3 research outputs found
Stem cell factor induces phosphatidylinositol 3'-kinase-dependent Lyn/Tec/Dok-1 complex formation in hematopoietic cells
Stem cell factor (SCF) has an important role in the proliferation,
differentiation, survival, and migration of hematopoietic cells. SCF
exerts its effects by binding to cKit, a receptor with intrinsic tyrosine
kinase activity. Activation of phosphatidylinositol 3'-kinase (PI3-K) by
cKit was previously shown to contribute to many SCF-induced cellular
responses. Therefore, PI3-K-dependent signaling pathways activated by SCF
were investigated. The PI3-K-dependent activation and phosphorylation of
the tyrosine kinase Tec and the adapter molecule p62Dok-1 are reported.
The study shows that Tec and Dok-1 form a stable complex with Lyn and 2
unidentified phosphoproteins of 56 and 140 kd. Both the Tec homology and
the SH2 domain of Tec were identified as being required for the
interaction with Dok-1, whereas 2 domains in Dok-1 appeared to mediate the
association with Tec. In addition, Tec and Lyn were shown to phosphorylate
Dok-1, whereas phosphorylated Dok-1 was demonstrated to bind to the SH2
domains of several signaling molecules activated by SCF, including Abl,
CrkL, SHIP, and PLCgamma-1, but not those of Vav and Shc. These findings
suggest that p62Dok-1 may function as an important scaffold molecule in
cKit-mediated signaling
Tyrosine kinase receptor RON functions downstream of the erythropoietin
Erythropoietin (EPO) is required for cell survival during differentiation
and for progenitor expansion during stress erythropoiesis. Although
signaling pathways may couple directly to docking sites on the EPO
receptor (EpoR), additional docking molecules expand the signaling
platform of the receptor. We studied the roles of the docking molecules
Grb2-associated binder-1 (Gab1) and Gab2 in EPO-induced signal
transduction and erythropoiesis. Inhibitors of phosphatidylinositide
3-kinase and Src kinases suppressed EPO-dependent phosphorylation of Gab2.
In contrast, Gab1 activation depends on recruitment and phosphorylation by
the tyrosine kinase receptor RON, with which it is constitutively
associated. RON activation induces the phosphorylation of Gab1,
mitogen-activated protein kinase (MAPK), and protein kinase B (PKB) but
not of signal transducer and activator of transcription 5 (Stat5). RON
activation was sufficient to replace EPO in progenitor expansion but not
in differentiation. In conclusion, we elucidated a novel mechanism
specifically involved in the expansion of erythroblasts involving RON as a
downstream target of the Epo
Protein kinase C alpha controls erythropoietin receptor signaling.
Protein kinase C (PKC) is implied in the activation of multiple targets of
erythropoietin (Epo) signaling, but its exact role in Epo receptor (EpoR)
signal transduction and in the regulation of erythroid proliferation and
differentiation remained elusive. We analyzed the effect of PKC inhibitors
with distinct modes of action on EpoR signaling in primary human
erythroblasts and in a recently established murine erythroid cell line.
Active PKC appeared essential for Epo-induced phosphorylation of the Epo
receptor itself, STAT5, Gab1, Erk1/2, AKT, and other downstream targets.
Under the same conditions, stem cell factor-induced signal transduction
was not impaired. LY294002, a specific inhibitor of phosphoinositol
3-kinase, also suppressed Epo-induced signal transduction, which could be
partially relieved by activators of PKC. PKC inhibitors or LY294002 did
not affect membrane expression of the EpoR, the association of JAK2 with
the EpoR, or the in vitro kinase activity of JAK2. The data suggest that
PKC controls EpoR signaling instead of being a downstream effector. PKC
and phosphoinositol 3-kinase may act in concert to regulate association of
the EpoR complex such that it is responsive to ligand stimulation. Reduced
PKC-activity inhibited Epo-dependent differentiation, although it did not
effect Epo-dependent "renewal divisions" induced in the presence of Epo,
stem cell factor, and dexamethasone