning and functional characterization of two novel fusion genes BCR-RET and FGFR1OP-RET in chronic myelomonocytic leukemia (CMML) cases generated by two balanced translocations t(10;22)(q11;q11) and t(6;10)(q27;q11), respectively. The two RET fusion genes leading to the aberrant activation of RET, are able to transform hematopoietic cells and skew the hematopoietic differentiation program towards the monocytic/macrophage lineage. The RET fusion genes seem to constitutively mimic the same signaling pathway as RAS mutations frequently involved in CMML. One patient was treated with Sorafenib, a specific inhibitor of the RET TK function, and demonstrated cytological and clinical remissions.

A MEK inhibitor abrogates myeloproliferative disease in Kras mutant mice

Chronic and juvenile myelomonocytic leukemias (CMML and JMML) are aggressive myeloproliferative neoplasms that are incurable with conventional chemotherapy. Mutations that deregulate Ras signaling play a central pathogenic role in both disorders, and Mx1-Cre, Kras(LSL-G12D) mice that express the Kras oncogene develop a fatal disease that closely mimics these two leukemias in humans. Activated Ras controls multiple downstream effectors, but the specific pathways that mediate the leukemogenic effects of hyperactive Ras are unknown. We used PD0325901, a highly selective pharmacological inhibitor of mitogen-activated or extracellular signal-regulated protein kinase kinase (MEK), a downstream component of the Ras signaling network, to address how deregulated Raf/MEK/ERK (extracellular signal-regulated kinase) signaling drives neoplasia in Mx1-Cre, Kras(LSL-G12D) mice. PD0325901 treatment induced a rapid and sustained reduction in leukocyte counts, enhanced erythropoiesis, prolonged mouse survival, and corrected the aberrant proliferation and differentiation of bone marrow progenitor cells. These responses were due to direct effects of PD0325901 on Kras mutant cells rather than to stimulation of normal hematopoietic cell proliferation. Consistent with the in vivo response, inhibition of MEK reversed the cytokine hypersensitivity characteristic of Kras(G12D) hematopoietic progenitor cells in vitro. Our data demonstrate that deregulated Raf/MEK/ERK signaling is integral to the growth of Kras-mediated myeloproliferative neoplasms and further suggest that MEK inhibition could be a useful way to ameliorate functional hematologic abnormalities in patients with CMML and JMML.

Regulation of the adaptor molecule LAT2, an in vivo target gene of AML1/ETO (RUNX1/RUNX1T1), during myeloid differentiation

The leukaemia-specific fusion oncoprotein RUNX1/RUNX1T1 (AML1/ETO), resulting from the chromosomal translocation (8;21) in acute myeloid leukaemia (AML), imposes a striking genotype-phenotype relationship upon this distinct subtype of AML, which is mediated by multiple, co-ordinate downstream effects induced by this chimeric transcription factor. We previously identified the LAT2 gene, encoding the adaptor molecule LAT2 (NTAL, LAB), which is phosphorylated by KIT and has a role in mast cell and B-cell activation, as a target of the repressor activity of RUNX1/RUNX1T1. These results were confirmed and extended by demonstrating downregulation of the LAT2 protein in response to conditional RUNX1/RUNX1T1 expression, and its absence in primary AML with the t(8;21). In contrast, in a cohort of 43 AML patients, higher levels of LAT2 were associated with myelomonocytic features. Differentiation of HL-60 and NB4 cells towards granulocytes by all trans-retinoic acid (ATRA) resulted in downregulation of LAT2; conversely, it was upregulated during phorbol ester-induced monocytic differentiation of HL-60 cells. Forced expression of LAT2 in Kasumi-1 cells resulted in a striking block of ATRA- and phorbol ester-induced differentiation, implicating disturbances of the graded expression of this adaptor molecule in the maturation block of myeloid leukaemia cells.

