JunB Deficiency Leads to a Myeloproliferative Disorder Arising from Hematopoietic Stem Cells

Delayed wound healing and epidermal hyperproliferation in mice lacking JunB in the skin

The cutaneous response to injury and stress comprises a temporary change in the balance between epidermal proliferation and differentiation as well as an activation of the immune system. Soluble factors play an important role in the regulation of these complex processes by coordinating the intercellular communication between keratinocytes, fibroblasts, and inflammatory cells. In this study, we demonstrate that JunB, a member of the activator protein-1 transcription factor family, is an important regulator of cytokine expression and thus critically involved in the cutaneous response to injury and stress. Mice lacking JunB in the skin develop normally, indicating that JunB is neither required for cutaneous organogenesis, nor homeostasis. However, upon wounding and treatment with the phorbol ester 12-O-decanoyl-phorbol-13-acetate, JunB-deficiency in the skin likewise resulted in pronounced epidermal hyperproliferation, disturbed differentiation, and prolonged inflammation. Furthermore, delayed tissue remodelling was observed during wound healing. These phenotypic skin abnormalities were associated with JunB-dependent alterations in expression levels and kinetics of important mediators of wound repair, such as granulocyte macrophage colony-stimulating factor, growth-regulated protein-1, macrophage inflammatory protein-2, and lipocalin-2 in both the dermal and epidermal compartment of the skin, and a reduced ability of wound contraction of mutant dermal fibroblasts in vitro.

The JAK2 V617F mutation occurs in hematopoietic stem cells in polycythemia vera and predisposes toward erythroid differentiation

Although a large proportion of patients with polycythemia vera (PV) harbor a valine-to-phenylalanine mutation at amino acid 617 (V617F) in the JAK2 signaling molecule, the stage of hematopoiesis at which the mutation arises is unknown. Here we isolated and characterized hematopoietic stem cells (HSC) and myeloid progenitors from 16 PV patient samples and 14 normal individuals, testing whether the JAK2 mutation could be found at the level of stem or progenitor cells and whether the JAK2 V617F-positive cells had altered differentiation potential. In all PV samples analyzed, there were increased numbers of cells with a HSC phenotype (CD34+CD38-CD90+Lin-) compared with normal samples. Hematopoietic progenitor assays demonstrated that the differentiation potential of PV was already skewed toward the erythroid lineage at the HSC level. The JAK2 V617F mutation was detectable within HSC and their progeny in PV. Moreover, the aberrant erythroid potential of PV HSC was potently inhibited with a JAK2 inhibitor, AG490.

Gene expression profiling of CD34+ cells identifies a molecular signature of chronic myeloid leukemia blast crisis

Despite recent success in the treatment of early-stage disease, blastic phase (BP) of chronic myeloid leukemia (CML) that is characterized by rapid expansion of therapy-refractory and differentiation-arrested blasts, remains a therapeutic challenge. The development of resistance upon continuous administration of imatinib mesylate is associated with poor prognosis pointing to the need for alternative therapeutic strategies and a better understanding of the molecular mechanisms underlying disease progression. To identify transcriptional signatures that may explain pathological characteristics and aggressive behavior of BP blasts, we performed comparative gene expression profiling on CD34+ Ph+ cells purified from patients with untreated newly diagnosed chronic phase CML (CP, n=11) and from patients in BP (n=9) using Affymetrix oligonucleotide arrays. Supervised microarray data analysis revealed 114 differentially expressed genes (P<10(-4)), 34 genes displaying more than two-fold transcriptional changes when comparing CP and BP groups. While 24 of these genes were downregulated, 10 genes, especially suppressor of cytokine signalling 2 (SOCS2), CAMPATH-1 antigen (CD52), and four human leukocyte antigen-related genes were strongly overexpressed in BP. Expression of selected genes was validated by real-time-polymerase chain reaction and flow cytometry. Our data suggest the existence of a common gene expression profile of CML-BP and provide new insight into the molecular phenotype of blasts associated with disease progression and high malignancy.

