Contribution of abnormal insulin secretion and insulin resistance to the pathogenesis of type 2 diabetes in myotonic dystrophy

New developments in myotonic dystrophies from a multisystemic perspective

PURPOSE OF REVIEW

The multisystemic involvement of myotonic dystrophies (DMs) intricates disease monitoring, patients' care and trial design. This update of the multifaceted comorbidities observed in DMs aims to assist neurologists in the complex management of patients and to encourage further studies for still under-investigated aspects of the disease.

RECENT FINDINGS

We reviewed the most recent studies covering pathogenesis and clinical aspects of extra-muscular involvement in DM1 and DM2. The largest body of evidence regards the cardiac and respiratory features, for which experts' recommendations have been produced. Gastrointestinal symptoms emerge as one of the most prevalent complaints in DMs. The alteration of insulin signaling pathways, involved in gastrointestinal manifestations, carcinogenesis, muscle function, cognitive and endocrinological aspects, gain further relevance in the light of recent evidence of metformin efficacy in DM1. Still, too few studies are performed on large DM2 cohorts, so that current recommendations mainly rely on data gathered in DM1 that cannot be fully translated to DM2.

SUMMARY

Extra-muscular manifestations greatly contribute to the overall disease burden. A multidisciplinary approach is the key for the management of patients. Consensus-based recommendations for DM1 and DM2 allow high standards of care but further evidence are needed to implement these recommendations.

Insulin Signaling as a Key Moderator in Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is an autosomal dominant genetic disease characterized by multi-system involvement. Affected organ system includes skeletal muscle, heart, gastro-intestinal system and the brain. In this review, we evaluate the evidence for alterations in insulin signaling and their relation to clinical DM1 features. We start by summarizing the molecular pathophysiology of DM1. Next, an overview of normal insulin signaling physiology is given, and evidence for alterations herein in DM1 is presented. Clinically, evidence for involvement of insulin signaling pathways in DM1 is based on the increased incidence of insulin resistance seen in clinical practice and recent trial evidence of beneficial effects of metformin on muscle function. Indirectly, further support may be derived from certain CNS derived symptoms characteristic of DM1, such as obsessive-compulsive behavior features, for which links with altered insulin signaling has been demonstrated in other diseases. At the basic scientific level, several pathophysiological mechanisms that operate in DM1 may compromise normal insulin signaling physiology. The evidence presented here reflects the importance of insulin signaling in relation to clinical features of DM1 and justifies further basic scientific and clinical, therapeutically oriented research.

Lipid mediators of insulin signaling in diabetic kidney disease

Diabetic kidney disease (DKD) affects ∼40% of patients with diabetes and is associated with high mortality rates. Among different cellular targets in DKD, podocytes, highly specialized epithelial cells of the glomerular filtration barrier, are injured in the early stages of DKD. Both clinical and experimental data support the role of preserved insulin signaling as a major contributor to podocyte function and survival. However, little is known about the key modulators of podocyte insulin signaling. This review summarizes the novel knowledge that intracellular lipids such as cholesterol and sphingolipids are major determinants of podocyte insulin signaling. In particular, the implications of these lipids on DKD development, progression, and treatment will be addressed.

Insulin Receptor Isoforms in Physiology and Disease: An Updated View

The insulin receptor (IR) gene undergoes differential splicing that generates two IR isoforms, IR-A and IR-B. The physiological roles of IR isoforms are incompletely understood and appear to be determined by their different binding affinities for insulin-like growth factors (IGFs), particularly for IGF-2. Predominant roles of IR-A in prenatal growth and development and of IR-B in metabolic regulation are well established. However, emerging evidence indicates that the differential expression of IR isoforms may also help explain the diversification of insulin and IGF signaling and actions in various organs and tissues by involving not only different ligand-binding affinities but also different membrane partitioning and trafficking and possibly different abilities to interact with a variety of molecular partners. Of note, dysregulation of the IR-A/IR-B ratio is associated with insulin resistance, aging, and increased proliferative activity of normal and neoplastic tissues and appears to sustain detrimental effects. This review discusses novel information that has generated remarkable progress in our understanding of the physiology of IR isoforms and their role in disease. We also focus on novel IR ligands and modulators that should now be considered as an important strategy for better and safer treatment of diabetes and cancer and possibly other IR-related diseases.

