Two pathways for serum regulation of the c-fos serum response element require specific sequence elements and a minimal domain of serum response factor

Expression of serum response factor in the lung mesenchyme is essential for development of pulmonary fibrosis

Extracellular matrix deposition characterizes idiopathic pulmonary fibrosis (IPF) and is orchestrated by myofibroblasts. The lung mesenchyme is an essential source of myofibroblasts in pulmonary fibrosis. Although the transcription factor serum response factor (SRF) has shown to be critical in the process of myofibroblast differentiation, its role in development of pulmonary fibrosis has not been determined in vivo. In this study, we observed that SRF expression localized to mesenchymal compartments, areas of dense fibrosis, and fibroblastic foci in human (IPF and normal) and bleomycin-treated mouse lungs. To determine the role of mesenchymal SRF in pulmonary fibrosis, we utilized a doxycycline-inducible, Tbx4 lung enhancer (Tbx4^LE)-driven Cre-recombinase to disrupt SRF expression in the lung mesenchyme in vivo. Doxycycline-treated Tbx4^LE-rtTA/TetO-Cre/tdTom/SRF^f,f (and controls) were treated with a single intratracheal dose of bleomycin to induce pulmonary fibrosis and examined for lung mesenchymal expansion, pulmonary fibrosis, and inflammatory response. Bleomycin-treated Tbx4^LE-rtTA/TetO-Cre/tdTom/SRF^f,f mice showed decreased numbers of Tbx4^LE-positive lung mesenchymal cells (LMCs) and collagen accumulation (via hydroxyproline assay) compared with controls. This effect was associated with SRF-null LMCs losing their proliferative and myofibroblast differentiation potential compared with SRF-positive controls. Together, these data demonstrate that SRF plays a critical role in LMC myofibroblast expansion during bleomycin-induced pulmonary fibrosis. This sets the stage for pharmacological strategies that specifically target SRF in the lung mesenchyme as a potential means of treating pulmonary fibrosis.

The Aging Vasculature: Glucose Tolerance, Hypoglycemia and the Role of the Serum Response Factor

The fastest growing demographic in the U.S. at the present time is those aged 65 years and older. Accompanying advancing age are a myriad of physiological changes in which reserve capacity is diminished and homeostatic control attenuates. One facet of homeostatic control lost with advancing age is glucose tolerance. Nowhere is this more accentuated than in the high proportion of older Americans who are diabetic. Coupled with advancing age, diabetes predisposes affected subjects to the onset and progression of cardiovascular disease (CVD). In the treatment of type 2 diabetes, hypoglycemic episodes are a frequent clinical manifestation, which often result in more severe pathological outcomes compared to those observed in cases of insulin resistance, including premature appearance of biomarkers of senescence. Unfortunately, molecular mechanisms of hypoglycemia remain unclear and the subject of much debate. In this review, the molecular basis of the aging vasculature (endothelium) and how glycemic flux drives the appearance of cardiovascular lesions and injury are discussed. Further, we review the potential role of the serum response factor (SRF) in driving glycemic flux-related cellular signaling through its association with various proteins.

Transcriptional outcomes and kinetic patterning of gene expression in response to NF-κB activation

Transcription factor nuclear factor kappa B (NF-κB) regulates cellular responses to environmental cues. Many stimuli induce NF-κB transiently, making time-dependent transcriptional outputs a fundamental feature of NF-κB activation. Here we show that NF-κB target genes have distinct kinetic patterns in activated B lymphoma cells. By combining RELA binding, RNA polymerase II (Pol II) recruitment, and perturbation of NF-κB activation, we demonstrate that kinetic differences amongst early- and late-activated RELA target genes can be understood based on chromatin configuration prior to cell activation and RELA-dependent priming, respectively. We also identified genes that were repressed by RELA activation and others that responded to RELA-activated transcription factors. Cumulatively, our studies define an NF-κB-responsive inducible gene cascade in activated B cells.

The nuclear factor kappa B (NF-κB) family of transcription factors regulates cellular responses to a wide variety of environmental cues. These could be extracellular stimuli that activate cell surface receptors, such as pathogens, or intracellular stress signals such as DNA damage or oxidative stress. In response to these triggers, NF-κB proteins accumulate in the cell nucleus, bind to specific DNA sequences in the genome, and thereby modulate gene transcription. Because of the diversity of signals that activate NF-κB and the ubiquity of this pathway in most cell types, cellular outcomes via NF-κB activation must be finely tuned to respond to the initiating stimulus. One mechanism by which NF-κB-dependent gene expression is regulated is by varying the duration of nuclear NF-κB; some signals lead to persistent nuclear NF-κB, while others lead to transient nuclear NF-κB. Consequently, time dependency of transcriptional responses is a unique signature of the initiating stimulus. Here we probed mechanisms that generate kinetic patterns of NF-κB-dependent gene expression in B lymphoma cells responding to a transient NF-κB-activating stimulus. By genetically manipulating NF-κB induction, we identified direct targets of RELA, a member of the NF-κB family, and provide evidence that kinetic patterns are established by a combination of factors that include the chromatin state of genes prior to cell activation and cofactors that work with RELA.

Cytosolic Prostaglandin E Synthase Is Involved in c-Fos Expression in Rat Fibroblastic 3Y1 Cells

Cytosolic prostaglandin (PG) E synthase (cPGES/p23) plays a role in the biosynthesis of PGE₂ and in the molecular chaperone machinery. Studies of knockout mice lacking cPGES/p23 have demonstrated that cPGES/p23 is essential in fetal mouse development. A cDNA microarray analysis revealed that a lack of cPGES/p23 decreases the expression of some immediate early genes, such as c-fos and activating transcription factor 3 (ATF3). Here we report the involvement of cPGES/p23 in c-Fos expression. A stable knockdown of cPGES/p23 in cultured fibroblasts not only reduced serum-induced c-Fos expression, but also decreased the phosphorylation of extracellular signal regulated kinase (ERK). These results suggest that cPGES/p23 is involved in the activation of ERK to promote c-Fos expression.

Cyclodextrin mediated delivery of NF-κB and SRF siRNA reduces the invasion potential of prostate cancer cells in vitro

Prostate cancer is the most common cancer in men of the western world. To date, no effective treatment exists for metastatic prostate cancer and consequently, there is an urgent need to develop new and improved therapeutics. In recent years, the therapeutic potential of RNA interference (RNAi) has been extensively explored in a wide range of diseases including prostate cancer using numerous gene delivery vectors. The aims of this study were to investigate the ability of a non-viral modified cyclodextrin (CD) vector to deliver siRNA to the highly metastatic PC-3 prostate cancer cell line, to quantify the resulting knockdown of the two target genes (RelA and SRF) and to study the effects of the silencing on metastasis. Data from a Matrigel in vitro invasion assay indicated that the silencing of the target genes achieved by the CD vector resulted in significant reductions (P=0.0001) in the metastatic potential of these cells. As the silencing of these target genes was shown not to have a negative impact on cell viability, we hypothesise that the mechanism of invasion inhibition is due, in part, to the significant reduction observed (P⩽0.0001) in the level of pro-inflammatory cytokine, MMP9, which is known to be implicated in the metastasis of prostate cancer.

Overexpression of p49/STRAP alters cellular cytoskeletal structure and gross anatomy in mice

Background

The protein p49/STRAP (SRFBP1) is a transcription cofactor of serum response factor (SRF) which regulates cytoskeletal and muscle-specific genes.

