Long-lived growth hormone receptor knockout mice: Interaction of reduced insulin-like growth factor I/insulin signaling and caloric restriction

Reduced IGF-I/insulin signaling and caloric restriction (CR) are known to extend the life span and delay age-related diseases. To address the interaction of these two interventions, we subjected normal (N) and long-lived GH receptor knockout (GHRKO) mice to CR for 20 months starting at weaning. We also used bovine GH transgenic (bGH Tg) mice, which overexpress GH and are short-lived and insulin resistant, for comparison. Circulating insulin and IGF-I levels were reduced by CR in N animals, whereas GHRKO animals exhibited very low insulin and undetectable IGF-I. Consistently, hepatic Akt phosphorylation was reduced by CR and was very low in GHRKO mice. bGH Tg mice exhibited increased active Akt. The forkhead box O1 (Foxo1) transcription factor was additively increased by CR and GHRKO at the mRNA level. However, Foxo1 protein levels were only elevated in GHRKO mice. The coactivator peroxisome proliferator-activated receptor-gamma coactivator 1alpha was increased at both gene and protein levels in GHRKO mice. N-CR and GHRKO mice also exhibited increased phosphorylated cAMP response element-binding protein and active p38 compared with the N ad libitum-fed mice, and the levels of these proteins were greatly diminished in bGH Tg mice. The protein levels of the deacetylase sirtuin 1 (SIRT1) were elevated in the two CR groups and, unexpectedly, also in bGH Tg mice. These results suggest a major role for the Akt/Foxo1 pathway in the regulation of longevity in rodents. An 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 GH receptor/GH-binding protein gene and CR act via overlapping, but distinct, mechanisms.