Autologous serum improves yield and metabolic capacity of monocyte-derived hepatocyte-like cells: possible implication for cell transplantation

Hepatocyte-transplantation is a therapeutic approach for diverse acute and chronic liver diseases. As availability of primary cells is limited, there is an increasing demand for hepatocyte-like cells (e.g., neohepatocytes generated from peripheral blood monocytes). The aim of this study was to evaluate the effects of six different human AB sera, fetal calf serum, or autologous serum on production of neohepatocytes. The yield and quality of neohepatocytes varied considerably depending on the different sera. Using autologous sera for the whole production process we constantly generated the highest amount of cells with the highest metabolic activity for phase I (e.g., CYP1A1/2, CYP3A4) and phase II enzymes (e.g., glutathione-S-transferase). Moreover, similar effects were seen examining glucose and urea metabolism. Especially, glucose-6-phosphatase and PAS staining showed distinct serum-dependent differences. The role of macrophage activation was investigated by measuring the secretion of TNF-α, TGF-β, and RANKL, MMP activity, as well as mRNA levels of different interleukins in programmable cells of monocytic origin (PCMO). Our data clearly demonstrate that the use of autologous serum reduced initial macrophage activation in PCMOs and subsequently improved both yield and function of differentiated neohepatocytes. The autologous approach presented here might also be useful in other stem cell preparation processes where cell activation during generation shall be kept to a minimum.

Ras-induced reactive oxygen species promote growth factor-independent proliferation in human CD34+ hematopoietic progenitor cells

Excessive production of reactive oxygen species (ROS) is a feature of human malignancy and is often triggered by activation of oncogenes such as activated Ras. ROS act as second messengers and can influence a variety of cellular process including growth factor responses and cell survival. We have examined the contribution of ROS production to the effects of N-Ras(G12D) and H-Ras(G12V) on normal human CD34(+) progenitor cells. Activated Ras strongly up-regulated the production of both superoxide and hydrogen peroxide through the stimulation of NADPH oxidase (NOX) activity, without affecting the expression of endogenous antioxidants or the production of mitochondrially derived ROS. Activated Ras also promoted both the survival and the growth factor-independent proliferation of CD34(+) cells. Using oxidase inhibitors and antioxidants, we found that excessive ROS production by these cells did not contribute to their enhanced survival; rather, ROS promoted their growth factor-independent proliferation. Although Ras-induced ROS production specifically activated the p38(MAPK) oxidative stress response, this failed to induce expression of the cell-cycle inhibitor, p16(INK4A); instead, ROS promoted the expression of D cyclins. These data are the first to show that excessive ROS production in the context of oncogene activation can promote proliferative responses in normal human hematopoietic progenitor cells.

Increased c-Jun expression and reduced GATA2 expression promote aberrant monocytic differentiation induced by activating PTPN11 mutants

Juvenile myelomonocytic leukemia (JMML) is characterized by myelomonocytic cell overproduction and commonly bears activating mutations in PTPN11. Murine hematopoietic progenitors expressing activating Shp2 undergo myelomonocytic differentiation, despite being subjected to conditions that normally support only mast cells. Evaluation of hematopoietic-specific transcription factor expression indicates reduced GATA2 and elevated c-Jun in mutant Shp2-expressing progenitors. We hypothesized that mutant Shp2-induced Ras hyperactivation promotes c-Jun phosphorylation and constitutive c-Jun expression, permitting, as a coactivator of PU.1, excessive monocytic differentiation and reduced GATA2. Hematopoietic progenitors expressing activating Shp2 demonstrate enhanced macrophage CFU (CFU-M) compared to that of wild-type Shp2-expressing cells. Treatment with the JNK inhibitor SP600125 or cotransduction with GATA2 normalizes activating Shp2-generated CFU-M. However, cotransduction of DeltaGATA2 (lacking the C-terminal zinc finger, needed to bind PU.1) fails to normalize CFU-M. NIH 3T3 cells expressing Shp2E76K produce higher levels of luciferase expression directed by the macrophage colony-stimulating factor receptor (MCSFR) promoter, which utilizes c-Jun as a coactivator of PU.1. Coimmunoprecipitation demonstrates increased c-Jun-PU.1 complexes in mutant Shp2-expressing hematopoietic progenitors, while chromatin immunoprecipitation demonstrates increased c-Jun binding to the c-Jun promoter and an increased c-Jun-PU.1 complex at the Mcsfr promoter. Furthermore, JMML progenitors express higher levels of c-JUN than healthy controls, substantiating the disease relevance of these mechanistic findings.

