Glucokinase and glucose transporter expression in liver and islets: implications for control of glucose homoeostasis.

The liver/islet high-K, facilitated glucose transporter (GLUT-2) and the glucose phosphorylating enzyme, glucokinase, are key regulators of the rate of glucose metabolism in liver and islets, the two ‘glucose-sensing’ tissues in mammals. Increases in circulating glucose stimulate insulin secretion from the ,%cells of the islets of Langerhans and convert the liver from an organ o f net glucose output to one of glucose storage, processes mediated by increased glucose metabolism in both tissues. Increases in islet glucose metabolism result in elevated levels of a number of intracellular compounds that are recognized as second messengers for insulin secretion, including intracellular Ca”, inositol phosphates, arachidonic acid, diacylglycerol and ATP/ADP ratios (reviewed in [ 1-51). The @-cell glucose transporter, which has a K , for glucose of 17 mM and which has the same primary sequence as and similar kinetics to liver GLUT-2 161, probably plays a permissive role, by allowing rapid equilibration of intracellular and extracellular glucose. The rate of glucose metabolism is then largely controlled by glucokinase [ 2, 7-91. Perhaps the most compelling single piece of evidence supporting a regulatory role for the enzyme is that the curves for glucose dependence o f glucose usage (metabolism) and glucokinase activity in islets are virtually superimposable (see [2]). The curve for glucokinase activity as a function of glucose concentration in both liver and islets is sigmoidal, and is particularly steep over the physiological range of glucose concentrations (410 mM). This means that even modest increments in circulating glucose can quickly be translated into substantial increases in glucose metabolism, through rapid transporter-mediated equilibration across the plasma membrane and phosphorylation by glucokinase. It has been known for some time that islets isolated from fasted animals exhibit both an attenuated insulin secretory response to glucose and a reduction in glucokinase activity that is thought to be sufficient to account for the dampened response [ lo , 111. Refeeding of fasted animals results in a return of glucokinase activity and glucose-stimulated insulin secretion to normal within a period of hours. Alterations in hepatic glucokinase activity as a consequence of dietary status are also well studied, and are similar in direction and magnitude to the changes observed in islets 1121. With the preparation of anti-glucokinase antibodies [ 131 and the cloning of the cDNA for hepatic glucokinase [14, 151, i t has become possible to analyse the effects of dietary manipulation on the level of expression of glucokinase mRNA and protein in liver and islets and to correlate these changes with the known alterations in enzyme activity. In a recent collaborative study between our laboratory and that of Dr. Patrick lynedjian, important differences in the regulation and structure of the glucokinase gene products in liver and islets were observed 1161. A glucokinase cDNA probe prepared from hepatic mRNA [ 141 hybridized with equal intensity to a