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 Previous Article | Table of Contents | Next Article 
Blood, Vol. 95 No. 1 (January 1), 2000:
pp. 19-29
REVIEW ARTICLE
The Jak-Stat pathway in normal and perturbed hematopoiesis
Alister C. Ward,
Ivo Touw, and
Akihiko Yoshimura
From the Institute of Hematology, Erasmus University, Rotterdam, The
Netherlands; and the Institute of Life Science, Kurume University,
Japan.
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Introduction |
The Janus kinase-signal transducer and
activator of transcription (Jak-Stat) pathway stands as a paradigm of
how diverse extracellular signals can elicit rapid changes in gene
expression in specific target cells. This pathway is widely used by
members of the cytokine receptor superfamily, including those for the
clinically important cytokines granulocyte colony-stimulating factor
(G-CSF), erythropoietin, thrombopoietin, the interferons, and numerous
interleukins, which makes it central to hematopoietic cell biology and
hematologic therapy alike. Indeed, study of the Jak-Stat pathway has
provided a wealth of information on hematopoiesis and hematopoietic
disease, and conversely, studies of hematopoietic disorders have
yielded new insights into the functions of Jaks and Stats. This review aims to detail the role of the Jak-Stat pathway in the normal development and function of hematopoietic cells and to describe how several hematopoietic disorders are caused, at least in part, by perturbations of this pathway.
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Jaks |
Jaks are cytoplasmic tyrosine kinases that participate in signaling
from a range of cell-surface receptors, particularly members of the
cytokine receptor superfamily, which lack intrinsic tyrosine kinase
activity.1,2 There are 4 mammalian Jaks: Jak1, Jak2, Jak3,
and Tyk2. These associate with the weakly conserved "box 1" and
"box 2" recognition motifs in the membrane-proximal region of
cytokine receptors3,4 and are responsible for a range of
phosphorylation events on stimulation of such receptors with their
specific ligand. In addition, some receptors that have tyrosine kinase
activity, such as those for macrophage colony-stimulating factor and
stem cell factor, also activate Jaks, though it is unclear what role
they play in these instances.5,6
Cell lines deficient for either Jak1 or Jak2 are unable to mediate a
response to interferon- , whereas those deficient in Tyk2 fail to
respond to interferon- / .7-9 In addition, the
expression of kinase-deficient Jaks or the introduction of mutations
that prevent Jak binding and activation abolishes the proliferative and
anti-apoptotic signaling from a number of other cytokine
receptors.10-14 The essential role of Jaks in mediating the
effects of these hematopoietic regulators was recently confirmed by
targeted disruption of the corresponding murine genes (Table
1). Jak1-deficient mice exhibited perinatal lethality, apparently because of defective neural function, and defective lymphoid development.15 Targeted disruption
of the Jak2 gene resulted in an embryonic lethal phenotype
caused by a block in definitive erythropoiesis but with intact lymphoid development.16,17 In both cases, a number of other
specific cytokine-induced biologic responses were absent or impaired.
Finally, Jak3 knockout mice exhibited severe combined
immunodeficiency, with markedly reduced numbers of functional T and B
lymphocytes,18,19 and dysregulated
myelopoiesis.20 Thus, the Jaks collectively are vital for
normal hematopoietic function, which can be explained by their
nonredundant role in the signaling of specific cytokines (Table
1).
