Purification and characterization of REKS from Xenopus eggs. Identification of REKS as a Ras-dependent mitogen-activated protein kinase kinase kinase

Epigenetic control of smooth muscle cell differentiation and phenotypic switching in vascular development and disease

The vascular smooth muscle cell (SMC) in adult animals is a highly specialized cell whose principal function is contraction. However, this cell displays remarkable plasticity and can undergo profound changes in phenotype during repair of vascular injury, during remodeling in response to altered blood flow, or in various disease states. There has been extensive progress in recent years in our understanding of the complex mechanisms that control SMC differentiation and phenotypic plasticity, including the demonstration that epigenetic mechanisms play a critical role. In addition, recent evidence indicates that SMC phenotypic switching in adult animals involves the reactivation of embryonic stem cell pluripotency genes and that mesenchymal stem cells may be derived from SMC and/or pericytes. This review summarizes the current state of our knowledge in this field and identifies some of the key unresolved challenges and questions that we feel require further study.

Fate-determining mechanisms in epithelial-myofibroblast transition: major inhibitory role for Smad3

Smad3 inhibits activation of the smooth muscle actin promoter and functions as a timer for myogenic programming in the epithelium.

Epithelial–myofibroblast (MF) transition (EMyT) is a critical process in organ fibrosis, leading to α–smooth muscle actin (SMA) expression in the epithelium. The mechanism underlying the activation of this myogenic program is unknown. We have shown previously that both injury to intercellular contacts and transforming growth factor β (TGF-β) are indispensable for SMA expression (two-hit model) and that contact disruption induces nuclear translocation of myocardin-related transcription factor (MRTF). Because the SMA promoter harbors both MRTF-responsive CC(A/T)-rich GG element (CArG) boxes and TGF-β–responsive Smad-binding elements, we hypothesized that the myogenic program is mobilized by a synergy between MRTF and Smad3. In this study, we show that the synergy between injury and TGF-β exclusively requires CArG elements. Surprisingly, Smad3 inhibits MRTF-driven activation of the SMA promoter, and Smad3 silencing renders injury sufficient to induce SMA expression. Furthermore, Smad3 is degraded under two-hit conditions, thereby liberating the myogenic program. Thus, Smad3 is a critical timer/delayer of MF commitment in the epithelium, and EMyT can be dissected into Smad3-promoted (mesenchymal) and Smad3-inhibited (myogenic) phases.

Regulation of the meiosis-inhibited protein kinase, a p38(MAPK) isoform, during meiosis and following fertilization of seastar oocytes

A p38(MAPK) homolog Mipk (meiosis-inhibited protein kinase) was cloned from seastar oocytes. This 40-kDa protein shares approximately 65% amino acid identity with mammalian p38-alpha isoforms. Mipk was one of the major tyrosine-phosphorylated proteins in immature oocytes arrested at the G(2)/M transition of meiosis I. The tyrosine phosphorylation of Mipk was increased in response to anisomycin, heat, and osmotic shock of oocytes. During 1-methyladenine-induced oocyte maturation, Mipk underwent tyrosine dephosphorylation and remained dephosphorylated in mature oocytes and during the early mitotic cell divisions until approximately 12 h after fertilization. At the time of differentiation and acquisition of G phases in the developing embryos, Mipk was rephosphorylated on tyrosine. In oocytes that were microinjected with Mipk antisense oligonucleotides and subsequently were allowed to mature and become fertilized, differentiation was blocked. Because MipK antisense oligonucleotides and a dominant-negative (K62R)Mipk when microinjected into immature oocytes failed to induce germinal vesicle breakdown, inhibition of Mipk function was not sufficient by itself to cause oocyte maturation. These findings point to a putative role for Mipk in cell cycle control as a G-phase-promoting factor.

