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【摘要】
Duchenne muscular dystrophy (DMD) is a progressive muscle-wasting disease resulting from lack of the sarcolemmal protein dystrophin. However, the mechanism leading to the final disease status is not fully understood. Several lines of evidence suggest a role for nuclear factor (NF)-B in muscle degeneration as well as regeneration in DMD patients and mdx mice. We investigated the effects of blocking NF-B by inhibition of oxidative stress/lipid peroxidation on the dystrophic process in mdx mice. Five-week-old mdx mice received three times a week for 5 weeks either IRFI-042 (20 mg/kg), a strong antioxidant and lipid peroxidation inhibitor, or its vehicle. IRFI-042 treatment increased forelimb strength (+22%, P < 0.05) and strength normalized to weight (+23%, P < 0.05) and decreased fatigue (C45%, P < 0.05). It also reduced serum creatine kinase levels (P < 0.01) and reduced muscle-conjugated diene content and augmented muscle-reduced glutathione (P < 0.01). IRFI-042 blunted NF-B DNA-binding activity and tumor necrosis factor- expression in the dystrophic muscles (P < 0.01), reducing muscle necrosis (P < 0.01) and enhancing regeneration (P < 0.05). Our data suggest that oxidative stress/lipid peroxidation represents one of the mechanisms activating NF-B and the consequent pathogenetic cascade in mdx muscles. Most importantly, these new findings may have clinical implications for the pharmacological treatment of patients with DMD.
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Duchenne muscular dystrophy (DMD) is a progressive muscle-wasting disease leading to loss of ambulation by the 13th year and to death, usually in early adulthood.1 The disease results from absence of the protein dystrophin, which is an essential component of the dystrophin-glycoprotein complex that maintains membrane integrity of muscle fibers by linking cytoskeleton to extracellular matrix.2-5 Although the primary genetic defect is known, how this mutation gives rise to the final disease status is not fully understood. The mechanisms responsible for the pathological hallmarks of the dystrophic process, such as necrosis, phagocytosis, infiltration of inflammatory cells, initial efficient regeneration followed by a decline and secondary fibrosis, have not been definitively identified. DMD pathogenesis is frequently studied in the genetically homologous animal, the mdx mouse, despite relevant clinical and pathological differences. The murine model exhibits late muscle weakness, a slow disease progression, similar extensive degeneration and regeneration occurring between 2 and 12 weeks of age, but no proliferation of connective tissue in limb muscles.6,7
Several lines of evidence suggest that oxidative stress might be involved in the dystrophic process. Free radical injury may contribute to loss of membrane integrity in muscular dystrophies8 and dystrophic muscle cells have an increased susceptibility to reactive oxygen intermediates.9-11 Markers of oxidative stress have been detected in muscles of either DMD patients or mdx mice.9,12,13 An involvement of reactive oxygen intermediates is also supported by observations of increased biological by-products of oxidative stress,14 reduced cellular antioxidants (glutathione and vitamin E), and altered concentrations of antioxidant enzymes.12,15
Recently, the role of nuclear factor-B (NF-B) in the skeletal muscle-wasting process is gaining increasing attention, mainly because NF-B is activated in response to several inflammatory molecules that cause muscle loss.16,17 NF-B is an ubiquitous transcription factor regulating the expression of a plethora of genes involved in inflammatory, immune, and acute stress responses.18 In fact NF-B, after proteasomal degradation of the inhibitory protein I-B (I-B), translocates to the nucleus and binds target DNA elements in the promoter of different genes expressing cytokines, chemokines, cell adhesion molecules, immunoreceptors, and inflammatory enzymes such as nitric oxide synthase, matrix metalloproteinases, and phospholipase A2.19-21 On the other hand, NF-B is activated in response to several inflammatory molecules, such as interleukin-1ß (IL-1ß), tumor necrosis factor- (TNF-), metalloproteinases, whose circulating levels have been found elevated in DMD and other types of muscular dystrophies.22-25 Moreover oxidative stress strongly activates NF-B,26-28 which is involved in the up-regulation of antioxidant enzymes such as glutathione peroxidase and catalase.28 An involvement of NF-B in myogenesis has been suggested because its activity was shown to be required by human and rat myoblasts to fuse into myotubes and to express muscle-specific proteins such as myosin heavy chain and caveolin 3.29 In addition, it has also been demonstrated that systemic administration of the NF-B inhibitor curcumin stimulates muscle regeneration after traumatic injury, suggesting that modulation of NF-B activity within muscle tissue could be beneficial for muscle repair.30
