bluuebunny/crossref_metadata_2025_split huggingface dataset:
GLUT4 activation: thoughts on possible mechanisms
Authors: Michelle Furtado L. , Poon V. , Klip A. 
Published: July 2003

AbstractA family of facilitative glucose transporters or GLUTs mediates glucose uptake by cells and tissues. The glucose transporter isoform GLUT4, which is the predominant isoform expressed in mature muscle and fat tissues, is primarily responsible for the increase in glucose uptake in response to insulin stimulation. Recent work in our laboratory suggests that there are two divergent responses initiated by insulin stimulation. The first response involves the recruitment of GLUT4 transporters from intracellular reserves and their subsequent insertion into the plasma membrane. The second pathway results in an increase in the intrinsic activity of the transporters. This review will discuss evidence supporting the divergence of the two pathways regulating glucose uptake and, in particular, evidence for the increased intrinsic activity of GLUT4 in response to insulin stimulation. Inhibitors of p38 mitogen‐activated protein kinase (MAPK) affected only the arm leading to the insulin‐stimulated activation of GLUT4. This implicates p38 MAPK involvement in the regulation of this pathway. There is further evidence that p38 MAPK is itself recruited to the plasma membrane. The role of the phosphorylation state of the glucose transporter in response to insulin stimulation has been studied and indicates that, contrary to what might be predicted, there is actually a decrease in its phosphorylation at the plasma membrane in response to insulin. The relationship of this change to glucose uptake remains to be established. Other possible mechanisms regulating GLUT4 activity include binding of (+) or (−) modulators of its function.
Peroxisome proliferator‐activated receptors (PPARS): regulators of gene expression in heart and skeletal muscle
Authors: Gilde A. J. , Van Bilsen M. 
Published: July 2003

AbstractThe peroxisome proliferator‐activated receptors (PPARs) are members of the nuclear hormone receptor superfamily. The three isoforms (PPARα, β/δ and γ) have been implicated in the regulation of the expression of genes involved in lipid metabolism. Although their prominent role in lipid homeostasis is well established, the way in which the activity of each of the PPAR isoforms is regulated under physiological and pathological conditions is still subject of intensive research. In skeletal as well as cardiac muscle cells it has been demonstrated that the expression of a large panel of proteins involved in the transport and metabolic conversion of fatty acids is under control of PPARs. The pivotal role of the PPARα isoform in cardiac fatty acid metabolism has been confirmed in PPARα‐null mice. The exact role of PPARβ/δ in the regulation of muscle metabolism is still a matter of debate. Whereas several studies provided evidence to support the notion that PPARα and PPARβ/δ have redundant roles, other studies suggest that PPARα activity is counteracted by PPARβ/δ. Marked effects of bona fide PPARγ ligands (the anti‐diabetic thiazolidinediones) on skeletal and cardiac muscle function and phenotype, have also been reported. However, next to activating PPARγ, the thiazolidinediones do affect other cellular processes as well. To date it is being realized that the control of the trans‐activating capacity of each of the PPAR isoforms is multi‐factorial and, in addition to ligand availability, depends on such factors as isoform‐specific phosphorylation and selective interaction with various proteins acting either as co‐activator or co‐repressor.
The role of intramuscular lipid in insulin resistance
Authors: Hegarty B. D. , Furler S. M. , Ye J. , Cooney G. J. , Kraegen E. W. 
Published: July 2003

AbstractThere is interest in how altered lipid metabolism could contribute to muscle insulin resistance. Many animal and human states of insulin resistance have increased muscle triglyceride content, and there are now plausible mechanistic links between muscle lipid accumulation and insulin resistance, which go beyond the classic glucose–fatty acid cycle. We postulate that muscle cytosolic accumulation of the metabolically active long‐chain fatty acyl CoAs (LCACoA) is involved, leading to insulin resistance and impaired insulin signalling or impaired enzyme activity (e.g. glycogen synthase or hexokinase) either directly or via chronic translocation/activation of mediators such as a protein kinase C (particularly PKC θ and ɛ). Ceramides and diacylglycerols (DAGs) have also been implicated in forms of lipid‐induced muscle insulin resistance. Dietary lipid‐induced muscle insulin resistance in rodents is relatively easily reversed by manipulations that lessen cytosolic lipid accumulation (e.g. diet change, exercise or fasting). PPAR agonists (both γ and α) also lower muscle LCACoA and enhance insulin sensitivity. Activation of AMP‐activated protein kinase (AMPK) by AICAR leads to muscle enhancement (especially glycolytic muscle) of insulin sensitivity, but involvement of altered lipid metabolism is less clear cut. In rodents there are similarities in the pattern of muscle lipid accumulation/PKC translocation/altered insulin signalling/insulin resistance inducible by 3–5‐h acute free fatty acid elevation, 1–4 days intravenous glucose infusion or several weeks of high‐fat feeding. Recent studies extend findings and show relevance to humans. Muscle cytosolic lipids may accumulate either by increased fatty acid flux into muscle, or by reduced fatty acid oxidation. In some circumstances muscle insulin resistance may be an adaptation to optimize use of fatty acids when they are the predominant available energy fuel. The interactions described here are fundamental to optimizing therapy of insulin resistance based on alterations in muscle lipid metabolism.
Push/pull mechanisms of GLUT4 traffic in muscle cells
Authors: Rudich A. , Klip A. 
Published: July 2003

