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Molecular and Cellular Biology, October 1998, p. 5852-5860, Vol. 18, No. 10
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
A Sequence of the CIS Gene Promoter Interacts Preferentially with
Two Associated STAT5A Dimers: a Distinct Biochemical Difference
between STAT5A and STAT5B
Frédérique
Verdier,1
Raquel
Rabionet,1
Fabrice
Gouilleux,1
Christian
Beisenherz-Huss,2
Paule
Varlet,1
Odile
Muller,1
Patrick
Mayeux,1
Catherine
Lacombe,1
Sylvie
Gisselbrecht,1 and
Stany
Chretien3,*
Institut Cochin de Génétique
Moléculaire (ICGM), Institut National de la Santé et de la
Recherche Médicale (INSERM U363), Hopital Cochin,
Université René Descartes, F75014
Paris,1 and
Institut National de la
Transfusion Sanguine (INTS-GIP), F75015 Paris,3
France, and
Tumor Biology Center, Institute for
Experimental Cancer Research and Department of Biology, University
of Freiburg, 79106 Freiburg, Germany2
Received 13 February 1998/Returned for modification 3 April
1998/Accepted 17 July 1998
 |
ABSTRACT |
Two distinct genes encode the closely related signal transducer and
activator of transcription proteins STAT5A and STAT5B. The molecular
mechanisms of gene regulation by STAT5 and, particularly, the
requirement for both STAT5 isoforms are still undetermined. Only a few
STAT5 target genes, among them the CIS (cytokine-inducible SH2-containing protein) gene, have been identified. We cloned the human
CIS gene and studied the human CIS gene promoter. This promoter
contains four STAT binding elements organized in two pairs. By
electrophoretic mobility shift assay studies using nuclear extracts of
UT7 cells stimulated with erythropoietin, we showed that these four
sequences bound to STAT5-containing complexes that exhibited different
patterns and affinities: the three upstream STAT binding sequences
bound to two distinct STAT5-containing complexes (C0 and C1) and the
downstream STAT box bound only to the slower-migrating C1 band.
Using nuclear extracts from COS-7 cells transfected with
expression vectors for the prolactin receptor, STAT5A, and/or
STAT5B, we showed that the C1 complex was composed of a STAT5 tetramer
and was dependent on the presence of STAT5A. STAT5B lacked this
property and bound with a stronger affinity than did STAT5A to the four
STAT sequences as a homodimer (C0 complex). This distinct biochemical
difference between STAT5A and STAT5B was confirmed with purified
activated STAT5 recombinant proteins. Moreover, we showed that the
presence on the same side of the DNA helix of a second STAT sequence
increased STAT5 binding and that only half of the palindromic STAT
binding sequence was sufficient for the formation of a STAT5 tetramer.
Again, STAT5A was essential for this cooperative tetrameric
association. This property distinguishes STAT5A from STAT5B and could
be essential to explain the transcriptional regulation diversity of
STAT5.
 |
INTRODUCTION |
STAT proteins are latent
transcription factors containing a Src homology 2 domain (SH2 domain)
that become activated by tyrosine phosphorylation. The binding of the
STAT SH2 domains to the phosphorylated cytokine receptors allows their
tyrosine phosphorylation by Jak kinases. After dimerization
and nuclear translocation, STAT dimers bind to specific DNA
sequences, thereby allowing downstream gene regulation. Seven
members of the STAT family have been described (STAT1
/
,
STAT2, STAT3
/
, STAT4, STAT5A, STAT5B, and STAT6), and specific
combinations are involved in the signaling pathways of different
cytokines. The association of STAT5 with various coactivators or
repressors and the binding of additional transcription factors to the
promoter region of target genes, together with various combinations of
STAT dimers, could contribute to the diversity and specificity of the
transcriptional regulation of target genes (for reviews, see references
4, 11, and 26).
STAT5, originally named mammary gland factor (MGF), has been described
as a positive regulator of the
-casein (
CAS) promoter by
prolactin (29). In addition to prolactin, STAT5 proteins have been shown to be activated by erythropoietin (Epo), growth hormone, interleukin 2 (IL-2), IL-3, IL-5, IL-7, IL-9, IL-15, granulocyte-macrophage colony-stimulating factor (GM-CSF),
thrombopoietin, and several other growth factors. However, only a few
STAT5 target sequences have been identified, among them the
CAS promoter (8), the IL-2 receptor alpha (IL-2R
)
chain enhancer (15), p21 (WAF1) (21),
oncostatin M (34), the serine protease inhibitor 2.1 (31), and the CIS gene promoter (20). Different
MGF boxes located inside these promoters are implicated in
transcriptional regulation following STAT5 activation.
Besides positive regulatory pathways, negative regulatory
pathways such as phosphatases modulate the response to cytokines. The phosphatase SHP-1 has been shown to bind to both the phosphorylated Epo receptor (Epo-R) and the Jak2 kinase and to dephosphorylate these
proteins, thereby leading to inactivation of Epo-R signaling (12). However, under certain conditions, STAT5 itself or
carboxy-terminally truncated isoforms of STAT5 act as negative
regulators of gene transcription (1, 19, 23, 24, 30).
Another negative regulatory mechanism involves the protein CIS. The CIS
protein is rapidly induced in hematopoietic cells by IL-2, IL-3,
GM-CSF, and Epo. CIS contains an SH2 domain in the central part
of the molecule and is associated with the cytoplasmic domains of the tyrosine-phosphorylated Epo-R and the
chain of IL-3R
(35). CIS overexpression reduces the activation of promoters
regulated by STAT5. Moreover, STAT boxes were shown to be involved in
the Epo-dependent promoter activation of the murine CIS promoter in HEK
293 cells (20).
In this report, we identified different nuclear factors which bound to
the human CIS gene promoter. This complex combination of
transcriptional factors comprised STAT5A, STAT5B, an Sp1-related family protein, and at least three GGAA binding proteins, among which
one showed an Epo-dependent DNA binding capacity. Among the four STAT5
boxes of the CIS promoter, only one bound to a STAT5 tetramer.
Moreover, we showed that two STAT5A dimers can interact in cooperative
binding with two STAT binding sequences of low or undetectable affinity
to form a tetramer. This structure involves STAT5A but not STAT5B.
The difference in the behavior of the two isoforms of STAT5 could be
essential for the differential transcriptional regulation of STAT5
target genes.
 |
MATERIALS AND METHODS |
Reagents.
Anti-STAT5 antibodies, directed against the amino
terminus of the sheep STAT5A and recognizing both STAT5A and
STAT5B, were produced as described previously (7). We
