Muscle Plasticity and Intramuscular signaling in the Insulin-resistant Obese Zucker Rat

The ability to increase skeletal muscle mass may have important implications for the treatment of insulin resistance (IR) and diabetes [1-3]. Recent data suggest that IR muscle may adapt differently than normal muscle; however, molecular mechanism(s) responsible for this finding are not well understood [4]. Herein, we investigate the molecular mechanisms underlying the skeletal muscle remodeling in the IR Obese Zucker (OZ) rat. The OZ rat is characterized by skeletal muscle insulin resistance, hyperglycemia, and hyperlipidemia. Compared to LZ rats, our data demonstrate that soleus muscle hypertrophy was significantly attenuated in the OZ rats after 3-weeks of muscle overload and that these findings appear to be accompanied by significant impairments in the ability of the soleus to undergo phosphorylation of mammalian target of rapamycin (mTOR), 70 kDa ribosomal protein S6 kinase (p70S6k), ribosomal protein S6 (rpS6) and protein kinase B (Akt). Recent in vitro and in vivo studies have suggested a role for AMP-activated protein kinase (AMPK) and dsRNA-dependent protein kinase (PKR) in skeletal muscle adaptation and their interactions with mTOR related signaling [5, 6]. Our data suggest that IR attenuates overload-induced skeletal muscle hypertrophy through the activation of AMPK and PKR, which appears to be associated with an inhibition of mTOR and eIF2α phosphorylation. This finding is consistent with the possible depression of protein synthesis. Other data demonstrate that IR resistance is associated with the PKR-mediated activation of p38 MAP kinase, which would be predicted to lead to increased protein degradation. Further, we demonstrated that the regulation of heat shock proteins (HSPs) and the mitogen-activated protein kinases (MAPKs) are altered during hypertrophy in OZ rat, which suggest that these molecules may play a role in explaining why IR may be associated with alterations in muscle plasticity. In addition to traditional biochemical signaling cascades, recent data have strongly suggested that muscle-specific miRNAs may participate in the regulation of load-induced skeletal muscle remodeling [7]. To this end, we demonstrate for the first time that miR-1 and miR133 expression levels are lower in IR muscle. Further, we also observed that overload decreased mir-1 expression in the LZ muscle to a greater extent to that measured in the OZ muscle. Combined, these results are the first to report evidence that overload-induced skeletal muscle remodeling in IR OZ rat is associated with multiple level decrements including changes in mTOR signaling, hyperphosphorylation of AMPK and PKR and altered regulation of muscle-specific miRNAs

Lean and Obese Zucker Rats Exhibit Different Patterns of p70S6kinase Regulation in the Tibialis Anterior Muscle in Response to High Force Muscle Contraction

Increased muscle loading results in phosphorylation of the 70 kDa ribosomal S6 kinase (p70s6k) signaling pathway and this event strongly correlates with degree of muscle adaptation following resistance exercise. Here, we compared the basal and contraction- induced phosphorylation of p70s6k, Akt and mTOR in tibialis anterior muscles of lean and obese Zucker rats. Immunoblotting demonstrated differences in level of basal p70S6k phosphorylation (Thr 389) in the normal and diabetic TA. HFES had an increase in p70S6k (Thr389) phosphorylation at 0-, 1- and 3-hr in lean TA and only at 3-hr in obese TA. mTOR (Ser 2448) Phosphorylation was elevated in lean TA immediately after HFES and remains unaltered in obese TA. HFES increased activity of both Akt (Thr 308) and Akt (Ser 473) in lean TA. These results suggest that diabetes is associated with alterations in the muscle content and ability to activate p70s6k signaling following an acute bout of exercise

Diabetes alters vascular mechanotransduction: pressure-induced regulation of mitogen activated protein kinases in the rat inferior vena cava

BACKGROUND: Diabetes mellitus is an important risk factor for increased vein graft failure after bypass surgery. However, the cellular and molecular mechanism(s) underlying vessel attrition in this population remain largely unexplored. Recent reports have suggested that the pathological remodeling of vein grafts may be mediated by mechanically-induced activation of the mitogen activated protein kinase (MAPK) signaling pathways and the MAPK-related induction of caspase-3 activity. On the basis of these findings, we hypothesized that diabetes may be associated with alterations in how veins "sense" and "respond" to altered mechanical loading. METHODS: Inferior venae cavae (IVC) from the non-diabetic lean (LNZ) and the diabetic obese (OSXZ) Zucker rats were isolated and incubated ex vivo under basal or pressurized conditions (120 mmHg). Protein expression, basal activation and the ability of increased pressure to activate MAPK pathways and apoptosis-related signaling was evaluated by immunoblot analysis. RESULTS: Immunoblot analyses revealed differential expression and activation of extracellular signal-regulated kinase (ERK1/2), p38 and c-Jun NH2-terminal kinase (JNK) MAPKs in the IVCs of diabetic rats as compared to non-diabetic rats. In particular, the expression and basal phosphorylation of p38β- (52.3 ± 11.8%; 45.8 ± 18.2%), JNK 1- (21.5 ± 9.3%; 19.4 ± 11.6%) and JNK3-MAPK (16.8 ± 3.3%; 29.5 ± 17.6%) were significantly higher (P < 0.05) in the diabetic vena cava. An acute increase in IVC intraluminal pressure failed to increase the phosphorylation of ERK1-, JNK-2, or any of the p38-MAPKs in the diabetic obese Zucker rats. Also, IVC loading in the LNZ led to a 276.0 ± 36.0% and 85.8 ± 25.1% (P < 0.05) increase in the cleavage of caspase-3 and caspase-9, respectively, with no effect on these molecules in the OSXZ. No differences were found in the regulation of Bax and Bcl-2 between groups. However, basal expression levels of Akt, phospho-Akt, PTEN, phospho-PTEN and phospho-Bad were higher in the diabetic venae cavae (P < 0.05). CONCLUSION: These data suggest that diabetes is associated with significant alteration in the ability of the vena cava to activate MAPK- and apoptosis-related signaling. Whether these changes are associated with the increased vein graft attrition seen in the diabetic population will require further investigation

