Koichiro Nabe

Src activation generates reactive oxygen species and impairs metabolism–secretion coupling in diabetic Goto–Kakizaki and ouabain-treated rat pancreatic islets

Aims/hypothesisNa+/K+-ATPase inhibition by ouabain suppresses ATP production by generating reactive oxygen species (ROS) and impairs glucose-induced insulin secretion from pancreatic islets. To clarify the signal-transducing function of Na+/K+-ATPase in decreasing ATP production by the generation of ROS in pancreatic islets, the involvement of Src was examined. In addition, the significance of Src activation in diabetic islets was examined.MethodsIsolated islets from Wistar rats and diabetic Goto–Kakizaki (GK) rats (a model for diabetes) were used. ROS was measured by 5-(and 6)-chloromethyl-2′,7′-dichlorofluorescein fluorescence using dispersed islet cells. After lysates were immunoprecipitated by anti-Src antibody, immunoblotting was performed.ResultsOuabain caused a rapid Tyr418 phosphorylation, indicating activation of Src in the presence of high glucose. The specific Src inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) restored the ouabain-induced decrease in ATP content and the increase in ROS production. Both PP2 and ROS scavenger restored the impaired insulin release and impaired ATP elevation in GK islets, but had no such effect in control islets. PP2 reduced the high glucose-induced increase in ROS generation in GK islet cells but had no effect on that in control islet cells. Moreover, ouabain had no effect on ATP content and ROS production in the presence of high glucose in GK islets.Conclusions/interpretationThese results indicate that Src plays a role in the signal-transducing function of Na+/K+-ATPase, in which ROS generation decreases ATP production in control islets. Moreover, ROS generated by Src activation plays an important role in impaired glucose-induced insulin secretion in GK islets, in which Src is endogenously activated independently of ouabain.

Effect of high dietary fat on insulin secretion in genetically diabetic Goto-Kakizaki rats

Introduction and Aim To clarify the effects of a high fat-diet on insulin secretion from genetically diabetic beta cells, Goto-Kakizaki rats and Wistar rats were subjected to oral glucose tolerance test (OGTT) after 12-week high-fat feeding. Methodology We compared Wistar and Goto-Kakizaki (GK) rats fed a high-fat diet (45% fat content) for 12 weeks, measuring insulin secretion and insulin release. Results Insulin secretion during oral glucose tolerance test (OGTT) was enhanced in high-fat diet–fed Wistar rats (WF) with normal glucose tolerance. Insulin secretion in high-fat diet–fed GK rats (GF) during OGTT also was enhanced together with deteriorated glucose tolerance. Basal insulin release from the isolated perfused pancreas at 3.3 m M glucose in WF was comparable to that in normal chow-fed Wistar rats (WN), but basal insulin release in GF was remarkably higher than in normal chow–fed GK rats (GN). Stimulated insulin release induced by 16.7 m M glucose was remarkably increased in WF compared with WN. Total insulin release at 16.7 m M glucose in both GK rat groups was similar and minimal. Conclusion These results indicate that normal pancreatic &bgr;-cells have the ability to secrete sufficient insulin to compensate for the insulin resistance induced by a high-fat diet. In contrast, glucose metabolism in diabetic rats after high-fat diet deteriorated partly because of insufficient insulin secretion caused by genetic defects and lipotoxicity due to chronically high FFA levels.

Factors influencing the durability of the glucose-lowering effect of sitagliptin combined with a sulfonylurea.

We analyzed the changes of glycemic control over 12 months and the factors influencing blood glucose in 162 Japanese patients with type 2 diabetes having inadequate glycemic control despite sulfonylurea‐based therapy who received add‐on sitagliptin. Hemoglobin A1c (HbA1c) decreased significantly after 4 weeks of treatment, and this improvement was maintained for 1 year, although HbA1c was slightly higher in week 52 than in week 24. Comparison of the patients showing a ≥0.4% increase of HbA1c between weeks 24 and 52 (n = 57) with the others (n = 105) showed a significant difference in the change of bodyweight, as well as the dose of glibenclamide (both P < 0.01). Although combined therapy with sitagliptin and a sulfonylurea seems to be effective for at least 1 year, blood glucose levels are more likely to increase again in patients who show greater weight gain after 24 weeks of treatment and those receiving a higher dose of glibenclamide.