produced specific antibodies directed against STAT5A and STAT5B by
immunizing rabbits with peptides corresponding to the 12 carboxy-terminal amino acids (AGLFTSARSSLS) of STAT5A or the 8 carboxy-terminal amino acids of STAT5B (QWIPHAQS) coupled to keyhole
limpet hemocyanin (data not published). Rabbit polyclonal anti-Elf-1
and anti-Ets-1/2 were purchased from Santa Cruz Biotechnology (Santa
Cruz, Calif.). Antibodies directed against the amino-terminal domain of
PU.1/Spi-1 were kindly provided by F. Moreau-Gachelin (Institut Curie,
Paris, France). Anti-GABP
and -
antibodies were kindly provided
by M. Negishi (National Institutes of Health, Bethesda, Md.)
(33). Highly purified recombinant human Epo (specific
activity, 120,000 U/mg) was a generous gift from M. Brandt (Boehringer
Mannheim), and prolactin was purchased from Sigma.
Isolation of human CIS promoter.
The murine CIS cDNA from
nucleotides 111 to 1092 (GenBank accession no. MUSSH2DC)
(35) was cloned by reverse transcriptase PCR with total RNA
from IW32 erythroleukemia cells with a sense primer
(5'-CTCCTTCCATCCCGCCGAA-3') and an antisense primer
(5'-CCTGCCTTGTTCTTGCTGGCA-3').
A human placenta genomic DNA library cloned in the cosmid pWE 15 (Clontech; reference no. HL 1095m) was screened with the murine CIS
cDNA PCR probe.
Cell cultures.
The GM-CSF-dependent human megakaryoblastic
cell line UT7 (13) was passaged by dilution twice weekly in
alpha minimal essential medium containing 10% fetal calf serum
containing 2.5 ng of GM-CSF per ml. COS-7 cells (ECCAC no. 87021302)
were maintained in Dulbecco's modified Eagle's medium supplemented
with 10% fetal calf serum.
Preparation of nuclear extracts.
Starved cells were
stimulated at 37°C with 10 U of Epo per ml and quickly chilled in
ice-cold phosphate-buffered saline. Cells were pelleted and solubilized
with buffer A (buffer A: 20 mM HEPES; 10 mM KCl; 1 mM EDTA; 1 mM
dithiothreitol [DTT]; 1 mM phenylmethylsulfonyl fluoride; 0.1 mM
Na2VO4; 0.2% Nonidet P-40; 10% glycerol; 1 µg each of aprotinin, pepstatin, and leupeptin per ml, pH 7.9). Cell lysates were centrifuged at 20,000 × g for 2 min, and
the pellets were extracted with buffer B (buffer B: 20 mM HEPES; 350 mM
NaCl; 10 mM KCl; 1 mM EDTA; 1 mM DTT; 1 mM phenylmethylsulfonyl
fluoride; 0.1 mM Na2VO4; 20% glycerol; 1 µg
each of aprotinin, pepstatin, and leupeptin per ml, pH 7.9) with 1 ml
of buffer B for 5 × 107 cells. The extracts were
centrifuged at 20,000 × g for 5 min, and supernatants
were quickly frozen and stored at
80°C.
Production of recombinant activated STAT5 protein in Sf9
cells.
Sf9 cells grown in a spinner flask suspension culture were
coinfected with baculoviruses encoding either STAT5A and Jak2 or STAT5B
and Jak2 with a multiplicity of infection of 10 PFU/cell for each
virus. At 60 h after infection, cells were harvested and lysed for
15 min on ice in a buffer containing 10% glycerol, 20 mM HEPES (pH
7.9), 10 mM KCl, 0.2% Nonidet P-40, 1 mM EDTA, 0.1 mM sodium vanadate,
2 mM DTT, 4-(2-aminoethyl)-benzesulfonyl fluoride, phenylmethylsulfonyl
fluoride, leupeptin, and aprotinin. Proteins were loaded on a heparin
column (Pharmacia) and eluted with an NaCl gradient in lysis buffer.
Electrophoretic mobility shift assay (EMSA).
Two microliters
of nuclear extracts was mixed with 20 µl of binding buffer [10 mM
Tris-HCl, 50 mM NaCl, 1 mM EDTA, 1 mM DTT, 0.1% Nonidet P-40, 5%
glycerol, 1 mg of bovine serum albumin per ml, 2 mg of poly(dI-dC) per
ml, pH 7.5] containing 60,000 cpm of end-labelled probe, and the
mixture was incubated for 30 min at 4°C. Complexes were separated on
6% nondenaturing polyacrylamide gels in 0.25× Tris-borate-EDTA and
detected by autoradiography.
UV cross-linking of CIS12 probe to complexes C0 and C1.
Two
probes (A and B) were used in cross-linking experiments.
Single-stranded oligonucleotides (25 ng) CIS12 (antisense) and CIS2
(sense) (probe A) or CIS12 (sense) and CIS1 (antisense) (probe B) were
annealed. Flanking sequences were transcribed by the Klenow fragment of
DNA polymerase under standard conditions in the presence of 250 µM
dATP-dGTP-5-bromo-2'-dUTP-80 µCi of [
-32P]dCTP
(3,000 Ci/mmol). Probes were purified by Sephadex G-25 chromatography,
and 2.5 ng of probes was incubated with UT7 nuclear extracts under EMSA
conditions. After separation of the different complexes on a 6%
nondenaturing polyacrylamide gel, the gel was irradiated for 15 min
with UV light and autoradiographed. The bands were cut out and placed
on the top of a sodium dodecyl sulfate (SDS)-10% polyacrylamide gel.
After migration of the gel, labelled proteins were detected by
autoradiography.
Transfections and luciferase assays.
All plasmids were
purified by the Qiagen kit method (Qiagen, Inc.), and two different
preparations of plasmid for each construct were tested. Plasmids (1 µg/transfection) of different promoter-luciferase constructs cloned
in the pGL2-basic vector (Promega) were introduced in COS-7 cells
together with 1 µg of the pCMV-gal plasmid (internal control for
transfection efficiency). After 24 h of culture with the
appropriate medium, half of the transfected cells were stimulated with
1 µg of prolactin per ml. Total cell extracts were prepared and used
for determination of luciferase and
-galactosidase activities according to the manufacturer's instructions (Promega kit). Final luciferase activity results were obtained after normalization with
-galactosidase activity.
 |
RESULTS |
Localization and structural analysis of the human CIS gene
5'-flanking region: constitutive and cytokine-induced nuclear
factors bound to the CIS gene promoter.
The murine CIS
cDNA probe was used to screen a human genomic library as described in
Materials and Methods. A cosmid, COS-CIS, that contained the
full-length CIS gene was isolated (data not shown). Eight hundred
sixty-five base pairs of the murine CIS gene promoter sequences have
been previously reported by Matsumoto et al. (20). The
nucleotide sequence of the 5'-flanking region of the human CIS gene was
compared with this sequence. Computer alignments revealed that regions
conserved between the two species began at position
646 of the murine
CIS sequence (Fig. 1) and displayed 70%
identity. Two blocks of 50 to 60 nucleotides were highly conserved:
nucleotides
376 to
324 and
262 to
198 (human sequence) had 94 and 91% identity, respectively. Each conserved region contained two
STAT binding consensus sequences (CIS3 and CIS4 and CIS1 and CIS2 at
positions
361 and
350 and
249 and
232, respectively [Fig.
1]). These STAT boxes and the distance between CIS3 and CIS4 (2 bases)
and between CIS1 and CIS2 (8 bases) were totally conserved between the
human and murine species.

