Motoyuki Tamaki

The diabetes-susceptible gene SLC30A8/ZnT8 regulates hepatic insulin clearance.

Recent genome-wide association studies demonstrated that common variants of solute carrier family 30 member 8 gene (SLC30A8) increase susceptibility to type 2 diabetes. SLC30A8 encodes zinc transporter-8 (ZnT8), which delivers zinc ion from the cytoplasm into insulin granules. Although it is well known that insulin granules contain high amounts of zinc, the physiological role of secreted zinc remains elusive. In this study, we generated mice with β cell-specific Slc30a8 deficiency (ZnT8KO mice) and demonstrated an unexpected functional linkage between Slc30a8 deletion and hepatic insulin clearance. The ZnT8KO mice had low peripheral blood insulin levels, despite insulin hypersecretion from pancreatic β cells. We also demonstrated that a substantial amount of the hypersecreted insulin was degraded during its first passage through the liver. Consistent with these findings, ZnT8KO mice and human individuals carrying rs13266634, a major risk allele of SLC30A8, exhibited increased insulin clearance, as assessed by c-peptide/insulin ratio. Furthermore, we demonstrated that zinc secreted in concert with insulin suppressed hepatic insulin clearance by inhibiting clathrin-dependent insulin endocytosis. Our results indicate that SLC30A8 regulates hepatic insulin clearance and that genetic dysregulation of this system may play a role in the pathogenesis of type 2 diabetes.

Human IAPP–induced pancreatic β cell toxicity and its regulation by autophagy

Pancreatic islets in patients with type 2 diabetes mellitus (T2DM) are characterized by loss of β cells and formation of amyloid deposits derived from islet amyloid polypeptide (IAPP). Here we demonstrated that treatment of INS-1 cells with human IAPP (hIAPP) enhances cell death, inhibits cytoproliferation, and increases autophagosome formation. Furthermore, inhibition of autophagy increased the vulnerability of β cells to the cytotoxic effects of hIAPP. Based on these in vitro findings, we examined the pathogenic role of hIAPP and its relation to autophagy in hIAPP-knockin mice. In animals fed a standard diet, hIAPP had no toxic effects on β cell function; however, hIAPP-knockin mice did not exhibit a high-fat-diet-induced compensatory increase in β cell mass, which was due to limited β cell proliferation and enhanced β cell apoptosis. Importantly, expression of hIAPP in mice with a β cell-specific autophagy defect resulted in substantial deterioration of glucose tolerance and dispersed cytoplasmic expression of p62-associated toxic oligomers, which were otherwise sequestrated within p62-positive inclusions. Together, our results indicate that increased insulin resistance in combination with reduced autophagy may enhance the toxic potential of hIAPP and enhance β cell dysfunction and progression of T2DM.

Downregulation of ZnT8 expression in pancreatic β-cells of diabetic mice.

Zinc transporter 8 (ZnT8) has been identified as a β-cell-specific Zinc transporter expressed in insulin-secretory granules. Recent genome wide association studies indicated that Arg325Trp polymorphism of Slc30a8 encoding ZnT8 is associated with susceptibility to type 2 diabetes. As a first step towards understanding the pathogenic role of ZnT8 in diabetes, we evaluated the expression of ZnT8 in mouse pancreas. A rabbit polyclonal antibody specific to ZnT8 was raised. The raised ZnT8 antibody reacted with mouse, rat and human ZnT8 expressed in β-cells without cross-reacting with other ZnTs. ZnT8 was expressed in α-, β- and PP-cells, but not in δ-cells, in adult mouse islets. During mouse pancreas development, ZnT8 expression was detected as early as embryonic day 15.5 when β-cells started to appear in large numbers. Finally, the expression level of ZnT8 was compared with pancreas of two diabetic model mice, db/db mice and Akita mice. In both animal models of diabetes, ZnT8 expression was remarkably downregulated in the early stage of diabetes. As a conclusion, ZnT8 is expressed in multiple lineages of endocrine cells in the pancreas. Our findings suggest that downregulation of ZnT8 may be associated with impaired function of β-cells in diabetes.

Efficacy and safety of modified Yale insulin infusion protocol in Japanese diabetic patients after open-heart surgery

To our knowledge, there is currently no insulin infusion protocol for critically ill patients especially designed for Asian diabetics although many such protocols are used in Western countries. In this study, we modified the Yale insulin infusion protocol taking into consideration the characteristics of Japanese diabetics and hospital environment. We tested the modified protocol in 40 type 2 diabetic patients after elective open-heart surgery (MY group) comparing with 35 type 2 diabetic patients under empirical blood glucose control (EC group). Analyses of 1656 blood glucose measurements during insulin infusion revealed that percentage of samples that showed achievement of target blood glucose level (80-140 mg/dl) was higher under MY (78+/-15%, n=870) than EC (57+/-23%, n=786, p<0.0001). On the other hand, the percentage of samples in which blood glucose was less than 60 mg/dl was comparable in the two groups (MY: 0.5+/-5.9 per thousand, EC: 5.1+/-18.5 per thousand). None of the patients with hypoglycemia showed significant clinical adverse effects. In conclusion, our modified Yale insulin infusion protocol is effective and safe for tight blood glucose control in Japanese diabetic patients after open-heart surgery.

