AP-1 binds to a putative cAMP response element of the MyoD1 promoter and negatively modulates MyoD1 expression in dividing myoblasts

Prediction and Experimental Verification of a Hierarchical Transcription Factor Regulatory Network of Porcine Myoglobin (Mb)

Myoglobin is a key chemical component that determines meat's color and affects consumers' purchase intentions. In this work, we firstly identified the promoter sequence of the Mb gene from the primary assembly of high-throughput genome sequencing in pigs, and predicted its potential transcription factors by LASAGNA. Through the data mining of the mRNA expression profile of longissimus dorsi muscle of different pig breeds, we constructed a hierarchical interplay network of Mb-TFs (Myoglobin-Transcription Factors), consisting of 16 adaptive transcription factors and 23 secondary transcription factors. The verification of gene expression in longissimus dorsi muscle showed that the Mb mRNA and encoded protein were significantly (p < 0.05) more abundant in Bama pigs than Yorkshire pigs. The qRT-PCR (Real-Time Quantitative Reverse Transcription PCR) validation on genes of the Mb-TFs network showed that FOS, STAT3, STAT1, NEFL21, NFE2L2 and MAFB were significant positive regulatory core transcription factors of Mb-TFs network in Bama pigs, whereas ATF3 was the secondary transcription factor most responsible for the activation of the above transcription factors. Our study provides a new strategy to unravel the mechanism of pork color formation, based on public transcriptome and genome data analysis.

JNK activation in TA and EDL muscle is load-dependent in rats receiving identical excitation patterns

As the excitation-contraction coupling is inseparable during voluntary exercise, the relative contribution of the mechanical and neural input on hypertrophy-related molecular signalling is still poorly understood. Herein, we use a rat in-vivo strength exercise model with an electrically-induced standardized excitation pattern, previously shown to induce a load-dependent increase in myonuclear number and hypertrophy, to study acute effects of load on molecular signalling. We assessed protein abundance and specific phosphorylation of the four protein kinases FAK, mTOR, p70S6K and JNK after 2, 10 and 28 min of a low- or high-load contraction, in order to assess the effects of load, exercise duration and muscle-type on their response to exercise. Specific phosphorylation of mTOR, p70S6K and JNK was increased after 28 min of exercise under the low- and high-load protocol. Elevated phosphorylation of mTOR and JNK was detectable already after 2 and 10 min of exercise, respectively, but greatest after 28 min of exercise, and JNK phosphorylation was highly load-dependent. The abundance of all four kinases was higher in TA compared to EDL muscle, p70S6K abundance was increased after exercise in a load-independent manner, and FAK and JNK abundance was reduced after 28 min of exercise in both the exercised and control muscles. In conclusion, the current study shows that JNK activation after a single resistance exercise is load-specific, resembling the previously reported degree of myonuclear accrual and muscle hypertrophy with repetition of the exercise stimulus.

Myofiber stretch induces tensile and shear deformation of muscle stem cells in their native niche

Muscle stem cells (MuSCs) are requisite for skeletal muscle regeneration and homeostasis. Proper functioning of MuSCs, including activation, proliferation, and fate decision, is determined by an orchestrated series of events and communication between MuSCs and their niche. A multitude of biochemical stimuli are known to regulate MuSC fate and function. However, in addition to biochemical factors, it is conceivable that MuSCs are subjected to mechanical forces during muscle stretch-shortening cycles because of myofascial connections between MuSCs and myofibers. MuSCs respond to mechanical forces in vitro, but it remains to be proven whether physical forces are also exerted on MuSCs in their native niche and whether they contribute to the functioning and fate of MuSCs. MuSC deformation in their native niche resulting from mechanical loading of ex vivo myofiber bundles was visualized utilizing mT/mG double-fluorescent Cre-reporter mouse and multiphoton microscopy. MuSCs were subjected to 1 h pulsating fluid shear stress (PFSS) with a peak shear stress rate of 6.5 Pa/s. After PFSS treatment, nitric oxide, messenger RNA (mRNA) expression levels of genes involved in regulation of MuSC proliferation and differentiation, ERK 1/2, p38, and AKT activation were determined. Ex vivo stretching of extensor digitorum longus and soleus myofiber bundles caused compression as well as tensile and shear deformation of MuSCs in their niche. MuSCs responded to PFSS in vitro with increased nitric oxide production and an upward trend in iNOS mRNA levels. PFSS enhanced gene expression of c-Fos, Cdk4, and IL-6, whereas expression of Wnt1, MyoD, Myog, Wnt5a, COX2, Rspo1, Vangl2, Wnt10b, and MGF remained unchanged. ERK 1/2 and p38 MAPK signaling were also upregulated after PFSS treatment. We conclude that MuSCs in their native niche are subjected to force-induced deformations due to myofiber stretch-shortening. Moreover, MuSCs are mechanoresponsive, as evidenced by PFSS-mediated expression of factors by MuSCs known to promote proliferation.

