Lean and obese Zucker rats exhibit different patterns of p70s6 kinase regulation in the tibialis anterior muscle in response to high-force muscle contraction

Pathways in Skeletal Muscle: Protein Signaling and Insulin Sensitivity after Exercise Training and Weight Loss Interventions in Middle-Aged and Older Adults

Aging and obesity contribute to insulin resistance with skeletal muscle being critically important for maintaining whole-body glucose homeostasis. Both exercise and weight loss are lifestyle interventions that can affect glucose metabolism. The purpose of this study was to examine the effects of a six-month trial of aerobic exercise training or weight loss on signaling pathways in skeletal muscle in the basal condition and during hyperinsulinemia during a glucose clamp in middle-aged and older adults. Overweight and obese men and women aged 50–70 years were randomly allocated and completed six months of either weight loss (WL) (n = 18) or 3x/week aerobic exercise training (AEX) (n = 17). WL resulted in 10% weight loss and AEX increased maximal oxygen consumption (VO₂max) (both p < 0.001). Insulin sensitivity (hyperinsulinemic-euglycemic 80 mU·m⁻²·min⁻¹ clamp) increased in WL and AEX (both p < 0.01). In vivo insulin stimulation increased phosphorylation/total protein ratio (P/T) of protein kinase B (Akt), glycogen synthase kinase 3 beta (GSK-β3), 70 kDa ribosomal protein S6 kinase (p70S6k), insulin receptor substrate 1 (IRS-1), and insulin receptor (IR) expression (all p < 0.05) but not P/T extracellular regulated kinase ½ (ERK1/2), c-jun N-terminal kinases (JNK), p38 mitogen-activated protein kinases (p38), or insulin-like growth factor 1 receptor (IGF-1R). There were differences between WL and AEX in the change in basal Akt P/T (p = 0.05), GSK-3β P/T ratio (p < 0.01), p70S6k (p < 0.001), ERK1/2 (p = 0.01) P/T ratio but not p38, JNK, IRS-1, and IGF-1R P/T ratios. There was a difference between WL and AEX in the insulin stimulation changes in GSK3 which increased more after WL than AEX (p < 0.05). In the total group, changes in M were associated with changes in basal total GSK-3β and basal total p70Sk as well as insulin stimulation of total p70Sk. Protein signaling in skeletal muscle provides insight as to mechanisms for improvements in insulin sensitivity in aging and obesity.

Leptin receptor defect with diabetes causes skeletal muscle atrophy in female obese Zucker rats where peculiar depots networked with mitochondrial damages

Tibialis anterior muscles of 45-week-old female obese Zucker rats with defective leptin receptor and non-insulin dependent diabetes mellitus (NIDDM) showed a significative atrophy compared to lean muscles, based on histochemical-stained section's measurements in the sequence: oxidative slow twitch (SO, type I) < oxidative fast twitch (FOG, type IIa) < fast glycolytic (FG, type IIb). Both oxidative fiber's outskirts resembled 'ragged' fibers and, in these zones, ultrastructure revealed small clusters of endoplasm-like reticulum filled with unidentified electron contrasted compounds, contiguous and continuous with adjacent mitochondria envelope. The linings appeared crenated stabbed by circular patterns resembling those found of ceramides. The same fibers contained scattered degraded mitochondria that tethered electron contrasted droplets favoring larger depots while mitoptosis were widespread in FG fibers. Based on other interdisciplinary investigations on the lipid depots of diabetes 2 muscles made us to propose these accumulated contrasted contents to be made of peculiar lipids, including acyl-ceramides, as those were only found while diabetes 2 progresses in aging obese rats. These could interfere in NIDDM with mitochondrial oxidative energetic demands and muscle functions.

