Reduced autophagy in livers of fasted, fat-depleted, ghrelin-deficient mice: Reversal by growth hormone

Plasma growth hormone (GH) and hepatic autophagy each have been reported to protect against hypoglycemia in the fasted state, but previous data have not linked the two. Here we demonstrate a connection using a mouse model of fasting in a fat-depleted state. Mice were subjected to 1 wk of 60% calorie restriction, causing them to lose nearly all body fat. They were then fasted for 23 h. During fasting, WT mice developed massive increases in plasma GH and a concomitant increase in hepatic autophagy, allowing them to maintain viable levels of blood glucose. In contrast, lethal hypoglycemia occurred in mice deficient in the GH secretagogue ghrelin as a result of knockout of the gene encoding ghrelin O-acyltransferase (GOAT), which catalyzes a required acylation of the peptide. Fasting fat-depleted Goat^-/- mice showed a blunted increase in GH and a marked decrease in hepatic autophagy. Restoration of GH by infusion during the week of calorie restriction maintained autophagy in the Goat^-/- mice and prevented lethal hypoglycemia. Acute injections of GH after 7 d of calorie restriction also restored hepatic autophagy, but failed to increase blood glucose, perhaps owing to ATP deficiency in the liver. These data indicate that GH stimulation of autophagy is necessary over the long term, but not sufficient over the short term to maintain blood glucose levels in fasted, fatdepleted mice.