Activators and effectors of ras p21 proteins

Mitosis-specific negative regulation of epidermal growth factor receptor, triggered by a decrease in ligand binding and dimerization, can be overcome by overexpression of receptor

The function of epidermal growth factor receptor (EGFR) was found to be negatively regulated in M phase in which it showed less phosphotyrosine content and reduced intrinsic kinase activity accompanied by retarded electrophoretic mobility owing to total hyperphosphorylation. Ligand-induced autophosphorylation and downstream signaling of EGFR were tightly suppressed in M phase due to a decrease in ligand binding affinity and the inability of epidermal growth factor (EGF) to induce receptor dimerization. There was no change in the number of surface-exposed EGF receptors between G0/G1 and M phases of the cell cycle. Hyperphosphorylation (due to serine and/or threonine phosphorylation) correlates with the unresponsiveness of cells to EGF-mediated stimulation of tyrosine phosphorylation in cells that express the normal or basal level of EGFR. This M phase-specific negative regulation was overcome by overexpression of EGFR, which was responsive to ligand throughout the cell cycle and revealed ligand-induced signaling in the M phase. These findings indicate that EGFR does not respond to ligand stimulation in M phase and suggest that a negative regulation of ligand-receptor interactions in M phase may control the normal function of receptor tyrosine kinase and that receptor overexpression will disrupt this cell cycle-dependent regulation of receptor tyrosine kinases.

Activation of R-Ras by Ras-guanine nucleotide-releasing factor

Ras-GRF/CDC25(Mm), mSos, and C3G have been identified as guanine nucleotide-releasing factors for Ras family proteins. We investigated in this study the guanine nucleotide-releasing activities of Ras-GRF, mSos, and C3G toward R-Ras, which shows high sequence similarity to Ras. Ras-GRF markedly stimulated the dissociation of GDP from R-Ras, and C3G also promoted the release of R-Ras-bound GDP. Under the same conditions, mSos little affected the reaction. When Ras-GRF and R-Ras were coexpressed in COS7 cells, the remarkable accumulation of the active GTP-bound form of R-Ras was observed. C3G also increased active R-Ras in COS7 cells, while mSos did not give any effect. These results indicated that Ras-GRF and C3G could activate R-Ras. Furthermore, the activation of R-Ras by Ras-GRF was enhanced when cells were treated with ionomycin, which is known to increase the intracellular calcium concentration. The examination of tissue distribution of R-Ras, Ras-GRF, and mSos by the reverse transcription-polymerase chain reaction revealed that Ras-GRF was expressed only in brain and testis, whereas R-Ras, C3G, and mSos were expressed rather ubiquitously. These findings raise the possibility that R-Ras is activated by Ras-GRF in brain and testis, and by C3G in other tissues, respectively.

Shc contains two Grb2 binding sites needed for efficient formation of complexes with SOS in B lymphocytes

Cross-linking of the B-cell antigen receptor (BCR) induces tyrosine phosphorylation of Shc, which is believed to lead to the activation of Ras. Previous work has shown that tyrosine-phosphorylated Shc forms complexes with another adapter protein, Grb2, and the Ras guanine nucleotide exchange factor SOS. Here, we demonstrate that phosphorylation of Shc by the hematopoietic cell-specific tyrosine kinase Syk induces binding of Grb2 to Shc, suggesting that Syk phosphorylates Shc in stimulated B cells. Surprisingly, Syk-phosphorylated Shc possesses two Grb2 binding sites rather than the one site that has been previously reported. Both of these sites are required for efficient formation of Shc-Grb2-SOS complexes in vitro and in vivo. We suggest that two Grb2 proteins anchored by a single Shc protein bind simultaneously to one SOS molecule, resulting in a complex that is more stable than a complex containing only a single Grb2 protein bound to one SOS molecule. This model is consistent with our observation that BCR stimulation greatly increases the amount of SOS associated with Grb2.

The complexity of signaling pathways activated by the BCR

Cross-linking of the B cell antigen receptor (BCR) leads to the activation of three types of intracellular protein tyrosine kinases. These tyrosine kinases then phosphorylate signaling components to activate a variety of signaling reactions, including phosphatidylinositol 4,5-bisphosphate hydrolysis, Ras activation, and phosphatidylinositol 3-kinase activation. Each of these signaling reactions, and also the signaling molecules Vav and HS1, appears to be important for at least some of the many types of B cell responses to antigen. The complexity of BCR signaling reactions may be required to allow the B cell to respond in a number of distinct ways to antigen (proliferation, survival, apoptosis, maturational arrest, etc.) depending on the maturation state of the B cell, the location in the body, the physical nature of the antigen, and the possible presence of the antigen in complex with antibody or complement components.

