Skeletal Muscle Protein Abundance Research Articles

Protein Abundance of NOR‐1 is Diminished During Aging and Skeletal Muscle Disuse

INTRODUCTION Skeletal muscle mass and function are crucial for metabolic health and quality of life. Mitochondrial defects and loss of proteostasis driven by inactivity and aging predispose skeletal muscle to apoptosis and greater degradative signaling. Nuclear orphan receptor NOR-1 has been shown to be responsive to exercise and promotes an oxidative phenotype. NOR-1 is also an important regulator of glucose uptake, fatty acid oxidation, and mitochondrial biogenesis. A gap in our knowledge is that we do not know if NOR-1 is altered by aging or muscle disuse. PURPOSE Exercise is beneficial for skeletal muscle and overall health. However, exercise participation is low among the elderly, which represent a clinically relevant population prone to inactivity and bed rest. Disuse due to prolonged immobility can result in significant atrophy and metabolic impairments to the immobilized limb. Identifying genes that are dysregulated during aging or disuse is crucial for discovering therapeutic targets that might be modulated to delay or mitigate detriments to skeletal muscle mass and function. Although NOR-1 is responsive to exercise and is associated with improved metabolic and mitochondrial function, the role of NOR-1 in disuse and aging-related muscle atrophy has yet to be investigated. Therefore, the purpose of this study was to determine if aging and limb immobilization impact skeletal muscle protein content of NOR-1. We hypothesized that both disuse and aging reduce skeletal muscle protein abundance of NOR-1 in mice. METHODS Aged (33 months) and adult (6-10 months) C57/BL6 mice were used in this study. To investigate the effect of age, the tibialis anterior (TA) of aged and adult control mice were harvested and analyzed for skeletal muscle protein content of NOR-1. In a separate set of experiments, adult mice were subjected to 14 days of unilateral cast immobilization to induce disuse associated atrophy in the hindlimb plantarflexors. After immobilization, the gastrocnemius and soleus muscles of both the immobilized and contralateral limb were harvested, weighed, and analyzed for NOR-1 protein content. Skeletal muscle abundance of NOR-1 protein was measured by western blot. Paired t-test were used to determine differences between immobilized and contralateral limbs. To compare aged and adult animals, data were analyzed via unpaired t-test. RESULTS Protein abundance of NOR-1 was reduced (72%; p<0.05) in aged mice when compared to adult control mice. Additionally, hindlimb immobilization in adult mice resulted in marked (35% and 18%; p<0.05) decrease in the wet weight of soleus and gastrocnemius muscles respectively and reduced NOR-1 protein abundance (51% and 42%; p<0.05) in soleus and gastrocnemius muscles respectively, as compared to the contralateral control muscles. Taken together, these data suggest muscle protein content of NOR-1 is reduced during disuse and aging.

Biochanin A, a soy isoflavone, diminishes insulin resistance by modulating insulin-signalling pathway in high-fat diet-induced diabetic mice

Aim The objective of this study was to evaluate the probable mechanism of action of Biochanin A (BCA) on high-fat diet-induced insulin resistance mice. Methods Twenty-four male C57/BL/6J mice were divided into two and fed either control diet or a high fat-high diet (HFD). After 6 weeks, mice were grouped into: Control, Control + BCA, HFD and HFD + BCA (n = 6). Mice were made diabetic by feeding them an HFD. Results Body weight, plasma glucose, insulin, leptin levels and HOMA values were significantly lower in the BCA supplemented HFD group when related to the HFD group. Furthermore, BCA supplementation significantly increased Insulin receptor substrate 1 (IRS 1), PI3K, Akt and Glucose transporter type 4 (GLUT-4) protein abundance in skeletal muscle when equated with HFD. Conclusion This study demonstrates that BCA can improve insulin sensitivity by activating insulin signalling, suggesting that it may possess antidiabetic activities.

RabGAPs in skeletal muscle function and exercise.

