Biomedical subjects

L J Goodyear

Publications and source records attributed to L J Goodyear.

At least 19 recordsLinked to original sources

PubMed2001

Altered extracellular signal-regulated kinase signaling and glycogen metabolism in skeletal muscle from p90 ribosomal S6 kinase 2 knockout mice.

The p90 ribosomal S6 kinase (RSK), a cytosolic substrate for the extracellular signal-regulated kinase (ERK), is involved in transcriptional regulation, and one isoform (RSK2) has been implicated in the activation of glycogen synthase by insulin. To determine RSK2 function in vivo, mice lacking a functional rsk2 gene were generated and studied in response to insulin and exercise, two potent stimulators of the ERK cascade in skeletal muscle. RSK2 knockout (KO) mice weigh 10% less and are 14% shorter than wild-type (WT) mice. They also have impaired learning and coordination. Hindlimb skeletal muscles were obtained from mice 10, 15, or 30 min after insulin injection or immediately after strenuous treadmill exercise for 60 min. While insulin and exercise significantly increased ERK phosphorylation in skeletal muscle from both WT and KO mice, the increases were twofold greater in the KO animals. This occurred despite 27% lower ERK2 protein expression in skeletal muscle of KO mice. KO mice had 18% less muscle glycogen in the fasted basal state, and insulin increased glycogen synthase activity more in KO than WT mice. The enhanced insulin-stimulated increases in ERK and glycogen synthase activities in KO mice were not associated with higher insulin receptor or with IRS1 tyrosine phosphorylation or with IRS1 binding to phosphatidylinositol 3-kinase. However, insulin-stimulated serine phosphorylation of Akt was significantly higher in the KO animals. c-fos mRNA was increased similarly in muscle from WT and KO mice in response to insulin (2. 5-fold) and exercise (15-fold). In conclusion, RSK2 likely plays a major role in feedback inhibition of the ERK pathway in skeletal muscle. Furthermore, RSK2 is not required for activation of muscle glycogen synthase by insulin but may indirectly modulate muscle glycogen synthase activity and/or glycogen content by other mechanisms, possibly through regulation of Akt. RSK2 knockout mice may be a good animal model for the study of Coffin-Lowry syndrome.

Animals↗

PubMed2000

Marathon running transiently increases c-Jun NH2-terminal kinase and p38 activities in human skeletal muscle.

We examined the pattern of activation and deactivation of the stress-activated protein kinase signalling molecules c-Jun NH2-terminal kinase (JNK) and p38 kinase in skeletal muscle in response to prolonged strenuous running exercise in human subjects. Male subjects (n = 14; age 32 +/- 2 years; VO2,max 60 +/- 2 ml kg-1 min-1) completed a 42.2 km marathon (mean race time 3 h 35 min). Muscle biopsies were obtained 10 days prior to the marathon, immediately following the race, and 1, 3 and 5 days after the race. The activation of JNK and p38, including both p38alpha and p38gamma, was measured with immune complex assays. The phosphorylation state of p38 (alpha and gamma) and the upstream regulators of JNK and p38, mitogen-activated protein kinase kinase 4 (MKK4) and mitogen-activated protein kinase kinase 6 (MKK6), were assessed using phosphospecific antibodies. JNK activity increased 7-fold over basal level immediately post-exercise, but decreased back to basal levels 1, 3 and 5 days after the exercise. p38gamma phosphorylation (4-fold) and activity (1.5-fold) increased immediately post-exercise and returned to basal levels at 1, 3 and 5 days following exercise. In contrast, p38alpha phosphorylation and activity did not change over the time course studied. MKK4 and MKK6 phosphorylation increased and decreased in a trend similar to that observed with JNK activity and p38gamma phosphorylation. Prolonged running exercise did not affect JNK, p38alpha, or p38gamma protein expression in the days following the race. This study demonstrates that both JNK and p38 intracellular signalling cascades are robustly, yet transiently increased following prolonged running exercise. The differential activation of the p38 isoforms with exercise in human skeletal muscle indicates that these proteins may have distinct functions in vivo.

Adult↗

PubMed2000

Exercise induces isoform-specific increase in 5'AMP-activated protein kinase activity in human skeletal muscle.