Cancer stem cells

Cancer stem cells (CSC) are recently proposed to be the cancer initiating cells responsible for tumorigenesis and contribute to cancer resistance. Advances have been made in identifying and enriching CSC in leukemia and several solid tumors, including breast, brain and lung cancers. These studies suggest that, like normal stem cells, CSCs should be rare, quiescent, and capable of self-renewing and maintaining tumor growth and heterogeneity. Although the concept of CSC originates from that of normal stem cells, CSCs are not necessarily aberrant counterparts of normal stem cells. In fact, they may arise from stem cells or committed progenitors of corresponding tissues, and even cells from other tissues. At the molecular level, the alteration of stem cell self-renewal pathway(s) has been recognized as an essential step for CSC transformation. Better understanding of CSC will no doubt lead to a new era of both basic and clinical cancer research, re-classification of human tumors and development of novel therapeutic strategies specifically targeting CSC.

Gene expression changes associated with progression and response in chronic myeloid leukemia

Chronic myeloid leukemia (CML) is a hematopoietic stem cell disease with distinct biological and clinical features. The biologic basis of the stereotypical progression from chronic phase through accelerated phase to blast crisis is poorly understood. We used DNA microarrays to compare gene expression in 91 cases of CML in chronic (42 cases), accelerated (17 cases), and blast phases (32 cases). Three thousand genes were found to be significantly (P < 10(-10)) associated with phase of disease. A comparison of the gene signatures of chronic, accelerated, and blast phases suggest that the progression of chronic phase CML to advanced phase (accelerated and blast crisis) CML is a two-step rather than a three-step process, with new gene expression changes occurring early in accelerated phase before the accumulation of increased numbers of leukemia blast cells. Especially noteworthy and potentially significant in the progression program were the deregulation of the WNT/beta-catenin pathway, the decreased expression of Jun B and Fos, alternative kinase deregulation, such as Arg (Abl2), and an increased expression of PRAME. Studies of CML patients who relapsed after initially successful treatment with imatinib demonstrated a gene expression pattern closely related to advanced phase disease. These studies point to specific gene pathways that might be exploited for both prognostic indicators as well as new targets for therapy.

Protein tyrosine phosphatase 1B negatively regulates macrophage development through CSF-1 signaling

Protein tyrosine phosphatase 1B (PTP-1B) is a ubiquitously expressed cytosolic phosphatase with the ability to dephosphorylate JAK2 and TYK2, and thereby down-regulate cytokine receptor signaling. Furthermore, PTP-1B levels are up-regulated in certain chronic myelogenous leukemia patients, which points to a potential role for PTP-1B in myeloid development. The results presented here show that the absence of PTP-1B affects murine myelopoiesis by modifying the ratio of monocytes to granulocytes in vivo. This bias toward monocytic development is at least in part due to a decreased threshold of response to CSF-1, because the PTP-1B -/- bone marrow presents no abnormalities at the granulocyte-monocyte progenitor level but produces significantly more monocytic colonies in the presence of CSF-1. This phenomenon is not due to an increase in receptor levels but rather to enhanced phosphorylation of the activation loop tyrosine. PTP-1B -/- cells display increased inflammatory activity in vitro and in vivo through the constitutive up-regulation of activation markers as well as increased sensitivity to endotoxin. Collectively, our data indicate that PTP-1B is an important modulator of myeloid differentiation and macrophage activation in vivo and provide a demonstration of a physiological role for PTP-1B in immune regulation.

Pten deletion leads to the expansion of a prostatic stem/progenitor cell subpopulation and tumor initiation

PTEN (phosphatase and tensin homolog deleted on chromosome 10) is a potent tumor suppressor gene frequently mutated in human prostate cancers. Deletion of Pten in a murine model of prostate cancer recapitulates the disease progression seen in humans. Using defined cell lineage markers, we demonstrate that PTEN negatively regulates p63-positive prostatic basal cell proliferation without blocking differentiation. Concomitant with basal cell proliferation is the expansion of a prostate stem/progenitor-like subpopulation as evidenced by the progressive increase of stem cell antigen-1 (Sca-1)- and BCL-2-positive cells. This observation provides strong evidence that basal cell proliferation can be an initiating event for precancerous lesions. Sca-1(+) and BCL-2(+) progenitors may serve as cancer-initiating cells in this model.