Changes in Components, Glycyrrhizin and Glycyrrhetinic Acid, in RawGlycyrrhiza uralensisFisch, Modify Insulin Sensitizing and Insulinotropic Actions

We hypothesized that roasted Glycyrrhizae Radix (Glycyrrhizin Radix Praeparata, GRP) might modify anti-diabetic action due to compositional changes. Then we examined the anti-diabetic effect and mechanism of raw Glycyrrhizae Radix (GR) and GRP extracts and their major respective components, glycyrrhizin and glycyrrhetinic acid. In partial pancreatectomized (Px) diabetic mice, both GR and GRP improved glucose tolerance, but only GRP enhanced glucose-stimulated insulin secretion as much as exendin-4. Both GR and GRP extracts enhanced insulin-stimulated glucose uptake through peroxisome proliferation-activated receptor (PPAR)-gamma activation in 3T3-L1 adipocytes. Consistently with the results of the mice study, only GRP and glycyrrhetinic acid enhanced glucose-stimulated insulin secretion in isolated islets. In addition, they induced mRNA levels of insulin receptor substrate-2, pancreas duodenum homeobox-1, and glucokinase in the islets, which contributed to improving beta-cell viability. In conclusion, GRP extract containing glycyrrhetinic acid improved glucose tolerance better than GR extract by enhancing insulinotropic action. Thus, GRP had better anti-diabetic action than GR.

Improvement of glucose tolerance by rhein with restored early-phase insulin secretion in db/db mice

AIMS

In the present study, we investigated whether rhein exerted hypoglycemic action and rhein's effect on the pancreatic β cell in db/db mice.

MATERIALS AND METHODS

Thirty 4-week-old db/db mice were randomized to treatment with rhein (120 mg/kg) (no.=15) and placebo (1% natrium cellulose solution) (no.=15) for 8 weeks, respectively. Fifteen age-matched non-diabetic littermates db/m mice treated with placebo were studied as non-diabetic control. After an 8-week treatment, ip glucose tolerance test (IPGTT) and arginine tolerance test were performed. Area under curve (AUC) of insulin levels in IPGTT was calculated to evaluate insulin secretory function. Immunohistochemical staining of insulin was performed to estimate β cell mass. TUNEL assay was performed to determine β cell apoptosis. Islet isolation and perifusion were performed to evaluate kinetics of insulin release in vitro, especially first-phase insulin.

RESULTS

Compared with control group, AUC of glucose concentrations significantly decreased in the rhein-treated group (p<0.05). Simultaneously, AUC of insulin levels increased in the rhein-treated group (p<0.05), especially in the first 30 min after glucose load. Perifusion showed that the rhein-treated group manifested a significantly increase of first-phase insulin secretion. Immunohistochemical study and TUNEL assay showed that rhein treatment greatly preserved β cell mass and inhibited β cell apoptosis.

CONCLUSIONS

Rhein treatment significantly improved glucose- dependent and independent insulin secretion by preservation of β cell mass and inhibition of β cell apoptosis in db/db mice. The characteristics of rhein may make it a novel therapeutic means for preventing from or curing diabetes in the near future.