The Switch from NF-YAl to NF-YAs Isoform Impairs Myotubes Formation

NF-YA, the regulatory subunit of the trimeric transcription factor (TF) NF-Y, is regulated by alternative splicing (AS) generating two major isoforms, “long” (NF-YAl) and “short” (NF-YAs). Muscle cells express NF-YAl. We ablated exon 3 in mouse C2C12 cells by a four-guide CRISPR/Cas9n strategy, obtaining clones expressing exclusively NF-YAs (C2-YAl-KO). C2-YAl-KO cells grow normally, but are unable to differentiate. Myogenin and—to a lesser extent, MyoD— levels are substantially lower in C2-YAl-KO, before and after differentiation. Expression of the fusogenic Myomaker and Myomixer genes, crucial for the early phases of the process, is not induced. Myomaker and Myomixer promoters are bound by MyoD and Myogenin, and Myogenin overexpression induces their expression in C2-YAl-KO. NF-Y inactivation reduces MyoD and Myogenin, but not directly: the Myogenin promoter is CCAAT-less, and the canonical CCAAT of the MyoD promoter is not bound by NF-Y in vivo. We propose that NF-YAl, but not NF-YAs, maintains muscle commitment by indirectly regulating Myogenin and MyoD expression in C2C12 cells. These experiments are the first genetic evidence that the two NF-YA isoforms have functionally distinct roles.

MiR-501-3p Forms a Feedback Loop with FOS, MDFI, and MyoD to Regulate C2C12 Myogenesis

Skeletal muscle plays an essential role in maintaining body energy homeostasis and body flexibility. Loss of muscle mass leads to slower wound healing and recovery from illness, physical disability, poor quality of life, and higher health care costs. So, it is critical for us to understand the mechanism of skeletal muscle myogenic differentiation for maintaining optimal health throughout life. miR-501-3p is a novel muscle-specific miRNA, and its regulation mechanism on myoblast myogenic differentiation is still not clear. We demonstrated that FOS was a direct target gene of miR-501-3p, and MyoD regulated miR-501-3p host gene Clcn5 through bioinformatics prediction. Our previous laboratory experiment found that MDFI overexpression promoted C2C12 myogenic differentiation and MyoD expression. The database also showed there is an FOS binding site in the MDFI promoter region. Therefore, we hypothesize that miR-501-3p formed a feedback loop with FOS, MDFI, and MyoD to regulate myoblast differentiation. To validate our hypothesis, we demonstrated miR-501-3p function in the proliferation and differentiation period of C2C12 cells by transfecting cells with miR-501-3p mimic and inhibitor. Then, we confirmed there is a direct regulatory relationship between miR-501-3p and FOS, MyoD and miR-501-3p, FOS and MDFI through QPCR, dual-luciferase reporter system, and ChIP experiments. Our results not only expand our understanding of the muscle myogenic development mechanism in which miRNA and genes participate in controlling skeletal muscle development, but also provide treatment strategies for skeletal muscle or metabolic-related diseases in the future.

Inhibition of the JNK/MAPK signaling pathway by myogenesis-associated miRNAs is required for skeletal muscle development

Skeletal muscle differentiation is controlled by multiple cell signaling pathways, however, the JNK/MAPK signaling pathway dominating this process has not been fully elucidated. Here, we report that the JNK/MAPK pathway was significantly downregulated in the late stages of myogenesis, and in contrast to P38/MAPK pathway, it negatively regulated skeletal muscle differentiation. Based on the PAR-CLIP-seq analysis, we identified six elevated miRNAs (miR-1a-3p, miR-133a-3p, miR-133b-3p, miR-206-3p, miR-128-3p, miR-351-5p), namely myogenesis-associated miRNAs (mamiRs), negatively controlled the JNK/MAPK pathway by repressing multiple factors for the phosphorylation of the JNK/MAPK pathway, including MEKK1, MEKK2, MKK7, and c-Jun but not JNK protein itself, and as a result, expression of transcriptional factor MyoD and mamiRs were further promoted. Our study revealed a novel double-negative feedback regulatory pattern of cell-specific miRNAs by targeting phosphorylation kinase signaling cascade responsible for skeletal muscle development.

Comparison of skeletal muscle miRNA and mRNA profiles among three pig breeds

The pig is an important source of animal protein, and is also an ideal model for human disease. There are significant differences in growth rate, muscle mass, and meat quality between different breeds. To understand the molecular mechanisms underlying porcine skeletal muscle phenotypes, we performed mRNA and miRNA profiling of muscle from three different breeds of pig, Landrace (lean-type), Tongcheng (obese-type), and Wuzhishan (mini-type) by Solexa sequencing. Forty-three genes and 106 miRNAs were differentially expressed between Landrace and Tongcheng pigs, 92 genes and 151 miRNAs were differentially expressed between Tongcheng and Wuzhishan pigs, and 145 genes and 156 miRNAs were differential expressed between Landrace and Wuzhishan pigs. Gene ontology analysis suggested that genes differentially expressed between Landrace and Tongcheng pigs were mainly involved in the biological processes of oxidative stress and muscle organ development. Meanwhile, for Tongcheng vs Wuzhishan and Landrace vs Wuzhishan pigs, the differentially expressed genes were involved in fatty acid metabolism, oxidative stress, muscle contraction, and muscle organ development, processes that are closely related to meat quality. To investigate the molecular mechanisms underlying meat quality diversity based on differentially expressed genes and miRNAs, interaction networks were constructed, according to target prediction results and integration analysis of up-regulated genes with down-regulated miRNAs or down-regulated genes with up-regulated miRNAs. Our findings identify candidate genes and miRNAs associated with muscle development and indicate their potential roles in muscle phenotype variance between different pig breeds. These results serve as a foundation for further studies on muscle development and molecular breeding.

