The kinase MSK1 is required for induction of c-fos by lysophosphatidic acid in mouse embryonic stem cells

Regulation of c-Fos gene transcription by stimulus-responsive protein kinases

The basic region leucin zipper (bZIP) protein c-Fos constitutes together with other bZIP proteins the AP-1 transcription factor complex. Expression of the c-Fos gene is regulated by numerous extracellular signaling molecules including mitogens, metabolites, and ligands for receptor tyrosine kinases, G protein-coupled receptors, and cytokine receptors. Here, we analyzed the effects of the stimulus-responsive MAP kinases ERK1/2 (extracellular signal-regulated protein kinase), JNK (c-Jun N-terminal protein kinase) and p38 protein kinase on transcription of the c-Fos gene. We used chromatin-integrated c-Fos promoter-luciferase reporter genes containing inactivating point mutations of DNA binding sites for distinct transcription factors. ERK1/2, JNK, and p38 protein kinases were specifically activated following expression of either a mutant of B-Raf, a truncated version of mitogen-activated/extracellular signal responsive kinase kinase kinase-1 (MEKK1), or a mutant of MAP kinase kinase-6 (MKK6), respectively. The results show that the DNA binding sites for serum response factor (SRF) and for the ternary complex factor (TCF) are of major importance for stimulating c-Fos promoter activity by MAP kinases. ERK1/2 and p38-induced stimulation of the c-Fos promoter additionally required the DNA binding site for the transcription factor AP-1. Mutation of the DNA binding site for STAT had no or only a small effect on c-Fos promoter activity. We conclude that MAP kinases do not activate distinct transcription factors involving distinct genetic elements. Rather, these kinases mainly target SRF and TCF proteins, leading to an activation of transcription of the c-Fos gene via the serum response element.

Revisiting the role of lysophosphatidic acid in stem cell biology

Stem cells possess unique biological characteristics such as the ability to self-renew and to undergo multilineage differentiation into specialized cells. Whereas embryonic stem cells (ESC) can differentiate into all cell types of the body, somatic stem cells (SSC) are a population of stem cells located in distinct niches throughout the body that differentiate into the specific cell types of the tissue in which they reside in. SSC function mainly to restore cells as part of normal tissue homeostasis or to replenish cells that are damaged due to injury. Cancer stem-like cells (CSC) are said to be analogous to SSC in this manner where tumor growth and progression as well as metastasis are fueled by a small population of CSC that reside within the corresponding tumor. Moreover, emerging evidence indicates that CSC are inherently resistant to chemo- and radiotherapy that are often the cause of cancer relapse. Hence, major research efforts have been directed at identifying CSC populations in different cancer types and understanding their biology. Many factors are thought to regulate and maintain cell stemness, including bioactive lysophospholipids such as lysophosphatidic acid (LPA). In this review, we discuss some of the newly discovered functions of LPA not only in the regulation of CSC but also normal SSC, the similarities in these regulatory functions, and how these discoveries can pave way to the development of novel therapies in cancer and regenerative medicine.

The Role of Lysophosphatidic Acid in Adult Stem Cells

Stem cells are undifferentiated multipotent precursor cells that are capable both of perpetuating themselves as stem cells (self-renewal) and of undergoing differentiation into one or more specialized types of cells. And these stem cells have been reported to reside within distinct anatomic locations termed “niches”. The long-term goals of stem cell biology range from an understanding of cell-lineage determination and tissue organization to cellular therapeutics for degenerative diseases. Stem cells maintain tissue function throughout an organism’s lifespan by replacing differentiated cells. To perform this function, stem cells provide a unique combination of multilineage developmental potential and the capacity to undergo self-renewing divisions. The loss of self-renewal capacity in stem cells underlies certain degenerative diseases and the aging process. This self-renewal regulation must balance the regenerative needs of tissues that persist throughout life. Recent evidence suggests lysophosphatidic acid (LPA) signaling pathway plays an important role in the regulation of a variety of stem cells. In this review, we summarize the evidence linking between LPA and stem cell regulation. The LPA-induced signaling pathway regulates the proliferation and survival of stem cells and progenitors, and thus are likely to play a role in the maintenance of stem cell population in the body. This lipid mediator regulatory system can be a novel potential therapeutics for stem cell maintenance.

Roles of lysophosphatidic acid and sphingosine-1-phosphate in stem cell biology

Stem cells are unique in their ability to self-renew and differentiate into various cell types. Because of these features, stem cells are key to the formation of organisms and play fundamental roles in tissue regeneration and repair. Mechanisms controlling their fate are thus fundamental to the development and homeostasis of tissues and organs. Lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are bioactive phospholipids that play a wide range of roles in multiple cell types, during developmental and pathophysiological events. Considerable evidence now demonstrates the potent roles of LPA and S1P in the biology of pluripotent and adult stem cells, from maintenance to repair. Here we review their roles for each main category of stem cells and explore how those effects impact development and physiopathology.

Resveratrol stimulates c-Fos gene transcription via activation of ERK1/2 involving multiple genetic elements

The polyphenol resveratrol is found in many plant and fruits and is a constituent of our diet. Resveratrol has been proposed to have chemopreventive and anti-inflammatory activities. On the cellular level, resveratrol activates stimulus-regulated transcription factors. To identify resveratrol-responsive elements within a natural gene promoter, the molecular pathway leading to c-Fos gene expression by resveratrol was dissected. The c-Fos gene encodes a basic region leucine zipper transcription factor and is a prototype of an immediate-early gene that is regulated by a wide range of signaling molecules. We analyzed chromatin-integrated c-Fos promoter-luciferase reporter genes where transcription factor binding sites were destroyed by point mutations or deletion mutagenesis. The results show that mutation of the binding sites for serum response factor (SRF), activator protein-1 (AP-1) and cAMP response element binding protein (CREB) significantly reduced reporter gene transcription following stimulation of the cells with resveratrol. Inactivation of the binding sites for signal transducer and activator of transcription (STAT) or ternary complex factors did not influence resveratrol-regulated c-Fos promoter activity. Thus, the c-Fos promoter contains three resveratrol-responsive elements, the cAMP response element (CRE), and the binding sites for SRF and AP-1. Moreover, we show that the transcriptional activation potential of the c-Fos protein is increased in resveratrol-stimulated cells, indicating that the biological activity of c-Fos is elevated by resveratrol stimulation. Pharmacological and genetic experiments revealed that the protein kinase ERK1/2 is the signal transducer that connects resveratrol treatment with the c-Fos gene.

