From the
Division of Molecular Genetics,
Institutes for Enzyme Research and
Department of
Ophthalmology, Graduate School of Medicine, University of Tokushima, 3-18-15
Kuramoto, Tokushima City, Tokushima 770-8503, Japan, the
Institute of Molecular and Cellular
Biosciences, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032,
Japan, and the ||Division of Signal Transduction,
Department of Medicine, Beth Israel Deaconess Medical Center, Boston,
Massachusetts 02215
Received for publication, February 27, 2003 , and in revised form, April 15, 2003.
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ABSTRACT |
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INTRODUCTION |
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Following insulin stimulation, insulin-receptor substrate proteins are phosphorylated, after which phosphoinositide 3-kinase (PI3-kinase) is activated. The pleckstrin homology (PH) domain of Akt binds to the lipid products of PI3-kinase and thereby mediates recruitment to the membrane in response to PI3-kinase activation (5, 7). The translocation of Akt allows for phosphorylation at Thr308 by another Ser/Thr protein kinase, 3-phosphoinositide-dependent protein kinase 1 (8, 9) and at Ser473 by Akt itself (autophosphorylation) (10) or an as yet unidentified protein kinase (11, 12).
With regard to insulin action, Akt was shown to phosphorylate and inhibit
glycogen synthase kinase 3 (GSK3) and subsequently promote glycogen synthesis
in response to insulin (13).
However, regarding another important insulin action, which is the stimulation
of glucose uptake by the GLUT4 translocation, the role of Akt remained
controversial (14,
15). In addition, although the
participation of atypical protein kinase C (i.e. PKC and
PKC
), which is also thought to be activated by phosphoinositides and
3-phosphoinositide-dependent protein kinase 1, in glucose uptake in 3T3-L1
adipocytes has been proposed
(16), the opposite result has
also been reported in the case of the same cell type
(17). Thus, the role of
atypical PKC also seems to be controversial.
An adenoviral gene transfer technique now in common use is thought to be a powerful strategy to introduce high levels of transgene expression into mammalian cells (18). The technique facilitates expression of inactive Akt in cells to overcome endogenous Akt activity, dominant negatively (14, 15). According to other authors who have examined the role of Akt in glucose metabolism (14, 15), Akt-2A (Akt-T308A/S473A) and Akt-AAA (Akt-K179A/T308A/S473A) have been used. We used Akt-MAA (Akt-K179M/T308A/S473A) in the present study, since we considered that Met (for Lys179; so-called catalytic Lys) has a relatively large side chain and is likely to impair kinase activity by occupying the ATP-binding pocket more efficiently compared with Ala as well as losing a positive charge for the stabilization of ATP binding and a hydrogen bond with the Glu in helix C.
RNA interference (RNAi) is a process whereby double-stranded RNA (dsRNA) induces the degradation of cognate mRNA for prevention of the expression of alien genes. Originally, the RNAi phenomenon was discovered in Caenorhabditis elegans; however, recent studies revealed that such phenomena are evolutionarily conserved from plants to mammals (1923). According to recent data, duplexes of 21-nucleotide short interference RNA (siRNA) with 2-nucleotide 3'-overhangs are the most efficient triggers of sequence-specific mRNA degradation (24, 25). One fascinating aspect of RNAi is its extreme efficiency, since only a few trigger dsRNA molecules introduced into the cells suffice to inactivate a continuously transcribed cognate target mRNA for long periods of time (22). Although the introduction of dsRNA into mammalian cells has been known to cause RNA-dependent protein kinase-mediated apoptosis, the siRNA (e.g. 21-nucleotide dsRNA) has been shown not to cause such an effect (24). This important observation revealed new possibilities whereby one could prove gene function in mammalian cells.
In attempts to elucidate the roles of Akt in insulin actions, we used both adenoviral overexpression and RNAi in both CHO cells and 3T3-L1 adipocytes. Since it is unclear which isoforms of Akt are responsible for each insulin action, we investigated the role of each isoform in insulin actions using a specific RNAi against each Akt isoform.