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FIG. 1.
Nucleotide sequence comparison of human and murine gene
5'-flanking regions. The Kanehisa program was used to compare the human
(upper) and murine (lower) 5'-flanking sequences of the CIS gene. The
murine sequence corresponds to the sequence published by Matsumoto et
al. (20). These sequences are numbered such that +1 refers
to the translation initiation codon of the CIS protein. Positions of
transcription start sites are indicated by arrows. The consensus
binding sites for STAT and SP1 are boxed.
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Double-stranded oligonucleotides corresponding to individual STAT boxes
or to a cluster of two STAT boxes were used in gel
retardation assays
to characterize the nuclear proteins able to
bind to these sequences.
All the names, sequences, and positions
of the oligonucleotides used in
this study are indicated in Fig.
2. The
four STAT boxes of the CIS gene promoter were compared
with the

CAS
oligonucleotide sequence: the CIS1 sequence TTCTAGGAA
was
identical to the

CAS STAT binding site while the three other
STAT
boxes contained different nucleotides at positions 4 and
5.

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FIG. 2.
(A) Synthetic oligonucleotides used in bandshift assays.
Double-stranded oligonucleotide probes containing STAT binding sites
were end labelled and tested for protein binding by EMSA. Sequences and
positions of the upper-strand oligonucleotides used as probes in EMSAs
are indicated. The consensus STAT binding sequence TTCNNNGAA is
underlined. (B) Schematic representation of the 5'-flanking region of
the human CIS gene. Boxes and circles indicate the positions of STAT
and SP1 potential binding sequences, respectively. Positions of
transcription start sites are indicated by arrows. CIS1 to CIS4
correspond to the names of the four STAT binding oligonucleotides
described for panel A.
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EMSAs were performed with nuclear extracts from resting and
Epo-stimulated UT7 cells. The same mobility complex (C0) was obtained
with the

CAS and the CIS1 probes. This complex, which is known
to
contain a dimer of STAT5, was supershifted with antibodies
against
STAT5. Thus, CIS1 bound specifically to STAT5 but with
a weaker
affinity than

CAS (Fig.
3A; compare
lanes 2 and 5).
CIS3, CIS4, and CIS34 oligonucleotides displayed
similar Epo-induced
retarded bands: a complex migrating like the
STAT5-

CAS complex
(C0) and a DNA-protein complex of lower mobility
(C1) (Fig.
3B,
lanes 8, 10, and 12). A different pattern of protein-DNA
complexes
was detected with the CIS2 and CIS12 probes: the C0 complex
was
absent, but the Epo-induced complex C1 could be visualized, the
CIS12 probe displaying a higher affinity for complex C1 than CIS2
(Fig.
3B, lanes 4 and 6). Two prominent bands of higher mobility
were also
detected with these two probes. One band contained a
constitutive C2
DNA-protein complex, and the lower band contained
at least two
complexes of similar mobilities, C3 and C4 (Fig.
3B, lanes 3 to 6; see
also Fig.
6A, lanes 2 [C4] and 3 [C3+C4]),
C3 being detected only
in nuclear extracts from Epo-stimulated
cells.

