Self-renewal of pluripotent embryonic stem cells is mediated via activation of STAT3

The ribosomal S6 kinases, cAMP-responsive element-binding, and STAT3 proteins are regulated by different leukemia inhibitory factor signaling pathways in mouse embryonic stem cells

Mouse embryonic stem (ES) cells remain "pluripotent" in vitro in the continuous presence of leukemia inhibitory factor (LIF). In the absence of LIF, ES cells are irreversibly committed to differentiate into various lineages. In this study we have set up an in vitro assay based on the anti-apoptotic activity of LIF to distinguish pluripotent from "differentiation-committed" ES cells. We have examined the phosphorylation profiles of known (STAT3 and ERKs) and identified new (ribosomal S6 kinases (RSKs) and cAMP-responsive element-binding protein (CREB)) LIF-regulated targets in ES and in ES-derived neuronal cells. We have demonstrated that although STAT3, a crucial player in the maintenance of ES cell pluripotency, is induced by LIF in all cell types tested, the LIF-dependent activation of RSKs is restricted to ES cells. We have shown that LIF-induced phosphorylation of RSKs in ES cells is dependent on ERKs, whereas STAT3 phosphorylation is not mediated by any known MAPK activities. Our results also demonstrate that the LIF-dependent phosphorylation of CREB is partially under the control of the RSK2 kinase.

The end of the beginning for pluripotent stem cells

Pluripotent stem cells can be expanded seemingly indefinitely in culture, maintain a normal karyotype and have the potential to generate any cell type in the body. As such they represent an incredible resource for the repair of diseased or damaged tissues in our bodies. These cells also promise to open a new window into the embryonic development of our species.

The regulation of embryonic stem cell differentiation by leukaemia inhibitory factor (LIF)

LIF (leukaemia inhibitory factor) is commonly used to maintain mouse embryonic stem cells in an undifferentiated state. These cells spontaneously differentiate when allowed to aggregate in the absence of LIF, forming embryoid bodies in which early embryonic cell lineages develop. Using embryoid bodies cultured in the presence and absence of LIF, we show that although LIF inhibited the development of visceral and parietal endodermal cells, it did not affect the differentiation of the primitive endodermal cell precursors of these extraembryonic cell lineages. Furthermore, deposition of the basement membrane produced by the primitive endodermal cells, which separates them from the remaining cells of the embryoid body, still occurred. The differentiation of primitive ectodermal cells and their progeny was inhibited by LIF, as evidenced by the lack of expression of FGF-5, muscle, and neuronal markers. However, cavitation of the embryoid body and maintenance of the cells in contact with the primitive endodermal basement membrane as an epiblast epithelium still occurred normally in the presence of LIF. These results indicate that cavitation and formation of the epiblast epithelium are regulated by mechanisms distinct from those controlling the differentiation of epiblast cell lineages. Furthermore, although epithelium formation and cavitation do not require the differentiation of visceral endodermal cells, the results are consistent with the hypothesis that the primitive endodermal basement membrane is sufficient to induce the epithelialization of undifferentiated embryonic stem cells necessary for cavitation.

Lymphocyte development from hematopoietic stem cells

The recent application of new techniques, such as multi-color cell sorting and the production of transgenic and gene-knockout mice, has contributed to a better understanding of lymphocyte development from hematopoietic stem cells. Now that we can purify progenitors at different maturational stages during lymphocyte development, the challenge is to understand the processes that govern each developmental stage transition.

Pluripotent cells (stem cells) and their determination and differentiation in early vertebrate embryogenesis

Mammalian embryonic stem cells can be obtained from the inner cell mass of blastocysts or from primordial germ cells. These stem cells are pluripotent and can develop into all three germ cell layers of the embryo. Somatic mammalian stem cells, derived from adult or fetal tissues, are more restricted in their developmental potency. Amphibian ectodermal and endodermal cells lose their pluripotency at the early gastrula stage. The dorsal mesoderm of the marginal zone is determined before the mid-blastula transition by factors located after cortical rotation in the marginal zone, without induction by the endoderm. Secreted maternal factors (BMP, FGF and activins), maternal receptors and maternal nuclear factors (beta-catenin, Smad and Fast proteins), which form multiprotein transcriptional complexes, act together to initiate pattern formation. Following mid-blastula transition in Xenopus laevis (Daudin) embryos, secreted nodal-related (Xnr) factors become important for endoderm and mesoderm differentiation to maintain and enhance mesoderm induction. Endoderm can be induced by high concentrations of activin (vegetalizing factor) or nodal-related factors, especially Xnr5 and Xnr6, which depend on Wnt/beta-catenin signaling and on VegT, a vegetal maternal transcription factor. Together, these and other factors regulate the equilibrium between endoderm and mesoderm development. Many genes are activated and/or repressed by more than one signaling pathway and by regulatory loops to refine the tuning of gene expression. The nodal related factors, BMP, activins and Vg1 belong to the TGF-beta superfamily. The homeogenetic neural induction by the neural plate probably reinforces neural induction and differentiation. Medical and ethical problems of future stem cell therapy are briefly discussed.

Tissue-specific autoregulation of the stat3 gene and its role in interleukin-6-induced survival signals in T cells

Signal transducer and activator of transcription 3 (STAT3) mediates signals of various growth factors and cytokines, including interleukin-6 (IL-6). In certain IL-6-responsive cell lines, the stat3 gene is autoregulated by STAT3 through a composite IL-6 response element in its promoter that contains a STAT3-binding element (SBE) and a cyclic AMP-responsive element. To reveal the nature and roles of the stat3 autoregulation in vivo, we generated mice that harbor a mutation in the SBE (stat3(mSBE)). The intact SBE was crucial for IL-6-induced stat3 gene activation in the spleen, especially in the red pulp region, the kidney, and both mature and immature T lymphocytes. The SBE was not required, however, for IL-6-induced stat3 gene activation in hepatocytes. T lymphocytes from the stat3(mSBE/mSBE) mice were more susceptible to apoptosis despite the presence of IL-6 than those from wild-type mice. Consistent with this, IL-6-dependent activation of the Pim-1 and junB genes, direct target genes for STAT3, was attenuated in T lymphocytes of the stat3(mSBE/mSBE) mice. Thus, the tissue-specific autoregulation of the stat3 gene operates in vivo and plays a role in IL-6-induced antiapoptotic signaling in T cells.

