Downregulation of SIRT1 signaling underlies hepatic autophagy impairment in glycogen storage disease type Ia

A deficiency in glucose-6-phosphatase-α (G6Pase-α) in glycogen storage disease type Ia (GSD-Ia) leads to impaired glucose homeostasis and metabolic manifestations including hepatomegaly caused by increased glycogen and neutral fat accumulation. A recent report showed that G6Pase-α deficiency causes impairment in autophagy, a recycling process important for cellular metabolism. However, the molecular mechanism underlying defective autophagy is unclear. Here we show that in mice, liver-specific knockout of G6Pase-α (L-G6pc-/-) leads to downregulation of sirtuin 1 (SIRT1) signaling that activates autophagy via deacetylation of autophagy-related (ATG) proteins and forkhead box O (FoxO) family of transcriptional factors which transactivate autophagy genes. Consistently, defective autophagy in G6Pase-α-deficient liver is characterized by attenuated expressions of autophagy components, increased acetylation of ATG5 and ATG7, decreased conjugation of ATG5 and ATG12, and reduced autophagic flux. We further show that hepatic G6Pase-α deficiency results in activation of carbohydrate response element-binding protein, a lipogenic transcription factor, increased expression of peroxisome proliferator-activated receptor-γ (PPAR-γ), a lipid regulator, and suppressed expression of PPAR-α, a master regulator of fatty acid β-oxidation, all contributing to hepatic steatosis and downregulation of SIRT1 expression. An adenovirus vector-mediated increase in hepatic SIRT1 expression corrects autophagy defects but does not rectify metabolic abnormalities associated with G6Pase-α deficiency. Importantly, a recombinant adeno-associated virus (rAAV) vector-mediated restoration of hepatic G6Pase-α expression corrects metabolic abnormalities, restores SIRT1-FoxO signaling, and normalizes defective autophagy. Taken together, these data show that hepatic G6Pase-α deficiency-mediated down-regulation of SIRT1 signaling underlies defective hepatic autophagy in GSD-Ia.

GSD-Ia is an autosomal recessive metabolic disorder caused by a deficiency in G6Pase-α, a key enzyme in maintaining blood glucose levels between meals. Despite strong compliance to dietary therapies, GSD-Ia patients continue manifesting metabolic aberrations including excessive accumulation of glycogen and lipid in the liver. Recently, G6Pase-α deficiency has been linked to impairment in autophagy, a recycling process essential for cellular homeostasis. However, the underlying mechanism is unclear. In this study, we show that hepatic G6Pase-α deficiency alters the activity and/or expression of several lipid regulators, leading to hepatic steatosis and reduced expression of SIRT1, an enzyme that regulates the activity of many proteins via deacetylation. The impaired SIRT1 signaling increases the acetylation of ATG proteins critical for autophagic vesicle elongation, and reduces the activity of FoxO factors that can induce autophagy genes. Consistently, the G6Pase-α-deficient liver exhibits autophagy impairment characterized by attenuated expression of many autophagy components, defective autophagic vesicle elongation, impaired autophagosome formation, and reduced autophagy flux. Importantly, SIRT1 overexpression in G6Pase-α-deficient liver corrects autophagy deficiency. Finally, restoration of hepatic G6Pase-α expression corrects metabolic abnormalities, restores SIRT1-FoxO signaling, and normalizes defective autophagy. Collectively, hepatic G6Pase-α deficiency-mediated down-regulation of SIRT1 signaling underlies defective hepatic autophagy in GSD-Ia.