Cyclic mechanical strain inhibits skeletal myogenesis through activation of focal adhesion kinase, Rac-1 GTPase, and NF-kappaB transcription factor

The vascular-disrupting agent combretastatin impairs splitting and sprouting forms of physiological angiogenesis

OBJECTIVE

Vascular-disrupting agents like combretastatin (CA-4-P), used to attenuate tumor blood flow in vivo, exert anti-mitotic and anti-migratory effects on endothelial cells in vitro. We tested whether anti-vascular or anti-angiogenic effects of CA-4-P are evident with physiological angiogenesis in skeletal muscle (EDL) due to sustained hyperemia (intraluminal splitting) and chronic muscle overload (abluminal sprouting).

METHODS

CA-4-P was given i.v. (25 mg/kg on alternate days for 14 days) to mice subjected to angiogenic stimuli (prazosin or synergist extirpation). The responses of femoral artery blood flow as well as capillarity, capillary ultrastructure, and levels of Rho GTPase were measured.

RESULTS

Blood flow was unaffected in the sprouting angiotype, but decreased in the splitting angiotype, by CA-4-P. In contrast, CA-4-P attenuated the capillarity increase in both models, associated with reduced lamellipodia and filopodia formation. Muscle overload, but not hyperemia, was accompanied by an increase in Rho GTPase with CA-4-P.

CONCLUSIONS

CA-4-P impaired the angiogenic response in both experimental models. This inhibitory effect was associated with a lower increase in femoral blood flow in splitting, whereas sprouting angiogenesis was accompanied by higher Rho activity consistent with the interruption of actin polymerization. Thus, CA-4-P may exert context-dependent anti-vascular and anti-angiogenic effects in vivo under physiological conditions.

Effect of cyclic hydrodynamic pressure-induced proliferation of human bladder smooth muscle through Ras-related C3 botulinum toxin substrate 1, mitogen-activated protein kinase kinase 1/2 and extracellular regulated protein kinases 1/2

OBJECTIVES

To examine the role of Ras-related C3 botulinum toxin substrate 1, mitogen-activated protein kinase kinase 1/2 and extracellular regulated protein kinases 1/2 in the cyclic hydrodynamic pressure-induced proliferation of human bladder smooth muscle cells.

METHODS

Human bladder smooth muscle cells were exposed to cyclic hydrodynamic pressures in vitro with defined parameters (static, 100 cmH(2) O, 200 cmH(2) O and 300 cmH(2) O pressure) for 24 h. The proliferation of cells was assessed by flow cytometry. Ras-related C3 botulinum toxin substrate 1, mitogen-activated protein kinase kinase 1/2 and extracellular regulated protein kinases 1/2 messenger ribonucleic acid, and protein expression was analyzed by real-time polymerase chain reaction and Western blot. Specificity of the Rac1 was determined with real-time polymerase chain reaction and Western blot technique with small interfering ribonucleic acid transfection and Rac1 inhibitor (NSC23766).

RESULTS

The proliferation of human bladder smooth muscle cells was increased. Ras-related C3 botulinum toxin substrate 1, mitogen-activated protein kinase kinase 1/2 and extracellular regulated protein kinases 1/2 were activated by 200 and 300 cmH(2) O cyclic hydrodynamic pressure compared with static and 100 cmH(2) O pressure. The "knockdown" of activation of Rac1 using target small interfering ribonucleic acid transfection and Rac1 inhibitor (NSC23766) decreased proliferation of human bladder smooth muscle cells, and downregulated mitogen-activated protein kinase kinase 1/2, extracellular regulated protein kinases 1/2.

CONCLUSION

The Rac1 pathway is activated in mechanotransduction and regulation of human bladder smooth muscle cell proliferation in response to cyclic hydrodynamic pressure.

Focal adhesion kinase regulation of mechanotransduction and its impact on endothelial cell functions

Vascular endothelial cells lining the blood vessels form the interface between the bloodstream and the vessel wall and as such they are continuously subjected to shear and cyclic stress from the flowing blood in the lumen. Additional mechanical stimuli are also imposed on these cells in the form of substrate stiffness transmitted from the extracellular matrix components in the basement membrane, and additional mechanical loads imposed on the lung endothelium as the result of respiration or mechanical ventilation in clinical settings. Focal adhesions (FAs) are complex structures assembled at the abluminal endothelial plasma membrane which connect the extracellular filamentous meshwork to the intracellular cytoskeleton and hence constitute the ideal checkpoint capable of controlling or mediating transduction of bidirectional mechanical signals. In this review we focus on focal adhesion kinase (FAK), a component of FAs, which has been studied for a number of years with regards to its involvement in mechanotransduction. We analyzed the recent advances in the understanding of the role of FAK in the signaling cascade(s) initiated by various mechanical stimuli with particular emphasis on potential implications on endothelial cell functions.

Loss of desmin triggers mechanosensitivity and up-regulation of Ankrd1 expression through Akt-NF-κB signaling pathway in smooth muscle cells

Muscle cells, including human airway smooth muscle cells (HASMCs) express ankyrin repeat protein 1 (Ankrd1), a member of ankyrin repeat protein family. Ankrd1 efficiently interacts with the type III intermediate filament desmin. Our earlier study showed that desmin is an intracellular load-bearing protein that influences airway compliance, lung recoil, and airway contractile responsiveness. These results suggest that Ankrd1 and desmin may play important roles on ASMC homeostasis. Here we show that small interfering (si)RNA-mediated knockdown of the desmin gene in HASMCs, recombinant HASMCs (reHASMCs), up-regulates Ankrd1 expression. Moreover, loss of desmin in HASMCs increases the phosphorylation of Akt, inhibitor of κB kinase (IKK)-α, and inhibitor of κB (IκB)-α proteins, leading to NF-κB activation. Treatment of reHASMCs with Akt, IKKα, IκBα, or NF-κB inhibitor inhibits the loss of desmin-induced Ankrd1 up-regulation, suggesting Akt/NF-κB-mediated Ankrd1 regulation. Transfection of reHASMCs with siRNA specific for p50 or p65 corroborates the NF-κB-mediated Ankrd1 regulation. Luciferase reporter assays show that NF-κB directly binds on Ankrd1 promoter and up-regulates Ankrd1 levels. Overall, our data provide a new link between desmin and Ankrd1 regulation, which may be important for ASMC homeostasis.