Ecklonia cava Inhibits Glucose Absorption and Stimulates Insulin Secretion in Streptozotocin-Induced Diabetic Mice

Aims of study. Present study investigated the effect of Ecklonia cava (EC) on intestinal glucose uptake and insulin secretion. Materials and methods. Intestinal Na⁺-dependent glucose uptake (SGU) and Na⁺-dependent glucose transporter 1 (SGLT1) protein expression was determined using brush border membrane vesicles (BBMVs). Glucose-induced insulin secretion was examined in pancreatic β-islet cells. The antihyperglycemic effects of EC, SGU, and SGLT1 expression were determined in streptozotocin (STZ)-induced diabetic mice. Results. Methanol extract of EC markedly inhibited intestinal SGU of BBMV with the IC₅₀ value of 345 μg/mL. SGLT1 protein expression was dose dependently down regulated with EC treatment. Furthermore, insulinotrophic effect of EC extract was observed at high glucose media in isolated pancreatic β-islet cells in vitro. We next conducted the antihyperglycemic effect of EC in STZ-diabetic mice. EC supplementation markedly suppressed SGU and SGLT1 abundance in BBMV from STZ mice. Furthermore, plasma insulin level was increased by EC treatment in diabetic mice. As a result, EC supplementation improved postprandial glucose regulation, assessed by oral glucose tolerance test, in diabetic mice. Conclusion. These results suggest that EC play a role in controlling dietary glucose absorption at the intestine and insulinotrophic action at the pancreas contributing blood glucose homeostasis in diabetic condition.

[A genetic systemic disease: clinical description of type 1 myotonic dystrophy in adults]

Type 1 myotonic dystrophy is an autosomal dominant inherited disorder related to the expansion of a trinucleotide (CTG) repeat in the exon 15 in the 3'-untranslated region of the myotonic dystrophy protein kinase (DMPK) gene. Mutant transcripts containing an expanded CUG repeat are retained in nuclear foci and cause numerous dysfunctions by interfering with biogenesis of other mRNAs. Prominent clinical features are progressive muscular weakness and myotonia, which affect skeletal muscles but also white muscles leading to digestive, urinary and obstetrical disorders. Functional prognosis correlates with motor handicap and vital prognosis is linked to cardiac rhythm disturbances and conduction defects due to progressive subendocardial fibrosis, and to complex respiratory dysfunctions, which associate restrictive lung disease, involvement of the central inspiratory pathway, and sleep apnea. Other clinical features are lens opacity, glucose intolerance, metabolic syndrome, several endocrine disorders (gonadal deficiency, hyperparathydoidism), or immunoglobulin deficiency due to immunoglobulin G hypercatabolism. Life expectancy is reduced in myotonic dystrophy, and death is mainly caused by respiratory complications, but also by cardiac arrhythmias. Moreover, an abnormal incidence of tumors has been reported. Therefore, myotonic dystrophy does not only concern neurologists but a multidisciplinary approach is necessary, including at least pneumologist, cardiologist, and physiotherapist. General internists should also be implicated, not only in the initial diagnosis step, but also in the diagnosis of complications and their treatments.

Glyceollin-containing fermented soybeans improve glucose homeostasis in diabetic mice

OBJECTIVE

Our previous in vitro study demonstrated that glyceollins help normalize glucose homeostasis by potentiating β-cell function and survival in insulinoma cells as well as improving glucose utilization in adipocytes. Here, we investigated whether fermented soybeans containing glyceollins had an antidiabetic action in type 2 diabetic animals.

METHODS

The diabetic mice, their diabetes induced by intraperitoneal injections of streptozotocin (20 mg/kg bw), were administered a high fat diet with no soybeans (control), 10% unfermented soybeans and 10% fermented soybeans containing glyceollins, respectively, (FSG) for 8 weeks. As positive controls, rosiglitazone (20 mg/kg/bw) was given to diabetic mice fed a no soybean diet and non-diabetic mice were also placed on the same diet.

