The carboxyl-terminal domains of gp130-related cytokine receptors are necessary for suppressing embryonic stem cell differentiation. Involvement of STAT3

Regulation of Embryonic Stem Cell Self-Renewal

Embryonic stem cells (ESCs) are a type of cells capable of self-renewal and multi-directional differentiation. The self-renewal of ESCs is regulated by factors including signaling pathway proteins, transcription factors, epigenetic regulators, cytokines, and small molecular compounds. Similarly, non-coding RNAs, small RNAs, and microRNAs (miRNAs) also play an important role in the process. Functionally, the core transcription factors interact with helper transcription factors to activate the expression of genes that contribute to maintaining pluripotency, while suppressing the expression of differentiation-related genes. Additionally, cytokines such as leukemia suppressor factor (LIF) stimulate downstream signaling pathways and promote self-renewal of ESCs. Particularly, LIF binds to its receptor (LIFR/gp130) to trigger the downstream Jak-Stat3 signaling pathway. BMP4 activates the downstream pathway and acts in combination with Jak-Stat3 to promote pluripotency of ESCs in the absence of serum. In addition, activation of the Wnt-FDZ signaling pathway has been observed to facilitate the self-renewal of ESCs. Small molecule modulator proteins of the pathway mentioned above are widely used in in vitro culture of stem cells. Multiple epigenetic regulators are involved in the maintenance of ESCs self-renewal, making the epigenetic status of ESCs a crucial factor in this process. Similarly, non-coding RNAs and cellular energetics have been described to promote the maintenance of the ESC's self-renewal. These factors regulate the self-renewal and differentiation of ESCs by forming signaling networks. This review focused on the role of major transcription factors, signaling pathways, small molecular compounds, epigenetic regulators, non-coding RNAs, and cellular energetics in ESC's self-renewal.

Identification of differentially expressed genes in mouse embryonic stem cell under hypoxia

BACKGROUND

Under hypoxia, mouse embryonic stem cells (mESCs) lose the ability to self-renew and begin to differentiate through down-regulation of LIFR-STAT3 pathway via hypoxia-inducible factor-1α (HIF-1α). However, it remains largely unknown what kinds of factors are involved in hypoxia-induced differentiation of mESCs.

PURPOSE

This study aims to identify the differentially expressed genes (DEGs) in early differentiation of mESCs under hypoxia.

METHODS

Here we utilized a Genefishing technique^TM to discover the new DEGs during hypoxia-induced early differentiation in CCE mESCs. Next, we investigated the role of DEGs using morphological observation, alkaline phosphatase (ALP) assay, STAT3 activation analysis, and biomarkers analysis for stemness.

RESULTS

We detected 19 DEGs under hypoxia and performed cloning with sequencing in six genes. We confirmed the expression patterns of five DEGs including H2afz and GOT1 by realtime PCR assay. Among them, H2afz was significantly decreased under hypoxia, depending on HIF-1α. H2afz-overexpressing CCE mESCs maintained their ALP activity and stem cell markers (Nanog and Rex1), even in hypoxic condition. On the other hand, the early differentiation markers such as FGF5 and STAT5a, which had been increased in hypoxic conditions, were reduced by H2afz overexpression.

CONCLUSION

We discovered that H2afz could be a new target gene that functions in hypoxia-induced differentiation in mESCs and have revealed that it is involved in maintaining the pluripotency of mESCs in the early stages of differentiation. These findings will provide insights into mechanisms of hypoxia-mediated differentiation of mESCs during early development.

GP130 Cytokines in Breast Cancer and Bone

Breast cancer cells have a high predilection for skeletal homing, where they may either induce osteolytic bone destruction or enter a latency period in which they remain quiescent. Breast cancer cells produce and encounter autocrine and paracrine cytokine signals in the bone microenvironment, which can influence their behavior in multiple ways. For example, these signals can promote the survival and dormancy of bone-disseminated cancer cells or stimulate proliferation. The interleukin-6 (IL-6) cytokine family, defined by its use of the glycoprotein 130 (gp130) co-receptor, includes interleukin-11 (IL-11), leukemia inhibitory factor (LIF), oncostatin M (OSM), ciliary neurotrophic factor (CNTF), and cardiotrophin-1 (CT-1), among others. These cytokines are known to have overlapping pleiotropic functions in different cell types and are important for cross-talk between bone-resident cells. IL-6 cytokines have also been implicated in the progression and metastasis of breast, prostate, lung, and cervical cancer, highlighting the importance of these cytokines in the tumor–bone microenvironment. This review will describe the role of these cytokines in skeletal remodeling and cancer progression both within and outside of the bone microenvironment.

STAT3-Inducible Mouse ESCs: A Model to Study the Role of STAT3 in ESC Maintenance and Lineage Differentiation

Studies have demonstrated that STAT3 is essential in maintaining self-renewal of embryonic stem cells (ESCs) and modulates ESC differentiation. However, there is still lack of direct evidence on STAT3 functions in ESCs and embryogenesis because constitutive STAT3 knockout (KO) mouse is embryonic lethal at E6.5-E7.5, prior to potential functional role in early development can be assessed. Therefore, in this study, two inducible STAT3 ESC lines were established, including the STAT3 knockout (InSTAT3 KO) and pSTAT3 overexpressed (InSTAT3 CA) using Tet-on inducible system in which STAT3 expression can be strictly controlled by doxycycline (Dox) stimulation. Through genotyping, deletion of STAT3 alleles was detected in InSTAT3 KO ESCs following 24 hours Dox stimulation. Western blot also showed that pSTAT3 and STAT3 protein levels were significantly reduced in InSTAT3 KO ESCs while dominantly elevated in InSTAT3 CA ECSs upon Dox stimulation. Likewise, it was found that STAT3-null ESCs would affect the differentiation of ESCs into mesoderm and cardiac lineage. Taken together, the findings of this study indicated that InSTAT3 KO and InSTAT3 CA ESCs could provide a new tool to clarify the direct targets of STAT3 and its role in ESC maintenance, which will facilitate the elaboration of the mechanisms whereby STAT3 maintains ESC pluripotency and regulates ESC differentiation during mammalian embryogenesis.