Short-term high-fat diet induces muscle fiber type-selective anabolic resistance to resistance exercise

Chronic obesity and insulin resistance are considered to inhibit contraction-induced muscle hypertrophy, through impairment of mammalian target of rapamycin complex 1 (mTORC1) and muscle protein synthesis (MPS). A high-fat diet is known to rapidly induce obesity and insulin resistance within a month. However, the influence of a short-term high-fat diet on the response of mTORC1 activation and MPS to acute resistance exercise (RE) is unclear. Thus the purpose of this study was to investigate the effect of a short-term high-fat diet on the response of mTORC1 activation and MPS to acute RE. Male Sprague-Dawley rats were randomly assigned to groups and fed a normal diet, high-fat diet, or pair feed for 4 wk. After dietary habituation, acute RE was performed on the gastrocnemius muscle via percutaneous electrical stimulation. The results showed that 4 wk of a high fat-diet induced intramuscular lipid accumulation and insulin resistance, without affecting basal mTORC1 activity or MPS. The response of RE-induced mTORC1 activation and MPS was not altered by a high-fat diet. On the other hand, analysis of each fiber type demonstrated that response of MPS to an acute RE was disappeared specifically in type I and IIa fiber. These results indicate that a short-term high-fat diet causes anabolic resistance to acute RE, depending on the fiber type.NEW & NOTEWORTHY A high-fat diet is known to rapidly induce obesity, insulin resistance, and anabolic resistance to nutrition within a month. However, the influence of a short-term high-fat diet on the response of muscle protein synthesis to acute resistance exercise is unclear. We observed that a short-term high-fat diet causes obesity, insulin resistance, intramuscular lipid droplet accumulation, and anabolic resistance to resistance exercise specifically in type I and IIa fibers.

Type 2 diabetes causes skeletal muscle atrophy but does not impair resistance training-mediated myonuclear accretion and muscle mass gain in rats

New Findings

What is the central question of this study?

Type 2 diabetes mellitus (T2DM) causes skeletal muscle atrophy; does it affect resistance training (RT)‐mediated molecular adaptations and subsequent muscle hypertrophy?

What is the main finding and its importance?

Although skeletal muscle mass and regulation were not preserved under conditions of T2DM, the response of RT‐induced skeletal muscle hypertrophy was not impaired in T2DM rat skeletal muscle. These findings suggest that the capacity of RT‐mediated muscle mass gain is not diminished in the T2DM condition.

Abstract

Type 2 diabetes mellitus (T2DM) is known to cause skeletal muscle atrophy. However, it is not known whether T2DM affects resistance training (RT)‐mediated molecular adaptations and subsequent muscle hypertrophy. Therefore, we investigated the effect of T2DM on response of skeletal muscle hypertrophy to chronic RT using a rat resistance exercise mimetic model. T2DM and healthy control rats were subjected to 18 bouts (3 times per week) of chronic RT on unilateral lower legs. RT significantly increased gastrocnemius muscle mass and myonuclei in both T2DM and healthy control rats to the same extent, even though T2DM caused muscle atrophy in the resting condition. Further, T2DM significantly reduced mechanistic target of rapamycin complex 1 (mTORC1) activity (phosphorylation of p70S6K^Thr389 and 4E‐BP1^Thr37/46) to insulin stimulation and the number of myonuclei in the untrained basal condition, but RT‐mediated adaptations were not affected by T2DM. These findings suggested that although the skeletal muscle mass and regulation were not preserved under basal conditions of T2DM, the response of RT‐induced skeletal muscle hypertrophy was not impaired in T2DM rat skeletal muscle.

Lean and Obese Zucker Rat Extensor Digitorum Longus Muscle high-frequency electrical stimulation (HFES) Data: Regulation of p70S6kinase Associated Proteins

Anaerobic exercise has been advocated as a prescribed treatment for the management of diabetes: however, alterations in exercise-induced signaling remain largely unexplored in the diabetic muscle. Here, we compare the basal and the in situ contraction-induced phosphorylation of the AKT, GSK3beta, mTor, p70s6K, Pten, and Shp2 in the lean and obese (fa/fa) Zucker rat Extensor Digitorum Longus (EDL) muscle following a single bout of contractile stimuli. This article represents data associated with prior publications from our lab (Katta et al., 2009a, 2009b; Tullgren et al., 1991) [1–3] and concurrent Data in Brief articles (Ginjupalli et al., 2017a, 2017b; Rice et al., 2017a, 2017b) [4–7].

