Clarifying the role of Stat5 in lymphoid development and Abelson-induced transformation

Acute lymphoblastic leukemia and developmental biology: a crucial interrelationship

The latest scientific findings in the field of cancer research are redefining our understanding of the molecular and cellular basis of the disease, moving the emphasis toward the study of the mechanisms underlying the alteration of the normal processes of cellular differentiation. The concepts best exemplifying this new vision are those of cancer stem cells and tumoral reprogramming. The study of the biology of acute lymphoblastic leukemias (ALLs) has provided seminal experimental evidence supporting these new points of view. Furthermore, in the case of B cells, it has been shown that all the stages of their normal development show a tremendous degree of plasticity, allowing them to be reprogrammed to other cellular types, either normal or leukemic. Here we revise the most recent discoveries in the fields of B-cell developmental plasticity and B-ALL research and discuss their interrelationships and their implications for our understanding of the biology of the disease.

IL-2 regulates expression of C-MAF in human CD4 T cells

Blockade of IL-2R with humanized anti-CD25 Abs, such as daclizumab, inhibits Th2 responses in human T cells. Recent murine studies have shown that IL-2 also plays a significant role in regulating Th2 cell differentiation by activated STAT5. To explore the role of activated STAT5 in the Th2 differentiation of primary human T cells, we studied the mechanisms underlying IL-2 regulation of C-MAF expression. Chromatin immunoprecipitation studies revealed that IL-2 induced STAT5 binding to specific sites in the C-MAF promoter. These sites corresponded to regions enriched for markers of chromatin architectural features in both resting CD4 and differentiated Th2 cells. Unlike IL-6, IL-2 induced C-MAF expression in CD4 T cells with or without prior TCR stimulation. TCR-induced C-MAF expression was significantly inhibited by treatment with daclizumab or a JAK3 inhibitor, R333. Furthermore, IL-2 and IL-6 synergistically induced C-MAF expression in TCR-activated T cells, suggesting functional cooperation between these cytokines. Finally, both TCR-induced early IL4 mRNA expression and IL-4 cytokine expression in differentiated Th2 cells were significantly inhibited by IL-2R blockade. Thus, our findings demonstrate the importance of IL-2 in Th2 differentiation in human T cells and support the notion that IL-2R-directed therapies may have utility in the treatment of allergic disorders.

Ebf1 or Pax5 haploinsufficiency synergizes with STAT5 activation to initiate acute lymphoblastic leukemia

As STAT5 is critical for the differentiation, proliferation, and survival of progenitor B cells, this transcription factor may play a role in acute lymphoblastic leukemia (ALL). Here, we show increased expression of activated signal transducer and activator of transcription 5 (STAT5), which is correlated with poor prognosis, in ALL patient cells. Mutations in EBF1 and PAX5, genes critical for B cell development have also been identified in human ALL. To determine whether mutations in Ebf1 or Pax5 synergize with STAT5 activation to induce ALL, we crossed mice expressing a constitutively active form of STAT5 (Stat5b-CA) with mice heterozygous for Ebf1 or Pax5. Haploinsufficiency of either Pax5 or Ebf1 synergized with Stat5b-CA to rapidly induce ALL in 100% of the mice. The leukemic cells displayed reduced expression of both Pax5 and Ebf1, but this had little effect on most EBF1 or PAX5 target genes. Only a subset of target genes was deregulated; this subset included a large percentage of potential tumor suppressor genes and oncogenes. Further, most of these genes appear to be jointly regulated by both EBF1 and PAX5. Our findings suggest a model whereby small perturbations in a self-reinforcing network of transcription factors critical for B cell development, specifically PAX5 and EBF1, cooperate with STAT5 activation to initiate ALL.

Signaling proteins and transcription factors in normal and malignant early B cell development

B cell development starts in bone marrow with the commitment of hematopoietic progenitors to the B cell lineage. In murine models, the IL-7 and preBCR receptors, and the signaling pathways and transcription factors that they regulate, control commitment and maintenance along the B cell pathway. E2A, EBF1, PAX5, and Ikaros are among the most important transcription factors controlling early development and thereby conditioning mice homeostatic B cell lymphopoiesis. Importantly, their gain or loss of function often results in malignant development in humans, supporting conserved roles for these transcription factors. B cell acute lymphoblastic leukemia is the most common cause of pediatric cancer, and it is characterized by unpaired early B cell development resulting from genetic lesions in these critical signaling pathways and transcription factors. Fine mapping of these genetic abnormalities is allowing more specific treatments, more accurately predicting risk profiles for this disease, and improving survival rates.

Tracking the activation of Stat5 through the expression of an inducible reporter gene in a transgenic mouse line

Signal transducer and activator of transcription 5 (Stat5), a latent cytoplasmic transcription factor, becomes activated by phosphorylation upon cytokine, hormone, and growth factor interactions with their appropriate receptors and induces the transcription of target genes. It plays crucial roles in principal cell fate decisions and regulates cell differentiation, development, proliferation, apoptosis, and inflammation. It is active in the mammary gland, the liver, hematopoietic cells, and other organs and has pleiotropic functions, depending on its activation pathway and its site of action. We derived transgenic mice in which the expression of a LacZ reporter gene is directed by Stat5-specific response elements and visualized the activation of Stat5 in cells of mouse organs at different developmental stages. The reporter gene activity reflects the timing and the location of Stat5 activation and was documented in mammary epithelial cells during developmental stages of the gland, cells of the liver, kidney, spleen, thymus, and uterus and in granulocytes and macrophages of the transgenic lines.

Sensitized phenotypic screening identifies gene dosage sensitive region on chromosome 11 that predisposes to disease in mice

The identification of susceptibility genes for human disease is a major goal of current biomedical research. Both sequence and structural variation have emerged as major genetic sources of phenotypic variability and growing evidence points to copy number variation as a particularly important source of susceptibility for disease. Here we propose and validate a strategy to identify genes in which changes in dosage alter susceptibility to disease-relevant phenotypes in the mouse. Our approach relies on sensitized phenotypic screening of megabase-sized chromosomal deletion and deficiency lines carrying altered copy numbers of ∼30 linked genes. This approach offers several advantages as a method to systematically identify genes involved in disease susceptibility. To examine the feasibility of such a screen, we performed sensitized phenotyping in five therapeutic areas (metabolic syndrome, immune dysfunction, atherosclerosis, cancer and behaviour) of a 0.8 Mb reciprocal chromosomal duplication and deficiency on chromosome 11 containing 27 genes. Gene dosage in the region significantly affected risk for high-fat diet-induced metabolic syndrome, antigen-induced immune hypersensitivity, ApoE-induced atherosclerosis, and home cage activity. Follow up studies on individual gene knockouts for two candidates in the region showed that copy number variation in Stat5 was responsible for the phenotypic variation in antigen-induced immune hypersensitivity and metabolic syndrome. These data demonstrate the power of sensitized phenotypic screening of segmental aneuploidy lines to identify disease susceptibility genes.

