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© 2007 American Society for Nutrition J. Nutr. 137:1908-1915, August 2007

Nutrition and Disease

Dietary Palatinose and Oleic Acid Ameliorate Disorders of Glucose and Lipid Metabolism in Zucker Fatty Rats1–3,

Kazusa Sato4,5, Hidekazu Arai4,5,*, Akira Mizuno6, Makiko Fukaya5, Tadatoshi Sato5, Megumi Koganei7, Hajime Sasaki7, Hironori Yamamoto5, Yutaka Taketani5, Toshio Doi6 and Eiji Takeda5

5 Department of Clinical Nutrition and 6 Department of Clinical Biology and Medicine, Institute of Health Biosciences, University of Tokushima Graduate School, Tokushima 770-8503, Japan; and 7 Division of Research and Development, Food Science Institute, Meiji Dairies Corporation, Odawara, Kanagawa 250-0862, Japan

* To whom correspondence should be addressed. E-mail: arai{at}nutr.med.tokushima-u.ac.jp.


   ABSTRACT
TOP
 ABSTRACT
Introduction
 Materials and Methods
 Results
 Discussion
 LITERATURE CITED
 
Excessive dietary intake of carbohydrates and fats has been linked to the development of obesity. However, the mechanism by which these dietary factors interact to bring about metabolic changes has not been elucidated. We examined the combined effects of different types of dietary carbohydrates and fats on the etiology of obesity and its complications in the Zucker fatty (fa/fa) rat, a model of obesity. Specifically, these rats were fed an isocaloric diet containing various combinations of carbohydrates [palatinose (P), an insulin-sparing sucrose analogue, and sucrose (S)] and fatty acids [oleic acid (O) and linoleic acid (L)]. After 8 wk, palatinose feeding (PO and PL) led to significant reductions in visceral fat mass, adipocyte cell size, hyperglycemia, and hyperlipidemia compared with sucrose feeding (SO and SL); pancreatic islet hypertrophy was also prevented by palatinose feeding. Linoleic-acid–fed rats (PL and SL) exhibited reduced insulin-immunoreactive staining of the pancreatic islets, enhanced macrophage infiltration in adipose tissue, and an elevated plasma tumor necrosis factor-{alpha} concentration when compared with oleic-acid–fed rats (PO and SO). Furthermore, sucrose and linoleic acid synergistically increased the expression of genes involved in hepatic gluconeogenesis and lipogenesis [sterol regulatory-element binding protein (SREBP)-1c and SREBP-2]. In conclusion, a diet containing palatinose and oleic acid may prevent diet-induced metabolic abnormalities. The combination of palatinose and oleic acid holds promise for a new approach to preventing and treating obesity and its complications.


   Introduction
TOP
 ABSTRACT
 Introduction
Materials and Methods
 Results
 Discussion
 LITERATURE CITED
 
Obesity, in particular visceral obesity, is strongly associated with the development of insulin resistance, type 2 diabetes, metabolic syndrome, and cardiovascular disease. Although a complex interaction between many genetic and environmental factors contributes to the development of obesity, above all, diet is considered to be the single most important etiological factor. Specifically, the consumption of a high-fat or high-carbohydrate (especially sucrose and fructose) diet is thought to be most closely associated with the accumulation of excess body fat and systemic insulin resistance (1–3).