Combination treatment of db/db mice with exendin-4 and gastrin preserves β-cell mass by stimulating β-cell growth and differentiation

Aim/Introduction:  Preservation of β‐cell mass is crucial for maintaining long‐term glucose homeostasis. Therapies based on incretin and its mimetics are expected to achieve this goal through various biological functions, particularly the restoration of β‐cell mass. Here we tested the effects of gastrin and exendin‐4 in type 2 diabetic animals.

Effects of changes in basal/total daily insulin ratio in type 2 diabetes patients on intensive insulin therapy including insulin glargine (JUN-LAN Study 6).

Intensive insulin therapy composed of bolus and basal insulin has been believed as the most powerful recipe for glycemic control of both type 1 and type 2 diabetes. In this study, we investigated the effects of changes in basal/total daily insulin ratio (B/TD ratio) in type 2 diabetes patients on intensive insulin therapy including insulin glargine. The B/TD ratio used in our Japanese patients was about 0.35, and the ratio was increased up to about 0.46+/-0.12 without change of total insulin daily dose. After 24-week-treatment, mean glycated albumin of the patients whose B/TD ratio was increased was significantly lower than those of the patients whose B/TD ratio was not changed. Our results suggest that adequate supplementation of basal insulin may be important for maximum effect of bolus insulin even in Japanese who have serious defect in postprandial rapid insulin secretion.

Efficacy of pioglitazone on glycemic control and carotid intima‐media thickness in type 2 diabetes patients with inadequate insulin therapy

Aims/Introduction:  The present study was designed to determine the effects of pioglitazone on glycemic control and atherosclerosis in patients with poorly controlled type 2 diabetes on insulin therapy.

Beneficial effects of vildagliptin combined with miglitol on glucose tolerance and islet morphology in diet-controlled db/db mice.

Dipeptidyl peptidase-4 (DPP-4) inhibitors improve glycemic control in patients with type 2 diabetes primarily by increasing plasma active glucagon-like peptide-1 (GLP-1) levels. While various combination therapies based on DPP-4 inhibitors have been proposed for treatment of type 2 diabetes, the effects of combination therapy of DPP-4 inhibitors and alpha-glucosidase inhibitors on β-cell function are less characterized. We evaluated the effects of long-term treatment with vildagliptin, a DPP-4 inhibitor, on metabolic parameters and β-cell function, in combination with miglitol, an alpha-glucosidase inhibitor, in diet-controlled db/db mice. In this study, 6-week-old male db/db mice were provided with standard chow twice a day for 6 weeks. Meal tolerance tests and glucose tolerance tests showed that the combination therapy of vildagliptin with miglitol, but not each alone, suppressed postprandial glycemic excursion, enhanced postprandial active GLP-1 levels and prevented deterioration of glucose tolerance in the db/db mice. The combination treatment did not alter β-cell mass, but resulted in preserved expression of glucose transporter 2, Zinc transporter 8 and MafA and reduced the number of α cells. These results suggest that the combination of vildagliptin and miglitol prevents the development of overt diabetes in diet-controlled pre-diabetic db/db mice by normalizing postprandial glucose and incretin response, and by preserving β-cell structure and the expression of factors essential for β-cell function.

2-Methoxyestradiol ameliorates glucose tolerance with the increase in β-cell mass in db/db mice.

Aims/Introduction:  2‐Methoxyestradiol (2ME) is an estradiol metabolite with little estrogenic activity. Previous data identified its anti‐carcinogenic properties and possible cardiovascular benefits. However, its effect on diabetes mellitus has not been fully elucidated. The aim of the present study was to determine the effects of 2ME on glucose metabolism in the diabetic state.

A case of false hypoglycemia by SMBG due to improper storage of glucometer test strips

Many types of glucometer test strips deteriorate in the presence of humidity. Storing test strips outside their container or removing them from their individual wrappers could worsen their measurement accuracy. While the measurement accuracy can gradually deteriorate under such conditions, some patients are unaware of this aspect of test strip care and handling. Although such a deterioration in glucometer accuracy is often seen in clinical practice, there are few reports that warn about this issue. This is the first case report of pseudohypoglycemia caused by the inappropriate storage of test strips, which resulted in inadequate use of medications and deterioration of glycemic control. In clinical practice, patients sometimes handle glucometers in unexpected ways. This can arise due to inadequate instruction. Patients should be advised to read the manual associated with the device carefully and to store the test strips in their original container, closing the cap tightly, and to take each test strip out of its individual wrapper just before it is used. Careful patient instruction is needed to ensure safe and stable diabetic control.