The p90 ribosomal S6 kinase 2 specifically affects mitotic progression by regulating the basal level, distribution and stability of mitotic spindles

RSK2, also known as RPS6KA3 (ribosomal protein S6 kinase, 90 kDa, polypeptide 3), is a downstream kinase of the mitogen-activated protein kinase (MAPK) pathway, which is important in regulating survival, transcription, growth and proliferation. However, its biological role in mitotic progression is not well understood. In this study, we examined the potential involvement of RSK2 in the regulation of mitotic progression. Interestingly, depletion of RSK2, but not RSK1, caused the accumulation of mitotic cells. Time-lapse analysis revealed that mitotic duration, particularly the duration for metaphase-to-anaphase transition was prolonged in RSK2-depleted cells, suggesting activation of spindle assembly checkpoint (SAC). Indeed, more BubR1 (Bub1-related kinase) was present on metaphase plate kinetochores in RSK2-depleted cells, and depletion of BubR1 abolished the mitotic accumulation caused by RSK2 depletion, confirming BubR1-dependent SAC activation. Along with the shortening of inter-kinetochore distance, these data suggested that weakening of the tension across sister kinetochores by RSK2 depletion led to the activation of SAC. To test this, we analyzed the RSK2 effects on the stability of kinetochore–microtubule interactions, and found that RSK2-depleted cells formed less kinetochore–microtubule fibers. Moreover, RSK2 depletion resulted in the decrease of basal level of microtubule as well as an irregular distribution of mitotic spindles, which might lead to observed several mitotic progression defects such as increase in unaligned chromosomes, defects in chromosome congression and a decrease in pole-to-pole distance in these cells. Taken together, our data reveal that RSK2 affects mitotic progression by regulating the distribution, basal level and the stability of mitotic spindles.

Altered ERK1/2 Signaling in the Brain of Learned Helpless Rats: Relevance in Vulnerability to Developing Stress-Induced Depression

Extracellular signal-regulated kinase 1/2- (ERK1/2-) mediated cellular signaling plays a major role in synaptic and structural plasticity. Although ERK1/2 signaling has been shown to be involved in stress and depression, whether vulnerability to develop depression is associated with abnormalities in ERK1/2 signaling is not clearly known. The present study examined ERK1/2 signaling in frontal cortex and hippocampus of rats that showed vulnerability (learned helplessness, (LH)) or resiliency (non-learned helplessness, (non-LH)) to developing stress-induced depression. In frontal cortex and hippocampus of LH rats, we found that mRNA and protein expressions of ERK1 and ERK2 were significantly reduced, which was associated with their reduced activation and phosphorylation in cytosolic and nuclear fractions, where ERK1 and ERK2 target their substrates. In addition, ERK1/2-mediated catalytic activities and phosphorylation of downstream substrates RSK1 (cytosolic and nuclear) and MSK1 (nuclear) were also lower in the frontal cortex and hippocampus of LH rats without any change in their mRNA or protein expression. None of these changes were evident in non-LH rats. Our study indicates that ERK1/2 signaling is differentially regulated in LH and non-LH rats and suggests that abnormalities in ERK1/2 signaling may be crucial in the vulnerability to developing depression.

G protein-coupled receptors in stem cell maintenance and somatic reprogramming to pluripotent or cancer stem cells

G protein-coupled receptors (GPCRs) are a large class of transmembrane receptors categorized into five distinct families: rhodopsin, secretin, adhesion, glutamate, and frizzled. They bind and regulate 80% of all hormones and account for 20-50% of the pharmaceuticals currently on the market. Hundreds of GPCRs integrate and coordinate the functions of individual cells, mediating signaling between various organs. GPCRs are crucial players in tumor progression, adipogenesis, and inflammation. Several studies have also confirmed their central roles in embryonic development and stem cell maintenance. Recently, GPCRs have emerged as key players in the regulation of cell survival, proliferation, migration, and self-renewal in pluripotent (PSCs) and cancer stem cells (CSCs). Our study and other reports have revealed that the expression of many GPCRs is modulated during the generation of induced PSCs (iPSCs) or CSCs as well as during CSC sphere formation. These GPCRs may have crucial roles in the regulation of selfrenewal and other biological properties of iPSCs and CSCs. This review addresses the current understanding of the role of GPCRs in stem cell maintenance and somatic reprogramming to PSCs or CSCs.

Lysophosphatidic acid-induced IL-8 secretion involves MSK1 and MSK2 mediated activation of CREB1 in human fibroblast-like synoviocytes

Lysophosphatidic acid (LPA) is a pleiotropic lipid mediator that promotes motility, survival, and the synthesis of chemokines/cytokines such as interleukin-8 (IL-8) and interleukin-6 by human fibroblast-like synoviocytes from patients with rheumatoid arthritis (RAFLS). In those cells LPA was reported to induce IL-8 secretion through activation of various signaling pathways including p38 mitogen-activated protein kinase (p38 MAPK), p42/44 MAPK, and Rho kinase. In addition to those pathways we report that mitogen- and stress-activated protein kinases (MSKs) known to be activated downstream of the ERK1/2 and p38 MAPK cascades and CREB are phosphorylated in response to LPA. The silencing of MSKs with small-interfering RNAs and the pharmacological inhibitor of MSKs SB747651A shows a role for both MSK1 and MSK2 in LPA-mediated phosphorylation of CREB at Ser-133 and secretion of IL-8 and MCP-1. Whereas CREB inhibitors have off target effects and increased LPA-mediated IL-8 secretion, the silencing of CREB1 with short hairpin RNA significantly reduced LPA-induced chemokine production in RAFLS. Taken together the data clearly suggest that MSK1 and MSK2 are the major CREB kinases in RAFLS stimulated with LPA and that phosphorylation of CREB1 at Ser-133 downstream of MSKs plays a significant role in chemokine production.

Molecular phylogeny of Koenigia L. (Polygonaceae: Persicarieae): implications for classification, character evolution and biogeography

To examine the phylogenetic relationships of Koenigia sensu lato (Polygonaceae), 43 samples representing all species of Koenigia and closely related taxa (e.g., Aconogonon, Bistorta, and Persicaria) were sequenced for nuclear ITS and four plastid regions (trnL-F, atpB-rbcL, rbcL, and rpl32-trnL((UAG))). Phylogenetic analyses indicate that Koenigia recognized by Hedberg is paraphyletic and that the basal species K. delicatula should be reassigned to a separate new genus. Based on these findings, we further propose that the genus Koenigia sensu lato be circumscribed to include five species. Ancestral state reconstruction showed that the pollen apertures likely evolved in parallel in the Aconogonon-Koenigia-Bistorta clade and Persicaria clade and that tricolpate pollen is most likely to be the ancestral state. Quincuncial aestivation likely evolved during the early evolution of Koenigia and its close relatives. Our findings suggest that the uplift of the Himalayas has played an important role in promoting species diversification of Koenigia. Koenigia islandica expanded its range during Pleistocene glacial cycles by tracking changes in newly available habitats.