Mechanical signaling through the cytoskeleton regulates cell proliferation by coordinated focal adhesion and Rho GTPase signaling

The notion that cell shape and spreading can regulate cell proliferation has evolved over several years, but only recently has this been linked to forces from within and upon the cell. This emerging area of mechanical signaling is proving to be wide-spread and important for all cell types. The microenvironment that surrounds cells provides a complex spectrum of different, simultaneously active, biochemical, structural and mechanical stimuli. In this milieu, cells probe the stiffness of their microenvironment by pulling on the extracellular matrix (ECM) and/or adjacent cells. This process is dependent on transcellular cell-ECM or cell-cell adhesions, as well as cell contractility mediated by Rho GTPases, to provide a functional linkage through which forces are transmitted through the cytoskeleton by intracellular force-generating proteins. This Commentary covers recent advances in the underlying mechanisms that control cell proliferation by mechanical signaling, with an emphasis on the role of 3D microenvironments and in vivo extracellular matrices. Moreover, as there is much recent interest in the tumor-stromal interaction, we will pay particular attention to exciting new data describing the role of mechanical signaling in the progression of breast cancer.

Can human mesenchymal stem cells survive on a NiTi implant material subjected to cyclic loading?

Nickel-titanium shape memory alloys (NiTi-SMAs) exhibit mechanical and chemical properties which make them attractive candidate materials for various types of biomedical applications. However, the high nickel content of NiTi-SMAs may result in adverse tissue reactions, especially when they are considered for load-bearing implants. It is generally assumed that a protective titanium oxide layer separates the metallic alloy from its environment and that this explains the good biocompatibility of NiTi. Cyclic loading may result in failure of the protective oxide layer. The scientific objective of this work was to find out whether cyclic dynamic strain, in a range relevant for orthopedic implants, diminishes the biocompatibility of NiTi-SMAs. In order to analyze the biocompatibility of NiTi-SMA surfaces subjected to cyclic loading, NiTi-SMA tensile specimens were preloaded with mesenchymal stem cells, transferred to a sterile cell culture system and fixed to the pull rods of a tensile testing machine. Eighty-six thousand and four hundred strain cycles at 2% pseudoelastic strain were performed for a period of 24 h or 7 days. Cytokines (IL-6, IL-8 and VEGF) and nickel ion release were determined within the cell culture medium. Adherent cells on the tensile specimens were stained with calcein-AM and propidium iodide to determine cell viability. Dynamic loading of the tensile specimens did not influence the viability of adherent human mesenchymal stem cells (hMSCs) after 24 h or 7 days compared with the non-strained control. Dynamic cycles of loading and unloading did not affect nickel ion release from the tensile specimens. The release of IL-6 from hMSCs cultured under dynamic conditions was significantly higher after mechanical load (873 pg ml(-1)) compared with static conditions (323 pg ml(-1)). The present work demonstrates that a new type of mechanical in vitro cell culture experiment can provide information which previously could only be obtained in large animal experiments.

S100B in myoblasts regulates the transition from activation to quiescence and from quiescence to activation and reduces apoptosis

S100B protein activates IKKβ/NF-κB within myoblasts, thereby inhibiting the expression of MyoD and the MyoD-downstream effectors, myogenin and p21(WAF1), and myoblast differentiation. Herein we show that myoblasts downregulate S100B expression once transferred from proliferation medium to differentiation medium via a p38 MAPK-driven transcriptional mechanism as well as a post-translational, proteasome-dependent mechanism, and that myoblasts that have not been committed to differentiation resume expressing S100B once transferred back to proliferation medium. Likewise, myoblasts downregulate S100B expression once transferred to quiescence medium, and interference with S100B downregulation as obtained by stable overexpression of the protein results in reduced acquisition of quiescence and a faster proliferation upon transfer of the cells from quiescence medium to proliferation medium, compared to controls. These latter effects are dependent on S100B-induced activation of JNK. Moreover, S100B reduces myoblast apoptosis in an MEK-ERK1/2, Akt, JNK, and NF-κB-dependent manner. However, myogenin(+) myoblasts (i.e., myocytes) and myotubes abundantly express S100B likely induced by myogenin. Our results suggest that (1) a timely repression of S100B expression is required for efficient myogenic differentiation; (2) S100B plays an important role in the expansion of the activated (i.e., proliferating) myoblast population; (3) under conditions associated with enhanced expression of S100B, the transition from proliferation to quiescence and from quiescence to proliferation might be altered; and (4) S100B exerts different regulatory effects in myoblasts and myocytes/myotubes/myofibers. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Nebulette mutations are associated with dilated cardiomyopathy and endocardial fibroelastosis

OBJECTIVES

Four variants (K60N, Q128R, G202R, and A592E) in the nebulette gene were identified in patients with dilated cardiomyopathy (DCM) and endocardial fibroelastosis. We sought to determine if these mutations are cardiomyopathy causing.

BACKGROUND

Nebulette aligns thin filaments and connects them with the myocardial Z-disk, playing a role in mechanosensation.

METHODS

We generated transgenic mice with cardiac-restricted overexpression of human wild-type or mutant nebulette. Chimera and transgenic mice were examined at 4, 6, and 12 months of age by echocardiography and cardiac magnetic resonance imaging. The hearts from embryos and adult mice were assessed by histopathologic, immunohistochemical, ultrastructural, and protein analyses. Rat H9C2 cardiomyoblasts with transient expression of nebulette underwent cyclic mechanical strain.

RESULTS

We identified lethal cardiac structural abnormalities in mutant embryonic hearts (K60N and Q128R). Founders of the mutant mouse lines developed DCM with severe heart failure. An irregular localization pattern for nebulette and impaired desmin expression were noted in the proband and chimeric Q128R mice. Mutant G202R and A592E mice exhibited left ventricular dilation and impaired function with specific changes in I-band and Z-disk proteins by 6 months of age. The mutations modulated distribution of nebulette in the sarcomere and Z-disk during stretch of H9C2 cells.

CONCLUSIONS

Nebulette is a new susceptibility gene for endocardial fibroelastosis and DCM. Different mutations in nebulette trigger specific mechanisms, converging to a common pathological cascade leading to endocardial fibroelastosis and DCM.

Skeletal muscle myoblasts possess a stretch-responsive local angiotensin signalling system

A paucity of information exists regarding the presence of local renin—angiotensin systems (RASs) in skeletal muscle and associated muscle stem cells. Skeletal muscle and muscle stem cells were isolated from C57BL/6 mice and examined for the presence of a local RAS using quantitative reverse transcriptase polymerase chain reaction (qRT-PCR), immunohistochemistry (IHC), Western blotting and liquid chromatography-mass spectrometry (LC-MS). Furthermore, the effect of mechanical stimulation on RAS member gene expression was analysed. Whole skeletal muscle, primary myoblasts and C2C12 derived myoblasts and myotubes differentially expressed members of the RAS including angiotensinogen, angiotensin-converting enzyme (ACE), angiotensin II (Ang II) type 1 (AT1) and type 2 (AT2). Renin transcripts were never detected, however, mRNA for the ‘renin-like’ enzyme cathepsin D was observed and Ang I and Ang II were identified in cell culture supernatants from proliferating myoblasts. AT1 appeared to co-localise with polymerised actin filaments in proliferating myoblasts and was primarily found in the nucleus of terminally differentiated myotubes. Furthermore, mechanical stretch of proliferating and differentiating C2C12 cells differentially induced mRNA expression of angiotensinogen, AT 1 and AT2. Proliferating and differentiated muscle stem cells possess a local stress-responsive RAS in vitro. The precise function of a local RAS in myoblasts remains unknown. However, evidence presented here suggests that Ang II may be a regulator of skeletal muscle myoblasts.