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Stats |
Stats are latent cytoplasmic transcription factors that become
activated after recruitment to an activated receptor complex. Subsequently, these active Stats translocate to the nucleus to affect
gene expression. Seven Stat proteins have been identified in mammalian
cells Stats1 to 6, including Stat5a and Stat5b, which are encoded by
distinct genes. In addition, different isoforms of several Stats have
been identified.21
Targeted inactivation of Stat genes in the mouse also resulted
in severe effects on the development and function of hematopoietic cells (Table 2). Stat1 knockout
mice showed defective innate immune responses to viruses and bacteria
because of the absence of interferon signaling,22,23
whereas Stat2 knockout mice showed defective responses to
interferon / (C. Schindler, personal communication,
1999). Targeted disruption of the Stat3 gene
produced early embryonic lethality.24 Subsequently,
T-cell-specific Stat3-deficient mice were generated that were severely
impaired in IL-6-induced proliferation because of enhanced
apoptosis.25 Stat4 knockout mice were defective in
the formation of Th1 cells, largely a result of disrupted IL-12
receptor function.26,27 Mice with both Stat5a and
Stat5b genes disrupted showed multiple defects, with responses to IL-2, IL-3, granulocyte-macrophage colony-stimulating factor (GM-CSF), and granulocyte colony-stimulating factor (G-CSF)
affected,28,29 whereas the respective single Stat5
knockouts also exhibited defective proliferative responses to specific
cytokines.30,31 Finally, a block in Th2 cell development
and IgE class switching was observed in Stat6 knockout
mice.32,33
Further evidence of the vital role of Stats in cytokine receptor
signaling has been obtained from studies in cell lines. For example, a
mutant cell line deficient in Stat1 expression showed a block in
interferon signaling similar to that in Stat1 knockout mice.34 Other studies using dominant-negative Stats or
specific receptor mutants have shown, for example, that Stat3
activation plays a key role in the differentiation responses to IL-6
and G-CSF,35-37 whereas Stat5 appears very important for
proliferative responses to IL-3, IL-5, G-CSF, and
GM-CSF38-40 and for neutrophilic differentiation in
response to G-CSF.40 Interestingly, these latter studies
differ from the relative mild effects on hematopoiesis seen in
Stat5ab double knockout mice,28 and they serve to
highlight some of the potential problems in interpreting both model
systems. For example, compensatory overlapping pathways can mask the
physiological consequences of a gene disruption in a whole animal,
whereas effects seen in cell lines may represent physiologically
irrelevant endpoints or nonspecific consequences of a
presumed dominant-negative protein acting on other pathways. However,
both types of studies have together yielded great insight into the
biologic function of Stats.
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Jak-Stat pathway |
A general model of Jak-Stat activation from cytokine receptors has
been proposed,2,21,41 though there are several exceptions and variations on the basic theme first elucidated for the interferon receptors.42 Binding of ligand to a cytokine receptor leads to the activation of Jaks, presumably through autophosphorylation on
tyrosines (Figure 1). Activated Jaks then
phosphorylate the receptor, creating docking sites for specific
signaling proteins, including Stat proteins, which the Jaks can then
phosphorylate on a conserved tyrosine residue at their C-terminus.
Subsequently, the Stats form stable homodimers and heterodimers by
interactions between the Src homology 2 (SH2) domain of one Stat
protein and the phosphotyrosine of another before translocation to the
nucleus, where they influence transcription of target genes by binding to specific regulatory sequences.21,41
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| Fig 1.
Activation of the Jak-Stat pathway by cytokine receptors
and its regulation by CIS family members and tyrosine phosphatases.
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Specificity in the Jak-Stat pathway |
Specificity in cytokine signaling is largely determined by the
combination of activated Jaks and Stats. A wide range of cytokines and
growth factors activate Jak1, Jak2, and Tyk, whereas Jak3 is only
activated by cytokines that have the common -chain as a component of
their receptor complex.43 Receptors show more specificity
in their ability to recruit and activate Stats. For example,
stimulation with interferon (IFN)- or IFN- leads to phosphorylation of Stats 1 and 2, which form a complex with a third
protein, p48.44 In contrast, G-CSF stimulation leads to the
activation of Stat3 and Stat5 homodimers, some Stat1 homodimers, and
Stat1/3 and Stat3/5 heterodimers.45,46 Such studies have led to the suggestion that diversity within the type of Stat complexes activated contributes to the nature of the cellular responses to a
given cytokine or growth factor.47 The subsequent gene knockout and dominant-negative mutant studies described above have
largely supported this hypothesis.
