Yoshifumi Tamura

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.

Gut Dysbiosis and Detection of “Live Gut Bacteria” in Blood of Japanese Patients With Type 2 Diabetes

OBJECTIVE Mounting evidence indicates that the gut microbiota are an important modifier of obesity and diabetes. However, so far there is no information on gut microbiota and “live gut bacteria” in the systemic circulation of Japanese patients with type 2 diabetes. RESEARCH DESIGN AND METHODS Using a sensitive reverse transcription–quantitative PCR (RT-qPCR) method, we determined the composition of fecal gut microbiota in 50 Japanese patients with type 2 diabetes and 50 control subjects, and its association with various clinical parameters, including inflammatory markers. We also analyzed the presence of gut bacteria in blood samples. RESULTS The counts of the Clostridium coccoides group, Atopobium cluster, and Prevotella (obligate anaerobes) were significantly lower (P < 0.05), while the counts of total Lactobacillus (facultative anaerobes) were significantly higher (P < 0.05) in fecal samples of diabetic patients than in those of control subjects. Especially, the counts of Lactobacillus reuteri and Lactobacillus plantarum subgroups were significantly higher (P < 0.05). Gut bacteria were detected in blood at a significantly higher rate in diabetic patients than in control subjects (28% vs. 4%, P < 0.01), and most of these bacteria were Gram-positive. CONCLUSIONS This is the first report of gut dysbiosis in Japanese patients with type 2 diabetes as assessed by RT-qPCR. The high rate of gut bacteria in the circulation suggests translocation of bacteria from the gut to the bloodstream.

Impact of Oxidative Stress and Peroxisome Proliferator–Activated Receptor γ Coactivator-1α in Hepatic Insulin Resistance

OBJECTIVE—Recent studies identified accumulation of reactive oxygen species (ROS) as a common pathway causing insulin resistance. However, whether and how the reduction of ROS levels improves insulin resistance remains to be elucidated. The present study was designed to define this mechanism. RESEARCH DESIGN AND METHODS—We investigated the effect of overexpression of superoxide dismutase (SOD)1 in liver of obese diabetic model (db/db) mice by adenoviral injection. RESULTS—db/db mice had high ROS levels in liver. Overexpression of SOD1 in liver of db/db mice reduced hepatic ROS and blood glucose level. These changes were accompanied by improvement in insulin resistance and reduction of hepatic gene expression of phosphoenol-pyruvate carboxykinase and peroxisome proliferator–activated receptor γ coactivator-1α (PGC-1α), which is the main regulator of gluconeogenic genes. The inhibition of hepatic insulin resistance was accompanied by attenuation of phosphorylation of cAMP-responsive element-binding protein (CREB), which is a main regulator of PGC-1α expression, and attenuation of Jun NH2-terminal kinase (JNK) phosphorylation. Simultaneously, overexpression of SOD1 in db/db mice enhanced the inactivation of forkhead box class O1, another regulator of PGC-1α expression, without the changes of insulin-induced Akt phosphorylation in liver. In hepatocyte cell lines, ROS induced phosphorylation of JNK and CREB, and the latter, together with PGC-1α expression, was inhibited by a JNK inhibitor. CONCLUSIONS—Our results indicate that the reduction of ROS is a potential therapeutic target of liver insulin resistance, at least partly by the reduced expression of PGC-1α.

Protein Kinase Cδ Plays a Non-redundant Role in Insulin Secretion in Pancreatic β Cells

Protein kinase C (PKC) is considered to modulate glucose-stimulated insulin secretion. Pancreatic β cells express multiple isoforms of PKCs; however, the role of each isoform in glucose-stimulated insulin secretion remains controversial. In this study we investigated the role of PKCδ, a major isoform expressed in pancreatic β cells on β cell function. Here, we showed that PKCδ null mice manifested glucose intolerance with impaired insulin secretion. Insulin tolerance test showed no decrease in insulin sensitivity in PKCδ null mice. Studies using islets isolated from these mice demonstrated decreased glucose- and KCl-stimulated insulin secretion. Perifusion studies indicated that mainly the second phase of insulin secretion was decreased. On the other hand, glucose-induced influx of Ca2+ into β cells was not altered. Immunohistochemistry using total internal reflection fluorescence microscopy and electron microscopic analysis showed an increased number of insulin granules close to the plasma membrane in β cells of PKCδ null mice. Although PKC is thought to phosphorylate Munc18-1 and facilitate soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors complex formation, the phosphorylation of Munc18-1 by glucose stimulation was decreased in islets of PKCδ null mice. We conclude that PKCδ plays a non-redundant role in glucose-stimulated insulin secretion. The impaired insulin secretion in PKCδ null mice is associated with reduced phosphorylation of Munc18-1.