Very recently, NF-B activity has been demonstrated to be increased in muscles of either DMD patients21 or mdx mice,16,17 but its effective role in DMD pathogenesis is not clear to date. Interestingly, we have reported the novel observation of increased immunoreactivity for NF-B in the cytoplasm of all regenerating fibers and in 20 to 40% of necrotic fibers in DMD as well as in inflammatory myopathies.21
Taken together, this evidence suggests that reactive oxygen intermediates might be involved in the dystrophic process, triggering an inflammatory cascade that leads to NF-B activation and to the subsequent release of inflammatory mediators. This work hypothesis would also indicate that the interruption of this cascade might have a therapeutic potential. To confirm and clarify this issue, we used as a pharmacological tool (??)-5-emisuccinoyl-2--2,3-dihydro-4,6,7-trimethylbenzofuran (IRFI-042), a synthetic, vitamin E analogue. Vitamin E has been suggested to act as potential inhibitor of NF-B activation31 ; nevertheless the marked lipophilicity of this vitamin limits its therapeutic potential with low circulating levels and poor tissue distribution after somministration. IRFI-042 is a less lipophilic compound with powerful antioxidant properties due to the combination in the same molecule of a chain-breaking moiety (characteristic of phenols related to -tocopherol) with the reducing ability of a thiol group (dual antioxidant). Moreover this compound shows no systemic toxicity even after high dosage (up to 1 g/kg).31 IRFI-042 possesses a strong inhibitory activity on both oxidative stress/lipid peroxidation and NF-B activation demonstrated in different experimental models, such as endotoxin-induced shock,32 organ ischemia/reperfusion injury,33 neurotoxicity,34 and impaired wound healing process.35 The aim of our study was to test the novel hypothesis that the modulation of NF-B activity by oxidative stress/lipid peroxidation inhibition may influence the skeletal muscle pathology in mdx mice, with respect to the functional, morphological, and biochemical patterns.
【关键词】 peroxidation inhibition factor-b activation skeletal degeneration enhances function
Materials and Methods
Animals
Male mdx and wild-type C57BJ/10 (WT) mice were obtained from The Jackson Laboratory (Bar Harbor, ME) and bred in our animal facilities. Mice were housed in plastic cages in a temperature-controlled environment with a 12-hour light/dark cycle and free access to food and water. The investigation conformed with the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH publication no.85-23, revised 1996). Five-week-old mdx and WT mice have been treated for 5 weeks with intraperitoneal injections with either IRFI 042 (n:8; 20 mg/kg three times a week) or vehicle (n:8; dimethyl sulfoxide/NaCl 0.9%; 0.1:1 v/v; 0.2 mg/kg three times a week). At the end of the experiments, animals were anesthetized with an intraperitoneal administration of sodium pentobarbital (80 mg/kg). Then, blood, collected by intracardiac puncture, was drawn to analyze creatine kinase (CK) levels and the biceps, quadriceps, and extensor digitorum longus (EDL) muscles were removed bilaterally and immediately frozen in liquid nitrogen-cooled isopentane and stored at C80??C for morphological and biochemical evaluations.
Animal Examinations
Mice were weighed and examined for forelimb strength at baseline and after 5 weeks of treatment. Strength testing consisted of five separate measurements using a grip meter attached to a force transducer that measures peak force generated (Stoelting Co., Wooddale, IL). The mouse grabs the trapeze bar as it is pulled backward and the peak pull force in grams is recorded on a display. The three highest measurements for each animal were averaged to give the strength score. We calculated also the degree of fatigue by comparing the first two pulls to the last two pulls. The decrement between pulls one and two and pulls four and five gives a measure of fatigue.6
Serum CK Evaluation
Blood samples were centrifuged at 6000 rpm and the serum was stored at C80??C until the day of analysis. Serum CK was evaluated at 37??C using a commercially available kit (Randox Laboratories Ltd., Antrim, UK). The results were expressed as U/L.
