Structure and function of the upstream promotor of the humanMafbx gene: The proximal upstream promotor modulates tissue-specificity

Constant heat stress reduces skeletal muscle protein deposition in broilers

BACKGROUND

This experiment was conducted to evaluate the effects of constant heat stress on growth performance and protein metabolism in skeletal muscle of Arbor Acres broilers.

RESULTS

Two hundred and seventy 21-day-old Arbor Acres broilers with similar body weight (1298 ± 28 g) were selected for a 3-week trial (29-49 days of age). The broilers were randomly assigned to three groups including the control group, constant heat stress group and pair-fed group. Up-regulation of the rectal temperature and the mRNA expression of heat shock protein 70 in liver indicate that the model for constant heat stress was success. The average daily gain, feed conversion ratio, breast and thigh muscle weight, percentage of breast muscle, crude protein content in breast and thigh muscle in constant heat stress group were significantly lower than in control group and pair-fed group. Serum uric acid content and the glutamic-oxaloacetic transaminase activity were significantly higher, while protein content and glutamic-pyruvate transaminase activity were significantly lower in liver of heat stress group than of the control and pair-fed groups. The expression of insulin-like growth factor 1, phosphatidylinositol 3-kinase and p70S6 kinase associated with protein synthesis were lower in breast muscle but higher in thigh muscle in heat stress group compared to the control or fed-pair groups. In thigh muscles, the expression of muscle ring-finger protein-1 and MAFbx associated with protein degradation were higher in the heat stress group than in the control and pair-fed groups.

CONCLUSION

Poor performance of the birds under heat stress may be due to lower synthesis and increased degradation of proteins.

Smad3 induces atrogin-1, inhibits mTOR and protein synthesis, and promotes muscle atrophy in vivo

Myostatin, a member of the TGF superfamily, is sufficient to induce skeletal muscle atrophy. Myostatin-induced atrophy is associated with increases in E3-ligase atrogin-1 expression and protein degradation and decreases in Akt/mechanistic target of rapamycin (mTOR) signaling and protein synthesis. Myostatin signaling activates the transcription factor Smad3 (Small Mothers Against Decapentaplegic), which has been shown to be necessary for myostatin-induced atrogin-1 expression and atrophy; however, it is not known whether Smad3 is sufficient to induce these events or whether Smad3 simply plays a permissive role. Thus, the aim of this study was to address these questions with an in vivo model. To accomplish this goal, in vivo transfection of plasmid DNA was used to create transient transgenic mouse skeletal muscles, and our results show for the first time that Smad3 expression is sufficient to stimulate atrogin-1 promoter activity, inhibit Akt/mTOR signaling and protein synthesis, and induce muscle fiber atrophy. Moreover, we propose that Akt/mTOR signaling is inhibited by a Smad3-induced decrease in microRNA-29 (miR-29) expression and a subsequent increase in the translation of phosphatase and tensin homolog (PTEN) mRNA. Smad3 is also sufficient to inhibit peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α) promoter activity and to increase FoxO (Forkhead Box Protein, Subclass O)-mediated signaling and the promoter activity of plasminogen activator inhibitor 1 (PAI-1). Combined, this study provides the first evidence that Smad3 is sufficient to regulate many of the events associated with myostatin-induced atrophy and therefore suggests that Smad3 signaling may be a viable target for therapies aimed at preventing myostatin-induced muscle atrophy.

MuRF-1 and p-GSK3β expression in muscle atrophy of cirrhosis

BACKGROUND

Chronic diseases, including cirrhosis, are often accompanied by protein-energy malnutrition and muscle loss, which in turn negatively affect quality of life, morbidity and mortality. Unlike other chronic conditions, few data are available on the molecular mechanisms underlying muscle wasting in this clinical setting.

AIMS

To assess mechanisms of muscle atrophy in patients with cirrhosis.

METHODS

Nutritional [subjective global assessment (SGA) and anthropometry] and metabolic assessment was performed in 30 cirrhotic patients awaiting liver transplantation. Rectus abdominis biopsies were obtained intraoperatively in 22 cirrhotic patients and in 10 well-nourished subjects undergoing elective surgery for non-neoplastic disease, as a control group. Total RNA was extracted and mRNA for atrogenes (MuRF-1, Atrogin-1/MAFbx), myostatin (MSTN), GSK3β and IGF-1 was assayed.

