From the Center of Tsukuba Advanced Research
Alliance, § Institute of Applied Biochemistry, University of
Tsukuba, Tsukuba, Ibaraki 305-8577, Japan
Received for publication, September 27, 2002, and in revised form, December 11, 2002
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ABSTRACT |
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Previous studies have shown that FKHR, a member
of the forkhead family of transcription factors, acts as a DNA
binding-independent cofactor of nuclear receptors, including estrogen,
retinoid, and thyroid hormone receptors, in addition to the original
function as a DNA binding transcription factor that redistributes from the nucleus to the cytoplasm by insulin-induced phosphorylation. Here,
we demonstrated the physical interaction of FKHR with hepatocyte nuclear factor (HNF)-4, a member of steroid/thyroid nuclear receptor superfamily, and the repression of HNF-4 transactivation by FKHR. FKHR
interacted with the DNA binding domain of HNF-4 and inhibited HNF-4
binding to the cognate DNA. Furthermore, the binding affinity of HNF-4
with phosphorylated FKHR significantly decreased in comparison to that
with unphosphorylated FKHR. Therefore, a phosphorylation of FKHR by
insulin followed by its dissociation from HNF-4 and the redistribution
of FKHR from the nucleus to the cytoplasm would expect to induce the
transcriptional activation of HNF-4 by facilitating to the access of
HNF-4 to its DNA element. Indeed, most intriguingly, insulin
stimulation reversed the repression of HNF-4 transcriptional activity
by phosphorylation-sensitive (wild-type) FKHR, but not by
phosphorylation-deficient FKHR. These results suggest that insulin
regulates the transcriptional activity of HNF-4 via FKHR as a
signal-regulated transcriptional inhibitor.
HNF-4,1 a
member of the steroid/thyroid nuclear receptor superfamily, is a
transcriptional factor which expresses in the liver, intestine, kidney,
and pancreatic FKHR, a forkhead family member, is a transcriptional factor
and regulates the expression of multiple genes, such as key enzymes of
gluconeogenesis (10, 11). Insulin has a dynamic effect on the
localization of FKHR, when phosphorylated by Akt at the three residues
of FKHR: Thr-24, Ser-253, and Ser-316. Once phosphorylated, the
cytoplasmic retention is induced, leading to inhibit the
transcriptional activity. On the other hand, in the absence of insulin,
FKHR is dephosphorylated and localized to the nucleus, where FKHR binds to the specific DNA element, resulting in transcriptional activation of
the target genes (12-15).
Recently, it has been reported that FKHR activates or
represses the transactivation by nuclear receptor family members as a
DNA binding-independent cofactor (16, 17). In the present study, we
analyzed the interaction of FKHR with HNF-4, the effect of FKHR on the
transactivation mediated by HNF-4, and its molecular mechanism. FKHR
associates with HNF-4 in vivo and in vitro and represses the transactivation by HNF-4 through the decrease in its DNA
binding affinity. Interestingly, the inhibitory effect is canceled by
insulin, resulting from the dissociation of HNF-4 from phosphorylated
FKHR that subsequently translocates to the cytoplasm. This suggests the
possibility that HNF-4 is a novel downstream target of insulin via FKHR
as a signal-regulated transcriptional inhibitor.
Cell Culture and Transfections and Reporter Gene
Assays--
HepG2 cells were cultured in Dalbecco's modified Eagle's
medium supplemented with 10% fetal bovine serum. Transfections were performed by FuGENE-6 (Roche Molecular Biochemicals). Twenty ng of
pCMV- Plasmids--
The hHNF-4 GST Pull-down Assay--
GST-FKHR and GST-HNF-4 full-length and
deletion mutant fusion proteins were prepared as described previously
(19). [35S]Methionine-labeled FKHR was prepared by
in vitro translation with the TNT-coupled reticulocyte
lysate system (Promega) with T7 RNA polymerase. 293T whole cell
extracts that transiently expressed full-length HNF-4 or its
deletion mutants were prepared with 1xlysis buffer (0.1% Tween 20, 200 mM NaCl, 1 mM phenylmethylsulfonyl fluoride, 20 mM HEPES, pH 7.9, 1 mM
DTT). For GST pull-down assay, [35S]methionine-labeled
FKHR or 200 µg of 293T whole cell extracts were incubated with about
20 µg of GST fusion protein bound to glutathione-Sepharose beads at
4 °C for 4 h in 1 ml of the HEPES binding buffer (20 mM HEPES, pH 7.9, 200-300 mM NaCl, 1 mM EDTA, 1 mM DTT, 0.1% Tween 20, 5%
glycerol, protease inhibitors). After washing the beads with HEPES
binding buffer, the pull-down complexes were fractionated by SDS-PAGE.
