Yukiyasu Toyoda

Glucokinase and IRS-2 are required for compensatory β cell hyperplasia in response to high-fat diet–induced insulin resistance

Glucokinase (Gck) functions as a glucose sensor for insulin secretion, and in mice fed standard chow, haploinsufficiency of beta cell-specific Gck (Gck(+/-)) causes impaired insulin secretion to glucose, although the animals have a normal beta cell mass. When fed a high-fat (HF) diet, wild-type mice showed marked beta cell hyperplasia, whereas Gck(+/-) mice demonstrated decreased beta cell replication and insufficient beta cell hyperplasia despite showing a similar degree of insulin resistance. DNA chip analysis revealed decreased insulin receptor substrate 2 (Irs2) expression in HF diet-fed Gck(+/-) mouse islets compared with wild-type islets. Western blot analyses confirmed upregulated Irs2 expression in the islets of HF diet-fed wild-type mice compared with those fed standard chow and reduced expression in HF diet-fed Gck(+/-) mice compared with those of HF diet-fed wild-type mice. HF diet-fed Irs2(+/-) mice failed to show a sufficient increase in beta cell mass, and overexpression of Irs2 in beta cells of HF diet-fed Gck(+/-) mice partially prevented diabetes by increasing beta cell mass. These results suggest that Gck and Irs2 are critical requirements for beta cell hyperplasia to occur in response to HF diet-induced insulin resistance.

Rare sugar D-psicose improves insulin sensitivity and glucose tolerance in type 2 diabetes Otsuka Long-Evans Tokushima Fatty (OLETF) rats

A rare sugar, D-psicose has progressively been evaluated as a unique metabolic regulator of glucose and lipid metabolism, and thus represents a promising compound for the treatment of type 2 diabetes mellitus (T2DM). The present study was undertaken to examine the underlying effector organs of D-psicose in lowering blood glucose and abdominal fat by exploiting a T2DM rat model, Otsuka Long-Evans Tokushima Fatty (OLETF) rats. Rats were fed 5% D-psicose or 5% D-glucose supplemented in drinking water, and only water in the control for 13 weeks and the protective effects were compared. A non-diabetic Long-Evans Tokushima Otsuka (LETO), fed with water served as a counter control of OLETF. After 13 weeks feeding, D-psicose treatment significantly reduced the increase in body weight and abdominal fat mass. Oral glucose tolerance test (OGTT) showed the reduced blood glucose and insulin levels suggesting the improvement of insulin resistance in OLETF rats. Oil-red-O staining elucidated that D-psicose significantly reduced lipid accumulation in the liver. Immunohistochemical analysis showed D-psicose induced glucokinase translocation from nucleus to cytoplasm of the liver which enhances glucokinase activity and subsequent synthesis of glycogen in the liver. D-psicose also protected the pathological change of the β-cells of pancreatic islets. These data demonstrate that D-psicose controls blood glucose levels by reducing lipotoxicity in liver and by preserving pancreatic β-cell function.

Co-localization of glucokinase with actin filaments

A portion of glucokinase appeared to be co‐localized with actin filaments in the cytoplasm of cultured rat hepatocytes incubated with 25 mM glucose. When liver‐ or islet‐type glucokinase was transiently expressed in COS‐7 cells, the expressed glucokinase was also co‐localized with actin filaments in the cytoplasm of these transfected cells. Although co‐localization of glucokinase with actin filaments was not clearly demonstrated in the pancreatic β‐cell line MIN6, islet glucokinase was found to be present in both the nucleus and the cytoplasm, though predominantly in the nucleus. These findings suggest that subcellular localization of glucokinase, including co‐localization with actin filaments, may have an important physiological role in metabolic regulation.

Glucokinase is concentrated in insulin-secretory granules of pancreatic B-cells.

Abstract We immunohistochemically examined the distribution of glucokinase (GK) in the B-cells of pancreatic islets of normal rats. GK was stained punctately in the cytoplasm of B-cells when examined under the light microscope. By use of a double-immunostaining technique, most of the GK immunoreactivity was observed to be colocalized with insulin immunoreactivity. Electron microscopic examination by the immunogold method revealed that GK immunoreactivity was predominantly located within insulin-secretory granules of pancreatic B-cells.

Tissue and subcellular distribution of glucokinase in rat liver and their changes during fasting-refeeding.

The distribution of glucokinase in rat liver under both normal feeding and fasting-refeeding conditions was investigated immunohistochemically. Under normal feeding conditions, glucokinase immunoreactivity was observed in both nuclei and cytoplasm of parenchymal cells. The nuclei were stained intensely and evenly, whereas the cytoplasm showed weak immunoreactivity of different degrees of staining intensity depending on the location of the cells. The cytoplasm of perivenous hepatocytes was stained more intensely, though not so much more, than that of periportal hepatocytes. The cytoplasm of hepatocytes surrounding the terminal hepatic venule (THV), of hepatocytes surrounding the portal triad, and of some other hepatocytes showed a stronger immunoreactivity than that of residual hepatocytes. The nuclear immunoreactivity in hepatocytes surrounding the portal triad and in some other hepatocytes was weak or absent, and positive immunoreactivity was detected at the plasma membrane of some of these cells. After 72 h of fasting, glucokinase immunoreactivity was markedly decreased in all hepatocytes. After the start of refeeding, the cytoplasmic immunoreactivity began to increase first in the parenchymal cells surrounding the THV and extended to those in the intermediate zone followed by those in the periportal zone. In contrast, the increase in nuclear immunoreactivity started in hepatocytes situated in the intermediate zone adjacent to the perivenous zone and then extended to those in the perivenous zone followed by those in the periportal zone. Hepatocytes surrounding either THV or portal triad showed a distinctive change in immunoreactivity during the refeeding period. After 10 h of refeeding, strong immunoreactivity was observed in both the cytoplasm and the nuclei of all hepatocytes, and appreciable glucokinase immunoreactivity was detected at the plasma membrane of some hepatocytes. These findings are discussed from the standpoint of a functional role of glucokinase in hepatic glucose metabolism.

