The p38 pathway regulates Akt both at the protein and transcriptional activation levels during myogenesis

Inhibition of sPLA2 enzyme activity by cell-permeable antioxidant EUK-8 and downregulation of p38, Akt, and p65 signals induced by sPLA2 in inflammatory mouse paw edema model

The arachidonic acid (AA) metabolic pathway, plays a vital role in the production of eicosanoids by the action of pro-inflammatory secretory phospholipase A₂ (PLA₂ ). Release of eicosanoids is known to be involved in many inflammatory diseases. Identification of the inhibitory molecules of this AA pathway enzyme along with the regulation of intracellular signaling cascades may be a finer choice to develop as a powerful anti-inflammatory drug. In this regard, we have screened few cell-permeable antioxidant molecules Tempo, Mito-TEMPO, N,N'-Bis(salicylideneamino)ethane-manganese(II) (EUK)-134, and EUK-8 against pro-inflammatory sPLA₂ s. Among these, we found EUK-8 is a potent inhibitor with its IC₅₀ value ranges 0.7-2.0 µM for sPLA₂ s isolated from different sources. Furthermore, docking studies confirm the strong binding of EUK-8 towards sPLA₂ . In vivo effect of EUK-8 was studied in HSF-sPLA₂ -induced edema in mouse paw model. In addition to neutralizing the edema, EUK-8 significantly reduces the phosphorylation level of inflammatory proteins such as p38 member of MAPK pathway, Akt, and p65 along with the suppression of pro-inflammatory cytokine (interleukin-6) and chemokine (CXCL1) in edematous tissue. This shows that EUK-8 not only inhibits the sPLA₂ activity, it also plays an important role in the regulation of sPLA₂ -induced cell signaling cascades. Apart from the sPLA₂ inhibition, we also examine the regulatory actions of EUK-8 with other downstream enzymes of AA pathway such as 5-LOX assay in human polymorphonuclear leukocytes (PMNs) and COX-2 expression in carrageenan-λ induced paw edema. Here EUK-8 significantly inhibits 5-LOX enzyme activity and downregulates COX-2 expression. These data indicate that EUK-8 found to be a promising multitargeted inhibitory molecule toward inflammatory pathway. In conclusion, mitochondrial targeted antioxidant EUK-8 is not only the powerful antioxidant, also a potent anti-inflammatory molecule and may be a choice of molecule for pharmacological applications.

Restoration of Lysosomal and Mitochondrial Function Through p38 Mitogen-Activated Protein Kinase Inhibition Ameliorates Senescence

Oncogene-induced senescence (OIS), characterized by irreversible cell cycle arrest by oncogene activation, plays an important role in the pathogenesis of aging and age-related diseases. Recent research indicates that OIS is driven by activation of mitogen-activated protein kinase (MAPK). However, it is not apparent whether MAPK inhibition helps to recover senescence. In our previous study, we uncovered p38 MAPK inhibitor, SB203580, as an effective agent to reduce reactive oxygen species and increase proliferation in premature senescent cells. In this study, we evaluated whether SB203580 could ameliorate senescence in normal senescent cells. The senescence-improving effect was observed in the results that SB203580 treatment restored lysosomal function, as evidenced by a decrease in lysosomal mass and an increase in autophagic vacuoles. Then, SB203580-mediated lysosomal function restoration triggered the clearance of damaged mitochondria, leading to metabolic reprogramming necessary for amelioration of senescence. Indeed, p38 MAPK inhibition by SB203580 improved key senescent phenotypes. Our findings suggest a novel mechanism by which modulation of p38 MAPK activity leads to senescence improvement through functional restoration of lysosome and mitochondria.

Lumican, an Exerkine, Protects against Skeletal Muscle Loss

Exerkines are soluble factors secreted by exercised muscles, mimicking the effects of exercise in various organs, including the muscle itself. Lumican is reportedly secreted from muscles; however, its roles in skeletal muscle remain unknown. Herein, we found that lumican mRNA expression in the extensor digitorum longus was significantly higher in exercised mice than in unloading mice, and lumican stimulated myogenesis in vitro. Additionally, lumican knockdown significantly decreased muscle mass and cross-sectional area (CSA) of the muscle fiber in the gastrocnemius muscle of exercised mice. Lumican upregulated phosphorylation of p38 mitogen-activated protein kinase (MAPK) and a p38 inhibitor near completely blocked lumican-stimulated myogenesis. Inhibitors for integrin α2β1 and integrin ανβ3 also prevented lumican-stimulated myogenesis. Systemic lumican treatment, administered via the tail vein for 4 weeks, significantly increased relative muscle masses by 36.1% in ovariectomized mice. In addition, intramuscular lumican injection into unloaded muscles for 2 weeks significantly increased muscle mass by 8.5%. Both intravenous and intramuscular lumican treatment significantly increased muscle CSA. Our in vitro and in vivo experiments indicate that lumican is a muscle-secreted exerkine that affords protection against muscle loss by activating p38 MAPK via integrin receptors.

Anti‑oncogenic and pro‑myogenic action of the MKK6/p38/AKT axis induced by targeting MEK/ERK in embryonal rhabdomyosarcoma

Insights into the molecular and cellular biology of embryonal rhabdomyosarcoma (ERMS), an aggressive paediatric tumour, are required in order to identify new targets for novel treatments that may benefit patients with this disease. The present study examined the functional effects of MKK3 and MKK6, two upstream kinases of p38, and found that the ectopic expression of MKK6 led to rapid p38 activation and the myogenic differentiation of ERMS cells, whereas MKK3 failed to induce differentiation, while maintaining the proliferation state. Myogenin and myosin heavy chain were induced in MKK6‑overexpressing ERMS cells and were inhibited by the p38 inhibitor, SB203580. The expression of Myc and ERK‑PO4 increased under the effect of SB203580, whereas it decreased in MKK6‑overexpressing cells. AKT activation was part of the myogenic program triggered by MKK6 overexpression alone. To the best of our knowledge, the present study demonstrates, for the first time, that the endogenous MKK6 pathway may be recovered by MEK/ERK inhibition (U0126 and trametinib) and that it concomitantly induces the reversal of the oncogenic pattern and the induction of the myogenic differentiation of ERMS cell lines. The effects of MEK/ERK inhibitors markedly increase the potential clinical applications in ERMS, particularly on account of the MEK inhibitor‑induced early MKK6/p38 axis activation and of their anti‑oncogenic effects. The findings presented herein lend further support to the antitumour effects of MKK6; MKK6 may thus represent a novel target for advanced personalised treatments against ERMS.

