title: Glycogen Repletion in Brown Adipose Tissue upon Refeeding Is Primarily Driven by Phosphorylation-Independent Mechanisms.
abstract: Glycogen storage in brown adipose tissue (BAT) is generally thought to take place through passive, substrate-driven activation of glycogenesis rather than programmatic shifts favoring or opposing the storage and/or retention of glycogen. This perception exists despite a growing body of evidence suggesting that BAT glycogen storage is actively regulated by covalent modification of key glycogen-metabolic enzymes, protein turnover, and endocrine hormone signaling. Members of one such class of covalent-modification regulators, glycogen-binding Phosphoprotein Phosphatase-1 (PP1)-regulatory subunits (PPP1Rs), targeting PP1 to glycogen-metabolic enzymes, were dynamically regulated in response to 24 hr of starvation and/or 24 hr of starvation followed by ad libitum refeeding. Over-expression of the PPP1R Protein Targeting to Glycogen (PTG), under the control of the aP2 promoter in mice, inactivated glycogen phosphorylase (GP) and enhanced basal- and starvation-state glycogen storage. Total interscapular BAT glycogen synthase and the constitutive activity of GS were conditionally affected. During starvation, glucose-6-phosphate (G-6-P) levels and the relative phosphorylation of Akt (p-Ser-473-Akt) were both increased in PTG-overexpressing (Tg) mice, suggesting that elevated glycogen storage during starvation modifies broader cellular metabolic pathways. During refeeding, Tg and WT mice reaccumulated glycogen similarly despite altered GS and GP activities. All observations during refeeding suggest that the phosphorylation states of GS and GP are not physiologically rate-controlling, despite there being a clear balance of endogenous kinase- and phosphatase activities. The studies presented here reveal IBAT glycogen storage to be a tightly-regulated process at all levels, with potential effects on nutrient sensing in vivo.
authors.affiliation: From the Committee on Molecular Metabolism and Nutrition, University of Chicago, Chicago, Illinois, 60637, United States of America.
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Carmean
authors.affiliation: Department of Medicine, Section of Endocrinology, Diabetes and Metabolism, University of Chicago, Chicago, Illinois, 60637, United States of America.
authors.forename: Y Hanna
authors.initials: YH
authors.lastname: Huang
authors.affiliation: From the Committee on Molecular Metabolism and Nutrition, University of Chicago, Chicago, Illinois, 60637, United States of America.|Department of Medicine, Section of Endocrinology, Diabetes and Metabolism, University of Chicago, Chicago, Illinois, 60637, United States of America.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Brady
mesh_terms: D002001:Adipose Tissue, Brown / Q000378:metabolism; D000818:Animals; D004435:Eating / Q000502:physiology*; D006003:Glycogen / Q000378:metabolism; D024981:Glycogen Phosphorylase / Q000378:metabolism; D006006:Glycogen Synthase / Q000378:metabolism; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D051058:Oncogene Protein v-akt / Q000378:metabolism; D010749:Phosphoprotein Phosphatases / Q000378:metabolism; D010766:Phosphorylation; D015398:Signal Transduction / Q000502:physiology; D013217:Starvation / Q000378:metabolism*
chemical_list: D047908:Intracellular Signaling Peptides and Proteins; C104748:Ppp1r3c protein, mouse; D006003:Glycogen; D024981:Glycogen Phosphorylase; D006006:Glycogen Synthase; D051058:Oncogene Protein v-akt; D010749:Phosphoprotein Phosphatases