Mice mutant for glucokinase regulatory protein exhibit decreased liver glucokinase: A sequestration mechanism in metabolic regulation

The importance of glucokinase (GK; EC 2.7.1.12) in glucose homeostasis has been demonstrated by the association of GK mutations with diabetes mellitus in humans and by alterations in glucose metabolism in transgenic and gene knockout mice. Liver GK activity in humans and rodents is allosterically in hibited by GK regulatory protein (GKRP). To further understand the role of GKRP in GK regulation, the mouse GKRP gene was inactivated. With the knocko ut of the GKRP gene, there was a parallel loss of GK protein and activity i n mutant mouse liver. The loss was primarily because of posttranscriptional regulation of GK, indicating a positive regulatory role for GKRP in mainta ining GK levels and activity. As in rat hepatocytes. both GK and GKRP were localized in the nuclei of mouse hepatocytes cultured in low-glucose-contai ning medium. In the presence of fructose or high concentrations of glucose, conditions known to relieve GK inhibition by GKRP in vitro, only GK was tr anslocated into the cytoplasm. In the GKRP-mutant hepatocytes, GK was not f ound in the nucleus under any tested conditions. We propose that GKRP funct ions as an anchor to sequester and inhibit GK in the hepatocyte nucleus, wh ere it is protected from degradation. This ensures that glucose phosphoryla tion is minimal when the liver is in the fasting. glucose-producing phase. This also enables the hepatocytes to rapidly mobilize GK into the cytoplasm to phosphorylate and store or metabolize glucose after the ingestion of di etary glucose. In GKRP-mutant mice, the disruption of this regulation and t he subsequent decrease in CK activity leads to altered glucose metabolism a nd impaired glycemic control.