The two closely related RabGAPs TBC1D1 and TBC1D4 are key signaling factors of skeletal muscle substrate utilization. In mice, deficiency in both RabGAPs leads to reduced skeletal muscle glucose transport in response to insulin and lower GLUT4 abundance. Conversely, Tbc1d1 and Tbc1d4 deficiency results in enhanced lipid use as fuel in skeletal muscle, through yet unknown mechanisms. In humans, variants in TBC1D1 and TBC1D4 are linked to obesity, insulin resistance and type 2 diabetes. While the specific function in metabolism of each of the two RabGAPs remains to be determined, TBC1D1 emerges to be controlling exercise endurance and physical capacity, whereas TBC1D4 may rather be responsible for maintaining muscle insulin sensitivity, muscle contraction, and exercise. There is growing evidence that TBC1D1 also plays an important role in skeletal muscle development, since it has been found to be associated to meat production traits in several livestock species. In addition, TBC1D1 protein abundance in skeletal muscle is regulated by both, insulin receptor and insulin-like growth factor-1 (IGF-1) receptor signaling. This review focuses on the specific roles of the two key signaling factors TBC1D1 and TBC1D4 in skeletal muscle metabolism, development and exercise physiology.

Nicotinamide riboside does not alter mitochondrial respiration, content or morphology in skeletal muscle from obese and insulin-resistant men.

This is the first long-term human clinical trial to report on effects of nicotinamide riboside (NR) on skeletal muscle mitochondrial function, content and morphology. NR supplementation decreases nicotinamide phosphoribosyltransferase (NAMPT) protein abundance in skeletal muscle. NR supplementation does not affect NAD metabolite concentrations in skeletal muscle. Respiration, distribution and quantity of muscle mitochondria are unaffected by NR. NAMPT in skeletal muscle correlates positively with oxidative phosphorylation Complex I, sirtuin 3 and succinate dehydrogenase. Preclinical evidence suggests that the nicotinamide adenine dinucleotide (NAD+ ) precursor nicotinamide riboside (NR) boosts NAD+ levels and improves diseases associated with mitochondrial dysfunction. We aimed to determine if dietary NR supplementation in middle-aged, obese, insulin-resistant men affects mitochondrial respiration, content and morphology in skeletal muscle. In a randomized, placebo-controlled clinical trial, 40 participants received 1000mg NR or placebo twice daily for 12weeks. Skeletal muscle biopsies were collected before and after the intervention. Mitochondrial respiratory capacity was determined by high-resolution respirometry on single muscle fibres. Protein abundance and mRNA expression were measured by Western blot and quantitative PCR analyses, respectively, and in a subset of the participants (placebo n=8; NR n=8) we quantified mitochondrial fractional area and mitochondrial morphology by laser scanning confocal microscopy. Protein levels of nicotinamide phosphoribosyltransferase (NAMPT), an essential NAD+ biosynthetic enzyme in skeletal muscle, decreased by 14% with NR. However, steady-state NAD+ levels as well as gene expression and protein abundance of other NAD+ biosynthetic enzymes remained unchanged. Neither respiratory capacity of skeletal muscle mitochondria nor abundance of mitochondrial associated proteins were affected by NR. Moreover, no changes in mitochondrial fractional area or network morphology were observed. Our data do not support the hypothesis that dietary NR supplementation has significant impact on skeletal muscle mitochondria in obese and insulin-resistant men. Future studies on the effects of NR on human skeletal muscle may include both sexes and potentially provide comparisons between young and older people.

Exercise twice-a-day potentiates markers of mitochondrial biogenesis in men.

Endurance exercise begun with reduced muscle glycogen stores seems to potentiate skeletal muscle protein abundance and gene expression. However, it is unknown whether this greater signaling responses is due to performing two exercise sessions in close proximity-as a first exercise session is necessary to reduce the muscle glycogen stores. In the present study, we manipulated the recovery duration between a first muscle glycogen-depleting exercise and a second exercise session, such that the second exercise session started with reduced muscle glycogen in both approaches but was performed either 2 or 15hours after the first exercise session (so-called "twice-a-day" and "once-daily" approaches, respectively). We found that exercise twice-a-day increased the nuclear abundance of transcription factor EB (TFEB) and nuclear factor of activated T cells (NFAT) and potentiated the transcription of peroxisome proliferator-activated receptor-ɣ coactivator 1-alpha (PGC-1α), peroxisome proliferator-activated receptor-alpha (PPARα), and peroxisome proliferator-activated receptor beta/delta (PPARβ/δ) genes, in comparison with the once-daily exercise. These results suggest that part of the elevated molecular signaling reported with previous "train-low" approaches might be attributed to performing two exercise sessions in close proximity. The twice-a-day approach might be an effective strategy to induce adaptations related to mitochondrial biogenesis and fat oxidation.

