Physical Exercise Reduces Circulating Lipopolysaccharide and TLR4 Activation and Improves Insulin Signaling in Tissues of DIO Rats
Abstract
OBJECTIVE Insulin resistance in diet-induced obesity (DIO) is associated with a chronic systemic low-grade inflammation, and Toll–like receptor 4 (TLR4) plays an important role in the link among insulin resistance, inflammation, and obesity. The current study aimed to analyze the effect of exercise on TLR4 expression and activation in obese rats and its consequences on insulin sensitivity and signaling.
RESEARCH DESIGN AND METHODS The effect of chronic and acute exercise was investigated on insulin sensitivity, insulin signaling, TLR4 activation, c-Jun NH2-terminal kinase (JNK) and IκB kinase (IKKβ) activity, and lipopolysaccharide (LPS) serum levels in tissues of DIO rats.
RESULTS The results showed that chronic exercise reduced TLR4 mRNA and protein expression in liver, muscle, and adipose tissue. However, both acute and chronic exercise blunted TLR4 signaling in these tissues, including a reduction in JNK and IKKβ phosphorylation and IRS-1 serine 307 phosphorylation, and, in parallel, improved insulin-induced IR, IRS-1 tyrosine phosphorylation, and Akt serine phosphorylation, and reduced LPS serum levels.
CONCLUSIONS Our results show that physical exercise in DIO rats, both acute and chronic, induces an important suppression in the TLR4 signaling pathway in the liver, muscle, and adipose tissue, reduces LPS serum levels, and improves insulin signaling and sensitivity. These data provide considerable progress in our understanding of the molecular events that link physical exercise to an improvement in inflammation and insulin resistance.
It has become increasingly evident that insulin resistance, induced by obesity, is associated with a chronic systemic low-grade inflammation (1–4). In this context, recent studies from our group and others show that the Toll–like receptor 4 (TLR4) may play a central role in the link among insulin resistance, inflammation, and obesity and that a point mutation in TLR4, which inactivates this receptor, prevents the diet-induced obesity (DIO) activation of IκB kinase (IKKβ) and c-Jun NH2-terminal kinase (JNK), and insulin resistance, suggesting that TLR4 is a key modulator in the cross-talk between inflammatory and metabolic pathways (5–10). TLR4 is an essential receptor for the recognition of lipopolysaccharide (LPS) (11). Moreover, a recent study demonstrated that LPS plasma concentrations increase significantly after the intake of high-fat, high-carbohydrate meals (12), suggesting that this LPS comes from the gastrointestinal tract because LPS is fat-soluble. In addition, it has recently been shown that fat intake leads to increased intestinal permeability for LPS (13).
On the other hand, evidence has emerged that exercise training has anti-inflammatory effects, with minimal side effects, which have been shown to occur in several tissues, including skeletal muscle (14), adipose tissue (15), and probably liver. In rats, exercise training lowers adipose inflammation (16), suggesting that exercise may be a useful therapy. Lifestyle interventions involving exercise clearly improve insulin sensitivity, and possibly inflammation, in obese individuals; yet the mechanisms for these effects are not well understood.
On the basis of data from these studies, we hypothesize that suppression of TLR4 signaling may play an important role in the exercise-induced improvement of insulin sensitivity. Thus, the current study aimed to analyze the effect of exercise on TLR4 expression and activation in obese rats, and its consequences on insulin sensitivity and signaling. We report that DIO induces the expression and activation of TLR4 in muscle, adipose tissue, and liver. Furthermore, both acute and chronic exercise strongly reverse the activation of this pathway and improve insulin signaling, providing a new mechanism by which exercise improves insulin action in obesity and type 2 diabetes. In addition, we show that exercise, both acute and chronic, promotes a reduction in serum LPS in DIO rats.
RESEARCH DESIGN AND METHODS
Male Wistar rats, C3H/HeJ mice, and C3H/HeN mice, their respective controls from the University of Campinas Central Animal Breeding Center, were used in the experiments. All antibodies were from Santa Cruz Technology (Santa Cruz, CA), with the exception of anti-Akt, anti–phospho-Akt, anti–phospho-IKKβ, anti–phospho-IRS-1 Ser307, and anti-TLR4, which were obtained from Cell Signaling Technology (Beverly, MA). TAK-242 (ethyl(6R)-6-[N-(2-chloro-4-fluorophenyl)sulfamoyl] cyclohex-1-ene-1-carboxylate) was synthesized at the Chemistry Institute of University of Campinas. LPS and routine reagents were purchased from Sigma Chemical (St. Louis, MO), unless specified elsewhere.
