Induction of progesterone target genes requires activation of Erk and Msk kinases and phosphorylation of histone H3

Hormone-dependent nuclear export of estradiol receptor and DNA synthesis in breast cancer cells

In breast cancer cells, cytoplasmic localization of the estradiol receptor alpha (ERalpha) regulates estradiol-dependent S phase entry. We identified a nuclear export sequence (NES) in ERalpha and show that its export is dependent on both estradiol-mediated phosphatidylinositol-3-kinase (PI3K)/AKT activation and chromosome region maintenance 1 (CRM1). A Tat peptide containing the ERalpha NES disrupts ERalpha-CRM1 interaction and prevents nuclear export of ERalpha- and estradiol-induced DNA synthesis. NES-ERalpha mutants do not exit the nucleus and inhibit estradiol-induced S phase entry; ERalpha-dependent transcription is normal. ERalpha is associated with Forkhead proteins in the nucleus, and estradiol stimulates nuclear exit of both proteins. ERalpha knockdown or ERalpha NES mutations prevent ERalpha and Forkhead nuclear export. A mutant of forkhead in rhabdomyosarcoma (FKHR), which cannot be phosphorylated by estradiol-activated AKT, does not associate with ERalpha and is trapped in the nucleus, blocking S phase entry. In conclusion, estradiol-induced AKT-dependent phosphorylation of FKHR drives its association with ERalpha, thereby triggering complex export from the nucleus necessary for initiation of DNA synthesis and S phase entry.

Estrogen facilitates both phosphatidylinositol 3-kinase/Akt and ERK1/2 mitogen-activated protein kinase membrane signaling required for long-term neuropeptide Y transcriptional regulation in clonal, immortalized neurons

It is established that increases in neuropeptide Y (NPY) expression are associated with hyperphagia and obesity. These effects can be reversed by estrogen, a recognized anorexigen. We found that 17beta-estradiol (E(2)) regulates biphasic NPY gene expression in a clonal, immortalized hypothalamic cell line, N-38, through estrogen receptor (ER) action at the level of the NPY promoter. However, rapid, nongenomic actions of estrogen, linked to the phosphatidylinositol 3-kinase (PI3-K)/Akt and ERK1/2 mitogen-activated protein kinase (MAPK) pathways, may also play a role. We therefore examined the changes in the phosphorylation status of Akt, ERK1/2, and cAMP response element-binding protein (CREB) after treatment with 10 nm E(2) in the N-38 neurons and found activation of these signaling proteins within 5-30 min. We also demonstrated possible cross talk between the estrogen-activated PI3-K/Akt and MAPK/extracellular signal-regulated kinase pathways using pharmacological inhibitors. We find that only ERalpha is involved in the early signaling events using the ERalpha agonist 4,4',4''-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol and the ERbeta agonist 2,3-bis(4-hydroxyphenyl)-propionitrile. Furthermore, we can detect colocalization of ERalpha and caveolin-1, a membrane-associated signaling protein. Remarkably, we find that the membrane-mediated events are critical for the long-term estrogen-mediated repression of NPY gene expression that can be mapped to within -97 bp of the NPY promoter. To link the early signaling events to downstream effectors, we detected induction of c-fos and inactivation of MSK-1 by estrogen and binding of CREB to this minimal promoter region. These observations suggest that rapid estrogen-mediated signaling is mediated by ERalpha, and the signal transduction events potentiate the genomic actions of estrogen on NPY gene expression in the N-38 NPY neurons.

Nuclear and extranuclear pathway inputs in the regulation of global gene expression by estrogen receptors

Whereas estrogens exert their effects by binding to nuclear estrogen receptors (ERs) and directly altering target gene transcription, they can also initiate extranuclear signaling through activation of kinase cascades. We have investigated the impact of estrogen-mediated extranuclear-initiated pathways on global gene expression by using estrogen-dendrimer conjugates (EDCs), which because of their charge and size remain outside the nucleus and can only initiate extranuclear signaling. Genome-wide cDNA microarray analysis of MCF-7 breast cancer cells identified a subset of 17beta-estradiol (E2)-regulated genes ( approximately 25%) as EDC responsive. The EDC and E2-elicited increases in gene expression were due to increases in gene transcription, as observed in nuclear run-on assays and RNA polymerase II recruitment and phosphorylation. Treatment with antiestrogen or ERalpha knockdown using small interfering RNA abolished EDC-mediated gene stimulation, whereas GPR30 knockdown or treatment with a GPR30-selective ligand was without effect, indicating ER as the mediator of these gene regulations. Inhibitors of MAPK kinase and c-Src suppressed both E2 and EDC stimulated gene expression. Of note, in chromatin immunoprecipitation assays, EDC was unable to recruit ERalpha to estrogen-responsive regions of regulated genes, whereas ERalpha recruitment by E2 was very effective. These findings suggest that other transcription factors or kinases that are downstream effectors of EDC-initiated extranuclear signaling cascades are recruited to regulatory regions of EDC-responsive genes in order to elicit gene stimulation. This study thus highlights the importance of inputs from both nuclear and extranuclear ER signaling pathways in regulating patterns of gene expression in breast cancer cells.

