Regulation of physiological and metabolic function of muscle by female sex steroids

The ability of female sex steroids to regulate tissue function has long been appreciated; however, their role in the regulation of striated muscle function has received considerably less attention. The purpose of this symposium review was to document recent evidence indicating the role female sex steroids have in defining the functional characteristics of striated muscle. The presentations provide substantial evidence indicating that estrogens are critical to the physiological and metabolic regulations of striated muscle; thus, when considering women's health issues, striated muscle must included as an important target tissue along with other classically thought of estrogen-sensitive tissues.

Peripheral nervous system insulin resistance in ob/ob mice

Background

A reduction in peripheral nervous system (PNS) insulin signaling is a proposed mechanism that may contribute to sensory neuron dysfunction and diabetic neuropathy. Neuronal insulin resistance is associated with several neurological disorders and recent evidence has indicated that dorsal root ganglion (DRG) neurons in primary culture display altered insulin signaling, yet in vivo results are lacking. Here, experiments were performed to test the hypothesis that the PNS of insulin-resistant mice displays altered insulin signal transduction in vivo. For these studies, nondiabetic control and type 2 diabetic ob/ob mice were challenged with an intrathecal injection of insulin or insulin-like growth factor 1 (IGF-1) and downstream signaling was evaluated in the DRG and sciatic nerve using Western blot analysis.

Results

The results indicate that insulin signaling abnormalities documented in other “insulin sensitive” tissues (i.e. muscle, fat, liver) of ob/ob mice are also present in the PNS. A robust increase in Akt activation was observed with insulin and IGF-1 stimulation in nondiabetic mice in both the sciatic nerve and DRG; however this response was blunted in both tissues from ob/ob mice. The results also suggest that upregulated JNK activation and reduced insulin receptor expression could be contributory mechanisms of PNS insulin resistance within sensory neurons.

Conclusions

These findings contribute to the growing body of evidence that alterations in insulin signaling occur in the PNS and may be a key factor in the pathogenesis of diabetic neuropathy.

Acute heat stress prior to downhill running may enhance skeletal muscle remodeling

Heat shock proteins (HSPs) are chaperones that are known to have important roles in facilitating protein synthesis, protein assembly and cellular protection. While HSPs are known to be induced by damaging exercise, little is known about how HSPs actually mediate skeletal muscle adaption to exercise. The purpose of this study was to determine the effects of a heat shock pretreatment and the ensuing increase in HSP expression on early remodeling and signaling (2 and 48 h) events of the soleus (Sol) muscle following a bout of downhill running. Male Wistar rats (10 weeks old) were randomly assigned to control, eccentric exercise (EE; downhill running) or heat shock + eccentric exercise (HS; 41°C for 20 min, 48 h prior to exercise) groups. Markers of muscle damage, muscle regeneration and intracellular signaling were assessed. The phosphorylation (p) of HSP25, Akt, p70s6k, ERK1/2 and JNK proteins was also performed. As expected, following exercise the EE group had increased creatine kinase (CK; 2 h) and mononuclear cell infiltration (48 h) compared to controls. The EE group had an increase in p-HSP25, but there was no change in HSP72 expression, total protein concentration, or neonatal MHC content. Additionally, the EE group had increased p-p70s6k, p-ERK1/2, and p-JNK (2 h) compared to controls; however no changes in p-Akt were seen. In contrast, the HS group had reduced CK (2 h) and mononuclear cell infiltration (48 h) compared to EE. Moreover, the HS group had increased HSP72 content (2 and 48 h), total protein concentration (48 h), neonatal MHC content (2 and 48 h), p-HSP25 and p-p70s6k (2 h). Lastly, the HS group had reduced p-Akt (48 h) and p-ERK1/2 (2 h). These data suggest that heat shock pretreatment and/or the ensuing HSP72 response may protect against muscle damage, and enhance increases in total protein and neonatal MHC content following exercise. These changes appear to be independent of Akt and MAPK signaling pathways.