Elucidation of binding determinants and functional consequences of Ras/Raf-cysteine-rich domain interactions

Raf-1 is a critical downstream target of Ras and contains two distinct domains that bind Ras. The first Ras-binding site (RBS1) in Raf-1 has been shown to be essential for Ras-mediated translocation of Raf-1 to the plasma membrane, whereas the second site, in the Raf-1 cysteine-rich domain (Raf-CRD), has been implicated in regulating Raf kinase activity. While recognition elements that promote Ras.RBS1 complex formation have been characterized, relatively little is known about Ras/Raf-CRD interactions. In this study, we have characterized interactions important for Ras binding to the Raf-CRD. Reconciling conflicting reports, we found that these interactions are essentially independent of the guanine nucleotide bound state, but instead, are enhanced by post-translational modification of Ras. Specifically, our findings indicate that Ras farnesylation is sufficient for stable association of Ras with the Raf-CRD. Furthermore, we have also identified a Raf-CRD variant that is impaired specifically in its interactions with Ras. NMR data also suggests that residues proximal to this mutation site on the Raf-CRD form contacts with Ras. This Raf-CRD mutant impairs the ability of Ras to activate Raf kinase, thereby providing additional support that Ras interactions with the Raf-CRD are important for Ras-mediated activation of Raf-1.

Hsp90 is required for c-Mos activation and biphasic MAP kinase activation in Xenopus oocytes

During Xenopus oocyte maturation, the Mos protein kinase is synthesized and activates the MAP kinase cascade. In this report, we demonstrate that the synthesis and activation of Mos are two separable processes. We find that Hsp90 function is required for activation and phosphorylation of Mos and full activation of the MAP kinase cascade. Once Mos is activated, Hsp90 function is no longer required. We show that Mos interacts with both Hsp90 and Hsp70, and that there is an inverse relationship between association of Mos with these two chaperones. We propose that Mos protein kinase is activated by a novel mechanism involving sequential association with Hsp70 and Hsp90 as well as phosphorylation. We also present evidence for a two-phase activation of MAP kinase in Xenopus oocytes.

Farnesylation of Ras is important for the interaction with phosphoinositide 3-kinase gamma

The correct functioning of Ras proteins requires post-translational modification of the GTP hydrolases (GTPases). These modifications provide hydrophobic moieties that lead to the attachment of Ras to the inner side of the plasma membrane. In this study we investigated the role of Ras processing in the interaction with various putative Ras-effector proteins. We describe a specific, GTP-independent interaction between post-translationally modified Ha- and Ki-Ras4B and the G-protein responsive phosphoinositide 3-kinase p110gamma. Our data demonstrate that post-translational processing increases markedly the binding of Ras to p110gamma in vitro and in Sf9 cells, whereas the interaction with p110alpha is unaffected under the same conditions. Using in vitro farnesylated Ras, we show that farnesylation of Ras is sufficient to produce this effect. The complex of p110gamma and farnesylated RasGTP exhibits a reduced dissociation rate leading to the efficient shielding of the GTPase from GTPase activating protein (GAP) action. Moreover, Ras processing affects the dissociation rate of the RasGTP complex with the Ras binding domain (RBD) of Raf-1, indicating that processing induces alterations in the conformation of RasGTP. The results suggest a direct interaction between a moiety present only on fully processed or farnesylated Ras and the putative target protein p110gamma.

Replacement of the H-Ras farnesyl group by lipid analogues: implications for downstream processing and effector activation in Xenopus oocytes

Ras proteins must undergo a series of posttranslational lipidation steps before they become biologically functional. While the fact that farnesylation is required for subsequent processing steps and indispensable for Ras function has been established, the significance of the isoprenoid structure per se in the context of fully processed Ras is unknown. Here, we describe a novel approach for studying the isoprenoid structure-function relationship in vivo by replacing the H-Ras farnesyl group with synthetic analogues and analyzing their biological functions following microinjection into Xenopus oocytes. We show that the H-Ras farnesyl group can be stripped of most of its isoprenoid features that distinguish it from a fatty acid without any apparent effect on its ability to induce oocyte maturation and activation of mitogen-activated protein kinase. In contrast, replacement by the less hydrophobic isoprenoid geranyl causes severely delayed oocyte activation. Analysis of posttranslational processing reveals a striking correlation between the kinetics of processing, membrane binding, and the onset of biological activity regardless of lipid structure and suggests that slow C-terminal proteolysis and/or methylation can become rate-limiting for H-Ras function. Thus, while our results suggest no stringent requirement for the H-Ras farnesyl structure for effector activation in Xenopus oocytes, they reveal an important role for the lipid present at the farnesylation site in promoting efficient proteolysis and/or methylation which allows rapid palmitoylation, membrane localization, and biological activity. Xenopus oocytes provide a useful in vivo system for the kinetic analysis of the function of the protein of interest present at the physiological dose, which is required for accurate determination of structure-function relationships.