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EXPERIMENTAL PROCEDURES |
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Synthetic siRNA and Its PreparationAll DNA/RNA chimeric oligonucleotides were purchased from Hokkaido System Science (Sapporo, Japan) and annealed to be double-stranded, as described (23). The sequences of control siRNA against GFP are as follows: 5'-CUGGAGUUGUCCCAAUUCCTT-3' and 5'-AGAAUUGGGACAACUCCAGTT-3' (the 2-nucleotide overhanging of 2'-deoxythymidine is indicated as TT and denoted by underlines) (23). The sequences of all other specific siRNAs against Akt are indicated in Fig. 3, A and C, and Fig. 6A.
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PlasmidsHA-tagged rat Akt2 and Akt3 were PCR-amplified
using a rat brain cDNA library as a template and the following
oligonucleotides as primers:
5'-GGACTAGTGCCATGTACCCATACGATGTGCCAGATTACGCCAATGAGGTATCTGTCATCAAG-3'
(HA tag denoted by
underline)/5'-GGATCGATCACTCTCGGATGCTGGCTGAAGTAGGAGAACTG-3'
(stop codon is denoted in boldface type) and
5'-GGACTAGTGCCACCATGTACCCATACGATGTGCCAGATTACGCCAGCGATGTTACCATCGTTAAAG-3'
(the region of HA tag denoted by
underline)/5'-GGATCGATTATAGTGGACACTTTTCAGGTGGTGTTATTG (stop codon
is denoted in boldface type), respectively. Amplified PCR fragments were
subcloned into pCR2.1-TOPO (Invitrogen) using the TA cloning method according
to the manufacturer's protocol, their DNA sequences were confirmed, and they
were subcloned into pCX2 (EcoRI site) and pEGFP-C1
(CloneTech; NheI and HindIII sites; enhanced green
fluorescence protein clone was removed), respectively. Mammalian expression
constructs encoding HA-tagged rat Akt1 wild type and rat Akt1-2A (T308A/S473A)
were kindly provided by Dr. W. Ogawa (Kobe University, Kobe, Japan)
(14). The dominant negative
form of rat Akt-MAA (Akt-MAA: K179M/T308A/S473A) was made using QuikChange
kits (Stratagene) with rat Akt1-2A (as a template) and the following
oligonucleotides:
5'-cgctactatgccatgatgatcctcaagaaggagg-3'/5'-cctccttcttgaggatcatcatggcatagtagcg-3'.
Constitutively active Akt was generated by inserting an oligonucleotide duplex
encoding the c-Src myristoylation signal (MGSSKSKPKDPSQRRS,
5'-GATCCACCATGGGGAGCAGCAAGAGCAAGCCCAAGGACCCCAGCCAGCGCA-3',
5'-GATCTGCGCTGGCTGGGGTCCTTGGGCTTGCTCTTGCTGCTCCCCATGGTG-3')
upstream of human Akt1. The oligo-DNA duplex was initially subcloned into
pCR-blunt (Invitrogen) at BamHI and BglII sites, and then a
fragment of human Akt1 (29),
digested with BamHI, was subcloned into the vector that had been
digested with BglII. Finally, myr-Akt1 was digested with
BamHI and subcloned into pAdTrackCMV, an adenovirus transfer vector,
for adenovirus generation. FLAG-tagged FKHRL1 was kindly provided by Dr. M.
Greenberg (Harvard Medical School, Boston, MA)
(30). BP1-luc was provided by
Dr. T. Unterman (31). pRL-SV40
vector was from Promega (Madison, WI). Mammalian expression plasmids for
PKC and PKC
were kindly provided by Dr. S. Ohno (Yokohama City
University, Yokohama, Japan)
(17).
