Elevated plasma glucose-dependent insulinotropic polypeptide associates with hyperinsulinemia in impaired glucose tolerance

Metabolic targets in the Postural Orthostatic Tachycardia Syndrome: A short thematic review

Postural Orthostatic Tachycardia Syndrome (POTS) is a chronic autonomic condition hallmarked by orthostatic intolerance and tachycardia in the upright position. POTS impacts approximately 1-3 million people in the U.S. alone, in which the majority of patients are premenopausal women. The etiology of POTS is multi-factorial with three primary clinical subtypes, including neuropathic, hyperadrenergic, and hypovolemic POTS. Recent evidence suggests potential metabolic associations with POTS pathophysiology, particularly involving insulin resistance and abnormal vasoactive gut hormones. This review aims to characterize POTS phenotypes and explore potential metabolic links, focusing on insulin resistance and vasoactive gut hormones. Understanding the metabolic aspects of POTS pathophysiology could provide novel insights into its mechanisms and guide therapeutic approaches.

Comparison of islet cell function, insulin sensitivity, and incretin axis between Asian-Indians with either impaired fasting glucose or impaired glucose tolerance, and normal healthy controls

AIMS

The objective of this study was to compare the islet cell function, insulin sensitivity, and incretin axis between Asian-Indian subjects with either impaired fasting glucose (IFG), or impaired glucose tolerance (IGT), and normal glucose tolerance (NGT).

MATERIALS AND METHODS

Prediabetes subjects underwent a mixed meal tolerance test(MMTT) after overnight fasting. Samples for glucose, insulin, glucagon, and glucagon-like peptide-1 (GLP-1) were collected at 0, 30, 60, and 120 min. Insulin secretion sensitivity index -2 (ISSI-2) for beta-cell function and Matsuda index for insulin sensitivity were assessed. Alpha cell function was assessed by measuring the area under the curve (AUC) 0-120 glucagon/AUC0-120 glucose.

RESULTS

A total of sixty subjects were recruited with 20 in each group. The beta-cell function represented by ISSI-2 was impaired in prediabetes subjects as compared to NGT group (IFG: 2.09 ± 0.44 vs. NGT: 3.04 ± 0.80, P < 0.0001, and IGT: 2.33 ± 0.59 vs. NGT: 3.04 ± 0.80, P = 0.002). Similarly, AUC0-120 glucagon/AUC0-120 glucose was also lower in prediabetes group as compared to healthy controls (IFG: 0.41(0.54) vs. NGT: 1.07(0.39), P = 0.003 and IGT: 0.57(0.38) vs. NGT: 1.07(0.39), P = 0.001).

CONCLUSION

Asian-Indian prediabetes subjects have reduced beta-cell function with lesser glucagon secretion during MMTT as compared to normal healthy controls.

Repositioning the Alpha Cell in Postprandial Metabolism

Glucose homeostasis is maintained in large part due to the actions of the pancreatic islet hormones insulin and glucagon, secreted from β- and α-cells, respectively. The historical narrative positions these hormones in opposition, with insulin primarily responsible for glucose-lowering and glucagon-driving elevations in glucose. Recent progress in this area has revealed a more complex relationship between insulin and glucagon, highlighted by data demonstrating that α-cell input is essential for β-cell function and glucose homeostasis. Moreover, the common perception that glucagon levels decrease following a nutrient challenge is largely shaped by the inhibitory effects of glucose administration alone on the α-cell. Largely overlooked is that a mixed nutrient challenge, which is more representative of typical human feeding, actually stimulates glucagon secretion. Thus, postprandial metabolism is associated with elevations, not decreases, in α-cell activity. This review discusses the recent advances in our understanding of how α-cells regulate metabolism, with a particular focus on the postprandial state. We highlight α- to β-cell communication, a term that describes how α-cell input into β-cells is a critical axis that regulates insulin secretion and glucose homeostasis. Finally, we discuss the open questions that have the potential to advance this field and continue to evolve our understanding of the role that α-cells play in postprandial metabolism.

Retinol-binding protein 2 (RBP2): biology and pathobiology

Retinol-binding protein 2 (RBP2; originally cellular retinol-binding protein, type II (CRBPII)) is a 16 kDa cytosolic protein that in the adult is localized predominantly to absorptive cells of the proximal small intestine. It is well established that RBP2 plays a central role in facilitating uptake of dietary retinoid, retinoid metabolism in enterocytes, and retinoid actions locally within the intestine. Studies of mice lacking Rbp2 establish that Rbp2 is not required in times of dietary retinoid-sufficiency. However, in times of dietary retinoid-insufficiency, the complete lack of Rbp2 gives rise to perinatal lethality owing to RBP2 absence in both placental (maternal) and neonatal tissues. Moreover, when maintained on a high-fat diet, Rbp2-knockout mice develop obesity, glucose intolerance and a fatty liver. Unexpectedly, recent investigations have demonstrated that RBP2 binds long-chain 2-monoacylglycerols (2-MAGs), including the canonical endocannabinoid 2-arachidonoylglycerol, with very high affinity, equivalent to that of retinol binding. Crystallographic studies establish that 2-MAGs bind to a site within RBP2 that fully overlaps with the retinol binding site. When challenged orally with fat, mucosal levels of 2-MAGs in Rbp2 null mice are significantly greater than those of matched controls establishing that RBP2 is a physiologically relevant MAG-binding protein. The rise in MAG levels is accompanied by elevations in circulating levels of the hormone glucose-dependent insulinotropic polypeptide (GIP). It is not understood how retinoid and/or MAG binding to RBP2 affects the functions of this protein, nor is it presently understood how these contribute to the metabolic and hormonal phenotypes observed for Rbp2-deficient mice.