RAS oncogene suppression induces apoptosis followed by more differentiated and less myelosuppressive disease upon relapse of acute myeloid leukemia

To study the oncogenic role of the NRAS oncogene (NRAS(G12V)) in the context of acute myeloid leukemia (AML), we used a Vav promoter-tetracycline transactivator (Vav-tTA)-driven repressible TRE-NRAS(G12V) transgene system in Mll-AF9 knock-in mice developing AML. Conditional repression of NRAS(G12V) expression greatly reduced peripheral white blood cell (WBC) counts in leukemia recipient mice and induced apoptosis in the transplanted AML cells correlated with reduced Ras/Erk signaling. After marked decrease of AML blast cells, myeloproliferative disease (MPD)-like AML relapsed characterized by cells that did not express NRAS(G12V). In comparison with primary AML, the MPD-like AML showed significantly reduced aggressiveness, reduced myelosuppression, and a more differentiated phenotype. We conclude that, in AML induced by an Mll-AF9 transgene, NRAS(G12V) expression contributes to acute leukemia maintenance by suppressing apoptosis and reducing differentiation of leukemia cells. Moreover, NRAS(G12V) oncogene has a cell nonautonomous role in suppressing erythropoiesis that results in the MPD-like AML show significantly reduced ability to induce anemia. Our results imply that targeting NRAS or RAS oncogene-activated pathways is a good therapeutic strategy for AML and attenuating aggressiveness of relapsed AML.

Src-family kinases play an essential role in differentiation signaling downstream of macrophage colony-stimulating factor receptors mediating persistent phosphorylation of phospholipase C-gamma2 and MAP kinases ERK1 and ERK2

Macrophage colony-stimulating factor (M-CSF) has been found to be involved in multiple developmental processes, especially production of cells belonging to the mononuclear phagocyte system. The decision of myeloid progenitor cells to commit to differentiation depends on activation levels of the mitogen-activated protein kinases (MAPK), ERK1 and ERK2. Using the murine myeloid progenitor cell line FD-Fms, we show here that persistent activity of Src-family kinases (SFK) is necessary for FD-Fms cell differentiation to macrophages in response to M-CSF. Chemical inhibition of SFK blocked FD-Fms cell differentiation while it caused strong inhibition of the late phosphorylation of phospholipase C (PLC)-gamma2 and MAPK. The PLC inhibitor U73122, previously shown to block M-CSF-induced differentiation, strongly decreased long-term MAPK phosphorylation. Interestingly, inhibiting SFK with SU6656 or the MAPK kinases MEK with U0126 significantly impaired development of mononuclear phagocytes in cultures of mouse bone marrow cells stimulated with M-CSF. Collectively, results support a model in which SFK are required for sustained PLC activity and MAPK activation above threshold required for commitment of myeloid progenitors to macrophage differentiation.

N-ras oncogene–induced gene expression in human hematopoietic progenitor cells: Upregulation of p16INK4a and p21CIP1/WAF1 correlates with myeloid differentiation

OBJECTIVES

Mutations in ras oncogenes occur at high frequency in acute myeloid leukemia and myelodysplastic syndromes; however, the role of ras genes in leukemogenesis has not been clearly defined. Our previous studies have shown that expression of mutant N-ras (N-rasG13R, G to C transversion) in human hematopoietic progenitor cells (HPC) promotes myeloid differentiation and proliferation both in vitro and in a NOD/SCID mouse model. In the present study, we performed expression profiling to identify the transcriptome induced by N-rasG13R in human HPC, and analyzed the effect of mutant N-ras in sorted specific subpopulations of HPC.