Chronic Paracetamol Treatment Influences Indices of Reactive Oxygen Species Accumulation in the Aging Fischer 344 X Brown Norway Rat Aorta

Previous reports have demonstrated that increased levels of reactive oxygen species (ROS) and alterations in cell signaling characterize aging in the Fischer 344 X Brown Norway (FBN) rat aorta. Other work has suggested that increases in ROS may be related to vascular wall thickening and the development of hypertension. Paracetamol (acetaminophen) is a potent antioxidant that has been found to diminish free radicals in ischemia-reperfusion studies. However, it remains unclear whether chronic paracetamol administration influences signaling or ROS accumulation in the aging aorta. FBN rats (27 months old; n=8) were subjected to 6 months of treatment with a therapeutic dose of paracetamol (30 mg/kg/day) and compared to age-matched untreated FBN rat controls (n=8). Compared to measurements in the aortae of 6-month old animals, tunica media thickness, tissue superoxide levels, and protein oxidation levels were 38 ± 7%, 92 ± 31%, and 7 ± 2% higher in the aortae of 33-month control animals (p ≤0.05). Chronic paracetamol treatment decreased tunica media thickness and the amount of oxidized protein by 13 ± 4% and 30 ± 1%, respectively (p ≤0.05). This finding of diminished aortic thickening was associated with increased phosphorylation (activation) of the mitogen activated protein kinases and diminished levels of the anti-apoptotic protein Bcl-2. Taken together, these data suggest that chronic paracetamol treatment may decrease the deleterious effects of aging in the FBN rat aorta

Altered Regulation of Contraction-Induced Akt/mTOR/p70S6k Pathway Signaling in Skeletal Muscle of the Obese Zucker Rat

Increased muscle loading results in the phosphorylation of the 70 kDa ribosomal S6 kinase (p70S6k), and this event is strongly correlated with the degree of muscle adaptation following resistance exercise. Whether insulin resistance or the comorbidities associated with this disorder may affect the ability of skeletal muscle to activate p70S6k signaling following an exercise stimulus remains unclear. Here, we compare the contraction-induced activation of p70S6k signaling in the plantaris muscles of lean and insulin resistant obese Zucker rats following a single bout of increased contractile loading. Compared to lean animals, the basal phosphorylation of p70S6k (Thr389; 37.2% and Thr421/Ser424; 101.4%), Akt (Thr308; 25.1%), and mTOR (Ser2448; 63.0%) was higher in obese animals. Contraction increased the phosphorylation of p70S6k (Thr389), Akt (Ser473), and mTOR (Ser2448) in both models however the magnitude and kinetics of activation differed between models. These results suggest that contraction-induced activation of p70S6k signaling is altered in the muscle of the insulin resistant obese Zucker rat

Acetaminophen Improves Protein Translational Signaling in Aged Skeletal Muscle

Background: Age-related muscle atrophy is characterized by increased oxidative stress, diminished Akt enzymatic function, and reduced phosphorylation of the mammalian target of rapamycin (mTOR), which can be attenuated by chronic acetaminophen ingestion. Here we hypothesize that age-related impairments in Akt/ mTOR function are associated with reduced protein translational signaling, and that these changes, if present, can be attenuated by acetaminophen treatment. Results: Compared to 6- and 27-month old animals, the expression of the mTOR-complex proteins raptor and GbL and the phosphorylation of tuberin/TSC2 (Thr1462) were reduced in the soleus muscles of very aged rats (33 months old). These changes in Akt/mTOR pathway signaling proteins were in turn associated with decreased phosphorylation of S6 kinase p85S6K (Thr412) and eukaryotic translation initiation factor-4E (eIF4E) binding protein-1 (4EBP1, Thr37/46), reduced phosphorylation of S6 ribosomal protein (Ser235/236), and increased inhibition of eIF4E by binding to 4EBP1. Age-associated alterations in the Akt/mTOR pathway signaling and in the phosphorylation of the stress-responsive eIF2a protein were attenuated by chronic acetaminophen treatment (30 mg/kg body weight per day). Ex vivo incubation of adult muscles with hydrogen peroxide mimicked the age-related decreases seen in eIF4E and 4EBP1 phosphorylation, whereas the inclusion of acetaminophen in the muscle bath attenuated this effect. Conclusion: Aging is associated with impairments in the regulation of proteins thought to be important in controlling mRNA translation, and acetaminophen may be useful for the treatment of age-related muscle atrophy by reducing oxidative stress

