Letter to the Editor Reduction of pancreaticb-cell dedifferentiation after gastric bypass surgery in diabetic rats

Dear Editor, Type 2 diabetes mellitus (T2DM) develops only in insulin-resistant subjects when pancreatic b-cell compensation fails (Matveyenko and Butler, 2006). Decreased insulin secretory function and reduced cell mass are traditionally viewed as major contributing factors in b-cell insufficiency. A recent study using a diabetic rodent model suggests that progressive b-cell dedifferentiation is an important underlying mechanism in b-cell failure (Talchai et al., 2012). b-cell dedifferentiation in diabetes refers to the loss by healthy b-cells of key components characteristic of the differentiated state (Dor and Glaser, 2013), including insulin (for its secretory product), Glut2 (for glucose intake), and PDX-1 (for critical insulin transcription factor). b-cell dedifferentiation may be largely responsible for not only b-cell secretory dysfunction but also impaired b-cell identity. In view of findings that bariatric surgery in a rodent T2DM model led to increased b-cell mass and improved islet morphology (Strader et al., 2009), we investigated the effects of gastric bypass surgery on dedifferentiated b-cells. Roux-en-Y gastric bypass (RYGB), a type of bariatric surgery, is an effective surgical treatment for patients with morbid obesity. RYGB surgery also improved secretion of b-cells in response to intravenous glucose (Salinari et al., 2013) and completely resolved T2DM in a significant number of patients (Schauer et al., 2003). In animal studies, novel surgical approaches relieved diabetes in a rapid and sustained manner, independent of weight loss effects (Strader et al., 2009). In view of the trend away from bariatric surgery and toward metabolic surgery, we performed RYGB surgery on spontaneous T2DM Goto-Kakizaki (GK) rats (GK-S), a non-obese model with inherited b-cell deficits, to study weight lossindependent effects of RYGB on islets. Dedifferentiated b-cells were examined in this experimental group (GK-S) and two control groups: (i) sham-operated rats pair-fed with the GK-S group (GK-PF-Sham), and (ii) normal Wistar rats (Wistar). Pancreatic b-cell function after 3 months of RYGB surgery was evaluated by intravenous glucose tolerance tests (IVGTT). Both blood glucose levels at individual time points and total glucose level determined by area under the curve (AUC) reflected improvement of glycemic controls following RYGB surgery (Figure 1A-i,ii). Basal insulin levels were similar in the two GK groups. Plasma insulin levels after 2 min and 5 min of intravenous glucose load were significantly higher in GK-S group than in GK-PF-Sham group. AUC-insulin level within 5 min was alsosignificantlyhigher inGK-Sgroup, indicating improvement of first-phase insulin secretion following RYGB surgery (Figure 1A-iii,iv). These findings indicate that pancreatic b-cell secretory function is improved after RYGB surgery, in agreement with previous reports (Salinari et al., 2013). We then performed immunohistochemical analyses to assess islet structure in the pancreas. GK-PF-Sham pancreases contained predominantly irregularly shaped ‘broken’ islets, in which weaker insulin immunoreactive staining revealed uneven hypoglycemic factor expression in b-cells, whereas GK-S pancreases contained many normal islets indistinguishable from those in Wistar group (Figure 1B-i). Insulin content data showed a partial but significant recovery of insulin storage after surgery in total pancreaticb-cells within equivalent amounts of pancreas (Figure 1B-ii). Glucagon content data were comparable between the two GK groups (Figure 1B-iii). Thus, the insulin/ glucagon ratio was significantly higher after bypass surgery (GK-PF-Sham: 23.7 + 0.6 vs. GK-S: 32.4 + 1.1; P , 0.05, n 1⁄4 6). These morphological and pancreatic hormone content data also reflect functional improvement of GK-S b-cells. In a study by Talchai et al. (2012), the endocrine cell marker secretagogin (SCGN) was still present after the insulin staining disappeared in dedifferentiated b-cells. To distinguish immunoreactive areas between SCGN-positive and pancreatic hormone-positive cells, we measured expressions of four pancreatic hormones (glucagon, insulin, somatostatin, pancreatic polypeptide) and SCGN in islets by confocal microscopy (Supplementary Figure S1). Cells with strongly positive hormone staining were regarded as healthy endocrine cells, cells with reduced hormone staining were regarded as degranulated endocrine cells, and those SCGN-positive but hormone-negative were regarded as dedifferentiated endocrine cells (Figure 1C-i; Supplementary Figure S1). Endocrine cells consist of 15% 2 20% a-cells and 65% 2 80% b-cells. Since a-cells determined by glucagon expression display hyperfunction and increased mass in T2DM (Elayat, 1995; Figure 1B-iii and C-iii), we presume that the hormone ‘empty’ and ‘pale-staining’ endocrine cells were primarily dedifferentiated and degranulated b-cells. Islets from GK-S group, in comparison with GK-PF-Sham group, showed a significant reduction in doi:10.1093/jmcb/mju042 Journal of Molecular Cell Biology (2014), 6(6), 531–534 | 531 Published online October 31, 2014