The serum response factor is extensively modified by phosphorylation following its synthesis in serum-stimulated fibroblasts

SRF phosphorylation by glycogen synthase kinase-3 promotes axon growth in hippocampal neurons

The growth of axons is an intricately regulated process involving intracellular signaling cascades and gene transcription. We had previously shown that the stimulus-dependent transcription factor, serum response factor (SRF), plays a critical role in regulating axon growth in the mammalian brain. However, the molecular mechanisms underlying SRF-dependent axon growth remains unknown. Here we report that SRF is phosphorylated and activated by GSK-3 to promote axon outgrowth in mouse hippocampal neurons. GSK-3 binds to and directly phosphorylates SRF on a highly conserved serine residue. This serine phosphorylation is necessary for SRF activity and for its interaction with MKL-family cofactors, MKL1 and MKL2, but not with TCF-family cofactor, ELK-1. Axonal growth deficits caused by GSK-3 inhibition could be rescued by expression of a constitutively active SRF. The SRF target gene and actin-binding protein, vinculin, is sufficient to overcome the axonal growth deficits of SRF-deficient and GSK-3-inhibited neurons. Furthermore, short hairpin RNA-mediated knockdown of vinculin also attenuated axonal growth. Thus, our findings reveal a novel phosphorylation and activation of SRF by GSK-3 that is critical for SRF-dependent axon growth in mammalian central neurons.

NADPH oxidase 4 mediates TGF-β-induced smooth muscle α-actin via p38MAPK and serum response factor

In contrast to other cell types, vascular smooth muscle cells modify their phenotype in response to external signals. NADPH oxidase 4 (Nox4) is critical for maintenance of smooth muscle gene expression; however, the underlying mechanisms are incompletely characterized. Using smooth muscle α-actin (SMA) as a prototypical smooth muscle gene and transforming growth factor-β (TGF-β) as a differentiating agent, we examined Nox4-dependent signaling. TGF-β increases Nox4 expression and activity in human aortic smooth muscle cells (HASMC). Transfection of HASMC with siRNA against Nox4 (siNox4) abolishes TGF-β-induced SMA expression and stress fiber formation. siNox4 also significantly inhibits TGF-β-stimulated p38MAPK phosphorylation, as well as that of its substrate, mitogen-activated protein kinase-activated protein kinase-2. Moreover, the p38MAPK inhibitor SB-203580 nearly completely blocks the SMA increase induced by TGF-β. Inhibition of either p38MAPK or NADPH oxidase-derived reactive oxygen species impairs the TGF-β-induced phosphorylation of Ser103 on serum response factor (SRF) and reduces its transcriptional activity. Binding of SRF to myocardin-related transcription factor (MRTF) is also necessary, because downregulation of MRTF by siRNA abolishes TGF-β-induced SMA expression. Taken together, these data suggest that Nox4 regulates SMA expression via activation of a p38MAPK/SRF/MRTF pathway in response to TGF-β.

PKA-dependent phosphorylation of serum response factor inhibits smooth muscle-specific gene expression

OBJECTIVE

Our goal was to identify phosphorylation sites that regulate serum response factor (SRF) activity to gain a better understanding of the signaling mechanisms that regulate SRF's involvement in smooth muscle cell (SMC)-specific and early response gene expression.

METHODS AND RESULTS

By screening phosphorylation-deficient and mimetic mutations in SRF(-/-) embryonic stem cells, we identified T159 as a phosphorylation site that significantly inhibits SMC-specific gene expression in an embryonic stem cell model of SMC differentiation. This residue conforms to a highly conserved consensus cAMP-dependent protein kinase (PKA) site, and in vitro and in vivo labeling studies demonstrated that it was phosphorylated by PKA. Results from gel shift and chromatin immunoprecipitation assays demonstrated that T159 phosphorylation inhibited SRF binding to SMC-specific CArG elements. Interestingly, the myocardin factors could at least partially rescue the effects of the T159D mutation under some conditions, but this response was promoter specific. Finally, PKA signaling had much less of an effect on c-fos promoter activity and SRF binding to the c-fos CArG.

CONCLUSIONS

Our results indicate that phosphorylation of SRF by PKA inhibits SMC-specific transcription suggesting a novel signaling mechanism for the control of SMC phenotype.

