Energy expenditure, body composition and insulin response to glucose in male twins discordant for the Trp64Arg polymorphism of the beta3-adrenergic receptor gene

AIM

The tryptophan to arginine change in position 64 (Trp64Arg) polymorphism of the beta3-adrenergic receptor (beta3AR) gene has been associated with an increased prevalence of obesity, insulin resistance and type 2 diabetes. In this, decreased rates of energy expenditure and impaired insulin secretion could play a role.

METHODS

In 10 male twin pairs discordant for the Trp64Arg polymorphism, we examined insulin response to glucose by an oral glucose tolerance test (OGTT), a frequently sampled intravenous glucose tolerance test (FSIGT), body composition by the bioimpedance method, dual-energy X-ray absorptiometry scanning and energy expenditure by indirect and direct calorimetry.

RESULTS

Twins heterozygous for the Trp64Arg polymorphism showed significantly lower fat mass independent of the method used, and significantly lower fasting insulin and glucose concentrations compared with their homozygous wild-type co-twins. Correspondingly, insulin resistance and insulin secretion determined by homeostasis model assessment were significantly lower in twins carrying the Trp64Arg polymorphism. However, there were no significant differences in adiponectin levels, insulinogenic index assessed by OGTT, or insulin sensitivity, acute insulin response to glucose, glucose effectiveness or insulin disposition index assessed by minimal modelling of the FSIGT. Furthermore, there were no differences in sleeping, resting or post-prandial energy expenditure.

CONCLUSIONS

In male twins with a high similarity in genetic and environmental background, the Trp64Arg polymorphism of the beta3AR gene is associated with lower fat mass, fasting insulin levels and an appropriate insulin response to glucose. Thus, heterozygosity for the Trp64Arg variant is unlikely to increase the risk of obesity, insulin resistance or type 2 diabetes.

Increased expression of 11beta-hydroxysteroid dehydrogenase type 1 in type 2 diabetic myotubes

BACKGROUND

Alterations in glucocorticoid hormone metabolism in skeletal muscle have been suggested to contribute to the pathogenesis of the metabolic syndrome. Circulating glucocorticoids consist of inactive cortisone and active cortisol interconverted in various tissues by the enzyme 11beta hydroxysteroid dehydrogenase (HSD). This study aims to investigate whether human myotubes established from healthy obese and matched obese type 2 diabetic (T2D) subjects reveal differences in the expression level of glucocorticoid receptor (GR) and 11beta hydroxysteroid dehydrogenase (HSD1 and HSD2), and to investigate whether chronic exposure to cortisone affects glucose transport.

METHODS

In myotubes established from T2D and healthy control subjects we determined the mRNA expression of HSD1, HSD2, GR and determined basal and insulin-stimulated glucose uptake in myotubes precultured with cortisone, cortisol and the HSD1 inhibitor, carbenoxolone for four days.

RESULTS

Myotubes established from T2D subjects showed an increased expression of HSD1 mRNA, but with no differences in mRNA of GRalpha, LXRalpha and LXRbeta, whereas HSD2 mRNA was not expressed. Cortisone reduced glucose uptake in diabetic myotubes and the cortisone effect could be abolished by the HSD1 inhibitor carbenoxolone.

CONCLUSIONS

Our study shows that cortisone reduces glucose uptake in diabetic myotubes and that this effect seems mediated by an increased mRNA HSD1 expression emphasizing that the local conversion of inactive to active glucocorticoids may be important in the pathogenesis of insulin resistance.

Rosiglitazone Evaluated for Cardiac Outcomes and Regulation of Glycaemia in Diabetes (RECORD): study design and protocol

AIMS/HYPOTHESIS

Studies suggest that in addition to blood glucose concentrations, thiazolidinediones such as rosiglitazone improve some cardiovascular (CV) risk factors and surrogate markers, that are abnormal in type 2 diabetes. However, fluid retention might lead to cardiac failure in a minority of people. The aim of the Rosiglitazone Evaluated for Cardiac Outcomes and Regulation of Glycaemia in Diabetes (RECORD) study is to evaluate the long-term impact of these effects on CV outcomes, as well as on long-term glycaemic control, in people with type 2 diabetes.

