Biology Reference
In-Depth Information
forms a homodimer [47, 48]. Biophysical studies suggest that EPO binds to
one of the two receptors with high affinity (approximately 1 nM) and to the
other with low affinity (approximately 1 µM) [49]. Dimerization of EPOR can
also be achieved from other molecules (different form and size) as long as two
recognition motifs are provided in each molecule. Such a molecule can be a
dimeric EPO, containing only one binding motif on each monomer (het-
erodimerization), a mutant EPOR which is cross-linked by a disulfide bound,
a bivalent antibody against the extracellular domain of EPOR, or an EPO
mimetic peptide [48, 50-55].
EPO mimetic peptides are a family of EPOR agonists which contain 9-22
amino acids. Although they do not share sequence homology with EPO, they
mimic its erythropoietic effects
in vivo
and
in vitro
[54, 56]. X-ray crystallog-
raphy gives evidence of their specific binding to EPOR and its dimerization.
The extracellular domain folds into a L-shape with the long axis and aligned
at approximately 90° to the other axis [55]. The three-dimensional structure of
the complex of two extracellular domains of EPOR bound to EPO shows that
the receptor orientation by the ligand is critical for the efficiency of signaling
[37, 55]. It has also been shown that a major fraction of the full-length
(murine) EPOR exists as a preformed dimer which is not constitutively active
and requires EPO-binding to induce signaling.
Once EPOR is dimerized, the intracellular signaling pathway becomes acti-
vated by a rapid tyrosine phosphorylation of numerous proteins that associate
with EPOR. EPOR itself possesses no endogenous tyrosine kinase activity [57,
58]. In this process, Janus kinase 2 (JAK-2) is the primary kinase responsible
for the phosphorylation of EPOR (Fig. 2). JAK-2 is associated with EPOR
before ligand binding,and rapidly activates it after the binding of the ligand
and the dimerization of EPOR [58]. The EPO signaling pathways, which are
induced following EPOR activation, include the JAK/STAT pathway,
Ras/MAP (Ras protein/mitogen-activated protein) kinase, phosphatidylinosi-
tol-3 kinase (PI-3 kinase), and protein kinase C (PKC) pathways. After signal-
ing is initiated, the receptor-ligand complex is subsequently internalized and
degraded. The signaling is terminated by dephosphorylation of EPOR and
JAK-2. In this process, negative regulatory loop mechanisms are included
(Fig. 1) [59]. The EPOR signaling pathways are described in detail by Ghaffari
et al. in Chapter 5.
Other factors binding to EPOR
Other members of the cytokine receptor super-family interact with EPOR,
which may be relevant for its tissue-specific activation and function: In ery-
throid progenitor cells, EPOR is phosphorylated by the tyrosine kinase activi-
ty of the activated receptor for stem cell factor (
c-kit
).
C-kit
physically inter-
acts with the extended box 2 region in the cytoplasmic domain of EPOR [13,
17, 60]. Moreover, the
-chain of the IL-3 receptor functionally interacts with
β
