Hideshi Kuzuya

Determination of free and total insulin and C-peptide in insulin-treated diabetics.

Serum levels of free and total insulin as well as total C-peptide immunoreactivity (C-peptide and proinsulin) and C-peptide were measured in insulin-treated diabetics with circulating insulin antibodies by the addition of polyethylene glycol (PEG) before and after acidification. PEG resulted in complete precipitation of insulin antibodies from serum and made it possible to measure free insulin in the supernatant. Incubation of serum at 37δ C. for two hours before addition of PEG resulted in values for free insulin that probably resembled the in-vivo levels most closely. The same method could also be used to remove proinsulin bound to circulating insulin antibodies and permitted the measurement of C-peptide in the supernatant. Clinical studies using this approach indicate that combined measurements of serum free and total insulin and C-peptide provide information that is helpful in understanding the contribution of endogenous and exogenous insulin to the course and metabolic control of insulin-requiring diabetic patients.

Increased Adiponectin Secretion by Highly Purified Eicosapentaenoic Acid in Rodent Models of Obesity and Human Obese Subjects

Objectives—Fish oil rich in n-3 polyunsaturated fatty acids (PUFAs) or n-3 PUFAs have been shown to reduce the incidence of coronary heart disease. Here we investigated the effect of highly purified eicosapentaenoic acid (EPA) on production of adiponectin, the only established antiatherogenic and antiinflammatory adipocytokine, in rodent models of obesity and human obese subjects. Methods and Results—We demonstrated that EPA increases adiponectin secretion in genetically obese ob/ob mice and high-fat diet–induced obese mice. In the in vitro coculture of adipocytes and macrophages, EPA reversed the coculture-induced decrease in adiponectin secretion at least in part through downregulation of tumor necrosis factor-&agr; in macrophages. We also showed significant increase in plasma adiponectin concentrations in human obese subjects after a 3-month treatment with EPA (1.8 g daily). Multivariate regression analysis revealed that EPA treatment is the only independent determinant of plasma adiponectin concentrations. Conclusion—This study demonstrates that EPA increases adiponectin secretion in rodent models of obesity and human obese subjects, possibly through the improvement of the inflammatory changes in obese adipose tissue. Because EPA has reduced the risk of major coronary events in a large-scale, prospective, randomized clinical trial, this study provides important insight into its therapeutic implication in obesity-related metabolic sequelae.

Characterization of Seven C-peptide Antisera

The plasma C-peptide immunoreactivity (CPR) in 10 normal subjects varied considerably when measured with different antisera in parallel assays. The CPR level correlated with the blank “CPR” value measured in plasma devoid of C-peptide and to a lesser degree with the sensitivity of the standard curves obtained with the individual antisera. Storage of plasma samples at different temperatures and for different lengths of time before the analyses were carried out resulted In further variation in the CPR results. This was caused by a time- and temperature-dependent fall in CPR, which was more pronounced with some antisera than with others. This sensitivity to storage of plasma did not correlate with the antigenk characteristics of the antisera as determined by their reactivity with 11 specific fragments of the C-peptide molecule. The contribution of human proinsulin to the CPR concentration in normal subjects was considered to be negligible even though the relative immunoreactivity of human proinsulin and C-peptide ranged from 11 to 143 per cent among these antisera. These results suggest that differences in C-peptide antisera are a major reason for the variation in the concentration of circulating CPR as measured in different, C-peptide immunoassays.

Trial of insulinlike growth factor I therapy for patients with extreme insulin resistance syndromes

Extreme insulin resistance occurs in patients with primary defects in insulin action at the receptor or postreceptor levels. The condition commonly is associated with acanthosis nigricans and ovarian masculinization. Despite a marked increase in insulin secretion, some patients develop frank diabetes mellitus that does not respond adequately to insulin therapy. Insulinlike growth factor I exerts metabolic effects similar to those of insulin. This study assessed the potential effectiveness of IGF-I as a blood glucose lowering agent in patients with extreme insulin resistance syndromes, including type A insulin resistance, congenital generalized lipodystrophy, and leprechaunism. Among the 11 patients studied, some exhibited mutated insulin receptors, whereas others were suspected to have defects in postreceptor sites. In each patient, plasma glucose levels decreased in response to subcutaneous injections of recombinant human IGF-I (0.1−0.3 mg/kg body wt). The degree of the decrease was roughly comparable with that observed in normal individuals. IGF-I also reduced plasma insulin concentrations. A long-term trial of IGF-I (up to 16 mo) showed that IGF-I (0.1−0.4 mg/kg body wt twice daily) is effective in lowering both fasting and postprandial plasma glucose concentrations with decreases in both fructosamine and HbA1c values. Improvement of acanthosis nigricans was observed in some of the patients. These results suggest that recombinant human IGF-I could be used clinically as a hypoglycemic agent in diabetic patients with extreme insulin resistance in whom insulin treatment is ineffective.