Animal care and experimental procedures.
All experiments were approved by the ethics committee at the State University of Campinas. Eight-week-old male Wistar rats were randomly divided into groups: control (C), fed standard rodent chow and water ad libitum; DIO-sedentary rats (DIO), fed a high-fat diet, as previously used (9), and water ad libitum for 20 weeks; DIO-chronic exercised rats (DIO+CE) and DIO-acute exercised rats (DIO+AE), fed a high-fat diet. Insulin and glucose tolerance tests were performed on these rats after 20 weeks of consumption of the diets and after both the exercise protocols, as described previously (17,18). Because individual rats can vary in their ability to perform swimming, some rats (10–15%) were rejected in the screening for the procedure.
Hyperinsulinemic-euglycemic clamps.
After an overnight fast (∼12 h), a 2-h hyperinsulinemic-euglycemic clamp was conducted in anesthetized catheterized rats with [3-3H]glucose and 2-deoxy-d-[1-14C]glucose to assess glucose metabolism in muscle, as previously described (18–20).
Assays.
Insulin and interleukin (IL)-6 concentrations were determined by an ELISA (Linco, St. Charles, MO). Serum free fatty acid (FFA) levels were analyzed using NEFA-kit-U (Wako Chemical, Neuss, Germany) with oleic acid as a standard. Glucose was measured from whole venous blood with a glucose monitor (Glucometer; Bayer Diagnostics, New York, NY).
Chronic exercise protocol.
Rats were adapted to swimming for 10 min for 2 days to reduce water-induced stress. Animals swam in groups of three in plastic barrels of 45 cm in diameter that were filled to a depth of 60 cm, and the water temperature was maintained at ∼34°C. The training consisted of daily swimming sessions (1 h/day, 5 days/week, for 8 weeks) with a progressive load increase up to 5% of body weight. These conditions were chosen on the basis of previous studies showing that swimming training with this load improved the physical condition of rats (21). The animals were killed with an overdose of anesthetic (sodium thiopental) at 24 and 36 h after the last session.
Acute exercise protocol.
Under the same conditions imposed as chronic exercise, the acute exercise protocol consisted of two 3-h bouts, separated by a 45-min rest period, as described previously (18). The animals were killed at 2 and 16 h after this protocol was carried out.
Tissue extraction and protein analysis by immunoblotting.
After exercise protocols, rats were anesthetized and used 10–15 min later, i.e., as soon as anesthesia was ensured by the loss of pedal and corneal reflexes, the abdominal cavity was opened, the portal vein was exposed, and 0.2 mL of normal saline was injected with or without insulin (10−6 mol/L). At 30 and 90 s after insulin injection, liver and gastrocnemius and adipose tissue were removed, minced coarsely, and homogenized immediately in extraction buffer, as previously described (22). Extracts were used for immunoprecipitation with MyD88 and Protein A-Sepharose 6MB (Pharmacia, Uppsala, Sweden). The precipitated proteins or whole-tissue extracts were subjected to SDS-PAGE and immunoblotting, as previously described (17,20).
LPS levels.
We diluted plasma samples to 20% with endotoxin-free water and then heated them to 70°C for 10 min to inactivate plasma proteins. We then quantified serum LPS with a commercially available Limulus Amebocyte assay from Cambrex (Walkersville, MD) according to the manufacturer’s protocol. We ran samples in duplicate and subtracted the background (23).
Real-time RT-PCR.
The mRNA was determined in the muscle, liver, and adipose tissue using RT-PCR, as previously reported (24). Primer sequences are shown in Supplementary Table 1. Results are expressed as relative expression values, as published previously (1).
Statistical analysis.
Data are expressed as means ± SEM, and the number of independent experiments is indicated. The results of blots are presented as direct comparisons of bands or spots in autoradiographs and quantified by optical densitometry (Scion Image; Scion Corporation, Frederick, MD). For statistical analysis, the groups were compared using a two-way ANOVA with the Bonferroni test for post hoc comparisons. The level of significance adopted was P < 0.05.
RESULTS
Physiologic and metabolic parameters.
