G proteins and regulation of pyruvate dehydrogenase activity by insulin in human circulating lymphocytes

The evaluation of intracellular energy metabolism in prediabetic patients and patients newly diagnosed with type 2 diabetes mellitus

Background/aim

Increased susceptibility to infections is a serious problem in diabetics. Impairment in the energy metabolism of the immune system is the main source of the problem. Early diagnosis of the impairment in energy metabolism is crucial. Our study aimed to investigate the energy metabolism in leukocytes in patient groups such as prediabetics and patients newly diagnosed with type 2 diabetes mellitus.

Materials and methods

Our study included 21 newly diagnosed type 2 diabetic patients (NDDP), 30 prediabetic patients, and 22 adult volunteers. 75 g oral glucose tolerance test (OGTT) was applied to all patients included in the study. Blood samples were taken after 9-16 h of fasting and fasting blood glucose (FBG), postprandial blood glucose (PBG) levels, total cholesterol (TC), triglyceride (TG), high- density lipoprotein (HDL), fasting serum insulin, and hemoglobin A1c (HbA1c) levels were evaluated. After the cells were completely lysed, citrate levels from the released mononuclear leukocyte cells (MNC) content were manually studied, and lactate levels were applied to the autoanalyzer with the lactate kit. Lactate and citrate results were calculated as μg/mL. Statistical comparisons were done using Chi-square test, Mann-Whitney U test and student’s t test, and P < 0.05 values were accepted as significant.

Results

A significant difference was found between the controls and the other groups (newly diagnosed diabetic patients (NDDP), impaired fasting glucose (IFG), and impaired glucose tolerance (IGT)) in terms of FBG levels (P < 0.001, P < 0.001 and P < 0.001, respectively). IFG and IGT patients had significantly higher PBG levels compared to the control group (P = 0.009 and P < 0.001, respectively). There was a significant difference between the IFG and IGT patients in terms of insulin levels (P = 0.019). There was a significant relationship between FBG levels and lactate production only in the NDDP group (r = 0.610, P = 0.003)

Conclusion

The metabolic effects of hyperglycemia on leukocytes is in direction of anaerobic glycolysis. The increased anaerobic pathway is closely related to blood glucose levels and insulin resistance.

Cellular glucose transport and glucotransporter 4 expression as a therapeutic target: clinical and experimental studies

INTRODUCTION

The normalization of cellular glucose assimilation is the basic aim of metabolic therapy in type 2 diabetes mellitus (T2DM). It requires parallel changes in the process of cellular glucose transport (CGT). Therefore the level of CGT could be regarded as a therapeutic target for oral hypoglycemic drugs in T2DM. To explore this hypothesis, CGT levels before and after sulfonylurea therapy were investigated. Peripheral blood lymphocytes were used as a cell model for testing CGT.

MATERIALS AND METHODS

CGT was assessed by experimental in vitro tests allowing timed comparative observation of the transport process during the incubation of lymphocytes with 2-[(3)H(G)] glucose under basal conditions and after the addition of sulfonylurea or sulfonylurea plus insulin. The incubation tests were performed at baseline in 28 persons with newly diagnosed, therapy-naive T2DM and in 20 control subjects. In the diabetic patients the tests for CGT were repeated after 3 months of sulfonylurea therapy. The level of glucotransporter 4 (GLUT4) expression was also assessed by flow cytometry before and after the therapy.

RESULTS

Before treatment, CGT was significantly lower in the subjects with T2DM. The cells responded to the addition of sulfonylurea by a moderate increase in CGT. This response was augmented by the addition of insulin to sulfonylurea in the culture medium.

CONCLUSIONS

The three-month therapy with sulfonylurea resulted in a significant increase in CGT in all types of culture tests. This sulfonylurea-related improvement in CGT was associated with a near normalization of GLUT4 expression in the cells.

Glucose transport in human peripheral blood lymphocytes influenced by type 2 diabetes mellitus

INTRODUCTION

The objective of this study was to evaluate glucose transport into lymphocytes in healthy subjects and patients with type 2 diabetes mellitus (DM) treated either with diet only or with insulin and to propose peripheral blood lymphocytes as a convenient model for cellular glucose transport studies.

MATERIALS AND METHODS

Sixty subjects with type 2 DM, 30 treated with diet only and 30 with insulin, were investigated. Thirty healthy subjects matched for age, weight, and sex served as a control group. Deoxy-D-glucose, 2-[(3)H(G)] transport was studied in isolated peripheral blood lymphocytes. Expression of glucose transporters was ascertained by immunocytochemical identification and by Western blotting.

RESULTS

In lymphocytes from the control group, deoxy-D-glucose uptake increased gradually with the duration of the experiment. In diabetics treated with insulin, the maximal increase in deoxy-D-glucose uptake was observed after 30 min of the investigation, followed by a plateau phase. In diabetics treated with diet, deoxy-D-glucose uptake increased slowly during the first 30 min. The presence of GLUT1 and GLUT3 in lymphocytes was confirmed in this study.

CONCLUSIONS

Glucose transport into lymphocytes is altered in type 2 DM. In lymphocytes from diabetics, the dynamics of deoxy-D-glucose uptake significantly differed from that in healthy subjects. There was also a significant difference between the diabetic groups, representing different modes of therapy and stages of the disease. Glucose transport into lymphocytes is apparently influenced by DM as well as by the mode of therapy. We suggest that peripheral blood lymphocytes may become a promising model for studies on glucose transport in diabetes.

