Cloning and characterization of Xenopus Rsk2, the predominant p90 Rsk isozyme in oocytes and eggs

Revisiting the multisite phosphorylation that produces the M-phase supershift of key mitotic regulators

The term M-phase supershift denotes the phosphorylation-dependent substantial increase in the apparent molecular weight of numerous proteins of varied biological functions during M-phase induction. Although the M-phase supershift of multiple key mitotic regulators has been attributed to the multisite phosphorylation catalyzed by the Cdk1/cyclin B/Cks complex, this view is challenged by multiple lines of paradoxical observations. To solve this problem, we reconstituted the M-phase supershift of Xenopus Cdc25C, Myt1, Wee1A, APC3 and Greatwall in Xenopus egg extracts and characterized the supershift-producing phosphorylations. Our results demonstrate that their M-phase supershifts are each due to simultaneous phosphorylation of a considerable portion of S/T/Y residues in a long intrinsically disordered region that is enriched in both S/T residues and S/TP motifs. Although the major mitotic kinases in Xenopus egg extracts, Cdk1, MAPK, Plx1 and RSK2, are able to phosphorylate the five mitotic regulators, they are neither sufficient nor required to produce the M-phase supershift. Accordingly, inhibition of the four major mitotic kinase activities in Xenopus oocytes did not inhibit the M-phase supershift in okadaic acid-induced oocyte maturation. These findings indicate that the M-phase supershift is produced by a previously unrecognized category of mitotic phosphorylation that likely plays important roles in M-phase induction.

Hydrogen Sulfide Impairs Meiosis Resumption in Xenopus laevis Oocytes

The role of hydrogen sulfide (H₂S) is addressed in Xenopus laevis oocytes. Three enzymes involved in H₂S metabolism, cystathionine β-synthase, cystathionine γ-lyase, and 3-mercaptopyruvate sulfurtransferase, were detected in prophase I and metaphase II-arrested oocytes and drove an acceleration of oocyte meiosis resumption when inhibited. Moreover, meiosis resumption is associated with a significant decrease in endogenous H₂S. On another hand, a dose-dependent inhibition was obtained using the H₂S donor, NaHS (1 and 5 mM). NaHS impaired translation. NaHS did not induce the dissociation of the components of the M-phase promoting factor (MPF), cyclin B and Cdk1, nor directly impacted the MPF activity. However, the M-phase entry induced by microinjection of metaphase II MPF-containing cytoplasm was diminished, suggesting upstream components of the MPF auto-amplification loop were sensitive to H₂S. Superoxide dismutase and catalase hindered the effects of NaHS, and this sensitivity was partially dependent on the production of reactive oxygen species (ROS). In contrast to other species, no apoptosis was promoted. These results suggest a contribution of H₂S signaling in the timing of amphibian oocytes meiosis resumption.

A Rising Cancer Prevention Target of RSK2 in Human Skin Cancer

RSK2 is a p90 ribosomal S6 kinase family (p90(RSK)) member regulating cell proliferation and transformation induced by tumor promoters such as epithelial growth factor (EGF) and 12-O-tetradecanoylphorbol-13-acetate. This family of p90(RSK) has classified as a serine/threonine kinase that respond to many growth factors, peptide hormones, neurotransmitters, and environmental stresses such as ultraviolet (UV) light. Our recent study demonstrates that RSK2 plays a key role in human skin cancer development. Activation of RSK2 by EGF and UV through extracellular-activated protein kinases signaling pathway induces cell cycle progression, cell proliferation, and anchorage-independent cell transformation. Moreover, knockdown of RSK2 by si-RNA or sh-RNA abrogates cell proliferation and cell transformation of non-malignant human skin keratinocyte, and colony growth of malignant melanoma (MM) cells in soft agar. Importantly, activated and total RSK2 protein levels are highly detected in human skin cancer tissues including squamous cell carcinoma, basal-cell carcinoma, and MM. Kaempferol and eriodictyol are natural substances to inhibit kinase activity of the RSK2 N-terminal kinase domain, which is a critical kinase domain to transduce their activation signals to the substrates by phosphorylation. In this review, we discuss the role of RSK2 in skin cancer, particularly in activation of signaling pathways and potent natural substances to target RSK2 as chemopreventive and therapeutic agents.