Tropomyosin-2 cDNA lacking the 3' untranslated region riboregulator induces growth inhibition of v-Ki-ras-transformed fibroblasts

The levels of high molecular weight isoforms of tropomyosin (TM) are markedly reduced in ras-transformed cells. Previous studies have demonstrated that the forced expression of tropomyosin-1 (TM-1) induces reversion of the transformed phenotype of ras-transformed fibroblasts. The effects of the related isoform TM-2 on transformation are less clear. To assess the effects of forced expression of the TM-2 protein on ras-induced tumorigenicity, we introduced a TM-2 cDNA lacking the 3' untranslated region riboregulator into ras-transformed NIH 3T3 fibroblasts. TM-2 expression resulted in a flatter cell morphology and restoration of stress fibers. TM-2 expression also significantly reduced growth rates in low serum, soft agar, and nude mice. The reduced growth rates were associated with a prolongation of G0-G1. To identify the mechanism of TM-2-induced growth inhibition, we analyzed the effects of TM-2 reexpression of ERK and c-jun N-terminal kinase (JNK) activities. Levels of ERK phosphorylation and activity in TM-2-transfected tumor cells were comparable to those in mock-transfected tumor cells. JNK activity was only modestly increased in ras-transformed cells relative to untransformed NIH 3T3 cells and only slightly reduced as result of forced TM-2 expression. We conclude that the partially restored expression of the TM-2 protein induces growth inhibition of ras-transformed NIH 3T3 cells without influencing ERK or JNK activities. Furthermore, the 3' untranslated region riboregulator of the alpha-tropomyosin gene is not needed for the inhibition of ras-induced growth.

A Ras GAP is essential for cytokinesis and spatial patterning in Dictyostelium

Using the yeast two-hybrid system, we have identified developmentally regulated Dictyostelium genes whose encoded proteins interact with Ras-GTP but not Ras-GDP. By sequence homology and biochemical function, one of these genes encodes a Ras GAP (DdRasGAP1). Cells carrying a DdRasGAP1 gene disruption (ddrasgap1 null cells) have multiple, very distinct growth and developmental defects as elucidated by examining the phenotypes of ddrasgap1 null strains. First, vegetative ddrasgap1 null cells are very large and highly multinucleate cells when grown in suspension, indicating a severe defect in cytokinesis. When suspension-grown cells are plated in growth medium on plastic where they attach and can move, the cells rapidly become mono- and dinucleate by traction-mediated cell fission and continue to grow vegetatively with a number of nuclei (1–2) per cell, similar to wild-type cells. The multinucleate phenotype, combined with results indicating that constitutive expression of activated Ras does not yield highly multinucleate cells and data on Ras null mutants, suggest that Ras may need to cycle between GTP- and GDP-bound states for proper cytokinesis. After starvation, the large null cells undergo rapid fission when they start to move at the onset of aggregation, producing mononucleate cells that form a normal aggregate. Second, ddrasgap1 null cells also have multiple developmental phenotypes that indicate an essential role of DdRasGAP1 in controlling cell patterning. Multicellular development is normal through the mid-slug stage, after which morphological differentiation is very abnormal and no culminant is formed: no stalk cells and very few spores are detected. lacZ reporter studies show that by the mid-finger stage, much of the normal cell-type patterning is lost, indicating that proper DdRasGAP1 function and possibly normal Ras activity are necessary to maintain spatial organization and for induction of prestalk to stalk and prespore to spore cell differentiation. The inability of ddrasgap1 null cells to initiate terminal differentiation and form stalk cells is consistent with a model in which Ras functions as a mediator of inhibitory signals in cell-type differentiation at this stage. Third, DdRasGAP1 and cAMP dependent protein kinase (PKA) interact to control spatial organization within the organism. Overexpression of the PKA catalytic subunit in ddrasgap1 cells yields terminal structures that are multiply branched but lack spores. This suggests that RasGAP and PKA may mediate common pathways that regulate apical tip differentiation and organizer function, which in turn control spatial organization during multicellular development. It also suggests that DdRasGAP1 either lies downstream from PKA in the prespore to spore pathway or in a parallel pathway that is also essential for spore differentiation. Our results indicate that DdRasGAP1 plays an essential role in controlling multiple, potentially novel pathways regulating growth and differentiation in Dictyostelium and suggest a role for Ras in these processes.

Formation of actin stress fibers and focal adhesions enhanced by Rho-kinase

The small guanosine triphosphatase (GTPase) Rho is implicated in the formation of stress fibers and focal adhesions in fibroblasts stimulated by extracellular signals such as lysophosphatidic acid (LPA). Rho-kinase is activated by Rho and may mediate some biological effects of Rho. Microinjection of the catalytic domain of Rho-kinase into serum-starved Swiss 3T3 cells induced the formation of stress fibers and focal adhesions, whereas microinjection of the inactive catalytic domain, the Rho-binding domain, or the pleckstrin-homology domain inhibited the LPA-induced formation of stress fibers and focal adhesions. Thus, Rho-kinase appears to mediate signals from Rho and to induce the formation of stress fibers and focal adhesions.

Expression of an activated rasD gene changes cell fate decisions during Dictyostelium development

It has been previously demonstrated that the expression of an activated rasD gene in wild-type Dictyostelium cells results in formation of aggregates with multitips, instead of the normal single tips, and a block in further development. In an attempt to better understand the role of activated RasD development, we examined cell-type-specific gene expression in a strain stably expressing high levels of RasD[G12T]. We found that the expression of prestalk cell-specific genes ecmA and tagB was markedly enhanced, whereas the expression of the prespore cell-specific gene cotC was reduced to very low levels. When the fate of cells in the multitipped aggregate was monitored with an ecmA/lacZ fusion, it appeared that most of the cells eventually adopted prestalk gene expression characteristics. When mixtures of the [G12T]rasD cells and Ax3 cells were induced to differentiate, chimeric pseudoplasmodia were not formed. Thus, although the [G12T]rasD transformant had a marked propensity to form prestalk cells, it could not supply the prestalk cell population when mixed with wild-type cells. Both stalk and spore cell formation occurred in low cell density monolayers of the [G12T]rasD strain, suggesting that at least part of the inhibition of stalk and spore formation during multicellular development involved inhibitory cell interactions within the cell mass. Models for the possible role of rasD in development are discussed.