Cell CultureCOS-7 cells and Chinese hamster ovary (CHO) cells stably expressing Myc-tagged rat glucose transporter 4 (GLUT4myc) were maintained in either Dulbecco's modified Eagle's medium (Invitrogen) with 10% fetal calf serum (Invitrogen) or F-12 culture medium (Biological Industries, Kibbutz Beit Haemek, Israel) with 10% fetal calf serum, respectively (32). A 3T3-L1 fibroblast was maintained in Dulbecco's modified Eagle's medium (Invitrogen) supplemented with 10% calf serum (Invitrogen) and differentiated into 3T3-L1 adipocytes stably expressing GLUT4myc (3T3-L1-GLUT4myc), as described (32). More than 90% of the cells expressed the adipocyte phenotype (i.e. lipid droplets). Human brain tumor-derived U251-MG cells were provided by the Health Science Research Resources Bank of Japan (Sen-nan, Osaka, Japan) and were maintained in Dulbecco's modified Eagle's medium with 10% fetal calf serum as above.
Preparation of Recombinant AdenovirusThe recombinant adenovirus encoding either dominant-negative (DN) Akt (Akt-MAA: K179M/T308A/S473A) or constitutively active (CA) Akt (myr-Akt1) was generated using the AdEasy system, as described (18). The recombinant adenoviruses were amplified in 293 cells. The 50% tissue culture infectious dose (TCID50) was determined as pfu/ml, and either CHO cells or 3T3-L1 adipocytes were infected at the multiplicities of infection (MOI; pfu/cell), as indicated.
Adenoviral Gene TransductionCHO-GLUT4myc cells or 3T3-L1 adipocytes (the 8th day after initiation of differentiation) were incubated with OPTI-MEM (Invitrogen) containing the indicated MOI of adenovirus for 2 h with tilting every 30 min; we then added the growth medium and incubated the preparation for 32 h followed by serum starvation with serum-free medium (i.e. F-12 or Dulbecco's modified Eagle's medium) for 16 h. Experiments were performed 48 h after the infection.
Akt in Vitro Kinase AssayAkt kinase activity was measured
using AKTide-2T (28) as a
substrate, as described (33).
Cells seeded on either six-well or 24-well plates were incubated in serum-free
medium for 16 h and then stimulated with 100 nM insulin for 10 min.
Cells were washed once with PBS and lysed in solubilizing buffer containing 20
mM Tris-HCl (pH 7.5), 140 mM NaCl, 1 mM
MgCl2, 1 mM CaCl2, 0.5 mM
Na3VO4, 50 mM -glycerophosphate, 20
mM Na4P2O7, 20 mM NaF,
10% (v/v) glycerol, 1% (v/v) Nonidet P-40, 1 mM dithiothreitol, and
1 mM phenylmethylsulfonyl fluoride. After centrifugation at 15,000
x g for 10 min at 4 °C, the obtained cell lysates were
immunoprecipitated with the anti-Akt antibody
(26). Immunoprecipitated
enzyme was incubated in 30 µl of reaction mixture containing 20
mM HEPES-NaOH (pH 7.4), 100 mM NaCl, 10 mM
NaF, 10 mM
-glycerophosphate, 0.1 mM
Na3VO4, 10 mM MgCl2, 0.5
mM EGTA, 1 µM protein kinase inhibitor, 1
mM dithiothreitol, 5 µM cold ATP, 3 µCi of
[
-32P]ATP, and 50 µM AKTide-2T for 10 min at
37 °C. The reactions were terminated by adding 10 µl of 8 N
HCl and spotted onto P81 phosphocellulose paper (Whatman, Rockland, MA) and
then washed five times with 0.5% phosphoric acid. Radioactivity was measured
using a liquid scintillation counter.
