Ursolic Acid Inhibits Adipogenesis in 3T3-L1 Adipocytes through LKB1/AMPK Pathway

Background
Ursolic acid (UA) is a triterpenoid compound with multiple biological functions. This compound has recently been reported to possess an anti-obesity effect; however, the mechanisms are less understood.
Objective
As adipogenesis plays a critical role in obesity, the present study was conducted to investigate the effect of UA on adipogenesis and mechanisms of action in 3T3-L1 preadipocytes.
Methods and Results
The 3T3-L1 preadipocytes were induced to differentiate in the presence or absence of UA for 6 days. The cells were determined for proliferation, differentiation, fat accumulation as well as the protein expressions of molecular targets that regulate or are involved in fatty acid synthesis and oxidation. The results demonstrated that ursolic acid at concentrations ranging from 2.5 µM to 10 µM dose-dependently attenuated adipogenesis, accompanied by reduced protein expression of CCAAT element binding protein β (C/EBPβ), peroxisome proliferator-activated receptor γ (PPARγ), CCAAT element binding protein α (C/EBPα) and sterol regulatory element binding protein 1c (SREBP-1c), respectively. Ursolic acid increased the phosphorylation of acetyl-CoA carboxylase (ACC) and protein expression of carnitine palmitoyltransferase 1 (CPT1), but decreased protein expression of fatty acid synthase (FAS) and fatty acid-binding protein 4 (FABP4). Ursolic acid increased the phosphorylation of AMP-activated protein kinase (AMPK) and protein expression of (silent mating type information regulation 2, homolog) 1 (Sirt1). Further studies demonstrated that the anti-adipogenic effect of UA was reversed by the AMPK siRNA, but not by the Sirt1 inhibitor nicotinamide. Liver kinase B1 (LKB1), the upstream kinase of AMPK, was upregulated by UA. When LKB1 was silenced with siRNA or the inhibitor radicicol, the effect of UA on AMPK activation was diminished.
Conclusions
Ursolic acid inhibited 3T3-L1 preadipocyte differentiation and adipogenesis through the LKB1/AMPK pathway. There is potential to develop UA into a therapeutic agent for the prevention or treatment of obesity.

研究背景
熊果酸（Ursolic acid, UA）是一类具有多种生物学功能的三萜类化合物。近期有研究报道该化合物具备抗肥胖作用，但其具体作用机制尚不明确。
研究目的
鉴于脂肪生成在肥胖进程中发挥关键作用，本研究旨在探讨熊果酸对3T3-L1前脂肪细胞脂肪生成的影响及其作用机制。
方法与结果
将3T3-L1前脂肪细胞置于含或不含熊果酸的环境中诱导分化6天，随后检测细胞的增殖、分化、脂肪蓄积情况，以及调控或参与脂肪酸合成与氧化的分子靶点的蛋白表达水平。结果显示，浓度范围为2.5 μM至10 μM的熊果酸可剂量依赖性地抑制脂肪生成，同时分别降低CCAAT元件结合蛋白β（CCAAT element binding protein β, C/EBPβ）、过氧化物酶体增殖物激活受体γ（peroxisome proliferator-activated receptor γ, PPARγ）、CCAAT元件结合蛋白α（C/EBPα）以及固醇调节元件结合蛋白1c（sterol regulatory element binding protein 1c, SREBP-1c）的蛋白表达水平。熊果酸可提升乙酰辅酶A羧化酶（acetyl-CoA carboxylase, ACC）的磷酸化水平以及肉碱棕榈酰转移酶1（carnitine palmitoyltransferase 1, CPT1）的蛋白表达，却降低脂肪酸合酶（fatty acid synthase, FAS）与脂肪酸结合蛋白4（fatty acid-binding protein 4, FABP4）的蛋白表达。此外，熊果酸可增强AMP活化蛋白激酶（AMP-activated protein kinase, AMPK）的磷酸化水平，并上调沉默信息调节因子2同源物1（silent mating type information regulation 2, homolog 1, Sirt1）的蛋白表达。进一步研究表明，熊果酸的抗脂肪生成作用可被AMPK小干扰RNA（siRNA）逆转，但无法被Sirt1抑制剂烟酰胺阻断。AMPK的上游激酶肝激酶B1（Liver kinase B1, LKB1）的表达可被熊果酸上调；当通过siRNA或抑制剂雷迪奇酚沉默LKB1后，熊果酸对AMPK的激活作用被削弱。
研究结论
熊果酸可通过LKB1/AMPK通路抑制3T3-L1前脂肪细胞的分化与脂肪生成，具备开发为预防或治疗肥胖症治疗药物的潜力。