Evidence is mounting for the role of as yet unidentified serine proteases in the proteolytic processing of STAT proteins, although at least one cysteine protease, calpain is also known to
Trang 1M I N I R E V I E W
Regulation of STAT signalling by proteolytic processing
Lisa Hendry and Susan John
Peter Gorer Department of Immunobiology, Programme in Infection and Immunity, King’s College London, UK
Interaction of cytokines with their cognate receptors leads
to the activation of latent transcription factors, the signal
transducer and activator of transcription (STAT) proteins
Numerous studies have identified the critical roles played
by STAT proteins in regulating cell proliferation,
differ-entiation and survival Consequently, the activity of STAT
proteins is negatively regulated by a variety of different
mechanisms, which include alternative splicing, covalent
modifications, protein–protein interactions with negative
regulatory proteins and proteolytic processing by
pro-teases Cleavage of STAT proteins by proteases results in
the generation of C-terminally truncated proteins, called
STATc, which lack the transactivation domain and behave
as functional dominant-negative proteins Currently,
STATc isoforms have been identified for Stat3, Stat5a, Stat5b and Stat6 in different cellular contexts and biolo-gical processes Evidence is mounting for the role of as yet unidentified serine proteases in the proteolytic processing
of STAT proteins, although at least one cysteine protease, calpain is also known to cleave these STATs in platelets and mast cells Recently, studies of acute myeloid leukae-mia and cutaneous T cell lymphoma patients have revealed important roles for the aberrant expression of Stat3c and Stat5c proteins in the pathology of these diseases To-gether, these findings indicate that proteolytic processing is
an important mechanism in the regulation of STAT pro-tein biological activity and provides a fertile area for future studies
Introduction
The Janus kinase-signal transducer and activator of
tran-scription (JAK-STAT) signalling pathway, first identified
for the interferon-a/b and c receptors, is now known to be
employed by many cytokine and growth factor receptors
and to be evolutionarily conserved [1,2] STAT proteins
have a common overall structure and are organized into
distinct functional modular domains (Fig 1)
After a decade of intense investigation into the structure
and biological functions of STAT proteins, their essential
roles in cell proliferation, differentiation and survival have
been firmly established [2] A number of studies have
iden-tified important negative regulatory mechanisms that exist to
curtail the activity of STAT proteins (Fig 2) These include
the activities of phosphatases, suppressors of cytokine
signalling (SOCS), interaction of inhibitory proteins such
as protein inhibitor of activated STATs (PIAS), and targeted
proteasome-dependent degradation of active STATs [2,3]
In addition to these direct protein–protein interaction methods of negative regulation, STATs are also regulated at the level of alternative splicing The STATb forms, gener-ated by alternative splicing, possess an altered carboxy-terminal (C-carboxy-terminal) lacking the natural transactivation domain and behave as functional dominant-negative pro-teins when overexpressed in cells [4–6] However, recent evidence from transgenic mice indicates that STATb proteins are not strict dominant-negatives, and actually contribute to transcriptional activation of selective target genes, despite the absence of the natural transactivation domain [7–9] The mechanism by which STATb isoforms achieve transactivation remains to be elucidated, but probably involves the differential interaction with other transcription factors
Another mechanism by which STAT signalling is regu-lated occurs at the level of limited proteolytic processing in cellular contexts where there is no evidence for alternative splicing [10] Proteolytic processing of STAT proteins also results in the generation of C-terminally truncated STAT proteins, referred to as STATc, but these proteins lack the transactivation domain, without the addition of any extra amino acid sequences at their C-termini Thus, multiple functional forms of STAT proteins, generated by distinct mechanisms exist in different cell lineages Here we review the generation and function of STATc proteins and their role in human diseases
Processing of Stat5 in haematopoietic progenitor cells
Stat5 is activated by a wide variety of haematological and nonhaematological cytokines and growth factors including those which regulate the proliferation and differentiation of
Correspondence to S John, Peter Gorer Department of
Immunobiol-ogy, Programme in Infection and Immunity, King’s College London,
2nd floor New Guy’s House, St Thomas Street, London SE1 9RT,
UK E-mail: susan.john@kcl.ac.uk
Abbreviations: AML, acute myeloid leukaemia; BMMC, bone
mar-row-derived mast cells; CTCL, cutaneous T cell lymphoma; G-CSF,
granulocyte colony stimulating factor; GM-CSF,
granulocyte-macrophage colony-stimulating factor; IL, interleukin; JAK, Janus
kinase; PBMC, peripheral blood mononuclear cell; PIAS, protein
inhibitor of activated STATs; PMSF, phenylmethanesulfonyl fluoride;
SOCS, suppressors of cytokine signalling; SS, Sezary syndrome;
STAT, signal transducer and activator of transcription.