Reduced RECK levels accelerate skeletal muscle differentiation, improve muscle regeneration, and decrease fibrosis

The muscle regeneration process requires a properly assembled extracellular matrix (ECM). Its homeostasis depends on the activity of different matrix-metalloproteinases (MMPs). The reversion-inducing-cysteine-rich protein with kazal motifs (RECK) is a membrane-anchored protein that negatively regulates the activity of different MMPs. However, the role of RECK in the process of skeletal muscle differentiation, regeneration, and fibrosis has not been elucidated. Here, we show that during skeletal muscle differentiation of C2C12 myoblasts and in satellite cells on isolated muscle fibers, RECK is transiently up regulated. C2C12 myoblasts with reduced RECK levels are more prone to enter the differentiation program, showing an accelerated differentiation process. Notch-1 signaling was reduced, while p38 and AKT signaling were augmented in myoblasts with decreased RECK levels. Overexpression of RECK restores the normal differentiation process but diminished the ability to form myotubes. Transient up-regulation of RECK occurs during skeletal muscle regeneration, which was accelerated in RECK-deficient mice (Reck±). RECK, MMPs and ECM proteins augmented in chronically damaged WT muscle, a model of muscle fibrosis. In this model, RECK ± mice showed diminished fibrosis compared to WT. These results strongly suggest that RECK is acting as a potential myogenic repressor during muscle formation and regeneration, emerging as a new player in these processes, and as a potential target to treat individuals with the muscle-wasting disease.

Key Genes Regulating Skeletal Muscle Development and Growth in Farm Animals

Simple Summary

Skeletal muscle mass is an important economic trait, and muscle development and growth is a crucial factor to supply enough meat for human consumption. Thus, understanding (candidate) genes regulating skeletal muscle development is crucial for understanding molecular genetic regulation of muscle growth and can be benefit the meat industry toward the goal of increasing meat yields. During the past years, significant progress has been made for understanding these mechanisms, and thus, we decided to write a comprehensive review covering regulators and (candidate) genes crucial for muscle development and growth in farm animals. Detection of these genes and factors increases our understanding of muscle growth and development and is a great help for breeders to satisfy demands for meat production on a global scale.

Abstract

Farm-animal species play crucial roles in satisfying demands for meat on a global scale, and they are genetically being developed to enhance the efficiency of meat production. In particular, one of the important breeders’ aims is to increase skeletal muscle growth in farm animals. The enhancement of muscle development and growth is crucial to meet consumers’ demands regarding meat quality. Fetal skeletal muscle development involves myogenesis (with myoblast proliferation, differentiation, and fusion), fibrogenesis, and adipogenesis. Typically, myogenesis is regulated by a convoluted network of intrinsic and extrinsic factors monitored by myogenic regulatory factor genes in two or three phases, as well as genes that code for kinases. Marker-assisted selection relies on candidate genes related positively or negatively to muscle development and can be a strong supplement to classical selection strategies in farm animals. This comprehensive review covers important (candidate) genes that regulate muscle development and growth in farm animals (cattle, sheep, chicken, and pig). The identification of these genes is an important step toward the goal of increasing meat yields and improves meat quality.

Epigenetic modifications in muscle regeneration and progression of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a multisystemic disorder that affects 1:5000 boys. The severity of the phenotype varies dependent on the mutation site in the DMD gene and the resultant dystrophin expression profile. In skeletal muscle, dystrophin loss is associated with the disintegration of myofibers and their ineffective regeneration due to defective expansion and differentiation of the muscle stem cell pool. Some of these phenotypic alterations stem from the dystrophin absence-mediated serine-threonine protein kinase 2 (MARK2) misplacement/downregulation in activated muscle stem (satellite) cells and neuronal nitric oxide synthase loss in cells committed to myogenesis. Here, we trace changes in DNA methylation, histone modifications, and expression of regulatory noncoding RNAs during muscle regeneration, from the stage of satellite cells to myofibers. Furthermore, we describe the abrogation of these epigenetic regulatory processes due to changes in signal transduction in DMD and point to therapeutic treatments increasing the regenerative potential of diseased muscles based on this acquired knowledge.

Protective role of renal proximal tubular alpha-synuclein in the pathogenesis of kidney fibrosis

Kidney fibrosis is a highly deleterious process and a final manifestation of chronic kidney disease. Alpha-(α)-synuclein (SNCA) is an actin-binding neuronal protein with various functions within the brain; however, its role in other tissues is unknown. Here, we describe the expression of SNCA in renal epithelial cells and demonstrate its decrease in renal tubules of murine and human fibrotic kidneys, as well as its downregulation in renal proximal tubular epithelial cells (RPTECs) after TGF-β1 treatment. shRNA-mediated knockdown of SNCA in RPTECs results in de novo expression of vimentin and α-SMA, while SNCA overexpression represses TGF-β1-induced mesenchymal markers. Conditional gene silencing of SNCA in RPTECs leads to an exacerbated tubulointerstitial fibrosis (TIF) in two unrelated in vivo fibrotic models, which is associated with an increased activation of MAPK-p38 and PI3K-Akt pathways. Our study provides an evidence that disruption of SNCA signaling in RPTECs contributes to the pathogenesis of renal TIF by facilitating partial epithelial-to-mesenchymal transition and extracellular matrix accumulation.

Role of supplemental UV-B in changing the level of ozone toxicity in two cultivars of sunflower: growth, seed yield and oil quality

Ultraviolet-B radiation (UV-B) is inherent part of solar spectrum and tropospheric ozone (O₃) is a potent secondary air pollutant. Therefore the present study was conducted to evaluate the responses of Helianthus annuus L. cvs DRSF 108 and Sungold (sunflower) to supplemental UV-B (sUV-B; ambient + 7.2 kJ m^-2 d^-1) and elevated ozone (O₃; ambient + 10 ppb), given singly and in combination under field conditions using open-top chambers. The individual and interactive effects of O₃ and sUV-B induced varying changes in both the cultivars of sunflower ranging from ultrastructural variations to growth, biomass, yield and oil composition. Reduction in leaf area of Sungold acted as a protective feature which minimized the perception of sUV-B as well as uptake of O₃ thus led to lesser carbon loss compared to DRSF 108. Number- and weight of heads plant^-1 decreased although more in Sungold with decline of oil content. Both the stresses when given singly and combination induced rancidification of oil and thus made the oil less suitable for human consumption.

Canadine from Corydalis turtschaninovii Stimulates Myoblast Differentiation and Protects against Myotube Atrophy

Cachexia and sarcopenia are the main causes of muscle atrophy. These result in a reduction in the muscle fiber area, myo-protein content, and muscle strength, with various molecular modulators being involved. Although several reports have proposed potential therapeutic agents, no effective treatments have been found for muscle atrophy. We searched for myogenic modulators from medicinal plants to treat muscle diseases. We isolated six alkaloids from Corydalis turtschaninovii and evaluated their myogenic potential by using the MyoD reporter gene assay in C2C12 cells. Among the tested compounds, canadine showed the strongest transactivation of MyoD and increased MHC expression during myogenesis. The activation of p38 MAP kinase and Akt are major mechanisms that contribute to the myogenesis by canadine. Canadine increased the number of multinucleated and cylinder-shaped myotubes during myogenesis of C2C12 myoblasts. To determine the preventive effect of canadine in cancer-induced muscle wasting, differentiated C2C12 myotubes were treated with conditioned media from CT26 colon carcinoma culture (CT26 CM) in the presence of canadine. Canadine ameliorated the muscle protein degradation caused by CT26-CM by down-regulating the muscle specific-E3 ligases, MAFbx/atrogin-1 and MuRF1. In this study, we found that canadine from C. turtschaninovii stimulates myogenesis and also inhibits muscle protein degradation. Therefore, we suggest canadine as a protective agent against muscle atrophy.