AbstractAims:  Understanding the mechanisms by which insulin regulates glucose transporter 4 (GLUT4) traffic in skeletal muscle has been a major challenge since the discoveries of glucose transporter's translocation and the cloning of GLUT4. Here we summarize our work of the past 5 years on the regulation of GLUT4 traffic in skeletal muscle cells.Methods:  L6 cells overexpressing GLUT4 harbouring an exofacial myc epitope gave us the opportunity to perform dynamic assessments of GLUT4 exocytosis, endocytosis, as well as a means to follow GLUT4 molecules along their journey through intracellular compartments.Results:  We found that insulin stimulation, which results in the expected gain in surface GLUT4, is mostly attributed to enhanced GLUT4 exocytosis, and does not significantly affect the initial rate of internalization. Two mechanisms by which insulin enhances GLUT4 exocytosis are described: ‘Pull’ relates to actin remodelling‐based segregation of the insulin signalling molecules and the directed recruitment of GLUT4/VAMP2 containing vesicles. ‘Push’ is the accelerated inter‐endosomal transit of endocytosed GLUT4 molecules through the recycling endosome. The interface between the two types of regulatory input by insulin is suggested to be the budding of GLUT4 from the transferrin receptor (TfR)‐containing, recycling endosome.Conclusions:  We propose a model on the identity of the GLUT4 pools responsible for GLUT4 recruitment to the plasma membrane in the basal state, or following insulin or hyperosmolarity stimuli.
Regulation of glycogen synthase in skeletal muscle during exercise
Authors: Nielsen J. N. , Richter E. A. 
Published: July 2003

AbstractGlycogen synthase (GS) catalyses the incorporation of uridine diphosphate‐glucose into glycogen in skeletal muscle. In concert with the glucose transport step, GS activity is thought to be rate‐limiting in the disposal of glucose as muscle glycogen. Glycogen synthase is regulated by both allosteric factors (primarily glucose 6‐phosphate) and covalent modification by reversible phosphorylation and dephosphorylation leading to inactivation and activation of GS, respectively. Exercise activates both stimulatory and inhibitory regulators of GS and it is thought that the resultant activity of GS during exercise depends on the relative strength of opposing signals. However, the mechanisms by which exercise regulates GS activity are not fully understood. Glycogen breakdown, the GM–protein phosphatase 1 complex and possibly cellular relocalization of GS may be considered important factors involved in the stimulation of GS activity during exercise, while adenosine monophosphate‐activated protein kinase and plasma adrenaline (via protein kinase A) can be considered as essential for the exercise‐induced inhibitory signals to GS.
Cytoplasmic fatty acid‐binding protein facilitates fatty acid utilization by skeletal muscle
Authors: Glatz J. F. C. , Schaap F. G. , Binas B. , Bonen A. , Van Der Vusse G. J. , Luiken J. J. F. P. 
Published: July 2003

AbstractThe intracellular transport of long‐chain fatty acids in muscle cells is facilitated to a great extent by heart‐type cytoplasmic fatty acid‐binding protein (H‐FABP). By virtue of the marked affinity of this 14.5‐kDa protein for fatty acids, H‐FABP dramatically increases their concentration in the aqueous cytoplasm by non‐covalent binding, thereby facilitating both the transition of fatty acids from membranes to the aqueous space and their diffusional transport from membranes (e.g. sarcolemma) to other cellular compartments (e.g. mitochondria). Striking features are the relative abundance of H‐FABP in muscle, especially in oxidative muscle fibres, and the modulation of the muscular H‐FABP content in concert with the modulation of other proteins and enzymes involved in fatty acid handling and utilization. Newer studies with mice carrying a homozygous or heterozygous deletion of the H‐FABP gene show that, in comparison with wild‐type mice, hindlimb muscles from heterozygous animals have a markedly lowered (−66%) H‐FABP content but unaltered palmitate uptake rate, while in hindlimb muscles from homozygous animals (no H‐FABP present) palmitate uptake was reduced by 45%. These findings indicate that H‐FABP is present in relative excess and plays a substantial, but merely permissive role in fatty acid uptake by skeletal muscles.
Fate of fatty acids at rest and during exercise: regulatory mechanisms
Authors: Jensen M. D. 
Published: July 2003