Jun signalling in the epidermis: From developmental defects to psoriasis and skin tumors

The Jun proteins Jun, JunB and JunD are core members of activator protein-1 (AP-1), a dimeric transcription factor complex consisting of homo- and heterodimers of the Jun, Fos, activating transcription factor (ATF) and musculoaponeurotic fibrosarcoma (Maf) families. Growth factors, hormones and a variety of environmental stresses activate mitogen activated protein kinase (MAPK) cascades that enhance Jun/AP-1 activity, e.g. through phosphorylation thereby regulating cell proliferation, differentiation, transformation and/or apoptosis. Embryonic lethality of various AP-1 knock-outs, e.g. for Jun, JunB, Fra-1 and Fra-2 largely prevented functional studies in vivo. Therefore, conditional knock-out strategies, in particular for the epidermis, have become an important model to study the regulation and function of AP-1 subunits in physiological and pathological processes in vivo. Jun is regarded as a positive regulator of keratinocyte proliferation/differentiation during development and in skin cancer through its direct transcriptional effect on epidermal growth factor receptor (EGFR) expression. In contrast, JunB can antagonize proliferation of keratinocytes and hematopoietic stem cells. Furthermore, it has been demonstrated in patient's samples and an inducible mouse model that down-regulation of JunB/AP-1 in keratinocytes is one initiating event in the aetiology of psoriasis which is characterized by increased cell proliferation and deregulated cytokine expression.

Two types of precursor cells in a multipotential hematopoietic cell line

The biochemistry of early stages of hematopoietic differentiation is difficult to study because only relatively small numbers of precursor cells are available. The murine EML cell line is a multipotential cell line that can be used to model some of these steps. We found that the lineage- EML precursor cells can be separated into two populations based on cell surface markers including CD34. Both populations contain similar levels of stem cell factor (SCF) receptor (c-Kit) but only the CD34+ population shows a growth response when treated with SCF. Conversely, the CD34- population will grow in the presence of the cytokine IL-3. The human beta-globin locus control region hypersensitive site 2 plays different roles on beta-globin transcription in the CD34+ and CD34- populations. The two populations are present in about equal amounts in culture, and the CD34+ population rapidly regenerates the mixed population when grown in the presence of SCF. We suggest that this system may mimic a normal developmental transition in hematopoiesis.

Biology of Normal and Acute Myeloid Leukemia Stem Cells

The substantial understanding that has been gained over the past 5 decades of the biology of blood formation is largely due to the development of functional quantitative assays for cells at all stages of differentiation, from multipotential stem cells to mature cells. The majority of studies have involved the mouse because the ease with which repopulation studies can be carried out with this animal model allows the assay of complete lineage development from stem cells. In the past decade, advances in repopulation assays for human stem cells using xenotransplantation have greatly enhanced our understanding of human stem cell biology. Importantly, the xenotransplantation methodology has also been used to identify the cancer stem cell that initiates and sustains leukemic proliferation, providing key evidence for the cancer stem cell hypothesis. This hypothesis argues that cancer cells are functionally heterogeneous and hierarchically organized such that only specific cells are capable of sustaining tumor growth and continuously producing the cells that make up the bulk of the tumor. Recent studies have also brought into focus the importance of the intimate relationship between the stem cell (normal or leukemic) and its microenvironment. Coming into view are the molecular players involved in stem cell homing, migration, and adhesion, as well as the cellular components of the microenvironmental niche. Here we review recent studies that have begun, to elucidate the interplay between normal and leukemic human stem cells and their microenvironment.

Toward an understanding of the physiological function of Mammalian stem cells

Stem cell biology has the potential to yield new therapies, new insights into disease, and a clearer understanding of tissue formation and maintenance. However, much of what we know about many stem cells is based upon experiments performed in culture. Stem cells sometimes exhibit critical differences in their properties or regulation between the culture and in vivo environments. Though cell lines with stem cell properties can be derived from the long-term culture of diverse tissues, it is not clear whether cells with similar properties exist in vivo. If the goal is to use differentiated cells for therapy or drug screening, it may not matter whether these stem cells exist in vivo. However, to understand tissue development/maintenance or the role of stem cells in disease, it is important to characterize progenitor function in vivo to evaluate physiological significance.

