The liver has major functions in the coordination of metabolism. Hepatocytes are involved in lipid metabolism and the release of inflammatory cytokines [1]. In obesity, inflammatory cytokines released from adipose tissue move outside the liver through the portal vein and can directly interfere with liver functions. Especially, Kupffer cells are important sources of inflammatory cytokines within the liver and are the inhabitant macrophages in the liver. During steatosis, the recruitment of macrophages into the liver can alter cell distribution, thereby also changing Kupffer cell morphology and function [2]. Macrophages have a highly plastic phenotype that allows them to specialize and display polarized functional properties, such as inflammatory or anti-inflammatory actions in response to cytokine products [3].

The so-called classical activated or “M1” macrophages secrete high amounts of pro-inflammatory mediators while the alternatively activated “M2” macrophages are low pro-inflammatory cytokine producers. In obesity, the balance between M1 and M2 macrophages is disturbed. The M1 macrophage specific marker, CD11c, is increased in high fat diet conditions, whereas the M2 macrophage specific marker, CD163, is decreased [4].

Previous reports showed that the expression of adhesion molecules in adipose tissue was increased in the obese condition [5,6] and a different study indicated that HFD induced obese mice exhibited higher expression of Intercellular adhesion molecule 1(ICAM-1) and Vascular cell adhesion molecule 1(VCAM-1) in adipose tissue. On the other hand, obese mice did not exhibit macrophage infiltration into adipose tissue after ICAM-1 antagonist treatment [7]. Thus, adhesion molecules affect obesity-induced macrophage infiltration into adipose tissue.

Regular exercise can prevent chronic inflammation [8,9] and reduce the state of fibrosis as a marker of hepatic injury [10,11]. A previous study showed that exercise training reduces inflammatory cytokines in adipose tissue by suppressing several adipocyte-differentiation genes [12]. Cellular molecule recognition by toll-like receptors such as Toll-like receptor 4 (TLR4) induces inflammatory cytokine production [13]. A review [14] on the topic demonstrated that TLR4 expression in exercise groups are lower than in control groups. One possible explanation is that exercise training suppresses inflammatory cytokines in obese mice, which might be controlled by the decrease in TLR4.

Resveratrol has been shown to possess anti-inflammatory and antioxidant properties [15,16]. Resveratrol also attenuates hepatic lipid metabolism [17]. A recent study demonstrated that resveratrol exerts inhibitory effects on adipocyte differentiation, which accompanies the down-regulation of several adipocyte-specific genes [18].

Although resveratrol or exercise has been suggested to reduce hepatic fat accumulation, the beneficial effect on macrophage infiltration is unclear. Additionally, the protective effect of high-fat induced polarization from M2 to M1 is unknown.

Therefore, the purpose of this study was to compare the effectiveness of either resveratrol supplementation or aerobic exercise training on macrophage infiltration, shifting from M1 to M2 and inflammation induced genes in high fat diet mice.

A high fat diet is known to increase body weight and fat mass and obesity is recognized as a social health problem. Moderate exercise increases energy expenditure and has been recommended for the treatment of obesity [21]. Also, it is known that resveratrol treatments reduce body weight and liver weight compared to non-treatment in animals [22]. Thus, it was meaningful to compare the effectiveness of exercise and resveratrol on obesity-induced metabolic complications.

In our results, body and liver weights were markedly increased in the HD and HR groups compared to the ND group (Fig. 1). Nevertheless, the exercise trained mice (HE and HRE) had a lower body and liver weight than the high fat diet fed (HD) mice (p < 0.05). In support of our results, a previous study demonstrated that exercise has positive effects on the reduction of body weight and fat mass [23]. Another study showed that regular exercise reduced liver mass, indicating that reduced liver mass might lead to reduced hepatic triglyceride accumulation [24]. Therefore, our results suggest that exercise training effectively regulated high fat diet-induced liver and body weight and reduced hepatic triglyceride accumulation. The previous study reported that a high-fat diet with resveratrol supplementation reduced fat content and body weight gain [25]. In contrast, this effect was not observed in other studies [26,27]. In our experiments, there was no significant change in the HR group compared to the HD group. It seems that the effect of resveratrol depends on the dose and the length of supplementation.

Especially, obese-induced macrophage infiltration is a major source of pro-inflammatory cytokine expression [24]. Previous studies have noted that the high levels of pro-inflammatory cytokine production after high-fat-diet treatment were decreased by the inhibition of macrophage infiltration in obese animals [28,29]. However, the effect of exercise or resveratrol with respect to the inhibition of macrophage infiltration in the liver is still not established.

