From the Department of Biological Chemistry, University of Michigan, Ann Arbor, Michigan 48109-0606
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ABSTRACT |
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Phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) is a key molecule involved in cell growth signaling. We demonstrated that overexpression of PTEN, a putative tumor suppressor, reduced insulin-induced PtdIns(3,4,5)P3 production in human 293 cells without effecting insulin-induced phosphoinositide 3-kinase activation. Further, transfection of the catalytically inactive mutant of PTEN (C124S) caused PtdIns(3,4,5)P3 accumulation in the absence of insulin stimulation. Purified recombinant PTEN catalyzed dephosphorylation of PtdIns(3,4,5)P3, specifically at position 3 on the inositol ring. PTEN also exhibited 3-phosphatase activity toward inositol 1,3,4,5-tetrakisphosphate. Our results raise the possibility that PTEN acts in vivo as a phosphoinositide 3-phosphatase by regulating PtdIns(3,4,5)P3 levels. As expected, the C124S mutant of PTEN was incapable of catalyzing dephosphorylation of PtdIns(3,4,5)P3 consistent with the mechanism observed in protein-tyrosine phosphatase-catalyzed reactions.
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INTRODUCTION |
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A recently identified candidate tumor suppressor gene, PTEN/MMAC1, shares sequence identity with the family of protein-tyrosine phosphatases (PTPases)1 (1). Deletions and mutations within the PTEN gene have been observed in several cancer cell types and tumor cell lines (2, 3). Additional evidence that PTEN functions as a tumor suppressor was obtained by Furnari et al. (4), who showed that PTEN had a growth suppressor activity in glioma cells. PTEN encodes the active site consensus motif HCXXGXXR(S/T) found in all PTPases. In contrast, the recombinant protein is a poor catalyst toward both phosphoproteins and peptide substrates with the highest activity of PTEN observed toward the highly negatively charged, multiply phosphorylated polymer of (Glu-Tyr)n (5, 6). Based on these observations we thought it possible that PTEN could catalyze the dephosphorylation of acidic nonproteinaceous substrate. Identification of possible in vivo substrates would not only suggest a possible physiological function of PTEN, but they might also provide insight into how PTEN functions as a tumor suppressor.
PtdIns(3,4,5)P3 is an important second messenger involved in cell growth signaling (7). PtdIns(3,4,5)P3 is specifically produced from PtdIns(4,5)P2 by PI 3-kinase upon stimulation by a variety of ligands (7). Recent studies have identified that PtdIns(3,4,5)P3 can directly activate Akt, which in turn activates p70 S6 kinase and inhibits glycogen synthase kinase-3 (8, 9). Although there are several phosphoinositide 5-phosphatases, the mechanism of regulation and particularly the degradation pathway of PtdIns(3,4,5)P3 in vivo is still unclear (10, 11). In the present study we demonstrate that recombinant PTEN has PtdIns(3,4,5)P3 3-phosphatase activity. In addition, we provide evidence that PTEN may act in vivo as a regulator of PtdIns(3,4,5)P3, which produces a substrate that can be recycled by PI 3-kinase.
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EXPERIMENTAL PROCEDURES |
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Determination of PtdIns(3,4,5)P3 in Vivo-- The coding sequence of human PTEN and the C124S mutant of PTEN (gift from Yi Zhao) were amplified by polymerase chain reaction using 5' primer (5'-CCGGTACCGCCACCATGGACTACAAGGACGACGATGACAAGACAGCCATCATCAAAGAG-3') and 3' primer (5'-CCGTCGACTCAGACTTTTGTAATTTGTG-3'). The product was cleaved with KpnI and SalI and ligated into the KpnI/SalI sites of pCMV5 (gift from David W. Russell) to produce FLAG-tagged PTEN/pCMV5 and PTEN(CS)/pCMV5. Human 293 cells were cultured on a 6-well plate, and transfection of the cells were performed as described (12) using 1 µg of the constructs. The efficiency of transfection was about 80% in this condition. 48 h after the transfection, the cells were serum-starved and labeled with [32P]Pi (100 µCi/ml) for 4 h. The cells were stimulated by incubation with insulin (0.1 µg/ml) for 2.5 min at 37 °C, and the stimulation was quenched by the addition of 0.93 ml of CH3OH/CHCl3/1% HClO4 (50/25/18, v/v/v). The lipids were extracted and separated on a TLC plate as described (13) to determine the amount of [32P]PtdIns(3,4,5)P3. To analyze the expression of FLAG-tagged PTEN protein, the transfected cells were lysed in Laemmli sample buffer and subjected to SDS-polyacrylamide gel electrophoresis. The samples were transferred to Immobilon filter (Millipore) and immunoblotted with anti-FLAG M2 antibody (Kodak), and the signal was visualized by Enhanced Chemiluminescence (Amersham Pharmacia Biotech) using the manufacturer's recommended protocols.
