Polarized insertion of an intracellular glycoprotein pool into the apical membrane of MDCK cells

Fasting potentiates insulin-mediated glucose uptake in rested and prior-contracted rat skeletal muscle

A single bout of exercise can potentiate the effect of insulin on skeletal muscle glucose uptake via activation of the AMPK-TBC1 domain family member 4 (TBC1D4) pathway, which suggests a positive correlation between AMPK activation and insulin sensitization. In addition, prolonged fasting in rodents is known to upregulate and thereby synergistically enhance the effect of exercise on muscle AMPK activation. Therefore, fasting may potentiate the insulin-sensitizing effect of exercise. In the present study, we mimicked exercise by in situ muscle contraction and evaluated the effect of a 36-h fast on muscle contraction-induced insulin sensitization. Male Wistar rats weighing 150-170 g were allocated to either a 36-h fasting or feeding group. The extensor digitorum longus (EDL) muscles were electrically contracted via the common peroneal nerve for 10 min followed by a 3-h recovery period. EDL muscles were dissected and incubated in the presence or absence of submaximal insulin. Our results demonstrated that acute muscle contraction and 36 h of fasting additively upregulated AMPK pathway activation. Insulin-stimulated muscle glucose uptake and site-specific TBC1D4 phosphorylation were enhanced by prior muscle contraction in 36-h-fasted rats, but not in fed rats. Moreover, enhanced insulin-induced muscle glucose uptake and Akt phosphorylation due to 36 h of fasting were associated with a decrease in tribbles homolog 3 (TRB3), a negative regulator of Akt activation. In conclusion, fasting and prior muscle contraction synergistically enhance insulin-stimulated TBC1D4 phosphorylation and glucose uptake, which is associated with augmented AMPK pathway activation in rodents.NEW & NOTEWORTHY In this study, we revealed that 36 h of fasting additively upregulated acute muscle contraction-induced AMPK pathway activation in rats. Besides, fasting and muscle contraction synergistically enhanced insulin-stimulated site-specific TBC1D4 phosphorylation and glucose uptake, which was associated with augmented AMPK pathway activation. These results contribute to understanding the regulation of muscle insulin sensitivity.

Fighting Diabetes Mellitus: Pharmacological and Non-pharmacological Approaches

BACKGROUND

The increasing worldwide prevalence of diabetes mellitus confers heavy public health issues and points to a large medical need for effective and novel anti-diabetic approaches with negligible adverse effects. Developing effective and novel anti-diabetic approaches to curb diabetes is one of the most foremost scientific challenges.

OBJECTIVES

This article aims to provide an overview of current pharmacological and non-pharmacological approaches available for the management of diabetes mellitus.

METHODS

Research articles that focused on pharmacological and non-pharmacological interventions for diabetes were collected from various search engines such as Science Direct and Scopus, using keywords like diabetes, glucagon-like peptide-1, glucose homeostasis, etc. Results: We review in detail several key pathways and pharmacological targets (e.g., the G protein-coupled receptors- cyclic adenosine monophosphate, 5'-adenosine monophosphate-activated protein kinase, sodium-glucose cotransporters 2, and peroxisome proliferator activated-receptor gamma signaling pathways) that are vital in the regulation of glucose homeostasis. The currently approved diabetes medications, the pharmacological potentials of naturally occurring compounds as promising interventions for diabetes, and the non-pharmacological methods designed to mitigate diabetes are summarized and discussed.

CONCLUSION

Pharmacological-based approaches such as insulin, metformin, sodium-glucose cotransporters 2 inhibitor, sulfonylureas, glucagon-like peptide-1 receptor agonists, and dipeptidyl peptidase IV inhibitors represent the most important strategies in diabetes management. These approved diabetes medications work via targeting the central signaling pathways related to the etiology of diabetes. Non-pharmacological approaches, including dietary modification, increased physical activity, and microbiota-based therapy are the other cornerstones for diabetes treatment. Pharmacological-based approaches may be incorporated when lifestyle modification alone is insufficient to achieve positive outcomes.

Countering impaired glucose homeostasis during catch-up growth with essential polyunsaturated fatty acids: is there a major role for improved insulin sensitivity?

Background/Objectives

Catch-up growth, an important risk factor for later obesity and type 2 diabetes, is often characterized by a high rate of fat deposition associated with hyperinsulinemia and glucose intolerance. We tested here the hypothesis that refeeding on a high-fat diet rich in essential polyunsaturated fatty acids (ePUFA) improves glucose homeostasis primarily by enhancing insulin sensitivity in skeletal muscles and adipose tissues.