Figure 1 shows comparative data regarding the controls, DIO-sedentary rats, and DIO rats submitted to chronic exercise when investigated at 24 h (DIO+CE24h) or 36 h (DIO+CE36h) after the last training session. All DIO animals, submitted to chronic exercise or not, exhibited a higher body weight and epididymal fat than the age-matched C group (Fig. 1A and B). The fasting plasma glucose concentrations were similar among all groups (Fig. 1C). During the glucose tolerance test, glucose and insulin levels were higher in DIO rats, compared with controls, and chronic exercise improved glucose tolerance and reduced insulin levels at all time points (Fig. 1C and D). Serum insulin levels were higher in DIO rats, and chronic exercise was able to reduce this hyperinsulinemia (Fig. 1D). The glucose disappearance rate was lower in the DIO group, and exercise training reversed these alterations (Fig. 1E). A hyperinsulinemic-euglycemic clamp with tracer infusions was also performed to examine the effects of training on glucose metabolism in the skeletal muscle. The glucose infusion rate was lower in the DIO group than in the C group and reestablished to control levels in DIO-exercised rats (Fig. 1F). As shown in Fig. 1G, DIO rats presented a significant reduction in glucose uptake in the skeletal muscle when compared with the control group. In contrast, chronic exercise improved insulin-induced glucose uptake in the muscle of DIO rats (Fig. 1G). Serum levels of FFA were also higher in DIO rats and significantly decreased after swimming training (Fig. 1H).
Physiologic, metabolic, and insulin tolerance parameters in control rats, obese rats, and obese rats submitted to a chronic exercise protocol. A: Body weight. B: Epididymal fat pad weight. C: Glucose tolerance test after 20 weeks of a high-fat diet. D: Serum insulin levels during the glucose tolerance test after 20 weeks of the diet. E: Rate constant for insulin tolerance test and glucose response curve during the insulin tolerance test after 20 weeks of a high-fat diet. F: Steady-state glucose infusion rates obtained from averaged rates of 90–120 min of 10% unlabeled glucose infusion during hyperinsulinemic-euglycemic clamp procedures in the control rats, DIO rats, and DIO rats submitted to chronic exercise. G: Glucose transport in gastrocnemius muscle was evaluated by 2-deoxy-d-glucose uptake during the last 45 min of the hyperinsulinemic-euglycemic clamp studies. H: Serum FFA concentrations. Data are presented as means ± SEM of 10 rats per group. #P < 0.001 vs. control. *P < 0.05 vs. DIO. **P < 0.001 vs. DIO.
Effect of chronic exercise on TLR4 expression.
The TLR4 protein content in muscle, liver, and adipose tissue was higher in the DIO group than in the controls (Fig. 2A–C). Results show that, at 24 h after the last training, there was a marked decrease in TLR4 expression and, after 36 h, this was still decreased when compared with nonexercised obese rats (Fig. 2A–C). Furthermore, the TLR4 mRNA expression was significantly reduced after chronic exercise in muscle, liver, and adipose tissue, compared with DIO rats (Fig. 2D–F).
Effects of chronic exercise on TLR4 in obese Wistar rats. Representative blots show the expressions of TLR4 in muscle (A), liver (B), and adipose (C) of control, DIO, DIO+CE24h, and DIO+CE36h rats. Determination of the relative TLR4 mRNA expression by real-time PCR in muscle (D), liver (E), and adipose tissue (F) of control, DIO, DIO+CE24h, and DIO+CE36h rats. TLR4/MyD88 interaction in muscle (G), liver (H), and adipose (I) of control, DIO, DIO+CE24h, and DIO+CE36h rats (G–I, top). Total protein expression of MyD88 (G–I, bottom). IRAK-1 protein expression in muscle (J), liver (K), and adipose (L) of control, DIO, DIO+CE24h, and DIO+CE36h rats. Total protein expression of β-actin (J–K, bottom). Data are presented as means ± SE of 10 rats per group. #P < 0.05 vs. control group. *P < 0.05 vs. DIO. **P < 0.001 vs. DIO. IB, immunoblot; IP, immunoprecipitate.
Chronic exercise-mediated suppression of TLR4 activity in the skeletal muscle, adipose tissue, and liver of DIO Wistar rats.
We next investigated TLR4 pathway activation in two steps: 1) as an early event, TLR4/MyD88 interaction was examined; and 2) for downstream signaling, JNK, IKKβ, and ERK1/2 phosphorylation were studied (25). Skeletal muscle, adipose tissue, and liver of DIO rats exhibited significant increases in the TLR4/MyD88 interaction, compared with the C group (Fig. 2G–I). Conversely, in exercised groups, the TLR4/MyD88 interaction decreased significantly in all tissues studied compared with obese sedentary rats (Fig. 2G–I). IRAK-1, another protein of the TLR4 pathway, showed an increased expression in DIO animals compared with the C group. Conversely, chronic exercise did not have any effect on this protein (Fig. 2J–L).