PI 3-Kinase Assay--
Transfection, starvation and
stimulation of human 293 cells were carried out as described above in
the absence of radiolabel. After the stimulation, the cells were lysed,
followed by immunoprecipitation as described (14) using 4G10
anti-phosphotyrosine antibody (Upstate Biotechnology, Inc.). To analyze
the PI 3-kinase activity of the immunoprecipitants, the sample was
incubated for 10 min at 37 °C in 33 mM Tris-HCl (pH
7.4), 2.5 mM EGTA, 5 mM MgCl2, 30 mM NaCl, 0.1 mM [-32P]ATP (20 µCi), 0.1 mg/ml PtdIns(4,5)P2, and 0.15 mg/ml
phosphatidylserine. The reaction was terminated by the addition of 0.47 ml of CH3OH/CHCl3/6% HClO4
(30/15/2, v/v/v), and the phospholipids were extracted and separated on
a TLC plate as described (13).
Bacterial Expression and Purification of PTEN--
The
expression vector for PTEN was constructed by ligating a blunted
NdeI/SalI fragment from PTEN/pT7-7 (12) into the
SmaI site of pGEX-KG (15). This vector was used to transform
Escherichia coli strain JM109. Protein expression in 4-liter
culture was carried out as described (16). All of the following
procedures were performed at 4 °C. Cells were harvested, resuspended
in 80 ml of lysis buffer (20 mM Tris-HCl (pH 8), 2 mM EDTA, 2 mM DTT, 300 mM NaCl, and
1 mM phenylmethylsulfonyl fluoride) and lysed by sonication. The crude lysate was diluted by the addition of 720 ml of
the lysis buffer containing 1% (w/v) Triton X-100 and stirred for 30 min. Cell debris was removed by centrifugation at 27,000 × g for 20 min. A 5-ml slurry of glutathione-Sepharose 4B
(Amersham Pharmacia Biotech) was then added to the supernatant. After
the incubation for 2 h, the resin was packed in a column and
washed with 100 ml of the lysis buffer, and the glutathione
S-transferase-fusioned PTEN was eluted with 10 ml of the
lysis buffer containing 10 mM glutathione. After overnight
incubation with thrombin, the eluate was dialyzed against the lysis
buffer and passed through glutathione-Sepharose 4B column, followed by
a p-aminobezamidine-agarose column for adsorption of
glutathione S-transferase and thrombin, respectively. Then
the eluate was diluted with equal volume of TED buffer (20 mM Tris-HCl (pH 8), 2 mM EDTA, 2 mM
DTT) and applied to MonoQ HR5/5 (Amersham Pharmacia Biotech) column
equilibrated with TED buffer containing 150 mM NaCl. PTEN
was eluted with a linear gradient of NaCl (150-500 mM, 20 ml), followed by concentration using Centricon-30 (Amicon) and stored
at 80 °C until use.
