Ca2+-induced p38/SAPK signalling inhibited by the immunosuppressant cyclosporin A in human peripheral blood mononuclear cells

Spontaneous calcium responses of SF2 rat dental epithelial cells stably expressing the calcium sensor G-GECO

Genetically-encoded calcium indicators such as G-GECO are useful for studying Ca²⁺ responses during long-term processes. In this study, we employed a lentiviral vector and established a rat dental epithelial cell line that stably expressed G-GECO (SF2-G-GECO). Ca²⁺ imaging analysis under cell culture conditions revealed that SF2-G-GECO cells exhibited spontaneous Ca²⁺ responses, which could be classified into the following three major patterns depending on the cell density: localized Ca²⁺ responses at cell protrusions at a low density, a cell-wide spread of Ca²⁺ responses at a medium density, and Ca²⁺ responses in clusters of 3-20 cells at a high density. The P2Y receptor inhibitor suramin (10 μM), the ATP-degrading enzyme apyrase (5 units/mL), and the fibroblast growth factor (FGF) receptor inhibitor FIIN-2 (1 μM) decreased the frequency of spontaneous Ca²⁺ responses. These results indicate that ATP and FGF are involved in the spontaneous Ca²⁺ responses. SF2 cells differentiate into ameloblasts via interactions with mesenchymal cells. Therefore, SF2-G-GECO cells are expected to be a useful tool for studying the functions of Ca²⁺ responses in regulating gene expression during tooth development.

Cyclophilin A as a target in the treatment of cytomegalovirus infections

BACKGROUND

Viruses are obligate parasites that depend on the cellular machinery of the host to regenerate and manufacture their proteins. Most antiviral drugs on the market today target viral proteins. However, the more recent strategies involve targeting the host cell proteins or pathways that mediate viral replication. This new approach would be effective for most viruses while minimizing drug resistance and toxicity.

METHODS

Cytomegalovirus replication, latency, and immune response are mediated by the intermediate early protein 2, the main protein that determines the effectiveness of drugs in cytomegalovirus inhibition. This review explains how intermediate early protein 2 can modify the action of cyclosporin A, an immunosuppressive, and antiviral drug. It also links all the pathways mediated by cyclosporin A, cytomegalovirus replication, and its encoded proteins.

RESULTS

Intermediate early protein 2 can influence the cellular cyclophilin A pathway, affecting cyclosporin A as a mediator of viral replication or anti-cytomegalovirus drug.

CONCLUSION

Cyclosporin A has a dual function in cytomegalovirus pathogenesis. It has the immunosuppressive effect that establishes virus replication through the inhibition of T-cell function. It also has an anti-cytomegalovirus effect mediated by intermediate early protein 2. Both of these functions involve cyclophilin A pathway.

Glibenclamide–sulfonylurea receptor 1 antagonist alleviates LPS-induced BV2 cell activation through the p38/MAPK pathway

We investigated the anti-neuroinflammatory activity and mechanism of glibenclamide, sulfonylurea receptor 1 (Sur1) antagonist, against LPS-induced microglial activationin vitro.

Effect of 1,25-dihydroxyvitamin D3 on spontaneous calcium responses in rat dental epithelial SF2 cells revealed by long-term imaging

Genetically encoded calcium indicators (GECIs) are suitable for long-term imaging studies. In this study, we employed a highly sensitive GECI, G-GECO, and achieved efficient gene delivery with an adenoviral vector. The adenoviral vector allowed us to express G-GECO in more than 80% of cells. More than 80% of G-GECO-expressing cells showed an ATP-induced increase in fluorescence intensity due to Ca²⁺ release from intracellular stores and subsequent Ca²⁺ entry. The fluorescence intensity of these cells was increased more than 2-fold by stimulation with 10 μM ATP. We applied long-term imaging (for ~10 h) to monitor Ca²⁺ responses in SF2, a rat dental epithelial cell line, in culture conditions. SF2 cells showed intermittent rises in the intracellular Ca²⁺ concentration in the presence of 100 nM 1,25-dihydroxyvitamin D₃. Many of these Ca²⁺ responses began at a specific location in the cytoplasm and spread throughout the entire cytoplasm. The combination of efficient gene delivery with an adenoviral vector and long-term imaging with a highly sensitive GECI enabled detection of intermittent Ca²⁺ responses that occur only 3-10 times/h/100 cells. This method could be useful to study the effects of Ca²⁺ responses for regulating longterm processes, such as gene expression, cell migration, and cell division, in many cell types.