Activation of NF-kappaB by fluid shear stress, but not TNF-alpha, requires focal adhesion kinase in osteoblasts

When bone is mechanically loaded fluid shear stress (FSS) is generated as a result of the movement of interstitial fluid across the membranes of osteoblasts and osteocytes. This external mechanical loading stimulates changes in the activity of cytoplasmic signaling molecules and alters gene expression in bone cells. This process, referred to as mechanotransduction, is vital for maintaining bone health in vivo by regulating the balance between bone formation and bone resorption. This current study focuses on the role of focal adhesions, sites of integrin-mediated cellular attachment to the extracellular matrix, and their proposed function as mechanosensors in bone cells. We examined the role of a key component of focal adhesions and of mechanotransduction, focal adhesion kinase (FAK) in regulation of FSS- and tumor necrosis factor-alpha (TNF-alpha)-induced activation of nuclear factor-kappa B (NF-kappaB) signaling in osteoblasts. Immortalized FAK(+/+) and FAK(-)(/)(-) osteoblasts were exposed to periods of oscillatory fluid shear stress (OFF) and NF-kappaB activation was analyzed. We determined that FAK is required for OFF-induced nuclear translocation and activation of NF-kappaB in osteoblasts. In addition we found that OFF-induced phosphorylation of the IkappaB kinases (IKKalpha/beta) in both FAK(+/+) and FAK(-/-) osteoblasts, but only FAK(+/+) osteoblasts demonstrated the resulting degradation of NF-kappaB inhibitors IkappaBalpha and IkappaBbeta. OFF did not induce the degradation of IkappaBepsilon or the processing of p105 in either FAK(+/+) and FAK(-/-) osteoblasts. To compare the role of FAK in mediating OFF-induced mechanotransduction to the well characterized activation of NF-kappaB by inflammatory cytokines, we exposed FAK(+/+) and FAK(-/-) osteoblasts to TNF-alpha. Interestingly, FAK was not required for TNF-alpha induced NF-kappaB activation in osteoblasts. In addition we determined that TNF-alpha treatment did not induce the degradation of IkappaBbeta as did OFF. These data indicate a novel relationship between FAK and NF-kappaB activation in osteoblast mechanotransduction and demonstrates that the mechanism of FSS-induced NF-kappaB activation in osteoblasts differs from the well characterized TNF-alpha-induced activation.

NF-kappaB signaling: a tale of two pathways in skeletal myogenesis

NF-kappaB is a ubiquitiously expressed transcription factor that plays vital roles in innate immunity and other processes involving cellular survival, proliferation, and differentiation. Activation of NF-kappaB is controlled by an IkappaB kinase (IKK) complex that can direct either canonical (classical) NF-kappaB signaling by degrading the IkappaB inhibitor and releasing p65/p50 dimers to the nucleus, or causes p100 processing and nuclear translocation of RelB/p52 via a noncanonical (alternative) pathway. Under physiological conditions, NF-kappaB activity is transiently regulated, whereas constitutive activation of this transcription factor typically in the classical pathway is associated with a multitude of disease conditions, including those related to skeletal muscle. How NF-kappaB functions in muscle diseases is currently under intense investigation. Insight into this role of NF-kappaB may be gained by understanding at a more basic level how this transcription factor contributes to skeletal muscle cell differentiation. Recent data from knockout mice support that the classical NF-kappaB pathway functions as an inhibitor of skeletal myogenesis and muscle regeneration acting through multiple mechanisms. In contrast, alternative NF-kappaB signaling does not appear to be required for myofiber conversion, but instead functions in myotube homeostasis by regulating mitochondrial biogenesis. Additional knowledge of these signaling pathways in skeletal myogenesis should aid in the development of specific inhibitors that may be useful in treatments of muscle disorders.

S100B protein in myoblasts modulates myogenic differentiation via NF-kappaB-dependent inhibition of MyoD expression

S100B, a Ca(2+)-binding protein of the EF-hand type, is expressed in myoblasts, the precursors of skeletal myofibers, and muscle satellite cells (this work). S100B has been shown to participate in the regulation of several intracellular processes including cell cycle progression and differentiation. We investigated regulatory activities of S100B within myoblasts by stable overexpression of S100B and by inhibition of S100B expression. Overexpression of S100B in myoblast cell lines and primary myoblasts resulted in inhibition of myogenic differentiation, evidenced by lack of expression of myogenin and myosin heavy chain (MyHC) and absence of myotube formation. S100B-overexpressing myoblasts showed reduced MyoD expression levels and unchanged Myf5 expression levels, compared with control myoblasts, and transient transfection of S100B-overexpressing myoblasts with MyoD, but not Myf5, restored differentiation and fusion in part. The transcriptional activity of NF-kappaB, a negative regulator of MyoD expression, was enhanced in S100B-overexpressing myoblasts, and blocking NF-kappaB activity resulted in reversal of S100B's inhibitory effects. Yin Yang1, a transcriptional repressor that is induced by NF-kappaB (p65) and mediates NF-kappaB inhibitory effects on several myofibrillary genes, also was upregulated in S100B-overexpressing myoblasts. Conversely, silencing S100B expression in myoblast cell lines by RNA interference resulted in reduced NF-kappaB activity and enhanced MyoD, myogenin and MyHC expression and myotube formation. Thus, intracellular S100B might modulate myoblast differentiation by interfering with MyoD expression in an NF-kappaB-dependent manner.

ERK regulates strain-induced migration and proliferation from different subcellular locations

Repetitive deformation like that engendered by peristalsis or villous motility stimulates intestinal epithelial proliferation on collagenous substrates and motility across fibronectin, each requiring ERK. We hypothesized that ERK acts differently at different intracellular sites. We stably transfected Caco-2 cells with ERK decoy expression vectors that permit ERK activation but interfere with its downstream signaling. Targeting sequences constrained the decoy inside or outside the nucleus. We assayed proliferation by cell counting and migration by circular wound closure with or without 10% repetitive deformation at 10 cycles/min. Confocal microscopy confirmed localization of the fusion proteins. Inhibition of phosphorylation of cytoplasmic RSK or nuclear Elk confirmed functionality. Both the nuclear-localized and cytosolic-localized ERK decoys prevented deformation-induced proliferation on collagen. Deformation-induced migration on fibronectin was prevented by constraining the decoy in the nucleus but not in the cytosol. Like the nuclear-localized ERK decoy, a Sef-overexpressing adenovirus that sequesters ERK in the cytoplasm also blocked the motogenic and mitogenic effects of strain. Inhibiting RSK or reducing Elk ablated both the mitogenic and motogenic effects of strain. RSK isoform reduction revealed isoform specificity. These results suggest that ERK must translocate to the nucleus to stimulate cell motility while ERK must act in both the cytosol and the nucleus to stimulate proliferation in response to strain. Selectively targeting ERK within different subcellular compartments may modulate or replace physical force effects on the intestinal mucosa to maintain the intestinal mucosal barrier in settings when peristalsis or villous motility are altered and fibronectin is deposited into injured tissue.

