Mechanisms of diet-induced leptin resistance in ARC and VTA

描述（由申请人提供）：饮食诱导的肥胖症（DIO）的一个潜在致病因素是瘦素抵抗。瘦素是大多数瘦动物体内的一种有效的促性腺激素。然而，瘦素的作用在肥胖动物中减弱，这种活性被称为瘦素抵抗。高脂肪和蔗糖的饮食导致瘦素抵抗和肥胖，但致病的饮食成分仍然难以捉摸。我们的数据表明，低脂肪饮食与高果糖导致瘦素抵抗，而没有果糖的饮食不会。瘦素通过信号转导和转录激活蛋白3（STAT 3）以及营养感测AMP激活激酶（AMPK）/乙酰辅酶A羧化酶（ACC）途径来进行信号传导。瘦素通过促进pAMPK/pACC的去磷酸化而通过后者发出信号。我们证明，果糖阻止瘦素介导的pAMPK/pACC去磷酸化，导致瘦素抵抗。营养过剩也会导致瘦素抵抗。我们认为，饮食导致过度消费，加剧了果糖，促进炎症，内质网（ER）的压力，和活性氧（ROS），这些特异性抑制瘦素介导的STAT 3磷酸化（P-STAT 3）。我们推测，高果糖饮食，而不是葡萄糖，削弱瘦素介导的pAMPK/pACC在下丘脑弓状核（ARC）和腹侧被盖区（VTA）的去磷酸化。此外，我们假设果糖与营养过剩协同作用，增加炎症，ER应激和ROS，从而抑制pSTAT 3信号传导。此外，这两种途径的中断对于促进体重增加是必要的。我们将在雄性Sprague道利大鼠中通过检查食物摄入和体重的神经调节、全身能量稳态和脑区域特异性瘦素信号传导来检验这些假设。实验1将检查与膳食葡萄糖或蔗糖相比，2周膳食果糖是否抑制瘦素介导的pAMPK/pACC的去磷酸化或损害STAT 3的磷酸化。实验2将确定果糖是否通过果糖或葡萄糖的中枢输注直接作用于中枢瘦素信号通路。实验3检查了STAT 3和AMPK瘦素信号传导途径两者的中断是否对于通过特异性抑制各个途径的瘦素抗性促进的体重增加是关键的。在实验4中，我们将检查是否过度营养加果糖（相对于葡萄糖）将诱导更大的脑炎症/ER应激/ROS，并因此导致ARC和VTA中瘦素介导的P-STAT 3信号传导的更严重抑制。实验5将通过输注这些应激源的相应抑制剂来检查炎症、ER应激或ROS产生对果糖/过度营养诱导的瘦素抵抗的个体贡献。我们的方法是独特的，因为我们认为果糖加上营养过剩构成了一个解释瘦素抵抗的两次打击模型。对这些机制的理解将为肥胖症提供新的治疗策略，包括靶向炎症/ER应激/ROS的药物与低果糖饮食结合，以恢复瘦素的作用。 公共卫生相关性：由于长期食用高糖和高脂肪饮食，美国人口可能特别容易患饮食性肥胖症，与肥胖相关的健康并发症加速了残疾和死亡的发生。这项建议旨在研究膳食果糖和脂肪在促进体重增加中的作用。特别是，它研究了果糖和过度消费阻止激素瘦素正常作用的方式，从而促进体重和脂肪增加。我们拟定的研究将在Sprague-Dawley大鼠中进行，Sprague-Dawley大鼠是人类饮食诱导肥胖的临床前啮齿动物模型。

DESCRIPTION (provided by applicant): One potential causative factor of diet-induced obesity (DIO) is leptin resistance. Leptin is a potent anorexic hormone in most lean animals. However, the effects of leptin are diminished in obese animals, and this refractiveness is known as leptin resistance. Diets high in fat and sucrose lead to leptin resistance and obesity, yet the causative dietary ingredient(s) remain elusive. Our data demonstrate that a low-fat diet with elevated fructose causes leptin resistance, whereas diets without fructose do not. Leptin signals via the signal transducer and activator of transcription protein 3 (STAT3) and the nutrient sensing AMP-activated kinase (AMPK)/acetyl CoA carboxylase (ACC) pathways. Leptin signals through the latter by promoting dephosphorylation of pAMPK/pACC. We demonstrated that fructose prevents leptin-mediated pAMPK/pACC dephosphorylation, leading to leptin resistance. Over-nutrition also leads to leptin resistance. We suggest that diets that lead to over-consumption, exacerbated by fructose, promote inflammation, endoplasmic reticulum (ER) stress, and reactive oxygen species (ROS) and these specifically inhibit leptin-mediated STAT3 phosphorylation (P-STAT3). We postulate that elevated dietary fructose, but not glucose, impairs the leptin- mediated dephosphorylation of pAMPK/pACC in the hypothalamic arcuate nucleus (ARC) and ventral tegmental area (VTA). Additionally, we postulate that fructose, in synergy with over-nutrition, increases inflammation, ER stress, and ROS, thus inhibiting pSTAT3 signaling. Furthermore, disruptions in both pathways are necessary to promote weight gain. We will test these hypotheses in male Sprague Dawley rats by examining the neural regulation of food intake and body weight, whole body energy homeostasis, and brain region-specific leptin signaling. Exp 1 will examine if 2-week dietary fructose inhibits the leptin-mediated dephosphorylation of pAMPK/pACC or impairs the phosphorylation of STAT3 compared with that of dietary glucose or sucrose. Exp 2 will determine if fructose acts directly on central leptin signaling pathways by central infusion of fructose or glucose. Exp 3 examines if disruptions in both the STAT3 and AMPK leptin signaling pathways are critical for the leptin-resistance promoted weight gain by specific inhibition of the individual pathways. In Exp 4, we will examine if over-nutrition plus fructose (vs. glucose) will induce a greater brain inflammation/ER stress/ROS, and consequently, result in more severe inhibition of leptin-mediated P-STAT3 signaling in the ARC and VTA. Exp 5 will examine the individual contribution of inflammation, ER stress or ROS production to fructose/over-nutrition-induced leptin resistance by infusion of respective inhibitors of these stressors. Our approach is unique because we suggest that fructose plus over-nutrition constitute a two-hit model explaining leptin resistance. An understanding of these mechanisms will provide new therapeutic strategies for obesity, including drugs that target inflammation/ER stress/ROS coupled with low fructose diets to resurrect leptin action. PUBLIC HEALTH RELEVANCE: The American population may be particularly vulnerable to dietary obesity due to chronic consumption of high sugar and fat diets, and obesity-related health complications hasten the onset of disability and death. This proposal seeks to examine the role of dietary fructose and fat in the promoting weight gain. In particular, it examines the way fructose and over-consumption prevents the hormone leptin from its normal action, thus promoting weight and fat gain. Our proposed studies will be conducted in Sprague-Dawley rats, a pre-clinical rodent model for diet-induced obesity in humans.