Polarization of primary human monocytes by IFN-gamma induces chromatin changes and recruits RNA Pol II to the TNF-alpha promoter

Monocyte polarization by IFN-gamma or IL-4 drives a complex series of cellular responses leading to increased intracellular killing (IFN-gamma) or enhanced healing (IL-4) among other functional responses. We studied the effect of IL-4 and IFN-gamma polarization on histone modifications at the TNF-alpha locus in human primary monocytes. IFN-gamma polarization markedly increased the expression of TNF-alpha, whereas IL-4 treatment decreased the expression. We found that IFN-gamma alone increased histone H4 acetylation at the TNF-alpha promoter. The effect of IFN-gamma on TNF-alpha expression was durable upon cytokine washout and even repolarization with IL-4. Concordantly, IFN-gamma-mediated H4 acetylation was also durable. IFN-gamma recruited activating transcription factor-2 via p38 to the TNF-alpha promoter, but inhibition of p38 had minimal effect on H4 acetylation. In a novel finding, we found that IFN-gamma recruited RNA Pol II to the human TNF-alpha promoter via ERK signaling, but did so without initiating transcription, leading to a poised condition. These studies provide an important perspective on monocyte polarization. Polarization by IFN-gamma has a durable effect on TNF-alpha expression, and histone acetylation may provide a mechanism for persistence of the effect.

Altered subcellular distribution of MSK1 induced by glucocorticoids contributes to NF-kappaB inhibition

Glucocorticoids are widely used anti-inflammatory and immunomodulatory agents, of which the action mechanism is mainly based on interference of hormone-activated glucocorticoid receptor (GR) with the activity of transcription factors, such as nuclear factor-kappaB (NF-kappaB). In addition to the well described interaction-based mutual repression mechanism between the GR and NF-kappaB, additional mechanisms are at play, which help to explain the efficacy of glucocorticoid-mediated gene repression. In this respect, we found that glucocorticoids counteract the recruitment of activated Mitogen- and Stress-activated protein Kinase-1 (MSK1) at inflammatory gene promoters resulting in the inhibition of NF-kappaB p65 transactivation and of concurrent histone H3 phosphorylation. Additionally, we observed that activated GR can trigger redistribution of nuclear MSK1 to the cytoplasm through a CRM1-dependent export mechanism, as a result of an interaction between liganded GR and activated MSK1. These findings unveil a novel aspect within the GR-mediated NF-kappaB-targeting anti-inflammatory mechanism.

The forced swimming-induced behavioural immobility response involves histone H3 phospho-acetylation and c-Fos induction in dentate gyrus granule neurons via activation of the N-methyl-D-aspartate/extracellular signal-regulated kinase/mitogen- and stress-activated kinase signalling pathway

The hippocampus is involved in learning and memory. Previously, we have shown that the acquisition of the behavioural immobility response after a forced swim experience is associated with chromatin modifications and transcriptional induction in dentate gyrus granule neurons. Given that both N-methyl-D-aspartate (NMDA) receptors and the extracellular signal-regulated kinases (ERK) 1/2 signalling pathway are involved in neuroplasticity processes underlying learning and memory, we investigated in rats and mice whether these signalling pathways regulate chromatin modifications and transcriptional events participating in the acquisition of the immobility response. We found that: (i) forced swimming evoked a transient increase in the number of phospho-acetylated histone H3-positive [P(Ser10)-Ac(Lys14)-H3(+)] neurons specifically in the middle and superficial aspects of the dentate gyrus granule cell layer; (ii) antagonism of NMDA receptors and inhibition of ERK1/2 signalling blocked forced swimming-induced histone H3 phospho-acetylation and the acquisition of the behavioural immobility response; (iii) double knockout (DKO) of the histone H3 kinase mitogen- and stress-activated kinases (MSK) 1/2 in mice completely abolished the forced swimming-induced increases in histone H3 phospho-acetylation and c-Fos induction in dentate granule neurons and the behavioural immobility response; (iv) blocking mineralocorticoid receptors, known not to be involved in behavioural immobility in the forced swim test, did not affect forced swimming-evoked histone H3 phospho-acetylation in dentate neurons; and (v) the pharmacological manipulations and gene deletions did not affect behaviour in the initial forced swim test. We conclude that the forced swimming-induced behavioural immobility response requires histone H3 phospho-acetylation and c-Fos induction in distinct dentate granule neurons through recruitment of the NMDA/ERK/MSK 1/2 pathway.