The role of STAT5 in the development, function, and transformation of B and T lymphocytes

The transcription factor signal transducer and activator of transcription 5 (STAT5) is activated by a number of cytokine and growth hormone receptors and plays a key role in the development and function of many organ systems. In this review, we focus on recent discoveries about the role of STAT5 in the development and function of B and T lymphocytes. Of particular interest is the growing appreciation for the function of STAT5 as a transcriptional repressor. Finally, we discuss recent discoveries about the role of STAT5 in transformation of B and T lymphocytes.

High STAT5 levels mediate imatinib resistance and indicate disease progression in chronic myeloid leukemia

In BCR-ABL1(+) leukemia, drug resistance is often associated with up-regulation of BCR-ABL1 or multidrug transporters as well as BCR-ABL1 mutations. Here we show that the expression level of the transcription factor STAT5 is another parameter that determines the sensitivity of BCR-ABL1(+) cells against tyrosine kinase inhibitors (TKIs), such as imatinib, nilotinib, or dasatinib. Abelson-transformed cells, expressing high levels of STAT5, were found to be significantly less sensitive to TKI-induced apoptosis in vitro and in vivo but not to other cytotoxic drugs, such as hydroxyurea, interferon-β, or Aca-dC. The STAT5-mediated protection requires tyrosine phosphorylation of STAT5 independent of JAK2 and transcriptional activity. In support of this concept, under imatinib treatment and with disease progression, STAT5 mRNA and protein levels increased in patients with Ph(+) chronic myeloid leukemia. Based on our data, we propose a model in which disease progression in BCR-ABL1(+) leukemia leads to up-regulated STAT5 expression. This may be in part the result of clonal selection of cells with high STAT5 levels. STAT5 then accounts for the resistance against TKIs, thereby explaining the dose escalation frequently required in patients reaching accelerated phase. It also suggests that STAT5 may serve as an attractive target to overcome imatinib resistance in BCR-ABL1(+) leukemia.

A novel Ncr1-Cre mouse reveals the essential role of STAT5 for NK-cell survival and development

We generated a transgenic mouse line that expresses the Cre recombinase under the control of the Ncr1 (p46) promoter. Cre-mediated recombination was tightly restricted to natural killer (NK) cells, as revealed by crossing Ncr1-iCreTg mice to the eGFP-LSLTg reporter strain. Ncr1-iCreTg mice were further used to study NK cell-specific functions of Stat5 (signal transducers and activators of transcription 5) by generating Stat5(f/f) Ncr1-iCreTg animals. Stat5(f/f) Ncr1-iCreTg mice were largely devoid of NK cells in peripheral lymphoid organs. In the bone marrow, NK-cell maturation was abrogated at the NK cell-precursor stage. Moreover, we found that in vitro deletion of Stat5 in interleukin 2-expanded NK cells was incompatible with NK-cell viability. In vivo assays confirmed the complete abrogation of NK cell-mediated tumor control against B16F10-melanoma cells. In contrast, T cell-mediated tumor surveillance against MC38-adenocarcinoma cells was undisturbed. In summary, the results of our study show that STAT5 has a cell-intrinsic role in NK-cell development and that Ncr1-iCreTg mice are a powerful novel tool with which to study NK-cell development, biology, and function.

The tumor suppressor hTid1 inhibits STAT5b activity via functional interaction

STAT5a and -5b (signal transducers and activators of transcription 5a and 5b) proteins play an essential role in hematopoietic cell proliferation and survival and are frequently constitutively active in hematologic neoplasms and solid tumors. Because STAT5a and STAT5b differ mainly in the carboxyl-terminal transactivation domain, we sought to identify new proteins that bind specifically to this domain by using a bacterial two-hybrid screening. We isolated hTid1, a human DnaJ protein that acts as a tumor suppressor in various solid tumors. hTid1 interacts specifically with STAT5b but not with STAT5a in hematopoietic cell lines. This interaction involves the cysteine-rich region of the hTid1 DnaJ domain. We also demonstrated that hTid1 negatively regulates the expression and transcriptional activity of STAT5b and suppresses the growth of hematopoietic cells transformed by an oncogenic form of STAT5b. Our findings define hTid1 as a novel partner and negative regulator of STAT5b.

The role of Stat5 transcription factors as tumor suppressors or oncogenes

Stat5 is constitutively activated in many human cancers affecting the expression of cell proliferation and cell survival controlling genes. These oncogenic functions of Stat5 have been elegantly reproduced in mouse models. Aberrant Stat5 activity induces also mitochondrial dysfunction and reactive oxygen species leading to DNA damage. Although DNA damage can stimulate tumorigenesis, it can also prevent it. Stat5 can inhibit tumor progression like in the liver and it is a tumor suppressor in fibroblasts. Stat5 proteins are able to regulate cell differentiation and senescence activating the tumor suppressors SOCS1, p53 and PML. Understanding the context dependent regulation of tumorigenesis through Stat5 function will be central to understand proliferation, survival, differentiation or senescence of cancer cells.

Epigenetic regulation of V(D)J recombination

Chromosome breaks are dangerous business, carrying the risk of loss of genetic information or, even worse, misrepair of the break, leading to outcomes such as dicentric chromosomes or oncogenic translocations. Yet V(D)J recombination, a process that breaks, rearranges and repairs chromosomes, is crucial to the development of the adaptive immune system, for it gives B- and T-cells the capacity to generate a virtually unlimited repertoire of antigen receptor proteins to combat an equally vast array of antigens. To minimize the risks inherent in chromosomal breakage, V(D)J recombination is carefully orchestrated at multiple levels, ranging from DNA sequence requirements all the way up to chromatin conformation and nuclear architecture. In the present chapter we introduce various regulatory controls, with an emphasis on epigenetic mechanisms and recent work that has begun to elucidate their interdependence.

Akt is required for Stat5 activation and mammary differentiation

INTRODUCTION

The Akt pathway plays a central role in regulating cell survival, proliferation and metabolism, and is one of the most commonly activated pathways in human cancer. A role for Akt in epithelial differentiation, however, has not been established. We previously reported that mice lacking Akt1, but not Akt2, exhibit a pronounced metabolic defect during late pregnancy and lactation that results from a failure to upregulate Glut1 as well as several lipid synthetic enzymes. Despite this metabolic defect, however, both Akt1-deficient and Akt2-deficient mice exhibit normal mammary epithelial differentiation and Stat5 activation.

METHODS

In light of the overlapping functions of Akt family members, we considered the possibility that Akt may play an essential role in regulating mammary epithelial development that is not evident in Akt1-deficient mice due to compensation by other Akt isoforms. To address this possibility, we interbred mice bearing targeted deletions in Akt1 and Akt2 and determined the effect on mammary differentiation during pregnancy and lactation.