The present study found that expression of the macrophage marker, F4/80, was significantly increased in the HD group compared to the ND group. However, the expression of F4/80 was significantly lower in the exercise groups than in the high fat diet group. The present finding that exercise training reduces the expression of adipose tissue F4/80 mRNA coincides with previous studies [13,29].

In this study, resveratrol supplementation was not enough to reduce F4/80 in the liver. The reason for this has not been fully explained. However, Bujanda et al., [30] reported that resveratrol inhibited lipid peroxidation and reduced inflammation while simultaneously increasing antioxidant enzymes in obese rats. Another study reported that over 20 weeks of dietary intake of polyphenol alleviated hepatic fat accumulation in obese mice [31]. Therefore, more research is needed to elucidate the in vitro and in vivo long term effects of resveratrol treatment at various doses in obese rodents. To summarize, 12 weeks of exercise training had a more positive effect than resveratrol treatment in ameliorating high fat diet induced macrophage infiltration.

M1 and M2 macrophage activation represent two ends of the macrophage polarization state [24], which may accelerate or decelerate metabolic disease. In this study, mRNA expression of the M1 macrophage marker, CD11c, markedly increased in the HD group compared to the ND group (P <0.01).

Previous research reported that exercise training may inhibit M1 macrophage infiltration into visceral adipose tissue [24]. Treatment with the CD11c antagonist attenuated inflammation via the inhibition of M1 macrophage infiltration in obese mice [32]. Nevertheless, the results of this study showed that there was no difference in CD11c mRNA expression between the exercise and resveratrol treatment groups in the liver. In contrast, mRNA expression of the M2 macrophage specific marker, CD163, was reduced in the HD group compared to the ND group (p < 0.05). In the HE and HRE groups, CD163 mRNA expression was significantly increased compared to the HD group (p < 0.05). Macrophage activation has been categorized into two separate polarization conditions known as M1 and M2 [33]. M1 macrophages powerfully express TLR4, which produces pro-inflammatory cytokines such as TNF-a. In contrast, M2 macrophages generate IL-10, which suppresses pro-inflammation states. In a previous study, obese animals exhibited a phenotypic switch from M2 macrophages to M1 macrophages in adipose tissue [34]. Another study demonstrated that exercise training reduced the expression of inflammatory cytokines and M1 macrophage markers in the adipose tissue of obese animals [24]. It is possible that exercise training suppressed macrophage infiltration in the liver by upregulating CD163 in obese mice.

In the present results, mRNA expression of TLR4, which induces pro-inflammatory cytokine production, were strongly decreased in HR, HE, and HRE compared to HD mice. Moderate exercise induces glucocorticoid secretion, which inhibits the expression of TLR2 and TLR4 [35]. The reduced level of TLR4 in the exercise trained group may have been caused by the stimulation of glucocorticoids.

Also, this study found that the HD group had higher mRNA expressions of ICAM-1 and VCAM-1 in the liver. Previous studies reported that the expression of adhesion molecules in visceral adipose tissue was increased by obesity [36,37]. In another study, obese mice did not exhibit macrophage infiltration into adipose tissue after ICAM-1 antagonist treatment [38]. A different study showed that plasma ICAM-1 concentrations were markedly decreased by moderate exercise training in metabolic disease patients without concomitant weight loss [39]. More research is required to further elucidate the effects of exercise or resveratrol on the weakening of adhesion molecules in the liver caused by the anti-inflammatory effects of exercise and resveratrol.

Exercise and resveratrol are currently believed to induce crucial events leading to the reduction of inflammatory cytokines. However, it is still unclear how exercise or resveratrol down-regulates the inflammatory state. The present study shows that the body and liver weights of the HD group were significantly increased compared to the ND group. However, the body and liver weights of the exercise groups were significantly decreased after long-term aerobics training, compared to the high fat diet group. In addition, it was identified that moderate exercise training controlled the downregulation of F4/80 and the upregulation of CD163. Also, the HE, HR, and HRE groups exhibited lower expression of TLR4, ICAM-1, and VCAM-1 compared to the HD group.

In conclusion, it seems that moderate exercise training might contribute to the inhibition of macrophage infiltration and the shifting of M1 to M2 macrophages by suppressing TLR 4 expression and inflammation in the liver. However, resveratrol supplementation alone is not enough to ameliorate high fat diet-induced obese complications.