NLS-RARα Inhibits the Effects of All-trans Retinoic Acid on NB4 Cells by Interacting with P38α MAPK

Nuclear localization signal retinoic acid receptor alpha(NLS-RARα), which forms from the cleavage of promyelocytic leukemia-retinoic acid receptor alpha(PML-RARα) protein by neutrophil elastase(NE), possesses an important role in the occurrence and development of acute promyelocytic leukemia(APL). However, the potential mechanism underlying the effects of NLS-RARα on APL is still not entirely clear. Here, we investigated the effects of NLS-RARα on APL NB4 cells and its mechanism. We found that all-trans retinoic acid(ATRA) could promote differentiation while inhibit proliferation of APL NB4 cells via upregulating the expression of phosphorylated p38α mitogen-activated protein kinase(p-p38α MAPK). We also found that NLS-RARα could inhibit differentiation while accelerate proliferation of NB4 cells via downregulating the expression of p-p38α protein in the presence of ATRA. Furthermore, immunofluorescence and co-immunoprecipitation assays confirmed NLS-RARα interacted with p38α protein directly. Finally, application of PD169316, an inhibitor of p38α protein, suggested that recruitment p38α-combinded NLS-RARα by ATRA eventually caused activation of p38α protein. In summary, our study demonstrated that ATRA cound promote differentiation while inhibit proliferation of APL NB4 cells via activating p38α protein after recruiting p38α-combinded NLS-RARα, while NLS-RARα could inhibit the effects of ATRA in the process.

Dupuytren's disease therapy: targeting the vicious cycle of myofibroblasts?

INTRODUCTION

Dupuytren's disease (DD) is a proliferative fibromatosis of the hand, which causes permanent flexion contracture of the digits and, ultimately, loss of function. The treatment of DD is complex and involves surgical and nonsurgical approaches, with the goal of removing the affected tissue. New biological targets are under investigation in order to develop innovative therapies.

AREAS COVERED

The etiology of DD is still unknown. Several authors who focused their studies on the genetics of DD recognized an inherited autosomal dominant pattern. Actually, DD is a multifactorial and complex disease. Myofibroblasts are thought to play a crucial role in its pathogenesis, although their origin is not clear.

EXPERT OPINION

There is a general consensus that a better understanding of cellular and molecular mechanisms of DD will lead to the design of more specific and effective treatment alternatives. In this review, the authors hypothesize a new biological model for DD pathology, where myofibroblasts enhance the reservoir of the disease acting as if in a vicious cycle. This could help, ultimately, in identifying new therapeutic strategies to treat this common and disabling fibroproliferative disorder.

Separating myoblast differentiation from muscle cell fusion using IGF-I and the p38 MAP kinase inhibitor SB202190

The p38 MAP kinases play critical roles in skeletal muscle biology, but the specific processes regulated by these kinases remain poorly defined. Here we find that activity of p38α/β is important not only in early phases of myoblast differentiation, but also in later stages of myocyte fusion and myofibrillogenesis. By treatment of C2 myoblasts with the promyogenic growth factor insulin-like growth factor (IGF)-I, the early block in differentiation imposed by the p38 chemical inhibitor SB202190 could be overcome. Yet, under these conditions, IGF-I could not prevent the later impairment of muscle cell fusion, as marked by the nearly complete absence of multinucleated myofibers. Removal of SB202190 from the medium of differentiating myoblasts reversed the fusion block, as multinucleated myofibers were detected several hours later and reached ∼90% of the culture within 30 h. Analysis by quantitative mass spectroscopy of proteins that changed in abundance following removal of the inhibitor revealed a cohort of upregulated muscle-enriched molecules that may be important for both myofibrillogenesis and fusion. We have thus developed a model system that allows separation of myoblast differentiation from muscle cell fusion and should be useful in identifying specific steps regulated by p38 MAP kinase-mediated signaling in myogenesis.

Phosphorylation of AKT induced by phosphorylated Hsp27 confers the apoptosis-resistance in t-AUCB-treated glioblastoma cells in vitro

The aim of this study is to determine whether phosphorylation of AKT could be effected by t-AUCB-induced p-Hsp27 and whether p-AKT inhibition sensitizes glioblastoma cells to t-AUCB, and to evaluate the effects of simultaneous inhibition of p-Hsp27 and p-AKT on t-AUCB treated glioblastoma cells. Cell growth was detected using CCK-8 assay; Caspase-3 activity assay kits and flow cytometry were used in apoptosis analysis; Western blot analysis was used to detect p-Hsp27 and p-AKT levels; RNA interference using the siRNA oligos of Hsp27 was performed to knockdown gene expression of Hsp27. All data were analyzed by the Student-Newman-Keul's test. We demonstrated that t-AUCB treatment induces AKT phosphorylation by activating Hsp27 in U251 and LN443 cell lines. Inhibition of AKT phosphorylation by AKT inhibitor IV sensitizes glioblastoma cells to t-AUCB, strengthens t-AUCB suppressing cell growth and inducing cell apoptosis. We also found inhibiting both p-Hsp27 and p-AKT synergistically strengthen t-AUCB suppressing cell growth. Thus, p-AKT induced by p-Hsp27 confers the apoptosis-resistance in t-AUCB-treated glioblastoma cells. Targeting p-Hsp27 and/or p-AKT may be a potential effective strategy for the treatment of glioblastoma.

Chicoric acid induces apoptosis in 3T3-L1 preadipocytes through ROS-mediated PI3K/Akt and MAPK signaling pathways

Chicoric acid has been reported to possess various bioactivities. However, the antiobesity effects of chicoric acid remain poorly understood. In this study, we investigated the effects of chicoric acid on 3T3-L1 preadipocytes and its molecular mechanisms of apoptosis. Chicoric acid inhibited cell viability and induced apoptosis in 3T3-L1 preadipocytes which was characterized by chromatin condensation and poly ADP-ribose-polymerase (PARP) cleavage. Mitochondrial membrane potential (MMP) loss, Bax/Bcl-2 dysregulation, cytochrome c release, and caspase-3 activation were observed, indicating mitochondria-dependent apoptosis induced by chicoric acid. Furthermore, PI3K/Akt and MAPK (p38 MAPK, JNK, and ERK1/2) signaling pathways were involved in chicoric acid-induced apoptosis. The employment of protein kinase inhibitors LY294002, SB203580, SP600125, and U0126 revealed that PI3K/Akt signaling pathway interplayed with MAPK signaling pathways. Moreover, chicoric acid induced reactive oxygen species (ROS) generation. Pretreatment with the antioxidant N-acetylcysteine (NAC) significantly blocked cell death and changes of Akt and MAPK signalings induced by chicoric acid. In addition, chicoric acid down regulated HO-1 and COX-2 via the PI3K/Akt pathway.

