The glucose dependent insulinotropic polypeptide response to oral glucose and mixed meals is increased in patients with type 2 (non-insulin-dependent) diabetes mellitus

Enhanced cAMP generation and insulin-releasing potency of two novel Tyr1-modified enzyme-resistant forms of glucose-dependent insulinotropic polypeptide is associated with significant antihyperglycaemic activity in spontaneous obesity-diabetes

Glucose-dependent insulinotropic polypeptide (GIP) is an important incretin hormone, which potentiates glucose-induced insulin secretion. Antihyperglycaemic actions of GIP provide significant potential in Type II diabetes therapy. However, inactivation of GIP by the enzyme dipeptidyl peptidase IV (DPP IV) and its consequent short circulating half-life limit its therapeutic use. Therefore two novel Tyr(1)-modified analogues of GIP, N-Fmoc-GIP (where Fmoc is 9-fluorenylmethoxycarbonyl) and N-palmitate-GIP, were synthesized and tested for metabolic stability and biological activity. Both GIP analogues were resistant to degradation by DPP IV and human plasma. In Chinese hamster lung (CHL) cells expressing the cloned human GIP receptor, both analogues exhibited a 2-fold increase in cAMP-generating potency compared with native GIP (EC(50) values of 9.4, 10.0 and 18.2 nM respectively). Using clonal BRIN-BD11 cells, both analogues demonstrated strong insulinotropic activity compared with native GIP ( P <0.01 to P <0.001). In obese diabetic ( ob / ob ) mice, administration of N-Fmoc-GIP or N-palmitate-GIP (25 nmol/kg) together with glucose (18 mmol/kg) significantly reduced the peak 15 min glucose excursion (1.4- and 1.5-fold respectively; P <0.05 to P <0.01) compared with glucose alone. The area under the curve (AUC) for glucose was significantly lower after administration of either analogue compared with glucose administered alone or in combination with native GIP (1.5-fold; P <0.05). This was associated with a significantly greater AUC for insulin (2.1-fold; P <0.001) for both analogues compared with native GIP. A similar pattern of in vivo responsiveness was evident in lean control mice. These data indicate that novel N-terminal Tyr(1) modification of GIP with an Fmoc or palmitate group confers resistance to degradation by DPP IV in plasma, which is reflected by increased in vitro potency and greater insulinotropic and antihyperglycaemic activities in an animal model of Type II diabetes mellitus.

Regulation of endogenous glucose production after a mixed meal in type 2 diabetes

The extent and time course of suppression of endogenous glucose production (EGP) in type 2 diabetes after a mixed meal have been determined using a new tracer methodology. Groups of age-, sex-, and weight-matched normal controls (n = 8) and diet-controlled type 2 diabetic subjects (n = 8) were studied after ingesting a standard mixed meal (550 kcal; 67% carbohydrate, 19% fat, 14% protein). There was an early insulin increment in both groups such that, by 20 min, plasma insulin levels were 266 +/- 54 and 190 +/- 53 pmol/l, respectively. EGP was similar basally [2.55 +/- 0.12 mg x kg(-1) x min(-1) in control subjects vs. 2.92 +/- 0.16 mg x kg(-1) x min(-1) in the patients (P = 0.09)]. After glucose ingestion, EGP declined rapidly in both groups to approximately 50% of basal within 30 min of the meal. Despite the initial rapid decrease, the EGP was significantly greater in the diabetic group at 60 min (1.75 +/- 0.12 vs. 1.05 +/- 0.14 mg x kg(-1) x min(-1); P < 0.01) and did not reach nadir until 210 min (0.96 +/- 0.17 mg x kg(-1) x min(-1)). Between 60 and 240 min, EGP was 47% higher in the diabetic group (0.89 +/- 0.09 vs. 1.31 +/- 0.13 mg x kg(-1) x min(-1), P < 0.02). These data quantitate the initial rapid suppression of EGP after a mixed meal in type 2 diabetes and the contribution of continuing excess glucose production to subsequent hyperglycemia.

