Inhibition of lipolysis by ilexgenin A via AMPK activation contributes to the prevention of hepatic insulin resistance

Potential lipolytic regulators derived from natural products as effective approaches to treat obesity

Epidemic obesity is contributing to increases in the prevalence of obesity-related metabolic diseases and has, therefore, become an important public health problem. Adipose tissue is a vital energy storage organ that regulates whole-body energy metabolism. Triglyceride degradation in adipocytes is called lipolysis. It is closely tied to obesity and the metabolic disorders associated with it. Various natural products such as flavonoids, alkaloids, and terpenoids regulate lipolysis and can promote weight loss or improve obesity-related metabolic conditions. It is important to identify the specific secondary metabolites that are most effective at reducing weight and the health risks associated with obesity and lipolysis regulation. The aims of this review were to identify, categorize, and clarify the modes of action of a wide diversity of plant secondary metabolites that have demonstrated prophylactic and therapeutic efficacy against obesity by regulating lipolysis. The present review explores the regulatory mechanisms of lipolysis and summarizes the effects and modes of action of various natural products on this process. We propose that the discovery and development of natural product-based lipolysis regulators could diminish the risks associated with obesity and certain metabolic conditions.

Toll-like receptor 3 ablation prevented high-fat diet-induced obesity and metabolic disorder

Inflammation in insulin-sensitive tissues (e.g., liver, visceral adipose tissue [VAT]) plays a major role in obesity and insulin resistance. Recruitment of innate immune cells drives the dysregulation of glucose and lipid metabolism. We aimed to seek the role of Toll like receptor 3 (TLR3), a pattern recognition receptor involved in innate immunity, obesity and the metabolic disorder. TLR3 expression in liver and VAT from diet induced obese mice and in VAT from overweight women was examined. Body weight, glucose homeostasis and insulin sensitivity were evaluated in TLR3 wild-type and knockout (KO) mice on a chow diet (CD) or high-fat diet for 15 weeks. At euthanasia, blood was collected, and plasma biochemical parameters and adipokines were determined with commercial kits. Flow cytometry was used to measure macrophage infiltration and activation in VAT. Standard western blot, immunohistochemistry and quantative PCR were used to assess molecules in pathways about lipid and glucose metabolism, insulin and inflammation in tissues of liver and VAT. Utilizing human and animal samples, we found that expression of TLR3 was upregulated in the liver and VAT in obese mice as well as VAT in overweight women. TLR3-deficiency protected against high-fat diet induced obesity, glucose intolerance, insulin resistance and lipid accumulation. Lipolysis was enhanced in VAT and hepatic lipogenesis was inhibited in TLR3 KO animals. Macrophages infiltration into adipose tissue was attenuated in TLR3 KO mice, accompanied with inhibition of NF-κB-dependent AMPK/Akt signaling pathway. These findings demonstrated that TLR3 ablation prevented obesity and metabolic disorders, thereby providing new mechanistic links between inflammation and obesity and associated metabolic abnormalities in lipid/glucose metabolism.

Ilexgenin A restrains CRTC2 in the cytoplasm to prevent SREBP1 maturation via AMP kinase activation in the liver

BACKGROUND AND PURPOSE

Ilexgenin A is a triterpenoid from ShanLv Cha with beneficial effects on metabolic homeostasis. We investigated whether ilexgenin A could inhibit hepatic de novo fatty acid synthesis via the interfering with SREBP1 maturation.

EXPERIMENTAL APPROACH

The effects of Ilexgenin A on CRTC2 translocation and SREBP1 maturation were investigated in the liver of fasted mice and hepatocytes exposed to saturated fatty acids. The effect of Iilexgenin A on hepatic lipid accumulation was also observed in high-fat diet fed mice.

