Dietary 1-monoolein decreases postprandial GIP release by

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Am J Physiol Gastrointest Liver Physiol 303: G298–G310, 2012.
First published May 30, 2012; doi:10.1152/ajpgi.00457.2011.
Dietary 1-monoolein decreases postprandial GIP release by reducing jejunal
transport of glucose and fatty acid in rodents
Akira Shimotoyodome,1 Noriko Osaki,1 Koji Onizawa,1 Tomohito Mizuno,2 Chika Suzukamo,1
Fumiaki Okahara,1 Daisuke Fukuoka,1 and Tadashi Hase1
1
Biological Science Laboratories, Kao Corporation, Tochigi, Japan; and 2Health Care Food Research Laboratories,
Kao Corporation, Tokyo, Japan
Submitted 1 November 2011; accepted in final form 28 May 2012
FAT/CD36; indirect calorimetry; insulin; intestinal transport; SGLT-1
THE WIDESPREAD PREVALENCE of obesity is now a worldwide
health problem. Excess adiposity, especially excess abdominal
fat accumulation, increases the risk of morbidity from a number of diseases, including diabetes, hypertension, and cardiovascular diseases and is also associated with a greater risk for
certain cancers (25). Therefore, improvement of lifestyle, particularly dietary content, is often recommended for the primary
prevention and treatment of these diseases.
Diacylglycerol (DAG), which consists of 1,3-DAG and
1,2(2,3)-DAG is contained in natural edible oils at a level of
2–10% (8) and is consumed in the daily human diet. Prior
studies in animals and humans have shown that dietary DAG
Address for reprint requests and other correspondence: A. Shimotoyodome,
2606 Akabane, Ichikai-machi, Haga-gun, Tochigi 321-3497, Japan (e-mail:
shimotoyodome.akira@kao.co.jp).
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oil composed mainly of 1,3-DAG leads to the suppression of
body fat accumulation, body weight loss, improved glucose
tolerance, and lower postprandial lipemia compared with that
of triacylglycerol (TAG), which has a similar fatty acid composition (32, 39). Recent studies have shown that the ingestion
of a 1,3-DAG-rich diet results in a lower postprandial insulin
response than that of TAG oil in humans (33, 40). Since a
high-level postprandial insulin response has been shown to be
associated with increased body fat (38) and the development of
insulin resistance (34) these findings suggest a possible mechanism for the beneficial effects of 1,3-DAG.
Postprandial insulin secretion is regulated by not only dietary carbohydrate but also gut-derived incretins, glucosedependent insulinotropic polypeptide (GIP), and glucagon-like
peptide-1 (GLP-1) (9). More recently, we have shown that
1,3-DAG-rich oil stimulates less GIP secretion compared with
TAG-rich oil of a similar fatty acid composition (35), supporting the lower postprandial insulin response after 1,3-DAG-rich
oil ingestion in combination with carbohydrate. There is
strong evidence for its anabolic characteristics and role in
the regulation of adipogenesis of GIP (14). Since increased
blood GIP level has been shown in obese patients (7) and
decreased GIP signaling attenuates obesity (1, 19), lower
postprandial GIP response also seems to contribute to the
antiobesity effect of 1,3-DAG.
The structural and metabolic characteristics of 1,3-DAG
compared with those of TAG are believed to be responsible for
its beneficial effects. It has been shown that 1,3-DAG is
digested into 1-monoacylglycerol (1-MO) and free fatty acids
(FFA) in the small intestine (17). However, the precise role of
1-MO in postprandial GIP release, the characteristic metabolite
of 1,3-DAG, has not been clearly elucidated.
Therefore, this study was designed to provide evidence that
1-MO (oleic acid-rich) reduced postprandial GIP response and
attenuated high-fat (HF) diet-induced obesity in mice. Furthermore, this study was carried out to clarify the underlying
mechanism of reduction in GIP secretion after 1-MO ingestion.
Recent studies have shown that glucose-stimulated GIP
secretion was blocked by phloridzin, an inhibitor of sodiumglucose cotransporter-1 (SGLT-1), in vitro (29) and in vivo
(20), indicating that intestinal glucose transport via SGLT-1
triggers GIP secretion. Since very little is known about the
apparent mechanism of fat-stimulated GIP secretion, we hypothesized that fat-induced GIP secretion may be triggered by
intestinal fatty acid transport. Previous studies indicated that
fat-stimulated GIP secretion was associated with chylomicron
formation (26, 35). Either chylomicron formation (24) or fatty
acid uptake (23) was shown to be mediated by fatty acid
translocase (FAT)/CD36 in the proximal intestine. Accord-
0193-1857/12 Copyright © 2012 the American Physiological Society
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Shimotoyodome A, Osaki N, Onizawa K, Mizuno T, Suzukamo C, Okahara F, Fukuoka D, Hase T. Dietary 1-monoolein
decreases postprandial GIP release by reducing jejunal transport of
glucose and fatty acid in rodents. Am J Physiol Gastrointest Liver
Physiol 303: G298 –G310, 2012. First published May 30, 2012;
doi:10.1152/ajpgi.00457.2011.—Postprandial secretion of insulin
and glucose-dependent insulinotropic polypeptide (GIP) is differentially regulated by not only dietary carbohydrate but also fat. Recent
studies have shown that the ingestion of diacylglycerol (DAG) results
in lower postprandial insulin and GIP release than that of triacylglycerol (TAG), suggesting a possible mechanism for the antiobesity
effect of DAG. The structural and metabolic characteristics of DAG
are believed to be responsible for its beneficial effects. This study was
designed to clarify the effect of 1-monoacylglycerol [oleic acid-rich
(1-MO)], the characteristic metabolite of DAG, on postprandial insulin and GIP secretion, and the underlying mechanism. Dietary 1-MO
dose dependently stimulated whole body fat utilization, and reduced
high-fat diet-induced body weight gain and visceral fat accumulation
in mice, both of which are consistent with the physiological effect of
dietary DAG. Although glucose-stimulated insulin and GIP release
was augmented by the addition of fat, coingestion of 1-MO reduced
the postprandial hormone release in a dose-dependent manner. Either
glucose or fatty acid transport into the everted intestinal sacs and
enteroendocrine HuTu-80 cells was also reduced by the addition of
1-MO. Reduction of either glucose or fatty acid transport or the
nutrient-stimulated GIP release by 1-MO was nullified when the
intestine was pretreated with sodium-glucose cotransporter-1
(SGLT-1) or fatty acid translocase (FAT)/CD36 inhibitor. We conclude that dietary 1-MO attenuates postprandial GIP and insulin
secretion by reducing the intestinal transport of the GIP secretagogues, which may be mediated via SGLT-1 and FAT/CD36. Reduced secretion of these anabolic hormones by 1-MO may be related
to the antiobesity effect of DAG.