Defective glucose-dependent insulinotropic polypeptide receptor expression in diabetic fatty Zucker rats

Glucose-dependent insulinotropic polypeptide (GIP) is a peptide hormone that is released postprandially from the small intestine and acts in concert with glucagon-like peptide (GLP)-1 to potentiate glucose-induced insulin secretion from the pancreatic beta-cell. In type 2 diabetes, there is a decreased responsiveness of the pancreas to GIP; however, the insulin response to GLP-1 remains intact. The literature suggests that the ineffectiveness of GIP in type 2 diabetes may be a result of chronic homologous desensitization of the GIP receptor. Yet, there has been no conclusive evidence suggesting that GIP levels are elevated in diabetes. The hypothesis of the present study is that one cause of decreased responsiveness to GIP in type 2 diabetes is an inappropriate expression of the GIP receptor in the pancreatic islet. This hypothesis was tested using a strain of diabetic fatty Zucker rats. The obese rats displayed basal GIP levels similar to the control animals; however, they were unresponsive to a GIP infusion (4 pmol.min(-1). kg(-1)), whereas the lean animals displayed a significant reduction in blood glucose (GIP levels, 50% control after 60 min, P < 0.05) as well as a significant increase in circulating insulin. GIP also potently stimulated first-phase insulin secretion from isolated perifused islets (10.3 +/- 3.0 x basal), and GIP and GLP-1 potentiated insulin secretion from the perfused pancreas (6 x control area under the curve [AUC]) from lean animals. GIP yielded no significant effect in the Vancouver diabetic fatty Zucker (VDF) rat pancreases, whereas GLP-1 elicited an eightfold increase of insulin secretion from the perfused VDF pancreas. Islets from lean animals subjected to static incubations with GIP showed a 2.2-fold increase in cAMP, whereas GIP failed to increase islet cAMP in the VDF islets. Finally, the expression of both GIP receptor mRNA and protein was decreased in islets from VDF rats. These data suggest that the decreased effectiveness of GIP in the VDF rat and in type 2 diabetes may be a result of a decreased receptor expression in the islet.

Role of regulator of G protein signaling in desensitization of the glucose-dependent insulinotropic peptide receptor

The glucose-dependent insulinotropic peptide receptor (GIP-R) is a member of the G protein-coupled receptors. Recent studies have indicated that elevated serum GIP concentrations in type II diabetic patients might induce desensitization of the GIP-R, and this mechanism could contribute to impaired insulin secretion. The cellular and molecular mechanisms governing GIP desensitization are unknown. Here, we report the results of studies on a new family of proteins known as regulators of G protein signaling (RGS) that have been shown to mediate the desensitization process of other receptors. GIP-R and RGS1, -2, -3, and -4 complementary DNAs were cotransfected into human embryonic kidney cells (L293). GIP-stimulated cAMP generation in control cells and in those coexpressing RGS1, -3, and -4 displayed a dose-dependent increase 10 min after GIP treatment. In contrast, RGS2 expression inhibited the GIP-induced cAMP response by 50%, a response similar to that of cells desensitized by preincubation with 10(-7) M GIP. In betaTC3 cells, preincubation of GIP attenuated GIP-induced insulin release by 45% at 15 min and by 55% at 30 min. Expression of RGS2 in the betaTC3 cells significantly decreased GIP-stimulated insulin secretion, whereas glucose-induced insulin release was not affected. RGS2 messenger RNA was identified by Northern blot analysis to be expressed endogenously in betaTC3 and L293 cells, and its level was significantly induced by GIP treatment in betaTC3 cells. Moreover, RGS2 bound Gs alpha protein in an in vitro system, suggesting that RGS2 attenuated the Gs-adenylate cyclase signaling pathway. These results suggest a potential role for RGS2 in modulating GIP-mediated insulin secretion in pancreatic islet cells.

The cysteine of the cytoplasmic tail of glucose-dependent insulinotropic peptide receptor mediates its chronic desensitization and down-regulation

