Regulation of Insulin sensitivity by Glucocorticoid-Angiopoietin-like 4-Ceramide Axis

摘要 糖皮质激素(GC)在应激状态下的代谢适应中起着关键作用。然而，长期接触 GC是有害的，会导致胰岛素抵抗等疾病。我们之前发现了血管生成素样蛋白4 (ANGPTL4)作为GC调控的基因，编码一种分泌蛋白，促进脂肪细胞脂解并抑制 细胞外脂蛋白脂酶(LPL)。我们证明了ANGPTL4是GC促进的白色脂肪分解所必需的 脂肪组织(Wat)和ANGPTL4基因缺失小鼠(ANGPTL4-/-)，过量GC诱导的脂肪肝和 高甘油三酯血症减轻。我们最近的初步研究进一步表明，GC诱导的 ANGPTL4-/-小鼠的胰岛素抵抗得到了缓解。耐人寻味的是，GC治疗增加了肝脏神经酰胺 但这种GC效应在没有ANGPTL4的情况下减弱。事实上，Myriocin，它抑制 神经酰胺合成可降低野生型(WT)小鼠的GC诱导的胰岛素抵抗，但不能降低ANGPTL4-/-小鼠。我们 此外，研究还发现，GC暴露增加了PP2A和PKCζ(两个下游效应因子)的活性。 神经酰胺)以及一系列神经酰胺合成基因在肝脏中的表达。在ANGPTL4-/-小鼠中， ζ活性降低，神经酰胺合成酶3-6(Cers3-6)表达减少。 特别是受损的。基于这些初步数据，我们假设GC增强了 ANGPTL4，促进WAT脂解动员脂肪酸(FA)到肝脏作为底物和信号 GC需要增加Cers3-6的表达。这将特定神经酰胺种类的产量提高到 激活PP2A和PKCζ抑制胰岛素的作用。在这份提案中，我们将研究这一模型。在目标1中， 我们的初步研究发现，ANGPTL4的纤维蛋白原样结构域(FLD)促进脂肪细胞脂解 而不抑制LPL。我们将测试FLD是否能恢复GC诱导的ANGPTL4-/-小鼠的胰岛素抵抗。 我们还将研究FA动员对肝脏在GC诱导的胰岛素抵抗中的作用。 两种主要的肝脏FA转运蛋白Fatp5和CD36的表达减少。此外，高胰岛素血症- 正常血糖钳将用于确定ANGPTL4在GC增强的肝脏葡萄糖中的作用 产生，肝脏胰岛素抵抗的一个标志。在目标2中，我们将确定哪个神经酰胺合成酶(S)， Cers3-6参与了GC诱导的胰岛素抵抗。PPARα可由FA激活，并已显示 以增加某些组织中神经酰胺的合成。因此，它在GC诱导的肝脏Cers3-6表达中的作用 并将审查神经酰胺的生产。在目标3中，我们将分析PP2A和PKCζ在GC中的作用。 诱导胰岛素抵抗并将鉴定神经酰胺合成酶(S)激活PP2A和PKCζ引起 胰岛素抵抗。总体而言，这项研究将是第一次建立ANGPTL4的FLD作为脂质动员 参与GC诱导的胰岛素抵抗的因子以及首次发现的特异性神经酰胺合成酶 激活PKCζ和PP2A将被识别。这些发现将促进我们对这一机制的理解 并将为改善胰岛素敏感性提供新的分子靶点。 好了！ 好了！

Abstract Glucocorticoids (GC) play a key role in metabolic adaptation during stress. However, chronic exposure to GC, is harmful and results in disorders such as insulin resistance. We previously identified angiopoietin-like 4 (Angptl4) as a GC-regulated gene encoding a secreted protein that promotes adipocyte lipolysis and inhibits extracellular lipoprotein lipase (LPL). We showed that Angptl4 is required for GC-promoted lipolysis in white adipose tissue (WAT) and in Angptl4 null mice (Angptl4-/-), excess GC-induced fatty liver and hypertriglyceridemia are diminished. Our recent preliminary studies further demonstrated that GC-induced insulin resistance was compromised in Angptl4-/- mice. Intriguingly, GC treatment increased hepatic ceramide levels, but such GC effect was attenuated in the absence of Angptl4. Indeed, myriocin, which inhibits ceramide synthesis, decreased GC-induced insulin resistance in wild type (WT) but not Angptl4-/- mice. We additionally found that GC exposure elevated the activity of PP2A and PKCζ (two downstream effectors of ceramides) as well as the expression of a list of ceramide synthetic genes in liver. In Angptl4-/- mice, the activity of PP2A and PKCζ was mitigated and the expression of ceramide synthase 3-6 (Cers3-6) was specifically impaired. Based on these preliminary data, we hypothesize that GC augment the expression of Angptl4, which promotes WAT lipolysis mobilizing fatty acids (FA) to liver to serve as substrates and signals required for GC to increase Cers3-6 expression. This elevates the production of specific ceramide species to activate PP2A and PKCζ to suppress insulin action. In this proposal, we will examine this model. In Aim 1, our preliminary study found that the fibrinogen-like domain (FLD) of Angptl4 promotes adipocyte lipolysis without inhibiting LPL. We will test whether FLD restores GC-induced insulin resistance in Angptl4-/- mice. We will also examine the role of FA mobilization to the liver in GC-induced insulin resistance using mice with reduced expression of two major hepatic FA transporters, Fatp5 and Cd36. Moreover, hyperinsulinemic- euglycemic clamps will be used to determine the role of Angptl4 in GC-augmented hepatic glucose production, a hallmark of hepatic insulin resistance. In Aim 2, we will determine which ceramide synthase(s), Cers3-6, are involved in GC-induced insulin resistance. PPARα can be activated by FA and has been shown to increase ceramide synthesis in certain tissues. Thus, its role in GC-induced hepatic Cers3-6 expression and ceramide production will be examined. In Aim 3, we will analyze the role of PP2A and PKCζ in GC- induced insulin resistance and will identify ceramide synthase(s) that activate PP2A and PKCζ to cause insulin resistance. Overall, this study will be the first to establish the FLD of Angptl4 as a lipid mobilization factor participating in GC-induced insulin resistance and for the first time, specific ceramide synthases that activate PKCζ and PP2A will be identified. These findings will advance our understanding of the mechanism of insulin resistance and will provide novel molecular targets for improving insulin sensitivity. ! !