From the Department of Pharmacology and Howard Hughes
Medical Institute, University of California San Diego, La Jolla,
California 92093-0647, the ¶ Division of Cell Biology, The
Hospital for Sick Children, Toronto, Ontario M5G 1X8, Canada, the
Institute of Organic Chemistry, University of Bremen UFT, 28359 Bremen, Germany, and the ** Department of Medicine, University of
California San Diego, La Jolla, California 92093
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ABSTRACT |
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Phosphoinositide 3-OH kinases and their products, D-3 phosphorylated phosphoinositides, are increasingly recognized as crucial elements in many signaling cascades. A reliable means to introduce these lipids into intact cells would be of great value for showing the physiological roles of this pathway and for testing the specificity of pharmacological inhibitors of the kinases. We have stereospecifically synthesized di-C8-PIP3/AM and di-C12-PIP3/AM, the heptakis(acetoxymethyl) esters of dioctanoyl- and dilauroylphosphatidylinositol 3,4,5-trisphosphate, in 14 steps from myo-inositol. The ability of these uncharged lipophilic derivatives to deliver phosphatidylinositol 3,4,5-trisphosphate across cell membranes was demonstrated on 3T3-L1 adipocytes and T84 colon carcinoma monolayers. Insulin stimulation of hexose uptake into adipocytes was inhibited by the kinase inhibitor wortmannin and was largely restored by di-C8-PIP3/AM, which had no effect in the absence of insulin. Thus phosphatidylinositol 3,4,5-trisphosphate or a metabolite was necessary but not sufficient for stimulation of hexose transport. In T84 epithelial monolayers, di-C12-PIP3/AM mimicked epidermal growth factor in inhibiting chloride secretion and potassium efflux, suggesting that phosphatidylinositol 3,4,5-trisphosphate was sufficient to modulate these fluxes and mediate epidermal growth factor's action.
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INTRODUCTION |
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The recent discovery of D-3 phosphorylated inositol lipids and
their biosynthesis by a family of phosphoinositide 3-OH kinases (PI3K)1 has opened a new area
in cell signal transduction research (1). These enzymes phosphorylate
phosphatidylinositol, phosphatidylinositol 4-phosphate, and
phosphatidylinositol 4,5-bisphosphate on the D-3 position of the
inositol ring to generate phosphatidylinositol 3-phosphate (PI(3)P),
phosphatidylinositol 3,4-bisphosphate (PI(3,4)P2), and
phosphatidylinositol 3,4,5-trisphosphate (PIP3, Fig.
1). Some forms of PI3K such as the yeast
Vps34p and homologues produce exclusively PI(3)P. In mammalian cells,
PI(3)P is usually constitutively present. PI(3,4)P2 and
PIP3 are normally undetectable in unstimulated cells, but
can become transiently elevated within seconds to minutes following
stimulation with a wide range of growth factors and cytokines. This
behavior is indicative of signaling roles for both
PI(3,4)P2 and PIP3. Various PI3Ks can be
activated through both tyrosine kinase and G-protein dependent pathways
and multiple putative downstream targets have been identified including
Ca2+-independent protein kinase C (PKC) isoforms ,
,
, and
, proteins with pleckstrin homology domains such as
Akt/PKB, as well as other proteins such as synaptotagmin.
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Because PIP3 seems to play such an important role in signal transduction but is difficult to purify from biological sources, several groups have synthesized PIP3 by various synthetic routes and tested its in vitro actions on purified enzymes. However, to reveal the role of PIP3 in intact cells, especially when its precise molecular target is uncertain, it would be very helpful to be able to deliver exogenous PIP3 to its site of action inside whole cells. Such delivery would also be valuable to test the pharmacological specificity of PI3K blockers such as wortmannin. The ability of wortmannin to inhibit a cellular response is often taken to suggest that the response involves PI3K. The best test to prove the specificity of wortmannin for PI3K would be to deliver PIP3 by other means and show that the cellular response is restored. Unfortunately, PIP3 has at least 4 negative charges at physiological pH, so it is extremely unlikely to diffuse into cells by itself. Therefore effective administration of PIP3 itself to intact cells is problematic. A possible strategy to deliver PIP3 across the plasma membrane would be to derivatize the phosphates as acetoxymethyl (AM) esters so that the resulting neutral molecule can cross the plasma membrane by passive diffusion. The desirable feature of AM esters is that they are readily hydrolyzed by intracellular esterases, which should regenerate PIP3 inside the cells. This approach has previously been successfully applied to antitumor nucleotides (2, 3), cyclic nucleotides (4, 5), and inositol polyphosphates (6-8), but syntheses of AM esters of phospholipids or other compounds with acyclic phosphodiester bonds have not yet been described. We now report the stereospecific total synthesis of the heptakis (acetoxymethyl) esters of dioctanoyl- and dilauroyl-PIP3 and tests of their biological effects on insulin-stimulated glucose transport into intact fat cells and epidermal growth factor (EGF)-stimulated chloride transport across monolayers of T84 colon carcinoma cells.