A role for the extracellular signal-regulated kinase and p38 mitogen-activated protein kinases in interleukin-1 beta-stimulated delayed signal tranducer and activator of transcription 3 activation, atrial natriuretic factor expression, and cardiac myocyte morphology

We have demonstrated that two hypertrophic agents, interleukin-1 beta (IL-1 beta) and leukemic inhibitory factor (LIF), altered cardiac myocyte morphology with striking similarity and prompted us to investigate the common actions of these cytokines. We compared the phosphorylation/activation of signal tranducer and activator of transcription 3 (STAT3), extracellular signal-regulated kinase (ERK), p38(MAPK), and c-Jun N-terminal kinase mitogen-activated protein kinases (MAPKs). The phosphorylation of STAT3 by IL-1 beta was delayed (>60 min), whereas the response to LIF was rapid (<10 min) and transient. We confirmed that IL-1 beta potently stimulated all three MAPK subfamilies. In contrast, LIF promoted strong activation of ERKs, marginal activation of p38(MAPK), and no c-Jun N-terminal kinase activation. To test the roles of ERKs and p38(MAPK), myocytes were pretreated with PD98059 and SB203580. Either inhibitor alone prevented STAT3 phosphorylation, implicating ERKs and p38(MAPK) in the delayed STAT3 response to IL-1 beta. The interplay of MAPKs and STAT3 phosphorylation in regulating IL-1 beta-stimulated hypertrophy was investigated by evaluating the effect of MAPK inhibitors on atrial natriuretic factor (ANF) expression and myocyte morphology. The specific inhibition of either ERK or p38(MAPK) attenuated the IL-1 beta- or LIF-stimulated ANF expression by up to 70%. Inhibition was not further increased in the presence of both inhibitors. Furthermore, although individual inhibition of ERK or p38(MAPK) did not affect morphology, co-treatment with both inhibitors abrogated the hypertrophic morphology stimulated by IL-1 beta but not by LIF. Taken together, our data indicate that the activation of ERK and p38(MAPK) is essential in regulating a delayed STAT3 phosphorylation as well as changes in ANF expression and morphology that follow IL-1 beta treatment. Thus, the role of MAPKs in the hypertrophic response can be dictated at least partly by the nature of the hypertrophic agent employed.

Defective gp130-mediated signal transducer and activator of transcription (STAT) signaling results in degenerative joint disease, gastrointestinal ulceration, and failure of uterine implantation

The receptor subunit gp130 transduces multiple cell type-specific activities of the leukemia inhibitory factor (LIF)/interleukin (IL)-6 family of cytokines through the signal transducer and activator of transcription (STAT) and src homology 2 domain-bearing protein tyrosine phosphatase (SHP)-2/ras/Erk pathways. To define STAT-dependent physiological responses, we generated mice with a COOH-terminal gp130(DeltaSTAT) "knock-in" mutation which deleted all STAT-binding sites. gp130(DeltaSTAT) mice phenocopyed mice deficient for IL-6 (impaired humoral and mucosal immune and hepatic acute phase responses) and LIF (failure of blastocyst implantation). However, unlike mice with null mutations in any of the components in the gp130 signaling pathway, gp130(DeltaSTAT) mice also displayed gastrointestinal ulceration and a severe joint disease with features of chronic synovitis, cartilaginous metaplasia, and degradation of the articular cartilage. Mitogenic hyperresponsiveness of synovial cells to the LIF/IL-6 family of cyto-kines was caused by sustained gp130-mediated SHP-2/ras/Erk activation due to impaired STAT-mediated induction of suppressor of cytokine signaling (SOCS) proteins which normally limits gp130 signaling. Therefore, the joint pathology in gp130(DeltaSTAT) mice is likely to arise from the disturbance of the otherwise balanced activation of the SHP-2/ras/Erk and STAT signaling cascades emanating from gp130.

Inner cell mass-specific expression of a cell adhesion molecule (PECAM-1/CD31) in the mouse blastocyst

Platelet/Endothelial Cell Adhesion Molecule-1 (PECAM-1 or CD31) is thought to be a vascular-specific protein, but its function has not been clearly defined. Here, we demonstrate by using confocal immunofluorescence microscopy that PECAM-1 is first detected in the mouse blastocyst, which contains no vascular cells, and its expression is restricted to the pluripotent inner cell mass (ICM) cells. Expression is localized to cell-cell borders of the ICM and is detected at the very first signs of blastocoel formation. Consistent with these observations is that embryonic transcripts of PECAM-1 mRNA, as detected by RT-PCR, greatly increase during the morula-to-blastocyst transition and seven of the eight known alternatively spliced isoforms of PECAM-1 are expressed in the blastocyst. The synthesis of PECAM-1 is independent of compaction, cytokinesis, and DNA replication, as it is detected in embryos that are chronologically at the blastocyst stage following culture of 8-cell embryos in Ca2+-free medium, or medium containing cytochalasin D or aphidicolin, respectively. By the late blastocyst stage, PECAM-1 expression is restricted to the pluripotent epiblast, at which point it has a mutually exclusive expression pattern to that of type IV collagen, a basement membrane marker. The reduction in PECAM-1 transcripts in retinoic acid-induced differentiation of F9 teratocarcinoma cells, a model of epiblast-to-primitive endoderm differentiation, confirmed the epiblast-specific expression of PECAM-1. By the egg cylinder stage of development, at which point the epiblast is no longer pluripotent, PECAM-1 is not detected. This ICM-specific pattern of expression suggests a novel developmental role of PECAM-1 that is independent of its function in vascular ontogeny.

Potential of embryonic stem cells

Embryonic stem (ES) cells are pluripotent cell lines established from undifferentiated embryonic cells characterized by nearly unlimited self-renewal and differentiation capacity. During differentiation in vitro, ES cells were found to be able to develop into specialized somatic cells types and to recapitulate processes of early embryonic development. These properties allow to use ES cells as model system for studying early embryonic development by gain- or loss-of-function approaches, or to investigate the effects of drugs and environmental factors on differentiation and cell function in embryotoxicity and pharmacology. Now, ES cells derived of human blastocysts may be used for the generation of somatic precursor or differentiated cells in cell and tissue therapy. The review presents data of mouse ES cell differentiation and gives an outlook on future perspectives and problems of using human ES cells in regenerative medicine.

Molecular mechanism to maintain stem cell renewal of ES cells

Embryonic stem (ES) cells are pluripotent cells directly derived from early stage embryos that retain the ability to differentiate into all cell types. This unique feature is the basis of various applications of ES cell technology such as in vitro models of mammalian development, germline transgenesis to make knockout mice, and a generic source for cell therapy in regenerative medicine. To achieve success in these applications, the pluripotency of ES cells has to be kept stable during long-term culture in vitro, leading to the necessity of determining the molecular basis for maintaining ES self-renewal. This paper summarizes the recent progress in this area, focusing mainly on the LIF signaling pathway and the transcription factor Oct-3/4. Although it is still unclear how these components works together, a model is presented here that provides a plan to solve this problem.

