Casein kinase II enhances the DNA binding activity of serum response factor

Striated muscle activator of Rho signalling (STARS) is reduced in ageing human skeletal muscle and targeted by miR-628-5p

AIM

The striated muscle activator of Rho signalling (STARS) is a muscle-specific actin-binding protein. The STARS signalling pathway is activated by resistance exercise and is anticipated to play a role in signal mechanotransduction. Animal studies have reported a negative regulation of STARS signalling with age, but such regulation has not been investigated in humans.

METHODS

Ten young (18-30 years) and 10 older (60-75 years) subjects completed an acute bout of resistance exercise. Gene and protein expression of members of the STARS signalling pathway and miRNA expression of a subset of miRNAs, predicted or known to target members of STARS signalling pathway, were measured in muscle biopsies collected pre-exercise and 2 h post-exercise.

RESULTS

For the first time, we report a significant downregulation of the STARS protein in older subjects. However, there was no effect of age on the magnitude of STARS activation in response to an acute bout of exercise. Finally, we established that miR-628-5p, a miRNA regulated by age and exercise, binds to the STARS 3'UTR to directly downregulate its transcription.

CONCLUSION

This study describes for the first time the resistance exercise-induced regulation of STARS signalling in skeletal muscle from older humans and identifies a new miRNA involved in the transcriptional control of STARS.

RhoA as a mediator of clinically relevant androgen action in prostate cancer cells

Recently, we have identified serum response factor (SRF) as a mediator of clinically relevant androgen receptor (AR) action in prostate cancer (PCa). Genes that rely on SRF for androgen responsiveness represent a small fraction of androgen-regulated genes, but distinguish benign from malignant prostate, correlate with aggressive disease, and are associated with biochemical recurrence. Thus, understanding the mechanism(s) by which SRF conveys androgen regulation to its target genes may provide novel opportunities to target clinically relevant androgen signaling. Here, we show that the small GTPase ras homolog family member A (RhoA) mediates androgen-responsiveness of more than half of SRF target genes. Interference with expression of RhoA, activity of the RhoA effector Rho-associated coiled-coil containing protein kinase 1 (ROCK), and actin polymerization necessary for nuclear translocation of the SRF cofactor megakaryocytic acute leukemia (MAL) prevented full androgen regulation of SRF target genes. Androgen treatment induced RhoA activation, increased the nuclear content of MAL, and led to MAL recruitment to the promoter of the SRF target gene FHL2. In clinical specimens RhoA expression was higher in PCa cells than benign prostate cells, and elevated RhoA expression levels were associated with aggressive disease features and decreased disease-free survival after radical prostatectomy. Overexpression of RhoA markedly increased the androgen-responsiveness of select SRF target genes, in a manner that depends on its GTPase activity. The use of isogenic cell lines and a xenograft model that mimics the transition from androgen-stimulated to castration-recurrent PCa indicated that RhoA levels are not altered during disease progression, suggesting that RhoA expression levels in the primary tumor determine disease aggressiveness. Androgen-responsiveness of SRF target genes in castration-recurrent PCa cells continued to rely on AR, RhoA, SRF, and MAL and the presence of intact SRF binding sites. Silencing of RhoA, use of Rho-associated coiled-coil containing protein kinase 1 inhibitors, or an inhibitor of SRF-MAL interaction attenuated (androgen-regulated) cell viability and blunted PCa cell migration. Taken together, these studies demonstrate that the RhoA signaling axis mediates clinically relevant AR action in PCa.

Nrf3-Pla2g7 interaction plays an essential role in smooth muscle differentiation from stem cells

OBJECTIVE

Phospholipase A2, group 7 (Pla2g7) is an important mediator in cardiovascular development and diseases because of its divergent physiological and pathological functions in inflammation and oxidative stress. However, little is known about the functional role of Pla2g7 in smooth muscle cell (SMC) differentiation from stem cells.

METHODS AND RESULTS

In the present study, embryonic stem cells were cultivated on collagen IV-coated plates to allow SMC differentiation. Pla2g7 gene expression and activity were upregulated significantly following 4 to 14 days of cell differentiation and colocalized with SMC differentiation markers in the differentiated SMCs. Knockdown of Pla2g7 resulted in downregulation of smooth muscle-specific markers in vitro and impairment of SMC differentiation in vivo, whereas enforced expression of Pla2g7 enhanced SMC differentiation and increased reactive oxygen species generation. Importantly, enforced expression of Pla2g7 significantly increased the binding of serum response factor to SMC differentiation gene promoters, resulting in SMC differentiation, which was abolished by free radical scavenger and flavoprotein inhibitor of NADPH oxidase but not hydrogen peroxide inhibitor. Moreover, we demonstrated that nuclear factor erythroid 2-related factor 3 (Nrf3) regulates Pla2g7 gene expression through direct binding to the promoter regions of Pla2g7 gene.

CONCLUSION

Our findings demonstrated that Pla2g7 plays a crucial physiological role in SMC differentiation from stem cells, and the fine interactions between Nrf3 and Pla2g7 are essential for SMC differentiation.

The essential function for serum response factor in T-cell development reflects its specific coupling to extracellular signal-regulated kinase signaling

Serum response factor (SRF) recruits members of two families of signal-regulated coactivators, the extracellular signal-regulated kinase (ERK)-regulated ternary complex factors (TCFs) and the actin-regulated myocardin-related transcription factors (MRTFs), to its target genes through its DNA-binding domain. Whether coactivator association is required for SRF function in vivo and whether particular SRF functions reflect specific coupling to one or the other signal pathway have remained largely unexplored. We show that SRF is essential for thymocyte positive selection and thymic T(reg) and NK T-cell development but dispensable for early thymocyte development and negative selection. Expression of wild-type SRF, or mutants lacking the N-terminal phosphorylation sites or C-terminal transcriptional activation domain, restores positive selection in SRF null thymocytes. In contrast, SRF.V194E, which cannot recruit TCF or MRTF family members, is inactive, although it is recruited to target genes. Fusion of a TCF C-terminal activation domain to SRF.V194E effectively restores ERK-dependent single-positive (SP) thymocyte development. The resulting SP thymocytes exhibit normal surface marker expression and proliferation following T-cell receptor cross-linking. Thus, ERK signaling through the TCF pathway to SRF is necessary and sufficient for SRF function in thymocyte positive selection.

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.

c-Fos as a proapoptotic agent in TRAIL-induced apoptosis in prostate cancer cells

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)/Apo-2L promotes apoptosis in cancer cells while sparing normal cells. Although many cancers are sensitive to TRAIL-induced apoptosis, some evade the proapoptotic effects of TRAIL. Therefore, differentiating molecular mechanisms that distinguish between TRAIL-sensitive and TRAIL-resistant tumors are essential for effective cancer therapies. Here, we show that c-Fos functions as a proapoptotic agent by repressing the antiapoptotic molecule c-FLIP(L). c-Fos binds the c-FLIP(L) promoter, represses its transcriptional activity, and reduces c-FLIP(L) mRNA and protein levels. Therefore, c-Fos is a key regulator of c-FLIP(L), and activation of c-Fos determines whether a cancer cell will undergo cell death after TRAIL treatment. Strategies to activate c-Fos or inhibit c-FLIP(L) may potentiate TRAIL-based proapoptotic therapies.