We also investigated whether this reduction was reflected in IKKβ and JNK phosphorylation, which are downstream of TLR4. As expected, an increase in IKKβ and JNK phosphorylation was found in the muscle, liver, and adipose tissue of DIO rats, compared with controls, and this effect was attenuated in the tissues of chronic-exercised obese rats, compared with sedentary DIO rats (Fig. 3A–F). With regard to ERK1/2, DIO rats exhibited high phosphorylation levels in the three tissues analyzed, compared with control animals (Supplementary Fig. 1). In contrast, we detected a marked reduction in ERK activation after chronic exercise (Supplementary Fig. 1).
Effects of chronic exercise on modulators of insulin signaling. Representative blots show the expressions of IKKβ phosphorylation in muscle (A), liver (B), and adipose (C) of control, DIO, DIO+CE24h, and DIO+CE36h rats (top). Total protein expression of IKKβ (A–C, bottom). Expression of JNK phosphorylation in muscle (D), liver (E), and adipose (F) of control, DIO, DIO+CE24h, and DIO+CE36h rats (D–F, top). Total protein expression of JNK (D–F, bottom). IRS-1 serine 307 phosphorylation in muscle (G), liver (H), and adipose (I) of control, DIO, DIO+CE24h, and DIO+CE36h rats (top). Total protein expression of IRS-1 (G–I, bottom). Data are presented as means ± SE of 10 rats per group. #P < 0.05 vs. control group. *P < 0.05 vs. DIO. IB, immunoblot.
We then evaluated the effect of exercise training on an important substrate of these kinases of insulin signaling pathway, namely, IRS-1 Ser307 phosphorylation. This phosphorylation was, on average, markedly upregulated in the muscle, liver, and adipose tissue of obese sedentary rats, compared with controls (Fig. 3G–I). In addition, exercise training was also able to reverse diet-induced IRS-1 Ser307 phosphorylation in the muscle, liver, and adipose tissue of DIO+CE24h and DIO+CE36h rats (Fig. 3G–I).
Chronic exercise improves insulin signaling in obese rats.
We then examined the consequences that this exercise-induced improved inflammatory profile exerts on the insulin signaling pathway. In the muscle, liver, and adipose tissue, insulin-induced IRβ, IRS-1 tyrosine phosphorylation, and Akt phosphorylation were reduced by 50–80% in DIO rats, compared with the C group (Fig. 4A–I). On the other hand, insulin-induced IRβ, IRS-1, and Akt phosphorylation were increased in the tissues of DIO+CE24h and DIO+CE36h rats, compared with obese sedentary rats, and approached levels of those found in the C group (Fig. 4A–I). No changes in basal phosphorylation or tissue protein levels were observed among groups (Fig. 4A–I).
Effects of chronic exercise on insulin signaling in rats fed a high-fat diet. Representative blots show tyrosine phosphorylation of IRβ in muscle (A), liver (B), and adipose (C) of control, DIO, DIO+CE24h, and DIO+CE36h rats (top). Total protein expression of IRβ (A–C, bottom). Tyrosine phosphorylation of IRS-1 in muscle (D), liver (E), and adipose (F) of control, DIO, DIO+CE24h, and DIO+CE36h rats (top). Total protein expression of IRS1 (D–F, bottom). Serine phosphorylation of Akt in muscle (G), liver (H), and adipose (I) of control, DIO, DIO+CE24h, and DIO+CE36h rats (top). Total protein expression of Akt (G–I, bottom). Data are presented as means ± SE of 10 rats per group. #P < 0.05 vs. control group. *P < 0.05 vs. DIO. IB, immunoblot.
Acute exercise and metabolic parameters.
Figure 5 shows comparative data for the DIO rats and DIO rats submitted to the acute exercise protocol (DIO+AE2h and DIO+AE16h). No significant variation was found in body weight, epididymal fat, and fasting blood glucose in DIO rats after a single session of exercise, when compared with sedentary obese rats (Fig. 5A–C). Acute exercise improved glucose tolerance and reduced serum insulin levels at all time points after the glucose load (Fig. 5D and E). The glucose disappearance rate was restored at both 2 h and 16 h after acute exercise (Fig. 5E). Acute exercise was also capable of restoring the glucose infusion rate during the hyperinsulinemic-euglycemic clamp (Fig. 5F), accompanied by a significant increase in glucose uptake in the skeletal muscle, compared with DIO rats (Fig. 5G). As expected, serum levels of FFA and IL-6 were higher in obese rats, and acute exercise induced a marked increase in the levels of this substrate, mainly in the DIO+AE2h group (Fig. 5H and I).