Regulation of differentiation/maturation in vascular smooth muscle cells by hormones and growth factors

Smooth muscle cells (SMC) within atherosclerotic lesions show marked alterations in their differentiated properties as compared to normal medial SMC. This process of de-differentiation of SMC has been referred to as "phenotypic modulation", and is characterized by increased growth responsiveness, altered lipid metabolism, increased matrix production, and loss of contractile proteins, all of which can contribute to the development and/or progression of atherosclerotic disease. As such there has been much interest in understanding mechanisms and factors that control the differentiation of the vascular SMC. This paper reviews the effects of growth factors, growth inhibitors, and other extrinsic factors on differentiation/maturation of SMC, with a particular emphasis on consideration of factors that may contribute to abnormal control of SMC differentiation in vascular disease. In addition, we will briefly summarize what is currently known regarding molecular mechanisms that control the coordinate expression of genes encoding for SMC-selective/specific proteins that are required for the differentiated function of the vascular SMC.

An intact Raf zinc finger is required for optimal binding to processed Ras and for ras-dependent Raf activation in situ

The function of the c-Raf-1 zinc finger domain in the activation of the Raf kinase was examined by the creation of variant zinc finger structures. Mutation of Raf Cys 165 and Cys 168 to Ser strongly inhibits the Ras-dependent activation of c-Raf-1 by epidermal growth factor (EGF). Deletion of the Raf zinc finger and replacement with a homologous zinc finger from protein kinase C gamma (PKC gamma) (to give gamma/Raf) also abrogates EGF-induced activation but enables a vigorous phorbol myristate acetate (PMA)-induced activation. PMA activation of gamma/Raf does not require endogenous Ras or PKCs and probably occurs through a PMA-induced recruitment of gamma/Raf to the plasma membrane. The impaired ability of EGF to activate the Raf zinc finger variants in situ is attributable, at least in part, to a major decrement in their binding to Ras-GTP; both Raf zinc finger variants exhibit decreased association with Ras (V12) in situ upon coexpression in COS cells, as well as diminished binding in vitro to immobilized, processed COS recombinant Ras(V12)-GTP. In contrast, Raf binding to unprocessed COS or prokaryotic recombinant Ras-GTP is unaffected by Raf zinc finger mutation. Thus, the Raf zinc finger contributes an important component to the overall binding to Ras-GTP in situ, through an interaction between the zinc finger and an epitope on Ras, distinct from the effector loop, that is present only on prenylated Ras.

The dominant negative effects of H-Ras harboring a Gly to Ala mutation at position 60

v-H-Ras harboring the Gly-60 to Ala mutation (G60A) lacks the ability to induce germinal vesicle breakdown in Xenopus oocytes. Moreover, this mutant is capable of inhibiting the activity of v-H-Ras to induce oocyte germinal vesicle breakdown when co-injected. The duration and the extent of inhibition depends on the molar ratio of v-H-Ras(G60A) to v-H-Ras. The inhibition is not due to a general toxicity of v-H-Ras(G60A) to oocytes because oocytes injected with v-H-Ras(G60A) can be readily induced to mature by other mitogenic agents, such as insulin, insulin-like growth factor 1, insulin-like growth factor 2, and phosphatidylcholine-specific phospholipase C. The dominant negative effect of v-H-Ras(G60A) requires proper membrane attachment of v-H-Ras(G60A). By using a competition assay, it was concluded that the dominant negative phenotype of v-H-Ras(G60A) resulted from sequestering H-Ras downstream effector(s). Raf-1 was identified as one of the sequestered targets.