RESULTS

Deletion of one allele of Akt2 in Akt1-deficient mice resulted in a severe defect in Stat5 activation during late pregnancy that was accompanied by a global failure of terminal mammary epithelial cell differentiation, as manifested by the near-complete loss in production of the three principal components of milk: lactose, lipid, and milk proteins. This defect was due, in part, to a failure of pregnant Akt1(-/-);Akt2(+/-) mice to upregulate the positive regulator of Prlr-Jak-Stat5 signaling, Id2, or to downregulate the negative regulators of Prlr-Jak-Stat5 signaling, caveolin-1 and Socs2.

CONCLUSIONS

Our findings demonstrate an unexpected requirement for Akt in Prlr-Jak-Stat5 signaling and establish Akt as an essential central regulator of mammary epithelial differentiation and lactation.

The gene encoding the hematopoietic stem cell regulator CCN3/NOV is under direct cytokine control through the transcription factors STAT5A/B

Cytokines control the biology of hematopoietic stem cells (HSCs) and progenitor cells in part through the transcription factors STAT5A/B. To investigate the target genes of STAT5A/B activated by cytokines in HSCs and progenitors, we performed microarray analyses using Lineage(-) Sca-1(+) c-Kit(+) (KSL) cells in the presence and absence of STAT5A/B. Stimulation with a mixture containing IL-3, IL-6, stem cell factor, thrombopoietin, and Flt3 ligand induced Ccn3/Nov mRNA over 100-fold in WT (control) but not Stat5a/b-null KSL cells. CCN3/NOV is a positive regulator of human HSC self-renewal and development of committed blood cells. Without stimulation, the Ccn3/Nov signal level was low in control KSL cells similar to Stat5a/b-null KSL cells. To determine which cytokine activates the Ccn3/Nov gene, we analyzed Lineage(-) c-Kit(+) (KL) and 32D cells using quantitative PCR and ChIP assays. Although stimulation with a mixture lacking IL-3 prevented the induction of Ccn3/Nov in control KL cells, IL-3 alone could induce Ccn3/Nov mRNA in control KL and 32D cells. ChIP assays using 32D cells revealed IL-3-induced binding of STAT5A/B to a γ-interferon-activated sequences site in the Ccn3/Nov gene promoter. This is the first report that Ccn3/Nov is directly induced by cytokines through STAT5A/B.

BCR-ABL but not JAK2 V617F inhibits erythropoiesis through the Ras signal by inducing p21CIP1/WAF1

BCR-ABL is a causative tyrosine kinase (TK) of chronic myelogenous leukemia (CML). In CML patients, although myeloid cells are remarkably proliferating, erythroid cells are rather decreased and anemia is commonly observed. This phenotype is quite different from that observed in polycythemia vera (PV) caused by JAK2 V617F, whereas both oncogenic TKs activate common downstream molecules at the level of hematopoietic stem cells (HSCs). To clarify this mechanism, we investigated the effects of BCR-ABL and JAK2 V617F on erythropoiesis. Enforced expression of BCR-ABL but not of JAK2 V617F in murine LSK (Lineage(-)Sca-1(hi)CD117(hi)) cells inhibited the development of erythroid cells. Among several signaling molecules downstream of BCR-ABL, an active mutant of N-Ras (N-RasE12) but not of STAT5 or phosphatidylinositol 3-kinase (PI3-K) inhibited erythropoiesis, while N-RasE12 enhanced the development of myeloid cells. BCR-ABL activated Ras signal more intensely than JAK2 V617F, and inhibition of Ras by manumycin A, a farnesyltransferase inhibitor, ameliorated erythroid colony formation of CML cells. As for the mechanisms of Ras-induced suppression of erythropoiesis, we found that GATA-1, an erythroid-specific transcription factor, blocked Ras-mediated mitogenic signaling at the level of MEK through the direct interaction. Furthermore, enforced expression of N-RasE12 in LSK cells derived from p53-, p16(INK4a)/p19(ARF)-, and p21(CIP1/WAF1)-null/wild-type mice revealed that suppressed erythroid cell growth by N-RasE12 was restored only by p21(CIP1/WAF1) deficiency, indicating that a cyclin-dependent kinase (CDK) inhibitor, p21(CIP1/WAF1), plays crucial roles in Ras-induced suppression of erythropoiesis. These data would, at least partly, explain why respective oncogenic TKs cause different disease phenotypes.

Effective targeting of STAT5-mediated survival in myeloproliferative neoplasms using ABT-737 combined with rapamycin

Signal transducer and activator of transcription-5 (STAT5) is a critical transcription factor for normal hematopoiesis and its sustained activation is associated with hematologic malignancy. A persistently active mutant of STAT5 (STAT5a^S711F) associates with Grb2 associated binding protein 2 (Gab2) in myeloid leukemias and promotes growth in vitro through AKT activation. Here we have retrovirally transduced wild-type or Gab2^−/− mouse bone marrow cells expressing STAT5a^S711F and transplanted into irradiated recipient mice to test an in vivo myeloproliferative disease (MPD) model. To target Gab2-independent AKT/mTOR activation, wild-type mice were treated separately with rapamycin. In either case, mice lacking Gab2 or treated with rapamycin displayed attenuated myeloid hyperplasia and modestly improved survival, but the effects were not cytotoxic and were reversible. To improve upon this approach, in vitro targeting of STAT5-mediated AKT/mTOR using rapamycin was combined with inhibition of the STAT5 direct target genes bcl-2 and bcl-X_L using ABT-737. Striking synergy with both drugs was observed in mouse BaF3 cells expressing STAT5a^S711F, TEL-JAK2, or BCR-ABL and in the relatively single agent-resistant human BCR-ABL positive K562 cell line. Therefore, targeting distinct STAT5 mediated survival signals, e.g. bcl-2/bcl-X_L and AKT/mTOR may be an effective therapeutic approach for human myeloproliferative neoplasms.

Stat5a serine 725 and 779 phosphorylation is a prerequisite for hematopoietic transformation

Stat5 transcription factors are essential gene regulators promoting proliferation, survival, and differentiation of all hematopoietic cell types. Mutations or fusions of oncogenic tyrosine kinases often result in constitutive Stat5 activation. We have modeled persistent Stat5 activity by using an oncogenic Stat5a variant (cS5). To analyze the hitherto unrecognized role of Stat5 serine phosphorylation in this context, we have generated cS5 constructs with mutated C-terminal serines 725 and 779, either alone or in combination. Genetic complementation assays in primary Stat5(null/null) mast cells and Stat5(DeltaN) T cells demonstrated reconstitution of proliferation with these mutants. Similarly, an in vivo reconstitution experiment of transduced Stat5(null/null) fetal liver cells transplanted into irradiated wild-type recipients revealed that these mutants exhibit biologic activity in lineage differentiation. By contrast, the leukemogenic potential of cS5 in bone marrow transplants decreased dramatically in cS5 single-serine mutants or was completely absent upon loss of both serine phosphorylation sites. Our data suggest that Stat5a serine phosphorylation is a prerequisite for cS5-mediated leukemogenesis. Hence, interference with Stat5a serine phosphorylation might provide a new therapeutic option for leukemia and myeloid dysplasias without affecting major functions of Stat5 in normal hematopoiesis.

