Biology Reference
In-Depth Information
present in the cytoplasm of unstimulated cells, and directly link activated
cytokine receptors to gene transcription. The STAT are recuited to the recep-
tor complexes through SH2 domain recognition of specific phosphorylated
tyrosine residues on the receptors (or JAK-2 or other proteins), are phospho-
rylated by JAK, undergo homodimerization or heterodimerization and translo-
cate to the nucleus where they drive the transcription of a wide range of genes.
Stimulation of EPOR specifically activates STAT5 [54, 65, 66]. EPOR
mutants that contain only one tyrosine, either Y343 (F7Y343) or Y401
(F7Y401), activate STAT5 to the same extent as the wild-type receptor [54,
67]. These mutant receptors can support differentiation of only 50% and 30%
of the normal numbers of fetal liver CFU-E [68], respectively,suggesting that
STAT5 activation is not sufficient to support normal erythroid differentiation.
STAT5 is activated at least to some extent (approximately 10%) after stimula-
tion of a mutant EPOR lacking all tyrosine residues (EPOR F8), indicating that
there are multiple pathways leading to EPOR activation of STAT5, and that
JAK-2 tyrosine kinase may directly activate STAT5 through STAT5 directly
binding to phosphotyrosines on JAK-2. Interestingly, the constitutively active
EPOR (R129C) mutant does not induce constitutive STAT5 activation in BaF
3
cells, indicating that signaling by EPOR and its constitutively active mutant
may be distinct [69].
STAT activity as a result of ligand stimulation is transient and is rapidly
downregulated by several mechanisms including ubiquitin-mediated degrada-
tion of STAT5 [70], competition by induced cytokine-inducible SH2-contain-
ing molecule for the STAT binding site on the cytokine receptor [71], or direct
binding of STAT5 to the negative regulator protein inhibitor of activated STAT
[72, 73]. A role for protein inhibitor of activated STAT in down-regulation of
STAT5 activity in particular has not been reported.
Known STAT5 target genes include tissue-specific genes and genes that reg-
ulate general cell growth. Overexpression studies using dominant negative and
constitutively active forms of STAT5 in erythroid cell lines and cultured fetal
liver cells have demonstrated an anti-apoptotic role for STAT5. In a number of
erythroid and hematopoietic cells, STAT5 induces the immediate early gene
expression of the anti-apoptotic Bcl2 family member gene
Bcl
X
l
,by directly
binding to STAT5 consensus binding site in the Bcl
XL
promoter [74-77]. A
role for STAT5 in both proliferation and differentiation of erythroid cells has
been suggested; however, strict correlation of STAT5 activation with EPOR
proliferative or differentiative potential appears to depend on the cell context
[54, 66, 68].
In addition to STAT5, STAT1 and STAT3 are activated in response to recep-
tor stimulation of EPO-dependent cells [78, 79]. Activation of STAT1 and
STAT3 may negatively regulate EPO-induced differentiation of the human
EPO-dependent UT7/EPO cells [79].
