Shuttling of Glucokinase between the Nucleus and the Cytoplasm in Primary Cultures of Rat Hepatocytes: Possible Involvement in the Regulation of the Glucose Metabolism

Glucokinase (GK) is believed to play a key role in the control of the hepatic glucose metabolism. To address the mechanism of the regulation of glucose metabolism through GK action, we immunohistochemically studied changes in GK distribution in primary cultures of rat hepatocytes. In hepatocyte monolayers incubated in 5mM glucose, GK staining by the immunoperoxidase method was observed predominantly in the nucleus. When cultured hepatocytes were incubated for 30 min in various concentrations (5-45mM)of glucose, there was an appreciable decrease in nuclear GK immunoreactivity, even at 10mM compared with that at 5mM. After the shift of glucose concentration from 5mM to 25mM, the GK distribution changed timedependently over 1h. A time-dependent change in GK distribution was also observed when the glucose concentration was shifted from 25mM to 5mM. Reversal of GK distribution in response to the change in glucose concentration from 5 to 25mM and vice versa was shown to repeatedly occur. Lower concentrations (0.05-5mM) of fructose, which is known to stimulate glucose phosphorylation by GK, in combination with 5mM glucose, induced the translocation of GK from the nucleus to the cytoplasm. Mannose (20mM), a substrate of GK, and sorbitol (1mM), a stimulator of glucose phosphorylation by GK, induced the translocation of GK from the nucleus to the cytoplasm in the presence of 5mM glucose. L-glucose, galactose, 3-O-methylglucose, and 2-deoxyglucose at 20mM each did not affect the GK distribution observed in the presence of 5mM glucose. The results suggest that GK is present mainly in the nucleus under conditions where GK action is not much needed, whereas the enzyme exists mainly in the cytoplasm under conditions where it must function extensively. Our findings indicate that the shuttling of GK between the nucleus and the cytoplasm is essential for the regulation of the glucose metabolism in the liver.