Biological aging is closely associated with a decline in the capacity for protein synthesis which has been hypothesized to contribute to the decline in tissue function and increased susceptibility to disease. Growth hormone and IGF-1 are two important anabolic hormones that regulate metabolic processes including protein synthesis in almost all tissues throughout the lifespan of mammals. Previous studies indicate that age-related perturbations in the neuroendocrine lead to a loss of growth hormone pulse amplitude and a subsequent decline in plasma IGF-1 concentration. However, despite an increase in growth hormone receptor (GHR) density with age, growth hormone induced plasma IGF-1 levels remain low in aged animals, suggesting that tissue resistance to growth hormone may be an additional manifestation of age. Moderate caloric restriction is one of the few regimens consistently reported to decrease the appearance of age-related pathologies, and increase mean and maximum lifespan. This regimen increases protein synthesis, enhances growth hormone secretion and prevents the age-related decline in IGF-1. The studies in this project were designed to: 1) determine whether alterations in tissue responsiveness to growth hormone contributes to the age-related decrease in plasma IGF-1 and protein synthesis 2) assess the precise mechanisms responsible for increased tissue resistance to growth hormone with age and 3) determine whether tissue resistance to growth hormone with age can be altered by moderate caloric restriction.

The first study examined the relationship between age-related alterations in GHR density, growth hormone induced IGF-1 gene expression, GHR and JAK2 kinase phosphorylation, and the basal expression and activity of cytosolic MAP kinase in liver from female C57BL/6 mice. The results of this study demonstrated that growth hormone induces phosphorylation of JAK2 kinase and GHR, increases cytosolic MAP kinase activity and IGF-1 gene expression. Despite a two fold increase in GHR density in old ad libitum animals, there was a marked decline in IGF-1 gene expression that was associated with a reduction in JAK2 kinase and GHR phosphorylation. These data suggest that the signal transduction pathway for growth hormone is impaired with age and that these deficits contribute to the decline in IGF-1 gene expression.

The second study assessed the effects of long term moderate caloric restriction on age-related changes in tissue responsiveness to growth hormone. Old caloric restricted animals demonstrated a significant increase in GHR, JAK2 kinase phosphorylation and MAP kinase activity in response to growth hormone stimulation compare to old ad libitum fed animals as well as growth hormone induced 3H-leucine incorporation, IGF-1 gene expression and secretion in female C57BL/6 mice. The results of this study indicated that moderate caloric restriction prevents the age-related decline in GHR signal transduction, suggesting that the effects of moderate caloric restriction are mediated, at least in part, by improve tissue sensitivity to growth hormone.

The spatial separation between the site of signal initiation and the nuclear targets of signal transduction indicates that information must be transferred between different subcellular compartments. Studies indicate that several intracellular proteins are phosphorylated in response to growth hormone, including STATs protein and MAP kinase, which then move into the nucleus and promote gene transcription. The results of activation of the GHR either through STATs or MAP kinase is an increase in IGF-1 gene expression and secretion. The third study examined Stat3 and MAP kinase nuclear translocation after growth hormone stimulation in ad libitum and moderate caloric restricted B6D2 male mice of different ages. Although growth hormone induced nuclear MAP kinase translocation did not change with age in ad libitum animals, growth hormone induced nuclear translocation of Stat3 decreased significantly, suggesting that the JAK2 kinase-Stat3 pathway is specifically impaired with age. The age-related decline in growth hormone induced nuclear Stat3 translocation was not evident in caloric restricted animals and 36 months old caloric restricted mice had a two fold greater growth hormone induced nuclear Stat3 than 36 month old ad libitum animals. In addition, caloric restricted animals exhibited an age-related increase in basal JAK2 kinase levels, suggesting that increased JAK2 levels may be part of the compensatory mechanisms that serve to maintain GHR signal transduction and IGF-1 gene expression in these animals.

We conclude that tissue responsiveness to growth hormone decreases with age and contributes to the decline in plasma IGF-1 levels. The mechanisms responsible for this age-related deficiency appear to be related to diminished capacity of growth hormone to phosphorylate JAK2 kinase resulting in a decline in Stat3 phosphorylation and translocation into nucleus. In addition, the effects of aging are reversed by moderate caloric restriction possibly by increased in JAK2 kinase levels and activities. Our results indicate that increased growth hormone resistance is an important manifestation of aging which contributes to the decline in tissue protein synthesis and function. Moderate caloric restriction appears to prevent or delay age associated declines in tissue function by maintaining the capacity for adequate receptor signal transduction.