SC1 inhibits the differentiation of F9 embryonic carcinoma cells induced by retinoic acid

The ability to self-renew is one of the most important properties of embryonic stem (ES) cells. Pluripotin (SC1), a small molecule with high activity and low toxicity, promotes self-renewal in mouse ES cells. SC1 can noticeably change the morphology of retinoic acid (RA)-induced F9 embryonic carcinoma cells (F9 cells). However, in the long term, RA and SC1 together cause cell apoptosis. When being added after 18-24 h of RA-induced F9 cell differentiation, SC1 transitorily activated Nanog and Oct4. Both Nanog and Oct4 were downregulated when SC1 and RA were added simultaneously. On the other hand, Klf4 was continually activated when SC1 was added between 6 and 24 h. Phosphorylated Erk1/2 protein levels were reduced from 6 to 24 h, whereas unphosphorylated Erk1 protein levels remained unchanged. A higher concentration of SC1 promoted cell self-renewal by strengthening the inhibition of Erk1/2 protein phosphorylation in F9 cells. Furthermore, SC1 and RA affect global DNA methylation by influencing the expressions of methylation-associated proteins, including Dnmt3b, Dnmt3l, Tet1, Tet2, and Tet3. In conclusion, SC1 inhibits the differentiation of RA-induced F9 cells mainly by reducing the levels of phosphorylated Erk1/2 and enhancing the expression of Klf4, although it also reduces DNA methylation, which may have an additional effect on ES cell differentiation.

Stat3 phosphorylation is required for embryonic stem cells ground state maintenance in 2i culture media

Embryonic stem cells (ES cells) can be maintained its undifferentiated state with feeder cells or LIF, which can activate Jak/Stat3 pathway. Recently, it has been reported a new culture condition comprising serum-free medium with ERK and GSK3β inhibitors (2i) could drive ES cells into a state of pluripotency more like inner cell mass (ICM) in mouse blastocysts called ground state. However, although 2i could sustain ES cells self-renewal, LIF is routinely added. The roles of Stat3 activation are still unclear now. Here we investigated whether Jak/Stat3 might also contribute to the induction of ground state pluripotency. We introduced a lentiviral construct with 7-repeat Stat3-binding sequence to drive Renilla luciferase into ES cells, which can be used as a reporter to detect Stat3 activation by noninvasive bioluminescence imaging. Using this ES cells, we investigated the role of Stat3 activation in ground state maintenance. The results showed that Stat3 could be activated by 2i. Stattic, a chemical inhibitor of Stat3 phosphorylation, could effectively inhibit Stat3 activation in ES cells. When Stat3 activation was suppressed, ground state related genes were down regulated, and ES cells could not be maintained the ground state pluripotency even in 2i medium. All of these results indicate Stat3 activation is required in ground state maintenance.

Soluble ligands and their receptors in human embryo development and implantation

Extensive evidence suggests that soluble ligands and their receptors mediate human preimplantation embryo development and implantation. Progress in this complex area has been ongoing since the 1980s, with an ever-increasing list of candidates. This article specifically reviews evidence of soluble ligands and their receptors in the human preimplantation stage embryo and female reproductive tract. The focus will be on candidates produced by the human preimplantation embryo and those eliciting developmental responses in vitro, as well as endometrial factors related to implantation and receptivity. Pathways to clinical translation, including innovative diagnostics and other technologies, are also highlighted, drawing from this collective evidence toward facilitating joint improvements in embryo quality and endometrial receptivity. This strategy could not only benefit clinical outcomes in reproductive medicine but also provide broader insights into the peri-implantation period of human development to improve fetal and neonatal health.

Galanin is an autocrine myelin and oligodendrocyte trophic signal induced by leukemia inhibitory factor

In order to further investigate the molecular mechanisms that regulate oligodendrocyte (OC) survival, we utilized microarrays to characterize changes in OC gene expression after exposure to the cytokines neurotrophin3, insulin, or leukemia inhibitory factor (LIF) in vitro. We identified and validated the induction and secretion of the neuropeptide galanin in OCs, specifically in response to LIF. We next established that galanin is an OC survival factor and showed that autocrine or paracrine galanin secretion mediates LIF-induced OC survival in vitro. We also revealed that galanin is up-regulated in OCs in the cuprizone model of central demyelination, and that oligodendroglial galanin expression is significantly regulated by endogenous LIF in this context. We also showed that knock-out of galanin reduces OC survival and exacerbates callosal demyelination in the cuprizone model. These findings suggest a potential role for the use of galanin agonists in the treatment of human demyelinating diseases.

Roles of TGF-β family signals in the fate determination of pluripotent stem cells

Members of the transforming growth factor-β (TGF-β) family have been implicated in embryogenesis as well as in the determination of the cell fates of mouse and human embryonic stem (ES) cells, which are characterized by their self-renewal and pluripotency. The cellular responses to TGF-β family signals are divergent depending on the cellular context and local environment. TGF-β family signals play critical roles both in the maintenance of the pluripotent state of ES cells by inducing the expression of Nanog, Oct4, and Sox2, and in their differentiation into various cell types by regulating the expression of master regulatory genes. Moreover, multiple lines of evidence have suggested the importance of TGF-β family signals in establishing induced pluripotent stem (iPS) cells. Since ES and iPS cells have great potential for applications in regenerative medicine, it is critical to figure out the mechanisms underlying their self-renewal, pluripotency, and differentiation. Here, we discuss the roles of TGF-β family ligands and their downstream signaling molecules, Smad proteins, in the maintenance of the pluripotency and lineage specification of mouse and human ES and iPS cells.

Transcription Factor STAT3 as a Novel Molecular Target for Cancer Prevention

Signal Transducers and Activators of Transcription (STATs) are a family of transcription factors that regulate cell proliferation, differentiation, apoptosis, immune and inflammatory responses, and angiogenesis. Cumulative evidence has established that STAT3 has a critical role in the development of multiple cancer types. Because it is constitutively activated during disease progression and metastasis in a variety of cancers, STAT3 has promise as a drug target for cancer therapeutics. Recently, STAT3 was found to have an important role in maintaining cancer stem cells in vitro and in mouse tumor models, suggesting STAT3 is integrally involved in tumor initiation, progression and maintenance. STAT3 has been traditionally considered as nontargetable or undruggable, and the lag in developing effective STAT3 inhibitors contributes to the current lack of FDA-approved STAT3 inhibitors. Recent advances in cancer biology and drug discovery efforts have shed light on targeting STAT3 globally and/or specifically for cancer therapy. In this review, we summarize current literature and discuss the potential importance of STAT3 as a novel target for cancer prevention and of STAT3 inhibitors as effective chemopreventive agents.