Evidence suggests that the types of dietary carbohydrate and fat influence the risk of developing diet-induced metabolic disorders. The glycemic responses of various dietary carbohydrates are dependent on the rate at which they are digested and absorbed. Research clearly indicates that diets rich in sucrose or fructose contribute to the development of obesity and insulin resistance (4). In contrast to these rapidly digestible saccharides, slowly digestible carbohydrates, such as oligosaccharides and resistant starches, have been shown to prevent postprandial hyperglycemia and hyperinsulinemia, both of which increase the risk of diabetes and atherosclerosis in animal and human studies (5). One such carbohydrate is palatinose [isomaltulose; 6-0-({alpha}-0-glucopyranosyl)-D-fructofuranose], a sucrose analogue composed of {alpha}-1,6-linked glucose and fructose, the ingestion of which has been reported to improve diabetic symptoms (6, 7). Similarly, the constituent fatty acids in dietary fats influence their biological roles. Linoleic acid and oleic acid are 2 major fatty acids in dietary fat and in plasma triglycerides. Linoleic acid, 18:2(n-6), an essential fatty acid, is converted into arachidonic acid, 20:4(n-6), which is an important precursor, via the cyclooxygenase pathway, of inflammatory eicosanoids such as prostaglandins and hydroxyeicosatetraenoic acids (HETE). Thus, linoleic acid has been implicated in the upregulation of inflammatory responses (8). On the other hand, the consumption of diets rich in monounsaturated fatty acids (MUFA), particularly oleic acid, 18:1(n-9), has been linked to a low prevalence of atherosclerosis and to beneficial effects on lipoprotein metabolism (9, 10). Previous studies have also shown that MUFA improve impaired pancreatic ß-cell secretory function resulting from chronic hyperglycemia and hyperlipidemia, respectively referred to lipotoxicity and glucotoxicity (11, 12).

Although diets are made up of a mixture of foods with a complex combination of nutrients that are interactive and synergistic (13), most studies have focused on individual nutrients. It is likely that the selective combination of functional carbohydrates and fatty acids can reduce the risk of developing metabolic disorders. Thus, it would be important to investigate the effects of combinations of multiple nutrients on systemic metabolism.

In a previous study, we reported that a novel liquid balanced formula (MHN-01/Inslow) containing palatinose and oleic acid suppresses postprandial hyperglycemia and hyperinsulinemia in humans and Sprague-Dawley rats, reduces visceral fat accumulation, and improves insulin sensitivity in Sprague-Dawley rats (14, 15). However, we have not obtained prospective data on the effects of this diet nor has the mechanism by which these dietary factors interact to bring about beneficial changes in glucose and lipid metabolism been elucidated. Finally, because our previous data were collected in healthy men and metabolically normal Sprague-Dawley rats, the potential benefits of the above diet in obesity, insulin resistance, and/or type-2 diabetes, were not assessed. The Zucker fatty (fa/fa) rat is a classic model of insulin resistance with features resembling human metabolic syndrome (16). Several investigators noted that, unlike lean rats, obese Zucker fatty rats preferentially convert available dietary substrates into lipids (17, 18). Because we intended to obtain precise and specific estimates of the combined effect of dietary carbohydrate and fat, as well as the types of carbohydrate and fat, we therefore selected the Zucker fatty (fa/fa) rat for our current study. Consequently, the purpose of this study was to examine the effects of combinations of dietary carbohydrates (palatinose and sucrose) and fats (oleic acid and linoleic acid) on the metabolic profile of obese Zucker fatty (fa/fa) rats.


   Materials and Methods
TOP
 ABSTRACT
 Introduction
 Materials and Methods
Results
 Discussion
 LITERATURE CITED
 
    Animals. Twenty male, 11-wk–old Zucker fatty (fa/fa) rats (Charles River), weighing 450–470 g, were individually caged in a climate-controlled room (23 ± 1°C with humidity 70–75%; specific pathogen-free) with a 12-h light:12-h dark cycle. Prior to the initiation of our study, the rats were allowed free access to standard rodent powder diet (MF; Oriental Yeast) and water.

    Diets. The experimental diets were freshly prepared each week, sealed in air-tight plastic bags, and stored at 4°C to avoid rancidity. The composition of each diet is included in Table 1. Safflower oil was used as the linoleic acid–rich oil (L); a mixture of high oleic sunflower oil and perilla oil as the oleic acid–rich oil (O). The final composition of the diet (g/kg) was 212.8 of total milk protein and casein, 353.2 of palatinose or sucrose, 157.5 of dextrin, 63.8 of indigestible dextrin, 72.3 of vitamin and mineral mixture, and 140.4 of test oil.