The specificity of Stat activation is partially mediated through their
recruitment to specific cytoplasmic tyrosines of particular receptors
through their SH2 domains. For example, tyrosine 440 in the cytoplasmic
domain of the IFN- receptor chain is responsible for recruitment
and activation of Stat1,48 whereas tyrosines 578 and 606 of
the IL-4-R are required for phosphorylation and activation of
Stat6.49 Similarly, numerous studies have identified the
YxxQ motif as a consensus Stat3 docking site, though it can bind to
other motifs.45,50,51 In addition, several examples of Stat
activation that do not require direct docking to receptor tyrosines
have been reported. For instance, full activation of Stat1 by G-CSF or
growth hormone and of Stat5 by G-CSF and GM-CSF occurs in the complete
absence of receptor tyrosines.38,45,51,52 It has been
proposed that Jak1 and Jak2 can specifically recruit and phosphorylate
Stat1 and Stat5, respectively,53,54 which could explain
their activation in these cases. However, there is growing evidence
that other receptor components can also act as docking sites for
Stats.45,55 For example, Stat1 is recruited to the
IFN-/ receptor complex by binding to a Stat2 molecule already
docked to the activated receptor.21 Thus, Stat specificity is determined by recruitment to the receptor complex in toto rather than simply to the linear sequence of each receptor. However, the
particular Jaks and Stats activated may also be dependent on the
cell-type or its state of differentiation,21,56-58 and receptor "cross-talk" may further modify the response
elicited. For example, IL-4 inhibits IL-2-mediated Stat5
activation,59 IL-10 suppresses
interferon-mediated Stat activation,60 and cyclic adenosine monophosphate impairs IL-2-dependent signaling by
downregulating levels of the Jak3 protein itself.61
Individual Stats bind to similar DNA response elements, mostly related
to a interferon-activated site, a regulatory element in the
promoter of IFN--inducible genes.42 However, the
recognition sites are not identical,62-64 and so different
genes are targeted for induction by different Stats (Table
3). In addition, Stats can mediate
transcriptional repression at specific promoters.65,66 Some
Stats are able to form both homodimers and heterodimers, which can
further broaden the range of Stat/DNA-binding
specificities.21 The duration of Stat activation is also
able to influence the transcriptional program induced.21,46
In addition, various Stat isoforms are differentially expressed in
specific cell types, which can also have an impact on the expression of
Stat-responsive genes.21,57,58,65 It has recently been
shown that Stats can interact with a range of other nuclear factors and
coactivators, including CBP, Nmi, the glucocorticoid receptor c-Jun,
and MCM5,21,67-69 which increases the range of
transcriptional responses in which Stats can participate. Finally,
though phosphorylation of the C-terminal tyrosine is critical for Stat
activation, serine phosphorylation probably also modulates the
transcriptional response.70-73 Indeed, it was shown that
Ser727 of Stat1 is directly involved in the recruitment of MCM5 as
part of IFN--induced transcriptional activation.68 Together, this complex control of specificity enables an individual hematopoietic cell to elicit the appropriate transcriptional response to incoming signals from cytokines, growth factors, and other stimuli.
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Independent functions of Jaks and Stats |
The major function of Jaks is generally considered to be Stat
activation. However, this is clearly not the only role that Jaks play
in signaling. For example, Jaks are directly implicated in the
activation of the kinase Pyk2,74 stimulation of the
Ras-MAPK pathway,13,75,76 and the induction of the
c-fos and c-myc genes.77 Conversely, there
is considerable evidence that some activation of Stats occurs
independently of Jaks. For example, cell lines deficient in Jak2 or
Tyk2 showed no effect on G-CSF-dependent Stat activation, and a cell
line deficient in Jak1 showed only partial reduction in Stat3
activation.55 Similar results were obtained with
Jak knockout mice for a range of factors.16
Furthermore, Stat6 activation after CD40 engagement occurs
independently of detectable Jak phosphorylation.78 Such
data suggest that other kinases are probably also involved in mediating
Stat activation. In support of this, Src has been shown to bind and
activate Stat3 directly,79 whereas Bcr-Abl can also recruit
and activate Stat5 by the interaction of Stat5 with the adaptor CrkL,
which itself docks to Bcr-Abl.80 Other work implicates a
number of non-Jak kinases as responsible for the serine phosphorylation
of Stats.73,81
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Negative regulation of the Jak-Stat pathway |
As further evidence of the importance of the Jak-Stat pathway,
negative feedback mechanisms have been identified that control its
activation (Table 4). These include
endosomal degradation of Jak/receptor complexes through
receptor-mediated endocytosis82,83 and the
dominant-negative effects of several naturally occurring Stat
variants.84,85 In addition, the PIAS proteins have been identified. They seem to bind directly to Stats and to inhibit DNA
binding, though their exact biologic role remains
unclear.72,86 Two other means of negative regulation, by
CIS/SOCS/SSI family members and by tyrosine phosphatases, have been
studied in more detail.