Up-regulated alpha-actin expression is associated with cell adhesion ability in 3-D cultured myocytes subjected to mechanical stimulation

This study was aimed to investigate the alteration of alpha-actin in three-dimensionally (3-D) cultured myocytes under cyclic tensile stress loading. Myocytes were collected from neonatal SD rat's lateral pterygoid muscle for primary cell culture. The third-passage cells were implanted and 3-D cultured in poly lactic-co-glycolic acid (PLGA) scaffold, and then subjected to cyclic tensile stress (0.5 Hz, 2,000 microstrain) for 0, 2, 4, 8, 12, and 24 h through a four-point bending strain system. The alpha-actin mRNA was investigated by semi-quantitative RT-PCR. The alpha-actin protein expression was examined by immunofluorescent cytochemistry, laser confocal scanning microscopy (LCSM), and image analysis technology. The dynamic adhesion of myocytes to PLGA scaffolds was investigated by fluorescence microscope and the viability of the myocytes was measured by MTT assay. After mechanical loading, the alpha-actin mRNA increased at 2 h and then declined. The alpha-actin protein expression kept increased until peaked at 12 h, but declined at 24 h. The time course changing of alpha-actin protein expression parallelled with that of cell adhesion ability. It is concluded that alpha-actin expression is probably associated with cell adhesion ability in myocytes subjected to mechanical stimulation.

Effect of Electromechanical Stimulation on the Maturation of Myotubes on Aligned Electrospun Fibers

Tissue engineering may provide an alternative to cell injection as a therapeutic solution for myocardial infarction. A tissue-engineered muscle patch may offer better host integration and higher functional performance. This study examined the differentiation of skeletal myoblasts on aligned electrospun polyurethane (PU) fibers and in the presence of electromechanical stimulation. Skeletal myoblasts cultured on aligned PU fibers showed more pronounced elongation, better alignment, higher level of transient receptor potential cation channel-1 (TRPC-1) expression, upregulation of contractile proteins and higher percentage of striated myotubes compared to those cultured on random PU fibers and film. The resulting tissue constructs generated tetanus forces of 1.1 mN with a 10-ms time to tetanus. Additional mechanical, electrical, or synchronized electromechanical stimuli applied to myoblasts cultured on PU fibers increased the percentage of striated myotubes from 70 to 85% under optimal stimulation conditions, which was accompanied by an upregulation of contractile proteins such as α-actinin and myosin heavy chain. In describing how electromechanical cues can be combined with topographical cue, this study helped move towards the goal of generating a biomimetic microenvironment for engineering of functional skeletal muscle.

Mechanical stretch decreases migration of alveolar epithelial cells through mechanisms involving Rac1 and Tiam1

Mechanical ventilation can overdistend the lungs or generate shear forces in them during repetitive opening/closing, contributing to lung injury and inflammation in patients with acute respiratory distress syndrome (ARDS). Repair of the injured lung epithelium is important for restoring normal barrier and lung function. In the current study, we investigated the effects of cyclic mechanical strain (CS), constant distention strain (CD), and simulated positive end-expiratory pressure (PEEP) on activation of Rac1 and wound closure of rat primary alveolar type 2 (AT2) cells. Cyclic stretch inhibited the migration of wounded AT2 cells in a dose-dependent manner with no inhibition occurring with 5% CS, but significant inhibition with 10% and 15% CS. PEEP conditions were investigated by stretching AT2 cells to 15% maximum strain (at a frequency of 10 cycles/min) with relaxation to 10% strain. AT2 cells were also exposed to 20% CD. All three types of mechanical strain inhibited wound closure of AT2 cells compared with static controls. Since lamellipodial extensions in migrating cells at the wound edge were significantly smaller in stretched cells, we measured Rac1 activity and found it to be decreased in stretched cells. We also demonstrate that Tiam1, a Rac1-specific guanine nucleotide exchange factor, was expressed mainly in the cytosol of AT2 cells exposed to mechanical strain compared with membrane localization in static cells. Downregulation of Tiam1 with 100 microM NSC-23766 inhibited activation of Rac1 and migration of AT2 cells, suggesting its involvement in repair mechanisms of AT2 cells subjected to mechanical strain.

Tumor-derived endothelial cells exhibit aberrant Rho-mediated mechanosensing and abnormal angiogenesis in vitro

Tumor blood vessels exhibit abnormal structure and function that cause disturbed blood flow and high interstitial pressure, which impair delivery of anti-cancer agents. Past efforts to normalize the tumor vasculature have focused on inhibition of soluble angiogenic factors, such as VEGF; however, capillary endothelial (CE) cell growth and differentiation during angiogenesis are also influenced by mechanical forces conveyed by the extracellular matrix (ECM). Here, we explored the possibility that tumor CE cells form abnormal vessels because they lose their ability to sense and respond to these physical cues. These studies reveal that, in contrast to normal CE cells, tumor-derived CE cells fail to reorient their actin cytoskeleton when exposed to uniaxial cyclic strain, exhibit distinct shape sensitivity to variations in ECM elasticity, exert greater traction force, and display an enhanced ability to retract flexible ECM substrates and reorganize into tubular networks in vitro. These behaviors correlate with a constitutively high level of baseline activity of the small GTPase Rho and its downstream effector, Rho-associated kinase (ROCK). Moreover, decreasing Rho-mediated tension by using the ROCK inhibitor, Y27632, can reprogram the tumor CE cells so that they normalize their reorientation response to uniaxial cyclic strain and their ability to form tubular networks on ECM gels. Abnormal Rho-mediated sensing of mechanical cues in the tumor microenvironment may therefore contribute to the aberrant behaviors of tumor CE cells that result in the development of structural abnormalities in the cancer microvasculature.

