Ubiquitin-mediated regulation of JAK-STAT signaling in embryonic stem cells

Abstract

LIF activates several intracellular signaling pathways including JAK-STAT, PI3K/AKT and MAPK pathways. LIF is an important cytokine for maintenance of pluripotency and self-renewal of mouse ES cells. The JAK-STAT signal plays a key role in maintenance of the pluripotency of ESCs. Recent evidence shows that several post-translational modifications regulate activation or inhibition of intracellular signal transductions. The JAK-STAT signal is also modulated by several modifications including phosphorylation, acetylation and ubiquitination. In this review, we discuss regulation of the LIF-mediated-JAK-STAT signaling pathway that contributes to self-renewal of pluripotent ESCs.

Keywords: ESC; Hsp90; STAT; TRIM; ubiquitin.

Figure 1. Regulation of the LIF-mediated signaling pathway in mouse ESCs. LIF causes heterodimerization of LIFR and gp130. LIF causes phosphorylation and activation of JAK, which is tethered to the intracellular region of gp130. Activated JAK phosphorylates gp130 and its tyrosine phosphorylations causes interaction with STAT3. Then STAT3 is also tyrosine-phosphorylated by JAK and is homodimerized via its SH2 domains. The dimerized STAT3 translocates to the nucleus and activates target gene transcription. JAK-STAT3 activates Klf4, which maintains Oct3/4 expression via Nanog. JAK activates PI3K, which acts as an activator for AKT. AKT then upregulates Tbx3 as another pluripotency gene. The JAK-STAT pathway is negatively regulated by several inhibitory systems: dephosphorylation by tyrosine phosphatases including SHP and physical inhibition or ubiquitin-mediated degradation of JAK by SOCS. JAK phosphorylates SHP and then SHP interacts with the Grb2/SOS complex to activate the MAPK pathway, leading to inhibition of Tbx3 and Nanog. TRIM8 likely negatively regulates Hsp90β-mediated translocation of STAT3 into the nucleus of ESCs. SLIM is an E3 ubiquitin ligase that negatively regulates the JAK-STAT signal pathway, but SLIM is expressed at a low level in mouse ESCs. P, phosphorylation.

Figure 2. Factors required for maintaining self-renewal and pluripotency of mouse and human ESCs. LIF is a key cytokine for maintaining self-renewal and pluripotency of mouse ESCs. In contrast to mouse ESCs, fibroblast growth factor 2 (FGF2), also called basic FGF (bFGF), and activin A are used to maintain self-renewal in human ESCs. Human ESCs are likely to be slightly differentiated cells in the “primed state” and mouse ESCs are fully undifferentiated cells in the “naïve state.”

Figure 3. Maintenance of self-renewal and pluripotency of mouse ESCs by chemical inhibitors. FGF4 autocrined by ESCs likely causes activation of SHP2-Ras-MAPK. Activation of the SHP2-Ras-MAPK cascade including MEK induces differentiation of ESCs. Moreover, activation of GSK3β causes phosphorylation of c-Myc followed by degradation of c-Myc and also inhibits Nanog expression, resulting in the differentiation of ESCs. LIF-mediated AKT activation causes phosphorylation of GSK3β, followed by inactivation of GSK3β (A). Without LIF, mouse ESCs are differentiated through unphosphorylated GSK3β and MAPK activated by autocrine FGF4 (B). The combination (2i) of a MEK inhibitor (PD184352/PD325901) and GSK3β inhibitor (CHIR99021) is sufficient for maintaining self-renewal and pluripotency of mouse ESCs without LIF (C).