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  		TABLE 1 Composition of the experimental diets1

 
    Experimental design. Rats were randomly assigned to 1 of 4 dietary groups, each containing 5 rats, that were fed either a diet rich in sucrose and linoleic acid (SL),8 sucrose and oleic acid (SO), palatinose and linoleic acid (PL), or palatinose and oleic acid (PO). To eliminate the difference in energy state due to altered palatability among these diets, PO-based pair-feeding was performed because, in a preliminary feeding experiment, rats displayed a reduced preference for consumption of the PO diet. Pair-feeding was conducted by assigning the rats into 5 sets that each consisted of 1 PO rat, 1 PL rat, 1 SO rat, and 1 SL rat. Rats in the PO group were allowed free access to food and water throughout the 8-wk feeding period. The amount of food consumed by the PO rat during a 24-h period was measured, and this amount was provided to the other 3 rats in the set in a single feed at 1800 on the following day. Food intake was monitored and the food itself was replaced daily, and body weights were recorded weekly throughout the feeding period.

After 8 wk, the rats were allowed access to their final feed for 14 h (1800–800) before the food pots were removed. Some food was left in the food pots of all rats. Twenty-four hours after the food pots were removed, the rats were killed for blood and tissue collection. Blood samples collected from the tail vein were used to determined plasma glucose and insulin levels. The rats were then anesthetized with diethylether and blood was withdrawn from their jugular vein that was used for all other measurements; these determinations were made after the rats were deprived of food for 24 h to avoid any influence of their intestinal contents, which were considerable after 12 h of food deprivation in a preliminary feeding experiment. After being killed by exsanguination, liver, visceral fat, and pancreatic samples were harvested and weighed.

The Institutional Animal Care and Oversight Committee approved the experimental protocols of the study, which were carried out according to the guidelines and principles for the care and use of animals at the University of Tokushima.

    Plasma glucose, insulin, lipid, and adipocytokine concentrations. Plasma glucose was determined by the glucose dehydrogenase method using an Accu-Chek blood glucose meter (Roche Diagnostics). Plasma insulin was determined by ELISA (Morinaga). Plasma triglyceride, cholesterol, and free fatty acid (FFA) concentrations were determined using Triglyceride-E-, Cholesterol-E-, and NEFA-C- tests (Wako Pure Chemical Industries), respectively. Plasma adiponectin and tumor necrosis factor-{alpha} (TNF{alpha}) concentrations were assessed by ELISA (Ostuka Seiyaku and Biosource International, respectively).

    Hepatic triglyceride concentration. Hepatic lipids were extracted as previously described (19). The lipid extracts were resuspended in methanol and used for measuring the triglyceride level with a commercial kit (Triglyceride-E-kit,Wako Pure Chemical Industries).

    RNA preparation and quantitative RT-PCR. To assess diet-induced changes in gene expressions, mRNA levels of genes involved in glucose and lipid homeostasis were determined by reverse transcription of total RNA followed by PCR analysis. Total RNA was isolated from snap-frozen liver and epididymal adipose tissue samples using a commercially available acid-phenol reagent, Trizol (Invitrogen) or RNeasy kit (QIAGEN), respectively. First-strand cDNA were reverse-transcribed at 42°C for 60 min and 95°C for 5 min from 5 µg of the extracted total RNA with M-MLV reverse transcriptase (Invitrogen) and oligo-dT primer. We performed real-time PCR using the primers described in Supplemental Table 1, and SYBR green dye (SYBR Premix Ex Taq; TAKARA BIO) in a LightCycler real-time PCR system (Roche Diagnostics), according to the manufacturer's instructions. The relative abundance of target transcripts was normalized to the constitutive expression of ß-actin. The ratio for the data from the SL group was set at 100%.

    Histological and immunohistochemical analyses. Epididymal adipose tissue and pancreatic samples were fixed in 4% buffered paraformaldehyde, embedded in paraffin, sectioned, deparaffinized in xylene, stained with Mayer's hematoxylin and eosin (Wako Pure Chemical Industries), and examined by light microscopy.