Endothelin-1 induces an increase in total protein synthesis and expression of the smooth muscle alpha-actin gene in vascular smooth muscle cells

The growth response of aortic vascular smooth muscle cells (VSMCs) to chronic hypertension includes vascular hypertrophy. We have shown previously that angiotensin II positively regulates the expression of the human vascular smooth muscle (SM) alpha-actin gene. To further expand our understanding of vasoactive peptide-induced vascular hypertrophy, we studied endothelin-1 (ET-1) regulation of total protein synthesis and cytoskeletal gene expression in VSMCs. In a concentration-dependent manner ET-1 increased [3H] leucine incorporation by VSMCs (122.4 +/- 5.5%, mean +/- SEM, n = 5). ET-1 (0.1 microM) induced expression of SM alpha-actin mRNA as detected by Northern blot analysis. Also, ET-1 in a concentration-dependent manner (0.1 nM-0.1 microM) induced expression of the chloramphenicol acetyl transferase gene driven by 896 bp of the human SM alpha-actin promoter when transiently transfected into rat aortic VSMCs by the calcium phosphate method (141.2 +/- 9.8%, mean +/- SEM, n = 10). These data suggest that part of ET-1-induced increase in protein synthesis is achieved through transcriptional regulation of the SM alpha-actin gene via activation of cis-acting element(s) in the promoter. Such findings help elucidate the role of ET-1 in regulation of vascular growth.

Different effects of various phospholipids on Ki-Ras-, Ha-Ras-, and Rap1B-induced B-Raf activation

We have recently purified a Ki-Ras- and Ha-Ras-dependent extracellular signal-regulated kinase kinase from bovine brain and identified it as B-Raf protein kinase complexed with 14-3-3 proteins (Yamamori, B., Kuroda, S., Shimizu, K., Fukui, K., Ohtsuka, T., and Takai, Y. (1995) J. Biol. Chem. 270, 11723-11726). Moreover, we found that Rap1B as well as Ki-Ras and Ha-Ras stimulate the B-Raf activity. Since B-Raf contains a cysteine-rich domain originally found in protein kinase C as a domain responsible for interaction with phosphatidylserine (PS) and diacylglycerol or 12-O-tetradecanoylphorbol-13-acetate, we have examined here the effect of these compounds on the Ki-Ras-, Ha-Ras-, and Rap1B-induced activation of bovine brain B-Raf. Bovine brain PS enhanced Ki-Ras-stimulated B-Raf activity. Phosphatidic acid was slightly active, but other phospholipids, such as phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol (PI), PI-4-monophosphate, PI-4,5-bisphosphate, and PI-3,4,5-trisphosphate, were inactive. However, none of the above phospholipids affected the Ha-Ras-stimulated B-Raf activity, whereas PI, PS, phosphatidylethanolamine, and phosphatidic acid inhibited the Rap1B-stimulated B-Raf activity. Phosphatidylcholine or PI-4-monophosphate did not show any effect on the Rap1B-stimulated B-Raf activity. Synthetic PS with two unsaturated fatty acids, such as 1,2-dioleoyl-PS or 1,2-dilinoleoyl-PS, showed the same effect toward the Ki-Ras- and Rap1B-stimulated B-Raf activities, but synthetic PS with two saturated fatty acids, such as 1, 2-distearoyl-PS, was inactive. 12-O-Tetradecanoylphorbol-13-acetate did not affect the stimulatory or inhibitory effect of PS on the Ki-Ras- and Rap1B-stimulated B-Raf activities, respectively. PS did not affect the Ki-Ras-, Ha-Ras-, or Rap1B-independent basal B-Raf activity or the mitogen-activated protein kinase kinase or extracellular signal-regulated kinase activity. These results indicate that various phospholipids differently affect Ki-Ras-, Ha-Ras, and Rap1B-induced B-Raf activation.