Retinol-binding protein 2 (RBP2) binds monoacylglycerols and modulates gut endocrine signaling and body weight

Expressed in the small intestine, retinol-binding protein 2 (RBP2) facilitates dietary retinoid absorption. Rbp2-deficient (Rbp2-/- ) mice fed a chow diet exhibit by 6-7 months-of-age higher body weights, impaired glucose metabolism, and greater hepatic triglyceride levels compared to controls. These phenotypes are also observed when young Rbp2-/- mice are fed a high fat diet. Retinoids do not account for the phenotypes. Rather, RBP2 is a previously unidentified monoacylglycerol (MAG)-binding protein, interacting with the endocannabinoid 2-arachidonoylglycerol (2-AG) and other MAGs with affinities comparable to retinol. X-ray crystallographic studies show that MAGs bind in the retinol binding pocket. When challenged with an oil gavage, Rbp2-/- mice show elevated mucosal levels of 2-MAGs. This is accompanied by significantly elevated blood levels of the gut hormone GIP (glucose-dependent insulinotropic polypeptide). Thus, RBP2, in addition to facilitating dietary retinoid absorption, modulates MAG metabolism and likely signaling, playing a heretofore unknown role in systemic energy balance.

Sustained effects of resistant starch on the expression of genes related to carbohydrate digestion/absorption in the small intestine

Resistant starch (RS) consumption has beneficial effects on health, such as reduced postprandial blood glucose levels. In this study, we evaluated the effect of a 14-day diet containing RS on α-glucosidase activity and the expression of genes related to carbohydrate digestion/absorption in rats. We examined whether the effects of RS persist when the rats were shifted to a control diet. The results suggest that RS consumption reduces α-glucosidase activity and Mgam, Si and Sglt1 mRNA levels in the proximal jejunum. In addition, RS consumption appeared to influence the serum GIP level, up to 2 days after the animals were shifted to a control diet. To our knowledge, this is the first report that RS has a sustained effect on gut hormone expression and the expression of genes related to carbohydrate digestion/absorption in the proximal jejunum.

IGFBP-2 and aging: a 20-year longitudinal study on IGFBP-2, IGF-I, BMI, insulin sensitivity and mortality in an aging population

Objective Insulin-like growth factor-binding protein-2 (IGFBP-2) concentrations are low in subjects with metabolic syndrome and type 2 diabetes. Intriguingly, recent studies have demonstrated an association between high IGFBP-2 concentrations and increased mortality not only in populations with certain types of cancer, but also in relatively healthy populations. We evaluated the role of IGFBP-2 in relation to BMI and mortality. Design and Participants BMI, insulin sensitivity, insulin-like growth factor 1 (IGF-I) and IGFBP-2 were assessed repeatedly in 539 participants of the Baltimore Longitudinal Study of Aging around the ages of 55, 65 and 75 years. Results IGFBP-2 concentrations positively correlated with insulin sensitivity and inversely with BMI, both at baseline and follow-up. Independent of IGF-I, sex, BMI and insulin sensitivity, circulating IGFBP-2 levels positively correlated with age (P < 0.001). Changes over time in BMI were associated with an inverse correlation in IGFBP-2 concentrations. Furthermore, we found indications of a relationship between low baseline IGFBP-2 levels and mortality. Remarkably, after adjustment for insulin sensitivity, the opposite association was found, as a unit increase of log(IGFBP2) was associated with an increase in the log hazard by 1.43 (95% CI: 0.3-2.6). This accounted for both baseline (P = 0.02) as well as serial (P < 0.001) measurements of IGFBP2. Finally, in this longitudinal study, we found that IGF-I concentrations increased with age (0.82 ± 0.2 (µg/L)/year, P < 0.001). Conclusion This is the first study investigating the relationship between IGFBP-2 levels and age in a longitudinal setting. Serum IGFBP-2 levels increase with age after the age of 50 years and evolve in parallel with insulin sensitivity. IGFBP-2 may therefore be a potential marker for insulin sensitivity. We further show that IGFBP-2 levels can predict mortality in this aging population. However, its predictive value for mortality can only be interpreted in relation to insulin sensitivity. After adjustment for insulin sensitivity, high IGFBP-2 levels are predictive of increased mortality.

Postmeal increment in intact glucagon-like peptide 1 level, but not intact glucose-dependent insulinotropic polypeptide levels, is inversely associated with metabolic syndrome in patients with type 2 diabetes

PURPOSE

Metabolic syndrome increases the risk of cardiovascular disease. Recently glucagon-like peptide 1 (GLP-1) agonists proved to be effective in preventing cardiovascular disease (CVD) in patients with type 2 diabetes. We investigated the association of blood incretin levels with metabolic syndrome in patients with type 2 diabetes.

MATERIALS AND METHODS

This is a cross-sectional study involving 334 people with type 2 diabetes. Intact GLP-1 (iGLP-1) and intact glucose-dependent insulinotropic polypeptide (iGIP) levels were measured in a fasted state and 30 min after ingestion of a standard mixed meal. Metabolic syndrome was diagnosed based on the criteria of the International Diabetes Federation.

RESULTS

Two hundred twenty-five (69%) of the subjects have metabolic syndrome. The fasting iGLP-1 level was no different between groups. Thirty-min postprandial iGLP-1 was non-significantly lower in the subjects who had metabolic syndrome. Incremental iGLP-1 (ΔiGLP-1, the difference between 30-min postmeal and fasting iGLP-1 levels) was significantly lower in those with metabolic syndrome. There were no significant differences in fasting iGIP, postprandial iGIP, and ΔiGIP between groups. The ΔiGLP-1, but not ΔiGIP levels decreased significantly as the number of metabolic syndrome components increased. In hierarchical logistic regression analysis, the ΔiGLP-1 level was found to be a significant contributor to metabolic syndrome even after adjusting for other covariates.

CONCLUSION

Taken together, the iGLP-1 increment in the 30 min after meal ingestion is inversely associated with metabolic syndrome in patients with type 2 diabetes. This suggests that postmeal iGLP-1 increment could be useful in assessing cardiovascular risk in type 2 diabetes.