The glucose-dependent insulinotropic peptide receptor (GIP-R) is a member of G-protein-coupled, seven transmembrane-spanning receptors. Recent studies have shown that elevated serum GIP level in diabetic patients may induce chronic desensitization of the GIP-R, and that this mechanism could contribute to impaired insulin secretion. The cellular basis of down-regulation and chronic desensitization of GIP-R is unclear. To explore the role of the carboxyl terminus of the GIP-R in mediating these processes, five truncated GIP-Rs (T395, T399, T420, T431, T455) were created to delete consecutive serines from the carboxyl end. All mutants except T395 exhibit an identical ligand-binding affinity to the WT receptor. The T395 mutant, which had the entire carboxyl tail removed, does not bind to ligand. Down-regulation and desensitization was assessed by measuring the receptor number and the ability of agonist-induced cAMP or [Ca2+] generation after pre-exposure to 10(-7) M GIP for 24 h. The wild-type (WT) and T421, T431, T455 mutant GIP-Rs are maximally down-regulated by GIP preincubation, whereas T399 mutant does not, indicating that the sequence between amino acids 399 and 420 is critical for this process. Mutation analysis of this area by alanine scanning mutagenesis reveals two critical residues: serine 406 and cysteine 411. Replacement of serine 406 with arginine (S406R) or alanine (S406A) partly attenuates agonist-induced down-regulation and desensitization. In contrast, mutation of the cysteine 411 to glycine (C411G) or alanine (C411A) markedly attenuates both processes. Mutant SCRG, in which both serine 406 and cysteine 411 are mutated, behaves similar to C411G or C4111A. The data suggest that chronic desensitization and down-regulation of the GIP-R may be mediated by similar mechanisms, and that the cysteine in the carboxyl terminus plays an essential role in regulating both processes.

Cell-specific expression of the glucose-dependent insulinotropic polypeptide gene in a mouse neuroendocrine tumor cell line

Glucose-dependent insulinotropic polypeptide (GIP) is a 42-amino acid gastrointestinal regulatory peptide that, in the presence of glucose, stimulates insulin secretion. GIP is expressed in K cells of the small intestine and in cells of the submandibular salivary gland. Using a rat GIP cDNA as a specific probe, we screened a number of established cell lines for the expression of GIP mRNA. STC-1 cells, a cell line derived from a mouse neuroendocrine tumor, were found to express high levels of GIP mRNA. GIP-specific transcripts were not detected in other cell lines tested, which included cells of intestinal, salivary, and endocrine origin. Analysis of GIP-luciferase fusions identified two promoters, a distal and a proximal promoter, upstream of the translation initiation codon for GIP. The distal promoter, located upstream of position +1, corresponds to the principal promoter of the GIP gene and can promote cell-specific transcription. Sequential deletion and site-directed mutational analysis of the distal promoter demonstrated that the sequence between -193 and -182 determines cell-specific expression of GIP. Contained in this region is a consensus GATA motif, suggesting that a member of the GATA family of DNA-binding proteins is involved in the cell-specific regulation of the GIP gene.

Direct assessment of liver glycogen storage by 13C nuclear magnetic resonance spectroscopy and regulation of glucose homeostasis after a mixed meal in normal subjects

Despite extensive recent studies, understanding of the normal postprandial processes underlying immediate storage of substrate and maintenance of glucose homeostasis in humans after a mixed meal has been incomplete. The present study applied 13C nuclear magnetic resonance spectroscopy to measure sequential changes in hepatic glycogen concentration, a novel tracer approach to measure postprandial suppression of hepatic glucose output, and acetaminophen to trace the pathways of hepatic glycogen synthesis to elucidate the homeostatic adaptation to the fed state in healthy human subjects. After the liquid mixed meal, liver glycogen concentration rose from 207 +/- 22 to 316 +/- 19 mmol/liter at an average rate of 0.34 mmol/liter per min and peaked at 318 +/- 31 min, falling rapidly thereafter (0.26 mmol/liter per min). The mean increment at peak represented net glycogen synthesis of 28.3 +/- 3.7 g (approximately 19% of meal carbohydrate content). The contribution of the direct pathway to overall glycogen synthesis was 46 +/- 5 and 68 +/- 8% between 2 and 4 and 4 and 6 h, respectively. Hepatic glucose output was completely suppressed within 30 min of the meal. It increased steadily from 60 to 255 min from 0.31 +/- 32 to 0.49 +/- 18 mg/kg per min then rapidly returned towards basal levels (1.90 +/- 0.04 mg/kg per min). This pattern of change mirrored precisely the plasma glucagon/insulin ratio. These data provide for the first time a comprehensive picture of normal carbohydrate metabolism in humans after ingestion of a mixed meal.