Repetitive deformation activates Src-independent FAK-dependent ERK motogenic signals in human Caco-2 intestinal epithelial cells

Repetitive deformation due to villous motility or peristalsis may support the intestinal mucosa, stimulating intestinal epithelial proliferation under normal circumstances and restitution in injured and inflamed mucosa rich in tissue fibronectin. Cyclic strain enhances Caco-2 and IEC-6 intestinal epithelial cell migration across fibronectin via ERK. However, the upstream mediators of ERK activation are unknown. We investigated whether Src and FAK mediate strain-induced ERK phosphorylation and migration in human Caco-2 intestinal epithelial cells on fibronectin. Monolayers on tissue fibronectin-precoated membranes were subjected to an average 10% repetitive deformation at 10 cycles/min. Phosphorylation of Src-Tyr 418, FAK-Tyr 397-Tyr 576-Tyr 925, and ERK were significantly increased by deformation. The stimulation of wound closure by strain was prevented by Src blockade with PP2 (10 micromol/l) or specific short interfering (si)RNA. Src inhibition also prevented strain-induced FAK phosphorylation at Tyr 397 and Tyr 576 but not FAK-Tyr 925 or ERK phosphorylation. Reducing FAK by siRNA inhibited strain-induced ERK phosphorylation. Transfection of NH2-terminal tyrosine phosphorylation-deficient FAK mutants Y397F, Y576F-Y577F, and Y397F-Y576F-Y577F did not prevent the activation of ERK2 by cyclic strain, but a FAK mutant at the COOH terminal (Y925F) prevented the strain-induced activation of ERK2. Although the Y397F-Y576F-Y577F FAK construct exhibited less basal FAK-Tyr 925 phosphorylation under static conditions, it nevertheless exhibited increased FAK-Tyr 925 phosphorylation in response to strain. These results suggest that repetitive deformation stimulates intestinal epithelial motility across fibronectin in a manner that requires both Src activation and a novel Src-independent FAK-Tyr 925-dependent pathway that activates ERK. This pathway may be an important target for interventions to promote mucosal healing in settings of intestinal ileus or fasting.

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.

Tumor necrosis factor-alpha augments matrix metalloproteinase-9 production in skeletal muscle cells through the activation of transforming growth factor-beta-activated kinase 1 (TAK1)-dependent signaling pathway

We have investigated the effect of tumor necrosis factor-alpha (TNF-alpha) on the production of extracellular matrix-degrading proteases in skeletal muscles. Using microarray, quantitative PCR, Western blotting, and zymography, we found that TNF-alpha drastically increases the production of matrix metalloproteinase (MMP)-9 from C2C12 myotubes. In vivo administration of TNF-alpha in mice increased the transcript level of MMP-9 in skeletal muscle tissues. Although TNF-alpha activated all the three MAPKs (i.e. ERK1/2, JNK, and p38), inhibition of ERK1/2 or p38 but not JNK blunted the TNF-alpha-induced production of MMP-9 from myotubes. Inhibition of Akt also inhibited the TNF-alpha-induced production of MMP-9. TNF-alpha increased the activation of transcription factors NF-kappaB and AP-1 but not SP-1 in myotubes. Overexpression of a dominant negative inhibitor of NF-kappaB or AP-1 blocked the TNF-alpha-induced expression of MMP-9 in myotubes. Similarly, point mutations in AP-1- or NF-kappaB-binding sites in MMP-9 promoter inhibited the TNF-alpha-induced expression of a reporter gene. TNF-alpha increased the activity of transforming growth factor-beta-activating kinase-1 (TAK1). Furthermore, overexpression of a dominant negative mutant of TAK1 blocked the TNF-alpha-induced expression of MMP-9 and activation of NF-kappaB and AP-1. Our results also suggest that TNF-alpha induces MMP-9 expression in muscle cells through the recruitment of TRAF-2, Fas-associated protein with death domain, and TNF receptor-associated protein with death domain but not NIK or TRAF-6 proteins. We conclude that TAK1-mediated pathways are involved in TNF-alpha-induced MMP-9 production in skeletal muscle cells.

In ovo temperature manipulation influences embryonic motility and growth of limb tissues in the chick (Gallus gallus)

The chick embryo, developing in the egg, is an ideal system in which to investigate the effects of incubation environment on the development of the embryo. We show that raising the temperature of the eggs by just one degree, from 37.5 degrees C to 38.5 degrees C, during embryonic days (ED) 4-7 causes profound changes in development. We demonstrate that embryonic movement is significantly increased in the chicks raised at 38.5 degrees C both during the period in which they are at the higher temperature but also 4 days after their return to the control temperature. Concomitant with this increase in embryonic activity, the embryos raised at higher temperature grow to significantly heavier weights and exhibit significantly longer leg bones (tibia and tarsus) than the controls from ED12 onwards, although mineralization occurs normally. Additionally, the number of leg myonuclei is increased from ED12 in the embryos raised at the higher temperature. This is likely to promote greater leg muscle growth later in development, which may provide postural stability to the chicks posthatch. These changes are similar to those seen when drugs are injected to increase embryonic activity. We therefore believe that the increased embryonic activity provides a mechanism that can explain the increased growth of leg muscle and bone seen when the eggs are incubated for 3 days at higher temperature.

Effect of cyclic stretch on β1D-integrin expression and activation of FAK and RhoA

Integrins play a pivotal role in proliferation, differentiation, and survival in skeletal and cardiac myocytes. The β1D-isoform of the β1-integrin is specifically expressed in striated skeletal muscle. However, little is known about the role and the mechanisms by which the splice variant β1D-integrin regulates myogenesis and mechanotransduction. We observed that cyclic mechanical stretch increases β1D-integrin protein levels and activates the downstream cytoskeletal signaling proteins focal adhesion kinase (FAK) and RhoA. Elimination of native β1D-integrin expression by RNA interference in immature developing myoblasts abolished stretch-induced increases in FAK phosphorylation and further downregulated RhoA activity. Blocking of β1D-integrin expression prevented myocellular fusion to form multinucleated mature myotubes. Restoration of human β1D-integrin expression in β1D-integrin-deficient cells partially restored myotube formation. The onset of myofusion also requires the generation of nitric oxide (NO). The release of NO affects cytoskeletal proteins by mediating RhoA activity and protein degradation. Our previous study demonstrated that stretch-induced NO positively modulates mechanical properties of differentiating skeletal myocytes. We found a significant decrease in NO production and apparent elastic modulus in β1D-integrin-deficient cells, suggesting signaling interactions between β1D-integrin and neuronal NO synthase to mediate mechanotransduction and myogenesis in skeletal myocytes. These results suggest that, in addition to regulating differentiation, the β1D-integrin isoform plays a critical role in the response of skeletal myoblasts to cyclic stretch by activating the downstream components of FAK and RhoA activity and affecting NO release.