Ingestion of a moderate high‐sucrose diet results in glucose intolerance with reduced liver glucokinase activity and impaired glucagon‐like peptide‐1 secretion

Aims/Introduction:  Excessive intake of sucrose can cause severe health issues, such as diabetes mellitus. In animal studies, consumption of a high‐sucrose diet (SUC) has been shown to cause obesity, insulin resistance and glucose intolerance. However, several in vivo experiments have been carried out using diets with much higher sucrose contents (50–70% of the total calories) than are typically ingested by humans. In the present study, we examined the effects of a moderate SUC on glucose metabolism and the underlying mechanism.

Rare sugar D-allulose: Potential role and therapeutic monitoring in maintaining obesity and type 2 diabetes mellitus.

Obesity and type 2 diabetes mellitus (T2DM) are the leading worldwide risk factors for mortality. The inextricably interlinked pathological progression from excessive weight gain, obesity, and hyperglycemia to T2DM, usually commencing from obesity, typically originates from overconsumption of sugar and high-fat diets. Although most patients require medications, T2DM is manageable or even preventable with consumption of low-calorie diet and maintaining body weight. Medicines like insulin, metformin, and thiazolidinediones that improve glycemic control; however, these are associated with weight gain, high blood pressure, and dyslipidemia. These situations warrant the attentive consideration of the role of balanced foods. Recently, we have discovered advantages of a rare sugar, D-allulose, a zero-calorie functional sweetener having strong anti-hyperlipidemic and anti-hyperglycemic effects. Study revealed that after oral administration in rats D-allulose readily entered the blood stream and was eliminated into urine within 24h. Cell culture study showed that D-allulose enters into and leaves the intestinal enterocytes via glucose transporters GLUT5 and GLUT2, respectively. In addition to D-alluloses short-term effects, the characterization of long-term effects has been focused on preventing commencement and progression of T2DM in diabetic rats. Human trials showed that D-allulose attenuates postprandial glucose levels in healthy subjects and in borderline diabetic subjects. The anti-hyperlipidemic effect of D-allulose, combined with its anti-inflammatory actions on adipocytes, is beneficial for the prevention of both obesity and atherosclerosis and is accompanied by improvements in insulin resistance and impaired glucose tolerance. Therefore, this review presents brief discussions focusing on physiological functions and potential benefits of D-allulose on obesity and T2DM.

Catabolite inactivation of fructose 1,6-bisphosphatase and cytoplasmic malate dehydrogenase in yeast

Catabolite inactivation of fructose 1,6-bisphosphatase and cytoplasmic malate dehydrogenase was studied using the protease-deficient and vacuole-defective yeast strain pep4-3. The catabolite inactivation of fructose 1,6-bisphosphatase in pep4-3 was found to have a normal first inactivation step but with a defective second proteolytic step. In contrast, catabolite inactivation of cytoplasmic malate dehydrogenase was normal in pep4-3. These results suggest that the proteolytic pathways utilized in the hydrolysis of the two enzymes may be different and that proteolysis of fructose 1,6-bisphosphatase may require functional vacuoles while proteolysis of cytoplasmic malate dehydrogenase may not.

Anomeric preference of glucose utilization in human erythrocytes loaded with glucokinase.

Human erythrocytes were loaded with homogeneous rat liver glucokinase by an encapsulation method based on hypotonic hemolysis and isotonic resealing. As assayed at 10 mM glucose, glucokinase and hexokinase activities in glucokinase-loaded erythrocytes were 218 and 384 nmol/min/gHb, respectively; whereas hexokinase activity in both intact and unloaded red cells, which contain no glucokinase activity, was about 400 nmol/min/gHb. No difference in the rate of lactate production from glucose anomers between intact and unloaded erythrocytes suggested that the encapsulation procedure itself did not affect glucose utilization in red cells. Alpha-anomeric preference in lactate production from glucose was observed in glucokinase-loaded erythrocytes, whereas the beta anomer of glucose was more rapidly utilized than the alpha anomer in intact and unloaded erythrocytes. The results indicate that the step of glucose phosphorylation determines the anomeric preference in glucose utilization by human erythrocytes, since glucokinase and hexokinase are alpha- and beta-preferential, respectively, in glucose phosphorylation.

Changes in subcellular and zonal distribution of glucokinase in rat liver during postnatal development

Subcellular and zonal distribution of glucokinase in rat liver during postnatal development was examined immunohistochemically. Before day 11 after birth, only some hepatocytes were immunostained, and a positive immunostaining was found in the cytoplasm but not in the nucleus. No zonal distribution of glucokinase was observed in livers of such pups. From day 15, at which time a dietary change from milk to laboratory chow begins to take place, glucokinase immunoreactivity increased; this increase was associated with increases in glucokinase activity and in glucokinase protein, and also the immunostaining was observed mainly in the nuclei. At day 21, the glucokinase immunoreactivity was found almost exclusively in the perivenous zone. At day 30, an intense immunostaining was seen both in the perivenous zone and in the periportal zone, being slightly predominant in the former. The present results indicate that dramatic changes in the distribution of glucokinase in developing rat liver may be related to dietary change.