Immunohistochemistry was performed to assess the degree of macrophage infiltration in white adipose tissue and the relative levels of insulin-secreting ß-cells in the pancreatic islets. Slides were microwaved in 10 mmol/L sodium citrate (pH 6.0) for 5 min to retrieve antigen. After a blocking step with 5% BSA for 2 h at room temperature, slides were incubated with primary antibodies [rabbit polyclonal IgG anti-F4/80 (Santa Cruz Biotechnology), a macrophage-restricted surface glycoprotein; rabbit polyclonal IgG anti-insulin (Nichirei Bioscience)] overnight at 4°C. Subsequently, they were washed in PBS and incubated with peroxidase-conjugated goat anti-rabbit IgG (EnVision+ System-HRP; DakoCytomation) for 1 h at room temperature. The slides were then developed with diaminobenzidine (Nakalai Tesk) and counterstained with Mayer's hematoxylin (Wako Pure Chemical).

Adipocyte, pancreatic islet, and insulin-immunoreactive ß-cell areas were measured using Image-Pro Plus 6.0 Software (Media-Cybernetics).

    Statistical analysis. Values are means ± SEM. Significant effects of dietary carbohydrate and fat were identified by 2-way ANOVA followed by the Tukey-Kramer post hoc test. For morphological analysis of pancreatic islets and epididymal fat cells, the nonparametric Kruskal-Wallis test, followed by Steel-Dwass' post hoc test, was performed. Signficance was determined at P < 0.05. Statistical tests were performed using StatView 5.0 (SAS) and Excel-Toukei 2006 (SSRI).


   Results
TOP
 ABSTRACT
 Introduction
 Materials and Methods
 Results
Discussion
 LITERATURE CITED
 
    Body and organ weights, hepatic triglyceride concentration, and blood chemistry. During the 8-wk feeding period, the energy intake was similar in all groups (379.0 ± 3.2 kJ/d). We observed neither diarrhea nor constipation among rats in any dietary group. After the experimental period, the palatinose-fed groups (PL and PO) weighed less than the sucrose-fed groups (SL and SO) (Table 2). Visceral fat accumulation was also markedly suppressed in the palatinose-fed groups. Relative weights of liver and pancreas and hepatic triglyceride concentrations were not affected by the treatments (Table 2). The palatinose-fed groups had significantly lower plasma glucose and insulin concentrations than the sucrose-fed groups, suggesting that these rats had improved glucose metabolism (Table 3). Palatinose-feeding also ameliorated hyperlipidemia in Zucker fatty rats. The diet treatments did not affect plasma FFA concentrations (Table 3).


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  		TABLE 2 Body and organ weights and hepatic triglyceride concentration of Zucker fatty rats fed combinations of carbohydrates and fats for 8 wk1

 

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  		TABLE 3 Plasma concentrations of glucose, insulin and lipids in Zucker fatty rats fed combinations of carbohydrates and fats for 8 wk1

 
    Diet-induced changes in hepatic gene expression. To investigate diet-induced alterations in glucose and lipid metabolism, we examined the expression of gluconeogenic [phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase)], lipogenic [sterol regulatory-element binding protein (SREBP)-1c, SREBP-2, liver X receptor-{alpha} (LXR{alpha}), fatty acid synthase (FAS), acetyl-coenzyme A carboxylase (ACC), steroyl-CoA desaturase 1 (SCD-1), 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase) and apolipoprotein B (apoB)] and proinflammatory [TNF{alpha} and cyclooxygenase-2 (COX-2)] genes in the liver using real-time PCR. As predicted by their plasma glucose concentrations, PEPCK expression was more pronounced in the SL group than in the other 3 groups (Table 4). SREBP-1c is a dominant transcription factor that regulates biosynthesis of fatty acid and triglyceride, whereas SREBP-2 and LXR{alpha} modulate cholesterol synthesis and homeostasis (20, 21). The mRNA levels of these key lipogenic transcription factors were significantly elevated in the sucrose-fed groups compared with the palatinose-fed groups. Furthermore, sucrose and linoleic acid synergistically increased the expression of PEPCK, SREBP-1c, and SREBP-2 (Table 4). The expression of the transcription factors ACC (fatty acid synthesis from acecyl-CoA) and apoB (a core component of VLDL) generally paralleled the rats' plasma lipid profile (Tables 3 and 4). On the other hand, the mRNA levels of the lipogenic enzymes FAS (fatty acid synthesis), SCD-1 (oleic acid synthesis and triglyceride accumulation), and HMG-CoA reductase (cholesterol biosynthesis), were significantly elevated in the linoleic-acid-fed groups (SL and PL) compared with the oleic-acid-fed groups (SO and PO). As markers of hepatic inflammation, which aggravates hepatic insulin resistance, we measured TNF{alpha} and COX-2 mRNA levels. We observed significant elevations in TNF{alpha} and COX-2 expression only in the linoleic-acid-fed groups (Table 4).