Ectopic expression of Oct-4 blocks progenitor-cell differentiation and causes dysplasia in epithelial tissues

The POU-domain transcription factor Oct-4 is normally expressed in pluripotent cells of the mammalian embryo. In addition, germ-cell tumors and a few somatic tumors show detectable expression of Oct-4. While Oct-4's role during preimplantation development is to maintain embryonic cells in a pluripotent state, little is known about its potential oncogenic properties. Here we investigate the effect of ectopic Oct-4 expression on somatic tissues of adult mice using a doxycycline-dependent expression system. Activation of Oct-4 results in dysplastic growths in epithelial tissues that are dependent on continuous Oct-4 expression. Dysplastic lesions show an expansion of progenitor cells and increased beta-catenin transcriptional activity. In the intestine, Oct-4 expression causes dysplasia by inhibiting cellular differentiation in a manner similar to that in embryonic cells. These data show that certain adult progenitors remain competent to interpret key embryonic signals and support the notion that progenitor cells are a driving force in tumorigenesis.

JunB expression is a common feature of CD30+ lymphomas and lymphomatoid papulosis

JunB is a member of the Jun family of proteins that are components of the AP-1 transcription factor complex. AP-1 is involved in cell proliferation and apoptosis. Recent evidence suggests that Hodgkin and Reed-Sternberg cells overexpress JunB and that JunB facilitates constitutive CD30 expression by binding to an AP-1 site in the CD30 promoter. In this study we surveyed JunB expression in a variety of CD30+ lymphoma types including 42 cases of anaplastic large cell lymphoma, 36 classical Hodgkin lymphoma, 15 cutaneous anaplastic large cell lymphoma, and 11 CD30+ diffuse large B-cell lymphoma. In addition, seven cases of nodular lymphocyte-predominant Hodgkin lymphoma and 42 diffuse large B-cell lymphoma, known to be CD30-, were analyzed. JunB expression was assessed using tissue microarrays, immunohistochemistry and a monoclonal antibody specific for JunB. Expression of JunB was observed in 41 of 42 cases of anaplastic large cell lymphoma, including all 21 cases positive for anaplastic lymphoma kinase and 20 of 21 (95%) negative for anaplastic lymphoma kinase. JunB was also expressed in all cases of classical Hodgkin lymphoma, cutaneous anaplastic large cell lymphoma and CD30+ diffuse large B-cell lymphoma, and in lymphomatoid papulosis. By contrast, all nodular lymphocyte-predominant Hodgkin lymphomas and diffuse large B-cell lymphomas that were CD30- were also JunB-. We conclude that JunB is expressed in virtually all CD30+ lymphomas and is a potential target for experimental therapy in patients with these tumors.

Psoriasis-like skin disease and arthritis caused by inducible epidermal deletion of Jun proteins

Psoriasis is a frequent, inflammatory disease of skin and joints with considerable morbidity. Here we report that in psoriatic lesions, epidermal keratinocytes have decreased expression of JunB, a gene localized in the psoriasis susceptibility region PSORS6. Likewise, inducible epidermal deletion of JunB and its functional companion c-Jun in adult mice leads (within two weeks) to a phenotype resembling the histological and molecular hallmarks of psoriasis, including arthritic lesions. In contrast to the skin phenotype, the development of arthritic lesions requires T and B cells and signalling through tumour necrosis factor receptor 1 (TNFR1). Prior to the disease onset, two chemotactic proteins (S100A8 and S100A9) previously mapped to the psoriasis susceptibility region PSORS4, are strongly induced in mutant keratinocytes in vivo and in vitro. We propose that the abrogation of JunB/activator protein 1 (AP-1) in keratinocytes triggers chemokine/cytokine expression, which recruits neutrophils and macrophages to the epidermis thereby contributing to the phenotypic changes observed in psoriasis. Thus, these data support the hypothesis that epidermal alterations are sufficient to initiate both skin lesions and arthritis in psoriasis.

