CCAAT/enhancer-binding protein beta promotes muscle stem cell quiescence through regulation of quiescence-associated genes.

Regeneration of skeletal muscle depends on resident muscle stem cells called satellite cells that in healthy, uninjured muscle remain quiescent (noncycling). After activation and expansion of satellite cells postinjury, satellite cell numbers return to uninjured levels and return to mitotic quiescence. Here, we show that the transcription factor CCAAT/enhancer-binding protein beta (C/EBPß) is required to maintain quiescence of satellite cells in uninjured muscle. We show that C/EBPß is expressed in quiescent satellite cells in vivo and upregulated in noncycling myoblasts in vitro. Loss of C/EBPß in satellite cells promotes their premature exit from quiescence resulting in spontaneous activation and differentiation of the stem cell pool. Forced expression of C/EBPß in myoblasts inhibits proliferation by upregulation of 28 quiescence-associated genes. Furthermore, we find that caveolin-1 is a direct transcriptional target of C/EBPß and is required for cell cycle exit in muscle satellite cells expressing C/EBPß. The induction of mitotic quiescence is considered necessary for the long-term maintenance of adult stem cell populations with dysregulation driving increased differentiation of progenitors and depletion of the stem cell pool. Our findings place C/EBPß as an important transcriptional regulator of muscle satellite cell quiescence.

Mdm2 promotes myogenesis through the ubiquitination and degradation of CCAAT/enhancer-binding protein ß.

Myogenesis is a tightly regulated differentiation process during which precursor cells express in a coordinated fashion the myogenic regulatory factors, while down-regulating the satellite cell marker Pax7. CCAAT/Enhancer-binding protein ß (C/EBPß) is also expressed in satellite cells and acts to maintain the undifferentiated state by stimulating Pax7 expression and by triggering a decrease in MyoD protein expression. Herein, we show that C/EBPß protein is rapidly down-regulated upon induction of myogenesis and this is not due to changes in Cebpb mRNA expression. Rather, loss of C/EBPß protein is accompanied by an increase in Mdm2 expression, an E3 ubiquitin ligase. We demonstrate that Mdm2 interacts with, ubiquitinates and targets C/EBPß for degradation by the 26 S proteasome, leading to increased MyoD expression. Knockdown of Mdm2 expression in myoblasts using a shRNA resulted in high C/EBPß levels and a blockade of myogenesis, indicating that Mdm2 is necessary for myogenic differentiation. Primary myoblasts expressing the shMdm2 construct were unable to contribute to muscle regeneration when grafted into cardiotoxin-injured muscle. The differentiation defect imposed by loss of Mdm2 could be partially rescued by loss of C/EBPß, suggesting that the regulation of C/EBPß turnover is a major role for Mdm2 in myoblasts. Taken together, we provide evidence that Mdm2 regulates entry into myogenesis by targeting C/EBPß for degradation by the 26 S proteasome.

Targeted ablation of the cellular inhibitor of apoptosis 1 (cIAP1) attenuates denervation-induced skeletal muscle atrophy.

BACKGROUND: Skeletal muscle atrophy is a pathological condition that contributes to morbidity in a variety of conditions including denervation, cachexia, and aging. Muscle atrophy is characterized as decreased muscle fiber cross-sectional area and protein content due, in part, to the proteolytic activities of two muscle-specific E3 ubiquitin ligases: muscle RING-finger 1 (MuRF1) and muscle atrophy F-box (MAFbx or Atrogin-1). The nuclear factor-kappa B (NF-κB) pathway has emerged as a critical signaling network in skeletal muscle atrophy and has become a prime therapeutic target for the treatment of muscle diseases. Unfortunately, none of the NF-κB targeting drugs are currently being used to treat these diseases, likely because of our limited knowledge and specificity, for muscle biology and disease. The cellular inhibitor of apoptosis 1 (cIAP1) protein is a positive regulator of tumor necrosis factor alpha (TNFα)-mediated classical NF-κB signaling, and cIAP1 loss has been shown to enhance muscle regeneration during acute and chronic injury. METHODS: Sciatic nerve transection in wild-type, cIAP1-null and Smac mimetic compound (SMC)-treated mice was performed to investigate the role of cIAP1 in denervation-induced atrophy. Genetic in vitro models of C2C12 myoblasts and primary myoblasts were also used to examine the role of classical NF-κB activity in cIAP1-induced myotube atrophy. RESULTS: We found that cIAP1 expression was upregulated in denervated muscles compared to non-denervated controls 14 days after denervation. Genetic and pharmacological loss of cIAP1 attenuated denervation-induced muscle atrophy and overexpression of cIAP1 in myotubes was sufficient to induce atrophy. The induction of myotube atrophy by cIAP1 was attenuated when the classical NF-κB signaling pathway was inhibited. CONCLUSIONS: These results demonstrate the cIAP1 is an important mediator of NF-κB/MuRF1 signaling in skeletal muscle atrophy and is a promising therapeutic target for muscle wasting diseases.