The gels were performed immunoblotting with rat anti-HA monoclonal
antibody or analyzed using the imaging analyzer.
Immunoprecipitation--
Approximately 200 µg of HepG2 nuclear
extracts were immunoprecipitated with anti-FKHR antibody in IP buffer
(20 mM HEPES, pH 7.9, 100 mM NaCl, 0.1%
Nonidet P-40, 1 mM EDTA, 1 mM DTT, 5% glycerol, protease inhibitors). Immunoblotting of immunoprecipitates was done using anti-FKHR antibody or anti-HNF-4 antibody as described previously (19).
In Vitro Kinase Assay--
293T cells were seeded and
transfected with 2 µg of the constitutive active (CA) or dominant
negative (DN) form Myc-Akt expression vectors (Upstate Biotechnology).
Cell extracts were immunoprecipitated using anti-Myc antibody (PL14,
Medical Biological Laboratory). The kinase reaction was
performed using CA or DN form Myc-Akt, 2 µg of purified GST fusion
protein as a substrate, 5 µCi of [
In vitro kinase assay for GST pull-down assay was performed
with [35S]methionine-labeled FKHR wt and 3A
mutant, 0.7 mM ATP, and activated Akt (Upstate
Biotechnology) in a kinase reaction buffer. After each reaction mixture
was incubated for 2 h at 30 °C, GST pull-down assay was
performed with HEPES binding buffer (20 mM HEPES, pH 7.9, 600 mM NaCl, 1 mM EDTA, 1 mM DTT,
0.1% Tween 20, 5% glycerol, protease inhibitors).
EMSA--
In vitro translated HNF-4 was synthesized
in the TNT-coupled reticulocyte lysate system (Promega) with T7 RNA
polymerase. GST-FKHR or GST was expressed in Escherichia
coli strains BL21, purified using glutathione-Sepharose beads,
eluted with an elution buffer (50 mM Tris-HCl, pH 8.0, 20 mM reduced glutathione), dialyzed to reaction buffer (22),
and concentrated using Aquacide II (Calbiochem). The C3P
double-stranded oligonucleotides were prepared as described previously
(22). Four microliters of in vitro translated HNF-4 were
incubated with 1 µg of poly[d(I-C)] (Roche Molecular Biochemicals),
10 µg of bovine serum albumin, and GST-FKHR or GST at 4 °C for 30 min in the presence or absence of 2 µl of anti-HNF-4 antibodies
(Santa Cruz Biotechnology), subsequently added end-labeled oligonucleotide (0.14 ng, 11,000 cpm) and incubated at 4 °C for 30 min. The binding reaction was carried out as described previously (22).
The reaction mixtures were directly loaded onto 5% nondenaturing polyacrylamide gels made in 1 × TBE (90 mM Tris-HCl,
pH 8.0, 89 mM boric acid, 2 mM EDTA). After
electrophoresis was performed at 130 V for 3 h at 4 °C, the
gels were dried and analyzed with a bio-imaging analyzer.
Immunofluorescence--
HepG2 cells were plated onto glass
coverslips at 20% confluence and transfected using FuGENE-6 regents
(Roche Molecular Biochemicals). Cells were incubated in Dulbecco's
modified Eagle's medium (DMEM) contained 10% fetal bovine serum for
24 h and sequentially serum-free DMEM for 4 h and then
stimulated with insulin (100 nM) for 45 min. Cells were
fixed and permeabilized as described previously (18). After blocking
with 1% bovine serum albumin, 0.1% Triton X-100 in phosphate-buffered
saline for 30 min, cells were incubated with rat anti-HA monoclonal
antibody (1:250 dilution; 3F10, Roche Molecular Biochemicals) and
anti-FLAG monoclonal antibody (1:500 dilution; M2, Sigma), followed by
staining with Cy5-conjugated anti-rat and Cy3-conjugated anti-mouse
secondary antibodies (1:1000 dilution each; Amersham
Biosciences).
Antibodies--
Anti-HNF-4 rabbit polyclonal antibody was
generated against a GST-HNF-4-(133-366). Anti-FKHR rabbit
polyclonal antibody was generated against a GST-FKHR-(541-652).