CIS/SOCS/SSI Family
This family, variously called CIS, SOCS, or SSI, is a group of small
proteins containing SH2 and CIS homology (CH) domains (also called SOCS
boxes). At least 8 family members have been identified (CIS1 to CIS7
and JAB),87,88 which are involved in the relatively
specific regulation of cytokine signaling.89
The first member of this family, originally denoted CIS (now called
CIS1) for cytokine-induced SH2-containing protein, was cloned as an
immediate early gene that was induced by IL-2, IL-3, and
erythropoietin.90,91 The induced CIS protein can
subsequently associate with specific tyrosines of the activated
receptors,92 including one of the major Stat5 binding sites
of the erythropoietin receptor.93 Forced expression of CIS1
partially suppresses IL-3 or erythropoietin-induced proliferation and
Stat5 activation.92 Conversely, Stat5 activates the CIS1
promoter through interaction with tandem TTCNNNGAA
motifs,92 with CIS1 expression absent in the ovaries of
Stat5ab double knockout mice.28 Thus, CIS1 appears
to act as a negative feedback regulator of the Jak-Stat5 pathway. This
is supported by recent observations in CIS1-transgenic mice that
exhibited a phenotype similar to that observed in Stat5ab double knockout mice.94
The second CIS/SOCS/SSI family member, JAB (also called SOCS-1, SSI-1,
and TIP-3), was identified independently because of its ability to
interact with the kinase domains of Jak2 and Tec,95,96 to
inhibit the IL-6-induced differentiation and growth arrest of a
leukemic cell line,97 and to be recognized by an antibody to Stat SH2 domains.98 Overexpression of JAB can inhibit
virtually any signal using Jaks, such as Stat5 activation by
erythropoietin, Stat3 activation by leukemia inhibitory factor or IL-6,
and c-fos induction by IL-2. However, JAB does not inhibit
fibroblast growth factor (FGF)-induced c-fos activation or
c-Kit phosphorylation, though it binds to the FGF receptor and
c-Kit.87 Although JAB was found to suppress Tec kinase
activity, this effect was marginal compared with its effect on Jaks.
Therefore, JAB-mediated kinase inhibition seems to be specific for Jak
tyrosine kinases; binding does not always imply inhibition. However,
recently JAB has been shown to bind inducibly to the c-Kit receptor
tyrosine kinase through its SH2 domain.95,99 Although JAB
did not inhibit the catalytic activity of the c-Kit tyrosine kinase, it
inhibited c-Kit-mediated proliferation signals, probably by
interaction with the SH3 domains of the signaling proteins Grb2 and
Vav, thereby suppressing their function.99 In certain
circumstances JAB may also be able to suppress signals from non-Jak
tyrosine kinases. JAB knockout mice displayed growth retardation, fatty
degeneration of the liver, and monocytic infiltration of several
organs. They died before weaning within 3 weeks of
birth.100,101 Lymphocytes in the thymus and spleen of these
mice exhibited accelerated apoptosis, and at 10 days of age their
numbers were 20% to 25% of those in wild-type mice.100
Among various pro-apoptotic and anti-apoptotic molecules examined, an
upregulation of Bax was found in lymphocytes of the spleen and thymus
of knockout mice.100 In addition, there was a progressive
loss of maturing B lymphocytes in the bone marrow, spleen, and
peripheral blood, whereas constitutive activation of Stat1 was found in
the liver of JAB knockout mice.101 Part of this phenotype
clearly resembles that found in IFN--transgenic mice.102 In addition, hematopoietic progenitor cells from
JAB knockout mice were hyperresponsive to IFN-, with the degree of inhibition varying markedly with the stimulating factor
used.103 It is important to note that many of the
pathologic conditions observed in JAB knockout mice can be eliminated
by antibody injections or by crossing them with IFN- knockout
mice.104 Therefore, JAB appears to act as a negative