Critical role of serum response factor in pulmonary myofibroblast differentiation induced by TGF-beta

Transforming growth factor-beta (TGF-beta) is a cytokine implicated in wound healing and in the pathogenesis of pulmonary fibrosis. TGF-beta stimulates myofibroblast differentiation characterized by expression of contractile smooth muscle (SM)-specific proteins such as SM-alpha-actin. In the present study, we examined the role of serum response factor (SRF) in the mechanism of TGF-beta-induced pulmonary myofibroblast differentiation of human lung fibroblasts (HLF). TGF-beta stimulated SM-alpha-actin expression in HLF, which paralleled with a profound induction of SRF expression and activity. Inhibition of SRF by the pharmacologic SRF inhibitor (CCG-1423), or via adenovirus-mediated transduction of SRF short hairpin RNA (shSRF), blocked the expression of both SRF and SM-alpha-actin in response to TGF-beta without affecting Smad-mediated signaling of TGF-beta. However, forced expression of SRF on its own did not promote SM-alpha-actin expression, whereas expression of the constitutively transactivated SRF fusion protein (SRF-VP16) was sufficient to induce SM-alpha-actin expression, suggesting that both expression and transactivation of SRF are important. Activation of protein kinase A (PKA) by forskolin or iloprost resulted in a significant inhibition of SM-alpha-actin expression induced by TGF-beta, and this was associated with inhibition of both SRF expression and activity, but not of Smad-mediated gene transcription. In summary, this is the first direct demonstration that TGF-beta-induced pulmonary myofibroblast differentiation is mediated by SRF, and that inhibition of myofibroblast differentiation by PKA occurs through down-regulation of SRF expression levels and SRF activity, independent of Smad signaling.

Serum response factor mediates NGF-dependent target innervation by embryonic DRG sensory neurons

Serum response factor (SRF) is a prototypic transcription factor that mediates stimulus-dependent gene expression. Here, we show that SRF mediates NGF signaling, axonal growth, branching, and target innervation by embryonic DRG sensory neurons. Conditional deletion of the murine SRF gene in DRGs results in no deficits in neuronal viability or differentiation but causes defects in extension and arborization of peripheral axonal projections in the target field in vivo, similar to the target innervation defects observed in mice lacking NGF. Moreover, SRF is both necessary and sufficient for NGF-dependent axonal outgrowth in vitro, and NGF regulates SRF-dependent gene expression and axonal outgrowth through activation of both MEK/ERK and MAL signaling pathways. These findings show that SRF is a major effector of both MEK/ERK and MAL signaling by NGF and that SRF is a key mediator of NGF-dependent target innervation by embryonic sensory neurons.

Endothelial-specific ablation of serum response factor causes hemorrhaging, yolk sac vascular failure, and embryonic lethality

Background

Serum response factor (SRF), a member of the MADS box family of nuclear transcription factors, plays an important role in cardiovascular development and function. Numerous studies demonstrate a central role for SRF in regulating smooth and cardiac muscle cell gene expression. Consistent with this, loss of SRF function blocks differentiation of coronary vascular smooth muscle cells from proepicardial precursors, indicating SRF is necessary for coronary vasculogenesis. The role of SRF in endothelial cell contribution during early vascular development, however, has not been addressed. To investigate this, we generated transgenic mice lacking expression of SRF in endothelial cells. Mice expressing Cre recombinase (Tie2Cre⁺) under Tie2 promoter control were bred to mice homozygous for Srf alleles containing loxP recombination sites within the Srf gene (Srf^f/f). Tie2 is a tyrosine kinase receptor expressed predominantly on endothelial cells that mediates signalling during different stages of blood vessel remodelling. Resulting embryos were harvested at specific ages for observation of physical condition and analysis of genotype.

