Down-regulation of AMP-activated protein kinase by calorie restriction in rat liver

AMP-activated protein kinase (AMPK) may act as a key enzyme for metabolic adaptation to calorie restriction (CR) or reduced growth hormone (GH)-insulin-like growth factor (IGF)-1 signaling, an experimental intervention for lifespan extension in animals. We investigated the protein levels of AMPKα and a downstream enzyme, acetyl-CoA carboxylase (ACC), by immunoblotting of liver and quadriceps femoris muscle (QFM) extracts from 6-month-old wild-type (W) and GH-suppressed transgenic (Tg) Wistar rats fed ad libitum (AL) or 30% CR diets from 6 weeks of age. A modified alternate-day feeding regimen for CR yielded a fed-fasted cycle in CR rats, and therefore the effects of overnight fasting in W-AL rats were also evaluated. CR decreased threonine-172-phosphorylated AMPKα (p-AMPKα; an activated form) levels in the liver, whereas the CR-fed-fasted cycle or overnight fasting did not significantly affect the p-AMPKα level. In the QFM, the p-AMPKα level was slightly elevated in the CR-fasted phase, but greatly increased in the AL-fasted phase. Suppression of GH did not affect the p-AMPKα level. The phosphorylated-ACC levels did not alter in parallel with the p-AMPKα level, particularly in the liver. The present results suggest that CR down-regulates the AMPK activity in the liver on a long-term basis.