Neuronal differentiation of mouse embryonic stem cells: lineage selection and forced differentiation paradigms

Primitive embryonic stem cells are an ideal starting cell population for studies of gene expression and lineage segregation during development. Despite their potential, it has been difficult to determine culture conditions that cause single-lineage differentiation of these pluripotent cells. Both genetic and epigenetic approaches have been taken to promote neuronal differentiation of embryonic stem cells, including aggregation, exposure to the nonspecific teratogen/morphogen retinoic acid, low-density culture, exposure to growth/differentiation factors, and forced differentiation following expression of lineage-restricted "developmental control" genes. In the current investigation, a hybrid approach involving genetic techniques of "lineage selection" or "forced differentiation" has been employed to develop primitive neural progenitor cell lines. These lines form an important starting point to examine the cascades of gene expression (and inhibition) during neuronal and glial lineage segregation, to study growth factor effects on neural differentiation, and ultimately to provide a source of cells for transplantation to a damaged nervous system.

Identification of a new gene family specifically expressed in chicken embryonic stem cells and early embryo

Chicken embryonic stem (CES) cells are pluripotent cells derived from chicken early blastoderm. In order to identify new genes specifically expressed in these pluripotent cells, we have used a gene trap strategy and cloned a novel gene family called cENS for chicken Embryonic Normal Stem cell gene. The cENS genes expression decreases after induction of CES cells differentiation in culture and is restricted in vivo to the very early embryo. We have characterized three different cENS genes. One, cENS-1, is composed of an open reading frame inserted between two terminal direct repeats which are the common point of the cENS genes. cENS-1 encodes a protein identical to cERNI, a recently described protein. cENS-2 is a truncated form of cENS-1. cENS-3 presents two adjacent open reading frames coding respectively for env and pol related proteins. The presence of conserved direct repeats, of retrovirus related genes and the absence of introns argue in favor of a retroviral origin of the cENS genes. In the cENS we identified a promoter region whose activity is strong in CES cells and decreases after induced differentiation showing a highly specific transcriptional activity specific of undifferentiated chicken embryonic stem cells.

Differential expression of multiple genes during articular chondrocyte redifferentiation

Articular chondrocytes undergo a rapid change in phenotype and gene expression, termed dedifferentiation, when isolated from cartilage tissue and cultured on tissue culture plastic. On the other hand, "redifferentiation" of articular chondrocytes in suspension culture is characterized by decreased cellular proliferation and the reinitiation of synthesis of hyaline articular cartilage extracellular matrix molecules. The molecular triggers for these events have yet to be defined. Subtracted cDNA libraries representing genes involved in the early events of adult human articular chondrocyte redifferentiation were generated from human articular chondrocytes that were first cultured in monolayer, and subsequently transferred to suspension culture at 10(6) cells/ml for redifferentiation. Differential regulation of genes involved in cellular organization, nuclear structure, cellular growth regulation, and extracellular matrix deposition and remodeling were observed within 48 hr of this transfer. Many of these genes had not been previously identified in the chondrocyte differentiation pathway and a number of the isolated cDNAs did not have homologies to sequences in the public data banks. Genes involved in IL-6 signal transduction including acute phase response factor (APRF), Mn superoxide dismutase, and IL-6 itself were up-regulated in suspension culture. Membrane glycoprotein gp130, a component of the IL-6 receptor, was down-regulated. Other genes involved in cell polarity, cell adherence, apoptosis, and possibly TGF-beta signaling were differentially regulated. The differential regulation of the cytokine connective tissue growth factor (CTGF) during the early stages of articular chondrocyte redifferentiation, decreasing within 48 hours of transfer to suspension culture, was particularly interesting given its reported role in the stimulation of cellular proliferation. CTGF was highly expressed in proliferative monolayer culture, and then greatly reduced by redifferentiation in standard high-density suspension culture. When articular chondrocytes were seeded in suspension at low-density (10(4) cells/ml), however, high levels of CTGF were observed along with increased levels of mature articular cartilage extracellular matrix protein RNAs, such as type II collagen and aggrecan. Although the role of CTGF in articular cartilage biology remains to be elucidated, the results described here demonstrate the potential utility of subtractive hybridization in understanding the process of articular chondrocyte redifferentiation.

Stem cells and diabetes

Diabetes mellitus is a metabolic disorder affecting 2-5% of the population. Transplantation of isolated islets of Langerhans from donor pancreata could be a cure for diabetes; however, such an approach is limited by the scarcity of the transplantation material and the long-term side effects of immunosuppressive therapy. These problems may be overcome by using a renewable source of cells, such as islet cells derived from stem cells. Stem cells are defined as clonogenic cells capable of both self-renewal and multilineage differentiation. This mean that these cells can be expanded in vivo or in vitro and differentiated to produce the desired cell type. There exist several sources of stem cells that have been demonstrated to give rise to pluripotent cell lines: 1) embryonic stem cells; 2) embryonic germ cells; 3) embryonic carcinoma cells; and 4) adult stem cells. By using in vitro differentiation and selection protocols, embryonic stem cells can be guided into specific cell lineages and selected by applying genetic selection when a marker gene is expressed. Recently, differentiation and cell selection protocols have been used to generate embryonic stem cell-derived insulin-secreting cells that normalise blood glucose when transplanted into diabetic animals. Some recent reports suggest that functional plasticity of adult stem cells may be greater than expected. The use of adult stem cells will circumvent the ethical dilemma surrounding embryonic stem cells and will allow autotransplantation. These investigations have increased the expectations that cell therapy could be one of the solutions to diabetes.

Versatile stem cells, young and old. A review

Both embryonic and somatic stem cells have been studied in recent years with particular regard to their differentiation potential. In vitro studies allow a considerable amplification of such cells in culture as well as the induction of commitment in different directions under proper stimulating factors. Moreover, a surprising versatility has been discovered,which makes possible a ;reprogramming' of stem cells into a lineage pathway which may be completely different from the expected direction: for instance, a production of brain cells from blood progenitors has been obtained. It is thus possible to envisage methods of producing in culture sufficient amounts of stem cells, committed to a certain pathway, which can be transplanted in vivo to replace damaged tissues and organs.

The role of Stat3 in apoptosis and mammary gland involution. Conditional deletion of Stat3

STATs (signal transducer and activator of transcription) are a family of latent transcription factors which are activated in response to a variety of cytokines and growth factors. This family of signalling molecules have been implicated in growth, differentiation, survival and apoptosis. In this article, we will review work which highlights the role of individual STAT factors in mammary gland and demonstrate the value of genetically modified mice in defining the function of STAT3. Involution of the mouse mammary gland is characterised by extensive apoptosis of the epithelial cells and the activation of STAT3. STATs 3 and 5 have reciprocal patterns of activation throughout a mammary developmental cycle suggesting that STAT5 may be a survival factor and STAT3 a death factor for differentiated mammary epithelium. To clarify the role of STAT3 in mammary epithelial apoptosis, we have generated a conditional knockout using the lox/Cre recombination system. Mammary glands from crosses of transgenic mice expressing Cre recombinase under the control of the beta-lactoglobulin milk protein gene promoter with mice harbouring one floxed STAT3 allele and one null STAT3 allele, showed a decrease in epithelial apoptosis and a dramatic delay of the involution process upon forced weaning. This was accompanied by precocious activation of STAT1 and increases in p53 and p21 levels--these may act as a compensatory mechanism for initiating the eventual involution which occurs in STAT3 null mammary glands. This demonstrates for the first time the importance of STAT factors in signalling the initiation of physiological apoptosis in vivo and highlights the utility of the lox/Cre system for addressing the function of genes, which have an embryonic lethal phenotype, specifically in mammary gland.