Wnt3a signal pathways activate MyoD expression by targeting cis-elements inside and outside its distal enhancer

Wnt proteins are secreted cytokines and several Wnts are expressed in the developing somites and surrounding tissues. Without proper Wnt stimulation, the organization of the dermomyotome and myotome can become defective. These Wnt signals received by somitic cells can lead to activation of Pax3/Pax7 and myogenic regulatory factors (MRFs), especially Myf5 and MyoD. However, it is currently unknown whether Wnts activate Myf5 and MyoD through direct targeting of their cis-regulatory elements or via indirect pathways. To clarify this issue, in the present study, we tested the regulation of MyoD cis-regulatory elements by Wnt3a secreted from human embryonic kidney (HEK)-293T cells. We found that Wnt3a activated the MyoD proximal 6.0k promoter (P6P) only marginally, but highly enhanced the activity of the composite P6P plus distal enhancer (DE) reporter through canonical and non-canonical pathways. Further screening of the intervening fragments between the DE and the P6P identified a strong Wnt-response element (WRE) in the upstream −8 to −9k region (L fragment) that acted independently of the DE, but was dependent on the P6P. Deletion of a Pax3/Pax7-targeted site in the L fragment significantly reduced its response to Wnt3a, implying that Wnt3a activates the L fragment partially through Pax3/Pax7 action. Binding of β-catenin and Pax7 to their target sites in the DE and the L fragment respectively was also demonstrated by ChIP. These observations demonstrated the first time that Wnt3a can directly activate MyoD expression through targeting cis-elements in the DE and the L fragment.

We found that Wnt3a can directly activate MyoD expression through targeting cis-elements in the distal enhancer and in the upstream −8 to −9k region. A novel Pax3/Pax7-involved pathway and both canonical and non-canonical Wnt pathways are involved in this activation.

Characterization of white spot syndrome virus immediate-early gene promoters

Twenty-one immediate-early (IE) genes of white spot syndrome virus (WSSV) have been identified so far. However, the transcriptional regulation of WSSV IE genes remains largely unknown. In this report, the 5′ flanking regions of 18 WSSV IE genes were cloned and eight functional promoter regions were identified. WSSV IE gene promoters normally contained a TATA box approximately 30 bp upstream of the transcriptional initiation site. Also, the cyclic AMP response element (CRE; TGACGTCA) was frequently found within the WSSV IE promoter regions. Mutations of the CREs of WSSV IE promoters P403 and P465 reduced their activity significantly, suggesting that these elements have a role in WSSV IE gene transcription. Our findings provide a more global view of WSSV IE gene promoters and will facilitate the in-depth investigation of viral gene transcriptional regulation.

Emerin inhibits Lmo7 binding to the Pax3 and MyoD promoters and expression of myoblast proliferation genes

X-linked Emery–Dreifuss muscular dystrophy (X-EDMD) is caused by mutations in the inner nuclear membrane protein emerin. Previous studies have shown that emerin binds to and inhibits the activity of LIM domain only 7 (Lmo7), a transcription factor that regulates the expression of genes implicated in X-EDMD. Here, we analyzed Lmo7 function in C2C12 myoblast differentiation and its regulation by emerin. We found that Lmo7 was required for proper myoblast differentiation. Lmo7-downregulated myoblasts exhibited reduced expression of Pax3, Pax7, Myf5 and MyoD, whereas overexpression of GFP–Lmo7 increased the expression of MyoD and Myf5. Upon myotube formation, Lmo7 shuttled from the nucleus to the cytoplasm, concomitant with reduced expression of MyoD, Pax3 and Myf5. Importantly, we show that Lmo7 bound the Pax3, MyoD and Myf5 promoters both in C2C12 myoblasts and in vitro. Because emerin inhibited Lmo7 activity, we tested whether emerin competed with the MyoD promoter for binding to Lmo7 or whether emerin sequestered promoter-bound Lmo7 to the nuclear periphery. Supporting the competition model, emerin binding to Lmo7 inhibited Lmo7 binding to and activation of the MyoD and Pax3 promoters. These findings support the hypothesis that the functional interaction between emerin and Lmo7 is crucial for temporally regulating the expression of key myogenic differentiation genes.

Folate deficiency regulates expression of DNA polymerase β in response to oxidative stress

Folate deficiency has been shown to influence carcinogenesis by creating an imbalance in the base excision repair (BER) pathway, affecting BER homeostasis. The inability to mount a BER response to oxidative stress in a folate-deficient environment results in the accumulation of DNA repair intermediates, i.e., DNA strand breaks. Our data indicate that upregulation of β-pol expression in response to oxidative stress is inhibited by folate deficiency at the level of gene expression. Alteration in the expression of β-pol in a folate-deficient environment is not due to epigenetic changes in the core promoter of the β-pol gene, i.e., the CpG islands within the β-pol promoter remain unmethylated in the presence or absence of folate. However, the promoter analysis studies show a differential binding of regulatory factors to the -36 to -7 region (the folic acid-response region, FARR) within the core promoter of β-pol. Moreover, we observe a tight correlation between the level of binding of regulatory factors with the FARR and inhibition of β-pol expression. Based on these findings, we propose that folate deficiency results in an upregulation/stability of negative regulatory factors interacting with FARR, repressing the upregulation of the β-pol gene in response to oxidative stress.