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FIG. 3.
Gel shift assays of DNA complexes bound to STAT binding
oligonucleotides of the human CIS gene promoter. Nuclear extracts from
Epo-stimulated UT7 cells were incubated with
32P-labelled probes. DNA-protein complexes were
analyzed in nondenaturing acrylamide gels and revealed by
autoradiography. Free probes are not visible on these autoradiographs.
(A) Identification of DNA-protein complexes which bind to the CIS1
probe (lanes 1 to 3). CAS probe was used as a control (lanes 4 and
5). An antibody directed against STAT5 (7) was added to
nuclear extracts in lane 3. The numbers above the panel show time in
minutes. (B) Identification of DNA-protein complexes which bind to the
CIS2 (lanes 3 and 4), CIS12 (lanes 5 and 6), CIS3 (lanes 7 and 8), CIS4
(lanes 9 and 10), and CIS34 (lanes 11 and 12) probes. CAS probe was
used as a control (lanes 1 and 2). The position and the name of each
specific DNA-protein complex (C) are indicated on the right by an
arrow. Numbers above the panel show time in minutes. (C) Nuclear
extracts from UT7 cells stimulated with Epo for 15 min were incubated
with the clustered STAT binding probes CIS12 (lanes 1, 4, and 6) and
CIS34 (lanes 2, 5, and 7). Competition assays with a 50-fold molar
excess of unlabelled double-stranded CAS probe were performed (lanes
4 and 5). For lanes 6 and 7, anti-STAT5 serum was added to the EMSA
reaction.
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Identification of STAT5 nuclear factors.
To identify the
nuclear factors bound to the CIS34 and CIS12 probes, we performed
EMSA in the presence of an excess of unlabelled oligonucleotides
and supershift assays with various antibodies. Competition assays with
unlabelled
CAS oligonucleotide showed that C2, C3, and C4 complexes
were not affected (Fig. 3C, lane 4). In contrast, Epo-induced C1 and C0
complexes totally disappeared, suggesting that the C1 and C0 complexes
both contained STAT5 (Fig. 3C, lanes 4 and 5), as confirmed by
supershift experiments with STAT5-specific antibody (Fig. 3C, lanes 6 and 7).
Two highly homologous STAT5 genes encoding STAT5A and STAT5B are
expressed in many tissues and cell lines (
16,
17,
25).
To
determine which STAT5 protein bound to the STAT binding sequences
of
the CIS gene promoter, COS-7 cells were cotransfected with
vectors
directing the expression of STAT5A or STAT5B and the prolactin
receptor. After prolactin stimulation, nuclear extracts
(COS-STAT5A
and COS-STAT5B) were tested for DNA binding activity.
As STAT5A
and STAT5B were independently transfected, we verified that
both
proteins were expressed and phosphorylated at similar levels.
When
EMSAs were performed with extracts from COS-7 cells transfected
with STAT5A, strong C0 and C1 complexes were detected with the
CIS34
probe (Fig.
4A, lane 1). In contrast,
STAT5B-transfected
cell extracts induced a C0-like complex (Fig.
4A,
lane 2). When
extracts from COS-7 cells transfected with STAT5A and
STAT5B were
mixed, we detected the same retardation pattern as that
with UT7
nuclear extracts (Fig.
4A, lane 3, and 3B, lane 12).