GLUT4 TranslocationInsulin-stimulated GLUT4 translocation to plasma membrane was examined by determination of the exposure of Myc epitope (GLUT4myc) to the outer surface of plasma membrane in response to insulin, as described (34). In brief, cells on 24-well plates were incubated in 500 µl of Krebs-Ringer-HEPES buffer for 20 min at 37 °C and then stimulated with 100 nM insulin for 10 min at 37 °C. After fixation with 2% paraformaldehyde, cells were washed with PBS and then incubated with 100 mM glycine/PBS for 15 min. After the blocking, cells were incubated with 300 µl of anti-Myc antibody (9E10; 1:1000 dilution) for 2 h, washed with PBS, and incubated with 300 µl of horseradish peroxidase-conjugated anti-mouse IgG (1:2000 dilution) at room temperature for 1 h. The wells were then washed with five times with PBS. GLUT4myc translocation was determined and analyzed by enhanced chemiluminescence and a Luminescencer-JNR luminometer (AB-2100; ATTO, Tokyo, Japan).
2-Deoxy-D-glucose UptakeInsulin-stimulated 2-deoxy-D-glucose uptake was measured based on the determination of 2-deoxy-D-[1,2-3H]glucose incorporation into the cell, as described (34).
Glycogen Synthesis AssayInsulin-stimulated glycogen synthesis was measured based on the determination of D-[6-3H]glucose incorporation into the glycogen fraction, as described (34).
ImmunoblottingFor immunoblotting, we used the indicated antibodies, and the signals were detected using horseradish peroxidase-mediated chemiluminescence, as described (35).
Luciferase AssayThe specific (against Akt; siAKTc) and nonspecific (against GFP; siGFP) synthetic 21-mer dsRNAs (total of 0.5 µg/well) were co-transfected with FKHRL1 (0.01 µg/well), BP1-luc (0.5 µg/well), and pRL-SV40 (0.1 µg/well) into either COS-7 cells or 3T3-L1 adipocytes plated on a 24-well plate using LipofectAMINE 2000 (Invitrogen). After 48 h of transfection, luciferase activity was measured using dual luciferase reporter assay kits (Promega, Madison, WI). A PI3-kinase inhibitor, LY294002 (100 µM), was also used for 1 h before cell harvest. Normalized relative luciferase activity (Photinus pyralis luciferase/Renilla reniformis luciferase activity) was expressed. The S.D. value of R. reniformis luciferase activity was within 5% of the average value.
Statistical AnalysisThe data are represented as mean ± S.E. p values were determined by Bonferroni-Dunncan's multiple comparison test, and a p < 0.05 value was considered to be statistically significant.
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RESULTS |
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We next examined the effect of DN-Akt on insulin actions in these cells. As shown in Figs. 1E and 2E, DN-Akt adenovirus efficiently inhibited insulin-stimulated Akt kinase activity and insulin-stimulated GLUT4 translocation, 2-DG uptake, and glycogen synthesis in an MOI-dependent manner in both cell lines. In the case of 3T3-L1 adipocytes, although DN-Akt (at MOI 80) inhibited Akt activity almost completely, insulin-stimulated metabolic activities, including 2-DG uptake and glycogen synthesis, still remained, which suggests an additional pathway in 3T3-L1 adipocytes (see "Discussion"). Expression of DN-Akt had little effect on either phosphorylation of Erk-1/2 (by immunoblotting with phosphospecific antibody) or activation of PI3-kinase activity (by in vitro kinase assay) in insulin-stimulated cells (data not shown).