Selective activation of STAT5 unveils its role in stem cell self-renewal in normal and leukemic hematopoiesis

Although the concept of a leukemic stem cell system has recently been well accepted, its nature and the underlying molecular mechanisms remain obscure. Constitutive activation of signal transducers and activators of transcription 3 (STAT3) and STAT5 is frequently detected in various hematopoietic tumors. To evaluate their role in normal and leukemic stem cells, we took advantage of constitutively active STAT mutants to activate STAT signaling selectively in hematopoietic stem cells (HSCs). Activation of STAT5 in CD34- c-Kit+ Sca-1+ lineage marker- (CD34- KSL) HSCs led to a drastic expansion of multipotential progenitors and promoted HSC self-renewal ex vivo. In sharp contrast, STAT3 was demonstrated to be dispensable for the HSC maintenance in vivo, and its activation facilitated lineage commitment of HSCs in vitro. In a mouse model of myeloproliferative disease (MPD), sustained STAT5 activation in CD34- KSL HSCs but not in CD34+ KSL multipotential progenitors induced fatal MPD, indicating that the capacity of STAT5 to promote self-renewal of hematopoietic stem cells is crucial to MPD development. Our findings collectively establish a specific role for STAT5 in self-renewal of normal as well as leukemic stem cells.

Identification of bronchioalveolar stem cells in normal lung and lung cancer

Injury models have suggested that the lung contains anatomically and functionally distinct epithelial stem cell populations. We have isolated such a regional pulmonary stem cell population, termed bronchioalveolar stem cells (BASCs). Identified at the bronchioalveolar duct junction, BASCs were resistant to bronchiolar and alveolar damage and proliferated during epithelial cell renewal in vivo. BASCs exhibited self-renewal and were multipotent in clonal assays, highlighting their stem cell properties. Furthermore, BASCs expanded in response to oncogenic K-ras in culture and in precursors of lung tumors in vivo. These data support the hypothesis that BASCs are a stem cell population that maintains the bronchiolar Clara cells and alveolar cells of the distal lung and that their transformed counterparts give rise to adenocarcinoma. Although bronchiolar cells and alveolar cells are proposed to be the precursor cells of adenocarcinoma, this work points to BASCs as the putative cells of origin for this subtype of lung cancer.

Normal and leukaemic stem cells

The blood-related cancer leukaemias were the first diseases where human cancer stem cells, or leukaemic stem cells (LSC), were isolated. The haematopoietic system is one of the best tissues for investigating cancer stem cells, because the developmental hierarchy of normal blood formation is well defined. Leukaemias can now be viewed as aberrant haematopoietic processes initiated by rare LSC that have maintained or reacquired the capacity for indefinite proliferation through accumulated mutations and/or epigenetic changes. Yet, despite their critical importance, much remains to be learned about the developmental origin of LSC and the mechanisms responsible for their emergence in the course of the disease. This report will review our current knowledge on normal and LSC development and examine the impact of these discoveries may have clinically and in our understanding of the leukaemogenic process.

Effect of transcription-factor concentrations on leukemic stem cells

Increasing evidence suggests that leukemias are sustained by leukemic stem cells. However, the molecular pathways underlying the transformation of normal cells into leukemic stem cells are still poorly understood. The involvement of a small group of key transcription factors into this process was suggested by their frequent mutation or down-regulation in patients with acute myeloid leukemia (AML). Recent findings in mice with hypomorphic transcription-factor genes demonstrated that leukemic stem-cell formation in AML could directly be caused by reduced transcription-factor activity beyond a critical threshold. Most interestingly, those experimental models and the paucity of biallelic null mutations or deletions in transcription-factor genes in patients suggest that AML is generally associated with graded down-regulation rather than complete disruption of transcription factors. Here, we discuss the effects of transcription-factor concentrations on hematopoiesis and leukemia, with a focus on the regulation of transcription-factor gene expression as a major mechanism that alters critical threshold levels during blood development and cancer.

Leukaemia stem cells and the evolution of cancer-stem-cell research

Many cancers seem to depend on a small population of 'cancer stem cells' for their continued growth and propagation. The leukaemia stem cell (LSC) was the first such cell to be described. The origins of these cells are controversial, and their biology - like that of their normal-tissue counterpart, the haematopoietic stem cell (HSC) - is still not fully elucidated. However, the LSC is likely to be the most crucial target in the treatment of leukaemias, and a thorough understanding of its biology - particularly of how the LSC differs from the HSC - might allow it to be selectively targeted, improving therapeutic outcome.