The 5'AMP-activated protein kinase (AMPK) is stimulated by contractile activity in rat skeletal muscle. AMPK has emerged as an important signaling intermediary in the regulation of cell metabolism being linked to exercise-induced changes in muscle glucose and fatty acid metabolism. In the present study, we determined the effects of exercise on isoform-specific AMPK activity (alpha1 and alpha2) in human skeletal muscle. Needle biopsies of vastus lateralis muscle were obtained from seven healthy subjects at rest, after 20 and 60 min of cycle ergometer exercise at 70% of VO(2)max, and 30 min following the 60 min exercise bout. In comparison to the resting state, AMPK alpha2 activity significantly increased at 20 and 60 min of exercise, and remained at a higher level with 30 min of recovery. AMPK alpha1 activity tended to slightly decrease with 20 min of exercise at 70%VO(2)max; however, the change was not statistically significant. AMPK alpha1 activities were at basal levels at 60 min of exercise and 30 min of recovery. On a separate day, the same subjects exercised for 20 min at 50% of VO(2)max. Exercise at this intensity did not change alpha2 activity, and similar to exercise at 70% of VO(2)max, there was no significant change in alpha1 activity. In conclusion, exercise at a higher intensity for only 20 min leads to increases in AMPK alpha2 activity but not alpha1 activity. These results suggest that the alpha2-containing AMPK complex, rather than alpha1, may be involved in the metabolic responses to exercise in human skeletal muscle.

Adenosine Triphosphate↗

PubMed2000

Targeted disruption of the glucose transporter 4 selectively in muscle causes insulin resistance and glucose intolerance.

The prevalence of type 2 diabetes mellitus is growing worldwide. By the year 2020, 250 million people will be afflicted. Most forms of type 2 diabetes are polygenic with complex inheritance patterns, and penetrance is strongly influenced by environmental factors. The specific genes involved are not yet known, but impaired glucose uptake in skeletal muscle is an early, genetically determined defect that is present in non-diabetic relatives of diabetic subjects. The rate-limiting step in muscle glucose use is the transmembrane transport of glucose mediated by glucose transporter (GLUT) 4 (ref. 4), which is expressed mainly in skeletal muscle, heart and adipose tissue. GLUT4 mediates glucose transport stimulated by insulin and contraction/exercise. The importance of GLUT4 and glucose uptake in muscle, however, was challenged by two recent observations. Whereas heterozygous GLUT4 knockout mice show moderate glucose intolerance, homozygous whole-body GLUT4 knockout (GLUT4-null) mice have only mild perturbations in glucose homeostasis and have growth retardation, depletion of fat stores, cardiac hypertrophy and failure, and a shortened life span. Moreover, muscle-specific inactivation of the insulin receptor results in minimal, if any, change in glucose tolerance. To determine the importance of glucose uptake into muscle for glucose homeostasis, we disrupted GLUT4 selectively in mouse muscles. A profound reduction in basal glucose transport and near-absence of stimulation by insulin or contraction resulted. These mice showed severe insulin resistance and glucose intolerance from an early age. Thus, GLUT4-mediated glucose transport in muscle is essential to the maintenance of normal glucose homeostasis.

Animals↗

PubMed2000

Insulin signaling and insulin sensitivity after exercise in human skeletal muscle.

Muscle glucose uptake, glycogen synthase activity, and insulin signaling were investigated in response to a physiological hyperinsulinemic (600 pmol/l)-euglycemic clamp in young healthy subjects. Four hours before the clamp, the subjects performed one-legged exercise for 1 h. In the exercised leg, insulin more rapidly activated glucose uptake (half activation time [t1/2] = 11 vs. 34 min) and glycogen synthase activity (t1/2 = 8 vs. 17 min), and the magnitude of increase was two- to fourfold higher compared with the rested leg. However, prior exercise did not result in a greater or more rapid increase in insulin-induced receptor tyrosine kinase (IRTK) activity (t1/2 = 50 min), serine phosphorylation of Akt (t1/2 = 1-2 min), or serine phosphorylation of glycogen synthase kinase-3 (GSK-3) (t1/2 = 1-2 min) or in a larger or more rapid decrease in GSK-3 activity (t1/2 = 3-8 min). Thirty minutes after cessation of insulin infusion, glucose uptake, glycogen synthase activity, and signaling events were partially reversed in both the rested and the exercised leg. We conclude the following: 1) physiological hyperinsulinemia induces sustained activation of insulin-signaling molecules in human skeletal muscle; 2) the more distal insulin-signaling components (Akt, GSK-3) are activated much more rapidly than the proximal signaling molecules (IRTK as well as insulin receptor substrate 1 and phosphatidylinositol 3-kinase [Wojtaszewski et al., Diabetes 46:1775-1781, 1997]); and 3) prior exercise increases insulin stimulation of both glucose uptake and glycogen synthase activity in the absence of an upregulation of signaling events in human skeletal muscle.