Physiologic, metabolic, and insulin tolerance parameters of obese sedentary rats and obese rats submitted to an acute exercise protocol. A: Body weight. B: Epididymal fat pad weight. C: Glucose tolerance test after 20 weeks of a high-fat diet. D: Serum insulin levels during the glucose tolerance test after 20 weeks of the diet. E: Rate constant for insulin tolerance test and glucose response curve during the insulin tolerance test after 20 weeks of a high-fat diet. F: Steady-state glucose infusion rates obtained from averaged rates of 90–120 min of 10% unlabeled glucose infusion during hyperinsulinemic-euglycemic clamp procedures in the DIO sedentary rats and DIO rats submitted to acute exercise. G: Glucose transport in gastrocnemius muscle was evaluated by 2-deoxy-d-glucose uptake during the last 45 min of the hyperinsulinemic-euglycemic clamp studies. H: Serum FFA concentrations. Data are presented as means ± SEM of 10 rats per group. *P < 0.05 vs. DIO. **P < 0.001 vs. DIO.
Acute physical exercise reverses obesity-induced TLR4 activation in obese rats.
In DIO animals submitted to acute exercise, no changes in TLR4 protein expression were observed in muscle, liver, and adipose tissue (data not shown). On the other hand, the acute exercised animals showed lower TLR4 mRNA only in muscle (Supplementary Fig. 2A–C). In contrast, we observed significant reductions in the TLR4/MyD88 interaction in all studied tissues (Fig. 6A–C).
Effects of acute exercise on the TLR4 pathway. Representative blots show the TLR4/MyD88 interaction in muscle (A), liver (B), and adipose (C) of control, DIO, DIO+AE2h, and DIO+AE16h rats (top). Total protein expression of MyD88 and TLR4 (A–C, bottom). Phosphorylation of IKKβ in muscle (D), liver (E), and adipose (F) of control, DIO, DIO+AE2h, and DIO+AE16h rats (top). Total protein expression of IKKβ (D–F, bottom). Expression of JNK phosphorylation in muscle (G), liver (H), and adipose (I) of control, DIO, DIO+AE2h, and DIO+AE16h rats (top). Total protein expression of JNK (G–I, bottom). IRS-1 serine 307 phosphorylation in muscle (J), liver (K), and adipose (L) of control, DIO, DIO+AE2h, and DIO+AE16h rats (top). Total protein expression of IRS-1 (J–L, bottom). Data are presented as means ± SE of 10 rats per group. #P < 0.05 vs. control group. *P < 0.05 vs. DIO. IB, immunoblot; IP, immunoprecipitate.
We also investigated the effect of acute exercise on TLR4 downstream signaling. As expected, significant decreases in IKKβ and JNK phosphorylation were found in the muscle, liver, and adipose tissue of acute exercised rats, compared with the DIO group (Fig. 6D–I). The same behavior was found for ERK1/2 phosphorylation after the acute exercise (Supplementary Fig. 1). Acute exercise was also able to reverse the diet-induced IRS-1 Ser307 phosphorylation in muscle, liver, and adipose tissue of both DIO+AE2h and DIO+AE16h rats (Fig. 6J–L).
Acute exercise improves insulin signaling in DIO rats.
Acute exercise, as observed in the chronic group, improved insulin-induced tyrosine phosphorylation of IRβ and IRS-1 in the insulin-sensitive tissues (Fig. 7A–F). With regard to Akt, the acute protocol stimulated an impressive increase in serine phosphorylation of this protein in all tissues studied that was similar to that observed in the C group (Fig. 7G–I).
Effects of a single session of exercise on insulin signaling in high-fat fed rats. Representative blots show tyrosine phosphorylation of IRβ in muscle (A), liver (B), and adipose (C) of control, DIO, DIOACE2h, and DIO+AE16h rats (top). Total protein expression of IRβ (A–C, bottom). Tyrosine phosphorylation of IRS-1 in muscle (D), liver (E), and adipose (F) of control, DIO, DIO+AE2h, and DIO+AE16h rats (top). Total protein expression of IRS-1 (D–F, bottom). Serine phosphorylation of Akt in muscle (G), liver (H), and adipose (I) of control, DIO, DIO+AE2h, and DIO+AE16h rats (top). Total protein expression of Akt (G–I, bottom). Data are presented as means ± SE of 10 rats per group. #P < 0.05 vs. control group. *P < 0.05 vs. DIO. IB, immunoblot.