High affinity binding of the pleckstrin homology domain of mSos1 to phosphatidylinositol (4,5)-bisphosphate

mSos1 has been implicated in coupling mammalian tyrosine kinases to the Ras GTPase. Because activation of Ras induced by growth factor stimulation likely requires the localization of mSos1 to the plasma membrane, we have investigated the possibility that the PH domain of mSos1 might mediate an interaction of mSos1 with phospholipid membranes. A glutathione S-transferase fusion protein containing the pleckstrin homology (PH) domain of mSos1 bound specifically and tightly to phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) with a Kd of 1.8 +/- 0.4 microM. This interaction was saturable and was competed away with the soluble head group of PI(4,5)P2, inositol 1,4, 5-triphosphate. Substitution of Arg452 within the PH domain with Ala had only a slight effect on binding to PI(4,5)P2, whereas substitution of Arg459 severely compromised the ability of the mSos1 PH domain to bind to PI(4,5)P2 containing vesicles. Purified full-length mSos1 and mSos1 complexed with Grb2 were also tested for binding to various phosphoinositol derivatives and demonstrated a specific interaction with PI(4,5)P2, although these interactions were weaker (Kd = approximately 53 and approximately 69 microM, respectively) than that of the PH domain alone. These findings suggest that the PH domain of mSos1 can interact in vitro with phospholipid vesicles containing PI(4,5)P2 and that this interaction is facilitated by the ionic interaction of Arg459 with the negatively charged head group of PI(4,5)P2. The association of the mSos1 PH domain with phospholipid may therefore play a role in regulating the function of this enzyme in vivo.

Differentiation and transdifferentiation of the retinal pigment epithelium

The retinal pigment epithelium (RPE) lies between the retina and the choroid of the eye and plays a vital role in ocular metabolism. The RPE develops from the same sheet of neuroepithelium as the retina and the two derivatives become distinguished by different expression patterns of a number of transcription factors during embryonic development. As the RPE layer differentiates it expresses a set of unique molecules, many of which are restricted to certain regions of the cell. PRE cells undergo both a loss of polarity and a loss of expression of many of these cell type-specific molecules when placed in monolayer culture. The RPE of many species, including mammals, can be induced to transdifferentiate by growth factors such as basic fibroblast growth factor. Under the influence of such factors the RPE is triggered to alter expression of a wide array of molecules and to take on a retinal epithelium fate, from which differentiated retinal cell types including rod photoreceptors can be produced.

The cellular and molecular response of cardiac myocytes to mechanical stress

External load plays a critical role in determining muscle mass and its phenotype in cardiac myocytes. Cardiac myocytes have the ability to sense mechanical stretch and convert it into intracellular growth signals, which lead to hypertrophy. Mechanical stretch of cardiac myocytes in vitro causes activation of multiple second messenger systems that are very similar to growth factor-induced cell signaling systems. Stretch of neonatal rat cardiac myocytes stimulates a rapid secretion of angiotensin II which, together with other growth factors, mediates stretch-induced hypertrophic responses in vitro. In this review, various cell signaling mechanisms initiated by mechanical stress on cardiac myocytes are summarized with emphasis on potential mechanosensing mechanisms and the relationship between mechanical loading and the cardiac renin-angiotensin system.

Regulation of phosphorylation pathways by p21 GTPases. The p21 Ras-related Rho subfamily and its role in phosphorylation signalling pathways

The oncogenic Ras p21 GTPases regulate phosphorylation pathways that underlie a wealth of activities, including growth and differentiation, in organisms ranging from yeast to human. In metazoa, growth factors trigger conversion of Ras from an inactive GDP-bound form to an active GTP-bound form. This activation of Ras leads to activation of Raf. Raf is one of the initial kinases in the cytoplasmic mitogen-activated protein kinase (MAPK) cascade, involving extracellular-signal-regulated kinases (ERK), which culminates in nuclear transcription. The Ras-related subfamily of Rho p21s, including Rho, Rac and Cdc42 are similarly active in their GTP-bound forms. These p21s mediate growth-factor-induced morphological changes involving actin-based cellular structures. For example, in mammalian fibroblasts, Rho mediates the formation of cytoskeletal stress fibres induced by lysophosphatidic acid, while Rac mediates the formation of membrane ruffles induced by platelet-derived growth factor, and Cdc42 mediates the formation of peripheral filopodia by bradykinin. In some cases, factor-induced Rac activation results in Rho activation, and factor-induced Cdc42 activation leads to Rac activation, as determined by specific morphological changes. Although separate Cdc42/Rac and Rac/Rho hierarchies exist, these might not extend into a linear form (i.e. Cdc42-->Rac-->Rho) since Cdc42 and Rho activities may be competitive or even antagonistic. Thus Cdc42-mediated formation of filopodia is accompanied by loss of stress fibres (whose formation is mediated by Rho). Recently, mammalian kinases that bind to the GTP-bound forms of Rho p21s have been isolated. These kinases include the p21-activated serine/threonine kinase (PAK), which is stimulated by binding to Cdc42 and Rac, and the Rho-binding serine/threonine kinase (ROK), which is not as strongly stimulated by binding. These kinases act as effectors for their p21 partners since they can directly affect the reorganization of the relevant actin-containing structures. ROK promotes the formation of Rho-induced actin-containing stress fibres and focal-adhesion complexes, to which the ends of the stress fibres attach. PAK stimulates the disassembly of stress fibres, which has been shown to accompany formation of Cdc42-induced peripheral-actin-containing structures, including filopodia, which with Rac-induced membrane ruffles play a role in cell movement. PAK also fosters loss of focal-adhesion complexes. Thus, there is cooperation between different Rho p21s as well as antagonism, with their associated kinases having a role in the integration of the reorganization of the actin cytoskeleton. The similarity of PAK to the Saccharomyces cerevisiae kinase Ste20p, which initiates the yeast mating/pheromone MAPK cascade, led to experiments showing that Cdc42 regulates Ste20p in this MAPK pathway. This similarity has also led to the demonstration that mammalian Cdc42 and Rac can signal to the nucleus through MAPK pathways. However, c-Jun N-terminal kinase (JNK, stress-activated protein kinase) rather than ERK, is involved. PAK have been implicated in the JNK pathway, but their exact roles are uncertain. Thus members of the Rho subfamily, and kinases that bind to these p21s are intimately involved in immediate morphological processes as well as long-term transcriptional events.

