**6. Functional role of the JAK/STAT pathway in stem cells**

#### **6.1. Stat3 maintains naïve pluripotency in mouse embryonic stem cells**

ESC pluripotency is regulated by transcriptional networks that maintain self-renewal and inhibit differentiation [172-174]. Stat3 and Myc are necessary to maintain mouse ESC (mESC) self-renewal andbindtomanyESC-enrichedgenes [175].Theirtargetgenes includepluripoten‐ cy-related transcription factors, polycomb group repressive proteins, and histone modifiers [176, 177]. The transcription factor Stat3 is a key pluripotency factor required for ESC selfrenewal [178, 179]. Mouse ESC require LIF-Stat3 (leukemia inhibitory factor) and Bmp4 (bone morphogenic protein 4) to remain pluripotent in *in vitro* cultures, whereas human ESC require FGF2/MAPK (fibroblast growth factor / mitogen-activated protein kinase) and TGFβ/Activin/ Nodal (transforming growth factor β) [180-183]. Nevertheless, the core circuitry of pluripoten‐ cy is conserved among species and includes OCT4, SOX2 and NANOG [174].

activated in a cell contact-dependent way, which is distinct from its cytokine activation. Cocultures of MSC (human mesenchymal stem cells) and APC (antigen-presenting cell) increase STAT3 signaling in both cell types in a cell contact-dependent way, which mediates the immune-modulatory effects of MSC to block APC maturation and induce T-cell tolerance [203]. MSC are high-proliferative non-hematopoietic stem cells with the ability to differentiate into multiple mesenchymal lineages [204-206]. They accumulate in tumor environments in response to NFκB signaling and produce cytokines [207]. MSCs are FDA-approved for the treatment of severe acute GVHD, due to their immunomodulatory properties [208]. STAT3 phosphorylation is induced by cell-cell contacts and inhibited in postconfluent cells that consequently become apoptotic. Therefore, STAT3 may represent a molecular junction that allows cell proliferation or growth arrest depending on the state of the cell. Increased STAT3

The Role of an NFκB-STAT3 Signaling Axis in Regulating the Induction and Maintenance of the Pluripotent State

http://dx.doi.org/10.5772/57602

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Constitutive STAT3 activation can by itself result in cellular transformation [210-214]. For example, contact-dependent STAT3 activation is known to play a promoting role in the interactions between tumor cells and their environment [215-218]. Cell transformation and the induction of pluripotency may share very similar signaling processes, and it is possible that STAT3 may represent a common axis [219, 220]. During early tumor development, certain cells have to acquire stem cell-like features that allow them to self-renew (tumor-initiating cells) and to produce cell progeny (tumor bulk) [221-224]. These tumor-initiating cells are very difficult to eradicate during chemotherapies and often re-establish the tumor seen as clinical relapse [225-227]. Tumor-initiating cells display strong inflammatory gene signatures with elevated IL6-STAT3-NFκB signaling to sustain their self-renewal [228-231]. A better under‐ standing of the mechanism by which STAT3 and NFκB regulates the acquisition of pluripo‐ tency and self-renewal might also give us crucial insight about tumor development, and may

Activation of Stat3 is a limiting factor for the induction of pluripotency, and its over-expression eliminates the requirement for additional factors to establish pluripotency [233]. These key properties have positioned Stat3 signaling as one of the master reprogramming factors that dominantly instructs naïve pluripotency [175]. Elevated Stat3 activity overcomes the pre-iPSC reprogramming block and enhances the establishment of pluripotency induced by SOKM [234]. Stat3 and Klf4 co-occupy genomic sites of *Oct4, Sox2* and *Nanog*. Klf4 and c-Myc are downstream targets of Stat3 signaling and part of the transcriptional network governing pluripotency. The Stat3 effect is combinatorial with other reprogramming factors, which

activity may promote cell survival during cell confluency [209].

lead to future novel therapies [171, 232].

**7.1. STAT3 is a master reprogramming factor**

**6.4. Cell contact-dependent STAT3 signaling during cell transformation**

**7. The role of STAT3 signaling during reprogramming**

implies that additional targets of Stat3 play a pivotal role [235].