Physical exercise blunted the high levels of cytokine mRNA expression in obese rats.
The tumor necrosis factor (TNF)-α and IL-6 mRNA levels were upregulated in the DIO group and were decreased after both exercise protocols in muscle, liver, and adipose tissue of almost all exercised groups (Supplementary Fig. 1D–I).
Physical exercise reduces serum LPS levels in DIO rats.
Serum LPS levels were significantly higher in obese sedentary rats when compared with control animals. Conversely, in animals of the DIO+AE2h group, acute exercise was able to completely reverse the high levels of LPS, and this was still observed at 16 h after the acute exercise (Fig. 8A). Notably, in chronic-exercised animals there was also a reduction in serum LPS levels, compared with the DIO group (Fig. 8A).
Serum LPS concentrations and influence of LPS challenge on the acute exercise effect in obese Wistar rats. A: Serum LPS levels in control, DIO, DIO+AE2h, DIO+AE16h, DIO+CE24h, and DIO+CE36h rats, and DIO+AE animals treated with LPS. Representative blots show the TLR4/MyD88 interaction in control, DIO, DIO+AE2h, and DIO+AE16h rats treated with saline or LPS in muscle (B) and adipose (C). Total protein expression of MyD88 (B–C, bottom). Phosphorylation of IKKβ in muscle (D) and adipose (E) of DIO+AE2h and DIO+AE16h rats with saline or LPS (top). Total protein expression of IKKβ (D–E, bottom). Expression of JNK phosphorylation in muscle (F) and adipose (G) of DIO+AE2h and DIO+AE16h rats treated with saline or LPS (top). Total protein expression of JNK (F–G, bottom). Serine phosphorylation of Akt in muscle (H) and adipose (I) in control, DIO, DIO+AE2h, and DIO+AE16h rats treated with saline or LPS (top). Total protein expression of Akt (H–I, bottom). Data are presented as means ± SE of 10 rats per group. #P < 0.05 vs. control group. ##P < 0.001 vs. control group. *P < 0.05 vs. DIO. **P < 0.001 vs. DIO. &P < 0.001 vs. DIO+AE2h. $P < 0.001 vs. DIO+AE16h. IB, immunoblot; IP, immunoprecipitate.
LPS infusion partially reverses the effects of exercise.
To examine whether the exercised-induced reduction in serum LPS plays an important role in the improvement of inflammatory status and insulin sensitivity, we infused LPS into the peritoneum of obese exercised rats immediately after acute exercise. Exercised animals treated with LPS showed serum levels of LPS similar to the DIO animals and presented significant increases in TLR4/MyD88 interaction, IKKβ, and JNK phosphorylation in the muscle and adipose tissue (DIO+AE2h and DIO+AE16h) (Fig. 8A–G), increase in TNF-α mRNA in muscle and adipose tissue, and increase in IL-6 mRNA only in muscle (Supplementary Fig. 2D–I). A significant decrease in Akt serine phosphorylation levels was also achieved in LPS-treated animals (Fig. 8H and I).
Physical exercise does not improve insulin signaling in DIO rats treated with a pharmacologic inhibitor of TLR4 or in C3H/HeJ mice fed with a high-fat diet.
We next investigated whether an inhibitor of TLR4 signaling (TAK-242) could mimic the effect of exercise on the inflammatory pathway and insulin signaling in DIO rats. The administration of TAK-242 for 3 days improved inflammatory status in the muscle and adipose tissue of DIO rats, as measured by the reduction of TLR4/MyD88 interaction and phosphorylation of IKKβ and JNK, in parallel with an improvement in insulin-induced Akt phosphorylation (Supplementary Fig. 3A). In DIO+AE2h animals treated with this drug, no additive effect was observed in inflammatory parameters or insulin signaling (Supplementary Fig. 3A). In a similar fashion, C3H/HeJ mice fed a high-fat diet did not present insulin resistance, and we did not observe any increase in IKKβ and JNK phosphorylation; furthermore, exercise had no effect on these parameters or on insulin signaling (Supplementary Fig. 3D–F).