Results

Two conserved domains were found in the p49/STRAP protein. The SRF-binding domain was at its N-terminus and was highly conserved among mammalian species, xenopus and zebrafish. A BUD22 domain was found at its C-terminus in three sequence databases. The BUD22 domain was conserved among mammalian p49/STRAP proteins, and yeast cellular morphogenesis proteins, which is involved in ribosome biogenesis that affects growth rate and cell size. The endogenous p49/SRAP protein was localized mainly in the nucleus but also widely distributed in the cytoplasm, and was in close proximity to the actin. Transfected GFP-p49/STRAP protein co-localized with nucleolin within the nucleolus. Overexpression of p49/STRAP reduced actin content in cultured cells and resulted in smaller cell size versus control cells. Increased expression of p49/STRAP in transgenic mice resulted in newborns with malformations, which included asymmetric abdominal and thoracic cavities, and substantial changes in cardiac morphology. p49/STRAP altered the expression of certain muscle-specific genes, including that of the SRF gene, which is a key regulator of cardiac genes at the developmental, structural and maintenance level and has two SRE binding sites.

Conclusions

Since p49/STRAP is a co-factor of SRF, our data suggest that p49/STRAP likely regulates cell size and morphology through SRF target genes. The function of its BUD22 domain warrants further investigation. The observed increase in p49/STRAP expression during cellular aging may contribute to observed morphological changes in senescence.

A novel ortholog of serum response factor (SRF) with immune defense function identified in Crassostrea hongkongensis

Serum response factor (SRF) function is essential for transcriptional regulation of numerous growth-factor-inducible genes and triggers proliferation, differentiation and apoptosis of the cells. In this report, the first mollusk serum response factor like homolog gene (designated ChSRF) was identified and characterized from the Hong Kong oyster, Crassostrea hongkongensis. The full-length cDNA of ChSRF was 1716 bp in length and encodes a putative protein of 434 amino acids respectively, and shares the MADS domain at the N-terminal. ChSRF is ubiquitously expressed in various tissues, with the highest expression level observed in muscle. Temporal expression of ChSRF following microbe infection shows that the expression of ChSRF in hemocytes increases from 3 to 24 h post-challenge. As a target gene of SRF, β-actin demonstrates a similar gene expression mode in constitutive tissue and pathogen infection. Furthermore, some protein profiles of ChSRF was revealed, fluorescence microscopy results show that ChSRF located in the nuclei of HeLa cells and over-expression of ChSRF activated the transcriptional activities of MAPK signal pathway in HEK293T cells. These results indicate that ChSRF maybe play an important role in signal transduction in the immunity and development response of oysters. Furthermore, we found that ChSRF could regulate the expression of β-actin gene, which indicate that ChSRF is a muscle differentiation regulator in the oyster and it will help us to improve aquaculture production.

Degeneration of the mouse retina upon dysregulated activity of serum response factor

PURPOSE

Our aim was to generate and phenotypically characterize a transgenic mouse line expressing a constitutively active variant of the transcription regulatory protein serum response factor (SRF), namely the SRF-VP16 protein. This new mouse strain has been registered under the designation Gt(ROSA)26Sor(tm1(SRF-VP16)Antu). We found phenotypic changes upon ectopic expression of SRF-VP16, especially in the mouse retina.

METHODS

Using homologous recombination, we integrated an SRF-VP16 conditional (i.e., "flox-STOP" repressed) expression transgene into the Rosa26 locus of murine embryonic stem (ES) cells. These engineered ES cells were used to derive the Gt(ROSA)26Sor(tm1(SRF-VP16)Antu) mouse strain. Semiquantitative real-time PCR was used to determine expression of the SRF-VP16 transgene at the mRNA level, both in young (P20 and P30) and adult (six months old) Gt(ROSA)26Sor(tm1(SRF-VP16)Antu) mice. We also investigated the transcript levels of endogenous Srf and several SRF target genes. Retinal function was tested by electroretinography in both young and adult mice. Morphological abnormalities could be visualized by hematoxylin and eosin staining of sectioned, paraffin-embedded eye tissue samples. Scanning-laser ophthalmoscopy was used to investigate retinal vascularization and degeneration in adult mice.

RESULTS

We show that the SRF-VP16 mRNA is expressed to a low but significant degree in the retinas of young and adult animals of the Gt(ROSA)26Sor(tm1(SRF-VP16)Antu) mouse strain, even in the absence of Cre-mediated deletion of the "flox-STOP" cassette. In the retinas of these transgenic mice, endogenous Srf displays elevated transcript levels. Ectopic retinal expression of constitutively active SRF-VP16 is correlated with the malfunction of retinal neurons in both heterozygous and homozygous animals of both age groups (P20 and adult). Additionally, mislamination of retinal cell layers and cellular rosette formations are found in retinas of both heterozygous and homozygous animals of young age. In homozygous individuals, however, the cellular rosettes are more widespread over the fundus. At adult age, retinas both from animals that are heterozygous and homozygous for the floxSTOP/SRF-VP16 transgene display severe degeneration, mainly of the photoreceptor cell layer. Wild-type age-matched littermates, however, do not show any degeneration. The severity of the observed effects correlates with dosage of the transgene.

CONCLUSIONS

This is the first report suggesting an influence of the transcription factor SRF on the development and function of the murine retina. Ectopic SRF-VP16 mRNA expression in the retinas of young animals is correlated with photoreceptor layer mislamination and impaired retinal function. At an advanced age of six months, degenerative processes are detected in SRF-VP16 transgenic retinas accompanied by impaired retinal function. The Gt(ROSA)26Sor(tm1(SRF-VP16)Antu) mouse strain represents a genetic SRF gain-of-function mouse model that will complement the current SRF loss-of-function models. It promises to provide new insight into the hitherto poorly defined role of SRF in retinal development and function, including potential contributions to ophthalmologic disorders. Furthermore, using conditional Cre-mediated activation of SRF-VP16, the described mouse strain will enable assessment of the impact of dysregulated SRF activity on the physiologic functions of various other organs.

Serum response factor regulates immediate early host gene expression in Toxoplasma gondii-infected host cells

Toxoplasma gondii is a wide spread pathogen that can cause severe and even fatal disease in fetuses and immune-compromised hosts. As an obligate intracellular parasite, Toxoplasma must alter the environment of its host cell in order to establish its replicative niche. This is accomplished, in part, by secretion of factors into the host cell that act to modulate processes such as transcription. Previous studies demonstrated that genes encoding transcription factors such as c-jun, junB, EGR1, and EGR2 were amongst the host genes that were the most rapidly upregulated following infection. In cells stimulated with growth factors, these genes are regulated by a transcription factor named Serum Response Factor. Serum Response Factor is a ubiquitously expressed DNA binding protein that regulates growth and actin cytoskeleton genes via MAP kinase or actin cytoskeletal signaling, respectively. Here, we report that Toxoplasma infection leads to the rapid activation of Serum Response Factor. Serum Response Factor activation is a Toxoplasma-specific event since the transcription factor is not activated by the closely related protozoan parasite, Neospora caninum. We further demonstrate that Serum Response Factor activation requires a parasite-derived secreted factor that signals via host MAP kinases but independently of the host actin cytoskeleton. Together, these data define Serum Response Factor as a host cell transcription factor that regulates immediate early gene expression in Toxoplasma-infected cells.