AbstractFatty acids are a major fuel source for humans both at rest and during exercise. Plasma free fatty acids (FFA), although present only in micromolar concentrations, are the major circulating lipid fuel. FFA availability can increase two‐ to four‐fold with moderate intensity exercise. Other potential sources of fatty acids include circulating very low‐density lipoprotein (VLDL) triglycerides (TGs) (∼1/5 the fuel availability of FFA) and intramyocellular TGs (∼2 mmol kg−1 muscle). At rest ∼40% of systemic FFA uptake occurs in the splanchnic bed and uptake in legs is ∼15–20%. During leg exercise the uptake of FFA in leg tissue increases to 30–60% of systemic uptake and splanchnic uptake decreases to 15%. The fate of VLDL TG fatty acids has not been adequately studied. Intramyocellular TG hydrolysis increases during exercise, but the factors that regulate this response are not clear. The fact that contraction of isolated muscles can stimulate the hydrolysis and oxidation of intramyocellular TGs (in the absence of hormonal or neural input) suggests an intracellular regulation of this process. Additional regulation from changes in catecholamines and insulin may also occur. During moderate intensity exercise circulating FFA and intramyocellular TG provide roughly equal portions of fatty acids for oxidation. In addition to endurance training, dietary factors have been shown to modulate the fatty acid oxidation response to exercise. Much remains to be learned about fatty acid trafficking during exercise. What role do VLDL TG play? How is the oxidation of intramyocellular TGs regulated? Techniques to address these questions in humans are only now becoming available.
Frequent association between MEN 2A and cutaneous lichen amyloidosis
Authors: Verga Uberta , Fugazzola Laura , Cambiaghi Stefano , Pritelli Chiara , Alessi Elvio , Cortelazzi Donatella , Gangi Emanuela , Beck‐Peccoz Paolo 
Published: July 2003

Summaryobjective Multiple endocrine neoplasia type 2A (MEN 2A) and familial medullary thyroid carcinoma (FMTC) are genetic diseases due to activating mutations of the RET proto‐oncogene. Affected patients develop medullary thyroid carcinoma (100%), in an isolated form (FMTC) or in association with phaeochromocytoma (30–50%), and primary hyperparathyroidism (10–20%) (MEN 2A). The presence of cutaneous lichen amyloidosis (CLA) has been anecdotally described in few families harbouring RET proto‐oncogene mutation in codon 634. The aim of the study was to evaluate the incidence of CLA in MEN 2A/FMTC families.patients and design Ten MEN 2A/FMTC families were studied and RET gene mutations identified in all. Complete dermatological assessment was carried out in each family member. Skin biopsy for histological studies was performed in patients with CLA.results Among 10 MEN 2A/FMTC families, the presence of CLA was found only in patients belonging to the three families with MEN 2A and RET mutation in codon 634. Nine of 25 patients (36%) with codon 634 mutation presented CLA, though two of them did not show CLA skin lesions but the typical neurological pruritus in the upper back. In all patients, neurological pruritus was present since infancy as a precocious marker of the disorder. The dermatological study of patients with CLA skin lesions added further evidence that pruritus has a pivotal role in the development of CLA, the amyloid deposition being the consequence of repeated scratching. Light microscopy revealed orthokeratotic hyperkeratosis, with elongation of the rete ridges, rare intramalpighian apoptic keratinocytes and deposits of amorphous material in the superficial dermis. Examination under ultraviolet light showed thioflavin T‐positive staining, confirming the presence of amyloid in the papillary dermis. The use of Capsaicin at the dilution of 0·025% had a mild efficacy on the cutaneous symptoms.conclusions Among the members of the three families with MEN 2A and RET 634 mutation, the incidence of CLA was 36%, a figure similar to that reported in the literature for phaeochromocytoma (30–50%) and even higher than that for hyperparathyroidism (10–20%). The present data confirm that CLA is linked to codon 634 RET mutations and is a precocious marker of the disorder.
Thyroglobulin epitope recognition in a post iodine‐supplemented Sri Lankan population
Authors: Okosieme O. E. , Premawardhana L. D. K. E. , Jayasinghe A. , De Silva D. G. H. , Smyth P. P. A. , Parkes A. B. , Lejeune P. J. , Ruf J. , Lazarus J. H. 
Published: July 2003