Lean and Obese Zucker Rat Extensor Digitorum Longus Muscle high-frequency electrical stimulation (HFES) Data: Regulation of MAPKs Associated Proteins

Anaerobic exercise has been advocated as a prescribed treatment for the management of diabetes: however, alterations in exercise-induced signaling remain largely unexplored in the diabetic muscle. Here, we compare the basal and the in situ contraction-induced phosphorylation of the mitogen-activated protein kinases (MAPKs) ERK 1/2, p38, and JNK in the lean and obese (fa/fa) Zucker rat extensor digitorum longus (EDL) muscle following a single bout of contractile stimuli. This article represents data associated with prior publications from our (Katta et al., 2009a, 2009b, 2008) [1-3] and concurrent Data in Brief articles (Ginjupalli et al., 2017a, 2017b; Rice et al., 2017a, 2017b) [4-7].

Diabetic Zucker rat Tibialis anterior muscle high-frequency electrical stimulation (HFES) data: Regulation of MAPKs associated proteins

Anaerobic exercise has been advocated as a prescribed treatment for the management of diabetes: however, alterations in exercise-induced signaling remain largely unexplored in the diabetic muscle. Here, we compare the basal and the in situ contraction-induced phosphorylation of the mitogen-activated protein kinases (MAPKs) ERK 1/2, p38, and JNK in the lean and obese (fa/fa) Zucker rat tibialus anterior (TA) muscle following a single bout of contractile stimuli. This article represents data associated with prior publications from our lab (Katta et al., 2009, Katta et al., 2009, Tullgren et al., 1991) [1–3] and concurrent Data in Brief articles (Ginjupalli et al., 2017, Rice et al., 2017, Rice et al., 2017, Rice et al., 2017) [4–7].

High-frequency electrical stimulation (HFES) data lean and obese Zucker rat tibialis anterior muscle: Regulation of glycogen synthase kinase 3 beta (GSK3B) associated proteins

Anaerobic exercise has been advocated as a prescribed treatment for the management of diabetes: however, alterations in exercise-induced signaling remain largely unexplored in the diabetic muscle. Here, we compare the basal and the in situ contraction-induced phosphorylation of the AMPK, GSK3beta, and Shp2 in the lean and obese (fa/fa) Zucker rat tibialis anterior (TA) muscle following a single bout of contractile stimuli. This article represents data associated with prior publications from our lab (Katta et al., 2009; Katta et al., 2009; Tullgren et al., 1991) [1–3] and concurrent Data in Brief articles (Ginjupalli et al., 2017; Rice et al., 2017; Rice et al., 2017; Rice et al., 2017) [4–7].

High-frequency electrical stimulation (HFES) Data Lean and Obese Zucker Rat Soleus Muscle: Regulation of p70S6kinase Associated Proteins

Anaerobic exercise has been advocated as a prescribed treatment for the management of diabetes: however, alterations in exercise-induced signaling remain largely unexplored in the diabetic muscle. Here, we compare the basal and the in situ contraction-induced phosphorylation of the AKT, GSK3beta, mTor, p70s6K, Pten, and Shp2 proteins in the lean and obese (fa/fa) Zucker rat soleus muscle following a single bout of contractile stimuli. This article represents data associated with prior publications from our lab (Katta et al., 2009a, 2009b; Tullgren et al., 1991) [1-3] and concurrent Data in Brief articles (Ginjupalli et al., 2017a, 2017b; Rice et al., 2017a, 2017b) [4-7].

