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Endocrinology Vol. 147, No. 10 4553-4558
Copyright © 2006 by The Endocrine Society

Endogenous Interleukin-10 Protects against Hepatic Steatosis but Does Not Improve Insulin Sensitivity during High-Fat Feeding in Mice

Marion A. M. den Boer, Peter J. Voshol, Janny P. Schröder-van der Elst, Elena Korsheninnikova, D. Margriet Ouwens, Folkert Kuipers, Louis M. Havekes and Johannes A. Romijn

Departments of Endocrinology and Metabolism (M.A.M.d.B., P.J.V., J.P.S.-v.d.E., J.A.R.), Molecular Cell Biology (E.K., D.M.O), and Cardiology (L.M.H.), Leiden University Medical Center, 2333 ZA Leiden, The Netherlands; Department of Pediatrics (F.K.), University Medical Center Groningen, 9700 AB Groningen, The Netherlands; and Netherlands Organization for Applied Scientific Research Quality of Life (L.M.H.), 2333 CK Leiden, The Netherlands

Address all correspondence and requests for reprints to: Marion A. M. den Boer, Leiden University Medical Center, Department of Endocrinology and Metabolism, C4-R, Albinusdreef 2, 2333 ZA, Leiden, The Netherlands. E-mail: A.M.den_Boer{at}lumc.nl.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Several studies have demonstrated an association in humans between plasma levels or production capacity of the antiinflammatory cytokine IL-10 and insulin sensitivity. The aim of our study was to investigate the protective role of endogenous IL-10 availability in the development of diet-induced insulin resistance. We compared parameters of glucose and lipid metabolism between IL-10–/– mice and wild-type (wt) mice fed a high-fat diet for 6 wk. This diet has previously been shown to induce steatosis and insulin resistance. After 6 wk on the high-fat diet, no differences in body weight, basal metabolism (measured by indirect calorimetry), or plasma levels of glucose, triglycerides, or cholesterol were observed between IL-10–/– and wt mice. Nonetheless, in IL-10–/– mice, plasma free fatty acid levels were 75% increased compared with wt mice after overnight fasting (P < 0.05). In addition, hepatic triglyceride content was 54% increased in IL-10–/– mice (P < 0.05). During a hyperinsulinemic euglycemic clamp, no differences were observed in whole-body or hepatic insulin sensitivity between both groups. We conclude that basal IL-10 production protects against hepatic steatosis but does not improve hepatic or whole-body insulin sensitivity, during high-fat feeding.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
IN EPIDEMIOLOGICAL STUDIES, insulin resistance is associated with chronic low-grade inflammation (1). This is reflected in associations between the degree of insulin sensitivity and plasma levels of several cytokines, such as TNF{alpha} and IL-6 (2, 3). In addition, administration of exogenous TNF{alpha} and IL-6 induces insulin resistance in vivo (4, 5). Conversely, IL-6 depletion improves hepatic insulin action in an animal model of obesity (6).

IL-10 is a potent antiinflammatory cytokine, which is produced by T cells, B cells, monocytes, and macrophages and plays a crucial role in the innate immune system (7, 8). IL-10 potently inhibits the production of proinflammatory cytokines, including TNF{alpha} and IL-6 (9). Several lines of evidence point to a beneficial effect of IL-10 on insulin sensitivity. A recent epidemiological study showed a positive correlation between IL-10 levels and insulin sensitivity in healthy subjects (10). In the Leiden 85-Plus Study, the IL-10 production capacity of whole blood was investigated using lipopolysaccharide as a stimulus. The IL-10 production capacity was found to be inversely associated with blood glucose and glycosylated hemoglobin levels (11). Finally, administration of IL-10 in mice prevented IL-6-induced defects in hepatic insulin action and signaling activity (12). Although these studies suggest a potentially beneficial role of IL-10 in insulin-resistant conditions, the beneficial role of endogenous IL-10 secretion in insulin-resistant states has not been proven.