RNAi-mediated Gene Silencing of AktWe then carried out RNAi-mediated gene silencing using a synthetic 21-mer oligonucleotide RNA duplex (siRNA) to determine the requirement of Akt for insulin actions. We designed a pair of oligonucleotides corresponding to the sequence of Akt1 that matches maximally with the other two Akt isoforms (rat, mouse, and human) and named "siAKTc," as shown in Fig. 3A. There were a few mismatches within the corresponding sequences of rat Akt3, human Akt3, and mouse Akt2 and Akt3. COS-7 cells were then co-transfected with the siAKTc and Akt expression plasmid that also expressed GFP driven by an independent CMV promoter as well as CMV-driven HA-tagged rat Akt1. As shown in Fig. 3B, the introduction of synthetic oligo-RNA (siAKTc) suppressed the expression of HA-tagged rat Akt1, but not GFP, in a dose-dependent manner. In our preliminary examination using fluorescein isothiocyanate-labeled siAKTc, siRNA was observed in the nucleus within 1 h after transfection (data not shown). To test the specificity of the siRNA, we also designed another two pairs of oligo-RNA duplexes that contain either single (siAKTc-1m) or triple (siAKTc-3m) mismatches against the target sequence of siAKTc, as shown in Fig. 3C. As shown in Fig. 3D, we then co-transfected siRNAs with either HA-tagged Akt1 or Akt2 or an Akt3 expression construct. Although siAKTc-1m had only a single mismatch, the siAKT-1m did not show any RNAi effect. The siAKT-3m as well as siAKT-1m also showed no effect. Consistent with this, since siAKTc has a single mismatch against rat Akt3, it showed no RNAi effect (Fig. 3D). Thus, we confirmed the specific silencing against the cognate target gene by siRNA.
To determine whether siRNA would affect endogenous Akt activity, we performed the FOXO3a/FKHRL1 reporter assay that has been shown to be a model system reflecting endogenous Akt activity (30, 31, 36). According to reported data, Akt phosphorylates and down-regulates the function of forkhead transcription factors, including FOXO3a/FKHRL1, and subsequently inhibits IGF-I-BP (insulin-like growth factor I-binding protein) reporter activity (37) shown as a scheme in Fig. 4A. As shown in Fig. 4, B and C, siAKTc as well as LY294002 (PI3-kinase inhibitor) enhanced luciferase activities of IGF-I-BP1 in COS-7 cells and 3T3-L1 adiopocytes in a dose-dependent manner. Thus, we also confirmed that the siRNA silenced endogenous Akt as well as exogenously transfected Akt.
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Effect of siRNA on Insulin ActionsTo elucidate the roles of Akt on insulin actions, we introduced siRNA into cultures of CHO cells and 3T3-L1 adipocytes. Before the experiments, since the half-life of protein levels of Akt was thought to be a key factor to determine the optimal incubation period from the transfection to biological assays, we estimated that the half-life of endogenous Akt protein was around 4 h in cycloheximide-treated CHO cells (data not shown). As shown in Fig. 5A, siAKTc, but not control siRNA (siGFP), down-regulates endogenous protein kinase activity of Akt in both quiescent and insulin-stimulated cells. In parallel with this, siAKTc inhibited insulin-stimulated GLUT4 translocation to plasma membrane, 2-DG uptake, and glycogen synthesis in CHO cells. We also observed the effect of siAKTc on GLUT-4 translocation in an immunocytochemical analysis (data not shown).
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Since 3T3-L1 cells were a mouse-derived line, siAKTc could silence only Akt1 and not Akt2, as shown in Fig. 8E, because of a mismatch (see Fig. 3A). However, as shown in Fig. 5, the suppression of Akt1 in 3T3-L1 adipocytes by siAKTc could suppress all of these insulin actions only weakly compared with CHO cells yet significantly.
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Regarding the expression of Akt3 in these cells, we did not observe the expression of Akt3 by immunoblotting using an anti-Akt3-specific antibody, whereas the expression could be seen in the lysate from brain-derived U251MG cells, as shown in Fig. 8I.
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Isoform-specific Gene Silencing of AKT by siRNAThree Akt
isoforms (Akt1/PKB, Akt2/PKB
, and Akt3/PKB
) are known, and
we carried out isoform-specific silencing using isoform-specific siRNA against
each Akt isoform, as summarized in Fig.
6A. We introduced each RNAi as well as nonspecific
control siRNA (i.e. siGFP) with the indicated expression constructs,
including serum- and glucocorticoid-inducible protein kinase, PKC
, and
PKC
, into COS-7 cells. As shown in Fig.
6B, each siRNA clearly silenced only each cognate gene.
