Bingwen Jin

TNF-α acts via p38 MAPK to stimulate expression of the ubiquitin ligase atrogin1/MAFbx in skeletal muscle

Atrogin1/MAFbx is an ubiquitin ligase that mediates muscle atrophy in a variety of catabolic states. We recently found that H2O2 stimulates atrogin1/MAFbx gene expression. Since the cytokine tumor necrosis factor‐α (TNF‐α) stimulates both reactive oxygen production and general activity of the ubiquitin conjugating pathway, we hypothesized that TNF‐α would also increase atrogin1/MAFbx gene expression. As with H2O2, we found that TNF‐α exposure up‐regulates atrogin1/MAFbx mRNA within2hin C2C12 myotubes. Intraperitoneal injection of TNF‐α increased atrogin1/MAFbx mRNA in skeletal muscle of adult mice within 4 h. Exposing myotubes to either TNF‐α or H2O2 also produced general activation of the mitogen‐activated protein kinases (MAPKs): p38, ERK1/2, and JNK. The increase in atrogin1/MAFbx gene expression induced by TNF‐α was not altered significantly by ERK inhibitor PD98059 or the JNK inhibitor SP600125. In contrast, atrogin1/MAFbx up‐regulation and the associated increase in ubiquitin conjugating activity were both blunted by p38 inhibitors, either SB203580 or curcumin. These data suggest that TNF‐α acts via p38 to increase atrogin1/MAFbx gene expression in skeletal muscle.—Li, Y.‐P., Chen, Y., John, J., Moylan, J., Jin, B., Mann, D. L., Reid, M. B. TNF‐α acts via p38 MAPK to stimulate expression of the ubiquitin ligase atrogin1/MAFbx in skeletal muscle. FASEB J. 19, 362–370 (2005)

Curcumin prevents lipopolysaccharide‐induced atrogin‐1/MAFbx upregulation and muscle mass loss

Because elevated ubiquitin ligase atrogin‐1/MAFbx and MuRF1 mediate skeletal muscle wasting associated with various catabolic conditions, the signaling pathways involved in the upregulation of these genes under pathological conditions are considered therapeutic targets. AKT and NF‐κB have been previously shown to regulate the expression of atrogin‐1/MAFbx or MuRF1, respectively. In addition, we recently found that p38 MAPK mediates TNF‐α upregulation of atrogin‐1/MAFbx expression, suggesting that multiple signaling pathways mediate muscle wasting in inflammatory diseases. To date, however, these advances have not resulted in a practical clinical intervention for disease‐induced muscle wasting. In the present study, we tested the effect of curcumin—a non‐toxic anti‐inflammatory reagent that inhibits p38 and NF‐κB—on lipopolysaccharide (LPS)‐induced muscle wasting in mice. Daily intraperitoneal (i.p.) injection of curcumin (10–60 µg/kg) for 4 days inhibited, in a dose‐dependent manner, the LPS‐stimulated (1 mg/kg, i.p.) increase of atrogin‐1/MAFbx expression in gastrocnemius and extensor digitorum longus (EDL) muscles, resulting in the attenuation of muscle protein loss. It should also be noted that curcumin administration did not alter the basal expression of atrogin‐1/MAFbx, nor did it affect LPS‐stimulated MuRF1 and polyubiquitin expression. LPS activated p38 and NF‐κB, while inhibiting AKT; whereas, curcumin administration inhibited LPS‐stimulated p38 activation, without altering the effect of LPS on NF‐κB and AKT. These results indicate that curcumin is effective in blocking LPS‐induced loss of muscle mass through the inhibition of p38‐mediated upregulation of atrogin‐1/MAFbx. J. Cell. Biochem. 100: 960–969, 2007.

TACE release of TNF-α mediates mechanotransduction-induced activation of p38 MAPK and myogenesis

Skeletal muscle responds to mechanical stimulation by activating p38 MAPK, a key signal for myogenesis. However, the mechanotransduction mechanism that activates p38 is unknown. Here we show that mechanical stimulation of myoblasts activates p38 and myogenesis through stimulating TNF-α release by TNF-α converting enzyme (TACE). In C2C12 or mouse primary myoblasts cultured in growth medium, static stretch activated p38 along with ERK1/2, JNK and AKT. Disrupting TNF-α signaling by TNF-α-neutralizing antibody or knocking out TNF-α receptors blocked stretch activation of p38, but not ERK1/2, JNK or AKT. Stretch also activated differentiation markers MEF2C, myogenin, p21 and myosin heavy chain in a TNF-α- and p38-dependent manner. Stretch stimulated the cleavage activity of TACE. Conversely, TACE inhibitor TAPI or TACE siRNA abolished stretch activation of p38. In addition, conditioned medium from stretched myoblast cultures activated p38 in unstretched myoblasts, which required TACE activity in the donor myoblasts, and TNF-α receptors in the recipient myoblasts. These results indicate that posttranscriptional activation of TACE mediates the mechanotransduction that activates p38-dependent myogenesis via the release of TNF-α.

