Modulating Growth Hormone Action as a Target for Improved Health and Longevity

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Abstract Our laboratory previously generated the growth hormone receptor gene disrupted mouse (GHR-/-) to study the physiological importance of growth hormone (GH). Of the many discoveries that have resulted from this mouse line, perhaps the most extraordinary, is that they are recognized as the longest-lived laboratory mouse. Numerous studies indicate that these mice also have improved long-term health as they are resistance to cognitive decline, accumulation of senescent cells, diabetes, and cancer. Humans with mutations in the GHR gene (Laron Syndrome) are also protected from diabetes and cancer, indicating that studies with GHR-/- mice have clinical relevance. Therefore, the long-term goal of our laboratory is to determine the molecular mechanisms that are responsible for these remarkable health and longevity benefits. The overall objectives in this application are to determine whether disrupting GH action “temporally” replicates the health benefits found in GHR-/- mice and if combining two separate life-extending interventions can increase lifespan to a greater extent than either intervention alone. To accomplish these objectives, we propose two Specific Aims: (1) to determine if disrupting GHR in young (2 week) or adult (4 month) mice improves health and longevity, and (2) to determine if rapamycin treatment of GHR-/- or Ames mice can improve health and further extend lifespan. Our hypotheses are that the health benefits and extended longevity seen in GHR-/- mice can be replicated by interventions applied after birth and may be further improved when combined with interventions whose mechanisms are not fully overlapping. Our preliminary data combined with recently published data demonstrate that: (1) Temporal disruption of GHR at 6 weeks of age is feasible and results in increased maximal lifespan in females; (2) GHR-/- mice have tissue specific alterations in respiration and mitochondrial function that could influence their favorable glucose metabolism; and (3) Rapamycin and GHR-/- have opposite effects on both glucose metabolism and the GH/IGF-1 axis, suggesting that combining these interventions may enhance lifespan additively or synergistically. Our approach is innovative in that it combines methodologies for manipulating gene expression to control the GH/IGF-1 axis (i.e. GHR gene disruption selectively after birth or in adulthood)(Aim 1) and for combining of GH deficiency or resistance with rapamycin (Aim 2). We will also evaluate longevity and multiple mechanisms associated with aging in these novel mice including components of the GH/IGF-1 axis, glucose homeostasis, mTOR activity, cellular senescence, and indices of energy metabolism (cellular respiration, mitochondrial function, metabolic flexibility). The proposed research is significant as it will provide a better understanding of the role of GH in aging and will reveal clues for effective strategies and interventions that improve health.