We also examined the effect of isoform-specific siRNA on endogenous isoforms
of Akt in both CHO cells and 3T3-L1 adipocytes as shown in
Fig. 8, A and
E. Although the expression of Akt3 in both lines was not
detectable (Fig. 8I),
we did detect a significant expression of both Akt1 and Akt2 in these lines,
and each isoform-specific siRNA did inhibit each cognate target gene (Figs.
7 and
8, A and E). The siAKTc silenced Akt2 in CHO cells but not in 3T3-L1 adipocytes. Thus, we
demonstrated the sequence-specific gene silencing of both exogenous and
endogenous Akt using isoform-specific siRNA.
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Specific Silencing of Akt Signaling by siRNAWe attempted to
confirm the effect of isoform-specific siRNA on insulin signaling in
CHO-GLUT4myc cells. Although single transfection revealed the effect of RNAi,
as shown in Fig. 4, to
interpret the roles of each isoform more clearly, we used dual transfection (6
and 24 h before the assay) in these experiments, and the effect could be
clearly observed on immunoblotting. As shown in
Fig. 7, siAKTc, siAKT1, and
siAKT2, but not siAKT3, down-regulated not only each endogenous cognate
isoform of Akt but also total Akt expression (the anti-Akt antibody recognized
all Akt isoforms) and insulin-stimulated phosphorylation
(Ser(P)473) of Akt compared with control siRNA (siGFP). In parallel
with this, phospho-Ser9/Ser21 of GSK3/
(known as the phosphorylation sites by Akt) was also inhibited, but the
expression of GSK3
/
itself was not measurably affected. On the
other hand, either the upstream (i.e. Tyr phosphorylation of insulin
receptor) or the distinct signaling (i.e. phosphorylation of Erk1/2)
was not measurably affected. Thus, the siRNA against Akt was likely to
specifically silence only Akt signalings.
Effect of Isoform-specific siRNA on Insulin Actions in CHO-GLUT4myc Cells or 3T3-L1-GLUT4myc AdipocytesFinally, we examined the effect of isoform-specific siRNA against Akt on insulin actions. As shown in Fig. 8, A and E, we observed a clear silencing effect of each specific siRNA on each cognate isoform of Akt, and then we did biological assays under the same conditions. The silencing of Akt2 affected both insulin-stimulated GLUT4 translocation (Fig. 8, B and F) and 2-DG uptake (Fig. 8, C and Fig. 8, G) most prominently in both lines. On the other hand, silencing of Akt1 and Akt2 equally affected insulin-stimulated glycogen synthesis (Fig. 8, D and Fig. 8, H). We did not observe any significant effect in control siRNA-transfected cells compared with either mock-transfected cells or untransfected cells (data not shown).
In the case of CHO-GLUT4myc cells, siAKTc silenced both Akt1 and Akt2 simultaneously and showed additive inhibitory effects on all of these insulin actions compared with either siAKT1 or siAKT2 alone as shown in Fig. 8, Fig. 8. By contrast, in the case of 3T3-L1-GLUT4myc adipocytes, siAKTc silenced only Akt1 because of single mismatch of siAKTc to mouse Akt2 (Fig. 3A). We also tested the effect of siAKT1 and siAKT2 simultaneously in 3T3-L1-GLUT4myc adipocytes and observed additive effects on insulin-stimulated GLUT4 translocation and 2-DG uptake but not significantly on glycogen synthesis (data not shown).
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DISCUSSION |
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Regarding the role of Akt in the translocation of GLUT4 to the plasma
membrane, there are arguments. In 1993, we reported that insulin-stimulated
glucose uptake was mediated by the PI3-kinase-dependent pathway
(38), and this observation was
given agreement. In 1996, Kohn et al.
(39) reported that
constitutively active Akt mimics insulin-stimulated glucose uptake, thereby
suggesting the participation of Akt in the action. However, over the next 7
years, the issue has been controversial. We speculate that the endogenous Akt
kinase activity seems to difficult to eradicate, especially in 3T3-L1
adipocytes. In addition, 3T3-L1 adipocytes express low levels of the
adenovirus receptor (i.e. Coxsackie virus and adenovirus receptor).