MG-132 Sensitizes TRAIL-Resistant Prostate Cancer Cells by Activating c-Fos/c-Jun Heterodimers and Repressing c-FLIP(L)

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising anticancer agent because it induces apoptosis in cancer cells but not in normal cells. Unfortunately, some cancer cells develop resistance to TRAIL-induced apoptosis. Therefore, it is clinically relevant to determine the molecular mechanisms that differentiate between TRAIL-sensitive and TRAIL-resistant tumors. Previously, we have shown that the antiapoptotic molecule cellular-FLICE-inhibitory protein long isoform [c-FLIP(L)] is necessary and sufficient to maintain resistance to TRAIL-induced apoptosis. We have found that c-FLIP(L) is transcriptionally regulated by the activator protein-1 (AP-1) family member protein c-Fos. Here, we report that MG-132, a small-molecule inhibitor of the proteasome, sensitizes TRAIL-resistant prostate cancer cells by inducing c-Fos and repressing c-FLIP(L). c-Fos, which is activated by MG-132, negatively regulates c-FLIP(L) by direct binding to the putative promoter region of the c-FLIP(L) gene. In addition to activating c-Fos, MG-132 activates another AP-1 family member, c-Jun. We show that c-Fos heterodimerizes with c-Jun to repress transcription of c-FLIP(L). Therefore, MG-132 sensitizes TRAIL-resistant prostate cancer cells by activating the AP-1 family members c-Fos and c-Jun, which, in turn, repress the antiapoptotic molecule c-FLIP(L).

130-kDa smooth muscle myosin light chain kinase is transcribed from a CArG-dependent, internal promoter within the mouse mylk gene

The 130-kDa smooth muscle myosin light chain kinase (smMLCK) is a Ca2+/CaM-regulated enzyme that plays a pivotal role in the initiation of smooth muscle contraction and regulation of cellular migration and division. Despite the critical importance of smMLCK in these processes, little is known about the mechanisms regulating its expression. In this study, we have identified the proximal promoter of smMLCK within an intron of the mouse mylk gene. The mylk gene encodes at least two isoforms of MLCK (130 and 220 kDa) and telokin. Luciferase reporter gene assays demonstrated that a 282-bp fragment (−167 to +115) of the smMLCK promoter was sufficient for maximum activity in A10 smooth muscle cells and 10T1/2 fibroblasts. Deletion of the 16 bp between −167 and −151, which included a CArG box, resulted in a nearly complete loss of promoter activity. Gel mobility shift assays and chromatin immunoprecipitation assays demonstrated that serum response factor (SRF) binds to this CArG box both in vitro and in vivo. SRF knockdown by short hairpin RNA decreased endogenous smMLCK expression in A10 cells. Although the SRF coactivator myocardin induced smMLCK expression in 10T1/2 cells, myocardin activated the promoter only two- to fourfold in reporter gene assays. Addition of either intron 1 or 6 kb of the 5′ upstream sequence did not lead to any further activation of the promoter by myocardin. The proximal smMLCK promoter also contains a consensus GATA-binding site that bound GATA-6. GATA-6 binding to this site decreased endogenous smMLCK expression, inhibited promoter activity in smooth muscle cells, and blocked the ability of myocardin to induce smMLCK expression. Altogether, these data suggest that SRF and SRF-associated factors play a key role in regulating the expression of smMLCK.

Serum response factor MADS box serine-162 phosphorylation switches proliferation and myogenic gene programs

Phosphorylation of a cluster of amino acids in the serum response factor (SRF) "MADS box" alphaI coil DNA binding domain regulated the transcription of genes associated with proliferation or terminal muscle differentiation. Mimicking phosphorylation of serine-162, a target of protein kinase C-alpha, with an aspartic acid substitution (SRF-S162D) completely inhibited SRF-DNA binding and blocked alpha-actin gene transcription even in the presence of potent myogenic cofactors, while preserving c-fos promoter activity because of stabilization of the ternary complex via Elk-1. Introduction of SRF-S162D into SRF null ES cells permitted transcription of the c-fos gene but was unable to rescue expression of myogenic contractile genes. Transition of proliferating C2C12 myoblasts to postfusion myocytes after serum withdrawal was associated with a progressive decline in SRF-S162 phosphorylation and an increase in alpha-actin gene expression. Hence, the phosphorylation status of serine-162 in the alphaI coil may constitute a novel switch that directs target gene expression into proliferation or differentiation programs.

Serum response factor function and dysfunction in smooth muscle

Tight control of smooth muscle cell (SM) proliferation, differentiation, and apoptosis requires a balance between signaling and transcriptional events. Recent developments in vascular research revealed that serum response factor (SRF) function is important for the regulation of each of these processes. The cloning and characterization of several SM specific genes and the discovery that SRF is central for their expression fueled studies aimed at understanding the role of molecular partners including co-activators and co-repressors. Perturbations of pathways involving SRF are associated with abnormalities in the myogenic program and aberrant phenotypic consequences. Surprisingly, studies on airway SM have remained an underrepresented area of investigation. Our laboratory described a novel regulatory mechanism of SRF function in airway myocytes by modulation of its subcellular localization. This review summarizes current knowledge on the structure and function of this essential transcription factor as well different modes of regulating SRF expression and activity that are becoming key players in directing SM function in health and disease.

Megakaryoblastic leukemia 1, a potent transcriptional coactivator for serum response factor (SRF), is required for serum induction of SRF target genes

Megakaryoblastic leukemia 1 (MKL1) is a myocardin-related transcription factor that we found strongly activated serum response element (SRE)-dependent reporter genes through its direct binding to serum response factor (SRF). The c-fos SRE is regulated by mitogen-activated protein kinase phosphorylation of ternary complex factor (TCF) but is also regulated by a RhoA-dependent pathway. The mechanism of this pathway is unclear. Since MKL1 (also known as MAL, BSAC, and MRTF-A) is broadly expressed, we assessed its role in serum induction of c-fos and other SRE-regulated genes with a dominant negative MKL1 mutant (DN-MKL1) and RNA interference (RNAi). We found that DN-MKL1 and RNAi specifically blocked SRE-dependent reporter gene activation by serum and RhoA. Complete inhibition by RNAi required the additional inhibition of the related factor MKL2 (MRTF-B), showing the redundancy of these factors. DN-MKL1 reduced the late stage of serum induction of endogenous c-fos expression, suggesting that the TCF- and RhoA-dependent pathways contribute to temporally distinct phases of c-fos expression. Furthermore, serum induction of two TCF-independent SRE target genes, SRF and vinculin, was nearly completely blocked by DN-MKL1. Finally, the RBM15-MKL1 fusion protein formed by the t(1;22) translocation of acute megakaryoblastic leukemia had a markedly increased ability to activate SRE reporter genes, suggesting that its activation of SRF target genes may contribute to leukemogenesis.

Identification of novel sites of O-N-acetylglucosamine modification of serum response factor using quadrupole time-of-flight mass spectrometry

The addition of a single N-acetylglucosamine moiety O-linked to serine and threonine residues of nuclear and cytoplasmic proteins is a widespread post-translational modification. The conventional method for detecting and locating sites of modification is through a multistep radioactivity-based approach. We have recently shown that sites of O-GlcNAc modification can be determined using quadrupole time-of-flight tandem mass spectrometry (Chalkley, R. J., and Burlingame, A. L. (2001) Identification of GlcNAcylation sites of peptides and alpha-crystallin using Q-TOF mass spectrometry. J. Am. Soc. Mass Spectrom. 12, 1106-1113). In this work utilization of this new approach has revealed previously undetected sites of O-GlcNAc modification of the transcription factor serum response factor.

Genome-wide analysis of yeast transcription upon calcium shortage

Several regulatory circuits related to important functions, like membrane excitation, immunoresponse, replication, control of the cell cycle and differentiation, among others, cause an increase in intracellular calcium level that finally has a consequence upon transcription of specific genes. The sequencing of the whole genome of eukaryotic cells enables genome-wide analysis of gene expression under many conditions not yet assessed by conventional methods. Using the array technology, the effect of calcium shortage in yeast cells was studied. Correspondence analysis of data showed that there is a response in transcription that is correlated to calcium shortage. The distribution of up-regulated-genes in functional categories suggests a regulatory connection between the cell-cycle progression and the energetic metabolic requirements for growth and division. In silico analysis of promoters reveals the frequent appearance of the Mlu I cell cycle box (MCB) cis element that binds the transcriptional regulatory factor Mcm1.

