Center for Lipid and Arteriosclerosis Studies, Department of Pathology and Laboratory Medicine, University of Cincinnati College of Medicine, Cincinnati, Ohio 45267
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ABSTRACT |
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Group 1B
phospholipase A2 (PLA2) is an abundant
lipolytic enzyme that is well characterized biochemically and
structurally. Because of its high level of expression in the pancreas,
it has been presumed that PLA2 plays a role in the
digestion of dietary lipids, but in vivo data have been lacking to
support this theory. Our initial study on mice lacking PLA2
demonstrated no abnormalities in dietary lipid absorption in mice
consuming a chow diet. However, the effects of PLA2
deficiency on animals consuming a high-fat diet have not been studied.
To investigate this, PLA2+/+ and
PLA2/
mice were fed a western
diet for 16 wk. The results showed that PLA2
/
mice were resistant to
high-fat diet-induced obesity. This observed weight difference was due
to decreased adiposity present in the PLA2
/
mice. Compared with
PLA2+/+ mice, the
PLA2
/
mice had 60% lower plasma
insulin and 72% lower plasma leptin levels after high-fat diet
feeding. The PLA2
/
mice also did
not exhibit impaired glucose tolerance associated with the development
of obesity-related insulin resistance as observed in the
PLA2+/+ mice. To investigate the
mechanism by which PLA2
/
mice
exhibit decreased weight gain while on a high-fat diet, fat absorption
studies were performed. The
PLA2
/
mice displayed 50 and 35%
decreased plasma [3H]triglyceride concentrations 4 and
6 h, respectively, after feeding on a lipid-rich meal containing
[3H]triolein. The PLA2
/
mice also displayed increased lipid content in the stool, thus indicating decreased fat absorption in these animals. These results suggest a novel role for PLA2 in the protection against
diet-induced obesity and obesity-related insulin resistance, thereby
offering a new target for treatment of obesity and diabetes.
phospholipase A2; lipase; pancreatic enzymes; animal models; lipid absorption
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INTRODUCTION |
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THE PHOSPHOLIPASE A2 (PLA2) lipolytic enzyme is an abundant protein secreted by the pancreas in response to food intake. It belongs to the Group 1 class B type of secretory PLA2 (7) and is capable of hydrolyzing the fatty acyl bond at the sn-2 position of phospholipids to generate free fatty acids and lysophospholipids in the intestinal lumen. In addition to its prominent expression in the pancreas, the Group IB PLA2 is also expressed in other tissues (14, 30, 31, 33). Phospholipase A2 has been one of the most extensively studied enzymes in terms of structure and mechanism of action, due to its abundant availability, stability, and ease of isolation. Despite the wealth of information known about the biochemical and structural characteristics of this enzyme (7, 8), the exact physiological function of PLA2 has not been completely delineated.
Phospholipids entering the digestive tract from the diet and bile comprise the second most abundant dietary lipid class found in the intestinal lumen (5). Therefore, it has been suggested that PLA2 functions in hydrolyzing these phospholipids to forms that can be absorbed by the enterocytes. Various in vitro and in vivo studies have also implied that PLA2 hydrolysis of phospholipids in the intestinal lumen is required for the efficient absorption of cholesterol from the diet. Studies in rats and humans have shown that intraduodenal infusion of phosphatidylcholine results in decreased cholesterol absorption compared with subjects infused with lower levels of phosphatidylcholine (2, 17). Mackay et al. (24) identified PLA2 as the major protein in pancreatic extract that mediates cholesterol transport in Caco-2 cells. The addition of PLA2 relieved the phosphatidylcholine inhibition of cholesterol transport from bile salt micelles to Caco-2 cells (18). In addition, our laboratory (39) has shown that PLA2 hydrolysis of phospholipids on the surface of lipid emulsions was required before pancreatic lipase digestion of triglycerides in the core of lipid emulsions, therefore suggesting a role for PLA2 in fat absorption.
