Aerobic Fitness, Mitochondrial Function, and Fatty Liver Disease.

低有氧能力（AC）是早期死亡和代谢性疾病（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、更长寿有关，即使面对肥胖。我们将利用选择性饲养的不同内在AC（高或低运行能力[HCR/LCR]）大鼠模型来揭示AC影响肝脏脂肪变性和代谢病理的机制。在久坐状态下，HCR大鼠的内在AC比HFD高40%，并且可以防止高脂肪/蔗糖（HFD）诱导的肝脏脂肪变性和胰岛素抵抗，而LCR则高度敏感。我们已经证明，HCR和LCR之间肝脏线粒体功能（MitoFX：这里定义为脂肪氧化和呼吸能力）的差异分别在它们对肝脏脂肪变性的保护或易感中起重要作用。新的数据表明，通过TCA循环和糖异生的肝脏代谢通量在HCR大鼠中也高于LCR大鼠，但这些途径尚未在防止脂肪变性的背景下进行研究。此外，新的初步数据表明，与LCR相比，HCR大鼠胆汁酸（BA）合成升高，粪便固醇和BA排泄增加。经过运动训练的小鼠也有升高的MitoFX并被保护免受脂肪变性，显示出类似的BA合成和排泄上调的证据。我们将测试肝脏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的假设，并通过以下方式促进肝脏MitoFx、代谢通量和脂肪变性的保护：1)将乙酰辅酶a拉出线粒体（最大限度地减少反馈抑制和线粒体蛋白乙酰化）；2)通过“虹吸机制”将乙酰辅酶a从积累和去新脂生成（DNL）转移到BA合成和随后的粪便损失。我们将利用药理学和分子工具，结合体内代谢示踪剂，在HCR/LCR大鼠和运动与久坐小鼠中调节CYP7a1活性和BA合成，测试这些机制。此外，与LCR相比，HCR肝脏对高脂肪饲料（HFD）的代谢和转录适应性更强。我们的初步数据表明，HCR肝脏中转录适应性的增强是由组蛋白（H3K9ac和H3K27ac）乙酰化的增加引起的，这些组蛋白协调参与线粒体代谢的基因的表达，特别是BA合成。因此，我们假设高AC和运动诱导代谢通量的增加和BA合成的增强可能会增加乙酰辅酶a从线粒体进入细胞质的通量，在那里它可以作为组蛋白乙酰化的底物。这一建议也将验证HCR大鼠和运动小鼠的肝脏可以通过肝脏MitoFX、BA合成和排泄以及表观遗传机制（组蛋白乙酰化）之间的关系来转录适应高脂肪饮食并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).