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FIG. 4.
Specific binding patterns of STAT5A and STAT5B for the
CIS probes. COS-7 cells transfected with expression vectors for the
prolactin receptor, STAT5A, or STAT5B were treated with prolactin, and
nuclear extracts were prepared (COS-STAT5A and COS-STAT5B,
respectively). Bandshift assays were carried out with
32P-labelled CIS34 probe (lanes 1 to 3 [A]), CIS12 probe
(lanes 4 to 6 [A] and 1 to 9 [B]), CIS1 probe (lanes 7 and 8 [A]), or CIS2 probe (lanes 9 and 10 [A]) in the presence of
different combinations of nuclear extracts (COS-STAT5A,
COS-STAT5B, and UT7) as indicated by a + in the figure (++
corresponds to a double quantity of nuclear extracts).
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With the CIS12 probe, different results were obtained under
the same EMSA conditions (COS-7 reconstituted model): STAT5A
bound
strongly to the CIS12 probe and appeared as a C1 complex (Fig.
4A, lane 4) whereas a C0-like complex was formed with
COS-STAT5B
nuclear extracts (Fig.
4A, lane 5). We also
observed a faint C1
complex with STAT5B. However, when EMSA was
performed with purified
recombinant STAT5B, C1 complex was not detected
(see below and
Fig.
5C, lane 5). COS-7 cells were then tested for
endogenous
STAT5 expression by Western blot analysis with specific
anti-STAT5A
and anti-STAT5B antibodies. Both proteins were detected at
a low
level (data not shown). Therefore, the weak C1 complex
observed
in COS-7 cells could result from endogenous STAT5A
expression.
When COS-STAT5A and COS-STAT5B extracts were mixed,
a main C1 complex was detected (Fig.
4A, lane 6), as observed
with UT7
nuclear extracts (Fig.
3B, lane 6).
In conclusion, the CIS12 probe preferentially bound to STAT5A and the
C1 complex seemed to depend on the presence of STAT5A.
With the CIS2
probe, we observed the same complexes, C1 with STAT5A
and C0 with
STAT5B, although STAT5B showed a stronger affinity
for this probe than
did STAT5A (Fig.
4A, lanes 9 and 10). In the
same conditions, the CIS1
probe bound only to STAT5B (Fig.
4A,
lanes 7 and 8).
Finally, addition of UT7 nuclear extracts which contain phosphorylated
STAT5A/B heterodimers to COS-STAT5B extracts led to
the
disappearance of the C0 complex and to an increase of the
C1
complex on the CIS12 probe (Fig.
4B, compare lanes 2 and 5).
When
COS-STAT5A nuclear extracts were mixed with UT7 nuclear extracts,
the intensity of the C1 band decreased while C2, C3, and C4 formation
increased (Fig.
4B, lanes 3 and 7). This could result from the
high
expression in UT7 cells of proteins forming C2, C3, and C4
complexes.
These proteins bind to a DNA sequence which overlaps
with a STAT
binding site and could compete with STAT5 proteins
for binding to the
CIS12 probe. Increasing the amount of COS-STAT5A
and STAT5B
nuclear extracts augmented the C1 complex (Fig.
4B,
compare lanes
5 with 6 and 7 with 8, respectively). On the other
hand, increasing the
amount of COS-STAT5B nuclear extracts resulted
in the appearance of
the C0 complex (Fig.
4B, lane 6). In conclusion,
these data suggest
that the C0 complex contains STAT5B homodimers
whereas the C1 complex
is dependent on the binding of STAT5A.
To demonstrate the tetrameric structure of C1 complexes, we first
analyzed the nature of the proteins bound to the CIS12 probe
by UV
cross-linking. Two kinds of CIS12 probes were used, probes
A and B,
which contained 5-bromo-2'-dUTP in regard to CIS2 or
CIS1, respectively
(see Materials and Methods and Fig.
5A). EMSAs
performed with
Epo-stimulated UT7 extracts revealed that probe
A bound the same
complexes as did the CIS2 probe and that probe
B was able to also bind
C0. This could be explained by the fact
that CIS1 but not CIS2 binds to
complex C0 (Fig.
3A and B). The
presence of 5-bromo-2'-dUTP in regard
to the CIS1 binding site
could stabilize C0 complexes on the CIS1
sequence. After UV irradiation
of the gel, the complexes C1-probe A,
C1-probe B, and C0-probe
A were resolved on an SDS-polyacrylamide gel.
In all cases, the
cross-linked complexes appeared as a single band of
about 120
kDa which corresponded to the molecular mass of STAT5 (90 kDa)
after subtraction of the mass of the DNA probe (about 30 kDa for
35 bp) (Fig.
5B). These data indicate
that the only protein of
the C1 complex that binds to CIS12 DNA is
STAT5.

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FIG. 5.
(A and B) UV cross-linking of STAT5 complexes to CIS12
probe. Two double-stranded DNA probes comprising CIS1 and CIS2 were
used. The first contained 5-bromo-2'-dUTP in regard to the CIS2 binding
site (probe A), and the other contained it in regard to CIS1 (probe B)
(see Materials and Methods). (A) EMSAs were carried out with UT7 cell
extracts stimulated for 15 min. The complexes C1-probe A, C1-probe B,
and C0-probe B were cut out after UV irradiation of the gel. (B) These
complexes were electrophoresed on an SDS-10% polyacrylamide gel, and
the cross-linked complexes were visualized by autoradiography.
Molecular masses of the marker proteins are shown in kilodaltons. (C)
Effect of the disruption of STAT5 protein interactions by deoxycholate
(DOC). EMSA reactions were performed with the CIS12 probe in the
presence of purified activated STAT5A (lanes 1 to 4) or STAT5B (lanes 5 to 8). Specific binding of STAT5 proteins to CIS12 probe was determined
by supershift assays with specific antibodies directed against STAT5A
(lane 1) and STAT5B (lane 8). Different concentrations of deoxycholate
were added in the binding reaction mixtures as indicated. DNA-protein
complexes were analyzed in nondenaturing acrylamide gels and revealed
by autoradiography.
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To confirm that C1 was composed of a tetramer of STAT5A, we then
performed EMSAs with purified recombinant STAT5A and STAT5B
proteins. Figure
5C shows that recombinant STAT5A induced C1
complexes
with the CIS12 probe, confirming that no additional proteins
were
necessary for the formation of C1. Only the dimeric C0 complex
was
detected with recombinant STAT5B. Finally, to demonstrate
that the C1
complex depended on interactions between two STAT5A
dimers
and did not result from the independent binding of two
STAT5A dimers,
we used deoxycholate, which disrupts protein interactions,
and tested
this compound on STAT5 complex formation. Increasing
the
concentration of this detergent reduced the amount of C1 complex
(Fig.
5C, lanes 2 to 4). Altogether, these data demonstrate that
STAT5
proteins are sufficient for the formation of a C1 complex
and that this
complex results from an interaction between two
STAT5A dimers.
Analysis of C2, C3, and C4 complexes.
The identity of the
constitutive C2 and C4 complexes and of the Epo-induced C3 complex was
first studied by competition assays with unlabelled oligonucleotides on
the CIS12 probe (Fig. 6A). A 50-fold
molar excess of CIS12 oligonucleotide resulted in the complete
disappearance of all C complexes (Fig. 6A, lane 4); similar results
were observed with an excess of CIS2 oligonucleotide, although
competition was weaker for C2 and C4 complexes (Fig. 6A, lane 6); in
contrast, the same amount of oligonucleotides CIS1 and
CAS competed
only for the binding of STAT5-containing complex C1 (Fig. 6A, lanes 5 and 7). The failure of oligonucleotides CIS1 and
CAS to compete for
the binding of C2, C3, and C4 to CIS12 confirmed that these three
complexes bound to the
240-to-
223 sequence of the human CIS gene
promoter (Fig. 2B). The CIS12 sequence contains two 5'-GGAA-3' or
5'-TTCC-3' motifs known to be the core binding site of Ets factors. To
determine if CIS12 was able to bind Ets proteins, an optimized Ets
probe from the Drosophila E74 promoter (3) was
used as a competitor. This oligonucleotide competed for the binding
of C2, C3, and C4 (Fig. 6A, lane 8); similar results were obtained with
the CIS2 probe (data not shown). As a control for specificity, we used
as a competitor the oligonucleotide EtsM, in which the Ets binding
sequence GGAA was mutated into a CCAA sequence (Fig. 6A, lane 9). In
conclusion, the three complexes C2 to C4 recognize the motif
5'-GGAA-3' and could be related to different members of the Ets family
or to GGAA binding proteins. The CIS2 STAT binding sequence overlaps
with two Ets sequences (Ets-b and Ets-c [Fig. 6B]) and is also
involved in Epo-induced STAT5 C1 complex formation as described above.
However, competition with the Ets E74 oligonucleotide did not affect
the formation of the C1 complex.