Results

Tie2Cre^+/-Srf^f/f embryos appeared to develop normally compared to wild-type littermates until embryonic day 10.5 (E10.5). Beginning at E11.5, Tie2Cre^+/-Srf^f/f embryos exhibited cerebrovascular hemorrhaging and severely disrupted vascular networks within the yolk sac. Hemorrhaging in mutant embryos became more generalized with age, and by E14.5, most Tie2Cre^+/-Srf^f/f embryos observed were nonviable and grossly necrotic. Hearts of mutant embryos were smaller relative to overall body weight compared to wild-type littermates. Immunohistochemical analysis revealed the presence of vascular endothelial cells; however, vessels failed to undergo appropriate remodelling. Initial analysis by electron microscopy suggested a lack of appropriate cell-cell contacts between endothelial cells. Consistent with this, disrupted E-cadherin staining patterns were observed in mutant embryos.

Conclusion

These results provide the first in vivo evidence in support of a role for SRF in endothelial cell function and strongly suggest SRF is required for appropriate vascular remodelling.

Transcription factor serum response factor is selectively involved in the mechanisms of long-term synapse-specific plasticity

Our previous studies demonstrated that acquisition of nociceptive sensitization in common snails is accompanied by long-term facilitation of the responses of defensive behavior command neurons LPl1 and RPl1 to sensory stimuli, this being dependent on the processes of translation and transcription. The mechanism of induction of long-term synaptic facilitation at the sensory inputs of neurons from chemoreceptors on the head involves cAMP and the immediate early gene transcription factor C/EBP (CCAAT-enhancer-binding protein), while regulation of the other sensory input of neurons LPl1 and RPl1 - from mechanoreceptors on the head - depends on protein kinase C. The present report describes studies of the involvement of the transcription factor serum response factor (SRF) in the processes of the synapse-specific plasticity of neuron LPl1 during the acquisition of sensitization in snails. The acquisition of sensitization during intracellular administration of oligonucleotides specifically inhibiting SRF led to the selective suppression of synaptic facilitation in the responses of neuron LPl1 to tactile stimulation of the snail's head. Synaptic facilitation of responses to chemical stimulation of the head and tactile stimulation of the foot developed just as in neurons in control sensitized animals. The results were assessed in relation to a hypothesis postulating that synapse-specific plasticity on learning may occur because of selective neurochemical "projection" of synaptic connections to various genes within neurons.

Persistent increased DNA-binding and expression of serum response factor occur with epilepsy-associated long-term plasticity changes

We have previously shown that NMDA receptor activation during status epilepticus (SE) is required to produce epilepsy in in vitro and in vivo models. As in human symptomatic epilepsy, the epilepsy in these models is permanent, suggesting that the pathological activation of NMDA receptors causes permanent plasticity changes in the brain. Ca(2+) influx through NMDA receptors is known to transiently activate a key transcription factor, serum response factor (SRF). Thus, we investigated whether this factor, in terms of its expression and ability to bind to the consensus serum response element, was altered long term in the pilocarpine model of epilepsy. In hippocampal nuclear extracts, SRF binding to DNA was significantly increased over saline-injected control rats at 24 hr and at 8 weeks after the onset of SE. This increase was shown to be the result of significantly elevated levels of SRF. DNA binding was also persistently increased in the cortical, but not in the cerebellar, extracts. Hippocampal expression of SRF was localized to neurons using immunohistochemistry. NMDA receptor activation during SE was required for these changes to take place, and the spontaneous seizures seen in epileptic rats did not appear to be responsible for the increase in SRF. The results demonstrate that SRF is persistently elevated after SE in the pilocarpine model of epilepsy and support the theory that long-term gene changes in this model occur and are associated with the long-lasting plasticity changes that are initiated during epileptogenesis.

Platelet-derived growth factor-stimulated expression of the MCP-1 immediate-early gene involves an inhibitory multiprotein complex

We have demonstrated previously that the seven-nucleotide (nt) motif TTTTGTA (the heptamer) that is present within the proximal 3' untranslated sequences of numerous immediate-early genes is essential for platelet-derived growth factor (PDGF)-stimulated induction of the MCP-1 immediate-early gene. On this basis, the heptamer was suggested to be a conserved regulatory element involved in immediate-early gene expression, although its mechanism of action was unknown. Herein, we demonstrate that the heptamer functions to remove an inhibition of PDGF induction of MCP-1 maintained by two independently acting inhibitory elements present in the MCP-1 5' flanking sequences (designated I* elements). PDGF treatment relieves the I*-mediated inhibition of MCP-1 expression only if the heptamer is also present. One inhibitory element is contained within a 59-nt portion of MCP-1 5' flanking sequences and functions in an orientation-independent and heptamer-regulated manner. Significantly, proteins binding to two DNA sequences contribute to the formation of a single multiprotein complex on the 59-nt I* element. The I*-binding complex contains Sp3, an Sp1-like protein, and a novel DNA-binding protein. Moreover, the complex does not form on two 59-nt sequences containing mutations that reverse the inhibition of PDGF induction maintained by the wild-type I* element. We propose to call the multiprotein I*-binding complex a repressosome and suggest that it acts to repress PDGF-stimulated transcription of MCP-1 in the absence of the heptamer TTTTGTA.