Adult↗

PubMed2000

Metabolic stress and altered glucose transport: activation of AMP-activated protein kinase as a unifying coupling mechanism.

5'AMP-activated protein kinase (AMPK) can be activated in response to cellular fuel depletion and leads to switching off ATP-consuming pathways and switching on ATP-regenerating pathways in many cell types. We have hypothesized that AMPK is a central mediator of insulin-independent glucose transport, which enables fuel-depleted muscle cells to take up glucose for ATP regeneration under conditions of metabolic stress. To test this hypothesis, rat epitrochlearis muscles were isolated and incubated in vitro under several conditions that evoke metabolic stress accompanied by intracellular fuel depletion. Rates of glucose transport in the isolated muscles were increased by all of these conditions, including contraction (5-fold above basal), hypoxia (8-fold), 2,4-dinotrophenol (11-fold), rotenone (7-fold), and hyperosmolarity (8-fold). All of these stimuli simultaneously increased both alpha1 and alpha2 isoform-specific AMPK activity. There was close correlation between alpha1 (r2 = 0.72) and alpha2 (r2 = 0.67) AMPK activities and the rate of glucose transport, irrespective of the metabolic stress used, all of which compromised muscle fuel status as judged by ATP, phosphocreatine, and glycogen content. 5-Aminoimidazole-4-carboxamide ribonucleoside, a pharmacological AMPK activator that is metabolized to an AMP-mimetic ZMP, also increased both glucose transport and AMPK activity but did not change fuel status. Insulin stimulated glucose transport by 6.5-fold above basal but did not affect AMPK activity. These results suggest that the activation of AMPK may be a common mechanism leading to insulin-independent glucose transport in skeletal muscle under conditions of metabolic stress.

2,4-Dinitrophenol↗

PubMed2000

AMP-activated protein kinase: a critical signaling intermediary for exercise-stimulated glucose transport?

Physical exercise increases the rate of glucose transport into contracting skeletal muscles by a mechanism that involves translocation of the GLUT4 glucose transporter to the cell surface. This brief report will discuss an emerging hypothesis that suggests that the AMP-activated protein kinase is a key signaling intermediary leading to exercise-stimulated GLUT4 translocation in skeletal muscle.

AMP-Activated Protein Kinases↗

PubMed1999

Insulin and exercise decrease glycogen synthase kinase-3 activity by different mechanisms in rat skeletal muscle.

Glycogen synthase activity is increased in response to insulin and exercise in skeletal muscle. Part of the mechanism by which insulin stimulates glycogen synthesis may involve phosphorylation and activation of Akt, serine phosphorylation and deactivation of glycogen synthase kinase-3 (GSK-3), leading to dephosphorylation and activation of glycogen synthase. To study Akt and GSK-3 regulation in muscle, time course experiments on the effects of insulin injection and treadmill running exercise were performed in hindlimb skeletal muscle from male rats. Both insulin and exercise increased glycogen synthase activity (%I-form) by 2-3-fold over basal. Insulin stimulation significantly increased Akt phosphorylation and activity, whereas exercise had no effect. The time course of the insulin-stimulated increase in Akt was closely matched by GSK-3alpha Ser(21) phosphorylation and a 40-60% decrease in GSK-3alpha and GSK-3beta activity. Exercise also deactivated GSK-3alpha and beta activity by 40-60%. However, in contrast to the effects of insulin, there was no change in Ser(21) phosphorylation in response to exercise. Tyrosine dephosphorylation of GSK-3, another putative mechanism for GSK-3 deactivation, did not occur with insulin or exercise. These data suggest the following: 1) GSK-3 is constitutively active and tyrosine phosphorylated under basal conditions in skeletal muscle, 2) both exercise and insulin are effective regulators of GSK-3 activity in vivo, 3) the insulin-induced deactivation of GSK-3 occurs in response to increased Akt activity and GSK-3 serine phosphorylation, and 4) there is an Akt-independent mechanism for deactivation of GSK-3 in skeletal muscle.