An efficient system for conditional gene expression in embryonic stem cells and in their in vitro and in vivo differentiated derivatives

We have developed a universally applicable system for conditional gene expression in embryonic stem (ES) cells that relies on tamoxifen-dependent Cre recombinase-loxP site-mediated recombination and bicistronic gene-trap expression vectors that allow transgene expression from endogenous cellular promoters. Two vectors were introduced into the genome of recipient ES cells, successively: (i) a bicistronic gene-trap vector encoding the beta-galactosidase/neo(R) fusion protein and the Cre-ER(T2) (Cre recombinase fused to a mutated ligand-binding domain of the human estrogen receptor) and (ii) a bicistronic gene-trap vector encoding the hygro(R) protein and the human alkaline phosphatase (hAP), the expression of which is prevented by tandemly repeated stop-of-transcription sequences flanked by loxP sites. In selected clones, hAP expression was shown to be regulated accurately by 4'hydroxy-tamoxifen. Strict hormone-dependent expression of hAP was achieved (i) in vitro in undifferentiated ES cells and embryoid bodies, (ii) in vivo in virtually all the tissues of the 10-day-old chimeric fetus (after injection of 4'hydroxy-tamoxifen to foster mothers), and (iii) ex vivo in primary embryonic fibroblasts isolated from chimeric fetuses. Therefore, this approach can be applied to drive conditional expression of virtually any transgene in a large variety of cell types, both in vitro and in vivo.

STAT proteins: novel molecular targets for cancer drug discovery

Signal Transducers and Activators of Transcription (STATs) are a family of cytoplasmic proteins with roles as signal messengers and transcription factors that participate in normal cellular responses to cytokines and growth factors. Frequently, however, abnormal activity of certain STAT family members, particularly Stat3 and Stat5, is associated with a wide variety of human malignancies, including hematologic, breast, head and neck, and prostate cancers. Application of molecular biology and pharmacology tools in disease-relevant models has confirmed Stat3 as having a causal role in oncogenesis, and provided validation of Stat3 as a target for cancer drug discovery and therapeutic intervention. Furthermore, a constitutively-active mutant form of Stat3 is sufficient to induce oncogenic transformation of cells, which form tumors in vivo. Constitutive activation of Stat3 signaling is accompanied by upregulation of cyclin D1, c-Myc, and Bcl-x, changes consistent with subversion of normal cellular growth and survival control mechanisms. Block of constitutive Stat3 signaling results in growth inhibition and apoptosis of Stat3-positive tumor cells in vitro and in vivo. The observed dependence of certain tumors on constitutive Stat3 signaling for growth and survival has wide implications for cancer therapy, offering the potential for preferential tumor cell killing. This review evaluates constitutive Stat3 activation as a 'cancer-causing' factor, and proposes a number of molecular strategies for targeting Stat3 signaling for therapeutic intervention.

Ankycorbin: a novel actin cytoskeleton-associated protein

BACKGROUND

Actin cytoskeleton structures are essential for a wide variety of cell functions, including cell shape change, cell motility, cell adhesion, cell polarity and cytokinesis. Many actin filament (F-actin)-binding proteins have been isolated and implicated in the maintenance and reorganization of actin cytoskeleton structures.

RESULTS

We purified here a novel protein with a molecular mass of about 125 kDa (p125) from rat liver. We cloned its cDNA from a mouse kidney cDNA library and determined its nucleotide and deduced amino acid sequences. p125 was a protein of 979 amino acids with a calculated Mr of 108 847. p125 contained six ankyrin repeats in the N-terminal region and a domain predicted to form a coiled-coil structure in the C-terminal region. We named p125 ankycorbin (ankyrin repeat- and coiled-coil structure-containing protein). Northern blot analysis indicated that ankycorbin was ubiquitously expressed in all the tissues examined. Immunofluorescence and immunoelectron microscope analyses revealed that ankycorbin was associated with the cortical actin cytoskeleton structures in terminal web and cell-cell adhesion sites and stress fibres. However, ankycorbin did not directly bind to F-actin as estimated by the F-actin co-sedimentation assay.

CONCLUSIONS

These results indicate that ankycorbin is indirectly associated with the actin cytoskeleton structures, presumably through an unidentified factor and suggest that it is involved in their maintenance and/or reorganization.

Identification and characterization of SKAT-2, a novel Th2-specific zinc finger gene

We have identified a novel Kruppel-type zinc finger (ZF) gene, SKAT-2, which is selectively expressed by murine Th2 cells. The protein encoded by this gene has 14 C2H2-type ZF tandemly arrayed at its C terminus and N-terminal SCAN box and KRAB domains. SKAT-2 is tissue restricted in expression at the RNA level, detectable only in brain and at low levels in kidney and spleen and few hematopoietic cell lines. By in situ hybridization, SKAT-2 expression was found to peak in antigen-stimulated CD4(+) T cells after 2-3 days of culture under Th2 but not Th1 biasing conditions. This pattern of expression closely mirrored that of GATA-3 in the same cells. In transient transfection experiments in phorbol 12-myristate 13-acetate/ionomycin-stimulated EL4 cells, SKAT-2 was found to up-regulate the activity of the IL-4 but not the IL-5 promoter, contrasting with the ability of GATA-3 to activate both promoters. This result was confirmed using clones of EL4 cells stably expressing an inducible form of SKAT-2, thus SKAT-2 is a novel Th2-specific gene that may play a role in selective regulation of cytokine genes in T cells.

STAT signalling in cell proliferation and in development

Recent advances in STAT signalling research include a better understanding of the roles of mammalian STAT proteins in cell proliferation and apoptosis, and of non-mammalian STAT proteins in morphogenesis. Two different ways in which STAT signalling pathways can interface with Smad signalling pathways significantly increasing combinatorial signalling possibilities, have also been described.