To determine whether endogenous IL-10 production can protect against diet-induced insulin resistance, we compared metabolic characteristics of IL-10–/– mice and wild-type (wt) control mice. We fed the mice a high-fat diet for 6 wk and subsequently analyzed parameters of lipid and glucose metabolism. Previous studies have documented that high-fat feeding induces accumulation of triglycerides (TG) in the liver and hepatic insulin resistance (13). We phenotyped the interaction between genotypes and diet by using the metabolic cages and by assessing insulin sensitivity with the hyperinsulinemic euglycemic clamp method. Our data indicate that, in contrast to our expectations, basal IL-10 production protects against hepatic steatosis during high-fat feeding but does not improve hepatic or whole-body insulin sensitivity.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Animals
Ten-week-old male C57BL6/J mice (wt) and IL-10–/– mice on the same background were purchased from Charles River (Maastricht, The Netherlands). Mice had free access to water and a normal chow diet (Technilab BMI, Someren, The Netherlands) until 12 wk of age. Subsequently, mice were fed a high-fat diet for 6 wk (40% of calories from bovine lard; Hope Farms, Woerden, The Netherlands). A previous study showed a 2.5-fold increased liver lipid content on this high-fat diet with a concurrent decrease in hepatic insulin sensitivity (13). Mice were weighed every week, and at time zero and after 6 wk on the high-fat diet, a blood sample was taken to determine plasma TG, cholesterol, and glucose levels. Principles of laboratory animal care were followed, and the animal ethics committee of our institute approved all animal experiments.

Plasma lipid and glucose analysis
In all experiments, tail vein blood was collected into chilled paraoxon-coated capillary tubes to prevent in vitro lipolysis (14). These tubes were placed on ice and immediately centrifuged at 4 C. Plasma was isolated, snap-frozen in liquid nitrogen, and stored at –20 C until analysis. The levels of plasma TG, total cholesterol, free fatty acids (FFA), and glucose were determined enzymatically using commercially available kits and standards (310-A Sigma GPO-Trinder kit and 315 Sigma NEFA-C kit from Sigma Chemical Co., St. Louis, MO; CHOL MPR3 from Boehringer, Mannheim, Germany; and hexokinase method from Instruchemie, Delfzijl, The Netherlands).

Metabolic cages
After 6 wk on the high-fat diet, basal metabolism in the IL-10–/– and wt mice was studied using the Comprehensive Laboratory Animal Monitoring System (CLAMS; Columbus Instruments, Columbus, OH). Metabolic rates were measured using an eight-chamber open-circuit system. Animals were maintained at approximately 24 C under a 12-h light, 12-h dark cycle. Food and water were freely available. The mice were housed individually in plexiglas cages through which 0.6 liter of air was passed per minute. Each chamber was sampled for 45 sec at 7-min intervals for a 24-h period. The O2 and CO2 content of the exhaust air was compared with the O2 and CO2 content of the standardized sample air. Before the start of the actual 24-h measurements, mice were weighed and acclimatized to the cages for 24 h.

Hyperinsulinemic euglycemic clamp experiments
After 6 wk on the high-fat diet, clamp experiments were performed as described previously (15, 16) after an overnight fast. Animals were anesthetized by ip injection with a combination of 6.25 mg/kg acetylpromazine (Sanofi Santé Nutrition Animale, Libourne Cedex, France) 6.25 mg/kg midazolam (Roche, Mijdrecht, The Netherlands), and 0.3125 mg/kg fentanyl (Janssen-Cilag, Tilburg, The Netherlands). An infusion needle was placed into the tail vein. After 45 min infusion of D-[3-3H]glucose at a rate of 0.8 µCi/h (specific activity, 620 GBq/mmol; Amersham, Little Chalfont, UK) to achieve steady-state levels, basal parameters were determined with 15-min intervals. Thereafter, a bolus of insulin (4.5 mU, Actrapid; Novo Nordisk, Chartres, France) was administered and the hyperinsulinemic clamp was started. Insulin was infused at a constant rate of 6.8 mU/h, and D-[3-3H]glucose was infused at a rate of 0.8 µCi/h. A variable infusion of 12.5% D-glucose (in PBS) was also started to maintain blood glucose at approximately 7 mM. Blood glucose was measured with the FreeStyle hand glucose measurer (Therasense; Disetronic Medical Systems, Vianen, The Netherlands) every 10 min to monitor glucose levels and adjust the glucose pump. After reaching steady state, blood samples were taken at 10-min intervals during 30 min to determine steady-state levels of [3H]glucose. After the last blood sample, mice were killed by cervical dislocation and the organs were dissected. An average clamp experiment took approximately 3 h, and anesthesia was maintained throughout the procedure.

Analysis of clamp samples
Plasma insulin concentrations were measured by ELISA (ALPCO Diagnostics, Windham, NH). To measure plasma [3H]glucose, trichloroacetic acid (final concentration 2%) was added to 7.5 µl plasma to precipitate proteins using centrifugation. The supernatant was dried to remove water and resuspended in milliQ. The samples were counted using scintillation counting (Packard Instruments, Dowers Grove, IL).