Stat5 is indispensable for the maintenance of bcr/abl-positive leukaemia

Tumourigenesis caused by the Bcr/Abl oncoprotein is a multi-step process proceeding from initial to tumour-maintaining events and finally results in a complex tumour-supporting network. A key to successful cancer therapy is the identification of critical functional nodes in an oncogenic network required for disease maintenance. So far, the transcription factors Stat3 and Stat5a/b have been implicated in bcr/abl-induced initial transformation. However, to qualify as a potential drug target, a signalling pathway must be required for the maintenance of the leukaemic state. Data on the roles of Stat3 or Stat5a/b in leukaemia maintenance are elusive. Here, we show that both, Stat3 and Stat5 are necessary for initial transformation. However, Stat5- but not Stat3-deletion induces G₀/G₁ cell cycle arrest and apoptosis of imatinib-sensitive and imatinib-resistant stable leukaemic cells in vitro. Accordingly, Stat5-abrogation led to effective elimination of myeloid and lymphoid leukaemia maintenance in vivo. Hence, we identified Stat5 as a vulnerable point in the oncogenic network downstream of Bcr/Abl representing a case of non-oncogene addiction (NOA).

Visualization of Stat3 and Stat5 transactivation activity with specific response element dependent reporter constructs integrated into lentiviral gene transfer vectors

BACKGROUND

Signal transducer and activator of transcription 3 and 5 (Stat3 and Stat5) play important roles in cell differentiation, proliferation, apoptosis and inflammation. They are transiently activated by ligand-receptor interactions in normal cells but are often found to be constitutively active in cancer cells. Analysis of their activation pattern is therefore important for the description of developmental processes and the understanding of cellular transformation.

MATERIALS AND METHODS

To visualize Stat3 and Stat5 transactivation activity in different cell types, we designed novel reporter constructs. These constructs comprise Stat3 or Stat5 specific promoter elements and reporter genes encoding β-galactosidase or fluorescent proteins. These constructs were integrated into lentiviral gene transfer vectors facilitating efficient transduction of most cell types.

RESULTS

The lentiviral reporter constructs were used to infect different cell types and their inducibility by activated Stat3 or Stat5 was measured. The Stat3-mCherry reporter was active in transduced tumor cells, which exhibit high levels of phosphorylated Stat3 and it was inducible in HepG2 liver cells by interleukin-6 treatment. The Stat5-LacZ reporter was active in cultured cells upon hormone induction of Stat5 and in primary mammary epithelial cells transplanted into cleared fat pads of mice during late pregnancy.

CONCLUSION

These novel reporter constructs are valuable tools to investigate and to distinguish between Stat3 and Stat5 activity in primary cells and cancer cells. They will also be useful in the discovery of drugs targeting Stat3 or Stat5. They can also be employed to generate transgenic mice and track Stat activity during development.

STAT5 requires the N-domain for suppression of miR15/16, induction of bcl-2, and survival signaling in myeloproliferative disease

Phosphorylated signal transducer and activator of transcription 5 (STAT5) is a biomarker and potential molecular target for hematologic malignancies. We have shown previously that lethal myeloproliferative disease (MPD) in mice mediated by persistently activated STAT5 (STAT5a(S711F)) requires the N-domain, but the mechanism was not defined. We now demonstrate by retrovirally complementing STAT5ab(null/null) primary mast cells that relative to wild-type STAT5a, STAT5a lacking the N-domain (STAT5aDeltaN) ineffectively protected against cytokine withdrawal-induced cell death. Both STAT5a and STAT5aDeltaN bound to a site in the bcl-2 gene and both bound near the microRNA 15b/16 cluster. However, only STAT5a could effectively induce bcl-2 mRNA and reciprocally suppress miR15b/16 leading to maintained bcl-2 protein levels. After retroviral complementation of STAT5ab(null/null) fetal liver cells and transplantation, persistently active STAT5a(S711F) lacking the N-domain (STAT5aDeltaN(S711F)) was insufficient to protect c-Kit(+)Lin(-)Sca-1(+) (KLS) cells from apoptosis and unable to induce bcl-2 expression, whereas STAT5a(S711F) caused robust KLS cell expansion, induction of bcl-2, and lethal MPD. Severe attenuation of MPD by STAT5aDeltaN(S711F) was reversed by H2k/bcl-2 transgenic expression. Overall, these studies define N-domain-dependent survival signaling as an Achilles heel of persistent STAT5 activation and highlight the potential therapeutic importance of targeting STAT5 N-domain-mediated regulation of bcl-2 family members.

Role of STAT5 in controlling cell survival and immunoglobulin gene recombination during pro-B cell development

STAT5 and interleukin 7 (IL-7) signaling are thought to control B lymphopoiesis by regulating the expression of key transcription factors and by activating variable (V(H)) gene segments at the immunoglobulin heavy-chain (Igh) locus. Using conditional mutagenesis to delete the gene encoding the transcription factor STAT5, we demonstrate that the development of pro-B cells was restored by transgenic expression of the prosurvival protein Bcl-2, which compensated for loss of the antiapoptotic protein Mcl-1. Expression of the genes encoding the B cell-specification factor EBF1 and the B cell-commitment protein Pax5 as well as V(H) gene recombination were normal in STAT5- or IL-7 receptor alpha-chain (IL-7Ralpha)-deficient pro-B cells rescued by Bcl-2. STAT5-expressing pro-B cells contained little or no active chromatin at most V(H) genes. In contrast, rearrangements of the immunoglobulin-kappa light-chain locus (Igk) were more abundant in STAT5- or IL-7Ralpha-deficient pro-B cells. Hence, STAT5 and IL-7 signaling control cell survival and the developmental ordering of immunoglobulin gene rearrangements by suppressing premature Igk recombination in pro-B cells.

Development of mammary luminal progenitor cells is controlled by the transcription factor STAT5A

Mammary alveologenesis is abrogated in the absence of the transcription factors STAT5A/5B, which mediate cytokine signaling. To reveal the underlying causes for this developmental block, we studied mammary stem and progenitor cells. While loss of STAT5A/5B did not affect the stem cell population and its ability to form mammary ducts, luminal progenitors were greatly reduced and unable to form alveoli during pregnancy. Temporally controlled expression of transgenic STAT5A in mammary epithelium lacking STAT5A/5B restored the luminal progenitor population and rescued alveologenesis in a reversible fashion in vivo. Thus, STAT5A is necessary and sufficient for the establishment of luminal progenitor cells.