Animals↗

PubMed1999

Insulin receptor substrate-2 is not necessary for insulin- and exercise-stimulated glucose transport in skeletal muscle.

Insulin receptor substrate-2-deficient (IRS2(-/-)) mice develop type 2 diabetes. The purpose of this study was to determine whether there is a defect in basal, insulin-, and exercise-stimulated glucose transport in the skeletal muscle of these animals. IRS2(-/-) and wild-type (WT) mice (male, 8-10 weeks) exercised on a treadmill for 1 h or remained sedentary. 2-Deoxyglucose (2DG) uptake was measured in isolated soleus muscles incubated in vitro in the presence or absence of insulin. Resting blood glucose concentration in IRS2(-/-) mice (10.3 mM) was higher than WT animals (4.1 mM), but there was a wide range among the IRS2(-/-) mice (3-25 mM). Therefore, IRS2(-/-) mice were divided into two subgroups based on blood glucose concentrations (IRS2(-/-)L < 7.2 mM, IRS2(-/-)H > 7.2 mM). Only IRS2(-/-)H had lower basal, exercise-, and submaximally insulin-stimulated 2DG uptake, while maximal insulin-stimulated 2DG uptake was similar among the three groups. The ED(50) for insulin to stimulate 2DG uptake above basal in IRS2(-/-)H was higher than WT and IRS2(-/-)L mice, suggesting insulin resistance in the skeletal muscle from the IRS2(-/-) mice with high blood glucose concentrations. Furthermore, resting blood glucose concentrations from all groups were negatively correlated to submaximally insulin-stimulated 2DG uptake (r(2) = 0.33, p < 0.01). Muscle GLUT4 content was significantly lower in IRS2(-/-)H mice compared with WT and IRS2(-/-)L mice. These results demonstrate that the IRS2 protein in muscle is not necessary for insulin- or exercise-stimulated glucose transport, suggesting that the onset of diabetes in the IRS2(-/-) mice is not due to a defect in skeletal muscle glucose transport; hyperglycemia may cause insulin resistance in the muscle of IRS2(-/-) mice.

Animals↗

PubMed1999

Skeletal muscle contractile activity in vitro stimulates mitogen-activated protein kinase signaling.

Physical exercise is a potent stimulator of mitogen-activated protein (MAP) kinase signaling. To determine if this activation is secondary to systemic responses to exercise or due to muscle contractile activity per se, an isolated muscle preparation was developed. Contractile activity in vitro significantly increased p44(MAPK) and p42(MAPK) phosphorylation by 2.9- and 2.4-fold, respectively. Contraction-stimulated MAP kinase phosphorylation was not decreased in the presence of D-tubocurarine or calphostin C, suggesting that neither neurotransmitter release nor diacylglycerol-sensitive protein kinase C mediates the contraction-induced activation of this signaling cascade. However, PD-98059, an inhibitor of MAP kinase kinase (MEK), inhibited the contraction-induced increases in MAP kinase phosphorylation. PD-98059 did not alter contraction-induced increases in glucose uptake or glycogen synthase activity, demonstrating that MAP kinase signaling is not necessary for these important metabolic effects of contractile activity in skeletal muscle. These data suggest that contractile activity of the skeletal muscle fibers per se, and not responses to neurotransmitter release, hormones, or other systemic factors, is responsible for the stimulation of MAP kinase signaling with physical exercise.

Animals↗

PubMed1999

Surgery-induced insulin resistance in human patients: relation to glucose transport and utilization.

