Regulation of Hepatic Glycogen Metabolism by Glycogen Targeting Subunits of Protein Phosphatase 1

Glycogen targeting subunits of protein phosphatase 1 play a critical role in fuel homeostasis through the regulation of glycogen metabolism. Adenovirus-mediated overexpression of the liver subunit GL, the muscle subunit GM, or a truncated version of GM, GMdeltaC increased hepatic glycogen content in high fat fed rats, a model of insulin resistance. Rats expressing GMdeltaC and GL had similar amounts of hepatic glycogen following an oral glucose tolerance test, but only GMdeltaC expression improved glucose tolerance. The explanation for this difference is that animals with overexpressed GMdeltaC experience a larger increment in hepatic glycogen storage during OGTT than animals with overexpressed GL, probably related to the much higher fasting liver glycogen levels in the latter group. Since it is possible to improve glucose tolerance via expression of glycogen targeting subunits, the remaining research focused on designing and testing a dominant-negative glycogen targeting subunit, PTG-VF. Overexpression of PTG-VF caused an 83% reduction in glycogen content in hepatocytes indicating that the activity of glycogen targeting subunits is necessary for glycogen accumulation. Further studies found that PTG-VF was more potent in blocking glycogen synthesis in hepatocytes with overexpressed GL than PTG. PTG-VF expression increases phosphoryalse a levels, which preferentially inhibits GL through its C-terminal phosphorylase a binding site that is lacking in PTG. Removal of the phosphorylase a binding site from GL renders the subunit less susceptible to inhibition by PTG-VF. PTG-VF was overexpressed in rats fed on standard chow (SC) or high fat (HF) diet to determine if suppression of glycogen targeting subunit activity could cause glucose intolerance or diabetes. Hepatic glycogen stores were decreased by PTG-VF, but this did not lead to alterations in glucose homeostasis.