The ability of insulin to stimulate uptake of glucose in muscle and fat tissue plays a central role in the maintenance of whole body glucose homeostasis (9). The signal transduction pathway utilized by insulin in promoting glucose transport has been shown to involve autophosphorylation of the insulin receptor with ensuing activation of its intrinsic receptor tyrosine kinase activity and phosphorylation of insulin receptor substrates such as IRS-1, IRS-2, and IRS-3 (10, 11). PI3K interacts with tyrosine-phosphorylated IRS proteins through an SH2 domain on its regulatory p85 subunit (12), thus activating its catalytic p110 subunit (13). The conclusion that PI3K is essential for insulin regulation of glucose transport is based largely on the use of the inhibitors wortmannin (14) and LY294002 (15). These studies have been supported by other approaches, such as overexpression of dominant negative or constitutively active PI3K mutants (16). The impact of PI3K stimulation on glucose transport is mediated either directly by the D-3 phosphorylated inositol phospholipid products of the enzyme, or alternatively by activation of intermediate molecules. Recently, two such downstream effectors of PI3K have been proposed, the serine/threonine kinase Akt or protein kinase B (17) and specific isoforms of protein kinase C (18, 19). Because of the known pharmacology of 3T3-L1 adipocytes and the importance of insulin-stimulated glucose transport in these cells, they are a suitable system to test the effects of the membrane-permeant PIP3 esters, especially their ability to bypass wortmannin blockade.
Regulation of chloride ion flux plays a key role in the control
of salt and fluid secretion across mucous membranes in addition to a
variety of other functions. An important model system for epithelial
transport is the T84 colon carcinoma cell line, which forms
monolayers that actively transport Cl in response to a
variety of agonists in much the same way as normal intestinal
epithelia. Cl
secretion in T84 cells can be
triggered through cyclic AMP (20) and calcium-dependent
signaling mechanisms (21). As in many other systems, these pathways
interact synergistically in T84 (22-24). More recently, we
have identified two receptor-activated pathways which limit
Cl
secretion through the calcium-dependent
but not through the cyclic AMP-dependent pathway: 1)
prolonged stimulation of the muscarinic M3 receptor on
T84 cells leads to accumulation of intracellular Ins(3,4,5,6)P4, which, in turn, inhibits
transepithelial Cl
efflux by restricting flow through
apically located Cl
channels (6). 2) Another pathway,
stimulated by EGF and inhibited by wortmannin, also restricts
transepithelial Cl
transport (25) by limiting basolateral
efflux through K+ channels (26). Moreover, the effects of
EGF and carbachol are additive (25), further arguing that the two
inhibitory pathways are independent. EGF probably works at least partly
through stimulation of PI3K, because EGF treatment elevates
PIP3, and the effect of EGF can be ablated by the PI3K
inhibitor wortmannin (25, 27). However, these results obtained with
standard techniques leave open the questions of whether the wortmannin
block is specific and whether EGF might also have other biochemical
effects that are also necessary for its inhibition of
carbachol-stimulated Cl
flux. Such effects would be
plausible because the EGF receptor is a powerful tyrosine kinase with
many targets other than PI3K. Therefore we tested whether either
PIP3/AM or PIP3 could mimic the effect of
EGF.
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MATERIALS AND METHODS |
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All chemicals from commercial sources were used without further purification. D-myo-Inositol (Aldrich) was dried at 80 °C under high vacuum overnight before use. sn-1,2-Dioctanoylglycerol and sn-1,2-dilauroylglycerol were purchased from Avanti Polar Lipids, Inc. Reagents were dried by mixing with activated molecular sieves at least 1 day before use. 1H NMR spectra were obtained on Varian 200 MHz or Bruker 300 MHz instruments. 13C NMR were obtained at 50 MHz. Mass spectra were recorded on a electrospray mass spectrometer (Hewlett Packard 59987A). Column chromatography was performed on silica gel (230-400 mesh from EM Science).
Compound 5: 730 mg (1 mmol) of diol 4 (28) in dry
CH2Cl2 was treated with 3 ml of
diisopropylethylamine (17 mmol) and 1 ml of benzyloxymethyl chloride
(Fluka, 60% purity, 4 mmol) and heated at 60 °C for 30 h. The
reaction mixture was allowed to cool and solvent was removed under
vacuum. The brown material was redissolved in
CH2Cl2 and purified by silica gel chromatography, eluting with 6:4 (v/v)
CH2Cl2:hexane. 950 mg of colorless oily
5 was obtained, 98% yield. 1H NMR
(CDCl3, ppm): 7.72-7.82 (m, 6H), 7.13-7.48 (m, 29H),
6.31 (t, 1H), 5.59 (t, 1H), 5.03 (dd, 1H), 4.92 (s, 4H), 4.80 (d, 1H), 4.68 (s, 4H), 4.52 (dd, 1H), 4.24 (dd, 1H), 1.11 (s, 9H);
13C NMR (CDCl3,
ppm): 166.2, 159.6, 138.2, 136.4, 133.3, 130.6, 130.4, 130.1, 128.9, 128.7, 128.6, 128.3, 128.2, 127.9, 127.8, 127.7, 97.2, 96.3, 91.8, 74.8, 73.0, 72.0, 71.4, 70.3,
70.1, 27.8, 19.6. MS: calculated for
[C59H58O11Si + H+] 972.2, found
972.2.