Coronary smooth muscle differentiation from proepicardial cells requires rhoA-mediated actin reorganization and p160 rho-kinase activity

We recently reported that the first detectable expression of SMC-specific proteins during coronary smooth muscle cell (CoSMC) differentiation from isolated proepicardial cells was restricted to cells undergoing epithelial-to-mesenchymal transformation (EMT). The objectives of this study were to examine more closely the relation between actin cytoskeletal rearrangements and serum response factor (SRF)-dependent transcription, and to specifically test whether rhoA-GTPase signaling is required for CoSMC differentiation. We report here that PDGF-BB stimulates EMT and promotes SRF-dependent expression of SMC marker genes calponin, SM22alpha, and SMgamma(actin) (SMgammaA) in proepicardial cells. C3 exoenzyme or rhoGDI, inhibitors of rhoA signaling, blocked PDGF-BB-induced EMT, prevented actin reorganization into stress fibers, and inhibited CoSMC differentiation. Incubation with the selective p160 rho-kinase (p160RhoK) inhibitor Y27632 (RKI) blocked EMT, prevented the appearance of calponin and SMgammaA-positive cells, and abolished expression and nuclear localization of SRF. To test the role of RhoK signaling for CoSMC differentiation in vivo, quail proepicardial organs (PEOs) were pretreated with RKI or vehicle and then grafted into age-matched host chick embryos to produce a chimeric epicardium. The ability of grafted cells to participate in coronary vessel formation was monitored by staining with antibodies for quail cell nuclear antigen and SMC marker proteins. Proepicardial cells pretreated with RKI failed to form CoSMCs in vivo. Time course studies traced this deficiency to a failure of epicardial-derived mesenchymal cells to migrate into or survive within the myocardium. In summary, these data point to important roles for rhoA-RhoK signaling in molecular pathways controlling cytoskeletal reorganization, SRF-dependent transcription, and cell survival that are required to produce CoSMCs from proepicardial cells.

Serum response factor cleavage by caspases 3 and 7 linked to apoptosis in human BJAB cells

Apoptosis involves the cessation of cellular processes, the breakdown of intracellular organelles, and, finally, the nonphlogistic clearance of apoptotic cells from the body. Important for these events is a family of proteases, caspases, which are activated by a proteolytic cleavage cascade and drive apoptosis by targeting key proteins within the cell. Here, we demonstrate that serum response factor (SRF), a transcription factor essential for proliferative gene expression, is cleaved by caspases and that this cleavage occurs in proliferating murine fibroblasts and can be induced in the human B-cell line BJAB. We identify the two major sites at which SRF cleavage occurs as Asp(245) and Asp(254), the caspases responsible for the cleavage and generate a mutant of SRF resistant to cleavage in BJAB cells. Investigation of the physiological and functional significance of SRF cleavage reveals that it correlates with the loss of c-fos expression, whereby neither SRF cleavage fragment retains transcriptional activity. Moreover, the expression of a noncleavable SRF in BJAB cells suppresses apoptosis induced by Fas cross-linking. These results suggest that for apoptosis to proceed, the transcriptional events promoting cell survival and proliferation, in which SRF is involved, must first be inactivated.

A nuclear casein kinase 2 activity is involved in early events of transcriptional activation induced by salicylic acid in tobacco

Salicylic acid (SA) activates immediate early transcription of genes controlled by a family of DNA promoter elements named as-1-like elements. These elements are functional in the promoter of glutathione S-transferase genes. We have previously shown that SA increases the binding of tobacco (Nicotiana tabacum cv Xanthi nc) nuclear factors to the as-1 sequence in a process mediated by protein phosphorylation. In this study we give evidence for the participation of a nuclear protein kinase CK2 (casein kinase 2) in the pathway activated by SA in tobacco. The first line of evidence comes from the evaluation of the CK2 activity in nuclear extracts prepared from tobacco plants treated with SA or water as a control. Results from these experiments indicate that SA increases the nuclear CK2 activity. The second line of evidence derives from the evaluation of the in vivo effect of 5,6-dichloro-1-(beta-D-ribofuranosyl) benzimidazole (DRB), a cell-permeable CK2 inhibitor, on the responsiveness of the as-1 sequence to SA. Results from these experiments indicate that DRB impairs the activating effect of SA on the transcription of both, the GUS reporter gene controlled by a tetramer of the as-1 element, and the endogenous gnt35 gene encoding a glutathione S-transferase, in transgenic tobacco plants. DRB also impaired the increasing effect of SA on the binding of nuclear factors to the as-1 element. Furthermore, transcription of the as-1/GUS reporter gene activated by the synthetic auxin 2,4-dichlorophenoxyacetic acid and by methyl jasmonate was also inhibited by DRB. To our knowledge, this is the first report in which activation of a CK2 enzyme by a plant hormone is reported.

Physiological control of smooth muscle-specific gene expression through regulated nuclear translocation of serum response factor

Prolonged serum deprivation induces a structurally and functionally contractile phenotype in about 1/6 of cultured airway myocytes, which exhibit morphological elongation and accumulate abundant contractile apparatus-associated proteins. We tested the hypothesis that transcriptional activation of genes encoding these proteins accounts for their accumulation during this phenotypic transition by measuring the transcriptional activities of the murine SM22 and human smooth muscle myosin heavy chain promoters during transient transfection in subconfluent, serum fed or 7 day serum-deprived cultured canine tracheal smooth muscle cells. Contrary to our expectation, SM22 and smooth muscle myosin heavy chain promoter activities (but not viral murine sarcoma virus-long terminal repeat promoter activity) were decreased in long term serum-deprived myocytes by at least 8-fold. Because serum response factor (SRF) is a required transcriptional activator of these and other smooth muscle-specific promoters, we evaluated the expression and function of SRF in subconfluent and long term serum-deprived cells. Whole cell SRF mRNA and protein were maintained at high levels in serum-deprived myocytes, but SRF transcription-promoting activity, nuclear SRF binding to consensus CArG sequences, and nuclear SRF protein were reduced. Furthermore, immunocytochemistry revealed extranuclear redistribution of SRF in serum-deprived myocytes; nuclear localization of SRF was restored after serum refeeding. These results uncover a novel mechanism for physiological control of smooth muscle-specific gene expression through extranuclear redistribution of SRF and consequent down-regulation of its transcription-promoting activity.

Binding of Delta1, Jagged1, and Jagged2 to Notch2 rapidly induces cleavage, nuclear translocation, and hyperphosphorylation of Notch2

Delta1, Jagged1, and Jagged2, commonly designated Delta/Serrate/LAG-2 (DSL) proteins, are known to be ligands for Notch1. However, it has been less understood whether they are ligands for Notch receptors other than Notch1. Meanwhile, ligand-induced cleavage and nuclear translocation of the Notch protein are considered to be fundamental for Notch signaling, yet direct observation of the behavior of the Notch molecule after ligand binding, including cleavage and nuclear translocation, has been lacking. In this report, we investigated these issues for Notch2. All of the three DSL proteins bound to endogenous Notch2 on the surface of BaF3 cells, although characteristics of Jagged2 for binding to Notch2 apparently differed from that of Delta1 and Jagged1. After binding, the three DSL proteins induced cleavage of the membrane-spanning subunit of Notch2 (Notch2(TM)), which occurred within 15 min. In a simultaneous time course, the cleaved fragment of Notch2(TM) was translocated into the nucleus. Interestingly, the cleaved Notch2 fragment was hyperphosphorylated also in a time-dependent manner. Finally, binding of DSL proteins to Notch2 also activated the transcription of reporter genes driven by the RBP-Jkappa-responsive promoter. Together, these data indicate that all of these DSL proteins function as ligands for Notch2. Moreover, the findings of rapid cleavage, nuclear translocation, and phosphorylation of Notch2 after ligand binding facilitate the understanding of the Notch signaling.