DISCUSSION
Our results show that physical exercise, both acute and chronic, induces an important suppression in the TLR4 signaling pathway as evidenced by the reduction in an early step of this pathway, i.e., TLR4/MyD88 interaction, and in main downstream targets, such as IKKβ and JNK phosphorylation. These alterations are associated with a significant improvement in insulin resistance, in glucose uptake in muscle, and in the insulin signaling pathway. In DIO rats, circulating levels of LPS were increased and acute and chronic exercise reduced the circulating levels of this TLR4 ligand. Taken together, these data suggest that exercise is effective in reducing chronic low-grade inflammation, because it downregulates TLR4 ligand and an important pathway of the inflammatory response.
The increase in TLR4 protein levels in the muscle, liver, and adipose tissue of DIO rats was partially reversed after chronic exercise. This reduction in TLR4, after chronic exercise, supports findings of previous studies, in which the deletion of TLR4 in mice has been reported to prevent high-fat diet–induced insulin resistance (9,26). There was also a decrease in TLR4 activation and a parallel improvement in insulin signaling and sensitivity. Accordingly, a recent study has shown that MyD88-deficient mice are protected from high-fat diet–induced weight gain and impairment of peripheral glucose metabolism induced by palmitate (27). In line with this, our study demonstrated that a point mutation or pharmacologic blocking of TLR4 protects DIO rats from inflammation and insulin resistance, but had no additive or synergic effects to exercise in insulin signaling.
In accordance with a reduction in TLR4 activation, our data demonstrate reductions in IKKβ and JNK activities after chronic exercise in obese animals, which culminates in downregulation of both TNF-α and IL-6 mRNA. This finding is in contrast with previous studies that demonstrated an increase in IL-6 mRNA with exercise (28,29). Nevertheless, these previous studies investigated IL-6 mRNA levels during exercise, in control animals, and with high or moderate intensities, whereas our data were obtained in obese rats, at least 2 h after the exercise and with low-intensity. It is also important to mention that these cytokines, possibly through dysregulation of the TNF-α converting enzyme/tissue inhibitor of metalloproteinase 3 proteolytic system, have been shown to play an important role in subclinical inflammation in obesity/type 2 diabetes insulin resistance (30). It is well established that interventions that inhibit or attenuate IKKβ or JNK activity significantly improve peripheral insulin sensitivity (31). In this context, a previous study revealed that endurance training in obese, diabetic subjects suppresses the activation of the IKK/NFκB pathway, as proven by an increased abundance of IκB-α and IκB-β (32). Moreover, Ropelle et al. (18) have demonstrated that a single session of exercise is able to reverse obese-induced JNK activation and consequently promote increased insulin sensitivity. The suppression induced by exercise in these pathways is important because these are overactivated by obesity and can negatively regulate insulin signaling through IRS-1 Ser307 phosphorylation (33). Moreover, the activation of the mitogen-activated protein kinase (MAPK) signaling pathway is a mechanism that may also increases IRS-1 Ser307 phosphorylation. Our data demonstrate that physical exercise was capable of decreasing ERK1/2 phosphorylation, which may contribute to improving insulin signaling. Previous studies have observed that MAPK activity increases during exhaustive acute exercise (34–36), but the exercise protocol may contribute in explaining these differences; furthermore, the exercise-induced increase in MAPK phosphorylation rapidly decreases on cessation of exercise session and is completely restored to resting levels at 60 min after exercise (37).
Thus, the decrease in TLR4 expression/activation induced by chronic exercise is intriguing and may contribute to explain the amelioration in inflammation status and insulin sensitivity after chronic exercise. These data are in agreement with a previous hypothesis that a physically active lifestyle promotes anti-inflammatory properties (14), as proven in two longitudinal studies showing that regular training may suppress systemic low–grade inflammation (38,39).
