Kazuki Tajima

Yokohama City University, Yokohama, Kanagawa, Japan

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Publications (14)56.06 Total impact

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    ABSTRACT: We investigated the role of insulin receptor substrate (Irs)-1 for diethylnitrosamine (DEN) plus high-fat (HF) diet-induced hepatic tumorigenesis in mice. We gave DEN by intraperitoneal injection at the dose of 80 mg/kg to 18-week-old wild-type (WT) and Irs1-knockout (Irs1−/−) mice, which were fed a HF diet from 8 weeks-of-age until they were killed (52 weeks). The Irs1−/− mice showed significantly lower plasma alanine aminotransferase levels, triglyceride contents in the liver and also lower expression levels of the genes encoding inflammatory cytokines than the WT mice. The incidence of DEN plus HF diet-induced hepatic tumors was 71.4% in the WT mice, whereas it was just 14.3% in the Irs1−/− mice. The present study showed that Irs1 played an important role in DEN plus HF diet-induced hepatic tumorigenesis.
    02/2014; 5(1). DOI:10.1111/jdi.12128
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    ABSTRACT: The glucokinase-induced upregulation of insulin receptor substrate 2 (IRS-2) plays an important role in β cell adaptive proliferation in response to high-fat diet-induced insulin resistance. This study aimed to investigate the role of IRS-2 in the proliferation of β cells following a 60% partial pancreatectomy. IRS-2 deficient (IRS-2(-/-)) mice or wild-type (WT) mice were subjected to a pancreatectomy (60% partial pancreatectomy) or a sham operation (Sham). The β cell proliferation and gene expression profiles of the islets were then assessed. Gene expression in islets from pancreatectomized and sham-operated C57BL/6J male mice was analyzed using a cDNA microarray analysis. To compare with β cell proliferation induced by a high-fat diet, Gck(+/-) mice subjected to a pancreatectomy were also analyzed. The IRS-2(-/-) mice exhibited β cell expansion and a significant increase in β cell proliferation after the pancreatectomy, compared with the Sham group. Although glucose stimulated insulin secretion from islets was not impaired, IRS-2(-/-) mice manifested severe hyperglycemia after the pancreatectomy. The expression levels of Aurora kinase B (AurkB), Cyclin A, and Cyclin B1 in the pancreatectomized islets were also enhanced in the IRS-2(-/-) mice. A gene set enrichment analysis (GSEA) suggested an association between the genes that were upregulated in the pancreatectomized islets and those involved in M phase progression in the cell cycle. β cell proliferation after a pancreatectomy was observed even in the Gck(+/-) mice. In conclusion, IRS-2 was not required for β cell proliferation, but might be needed for functional β cell mass, after a pancreatectomy. A partial pancreatectomy in mice may be an attractive model for the development of new strategy for exploring the unique nature of β cell proliferation.
    Endocrinology 02/2014; 155(5):en20131796. DOI:10.1210/en.2013-1796 · 4.64 Impact Factor
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    ABSTRACT: We previously reported that combination therapy with an α-glucosidase inhibitor (αGI) and a dipeptidyl peptidase-4 (DPP-4) inhibitor increased active glucagon-like peptide-1 (GLP-1) levels and decreased total glucose-dependent insulinotropic polypeptide (GIP) levels, compared with monotherapy, in non-diabetic men. However, the peptide YY (PYY), cholecystokinin (CCK), ghrelin, and obestatin levels in patients receiving a combination of αGIs and DPP-4 inhibitors have not been previously reported. We evaluated the effect of miglitol, vildagliptin, or their combination on these parameters. Miglitol and/or vildagliptin were administered according to four different intake schedules in eleven non-diabetic men (C: no drug, M: miglitol; V: vildagliptin, M+V: miglitol+vildagliptin). Blood samples were collected at 0, 30, 60, and 120 min after the start of breakfast. The plasma glucose, serum insulin, serum total PYY (PYY1-36 and PYY3-36), plasma CCK, plasma active ghrelin, and plasma obestatin levels were measured. The area under the curve (AUC) of the serum total PYY level in the M group was significantly greater than that in the C group, and the AUC of the serum total PYY level in the M+V group was significantly lower than that in the M group. The combination therapy did not change the AUC of the plasma CCK, plasma active ghrelin, plasma obestatin, and ghrelin/obestatin levels, compared with the control. The results of our study suggested that combination therapy with miglitol and vildagliptin had no effect on appetite regulation hormones, such as total PYY, CCK, active ghrelin, and obestatin, compared with the levels in the control group.