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  		TABLE 4 Hepatic and adipose gene expression in Zucker fatty rats fed combinations of carbohydrates and fats for 8 wk12

 
    Pancreatic islet morphology. HE-stained pancreatic sections revealed altered islet morphology (Fig. 1 and Supplemental Fig. 1). After treatment, PL and PO rats were found to have normal islets that were round or oval, although some irregularity in shape was apparent in PL islets. In contrast, SO and SL rats had large, irregular, and often bilobed islets, suggesting that they were hypertrophic; these abnormal and irregular islets were more frequent in SL rats. Mean islet area in the sucrose-fed groups was significantly larger than in the palatinose-fed groups (Table 5). Thus, islet hypertrophy was associated with sucrose feeding as opposed to palatinose feeding. A core of uniform, intensely brown (i.e., insulin positive) staining islets was seen in SO and PO rats, whereas SL and PL rats exhibited islets that had attenuated and variable staining suggestive of ß-cell loss (Fig. 1). The mean proportion of ß-cell area to islet area was consistently significantly higher in the oleic-acid–fed groups than in the linoleic-acid–fed groups (Table 5).


Figure 1
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  		FIGURE 1  Differential effects of dietary carbohydrate and fat on pancreatic islet morphology in Zucker fatty rats fed combinations of carbohydrates and fats for 8 wk. Paraffin-embedded serial sections of pancreas from SL, SO, PL, or PO rats were stained with either HE (upper row) or with antiinsulin antibody (lower row) as described in Materials and Methods. Magnification bar, 100 µm.

 

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  		TABLE 5 Islet area and proportion of insulin-immunoreactive area to islet area in pancreas, and adipocyte area in the epididymal adipose tissue in Zucker fatty rats fed combinations of carbohydrates and fats for 8 wk12

 
    Dietary modification of adipocyte size. Although rats in the sucrose-fed groups displayed numerous large adipocytes, palatinose-fed rats had greater numbers of smaller cells and fewer large cells (Fig. 2 and Supplemental Fig. 2). The mean adipocyte area in the palatinose-fed groups was significantly smaller than in the sucrose-fed groups (Table 5); within each carbohydrate group, the fat groups did not differ, consistent with the adipose tissue weight data (Table 2).


Figure 2
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  		FIGURE 2  Morphology of adipose tissue in Zucker fatty rats fed combinations of carbohydrates and fats for 8 wk. HE-stained paraffin-embedded sections of epididymal adipose tissue from SL, SO, PL, or PO rats and the size distribution of adipocytes in these rats. Magnification bar, 100 µm.

 
    Dietary regulation of adipose tissue inflammation. In addition to size differences, there were clusters of small, nucleated, F4/80-positive cells in the interstices between adipocytes in white adipose tissue in the SL and PL groups, in contrast to the near absence of such cells in the PO group (Fig. 3 and Supplemental Fig. 3). In accordance with these morphological findings, mRNA expression levels of EGF-like module containing mucin-like, hormone receptor-like sequence 1 (Emr1) (F4/80) and COX-2 were significantly elevated in the linoleic-acid–fed groups compared with the oleic-acid–fed groups (Table 4). Plasma adiponectin concentrations were significantly elevated in the palatinose-fed groups (7.29 ± 0.41 mg/L) compared with the sucrose-fed groups (5.80 ± 0.33 mg/L, P = 0.011 for carbohydrate effect), whereas a significant elevation in plasma TNF{alpha} concentrations was found in the linoleic-acid–fed groups (65.30 ± 8.76 ng/L) compared with the oleic-acid–fed groups (35.97 ± 3.37 ng/L, P = 0.009 for fat effect).