Identification and characterization of the Mustang promoter: regulation by AP-1 during myogenic differentiation

We previously identified Mustang (musculoskeletal temporally activated novel gene) with expression exclusively in the musculoskeletal system. Although its expression is almost undetectable in intact bone, it is robustly upregulated during bone regeneration. It is also abundantly expressed in adult skeletal muscle and tendon. As such, Mustang represents a marker for these cells and thus identifying its promoter would enable us to characterize its transcriptional regulation. To this end, we have isolated and characterized a 1512-bp mouse genomic clone representing the Mustang 5'-flanking region and identified a transcription start site, a TATA box, and multiple putative transcription factor binding sites (including AP-1 and AP-2). The activity of this promoter was detected in musculoskeletal cells and embryonic fibroblasts, even exceeding levels (145%) of the control SV40 promoter (in C2C12 cells). Further, the contribution of specific AP-1 and AP-2 sites was determined with serially deleted and mutated promoter constructs. Results indicate that one of the four AP-1 sites is required for substantial transcriptional activation, as its specific deletion or mutation decreases promoter activity by 32% and 40%, respectively. In contrast, deletion of both identified AP-2 sites results in only a 12% decrease in promoter activity. We further characterized the key AP-1 site by EMSA and determined that in both proliferating and differentiating C2C12 cells, only c-Fos, Fra-2 and JunD were required for transcriptional activation. Mustang's restricted tissue specificity and strong promoter makes this gene an ideal candidate for utilization in cell lineage studies that could unveil cellular/molecular mechanisms responsible for musculoskeletal development and regeneration.

Gene expression in skeletal tissues: application of laser capture microdissection

Tissue differentiation is based on the expression of transcription factors, receptors for cytokines, and nuclear receptors that regulate a specific phenotype. The purpose of this study was to select cells from various skeletal tissues in order to analyse differential gene expression of cells in the native environment in vivo. It is a difficult task to obtain cells from skeletal tissues, such as cartilage, periost, bone and muscle, that are structured together and do not exist as individual organs. We used laser capture microdissection which permits the selection and isolation of individual cells from tissue sections. The RNA isolated from these tissues was used for reverse transcriptase-polymerase chain reactions for molecular analysis. We analysed the expression of transcription factors (cFOS, cbfa1, MyoD), receptors for cytokines, nuclear receptors, alkaline phosphatase and the structural proteins osteocalcin and collagen II. The results obtained demonstrate differential patterns of gene expression according to the tissue arrangement in their native in vivo environment, with reliable interpretation of the functions of the analysed genes in the context of intact skeletal tissue physiology.

Inhibition of MuSK expression by CREB interacting with a CRE-like element and MyoD

The type I receptor-like protein tyrosine kinase MuSK is essential for the neuromuscular junction formation. MuSK expression is tightly regulated during development, but the underlying mechanisms were unclear. Here we identified a novel mechanism by which MuSK expression may be regulated. A cyclic AMP response element (CRE)-like element in the 5'-flanking region of the MuSK gene binds to CREB1 (CRE-binding protein 1). Mutation of this element increases the MuSK promoter activity, suggesting a role for CREB1 in attenuation of MuSK expression. Interestingly, CREB mutants unable to bind to DNA also inhibit MuSK promoter activity, suggesting a CRE-independent inhibitory mechanism. In agreement, CREB1 could inhibit a mutant MuSK transgene reporter whose CRE site was mutated. We provide evidence that CREB interacts directly with MyoD, a myogenic factor essential for MuSK expression in muscle cells. Suppression of CREB expression by small interfering RNA increases MuSK promoter activity. These results demonstrate an important role for CREB1 in the regulation of MuSK expression.

An initial blueprint for myogenic differentiation

We have combined genome-wide transcription factor binding and expression profiling to assemble a regulatory network controlling the myogenic differentiation program in mammalian cells. We identified a cadre of overlapping and distinct targets of the key myogenic regulatory factors (MRFs)--MyoD and myogenin--and Myocyte Enhancer Factor 2 (MEF2). We discovered that MRFs and MEF2 regulate a remarkably extensive array of transcription factor genes that propagate and amplify the signals initiated by MRFs. We found that MRFs play an unexpectedly wide-ranging role in directing the assembly and usage of the neuromuscular junction. Interestingly, these factors also prepare myoblasts to respond to diverse types of stress. Computational analyses identified novel combinations of factors that, depending on the differentiation state, might collaborate with MRFs. Our studies suggest unanticipated biological insights into muscle development and highlight new directions for further studies of genes involved in muscle repair and responses to stress and damage.

Rhabdomyosarcoma development in mice lacking Trp53 and Fos: tumor suppression by the Fos protooncogene

The Fos protein, a major component of the AP-1 transcription factor, is essential for osteoclast differentiation, acts as an oncogene, potentiates transforming signals, and controls invasive growth and angiogenesis during tumor progression. To investigate a potential genetic interaction between the Trp53 and Fos pathways, Trp53/Fos double knockout mice were generated. These mice develop highly proliferative and invasive rhabdomyosarcomas of the facial and orbital regions, with more than 90% penetrance at 6 months of age. Rhabdomyosarcoma cell lines established from the primary tumors express characteristic muscle-specific markers, and reexpression of Fos is associated with enhanced apoptosis in vitro. Moreover, Fos is able to repress Pax7 expression in rhabdomyosarcoma cell lines and primary myoblasts, suggesting a molecular link to genetic alterations involved in human rhabdomyosarcomas.

