Biomedical Engineering Reference
In-Depth Information
trophectoderm accompanied by upregulation of Cdx2 and Eomes. However,
overexpression of Oct4 promotes hypoblast differentiation from ESCs (Niwa
et al., 2000). Additionally, Nanog antagonizes Gata6 to prevent differentiation
toward mature hypoblast (Chazaud et al., 2006). Thus, the state of ESCs is
under tight regulation by multiple factors acting either together or in antagon-
ism. In this chapter we address how pluripotency of ESCs is maintained.
2 Extrinsic Factors
Mitotically inactivated MEFs sustain the ESC state. Numerous studies have
sought to identify factors secreted from MEFs. One of the critical factors for
mESCs is LIF (Leukemia inhibitory factor), a member of the IL-6 (interleukin-6)
cytokine family. LIF, first described as an inducer of differentiation of M1
myeloid leukemia cells, supports survival and proliferation of mESCs without
inducing differentiation (Smith et al., 1988; Williams et al., 1988). mESCs express
the LIF receptor (LIFR), which consists of the LIF-specific receptor subunit
LIFRb and the common signal transducer gp130 (glycoprotein-130) (Yoshida
et al., 1994). Binding of LIF to LIFR activates JAK (Janus-associate tyrosine
kinase) which then phosphorylates gp130, establishing a docking site for proteins
bearing SH2 (Src homology 2) domains, including the STAT (signal transducer
and activator of transcription) family of transcription factors. STAT proteins are
a family of transcription factors that are normally inactive within the cytoplasm
and accumulate in the nucleus upon growth factor stimulation. Several reports
indicate that Stat3 is a crucial downstream transcription factor of the LIF/gp130
pathway. Similar to other STAT family proteins, STAT3 activation is transient
in normal cells. However, STAT3 is persistently activated in many tumors as a
consequence of aberrant growth factor or tyrosine kinase signaling. In mESCs, the
binding of LIF to LIFR triggers STAT3 phosphorylation by JAK. Thereafter,
phosphorylated STAT3 forms homodimers that translocate to the nucleus to
promote gene activation. Experiments validate STAT3's intimate involvement
in LIF signaling. Forced expression of dominant-negative STAT3 causes sponta-
neous differentiation of ES cells, even in the presence of LIF (Niwa et al., 1998).
ESCs expressing a fusion molecule consisting of STAT3 and the estrogen receptor
ligand-binding domain can be maintained in a pluripotent state in the presence
of estrogen without LIF (Matsuda et al., 1999). The basic helix-loop-helix tran-
scription factor c-myc, an established accelerator of the cell cycle, appears to lie
downstream of activated STAT3. By ChIP (chromatin immunoprecipitation)
analysis, STAT3 has been detected at the promoter of c-myc in mESCs. LIF
withdrawal is associated with decreased DNA-binding activity of STAT3 and
concomitant downregulation of c-myc expression (Cartwright et al., 2005). Over-
expression of a dominant-active form of c-myc is sufficient to maintain self-
renewal of ESCs independent of LIF and bypasses the dominant-negative effect
of STAT3. Expression of a dominant-negative form of c-myc antagonizes