    Endocrine Journal 12/2013; DOI:10.1507/endocrj.EJ13-0399 · 2.02 Impact Factor
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    ABSTRACT: The prevalence of nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) is increasing with the growing epidemics of obesity and diabetes. NAFLD encompasses a clinicopathologic spectrum of disease ranging from isolated hepatic steatosis to NASH, which is a more aggressive form of fatty liver disease, to cirrhosis and finally, hepatocellular carcinoma (HCC). The exact mechanism behind the development of HCC in NASH remains unclear; however it has been established that hepatic steatosis is the important risk factor in the development of HCC. Metformin has recently drawn attention because of its potential antitumor effect. Here, we investigated the effects of metformin on high-fat diet (HFD)-induced liver tumorigenesis using a mouse model of NASH and liver tumor. Metformin prevented long-term HFD-induced liver tumorigenesis in C57Bl/6 mice. Of note, metformin failed to protect against liver tumorigenesis in mice that had already begun to develop NAFLD. Metformin improved short-term HFD-induced fat accumulation in the liver, associated with the suppression of adipose tissue inflammation. Collectively, these results suggest that metformin may prevent liver tumorigenesis via suppression of liver fat accumulation in the early stage, before the onset of NAFLD, which seems to be associated with a delay in the development of inflammation of the adipose tissue.
    AJP Endocrinology and Metabolism 08/2013; 305(8). DOI:10.1152/ajpendo.00133.2013 · 4.09 Impact Factor
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    ABSTRACT: The derangement of endoplasmic reticulum (ER) homeostasis triggers β-cell apoptosis, leading to diabetes. Glucokinase up-regulates IRS-2 expression in β cell, but the role of glucokinase and IRS-2 in ER stress has been unclear. In this study, we investigated the impact of glucokinase activation by glucokinase activator (GKA) on ER stress in β cells. GKA administration improved β-cell apoptosis in Akita mice, a model of ER stress-mediated diabetes. GKA increased the expression of IRS-2 in β cells, even under ER stress. Both glucokinase-deficient Akita mice and IRS-2-deficient Akita mice exhibited an increase in β-cell apoptosis, compared with Akita mice. β-cell-specific IRS-2-overexpressing (βIRS2Tg) Akita mice showed less β-cell apoptosis than Akita mice. IRS-2-deficient islets were vulnerable, but βIRS2Tg islets were resistant to ER stress-induced apoptosis. Meanwhile, GKA regulated the expressions of CHOP and other ER stress-related genes in an IRS-2-independent fashion in islets. GKA suppressed the expressions of CHOP and Bax and protected against β-cell apoptosis under ER stress in an ERK1/2-dependent, IRS-2-independent manner. Taken together, GKA ameliorated ER stress-mediated apoptosis by harmonizing IRS-2 upregulation and the IRS-2-independent control of apoptosis in β cells.
    Diabetes 06/2013; 62(10). DOI:10.2337/db13-0052 · 8.47 Impact Factor
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    ABSTRACT: The precise role of AMP-activated protein kinase (AMPK), a target of metformin, in pancreatic β cells remains controversial, even though metformin was recently shown to enhance the expression of incretin receptors (GLP-1 and GIP receptors) in pancreatic β cells. In this study, we investigated the effect of AMPK in the regulation of incretin receptors expression in pancreatic islets. The phosphorylation of AMPK in the mouse islets was decreased by increasing glucose concentrations. We showed the expression of incretin receptors in bell-shaped response to glucose; Expression of the incretin receptors in the isolated islets showed higher levels under a medium glucose concentration (11.1 mM) than that under a low glucose concentration (2.8 mM), but was suppressed under a high glucose concentration (22.2 mM). Both treatment with an AMPK inhibitor and DN-AMPK expression produced a significant increase of the incretin receptors expression under a low glucose concentration. By contrast, in hyperglycemic db/db islets, the enhancing effect of the AMPK inhibitor on the expression of incretin receptors was diminished under a low glucose concentration. Taken together, AMPK is involved in the regulation of incretin receptors expression in pancreatic islets under a low glucose concentration.