The Arabidopsis floral homeotic gene PISTILLATA is regulated by discrete cis-elements responsive to induction and maintenance signals

PISTILLATA is a B-class floral organ identity gene required for the normal development of petals and stamens in Arabidopsis. PISTILLATA expression is induced in the stage 3 flowers (early expression) and is maintained until anthesis (late expression). To explore in more detail the developmentally regulated gene expression of PISTILLATA, we have analyzed the PISTILLATA promoter using uidA (beta)-glucuronidase gene) fusion constructs (PI::GUS) in transgenic Arabidopsis. Promoter deletion analyses suggest that early PISTILLATA expression is mediated by the distal region and that late expression is mediated by the proximal region. Based on the PI::GUS expression patterns in the loss- and gain-of-function alleles of meristem or organ identity genes, we have shown that LEAFY and UNUSUAL FLORAL ORGANS induce PISTILLATA expression in a flower-independent manner via a distal promoter, and that PISTILLATA and APETALA3 maintain PISTILLATA expression (autoregulation) in the later stages of flower development via a proximal promoter. In addition, we have demonstrated that de novo protein synthesis is required for the PISTILLATA autoregulatory circuit.

A naturally occurring sequence variation that creates a YY1 element is associated with increased cystic fibrosis transmembrane conductance regulator gene expression

We have identified previously a novel complex mutant allele in the cystic fibrosis transmembrane conductance regulator (CFTR) gene in a patient affected with cystic fibrosis (CF). This allele contained a mutation in CFTR exon 11 known to cause CF (S549R(T>G)), associated with the first alteration described so far in the minimal CFTR promoter region (-102T>A). Studies on genotype-phenotype correlations revealed striking differences between patients carrying mutation (S549R(T>G)) alone, who had a severe disease, and patients carrying the complex allele (-102(T>A)+S549R(T>G)), who exhibited milder forms of CF. We thus postulated that the sequence change (-102T>A) may attenuate the effects of the severe (S549R(T>G)) mutation through regulation of CFTR expression. Analysis of transiently transfected cell lines with wild-type and -102A variant human CFTR-directed luciferase reporter genes demonstrates that constructs containing the -102A variant (which creates a Yin Yang 1 (YY1) core element) increases CFTR expression significantly. Electrophoretic mobility shift assays indicate that the -102 site is located in a region of multiple DNA-protein interactions and that the -102A allele recruits specifically an additional nuclear protein related to YY1. The finding that the YY1-binding allele causes a significant increase in CFTR expression in vitro may allow a better understanding of the milder phenotype observed in patients who carry a severe CF mutation within the same gene.

Casein kinase 2 as a potentially important enzyme in the nervous system

Protein kinase CK2 is a ubiquitous and pleiotropic seryl/threonyl protein kinase which is highly conserved in evolution indicating a vital cellular role for this kinase. The holoenzyme is generally composed of two catalytic (alpha and/or alpha') and two regulatory (beta) subunits, but the free alpha/alpha' subunits are catalytically active by themselves and can be present in cells under some circumstances. Special attention has been devoted to phosphorylation status and structure of these enzymic molecules, however, their regulation and roles remain intriguing. Until recently, CK2 was believed to represent a kinase especially required for cell cycle progression in non-neural cells. At present, with respect to recent findings, four essential features suggest potentially important roles for this enzyme in specific neural functions: (1) CK2 is much more abundant in brain than in any other tissue; (2) there appear to be a myriad of substrates for CK2 in both synaptic and nuclear compartments that have clear implications in development, neuritogenesis, synaptic transmission, synaptic plasticity, information storage and survival; (3) CK2 seems to be associated with mechanisms underlying long-term potentiation in hippocampus; and (4) neurotrophins stimulate activity of CK2 in hippocampus. In addition, some data are suggestive that CK2 might play a role in processes underlying progressive disorders due to Alzheimer's disease, ischemia, chronic alcohol exposure or immunodeficiency virus HIV. The present review focuses mainly on the latest data concerning the regulatory mechanisms and the possible neurophysiological functions of this enzyme.

Regulated interaction of protein kinase Cdelta with the heterogeneous nuclear ribonucleoprotein K protein

The heterogeneous nuclear ribonucleoprotein (hnRNP) K protein recruits a diversity of molecular partners that are involved in signal transduction, transcription, RNA processing, and translation. K protein is phosphorylated in vivo and in vitro by inducible kinase(s) and contains several potential sites for protein kinase C (PKC) phosphorylation. In this study we show that K protein is phosphorylated in vitro by PKCdelta and by other PKCs. Deletion analysis and site-directed mutagenesis revealed that Ser302 is a major K protein site phosphorylated by PKCdelta in vitro. This residue is located in the middle of a short amino acid fragment that divides the two clusters of SH3-binding domains. Mutation of Ser302 decreased the level of phosphorylation of exogenously expressed K protein in phorbol 12-myristate 13-acetate-treated COS cells, suggesting that Ser302 is also a site for PKC-mediated phosphorylation in vivo. In vitro, PKCdelta binds K protein via the highly interactive KI domain, an interaction that is blocked by poly(C) RNA. Mutation of Ser302 did not alter the K protein-PKCdelta interaction in vitro, suggesting that phosphorylation of this residue alone is not sufficient to alter this interaction. Instead, binding of PKCdelta to K protein in vitro and in vivo was greatly increased by K protein phosphorylation on tyrosine residues. The ability of PKCdelta to bind and phosphorylate K protein may serve not only to alter the activity of K protein itself, but K protein may also bridge PKCdelta to other K protein molecular partners and thus facilitate molecular cross-talk. The regulated nature of the PKCdelta-K protein interaction may serve to meet cellular needs at sites of active transcription, RNA processing and translation in response to changing extracellular environment.

Role of protein kinase C and 72 kDa heat shock protein in ischemic tolerance following heat stress in the rat heart

Heat stress (HS) and the subsequent expression of 72 kDa heat shock protein (HSP 72) has been shown to enhance post-ischemic functional recovery and reduce infarct size. Because the synthesis of heat shock proteins involves activation of heat shock transcription factors through phosphorylation, we hypothesized that inhibition of protein kinase C (PKC) would block HS mediated protection and expression of HSP 72 in the heart. Five groups of rats were studied (1) Sham anesthetized, (2) HS group--animals were heat shocked by raising the whole body core temperature to 42 degrees C for 15 min, (3) Vehicle group--HS rats treated with 50% DMSO in saline, (4) PKC inhibitor-treated group--specific PKC antagonist, chelerythrine chloride (5 mg/kg, i.p) given 30 min prior to HS and (5) Vehicle treated control--non-HS rats treated with vehicle prior to ischemia/reperfusion. Hearts were subjected to 30 min of regional ischemia and 90 min of reperfusion 24 h after HS. Risk area was delineated by injection of 10% Evan's blue and infarct size determined using computer morphometry of tetrazolium stained sections. Infarct size (% area at risk) reduced significantly from 49.4 +/- 2.3% (n = 7) in sham to 10.0 +/- 2.5% (p < 0.01) and 9.1 +/- 3.0% in HS and vehicle treated HS groups respectively (p < 0.05) Treatment with chelerythrine prior to HS increased infarct size to 49.4 +/- 2.3% (p < 0.05). Infarct size in chelerythrine-treated non-HS ischemic/reperfused heart was 40.7 +/- 5.4%, which did not differ significantly from vehicle-treated sham group. Western blot analysis demonstrated marked increase in HSP 72 in HS groups (with or without vehicle treatment) and pretreatment with chelerythrine chloride failed to inhibit the expression of HSP 72. The results suggest that HS-induced ischemic tolerance is mediated via PKC pathway and this protection does not appear to be directly related to the expression of HSP 72 in rat heart.