regulator of IFN-/Stat1 and probably functions by preventing
apoptosis induced by Stat1. However, other studies suggest that JAB may
also inhibit other pathways that induce Bax expression, thereby
preventing Bax-induced apoptosis in vivo.100
So how do the CIS family members exert their negative effects on
Jak-Stat signaling? Several possibilities, which are not mutually
exclusive, are shown in Figure 1. Among CIS family members, JAB and
CIS3 are able to bind the Jak2 catalytic (JH1) domain,87 leading to direct inhibition of the Jak kinase.105 Binding
requires the SH2 domain plus an additional N-terminal 12 amino acids
(extended SH2 subdomain) containing 2 residues (Ile68 and Leu75) that
are conserved in the CIS family.14,105 This subdomain
interacts with the tyrosine residue Y1007 in the activation loop of
Jak2, whose phosphorylation is critical for the induction of kinase activity.106,107 Other CIS family members bind directly to
receptors, where they may function by preventing stimulatory signaling
pathways coupled to specific phosphotyrosine motifs on
receptors.92,93 Alternatively, given that CIS proteins have
a relatively short half-life, they may act as scavengers of activated
receptor complexes, targeting them for degradation.93 In
support of this, it has been shown that CIS1 itself is
ubiquitinylated,93 whereas the CH domain appears to
interact with components of the proteasomal degradation
pathway.108-110 However, other studies have shown that the
CH domain of JAB may actually protect this molecule from
degradation.108,111 Clearly more work is required to
unravel the various intracellular functions of the CIS family members.
Tyrosine phosphatases
Several studies have shown that an important negative regulatory
mechanism of the Jak-Stat pathway involves the recruitment of tyrosine
phosphatases containing tandem SH2 domains (SHP-1 and SHP-2) to
receptor complexes. Both phosphatases can bind either activated
receptors or to Jak family members themselves, leading to
dephosphorylation of the kinase (Figure 1).112-115 This, in
turn, leads to reduced activation of Jak-Stat pathway
components.114,116 The potential in vivo importance of this
mechanism is strongly suggested by the phenotype of motheaten
(me/me) mice lacking SHP-1, which die of a disease with
components of autoimmunity and inflammation.117 However, it
remains to be elucidated whether enhanced Jak kinase activity is
entirely responsible for the motheaten phenotype because SHP-1
has also been shown to regulate negatively a number of receptor and
nonreceptor tyrosine kinases.118,119 In addition, though it
is clear that the major means by which Stat activity is attenuated is
through dephosphorylation by protein tyrosine
phosphatases,21 it is unknown whether SHP-1 or SHP-2 or
some other phosphatase is responsible. However, it appears that SHP-1
can associate directly with Stat5, implicating it in the direct
dephosphorylation and deactivation of this
Stat.120
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Jaks, Stats, and hematopoietic diseases |
The wide use of the Jak-Stat pathway by hematologically important
factors, the severity of artificially disrupting the Jak-Stat pathway
on hematopoiesis, and the number of key genes with Stat-response elements already provides some appreciation of the importance of this
pathway in hematopoiesis and the regulation of hematopoietic cell
function. We will now summarize the studies showing that several
diverse hematopoietic disorders exhibit perturbations in the Jak-Stat
pathway. Indeed, in a number of these cases, experiments have directly
implicated the altered Jak or Stat signaling, or both, in the
pathogenesis of the disease. Such molecular investigations provide a
foundation on which to build an understanding of these conditions and a
framework for rational improvements in therapy.