Suppression of HIF-1α by Valproic Acid Sustains Self-Renewal of Mouse Embryonic Stem Cells under Hypoxia In Vitro

The developing embryo naturally experiences relatively low oxygen conditions in vivo. Under in vitro hypoxia, mouse embryonic stem cells (mESCs) lose their self-renewal activity and display an early differentiated morphology mediated by the hypoxia-inducible factor-1α (HIF-1α). Previously, we demonstrated that histone deacetylase (HDAC) is activated by hypoxia and increases the protein stability and transcriptional activity of HIF-1α in many human cancer cells. Furthermore HDAC1 and 3 mediate the differentiation of mECSs and hematopoietic stem cells. However, the role of HDACs and their inhibitors in hypoxia-induced early differentiation of mESCs remains largely unknown. Here, we examined the effects of several histone deacetylase inhibitors (HDA-CIs) on the self-renewal properties of mESCs under hypoxia. Inhibition of HDAC under hypoxia effectively decreased the HIF-1α protein levels and substantially improved the expression of the LIF-specific receptor (LIFR) and phosphorylated-STAT3 in mESCs. In particular, valproic acid (VPA), a pan HDACI, showed dramatic changes in HIF-1α protein levels and LIFR protein expression levels compared to other HDACIs, including sodium butyrate (SB), trichostatin A (TSA), and apicidin (AP). Importantly, our RT-PCR data and alkaline phosphatase assays indicate that VPA helps to maintain the self-renewal activity of mESCs under hypoxia. Taken together, these results suggest that VPA may block the early differentiation of mESCs under hypoxia via the destabilization of HIF-1α.

Differential STAT3 signaling in the heart: Impact of concurrent signals and oxidative stress

Multiple lines of evidence suggest that the transcription factor STAT3 is linked to a protective and reparative response in the heart. Thus, increasing duration or intensity of STAT3 activation ought to minimize damage and improve heart function under conditions of stress. Two recent studies using genetic mouse models, however, report findings that appear to refute this proposition. Unfortunately, studies often approach the question of the role of STAT3 in the heart from the perspective that all STAT3 signaling is equivalent, particularly when it comes to signaling by IL-6 type cytokines, which share the gp130 signaling protein. Moreover, STAT3 activation is typically equated with phosphorylation of a critical tyrosine residue. Yet, STAT3 transcriptional behavior is subject to modulation by serine phosphorylation, acetylation, and redox status of the cell. Unphosphorylated STAT3 is implicated in gene induction as well. Thus, how STAT3 is activated and also what other signaling events are occurring at the same time is likely to impact on the outcome ultimately linked to STAT3. Notably STAT3 may serve as a scaffold protein allowing it to interact with other singling pathways. In this context, canonical gp130 cytokine signaling may function to integrate STAT3 signaling with a protective PI3K/AKT signaling network via mutual involvement of JAK tyrosine kinases. Differences in the extent of integration may occur between those cytokines that signal through gp130 homodimers and those through heterodimers of gp130 with a receptor α chain. Signal integration may have importance not only for deciding the particular gene profile linked to STAT3, but for the newly described mitochondrial stabilization role of STAT3 as well. In addition, disruption of integrated gp130-related STAT3 signaling may occur under conditions of oxidative stress, which negatively impacts on JAK catalytic activity. For these reasons, understanding the importance of STAT3 signaling to heart function requires a greater appreciation of the plasticity of this transcription factor in the context in which it is investigated.

Role of STAT3 in Transformation and Drug Resistance in CML

Chronic myeloid leukemia (CML) is initially driven by the bcr-abl fusion oncoprotein. The identification of bcr-abl led to the discovery and rapid translation into the clinic of bcr-abl kinase inhibitors. Although, bcr-abl inhibitors are efficacious, experimental evidence indicates that targeting bcr-abl is not sufficient for elimination of minimal residual disease found within the bone marrow (BM). Experimental evidence indicates that the failure to eliminate the leukemic stem cell contributes to persistent minimal residual disease. Thus curative strategies will likely need to focus on strategies where bcr-abl inhibitors are given in combination with agents that specifically target the leukemic stem cell or the leukemic stem cell niche. One potential target to be exploited is the Janus kinase (JAK)/signal transducers and activators of transcription 3 (STAT3) pathway. Recently using STAT3 conditional knock-out mice it was shown that STAT3 is critical for initiating the disease. Interestingly, in the absence of treatment, STAT3 was not shown to be required for maintenance of the disease, suggesting that STAT3 is required only in the tumor initiating stem cell population (Hoelbl et al., 2010). In the context of the BM microenvironment, STAT3 is activated in a bcr-abl independent manner by the cytokine milieu. Activation of JAK/STAT3 was shown to contribute to cell survival even in the event of complete inhibition of bcr-abl activity within the BM compartment. Taken together, these studies suggest that JAK/STAT3 is an attractive therapeutic target for developing strategies for targeting the JAK-STAT3 pathway in combination with bcr-abl kinase inhibitors and may represent a viable strategy for eliminating or reducing minimal residual disease located in the BM in CML.

PKC-delta inhibitors sustain self-renewal of mouse embryonic stem cells under hypoxia in vitro

Under hypoxia, mouse embryonic stem cells (mESCs) lose their self-renewal activity and display an early differentiated morphology mediated by the hypoxia-inducible factor-1 alpha (HIF-1 alpha). Previous studies have demonstrated that PKC-delta is activated by hypoxia and increases the protein stability and transcriptional activity of HIF-1 alpha in human cancer cells. Furthermore, activation of PKC-delta mediates cardiac differentiation of ESCs and hematopoietic stem cells. However, the role of PKC-delta in hypoxia-induced early differentiation of mESCs remains largely unknown. Here, we show the inhibition of PKC-delta activity prevents the early differentiation of mESCs under hypoxia using PKC-delta inhibitors, GF 109203X and rottlerin. Reduction of PKC-delta activity under hypoxia effectively decreased HIF-1 alpha protein levels and substantially recovered the expression of LIF-specific receptor (LIFR) and phosphorylated-STAT3 in mESCs. Furthermore, PKC-delta inhibitors aid to sustain the expression of self-renewal markers and suppress the expression of early differentiation markers in mESCs under hypoxia. Taken together, these results suggest that PKC-delta inhibitors block the early differentiation of mESCs via destabilization of HIF-1 alpha under hypoxia.