MATERIALS AND METHODS

RECORD is a 6-year, randomised, open-label study in type 2 diabetic patients with inadequate blood glucose control (HbA1c 7.1-9.0%) on metformin or sulphonylurea alone. The study is being performed in 327 centres in Europe and Australasia. After a 4-week run-in, participants were randomised by current treatment stratum to add-on rosiglitazone, metformin or sulphonylurea, with dose titration to a target HbA1c of < or = 7.0%. If confirmed HbA1c rises to > or = 8.5%, either a third glucose-lowering drug is added (rosiglitazone-treated group) or insulin is started (non-rosiglitazone group). The same criterion for failure of triple oral drug therapy in the rosiglitazone-treated group is used for starting insulin in this group. The primary endpoint is the time to first CV hospitalisation or death, blindly adjudicated by a central endpoints committee. The study aim is to evaluate non-inferiority of the rosiglitazone group vs the non-rosiglitazone group with respect to CV outcomes. Safety, tolerability and study conduct are monitored by an independent board. All CV endpoint and safety data are held and analysed by a clinical trials organisation, and are not available to the study investigators while data collection is open.

RESULTS

Over a 2-year period a total of 7,428 people were screened in 25 countries. Of these, 4,458 were randomised; 2,228 on background metformin, 2,230 on background sulphonylurea. Approximately half of the participants are male (52%) and almost all are Caucasian (99%).

CONCLUSIONS/INTERPRETATION

The RECORD study should provide robust data on the extent to which rosiglitazone, in combination with metformin or sulphonylurea therapy, affects CV outcomes and progression of diabetes in the long term.

Skeletal changes in osteoprotegerin and receptor activator of nuclear factor-kappab ligand mRNA levels in primary hyperparathyroidism: effect of parathyroidectomy and association with bone metabolism

The effect of parathyroid hormone (PTH) on the production of osteoprotegerin (OPG) and ligand of receptor activator of NF-kappaB (RANKL) in human bone is incompletely understood. Most in vitro studies indicate that PTH decreases OPG and increases RANKL production. In primary hyperparathyroidism (PHPT), hypersecretion of PTH leads to enhanced bone resorption and formation with increased risk of fracture. Decreasing PTH levels by surgery normalizes bone metabolism, but the effects on skeletal OPG and RANKL production are unknown. In this study, 24 patients referred to our clinic for evaluation, and treatment of PHPT were included. A transiliac bone biopsy was done before (n = 24) and 12 months after parathyroidectomy (PTX) (n = 21). Biopsies were frozen in liquid nitrogen and RNA extracted using Trizol. A competitive RT-PCR assay for RANKL and OPG mRNA using artificial cDNA standards was developed and used for quantification. Results were normalized for GAPDH mRNA content. Before surgery, the RANKL/GAPDH gene expression ratio showed positive correlations with serum osteocalcin (r = 0.42, P < 0.05) and urinary NTX (r = 0.43, P < 0.05). The OPG/GAPDH mRNA levels in iliac bone before surgery correlated with serum osteocalcin (r = 0.52, P < 0.01), but not with bone resorption markers. The mRNA ratio of RANKL/OPG decreased significantly (P < 0.05) after surgery. In conclusion, RANKL and OPG gene expression within the human bone microenvironment are influenced by PTH, as the ratio RANKL/OPG decreased upon PTX. In addition, locally produced RANKL appears to affect bone turnover in the hyperparathyroid state.

GLUT11, but not GLUT8 or GLUT12, is expressed in human skeletal muscle in a fibre type-specific pattern

Nine novel sugar transporter-like proteins have been discovered in the past 5 years. The mRNA for three of these, the glucose transporters (GLUT) GLUT8, GLUT11 and GLUT12, have been detected in human skeletal muscle. In the present study, we examined the pattern of expression and localization of the GLUT isoforms 8, 11 and 12 in human skeletal muscle using an immunohistochemical approach. Biopsies of human skeletal muscle from sedentary or trained healthy adults, from fetal muscle (24 weeks of gestation), from obese type-2 diabetic subjects, and from patients suffering from polymyositis or amyotrophic lateral sclerosis (ALS) were studied. GLUT8 and 12 immunoreactivity was below detection level in both developing and adult muscle fibres. GLUT11 immunoreactivity, however, was present in slow-twitch muscle fibres, but not in fast twitch fibres. Since, in contrast, GLUT4 was expressed in all investigated muscle fibres, the pattern of expression of GLUT11 differs from that of GLUT4, suggesting a specialized function for GLUT11 with a regulation independent from that of GLUT4. Obesity, type-2 diabetes, training, conditions of de- and reinnervation (ALS) and regeneration (polymyositis) failed to induce GLUT8 or -12 expression. Likewise, the fibre type-dependent pattern of GLUT11 immunoreactivity was unaltered. However, some slow muscle fibres lose their GLUT11 immunoreactivity under regeneration. Our results indicate that GLUT11 immunoreactivity, in contrast to that of GLUT4, is expressed exclusively in slow-twitch muscle fibres and is unaffected by physiological and pathophysiological conditions except in primary myopathy. GLUT8 and GLUT12 do not appear to be of importance in human muscle under physiological and pathophysiological conditions.