Basic fibroblast growth factor activates serum response factor gene expression by multiple distinct signaling mechanisms

Serum response factor (SRF) plays a central role in the transcriptional response of mammalian cells to a variety of extracellular signals. It is a key regulator of many cellular early response genes which are believed to be involved in cell growth and differentiation. The mechanism by which SRF activates transcription in response to mitogenic agents has been extensively studied; however, significantly less is known about regulation of the SRF gene itself. Previously, we identified distinct regulatory elements in the SRF promoter that play a role in activation, including a consensus ETS domain binding site, a consensus overlapping Sp/Egr-1 binding site, and two SRF binding sites. We further showed that serum induces SRF by a mechanism that requires an intact SRF binding site, also termed a CArG box. In the present study we demonstrate that in response to stimulation of cells by a purified growth factor, basic fibroblast growth factor (bFGF), the SRF promoter is upregulated by a complex pathway that involves at least two independent mechanisms: a CArG box-independent mechanism that is mediated by an ETS binding site, and a novel CArG box-dependent mechanism that requires both an Sp factor binding site and the CArG motifs for maximal stimulation. Our analysis indicates that the CArG/Sp element activation mechanism is mediated by distinct signaling pathways. The CArG box-dependent component is targeted by a Rho-mediated pathway, and the Sp binding site-dependent component is targeted by a Ras-mediated pathway. Both SRF and bFGF have been implicated in playing an important role in mediating cardiogenesis during development. The implications of our findings for SRF expression during development are discussed.

The thrombopoietin receptor can mediate proliferation without activation of the Jak-STAT pathway

Cytokine receptors of the hematopoietic receptor superfamily lack intrinsic tyrosine kinase domains for the intracellular transmission of their signals. Instead all members of this family associate with Jak family nonreceptor tyrosine kinases. Upon ligand stimulation of the receptors, Jaks are activated to phosphorylate target substrates. These include STAT (signal transducers and activators of transcription) proteins, which after phosphorylation translocate to the nucleus and modulate gene expression. The exact role of the Jak-STAT pathway in conveying growth and differentiation signals remains unclear. Here we describe a deletion mutant of the thrombopoietin receptor (c-mpl) that has completely lost the capacity to activate Jaks and STATs but retains its ability to induce proliferation. This mutant still mediates TPO-induced phosphorylation of Shc, Vav, mitogen-activated protein kinase (MAPK) and Raf-1 as well as induction of c-fos and c-myc, although at somewhat reduced levels. Furthermore, we show that both wild-type and mutant receptors activate phosphatidylinositol (PI) 3-kinase upon thrombopoietin stimulation and that thrombopoietin-induced proliferation is inhibited in the presence of the PI 3-kinase inhibitor wortmannin. These results demonstrate that the Jak-STAT pathway is dispensable for the generation of mitogenic signals by a cytokine receptor.

Nuclear localization of basonuclin in human keratinocytes and the role of phosphorylation

Basonuclin is a zinc-finger protein found in basal cells of the epidermis. In human keratinocyte cultures, basonuclin is susceptible to serine-phosphorylation and the addition of the phosphatase inhibitor, okadaic acid, promotes accumulation of basonuclin in the cytoplasm. The region of basonuclin containing the nuclear localization signal of basonuclin is necessary for nuclear localization of the protein and Ser-541, located immediately C-terminal to the nuclear localization signal, is the principal phosphorylation site in vitro. A nearly complete basonuclin transiently expressed in cultured keratinocytes localizes predominantly in the nucleus, but substitution of aspartic acid for Ser-541 promotes cytoplasmic localization. The same substitution of Ser-537 has a similar but weaker effect. Substitution of both serine residues by alanine leads to nuclear localization. These results show that nuclear localization of basonuclin depends on serine dephosphorylation, primarily of Ser-541. Different subcellular locations of basonuclin in different keratinocyte subtypes are therefore most likely to be controlled by the state of phosphorylation of Ser-541.