GPCRs in stem cell function

Many tissues of the body cannot only repair themselves, but also self-renew, a property mainly due to stem cells and the various mechanisms that regulate their behavior. Stem cell biology is a relatively new field. While advances are slowly being realized, stem cells possess huge potential to ameliorate disease and counteract the aging process, causing its speculation as the next panacea. Amidst public pressure to advance rapidly to clinical trials, there is a need to understand the biology of stem cells and to support basic research programs. Without a proper comprehension of how cells and tissues are maintained during the adult life span, clinical trials are bound to fail. This review will cover the basic biology of stem cells, the various types of stem cells, their potential function, and the advantages and disadvantages to their use in medicine. We will next cover the role of G protein-coupled receptors in the regulation of stem cells and their potential in future clinical applications.

Characterization of the cellular action of the MSK inhibitor SB-747651A

MSK1 (mitogen- and stress-activated kinase 1) and MSK2 are nuclear protein kinases that regulate transcription downstream of the ERK1/2 (extracellular-signal-regulated kinase 1/2) and p38α MAPKs (mitogen-activated protein kinases) via the phosphorylation of CREB (cAMP-response-element-binding protein) and histone H3. Previous studies on the function of MSKs have used two inhibitors, H89 and Ro 31-8220, both of which have multiple off-target effects. In the present study, we report the characterization of the in vitro and cellular properties of an improved MSK1 inhibitor, SB-747651A. In vitro, SB-747651A inhibits MSK1 with an IC50 value of 11 nM. Screening of an in vitro panel of 117 protein kinases revealed that, at 1 μM, SB-747651A inhibited four other kinases, PRK2 (double-stranded-RNA-dependent protein kinase 2), RSK1 (ribosomal S6 kinase 1), p70S6K (S6K is S6 kinase) (p70RSK) and ROCK-II (Rho-associated protein kinase 2), with a similar potency to MSK1. In cells, SB-747651A fully inhibited MSK activity at 5-10 μM. SB-747651A was found to inhibit the production of the anti-inflammatory cytokine IL-10 (interleukin-10) in wild-type, but not MSK1/2-knockout, macrophages following LPS (lipopolysaccharide) stimulation. Both SB-747651A and MSK1/2 knockout resulted in elevated pro-inflammatory cytokine production by macrophages in response to LPS. Comparison of the effects of SB-747651A, both in vitro and in cells, demonstrated that SB-747651A exhibited improved selectivity over H89 and Ro 31-8220 and therefore represents a useful tool to study MSK function in cells.

Role of IGF signaling in catch-up growth and accelerated temporal development in zebrafish embryos in response to oxygen availability

Animals respond to adverse environments by slowing down or arresting growth and development. Upon returning to normal conditions, they often show compensatory acceleration in growth and developmental rate. This phenomenon, known as `catch-up' growth, is widely documented in the animal kingdom. The underlying molecular mechanisms, however, are poorly understood. Using the zebrafish embryo as an experimental model system, we tested the hypothesis that changes in IGF signaling activities play an important role in the accelerated growth and temporal development resulting from re-oxygenation following hypoxia. We show that chronic hypoxia reduced, and re-oxygenation accelerated, embryonic growth and developmental rate. Whereas hypoxia repressed the Igf1 receptor and its downstream Erk1/2 and Akt signaling activities, re-oxygenation restored their activities. Specific inhibition of Igf1 receptor signaling during re-oxygenation by genetic and pharmacological approaches attenuated catch-up growth. Further analysis showed that whereas PI3K-Akt is required in both normal and catch-up growth, Mek1/2-Erk1/2 activation induced by elevated IGF signaling during re-oxygenation is particularly crucial for catch-up growth. These results suggest that the evolutionarily conserved IGF signaling pathway coordinates growth and temporal development in zebrafish embryos in response to oxygen availability.

G-protein coupled receptors in stem cell self-renewal and differentiation

Stem cells have great potential for understanding early development, treating human disease, tissue trauma and early phase drug discovery. The factors that control the regulation of stem cell survival, proliferation, migration and differentiation are still emerging. Some evidence now exists demonstrating the potent effects of various G-protein coupled receptor (GPCR) ligands on the biology of stem cells. This review aims to give an overview of the current knowledge of the regulation of embryonic and somatic stem cell maintenance and differentiation by GPCR ligands.

MiR-148a attenuates paclitaxel resistance of hormone-refractory, drug-resistant prostate cancer PC3 cells by regulating MSK1 expression

MicroRNAs are involved in cancer pathogenesis and act as tumor suppressors or oncogenes. It has been recently reported that miR-148a expression is down-regulated in several types of cancer. The functional roles and target genes of miR-148a in prostate cancer, however, remain unknown. In this report, we showed that miR-148a expression levels were lower in PC3 and DU145 hormone-refractory prostate cancer cells in comparison to PrEC normal human prostate epithelial cells and LNCaP hormone-sensitive prostate cancer cells. Transfection with miR-148a precursor inhibited cell growth, and cell migration and invasion, and increased the sensitivity to anti-cancer drug paclitaxel in PC3 cells. Computer-aided algorithms predicted mitogen- and stress-activated protein kinase, MSK1, as a potential target of miR-148a. Indeed, miR-148a overexpression decreased expression of MSK1. Using luciferase reporter assays, we identified MSK1 as a direct target of miR-148a. Suppression of MSK1 expression by siRNA, however, showed little or no effects on malignant phenotypes of PC3 cells. In PC3PR cells, a paclitaxel-resistant cell line established from PC3 cells, miR-148a inhibited cell growth, and cell migration and invasion, and also attenuated the resistance to paclitaxel. MiR-148a reduced MSK1 expression by directly targeting its 3'-UTR in PC3PR cells. Furthermore, MSK1 knockdown reduced paclitaxel-resistance of PC3PR cells, indicating that miR-148a attenuates paclitaxel-resistance of hormone-refractory, drug-resistant PC3PR cells in part by regulating MSK1 expression. Our findings suggest that miR-148a plays multiple roles as a tumor suppressor and can be a promising therapeutic target for hormone-refractory prostate cancer especially for drug-resistant prostate cancer.

