IGF-I Signaling and Aging

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Holzenberger et al (2003) recently reported that mice heterozygous for the IGF-I receptor gene in all tissues (Igflr^' mice) have extended life span in association with resistance to paraquat toxicity. This finding would be seminal since it shows that the increased life span and resistance to oxidative stress observed in invertebrates models with reduced insulin/IGF-l signaling extends to mammals. This result also provides elegant support for the hypothesis that the mechanism of extended life span of calorie-restricted (CR) animals and GH-deficient dwarf mice is, at least in part, reduced IGF-I action. However, there are major conceptual and technical concerns about the report of Holzenberger et al. First, the literature provides abundant evidence that IGF-I signaling protects against oxidative stress. Secondly, there is no evidence yet available that oxidative damage is reduced over the life span of Igf1r+/' mice, nor is there any evidence of reduced age-related pathology and other markers of biological aging. Third, the life span of the control wild- type mice in the study of Holzenberger et al was short, suggesting the possibility that enhanced life span of the lgf1r+/~ mice was due to resistance to a stress of their particular housing environment rather than reduced biological aging. Related to the problem of poor overall survival was the observation that male lgf1r+/~ mice had a 16% extension of life span that was not statistically significant and that male mice were not resistant to paraquat. In the facilities at UTHSCSA, a 16% increase in life span using the proper number of mice would be statistically significant. In view of these deficiencies in the report of Holzenberger et al, it is essential to use the combined resources and expertise available at UTHSCSA in the areas of IGF-I signaling, oxidative stress and damage, and biological aging to test the hypothesis that Igflr^' mice have extended life span in association with reduced oxidative damage and reduced biological, aging. The following Specific Aims will be pursued to test this hypothesis by determining over the lifespan of the lgf1r+/~ mice and wild-type control mice whether: 1) the lgf1r+/~ phenotypes of reduced numbers of IGF-I receptors and reduced activation of IGF-I signaling pathways are maintained and the effects of these changes on the GH/IGF-I axis and insulin and glucose tolerance; 2) lgf1r+/~ mice are more resistant to paraquat-induced oxidative stress and have reduced oxidative damage to macromolecules and increased anti-oxidant enzymes over the lifespan; and 3) Igflr^' mice exhibit reduced age-related pathology and markers of biological aging and have extended life span. Outcomes of this research will provide the first definitive data whether a reduction in IGF-I receptor signaling decreases oxidative stress and damage and leads to a broad reduction in biological aging with extended life span in mammals.