To investigate the role of PLA2 in intestinal lipid digestion and transport, we recently generated mice lacking PLA2. Using both lymph fistula and single-dose, dual-isotope fecal recovery methods, we demonstrated that mice deficient in PLA2 had no differences in the absorption of dietary lipids compared with wild-type mice (30). We concluded that, although phospholipid digestion in the intestinal lumen is a prerequisite for efficient absorption of dietary lipids, additional enzyme(s) in the digestive tract can compensate for the lack of PLA2 in catalyzing phospholipid digestion and facilitating lipid absorption in the PLA2 knockout mice (29). However, these studies were performed under basal chow-fed dietary conditions; therefore, dietary lipid absorption in the PLA2 knockout mice consuming a high-fat diet is not known.
The goal of the present study was to assess the lack of PLA2 in mice fed a western-type, high-fat/high-cholesterol diet. We hypothesize that the compensatory mechanisms present to compensate for the absence of PLA2 under basal chow-fed conditions may be overcome by feeding mice a high-fat diet. We report that PLA2-deficient mice are resistant to high-fat diet-induced obesity and that the mechanism for this effect is most likely due to decreased dietary fat absorption.
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EXPERIMENTAL PROCEDURES |
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Generation and maintenance of PLA2-deficient mice. The strategy used to disrupt the PLA2 gene to generate PLA2-deficient mice was described previously (29). All animals used in these studies were back-crossed seven times into the C57BL/6 background and were genotyped by PCR as previously described (29). The mouse colony was maintained in a temperature- and humidity-controlled room with a 12:12-h light-dark cycle and fed a rodent chow (LM485; Harlan-Teklad, Madison, WI) with free access to water. All animal protocols used in this study were approved by the Institutional Animal Care and Use Committee at the University of Cincinnati.
Diet-induced obesity study.
Wild-type (PLA2+/+) and
PLA2-deficient
(PLA2/
) mice were fed a standard
mouse chow (LM485) or a western-type, high-fat/high-cholesterol diet
containing 21% fat and 0.15% cholesterol by weight (TD88137, Harlan
Teklad) for 16 wk. For the insulin tolerance test studies, PLA2+/+ and
PLA2
/
mice were fed a
high-fat/high carbohydrate diet (no. F3282; Bioserve Industries,
Frenchtown, NJ), which contained 35.5% (wt/wt) fat (primarily lard)
and 36.6% carbohydrate (primarily sucrose), for 14 wk. Mice had free
access to water during the study period. At the beginning of the diet
study, mice were 8-10 wk of age. Body weights were recorded
throughout the experimental feeding period.
Food consumption studies. At week 14 of the experimental diet period, individually caged mice were given preweighed food, and the amount of food consumed was determined over a 24-h period for 5 days. The results are expressed as grams of food consumed per day.
Glucose tolerance tests.
After an overnight fast, PLA2+/+ and
PLA2/
male mice consuming either
the basal low-fat or the western-type diet for 15 wk were injected
intraperitoneally with a bolus load of glucose (2 g/kg body wt). Blood
was obtained from the tail vein before and 15, 30, 60, and 120 min
after glucose administration. Blood glucose was measured using an
automated glucose analyzer (Elite XL; Bayer, Elkhart, IN).
Insulin tolerance tests.
After a 4-h fast, PLA2+/+ and
PLA2/
female mice consuming
either the basal low-fat or high-fat/high-carbohydrate diet for 14 wk were injected intraperitoneally with bovine insulin (1 U/kg body wt;
Sigma Chemical, St. Louis, MO). Blood was obtained for glucose determination from the tail vein before and 15, 30, and 60 min after
insulin administration.
Euthanasia. After the 16-wk experimental feeding period, the mice were fasted for 4 h and then anesthetized by intraperitoneal injection with a solution composed of ketamine (80 mg/kg body wt; Fort Dodge Laboratories, Fort Dodge, IA) and xylazine (16 mg/kg; Butler, Columbus, OH) diluted in 0.9% saline. Body temperature was determined using a rectal thermometer. Blood was removed by cardiac puncture into tubes containing 1 mM EDTA. Adipose (epididymal and uterine fat pads) and brown adipose (intrascapular) tissue, and liver, heart, spleen, lungs, and kidneys were removed, and wet weight was recorded.
Plasma chemistries. Plasma was obtained by low-speed centrifugation of the blood samples. Plasma triglyceride, cholesterol, and free fatty acid concentrations were determined by colorimetric assays from Wako Chemicals (Richmond, VA). Plasma leptin and insulin concentrations were measured using radioimmunoassay kits from Linco Research (St. Charles, MO). Blood glucose was measured as described in Glucose tolerance tests. Results from male and female mice were averaged together because there were no apparent differences based on the sex of the animal. Free fatty acid concentrations in plasma were determined only from the female mice.