Chemical genetics identifies c-Src as an activator of primitive ectoderm formation in murine embryonic stem cells

Multiple Src family kinases (SFKs) are present in murine embryonic stem (mES) cells. Whereas complete inhibition of SFK activity blocks mES cell differentiation, sole inhibition of the SFK member c-Yes induces differentiation. Thus, individual SFKs may have opposing roles in the regulation of mES cell fate. To test this possibility, we generated SFK mutants with engineered resistance to a nonselective SFK inhibitor. The presence of an inhibitor-resistant c-Src mutant, but not analogous mutants of Hck, Lck, c-Yes, or Fyn, reversed the differentiation block associated with inhibitor treatment, resulting in the formation of cells with properties of primitive ectoderm. These results show that distinct SFK signaling pathways regulate mES cell fate and demonstrate that the formation of primitive ectoderm is regulated by the activity of c-Src.

Cyclin-dependent kinase 2-associating protein 1 commits murine embryonic stem cell differentiation through retinoblastoma protein regulation

Mouse embryonic stem cells (mESCs) maintain pluripotency and indefinite self-renewal through yet to be defined molecular mechanisms. Leukemia inhibitory factor has been utilized to maintain the symmetrical self-renewal and pluripotency of mESCs in culture. It has been suggested that molecules with significant cellular effects on retinoblastoma protein (pRb) or its related pathways should have functional impact on mESC proliferation and differentiation. However, the involvement of pRb in pluripotent differentiation of mESCs has not been extensively elaborated. In this paper, we present novel experimental data indicating that Cdk2ap1 (cyclin-dependent kinase 2-associating protein 1), an inhibitor of G(1)/S transition through down-regulation of CDK2 and an essential gene for early embryonic development, confers competency for mESC differentiation. Targeted disruption of Cdk2ap1 in mESCs resulted in abrogation of leukemia inhibitory factor withdrawal-induced differentiation, along with altered pRb phosphorylation. The differentiation competency of the Cdk2ap1(-/-) mESCs was restored upon the ectopic expression of Cdk2ap1 or a nonphosphorylatable pRb mutant (mouse Ser(788) --> Ala), suggesting that the CDK2AP1-mediated differentiation of mESCs was elicited through the regulation of pRb. Further analysis on mESC maintenance or differentiation-related gene expression supports the phosphorylation at serine 788 in pRb plays a significant role for the CDK2AP1-mediated differentiation of mESCs. These data clearly demonstrate that CDK2AP1 is a competency factor in the proper differentiation of mESCs by modulating the phosphorylation level of pRb. This sheds light on the role of the establishment of the proper somatic cell type cell cycle regulation for mESCs to enter into the differentiation process.

Characterization of embryonic stem cell lines derived from New Zealand white rabbit embryos

The purposes of this study were to examine technical details in deriving and maintaining rabbit embryonic stem (rES) cell lines and to analyze their characteristics. When STO cells were used as feeder cells, no rES cell lines were established using either intact blastocysts or inner cell masses (ICMs). On the mouse embryonic fibroblasts (MEF) feeder, rES cell lines were efficiently (24%) derived. Addition of leukemia inhibitory factor (LIF) to the cells cultured on the MEF feeders further increased the derivation efficiency (57%) of rES cells. The fact that LIF induced serine-phosphorylation of STAT3 suggested LIF-dependent maintenance of rES cells. Most of the rES cell lines expressed AP, SSEA-4, Oct4, TRA-1-60, and TRA-1-81. Western blot or RT-PCR analysis also confirmed the expression of Oct4, Nanog, and Sox2. When induced to form EBs in vitro or injected to the severe combined immunodeficiency (SCID) mice, the rES cells generated embryoid bodies (EBs) and teratomas with three germ layers expressing the marker genes including MAP2, Desmin, and GATA4, respectively. In conclusion, rabbit ES cell lines can be efficiently established using our current protocols with LIF supplement. These ES cells express pluripotent stem cell markers and retain their capability to differentiate into different tissue cells. Furthermore, rES cells depend on LIF for self-renewal, likely via the JAK-STAT pathway.

Imaging of STAT3 signaling pathway during mouse embryonic stem cell differentiation

Signal transducers and activators of transcription 3 (STAT3) is a pleiotropic transcription factor involved in a variety of physiological processes. STAT3 acts as a key transcriptional determinant of mouse embryonic stem (ES) cell self-renewal and plays a pivotal function in early mammalian embryogenesis because the development of many organs requires STAT3 activation. However, little is known about the role of STAT3 function during ES cell differentiation. To answer this question, we built a lentiviral construct with 7-repeat STAT3-binding sequence (enhancer) and minimal TA (promoter) driving renilla luciferase and monomeric red fluorescence protein (Rluc-mRFP), followed by a constitutive cytomegalovirus promoter driving green fluorescent protein as a selection marker. The specificity of our custom-designed 7-repeat STAT3 reporter construct was first confirmed by cotransfection with constitutively active version of STAT3 (STAT3C) into human embryonic kidney 293T cells. Next, a mouse ES cell line stably transduced with STAT3 reporter construct was isolated. This ES cell line showed a tight response in reporter gene expression with leukemia inhibitory factor (LIF) induction and was chosen as a developmental model for the STAT3 functional study. Using serial noninvasive bioluminescence imaging, we showed that the onset of embryoid body (EB) formation involved inhibition of STAT3 activity. However, during differentiation, STAT3 activity steadily increased from day 5 to 14 and was reduced by day 21. STAT3 activity was also confirmed separately by Western blots. Finally, phosphorylation of STAT3 was also found to correspond with cardiomyocyte differentiation. In summary, this is the first study to monitor real-time STAT3 activity during ES cell differentiation. This genetically modified line can be used to study the biological role of STAT3 during ES cell differentiation into different derivatives.

Tyrosine kinase signalling in embryonic stem cells

Pluripotent ES (embryonic stem) cells can be expanded in culture and induced to differentiate into a wide range of cell types. Self-renewal of ES cells involves proliferation with concomitant suppression of differentiation. Some critical and conserved pathways regulating self-renewal in both human and mouse ES cells have been identified, but there is also evidence suggesting significant species differences. Cytoplasmic and receptor tyrosine kinases play important roles in proliferation, survival, self-renewal and differentiation in stem, progenitor and adult cells. The present review focuses on the role of tyrosine kinase signalling for maintenance of the undifferentiated state, proliferation, survival and early differentiation of ES cells.

Mechanisms that mediate stem cell self-renewal and differentiation

Stem cells have two common properties: the capacity for self-renewal and the potential to differentiate into one or more specialized cell types. In general, stem cells can be divided into two broad categories: adult (somatic) stem cells and embryonic stem cells. Recent evidence suggested that tumors may contain "cancer stem cells" with indefinite potential for self-renewal. In this review, we will focus on the molecular mechanisms regulating embryonic stem cell self-renewal and differentiation, and discuss how these mechanisms may be relevant in cancer cells.