RSK promotes G2/M transition through activating phosphorylation of Cdc25A and Cdc25B

Activation of the mitogen-activated protein kinase (MAPK) cascade in mammalian cell lines positively regulates the G2/M transition. The molecular mechanism underlying this biological phenomenon remains poorly understood. Ribosomal S6 kinase (RSK) is a key downstream element of the MAPK cascade. Our previous studies established roles of RSK2 in Cdc25C activation during progesterone-induced meiotic maturation of Xenopus oocytes. In this study we demonstrate that both recombinant RSK and endogenous RSK in Xenopus egg extracts phosphorylate all three isoforms of human Cdc25 at a conserved motif near the catalytic domain. In human HEK293 and PC-3mm2 cell lines, RSK preferentially phosphorylates Cdc25A and Cdc25B in mitotic cells. Phosphorylation of the RSK sites in these Cdc25 isoforms increases their M-phase-inducing activities. Inhibition of RSK-mediated phosphorylation of Cdc25 inhibits G2/M transition. Moreover, RSK is likely to be more active in mitotic cells than in interphase cells, as evidenced by the phosphorylation status of T359/S363 in RSK. Together, these findings indicate that RSK promotes G2/M transition in mammalian cells through activating phosphorylation of Cdc25A and Cdc25B.

Oncogenic Ras suppresses Cdk1 in a complex manner during the incubation of activated Xenopus egg extracts

The activity of Cdk1 is the driving force for entry into M-phase during the cell cycle. Activation of Cdk1 requires synthesis and accumulation of cyclin B, binding of cyclin B to Cdk1, and removal of the inhibitory tyr-15-Cdk1 phosphorylation. It was previously shown that oncogenic Ras suppresses Cdk1 activation during the incubation of activated Xenopus egg extracts. However, how oncogenic Ras suppresses Cdk1 remained unclear. Using the histone H1 kinase assay to follow Cdk1 activity and Western blot analysis to assess levels of both cyclin B2 and phosphorylated-tyr-15-Cdk1, how oncogenic Ras suppresses Cdk1 is studied. The results indicate that oncogenic Ras suppresses Cdk1 via induction of persistent phosphorylation of tyr-15-Cdk1. Interestingly, the results reveal that, compared with cyclin B2 in control activated egg extracts, which increased, peaked and then declined during the incubation, oncogenic Ras induced continuous accumulation of cyclin B2. The results also indicate that oncogenic Ras induces continuous accumulation of cyclin B2 primarily through stabilization of cyclin B2, which is mediated by constitutive activation of the Raf-Mek-Erk-p90(rsk) pathway. Taken together, these results indicate that oncogenic Ras suppresses Cdk1 in a complex manner: It induces continuous accumulation of cyclin B2, but also causes persistent inhibitory phosphorylation of tyr-15-Cdk1.

Regulation and function of the RSK family of protein kinases

The RSK (90 kDa ribosomal S6 kinase) family comprises a group of highly related serine/threonine kinases that regulate diverse cellular processes, including cell growth, proliferation, survival and motility. This family includes four vertebrate isoforms (RSK1, RSK2, RSK3 and RSK4), and single family member orthologues are also present in Drosophila and Caenorhabditis elegans. The RSK isoforms are downstream effectors of the Ras/ERK (extracellular-signal-regulated kinase) signalling pathway. Significant advances in the field of RSK signalling have occurred in the past few years, including several new functions ascribed to the RSK isoforms, the discovery of novel protein substrates and the implication of different RSK isoforms in cancer. Collectively, these new findings increase the diversity of biological functions regulated by RSK, and highlight potential new directions of research. In the present paper, we review the structure, expression and activation mechanisms of the RSK isoforms, and discuss their physiological roles on the basis of established substrates and recent discoveries.

Possible involvement of mitogen- and stress-activated protein kinase 1, MSK1, in metaphase-II arrest through phosphorylation of EMI2 in mouse oocytes

Ovulated oocytes are arrested at the metaphase of second meiotic division. The metaphase-II arrest in Xenopus oocytes is regulated by RSKs located downstream of the Mos-MAPK pathway. In mice, other kinase(s) besides RSKs may be responsible for the metaphase-II arrest, because RSK1/RSK2/RSK3-triple knockout mice exhibit no obvious phenotype. Here, we show the subcellular localization and possible role of mitogen- and stress-activated kinase 1, MSK1 known as another downstream kinase of the Mos-MAPK pathway, in the mouse oocytes. Immunostaining analysis indicated that MSK1 is present in the germinal vesicle (GV) and cytoplasm of oocytes at the GV and metaphase-II stages, respectively. An active, phosphorylated form of MSK1 was predominantly localized to the metaphase-II spindle. The inhibition of the MSK1 activity failed to maintain the sister chromatid alignment within the metaphase-II plate. Importantly, MSK1 exhibited the ability to phosphorylate four Ser/Thr residues of meiotic cell-cycle regulator EMI2. The phosphorylation was required for up-regulation of the EMI2 activity in the oocytes. These results suggest that mouse MSK1 may play a key role in the metaphase-II arrest through phosphorylation of EMI2.