Phosphatase Assays--
PtdIns(3,4,5)P3 phosphatase
assay was performed at 37 °C in a buffer (20 µl) consisting of 100 mM Tris-HCl (pH 8), 10 mM DTT, [32P]PtdIns(3,4,5)P3 and 1 µg of purified
PTEN. The reaction was terminated by the addition of 0.47 ml of
CH3OH/CHCl3/6% HClO4 (30/15/2,
v/v/v). Then the phospholipids were extracted and separated on a TLC
plate as described (13). To prepare
[32P]PtdIns(3,4,5)P3, the phosphorylation
of PtdIns(4,5)P2 by PI 3-kinase using
[-32P]ATP was carried out as described above. Then the
phospholipids were extracted as described (13) and stored at
20 °C
until use. The PI 3-kinase used was prepared by immunoprecipitation from the 293 cell lysate with anti-p85 antibody (Upstate Biotechnology, Inc.) as described (14). For identification of the dephosphorylation site (see Fig. 2B), dephosphorylation of
PtdIns(3,4,5)P3 by PTEN was carried out in a buffer (20 µl) consisting of 100 mM Tris-HCl (pH 8), 10 mM DTT, 0.1 mg/ml of PtdIns(3,4,5)P3 (BIOMOL),
0.15 mg/ml of phosphatidylserine, and 1 µg of purified PTEN. The
reaction was terminated by the addition of 0.47 ml of
CH3OH/CHCl3/6% HClO4 (30/15/2,
v/v). The phospholipids were extracted as described (13), dried, and
then used for PI 3-kinase-catalyzed reaction. Inositol phosphatase
assays were performed using commercially available
[3H]inositol phosphate (NEN) as substrates. Assay was
carried out at 37 °C in a buffer (20 µl) consisting of 100 mM Tris-HCl (pH 8), 10 mM DTT, 60 µM [3H]inositol phosphate (0.01 µCi) and
1 µg of enzyme. After an incubation of 30 min, the reaction was
terminated by the addition of 1 ml of stop solution. Then, to separate
the dephosphorylated product from the substrate, the sample was applied
to AG1-X8 column (0.5 ml) equilibrated with the stop solution.
Dephosphorylated [3H]inositol phosphate was eluted with 5 ml of the stop solution, whereas the substrate remained in the column,
and the radioactivity of the eluate was measured. The stop solutions
used were 0.2 M HCOONH4/0.1 M
HCOOH, 0.4 M HCOONH4/0.1 M HCOOH,
and 0.7 M HCOONH4/0.1 M HCOOH for
[3H]inositol 1, 4-bisphosphate,
[3H]Ins(1,4,5)P3, and
[3H]Ins(1,3,4,5)P4, respectively. Recombinant
human VHR, Cdc25B, and PTP1D were kindly gifts from Harris Vikis,
Elizabeth Gottlin, and Jin Zhou, respectively. For kinetic analysis
(see Fig. 3B), inositol phosphatase activity was assayed in
the same buffer as described above using 10-150 µM
Ins(1,3,4,5)P4 instead of [3H]inositol
phosphate. After an incubation of 1 min at 37 °C, the reaction was
terminated by the addition of 1 ml of ice-cold water, and the amount of
Ins(1,4,5)P3 produced was estimated using BIOTRAK Ins(1,4,5)P3 detection kit (Amersham Pharmacia Biotech)
following the manufacturer's recommended protocol. Kinetic constants
were determined using KaleidaGraph software (Abelbech).
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RESULTS AND DISCUSSION |
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Although PTEN has the consensus sequence of a PTPase, it dephosphorylates p-nitrophenylphosphate and other artificial protein substrates poorly, having the highest catalytic activity with the highly negatively charged, multiply phosphorylated polymer of (Glu-Tyr)n (5, 6). This observation raised the distinct possibility that PTEN might utilize acidic substrates other than Tyr or Ser/Thr phosphoproteins. In order to explore this possibility, we transfected PTEN into 293 cells and analyzed the changes in cellular phospholipids.
Suppression of Insulin-induced PtdIns(3,4,5)P3 Production by Overexpression of PTEN-- PtdIns(3,4,5)P3 is an important second messenger in the regulation of cell growth (7). In human 293 cells, insulin stimulates PI 3-kinase activity (Fig. 1C, lanes 1 and 2), resulting in an increase in PtdIns(3,4,5)P3 (Fig. 1A, lanes 1 and 2). When PTEN was overexpressed in the 293 cells, the insulin-induced PtdIns(3,4,5)P3 levels were significantly reduced in a dose-dependent manner (Fig. 1, A and B), whereas no effect on the activation of PI 3-kinase was observed (Fig. 1C, lanes 2 and 4). Because PtdIns(3,4,5)P3 is specifically produced by PI 3-kinase, this result suggests that PTEN directly effects the turnover of PtdIns(3,4,5)P3. Additionally, overexpression of the catalytically inactive mutant (C124S) of PTEN (see Fig. 3A) caused an accumulation of PtdIns(3,4,5)P3 in the absence of insulin stimulation (Fig. 1D, lanes 1 and 3), whereas overexpression of the mutant did not affect PI 3-kinase activity (data not shown). These results suggest that PTEN potentially regulates PtdIns(3,4,5)P3 levels in cells without alteration of the insulin-stimulated PI 3-kinase activity.