RESULTS

A total of 50% of cirrhotic patients were malnourished based on SGA, while 53% were muscle-depleted according to mid-arm muscle area (MAMA<5th percentile). MuRF-1 RNA expression was significantly increased in malnourished cirrhotic patients (SGA-B/C) vs. well-nourished patients (SGA-A) (P = 0.01). The phosphorylation of GSK3β was up-regulated in cirrhotic patients with hepatocellular carcinoma (HCC) vs. patients without tumour (P < 0.05).

CONCLUSIONS

Muscle loss is frequently found in end-stage liver disease patients. Molecular factors pertaining to signalling pathways known to be involved in the regulation of muscle mass are altered during cirrhosis and HCC.

Gastrocnemius transcriptome analysis reveals domestication induced gene expression changes between wild and domestic chickens

Artificial selection of chicken for human-preferred traits has manifested great phenotypic differences between wild and domestic chickens. Study on the formation of these phenotypic variations will contribute to comprehensive understanding of the molecular mechanism of animal domestication. We used three kinds of chicken breeds for transcriptome analysis, including the red jungle fowl which was the wild ancestor of chickens, and two other domestic breeds, the chahua chicken and the avian broiler. More than 12,000 genes' expression levels were compared between different chicken breeds, and hundreds of genes displayed differential expression levels compared with wild chicken. Gene ontology analysis showed that differentially expressed genes in domestic chickens tended to be enriched in extracellular matrix, DNA binding and immune system development, etc. Some genes with important biological functions were differentially expressed in the domestic chickens, including titin, myostatin ubiquitin related genes, and transforming growth factor-beta receptor III, indicating possible selection pressures on these genes.

Effects of fasting and refeeding on expression of MAFbx and MuRF1 in chick skeletal muscle

The present study investigated the effects of fasting and refeeding on the expression of proteasome-related genes and their downstream targets in the skeletal muscles of chicks. Seven-day-old chicks were fasted for 24 or 48 h and then refed for 4 h. The expression levels of MAFbx and MuRF1, which function as E3 ligases in the ubiquitin-proteasome system, were investigated at the mRNA and protein levels. MAFbx and MuRF1 expression were increased by fasting and these increases were downregulated by refeeding. The expression of the target proteins of these E3 ligases, MyoD and M-CK, was also analyzed. The levels of these proteins were downregulated by fasting, and these decreases were rescued by refeeding. The results of this study indicate that fasting stimulates MAFbx and MuRF1 expression in chicks, possibly leading to increased degradation of their corresponding target proteins.

Mitogen-activated protein kinase kinase 1 (MEK1) stabilizes MyoD through direct phosphorylation at tyrosine 156 during myogenic differentiation

Previously, we reported that mitogen-activated protein kinase kinase 1 (MEK1) activated in the mid-stage of skeletal muscle differentiation promotes myogenic differentiation. To elucidate the molecular mechanism, we investigated an activity of MEK1 for MyoD. Activated MEK1 associates with MyoD in the nucleus of differentiating myoblasts. In vitro kinase assay using active MEK1, a (32)P-labeled protein band corresponding to GST-MyoD was observed but not to mutant GST-MyoD-Y156F. Tyrosine phosphorylation of endogenous MyoD was detected with a specific anti-pMyoD-Y156 antibody; however, this response was blocked by PD184352, a MEK-specific inhibitor. These results indicate that activated MEK1 phosphorylates the MyoD-Y156 residue directly. Interestingly, the protein level of mutant MyoD-Y156F decreased compared with that of wild type but was recovered in the presence of lactacystin, a proteasome inhibitor. The protein level of MyoD-Y156E, which mimics phosphorylation at Tyr-156, was above that of wild type, indicating that the phosphorylation protects MyoD from the ubiquitin proteasome-mediated degradation. In addition, the low protein level of MyoD-Y156F was recovered over that of wild type by an additional mutation at Leu-164, a critical binding residue of MAFbx/AT-1, a Skp, Cullin, F-box (SCF) E3-ubiquitin ligase. The amount of MyoD co-precipitated with MAFbx/AT-1 also was reduced in the presence of active MEK1. Thus, these results suggested that the phosphorylation probably interrupts the binding of MAFbx/AT-1 to MyoD and thereby increases its stability. Collectively, our results suggest that MEK1 activated in differentiating myoblasts stimulates muscle differentiation by phosphorylating MyoD-Y156, which results in MyoD stabilization.