Effects of unilateral nigrostriatal dopamine depletion on peripheral glucose tolerance and insulin signaling in middle aged rats

Clinical studies indicate an increased incidence of impaired glucose tolerance in individuals with Parkinson's disease (PD). The mechanisms that underlie this co-morbidity are currently unknown. The purpose of this study was to analyze peripheral glucose tolerance following severe unilateral nigrostriatal dopamine (DA) depletion, and to determine whether central and peripheral insulin signaling was affected in the 6-hydroxydopamine (6-OHDA) middle-aged rat model of PD. Although serum insulin levels differed significantly between the 6-OHDA and sham groups over the course of a glucose tolerance test six weeks post-lesion, no significant effect on glucose tolerance or insulin signaling in skeletal muscle was observed. In contrast, markers of striatal insulin resistance were evident in the rats. These data suggest that while 6-OHDA may affect serum insulin levels and striatal insulin signaling, the unilateral 6-OHDA lesion model does not induce glucose intolerance or peripheral insulin resistance, at least at the six-week post-lesion timepoint.

Insulin resistance impairs nigrostriatal dopamine function

Clinical studies have indicated a link between Parkinson's disease (PD) and Type 2 Diabetes. Although preclinical studies have examined the effect of high-fat feeding on dopamine function in brain reward pathways, the effect of diet on neurotransmission in the nigrostriatal pathway, which is affected in PD and parkinsonism, is less clear. We hypothesized that a high-fat diet, which models early-stage Type 2 Diabetes, would disrupt nigrostriatal dopamine function in young adult Fischer 344 rats. Rats were fed a high fat diet (60% calories from fat) or a normal chow diet for 12 weeks. High fat-fed animals were insulin resistant compared to chow-fed controls. Potassium-evoked dopamine release and dopamine clearance were measured in the striatum using in vivo electrochemistry. Dopamine release was attenuated and dopamine clearance was diminished in the high-fat diet group compared to chow-fed rats. Magnetic resonance imaging indicated increased iron deposition in the substantia nigra of the high fat group. This finding was supported by alterations in the expression of several proteins involved in iron metabolism in the substantia nigra in this group compared to chow-fed animals. The diet-induced systemic and basal ganglia-specific changes may play a role in the observed impairment of nigrostriatal dopamine function.

In vivo stimulation of oestrogen receptor α increases insulin-stimulated skeletal muscle glucose uptake

Previous studies suggest oestrogen receptor α (ERα) is involved in oestrogen-mediated regulation of glucose metabolism and is critical for maintenance of whole body insulin action. Despite this, the effect of direct ERα modulation in insulin-responsive tissues is unknown. The purpose of the current study was to determine the impact of ERα activation, using the ER subtype-selective ligand propylpyrazoletriyl (PPT), on skeletal muscle glucose uptake. Two-month-old female Sprague-Dawley rats, ovariectomized for 1 week, were given subcutaneous injections of PPT (10 mg kg⁻¹), oestradiol benzoate (EB; 20 μg kg⁻¹), the ERβ agonist diarylpropionitrile (DPN, 10 mg kg⁻¹) or vehicle every 24 h for 3 days. On the fourth day, insulin-stimulated skeletal muscle glucose uptake was measured in vitro and insulin signalling intermediates were assessed via Western blotting.Activation of ERα with PPT resulted in increased insulin-stimulated glucose uptake into the slow-twitch soleus and fast-twitch extensor digitorum longus (EDL)muscles, activation of insulin signalling intermediates (as measured by phospho-Akt (pAkt) and pAkt substrate (PAS)) and phosphorylation of AMP-activated protein kinase (AMPK). GLUT4 protein was increased only in the EDL muscle. Rats treated with EB or DPN for 3 days did not show an increase in insulin-stimulated skeletal muscle glucose uptake compared to vehicle-treated animals. These new findings reveal that direct activation of ERα positively mediates glucose uptake and insulin action in skeletal muscle. Evidence that oestrogens and ERα stimulate glucose uptake has important implications for understanding mechanisms of glucose homeostasis, particularly in postmenopausal women.