### **6.2. The LIF-IL6-STAT3 circuitry**

LIF belongs to the IL-6 family of cytokines and acts in parallel through the Jak/Stat3 and PI3K/ Akt (Phosphatidylinositide 3-kinase) pathways to maintain *Oct4, Sox2* and *Nanog* expression viaKruppel-like factor 4 (Klf4) andT-box factor 3 [184, 185].Lif andIL-6 arenecessaryfor STAT3 phosphorylation mediated by Jak1 [186]. Stat3 phosphorylation positively regulates *Klf4* and *Nanog* transcripts andfacilitatesLif-dependent maintenance ofpluripotency in a signaling loop [106].Stat3directlybinds togenomic sitesof*Oct4*and*Nanog*,regulates theOct4-Nanogcircuitry and is necessary to maintain the self-renewal and pluripotency of mESC [187-189]. Overexpres‐ sion of *Stat3* maintains mESC self-renewal even in the absence of Lif [190]. Withdrawal of LIF up-regulates the NFκB pathway and results in ESC differentiation as well as STAT3 disrup‐ tion [191-193]. The interleukin 6 (IL-6)response element (IRE) is activated by STAT3, vice versa IL-6 stimulation leads to STAT3 phosphorylation and transactivation of IRE- containing promoters providing a positively regulated STAT3-IL6 loop. STAT3 directly associates with c-Jun and c-Fos in response to IL-6 [194]. c-Jun and c-Fos are DNA binding proteins and compo‐ nentsoftheAP-1 (activationprotein-1)transcriptionfactor complex [195].AP-1 canbe activated by TLR2/4, IL-10 or STAT3 to regulate inflammatory responses or drive keratinocyte differen‐ tiation in interplay with STAT3 and c-MYC [196]. Tlr2 also plays an important role in the maintenance of mESC [107]. STAT3 is important to tune appropriate amounts of AP-1 pro‐ teins requiredforproperdifferentiation.DNAbinding sites for both*AP-1* and*STAT3*have been found in many gene promoters [194, 197]. It is important to note that c-Jun is able to capture or release the NuRD (nucleosome remodeling and deacetylation) repressor complex, an impor‐ tant epigenetic modulator of gene silencing [198, 199]. STAT3 is able to bind to bivalent histone modifications enabling a quick switchbetweenthe activationofpluripotency genesduringESC maintenance and their inhibition during cell differentiation [193].

#### **6.3. STAT3 signaling in immune cells**

STAT3 also has complex functions during hematopoietic development, immune regulation, cell growth, and leukemic transformation [200-202]. It is critically important for the survival and differentiation of lymphocytes and myeloid progenitors [171]. STAT3 signaling can be activated in a cell contact-dependent way, which is distinct from its cytokine activation. Cocultures of MSC (human mesenchymal stem cells) and APC (antigen-presenting cell) increase STAT3 signaling in both cell types in a cell contact-dependent way, which mediates the immune-modulatory effects of MSC to block APC maturation and induce T-cell tolerance [203]. MSC are high-proliferative non-hematopoietic stem cells with the ability to differentiate into multiple mesenchymal lineages [204-206]. They accumulate in tumor environments in response to NFκB signaling and produce cytokines [207]. MSCs are FDA-approved for the treatment of severe acute GVHD, due to their immunomodulatory properties [208]. STAT3 phosphorylation is induced by cell-cell contacts and inhibited in postconfluent cells that consequently become apoptotic. Therefore, STAT3 may represent a molecular junction that allows cell proliferation or growth arrest depending on the state of the cell. Increased STAT3 activity may promote cell survival during cell confluency [209].

#### **6.4. Cell contact-dependent STAT3 signaling during cell transformation**

Constitutive STAT3 activation can by itself result in cellular transformation [210-214]. For example, contact-dependent STAT3 activation is known to play a promoting role in the interactions between tumor cells and their environment [215-218]. Cell transformation and the induction of pluripotency may share very similar signaling processes, and it is possible that STAT3 may represent a common axis [219, 220]. During early tumor development, certain cells have to acquire stem cell-like features that allow them to self-renew (tumor-initiating cells) and to produce cell progeny (tumor bulk) [221-224]. These tumor-initiating cells are very difficult to eradicate during chemotherapies and often re-establish the tumor seen as clinical relapse [225-227]. Tumor-initiating cells display strong inflammatory gene signatures with elevated IL6-STAT3-NFκB signaling to sustain their self-renewal [228-231]. A better under‐ standing of the mechanism by which STAT3 and NFκB regulates the acquisition of pluripo‐ tency and self-renewal might also give us crucial insight about tumor development, and may lead to future novel therapies [171, 232].