Activation and function of the MAPKs and their substrates, the MAPK-activated protein kinases

The mitogen-activated protein kinases (MAPKs) regulate diverse cellular programs by relaying extracellular signals to intracellular responses. In mammals, there are more than a dozen MAPK enzymes that coordinately regulate cell proliferation, differentiation, motility, and survival. The best known are the conventional MAPKs, which include the extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun amino-terminal kinases 1 to 3 (JNK1 to -3), p38 (α, β, γ, and δ), and ERK5 families. There are additional, atypical MAPK enzymes, including ERK3/4, ERK7/8, and Nemo-like kinase (NLK), which have distinct regulation and functions. Together, the MAPKs regulate a large number of substrates, including members of a family of protein Ser/Thr kinases termed MAPK-activated protein kinases (MAPKAPKs). The MAPKAPKs are related enzymes that respond to extracellular stimulation through direct MAPK-dependent activation loop phosphorylation and kinase activation. There are five MAPKAPK subfamilies: the p90 ribosomal S6 kinase (RSK), the mitogen- and stress-activated kinase (MSK), the MAPK-interacting kinase (MNK), the MAPK-activated protein kinase 2/3 (MK2/3), and MK5 (also known as p38-regulated/activated protein kinase [PRAK]). These enzymes have diverse biological functions, including regulation of nucleosome and gene expression, mRNA stability and translation, and cell proliferation and survival. Here we review the mechanisms of MAPKAPK activation by the different MAPKs and discuss their physiological roles based on established substrates and recent discoveries.

Activation and Function of the MAPKs and Their Substrates, the MAPK-Activated Protein Kinases

The mitogen-activated protein kinases (MAPKs) regulate diverse cellular programs by relaying extracellular signals to intracellular responses. In mammals, there are more than a dozen MAPK enzymes that coordinately regulate cell proliferation, differentiation, motility, and survival. The best known are the conventional MAPKs, which include the extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun amino-terminal kinases 1 to 3 (JNK1 to -3), p38 (α, β, γ, and δ), and ERK5 families. There are additional, atypical MAPK enzymes, including ERK3/4, ERK7/8, and Nemo-like kinase (NLK), which have distinct regulation and functions. Together, the MAPKs regulate a large number of substrates, including members of a family of protein Ser/Thr kinases termed MAPK-activated protein kinases (MAPKAPKs). The MAPKAPKs are related enzymes that respond to extracellular stimulation through direct MAPK-dependent activation loop phosphorylation and kinase activation. There are five MAPKAPK subfamilies: the p90 ribosomal S6 kinase (RSK), the mitogen- and stress-activated kinase (MSK), the MAPK-interacting kinase (MNK), the MAPK-activated protein kinase 2/3 (MK2/3), and MK5 (also known as p38-regulated/activated protein kinase [PRAK]). These enzymes have diverse biological functions, including regulation of nucleosome and gene expression, mRNA stability and translation, and cell proliferation and survival. Here we review the mechanisms of MAPKAPK activation by the different MAPKs and discuss their physiological roles based on established substrates and recent discoveries.

Targeting the purinome

Purines are critical cofactors in the enzymatic reactions that create and maintain living organisms. In humans, there are approximately 3,266 proteins that utilize purine cofactors and these proteins constitute the so-called purinome. The human purinome encompasses a wide-ranging functional repertoire and many of these proteins are attractive drug targets. For example, it is estimated that 30% of modern drug discovery projects target protein kinases and that modulators of small G-proteins comprise more than 50% of currently marketed drugs. Given the importance of purine-binding proteins to drug discovery, the following review will discuss the forces that mediate protein:purine recognition, the factors that determine druggability of a protein target, and the process of structure-based drug design. A review of purine recognition in representatives of the various purine-binding protein families, as well as the challenges faced in targeting members of the purinome in drug discovery campaigns will also be given.