Methods

Rats were caloric restricted for 2 weeks followed by 1–2 weeks of isocaloric refeeding on either a low-fat (LF) diet, a high-fat (HF) diet based on animal fat and high in saturated and monounsaturated fatty acids (HF SMFA diet), or a HF diet based on vegetable oils (1:1 mixture of safflower and linseed oils) and rich in the essential fatty acids linoleic and α-linolenic acids (HF ePUFA diet). In addition to measuring body composition and a test of glucose tolerance, insulin sensitivity was assessed during hyperinsulinemic-euglycemic clamps at the whole-body level and in individual skeletal muscles and adipose tissue depots.

Results

Compared to animals refed the LF diet, those refed the HF-SMFA diet showed a higher rate of fat deposition, higher plasma insulin and glucose responses during the test of glucose tolerance, and markedly lower insulin-stimulated glucose utilization at the whole body level (by a-third to a-half) and in adipose tissue depots (by 2–5 folds) during insulin clamps. While refeeding on the ePUFA diet prevented the increases in fat mass and in plasma insulin and glucose, the results of insulin clamps revealed that insulin-stimulated glucose utilization was not increased in skeletal muscles and only marginally higher in adipose tissues and at the whole-body level.

Conclusions

These results suggest only a minor role for enhanced insulin sensitivity in the mechanisms by which diets high in ePUFA improves glucose homeostasis during catch-up growth.

Enhanced skeletal muscle insulin sensitivity after acute resistance-type exercise is upregulated by rapamycin-sensitive mTOR complex 1 inhibition

Acute aerobic exercise (AE) increases skeletal muscle insulin sensitivity for several hours, caused by acute activation of AMP-activated protein kinase (AMPK). Acute resistance exercise (RE) also activates AMPK, possibly improving insulin-stimulated glucose uptake. However, RE-induced rapamycin-sensitive mechanistic target of rapamycin complex 1 (mTORC1) activation is higher and has a longer duration than after AE. In molecular studies, mTORC1 was shown to be upstream of insulin receptor substrate 1 (IRS-1) Ser phosphorylation residue, inducing insulin resistance. Therefore, we hypothesised that although RE increases insulin sensitivity through AMPK activation, prolonged mTORC1 activation after RE reduces RE-induced insulin sensitising effect. In this study, we used an electrical stimulation-induced RE model in rats, with rapamycin as an inhibitor of mTORC1 activation. Our results showed that RE increased insulin-stimulated glucose uptake following AMPK signal activation. However, mTORC1 activation and IRS-1 Ser632/635 and Ser612 phosphorylation were elevated 6 h after RE, with concomitant impairment of insulin-stimulated Akt signal activation. By contrast, rapamycin inhibited these prior exercise responses. Furthermore, increases in insulin-stimulated skeletal muscle glucose uptake 6 h after RE were higher in rats with rapamycin treatment than with placebo treatment. Our data suggest that mTORC1/IRS-1 signaling inhibition enhances skeletal muscle insulin-sensitising effect of RE.

The Emerging Roles of Nicotinamide Adenine Dinucleotide Phosphate Oxidase 2 in Skeletal Muscle Redox Signaling and Metabolism

Significance: Skeletal muscle is a crucial tissue to whole-body locomotion and metabolic health. Reactive oxygen species (ROS) have emerged as intracellular messengers participating in both physiological and pathological adaptations in skeletal muscle. A complex interplay between ROS-producing enzymes and antioxidant networks exists in different subcellular compartments of mature skeletal muscle. Recent evidence suggests that nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOXs) are a major source of contraction- and insulin-stimulated oxidants production, but they may paradoxically also contribute to muscle insulin resistance and atrophy. Recent Advances: Pharmacological and molecular biological tools, including redox-sensitive probes and transgenic mouse models, have generated novel insights into compartmentalized redox signaling and suggested that NOX2 contributes to redox control of skeletal muscle metabolism. Critical Issues: Major outstanding questions in skeletal muscle include where NOX2 activation occurs under different conditions in health and disease, how NOX2 activation is regulated, how superoxide/hydrogen peroxide generated by NOX2 reaches the cytosol, what the signaling mediators are downstream of NOX2, and the role of NOX2 for different physiological and pathophysiological processes. Future Directions: Future research should utilize and expand the current redox-signaling toolbox to clarify the NOX2-dependent mechanisms in skeletal muscle and determine whether the proposed functions of NOX2 in cells and animal models are conserved into humans.