Acute heat treatment improves insulin-stimulated glucose uptake in aged skeletal muscle

Aging is associated with insulin resistance and decreased insulin-stimulated glucose uptake into skeletal muscle. Although the mechanisms underlying age-related insulin resistance are not clearly defined, impaired defense against inflammation and tissue oxidative stress are likely causes. Heat shock proteins (HSPs) have been shown to protect tissue from oxidative stress and inhibit the activation of stress kinases such as JNK, known to interfere with the insulin signaling pathway. While the induction of HSPs via chronic heat treatment has been shown to protect skeletal muscle from obesity-related insulin resistance, the ability of heat treatment to improve insulin action in aged skeletal muscle is not known. In the present study, one bout of in vivo heat treatment applied to 24-mo-old Fischer 344 rats improved insulin-stimulated glucose uptake after 24 h in slow-twitch soleus muscles. In vitro heat treatment applied to young (3-mo-old) and aged (24-mo-old) soleus muscles increased expression of HSP72 and inhibited anisomycin-induced activation of JNK. In contrast, heat treatment had no effect on p38 MAPK, a MAPK strongly activated with anisomycin. Prior inhibition of HSP72 transcription with the pharmacological inhibitor KNK437 eliminated the ability of heat treatment to blunt JNK activation. This suggests that the ability of heat treatment to inhibit JNK activation in skeletal muscle is dependent on increased HSP72 expression. In conclusion, an acute bout of heat treatment can increase insulin-stimulated glucose uptake in aged skeletal muscle, with the underlying mechanism likely to be HSP72-mediated JNK inhibition.

Altered estrogen receptor expression in skeletal muscle and adipose tissue of female rats fed a high-fat diet

Estrogen receptors (ERs) are expressed in adipose tissue and skeletal muscle, with potential implications for glucose metabolism and insulin signaling. Previous studies examining the role of ERs in glucose metabolism have primarily used knockout mouse models of ERα and ERβ, and it is unknown whether ER expression is altered in response to an obesity-inducing high-fat diet (HFD). The purpose of the current study was to determine whether modulation of glucose metabolism in response to a HFD in intact and ovariectomized (OVX) female rats is associated with alterations in ER expression. Our results demonstrate that a 6-wk HFD (60% calories from fat) in female rats induces whole body glucose intolerance with tissue-specific effects isolated to the adipose tissue, and no observed differences in insulin-stimulated glucose uptake, GLUT4, or ERα protein expression levels in skeletal muscle. In chow-fed rats, OVX resulted in decreased ERα with a trend toward decreased GLUT4 expression in adipose tissue. Sham-treated and OVX rats fed a HFD demonstrated a decrease in ERα and GLUT4 in adipose tissue. The HFD also increased activation of stress kinases (c-jun NH₂-terminal kinase and inhibitor of κB kinase β) in the sham-treated rats and decreased expression of the protective heat shock protein 72 (HSP72) in both sham-treated and OVX rats. Our findings suggest that decreased glucose metabolism and increased inflammation in adipose tissue with a HFD in female rats could stem from a significant decrease in ERα expression.

Neurodegeneration in an animal model of Parkinson's disease is exacerbated by a high-fat diet

Despite numerous clinical studies supporting a link between type 2 diabetes (T2D) and Parkinson's disease (PD), the clinical literature remains equivocal. We, therefore, sought to address the relationship between insulin resistance and nigrostriatal dopamine (DA) in a preclinical animal model. High-fat feeding in rodents is an established model of insulin resistance, characterized by increased adiposity, systemic oxidative stress, and hyperglycemia. We subjected rats to a normal chow or high-fat diet for 5 wk before infusing 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle. Our goal was to determine whether a high-fat diet and the resulting peripheral insulin resistance would exacerbate 6-OHDA-induced nigrostriatal DA depletion. Prior to 6-OHDA infusion, animals on the high-fat diet exhibited greater body weight, increased adiposity, and impaired glucose tolerance. Two weeks after 6-OHDA, locomotor activity was tested, and brain and muscle tissue was harvested. Locomotor activity did not differ between the groups nor did cholesterol levels or measures of muscle atrophy. High-fat-fed animals exhibited higher homeostatic model assessment of insulin resistance (HOMA-IR) values and attenuated insulin-stimulated glucose uptake in fast-twitch muscle, indicating decreased insulin sensitivity. Animals in the high-fat group also exhibited greater DA depletion in the substantia nigra and the striatum, which correlated with HOMA-IR and adiposity. Decreased phosphorylation of HSP27 and degradation of IκBα in the substantia nigra indicate increased tissue oxidative stress. These findings support the hypothesis that a diet high in fat and the resulting insulin resistance may lower the threshold for developing PD, at least following DA-specific toxin exposure.

