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Submitted on August 24, 2004
Accepted on October 15, 2004
Departments of Physiology and Internal Medicine, Southern Illinois University School of Medicine, Springfield, Illinois 62794
* To whom correspondence should be addressed. E-mail: abartke{at}siumed.edu.
Reduced IGF-1/insulin signaling and caloric restriction (CR) are known to extend life span and delay age-related diseases. To address the interaction of these two interventions, we subjected normal (N) and the long-lived growth hormone receptor knockout (GHRKO) mice to CR starting at weaning for 20 months. We also used bovine growth hormone transgenic (bGH Tg) mice which overexpress GH and are short-lived and insulin resistant for comparison. Circulating insulin and IGF-1 levels were reduced by CR in N animals while GHRKO animals exhibited very low insulin and undetectable IGF-1. Consistently, hepatic Akt phosphorylation was reduced by CR and very low in GHRKO mice. bGH Tg mice exhibited increased active Akt. The transcription factor Foxo1 was additively increased by CR and GHRKO at the mRNA level. However, Foxo1 protein levels were only elevated in GHRKO mice. The coactivator PGC-1
was increased at both gene and protein levels in GHRKO mice. N-CR and GHRKO mice also exhibited increased phosphorylated CREB and active p38 compared with the N-AL mice and the levels of these proteins were greatly diminished in bGH Tg mice. The protein levels of the deacetylase SIRT1 were elevated in the two CR groups and unexpectedly, also in bGH Tg mice. These results suggest a major role for Akt/Foxo1 pathway in the regulation of longevity in rodents. Activated gluconeogenic pathway and increased fat metabolism may be involved in mediating the effects of reduced somatotropic and insulin signaling on longevity. These results also add to the evidence that targeted disruption of the GHR/GHBP gene and CR act via overlapping but distinct mechanisms.
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