Functional and immunohistochemical evaluation of porcine neonatal islet-like cell clusters

Porcine neonatal islet-like cell clusters (NICCs) may be an attractive source of insulin-producing tissue for xenotransplantation in type I diabetic patients. We examined the functional and immunohistochemical outcome of the islet grafts in vitro during long-term culture and in vivo after transplantation to athymic nude mice. On average we obtained 29,000 NICCs from each pancreas. In a perifusion system, NICCs responded poorly to a glucose challenge alone, but 10 mmol/L arginine elicited a fourfold increase in insulin secretion and 16.7 mmol/L glucose + 10 mmol/L arginine caused a sevenfold increase in insulin section indicating some sensitivity towards glucose. Hormone content as well as the number of hormone-containing cells increased for the first 14 days of culture. When NICCs were stained for hormones, proliferation (Ki67), and duct cells (CK7), some insulin- and glucagon-positive cells co-stained for proliferation. However no co-staining was observed between insulin- and glucagon-positive cells or between hormone-and CK-positive cells. Following transplantation of 2000 NICCs under the renal capsule of diabetic nude mice, BG levels were normalized within an average of 13 weeks. Oral and IP glucose tolerance tests revealed a normal or even faster clearance of a glucose load compared with normal controls. Immunohistochemical examination of the grafts revealed primarily insulin-positive cells. In summary, in vitro, NICCs responded to a challenge including glucose and arginine. There was a potential for expansion of the beta-cell mass of NICCs in vitro as well as in vivo where NICCs eventually may normalize blood glucose of diabetic mice.

Does overnight normalization of plasma glucose by insulin infusion affect assessment of glucose metabolism in Type 2 diabetes?

AIMS

In order to perform euglycaemic clamp studies in Type 2 diabetic patients, plasma glucose must be reduced to normal levels. This can be done either (i) acutely during the clamp study using high-dose insulin infusion, or (ii) slowly overnight preceding the clamp study using a low-dose insulin infusion. We assessed whether the choice of either of these methods to obtain euglycaemia biases subsequent assessment of glucose metabolism and insulin action.

METHODS

We studied seven obese Type 2 diabetic patients twice: once with (+ ON) and once without (- ON) prior overnight insulin infusion. Glucose turnover rates were quantified by adjusted primed-constant 3-3H-glucose infusions, and insulin action was assessed in 4-h euglycaemic, hyperinsulinaemic (40 mU m-2 min-1) clamp studies using labelled glucose infusates (Hot-GINF).

RESULTS

Basal plasma glucose levels (mean +/- sd) were 5.5 +/- 0.5 and 10.7 +/- 2.9 mmol/l in the + ON and - ON studies, respectively, and were clamped at -5.5 mmol/l. Basal rates of glucose production (GP) were similar in the + ON and - ON studies, 83 +/- 13 vs. 85 +/- 14 mg m-2 min-1 (NS), whereas basal rates of glucose disappearance (Rd) were lower in the + ON than in the - ON study, 84 +/- 8 vs. 91 +/- 11 mg m-2 min-1 (P = 0.02). During insulin infusion in the clamp period, rates of GP, 23 +/- 11 vs. 25 +/- 10 mg m-2 min-1, as well as rates of Rd, 133 +/- 32 vs. 139 +/- 37 mg m-2 min-1, were similar in the + ON and - ON studies, respectively (NS).

CONCLUSIONS

Apart from basal rates of Rd, assessment of glucose turnover rates in euglycaemic clamp studies of Type 2 diabetic patients is not dependent on the method by which plasma glucose levels are lowered.

