NAD+ and metabolic flexibility

肥胖与严重代谢异常的风险增加有关，如2型糖尿病、胰岛素抵抗和非酒精性脂肪性肝病(NAFLD)。从对人类和啮齿动物进行的研究中获得的数据表明，在这种代谢异常的病理生理学中，“代谢不灵活”是至关重要的。肥胖导致代谢不灵活的机制(S)尚不清楚，但可能涉及白色脂肪组织(WAT)代谢活动的改变。在这笔赠款的当前资金周期中，我们专注于研究脂肪组织NAD+生物学，并进行了一系列研究，证明Wat NAD+代谢缺陷与代谢不灵活之间的因果关系。我们发现：1)NAD+生物合成酶NAMPT的缺失，损害了WAT中细胞胰岛素信号、线粒体功能和肾上腺素能刺激的脂解活性；2)脂肪细胞特异性NAMPT基因敲除(ANKO)小鼠具有多器官(骨骼肌、肝脏、WAT)胰岛素抵抗、低脂联素血症、适应性产热和全身能量代谢受损，以及对高热量和低热量挑战的燃料选择受损；3)NAD+代谢与全身代谢灵活性的关键调节因子Cavelin-1(CAV1)之间的新的分子联系；4)饮食限制和运动，众所周知的代谢灵活性的增强剂，刺激NAMPT介导的NAD+生物合成；5)与我们的啮齿动物数据一致，肥胖者的Wat NAMPT表达和NAD+浓度降低。基于这些发现，这一新的应用将检验以下假设：NAMPT介导的NAD+生物合成调节代谢灵活性和葡萄糖代谢的关键效应因子Wat中的CAV1和线粒体功能，以及Wat NAD+代谢缺陷是肥胖诱导的代谢不灵活的新机制和治疗靶点。在目标1中，我们建议建立两个新的小鼠模型，选择性地在内脏Wat(使用新的AAV系统)和他莫昔芬诱导的anko(Ianko)小鼠中过度表达NAMPT，并评估对高脂饮食喂养和改变生活方式(饮食限制，运动)的代谢反应。在目标2中，我们建议使用体外系统，探索将NAD+代谢与CAV1和线粒体功能联系起来的四种机制(CAV1的赖氨酸乙酰化、PPARG、sirtuins和氧化还原代谢)。最后，在目标3中，我们建议确定我们在小鼠模型(目标1)和细胞培养系统(目标2)中进行的研究的潜在临床相关性。具体地说，我们将评估两种潜在的“NAD+增强剂”，即改变生活方式(低卡路里饮食±监督运动训练)和补充烟酰胺单核苷酸(NMN)(每天250毫克，8周)，对超重者的Wat NAD+代谢、CAV1和线粒体生物学的影响。从这项提议中获得的预期结果将为脂肪组织NAD+生物学在调节代谢灵活性和葡萄糖代谢中的重要性提供新的见解。

Obesity is associated with an increased risk of serious metabolic abnormalities, such as type 2 diabetes, insulin resistance, and nonalcoholic fatty liver disease (NAFLD). Data obtained from studies conducted in humans and rodents have suggested that “metabolic inflexibility” is critically involved in the pathophysiology of such metabolic abnormalities. The mechanism(s) responsible for obesity-induced metabolic inflexibility is not clear but could involve altered metabolic activity in white adipose tissue (WAT). In the current funding cycle of this grant, we have focused on studying adipose tissue NAD+ biology and conducted a series of studies that demonstrate the causal relationship between defective WAT NAD+ metabolism and metabolic inflexibility. We found: 1) loss of NAMPT, a key NAD+ biosynthetic enzyme, impairs cellular insulin signaling, mitochondrial function, and adrenergic-stimulated lipolytic activity in WAT; 2) adipocyte-specific Nampt knockout (ANKO) mice have multi-organ (skeletal muscle, liver, WAT) insulin resistance, hypoadiponectinemia, impaired adaptive thermogenesis and whole-body energy metabolism, and impaired fuel selection to hypercaloric and hypocaloric challenges; 3) a novel molecular link between NAD+ metabolism and Caveolin-1 (CAV1), a key regulator of whole-body metabolic flexibility; 4) dietary restriction and exercise, well-known enhancers of metabolic flexibility, stimulate NAMPT-mediated NAD+ biosynthesis in WAT; and 5) consistent with our rodent data, people with obesity have decreases in WAT NAMPT expression and NAD+ concentration. Based on these findings, this renewal application will test the hypotheses that NAMPT-mediated NAD+ biosynthesis regulates CAV1 and mitochondrial function in WAT, key effectors of metabolic flexibility and glucose metabolism, and that defective WAT NAD+ metabolism is a novel mechanism and therapeutic target for obesity-induced metabolic inflexibility. In Aim 1, we propose to generate two novel mouse models, mice overexpressing NAMPT selectively in visceral WAT (using the novel AAV system) and tamoxifen-inducible ANKO (iANKO) mice, and evaluate metabolic responses to high-fat diet feeding and lifestyle modification (dietary restriction, exercise). In Aim 2, we propose to use in vitro systems and explore four mechanisms (lysine acetylation of CAV1, PPARG, sirtuins, and redox metabolism) that link NAD+ metabolism with CAV1 and mitochondrial function. Finally, in Aim 3, we propose to determine the potential clinical relevance of the studies we conducted in the mouse model (Aim 1) and cell culture systems (Aim 2). Specifically, we will evaluate the effects of two potential “NAD+ enhancers”, namely lifestyle modification (low- calorie diet ± supervised exercise training) and nicotinamide mononucleotide (NMN) supplementation (250 mg/day, 8 weeks), on WAT NAD+ metabolism, CAV1, and mitochondrial biology in overweight people. The anticipated results obtained from this proposal will provide novel insight into the importance of adipose tissue NAD+ biology in regulating metabolic flexibility and glucose metabolism.