Biology Reference
In-Depth Information
(SHIP) [92]. EPOR-associated IRS-2 may provide an alternative mechanism
for activation of PI3-kinase in response to EPO in UT7 cells. This mechanism
of PI3-kinase activation may be used in a subset of EPO-dependent cells
expressing IRS-2 [92]. Alternatively in K562 cells, PI3-kinase is activated by
binding of its P85 sub-unit to EPOR phosphorylated by Src tyrosine kinase. In
these cells, Src also associates directly with P85 sub-unit of PI3-kinase and
may enhance its activation [93].
Activation of AKT pathway
More recent work has clearly demonstrated activation of the AKT pathway in
a PI3-kinase-dependent manner in response to EPO [90, 94]. Upon EPO stim-
ulation, AKT serine threonine kinase is phosphorylated and activated in both
primary human [91] and cultured erythroid cells [90, 94]. The activation of the
PI3-kinase/AKT pathway has been shown to over-ride the DNA damage-
induced cell cycle arrest of hematopoietic cells [95].
EPO also induces phosphorylation of the pro-apoptotic FKHRL-1 tran-
scription factor, a known downstream target of AKT, and induces its cytoplas-
mic relocalization [90, and Ghaffari,Kitidis, and Lodish, unpublished data].
Whether phosphorylation of FKHRL-1 in response to EPO is through AKT or
other PI3-kinase-dependent kinases is not clear. In addition, the role of AKT
in erythroid development is unknown.
PI3-kinase/AKT knock-outs
Although PI3-kinase/AKT activation seems to be important for erythroid
development, mice lacking AKT1 or AKT2 do not exhibit any obvious ery-
throid phenotype [96-98], possibly due to functional redundancy of AKT iso-
forms. Mice lacking either AKT3 alone or all AKT isoforms have not been
generated and hematopoietic development in AKT1 and AKT2 knock-outs
have not been closely examined. The status of red cell development in PI3-
kinase
-/-
mice is also unknown.
Ras/MAPK pathways
Signaling
In mammalian cells, Ras activates three groups of MAPK, including ERK,
JNK, or SAPK, and p38 [99]. In various EPO-dependent cell lines, different
groups of MAPK are activated in response to EPO [100-104]. Activation of
the Ras/ERK pathway may be through the canonical Grb2 binding site at
(P)Y464 of EPOR [105], as cell proliferation promoted by this pathway in
response to EPO is inhibited by Raf-1 antisense oligonucleotides [106, 107].
Synergestic activation of ERK1/2 by EPO and SCF may be important for pro-
liferation of human erythroid progenitors [108]. Alternatively, the ERK path-
way may be activated independent of Ras through the PI3-K pathway [86].
JNK and p38 are implicated in induction of erythroid differentiation of EPO-