Osteoprotegerin levels in primary hyperparathyroidism: effect of parathyroidectomy and association with bone metabolism

The effect of parathyroid hormone (PTH) on the production of osteoprotegerin (OPG) remains controversial. Most in vitro studies indicate that PTH decreases OPG secretion by the osteoblast, but in vivo observations are conflicting. In primary hyperparathyroidism (PHPT), hypersecretion of PTH leads to enhanced bone resorption and formation with increased risk of fracture. Patients with PHPT are cured by surgery, resulting in normalization of PTH levels and bone metabolism, but the concomitant effects on OPG production are not known. The hypothesis of the present study was that the circulating level of OPG is diminished in patients with PHPT and increases with successful parathyroidectomy. We also speculated that serum OPG may determine the magnitude of bone loss up to the time of surgery. In the present study, 20 patients (17 women and 3 men, mean age 62 y) with PHPT who were candidates for surgical cure were examined before and 12 months after surgery. Bone turnover markers decreased and BMD increased significantly after surgery. Serum OPG did not correlate with PTH before surgery (r = 0.07, P = 0.77) and was not affected by parathyroidectomy (P = 0.79). After normalization of PTH, bone formation markers showed significant (P1NP) and near-significant (osteocalcin) correlations with serum OPG. In conclusion, serum OPG is not decreased in patients with PHPT, nor is serum OPG to any demonstrable extent regulated by PTH pre- or postoperatively.

GLUT4 expression in human muscle fibres is not correlated with intracellular triglyceride (TG) content. Is TG a maker or a marker of insulin resistance?

We have recently reported a progressive decline in the expression of glucose transporter isoform 4 (GLUT4) from control subjects through obese non-diabetics to obese type 2 diabetic subjects, indicating that the reduced GLUT4 in slow twitch fibres could be secondary to obesity. In this study we investigate the association of GLUT4 expression with the intracellular triglyceride (TG) content in the same muscle fibres and with plasma lipid parameters. We used histochemistry and stereology to study the relationship between TG content and GLUT4 expression in muscle fibres from obese, obese type 2 diabetic subjects, and young lean controls. TG density was significantly higher in slow compared to fast fibres in all studied subjects (p<0.05). We found an increased TG density in slow twitch fibres of obese diabetic subjects compared to obese (p<0.05) and lean controls (p<0.008). Intracellular TG densities in slow and fast fibres did not correlate with the corresponding GLUT4 density in the same fibres in our study groups (p>0.05). Plasma TG and FFA did not correlate with GLUT4 expression in slow or fast fibres (p>0.05). In conclusion, TG content was increased in diabetic slow fibres with a reduced GLUT4 expression. The GLUT4 expression was not associated with an increased intracellular triglyceride content or with increased plasma FFA levels. Thus, intracellular TG content and circulating FFA may not influence glucose transport directly through GLUT4 expression.

Hormone replacement therapy dissociates fat mass and bone mass, and tends to reduce weight gain in early postmenopausal women: a randomized controlled 5-year clinical trial of the Danish Osteoporosis Prevention Study

The aim of this study was to study the influence of hormone replacement therapy (HRT) on weight changes, body composition, and bone mass in early postmenopausal women in a partly randomized comprehensive cohort study design. A total of 2016 women ages 45-58 years from 3 months to 2 years past last menstrual bleeding were included. One thousand were randomly assigned to HRT or no HRT in an open trial, whereas the others were allocated according to their preferences. All were followed for 5 years for body weight, bone mass, and body composition measurements. Body weight increased less over the 5 years in women randomized to HRT (1.94 +/- 4.86 kg) than in women randomized to no HRT (2.57 +/- 4.63, p = 0.046). A similar pattern was seen in the group receiving HRT or not by their own choice. The smaller weight gain in women on HRT was almost entirely caused by a lesser gain in fat. The main determinant of the weight gain was a decline in physical fitness. Women opting for HRT had a significantly lower body weight at inclusion than the other participants, but the results in the self-selected part of the study followed the pattern found in the randomized part. The change in fat mass was the strongest predictor of bone changes in untreated women, whereas the change in lean body mass was the strongest predictor when HRT was given. Body weight increases after the menopause. The gain in weight is related to a decrease in working capacity. HRT is associated with a smaller increase in fat mass after menopause. Fat gain protects against bone loss in untreated women but not in HRT-treated women. The data suggest that women's attitudes to HRT are more positive if they have low body weight, but there is no evidence that the conclusions in this study are skewed by selection bias.