Growth and differentiation of C2 myogenic cells are dependent on serum response factor

In order to study to what extent and at which stage serum response factor (SRF) is indispensable for myogenesis, we stably transfected C2 myogenic cells with, successively, a glucocorticoid receptor expression vector and a construct allowing for the expression of an SRF antisense RNA under the direction of the mouse mammary tumor virus long terminal repeat. In the clones obtained, SRF synthesis is reversibly down-regulated by induction of SRF antisense RNA expression by dexamethasone, whose effect is antagonized by the anti-hormone RU486. Two kinds of proliferation and differentiation patterns have been obtained in the resulting clones. Some clones with a high level of constitutive SRF antisense RNA expression are unable to differentiate into myotubes; their growth can be blocked by further induction of SRF antisense RNA expression by dexamethasone. Other clones are able to differentiate and are able to synthesize SRF, MyoD, myogenin, and myosin heavy chain at confluency. When SRF antisense RNA expression is induced in proliferating myoblasts by dexamethasone treatment, cell growth is blocked and cyclin A concentration drops. When SRF antisense RNA synthesis is induced in arrested confluent myoblasts cultured in a differentiation medium, cell fusion is blocked and synthesis of not only SRF but also MyoD, myogenin, and myosin heavy chain is inhibited. Our results show, therefore, that SRF synthesis is indispensable for both myoblast proliferation and myogenic differentiation.

Serum induction of MEF2/RSRF expression in vascular myocytes is mediated at the level of translation

Vascular smooth muscle cells (VSMCs) reversibly coordinate the expression of VSMC-specific genes and the genes required for cell cycle progression. Here we demonstrate that isoforms of the MEF2/RSRF transcription factor are expressed in VSMCs and in vascular tissue. The MEF2A DNA-binding activity was upregulated when quiescent VSMCs were stimulated to proliferate with serum mitogens. The serum-induction of MEF2A DNA-binding activity occurred approximately 4 h following serum activation, and this correlated with an increase in the level of MEF2A protein without changes in the level of MEF2A mRNA or protein stability. These results indicate that MEF2A induction by serum is regulated at the level of translation.

A new platelet-derived growth factor-regulated genomic element which binds a serine/threonine phosphoprotein mediates induction of the slow immediate-early gene MCP-1

The MCP-1 chemokine gene belongs to a cohort of immediate-early genes that are induced with slower kinetics than c-fos. In this study, we identified a cluster of four platelet-derived growth factor (PDGF)-responsive elements within a 240-bp enhancer found in the distal 5' flanking MCP-1 sequences. Two of the elements bind one or more forms of the transcription factor NF-kappa B. We focused on the other two elements which are hitherto unreported, PDGF-regulated genomic motifs. One of these novel elements, detected as a 28-mer by DNase I footprinting, restores PDGF inducibility when added in two copies to a 5' truncated MCP-1 gene. A single copy of the second novel element, a 27-mer, restores PDGF inducibility to a 5' truncated MCP-1 gene. The 27-base element interacts with a PDGF-activated serine/threonine phosphoprotein that is detected only within the nucleus of PDGF-treated 3T3 cells. DNA binding of this phosphoprotein is activated by PDGF treatment with slow kinetics that match the time course of MCP-1 gene expression, and activation is not inhibited by cycloheximide. PDGF-activated binding to the 27-mer is shown to involve a single 30-kDa protein by UV-cross-linking analysis.