Weak up-regulation of serum response factor in gastric ulcers in patients with co-morbidities is associated with increased risk of recurrent bleeding

Background

Serum response factor (SRF) is crucial for gastric ulcer healing process. The study determined if gastric ulcer tissues up-regulate SRF and if such up-regulation correlated with co-morbidities and the risk of recurrent bleeding.

Methods

Ulcer and non-ulcer tissues were obtained from 142 patients with active gastric ulcers for SRF expression assessed by immunohistochemistry. Based on the degree of SRF expression between these two tissue types, SRF up-regulation was classified as strong, intermediate, and weak patterns. The patients were followed-up to determine if SRF up-regulation correlated to recurrent bleeding.

Results

Gastric ulcer tissues had higher SRF expression than non-ulcer tissues (p < 0.05). Patients with strong SRF up-regulation had lower rates of stigmata of recent hemorrhage (SRH) on the ulcer base than the others (p < 0.05). Multivariate logistic regression confirmed that co-morbidities and weak SRF up-regulation were two independent factors of recurrent gastric ulcer bleeding (p < 0.05). Combining both factors, there was an 8.29-fold (95% CI, 1.31~52.62; p = 0.03) higher risk of recurrent gastric ulcer bleeding.

Conclusions

SRF expression is higher in gastric ulcer tissues than in non-ulcer tissues. Weak SRF up-regulation, combined with the presence of co-morbidities, increase the risk of the recurrent gastric ulcer bleeding.

Regulation of serum response factor activity and smooth muscle cell apoptosis by chromodomain helicase DNA-binding protein 8

Serum response factor (SRF) is a widely expressed protein that plays a key role in the regulation of smooth muscle differentiation, proliferation, migration, and apoptosis. It is generally accepted that one mechanism by which SRF regulates these diverse functions is through pathway-specific cofactor interactions. A novel SRF cofactor, chromodomain helicase DNA binding protein 8 (CHD8), was isolated from a yeast two-hybrid screen using SRF as bait. CHD8 is highly expressed in adult smooth muscle tissues. Coimmunoprecipitation assays from A10 smooth muscle cells demonstrated binding of endogenous SRF and CHD8. Data from GST-pulldown assays indicate that the NH(2)-terminus of CHD8 can interact directly with the MADS domain of SRF. Adenoviral-mediated knockdown of CHD8 in smooth muscle cells resulted in attenuated expression of SRF-dependent, smooth muscle-specific genes. Knockdown of CHD8, SRF, or CTCF, a previously described binding partner of CHD8, in A10 VSMCs also resulted in a marked induction of apoptosis. Mechanistically, apoptosis induced by CHD8 knockdown was accompanied by attenuated expression of the anti-apoptotic proteins, Birc5, and CARD10, whereas SRF knockdown attenuated expression of CARD10 and Mcl-1, but not Birc5, and CTCF knockdown attenuated expression of Birc5. These data suggest that CHD8 plays a dual role in smooth muscle cells modulating SRF activity toward differentiation genes and promoting cell survival through interactions with both SRF and CTCF to regulate expression of Birc5 and CARD10.

The RhoA-specific guanine nucleotide exchange factor p63RhoGEF binds to activated Galpha(16) and inhibits the canonical phospholipase Cbeta pathway

Heterotrimeric G proteins regulate diverse physiological processes by modulating the activities of intracellular effectors. Members of the Galpha(q) family link G protein-coupled receptor activation to phospholipase Cbeta (PLCbeta) activity and intracellular calcium signaling cascades. However, they differ markedly in biochemical properties as well as tissue distribution. Recent findings have shown that some of the cellular activities of Galpha(q) family members are independent of PLCbeta activation. A guanine nucleotide exchange factor, p63RhoGEF, has been shown to interact with Galpha(q) proteins and thus provides linkage to RhoA activation. However, it is not known if p63RhoGEF can associate with other Galpha(q) family members such as Galpha(16). In the present study, we employed co-immunoprecipitation studies in HEK293 cells to demonstrate that p63RhoGEF can form a stable complex with the constitutively active mutant of Galpha(16) (Galpha(16)QL). Interestingly, overexpression of p63RhoGEF inhibited Galpha(16)QL-induced IP(3) production in a concentration-dependent manner. The binding of PLCbeta(2) to Galpha(16)QL could be displaced by p63RhoGEF. Similarly, p63RhoGEF inhibited the binding of tetratricopeptide repeat 1 to Galpha(16)QL, leading to a suppression of Galpha(16)QL-induced Ras activation. In the presence of p63RhoGEF, Galpha(16)QL-induced STAT3 phosphorylation was significantly reduced and Galpha(16)QL-mediated SRE transcriptional activation was attenuated. Taken together, these results suggest that p63RhoGEF binds to activated Galpha(16) and inhibits its signaling pathways.

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.

Activation of serum response element by D2 dopamine receptor is governed by Gbetagamma-mediated MAPK and Rho pathways and regulated by RGS proteins

In this study, we investigated the activation of the serum response element (SRE) by the D2 dopamine receptor (D2R) agonist quinpirole. Stimulation of CHO cells expressing the D2R by quinpirol evoked a dose-dependent SRE activation, which was completely blocked by overnight treatment of pertussis toxin or by co-expression of the beta-adrenergic receptor kinase C-terminus, implicating the involvement of Galpha(i )and Gbetagamma in the signal transduction. Furthermore, using MEK inhibitors and dominant negative mutants of RhoA, Rac1, and Cdc42, we showed that the Gbetagamma-mediated activation of the SRE in CHO cells utilizes both MAPK and Rho pathways. Expression of either regulator of G protein signaling 2 or 4 (RGS2 or RGS4) proteins significantly attenuated the quinpirole-induced SRE activation. These results delineate the signaling pathways which couple D2 receptor to the transcriptional activation of SRE and demonstrate a modulatory role for RGS proteins in these processes.

Involvement of phosphatidylcholine-selective phospholipase C in activation of mitogen-activated protein kinase pathways in imidazoline receptor antisera-selected protein

Imidazoline receptor antisera-selected protein (IRAS) is considered as a candidate for the I1-imidazoline receptor (I1R), but the signaling pathway mediated by IRAS remains unknown. In our study, the signal transduction pathways of IRAS were investigated in CHO cells stably expressing IRAS (CHO-IRAS), and compared to the native I1R signaling pathways. Rilmenidine or moxonidine (10 nM-100 microM), I1R agonists, failed to stimulate [35S]-GTPgammaS binding in CHO-IRAS cell membrane preparations, suggesting that G protein may not be involved in IRAS signaling pathway. However, incubation of CHO-IRAS with rilmenidine or moxonidine for 5 min could induce an upregulation of phosphatidylcholine-selective phospholipase C (PC-PLC) activity, and an increase in the accumulation of diacylglycerol (DAG), the hydrolysate of PC-PLC, in a concentration-dependent manner. The elevated activation of PC-PLC by rilmenidine or moxonidine (100 nM) could be blocked by efaroxan, a selective I1R antagonist. Cells treated with rilmenidine or moxonidine showed an increased level of extracellular signal-regulated kinase (ERK) phosphorylation in a concentration-dependent manner, which could be reversed by efaroxan or D609, a selective PC-PLC inhibitor. These results suggest that the signaling pathway of IRAS in response to I1R agonists coupled with the activation of PC-PLC and its downstream signal transduction molecule, ERK. These findings are similar to those in the signaling pathways of native I1R, providing some new evidence for the relationship between I1R and IRAS.