    PLoS ONE 05/2013; 8(5):e64633. DOI:10.1371/journal.pone.0064633 · 3.53 Impact Factor
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    ABSTRACT: AIMS: To compare the efficacy and safety of these two agents and the presence of surrogate markers related to diabetic complications in Japanese type 2 diabetic patients. MATERIALS AND METHODS: In a multicenter, open-label trial, 130 patients whose diabetes had been inadequately controlled (HbA1c, 6.9%-9.5%) with metformin and/or sulfonylurea were randomly assigned to a sitagliptin group (50 mg/day) or a pioglitazone group (15 mg/day) and were followed up for 24 weeks. At 16 weeks, if the patient's HbA1c level was ≥ 6.5%, the dose of sitagliptin or pioglitazone was increased up to 100 mg/day or 30 mg/day, respectively. Main outcome measure was the difference in the mean changes in the HbA1c level from baseline at 24 weeks between these two groups. RESULTS: Of the 130 patients who were enrolled, 115 subjects (sitagliptin group: 58 patients, pioglitazone group: 57 patients) completed this trial. At 0 weeks, the mean HbA1c level was 7.47 ± 0.66% in the sitagliptin group and 7.40 ± 0.61% in the pioglitazone group. At 24 weeks, the mean changes in the HbA1c level from baseline were -0.86 ± 0.63% vs. -0.58 ± 0.68% (P = 0.024). Hypoglycemia (2 patients, 3.4% vs. 2 patients, 3.5%), gastrointestinal symptoms (3 patients, 5.2% vs. 1 patient, 1.8%), and pretibial edema (0 patients, 0% vs. 39 patients, 68.4%, P < 0.001) were observed for 24 weeks. CONCLUSIONS: Sitagliptin was not only more tolerable, but more effective than pioglitazone in Japanese type 2 diabetic patients who had been treated with metformin and/or sulfonylurea.
    Diabetes Obesity and Metabolism 12/2012; 15(5). DOI:10.1111/dom.12055 · 5.46 Impact Factor
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    ABSTRACT: Decreased β-cell mass is a hallmark of type 2 diabetes, and therapeutic approaches to increase the pancreatic β-cell mass have been expected. In recent years, gastrointestinal incretin peptides have been shown to exert a cell-proliferative effect in pancreatic β-cells. Trefoil factor 2 (TFF2), which is predominantly expressed in the surface epithelium of the stomach, plays a role in antiapoptosis, migration, and proliferation. The TFF family is expressed in pancreatic β-cells, whereas the role of TFF2 in pancreatic β-cells has been obscure. In this study, we investigated the mechanism by which TFF2 enhances pancreatic β-cell proliferation. The effects of TFF2 on cell proliferation were evaluated in INS-1 cells, MIN6 cells, and mouse islets using an adenovirus vector containing TFF2 or a recombinant TFF2 peptide. The forced expression of TFF2 led to an increase in bromodeoxyuridine (BrdU) incorporation in both INS-1 cells and islets, without any alteration in insulin secretion. TFF2 significantly increased the mRNA expression of cyclin A2, D1, D2, D3, and E1 in islets. TFF2 peptide increased ERK1/2 phosphorylation and BrdU incorporation in MIN6 cells. A MAPK kinase inhibitor (U0126) abrogated the TFF2 peptide-mediated proliferation of MIN6 cells. A CX-chemokine receptor-4 antagonist also prevented the TFF2 peptide-mediated increase in ERK1/2 phosphorylation and BrdU incorporation in MIN6 cells. These results indicated that TFF2 is involved in β-cell proliferation at least partially via CX-chemokine receptor-4-mediated ERK1/2 phosphorylation, suggesting TFF2 may be a novel target for inducing β-cell proliferation.