The transcription factors STAT5A/B regulate GM-CSF-mediated granulopoiesis

Neutrophils play a vital role in the immune defense, which is evident by the severity of neutropenia causing life-threatening infections. Granulocyte macrophage-colony stimulating factor (GM-CSF) controls homeostatic and emergency development of granulocytes. However, little is known about the contribution of the downstream mediating transcription factors signal transducer and activator of transcription 5A and 5B (STAT5A/B). To elucidate the function of this pathway, we generated mice with complete deletion of both Stat5a/b genes in hematopoietic cells. In homeostasis, peripheral neutrophils were markedly decreased in these animals. Moreover, during emergency situations, such as myelosuppression, Stat5a/b-mutant mice failed to produce enhanced levels of neutrophils and were unable to respond to GM-CSF. Both the GM-CSF-permitted survival of mature neutrophils and the generation of granulocytes from granulocyte-macrophage progenitors (GMPs) were markedly reduced in Stat5a/b mutants. GMPs showed impaired colony-formation ability with reduced number and size of colonies on GM-CSF stimulation. Moreover, continuous cell fate analyses by time-lapse microscopy and single cell tracking revealed that Stat5a/b-null GMPs showed both delayed cell-cycle progression and increased cell death. Finally, transcriptome analysis indicated that STAT5A/B directs GM-CSF signaling through the regulation of proliferation and survival genes.

Modeling the functional heterogeneity of leukemia stem cells: role of STAT5 in leukemia stem cell self-renewal

Although the cancer stem cell (CSC) concept implies that CSCs are rare, recent reports suggest that CSCs may be frequent in some cancers. We hypothesized that the proportion of leukemia stem cells would vary as a function of the number of dysregulated pathways. Constitutive expression of MN1 served as a 1-oncogene model, and coexpression of MN1 and a HOX gene served as a 2-oncogene model. Leukemia-initiating cell (LIC) number and in vitro expansion potential of LICs were functionally assessed by limiting dilution analyses. LIC expansion potential was 132-fold increased in the 2- compared with the 1-oncogene model, although phenotypically, both leukemias were similar. The 2-oncogene model was characterized by granulocyte-macrophage colony-stimulating factor (GM-CSF) hypersensitivity and activated STAT/ERK signaling. GM-CSF hypersensitivity of the 2-oncogene model (MN1/HOXA9) was lost in Stat5b(-/-) cells, and the LIC expansion potential was reduced by 86- and 28-fold in Stat5b(-/-) and Stat1(-/-) cells, respectively. Interestingly, in 201 acute myeloid leukemia (AML) patients, coexpression of MN1 and HOXA9 was restricted to patients with the poorest prognosis and was associated with highly active STAT signaling. Our data demonstrate the functional heterogeneity of LICs and show that STAT signaling is critical for leukemia stem cell self-renewal in MN1- and HOXA9-expressing leukemias.

Loss of STAT5 causes liver fibrosis and cancer development through increased TGF-{beta} and STAT3 activation

The molecular mechanisms underlying the development of hepatocellular carcinoma are not fully understood. Liver-specific signal transducer and activator of transcription (STAT) 5A/B–null mice (STAT5-LKO) were treated with carbon tetrachloride (CCl₄), and histological analyses revealed liver fibrosis and tumors. Transforming growth factor (TGF)–β levels and STAT3 activity were elevated in liver tissue from STAT5-LKO mice upon CCl₄ treatment. To define the molecular link between STAT5 silencing and TGF-β up-regulation, as well as STAT3 activation, we examined STAT5-null mouse embryonic fibroblasts and primary hepatocytes. These cells displayed elevated TGF-β protein levels, whereas messenger RNA levels remained almost unchanged. Protease inhibitor studies revealed that STAT5 deficiency enhanced the stability of mature TGF-β. Immunoprecipitation and immunohistochemistry analyses demonstrated that STAT5, through its N-terminal sequences, could bind to TGF-β and that retroviral-mediated overexpression of STAT5 decreased TGF-β levels. To confirm the in vivo significance of the N-terminal domain of STAT5, we treated mice that expressed STAT5 lacking the N terminus (STAT5-ΔN) with CCl₄. STAT5-ΔN mice developed CCl₄-induced liver fibrosis but no tumors. In conclusion, loss of STAT5 results in elevated TGF-β levels and enhanced growth hormone–induced STAT3 activity. We propose that a deregulated STAT5–TGF-β–STAT3 network contributes to the development of chronic liver disease.

Molecular biology of bcr-abl1-positive chronic myeloid leukemia

Chronic myeloid leukemia (CML) has been regarded as the paradigmatic example of a malignancy defined by a unique molecular event, the BCR-ABL1 oncogene. Decades of research zeroing in on the role of BCR-ABL1 kinase in the pathogenesis of CML have culminated in the development of highly efficacious therapeutics that, like imatinib mesylate, target the oncogenic kinase activity of BCR-ABL1. In recent years, most research efforts in CML have been devoted to developing novel tyrosine kinase inhibitors (TKIs) as well as to elucidating the mechanisms of resistance to imatinib and other TKIs. Nonetheless, primordial aspects of the pathogenesis of CML, such as the mechanisms responsible for the transition from chronic phase to blast crisis, the causes of genomic instability and faulty DNA repair, the phenomenon of stem cell quiescence, the role of tumor suppressors in TKI resistance and CML progression, or the cross-talk between BCR-ABL1 and other oncogenic signaling pathways, still remain poorly understood. Herein, we synthesize the most relevant and current knowledge on such areas of the pathogenesis of CML.

Following the cytokine signaling pathway to leukemogenesis: a chronology

Studies over the past 50 years revealing the molecular events that promote normal T lymphocyte cycle competence and progression led to a detailed understanding of how cytokines function to regulate normal hematopoietic cell proliferation. During that same period, the molecular and genetic changes introduced by the Philadelphia chromosome in chronic myelogenous leukemia were unraveled, and these have led to an understanding of how mutations that constitutively activate normal cytokine signaling pathways can cause unregulated cell proliferation and malignant transformation. Based on the paradigm established by these data, it is inescapable that going forward, investigators will operate under the hypothesis that transformation of additional cells and tissues will have a similar pathogenesis.

Expression of P190 and P210 BCR/ABL1 in normal human CD34(+) cells induces similar gene expression profiles and results in a STAT5-dependent expansion of the erythroid lineage

OBJECTIVE

The P190 and P210 BCR/ABL1 fusion genes are mainly associated with different types of hematologic malignancies, but it is presently unclear whether they are functionally different following expression in primitive human hematopoietic cells.

MATERIALS AND METHODS

We investigated and systematically compared the effects of retroviral P190 BCR/ABL1 and P210 BCR/ABL1 expression on cell proliferation, differentiation, and global gene expression in human CD34(+) cells from cord blood.

RESULTS

Expression of either P190 BCR/ABL1 or P210 BCR/ABL1 resulted in expansion of erythroid cells and stimulated erythropoietin-independent burst-forming unit-erythroid colony formation. By using a lentiviral anti-signal transducer and activator of transcription 5 (STAT5) short-hairpin RNA, we found that both P190 BCR/ABL1- and P210 BCR/ABL1-induced erythroid cell expansion were STAT5-dependent. Under in vitro conditions favoring B-cell differentiation, neither P190 nor P210 BCR/ABL1-expressing cells formed detectable levels of CD19-positive cells. Gene expression profiling revealed that P190 BCR/ABL1 and P210 BCR/ABL1 induced almost identical gene expression profiles.