The Regulation of Peripheral Metabolism by Gut-Derived Hormones

Enteroendocrine cells lining the gut epithelium constitute the largest endocrine organ in the body and secrete over 20 different hormones in response to cues from ingested foods and changes in nutritional status. Not only do these hormones convey signals from the gut to the brain via the gut-brain axis, they also act directly on metabolically important peripheral targets in a highly concerted fashion to maintain energy balance and glucose homeostasis. Gut-derived hormones released during fasting tend to be orexigenic and have hyperglycaemic potential. Conversely, gut hormones secreted postprandially generally promote satiety and facilitate glucose clearance. Although some of the metabolic benefits conferred by bariatric surgeries have been ascribed to changes in the secretory profiles of various gut hormones, the therapeutic potential of the enteroendocrine system as a viable target against metabolic diseases remain largely underexploited, except for incretin-mimetics. This review provides a brief overview of the physiological importance and highlights the therapeutic potential of the following gut hormones: serotonin, glucose-dependent insulinotropic peptide, glucagon-like peptide 1, oxyntomodulin, peptide YY, insulin-like peptide 5, and ghrelin.

Modeling the oral glucose tolerance test in normal and impaired glucose tolerant states: a population approach

OBJECTIVE

The conventional approach to analyzing data from oral glucose tolerance testing (OGTT) requires model identification in each individual separately (standard two stage, STS), ignoring knowledge about the population as a whole. In practice, however, the OGTT is sparsely sampled and individual estimates are often not resolvable from available data. This weakness is often encountered in large scale trials or epidemiological studies, leading to either multiple imputations or simply much less data available for analysis.

METHODS

We have applied a population approach, nonlinear mixed effects modeling, to plasma glucose, insulin and C-peptide data obtained from a 120 minute OGTT undertaken by 106 subjects with varying glucose tolerance. This method provides estimates of population means, variances and covariances of model parameters and empirical Bayes estimates of individual parameter values, as well as measures of intra-individual (within-subject) and inter-individual (between-subject) variability. The recently developed oral glucose minimal model was used to evaluate insulin sensitivity, and a combined model approach was used to assess β-cell secretion.

RESULTS

Applying these models allowed for the reconstruction of insulin secretion and glucose absorption profiles and gave population indexes of insulin sensitivity (S_I = 6.51 ± 1.20 × 10^-4 min^-1·μU^-1·ml), fractional hepatic extraction of insulin (F = 0.522 ± 0.291) and fractional insulin clearance (k_I = 0.258 ± 0.151 min^-1). Whereas the traditional approach to parameter estimation failed to recover estimates in more than one third of the population, the population approach provided individual estimates in all subjects. Examination of the empirical Bayes estimates showed that individual parameter estimates were able to differentiate well between individuals at glucose tolerant states ranging from euglycemia to overt type 2 diabetes.

CONCLUSIONS

Our findings suggest that population analysis is a powerful tool for obtaining accurate assessments of indexes of insulin sensitivity and β-cell function from the OGTT, especially in epidemiological studies with large numbers of sparsely sampled subjects.

High prevalence of cardiometabolic risk factors in Hispanic adolescents: correlations with adipocytokines and markers of inflammation

This study assessed the association of cardiometabolic risk factors with systemic inflammation, insulin resistance, and adypocytokines in a Hispanic adolescent subgroup. A clinic-based sample of 101 Puerto Rican adolescents, 48 of whom were overweight or obese based on body mass index percentiles for age and sex, was recruited during 2010. Data were collected through interviews, blood pressure and anthropometric measurements, and blood drawing. Overall prevalence of the metabolic syndrome was 16.8 % and increased to 37.5 % among overweight/obese youth. The overweight/obese group exhibited significantly (p < 0.05) higher values for abdominal obesity measures, systolic blood pressure, triglycerides, insulin resistance, C peptide, high-sensitivity C reactive protein, fibrinogen, leptin, and IL-6 and lower levels of high density lipoprotein cholesterol, adiponectin, and IGF-1. Total adiponectin significantly correlated with most cardiovascular risk factors independent of sex, Tanner stage, and adiposity. Altered cardiometabolic and adipocytokine profiles were present in this Hispanic subgroup, reinforcing the need to strengthen strategies addressing childhood obesity.

Insulin drives glucose-dependent insulinotropic peptide expression via glucose-dependent regulation of FoxO1 and LEF1/β-catenin

Minutes after ingestion of fat or carbohydrates, vesicles stored in enteroendocrine cells release their content of incretin peptide hormones that, together with absorbed glucose, enhance insulin secretion by beta-pancreatic cells. Freshly-made incretins must therefore be packed into new vesicles in anticipation of the next meal with cells adjusting new incretin production to be proportional to the level of previous insulin release and absorbed blood glucose. Here we show that insulin stimulates the expression of the major human incretin, glucose-dependent insulinotropic peptide (GIP) in enteroendocrine cells but requires glucose to do it. Akt-dependent release of FoxO1 and glucose-dependent binding of LEF1/β-catenin mediate induction of Gip expression while insulin-induced phosphorylation of β-catenin does not alter its localization or transcriptional activity in enteroendocrine cells. Our results reveal a glucose-regulated feedback loop at the entero-insular axis, where glucose levels determine basal and insulin-induced Gip expression; GIP stimulation of insulin release, physiologically ensures a fine control of glucose homeostasis. How enteroendocrine cells adjust incretin production to replace incretin stores for future use is a key issue because GIP malfunction is linked to all forms of diabetes.

Subacute antidiabetic and in vivo antioxidant effects of methanolic extract of Bridelia micrantha (Hochst Baill) leaf on alloxan-induced hyperglycaemic rats

The methanolic leaf extract of Bridelia micrantha was tested for subacute antidiabetic and in vivo antioxidant effects in alloxan-induced hyperglycaemic rats. The subacute treatment of the extract (125, 250 and 500 mg/kg) produced 75, 68 and 63% reduction in fasting blood sugar level respectively, on day 14 of treatment. The extract produced time-dependent effect, but did not show a dose-dependent effect. Its optimum antidiabetic activity was noted at the dose of 125 mg/kg and this was comparable to glibenclamide 2 mg/kg (positive control). The extract (125 mg/kg) showed good oral glucose tolerance test (OGTT) effect in both normoglycaemic and hyperglycaemic rats. The OGTT effect of the extract (125 mg/kg) did not differ significantly (p>0.05) from glibenclamide (2 mg/kg). The antioxidant effect of the extract was assayed through the determination of the level of thiobarbituric acid reactive substance (TBARS) and catalase activity. The extract produced a dose-dependent decrease in the serum level of TBARS and gave its optimum catalase activity at the dose of 500 mg/kg. This study suggests that the B. micrantha extract has antihyperglycaemic and antioxidant activities. Therefore, could be a potential source of novel antidiabetic and antioxidant agent for the management of diabetes mellitus.