WNK1 regulates skeletal muscle cell hypertrophy by modulating the nuclear localization and transcriptional activity of FOXO4

With-no-lysine (K) (WNK) kinases, which are mutated in the inherited form of hypertension pseudohypoaldosteronism type II, are essential regulators of membrane ion transporters. Here, we report that WNK1 positively regulates skeletal muscle cell hypertrophy via mediating the function of the pro-longevity transcription factor forkhead box protein O4 (FOXO4) independent of the conventional WNK signaling pathway linking SPS/STE20-related proline-alanine–rich kinase (SPAK)/oxidative stress response kinase 1 (OSR1) to downstream effector ion transporters. Small interfering RNA (siRNA)-mediated silencing of WNK1, but not SPAK/OSR1 kinases, induced myotube atrophy and remarkable increases in the mRNA expression of the muscle atrophy ubiquitin ligases MAFbx and MuRF1 in C2C12 mouse skeletal muscle cells. WNK1 silencing also increased FOXO4 nuclear localization, and co-transfection of Foxo4-targeted siRNA completely reversed the myotube atrophy and upregulation of atrogene transcription induced by WNK1 silencing. We further illustrated that WNK1 protein abundance in skeletal muscle was increased by chronic voluntary wheel running exercise (hypertrophic stimulus) and markedly decreased by adenine-induced chronic kidney disease (atrophic stimulus) in mice. These findings suggest that WNK1 is involved in the physiological regulation of mammalian skeletal muscle hypertrophy and atrophy via interactions with FOXO4. The WNK1-FOXO4 axis may be a potential therapeutic target in human diseases causing sarcopenia.

Maternal bisphenol A exposure alters rat offspring hepatic and skeletal muscle insulin signaling protein abundance

The obesogenic and diabetogenic effects of the environmental toxin bisphenol A during critical windows of development are well recognized. Liver and skeletal muscle play a central role in the control of glucose production, utilization, and storage. We hypothesized that maternal bisphenol A exposure disrupts insulin signaling in rat offspring liver and skeletal muscle. We determined the protein expression of hepatic and skeletal muscle insulin signaling molecules including insulin receptor beta, its downstream target insulin receptor substrate 1 and glucose transporters (glucose transporter 2, glucose transporter 4), and hepatic glucose-regulating enzymes phosphoenolpyruvate carboxykinase and glucokinase. Rat dams had ad libitum access to filtered drinking water (control) or drinking water with bisphenol A from 2 weeks prior to mating and through pregnancy and lactation. Offspring litters were standardized to 4 males and 4 females and nursed by the same dam. At weaning, bisphenol A exposure was removed from all offspring. Glucose tolerance was tested at 6 weeks and 6 months. Liver and skeletal muscle was collected from 3 week old and 10 month old offspring for protein expression (Western blot) of insulin receptor beta, insulin receptor substrate 1, glucose transporter 2, glucose transporter 4, phosphoenolpyruvate carboxykinase, and glucokinase. Male, but not female, bisphenol A offspring had impaired glucose tolerance at 6 weeks and 6 months. Both male and female adult offspring had higher glucose-stimulated insulin secretion as well as the ratio of stimulated insulin to glucose. Male bisphenol A offspring had higher liver protein abundance of the 200 kDa insulin receptor beta precursor (2-fold), and insulin receptor substrate 1 (1.5-fold), whereas glucose transporter 2 was 0.5-fold of the control at 3 weeks of age. In adult male bisphenol A offspring, the abundance of insulin receptor beta was higher (2-fold) and glucose transporter 4 was 0.8-fold of the control in skeletal muscle. In adult female bisphenol A offspring, the skeletal muscle protein abundance of glucose transporter 4 was 0.4-fold of the control. Maternal bisphenol A had sex- and tissue-specific effects on insulin signaling components, which may contribute to increased risk of glucose intolerance in offspring. Glucose transporters were consistently altered at both ages as well as in both sexes and may contribute to glucose intolerance. These data suggest that maternal bisphenol A exposure should be limited during pregnancy and lactation.

Proteogenomic Analysis of a Hibernating Mammal Indicates Contribution of Skeletal Muscle Physiology to the Hibernation Phenotype.