(Received 17 August 2004, accepted 7 October 2004)
Trang 2myeloid [interleukin (IL)-3, IL-5, granulocyte-macrophage
colony-stimulating factor (GM-CSF) and thrombopoietin],
erythroid (erythropoietin) and lymphoid lineages (the
gamma-c family of cytokines, IL-2, IL-7 and IL-15)
[11,12] Targeted deletions in mice of genes encoding Stat5
results in defects in myeloid cell differentiation through
effects on early haematopoietic progenitor cells [13] The two Stat5 proteins, Stat5a and Stat5b, are encoded by separate genes and are expressed as both full-length (Stat5a) and shorter, C-terminally truncated proteins [5,14] Although alternative splicing generates Stat5b in certain cellular contexts, the lack of abundance of the alternatively
Fig 2 Negative regulation of STAT signalling Cytokine-induced STAT activation can be inhibited by suppressors of cytokine signalling (SOCS) proteins, whose gene expression is regulated by STAT proteins, thus fulfilling a negative feedback loop SOCS proteins inhibit STAT activation either by inhibition of the activating JAKs or by competition with STATs for receptor binding Activated STAT proteins can be dephosphorylated
by cytoplasmic and/or nuclear phosphatases C-terminally truncated STAT proteins, STATb and STATc, behave as dominant-negative proteins to functionally compete with their full-length counterparts to alter or inhibit gene expression, respectively Protein inhibitor of activated STATs (PIAS) proteins interact with STAT proteins to inhibit their DNA binding and/or potentially facilitate their covalent modification by sumoylation and subsequent degradation Ub, ubiquitin; SUMO, small ubiquitin-like modifier.
Fig 1 Modular structure of STAT proteins All STAT proteins share a common molecular topology and are organized into distinct functional domains The NH 2 -terminus (N-domain) is involved in protein–protein interactions between adjacent STAT dimers on DNA, facilitating the formation of STAT tetramers It is also involved in the formation of dimers between nonphosphorylated STAT monomers, which is important for receptor-mediated activation and nuclear translocation of certain STAT proteins Interactions with STAT cofactors, which positively or negatively modulate their transcriptional activity, occur via the N-domain, the adjacent coiled-coil domain and the carboxy-terminal transactivation domain (TAD) The conserved serine residue (p-S), which is phosphorylated upon cytokine stimulation and is important for maximal transcriptional activation, is located within the transactivation domain The conserved tyrosine residue (p-Y), that becomes phosphorylated upon activation is located immediately preceeding the transactivation domain.
Trang 3spliced message in haematopoetic progenitor cells led
investigators to evaluate other mechanisms for the
genera-tion of C-terminally truncated Stat5 proteins
It was noted that distinct forms of Stat5 proteins were
activated upon IL-3 treatment of specific myeloid cell
lineages Thus, in myeloid progenitor cell lineages
stimula-tion with IL-3, GM-CSF or erythropoietin activates a
shorter, C-terminally truncated isoform of Stat5a (77 kDa)
and Stat5b (80 kDa), while full-length Stat5a (96 kDa) and
Stat5b (94 kDa) are only activated in differentiated mature
myeloid cells [10,15] The Stat5c proteins in myeloid
progenitor cells are generated by a putative Stat5 protease,
which is primarily located in the nucleus and cleaves Stat5
proteins independently of their tyrosine-phosphorylation
states [10,16] The protease is an endopeptidase and is
inhibited by the broad-spectrum serine protease inhibitor,
phenylmethanesulfonyl fluoride (PMSF) Cellular
fraction-ation and chromatography studies indicate that the protease
has an approximate molecular mass of 25 kDa and cleaves
murine Stat5a between amino acids 719 (tyrosine; Y) and
720 (methionine; M) and Stat5b between Y724 and M725
[17] Mutant Stat5 proteins bearing amino acid substitutions
at these positions were resistant to cleavage by the protease
Importantly, the Stat5-proteolytic activity was absent in
mature myeloid cells suggesting that either the expression of
protease is down-regulated or alternatively inactivated upon
myeloid cell differentiation [10,16,17]