Stac3 is required for myotube formation and myogenic differentiation in vertebrate skeletal muscle

Stac3 was identified as a nutritionally regulated gene from an Atlantic salmon subtractive hybridization library with highest expression in skeletal muscle. Salmon Stac3 mRNA was highly correlated with myogenin and myoD1a expression during differentiation of a salmon primary myogenic culture and was regulated by amino acid availability. In zebrafish embryos, stac3 was initially expressed in myotomal adaxial cells and in fast muscle fibers post-segmentation. Morpholino knockdown resulted in defects in myofibrillar protein assembly, particularly in slow muscle fibers, and decreased levels of the hedgehog receptor patched. The function of Stac3 was further characterized in vitro using the mammalian C2C12 myogenic cell line. Stac3 mRNA expression increased during the differentiation of the C2C12 myogenic cell line. Knockdown of Stac3 by RNAi inhibited myotube formation, and microarray analysis revealed that transcripts involved in cell cycle, focal adhesion, cytoskeleton, and the pro-myogenic factors Igfbp-5 and Igf2 were down-regulated. RNAi-treated cells had suppressed Akt signaling and exogenous insulin-like growth factor (Igf) 2 was unable to rescue the phenotype, however, Igf/Akt signaling was not blocked. Overexpression of Stac3, which results in increased levels of Igfbp-5 mRNA, did not lead to increased differentiation. In synchronized cells, Stac3 mRNA was most abundant during the G(1) phase of the cell cycle. RNAi-treated cells were smaller, had higher proliferation rates and a decreased proportion of cells in G(1) phase when compared with controls, suggesting a role in the G(1) phase checkpoint. These results identify Stac3 as a new gene required for myogenic differentiation and myofibrillar protein assembly in vertebrates.

Notch3 and Mef2c proteins are mutually antagonistic via Mkp1 protein and miR-1/206 microRNAs in differentiating myoblasts

BACKGROUND

Notch3 is expressed in myogenic precursors, but its function is not well known.

RESULTS

Notch3 represses the activity of Mef2c and is in turn inhibited by the microRNAs-1 and -206.

CONCLUSION

Notch3 serves as a regulator for preventing premature myogenic differentiation.

SIGNIFICANCE

Understanding how precocious differentiation is prevented is critical for designing therapy for skeletal muscle regeneration. The Notch signaling pathway is a well known regulator of skeletal muscle stem cells known as satellite cells. Loss of Notch1 signaling leads to spontaneous myogenic differentiation. Notch1, normally expressed in satellite cells, is targeted for proteasomal degradation by Numb during differentiation. A homolog of Notch1, Notch3, is also expressed in these cells but is not inhibited by Numb. We find that Notch3 is paradoxically up-regulated during the early stages of differentiation by an enhancer that requires both MyoD and activated Notch1. Notch3 itself strongly inhibits the myogenic transcription factor Mef2c, most likely by increasing the p38 phosphatase Mkp1, which inhibits the Mef2c activator p38 MAP kinase. Active Notch3 decreases differentiation. Mef2c, however, induces microRNAs miR-1 and miR-206, which directly down-regulate Notch3 and allow differentiation to proceed. Thus, the myogenic differentiation-induced microRNAs miR-1 and -206 are important for differentiation at least partly because they turn off Notch3. We suggest that the transient expression of Notch3 early in differentiation generates a temporal lag between myoblast activation by MyoD and terminal differentiation into myotubes directed by Mef2c.

Microarray analysis of Dupuytren's disease cells: the profibrogenic role of the TGF-β inducible p38 MAPK pathway

BACKGROUND

Dupuytren's disease (DD) is a nodular palmar fibromatosis that causes irreversible permanent contracture of fingers and results in the loss of hand function. Surgery still remains the only available solution for DD patients but cannot permanently cure the disease nor reduce high recurrence rates. With this rationale, we designed a study aimed at an improved understanding of the molecular mechanisms underlying DD. Our major focus was an analysis of the global gene expression profile and signalling pathways in DD cells with the aim of identifying novel biomarkers and/or therapeutic targets.

METHODS

Primary cells were cultured from surgically removed diseased and healthy tissue. Microarray expression analysis (HG-U133A array, Affymetrix) and qPCR was performed with total RNA isolated from primary DD cells. Mechanistic studies involving inhibition of p38 phosphorylation were performed on normal human fibroblasts' and primary DD cells' in vitro models. Expression of stem cell markers in primary fibroblasts/myofibroblasts was assessed as well.

RESULTS

We identified 3 p38MAPK signalling pathway regulatory genes, THBS1, GADD45α and NUAK1, all involved in cellular proliferation and production of the extracellular matrix proteins. Inhibition of the p38MAPK signalling pathway induced down-regulation of myofibroblast markers, α-smooth muscle actin and palladin. A stem-cell like subpopulation positive for CD90 marker was identified among primary DD cells.

CONCLUSION

The study reveals involvement of the p38 MAPK pathway as a possible signalling cascade in the pathogenesis of Dupuytren's disease. Moreover, a particular stem cell-like CD90(+) subpopulation was identified that might contribute to DD development.

AKT and PAX3-FKHR cooperation enforces myogenic differentiation blockade in alveolar rhabdomyosarcoma cell

The chimeric PAX3-FKHR transcription factor is present in a majority of alveolar rhabdomyosarcoma (ARMS), an aggressive skeletal muscle cancer of childhood. PAX3-FKHR-mediated aberrant myogenic gene expression resulting in escape from terminal differentiation program is believed to contribute in ARMS development. In skeletal muscle differentiation, activation of AKT pathway leads to myogenic gene activation and terminal differentiation. Here, we report that AKT acts, in part, by modulating PAX3-FKHR transcriptional activity via phosphorylation in the maintenance of the myogenic differentiation blockade in established mouse models of ARMS cells. We observed that low levels of AKT activity are associated with elevated levels of PAX3-FKHR transcriptional activity, and AKT hyperactivation results in PAX3-FKHR phosphorylation coupled with decreased activity once cells are under differentiation-permissible conditions. Subsequent data shows that attenuated AKT activity-associated PAX3-FKHR activity is required to suppress the function of MyoD, a key myogenic regulator of muscle differentiation. Conversely, decreased PAX3-FKHR activity results in the eradication of MyoD expression and subsequent suppression of the myogenic differentiation. Thus, AKT regulation of the PAX3- FKHR suppresses myogenic gene expression in ARMS cells, causing a failure in differentiation. Evidence is presented that provides a novel molecular link between AKT and PAX3-FKHR in maintaining myogenic differentiation blockade in ARMS.

Use of p38 MAPK inhibitor reduces resistance of colon cancer cells to doxorubicin

AIM: To evaluate whether SB203580, a p38MAPK inhibitor, reduces resistance of colon cancer HCT-116 cells to doxorubicin and to explore possible mechanisms involved.

METHODS: Doxorubicin alone or in combination with SB203580 was used to treat subcultured HCT-116 cells. After treatment, cell growth was determined by MTT assay, and Akt and p-AKt expression was detected by Western blotting.

RESULTS: Either doxorubicin or SB203580 inhibited HCT-116 cell growth, and the latter had a stronger inhibitory effect than the former (26.60% vs 33.87%). Combined use of doxorubicin and SB203580 had more strong inhibitory effect than treatment with either of them alone (56.04%). Expression of Akt showed no significant difference between cells treated with doxorubicin alone and those treated with combined doxorubicin and SB203580, while the phosphorylation of Akt was significantly higher in the doxorubicin group than in the combination group.

