Abnormalities of Glucose Homeostasis and the Hypothalamic-Pituitary-Adrenal Axis in Mice Lacking Hexose-6-Phosphate Dehydrogenase

Abstract

Hexose-6-phosphate dehydrogenase (EC 1.1.1.47) catalyzes the conversion of glucose 6-phosphate to 6-phosphogluconolactone within the lumen of the endoplasmic reticulum, thereby generating reduced nicotinamide adenine dinucleotide phosphate. Reduced nicotinamide adenine dinucleotide phosphate is a necessary cofactor for the reductase activity of 11β-hydroxysteroid dehydrogenase type 1 (EC 1.1.1.146), which converts hormonally inactive cortisone to active cortisol (in rodents, 11-dehydrocorticosterone to corticosterone). Mice with targeted inactivation of hexose-6-phosphate dehydrogenase lack 11β-hydroxysteroid dehydrogenase type 1 reductase activity, whereas dehydrogenase activity (corticosterone to 11-dehydrocorticosterone) is increased. We now report that both glucose output and glucose use are abnormal in these mice. Mutant mice have fasting hypoglycemia. In mutant primary hepatocytes, glucose output does not increase normally in response to glucagon. Mutant animals have lower hepatic glycogen content when fed and cannot mobilize it normally when fasting. As assessed by RT-PCR, responses of hepatic enzymes to fasting are blunted; enzymes involved in gluconeogenesis (phosphoenolpyruvate carboxykinase, tyrosine aminotransferase) are not appropriately up-regulated, and expression of glucokinase, an enzyme required for glycolysis, is not suppressed. Corticosterone has attenuated effects on expression of these enzymes in cultured mutant primary hepatocytes. Mutant mice have increased sensitivity to insulin, as assessed by homeostatic model assessment values and by increased glucose uptake by the muscle. The hypothalamic-pituitary-adrenal axis is also abnormal. Circulating ACTH, deoxycorticosterone, and corticosterone levels are increased in mutant animals, suggesting decreased negative feedback on the hypothalamic-pituitary-adrenal axis. Comparison with other animal models of adrenal insufficiency suggests that many of the observed abnormalities can be explained by blunted intracellular corticosterone actions, despite elevated circulating levels of this hormone.