Composition and function of AP-1 transcription complexes during muscle cell differentiation

The role of activating protein-1 (AP-1) in muscle cells is currently equivocal. While some studies propose that AP-1 is inhibitory for myogenesis, others implicate a positive role in this process. We tested whether this variation may be due to different properties of the AP-1 subunit composition in differentiating cells. Using Western analysis we show that c-Jun, Fra-2, and JunD are expressed throughout the time course of differentiation. Phosphatase assays indicate that JunD and Fra-2 are phosphorylated in muscle cells and that at least two isoforms of each are expressed in muscle cells. Electrophoretic mobility shift assays combined with antibody supershifts indicate the appearance of Fra-2 as a major component of the AP-1 DNA binding complex in differentiating cells. In this context it appears that Fra-2 heterodimerizes with c-Jun and JunD. Studying the c-jun enhancer in reporter gene assays we observed that the muscle transcription factors MEF2A and MyoD can contribute to robust transcriptional activation of the c-jun enhancer. In differentiating muscle cells mutation of the MEF2 site reduces transactivation of the c-jun enhancer and MEF2A is the predominant MEF2 isoform binding to this cis element. Transcriptional activation of an AP-1 site containing reporter gene (TRE-Luc) is enhanced under differentiation conditions compared with growth conditions in C2C12 muscle cells. Further studies indicate that Fra-2 containing AP-1 complexes can transactivate the MyoD enhancer/promoter. Thus, an AP-1 complex containing Fra-2 and c-Jun or JunD is consistent with muscle differentiation, indicating that AP-1 function during myogenesis is dependent on its subunit composition.

Indications of associations of the porcine FOS proto-oncogene with skeletal muscle fibre traits

Skeletal muscle fibre characteristics are key determinants of meat quality. High fibre diameters and shifting towards higher white fibre proportions lead to increasing R-values (degree of desamination of adenosine) and lactate-production, resulting in high incidences of pale, soft, exudative (PSE) meat and stress susceptibility in European and American pig breeds. Development of muscle fibres including their enzymes, is regulated by the MyoD-gene family together with transcription factors like FOS. We report on the associations between the chromosomal region of FOS with skeletal muscle fibre and metabolism traits. The BB genotype representing the European Pietrain breed had 10.9% more white fibres with fibre diameters decreased by 6.1%, with 3.9% higher R-values and 8.5% higher lactate levels than the AA genotype of the Chinese Meishan. Lactate levels and R-values per microm of fibre diameter were increased to 18.4 and 11.6% in the BB genotype. The contrast between the two quantitative trait loci (QTL) alleles associated with a polymorphism in the FOS gene explained up to 5.13% of the total variance. A new TaiI-restriction fragment length polymorphism (RFLP) connected to a Asn258/Ser mutation, located in a transcription activator region, was used to map FOS between markers S0115 and Sw581 on SSC7. The QTLs for skeletal muscle fibre and metabolism traits have been mapped to the marker interval around FOS. The present data suggest that variability in FOS gene may underlie phenotypic variation in skeletal muscle fibre and metabolism traits in the pig.

Opposing functions of ATF2 and Fos-like transcription factors in c-Jun-mediated myogenin expression and terminal differentiation of avian myoblasts

With the aim to identify the oncoprotein partners implicated in the c-Jun myogenic influence, we carried out stable transfection experiments of c-Jun and/or ATF2, Fra2, c-Fos overexpression in avian myoblasts. Before induction of differentiation, c-Jun repressed myoblast withdrawal from the cell cycle, as did a TPA treatment. However, after serum removal, unlike TPA, c-Jun significantly stimulated myoblast differentiation. In search for specific partners involved in this dual influence, we found that a reduction in the amounts of c-Fos and Fra2 and an increase in c-Jun proteins occurred at cell confluence, a situation likely to favor cooperation between c-Jun and ATF2 during terminal differentiation. Whereas c-Fos and Fra2 cooperated with c-Jun to abrogate myoblast withdrawal from the cell cycle and terminal differentiation, ATF2 co-expression potentiated the positive myogenic c-Jun influence. In addition, myogenin expression was a positive target of this cooperation and this regulation occurred through a stimulation of myogenin promoter activity: (1) whereas c-Fos or Fra2 co-expression abrogated c-Jun stimulatory activity on this promoter, ATF2 co-expression potentiated this influence; (2) using a dominant negative ATF2 mutant, we established that c-Jun transcriptional activity required functionality of endogenous ATF2. These data suggest that through this dual myogenic influence due to cooperations with different partners, c-Jun is involved in the control of duration of myoblast proliferation and thereafter of fusion efficiency.