In Vivo and in Vitro Binding of HNF-4 to FKHR--
GST pull-down
assay was conducted to test whether FKHR could interact with HNF-4.
293T whole cell extracts, transiently expressed HA-HNF-4, were
incubated with GST or GST-FKHR fusion protein. As seen in Fig.
1A, HA-HNF-4 associated with
GST-FKHR but failed to bind to GST protein. Furthermore,
coimmunoprecipitation assay was carried out to confirm the interaction
between HNF-4 and FKHR in HepG2. FKHR was immunoprecipitated with
anti-FKHR antibody from HepG2 cells, and the immune complexes were
subsequently resolved by SDS-PAGE, followed by Western blotting with
antibodies against FKHR or HNF-4. The FKHR-precipitated complexes
included HNF-4 (Fig. 1B), demonstrating the in
vivo interaction between FKHR and HNF-4.
To determine the domain of HNF-4 involved in the
interaction with FKHR, each of the HA-tagged HNF-4 N-, M-, and
C-fragments expressed in 293T was incubated with GST or GST FKHR fusion
protein (Fig. 1C). FKHR associated with the N-region but
neither with the M- nor C-region of HNF-4 (Fig. 1D). The
N-terminal fragment contains several functional domains, such as AF1, a
DNA binding domain, and a part of AF2. To define which domains of HNF-4
could associate with FKHR, it was expressed using in vitro
transcription/translation and tested for the interaction with GST-HNF-4
subregions using GST pull-down assay. As shown in Fig. 1E,
FKHR interacted with the NM fragment of HNF-4 containing the DNA
binding domain.
Mechanism of the Repression of HNF-4-mediated Transactivation by
FKHR--
To investigate the functional significance of this
association, a luciferase reporter assay using the eight copies of the HNF-4 binding element was conducted. HNF-4 enhanced promoter activity, but FKHR did not alter it (Fig.
2A). When FKHR wt was
cotransfected along with HNF-4, luciferase activity was strongly
decreased in a dose-dependent manner. The FKHR 3A mutant,
in which three putative Akt phosphorylation sites (Thr-24, Ser-253, and
Ser-316) were replaced by a nonphosphorylatable alanine residue,
resulting in the predominant localization in the nucleus, repressed the
reporter activity more dramatic than FKHR wt. These data
prompted us to propose the hypothesis that FKHR may inhibit the binding
of HNF-4 to the cognate DNA and repress the transactivation. To
determine whether FKHR could inhibit the transactivation by an HNF-4
tethered to DNA by a heterologous DNA binding domain, the luciferase
reporter assay using Gal4-HNF-4 fusion was conducted. The
transactivation by Gal4-HNF-4 fusion was not repressed by FKHR (Fig.
2B). EMSA indicated that GST-FKHR prevented HNF-4 from
binding to its recognition motif in a dose-dependent
manner, whereas a control GST protein did not (Fig. 2C).
These data suggested that the transcriptional reduction of HNF-4
results from the inhibitory effect of FKHR on HNF-4 binding to target
DNA elements.
Effects of Insulin on the Repression of HNF-4 Transactivation by
FKHR--
We tested a possible role of insulin in the repression of
HNF-4-mediated transcriptional activity by FKHR. Whereas insulin did
not alter the HNF-4-mediated transcriptional activity, insulin restored
that activity, in the presence of cotransfected FKHR wt, to 80%
of the HNF-4-mediated transcriptional activity, but not FKHR 3A mutant
(Fig. 3A). As a step toward
understanding the molecular mechanism of this restoration, we first
investigated the effect of Akt phosphorylation on HNF-4 because of
being a putative Akt phosphorylation site
(RXRXXS/T) in the NM region of HNF-4 (RDRIST:
125-130 amino acids). To determine whether HNF-4 is a substrate of Akt
in vitro, GST-full-length HNF-4 or GST-HNF4 NM region
was bacterially expressed for this assay. The CA, but not DN, form of
Akt immunoprecipitated from 293T cells effectively phosphorylated GST
FKHR as a positive control. However, CA-Akt failed to phosphorylate
GST-full-length HNF-4 or GST-HNF4 NM region in an in
vitro kinase assay (Fig. 3B), suggesting that insulin stimulation via the PI3K/Akt pathway induced the phosphorylation of
FKHR, but not that of HNF-4. We next investigated the subcellular localization of FKHR in the presence of HNF-4. We coexpressed HNF-4 and
FKHR in HepG2 cells in the absence or presence of insulin. Whereas
HNF-4 was exclusively in the nucleus when exposed to insulin, FKHR was
found to be predominantly in the cytoplasm (Fig. 3C).