RNPC1, an RNA-binding protein and a target of the p53 family, is required for maintaining the stability of the basal and stress-induced p21 transcript

p21, a cyclin-dependent kinase inhibitor, is transcriptionally regulated by the p53 family to induce cell cycle arrest. p21 is also regulated post-transcriptionally upon DNA damage in a p53-dependent manner, but the mechanism is uncertain. Here, we found that RNPC1, an RNA-binding protein and a target of the p53 family, is required for maintaining the stability of the basal and stress-induced p21 transcript. Specifically, we showed that RNPC1 is induced by the p53 family and DNA damage in a p53-dependent manner. The RNPC1 gene encodes at least two alternative spliced isoforms, RNPC1a and RNPC1b, both of which contain an intact RNA recognition motif. Interestingly, we found that RNPC1a, but not RNPC1b, induces cell cycle arrest in G1, although both isoforms are expressed in the nucleus and cytoplasm. In addition, we found that while both isoforms directly bind to the 3' untranslated region in p21 transcript, only RNPC1a is able to stabilize both the basal and stress-induced p21 transcripts. Conversely, RNPC1a knockdown destabilizes p21 transcript. Finally, we found that RNPC1a is required to maintain the stability of p21 transcript induced by p53.

Phosphorylation of carboxyl-terminal tyrosines modulates the specificity of Sprouty-2 inhibition of different signaling pathways

Sprouty proteins are evolutionarily conserved negative feedback regulators of multiple receptor tyrosine kinases. Mammalian versions of these proteins differentially regulate signaling induced by the fibroblast and the epidermal growth factors (FGF and EGF, respectively). Herein we show that, although both growth factors elevate expression of Sprouty-2, FGF- and not EGF-induced activation of the Erk/MAPK pathway is inhibited by Sprouty-2. Attenuation of FGF-signaling is accompanied by the induction of Sprouty-2 phosphorylation on the amino-terminal as well as carboxyl-terminal tyrosine residues, which are less effectively modified upon EGF treatment. Mutagenesis of carboxyl-terminal tyrosines, especially a newly identified phosphorylation site, tyrosine 227, impaired the inhibitory activity of Sprouty-2. These results attribute a novel role for carboxyl-terminal tyrosine residues and yet unidentified phosphotyrosine-binding proteins in the differential regulation of Sprouty-2 activity.

Identification of a novel serum response factor cofactor in cardiac gene regulation

The transcription factor serum response factor (SRF) plays an important role in the regulation of a variety of cardiac genes during development and during adult aging. A novel SRF cofactor, herein called p49/STRAP, for SRF-dependent transcription regulation-associated protein, was recently identified in our laboratory. This protein interacted mainly with the transcriptional activation domain of the SRF protein and was found to bind to SRF or to the complex of SRF and another cofactor, such as myocardin or Nkx2.5. The expression of p49/STRAP affected the promoter activity of SRF target genes in a non-uniform manner. For example, p49 activated MLC2v and cardiac actin promoters when it was co-transfected with SRF, but it repressed atrial natriuretic factor promoter activity, which was strongly induced by myocardin. The p49/STRAP mRNA was observed to be highly expressed in fetal, adult, and senescent human hearts, and also in hearts of young adult and old mice, suggesting that p49/STRAP may be an important SRF cofactor in the transcriptional regulation of mammalian cardiac muscle genes throughout the life span.

Role of MADS box proteins and their cofactors in combinatorial control of gene expression and cell development

In all organisms, correct development, growth and function depends on the precise and integrated control of the expression of their genes. Often, gene regulation depends upon the cooperative binding of proteins to DNA and upon protein-protein interactions. Eukaryotes have widely exploited combinatorial strategies to create gene regulatory networks. MADS box proteins constitute the perfect example of cellular coordinators. These proteins belong to a large family of transcription factors present in most eukaryotic organisms and are involved in diverse and important biological functions. MADS box proteins are combinatorial transcription factors in that they often derive their regulatory specificity from other DNA binding or accessory factors. This review is aimed at analyzing how MADS box proteins combine with a variety of cofactors to achieve functional diversity.

Transcriptional regulators and myelopoiesis: the role of serum response factor and CREB as targets of cytokine signaling

Hematopoiesis is a complex process in which mature myeloid and lymphoid cells are produced from a small population of pluripotent stem cells within the bone marrow. Blood cell formation occurs, in part, by progenitor cell exposure to humoral growth regulators, known as hematopoietic cytokines, as well as by the regulated expression of genes by transcription factors. In this paper, we review two important nuclear proteins, the serum response factor and the cyclic adenosine monophosphate response element-binding protein, as downstream targets of mitogens, with a specific focus on hematopoietic cytokine signaling and the role these proteins play in gene regulation.

CrkII induces serum response factor activation and cellular transformation through its function in Rho activation

CrkII belongs to the adaptor protein family that plays a crucial role in signal transduction. In order to better understand the biological functions of CrkII, we focused on the regulation of gene expression by CrkII. Various transcriptional control elements were examined for their activation by CrkII-expression, and we found that CrkII selectively activates the serum response element (SRE), a transcriptional control element of immediate-early genes. This SRE activation induced by CrkII-overexpression was mediated by the serum response factor (SRF) via Rho. Indeed, we confirmed that the amount of activated Rho was increased in the CrkII-expressing cells. Moreover, we showed that when overexpressed, CrkII induces the cellular transformation of NIH 3T3 cells and that a dominant negative mutant of Rho suppresses this transformation, strongly suggesting that activation of Rho is essential for the transforming activity by CrkII. Furthermore, we also found that CrkII and Galpha12, a member of the heterotrimeric G proteins, synergistically activates Rho as well as the SRF, and that an SH3 mutant of CrkII can inhibit the Galpha12-induced activation of SRF. These results strongly suggest that CrkII is involved in the activation of Rho and SRF by Galpha12. Our study provides strong evidence that Rho activation plays a crucial role in CrkII-mediated signals to induce gene expression and cellular transformation.

Brain-derived neurotrophic factor protection of cortical neurons from serum withdrawal-induced apoptosis is inhibited by cAMP

Programmed cell death plays an important role both during the development of the CNS and in its homeostasis throughout adulthood. A complex balance between cell death- and survival-inducing signals determines the fate of individual neurons. Intracellular cAMP is thought to regulate neuronal survival, and previous studies have shown that the survival of retinal ganglion cells by brain-derived neurotrophic factor (BDNF) is dependent on cAMP. Here we report the surprising observation that cAMP attenuates the ability of BDNF to rescue cortical neurons from apoptosis after serum deprivation, a process mediated via the phosphatidylinositol 3 (PI3)-kinase signal transduction cascade. Depolarization by KCl, which increases cAMP in cortical neurons, also attenuates BDNF protection against serum withdrawal. Our data indicate that cAMP antagonizes neurotrophin protection from serum withdrawal by inhibiting the PI3-kinase signal transduction cascade. This study indicates that cAMP may inhibit some forms of neurotrophin-mediated neuronal survival and suggests that a number of PI3-kinase-regulated processes in neurons may be inhibited by cAMP.

