Long-Lived Growth Hormone Receptor Knockout Mice: Interaction of Reduced Insulin-Like Growth Factor I/Insulin Signaling and Caloric Restriction

Abstract

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.

Caloric restriction and growth hormone receptor knockout: Effects on expression of genes involved in insulin action in the heart

“…We have previously reported that this regimen of CR produces the expected reduction in body weight in both KO and normal mice (35). In the same study, CR down regulated circulating IGF1 levels in normal animals, while the extremely low levels of circulating IGF1 in both KO-AL and KO-CR animals were below the detection limits of the employed ELISA kit used in these studies (35).…”

“…The detailed protocol of the experiment was described previously (34,35). Because differences between genotypes and/or after CR treatment tend to be more pronounced in older animals, we continued caloric restriction for the period of 19 months till the animals reached the age of 21 months.…”

“…After crushing the hearts in liquid nitrogen, adding equal volume of PBS and removing 250 μl for RNA extraction, the remaining homogenate was used for protein extraction (35). Western blotting was performed using PPARγ (Cell Signalling, Beverly, MA), PPARα and PPARδ antibodies (Santa Cruz Biotechnologies, Santa Cruz, CA) and protocol described earlier (35).…”

“…Western blotting was performed using PPARγ (Cell Signalling, Beverly, MA), PPARα and PPARδ antibodies (Santa Cruz Biotechnologies, Santa Cruz, CA) and protocol described earlier (35). After stripping the membrane, a monoclonal β-actin antibody (Sigma, St. Louis, MO) was used as a control for protein loading.…”

Connecting serum IGF-1, body size, and age in the domestic dog

“…Studies in the mouse demonstrate that different dwarf mice, smaller than normal littermates by weight and characterized by decreased levels of IGF-1, live longer than littermates (Al-Regaiey et al 2005). The current study evaluated serum IGF-1 levels in dogs of highly variable size on the basis of body weight.…”