Consistent with the distinct function of truncated Stat5
proteins in immature myeloid progenitors, they fail to
activate several known IL-3-induced target genes that are
activated by the full-length proteins in differentiated mature
myeloid cells [10] The functional significance of truncated
Stat5 proteins in maintaining an undifferentiated immature
phenotype of myeloid cells was convincingly demonstrated
by studies using stable enforced expression of mutant,
noncleavable forms of Stat5 in undifferentiated myeloid
cells [18] The mutant cell lines developed a partially
differentiated phenotype and were resistant to further
differentiation by cytokine treatment Thus, proteolytic
cleavage of Stat5 is an important physiological mechanism
in regulating myeloid cell differentiation
Proteolytic cleavage of Stat5 in peripheral
T cells
Despite the clear role of proteases in regulating myeloid cell
development, Schindler and colleagues were unable to
demonstrate an analogous situation for lymphoid cell
development in murine thymic T cells [17] However, studies
of human peripheral blood mononuclear cells (PBMCs)
indicate that naı¨ve T cells in the peripheral immune system
possess a similar mechanism for regulating Stat5 as myeloid
progenitor cells Activation of naı¨ve T cells by antigenic or
mitogenic stimulation leads to cell proliferation and
differ-entiation into effector T cells, mediated by the action of
immunologically important cytokines, which signal via
Type I and Type II cytokine receptors Stat5 activation,
mediated by IL-2 signalling upon T cell activation, is an
important regulator of cell proliferation and survival [19,20]
Recently, studies on Stat5 expression and activation in
normal human PBMC and peripheral T cells revealed that
Stat5 is expressed exclusively as a truncated protein in the
nucleus of naı¨ve PBMC/T cells [21] Analysis of the truncated Stat5 proteins using N- and C-terminal Stat5 antibodies revealed that the truncation is at the C-terminus
of the Stat5 protein, as previously noted in myeloid cells The expression of the truncated protein in the nucleus is independent of the phosphorylation state of Stat5a and Stat5b Unlike myeloid progenitor cells, the cytoplasmic fraction expresses both the full-length and the truncated Stat5 protein, although at present we cannot exclude the possibility that the truncated protein is exclusively generated
in the nucleus but is present in the cytoplasmic fraction due
to protein shuttling, which has been shown to occur in a cytokine-dependent and independent manner for STAT proteins [22,23]
Upon activation of naı¨ve T cells by mitogenic stimula-tion, the expression of truncated Stat5a and Stat5b proteins disappears and is replaced by the expression and activation
of the full-length Stat5 proteins [21] Significantly, the normal regulation of truncated vs full-length Stat5 is dysregulated in cutaneous T cell lymphoma (CTCL) patients and will be described in a later section Ongoing studies indicate that the truncated Stat5 protein is generated
by the activity of a protease, which is down-regulated or inactivated upon mitogenic stimulation (Fig 3) Future biochemical characterization and purification of the prote-ase(s) and the identification of the exact cleavage site on Stat5 will be important in enhancing our understanding of the regulation of Stat5 function by proteolytic cleavage in peripheral T cells
Proteolytic regulation of Stat5 and Stat3
in mature human neutrophils Stat3 and Stat5 isoforms have been identified in differen-tiated human peripheral blood monocytes and
polymor-Fig 3 Stat5a protein is cleaved to Stat5c by the activity of a protease
present in peripheral blood mononuclear cell (PBMC) extract The presence of Stat5-proteolytic activity was evaluated by coincubation assay Extracts prepared from either PBMC (lane 2) or PBMC mito-genically stimulated with phytohaemagglutinin (PHA-Blasts, lane 3),
or a buffer control containing no cell extract (lane 1), were incubated with FLAG-tagged Stat5a protein at 37 °C for 15 min Samples were then analyzed by Western blot analysis using an anti-FLAG IgG Cleavage of the FLAG-Stat5a input protein was obtained specifically with fresh PBMC extrcats and not with extracts made from PHA-Blasts.