Calculations
The glucose turnover rate (µmol/min·kg) was calculated during the basal period and under steady-state clamp conditions as the rate of tracer infusion (dpm/min) divided by the plasma specific activity of [3H]glucose (dpm/µmol). The ratio was corrected for body weight. The hyperinsulinemic hepatic glucose production (HGP) was calculated as the difference between the tracer-derived rate of glucose appearance and the glucose infusion rate.

Determination of Akt phosphorylation in liver samples
To investigate hepatic insulin signaling, liver samples (100 mg) from clamped mice (n = 4–5 mice per group) were homogenized in a buffer containing 30 mM Tris, 2.5 mM EDTA, 150 mM NaCl, 0.5 mM Na3VO4, 5 mM NaF, 5 mM MgCl2, glycerol, Nonidet P-40, and protease inhibitors. The samples were homogenized using Ultra-Turrax for 20 sec. After centrifugation (14,000 rpm for 15 min at 4 C), the supernatant was clarified from the pellet and its protein content was determined (Pierce, Rockford, IL). For detecting protein levels of phosphorylated protein kinase B (pAkt), Akt, and insulin receptor (IR), equal amounts of protein (25 µg) were solubilized in 5x Laemmli sample buffer. Proteins were separated by SDS-PAGE, transferred to Immobilon-P membranes, blocked, incubated with polyclonal anti-IR (Santa Cruz Biotechnology, Santa Cruz, CA), anti-pAkt, -Akt, and -IR (Cell Signaling, Beverly, MA) primary antibodies (1:1000), and detected by enhanced chemiluminescence after incubation with horseradish peroxidase-linked secondary antibodies (1:5000). The protein bands were quantified using ImageGauge software (version 3.12; Fuji Photo Film, Tokyo, Japan).

Liver lipid analysis using high-performance thin layer chromatography
For analysis of lipid content, livers were homogenized in PBS. Lipids were extracted with Bligh and Dyer’s method as described (17). Lipids were separated by high-performance thin-layer chromatography on silica-gel-60 precoated plates (Alltech, Breda, The Netherlands) as described (18). The amount of lipid (free cholesterol, TG, and cholesteryl esters) was determined with TINA software (Raytest Isotopen mebgeräte GmbH, Straubenhardt, Messgeräte, Germany).

Determination of fibrinogen and serum amyloid-A (SAA)
Plasma fibrinogen and SAA levels were determined after 6 wk on the high-fat diet by ELISA as previously described (19).

Statistical analysis
Results are presented as means ± SD for the number of animals indicated. Differences between experimental groups were determined by the Mann-Whitney U test. The level of statistical significance of the differences was set at P < 0.05. Analyses were performed using SPSS 12.0.1 for Windows software (SPSS, Chicago, IL).


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Plasma lipid parameters and basal energy metabolism
We observed no differences in body weight between the IL-10–/– and wt mice before and after 6 wk on a high-fat diet. Blood samples taken after 4 h fasting showed no differences between the two groups in plasma TG and cholesterol levels before and after 6 wk on a high-fat diet (Fig. 1Go). To study basal energy metabolism, IL-10–/– mice and wt controls were studied in the metabolic cages after 6 wk on the high-fat diet. Figure 2Go shows metabolic characteristics during both the active (night) and inactive (day) periods. We observed no differences in O2 consumption (3394 ± 636 vs. 3201 ± 635 ml/kg·h at night), heat production (0.45 ± 0.09 vs. 0.44 ± 0.09 kcal/h at night), or respiratory exchange ratio (0.83 ± 0.05 vs. 0.82 ± 0.05 at night) after 6 wk on the high-fat diet.


Figure 1
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  		FIG. 1. IL-10 deficiency does not affect plasma lipid levels. Plasma lipid levels were measured after 4 h fasting. A, Plasma TG; B, plasma cholesterol. n = 12 mice per group.

 

Figure 2
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  		FIG. 2. IL-10 deficiency does not affect basal energy metabolism. A, VO2 of IL-10–/– mice and wt controls after 6 wk on the high-fat diet (n = 4); B, heat production of IL-10–/– mice and wt controls after 6 wk on the high-fat diet (n = 4); C, respiratory exchange ratio (RER) of the IL-10–/– mice and wt controls after 6 wk on the high-fat diet as measured by indirect calorimetry (n = 4 mice per group).