1-MO ATTENUATES POSTPRANDIAL GIP RELEASE
ingly, we investigated how 1-MO affected intestinal transport
of GIP secretagogues (glucose and fatty acid) via SGLT-1 and
FAT/CD36 to elucidate the mechanism of reduction of GIP
secretion by 1-MO.
MATERIALS AND METHODS
Table 1. Composition of the semipurified experimental diet
Group
LF
HF
HF ⫹3% 1-MO
HF ⫹6% 1-MO
Potato starch, %
1-MO, %
Sucrose, %
Triglycerides, %*
Lard, %
Casein, %
Cellulose, %
Mineral mixture, %
Vitamin mixture, %
Energy, kcal/ g
66.5
28.5
0
13
25
5
20
4
3.5
1
5.2
25.5
3
13
25
5
20
4
3.5
1
5.4
22.5
6
13
25
5
20
4
3.5
1
5.5
5
20
4
3.5
1
4.0
LF, low-fat diet; HF, high-fat diet; MO, monoacylglycerol. *Fatty acid
composition of triglycerides was oleic/stearic/linoleic acid/others ⫽ 35.24/
2.00/48.15/14.61 (%).
system (36) equipped with a sixteen-chamber airtight metabolic cage
(ARCO2000-RAT/ANI 16 Chamber System; ARCOSYSTEMS,
Chiba, Japan). Air flow through the metabolic cage was adjusted to
0.4 l/min.
Mice were acclimated to metabolic cages for 24 h with free access
to food and water. Data were collected continuously for 3 days with
a settling time of 15 s and a measuring time of 15 s, and the reference was
room air. Therefore, data for each chamber could be obtained every 4.25 min.
The respiratory exchange ratio (RER) and substrate utilization
were calculated from the measured values of oxygen consumption
(V̇O2) and carbon dioxide production (V̇CO2) according to a previous study (36).
Oral glucose and fat load study. An oral glucose and fat load study
was conducted as described previously (35). Briefly, overnight-fasted
mice were gastrically administered D-(⫹)-glucose (2 mg/g body wt)
alone, or glucose plus glyceryl trioleate (TO; 2 mg/g body wt each)
with or without 1-oleoyl-rac-glycerol (1-MO). Lecithin (from egg
yolk, 0.08 mg/g body wt) and albumin (from bovine serum, 0.04 mg/g
body wt) were included in all test solutions containing oil, and these
were subsequently sonicated to obtain stable emulsions. The addition
of a minimal amount of albumin was necessary to obtain stable
emulsions with 1-MO. After being determined by laser light-scattering spectrometry (model SALD-2100; Shimadzu, Kyoto, Japan), the
particle size distributions of the two emulsions were found to be
similar, with mean diameters of 2.221 ⫾ 0.203 and 2.391 ⫾ 0.293 ␮m
in control and 1-MO-containing emulsion, respectively. We preliminarily confirmed that the minimal amount of lecithin and albumin did
not affect blood glucose and insulin responses. Accordingly, we used
the glucose solution as a control sample (35). Blood samples (⬃50 ␮l)
were collected from the orbital sinus under anesthesia using a heparinized capillary tube. They were kept on ice until plasma preparation.
After centrifugation, plasma was stored at ⫺80 °C until analysis.
For the first GIP analysis, we collected whole blood samples via the
abdominal vein for single blood time points (0, 10 min) to estimate the
peak GIP levels circulating in the whole body (35). Overnight-fasted
mice were divided into six groups. One group was designated the 0
min group, and blood samples were taken from the abdominal vein.
One group was administered glucose alone, and the remaining four
were administered glucose plus TO with or without 1-MO. Blood
samples were taken from the administered groups at 10 min after
gastric gavage via the abdominal vein. Blood samples were collected
into capillary blood collection tubes containing dipeptidyl peptidase
IV inhibitor (Millipore, Tokyo, Japan) and maintained on ice. After
centrifugation, plasma was stored at ⫺80°C until total GIP analysis.
Primary crypt culture. Primary duodenal crypt culture was prepared following the method described by Parker et al. (29) Freshly
removed mouse duodenum was digested with collagenase XI (0.4
mg/ml) for 40 min at 37°C. Digested tissue was centrifuged and
resuspended in DMEM (25 mmol/l glucose) supplemented with 10%
FCS, 2 mmol/l L-glutamine, penicillin, and streptomycin. The tissue
suspension was incubated in Matrigel (BD Biosciences, Bedford,
MA)-coated 24-well plates for 24 h at 37°C under 5% CO2. Viability
of the crypt culture was verified using LDH assay kit (cat. no. TOX7,
In Vitro Toxicology Assay Kit, lactate dehydrogenase-based; Sigma).
In vitro GIP secretion study. GIP secretion study from primary
crypt culture was also performed as described previously (29). Briefly,
the primary crypt culture was washed twice and incubated in the lipid
micelle [sodium taurocholate (2 mmol/l), oleic acid (0.4 mmol/l), and
either 1-MO or 2-MO (0.2 mmol/l each)] suspended in bath solution
(29) containing 0.1% fatty acid-free BSA for 2 h at 37°C. The
composition of the lipid micelle was in accordance with that of a
previous study (3), but slightly modified because of cell toxicity of the
micelle components. The medium was collected and centrifuged to
remove any contaminating cells. The plated cells were disrupted with
lysis buffer (29) to extract intracellular GIP. Total GIP was determined in the supernatant fraction and cell lysis sample using a total
GIP ELISA kit. GIP secretion in each well is expressed as a fraction
AJP-Gastrointest Liver Physiol • doi:10.1152/ajpgi.00457.2011 • www.ajpgi.org
Materials. All reagents for the experiments were from SigmaAldrich Japan (Tokyo, Japan), unless otherwise stated. Commercially
available 1-MO of food additive grade was from Kao Corporation
(cat. no. O-95R; Tokyo, Japan). The ester distributions and fatty acid
compositions of 1-MO were determined by gas chromatography. The
fatty acid composition of 1-MO was oleic/stearic/linoleic acid/others ⫽
71.33/4.39/13.48/10.80 (%).