Beta and alpha cell function in metabolically healthy but obese subjects: relationship with entero-insular axis

OBJECTIVE

Obesity is widely acknowledged as a critical risk factor for metabolic complications. Among obese subjects, there is a phenotype of metabolically healthy but obese (MHO) individuals that shows a favorable cardiometabolic risk profile. We aimed to evaluate the potential mechanisms underlying the metabolic profile of this subset, including alpha and beta cell function and entero-insular axis.

DESIGN AND METHODS

One hundred twenty-nine obese and 24 nonobese subjects were studied. Obese participants were defined as MHO or at-risk obese, according to the homeostasis model of assessment-insulin resistance (HOMA-IR) index (MHO: lower tertile of HOMA-IR, n = 43; at-risk: upper tertile of HOMA-IR index, n = 41). Insulin, glucagon, and incretin responses after a 120' oral glucose tolerance test (75-g OGTT) were investigated.

RESULTS

During OGTT, MHO individuals showed in comparison with at-risk subjects: lower fasting and afterloads plasma levels of glucose, insulin, and C-peptide; higher disposition index; lower fasting (P = 0.004) and at 30' (P = 0.01) plasma glucose-dependent insulinotropic polypeptide (GIP) levels; lower area under the curve (AUC) (0-30) for GIP (P = 0.008); higher glucagon-like peptide-1 (GLP-1) plasma levels at 90' (P = 0.02) and 120' (P = 0.02); lower glucagon plasma levels at baseline (P = 0.04) and at 30' (P = 0.03); and appropriate glucagon suppression after the oral glucose load.

CONCLUSIONS

MHO subjects show, as well as normal-weight individuals, a lower diabetogenic profile by virtue of higher disposition index and unaffected entero-insular axis. At-risk obese individuals present increased GIP levels that might play a role in determining increased glucagon secretion and inappropriate glucagon responses after glucose load, thus contributing to impaired glucose homeostasis.

Increased secretion of insulin and proliferation of islet β-cells in rats with mesenteric lymph duct ligation

BACKGROUND & AIMS

It has been suggested that intestinal lymph flow plays an important role in insulin secretion and glucose metabolism after meals. In this study, we investigated the influence of ligation of the mesenteric lymph duct on glucose metabolism and islet β-cells in rats.

METHODS

Male Sprague-Dawley rats (10 weeks old) were divided into two groups: one underwent ligation of the mesenteric lymph duct above the cistern (ligation group), and the other underwent a sham operation (sham group). After 1 and 2 weeks, fasting plasma concentrations of glucose, insulin, triglyceride, glucose-dependent insulinotropic polypeptide (GIP), and the active form of glucagon-like peptide-1 (GLP-1) were measured. At 2 weeks after the operation, the oral glucose tolerance test (OGTT) and intravenous glucose tolerance test (IVGTT) were performed. After the rats had been sacrificed, the insulin content of the pancreas was measured and the proliferation of β-cells was assessed immunohistochemically using antibodies against insulin and Ki-67.

RESULTS

During the OGTT, the ligation group showed a significant decrease in the plasma glucose concentration at 120 min (p<0.05) and a significant increase in the plasma insulin concentration by more than 2-fold at 15 min (p<0.01). On the other hand, the plasma GIP concentration was significantly decreased at 60 min (p<0.01) in the ligated group, while the active form of GLP-1 showed a significantly higher level at 90 min (1.7-fold; p<0.05) and 120 min (2.5-fold; p<0.01). During the IVGTT, the plasma insulin concentration in the ligation group was significantly higher at 2 min (more than 1.4-fold; p<0.05). Immunohistochemistry showed that the ratios of β-cell area/acinar cell area and β-cell area/islet area, and also β-cell proliferation, were significantly higher in the ligation group than in the sham group (p<0.05, p<0.01 and p<0.01, respectively). The insulin content per unit wet weight of pancreas was also significantly increased in the ligation group (p<0.05).

CONCLUSIONS

In rats with ligation of the mesenteric lymph duct, insulin secretion during the OGTT or IVGTT was higher, and the insulin content and β-cell proliferation in the pancreas were also increased. Our data show that mesenteric lymph duct flow has a role in glucose metabolism.

Elevated plasma glucose-dependent insulinotropic polypeptide associates with hyperinsulinemia in metabolic syndrome

BACKGROUND AND AIMS

Metabolic syndrome (MS) is a high-risk condition for type 2 diabetes, a disease characterized by insulin resistance and insulin secretion abnormalities. Insulin resistance has been widely characterized in MS subjects while insulin secretion has been poorly investigated. The present study was hence undertaken to further investigate the α and β cell function and entero-insular axis in this pre-diabetic condition.

MATERIALS AND METHODS

Using 120' oral glucose tolerance test (OGTT, 75  g) and 60' intravenous glucose tolerance test (IVGTT, 0.3  g/kg), we studied α and β cell function, insulin resistance, and incretin levels in 96 subjects with normal fasting glucose and normal glucose tolerance to OGTT, with (MS+, n=29) and without MS (MS-, n=67).

RESULTS

MS+ individuals showed in comparison with MS-: higher insulinogenic index (IG30) and higher area under the curve (AUC) (0-120) for glucose and insulin during the OGTT, P<0.05; higher AUC (0-10) for glucose (P<0.05) but similar first phase insulin secretion (P=NS) as measured by ΔAIRG and AUC (0-10) for insulin during the IVGTT; increased AUC (0-60) for insulin during the IVGTT (P=0.04); higher GIP levels at 30' (P=0.03), 60' (P=0.01), 90' (P=0.003), and 120' (P=0.004); higher AUC (0-120) for GIP (P=0.007); similar AUC (0-120) for GLP-1 during the OGTT; and delayed glucagon suppression after the OGTT.

CONCLUSION

NGT subjects with MS showed increased GIP secretion that could be responsible for the delayed glucagon suppression during the OGTT, thereby suggesting a role for incretins in regulating glucose homeostasis in this condition.