Kitamura et al. (14)
reported that dominant negative Akt (Akt-2A; Akt-T308A/S473A) inhibited the
kinase activity of Akt by 80% but did not impair insulin-stimulated
glucose uptake in 3T3-L1 adipocytes. However, there was residual Akt activity,
which may be sufficient to support insulin-stimulated glucose uptake
(15). On the other hand, Wang
et al. (15) reported
that dominant-negative Akt (Akt-AAA; Akt-K179A/T308A/S473A) did impair both
endogenous Akt kinase activity and insulin-stimulated glucose uptake in L6
myoblasts. In our present study, we used Akt-K179M/T308A/S473A (Akt-MAA),
which impaired endogenous kinase activity dominant negatively as did the other
dominant negative form of Akt-AAA. This DN-Akt adenovirus was recognized to
inhibit signalings via all Akt isoforms, because the Akt kinase assay (Figs.
1E and
2E) reflected total
Akt activity. The antibody we used in this assay recognized all isoforms of
Akt (data not shown). In addition, we also confirmed that insulin-stimulated
Ser473 phosphorylation of Akt2 in 3T3-L1-GLUT4myc adipocytes was
attenuated by the DN-Akt (data not shown). In our study, although DN-Akt (at
MOI 80) inhibited Akt activity almost completely, residual metabolic
activities could still be slightly observed
(Fig. 2, EH).
Therefore, there may be an additional pathway in 3T3-L1 adipocytes
(e.g. atypical PKC). Although we observed little effect of adenoviral
overexpression on either upstream (PI3-kinase) or distinct signaling (Erk), we
further used the RNAi technique that was considered to be more specific.
With RNAi targeting each Akt isoform (i.e. Akt13), isoform-specific silencing was observed in both CHO-GLUT4myc cells and 3T3-L1-GLUT4myc adipocytes (Fig. 8, A and E). The inhibitory effects of siRNA on insulin actions were modest compared with that of adenovirus but were significant. We speculate this is because the transfection efficiency of siRNA by lipofection is limited, whereas adenovirus can infect all cells with multiple copy number.
Celera et al. (40) reported that insulin increased the association of Akt2 with GLUT4-containing vesicles in rat adipocytes. Hill et al. also suggested the importance of Akt2 in insulin-stimulated glucose uptake in adipocytes using interfering antibodies and substrate peptides (41). Recently, Cho et al. (42) have reported that targeting disruption of Akt2 causes insulin resistance and a type II diabetes-like phenotype. On the other hand, Akt1-deficient mice display growth reduction but no metabolic defect, including glucose metabolism (43, 44). In an intact body, Akt1 and Akt2 are likely to be key molecules for growth and metabolism, respectively. In accordance with their data, our data also suggested that Akt2 is a dominant isoform among the Akt family in insulin-stimulated glucose uptake as well as GLUT4 translocation but also suggested a modest role for Akt1 at least at the cell level. The suppression of Akt1 also affected insulin-stimulated GLUT4 translocation and glucose uptake.
The roles of Akt3 in these cells are unclear. Although Akt1 and Akt2 have been shown to be expressed ubiquitously even in insulin-responsible tissues (i.e. liver, skeletal muscle, and adipose tissue) (45), the distribution of Akt3 seems to be relatively limited (i.e. testis and brain). We also observed high levels of Akt3 expression in mouse tissues, including brain and testis, but extremely lower levels in adipose tissues, only trace amounts in the liver, and nondetectable levels in skeletal muscles (data not shown). As shown Fig. 8I, we did detect the expression of Akt3 in lysates from a brain tumor-derived cell line (i.e. U251MG cells) but did not observe any significant signal corresponding to Akt3 in lysates from either CHO cells or 3T3-L1 adipocytes. Thus, expressions of Akt3 in these cells are estimated to be at least extremely low level in these two cell lines.