Two cell adhesion molecules, nectin and cadherin, interact through their cytoplasmic domain-associated proteins

We have found a new cell-cell adhesion system at cadherin-based cell-cell adherens junctions (AJs) consisting of at least nectin and l-afadin. Nectin is a Ca(2+)-independent homophilic immunoglobulin-like adhesion molecule, and l-afadin is an actin filament-binding protein that connects the cytoplasmic region of nectin to the actin cytoskeleton. Both the trans-interaction of nectin and the interaction of nectin with l-afadin are necessary for their colocalization with E-cadherin and catenins at AJs. Here, we examined the mechanism of interaction between these two cell-cell adhesion systems at AJs by the use of alpha-catenin-deficient F9 cell lines and cadherin-deficient L cell lines stably expressing their various components. We showed here that nectin and E-cadherin were colocalized through l-afadin and the COOH-terminal half of alpha-catenin at AJs. Nectin trans-interacted independently of E-cadherin, and the complex of E-cadherin and alpha- and beta-catenins was recruited to nectin-based cell-cell adhesion sites through l-afadin without the trans-interaction of E-cadherin. Our results indicate that nectin and cadherin interact through their cytoplasmic domain-associated proteins and suggest that these two cell-cell adhesion systems cooperatively organize cell-cell AJs.

AML1-MTG8 leukemic protein induces the expression of granulocyte colony-stimulating factor (G-CSF) receptor through the up-regulation of CCAAT/enhancer binding protein epsilon

The t(8;21) translocation is one of the most frequent chromosomal abnormalities associated with acute myeloid leukemia (AML). In this translocation, the AML1 (CBFA2/PEBP2aB) gene is disrupted and fused to the MTG8 (ETO) gene. The ectopic expression of the resulting AML1-MTG8 fusion gene product in L-G and 32Dcl3 murine myeloid precursor cells stimulates cell proliferation without inducing morphologic terminal differentiation into mature granulocytes in response to granulocyte-colony stimulating factor (G-CSF). This study found that the ectopic expression of AML1-MTG8 elevates the expression of the G-CSF receptor (G-CSFR). Analysis of the promoter region of the G-CSFR gene revealed that up-regulation of G-CSFR expression by AML1-MTG8 does not depend on the AML1-binding sequence, but on the C/EBP (CCAAT/enhancer binding protein) binding site. The results suggest that the overproduction of G-CSFR is at least partly mediated by C/EBPɛ, whose expression is activated by AML1-MTG8. The ectopic expression of G-CSFR in L-G cells induced cell proliferation in response to G-CSF, but did not inhibit cell differentiation into mature neutrophils. Overexpression of C/EBPɛ in L-G cells also stimulated G-CSF–dependent cell proliferation. High expression levels of G-CSFR were also found in the leukemic cells of AML patients with t(8;21). Therefore, G-CSF–dependent cell proliferation of myeloid precursor cells may be implicated in leukemogenesis.

STAT signalling in the mature and aging brain

Activation of the Janus kinases (JAK) and signal transducers and activator of transcription (STAT) proteins in response to specific cytokines and growth factors has been investigated primarily in cells of non-neuronal origin. More recently, the JAKs and the STATs have also been found to be active in the developing and mature brain, providing evidence for important roles played by these molecules in the control of neuronal proliferation, survival and differentiation. Nothing, however, is known about their occurrence and role(s) in the aged brain. We, therefore, investigated the presence of Stat3 and Stat1 in aged-rat brain, and have found that the Stat3 protein was markedly down regulated with respect to adult tissue, while Stat1 remained invariant. We also investigated the potential role of some growth factors in the activation of the JAK/STAT in mature neurons, exposing primary neuronal cells to ciliary neurotrophic factor (CNTF), basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF). Besides CNTF, which is known to recruit Stat3, we found that Stat3 was also tyrosine phosphorylated by bFGF. These data are indicative of an important role of Stat3 and Stat1 in regulating the physiological status of mature neurons.

Expression of the leptin receptor during germ cell development in the mouse testis

Leptin, a recently identified hormonal product of the ob gene, is known to regulate appetite, body metabolism, and reproductive functions. We investigated the expression of the leptin receptor (Ob-R) in testes from different age groups. The messenger RNA for Ob-R was found in testes from all age groups using RT-PCR. Using immunohistochemistry, we observed age- and stage-dependent distribution of the Ob-R in mouse testis. In testis of 5-day-old mice, its expression was mainly in type A spermatogonia. In the 20- and 30-day-old testis, Ob-R expression was in the spermatocytes; in the adult testis, it was specific to spermatocytes in stages IX and X of the cycle of the seminiferous epithelium. Five main immunoreactive proteins were detected using Western blot (220, 120, 90, 66, and 46 kDa). The 120-kDa protein was evident only in 20-day-old and older testes, whereas the 90-kDa band was present only in the 5- and 10-day-old testis. Leptin treatment induced phosphorylation of signal transducer and activator of transcription-3 in cultured seminiferous tubules from adult and 5-day-old testes. Our results show for the first time age- and stage-specific localization of a functional Ob-R in testicular germ cells. We hypothesize a direct role for leptin, through phosphorylation of signal transducer and activator of transcription-3, in proliferation and differentiation of germ cells, which may partially explain the infertility observed in leptin-deficient mice.

AML1-MTG8 leukemic protein induces the expression of granulocyte colony-stimulating factor (G-CSF) receptor through the up-regulation of CCAAT/enhancer binding protein epsilon

The t(8;21) translocation is one of the most frequent chromosomal abnormalities associated with acute myeloid leukemia (AML). In this translocation, the AML1 (CBFA2/PEBP2aB) gene is disrupted and fused to the MTG8 (ETO) gene. The ectopic expression of the resulting AML1-MTG8 fusion gene product in L-G and 32Dcl3 murine myeloid precursor cells stimulates cell proliferation without inducing morphologic terminal differentiation into mature granulocytes in response to granulocyte-colony stimulating factor (G-CSF). This study found that the ectopic expression of AML1-MTG8 elevates the expression of the G-CSF receptor (G-CSFR). Analysis of the promoter region of the G-CSFR gene revealed that up-regulation of G-CSFR expression by AML1-MTG8 does not depend on the AML1-binding sequence, but on the C/EBP (CCAAT/enhancer binding protein) binding site. The results suggest that the overproduction of G-CSFR is at least partly mediated by C/EBPɛ, whose expression is activated by AML1-MTG8. The ectopic expression of G-CSFR in L-G cells induced cell proliferation in response to G-CSF, but did not inhibit cell differentiation into mature neutrophils. Overexpression of C/EBPɛ in L-G cells also stimulated G-CSF–dependent cell proliferation. High expression levels of G-CSFR were also found in the leukemic cells of AML patients with t(8;21). Therefore, G-CSF–dependent cell proliferation of myeloid precursor cells may be implicated in leukemogenesis.

