Implications for the insulin signaling pathway in Snell dwarf mouse longevity: a similarity with the C. elegans longevity paradigm.

Mutation analyses in the nematode, Caenorhabditis elegans, and mice have identified genes that increase their life-span via hormonal signal transduction, i.e. the insulin/insulin-like growth factor-1 (IGF-1) pathway in nematodes, and the growth hormone (GH)-thyriod stimulating hormone (TSH)-prolactin system in Snell dwarf mouse mutants. We have shown that the GH deficiency due to Pit1 mutation in the long-lived Snell dwarf mice may decrease circulating insulin levels, thereby resulting in a decreased activity of the insulin/IGF-1 signaling pathway. The data presented are consistent with our hypothesis that the decreased circulating insulin levels resulting from the Pit1 mutation mimics a physiological state similar to that proposed to occur in the long-lived C. elegans, daf-2 mutant. Our studies demonstrate a series of changes in components of the insulin/IGF-1-signaling pathway that suggest a reduction-of-function of this pathway in the aged dwarf. These include a decreased IRS-2 pool level, a decrease in PI3K activity and its association with IRS-2 and decreased docking of p85alpha to IRS-2. Our data also suggest a preferential docking of IRS-2-p85 alpha -p110 alpha in the aged dwarf liver and IRS-2-p85 alpha -p110 beta in the aged control. We speculate that the preference for the p110 alpha-containing complex may be a specific characteristic of a downstream segment of the longevity-signaling cascade. We conclude that the Pit1 mutation may result in physiological homeostasis that favors longevity, and that the Snell dwarf mutant conforms to the nematode longevity paradigm.