    Endocrinology 11/2012; 154(1). DOI:10.1210/en.2012-1814 · 4.64 Impact Factor
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    ABSTRACT: AIMS/HYPOTHESIS: Epidemiological studies have revealed that obesity and diabetes mellitus are independent risk factors for the development of non-alcoholic steatohepatitis (NASH) and hepatocellular carcinoma. However, the debate continues on whether insulin resistance as such is directly associated with NASH and liver tumourigenesis. Here, we investigated the incidence of NASH and liver tumourigenesis in Irs1 ( -/- ) mice subjected to a long-term high-fat (HF) diet. Our hypothesis was that hepatic steatosis, rather than insulin resistance may be related to the pathophysiology of these conditions. METHODS: Mice (8 weeks old, C57Bl/6J) were given free access to standard chow (SC) or an HF diet. The development of NASH and liver tumourigenesis was evaluated after mice had been on the above-mentioned diets for 60 weeks. Similarly, Irs1 ( -/- ) mice were also subjected to an HF diet for 60 weeks. RESULTS: Long-term HF diet loading, which causes obesity and insulin resistance, was sufficient to induce NASH and liver tumourigenesis in the C57Bl/6J mice. Obesity and insulin resistance were reduced by switching mice from the HF diet to SC, which also protected these mice against the development of NASH and liver tumourigenesis. However, compared with wild-type mice fed the HF diet, Irs1 ( -/- ) mice fed the HF diet were dramatically protected against NASH and liver tumourigenesis despite the presence of severe insulin resistance and marked postprandial hyperglycaemia. CONCLUSIONS/INTERPRETATION: IRS-1 inhibition might protect against HF diet-induced NASH and liver tumourigenesis, despite the presence of insulin resistance.
    Diabetologia 09/2012; 55(12). DOI:10.1007/s00125-012-2703-1 · 6.88 Impact Factor
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    ABSTRACT: The glucagon-like peptide-1 receptor agonist liraglutide is used to treat diabetes. A hallmark of liraglutide is the glucose-dependent facilitation of insulin secretion from pancreatic β-cells. In β-cells, the glycolytic enzyme glucokinase plays a pivotal role as a glucose sensor. However, the role of glucokinase in the glucose-dependent action of liraglutide remains unknown. We first examined the effects of liraglutide on glucokinase haploinsufficient (Gck(+/-)) mice. Single administration of liraglutide significantly improved glucose tolerance in Gck(+/-) mice without increase of insulin secretion. We also assessed the effects of liraglutide on the survival rates, metabolic parameters, and histology of liver or pancreas of β-cell-specific glucokinase-deficient (Gck(-/-)) newborn mice. Liraglutide reduced the blood glucose levels in Gck(-/-) neonates but failed to prolong survival, and all the mice died within 1 wk. Furthermore, liraglutide did not improve glucose-induced insulin secretion in isolated islets from Gck(-/-) neonates. Liraglutide initially prevented increases in alanine aminotransferase, free fatty acids, and triglycerides in Gck(-/-) neonates but not at 4 d after birth. Liraglutide transiently prevented liver steatosis, with reduced triglyceride contents and elevated glycogen contents in Gck(-/-) neonate livers at 2 d after birth. Liraglutide also protected against reductions in β-cells in Gck(-/-) neonates at 4 d after birth. Taken together, β-cell glucokinase appears to be essential for liraglutide-mediated insulin secretion, but liraglutide may improve glycemic control, steatosis, and β-cell death in a glucokinase-independent fashion.
    Endocrinology 05/2012; 153(7):3066-75. DOI:10.1210/en.2012-1165 · 4.64 Impact Factor
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    ABSTRACT: We investigated a possible association between serum plasminogen activator inhibitor-1 (PAI-1) levels and renal dysfunction in 124 type 2 diabetes patients. Multiple linear regression analyses indicated that the PAI-1 levels were significantly inversely correlated with estimated glomerular filtration rate (eGFR) independent of albuminuria, BMI, LDL-C, and triglyceride.