The hormonal herbicide, 2,4-dichlorophenoxyacetic acid, inhibits Xenopus oocyte maturation by targeting translational and post-translational mechanisms

The widely used hormonal herbicide, 2,4-dichlorophenoxyacetic acid, blocks meiotic maturation in vitro and is thus a potential environmental endocrine disruptor with early reproductive effects. To test whether maturation inhibition was dependent on protein kinase A, an endogenous maturation inhibitor, oocytes were microinjected with PKI, a specific PKA inhibitor, and exposed to 2,4-D. Oocytes failed to mature, suggesting that 2,4-D is not dependent on PKA activity and likely acts on a downstream target, such as Mos. De novo synthesis of Mos, which is triggered by mRNA poly(A) elongation, was examined. Oocytes were microinjected with radiolabelled in vitro transcripts of Mos RNA and exposed to progesterone and 2,4-D. RNA analysis showed progesterone-induced polyadenylation as expected but none with 2,4-D. 2,4-D-activated MAPK was determined to be cytoplasmic in localization studies but poorly induced Rsk2 phosphorylation and activation. In addition to inhibition of the G2/M transition, 2,4-D caused abrupt reduction of H1 kinase activity in MII phase oocytes. Attempts to rescue maturation in oocytes transiently exposed to 2,4-D failed, suggesting that 2,4-D induces irreversible dysfunction of the meiotic signaling mechanism.

Mammalian egg activation: from Ca2+ spiking to cell cycle progression

Mammalian eggs arrest at metaphase of the second meiotic division (MetII). Sperm break this arrest by inducing a series of Ca2+spikes that last for several hours. During this time cell cycle resumption is induced, sister chromatids undergo anaphase and the second polar body is extruded. This is followed by decondensation of the chromatin and the formation of pronuclei. Ca2+spiking is both the necessary and solely sufficient sperm signal to induce full egg activation. How MetII arrest is established, how the Ca2+spiking is induced and how the signal is transduced into cell cycle resumption are the topics of this review. Although the roles of most components of the signal transduction pathway remain to be fully investigated, here I present a model in which a sperm-specific phospholipase C (PLCζ) generates Ca2+spikes to activate calmodulin-dependent protein kinase II and so switch on the Anaphase-Promoting Complex/Cyclosome (APC/C). APC/C activation leads to securin and cyclin B1 degradation and in so doing allows sister chromatids to be segregated and to decondense.

Features of Selective Kinase Inhibitors

Small-molecule inhibitors of protein and lipid kinases have emerged as indispensable tools for studying signal transduction. Despite the widespread use of these reagents, there is little consensus about the biochemical criteria that define their potency and selectivity in cells. We discuss some of the features that determine the cellular activity of kinase inhibitors and propose a framework for interpreting inhibitor selectivity.

Kinetic analysis of RSK2 and Elk-1 interaction on the serum response element and implications for cellular engineering

Immediate early gene activation upon mitogenic activation occurs through the serum response element (SRE), which makes the delineation of the upstream pathways a powerful means to engineer cellular responses. The malfunctioning of this system leads to a variety of disorders, ranging from neurological disorders such as Coffin-Lowry syndrome (RSK2 mutations) to cancer (c-fos mutations). We therefore investigated the SRE activation mechanism in a typical mammalian cell. Mitogenic signaling uses the mitogen-activated protein kinase (MAPK) module through increased binding of the ternary complex factor (TCF), such as Elk-1, to the promoter DNA (the SRE element) and subsequent transcriptional activation, as well as through activation of a histone kinase, such as the MAPK-activated protein kinase (MAPKAP-K) ribosomal S6 kinase (RSK2). This computational model uses the biochemical simulation environment GEPASI 3.30 to investigate three major models of interaction for Elk-1 and RSK2, and to study the effect of histone acetyl transferase (HAT) recruitment in each of these models on the local chromatin modifications in the presence and absence of MAPK activation. We show that the quickest response on the chromatin can be achieved in the presence of a preformed complex of RSK2, Elk-1 and HAT, with HAT being activated upon dissociation from the complex upon activation of the MAPK cascade. This study presents critical components in the pathway that can be targeted for engineering of specific inhibitors or activators of the system.