Serum-stimulated cell cycle entry of fibroblasts requires undisturbed phosphorylation and non-phosphorylation interactions of the catalytic subunits of protein kinase CK2

Protein kinase CK2 is a pleiotropic Ser/Thr kinase occurring as alpha2beta2, alpha'2beta2, or alphaalpha'beta2 tetramers. A requirement in serum-stimulated cell cycle entry in both the cytoplasm and the nucleus of human fibroblasts for phosphorylation(s) by CK2 has been concluded from stimulation inhibition by microinjected antibodies against the regulatory subunit (beta). We have now examined this idea more directly by microinjection-mediated perturbation of phosphorylation and non-phosphorylation interactions of the catalytic subunits (alpha and alpha'), and by verifying the supposed matching of the cellular partition of CK2 subunits in the fibroblasts employed. While immunostaining and cell fractionation indicate that the partitions of subunits indeed match each other (with their predominant location in the nucleus in both quiescent and serum-stimulated cells), microinjection of substrate or pseudosubstrate peptides competing for the CK2-mediated phosphorylation in vitro resulted in significant inhibition of serum stimulation when placed into the nucleus but not when placed into the cytoplasm. Also inhibitory were nuclear but not cytoplasmic injections of antibodies against alpha and alpha' that affect neither their kinase activity in vitro nor their complexing to beta. The data indicate that the role played by CK2 in serum-stimulated cell cycle entry is predominantly nuclear and more complex than previously assumed, involving not only phosphorylation but also experimentally separable non-phosphorylation interactions by the catalytic subunits.

CD28-mediated regulation of the c-jun promoter involves the MEF2 transcription factor in Jurkat T cells

Within a few minutes of T-cell activation, transcription of a set of genes including c-fos and c-jun is activated. For maximal induction of c-jun, at least two major signal pathways are required. One can be triggered by T-cell receptor engagement or phorbol esters and the other by anti-CD28 engagement. The c-jun promoter region between -117 and -50 contains binding sites for the transcription factors Spl, CTF, ATF/CREB, and MEF2. In this study, we sought to map the sequences in the c-jun promoter responsible for CD28-mediated induction in activated Jurkat T cell by point mutational analysis. We found that mutation of the c-jun MEF2 site strongly reduces CD28 induction of the promoter in Jurkat T cells and that MEF2D is the major binding molecule to the c-jun MEF2 site in Jurkat T cells. Mutation of the c-jun ATF site also partially reduced CD28 induction of the promoter. In addition, pretreatment with an endolysomotropic agent NH4Cl, an acidic sphingomyelinase inhibitor, completely inhibited the activation of the c-jun promoter by anti-CD28 antibody treatment, whereas pretreatment with wortmannin, a PI3-kinase inhibitor, did not affect the induction of the c-jun promoter. These results suggest that CD28 signaling leading to the c-jun promoter involves acidic sphingomyelinase, but not PI3-kinase, to activate factors binding to the MEF2 and ATF sites.

Novel roles of specific isoforms of protein kinase C in activation of the c-fos serum response element

Protein kinase C (PKC) is a multigene family of enzymes consisting of at least 11 isoforms. It has been implicated in the induction of c-fos and other immediate response genes by various mitogens. The serum response element (SRE) in the c-fos promoter is necessary and sufficient for induction of transcription of c-fos by serum, growth factors, and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). It forms a complex with the ternary complex factor (TCF) and with a dimer of the serum response factor (SRF). TCF is the target of several signal transduction pathways and SRF is the target of the rhoA pathway. In this study we generated dominant-negative and constitutively active mutants of PKC-alpha, PKC-delta, PKC-epsilon, and PKC-zeta to determine the roles of individual isoforms of PKC in activation of the SRE. Transient-transfection assays with NIH 3T3 cells, using an SRE-driven luciferase reporter plasmid, indicated that PKC-alpha and PKC-epsilon, but not PKC-delta or PKC-zeta, mediate SRE activation. TPA-induced activation of the SRE was partially inhibited by dominant negative c-Raf, ERK1, or ERK2, and constitutively active mutants of PKC-alpha and PKC-epsilon activated the transactivation domain of Elk-1. TPA-induced activation of the SRE was also partially inhibited by a dominant-negative MEKK1. Furthermore, TPA treatment of serum-starved NIH 3T3 cells led to phosphorylation of SEK1, and constitutively active mutants of PKC-alpha and PKC-epsilon activated the transactivation domain of c-Jun, a major substrate of JNK. Constitutively active mutants of PKC-alpha and PKC-epsilon could also induce a mutant c-fos promoter which lacks the TCF binding site, and they also induce transactivation activity of the SRF. Furthermore, rhoA-mediated SRE activation was blocked by dominant negative mutants of PKC-alpha or PKC-epsilon. Taken together, these findings indicate that PKC-alpha and PKC-epsilon can enhance the activities of at least three signaling pathways that converge on the SRE: c-Raf-MEK1-ERK-TCF, MEKK1-SEK1-JNK-TCF, and rhoA-SRF. Thus, specific isoforms of PKC may play a role in integrating networks of signal transduction pathways that control gene expression.

Serum response factor mRNA induction in the hypertrophying chicken patagialis muscle

Gene expression in the stretched chicken patagialis (Pat) muscle has not been extensively examined. This study's purpose was to determine the Pat muscle's expression pattern of serum response factor (SRF), skeletal alpha-actin, and MyoD mRNAs after 3 days (onset of stretch), 6 days (end of first week of rapid growth), and 14 days (slowed rate of stretch-induced growth) of stretch. SRF mRNA demonstrated two species (B1 and B2), with B2 being more prevalent in the predominantly fast-twitch Pat muscle, compared with the slow-tonic muscle. Stretch overload increased B1 and B2 SRF mRNA concentrations, and the increase in B1 SRF mRNA concentration was greater at day 6 compared with days 3 or 14. MyoD mRNA concentration was greater in 3-day-stretched Pat muscles, compared with days 6 or 14. Skeletal alpha-actin mRNA concentration was not changed during the study. Gel mobility shift assays demonstrated that SRF binding with serum response element 1 of the skeletal alpha-actin promoter had no altered binding patterns from 6-day-stretched Pat nuclear extracts. It appears that SRF and MyoD mRNAs are induced in the stretch-overloaded Pat muscle but at different time points.

Effect of serum and mechanical stretch on skeletal alpha-actin gene regulation in cultured primary muscle cells

The purpose of this study was to determine whether mechanical stretch or serum availability alters pretranslational regulation of skeletal alpha-actin (SkA) in cultured striated muscle cells. Chicken primary skeletal myoblasts and cardiac myocytes were plated on collagenized Silastic membranes adherent to nylon supports and stretched 8-20% of initial length 96 h postplating. Serum dependence of SkA gene regulation was determined by maintaining differentiated muscle cells in growth/differentiation (G/D; skeletal myotubes, 10% horse serum-2% chick embryo extract; cardiac myocytes, 10% horse serum) or growth-limiting (G-L; 0.5% horse serum) medium. Skeletal myotubes had higher SkA mRNA and SkA promoter activity in G/D than in G-L medium. Cardiac myocyte SkA mRNA was higher in G-L than in G/D medium. Serum response factor (SRF) protein binding to serum response element 1 (SRE1) of SkA promoter increased in skeletal cultures in G/D compared with G-L medium. Western blot analysis demonstrated that increased SRF-SRE1 binding was due, in part, to increased SRF protein. Stretching skeletal myotubes in G-L medium reduced SkA mRNA and repressed SkA promoter activity. The first 100 bp of SkA promoter were sufficient for stretch-induced repression of SkA promoter activity, and an intact transcriptional enhancer factor 1 (TEF-1) binding site was necessary for this response. Serum and stretch appear to repress SkA promoter activity in skeletal myotubes through different DNA binding elements, the SRE1 and TEF-1 sites, respectively. Stretching increased SkA mRNA in cardiac myocytes in G-L medium but did not alter SkA mRNA level in cardiac cells in G/D medium. These results demonstrate that stretch and serum interact differently to alter SkA expression in cultured cardiac and skeletal muscle cells.