RESULTS

Among the diabetic mice, FSG-treated mice exhibited the lowest peak for blood glucose levels with an elevation of serum insulin levels during the first part of oral glucose tolerance testing. FSG also made blood glucose levels drop quickly after the peak and it decreased blood glucose levels more than the control during insulin tolerance testing. This improvement was associated with increased hepatic glycogen accumulation and decreased triglyceride storage. The phosphorylation of Akt, AMP-kinase, and acetyl-CoA carboxylase in the liver was potentiated by FSG, whereas phosphoenolpyruvate carboxykinase expression decreased. The enhancement of glucose homeostasis was comparable to the effect induced by rosiglitazone, a commercial peroxisome proliferator-activated receptor-γ agonist, but it did not match the level of glucose homeostasis in the non-diabetic mice.

CONCLUSION

Glyceollin-containing FSG improves glucose homeostasis, partly by enhancing hepatic insulin sensitivity in type 2 diabetic mice.

Muscular dystrophies at different ages: metabolic and endocrine alterations

Common metabolic and endocrine alterations exist across a wide range of muscular dystrophies. Skeletal muscle plays an important role in glucose metabolism and is a major participant in different signaling pathways. Therefore, its damage may lead to different metabolic disruptions. Two of the most important metabolic alterations in muscular dystrophies may be insulin resistance and obesity. However, only insulin resistance has been demonstrated in myotonic dystrophy. In addition, endocrine disturbances such as hypogonadism, low levels of testosterone, and growth hormone have been reported. This eventually will result in consequences such as growth failure and delayed puberty in the case of childhood dystrophies. Other consequences may be reduced male fertility, reduced spermatogenesis, and oligospermia, both in childhood as well as in adult muscular dystrophies. These facts all suggest that there is a need for better comprehension of metabolic and endocrine implications for muscular dystrophies with the purpose of developing improved clinical treatments and/or improvements in the quality of life of patients with dystrophy. Therefore, the aim of this paper is to describe the current knowledge about of metabolic and endocrine alterations in diverse types of dystrophinopathies, which will be divided into two groups: childhood and adult dystrophies which have different age of onset.

Peripheral nerve axon involvement in myotonic dystrophy type 1, measured using the automated nerve excitability test

Background and Purpose

Primary involvement of the peripheral nerves in myotonic dystrophy type I (MyD1) is controversial. We investigated whether the involvement of peripheral nerves is a primary event of MyD1 or secondary to another complication such as diabetes mellitus (DM).

Methods

The subjects comprised 12 patients with MyD1, 12 with DM and no peripheral nerve involvement, and 25 healthy volunteers. We measured multiple excitability indices in the median motor axons. The strength-duration time constant was calculated from the duration-charge curve, the threshold electrotonus and current-threshold relationships were calculated from the sequential subthreshold current, and the recovery cycle was derived from double suprathreshold stimulation.

Results

The depolarizing and hyperpolarizing threshold electrotonus were significantly reduced and exhibited increased refractoriness in the MyD1 group compared with the DM and control groups. The SDTC, superexcitability, and subexcitability were not significantly altered in the MyD1 group.

Conclusions

The MyD1 group exhibited a depolarized axonal membrane potential. The significant differences in peripheral nerve excitability between the MyD1 group and the DM and normal control groups suggest that peripheral neuropathy is a primary event in MyD1 rather than a secondary complication of DM.

Myotonic dystrophy types 1 and 2

Myotonic dystrophies (dystrophia myotonica, or DM) are inherited disorders characterized by myotonia and progressive muscle degeneration, which are variably associated with a multisystemic phenotype. To date, two types of myotonic dystrophy, type 1 (DM1) and type 2 (DM2), are known to exist; both are autosomal dominant disorders caused by expansion of an untranslated short tandem repeat DNA sequence (CTG)(n) and (CCTG)(n), respectively. These expanded repeats in DM1 and DM2 show different patterns of repeat-size instability. Phenotypes of DM1 and DM2 are similar but there are some important differences, most conspicuously in the severity of the disease (including the presence or absence of the congenital form), muscles primarily affected (distal versus proximal), involved muscle fiber types (type 1 versus type 2 fibers), and some associated multisystemic phenotypes. The pathogenic mechanism of DM1 and DM2 is thought to be mediated by the mutant RNA transcripts containing expanded CUG and CCUG repeats. Strong evidence supports the hypothesis that sequestration of muscle-blind like (MBNL) proteins by these expanded repeats leads to misregulated splicing of many gene transcripts in corroboration with the raised level of CUG-binding protein 1. However, additional mechanisms, such as changes in the chromatin structure involving CTCN-binding site and gene expression dysregulations, are emerging. Although treatment of DM1 and DM2 is currently limited to supportive therapies, new therapeutic approaches based on pathogenic mechanisms may become feasible in the near future.