Summaryobjective We previously reported a high prevalence of raised thyroglobulin autoantibodies (TgAb) in apparently healthy Sri Lankan schoolgirls following salt iodination. To characterize these antibodies further we determined the epitopes on thyroglobulin (Tg) with which they react and compared these with serum obtained from both healthy subjects and established autoimmune thyroid disease (AITD) patients from the UK. To extend our study to a wider population within Sri Lanka, we in addition determined the epitopes recognized by a group of AITD patients selected from a thyroid clinic in Sri Lanka, as well as apparently healthy female Sri Lankan tea workers of distinct ethnicity from the schoolgirls and AITD patients.design Sri Lankan schoolgirls (n = 282) and adult female tea estate workers (n = 208) were examined for thyroid autoimmune markers. Sera with high TgAb (> 98 kIU/l) were selected from these two groups (n = 36 and 45, respectively) to study epitope‐binding patterns. We also examined the sera from 16 AITD patients attending a thyroid clinic in Colombo, 16 patients with AITD from the thyroid clinic at the University Hospital of Wales and 16 sera from healthy control UK women with no evidence of thyroid disease. To determine the epitopes on Tg recognized by the subjects’ TgAb, we employed a panel of Tg mouse monoclonal antibodies labelled with alkaline phosphatase in a competitive enzyme‐linked immunosorbent assay reaction with the subjects’ serum.results and conclusions A majority of the Sri Lankan schoolgirls did not react with the immunodominant epitopes and did not differ significantly from healthy subjects from the UK in their Tg epitope recognition pattern. On the other hand, tea estate workers and Sri Lankan AITD patients recognized typical autoimmune thyroid disease epitopes and, in addition, recognized a separate cluster not previously associated with either the autoimmune state or the healthy state. The significance of this cluster requires further clarification.
Partial iodide organification defect caused by a novel mutation of the thyroid peroxidase gene in three siblings
Authors: Kotani Tomio , Umeki Kazumi , Kawano Jun–ichi , Suganuma Tatsuo , Hishinuma Akira , Ieiri Tamio , Harada Shohei 
Published: July 2003

Summarybackground Three siblings with goitre and latent to mild hypothyroidism were suspected of having thyroid peroxidase (TPO) abnormality. Direct sequencing of their genomic DNAs showed two novel mutations of the TPO gene, one of which was G1687T (Gly533Cys; exon 9) and the other 1808–13del (Asp574/Leu575del; exon 10). The two mutations were compound heterozygous, as the former was found in their father's DNA as heterozygous, and the latter was found in DNA from their mother, also as heterozygous. As Gly533 and Asp574/Leu575 were well‐conserved amino acids in the peroxidase superfamily, Gly533Cys‐ and Asp574/Leu575del‐TPOs were thought to be affected structurally or functionally. In expression studies using CHO‐Kl cells and mRNAs introduced with individual mutations, both mutated TPO proteins were expressed at the same molecular size as wild‐type TPO and had enzyme activity, although Gly533Cys‐TPO was slightly lower in efficiency of expression and more degenerative than wild‐type TPO.methods We examined the localization of both mutated TPOs. Gly533Cys‐TPO was located on the endoplasmic reticulum (ER) and nuclear envelope but not on the plasma membrane, whereas Asp574/Leu575del‐TPO was located not only on the ER and nuclear envelope but also on the plasma membrane, as wild‐type TPO. Nevertheless, only one point differed between Asp574/Leu575del‐ and wild‐type TPOs: the mutated TPO was expressed on the plasma membrane surface at less than half the rate of wild‐type TPO.results Gly533Cys‐TPO synthesized almost no thyroid hormone because of its defective localization on the apical membrane surface of thyrocytes, whereas Asp574/Leu575del‐TPO performed thyroid hormone synthesis at a rate of less half that of wild‐type TPO. In cotransfection experiments using three combinations of wild‐type and G1687T‐mRNAs, wild‐type and 1808–13del‐mRNAs, and G1687T‐, 1808–13del‐mRNAs, the three kinds of mRNAs were considered to have no influence on cell surface TPO expression of another mRNA when a 50%‐maximal amount of each mRNA was transfected. When a larger amount of each mRNA was transfected, the former two combinations showed the level of cell surface TPO expression obtained from the saturating amount of wild‐type mRNA, whereas the last combination of mutated mRNAs covered only about half of the expression level.conclusion Defective thyroid hormone production resulting from the abnormal TPOs was at a level that caused latent hypothyroidism when the patients were born. With their growth, thyroid hormone volume gradually became inadequate and their thyroid gland enlarged compensatorily.
Grey‐scale analysis allows a quantitative evaluation of thyroid echogenicity in the patients with Hashimoto's thyroiditis
Authors: Mazziotti Gherardo , Sorvillo Francesca , Iorio Sergio , Carbone Antonella , Romeo Antonio , Piscopo Marco , Capuano Salvatore , Capuano Ermanno , Amato Giovanni , Carella Carlo 
Published: July 2003