To investigate the underlying molecular mechanisms for surgery-induced insulin resistance in skeletal muscle, six otherwise healthy patients undergoing total hip replacement were studied before, during, and after surgery. Patients were studied under basal conditions and during physiological hyperinsulinemia (60 microU/ml). Biopsies of vastus lateralis muscle were used to measure GLUT-4 translocation, glucose transport, and glycogen synthase activities. Surgery reduced insulin-stimulated glucose disposal (P < 0.05) without altering the insulin-stimulated increase in glucose oxidation or suppression of endogenous glucose production. Preoperatively, insulin infusion increased plasma membrane GLUT-4 in all six subjects (P < 0.05), whereas insulin-stimulated GLUT-4 translocation only occurred in three patients postoperatively (not significant). Moreover, nonoxidative glucose disposal rates and basal levels of glycogen synthase activities in muscle were reduced postoperatively (P < 0.05). These findings demonstrate that peripheral insulin resistance develops immediately postoperatively and that this condition might be associated with perturbations in insulin-stimulated GLUT-4 translocation as well as nonoxidative glucose disposal, presumably at the level of glycogen synthesis.

Arthroplasty, Replacement, Hip↗

PubMed1999

Differential regulation of MAP kinase, p70(S6K), and Akt by contraction and insulin in rat skeletal muscle.

To study the effects of contractile activity on mitogen-activated protein kinase (MAP kinase), p70 S6 kinase (p70(S6K)), and Akt kinase signaling in rat skeletal muscle, hindlimb muscles were contracted by electrical stimulation of the sciatic nerve for periods of 15 s to 60 min. Contraction resulted in a rapid and transient activation of Raf-1 and MAP kinase kinase 1, a rapid and more sustained activation of MAP kinase and the 90-kDa ribosomal S6 kinase 2, and a dramatic increase in c-fos mRNA expression. Contraction also resulted in an apparent increase in the association of Raf-1 with p21Ras, although stimulation of MAP kinase signaling occurred independent of Shc, IRS1, and IRS2 tyrosine phosphorylation or the formation of Shc/Grb2 or IRS1/Grb2 complexes. Insulin was considerably less effective than contraction in stimulating the MAP kinase pathway. However, insulin, but not contraction, increased p70(S6K) and Akt activities in the muscle. These results demonstrate that contraction-induced activation of the MAP kinase pathway is independent of proximal steps in insulin and/or growth factor-mediated signaling, and that contraction and insulin have discordant effects with respect to the activation of the MAP kinase pathway vs. p70(S6K) and Akt. Of the numerous stimulators of MAP kinase in skeletal muscle, contractile activity emerges as a potent and physiologically relevant activator of MAP kinase signaling, and thus activation of this pathway is likely to be an important molecular mechanism by which skeletal muscle cells transduce mechanical and/or biochemical signals into downstream biological responses.

Animals↗

PubMed1999

Differential regulation of MAP kinase by contraction and insulin in skeletal muscle: metabolic implications.

We have investigated the activation of the extracellular signal-regulated kinases (ERK1 and ERK2) by muscle contraction and insulin in perfused rat skeletal muscle. Both stimuli activated ERK1 and ERK2 by an upstream kinase MAP/ERK kinase (MEK)-dependent mechanism, as the MEK inhibitor PD-98059 inhibited ERK phosphorylation. The presence of the phosphatidylinositol (PI) 3-kinase inhibitors LY-294002 and wortmannin totally eradicated ERK1 and ERK2 phosphorylation in response to insulin but not contraction. Insulin and muscle contraction activated muscle glucose transport, glycogen synthase, and amino acid transport independently of ERK signaling, whereas the PI 3-kinase inhibitors abolished the stimulatory effects of insulin but not those of contraction on these three cellular processes. We conclude that 1) insulin and contraction activate ERK signaling in skeletal muscle; 2) ERK signaling is not necessary for activation of glucose and amino acid transport or glycogen synthase activity by contraction and insulin in skeletal muscle; and 3) insulin-induced activation of MEK, the upstream activator of ERK, is dependent on PI 3-kinase, whereas contraction utilizes a different mechanism.