The crystal structure of the active form of the C-terminal kinase domain of mitogen- and stress-activated protein kinase 1

Mitogen- and stress-activated protein kinase 1 (MSK1) is a growth-factor-stimulated serine/threonine kinase that is involved in gene transcription regulation and proinflammatory cytokine stimulation. MSK1 is a dual kinase possessing two nonidentical protein kinase domains in one polypeptide. We present the active conformation of the crystal structures of its C-terminal kinase domain in apo form and in complex with a nonhydrolyzable ATP analogue at 2.0 A and 2.5 A resolutions, respectively. Structural analysis revealed substantial differences in the contacts formed by the C-terminal helix, which is responsible for the inactivity of other autoinhibited kinases. In the C-terminal kinase domain of MSK1, the C-terminal alphaL-helix is located in the surface groove, but forms no hydrogen bonds with the substrate-binding loop or nearby helices, and does not interfere with the protein's autophosphorylation activity. Mutational analysis confirmed that the alphaL-helix is inherently nonautoinhibitory. Overexpression of the single C-terminal kinase domain in JB6 cells resulted in tumor-promoter-induced neoplastic transformation in a manner similar to that induced by the full-length MSK1 protein. The overall results suggest that the C-terminal kinase domain of MSK1 is regulated by a novel alphaL-helix-independent mechanism, suggesting that a diverse mechanism of autoinhibition and activation might be adopted by members of a closely related protein kinase family.

MSK1 regulates the transcription of IL-1ra in response to TLR activation in macrophages

The activity of the pro-inflammatory cytokine IL (interleukin)-1 is closely regulated in vivo via a variety of mechanisms, including both the control of IL-1 production and secretion as well as naturally occurring inhibitors of IL-1 function, such as IL-1ra (IL-1 receptor antagonist). IL-1ra is homologous with IL-1, and is able to bind but not activate the IL-1 receptor. IL-1ra can be produced by a variety of cell types, and its production is stimulated by inflammatory signals. In the present study, we show that in macrophages the TLR (Toll-like receptor)-mediated induction of IL-1ra from both its proximal and distal promoters involves the p38 and ERK1/2 (extracellular-signal-regulated kinase 1/2) MAPK (mitogen-activated protein kinase) cascades. In addition, we show that MSK1 and 2 (mitogen- and stress-activated kinase 1 and 2), kinases activated by either ERK1/2 or p38 in vivo, are required for the induction of both IL-1ra mRNA and protein. MSKs regulate IL-1ra transcription via both IL-10-dependent and -independent mechanisms in cells. Consistent with this, knockout of MSK in mice was found to result in a decrease in IL-1ra production following LPS (lipopolysaccharide) injection. MSKs therefore act as important negative regulators of inflammation following TLR activation.

The versatile role of MSKs in transcriptional regulation

Among the mitogen-activated protein kinase (MAPK) targets, MSKs (mitogen- and stress-activated protein kinases) comprise a particularly interesting protein family. Because MSKs can be activated by both extracellular-signal-regulated kinase and p38 MAPKs, they are activated by many physiological and pathological stimuli. About ten years after their original discovery, they have been recognized as versatile kinases regulating gene transcription at multiple levels. MSKs directly target transcription factors, such as cAMP-response-element-binding protein and nuclear factor-kappaB, thereby enhancing their transcriptional activity. They also induce histone phosphorylation, which is accompanied by chromatin relaxation and facilitated binding of additional regulatory proteins. Here, we review the current knowledge on MSK activation and its molecular targets, focusing on recent insights into the role of MSKs at multiple levels of transcriptional regulation.

The RSK family of kinases: emerging roles in cellular signalling

The 90 kDa ribosomal S6 kinase (RSK) family of proteins is a group of highly conserved Ser/Thr kinases that regulate diverse cellular processes, such as cell growth, cell motility, cell survival and cell proliferation. RSKs are downstream effectors of the Ras-extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) signalling cascade. Significant advances in the field of RSK and ERK/MAPK signalling have occurred in the past few years, including biological insights and the discovery of novel substrates and new RSK regulatory mechanisms. Collectively, these data expand the current models of RSK signalling and highlight potential directions of research in RSK-mediated survival, growth, proliferation and migration.

Lysophosphatidic acid-induced c-fos up-regulation involves cyclic AMP response element-binding protein activated by mitogen- and stress-activated protein kinase-1

Lysophosphatidic acid (LPA) is a lipid growth factor that exerts diverse biological effects through its cognate receptor-mediated signaling cascades. Recently, we reported that LPA stimulates cAMP response element-binding protein (CREB) through mitogen- and stress-activated protein kinase-1 (MSK1). Previously, LPA has been shown to stimulate c-fos mRNA expression in Rat-2 fibroblast cells via a serum response element binding protein (SRF). However, involvement of CREB in LPA-stimulated c-fos gene expression is not elucidated yet. To investigate the CREB-mediated c-fos activation by LPA, various c-fos promoter-reporter constructs containing wild-type and mutated SRE and CRE were tested for their inducibility by LPA in transient transfection assays. LPA-stimulated c-fos promoter activation was markedly decreased when SRE and CRE were mutated. A dominant negative CREB significantly down-regulated the LPA-stimulated c-fos promoter activation. Chromatin immunoprecipitation assay revealed that LPA induced an increased binding of phosphorylated CREB and CREB-binding protein (CBP) to the CRE region of the endogenous c-fos promoter. Immunoblot analyses with various pharmacological inhibitors further showed that LPA induces up-regulation of c-fos mRNA level by activation of ERK, p38 MAPK, and MSK1. Taken together, our results suggest that CREB plays an important role in up-regulation of c-fos mRNA level in LPA-stimulated Rat-2 fibroblast cells.