Figure 3
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  		FIGURE 3  Effects of dietary carbohydrates and fats on macrophage infiltration to adipose tissue in Zucker fatty rats fed combinations of carbohydrates and fats for 8 wk. To visualize accumulation of macrophages in adipose tissue, paraffin-embedded sections of epididymal adipose tissue from PO, PL, SO, or SL rats were stained with anti-F4/80 antibody, and were counterstained with hematoxylin. Magnification bar, 50 µm.

 

   Discussion
TOP
 ABSTRACT
 Introduction
 Materials and Methods
 Results
 Discussion
LITERATURE CITED
 
To our knowledge, this is the first study to examine the role of combinations of the different types of dietary carbohydrates (palatinose and sucrose) and fats (oleic acid and linoleic acid) on the metabolic profile of obese Zucker fatty (fa/fa) rats. The findings reported herein demonstrate that sucrose but not palatinose feeding exacerbated hyperglycemia, hyperinsulinemia, and hyperlipidemia in these rats. Pancreatic islet hypertrophy and visceral fat accumulation were more severe in the sucrose-fed groups than in the palatinose-fed groups. Sucrose feeding also increased the expression of gluconeogenic and lipogenic genes in the liver. In addition, linoleic acid but not oleic acid–feeding resulted in reduced insulin-immunoreactive ß-cells in the pancreas, elevated hepatic inflammatory gene expression, and enhanced adipose tissue inflammation. Taken together, palatinose, combined with oleic acid feeding, appears to prevent diet-induced metabolic abnormalities.

The principal difference between palatinose and sucrose vis-à-vis their role in the induction of obesity relates to their digestibility. Palatinose is a naturally occurring disaccharide composed of {alpha}-1,6-linked glucose and fructose, which takes longer to absorb than other disaccharides and sucrose. Although the hydrolysis rate of palatinose at brush border membranes in rat and human intestine averages only 11–25%, palatinose is completely hydrolyzed by the intestinal sucrase/isomaltase complex and absorbed (22, 23). Macdonald and Daniel showed, using 14C-palatinose and 14C-sucrose, that the rate of excretion of 14CO2 in the expired air of rats is similar for both disaccharides, indicating that the net absorptions are similar (23). This property makes palatinose a beneficial sugar for subjects with diabetes and prediabetic conditions such as obesity or insulin resistance. In this study, palatinose-fed rats had lower plasma glucose and insulin levels and exhibited less hyperlipidemia than sucrose-fed rats; these findings are consistent with our previous report (14).

We found that dietary palatinose had an antihypertrophyc effect on the islets of Zucker fatty rats. Islet hypertrophy develops in response to hyperglycemia and insulin resistance and has been linked to the ingestion of a high sucrose diet (24). Del Zotto et al. (25) reported that prolonged sucrose feeding induces elevated blood glucose level and pancreatic islet hypertrophy because of enhanced replication of ß-cells and a decrease in the rate of ß-cell apoptosis. The islets in palatinose-fed rats were smaller than those in sucrose-fed rats; this finding was consistent with their plasma glucose and insulin concentrations. We also found that linoleic acid consumption reduced insulin-immunoreactive staining of the pancreatic islets, perhaps as a result of an alteration in their fatty acid composition. Several studies reported that the phospholipid fatty acid composition of various tissues, including the pancreas and adipose tissue, is modified by varying the composition of dietary fatty acids (26, 27). In many animal tissues, linoleic acid is converted to arachidonic acid by an alternating sequence of {Delta}6 desaturase, chain elongation, and {Delta}5 desaturation. Arachidonic acid in membrane phospholipids is a precursor in the biosynthesis of prostaglandins, HETEs, and leukotrienes, which play a role in the initiation and regulation of inflammation (28). Dietary linoleic acid has been reported to increase phospholipid linoleic and arachidonic acid levels in various rat tissues (27, 29). In the pancreas, arachidonic acid is converted to 12-hydroxyeicosatetraenoic acid (HETE), which impairs ß-cell function and viability (30, 31). From these reports, we speculate that the production of inflammatory eicosanoids was augmented in the tissues of SL and PL rats due to alteration of tissue membrane fatty acid composition after 8 wk of feeding. Coupled with sucrose-feeding–induced islet hypertrophy and ß-cell dysfunction, linoleate-derived inflammatory eicosanoids might induce overt diabetes. The PO diet, on the other hand, prevented both sucrose-mediated islet hypertrophy and the linoleate-mediated decrease in insulin-positive ß-cells, which could suggest ß-cell dysfunction.