The Grb2-mSos1 complex binds phosphopeptides with higher affinity than Grb2

Epidermal growth factor (EGF) stimulation leads to autophosphorylation of the epidermal growth factor receptor (EGFR) and tyrosine phosphorylation of Shc. The Grb2 SH2 domain binds to Tyr1068 of EGFR and Tyr317 of Shc while its SH3 domains bind to mSos1. Therefore, EGF treatment potentially results in the formation of several multimeric signaling complexes, including EGFR-Grb2-mSos1, EGFR-Shc-Grb2-mSos1, and Shc-Grb2-mSos1, linking the receptor to activation of the Ras GTPase. We have purified Grb2, mSos1, and the Grb2-mSos1 complex to high homogeneity, and used these isolated proteins to obtain binding affinities of mSos1 for Grb2 and of either Grb2 or Grb2-mSos1 for phosphotyrosine-containing peptides. mSos1 bound Grb2 with a KD of 0.4 microM; the stoichiometry of the Grb2-mSos1 complex was 1:1. An EGFR-derived phosphopeptide bound Grb2 with a KD of 0.7 microM, whereas the Shc-derived phosphopeptide bound Grb2 with a KD of 0.2 microM. Since Grb2 exists in a stable complex with mSos1, and both proteins can exist in a constitutive complex in unstimulated cells, we performed phosphopeptide binding studies on the Grb2-mSos1 complex to gain a better understanding of binding events in the intact cell. Grb2-mSos1 bound to both EGFR- and Shc-derived phosphopeptides with higher affinities (KD of 0.3 microM and 31 nM, respectively) than Grb2 alone. These findings suggest that the proximity of mSos1 to Grb2 in the complex can influence the interactions of the Grb2 SH2 domain with phosphopeptides and raise the possibility that in the Grb2-mSos1 complex the SH2 and SH3 domains of Grb2 are not independent of each other but may be indirectly linked by mSos1.

Immunohistochemical localization of signal transduction pathways during amelogenesis: an initial exploration

This study was undertaken to map signal transduction pathway (STP) components uniquely associated with the four major receptor groups and their related STPs in association with the events involved in amelogenesis in the rat. Whole-head, freeze-dried sagittal sections were obtained at the level of the maxillary first molars and picked up on transparent adhesive tape. The sections were not decalcified or fixed, providing optimum conditions for immunohistochemical (IHC) localization. Antibodies to pathway components Gs alpha, Gi alpha, Gq alpha, Sos-1, Grb-2, p125Fak, Jak2, and Vav were localized. The respective patterns of localization indicate that the Gq alpha-linked, the receptor tyrosine kinase-initiated, and the integrin receptor-initiated pathways are involved in the proliferating pre-ameloblast cells. In the differentiating and differentiated ameloblasts, the Gs alpha-linked cAMP pathway is involved, apparently reading a factor(s) released by the dentin matrix. The Gq alpha-linked, the receptor tyrosine kinase-initiated, the integrin receptor-initiated, and the cytokine receptor-initiated pathways are also up-regulated in the proximal ends of the ameloblasts. These observations indicate that all four of the major receptor groups are involved in amelogenesis and that the role of classes of ligands not previously implicated in enamel formation must now be considered. It seems that the cells of the enamel organ respond to the appearance and disappearance of autocrine and paracrine growth factors, but they also up-regulate specific STPs to enable them to respond to circulating hormones and growth factors whose concentrations in the extracellular fluids remain relatively constant.

Signaling from Neural Impulses to Genes

Nerve impulses regulate expression of genes that control receptors, channels, enzymes, and structural proteins. This activity-dependent feedback allows adaptation to changing requirements and environmental conditions. The signal transduction mechanisms carrying information from the cell membrane to the nucleus are becoming well characterized, but a more dynamic view of intracellular signaling is emerging to explain cellular responses to specific patterns of neural impulses. This review analyzes this interface between electrophysiology and molecular cell biology to examine the signals, substrates, and processes that enable the nervous system to regulate its structure and function as a consequence of its own operation.