Heat treatment improves glucose tolerance and prevents skeletal muscle insulin resistance in rats fed a high-fat diet

OBJECTIVE

Heat treatment and overexpression of heat shock protein 72 (HSP72) have been shown to protect against high-fat diet-induced insulin resistance, but little is known about the underlying mechanism or the target tissue of HSP action. The purpose of this study is to determine whether in vivo heat treatment can prevent skeletal muscle insulin resistance.

RESEARCH DESIGN AND METHODS

Male Wistar rats were fed a high-fat diet (60% calories from fat) for 12 weeks and received a lower-body heat treatment (41 degrees C for 20 min) once per week.

RESULTS

Our results show that heat treatment shifts the metabolic characteristics of rats on a high-fat diet toward those on a standard diet. Heat treatment improved glucose tolerance, restored insulin-stimulated glucose transport, and increased insulin signaling in soleus and extensor digitorum longus (EDL) muscles from rats fed a high-fat diet. Heat treatment resulted in decreased activation of Jun NH2-terminal kinase (JNK) and inhibitor of kappaB kinase (IKK-beta), stress kinases implicated in insulin resistance, and upregulation of HSP72 and HSP25, proteins previously shown to inhibit JNK and IKK-beta activation, respectively. Mitochondrial citrate synthase and cytochrome oxidase activity decreased slightly with the high-fat diet, but heat treatment restored these activities. Data from L6 cells suggest that one bout of heat treatment increases mitochondrial oxygen consumption and fatty acid oxidation.

CONCLUSIONS

Our results indicate that heat treatment protects skeletal muscle from high-fat diet-induced insulin resistance and provide strong evidence that HSP induction in skeletal muscle could be a potential therapeutic treatment for obesity-induced insulin resistance.

Lipoic acid increases heat shock protein expression and inhibits stress kinase activation to improve insulin signaling in skeletal muscle from high-fat-fed rats

The antioxidant alpha-lipoic acid (LA) has been shown to improve insulin action in high-fat (HF)-fed animal models, yet little is known about its underlying mechanisms of action. We hypothesize that LA acts by inducing heat shock proteins (HSPs), which then inhibit stress kinases known to interfere with insulin signaling intermediates. Male Wistar rats were fed a HF diet (60% calories from fat) for 6 wk, while controls received a chow diet (10% calories from fat). One-half of the rats in each group received daily LA injections (30 mg/kg body wt). In rats fed a HF diet, LA increased expression of HSP72 and activation of HSP25 in soleus muscle, but it had no effect on HSPs in muscle from chow-fed rats. LA treatment reduced phosphorylation of c-Jun NH(2)-terminal kinase (JNK) and inhibitor of kappaB kinase-beta (IKKbeta) activity (IkappaBalpha protein levels) in rats fed a HF diet and effectively restored insulin responsiveness, as seen by insulin-stimulated phosphorylated Akt/Akt and 2-deoxyglucose uptake in soleus muscle. LA also induced activation of p38 MAPK and AMP-activated protein kinase, proteins previously implicated in insulin-independent glucose uptake. In addition, acute LA treatment induced HSPs in vitro in L6 muscle cells and prevented the activation of JNK and IKKbeta with stimulants such as anisomycin and TNF-alpha, respectively. In conclusion, our results suggest chronic LA treatment results in stress kinase inhibition and improved insulin signaling through a HSP-mediated mechanism.

