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In-Depth Information
JAK-2-dependent but receptor phosphotyrosine-independent pathways in
EPOR signaling. For instance, JAK-2 may directly phosphorylate STAT5 inde-
pendently from STAT5-binding to EPOR, since EPOR (F8) where all EPOR
cytosolic tyrosines are mutated to phenylalanine, can still support 10% of the
STAT5 activity that is normally generated by EPO stimulation of wild-type
EPOR [54].
Lyn tyrosine kinase
Three protein tyrosine kinases (Lyn, Syk, and Tec) have been reported to asso-
ciate with activated EPOR complexes [55-57]. Among them, Lyn tyrosine
kinase has been studied most extensively. In a mutant erythroleukemic J2E cell
line that fails to differentiate in response to EPO, mRNA and protein expres-
sions of Lyn are reduced and wild-type Lyn rescues the mutant defect [58]. In
32D cells, Lyn associates with EPOR at (P)Y464 and/or (P)Y479 and phos-
phorylates both EPOR and STAT5 [55]. Additional signaling proteins have
been identified that bind Lyn in response to EPO, including SHP-1, SHP-2,
ras-GAP, CrkL, Raf-1, MAPK, and HS1 [59-62]. A dominant-negative Lyn
mutant that inhibits the association of Lyn to most of these signaling proteins
also blocks erythroid differentiation of cultured cells [60]. In normal erythrob-
lasts, EPO enhances phosphorylation of Lyn; moreover, overexpression of Lyn
increases the number of CFU-E-derived colonies but has no effect on BFU-E
colonies [60]. Although Lyn has been shown to phosphorylate EPOR in the
presence of EPO, it is considered as a secondary kinase in EPOR signaling
[60]. Consistent with this notion, Lyn
-/-
mice, in marked contrast to EPO
-/-
,
EPOR
-/-
, and JAK-2
-/-
mice, do not die with embryonic anemia and do not
exhibit major erythroid defects [1, 43, 44, 63]. Detailed analyses of Lyn-defi-
cient mice are required to determine the precise role of Lyn in EPOR signal-
ing
in vivo
.
Phosphorylation of EPOR and activation of downstream signaling
pathways
Binding of EPO to its receptor activates many signaling pathways. All these
pathways are activated by other cytokines and to date, the specificity of these
pathways in terms of cellular outcome is not clear. In particular, we do not
know if EPOR activates different pathways to different extents in CFU-E, ery-
throblasts, and neuronal cells.
EPOR and STAT5
STAT proteins
STAT proteins are identified as major players in cytokine signaling [64]. They
were first identified as interferon-responsive transcription factors. STATs are
