Abstract
Recent reports point out the importance of the complex GK-GKRP in controlling glucose and lipid homeostasis. Several GK mutations affect GKRP binding, resulting in permanent activation of the enzyme. We hypothesize that hepatic overexpression of a mutated form of GK, GKA456V, described in a patient with persistent hyperinsulinemic hypoglycemia of infancy (PHHI) and could provide a model to study the consequences of GK-GKRP deregulation in vivo. GKA456V was overexpressed in the liver of streptozotocin diabetic mice. Metabolite profiling in serum and liver extracts, together with changes in key components of glucose and lipid homeostasis, were analyzed and compared to GK wild-type transfected livers. Cell compartmentalization of the mutant but not the wild-type GK was clearly affected in vivo, demonstrating impaired GKRP regulation. GKA456V overexpression markedly reduced blood glucose in the absence of dyslipidemia, in contrast to wild-type GK-overexpressing mice. Evidence in glucose utilization did not correlate with increased glycogen nor lactate levels in the liver. PEPCK mRNA was not affected, whereas the mRNA for the catalytic subunit of glucose-6-phosphatase was upregulated ~4 folds in the liver of GKA456V-treated animals, suggesting that glucose cycling was stimulated. Our results provide new insights into the complex GK regulatory network and validate liver-specific GK activation as a strategy for diabetes therapy.
Highlights
Glucokinase (GK) (EC 2.7.1.1) is expressed in glucosensitive cells of the pancreas, liver, hypothalamus, anterior pituitary gland, and enteroendocrine K and L cells [1,2,3]
Several naturally occurring activating mutations have been described that induce a conformational change in GK, which results in higher affinity for glucose, and reduce the interaction between GK and glucokinase regulatory protein (GKRP) [6, 7]
We aimed to study the metabolic consequences of GK-GKRP deregulation by overexpressing a GK activating
Summary
Glucokinase (GK) (EC 2.7.1.1) is expressed in glucosensitive cells of the pancreas, liver, hypothalamus, anterior pituitary gland, and enteroendocrine K and L cells [1,2,3]. GK is controlled by an endogenous inhibitor, the glucokinase regulatory protein (GKRP) [4, 5]. Several naturally occurring activating mutations have been described that induce a conformational change in GK, which results in higher affinity for glucose, and reduce the interaction between GK and GKRP [6, 7]. One such mutation, A456V, completely impairs GK-GKRP complex. Synthetic GK activators (GKAs) enhance GK activity and induce its translocation to the cytosol. [8,9,10,11,12]
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