Regulation of Cbl molecular interactions by the co-receptor molecule CD43 in human T cells

CD43, one of the most abundant glycoproteins on the T cell surface, has been implicated in selection and maturation of thymocytes and migration, adhesion, and activation of mature T cells. The adapter molecule Cbl has been shown to be a negative regulator of Ras. Furthermore, it may also regulate intracellular signaling through the formation of several multi-molecular complexes. Here we investigated the role of Cbl in the CD43-mediated signaling pathway in human T cells. Unlike T cell receptor signaling, the interaction of the adapter protein Cbl with Vav and phosphatidylinositol 3-kinase, resulting from CD43-specific signals, is independent of Cbl tyrosine phosphorylation, suggesting an alternative mechanism of interaction. CD43 signals induced a Cbl serine phosphorylation-dependent interaction with the tau-isoform of 14-3-3. protein. Protein kinase C-mediated Cbl serine phosphorylation was required for this interaction, because the PKC inhibitor RO-31-8220 prevented it, as well as 14-3-3 dimerization. Moreover, mutation of Cbl serine residues 619, 623, 639, and 642 abolished the interaction between Cbl and 14-3-3. Overexpression of Cbl in Jurkat cells inhibited the CD43-dependent activation of the mitogen-activated protein kinase (MAPK) pathway and AP-1 transcriptional activity, confirming nevertheless a negative role for Cbl in T cell signaling. However, under normal conditions, PKC activation resulting from CD43 engagement was required to activate the MAPK pathway, suggesting that phosphorylation of Cbl on serine residues by PKC and its association with 14-3-3 molecules may play a role in preventing the Cbl inhibitory effect on the Ras-MAPK pathway. These data suggest that by inducing its phosphorylation on serine residues, CD43-mediated signals may regulate the molecular associations and functions of the Cbl adapter protein.

CD43-mediated signals induce DNA binding activity of AP-1, NF-AT, and NFkappa B transcription factors in human T lymphocytes

Although numerous reports document a role for CD43 in T cell signaling, the direct participation of this molecule in cell activation has been questioned. In this study we show that CD43 ligation on human normal peripheral T cells was sufficient to induce interleukin-2, CD69, and CD40-L gene expression, without requiring signals provided by additional receptor molecules. This response was partially inhibited by cyclosporin A and staurosporine, suggesting the participation of both the Ca(2+) and the protein kinase C pathways in CD43 signaling. Consistent with the transient CD43-dependent intracellular Ca(2+) peaks reported by others, signals generated through the CD43 molecule resulted in the induction of NF-AT DNA binding activity. CD43-dependent signals resulted also in AP-1 and NFkappaB activation, probably as a result of protein kinase C involvement. AP-1 complexes bound to the AP-1 sequence contained c-Jun, and those bound to the NF-AT-AP-1 composite site contained c-Jun and Fos. NFkappaB complexes containing p65 could be found as early as 1 h after CD43 cross-linking, suggesting that CD43 participates in early events of T cell activation. The induction of the interleukin-2, CD69, and CD-40L genes and the participation of AP-1, NF-AT, and NFkappaB in the CD43-mediated signaling cascade implicate an important role for this molecule in the regulation of gene expression and cell function.

Regulation of mouse SP-B gene promoter by AP-1 family members

The regulatory role of activator protein-1 (AP-1) family members in mouse surfactant protein (SP) B (mSP-B) promoter function was assessed in the mouse lung epithelial cell line MLE-15. Expression of recombinant Jun B and c-Jun inhibited mSP-B promoter activity by 50-75%. Although c-Fos expression did not alter mSP-B transcription, Jun D enhanced mSP-B promoter activity and reversed inhibition of mSP-B by c-Jun or Jun B. A proximal AP-1 binding site (-18 to -10 bp) was identified that overlaps a thyroid transcription factor-1 binding site. Mutation of this proximal AP-1 site blocked both Jun B inhibition and Jun D enhancement and partially blocked c-Jun inhibition of promoter activity. Promoter deletion mutants were used to identify additional sequences mediating the inhibitory effects of c-Jun in the distal region from -397 to -253 bp. The AP-1 element in this distal site (-370 to -364 bp) is part of a composite binding site wherein AP-1, cAMP response element binding protein, thyroid transcription factor-1, and nuclear factor I interact. Point mutation of the distal AP-1 binding site partially blocked c-Jun-mediated inhibition of the SP-B promoter. Both stimulatory (Jun D) and inhibitory (c-Jun/Jun B) effects of AP-1 family members on mSP-B promoter activity are mediated by distinct cis-acting elements in the mSP-B 5'-flanking region.

T cell activation through the CD43 molecule leads to Vav tyrosine phosphorylation and mitogen-activated protein kinase pathway activation

CD43, the most abundant membrane protein of T lymphocytes, is able to initiate signals that lead to Ca2+ mobilization and interleukin-2 production, yet the molecular events involved in signal transduction pathway of the CD43 molecule are only beginning to be understood. We have shown recently that cross-linking CD43 on the cell surface of human T lymphocytes with the anti-CD43 monoclonal antibody L10 leads to CD43-Fyn kinase interactions and to Fyn phosphorylation on tyrosine residues. This interaction seems to be mediated by the SH3 domain of Fyn and a proline-rich sequence located in the cytoplasmic domain of CD43. Here we show that CD43-specific activation of human T lymphocytes induced tyrosine phosphorylation of the adaptor protein Shc and of the guanine exchange factor Vav, as well as the formation of a macromolecular complex that comprises Shc, GRB2, and Vav. CD43 ligation resulted in enhanced formation of Vav.SLP-76 complexes and in the activation and nuclear translocation of ERK2. Cross-linking of the CD43 molecule in 3T3-CD43(+) cells induced luciferase activity from a construct under the control of the Fos serum responsive element. Altogether, these data suggest that the mitogen-activated protein kinase pathway is involved in CD43-dependent interleukin-2 gene expression.