Postprandial fat absorption. Mice maintained on the basal low-fat diet or the western diet for 4 wk were fasted overnight. The following morning, the mice were injected with 12.5 mg of Triton WR-1339 to block lipolysis (1). Ten minutes later, the mice received an intragastric load of 1 µCi of [3H]triolein (Amersham Pharmacia Biotech, Piscataway, NJ) in 50 µl of olive oil. The mice were allowed access to water but not food during the course of the experiment. Blood samples were taken 1, 2, 4, and 6 h after gavage by tail bleeding. Radioactivity appearing in plasma was determined by liquid scintillation counting.
Fecal lipid analysis. Feces were collected from mice fed the western diet for 4 wk over a 24-h period. Mice in each group were housed four per cage; therefore, results represent data from pooled fecal samples. The stool samples were dried to a constant weight, and the lipids were extracted from 100 mg of dried feces as described (32) and analyzed by thin-layer chromatography as described (3, 23).
Statistical analysis. All results are presented as means ± SD. Differences between the two genotypes were determined by Student's t-test or the Mann-Whitney rank sum test. Differences in the body weight growth curves, glucose tolerance tests, and insulin tolerance tests were determined by one-way ANOVA followed by the Tukey-Kramer tests. P < 0.05 was accepted as statistically significant. All statistical analysis was completed using the SigmaStat software from Jandel (San Rafael, CA).
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RESULTS |
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Mice lacking PLA2 have normal growth, lipid
metabolism, and reproductive functions while maintained on a basal
low-fat diet (Ref. 29 and unpublished
observations). To assess the lack of PLA2 on mice
fed a western-type high-fat/high-cholesterol diet for an extended
period of time, wild-type (PLA2+/+)
and PLA2 knockout
(PLA2/
) male and female mice
were fed either a standard mouse chow or a high-fat/high-cholesterol
(21% fat, 0.15% cholesterol) diet for 16 wk. There was no difference
in body weight between PLA2+/+ and
PLA2
/
mice when fed the basal
low-fat diet (males: 25.3 ± 3.4 vs. 23.8 ± 4.0 g;
females: 23.9 ± 4.0 vs. 21.6 ± 0.60 g, respectively). In contrast, the PLA2+/+ male and
female mice gained significantly more weight than the PLA2
/
mice upon being fed the
western diet (Fig. 1). This resulted in
~57 and 40% more weight gained by the male (Fig. 1A,
inset) and female (Fig. 1B, inset)
PLA2+/+ mice compared with the
PLA2
/
mice after feeding on the
high-fat diet for 16 wk.
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To account for the differences in weight gain between the
PLA2+/+ and
PLA2/
mice, various tissues were
removed and weighed. Under low-fat dietary conditions, there were no
differences in epididymal and uterine fat pad, brown fat, and liver
weights between male and female
PLA2+/+ and
PLA2
/
mice (Fig.
2, A and B). In
contrast, the PLA2
/
mice had
~50% lower epididymal and uterine fat pad weight compared with
PLA2+/+ mice (Fig. 2, C
and D). The increased adiposity was specific for white fat,
as there was no difference in brown adipose mass between the two
genotypes after high-fat dietary treatment (Fig. 2, C and
D). Male PLA2
/
mice
also had ~36% lower liver weight compared with the
PLA2+/+ mice (Fig. 2C).
However, this difference in liver weight was not apparent in the female
mice (Fig. 2D). There were also no significant differences
in heart, spleen, lung, and kidney weights between the
PLA2+/+ and
PLA2
/
mice fed the western diet
(data not shown). These data suggest that the observed weight
difference among the mice fed the western diet was due to increased
adiposity in the wild-type mice.
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In addition to differences in body weight gain and adiposity upon
feeding on a high-fat/high-cholesterol diet, the
PLA2/
mice had decreased fasting
plasma leptin and insulin concentrations compared with the
PLA2+/+ mice (Table
1). The difference in leptin levels is
most likely due to changes in adipose tissue observed between the two
groups of mice. In contrast, no significant difference was observed in fasting plasma glucose and free fatty acid concentrations (Table 1).