Ouabain exacerbates botulinum neurotoxin-induced muscle paralysis via progression of muscle atrophy in mice

Botulinum neurotoxin serotype A (BoNT/A) inhibits acetylcholine release at the neuromuscular junction in isolated muscles, and ouabain can partially block its effect. However, it is not clear whether ouabain attenuates BoNT/A-induced neuromuscular paralysis in vivo. In this work, we investigated the effects of ouabain on BoNT/A-induced neuromuscular paralysis in mice. Ouabain was administered to mice intraperitoneally immediately after a single injection of BoNT/A into skeletal muscle. The effects of ouabain on BoNT/A-induced muscle paralysis were assessed by quantitative monitoring of muscle tension and digit abduction via the digit abduction scoring (DAS) assay. A single administration of ouabain significantly prolonged BoNT/A-induced neuromuscular paralysis. Moreover, consecutive daily injection of ouabain exacerbated BoNT/A-induced neuromuscular paralysis, and led to a significant decrease in both twitch and tetanic forces as assayed in isolated BoNT/A-injected muscles. We next looked at the effects of ouabain on BoNT/A-induced muscle atrophy. Administration of ouabain led to a decrease in the myofibrillar cross-sectional area (CSAs) by 14 post-BoNT/A injection. In addition, repeated administration of ouabain increased mRNA expression levels of ubiquitin ligases, which are markers of muscle atrophy, in BoNT/A-injected muscle. These results suggest that ouabain exacerbates BoNT/A-induced neuromuscular paralysis via a marked progression of BoNT/A-induced muscle atrophy.

Traf7, a MyoD1 transcriptional target, regulates nuclear factor-κB activity during myogenesis

We have identified the E3 ligase Traf7 as a direct MyoD1 target and show that cell cycle exit-an early event in muscle differentiation-is linked to decreased Traf7 expression. Depletion of Traf7 accelerates myogenesis, in part through downregulation of nuclear factor-κB (NF-κB) activity. We used a proteomic screen to identify NEMO, the NF-κB essential modulator, as a Traf7-interacting protein. Finally, we show that ubiquitylation of NF-κB essential modulator is regulated exclusively by Traf7 activity in myoblasts. Our results suggest a new mechanism by which MyoD1 function is coupled to NF-κB activity through Traf7, regulating the balance between cell cycle progression and differentiation during myogenesis.

Expression of the muscle atrophy factor muscle atrophy F-box is suppressed by testosterone

The ubiquitin ligase muscle atrophy F-box (MAFbx; also called atrogin-1) is thought to play important roles in muscle loss. Conversely, testosterone reduces atrophy from glucocorticoids or denervation associated with repression of MAFbx. To characterize mechanisms of such repression, the effects of testosterone on MAFbx expression in C2C12 cells were tested. Testosterone reduced MAFbx mRNA levels as well as expression of a reporter gene under the control of 3.1 kb of the human MAFbx promoter. Repression required the androgen receptor (AR) as well as sequences within the first 208 bases upstream of the first codon of the MAFbx gene. This sequence is downstream of known forkhead transcription factor binding sites and testosterone did not alter Forkhead box O 3A phosphorylation. The AR associated with sequences conferring repression in a manner that was stimulated by testosterone and was independent of DNA binding. In gel shift studies, octamer binding transcription factor (Oct)-1 bound two predicted Oct-1 sites within these sequences. Deletion of Oct-1 sites from reporter genes prevented repression by testosterone. Gene knockdown of Oct-1 blocked repression of MAFbx reporter gene activity by testosterone and binding of AR to sequences conferring repression. In conclusion, testosterone represses MAFbx expression via interactions of the AR with Oct-1 that are associated with sequences within the 5' untranslated region of the MAFbx promotor located just upstream of the first codon. This action of testosterone may contribute to beneficial actions of testosterone on muscle.