Raf regulates positive selection

T cell development is regulated by extracellular signals that mediate cellular proliferation and differentiation via specific signal transduction pathways. To determine the importance of the mitogen-activated protein kinase (MAP kinase) pathway in thymocyte development, we analyzed transgenic mice expressing dominant negative Raf (DN Raf) and a constitutively active v-Raf under the control of the p56lck proximal promoter. DN Raf had a profound effect on T cell receptor (TCR)-mediated signaling events as assessed by the inhibition of mitogen-induced proliferation of thymocytes in vitro. Overall thymocyte numbers were decreased by at most twofold from nontransgenic littermates. Positive selection was inhibited in DN Raf transgenic mice, as evidenced by both reduced numbers of mature thymocytes and a decrease in CD8+ thymocytes in female mice doubly transgenic for DN-Raf and a class I-restricted H-Y TCR. In contrast, the differentiation of double-positive thymocytes to single-positive thymocytes was enhanced in H-YTCR transgenic mice expressing constitutively active Raf (v-Raf). Thus, Raf regulates positive selection in the thymus.

What's new in ras genes? Physiological role of ras genes in signal transduction and significance of ras gene activation in tumorigenesis

Ras gene mutations have been found with variable prevalence in different tumor types. While during the past decade a lot of information has been accumulated on the frequency of ras oncogene activation in tumors, the last two years brought considerable progress in elucidating molecular mechanisms of signal transduction for which cellular ras proteins are key elements. They transmit signals from upstream tyrosine kinases to downstream serine/threonine kinases ultimately leading to changes of gene expression cytoskeletal architecture, cell-to-cell interactions and metabolism. These signalling pathways are of interest for the physiological regulation of proliferation and differentiation in normal, as well as in cancer tissue. Mutational activation of cellular ras genes to transforming oncogenes is thought to promote cell growth even in the absence of extracellular stimuli, and may thereby contribute to the initiation and/or progression of tumors.

Phosphorylation and activation of myosin by Rho-associated kinase (Rho-kinase)

The small GTPase Rho is implicated in physiological functions associated with actin-myosin filaments such as cytokinesis, cell motility, and smooth muscle contraction. We have recently identified and molecularly cloned Rho-associated serine/threonine kinase (Rho-kinase), which is activated by GTP Rho (Matsui, T., Amano, M., Yamamoto, T., Chihara, K., Nakafuku, M., Ito, M., Nakano, T., Okawa, K., Iwamatsu, A., and Kaibuchi, K. (1996) EMBO J. 15, 2208-2216). Here we found that Rho-kinase stoichiometrically phosphorylated myosin light chain (MLC). Peptide mapping and phosphoamino acid analyses revealed that the primary phosphorylation site of MLC by Rho-kinase was Ser-19, which is the site phosphorylated by MLC kinase. Rho-kinase phosphorylated recombinant MLC, whereas it failed to phosphorylate recombinant MLC, which contained Ala substituted for both Thr-18 and Ser-19. We also found that the phosphorylation of MLC by Rho-kinase resulted in the facilitation of the actin activation of myosin ATPase. Thus, it is likely that once Rho is activated, then it can interact with Rho-kinase and activate it. The activated Rho-kinase subsequently phosphorylates MLC. This may partly account for the mechanism by which Rho regulates cytokinesis, cell motility, or smooth muscle contraction.

Regulation of myosin phosphatase by Rho and Rho-associated kinase (Rho-kinase)

The small guanosine triphosphatase Rho is implicated in myosin light chain (MLC) phosphorylation, which results in contraction of smooth muscle and interaction of actin and myosin in nonmuscle cells. The guanosine triphosphate (GTP)-bound, active form of RhoA (GTP.RhoA) specifically interacted with the myosin-binding subunit (MBS) of myosin phosphatase, which regulates the extent of phosphorylation of MLC. Rho-associated kinase (Rho-kinase), which is activated by GTP.RhoA, phosphorylated MBS and consequently inactivated myosin phosphatase. Overexpression of RhoA or activated RhoA in NIH 3T3 cells increased phosphorylation of MBS and MLC. Thus, Rho appears to inhibit myosin phosphatase through the action of Rho-kinase.

Role of the Raf/mitogen-activated protein kinase pathway in p21ras desensitization

Desensitization of p21(ras) after stimulation of cells by growth factors and phorbol 12-myristate 13-acetate (PMA) correlates with hyperphosphorylation of the guanine nucleotide exchange factor Son-of-sevenless (Sos) and its dissociation from the adaptor protein Grb2 (Cherniack, A., Klarlund, J. K., Conway, B. R., and Czech, M. P. (1995) J. Biol. Chem. 270, 1485-1488). To test the role of the Raf/mitogen-activated protein (MAP) kinase pathway, we utilized cells expressing a chimera composed of the catalytic domain of p74Raf-1 and the hormone binding domain of the estradiol receptor (DeltaRaf-1:ER). Estradiol markedly stimulated DeltaRaf-1:ER and the downstream MEK and MAP kinases in these cells as well as Sos phosphorylation. However, the dissociation of Grb2 from Sos observed in response to PMA was not apparent upon DeltaRaf-1:ER activation. Furthermore, stimulation of DeltaRaf-1:ER did not impair GTP loading of p21(ras) in response to platelet-derived growth factor or epidermal growth factor. We conclude that activation of the Raf/MAP kinase pathway alone in these cells is insufficient to cause disassembly of Sos from Grb2 or to interrupt the ability of Sos to catalyze activation of p21(ras).