    Diabetes research and clinical practice 04/2012; 97(1):e9-12. DOI:10.1016/j.diabres.2012.03.017 · 2.54 Impact Factor
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    ABSTRACT: Aims/Introduction: We investigated the relationship between non‐alcoholic steatohepatitis (NASH) and different stages of fasting plasma glucose (FPG) concentrations, and the association between factors related to glucose tolerance and severity of NASH. Materials and Methods: A total of 147 patients with non‐alcoholic fatty liver disease (NAFLD) who had undergone a liver biopsy were divided into three groups: a normal glucose tolerance (NGT) group, an impaired fasting glucose (IFG) group and a diabetes (DM) group. In addition, to investigate progression factors of NASH in the DM group, we divided the diabetic patients into two groups: a group with NASH (NASH group) and a group without NASH, the simple steatosis (SS) group. The relationship between the patients’ clinical parameters and the severity of NAFLD/NASH were analyzed. Results: In the patients with liver biopsies, the IFG group had the highest percentage of NASH. There was no correlation between FPG and either total NAFLD activity scores (NAS) or staging of NASH, but the fasting serum insulin was correlated significantly with both, even after adjusting for age, sex and body mass index. Among the diabetic patients, the fasting insulin values in the NASH group were significantly higher than in the SS group, but there were no differences in FPG or A1c values between the two groups. The fasting serum insulin correlated significantly with total NAS, but the FPG and A1c values did not. Conclusions: A high percentage of the IFG group developed NASH. Hyperinsulinemia, but not hyperglycemia, was associated with severity of NASH. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2011.00134.x, 2011)
    11/2011; 2(6):483-9. DOI:10.1111/j.2040-1124.2011.00134.x
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    ABSTRACT: Chronic exposure to high glucose and fatty acid levels caused by dietary sugar and fat intake induces β cell apoptosis, leading to the exacerbation of type 2 diabetes. Oleic acid and linoleic acid are two major dietary fatty acids, but their effects in diabetes are unclear. We challenged β cell-specific glucokinase haploinsufficient (Gck(+/-)) mice with a diet containing sucrose and oleic acid (SO) or sucrose and linoleic acid (SL) and analyzed β cell apoptosis. In Gck(+/-) but not wild-type mice, SL significantly decreased the β cell mass and β cell proportion in islet cells arising from increased apoptosis to a greater degree than did SO. The mRNA expression of SREBP-1c was significantly higher, and that of E-cadherin was significantly lower in the islets of Gck(+/-) mice fed SL compared with mice fed SO. We next evaluated monotherapy with desfluorositagliptin, a dipeptidyl peptidase-4 (DPP-4) inhibitor, in these mouse groups. DPP-4 inhibitor protected against β cell apoptosis, restored the β cell mass, and normalized islet morphology in Gck(+/-) mice fed SL. DPP-4 inhibition normalized the changes in the islet expression of SREBP-1c and E-cadherin mRNA induced by the SL diet. Furthermore, linoleic acid induced β cell apoptosis to a greater degree in the presence of high glucose levels than in the presence of low glucose levels in vitro in islets and MIN6 cells, whereas a GLP-1 receptor agonist prevented apoptosis. In conclusion, SL exacerbated β cell apoptosis in diabetic Gck(+/-) mice but not in euglycemic wild-type mice, and DPP-4 inhibition protected against these effects.
    Journal of Biological Chemistry 05/2011; 286(29):25467-76. DOI:10.1074/jbc.M110.217216 · 4.57 Impact Factor
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    ABSTRACT: Chronic exposure to high glucose and fatty acid levels caused by dietary sugar and fat intake induces β cell apoptosis, leading to the exacerbation of type 2 diabetes. Oleic acid and linoleic acid are two major dietary fatty acids, but their effects in diabetes are unclear. We challenged β cell-specific glucokinase haploinsufficient (Gck+/-) mice with a diet containing sucrose and oleic acid (SO) or sucrose and linoleic acid (SL) and analyzed β cell apoptosis. In Gck+/-, but not wild-type, mice, SL significantly decreased the β cell mass and β cell proportion in islet cells arising from increased apoptosis to a greater degree than SO. The mRNA expression of SREBP-1c was significantly higher and that of E-cadherin was significantly lower in the islets of Gck+/- mice fed SL, compared with mice fed SO. We next evaluated monotherapy with des-fluoro-sitagliptin, a dipeptidyl peptidase-4 (DPP-4) inhibitor, for them. DPP-4 inhibitor protected against β cell apoptosis, restored the β cell mass, and normalized islet morphology in Gck+/- mice fed SL. DPP-4 inhibition normalized the changes in the islet expression of SREBP-1c and E-cadherin mRNA induced by the SL diet. Furthermore, linoleic acid induced β cell apoptosis to a greater degree in the presence of high glucose levels than in the presence of low glucose levels in vitro in islets and MIN6 cells, whereas a GLP-1 receptor agonist prevented apoptosis. In conclusion, SL exacerbated β cell apoptosis in diabetic Gck+/- mice but not in euglycemic wild-type mice, and DPP-4 inhibition protected against these effects.
    Journal of Biological Chemistry 05/2011; · 4.57 Impact Factor