Animals. Male C57BL/6J mice (7– 8 wk old; CLEA Japan, Tokyo,
Japan) were housed at four or five per cage in a temperature- and
relative humidity-controlled (23 ⫾ 2.0°C, 55 ⫾ 10%) room with a
12:12-h light-dark cycle with lights on at 0700. Mice were fed standard
chow (cat. no. CE-2, CLEA Japan) consisting of 3.47 kcal/g, with 4.6%
fat, 51.4% carbohydrate, and 24.9% protein. Food and water were
provided ad libitum. All animal experiments were conducted in the
Experimental Animal Facility of Kao Tochigi Institute. The Animal Care
Committee of Kao Tochigi Institute approved the present study.
HF diet-induced obesity. Eight mice were assigned to each of the
indicated groups (Table 1). The average body weight was adjusted
among each group. Each group had free access to the semipurified
powder diet (Table 1). The composition of the respective diets was
based on our previous study on the antiobesity effect of DAG (21). A
dome-type cover on the feeding dish (model Roden CAFE; Oriental
Yeast) was used to avoid the scattering of the powdered diet in the
cage. The energy values for each diet were calculated from the
macronutrient composition using values of 4, 4, and 9 kcal/g for
carbohydrate, protein, and oil, respectively.
Mice were maintained for 9 wk. Individual body weights were
monitored weekly. Food intake was measured on a per-cage basis
throughout the study every 2 or 3 days. Food intake was determined
by subtracting the remaining food weight from the initial food weight
on the previous feeding day. The energy intake was calculated from
the food intake and macronutrient composition of each diet. Mice were
anesthetized by the inhalation of sevoflurane (Maruishi Pharmaceutical,
Osaka, Japan). Blood samples were collected from the abdominal vein
into capillary blood collection tubes (model CAPIJECT with EDTA2Na; Terumo Medical, Tokyo, Japan) and maintained on ice until
plasma preparation. The epididymal, mesenteric, perirenal, and retroperitoneal white adipose tissues (WAT) and liver were removed and
weighed. After centrifugation at 3,500 g for 15 min at 4°C, plasma
samples were stored at ⫺80°C until analysis.
Indirect calorimetry. Respiratory metabolic performance in the
sedentary condition was measured using an indirect calorimetric
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of the total GIP measured in that well and is normalized to the basal
secretion measured in parallel on the same day.
Western blot analysis. The jejunum was removed, and mucosa was
scraped off from mice. Fifteen milligrams of protein were separated
by SDS-PAGE and transferred to a polyvinylidene difluoride membrane (Immun-Blot polyvinylidene difluoride membrane; Bio-Rad,
Hercules, CA). Immunoblotting was performed using goat anti-CD36
(mouse) IgG (cat. no. AF2519, 1:2000; R&D Systems, Minneapolis,
MN). Antibody labeling was detected by chemiluminescence (Pierce
Western Blotting Substrate Plus; Thermo Scientific, Rockford, IL)
and visualized using a ChemiDoc XRS plus imager (Bio-Rad).
Immunohistochemistry. Mouse duodenum was fixed in 4% paraformaldehyde. Frozen sections were blocked with 10% donkey serum
in PBS and incubated with a mixture of rabbit anti-GIP (porcine)
serum (cat. no. T-4340, 1:400; Peninsula Laboratories) and goat
anti-CD36 (mouse) IgG (cat. no. AF2519, 1:200; R&D Systems,
Minneapolis, MN) for 2 h at room temperature. Bound GIP and
FAT/CD36 antibodies were detected using an Alexa Fluor 488conjugated donkey anti-rabbit IgG and Alexa Fluor 555-conjugated
donkey anti-goat IgG secondary antibody (1:500; Invitrogen, Carlsbad, California). Tissue samples stained with the normal rabbit IgG
(10 ␮g/ml; Dako, Glostrup, Denmark) served as a negative control.
Prolong Gold Antifade Reagent with 4,6-diamidino-2-phenylindole
(Invitrogen) was used for staining nuclei. All fluorescence images
were captured using an all-in-one fluorescence microscope (model
BIOREVO BZ-9000; Keyence, Osaka, Japan) with a ⫻100/NA 1.4
objective, at an optical slice thickness of 4 ␮m.
Glucose and fatty acid transport. Rat jejunal everted sacs were
prepared as described previously (37). The functional integrity of the
everted sac preparation was assessed on the basis of its ability to
accumulate radiolabeled D-glucose and oleate in the tissue and transfer
it to the serosal solution in a time-dependent manner. Glucose (5%), TO
Fig. 1. . Average (A) and final body weights (B),
white adipose tissue (WAT) weights (C), dietary intake (D), body weight gain (E), and
WAT weights per calorie intake (F) in
C57BL/6J mice fed on 5 or 30% triglycerides
(TG)- or 1-monoacylglycerol (1-MO)-supplemented diets for 9 wk. Data are expressed as
means ⫾ SE; n ⫽ 8 in each group. Mice were
fed a low-fat diet (Œ), high-fat (HF) diet (), or
HF diet supplemented with 3 (Œ) or 6% ()
1-MO. B–F: data are expressed as means ⫾ SE;
n ⫽ 8 in each group. LF, low-fat diet. Statistical
analysis was conducted using 1-way ANOVA
followed by Dunnett post hoc test. *P ⬍ 0.05
(for the difference from the HF diet group).
N.S., not significant.
Glucose and fatty acid uptake study with HuTu-80 cells. The
HuTu-80/SGLT-1 cells were grown to 90% confluence. The cells
were then split and seeded into 12-well plates (5 ⫻ 105 cells/well).
The cells were cultured in glucose-free and FBS-free DMEM medium
for 1 h, before nutrient uptake.
To examine the effect of phloridzin on glucose uptake, the cells
were washed twice with PBS containing CaCl2 (1.8 mM) and MgCl2
(1 mM) [PBS (⫹)] buffer and then incubated in 0.5 ml of PBS (⫹)
buffer containing [14C]␣-methyl glucopyronoside (␣-MG) (0.5 ␮Ci/
ml, cat. no. NEC659; PerkinElmer, Waltham, MA) and 100 ␮M
␣-MG with or without 0.5 mM phloridzin for 10 or 30 min at 37°C.
In the case of SSO inhibition study, the cells were incubated in 0.5
ml of PBS (⫹) buffer with or without 0.5 mM SSO for 30 min at 37°C
followed by incubation in [3H]oleate (0.5 ␮Ci/ml, cat. no. NET289,
PerkinElmer) and 100 ␮M oleate for 30 min at 37°C.
To determine the effect of 1-MO on nutrient uptake, the cells were
incubated in 0.5 ml of PBS (⫹) buffer containing [14C] ␣-MG (0.5
␮Ci/ml), 100 ␮M ␣-MG, [3H]oleate (0.5 ␮Ci/ml), 100 ␮M oleate and
1% TO with or without 0.2% 1-MO for 10 or 30 min at 37°C.