Amino Acids↗

PubMed1999

Exercise and insulin cause GLUT-4 translocation in human skeletal muscle.

Studies in rodents have established that GLUT-4 translocation is the major mechanism by which insulin and exercise increase glucose uptake in skeletal muscle. In contrast, much less is known about the translocation phenomenon in human skeletal muscle. In the current study, nine healthy volunteers were studied on two different days. On one day, biopsies of vastus lateralis muscle were taken before and after a 2-h euglycemic-hyperinsulinemic clamp (0.8 mU. kg(-1). min(-1)). On another day, subjects exercised for 60 min at 70% of maximal oxygen consumption (VO(2 max)), a biopsy was obtained, and the same clamp and biopsy procedure was performed as that during the previous experiment. Compared with insulin treatment alone, glucose infusion rates were significantly increased during the postexercise clamp for the periods 0-30 min, 30-60 min, and 60-90 min, but not during the last 30 min of the clamp. Plasma membrane GLUT-4 content was significantly increased in response to physiological hyperinsulinemia (32% above rest), exercise (35%), and the combination of exercise plus insulin (44%). Phosphorylation of Akt, a putative signaling intermediary for GLUT-4 translocation, was increased in response to insulin (640% above rest), exercise (280%), and exercise plus insulin (1,000%). These data demonstrate that two normal physiological conditions, moderate intensity exercise and physiological hyperinsulinemia approximately 56 microU/ml, cause GLUT-4 translocation and Akt phosphorylation in human skeletal muscle.

Adult↗

PubMed1999

Eccentric exercise markedly increases c-Jun NH(2)-terminal kinase activity in human skeletal muscle.

Eccentric contractions require the lengthening of skeletal muscle during force production and result in acute and prolonged muscle injury. Because a variety of stressors, including physical exercise and injury, can result in the activation of the c-Jun NH(2)-terminal kinase (JNK) intracellular signaling cascade in skeletal muscle, we investigated the effects of eccentric exercise on the activation of this stress-activated protein kinase in human skeletal muscle. Twelve healthy subjects (7 men, 5 women) completed maximal concentric or eccentric knee extensions on a KinCom isokinetic dynamometer (10 sets, 10 repetitions). Percutaneous needle biopsies were obtained from the vastus lateralis muscle 24 h before exercise (basal), immediately postexercise, and 6 h postexercise. Whereas both forms of exercise increased JNK activity immediately postexercise, eccentric contractions resulted in a much higher activation (15.4 +/- 4.5 vs. 3.5 +/- 1.4-fold increase above basal, eccentric vs. concentric). By 6 h after exercise, JNK activity decreased back to baseline values. In contrast to the greater activation of JNK with eccentric exercise, the mitogen-activated protein kinase kinase 4, the immediate upstream regulator of JNK, was similarly activated by concentric and eccentric exercise. Because the activation of JNK promotes the phosphorylation of a variety of transcription factors, including c-Jun, the results from this study suggest that JNK may be involved in the molecular and cellular adaptations that occur in response to injury-producing exercise in human skeletal muscle.

Adult↗

PubMed1999

Exercise modulates postreceptor insulin signaling and glucose transport in muscle-specific insulin receptor knockout mice.

Physical exercise promotes glucose uptake into skeletal muscle and makes the working muscles more sensitive to insulin. To understand the role of insulin receptor (IR) signaling in these responses, we studied the effects of exercise and insulin on skeletal muscle glucose metabolism and insulin signaling in mice lacking insulin receptors specifically in muscle. Muscle-specific insulin receptor knockout (MIRKO) mice had normal resting 2-deoxy-glucose (2DG) uptake in soleus muscles but had no significant response to insulin. Despite this, MIRKO mice displayed normal exercise-stimulated 2DG uptake and a normal synergistic activation of muscle 2DG uptake with the combination of exercise plus insulin. Glycogen content and glycogen synthase activity in resting muscle were normal in MIRKO mice, and exercise, but not insulin, increased glycogen synthase activity. Insulin, exercise, and the combination of exercise plus insulin did not increase IR tyrosine phosphorylation or phosphatidylinositol 3-kinase activity in MIRKO muscle. In contrast, insulin alone produced a small activation of Akt and glycogen synthase kinase-3 in MIRKO mice, and prior exercise markedly enhanced this insulin effect. In conclusion, normal expression of muscle insulin receptors is not needed for the exercise-mediated increase in glucose uptake and glycogen synthase activity in vivo. The synergistic activation of glucose transport with exercise plus insulin is retained in MIRKO mice, suggesting a phenomenon mediated by nonmuscle cells or by downstream signaling events.