Histological Studies
Ten-µm-thick transverse cryostat sections were obtained from the midpoint of the biceps and EDL muscle body. The whole muscle cross-sections (corresponding to a mean area of 2.15 mm2 in biceps and 1.69 mm2 in EDL), stained with hematoxylin and eosin (H&E), were examined by a blinded observer, using the AxioVision 2.05 image analysis system equipped with Axiocam camera scanner (Zeiss, Munchen, Germany). The following four areas were recognized with patchy distribution: 1) normal fibers, identified by the presence of peripheral nuclei; 2) centrally nucleated fibers, identified by normal size but with central nuclei; 3) regenerating fibers, identified by small size, basophilic cytoplasm, and central nuclei; 4) necrotic fibers, identified by pale cytoplasm and phagocytosis. The results were expressed as the ratio of the area occupied by normal fibers, centrally nucleated fibers, regenerating fibers, or necrotic fibers divided by the total surface area as a percentage.
Immunocytochemistry
Seven-µm-thick transverse cryostat sections from biceps and EDL muscles were incubated for 120 minutes at 37??C in rabbit polyclonal antibody against phospho-NF-B p65 subunit (Ser276) (1:50; Cell Signaling Technology, Beverly, MA). It selectively binds to the NF-B p65 only when phosphorylated at serine 276, ie, it is activated and can then undergo nuclear translocation. Nonspecific binding of immunoglobulin was blocked with 5% normal horse serum. Immunodetection was performed using a biotin-avidin system (DAKO, Milan, Italy) followed by horseradish peroxidase staining with 3,3-diaminobenzidine tetrahydrochloride.
Evaluation of Conjugated Dienes (CDs) Content
Estimation of the tissue content of CDs was performed to evaluate the extent of lipid peroxidation in tissue as previously shown.31 Samples of biceps muscle were collected in polyethylene tubes and then washed with 1 ml of butylated hydroxytoluene (BHT) (1 mg/ml in phosphate buffer). The samples, after drying in absorbent paper, were frozen at 4??C until the analysis. The biochemical assay of CDs required previous lipid extraction from the tissue samples by chloroform/methanol (2:1). The lipid layer was dried under nitrogen atmosphere and then dissolved in cyclohexane. Muscle contents of CDs was measured at 232 nm by using a spectrophotometric technique. The amount of muscle CDs was expressed as ABS/mg protein.
Evaluation of Reduced Glutathione (GSH) Levels
GSH activity was evaluated to estimate endogenous defenses against oxidative stress. The levels in biceps muscles were determined as previously described.31 Briefly, tissue samples were homogenized with a Ultra-turrax (IKA, Staufen, Germany) homogenizer in a solution containing 5% trichloroacetic acid and 5 mmol/L ethylenediamine tetraacetic acid at 4??C. Then each sample was centrifuged at 15,000 x g for 10 minutes at 4??C. Homogenate supernatant (0.4 ml) was added in polyethylene dark tubes containing 1.6 ml of Tris-ethylenediamine tetraacetic acid buffer 0.4 mol/L, pH 8.9. After vortexing, 40 µl of 10 mmol/L dithiobisnitrobenzoic acid were added. The samples were vortexed again and the absorbance was read after 5 minutes at 412 nm. The values of unknown samples were drawn from a standard curve plotted by assaying different known concentrations of GSH. The amount of muscle GSH was expressed as µmol/g protein.
Electrophoretic Mobility Shift Assay
NF-B binding activity in quadriceps muscle specimens was performed in a 15-µl binding reaction mixture containing 1% binding buffer ATP (3000 Ci/mmol at 10 mCi/ml; Amersham Life Sciences, Arlington Heights, IL) using T4 polynucleotide kinase. The binding reaction mixture was incubated at room temperature for 20 minutes and analyzed by electrophoresis on 5% nondenaturing polyacrylamide gels. After electrophoresis, the gels were dried using a gel-drier and exposed to Kodak X-ray films at C70??C. The binding bands were quantified by scanning densitometry of a bio-image analysis system (Bio-Profil; Celbio, Milan, Italy). The results were expressed as relative integrated intensity compared with normal controls, considering exposure time, background levels, and known protein concentration of an Epstein-Barr virus nuclear antigen-1 extract, which was used as electrophoretic mobility shift assay control.