Uncoupling protein 2 negatively regulates glucose-induced glucagon-like peptide 1 secretion

It is known that endogenous levels of the incretin hormone glucagon-like peptide 1 (GLP1) can be enhanced by various secretagogues, but the mechanism underlying GLP1 secretion is still not fully understood. We assessed the possible effect of uncoupling protein 2 (UCP2) on GLP1 secretion in mouse intestinal tract and NCI-H716 cells, a well-characterized human enteroendocrine L cell model. Localization of UCP2 and GLP1 in the gastrointestinal tract was assessed by immunofluorescence staining. Ucp2 mRNA levels in gut were analyzed by quantitative RT-PCR. Human NCI-H716 cells were transiently transfected with siRNAs targeting UCP2. The plasma and ileum tissue levels of GLP1 (7-36) amide were measured using an ELISA kit. UCP2 was primarily expressed in the mucosal layer and colocalized with GLP1 in gastrointestinal mucosa. L cells secreting GLP1 also expressed UCP2. After glucose administration, UCP2-deficient mice showed increased glucose-induced GLP1 secretion compared with wild-type littermates. GLP1 secretion increased after NCI-H716 cells were transfected with siRNAs targeting UCP2. UCP2 was markedly upregulated in ileum tissue from ob/ob mice, and GLP1 secretion decreased compared with normal mice. Furthermore, GLP1 secretion increased after administration of genipin by oral gavage. Taken together, these results reveal an inhibitory role of UCP2 in glucose-induced GLP1 secretion.

Physiology of weight loss surgery

The clinical outcomes achieved by bariatric surgery have been impressive. However, the physiologic mechanisms and complex metabolic effects of bariatric surgery are only now beginning to be understood. Ongoing research has contributed a large amount of data and shed new light on the science behind obesity and its treatment, and this article reviews the current understanding of metabolic and bariatric surgery physiology.

The role of incretin therapy at different stages of diabetes

The pathogenetic mechanisms causing type 2 diabetes are complex, and include a significant reduction of the incretin effect. In patients with type 2 diabetes, GLP-1 secretion may be impaired, while GIP secretion seems unaffected. In contrast, the insulinotropic activity of GIP is severely altered, whereas that of GLP-1 is maintained to a great extent. Better understanding of the role of incretin hormones in glucose homeostasis has led to the development of incretin-based therapies that complement and offer important advantages over previously used agents. Incretin-based agents have significant glucose-lowering effects, promote weight loss (or are weight-neutral), inhibit glucagon secretion while maintaining counter-regulatory mechanisms, exhibit cardiovascular benefits, and protect β-cells while possessing a low risk profile. At present, incretin-based therapies are most widely used as add on to metformin to provide sufficient glycemic control after metformin failure. However, they are also recommended as monotherapy early in the disease course, and later in triple combination. These agents may also be a promising therapeutic tool in prediabetic subjects. Therefore, a therapeutic algorithm is needed for their optimal application at different stages of diabetes, as suggested in this article.

Glucose-dependent insulinotropic polypeptide: from pathophysiology to therapeutic opportunities in obesity-associated disorders

Glucose-dependent insulinotropic polypeptide (GIP) is a hormone secreted from the intestinal K-cells with established insulin-releasing actions. However, the GIP receptor is widely distributed in peripheral organs, including the adipose tissue, gut, bone and brain, where GIP modulates energy intake, cell metabolism and proliferation, and lipid and glucose metabolism, eventually promoting lipid and glucose storage. In diabetes and obesity, the incretin effect of GIP is blunted, while the extrapancreatic tissues keep a normal sensitivity to this hormone. As GIP levels are normal or elevated in obesity and diabetes, mounting evidence from chemical or genetic GIP deletion in animal models of obesity-related diabetes suggests that GIP may have a pro-obesogenic action and that a strategy antagonizing GIP action may be beneficial in these conditions, clearing triglyceride deposits from adipose tissue, liver and muscle, and restoring normal insulin sensitivity. Emerging evidence also suggests that the metabolic benefits of bypass surgery are mediated, at least in part, by surgical removal of GIP-secreting K-cells in the upper small intestine.

GIP and bariatric surgery

Bariatric surgery is the most effective modality of achieving weight loss as well as the most effective treatment for type 2 diabetes mellitus (T2DM). Glucose-dependent insulinotropic polypeptide (GIP) is an incretin and is implicated in the pathogenesis of obesity and T2DM. Its role in weight loss and resolution of T2DM after bariatric surgery is very controversial. We have made an attempt to review the physiology of GIP and its role in weight loss and resolution of T2DM after bariatric surgery. We searched PubMed and included all relevant original articles (both human and animal) in the review. Whereas most human studies have shown a decrease in GIP post-malabsorptive bariatric surgery, the role of GIP in bariatric surgery done in animal experiments remains inconclusive.

Collection and handling of blood specimens for peptidomics

Pre-analytical variables can alter the analysis of blood-derived samples. In particular, sample collection and specimen preparation can alter the validity of results obtained by modern multiplex assays (e.g., LC-MS). Low-molecular weight proteins (peptides) as products of proteolytic cleavage events exhibit a close connection to protease activity and function. Altered proteolytic activity during sample collection, preparation, handling, and storage is mirrored by alterations in abundance of specific peptides. Awareness of clinical practices in medical laboratories allows for the identification of specific variables that may affect the results of a peptidomic study. Knowledge of pre-analytical variables is a prerequisite to understand and control their impact.

Immunoassays for the incretin hormones GIP and GLP-1

The measurement of the incretin hormones, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), using immunologically based assays is made difficult by the fact that the processing of the precursor molecules gives rise to a number of different peptides which cross-react with antisera raised against the two hormones. For GLP-1, the picture is further complicated because of the necessity to differentiate between the intestinal and pancreatic proglucagon products. Finally, once secreted, both incretins are rapidly degraded by the enzyme dipeptidyl peptidase-4 (DPP-4) to generate metabolites which have lost their insulinotropic activities. These metabolites are the major circulating forms of the incretins, accounting for 60-80% of total immunoreactive GLP-1 and GIP in the peripheral plasma, while concentrations of the intact forms can be very low and, in some cases, barely detectable. The use of highly specific assays using well-characterised antisera and careful sample handling is therefore required for a reliable determination of incretin hormone concentrations.

