Mitsuhisa Komatsu

Dual Functional Role of Membrane Depolarization/Ca2+ Influx in Rat Pancreatic B-Cell

Transient exposure of rat pancreatic B-cell to 50 mM K+ ([K+50]) makes exocytosis unresponsive to further depolarization, i.e., stimulation with 100 mM K+ or 1 uM glyburide, which closes the ATP-sensitive K+ (K+ATP) channel, simultaneously with [K+50] does not produce any greater insulin secretion compared with [K+50] alone. In sharp contrast, 16.7 mM glucose ([G16.7]) applied simultaneously with [K+50] elicits an insulin response markedly greater than that produced by [K+50] alone, which is not attenuated by 100 uM diazoxide, an inhibitor of K+ATP channel closure. [G16.7]-Induced Insulin secretion at the basal K+ concn of 4.7 mM was greatly (93%) suppressed by 100 uM diazoxide. Insulin secretion induced by [K+50] plus [G16.7] ([K+50 + G16.7]) was markedly suppressed (70%) by 1 uM nifedipine, a Ca2+-channel blocker and was completely abolished by 2 mM 2-cyclohexen-1-one, which reportedly decreases reduced glutathione level and blocks glucokinase. This finding indicates that insulin release induced by [K+50 + G16.7] is not due to leakage produced by toxic stimuli but to activation of exocytosis. When graded concentrations (25 and 50 mM) of K+ were applied simultaneously with [G16.7] in the presence of 100 uM diazoxide, insulin response was clearly dependent on K+ concentration, indicating that the physiological range of membrane depolarization also activates the glucose-responsive effector. Membrane depolarization/Ca2+ influx directly stimulates hormone exocytosis on one hand and activates the K+ATP channel-independent glucose-responsive effector or effectors on the other in the B-cell. The nature of the glucose-responsive effector or effectors remains to be established.

Augmentation of Insulin Release by Glucose in the Absence of Extracellular Ca2+: New Insights Into Stimulus-Secretion Coupling

Glucose stimulates insulin secretion in the pancreatic β-cell by means of a synergistic interaction between at least two signaling pathways. One, the KATP channel-dependent pathway, increases the entry of Ca2+ through voltage-gated channels by closure of the KATP channels and depolarization of the β-cell membrane. The resulting increase in [Ca2+]i stimulates insulin exocytosis. The other, a KATP channel-independent pathway, requires that [Ca2+]i be elevated and augments the Ca2+-stimulated release. These mechanisms are in accord with the belief that glucose-stimulated insulin secretion has an essential requirement for extracellular Ca2+ and increased [Ca2+]i. However, when protein kinases A and C are activated simultaneously, a large effect of glucose to augment insulin release can be seen in the absence of extracellular Ca2+, under conditions in which [Ca2+]i is not increased, and even when [Ca2+]i is decreased to low levels by intracellular chelation with BAPTA. In the presence or absence of Ca2+, there are similarities in the characteristics of augmentation of insulin release that suggest that only one augmentation mechanism may be involved. These similarities include time course, glucose dose-responses, augmentation by nutrients other than glucose such as α-ketoisocaproate (α-KIC), and augmentation by the fatty acids palmitate and myristate. However, augmentation in the presence and absence of Ca2+ is distinctly different in GTP dependency. Therefore, exocytosis under these two conditions appears to be triggered differently—one by Ca2+ and the other by GTP or a GTPdependent mechanism. The augmentation pathways are likely responsible for time-dependent potentiation of secretion and for the second phase of glucose-stimulated insulin release.

Selective Attenuation of Metabolic Branch of Insulin Receptor Down-signaling by High Glucose in a Hepatoma Cell Line, HepG2 Cells