 
Hyperinsulinemic euglycemic clamp studies
We performed hyperinsulinemic euglycemic clamp studies in IL-10–/– mice and wt controls after an overnight fast after 6 wk on a high-fat diet. After overnight fasting, no difference in body weight was observed (Table 1Go). The plasma values of glucose, insulin, and FFA before and during hyperinsulinemia are shown in Table 1Go. During hyperinsulinemia, glucose levels were maintained at approximately 7 mM, and plasma insulin levels were approximately 5- to 10-fold higher when compared with basal conditions. Strikingly, during the hyperinsulinemic period, plasma insulin concentrations were approximately 55% lower in IL-10–/– mice compared with wt control mice (1.8 ± 0.8 vs. 4.1 ± 1.5 ng/ml; P < 0.05) despite the infusion of identical amounts of insulin.


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  		TABLE 1. Plasma parameters and glucose infusion rate during the clamp

 
Basal HGP, which equals whole-body glucose uptake in the basal state, was not different between IL-10–/– mice and wt controls (47.3 ± 7.0 vs. 50.1 ± 4.0 µmol/min·kg; Fig. 3Go, A and B). During hyperinsulinemia, whole-body glucose disposal increased to a similar level in the two groups (74.6 ± 17.1 vs. 83.8 ± 25.9 µmol/min·kg; Fig. 3AGo). No differences in HGP were observed during hyperinsulinemia (26.5 ± 9.6 vs. 26.5 ± 12.0 µmol/min·kg; Fig. 3BGo). However, after correction for the 55% lower hyperinsulinemic plasma insulin levels, IL-10–/– mice showed a larger increase in insulin-stimulated whole-body glucose uptake. The corrected insulin-mediated decrease in HGP was significantly larger in IL-10–/– mice compared with wt control mice (14.5 ± 6.5 vs. 5.0 ± 2.6 µmol/min·kg·ng plasma insulin; P < 0.05; Fig. 3CGo).


Figure 3
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  		FIG. 3. IL-10 deficiency does not affect peripheral or hepatic insulin sensitivity as measured during a hyperinsulinemic euglycemic clamp. A and B, Whole-body glucose disposal (WGD) (A) and HGP (B) were measured during the basal period and under hyperinsulinemic conditions using the hyperinsulinemic euglycemic clamp method in both groups; C, the insulin-mediated stimulation of whole-body glucose disposal and the inhibition of HGP were corrected for the plasma insulin levels (CID) because in the IL-10–/– mice, the plasma insulin levels were approximately 55% lower compared with wt mice. *, P < 0.05; n = 6–7 mice per group.

 
Basal plasma FFA levels were significantly increased in the IL-10–/– mice compared with the wt mice after overnight fasting (1.14 ± 0.14 vs. 0.65 ± 0.09 mM; P < 0.05; Table 1Go). During hyperinsulinemia, plasma FFA levels decreased in both groups by about 40% compared with the respective levels under basal conditions. Nonetheless, plasma FFA levels remained significantly higher in IL-10–/– mice (0.52 ± 0.31 vs. 0.29 ± 0.05 mM; P < 0.05).

Hepatic pAkt protein expression levels
To investigate the effect of the hyperinsulinemic euglycemic clamp conditions on insulin signaling in the liver, we measured phosphorylation and protein expression of Akt and IR protein expression. We performed immunoblotting on liver samples from mice that had undergone the euglycemic hyperinsulinemic clamp. Despite decreased plasma insulin levels, we found increased phosphorylation of Akt in IL-10–/– mice upon insulin stimulation during the clamp compared with wt mice (Fig. 4Go; 12.5 ± 1.4 vs. 9.3 ± 2.0 arbitrary units; P < 0.05), whereas Akt and IR protein levels were not changed (2.7 ± 0.3 vs. 2.2 ± 0.1 and 8.4 ± 0.5 vs. 7.8 ± 1.4 arbitrary units, respectively).


Figure 4
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  		FIG. 4. Increased hepatic Akt phosphorylation in IL-10–/– mice during euglycemic hyperinsulinemic clamp conditions. pAkt protein levels were determined using Western blotting. Equal amounts of protein (25 µg) for pAkt and Akt expression were loaded, quantified, and corrected for loading differences. A, Western blot; B, quantification of pAkt protein levels corrected for loading differences. *, P < 0.05; n = 4.