Leukemia-inhibitory factor-neuroimmune modulator of endocrine function

Leukemia-inhibitory factor (LIF) is a pleiotropic cytokine expressed by multiple tissue types. The LIF receptor shares a common gp130 receptor subunit with the IL-6 cytokine superfamily. LIF signaling is mediated mainly by JAK-STAT (janus-kinase-signal transducer and activator of transcription) pathways and is abrogated by the SOCS (suppressor-of cytokine signaling) and PIAS (protein inhibitors of activated STAT) proteins. In addition to classic hematopoietic and neuronal actions, LIF plays a critical role in several endocrine functions including the utero-placental unit, the hypothalamo-pituitary-adrenal axis, bone cell metabolism, energy homeostasis, and hormonally responsive tumors. This paper reviews recent advances in our understanding of molecular mechanisms regulating LIF expression and action and also provides a systemic overview of LIF-mediated endocrine regulation. Local and systemic LIF serve to integrate multiple developmental and functional cell signals, culminating in maintaining appropriate hormonal and metabolic homeostasis. LIF thus functions as a critical molecular interface between the neuroimmune and endocrine systems.

Receptor recognition by gp130 cytokines

Cytokines of the gp130 family exert their diverse biological effects by formation of stable high affinity transmembrane receptor complexes that are characterized by the presence of the shared transmembrane signalling receptor gp130. Different gp130 ligands form signalling complexes that vary in both composition and stoichiometry. Analysis of the three-dimensional structure of selected ligands and receptor elements indicates that ligands display three topologically conserved receptor recognition epitopes that interact with complementary ligand recognition elements. The composition of the signalling complex and downstream biological responses is defined by the relative affinity of different receptor components for these epitopes. The detailed structure of receptor recognition epitopes indicates that the generation of small molecule cytokine mimetics may be a feasible objective.

Roles of STAT3 defined by tissue-specific gene targeting

The physiological role of each individual STAT protein is now being examined through the study of 'knockout' (KO) mice, harboring a null allele for the particular gene. In contrast to other STATs deficient mice that are born alive, STAT3-deficient mice die during early embryogenesis. However, the role of STAT3 in adult tissues can be assessed by utilizing the Cre-loxP recombination system to ablate the gene in later life. Analyses of tissue-specific STAT3-deficient mice indicate that STAT3 plays a crucial role in a variety of biological functions including cell growth, suppression and induction of apoptosis, and cell motility. Oncogene (2000).

Roles of STAT3 in mediating the cell growth, differentiation and survival signals relayed through the IL-6 family of cytokine receptors

Members of the IL-6 cytokine family are involved in a variety of biological responses, including the immune response, inflammation, hematopoiesis, and oncogenesis by regulating cell growth, survival, and differentiation. These cytokines use gp130 as a common receptor subunit. The binding of ligand to gp130 activates the JAK/STAT signal transduction pathway, where STAT3 plays a central role in transmitting the signals from the membrane to the nucleus. STAT3 is essential for gp130-mediated cell survival and G1 to S cell-cycle-transition signals. Both c-myc and pim have been identified as target genes of STAT3 and together can compensate for STAT3 in cell survival and cell-cycle transition. STAT3 is also required for gp130-mediated maintenance of the pluripotential state of proliferating embryonic stem cells and for the gp130-induced macrophage differentiation of M1 cells. Furthermore, STAT3 regulates cell movement, such as leukocyte, epidermal cell, and keratinocyte migration. STAT3 also appears to regulate B cell differentiation into antibody-forming plasma cells. Since the IL-6/gp130/STAT3 signaling pathway is involved in both B cell growth and differentiation into plasma cells it is likely to play a central role in the generation of plasma cell neoplasias. Oncogene (2000).

Control of myeloid differentiation and survival by Stats

Hematopoiesis involves a complex array of growth factors that regulate the survival and proliferation of immature progenitors, influence differentiation commitment, and modulate end-stage cell functions. This mini-review is focused on the role of Stat activation in the development of myeloid cells in response to hematopoietic cytokines. Much of the evidence implicating Stats in these cellular processes comes from studies of mutant cytokine receptors selectively uncoupled from Stat activation, dominant-inhibitory Stat mutants, and mice with targeted disruptions of Stat genes. Together these approaches provide strong evidence that Stat activation, particularly of Stat3 and Stat5, plays an important role in myeloid differentiation and survival. Oncogene (2000).

Induction of apoptosis by extracellular ubiquitin in human hematopoietic cells: possible involvement of STAT3 degradation by proteasome pathway in interleukin 6-dependent hematopoietic cells

The ubiquitin–proteasome pathway is responsible for selective degradation of short-lived cellular proteins and is critical for the regulation of many cellular processes. We previously showed that ubiquitin (Ub) secreted from hairy cell leukemia cells had inhibitory effects on clonogenic growth of normal hematopoietic progenitor cells. In this study, we examined the effects of exogenous Ub on the growth and survival of a series of human hematopoietic cells, including myeloid cell lines (HL-60 and U937), a B-cell line (Daudi), and T-cell lines (KT-3, MT-4, YTC-3, and MOLT-4). Exogenous Ub inhibited the growth of various hematopoietic cell lines tested, especially of KT-3 and HL-60 cells. The growth-suppressive effects of Ub on KT-3 and HL-60 cells were almost completely abrogated by the proteasome inhibitor PSI or MG132, suggesting the involvement of the proteasome pathway in this process. Furthermore, exogenous Ub evoked severe apoptosis of KT-3 and HL-60 cells through the activation of caspase-3. In interleukin-6 (IL-6)-dependent KT-3 cells, STAT3 was found to be conjugated by exogenous biotinylated Ub and to be degraded in a proteasome-dependent manner, whereas expression levels of STAT1, STAT5, or mitogen-activated protein kinase were not affected. Moreover, IL-6-induced the up-regulation of Bcl-2 and c-myc, and JunB was impaired in Ub-treated KT-3 cells, suggesting that the anti-apoptotic and mitogenic effects of IL-6 were disrupted by Ub. These results suggest that extracellular Ub was incorporated into hematopoietic cells and mediated their growth suppression and apoptosis through proteasome-dependent degradation of selective cellular proteins such as STAT3.

Induction of apoptosis by extracellular ubiquitin in human hematopoietic cells: possible involvement of STAT3 degradation by proteasome pathway in interleukin 6-dependent hematopoietic cells

The ubiquitin–proteasome pathway is responsible for selective degradation of short-lived cellular proteins and is critical for the regulation of many cellular processes. We previously showed that ubiquitin (Ub) secreted from hairy cell leukemia cells had inhibitory effects on clonogenic growth of normal hematopoietic progenitor cells. In this study, we examined the effects of exogenous Ub on the growth and survival of a series of human hematopoietic cells, including myeloid cell lines (HL-60 and U937), a B-cell line (Daudi), and T-cell lines (KT-3, MT-4, YTC-3, and MOLT-4). Exogenous Ub inhibited the growth of various hematopoietic cell lines tested, especially of KT-3 and HL-60 cells. The growth-suppressive effects of Ub on KT-3 and HL-60 cells were almost completely abrogated by the proteasome inhibitor PSI or MG132, suggesting the involvement of the proteasome pathway in this process. Furthermore, exogenous Ub evoked severe apoptosis of KT-3 and HL-60 cells through the activation of caspase-3. In interleukin-6 (IL-6)-dependent KT-3 cells, STAT3 was found to be conjugated by exogenous biotinylated Ub and to be degraded in a proteasome-dependent manner, whereas expression levels of STAT1, STAT5, or mitogen-activated protein kinase were not affected. Moreover, IL-6-induced the up-regulation of Bcl-2 and c-myc, and JunB was impaired in Ub-treated KT-3 cells, suggesting that the anti-apoptotic and mitogenic effects of IL-6 were disrupted by Ub. These results suggest that extracellular Ub was incorporated into hematopoietic cells and mediated their growth suppression and apoptosis through proteasome-dependent degradation of selective cellular proteins such as STAT3.