The effects of a high concentration of glucose on the insulin receptor-down signaling were investigated in human hepatoma (HepG2) cells in vitro to delineate the molecular mechanism of insulin resistance under glucose toxicity. Treatment of the cells with high concentrations of glucose (15–33 mm) caused phosphorylation of serine residues of the insulin receptor substrate 1 (IRS-1), leading to reduced electrophoretic mobility of it. The phosphorylation of IRS-1 with high glucose treatment was blocked only by protein kinase C (PKC) inhibitors. The high glucose treatment attenuated insulin-induced association of IRS-1 and phosphatidylinositol 3-kinase and insulin-stimulated phosphorylation of Akt. A metabolic effect of insulin, stimulation of glycogen synthesis, was also inhibited by the treatment. In contrast, insulin-induced association of Shc and Grb2 was not inhibited. Treatment of the cells with high glucose promoted the translocation of PKCε and PKCδ from the cytosol to the plasma membrane but not that of other PKC isoforms. Finally, PKCε and PKCδ directly phosphorylated IRS-1 under cell-free conditions. We conclude that a high concentration of glucose causes phosphorylation of IRS-1, leading to selective attenuation of metabolic signaling of insulin. PKCε and PKCδ are involved in the down-regulation of insulin signaling, and they may lie in a pathway regulating the phosphorylation of IRS-1.

Mastoparan, a wasp venom, stimulates insulin release by pancreatic islets through pertussis toxin sensitive GTP-binding protein

A wasp venom, mastoparan, rapidly stimulated insulin release by rat pancreatic islets in a dose-related manner. The amount of insulin released in response to 58 microM mastoparan far exceeded that induced by 27.8 mM glucose. Mastoparan stimulated insulin release to similar degrees at ambient glucose concentrations of 1.7 mM and 5.6 mM. The islets obtained from pertussis toxin-treated rats showed unequivocally less response to mastoparan. Pretreatment of islets with bromophenacyl bromide, a phospholipase A2 inhibitor, abolished their responsiveness to mastoparan. Pretreatment of islets with nifedipine, a Ca2+ channel blocker, was without effect. Mastoparan is a unique stimulator of insulin release by the pancreatic islets, which acts through GTP-binding protein(s) and phospholipase A2.

Glucose‐stimulated insulin secretion: A newer perspective

Existing concepts and models for glucose‐stimulated insulin secretion (GSIS) are overviewed and a newer perspective has been formulated toward the physiological understanding of GSIS. A conventional model has been created on the basis of in vitro data on application of a square wave high glucose in the absence of any other stimulatory inputs. Glucose elicits rapid insulin release through an adenosine triphosphate‐sensitive K+ channel (KATP channel)‐dependent mechanism, which is gradually augmented in a KATP channel‐independent manner. Biphasic GSIS thus occurs. In the body, the β‐cells are constantly exposed to stimulatory signals, such as glucagon‐like peptide 1 (GLP‐1), parasympathetic inputs, free fatty acid (FFA), amino acids and slightly suprathreshold levels of glucose, even at fasting. GLP‐1 increases cellular cyclic adenosine monophosphate, parasympathetic stimulation activates protein kinase C, and FFA, amino acids and glucose generate metabolic amplification factors. Plasma glucose concentration gradually rises postprandially under such tonic stimulation. We hypothesize that these stimulatory inputs together make the β‐cells responsive to glucose independently from its action on KATP channels. Robust GSIS in patients with a loss of function mutation of the sulfonylurea receptor, a subunit of KATP channels, is compatible with this hypothesis. Furthermore, pre‐exposure of the islets to an activator of protein kinase A and/or C makes β‐cells responsive to glucose in a KATP channel‐ and Ca2+‐independent manner. We hypothesize that GSIS occurs in islet β‐cells without glucose regulation of KATP channels in vivo, for which priming with cyclic adenosine monophosphate, protein kinase C and non‐glucose nutrients are required. To understand the physiology of GSIS, comprehensive integration of accumulated knowledge is required.

Clinical features of a new disease concept, IgG4-related thyroiditis

Objectives: Immunoglobulin (Ig)G4-related disease is a recently proposed systemic disorder that includes autoimmune pancreatitis (AIP), Mikulicz’s disease, and various other organ lesions. In the present retrospective study, we examined whether thyroid lesions should also be included in IgG4-related disease (Ig4-RD) under the new term IgG4-related thyroiditis. Method: We enrolled 114 patients with Ig4-RD, including 92 patients with AIP, 15 patients with Mikulicz’s disease, and seven patients with IgG4-related cholangitis, and analysed clinical findings, function, serum values of activity markers, computed tomography (CT) images, and histology of the thyroid gland. Results: Among the 22 patients (19%) in our cohort who were found to have hypothyroidism [thyroid stimulating hormone (TSH) > 4 mIU/L], 11 patients had clinical hypothyroidism [free thyroxine (FT4) < 1 ng/dL] and 11 patients had subclinical hypothyroidism (FT4 ≥ 1 ng/dL). Serum concentrations of IgG, IgG4, circulating immune complex (CIC), and β2-microglobulin (β2-MG) were significantly higher in the hypothyroidism group compared with the remaining 92 euthyroid patients, and serum C3 concentration was significantly lower. After prednisolone treatment, TSH values had decreased significantly (p = 0.005) in this group and FT4 values had increased significantly (p = 0.047). CT images showed that the thyroid glands of patients with clinical hypothyroidism had a significantly greater volume than those of the euthyroid and other groups. Pathological analysis of one resected thyroid gland disclosed a focused lesion with infiltration of lymphocytes and IgG4-bearing plasma cells and loss of thyroid follicles. Conclusions: Thyroid lesions associated with hypothyroidism can be considered as a new disease termed IgG4-related thyroiditis. Awareness of this condition should lead to appropriate corticosteroid treatment that may prevent progression to a fibrous state.