Uniaxial stretch-induced regulation of mitogen-activated protein kinase, Akt and p70 S6 kinase in the ageing Fischer 344 x Brown Norway rat aorta

The effects of ageing on the cardiovascular system contribute to substantial alterations in cellular morphology and function. The variables regulating these changes are unknown; however, one set of signalling molecules that may be of particular importance in mediating numerous cellular responses, including control of cell growth, differentiation and adaptation, are the proteins associated with the mitogen-activated protein kinase (MAPK) signalling systems. The MAPKs, in conjunction with the p70 S6k signalling cascade, have emerged as critical components for regulating numerous mechanotransduction-related cellular responses. Here we investigate the ability of uniaxial stretch to activate the MAPK and p70 S6k pathways in adult (6-month-old), aged (30-month-old) and very aged (36-month-old) Fischer 344/NNiaHSd x Brown Norway/BiNia (FBN) rats. Western blotting of the MAPK family proteins extracellular signal-regulated kinase (Erk) 1/2, p38- and c-Jun NH(2)-terminal kinase (Jnk)-MAPKs showed differential expression and activation between these proteins with age. An acute 15 min interval of 20% uniaxial stretch using an ex vivo aortic preparation demonstrated similar regulation of Erk1/2, p38- and Jnk-MAPK. However, ageing altered uniaxial induced p70 S6k pathway signalling. These observations confirm previous data demonstrating that MAPK proteins are mechanically regulated and also suggest that p70 S6k signalling expression and activation are controlled differently with ageing. Taken together, these data may help to explain, in part, the age-related changes in vascular morphology, function and response to injury.

Fibroblast growth factor inducible 14 (Fn14) is required for the expression of myogenic regulatory factors and differentiation of myoblasts into myotubes. Evidence for TWEAK-independent functions of Fn14 during myogenesis

Fibroblast growth factor-inducible 14 (Fn14), distantly related to tumor necrosis factor receptor superfamily and a receptor for TWEAK cytokine, has been implicated in several biological responses. In this study, we have investigated the role of Fn14 in skeletal muscle formation in vitro. Flow cytometric and Western blot analysis revealed that Fn14 is highly expressed on myoblastic cell line C2C12 and mouse primary myoblasts. The expression of Fn14 was decreased upon differentiation of myoblasts into myotubes. Suppression of Fn14 expression using RNA interference inhibited the myotube formation in both C2C12 and primary myoblast cultures. Fn14 was required for the transactivation of skeletal alpha-actin promoter and the expression of specific muscle proteins such as myosin heavy chain fast type and creatine kinase. RNA interference-mediated knockdown of Fn14 receptor in C2C12 myoblasts decreased the levels of myogenic regulatory factors MyoD and myogenin upon induction of differentiation. Conversely, overexpression of MyoD increased differentiation in Fn14-knockdown C2C12 cultures. Suppression of Fn14 expression in C2C12 myoblasts also inhibited the differentiation-associated increase in the activity of serum response factor and RhoA GTPase. In addition, our data suggest that the role of Fn14 during myogenic differentiation could be independent of TWEAK cytokine. Collectively, our study suggests that the Fn14 receptor is required for the expression of myogenic regulatory factors and differentiation of myoblasts into myotubes.

Src and focal adhesion kinase mediate mechanical strain-induced proliferation and ERK1/2 phosphorylation in human H441 pulmonary epithelial cells

Pulmonary epithelial cells are exposed to repetitive deformation during physiological breathing and mechanical ventilation. Such deformation may influence pulmonary growth, development, and barotrauma. Although deformation stimulates proliferation and activates extracellular signal-regulated kinases (ERK1/2) in human pulmonary epithelial H441 cells, the upstream mechanosensors that induce ERK activation are poorly understood. We investigated whether c-Src or focal adhesion kinase (FAK) mediates cyclic mechanical strain-induced ERK1/2 activation and proliferation in human pulmonary epithelial (NCI-H441) cells. The H441 and A549 cells were grown on collagen I-precoated membranes and were subjected to an average 10% cyclic mechanical strain at 20 cycles/min. Cyclic strain activated Src within 2 min by increasing phosphorylation at Tyr(418), followed by rapid phosphorylation of FAK at Tyr(397) and Tyr(576) and ERK1/2 at Thr(202)/Tyr(204) (n = 5, P < 0.05). Twenty-four (A549 cells) and 24-72 h (H441 cells) of cyclic mechanical strain increased cell numbers compared with static culture. Twenty-four hours of cyclic strain also increased H441 FAK, Src, and ERK phosphorylation without affecting total FAK, Src, or ERK protein. The mitogenic effect was blocked by Src (10 micromol/l PP2 or short interfering RNA targeted to Src) or MEK (50 micromol/l PD-98059) inhibition. PP2 also blocked strain-induced phosphorylation of FAK-Tyr(576) and ERK-Thr(202)/Tyr(204) but not FAK-Tyr(397). Reducing FAK by FAK-targeted short interfering RNA blocked mechanical strain-induced mitogenicity and significantly attenuated strain-induced ERK activation but not strain-induced Src phosphorylation. Together, these results suggest that repetitive mechanical deformation induced by ventilation supports pulmonary epithelial proliferation by a pathway involving Src, FAK, and then ERK signaling.

Repetitive deformation activates focal adhesion kinase and ERK mitogenic signals in human Caco-2 intestinal epithelial cells through Src and Rac1

Intestinal epithelial cells are subject to repetitive deformation during peristalsis and villous motility, whereas the mucosa atrophies during sepsis or ileus when such stimuli are abnormal. Such repetitive deformation stimulates intestinal epithelial proliferation via focal adhesion kinase (FAK) and extracellular signal-regulated kinases (ERK). However, the upstream mediators of these effects are unknown. We investigated whether Src and Rac1 mediate deformation-induced FAK and ERK phosphorylation and proliferation in human Caco-2 and rat IEC-6 intestinal epithelial cells. Cells cultured on collagen-I were subjected to an average 10% cyclic strain at 10 cycles/min. Cyclic strain activated Rac1 and induced Rac1 translocation to cell membranes. Mechanical strain also induced rapid sustained phosphorylation of c-Src at Tyr(418), Rac1 at Ser(71), FAK at Tyr(397) and Tyr(576), and ERK1/2 at Thr(202)/Tyr(204). The mitogenic effect of cyclic strain was blocked by inhibition of Src (PP2 or short interfering RNA) or Rac1 (NSC23766). Src or Rac1 inhibition also prevented strain-induced FAK phosphorylation at Tyr(576) and ERK phosphorylation but not FAK phosphorylation at Tyr(397). Reducing FAK using short interfering RNA blocked strain-induced mitogenicity and attenuated ERK phosphorylation but not Src or Rac1 phosphorylation. Src inhibition blocked strain-induced Rac1 phosphorylation, but Rac inhibition did not alter Src phosphorylation. Transfection of a two-tyrosine phosphorylation-deficient FAK mutant Y576F/Y577F prevented activation of cotransfected myc-ERK2 by cyclic strain. Repetitive deformation induced by peristalsis or villus motility may support the gut mucosa by a pathway involving Src, Rac1, FAK, and ERK. This pathway may present important targets for interventions to prevent mucosal atrophy during prolonged ileus or fasting.