ARIA/HRG regulates AChR epsilon subunit gene expression at the neuromuscular synapse via activation of phosphatidylinositol 3-kinase and Ras/MAPK pathway

AChR-inducing activity (ARIA)/heregulin, a ligand for erbB receptor tyrosine kinases (RTKs), is likely to be one nerve-supplied signal that induces expression of acetylcholine receptor (AChR) genes at the developing neuromuscular junction. Since some RTKs act through Ras and phosphatidylinositol 3-kinase (PI3K), we investigated the role of these pathways in ARIA signaling. Expression of activated Ras or Raf mimicked ARIA-induction of AChR epsilon subunit genes in muscle cells; whereas dominant negative Ras or Raf blocked the effect of ARIA. ARIA rapidly activated erk1 and erk2 and inhibition of both erks also abolished the effect of ARIA. ARIA stimulated association of PI3K with erbB3, expression of an activated PI3K led to ARIA-independent AChR epsilon subunit expression, and inhibition of PI3K abolished the action of ARIA. Thus, synaptic induction of AChR genes requires activation of both Ras/MAPK and PI3K signal transduction pathways.

Constitutive phosphorylation of TrkC receptors in cultured cerebellar granule neurons might be responsible for the inability of NT-3 to increase neuronal survival and to activate p21 Ras

The neurotrophins brain derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) are both expressed in developing cerebellum in addition to their tyrosine kinase receptors. TrkB and TrkC. In contrast to BDNF.NT-3 has only a negligible or a transient survival activity on cultured cerebellar granule neurons. The granule neurons however, express both TrkC and Trk B receptors which suggests a basic difference in signaling between BDNF and NT-3 in these neurons. Here we have studied whether this difference can be attributed to the presence of alternative TrkC receptor variants on the granule neurons and which signaling pathway is specifically activated by BDNF but not by NT-3 in these neurons. Using RT-PCR it was shown that the cerebellar granule neurons express the full length TrkC receptor, in addition to variant receptors containing small inserts in the receptor tyrosine kinase domain. There was no dramatic change in the relative amounts of different TrkC receptors during development. However, we found the TrkC receptor constitutively phosphorylated even in the absence of added ligand suggesting an interaction of TrkC with endogenously produced NT-3. In addition, NT-3 was able to phosphorylate the BDNF receptor, TrkB but only at higher concentration (50 ng/ml). There were also distinct differences in the activation of intracellular molecules by BDNF and NT-3. Thus, p21 Ras and PLC gamma were activated by BDNF but not by NT-3 whereas both BDNF and NT-3 increased calcium and c-fos mRNA in the granule neurons. These results show that differential activation of specific intracellular pathways such as that of p21 Ras determines the specific effects of BDNF and NT-3 on granule neuron survival. In addition, since calcium is increased by NT-3 in the cerebellar granule neurons, this neurotrophin might have some unknown important effects on these neurons.

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.

DOS, a novel pleckstrin homology domain-containing protein required for signal transduction between sevenless and Ras1 in Drosophila

The specification of the R7 photoreceptor cell in the developing eye of Drosophila is dependent upon activation of the Sevenless (SEV) receptor tyrosine kinase. By screening for mutations that suppress signaling via a constitutively activated SEV protein, we have identified a novel gene, daughter of sevenless (dos). DOS is required not only for signal transduction via SEV but also in other receptor tyrosine kinase signaling pathways throughout development. The presence of an amino-terminally located pleckstrin homology domain and many potential tyrosine phosphorylation sites suggests that DOS functions as an adaptor protein able to interact with multiple signaling molecules. Our genetic analysis demonstrates that DOS functions upstream of Ras1 and defines a signaling pathway that is independent of direct binding of the DRK SH2/SH3 adaptor protein to the SEV receptor tyrosine kinase.

Ras2 signals via the Cdc42/Ste20/mitogen-activated protein kinase module to induce filamentous growth in Saccharomyces cerevisiae

RAS2val19, a dominant activated form of Saccharomyces cerevisiae Ras2, stimulates both filamentous growth and expression of a transcriptional reporter FG(TyA)::lacZ but does not induce the mating pathway reporter FUS1::lacZ. This induction depends upon elements of the conserved mitogen-activated protein kinase (MAPK) pathway that is required for both filamentous growth and mating, two distinct morphogenetic events. Full induction requires Ste20 (homolog of mammalian p65PAK protein kinases), Ste11 [an MEK kinase (MEKK) or MAPK kinase (MEK) kinase], Ste7 (MEK or MAPK kinase), and the transcription factor Ste12. Moreover, the Rho family protein Cdc42, a conserved morphogenetic G protein, is also a potent regulator of filamentous growth and FG(TyA)::lacZ expression in S. cerevisiae. Stimulation of both filamentous growth and FG(TyA)::lacZ by Cdc42 depends upon Ste20. In addition, dominant negative CDC42Ala118 blocks RAS2val19 activation, placing Cdc42 downstream of Ras2. Our results suggest that filamentous growth in budding yeast is regulated by an evolutionarily conserved signaling pathway that controls cell morphology.