Effects of the natural and artificial menstrual cycle on the production of osteoprotegerin and the bone resorptive cytokines IL-1beta and IL-6

Bone remodelling changes within the menstrual cycle. Though the luteal phase is accompanied by decreased bone resorption, it is also paradoxically a time of increased production of bone resorptive cytokines. The present study examined the hypothesis that changes in serum osteoprotegerin (OPG) within the menstrual cycle prevent the increase in bone remodelling, which would otherwise have been the result of the luteal increase in the capacity for producing resorptive cytokines. The study population consisted of healthy female volunteers: premenopausal women (n = 11, mean age 39.4 y +/- 6.1) without cycle irregularities. Postmenopausal women (n = 11, mean age 56.8 y +/- 3.6) receiving cyclic HRT (estradiol and noretisterone acetate). Luteal and follicular phase blood samples were diluted and cultured for 24 hours with and without lipopolysaccharide (LPS). The supernatant was assayed for IL-1 beta and IL-6 by ELISA. Serum OPG was measured by ELISA. The LPS-stimulated production of IL-1 and IL-6 was significantly higher in the luteal phase. When the analysis was restricted to the natural menstrual cycle, only the increase in IL-1 production remained statistically significant. NTX excretion was similar in the two phases of HRT, but decreased nonsignificantly (p = 0.05) in the luteal phase in the premenopausal women. OPG levels did not exhibit any menstrual cycle-dependent changes. In conclusion, bone resorption is suppressed in the luteal phase through a mechanism that does not involve increases in serum OPG. An increased cytokine secretory capacity of blood cells may be an epiphenomenon particular to the luteal phase but unrelated to bone metabolism.

Proposed diagnostic thresholds for gestational diabetes mellitus according to a 75-g oral glucose tolerance test. Maternal and perinatal outcomes in 3260 Danish women

AIMS

To study if established diagnostic threshold values for gestational diabetes based on a 75-g, 2-h oral glucose tolerance test can be supported by maternal and perinatal outcomes.

METHODS

Historical cohort study of 3260 pregnant women examined for gestational diabetes on the basis of risk indicators. Information on oral glucose tolerance test results and clinical outcomes were collected from medical records.

RESULTS

There was an increased risk of delivering a macrosomic infant in women with 2-h capillary blood glucose of 7.8-8.9 mmol/l compared with women with 2-h glucose < 7.8 mmol/l. Despite treatment, 2-h glucose of 9.0-11.0 mmol/l and > or = 11.1 mmol/l were both associated with increased rates of macrosomia, spontaneous preterm delivery, hypertensive complications, and neonatal hypoglycaemia. Adverse outcomes tended to be more frequent in women with 2-h glucose > or = 11.1 mmol/l than in women with 2-h glucose of 9.0-11.0 mmol/l.

CONCLUSIONS

The risk for several maternal and perinatal complications increased with the diagnostic threshold for 2-h glucose. Large-scale blinded studies are needed to clarify the question of a clinically meaningful diagnosis of gestational diabetes mellitus. Until these results are available, a 2-h threshold level of 9.0 mmol/l after a 75-g oral glucose tolerance test seems acceptable.

Effect of insulin on protein phosphatase 2A expression in muscle in type 2 diabetes

BACKGROUND

Protein phosphatase 2A (PP2A) acts on a number of enzymes involved in the insulin regulation of glucose uptake and glycogen synthesis. This study was carried out to investigate the effect of insulin on PP2A expression in skeletal muscles of type 2 diabetic and control subjects.

MATERIAL AND METHODS

Ten type 2 diabetic and 10 matched, control subjects were studied using the euglycaemic-hyperinsulinaemic clamp technique combined with indirect calorimetry. Immunoreactive protein levels of the catalytic alpha subunit of PP2A (PP2A-C alpha) were measured in biopsies from the vastus lateralis muscle obtained in the basal and insulin-stimulated state.

RESULTS

In type 2 diabetic subjects insulin-mediated glucose disposal, glucose oxidation and nonoxidative glucose metabolism were reduced, whereas lipid oxidation was increased (all P < 0.05). Insulin down-regulated PP2A-C alpha expression in skeletal muscle of the control subjects (P < 0.05) but not in the type 2 diabetic subjects. In the control subjects, the insulin-mediated decrease in PP2A-C alpha correlated with the insulin-mediated increase in glucose disposal, glucose oxidation, nonoxidative glucose metabolism (all P < 0.05) and decrease in lipid oxidation (P < 0.01). In the type 2 diabetic subjects these relationships were absent.