CONCLUSIONS

Our data suggest that the early cellular and transcriptional effects of P190 BCR/ABL1 and P210 BCR/ABL1 expression are very similar when they are expressed in the same human progenitor cell population, and that STAT5 is an important regulator of BCR/ABL1-induced erythroid cell expansion.

Leukemic challenge unmasks a requirement for PI3Kδ in NK cell–mediated tumor surveillance

Specific inhibitors of PI3K isoforms are currently evaluated for their therapeutic potential in leukemia. We found that BCR/ABL+ human leukemic cells express PI3Kδ and therefore explored its impact on leukemia development. Using PI3Kδ-deficient mice, we define a dual role of PI3Kδ in leukemia. We observed a growth-promoting effect in tumor cells and an essential function in natural killer (NK) cell–mediated tumor surveillance: Abelson-transformed PI3Kδ-deficient cells induced leukemia in RAG2-deficient mice with an increased latency, indicating that PI3Kδ accelerated leukemia progression in vivo. However, the absence of PI3Kδ also affected NK cell–mediated tumor surveillance. PI3Kδ-deficient NK cells failed to lyse a large variety of target cells because of defective degranulation, as also documented by capacitance recordings. Accordingly, transplanted leukemic cells killed PI3Kδ-deficient animals more rapidly. As a net effect, no difference in disease latency in vivo was detected if both leukemic cells and NK cells lack PI3Kδ. Other tumor models confirmed that PI3Kδ-deficient mice succumbed more rapidly when challenged with T- or B-lymphoid leukemic or B16 melanoma cells. Thus, the action of PI3Kδ in the NK compartment is as relevant to survival of the mice as the delayed tumor progression. This dual function must be taken into account when using PI3Kδ inhibitors as antileukemic agents in clinical trials.

Maximal STAT5-induced proliferation and self-renewal at intermediate STAT5 activity levels

The level of transcription factor activity critically regulates cell fate decisions, such as hematopoietic stem cell (HSC) self-renewal and differentiation. We introduced STAT5A transcriptional activity into human HSCs/progenitor cells in a dose-dependent manner by overexpression of a tamoxifen-inducible STAT5A(1*6)-estrogen receptor fusion protein. Induction of STAT5A activity in CD34(+) cells resulted in impaired myelopoiesis and induction of erythropoiesis, which was most pronounced at the highest STAT5A transactivation levels. In contrast, intermediate STAT5A activity levels resulted in the most pronounced proliferative advantage of CD34(+) cells. This coincided with increased cobblestone area-forming cell and long-term-culture-initiating cell frequencies, which were predominantly elevated at intermediate STAT5A activity levels but not at high STAT5A levels. Self-renewal of progenitors was addressed by serial replating of CFU, and only progenitors containing intermediate STAT5A activity levels contained self-renewal capacity. By extensive gene expression profiling we could identify gene expression patterns of STAT5 target genes that predominantly associated with a self-renewal and long-term expansion phenotype versus those that identified a predominant differentiation phenotype.

Stat5 regulates cellular iron uptake of erythroid cells via IRP-2 and TfR-1

Erythropoiesis strictly depends on signal transduction through the erythropoietin receptor (EpoR)-Janus kinase 2 (Jak2)-signal transducer and activator of transcription 5 (Stat5) axis, regulating proliferation, differentiation, and survival. The exact role of the transcription factor Stat5 in erythropoiesis remained puzzling, however, since the first Stat5-deficient mice carried a hypomorphic Stat5 allele, impeding full phenotypical analysis. Using mice completely lacking Stat5--displaying early lethality--we demonstrate that these animals suffer from microcytic anemia due to reduced expression of the antiapoptotic proteins Bcl-x(L) and Mcl-1 followed by enhanced apoptosis. Moreover, transferrin receptor-1 (TfR-1) cell surface levels on erythroid cells were decreased more than 2-fold on erythroid cells of Stat5(-/-) animals. This reduction could be attributed to reduced transcription of TfR-1 mRNA and iron regulatory protein 2 (IRP-2), the major translational regulator of TfR-1 mRNA stability in erythroid cells. Both genes were demonstrated to be direct transcriptional targets of Stat5. This establishes an unexpected mechanistic link between EpoR/Jak/Stat signaling and iron metabolism, processes absolutely essential for erythropoiesis and life.

New insights into the roles of Stat5a/b and Stat3 in T cell development and differentiation

T cell development and differentiation is carefully orchestrated by a series of cytokines. The importance of STAT family proteins in mediating signals by these cytokines is well-known, but new information on the role of STATs in novel aspects of T cell function and T cell subsets continues to accumulate. Recent studies have placed Stat5a/b and Stat3 center stage in T cell development and differentiation. Stat5a/b are indispensable in T regulatory (Treg) cell development and maintenance, and negatively regulate T helper 17 (Th17) cell differentiation. Conversely, Stat3 is essential for Th17 differentiation and inhibits Treg cells. The balance of Treg and Th17 cells is thought to be critical in maintaining immune tolerance, while preserving effective host defense. Therefore, Stat5a/b and Stat3 are emerging to be key players in T cell differentiation and homeostasis.

The chemokine interleukin-8 and the surface activation protein CD69 are markers for Bcr-Abl activity in chronic myeloid leukemia

We have identified differentially regulated genes in chronic myeloid leukemia (CML) cells upon short treatment with the broad-spectrum Bcr-Abl inhibitor dasatinib. The highly specific Bcr-Abl inhibitor nilotinib caused a very similar gene expression signature, validating the identified differentially regulated genes as a read-out of Bcr-Abl activity and implying that Bcr-Abl is the functionally central target of dasatinib in CML cells. Among the strongest downregulated genes, we have further validated the activation marker CD69 and the chemokine interleukin (IL)-8. Expression of both proteins is upregulated upon Bcr-Abl expression and inhibited by dasatinib and nilotinib. IL-8 may thus be a useful marker for the monitoring of CML inhibitor efficacy and play a potential pathophysiological role in CML.

Human and simian immunodeficiency viruses deregulate early hematopoiesis through a Nef/PPARgamma/STAT5 signaling pathway in macaques

Infection of primates by HIV-1 and SIV induces multiple hematological abnormalities of central hematopoietic origin. Although these defects greatly contribute to the pathophysiology of HIV-1 infection, the molecular basis for altered BM function remains unknown. Here we show that when cynomolgus macaques were infected with SIV, the multipotent potential of their hematopoietic progenitor cells was lost, and this correlated with downregulation of STAT5A and STAT5B expression. However, forced expression of STAT5B entirely rescued the multipotent potential of the hematopoietic progenitor cells. In addition, an accessory viral protein required for efficient SIV and HIV replication and pathogenicity, "Negative factor" (Nef), was essential for SIV-mediated impairment of the multipotent potential of hematopoietic progenitors ex vivo and in vivo. This newly uncovered property of Nef was both conserved between HIV-1 and SIV strains and entirely dependent upon the presence of PPARgamma in targeted cells. Further, PPARgamma agonists mimicked Nef activity by inhibiting STAT5A and STAT5B expression and hampering the functionality of hematopoietic progenitors both ex vivo and in vivo. These findings have extended the role of Nef in the pathogenicity of HIV-1 and SIV and reveal a pivotal role for the PPARgamma/STAT5 pathway in the regulation of early hematopoiesis. This study may provide a basis for investigating the potential therapeutic benefits of PPARgamma antagonists in both patients with AIDS and individuals with hematopoietic disorders.