p38 Mitogen-activated protein kinase mediates the transcriptional induction of the atrial natriuretic factor gene through a serum response element. A potential role for the transcription factor ATF6

In various cell types certain stresses can stimulate p38 mitogen-activated protein kinase (p38 MAPK), leading to the transcriptional activation of genes that contribute to appropriate compensatory responses. In this report the mechanism of p38-activated transcription was studied in cardiac myocytes where this MAPK is a key regulator of the cell growth and the cardiac-specific gene induction that occurs in response to potentially stressful stimuli. In the cardiac atrial natriuretic factor (ANF) gene, a promoter-proximal serum response element (SRE), which binds serum response factor (SRF), was shown to be critical for ANF induction in primary cardiac myocytes transfected with the selective p38 MAPK activator, MKK6 (Glu). This ANF SRE does not possess sequences typically required for the binding of the Ets-related ternary complex factors (TCFs), such as Elk-1, indicating that p38-mediated induction through this element may take place independently of such TCFs. Although p38 did not phosphorylate SRF in vitro, it efficiently phosphorylated ATF6, a newly discovered SRF-binding protein that is believed to serve as a co-activator of SRF-inducible transcription at SREs. Expression of an ATF6 antisense RNA blocked p38-mediated ANF induction through the ANF SRE. Moreover, when fused to the Gal4 DNA-binding domain, an N-terminal 273-amino acid fragment of ATF6 was sufficient to support trans-activation of Gal4/luciferase expression in response to p38 but not the other stress kinase, N-terminal Jun kinase (JNK); p38-activating cardiac growth promoters also stimulated ATF6 trans-activation. These results indicate that through ATF6, p38 can augment SRE-mediated transcription independently of Ets-related TCFs, representing a novel mechanism of SRF-dependent transcription by MAP kinases.

Interaction of ATF6 and serum response factor

Serum response factor (SRF) is a transcription factor which binds to the serum response element (SRE) in the c-fos promoter. It is required for regulated expression of the c-fos gene as well as other immediate-early genes and some tissue-specific genes. To better understand the regulation of SRF, we used a yeast interaction assay to screen a human HeLa cell cDNA library for SRF-interacting proteins. ATF6, a basic-leucine zipper protein, was isolated by binding to SRF and in particular to its transcriptional activation domain. The binding of ATF6 to SRF was also detected in vitro. An ATF6-VP16 chimera activated expression of an SRE reporter gene in HeLa cells, suggesting that ATF6 can interact with endogenous SRF. More strikingly, an antisense ATF6 construct reduced serum induction of a c-fos reporter gene, suggesting that ATF6 is involved in activation of transcription by SRF. ATF6 was previously partially cloned as a member of the ATF family. The complete cDNA of ATF6 was isolated, and its expression pattern was described.

Serine phosphorylation within a concise amino-terminal domain in nuclear respiratory factor 1 enhances DNA binding

Nuclear respiratory factor 1 (NRF-1) is a transcriptional activator that acts on a diverse set of nuclear genes required for mitochondrial respiratory function in mammalian cells. These genes encode respiratory proteins as well as components of the mitochondrial transcription, replication, and heme biosynthetic machinery. Here, we establish that NRF-1 is a phosphoprotein in vivo. Phosphorylation occurs on serine residues within a concise NH2-terminal domain with the major sites of phosphate incorporation at serines 39, 44, 46, 47, and 52. The in vivo phosphorylation pattern can be approximated in vitro by phosphorylating recombinant NRF-1 with purified casein kinase II. Phosphate incorporation at the sites utilized in vivo results in a marked stimulation of DNA binding activity which is not observed in mutated proteins lacking these sites. Pairwise expression of the wild-type protein with each of a series of truncated derivatives in transfected cells results in the formation of a dimer between wild-type and mutant forms demonstrating that a homodimer is the active binding species. Although NRF-1 can dimerize in the absence of DNA, phosphorylation does not enhance the formation of these dimers. These findings suggest that phosphorylation results in an intrinsic change in the NRF-1 dimer enhancing its ability to bind DNA.

A role for phosphorylation by casein kinase II in modulating Antennapedia activity in Drosophila

We present evidence that the in vivo activity of the HOX protein Antennapedia (ANTP) is modified because of phosphorylation by the serine/threonine kinase casein kinase II (CKII). Using an in vivo assay a form of ANTP that has alanine substitutions at its CKII target sites has, in addition to wild-type ANTP functions, the ability to alter severely thoracic and abdominal development. The novel functions of this protein suggest that this form of ANTP is not suppressed phenotypically by the more posterior homeotic proteins. In contrast, the in vivo activity of a form of ANTP that contains acidic amino acid substitutions at its CKII target sites, thereby mimicking a constitutively phosphorylated ANTP protein, is greatly reduced. This hypoactive form of ANTP, but not the alanine-substituted form, is also reduced in its ability to bind to DNA cooperatively with the homeodomain protein Extradenticle. Our results suggest that phosphorylation of ANTP by CKII is important for preventing inappropriate activities of this homeotic protein during embryogenesis.

GATA-4 activates transcription via two novel domains that are conserved within the GATA-4/5/6 subfamily

GATA-4 is one of the earliest developmental markers of the precardiac mesoderm, heart, and gut and has been shown to activate regulatory elements controlling transcription of genes encoding cardiac-specific proteins. To elucidate the molecular mechanisms underlying the transcriptional activity of the GATA-4 protein, structure-function analyses were performed. These analyses revealed that the C-terminal zinc finger and adjacent basic domain of GATA-4 is bifunctional, modulating both DNA-binding and nuclear localization activities. The N terminus of the protein encodes two independent transcriptional Activation Domains (amino acids 1-74 and amino acids 130-177). Amino acid residues were identified within each domain that are required for transcriptional activation. Finally, we have shown that regions of Xenopus GATA-5 and -6 corresponding to Activation Domains I and II, respectively, function as potent transcriptional activators. The identification and functional characterization of two evolutionarily conserved transcriptional Activation Domains within the GATA-4/5/6 subfamily suggests that each of these domains modulates critical functions in the transcriptional regulatory program(s) encoded by GATA-4, -5, and -6 during vertebrate development. As such these data provide novel insights into the molecular mechanisms that control development of the heart.

Phosphorylation of the MADS-Box transcription factor MEF2C enhances its DNA binding activity

Members of the myocyte enhancer factor-2 (MEF2) family of transcription factors activate muscle gene expression by binding an A/T-rich DNA sequence in the control regions of muscle-specific genes. There are four MEF2 factors in vertebrates, MEF2A-D, which share homology in an amino-terminal MADS domain and an adjacent region known as the MEF2 domain, that together mediate DNA binding and dimerization. We show that serine 59 located between the MADS and MEF2 domains of MEF2C is phosphorylated in vivo and can be phosphorylated in vitro by casein kinase-II (CKII). Phosphorylation of this site enhanced the DNA binding and transcriptional activity of MEF2C by increasing its DNA binding activity 5-fold. In vivo 32P labeling experiments showed that serine 59 is the only phosphorylation site in the MADS and MEF2 domains. Mutagenesis of this serine to an aspartic acid resulted in an increase in DNA binding and transcriptional activity of MEF2C comparable to that observed when this site was phosphorylated, suggesting that phosphorylation augments DNA binding activity by introducing negative charge. This phosphorylation site, which corresponds to a CKII recognition site, is conserved in all known MEF2 factors in organisms ranging from flies to humans, consistent with its importance for the functions of MEF2C.