However, there was a consistent trend toward decreased (20%) fasting
glucose levels in the PLA2
/
mice. Plasma cholesterol and triglyceride levels were similar between
the PLA2+/+ and
PLA2
/
mice after high-fat
dietary treatment (Table 1).
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It appears that the high-fat-fed
PLA2+/+ mice are more insulin
resistant than the high-fat-fed
PLA2/
mice because the increased
fasting plasma insulin concentrations are necessary to maintain normal
plasma glucose concentrations. To test glucose metabolism
directly in these animals, glucose tolerance tests were performed on
PLA2+/+ and
PLA2
/
mice under basal low-fat
dietary conditions when the animals were similar in weight and
adiposity and after feeding on a high-fat diet. There was no difference
in glucose metabolism between the PLA2+/+ and
PLA2
/
mice consuming the chow
diet (Fig. 3A). In contrast,
after high-fat feeding, the
PLA2
/
mice displayed lower blood
glucose concentrations 15 and 30 min after intraperitoneal glucose
administration compared with those observed in the
PLA2+/+ mice (Fig. 3B).
In addition, insulin levels 30 min after glucose injection were 45%
lower (1.15 ± 0.28 vs. 0.63 ± 0.08 ng/ml; P < 0.05) in the PLA2
/
mice.
These data demonstrate that high-fat-fed
PLA2+/+ mice have impaired glucose
tolerance due to the development of obesity-related insulin resistance,
whereas high-fat-fed PLA2
/
mice
maintained normal glucose tolerance.
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To test directly the development of obesity-related insulin resistance
in these animals, insulin tolerance tests were performed. Both the
PLA2+/+ and
PLA2/
mice were fed a low-fat or
a high-fat/high-carbohydrate diet for 14 wk. The latter diet
has been shown previously to induce obesity and insulin resistance in
C57BL/6 mice (36, 37). Interestingly, female
PLA2
/
mice exhibited increased
glucose disposal 30 min after insulin injection compared with
PLA2+/+ mice under low-fat feeding
conditions (Fig. 4A). After
feeding on the high-fat/high-carbohydrate diet for 14 wk, the
PLA2
/
mice had lower blood
glucose levels 15, 30, and 60 min after insulin injection compared with
PLA2+/+ mice (Fig. 4B).
These results confirmed that
PLA2
/
mice were protected
against the development of high-fat-induced insulin resistance. The
data also suggest that PLA2
/
mice have improved insulin sensitivity compared with
PLA2+/+ mice, even under low-fat
feeding conditions.
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To examine potential mechanisms for the resistance to diet-induced
obesity in the PLA2/
mice, we
performed studies to measure food consumption, body temperature, and
postprandial fat absorption. There was no difference in the amount of
food consumed per day or resting body temperature between the
PLA2+/+ and
PLA2
/
mice (Table 1). These data
suggest that caloric intake and energy expenditure are not contributing
to the resistance to diet-induced obesity in the
PLA2
/
mice. To examine
postprandial fat absorption, we injected the mice fed the basal low-fat
diet or the high-fat diet for 4 wk with Triton WR-1339 to inhibit
lipolysis and suppress lipoprotein clearance from circulation. A bolus
load of olive oil containing [3H]triolein was then fed to
each mouse by gastric gavage. Lipid absorption efficiency was
determined on the basis of the appearance of
[3H]triglyceride in the plasma. Results, as shown in Fig.
5A, indicated no difference in
the appearance of [3H]triglyceride in the plasma of
PLA2+/+ and
PLA2
/
mice fed the chow diet.
Interestingly, PLA2
/
mice
previously maintained on a high-fat diet had decreased appearance of
[3H]triglyceride in the plasma 4 and 6 h after oil
administration (Fig. 5B). In addition, there were no
significant differences in [3H]triglyceride present in
the intestinal wall between PLA2+/+
and PLA2
/
mice (data not
shown). These data suggest that the decreased postprandial fat
absorption observed in the PLA2
/
mice was not due to increased intestinal retention of radiolabel, thus
suggesting suboptimal lipid digestion and/or uptake in the intestinal
lumen of PLA2
/
mice.