The cells were washed three times with PBS (⫹) buffer and solubilized with 0.5 ml of 0.1 N NaOH, which was added to 10 ml of
Scintillation Cocktail (Hionic-Fluor, PerkinElmer). The radioactivity of
3
H and 14C in the cell lysate was measured using an automatic liquidscintillation spectrometer (Tri-Carb 2550 TR/LL; Packard, Rungis,
France).
Plasma analysis. Blood glucose was determined using a blood
glucose selfmonitoring device (Accu-Check Comfort; Roche Diagnostics, Tokyo, Japan) immediately after blood collection. Plasma
insulin was determined using a rat/mouse insulin ELISA kit and rat
insulin as the standard (Morinaga Institute of Biological Science,
Yokohama, Japan). Triglycerides (TG) were assessed with the triglyceride E-test (Wako Pure Chemical Industries, Osaka, Japan). Total
GIP was measured using a total GIP ELISA kit (Merck Millipore).
Statistical analysis. Values are means ⫾ SE. Glycemic, insulinemic, and GIP responses were assessed by calculating the incremental
area under the curve (AUC) using the trapezoid rule. The glucose
elimination constant (KG) was estimated as the glucose elimination
rate between 10 and 30 min: KG(%/min) ⫽ ([glucose]10 min ⫺
[glucose]30 min)/[glucose]10 min/20 min.
Pearson’s correlation coefficients were obtained to estimate the
linear correlation. Student’s t-tests after a preliminary F-test of the
homogeneity of within-group variance were used when comparing
values between the groups. In indirect calorimetry, two-way repeatedmeasures ANOVA was adopted to compare V̇O2 and RER between
the two groups. When more than two groups were compared, statistical analysis was conducted using a one-way ANOVA followed by
Dunnett or Bonferroni/Dunn post hoc tests (STATVIEW for Windows version 5.0; SAS Institute, Cary, NC).
Fig. 2. Energy homeostasis of 1-MO-fed mice. Mice
were housed in metabolic chambers for 4 days (days
0 –3, including a 24-h adaptation period) and calorimetric data were collected from days 1–3. Total energy
expenditure (A) and fat utilization (B) of mice for 70 h.
Data are expressed as means ⫾ SE; n ⫽ 5 in each group.
Statistical analysis was conducted using 1-way ANOVA
and, subsequently, Bonferroni/Dunn multiple comparison. a,bMeans not sharing a given letter differ significantly (P ⬍ 0.0167).
(5%), lecithin (from egg yolk, 0.2%), and [14C]D-glucose (0.2 ␮Ci/ml,
CFB96), [14C]D-maltose (0.2 ␮Ci/ml, CFB182), or [14C]mannitol (0.2
␮Ci/ml, CFB238), as well as [3H]oleic acid (0.2 ␮Ci/ml, TRK140)
(GE Healthcare, Buckinghamshire, UK) with or without 1-MO (1%)
were subsequently sonicated to obtain stable emulsions. Everted sacs
were incubated in the emulsion (4 ml). Incubations were carried out
for 10 min at 37°C, after which serosal solutions were removed; the
sacs were rinsed with 0.1% N-acetyl cysteine and 0.1% taurocholate,
and transferred to preweighed glass vials. The radioactivity of 3H and
14
C in the tissue samples was measured using an automatic liquidscintillation spectrometer (Tri-Carb 2550 TR/LL; Packard, Rungis,
France).
Effect of phloridzin on glucose-induced GIP secretion and glucose
transport. Overnight-fasted Sprague-Dawley rats (8 wk old; Japan
SLC, Shizuoka, Japan) were gastrically administered 5% glucose
(1 mg/g body wt) with or without phloridzin (10 mM), and blood
samples were collected from the tail vein.
In the glucose and fatty acid transport experiment with phloridzin, the
everted sacs were treated in glucose plus TO emulsion with or without 10
mM phloridzin at 37 °C for 10 min, followed by rinsing with 0.1%
N-acetyl cysteine and 0.1% taurocholate.
Effect of sulfo-N-succinimidyl oleate on fat-induced GIP secretion
and fatty acid transport. Sulfo-N-succinimidyl oleate (SSO), an inhibitor of FAT/CD36 (6, 30), was synthesized as previously reported
(12). Overnight-fasted rats were anesthetized by the intraperitoneal
injection of 7% urethane and 1.5% chloral hydrate. Under anesthesia,
the duodenum was cannulated using a polyethylene tube and infused
with Krebs buffer containing 0.2% BSA with or without 200 ␮M SSO
for 60 min at a rate of 3 ml/h. The solutions were replaced with 10%
TO emulsion containing 0.2% lecithin for 30 min, followed by Krebs
buffer containing 0.2% BSA for 30 min. Blood samples were collected from the tail vein.
In the glucose and fatty acid transport experiment with SSO, the
everted sacs were pretreated in Krebs buffer containing 0.2% BSA with
or without 200 ␮M SSO at 37°C for 25 min followed by rinsing twice
with 4 ml of Krebs buffer containing 0.2% BSA for 20 s. The sacs were
treated in glucose plus TO emulsion at 37 °C for 10 min, followed by
rinsing with 0.1% N-acetyl cysteine and 0.1% taurocholate.
HuTu-80 cells and transfection with human SGLT-1. The cDNA of
human SGLT-1 was purchased from GeneCopoeia (cat. no. EXC0294-M02; Germantown, MD). HuTu-80 human duodenal endocrine cell line (31) (Cell Lines Service, Eppelheim, Germany) was
cultured in DMEM/Ham’s F12 medium supplemented with 2 mM
L-glutamine and 10% FBS. Prior to transfection, cells were split and
seeded for 24 h, and transfection was carried out using Lipofectamine
2000 (Invitrogen, Carlsbad, California) according to the manufacturer’s
instructions. Stable expression was achieved by splitting the cells 48 h
after transfection into medium supplemented with 0.6 mg/ml Geneticin (G418). Clonal SGLT-1-expressing cell line (HuTu-80/SGLT-1)
was obtained by serial dilution.
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RESULTS
HF diet-induced obesity. The body and visceral WAT
weights of mice fed a HF diet were significantly higher than
those of mice fed an low-fat diet. The mice fed the HF diet
containing 3 or 6% 1-MO weighed less than those fed the HF
diet (Fig. 1, A and B). The body and total WAT weights were
significantly lower in the 6% 1-MO diet-fed mice than in those
on the HF diet (Fig. 1, B and C). Since dietary intake tended to
be lower (P ⫽ 0.17) in the 6% 1-MO diet-fed than in the HF
diet-fed groups (Fig. 1D), we calculated the food efficiency by
dividing the body weight gain and total WAT weight by the
food intake. Dietary supplementation with 6% 1-MO reduced
the food efficiency for either the body or the visceral WAT
weight in a dose-dependent manner (Fig. 1, E and F). Body and
visceral WAT weights per food intake were significantly lower
in the 6% 1-MO-fed than in the HF diet groups.
