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TGR5 (also known as G protein–coupled bile acid receptor 1, GPBAR1) is a G protein–coupled bile acid receptor that is expressed in many diverse tissues. TGR5 is involved in various metabolic processes, including glucose metabolism and energy expenditure; however, TGR5's function in skeletal muscle is not fully understood. Using both gain- and loss-of-function mouse models, we demonstrate here that Tgr5 activation promotes muscle cell differentiation and muscle hypertrophy. Both young and old transgenic mice with muscle-specific Tgr5 expression exhibited increased muscle strength. Moreover, we found that Tgr5 expression is increased by the unfolded protein response (UPR), which is an adaptive response required for maintenance of endoplasmic reticulum (ER) homeostasis. Both ER stress response element (ERSE)- and unfolded protein response element (UPRE)-like sites are present in the 5′ upstream region of the Tgr5 gene promoter and are essential for Tgr5 expression by Atf6α (activating transcription factor 6α), a well known UPR-activated transcriptional regulator. We observed that in the skeletal muscle of mice, exercise-induced UPR increases Tgr5 expression, an effect that was abrogated in Atf6α KO mice, indicating that Atf6α is essential for this response. These findings indicate that the bile acid receptor Tgr5 contributes to improved muscle function and provide an additional explanation for the beneficial effects of exercise on skeletal muscle activity.

Obacunone is a limonoid that is predominantly found in Citrus. Although various biological activities of limonoids have been reported, little is known about the beneficial effects of obacunone on metabolic disorders. In the present study, we examined the effects of dietary obacunone supplementation on obese KKAy mice, to clarify the function of obacunone in metabolic regulation. Mice were pair-fed a normal diet either alone or supplemented with 0.1% w/w obacunone for 28 days. Compared with the control, obacunone-fed mice had lower glycosylated hemoglobin, blood glucose, and white adipose tissue weight, although there was no significant difference in body weight. Obacunone treatment also significantly increased the weight of the gastrocnemius and quadriceps muscles. Reporter gene assays revealed that obacunone stimulated the transcriptional activity of the bile acids-specific G protein-coupled receptor, TGR5, in a dose-dependent manner. In addition, obacunone inhibited adipocyte differentiation in 3T3-L1 cells and antagonized ligand-stimulated peroxisome proliferator-activated receptor γ (PPARγ) transcriptional activity. These results suggest that obacunone stimulates muscle hypertrophy and prevents obesity and hyperglycemia, and that these beneficial effects are likely to be mediated through the activation of TGR5 and inhibition of PPARγ transcriptional activity.