 
Liver lipid content
Hepatic TG content is inversely related to hepatic insulin sensitivity in some mouse models (20). In IL-10–/– mice, we observed an approximately 54% increase in hepatic TG content compared with wt mice (168.7 ± 42.3 vs. 109.4 ± 42.3 µg TG/mg protein; P < 0.05; Fig. 5Go), even though IL-10 deficiency does not alter plasma TG levels (see Fig. 1Go). Liver free cholesterol content was also increased in IL-10–/– mice (17.7 ± 5.2 vs. 11.9 ± 2.7 µg free cholesterol/mg protein; P < 0.05), whereas the amount of cholesteryl esters was decreased (3.8 ± 1.8 vs. 6.3 ± 2.8 µg cholesteryl ester/mg protein; P = 0.063). Consequently, the free cholesterol/cholesteryl ester ratio was significantly larger in the IL-10–/– mice (5.9 ± 3.3 vs. 2.3 ± 1.4; P < 0.05).


Figure 5
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  		FIG. 5. IL-10 protects against hepatic steatosis. Hepatic TG content was determined using high-performance thin-layer chromatography. *, P < 0.05; n = 6–7 mice per group.

 
Adipose tissue mass
Fatty liver and increased plasma FFA are associated with increased visceral adipose tissue mass (21). Therefore, we measured sc and visceral adipose tissue mass in the IL-10–/– and wt mice and related it to the body weight of the mice. We found that visceral adipose tissue mass was significantly increased in IL-10–/– compared with wt mice (2.0 ± 0.5 vs. 1.5 ± 0.7% of total body weight; P < 0.05; Fig. 6Go). The sc adipose tissue mass was not different between IL-10–/– and wt mice (0.8 ± 0.2 vs. 0.7 ± 0.2% of total body weight).


Figure 6
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  		FIG. 6. Increased visceral adipose tissue mass in IL-10–/– mice. Visceral (visc) and sc (subc) adipose tissue was quantified as a percentage of total body weight (BW). *, P < 0.05; n = 6–7 mice per group.

 
Plasma fibrinogen and SAA
To exclude differences in systemic or hepatic inflammation, we measured plasma fibrinogen and SAA levels after 6 wk on the high-fat diet in both mouse groups. No differences in fibrinogen (2.9 ± 1.1 vs. 2.6 ± 0.8 mg/ml) or SAA levels (182 ± 170 vs. 217 ± 291 µg/ml) were observed.


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Our study is the first to establish the direct consequences of IL-10 deficiency on hepatic and peripheral insulin sensitivity. Our data show that basal IL-10 production protects against hepatic steatosis during high-fat feeding. However, endogenous IL-10 secretion does not improve hepatic or whole-body insulin sensitivity during high-fat feeding as assessed by the hyperinsulinemic euglycemic clamp technique. These observations argue against a simple protective role of endogenous IL-10 secretion in insulin-resistant states, at least within our mouse model. Nonetheless, our data also indicate that endogenous IL-10 secretion is not metabolically inert, because we documented clear effects of IL-10 deficiency on hepatic and peripheral lipid metabolism.

We observed no differences in the plasma levels of TG and total cholesterol between high-fat-fed IL-10–/– and wt mice. This is in concordance with a previous study (22) in IL-10–/– mice on an apolipoprotein E-deficient background. In those mice, a shift of cholesterol from very-low-density lipoprotein to low-density lipoprotein was observed, although total cholesterol levels remained unchanged. Conversely, overexpression of IL-10 in mice on a LDLr–/– background led to a significant decrease in total cholesterol (23). In that study, a high correlation between plasma total cholesterol levels and plasma IL-10 concentration was found. In accordance, several studies in humans documented an inverse association between plasma IL-10 and lipid levels (11, 24). In contrast, this association does not hold in the complete absence of IL-10, as we show in our study in IL-10–/– mice on a Black6 background and is shown by others in apolipoprotein E knockout mice (22). We cannot exclude the possibility that in the absence of any IL-10 production capacity, compensatory mechanisms prevent dysregulation of the lipid metabolism.

We expected mice lacking IL-10 to be more catabolic in comparison with wt mice, because they lack this antiinflammatory cytokine. Interestingly, when we compared basal metabolic characteristics, we found absolutely no differences in heat production, food intake, VO2, VCO2, and respiratory exchange ratio. Apparently, under basal conditions, IL-10 is not a crucial cytokine in energy metabolism. Lipopolysaccharide-mediated activation of the immune system may elucidate a more important role for IL-10. However, that would be a model of infection rather than a model of metabolic regulation per se.