Compound 6: 3.5 g of 5 (3.61 mmol)
was dissolved in tetrahydrofuran and 1.2 g of tetrabutylammonium
fluoride (4.6 mmol, 1.27 equivalents) was added slowly. After stirring for 20 min at room temperature, the reaction was completed and solvent
was removed. Silica gel column chromatography eluting with 98:2 (v/v)
CHCl3:CH3OH furnished 2.48 g of
6, 94% yield. 1H NMR (CDCl3, ppm): 7.65-7.88 (m, 6H), 7.01-7.48 (m, 19H), 6.08 (t, 1H), 5.53 (t,
1H), 5.22 (dd, 1H), 4.92 (d, 1H), 4.82 (d, 1H), 4.34-4.72 (m, 10H),
4.08 (t, 1H). 13C NMR (CDCl3,
ppm): 166.0, 165.8, 165.7, 137.3, 137.1, 133.4, 133.2, 133.1, 129.8, 129.7, 128.5,
128.4, 128.3, 127.9, 127.8, 127.7, 127.6, 96.2, 80.6, 72.5, 72.0, 71.3, 70.5, 70.3, 70.0. MS: calculated for
[C43H40O11 + H+] 733.8, found
733.7.
Compound 7: 732 mg of 6 (1 mmol) was
dissolved in dry dimethylformamide with 170 mg of imidazole (2.5 mmol). 250 µl of dimethylisopropylsilyl chloride (1.6 mmol) was added and
the reaction mixture stirred under argon at room temperature for 4 h. Dimethylformamide was removed under vacuum and the product purified
on a silica gel column with CHCl3 as eluant. 7 was obtained as 800 mg of colorless oil, 96% yield. 1H NMR
(CDCl3, ppm): 7.80-7.98 (m, 6H), 7.25-7.48 (m, 19H),
6.24 (t, 1H), 5.68 (t, 1H), 5.28 (dd, 1H), 5.10 (dd, 2H), 4.91 (s, 1H),
4.84 (d, 1H), 4.79 (d, 1H), 4.66 (s, 2H), 4.57 (s, 2H), 4.31 (s, 2H),
4.12 (dd, 1H), 0.96-0.98 (m, 7H), 0.16 (d, 6H). 13C NMR
(CDCl3,
ppm): 166.2, 138.2, 134.0, 133.6, 130.3, 130.1, 128.9, 128.8, 128.7, 128.6, 127.9, 127.8, 96.4, 96.0, 74.3, 72.8, 72.5, 71.3, 70.2, 69.8, 18.4, 16.5,
2.2. MS: calculated for
[C48H52O11Si + H+] 834.0, found
834.0.
Compound 8: 290 mg of 7 (0.35 mmol) was
dissolved in dry methanol. 100 mg of KCN (1.53 mmol, dried over KOH under vacuum) was added and the reaction mixture stirred at room temperature for 9 h. Ater removing solvent, the reaction mixture was redissolved in CHCl3 and purified on a silica gel
column, eluting with 95:5 (v/v) CHCl3:MeOH. 200 mg of
8 was obtained, 89% yield. 1H NMR
(CDCl3, ppm): 7.34-7.37 (m, 10H), 5.01 (d, 1H),
4.93(d, 1H), 4.85 (d, 1H), 4.83 (s, 2H), 4.78 (m, 6H), 4.63 (d, 1H),
4.57 (d, 1H), 4.09 (d, 1H), 3.3-3.7 (m, 3H), 0.96 (s, 3H), 0.93 (s, 3H), 0.85 (m, 1H), 0.06 (s, 6H). 13C NMR
(CDCl3,
ppm): 129.1, 128.5, 97.3, 97.2, 85.0, 82.8, 74.3, 74.2, 72.6, 71.5, 70.8, 17.5, 15.3,
2.4. MS: calculated for
[C27H40O8Si +H+] 521.7, found 521.8.