Western Blot Analysis
Samples from quadriceps muscles were homogenized in lysis buffer (1% Triton X-100, 20 mmol/L Tris/HCl, pH 8.0, 137 mmol/L NaCl, 10% glycerol, 5 mmol/L ethylenediamine tetraacetic acid, 1 mmol/L phenylmethyl sulfonyl fluoride, 1% aprotinin, 15 µg ml leupeptin). Protein samples (40 µg) were denatured in reducing buffer (62 mmol/L Tris, pH 6.8, 10% glycerol, 2% sodium dodecyl sulfate, 5% ß-mercaptoethanol, 0.003% bromophenol blue) and separated by electrophoresis on sodium dodecyl sulfate (12%) polyacrylamide gel with prestained standard proteins (Bio-Rad, Milan, Italy) to achieve a more accurate molecular weight determination. The separated proteins were transferred onto a nitrocellulose membrane using the transfer buffer (39 mmol/L glycine, 48 mm Tris, pH 8.3, 20% methanol) at 200 mA for 1 hour. The membranes were stained with Ponceau S (0.005% in 1% acetic acid) to confirm equal amounts of protein and were blocked with 5% non-fat dry milk in Tris-buffered saline-0.1% Tween for 1 hour at room temperature, washed three times for 10 minutes each in Tris-buffered saline-0.015% Tween, and incubated with rabbit monoclonal antibody against TNF- (Chemicon, Temecula, CA) in Tris-buffered saline-0.1% Tween overnight at 4??C. After washing three times for 10 minutes each in Tris-buffered saline-0.15% Tween, the membranes were incubated with peroxidase-conjugated goat anti-rabbit IgG (Pierce, Milan, Italy) for 1 hour at room temperature. After washing, the membranes were analyzed by the enhanced chemiluminescence system according to the manufacturer??s protocol (Amersham). The protein signals were quantified by scanning densitometry using a bio-image analysis system (Bio-Profil, Celbio). The results from each experimental group were expressed as relative integrated intensity compared with control muscle measured with the same batch. Equal loading of protein was assessed on stripped blots by immunodetection of ß-actin with a rabbit monoclonal antibody (Cell Sig-naling, Celbio) diluted 1:500 and peroxidase-conjugated goat anti-rabbit IgG (Pierce) diluted 1:15,000. All anti-bodies are purified by protein A and peptide affinity chromatography.
Drug
IRFI 042 was supplied by Biomedica Foscama Research Centre, Ferentino, Italy. All substances were prepared fresh daily and administered in a volume of 1 ml/kg.
Statistical Analysis
Results are expressed as mean ?? SD. Statistical evaluation was performed by using one-way analysis of variance followed by Dunnett??s post hoc tests and paired Student??s t-test with the use of the InPlotPrism software version 3.0 (GraphPad Software, San Diego, CA). P values <0.05 were considered significant.
Results
Body Weight, Forelimb Strength, and Fatigue
Body weight was not significantly different among the animal groups at baseline as well as after treatment. Comparing the values longitudinally, at the end of the experiment all groups had an increased body weight (P < 0.01) (Figure 1A) . At baseline, strength and strength normalized to weight were significantly lower in both mdx mice groups (assigned to IRFI 042 or to vehicle treatment) compared to WT groups (allocated to IRFI 042 or to vehicle treatment) (P < 0.05). At the end of treatment, IRFI 042-treated mdx mice had higher forelimb strength (+22%, P < 0.05) and strength normalized to weight (+23%, P < 0.05) compared to vehicle-treated mdx mice (Figure 1, B and C) . In all groups the somatic growth paralleled with an increment of strength if compared with baseline values (P < 0.01 in mdx + IRFI 042, P < 0.05 in the other groups) (Figure 1B) , but when normalized to weight only the IRFI 042-treated mdx mice showed a significant amelioration in strength (P < 0.05) (Figure 1C) .