Dihydroxy bile acids activate the transcription of cyclooxygenase-2

Bile acids, endogenous promoters of gastrointestinal cancer, activate protein kinase C (PKC) and the activator protein-1 (AP-1) transcription factor. Because other activators of PKC and AP-1 induce cyclooxygenase-2 (COX-2), we determined the effects of bile acids on the expression of COX-2 in human esophageal adenocarcinoma cells. Treatment with the dihydroxy bile acids chenodeoxycholate and deoxycholate resulted in an approximately 10-fold increase in the production of prostaglandin E2 (PGE2). Enhanced synthesis of PGE2 was associated with a marked increase in the levels of COX-2 mRNA and protein, with maximal effects at 8-12 and 12-24 h, respectively. In contrast, neither cholic acid nor conjugated bile acids affected the levels of COX-2 or the synthesis of PGE2. Nuclear run-off assays and transient transfections with a human COX-2 promoter construct showed that induction of COX-2 mRNA by chenodeoxycholate and deoxycholate was due to increased transcription. Bile acid-mediated induction of COX-2 was blocked by inhibitors of PKC activity, including calphostin C and staurosporine. Treatment with bile acid enhanced the phosphorylation of c-Jun and increased binding of AP-1 to DNA. These data are important because dihydroxy bile acid-mediated induction of COX-2 may explain, at least in part, the tumor-promoting effects of bile acids.

A protein kinase C-activating phorbol ester enhances transcription of the human DBH gene through a cyclic AMP response element in SK-N-BE(2)C cells

Protein kinase C (PKC) activation after treatment of human neuroblastoma SK-N-BE(2)C cells with phorbol 12-myristate 13-acetate (PMA) was found to enhance transcription of the human dopamine beta-hydroxylase (DBH) in those cells. To identify which cis-acting element is responsive to the PMA treatment during DBH gene expression, we employed transient transfection assays with serially deleted constructs of the human DBH gene's 5' upstream region fused to the chloramphenicol acetyltransferase (CAT) gene. Treatment of transfected cells with PMA resulted in an approximate threefold increase in CAT expression for all deletion constructs ranging from -978 bp to -262 bp, while the enhancement did not occur with a construct shortened to -114 bp. The region between -262 and -114 bp from the initiation site of transcription contains several cis-regulatory elements including a cyclic AMP response element (CRE) and putative AP1 and YY1 sequences. Site-directed mutagenesis of those cis-acting elements were performed to identify which of the elements mediated the PMA-induced transcriptional enhancement. Substitution of bases in the putative AP1 site containing in part a putative YY1 sequence did not effect the PMA inducibility. However, specific mutations in the CRE sequence abolished the PMA-inducible effect. Changing the CRE sequence into an authentic AP1 sequence (TGACGTCC --> TGACTCA) did not affect the PMA inducibility, suggesting that AP1 factors might interact with the new AP1 site upon PKC activation. A specific PKC inhibitor, GF109203X, completely inhibited the stimulatory effect of PMA on the expression of the human DBH gene. PMA induced an increase in the DBH mRNA level as detected by Northern blot analysis. Gel retardation showed that the binding of nuclear factors to CRE, putative YY1, and AP1 was sequence specific. Our data suggest that the enhancement of the human DBH gene expression by PMA treatment is mediated by the CRE motif in the 5' upstream region of the gene, and occurs via a PKC-dependent pathway.

Glucocorticoid effects on the skeletal muscle differentiation program: analysis of clonal proliferation, morphological differentiation and the expression of muscle-specific and regulatory genes

We examined the effect of glucocorticoids on the proliferation and differentiation of skeletal muscle cells using the C2C12 cell line. We found that treatment with glucocorticoids enhanced muscle cell differentiation but had only minor effects on the clonal growth rate of C2C12 cells. The stimulatory effect of glucocorticoids on myogenic differentiation was reflected in the increased expression of muscle-specific genes, creatine kinase (CK) and acetylcholine receptor gamma subunit (AChR). Dexamethasone had no effect on CK and AChR mRNA stability and enhanced transcription from a CAT reporter genes containing the 3.3kb 5' flanking region of the murine CK gene (-3300MCK-CAT). Since dexamethasone did not affect the expression levels of the myogenic regulatory genes such as myoD and myogenin, the enhancement of muscle-specific transcription might reflect an increase in the functional activity of the regulatory proteins. Other possible mechanisms involved in the differentiation-enhancing effect of glucocorticoids are discussed.

v-src induces prostaglandin synthase 2 gene expression by activation of the c-Jun N-terminal kinase and the c-Jun transcription factor