Alzheimer's disease and type 2 diabetes-related alterations in brain mitochondria, autophagy and synaptic markers

We aimed to investigate mitochondrial function, biogenesis and autophagy in the brain of type 2 diabetes (T2D) and Alzheimer's disease (AD) mice. Isolated brain mitochondria and homogenates from cerebral cortex and hippocampus of wild-type (WT), triple transgenic AD (3xTg-AD) and T2D mice were used to evaluate mitochondrial functional parameters and protein levels of mitochondrial biogenesis, autophagy and synaptic integrity markers, respectively. A significant decrease in mitochondrial respiration, membrane potential and energy levels was observed in T2D and 3xTg-AD mice. Also, a significant decrease in the levels of autophagy-related protein 7 (ATG7) and glycosylated lysosomal membrane protein 1 (LAMP1) was observed in cerebral cortex and hippocampus of T2D and 3xTg-AD mice. Moreover, both brain regions of 3xTg-AD mice present lower levels of nuclear respiratory factor (NRF) 1 while the levels of NRF2 are lower in both brain regions of T2D and 3xTg-AD mice. A decrease in mitochondrial encoded, nicotinamide adenine dinucleotide dehydrogenase subunit 1 (ND1) was also observed in T2D and 3xTg-AD mice although only statistically significant in T2D cortex. Furthermore, a decrease in the levels of postsynaptic density protein 95 (PSD95) in the cerebral cortex of 3xTg-AD mice and in hippocampus of T2D and 3xTg-AD mice and a decrease in the levels of synaptosomal-associated protein 25 (SNAP 25) in the hippocampus of T2D and 3xTg-AD mice were observed suggesting synaptic integrity loss. These results support the idea that alterations in mitochondrial function, biogenesis and autophagy cause synaptic damage in AD and T2D.

Leucine-enriched essential amino acids attenuate muscle soreness and improve muscle protein synthesis after eccentric contractions in rats

Eccentric exercise results in prolonged muscle weakness and muscle soreness, which are typical symptoms of muscle damage. Recovery from muscle damage is related to mammalian target of rapamycin (mTOR) activity. Leucine-enriched essential amino acids (LEAAs) stimulate muscle protein synthesis via activation of the mTOR pathway. Therefore, we investigated the effect of LEAAs on muscle protein synthesis and muscle soreness after eccentric contractions (EC). Male Sprague–Dawley rats (9–11 weeks old) were administered an LEAA solution (AminoL40; containing 40 % leucine and 60 % other essential amino acids) at 1 g/kg body weight or distilled water (control) 30 min before and 10 min after EC. Tibialis anterior (TA) muscle was exposed to 500 EC by electrical stimulation under anesthesia. The fractional synthesis rate (FSR; %/h) in the TA muscle was measured by incorporating l-[ring-²H₅] phenylalanine into skeletal muscle protein. Muscle soreness was evaluated by the paw withdrawal threshold using the Randal–Selitto test with some modifications from 1 to 3 days after EC. The FSR in the EC-control group (0.147 ± 0.016 %/h) was significantly lower than in the sedentary group (0.188 ± 0.016 %/h, p < 0.05). AminoL40 administration significantly mitigated the EC-induced impairment of the FSR (0.172 ± 0.018 %/h). EC decreased the paw withdrawal threshold at 1 and 2 days after EC, which indicated that EC induced muscle soreness. Furthermore, AminoL40 administration alleviated the decreased paw withdrawal threshold. These findings suggest that LEAA supplementation improves the rate of muscle protein synthesis and ameliorates muscle soreness after eccentric exercise.

Eccentric exercise leads to performance decrease and insulin signaling impairment

PURPOSE

This study aimed to evaluate the effects of an overtraining (OT) protocol based on eccentric exercise (EE) sessions on the insulin and inflammatory signaling pathways in the skeletal muscles of Swiss mice.

METHODS

Rodents were divided into control (C; sedentary mice), trained (TR; performed the aerobic training protocol), and overtrained (OTR; performed the OT protocol). The incremental load test and exhaustive test were used to measure performances before and after exercise protocols. Twenty-four hours after the exhaustive test performed at the end of week 8, the extensor digitorum longus (EDL) and soleus muscles were removed for subsequent protein analysis by immunoblotting.

RESULTS

The phosphorylation of insulin receptor beta (pIRbeta; Tyr1146) diminished for EDL and soleus muscles in OTR compared with C. The phosphorylation of insulin receptor substrate 1 (pIRS-1; Ser307) increased for EDL and soleus muscles in OTR compared with C and TR. The phosphorylation of protein kinase B (pAkt; Ser473) diminished for EDL and soleus muscles in OTR compared with C and TR. The phosphorylation of IκB kinase alpha and beta (pIKKalpha/beta; Ser176/180), stress-activated protein kinases/Jun amino-terminal kinases (pSAPK/JNK; Thr183/Tyr185), and the protein levels of suppressor of cytokine signaling 3 (SOCS3) increased for EDL and soleus muscles in OTR compared with C and TR.