To examine whether the FKHR phosphorylation by Akt affects its binding
affinity with HNF-4, we performed the GST pull-down assay using FKHR
mutants (3A, 3D, and S253D). HNF-4 bound to unphosphorylated FKHR
wt with the same affinity as the FKHR 3A mutant (Fig.
3D, left). However, the binding affinity of FKHR
wt phosphorylated by Akt with HNF-4 significantly decreased in
comparison to that of FKHR 3A mutant (Fig. 3D,
right). Next, we tested FKHR mutants, in which
Thr-24/Ser-253/Ser-316 or Ser-253 was replaced by an aspartic acid
residue, to mimic the effect of phosphorylation. The FKHR mutants (3D
and S253D) interacted with HNF-4 weaker than FKHR wt (Fig. 3,
E and F). Taken together, these findings suggest that insulin stimulation promotes the dissociation of HNF-4 from FKHR,
resulting in its subsequent retention into the cytoplasm, and reverses
the inhibitory effects of FKHR on the transactivation by HNF-4 through
facilitating the access to the target elements.
FKHR binds to specific DNA elements and activates its
transcription in the nucleus. Once cells are exposed to extracellular signals, such as insulin and insulin-like growth factor 1 (IGF1), PI3K-regulated kinase Akt phosphorylates FKHR, thereby inducing the
exit of FKHR from the nucleus and the repression of
FKHR-dependent transcription (12-15, 23-26). It has been
shown that FKHR is able to function as a DNA binding-independent
transcriptional cofactor of nuclear receptors such as estrogen,
retinoid, and thyroid hormone receptors in addition to the original
function as a DNA binding transcription factor (16, 17). In this
report, FKHR was identified as a cofactor for HNF-4. Our data showed
that FKHR associated with the DNA binding domain of HNF-4 and inhibited
the access of HNF-4 to its binding element, leading to repress HNF-4
mediated transactivation. Moreover, one striking finding is that
insulin stimulation reversed the repression by FKHR wt, but not
by the FKHR 3A mutant, suggesting that the reverse regulation might
result from insulin dependent shuttling of FKHR from the nucleus to the cytoplasm. It has been reported that the localization of FKHR is
altered through a phosphorylation by various kinases, such as CK1,
DYRA1A, and SGK in addition to Akt (23, 24, 27-29). These observations
raise the possibility that HNF-4 may be a "downstream target" of
diverse signals mediated by the above kinases via FKHR.
Recent reports have shown that the DNA binding activity of HNF-4 is
modulated by diet in vivo. Namely, the HNF-4 DNA binding activity in nuclear extracts of fasted rat liver was markedly reduced
in comparison with that of refed rat liver in EMSA, at least resulting
from the phosphorylation of HNF-4 by PKA (9). We illustrated that the
HNF-4 DNA binding activity was decreased by the interaction of HNF-4
with FKHR. It is known that the physiological effects of fasting are
mediated by the activation of cAMP-dependent PKA,
coincident with the inactivation of Akt by the decrease in insulin
secretion. Therefore, because of not only the phosphorylation of HNF-4
by PKA but also the reinforced interaction of HNF-4 with FKHR resulting
from the translocation of FKHR to the nucleus, the DNA binding activity
of HNF-4 to the target DNA elements might be decreased in the fasted states.
The primary target tissues for insulin are muscle, adipose tissue, and
liver. Tissue-specific knock-out of the insulin receptor in muscle
failed to produce diabetes, but disruption of the gene in the liver
exhibited a diabetic phenotype, suggesting that insulin actions in the
liver play central roles in glucose homeostasis (30, 31). Insulin
decreases transcription of the genes encoding gluconeogenic enzymes and
increases transcription of those encoding glycolytic enzymes in the
liver. A number of studies have established that FKHR activates the
transcription of gluconeogenic enzymes in the absence of insulin and
inhibits the transcription in the presence of insulin. On the other
hand, HNF-4 positively regulates the genes involved in glucose
transport and glycolysis (4, 6). Here, we provided a novel mechanism
that insulin reversed the repression of HNF-4 transcriptional activity
by FKHR. One might expect that in the fasted state, FKHR as an
inhibitor of HNF-4 represses glycolysis, whereas FKHR as a DNA
binding transcription factor enhanced gluconeogenesis, leading to
glucose secretion. In contrast, in the fed state, because FKHR
phosphorylated by insulin translocates into the cytoplasm following the
dissociation from HNF-4, HNF-4 that is released from FKHR promotes the
glucose transport and glycolysis, leading to glucose uptake. The
identification of an insulin-regulated transcriptional inhibitor, FKHR,
might assist in clarifying a molecular mechanism of glucose homeostasis.