p90Rsk is not involved in cytostatic factor arrest in mouse oocytes

Vertebrate oocytes arrest in metaphase of the second meiotic division (MII), where they maintain a high cdc2/cyclin B activity and a stable, bipolar spindle because of cytostatic factor (CSF) activity. The Mos-MAPK pathway is essential for establishing CSF. Indeed, oocytes from the mos-/- strain do not arrest in MII and activate without fertilization, as do Xenopus laevis oocytes injected with morpholino oligonucleotides directed against Mos. In Xenopus oocytes, p90Rsk (ribosomal S6 kinase), a MAPK substrate, is the main mediator of CSF activity. We show here that this is not the case in mouse oocytes. The injection of constitutively active mutant forms of Rsk1 and Rsk2 does not induce a cell cycle arrest in two-cell mouse embryos. Moreover, these two mutant forms do not restore MII arrest after their injection into mos-/- oocytes. Eventually, oocytes from the triple Rsk (1, 2, 3) knockout present a normal CSF arrest. We demonstrate that p90Rsk is not involved in the MII arrest of mouse oocytes.

Functional interaction between p90Rsk2 and Emi1 contributes to the metaphase arrest of mouse oocytes

Vertebrate eggs arrest at metaphase of the second meiotic division before fertilization under the effect of a cytostatic factor (CSF). This arrest is established during oocyte maturation by the MAPK kinase module, comprised of Mos, MEK, MAPKs and p90Rsk. Maintenance of CSF arrest at metaphase requires inhibitors of the anaphase-promoting complex (APC) like Emi1, which sequesters the APC activator Cdc20. Although it was proposed that the Mos pathway and Emi1 act independently, neither one alone is sufficient to entirely reproduce CSF arrest. Herein we demonstrate that p90Rsk2 associates with and phosphorylates Emi1 upstream of the binding region for Cdc20, thus stabilizing their interaction. Experiments in transfected cells and two-cell embryos indicate that Emi1 and p90Rsk2 cooperate to induce the metaphase arrest. Moreover, oocyte maturation was impaired by interfering with the interaction between p90Rsk2 and Emi1 or by RNA interference of Emi1. Our results indicate that p90Rsk2 and Emi1 functionally interact during oocyte maturation and that the Mos pathway establishes CSF activity through stabilization of an APC-inhibitory complex composed by Emi1 and Cdc20 before fertilization.

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.

A requirement for MAP kinase in the assembly and maintenance of the mitotic spindle

Circumstantial evidence has suggested the possibility of microtubule-associated protein (MAP) kinase's involvement in spindle regulation. To test this directly, we asked whether MAP kinase was required for spindle assembly in Xenopus egg extracts. Either the inhibition or the depletion of endogenous p42 MAP kinase resulted in defective spindle structures resembling asters or half-spindles. Likewise, an increase in the length and polymerization of microtubules was measured in aster assays suggesting a role for MAP kinase in regulating microtubule dynamics. Consistent with this, treatment of extracts with either a specific MAP kinase kinase inhibitor or a MAP kinase phosphatase resulted in the rapid disassembly of bipolar spindles into large asters. Finally, we report that mitotic progression in the absence of MAP kinase signaling led to multiple spindle abnormalities in NIH 3T3 cells. We therefore propose that MAP kinase is a key regulator of the mitotic spindle.

Nuclear translocation of p90Rsk and phosphorylation of CREB is induced by ionomycin in a Ras-independent manner in PC12 cells

In the present study we examined the possible role of p90Rsk in pathways leading to neuronal differentiation of PC12 cells induced by nerve growth factor (NGF) and the calcium ionophore ionomycin. PC12-M17 cells, expressing a dominant inhibitory Ras protein, do not undergo neuronal differentiation in response to NGF like wild-type PC12 cells, but exhibit neurite outgrowth when treated with NGF in combination with ionomycin. However, the blockade of Ras in these cells results in failure of activation of mitogen-activated protein kinase (MAPK)/extracellular signal regulation kinase (ERK) (MEK) and ERK activation as well, therefore kinases other than those of the ERK pathway might play a role in the induction of neuronal differentiation in this case. Here we show that p90Rsk translocates to the nucleus in response to ionomycin in both wild-type PC12 and PC12-M17 cells, and this spatial distribution is followed by increased phosphorylation of the cAMP response element binding protein (CREB). Since CREB is believed to be the transcription factor that can integrate Ca2+, growth factor and cAMP-induced signals, we suggest that p90Rsk may be one of the kinases which is able to replace ERKs under certain circumstances, thereby participating in Ras-independent neuronal differentiation induced by NGF plus ionomycin.