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Human insulin (INS) gene diverged from the ancestral genes of invertebrate and mammalian species millions of years ago. We previously found that mouse insulin gene (Ins2) isoforms are expressed in brain choroid plexus (ChP) epithelium cells, where insulin secretion is regulated by serotonin and not by glucose. We further compared human INS isoform expression in postmortem ChP and islets of Langerhans. We uncovered novel INS upstream open reading frame isoforms and their protein products. In addition, we found a novel alternatively spliced isoform that translates to a 74-amino acid (AA) proinsulin containing a shorter 19-AA C-peptide sequence, herein designated Cα-peptide. The middle portion of the conventional C-peptide contains β-sheet (GQVEL) and hairpin (GGGPG) motifs that are not present in Cα-peptide. Islet amyloid polypeptide (IAPP) is not expressed in ChP, and its amyloid formation was inhibited in vitro more efficiently by Cα-peptide than by C-peptide. Of clinical relevance, the ratio of the 74-AA proinsulin to proconvertase-processed Cα-peptide was significantly increased in islets from type 2 diabetes mellitus autopsy donors. Intriguingly, 100 years after the discovery of insulin, we found that INS isoforms are present in ChP from insulin-deficient autopsy donors. [ABSTRACT FROM AUTHOR]

Eating a "Westernized" diet high in fat and sugar leads to weight gain and numerous health problems, including the development of type 2 diabetes mellitus (T2DM). Rodent studies have shown that resveratrol supplementation reduces blood glucose levels, preserves β-cells in islets of Langerhans, and improves insulin action. Although rodent models are helpful for understanding β-cell biology and certain aspects of T2DM pathology, they fail to reproduce the complexity of the human disease as well as that of nonhuman primates. Rhesus monkeys were fed a standard diet (SD), or a high-fat/high-sugar diet in combination with either placebo (HFS) or resveratrol (HFS+Resv) for 24 months, and pancreata were examined before overt dysglycemia occurred. Increased glucose-stimulated insulin secretion and insulin resistance occurred in both HFS and HFS+Resv diets compared with SD. Although islet size was unaffected, there was a significant decrease in β-cell and an increase in α-cells containing glucagon and glucagon-like peptide 1 with HFS diets. Islets from HFS+Resv monkeys were morphologically similar to SD. HFS diets also resulted in decreased expression of essential β-cell transcription factors forkhead box O1 (FOXO1), NKX6-1, NKX2-2, and PDX1, which did not occur with resveratrol supplementation. Similar changes were observed in human islets where the effects of resveratrol were mediated through Sirtuin 1. These findings have implications for the management of humans with insulin resistance, prediabetes, and diabetes. [ABSTRACT FROM AUTHOR]

OBJECTIVE--Glucose-dependent insulinotropic polypeptide (GIP), unlike glucagon-like peptide (GLP)-1, lacks glucose-lowering properties in patients with type 2 diabetes. We designed this study to elucidate the underlying pathophysiology. RESEARCH DESIGN AND METHODS--Twenty-two insulin-naïve subjects with type 2 diabetes were given either synthetic human GIP (20 ng ⋅ kg-1 ⋅ min-1) or placebo (normal saline) over 180 min, starting with the first bite of a mixed meal (plus 1 g of acetaminophen) on two separate occasions. Frequent blood samples were obtained over 6 h to determine plasma GIP, GLP-1, glucose, insulin, glucagon, resistin, and acetaminophen levels. RESULTS--Compared with placebo, GIP induced an early postprandial increase in insulin levels. Intriguingly, GIP also induced an early postprandial augmentation in glucagon, a significant elevation in late postprandial glucose, and a decrease in late postprandial GLP-1 levels. Resistin and acetaminophen levels were comparable in both interventions. By immunocytochemistry, GIP receptors were present on human and mouse α-cells. In αTC1 cell line, GIP induced an increase in intracellular cAMP and glucagon secretion. CONCLUSIONS--GIP, given to achieve supraphysiological plasma levels, still had an early, short-lived insulinotropic effect in type 2 diabetes. However, with a concomitant increase in glucagon, the glucose-lowering effect was lost. GIP infusion further worsened hyperglycemia postprandially, most likely through its suppressive effect on GLP-1. These findings make it unlikely that GIP or GIP receptor agonists will be useful in treating the hyperglycemia of patients with type 2 diabetes. Diabetes 58:1342-1349, 2009 [ABSTRACT FROM AUTHOR]

Glucose-dependent insulinotropic polypeptide (GIP) is an incretin secreted from the enteroendocrine K-cells in response to food. Unlike the other incretin, glucagon-like peptide-1 (GLP-1), GIP is reported to have lost its insulinotropic effect and not have glucose-lowering properties in patients with type 2 diabetes mellitus (T2DM). We attempted to gain insight into the pathophysiology underlying the seeming lack of effects of GIP on glucose homeostasis in T2DM. We therefore administered synthetic human GIP (with a meal) to subjects who had T2DM, and then, for 6hr, took frequent blood samples to measure various factors known to be involved in glucose homeostasis. 22 insulin-naive subjects with T2DM (ages 52.6±9.3years; BMI 36.7±7.3kg/m²; HbA[sub 1c] 7.4±1.5%; fasting plasma glucose 155±46mg/dl; diabetes duration 4.3±4.2years; f/m ratio 13/9) that had been treated with diet alone, metformin and/or sulfonylurea were recruited. They stopped oral hypoglycemic medications for 5 days prior to testing and fasted for 8hr prior to 2 study visits, each visit separated by not less than 6 weeks. After 3 baseline blood draws, the subjects received continuous intravenous administration of either placebo (0.9%NaCl) or GIP (20ng/kg/min) for 180min starting with the first bite of a 550calorie mixed-meal, and 1g of acetaminophen to study gastric emptying. Frequent blood sampling took place over the subsequent 6hr (every 5min for the first 75min then every 15min until 180min and every 20min until 360min). Compared to placebo, GIP induced an increase in plasma insulin from 5-30min but a transient decrease in plasma glucose at the 10min time-point only (p<0.05). Intriguingly, and hitherto undescribed, GIP induced a later augmentation in glucagon secretion with a peak level at approximately 30min (p<0.05), a significant elevation in plasma glucose from 120-220min (p<0.05) and a decrease in GLP-I secretion at 135min that lasted for the remainder of the 6hr study period (p<0.05). Based on plasma acetaminophen levels, GIP did not affect gastric emptying and NEFAs were similar during both studies. In T2DM therefore, GIP, given at a pharmacological dose with a meal, still has an early, short-lived insulinotropic effect, resulting in a transient, clinically irrelevant plasma glucose drop. However, this was subsequently followed by increased plasma glucagon levels and worsening hyperglycemia. These findings make it unlikely that GIP or GIP receptor agonists will ever be useful as glucose-lowering agents. [ABSTRACT FROM AUTHOR]