CONCLUSIONS

Down-regulation of PP2A-C alpha expression by insulin in skeletal muscle seems to be associated with a normal insulin action on glucose storage, glucose and lipid oxidation. Impaired down-regulation of PP2A-C alpha expression by insulin may be a marker for insulin resistance and contribute to the pathogenesis of type 2 diabetes.

Age-dependent impact of zygosity and birth weight on insulin secretion and insulin action in twins

AIMS/HYPOTHESIS

Study the influence of age, zygosity and birth weight on insulin action and insulin secretion in twins.

METHODS

In vivo insulin action and insulin secretion were measured using the euglycaemic, hyperinsulinaemic clamp technique and intravenous glucose tolerance test, respectively. We examined 104 monozygotic (MZ) twins and 88 dizygotic (DZ) twins in two age groups (25-34 and 57-66 years).

RESULTS

There were no differences in birth weight, body mass index, waist to hip ratio or lean body mass between monozygotic and dizygotic twins. Younger monozygotic twins had a slightly higher insulin-stimulated glucose uptake (M) than younger dizygotic twins. In contrast, elderly monozygotic twins had a lower insulin-stimulated glucose uptake value compared with elderly dizygotic twins. Elderly monozygotic twins had a reduced insulin secretion relative to insulin resistance compared to dizygotic twins during oral glucose tolerance test (OGTT). Birth weight per se was not associated with insulin secretion or action in the twins. However, correcting for the genetic influence on birth weight using intrapair differences among monozygotic twin pairs, low birth weight was associated with insulin resistance and low insulin secretion after both oral and intravenous glucose administration in elderly MZ twins.

CONCLUSION/INTERPRETATION

Zygosity status has a major age (or time) dependent impact on in vivo insulin secretion and insulin action in twins independent of birth weight and adult antropometry. An additional non-genetic impact of low birth weight on insulin secretion and insulin action was found in elderly monozygotic twins. Ageing could play an important role by unmasking the influence of an adverse intrauterine environment on insulin resistance and low insulin secretion in twins.

Is low birth weight a risk factor for the development of diabetic nephropathy in patients with type 1 diabetes? A population-based case-control study

OBJECTIVES

To investigate if low birth weight as a consequence of intrauterine malnutrition is a risk factor for the later development of diabetic nephropathy.

DESIGN AND SUBJECTS

In a case-control set-up a group of type 1 diabetic subjects with diabetic nephropathy (n = 51) and a matched control group with normal kidney function (n = 51) were compared. Diabetic nephropathy and normal kidney function were defined as urinary albumin excretion rate above 200 microg min-1 and below 20 microg min-1, respectively. The birth weights were all obtained from the midwives' original records.

SETTING

The patients were identified from a population-based study of chronic diabetic complications in the Funen County, Denmark.

MAIN OUTCOMES

Birth weights according to the presence of diabetic nephropathy.

RESULTS

The median (10-90 percentile) birth weights were 3,600 g (2,960-4,274) in the group with diabetic nephropathy and 3,600 g (2,880-4,220) in the group without nephropathy, P = 0.52. In the lower quartile of birth weights the median (10-90 percentile) birth weights were 3,000 g (2,780-3,200) in the group with nephropathy versus 2,850 g (2,250-3,175) in the group without nephropathy, P = 0.07. In the upper quartile the median (10-90 percentile) birth weights were 4,225 g (4,000-4,741) in the nephropathy group and 4,000 g in the group without nephropathy, P = 0.13. We found no significant correlation between birth weights and log urinary albumin excretion rate (r = 0.148, P = 0.14) and no difference in the number of patients with nephropathy in the lower versus upper quartiles of birth weights.

CONCLUSION

We found no evidence of low birth weight as a risk factor for the development of diabetic nephropathy.

Regenerating human muscle fibres express GLUT3 protein

The presence of the GLUT3 glucose transporter protein in human muscle cells is a matter of debate. The present study was designed to establish whether GLUT3 is expressed in mature human skeletal muscle fibres and, if so, whether its expression changes under different conditions, such as metabolic stress (obesity, obese non-insulin-dependent diabetes mellitus), hypertrophy (training), de- and reinnervation (amyotrophic lateral sclerosis) or regeneration (polymyositis). We used an immunohistochemical approach to detect and localise GLUT3. GLUT3 immunoreactivity was not detectable in adult skeletal muscle fibres, nor did metabolic stress, training or de- and re-innervation induce GLUT3 expression, while a few GLUT3 expressing fibres were seen in some cases of polymyositis. In contrast, GLUT4 was expressed in all investigated muscle fibres. GLUT3 immunoreactivity was found in perineural and endoneural cells, indicating that GLUT3 is important for glucose transport into nerves through the perineurium. Taken together, these data suggest that GLUT3 expression is restricted to regenerating muscle fibres and nerves in adult human muscle. Although the significance of GLUT3 in adult human muscle fibres appears limited, GLUT3 may be of importance for the glucose supply in fetal muscle fibres and regenerating adult muscle fibres.