The regulation mechanism of c-jun and junB by human papillomavirus type 16 E5 oncoprotein

In this study, we show that HPV-16 E5 induced anchorage-independent growth in immortalized human epidermal keratinocytes and that HPV-16 E5 in human keratinocytes had higher expression of c-jun and junB; also, we investigated the role of transcriptional initiation pathways in the expression elevation. In addition, Ras-dependent pathway, as well as PKC-dependent pathway, leads to HPV-16 E5-induced c-jun gene expression.

Geranylgeranyl diphosphate-based inhibitors of post-translational geranylgeranylation of cellular proteins

A novel series of stable analogs of geranylgeranyl diphosphate (GGdP) are described in which the biologically labile diphosphate moiety of GGdP is replaced by portions that can act as stable isosters. The compounds inhibited the geranylgeranyltransferase activity in whole PC-3 prostate cancer cells, as determined by the inhibition of post-translational isoprenylation of the small GTP-binding protein p21rap 1 and the accumulation of unprocessed p21rap 1 in the cytosolic fraction. However, the compounds did not affect the farnesylation of p21ras, as shown by protein immunoprecipitation after whole cell labeling with [3 H]-(R,S)-mevalonolactone. Despite the absence of effects of post-translational processing of p21ras, these compounds proved to be cytotoxic for prostate cancer cells, with half-maximal inhibition of cell growth obtained in the range 18.5-35.1 microM. The GGdP analogs described in the this study are novel, non-peptidic inhibitors of geranylgeranylation that may be active as antitumor agents.

Stimulation of growth factor receptor signal transduction by activation of voltage-sensitive calcium channels

To understand the mechanisms by which electrical activity may generate long-term responses in the nervous system, we examined how activation of voltage-sensitive calcium channels (VSCCs) can stimulate the Ras/mitogen-activated protein kinase (MAPK) signaling pathway. Calcium influx through L-type VSCCs leads to tyrosine phosphorylation of the adaptor protein Shc and its association with the adaptor protein Grb2, which is bound to the guanine nucleotide exchange factor Sos1. In response to calcium influx, Shc, Grb2, and Sos1 inducibly associate with a 180-kDa tyrosine-phosphorylated protein, which was determined to be the epidermal growth factor receptor (EGFR). Calcium influx induces tyrosine phosphorylation of the EGFR to levels that can activate the MAPK signaling pathway. Thus, ion channel activation stimulates growth factor receptor signal transduction.

Farnesyltransferase inhibitors and anti-Ras therapy

The oncoprotein encoded by mutant ras genes is initially synthesized as a cytoplasmic precursor which requires posttranslational processing to attain biological activity; farnesylation of the cysteine residue present in the CaaX motif located at the carboxy-terminus of all Ras proteins is the critical modification. Once farnesylated and further modified, the mature Ras protein is inserted into the cell's plasma membrane where it participates in the signal transduction pathways that control cell growth and differentiation. The farnesylation reaction that modifies Ras and other cellular proteins having an appropriate CaaX motif is catalyzed by a housekeeping enzyme termed farnesyl-protein transferase (FPTase). Inhibitors of this enzyme have been prepared by several laboratories in an effort to identify compounds that would block Ras-induced cell transformation and thereby function as Ras-specific anticancer agents. A variety of natural products and synthetic organic compounds were found to block farnesylation of Ras proteins in vitro. Some of these compounds exhibit antiproliferative activity in cell culture, block the morphological alterations associated with Ras-transformation, and can block the growth of Ras-transformed cell lines in tumor colony-forming assays. By contrast, these compounds do not affect the growth or morphology of cells transformed by the Raf or Mos oncoproteins, which do not require farnesylation to achieve biological activity. The efficacy and lack of toxicity observed with FPTase inhibitors in an animal tumor model suggest that specific FPTase inhibitors may be useful for the treatment of some types of cancer.

An X chromosome-linked gene encoding a protein with characteristics of a rhoGAP predominantly expressed in hematopoietic cells

An increasingly large number of proteins involved in signal transduction have been identified in recent years and shown to control different steps of cell survival, proliferation, and differentiation. Among the genes recently identified at the tip of the long arm of the human X chromosome, a novel gene, C1, encodes a protein that appears to represent a newly discovered member of the group of signaling proteins involved in regulation of the small GTP binding proteins of the ras superfamily. The protein encoded by C1, p115, is synthesized predominantly in cells of hematopoietic origin. It is characterized by two regions of similarity to motifs present in known proteins: GAP and SH3 homologous regions. Its localization in a narrow cytoplasmic region just below the plasma membrane and its inhibitory effect on stress fiber organization indicate that p115 may down regulate rho-like GTPases in hematopoietic cells.

Identification of AF-6 and canoe as putative targets for Ras

Ras (Ha-Ras, Ki-Ras, N-Ras) is implicated in the regulation of various cell functions such as gene expression and cell proliferation downstream from specific extracellular signals. Here, we partially purified a Ras-interacting protein with molecular mass of about 180 kDa (p180) from bovine brain membrane extract by glutathione S-transferase (GST)-Ha-Ras affinity column chromatography. This protein bound to the GTP gamma S (guanosine 5'-(3-O-thio)triphosphate, a nonhydrolyzable GTP analog).GST-Ha-Ras affinity column but not to those containing GDP.GST-Ha-Ras or GTP gamma S.GST-Ha-Ras with a mutation in the effector domain (Ha-RasA38). The amino acid sequences of the peptides derived from p180 were almost identical to those of human AF-6 that is identified as the fusion partner of the ALL-1 protein. The ALL-1/AF-6 chimeric protein is the critical product of the t (6:11) abnormality associated with some human leukemia. AF-6 has a GLGF/Dlg homology repeat (DHR) motif and shows a high degree of sequence similarity with Drosophila Canoe, which is assumed to function downstream from Notch in a common developmental pathway. The recombinant N-terminal domain of AF-6 and Canoe specifically interacted with GTP gamma S.GST-Ha-Ras. The known Ras target c-Raf-1 inhibited the interaction of AF-6 with GTP gamma S.GST-Ha-Ras. These results indicate that AF-6 and Canoe are putative targets for Ras.