INTRODUCTION
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ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
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DISCUSSION
REFERENCES
-cells (1, 2). It contains several functional
domains: a ligand-independent activation domain (AF1), a zinc finger
DNA binding domain, and a ligand-dependent activation
domain (AF2) (3). HNF-4 binds to a specific DNA element as a homodimer
and regulates the expression of many genes, involved in glucose, fatty
acid, and cholesterol metabolisms (4-6). The blood glucose levels are
controlled by the balance of two opposing hormones, glucagon and
insulin. Whereas glucagon decreases the activity of HNF-4, the effect
of insulin on that of HNF-4 has not fully been understood yet
(7-9).
EXPERIMENTAL PROCEDURES
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ABSTRACT
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EXPERIMENTAL PROCEDURES
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DISCUSSION
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gal plasmid were included in each transfection experiment to
control for the efficiency of transfection. To ensure equal DNA
amounts, empty plasmids were added in each transfection. The luciferase
activity was measured with an ARVOTMSX (Wallac Berthold). The
values were normalized to
-galactosidase activity as an internal control.
2 cDNA was subcloned into
pcDNA3 tagged with the HA epitope (pcDNA3HA). A series of HNF-4
deletion fragments were generated by PCR and subcloned into pGEX 4T-1
(Amersham Biosciences) or pcDNA3HA. The pGAL4-HNF-4 vector
was made by subcloning hHNF-4
2 cDNA tagged with the Gal4
DNA binding domain (amino acids 1-147) of pGBT9
(Clontech) at the N terminus into pcDNA3. pGAL4
control vector, pHNF4-tk-Luc, pG5b-Luc, pcDNA3-FLAG-FKHR-wild-type,
and 3A mutant were described previously (18-20). A new versatile PCR strategy was used to generate pcDNA3-FLAG-FKHR-3D mutant and
pcDNA3-FLAG-FKHR-S253D mutant (21).
-32P]ATP, and 20 µM ATP in a kinase reaction buffer (20 mM
HEPES, pH 7.9, 20 mM
glycerophosphate, 10 mM
MgCl2, 1 mM DTT, 50 µM Na3VO4)
for 1 h at 30 °C. The reaction products were resolved by
SDS/PAGE and analyzed with a bio-imaging analyzer.
RESULTS
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ABSTRACT
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DISCUSSION
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Fig. 1.
In vivo and in vitro
binding of HNF-4 to FKHR. A, in vitro
binding assay, using 293T whole cell extracts that expressed HA-tagged
full-length HNF-4 or an empty control plasmid, with GST protein alone
or GST FKHR fusion protein. Bound proteins were analyzed by SDS-PAGE
gel electrophoresis and subjected to immunoblotting with anti-HA
antibody (12CA5, Roche Molecular Biochemicals). B,
coimmunoprecipitation of endogenous HNF-4 using anti-FKHR
antibody. Nuclear extracts from HepG2 cells were
immunoprecipitated with anti-FKHR antibody or preimmune as a control.
C, a schematic diagram of HNF-4 and its deletion mutants
used in this study. Hatched area, activation function 1;
DBD, DNA binding domain. D, in vitro
interaction of GST-FKHR with HNF-4. Whole cell extracts from 293T cells
transfected with HA tagged HNF-4 deletion mutants were incubated with
GST protein alone or GST FKHR fusion protein. Bound proteins were
analyzed as described in A. E, in
vitro binding assay using in vitro translated
35S-labeled FKHR and GST or GST-HNF-4 deletion
mutants. Bound proteins were analyzed by SDS-PAGE using the imaging
analyzer.
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Fig. 2.
Mechanism of the repression of HNF-4 mediated
transactivation by FKHR. A, repression of HNF-4
activity by FKHR. HepG2 cells were cotransfected with 100 ng of
HNF4-tk-Luc reporter plasmid and 3 ng of HNF-4 expression plasmid
together with 10 or 20 ng of the expression vectors for either FKHR
wt or 3A mutant. The results are presented as arbitrary units (1 × 105). All values represent the mean of triple samples.