METHODS

cDNA microarray analysis was performed on cord blood CD34(+) cells transduced with a retrovirus containing GFP alone or in combination with mutant N-ras. Transduced cells were also sorted into factorial subpopulations according to CD34 and transgene expression, and analyzed in suspension or semi-solid methylcellulose culture.

RESULTS

Among a variety of changes, including upregulation of cytokine genes, we found that N-rasG13R induced expression of the cyclin-dependent kinase inhibitors p16(INK4a) and p21(CIP1/WAF1). Analysis by RT-PCR revealed that increased p16(INK4a) and p21(CIP1/WAF1) occurred in the most primitive, CD34(+)/Ras(+) population but not in the more mature CD34(-)/Ras(+) cells or in the CD34(+)/Ras(-) cells. Moreover, N-rasG13R inhibited the proliferation of the primitive CD34(+)/Ras(+) cells, both in liquid culture and in colony assays. This growth suppression correlated with an increased proportion of myelomonocytic colonies and a decrease of erythroid colonies. In contrast, the growth of CD34(-)/Ras(+) cells and CD34(+)/Ras(-) HPC was not inhibited.

CONCLUSIONS

These findings demonstrated the mutant N-ras induced transcriptome, and that this is associated with HPC growth suppression/myelomonocytic differentiation, and identify upregulation of cyclin inhibitors as key events in this process. The results indicate that ras mutation alone is not sufficient to induce leukemogenesis; collaborative secondary event(s) are involved in the process.

The role of PKC and PDK1 in monocyte lineage specification by Ras

Although hyperactivation of Ras is a common feature of myeloid malignancy, its role in subverting hematopoiesis is unclear. We have examined the influence of Ras on normal human uncommitted myeloid subsets and show that expression of this oncogene strongly favors monocyte lineage selection in bipotential granulocyte/macrophage progenitors while inhibiting colony formation in other uncommitted subsets. Ras also promoted monocytic differentiation but not the proliferation of these cells. The mechanism through which Ras drives monocyte lineage selection was dependent on PKC activity and Ras was found to promote the expression, membrane translocation, and phosphorylation of conventional and novel PKC isoforms. We further show that Ras promoted the expression of the AGC kinase master regulator, PDK1, which maintains the stability and activity of PKC isoforms. Consistent with this, overexpression of PDK1 itself promoted monocyte colony formation and translocation of PKC. Overexpression of PDK1 was found to be a common feature of acute myeloid leukemia (45% of patients) and was closely associated with hyperphosphorylation of PKC. These data demonstrate that Ras is able to promote monocyte lineage selection via PKC and show for the first time the involvement of the kinase master regulator, PDK1, in both lineage specification and in human leukemia.

Ral is both necessary and sufficient for the inhibition of myeloid differentiation mediated by Ras

Hyperactivation of Ras is one of the most common abnormalities in acute myeloid leukemia. In experimental models, Ras inhibits myeloid differentiation, which is characteristic of leukemia; however, the mechanism through which it disrupts hematopoiesis is poorly understood. In multipotent FDCP-mix cells, Ras inhibits terminal neutrophil differentiation, thereby indefinitely extending their proliferative potential. Ras also strongly promotes the sensitivity of these cells to granulocyte-macrophage colony-stimulating factor (GM-CSF). Using this model, we have dissected the signaling elements downstream of Ras to determine their relative contribution to the dysregulation of hematopoiesis. Cells expressing Ras mutants selectively activating Raf (Ras*T35S) or phosphatidylinositol 3-kinase (Ras*Y40C) did not significantly affect differentiation or proliferative capacity, whereas Ras*E37G (which selectively activates RalGEFs) perpetuated proliferation and blocked neutrophil development in a manner similar to that of Ras. Correspondingly, expression of constitutively active versions of these effectors confirmed the overriding importance of Ral guanine nucleotide exchange factors. Cells expressing Ras demonstrated hyperactivation of Ral, which itself was able to exactly mimic the phenotype of Ras, including hypersensitivity to GM-CSF. Conversely, dominant negative Ral promoted spontaneous neutrophil development. Ral, in turn, appears to influence differentiation through multiple effectors. These data show, for the first time, the importance of Ral in regulating differentiation and self-renewal in hematopoietic cells.