Role of AMP-Activated Protein Kinase for Regulating Post-exercise Insulin Sensitivity

Skeletal muscle insulin resistance precedes development of type 2 diabetes (T2D). As skeletal muscle is a major sink for glucose disposal, understanding the molecular mechanisms involved in maintaining insulin sensitivity of this tissue could potentially benefit millions of people that are diagnosed with insulin resistance. Regular physical activity in both healthy and insulin-resistant individuals is recognized as the single most effective intervention to increase whole-body insulin sensitivity and thereby positively affect glucose homeostasis. A single bout of exercise has long been known to increase glucose disposal in skeletal muscle in response to physiological insulin concentrations. While this effect is identified to be restricted to the previously exercised muscle, the molecular basis for an apparent convergence between exercise- and insulin-induced signaling pathways is incompletely known. In recent years, we and others have identified the Rab GTPase-activating protein, TBC1 domain family member 4 (TBC1D4) as a target of key protein kinases in the insulin- and exercise-activated signaling pathways. Our working hypothesis is that the AMP-activated protein kinase (AMPK) is important for the ability of exercise to insulin sensitize skeletal muscle through TBC1D4. Here, we aim to provide an overview of the current available evidence linking AMPK to post-exercise insulin sensitivity.

Effects of a brief high-fat diet and acute exercise on the mTORC1 and IKK/NF-κB pathways in rat skeletal muscle

One exercise session can improve subsequent insulin-stimulated glucose uptake by skeletal muscle in healthy and insulin-resistant individuals. Our first aim was to determine whether a brief (2 weeks) high-fat diet (HFD) that caused muscle insulin resistance would activate the mammalian target of rapamycin complex 1 (mTORC1) and/or inhibitor of κB kinase/nuclear factor κB (IKK/NF-κB) pathways, which are potentially linked to induction of insulin resistance. Our second aim was to determine whether acute exercise that improved insulin-stimulated glucose uptake by muscles would attenuate activation of these pathways. We compared HFD-fed rats with rats fed a low-fat diet (LFD). Some animals from each diet group were sedentary and others were studied 3 h postexercise, when insulin-stimulated glucose uptake was increased. The results did not provide evidence that brief HFD activated either the mTORC1 (including phosphorylation of mTOR(Ser2448), TSC2(Ser939), p70S6K(Thr412), and RPS6(Ser235/236)) or the IKK/NF-κB (including abundance of IκBα or phosphorylation of NF-κB(Ser536), IKKα/β(Ser177/181), and IκB(Ser32)) pathway in insulin-resistant muscles. Exercise did not oppose the activation of either pathway, as evidenced by no attenuation of phosphorylation of key proteins in the IKK/NF-κB pathway (NF-κB(Ser536), IKKα/β(Ser177/181), and IκB(Ser32)), unaltered IκBα abundance, and no attenuation of phosphorylation of key proteins in the mTORC1 pathway (mTOR(Ser2448), TSC2(Ser939), and RPS6(Ser235/236)). Instead, exercise induced greater phosphorylation of 2 proteins of the mTORC1 pathway (PRAS40(Thr246) and p70S6K(Thr412)) in insulin-stimulated muscles, regardless of diet. Insulin resistance induced by a brief HFD was not attributable to greater activation of the mTORC1 or the IKK/NF-κB pathway in muscle, and exercise-induced improvement in insulin sensitivity was not attributable to attenuated activation of these pathways in muscle.

Effects of gender and prior swim exercise on glucose uptake in isolated skeletal muscles from mice

The purpose of this study was to characterize the effects of prior swim exercise on glucose uptake in isolated skeletal muscles of mice. Male and female mice (C57BL/6) performing 180 min of swimming had significantly decreased glycogen concentration compared to resting controls in soleus, extensor digitorum longus (EDL), and epitrochlearis muscles, regardless of gender. Glucose uptake by isolated muscles was measured using [(3)H]-2-deoxyglucose without insulin or with 180 pmol/l insulin (20, 75, or 200 min post-exercise and sedentary) or 12,000 pmol/l (20 or 200 min post-exercise and sedentary) in the soleus and EDL and without insulin or with 12,000 pmol/l insulin (20 or 200 min post-exercise and sedentary) in the epitrochlearis. Glucose uptake was higher (P < or = 0.01) for female versus male mice at each insulin concentration in the soleus and EDL, but not the epitrochlearis. Although prolonged (180 min) swim exercise did not alter subsequent glucose uptake, a shorter duration exercise protocol (60 min) tested in male mice (20 min post-exercise) led to a 1.5-fold increase in insulin-independent glucose uptake in EDL muscles. However, insulin-stimulated (180 pmol/l) glucose uptake was not altered by 60 min exercise in EDL or soleus. In light of these results, swim exercise is not recommended to evaluate the exercise-induced improvement in insulin-stimulated glucose uptake of muscles of C57BL/6 mice.