C/EBPβ mediates tumour‐induced ubiquitin ligase atrogin1/MAFbx upregulation and muscle wasting

Upregulation of ubiquitin ligase atrogin1/MAFbx and muscle wasting are hallmarks of cancer cachexia; however, the underlying mechanism is undefined. Here, we describe a novel signalling pathway through which Lewis lung carcinoma (LLC) induces atrogin1/MAFbx upregulation and muscle wasting. C2C12 myotubes treated with LLC‐conditioned medium (LCM) rapidly activates p38 MAPK and AKT while inactivating FoxO1/3, resulting in atrogin1/MAFbx upregulation, myosin heavy chain loss, and myotube atrophy. The p38α/β MAPK inhibitor SB202190 blocks the catabolic effects. Upon activation, p38 associates with C/EBPβ resulting in its phosphorylation and binding to a C/EBPβ‐responsive cis‐element in the atrogin1/MAFbx gene promoter. The promoter activity is stimulated by LCM via p38β‐mediated activation of the C/EBPβ‐responsive cis‐element, independent of the adjacent FoxO1/3‐responsive cis‐elements in the promoter. In addition, p38 activation is observed in the muscle of LLC tumour‐bearing mice, and SB202190 administration blocks atrogin1/MAFbx upregulation and muscle protein loss. Furthermore, C/EBPβ−/− mice are resistant to LLC tumour‐induced atrogin1/MAFbx upregulation and muscle wasting. Therefore, activation of the p38β MAPK–C/EBPβ signalling pathway appears a key component of the pathogenesis of LLC tumour‐induced cachexia.

Proteasome inhibition improves diaphragm function in congestive heart failure rats.

In congestive heart failure (CHF), diaphragm weakness is known to occur and is associated with myosin loss and activation of the ubiquitin-proteasome pathway. The effect of modulating proteasome activity on myosin loss and diaphragm function is unknown. The present study investigated the effect of in vivo proteasome inhibition on myosin loss and diaphragm function in CHF rats. Coronary artery ligation was used as an animal model for CHF. Sham-operated rats served as controls. Animals were treated with the proteasome inhibitor bortezomib (intravenously) or received saline (0.9%) injections. Force generating capacity, cross-bridge cycling kinetics, and myosin content were measured in diaphragm single fibers. Proteasome activity, caspase-3 activity, and MuRF-1 and MAFbx mRNA levels were determined in diaphragm homogenates. Proteasome activities in the diaphragm were significantly reduced by bortezomib. Bortezomib treatment significantly improved diaphragm single fiber force generating capacity (approximately 30-40%) and cross-bridge cycling kinetics (approximately 20%) in CHF. Myosin content was approximately 30% higher in diaphragm fibers from bortezomib-treated CHF rats than saline. Caspase-3 activity was decreased in diaphragm homogenates from bortezomib-treated rats. CHF increased MuRF-1 and MAFbx mRNA expression in the diaphragm, and bortezomib treatment diminished this rise. The present study demonstrates that treatment with a clinically used proteasome inhibitor improves diaphragm function by restoring myosin content in CHF.

TIMP3: a physiological regulator of adult myogenesis

Myogenic differentiation in adult muscle is normally suppressed and can be activated by myogenic cues in a subset of activated satellite cells. The switch mechanism that turns myogenesis on and off is not defined. In the present study, we demonstrate that tissue inhibitor of metalloproteinase 3 (TIMP3), the endogenous inhibitor of TNFα-converting enzyme (TACE), acts as an on–off switch for myogenic differentiation by regulating autocrine TNFα release. We observed that constitutively expressed TIMP3 is transiently downregulated in the satellite cells of regenerating mouse hindlimb muscles and differentiating C2C12 myoblasts. In C2C12 myoblasts, perturbing TIMP3 downregulation by overexpressing TIMP3 blocks TNFα release, p38 MAPK activation, myogenic gene expression and myotube formation. TNFα supplementation at a physiological concentration rescues myoblast differentiation. Similarly, in the regenerating soleus, overexpression of TIMP3 impairs release of TNFα and myogenic gene expression, and delays the formation of new fibers. In addition, downregulation of TIMP3 is mediated by the myogenesis-promoting microRNA miR-206. Thus, TIMP3 is a physiological regulator of myogenic differentiation.

Src mediates the mechanical activation of myogenesis by activating TNFα-converting enzyme

Summary Mechanical stimulation affects many biological aspects in living cells through mechanotransduction. In myogenic precursor cells (MPCs), mechanical stimulation activates p38 mitogen-activated protein kinase (MAPK), a key regulator of myogenesis, via activating TNF&agr;-converting enzyme (TACE, also known as ADAM17), to release autocrine TNF&agr;. However, the signaling mechanism of mechanical activation of TACE is unknown. Because TACE possesses the structural features of substrates of the non-receptor tyrosine kinase Src, we tested the hypothesis that Src mediates mechanical activation of TACE in MPCs. We observed that mechanical stretch of C2C12 or primary rat myoblasts rapidly activates Src, which in turn interacts and colocalizes with TACE, resulting in tyrosine phosphorylation and activation of TACE. Particularly, Src activates TACE via the phosphorylation of amino acid residue Tyr702 in the intracellular tail of TACE, resulting in increased TNF&agr; release and p38 activation. Src inhibition or deficiency blocks stretch activation of the TACE–p38-MAPK signaling, resulting in impaired myogenic gene expression. In response to functional overloading, Src and TACE are activated in mouse soleus muscle. Further, overloading-induced myogenesis and regeneration are impaired in the soleus of Src+/− mice. Therefore, Src mediates mechano-activation of TACE and myogenesis.