CCAAT/Enhancer Binding Protein ß inhibits myogenic differentiation via ID3.

Myogenesis is regulated by the coordinated expression of muscle regulatory factors, a family of transcription factors that includes MYOD, MYF5, myogenin and MRF4. Muscle regulatory factors are basic helix-loop-helix transcription factors that heterodimerize with E proteins to bind the regulatory regions of target genes. Their activity can be inhibited by members of the Inhibitor of DNA binding and differentiation (ID) family, which bind E-proteins with high affinity, thereby preventing muscle regulatory factor-dependent transcriptional responses. CCAAT/Enhancer Binding protein beta (C/EBPß) is a transcription factor expressed in myogenic precursor cells that acts to inhibit myogenic differentiation, though the mechanism remains poorly understood. We identify Id3 as a novel C/EBPß target gene that inhibits myogenic differentiation. Overexpression of C/EBPß stimulates Id3 mRNA and protein expression, and is required for C/EBPß-mediated inhibition of myogenic differentiation. Misexpression of C/EBPß in myogenic precursors, such as in models of cancer cachexia, prevents the differentiation of myogenic precursors and we show that loss of Id3 rescues differentiation under these conditions, suggesting that the stimulation of Id3 expression by C/EBPß is an important mechanism by which C/EBPß inhibits myogenic differentiation.

SOX7 Is Required for Muscle Satellite Cell Development and Maintenance.

Satellite cells are skeletal-muscle-specific stem cells that are activated by injury to proliferate, differentiate, and fuse to enable repair. SOX7, a member of the SRY-related HMG-box family of transcription factors is expressed in quiescent satellite cells. To elucidate SOX7 function in skeletal muscle, we knocked down Sox7 expression in embryonic stem cells and primary myoblasts and generated a conditional knockout mouse in which Sox7 is excised in PAX3+ cells. Loss of Sox7 in embryonic stem cells reduced Pax3 and Pax7 expression. In vivo, conditional knockdown of Sox7 reduced the satellite cell population from birth, reduced myofiber caliber, and impaired regeneration after acute injury. Although Sox7-deficient primary myoblasts differentiated normally, impaired myoblast fusion and increased sensitivity to apoptosis in culture and in vivo were observed. Taken together, these results indicate that SOX7 is dispensable for myogenesis but is necessary to promote satellite cell development and survival.

Induction of CCAAT/Enhancer-Binding Protein ß Expression With the Phosphodiesterase Inhibitor Isobutylmethylxanthine Improves Myoblast Engraftment Into Dystrophic Muscle.

UNLABELLED: Duchenne muscular dystrophy (DMD), caused by mutations in the dystrophin gene, is the most common muscular dystrophy. Characterized by rounds of muscle degeneration and regeneration, DMD features progressive muscle wasting and is fatal. One approach for treatment is transplantation of muscle progenitor cells to repair and restore dystrophin expression to damaged muscle. However, the success of this approach has been limited by difficulties in isolating large numbers of myogenic progenitors with strong regenerative potential, poor engraftment, poor survival of donor cells, and limited migration in the diseased muscle. We demonstrate that induction of the transcription factor CCAAT/enhancer-binding protein ß (C/EBPß) using the cyclic adenosine monophosphate phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) results in enhanced myoblast expansion in culture and increased satellite cell marker expression. When equal numbers of IBMX-treated cells were transplanted into dystrophic muscle, they contributed to muscle repair more efficiently than did vehicle-treated cells and engrafted into the satellite cell niche in higher numbers, demonstrating improved cell migration from the site of injury and enhanced survival after transplantation. Thus, pharmacologic stimulation of C/EBPß expression reprograms myoblasts to a more stem cell-like state, promotes expansion in culture, and improves engraftment such that better transplantation outcomes are achieved. SIGNIFICANCE: Duchenne muscular dystrophy is a genetic disorder for which no cure exists. One therapeutic approach is transplantation of myogenic progenitors to restore dystrophin to damaged muscle, but this approach is limited by poor engraftment of cultured myoblasts. Transient upregulation of CCAAT/enhancer-binding protein ß in primary myoblasts using the phosphodiesterase isobutylmethylxanthine (IBMX) increases satellite cell marker expression in cultured myoblasts, improves their migration, and increases their survival after transplantation. When transplanted into C57BL/10ScSn-mdx/J mice , IBMX-treated myoblasts restored dystrophin expression and were able to occupy the satellite cell niche more efficiently than controls. A myoblast culture approach that reprograms myoblasts to a more primitive state, resulting in improved transplantation outcomes and reinvigorating research into myoblast transplantation as a viable therapeutic approach, is described.

CCAAT/enhancer binding protein ß is required for satellite cell self-renewal.