Involvement of tyrosine kinase signaling in maintaining murine embryonic stem cell functionality

OBJECTIVE

We previously demonstrated that c-kit expression decreases during murine embryonic stem cell (ESC) differentiation induced by leukemia inhibitory factor removal. In this study, we addressed the possibility that c-kit is a marker of undifferentiated murine ESC and, moreover, that it plays a role in maintaining the undifferentiated state of these cells.

MATERIALS AND METHODS

c-kit expression was analyzed under various differentiation conditions by flow cytometry and quantitative reverse transcription polymerase chain reaction. ESC were then sorted on the basis of c-kit expression and functionality was investigated using embryoid body and colony-forming cell assays. Imatinib (Gleevec) and ACK2 were used to block, and stem cell factor was used to stimulate, c-kit activity.

RESULTS

c-kit expression decreased in two murine ESC lines under various differentiation conditions. Sorting of ESC populations on the basis of c-kit expression revealed significant differences in the functional capacities and gene expression profiles of the sorted populations. The inhibition studies revealed an important role for tyrosine kinase activity in maintaining ESC viability and differentiation capacity, at least in part by preventing apoptosis and enhancing cell cycle progression. However, activation of c-kit alone is not sufficient for maintaining undifferentiated ESC.

CONCLUSION

The results suggest that c-kit may represent a useful marker for monitoring ESC functionality. Moreover, tyrosine kinase signaling plays an important role in maintaining undifferentiated ESC. This work provides valuable insights into the complex signaling pathways that synergize to maintain the undifferentiated state of murine ESC.

A novel role for Fyn: Change in sphere formation ability in murine embryonic stem cells

Fyn, a member of the Src-family protein tyrosine kinase (PTK), is an essential factor in myelination in the CNS and is involved in murine embryonic stem (ES) cell growth and differentiation. Although dysfunctions of Fyn have been comparatively studied, the gain of function by ectopic expression, especially using ES cells, has seldom been investigated. In this article, we give the first report of the involvement of Fyn alteration in the sphere formation ability of murine ES cells. First, transient transfection of Fyn hardly affected multiplication and specialization. Then, we investigated Fyn overexpression using ES cells, which stably express Fyn. As a result, altered sphere formation capability was observed in all clones stably expressing Fyn. These results may provide important information for reproduction medical treatment using ES cells.

Identification of Jmjd1a as a STAT3 downstream gene in mES cells

Mouse embryonic stem (mES) cells can be maintained in undifferentiated state in the presence of a cytokine, leukemia inhibitory factor (LIF). Many investigators found that STAT3 activation is important for the maintenance of pluripotency by LIF. However, the downstream pathways of STAT3 activation are still unknown. To look for STAT3-downstream target genes, we performed DD-RT PCR in the presence or absence of LIF. Through further confirmation, we finally selected 8 genes whose expressions were significantly dependent upon the presence of LIF. Among them, Jmjd1a was down-regulated after LIF withdrawal, and it was selected for further investigation. Its expression started to decrease 1 day after the removal of LIF, and disappeared on day 3. It was also shown that STAT3 could bind to the promoter region of Jmjd1a gene. These data demonstrate that Jmjd1a might be a critical signaling molecule underlying the maintenance of pluripotency in mES cells.

Immunogenicity and Engraftment of Mouse Embryonic Stem Cells in Allogeneic Recipients

Embryonic stem cells (ESCs) are pluripotent and therefore able to differentiate both in vitro and in vivo into specialized tissues under appropriate conditions, a property that could be exploited for cellular therapies. However, the immunological nature of these cells in vivo has not been well understood. In vitro, mouse-derived ESCs fail to stimulate T cells, but they abrogate ongoing alloresponses by a process that requires cell-cell contact. We further show that despite a high expression of the NKG2D ligand retinoic acid early inducible-1 by mouse ESCs, they remain resistant to natural killer cell lysis. In vivo, allogeneic mouse ESCs populate the thymus, spleen, and liver of sublethally irradiated allogeneic host mice, inducing apoptosis to T cells and establishing multilineage mixed chimerism that significantly inhibits alloresponses to donor major histocompatibility complex antigens. Immunohistochemical imaging revealed a significant percentage of ESC-derived cells in the splenic marginal zones, but not in the follicles. Taken together, the data presented here reveal that nondifferentiated mouse embryonic stem cells are non-immunogenic and appear to populate lymphoid tissues in vivo, leading to T-cell deletion by apoptosis.

Stem-cell-based approaches for regenerative medicine

Recent success in transplantation of islets raises the hopes of diabetic patients that replacement therapies may be a feasible treatment of their disease. Although several lines of evidence suggest that stem cells exist in the pancreas, it is still technically hard for us to isolate or maintain the stem cells in vitro. The establishment of human embryonic stem (ES) cells has excited scientists regarding their potential medical use in tissue replacement therapy. When applied with appropriate signals, ES cells can be directed to differentiate into a specific cell lineage. Therefore, ES cells are no doubt an excellent source not only for regenerative medicine but also for studies of early events of pancreatic development, and to portray the pancreatic progenitor cells. Despite many attempts that have been tried, the efficiency of differentiation of ES cells into islets is still very low. This low efficiency reflects our lack of understanding of the intrinsic and extrinsic signals which regulate the developmental processes of the pancreas. In this review, I present a summary of recent works on ES cells, the identification of pancreatic progenitor cells from the adult pancreas, and refer to the possibilities of transdifferentiation from adult stem cells derived from other tissues.

Live long and prosper

In this issue of Molecular Pharmacology, Meyn et al. (p. 1320) provide the results of the first comprehensive investigation of the expression pattern of the Src family of nonreceptor tyrosine kinases (SFK) in mouse embryonic stem (ES) cells. They found that self-renewing ES cells express seven of the eight mammalian SFK members and that some undergo distinct expression changes during early differentiation events. One of the most dramatic changes was in Hck transcript levels, which decreased almost 30-fold during the first 3 days of embryoid body formation, a culture system model of early embryogenesis and differentiation. Other SFKs, such as Fyn and Src, remain present and active as ES cells differentiate. Of particular interest was the observation that ES cell self-renewal or differentiation can be manipulated through the selective pharmacological inhibition of SFK members. This information should help in the expanding efforts to exploit ES cells for basic and clinical purposes.