Actin is required for endocytosis at the apical surface of Madin-Darby canine kidney cells where ARF6 and clathrin regulate the actin cytoskeleton

In epithelial cell lines, apical but not basolateral clathrin-mediated endocytosis has been shown to be affected by actin-disrupting drugs. Using electron and fluorescence microscopy, as well as biochemical assays, we show that the amount of actin dedicated to endocytosis is limiting at the apical surface of epithelia. In part, this contributes to the low basal rate of clathrin-dependent endocytosis observed at this epithelial surface. ARF6 in its GTP-bound state triggers the recruitment of actin from the cell cortex to the clathrin-coated pit to enable dynamin-dependent endocytosis. In addition, we show that perturbation of the apical endocytic system by expression of a clathrin heavy-chain mutant results in the collapse of microvilli. This phenotype was completely reversed by the expression of an ARF6-GTP-locked mutant. These observations indicate that concomitant to actin recruitment, the apical clathrin endocytic system is deeply involved in the morphology of the apical plasma membrane.

Gp135/podocalyxin and NHERF-2 participate in the formation of a preapical domain during polarization of MDCK cells

Epithelial polarization involves the segregation of apical and basolateral membrane domains, which are stabilized and maintained by tight junctions and membrane traffic. We report that unlike most apical and basolateral proteins in MDCK cells, which separate only after junctions have formed, the apical marker gp135 signifies an early level of polarized membrane organization established already in single cells. We identified gp135 as the dog orthologue of podocalyxin. With a series of domain mutants we show that the COOH-terminal PSD-95/Dlg/ZO-1 (PDZ)-binding motif is targeting podocalyxin to the free surface of single cells as well as to a subdomain of the terminally polarized apical membrane. This special localization of podocalyxin is shared by the cytoplasmic PDZ-protein Na+/H+ exchanger regulatory factor (NHERF)-2. Depleting podocalyxin by RNA interference caused defects in epithelial polarization. Together, our data suggest that podocalyxin and NHERF-2 function in epithelial polarization by contributing to an early apical scaffold based on PDZ domain-mediated interactions.

Par-1 promotes a hepatic mode of apical protein trafficking in MDCK cells

Simple (i.e., nonstratified) epithelial cells use two different routes to target their newly synthesized luminal plasma membrane proteins to the cell surface: a direct route from the Golgi complex, as in the kidney-derived MDCK cell line, or an indirect route that involves a intermediate stop at the ab-luminal (basolateral) membrane, as in hepatocytes. The mechanisms or proteins responsible for these different protein targeting strategies are not known. Here, we show that increased expression of EMK1, a mammalian ortholog of Caenorhabditis elegans Par-1, in MDCK cells promotes a switch from a direct to a transcytotic mode of apical protein delivery and other trafficking changes typical of hepatocytes. These results, together with our recent demonstration that PAR-1 promotes morphological features of hepatocytes in MDCK cells, indicate that Par-1 modulates the developmental decision to build a columnar versus a hepatic epithelial cell. To our knowledge, Par-1 is the first gene assigned to this task in epithelial morphogenesis.

RalA but not RalB enhances polarized delivery of membrane proteins to the basolateral surface of epithelial cells

RalA and RalB constitute a family of highly similar (85% identity) Ras-related GTPases. Recently, active forms of both RalA and RalB have been shown to bind to the exocyst complex, implicating them in the regulation of cellular secretion. However, we show here that only active RalA enhances the rate of delivery of E-cadherin and other proteins to their site in the basolateral membrane of MDCK cells, consistent with RalA being a regulator of exocyst function. One reason for this difference is that RalA binds more effectively to the exocyst complex than active RalB does both in vivo and in vitro. Another reason is that active RalA localizes to perinuclear recycling endosomes, where regulation of vesicle sorting is thought to take place, while active RalB does not. Strikingly, analysis of chimeras made between RalA and RalB reveals that high-affinity exocyst binding by RalA is due to unique amino acid sequences in RalA that are distal to the common effector-binding domains shared by RalA and RalB. Moreover, these chimeras show that the perinuclear localization of active RalA is due in part to its unique variable domain near the C terminus. This distinct localization appears to be important for RalA effects on secretion because all RalA mutants tested that failed to localize to the perinuclear region also failed to promote basolateral delivery of E-cadherin. Interestingly, one of these inactive mutants maintained binding to the exocyst complex, suggesting that RalA binding to the exocyst is necessary but not sufficient for RalA to promote basolateral delivery of membrane proteins.

Apical surface formation in MDCK cells: regulation by the serine/threonine kinase EMK1

It has recently become evident that basic mechanisms for the establishment of cell polarity are conserved between epithelial and nonepithelial systems. The vast catalogue of known gene products involved in various aspects of invertebrate and yeast cell polarity provides a repertoire of candidate proteins that can be tested for their roles in the organization of mammalian epithelia. Here, we describe cell biological approaches to study the development and maintenance of cell polarity in Mardin-Darby canine kidney (MDCK) cells, an established mammalian model cell line for simple epithelia. The assays allowed us to characterize the Caenorhabditis elegans PAR-1 homologue EMK1 as a novel regulator of apical surface formation in epithelial cells.