Mitogen-induced recruitment of ERK and MSK to SRE promoter complexes by ternary complex factor Elk-1

Many eukaryotic genes are acutely regulated by extra-cellular signals. The c-fos serum response element (SRE) mediates transcriptional activation in response to mitogens through serum response factor (SRF)-dependent recruitment of Elk-1, a mitogen-activated protein kinase (MAPK)-responsive transcription factor. How subsequent events at SRE promoters stimulate initiation of transcription has yet to be fully resolved. Here we show that extra-cellular signal-regulated kinase (ERK) and mitogen and stress-activated kinase (MSK) are recruited to SRE promoter complexes in vitro and in vivo. Their recruitment in vitro correlates with Elk-1 binding and for ERK the D domain/KIM of Elk-1 is specifically involved. In vivo, recruitment of ERK and MSK is stimulated by mitogens, correlates with histone H3 phosphorylation and is impaired by Elk-1 knockdown. Immunocytochemistry and confocal microscopy reveal that ERK appears to associate to some extent with initiating rather than elongating RNA polymerase II. Taken together, our data add to the body of evidence implying that ERK and related MAPKs may fulfil a generic role at the promoters of acutely regulated genes.

Farnesoid X receptor protects liver cells from apoptosis induced by serum deprivation in vitro and fasting in vivo

The farnesoid X receptor (FXR) is a key metabolic regulator in the liver by maintaining the homeostasis of liver metabolites. Recent findings suggest that FXR may have a much broader function in liver physiology and pathology. In the present work, we identify a novel role of FXR in protecting liver cell from apoptosis induced by nutritional withdrawal including serum deprivation in vitro or starvation in vivo. Two FXR ligands, chenodeoxycholic acid (CDCA) and GW4064, rescued HepG2 cells from serum deprivation-induced apoptosis in a dose-dependent manner. This effect of FXR on apoptotic suppression was compromised when FXR was knocked down by short interfering RNA. Similarly, the effects of both CDCA and GW4064 were abolished after inhibition of the MAPK pathway by a specific inhibitor of MAPK kinase 1/2. Immunoblotting results indicated that FXR activation by CDCA and GW4064 induced ERK1/2 phosphorylation, which was attenuated by serum deprivation. In vivo, FXR(-/-) mice exhibited an exacerbated liver apoptosis and lower levels of phosphorylated-ERK1/2 compared to wild-type mice after starvation. In conclusion, our results suggest a novel role of FXR in modulating liver cell apoptosis.

Fission yeast MAP kinase Sty1 is recruited to stress-induced genes

The stress-induced expression of many fission yeast genes is dependent upon the Sty1 mitogen-activated protein kinase (MAPK) and Atf1 transcription factor. Atf1 is phosphorylated by Sty1 yet this phosphorylation is not required for stress-induced gene expression, suggesting another mechanism exists whereby Sty1 activates transcription. Here we show that Sty1 associates with Atf1-dependent genes and is recruited to both their promoters and coding regions. This occurs in response to various stress conditions coincident with the kinetics of the activation of Sty1. Association with promoters is not a consequence of increased nuclear accumulation of Sty1 nor does it require the phosphorylation of Atf1. However, recruitment is completely abolished in a mutant lacking Sty1 kinase activity. Both Atf1 and its binding partner Pcr1 are required for association of Sty1 with Atf1-dependent promoters, suggesting that this heterodimer must be intact for optimal recruitment of the MAPK. However, many Atf1-dependent genes are still expressed in a pcr1Delta mutant but with significantly delayed kinetics, thus providing an explanation for the relatively mild stress sensitivity displayed by pcr1Delta. Consistent with this delay, Sty1 and Atf1 cannot be detected at these promoters in this condition, suggesting that their association with chromatin is weak or transient in the absence of Pcr1.

