Anti-obesity properties of selected phytochemicals in cell culture and animal models

Abstract

Obesity, as a widespread epidemic disease, is one of the main health risks worldwide. Extensive studies have therefore been conducted to explore preventive and healing strategies. One promising approach is to seek naturally occurring components with anti-obesity properties. Phytochemicals are chemical compounds derived from natural plants and many of them have been verified to possess anti-obesity activities. In this thesis, I study five phytochemicals (oxyresveratrol, cyanomaclurin, cynarin, pseudoprotodioscin and 5-hydroxytryptophan) for their anti-obesity properties, whose bioactivities in obesity resistance are previously unexplored.

Firstly, in vitro studies with 3T3-L1 cells were conducted to identify the efficacy of these five phytochemicals in suppressing adipogenesis. Adipogenesis is a progress through which mesenchymal stem cells develop into mature adipocytes. The exposure of oxyresveratrol (0-100 μM), cyanomaclurin (0-600 μM) or 5-hydroxytryptophan (0-400 μM) led to a dose-dependent decrease in triglyceride accumulation of 3T3-L1 cells. Their effects in suppressing adipogenesis were attributed in part to the induction of cell cycle arrest by retaining preadipocytes in the G1 phase during the first two days post-confluent through regulating cyclin D1, cyclin-dependent kinase 4 (CDK4) and cyclin-dependent kinase inhibitor 1B (P27/kip1) expressions. Their anti-adipogenic properties might also be induced by restraining major transcriptional factors expression in adipogenesis including peroxisome proliferator-activated receptor γ (PPARγ) and CCAAT/Enhancer-binding proteins α (C/EBPα).

After preliminary in vitro search, oxyresveratrol was further examined for its anti-obesity effect in C57bl/6 mice. Male C57bl/6 mice were randomly assigned to control (5% fat by weight), high-fat (30% fat by weight) and high-fat supplemented with 0.25% and 0.5% oxyresveratrol diet groups for 8 weeks. Oxyresveratrol effectively ameliorated obesity-associated symptoms in mice, such as insulin resistance, hepatic steatosis and hyperglycemia. Study on specific mRNA and protein expressions in liver suggested that the anti-obesity effect of oxyresveratrol is presumably attributed to stimulating sirtuin 1 (SIRT1) expression to relieve insulin resistance in liver, suppressing glucose-6-phosphatase (G6Pase) expression and promoting AMP-activated protein kinase α2 (AMPKα2) level to decease hepatic glucose synthesis, as well as restraining sterol regulatory element-binding proteins 1 (SREBP-1), fatty acid synthase (FAS) and C/EBPα expression, and increasing AMPKα2 level to inhibit hepatic fatty acids synthesis.

I further examined critical mRNA expressions in visceral fat, muscle and brain. Results showed that the anti-obesity effect of oxyresveratrol might also be induced by its regulation on glucose and lipid homeostasis in adipose tissue and muscle. Oxyresveratrol supplementation stimulated insulin sensitivity in adipose tissue and muscle by up-regulating insulin receptor substrate 1 (IRS-1) and/or glucose transporter type 4 (GLUT4) expressions in these tissues. This effect might promote blood glucose uptake into adipocytes and muscle cells, contributing to the reduced serum glucose level. Also, oxyresveratrol might promote metabolic fitness in high-fat diet-fed mice by raising adiponectin level in adipose tissue and elevating the expressions of SIRT1, AMPKα, peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) and carnitine palmitoyltransferase I (CPT-1) in muscle.

In summary, the present study demonstrated the anti-adipogenic properties of three phytochemicals (oxyresveratrol, cyanomaclurin and 5-hydroxytryptophan) in vitro in 3T3-L1 cell model, and further verified the anti-obesity effect of oxyresveratrol in C57bl/6 mice.