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 5 mM glucose, GK staining by the immunoperoxidase method was observed predominantly in the nucleus. When cultured hepatocytes were incubated for 30 min in various concentrations (5-45 mM) of glucose, there was an appreciable decrease in nuclear GK immunoreactivity, even at 10 mM compared with that at 5 mM. After the shift of glucose concentration from 5 mM to 25 mM, the GK distribution changed time-dependently over 1 h. A time-dependent change in GK distribution was also observed when the glucose concentration was shifted from 25 mM to 5 mM. Reversal of GK distribution in response to the change in glucose concentration from 5 to 25 mM and vice versa was shown to repeatedly occur. Lower concentrations (0.05-5 mM) of fructose, which is known to stimulate glucose phosphorylation by GK, in combination with 5 mM glucose, induced the translocation of GK from the nucleus to the cytoplasm. Mannose (20 mM), a substrate of GK, and sorbitol (1 mM), a stimulator of glucose phosphorylation by GK, induced the translocation of GK from the nucleus to the cytoplasm in the presence of 5 mM glucose. L-glucose, galactose, 3-O-methylglucose, and 2-deoxyglucose at 20 mM each did not affect the GK distribution observed in the presence of 5 mM 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.