Mitogen- and stress-activated kinase 1-mediated histone H3 phosphorylation is crucial for cell transformation

Mitogen- and stress-activated kinase 1 (MSK1) belongs to a family of dual protein kinases that are activated by either extracellular signal-regulated kinase or p38 mitogen-activated protein kinases in response to stress or mitogenic extracellular stimuli. The physiologic role of MSK1 in malignant transformation and cancer development is not well understood. Here, we report that MSK1 is involved in 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced or epidermal growth factor (EGF)-induced neoplastic transformation of JB6 Cl41 cells. H89, a potent inhibitor of MSK1, strongly suppressed TPA-induced or EGF-induced cell transformation. When cells overexpressing wild-type MSK1 were treated with TPA or EGF, colony formation increased substantially compared with untreated cells or cells that did not overexpress MSK1. In contrast, MSK1 COOH terminal or NH(2) terminal dead dominant negative mutants dramatically suppressed cell transformation. Introduction of small interfering RNA-MSK1 into JB6 Cl41 cells resulted in suppressed TPA-induced or EGF-induced cell transformation. In addition, cell proliferation was inhibited in MSK1 knockdown cells compared with MSK1 wild-type cells. In wild-type MSK1-overexpressing cells, activator protein (AP-1) activation increased after TPA or EGF stimulation, whereas AP-1 activation decreased in both MSK1 dominant-negative mutants and in MSK1 knockdown cells. Moreover, TPA-induced or EGF-induced phosphorylation of histone H3 at Ser(10) was increased in wild-type cells but the induced phosphorylation was abolished in MSK1 dominant-negative mutant or MSK1 knockdown cells. Thus, MSK1 is required for tumor promoter-induced cell transformation through its phosphorylation of histone H3 at Ser(10) and AP-1 activation.

Neuroplasticity in brain reward circuitry following a history of ethanol dependence

Mitogen-activated and extracellular regulated kinase (MEK) and extracellular signal-regulated protein kinase (ERK) pathways may underlie ethanol-induced neuroplasticity. Here, we used the MEK inhibitor 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene (UO126) to probe the role of MEK/ERK signaling for the cellular response to an acute ethanol challenge in rats with or without a history of ethanol dependence. Ethanol (1.5 g/kg, i.p.) induced expression of the marker genes c-fos and egr-1 in brain regions associated with both rewarding and stressful ethanol actions. Under non-dependent conditions, ethanol-induced c-fos expression was generally not affected by MEK inhibition, with the exception of the medial amygdala (MeA). In contrast, following a history of dependence, a markedly suppressed c-fos response to acute ethanol was found in the medial pre-frontal/orbitofrontal cortex (OFC), nucleus accumbens shell (AcbSh) and paraventricular nucleus (PVN). The suppressed ethanol response in the OFC and AcbSh, key regions involved in ethanol preference and seeking, was restored by pre-treatment with UO126, demonstrating a recruitment of an ERK/MEK-mediated inhibitory regulation in the post-dependent state. Conversely, in brain areas involved in stress responses (MeA and PVN), an MEK/ERK-mediated cellular activation by acute ethanol was lost following a history of dependence. These data reveal region-specific neuroadaptations encompassing the MEK/ERK pathway in ethanol dependence. Recruitment of MEK/ERK-mediated suppression of the ethanol response in the OFC and AcbSh may reflect devaluation of ethanol as a reinforcer, whereas loss of an MEK/ERK-mediated response in the MeA and PVN may reflect tolerance to its aversive actions. These two neuroadaptations could act in concert to facilitate progression into ethanol dependence.

MSK regulate TCR-induced CREB phosphorylation but not immediate early gene transcription

Stimulation of the T cell receptor activates the ERK1/2 and p38 mitogen-activated protein kinase (MAPK) cascades. We demonstrate that TCR stimulation also activates the mitogen- and stress-activated kinases (MSK) downstream of ERK1/2 and p38 in both a T cell line and primary peripheral T cells. MSK1/2-knockout mice were found to have normal numbers of T cells in the thymus, and development of these cells appeared unaffected. Using naive T cells and T lymphoblasts from MSK1/2-knockout mice, it was found that MSK was the kinase responsible for phosphorylation of the transcription factor CREB in response to TCR stimulation. Phosphorylation of CREB by MSK has been linked to the transcription of nur77, nor1 and c-fos downstream of MAPK signalling in various cell types. In T cells, the TCR-dependent transcription of these genes was found to require a MAPK-dependent but MSK-independent signalling pathway. Nevertheless, the number of T cells present in the spleens of MSK1/2-knockout mice and the IL-2-induced proliferation of these cells was reduced compared to wild-type mice. This correlated to a reduction in the TCR-induced up-regulation of the IL-2 receptor CD25 and a requirement for MSK in IL-2-induced CREB phosphorylation.

Stem cell regulation by lysophospholipids

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) regulate a diverse range of mammalian cell processes, largely through engaging multiple G protein-coupled receptors specific for these lysophospholipids. LPA and S1P have been clearly identified to have widespread physiological and pathophysiological actions, controlling events within the reproductive, gastrointestinal, vascular, nervous and immune systems, and also having a prominent role in cancer. Here we review the recent literature showing the additional emerging role for LPA and S1P in the regulation of stem cells and their progenitors. We discuss the role of these lysophospholipids in regulating the proliferation, survival, differentiation and migration of a range of adult and embryonic stem cells and progenitors, and thus are likely to play a substantial role in the maintenance, generation, mobilisation and homing of stem cell and progenitor populations in the body.

Identification of novel phosphorylation sites in MSK1 by precursor ion scanning MS

MSK1 (mitogen- and stress-activated kinase 1) is a dual kinase domain protein that acts downstream of the ERK1/2 (extracellular-signal-regulated kinase 1/2) and p38 MAPK (mitogen-activated protein kinase) signalling pathways in cells. MSK1, and its related isoform MSK2, phosphorylate the transcription factors CREB (cAMP-response-element-binding protein) and ATF1 (activating transcription factor 1), and the chromatin proteins histone H3 and HMGN1 (high-mobility-group nucleosomal-binding protein 1) in response to either mitogenic stimulation or cellular stress. MSK1 activity is tightly regulated in cells, and activation requires the phosphorylation of MSK1 by either ERK1/2 or p38a. This results in activation of the C-terminal kinase domain, which then phosphorylates further sites in MSK1, leading to the activation of the N-terminal kinase domain and phosphorylation of substrates. Here, we use precursor ion scanning MS to identify five previously unknown sites in MSK1: Thr630, Ser647, Ser657, Ser695 and Thr700. One of these sites, Thr700, was found to be a third site in MSK1 phosphorylated by the upstream kinases ERK1/2 and p38a. Mutation of Thr700 resulted in an increased basal activity of MSK1, but this could be further increased by stimulation with PMA or UV-C radiation. Surprisingly, however, mutation of Thr700 resulted in a dramatic loss of Thr581 phosphorylation, a site essential for activity. Mutation of Thr700 and Thr581 to an alanine residue resulted in an inactive kinase, while mutation of both sites to an aspartic acid residue resulted in a kinase with a significant basal activity that could not be further stimulated. Together these results are consistent with a mechanism by which Thr700 phosphorylation relieves the inhibition of MSK1 by a C-terminal autoinhibitory helix and helps induce a conformational shift that protects Thr581 from dephosphorylation.