Comparative transcriptional and biochemical studies in muscle of myotonic dystrophies (DM1 and DM2)

Myotonic dystrophy type 1 (DM1) and myotonic dystrophy type 2 (proximal muscular myopaty/DM2) are caused by similar dynamic mutations at two distinct genetic loci. The two diseases also lead to similar phenotypes but different clinical severity. Dysregulation of alternative splicing has been suggested as the common pathogenic mechanism. Here, we investigate the molecular differences between DM1 and DM2 using reverse transcriptase-polymerase chain reaction of troponin T (TnT) and the insulin receptor (IR), as well as immunoblotting of TnT in muscle biopsies from DM1 and DM2 patients. We found that: (a) slow TnT was encoded by two different transcripts in significantly different ratios in DM1 and DM2 muscles; (b) DM2 muscles exhibited a higher degree of alternative splicing dysregulation for fast TnT transcripts when compared to DM1 muscles; (c) the distribution of TnT proteins was significantly skewed towards higher molecular weight species in both diseases; (d) the RNA for the insulin-independent IR-A isoform was significantly increased and appeared related to the fibre-type composition in the majority of the cases examined. On the whole, these data should give a better insight on pathogenesis of DM1 and DM2.

A cross-sectional study for glucose intolerance of myotonic dystrophy

We made a cross-sectional study to analyze glucose intolerance of myotonic dystrophy type 1 (DM1) with several examination including oral glucose tolerance test (OGTT), insulin tolerance test (ITT) and adiponectin. Ninety-five DM1 patients participated in this study. Health examination data from general people were used as controls. In DM1, homeostasis model assessment-insulin resistance (HOMA-IR) was higher than control even in the lowest fasting blood sugar (FBS) stage (<80 mg/dl) and insulin sensitivity assessed by ITT was low regardless of their FBS. Insulinogenic index of DM1 was positively correlated to HOMA-IR. Insulinogenic index and sum of IRI in OGTT were markedly elevated in the lowest FBS stage and declined along with elevation of FBS. Consequently, as many as 13.3% of DM1 patients with 90-110 mg/dl of FBS exhibited DM pattern, while only 1.9% in control. Adiponectin was higher in DM1 than control. Although age correlated with adiponectin in both control and DM1, its impact was stronger in DM1. DM1 predisposes insulin resistance and compensatory hyperinsulinemia exist even in patients with low FBS. We should pay attention to glucose intolerance of DM1 patients earlier than that of the general population. It seemed that 90 mg/dl of FBS is an important index as an indication of careful managements.

Beneficial effect of chungkukjang on regulating blood glucose and pancreatic beta-cell functions in C75BL/KsJ-db/db mice

The current study investigated the antidiabetic effect of chungkukjang, a widely used traditional Korean soybean fermentation food, in a type 2 diabetic animal model, C57BL/KsJ-db/db mice. After a 2-week acclimation period, the db/db mice (male, 5 weeks old) were divided into three groups: diabetic control (AIN-76 diet), chungkukjang (5 g/100 g of diet), and rosiglitazone (0.005 g/100 g of diet). The supplementation of chungkukjang induced a significant reduction of blood glucose and glycosylated hemoglobin level, and it improved insulin tolerance compared to the diabetic control group. Plasma and pancreatic insulin levels of the chungkukjang-supplemented group were significantly higher than those of the diabetic control mice, and the plasma glucagon level was also significantly different. The supplementation of chungkukjang and rosiglitazone significantly elevated hepatic glucokinase activity with a simultaneous reduction of glucose-6-phosphatase and phosphoenolpyruvate carboxykinase activity in the db/db mice compared to the diabetic control mice. In addition, the chungkukjang-supplemented group had an increased hepatic glycogen content compared to the diabetic control and rosiglitazone-supplemented groups. Consequently, these results suggest that chungkukjang may be beneficial in improving insulin resistance and hyperglycemia in type 2 diabetic animals that are partly medicated by the regulation of hepatic glucose enzymes and insulin sensitivity in peripheral tissues.