Fig. 3. Plasma concentrations of glucose (A), insulin
(C), and TG (E) and areas under the curve (AUCs)
immediately before and 10, 30, 60, and 120 min after
the administration of glucose alone [2 mg/g body wt
(BW), Œ], glucose plus glyceryl trioleate (TO; 2 mg/g
body wt each, ), or 1-MO [0.08 (gray diamonds), 0.2
(gray triangles), or 0.4 mg/g body wt (gray squares)]
together with glucose plus TO (2 mg/g body wt each)
through gastric gavage in overnight-fasted anesthetized
male C57BL/6J mice. AUCs are calculated from 0 –120
min for glucose (B), and from 0 –30 min for insulin (D).
Data are expressed as means ⫾ SE; n ⫽ 8 in each group.
Increase in the plasma concentration of glucose-dependent insulinotropic polypeptide (GIP) at 10 min after the
administration of glucose alone (2 mg/g body wt) or
glucose ⫹ TO (2 mg/g body wt each) with 1-MO
(0 – 0.4 mg/g body wt) through gastric gavage in overnight-fasted anesthetized male C57BL/6J mice (F).
Whole blood samples were collected via the abdominal
vein for single blood time points (0, 10 min) to estimate
total GIP circulating in the whole body. Data are calculated by subtracting the average initial concentration
from that after 10 min, and expressed as means ⫾ SE;
n ⫽ 12 in each group. Effect of monoolein on GIP
secretion from mouse duodenum crypt culture (G). The
crypt culture was incubated for 2 h in the lipid micelle
[sodium taurocholate (2 mmol/l), oleic acid (0.4 mmol/l), and
either 1-MO or 2-MO (0.2 mmol/l each)] suspended in
control buffer. Data are normalized to the basal secretion after control buffer, and expressed as means ⫾ SE;
n ⫽ 6 in each group. Statistical analysis was conducted
using 1-way ANOVA followed by Dunnett post hoc test
[for A–F; *P ⬍ 0.05 (for the difference from the
high-fat diet group)] or Bonferroni/Dunn multiple comparison [for G; a,b,cMeans not sharing a given letter
differ significantly (P ⬍ 0.0167)].
glucose administration and thereafter declined, reaching the
baseline after 30 min (Fig. 3C). When TO was administered
together with glucose, the insulin level was significantly increased at 10 min, followed by a marked decline of blood glucose
at 30 min (Fig. 3, A and C). The glucose response was significantly decreased after glucose plus TO compared with that after
glucose (⬃30% decrease in AUCGlucose120 min, Fig. 3B).
The coadministration of 1-MO with glucose plus TO decreased peak insulin levels and increased blood glucose at 30
min in a dose-dependent manner (Fig. 3, C and A, respectively)
but did not affect peak blood glucose levels. Blood glucose at
30 min was negatively (R ⫽ ⫺0.636, P ⬍ 0.0001) and KG (the
glucose elimination rate between 10 and 30 min) was positively (R ⫽ 0.495, P ⬍ 0.01) correlated with the peak insulin
level at 10 min. The postprandial insulin response (AUC) for
30 min was decreased dose dependently upon 1-MO administration (Fig. 3D), which was associated with an increased glucose
response for 120 min (Fig. 3B). Peak insulin and triglycerides
Fig. 4. Plasma concentrations of glucose (A and D),
insulin (B and E), and GIP (C and F) immediately before
and 10, 30, 60, and 120 min after the administration of
glucose alone (2 mg/g body wt, Œ), glucose (G) ⫹ TO
(2 mg/g body wt each, ), or 1-MO (0.4 mg/g body wt,
gray squares) together with G ⫹ TO (2 mg/g body wt
each) through gastric gavage in overnight-fasted normal
(A–C) or obese (D–F) mice. Inset: AUCs are calculated
from 0 –120 min for glucose, and from 0 –30 min for
insulin and GIP. Data are expressed as means ⫾ SE; n ⫽
9 (A–C) or 5 (D–F) in each group. Statistical analysis of
the AUC was conducted using Student’s t-tests after a
preliminary F-test of the homogeneity of within-group
variance (A–C; *P ⬍ 0.05; **P ⬍ 0.01) or 1-way
ANOVA followed by Dunnett post hoc test [D–F; *P ⬍
0.05 (for the difference from the G ⫹ TO group)].
Energy expenditure and fat utilization. In the HF diet-fed
condition, RER was significantly lower in the 1-MO-fed
groups (0.844 ⫾ 0.005 and 0.829 ⫾ 0.003 for 3 and 6%,
respectively) than in the control group (0.856 ⫾ 0.006), while
energy expenditure was similar between the groups. Thus, fat
utilization was significantly higher in the 6% 1-MO, and
tended to be higher (P ⫽ 0.084) in the 3% 1-MO groups, than
in the control group (Fig. 2B), although total energy expenditure did not differ between the groups (Fig. 2A). Dietary intake
of the mice housed in metabolic chambers was not different
between the groups (P ⬎ 0.05). Body weight gains of the mice
housed in metabolic chambers were 1.04 ⫾ 0.27, 0.93 ⫾ 0.24,
and 0.58 ⫾ 0.24 g in the control, 3%, and 6% 1-MO groups,
respectively, and they were not significantly different (P ⬎
0.05).
Postprandial blood responses. Glucose administration increased plasma glucose levels at 10 min, which thereafter
declined (Fig. 3A). Insulin levels peaked at 10 min after
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insulin, and GIP levels after either glucose or glucose plus TO
administration were higher in the HF diet-fed obese mice (44.5 ⫾
0.7 g in body wt, Fig. 4, E and F) than in the standard chow
(CE-2)-fed normal mice (18.5 ⫾ 0.1 g in body wt, Fig. 5, A–C).
When 1-MO was administered with glucose plus TO, peak insulin
and GIP levels decreased. Postprandial insulin (AUCInsulin30 min)
and GIP (AUCGIP60 min) responses were significantly lower in
the 1-MO-administered mice than in the control (glucose plus
TO-administered) mice (Fig. 4, E and F). The blood glucose
response did not differ between the 1-MO-administered and
control groups (Fig. 4E). Fasting blood triglycerides levels and
postprandial blood TG responses were lower in the obese mice
than in the normal mice. The blood TG response did not differ
between the 1-MO-administered and control groups (data not
shown).