Nutrient-driven incretin secretion into intestinal lymph is different between diabetic Goto-Kakizaki rats and Wistar rats

The incretin hormones gastric inhibitory polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) augment postprandial glucose-mediated insulin release from pancreatic beta-cells. The Goto-Kakizaki (GK) rat is a widely used, lean rodent model of Type 2 diabetes; however, little is known regarding the incretin secretion profile to different nutrients in these rats. We have recently shown that lymph is a sensitive medium to measure incretin secretion in rodents and probably the preferred compartment for GLP-1 monitoring. To characterize the meal-induced incretin profile, we compared lymphatic incretin concentrations in the GK and Wistar rat after enteral macronutrient administration. After cannulation of the major mesenteric lymphatic duct and duodenum, each animal received an intraduodenal bolus of either a fat emulsion, dextrin, a mixed meal, or saline. Lymph was collected for 3 h and analyzed for triglyceride, glucose, GLP-1, and GIP content. There was no statistical difference in GIP or GLP-1 secretion after a lipid bolus between GK and Wistar rats. Dextrin and a mixed meal both increased incretin concentration area under the curve, however, significantly less in GK rats compared with Wistar rats (dextrin GIP: 707 +/- 106 vs. 1,373 +/- 114 pg ml(-1) h, respectively, P < 0.001; dextrin GLP-1: 82.7 +/- 24.3 vs. 208.3 +/- 26.3 pM/h, respectively, P = 0.001). After administration of a carbohydrate-containing meal, GK rats were unable to mount as robust a response of both GIP and GLP-1 compared with Wistar rats, a phenomenon not seen after a lipid meal. We propose a similar, glucose-mediated incretin secretion pathway defect of both K and L cells in GK rats.

The role of incretins in glucose homeostasis and diabetes treatment

Incretins are gut hormones that are secreted from enteroendocrine cells into the blood within minutes after eating. One of their many physiological roles is to regulate the amount of insulin that is secreted after eating. In this manner, as well as others to be described in this review, their final common raison d'être is to aid in disposal of the products of digestion. There are two incretins, known as glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide-1 (GLP-1), that share many common actions in the pancreas but have distinct actions outside of the pancreas. Both incretins are rapidly deactivated by an enzyme called dipeptidyl peptidase 4 (DPP4). A lack of secretion of incretins or an increase in their clearance are not pathogenic factors in diabetes. However, in type 2 diabetes (T2DM), GIP no longer modulates glucose-dependent insulin secretion, even at supraphysiological (pharmacological) plasma levels, and therefore GIP incompetence is detrimental to beta-cell function, especially after eating. GLP-1, on the other hand, is still insulinotropic in T2DM, and this has led to the development of compounds that activate the GLP-1 receptor with a view to improving insulin secretion. Since 2005, two new classes of drugs based on incretin action have been approved for lowering blood glucose levels in T2DM: an incretin mimetic (exenatide, which is a potent long-acting agonist of the GLP-1 receptor) and an incretin enhancer (sitagliptin, which is a DPP4 inhibitor). Exenatide is injected subcutaneously twice daily and its use leads to lower blood glucose and higher insulin levels, especially in the fed state. There is glucose-dependency to its insulin secretory capacity, making it unlikely to cause low blood sugars (hypoglycemia). DPP4 inhibitors are orally active and they increase endogenous blood levels of active incretins, thus leading to prolonged incretin action. The elevated levels of GLP-1 are thought to be the mechanism underlying their blood glucose-lowering effects.

Laparoscopic ileal interposition associated to a diverted sleeve gastrectomy is an effective operation for the treatment of type 2 diabetes mellitus patients with BMI 21-29

BACKGROUND

The objective of this study is to evaluate the clinical results of the laparoscopic interposition of a segment of ileum into the proximal duodenum associated to a sleeve gastrectomy (II-DSG) in order to treat patients with type 2 diabetes mellitus (T2DM) and body mass index (BMI) 21-29 kg/m2.

PATIENTS AND METHODS

The laparoscopic procedure was performed in 69 patients, 22 female and 47 male. Mean age was 51 years (range 41-63 years). Mean BMI was 25.7 (21.8-29.2) kg/m2. All patients had the diagnosis of T2DM for at least 3 years and evidence of stable treatment with oral hypoglycemic agents and or insulin for at least 12 months. Insulin therapy was used by 44% of the patients. Mean duration of T2DM was 11 years (range 3-18 years). Dyslipidemia was diagnosed in 72.5% and hypertension in 66.7%. Nephropathy was characterized in 29% of the patients, retinopathy in 26.1%, and neuropathy in 24.6%.

RESULTS

Overall, 95.7% of the patients achieved adequate glycemic control (Hb(A1c) < 7%) without antidiabetic medication. Hb(A1c) below 6% was achieved by 65.2%. Mean postoperative follow-up was 21.7 months (range 7-42 months). Mean postoperative BMI was 21.8 kg/m2. There was no conversion to open surgery. Median hospital stay was 3.4 days (range 2-58 days). Major postoperative complications were diagnosed in 7.3%. There was no mortality. Fasting glycemia decreased from a mean of 218 to 102 mg/dl, postprandial glycemia from 305 to 141 mg/dl, and homeostasis model assessment of insulin resistance (Homa-IR) from 5.2 to 0.77. All associated comorbidities and complications related to T2DM had significant improvement or control. Arterial hypertension was controlled in 91.3%. Macroalbuminuria was no longer observed. Microalbuminuria resolved in 87.5% of patients. Hypercholesterolemia was normalized in 95% and hypertriglyceridemia in 92% of patients.

CONCLUSIONS

Laparoscopic II-DSG was an effective operation in controlling T2DM in a nonobese (BM < 30 kg/m2) population. Associated diseases and related complications were also improved. A longer follow-up period is needed.