Src Family Kinase Activity Is Required for Murine Embryonic Stem Cell Growth and Differentiation

Self-renewal and differentiation of embryonic stem (ES) cells are regulated by cytokines and growth factors through tyrosine kinase-dependent signaling pathways. In murine ES cells, signals for self-renewal are generated by the leukemia inhibitory factor (LIF). LIF and other growth factors are linked to the activation of the Src family of cytoplasmic protein-tyrosine kinases (SFKs), which consists of eight members having shared structural architecture. In this article, we show that murine ES cells express seven SFKs, three of which (Hck, Src, and Fyn) exhibit constitutive activity in self-renewing ES cells. Differentiation of ES cells to embryoid bodies was associated with rapid transcriptional silencing of Hck and Lck and with the loss of the corresponding kinase proteins. The expression of other family members remained relatively constant, although some loss of Fgr and Lyn proteins was observed during differentiation. Like ES cells, embryoid bodies maintained constitutive Src and Fyn kinase activity. Partial inhibition of endogenous SFK activity with the ATP-competitive inhibitors 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine or Src kinase inhibitor-1 induced differentiation of ES cells in the presence of LIF. In contrast, suppression of all SFK activity with higher concentrations of these inhibitors, or with the more potent compound A-419259 (Bioorg Med Chem Lett 12:1683-1686, 2002) blocked differentiation in response to LIF withdrawal. It is surprising that these inhibitor-treated cells remained pluripotent despite the absence of LIF. Our results implicate individual members of the Src kinase family in distinct ES cell renewal and differentiation pathways and show that small-molecule SFK inhibitors can control ES cell fate.

Lineage allocation and cell polarity during mouse embryogenesis

The first developmental lineage allocation during the generation of the mouse blastocyst is to outer trophoblast or to inner pluriblast (inner cell mass; ICM) cells. This allocation seems to be initiated at the 8-cell stage, when blastomeres polarise. Polarisation is followed by differentiative divisions at the subsequent two cleavage divisions to generate polar outer and non-polar inner 16- and 32-cells. The key events in polarisation are regulated post-translationally through a cell contact-mediated pathway, which imposes a heritable determinant-like organisation on the blastomere cortex. Two proteins in particular, E-cadherin and ezrin, are intimately involved in the generation and stabilisation of developmentally significant information. Transcriptional differences between lineages appear to follow and may coincide with the lineage commitment of cells.

A GTPase-activating protein binds STAT3 and is required for IL-6-induced STAT3 activation and for differentiation of a leukemic cell line

We previously identified a guanosine triphosphatase (GTPase)-activating protein (GAP) male germ cell Rac GAP (MgcRacGAP) that enhanced interleukin-6 (IL-6)-induced macrophage differentiation of murine M1 leukemia cells. Later, MgcRacGAP was found to play crucial roles in cell division. However, how MgcRacGAP enhanced IL-6-induced differentiation remained elusive. Here we show that MgcRacGAP enhances IL-6-induced differentiation through enhancement of signal transducer and activator of transcription-3 (STAT3) activation. MgcRacGAP, Rac, and STAT3 formed a complex in IL-6-stimulated M1 cells, where MgcRacGAP interacted with Rac1 and STAT3 through its cysteine-rich domain and GAP domain. In reporter assays, the wild-type MgcRacGAP enhanced transcriptional activation of STAT3 while a GAP-domain deletion mutant (DeltaGAP) did not significantly enhance it, suggesting that the GAP domain was required for enhancement of STAT3-dependent transcription. Intriguingly, M1 cells expressing DeltaGAP had no effect on the differentiation signal of IL-6, while forced expression of MgcRacGAP rendered M1 cells hyperresponsive to the IL-6-induced differentiation. Moreover, knockdown of MgcRacGAP by RNA interference profoundly suppressed STAT3 activation, implicating MgcRacGAP in the STAT3-dependent transcription. All together, our data not only reveal an important role for MgcRacGAP in STAT3 activation, but also demonstrate that MgcRacGAP regulates IL-6-induced cellular differentiation in which STAT3 plays a pivotal role.

Stem cells with multilineage potential derived from porcine skin

Stem cells from farm animals are valuable cell models for the study of development, differentiation, and are potential efficient donors for nuclear transfer. Here we report the isolation and characterization of stem cells from porcine skin. These porcine skin-originated sphere (PSOS) cells expressed the neural progenitor marker, nestin, as well as genes that are critical for pluripotency such as Oct4 and Stat3. The PSOS cells proliferated actively in vitro and retained normal karyotype after long-term culture. When cultured in defined medium, they generated cells with characteristics of neurons and astrocytes. A subpopulation of cells differentiated into adipocyte-like cells when cultured in 10% fetal bovine serum. Clonal study demonstrated that PSOS exhibited clonal-generating capability. Clonal populations from individual stem cells could form neuron-, astrocyte-, and adipocyte-like cells upon inducted differentiation. Our findings represent the first report of skin-originated stem cells isolated from non-rodent animals.

Imbalanced gp130-dependent signaling in macrophages alters macrophage colony-stimulating factor responsiveness via regulation of c-fms expression

The mechanisms by which interleukin-6 (IL-6) family cytokines, which utilize the common receptor signaling subunit gp130, influence monocyte/macrophage development remain unclear. Here we have utilized macrophages devoid of either gp130-dependent STAT1/3 (gp130(Delta STAT/Delta STAT)) or extracellular signal-regulated kinases 1 and 2 (ERK1/2) mitogen-activated protein (MAP) kinase (gp130(Y757F/Y757F)) activation to assess the individual contribution of each pathway to macrophage formation. While the inhibition by IL-6 of macrophage colony-stimulating factor (M-CSF)-induced colony formation observed in gp130(wt/wt) mice was abolished in gp130(Delta STAT/Delta STAT) mice, inhibition of macrophage colony formation was enhanced in gp130(Y757F/Y757F) mice. In gp130(Delta STAT/Delta STAT) bone marrow-derived macrophages (BMMs), both IL-6- and M-CSF-induced ERK1/2 tyrosine phosphorylation was enhanced. By contrast, tyrosine phosphorylation of ERK1/2 in response to M-CSF was reduced in gp130(Y757F/Y757F) BMMs, and the pattern of ERK1/2 activation in gp130 mutant BMMs correlated with their opposing responsiveness to M-CSF-induced proliferation. When compared to the level of expression in gp130(wt/wt) BMMs, c-fms expression was elevated in gp130(Delta STAT/Delta STAT) BMMs but reduced in gp130(Y757F/Y757F) BMMs. Finally, an ERK1/2 inhibitor suppressed M-CSF-induced BMM proliferation, and this result corresponded to a reduction in c-fms expression. Collectively, these results provide a functional and causal correlation between gp130-dependent ERK MAP kinase signaling and c-fms gene activation, a finding that provides a potential mechanism underlying the inhibition of M-CSF-dependent macrophage development by IL-6 family cytokines in mice.