Summaryobjective In the present study we have performed a grey‐scale quantitative analysis of thyroid echogenicity in the patients affected by Hashimoto's thyroiditis (HT), obtaining a numerical estimate of the degree of hypoecogenicity associated with the appearance of thyroid dysfunction.materials and methods The study group included 89 patients with serum positivity for thyroglobulin (TgAb) and/or peroxidase (TPOAb) antibodies. Ultrasound (US) evaluation of thyroid gland and biochemical assay of serum thyrotropin (TSH), free‐thyroxine (FT4) and free‐triiodiothyronyne (FT3) were performed in all patients, and in 40 healthy subjects enrolled as control group. Thyroid echogenicity was compared with that of the surrounding neck muscles, using the grey‐scale histogram analysis. The echogenicity was expressed in grey‐scales (GWE).results In HT patients, the mean of thyroid echogenicity was lower when compared to the normal thyroid (61·9 ± 8·3 GWE vs. 71·9 ± 3·1 GWE; P = 0·01). In all HT patients the lowest limit of thyroid echo distribution was in the echogenicity range of the surrounding muscle, the overlapping ranging between 3·4% and 95·0% (mean ± SD 48·4 ± 20·9%). The extension of like‐muscle hypoechogenicity into the thyroid gland was significantly correlated with serum TSH values (r = 0·37; P < 0·001), serum FT4 values (r = –0·60; P < 0·001), and serum TPOAb values (r = 0·31; P = 0·004). Nobody was hypothyroid when the hypoechogenicity was less than 38·0%, whereas hypothyrodism occurred in all cases with hypoechogenicity of more than 68·9%. The receiving operating characteristic curve demonstrated that 48·3% was the best cut‐off for identifying hypothyroid patients with sensitivity, specificity and diagnostic accuracy of 88·9%, 86·3% and 87·6%, respectively.conclusions In conclusion, the grey‐scale quantitative analysis has provided a measure of thyroid hypoechogenicity associated with the appearance of hypothyroidism during the course of HT. The results of the present study would encourage the application of the computerized grey‐scale analysis as complementary tool to US evaluation in the patients affected by HT.
The N363S polymorphism in the glucocorticoid receptor gene is associated with overweight in subjects with type 2 diabetes mellitus
Authors: Roussel Ronan , Reis André F. , Dubois‐Laforgue Danièle , Bellanné‐Chantelot Christine , Timsit José , Velho Gilberto 
Published: July 2003

Summaryobjective A single nucleotide polymorphism (A1220G; N363S) in exon 2 of the glucocorticoid receptor gene (NR3C1), associated with increased sensitivity to glucocorticoids, was shown to be associated with obesity in nondiabetic subjects. Here, we investigated the impact of this variant on traits related to obesity and hyperglycaemia in subjects with type 2 diabetes mellitus.patients and measurements The N363S variant was screened by restriction fragment length polymorphism technique following DNA amplification by polymerase chain reaction in 369 French Caucasians with type 2 diabetes mellitus.results Twenty subjects were found to be heterozygous for the variant (AG genotype frequency 0·0542). The prevalence of overweight [body mass index (BMI) > 25 kg/m2] was higher in AG carriers than in AA carriers (100%vs. 73%, P = 0·003). Moreover, the mean body weight and the BMI were higher in AG as compared to AA carriers, although only the body weight was significantly different between groups. However, when only the men were considered, a significantly higher BMI was observed in AG as compared to AA carriers: 30·0 ± 4·8 vs. 27·3 ± 4·6 kg/m2 (BMI Z‐score 1·28 ± 1·38 vs. 0·55 ± 1·17; P = 0·035). No evidence was found for an association of the N363S variant with parameters related to the severity of hyperglycaemia.conclusions The 363S allele of the N363S variant of NR3C1 is associated with the susceptibility to overweight in subjects with type 2 diabetes mellitus.