We sought to determine whether metformin treatment reverses a high-fat diet (HFD)-induced hepatic insulin resistance (IRes) and to identify lipid intermediates involved in induction of liver IRes. The experiments were conducted on male Wistar rats divided into three groups: 1. Control, 2. fed HFD and 3. fed HFD and treated with metformin. The animals were infused with a [U-¹³C]palmitate to measure fractional lipid synthesis rate. This allowed for the calculation of fractional synthesis rate of signaling lipids (FSR) through the estimation of their isotopic enrichment. Liver ceramide (Cer), diacylglycerol (DAG) and acyl-carnitine concentration and enrichment were analyzed by LC/MS/MS. The content of proteins involved in lipid metabolism and insulin signaling were analyzed by Western Blot. HFD treatment increased the content and FSR of DAG and Cer in the liver which was accompanied by systemic insulin resistance and inhibition of hepatic insulin signaling pathway under insulin stimulation. Metformin treatment ameliorated systemic insulin resistance and augmented the hepatic insulin signaling cascade. It reduced both the concentration and FSR of Cer, DAG, and increased acyl-carnitine content and the expression of mitochondrial markers. We postulate, that in liver, the insulin sensitizing effect of metformin depends on augmentation of mitochondrial β-oxidation, which protects from hepatic accumulation of both the Cer and DAG and preserves insulin sensitivity under HFD consumption. Moreover, we showed that hepatic content of Cer and DAG corresponds with their respective FSR.

中文翻译：

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高脂饮食和二甲双胍治疗对肝脏脂质蓄积的影响及其对胰岛素作用的影响。

我们试图确定二甲双胍治疗是否能逆转高脂饮食（HFD）诱导的肝胰岛素抵抗（IRes），并确定参与诱导肝IRes的脂质中间体。实验在雄性Wistar大鼠上进行，雄性Wistar大鼠分为三组：1。对照组，2。喂HFD和3.喂HFD并用二甲双胍治疗。给动物注入[U- ¹³C]棕榈酸酯以测量脂质合成分数。这允许通过估计其同位素富集来计算信号脂质（FSR）的分数合成速率。肝神经酰胺（Cer），二酰基甘油（DAG）和酰基肉碱的浓度和富集通过LC / MS / MS分析。通过蛋白质印迹法分析参与脂质代谢和胰岛素信号传导的蛋白质的含量。HFD处理增加了肝脏中DAG和Cer的含量和FSR，并伴有全身性胰岛素抵抗和胰岛素刺激下肝胰岛素信号通路的抑制。二甲双胍治疗可改善全身胰岛素抵抗并增强肝胰岛素信号传导级联。它降低了Cer，DAG的浓度和FSR，并增加了酰基肉碱含量和线粒体标记物的表达。我们推测，在肝脏中，二甲双胍对胰岛素的增敏作用取决于线粒体β-氧化的增强，从而防止肝细胞中Cer和DAG的积累，并在食用HFD时保持胰岛素敏感性。此外，我们表明Cer和DAG的肝含量与其各自的FSR相符。