Reduced Expression of the Leptin Gene (ob) by Catecholamine Through a GS Protein-Coupled Pathway in 3T3-L1 Adipocytes

Leptin, a recently identified hormone, is believed to reduce appetite and maintain body weight. The mRNA of leptin is expressed only in mature adipose cells. To clarify the regulation of leptin gene expression in adipocytes, 3T3-L1 adipocytes were treated for 16 h with various agents known to modulate lipid metabolism, and then the leptin mRNA was measured by the reverse transcription-polymerase chain reaction method. Interestingly, both norepinephrine and isoproterenol reduced the level of leptin mRNA to about 20% of that found in untreated control cells in a dose-dependent fashion. The maximum reduction occurred at 100 nmol/l of either norepinephrine or isoproterenol, and the half-maximal effect was observed at ∼ 3 nmol/l norepinephrine and ∼ 1 nmol/l isoproterenol. Propranolol reversed about 50% of the reduction by either norepinephrine or isoproterenol. In contrast, phentolamine did not inhibit the reduction by either norepinephrine or isoproterenol. Moreover, both cholera toxin and dibutyryl cAMP decreased the level of leptin mRNA to about 10% of that in control cells in a dose-dependent fashion. The maximum effect was elicited at 100 ng/ml cholera toxin and 100 micromol/l dibutyryl cAMP. The concentration producing the half-maximal effect was ∼ 1 ng/ml cholera toxin and ∼ 50 μmol/l dibutyryl cAMP. Dibutyryl cGMP, however, did not affect leptin gene expression. These results suggest that a signaling pathway that results in the activation of protein kinase A regulates leptin gene expression in 3T3-L1 adipocytes.

14-3-3β Protein Associates with Insulin Receptor Substrate 1 and Decreases Insulin-stimulated Phosphatidylinositol 3′-Kinase Activity in 3T3L1 Adipocytes

The 14-3-3 protein family has been implicated in growth factor signaling. We investigated whether 14-3-3 protein is involved in insulin signaling in 3T3L1 adipocytes. A significant amount of insulin receptor substrate 1 (IRS-1) was immunodetected in the immunoprecipitate with anti-14-3-3β antibody at the basal condition. 100 nm insulin increased the amount of IRS-1 in the immunoprecipitate 2.5-fold. The effect of insulin was abolished by 100 nm wortmannin. An in vitro binding study revealed that glutathione S-transferase-14-3-3β fusion protein directly associates with recombinant IRS-1. Pretreatment of recombinant IRS-1 with alkaline phosphatase clearly decreased this association. Because the recombinant IRS-1 was not phosphorylated on its tyrosine residues, the results suggest that serine/threonine phosphorylation of IRS-1 is responsible for the association. When the cells are treated with insulin, phosphatidylinositol 3′-kinase (PI3K) is supposed to complex either 14-3-3β-IRS-1 or IRS-1. The 14-3-3β-IRS-1-PI3K and IRS-1-PI3K complexes were separately prepared by a sequential immunoprecipitation, first with anti-14-3-3β and then with anti-IRS-1 antibodies. The specific activity of the PI3K in the former was approximately half of that in the latter, suggesting that 14-3-3β protein bound to IRS-1 inhibits insulin-stimulated lipid kinase activity of PI3K in 3T3L1 adipocytes.

Hypoglycemia and Endogenous Hyperinsulinism Complicating Diabetes Mellitus Application of the C-Peptide Assay to Diagnosis and Therapy

Described here is a patient with insulin-requiring diabetes mellitus in whom spontaneous fasting hypoglycemia developed. Endogenous hyperinsulinism was considered after the systematic exclusion of other causes of hypoglycemia, and this possibility was confirmed by measurement of serum C-peptide reactivity (CPR), an indicator of beta cell secretory function. Subtotal pancreatectomy relieved the fasting hypoglycemia, and was associated with a marked decline in CPR levels. The pancreatic islets showed hyperplasia and the beta cells were degranulated.

Starvation-induced Changes of Somatostatin, Glucagon, and Insulin Secretion from the Isolated Perfused Rat Pancreas

The effect of 16- and 48-h fasting on pancreatic somatostatin, insulin, and glucagon secretion was studied, using the isolated perfused rat pancreas. In the presence of 4.4 mM glucose, basal somatostatin and insulin concentrations in the perfusate were significantly lower in 48-h fasted rats than in fed animals, whereas basal glucagon secretion was significantly elevated in fasted rats. The infusion of 19 mM arginine significantly augmented secretion of somatostatin and glucagon and attenuated insulin secretion in 48-h fasted rats. It is concluded that fasting causes a decrease in basal pancreatic somatostatin secretion in vitro, although the response to arginine is rather exaggerated. Insulin and glucagon secretion also changed during the fasting. These results suggest that not only insulin and glucagon, but also somatostatin contribute to nutrient homeostasis.

Circulating serum C-peptide. A brief review of diagnostic implications.

The discovery of proinsulin by Steiner and Oyer1 established that insulin is not synthesized as such, but as a larger precursor molecule in which the insulin A and B chains are connected by an addi...

Somatostatin-like Immunoreactivity in Human Peripheral Plasma Measured by Radioimmunoassay Following Affinity Chromatography

Somatostatin-like immunoreactivity (SLI) concentrations were determined in human peripheral plasma using affinity chromatography followed by radioimmunoassay. In normal subjects, fasting SLI ranged from 2.9 to 22.0 pg/ml with a mean ± SE value of 10.2 ± 2.1 pg/ml. In totally pancreatectomized or gastrectomized patients, fasting SLI levels were not different from the values in normal subjects. In patients with medullary thyroid carcinoma, fasting SLI ranged from 11.8 to 71.0 pg/ml with a mean of 29.3 ± 12.3 pg/ml, which was significantly higher than normal values (P < 0.01). Following meal ingestion, plasma SLI increased significantly in normal subjects from a basal level of 9.1 ± 2.1 pg/ml to a peak value of 15.4 ± 2.9 pg/ml (P < 0.02). These results indicate that radioimmunoassay combined with affinity chromatography provides an accurate method of measuring SLI in human plasma.