Oncogenic Kit controls neoplastic mast cell growth through a Stat5/PI3-kinase signaling cascade

The D816V-mutated variant of Kit triggers multiple signaling pathways and is considered essential for malignant transformation in mast cell (MC) neoplasms. We here describe that constitutive activation of the Stat5-PI3K-Akt-cascade controls neoplastic MC development. Retrovirally transduced active Stat5 (cS5(F)) was found to trigger PI3K and Akt activation, and to transform murine bone marrow progenitors into tissue-infiltrating MCs. Primary neoplastic Kit D816V(+) MCs in patients with mastocytosis also displayed activated Stat5, which was found to localize to the cytoplasm and to form a signaling complex with PI3K, with consecutive Akt activation. Finally, the knock-down of either Stat5 or Akt activity resulted in growth inhibition of neoplastic Kit D816V(+) MCs. These data suggest that a downstream Stat5-PI3K-Akt signaling cascade is essential for Kit D816V-mediated growth and survival of neoplastic MCs.

Interpretation of cytokine signaling through the transcription factors STAT5A and STAT5B

Transcription factors from the family of Signal Transducers and Activators of Transcription (STAT) are activated by numerous cytokines. Two members of this family, STAT5A and STAT5B (collectively called STAT5), have gained prominence in that they are activated by a wide variety of cytokines such as interleukins, erythropoietin, growth hormone, and prolactin. Furthermore, constitutive STAT5 activation is observed in the majority of leukemias and many solid tumors. Inactivation studies in mice as well as human mutations have provided insight into many of STAT5's functions. Disruption of cytokine signaling through STAT5 results in a variety of cell-specific effects, ranging from a defective immune system and impaired erythropoiesis, the complete absence of mammary development during pregnancy, to aberrant liver function. On a molecular level, STAT5 has been linked to cell specification, proliferation, differentiation, and survival. Evidence is growing that the diverse outcomes of STAT5 signaling are not only determined by the expression of specific receptors but also by the interaction of STAT5 with cofactors and the cell-specific activity of members of the SOCS family, which negatively regulate STAT function. In this review, we focus on emerging concepts and challenges in the field of Janus kinase (JAK)-STAT5 signaling. First, we discuss unique functions of STAT5 in three distinct systems: mammary epithelial cells, hepatocytes, and regulatory T cells. Second, we present an example of how STAT5 can achieve cell specificity in hepatocytes through a physical and functional interaction with the glucocorticoid receptor. Third, we focus on the relevance of STAT5 in the development and progression of leukemia. Next, we discuss lessons derived from human mutations and disease. Finally, we address an emerging issue that the interpretation of experiments from STAT5-deficient mice and cells might be compromised as these cells might reroute and reprogram cytokine signals to the "wrong" STATs and thus acquire inappropriate cues. We propose that mice with mutations in various components of the JAK-STAT signaling pathway are living laboratories, which will provide insight into the versatility of signaling hardware and the adaptability of the software.

Genomic dissection of the cytokine-controlled STAT5 signaling network in liver

Growth hormone (GH) controls the physiology and pathophysiology of the liver, and its signals are conducted by two members of the family of signal transducers and activators of transcription, STAT5A and STAT5B. Mice in which the Stat5a/b locus has been inactivated specifically in hepatocytes display GH resistance, the sex-specific expression of genes associated with liver metabolism and the cytochrome P-450 system is lost, and they develop hepatosteatosis. Several groups have shown by global gene expression profiling that a cadre of STAT5A/B target genes identify genetic cascades induced by GH and other cytokines. Evidence is accumulating that in the absence of STAT5A/B GH aberrantly activates STAT1 and STAT3 and their downstream target genes and thereby offers a partial explanation of some of the physiological alterations observed in Stat5a/b-null mice and human patients. We hypothesize that phenotypic changes observed in the absence of STAT5A/B are due to two distinct molecular consequences: first, the failure of STAT5A/B target genes to be activated by GH and second, the rerouting of GH signaling to other members of the STAT family. Rerouting of GH signaling to STAT1 and STAT3 might partially compensate for the loss of STAT5A/B, but it certainly activates biological programs distinct from STAT5A/B. Here we discuss the extent to which studies on global gene expression profiling have fostered a better understanding of the biology behind cytokine-STAT5A/B networks in hepatocytes. We also explore whether this wealth of information on gene activity can be used to further understand the roles of cytokines in liver disease.

Rac GTPases as key regulators of p210-BCR-ABL-dependent leukemogenesis

Chronic myelogenous leukemia (CML) is a malignant disease characterized by expression of p210-BCR-ABL, the product of the Philadelphia chromosome. Survival of CML patients has been significantly improved with the introduction of tyrosine kinase inhibitors that induce long-term hematologic remissions. However, mounting evidence indicates that the use of a single tyrosine kinase inhibitor does not cure this disease due to the persistence of p210-BCR-ABL at the molecular level or the acquired resistance in the stem cell compartment to individual inhibitors. We have recently shown in a murine model that deficiency of the Rho GTPases Rac1 and Rac2 significantly reduces p210-BCR-ABL-mediated proliferation in vitro and myeloproliferative disease in vivo, suggesting Rac as a potential therapeutic target in p210-BCR-ABL-induced disease. This target has been further validated using a first-generation Rac-specific small molecule inhibitor. In this review we describe the role of Rac GTPases in p210-BCR-ABL-induced leukemogenesis and explore the possibility of combinatorial therapies that include tyrosine kinase inhibitor(s) and Rac GTPase inhibitors in the treatment of CML.

Csf3r mutations in mice confer a strong clonal HSC advantage via activation of Stat5

A fundamental property of leukemic stem cells is clonal dominance of the bone marrow microenvironment. Truncation mutations of CSF3R, which encodes the G-CSF receptor (G-CSFR), are implicated in leukemic progression in patients with severe congenital neutropenia. Here we show that expression of a truncated mutant Csf3r in mice confers a strong clonal advantage at the HSC level that is dependent upon exogenous G-CSF. G-CSF-induced proliferation, phosphorylation of Stat5, and transcription of Stat5 target genes were increased in HSCs isolated from mice expressing the mutant Csf3r. Conversely, the proliferative advantage conferred by the mutant Csf3r was abrogated in myeloid progenitors lacking both Stat5A and Stat5B, and HSC function was reduced in mice expressing a truncated mutant Csf3r engineered to have impaired Stat5 activation. These data indicate that in mice, inappropriate Stat5 activation plays a key role in establishing clonal dominance by stem cells expressing mutant Csf3r.