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The possible difference in fat absorption efficiency between
PLA2+/+ and
PLA2/
mice was addressed
directly by measuring their fecal lipid output after they were fed the
western diet for 4 wk. Fecal lipids were extracted and analyzed by TLC
analysis. There was increased lipid in the form of triglycerides, fatty
acids, and a lipid band migrating with cholesteryl ester in the
PLA2
/
mice compared with that
observed in PLA2+/+ mice (Fig.
6). Because the diet contains relatively
small amounts of cholesteryl ester, the identity of the fastest
migrating band may be retinyl ester, which is known to co-migrate with
cholesteryl ester. These data provided additional supporting evidence
to document that mice lacking PLA2 have increased fecal
lipid output when maintained on a high-fat diet.
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DISCUSSION |
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Obesity has developed into a significant health problem in westernized societies over the past 20 years due to its association with various chronic diseases such as non-insulin-dependent diabetes (type 2 diabetes), cardiovascular disease, and cancer (10, 19). The increased prevalence of obesity has been attributed to the increased availability and consumption of fat-rich foods and reduced physical activity (35). This has led many researchers to develop animal models that will allow for the study of the mechanisms by which diet-induced obesity contributes to various disease states.
The C57BL/6 mouse has been shown previously to be a good model for
studying diet-induced obesity and diabetes. It develops obesity,
insulin resistance, and hyperlipidemia resembling human type 2 diabetes
after feeding on a western-type, high-fat diet (22,
36-38). Results obtained from our wild-type mice are
consistent with these previous reports. Interestingly, C57BL/6 mice
with a PLA2-null mutation were resistant to diet-induced
obesity. This resulted in these animals being hypoinsulinemic,
hypoleptinemic, and more insulin sensitive compared with their obese
wild-type counterparts. The phenotype of these
PLA2/
mice is similar to that
observed in mice lacking the acyl-CoA:diacylglycerol transferase (Dgat)
gene (34) and to the phenotype of animals lacking the
protein tyrosine phosphatase-1B (PTP-1B) gene (9). In the
Dgat
/
mice, obesity resistance was
attributed to increased energy expenditure and increased activity
(34). Obesity resistance and increased insulin sensitivity
in the PTP-1B
/
mice was due to alteration in
fat metabolism and increased energy expenditure as a consequence of
alterations in the insulin-signaling pathway in muscle
(20). Because the amount of food eaten and resting body
temperature were similar between
PLA2+/+ and
PLA2
/
mice, we conclude that
decreased caloric intake and increased energy expenditure are not
contributing to the resistance to diet-induced obesity in the
PLA2
/
mice. In contrast, the
increased fecal lipid output observed in the
PLA2
/
mice suggests that the
most likely mechanism to account for the observed difference in
diet-induced obesity is reduced fat absorption in the
PLA2
/
mice after high-fat feeding.
Previous studies from our laboratory (29) have shown that
PLA2/
mice display normal lipid
absorption efficiency when fed a low-fat diet and respond to high-fat
feeding with increased plasma cholesterol and triglyceride levels to
the same extent as that observed in the
PLA2+/+ mice. These results are
consistent with previous observations that additional phospholipase(s)
in the intestine can partially compensate for the lack of
PLA2 in the PLA2
/
mice (29). The reduced fat absorption efficiency in these
animals after chronic feeding on a high-fat diet suggested that these compensatory phospholipases are not sufficient for complete fat digestion under high-fat loading conditions and that PLA2
is required for optimal lipid digestion and absorption. The mechanism
by which PLA2 inhibits fat absorption upon high-fat feeding
is likely related to the requirement for both pancreatic triglyceride
lipase and PLA2 in complete fat digestion before its
absorption by intestinal cells (18, 24, 39). Previous
studies have clearly documented that inhibition of fat absorption by
pancreatic triglyceride lipase inhibitors is effective in weight
reduction in obese patients (25, 28). The present study
reveals that reducing the level and activity of PLA2 is
also effective in reducing fat absorption and weight gain.
An explanation to account for these data is that PLA2 activity and/or its presence is necessary for the upregulation of fat absorption pathways that are induced by high-fat feeding. One possible mechanism by which this may occur is its putative role in the release of digestive enzymes and/or hormones from the pancreas. It is well established that high-fat feeding results in an increase in pancreatic lipase synthesis and secretion from pancreatic acinar cells (4). This presumably occurs because more lipase is needed to digest the increased amounts of fat entering the small intestine. PLA2 may be necessary for the upregulation of lipase from pancreatic acinar cells under high-fat conditions. This may occur either through hydrolysis of membrane phospholipids, thereby modifying the properties of the membranes favoring the secretory process, or, alternatively, PLA2-catalyzed hydrolysis of phospholipids may generate lipid-signaling molecules, such as lysophospholipids, that are mediators of the secretory process.
