Metabolic activities of butyric acid were examined in this study in diet-induced obese mice. The most important observation is that butyrate supplementation at 5% wt/wt in high-fat diet prevented development of dietary obesity and insulin resistance. It also reduced obesity and insulin resistance in obese mice. In butyrate-treated mice, the plasma butyrate concentration was increased, and blood lipids (triglycerides, cholesterol, and total fatty acids) were decreased (H–I
and Supplement 1). The change in insulin sensitivity may be a consequence of a reduction in adiposity in our model. The increase in energy expenditure and fatty acid oxidation may be responsible for the antiobesity effect of butyrate. Butyrate supplementation did not reduce food intake, fat absorption, or locomotor activity, suggesting that there was no toxicity from butyrate. Butyrate was tested at 5 and 2.5% wt/wt in the high-fat diet in this study. At the lower (2.5% wt/wt) dosage, similar metabolic activity was observed (Supplement 3). At 5% in the high-fat diet, butyrate increased the calorie content from 58 to 64.4% in the fat. The increase in fat calories may not contribute to our observation of the antiobesity activity for butyrate. A recent study of weight-loss diets suggests that total calorie intake, not diet composition, is responsible for weight reduction in humans (30
). At the cellular level, butyrate increased mitochondrial respiration, as indicated by the increase in oxygen consumption and CO2
production. At the molecular level, increased expression of PGC-1α, PPAR-δ, and CPT1b may be involved in the stimulation of mitochondrial function by butyrate.
The current study indicates that in vivo butyrate is a novel activator of PGC-1α. PGC-1α activity may be regulated by butyrate at three levels. PGC-1α expression was increased in both mRNA and protein. The protein elevation was observed in brown fat, skeletal muscle, and liver in butyrate-treated mice. It may be a result of increased mRNA expression or extended half-life of the PGC-1α protein. The change in protein stability is supported by the activities of AMPK and p38 in tissues and cells after butyrate treatment. These kinases phosphorylate the PGC-1α protein and inhibit its degradation (27
). As a transcriptional coactivator, PGC-1α transcription activity may be induced by phosphorylation, which leads to removal of a suppressor protein (p160 myb) that is associated with PGC-1α in the basal condition (35
). P38 acts downstream of AMPK in the phosphorylation of PGC-1α (36
). Therefore, AMPK may increase PGC-1α phosphorylation through direct and indirect (p38) mechanisms. It is not clear how AMPK is activated by butyrate. Butyrate may act through induction of AMP levels in cells from increased consumption of ATP. It was reported that butyrate increases ATP consumption (37
). Induction of PGC-1α activity may be a molecular mechanism by which butyrate stimulates mitochondrial function.
Inhibition of histone deacetylase may contribute to increased mRNA expression of PGC-1α, PPAR-δ, and CPT1b. Histone deacetylase inhibition promotes gene expression through transcriptional activation, which is determined by gene promoter activity. Promoter activation requires histone acetylation, which opens chromatin DNA to the general transcription factors for transcription initiation and mRNA elongation. Histone deacetylase inhibits gene promoter activity through deacetylation of histone proteins. In the presence of butyrate, promoter inhibition is prevented by butyrate suppression of histone deacetylase. Histone deacetylase suppression will enhance histone acetylation. This chromatin modification may occur in the gene promoters for PGC-1α, PPAR-δ, and CPT1b for the upregulation of gene transcription.
Butyrate induces type I fiber differentiation in skeletal muscle. In skeletal muscle cells, inhibition of histone deacetylase enhances myotube differentiation in vitro (28
) and protects muscle from dystrophy in vivo (29
). In transgenic mice, knockout of class II histone deacetylases was shown to promote differentiation of type I (oxidative) fibers in skeletal muscle (32
). This is consistent with our data that type I fiber was increased by butyrate, which inhibits histone deacetylase activities in the skeletal muscle of butyrate-treated mice. TSA, a typical histone deacetylase inhibitor, was tested in parallel treatment with butyrate. TSA exhibited activity similar to that of butyrate in mice (Supplement 2). TSA prevented dietary obesity, insulin resistance, and increased the type I fiber in the skeletal muscle. The activity was associated with elevation of PGC-1α protein. The current study suggests that the metabolic activities of butyrate may be dependent on the inhibition of histone deacetylase.
In summary, dietary supplementation of butyrate can prevent and treat diet-induced obesity and insulin resistance in mouse models of obesity. These activities of butyrate are similar to those of resveratrol (1
). The mechanism of butyrate action is related to promotion of energy expenditure and induction of mitochondrial function. Stimulation of PGC-1α activity may be a molecular mechanism of butyrate activity. Activation of AMPK and inhibition of histone deacetylases may contribute to the PGC-1α regulation. Butyrate and its derivatives may have potential application in the prevention and treatment of metabolic syndrome in humans.