CONCLUSION

In summary, the current used OT protocol based on eccentric exercise sessions impaired the insulin signaling pathway with concomitant increases of IKK, SAPK/JNK, and SOCS3 protein levels.

Muscle p70S6K phosphorylation in response to soy and dairy rich meals in middle aged men with metabolic syndrome: a randomised crossover trial

Background

The mammalian target of rapamycin (mTOR) pathway is the primary regulator of muscle protein synthesis. Metabolic syndrome (MetS) is characterized by central obesity and insulin resistance; little is known about how MetS affects the sensitivity of the mTOR pathway to feeding.

Methods

The responsiveness of mTOR pathway targets such as p706Sk to a high protein meal containing either dairy or soy foods was investigated in healthy insulin sensitive middle-aged men and those presenting with metabolic syndrome (MetS). Twenty male subjects (10 healthy controls, 10 MetS) participated in a single-blinded randomized cross-over study. In a random sequence, subjects ingested energy-matched breakfasts composed predominately of either dairy-protein or soy-protein foods. Skeletal muscle biopsies were collected in the fasted state and at 2 and 4 h post-meal ingestion for the analysis of mTOR- and insulin-signalling kinase activation.

Results

Phosphorylated Akt and Insulin receptor substrate 1 (IRS1) increased during the postabsorptive period with no difference between groups. mTOR (Ser448) and ribosomal protein S6 phosphorylation increased 2 h following dairy meal consumption only. p70S6K (Thr389) phosphorylation was increased after feeding only in the control subjects and not in the MetS group.

Conclusions

These data demonstrate that the consumption of a dairy-protein rich but not a soy-protein rich breakfast activates the phosphorylation of mTOR and ribosomal protein S6, required for protein synthesis in human skeletal muscle. Unlike healthy controls, subjects with MetS did not increase muscle p70S6K(Thr389) phosphorylation in response to a mixed meal.

Trial registration

This trial was registered with the Australian New Zealand Clinical Trials Registry (ANZCTR) as ACTRN12610000562077.

Abnormal protein turnover and anabolic resistance to exercise in sarcopenic obesity

Obesity may impair protein synthesis rates and cause anabolic resistance to growth factors, hormones, and exercise, ultimately affecting skeletal muscle mass and function. To better understand muscle wasting and anabolic resistance with obesity, we assessed protein 24-h fractional synthesis rates (24-h FSRs) in selected hind-limb muscles of sedentary and resistance-exercised lean and obese Zucker rats. Despite atrophied hind-limb muscles (-28% vs. lean rats), 24-h FSRs of mixed proteins were significantly higher in quadriceps (+18%) and red or white gastrocnemius (+22 or +38%, respectively) of obese animals when compared to lean littermates. Basal synthesis rates of myofibrillar (+8%) and mitochondrial proteins (-1%) in quadriceps were not different between phenotypes, while manufacture of cytosolic proteins (+12%) was moderately elevated in obese cohorts. Western blot analyses revealed a robust activation of p70S6k (+178%) and a lower expression of the endogenous mTOR inhibitor DEPTOR (-28%) in obese rats, collectively suggesting that there is an obesity-induced increase in net protein turnover favoring degradation. Lastly, the protein synthetic response to exercise of mixed (-7%), myofibrillar (+6%), and cytosolic (+7%) quadriceps subfractions was blunted compared to the lean phenotype (+34, +40, and +17%, respectively), indicating a muscle- and subfraction-specific desensitization to the anabolic stimulus of exercise in obese animals.