Figure 1. Effects of IRFI 042 and vehicle treatment on body weight (A), forelimb strength (B), forelimb strength normalized to weight (C), and fatigue (D) in WT and mdx mice (n = 8 in each group). *P < 0.05 and P < 0.01 versus baseline value; **P < 0.05 IRFI 042-treated versus vehicle-treated mdx mice. For statistical analysis between mdx and WT mice at baseline and at the end of treatment, see text.
At baseline, the percentage of fatigue was significantly higher in both mdx mice groups compared to WT groups (P < 0.01); furthermore, there was not any significant difference between the two mdx groups (Figure 1D) . After treatment we found in both mdx groups a higher level of fatigue compared to WT groups (P < 0.001 in mdx+ vehicle, P < 0.01 in mdx+ IRFI 042), but the value was significantly lower in IRFI 042-treated compared to vehicle-treated mdx (C45%, P < 0.05). Comparing the data longitudinally, the percentage of fatigue increased in the vehicle-treated (P < 0.05) and remained stable in the IRFI 042-treated mdx mice (Figure 1D) . IRFI 042 did not cause any significant change in the forelimb strength, strength normalized to body weight, and fatigue of WT animals.
CK Level Evaluation
Low CK levels were observed in WT animals treated either with vehicle or IRFI 042 (WT+ vehicle = 221 ?? 33 U/L, WT+ IRFI 042 = 145 ?? 28 U/L). Mdx mice showed a significant increase in serum CK levels (mdx + vehicle = 2662 ?? 79 U/L, P < 0.01 versus WT+ vehicle). IRFI 042 administration resulted in a marked reduction of the enzyme levels (mdx + IRFI 042 = 681 ?? 173 U/L, P < 0.01 versus mdx + vehicle) (Figure 2) .
Figure 2. Effects of IRFI 042 and vehicle treatment on serum CK levels (n = 8 in each group). *P < 0.01 versus vehicle-treated WT mice; P < 0.01 versus vehicle-treated mdx mice.
Histological Studies
Wild-type animals showed a normal architecture of the biceps and EDL muscles that was not modified by treatment with IRFI 042 (Figure 3) . Both biceps and EDL muscles from vehicle-treated mdx mice showed necrosis and regeneration (Figures 3, 4, and 5) . Quantitative morphological evaluation of biceps muscle from IRFI 042-treated mdx mice revealed a significant increase in regenerating area (P < 0.05) and a decrease in necrotic area (P < 0.01) (Figures 3 and 5) . Similarly, EDL muscle from IRFI 042-treated mdx mice showed an increase in regenerating area (P < 0.05) and a decrease in necrotic area (P < 0.01) (Figures 4 and 5) .
Figure 3. Surface area of biceps muscles occupied by normal area (A), necrotic area (B), regenerating area (C), and centrally nucleated fiber area (D) in the different animal groups (n = 8 in each group). *P < 0.05 and P < 0.01 versus vehicle-treated mdx mice.
Figure 4. Surface area of EDL muscles occupied by normal area (A), necrotic area (B), regenerating area (C), and centrally nucleated fiber area (D) in the different animal groups (n = 8 in each group). *P < 0.05 and P < 0.01 versus vehicle-treated mdx mice.
Immunocytochemistry
In vehicle-treated mdx mice muscles, a strong nuclear NF-B immunoreactivity was seen in 3 to 5% of normal fibers, 70 to 80% of regenerating fibers, and 90 to 95% of centrally nucleated fibers. NF-B immunoreactivity was absent in necrotic fibers with or without myophagia. In IRFI 042-treated mdx mice, NF-B immunoreactivity was markedly reduced (Figure 6) .
Figure 6. H&E staining (A, B) and NF-B immunoreactivity (C, D) on serial sections of biceps muscles in vehicle-treated (left column) and IRFI 042-treated (right column) mdx mice. A strong nuclear immunoreactivity for the activated form of NF-B was found in fibers with central nuclei in vehicle-treated mdx mice (A, C); necrotic fibers and macrophages (top right corner) were negative. IRFI 042 treatment drastically reduced NF-B immunoreactivity (B, D). Original magnifications, x55.