Cell cycle regulation by the Wee1 inhibitor PD0166285, pyrido [2,3-d] pyimidine, in the B16 mouse melanoma cell line

Background

Wee1 kinase plays a critical role in maintaining G2 arrest through its inhibitory phosphorylation of cdc2. In previous reports, a pyridopyrimidine molecule PD0166285 was identified to inhibit Wee1 activity at nanomolar concentrations. This G2 checkpoint abrogation by PD0166285 was demonstrated to kill cancer cells, there at a toxic highest dose of 0.5 μM in some cell lines for exposure periods of no longer than 6 hours. The deregulated cell cycle progression may have ultimately damaged the cancer cells. We herein report one of the mechanism by which PD0166285 leads to cell death in the B16 mouse melanoma cell line.

Methods

Tumor cell proliferation was determined by counting cell numbers. Cell cycle distribution was determined by flow cytometry. Morphogenesis analysis such as microtubule stabilization, Wee1 distribution, and cyclin B location were observed by immunofluorescence confocal microscopy. An immunoblot analysis of cdc2-Tyr15, cyclin D, E, p16, 21, 27, and Rb. A real-time PCR of the mRNA of cyclin D were completed.

Results

In our experiment, B16 cells also dramatically abrogated the G2 checkpoint and were found to arrest in the early G1 phase by treatment with 0.5 μM for 4 hours observed by flow cytometry. Cyclin D mRNA decreased within 4 hours observed by Real-time PCR. Rb was dephosphrylated for 24 hours. However, B16 cells did not undergo cell death after 0.5 μM treatment for 24 hours. Immnofluoscence microscopy showed that the cells become round and small in the morphogenesis. More interesting phenomena were that microtubule stabilization was blocked, and Wee1 distribution was restricted after treatment for 4 hours.

Conclusion

We analyzed the effect of Wee1 inhibitor PD0166285 described first by Wang in the G2 transition in the B16 melanoma cell line. The inhibitor PD0166285 abrogated G2/M checkpoint inducing early cell division. Moreover, we found that the treatment of cells with the inhibitor is related to microtubule stabilization and decrease in cyclin D transcription. These effects together suggest that Wee1 inhibitor may thus be a potentially useful anti-cancer therapy.

The motogenic effects of cyclic mechanical strain on intestinal epithelial monolayer wound closure are matrix dependent

BACKGROUND & AIMS

Complex deformation during normal digestion due to peristalsis or villous motility may be trophic for the intestinal mucosa. Because tissue fibronectin is increased in inflammatory states that may accompany mucosal injury, we evaluated the effects of cyclic mechanical strain and fibronectin on intestinal epithelial monolayer wound closure in Caco-2 and IEC-6 intestinal epithelial cells.

METHODS

Wounds created in intestinal epithelial monolayers were subjected to cyclic deformation. Wound closure was assessed by morphometry using microscopic imaging. Cell signals were assessed by Western blot and confocal microscopy.

RESULTS

Mechanical strain stimulated wound closure on fibronectin but inhibited closure on collagen in Caco-2 and IEC-6 cells. The effect was independent of proliferation or cell spreading. Myosin light chain (MLC) and extracellular signal-regulated kinase (ERK) were phosphorylated in response to strain in confluent monolayers on both collagen and fibronectin. Blocking MLC or ERK phosphorylation inhibited the motogenic effect of strain on fibronectin. Although phosphorylated MLC was redistributed to the leading edge of migrating cells following 6 hours of strain on collagen and fibronectin, phosphorylated ERK was redistributed to the lamellipodial edge only on fibronectin.

CONCLUSIONS

Strain promotes intestinal epithelial wound closure by a pathway requiring ERK and MLC kinase. Fibronectin-dependent ERK redistribution in response to strain in confluent migrating cells may explain the matrix dependence of the motogenic effect. Repetitive deformation stimulates intestinal epithelial proliferation on a collagen substrate, but not fibronectin. Deformation may exert matrix-dependent effects on intestinal epithelial cells, promoting epithelial restitution in fibronectin-rich tissue and proliferation in fibronectin-poor mucosa.

Tumor Necrosis Factor-like Weak Inducer of Apoptosis Inhibits Skeletal Myogenesis through Sustained Activation of Nuclear Factor-κB and Degradation of MyoD Protein

In this study we have investigated the effect and the mechanisms by which tumor necrosis factor-like weak inducer of apoptosis (TWEAK) modulates myogenic differentiation. Treatment of C2C12 myoblasts with TWEAK inhibited their differentiation evident by a decrease in the expression of creatine kinase, myosin heavy chain-fast twitch, myogenin, and the formation of multinucleated myotubes. TWEAK also inhibited the differentiation of mouse primary myoblasts. Conversely, the proliferation of C2C12 myoblasts and the expression of a cell-cycle regulator cyclin D1 were increased in response to TWEAK treatment. Inhibition of cellular proliferation using hydroxyurea only partially reversed the inhibitory effect of TWEAK on myogenic differentiation. Treatment of C2C12 myoblasts with TWEAK resulted in the activation of nuclear factor-kappaB (NF-kappaB), the (IkappaB) IkappaB kinase (IKK) complex, and the phosphorylation and degradation of IkappaBalpha protein. Inhibition of NF-kappaB activity by overexpression of a dominant negative mutant of IkappaBalpha (IkappaBalphaDeltaN) significantly increased the myogenic differentiation in TWEAK-treated C2C12 cultures. Furthermore, overexpression of a dominant negative mutant of IKKbeta (IKKbetaK44A) but not IKKalpha (IKKalphaK44M) reversed the inhibitory effect of TWEAK on myogenesis. TWEAK inhibited the expression of myogenic regulatory factors MyoD and myogenin and also induced the degradation of MyoD protein. Finally, inhibition of NF-kappaB activation through overexpression of IKKbetaK44A prevented the degradation of MyoD protein. Overall, our data suggest that TWEAK inhibits myogenesis through the activation of NF-kappaB signaling pathway and degradation of MyoD protein.

Inhibition of human embryonic stem cell differentiation by mechanical strain

Mechanical forces have been reported to induce proliferation and/or differentiation in many cell types, but the role of mechanotransduction during embryonic stem cell fate decisions is unknown. To ascertain the role of mechanical strain in human embryonic stem cell (hESC) differentiation, we measured the rate of hESC differentiation in the presence and absence of biaxial cyclic strain. Above a threshold of 10% cyclic strain, applied to a deformable elastic substratum upon which the hESC colonies were cultured, hESC differentiation was reduced and self-renewal was promoted without selecting against survival of differentiated or undifferentiated cells. Frequency of mechanical strain application had little effect on extent of differentiation. hESCs cultured under cyclic strain retained pluripotency, evidenced by their ability to differentiate to cell lineages in all three germ layers. Mechanical inhibition of hESC differentiation could not be traced to secretion of chemical factors into the media suggesting that mechanical forces may directly regulate hESC differentiation. Mechanical strain is not sufficient to inhibit differentiation, however, in unconditioned medium, hESCs grown under strain differentiated at the same rate as cells cultured in the absence of strain. Thus, while mechanical forces play a role in regulating hESC self-renewal and differentiation, they must act synergistically with chemical signals. These findings imply that application of mechanical forces may be useful, in combination with chemical and matrix-encoded signals, towards controlling differentiation of hESCs for therapeutic applications.