B, effects of FKHR on the transactivation of GAL4-HNF-4.
HepG2 cells were transfected with 100 ng of pG5b-Luc reporter plasmid
and 3.3 ng of GAL4 or GAL4-HNF-4 expression plasmid together with 10 or
20 ng of FKHR expression plasmid. The results are presented as
arbitrary units (1 × 104). All values represent the mean of
triple samples. C, inhibition of HNF-4 binding to its
cognate DNA by FKHR. In vitro translated HNF-4 was incubated
with the 32P-labeled probe in the presence or absence of
bacterially expressed GST-FKHR.
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Fig. 3.
Effects of insulin on the repression of HNF-4
transactivation by FKHR. A, the reverse
regulation of repression of HNF-4 activity by FKHR. HepG2 cells were
cotransfected with 100 ng of HNF4-tk-Luc reporter plasmid and 3.3 ng of
HNF-4 expression plasmid together with 2.5 ng of the expression vectors
for either FKHR wt or 3A mutant. Following transfection, cells
were incubated in DMEM supplemented with 10% fetal bovine serum for
24 h and then serum-starved for 9 h in the presence or
absence of 100 nM insulin. Results are presented as
relative to the reporter activity observed in the presence of HNF-4.
B, phosphorylation by Akt. A constitutive active CA or DN
form of Akt expressed in 293T cells was incubated in the presence of
[32P]ATP and 2 µg of GST-FKHR, GST-full-length
HNF-4, or GST-HNF4 NM region. C, subcellular
localization of FKHR and HNF-4 in the presence or absence of insulin.
HepG2 was cotransfectioned with HA-HNF-4 and FLAG-FKHR WT or 3A mutant.
Sublocalization of HNF-4 and FKHR were analyzed by immunofluorescent
using anti-HA (3F10) and anti-FLAG (M2) antibody. D,
in vitro binding of phosphorylated or
phosphorylated-deficient FKHR to HNF-4. Left,
35S-labeled in vitro translated FKHR wt
or 3A mutant was subjected to GST pull-down assay. Results are
presented as relative to the intensity observed in 2% input.
Right, after 35S-labeled in vitro
translated FKHR wt or 3A mutant was incubated with ATP and Akt,
they were subjected to GST pull-down assay. FKHR phosphorylation by Akt
induces a shift up in the mobility of FKHR in SDS-PAGE gel. The
phosphorylated and unphosphorylated FKHR are indicated by * and **,
respectively. Results are presented as relative to the intensity
observed in 2% input. E and F, in
vitro binding of FKHR 3D or S253D mutant to HNF-4. Upper
panel, 35S-labeled in vitro translated FKHR
wt or 3D or S253D mutant was subjected to GST pull-down assay.
Lower panel, results are presented as relative to the
intensity observed in 1% input.
DISCUSSION
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ABSTRACT
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ACKNOWLEDGEMENTS |
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We thank the Fukamizu laboratory members for helpful discussion and encouragement.
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FOOTNOTES |
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* This work was supported by grants from The 21st Century COE Program, grant-in-aid for Scientific Research (A), and grant-in-aid for Scientific Research on Priority Areas from the Ministry of Education, Science, Sports and Technology of Japan, "Research for the Future" Program (The Japan Society for the Promotion of Science: JSPS-RFTF 97L00804), and by a Research Grant for Cardiovascular Diseases (11C-1) and Comprehensive Research on Aging and Health from the Ministry of Health, Labor, and Welfare.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
¶ Research fellow of the Japan Society for the Promotion of Science.
To whom correspondence should be addressed: Center for Tsukuba
Advanced Research Alliance, Inst. of Applied Biochemistry, University
of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan. Tel.:/Fax: 81-298-53-6070; E-mail: akif@tara.tsukuba.ac.jp.
Published, JBC Papers in Press, January 7, 2003, DOI 10.1074/jbc.C200553200
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ABBREVIATIONS |
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The abbreviations used are: HNF, hepatocyte nuclear factor; HA, hemagglutinin; GST, glutathione S-transferase; DTT, dithiothreitol; CA, constitutive active; DN, dominant negative; EMSA, electrophoretic mobility shift assay; DMEM, Dulbecco's modified Eagle's medium; PI3K, phosphatidylinositol 3-kinase; AF2, activation function 2.
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