CD and GSH Level Evaluations
Low CD content and a normal basal amount of GSH levels were observed in WT animals treated either with vehicle or IRFI 042 (Figure 7) . Mdx mice showed markers of oxidative stress damage, characterized by a significant increase in the tissue content of CD, accompanied by a concomitant decrease in the muscle levels of GSH, when compared to wild-type animals (P < 0.01) (Figure 7) . IRFI 042 administration in mdx mice resulted in a reduction of CD level and an increase in GSH value (P < 0.01) (Figure 7) .
Figure 7. Levels of muscular CD (A) and GSH (B) (n = 8 in each group). *P < 0.01 versus WT mice; P < 0.01 versus vehicle-treated mdx mice.
NF-B Binding Activity
NF-B DNA binding activity revealed by electrophoretic mobility shift assay was markedly increased in mdx mice administered with vehicle, when compared with WT mice (P < 0.01). Treatment with IRFI 042 drastically reduced NF-B binding activity in dystrophic mice (P < 0.01) (Figure 8A) .
Figure 8. A: Electrophoretic mobility shift assay of muscular NF-B binding activity. B: Western blot analysis of muscular TNF-. On the left of each figure are graphs with quantitative data and on the right, representative autoradiograms (n = 8 in each group). *P < 0.01 versus WT mice; P < 0.01 versus vehicle-treated mdx mice.
TNF- Expression
TNF- expression was very low in WT animals treated either with vehicle or IRFI 042. By contrast a marked increase in the expression of TNF- was observed in vehicle-treated mdx mice (P < 0.01). The administration of IRFI 042 significantly reduced TNF- expression in mdx mice (P < 0.01) (Figure 8B) .
Discussion
In this study we contributed to clarify the relationship among oxidative stress/lipid peroxidation, NF-B activation, and dystrophic process in dystrophin-deficient mdx mouse. The ameliorated functional parameters and the reduced dystrophic pathology in IRFI 042-treated animals support the hypothesis that NF-B contributes to the progression of the dystrophic damage.
Recently, Nakae and colleagues36 further supported the role of oxidative stress in DMD pathogenesis. They demonstrated an accumulation of lipofuscin, a product of oxidative degradation of cellular macromolecules caused by free radicals and redox-active metal ions, in muscles of DMD patients and mdx mice. In our study, the elevated CD levels in vehicle-treated mdx mice demonstrate an increased lipid peroxidation during the development of skeletal muscle damage. Moreover the low levels of reduced glutathione, an essential tripeptide that reacts with free radicals, indicate that the antioxidant defense mechanism is most likely unable to blunt the increased oxygen radical formation. These findings are in keeping with the reported increase of glutathione cycling components in DMD muscle fibers15 and of other lipid peroxidation products (thiobarbituric acid-reactive substances) in mdx mice9 associated with an up-regulation of several antioxidant enzymes activity, such as superoxide dismutase, catalase, and glutathione peroxidase.10 Interestingly, IRFI 042 treatment resulted in a significant reduction of CD levels and an increase in GSH content, accompanied by enhanced muscle function, blunted serum CK levels, and reduced myofiber degeneration. This suggests that lipid peroxidation inhibition by IRFI 042 could have induced a significant attenuation of membrane injury, restoring the defense mechanism in the muscle cells. In previous studies, cell oxidative state has been shown to influence the induction of NF-B activation, and in fact reactive oxygen intermediates can induce I-B phosphorylation by influencing the activity of tyrosine kinases.35 Moreover, treatment of muscle cells with N-acetyl-L-cysteine, a free radical scavenger, completely inhibits stress-induced activation of NF-B.27 In our experiment we confirmed that the augmented oxidative stress parallels an increased activation of NF-B in vehicle-administered mdx mice and consistent with these findings, we demonstrated that IRFI 042 treatment was able to strongly reduce this pathological cascade. Furthermore, this strong activation of NF-B in mdx mice muscles mostly occurs in regenerating fibers, since we reported the novel observation of a strong NF-B immunoreactivity in the nuclei of muscle cells at different levels of differentiation. IRFI 042 treatment almost completely blunted the NF-B immunoreactivity.