Another possible route by which PLA2 may influence dietary
lipid absorption is through an indirect mechanism mediated by its regulation of secretin release. This gastrointestinal hormone is known
to be important in the pancreatic adaptation to dietary fat
(4). It has also been shown to stimulate insulin secretion and enhance the insulin response to glucose (13). Recent
studies have identified the Group 1B PLA2 as the
secretin-releasing factor in the intestinal lumen (6, 21).
Because secretin release is stimulated by fat feeding, it is possible
that PLA2/
mice have a reduced
secretin level due to their lack of this secretin-releasing factor.
This, in turn, may influence the fat-stimulated release of lipase
leading to reduced fat absorption in these animals. Additional studies
will need to be conducted to test this hypothesis.
Although the decreased level of fat absorption is the most likely cause
of the protection against diet-induced obesity and obesity-related
insulin resistance observed in the
PLA2/
mice, other mechanisms may
also contribute to this observed phenotype. The increased glucose
disposal in response to insulin challenge observed in the
PLA2
/
mice under low-fat-feeding
conditions suggests that PLA2 gene inactivation may also
alter insulin-signaling pathways in a manner similar to that observed
in the PTP-1B
/
mice (9, 20).
Previous studies have shown that the Group 1B PLA2 is also
expressed in other tissues in addition to its prominent expression in
the pancreas (14, 30, 31, 33). The peripheral
PLA2 interacts with specific receptors and modulates cell
functions (15). Thus it is possible that defects in
PLA2 interaction with PLA2 receptors, such as
those observed in PLA2-deficient mice, may influence
insulin sensitivity in a favorable manner. In support of this
hypothesis is the report that PLA2 receptor-defective mice
are more resistant to endotoxic shock due to the reduced plasma level
of tumor necrosis factor-
(TNF-
) (16). This cytokine confers insulin resistance in cells by regulating glucose transporter synthesis and by interfering with insulin signaling (26).
Accordingly, Group 1B PLA2 deficiency may lower TNF-
production and increase insulin sensitivity in this manner. Also, cell
culture data have suggested that PLA2 participates directly
in insulin secretion from pancreatic islets (11, 12, 27).
Challenging the PLA2
/
mice with
a high-carbohydrate diet without fat may help to elucidate a role for
PLA2 in insulin signaling.
In summary, we report that mice lacking Group 1B PLA2 are resistant to diet-induced obesity. This most likely occurs through suppression of dietary fat absorption under high-fat-feeding conditions. Breeding the PLA2-deficient mice to various genetic models of obesity will provide exciting tools to understand the mechanisms of PLA2 on energy metabolism. Finally, regardless of the precise mechanism by which PLA2 deficiency results in protection against diet-induced obesity and obesity-related insulin resistance, these results offer a novel therapeutic strategy, i.e., the inhibition of Group 1B PLA2 activity, for the treatment of obesity and type 2 diabetes.
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ACKNOWLEDGEMENTS |
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We thank Drs. Patrick Tso, Laura Woollett, and Phillip Howles for valuable discussions, and Dr. David D'Alessio for assistance with the insulin assay. Nick Schildmeyer and Tara Riddle provided excellent technical assistance to this study.
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FOOTNOTES |
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This research was supported by a Program Project Grant (DK-54504) from the National Institutes of Health (NIH). K. W. Huggins was the recipient of a National Research Service Award from the NIH (F32 DK-10065), and A. C. Boileau received a Post-Doctoral Fellowship in Interdisciplinary Nutrition Science from the Dannon Institute.
Address for reprint requests and other correspondence: D. Y. Hui, Dept. of Pathology and Laboratory Medicine, Univ. of Cincinnati College of Medicine, 231 Albert Sabin Wy, Cincinnati, OH 45267-0529 (E-mail: Huidy{at}emailuc.edu).
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.
July 30, 2002;10.1152/ajpendo.00110.2002
Received 11 March 2002; accepted in final form 23 July 2002.
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