Regulation of transcription factor activity during cellular aging

Several lines of evidence suggest that the limited replication potential of normal human cells is due to the presence of an intrinsic genetic programme. This "senescence programme" is believed to reduce the incidence of cancer by limiting the growth of most of the transformed cells arising in vivo, although some cells do escape senescence becoming both immortalized and transformed. Here we review the literature that describes the senescence process in terms of gene expression and the regulation of gene expression by a variety of mechanisms affecting transcription factor activity. We focus on regulation of the c-fos gene through posttranslational modification of the serum response factor (SRF) as an example of altered gene expression during cellular aging.

Casein kinase II associates with Egr-1 and acts as a negative modulator of its DNA binding and transcription activities in NIH 3T3 cells

Although the activation domains within early growth response gene protein 1 (Egr-1) have been mapped, little is known of the kinases which phosphorylate Egr-1 and how phosphorylation correlates with the transcriptional activity of Egr-1. In this study we report that casein kinase II (CKII) co-immunoprecipitates with Egr-1 from NIH 3T3 cell lysates. The association of Egr-1 and CKII requires the C terminus of Egr-1 and CKII phosphorylates Egr-1 in vitro. The in vitro phosphorylation of Egr-1 by CKII and that induced by serum in vivo was compared by examining the CNBr-digested fragments of the phosphorylated Egr-1. CKII strongly phosphorylates fragments 7 and 10 which cover part of the activation/nuclear localization and DNA binding domains of Egr-1. CKII also phosphorylates, albeit weakly, fragments 5 and 8 which cover part of activation domain and the entire repression domain of Egr-1, respectively. Strong phosphorylation on fragment 10 as well as fragment 5 was also observed in Egr-1 immunoprecipitated from serum-induced, 32P-labeled cells. CKII phosphorylation of Egr-1 resulted in a decrease of its DNA binding as well as its transcriptional activities.

Multiple enhancer elements mediate induction of c-fos in vascular smooth muscle cells

Previous work from this and other laboratories has demonstrated that the vasoconstrictor peptide angiotensin II results in hypertrophy of rat aortic smooth muscle cells that is associated with an increase in transcription of the early growth response gene c-fos. To explore the molecular mechanism responsible for c-fos induction in rat aortic smooth muscle cells, we used a series of reporter constructs linked to the chloramphenicol acetyl transferase gene in transient transfection experiments in rat aortic smooth muscle cells. Constructs containing both the serum response element and cAMP response element exhibited a 20-fold increase in chloramphenicol acetyl transferase activity in response to either serum or angiotensin II, whereas no increase was seen in vehicle-treated cells. Mutations in either the serum response element or cAMP response element alone, which have been demonstrated to inactivate these elements in other cell types, had no effect on chloramphenicol acetyl transferase inducibility. In contrast, if both elements were mutated, inducibility was almost abolished. Electrophoretic mobility shift assays with oligonucleotides corresponding to either serum response element or cAMP response element demonstrated that these oligonucleotides are capable of forming specific complexes with proteins from rat aortic smooth muscle cell nuclear extracts. One of the proteins binding to the serum response element is the previously described serum response factor, since it was supershifted by a monospecific antibody. These studies demonstrate that c-fox induction in smooth muscle occurs by a dual mechanism that can activate transcription via the serum response element or cAMP response element. These elements appear to act equally and independently, involving a distinct set of transacting factors.

Casein kinase II phosphorylates I kappa B alpha at S-283, S-289, S-293, and T-291 and is required for its degradation

The phosphoprotein I kappa B alpha exists in the cytoplasm of resting cells bound to the ubiquitous transcription factor NF-kappa B (p50-p65). In response to specific cellular stimulation, I kappa B alpha is further phosphorylated and subsequently degraded, allowing NF-kappa B to translocate to the nucleus and transactivate target genes. To identify the kinase(s) involved in I kappa B alpha phosphorylation, we first performed an I kappa B alpha in-gel kinase assay. Two kinase activities of 35 and 42 kDa were identified in cellular extracts from Jurkat T and U937 promonocytic cell lines. Specific inhibitors and immunodepletion studies identified the I kappa B alpha kinase activities as those of the alpha and alpha' subunits of casein kinase II (CKII). Immunoprecipitation studies demonstrated that CKII and I kappa B alpha physically associate in vivo. Moreover, phosphopeptide maps of I kappa B alpha phosphorylated in vitro by cellular extracts and in vivo in resting Jurkat T cells contained the same pattern of phosphopeptides as observed in maps of I kappa B alpha phosphorylated in vitro by purified CKII. Sequence analysis revealed that purified CKII and the kinase activity within cell extracts phosphorylated I kappa B alpha at its C terminus at S-283, S-288, S-293, and T-291. The functional role of CKII was tested in an in vitro I kappa B alpha degradation assay with extracts from uninfected and human immunodeficiency virus (HIV)-infected U937 cells. Immunodepletion of CKII from these extracts abrogated both the basal and enhanced HIV-induced degradation of I kappa B alpha. These studies provide new evidence that the protein kinase CKII physically associates with I kappa B alpha in vivo, induces multisite (serine/threonine) phosphorylation, and is required for the basal and HIV-induced degradation of I kappa B alpha in vitro.

DNA binding specificity of the CCAAT/enhancer-binding protein transcription factor family

CCAAT/enhancer-binding protein (C/EBP) transcription factor family members are related by a high degree of amino acid sequence identity to the basic leucine zipper DNA-binding domain and show distinct but overlapping patterns of tissue- and stage-restricted expression. Although C/EBPalpha and C/EBPbeta have been shown to recognize a consensus sequence derived from regulatory elements in virus and acute-phase response genes, the potential for more subtle differences in the binding preference of the C/EBP family has not been previously addressed. The consensus sequence of C/EBPdelta has not been reported. By using the method of polymerase chain reaction-mediated random site selection to assess the DNA binding specificity of the C/EBP family in an unbiased manner, we demonstrated the sequence preferences for C/EBP family members. With small variations, these C/EBP family members showed similar sequence preferences, and the consensus sequence was identified as RTTGCGYAAY (R = A or G, and Y = C or T). The phosphorylation of C/EBPdelta by casein kinase II increased the binding activity, but did not affect the binding specificity, whereas it was reported that the phosphorylation of C/EBPalpha and C/EBPbeta decreased the binding affinity. The specificity of action of C/EBP family members may be derived from the characteristics of each factor, including the expression profiles, the DNA binding affinities, the cofactors, and so on, in addition to the DNA binding specificities.

Activation of the cardiac alpha-actin promoter depends upon serum response factor, Tinman homologue, Nkx-2.5, and intact serum response elements

A murine cardiac specific homeoboxgene, Nkx-2.5/CSX, a potential Drosophila tinman homologue, may have a fundamental role in cardiac myocyte differentiation. DNA binding targets for Nkx-2.5 were recently shown to represent novel homeodomain binding sequences, some of which resembled serum response elements (SREs); [Chen CY, Schwartz RJ (1995): J Biol Chem 270: 15628-15633]. In this study, Nkx-2.5 facilitated serum response factor (SRF) DNA-binding activity to the multiple SREs found on the cardiac alpha-actin promoter and together stimulated cardiac alpha-actin promoter dependent transcription in 10T1/2 fibroblasts. Analysis of cardiac alpha-actin promoter mutants demonstrated the importance of the multiple upstream SREs and an obligatory requirement for an intact proximal SRE1, for providing high levels of activity in the presence of Nkx-2.5 and SRF coexpression. Transfection assays with mutant SRF species indicated that the C-terminal activation domain and DNA-binding MADS box were necessary for transcriptional activity in the presence of Nkx-2.5. Expression of Nkx-2.5 mutants also demonstrated that the homeodomain alone was insufficient for directing promoter activity in the presence of SRF. The central role of SRF in regulating striated alpha-actin gene activity also was revealed by its embryonic expression restricted primarily to myocardium of the developing heart and the myotomal portion of somites. Thus the function of the cardiac actin promoter SREs appeared to provide binding sites for SRF and Nkx-2.5 to interact and elicit striated muscle specific transcription that was independent of the MyoD family.