Effect of 1-MO on intestinal glucose and fatty acid
transport. Transport of radiolabeled glucose, maltose, and
oleate from the emulsion into the jejunal sacs for 10 min was
significantly lower when the emulsion was supplemented with
1-MO (Fig. 5, A–C). Mannitol transport into the jejunal sacs
from the emulsion was not changed by the addition of 1-MO
(Fig. 5D).
Effects of inhibitors of glucose and fatty acid transport on
blood GIP response. The coadministration of phloridzin with
glucose significantly decreased the peak levels and postprandial increases of blood glucose (Fig. 6, A and B), insulin (Fig.
6, C and D), and GIP (Fig. 6, E and F) compared with those
with glucose alone in rats.
In addition, the peak GIP levels during duodenal TO infusion
were significantly reduced when TO was infused with SSO (Fig.
6G). The GIP response during the infusion period (60 min) was
significantly lower in SSO-treated than in control groups.
Fig. 5. Nutrient transport from emulsion to jejunal sacs.
[14C]glucose (A), [14C]maltose (B), [3H]oleate (C), and
[14C]mannitol (D) transported from emulsion with or
without 1-MO were determined from radioactivity accumulated in the jejunal tissue. Data are expressed as
means ⫾ SE; n ⫽ 8 in each group. Student’s t-tests after
a preliminary F-test of the homogeneity of within-group
variance were used when comparing values between the
groups. Probability level of random differences between
groups: *P ⬍ 0.05; **P ⬍ 0.01.
levels after glucose plus TO administration were significantly
decreased by the addition of 0.4% 1-MO to the levels after the
administration of glucose alone (Fig. 3, C and E).
To study the underlying mechanism of the inhibitory effect
of 1-MO on the fat-promoted blood insulin response that
occurred at 10 min after gavage, we examined the postprandial
response of GIP. In our previous study, the blood GIP level
was shown to peak at 10 min after gastric gavage (35).
Glucose administration increased circulating blood GIP levels at 10 min after gastric gavage (Fig. 3F). When TO was
administered together with glucose, the GIP level was significantly increased. However, the addition of 1-MO significantly
decreased the circulating GIP levels after glucose plus TO
administration in a dose-dependent manner (Fig. 3F). The
fat-promoted GIP response for 10 min was significantly inhibited by 0.4% 1-MO administration (P ⫽ 0.011) (Fig. 3F).
Effect of 1-MO on GIP secretion in vitro. To study how
1-MO affects the nutrient-induced GIP release from intestine,
we examined the effect of lipid micelle containing either 1-MO
or 2-MO on GIP secretion from duodenum crypt culture.
Whereas each lipid micelle stimulated GIP secretion from crypt
culture into medium, significantly less GIP was secreted (P ⬍
0.001) for lipid micelle containing 1-MO compared with that
containing 2-MO (Fig. 3G). The GIP release for 1-MO lipid
micelle was only half of that for 2-MO lipid micelle (Fig. 3G).
Effect of 1-MO on postprandial blood responses in HF
diet-fed obese mice. Several studies have shown that postprandial GIP secretion was increased after chronic HF feeding (10)
and in obese subjects (7). To examine the effect of 1-MO on
GIP hypersecretion, mice were maintained on a semipurified
HF diet for 23 wk to induce obesity, and we conducted glucose
and fat loading tests. Fasting and postprandial blood glucose,
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Localization of FAT/CD36 in GIP-producing K cells. Goat
anti-CD36 (mouse) IgG (cat. no. AF2519; R&D Systems) reacted
with a single band of FAT/CD36 in Western blot analysis of
mouse duodenal protein (Fig. 7A). Immunohistochemistry using
the antibody showed the apical localization of FAT/CD36 in
epithelial cells of the villus. GIP-positive staining was observed in
the cytoplasmic area of isolated epithelial cells. Continual FAT/
CD36-positive staining of the apical area and the GIP-positive
cytoplasmic area were colocated in the isolated, spindle-shaped
epithelial cells (Fig. 7, B–F).
Effect of glucose or fatty acid transport inhibition on 1-MO
activity. Figure 8, A and D shows the effects of 1-MO on glucose
(Fig. 8A) and oleate (Fig. 8D) transport with SGLT-1 and FAT/
CD36 inhibitors, respectively. Phloridzin reduced glucose transport (Fig. 8A), but did not affect oleate transport (Fig. 8B), from
the emulsion into the everted sacs. Glucose transport into the
Fig. 6. Plasma concentrations of glucose (A), insulin (C),
and GIP (E) after the administration of TO with () or
without (Œ) phloridzin through gastric gavage in male
Sprague-Dawley rats. AUCs are calculated from 0 –120
min for glucose (B) and from 0 – 60 min for insulin (D) and
GIP (F). Data are expressed as means ⫾ SE; n ⫽ 5 in each
group. G: plasma concentrations of GIP after the duodenal
infusion of TO emulsion with () or without (Œ) pretreatment with sulfo-N-succinimidyl oleate (SSO), a fatty acid
translocase (FAT)/CD36 inhibitor, in anesthetized male
Sprague-Dawley rats. The bar means the infusion of TO
emulsion. N ⫽ 8 in each group. Student’s t-tests after a
preliminary F-test of the homogeneity of within-group
variance were used when comparing values among the
groups. Probability level of random differences between
groups: *P ⬍ 0.05; **P ⬍ 0.01; ***P ⬍ 0.001.
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tissue was significantly reduced by 1-MO when the emulsion did
not contain phloridzin. However, glucose transport was not reduced further by adding 1-MO when the emulsion was supplemented with phloridzin (Fig. 8A). 1-MO reduced intestinal oleate
transport from the emulsion either with (P ⫽ 0.0051) or without
(P ⫽ 0.029) phloridzin (Fig. 8B).
Figure 8, C and D shows the effect of 1-MO on glucose (Fig.
8C) and oleate (Fig. 8D) transport with or without pretreatment
of SSO. SSO reduced oleate transport (P ⫽ 0.023) (Fig. 8D),
but did not affect glucose transport (Fig. 8C), from the emulsion into the tissue. Oleate transport was not reduced further by
adding 1-MO when the intestine was pretreated with SSO (Fig.
8D). Glucose transport tended to be reduced by 1-MO whether
the sacs were pretreated with SSO (P ⫽ 0.082) or not (P ⫽
0.0274) (Fig. 8C).