Memory T and memory B cells share a transcriptional program of self-renewal with long-term hematopoietic stem cells

The only cells of the hematopoietic system that undergo self-renewal for the lifetime of the organism are long-term hematopoietic stem cells and memory T and B cells. To determine whether there is a shared transcriptional program among these self-renewing populations, we first compared the gene-expression profiles of naïve, effector and memory CD8(+) T cells with those of long-term hematopoietic stem cells, short-term hematopoietic stem cells, and lineage-committed progenitors. Transcripts augmented in memory CD8(+) T cells relative to naïve and effector T cells were selectively enriched in long-term hematopoietic stem cells and were progressively lost in their short-term and lineage-committed counterparts. Furthermore, transcripts selectively decreased in memory CD8(+) T cells were selectively down-regulated in long-term hematopoietic stem cells and progressively increased with differentiation. To confirm that this pattern was a general property of immunologic memory, we turned to independently generated gene expression profiles of memory, naïve, germinal center, and plasma B cells. Once again, memory-enriched and -depleted transcripts were also appropriately augmented and diminished in long-term hematopoietic stem cells, and their expression correlated with progressive loss of self-renewal function. Thus, there appears to be a common signature of both up- and down-regulated transcripts shared between memory T cells, memory B cells, and long-term hematopoietic stem cells. This signature was not consistently enriched in neural or embryonic stem cell populations and, therefore, appears to be restricted to the hematopoeitic system. These observations provide evidence that the shared phenotype of self-renewal in the hematopoietic system is linked at the molecular level.

Bypass of senescence, immortalization, and transformation of human hematopoietic progenitor cells

We attempted to extend the lifespan of CD34+ stem/progenitor cells in human cord blood (CB) by transduction with lentiviral vectors carrying the human telomerase catalytic subunit (hTERT) and/or the human papillomavirus type 16 (HPV16) E6 and E7 oncogenes. We found that hTERT was incapable of prolonging the replicative capacity of CB cells maintained under serum-free conditions in the presence of stem cell factor, Flt3 ligand, thrombopoietin, and interleukin-3 beyond 4 months (n=3). However, transduced CB cells cultured in the same cytokine cocktail constitutively expressing HPV16 E6/E7 alone (n=2) or in concert with hTERT (n=9) continued to proliferate, giving rise to permanent (>2 years) cell lines with a CD45+ CD34- CD133+/- CD44+ CD235a+ CD71+ CD203+ CD33+ CD13+ myeloerythroid/mast cell progenitor phenotype. Notably, CB cell cultures expressing only HPV16 E6/E7 went through a crisis period, and the resulting oligoclonal cell lines were highly aneuploid. By comparison, the CB cell lines obtained by coexpression of HPV16 E6/E7 plus hTERT exhibited near-diploid karyotypes with minimal chromosomal aberrations, concomitant with stabilization of telomere length, yet were clonally derived. The immortalized E6/E7 plus hTERT-expressing CB cells were not tumorigenic when injected intravenously or subcutaneously into sublethally irradiated immunodeficient nonobese diabetic/severe combined immunodeficient mice but could be converted to a malignant state by ectopic expression of a v-H-ras or BCR-ABL oncogene. These findings provide new insights into the mechanisms governing the senescence checkpoint of primitive human hematopoietic precursors and establish a paradigm for studies of the multistep process of human leukemogenesis.