Human c-fos promoter mediates high-level, inducible expression in various mammalian cell lines

The promoter activity of the human c-fos and human cytomegalovirus (CMV) immediate early promoter was compared in transient and stable transfection experiments with six cell lines of mouse, human, and hamster origin which are all of commercial importance. The c-fos promoter was 1.8-5.6-fold stronger than the CMV promoter in BHK-A, BHK-B, CHO-DHFR(-), and mouse NIH-3T3 in stable transfectants and less effective in mouse myeloma or human 293 cells, suggesting a new transcriptional control element for high-level expression and protein production in mammalian cells. The induction profiles determined in the presence and absence of serum are dependent on the cell line used. Induction levels of up to 8-fold could be achieved in preselected cell pools.

Roles of Rac and cytosolic phospholipase A2 in the intracellular signalling in response to titanium particles

Titanium (Ti) particle is one of the prosthetic materials commonly used in implantation and has frequently been implicated in pathogenesis such as periprosthetic osteolysis. In the present study, we undertook to understand the intracellular signalling pathway stimulated by exogenous Ti at Rat-2 fibroblasts. By reporter gene analysis following transient transfections, exogenous Ti was shown to stimulate c-fos serum response element (SRE)-dependent luciferase activities in a dose-dependent manner. In addition, Ti-induced SRE activation was shown to be dramatically repressed by RacN17, a dominant negative mutant of Rac1, suggesting that Rac GTPase is essential for the signalling of Ti to c-fos SRE. Furthermore, pretreatment with MAFP, an inhibitor of cytosolic phospholipase A(2) (cPLA(2)), MK886, an inhibitor of 5-lipoxygenase (5-LO), or indomethacin, a general inhibitor of cyclooxygenase (COX), also significantly repressed Ti-induced SRE activation, suggesting mediatory roles of cPLA(2) and subsequent arachidonic acid (AA) metabolisms to leukotrienes (LTs) and prostaglandins (PGs) in the Ti signalling to c-fos SRE. Consistent with these results, intracellular levels of leukotriene B(4) (LTB(4)) and prostaglandin E(2) (PGE(2)) were Rac-dependently elevated in cells exposed to Ti particles.

Sprouty fine-tunes EGF signaling through interlinked positive and negative feedback loops

BACKGROUND

Growth factors and their receptor tyrosine kinases play pivotal roles in development, normal physiology, and pathology. Signal transduction is regulated primarily by receptor endocytosis and degradation in lysosomes ("receptor downregulation"). c-Cbl is an adaptor that modulates this process by recruiting binding partners, such as ubiquitin-conjugating enzymes. The role of another group of adaptors, Sprouty proteins, is less understood; although, studies in insects implicated the founder protein in the negative regulation of several receptor tyrosine kinases.

RESULTS

By utilizing transfection of living cells, as well as reconstituted in vitro systems, we identified a dual regulatory mechanism that combines human Sprouty2 and c-Cbl. Upon activation of the receptor for the epidermal growth factor (EGFR), Sprouty2 undergoes phosphorylation at a conserved tyrosine that recruits the Src homology 2 domain of c-Cbl. Subsequently, the flanking RING finger of c-Cbl mediates poly-ubiquitination of Sprouty2, which is followed by proteasomal degradation. Because phosphorylated Sprouty2 sequesters active c-Cbl molecules, it impedes receptor ubiquitination, downregulation, and degradation in lysosomes. This competitive interplay occurs in endosomes, and it regulates the amplitude and longevity of intracellular signals.

CONCLUSIONS

Sprouty2 emerges as an inducible antagonist of c-Cbl, and together they set a time window for receptor activation. When incorporated in signaling networks, the coupling of positive (Sprouty) to negative (Cbl) feedback loops can greatly enhance output diversification.

Vav1 couples T cell receptor to serum response factor-dependent transcription via a MEK-dependent pathway

The Vav family of guanine nucleotide exchange factors for Rho family GTPases plays a critical role in lymphocyte proliferation, gene transcription, and cytoskeleton reorganization following immunoreceptor stimulation. However, its role in immediate early gene activation is unclear. In this study, we have investigated the mechanisms by which Vav1 can regulate c-fos serum response element transcriptional activity. We show that T cell antigen receptor (TCR) stimulation induces the phosphorylation of serum response factor (SRF) on serine 103 and increases the binding of SRF complexes on serum response element in a MEK- and p38-dependent pathway. The physiological relevance of our findings is supported by the inhibition of the interleukin-2 gene transcriptional activity by a dominant negative SRF mutant. Overexpression of Vav1, which partially mimics TCR stimulation, promotes SRF-dependent transcription, and dominant negative Vav1 mutants block SRF activation by TCR. SRF activation by Vav1 occurs through a signaling cascade consisting of Rac1/Cdc42 and the serine/threonine kinases Pak1 and MEK, but independently of the phosphatidylinositol 3-kinase pathway. Interestingly, Vav2 also enhances SRF through Rho GTPases, suggesting that Vav proteins are general regulators of SRF activation in lymphocytes. This report establishes Vav proteins as a direct link between antigen receptors and SRF-dependent early gene expression.

Dynamic characterization of recombinant Chinese hamster ovary cells containing an inducible c-fos promoter GFP expression system as a biomarker

An inducible reporter gene system for Chinese Hamster Ovary (CHO-DHFR(-)) cells has been developed and characterized with respect to its dynamic properties. The reporter gene system consists of the human c-fos promoter and variants of the green fluorescence protein (GFP), either EGFP with enhanced fluorescence or its destabilized form d2EGFP. The expression of wild-type EGFP or its destabilized form was studied in CHO-DHFR(-) cells in response to serum addition or deprivation. It was shown that serum-induced c-fos promoter mediated EGFP expression was considerably higher than expression from the human CMV promoter, a strong, constitutive promoter preferentially used for high-level expression in CHO cells. However, EGFP was less suitable for studying expression dynamics than d2EGFP due to the protein's long half-life in mammalian cells. The use of d2EGFP resulted in a significant improvement in the dynamic characteristics of the biomarker, particularly when the recombinant cells were selected for high-level GFP expression by subcloning or fluorescence activated cell/sorting (FACS). GFP expression in different subclones and cell populations sorted by FACS was characterized with respect to its dynamic responses in the presence or absence of serum in the culture medium. Significant differences in the GFP expression dynamics were observed for the isolated cell populations. The experimental results indicate that cells with high-level GFP expression also have a faster dynamic response and are thus, desirable for practical application of the reporter gene system e.g. in toxicity monitoring.

Serum response factor is crucial for actin cytoskeletal organization and focal adhesion assembly in embryonic stem cells

The activity of serum response factor (SRF), an essential transcription factor in mouse gastrulation, is regulated by changes in actin dynamics. Using Srf(-/-) embryonic stem (ES) cells, we demonstrate that SRF deficiency causes impairments in ES cell spreading, adhesion, and migration. These defects correlate with defective formation of cytoskeletal structures, namely actin stress fibers and focal adhesion (FA) plaques. The FA proteins FA kinase (FAK), beta1-integrin, talin, zyxin, and vinculin were downregulated and/or mislocalized in ES cells lacking SRF, leading to inefficient activation of the FA signaling kinase FAK. Reduced overall actin expression levels in Srf(-/-) ES cells were accompanied by an offset treadmilling equilibrium, resulting in lowered F-actin levels. Expression of active RhoA-V14 rescued F-actin synthesis but not stress fiber formation. Introduction of constitutively active SRF-VP16 into Srf(-/-) ES cells, on the other hand, strongly induced expression of FA components and F-actin synthesis, leading to a dramatic reorganization of actin filaments into stress fibers and lamellipodia. Thus, using ES cell genetics, we demonstrate for the first time the importance of SRF for the formation of actin-directed cytoskeletal structures that determine cell spreading, adhesion, and migration. Our findings suggest an involvement of SRF in cell migratory processes in multicellular organisms.