The bioactive lipid sphingosylphosphorylcholine induces differentiation of mouse embryonic stem cells and human promyelocytic leukaemia cells

Sphingosylphosphorylcholine (SPC) is the major component of high-density lipoproteins (HDL) in blood plasma. The bioactive lipid acts mainly via G protein coupled receptors (GPCRs). Similar to ligands of other GPCRs, SPC has multiple biological roles including the regulation of proliferation, migration, angiogenesis, wound healing and heart rate. Lysophospholipids and their receptors have also been implicated in cell differentiation. A potential role of SPC in stem cell or tumour cell differentiation has been elusive so far. Here we examined the effect of SPC on the differentiation of mouse embryonic stem (ES) cells and of human NB4 promyelocytic leukemia cells, a well established tumour differentiation model. Our data show that mouse embryonic stem cells and NB4 cells express the relevant GPCRs for SPC. We demonstrate both at the level of morphology and of gene expression that SPC induces neuronal and cardiac differentiation of mouse ES cells. Furthermore, SPC induces differentiation of NB4 cells by a mechanism which is critically dependent on the activity of the MEK-ERK cascade. Thus, the bioactive lipid SPC is a novel differentiation inducing agent both for mouse ES cells, but also of certain human tumour cells.

Glutamate induces histone H3 phosphorylation but not acetylation in striatal neurons: role of mitogen- and stress-activated kinase-1

Chromatin remodelling is thought to play a key role in gene regulation that underlies long-term synaptic plasticity and memory formation. The dynamic process of chromatin remodelling requires post-translational modifications of histones, a group of highly basic proteins that are tightly linked to DNA. In the present study, we investigated histone H3 modifications in response to glutamate stimulation leading to c-Fos and c-Jun induction in an in vitro model system of striatal neurons in culture. Intracellular signalling pathways implicated in these modifications were analysed. Histone H3 acetylation was strong in basal conditions and unmodified by glutamate treatment. By contrast, glutamate induced a strong phosphorylation of histone H3 that was inhibited by selective inhibitors of the extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (p38 MAPK) pathways, U0126 and SB203580, respectively. Blocking activation of mitogen- and stress-activated kinase 1 (MSK1), a kinase downstream ERK and p38 MAPK, by pharmacological approach or using striatal cells from MSK1 deficient mice, totally abolished H3 phosphorylation, as well as c-Fos and c-Jun induction. Chromatin immunoprecipitation assays confirmed increased levels of phosphorylated H3 at the c-jun promoter. Altogether, our data highlight the crucial role of MSK1 in the nucleosomal response necessary for gene induction in neuronal cells.

Neuron-specific phosphorylation of mitogen- and stress-activated protein kinase-1 involved in cerebral hypoxic preconditioning of mice

Studies have demonstrated the involvement of mitogen-activated protein kinase (MAPK) cascade pathways in the development of cerebral ischemic/hypoxic preconditioning (I/HPC). However, the role of mitogen- and stress-activated protein kinase 1 (MSK1), an important downstream kinase of MAPK signaling pathways, in cerebral I/HPC is unclear. By using Western blot and immunostaining methods, we applied our unique "autohypoxia"-induced I/HPC mouse model to investigate the effects of repetitive hypoxic exposure (H0-H6, n=6 for each group) on phosphorylation and protein expression levels of MSK1 in the brain of mice. We found that the levels of phosphorylation on threonine 645 (Thr645) and serine 375 (Ser375) of MSK1, but not the protein expression, increased significantly both in hippocampus and in cortex of mice from H1-H6 groups (P<0.05) over that of the normoxic group (H0, n=6). Similarly, enhanced phosphorylations on Thr645 and Ser375 of MSK1 were also observed by immunostaining in both the cortex and the hippocampus of mice following three series of hypoxic exposures (H3). In addition, we found by using double-immunofluorescence labeling that phosphorylated Thr645-MSK1 colocalized with a neuron-specific protein, neurogranin, in both cortex and hippocampus of I/HPC mice (H3). These results suggest that the increased neuron-specific phosphorylation of MSK1 on Thr645 and Ser375, not protein expression, might be involved in the development of cerebral I/HPC in mice.

Modulation of collagen and MMP-1 gene expression in fibroblasts by the immunosuppressive drug rapamycin. A direct role as an antifibrotic agent?

We have examined whether rapamycin, an immunosuppressive drug, may exert part of its antifibrotic activity by directly targeting fibroblast extracellular matrix deposition. Incubation of human lung fibroblast (WI-26) cultures with rapamycin led to dose- and time-dependent reduction in the expression of types I and III collagens, both at the protein and mRNA levels. Rapamycin had no effect on collagen promoter activity but accelerated mRNA decay, indicating post-transcriptional control of collagen gene expression. In contrast, rapamycin significantly enhanced the expression of interstitial collagenase (MMP-1) at the protein and mRNA levels and transcriptionally. We determined that rapamycin efficiently activates AP-1-driven transcription by rapidly inducing c-jun/AP-1 phosphorylation with activation of the c-Jun N-terminal kinase (JNK) cascade, resulting in enhanced binding of AP-1.DNA complex formation and AP-1-dependent gene transactivation. Conversely, the JNK inhibitor SP600125 inhibited rapamycin-induced MMP-1 gene transactivation and AP-1/DNA interactions. A c-jun antisense expression vector efficiently prevented rapamycin-induced MMP-1 gene transcription. Pharmacological inhibition of either ERK or p38 MAPK pathways was without effect on rapamycin-induced MMP-1 gene expression. It thus appears that rapamycin may exert direct antifibrotic activities independent from its immunosuppressive action.

Insulin-like growth factor-1 (IGF-1) induces the activation/phosphorylation of Akt kinase and cAMP response element-binding protein (CREB) by activating different signaling pathways in PC12 cells

Background

Insulin-like growth factor-1 (IGF-1) is a polypeptide growth factor with a variety of functions in both neuronal and non-neuronal cells. IGF-1 plays anti-apoptotic and other functions by activating multiple signaling pathways including Akt kinase, a serine/threonine kinase essential for cell survival. The nuclear transcription factor cAMP response element-binding protein (CREB) may also be involved although relationships between these two proteins in IGF-1 receptor signaling and protection is not clear, especially in neuronal cells.