Compound 9: 0.5 g (2.1 mmol) of 2-cyanoethyl
diisopropylchlorophosphoramidite in dry CH2Cl2
was mixed with 0.4 ml of diisopropylethylamine (1.1 equivalent) and 160 µl (2.34 mmol) of 2-cyanoethanol. After stirring for 30 min at room
temperature, solvent was removed and dry ethyl ether added to
precipitate diisopropylethylammonium chloride. The ether extract
containing bis(2-cyanoethyl)diisopropylphosphoramidite was mixed with
110 mg of triol 8 (0.17 mmol), then the ether was removed
under vacuum. The mixture was redissolved in dry
CH2Cl2 and 160 mg of 1H-tetrazole (1.1 equivalents) added. After stirring overnight at room temperature under
argon, 1 ml of 5 M tert-butyl hydroperoxide in
hexane was added at 0 °C and stirred 10 min at that temperature and
then for 2 h at room temperature. The product was purified on a
silica gel column, eluting with 95:5 (v/v)
CHCl3:CH3OH. 220 mg of 9 was
obtained, 97% yield. 1H NMR (CDCl3, ppm):
7.32-7.42 (m, 10H), 4.55-5.22 (m, 10H), 4.18-4.50 (m, 15H), 4.08 (t,
1H), 2.82 (t, 12H). MS: calculated for
[C45H61N6O17P3Si
+Na+] 1102, found 1102.
Compound 10: 100 mg of 9 was added to 5 ml
of acetonitrile containing 2.5% aqueous HF and stirred at room
temperature for 2 h. After removing solvent, the product was
purified on a silica gel column eluted with 95:5 (v/v)
CHCl3:CH3OH. 85 mg of 10 was
obtained, 94% yield. 1H NMR (CDCl3, ppm):
7.30-7.40 (m, 10H), 5.05 (dd, 2H), 4.86 (s, 2H), 4.84 (d, 2H), 4.82 (s, 2H), 4.78 (t, 1H), 4.64 (s, 2H), 4.63 (d, 2H), 4.22-4.48 (m, 13H),
3.89 (t, 1H), 2.78-2.80 (m, 12H). 13C (CDCl3,
ppm): 138.2, 137.4, 129.1, 129.0, 128.5, 117.8, 117.6, 117.0, 97.8, 97.5, 82.2, 67.9, 67.6, 67.3, 63.2, 63.1, 20.2, 20.0. MS: calculated
for [C40H49N6O17P3 + H+]979.4, found 979.4.
Compound 11a: 80 mg of
sn-1,2-dioctanoylglycerol (0.23 mmol) in
CH2Cl2 was mixed with 45 µl of
diisopropylethylamine and 55 µl of 2-cyanoethyl
diisopropylchlorophosphoramidite. After stirring for 8 h at room
temperature, solvent was removed and dry ether was added. The crude
ether extract of 2-cyanoethyl (1,2-dioctanoyl)glyceryl diisopropylchlorophosphoramidite was mixed with 100 mg of
5 (0.1 mmol) and 100 mg of tetrazole (1.4 mmol) and kept at
room temperature overnight. 150 µl of 5 M
tert-butyl hydroperoxide was then added. 2 h later,
solvent was removed and product was purified on a silica gel column
eluted with 98:2 (v/v) CHCl3:CH3OH. 120 mg of
11a was obtained, 82% yield. 1H NMR (CDCl3,
, ppm): 7.32-7.42 (m, 10H), 5.42 (q, 1H), 5.05 (dd, 4H), 4.82 (d,
1H), 4.65-4.80 (m, 7H), 4.20-4.50 (m, 15H), 3.91 (t, 1H), 2.70-2.95
(m, 14H), 2.35-2.48 (m, 4H), 1.50-1.72 (m, 8H), 1.18-1.38 (m, 16H),
0.80-0.96 (m, 6H). 13C (CDCl3,
ppm): 173.8, 173.6, 138.2, 137.6, 128.9, 128.5, 128.2, 117.7, 117.5, 117.2, 97.6, 97.4,
71.1, 68.7, 66.7, 63.5, 63.2, 62.1, 34.2, 32.0, 29.4, 29.3, 25.2, 22.9,
20.1, 20.0, 19.9, 14.3. MS: calculated for
[C62H87N7O24P4 + Na+]1461, found 1461.
Compound 13a: 50 mg of 11a (35 µmol) in
CH2Cl2 was stirred overnight with 50 µl of
Et3N at room temperature, then solvent was removed under
vacuum. The crude product (12a) was used directly for the
next step by dissolving in 1 ml of dry acetonitrile and adding 100 µl
of bromomethyl acetate (1.02 mmol) and 200 µl of
diisopropylethylamine (1.15 mmol). The reaction mixture was stirred
overnight at room temperature. Solvent was removed and the residue was
extracted with dry ethyl ether. The ether supernatant was evaporated
and the resulting yellow oil chromatographed on a silica gel column
with 98:2 (v/v) CHCl3:CH3OH. 30 mg of
13a was obtained, 55% yield. 1H NMR
(CDCl3, ppm): 7.28-7.42 (m, 10H), 5.52-5.82 (m, 14H), 5.18 (q, 1H), 4.62-4.98 (m, 5H), 4.04-4.42 (m, 8H), 2.04-2.38 (m,
25H), 1.46-1.72 (m, 8H), 1.20-1.42 (m, 16H), 0.82-0.98 (m, 6H). MS:
calculated for [C62H93O38P4 + Na+]1594, found 1594.