Involvement of p38MAPK-ATF2 signaling pathway in alternariol induced DNA polymerase β expression

Base excision repair (BER) systems are important for maintaining the integrity of genomes in mammalian cells. Aberrant DNA bases or broken single strands can be repaired by BER. Consequently, DNA lesions, which may be caused by cancer and aging, have a close association with BER procedure. DNA polymerase β (polβ) is a critical BER enzyme that can excise 5'-sugar phosphate prior to adding a nucleotide in the gap by its function as a DNA polymerase in the BER process. However, DNA polβ is an error-prone DNA polymerase, and overexpressing polβ increases the cellular spontaneous mutation rate. DNA polβ overexpression has been identified in various human tumors, which implies that DNA polβ overexpression has a close association with tumorigenesis. The present study showed that alternariol (AOH), a secondary product of a fungus that is found in grains and fruits, could cause DNA damage to NIH3T3 cells in a single cell gel electrophoresis, and that 2, 10 and 20 µM AOH induced DNA polβ overexpression in a dose-dependent manner. In the process, the level of phosphorylation of mitogen-activated protein kinase 14 (p38) mitogen-activated protein kinase (MAPK) and activating transcription factor 2 (ATF2) was increased. In addition, SB203580, a p38MAPK inhibitor, resulted in decreased DNA polβ expression. Small hairpin RNA-p38MAPK had the same effect; notably, DNA polβ expression was downregulated in p38MAPK knockdown cells. These data suggest that the p38MAPK-ATF2 signaling pathway may be involved in DNA polβ expression induced by AOH.

SOCE and cancer: Recent progress and new perspectives

Ca²⁺ acts as a universal and versatile second messenger in the regulation of a myriad of biological processes, including cell proliferation, differentiation, migration and apoptosis. Store‐operated Ca²⁺ entry (SOCE) mediated by ORAI and the stromal interaction molecule (STIM) constitutes one of the major routes of calcium entry in nonexcitable cells, in which the depletion of intracellular Ca²⁺ stores triggers activation of the endoplasmic reticulum (ER)‐resident Ca²⁺ sensor protein STIM to gate and open the ORAI Ca²⁺ channels in the plasma membrane (PM). Accumulating evidence indicates that SOCE plays critical roles in cancer cell proliferation, metastasis and tumor neovascularization, as well as in antitumor immunity. We summarize herein the recent advances in our understanding of the function of SOCE in various types of tumor cells, vascular endothelial cells and cells of the immune system. Finally, the therapeutic potential of SOCE inhibitors in the treatment of cancer is also discussed.

Bone mechanotransduction may require augmentation in order to strengthen the senescent skeleton

Physical exercise is thought to hold promise as a non-invasive countermeasure against skeletal fragility arising from post-menopausal and age-related osteoporosis. Importantly, mechanical loading and exercise are capable of increasing bone size via periosteal expansion, which by far, is the most effective means of strengthening the structure of a given bone. The focus of this review was to therefore explore whether exercise has the potential to increase periosteal modeling and bone size in the senescent skeleton. A survey of exercise trials in humans suggests that exercise interventions that enhance periosteal modeling in the young skeleton fail to do the same in the elderly skeleton. Underlying this ineffectiveness, in vitro studies indicate that aging lowers basal levels of cell function and degrades bone mechanotransduction at a variety of levels from altered second messenger signaling to gene expression driving proliferation and/or differentiation. Given these age-related alterations, the ultimate efficacy of an exercise intervention may depend upon concurrent supplementation that directly address deficits in signaling and/or cell function. In this context, in vivo animal models of mechanical loading that simulate the muted periosteal adaptation in the elderly hold potential to examine the efficacy of countermeasures. Preliminary in vivo experiments suggest that pharmacologically counteracting age-related deficits in cellular function can restore exercise induced periosteal modeling in the senescent skeleton to levels observed in young animals. If the safety and efficacy of this strategy were to be confirmed for human use, it would enable the utilization of exercise as a viable countermeasure against skeletal fragility at senescence.

