Glucose phosphorylation as a therapeutic target for diabetes
Early experiments using genetic overexpression of glucokinase in liver of rodent models showed promise for the protection against hyperglycemia during diabetes, spawning the development of glucokinase activators . These drugs interact with allosteric sites of glucokinase, thus promoting a conformation that exposes the catalytic domain and potentiates the activity of the enzyme . Glucokinase activators have achieved beneficial effects on glycemia in human trials, in part by the putative activation of hepatic glycogen storage (a pathway that is impaired in the diabetic state). However, chronic genetic gain of function also causes fatty liver, hyperlipidemia, and increased insulin resistance . These undesired effects appear to be linked to the overstimulation of the lipogenic pathway by increased glucose uptake. Unfortunately, some of these long-term side effects have been recapitulated in clinical trials, leading to tempered enthusiasm for glucokinase activation . A related approach is to reduce hepatic glucose output by inhibition of glucose-6-phosphatase. Pharmacologic inhibitors of glucose-6-phosphatase have not been widely tested, but mice with a liver-specific loss of function have a dyslipidemia phenotype similar to glucokinase activation, in addition to glycogen storage disease . The negative effect of these interventions on lipidemia raises concerns that sequestering carbohydrate in liver may improve glycemia at the expense of promoting dyslipidemia.
The normal postabsorptive human liver stores roughly 100 g or approximately 300 mM of glucose as glycogen which it releases during fasting to maintain blood glucose concentration. By 48 h of fasting, the human will have depleted most of its liver glycogen. Following a meal, when hepatic glucose-6-phosphate is plentiful, glycogen can be replenished within several hours. The synthesis of glycogen begins with conversion of glucose-6-phosphate to glucose-1-phosphate by phosphoglucomutase, followed by the “activation” of glucose-1-phosphate to UDP-glucose by UDP-glucose pyrophosphorylase. Finally, glycogen synthase uses UDP-glucose to add glucose molecules one at a time to a growing glycogen particle. The mature glycogen particle has a highly branched structure, which is created by a branching enzyme that moves blocks of glucose residues and links them in β-1,6-glycosidic linkages. Approximately half of hepatic glycogen synthesized after a mixed meal originates by the “direct” conversion of glucose to glycogen. The remainder is synthesized by an “indirect” pathway, which refers to the observation that some exogenous glucose is metabolized to trioses (i.e., at least DHAP and GA3P but perhaps all the way to pyruvate) before being converted to glycogen. The indirect pathway of glycogen synthesis is the consequence of continued gluconeogenesis during the fasted-to-fed transition , and may be reinforced by Cori cycling. The indirect pathway can account for approximately 30–60% of liver glycogen synthesis in humans, with the exact percentage dependent on the time elapsed between meals .
Glycogen synthase exists as two major isoforms and is regulated by both allosteric and covalent modification . GYS1 is expressed in muscle and other cells which store glycogen, while hepatocytes express mainly GYS2. Glycogen synthase was one of the first enzymes to be identified whose activity is modified by phosphorylation, which occurs on multiple serine residues by several different kinases, notably PKA and GSK3α. Glycogen synthase activity is generally suppressed by phosphorylation, especially at site 2 (ser 7) . PKA phosphorylation (glycogen synthase deactivation) is triggered by glucagon receptor activation, while GSK3 activity is suppressed (glycogen synthase activation) by insulin. The latter is mediated by Akt phosphorylation of GSK3, though GSK3 independent mechanisms are also involved  and are likely mediated by allosteric regulation. Glycogen synthase is potently activated by allosteric interaction with glucose-6-phosphate . Thus, conditions such as elevated portal glucose and insulin lead to increased glucose transport and glucokinase activity, which favor increased glucose-6-phosphate concentration and induction of glycogen synthesis (Figure 13.8).
Phosphorylated glycogen synthase (deactivated) requires regulatory phosphatases for reactivation. Phosphatase activity is organized by scaffolding proteins known as glycogen targeting subunits . These proteins are part of a large family of more than 50 protein phosphatase-1 (PP1) binding proteins that deliver the enzyme to a wide array of substrates and cellular addresses, allowing PP-1 to participate in diverse cellular processes such as glycogen metabolism, cell division, vesicle fusion, and ion channel function. GL is a 35-kDa glycogen targeting protein that is preferentially expressed in liver. Protein targeting to glycogen (PTG), also known as PPP1R5, and a fourth form, PPP1R6, are similar in size to GL but are expressed in a wide variety of tissues. All of the targeting subunits are able to bind glycogen and PP-1, and exhibit varying capacities for binding of glycogen synthase, glycogen phosphorylase, and phosphorylase kinase . Overexpression of PTG inhibits activation of glycogenolysis by the glycogenolytic cascade, and protects diabetic animals from hyperglycemia. However, constitutive activation of PTG also causes a form of glycogen storage disease in nondiabetic animals. PTG is particularly important for the regulation of indirect glycogen synthesis from gluconeogenic substrates . Although the mechanism of PTG-mediated indirect synthesis of glycogen is not entirely clear, it appears to be related to accelerated disposal of glucose-6-P into glycogen, and a stimulation of gluconeogenesis. Thus PTG stimulates glycogen synthesis independent of glucose transport and GK activity.
Glycogen metabolism as a therapeutic target for diabetes
One approach to reducing hepatic glucose production during diabetes is to improve glycogen storage by activation of glycogen synthase and/or inhibition of glycogen phosphorylase. Glycogen synthase has been targeted by inhibition of GSK-3. GSK-3 inhibition increases glycogen storage and improves glycemia in a rodent model of type 2 diabetes . However, glycogen synthase kinase also phosphorylates many other targets, making its mechanism difficult to decipher. Alternatively, glycogen phosphorylase can be inhibited directly by several classes of glucose analogues that either impair its allosteric activation or repress its ATP binding capability. These compounds improve glycemia in animal models of diabetes by increasing direct and indirect glycogen synthesis . The adverse effects of pharmacologic promotion of glycogen storage are not completely known, but long term glycogen phosphorylase inhibition in animal models of type 2 diabetes can cause hepatomegaly, glycogenosis, and steatosis, side effects also common to glycogen storage diseases.