Early postnatal cardiac changes and premature death in transgenic mice overexpressing a mutant form of serum response factor

Serum response factor (SRF) is a key regulator of a number of extracellular signal-regulated genes important for cell growth and differentiation. A form of the SRF gene with a double mutation (dmSRF) was generated. This mutation reduced the binding activity of SRF protein to the serum response element and reduced the capability of SRF to activate the atrial natriuretic factor promoter that contains the serum response element. Cardiac-specific overexpression of dmSRF attenuated the total SRF binding activity and resulted in remarkable morphologic changes in the heart of the transgenic mice. These mice had dilated atrial and ventricular chambers, and their ventricular wall thicknesses were only 1/2 to 1/3 the thickness of that of nontransgenic mice. Also these mice had smaller cardiac myocytes and had less myofibrils in their myocytes relative to nontransgenic mice. Altered gene expression and slight interstitial fibrosis were observed in the myocardium of the transgenic mice. All the transgenic mice died within the first 12 days after birth, because of the early onset of severe, dilated cardiomyopathy. These results indicate that dmSRF overexpression in the heart apparently alters cardiac gene expression and blocks normal postnatal cardiac growth and development.

ERK2- and p90(Rsk2)-dependent pathways regulate the CCAAT/enhancer-binding protein-beta interaction with serum response factor

The serum response element (SRE) of the c-fos promoter is a convergence point for mitogenic signaling pathways. Several transcription factors regulate SRE, including serum response factor (SRF), ternary complex factors, and CCAAT/enhancer-binding protein-beta (C/EBPbeta). C/EBPbeta can interact with both SRF and the ternary complex factor family member Elk-1, but only in response to activated Ras. Transactivation of the SRE by C/EBPbeta is also greatly stimulated by Ras. The Ras effectors that signal to C/EBPbeta are unknown. In this report, we demonstrate that a consensus MAPK site in C/EBPbeta is necessary for Ras stimulation of both C/EBPbeta-SRF interaction and transactivation of the SRE by C/EBPbeta. To dissect signaling pathways activated downstream of Ras, different Ras effector constructs were analyzed. We show that activated forms of Raf and phosphatidylinositol 3-kinase stimulate C/EBPbeta-SRF interaction. We also show a novel selectivity for the MAPK family member ERK2, where dominant-negative ERK2, but not dominant-negative ERK1, blocks Ras stimulation of C/EBPbeta-SRF interaction. In addition, recombinant C/EBPbeta protein is phosphorylated by ERK2, but not by ERK1, in vitro. Finally, we demonstrate a requirement for p90(Rsk2) in regulation of C/EBPbeta-SRF interaction. These data show that multiple Ras effectors are required to regulate C/EBPbeta and SRF association.

Receptor tyrosine kinase EphA2 is regulated by p53-family proteins and induces apoptosis

The p53 tumor suppressor protein is mutated in more than 50% of all human cancers, which makes the study of its functions and activities critical for the understanding and management of cancer. In response to cellular stresses, p53 is activated and can mediate cell cycle arrest and/or apoptosis via the upregulation of numerous target genes. Here, we have identified EphA2 as a target gene of the p53 family, that is, p53, p73, and p63. We also found that an increase of EphA2 transcript levels correlated with an increase of EphA2 protein expression, and induction of EphA2 in response to DNA damage corresponded with p53 activation. Furthermore, we identified a p53 response element located within the EphA2 promoter that is responsive to wild-type p53, p73, and p63, but not mutant p53. Interestingly, the ligand for EphA2, ephrin-A1, is also regulated by p53. EphA2 and ephrin-A1 are members of the Eph family of receptor tyrosine kinases and ligands, which are implicated in a number of developmental processes. To analyse the role of EphA2 in p53-mediated tumor suppression, we generated stable cell lines capable of expressing exogenous EphA2 in a tetracycline-repressible system. We found that EphA2 expression resulted in an increase in apoptosis. Thus, we hypothesize that the activated EphA2 may serve to impair anti-apoptotic signaling, perhaps by disrupting focal adhesions and thereby sensitize cells to pro-apoptotic stimuli.

Retinoid-dependent antagonism of serum response factor transactivation mediated by transcriptional coactivator proteins

Transcriptional coactivators SRC-1 and p300 specifically interact with liganded-nuclear receptors and also modulate other transcription factors, including serum response factor (SRF). Here, we report that retinoids repress transactivation by SRF and specific interactions exist between the DNA binding domains of SRF and retinoic acid and retinoid X receptors. We further demonstrate that the repression may involve retinoid-dependent competition for a limiting amount of SRC-1 and p300 between SRF and retinoid receptors. We propose that the well-defined anti-proliferative action of retinoids could, at least in part, result from this novel transrepressive action on the mitogenic transcription factor SRF.

Disruption of the actin cytoskeleton regulates cytokine-induced iNOS expression

Interleukin-1beta (IL-1beta) induces the inducible nitric oxide synthase (iNOS), resulting in the release of nitric oxide (NO) from glomerular mesangial cells. In this study, we demonstrated that disruption of F-actin formation by sequestration of G-actin with the toxin latrunculin B (LatB) dramatically potentiated IL-1beta-induced iNOS protein expression in a dose-dependent manner. LatB by itself had little or no effect on iNOS expression. Staining of F-actin with nitrobenzoxadiazole (NBD)-phallacidin demonstrated that LatB significantly impaired F-actin stress fiber formation. Jasplakinolide (Jasp), which binds to and stabilizes F-actin, suppressed iNOS expression enhanced by LatB. These data strongly suggest that actin cytoskeletal dynamics regulates IL-1beta-induced iNOS expression. We demonstrated that LatB decreases serum response factor (SRF) activity as determined by reporter gene assays, whereas Jasp increases SRF activity. The negative correlation between SRF activity and iNOS expression suggests a negative regulatory role for SRF in iNOS expression. Overexpression of a dominant negative mutant of SRF increases the IL-1beta-induced iNOS expression, providing direct evidence that SRF inhibits iNOS expression.