Several evidences demonstrated that the activation of NF-B can lead to an augmented expression of several inflammatory molecules such as IL-1ß, IL-6, TNF-, cell adhesion molecules, and matrix metalloproteinase-9,19,16,37 furthermore increased levels of some of them have been observed in inflammatory myopathies, DMD, and mdx mice.16,22,25 The abnormal increase of IL-1ß and TNF- has been suggested as a mechanism that promotes muscle wasting also in other cachexia-associated diseases.38,39 TNF- is also one of the most important NF-B inducers, contributing to a positive feedback loop. Therefore it might be postulated that a positive feedback perpetuates the effects of the activation of NF-B signaling pathway in the context of the dystrophic process. In our study we found a marked enhancement of TNF- expression in mdx muscle, strongly reduced by IRFI 042 treatment. These data are in agreement with the recently reported delay and reduction of the breakdown of dystrophic muscle in young mdx mice after pharmacological blockade of TNF- activity with Remicade.40 Moreover, our data would suggest that the protective effect of Remicade against muscle damage might be induced, at least in part, by inhibition of NF-B activity.
In mdx mice, muscle weakness and necrosis are present at 4 to 5 weeks of age, and then a morphological recovery begins with an apparent stabilization of myopathy.6,41-43 For this reason we chose to start the study at the 5th week of age and to end it by the 10th week to better verify the effect of treatment on both functional and morphological patterns. In our study, we confirmed in mdx mice the presence of weakness and fatigue already at 5 weeks of age. At 10 weeks of age, after somatic growth the strength was increased, but it was evident a trend toward a decline in strength normalized to weight and a significant increment of fatigue; these data were accompanied by the presence of muscle necrosis and regeneration. On the contrary, IRFI 042-treated mdx mice showed an increment of strength, strength normalized to weight, and a stabilization of the levels of fatigue throughout the 5-week period. This beneficial effect was also supported by the histological findings, consisting in a significant decrease in the area occupied by necrosis and an increase in the area occupied by regenerating fibers in IRFI 042-treated compared to vehicle-treated mdx mice. The IRFI 042 effects on blunting NF-B immunoreactivity in regenerating cells and on promoting regeneration are also consistent with the results obtained by Thaloor and colleagues30 of an enhanced muscle repair after NF-B inhibition in a different model of muscle damage. Other NF-B inhibitors, such as corticosteroids, have been shown to reduce necrosis and to enhance regeneration.44,45 It can be hypothesized that the well known positive effect of corticosteroids in the treatment of DMD might be partially mediated through a NF-B activity inhibition, which in turn down-regulates expression of cytokines and adhesion molecules. Moreover other drugs with demonstrated positive effects in mdx mice, such as creatine, which improves intracellular Ca++ handling46 and green tea, with antioxidant effects,47 could inhibit at different steps the NF-B signaling pathway.
In several chronic inflammatory diseases, such as rheumatoid arthritis48 and asthma,49 the role of NF-B has been demonstrated in the amplification and perpetuation of the inflammatory process. In the mdx mouse model, Kumar and colleagues demonstrated a skeletal muscle-specific activation of NF-B even before the onset of muscular dystrophy and postulated that this might lead to an augmented level of TNF- and IL-1ß.16 Herein we have obtained data showing that a cross-talk between oxidative stress/lipid peroxidation and NF-B activation is likely to occur in mdx mice, in turn triggering an inflammatory cascade contributing to muscle damage (Figure 9) . Finally modulation of this cascade, obtained through IRFI 042 treatment, might represent a rational pharmacological approach to limit muscle damage in dystrophinopathies. However this hypothesis deserves further experiments to delineate possible therapeutic implications in DMD.
Figure 9. Synthetic scheme of the dystrophic process pathological cascade, showing hypothetical interactions between oxidative stress/lipid peroxidation, and NF-B activation in mdx mice.
Figure 5. Histological appearance of biceps (A) and EDL (B) muscles in vehicle-treated (left column) and IRFI 042-treated (right column) mdx mice. IRFI 042-treated mdx mice showed decreased necrosis and enhanced muscle fiber regeneration in both muscles. H&E staining. Original magnifications, x55.
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作者单位：From the Departments of Neuroscience, Psychiatry, and Anaesthesiology* and Experimental Medicine and Pharmacology, University of Messina, Messina, Italy