A comparison of methods for analyzing glucose and insulin areas under the curve following nine months of exercise in overweight adults

OBJECTIVE

We examined three methods for calculating the area under the curve (AUC) following an oral glucose tolerance test (OGTT) in overweight adults prior to and after 9 months of exercise.

METHOD

Subjects (n=27) were randomly assigned to a control (CON, n=9) or intervention (INT, n=18) group. INT performed supervised exercise 5 days per week, 45 min per session, at 65% of heart rate reserve. OGTTs were administered pre- and post-training. Blood was collected during a 75 g OGTT and analyzed for glucose (GLU) and insulin (INS) concentrations. AUCs were calculated using the incremental, positive incremental, and total AUC methods and the difference scores for pre- and post-training were determined.

RESULTS

No differences were observed among the methods for glucose AUC for either group. Significant differences were observed for INT insulin AUC with total AUC (1525+/-3291 microU/1/180 min) significantly greater than incremental AUC (1112+/-3229 microU/1/180 min) or positive incremental AUC (1085+/-3195 microU/I/180 min). Total insulin AUC was significantly reduced following training for INT, while incremental and positive incremental insulin AUCs showed no change.

CONCLUSION

These data suggest that the method of used to calculate AUC may affect the interpretation of whether or not an intervention was effective.

Mechanisms of vanadium action: insulin-mimetic or insulin-enhancing agent?

The demonstration that the trace element vanadium has insulin-like properties in isolated cells and tissues and in vivo has generated considerable enthusiasm for its potential therapeutic value in human diabetes. However, the mechanisms by which vanadium induces its metabolic effects in vivo remain poorly understood, and whether vanadium directly mimics or rather enhances insulin effects is considered in this review. It is clear that vanadium treatment results in the correction of several diabetes-related abnormalities in carbohydrate and lipid metabolism, and in gene expression. However, many of these in vivo insulin-like effects can be ascribed to the reversal of defects that are secondary to hyperglycemia. The observations that the glucose-lowering effect of vanadium depends on the presence of endogenous insulin whereas metabolic homeostasis in control animals appears not to be affected, suggest that vanadium does not act completely independently in vivo, but augments tissue sensitivity to low levels of plasma insulin. Another crucial consideration is one of dose-dependency in that insulin-like effects of vanadium in isolated cells are often demonstrated at high concentrations that are not normally achieved by chronic treatment in vivo and may induce toxic side effects. In addition, vanadium appears to be selective for specific actions of insulin in some tissues while failing to influence others. As the intracellular active forms of vanadium are not precisely defined, the site(s) of action of vanadium in metabolic and signal transduction pathways is still unknown. In this review, we therefore examine the evidence for and against the concept that vanadium is truly an insulin-mimetic agent at low concentrations in vivo. In considering the effects of vanadium on carbohydrate and lipid metabolism, we conclude that vanadium acts not globally, but selectively and by enhancing, rather than by mimicking the effects of insulin in vivo.

Redox regulation of skeletal muscle glucose transport

Although the control of carbohydrate metabolism may be regulated by numerous factors, the redox state of the cell is of primary importance. The redox state may be influenced by a number of different factors, including different reactive oxygen species (ROS) and reactive nitrogen species (RNS) collectively, called reactive oxygen/nitrogen species (RONS). This review attempts to summarize the importance of redox regulation in relation to glucose transport and regulation of carbohydrate metabolism in skeletal muscle. In addition, prior studies implicating the role of different RONS in the control of glucose transport in skeletal muscle will be presented. Finally, the possible involvement of the cGMP, p21ras, and mean arterial pressure (MAP) kinase signal transduction cascades, which have been implicated with redox-sensitive alterations in glucose transport, will also be discussed.

Effect of acidification on the location of H+-ATPase in cultured inner medullary collecting duct cells

In previous studies, our laboratory has utilized a cell line derived from the rat inner medullary collecting duct (IMCD) as a model system for mammalian renal epithelial cell acid secretion. We have provided evidence, from a physiological perspective, that acute cellular acidification stimulates apical exocytosis and elicits a rapid increase in proton secretion that is mediated by an H+-ATPase. The purpose of these experiments was to examine the effect of acute cellular acidification on the distribution of the vacuolar H+-ATPase in IMCD cells in vitro. We utilized the 31-kDa subunit of the H+-ATPase as a marker of the complete enzyme. The distribution of this subunit of the H+-ATPase was evaluated by immunohistochemical techniques (confocal and electron microscopy), and we found that there is a redistribution of these pumps from vesicles to the apical membrane. Immunoblot evaluation of isolated apical membrane revealed a 237 +/- 34% (P < 0.05, n = 9) increase in the 31-kDa subunit present in the membrane fraction 20 min after the induction of cellular acidification. Thus our results demonstrate the presence of this pump subunit in the IMCD cell line in vitro and that cell acidification regulates the shuttling of cytosolic vesicles containing the 31-kDa subunit into the apical membrane.

