Glucose-induced dissociation of glucokinase from its regulatory protein in the nucleus of hepatocytes prior to nuclear export

The glucose phosphorylating enzyme glucokinase regulates glucose metabolism in the liver. Glucokinase activity is modulated by a liver-specific competitive inhibitor, the glucokinase regulatory protein (GRP), which mediates sequestration of glucokinase to the nucleus at low glucose concentrations. However, the mechanism of glucokinase nuclear export is not fully understood. In this study we investigated the dynamics of glucose-dependent interaction and translocation of glucokinase and GRP in primary hepatocytes using fluorescence resonance energy transfer, selective photoconversion and fluorescence recovery after photobleaching. The formation of the glucokinase:GRP complex in the nucleus of primary hepatocytes at 5 mmol/l glucose was significantly reduced after a 2 h incubation at 20 mmol/l glucose. The GRP was predominantly localized in the nucleus, but a mobile fraction moved between the nucleus and the cytoplasm. The glucose concentration only marginally affected GRP shuttling. In contrast, the nuclear export rate of glucokinase was significantly higher at 20 than at 5 mmol/l glucose. Thus, glucose was proven to be the driving-force for nuclear export of glucokinase in hepatocytes. Using the FLII12Pglu-700μ-δ6 glucose nanosensor it could be shown that in hepatocytes the kinetics of nuclear glucose influx, metabolism or efflux were significantly faster compared to insulin-secreting cells. The rapid equilibration kinetics of glucose flux into the nucleus facilitates dissociation of the glucokinase:GRP complex and also nuclear glucose metabolism by free glucokinase enzyme. In conclusion, we could show that a rise of glucose in the nucleus of hepatocytes releases active glucokinase from the glucokinase:GRP complex and promotes the subsequent nuclear export of glucokinase.