Inhibition of mechanosensitive cation channels inhibits myogenic differentiation by suppressing the expression of myogenic regulatory factors and caspase-3 activity

Mechanosensitive cation channels (MSC) are ubiquitous in eukaryotic cell types. However, the physiological functions of MSC in several tissues remain in question. In this study we have investigated the role of MSC in skeletal myogenesis. Treatment of C2C12 myoblasts with gadolinium ions (MSC blocker) inhibited myotube formation and the myogenic index in differentiation medium (DM). The enzymatic activity of creatine kinase (CK) and the expression of myosin heavy chain-fast twitch (MyHCf) in C2C12 cultures were also blocked in response to gadolinium. Treatment of C2C12 myoblasts with gadolinium ions did not affect the expression of either cyclin A or cyclin D1 in DM. Other inhibitors of MSC such as streptomycin and GsTMx-4 also suppressed the expression of CK and MyHCf in C2C12 cultures. The inhibitory effect of gadolinium ions on myogenic differentiation was reversible and independent of myogenic cell type. Real-time-polymerase chain reaction analysis revealed that inhibition of MSC decreases the expression of myogenic transcription factors MyoD, myogenin, and Myf-5. Furthermore, the activity of skeletal alpha-actin promoter was suppressed on MSC blockade. Treatment of C2C12 myoblasts with gadolinium ions prevented differentiation-associated cell death and inhibited the cleavage of poly (ADP-ribose) polymerase and activation of caspase-3. On the other hand, delivery of active caspase-3 protein to C2C12 myoblasts reversed the inhibitory effect of gadolinium ions on myogenesis. Our data suggest that inhibition of MSC suppresses myogenic differentiation by inhibiting the caspase-3 activity and the expression of myogenic regulatory factors.

Regulation of phosphatidylinositol 3-kinase (PI3K)/Akt and nuclear factor-kappa B signaling pathways in dystrophin-deficient skeletal muscle in response to mechanical stretch

Phosphatidylinositol 3-kinase (PI3K)/Akt and nuclear factor-kappa B (NF-kappaB) signaling pathways play a critical role in mediating survival signals. In this study we have investigated how loss of dystrophin (the primary cause of Duchenne muscular dystrophy) modulates the activation of PI3K/Akt and NF-kappaB signaling pathways in skeletal muscle in response to mechanical stimulation. Activation of Akt was significantly higher in diaphragm muscle from dystrophin-deficient mdx mice compared to normal mice at both prenecrotic and necrotic states. Higher activation of Akt was also observed in cultured dystrophin-deficient primary myotubes differentiated in vitro. Application of passive mechanical stretch ex vivo synergistically increased the activation of Akt in diaphragm of mdx mice. Stretch-induced activation of PDK-1 and PI3K were also higher in diaphragm of mdx mice compared to normal mice. Pretreatment of diaphragm with PI3K inhibitor LY294002 blocked the activation of Akt in normal and mdx mice. Higher activation of Akt was associated with increased phosphorylation of its downstream targets glycogen synthase kinase 3beta (GSK3beta), FKHR, and mammalian target of rapamycin (mTOR). Treatment of diaphragm muscle with LY294002 inhibited the stretch-induced activation of IkappaB (IkappaB) kinase (IKK) and NF-kappaB transcription factor in normal and mdx mice. Mechanical stretch also reduced the interaction of HDAC1 with RelA subunit of NF-kappaB in diaphragm muscle. Finally, cellular levels of Bcl-2, cIAP1, and integrin beta1 and activation of integrin linked kinase were higher in diaphragm muscle of mdx mice compared to normal mice. Taken together, our data suggest that loss of dystrophin and/or mechanical stretch results in the up-regulation of P13K/Akt and NF-kappaB signaling pathways in skeletal muscle.

Regulation of growth factor signaling and cell cycle progression by cell adhesion and adhesion-dependent changes in cellular tension

The proliferation of most non-transformed cell types requires cell adhesion and cellular tension as well as exposure to mitogenic growth factors. Integrins and cadherins provide the adhesion signals, which ultimately allow for the cytoskeletal changes that control cellular tension. This review discusses the roles of integrins, cadherins, and the actin cytoskeleton as mediators of the mechanical tension critical for growth factor-dependent signaling and cell cycle progression.

Stretch-induced myoblast proliferation is dependent on the COX2 pathway

Skeletal muscle increases in size due to weight bearing loads or passive stretch. This growth response is dependent in part upon myoblast proliferation. Although skeletal muscles are responsive to mechanical forces, the effect on myoblast proliferation remains unknown. To investigate the effects of mechanical stretch on myoblast proliferation, primary myoblasts isolated from Balb/c mice were subjected to 25% cyclical uniaxial stretch for 5 h at 0.5 Hz. Stretch stimulated myoblast proliferation by 32% and increased cell number by 41% 24 and 48 h after stretch, respectively. COX2 mRNA increased 3.5-fold immediately poststretch. Prostaglandin E2 and F2alpha increased 2.4- and 1.6-fold 6 h after stretch, respectively. Because COX2 has been implicated in regulating muscle growth and regeneration, we hypothesized that stretched myoblasts may proliferate via a COX2-dependent mechanism. We employed two different models to disrupt COX2 activity: (1) treatment with a COX2-selective drug, and (2) transgenic mice null for COX2. Treating myoblasts with a COX2-specific inhibitor blocked stretch-induced proliferation. Likewise, stretched COX2-/- myoblasts failed to proliferate compared to controls. However, supplementing stretched, COX2-/- myoblasts with prostaglandin E2 or fluprostenol increased proliferation. These data suggest that the COX2 pathway is critical for myoblast proliferation in response to stretch.

Single-cell-bioreactors as end of miniaturization approaches in biotechnology: progresses with characterised bioreactors and a glance into the future

Incidents with single cells and their genesis have not been the major focus of science up to now. This fact is supported by the difficulties one faces when wanting to monitor and cultivate small populations of cells in a defined compartment under controlled conditions, in vitro. Several approaches of up- and down-scaling have often led to poorly understood results which might be better elucidated by understanding the cellular genesis as a function of its microenvironment. This review of the approaches of scale-up and scale-down processes illustrates technical possibilities and shows up their limitations with regard to obtainable data for the characterisation of cellular genesis and impact of the cellular microenvironment. For example, stem cell research advances underline the lack of information about the impact of the microenvironment on cellular development. Finally, a proposal of future research efforts is given on how to overcome this lack of data via a novel bioreactor setup.