Application of Poly(amidoamine) Dendrimers for Use in Bionanomotor Systems

The study and utilization of bionanomotors represents a rapid and progressing field of nanobiotechnology. Here, we demonstrate that poly(amidoamine) (PAMAM) dendrimers are capable of supporting heavy meromyosin dependent actin motility of similar quality to that observed using nitrocellulose, and that microcontact printing of PAMAM dendrimers can be exploited to produce tracks of active myosin motors leading to the restricted motion of actin filaments across a patterned surface. These data suggest that the use of dendrimer surfaces will increase the applicability of using protein biomolecular motors for nanotechnological applications

Diabetes Alters Contraction-Induced Mitogen Activated Protein Kinase Activation in the Rat Soleus and Plantaris

The prescription of anaerobic exercise has recently been advocated for the management of diabetes; however exercise-induced signaling in diabetic muscle remains largely unexplored. Evidence from exercise studies in nondiabetics suggests that the extracellular-signal-regulated kinases (Erk1/2), p38, and c-JUN NH2-terminal kinase (Jnk) mitogen-activated protein kinases (MAPKs) are important regulators of muscle adaptation. Here, we compare the basal and the in situ contraction-induced phosphorylation of Erk1/2- p38- and Jnk-MAPK and their downstream targets (p90rsk and MAPKAP-K2) in the plantaris and soleus muscles of normal and obese (fa/fa) Zucker rats. Compared to lean animals, the time course and magnitude of Erk1/2, p90rsk and p38 phosphorylation to a single bout of contractile stimuli were greater in the plantaris of obese animals. Jnk phosphorylation in response to contractile stimuli was muscle-type dependent with greater increases in the plantaris than the soleus. These results suggest that diabetes alters intramuscular signaling processes in response to a contractile stimulus

Acetaminophen prevents aging-associated hyperglycemia in aged rats: effect of aging-associated hyperactivation of p38-MAPK and ERK1/2

Background Aging-related hyperglycemia is associated with increased oxidative stress and diminished muscle glucose transporter-4 (Glut4) that may be regulated, at least in part, by the mitogen-activated protein kinases (MAPK). Methods To test the possibility that aging-related hyperglycemia can be prevented by pharmacological manipulation of MAPK hyperactivation, aged (27-month old) Fischer 344/NNiaHSD × Brown Norway/BiNia F1 (F344BN) rats were administered acetaminophen (30 mg/kg body weight/day) for 6 months in drinking water. Results Hepatic histopathology, serum aspartate aminotransferase and alanine aminotransferase analyses suggested that chronic acetaminophen did not cause hepatotoxicity. Compared with adult (6-month) and aged (27-month) rats, very aged rats (33-month) had higher levels of blood glucose, phosphorylation of soleus p38-MAPK and extracellular-regulated kinase 1/2 (ERK1/2), superoxide and oxidatively modified proteins (p \u3c 0.05), and these changes were associated with decreased soleus Glut4 protein abundance (p \u3c 0.05). Chronic acetaminophen treatment attenuated age-associated increase in blood glucose by 61.3% (p \u3c 0.05) and increased soleus Glut4 protein by 157.2% (p \u3c 0.05). These changes were accompanied by diminished superoxide levels, decrease in oxidatively modified proteins (−60.8%; p \u3c 0.05) and reduced p38-MAPK and ERK1/2 hyperactivation (−50.4% and − 35.4%, respectively; p \u3c 0.05). Conclusions These results suggest that acetaminophen may be useful for the treatment of age-associated hyperglycemia

Intratracheal Instillation of Cerium Oxide Nanoparticles Induces Hepatic Toxicity in Male Sprague-Dawley Rats

Background: Cerium oxide (CeO2) nanoparticles have been posited to have both beneficial and toxic effects on biological systems. Herein, we examine if a single intratracheal instillation of CeO2 nanoparticles is associated with systemic toxicity in male Sprague-Dawley rats. Methods and results: Compared with control animals, CeO2 nanoparticle exposure was associated with increased liver ceria levels, elevations in serum alanine transaminase levels, reduced albumin levels, a diminished sodium-potassium ratio, and decreased serum triglyceride levels (P \u3c 0.05). Consistent with these data, rats exposed to CeO2nanoparticles also exhibited reductions in liver weight (P \u3c 0.05) and dose-dependent hydropic degeneration, hepatocyte enlargement, sinusoidal dilatation, and accumulation of granular material. No histopathological alterations were observed in the kidney, spleen, and heart. Analysis of serum biomarkers suggested an elevation of acute phase reactants and markers of hepatocyte injury in the rats exposed to CeO2 nanoparticles. Conclusion: Taken together, these data suggest that intratracheal instillation of CeO2nanoparticles can result in liver damage