Evaluation of Beta Cell Dysfunction by Mixed Meal Tolerance Test and Oral L-arginine in Patients with Newly Diagnosed Type 2 Diabetes Mellitus

Effects of arginine on coenzyme-Q10 micelle uptake for mitochondria-targeted nanotherapy in phenylketonuria

Phenylketonuria (PKU) is a rare inherited metabolic disease characterized by phenylalanine hydroxylase enzyme deficiency. In PKU patients, coenzyme Q10 (CoQ10) levels were found low. Therefore, we focused on the modification of CoQ10 to load the micelles and increase entry of micelles into the cell and mitochondria, and it is taking a part in ATP turnover. Micelles had produced by comparing two different production methods (thin-film layer and direct-dissolution), and characterization studies were performed (zeta potential, size, and encapsulation efficiency). Then, L-arginine (LARG) and poly-arginine (PARG) were incorporated with the micelles for subsequential release and PKU cell studies. The effects of these components on intracellular uptake and their use in the cellular cycle were analyzed by ELISA, Western blot, membrane potential measurement, and flow cytometry methods. In addition, both effects of LARG and PARG micelles on pharmacokinetics at the cellular level and their cell binding rate were determined. The thin-film method was found superior in micelle preparation. PARG/LARG-modified micelles showed sustained release. In the cellular and mitochondrial uptake of CoQ10, CoQ10-micelle + PARG > CoQ10-micelle + LARG > CoQ10-micelle > CoQ10 was found. This increased localization caused lowering of oxygen consumption rates, but maintaining mitochondrial membrane potential. The study results had showed that besides micelle formulation, PARG and LARG are effective in cellular and mitochondrial targeting.

Diet and Physical Activity as Determinants of Continuously Measured Glucose Levels in Persons at High Risk of Type 2 Diabetes

We examined how dietary and physical activity behaviors influence fluctuations in blood glucose levels over a seven-day period in people at high risk for diabetes. Twenty-eight participants underwent a mixed meal tolerance test to assess glucose homeostasis at baseline. Subsequently, they wore an accelerometer to assess movement behaviors, recorded their dietary intakes through a mobile phone application, and wore a flash glucose monitoring device that measured glucose levels every 15 min for seven days. Generalized estimating equation models were used to assess the associations of metabolic and lifestyle risk factors with glycemic variability. Higher BMI, amount of body fat, and selected markers of hyperglycemia and insulin resistance from the meal tolerance test were associated with higher mean glucose levels during the seven days. Moderate- to vigorous-intensity physical activity and polyunsaturated fat intake were independently associated with less variation in glucose levels (CV%). Higher protein and polyunsaturated fatty acid intakes were associated with more time-in-range. In contrast, higher carbohydrate intake was associated with less time-in-range. Our findings suggest that dietary composition (a higher intake of polyunsaturated fat and protein and lower intake of carbohydrates) and moderate-to-vigorous physical activity may reduce fluctuations in glucose levels in persons at high risk of diabetes.

Effects of mixed meal tolerance test on gastric emptying, glucose and lipid homeostasis in obese nonhuman primates

Meal ingestion elicits a variety of neuronal, physiological and hormonal responses that differ in healthy, obese or diabetic individuals. The mixed meal tolerance test (MMTT) is a well-established method to evaluate pancreatic β-cell reserve and glucose homeostasis in both preclinical and clinical research in response to calorically defined meal. Nonhuman primates (NHPs) are highly valuable for diabetic research as they can naturally develop type 2 diabetes mellitus (T2DM) in a way similar to the onset and progression of human T2DM. The purpose of this study was to investigate the reproducibility and effects of a MMTT containing acetaminophen on plasma glucose, insulin, C-peptide, incretin hormones, lipids, acetaminophen appearance (a surrogate marker for gastric emptying) in 16 conscious obese cynomolgus monkeys (Macaca fascicularis). Plasma insulin, C-peptide, TG, aGLP-1, tGIP, PYY and acetaminophen significantly increased after meal/acetaminophen administration. A subsequent study in 6 animals showed that the changes of plasma glucose, insulin, C-peptide, lipids and acetaminophen were reproducible. There were no significant differences in responses to the MMTT among the obese NHPs with (n = 11) or without (n = 5) hyperglycemia. Our results demonstrate that mixed meal administration induces significant secretion of several incretins which are critical for maintaining glucose homeostasis. In addition, the responses to the MMTTs are reproducible in NHPs, which is important when the MMTT is used for evaluating post-meal glucose homeostasis in research.