Mammalian hibernation is a strategy employed by many species to survive fluctuations in resource availability and environmental conditions. Hibernating mammals endure conditions of dramatically depressed heart rate, body temperature, and oxygen consumption yet do not show the typical pathological response. Because of the high abundance and metabolic cost of skeletal muscle, not only must it adjust to the constraints of hibernation, but also it is positioned to play a more active role in the initiation and maintenance of the hibernation phenotype. In this study, MS/MS proteomic data from thirteen-lined ground squirrel skeletal muscles were searched against a custom database of transcriptomic and genomic protein predictions built using the platform Galaxy-P. This proteogenomic approach allows for a thorough investigation of skeletal muscle protein abundance throughout their circannual cycle. Of the 1563 proteins identified by these methods, 232 were differentially expressed. These data support previously reported physiological transitions, while also offering new insight into specific mechanisms of how their muscles might be reducing nitrogenous waste, preserving mass and function, and signaling to other tissues. Additionally, the combination of proteomic and transcriptomic data provides unique opportunities for estimating post-transcriptional regulation in skeletal muscle throughout the year and improving genomic annotation for this nonmodel organism.

Unchanged [3H]ouabain binding site content but reduced Na+-K+ pump α2-protein abundance in skeletal muscle in older adults

Aging is associated with reduced muscle mass, weakness, and increased fatigability. In skeletal muscle, the Na(+)-K(+) pump (NKA) is important in regulating Na(+)-K(+) gradients, membrane excitability, and thus contractility, but the effects of aging on muscle NKA are unclear. We investigated whether aging is linked with reduced muscle NKA by contrasting muscle NKA isoform gene expression and protein abundance, and NKA total content in 17 Elderly (66.8 ± 6.4 yr, mean ± SD) and 16 Young adults (23.9 ± 2.2 yr). Participants underwent peak oxygen consumption assessment and a vastus lateralis muscle biopsy, which was analyzed for NKA α(1)-, α(2)-, α(3)-, β(1)-, β(2)-, and β(3)-isoform gene expression (real-time RT-PCR), protein abundance (immunoblotting), and NKA total content ([(3)H]ouabain binding sites). The Elderly had lower peak oxygen consumption (-36.7%, P = 0.000), strength (-36.3%, P = 0.001), NKA α(2)- (-24.4%, 11.9 ± 4.4 vs. 9.0 ± 2.7 arbitrary units, P = 0.049), and NKA β(3)-protein abundance (-23.0%, P = 0.041) than Young. The β(3)-mRNA was higher in Elderly compared with Young (P = 0.011). No differences were observed between groups for other NKA isoform mRNA or protein abundance, or for [(3)H]ouabain binding site content. Thus skeletal muscle in elderly individuals was characterized by decreased NKA α(2)- and β(3)-protein abundance, but unchanged α(1) abundance and [(3)H]ouabain binding. The latter was likely caused by reduced α(2) abundance with aging, preventing an otherwise higher [(3)H]ouabain binding that might occur with a greater membrane density in smaller muscle fibers. Further study is required to verify reduced muscle NKA α(2) with aging and possible contributions to impaired exercise capability and daily living activities.

The proteomic signature of insulin-resistant human skeletal muscle reveals increased glycolytic and decreased mitochondrial enzymes

The molecular mechanisms underlying insulin resistance in skeletal muscle are incompletely understood. Here, we aimed to obtain a global picture of changes in protein abundance in skeletal muscle in obesity and type 2 diabetes, and those associated with whole-body measures of insulin action. Skeletal muscle biopsies were obtained from ten healthy lean (LE), 11 obese non-diabetic (OB), and ten obese type 2 diabetic participants before and after hyperinsulinaemic-euglycaemic clamps. Quantitative proteome analysis was performed by two-dimensional differential-gel electrophoresis and tandem-mass-spectrometry-based protein identification. Forty-four protein spots displayed significant (p < 0.05) changes in abundance by at least a factor of 1.5 between groups. Several proteins were identified in multiple spots, suggesting post-translational modifications. Multiple spots containing glycolytic and fast-muscle proteins showed increased abundance, whereas spots with mitochondrial and slow-muscle proteins were downregulated in the OB and obese type 2 diabetic groups compared with the LE group. No differences in basal levels of myosin heavy chains were observed. The abundance of multiple spots representing glycolytic and fast-muscle proteins correlated negatively with insulin action on glucose disposal, glucose oxidation and lipid oxidation, while several spots with proteins involved in oxidative metabolism and mitochondrial function correlated positively with these whole-body measures of insulin action. Our data suggest that increased glycolytic and decreased mitochondrial protein abundance together with a shift in muscle properties towards a fast-twitch pattern in the absence of marked changes in fibre-type distribution contribute to insulin resistance in obesity with and without type 2 diabetes. The roles of several differentially expressed or post-translationally modified proteins remain to be elucidated.

