Saponins From Boussingaultia Gracilis Prevent Obesity And Related Metabolic Impairments In Diet-induced Obese Mice

Objective: Obesity is a severe public health problem worldwide.Our study aims to assess whether a saponin-enriched extract from the leaves of Boussingaultia gracilis(SBG)could attenuate obesity and its related metabolic disorders in mice and to explore the potential mechanism of this effect.Methods: Three-week old male mice were fed with HFD for 7 wk followed by a 10-wk period in which the mice were supplemented with distilled water or SBG(250 mg/kg bodyweight).During this treatment period,the fasting blood glucose(FBG)levels and body weights of the mice were measured on a weekly basis.OGTT and ITT were used to evaluate the glucose tolerance and insulin sensitivity of mice in each group.FBG and insulin levels were tested to evaluate the insulin resistance of mice in each group.The serum total cholesterol(TC),triglycerides(TG),highdensity lipoprotein cholesterol(HDL-c),low-density lipoprotein cholesterol(LDL-c),aspartate aminotransferase(AST)and alanine aminotransferase(ALT)were measured by using an automatic biochemical analyzer.The degree of lipid deposition in liver tissue and the size of adipocytes in subcutaneous adipose tissue were observed by H&E staining.Western blotting(WB)and RT-PCR were used to detect the protein expression levels of PPAR? and SREBP-1 and the gene expression levels of lipid synthesis regulator enzymes(ACC1,FAS,SCD1)in liver and iWAT.Immunohistochemical staining was used to observe the expression levels of UCP1 in iWAT and brown adipose tissue(BAT).The expression levels of UCP1,PRDM16 and PGC1 in iWAT were determined to evaluate the browning and thermogenic activity of WAT in mice of each group.Expression levels of key proteins of the insulin signaling pathway(p-IRS/IRS,p-AKT/AK,GLUT2,GLUT4)were analyzed in liver,iWAT and muscle tissues to assess the uptake and utilization of glucose by major energy metabolism organs.The expression level of F4/80 in liver and adipose tissue was observed by fluorescence immunoassay to analyze the infiltration degree of macrophages.The expression levels of NF-?B p65,TNF?,IL-1? and IL-6 were measured.Finally,non-targeted metabolomics was used to analyze the metabolites in liver tissue extracts,select the differential metabolites related to SBG intervention,and conduct pathway enrichment analysis to explore the broad effect mechanism of SBG.Results: We demonstrated that SBG supplementation for 10 weeks prevented body weight gain,decreased total body fat accretion,improved blood lipids and reduced hepatic injury in diet-induced obesity(DIO)mice.The results of H&E staining showed that administration with SBG effectively attenuated HFD-induced lipid deposits in the liver and adipocyte hypertrophy.Compared with the HFD group,the expression levels of PPAR in the liver and gastriculus muscle of SBG group were significantly increased,while the expression levels of SREBP1 and lipid synthesis regulator enzyme(ACC1,FAS,SCD1)were decreased(P<0.05).The interscapular BAT showed no difference in UCP1 positivity in response to HFD stress compared to that in the NDC mice fed with a standard diet.However,SBG treatment resulted in the emergence of more abundant UCP1 positivity.In addition,the expression levels of PRDM16 and PGC1 in subcutaneous adipose tissue of SBG intervention group were increased,and the degree of browning and the thermogenic activity were enhanced.Compared with the HFD group,HOMA-IR decreased by 34% after SBG supplementation(P<0.05).Meanwhile,the glucose AUCs of OGTT and ITT were both lower in the SBG group than in the HFD group(P<0.05).And the signal transduction of insulin signaling pathways(p-IRS/IRS,p-AKT/AKT,GLUT2,GLUT4)was enhanced after SBG treatment.HFD feeding increased F4/80 positivity(macrophage infiltration indicator)by IF staining in the liver and iWAT,which was effectively ameliorated by administering SBG,almost recovering to the normal control level.Meanwhile,the inflammatory signaling pathways(NF-?B p65,TNF?,IL-1?,IL-6)were decreased in the SBG group than in the HFD group.Finally,nontargeted metabolomics of liver extracts revealed that 39 metabolites were related to SBG effect,and the main pathways included biological synthesis of branched chain amino acids,synthesis and degradation of ketone bodies,taurine and taurine metabolism,arachidonic acid metabolism,and histidine metabolism.Conclusion: In conclusion,the novel contribution of this work is that SBG might decrease HFD-induced lipid deposits through the regulation of PPAR? and SREBP-1 and promote adipocyte browning via the action of UCP1,PRDM16 and PGC1.Within the liver,skeletal muscle and adipose tissues,SBG might ameliorate HFD-induced IR by enhancing the IRS/PI3K/AKT/GLUT pathway and control HFD-induced inflammation by blocking NF-?B p65 and multiple cytokines.Moreover,in liver-specific metabolomics,the target point of SBG varies widely.Our study thus provides a novel insight into both the effects of SBG on HFD-induced obesity and the underlying mechanisms.Indeed,the consumption of saponins enriched in B.gracilis,combined with appropriate physical activity and a balanced diet,could be considered as an available strategy for the prevention or treatment of obesity and its related metabolic syndrome.