Trang 4phonuclear neutrophils [24–27] During terminal
differen-tiation of neutrophils, induced by granulocyte colony
stimulating factor (G-CSF), the main STAT that is
activated is Stat3 and it is predominantly expressed as
Stat3c, generated by proteolytic cleavage of Stat3a [25,28]
Unlike the progenitor myeloid Stat5 protease, the Stat3
protease, activated by G-CSF can only cleave the active,
phosphorylated form of Stat3a [25] The exact specificity
of the Stat3 protease appears to be less clear, as the
proteolytic activity was shown to be inhibited by
di-isopropylfluorophosphate and not PMSF in living cells,
but neither was effective at inhibiting the protease in vitro
[25] The relationship between the Stat3 protease from
mature neutrophils and the Stat5 protease from immature
myeloid cells is also unknown at present, but the activation
of these proteases in different developmental contexts may
suggest that they are distinct proteases
More recently, investigators have shown that Stat5 is
also similarly regulated by proteolytic processing in mature
human neutrophils [26] Stat5 is activated in human
neutrophils by the cytokines IL-2 and GM-CSF, which
are both potent modulators of neutrophil activity [29] In a
now familiar theme, these cytokines activate nuclear
expression of a C-terminally truncated form of Stat5 in
neutrophils, which results in a failure to induce expression of
known Stat5-regulated genes, such as osm and pim-1,
consistent with the inability of these cytokines to induce
proliferation of these cells [26] No evidence was found for
alternative splicing of Stat5 in these cells and instead
truncated Stat5 proteins were generated by the activity of a
nuclear, PMSF-sensitive serine protease The exact
rela-tionship between the various Stat5-serine proteases derived
from myeloid progenitors, human PBMC and mature
neutrophils awaits identification by future molecular
clo-ning studies
Regulation of Stat6 activity by proteolytic
cleavage in mast cells
Unlike Stat3 and Stat5, which are activated by a wide
variety of cytokines and growth factors, Stat6 is very
selectively activated by IL-4 and the related cytokine, IL-13
[30] Stat6 deficient mice reveal defects in such crucial
aspects of normal immune function as Th2 cell
differenti-ation, B cell isotype switching and the loss of contact
hypersensitivity [30] While IL-4 induced Stat6 signalling is
an activating signal in murine B and T cells, its role in bone
marrow-derived mast cells (BMMC) is less clear [31,32]
Analysis of Stat6 expression in murine BMMC provided a
clue to these apparent cellular differences in response to
IL-4 Brown and colleagues first observed that, whereas
Stat6 is expressed as a 100 kDa full-length protein in B and
T cells, it is expressed as a 65 kDa protein in murine BMMC
[33] A similarly truncated Stat6 protein has not been
identified in human mast cells and it is possible that this
mechanism of regulation of Stat6 has been lost during
evolution Studies revealed that Stat6 is truncated at its
C-terminus and is lacking the transactivation domain in
murine BMMCs [33] While no evidence for alternative
splicing of Stat6 was obtained in mast cells, several groups
have established that truncated Stat6 protein is generated
by proteolytic processing in these cells [33–35]
The activity of the murine Stat6 protease is exclusively nuclear and can be inhibited by the serine protease inhibitors, PMSF and 4-(2-aminoethyl)-benzenesulfonyl-fluoride [35] Moreover, the activity of the Stat6 protease is not dependent on the expression of Stat6, as Stat6-deficient BMMC also contained Stat6-specific proteolytic activity [35,36] More recently Iwamoto and colleagues have further characterized the serine protease to be inhibited by an elastase inhibitor ONO-5046, suggesting that this protease may belong to an elastase family [36,37] The Stat6 protease cleaves Stat6 between amino acids 685 (aspartic acid; D) and 686 (methionine; M) The amino acid sequences surrounding the cleavage site are not conserved in the human Stat6 protein, providing an explanation for the lack
of observation of truncated Stat6 in human mast cells While cleavage-resistant point mutants of Stat6 (Stat6 D685A and M686A) have similar transcriptional activity as their wild-type counterpart in cell transfection assays, the stable expression of these Stat6 mutants in cell lines results
in prolonged nuclear accumulation of Stat6 and enhanced IL-4-induced apoptosis and growth inhibition of the mutant mast cell lines [35] Furthermore, enforced coexpression of truncated Stat6 with Stat6 D685A reverses the functional effect of the latter mutant indicating that the truncated Stat6 protein can potentially function as a dominant-negative in BMMC [35]