Bioactives from bitter melon enhance insulin signaling and modulate acyl carnitine content in skeletal muscle in high-fat diet-fed mice

Bioactive components from bitter melon (BM) have been reported to improve glucose metabolism in vivo, but definitive studies on efficacy and mechanism of action are lacking. We sought to investigate the effects of BM bioactives on body weight, muscle lipid content and insulin signaling in mice fed a high-fat diet and on insulin signaling in L6 myotubes. Male C57BL/6J mice were randomly divided into low-fat diet control (LFD), high-fat diet (HFD) and HFD plus BM (BM) groups. Body weight, body composition, plasma glucose, leptin, insulin and muscle lipid profile were determined over 12 weeks. Insulin signaling was determined in the mouse muscle taken at end of study and in L6 myotubes exposed to the extract. Body weight, plasma glucose, insulin, leptin levels and HOMA-IR values were significantly lower in the BM-fed HFD group when compared to the HFD group. BM supplementation significantly increased IRS-2, IR β, PI 3K and GLUT4 protein abundance in skeletal muscle, as well as phosphorylation of IRS-1, Akt1 and Akt2 when compared with HFD ( P<.05 and P<.01). BM also significantly reduced muscle lipid content in the HFD mice. BM extract greatly increased glucose uptake and enhanced insulin signaling in L6 myotubes. This study shows that BM bioactives reduced body weight, improved glucose metabolism and enhanced skeletal muscle insulin signaling. A contributing mechanism to the enhanced insulin signaling may be associated with the reduction in skeletal muscle lipid content. Nutritional supplementation with this extract, if validated for human studies, may offer an adjunctive therapy for diabetes.

Proteomic analysis of rat skeletal muscle submitted to one bout of incremental exercise

Exercise can alter gene transcriptional and protein translational rates leading to changes in protein abundance toward adaptation to exercise. We investigated the alterations in protein abundance in skeletal muscle after one bout of an exhaustive exercise through proteomic analysis. Gastrocnemius muscles were sampled from non-exercised control rats and from rats exercised on a treadmill with incremental increases in speed until exhaustion (approximately 30 min). Rats were sacrificed 3 and 24 h after exercise cessation. Two-dimensional gel electrophoresis was performed and spots with a significant alteration in relative volume were identified by mass spectrometry. Six spots presented statistically significant altered abundances after exercise. The spots identified as the metabolic related proteins triosephosphate isomerase 1, glyceraldehyde-3-phosphate dehydrogenase, the β subunit of pyruvate dehydrogenase E(1) and carnitine palmitoyltransferase 2 were all more abundant after exercise. One spot identified as heat shock cognate 70 was also more abundant after exercise. One spot demonstrated a decreased abundance after exercise and was identified as α-actin. These results suggest that a single session of exhaustive incremental exercise in untrained muscle can alter thin filaments synthesis/degradation rate and enhance cytosolic and mitochondrial proteins synthesis. The identified proteins may be important to a general preconditioning of skeletal muscle for subsequent exercise sessions.

The cAMP-HMGA1-RBP4 system: a novel biochemical pathway for modulating glucose homeostasis