Hormonal evaluation following laparoscopic treatment of type 2 diabetes mellitus patients with BMI 20-34

BACKGROUND

A group of patients with type 2 diabetes mellitus (T2DM) and body mass index (BMI) 20-34 kg/m(2) were submitted to laparoscopic interposition of a segment of ileum into the proximal jejunum or into the proximal duodenum associated to a sleeve gastrectomy. The objective of this study is to evaluate the hormonal changes in the pre- and postoperative period.

MATERIALS AND METHODS

Hormonal evaluation was done in 58 patients operated between April 2005 and July 2006. Mean age was 51.4 years (40-66 years). Mean BMI was 28.2 (20-34.8) kg/m(2). All patients had had the diagnosis of T2DM for at least 3 years. Mean duration of T2DM was 9.6 years (3-22 years). Two techniques were performed, consisting of different combinations of ileal interposition (II) associated to a sleeve gastrectomy (SG). The following hormones were assayed in the pre- and postoperative period (mean 16 months) at the baseline and following specific food stimulation (30, 60, 120 min): glucogen-like protein 1 (GLP-1), glucose-dependent insulinotropic peptide (GIP), insulin, glucagon, C-peptide, amylin, cholecystokinin (CCK), pancreatic polypeptide (PPP), somatostatin, peptide YY (PYY), ghrelin, adiponectin, resistin, leptin, and interleukin-6 (IL-6).

RESULTS

Thirty patients had II associated to sleeve gastrectomy (II-SG) and 28 had II with diverted sleeve gastrectomy (II-DSG). GLP1 exhibited an important rise following the two operations, especially after II-DSG (p < 0.001). GIP also exhibited an important rise, with both II-SG and II-DSG being equally effective (p < 0.001). Insulin and amylin showed a significant rise at 30 min. Glucagon decreased slightly. CCK measurements were very low after II-DSG. PPP was also slightly altered by the II-DSG. PYY showed an important increase with both operations (p < 0.001). Ghrelin showed a significant decrease following the two operations (p < 0.001). Somatostatin and IL-6 were not affected (p = 0.632). Both leptin and resistin blood levels decreased. Adiponectin showed a slight increase. Mean postoperative follow-up was 19.2 months. Both II-SG and II-DSG were effective in achieving adequate glycemic control (91.2%).

CONCLUSIONS

There was a significant hormonal change following laparoscopic ileal interposition. These alterations may explain the promising good results associated to these operations for the treatment of T2DM in the nonmorbidly obese population.

Dietary resistant starch reduces levels of glucose-dependent insulinotropic polypeptide mRNA along the jejunum-ileum in both normal and type 2 diabetic rats

It has been reported that the circulating glucose-dependent insulinotropic polypeptide (GIP) levels were reduced by an intake of some foods/drugs capable of delaying carbohydrate digestion/absorption. In this study, we revealed that feeding rats with dietary resistant starch reduced the GIP mRNA levels along the entire length of the jejunoileum in both Wistar and type 2 diabetic GK rats.

Contribution of nonesterified fatty acids to insulin resistance in the elderly with normal fasting but diabetic 2-hour postchallenge plasma glucose levels: the Baltimore Longitudinal Study of Aging

Isolated postchallenge hyperglycemia (IPH) with normal fasting plasma glucose <100 mg/dL and plasma glucose with diabetic 2-hour plasma glucose >or=200 mg/dL after an oral glucose tolerance test (OGTT) is a common occurrence in the elderly. We sought to understand what unique characteristics this population might have that puts it at risk for this particular metabolic finding. We therefore conducted a longitudinal study of volunteers in the Baltimore Longitudinal Study of Aging (BLSA). All volunteers had an OGTT performed (75 g) on 2 or more occasions. We measured plasma levels of glucose, insulin, C-peptide, glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic peptide (GIP), ghrelin, leptin, adiponectin, resistin, C-reactive protein, cytokines, and their soluble receptors, as well as nonesterified free fatty acids (NEFAs). We determined that 22 subjects in BLSA had IPH, accounting for 2.1% of the BLSA population. All 22 were older than 65 years. They were then matched by age, sex, and body mass index to 12 subjects who had isolated impaired glucose tolerance (IGT) and 15 subjects with normal glucose tolerance (NGT). All subjects had normal fasting glucose levels <100 mg/dL in accordance with the American Diabetes Association Expert Committee on the Classification and Diagnosis of Diabetes Mellitus criteria (2003). We found that subjects with IPH had similar plasma insulin levels to the other 2 groups, except at the 2-hour time when their insulin levels were higher than NGT (P < .05). Although there was a clear trend for differences in the insulinogenic index, the areas under the curves for insulin, systolic blood pressure, adiponectin, and C-reactive protein across the glucose tolerance categories revealed no statistical significance. Cytokines and their soluble receptors, gut hormones, and adipokines were similar in all 3 groups. The NEFA levels were significantly elevated in the fasting state (P < .05) in the IPH compared with NGT, with IGT intermediate between the other 2 groups. The rate of clearance of NEFAs after the OGTT decreased progressively from the NGT to the IPH group (in micromoles per liter per minute: NGT, 11.9 vs IGT, 7.6 vs IPH, 3.0). We conclude that the rate of suppression of lipolysis in the elderly determines the sensitivity of glucose uptake to insulin after OGTT.

Gut-expressed gustducin and taste receptors regulate secretion of glucagon-like peptide-1

Glucagon-like peptide-1 (GLP-1), released from gut endocrine L cells in response to glucose, regulates appetite, insulin secretion, and gut motility. How glucose given orally, but not systemically, induces GLP-1 secretion is unknown. We show that human duodenal L cells express sweet taste receptors, the taste G protein gustducin, and several other taste transduction elements. Mouse intestinal L cells also express alpha-gustducin. Ingestion of glucose by alpha-gustducin null mice revealed deficiencies in secretion of GLP-1 and the regulation of plasma insulin and glucose. Isolated small bowel and intestinal villi from alpha-gustducin null mice showed markedly defective GLP-1 secretion in response to glucose. The human L cell line NCI-H716 expresses alpha-gustducin, taste receptors, and several other taste signaling elements. GLP-1 release from NCI-H716 cells was promoted by sugars and the noncaloric sweetener sucralose, and blocked by the sweet receptor antagonist lactisole or siRNA for alpha-gustducin. We conclude that L cells of the gut "taste" glucose through the same mechanisms used by taste cells of the tongue. Modulating GLP-1 secretion in gut "taste cells" may provide an important treatment for obesity, diabetes and abnormal gut motility.