DM1 CTG expansions affect insulin receptor isoforms expression in various tissues of transgenic mice

Myotonic dystrophy (DM1) is a dominant autosomal multisystemic disorder caused by the expansion of an unstable CTG trinucleotide repeat in the 3' untranslated region of the DMPK gene. Nuclear accumulation of the enlarged CUG-containing DMPK transcripts has a deleterious effect on the regulation of alternative splicing of some RNAs and has a central role in causing the symptoms of DM1. In particular, Insulin Receptor (IR) mRNA splicing defects have been observed in the muscle of DM1 patients. In this study, we have investigated IR splicing in insulin-responsive tissues (i.e. skeletal muscles, adipose tissue, liver) and pancreas and we have studied glucose metabolism in mice carrying the human genomic DM1 region with expanded (>350 CTG) or normal (20 CTG) repeats and in wild-type mice. Mice carrying DM1 expansions displayed a tissue- and age-dependent abnormal regulation of IR mRNA splicing in all the tissues that we investigated. Furthermore, these mice showed a basal hyperglycemia and glucose intolerance which disappeared with age. Our findings show that deregulation of IR splicing due to the DM1 mutation can occur in different mouse tissues, suggesting that CTG repeat expansions might also result in IR misplicing not only in muscles but also in other tissues in DM1 patients.

Gender-specific links between hepatic 11beta reduction of cortisone and adipokines

OBJECTIVE

Reduction of cortisone to cortisol is mediated by 11beta-hydroxysteroid dehydrogenase type 1 (11betaHSD1), a putative key enzyme in obesity-related complications. Experimental studies suggest that adipokines, notably leptin and tumor necrosis factor-alpha (TNF-alpha), are of importance for 11betaHSD1 activity. We hypothesized that the regulation of hepatic preceptor glucocorticoid metabolism is gender-specific and associated with circulating levels of leptin and TNF-alpha receptors and/or sex hormones.

RESEARCH METHODS AND PROCEDURES

A total of 34 males and 38 women (14 premenopausal and 22 postmenopausal) underwent physical examination and fasting blood sampling. Insulin sensitivity was tested by euglycemic hyperinsulinemic clamps, and hepatic 11betaHSD1 enzyme activity was estimated by the conversion of orally-ingested cortisone to cortisol.

RESULTS

Hepatic 11betaHSD1 activity was negatively associated with leptin and soluble TNF (sTNF) r1 and sTNFr2 in males. These correlations remained significant after adjustment for age and insulin sensitivity, and for sTNF-alpha receptors also after adjustment of BMI and waist circumference. In contrast, 11beta reduction of cortisone was positively associated to leptin in females after adjustment for BMI and waist circumference.

DISCUSSION

Hepatic 11beta reduction shows different links to circulating adipocyte-derived hormones in males and females. This emphasizes the need for further studies on tissue-specific regulation of 11betaHSD1 in both genders.