Promotion of BRCA1-associated triple-negative breast cancer by ovarian hormones

PURPOSE OF REVIEW

Mammary epithelial proliferation is controlled by the ovarian hormones estrogen and progesterone. Although BRCA1 (breast cancer 1, early onset) is ubiquitously expressed, women with BRCA1 mutations have a propensity to develop tumors in tissues sensitive to ovarian hormone. An understanding of the tissue-specific function of the BRCA1-encoded protein (BRCA1) provides additional insight that may improve cancer risk reduction in BRCA1 mutation carriers.

RECENT FINDINGS

Studies using mouse models have shown that BRCA1 regulates the abundance of progesterone receptor. The half-life of progesterone receptor is extended in cells harboring mutations in BRCA1. Reduced ubiquitination of progesterone receptor contributes to its stabilization and is correlated with increased cell proliferation in response to progesterone. Treatment of mutant mice with antiprogesterone prevents/delays tumor development. In vitro, BRCA1 and its interacting protein BARD1 (BRCA1-associated RING domain) serve as an ubiquitin ligase for the monoubiquitination of estrogen receptor-alpha, which may lead to alterations in estrogen receptor-alpha activity. Furthermore, the ubiquitin ligase activities of BRCA1/BARD1 may be determined by the ubiquitin-conjugating enzyme E2.

SUMMARY

BRCA1 exerts its tissue-specific function through the regulation of progesterone receptor and estrogen receptor-alpha. Interference with progesterone receptor, in addition to estrogen receptor-alpha, may be effective in reducing cancer risk in BRCA1 mutation carriers.

Convergence on chromatin of non-genomic and genomic pathways of hormone signaling

Gene regulation by steroid hormones involves genomic and non-genomic signaling pathways and the relationship between these two pathways is unknown. Genomic actions are often mediated by binding of the ligand-activated hormone receptors to hormone responsive elements (HREs) followed by recruitment of co-regulators, remodeling of chromatin and formation of the transcription initiation complex. The non-genomic effects of steroid hormones involve the rapid and transient activation of several kinase cascades often mediated by a subpopulation of "nuclear" receptors located in the cytoplasmic side of the cell membrane. The progesterone effect on breast cancer cell proliferation involves activation of the Src/Ras/Erk cascade mediated by a specific interaction between two domains of the N-terminal half of PR and the ligand-binding domain of ERalpha. Unexpectedly, selective inhibition of Erk, or its target kinase Msk1, interferes with chromatin remodeling and blocks MMTV transcriptional activation. A complex of activated PR, Erk and Msk1 is recruited to promoter already 5 min after hormone treatment and phosphorylates histone H3 at serine 10, leading to displacement of HP1gamma, as a requisite for recruitment of Src1, chromatin remodeling complexes (hSnf2h and Brg1) and RNA polymerase II. Thus, activation of signaling cascades in the cytoplasm is essential for chromatin remodeling and transcriptional activation of a subset of steroid hormone target genes.

Progesterone induction of the 11beta-hydroxysteroid dehydrogenase type 2 promoter in breast cancer cells involves coordinated recruitment of STAT5A and progesterone receptor to a distal enhancer and polymerase tracking

Steroid hormone receptors regulate gene expression, interacting with target DNA sequences but also activating cytoplasmic signaling pathways. Using the human 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2) gene as a model, we have investigated the contributions of both effects on a human progesterone-responsive promoter in breast cancer cells. Chromatin immunoprecipitation has identified two different mechanisms of hormone-induced progesterone receptor (PR) recruitment to the 11beta-HSD2 promoter: (i) direct PR binding to DNA at the proximal promoter, abrogated when PR contains a mutated DNA binding domain (DBD), and (ii) STAT5A (signal transducer and activator of transcription 5A)-mediated recruitment of PR to an upstream distal region, impaired by AG490, a JAK/STAT pathway inhibitor. The JAK/STAT inhibitor, as well as expression of dominant-negative STAT5A, impairs hormone induction of 11beta-HSD2. On the other hand, the DBD-mutated PR fully supports 11beta-HSD2 expression. These results, along with data from a deletion analysis, indicate that the distal region is crucial for hormone regulation of 11beta-HSD2. We show active RNA polymerase II tracking from the distal region upon PR and STAT5A binding, concomitant with synthesis of noncoding, hormone-dependent RNAs, suggesting that this region works as a hormone-dependent transcriptional enhancer. In conclusion, coordination of PR transcriptional effects and cytoplasmic signaling activation, in particular the JAK/STAT pathway, are critical in regulating progestin-induced endogenous 11beta-HSD2 gene expression in breast cancer cells. This is not unique to this promoter, as AG490 also alters the expression of other progesterone-regulated genes.