Immunodeficiency due to defects in store-operated calcium entry

Mutations in genes encoding the calcium-release activated calcium (CRAC) channel abolish calcium influx in cells of the immune system and cause severe congenital immunodeficiency. Patients with autosomal recessive mutations in the CRAC channel gene ORAI1, its activator stromal interaction molecule 1 (STIM1), and mice with targeted deletion of Orai1, Stim1, and Stim2 genes reveal important roles for CRAC channels in adaptive and innate immune responses to infection and in autoimmunity. Because CRAC channels have important functions outside the immune system, deficiency of either ORAI1 or STIM1 is associated with a unique clinical phenotype. This review will give an overview of CRAC channel function in the immune system, examine the consequences of CRAC channel deficiency for immunity in human patients and mice, and discuss genetic defects in immunoreceptor-associated signaling molecules that compromise calcium influx and cause immunodeficiency.

ATP receptors gate microglia signaling in neuropathic pain

Microglia were described by Pio del Rio-Hortega (1932) as being the 'third element' distinct from neurons and astrocytes. Decades after this observation, the function and even the very existence of microglia as a distinct cell type were topics of intense debate and conjecture. However, considerable advances have been made towards understanding the neurobiology of microglia resulting in a radical shift in our view of them as being passive bystanders that have solely immune and supportive roles, to being active principal players that contribute to central nervous system pathologies caused by disease or following injury. Converging lines of evidence implicate microglia as being essential in the pathogenesis of neuropathic pain, a debilitating chronic pain condition that can occur after peripheral nerve damage caused by disease, infection, or physical injury. A key molecule that modulates microglial activity is ATP, an endogenous ligand of the P2-purinoceptor family consisting of P2X ionotropic and P2Y metabotropic receptors. Microglia express several P2 receptor subtypes, and of these the P2X4, P2X7, and P2Y12 receptor subtypes have been implicated in neuropathic pain. The P2X4 receptor has emerged as the core microglia-neuron signaling pathway: activation of this receptor causes release of brain-derived neurotrophic factor (BDNF) which causes disinhibition of pain-transmission neurons in spinal lamina I. The present review highlights recent advances in understanding the signaling and regulation of P2 receptors expressed in microglia and the implications for microglia-neuron interactions for the management of neuropathic pain.

Sensitization of voltage activated calcium channel currents for capsaicin in nociceptive neurons by tumor-necrosis-factor-alpha

It is known that application of tumor-necrosis-factor-alpha (TNF-alpha) sensitizes neuronal calcium channels for heat stimuli in rat models of neuropathic pain. This study examines whether TNF-alpha modulates the capsaicin-induced effects after transient receptor potential vanilloid (TRPV)-1 receptor activation on voltage activated calcium channel currents (I(Ca(V))). TRPV-1 receptors are activated by heat and play an important role in the pathogenesis of thermal hyperalgesia in neuropathic pain syndromes, while voltage activated channels are essential for transmission of neuronal signals. Eliciting I(Ca(V)) in DRG neurons of rats by a depolarization from the resting potential to 0 mV, TNF-alpha (100 ng/ml) reduces I(Ca(V)) by 16.9+/-2.2%, while capsaicin (0.1 microM) decreases currents by 27+/-4.3%. Pre-application of TNF-alpha (100 ng/ml) for 24h results in a sensitization of I(Ca(V)) to capsaicin (0.1 microM) with a reduction of 42.8+/-4.4% mediated by TRPV-1. While L-type (36.6+/-5.2%) and P/Q-type currents (35.6+/-4.1%) are also sensitized by TRPV-1 activation, N-type channel currents are most sensitive (74.5+/-7.3%). The capsaicin-induced shift towards the hyperpolarizing voltage range does not occur when TNF-alpha is applied. Summarizing, TNF-alpha sensitizes nociceptive neurons for capsaicin.

P2X4-receptor-mediated synthesis and release of brain-derived neurotrophic factor in microglia is dependent on calcium and p38-mitogen-activated protein kinase activation

Microglia in the dorsal horn of the spinal cord are increasingly recognized as being crucial in the pathogenesis of pain hypersensitivity after injury to a peripheral nerve. It is known that P2X4 purinoceptors (P2X4Rs) cause the release of brain-derived neurotrophic factor (BDNF) from microglia, which is necessary for maintaining pain hypersensitivity after nerve injury. However, there is a critical gap in understanding how activation of microglial P2X4Rs leads to the release of BDNF. Here, we show that stimulating P2X4Rs with ATP evokes a biphasic release of BDNF from microglia: an early phase occurs within 5 min, whereas a late phase peaks 60 min after ATP stimulation. Concomitant with the late phase of release is an increased level of BDNF within the microglia. Both phases of BDNF release and the accumulation within the microglia are dependent on extracellular Ca(2+). The late phase of BDNF release and accumulation, but not the early phase of release, are suppressed by inhibiting transcription and translation, indicating that activation of P2X4R causes an initial release of a pre-existing pool of BDNF followed by an increase in de novo synthesis of BDNF. The release of BDNF is abolished by inhibiting SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor)-mediated exocytosis. Furthermore, we find that the P2X4R-evoked release and synthesis of BDNF are dependent on activation of p38-mitogen-activated protein kinase (MAPK). Together, our findings provide a unifying mechanism for pain hypersensitivity after peripheral nerve injury through P2X4R-evoked increase in Ca(2+) and activation of p38-MAPK leading to the synthesis and exocytotic release of BDNF from microglia.