An overview of muscle glucose uptake during exercise. Sites of regulation

The uptake of blood glucose by skeletal muscle is a complex process. In order to be metabolized, glucose must travel the path from blood to interstitium to intracellular space and then be phosphorylated to glucose 6-phosphate (G6P). Movement of glucose from blood to interstitium is determined by skeletal muscle blood flow, capillary recruitment and the endothelial permeability to glucose. The influx of glucose from the interstitium to intracellular space is determined by the number of glucose transporters in the sarcolemma and the glucose gradient across the sarcolemma. The capacity to phosphorylate glucose is determined by the amount of skeletal muscle hexokinase II, hexokinase II compartmentalization within the cell, and the concentration of the hexokinase II inhibitor G6P. Any change in glucose uptake occurs due to an alteration in one or more of these steps. Based on the low calculated intracellular glucose levels and the higher affinity of glucose for phosphorylation relative to transport, glucose transport is generally considered rate-determining for basal muscle glucose uptake. Exercise increases both the movement of glucose from blood to sarcolemma and the permeability of the sarcolemma to glucose. Whether the ability to phosphorylate glucose is increased in the working muscle remains to be clearly shown. It is possible that the accelerated glucose delivery and transport rates during exercise bias regulation so that muscle glucose phosphorylation exerts more control on muscle glucose uptake. Conditions that alter glucose uptake during exercise, such as increased NEFA concentrations, decreased oxygen availability and adrenergic stimulation, must work by altering one or more of the three steps involved in glucose uptake. This review describes the regulation of glucose uptake during exercise at each of these sites under a number of conditions, as well as describing muscle glucose uptake in the post-exercise state.

Restoration of hypoxia-stimulated glucose uptake in GLUT4-deficient muscles by muscle-specific GLUT4 transgenic complementation

To investigate whether GLUT4 is required for exercise/hypoxia-induced glucose uptake, we assessed glucose uptake under hypoxia and normoxia in extensor digitorum longus (EDL) and soleus muscles from GLUT4-deficient mice. In EDL and soleus from wild type control mice, hypoxia increased 2-deoxyglucose uptake 2-3-fold. Conversely, hypoxia did not alter 2-deoxyglucose uptake in either EDL or soleus from either male or female GLUT4-null mice. Next we introduced the fast-twitch skeletal muscle-specific MLC-GLUT4 transgene into GLUT4-null mice to determine whether changes in the metabolic milieu accounted for the lack of hypoxia-mediated glucose transport. Transgenic complementation of GLUT4 in EDL was sufficient to restore hypoxia-mediated glucose uptake. Soleus muscles from MLC-GLUT4-null mice were transgene-negative, and hypoxia-stimulated 2-deoxyglucose uptake was not restored. Although ablation of GLUT4 in EDL did not affect normoxic glycogen levels, restoration of GLUT4 to EDL led to an increase in glycogen under hypoxic conditions. Male GLUT4-null soleus displayed reduced normoxic glycogen stores, but female null soleus contained significantly more glycogen under normoxia and hypoxia. Reduced normoxic levels of ATP and phosphocreatine were measured in male GLUT4-null soleus but not in EDL. However, transgenic complementation of GLUT4 prevented the decrease in hypoxic ATP and phosphocreatine levels noted in male GLUT4-null and control EDL. In conclusion, we have demonstrated that GLUT4 plays an essential role in the regulation of muscle glucose uptake in response to hypoxia. Because hypoxia is a useful model for exercise, our results suggest that stimulation of glucose transport in response to exercise in skeletal muscle is totally dependent upon GLUT4. Furthermore, the compensatory glucose transport system that exists in GLUT4-null soleus muscle is not sensitive to hypoxia/muscle contraction.

Exercise, glucose transport, and insulin sensitivity

Physical exercise can be an important adjunct in the treatment of both non-insulin-dependent diabetes mellitus and insulin-dependent diabetes mellitus. Over the past several years, considerable progress has been made in understanding the molecular basis for these clinically important effects of physical exercise. Similarly to insulin, a single bout of exercise increases the rate of glucose uptake into the contracting skeletal muscles, a process that is regulated by the translocation of GLUT4 glucose transporters to the plasma membrane and transverse tubules. Exercise and insulin utilize different signaling pathways, both of which lead to the activation of glucose transport, which perhaps explains why humans with insulin resistance can increase muscle glucose transport in response to an acute bout of exercise. Exercise training in humans results in numerous beneficial adaptations in skeletal muscles, including an increase in GLUT4 expression. The increase in muscle GLUT4 in trained individuals contributes to an increase in the responsiveness of muscle glucose uptake to insulin, although not all studies show that exercise training in patients with diabetes improves overall glucose control. However, there is now extensive epidemiological evidence demonstrating that long-term regular physical exercise can significantly reduce the risk of developing non-insulin-dependent diabetes mellitus.