Radiosensitization by inhibition of IkappaB-alpha phosphorylation in human glioma cells

To assess the role of nuclear factor kappaB (NFKB) in cellular radiosensitivity, three different IkappaB-alpha (also known as NFKBIA) expression plasmids, i.e., S-IkappaB (mutations at (32, 36)Ser), Y-IkappaB (a mutation at (42)Tyr), and SY-IkappaB, were constructed and introduced into human brain tumor M054 cells. The clones were named as M054-S8, M054-Y2 and M054-SY4, respectively. Compared to the parental cell line, M054-S8 and M054-Y2 cells were more sensitive to X rays while M054-SY4 cells exhibited the greatest sensitivity. After treatment with N-acetyl-Leu-Leu-norleucinal, a proteasome inhibitor, the X-ray sensitivity of M054-S8 and M054-SY4 cells did not change, while that of M054-Y2 cells and the parental cells was enhanced. An increase in X-ray sensitivity accompanied by a decrease in translocation of NFKB to the nucleus in parental cells was observed after treatment with pervanadate, an inhibitor of tyrosine phosphatase, as well as in M054-S8 and M054-SY4 cells. Repair of potentially lethal damage (PLD) was observed in the parental cells but not in the clones. Four hours after irradiation (8 Gy), the expression of TP53 and phospho-p53 ((15)Ser) was induced in the parental cells but not in M054-S8, M054-Y2 or M054-SY4 cells. Our data suggest that inhibition of IkappaB-alpha phosphorylation at serine or tyrosine acts independently in sensitizing cells to X rays. NFKB may play a role in determining radiosensitivity and PLD repair in malignant glioma cells; TP53 may also be involved.

A dual, non-redundant, role for LIF as a regulator of development and STAT3-mediated cell death in mammary gland

STAT3 is the key mediator of apoptosis in mammary gland. We demonstrate here that LIF is the physiological activator of STAT3, because in involuting mammary glands of Lif(-/-) mice, pSTAT3 is absent and the STAT3 target, C/EBPdelta, is not upregulated. Similar to Stat3 knockouts, Lif(-/-) mammary glands exhibit delayed involution, reduced apoptosis and elevated levels of p53. Significantly, Lif(-/-) glands display precocious development during pregnancy, when pSTAT3 is not normally detected. We show that pERK1/2 is significantly reduced in Lif(-/-) glands at this time, suggesting that at this stage LIF mediates its effects through pERK1/2. Inhibition of LIF-mediated ERK1/2 phosphorylation potentiates the proapoptotic effects of STAT3. LIF therefore signals alternately through ERK1/2, then STAT3, to regulate mammary growth and apoptosis.

Src family kinase-independent signal transduction and gene induction by leukemia inhibitory factor

Members of the interleukin-6 (IL-6) family of cytokines exert their biological effects via binding to their cognate ligand-binding receptor subunit on a target cell. The subsequent recruitment of the common signal transducer glycoprotein 130 and activation of the JAK/STAT and SHP-2/Ras/mitogen-activated protein kinase (MAPK) pathways are responsible for the majority of cellular responses elicited by IL-6 cytokines. Several types of experiments suggest that the Src family of kinases (SFK) also participates in IL-6 family cytokine-mediated signaling events. SYF cells, which lack expression of SFKs Src, Yes, and Fyn, were used to determine the role of SFKs in IL-6 family cytokine signaling and gene induction. SYF and wild type (WT) control fibroblasts displayed similar activation of signaling intermediates following stimulation with leukemia inhibitory factor (LIF). LIF-stimulated tyrosine phosphorylation of SHP-2 and subsequent activation of MAPK in SYF cells were identical to that seen in LIF-stimulated WT cells. Both LIF-stimulated tyrosine phosphorylation of STAT1 and STAT3, as well as LIF-stimulated DNA binding activity of STAT-containing nuclear complexes were indistinguishable when compared in SYF and WT cells. In addition, the phosphatidylinositol 3-kinase-sensitive Akt kinase and p38 MAPK were activated by LIF in both SYF and WT cells. Furthermore, LIF-stimulated expression of c-fos, egr-1, and suppressor of cytokine signaling-3 was retained in SYF cells. The IL-6 family cytokine oncostatin M was also capable of activating MAPK, STAT3, STAT1, Akt, and p38 in both WT and SYF cells. These results demonstrate that IL-6 family cytokines can activate a full repertoire of signaling pathways and induce gene expression independent of SFKs.

Constitutive and specific activation of STAT3 by BCR-ABL in embryonic stem cells

BCR-ABL oncogene, the molecular hallmark of chronic myelogenous leukemia (CML) arises in a primitive hematopoietic stem cell with both differentiation and self-renewal ability. To study the phenotypic effects of BCR-ABL in a clonal in vitro self-renewal and differentiation model, we have introduced BCR-ABL in the ES cell line CCE. The major effect of BCR-ABL expression was the persistence of primitive morphology of ES cells despite LIF deprivation, correlated with a constitutive activation of STAT3, the major self-renewal factor of ES cells, but no evidence of activation of STAT5. The enforced expression of BCR-ABL in an ES cell line, engineered to express a tetracycline-inducible dominant-negative form of a STAT3, triggered ES cell differentiation with an increased generation of hematopoietic cells expressing erythroid and megakaryocytic phenotypes. RT-PCR analysis for Oct4, Brachyury and beta-globin expression confirmed a delay of differentiation in BCR-ABL expressing clones, which could be entirely reversed upon activation of the dominant-negative form of STAT3. To study the possible relevance of STAT3 activation by BCR-ABL in human CML, Western blot analyses performed on the CD34+ cells, purified from CML patients at different stages of their disease, also demonstrated increased levels of STAT3 proteins phosphorylated both on tyrosine and serine residues. These results represent to our knowledge the first functional link between BCR-ABL oncogene and a self-renewal in the context of ES cells through constitutive activation of STAT3. Thus, the BCR-ABL embryonic stem cell model that we developed as well as the results obtained in human CML samples suggests a role for STAT3 in the pathogenesis of human CML.