Compound 14a: 15 mg of 13a in tetrahydrofuran was hydrogenated with 5 mg of palladium black for 4 h at room temperature and atmospheric pressure. After filtering off the catalyst and removing solvent, 4 mg of 14a (di-C8-PIP3/AM) was obtained, 96% yield. 1H NMR: 5.52-5.80 (m, 14H), 5.18 (q, 1H), 4.64-4.98 (m, 5H), 2.06-2.38 (m, 25H), 1.44-1.74 (m, 8H), 1.20-1.42 (m, 16H), 0.84-0.94 (m, 6H). MS: calculated for [C46H79O36P4 + Na+]1353, found 1353.
Compound 11b was prepared in the same manner as
11a, 79% yield. 1H NMR (CDCl3,
, ppm): 7.31-7.42 (m, 10H), 5.41 (q, 1H), 5.06 (dd, 4H),
4.80 (d, 1H), 4.64-4.82 (m, 7H), 4.20-4.48 (m, 15H), 3.92 (t, 1H),
2.68-2.95 (m, 14H), 2.34-2.46 (m, 4H), 1.52-1.72 (m, 12H),
1.17-1.40 (m, 28H), 0.81-0.95 (m, 6H). 13C NMR
(CDC13,
, ppm): 174.1, 173.8, 138.3, 137.5, 128.8, 128.5, 128.3, 117.8, 117.4, 117.2, 97.5, 97.3, 71.2, 68.8, 66.8, 63.4, 63.2, 62.1, 34.1, 31.9, 29.6, 29.4, 25.3, 22.9, 20.2, 20.1, 20.0, 19.9, 19.8, 14.3. MS: calculated for
[C70H103N7O24P4 + Na+]1573, found 1573.
Compounds 12b and 13b were prepared in the
same manner as 12a and 13a. 12b was used directly
for synthesis of 13b without purification. 52 mg of
13b was obtained in 48% yield. 1H NMR
(CDCl3, ppm): 7.26-7.41 (m, 10H), 5.52-5.83 (m, 14H), 5.17 (q, 1H), 4.63-5.00 (m, 5H), 4.05-4.45 (m, 8H), 2.05-2.40 (m,
25H), 1.44-1.74 (m, 12H), 1.19-1.44 (m, 28H), 0.82-0.98 (m, 6H). MS:
calculated for [C70H109O38P4 + Na+]1706, found 1706.
Compound 14b was prepared in the same manner as 14a. 40 mg of 14b (di-C12-PIP3/AM) was obtained in 95% yield. 1H NMR: 5.51-5.79 (m, 14H), 5.16 (q, 1H), 4.65-4.96 (m, 5H), 2.02-2.41 (m, 25H), 1.45-1.74 (m, 12H), 1.17-1.46 (m, 28H), 0.83-0.92 (m, 6H). MS: calculated for [C54H93O36P4 + Na+]1465, found 1465.
Adipocyte Cell Culture-- All cell culture solutions and supplements were obtained from Life Technologies, Inc. (Burlington, ON, Canada). 3T3-L1 cells were a kind gift from Dr. G. Holman (University of Bath, United Kingdom) and were grown in monolayer culture in 12-well plates, bathed in Dulbecco's modified Eagle's medium supplemented with 10% (v/v) calf serum and 1% (v/v) antibiotic solution (10,000 units/ml penicillin and 10 mg/ml streptomycin) in an atmosphere of 5% CO2 at 37 °C and this medium was replenished every 48 h. Prior to experimental manipulation, the cells were depleted of serum for 3 h.
Determination of 2-Deoxyglucose Uptake in 3T3-L1
Adipocytes--
3T3-L1 adipocyte monolayers were rinsed with 140 mM NaCl, 2.4 mM MgSO4, 5 mM KCl, 1 mM CaCl2, and 20 mM Na-HEPES, pH 7.4. Glucose uptake was measured in 0.25-ml
incubation volumes using 10 µM
2-[3H]deoxyglucose (1 µCi/ml; NEN Life Science
Products) for 5 min. Previous studies have demonstrated 2-deoxyglucose
uptake to be linear in this time period. The radioactive solution was
aspirated, and the cells were rinsed three times with ice-cold isotonic
saline solution. Cells were disrupted with 1.0 ml of 0.05 N
NaOH, and the radioactivity of a 0.75-ml aliquot of the cell lysate was quantitated by liquid scintillation counting using an LKB 1217 -counter. Protein concentration of the lysate was determined using
the Bradford method (29). Nonspecific uptake was determined in the
presence of 10 µM cytochalasin B (Sigma) and was
subtracted from total uptake.