Diminished muscle growth in the obese Zucker rat following overload is associated with hyperphosphorylation of AMPK and dsRNA-dependent protein kinase

Previous data have suggested that insulin-resistant skeletal muscle may exhibit a diminished ability to undergo hypertrophy and that this result may be mediated, at least in part, from decrements in mammalian target of rapamycin (mTOR) signaling (Katta A, Kundla S, Kakarla SK, Wu M, Fannin J, Paturi S, Liu H, Addagarla HS, Blough ER. Am J Physiol Regul Integr Comp Physiol 299: R1666-R1675, 2010). Herein, we attempt to extend these observations by determining if this attenuation in muscle growth is associated with alterations in AMP-activated protein kinase (AMPK) signaling, an upstream mediator of mTOR, and changes in the activation of dsRNA-dependent protein kinase (PKR), which functions as an inhibitor of protein synthesis and potential mediator of protein degradation. Compared with that observed in lean Zucker (LZ) rats, the phosphorylation of AMPKα at Thr172 was higher after 3 wk of overload in the insulin-resistant obese Zucker (OZ) soleus (P < 0.05). This change in AMPKα phosphorylation was accompanied by increases in the amount of phosphorylated PKR (Thr446), elevations in the PKR-dependent phosphorylation of eukaryotic initiation factor (eIF)-2α (Ser51), augmented p38 MAP kinase (Thr180/Tyr182) phosphorylation, and increases in the amount of protein ubiquitination (P < 0.05). Taken together, these results suggest that the diminished hypertrophic response we observe in the OZ rat may be mediated, at least in part, by the hyperactivation of AMPK- and PKR-related signaling.

Body weight-dependent troponin T alternative splicing is evolutionarily conserved from insects to mammals and is partially impaired in skeletal muscle of obese rats

Do animals know at a physiological level how much they weigh, and, if so, do they make homeostatic adjustments in response to changes in body weight? Skeletal muscle is a likely tissue for such plasticity, as weight-bearing muscles receive mechanical feedback regarding body weight and consume ATP in order to generate forces sufficient to counteract gravity. Using rats, we examined how variation in body weight affected alternative splicing of fast skeletal muscle troponin T (Tnnt3), a component of the thin filament that regulates the actin-myosin interaction during contraction and modulates force output. In response to normal growth and experimental body weight increases, alternative splicing of Tnnt3 in rat gastrocnemius muscle was adjusted in a quantitative fashion. The response depended on weight per se, as externally attached loads had the same effect as an equal change in actual body weight. Examining the association between Tnnt3 alternative splicing and ATP consumption rate, we found that the Tnnt3 splice form profile had a significant association with nocturnal energy expenditure, independently of effects of weight. For a subset of the Tnnt3 splice forms, obese Zucker rats failed to make the same adjustments; that is, they did not show the same relationship between body weight and the relative abundance of five Tnnt3 β splice forms (i.e. Tnnt3 β2-β5 and β8), four of which showed significant effects on nocturnal energy expenditure in Sprague-Dawley rats. Heavier obese Zucker rats displayed certain splice form relative abundances (e.g. Tnnt3 β3) characteristic of much lighter, lean animals, resulting in a mismatch between body weight and muscle molecular composition. Consequently, we suggest that body weight-inappropriate skeletal muscle Tnnt3 expression in obesity is a candidate mechanism for muscle weakness and reduced mobility. Weight-dependent quantitative variation in Tnnt3 alternative splicing appears to be an evolutionarily conserved feature of skeletal muscle and provides a quantitative molecular marker to track how an animal perceives and responds to body weight.

Chronic central leptin infusion modifies the response to acute central insulin injection by reducing the interaction of the insulin receptor with IRS2 and increasing its association with SOCS3