BackgroundWe previously showed that mice lacking the high mobility group A1 gene (Hmga1-knockout mice) developed a type 2-like diabetic phenotype, in which cell-surface insulin receptors were dramatically reduced (below 10% of those in the controls) in the major targets of insulin action, and glucose intolerance was associated with increased peripheral insulin sensitivity. This particular phenotype supports the existence of compensatory mechanisms of insulin resistance that promote glucose uptake and disposal in peripheral tissues by either insulin-dependent or insulin-independent mechanisms. We explored the role of these mechanisms in the regulation of glucose homeostasis by studying the Hmga1-knockout mouse model. Also, the hypothesis that increased insulin sensitivity in Hmga1-deficient mice could be related to the deficit of an insulin resistance factor is discussed.ResultsWe first show that HMGA1 is needed for basal and cAMP-induced retinol-binding protein 4 (RBP4) gene and protein expression in living cells of both human and mouse origin. Then, by employing the Hmga1-knockout mouse model, we provide evidence for the identification of a novel biochemical pathway involving HMGA1 and the RBP4, whose activation by the cAMP-signaling pathway may play an essential role for maintaining glucose metabolism homeostasis in vivo, in certain adverse metabolic conditions in which insulin action is precluded. In comparative studies of normal and mutant mice, glucagon administration caused a considerable upregulation of HMGA1 and RBP4 expression both at the mRNA and protein level in wild-type animals. Conversely, in Hmga1-knockout mice, basal and glucagon-mediated expression of RBP4 was severely attenuated and correlated inversely with increased Glut4 mRNA and protein abundance in skeletal muscle and fat, in which the activation state of the protein kinase Akt, an important downstream mediator of the metabolic effects of insulin on Glut4 translocation and carbohydrate metabolism, was simultaneously increased.ConclusionThese results indicate that HMGA1 is an important modulator of RBP4 gene expression in vivo. Further, they provide evidence for the identification of a novel biochemical pathway involving the cAMP-HMGA1-RBP4 system, whose activation may play a role in glucose homeostasis in both rodents and humans. Elucidating these mechanisms has importance for both fundamental biology and therapeutic implications.

Characterization of the Human Skeletal Muscle Proteome by One-dimensional Gel Electrophoresis and HPLC-ESI-MS/MS

Changes in protein abundance in skeletal muscle are central to a large number of metabolic and other disorders, including, and perhaps most commonly, insulin resistance. Proteomics analysis of human muscle is an important approach for gaining insight into the biochemical basis for normal and pathophysiological conditions. However, to date, the number of proteins identified by this approach has been limited, with 107 different proteins being the maximum reported so far. Using a combination of one-dimensional gel electrophoresis and high performance liquid chromatography electrospray ionization tandem mass spectrometry, we identified 954 different proteins in human vastus lateralis muscle obtained from three healthy, nonobese subjects. In addition to a large number of isoforms of contractile proteins, we detected all proteins involved in the major pathways of glucose and lipid metabolism in skeletal muscle. Mitochondrial proteins accounted for 22% of all proteins identified, including 55 subunits of the respiratory complexes I-V. Moreover, a number of enzymes involved in endocrine and metabolic signaling pathways as well as calcium homeostasis were identified. These results provide the most comprehensive characterization of the human skeletal muscle proteome to date. These data hold promise for future global assessment of quantitative changes in the muscle proteome of patients affected by disorders involving skeletal muscle.

Serum- and Glucocorticoid-Inducible Kinase 1 Mediates Salt Sensitivity of Glucose Tolerance

Excess salt intake decreases peripheral glucose uptake, thus impairing glucose tolerance. Stimulation of cellular glucose uptake involves phosphatidylinositide-3-kinase (PI-3K)-dependent activation of protein kinase B/Akt. A further kinase downstream of PI-3K is serum- and glucocorticoid-inducible kinase (SGK)1, which is upregulated by mineralocorticoids and, thus, downregulated by salt intake. To explore the role of SGK1 in salt-dependent glucose uptake, SGK1 knockout mice (sgk1(-/-)) and their wild-type littermates (sgk1(+/+)) were allowed free access to either tap water (control) or 1% saline (high salt). According to Western blotting, high salt decreased and deoxycorticosterone acetate (DOCA; 35 mg/kg body wt) increased SGK1 protein abundance in skeletal muscle and fat tissue of sgk1(+/+) mice. Intraperitoneal injection of glucose (3 g/kg body wt) into sgk1(+/+) mice transiently increased plasma glucose concentration approaching significantly higher values ([glucose]p,max) in high salt (281 +/- 39 mg/dl) than in control (164 +/- 23 mg/dl) animals. DOCA did not significantly modify [glucose]p,max in control sgk1(+/+) mice but significantly decreased [glucose]p,max in high-salt sgk1(+/+) mice, an effect reversed by spironolactone (50 mg/kg body wt). [Glucose]p,max was in sgk1(-/-) mice insensitive to high salt and significantly higher than in control sgk1(+/+) mice. Uptake of 2-deoxy-d-[1,2-(3)H]glucose into skeletal muscle and fat tissue was significantly smaller in sgk1(-/-) mice than in sgk1(+/+) mice and decreased by high salt in sgk1(+/+) mice. Transfection of HEK-293 cells with active (S422D)SGK1, but not inactive (K127N)SGK, stimulated phloretin-sensitive glucose uptake. In conclusion, high salt decreases SGK1-dependent cellular glucose uptake. SGK1 thus participates in the link between salt intake and glucose tolerance.