14-3-3 PROTEINS AND SIGNAL TRANSDUCTION

Perhaps in keeping with their enigmatic name, 14-3-3 proteins offer a seemingly bewildering array of opportunities for interaction with signal transduction pathways. In each organism there are many isoforms that can form both homo- and heterodimers, and many biochemical activities have been attributed to the 14-3-3 group. The potential for diversity-and also confusion-is high. The mammalian literature on 14-3-3 proteins provides an appropriate context to appreciate the potential roles of 14-3-3s in plant signal transduction pathways. In addition, functional and structural themes emerge when 14-3-3s are examined and compiled in ways that draw attention to their participation in protein phosphorylation and protein-protein interactions. These themes allow examination of plant 14-3-3s from two perspectives: the ways in which plant 14-3- 3s contribute to and extend ideas already described in animals, and the ways that plant 14-3-3s present unique contributions to the field. The crystal structure of an animal 14-3- 3 has been solved. When considered with the evolutionary stability of large segments of the 14-3-3 protein, the structure illuminates several aspects of 14-3-3 function. However, diversity in other regions of the 14-3-3s and their presence as multigene families offer many opportunities for cell-specific specialization of individual functions.

Palmitoylation of Ha-Ras facilitates membrane binding, activation of downstream effectors, and meiotic maturation in Xenopus oocytes

Ras proteins serve as critical relays in signal transduction pathways that control growth and differentiation and must undergo posttranslational modifications before they become functional. While it is established that farnesylation is necessary for membrane binding and cellular functions of all Ras proteins, the significance of palmitoylation is unclear. We have studied the contribution of Ha-Ras palmitoylation for biological activity in Xenopus oocytes. In contrast to wild-type Ha-Ras, which binds to membranes and induces meiosis when microinjected into oocytes, a nonpalmitoylated but farnesylated and methylated mutant mislocalizes to the cytosol and fails to promote maturation. This lack of responsiveness correlates with the inability of the mutant to induce phosphorylation and activation of mitogen-activated protein kinase and maturation promoting factor, which are both strongly activated by wild-type Ha-Ras. Costimulation of oocytes with insulin increases their responsiveness to Ras and partially rescues the biological activity of the palmitoylation-resistant mutant. However, 25-50 times higher doses of mutant were required to elicit responses equivalent to wild-type Ha-Ras. These results suggest that palmitoylation and membrane association of Ha-Ras is necessary for efficient activation of the mitogen-activated protein kinase cascade in vivo and are consistent with a biochemical function for Ras as a membrane targeting signal for downstream effectors in this pathway.

Mos is required for MAP kinase activation and is involved in microtubule organization during meiotic maturation in the mouse

Mos is normally expressed during oocyte meiotic maturation in vertebrates. However, apart from its cytostatic factor (CSF) activity, its precise role during mouse meiosis is still unknown. First, we analyzed its role as a MAP kinase kinase kinase. Mos is synthesized concomitantly with the activation of MAP kinase in mouse oocytes. Moreover, MAP kinase is not activated during meiosis in oocytes from mos −/− mice. This result implies that Mos is necessary for MAP kinase activation in mouse oocytes. Raf-1, another MAP kinase kinase kinase, is already present in immature oocytes, but does not seem to be active when MAP kinase is activated. Moreover, the absence of MAP kinase activation in mos −/− oocytes demonstrates that Raf-1 cannot compensate for the lack of Mos. These results suggest that Raf-1 is not involved in MAP kinase activation. Second, we analyzed the organization of the microtubules and chromosomes in oocytes from mos −/− mice. We observed that during the transition between two meiotic M-phases, the microtubules and chromosomes evolve towards an interphase-like state in mos −/− oocytes, while in the control mos +/− oocytes they remain in an M-phase configuration, as in the wild type. Moreover, after spontaneous activation, the majority of mos −/− oocytes are arrested for at least 10 hours in a third meiotic M-phase where they exhibit monopolar half-spindles. These observations present the first evidence, in intact oocytes, of a role for the Mos/…/MAP kinase cascade in the control of microtubule and chromatin organization during meiosis.