A consensus cyclic AMP response element (CRE) in the murine prostaglandin synthase-2 (PGS2) promoter is essential for pgs2 gene expression induced by pp60v-src, the v-src oncogene product. In this study, we investigate (i) the transcription factors active at the PGS2 "CRE site" in response to v-src activation and (ii) the signal transduction pathways by which pp60v-src activates these transcription factors. Transient transfection assays with pgs2 promoter/luciferase reporter chimeric genes suggest that c-Jun mediates v-src-induced pgs2 gene expression. Antibody supershift experiments demonstrate that c-Jun can participate in a complex with the pgs2 promoter CRE site. Moreover, in vitro immuno-complex assays demonstrate that pp60v-src expression strongly activates c-Jun N-terminal kinase (JNK1) enzyme activity. Serines 63 and 73, the sites of c-Jun phosphorylation by JNK, are essential for v-src-induced, pgs2 promoter-mediated luciferase expression. Cotransfection studies with plasmids expressing wild-type JNK, dominant-negative JNK, and dominant-negative MEKK-1 confirm that activation of the Ras/MEKK-1/JNK/c-Jun pathway is required for v-src-induced pgs2 gene expression. Overexpression of either wild-type ERK-1 or ERK-2 proteins also potentiate v-src-mediated luciferase expression driven by the pgs2 promoter, and expression of dominant-negative mutants of ERK-1, ERK-2, or Raf-1 attenuate this response. Thus, in response to v-src expression, a Ras/MEKK-1/JNK signal transduction pathway activating c-Jun and a Ras/Raf-1/ERK pathway converge to mediate pgs2 gene expression via the CRE site in the pgs2 promoter.

Cell death in the failing heart: role of an unnatural growth response to overload

Hypertrophy of the overloaded heart, characterized by an increased number of sarcomeres, provides an adaptive, short-term response. However, when cardiac overload is long-standing, the hypertrophic response appears to cause shortened myocyte survival. The mechanisms responsible for the deleterious effects of chronic myocardial hypertrophy may include a maladaptive growth response of the mature heart. Because terminally differentiated adult cardiac myocytes have little or no capacity to divide, stimuli that promote growth in the overloaded adult heart cannot lead to normal cell division. Instead, overload initiates an unnatural growth response that appears to shorten cardiac myocyte survival, possibly because the same growth factors that mediate the hypertrophic response of the adult heart can also induce programmed cell death (apoptosis). The converting enzyme inhibitors and nitrates, which have growth-inhibitory as well as vasodilator effects, may improve prognosis in heart failure by inhibiting the production of transcription factors. These transcription factors stimulate both the unnatural growth response to overload and stimuli that lead to apoptosis. Since both beta-adrenergic agonists and cytokines, such as tumor necrosis factor-alpha, can stimulate production of similar transcription factors, evidence suggests that beta blockers and vesnarinone improve the prognosis in patients with heart failure possibly because of their ability to inhibit maladaptive growth.

Differential regulation of bcl-2, bax, c-fos, junB, and krox-24 expression in the cerebellum of Purkinje cell degeneration mutant mice

Purkinje cell degeneration (pcd) is an autosomal recessive mutation in the mouse characterized by an almost complete loss of cerebellar Purkinje neurons between postnatal days 22 and 28. The pcd gene has not been identified, however, a relationship between activation of specific genes and cell death has been suggested in other models of neuronal cell death. In the present study we analyzed the expression of several candidate cell death effector genes (bax, c-fos, junB, krox-24) and a cell death repressor gene (bcl-2) in the cerebellum of pcd homozygotes and wild-type mice. At postnatal day 22, when Purkinje cells start to degenerate, levels of c-fos, junB, and krox-24 mRNA increased about 5-fold in mutants. To the contrary, the amount of bcl-2 mRNA declined and bax transcripts remained unchanged compared to wild-type animals. Immunoreactivity for c-Fos and Jun could be detected exclusively in cerebellar Purkinje neurons of pcd mice but not in wild-types, whereas the number of Bcl-2 immunopositive Purkinje cells decreased significantly in mutants. Both double labeling experiments and immunostaining of consecutive sections revealed lack of colocalization of Jun with Bcl-2. These results demonstrate an induction of members of the fos and jun family and a downregulation of antiapoptotic bcl-2 in cerebellar Purkinje neurons that are destined to die. Fos and Jun transcription factor proteins may be implicated in the regulation of bcl-2 expression and in the signal cascade leading to Purkinje cell death.

Regulation of vertebrate muscle differentiation by thyroid hormone: the role of the myoD gene family

Skeletal myoblasts have their origin early in embryogenesis within specific somites. Determined myoblasts are committed to a myogenic fate; however, they only differentiate and express a muscle-specific phenotype after they have received the appropriate environmental signals. Once proliferating myoblasts enter the differentiation programme they withdraw from the cell cycle and form post-mitotic multinucleated myofibres (myogenesis); this transformation is accompanied by muscle-specific gene expression. Muscle development is associated with complex and diverse protein isoform transitions, generated by differential gene expression and mRNA splicing. The myofibres are in a state of dynamic adaptation in response to hormones, mechanical activity and motor innervation, which modulate differential gene expression and splicing during this functional acclimatisation. This review will focus on the profound effects of thyroid hormone on skeletal muscle, which produce alterations in gene and isoform expression, biochemical properties and morphological features that precipitate in modified contractile/mechanical characteristics. Insight into the molecular events that control these events was provided by the recent characterisation of the MyoD gene family, which encodes helix-loop-helix proteins; these activate muscle-specific transcription and serve as targets for a variety of physiological stimuli. The current hypothesis on hormonal regulation of myogenesis is that thyroid hormones (1) directly regulate the myoD and contractile protein gene families, and (2) induce thyroid hormone receptor-transcription factor interactions critical to gene expression.