In the present study, we found that in obese mice, the same degree of weight loss with ND ad libitum or with restriction of HFD resulted in differential effects on hepatic steatosis, hepatic and AT inflammation, and proportions of blood CD11c+
monocytes. Weight loss with ND, but not with HFD restriction, markedly reduced hepatic steatosis and inflammation, and normalized adiponectin levels and blood monocyte proportions. Unexpectedly, we also observed for the first time that weight loss with calorie restriction of HFD tended to decrease AT chemokines and CD11c+
cells more than weight loss with ND ad libitum. Therefore, compared with previous weight loss studies [3
], our current study provided novel information on body weight–independent effects of weight loss with a low-fat ND vs. restriction of HFD on AT and liver inflammation, hepatic steatosis, and blood monocytes.
Despite the same degree of weight loss and comparable body weight, mice with ND-induced weight loss had less fat mass than those with HFD restriction–induced weight loss. The low fat content of the ND was most likely the major contributor to this difference. Another possible mechanism was that EPA and DHA, present in the ND only, have been shown to reduce fat mass [21
]. Less dietary fat consumption may also have contributed to the greater improvement in hepatic steatosis in ND-induced weight loss. Greater reduction in hepatic lipogenesis as indicated by lower levels of lipogenic molecules may be another contributor to the lower hepatic TGs in mice with ND-induced weight loss than in mice with HFD restriction–induced weight loss. Dietary PUFAs have been shown to suppress hepatic expression of SREBP-1 and its target genes, such as FAS
]. Therefore, inclusion of EPA and DHA in the ND may have contributed to the lower hepatic levels of lipogenic genes, particularly SCD1
, in ND-induced weight loss. Our current study supports a beneficial effect of SCD1
inhibition through dietary PUFA supplementation in improving obesity-associated hepatic steatosis.
Weight loss with ND, but not HFD restriction, ameliorated hepatic inflammation as indicated by lower hepatic levels of chemokines and CD11c, a marker for activated macrophages or DCs [3
]. However, the hepatic levels of F4/80, a marker for total macrophages/DCs, were not significantly different among obese controls, mice with ND-induced weight loss, and mice with HFD restriction–induced weight loss. These data suggest plastic attributes of hepatic macrophages/DCs. Weight loss with ND, but not HFD restriction, resulted in conversion of these hepatic macrophages/DCs from “proinflammatory” phenotypes, with high expression of chemokines and CD11c, to a “resting” state, with reduction in chemokine and CD11c expression. Considering the proinflammatory properties of fat, particularly saturated fat [13
], we postulate that the lower hepatic fat content may play important roles in conversion of hepatic macrophage/DC phenotypes and resolution of hepatic inflammation in ND-induced weight loss. In addition, higher levels of adiponectin, a molecule with anti-inflammatory properties [24
], may also contribute to these effects of ND-induced weight loss.
Obesity and increased fat mass are associated with AT inflammation [2
]. Because of lower fat mass with ND-induced weight loss, we had predicted that ND would reduce AT inflammation more than HFD restriction. Unexpectedly, we found that weight loss with HFD restriction tended to decrease AT MCP-1 and MIP-2 levels and CD11c+
cells more than weight loss with ND. On the other hand, weight loss with ND, but not HFD restriction, normalized adiponectin levels. These data indicated that AT adiponectin expression was regulated by pathways different from those for regulation of other AT inflammatory markers, and factors other than adiponectin may play important roles in regulation of chemokine expression and CD11c+
cells in AT of these mouse models.