Behavioral, metabolic, and molecular correlates of lower insulin sensitivity in Mexican-Americans.

We determined whether lower insulin sensitivity persists in young, nonobese, nondiabetic Mexican-American [MA; n = 13, 27.0 +/- 2.0 yr, body mass index (BMI) 23.0 +/- 0.7] compared with non-Hispanic white (NHW; n = 13, 24.8 +/- 1.5 yr, BMI 22.8 +/- 0.6) males and females after accounting for cardiorespiratory fitness (maximal O(2) uptake), abdominal fat distribution (computed tomography scans), dietary intake (4-day records), and skeletal muscle insulin-signaling protein abundance from muscle biopsies (Western blot analysis). MA were significantly less insulin sensitive compared with their NHW counterparts when estimated by homeostatic model assessment of insulin resistance (MA: 1.53 +/- 0.22 vs. NHW: 0.87 +/- 0.16, P < 0.05) and the revised quantitative insulin sensitivity check index (MA: 0.45 +/- 0.08 vs. NHW: 0.58 +/- 0.19, P = 0.05). However, skeletal muscle protein abundance of insulin receptor-beta (IRbeta), phosphatidylinositol 3-kinase p85 subunit, Akt1, Akt2, and GLUT4 were not significantly different. Differences in indexes of insulin sensitivity lost significance after percent dietary intake of palmitic acid, palmitoleic acid, and skeletal muscle protein abundance of IRbeta were accounted for. We conclude that differences in insulin sensitivity between nonobese, nondiabetic MA and NHW persist after effects of chronic and acute exercise and total and abdominal fat distribution are accounted for. These differences may be mediated, in part, by dietary fat intake.

Exercise, diet, and skeletal muscle gene expression.

Skeletal muscle, as a consequence of its mass and great capacity for altered metabolism, has a major impact on whole-body metabolic homeostasis and is capable of remarkable adaptation in response to various physiological stimuli, including exercise and dietary intervention. Exercise-induced increases in skeletal muscle mRNA levels of a number of genes have been reported, due to transcriptional activation and/or increased mRNA stability. The cellular adaptations to exercise training appear to be due to the cumulative effects of transient increases in gene transcription after repeated exercise bouts. The relative importance of transcriptional (mRNA synthesis) and translational (mRNA stability or translational efficiency) mechanisms for the training-induced increases in skeletal muscle protein abundance remains to be fully elucidated. Dietary manipulation, and the associated alterations in nutrient availability and hormone levels, can also modify skeletal muscle gene expression, although fewer studies have been reported. A major challenge is to understand how exercise and diet exert their effects on gene and protein expression in skeletal muscle. In relation to exercise, potential stimuli include stretch and muscle tension, the pattern of motor nerve activity and the resultant calcium transients, the energy charge of the cell and substrate availability, oxygen tension and circulating hormones. These are detected by various cellular signaling mechanisms, acting on a range of downstream targets and a wide range of putative transcription factors. A key goal in the years ahead is to identify how alterations at the level of gene expression are coupled to the changes in skeletal muscle phenotype. It is clear that gene expression, although representing a specific site of regulation, is only one step in a complex cascade from the initial stimulus to the final phenotypic adaptation and integrated physiological response.

Divergence between GLUT4 mRNA and protein abundance in skeletal muscle of insulin resistant rats

Hyperglycemia and skeletal muscle insulin resistance coexist in uncontrolled type 2 diabetes mellitus. Similar defects in insulin action were observed in glucose-infused, normal rats, a model of glucose toxicity. In these rats insulin-stimulated glucose uptake by skeletal muscle was decreased due to a post-receptor defect. We investigated whether the impaired glucose uptake resulted from a decrease in the abundance of the predominant muscle glucose transporter (GLUT4) mRNA and/or protein. GLUT4 protein abundance in the hyperglycemic rats was not different from the control group despite a 50% decrease in muscle glucose uptake. GLUT4 mRNA abundance was 2.5-fold greater in the hyperglycemic rats as compared to the control animals. We conclude that the coexistence of hyperglycemia and hyperinsulinemia results in (1) a defect in GLUT4 compartmentalization and/or functional activity and (2) a divergence between GLUT4 mRNA levels and translation.