GLUT4 expression at the plasma membrane is related to fibre volume in human skeletal muscle fibres

In this study we examined the relationship between GLUT4 expression at the plasma membrane and muscle fibre size in fibre-typed human muscle fibres by immunocytochemistry and morphometry in order to gain further insight into the regulation of GLUT4 expression. At the site of the plasma membrane, GLUT4 was more abundantly expressed in slow as compared to fast fibres at the same fibre diameter (p < 0.01) and the GLUT4 expression increased with increasing fibre radius independently of fibre type (p < 0.01). The GLUT4 density at the surface of slow fibres of both diabetic and obese was reduced compared to control subjects at the same diameter (p < 0.001). Fast fibres in obese and type 2 diabetic subjects expressed a fibre-volume-dependent GLUT4 expression (p < 0.001), while this did not reach significance in slow fibres (obese p = 0.18 and diabetic p = 0.06). Our results show that increasing fibre volume is associated with increasing GLUT4 expression in both slow and fast fibres. Based on the possible dependency of GLUT4 expression on volume, we hypothesize that the reduced GLUT4 expression in obesity and type 2 diabetes may partly be compensated for by physical activity.

Insulin signalling in skeletal muscle of subjects with or without Type II-diabetes and first degree relatives of patients with the disease

AIMS/HYPOTHESIS

Alterations in insulin signalling could contribute to insulin resistance in Type II (non-insulin-dependent) diabetes mellitus. Some of these alterations could be secondary to the diabetic state, ie. the hyperglycaemia or increased NEFA concentrations. We sought to exclude such secondary factors and to investigate whether Type II diabetes in itself is associated with altered insulin signalling in skeletal muscle.

METHODS

Hyperinsulinaemic-euglycaemic clamps were performed in 10 obese Type II diabetic patients whose glucose concentrations had been normalised for 8 h by plasma glucose-adapted insulin infusion, 10 BMI-matched first-degree relatives of Type II diabetic patients, and 10 BMI-matched non-diabetic subjects. Muscle biopsies were obtained before and at the end of the clamps, and insulin receptor kinase activity, phosphatidylinositol-3'-kinase activity, Akt-Thr(308)-phosphorylation, and glycogen synthase activity determined.

RESULTS

At similar steady-state clamp insulin concentrations (approximately 400 pmol/l) similar receptor kinase activities, phosphatidylinositol-3'-kinase activities, Akt-Thr(308)-phosphorylation, and glycogen synthase activities were found in all subject groups although glucose disposal was reduced in the diabetic subjects and relatives. Pre-clamp signalling levels were different between subject groups, most likely due to different pre-clamp insulin concentrations.

CONCLUSION/INTERPRETATION

Our results in subjects at risk for the development of diabetes and Type II diabetic patients with normalized glucose concentrations suggest that Type II diabetes in itself is not associated with reduced signalling intensity at the studied signalling molecules, at least not at the chosen clamp insulin concentration and under the chosen conditions. Alterations responsible for the reduced glucose disposal could be located downstream of the investigated steps or in alternative insulin signalling pathways. A different spatial organisation of the investigated signalling molecules can also not be excluded.

Delays in insulin signaling towards glucose disposal in human skeletal muscle

We explored whether the delay that occurs between a rise in plasma insulin and the increase of glucose disposal occurs before, at, or downstream of steps that are believed to be part of the insulin signaling cascade. Skeletal muscle biopsies were obtained from 16 nondiabetic subjects before, and 20 and 180 min after plasma insulin levels had been augmented in euglycemic hyperinsulinemic glucose clamps. Although plasma insulin had reached 98% of its final concentration within 10 min, insulin receptor kinase (IRK) activity, p85 associated with insulin receptor substrate-1 (IRS-1), IRS-1-associated phosphatidylinositol 3-kinase (PI3K) activity, and Thr(308)-protein kinase B (PKB) phosphorylation in the muscle biopsies at 20 min had reached only 60, 48, 34 and 47% respectively of those at 180 min. This suggests a delay before the level of IRK and little or no delay between IRK and PKB activation. The observation that glycogen synthase activity and glucose disposal at 20 min had both only reached 25% of the respective values at 180 min suggests an additional delay downstream of the investigated signaling steps.