Secretory units of islets in transplantation index is a useful clinical marker to evaluate the efficacy of sitagliptin in treatment of type 2 diabetes mellitus

We carried out a retrospective analysis of 40 Japanese patients with type 2 diabetes mellitus who received sitagliptin. Glycated hemoglobin (HbA1c) and fasting plasma glucose were significantly decreased from 7.53 ± 0.65% and 155.2 ± 29.4 mg/dL at baseline to 6.80 ± 0.60% (P < 0.01) and 131.2 ± 22.3 mg/dL (P < 0.01) at week 20, respectively. β‐Cell function was evaluated by the secretory units of islets in transplantation (SUIT) index, which was significantly increased from 28.5 ± 14.0 at baseline to 38.6 ± 17.0 at week 20 (P < 0.01). Multivariate analysis was carried out between ΔHbA1c and several parameters (age, the duration of diabetes, body mass index, triglyceride [TG], C‐peptide [CPR], ΔCPR, HbA1c [baseline] and ΔSUIT), which showed HbA1c (baseline; β = 0.580, P < 0.001) and ΔSUIT (β = 0.308, P < 0.05) as significant independent determinants of ΔHbA1c. These two variables explained 53% of the variance in HbA1c response. These results suggest that SUIT index can be a clinical marker for the efficacy of sitagliptin in treatment of diabetes mellitus. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2011.00109.x, 2011)

Olmesartan reduced microalbuminuria in Japanese subjects with type 2 diabetes

We investigated the effect of olmesartan on early diabetic nephropathy in hypertensive patients with type 2 diabetes by its replacement of other angiotensin II type 1 receptor blockers (ARBs) by olmesartan. The urinary albumin/creatinine ratio significantly decreased. The present result suggests a possibility that olmesartan may have more effect on early nephropathy than other ARBs.

Usefulness of serum cystatin C in Japanese patients with type 2 diabetes mellitus and nephropathy

We examined usefulness of serum cystatin C to detect chronic kidney disease (CKD) stage >or=3, which was defined by Modification of Diet in Renal Disease formula. Serum cystatin C could detect CKD stage >or=3 with high efficacy in 289 Japanese patients with type 2 diabetes and nephropathy.

A case of lymphocytic panhypophysitis (LPH) during pregnancy

A 37-year-old pregnant woman developed continuous headache in the 10th week of pregnancy, followed by bilateral visual field defect and general malaise in the 24th week. The brain magnetic resonance imaging showed a pituitary mass. In laboratory examination, plasma concentration of free thyroxine, thyroid stimulating hormone (TSH), cortisol, and adrenocorticotropic hormone (ACTH) was low. General malaise vanished shortly after the replacement therapy of glucocorticoid and thyroid hormone, but partial central diabetes insipidus (CDI) appeared, which could be treated with desmopressin acetate (DDAVP). The visual field defect having enlarged, transsphenoidal surgery was performed in the 31st week of pregnancy. Adenohypophysis could be resected, and it showed infiltration of mature lymphocytes. After the surgery, the visual defect had improved, but hormone replacement was still necessary. She delivered a baby in the 38th week without any trouble. Provocative tests after delivery revealed a low response in TSH, prolactin (PRL), and follicle stimulating hormone (FSH). Hormone replacement and DDAVP administration was necessary in the same doses after delivery. The diagnosis was lymphocytic panhypophysitis (LPH). In the case of pregnant woman, LPH should be included in the differential diagnosis of pituitary mass for the fetomaternal safety.