JAKing up hematopoietic proliferation

Mutations that deregulate proliferation and survival pathways have emerged as a common molecular theme in the pathogenesis of myeloproliferative disorders (MPDs). Three studies now report an amino acid substitution in the JAK2 kinase in most patients with polycythemia vera as well as in some cases of essential thrombocythemia and chronic idiopathic myelofibrosis. Functional analysis demonstrates that this mutation confers erythropoietin-independent growth in vitro, deregulates signaling pathways downstream of JAK2, and causes polycythemia in mice. These results open new avenues for diagnosing and classifying patients with these disorders, and identify a new molecular target for drug discovery.

Mechanisms of BCR-ABL in the pathogenesis of chronic myelogenous leukaemia

Imatinib, a potent inhibitor of the oncogenic tyrosine kinase BCR-ABL, has shown remarkable clinical activity in patients with chronic myelogenous leukaemia (CML). However, this drug does not completely eradicate BCR-ABL-expressing cells from the body, and resistance to imatinib emerges. Although BCR-ABL remains an attractive therapeutic target, it is important to identify other components involved in CML pathogenesis to overcome this resistance. What have clinical trials of imatinib and studies using mouse models for BCR-ABL leukaemogenesis taught us about the functions of BCR-ABL beyond its kinase activity, and how these functions contribute to CML pathogenesis?

Mouse models of human non-small-cell lung cancer: raising the bar

Lung cancer is a devastating disease that presents a challenge to basic research to provide new steps toward therapeutic advances. The cell-type-specific responses to oncogenic mutations that initiate and regulate lung cancer remain poorly defined. A better understanding of the relevant signaling pathways and mechanisms that control therapeutic outcome could also provide new insight. Improved conditional mouse models are now available as tools to improve the understanding of the cellular and molecular origins of adenocarcinoma. These models have already proven their utility in proof-of-principle experiments with new technologies including genomics and imaging. Integrated thinking to apply technological advances while using the appropriate mouse model is likely to facilitate discoveries that will significantly improve lung cancer detection and intervention.

Prostate stem cells and prostate cancer

Understanding prostate stem cells (PSCs) may provide insight for the design of therapeutics for prostate cancer. We have developed a quantitative in vivo colony-forming assay and have demonstrated that the Sca-1 antigen is present on the surface of a prostate cell subpopulation that possesses multiple stem cell properties. Immunofluorescent analysis demonstrates that Sca-1 is expressed by both basal and luminal cells in the proximal region of the adult prostate, but is not expressed by either lineage in more distal regions. The proximal region has been suggested as the PSC niche based on BrdU label-retention studies and the presence of distinct smooth-muscle cells that produce high levels of TGF-beta. Sca-1 is also expressed by nearly all cells within fetal prostate epithelial chords, suggesting Sca-1 may be conserved on PSCs throughout development. Malignant epithelial cells from TRAMP mice, as well as normal prostate cells with lentiviral-mediated alteration of the PTEN/AKT signaling pathway, give rise to PIN lesions and prostate cancer in vivo. Alteration of PTEN/AKT signaling in Sca-1-enriched PSCs also results in PIN lesions, suggesting that PSCs can serve as one target for prostate carcinogenesis.

Chronic myeloid leukemia: proving ground for cancer stem cells

A central question in cancer biology is whether malignancy arises in self-renewing tissue stem cells that suffer oncogene activation or in differentiated cells that acquire properties of unremitting self-renewal? In two papers, Weissman and colleagues document both mechanisms: chronic leukemia arising by mutation affecting the hematopoietic stem cell (HSC) and acute leukemia evolving from committed granulocyte-macrophage progenitors that have acquired the self-renewal machinery of HSCs.

Chronic versus acute myelogenous leukemia: a question of self-renewal

Leukemia stem cells are defined as transformed hematopoietic stem cells or committed progenitor cells that have amplified or acquired the stem cell capacity for self-renewal, albeit in a poorly regulated fashion. In this issue of Cancer Cell, Huntly and colleagues report a striking difference in the ability of two leukemia-associated fusion proteins, MOZ-TIF2 and BCR-ABL, to transform myeloid progenitor populations. This rigorous study supports the idea of a hierarchy among leukemia-associated protooncogenes for their ability to endow committed myeloid progenitors with the self-renewal capacity driving leukemic stem cell propagation, and sheds new light on the pathogenesis of chronic and acute myelogenous leukemias.