Importance of hypercoagulability over hyperglycemia for vascular complication in type 2 diabetes

To critically evaluate the relative importance of coagulation abnormalities over other clinical variables for micro- and macrovascular diabetic complications, prothrombin fragment (F1+2), thrombin-antithrombin III complex (TAT), fibrin degradation product d-dimer, and alpha2 plasmin inhibitor complex were determined in 101 stable, relatively well controlled patients with Type 2 diabetes (the mean HbA1c, age and duration of diabetes, 7.1%, 61 and 7.5 years, respectively). First, incidence and severity of diabetic micro- and macroangiopathies were progressively increased with the severity of coagulation abnormalities. Next, correlation of the four values with the presence of micro- and macrovascular complications, respectively, was analyzed by the multiple logistic regression analysis, with the inclusion of other variables such as age, duration of diabetes, HbA1c, blood pressure, and urinary albumin excretion. With the presence of microangiopathies, F1+2 and systolic blood pressure were significantly related, with the relationship being very strong for the former (P=0.003) and weak for the latter (P=0.035). On the other hand, with the presence of macroangiopathies, F1+2 (P=0.003), TAT (P=0.002), duration of diabetes (P=0.015), and age (P=0.013) were related. Other clinical variables were not significantly related with the presence of complications. Coagulation and fibrinolytic abnormalities are stronger determinants of the presence of diabetic vascular complications than other clinical variables including the degree of glycemia, in stable, relatively well controlled patients with Type 2 diabetes.

Gender-specific regulation of response to thyroid hormone in aging

BackgroundSimilar to other systems, the endocrine system is affected by aging. Thyroid hormone, the action of which is affected by many factors, has been shown to be associated with longevity. The most useful marker for the assessment of thyroid hormone action is TSH level. Although age and gender are believed to modify the pituitary set point or response to free thyroid hormone concentration, the precise age- and gender-dependent responses to thyroid hormone have yet to be reported.MethodsWe analyzed the results of 3564 thyroid function tests obtained from patients who received medication at both out- and inpatient clinics of Shinshu University Hospital. Subjects were from among those with thyroid function test results in the normal or mildly abnormal range. Based on a log-linear relationship between the concentrations of FHs and TSH, we established the putative resistance index to assess the relation between serum FH and TSH levels.ResultsFree thyroid hormone and TSH concentration showed an inverse log-linear relation. In males, there was a negative relationship between the free T3 resistance index and age. In females, although there were no relationships between age and FHs, the indices were positively related to age.ConclusionsThese findings indicated that there is a gender-specific response to thyroid hormone with aging. Although the TSH level is a useful marker for the assessment of peripheral thyroid hormone action, the values should be interpreted carefully, especially with regard to age- and gender-related differences.

Rab27a: a new face in β cell metabolism-secretion coupling

In pancreatic β cells, not only insulin exocytosis per se, but translocation of β granules toward the plasma membrane — an event upstream of exocytosis — are under the control of glucose. However, the molecular basis of this translocation has been poorly understood. Rab27a-mediated translocation of glucose-induced β granules is reported in this issue of the JCI. Rab27a or its effector molecule may constitute a novel pharmacological target because potentiation of the Rab27a pathway is expected to restore β cell glucose competency in patients with diabetes mellitus.

Diminution of early insulin response to glucose in subjects with normal but minimally elevated fasting plasma glucose. Evidence for early beta-cell dysfunction

Aim Systematic analysis of β‐cell function in Japanese health examinees.