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Hematopoietic malignancies |
Aberrant activation of Jaks and Stats
The most direct evidence implicating dysregulation of the Jak-Stat
pathway in hematopoietic malignancies was the identification of
Tel-Jak2 fusions in lymphoid and myeloid leukemias.121,122 In early B-precursor acute lymphoblastic leukemia, t(9;12)(p24;p13) translocations were responsible, whereas in the case of atypical chronic myeloid leukemia there was a complex t(9;15;12)(p24;q15;p13) translocation. In each case, the helix-loop-helix oligomerization domain of the transcription factor Tel is fused to the catalytic JH1
domain of Jak2 (Figure 2), which leads to
constitutive association and hence activation of the kinase and
constitutive activation of Stat proteins.123,124 However,
Jaks and Stats are also known to be constitutively activated in
hematopoietic cells transformed by diverse oncogenic tyrosine kinases
(Table 5), as well as in a variety of
lymphomas and leukemias, including those transformed by oncogenic
viruses (Table 6). For the oncogenic
tyrosine kinases, the activation of Stats may be direct or occur
through Jaks, whereas the oncogenic viruses activate a number of
cytoplasmic kinases to mediate the constitutive Stat activation
observed (Figure 3).125
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| Fig 2.
Tel-Jak2 fusions observed in myeloid (CML; chronic
myeloid leukemia) and lymphoid (ALL; acute lymphoblastic leukemia)
leukemias.121,122
The relative position of the fusions, as well as the helix-loop-helix
(HLH) and Jak homology (JH) domains are shown.
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Evidence for Jak-Stat involvement
Although the data above are suggestive of a positive role for
constitutive activation of the Jak-Stat pathway in leukemia, the
results are largely correlative. However, other studies have provided
more direct evidence for this hypothesis. For example, overexpression
of a Tel-Jak2 fusion is sufficient to render Ba/F3 cells
factor-independent,121,123 and mice transplanted with
retrovirus expressing this fusion develop a fatal mixed
myeloproliferative and T-cell lymphoproliferative disorder with a
latency of 2 to 10 weeks.123 In addition, 2 mutants of the
Drosophila Jak kinase have been identified that lead to
leukemia-like defects through hyperactivation of the
kinase.126,127 Murine homologues of this Jak mutant have
been further shown to induce leukemia in mice.127 Finally,
inhibition of constitutive Jak2 phosphorylation in primary pre-B
leukemic cells with the Jak2 inhibitor AG490 is able to inhibit cell
proliferation.128 However, other studies suggest that the
constitutive Jak activation seen in transformed cells is not actually
required for transformation. Thus, dominant-negative Jaks are unable to
inhibit either Stat5 activation or factor-independent cell
proliferation induced by Bcr-Abl.129 In addition, v-Src can
directly activate Stat3,130 whereas the Herpesvirus Tip
protein co-associates Lck and Stat3, leading to constitutive Stat3
activation in T cells transformed by this virus,131
suggesting that in these cases Jaks are also superfluous.
In contrast, numerous recent studies have provided strong evidence for
a role of Stats in the transformation process. For example, a
dominant-negative Stat5 was able to inhibit apoptosis-resistant, growth
factor-independent proliferation and leukemic potential of Bcr-Abl
transformed cells132,133 and of the growth
factor-independent colony formation of primary mouse bone marrow
progenitor cells transduced with Bcr-Abl retrovirus.132 In
addition, a constitutively active Stat5 mutant could restore these
functions to a mutant Bcr-Abl deficient in Stat
activation.132 Similarly, the abrogation of IL-3 dependence
of myeloid cells by v-Src requires the SH2 and SH3 domains, which
specifies the activation of Stats,130 and dominant-negative
Stat3 has been shown specifically to block v-Src transformation in
other cell systems.134,135 Furthermore, studies in multiple
myeloma cells that show constitutive Stat activation have revealed that
dominant-negative Stat3 induces apoptosis,136 again
implicating Stat3 in the transformation process. Finally, a
constitutively active mutant of Stat5 is sufficient to induce factor
independence of Ba/F3 cells.137 Thus, constitutive Stat
activation appears necessary, and perhaps sufficient, for the
transformation process.