CONCLUSION: The p38 MAPK inhibitor SB203580 could block the Akt signaling pathways and increase the sensitivity of colon cancer cells to doxorubicin.

The myogenic kinome: protein kinases critical to mammalian skeletal myogenesis

Myogenesis is a complex and tightly regulated process, the end result of which is the formation of a multinucleated myofibre with contractile capability. Typically, this process is described as being regulated by a coordinated transcriptional hierarchy. However, like any cellular process, myogenesis is also controlled by members of the protein kinase family, which transmit and execute signals initiated by promyogenic stimuli. In this review, we describe the various kinases involved in mammalian skeletal myogenesis: which step of myogenesis a particular kinase regulates, how it is activated (if known) and what its downstream effects are. We present a scheme of protein kinase activity, similar to that which exists for the myogenic transcription factors, to better clarify the complex signalling that underlies muscle development.

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.

Autophagy induction by capsaicin in malignant human breast cells is modulated by p38 and extracellular signal-regulated mitogen-activated protein kinases and retards cell death by suppressing endoplasmic reticulum stress-mediated apoptosis

In our previous study, we showed that capsaicin induces autophagy in several cell lines. Here, we investigated the molecular mechanisms of capsaicin-induced autophagy in malignant (MCF-7 and MDA-MB-231) and normal (MCF10A) human breast cells. Capsaicin caused nonapoptotic cell cycle arrest of MCF-7 and MDA-MB-231 cells but induced apoptosis in MCF10A cells. In MCF-7 and MDA-MB-231 cells, capsaicin induced endoplasmic reticulum (ER) stress via inositol-requiring 1 and Chop and induced autophagy, as demonstrated by microtubule-associated protein 1 light chain-3 (LC3) conversion. Autophagy blocking by 3-methyladenine (3MA) or bafilomycin A1 (BaF1) activated caspase-4 and -7 and enhanced cell death. In MCF-7 and MDA-MB-231 cells, p38 was activated for more than 48 h by capsaicin treatment, but extracellular signal-regulated kinase (ERK) activation decreased after 12 h, and LC3II levels continuously increased. Furthermore, treatment with 3MA markedly down-regulated capsaicin-induced p38 activation and LC3 conversion, and BaF1 completely down-regulated ERK activation and led to LC3II accumulation. In addition, pharmacological blockade or knockdown of the p38 gene down-regulated Akt activation and LC3II levels but did not affect ERK, and pharmacological blockade or knockdown of the ERK gene up-regulated LC3II induction by capsaicin. Knockdown of inositol-requiring 1 down-regulated p38-Akt signaling. In MCF10A cells, capsaicin did not elicit p38 activation and LC3 conversion and caused the sustained activation of caspase-4. Collectively, capsaicin-induced autophagy is regulated by p38 and ERK; p38 controls autophagy at the sequestration step, whereas ERK controls autophagy at the maturation step, and that autophagy is involved in the retardation of cell death by blocking capsaicin-induced ER stress-mediated apoptosis in MCF-7 and MDA-MB-321 cells.

Hypoxia converts the myogenic action of insulin-like growth factors into mitogenic action by differentially regulating multiple signaling pathways

Insulin-like growth factors (IGFs) stimulate myoblast proliferation and differentiation. It remains elusive how these mutually exclusive cellular responses are elicited by the same growth factor. Here we report that whereas IGF promotes myoblast differentiation under normoxia, it stimulates proliferation under hypoxia. Hypoxia activates the HIF-1 transcriptional program and knockdown of HIF-1alpha changes the mitogenic action of IGF into myogenic action under hypoxia. Conversely, overexpression of HIF-1alpha abolishes the myogenic effect of IGF under normoxia. Under normoxia, IGF activates the Akt-mTOR, p38, and Erk1/2 MAPK pathways. Hypoxia suppresses basal and IGF-induced Akt-mTOR and p38 activity, whereas it enhances and prolongs IGF-induced Erk1/2 activation in a HIF-1-dependent fashion. Activation of Akt-mTOR and p38 promotes myogenesis, and p38 also inhibits proliferation. Activation of Erk stimulates myoblast proliferation but inhibits differentiation. These results suggest that hypoxia converts the myogenic action of IGFs into mitogenic action by differentially regulating multiple signaling pathways via HIF-1-dependent mechanisms. Our findings provide a mechanistic explanation for the paradoxical actions of IGFs during myogenesis and reveal a novel mechanism by which cells sense and integrate growth factor signals and oxygen availability in their microenvironments.

Regulation of the intracellular localization of Foxo3a by stress-activated protein kinase signaling pathways in skeletal muscle cells

Muscle atrophy is a debilitating process associated with many chronic wasting diseases, like cancer, diabetes, sepsis, and renal failure. Rapid loss of muscle mass occurs mainly through the activation of protein breakdown by the ubiquitin proteasome pathway. Foxo3a transcription factor is critical for muscle atrophy, since it activates the expression of ubiquitin ligase Atrogin-1. In several models of atrophy, inhibition of the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway induces nuclear import of Foxo3a through an Akt-dependent process. This study aimed to identify signaling pathways involved in the control of Foxo3a nuclear translocation in muscle cells. We observed that after nuclear import of Foxo3a by PI3K/Akt pathway inhibition, activation of stress-activated protein kinase (SAPK) pathways induced nuclear export of Foxo3a through CRM1. This mechanism involved the c-Jun NH(2)-terminal kinase (JNK) signaling pathway and was independent of Akt. Likewise, we showed that inhibition of p38 induced a massive nuclear relocalization of Foxo3a. Our results thus suggest that SAPKs are involved in the control of Foxo3a nucleocytoplasmic translocation in C2C12 cells. Moreover, activation of SAPKs decreases the expression of Atrogin-1, and stable C2C12 myotubes, in which the p38 pathway is constitutively activated, present partial protection against atrophy.

Peroxiredoxin 6 promotes lung cancer cell invasion by inducing urokinase-type plasminogen activator via p38 kinase, phosphoinositide 3-kinase, and Akt

The peroxiredoxin family of peroxidase has six mammalian members (Prx 1-6). Considering their frequent up-regulation in cancer cells, Prxs may contribute to cancer cells' survival in face of oxidative stress. Here, we show that Prx 6 promotes the invasiveness of lung cancer cells, accompanied by an increase in the activity of phosphoinositide 3-kinase (PI3K), the phosphorylation of p38 kinase and Akt, and the protein levels of uPA. Functional studies reveal that these components support Prx 6-induced invasion in the sequence p38 kinase/PI3K, Akt, and uPA. The findings provide a new understanding of the action of Prx 6 in cancer.

