Shengmai San formula alleviates high-fat diet-induced obesity in mice through gut microbiota-derived bile acid promotion of M2 macrophage polarization and thermogenesis

Background: Shengmai San Formula (SMS) is a traditional Chinese medicine (TCM) that has been used to treat wasting-thirst regarded as diabetes mellitus, which occurs disproportionately in obese patients. Therefore, we investigated whether SMS could be used to treat obesity, and explored possible mechanisms by which it might improve glucose and fat metabolism.

Methods: To investigate the effects of SMS on a high-fat diet (HFD)-induced obesity (DIO) model, we studied glucose metabolism via glucose tolerance testing (GTT) and insulin tolerance testing (ITT). Browning of white adipose tissue (WAT) was evaluated using H&E staining, along with browning-related gene and protein expression. Changes in bile acid (BA) levels in serum, liver, ileum, and inguinal white adipose tissue were detected by Ultra performance liquid chromatography tandem quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS). In addition, antimicrobial mixture (ABX) and fecal microbial transplantation (FMT) experiments were used to verify the role of gut flora in the effects produced by SMS on HFD-induced obesity model.

Results: SMS ameliorated diet-induced dyslipidemia in a dose-dependent manner and reduced glucose intolerance and insulin resistance in DIO mice, helping to restore energy metabolism homeostasis. SMS significantly altered the structure of intestinal microbiome composition, decreasing the abundance of Lactobacillus carrying bile salt hydrolase (BSH) enzymes and thereby increasing the level of conjugated BAs in the blood, ileum, and iWAT. Increased TCA content promoted the secretion of Slit3 from M2 macrophages in iWAT, which activates the protein kinase A/calmodulin-dependent protein kinase II (PKA/CaMKII) signaling pathway in sympathetic neurons via the roundabouts receptor 1(ROBO1). This pathway promotes the synthesis and release of norepinephrine (NE), inducing cyclic adenosine monophosphate (cAMP) release in adipose tissue that activates the cyclic adenosine monophosphate/protein kinase A/phosphorylated hormone-sensitive lipase (cAMP/PKA/pHSL) pathway and enhances WAT browning. ABX treatment eliminated SMS effects on glucose and lipid metabolism in DIO mice, whereas glucose and lipid metabolism in obese mice improved following SMS-FMT and increased the level of serum bile acids.

Conclusion: SMS affects intestinal flora and bile acid composition in vivo and increased TCA promotes M2 macrophage polarization and Slit3 release in adipose tissue. This induces NE release and increases WAT browning in obese mice, which may be a mechanism by which SMS could be used to treat obesity.