Nateglinide Reduces Carotid Intima-Media Thickening in Type 2 Diabetic Patients Under Good Glycemic Control

Objective—Postprandial hyperglycemia observed in type 2 diabetes mellitus is a risk factor for atherosclerosis. The aim of this study was to investigate the effect of strict glycemic control by nateglinide on common carotid far wall intima-media thickness in type 2 diabetic patients who were already under good glycemic control. Methods and Results—We performed an open labeled randomized prospective trial on 78 drug-naive type 2 diabetic patients whose HbA1c was less than 6.5%. Thirty-eight patients were randomly assigned to receive nateglinide (270 mg/dL) and 40 to control group (no treatment). After 12 months, a significant reduction in HbA1c was observed in the nateglinide group, whereas a significant increase of HbA1c was observed in the untreated group. The carotid intima-media thickness at the end of 1-year follow-up was significantly reduced in the nateglinide group compared with the untreated group (−0.017±0.054 mm/year versus 0.024±0.066 mm/year, P=0.0064). Whereas nateglinide treatment also reduced triglyceride, highly-sensitive C-reactive protein, and E-selectin, multiple regression analysis identified HbA1c as the only significant independent determinant of the change in carotid intima-media thickness. Conclusion—In type 2 diabetic patients with good glycemic control, further strict glycemic control by nateglinide results in regression of carotid intima-media thickness.

Blood flow restriction by low compressive force prevents disuse muscular weakness

Repetitive blood flow restriction prevents muscular atrophy and weakness induced by chronic unloading. However, it was unclear which external compressive force for blood flow restriction was optimal to prevent muscular dysfunction. The present study was intended to investigate the effects of repeated muscle blood flow restriction at low pressure on muscular weakness induced by immobilization without weight bearing. Using casts, the left ankles of 11 healthy males were immobilized for 2 weeks. Subjects were instructed to walk using crutches with no weight bearing during the period. Subjects were divided randomly into two groups: a restriction of blood flow (RBF) group (application of external compressive force of 50 mm Hg) and a control (CON) group (no intervention). We measured changes in the muscle strength of the knee extensor-flexor and ankle plantar flexor. The percent changes in knee extensor torque at 60°/s under eccentric contraction in the RBF group were significantly smaller than in the CON group (-12.5±10.7% and -30.1±10.9%, p<0.05). The percent changes in knee flexor torque when performing an eccentric contraction at 60°/s, an isometric contraction, or a concentric contraction at both 60 and 300°/s in the RBF group were significantly smaller than those in the CON group (p<0.05). In conclusion, our results show that repetitive restriction of blood flow with 50 mm Hg cuff pressure to the lower extremity reduces muscular weakness induced by chronic unloading.

Amelioration of glucose tolerance by hepatic inhibition of nuclear factor κB in db/db mice

Aims/hypothesisRecent studies have identified the involvement of inhibitor IκB kinase (IKK) in the pathogenesis of insulin resistance. To investigate the mechanism involved, we examined the role of nuclear factor κB (NF-κB), the distal target of IKK, in hepatic glucose metabolism.MethodsTo inhibit NF-κB activity, db/db mice were infected with adenovirus expressing the IκBα super-repressor.ResultsThe IκBα super-repressor adenovirus infection caused a moderate reduction of NF-κB activity in liver. The treatment was associated with improved glucose tolerance, reduction in the serum insulin level, and increased hepatic triacylglycerol and glycogen contents, but had no effect on insulin-stimulated phosphorylation of Akt. On the other hand, quantification of mRNA in the liver revealed marked reduction of expression of gluconeogenic genes, such as those encoding phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase, concurrent with reduced expression of gene encoding peroxisome proliferator-activated receptor gamma coactivator-1α (PPARGC1A, also known as PGC-1α). Furthermore, the production of super-repressor IκBα suppressed the increase in blood glucose level after pyruvate injection.Conclusions/interpretationOur results indicate that moderate inhibition of NF-κB improved glucose tolerance through decreased gluconeogenesis associated with reduced PGC-1α gene expression in db/db mice, and suggest that inhibition of NF-κB activity in liver is a potentially suitable strategy for the normalisation of blood glucose concentration in type 2 diabetes.

