Bioactive Compounds in Fenugreek Ameliorate ER Stress and VLDL Overproduction via the Mediation of Insig Signaling

A number of health-enhancing bioactive compounds have been identified in the seed of fenugreek, an annual legume. These compounds have been shown to exert multiple health beneficial effects on anti-obesity and type 2 diabetes. In this study, we characterized the molecular mechanisms of fenugreek seed in regulating lipid and lipoprotein metabolism and metabolic inflammatory stress. Two groups of hyperlipidemic mice induced by depletion of cAMP responsive element binding protein H were fed either a chow containing 2% fenugreek seed or vehicle control for 7 weeks. Q-RT-PCR and immunoblotting analysis demonstrated that fenugreek seed containing diet inhibited hepatic SREBP-1c activation and the subsequent de novo lipogenesis by enhancing expression of insulin-inducible gene-1 (Insig-1) and gene-2 (Insig-2). mRNA expression of PPARα and its target genes that were involved in fatty acid β-oxidation were also upregulated in the fenugreek seed treated-mice which was accompanied by significantly reduced hepatic lipid accumulation and VLDL secretion and improved endoplasmic reticulum (ER) stress. These actions ameliorated hepatic steatosis and systemic hyperlipidemia. Fenugreek seed further improved insulin sensitivity by upregulating expression of glucose transporters Glut-2 and Glut-4 and enhancing the activity of hepatic inulin signaling molecules, insulin receptor and AKT. In vitro, treating a rat hepatoma cell line, McA-RH7777, with trigonelline, a phytochemical compound in fenugreek seed, increased expression of Insig-2 which prevented the activation of SREBP-1c and the subsequent de novo lipid synthesis, resulting in less secretion of VLDL. Trigonelline treatment also attenuated ER stress induced by a free fatty acid, palmitic acid, indicated by the downregulation of GRP94/78 and reduced phosphorylation of JNK and eIF2α. This study unveiled a novel mechanism of the bioactive compound trigonelline in ameliorating metabolic inflammatory stress, hepatic steatosis, VLDL overproduction and insulin resistance via the mediation of Insig signaling. This evidence lands support for developing fenugreek seed as a nutraceutical supplement for the prevention and treatment of nutrient-surplus associated metabolic disorders.