Aerobic Fitness, Mitochondrial Function, and Fatty Liver Disease.

低有氧能力(AC)是早期死亡率和代谢性疾病风险的有力预测指标，包括肝脏脂肪过度储存(脂肪变性)。相反，在临床上，高AC与防止肝脏脂肪变性和更健康、更长的寿命有关，即使面对肥胖。我们将利用选择性地培育出不同的固有AC(高或低运行能力[HCR/LCR])的大鼠模型来揭示AC影响肝脏脂肪变性和代谢病理的机制。在久坐的条件下，HCR大鼠的固有AC增加了40%，并对高脂/蔗糖(HFD)诱导的肝脏脂肪变性和胰岛素抵抗具有保护作用，而LCR则高度敏感。我们已经证明，肝线粒体功能(这里定义为脂肪氧化和呼吸能力)在HCR和LCR之间的差异在它们对肝脏脂肪变性的保护或易感性方面分别起着重要的作用。新的数据表明，在LCR大鼠的HCR中，通过TCA循环和糖异生的肝脏代谢流量也增加，但这些途径尚未在防止脂肪变性的背景下进行检验。此外，新的初步数据表明，与LCR相比，HCR大鼠胆汁酸(BA)合成增加，同时粪便类固醇和BA排泄增加。运动训练的小鼠也有升高的MitoFX，并被保护免受脂肪变性的保护，显示出类似的BA合成和排泄上调的证据。我们将检验这一假设，即肝脏BA合成和粪便排泄的增加对高AC和慢性运动表型(S)至关重要，并有助于肝脏MitoFx、代谢流量和脂肪变性的保护：1)将乙酰辅酶A从线粒体中取出(将反馈抑制和线粒体蛋白乙酰化降至最低)；2)通过“虹吸机制”将乙酰辅酶A从蓄积和脱脂(DNL)转移到BA合成和随后的粪便丢失。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测试这些机制，以调节CYP7a1的活性和BA的合成。此外，与LCR相比，HCR肝脏对高脂饮食(HFD)喂养表现出更强的代谢和转录适应能力。我们的初步数据表明，HCR肝脏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，组蛋白乙酰化协调参与线粒体代谢的基因的表达，特别是BA合成。因此，我们推测，高AC和运动导致代谢通量增加，BA合成增加可能会增加乙酰辅酶A从线粒体到胞浆的通量，在那里它可以作为组蛋白乙酰化的底物。这项提议还将检验这一假设，即来自HCR大鼠和运动小鼠的肝脏可以转录适应高脂肪饮食，并通过将肝脏MitoFX、BA的合成和排泄与表观遗传机制(组蛋白乙酰化)联系起来而避免脂肪变性。

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).