Transforming growth factor-beta-activated kinase 1 is an essential regulator of myogenic differentiation

Satellite cells/myoblasts account for the majority of muscle regenerative potential in response to injury and muscular adaptation to exercise. Although the ability to influence this process would provide valuable benefits for treating a variety of patients suffering from muscle loss, the regulatory mechanisms of myogenesis are not completely understood. We have tested the hypothesis that transforming growth factor-beta-activated kinase 1 (TAK1) is an important regulator of skeletal muscle formation. TAK1 is expressed in proliferating C2C12 myoblasts, and its levels are reduced upon differentiation of myoblasts into myotubes. In vivo, TAK1 is predominantly expressed in developing skeletal muscle of young mice. However, the expression of TAK1 was significantly up-regulated in regenerating skeletal muscle of adult mice. Overexpression of a dominant negative mutant of TAK1 or knockdown of TAK1 inhibited the proliferation and differentiation of C2C12 myoblasts. TAK1 was required for the expression of myogenic regulatory factors in differentiating myoblasts. Genetic ablation of TAK1 also inhibited the MyoD-driven transformation of mouse embryonic fibroblasts into myotubes. Inhibition of TAK1 suppressed the differentiation-associated activation of p38 mitogen-activated protein kinase (MAPK) and Akt kinase. Overexpression of a constitutively active mutant of MAPK kinase 6 (MKK6, an upstream activator of p38 MAPK) but not constitutive active Akt restored the myogenic differentiation in TAK1-deficient mouse embryonic fibroblasts. Insulin growth factor 1-induced myogenic differentiation was also found to involve TAK1. Collectively, our results suggest that TAK1 is an important upstream regulator of skeletal muscle cell differentiation.

17β-Estradiol regulates the first steps of skeletal muscle cell differentiation via ER-α-mediated signals

17β-Estradiol (E2) mediates a wide variety of complex biological processes determining the growth and development of reproductive tract as well as nonreproductive tissues of male and female individuals. While E2 effects on the reproductive system, bone, and cardiovascular system are quite well established, less is known about how it affects the physiology of other tissues. Skeletal muscle is a tissue that is expected to be E2 responsive since both isoforms of estrogen receptor (ER-α and ER-β) are expressed. Significant sex-related differences have been described in skeletal muscle, although the role played by E2 and the mechanisms underlying it remain to be determined. Here, we demonstrate that E2 increases the glucose transporter type 4 translocation at membranes as well as the expression of well-known differentiation markers of myogenesis (i.e., myogenin and myosin heavy chain) in rat myoblast cells (L6). These E2-induced effects require rapid extranuclear signals and the presence of ER-α, whereas no contribution of IGF-I receptor has been observed. In particular, ER-α-dependent Akt activation participates in regulating the first step of myogenic differentiation. Moreover, both receptors mediate the E2-induced activation of p38, which, in turn, affects the expression of myogenin and myosin heavy chain. All together, these data indicate that E2 should be included in the list of skeletal muscle trophic factors.

Engineered early embryonic cardiac tissue increases cardiomyocyte proliferation by cyclic mechanical stretch via p38-MAP kinase phosphorylation

Cardiomyocyte (CM) transplantation is one therapeutic option for cardiac repair. Studies suggest that fetal CMs display the best cell type for cardiac repair, which can finitely proliferate, integrate with injured host myocardium, and restore cardiac function. We have recently developed an engineered early embryonic cardiac tissue (EEECT) using embryonic cardiac cells and have shown that EEECT contractile properties and cellular proliferative response to cyclic mechanical stretch stimulation mimic developing fetal myocardium. However, it remains unknown whether cyclic mechanical stretch-mediated high cellular proliferation activity within EEECT reflects CM or non-CM population. Studies have shown that p38-mitogen-activated protein kinase (p38MAPK) plays an important role in both cyclic mechanical stretch stimulation and cellular proliferation. Therefore, in the present study, we tested the hypothesis that cyclic mechanical stretch (0.5 Hz, 5% strain for 48 h) specifically increases EEECT CM proliferation mediated by p38MAPK activity. Cyclic mechanical stretch increased CM, but not non-CM, proliferation and increased p38MAPK phosphorylation. Treatment of EEECT with the p38MAPK inhibitor, SB202190, reduced CM proliferation. The negative CM proliferation effects of SB202190 were not reversed by concurrent stretch stimulation. Results suggest that immature CM proliferation within EEECT can be positively regulated by mechanical stretch and negatively regulated by p38MAPK inhibition.

Nuclear exclusion of forkhead box O and Elk1 and activation of nuclear factor-kappaB are required for C2C12-RasV12C40 myoblast differentiation

Activating ras point mutations are frequently found in skeletal muscle tumors such as rhabdomyosarcomas. In this study we investigated the impact of two different H-ras mutants in skeletal muscle differentiation: RasV12, a constitutively active form, and RasV12C40, a mutant deficient in Raf1 activation. Stably transfected C2C12-RasV12 myoblasts actively proliferated as indicated by the sustained expression of proliferating cell nuclear antigen and retinoblastoma at the hyperphosphorylated state and failed to express differentiation markers. This differentiation-defective phenotype was a consequence of the chronic p44/p42MAPK phosphorylation and the inability of the cells to activate AKT. Moreover, we observed that p44/p42MAPK activation in C2C12-RasV12 myoblasts phosphorylated the ETS-like transcription factor (ELK) 1, which translocates to the nuclei and seemed to be involved in maintaining myoblast proliferation. C2C12-RasV12C40 myoblasts cultured in low serum repressed phosphorylation of p44/p42MAPK and ELK1, resulting in cell cycle arrest and myogenic differentiation. Under this condition, activation of AKT, p70S6K, and p38MAPK was produced, leading to formation of myotubes in 3 d, 1 d earlier than in control C2C12-AU5 cells. Moreover, the expression of muscle-specific proteins, mainly the terminal differentiation markers caveolin-3 and myosin heavy chain, also occurred 1 d earlier than in control cells. Furthermore, AKT activation produced phosphorylation of Forkhead box O that led to nuclear exclusion and inactivation, allowing myogenesis. In addition, we found an induction of nuclear factor-kappaB activity in the nucleus in C2C12-RasV12C40 myotubes attributed to p38MAPK activation. Accordingly, muscle differentiation is associated with a pattern of transcription factors that involves nuclear exclusion ELK1 and Forkhead box O and the increase in nuclear factor-kappaB DNA binding.

Apoptosis signal-regulating kinase 1/p38 signaling pathway negatively regulates physiological hypertrophy

BACKGROUND

Mechanical stress on the heart can lead to crucially different outcomes. Physiological stimuli such as exercise cause adaptive cardiac hypertrophy, characterized by a normal cardiac structure and normal or enhanced cardiac function. Pathological stimuli such as hypertension and aortic valvular stenosis cause maladaptive cardiac remodeling and ultimately heart failure. Apoptosis signal-regulating kinase 1 (ASK1) is known to be involved in pathological cardiac remodeling, but it has not been determined whether ASK1 pathways coordinate the signaling cascade leading to physiological type cardiac growth.