Strikingly, we found decreased hyperinsulinemic plasma insulin concentrations in the IL-10–/– mice compared with the wt controls, although we infused identical amounts of exogenous insulin. The amount of insulin infused in our study protocols always results in plasma insulin levels of approximately 4–6 ng/ml as were observed in our wt control mice (25, 26). Thus, the absence of any difference in HGP and peripheral glucose uptake between IL-10–/– mice and wt controls during the clamp experiment occurred despite lower plasma insulin levels in the IL-10–/– mice. This combination of data suggests improved insulin sensitivity in IL-10–/– mice rather than the initially hypothesized decreased insulin sensitivity. In addition, the data indicate that IL-10 deficiency is associated with a higher rate of plasma clearance of insulin, for reasons presently unknown.

We subsequently evaluated the activity of important markers of the hepatic insulin signaling cascade in livers obtained from hyperinsulinemic IL-10–/– mice and wt controls. We found that phosphorylation of Akt was significantly increased in IL-10–/– mice despite lower plasma insulin concentrations under hyperinsulinemia, although Akt and insulin receptor expression were not changed. Therefore, both the in vivo glucose kinetic data obtained during hyperinsulinemia as well as these markers of the insulin signaling cascade point to increased hepatic insulin sensitivity rather than the expected hepatic insulin resistance in IL-10–/– mice.

IL-10 deficiency is associated with major changes in hepatic lipid content, reflected in increased TG content upon high-fat feeding. In many mouse models and in humans, positive correlations exist between hepatic steatosis and hepatic insulin resistance (20, 27, 28, 29, 30). However, there are also many examples of steatosis that are not associated with hepatic insulin resistance, including the treatment of mice with thiazolidinediones or liver X receptor agonists or the inhibition of fatty acid oxidation (31, 32, 33). Obviously, the relation between steatosis and hepatic insulin resistance is not straightforward, because other factors with complex interactions may be involved. The increased liver TG content may result from increased plasma FFA flux into the liver after overnight fast. Plasma FFA levels were significantly increased in the IL-10–/– mice both in the basal state and under hyperinsulinemia (Table 1Go). This may result from increased lipolysis and release of FFA from the increased visceral adipose tissue store in the IL-10–/– mice compared with control mice. In both groups of mice, plasma FFA as a measure of adipose tissue lipolysis is decreased by approximately 40% under hyperinsulinemia, suggesting no change in adipose tissue insulin sensitivity. However, in the IL-10–/– mice, the plasma FFA level remains significantly increased compared with controls. Increased visceral adipose tissue mass is associated with increased plasma FFA and fatty liver in humans (21). A potential explanation for this association may be the portal delivery of FFA to the liver (34). Subsequently, upon uptake by the liver, these FFA may be esterified into TG that may accumulate within the liver, because hepatic very-low-density lipoprotein TG production is not increased in IL-10–/– mice (23). Alternatively, we cannot exclude the involvement of other changes in intrahepatic fatty acid metabolism like an increase in the expression of lipogenic enzymes or a decrease in fatty acid oxidation. Although the increase in hepatic cholesterol content could result from increased cholesterol synthesis in the liver, the increased free cholesterol/cholesteryl ester ratio indicates an impairment of the esterification of cholesterol into cholesteryl esters. The mechanism behind this observation is beyond the scope of this paper.

We measured plasma fibrinogen and SAA in the IL-10–/– and the wt control mice. Although fibrinogen and SAA levels increased in both groups in time on the high-fat diet, no difference in the plasma levels of these markers of systemic and hepatic inflammation was observed between the two genotypes. Therefore, we conclude that the effects in IL-10-deficient mice do not simply reflect a higher state of chronic (hepatic) inflammation.

In summary, IL-10 deficiency alters peripheral and hepatic lipid metabolism. However, this study does not support a causal role of IL-10 in the protection against diet-induced hepatic insulin resistance and other metabolic disturbances.


   Footnotes
 
This work was supported by Netherlands Organization for Scientific Research (NWO) Grant 903-39-291.

Disclosure statement: The authors have nothing to disclose.

First Published Online May 18, 2006

Abbreviations: FFA, Free fatty acids; HGP, hepatic glucose production; IR, insulin receptor; pAkt, phosphorylated protein kinase B; SAA, serum amyloid-A; TG, triglycerides; wt, wild type.

Received March 31, 2006.

Accepted for publication May 10, 2006.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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