Islet amyloid polypeptide (IAPP) transgenic rodents as models for type 2 diabetes

Blood glucose concentrations are maintained by insulin secreted from beta-cells located in the islets of Langerhans. There are approximately 2000 beta-cells per islet, and approximately one million islets of Langerhans scattered throughout the pancreas. The islet in type 2 diabetes mellitus (T2D) has deficient beta-cell mass due to increased beta-cell apoptosis and islet amyloid derived from islet amyloid polypeptide (IAPP). Accumulating evidence implicates toxic IAPP oligomers in the mediation of beta-cell apoptosis in T2D. Humans, monkeys, and cats express an amyloidogenic toxic form of IAPP and spontaneously develop diabetes characterized by islet amyloid deposits. However, longitudinal studies of islet pathology in humans are impossible, and studies in nonhuman primates and cats are costly and impractical. Rodent IAPP is not amyloidogenic, thus commonly used rodent models of diabetes do not recapitulate islet pathology in humans. To investigate the diabetogenic role of human IAPP (h-IAPP), several mouse models and, more recently, a rat model transgenic for h-IAPP have been developed. Studies in these models have revealed that the toxic effect of h-IAPP on beta-cell apoptosis demonstrates a threshold-dependent effect. Specifically, increasing h-IAPP transgene expression by breeding or induction of insulin resistance leads to increased beta-cell apoptosis and diabetes. These transgenic rodent models for h-IAPP provide an opportunity to elucidate the mechanisms responsible for h-IAPP-induced beta-cell apoptosis further and to test novel approaches to the prevention and treatment of T2D.

Axonal function and activity-dependent excitability changes in myotonic dystrophy

To investigate peripheral nerve function and its potential contribution to symptoms of weakness in myotonic dystrophy type 1 (MD), nerve excitability was assessed in 12 MD patients. Compound muscle action potentials (CMAPs) were recorded at rest from abductor pollicis brevis (APB) following stimulation of the median nerve. Stimulus-response behavior, threshold electrotonus, a current-threshold relationship, and recovery cycles were successfully recorded in each patient. Compared with controls, there was significant reduction in CMAP amplitude in MD patients. This was accompanied by reduction in depolarizing threshold electrotonus and an increase in refractoriness and in the duration of the relative refractory period. To determine whether alteration in axonal resting membrane potential was a factor underlying these changes, axonal excitability was assessed following maximal contraction of APB for 60 seconds. Following contraction, there was reduction in CMAP amplitude for a submaximal stimulus (by 51.5+/-11.8%) and an increase in super-excitability (of 22.2+/-12.0%), consistent with activity-dependent hyperpolarization, with a greater increase in threshold for MD patients compared to controls (MD group, 22.3+/-5.1%; controls, 11.7+/-2.1%; P<0.04) and prolonged recovery to baseline. The present study has established that greater activity-dependent changes in excitability may be induced in MD patients by maximal voluntary contraction when compared to controls. The excitability changes and prolonged recovery of threshold following contraction are likely to contribute to symptoms of fatigue and weakness in MD patients.

The isoflavonoid aglycone-rich fractions of Chungkookjang, fermented unsalted soybeans, enhance insulin signaling and peroxisome proliferator-activated receptor-gamma activity in vitro

We investigated anti-diabetic candidates and their mechanisms from the fractions of Chungkookjang (CKJ), a traditional fermented unsalted soybean, by investigating insulin signaling, peroxisome proliferator-activated receptor (PPAR)-gamma activity and glucose-stimulated insulin secretion, in vitro. Cooked soybeans (CSB) and CKJ, fermented predominantly with Bacillus subtilis, were extracted by 70% EtOH followed by an XAD-4 column chromatography with a serial mixture of solvents comprised of MeOH and water. During fermentation, the contents of isoflavonoid aglycones were elevated, and the fractions enriched with aglycones enhanced insulin-stimulated glucose uptake in 3T3-L1 adipocytes. This increase in glucose uptake resulted from stimulating a translocation of the glucose transporter (GLUT)-4 into the plasma membrane through the phosphorylation of insulin receptor substrate (IRS)-1 and Akt. Especially, daidzein enriched fractions elevated insulin-stimulated glucose uptake by acting as PPAR-gamma agonist up to levels exhibited when 10 nM insulin is administered. Fractions containing small peptides with low polarity in CKJ slightly increased glucose-stimulated insulin secretion. The data suggest that an increase in isoflavonoid aglycones in CKJ, in comparison to CSB, enhances glucose utilization via activating insulin signaling and stimulates PPAR-gamma activity in adipocytes. In addition, CKJ contains small peptides improving glucose-stimulated insulin secretion in insulinoma cells. Overall, CKJ is superior to CSB in anti-diabetic action.

