The Mechanisms Of The Natural Compound Oleic Acid On Weight Maintenance By Its Receptor TGR5

Overweight and obesity are now viewed as chronic diseases in the whole world with an alarming epidemic prevalence. Increasing energy consumption has been the safest and most promising method of non-surgical weight loss.The G-protein coupled receptor TGR5 and the Farnesoid X receptor(FXR) are both receptors of bile acids, co-regulating homeostasis and signal transduction of bile acids as well as glucose metabolism. Body fat is stored in white adipose tissue(WAT) and brown adipose tissue(BAT), which play important roles in energy storage, and body heat generation and energy consumption, respectively. TGR5 is highly expressed in mitochondrials of BAT, and down-regulates ATP synthesis by bile acids-TGR5-CAMP-D2-T3-UCP pathways, thus regulating energy homeostasis and reduce body weight. Oleic acid(OA) is a natural ligand of TGR5, which was originally used for hepatitis. Thereafter, OA was found to reduce high-fat diet-induced obesity and improve glucose tolerance. However, its detailed relationship with TGR5 has not been characterized. Recently, mi RNAs have attracted considerable attention as potential new therapeutic targets for different human diseases. One of these mi RNAs, mi R-26 a plays pleiotropic roles in preventing obesity-induced insulin resistance and excessive lipid synthesis in the liver, which suggests it has potential as a therapeutic target for improving insulin sensitivity and inhibiting lipogenesis and gluconeogenesis. This study is to explore the mechanisms of the natural compound OA on regulating energy metabolisms by its receptor TGR5 in BAT in spontaneous and diet-induced obesity animal models.Following are several results we obtained from this study:TGR5 is required to mediate the effect of OA on obesity and glucose toleranceWide-type and TGR5-/- mice were fed with NF 、 HFD and HFD+OA.We took specimens for interrelated indicators after 20 weeks.GGT and insulin resistance were bedetermined as well.The results indicate that OA shows activities to activate TGR5 in vitro and display effect on body weight control and anti-hyperglycemia in mice. OA feeding significantly reduced BW to the level comparable to the Chow diet control group. However, although OA still significantly decreased the BW of HFD group, there was still significant difference between this group and chow diet-feeding group. We measured the weight of visceral fat and the results indicate a similar effect of OA on body fat formation during HFD feeding. This result was further confirmed by the reduced fat accumulation in wild type livers, but not in TGR5-/- livers, after OA treatment. These results thus clearly indicate that TGR5 mediates the OA-induced anti-obesity effect. The slight effect of OA in TGR5-/- mice could be due to the weak activity of OA on farnesoid X receptor or other proteins. We then compared the glucose tolerance on wild type and TGR5-/- mice. As expected, HFD caused higher glucose tolerance in both wild type and TGR5-/- mice. However, OA treatment significantly improved the glucose tolerance in wild type but not TGR5-/- mice, indicating a TGR5-dependent effect of improving glucose regulation by OA.TGR5 activation increases the expression of miR-26aThe results indicate that OA could be used as a tool to screen for potential downstream liver mi RNAs regulated by TGR5 activation. Among the mi RNAs we have searched, mi R-26 a was highly upregulated by OA treatment only in wild type but not in TGR5-/- mice. In comparison, mi R-16 a was not affected by OA or TGR5 status. CA feeding also significantly upregulated mi R-26 a levels.And the results indicated a FXR independent effect of CA on mi R-26 a expression.TGR5 activation upregulates mi R-26 a expression in macrophagesTreatment of RAW264.7 cells and Kupffer with TGR5 agonist: oleanolic acid(OA, 10 μM) indicate that TGR5 activation indeed induced mi R-26 a expression in both macrophage cell line and Kupffer cells. These results thus support a novel role of TGR5 in up-regulating mi R-26 a expression in macrophages. In contrast, other mi RNAs, including mi R-16, mi R-21 and mi R-126, were not affected by OA treatment in Kupffer cells. And other cell lines treated with OA had no effect on mi R-26 a expression.JNK pathway is involved downstream of TGR5 in mi R-26 a inductionJNK phosphorylation was increased by OA treatment rapidly in both RAW 264.7 cells and Kupffer cells. The phosphorylation levels of JNK in wild-type peritoneal macrophages were increased by OA stimulation, which was not observed in TGR5-/- peritoneal macrophages. These results indicate that JNK is specifically activated after TGR5 activation. We further examined whether inhibition of the JNK pathway could block the TGR5-dependent pro-inflammatory cytokine induction. JNK inhibitor, SP600125(10 μM), dramatically reduced the OA-induced increase of mi R-26 a transcription in RAW 264.7 cells. Moreover, mi R-26 a induction was increased in normal livers but impaired in Jnk1-/- livers after CA diet feeding. These results strong indicate that JNK is a critical downstream effector in TGR5-mediated cytokine induction in the liver.Promoter analysis identified the proximal responsive DNA sequences of miR-26 a to OAwe also located the TGR5 response DNA element to a proximal region of mi R-26 precursor, which is independent of the transcription of its host genes. These results, for the first time, suggest a novel mechanism by which TGR5 regulates mi R-26 a expression. The results also link the metabolic effect of BA signaling to a mi RNA-mediated pathway. In summary, the present work demonstrates that OA indeed gegulated energy metabolism to combat obesity by its receptor TGR5,which can increase mi R-26 a expression through a JNK-dependent pathway. Our results thus provide a novel link between BA signaling and mi RNAs in metabolic regulation. These findings also suggest the activation of TGR5 by small molecules can modulate the expression of mi RNAs for the treatment of obesity-associated metabolic diseases.we also located the TGR5 response DNA element to a proximal region of mi R-26 precursor, which is independent of the transcription of its host genes. These results, for the first time, suggest a novel mechanism by which TGR5 regulates mi R-26 a expression. The results also link the metabolic effect of BA signaling to a mi RNA-mediated pathway.In summary, the present work demonstrates that OA indeed gegulated energy metabolism to combat obesity by its receptor TGR5,which can increase mi R-26 a expression through a JNK-dependent pathway. Our results thus provide a novel link between BA signaling and mi RNAs in metabolic regulation. These findings also suggest the activation of TGR5 by small molecules can modulate the expression of mi RNAs for the treatment of obesity-associated metabolic diseases.