Loss of serum response element-binding activity and hyperphosphorylation of serum response factor during cellular aging

Human diploid fibroblasts undergo a limited number of population doublings in vitro and are used widely as a model of cellular aging. Despite growing evidence that cellular aging occurs as a consequence of altered gene expression, little is known about the activity of transcription factors in aging cells. Here, we report a dramatic reduction in the ability of proteins extracted from the nuclei of near-senescent fibroblasts to bind the serum response element which is necessary for serum-induced transcription of the c-fos gene. In contrast, the activities of proteins binding to the RNA polymerase core element, TATA, as well as to the cyclic AMP response element were maintained during cellular aging. While no major differences in the expression of the serum response factor (SRF) that binds the serum response element were seen between early-passage and late-passage cells, hyperphosphorylation of SRF was observed in near-senescent cells. Furthermore, removal of phosphatase inhibitors during the isolation of endogenous nuclear proteins restored the ability of SRF isolated from old cells to bind the SRE. These data, therefore, indicate that hyperphosphorylation of SRF plays a role in altering the ability of this protein to bind to DNA and regulate gene expression in senescent cells.

Loss of serum response element-binding activity and hyperphosphorylation of serum response factor during cellular aging

Human diploid fibroblasts undergo a limited number of population doublings in vitro and are used widely as a model of cellular aging. Despite growing evidence that cellular aging occurs as a consequence of altered gene expression, little is known about the activity of transcription factors in aging cells. Here, we report a dramatic reduction in the ability of proteins extracted from the nuclei of near-senescent fibroblasts to bind the serum response element which is necessary for serum-induced transcription of the c-fos gene. In contrast, the activities of proteins binding to the RNA polymerase core element, TATA, as well as to the cyclic AMP response element were maintained during cellular aging. While no major differences in the expression of the serum response factor (SRF) that binds the serum response element were seen between early-passage and late-passage cells, hyperphosphorylation of SRF was observed in near-senescent cells. Furthermore, removal of phosphatase inhibitors during the isolation of endogenous nuclear proteins restored the ability of SRF isolated from old cells to bind the SRE. These data, therefore, indicate that hyperphosphorylation of SRF plays a role in altering the ability of this protein to bind to DNA and regulate gene expression in senescent cells.

A growth factor-induced kinase phosphorylates the serum response factor at a site that regulates its DNA-binding activity

A signaling pathway by which growth factors may induce transcription of the c-fos proto-oncogene has been characterized. Growth factor stimulation of quiescent fibroblasts activates a protein kinase cascade that leads to the rapid and transient phosphorylation of the serum response factor (SRF), a regulator of c-fos transcription. The in vivo kinetics of SRF phosphorylation and dephosphorylation parallel the activation and subsequent repression of c-fos transcription, suggesting that this phosphorylation event plays a critical role in the control of c-fos expression. The ribosomal S6 kinase pp90rsk, a growth factor-inducible kinase, phosphorylates SRF in vitro at serine 103, the site that becomes newly phosphorylated upon growth factor stimulation in vivo. Phosphorylation of serine 103 significantly enhances the affinity and rate with which SRF associates with its binding site, the serum response element, within the c-fos promoter. These results suggest a model in which the growth factor-induced phosphorylation of SRF at serine 103 contributes to the activation of c-fos transcription by facilitating the formation of an active transcription complex at the serum response element.

A growth factor-induced kinase phosphorylates the serum response factor at a site that regulates its DNA-binding activity

A signaling pathway by which growth factors may induce transcription of the c-fos proto-oncogene has been characterized. Growth factor stimulation of quiescent fibroblasts activates a protein kinase cascade that leads to the rapid and transient phosphorylation of the serum response factor (SRF), a regulator of c-fos transcription. The in vivo kinetics of SRF phosphorylation and dephosphorylation parallel the activation and subsequent repression of c-fos transcription, suggesting that this phosphorylation event plays a critical role in the control of c-fos expression. The ribosomal S6 kinase pp90rsk, a growth factor-inducible kinase, phosphorylates SRF in vitro at serine 103, the site that becomes newly phosphorylated upon growth factor stimulation in vivo. Phosphorylation of serine 103 significantly enhances the affinity and rate with which SRF associates with its binding site, the serum response element, within the c-fos promoter. These results suggest a model in which the growth factor-induced phosphorylation of SRF at serine 103 contributes to the activation of c-fos transcription by facilitating the formation of an active transcription complex at the serum response element.