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PtdIns(3,4,5)P3 3-Phosphatase Activity of PTEN--
To
investigate the possibility that PTEN has PtdIns(3,4,5)P3
phosphatase activity, the recombinant enzyme was expressed
in E. coli and purified to homogeneity (data not shown).
Radiolabel was incorporated in position 3 of the substrate,
[32P]PtdIns(3,4,5)P3, using
PtdIns(4,5)P2, PI 3-kinase and [-32P]ATP.
When [32P]PtdIns(3,4,5)P3 was incubated with
the purified PTEN, the radiolabel of
[32P]PtdIns(3,4,5)P3 rapidly disappeared from
the lipid phase (Fig. 2A,
inset) while coincidentally appearing in the aqueous phase (Fig. 2A), suggesting the release of inorganic phosphate. In
order to conclusively prove that the only phosphate that had been
cleaved was at position 3 of the phosphoinositide, as opposed to other possible cleavages that could also generate a water-soluble radiolabel, we used the product of the reaction of PTEN as a substrate for PI
3-kinase. When the product was treated with PI 3-kinase,
[32P]PtdIns(3,4,5)P3 was reformed, thereby
providing further evidence that the two products generated by PTEN were
inorganic phosphate and PtdIns(4,5)P2 (Fig. 2B).
Under similar conditions, PTEN also exhibited 3-phosphatase activity on
phosphatidylinositol 3-monophosphate and phosphatidylinositol
3,4-bisphosphate; however, dephosphorylation of these phosphoinositides
occurred at ~20% the rate observed with PtdIns(3,4,5)P3
(data not shown). PTPases including PTP1D and dual-specific
phosphatases (VHR, Cdc25B) exhibited no phosphoinositide phosphatase
activity (data not shown).
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Inositol Phosphate 3-Phosphatase Activity of PTEN--
To
more carefully dissect the specific nature of the catalytic
activity of PTEN toward PtdIns(3,4,5)P3, we asked if PTEN
displayed activity toward inositol phosphates. PTEN can dephosphorylate Ins(1,3,4,5)P4, whereas tyrosine-specific (PTP1D) and the
dual-specific phosphatases (Cdc25B, VHR) exhibited no activity toward
this inositol phosphate (Fig.
3A). Again, the PTEN-catalyzed
reaction was specific for the position 3 of Ins(1,3,4,5)P4.
Other inositol phosphates that do not have a phosphate at the position
3 on the inositol ring were not dephosphorylated by PTEN (Fig.
3A). The dephosphorylated product was identified as
Ins(1,4,5)P3 using the Ins(1,4,5)P3-binding protein (Fig. 3B). These results demonstrate that PTEN also
has 3-phosphatase activity toward inositol phosphate. Both
Ins(1,4,5)P3 and Ins(1,3,4,5)P4 have been
proposed to be a functional second messenger responsible for the
intracellular calcium signaling (17). Interestingly,
Ins(1,3,4,5)P4 can associate and activate a
GTPase-activating protein (18). In contrast to
PtdIns(3,4,5)P3, Ins(1,3,4,5)P4 is
water-soluble and therefore was used to assess the significance of our
in vitro observations by determining the kinetic parameters
for the PTEN-catalyzed dephosphorylation occurred at position 3 on the
inositol ring. The Km and
Vmax values for Ins(1,3,4,5)P4 were
98.9 µM and 8.49 nmol/min/mg
(kcat, 0.49 min1), respectively
(Fig. 3B). The Km value of 98.9 µM is 250-fold lower than the Km of
p-nitrophenylphosphate, which is 25.6 mM.