T84 Colon Carcinoma Cell Culture-- T84 cells (passages 15-45) were grown and maintained as described previously (30) in Dulbecco's modified Eagle's medium/F-12 media (JRH Biosciences, Lexena, KS) supplemented with 5% newborn calf serum, 2 mM glutamine, and 50 units/ml each of penicillin/streptomycin (Core Cell Culture Facility, University of California, San Diego). Cells used in experiments were plated on Costar "snap-well" inserts and maintained in culture for 6-10 days to allow for formation of tight junctions prior to the experiment.
Short Circuit Current Measurements--
Snapwell inserts
containing confluent T84 monolayers were incubated for
0.5 h at 37 °C with 0.1 ml of PIP3 derivatives (200 µM) or vehicle applied to the apical side. The monolayers
were then mounted into modified Ussing chambers (Physiologic
Instruments, San Diego, CA), whose basolateral side was bathed with
Ringers solution warmed to 37 °C and gassed continuously with 95%
O2, 5% CO2 at a rate of 30-35 ml/min. The
spontaneous potential difference across the monolayer was
short-circuited with a voltage clamp (Model VCC MC6, Physiologic
Instruments, San Diego, CA). Short circuit current
(Isc) and conductances were recorded at 4-s
intervals using Acquire and Analyze Software 1.1. (Physiologic
Instruments, San Diego, CA). Increased Isc in
T84 has been demonstrated to reflect transepithelial
Cl secretion (21).
Rb+ Efflux Measurements--
Rb+ flux
measurements were a modification of a method previously published by
Venglarik et al. (31). Monolayers grown on Costar snap-well
inserts (Cambridge, MA) were rinsed in Hank's balanced salt solution
containing (in mM) Na+, 137.6;
Cl, 146.3; K+ 5.8;
H2PO4
, 0.44;
HPO42
, 0.34; Ca2+, 1;
Mg2+, 1; HEPES (pH 7.2), 15; and D-glucose, 10. The cells were loaded for 30 min with 5 µCi/0.25 ml at 37 °C added
to the basolateral surface. Simultaneously, the apical surface was
bathed with cell permeant esters of PIP3 (200 µM) or vehicle. Following 4 rinses of apical and
basolateral surfaces with Hank's buffer over a period of 6 min, the
inserts were sequentially transferred at 2-min intervals to fresh wells
of a 24-well culture dish floating in a 37 °C water bath. After 12 min, the inserts were transferred to wells containing 0.1 mM carbachol for the remainder of the experiment. After the experiment, the contents of the wells and the inserts were transferred to vials containing Ecoscint which were counted in a Packard
scintillation counter. The data was analyzed as described by Venglarik
et al. (31) yielding rate constants of nuclide efflux at
2-min intervals.
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RESULTS |
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Synthesis--
The synthesis of acetoxymethyl esters of
PIP3 (Fig. 2) started with
enantiomerically pure
D-1-O-(tert-butyldiphenylsilyl)-3,4,5-O-tribenzoyl-myo-inositol (4) which was prepared from myo-inositol in 4 steps with 30% yield. The synthesis of 4 was developed by
Bruzik and Tsai (28), and their method offers many advantages including enantiomeric purity and good yield. The NMR spectra, MS, and optical rotation of the diol 4 that we prepared are all consistent with their report. Diol 4 was then protected with
benzyloxymethyl ethers, which can be easily removed by hydrogenolysis
at the end of the synthesis without affecting the other groups on the
product. At this stage, the myo-inositol 1-hydroxyl was
still protected as a tert-butyldiphenylsilyl ether, whose
bulk was essential for regioselectivity in the synthesis of
4. But we found that tert-butyldiphenylsilyl
could not be removed at a late stage without cleavage of other
protecting groups, so it had to be replaced in 7 by
dimethylisopropylsilyl. The benzoate groups on 7 were
removed by KCN in methanol to give triol 8, which was
phosphitylated on the 3, 4, and 5 positions and oxidized to
9 with phosphates protected as -cyanoethyl esters. At
this stage, the dimethylisopropylsilyl chloride protecting the
1-hydroxyl could be cleaved by 2.5% HF to give 10 without affecting the other protecting groups. The 1-hydroxyl of 10 was phosphitylated and linked to sn-1,2-dioctanoylglycerol
or the analogous dilauroylglycerol to 11a or 11b,
respectively. The
-cyanoethyl protection on the phosphates was
removed with anhydrous triethylamine to 12a or
12b without affecting the diacylglycerol esters. The
PIP3's with 2,6-benzyloxymethyl ethers were esterified
with bromomethyl acetate to 13a or 13b to mask
all seven potential negative charges as acetoxymethyl (AM) esters. The
final products 14a and 14b were obtained by
hydrogenolysis of the benzyloxymethyl groups to free the
2,6-hydroxyls. For biological comparison, the corresponding di-C8-PIP3 lacking the AM esters was prepared
by hydrogenolysis of 12a.
