The author of this open access review of the study of growth hormone in aging is one of the eminent experts in this part of the field, noted for work on various loss of function mutant mice, lacking either functional growth hormone or functional growth hormone receptor genes. The current record for mouse longevity is held by a growth hormone knockout variant: these mice wouldn't survive in the wild, as they are small and vulnerable to cold, but they live 60-70% longer than their unmodified peers in the laboratory.
It is well documented that circulating levels of GH decline with age in various mammalian species, including humans, domestic dogs, and laboratory rodents. Yet in laboratory mice, disruption of growth hormone (GH) signaling leads to a remarkable extension of longevity. These findings were hard to interpret and were originally received with some skepticism because they implied that normal actions of a hormone have significant 'costs' in terms of longevity, and that a gross defect in the functioning of the endocrine system can have striking benefits for healthy survival. However, the evidence that absence of GH signaling extends longevity of mice is strong, reproducible, and now generally accepted.
Several aspects of the findings in GH-deficient and GH-resistant mice deserve particular emphasis. First, the significant extension of longevity in these animals is reproducible and not limited to a particular laboratory, diet, or genetic background. Second, lifespan is extended in both females and males. Third, extension of longevity is associated with a similarly striking extension of healthspan. Fourth, the magnitude of the increase in longevity exceeds the effects of most genetic, pharmacological, or dietary interventions that have anti-aging effects in mice.
A recent study examined longevity of mice lacking both GH and functional GH receptors. While these tiny 'double mutants' were remarkably long-lived compared to their normal siblings, they did not live significantly longer than mice lacking only GH or only GH receptors. In females, survival curves of GH-deficient Ames dwarf, GH-resistant GHRKO, and 'double mutant' (df/KO) animals were nearly identical.
The importance of GH signaling in the control of murine lifespan is further emphasized by the evidence that disruption of signaling events 'downstream' from GH and its receptor also extends longevity. Early findings of extended longevity of female mice heterozygous for the deletion of IGF-1 receptor were confirmed and extended in further studies. Major increase of longevity was seen in mice in which amount of bioavailable IGF-1 was reduced at the tissue level by germline or adult disruption of the gene coding for pregnancy associated plasma protein A, an enzyme degrading IGF-1 binding protein. Significant and reproducible extension of longevity was also produced by pharmacological suppression of the activity of mechanistic target of rapamycin, a kinase regulated by GH and IGF1.
Importantly, conclusions concerning pro-aging effects of normal or elevated GH based on studies in mutant, gene knockout, transgenic, or drug treated mice appear to apply to genetically normal mice and to other mammalian species. Multiple studies reported negative association of adult body size (a strongly GH- and IGF-1-dependent trait) with longevity in comparisons of different mouse strains, selected lines, and individual animals.