Insulin sensitizing and alpha-glucoamylase inhibitory action of sennosides, rheins and rhaponticin in Rhei Rhizoma

Extracts from Rhei Rhizoma extracts (RR) have been reported to attenuate metabolic disorders such as diabetic nephropathy, hypercholesterolemia and platelet aggregation. With this study we investigated the anti-diabetic action of 70% ethanol RR extract in streptozotocin-induced diabetic mice, and determined the action mechanism of active compounds of RR in vitro. In the diabetic mice, serum glucose levels at fasting and post-prandial states and glucose area under the curve at modified oral glucose tolerance tests were lowered without altering serum insulin levels, indicating that RR contained potential anti-diabetic agents. The fractions fractionated from RR extracts by XAD-4 column revealed that 60%, 80% and 100% methanol fractions enhanced insulin sensitivity and inhibited alpha-glucoamylase activity. The major compounds of these fractions were sennosides, rhein and rhaponticin. Rhaponticin and rhein enhanced insulin-stimulated glucose uptake in 3T3-L1 adipocytes. Rhaponticin increased adipocytes with a differentiating effect similar to pioglitazone, but rhein and sennoside B decreased triglyceride accumulation. Sennoside A and B inhibited alpha-glucoamylase activity as much as acarbose. In conclusion, a crude extract of RR improves glucose intolerance by enhancing insulin-stimulated glucose uptake and decreasing carbohydrate digestion via inhibiting alpha-glucoamylase activity. Rhein and rhaponticin are potential candidates for hypoglycemic agents.

Postabsorptive and insulin-stimulated energy and protein metabolism in patients with myotonic dystrophy type 1

BACKGROUND

Exaggerated insulin resistance was described as the major metabolic abnormality in myotonic dystrophy type 1 (DM1). We reported recently that the severity of the impairment in insulin-stimulated glucose metabolism in these patients was overestimated.

OBJECTIVE

The aim was to dissect out insulin action with respect to whole-body energy homeostasis and glucose, protein, and lipid metabolism in patients with DM1 to assess the relevance of insulin resistance to the heterogeneous clinical manifestations of this syndrome.

DESIGN

Ten nondiabetic patients with DM1 and 10 matched healthy control subjects were studied by means of 1) dual-energy X-ray absorptiometry; 2) a euglycemic-hyperinsulinemic clamp (40 mU. m(-2). min(-1)) combined with a primed, continuous infusion of [6,6-d(2)]glucose and [1-(13)C]leucine; 3) indirect calorimetry; and 4) localized (1)H magnetic resonance spectroscopy of the calf muscles.

RESULTS

Patients with DM1 had less lean body mass, greater fat mass, and greater intramyocellular lipid contents than did healthy control subjects. Energy expenditure and glucose and lipid metabolism did not differ significantly between the groups. In contrast, markers of proteolysis were higher in DM1 patients in the postabsorptive and insulin-stimulated conditions and were associated with lower plasma concentrations of insulin-like growth factor 1 (P < 0.03) and higher plasma concentrations of tumor necrosis factor alpha receptor 2 (P = 0.04).

CONCLUSIONS

Despite greater body fat and intramyocellular lipid contents in patients with DM1, insulin sensitivity was not significantly different between patients and control subjects. In contrast, the loss of lean body mass in patients with DM1 was associated with abnormal postabsorptive and insulin-stimulated regulation of protein breakdown. Lower plasma insulin-like growth factor 1 concentrations and higher tumor necrosis factor system activity might be involved in the muscle wasting of DM1.