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Effects on Hexose Uptake into 3T3-L1 Adipocytes-- Hexose uptake into adipocytes was markedly stimulated by a maximally effective dose of insulin (Fig. 3). As shown previously, this increase could be mostly prevented by the PI3K inhibitor wortmannin. The crucial new result is that wortmannin inhibition could be largely circumvented by PIP3/AM. The di-C8 version (14a) restored a greater percentage (87%) of the insulin stimulation than that (56%) produced by di-C12-PIP3/AM (14b). Interestingly, neither PIP3/AM had a significant effect on hexose uptake in the absence of insulin.
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Effects on Cl Transport Across T84
Monolayers--
In contrast to the above results with adipocytes,
PIP3/AM by itself was able to mimic the action of EGF on a
model of colonic epithelia. The di-C12 version
(14b) was more effective than the di-C8
(14a), the opposite ranking from that seen with the
adipocytes. Extracellular nonesterified PIP3 had no effect, a finding that confirmed that the site of action is intracellular and
that esterification is necessary for effective transmembrane delivery
of PIP3 in this system. Fig.
4 shows the large
Isc stimulated by carbachol (dotted
line) and its nearly complete inhibition by pretreatment either
with 1 µM EGF for 15 min (circles) or with 200 µM di-C12-PIP3/AM
(14b) for 30 min (dashed line). EGF and
PIP3/AM were equally effective in reducing
carbachol-stimulated peak Isc to 15% of
control. The combination of maximal doses of both EGF and
PIP3/AM (solid line) was no more effective than
either alone. These results argue that both agents are working through the same mechanism, namely generation of intracellular PIP3
or a metabolite thereof, which is not only necessary but sufficient to
mediate the effects of EGF in this response.
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DISCUSSION |
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Synthetic Strategy and Choice of Fatty Acid Chain Length-- The synthesis proceeded fairly smoothly from the cheap starting material myo-inositol via 1-O-tert-butyldiphenylsilyl-myo-inositol-3,4,5-tri-O-benzoate, in which the extreme steric bulk of the tert-butyldiphenylsilyl group was used to differentiate the 2- and 6-positions from the 3-, 4-, and 5-positions. Unfortunately, the very same bulk prevented deprotection under conditions that preserved the protecting groups on the phosphates. Therefore once the 2,6-positions were blocked, the tert-butyldiphenylsilyl group had to be replaced by a less hindered analog, dimethylisopropylsilyl. We initially chose dioctanoyl groups in the diacylglycerol moiety because of the pioneering work of Reddy et al. (32), who showed that di-C8-PIP3 was more soluble and tractable than PIP3's with more physiological fatty acids on the order of C18 or C20. Because di-C8-PIP3/AM proved easy enough to handle and because longer chain lengths might well simulate the natural PIP3 more closely, we eventually also synthesized di-C12-PIP3/AM. A virtue of the present synthetic route is that the diacylglycerol group is added intact at a late stage, so that variations in this part of the molecule are relatively easy. The C8 version proved more effective than its C12 analog on adipocytes, perhaps because the prominent fat droplets in those cells provided a competitive sink for the more hydrophobic analog. By contrast, the C12 version was more potent than C8 on the colonic epithelia, where the stronger membrane binding and more physiological chain length of the C12 might be decisive.
Advantages and Potential Problems of Masking the Polar Groups of PIP3-- Our strategy of esterifying all the phosphate negative charges was based on extensive prior experience with the transmembrane delivery of phosphate-containing second messengers. Masking of all charges is highly beneficial for cyclic nucleotides (4, 5), which carry only one charge, and are essential for inositol polyphosphates (6-8), which have multiple charges. However, after the completion of our synthesis of PIP3/AMs, it was reported that di-C16-PI(3,4)P2, di-C8-PIP3, and di-C16-PIP3 activate the kinase Akt and stimulate motility and chemotaxis when added extracellularly as sonicated vesicles to intact NIH 3T3 fibroblasts (33, 34). It was suggested that the PI(3,4)P2 or PIP3-containing vesicles fuse with the plasma membrane and deliver the free lipid to the intracellular leaflet. In our hands, di-C8-PIP3 was ineffective at mimicking PI3K activation in T84 epithelia, whereas the AM ester was fully effective. Thus in cell types in which fusion with PIP3 liposomes is not as facile as in NIH 3T3, the uncharged hydrolyzable PIP3 esters may be a more reliable means of delivery.
