Effects of Caloric Restriction on Insulin Pathway Gene Expression in the Skeletal Muscle and Liver of Normal and Long-lived GHR-KO Mice

Growth hormone receptor/binding protein knockout (GHR-KO) mice are characterized by resistance to growth hormone (GH), reduced insulin like growth factor 1 (IGF1) levels and enhanced insulin sensitivity and markedly increased lifespan. Findings in these and other long-lived mutant mice, and in normal animals subjected to caloric restriction (CR) indicate that insulin signaling is importantly involved in the control of longevity. We have examined the mRNA expression level of genes involved in insulin/IGF1 action in the skeletal muscle and liver of normal and GHR-KO mice fed ad libitum or subjected to long term 30% CR. The levels of IR, IRS1, IRS2, GLUT4 and IGF1 message in the skeletal muscle were reduced by CR in both normal and GHR-KO mice. In the liver, the results indicate that in GHR-KO mice mRNA expression of genes related to early steps of insulin signaling is up-regulated in the liver but not in the muscle. The results also show that improved insulin sensitivity in response to CR is not due to increased mRNA expression of the above genes in either normal or GHR-KO animals.