A Specific Gut Microbiota Dysbiosis of Type 2 Diabetic Mice Induces GLP-1 Resistance through an Enteric NO-Dependent and Gut-Brain Axis Mechanism

Glucagon-like peptide-1 (GLP-1)-based therapies control glycemia in type 2 diabetic (T2D) patients. However, in some patients the treatment must be discontinued, defining a state of GLP-1 resistance. In animal models we identified a specific set of ileum bacteria impairing the GLP-1-activated gut-brain axis for the control of insulin secretion and gastric emptying. Using prediction algorithms, we identified bacterial pathways related to amino acid metabolism and transport system modules associated to GLP-1 resistance. The conventionalization of germ-free mice demonstrated their role in enteric neuron biology and the gut-brain-periphery axis. Altogether, insulin secretion and gastric emptying require functional GLP-1 receptor and neuronal nitric oxide synthase in the enteric nervous system within a eubiotic gut microbiota environment. Our data open a novel route to improve GLP-1-based therapies.

A systems biology preview of the relationships between mineral and metabolic complications in chronic kidney disease

There are emerging data that the skeleton is connected to systemic biological functions through the release of two osteoblast-/osteocyte-derived hormones, fibroblastic growth factor 23 (FGF23) and undercarboxylated osteocalcin (Ocn). FGF23 is important in the regulation of phosphate and vitamin D metabolism, whereas Ocn participates in endocrine networks, coordinating bone and fat mass, energy metabolism, and sex hormone production. Bone remodeling and mineralization per se, along with the hormones leptin, insulin, glucocorticoids, PTH, and 1,25(OH)2D, regulate the release of FGF23 and Ocn, leading to complex cross-talk and coordination between endocrine networks previously thought to be distinct. These pathways are particularly important in chronic kidney disease, in which both FGF23 and Ocn are increased. Although these hormones initially serve an adaptive role, with progressive loss of renal function they show maladaptive effects, particularly on the cardiovascular system, through multiple mechanisms, including possible cross-talk with the renin angiotensin system. The complex interconnections between the various endocrine networks in chronic kidney disease may account for the difficulty in treating the uremic state.

Indices of insulin secretion during a liquid mixed-meal test in obese youth with diabetes

OBJECTIVE

To compare indices of insulin secretion, insulin sensitivity (IS), and oral disposition index (oDI) during the liquid mixed-meal test in obese youth with clinically diagnosed type 2 diabetes mellitus (T2DM) and negative autoantibodies (Ab(-)) versus those with T2DM and positive autoantibodies (Ab(+)) to examine whether differences in β-cell function can be detected between the 2 groups.

STUDY DESIGN

Twenty-seven youth with Ab(-) and 15 youth with Ab(+) clinically diagnosed T2DM underwent a mixed-meal test (Boost; 55% carbohydrate, 25% protein, and 20% fat). Fasting and mixed-meal-derived insulin and C-peptide indices of IS, secretion (30-minute insulinogenic [ΔI(30)/ΔG(30)] and C-peptide [ΔC(30)/ΔG(30)]), and oDI were calculated.

RESULTS

Indices of insulin secretion were ~40%-50% lower in patients with Ab(+) T2DM compared with those with Ab(-) T2DM. After controlling for body mass index, ΔI(30)/ΔG(30), ΔC(30)/ΔG(30), C-peptide area under the curve (AUC)/glucose AUC, and insulin AUC/glucose AUC were significantly (P < .05) lower in the Ab(+) group compared with the Ab(-) group. Sensitivity indices were significantly higher in the Ab(+) group. The oDI, 1/fasting insulin × ΔI(30)/ΔG(30) (0.04 ± 0.02 vs 0.12 ± 0.02 mg/dL(-1); P = .005), and 1/fasting C-peptide × ΔC(30)/ΔG(30) (0.02 ± 0.009 vs 0.05 ± 0.006 mg/dL(-1); P = .018) were lower in the Ab(+) group. Receiver operating characteristic curve analyses revealed that fasting C-peptide <3.2 ng/mL had 87% sensitivity and 74% specificity and ΔC(30)/ΔG(30) <0.075 ng/mL per mg/dL had 93% sensitivity and 80% specificity for identifying youth with Ab(+) T2DM.