Down-regulation of pyruvate dehydrogenase phosphatase in obese subjects is a defect that signals insulin resistance

OBJECTIVE

The objective of this study was to determine whether down-regulation of pyruvate dehydrogenase phosphatase (PDP) is responsible for poorly active pyruvate dehydrogenase (PDH) in circulating lymphocytes (CLs) of obese subjects (ObS), and if so, whether it improves when their plasma insulin rises.

RESEARCH METHODS AND PROCEDURES

PDH activity was compared in lysed CLs of 10 euglycemic ObS and 10 sex- and age-matched controls before and during plasma insulin enhancement in an oral glucose tolerance test. It was evaluated without (PDHa) or with Mg/Ca or Mg at various concentrations to assess PDP1 or PDP2 activities or with Mg/Ca and exogenous PDP to determine total PDH activity (PDHt), which is an indirect measure of the amount of PDH. The insulin sensitivity index was calculated, and PDP1 and PDP2 mRNA was sought in the CLs.

RESULTS

At T0 in ObS, PDHt was normal, whereas PDHa and PDP1 activity was below normal at all Mg/Ca concentrations. PDP2 activity was undetectable in both groups. PDP1 and PDP2 mRNA was identified, and insulin sensitivity index and PDHa were directly correlated. During the oral glucose tolerance test, plasma insulin rose considerably more in ObS than in controls; PDHa and PDP1 activity also increased but remained significantly below normal, and PDHt was unvaried in both groups.

DISCUSSION

PDP1 is down-regulated in CLs of ObS because it is poorly sensitive to Mg/Ca; this defect is attenuated when plasma insulin is greatly enhanced.

In obese individuals dexfenfluramine corrects molecular derangements reflecting insulin resistance

BACKGROUND

Circulating lymphocytes of obese individuals with and without type 2 diabetes have derangements of pyruvate dehydrogenase (PDH) that are described as reflecting a disorder underlying systemic insulin resistance, namely basal activity below normal and, in vitro, unresponsiveness to insulin at 33 pmol/l and activation at 330 pmol/l instead of activation and inhibition as in controls.

OBJECTIVE

To explore whether the above enzyme derangements are overcome in obese individuals on dexfenfluramine treatment, known to improve poor peripheral insulin sensitivity.

METHODS

Fifteen obese diabetic patients and 15 age-matched euglycaemic obese subjects with normal glucose tolerance were enrolled for a trial composed of two 21-day periods; in the first (D-21-D0), participants received a placebo, and in the second (D0-D21), dexfenfluramine (30 mg/day). At D-21, D0 and D21 participants were evaluated for weight, BMI, fasting glycaemia (FG), fasting insulinaemia (FI), fasting insulin resistance index (FIRI), area under the glycaemic (G-AUC) and insulinaemic (I-AUC) curves from an OGT test, and for PDH activity assayed in their circulating lymphocytes before (basal activity) and after incubation with 33 or 330 pmol/l insulin. At D2, basal PDH activity and clinical parameters were assayed.

RESULTS

In both groups of participants at D0 all parameters tested were constant with respect to D-21; at D2, only basal PDH activity rose significantly; at D21, basal and insulin stimulated PDH activities were normalized and weight decreased significantly, as did FG, FI, FIRI and G-AUC in the diabetic, and FI, FIRI, G-AUC and I-AUC in the non-diabetic participants.

CONCLUSION

In obese, non-diabetic and diabetic individuals on dexfenfluramine treatment, amelioration of clinical parameters and indexes of poor insulin sensitivity of blood glucose homeostasis are preceded by correction, in their circulating lymphocytes, of PDH derangements described as reflecting a disorder underlying insulin resistance.

Association of heterotrimeric G(i) with the insulin-like growth factor-I receptor. Release of G(betagamma) subunits upon receptor activation

The insulin-like growth factor-I receptor (IGF-IR) is a key regulator of cell proliferation and survival. Activation of the IGF-IR induces tyrosine autophosphorylation and the binding of a series of adaptor molecules, thereby leading to the activation of MAPK. It has been demonstrated that pertussis toxin, which inactivates the G(i) class of GTP-binding proteins, inhibits IGF-I-mediated activation of MAPK, and a specific role for G(betagamma) subunits in IGF-I signaling was shown. In the present study, we have investigated the role of heterotrimeric G(i) in IGF-IR signaling in neuronal cells. Pertussis toxin inhibited IGF-I-induced activation of MAPK in rat cerebellar granule neurons and NG-108 neuronal cells. G(alphai) and G(beta) subunits were associated with IGF-IR immunoprecipitates. Similarly, in IGF-IR-null mouse embryo fibroblasts transfected with the human IGF-IR, G(i) was complexed with the IGF-IR. G(alphas) was not associated with the IGF-IR in any cell type. IGF-I induced the release of the G(beta) subunits from the IGF-IR but had no effect on the association of G(alphai). These results demonstrate an association of heterotrimeric G(i) with the IGF-IR and identify a discrete pool of G(betagamma) subunits available for downstream signaling following stimulation with IGF-I.