With regard to the role of Akt in insulin-stimulated glycogen synthesis,
Cross et al. (13)
have reported that Akt phosphorylates and inhibits GSK3 (glycogen synthase
kinase 3). In accordance with this report, our data obtained using two
different strategies (adenoviral overexpression and RNAi-mediated silencing)
demonstrated the necessity of Akt for glycogen synthesis. As shown in Figs.
1 and
2, infection of adenovirus
encoding CA-AKT stimulated basal glycogen synthesis in both cell lines. On the
other hand, the enhancement of glycogen synthesis in response to insulin was
blunted compared with findings with control cells. Regarding this point, Kohn
et al. (39) observed
a similar result in CA-Akt-infected 3T3-L1 adipocytes. We reported that
membrane-targeted 3-phosphoinositide-dependent protein kinase 1 inhibits
insulin-stimulated glycogen synthesis, despite the constitutive
phosphorylation of GSK3 (46),
and Egawa et al. (47)
reported that the constitutively active form of PI3-kinase also inhibited
glycogen synthesis despite the constitutive activation of Akt. Thus, the
constitutive activation of the PI3-kinase-Akt pathway is likely to inhibit
insulin-stimulated glycogen synthesis because of desensitization
(46). Enhancement of basal
glycogen synthesis seems to induce increments of total glycogen content in
cells and provides a feedback signal to blunt the synthesis rate in response
to insulin (48,
49). We also measured glycogen
content in CA-Akt adenovirus-infected CHO-GLUT4myc cells and found 50%
increase in the content compared with that in control adenovirus-infected
cells (data not shown).
According to our previous report with use of peptide library screen, the substrate specificity of each Akt isoform is likely to be similar (28). Kim et al. reported that Akt1, but not Akt2, co-localized with JIP1, a scaffold protein of the c-Jun N-terminal kinase pathway, through the PH domain of Akt1 (50). We reported that TCL1 is a co-activator of Akt by promotion of oligomerization (51). The PH domain of Akt (52) mediates the binding of Akt to TCL1, and Akt1 and Akt2 bind to all of the known TCL1 family, including TCL1, MTCP, and TCL1b, yet Akt3 binds to only TCL1 (53). Thus, the cellular function of each Akt isoform may be determined by its binding partner, not by its substrate specificity. Yet to be identified proteins (40, 54) on the GLUT4 vesicle that bind dominantly to Akt2 may be candidates to direct GLUT4 translocation in response to insulin.
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FOOTNOTES |
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¶ These authors contributed equally to this work.
** Supported by the Uehara Memorial Foundation and Ministry of Education,
Science, Technology, Sports, and Culture of Japan Grant 13680691.
Supported by National Institutes of Health Grant GM41890.
¶¶ Supported by a grant from the Ministry of Education, Science, Technology, Sports, and Culture of Japan. To whom correspondence should be addressed. Tel.: 81-88-633-7436; Fax: 81-88-633-7437; E-mail: ebina{at}ier.tokushima-u.ac.jp.
1 The abbreviations used are: PKB, protein kinase B; CHO, Chinese hamster
ovary; GLUT4, glucose transporter 4; 2-DG, 2-deoxyglucose; siRNA, short
interference-RNA; RNAi, RNA interference; PKC, protein kinase C; PI3-kinase,
phosphoinositide 3'-kinase; GSK3, glycogen synthase kinase 3; Erk,
extracellular signal-regulated kinase; CMV, cytomegalovirus; FOXO, Forkhead
box group O; PH, pleckstrin homology; dsRNA, double-stranded RNA; DN, dominant
negative; CA, constitutively active; pfu, plaque-forming units; MOI,
multiplicity of infection; PBS, phosphate-buffered saline; GFP, green
fluorescent protein; HA, hemagglutinin; IGF, insulin-like growth factor.
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ACKNOWLEDGMENTS |
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REFERENCES |
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