Testosterone protects against dexamethasone-induced muscle atrophy, protein degradation and MAFbx upregulation

Administration of glucocorticoids in pharmacological amounts results in muscle atrophy due, in part, to accelerated degradation of muscle proteins by the ubiquitin-proteasome pathway. The ubiquitin ligase MAFbx is upregulated during muscle loss including that caused by glucocorticoids and has been implicated in accelerated muscle protein catabolism during such loss. Testosterone has been found to reverse glucocorticoid-induced muscle loss due to prolonged glucocorticoid administration. Here, we tested the possibility that testosterone would block muscle loss, upregulation of MAFbx, and protein catabolism when begun at the time of glucocorticoid administration. Coadministration of testosterone to male rats blocked dexamethasone-induced reduction in gastrocnemius muscle mass and upregulation of MAFbx mRNA levels. Administration of testosterone together with dexamethasone also prevented glucocorticoid-induced upregulation of MAFbx mRNA levels and protein catabolism in C2C12 myotube expressing the androgen receptor. Half-life of MAFbx was not altered by testosterone, dexamethasone or the combination. Testosterone blocked dexamethasone-induced increases in activity of the human MAFbx promotor. The findings indicate that administration testosterone prevents glucocorticoid-induced muscle atrophy and suggest that this results, in part at least, from reductions in muscle protein catabolism and expression of MAFbx.

Understanding the extrinsic and intrinsic signals involved in pancreas and β-cell development: from endoderm to β cells

PURPOSE OF REVIEW

To summarize recent progress in understanding of the extrinsic and intrinsic signals directing pancreas development from early endoderm.

RECENT FINDINGS

The pancreatic mesoderm was shown not only to play a permissive role in pancreas determination but also to control endocrine commitment and maturation through the interplay between Notch and fibroblast growth factor signaling. The requirement of Wnt (wingless-type)/β-catenin signaling in the expansion of the acinar cell lineage, and the spatial-temporal specificity of PDX1 (pancreatic and duodenal homeobox) activity, which is needed for proper acinar development, were also demonstrated. A novel factor, IA1 (insulinoma-associated 1), was identified as an endocrine marker downstream of Ngn3 (neurogenin); MAFB (musculo-aponeurotic fibrosarcoma) was shown to be a marker of α-cell and β-cell precursors, and ARX (aristaless-related homeobox), a marker of α-cell progenitors, was revealed to directly antagonize PAX4 (paired homeobox) in determining α-cell and β-cell lineages.

SUMMARY

Cell fate specification results from combined effects of extrinsic and intrinsic regulators and sensitivity of target cells to them, which vary depending on the precise stage of cell commitment or differentiation. Knowledge of the hierarchy of the different factors influencing pancreas development will aid in developing new cell therapies to treat diabetes.

Mechanical strain promotes fibroblast gene expression in presence of corticosteroid

Posterior tibial tendon (PTT) dysfunction has commonly been treated with local corticosteroid injections to reduce inflammation. However, a concern with this treatment is potential degeneration and spontaneous rupture of the PTT. This study set out to determine whether mechanical strain may counteract the potentially deleterious effect of corticosteroid treatment on fibroblasts and therefore improve outcomes during recovery from tendinitis. In this study, PTT fibroblasts in vitro were treated with 0 M, 10(-7) M, 10(-6) M, and 10(-5) M triamcinolone acetonide (TA) while incubated under cyclic strains of 0% or 5% for 24 hr. Type I collagen and decorin mRNA expressions were determined by RT-PCR. The results indicated that mechanical strain significantly increased type I collagen and decorin gene expression in the PTT fibroblasts and TA decreased type I collagen and decorin gene expression. Therefore, mechanical strain might be beneficial to PTT after corticosteroid treatment by direct stimulation of fibroblast synthesis.