Treadmill running increases phosphatidylinostol 3-kinase activity in rat skeletal muscle

Exercise has been shown to increase insulin-stimulated skeletal muscle glucose transport. Activation of phosphatidylinostol 3-kinase (PI 3-kinase) is required for insulin to stimulate glucose transport. The present study was designed to investigate whether treadmill running (60 min, 8% grade, 30 m/min) augments insulin-stimulated activation of PI 3-kinase. Insulin dramatically increased insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation (p < 0.05). Treadmill running did not induce IRS-1 tyrosine phosphorylation, not did it alter insulin-stimulated IRS-1 tyrosine phosphorylation. Insulin increased PI 3-kinase activity by 3.0-fold (over basal activity) in white muscle and 5.2-fold in red muscle (p < 0.05). Exercise did not alter basal PI 3-kinase activity in either white or red muscle. However, in response to exercise, insulin-stimulated PI 3-kinase activity was significantly increased in both muscle fibers (p < 0.05). These results suggest that increased insulin responsiveness induced by exercise may be due, in part, to enhanced insulin-stimulated PI 3-kinase activity.

C-peptide stimulates glucose transport in isolated human skeletal muscle independent of insulin receptor and tyrosine kinase activation

We have previously demonstrated that C-peptide stimulates glucose transport in skeletal muscle from non-diabetic subjects in a dose-dependent manner. To further elucidate the mechanism by which C-peptide activates glucose transport, we investigated the influence of human recombinant C-peptide on receptor and post-receptor events involved in the glucose transport process. Human skeletal muscle specimens were obtained from the vastus lateralis by means of an open biopsy procedure. Stimulation of isolated muscle strips from healthy control subjects with supra-physiological concentrations of insulin (6,000 pmol/l) and C-peptide (2,500 pmol/l), did not further augment the twofold increase in the rate of 3-o-methylglucose transport induced by either stimulus alone. C-peptide did not displace 125I-insulin binding from partially purified receptors, nor did it activate receptor tyrosine kinase activity. Tyrosine-labelled 125I-C-peptide did not bind specifically to crude membranes prepared from skeletal muscle, or to any serum protein other than albumin. The beta-adrenergic receptor stimulation with isoproterenol inhibited insulin- but not C-peptide-mediated 3-o-methylglucose transport by 63 +/- 18% (p < 0.01), whereas the cyclic AMP analogue, Bt2cAMP, abolished the insulin- and C-peptide-stimulated 3-o-methylglucose transport. C-peptide (600 pmol/l) increased 3-o-methylglucose transport 1.8 +/- 0.2-fold in skeletal muscle specimens from patients with insulin-dependent diabetes mellitus. In conclusion, C-peptide stimulates glucose transport by a mechanism independent of insulin receptor and tyrosine kinase activation. In contrast to the effect on insulin-stimulated glucose transport, catecholamines do not appear to have a counter regulatory action on C-peptide-mediated glucose transport.

Identification and characterization of an exercise-sensitive pool of glucose transporters in skeletal muscle

Augmentation of glucose transport into skeletal muscle by GLUT4 translocation to the plasma and T-tubule membranes can be mediated independently by insulin and by contraction/exercise. Available data suggest that separable pools of intracellular GLUT4 respond to these two stimuli. To identify and characterize these pools, we fractionated skeletal muscle membranes in a discontinuous sucrose density gradient. Fractions of 32 and 36% sucrose exhibited the highest enrichment of GLUT4 and were independently responsive to insulin and exercise, respectively. The combination of the two stimuli depleted both GLUT4 fractions simultaneously. Both vesicle populations contained the gp160 aminopeptidase, whose expression had previously been shown to be specific to muscle and fat and restricted to GLUT4 vesicles in the latter tissue. In muscle, gp160 translocates exactly as does GLUT4 in response to insulin and exercise. The contraction- and insulin-sensitive GLUT4 pools also contained secretory component-associated membrane protein/glucose transporter vesicle triplet but not GLUT1 and caveolin. Immunoadsorption of the two pools followed by silver staining did not reveal any obvious difference in their major protein components. On the other hand, sedimentational analysis in sucrose velocity gradients revealed that the insulin-sensitive GLUT4 vesicles had a larger sedimentation coefficient than the exercise-sensitive vesicles. Thus, the separation of the two intracellular GLUT4 pools should be useful in dissecting what are likely to be different signal transduction pathways that mediate their translocation to the cell surface.

