Systemic Factors that Maintain a Young Liver Phenotype

描述（由申请人提供）：本提案的长期目标是开发在衰老过程中维持生活质量和独立性的治疗方法。热量限制（CR）在衰老过程中保持健康并延长寿命;然而，分子介体尚不清楚。在CR和禁食期间，生长激素释放肽的产生增加，下丘脑中表达的生长激素释放肽受体（GHS-R1 a）上调8倍。生长激素释放肽调节途径的表征导致了这样的假设，即衰老表型与内源性生长激素释放肽信号传导受损相关，并且生长激素释放肽是CR益处的介体。在CR期间，动物必须通过改变外周组织中的代谢来补偿减少的能量摄入。据推测，调节是通过ghrelin对下丘脑弓状神经元（ARC）上表达的GHS-R1 a的作用中枢介导的，该神经元调节能量平衡。通过比较CR对野生型小鼠与Ghrelin-/-和Ghsr-/-小鼠的ARC神经肽Y（NPY）、刺鼠相关肽（AGRP）和前阿黑皮素（POMC）神经元的分子作用，可以鉴定Ghrelin介导的信号。通过在野生型和ghrelin-/-和Ghsr-/-小鼠中的类似比较，可以定义老化肝脏中CR诱导的ghrelin依赖性代谢变化。这一点很重要，因为肝脏是调节代谢的关键，并受到来自大脑、胰腺、脂肪和肌肉的输入的调节。除了定义生长素释放肽依赖性代谢作用外，还将比较CR野生型、生长素释放肽-/-和Ghsr-/-小鼠的寿命。据推测，CR不会延长突变小鼠的寿命。每天用稳定的生长素释放肽模拟物处理临时喂食的野生型小鼠，以测试以下假设：干预以拯救衰老期间受损的生长素释放肽信号传导将增加寿命并维持年轻的肝脏表型。总之，这项研究将确定CR对选定基因、蛋白质和蛋白质修饰的影响，这些基因、蛋白质和蛋白质修饰调节胰岛素敏感性、葡萄糖代谢、脂肪生成和肝脏中的脂肪积累（脂肪变性），这些都是生长激素释放肽依赖性的。具体目标1：在野生型而非ghrelin-/-和Ghsr-/-小鼠中，验证CR通过FoxO 1介导的途径增加弓状核（ARC）神经元中Npy和Agrp的表达并抑制Pomc的表达的假设，并阐明分子机制。具体目标二：确定野生型、ghrelin-/-和Ghsr-/-小鼠对与肝脏代谢、葡萄糖耐量、胰岛素敏感性、脂肪变性和寿命有关的长期热量限制的反应差异，并确定分子机制;检验年轻表型可以随意维持的假设。通过用稳定的生长素释放肽模拟物处理来喂养衰老期间的野生型小鼠。确定CR对由生长素释放肽介导的衰老的有益作用是非常重要的，因为口服活性的耐受性良好的长效生长素释放肽模拟物的可用性，迄今为止还没有发现一个临床利基。 公共卫生相关性：显著减少实验动物的食物摄入量可以预防糖尿病，降低癌症的发病率，延长寿命。这些好处与血液中一种叫做生长激素释放肽的激素水平的增加有关。Ghrelin水平随着年龄的增长而下降。我们的研究旨在测试用更稳定的形式取代这种激素是否会在衰老过程中提供健康和生活质量的好处。

DESCRIPTION (provided by applicant): The long-term objective of this proposal is to develop therapy that will maintain quality of life and independence during aging. Caloric restriction (CR) maintains health during aging and increases longevity; however, the molecular mediator(s) is unknown. During CR and fasting, production of the hormone ghrelin increases and the ghrelin receptor (GHS-R1a) expressed in the hypothalamus is upregulated 8-fold. Characterization of ghrelin regulated pathways led to the hypothesis that the aging phenotype is associated with impaired endogenous ghrelin signaling and that ghrelin is a mediator of the benefits of CR. During CR, the animal must compensate for reduced energy intake by modifying metabolism in peripheral tissues. It is speculated that accommodation is mediated centrally by the action of ghrelin on GHS-R1a expressed on hypothalamic arcuate neurons (ARC) that regulate energy balance. By comparing the molecular effects of CR on ARC neuropeptide Y (NPY), agouti-related peptide (AGRP) and pro-opiomelanocortin (POMC) neurons of wild type mice with ghrelin-/- and Ghsr-/- mice the ghrelin mediated signals can be identified. By similar comparisons in wildtype and ghrelin-/- and Ghsr-/- mice, CR-induced ghrelin dependent metabolic changes in the aging liver can be defined. This is important because the liver is pivotal for regulating metabolism and is modulated by input from the brain, pancreas, fat and muscle. In addition to defining ghrelin dependent metabolic effects, longevity of CR wildtype, ghrelin-/- and Ghsr-/- mice will be compared. It is speculated that CR will not extend the lifespan of the mutant mice. Ad lib fed wildtype mice will be treated daily with a stable ghrelin mimetic to test the hypothesis that intervening to rescue impaired ghrelin signaling during aging will increase longevity and maintain a young liver phenotype. In summary, this research will identify changes resulting from CR on selected genes, proteins and protein modifications that regulate insulin sensitivity, glucose metabolism, lipogenesis and fat accumulation in the liver (steatosis), that are ghrelin dependent. Specific Aim 1: Test the hypothesis that in wildtype, but not in ghrelin-/- and Ghsr-/- mice, that CR increases expression of Npy and Agrp and suppresses expression of Pomc in neurons of the arcuate nucleus (ARC) via FoxO1 mediated pathway and elucidate the molecular mechanisms. Specific Aim 2: Determine the differences in responses of wildtype, ghrelin-/- and Ghsr-/- mice to long-term caloric restriction that pertain to liver metabolism, glucose tolerance, insulin sensitivity, steatosis and longevity and define the molecular mechanisms; test the hypothesis that a young phenotype can be sustained in ad lib. fed wildtype mice during aging by treatment with a stable ghrelin mimetic. Identifying the beneficial effects of CR on aging that are mediated by ghrelin is fundamentally important because of the availability of orally active well tolerated long-acting ghrelin mimetics that have so far not found a clinical niche. Public Health Relevance: Markedly reducing food intake in experimental animals prevents diabetes, lowers the incidence of cancer and prolongs lifespan. These benefits are associated with increases in blood levels of a hormone called ghrelin. Ghrelin levels decline as a function of age. Our research is designed to test whether replacing this hormone with a more stable form will provide health and quality of life benefits during aging.