Despite the finding that both the Stat5 protease and the Stat6 protease are serine proteases, the similarity apparently does not extend any further The Stat5 protease from myeloid cells does not cleave Stat6 and is not inhibited by ONO-5046, and the Stat6 protease from BMMC does not cleave Stat5 [35–37] Thus, the serine proteases that regulate STAT activity show STAT and cell-type specificity Processing of Stat3, Stat5 and Stat6 by calpain While the most common mechanism of proteolytic process-ing of STAT proteins is mediated by the action of serine proteases, at least one other cellular protease is known to specifically cleave certain STAT proteins The calcium-dependent cysteine protease calpain was demonstrated to cleave Stat3 and Stat5 in platelets and Stat6 in mast cells to generate C-terminally truncated proteins [37,38] Activation
of intracellular calpain by thrombin treatment of platelets resulted in a significant increase in the levels of C-terminally truncated Stat3 and Stat5 [38] Similarly, Stat6 was cleaved upon activation of calpain by dibucaine treatment of BMMC [37] However, the truncated Stat6 protein that is generated as a result of cleavage by calpain is a 70 kDa protein as compared to the 65 kDa protein generated by the Stat6 protease Furthermore, the generation of the 70 kDa but not the 65 kDa Stat6 protein is inhibited by the calpain inhibitor calpeptin [37] Thus, multiple different STATc isoforms can be generated by the activation of different cellular proteases in BMMCs It is unclear whether the calpain cleaved Stat5 in platelets is identical in size and function to the Stat5c proteins generated by proteolytic processing by the Stat5 proteases from myeloid progenitor
or mature neutrophil cells The physiological importance of STATc isoforms generated by calpain is unknown at present but, as calpain is potently activated by increased intracellular calcium concentrations following cellular
Trang 5activation, it is plausible that calpain mediated processing of
STAT proteins may be an important mechanism for
regulating STAT-dependent gene expression [39]
Dysregulated expression of proteolytically
processed STAT proteins in human diseases
The constitutive activation of full-length Stat3 and Stat5 is a
common feature of many primary human tumours of
haematopoietic and nonhaematopoeitic origins and is
extensively reviewed elsewhere [40,41] Recent studies of
acute myeloid leukaemia (AML) and CTCL patients
indicate that C-terminally truncated STAT proteins also
contribute to the pathology of these diseases
Acute myeloid leukaemia
AML is characterized by the clonal expansion of myeloid
cells that have been arrested in their maturation Like their
normal counterparts, AML blasts can proliferate in
response to haematopoietic cytokines such as GM-CSF,
G-CSF, thrombopoietin and IL-3, which signal via the
JAK-STAT pathway [42] However, unlike normal myeloid
cells, which undergo differentiation in response to specific
cytokine treatment, the leukaemic cells proliferate but do
not differentiate, suggesting that crucial signalling pathways
that regulate cell proliferation and differentiation may be
dysregulated in this disease Analysis of a number of bone
marrow samples from pretreatment AML patients revealed
that 20–30% of AML blasts expressed constitutively
activated full-length Stat3 and Stat5 proteins but a much
higher proportion ( 80%) expressed C-terminally
trun-cated Stat3 and Stat5 proteins [43] Moreover, 94% of
patients in relapse expressed truncated STAT proteins
compared to 35% of patients with constitutively active
full-length STAT proteins, suggesting that the expression of
truncated Stat3 and Stat5 proteins may contribute adversely
to disease progression [44] Nevertheless, the shortest
disease-free survival rate and overall survival was seen in
patients that had both constitutive activation of full-length
Stat3 and concurrent aberrant expression of truncated Stat3
[45] These studies suggest that the relative ratio of
full-length : truncated STAT protein may influence the
out-come of disease progression Constitutive expression of
C-terminally truncated Stat5 proteins have also been
described previously in CD4 T cells from HIV patients
undergoing antiretroviral monotherapy or IL-2 treatment
and was associated with good response to therapy [46]
However, it is not known whether the truncated Stat5
protein in patient cells is generated by proteolytic activity or
by alternative splicing
Biochemical characterization of the AML samples that
contained truncated STAT proteins, revealed that a
pro-teolytic activity was expressed in these samples, which could
selectively cleave Stat3 and Stat5, but not Stat6 [47] The
serine protease inhibitor PMSF was able to inhibit the
activity of the Stat3/5 protease from AML blasts, as
previously observed for progenitor myeloid cells However,
this serine protease differed from that present in immature
myeloid cells in that it was present in both cytoplasmic and
nuclear fractions and chromatographic analysis of the
protease from AML blasts yielded a protein of approximate
molecular mass of 40 kDa Thus, the active protease in AML blasts may either represent yet another member of the STAT-serine protease family or alternatively may be aberrantly post-translationally modified Given the clearly established dominant-negative functions of C-terminally truncated STAT proteins, the aberrant constitutive expres-sion of truncated Stat3 and Stat5 proteins in AML blasts has important physiological implications for the pathology