Modulation of 14-3-3 interaction with phosphorylated histone H3 by combinatorial modification patterns

Post-translational modifications of histones are determining factors in the global and local regulation of genome activity. Phosphorylation of histone H3 is globally associated with mitotic chromatin compaction but occurs in a much more restricted manner during interphase transcriptional regulation of a limited subset of genes. In the course of gene regulation, serine 10 phosphorylation at histone H3 is targeted to a very small fraction of nucleosomes that is highly susceptible to additional acetylation events. Recently, we and others have identified 14-3-3 as a binding protein that recognizes both phosphorylated serine 10 and phosphorylated serine 28 on histone H3. In vitro, the affinity of 14-3-3 for phosphoserine 10 is weak but becomes significantly increased by additional acetylation of either lysine 9 or lysine 14 on the same histone tail. In contrast, the histone H3S28 site matches elements of 14-3-3 high affinity consensus motifs. This region mediates an initial stronger interaction that is less susceptible to modulation by "auxiliary" modifications. Here we discuss the binding of 14-3-3 proteins to histone H3 in detail and putative biological implications of these interactions.

Regulation of an inducible promoter by an HP1beta-HP1gamma switch

The mammalian heterochromatin protein 1 (HP1) family of proteins was recently shown to be involved in transient repression of inducible promoters. One of these promoters is the HIV1 long terminal repeat, which, during viral latency, recruits a non-processive RNA polymerase II (RNAPII) that synthesizes a short regulatory transcript. Here, we have used this promoter to examine the interplay of HP1alpha, HP1beta and HP1gamma with RNAPII. We find that, in the absence of stimulation, HP1beta is present on the promoter together with the non-processive RNAPII and functions as a negative regulator. On activation, HP1beta bound to methylated H3K9 is rapidly released concurrent with histone H3 phospho-acetylation, and is replaced by HP1gamma. This isoform localizes to the promoter but also inside the coding region, together with the processive RNAPII. Our data show that HP1 recruitment-release is a sequential mechanism that is precisely regulated and highly dependent on transcription.

Microplate-based chromatin immunoprecipitation method, Matrix ChIP: a platform to study signaling of complex genomic events

The chromatin immunoprecipitation (ChIP) assay is a major tool in the study of genomic processes in vivo. This and other methods are revealing that control of gene expression, cell division and DNA repair involves multiple proteins and great number of their modifications. ChIP assay is traditionally done in test tubes limiting the ability to study signaling of the complex genomic events. To increase the throughput and to simplify the assay we have developed a microplate-based ChIP (Matrix ChIP) method, where all steps from immunoprecipitation to DNA purification are done in microplate wells without sample transfers. This platform has several important advantages over the tube-based assay including very simple sample handling, high throughput, improved sensitivity and reproducibility, and potential for automation. 96 ChIP measurements including PCR can be done by one researcher in one day. We illustrate the power of Matrix ChIP by parallel profiling 80 different chromatin and transcription time-course events along an inducible gene including transient recruitment of kinases.

Progesterone receptor rapid signaling mediates serine 345 phosphorylation and tethering to specificity protein 1 transcription factors