To achieve the same body weight in both weight loss groups required not only restricting the amount of HFD consumed in the WL-HFDr group but also reducing the calorie intake below that of the WL-ND group. This is most likely due to the fact that animals on HFD are able to store the energy derived from dietary fat more efficiently in AT, thereby maintaining their body weight, than mice on a low-fat ND. As we provided HFD to the mice with HFD restriction–induced weight loss once a day, we observed that these mice consumed the HFD quickly and then had a long fasting period until the next HFD provision. Kosteli et al recently reported an increase in AT macrophage recruitment initially (days 3–7) during caloric restriction of HFD in obese mice, probably due to increased lipolysis induced by caloric restriction (fasting) [25
]. They also found reduced AT macrophages at a later stage of caloric restriction (day 21 and afterwards) [25
]. We analyzed AT macrophages on day 28 after dietary intervention, in samples collected after all the groups of mice had fasted for the same period of time; therefore, we did not expect significant confounding effects of an “acute” fasting-induced lipolysis on AT macrophages. Furthermore, the comparable plasma levels of total FFA in WL-ND and HFDr mice did not support a potentially confounding effect of lipolysis on AT macrophages in our mouse models. In contrast, our data suggested that low calorie intake and possibly periods of “chronic” fasting may be important in regulation of AT chemokines and CD11c+
cells, whereas type of diet may be more important in regulation of AT adiponectin (and leptin) expression. Because dietary EPA and DHA induce adiponectin expression [26
], inclusion of EPA and DHA in ND may play important roles in raising AT adiponectin. However, the favorable effects of EPA and DHA on other AT inflammatory markers [27
] may be counteracted by the greater calorie intake in mice with ND-induced weight loss than in mice with HFD restriction–induced weight loss.
Compared to HFD restriction–induced weight loss, weight loss with ND resulted in greater reductions in blood CD11c+
monocytes. Based on the role of CD11c+
monocytes in atherogenesis [16
], the reduction in CD11c+
monocytes would favor protective effects against obesity-associated atherosclerosis. MCP-1 increases CD11c expression on monocytes [28
]. The lower plasma MCP-1 levels in ND-induced weight loss may have contributed to the lower blood CD11c+
monocytes. Recent studies indicated that saturated FAs induced monocyte inflammation [29
]. Mice with ND-induced weight loss had lower levels of saturated FA, but higher levels of PUFAs, than mice with HFD restriction–induced weight loss. The potential effects of various types of FAs and also adiponectin on monocyte CD11c expression warrant further investigation.
The comparable improvements in glucose clearance as assessed by GTT between ND- and HFD restriction–induced weight loss may be explained by the dissociation of hepatic steatosis and insulin resistance [30
] and a trend toward higher AT inflammatory markers with ND-induced weight loss than HFD restriction–induced weight loss.
Li et al reported that switching from HFD to ND for 3 weeks improved insulin sensitivity and ameliorated hepatic steatosis and inflammation in obese mice [20
], which was consistent with our observation in WL-ND mice. However, they observed more pronounced reduction in AT inflammatory markers in their ND-induced weight loss group than we did in our WL-ND mice. Kalupahana et al reported that HFD restriction for 2 months in obese mice reduced AT inflammation, including decreased MCP-1, reduced plasma triglyceride levels, and improved hepatic steatosis [19
], observations consistent with ours in WL-HFDr mice. However, in contrast to our observations, they found that obese mice with HFD restriction maintained high levels of FFAs in plasma but had significantly decreased plasma insulin levels and HOMA-IR [19
]. Differences in types of diets and periods of dietary interventions may have contributed to these discrepancies. Whereas these studies examined the effect of either ND-induced weight loss alone [20
] or HFD restriction–induced weight loss alone [19
], we are the first to report the differential effects of ND- and HFD restriction–induced weight loss on inflammation in mice with diet-induced obesity.
In summary, using a mouse model of HFD-induced obesity, we demonstrated differential effects of ND- and HFD restriction–induced weight loss. In particular, weight loss with ND, but not with HFD restriction, markedly alleviated hepatic steatosis and inflammation as assessed by hepatic TGs and chemokine and CD11c expression. ND-induced weight loss normalized adiponectin expression in AT and tended to attenuate AT expression of chemokines and CD11c, whereas HFD restriction–induced weight loss significantly attenuated AT expression of chemokines and CD11c and tended to increase AT adiponectin. Weight loss with ND, but not with HFD restriction, normalized blood CD11c+
monocytes. Although improvements in glucose tolerance and insulin resistance were not significantly different between ND- and HFD restriction–induced weight loss in this short-term study, we postulate that long-term weight loss with ND would be more beneficial considering its greater ability to alleviate hepatic inflammation and steatosis, a potentially important contributor to obesity-linked insulin resistance and a risk factor for hepatic cirrhosis and carcinoma [9
], and to normalize blood CD11c+
monocytes, a contributor to atherosclerosis [16
]. Also, given the fact that HFD calorie restriction–induced weight loss tended to reduce AT inflammation more than ND-induced weight loss, we propose that long-term weight loss with low-fat ND along with calorie restriction would result in the most beneficial effects in obesity treatment.