Measures of striatal insulin resistance in a 6-hydroxydopamine model of Parkinson's disease

Clinical evidence has shown a correlation between Parkinson's disease (PD) and Type 2 Diabetes (T2D), as abnormal glucose tolerance has been reported in >50% of PD patients. The development of insulin resistance and the degeneration of nigrostriatal dopamine (DA) neurons are both mediated by oxidative mechanisms, and oxidative stress is likely a mechanistic link between these pathologies. Although glucose uptake in neuronal tissues is primarily non-insulin dependent, proteins involved in insulin signaling, such as insulin receptor substrate 2 (IRS2) and glucose transporter 4 (GLUT4), are present in the basal ganglia. The purpose of this study was to determine whether nigrostriatal DA depletion affects measures of insulin resistance in the striatum. Six weeks after 6-hydroxydopamine (6-OHDA) infusion into the medial forebrain bundle, rats were classified as having either partial (20-65%) or severe (90-99%) striatal DA depletion. Increased IRS2 serine phosphorylation, a marker of insulin resistance, was observed in the DA-depleted striatum. Additionally, severe depletion resulted in decreased total IRS2, indicating possible degradation of the protein. Decreased phosphorylation of AKT and expression of the kinase glycogen synthase kinase-3 alpha (GSK3-alpha) was also measured in the striatum of severely DA-depleted animals. Finally, expression of heat shock protein 25 (Hsp25), which is protective against oxidative damage and can decrease stress kinase activity, was decreased in the striatum of lesioned rats. Together, these results support the hypothesis that nigrostriatal DA depletion impairs insulin signaling in the basal ganglia.

Age-related differences in skeletal muscle insulin signaling: the role of stress kinases and heat shock proteins

Aging is associated with an increase in insulin resistance in skeletal muscle, yet the underlying mechanism is not well established. We hypothesize that with aging, a chronic increase in stress kinase activation, coupled with a decrease in oxidative capacity, leads to insulin resistance in skeletal muscle. In aged (24 mo old) and young (3 mo old) Fischer 344 rats, 2-deoxyglucose uptake and insulin signaling [as measured by phosphorylation of insulin receptor substrate-1 (IRS-1), Akt (protein kinase B), and Akt substrate of 160 kDa (AS160)] decreased significantly with age. Activation of, c-Jun NH(2)-terminal kinase (JNK), glycogen serine kinase-3beta (GSK-3beta), and degradation of IkappaBalpha by the upstream inhibitor of kappa B kinase (IKKbeta), as measured by Western blot analysis, were increased with age in both soleus and epitrochlearis (Epi) muscles. However, much higher activation of these kinases in Epi muscles from young rats compared with soleus results in a greater effect of these kinases on insulin signaling in fast-twitch muscle with age. Heat shock protein (HSP) 72 expression and phosphorylation of HSP25 were higher in soleus compared with Epi muscles, and both parameters decreased with age. Age and fiber type differences in cytochrome oxidase activity are consistent with observed changes in HSP expression and activation. Our results demonstrate a significant difference in the ability of slow-twitch and fast-twitch muscles to respond to insulin and regulate glucose with age. A greater constitutive HSP expression and lower stress kinase activation may account for the ability of slow-twitch muscles to preserve the capacity to respond to insulin and maintain glucose homeostasis with age.

IL-6 increases muscle insulin sensitivity only at superphysiological levels

Exercise induces an increase in glucose transport in muscle. As the acute increase in glucose transport reverses, it is replaced by an increase in insulin sensitivity. Interleukin-6 (IL-6) increases with exercise and has been reported to activate AMP-activated protein kinase (AMPK). Based on this information, we hypothesized that IL-6 would result in an increase in muscle insulin sensitivity. Rat epitrochlearis and soleus muscles were incubated with 120 ng/ml IL-6. Exposure to IL-6 induced a modest acute increase in glucose transport and was followed 3.5 h later by an increase in insulin sensitivity in epitrochlearis but not soleus muscles. IL-6 also brought about an increase in AMPK phosphorylation in epitrochlearis muscles. We conclude that exposure of fast-twitch muscle to 120 ng/ml IL-6 increases insulin sensitivity by activating AMPK. However, exposure of epitrochlearis muscles to 10 ng/ml IL-6, a concentration >100-fold higher than that attained in plasma during exercise, had no effect on glucose transport or insulin sensitivity. These findings provide evidence that the increases in glucose transport and insulin sensitivity induced by IL-6 are pharmacological rather than physiological effects. We interpret our results as evidence that the increase in IL-6 during exercise does not play a role in the exercise-induced increases in muscle glucose uptake and insulin sensitivity.