Animals↗

PubMed1999

Effects of streptozocin-induced diabetes and islet cell transplantation on insulin signaling in rat skeletal muscle.

Streptozocin-induced diabetes is associated with alterations in insulin signaling in rat skeletal muscle, including increased insulin receptor substrate-1 phosphorylation and phosphotidylinositol 3-kinase activity. In the current study, we determined the effects of streptozocin-induced diabetes and treatment of diabetes by islet cell transplantation on several proximal insulin-activated signaling proteins. Three groups of male Lewis rats (untreated streptozocin-diabetic animals, islet cell-transplanted diabetic rats, and nondiabetic control rats) were studied in the basal state or 30 min after i.p. insulin injection (20 U/rat). Mixed hindlimb skeletal muscle lysates were used to determine the expression and enzymatic activities of the extracellular regulated kinase 2 (ERK2), p90 ribosomal S6 kinase (RSK2), Akt, and p70 S6 kinase (p70S6k). In all three groups of rats, insulin significantly increased ERK2, RSK2, Akt, and p70S6k activities. There was no effect of diabetes on insulin-stimulated ERK2 activity or ERK2 protein levels. RSK2 expression and insulin-stimulated RSK2 activity were significantly elevated in diabetic rats compared with those in the control animals. Insulin-stimulated Akt activity was also significantly greater in the diabetic animals, but there was no change in protein expression. In contrast, there was a decrease in insulin-stimulated p70S6k activity with no change in protein expression in the diabetic rats. Islet transplantation partially (RSK2) or fully (Akt, p70S6k) normalized these diabetes-induced changes in insulin signaling proteins. We conclude that streptozocin diabetes results in the dysregulation of several critical insulin-activated proteins in rat skeletal muscle, but islet cell transplantation is an effective therapy to partially correct these alterations in insulin signaling.

Animals↗

PubMed1999

Acute exercise induces GLUT4 translocation in skeletal muscle of normal human subjects and subjects with type 2 diabetes.

Total GLUT4 content in skeletal muscle from individuals with type 2 diabetes is normal; however, recent studies have demonstrated that translocation of GLUT4 to the plasma membrane is decreased in response to insulin stimulation. It is not known whether physical exercise stimulates GLUT4 translocation in skeletal muscle of individuals with type 2 diabetes. Five subjects (two men, three women) with type 2 diabetes and five normal control subjects (5 men), as determined by a standard 75-g oral glucose tolerance test, were recruited to determine whether an acute bout of cycle exercise activates the translocation of GLUT4 to the plasma membrane in skeletal muscle. Each subject had two open biopsies of vastus lateralis muscle; one at rest and one 3-6 weeks later from the opposite leg after 45-60 min of cycle exercise at 60-70% of VO2max. Skeletal muscle plasma membranes were prepared by subcellular fractionation, and GLUT4 content was determined by Western blotting. Plasma membrane GLUT4 increased in each subject in response to exercise. The mean increase in plasma membrane GLUT4 for the subjects with type 2 diabetes was 74 +/-20% above resting values, and for the normal subjects the increase was 71+/-18% above resting values. Although plasma membrane GLUT4 content was approximately 32% lower at rest and after exercise in the muscle of the subjects with type 2 diabetes, the differences were not statistically significant. We conclude that in contrast to the previously reported defect in insulin-stimulated GLUT4 translocation in skeletal muscle of individuals with type 2 diabetes, a single bout of exercise results in the translocation of GLUT4 to the plasma membrane in skeletal muscle of individuals with type 2 diabetes. These data provide the first direct evidence that GLUT4 translocation is an important cellular mechanism through which exercise enhances skeletal muscle glucose uptake in individuals with type 2 diabetes.

5'-Nucleotidase↗