Quantitative expression of Oct-3/4 defines differentiation, dedifferentiation or self-renewal of ES cells

Cell fate during development is defined by transcription factors that act as molecular switches to activate or repress specific gene expression programmes. The POU transcription factor Oct-3/4 (encoded by Pou5f1) is a candidate regulator in pluripotent and germline cells and is essential for the initial formation of a pluripotent founder cell population in the mammalian embryo. Here we use conditional expression and repression in embryonic stem (ES) cells to determine requirements for Oct-3/4 in the maintenance of developmental potency. Although transcriptional determination has usually been considered as a binary on-off control system, we found that the precise level of Oct-3/4 governs three distinct fates of ES cells. A less than twofold increase in expression causes differentiation into primitive endoderm and mesoderm. In contrast, repression of Oct-3/4 induces loss of pluripotency and dedifferentiation to trophectoderm. Thus a critical amount of Oct-3/4 is required to sustain stem-cell self-renewal, and up- or downregulation induce divergent developmental programmes. Our findings establish a role for Oct-3/4 as a master regulator of pluripotency that controls lineage commitment and illustrate the sophistication of critical transcriptional regulators and the consequent importance of quantitative analyses.

Out of Eden: stem cells and their niches

Stem cells are currently in the news for two reasons: the successful cultivation of human embryonic stem cell lines and reports that adult stem cells can differentiate into developmentally unrelated cell types, such as nerve cells into blood cells. Both intrinsic and extrinsic signals regulate stem cell fate and some of these signals have now been identified. Certain aspects of the stem cell microenvironment, or niche, are conserved between tissues, and this can be exploited in the application of stem cells to tissue replacement therapy.

Signaling mechanisms of cytokine receptors and their perturbances in disease

Cytokines regulate the proliferation and differentiation of cells through their interaction with specific receptors on the surface of target cells which are coupled to intracellular signal transduction pathways. The cytokine receptor class I superfamily, characterized by structural homology in the extracellular domain, includes receptors for many interleukins and hematopoietic growth factors, but also those of growth hormone, leptin, ciliary neurotrophic factor (CNTF), oncostatin M (OSM), leukemia inhibitory factor (LIF) and cardiotrophin-1 (CT-1). The receptors for interferons are structurally distinct and have therefore been categorized separately (class II cytokine receptors). The discovery of the JAK/STAT pathway in the early 1990s has been an important step forward in deciphering cytokine mediated signaling. This pathway connects activation of the receptor complexes directly to transcription of genes. Studies of humans and mice, deficient for one of the JAKs or STATs, have revealed crucial roles of these molecules in embryonic development, blood cell formation and immune responses. In addition, recent studies have revealed some of the mechanisms that control the activation of the JAKs and STATs, which contribute to signal intensity and specificity. In this review we will summarize these recent insights and discuss their implications for a variety of pathological conditions.

Regulated expression of P210 Bcr-Abl during embryonic stem cell differentiation stimulates multipotential progenitor expansion and myeloid cell fate

P210 Bcr-Abl is an activated tyrosine kinase oncogene encoded by the Philadelphia chromosome associated with human chronic myelogenous leukemia (CML). The disease represents a clonal disorder arising in the pluripotent hematopoietic stem cell. During the chronic phase, patients present with a dramatic expansion of myeloid cells and a mild anemia. Retroviral gene transfer and transgenic expression in rodents have demonstrated the ability of Bcr-Abl to induce various types of leukemia. However, study of human CML or rodent models has not determined the direct and immediate effects of Bcr-Abl on hematopoietic cells from those requiring secondary genetic or epigenetic changes selected during the pathogenic process. We utilized tetracycline-regulated expression of Bcr-Abl from a promoter engineered for robust expression in primitive stem cells through multilineage blood cell development in combination with the in vitro differentiation of embryonal stem cells into hematopoietic elements. Our results demonstrate that Bcr-Abl expression alone is sufficient to increase the number of multipotent and myeloid lineage committed progenitors in a dose-dependent manner while suppressing the development of committed erythroid progenitors. These effects are reversible upon extinguishing Bcr-Abl expression. These findings are consistent with Bcr-Abl being the sole genetic change needed for the establishment of the chronic phase of CML and provide a powerful system for the analysis of any genetic change that alters cell growth and lineage choices of the hematopoietic stem cell.

Activity of growth factors in the IL-6 group in the differentiation of human lung adenocarcinoma

The role of the interleukin-6 (IL-6) group of cytokines in differentiation of two lung adenocarcinoma cell lines has been examined using induction of alkaline phosphatase and expression of surfactant protein A. Oncostatin M was the most active and potent for alkaline phosphatase in A549 cells, with IL-6 having similar activity but less potency. Neither cytokine induced alkaline phosphatase in NCI-H441 cells, although induction was obtained with lung fibroblast-conditioned medium. Surfactant protein A was induced in NCI-H441 cells by conditioned medium and dexamethasone and, to a much lesser extent, by oncostatin M or IL-6. Induction of alkaline phosphatase and surfactant protein A were both dexamethasone-dependent, though some induction of surfactant protein A was obtained with interferon-alpha in the absence of dexamethasone. The activity present in lung fibroblast-conditioned medium suggests paracrine control, but this appears not to be due to oncostatin M or IL-6 as disabling antibodies to either cytokine were not inhibitory, and, although alkaline phosphatase was induced in A549 by both cytokines, it was only induced by conditioned medium in NCI-H441 cells. Furthermore, surfactant protein A was induced in H441 by conditioned medium to a much greater extent than by oncostatin M or IL-6. These data demonstrate that cytokines of the IL-6 group have potential as differentiation inducers in lung adenocarcinoma cells and that there is an equivalent paracrine factor(s) in lung fibroblast conditioned medium. As the production of this factor by fibroblasts is not enhanced by glucocorticoid, although the response of the target cell is, it would appear to be distinct from the fibrocyte pneumocyte factor previously described by Post et al 1984.