The liver of palatinose-fed rats had significant reductions in the expression of genes that encode gluconeogenic and lipogenic enzymes, which complemented their plasma glucose and lipid profiles. Although fasting can affect the expression of the genes examined here, our observations are unlikely to reflect the rats' fasting state because the expression levels of peroxisome proliferator-activated receptor-{alpha} (PPAR{alpha}), which is the typical fasting-responsive gene (32, 33), were similar among groups (data not shown). Unexpectedly, the livers of PL and SL rats exhibited markedly increased expression of FAS, SCD-1, and HMG-CoA reductase, independent of the effects on their transcription factors. These data contrast with previous reports that found that dietary PUFA, including linoleic acid, suppressed lipogenic genes (34, 35). These conflicting results may reflect linoleic-acid-induced macrophage accumulation in adipose tissue. Liver inflammation induced by humoral mediators derived from adipose tissue has a causal role in systemic insulin resistance (36, 37) and hepatic lipogenesis is increased under conditions of chronic inflammation (38). Macrophage-derived inflammatory cytokines such as TNF{alpha}, IL-1, and IL-6 have been reported to accelerate hepatic de novo fatty acid and cholesterol synthesis via induction of FAS and HMG-CoA reductase, respectively (39, 40). The effects of cytokines on hepatic lipogenesis are probably direct and not mediated by insulin (41, 42). In the present study, the elevated plasma TNF{alpha} levels in the PL and SL groups might be responsible for the elevated hepatic TNF{alpha} and COX-2 mRNA expression and, at least in part, the increased expression of hepatic lipogenic enzymes in these groups. The discrepancy between blood lipid levels and hepatic mRNA expression of FAS, SCD-1, and HMG-CoA reductase would indicate that transcription factor–mediated (i.e., insulin-mediated) regulation, rather than inflammation-mediated regulation, has a profound effect on blood lipid levels. This possibility is consistent with the previous report that TNF treatment stimulates hepatic fatty acid synthesis but not an increase in blood triglyceride levels in rats fed a high sucrose/high-fat corn oil diet (43). The changes in expression of FAS, SCD-1, and HMG-CoA reductase, which are probably regulated by TNF{alpha}, are not proportional to hepatic triglyceride accumulation or to blood lipid profiles. This observation corresponds with the ACC and apoB mRNA levels indicative of the hepatic VLDL production, and thus supports our speculation that insulin-mediated hepatic lipogenesis might overwhelm that which is probably mediated by inflammation. However, additional studies are needed to investigate the underlying mechanisms. Recently, Rivera et al. (44) reported that a linoleic-acid–rich corn oil and/or sucrose-enriched diet exacerbated systemic and hepatic inflammation in sedentary normal rats, which supports our observations in SL rats.

Cell hypertrophy coupled with macrophage accumulation was evident in the adipose tissue from SL rats, findings that were not evident in PO rats. Sucrose-induced hyperinsulinemia results in body fat accumulation and adipocyte hypertrophy (45, 46). Consistent with this, SL and SO rats had increased visceral fat mass and larger adipocytes compared with PL and PO rats. Obese adipose tissue is characterized by the infiltration of macrophages, which is an important source of inflammatory cytokines that aggravate insulin resistance (47, 48). In our study, immunohistochemical analysis revealed the presence of F4/80-positive macrophages in the epididymal adipose tissues of SL and PL rats. Moreover, the mRNA expression levels of Emr1 (also known as F4/80) and COX-2, which catalyzes arachidonic acid into prostaglandins in response to proinflammatory stimuli (49), were markedly elevated in linoleic-acid–fed rats compared with oleic-acid–fed rats. These results suggest that dietary linoleic acid enhances macrophage infiltration and inflammation in adipose tissues. On the other hand, oleic-acid–rich diets reportedly reduce arachidonic acid mobilization and the production of prostaglandin E2 in rat macrophages (50), which may be associated with suppressed plasma TNF{alpha} concentrations in our oleic-acid–fed rats. In addition to sucrose-induced adipocyte hypertrophy, increased inflammation, partly due to linoleic-acid feeding, might have contributed to severe hyperglycemina associated with hyperinsulinemia, hypertriglyceridemia, and hypercholesterolemia, which is common in insulin-resistant states, in SL but not in PO rats.