Human progesterone receptors (PR) rapidly activate cytosolic signaling pathways, in addition to their classical function as ligand-activated transcription factors. Using ER+/PR-B+ T47D breast cancer cells, we probed the role of progestin-stimulated rapid PR signaling in the transcriptional regulation of target genes involved in breast cancer cell proliferation. Epidermal growth factor receptor (EGFR) was rapidly activated after a 10-min treatment with R5020. Progestin induced EGFR-, c-Src-, and MAPK-dependent phosphorylation of PR-B on the MAPK consensus site, Ser345. Ser345-phosphorylated PR-B receptors strongly associated with specificity protein 1 (Sp1) transcription factors to regulate PR cell cycle (p21) and growth-promoting (EGFR) target genes whose promoters lack canonical progesterone response element sequences. Inhibitors of EGFR, c-Src, or MAPK activities blocked PR tethering to Sp1 and progestin-stimulated S-phase entry. Mutant PR-B receptors defective for c-Src binding (mPro) were not phosphorylated on Ser345 in response to progestin and failed to interact with Sp1. Hormone-induced complexes containing Sp1 and wild-type PR-B, but not S345A or mPro PR-B, were recruited to Sp1 sites within the endogenous p21 promoter. Progestin-induced S-phase entry was attenuated in T47D cells containing wild-type PR-B and treated with EGFR, c-Src, or MAPK kinase inhibitors or in T47D cells stably expressing mPro or mutant DNA-binding domain PR-B. In sum, rapid progestin-activated PR signaling leads to PR Ser345 phosphorylation and tethering to Sp1. These events are critical for progestin-stimulated regulation of Sp1 target genes and breast cancer cell proliferation. Our data demonstrate the therapeutic potential for PR-targeted breast cancer treatment by exploiting multiple nodes along the PR signaling pathway, including PR-B, EGFR, c-Src, MAPK, or Sp1.

14-3-3 proteins recognize a histone code at histone H3 and are required for transcriptional activation

Interphase phosphorylation of S10 at histone H3 is linked to transcriptional activation of a specific subset of mammalian genes like HDAC1. Recently, 14-3-3 proteins have been described as detectors for this phosphorylated histone H3 form. Here, we report that 14-3-3 binding is modulated by combinatorial modifications of histone H3. S10 phosphorylation is necessary for an interaction, but additional H3K9 or H3K14 acetylation increases the affinity of 14-3-3 for histone H3. Histone H3 phosphoacetylation occurs concomitant with K9 methylation in vivo, suggesting that histone phosphorylation and acetylation can synergize to overcome repressive histone methylation. Chromatin immunoprecipitation experiments reveal recruitment of 14-3-3 proteins to the HDAC1 gene in an H3S10ph-dependent manner. Recruitment of 14-3-3 to the promoter is enhanced by additional histone H3 acetylation and correlates with dissociation of the repressive binding module HP1gamma. Finally, siRNA-mediated loss of 14-3-3 proteins abolishes the transcriptional activation of HDAC1. Together our data indicate that 14-3-3 proteins are crucial mediators of histone phosphoacetylation signals.

Phosphorylation of histone H3 at Ser10 facilitates RNA polymerase II release from promoter-proximal pausing in Drosophila

The Drosophila JIL-1 kinase is known to phosphorylate histone H3 at Ser10 (H3S10) during interphase. This modification is associated with transcriptional activation, but its function is not well understood. Here we present evidence suggesting that JIl-1-mediated H3S10 phosphorylation is dependent on chromatin remodeling by the brahma complex and is required during early transcription elongation to release RNA polymerase II (Pol II) from promoter-proximal pausing. JIL-1 localizes to transcriptionally active regions and is required for activation of the E75A ecdysone-responsive and hsp70 heat-shock genes. The heat-shock transcription factor, the promoter-paused form of Pol II (Pol IIo(ser5)), and the pausing factor DSIF (DRB sensitivity-inducing factor) are still present at the hsp70 loci in JIL-1-null mutants, whereas levels of the elongating form of Pol II (Pol IIo(ser2)) and the P-TEFb kinase are dramatically reduced. These observations suggest that phosphorylation of H3S10 takes place after transcription initiation but prior to recruitment of P-TEFb and productive elongation. Western analyses of global levels of both forms of Pol II further suggest that JIL-1 plays a general role in early elongation of a broad range of genes. Taken together, the results introduce H3S10 phosphorylation by JIL-1 as a hallmark of early transcription elongation in Drosophila.

Epigenetic mechanisms in stress-related memory formation

Coping with stressful events is part of everyone's daily life. The organism's response to stress is a complex array of physiological and behavioral changes aimed at the preservation/protection of the organism during the stressful event as well as at stimulating adaptive and mnemonic processes in case the event would re-occur in the future. The hippocampus including its 'gate', the dentate gyrus, is highly involved in these processes. We have been collecting evidence suggesting that the transcriptional activation seen in dentate gyrus neurons, which are involved in the encoding of memories of a psychologically stressful event, requires chromatin remodeling in these neurons driven by the phosphorylation (at Serine10) and acetylation (at Lysine14) of histone H3. These particular epigenetic mechanisms are potentially of special interest for neuronal functioning as they are associated with the induction of hitherto silent genes. The phospho-acetylation of histone H3 is brought about by the concurrent activation of two, possibly converging, signaling pathways, being the glucocorticoid receptor and the NMDA/MAPK/ERK/MSK signaling pathways. Thus, we present a new model about how signaling to the chromatin can shape a specific gene transcriptional response in dentate granule neurons required for the encoding of memory of the stressful event.