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FIG. 6.
Analysis of DNA-protein complexes which bind to the
CIS12 probe. Nuclear extracts from Epo-stimulated UT7 cells were
incubated with a 32P-labelled CIS12 probe. (A)
Identification of specific DNA-protein complexes by competition
experiments. Unlabelled competitors (50-fold molar excess) were added
to EMSA mixtures as follows: no competitor (lane 3) and double-stranded
oligonucleotides CIS12, CIS1, and CIS2; CAS; an optimized Ets probe
from the Drosophila E74 promoter (3); and EtsM,
in which the Ets binding sequence GGAA was mutated to CCAA (lanes 4 to
9, respectively). A CAS probe was used as a control (lane 1). Lane 2 contains nuclear extracts from unstimulated UT7 cells. (B) Mutated
CIS12 oligonucleotide bandshift assays. (Top) Nuclear extracts from
Epo-stimulated UT7 cells were incubated with the
32P-labelled CIS12 probe (lanes 1 and 7 to 13) and with
32P-labelled mutated CIS12 M1 to M5 probes (lanes 2 to 6, respectively). Competition assays were performed for lanes 8 to 13 with
a 50-fold molar excess of probes as indicated in the figure. (Bottom)
Alignment of wild-type CIS12 and mutated M1 to M5 sequences. The
consensus STAT binding sequence TTCNNNGAA is underlined, and the Ets
domains are boxed.
|
|
Identification of the C1 to C4 binding sites.
In order to
determine the sites required for each protein-DNA interaction, we
examined the effects of selected nucleotide modifications in the two
STAT binding sequences (CIS1 and CIS2) or in the three Ets sites
(Ets-a, -b, and -c). For this purpose, mutated oligonucleotides,
shown at the bottom of Fig. 6B, were used as probes or competitors in
bandshift assays with the CIS12 probe (Fig. 6B). As a result, C1
binding was not affected by mutations which modified the NNN sequence
of STAT5 consensus (M1 mutant) or destroyed the 3' part of the
palindromic proximal CIS2 STAT box and the Ets-c sequence (M2 mutant)
(Fig. 6B, lanes 2, 3, 9, and 10). However, the M3 and M5 mutants,
containing GG-to-TT and TT-to-AA substitutions that affected either the
3' binding site of the distal CIS1 STAT box and the Ets-a sequence (M3
mutant) or the 5' CIS1 STAT-binding sequence (M5 mutant), evidenced a markedly decreased affinity for the C1 complex (Fig. 6B, lanes 4, 6, 11, and 13). This M4 mutant, carrying a TT-to-CC substitution that
affected the 5' binding part of the proximal STAT box (CIS2) and the
Ets-b site, was unable to bind C1 (Fig. 6B, lane 5). The faint C0 band
detected with this mutant probably corresponded to the binding of STAT5
to the CIS1 sequence (Fig. 6B, lanes 5 and 12). In conclusion, both the
CIS1 distal STAT box and the 5' half of the proximal STAT site seemed
to cooperate to bind C1 with high affinity. Moreover, Ets-b was also
involved in the binding of C2 and C3: the M4 mutant did not bind C2 or
C3 (Fig. 6B, lanes 5 and 12), and the M1 and M2 oligonucleotides, in
which the Ets-b environment or the Ets-b site itself was changed, did not compete for the binding of C2 and C3 to the Ets-b sequence (Fig.
6B, lanes 9 and 12). Interestingly, the M2 mutation, in which the
second guanine of the Ets-c motif was changed to an adenine, restored a
new Ets sequence with increased affinity for C2, C3, and C4 (Fig. 6B,
lane 3). Using several anti-Ets antibodies, we identified C2 as
GABP
/
-containing complex (data not shown).
A 500-bp human CIS gene promoter shows a basal transcriptional
activity and is transactivated by STAT5.
To determine the role of
STAT5A and STAT5B on the minimal CIS promoter, the
404CIS-LUC
construct was transfected in COS-7 cells in combination with
different expression vectors carrying the prolactin receptor, STAT5A,
and STAT5B (Fig. 7). We compared the
activity of the CIS gene promoter with that of the
CAS gene promoter
(
CAS-LUC) (5). Promoter activities were examined in the
absence or presence of prolactin stimulation. In the absence of STAT5,
the
CAS-LUC construct was inactive; in contrast, the
404CIS-LUC
construct evidenced a basal activity which was not increased by
prolactin (Fig. 7, lane 1). In the presence of STAT5, prolactin
treatment stimulated the luciferase activity of both reporter
constructs to approximately the same level; however, if results were
expressed in fold induction, the
CAS gene promoter was much more
inducible than the CIS gene promoter. This difference results from the
basal activity of the
404CIS promoter in the absence of prolactin.
Similar results were obtained for UT7 cells grown in the absence or
presence of Epo (data not shown). Both STAT5A and STAT5B enhanced
reporter gene transcription, and STAT5B alone was more efficient than
STAT5A (Fig. 8, lanes 2 and 3). The
luciferase activity was identical in cells transfected with STAT5A and
STAT5B and in those with STAT5A alone (Fig. 7, lane 4). The
specificity of STAT5 transactivation was demonstrated by the
expression of STAT5 dominant-negative mutants which lacked the
C-terminal transactivation region (STAT5-
749 and STAT5-
754) (23). As shown in Fig. 7, lanes 5 and 6, basal expression of
404CIS-LUC was still observed in the presence of these STAT5 dominant-negative mutants. Altogether, these data show that the
404-to-
1 CIS promoter region is constitutively active in the absence of cytokine stimulation and that STAT proteins are responsible for the increased activity induced by prolactin or Epo stimulation.