Calcium induces increases in peroxisome proliferator-activated receptor gamma coactivator-1alpha and mitochondrial biogenesis by a pathway leading to p38 mitogen-activated protein kinase activation

Previous studies have shown that raising cytosolic calcium in myotubes induces increases in peroxisome proliferator-activated receptor gamma coactivator-1alpha expression and mitochondrial biogenesis. This finding suggests that the increases in cytosolic calcium in skeletal muscle during exercise may mediate the exercise-induced increase in mitochondria. The initial aim of this study was to determine whether raising calcium in skeletal muscle induces the same adaptations as in myotubes. We found that treatment of rat epitrochlearis muscles with a concentration of caffeine that raises cytosolic calcium to a concentration too low to cause contraction induces increases in peroxisome proliferator-activated receptor gamma coactivator-1alpha expression and mitochondrial biogenesis. Our second aim was to elucidate the pathway by which calcium induces these adaptations. Raising cytosolic calcium has been shown to activate calcium/calmodulin-dependent protein kinase in muscle. In the present study raising cytosolic calcium resulted in increases in phosphorylation of p38 mitogen-activated protein kinase and activating transcription factor-2, which were blocked by the calcium/calmodulin-dependent protein kinase inhibitor KN93 and by the p38 mitogen-activated protein kinase inhibitor SB202190. The increases in peroxisome proliferator-activated receptor gamma coactivator-1alpha expression and mitochondrial biogenesis were also prevented by inhibiting p38 activation. We interpret these findings as evidence that p38 mitogen-activated protein kinase is downstream of calcium/calmodulin-dependent protein kinase in a signaling pathway by which increases in cytosolic calcium lead to increases in peroxisome proliferator-activated receptor gamma coactivator-1alpha expression and mitochondrial biogenesis in muscle.

How muscle insulin sensitivity is regulated: testing of a hypothesis

Muscle contractions induce an increase in glucose transport. The acute effect of muscle contractions on glucose transport is independent of insulin and reverses rapidly after cessation of exercise. As the acute increase in glucose transport reverses, a marked increase in the sensitivity of muscle to insulin occurs. The mechanism for this phenomenon is unknown. We hypothesize that an increase in insulin sensitivity is a general phenomenon that occurs during reversal of an increase in cell surface GLUT4 induced by any stimulus, not just exercise. To test this hypothesis, epitrochlearis, rat soleus, and flexor digitorum brevis muscles were incubated for 30 min with a maximally effective insulin concentration (1.0 mU/ml). Muscles were allowed to recover for 3 h in the absence of insulin. Muscles were then exposed to 60 microU/ml insulin for 30 min followed by measurement of glucose transport. Preincubation with 1.0 mU/ml insulin resulted in an approximately 2-fold greater increase in glucose transport 3.5 h later in response to 60 microU/ml insulin than that which occurred in control muscles treated with 60 microU/ml insulin. Pretreatment of muscles with combined maximal insulin and exercise stimuli greatly amplified the increase in insulin sensitivity. The increases in glucose transport were paralleled by increases in cell surface GLUT4. We conclude that stimulation of glucose transport by any agent is followed by an increase in sensitivity of glucose transport to activation that is mediated by translocation of more GLUT4 to the cell surface.