Src-dependent signalling pathway regulation by sex-steroid hormones: Therapeutic implications

Sex-steroid hormones trigger association of their receptors with signalling effectors, and activate complex networks. These effectors include Src and p85alpha, the PI3-kinase (PI3K) regulatory subunit. Remarkably, various hormonal effects, such as DNA synthesis of mammary and prostate cancer cells, vasorelaxation and migration of different cell types are evoked by this activation. In addition, there are reports on a limited but increasing number of cells responding to hormones through signalling activation in the absence of receptor-dependent transcriptional activity. Altogether these findings indicate that further study is required on signalling inhibitors to control progression of tumors expressing steroid receptors. In addition, new molecules interfering in recruitment of signalling effectors by steroid receptors and leaving unaffected the receptor transcriptional activity could be employed to reduce cell proliferation. Inhibitors of steroid receptor-dependent signal transduction might emerge as a new category of steroid receptor antagonists.

MAP kinases and the control of nuclear events

The mitogen-activated protein kinases (MAPKs) are a family of serine/threonine kinases that play an essential role in signal transduction by modulating gene transcription in the nucleus in response to changes in the cellular environment. They include the extracellular signal-regulated protein kinases (ERK1 and ERK2); c-Jun N-terminal kinases (JNK1, JNK2, JNK3); p38s (p38alpha, p38beta, p38gamma, p38delta) and ERK5. The molecular events in which MAPKs function can be separated in discrete and yet interrelated steps: activation of the MAPK by their upstream kinases, changes in the subcellular localization of MAPKs, and recognition, binding and phosphorylation of MAPK downstream targets. The resulting pattern of gene expression will ultimately depend on the integration of the combinatorial signals provided by the temporal activation of each group of MAPKs. This review will focus on how the specificity of signal transmission by MAPKs is achieved by scaffolding molecules and by the presence of structural motifs in MAPKs that are dynamically regulated by phosphorylation and protein-protein interactions. We discuss also how MAPKs recognize and phosphorylate their target nuclear proteins, including transcription factors, co-activators and repressors and chromatin-remodeling molecules, thereby affecting an intricate balance of nuclear regulatory molecules that ultimately control gene expression in response to environmental cues.

Proteins kinases: chromatin-associated enzymes?

Protein kinases contribute to the regulation of gene expression by interacting with transcription factors that are recruited to the regulatory regions of genes. Previous studies investigated the role of protein kinases in transcription initiation. Here, we discuss new insights gleaned from recent work showing that kinases can also interact with chromatin throughout the entire transcribed region of target genes (Pokholok et al., 2006; Proft et al., 2006).

Osmotic stress-dependent repression is mediated by histone H3 phosphorylation and chromatin structure

Histone H3 phosphorylation has been linked to various environmental stress responses and specific chromatin structure. The role of H3 phosphorylation in the osmotic stress response was investigated on the mouse mammary tumor virus (MMTV) promoter in different chromatin configurations. Hormone-dependent transcription from the MMTV promoter is repressed by osmotic stress when the promoter is integrated and has a normal chromatin structure. However, when the MMTV promoter is transiently transfected, the chromatin structure is less organized, and hormone induction is not affected by osmotic stress. On the integrated MMTV promoter, phosphorylation of histone H3 serine 10 and 28 increases in response to osmotic stress, but the transient promoter shows no change. Hormone-dependent glucocorticoid receptor binding is reduced on the repressed promoter, and elevated H3 phosphorylation is temporally correlated with maximal MMTV repression Additionally, the protein kinase C inhibitor rottlerin, but not other kinase inhibitors, blocks both histone H3 phosphorylation and osmotic repression of MMTV transcription. Glucocorticoid receptor binding is inversely correlated with H3 phosphorylation, suggesting that displacement of the glucocorticoid receptor from the promoter is due to H3 phosphorylation and is the mechanism for the osmotic repression of hormone-dependent transcription.