Mechanisms mediating the anti-inflammatory effects of dietary methionine restriction

Introduction Obesity is associated with inflammation, which contributes to several obesity-related comorbidities. This inflammation is characterized by increased inflammatory cytokine expression in adipose tissue (AT) depots and the liver. Dietary restriction of the essential amino acid methionine, in rodents, has been shown to improve body weight and adiposity, decrease high-fat diet- and age-induced inflammation, reduce hepatic steatosis, and increase insulin sensitivity despite inducing hyperphagia. Recent studies indicate that dietary methionine restriction (MR) rapidly and persistently increases tissue and circulating concentrations of the hormone fibroblast growth factor 21 (FGF21). In rodents, FGF21 has been shown to reduce body weight, increase insulin sensitivity, and recent reports indicate it may also decrease inflammation. STAT3 is a generally pro-inflammatory transcription factor which microarray analyses have identified as a potential “master regulator” of inflammatory processes in MR. We hypothesized that MR decreases inflammation by increasing FGF21 and downregulating STAT3 signaling. Methods 24 wild type (WT) C57BL/6J and 24 Fgf21 knock-out (KO) mice were fed a high-fat diet for four weeks, then randomized to a high-fat control (CON) diet or high-fat MR diet for 13 weeks. RT-PCR was used to measure markers of inflammation in the liver and epididymal white adipose tissue (EWAT). Western blot analyses were conducted to determine whether STAT3 activity was affected by MR. Results Fgf21KO mice had elevated hepatic expression of Ccl2 (MCP-1), and MR significantly decreased its expression in Fgf21KO, but not WT mice. In EWAT, Fgf21KO mice had elevated expression of Emr1 (F4/80), and MR reduced its expression in Fgf21KO mice, but not WT mice. MR reduced Ccl2 expression in EWAT of both genotypes. Expression levels of Itgam (CD11b) and IL-6 in EWAT did not differ amongst groups. MR decreased STAT3 phosphorylation at Tyr705 in both genotypes, but had no effect on Ser727 phosphorylation. Conclusion The RT-PCR results suggest that mice lacking FGF21 may be more susceptible to HFD-induced inflammation. Since MR induced comparable reductions in inflammatory markers in both genotypes, at first glance it appears that FGF21 does not play a role in the mechanism by which MR decreases inflammation. However, it should be noted that in Fgf21KO mice, MR induced an unexpected decrease in food intake that resulted in reduced body weight and adiposity compared to control-fed Fgf21KO mice. Western blot results support the hypothesis that MR decreases inflammation by downregulating STAT3 signaling