Genetic disruption of p38alpha Tyr323 phosphorylation prevents T-cell receptor-mediated p38alpha activation and impairs interferon-gamma production

T cells possess a p38 activation alternative pathway in which stimulation via the antigen receptor (T-cell receptor [TCR]) induces phosphorylation of p38alpha and beta on Tyr323. To assess the contribution of this pathway to normal T-cell function, we generated p38alpha knockin mice in which Tyr323 was replaced with Phe (p38alpha(Y323F)). TCR-mediated stimulation failed to activate p38alpha(Y323F) as measured by phosphorylation of the Thr-Glu-Tyr activation motif and p38alpha catalytic activity. Cell-cycle entry was delayed in TCR-stimulated p38alpha(Y323F) T cells, which also produced less interferon (IFN)-gamma than wild-type T cells in response to TCR-mediated but not TCR-independent stimuli. p38alpha(Y323F) mice immunized with T-helper 1 (Th1)-inducing antigens generated normal Th1 effector cells, but these cells produced less IFN-gamma than wild-type cells when stimulated through the TCR. Thus, the Tyr323-dependent pathway and not the classic mitogen-activated protein (MAP) kinase cascade is the physiologic means of p38alpha activation through the TCR and is necessary for normal Th1 function but not Th1 generation.

Protein phosphatase-1 is a novel regulator of the interaction between IRBIT and the inositol 1,4,5-trisphosphate receptor

IRBIT is an IP3R [IP3 (inositol 1,4,5-trisphosphate) receptor]-binding protein that competes with IP3 for binding to the IP3R. Phosphorylation of IRBIT is essential for the interaction with the IP3R. The unique N-terminal region of IRBIT, residues 1-104 for mouse IRBIT, contains a PEST (Pro-Glu-Ser-Thr) domain with many putative phosphorylation sites. In the present study, we have identified a well-conserved PP1 (protein phosphatase-1)-binding site preceeding this PEST domain which enabled the binding of PP1 to IRBIT both in vitro and in vivo. IRBIT emerged as a mediator of its own dephosphorylation by associated PP1 and, hence, as a novel substrate specifier for PP1. Moreover, IRBIT-associated PP1 specifically dephosphorylated Ser68 of IRBIT. Phosphorylation of Ser68 was required for subsequent phosphorylation of Ser71 and Ser74, but the latter two sites were not targeted by PP1. We found that phosphorylation of Ser71 and Ser74 were sufficient to enable inhibition of IP3 binding to the IP3R by IRBIT. Finally, we have shown that mutational inactivation of the docking site for PP1 on IRBIT increased the affinity of IRBIT for the IP3R. This pinpoints PP1 as a key player in the regulation of IP3R-controlled Ca2+ signals.

Cyclosporin A elicits dose-dependent biphasic effects on osteoblast differentiation and bone formation

Cyclosporin A (CsA) is thought to prevent immune reactions after organ transplantation by inhibiting calcineurin (Cn) and its substrate, the Nuclear Factor of Activated T Cells (NFAT). A dichotomy exists in describing the effects of CsA on bone formation. The concept that the suppression of Cn/NFAT signaling by CsA inhibits bone formation is not entirely supported by many clinical reports and laboratory animal studies. Gender, dosage and basal inflammatory activity have all been suggested as explanations for these seemingly contradictory reports. Here we examine the effects of varying concentrations of CsA on bone formation and osteoblast differentiation and elucidate the role of NFATc1 in this response. We show that low concentrations of CsA (<1 microM in vitro and 35.5 nM in vivo) are anabolic as they increase bone formation, osteoblast differentiation, and bone mass, while high concentrations (>1 microM in vitro and in vivo) elicit an opposite and catabolic response. The overexpression of constitutively active NFATc1 inhibits osteoblast differentiation, and treatment with low concentrations of CsA does not ameliorate this inhibition. Treating osteoblasts with low concentrations of CsA (<1 microM) increases fra-2 gene expression and protein levels in a dose-dependent manner as well as AP-1 DNA-binding activity. Finally, NFATc1 silencing with siRNA increases Fra-2 expression, whereas NFATc1 overexpression inhibits Fra-2 expression. Therefore, NFATc1 negatively regulates osteoblast differentiation, and its specific inhibition may represent a viable anabolic therapy for osteoporosis.