Signal transducer and activator of transcription proteins in leukemias

Signal transducer and activator of transcription (STAT) proteins are a 7-member family of cytoplasmic transcription factors that contribute to signal transduction by cytokines, hormones, and growth factors. STAT proteins control fundamental cellular processes, including survival, proliferation, and differentiation. Given the critical roles of STAT proteins, it was hypothesized that inappropriate or aberrant activation of STATs might contribute to cellular transformation and, in particular, leukemogenesis. Constitutive activation of mutated STAT3 has in fact been demonstrated to result in transformation. STAT activation has been extensively studied in leukemias, and mechanisms of STAT activation and the potential role of STAT signaling in leukemogenesis are the focus of this review. A better understanding of mechanisms of dysregulation of STAT signaling pathways may serve as a basis for designing novel therapeutic strategies that target these pathways in leukemia cells.

Stemness, fusion and renewal of hematopoietic and embryonic stem cells

Development of replacement cell therapies awaits the identification of factors that regulate nuclear reprogramming and the mechanisms that control stem cell renewal and differentiation. Once such factors and signals will begin to be elucidated, new technologies will have to be envisaged where uniform differentiation of adult or embryonic stem cells along one differentiation pathway can be induced. Controlled differentiation of stem cells will require the engineering of niches and extracellular signal combinations that would amplify a particular signaling network and allow uniform and selective differentiation. Three recent advances in stem cell research open the possibility to approach engineering studies for cell replacement therapies. Fusion events between stem cells and adult cells or between adult and embryonic stem cells have been shown to result in altered fates and nuclear reprogramming of cell hybrids. Hematopoietic stem cells were shown to require Wnt signaling in order to renew. The purification of Wnt proteins would allow their use as exogenous purified cytokines in attempts to amplify stem cells before bone marrow transplantation. The homeodomain protein Nanog has been shown to be crucial for the embryonic stem cell renewal and pluripotency. However, the cardinal question of how stemness is preserved in the early embryo and adult stem cells remains opened.

Differential expression of protein tyrosine kinase genes during microglial activation

Protein tyrosine kinase (PTK) activity is abundant in microglia, but the PTKs that participate in their activation have not been identified. For these studies, we used three paradigms to characterize PTK expression during microglial activation: resting and activated microglia were bulk fractionated from the adult brain, cultured newborn microglia were treated with lipopolysaccharide (LPS) to model the transition from activated toward phagocytic microglia, and PTK expression was examined in activated microglia in situ after facial nerve axotomy. Two PCR-based strategies were used to show that 21 different PTK genes are expressed by rat brain microglia: 5 receptor PTKs, 10 nonreceptor PTKs, and 6 members of the src family. Seven of the 21 PTKs were examined in greater detail. Five PTK mRNAs (fgr, hck, fak, jak-2, and flk-1) increased expression across all three models of activation. We conclude that they represent key components in the cascades that participate in microglial activation. In contrast, expression of fes and fms correlated with stimuli that affect microglial proliferation. Four of the PTKs (hck, fgr, fes, and fms) are believed to be myeloid cell specific and were not expressed by cultured astrocytes. HCK and FAK protein were also not expressed in lysates of immature astrocytes and oligodendrocytes. Because of their putative specificity, these kinases represent potential targets for inhibitors of microglial activation. Because reactive microglia can exacerbate the severity of neurological diseases, the identification of specific kinases that participate in microglial activation represents an important advance toward the development of new therapeutics.

Reciprocal regulation of gastrointestinal homeostasis by SHP2 and STAT-mediated trefoil gene activation in gp130 mutant mice

The intracellular signaling mechanisms that specify tissue-specific responses to the interleukin-6 (IL-6) family of cytokines are not well understood. Here, we evaluated the functions of the two major signaling pathways, the signal transducers and activators of transcription 1 and 3 (STAT1/3) and the Src-homology tyrosine phosphatase 2 (SHP2)-Ras-ERK, emanating from the common signal transducer, gp130, in the gastrointestinal tract. Gp130(757F) mice, with a 'knock-in' mutation abrogating SHP2-Ras-ERK signaling, developed gastric adenomas by three months of age. In contrast, mice harboring the reciprocal mutation ablating STAT1/3 signaling (gp130(Delta STAT)), or deficient in IL-6-mediated gp130 signaling (IL-6(-/-) mice), showed impaired colonic mucosal wound healing. These gastrointestinal phenotypes are highly similar to the phenotypes exhibited by mice deficient in trefoil factor 1 (pS2/TFF1) and intestinal trefoil factor (ITF)/TFF3, respectively, and corresponded closely with the capacity of the two pathways to stimulate transcription of the genes encoding pS2/TFF1 and ITF/TFF3. We propose a model whereby mucosal wound healing depends solely on activation of STAT1/3, whereas gastric hyperplasia ensues when the coordinated activation of the STAT1/3 and SHP2-Ras-ERK pathways is disrupted.

Signalling, cell cycle and pluripotency in embryonic stem cells

Pluripotent mouse embryonic stem (ES) cells can be expanded in large numbers in vitro owing to a process of symmetrical self-renewal. Self-renewal entails proliferation with a concomitant suppression of differentiation. Here we describe how the cytokine leukaemia inhibitory factor (LIF) sustains self-renewal through activation of the transcription factor STAT3, and how two other signals - extracellular-signal-related kinase (ERK) and phosphatidylinositol-3-OH kinase (PI3K) - can influence differentiation and propagation, respectively. We relate these observations to the unusual cell-cycle properties of ES cells and speculate on the role of the cell cycle in maintaining pluripotency.

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.

Signaling through a novel domain of gp130 mediates cell proliferation and activation of Hck and Erk kinases

Interleukin-6 (IL-6) induces the activation of the Src family kinase Hck, which is associated with the IL-6 receptor beta-chain, gp130. Here we describe the identification of an "acidic" domain comprising amino acids 771 to 811 of gp130 as a binding region for Hck, which mediates proliferative signaling. The deletion of this region of gp130 (i.e., in deletion mutant d771-811) resulted in a significant reduction of Hck kinase activity and cell proliferation upon stimulation of gp130 compared to wild-type gp130. In addition, d771-811 disrupted the growth factor-stimulated activation of Erk and the dephosphorylation of Pyk2. Based on these findings, we propose a novel, acidic domain of gp130, which is responsible for the activation of Hck, Erk, and Pyk2 and signals cell proliferation upon growth factor stimulation.

Mouse ES cells: experimental exploitation of pluripotent differentiation potential

Pluripotent ES cells can be used to generate a wide variety of cell populations in vitro in a manner resembling embryonic development. Recent advances in controlling ES cell differentiation, combined with the power of genetic and biochemical manipulation, are providing insights into cell biology and the determination of cell fate.

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.