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FIG. 7.
STAT5 transactivates the minimal CIS gene promoter in a
reconstituted COS-7 model. COS-7 cells were cotransfected with
the CIS gene promoter ( 404CIS-LUC) or with the CAS gene
promoter-luciferase reporter constructs ( CAS-LUC) (lanes 1 and 2)
together with vectors encoding the prolactin receptor and STAT5A,
STAT5B, or two dominant-negative forms of STAT5 which lack the
C-terminal transactivation region (STAT5- 749 and STAT5- 754)
(23), as indicated in the figure (lanes 1 to 6). To
normalize the transfections, a -galactosidase gene was cotransfected
in all cases. Luciferase activities were measured after cell
stimulation with (COS-PRL-R Prolactin) or without (COS-PRL-R)
prolactin. Values were normalized to the -galactosidase activities
and were the means of three independent transfection assays.
|
|

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FIG. 8.
Schematic representation of the binding of STAT5A and
STAT5B to the CIS1 and CIS2 proximal STAT sequences of the CIS gene
promoter. This figure summarizes the abilities of STAT5A and STAT5B to
bind to CIS1, CIS2, or CIS12. Each STAT sequence is composed of the
palindromic sequence TTCNNNGAA and is represented by two fused
boxes. The affinity of the DNA-protein interactions is represented by
small vertical bars, and the level of this affinity is determined as
follows: no bar, no interaction; one bar, weak interaction; two bars,
strong interaction. The figure represents seven EMSA experiments (lanes
1 to 7) performed with nuclear extracts from COS-7 cells that were
transfected with expression vectors for prolactin, STAT5A, or STAT5B
and induced with prolactin. The left column represents the DNA
complexes with their affinity-binding levels, and the right column
indicates the corresponding complexes referenced in previous EMSA
figures. STAT5A binding to CIS12 M1 and M2 mutants, in which the 3' end
of the CIS2 palindromic sequence was destroyed, is represented in lane
7. The differences in the DNA complexes observed in the presence of
STAT5A and those observed with STAT5B and other information developed
in the discussion led us to hypothesize an association between two
STAT5A homodimers.
|
|
Similar studies were performed with the

292CIS-LUC construct, which
contained only the two proximal STAT5 sites (CIS1 and
CIS2): in both
UT7 and COS-7 cell lines, the level of luciferase
activity was
reduced by about 50% in comparison with the activity
of the

404CIS-LUC construct. We observed a basal activity without
hormone
stimulation and similar levels of STAT5 transactivation
as observed
with the