Direct targeting of neutral endopeptidase (EC 3.4.24.11) to the apical cell surface of transfected LLC-PK1 cells and unpolarized secretion of its soluble form

LLC-PK1 cells were transfected with a cDNA encoding rabbit neutral endopeptidase (NEP; EC 3.4.24.11), an abundant enzyme of the kidney proximal brush border. Clones of cells expressing high levels of the protein were isolated. Selective biotinylation and radioimmunolabelling were used to determine that 85-95% of NEP was localized in the apical domain of filter-grown LLC-PK1 cells. Pulse-chase and selective biotinylation studies revealed that the majority (85%) of newly made NEP was directly targeted to the apical membrane. However, a soluble form of NEP was found to be secreted in approximately equal amounts from both sides of the monolayer when expressed in LLC-PK1 cells. Transfected pro-opiomelanocortin, a pituitary hormone precursor, was secreted almost exclusively into the basolateral medium, suggesting that the bulk flow is to the basolateral membrane. This behaviour contrasts with that observed in MDCK cells, where both the transmembrane and secreted forms of NEP are directly targeted to the apical membrane and where the secretion of pro-opiomelanocortin is unpolarized.

Integrin expression and localization in normal MDCK cells and transformed MDCK cells lacking apical polarity

Epithelial cells polarize in response to contacts with the extracellular matrix and with neighboring cells. Interactions of cells with the extracellular matrix are mediated mainly by the integrin family of receptors. To begin to understand the role of integrins in polarization, we have investigated the expression and localization of three integrin families in the polarized Madin-Darby canine kidney (MDCK) epithelial cell line and in transformed MDCK cells lacking apical polarity. We find that MDCK cells express several beta 1 integrins, including alpha 2 beta 1, alpha 3 beta 1, and an unidentified integrin designated alpha × beta 1. The beta 1 integrins are the major receptors for collagens I and IV and laminin in MDCK cells, since a blocking anti-beta 1 antibody almost totally abolishes adhesion to these proteins. They also express a vitronectin receptor tentatively identified as alpha v beta 3, and the epithelial-specific integrin alpha 6 beta 4. The latter is not a laminin receptor in MDCK cells because a function blocking anti-alpha 6 antibody has no effect on cell adhesion to laminin. All three integrin families are expressed exclusively on both the basal and lateral surfaces, as determined by immunofluorescence microscopy and surface biotinylation. Transformed MDCK cells express beta 1 integrins as well as alpha v beta 3 and alpha 6 beta 4, but show alterations in the beta 1 family. Expression of alpha × is lacking, and the relative amount of the beta 1 subunit is diminished, resulting in the accumulation of Endo-H-sensitive alpha 3. In addition, surface biotinylation and immunofluorescence indicate that significant amounts of both alpha 2 beta 1 and alpha 3 beta 1 appear on not only the basolateral but also the apical plasma membrane. These results indicate that integrins are the major receptors for the extracellular matrix in MDCK cells, and that they may affect epithelial cell polarization by mediating not only cell-substratum but also cell-cell contacts.

Antimicrotubule drugs inhibit the polarized insertion of an intracellular glycoprotein pool into the apical membrane of Madin-Darby canine kidney (MDCK) cells

Previous experiments on MDCK cells have demonstrated that the polarized appearance of a 135 kDa glycoprotein (gp135) on the apical plasma membrane can occur through the insertion of both newly synthesized gp135 as well as a pre-existing gp135 intracellular pool. In this study, anticytoskeletal drugs were utilized to determine the role of the cytoskeleton in the polarized delivery of gp135. Colchicine and nocodazole produced a 15–20% inhibition in the apical surface accumulation of newly synthesized gp135 and inhibited the appearance of the gp135 pool by approximately 33%, while cytochalasin D had no affect on the apical accumulation of either newly synthesized gp135 or the gp135 pool. These results indicate that microtubules, but not microfilaments, are involved in the intracellular targeting of gp135. Quantitative immunogold electron microscopy of nocodazole-treated cells demonstrated that gp135 was not mistargeted to the basolateral membrane, suggesting the possibility that some vesicles containing gp135 did not fuse with the apical membrane and remained in the cells. These experiments demonstrate that microtubules are an important component of gp135 insertion into the apical membrane. They also suggest that gp135 resides within vesicles which have an apical membrane recognition signal and cannot fuse with the basolateral membrane. The possibility that these data, and those of others, could support a hypothesis for the presence of two constitutive apical transport pathways is discussed.