Similar comparisons with the phosphorylated polymer (Glu-Tyr)n
were difficult to assess because a detailed kinetic analysis was not
performed with this substrate (6), and it is likely to be
phosphorylated at more than one site. The low
Vmax for the PTEN-catalyzed dephosphorylation of Ins(1,3,4,5)P4 implies that this may not be the preferred
substrate in vitro for this enzyme. These in
vitro assays may not be reflective of the in vivo
activity of PTEN because the phosphatase could be regulated by
phosphorylation, subcellular localization, and/or interaction with
other cellular proteins. It is noteworthy that the C terminus of the
phosphatase has a consensus PDZ binding site, and we have recently
shown that PTEN interacts with several PDZ-containing
proteins.2
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Potential Role of PTEN in Intracellular PtdIns(3,4,5)P3 Regulation-- We have established that the recombinant PTEN has phosphoinositide 3-phosphatase and inositol phosphate 3-phosphatase activities. The data shown in Fig. 1 suggest that suppression of insulin-induced PtdIns(3,4,5)P3 production by overexpression of PTEN is due to its phosphoinositide phosphatase activity. In addition, as shown in Fig. 1D, overexpression of the CS mutant caused PtdIns(3,4,5)P3 accumulation without insulin stimulation. These results strongly suggest that PTEN can act as a regulator of PtdIns(3,4,5)P3 in vivo. Insulin activates PI 3-kinase via tyrosine phosphorylation of insulin receptor substrate-1 catalyzed by the insulin receptor (Fig. 4). PtdIns(3,4,5)P3 produced by the PI 3-kinase can then activate Akt-mediated signals (Fig. 4). PtdIns(3,4,5)P3 levels reached a plateau within 3-5 min after the stimulation, and then PtdIns(3,4,5)P3 is degraded by unknown mechanisms (data not shown). Although our results suggest that PTEN can alter PtdIns(3,4,5)P3 levels in 293 cells, it is clear that there are other cellular mechanisms that can also alter phosphoinositide concentrations. For example, Guilherme et al. (11) reported that a phosphoinositide 5-phosphatase was activated by insulin stimulation, and SHIP (SH2 domain-containing inositol 5-phosphatase) is also well known as a regulator of PtdIns(3,4,5)P3 (10, 19). If PTEN functions in vivo as a PtdIns(3,4,5)P3 phosphatase, it follows that a homozygous deletion/mutation of this tumor suppressor gene could lead to a tumorigenic state through activation of the proto-oncogene, Akt. Specifically, loss of PTEN function would increase cellular levels of PtdIns(3,4,5)P3, thereby resulting in enhanced activation of Akt. In conclusion, although the physiological function of PTEN needs further clarification, we propose that: (i) members of the PTPase family of enzymes having an active site motif HCXXGXXR(S/T) such as PTEN are candidates to regulate intracellular levels of nonproteinaceous substrates as has also been reported for the RNA 5'-triphosphatase activity of CEL-1 (20); (ii) the dephosphorylation of phosphoinositide and inositol phosphate by PTEN is specific for position 3 on the inositol ring; and (iii) there are likely to be additional activating and/or localization mechanisms for PTEN within the cell regulating the catalytic activity of this enzyme.
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ACKNOWLEDGEMENT |
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We thank Dr. Kim Orth for critical reading of the manuscript.
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FOOTNOTES |
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* This work was supported by Grant 18849 from the NIDDKD, National Institutes of Health (to J. E. D.) and by the Walther Cancer Institute.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.
1 The abbreviations used are: PTPase, protein-tyrosine phosphatase; PI, phosphoinositide; PtdIns(3,4,5)P3, phosphatidylinositol 3,4,5-trisphosphate; PtdIns(4,5)P2, phosphatidylinositol 4,5-bisphosphate; Ins(1,3,4,5)P4, inositol 1,3,4,5-tetrakisphosphate; Ins(1,4,5)P3, inositol 1,4,5-trisphosphate; DTT, dithiothreitol; VHR, VH1-related.
2 Y. Zhao and J. E. Dixon, unpublished data.
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REFERENCES |
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