Effect of 1-MO on glucose or fatty acid uptake into human
enteroendocrine cells. Preliminary analysis by semiquantitive
RT-PCR showed that Hutu-80, human duodenal endocrine cell
line, expressed Gip and Cd36, but not Sglt1 mRNA. Therefore,
the cells were stably transfected with human SGLT-1 (HuTu-80/
SGLT-1) to examine the effect of 1-MO on cellular uptake of
glucose and fatty acid. HuTu-80/SGLT-1 cells had 10-fold greater
glucose uptake activity than original HuTu-80 cells. Cellular
uptake of radiolabeled ␣-MG and oleate was increased in a
time-dependent manner, and significantly blocked by phloridzin
and SSO (data not shown). Addition of 1-MO significantly reduced radiolabeled ␣-MG and oleate transported into the cells for
30 min to ⬃60% compared with those of control (Fig. 9, A and B).
DISCUSSION
This study had four major findings. First, this study demonstrated that dietary 1-MO stimulated fat utilization and attenuated HF diet-induced obesity and postprandial lipemia in
mice, either of which is consistent with the physiological effect
of dietary 1,3-DAG. Second, and more importantly, was that
1-MO attenuated postprandial GIP and insulin responses either
in normal or in HF diet-fed obese mice. Third, and the most
Fig. 7. FAT/CD36 protein detected by Western blot
analysis and localization of GIP and FAT/CD36 in the
mouse duodenum. A: a single band at 88 kDa was
detected by Western blot analysis of mouse jejunum
(a) and jejunal mucosa (b) using goat anti-CD36
(mouse) IgG (cat. no. AF2519, 1:2,000; R&D Systems). Immunohistochemistry of mouse duodenum
for (B), GIP (green) using rabbit anti-GIP (porcine)
serum (cat. no. T-4340, 1:200; Peninsula Laboratories) and (C), FAT/CD36 (red) using goat anti-CD36
(mouse) IgG (cat. no. AF2519, 1:200; R&D Systems).
D: nuclei were stained with 4,6-diamidino-2-phenylindole (blue). Merged 2D (E) and 3D (F) images
show colocalization of GIP-positive cytoplasmic area
(green) and FAT/CD36-positive apical area (red) in
the isolated, spindle-shaped epithelial cell. White arrowheads indicate GIP-positive K cells.
importantly, was the novel finding that FAT/CD36 was located
on the apical membrane of GIP-producing K cells in the mouse
duodenum, and fat-stimulated GIP secretion was reduced by
SSO, suggesting that fatty acid transport via FAT/CD36 plays
a significant role in fat-stimulated GIP secretion from K cells.
Finally, this study provides evidence that the luminal 1-MO
decreased the intestinal transport of GIP secretagogues via
SGLT-1 and FAT/CD36, but not passive transport of mannitol.
The increased fat utilization is thought to drain fatty acids
from the body, reduce VLDL formation, and exert antiobesity
effects (4). Several studies demonstrated that the upregulation
of fat catabolism is associated with reduced fat accumulation in
rodent models of HF diet-induced or genetic insulin resistance
(2, 11, 22). The results in the present study suggest that the
antiobesity effect of 1,3-DAG is attributable to 1-MO, its
characteristic metabolite in the intestinal lumen. Coingestion of
1-MO also lowered the postprandial blood triglycerides response, suggesting that the decreased postprandial triglyceridemia after 1,3-DAG ingestion (32) may also be explained by
the specific metabolism of 1,3-DAG in the small intestine.
However, the effect of 1-MO of other fatty acids (e.g., linoleic
acid) still remains to be studied.
Although not vitiating the significance of the findings, dietary supplementation with 1-MO reduced the cumulative
energy intake of mice by ⬃9%. Although we cannot rule out
the possibility that 1-MO reduced diet-induced obesity by a
tendency of lowering the energy intake, body weight gain
could not be explained entirely by reduced energy intake since
feed efficiency (body weight gain per diet intake) was significantly lowered by 1-MO ingestion. We preliminarily confirmed that dietary 1-MO did not affect meal ingestion in the
short feeding period, suggesting that 1-MO does not induce
taste aversion in mice. Whereas calorie restriction has been
shown to decrease the metabolic rate in rodents (2, 18), 1-MO
did not affect the dietary intake and total energy expenditure,
and rather increased fat utilization in our indirect calorimetry.
The tendency of reduced cumulative energy intake in 1-MOfed mice in the longer feeding period may be attributable to a
lower energy requirement caused by a reduction of the body
weight compared with that of the HF diet-fed mice. As another
possibility, the long-term ingestion of 1-MO may directly
regulate appetite and satiety, for example, by affecting appetite-regulating gut hormones, such as ghrelin, CCK, PYY, or
GLP-1. Further studies are needed to clarify the effect of
chronic 1,3-DAG or 1-MO ingestion on gut-derived satiety
hormones and appetite/satiety control in the brain.
Although coingestion with 1-MO did not affect postprandial
peak blood glucose levels, it decreased peak insulin levels and
lowered the subsequent decrease in blood glucose. Blood
insulin level after 1-MO ingestion does not fall below, and
thereby blood glucose does not exceed, the level after glucose
alone. Thus, higher blood glucose caused by reduced insulin
response after 1-MO ingestion does not seem to be undesirable
from a therapeutic perspective.
Decreased levels of the anabolic hormones seem to be
beneficial to prevent the progression of obesity. Despite the
lack of a change in the activity of GIP or active GIP secretion,
this study suggests the intriguing possibility that luminal 1-MO
reduces the secretion of GIP and thereby lowers postprandial
insulin secretion. While we cannot exclude the possibility that
1-MO ingestion alters the renal or hepatic extraction of GIP or
insulin, respectively, the present results are consistent with our
previous findings (35), and suggest that lower GIP and insulin
levels after 1,3-DAG ingestion than those after TAG may be
explained by 1-MO.
From the result of indirect calorimetry, the magnitude of
increased fat utilization by 6% 1-MO ingestion was estimated
to be ⬃3 mg/g body wt/day. This increase in fat utilization
explains half of the decrease of visceral fat weight after 1-MO
ingestion for 9 wk. Furthermore, the inhibition of GIP signaling has been shown to increase fat oxidation and prevent the
onset of obesity induced by a HF diet (19). Our previous study
has shown that increased levels of circulating GIP or insulin
decreased the hepatic gene expression of ACO, the rate-
Fig. 8. Effect of 1-MO on nutrient transport from emulsion to jejunal sacs.