Intracellular protein phosphorylation in adherent U937 monocytes mediated by various culture conditions and fibronectin-derived surface ligands

Macrophages play a central role in the normal healing process after tissue injury and the host response to foreign objects such as biomaterials. The process leading to macrophage adhesion and activation on protein-adsorbed substrates is complex and unresolved. While the use of primary cells offers clinical relevancy, macrophage cell lines offer unique advantages such as availability and relatively homogeneous phenotype as models to probe the molecular mechanism of cell-surface interaction. Our goal was to better characterize the effect of the culture condition and surface-associated ligands on the extent of U937 adhesion. Tyrosine phosphorylation of intracellular proteins was surveyed as a basis to seek a greater understanding of the molecular mechanism involved in mediating U937 adhesion on various ligand-adsorbed surfaces. U937 viability and adhesion on tissue culture polystyrene (TCPS) increased with (i) increasing serum level, (ii) decreasing tyrosine phosphorylation inhibitor AG18 concentration, or (iii) increasing culture time. The adsorption of various adhesion proteins such as fibronectin and peptide ligands (i.e., RGD, PHSRN) on TCPS did not significantly increase the adherent density of U937 when compared with albumin and PBS ligand controls. However, ligand identity and the presence of phorbol myristate acetate dramatically affected the extent (i.e., increase or decrease) and the identity (i.e., molecular weight) of phosphotyrosine proteins in adherent U937 in a time-dependent manner. The extent and identity of phosphotyrosine proteins did not exhibit a clear AG18 dose dependency, rather the level of tyrosine phosphorylation for a distinct group of proteins was either increased or decreased for a given AG18 concentration.

DNA repair in mononuclear cells: role of serine/threonine phosphatases

Treatment with cyclosporin A (CsA) in kidney-transplant recipients is associated with reduced DNA repair and enhanced cancer incidence. CsA is an inhibitor of the serine/threonine phosphatase calcineurin, also termed PP2B, which is a Ca(2+)/calmodulin-dependent phosphatase. In this study we sought to elucidate the role of calcineurin in DNA repair using CsA and tacrolimus; examine whether UV-induced DNA repair is associated with dephosphorylation; and investigate whether phosphatases other than calcineurin are active in DNA repair, in light of the fact that calcineurin inhibition only partially suppressed DNA repair. Peripheral blood mononuclear cells from healthy donors were used. In vitro, we assayed UV-induced DNA repair by measuring the incorporation of tritiated thymidine in UV-irradiated cells. We gauged phosphatase activity indirectly by measuring free inorganic phosphate (Pi) excreted into the medium. The phosphatase assay was performed under the same conditions and in parallel to the DNA-repair assay. Tacrolimus, like CsA, inhibited DNA repair in a dose-dependent fashion. DNA repair was associated with production of Pi, which correlated with the number of cells performing DNA repair. Phosphatase activity increased after UV irradiation. DNA repair correlated directly with phosphatase activity, whereas CsA reduced both DNA repair and Pi production. Inhibition of calmodulin by trifluoperazine and W7 [N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide] reduced DNA repair in part. We investigated the role of the Ca(2+)-independent phosphatases PP1 and PP2A using specific inhibitors. Calyculin A, which inhibits both phosphatases, reduced DNA repair. Endothall, a PP2A inhibitor, had no effect on DNA repair. Okadaic acid, which is mostly a PP2A inhibitor but also a weak inhibitor of PP1, reduced DNA repair only slightly. We suggest that DNA repair is mediated by way of Ca(2+)-dependent and Ca(2+)-independent pathways, with calcineurin and PP1 being the respective phosphatases involved in each pathway.