CONCLUSION

During a liquid mixed-meal test, indices of β-cell function were lower and IS was higher in patients with Ab(+) T2DM versus those with Ab(-) T2DM, with high sensitivity and specificity for fasting and stimulated C-peptide as markers of Ab(+) status. Indices of insulin secretion during this standardized mixed-meal test could be used to assess β-cell function in therapeutic trials of β-cell restoration in youth with T2DM.

GPRC6A mediates the effects of L-arginine on insulin secretion in mouse pancreatic islets

L-arginine (l-Arg) is an insulin secretagogue, but the molecular mechanism whereby it stimulates insulin secretion from β-cells is not known. The possibility that l-Arg regulates insulin secretion through a G protein-coupled receptor (GPCR)-mediated mechanism is suggested by the high expression of the nutrient receptor GPCR family C group 6 member A (GPRC6A) in the pancreas and TC-6 β-cells and the finding that Gprc6a(-/]minus]) mice have abnormalities in glucose homeostasis. To test the direct role of GPRC6A in regulating insulin secretion, we evaluated the response of pancreatic islets derived from Gprc6a(-/]minus]) mice to L-Arg. We found that the islet size and insulin content were decreased in pancreatic islets from Gprac6a(-/]minus]) mice. These alterations were selective for β-cells, because there were no abnormalities in serum glucagon levels or glucagon content of islets derived from Gprac6a(-/]minus]) mice. Significant reduction was observed in both the pancreatic ERK response to L-Arg administration to Gprc6a(-/]minus]) mice in vivo and L-Arg-induced insulin secretion and production ex vivo in islets isolated from Gprc6a(-/]minus]) mice. L-Arg stimulation of cAMP accumulation in isolated islets isolated from Gprc6a(-/]minus]) mice was also diminished. These findings suggest that l-Arg stimulation of insulin secretion in β-cells is mediated, at least in part, through GPRC6A activation of cAMP pathways.

Multiligand specificity and wide tissue expression of GPRC6A reveals new endocrine networks

Emerging evidence supports the hypothesis that the skeleton is an endocrine organ that regulates energy metabolism through the release of the osteoblast-derived hormone, osteocalcin (Ocn). This bone-pancreas endocrine network is controversial because important gaps remain to be filled in our knowledge of the physiological effects of Ocn in multiple organs and the complex alterations in other hormonal networks induced by Ocn administration. A key step toward understanding the integrative regulation of energy metabolism by bone is the identification of GPCR family C group 6 member A (GPRC6A) as the Ocn receptor. GPRC6A is an amino acid-sensing G protein-coupled receptor highly expressed in β-cells and is activated by recombinant Ocn in vitro and in vivo but that is widely expressed in tissues other than the pancreas and is capable of sensing multiple structurally unrelated ligands, including l-amino acids, cations, and anabolic steroids in addition to Ocn. The broad expression and multiligand specificity of GPRC6A is identifying both systemic and paracrine regulation of seemingly disparate biological processes, ranging from energy metabolism, sexual reproduction, hypothalamic-pituitary function, bone formation, and prostate cancer. Consistent with the existence of more complex endocrine networks, ablation of GPRC6A in Gprc6a(-/-) mice results in complex metabolic abnormalities, including obesity, glucose intolerance, hepatic steatosis, insulin resistance, hyperphosphatemia, osteopenia, plus several hormonal abnormalities, including decreased circulating testosterone, IGF-I, and insulin and increased estradiol, LH, GH, and leptin. Recombinant Ocn also regulates testosterone production by the testes and male fertility through a GPRC6A-dependent mechanism, and testosterone regulation of LH secretion is abnormal in Gprc6a(-/-) mice. Thus, GPRC6A, as the biologically relevant receptor for Ocn, defines not only a molecular mechanism for linking bone metabolism with metabolic regulation of β-cells and sexual reproduction but also as a receptor shared by testosterone and dietary factors, and it is also involved in multiple endocrine networks integrating the functions of pancreas, muscle, liver, fat, testes, bone, and the hypothalamic-pituitary axis with alterations in both environmental and endogenous ligands.

Assessment of β-cell function in human patients

This review focuses on the methods accessing β-cell function. β-cell failure is the critical step in the development of type 2 diabetes. Therefore, assessment of β-cell function is an important part of the evaluation and treatment of diabetic patients. However, it is not easy because of complex interaction between multiple tissues. Several parameters should be considered, such as glucose level and insulin sensitivity of diverse insulin target tissues to assess β-cell function. To overcome these difficulties, several invasive or non-invasive methods have been developed to assess β-cell function for clinical or research purposes.