Results

IGF-1, in a concentration- and time-dependent manner, induces the activation/phosphorylation of Akt and CREB in PC12 cells by activating different signaling pathways. IGF-1 induced a sustained phosphorylation of Akt while only a transient one was seen for CREB. The phosphorylation of Akt is mediated by the PI3 kinase pathway while that of CREB is dependent on the activation of both MAPK kinase and p38 MAPK. Moreover, the stimulation of PKC attenuated the phosphorylation of Akt induced by IGF-1 while enhancing that of CREB. Survival assays with various kinase inhibitors suggested that the activation/phosphorylation of both Akt and CREB contributes to IGF-1 mediated cell survival in PC12 cells.

Conclusion

These data suggest that IGF-1 induced the activation of Akt and CREB using distinct pathways in PC12 cells.

Mitogen- and stress-activated protein kinase 1 is critical for interleukin-1-induced, CREB-mediated, c-fos gene expression in keratinocytes

c-fos, which encodes a transcription factor of the AP-1 family, is a prototypical immediate-early gene induced by a number of proinflammatory cytokines including interleukin-1 (IL-1), the latter being an important regulator of skin homeostasis. Using the human keratinocyte cell line HaCaT as an in vitro model, we dissected the molecular pathways leading to IL-1-induced c-fos gene induction. Phosphorylation of the transcription factor cAMP response element binding protein (CREB) at Ser133 was found to be essential for IL-1-induced c-fos gene induction and was closely paralleled by protein kinase A (PKA) activation. In contrast to other cell types, the cyclooxygenase/prostaglandin pathway, known to activate the cAMP/PKA cascade, plays little, if any, role in c-fos expression downstream of the IL-1 receptor in keratinocytes. Simultaneous activation of several of the mitogen-activated protein kinase (MAPK) cascades occurred in response to IL-1, but each differentially contributed to c-fos induction by IL-1, with the p38/MAPK being the most crucial of all, the extracellular signal-regulated kinase pathway contributing in an additive manner and the Jun N-terminal kinase pathway playing little, if any, role. We also demonstrate that p38-dependent activation of mitogen- and stress-activated kinase 1 (MSK1), a CREB kinase, is a key step for c-fos gene activation by IL-1. Finally, we identify MSK1 as playing a positive role in the control of cell proliferation of both HaCaT keratinocytes and the A431 human epidermoid carcinoma line.

MSKs are required for the transcription of the nuclear orphan receptors Nur77, Nurr1 and Nor1 downstream of MAPK signalling

MSK (mitogen- and stress-activated protein kinase) 1 and MSK2 are kinases activated downstream of either the ERK (extracellular-signal-regulated kinase) 1/2 or p38 MAPK (mitogen-activated protein kinase) pathways in vivo and are required for the phosphorylation of CREB (cAMP response element-binding protein) and histone H3. Here we show that the MSKs are involved in regulating the transcription of the immediate early gene Nur77. Stimulation of mouse embryonic fibroblasts with PMA, EGF (epidermal growth factor), TNF (tumour necrosis factor) or anisomycin resulted in induction of the Nur77 mRNA. The induction of Nur77 by TNF and anisomycin was abolished in MSK1/2 double-knockout cells, whereas induction was significantly reduced in response to PMA or EGF. The MSK responsive elements were mapped to two AP (activator protein)-1-like elements in the Nur77 promoter. The induction of Nur77 was also blocked by A-CREB, suggesting that MSKs control Nur77 transcription by phosphorylating CREB bound to the two AP-1-like elements. Consistent with the decrease in Nur77 mRNA levels in the MSK1/2-knockout cells, it was also found that MSKs were required for the induction of Nur77 protein by PMA and TNF. MSKs were also found to be required for the transcription of two genes related to Nur77, Nurr1 and Nor1, which were also transcribed in a CREB- or ATF1 (activating transcription factor-1)-dependent manner. Downstream of anisomycin signalling, a second ERK-dependent pathway, independent of MSK and CREB, was also required for the transcription of Nurr1 and Nor1.

MSK1 activity is controlled by multiple phosphorylation sites

MSK1 (mitogen- and stress-activated protein kinase) is a kinase activated in cells downstream of both the ERK1/2 (extracellular-signal-regulated kinase) and p38 MAPK (mitogen-activated protein kinase) cascades. In the present study, we show that, in addition to being phosphorylated on Thr-581 and Ser-360 by ERK1/2 or p38, MSK1 can autophosphorylate on at least six sites: Ser-212, Ser-376, Ser-381, Ser-750, Ser-752 and Ser-758. Of these sites, the N-terminal T-loop residue Ser-212 and the 'hydrophobic motif' Ser-376 are phosphorylated by the C-terminal kinase domain of MSK1, and their phosphorylation is essential for the catalytic activity of the N-terminal kinase domain of MSK1 and therefore for the phosphorylation of MSK1 substrates in vitro. Ser-381 is also phosphorylated by the C-terminal kinase domain, and mutation of Ser-381 decreases MSK1 activity, probably through the inhibition of Ser-376 phosphorylation. Ser-750, Ser-752 and Ser-758 are phosphorylated by the N-terminal kinase domain; however, their function is not known. The activation of MSK1 in cells therefore requires the activation of the ERK1/2 or p38 MAPK cascades and does not appear to require additional signalling inputs. This is in contrast with the closely related RSK (p90 ribosomal S6 kinase) proteins, whose activity requires phosphorylation by PDK1 (3-phosphoinositide-dependent protein kinase 1) in addition to phosphorylation by ERK1/2.

Differential localization of MAPK-activated protein kinases RSK1 and MSK1 in mouse brain

RSK1 and MSK1 are closely related members of the MAP kinase-activated kinase family and are direct substrates and effectors of the well-studied mitogen-activated protein kinases. Although extensively characterized at the biochemical level, little is known about the localization of these protein kinases in the brain. We utilized immunohistochemistry to determine the cellular and subcellular localization of RSK1 and MSK1 in the adult mouse brain. RSK1 is expressed at highest levels in cerebellum, especially in granule neurons and within neuropil of the molecular layer. RSK1 is also expressed in microglia throughout the brain. In a focal trauma model, RSK1 immunoreactivity is increased in activated microglia. RSK1 expression is also prominent in many large pyramidal neurons throughout the brain. At the subcellular level, RSK1 is highly concentrated in the golgi apparatus of both neurons and astroglia. In contrast, MSK1 is expressed at highest levels in striatal and olfactory tubercle neurons and to a lesser degree in cerebellar Purkinje cells. MSK1 is also expressed in a subset of astroglia. At the subcellular level, MSK1 is confined to the nucleus of all expressing cell types. The differential cellular and subcellular localizations of RSK1 and MSK1 suggest divergent functional roles in the brain, with RSK1 poised to regulate membrane trafficking or membrane-localized signaling, and MSK1 involved in modification of nuclear histones and transcription factors.