The influence of the polymorphism in apolipoprotein B codon 2488 on insulin and lipid levels in a Danish twin population

AIMS

The apolipoprotein B codon 2488 polymorphism has been associated with the metabolism of lipoproteins in subjects with Type 2 diabetes. However, no data are available on the influence of the polymorphism on insulin or glucose metabolism. This study examines the impact of the polymorphism on parameters associated with the insulin resistance syndrome in Danish twins.

METHODS

The effect of the polymorphism on lipid, glucose and insulin measures was studied in 548 same sex twins aged 55-74 years.

RESULTS

The codon 2488 polymorphism influenced fasting triglyceride levels, as well as insulin, as measured at 120 min in an oral glucose tolerance test. Subjects with the genotype T2488T had 14% higher triglyceride levels (P = 0.02) and 31% higher insulin levels (P = 0.004) than subjects with genotype C2488C. In twins discordant for genotype, the T-allele was associated with higher levels of triglyceride (P = 0.04) and insulin (P = 0.02) and lower levels of HDL-cholesterol (P = 0.04).

CONCLUSION

The T-allele of the codon 2488 polymorphism influenced parameters related to the insulin resistance syndrome, i.e. increased levels of insulin, increased levels of triglyceride and decreased levels of HDL. As the polymorphism is silent, these effects must be mediated through linkage to other polymorphisms in apolipoprotein B or other genes on chromosome 2.

[Hormone replacement therapy reduces the risk of forearm fracture in postmenopausal women. Results of the Danish Osteoporosis Prevention Study]

In a prospective, controlled, comprehensive cohort trial of 2,016 healthy early postmenopausal women aged 45-58 years we studied fracture prevention through the use of oestrogen. There were two main study arms: a randomised arm (randomised to HRT [n = 502] or not [n = 504]) and a non-randomised arm (on HRT [n = 221] or not [n = 789] by own choice). After five years, an intention-to-treat analysis (n = 2,016) showed a reduction in the overall fracture risk (RR = 0.73, 95% CI: 0.50-1.05) and in the forearm fracture risk (RR = 0.45, 95% CI: 0.22-0.90) with oestrogen. Restriction of the analysis to women who had adhered to their initial allocation of either oestrogen (n = 395) or no oestrogen (n = 977) showed a significant reduction in both the overall fracture risk (RR = 0.61, 95% CI: 0.39-0.97) and the risk of forearm fractures (RR = 0.24, 95% CI: 0.09-0.69). We conclude that it is possible to reduce the number of forearm fractures in early postmenopausal women by the use of oestrogen as primary prevention.

The basal kinetic parameters of glycogen synthase in human myotube cultures are not affected by chronic high insulin exposure

There is no consensus regarding the results from in vivo and in vitro studies on the impact of chronic high insulin and/or high glucose exposure on acute insulin stimulation of glycogen synthase (GS) kinetic parameters in human skeletal muscle. The aim of this study was to evaluate the kinetic parameters of glycogen synthase activity in human myotube cultures at conditions of chronic high insulin combined or not with high glucose exposure, before and after a subsequent acute insulin stimulation. Acute insulin stimulation significantly increased the fractional activity (FV(0.1)) of GS, increased the sensitivity of GS to the allosteric activator glucose 6-phosphate (A(0.5)) and increased the sensitivity of GS to its substrate UDPG (K(m(0.1))) when myotubes were precultured at low insulin with/without high glucose conditions. However, this effect of acute insulin stimulation was abolished in myotubes precultured at high insulin with or without high glucose. Furthermore, we found significant correlations between the fractional velocities FV(0.1) of GS and K(m(0.1)) (rho=-0.72, P<0.0001), between FV(0.1) and A(0.5) (rho=-0.82, P<0.0001) and between K(m(0.1)) and A(0.5) values (rho=0.71, P<0.0001). Our results show that chronic exposure of human myotubes to high insulin with or without high glucose did not affect the basal kinetic parameters but abolished the reactivity of GS to acute insulin stimulation. We suggest that insulin induced insulin resistance of GS is caused by a failure of acute insulin stimulation to decrease A(0.5) and K(m(0.1)) in human skeletal muscle.