BACKGROUND: Postnatal growth and repair of skeletal muscle relies upon a population of quiescent muscle precursor cells, called satellite cells that can be activated to proliferate and differentiate into new myofibers, as well as self-renew to replenish the satellite cell population. The balance between differentiation and self-renewal is critical to maintain muscle tissue homeostasis, and alterations in this equilibrium can lead to chronic muscle degeneration. The transcription factor CCAAT/enhancer binding protein beta (C/EBPß) is expressed in Pax7+ satellite cells of healthy muscle and is downregulated during myoblast differentiation. Persistent expression of C/EBPß upregulates Pax7, inhibits MyoD, and blocks myogenic differentiation. METHODS: Using genetic tools to conditionally abrogate C/EBPß expression in Pax7+ cells, we examined the role of C/EBPß in self-renewal of satellite cells during muscle regeneration. RESULTS: We find that loss of C/EBPß leads to precocious differentiation at the expense of self-renewal in primary myoblast and myofiber cultures. After a single muscle injury, C/EBPß-deficient satellite cells fail to self-renew resulting in a reduction of satellite cells available for future rounds of regeneration. After a second round of injury, muscle regeneration is impaired in C/EBPß conditional knockout mice compared to wild-type control mice. We find that C/EBPß can regulate Notch2 expression and that restoration of Notch activity in myoblasts lacking C/EBPß prevents precocious differentiation. CONCLUSIONS: These findings demonstrate that C/EBPß is a novel regulator of satellite cell self-renewal during muscle regeneration acting at least in part through Notch2.

Expression of CCAAT/Enhancer Binding Protein Beta in Muscle Satellite Cells Inhibits Myogenesis in Cancer Cachexia.

Cancer cachexia is a paraneoplastic syndrome that causes profound weight loss and muscle mass atrophy and is estimated to be the cause of up to 30% of cancer deaths. Though the exact cause is unknown, patients with cancer cachexia have increased muscle protein catabolism. In healthy muscle, injury activates skeletal muscle stem cells, called satellite cells, to differentiate and promote regeneration. Here, we provide evidence that this mechanism is inhibited in cancer cachexia due to persistent expression of CCAAT/Enhancer Binding Protein beta (C/EBPß) in muscle myoblasts. C/EBPß is a bzip transcription factor that is expressed in muscle satellite cells and is normally downregulated upon differentiation. However, in myoblasts exposed to a cachectic milieu, C/EBPß expression remains elevated, despite activation to differentiate, resulting in the inhibition of myogenin expression and myogenesis. In vivo, cancer cachexia results in increased number of Pax7+ cells that also express C/EBPß and the inhibition of normal repair mechanisms. Loss of C/EBPß expression in primary myoblasts rescues differentiation under cachectic conditions without restoring myotube size, indicating that C/EBPß is an important inhibitor of myogenesis in cancer cachexia.

Retinoic acid promotes myogenesis in myoblasts by antagonizing transforming growth factor-beta signaling via C/EBPß.

BACKGROUND: The effects of transforming growth factor-beta (TGFß) are mediated by the transcription factors Smad2 and Smad3. During adult skeletal myogenesis, TGFß signaling inhibits the differentiation of myoblasts, and this can be reversed by treatment with retinoic acid (RA). In mesenchymal stem cells and preadipocytes, RA treatment can function in a non-classical manner by stimulating the expression of Smad3. Smad3 can bind to and prevent the bzip transcription factor CCAAT/enhancer-binding protein beta (C/EBPß) from binding DNA response elements in target promoters, thereby affecting cell differentiation. In skeletal muscle, C/EBPß is highly expressed in satellite cells and myoblasts and is downregulated during differentiation. Persistent expression of C/EBPß in myoblasts inhibits their differentiation. METHODS: Using both C2C12 myoblasts and primary myoblasts, we examined the regulation of C/EBPß expression and activity following treatment with TGFß and RA. RESULTS: We demonstrate that treatment with RA upregulates Smad3, but not Smad2 expression in myoblasts, and can partially rescue the block of differentiation induced by TGFß. RA treatment reduces C/EBPß occupancy of the Pax7 and Smad2 promoters and decreased their expression. RA also inhibits the TGFß-mediated phosphorylation of Smad2, which may also contribute to its pro-myogenic activities. TGFß treatment of C2C12 myoblasts stimulates C/EBPß expression, which in turn can stimulate Pax7 and Smad2 expression, and inhibits myogenesis. Loss of C/EBPß expression in myoblasts partially restores differentiation in the presence of TGFß. CONCLUSIONS: TGFß acts, at least in part, to inhibit myogenesis by upregulating the expression of C/EBPß, as treatment with RA or loss of C/EBPß can partially rescue differentiation in TGFß-treated cells. This work identifies a pro-myogenic role for Smad3, through the inhibition of C/EBPß's actions in myoblasts, and reveals mechanisms of crosstalk between RA and TGFß signaling pathways.