Systemic Factors that Maintain a Young Liver Phenotype

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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.