KEY RESULTS

Sec23A and Sec31A are two subunits of COPII complex and their interaction is essential for the processing of SREBP1 maturation. Ilexgenin A activates AMPK by reducing cellular energy and preventing cytoplasmic CRTC2 to compete with Sec23A for binding to Sec31A under nutrient-rich conditions. Consequently, ilexgenin A impaired COPII-dependent SREBP1 maturation via disrupting Sec31A-Sec23A interaction, leading to the inhibition of de novo fatty acid synthesis in the liver. In contrast, mTORC1 phosphorylated Ser136 of CRTC2, facilitating the formation of Sec31A-Sec23A interaction to promote SREBP1 maturation, whereas this action was reversed by ilexgenin A in an AMPK-dependent manner. Ilexgenin A protected CRTC2 function and restrained hepatic lipogenic response in high fat diet-fed mice, providing in vivo evidence to support the beneficial effects of ilexgenin A on lipid metabolism.

CONCLUSIONS AND IMPLICATIONS

Ilexgenin A activated AMPK and restrained CRTC2 to the cytoplasm to prevent SREBP1 maturation via impairing COPII function in the liver. This suggests that CRTC2 might be a potential target for pharmacological intervention to prevent hepatic lipid deposition.

The Effect of Simvastatin on Gut Microbiota and Lipid Metabolism in Hyperlipidemic Rats Induced by a High-Fat Diet

The objective of this study was to investigate the effects of simvastatin (SIM) on lipid metabolism disorders and gut microbiota in high-fat diet-induced hyperlipidemic rats. The obtained results revealed that feeding rats with SIM (20 mg/kg/day) significantly decreased serum lipid level and inhibited hepatic lipid accumulation and steatosis. Histological analysis further indicated that SIM reduced lipid deposition in adipocytes and hepatocytes in comparison with that of the HFD group. The underlying mechanisms of SIM administration against HFD-induced hyperlipidemia were also studied by UPLC-Q-TOF/MS-based liver metabonomics coupled with pathway analysis. Metabolic pathway enrichment analysis of liver metabolites with significant difference in abundance indicated that fatty acids metabolism and amino acid metabolism were the main metabolic pathways altered by SIM administration. Meanwhile, operational taxonomic units (OTUs) analysis revealed that oral administration of SIM altered the composition of gut microbiota, including Ruminococcaceae (OTU960) and Lactobacillus (OTU152), and so on. Furthermore, SIM treatment also regulated the mRNA levels of the genes involved in lipid and cholesterol metabolism. Immunohistochemistry (IHC) analysis of the liver-related proteins (CD36, CYP7A1 and SREBP-1C) showed that oral administration of SIM could regulate the levels of the protein expression related to hepatic lipid metabolism.

TRB3 stimulates SIRT1 degradation and induces insulin resistance by lipotoxicity via COP1

Fatty acid-induced lipotoxicity plays an important role in the pathogenesis of diabetes mellitus. Our previous studies have documented that lipotoxicity contributes to the onset and development of diabetes via insulin resistance and/or compromised function of the pancreatic β-cells. However, the underlying molecular mechanisms associating lipotoxicity with insulin resistance remain to be fully elucidated. In this study, we explored the role of TRB3-COP1-SIRT1 in lipotoxicity leading to insulin resistance in hepatocytes. High fat diet (HFD)-fed mice and hepG2 cells stimulated with palmitate were utilized as models of lipid metabolism disorders. We analyzed the interactions of SIRT1 and COP1 with each other and with TRB3 using co-immunoprecipitation, western blotting. SIRT1 ubiquitination was also explored. Animal and cell experiments showed that lipotoxicity induced SIRT1 down-regulation at the protein level without altering the mRNA level, whereas, lipotoxicity led to up-regulation of TRB3 and COP1 at both the gene and protein levels. Mechanistic analysis indicated that COP1 functioned as an E3 Ub-ligase of SIRT1, responsible for its proteasomal degradation under lipotoxic conditions. TRB3 recruited COP1 to SIRT1 to promote its ubiquitination. Our data indicated for the first time that TRB3-COP1-SIRT1 pathway played an important role in lipotoxicity leading to insulin resistance in hepatocytes, and suggested that COP1 could be a potential therapeutic choice for the treatment of diabetes mellitus, with lipotoxicity being the important pathomechanism.