The results of the above studies imply that a permanent alteration in
the genetic program of transformed cells, achieved by the constitutive
activation of Stat proteins, is a critical step in the transformation
process. Recent studies have begun to shed light on those changes that
may be important. Thus, constitutive expression of Jak2 in Ba/F3 cells
has been shown to lead to the induction of Bcl-2, resulting in delayed
cell death,138 but the constitutively activated Stat3
observed in bone marrow mononuclear cells from patients with multiple
myeloma also confers resistance to apoptosis, this time through the
induction of Bcl-xL.136 Identification of other
genes involved in the transformation process remains an important goal
for future research.
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Other hematologic disorders |
Alterations in the Jak-Stat pathway have been associated either
directly or indirectly with other hematologic disease states.
Severe combined immunodeficiency
In the most common form of severe combined immunodeficiency (SCID),
X-linked SCID, both cellular and humoral immunity are severely
affected: T-cell development is arrested in the thymic cortex, there is
an almost complete lack of circulating T lymphocytes, and, though B
lymphocytes are present, they do not undergo class switching.139 In X-linked SCID, mutations have been
identified in the gene encoding the common chain (c), a
constituent of a number of cytokine receptor
complexes.140,141 Although these mutations occur at
multiple positions, all mutant receptors are defective in the
activation of Jak3.142,143 Indeed, Jak3 knockout mice display a SCID phenotype that is virtually indistinguishable from
that of c null mice.18,144 Moreover, in a less common autosomal-recessive form of SCID, patients have been reported with
inactivating mutations in the Jak3 gene
itself.4,145,146 Together these findings show that
abrogation of the Jak-Stat pathway is sufficient to account for SCID in humans.
Severe congenital neutropenia/acute myeloid leukemia
Patients with severe congenital neutropenia (SCN) exhibit a severe
reduction in circulating neutrophils and a maturation arrest of bone
marrow progenitor cells at the promyelocyte/myeloid
stage.147,148 Such patients have an increased risk for
myelodysplasia, acute myeloid leukemia, or both, and a poor prognosis
for survival.149,150 A subset of patients with SCN has been
identified with acquired nonsense mutations in the gene encoding the
G-CSF receptor, which truncate its
carboxyl-terminus.151,152 This subset has a strong (around
50%) predisposition to acute myeloid leukemia. Mice carrying a similar
G-CSF-R truncation also show reduced basal levels of circulating
neutrophils, but on continuous G-CSF treatment, neutrophil counts
become elevated to above those of wild-type controls because of the
increased proliferation of myeloid progenitors.154 This suggests that the G-CSF-R truncation may contribute to SCN and to the
subsequent development of acute myeloid leukemia in these patients.
Bone marrow cells from these mutant mice show reduced Stat3 activation
in response to G-CSF, even under saturating conditions. In addition,
there is an altered dose-response of Stat3 compared to Stat5
activation, such that at lower G-CSF concentrations the Stat3
deficiency is even more pronounced, a result confirmed in myeloid 32D
cells.45,155 Because Stat3 appears indispensable for
differentiation responses to G-CSF, the reduced Stat3:Stat5 ratio in
cells with truncated receptors at low G-CSF concentrations may
contribute to the reduced maturation observed.45,154,155 In
addition, molecular mechanisms have been identified recently that
explain the hyperproliferative function of truncated G-CSF-R. Such
receptors show defective internalization compared with wild-type receptors46,155,156 and have a concomitant extension in the activation of Stats, particularly Stat5,46 consistent with
a previous report of enhanced Jak2 activation in patients with SCN (Figure 3).157 It has recently been shown in 32D cells
expressing truncated G-CSF-R that dominant-negative Stat5 inhibits
whereas dominant-negative Stat3 actually enhances G-CSF-mediated
growth, implicating perturbed Stat5 activation as a key molecular
determinant of the hyperproliferative responses elicited from truncated
G-CSF-R (Ward et al, manuscript in preparation). In addition, a novel G-CSF-R mutation has been identified in a patient with SCN who was
unresponsive to G-CSF therapyin this case Stat5 activation was
substantially reduced,158 again consistent with an
important role of Stat5 in controlling proliferative responses to
G-CSF.