404CIS-LUC construct. Different mutants
of this construct in
which the two STAT boxes were mutated (separately
or together) were
also tested. However, no significant difference
could be observed due
to the low level of activity of these constructs
(data not shown).
 |
DISCUSSION |
A specific combination of transcriptional factors plays an
essential role in the activation of genes induced by cytokines. The
study of the CIS gene promoter has revealed the binding of a specific
association of ubiquitous and cytokine-induced transcription factors,
among which we identified STAT5A and STAT5B in close association with
ubiquitous GABP
and -
members (band C2) (data not shown) and an
Epo-induced protein (band C3). These GGAA proteins and STAT5 bind to
the same DNA region, and their binding sites overlap in the 5' part of
the CIS2 STAT box.
The promoter sequence at position
258 to
224 (CIS12), which
contains two MGF boxes localized in the same DNA helix face (8 bp
between each element), is the most interesting because this DNA
fragment is able to discriminate between STAT5A and STAT5B binding.
Indeed, in a reconstituted COS-7 model, we showed that the distal
STAT consensus sequence (CIS1) bound with a low affinity to STAT5B
homodimer (C0) but not to STAT5A (Fig. 8, lanes 1 and 2); in
contrast, the second element (CIS2) bound to a STAT5B homodimer with
good affinity (C0) and weakly to a STAT5A tetramer (C1) (Fig. 8, lanes
3 and 4). We show for the first time that a single STAT box can bind
STAT5 only in a tetrameric structure. When EMSAs were performed with an
oligonucleotide containing these two clustered STAT elements (CIS12
probe), we observed an increase in STAT5A binding (C1) revealing
cooperative binding of two STAT5A dimers (Fig. 8, lane 5). This CIS12
probe bound with high affinity to STAT5A and to STAT5B, and specific
shifts were observed with each STAT5 member: there was a low-mobility
DNA complex (C1 band) for STAT5A, whereas the fast-migrating C0 protein
complex contained STAT5B, demonstrating that STAT5B was in a dimer
organization (Fig. 8, lane 6). The formation of C1 with STAT5A strongly
suggested that only STAT5A tetramers bound to this probe with high
affinity. Competition experiments as well as the use of recombinant
STAT5 proteins favored this STAT5A tetramerization. Moreover,
disruption of the C1 complex generated with recombinant STAT5A proteins
by deoxycholate indicated a direct interaction between two dimers of
STAT5A.
To understand how STAT5 interacted with the CIS12 DNA sequence, we
determined the sequences required for the formation of the C1 complex
with mutated DNA sequences. These studies indicated that the 3' part of
the proximal CIS2 STAT element was not necessary, while the CIS1 STAT
box increased STAT5 binding on the CIS12 probe (Fig. 8, lane 7). In
conclusion, two neighboring MGF boxes binding (CIS2) or not binding
(CIS1) to STAT5A could constitute a high-affinity binding site for a
tetramer of STAT5A. The different patterns of STAT5 shifts associated
with CIS12, CIS1, or CIS2 probes, indicated in Fig. 8, could be
explained by the stronger DNA binding affinity of STAT5B. Thus, the
absence of binding of STAT5A to the CIS1 probe, which contains a
consensus STAT binding motif, emphasizes the importance of the
neighboring sequences of the STAT binding site (10, 14).
Because we used an excess of probe, the faintness of the C1 complex
observed with the CIS2 probe could result from a competition between
STAT5 and Ets-related proteins for binding to the same sequence. STAT5A
tetramer formation was strongly increased by the presence of the CIS1
sequence. Cooperative binding has been described for STAT4 and STAT1:
Xu et al. (32) and Vinkemeier et al. (28) have
shown that the amino-terminal domains of these STATs mediated STAT
interaction and led to cooperative binding on clustered nonconsensus
STAT boxes. More recently, Meyer et al. (22) described the
binding of STAT5A dimers and tetramers on two adjacent STAT boxes of
the IL-2R
enhancer sequence; moreover, by proteolytic clipping of
recombinant STAT5A proteins, they demonstrated that the N-terminal
domain of STAT5A was required for tetramerization. Our own data
indicate that STAT5B did not participate in tetramerization. Alignment of the 190 N-terminal amino acids of STAT5A and STAT5B showed
only 17 differences. Creation of chimeric STAT5A-STAT5B proteins would
be informative in localizing the amino acids crucial for STAT5A
tetramerization.
In this paper, we demonstrate, for the first time, a functional
difference in the binding properties of STAT5A and STAT5B. The
cooperative binding of STAT5A, leading to a new molecular complex,
could contribute to creating a functional diversity of STAT5
regulation: (i) the balance between the activated forms of STAT5A and
STAT5B in the cell could modify the combination of the DNA binding
complexes on target STAT5 genes, thereby explaining the modulation of
cytokine-activated genes during cell differentiation or their
tissue-specific expression; and (ii) a low-STAT5-affinity site near a
half-STAT consensus site or a STAT sequence which does not bind STAT5
could allow cooperative STAT5 binding to these sites. Among published
target STAT5 sequences, either a second STAT sequence or half a
sequence (TTC or GAA) is frequently observed, as in the IL-2R
enhancer (see above and the work of Meyer et al. [22])
and the hepatic serine protease inhibitor 2.1 promoter (2).
A TTC motif close to the proximal MGF box of the
CAS promoter is
conserved among species (9) and allows the formation of
STAT5 tetramer (2).
Individual target disruptions of STAT5A and STAT5B have been realized
(6, 18, 27). In EMSAs performed by Feldman et al.
(6), disruption of STAT5A led to the disappearance of the C1
tetramer protein complex on the
CAS probe. This observation is in
accordance with our own results and implicates STAT5A, but not STAT5B,
in C1 complex generation. This change in protein-DNA binding pattern
was correlated in vivo with the inhibition of CIS expression in
STAT5A-null mice (6). However, our studies with the minimal
404CIS and
292CIS promoter region did not reflect endogenous CIS
expression regulation. Indeed, promoter studies showed that the
404CIS or
292CIS promoter was active without STAT activation and
that a modest transactivation by STAT5 could be observed (Fig. 7 and
data not shown). Therefore, the decrease in CIS expression in
STAT5A-null mice cannot be reconstituted in vitro by promoter studies,
indicating that other domains could regulate the expression of the CIS
gene in vivo: one of these could be a sequence containing four
potential STAT boxes found 8 kb downstream of the CIS gene (data not
shown). Studies of the CIS promoter and particularly of the CIS12
binding proteins have been worthwhile. However, functional promoter
studies with
404CIS-LUC (Fig. 7),
292CIS-LUC, or mutants of these
constructions (data not shown) transfected in different cell lines did
not clearly demonstrate the functional role of STAT5 tetramerization
mediated by STAT5A. Interestingly, however, functional studies of other promoters strongly transactivated by STAT5 have shown that
disruption of one of the two STAT5 binding sites led to a decrease or
an absence of their transactivation (2, 22). Therefore,
these data and ours showing a distinct biochemical difference between STAT5A and STAT5B are in agreement in demonstrating a functional role
for STAT5 tetramerization mediated by STAT5A.
 |
ACKNOWLEDGMENTS |
This work was supported by grants from the Agence Française
du Sang (AFS contract no. 3FS08), from the Association pour la Recherche sur le Cancer (ARC contract no. 1373), and from the Ligue
contre le Cancer. Frédérique Verdier is supported by Glaxo Wellcome Laboratories.
We acknowledge Xavier Tatare (ESTBA student) for excellent technical
assistance. We are grateful to Emmanuel Gomas and Franck Letourneur for DNA sequencing. We thank M. Negishi for
anti-GABP
and -
antibodies.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Institut
National de la Transfusion Sanguine (INTS-GIP), 6 rue Alexandre
Cabanel, F75015 Paris, France. Phone: 33 1 46 33 14 09. Fax: 33 1 46 33 92 97. E-mail: chretien{at}cochin.inserm.fr.
 |
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Molecular and Cellular Biology, October 1998, p. 5852-5860, Vol. 18, No. 10
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
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