Glycogen autophagy in glucose homeostasis

Department of Anatomy, Histology and Embryology, Medical School, University of Ioannina, Ioannina, Greece.
Pathology - Research and Practice (Impact Factor: 1.4). 02/2006; 202(9):631-8. DOI: 10.1016/j.prp.2006.04.001
Source: PubMed


Glycogen autophagy, the sequestration and degradation of cell glycogen in the autophagic vacuoles, is a selective, hormonally controlled and highly regulated process, representing a mechanism of glucose homeostasis under conditions of demand for the production of this sugar. In the newborn animals, this process is induced by glucagon secreted during the postnatal hypoglycemia and inhibited by insulin and parenteral glucose, which abolishes glucagon secretion. Hormonal action is mediated by the cAMP/protein kinase A (induction) and phosphoinositides/mTOR (inhibition) pathways that converge on common targets, such as the protein phosphatase 2A to regulate autophgosomal glycogen-hydrolyzing acid glucosidase and glycogen autophagy. Intralysosomal phosphate exchange reactions, which are affected by changes in the calcium levels and acid mannose 6- and acid glucose 6-phosphatase activities, can modify the intralysosomal composition in phosphorylated and nonphosphorylated glucose and promote the exit of free glucose through the lysosomal membrane. Glycogen autophagy-derived nonphosphorylated glucose assists the hyaloplasmic glycogen degradation-derived glucose 6-phosphate to combat postnatal hypoglycemia and participates in other metabolic pathways to secure the fine tuning of glucose homeostasis during the neonatal period.

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    • "Gnmt- KO livers showed higher glycogen accumulation than WT mice after 24h of starvation, as shown by PAS staining (Fig. 2B and C). This suggests an impairment in autophagy functionality since functional autophagy is required for glucose release from glycogen[29]. "
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    ABSTRACT: Background & aims: Glycine N-methyltransferase (GNMT) expression is decreased in some patients with severe NAFLD. Gnmt deficiency in mice (Gnmt-KO) results in abnormally elevated serum levels of methionine and its metabolite S-adenosylmethionine (SAMe), and this leads to rapid liver steatosis development. Autophagy plays a critical role in lipid catabolism (lipophagy), and defects in autophagy have been related to liver steatosis development. Since methionine and its metabolite SAMe are well known inactivators of autophagy, we aimed to examine whether high levels of both metabolites could block autophagy-mediated lipid catabolism. Methods: We examined methionine levels in a cohort of 358 serum samples from steatotic patients. We used hepatocytes cultured with methionine and SAMe, and hepatocytes and livers from Gnmt-KO mice. Results: We detected a significant increase in serum methionine levels in steatotic patients. We observed that autophagy and lipophagy were impaired in hepatocytes cultured with high methionine and SAMe, and that Gnmt-KO livers were characterized by an impairment in autophagy functionality, likely caused by defects at the lysosomal level. Elevated levels of methionine and SAMe activated PP2A by methylation, while blocking PP2A activity restored autophagy flux in Gnmt-KO hepatocytes, and in hepatocytes treated with SAMe and Methionine. Finally, normalization of methionine and SAMe levels in Gnmt-KO mice using a methionine deficient diet normalized the methylation capacity, PP2A methylation, autophagy, and ameloriated liver steatosis. Conclusions: These data suggest that elevated levels of methionine and SAMe can inhibit autophagic catabolism of lipids contributing to liver steatosis.
    Journal of Hepatology 09/2015; DOI:10.1016/j.jhep.2015.08.037 · 11.34 Impact Factor
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    • "As a regulator of lipid and glucose metabolism [104], loss of autophagy caused abnormal accumulation of lipids in mouse hepatocytes and a significant increase in plasma triglycerides, with reductions in fatty acid beta-oxidation [105] and pancreatic β-cell mass [106]. Coincidently, saikosaponins increased hepatic uptake of cholesterol and decreased plasma levels of cholesterol and triglycerides [107]. "
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    ABSTRACT: Chaihu, prepared from the dried roots of Bupleurum Chinense DC (also known as bei Chaihu in Chinese) or Bupleurum scorzoneraefolium WILD (also known as nan Chaihu in Chinese), is a herbal medicine for harmonizing and soothing gan (liver) qi stagnation. Substantial pharmacological studies have been conducted on Chaihu and its active components (saikosaponins). One of the active components of Chaihu, saikosaponin-d, exhibited anticancer effects via autophagy induction. This article reviews the pharmacological findings for the roles of autophagy in the pharmacological actions of Chaihu and saikosaponins.
    Chinese Medicine 09/2014; 9(1):21. DOI:10.1186/1749-8546-9-21 · 1.49 Impact Factor
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    • "The association between autophagic buildup and resistance to therapy in Pompe skeletal muscle suggested that suppression of autophagy would be helpful. Since autophagy is a presumed mechanism of glycogen transport to the lysosomes (Schiaffino and Hanzlikova, 1972; Kotoulas et al., 2006; Schiaffino et al., 2008), this approach also had a potential to reduce or even eliminate lysosomal glycogen accumulation. The genetic suppression of autophagy in GAA-KO mice was achieved by selective inactivation of a critical autophagic gene, Atg7, in skeletal muscles (Atg7/GAA double knockout). "
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    ABSTRACT: Pompe disease is a lysosomal storage disorder in which acid alpha-glucosidase is deficient or absent. Deficiency of this lysosomal enzyme results in progressive expansion of glycogen-filled lysosomes in multiple tissues, with cardiac and skeletal muscle being the most severely affected. The clinical spectrum ranges from fatal hypertrophic cardiomyopathy and skeletal muscle myopathy in infants to relatively attenuated forms, which manifest as a progressive myopathy without cardiac involvement. The currently available enzyme replacement therapy proved to be successful in reversing cardiac but not skeletal muscle abnormalities. Although the overall understanding of the disease has progressed, the pathophysiology of muscle damage remains poorly understood. Lysosomal enlargement/rupture has long been considered a mechanism of relentless muscle damage in Pompe disease. In past years, it became clear that this simple view of the pathology is inadequate; the pathological cascade involves dysfunctional autophagy, a major lysosome-dependent intracellular degradative pathway. The autophagic process in Pompe skeletal muscle is affected at the termination stage - impaired autophagosomal-lysosomal fusion. Yet another abnormality in the diseased muscle is the accelerated production of large, unrelated to ageing, lipofuscin deposits - a marker of cellular oxidative damage and a sign of mitochondrial dysfunction. The massive autophagic buildup and lipofuscin inclusions appear to cause a greater effect on muscle architecture than the enlarged lysosomes outside the autophagic regions. Furthermore, the dysfunctional autophagy affects the trafficking of the replacement enzyme and interferes with its delivery to the lysosomes. Several new therapeutic approaches have been tested in Pompe mouse models: substrate reduction therapy, lysosomal exocytosis following the overexpression of transcription factor EB and a closely related but distinct factor E3, and genetic manipulation of autophagy.
    Frontiers in Aging Neuroscience 07/2014; 6:177. DOI:10.3389/fnagi.2014.00177 · 4.00 Impact Factor
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