Cani et al. (13) recently hypothesized that bacterial LPS, derived from Gram-negative bacteria residing in the gut microbiota, acts as a triggering factor, linking inflammation to high-fat diet–induced diabetes and obesity. They found that high-fat diet feeding resulted in a significant modulation of the dominant bacterial populations within the gut microflora. A reduction in the number of bifidobacteria, Eubacterium rectal-Clostridium coccoides group and Bacteroides, favoring an increase in the Gram-negative to Gram-positive ratio was observed. This modulation of gut microflora was associated with a significant increase in plasma LPS, fat mass, body weight gain, liver hepatic triglyceride accumulation, insulin resistance, and diabetes. Another study has shown that the treatment of rats with polymyxin B, an antibiotic that specifically targets Gram-negative organisms, reduced LPS expression and hepatic steatosis (40). As expected, an impressive increase in LPS serum level in DIO rats was observed. In contrast, for the first time, we showed a significant reduction in serum LPS levels in chronic-exercised obese rats compared with DIO rats. The reason for this reduction in LPS serum levels in chronic exercised obese rats is not known, but we can speculate that a reduction in gut blood flow during exercise, alterations in intestinal barrier, or reduced LPS absorption by a reduced expression of TLR4 in intestinal cells should be considered (41). On the other hand, the decrease in circulating levels of LPS could also play a role in the reduction of TLR4 mRNA and protein expression induced by chronic exercise, because it is well established that an increase in LPS is associated with increases in TLR4 expression (42). During sepsis, with very high levels of LPS, the modulation of TLR4 mRNA and protein expression is tissue-specific (43). Our findings show that the modulation of TLR4 induced by chronic exercise was similar in the three tissues investigated and probably has a transcriptional regulation, because our data showed a parallel decrease in TLR4 mRNA and protein expression. On the other hand, acute exercise modulated TLR4 mRNA only in muscle. It may be speculated that exercise and other factors, in addition to LPS levels, may also be involved in the modulation of TLR4 expression or signaling, such as intracellular lipid levels and reactive oxygen species levels (44–46), which are increased in DIO rats and decreased after exercise. In this context, earlier studies showed that reactive oxygen species levels increase during exercise and may participate in mechanisms that increase glucose uptake during activity (36). As such, this point deserves further exploration.
Modest weight loss, achieved by diet and exercise, can enhance insulin sensitivity and even reverse insulin resistance (47,48). In our study, obese trained animals showed a slight reduction in weight, which puts in doubt whether improvement in inflammation and insulin sensitivity are consequences of exercise per se or just weight loss effects. To elucidate this, obese animals were submitted to acute exercise, which had no effect on body weight or epididymal fat pads. Our data showed that acute exercise reduced TLR4 signaling and downstream kinases, such as IKKβ and JNK, and, consequently, also reduced IRS-1 Ser307 phosphorylation and improved insulin signaling and sensitivity, as well as induced increased muscle glucose uptake. The main difference between acute and chronic exercise was related to TLR4 protein levels, which did not change after an acute bout of exercise. However, the net effect on TLR4 signaling and insulin resistance was similar. Acute exercise also reduced circulating levels of LPS. This reduction in serum levels of LPS, after an acute bout of exercise, may be related to reduced gut blood flow (49,50). Whether reduced LPS levels may contribute to the reduction in TLR4 signaling and insulin sensitivity is not known, but when we infused a low dose of LPS in obese animals after an acute bout of exercise (to increase circulating levels close to those of obese sedentary animals), the reduction in inflammatory pathways and the improvement in insulin signaling and sensitivity were blunted.
In summary, our results show that physical exercise in DIO rats, both acute and chronic, induces an important suppression in TLR4 signaling pathway in the liver, muscle, and adipose tissue, reduces LPS serum levels, and improves insulin signaling and sensitivity. These data provide considerable progress in our understanding of the molecular events that link physical exercise to an improvement in inflammation and insulin resistance.
ACKNOWLEDGMENTS
This work was supported by grants from Fundação de Amparo à Pesquisa do Estado de SÃO PAULO, Conselho Nacional de Pesquisa, and INCT---Obesidade e Diabetes.
No potential conflicts of interest relevant to this article were reported.
A.G.O. researched data, contributed to discussion, wrote the article, and reviewed and edited the article. B.M.C. researched data and reviewed and edited the article. N.T. researched data and reviewed and edited the article. E.R.R. reviewed and edited the article. J.R.P. researched data. R.A.B. researched data. D.G. researched data. J.B.C.C. contributed to discussion and reviewed and edited the article. M.J.A.S. contributed to discussion, wrote the article, and reviewed and edited the article.
The authors thank L. Janeri and J. Pinheiro (Department of Internal Medicine, UNICAMP, Campinas, São Paulo) for technical assistance.
Footnotes
This article contains Supplementary Data online at http://diabetes.diabetesjournals.org/lookup/suppl/doi:10.2337/db09-1907/-/DC1.
- Received December 30, 2009.
- Accepted November 13, 2010.
- © 2011 by the American Diabetes Association.
Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work is not altered. See http://creativecommons.org/licenses/by-nc-nd/3.0/ for details.
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