Nutrient control of phosphorylation and translocation of Foxo1 in C57BL/6 and db/db mice

Abstract

The nutrient response mediated by feeding or fasting plays an important role in controlling gluconeogenic gene expression such as glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate carboxylase (PEPCK). The FOXO family of forkhead transcription factor Foxo1 (mouse FOXO1) is a key regulator that stimulates the expression of gluconeogenic genes in the nucleus but is phosphorylated by Akt (also known as protein kinase B; PKB) and translocated to the cytoplasm in response to insulin. Although it has been widely accepted that the cellular signaling of insulin represses Foxo1 function through Akt-dependent phosphorylation, the molecular mechanism behind the modulation of Foxo1 function by nutrient responses, including feeding or fasting, remains unknown in vivo. We investigated the consequences of the nutritional changes in Akt-mediated Foxo1 phosphorylation and translocation in the liver using control C57BL/6 and diabetic db/db mice. We found that feeding promotes the phosphorylation and nuclear exclusion of Foxo1, whereas fasting counteracted them in C57BL/6 mice. Notably, db/db mice exhibited constitutive phosphorylation but dominant nuclear accumulation of Foxo1, even though CREB phosphorylation usually occurred in the fasted status. Furthermore, in contrast to C57BL/6 mice, the expression of G6Pase, PEPCK and PGC-1alpha genes during feeding was not down-regulated in db/db mice. Thus, we suggest that the accurate regulation of Foxo1 via Akt-dependent phosphorylation is required for physiological adaptation to different nutritional statuses.