Leptin and insulin have overlapping intracellular signaling mechanisms and exert anorexigenic actions in the hypothalamus. We aimed to determine how chronic exposure to increased leptin affects the hypothalamic response to a rise in insulin. We analyzed the activation and interactions of components of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway in the hypothalamus of rats treated icv for 14 days with leptin followed by a central injection of insulin and killed 15 min later. Insulin increased glycemia and chronic leptin reduced this insulin induced rise in glucose. Leptin decreased the association between the insulin receptor beta chain (IRβ) and insulin receptor substrate 2 (IRS2), augmented the association between Janus kinase 2 and IRS2, increased levels of the catalytic subunit of PI3K and pAkt-Ser473 and decreased forkhead box O number 1 levels. Insulin reduced the association between suppressor of the cytokine signaling 3 and IRβ, increased IRβ-IRS2 association and pAkt-Thr308 levels, with chronic leptin exposure blunting these effects. In conclusion, chronic exposure to leptin decreases the central response to insulin by increasing suppressor of the cytokine signaling 3 association to IR, which inhibits insulin signaling at the level of interaction of its receptor with IRS2 and activates PI3K by promoting Janus kinase 2-IRS2 association. Thus, these results suggest that this mechanism could be a target for the treatment of insulin resistance.

Is protein metabolism changed with obesity?

PURPOSE OF REVIEW

Although it is well established that obesity is accompanied by various degrees of metabolic impairments especially in the regulation of carbohydrate and lipid metabolism, the influence of obesity on protein metabolism is not clearly understood. The purpose of this review is to present data describing the modification in protein metabolism that have been reported in the clinical setting of obesity.

RECENT FINDINGS

Recent findings suggest that protein metabolism at the whole-body level is less sensitive to insulin action. Impairments in skeletal muscle protein synthesis rates in the postabsorptive state and in response to anabolic factors are reported in obese human. Finally, chronic excessive energy intake and increased adiposity in rats, without the appearance of other metabolic disturbances, do not induce any changes in tissue protein synthesis rates.

SUMMARY

Body composition in obesity is characterized by elevated fat mass but also lean body mass which is considered either increased or decreased (in the case of sarcopenic obesity). Thus protein metabolism as reflected by changes in protein synthesis and breakdown might be modified in obese individuals but it is still largely debated. Only a few studies have investigated muscle protein kinetics during obesity and do not lead to the same conclusions prolonging the controversies. Indeed, obesity is associated with many metabolic disturbances which might constitute confounding factors differently affecting muscle protein metabolism.

Akt/protein kinase B in skeletal muscle physiology and pathology

The Akt/protein kinase B is critical regulator of cellular homeostasis with diminished Akt activity being associated with dysregulation of cellular metabolism and cell death while Akt over-activation has been linked to inappropriate cell growth and proliferation. Although the regulation of Akt function has been well characterized in vitro, much less is known regarding the function of Akt in vivo. Here we examine how skeletal muscle Akt expression and enzymatic activity are controlled, the role of Akt in the regulation of skeletal muscle contraction, stress response glucose utilization, and protein metabolism, and the potential participation of this important molecule in skeletal muscle atrophy, aging, and cancer.

The unfolded protein response is activated in skeletal muscle by high-fat feeding: potential role in the downregulation of protein synthesis

High-fat diets are known to decrease muscle protein synthesis, the adaptation to overload, and insulin sensitivity. Conditions that disrupt endoplasmic reticulum (ER) homeostasis lead to the activation of the unfolded protein response (UPR) that is associated with decreases in protein synthesis, chronic inflammation, and insulin resistance. The purpose of the present study was to establish whether ER stress is induced by a high-fat diet in skeletal muscle and whether ER stress can decrease mTORC1 activity and protein synthesis in muscle cells. Two independent protocols of high-fat feeding activated the UPR in mice. In the first study, mice consuming a high-fat diet containing 70% fat and <1% carbohydrates for 6 wk showed higher markers of the UPR (BiP, IRE1α, and MBTPS2) in the soleus and in the tibialis anterior muscles and ATF4 in the tibialis anterior (P < 0.05). In the second study, a 20-wk high-fat diet containing 46% fat and 36% carbohydrates also increased BiP, IRE1α, and phospho-PERK protein and the expression of ATF4, CHOP, and both the spliced and unspliced forms of XBP1 in the plantar flexors (P < 0.05). In C(2)C(12) muscle cells, tunicamycin, thapsigargin, and palmitic acid all increased UPR markers and decreased phosphorylation of S6K1 (P < 0.05). Collectively, these data show that a high-fat diet activates the UPR in mouse skeletal muscle in vivo. In addition, in vitro studies indicate that palmitic acid, and other well-known ER stress inducers, triggered the UPR in myogenic cells and led to a decrease in protein synthesis and mTORC1 activity.