Elevated lipolysis in adipose tissue by doxorubicin via PPARα activation associated with hepatic steatosis and insulin resistance

Adipose dysfunction is tightly associated with hepatic insulin resistance and steatosis condition. Doxorubicin would disturb the lipid metabolism both in adipose and liver. Here we projected that doxorubicin would impede lipogenesis and elevated lipolysis in adipose tissue would elevate the circulatory lipid profile and leads to insulin resistance. Further exacerbated lipid profile in circulation would impair the lipid metabolism in hepatic tissue which leads to fatty liver condition and consequently related disease during doxorubicin treatment. Doxorubicin impairs the lipogenesis through PPARγ and augments lipolysis and fatty acid oxidation through ATGL and PPARα in adipose tissue. Increased fatty acid level by adipose tissue in circulation would translocate into the liver and dysregulates AHR, PXR, PPARγ, ATGL and Apo B,which further develop insulin resistance and hepatic steatosis condition. The findings add to the mechanistic role of association between adipose tissue dysfunction and hepatic dysfunction.

Ilexgenin A enhances the effects of simvastatin on non-alcoholic fatty liver disease without changes in simvastatin pharmacokinetics

Cardiovascular disease (CVD) is the most common cause of death in patients with non-alcoholic fatty liver disease (NAFLD). New therapeutic strategies which have the potential for slowing down the evolution of NAFLD and reducing CVD-related mortality are urgently needed. Statins are well recognized in the treatment of dyslipidemia, but their use in the treatment of NAFLD is limited due to the safety concerns. Ilexgenin A (IA) is one of the main bioactive compounds in 'Shan-lv-cha', an herbal tea commonly used in China. In the present study, we investigated the possible synergistic therapeutic effects of IA and simvastatin (SV) on NAFLD. IA or SV showed beneficial effects on the rats with NAFLD by lowering the liver weight, liver index and plasma levels of alanine aminotransferase and aspartate aminotransferase, regulating abnormal metabolism of lipids and ameliorating steatosis in liver. IA significantly enhanced the hypolipidemic and anti-inflammation effects of SV. Furthermore, a sensitive, accurate, convenient and reproducible LC-MS method was developed to investigate the effects of IA on the pharmacokinetics of SV. No significant changes were observed in pharmacokinetic parameters of SV and simvastatin hydroxy acid in the IA plus SV co-treated group in comparison with those in the group treated with SV alone. The mRNA levels and activity of CYP3A1 were not altered by IA. In conclusion, the results obtained from the present study should be helpful for further clinical application of SV and IA alone or in combination.

Er-Miao-Fang Extracts Inhibits Adipose Lipolysis and Reduces Hepatic Gluconeogenesis via Suppression of Inflammation

High-fat-diet (HFD) feeding induces adipose dysfunction. This study aims to explore whether the Traditional Chinese Medical prescription Er-Miao-Fang could ameliorate adipose dysfunction and prevent hepatic glucose output. Short-term HFD feeding induced adipose lipolysis accompanied with enhanced hepatic glucose output in mice. Adipose lipolysis is initiated by cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) signaling. Oral administration Er-Miao-Fang inhibited inflammation in adipose tissue by dephosphorylation of JNK and reducing TNF-α and IL-1β production, and thus preserved phosphodiesterase 3B (PDE3B) induction, contributing to preventing cAMP accumulation. As a result, from suppression of PKA activation, Er-Miao-Fang reduced fatty acids and glycerol release from adipose tissue due to the inhibition hormone-sensitive lipase (HSL). By blocking the traffic of fatty acids and inflammatory mediators from adipose tissue to the liver, Er-Miao-Fang attenuated hepatic cAMP/PKA signaling by protecting phosphodiesterase 4B (PDE4B) induction from inflammatory insult, and thereby reduced hepatic glucose production by suppression of hepatic glucagon response in HFD-fed mice. In conclusion, Er-Miao-Fang prevented adipose lipolysis by suppression of inflammation, contributing to reducing excessive hepatic glucose output. These findings present a new view of regulating gluconeogenesis and provide the guiding significance for the regulation of multi-link targets with Traditional Chinese Medicine.