Exercise-induced mitochondrial biogenesis begins before the increase in muscle PGC-1alpha expression

Exercise results in rapid increases in expression of the transcription coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) and in mitochondrial biogenesis in skeletal muscle. PGC-1alpha regulates and coordinates mitochondrial biogenesis, and overexpression of PGC-1alpha in muscle cells results in increases in mitochondrial content. In this context, it has been proposed that the increase in PGC-1alpha protein expression mediates the exercise-induced increase in mitochondrial biogenesis. However, we found that mitochondrial proteins with a short half-life increase as rapidly as, or more rapidly than, PGC-1alpha protein. This finding led us to hypothesize that activation, rather than increased expression, of PGC-1alpha mediates the initial phase of the exercise-induced increase in mitochondria. In this study, we found that most of the PGC-1alpha in resting skeletal muscle is in the cytosol. Exercise resulted in activation of p38 MAPK and movement of PGC-1alpha into the nucleus. In support of our hypothesis, binding of the transcription factor nuclear respiratory factor 1 (NRF-1) to the cytochrome c promoter and NRF-2 to the cytochrome oxidase subunit 4 promoter increased in response to exercise prior to an increase in PGC-1alpha protein. Furthermore, exercise-induced increases in the mRNAs of cytochrome c, delta-aminolevulinate synthase, and citrate synthase also occurred before an increase in PGC-1 protein. Thus, it appears that activation of PGC-1alpha may mediate the initial phase of the exercise-induced adaptive increase in muscle mitochondria, whereas the subsequent increase in PGC-1alpha protein sustains and enhances the increase in mitochondrial biogenesis.

Mechanisms underlying myosin heavy chain expression during development of the rat diaphragm muscle

During early postnatal development in rat diaphragm muscle (Dia(m)), significant transitions in myosin heavy chain (MHC) isoform expression occur that are associated with fiber growth and increased MHC protein. At present, there is no direct information regarding the transcriptional regulation of MHC isoform expression during postnatal Dia(m) development. We hypothesized postnatal changes in MHC isoform mRNA expression are followed by concomitant changes in MHC protein expression. The Dia(m) was removed at postnatal days 0, 14, 28, and 84 (adult). MHC mRNA expression was determined by real-time RT-PCR. MHC protein expression was determined by SDS-PAGE. There was a significant effect of postnatal age on MHC isoform mRNA and protein expression. At birth, the MHC(Neo) isoform accounted for 28% of MHC mRNA and 54% of total MHC protein. By postnatal day 14, MHC(Neo) mRNA and protein increased significantly, and both decreased significantly by day 28, consistent with transcriptional control of the expression of this developmental isoform. By postnatal day 28, there were minimal changes in mRNA expression for MHC(Slow) and MHC(2X), yet protein expression increased significantly. MHC(2A) mRNA and protein expression did not change during this time. Thus changes in MHC protein expression did not follow (or parallel) changes in MHC mRNA for the adult MHC isoforms. The present findings indicate that changes in MHC expression in the developing rat Dia(m) are not driven solely by changes in mRNA expression. Knowledge of isoform-specific MHC mRNA expression only yields predictive information on MHC protein expression for the MHC(Neo) isoform.

Are tyrosine kinases involved in mediating contraction-stimulated muscle glucose transport?

Muscle contractions and insulin stimulate glucose transport into muscle by separate pathways. The contraction-mediated increase in glucose transport is mediated by two mechanisms, one involves the activation of 5'-AMP-activated protein kinase (AMPK) and the other involves the activation of calcium/calmodulin-dependent protein kinase II (CAMKII). The steps leading from the activation of AMPK and CAMKII to the translocation of GLUT4 to the cell surface have not been identified. Studies with the use of the tyrosine kinase inhibitor genistein suggest that one or more tyrosine kinases could be involved in contraction-stimulated glucose transport. The purpose of the present study was to determine the involvement of tyrosine kinases in contraction-stimulated glucose transport in rat soleus and epitrochlearis muscles. Contraction-stimulated glucose transport was completely prevented by pretreatment with genistein (100 microM) and the related compound butein (100 microM). However, the structurally distinct tyrosine kinase inhibitors 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyridine and herbimycin did not reduce contraction-stimulated glucose transport. Furthermore, genistein and butein inhibited glucose transport even when muscles were exposed to these compounds after being stimulated to contract. Muscle contractions did not result in increases in tyrosine phosphorylation of proteins such as proline-rich tyrosine kinase and SRC. These results provide evidence that tyrosine kinases do not mediate contraction-stimulated glucose transport and that the inhibitory effects of genistein on glucose transport result from direct inhibition of the glucose transporters at the cell surface.