White Matter Alteration in Metabolic Syndrome Diffusion tensor analysis

OBJECTIVE We explored the regional pattern of white matter alteration in subjects with metabolic syndrome. We also investigated whether white matter alteration was correlated with BMI. RESEARCH DESIGN AND METHODS Seven middle-aged men with metabolic syndrome and seven without metabolic syndrome underwent diffusion tensor imaging with a 3T magnetic resonance imaging imager. We analyzed the fractional anisotropy (FA) values by using a tract-based spatial statistics technique (whole-brain analysis). We subsequently focused on measuring the mean FA values of the right inferior fronto-occipital fasciculus (IFOF) of all subjects by tract-specific analysis (regional brain analysis). We used a Pearson correlation coefficient to evaluate the relationship between BMI and mean FA values of the right IFOF. RESULTS In the whole-brain analysis, subjects with metabolic syndrome had significantly lower FA values than control subjects in part of the right external capsule (part of the right IFOF), the entire corpus callosum, and part of the deep white matter of the right frontal lobe. In the regional brain analysis, the mean FA value of the right IFOF was 0.41 ± 0.03 for subjects with metabolic syndrome and 0.44 ± 0.05 for control subjects. A significant negative correlation was observed between BMI and FA values in the right IFOF (r = −0.56, P < 0.04). CONCLUSIONS Our results show that microstructural white matter changes occur in patients with metabolic syndrome. FA values may be useful indices of white matter alterations in patients with metabolic syndrome.

Short-term effects of dietary fat on intramyocellular lipid in sprinters and endurance runners.

The effect of short-term fat loading on intramyocellular lipid (IMCL) in different types of muscle in endurance runners and sprinters has not been fully elucidated yet. The purpose of this study was to investigate the effect of dietary lipid on IMCL in soleus muscle (SOL) and tibialis anterior muscle (TA) during training period in endurance runners and sprinters. Seven male endurance runners and 7 male sprinters were selected to participate in the study. We measured TA- and SOL-IMCL levels after 3-day course of isocaloric normal- (25%), high- (60%), and low-fat (10%) diet during training period by (1)H-magnetic resonance spectroscopy in each subject. In sprinters, TA- and SOL-IMCL levels were comparable after each diet protocol. However, in endurance runners, TA-IMCL levels after normal-fat and high-fat diets were 1.7 times and 3.0 times higher than that after low-fat diet, respectively. The SOL-IMCL values after normal-fat diet and high-fat diet were 1.5 times and 1.6 times higher than that after low-fat diet, respectively. In addition, the TA-IMCL level after high-fat diet, but not SOL-IMCL, was significantly higher compared with that after normal-fat diet. Our data suggested that short-term dietary fat challenge during training period significantly altered IMCL level in endurance runners, but not in sprinters. In addition, response to fat loading on IMCL was influenced by variation of muscle type in endurance runners. These phenotypic and regional differences might be explained by differences in type of exercise training and muscle fiber composition.

Effects of metformin on peripheral insulin sensitivity and intracellular lipid contents in muscle and liver of overweight Japanese subjects

Aim:  Recent studies suggest that insulin resistance is associated with increased intrahepatic lipid (IHL) and intramyocellular lipid (IMCL) contents. While metformin improves insulin resistance mainly in liver, its effects on IHL and IMCL have not been clarified yet. The aim of this study was to investigate the effects of low‐dose metformin (750 mg/day) on peripheral insulin sensitivity, IHL and IMCL.