Interaction of nectin with afadin is necessary for its clustering at cell-cell contact sites but not for its cis dimerization or trans interaction

We have recently found a novel functional unit of cell-cell adhesion at cadherin-based adherens junctions, consisting of at least nectin, a homophilic cell adhesion molecule, and afadin, an actin filament-binding protein, which connects nectin to the actin cytoskeleton. Here we studied a mechanism of cell-cell adhesion of the nectin-afadin system by use of a cadherin-deficient L cell line stably expressing the intact form of mouse nectin-2alpha, a truncated form of nectin-2alpha incapable of interacting with afadin (nectin-2alpha-DeltaC), or a point-mutated form of nectin-2alpha capable of interacting with afadin and a cadherin-expressing EL cell line, which transiently expressed the point-mutated form of nectin-2alpha. We found that the interaction of nectin-2alpha with afadin was necessary for their clustering at cell-cell contact sites. However, nectin-2alpha-DeltaC showed cis dimerization and trans interaction, both of which did not require the interaction of nectin-2alpha with afadin. We have previously shown in EL cells that the interaction of nectin-1 with afadin is necessary for its recruitment to adherens junctions. We found that the trans interaction of nectin-2alpha was furthermore necessary for this recruitment. On the basis of these observations, we propose a model for the mechanism of cell-cell adhesion of nectin and roles of afadin in this mechanism.

Dissection of signaling cascades through gp130 in vivo: reciprocal roles for STAT3- and SHP2-mediated signals in immune responses

We generated a series of knockin mouse lines, in which the cytokine receptor gp130-dependent STAT3 and/or SHP2 signals were disrupted, by replacing the mouse gp130 gene with human gp130 mutant cDNAs. The SHP2 signal-deficient mice (gp130F759/F759 were born normal but displayed splenomegaly and lymphadenopathy and an enhanced acute phase reaction. In contrast, the STAT3 signal-deficient mice (gp130FXQ/FXXQ) died perinatally, like the gp130-deficient mice (gp130D/D). The gp130F759/F759 mice showed prolonged gp130-induced STAT3 activation, indicating a negative regulatory role for SHP2. Th1-type cytokine production and IgG2a and IgG2b production were increased in the gp130F759/F759 mice, while they were decreased in the gp130FXXQ/FXXQ immune system. These results indicate that the balance of positive and negative signals generated through gp130 regulates the immune responses.

The role of SOCS-3 in leptin signaling and leptin resistance

We earlier demonstrated that leptin induces expression of SOCS-3 mRNA in the hypothalamus. Furthermore, transfection data suggest that SOCS-3 is an inhibitor of leptin signaling. However, little is known about the regulation of SOCS-3 expression by leptin and the mechanism by which SOCS-3 inhibits leptin action. We here show that in CHO cells stably expressing the long form of the leptin receptor (CHO-OBRl), leptin induces transient expression of endogenous SOCS-3 mRNA but not of CIS, SOCS-1, or SOCS-2 mRNA. SOCS-3 protein levels were maximal after 2-3 h of leptin treatment and remained elevated at 20 h. Furthermore, in leptin-pretreated CHO-OBRl cells, proximal leptin signaling was blocked for more than 20 h after pretreatment, thus correlating with increased SOCS-3 expression. Leptin pretreatment did not affect cell surface expression of leptin receptors as measured by (125)I-leptin binding assays. In transfected COS cells, forced expression of SOCS-3 results in inhibition of leptin-induced tyrosine phosphorylation of JAK2. Finally, JAK2 co-immunoprecipitates with SOCS-3 in lysates from leptin-treated COS cells. These results suggest that SOCS-3 is a leptin-regulated inhibitor of proximal leptin signaling in vivo. Excessive SOCS-3 activity in leptin-responsive cells is therefore a potential mechanism for leptin resistance, a characteristic feature in human obesity.

Removal of LIF (leukemia inhibitory factor) results in increased vitamin A (retinol) metabolism to 4-oxoretinol in embryonic stem cells

Retinoids, vitamin A (retinol) and its metabolic derivatives, are required for normal vertebrate development. In murine embryonic stem (ES) cells, which remain undifferentiated when cultured in the presence of LIF (leukemia inhibitory factor), little metabolism of exogenously added retinol takes place. After LIF removal, ES cells metabolize exogenously added retinol to 4-hydroxyretinol and 4-oxoretinol and concomitantly differentiate. The conversion of retinol to 4-oxoretinol is a high-capacity reaction because most of the exogenous retinol is metabolized rapidly, even when cells are exposed to physiological ( approximately 1 microM) concentrations of retinol in the medium. No retinoic acid or 4-oxoRA synthesis from retinol was detected in ES cells cultured with or without LIF. The cytochrome P450 enzyme CYP26 (retinoic acid hydroxylase) is responsible for the metabolism of retinol to 4-oxoretinol, and CYP26 mRNA is greatly induced (>15-fold) after LIF removal. Concomitant with the expression of CYP26, differentiating ES cells grown in the absence of LIF activate the expression of the differentiation marker gene FGF-5 whereas the expression of the stem cell marker gene FGF-4 decreases. The strong correlation between the production of polar metabolites of retinol and the differentiation of ES cells upon removal of LIF suggests that one important action of LIF in these cells is to prevent retinol metabolism to biologically active, polar metabolites such as 4-oxoretinol.

Leukemia inhibitory factor and its receptor promote adipocyte differentiation via the mitogen-activated protein kinase cascade

Extracellular factors and intracellular signaling pathways involved in early events of adipocyte differentiation are poorly defined. It is shown herein that expression of leukemia inhibitory factor (LIF) and LIF receptor is developmentally regulated during adipocyte differentiation. Preadipocytes secrete bioactive LIF, and an antagonist of LIF receptor inhibits adipogenesis. Genetically modified embryonic stem (ES) cells combined with culture conditions to commit stem cells into the adipocyte lineage were used to examine the requirement of LIF receptor during in vitro development of adipose cells. The capacity of embryoid bodies derived from lifr(-/-) ES cells to undergo adipocyte differentiation is dramatically reduced. LIF addition stimulates adipocyte differentiation of Ob1771 and 3T3-F442A preadipocytes and that of peroxisome proliferator-activated receptor gamma2 ligand-treated mouse embryonic fibroblasts. Expression of the early adipogenic transcription factors C/EBPbeta and C/EBPdelta is rapidly stimulated following exposure of preadipose cells to LIF. The selective inhibitors of mitogen-activated protein kinase kinase, i.e. PD98059 and U0126, inhibit LIF-induced C/EBP gene expression and prevent adipocyte differentiation induced by LIF. These results are in favor of a model that implicates stimulation of LIF receptor in the commitment of preadipocytes to undergo terminal differentiation by controlling the early expression of C/EBPbeta and C/EBPdelta genes via the mitogen-activated protein kinase cascade.