Activation of brain B-Raf protein kinase by Rap1B small GTP-binding protein

Rap1 small GTP-binding protein has the same amino acid sequence at its effector domain as that of Ras. Rap1 has been shown to antagonize the Ras functions, such as the Ras-induced transformation of NIH 3T3 cells and the Ras-induced activation of the c-Raf-1 protein kinase-dependent mitogen-activated protein (MAP) kinase cascade in Rat-1 cells, whereas we have shown that Rap1 as well as Ras stimulates DNA synthesis in Swiss 3T3 cells. We have established a cell-free assay system in which Ras activates bovine brain B-Raf protein kinase. Here we have used this assay system and examined the effect of Rap1 on the B-Raf activity to phosphorylate recombinant MAP kinase kinase (MEK). Recombinant Rap1B stimulated the activity of B-Raf, which was partially purified from bovine brain and immunoprecipitated by an anti-B-Raf antibody. The GTP-bound form was active, but the GDP-bound form was inactive. The fully post-translationally lipid-modified form was active, but the unmodified form was nearly inactive. The maximum B-Raf activity stimulated by Rap1B was nearly the same as that stimulated by Ki-Ras. Rap1B enhanced the Ki-Ras-stimulated B-Raf activity in an additive manner. These results indicate that not only Ras but also Rap1 is involved in the activation of the B-Raf-dependent MAP kinase cascade.

The farnesyl group of H-Ras facilitates the activation of a soluble upstream activator of mitogen-activated protein kinase

To study the function of the farnesyl modification of Ras, the farnesyl group and a variety of its structural analogs, which lack one or more double bonds and/or the methyl groups, were enzymatically incorporated into recombinant H-Ras in vitro. These proteins were used in a cell- and membrane-free, Ras-dependent mitogen-activated protein kinase (MAP kinase) activation system derived from Xenopus laevis eggs to examine the contribution of the farnesyl group toward the activation of the kinase. Whereas non-farnesylated H-Ras is unable to activate MAP kinase, farnesylation of H-Ras alone, in the absence of further processing, is sufficient to cause the activation of MAP kinase in this system. All of the analogs of the farnesyl group, when incorporated into H-Ras, support the activation of the kinase to variable extents. These results suggest a direct but fairly nonspecific interaction of the farnesyl moiety of H-Ras with a soluble upstream activator of MAP kinase.

Ras-effector interactions, the problem of specificity

Ras plays the role of a molecular switch in many cellular signalling pathways. The Raf-kinase has been identified as the direct target molecule of Ras in mammalian cells. However, in recent reports other proteins have been characterised as putative Ras effectors which have neither a functional nor a structural relationship to each other. In addition it has been shown that also other members of the Ras family like Rap and R-Ras can interact with some of these proteins. To address the problem of specificity and of biological relevance of the interactions, they have to be carefully quantified and the cellular localisation of the proteins involved taken into account.

Purification of a Ras-dependent mitogen-activated protein kinase kinase kinase from bovine brain cytosol and its identification as a complex of B-Raf and 14-3-3 proteins

We previously purified a protein factor, named REKS (Ras-dependent Extracellular Signal-regulated Kinase (ERK)/mitogen-activated protein kinase Kinase (MEK) Stimulator), from Xenopus eggs by use of a cell-free assay system in which recombinant GTP gamma S (guanosine 5'-(3-O-thio)triphosphate)-Ki-Ras activates recombinant MEK. By use of this assay system, we purified here bovine REKS to near homogeneity from the cytosol fraction of bovine brain by successive chromatographies of Mono S, Mono Q, GTP gamma S-glutathione S-transferase-Ha-Ras-coupled glutathione-agarose, and Mono Q columns. It was composed of three proteins with masses of about 95, 32, and 30 kDa as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The 95-, 32-, and 30-kDa proteins were identified by immunoblot analysis to be B-Raf protein kinase, 14-3-3 protein, and 14-3-3 protein, respectively. Moreover, the REKS activity was specifically immunoprecipitated by an anti-B-Raf antibody. Bovine REKS was activated by lipid-modified GTP gamma S-Ki-Ras far more effectively than by a lipid-unmodified one. Lipid-modified GDP-Ki-Ras was inactive. Exogenous addition of 14-3-3 proteins stimulated further the REKS activity both in the presence and absence of GTP gamma S-Ki-Ras. These results indicate that at least one of the direct targets of Ras is B-Raf complexed with 14-3-3 proteins in bovine brain.