Introduction Embryonic pluripotent cells can give rise to

Introduction Embryonic pluripotent cells can give rise to differentiated progeny representing all three embryonic germ layers (Pera et al., 2000). Thus, embryonic stem (ES) cells are special primal structures in the body that retain two distinctive properties: the ability to self-renew through mitotic cell division to remain in an undifferentiated state and the ability to differentiate into a specific cell type (Patel and Yang, 2010). Stem cell factors, including Oct3/4, Sox2, Klf4, Nanog, c-Myc and Lin28, are crucial transcription factors known to regulate ES cells\' self-renewal activity and/or to generate induced pluripotent stem (iPS) cells (Takahashi et al., 2007; Takahashi and Yamanaka, 2006; Yu et al., 2007). Nanog occupies a central position in the transcriptional network of pluripotency (Boyer et al., 2005; Cole et al., 2008; Loh et al., 2006) and plays an essential role in early embryonic development (Mitsui et al., 2003). It is expressed in pluripotent embryonic cells, derivative ES cells, and the developing germ line of mammals and ciprofloxacin (Chambers et al., 2003; Lavial et al., 2007; Mitsui et al., 2003; Yamaguchi et al., 2005). Mouse Nanog-deficient inner cell mass (ICM) fails to generate epiblasts and only produces parietal endoderm-like cells (Mitsui et al., 2003). Conversely, ectopic expression of Nanog is sufficient to drive leukemia inhibitory factor (LIF; a key cytokine for maintenance of ES cell pluripotency)-independent self-renewal of undifferentiated ES cells (Chambers et al., 2003). Furthermore, 4 factors including Oct3/4, Sox2, Nanog and Lin28, are sufficient to reprogram human somatic cells to pluripotent stem cells that exhibit identical and essential characteristics of ES cells (Yu et al., 2007). Thus, Nanog plays a central role in ES cell self-renewal in developmental and de-differentiation processes. The maintenance of self-renewal and pluripotency in ES cells is controlled by extrinsic signaling pathways, intrinsic self-renewal factors and epigenetic modifications (Boiani and Scholer, 2005; Stewart et al., 2006). LIF, a member of the IL-6 cytokine family (Smith et al., 1988) provided by mitotic-inactivated feeder cells, stimulates ES cells through gp130-JAK-STAT3 signaling (Niwa et al., 1998), resulting in prevention of ES cell differentiation in the presence of serum (Matsuda et al., 1999). Besides the activation of STAT3, LIF also activates mitogen-activated protein (MAP) kinases, whose activation promotes differentiation of ES cells in mice (Burdon et al., 1999). Suppression of ERKs (extracellular-signal regulated kinases) signaling can promote ES cell self-renewal in mice (Burdon et al., 1999). Thus, mouse ES (mES) cells might maintain their self-renewal activity in the presence of LIF due to the balance of STAT3 activation and ERKs. Also, growth factors, such as LIF and bFGF, activate the phosphoinositide 3-kinase (PI3-K)/Akt signaling pathway (Jirmanova et al., 2002) by inhibiting glutathione synthase kinase 3 β (GSK3β), p53 and Nanog (Takahashi et al., 2003), as well as suppressing apoptosis in ES cells (Gross et al., 2005). In contrast to LIF-STAT3 signaling, activation of the Wnt pathway by 6-bromoindirubin-3-oxime (BIO), a specific inhibitor of GSK3, maintains the undifferentiated phenotype in ES cells and sustains expression of ES cell specific markers (Sato et al., 2004) through a synergistic effect with LIF-STAT3 signaling (Hao et al., 2006). Transforming growth factor beta (TGFβ)/activin/nodal signaling plays an important role in ES cell self-renewal. Gene expression analysis indicates that TGFβ and its correlate factors, including Nodal Cripto Lefty1 and Lefty 2, are expressed at high levels in undifferentiated human ES cells (Sato et al., 2003). The TGFβ/activin/nodal pathway is activated through the Smad2/3 transcription factors in undifferentiated cells and Smad2/3 activation suppresses Smad1/5 activity, suggesting that TGFβ/activin/nodal signaling can negatively regulate bone morphogenetic protein 4 (BMP4) in human ES cells (Beattie et al., 2005; James et al., 2005). However, the mechanisms are not understood.