[14C]glucose (A and C) and [3H]oleate (B and D) transported from emulsion
with or without phloridzin (A and B), or pretreatment with SSO (C and D),
were determined from radioactivity accumulated in the jejunal tissue. Data are
expressed as means ⫾ SE; n ⫽ 10 (for A and B) or 12 (for C and D) in each
group. Statistical analysis of the AUC was conducted using 1-way ANOVA
and, subsequently, Bonferroni/Dunn multiple comparison. a,bMeans not sharing a given letter differ significantly (P ⬍ 0.0083).
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Fig. 9. Cellular uptake of nutrients by HuTu-80/SGLT-1
cells. A and B: uptake of [14C]-labeled [14C]␣-methyl
glucopyronoside (␣-MG) (A) or [3H]-labeled oleate (B)
into the cells (5 ⫻ 105 cells) was determined by radioactivity with () or without (Œ) treatment of either phloridzin
(A) or SSO (B). C and D: uptake of [14C]-labeled ␣-MG
(C) or [3H]-labeled oleate (D) with (black bars) or without
(white bars) 1-MO. Data are expressed as means ⫾ SE;
n ⫽ 3 in each group. Statistical analysis of the AUC was
conducted using Student’s t-tests after a preliminary F-test
of the homogeneity of within-group variance. *P ⬍ 0.05;
***P ⬍ 0.001.
activity. The results of cellular uptake of the GIP secretagogues, which was reduced by phloridzin, SSO, or 1-MO, were
consistent with those of nutrient-induced GIP secretion in vivo.
Taken together, these results suggest the intriguing possibility
that decreased GIP secretion due to 1-MO is attributable to
reduced or retarded active transport of the GIP secretagogues
via the transcellular pathway mediated by SGLT-1 or FAT/
CD36.
The possibility that 1-MO reduced fat absorption as the
mechanism behind its antiobesity effect cannot be excluded
since we did not examine overall lipid absorption from the
intestine. However, it seems to be unlikely that 1-MO ingestion
decreases whole-intestine lipid absorption, since 1,3-DAG-rich
oil did not increase lipid fecal excretion in rodents (27) even
though 1,3-DAG reduced fatty acid transport from the jejunal
sacs (Shimotoyodome A et al., unpublished data). In addition,
lipid administration to Cd36-deficient mice did not alter the
fecal lipid output (24) since FAT/CD36 mainly contributes to
fatty acid uptake by the proximal intestine (23). Taking
these results into account, dietary 1-MO does not seem to
inhibit whole-gut lipid absorption enough to increase fecal
lipid output.
Although intestinal uptake of glucose and fatty acid via
SGLT-1 and FAT/CD36 is basically common in rodents and
humans, there may be a limitation in this study due to the
difference of species since this study relied on different species
for postprandial GIP response, intestinal and cellular uptake of
nutrients. The molecular and cellular mechanism of reduced
GIP secretion by 1-MO remains to be definitively characterized.
In conclusion, this study provides evidence that 1-MO ingestion lowers nutrient-stimulated GIP secretion by reducing
glucose and fatty acid transport via SGLT-1 and FAT/CD36.
The present study suggests that 1-MO serves as an exogenous
physiological regulator of hormone secretion with an influence
on energy homeostasis. This should be important for the
further development of GIP-based therapy for the treatment of
obesity. Therefore, more detailed studies of GIP kinetics
should be undertaken in relation to energy homeostasis in
larger animals and humans.
ACKNOWLEDGMENTS
The authors thank Nanami Sugino for excellent technical support. The
authors also thank Kojiro Hashizume for support in the chemical synthesis of
sulfo-N-succinimidyl oleate.
limiting enzyme involved in peroxisomal fatty acid oxidation
and fat oxidation capacity, and whole body fat utilization (37).
Recently, Althage et al. (1) reported that the targeted ablation
of GIP-producing cells in transgenic mice enhanced energy
expenditure and reduced HF diet-induced obesity. The results
in this study are consistent with the above findings, and suggest
that lower blood GIP and insulin level after 1-MO ingestion
enhanced fat oxidation, and thereby prevented diet-induced
obesity.
Gastric emptying was shown to contribute to the postprandial glucose concentrations in healthy subjects and patients
with type 2 diabetes (16). However, it is unlikely that delayed
gastric emptying is responsible for the acute changes in GIP
secretion after the ingestion of 1-MO because the coingestion
of 1-MO did not delay the peak blood glucose and triglycerides
levels.
The results in this study suggested that fat-induced GIP
secretion was triggered by fatty acid transport via FAT/CD36
located in GIP-producing K cells. Parker et al. (29) have shown
the expression of Sglt1 mRNA and the apical localization of
SGLT-1 protein in K cells and inhibition of glucose-induced
GIP secretion by phloridzin. The inhibitory effect of phloridzin
on GIP secretion in this study was consistent with their results,
suggesting that glucose-induced GIP secretion is triggered by
glucose transport via SGLT-1 into K cells.
Recent reports suggest that G protein-coupled fatty acid
receptors, such as GPR40, 119, and 120, may play a role in the
regulation of enteroendocrine hormone release (5, 13, 28).
Hansen et al. (11) showed that 2-monoacylglycerol formed
during fat digestion can activate GPR119 and cause GLP-1
release from the human intestine. Hirasawa et al. (13) have
reported that long-chain fatty acids evoked ligand-induced
internalization of GPR120 and stimulated GLP-1 secretion
triggered by intracellular Ca2⫹ increase. Parker et al. (29) have
shown that murine K cells expressed these fatty acid receptors
including GPR119 and GPR120. Taking these findings together, long-chain fatty acids seem to stimulate GIP release by
a similar mechanism to that of GLP-1 release. However, the
molecular mechanism behind the relationship between GIP
secretion and the intracellular metabolism of fat in K cells
remains to be elucidated.
The reduction of glucose or fatty acid transport by 1-MO
was nullified when it was blocked by phloridzin (Fig. 8A) or
SSO (Fig. 8D), suggesting that 1-MO competes with each
specific inhibitor for the inhibition of SGLT-1 or FAT/CD36
GRANTS
The present study was supported financially by Kao Corporation.
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the author(s).
AUTHOR CONTRIBUTIONS
Author contributions: A.S. and T.H. conception and design of research;
A.S., N.O., K.O., T.M., C.S., F.O., and D.F. performed experiments; A.S.,
N.O., K.O., T.M., C.S., F.O., and D.F. analyzed data; A.S., N.O., K.O., T.M.,
C.S., F.O., D.F., and T.H. interpreted results of experiments; A.S. prepared
figures; A.S. drafted manuscript; A.S. edited and revised manuscript; A.S.,
N.O., K.O., T.M., C.S., F.O., D.F., and T.H. approved final version of
manuscript.
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