Direct measurement of change in muscle glycogen concentration after a mixed meal in normal subjects

Postprandial storage of carbohydrate as glycogen in muscle was quantitated in normal subjects (n = 8) by natural abundance 13C-nuclear magnetic resonance spectroscopy with proton decoupling in a 4.7-tesla magnet. After an overnight fast three basal measurements of gastrocnemius muscle glycogen were made and a mixed meal was given. Muscle glycogen concentration rose from 83.3 +/- 5.2 to a maximum of 100.2 +/- 6.7 mmol/l muscle at 4.9 h (P < 0.01) and fell thereafter to 90.6 +/- 5.9 mmol/l muscle at 7 h postprandially (P < 0.006). The meal brought about an increase in plasma glucose from 5.4 +/- 0.2 to 7.3 +/- 0.4 mmol/l at 30 min but this was followed by a rapid fall to 6.2 +/- 0.4 mmol/l at 75 min. Plasma insulin rose from 62.4 +/- 11.4 to 900 +/- 216 pmol/l at 30 min and declined steadily thereafter. It was calculated from total muscle mass measurements and estimation of carbohydrate absorption rates that at peak muscle glycogen concentrations between 26 and 35% of the absorbed carbohydrate was stored as muscle glycogen. These data quantitate the role of skeletal muscle glycogen synthesis in postprandial carbohydrate storage and demonstrate that this tissue acts as a dynamic buffer to maintain glucose homeostasis during postprandial substrate storage.

Gastric inhibitory polypeptide (GIP) responses in type 2 diabetes using three different antibodies

Contradictory reports of gastric inhibitory polypeptide (GIP) responses in diabetes have been published by different workers using different radioimmunoassay systems. The present study was undertaken to assess GIP responses in type 2 diabetes using three antibodies (S100, GP01 and GP24). Seven untreated diabetics and seven healthy control subjects had a standard 50 g oral glucose tolerance test. An alcohol extract of plasma of each sample was assayed using these three different antibodies. Using S100, GIP responses in the diabetic group were significantly lower at 30' (P<0.025) and at 120' (P<0.01) and the integrated incremental GIP responses also were significantly lower in the diabetic group (P< 0.025). Using GP01, GIP responses in the diabetic group were significantly lower only 120' (P<0.05) but there was no significant difference in the integrated incremental GIP responses. Using GP24, there was no significant difference between the diabetic and control groups at any time intervals or in the integrated incremental responses. However, three to sixfold higher levels of GIP were recorded when using GP24 as compared with the other two antibody systems which gave similar absolute values. Structurally abnormal variable cross-reacting 5000 dalton (5 kDa) and 8 kDa GIP forms or still unidentified structurally GIP related peptides associated with type 2 diabetes might be responsible for these conflicting results.

Conflicting gastric inhibitory polypeptide data: possible causes

The literature with respect to GIP is flooded with conflicting data especially with respect to its role in type 2 diabetes mellitus, obesity, type 1 diabetes mellitus and chronic pancreatitis. This review describes possible reasons for the discrepancies which include species variation of GIP, heterogeneity of molecules with different immunoreactivity and bioactivity, deterioration of immunoreactivity of standard and sample on prolonged storage and the effect of the preceding intake of type and quantity of food. The problems can be resolved by raising antibodies to synthetic human GIP and its fragments, the chemical characterization of and the raising of antibodies to immunoreactive GIP 8000, the correct storage of samples and the standard preparation of subjects prior to experimental procedures.

Gastric inhibitory polypeptide (GIP) response in diabetes using a highly specific antiserum

Gastric inhibitory polypeptide (GIP), a hormone secreted from the proximal small gut, is recognized as a major component of the enteroinsular axis. However, circulating levels of GIP in diabetes have been reported to be exaggerated, normal or decreased following glucose ingestion, which may be due to the presence of variable crossreacting immunoreactive GIP forms in the circulation. We have raised an antibody (S705) which recognizes only 5 kDa GIP. Using this antiserum we have measured circulating GIP levels in 18 healthy volunteers, and 13 Type 2 diabetic and 9 Type 1 diabetic patients following ingestion of 75 g glucose. As expected, blood glucose levels and blood insulin levels are significantly abnormal in the diabetic groups. On the other hand, circulating GIP levels at all time-points and integrated incremental GIP over 120 min were not different from the control group. However, we cannot exclude the possibility that apparently normal immunoreactive GIP levels in diabetes might conceal subtle alterations in biological activity which could play a role in the pathogenesis of the disease.