Impaired overload-induced hypertrophy is associated with diminished mTOR signaling in insulin-resistant skeletal muscle of the obese Zucker rat

Recent data have suggested that insulin resistance may be associated with a diminished ability of skeletal muscle to undergo hypertrophy (Paturi S, Gutta AK, Kakarla SK, Katta A, Arnold EC, Wu M, Rice KM, Blough ER. J Appl Physiol 108: 7-13, 2010). Here we examine the effects of insulin resistance using the obese Zucker (OZ) rat with increased muscle loading on the regulation of the mammalian target of rapamycin (mTOR) and its downstream signaling intermediates 70-kDa ribosomal protein S6 kinase (p70S6k), ribosomal protein S6 (rpS6), eukaryotic elongation factor 2 (eEF2), and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1). Compared with that observed in lean Zucker (LZ) rats, the degree of soleus muscle hypertrophy as assessed by changes in muscle wet weight (LZ: 35% vs. OZ: 16%) was significantly less in the OZ rats after 3 wk of muscle overload (P < 0.05). This diminished growth in the OZ rats was accompanied by significant impairments in the ability of the soleus to undergo phosphorylation of mTOR (Ser(2448)), p70S6k (Thr(389)), rpS6 (Ser(235/236)), and protein kinase B (Akt) (Ser(473) and Thr(308)) (P < 0.05). Taken together, these data suggest that impaired overload-induced hypertrophy in insulin-resistant skeletal muscle may be related to decreases in the ability of the muscle to undergo mTOR-related signaling.

Insulin resistance syndrome blunts the mitochondrial anabolic response following resistance exercise

Metabolic risk factors associated with insulin resistance syndrome may attenuate augmentations in skeletal muscle protein anabolism following contractile activity. The purpose of this study was to investigate whether or not the anabolic response, as defined by an increase in cumulative fractional protein synthesis rates (24-h FSR) following resistance exercise (RE), is blunted in skeletal muscle of a well-established rodent model of insulin resistance syndrome. Four-month-old lean (Fa/?) and obese (fa/fa) Zucker rats engaged in four lower body RE sessions over 8 days, with the last bout occurring 16 h prior to muscle harvest. A priming dose of deuterium oxide ((2)H(2)O) and (2)H(2)O-enriched drinking water were administered 24 h prior to euthanization for assessment of cumulative FSR. Fractional synthesis rates of mixed (-5%), mitochondrial (-1%), and cytosolic (+15%), but not myofibrillar, proteins (-16%, P = 0.012) were normal or elevated in gastrocnemius muscle of unexercised obese rats. No statistical differences were found in the anabolic response of cytosolic and myofibrillar subfractions between phenotypes, but obese rats were not able to augment 24-h FSR of mitochondria to the same extent as lean rats following RE (+14% vs. +28%, respectively). We conclude that the mature obese Zucker rat exhibits a mild, myofibrillar-specific suppression in basal FSR and a blunted mitochondrial response to contractile activity in mixed gastrocnemius muscle. These findings underscore the importance of assessing synthesis rates of specific myocellular subfractions to fully elucidate perturbations in basal protein turnover rates and differential adaptations to exercise stimuli in metabolic disease.

High-frequency electrically stimulated skeletal muscle contractions increase p70s6k phosphorylation independent of known IGF-I sensitive signaling pathways

Insulin-like growth factor (IGF-I) is hypothesized to be a critical upstream regulator of mammalian target of rapamycin (mTOR)-regulated protein synthesis with muscle contraction. We utilized a mouse model that expresses a skeletal muscle specific dominant-negative IGF-I receptor to investigate the role of IGF-I signaling of protein synthesis in response to unilateral lengthening contractions (10 sets, 6 repetitions, 100 Hz) at 0 and 3 h following the stimulus. Our results indicate that one session of high frequency muscle contractions can activate mTOR signaling independent of signaling components directly downstream of the receptor.