Role of the cytosolic phospholipase A2-linked cascade in signaling by an oncogenic, constitutively active Ha-Ras isoform

Activation of Ras signaling by growth factors has been associated with gene regulation and cell proliferation. Here we characterize the contributory role of cytosolic phospholipase A(2) in the oncogenic Ha-Ras(V12) signaling pathway leading to activation of c-fos serum response element (SRE) and transformation in Rat-2 fibroblasts. Using a c-fos SRE-luciferase reporter gene, we showed that the transactivation of SRE by Ha-Ras(V12) is mainly via a Rac-linked cascade, although the Raf-mitogen-activated protein kinase cascade is required for full activation. In addition, Ha-Ras(V12)-induced DNA synthesis was significantly attenuated by microinjection of recombinant Rac(N17), a dominant negative mutant of Rac1. To identify the mediators downstream of Rac in the Ha-Ras(V12) signaling, we investigated the involvement of cytosolic phospholipase A(2). Oncogenic Ha-Ras(V12)-induced SRE activation was significantly inhibited by either pretreatment with mepacrine, a phospholipase A(2) inhibitor, or cotransfection with the antisense oligonucleotide of cytosolic phospholipase A(2). We also found cytosolic phospholipase A(2) to be situated downstream of Ha-Ras(V12) in a signal pathway leading to transformation. Together, these results are indicative of mediatory roles of Rac and cytosolic phospholipase A(2) in the signaling pathway by which Ha-Ras(V12) transactivates c-fos SRE and transformation. Our findings point to cytosolic phospholipase A(2) as a novel potential target for suppressing oncogenic Ha-Ras(V12) signaling in the cell.

p63alpha and DeltaNp63alpha can induce cell cycle arrest and apoptosis and differentially regulate p53 target genes

The p53 tumor suppressor protein plays a critical role in the regulation of the cell cycle and apoptosis. The importance of p53's functions is underscored by the high incidence of p53 mutations in human cancers. Recently, two p53-related proteins, p73 and p63, were identified as members of the p53 gene family. Multiple isoforms of p73 have been found, including DeltaN variants in which the N-termini are truncated. p63 is expressed as three major forms, p63alpha, p63beta and p63gamma, each of which differ in their C-termini. All three forms can be alternatively transcribed from a cryptic promoter located within intron 3, producing DeltaNp63alpha, DeltaNp63beta and DeltaNp63gamma. The high degree of similarity of p73 and p63 to evolutionarily conserved regions of p53 suggests that these proteins play an important and potentially redundant role in regulating cell cycle arrest and apoptosis. Here we describe the characterization of cell lines generated to inducibly express p63alpha and DeltaNp63alpha. We have found that p63alpha and DeltaNp63alpha can differentially regulate endogenous p53 target genes and induce cell cycle arrest and apoptosis. Deletion of the N-terminal 26 amino acids of DeltaNp63alpha abolished its ability to transactivate p53 target genes and induce cell cycle arrest and apoptosis. This indicates that a putative transactivation domain exists within the N-terminus of the DeltaN variants of p63. Furthermore, the differential regulation of p53 target genes by p63alpha and DeltaNp63alpha suggests that p63 and p53 utilize both similar and different signaling pathways to execute their cellular functions.

p73 is transcriptionally regulated by DNA damage, p53, and p73

p73 is a member of the p53 family. Recent studies have shown that DNA damage can stabilize p73 protein and enhance p73-mediated apoptosis in a c-Abl dependent manner. To determine what regulates p73 transcriptionally, we analysed the expression of p73 in several cell lines following genotoxic stresses. We found that p73 is induced in certain cell lines when treated with therapeutic DNA damaging agents. We also found that p53 and p73, but not mutant p53(R249S) and p73beta292, directly induce the expression of the p73 gene. In addition, we found one potential p53-binding site in the promoter of the p73 gene. This binding site is responsive to p53, p73, and DNA damage. Taken together, these data suggest that p73 is transcriptionally regulated by DNA damage and p53, and is autoregulated.

Aquaporin 3, a glycerol and water transporter, is regulated by p73 of the p53 family

p73, a member of the p53 family, has been shown to exhibit similar biochemical activities to that of p53. However, in contrast to p53, p73 is rarely mutated in human tumors and p73 mutant mice develop neurological, pheromonal, and inflammatory defects, but not spontaneous tumors. Furthermore, p73 mutant mice are deficient in the physiological control of cerebral spinal fluid. To determine what mediates these p73 activities, cDNA subtraction assay was performed to identify cellular genes that are regulated by p73. We found that aquaporin 3 (AQP3), a glycerol and water transporter, is regulated by p73. In addition, we identified a potential p53 response element in the promoter of the AQP3 gene, which is responsive to p73. This suggests that AQP3 may mediate the activity of p73 in maintaining cerebral spinal fluid dynamics.

c-Cbl is a suppressor of the neu oncogene

A rodent oncogenic mutant of the Neu receptor tyrosine kinase is a useful experimental model because overexpression of the respective receptor, namely HER2/ErbB-2, in human malignancies is associated with relatively aggressive diseases. Here we show that the oncogenic form of Neu is constitutively associated with the product of the c-cbl proto-oncogene and is part of a large complex that includes the phosphoinositide 3-kinase and Shc. Ectopic expression of c-Cbl, a ubiquitin-protein isopeptide ligase specific to activated tyrosine kinases, causes rapid removal of Neu from the cell surface and severely reduces signaling downstream of oncogenic Neu. c-Cbl-induced down-regulation of Neu involves covalent attachment of ubiquitin molecules and requires the carboxyl-terminal domain of Neu. The negative effect of c-Cbl is antagonized by v-Cbl, a virus-encoded oncogenic truncated form of c-Cbl. In an in vivo model, infection of a Neu-transformed neuroblastoma with a c-Cbl-encoding retrovirus caused enhanced down-regulation of Neu and correlated with tumor retardation. Our results implicate c-Cbl in negative regulation of Neu and offer a potential target for treatment of HER2/ErbB-2-positive human malignancies.

Regulation of mitogen-activated protein kinase signaling networks by G protein-coupled receptors

The family of receptors that transmit signals through the activation of heterotrimeric GTP-binding proteins (G proteins) constitutes the largest group of cell surface proteins involved in signal transduction. These receptors participate in a broad range of important biological functions and are implicated in a number of disease states. More than half of all drugs currently available influence G protein-coupled receptors (GPCRs). These receptors affect the generation of small molecules that act as intracellular mediators or second messengers, and can regulate a highly interconnected network of biochemical routes controlling the activity of several members of the mitogen-activated protein kinase (MAPK) superfamily. They include extracellular signal-regulated kinase 1 (ERK1) and ERK2 (or p44(MAPK) and p42(MAPK)), c-Jun NH(2)-terminal kinases (JNKs), ERK5 (or BMK), and p38 MAPKs, including p38alpha (or CSBP-1), p38beta, p38gamma (or SAPK3 or ERK6), and p38delta?(or SAPK4). This review will focus on the molecular mechanisms by which GPCRs signal to the nucleus through this intricate network of second messenger-generating systems and MAPK signaling pathways, thereby affecting the expression of genes whose products influence many biological processes, including normal and aberrant cell growth.

