Activation of direct and indirect pathways of glycogen synthesis by hepatic overexpression of protein targeting to glycogen
Robert M. O’Doherty, Per B. Jensen, Paul Anderson, John G. Jones, Hal K. Berman, Denise Kearney, Christopher B. Newgard
Robert M. O’Doherty, Per B. Jensen, Paul Anderson, John G. Jones, Hal K. Berman, Denise Kearney, Christopher B. Newgard
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Activation of direct and indirect pathways of glycogen synthesis by hepatic overexpression of protein targeting to glycogen

Abstract

Glycogen-targeting subunits of protein phosphatase-1, such as protein targeting to glycogen (PTG), direct the phosphatase to the glycogen particle, where it stimulates glycogenesis. We have investigated the metabolic impact of overexpressing PTG in liver of normal rats. After administration of PTG cDNA in a recombinant adenovirus, animals were fasted or allowed to continue feeding for 24 hours. Liver glycogen was nearly completely depleted in fasted control animals, whereas glycogen levels in fasted or fed PTG-overexpressing animals were 70% higher than in fed controls. Nevertheless, transgenic animals regulated plasma glucose, triglycerides, FFAs, ketones, and insulin normally in the fasted and fed states. Fasted PTG-overexpressing animals receiving an oral bolus of [U-13C]glucose exhibited a large increase in hepatic glycogen content and a 70% increase in incorporation of [13C]glucose into glycogen. However, incorporation of labeled glucose accounted for only a small portion of the glycogen synthesized in PTG-overexpressing animals, consistent with our earlier finding that PTG promotes glycogen synthesis from gluconeogenic precursors. We conclude that hepatic PTG overexpression activates both direct and indirect pathways of glycogen synthesis. Because of its ability to enhance glucose storage without affecting other metabolic indicators, the glycogen-targeting subunit may prove valuable in controlling blood glucose levels in diabetes.

Authors

Robert M. O’Doherty, Per B. Jensen, Paul Anderson, John G. Jones, Hal K. Berman, Denise Kearney, Christopher B. Newgard

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Expression of the mouse PTG transgene in liver, OGTT protocol. Animals r...
Expression of the mouse PTG transgene in liver, OGTT protocol. Animals received either the AdCMV-PTG or AdCMV-βGAL adenoviruses and were allowed to feed ad libitum for 90 hours after viral administration. Animals were then fasted for 24-hours before receiving an oral bolus of [13C]-labeled glucose (2 g/kg). Animals were sacrificed for collection of liver samples 180 minutes after the oral glucose challenge, and a portion of these samples was used for preparation of RNA and multiplex RT-PCR analysis as described in the legend to Figure 3. Inspection of the data revealed that AdCMV-PTG–treated animals could be segregated into high-expresser and low-expresser groups. (a) Three representative samples for each group are displayed, showing amplification products for the mouse PTG transgene and the internal standard, EF-1α. (b) Quantitative analysis of the ratio of PTG/EF-1α signals was performed on all samples by exposing gels to a phosphorimager screen and processing the resulting scan with ImageQuant. Data represent the mean ± SEM for the following number of samples: PTG high expressers, n = 9; PTG low expressers, n = 7; βGAL controls, n = 9. *PTG was expressed at a higher level in the high expressers than in either the low expressers or βGAL controls (P < 0.001). #The values were greater in PTG low expressers than in βGAL controls (P = 0.023).