Initiation binding repressor, a factor that binds to the transcription initiation site of the histone h5 gene, is a glycosylated member of a family of cell growth regulators [corrected]

Initiation binding repressor [corrected] (IBR) is a chicken erythrocyte factor (apparent molecular mass, 70 to 73 kDa) that binds to the sequences spanning the transcription initiation site of the histone h5 gene, repressing its transcription. A variety of other cells, including transformed erythroid precursors, do not have IBR but a factor referred to as IBF (68 to 70 kDa) that recognizes the same IBR sites. We have cloned the IBR cDNA and studied the relationship of IBR and IBF. IBR is a 503-amino-acid-long acidic protein which is 99.0% identical to the recently reported human NRF-1/alpha-Pal factor and highly related to the invertebrate transcription factors P3A2 and erected wing gene product (EWG). We present evidence that IBR and IBF are most likely identical proteins, differing in their degree of glycosylation. We have analyzed several molecular aspects of IBR/F and shown that the factor associates as stable homodimers and that the dimer is the relevant DNA-binding species. The evolutionarily conserved N-terminal half of IBR/F harbors the DNA-binding/dimerization domain (outer limits, 127 to 283), one or several casein kinase II sites (37 to 67), and a bipartite nuclear localization signal (89 to 106) which appears to be necessary for nuclear targeting. Binding site selection revealed that the alternating RCGCRYGCGY consensus constitutes high-affinity IBR/F binding sites and that the direct-repeat palindrome TGCGCATGCGCA is the optimal site. A survey of genes potentially regulated by this family of factors primarily revealed genes involved in growth-related metabolism.

YY1 facilitates the association of serum response factor with the c-fos serum response element

YY1 is a multifunctional transcription factor that acts as an activator or repressor in different contexts. YY1 binds to multiple sites in the mouse c-fos promoter, inducing at each site a sharp DNA bend. Binding of YY1 to a site situated between the cyclic AMP response element (CRE) and the TATA box bends the DNA in a way that interferes with the interaction of proteins bound at the CRE and TATA elements, resulting in repression of transcription. Here, we show that binding of YY1 to a different site in the c-fos promoter has a different result. Binding of YY1 to the c-fos serum response element (SRE) enhances the binding of serum response factor (SRF). This enhancement requires the binding of YY1 to SRE DNA. YY1 and SRF can cooccupy the SRE at least transiently. In the region of overlapping contact, YY1 contacts DNA in the major groove, while SRF contacts DNA in the minor groove. YY1 also enhances the association of SRF with the SRE in transfected insect cells. Thus, although YY1 induces similar structural changes in DNA at different binding sites, it can have distinct local effects on protein-DNA and protein-protein interactions. These data support a general role for YY1 in the building of highly organized promoter complexes.

The yeast Mcm1 protein is regulated posttranscriptionally by the flux of glycolysis

Mcm1 is a multifunctional protein which plays a role both in the initiation of DNA replication and in the transcriptional regulation of diverse genes in Saccharomyces cerevisiae. The mcm1-1 mutation results in instability of minichromosomes and alpha-specific sterility. Second-site suppressors that restore minichromosome stability but not fertility to the mcm1-1 mutant were isolated. Two of the suppressors, pgm1-1 and pgm1-2, are mutant alleles of PGM1 which encodes a glycolytic enzyme, phosphoglycerate mutase. We show that the pgm1-1 mutation suppresses the minichromosome maintenance (Mcm) defect by increasing the protein activity or level of Mcm1-1 posttranscriptionally. This increase in the intracellular Mcm1-1 activity is sufficient to suppress the Mcm defect but only minimally suppresses the mating defect. Mutations in genes encoding other glycolytic enzymes, such as eno2::URA3, can also suppress the Mcm phenotype of mcm1-1. Suppression by these glycolytic enzyme mutations correlates with a reduced rate of glycolysis rather than a reduced rate of cell growth. This study suggests that in response to changes in their nutritional states yeast cells may attain homeostasis by modulating the activity of global regulators like Mcm1, which plays a central role in the regulation of energy-expensive anabolic processes.

Two distinct signaling pathways activate the latent DNA binding function of p53 in a casein kinase II-independent manner

Post-translational modification of a carboxyl-terminal negative regulatory domain in vitro by either casein kinase II or protein kinase C allosterically activates the latent sequence-specific DNA binding function of p53. Reported here is a biochemical approach to determine the types of signaling pathways and enzymes that are involved in p53 activation in cells. Using a novel chromatographic method, we have been able to separate three distinct biochemical forms of p53 that have been synthesized in vivo; two are in an activated state, and one is in a latent state for sequence-specific DNA binding. The two activated forms of p53 appear to be controlled individually by either a constitutive or a UV-inducible signaling pathway. p53 lacking the COOH-terminal casein kinase II site (p53 delta 4) was characterized biochemically and used to determine the affects of deletion of the casein kinase II motif on the production of the two activated forms of p53 in vivo. As observed with full-length p53, the production of two distinct chromatographic forms of activated p53 delta 4 occurs in vivo, indicating that p53 activation can occur through a casein kinase II-independent pathway and suggesting that two other factors are involved in activation of p53 in vivo.

Activation of p53 sequence-specific DNA binding by short single strands of DNA requires the p53 C-terminus

Upon cellular DNA damage, the p53 tumor suppressor protein transmits a signal to genes that control the cell cycle and apoptosis. One function of p53 that is important for its role in this pathway is its ability to function as a sequence-specific transcriptional activator. We demonstrate here that short single DNA strands can markedly stimulate the ability of human and murine p53 proteins to bind specifically to a p53 response element in supercoiled DNA. We also show that single-stranded DNA does not stimulate binding by a truncated p53 that lacks the C-terminal domain. Finally, we establish that a peptide spanning the p53 C-terminus has the ability in trans to stimulate sequence-specific DNA binding by p53 dramatically. These data taken together suggest a model in which the p53 C-terminus can recognize DNA structures resulting from damage-induced lesions, and this interaction can be propagated to regulate positively p53 sequence-specific DNA binding.

ZEMa, a member of a novel group of MADS box genes, is alternatively spliced in maize endosperm

The identification of a number of cis-elements which direct gene expression in maize endosperm, and the characterization of corresponding DNA binding proteins, point to the interaction of different classes of transcription factors in this tissue. To assess whether MADS box genes are also involved in maize endosperm development, cDNA and genomic MADS box clones have been isolated. The three cDNA clones ZEM1, ZEM2 and ZEM3 were cloned from a maize endosperm cDNA library using a probe based on sequences conserved in plant MADS box genes. Further transcripts were cloned by RT-PCR experiments and designated ZEM4 and ZEM5. Analysis of the corresponding genomic clones led to the identification of the ZEM2 MADS box gene family, three members of which were characterized sharing 97% sequence identity in corresponding domains. 100% sequence identities between cDNA and one of the genomic clones, conserved exon-intron boundaries and the demonstration of in vivo splicing in a maize endosperm transient expression system, show that the transcripts ZEM1-5 are derived by alternative splicing of ZEMa, one ZEM2 member. The ZEMa transcripts are present in almost all maize tissues, but specific differentially spliced forms accumulate preferentially in maturing endosperm and leaf. The function of the ZEMa gene is discussed in the light of similarities in the expression pattern with members of the human MEF2/RSRF gene family.