Aerobic Fitness, Mitochondrial Function, and Fatty Liver Disease.

低有氧代谢能力（AC）是早期死亡率和代谢性疾病（包括过度肝脏脂肪储存（脂肪变性））风险的有力预测因子。相反，高AC在临床上与防止肝脂肪变性和更健康，更长的寿命相关，即使在肥胖的情况下。我们将利用针对不同的内在AC（高或低运行能力[HCR/LCR]）选择性饲养的大鼠模型来阐明AC影响肝脂肪变性和代谢病理的机制。在久坐的条件下，HCR大鼠具有高40%的固有AC，并且保护免受高脂肪/蔗糖（HFD）诱导的肝脂肪变性和胰岛素抵抗，而LCR是高度易感的。我们已经表明，HCR和LCR之间的肝线粒体功能（MitoFX：在这里定义为脂肪氧化和呼吸能力）的差异分别在其对肝脂肪变性的保护或易感性中发挥重要作用。新的数据表明，通过TCA循环的肝代谢通量和肝再生在HCR中也高于LCR大鼠，但这些途径尚未在预防脂肪变性的背景下进行检查。此外，新的初步数据表明，与LCR相比，HCR大鼠的胆汁酸（BA）合成增加，粪便固醇和BA排泄增加。也具有升高的MitoFX并被保护免于脂肪变性的运动训练的小鼠显示出BA合成和排泄的类似上调的证据。我们将通过以下方式检验以下假设：肝脏BA合成和粪便排泄的增加对高AC和慢性运动表型至关重要，并有助于肝脏MitoFx、代谢通量和脂肪变性保护：1）将乙酰辅酶A从线粒体中拉出（最小化反馈抑制和线粒体蛋白乙酰化）和2）将乙酰辅酶A从积累和从头脂肪生成（DNL）转移并通过“虹吸机制”导致BA合成和随后的粪便损失。我们将在HCR/LCR大鼠和运动与久坐小鼠中，利用药理学和分子工具结合体内代谢示踪剂来调节CYP 7a 1活性和BA合成，以测试这些机制。此外，HCR肝脏显示出比LCR更大的代谢和转录适应性，以响应高脂饮食（HFD）喂养。我们的初步数据表明，在HCR肝脏转录适应性增强是由组蛋白（H3 K9 ac和H3 K27 ac）的乙酰化增加，协调参与线粒体代谢，特别是BA合成的基因的表达。因此，我们认为高AC和运动诱导的代谢通量增加和增强的BA合成可能增加乙酰辅酶A流出线粒体并进入细胞质，在那里它可以作为组蛋白乙酰化的底物。该提案还将测试HCR大鼠和运动小鼠的肝脏可以转录适应高脂肪饮食并通过连接肝脏MitoFX，BA合成和排泄以及表观遗传机制（组蛋白乙酰化）的关系避免脂肪变性的假设。

Low aerobic capacity (AC) is a powerful predictor of early mortality and risk for metabolic disease including excessive hepatic fat storage (steatosis). Conversely, high AC is clinically associated with protection against hepatic steatosis and a healthier, longer lifespan even in the face of obesity. We will utilize a rat model selectively bred for divergent intrinsic AC (high or low running capacity [HCR/LCR]) to unravel mechanisms by which AC impacts hepatic steatosis and metabolic pathologies. In a sedentary condition, HCR rats have a 40% higher intrinsic AC and are protected against high fat/sucrose (HFD)-induced hepatic steatosis and insulin resistance while LCR are highly susceptible. We have shown that differences in hepatic mitochondrial function (MitoFX: defined here as fat oxidation, and respiratory capacity) between the HCR and LCR play an important role in their protection or susceptibility for hepatic steatosis, respectively. New data suggests that hepatic metabolic flux through TCA cycle and gluconeogenesis are also elevated in the HCR over the LCR rat, but these pathways have yet to be examined in the context of protection against steatosis. In addition, novel preliminary data suggests HCR rats have elevated bile acid (BA) synthesis paired with increased fecal sterol and BA excretion compared to LCR. Exercise trained mice which also have elevated MitoFX and are protected from steatosis show evidence of a similar upregulation of BA synthesis and excretion. We will test the hypothesis that increases in hepatic BA synthesis and fecal excretion is critical to the high AC and chronic exercise phenotype(s) and contributes to hepatic MitoFx, metabolic flux, and protection of steatosis by: 1) Pulling acetyl-CoA out of the mitochondria (minimizing feedback inhibition and mitochondrial protein acetylation) and 2) diverting acetyl-CoA away from accumulation and de-novo-lipogenesis (DNL) and towards BA synthesis and subsequent fecal loss via a “siphoning mechanism”. We will test these mechanisms utilizing pharmacological and molecular tools to modulate CYP7a1 activity and BA synthesis combined with in-vivo metabolic tracers in HCR/LCR rats and exercise vs. sedentary mice. Additionally, HCR livers display greater metabolic and transcriptional adaptability in response to high-fat diet (HFD) feeding than LCR. Our preliminary data suggests that enhanced transcriptional adaptability in the HCR livers is caused by increases in the acetylation of histones (H3K9ac and H3K27ac) that coordinate expression of genes involved in mitochondrial metabolism and specifically for BA synthesis. Thus, we posit that high AC and exercise induced increases in metabolic flux and enhanced BA synthesis likely increase acetyl CoA flux out of the mitochondria and into the cytosol where it can serve as a substrate for histone acetylation. This proposal will also test the hypothesis that livers from HCR rats and from exercised mice can transcriptionally adapt to high fat diets and avoid steatosis through a relationship linking hepatic MitoFX, BA synthesis and excretion, and epigenetic mechanisms (histone acetylation).