Activation of the RAS pathway is predictive for a chemosensitive phenotype of acute myelogenous leukemia blasts

PURPOSE

Activation of the RAS pathway plays a major role in cancer cells. In acute myeloid leukemia (AML), mutations of the RAS genes cause an intrinsic activation of this pathway. Until now, clinical studies could not find clear association of RAS mutations with the clinical outcome after AML therapy. This could be due to alternative initiating events for activation of the RAS pathway like constitutive tyrosine kinase activation or mutations in Ras-regulating genes.

EXPERIMENTAL DESIGN

In total, 191 AML patients (126 as training population and 65 as test population) were studied for Ras activity with a glutathione S-transferase pull-down assay using Raf binding of activated Ras.

RESULTS

AML samples showed a wide range of Ras activity values, which was in contrast to normal bone marrow donors who showed no or very limited Ras activity. Using a Ras binding score based on semiquantitative Western blotting, we defined patients with strong Ras activity and compared Ras activity with RAS mutation. Surprisingly, only a minority of RAS mutated AML samples (22.2%) showed strong Ras activity, whereas 25 patients presented strong Ras activity in the absence of RAS mutations. Clinical outcome did not show differences according to RAS mutations. In contrast, Ras activity predicted for a high response rate (P <0.05) and proved to be an independent factor for overall survival rate (P <0.05) in younger AML patients receiving high-dose 1-beta-D-arabinofuranosylcytosine as induction therapy.

CONCLUSION

The data highlight the role for alternative pathways of Ras activation without RAS mutations. Intrinsically activated Ras seems to increase sensitivity of the AML blast to high-dose 1-beta-D-arabinofuranosylcytosine therapy.

Lonafarnib reduces the resistance of primitive quiescent CML cells to imatinib mesylate in vitro

Recent studies indicate that a rare population of primitive quiescent BCR-ABL(+) cells are innately insensitive to imatinib mesylate (IM) and persist after IM therapy of patients with chronic myeloid leukemia (CML). New approaches to the eradication of these cells are therefore likely to be crucial to the development of curative therapies for CML. We have now found that Ara-C, LY294002 (a PI-3 (phosphatidylinositol-3' kinase) kinase inhibitor), 17AAG (a heat-shock protein (HSP)-90 antagonist) and lonafarnib (a farnesyltransfease inhibitor) all enhance the toxicity of IM on K562 cells and on the total CD34(+) leukemic cell population from chronic phase CML patients. However, for quiescent CD34(+) leukemic cells, this was achieved only by concomitant exposure of the cells to lonafarnib. Ara-C or LY294002 alone blocked the proliferation of these cells but did not kill them, and Ara-C, LY294002 or 17AAG in combination with IM enhanced the cytostatic effect of IM but did not prevent the subsequent regrowth of the surviving leukemic cells. These studies demonstrate the importance of in vitro testing of novel agents on the subset of primary leukemic cells most likely to determine long-term treatment outcomes in vivo.

Mammary stem cells, self-renewal pathways, and carcinogenesis

The mammary gland epithelial components are thought to arise from stem cells that undergo both self-renewal and differentiation. Self-renewal has been shown to be regulated by the Hedgehog, Notch, and Wnt pathways and the transcription factor B lymphoma Mo-MLV insertion region 1 (Bmi-1). We review data about the existence of stem cells in the mammary gland and the pathways regulating the self-renewal of these cells. We present evidence that deregulation of the self-renewal in stem cells/progenitors might be a key event in mammary carcinogenesis. If 'tumor stem cells' are inherently resistant to current therapies, targeting stem cell self-renewal pathways might provide a novel approach for breast cancer treatment.