Stat5 activation enables erythropoiesis in the absence of EpoR and Jak2

Erythropoiesis requires erythropoietin (Epo) and stem cell factor (SCF) signaling via their receptors EpoR and c-Kit. EpoR, like many other receptors involved in hematopoiesis, acts via the kinase Jak2. Deletion of EpoR or Janus kinase 2 (Jak2) causes embryonic lethality as a result of defective erythropoiesis. The contribution of distinct EpoR/Jak2-induced signaling pathways (mitogen-activated protein kinase, phosphatidylinositol 3-kinase, signal transducer and activator of transcription 5 [Stat5]) to functional erythropoiesis is incompletely understood. Here we demonstrate that expression of a constitutively activated Stat5a mutant (cS5) was sufficient to relieve the proliferation defect of Jak2(-/-) and EpoR(-/-) cells in an Epo-independent manner. In addition, tamoxifen-induced DNA binding of a Stat5a-estrogen receptor (ER)* fusion construct enabled erythropoiesis in the absence of Epo. Furthermore, c-Kit was able to enhance signaling through the Jak2-Stat5 axis, particularly in lymphoid and myeloid progenitors. Although abundance of hematopoietic stem cells was 2.5-fold reduced in Jak2(-/-) fetal livers, transplantation of Jak2(-/-)-cS5 fetal liver cells into irradiated mice gave rise to mature erythroid and myeloid cells of donor origin up to 6 months after transplantation. Cytokine- and c-Kit pathways do not function independently of each other in hematopoiesis but cooperate to attain full Jak2/Stat5 activation. In conclusion, activated Stat5 is a critical downstream effector of Jak2 in erythropoiesis/myelopoiesis, and Jak2 functionally links cytokine- with c-Kit-receptor tyrosine kinase signaling.

Loss of Bcl-x in Ph+ B-ALL increases cellular proliferation and does not inhibit leukemogenesis

The kinase inhibitors imatinib mesylate and dasatinib are the preferred treatment for Philadelphia chromosome-positive (Ph+) leukemias, and they are highly successful in the chronic phase of chronic myeloid leukemia (CML). However, they are not efficient in Ph+ B-cell acute lymphoblastic leukemia (B-ALL). Ph+ leukemia cells are highly resistant to apoptosis, and evidence from cell lines and primary cells suggest Bcl-xL as a critical mediator of resistance to apoptosis: however, this concept has never been rigorously tested in an animal model. To clarify the role of Bcl-xL in Ph+ B-ALL, we generated 2 mouse models. In the first model, Ph+ B-ALL and loss of Bcl-xL expression are coinduced; in the second model, leukemia is induced with expression of Bcl-xL protein well above the levels found in wild-type lymphoblasts. Deletion of Bcl-xL did not inhibit leukemogenesis or affect apoptosis, but increased cellular proliferation. Consistent with this result, overexpression of Bcl-xL led to decreased cellular proliferation. These models reveal an unexpected role for Bcl-xL in cell-cycle entry and the proliferation of tumor cells.

Stat5 as a diagnostic marker for leukemia

The Jak-Stat-Socs pathway is an important component of cytokine receptor signaling. Not surprisingly, perturbation of this pathway is implicated in diseases of hematopoietic and immune origin, including leukemia, lymphoma and immune deficiencies. This review examines the role of a key component of this pathway, Stat5. This has been shown to be activated in a variety of leukemias and myeloproliferative disorders, including downstream of a range of key oncogenes where it has been shown to play an important role in mediating their effects. Therefore, Stat5 represents a useful pan-leukemia/myeloproliferative disorder diagnostic marker and key therapeutic end point, as well as representing an attractive therapeutic target for these disorders.

STAT5 requires the N-domain to maintain hematopoietic stem cell repopulating function and appropriate lymphoid-myeloid lineage output

OBJECTIVE

Signal transducer and activator of transcription 5 (STAT5) is a critical regulator of hematopoietic development and its impaired activation is associated with hematopoietic and immune cell defects. However, much of this information has been learned from knockout mice that still retain the potential for expression of STAT5 proteins that are N-terminally truncated due to alternative internal translation initiation codons. The goal of these studies was to use transplantation-based assays to analyze the degree of STAT5 deltaN activity in hematopoietic stem cells (HSC) and throughout lymphomyeloid development.

METHODS

We have directly compared E14.5 fetal liver cells from mice with potential to express STAT5ab deltaN (STAT5ab(deltaN/deltaN)) with mice completely lacking STAT5a and STAT5b (STAT5abnull/null). We have also utilized retroviral complementation of STAT5abnull/null fetal liver HSC to enforce expression of full-length STAT5a or STAT5a lacking the first 136 amino acids (STAT5a deltaN).

RESULTS

We report that STAT5 is required for HSC, lymphocyte, and erythrocyte development. We demonstrate that restored expression of STAT5a in STAT5abnull/null HSC provides a strong selective advantage, correcting T- and B-lymphocyte and erythrocyte development. Interestingly, Gr-1(+) blood cells were inversely correlated with B lymphocytes and both were normalized by STAT5a expression. In contrast, transduction of STAT5a deltaN only provided partial B-lymphocyte development.

CONCLUSIONS

These studies define the role of STAT5 in maintaining normal lymphoid vs myeloid balance during hematopoiesis and highlight a major role for the N-domain in HSC function. The platform of retroviral complementation described here will be particularly useful for future studies to subdefine the N-domain regions that are critical for hematopoiesis.

Direct glucocorticoid receptor-Stat5 interaction in hepatocytes controls body size and maturation-related gene expression

The glucocorticoid receptor regulates transcription through DNA binding as well as through cross-talk with other transcription factors. In hepatocytes, the glucocorticoid receptor is critical for normal postnatal growth. Using hepatocyte-specific and domain-selective mutations in the mouse we show that Stat5 in hepatocytes is essential for normal postnatal growth and that it mediates the growth-promoting effect of the glucocorticoid receptor through a direct interaction involving the N-terminal tetramerization domain of Stat5b. This interaction mediates a selective and unexpectedly extensive part of the transcriptional actions of these molecules since it controls the expression of gene sets involved in growth and sexual maturation.

Chromosomal position of a VH gene segment determines its activation and inactivation as a substrate for V(D)J recombination

Complete IgHC gene rearrangement occurs only in B cells in a stage-specific and ordered manner. We used gene targeting to reposition a distal V(H) gene segment to a region just 5' of the D(H) gene cluster and found its activation to be highly dependent on the chromosomal domain within which it resides. The targeted V(H) gene segment rearranged at a higher frequency than its endogenous counterpart, its rearrangement was no longer ordered, and its ability to be silenced by allelic exclusion was lost. Additionally, the targeted V(H) gene segment lost lineage specificity, as VDJ(H) rearrangement was observed in thymocytes. These data suggest that locus contraction, mimicked by proximal targeting, can override any regulation imposed by DNA sequences immediately surrounding V(H) gene segments.