One important choice in the design of a membrane-permeant PIP3 derivative is whether to protect the 2- and 6-hydroxyls and if so, with what. In the present molecules, those hydroxyls have been left free. The advantage is that neither the cell nor the experimenter need to do anything to unmask the OH groups on those positions. The main disadvantage of free hydroxyls is that they permit extensive migration of phosphate triesters. Such migration produces unwanted isomers and necessitates much higher concentrations of the permeant ester (7). Based on previous problems with hydrolysis of 6-O-butyrate esters of inositol-1,4,5-trisphosphate, we feared that esters on the 2- and 6-positions of PIP3 would similarly refuse to hydrolyze quickly enough because they are similarly sandwiched between flanking phosphate groups. However, 2,6-di-O-butyryl-PIP3/AM does have biological activity (27), so this concern may have been overcautious. Yet another possibility would be to mask the 2- and 6-hydroxyls with UV-photolyzable caging groups such as 3,4-dimethoxy-2-nitrobenzyl ethers. This group has proven to be the ideal way to protect the 6-hydroxyl of inositol 1,4,5-trisphosphate because it prevents migration yet can be instantaneously removed with a flash of UV, ideal for unleashing the immediate actions of this fast-acting, rapidly metabolized messenger (8). Although it is not clear whether any important physiology may require such rapid delivery of PIP3, a caged membrane-permeant PIP3 would be an ideal way to find out.Generalization to Other Polyphosphoinositides and Acyclic Phosphodiesters-- Now that permeant esters have been shown to deliver the extremely polar phospholipid PIP3 across the plasma membrane, it would be interesting to synthesize analogous esters of related phospholipids such as phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 4,5-bisphosphate. PI(3,4)P2 could be produced intracellularly by dephosphorylation of PIP3 and may be more potent than PIP3 at activating certain isoforms of protein kinase C. The 4,5-isomer is not only the classical substrate for phospholipase C but also is important for membrane-cytoskeletal interaction. Esters of such phospholipids might help reveal which interconversions occur inside cells and which lipids are the proximal agonists for the many downstream targets. Thiophosphate analogs might be particularly helpful because the thiophosphate groups are generally nonmetabolizable.
At present we do not have analytical means to quantify how much PIP3 is actually being delivered inside the cell. Probably the PIP3/AM would have to be radiolabeled, which would be a significant synthetic challenge for the future, or better analytical methods to measure unlabeled PIP3 would have to be developed.Mechanism of Insulin Signaling in Adipocytes and Additional Signals Provided by Insulin Receptors-- PIP3/AMs of two different chain lengths were capable of partially overcoming the inhibitory effect of wortmannin on insulin-stimulated glucose transport. However, PIP3/AMs alone did not stimulate basal glucose transport. Thus, PIP3 appears to be necessary but not sufficient for maximal insulin-stimulated glucose transport. We propose that a bifurcation of the insulin induced signal may occur: one signal involves generation of PIP3, while the other is independent of this product. Full stimulation of glucose transport would require activation of both signals.
The nature of the signal that is dependent on PIP3 is currently being investigated, and could include two enzymes whose activity was recently shown to depend on prior PI3K activation: the protein kinase c-Akt (also known as PKB) and protein kinase C-EGF Signaling via PIP3 in T84
Cells--
Membrane permeant esters of PIP3 mimic the
inhibitory effects of EGF both on Cl secretion and efflux
through K+ channels. Moreover, the EGF- and
PIP3/AM-induced inhibitions, but not that due to carbachol,
could be reversed by pretreatment with
Bt2Ins(1,4,5,6)P4/AM (27). Although
di-C8- and di-C12-PIP3/AM had the
same basic effects, the latter was somewhat more potent. We attempted
to test whether PIP3/AM would overcome wortmannin reversal
of EGF inhibition of Cl
secretion, but unfortunately this
experiment is greatly complicated by the ability of wortmannin to
augment calcium-dependent Cl
secretion in the
absence of EGF. However, pretreatment with PIP3/AM dramatically reduced calcium-mediated chloride secretion in the presence of
wortmannin,2
consistent with actions of PIP3/AM that bypass
wortmannin. Together, these data strongly suggest that a lipid product
of PI-3 kinase mediates EGF-induced inhibition of Cl
secretion in T84 colonic epithelia. However, the current
studies do not exclude the possibility that PI(3,4)P2 or
other PIP3 metabolite is the ultimate signal.
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ACKNOWLEDGEMENT |
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We thank Eleanor Wolfson for expert technical assistance.
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FOOTNOTES |
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* This work was supported by National Institutes of Health Grants NS27177 (to R. Y. T.) and DK47240 (to A. T.-K.), the University-wide AIDS Research Program (to A. T.-K.), and Medical Research Council (Canada) Grant MT-7307 (to A. K.).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.
§ Present address: Aurora Biosciences Corp., 11010 Torreyana Rd., San Diego, CA 92121.
Present address: Inologic Inc., 43012 S.E. 108th St., North
Bend, WA 98045.
§§ To whom correspondence should be addressed. Tel.: 619-534-4891; Fax: 619-534-5270.
1 The abbreviations used are: PI3K, phosphoinositide 3-OH kinases; PIP3, phosphatidylinositol 3,4,5-trisphosphate; PI(3)P, phosphatidylinositol 3-phosphate; PI(3,4)P2, phosphatidylinositol 3,4-bisphosphate; PKC, protein kinase C; AM, acetoxymethyl; EGF, epidermal growth factor; IRS, insulin receptor substrate; SH2, Src homology domain 2.
2 A. Traynor-Kaplan, unpublished observations.
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REFERENCES |
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