The ratio of (n-6):(n-3) PUFA could be an important determinant of insulin resistance and thus glucose and lipid metabolism. In our current study, oleic-acid–rich diets (PO and SO) and linoleic-acid–rich diets (PL and SL) considerably differed in (n-6) (linoleic acid) to (n-3) ({alpha}-linolenic acid) PUFA ratios, which were 2.4 and 382.0 in the oleic acid and linoleic acid diets, respectively. Ghafoorunissa et al. (51) found that the substitution of dietary linoleic acid with {alpha}-linolenic acid, i.e., decreasing the (n-6)/(n-3) ratio from 220 to 2, resulted in lowered blood lipid levels and increased the antilipolytic effects of insulin in adipocytes in sucrose-fed insulin-resistant rats. Therefore, the increased (n-6)/(n-3) ratio in the PL and SL diets might play a part in the negative effects of these diets.

In conclusion, Zucker fatty rats that consumed a diet containing palatinose and oleic acid for 8 wk presented both palatinose-mediated improvement of plasma glucose, insulin, lipid profiles, and a decrease in body fat accumulation and oleic-acid–mediated preservation of pancreatic ß-cells and a decrease in macrophage infiltration into adipose tissue. Our findings suggest that the selective combination of dietary carbohydrates and fats may reduce the risk of developing obesity, type 2 diabetes, and metabolic syndrome.


   ACKNOWLEDGMENTS
 
We thank Fumiaki Imamura (Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University) for statistical advice. We also thank Hiromi Sakata-Haga (Department of Anatomy and Development Neurobiology) for excellent technical assistance, and Kaoru Matsuo, Emi Shuto, and Asami Toya for essential experimental assistance.


   FOOTNOTES
 
1 Supported in part by Grants-in-Aid for Scientific Research and Knowledge Cluster Initiative from the Ministry of Education, Culture, Sports, Science, and Technology in Japan (to K. M., H. A., Y. T., and E. T.), and from the 21st-Century COE Program, Human Nutrition Science on Stress Control, Tokushima, Japan. Back

2 Author disclosures: K. Muto, H. Arai, A. Mizuno, M. Fukaya, T. Sato, M. Koganei, H. Sasaki, H. Yamamoto, Y. Taketani, T. Doi, and E. Takeda, no conflicts of interest. Back

3 Supplemental Table 1 and Supplemental Figures 1–3 are available as Online Supporting Material with the online posting of this paper at http://jn.nutrition.org. Back

4 These authors contributed equally to this work. Back

8 Abbreviations used: ACC, acetyl-coenzyme A carboxylase; apoB, apolipoprotein B; COX-2, cyclooxygenase-2; Emr1, EGF-like module containing mucin-like, hormone receptor-like sequence 1; FAS, fatty acid synthase; HMG-CoA reductase, 3-hydroxy-3-methylglutaryl-CoA reductase; LXR{alpha}, liver X receptor-{alpha}; PEPCK, phosphoenolpyruvate carboxykinase; PL, palatinose and linoleic acid; PO, palatinose and oleic acid; SCD-1, steroyl-CoA desaturase 1; SL, sucrose and linoleic acid; SO, sucrose and oleic acid; SREBP, sterol regulatory-element binding protein; TNF{alpha}, tumor necrosis factor-{alpha}. Back

Manuscript received 2 March 2007. Initial review completed 3 April 2007. Revision accepted 19 May 2007.


   LITERATURE CITED
TOP
 ABSTRACT
 Introduction
 Materials and Methods
 Results
 Discussion
 LITERATURE CITED
 

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T. Uebanso, Y. Taketani, M. Fukaya, K. Sato, Y. Takei, T. Sato, N. Sawada, K. Amo, N. Harada, H. Arai, et al.
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Am J Physiol Endocrinol Metab, July 1, 2009; 297(1): E76 - E84.
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