of the disease As cleavage-resistant mutant Stat5 proteins induce differentiation and apoptosis of myeloid cells when artificially expressed, it is plausible to speculate that the selective expression of truncated Stat5 and Stat3 proteins may enhance survival of leukaemic blasts cells in AML, while at the same time preventing cellular differentiation [18]
Cutaneous T cell Lymphoma Primary CTCLs are one of the most frequent extranodal lymphomas affecting the skin, and include mycosis fun-goides and its leukaemic variant Sezary syndrome (SS) [48] Tumour cells are typically CD4 T cells, which display a memory activated phenotype, and express Th2-like cyto-kines (IL-4, IL-5 and IL-10) [49] While the Jak3-Stat3 pathway is constitutively activated in SS, Stat5 activation is inducible [21,50] Recently, a study of SS patients with advanced stage disease identified a different form of dysregulation of Stat5 [21] As mentioned earlier, Stat5 is regulated by proteolytic processing in normal PBMC Analysis of PBMC from SS patients showed that, unlike
in healthy controls, there was elevated or exclusive expres-sion of the C-terminally truncated Stat5 protein even in potently activated cells DNA binding studies revealed that
in SS patients, truncated Stat5 proteins are activated upon IL-2 stimulation and preferentially bind to known Stat5 binding sites, even in patients where a mixture of full-length and truncated Stat5 proteins are expressed Consistent with these findings, there was a loss of IL-2-induced Stat5-dependent gene expression of target genes such as pim-1, cis, and bcl-2 in patient samples However, the Stat5-regulated gene CD25 was still inducible by IL-2, consistent with findings from other studies, which indicated that constitu-tively activated Stat3 aberrantly regulates CD25 expression
in SS [50] Thus, it seems likely that the regulation of other important target genes, which are shared by Stat5 and Stat3 may be similarly dysregulated in SS Future studies investigating the repertoire of target genes activated by full-length vs truncated Stat5 proteins in T cells will enable
us to better understand the functional differences between the different forms of Stat5 and their potential dysregulation
in SS Nevertheless, the preferential DNA binding of truncated Stat5 proteins and the concomitant loss of Stat5-dependent gene expression in SS patients demon-strates that truncated Stat5 proteins can behave as physio-logical dominant-negatives
Ongoing studies indicate that the dysregulated activity of
a Stat5 protease may be responsible for the elevated expression of C-terminally truncated Stat5 proteins in SS (L Hendry and S John, unpublished observations) Given the critical role of IL-2 induced Stat5 signalling in normal immune homeostasis and the maintenance of peripheral tolerance, the loss of this pathway has important
Trang 6implications for the pathogenesis of SS [20,51] Thus,
sustained expression of C-terminally truncated Stat5
pro-teins may be one mechanism adopted by indolent malignant
T cells in SS to escape apoptosis
Conclusions and perspectives
Evidence is accumulating to suggest that proteolytic
processing is a general mechanism for the negative
regula-tion of STAT protein funcregula-tion (Fig 4) Truncated forms of
Stat3, Stat5a, Stat5b and Stat6, generated by the proteolytic
cleavage of the C-termini, have been identified in progenitor
myeloid cells, mature neutrophils, mast cells and peripheral
T cells STATc proteins have different C-termini than
STATb proteins and behave as functional
dominant-negative proteins Of the STAT proteases that have been
characterized most are serine proteases, whose activities are
regulated by the developmental or activation state of cells
depending on the cellular context However, their
expres-sion and functional activities are not dependent on the
presence of the target STAT protein itself and it is likely that
other cellular targets exist for these proteases The exact
identities and mechanism of action of the individual
proteases are currently unknown but they show STAT
and cell-type specificity Future cloning of the proteases
from the different cell sources will reveal whether they
belong to a family of related serine proteases In addition,
the cysteine protease, calpain, has also been shown to
process Stat3 and Stat5 in platelets and Stat6 proteins in
mast cells, respectively, although the physiological import-ance of these findings are unknown Truncated Stat3c and Stat5c proteins generated by proteases have been shown to contribute significantly to the pathology of AML and CTCL Thus, future identification of the relevant serine proteases and their natural inhibitors from myeloid cells and
T cells will enhance our understanding of these diseases and also provide potential targets for therapeutic intervention by the rational design of drugs based on these proteins References
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