The Ras-MAPK signal transduction pathway, cancer and chromatin remodeling

Stimulation of the Ras-mitogen-activated protein kinase (MAPK) signal transduction pathway results in a multitude of events including expression of the immediate-early genes, c-fos and c-myc. Downstream targets of this stimulated pathway are the mitogen- and stress-activated protein kinases (MSK) 1 and 2, which are histone H3 kinases. In chromatin immunoprecipitation assays, it has been shown that the mitogen-induced phosphorylated H3 is associated with the immediate-early genes and that MSK1/2 activity and H3 phosphorylation have roles in chromatin remodeling and transcription of these genes. In oncogene-transformed fibroblasts in which the Ras-MAPK pathway is constitutively active, histone H1 and H3 phosphorylation is increased and the chromatin of these cells has a more relaxed structure than the parental cells. In this review we explore the deregulation of the Ras-MAPK pathway in cancer, with an emphasis on breast cancer. We discuss the features of MSK1 and 2 and the impact of a constitutively activated Ras-MAPK pathway on chromatin remodeling and gene expression.Key words: Ras, mitogen-activated protein kinase signal transduction pathway, histone H3 phosphorylation, MSK1, breast cancer.

What turns CREB on?

The transactivation domain of the cAMP response element-binding protein (CREB) consists of two major domains. The glutamine-rich Q2 domain, which interacts with the general transcription factor TAFII130/135, is sufficient for the recruitment of a functional RNA polymerase II complex and allows basal transcriptional activity. The kinase-inducible domain, however, mediates signal-induced activation of CREB-mediated transcription. It is generally believed that recruitment of the coactivators CREB-binding protein (CBP) and p300 after signal-induced phosphorylation of this domain at serine-133 strongly enhances CREB-dependent transcription. Transcriptional activity of CREB can also be potentiated by phosphoserine-133-independent mechanisms, and not all stimuli that provoke phosphorylation of serine-133 stimulate CREB-dependent transcription. This review presents an overview of the diversity of stimuli that induce CREB phosphorylation at Ser-133, focuses on phosphoserine-133-dependent and -independent mechanisms that affect CREB-mediated transcription, and discusses different models that may explain the discrepancy between CREB Ser-133 phosphorylation and activation of CREB-mediated transcription.

Mitogen- and stress-activated protein kinase 1 mediates cAMP response element-binding protein phosphorylation and activation by neurotrophins

Activation of the transcription factor cAMP response element-binding protein (CREB) by neurotrophins is believed to regulate the survival, differentiation, and maturation of neurons in the CNS and PNS. Although phosphorylation of Ser133 is critical for the expression of CREB-regulated genes, the identity of neurotrophin-regulated Ser133 kinases has remained controversial. We show here that neurotrophin-induced CREB phosphorylation in CNS neurons depends exclusively on the extracellular signal-regulated kinase 1/2-activated kinase mitogen- and stress-activated protein kinase 1 (MSK1). Small interfering RNA directed against ribosomal S6 kinase 1 (RSK1) and RSK2 reduced phosphorylation of a RSK substrate but did not effect CREB-dependent transcription. However, expression of a selective inhibitory MSK1 mutant markedly attenuated BDNF-stimulated CREB phosphorylation and CREB-mediated transcription. Moreover, the ability of neurotrophins to stimulate CREB phosphorylation was abolished in CNS neurons from MSK1 knock-out mice. Consistent with a role for MSK1 in Ser133 phosphorylation, neurotrophin-induced expression of CREB-regulated genes was attenuated in MSK-deficient neurons. These results indicate that MSK1 is the major neurotrophin-activated Ser133 kinase in CNS neurons.

ERK and p38 MAPK-activated protein kinases: a family of protein kinases with diverse biological functions

Conserved signaling pathways that activate the mitogen-activated protein kinases (MAPKs) are involved in relaying extracellular stimulations to intracellular responses. The MAPKs coordinately regulate cell proliferation, differentiation, motility, and survival, which are functions also known to be mediated by members of a growing family of MAPK-activated protein kinases (MKs; formerly known as MAPKAP kinases). The MKs are related serine/threonine kinases that respond to mitogenic and stress stimuli through proline-directed phosphorylation and activation of the kinase domain by extracellular signal-regulated kinases 1 and 2 and p38 MAPKs. There are currently 11 vertebrate MKs in five subfamilies based on primary sequence homology: the ribosomal S6 kinases, the mitogen- and stress-activated kinases, the MAPK-interacting kinases, MAPK-activated protein kinases 2 and 3, and MK5. In the last 5 years, several MK substrates have been identified, which has helped tremendously to identify the biological role of the members of this family. Together with data from the study of MK-knockout mice, the identities of the MK substrates indicate that they play important roles in diverse biological processes, including mRNA translation, cell proliferation and survival, and the nuclear genomic response to mitogens and cellular stresses. In this article, we review the existing data on the MKs and discuss their physiological functions based on recent discoveries.

Generation of single and double knockdowns in polarized epithelial cells by retrovirus-mediated RNA interference

RNA interference (RNAi) is a ubiquitous mechanism of eukaryotic gene regulation that can be exploited for specific gene silencing. Retroviruses have been successfully used for stable expression of short hairpin RNAs in mammalian cells, leading to persistent inhibition of gene expression by RNAi. Here, we apply retrovirus-mediated RNAi to epithelial Madin-Darby canine kidney cells, whose properties limit the applicability of other RNAi methods. We demonstrate efficient suppression of a set of 13 target genes by retroviral coexpression of short hairpin RNAs and a selectable marker. We characterize the resulting knockdown cell populations with regard to composition and stability, and examine the usefulness of proposed guidelines for choosing RNAi target sequences. Finally, we show that this system can be used to simultaneously target two genes, giving rise to double knockdowns. Thus, retrovirus-mediated RNAi is a convenient method for gene silencing in Madin-Darby canine kidney cells, and is likely to be applicable to virtually any mammalian cell.