M-CSF stimulated differentiation requires persistent MEK activity and MAPK phosphorylation independent of Grb2-Sos association and phosphatidylinositol 3-kinase activity

Macrophage colony-stimulating factor (M-CSF) is a physiological regulator of monocyte-macrophage lineage. Ectopic expression of the M-CSF receptor (M-CSFR, or Fms) in murine myeloid cell line FDC-P1 (FD/Fms cells) results in M-CSF-dependent macrophage differentiation. Previously, we observed that M-CSF induces two temporally distinct phases of mitogen-activated protein kinase (MAPK) phosphorylation. Here we show that levels of phosphorylated MAPK kinase MEK1 follow the same kinetics as MAPK phosphorylation, characterized by an early and transient phase (the first 30 min of M-CSF stimulation) and a late and persistent phase from 4 h of stimulation. The MEK inhibitor U0126 strongly inhibited both phases of MAPK phosphorylation as well as FD/Fms cell differentiation, indicating that MAPK may relay M-CSF differentiation signaling downstream of M-CSFR. Treatment of FD/Fms cells with U0126 during the first hour of M-CSF stimulation reversibly blocked the early phase of MAPK phosphorylation but did not affect differentiation. In contrast, U0126 still inhibited FD/Fms cell differentiation when its addition was delayed by 24 h. This demonstrated that late and persistent MEK activity is specifically required for macrophage differentiation to occur. Furthermore, disrupting Grb2-Sos complexes with a specific blocking peptide did not prevent FD/Fms cells differentiation in response to M-CSF, nor did it abolish MAPK phosphorylation. The role of phosphatidylinositol 3-kinase (PI 3-kinase), another potential regulator of the MAPK pathway, was examined using the specific inhibitor LY294002. This compound could not impede FD/Fms cell commitment to macrophage differentiation and did not significantly affect MAPK phosphorylation in response to M-CSF. Therefore, M-CSF differentiation signaling in myeloid progenitor cells is mediated through persistent MEK activity but it is not strictly dependent upon Grb2-Sos interaction or PI 3-kinase activity.

Functional analysis of leukemia-associated PTPN11 mutations in primary hematopoietic cells

PTPN11 encodes the protein tyrosine phosphatase SHP-2, which relays signals from growth factor receptors to Ras and other effectors. Germline PTPN11 mutations underlie about 50% of Noonan syndrome (NS), a developmental disorder that is associated with an elevated risk of juvenile myelomonocytic leukemia (JMML). Somatic PTPN11 mutations were recently identified in about 35% of patients with JMML; these mutations introduce amino acid substitutions that are largely distinct from those found in NS. We assessed the functional consequences of leukemia-associated PTPN11 mutations in murine hematopoietic cells. Expressing an E76K SHP-2 protein induced a hypersensitive pattern of granulocyte-macrophage colony-forming unit (CFU-GM) colony growth in response to granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin 3 (IL-3) that was dependent on SHP-2 catalytic activity. E76K SHP-2 expression also enhanced the growth of immature progenitor cells with high replating potential, perturbed erythroid growth, and impaired normal differentiation in liquid cultures. In addition, leukemia-associated SHP-2 mutations conferred a stronger phenotype than a germline mutation found in patients with NS. Mutant SHP-2 proteins induce aberrant growth in multiple hematopoietic compartments, which supports a primary role of hyperactive Ras in the pathogenesis of JMML.

Recent advances in the development of small-molecule inhibitors for the treatment of acute myeloid leukemia

PURPOSE OF REVIEW

This review outlines recent advances in the development of small-molecule inhibitors of molecular signaling pathways for the treatment of acute myeloid leukemia (AML). These compounds are typically targeted against components of the tyrosine kinase-Ras-Map kinase pathway that have been activated by mutation.

RECENT FINDINGS

Several agents have been tested in phase 2 trials, with only modest clinical results thus far. Careful correlative studies have allowed a clearer understanding of the reasons for the success or failure of these agents and have refined our approach to clinical trial design. In some cases, the target molecule has been successfully inhibited, but for an inadequate duration, and in other cases, inhibiting the target has little correlation with clinical effect.

SUMMARY

Small-molecule inhibitors of these molecular pathways clearly have significant promise for the treatment of AML, but several obstacles remain, and this field of pharmacotherapy is still quite new. These inhibitors seem unlikely to be curative when administered as monotherapy but rather will have to be used in combination with one another or with conventional chemotherapy. In addition, pharmacokinetic problems must be overcome with many of them.