METHODS AND RESULTS

To evaluate the role of ASK1 in the physiological form of cardiac growth, mice lacking ASK1 (ASK1-/-) were exercised by swimming for 4 weeks. ASK1-/- mice showed exaggerated growth of the heart accompanied by typical characteristics of physiological hypertrophy. Their swimming-induced activation of Akt, a key molecule in the signaling cascade of physiological hypertrophy, increased more than that seen in wild-type controls. The activation of p38, a downstream kinase of ASK1, was suppressed selectively in the swimming-exercised ASK1-/- mice. Furthermore, the inhibition of ASK1 or p38 activity enhanced insulin-like growth factor 1-induced protein synthesis in rat neonatal ventricular cardiomyocytes, and the treatment with a specific inhibitor of p38 resulted in enhancement of Akt activation and suppression of protein phosphatase 2A activation. The cardiac-specific p38alpha-deficient mice developed an exacerbated form of cardiac hypertrophy in response to swimming exercise.

CONCLUSIONS

These results indicate that the ASK1/p38 signaling pathway negatively regulates physiological hypertrophy.

PKR is a novel functional direct player that coordinates skeletal muscle differentiation via p38MAPK/AKT pathways

Myogenic differentiation is a highly orchestrated multistep process controlled by extracellular growth factors that modulate largely unknown signals into the cell affecting the muscle-transcription program. P38MAPK-dependent signalling, as well as PI3K/Akt pathway, has a key role in the control of muscle gene expression at different stages during the myogenic process. P38MAPK affects the activities of transcription factors, such as MyoD and myogenin, and contributes, together with PI3K/Akt pathway, to control the early and late steps of myogenic differentiation. The aim of our work was to better define the role of PKR, a dsRNA-activated protein kinase, as potential component in the differentiation program of C2C12 murine myogenic cells and to correlate its activity with p38MAPK and PI3K/Akt myogenic regulatory pathways. Here, we demonstrate that PKR is an essential component of the muscle development machinery and forms a functional complex with p38MAPK and/or Akt, contributing to muscle differentiation of committed myogenic cells in vitro. Inhibition of endogenous PKR activity by a specific (si)RNA and a PKR dominant-negative interferes with the myogenic program of C2C12 cells, causing a delay in activation of myogenic specific genes and inducing the formation of thinner myofibers. In addition, the construction of three PKR mutants allowed us to demonstrate that both N and C-terminal regions of PKR are critical for the interaction with p38MAPK and Akt. The novel discovered complex permits PKR to timely regulate the inhibition/activation of p38MAPK and Akt, controlling in this way the different steps characterizing skeletal muscle differentiation.

Ionizing radiation enhances matrix metalloproteinase-2 secretion and invasion of glioma cells through Src/epidermal growth factor receptor-mediated p38/Akt and phosphatidylinositol 3-kinase/Akt signaling pathways

Glioblastoma is a severe type of primary brain tumor, and its highly invasive character is considered to be a major therapeutic obstacle. Several recent studies have reported that ionizing radiation (IR) enhances the invasion of tumor cells, but the mechanisms for this effect are not well understood. In this study, we investigated the possible signaling mechanisms involved in IR-induced invasion of glioma cells. IR increased the matrix metalloproteinase (MMP)-2 promoter activity, mRNA transcription, and protein secretion along with the invasiveness of glioma cells lacking functional PTEN (U87, U251, U373, and C6) but not those harboring wild-type (WT)-PTEN (LN18 and LN428). IR activated phosphatidylinositol 3-kinase (PI3K), Akt, and mammalian target of rapamycin, and blockade of these kinases by specific inhibitors (LY294002, Akt inhibitor IV, and rapamycin, respectively) and transfection of dominant-negative (DN) mutants (DN-p85 and DN-Akt) or WT-PTEN suppressed the IR-induced MMP-2 secretion in U251 and U373 cells. In addition, inhibitors of epidermal growth factor receptor (EGFR; AG490 and AG1478), Src (PP2), and p38 (SB203580), EGFR neutralizing antibody, and transfection of DN-Src and DN-p38 significantly blocked IR-induced Akt phosphorylation and MMP-2 secretion. IR-induced activation of EGFR was suppressed by PP2, whereas LY294002 and SB203580 did not affect the activations of p38 and PI3K, respectively. Finally, these kinase inhibitors significantly reduced the IR-induced invasiveness of these cells on Matrigel. Taken together, our findings suggest that IR induces Src-dependent EGFR activation, which triggers the p38/Akt and PI3K/Akt signaling pathways, leading to increased MMP-2 expression and heightened invasiveness of PTEN mutant glioma cells.

Creatine enhances differentiation of myogenic C2C12cells by activating both p38 and Akt/PKB pathways

In myogenic C2C12cells, 5 mM creatine increased the incorporation of labeled [35S]methionine into sarcoplasmic (+20%, P < 0.05) and myofibrillar proteins (+50%, P < 0.01). Creatine also promoted the fusion of myoblasts assessed by an increased number of nuclei incorporated within myotubes (+40%, P < 0.001). Expression of myosin heavy chain type II (+1,300%, P < 0.001), troponin T (+65%, P < 0.01), and titin (+40%, P < 0.05) was enhanced by creatine. Mannitol, taurine, and β-alanine did not mimic the effect of creatine, ruling out an osmolarity-dependent mechanism. The addition of rapamycin, the inhibitor of mammalian target of rapamycin/70-kDa ribosomal S6 protein kinase (mTOR/p70s6k) pathway, and SB 202190, the inhibitor of p38, completely blocked differentiation in control cells, and creatine did not reverse this inhibition, suggesting that the mTOR/p70s6kand p38 pathways could be potentially involved in the effect induced by creatine on differentiation. Creatine upregulated phosphorylation of protein kinase B (Akt/PKB; +60%, P < 0.001), glycogen synthase kinase-3 (+70%, P < 0.001), and p70s6k(+50%, P < 0.001). Creatine also affected the phosphorylation state of p38 (−50% at 24 h and +70% at 96 h, P < 0.05) as well as the nuclear content of its downstream targets myocyte enhancer factor-2 (−55% at 48 h and +170% at 96 h, P < 0.05) and MyoD (+60%, P < 0.01). In conclusion, this study points out the involvement of the p38 and the Akt/PKB-p70s6kpathways in the enhanced differentiation induced by creatine in C2C12cells.

Hsp27 regulates Akt activation and polymorphonuclear leukocyte apoptosis by scaffolding MK2 to Akt signal complex

We have shown previously that Akt exists in a signal complex with p38 MAPK, MAPK-activated protein kinase-2 (MK2), and heat shock protein 27 (Hsp27) and MK2 phosphorylates Akt on Ser-473. Additionally, dissociation of Hsp27 from Akt, prior to Akt activation, induced polymorphonuclear leukocyte (PMN) apoptosis. However, the role of Hsp27 in regulating Akt activation was not examined. This study tested the hypothesis that Hsp27 regulates Akt activation and promotes cell survival by scaffolding MK2 to the Akt signal complex. Here we show that loss of Akt/Hsp27 interaction by anti-Hsp27 antibody treatment resulted in loss of Akt/MK2 interaction, loss of Akt-Ser-473 phosphorylation, and induced PMN apoptosis. Transfection of myristoylated Akt (AktCA) in HK-11 cells induced Akt-Ser-473 phosphorylation, activation, and Hsp27-Ser-82 phosphorylation. Cotransfection of AktCA with Hsp27 short interfering RNA, but not scrambled short interfering RNA, silenced Hsp27 expression, without altering Akt expression in HK-11 cells. Silencing Hsp27 expression inhibited Akt/MK2 interaction, inhibited Akt phosphorylation and Akt activation, and induced HK-11 cell death. Deletion mutagenesis studies identified acidic linker region (amino acids 117-128) on Akt as an Hsp27 binding region. Deletion of amino acids 117-128 on Akt resulted in loss of its interaction with Hsp27 and MK2 but not with Hsp90 as demonstrated by immunoprecipitation and glutathione S-transferase pulldown studies. Co-transfection studies demonstrated that constitutively active MK2 (MK2EE) phosphorylated Aktwt (wild type) on Ser-473 but failed to phosphorylate Akt(Delta117-128) mutant in transfixed cells. These studies collectively define a novel role of Hsp27 in regulating Akt activation and cellular apoptosis by mediating interaction between Akt and its upstream activator MK2.