Benign erythrocytosis
Benign erythrocytosis is a dominant autosomal condition
characterized by a mild increase in red blood cell counts and normal serum levels of erythropoietin because of hypersensitivity to erythropoietin.159,160 In addition, there is an increased
and a sustained activation of Jak2 and Stat5 after erythropoietin stimulation.112,160 A number of pedigrees have been
identified, all of which lead to erythropoietin (EPO)-R
truncations161,162 that invariably result in the loss of
the binding site for SHP-1 at Tyr 449 of the EPO-R.112
Because SHP-1 is a negative regulator of Jak2 activation by EPO, it
appears that lack of SHP-1 activation is responsible for the altered
Jak-Stat kinetics and enhanced EPO responses in these patients (Figure
3).
Fanconi anemia
Fanconi anemia (FA) is an autosomal recessive chromosome instability
syndrome characterized by progressive bone marrow failure and an
increased susceptibility to malignancy.163,164 The FA group
C gene (FAC) has been identified, with its disruption leading to
profound hypersensitivity of hematopoietic precursor cells to IFN-
in mice165 and in patients with FA group C.166
This appears to be the result of sustained Stat1 activation leading to
apoptosis of these cells.166 Other researchers have
reported that the FAC protein is involved in the recruitment of Stat1
to the IFN- receptor complex,167 which further suggests
that perturbed Stat1 activation contributes to the phenotype of this disease.
Interferon resistance
Interferons, particularly IFN-, have important therapeutic
applications in the treatment of hematologic malignancies, including CML, hairy cell leukemia, and cutaneous T-cell lymphoma
(CTCL).168,169 However, the efficacy is limited by the
development of clinical resistance to IFN therapy in these
patients.169 Efforts to understand the molecular basis of
IFN resistance have been made by generating somatic cell mutants
resistant to IFN, which showed that defects in the IFN receptor, Jaks,
or Stats could contribute to this phenomenon.21,42 Similar
analysis of IFN--resistant derivatives of CTCL cells also revealed
a defect in normal Jak-Stat responses caused by a total absence of
Stat1 expression,170 as previously observed in patients
with IFN-resistant melanoma.171 However, a recent study
also suggests a possible role for JAB in IFN resistance, especially for
patients with a dominant phenotype.172 Stable expression of
JAB in either NIH-3T3 or M1 leukemic cells leads to resistance to
IFN-- and IFN--induced growth arrest. In both cell systems,
IFN- did not induce tyrosine phosphorylation and DNA-binding
activity of Stat1. In addition, IFN-resistant clones derived from LoVo
cells and Daudi cells were found to express high endogenous levels of
JAB without stimulation, with a concomitant reduction in IFN-induced
Stat1 and Jak phosphorylation.172
Other diseases with altered Jak-Stat activation
Other hematologic diseases also show defects in the normal
activation and regulation of Jak-Stat pathway components. For example, bone marrow cells from patients with myelodysplastic syndrome show
impaired erythropoietin-induced Stat5 activation,173
whereas reduced Tyk2/SHP-1 interaction has been observed in a kindred of familial hemophagocytic lymphohistiocytosis.174 However,
additional experiments will be required to identify mechanisms by which
these perturbations in the Jak-Stat pathway may contribute to the
pathogenesis of disease.
| |
Future directions |
It is clear from this review that the Jak-Stat pathway is perturbed
in a variety of malignancies and hematopoietic disorders. There is also
now solid evidence that constitutive activation of Jak-Stat pathway
components plays an important role in transformation by Tel-Jak,
Bcr-Abl, and v-Src and in multiple myeloma. Furthermore, the importance
of defective Jak3 activation in SCID and of extended Stat5 activation
in the hyperproliferative responses of truncated G-CSF-R is now
established. However, the significance of altered Jak-Stat activation
in the other disorders remains less clear. In each case, the judicious
expression of dominant-negative or constitutively a |
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Introduction
Jaks