Calcineurin and hypertrophic heart disease: novel insights and remaining questions

In the past 2 years, an emerging body of research has focused on a novel transcriptional pathway involved in the cardiac hypertrophic response. Ever since its introduction, the significance of the calcineurin-NFAT module has been subject of controversy. The aim of this review is to provide both an update on the current status of knowledge and discuss the remaining issues regarding the involvement of calcineurin in hypertrophic heart disease. To this end, the molecular biology of calcineurin and its direct downstream transcriptional effector NFAT are discussed in the context of the genetic studies that established the existence of this signaling paradigm in the heart. The pharmacological mode-of-action and specificity of the calcineurin inhibitors cyclosporine A (CsA) and FK506 is discussed, as well as their inherent limitations to study the biology of calcineurin. A critical interpretation is given on studies aimed at analyzing the role of calcineurin in cardiac hypertrophy using systemic immunosuppression. To eliminate the controversy surrounding CsA/FK506 usage, recent studies employed genetic inhibitory strategies for calcineurin, which confirm the pivotal role for this signal transduction pathway in the ventricular hypertrophy response. Finally, unresolved issues concerning the role of calcineurin in cardiac pathobiology are discussed based upon the information available, including its controversial role in cardiomyocyte viability, the reciprocal relationship between myocyte Ca(2+) homeostasis and calcineurin activity and the relative importance of calcineurin in relation to other hypertrophic signaling cascades.

Retinoblastoma protein interacts with ATF2 and JNK/p38 in stimulating the transforming growth factor-beta2 promoter

Two highly related transcription factors, ATF2 and ATFa, enhance the activity of the Transforming Growth Factor beta2 (TGF-beta2) promoter via a partial cAMP response element in transfected CHO cells. The retinoblastoma protein (Rb) also activates this promoter and enhances the stimulatory effects of ATF2 but causes near extinction of the effects of ATFa. The site on Rb required for its effects alone and in combination with the ATFs has been mapped mainly to the A/B pockets but the C pocket is also implicated. Whereas MKK7 or JNK expression enhances the actions of both ATFs, MKK6 or p38 expression only augments the effects of ATF2. Immunoprecipitation with Rb antibodies of lysates from transfected cells brings down expressed ATF2 but not ATFa. Expressed JNK and p38 are also found in the anti-Rb immunoprecipitates. ATF2 antibodies bring down expressed Rb, JNK and p38 and expression of Rb enhances the immunoprecipitation of both JNK and p38 by ATF2 antibodies. The results suggest that Rb is acting as a matchmaker by bridging either JNK or p38 with their common substrate ATF2 and, hence, facilitating its activation. Consistent with this suggestion, expression of Rb enhances the phosphorylation of ATF2 in CHO cells.

Effects of concentric and eccentric contractions on phosphorylation of MAPK(erk1/2) and MAPK(p38) in isolated rat skeletal muscle

1. Exercise and contractions of isolated skeletal muscle induce phosphorylation of mitogen-activated protein kinases (MAPKs) by undefined mechanisms. The aim of the present study was to determine exercise-related triggering factors for the increased phosphorylation of MAPKs in isolated rat extensor digitorum longus (EDL) muscle. 2. Concentric or eccentric contractions, or mild or severe passive stretches were used to discriminate between effects of metabolic/ionic and mechanical alterations on phosphorylation of two MAPKs: extracellular signal-regulated kinase 1 and 2 (MAPK(erk1/2)) and stress-activated protein kinase p38 (MAPK(p38)). 3. Concentric contractions induced a 5-fold increase in MAPK(erk1/2) phosphorylation. Application of the antioxidants N-acetylcysteine (20 mM) or dithiothreitol (5 mM) suppressed concentric contraction-induced increase in MAPK(erk1/2) phosphorylation. Mild passive stretches of the muscle increased MAPK(erk1/2) phosphorylation by 1.8-fold, whereas the combination of acidosis and passive stretches resulted in a 2.8-fold increase. Neither concentric contractions, nor mild stretches nor acidosis significantly affected phosphorylation of MAPK(p38). 4. High force applied upon muscle by means of either eccentric contractions or severe passive stretches resulted in 5.7- and 9.5-fold increases of phosphorylated MAPK(erk1/2), respectively, whereas phosphorylation of MAPK(p38) increased by 7.6- and 1.9-fold (not significant), respectively. 5. We conclude that in isolated rat skeletal muscle an increase in phosphorylation of both MAPK(erk1/2) and MAPK(p38) is induced by mechanical alterations, whereas contraction-related metabolic/ionic changes (reactive oxygen species and acidosis) cause increased phosphorylation of MAPK(erk1/2) only. Thus, contraction-induced phosphorylation can be explained by the combined action of increased production of reactive oxygen species, acidification and mechanical perturbations for MAPK(erk1/2) and by high mechanical stress for MAPK(p38).