Dickkopf-1, an inhibitor of the Wnt signaling pathway, is induced by p53

Dickkopf-1 (Dkk-1), a secreted glycoprotein, has been found to be necessary and sufficient for inducing amphibian head formation. Interestingly, the mechanism by which Dkk-1 does this is the ability of Dkk-1 to antagonize the Wnt signaling pathway. Wnt, itself a proto-oncoprotein, can promote cell proliferation and transformation when mutated or overexpressed, leading to tumor formation. p53 is a tumor suppressor and loss of p53 function accelerates mammary tumorigenesis by Wnt. In this study, we found that Dkk-1 is induced by wild-type p53 but not mutant p53(R249S). In addition, DNA damage upregulates Dkk-1 in cell lines that harbor an endogenous wild-type p53 gene but not in cell lines that are p53-null or harbor an endogenous mutant p53 gene. We also found a potential p53 responsive element located approximately 2100 nucleotides upstream of the Dkk-1 transcription start site and we show that p53 binds specifically to this element both in vitro and in vivo. Furthermore, we have established several cell lines derived from H1299 lung carcinoma and U118 glioma cells that inducibly express Dkk-1 under a tetracycline-regulated promoter. We found that Dkk-1 has no effect on proliferation of cells that are not transformed by Wnt. Taken together, these results suggest that Dkk-1 may mediate p53 tumor suppression by antagonizing the Wnt signaling pathway.

p53 induces TAP1 and enhances the transport of MHC class I peptides

The transporter associated with antigen processing (TAP) 1 is required for the major histocompatibility complex (MHC) class I antigen presentation pathway, which plays a key role in host tumor surveillance. Since more than 50% of tumors have a dysfunctional p53, evasion of tumor surveillance by tumor cells may be linked to loss of p53 function. Here we found that TAP1 is strongly induced by p53 and DNA-damaging agents through a p53-responsive element. We also found that p73, which is homologous to p53, is capable of inducing TAP1 and cooperates with p53 to activate TAP1. Furthermore, we found that by inducing TAP1, p53 enhances the transport of MHC class I peptides and expression of surface MHC-peptide complexes, and cooperates with interferon gamma to activate the MHC class I pathway. These results suggest that tumor surveillance may be a mechanism by which p53 and/or p73 function as tumor suppressors.

Requirement of MEF2D in the induced differentiation of HL60 promyeloid cells

The regulatory role of MEF2 (myocyte enhancer binding factor 2) proteins in nonmuscle tissues has not been well characterized. We examined the expression of MEF2 family members, namely, MEF2A, -B, -C, and -D, in the differentiation of HL60 promyeloid cells and observed the remarkable increase in the expressions of MEF2A and MEF2D proteins during the differentiation process into monocytes. To examine the role of MEF2, we expressed a dominant-negative form of MEF2D, without its transactivation domain, in HL60 cells. When the HL60 cell line expressing the mutant MEF2D was induced to differentiate by VitD(3) treatment, cell surface expression of CD14 and the ability to reduce NBT, which are important characteristics of differentiated monocytes, were significantly decreased compared with control HL60 cells. These results show that MEF2D is required in the differentiation process along the monocyte/macrophage lineage,

Sequences at the 3' side of the c-fos SRE mediate gene expression via an Sob1-dependent, TCF-independent pathway

We previously described a 110-kDa tyrosine phosphoprotein, Sob 1, that regulates formation of the DNA binding complex Band A at the c-fos serum response element (SRE) during T cell activation. Using competition and mutant oligonucleotide analysis, we have determined that both the core CArG box of the c-fos SRE and the 3' sequences flanking the CArG box are necessary for stable Band A complex formation. Moreover, using transient transfection and reporter assays, we show that mutations affecting Band A complex formation in vitro also impaired serum induction of c-fos gene expression in vivo. Since mutation at this site has no effect on SRF binding, our results suggest that in combination with SRE/SRF, Sob 1-regulated factor(s) bind at the 3' side of SRE to form Band A, and this confers maximal serum induction of c-fos gene expression via the SRE.

The RING finger of c-Cbl mediates desensitization of the epidermal growth factor receptor

Ligand-induced activation of surface receptors, including the epidermal growth factor receptor (EGFR), is followed by a desensitization process involving endocytosis and receptor degradation. c-Cbl, a tyrosine phosphorylation substrate shared by several signaling pathways, accelerates desensitization by recruiting EGFR and increasing receptor polyubiquitination. Here we demonstrate that the RING type zinc finger of c-Cbl is essential for ubiquitination and subsequent desensitization of EGFR. Mutagenesis of a single cysteine residue impaired the ability of c-Cbl to enhance both down-regulation and ubiquitination of EGFR in living cells, although the mutant retained binding to the activated receptor. Consequently, the mutant form of c-Cbl acquired a dominant inhibitory function and lost the ability to inhibit signaling downstream to EGFR. In vitro reconstitution of EGFR ubiquitination implies that the RING finger plays an essential direct role in ubiquitin ligation. Our results attribute to the RING finger of c-Cbl a causative role in endocytic sorting of EGFR and desensitization of signal transduction.

Role of diacylglycerol (DAG) in hormonal induction of S phase in hepatocytes: the DAG-dependent protein kinase C pathway is not activated by epidermal growth factor (EGF), but is involved in mediating the enhancement of responsiveness to EGF by vasopressin, angiotensin II, and norepinephrine

The role of diacylglycerol (DAG) in hormonal induction of S phase was investigated in primary cultures of rat hepatocytes. In this model, several agonists that bind to G protein-coupled receptors act as comitogens when added to the cells soon after plating (i.e., in Go/early Gl phase), while the cells are most responsive to the mitogenic effect of epidermal growth factor (EGF) at 24-48 h of culturing (i.e., mid/late Gl). It was found that the cellular concentration of DAG rose markedly and progressively during the first 24 h of culturing. Exposure of the hepatocytes at 3 h to alpha1-adrenergic stimulation (norepinephrine with timolol), vasopressin, or angiotensin II further increased this rise, producing a sustained increase in the DAG level. Norepinephrine, which was the most efficient comitogen, produced the most prolonged DAG elevation. In contrast, no significant increase of DAG was found in response to EGF, neither at 3 nor at 24 h, using concentrations that markedly stimulated the ERK subgroup of the mitogen-activated protein kinases (MAPK) and DNA synthesis. Addition of Bacillus cereus phosphatidylcholine-specific phospholipase C (PC-PLC) strongly elevated DAG, while Streptomyces phospholipase D (PLD) increased phosphatidic acid (PA) but not DAG. B. cereus PC-PLC and the protein kinase C (PKC) activator tetradecanoyl phorbol-acetate (TPA), like norepinephrine, vasopressin, and angiotensin II, stimulated MAPK and enhanced the stimulatory effect of EGF on DNA synthesis. The PKC inhibitor GF109203X did not diminish the effect of EGF on MAPK or DNA synthesis, but strongly inhibited the effects of norepinephrine, vasopressin, angiotensin II, TPA and B. cereus PC-PLC on MAPK and almost abolished the enhancement by these agents of EGF-stimulated DNA synthesis. These results suggest that although generation of DAG is not a direct downstream response mediating the effects of the EGF receptor in hepatocytes, a sustained elevation of DAG with activation of PKC markedly increases the responsiveness to EGF. Mechanisms involving DAG and PKC seem to play a role in the comitogenic effects of various agents that bind to G protein-coupled receptors and activate the cells early in Gl, such as norepinephrine, angiotensin II, and vasopressin.

Signal-regulated activation of serum response factor is mediated by changes in actin dynamics

Serum response factor (SRF) regulates transcription of many serum-inducible and muscle-specific genes. Using a functional screen, we identified LIM kinase-1 as a potent activator of SRF. We show that SRF activation by LIM kinase-1 is dependent on its ability to regulate actin treadmilling. LIM kinase activity is not essential for SRF activation by serum, but signals depend on alterations in actin dynamics. Studies with actin-binding drugs, the actin-specific C2 toxin, and actin overexpression demonstrate that G-actin level controls SRF. Regulation of actin dynamics is necessary for serum induction of a subset of SRF target genes, including vinculin, cytoskeletal actin, and srf itself, and also suffices for their activation. Actin treadmilling provides a convergence point for both serum- and LIM kinase-1-induced signaling to SRF.