OBJECTIVEDeficits of b-cells characterize the islet pathology in type 2 diabetes. It is yet to be clear how the b-cell loss develops in type 2 diabetes. We explored the implication of oxidative stress, endoplasmic reticulum (ER)-induced stress, and autophagy deficit in the b-cell decline in Japanese type 2 diabetic patients. RESEARCH DESIGN AND METHODSPancreases from recent autopsy cases of 47 type 2 diabetic and 30 nondiabetic subjects were investigated on the islet structure with morphometric analysis. Volume densities of islet (Vi), b-cell (Vb), and a-cell (Va) were measured. To evaluate cell damage of endocrine cells, immunohistochemical expressions of oxidative stress-related DNA damage as expressed by gH2AX, ER stress-related cell damage as CCAAT/enhancer 1 binding protein-b (C/EBP-b), and autophagy deficit as P62 were semiquantified, and their correlations to islet changes were sought. RESULTSCompared with nondiabetic subjects, Vb was reduced in diabetic subjects. Contrariwise, there was an increase in Va. There was a significant link between reduced Vb and increased HbA 1c levels (P < 0.01) and a trend of inverse correlation between Vb and duration of diabetes (P = 0.06). Expressions of gH2AX, P62, and C/EBP-b were all enhanced in diabetic islets, and reduced Vb correlated with the intensity of gH2AX expression but not with C/EBP-b or P62 expressions. Combined expressions of gH2AX, P62, and C/EBP-b were associated with severe reduction of Vb. CONCLUSIONSb-Cell deficit in type 2 diabetes was associated with increased oxidative stress and may further be augmented by autophagic deficits and ER stress.

Aims/Introduction: Impaired growth and premature death of b-cells are implicated in the progression of islet pathology in type 2 diabetes. It remains unclear, however, how aging affects islet cells, or whether the islet change in diabetes is an augmented process of aging. We studied age-related changes of the islet structure in Japanese non-diabetic subjects and explored the underlying mechanism of the changes. Materials and Methods: A total of 115 non-diabetic autopsy cases were subjected to morphometric analysis for volume densities of islets, b-and non-b-cells, as well as their masses. Proliferation activity identified by Ki67, and expressions of pancreatic and duodenal homeobox (PDX)-1, cell cycle inhibitor P16, and oxidative stress marker cH2AX were also examined.Results: There was a gradual and marginal decline of volume densities of islets, b-and non-b-cells with aging, while masses of these components were increased during maturation and slowly decreased after the 40s. Islet density was high in the young, but reduced after maturation. There was only a minimal influence of increased body mass index (BMI) on the increase in b-cell mass, but not on the other variables. Ki67 positivity and PDX-1 expressions were high in the young, but low after maturation, whereas expressions of P16 and cH2AX were elevated in the aged. Conclusions: Age-associated decline of b-cell mass is marginal after maturation, and the reduction of b-cell mass could be a specific process in diabetes. The impact of BMI on the islet structure is limited in Japanese with normal glucose tolerance.

Prolactin (PRL) has roles in various physiological functions. Although experimental studies showed that PRL has both beneficial and adverse effects on type 2 diabetes mellitus, clinical findings in subjects with hyperprolactinemia indicate adverse effects on glucose metabolism. However, effects of PRL within the physiological range in human are controversial. A population-based study of 370 Japanese men enrolled in the 2014 Iwaki study (aged 52.0 ± 14.8 years). In this cross-sectional study, associations between serum PRL levels and homeostatic model assessment (HOMA) indices representing glucose metabolism in a physiological setting were examined using multivariable regression analysis. Although univariate linear regression analyses showed significant associations between serum PRL levels and HOMA indices, adjustment with multiple factors made the association with HOMA-ß (insulin secretion) insignificant, while those with HOMA-R (insulin resistance) remained significant (ß = 0.084, p = 0.035). Non-linear regression analyses showed a regression curve with a peak at serum PRL level, 12.4 ng/mL and a positive association of serum PRL level with HOMA-R below the peak (ß = 0.119, p = 0.004). Higher serum PRL levels within the physiological range seem to be associated with insulin resistance in men.

A sufficient b-cell mass is crucial for preventing diabetes, and perinatal b-cell proliferation is important in determining the adult b-cell mass. However, it is not yet known how perinatal b-cell proliferation is regulated. Here, we report that serotonin regulates b-cell proliferation through serotonin receptor 2B (HTR2B) in an autocrine/paracrine manner during the perinatal period. In b-cell-specific Tph1 knockout (Tph1 bKO) mice, perinatal b-cell proliferation was reduced along with the loss of serotonin production in b-cells. Adult Tph1 bKO mice exhibited glucose intolerance with decreased b-cell mass. Disruption of Htr2b in b-cells also resulted in decreased perinatal b-cell proliferation and glucose intolerance in adulthood. Growth hormone (GH) was found to induce serotonin production in b-cells through activation of STAT5 during the perinatal period. Thus, our results indicate that GH-GH receptor-STAT5-serotonin-HTR2B signaling plays a critical role in determining the b-cell mass by regulating perinatal b-cell proliferation, and defects in this pathway affect metabolic phenotypes in adults.

Long-term metabolic aberrations contribute to the development of diabetic neuropathy but the precise mechanism or mechanisms remains elusive. We have previously shown that aldose reductase deficient mice exhibit delayed onset and progression of neuropathy following induction of diabetes, suggesting a role both for downstream metabolites of this enzyme and also for other unrelated pathways. In the present study we have utilized comprehensive metabolomics analyses to identify potential neurotoxic metabolites in nerve of diabetic mice and explored the mechanism of peripheral nerve injury. Aldose reductase knockout and control C57Bl/6J mice were made diabetic by injection of streptozotocin and followed for 8-16 weeks. Diabetic aldose reductase knockout mice exhibited delayed onset of nerve conduction slowing compared to diabetic wild type mice. The sciatic nerves from aldose reductase knockout mice exposed to 12 weeks of diabetes were used for metabolomics analysis and compared with analyses of nerves from age-matched diabetic wild-type mice as well as non-diabetic aldose reductase knockout and wild-type mice. Neurotoxicity of candidate metabolites was evaluated using cultured Schwann cells and dorsal root ganglion neurons, and further confirmed in vivo. Metabolomics analysis identified elevated glucosamine levels in both diabetic aldose reductase knockout and diabetic wild mice. Exposure to glucosamine reduced survival of cultured Schwann cells and neurons accompanied by increased expression of cleaved caspase 3, CCAT-enhancer-binding homologous protein, and mitochondrial hexokinase-I, along with ATP depletion. These changes were suppressed by siRNA to hexokinase-I or the ATP donor, inosine, but not by the antioxidant N-acetylcysteine or the endoplasmic reticulum-stress inhibitor 4-phenylbutyrate. The O-GlcNAcylation enhancer, PUGNAc, did not augment glucosamine neurotoxicity. Single dose glucosamine injection into mice caused a reduction of sciatic nerve Na, K-ATPase activity, ATP content, and augmented expression of hexokinase-I, which were suppressed by pretreatment with inosine but not with 4-phenylbutyrate. Mice implanted with a subcutaneous pump to infuse glucosamine for 12 weeks developed nerve conduction slowing and intraepidermal nerve fiber loss, recapitulating prominent indices of diabetic neuropathy. While acute glucosamine neurotoxicity is unlikely to contribute substantially to the slowly-developing neuropathy phenotype in humans, sustained energy deprivation induced by glucosamine may well contribute to the pathogenesis of diabetic neuropathy. Our data thus identifies a novel pathway for diabetic neuropathy that may offer a potential new therapeutic target.