Ubiquitination is involved in glucose-mediated downregulation of GIP receptors in islets

Glucose-dependent insulinotropic polypeptide (GIP) is a gastrointestinal hormone that has a potent stimulatory effect on insulin release under conditions of normal glucose tolerance. However, its insulinotropic effect is reduced or even absent entirely in type 2 diabetic patients. In this study, we addressed the role of glucose concentration in the diabetic range of >or=11 mM, i.e., hyperglycemia per se, as a cause of the lack of response to GIP. Culturing rat and human pancreatic islets in >or=11 mM glucose for up to 24 h resulted in prevention of GIP-mediated intracellular cAMP increase compared with culturing in 5 mM glucose. Western blot analysis revealed a selective 67 +/- 2% (rat) and 60 +/- 8% (human) decrease of GIP-R expression in islets exposed to >or=11 mM glucose compared with 5 mM glucose (P < 0.001). We further immunoprecipitated GIP-R from islets and found that GIP-R was targeted for ubiquitination in a glucose- and time-dependent manner. Downregulation of GIP-R was rescued by treating isolated islets with proteasomal inhibitors lactacystin and MG-132, and the islets were once again capable of increasing intracellular cAMP levels in response to GIP. These results suggest that the GIP-R is ubiquitated, resulting in downregulation of the actions of GIP.

Incretins and the development of type 2 diabetes

The incretin hormones gastric inhibitory polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) are released in response to nutrient ingestion and potentiate glucose-stimulated insulin secretion from pancreatic beta cells. The augmentation of postprandial insulin secretion by such gastrointestinal hormones is called the incretin effect. The incretin effect is almost completely absent in patients with type 2 diabetes. This is due to 1) an approximate 15% reduction in postprandial GLP-1 secretion and 2) a near total loss of insulinotropic activity of GIP. This review article summarizes clinical studies on abnormalities in the secretion and insulinotropic effects of GIP and GLP-1 in patients with type 2 diabetes as well as in individuals at high risk. A significant proportion of first-degree relatives are characterized by a reduced insulinotropic response to exogenous GIP. Nevertheless, this phenomenon does not predispose to a more rapid deterioration in glucose tolerance or conversion to impaired glucose tolerance or diabetes. Therefore, although there are hints of early abnormalities in incretin secretion and action in prediabetic populations, it has not been proven that such phenomena are central to the pathogenesis of type 2 diabetes.

Human duodenal enteroendocrine cells: source of both incretin peptides, GLP-1 and GIP

Among the products of enteroendocrine cells are the incretins glucagon-like peptide-1 (GLP-1, secreted by L cells) and glucose-dependent insulinotropic peptide (GIP, secreted by K cells). These are key modulators of insulin secretion, glucose homeostasis, and gastric emptying. Because of the rapid early rise of GLP-1 in plasma after oral glucose, we wished to definitively establish the absence or presence of L cells, as well as the relative distribution of the incretin cell types in human duodenum. We confirmed the presence of proglucagon and pro-GIP genes, their products, and glucosensory molecules by tissue immunohistochemistry and RT-PCR of laser-captured, single duodenal cells. We also assayed plasma glucose, incretin, and insulin levels in subjects with normal glucose tolerance and type 2 diabetes for 120 min after they ingested 75 g of glucose. Subjects with normal glucose tolerance (n=14) had as many L cells (15+/-1), expressed per 1,000 gut epithelial cells, as K cells (13+/-1), with some containing both hormones (L/K cells, 5+/-1). In type 2 diabetes, the number of L and L/K cells was increased (26+/-2; P<0.001 and 9+/-1; P < 0.001, respectively). Both L and K cells contained glucokinase and glucose transporter-1, -2, and -3. Newly diagnosed type 2 diabetic subjects had increased plasma GLP-1 levels between 20 and 80 min, concurrently with rising plasma insulin levels. Significant coexpression of the main incretin peptides occurs in human duodenum. L and K cells are present in equal numbers. New onset type 2 diabetes is associated with a shift to the L phenotype.

cDNA array reveals increased expression of glucose-dependent insulinotropic polypeptide following chronic clozapine treatment: role in atypical antipsychotic drug-induced adverse metabolic effects

Clozapine is an atypical antipsychotic drug with unique pharmacological and therapeutic properties. Unlike the typical antipsychotic drug, haloperidol, clozapine does not cause extrapyramidal side effects; however, weight gain, dyslipidemia, and type II diabetes are commonly associated with the use of this drug in subjects with schizophrenia. The aim of this study was to profile gene expression in the rat striatum following clozapine treatment. Chronic treatment with clozapine revealed upregulation of several genes including the glucose-dependent insulinotropic polypeptide (GIP) gene by over 200% in the rat striatum. The cDNA array results for the GIP gene were confirmed by real-time RT-PCR as well as by radioimmunoassay. Expression of the GIP gene in the central nervous system is consistent with the results of retinal GIP gene expression as reported by other investigators. Taken together, these findings implicate the possible role of GIP as a neuromodulator in the central nervous system. GIP is an insulinotropic agent with stimulatory effects on insulin synthesis and release from the pancreas. However, changes in brain GIP levels are most likely unrelated to the metabolic adverse effects (dyslipidemia, type II diabetes, weight gain) associated with clozapine treatment. Therefore, we also measured GIP gene expression in the K-cell-rich regions, duodenum and jejunum (small intestine), and plasma GIP levels using radioimmunoassay following chronic treatment with clozapine. GIP mRNA levels in the small intestine and the plasma GIP at the protein level were significantly elevated in clozapine-treated subjects. Furthermore, as observed in humans, chronic clozapine treatment also caused weight gain, and increased levels of insulin, triglycerides and leptin in the plasma. These results suggest that adverse metabolic effects associated with clozapine treatment may be related to its ability to increase intestinal gene expression for GIP.