TNF-alpha regulates myogenesis and muscle regeneration by activating p38 MAPK

Although p38 MAPK activation is essential for myogenesis, the upstream signaling mechanism that activates p38 during myogenesis remains undefined. We recently reported that p38 activation, myogenesis, and regeneration in cardiotoxin-injured soleus muscle are impaired in TNF-alpha receptor double-knockout (p55(-/-)p75(-/-)) mice. To fully evaluate the role of TNF-alpha in myogenic activation of p38, we tried to determine whether p38 activation in differentiating myoblasts requires autocrine TNF-alpha, and whether forced activation of p38 rescues impaired myogenesis and regeneration in the p55(-/-)p75(-/-) soleus. We observed an increase of TNF-alpha release from C2C12 or mouse primary myoblasts placed in low-serum differentiation medium. A TNF-alpha-neutralizing antibody added to differentiation medium blocked p38 activation and suppressed differentiation markers myocyte enhancer factor (MEF)-2C, myogenin, p21, and myosin heavy chain in C2C12 myoblasts. Conversely, recombinant TNF-alpha added to differentiation medium stimulated myogenesis at 0.05 ng/ml while inhibited it at 0.5 and 5 ng/ml. In addition, differentiation medium-induced p38 activation and myogenesis were compromised in primary myoblasts prepared from p55(-/-)p75(-/-) mice. Increased TNF-alpha release was also seen in cardiotoxin-injured soleus over the course of regeneration. Forced activation of p38 via the constitutive activator of p38, MKK6bE, rescued impaired myogenesis and regeneration in the cardiotoxin-injured p55(-/-)p75(-/-) soleus. These results indicate that TNF-alpha regulates myogenesis and muscle regeneration as a key activator of p38.

The p38 MAPK signaling pathway: a major regulator of skeletal muscle development

Skeletal muscle development is regulated by extracellular growth factors that transmit largely unknown signals into the cell affecting the muscle-transcription program. One intracellular signaling pathway activated during the differentiation of myogenic cell lines is p38 mitogen-activated protein kinase (MAPK). As a result of modifying the activity of p38 in myoblasts, the pathway proved essential for the expression of muscle-specific genes. P38 affects the activities of transcription factors from the MyoD and MEF2 families and participates in the remodeling of chromatin at specific muscle-regulatory regions. P38 cooperates with the myogenic transcription factors in the activation of a subset of late-transcribed genes, hence contributing to the temporal expression of genes during differentiation. Recent developmental studies with mouse and Xenopus embryos, substantiated and further extended the essential role of p38 in myogenesis. Evidence exists supporting the crucial role for p38 signaling in activating MEF2 transcription factors during somite development in mice. In Xenopus, p38 signaling was shown to be needed for the early expression of Myf5 and for the expression of several muscle structural genes. The emerging data indicate that p38 participates in several stages of the myogenic program.

The phosphoinositide 3-kinase/Akt pathway is essential for the retinoic acid-induced differentiation of F9 cells

Retinoic acid (RA) induces cell growth arrest and differentiation through two families of nuclear receptors, the RARs and the RXRs. The phosphoinositide 3-kinase (PI3K)/Akt pathway also plays key roles in these processes, that is, cell cycle progression, cell differentiation and cell survival. We report that, in mouse embryocarcinoma cells (F9 cells), RA induces an early activation of PI3K and Akt via an increase in the expression of the p85alpha regulatory subunit. This effect is followed by an inhibition of Akt. Both effects require the integrity of the RA pathway as they are not observed in RA-resistant RARgamma null cells. We propose a model through which RA induces a biphasic regulation of Akt with an activation participating to the differentiation process, followed by an inhibition, which has been correlated to the RA-induced growth arrest.

Myostatin regulates cardiomyocyte growth through modulation of Akt signaling

Myostatin is a highly conserved, potent negative regulator of skeletal muscle hypertrophy in many species, from rodents to humans, although its mechanisms of action are incompletely understood. Transcript profiling of hearts from a genetic model of cardiac hypertrophy revealed dramatic upregulation of myostatin, not previously recognized to play a role in the heart. Here we show that myostatin abrogates the cardiomyocyte growth response to phenylephrine in vitro through inhibition of p38 and the serine-threonine kinase Akt, a critical determinant of cell size in many species from drosophila to mammals. Evaluation of male myostatin-null mice revealed that their cardiomyocytes and hearts overall were slightly smaller at baseline than littermate controls but exhibited more exuberant growth in response to chronic phenylephrine infusion. The increased cardiac growth in myostatin-null mice corresponded with increased p38 phosphorylation and Akt activation in vivo after phenylephrine treatment. Together, these data demonstrate that myostatin is dynamically regulated in the heart and acts more broadly than previously appreciated to regulate growth of multiple types of striated muscle.

Muc4-ErbB2 complex formation and signaling in polarized CACO-2 epithelial cells indicate that Muc4 acts as an unorthodox ligand for ErbB2

Muc4 serves as an intramembrane ligand for the receptor tyrosine kinase ErbB2. The time to complex formation and the stoichiometry of the complex were determined to be <15 min and 1:1 by analyses of Muc4 and ErbB2 coexpressed in insect cells and A375 tumor cells. In polarized CACO-2 cells, Muc4 expression causes relocalization of ErbB2, but not its heterodimerization partner ErbB3, to the apical cell surface, effectively segregating the two receptors. The apically located ErbB2 is phosphorylated on tyrosines 1139 and 1248. The phosphorylated ErbB2 in CACO-2 cells recruits the cytoplasmic adaptor protein Grb2, consistent with previous studies showing phosphotyrosine 1139 to be a Grb2 binding site. To address the issue of downstream signaling from apical ErbB2, we analyzed the three MAPK pathways of mammalian cells, Erk, p38, and JNK. Consistent with the more differentiated phenotype of the CACO-2 cells, p38 phosphorylation was robustly increased by Muc4 expression, with a consequent activation of Akt. In contrast, Erk and JNK phosphorylation was not changed. The ability of Muc4 to segregate ErbB2 and other ErbB receptors and to alter downstream signaling cascades in polarized epithelial cells suggests that it has a role in regulating ErbB2 in differentiated epithelia.