A novel 7-nucleotide motif located in 3' untranslated sequences of the immediate-early gene set mediates platelet-derived growth factor induction of the JE gene

A cohort of the serum and growth factor regulated immediate-early gene set is induced with slower kinetics than c-fos. Two of the first immediate-early genes characterized as such, c-myc and JE, are contained within this subset. cis-acting genomic elements mediating induction of the slower responding subset of immediate-early genes have never been characterized. Herein we characterize two widely separated genomic elements which are together essential for induction of the murine JE gene by platelet-derived growth factor, serum, interleukin-1, and double-stranded RNA. One of these elements is novel in several regards. It is a 7-mer, TTTTGTA, found in the proximal 3' sequences downstream of the JE stop codon. The 3' element is position dependent and orientation independent. It does not function in polyadenylation, splicing, or destabilization of the JE transcript. Copies of the 7-mer or its inverse are found at comparable 3' sites in 25 immediate-early genes that encode transcription factors or cytokines. Given its general occurrence, the 7-mer may be a required cis-acting control element mediating induction of the immediate-early gene set.

A novel 7-nucleotide motif located in 3' untranslated sequences of the immediate-early gene set mediates platelet-derived growth factor induction of the JE gene

A cohort of the serum and growth factor regulated immediate-early gene set is induced with slower kinetics than c-fos. Two of the first immediate-early genes characterized as such, c-myc and JE, are contained within this subset. cis-acting genomic elements mediating induction of the slower responding subset of immediate-early genes have never been characterized. Herein we characterize two widely separated genomic elements which are together essential for induction of the murine JE gene by platelet-derived growth factor, serum, interleukin-1, and double-stranded RNA. One of these elements is novel in several regards. It is a 7-mer, TTTTGTA, found in the proximal 3' sequences downstream of the JE stop codon. The 3' element is position dependent and orientation independent. It does not function in polyadenylation, splicing, or destabilization of the JE transcript. Copies of the 7-mer or its inverse are found at comparable 3' sites in 25 immediate-early genes that encode transcription factors or cytokines. Given its general occurrence, the 7-mer may be a required cis-acting control element mediating induction of the immediate-early gene set.

Growth and differentiation of embryonic stem cells that lack an intact c-fos gene

The c-fos protooncogene encodes a transcription factor that is thought to play a critical role in proliferation and differentiation as well as in the physiological response of mature cells to their environment. To test directly the role of c-fos in growth and differentiation, we generated mouse embryonic stem cell lines in which both copies of the c-fos gene were specifically disrupted by homologous recombination. Remarkably, the disruption of both copies of c-fos in these cells has no detectable effect on embryonic stem cell viability, growth rate, or differentiation potential. Embryonic stem cells lacking c-fos can differentiate into a wide range of cell types in tissue culture and also in chimeric mice. We conclude that despite a large body of literature suggesting an important role for c-fos in cell growth and differentiation, in at least some cell types this gene is not essential for these processes.

Serum response factor p67SRF is expressed and required during myogenic differentiation of both mouse C2 and rat L6 muscle cell lines

The 67-kD serum response factor (p67SRF) is a ubiquitous nuclear transcription factor that acts by direct binding to a consensus DNA sequence, the serum response element (SRE), present in the promoter region of numerous genes. Although p67SRF was initially implicated in the activation of mitogen-stimulated genes, the identification of a sequence similar to SRE, the CArG box motif, competent to interact with SRE binding factors in many muscle-specific genes, has led to speculation that, in addition to its function in cell proliferation, p67SRF may play a role in muscle differentiation. Indirect immunofluorescence using affinity-purified antibodies specifically directed against p67SRF reveals that this factor is constitutively expressed and localized in the nucleus of two skeletal muscle cell lines: rat L6 and mouse C2 myogenic cells during myogenic differentiation. This result was further confirmed through immunoblotting and Northern blot analysis. Furthermore, specific inhibition of p67SRF in vivo through microinjection of purified p67SRF antibodies prevented the myoblast-myotube transition and the expression of muscle-specific genes such as the protein troponin T. We further showed that anti-p67SRF injection also inhibited the expression of the myogenic factor myogenin, implying an early requirement for p67SRF in muscle differentiation. These results demonstrate that p67SRF is involved in the process of skeletal muscle differentiation. The potential action of p67SRF via CArG sequences is discussed.