The yeast growth control gene GRC5 is highly homologous to the mammalian putative tumor suppressor gene QM

We isolated the Saccharomyces cerevisiae GRC5 (growth control) gene by functional complementation in vivo of a ts (temperature sensitive) mutation. Phenotypic analysis suggested involvement of GRC5 in cell growth and proliferation. Mutant cells arrest their cell cycles after one to three cell divisions predominantly as mother cells with a large bud. In the region of the septum, a massive accumulation of cell wall material is observed. The mother and daughter nuclei are well separated and spindles are no longer present, while the cytoskeleton is of aberrant appearance. Arrested cells do not perform protein synthesis and are unable to mate. Furthermore, grc5-1ts cells rapidly lose viability at the restrictive temperature (37 degrees C) only on full media, but not under nitrogen-starvation conditions, indicating that proper response to this nutrient limitation is still intact in mutant cells after cell cycle arrest. The sequence of GRC5 translates into a basic protein of 221 amino acid with a corresponding Mr of 25.4 kDa. GRC5 is a member of the highly conserved QM gene family, members of which have been reported from plants, invertebrates and vertebrates. The amino acid sequence of GRC5 over its entire length is more than 60% identical to the human QM protein, expression of which is associated with loss of the tumorigenic phenotype in a cell line derived from Wilms' tumor, a malignancy of the embyronic kidney. Here, we show that GRC5 is an essential yeast gene, the function of which as inferred from analysis of the grc5-1ts mutant is crucial for establishment of proper cytoskeletal structure and regulation of growth in yeast cells.

A novel GTPase-activating protein for R-Ras

R-Ras, belonging to the Ras small GTP-binding protein superfamily, has been implicated in regulation of various cell functions such as gene expression, cell proliferation, and apoptotic cell death. In the present study, we purified an R-Ras-interacting protein with molecular mass of about 98 kDa (p98) from bovine brain cytosol by glutathione S-transferase (GST)-R-Ras affinity column chromatography. This protein bound to GTP gamma S (guanosine 5'-(3-O-thio)triphosphate, a nonhydrolyzable GTP analog).R-Ras but not to GDP.R-Ras, GTP gamma S.R-Ras with a mutation in the effector domain (R-RasA64), GTP gamma S.Ha-Ras, or GTP gamma S.RalA. We obtained a cDNA encoding p98 on the basis of its partial amino acid sequences. The predicted protein consists of 834 amino acids whose calculated mass, 95,384 Da, is close to the apparent molecular mass of p98. The amino acid sequence shows a high degree of sequence similarity to the entire sequence of Gap1m, one of the GTPase-activating proteins (GAP) for Ha-Ras. A recombinant protein consisting of the GAP-related domain of p98 fused to maltose-binding protein stimulated GTPase activity of R-Ras, and showed a weak effect on that of Ha-Ras but not that of Rap1 or Rho. These results clearly indicate that p98 is a novel GAP for R-Ras. Thus, we designated this protein as R-Ras GAP.

KSR, a novel protein kinase required for RAS signal transduction

We have identified and characterized two genes in Drosophila whose products are required for activated RAS to signal with normal efficiency, but do not appear to effect signaling by activated RAF. One encodes the beta subunit of type I geranylgeranyl transferase, a prenylation enzyme essential for targeting RAS to the plasma membrane. The other encodes a protein kinase that we have named kinase suppressor of ras (ksr). By genetic criteria, we show that KSR functions in multiple receptor tyrosine kinase pathways. We have isolated mammalian homologs of KSR that, together with the Drosophila gene, define a novel class of kinases. Our results suggest that KSR is a general and evolutionarily conserved component of the RAS signaling pathway that acts between RAS and RAF.

Activation and nuclear translocation of mitogen-activated protein kinases by polyomavirus middle-T or serum depend on phosphatidylinositol 3-kinase

Several cellular signal transduction pathways activated by middle-T in polyomavirus-transformed cells are required for viral oncogenicity. Here we focus on the role of phosphatidylinositol 3-kinase (PI 3-kinase) and Ras and address the question how these signaling molecules cooperate during cell cycle activation. Ras activation is mediated through association with SHC.GRB2.SOS and leads to increased activity of several members of the mitogen-activated protein (MAP) kinase family, while activation of PI 3-kinase results in the generation of D3-phosphorylated phosphatidylinositides whose downstream targets remain elusive. PI 3-kinase activation might also ensue as a direct consequence of Ras activation. Oncogenicity of middle-T requires stimulation of both Ras- and PI 3-kinase-dependent pathways. Mutants of middle-T incapable to bind either SHC.GRB2.SOS or PI 3-kinase are not oncogenic. Sustained activation and nuclear localization of one of the MAP kinases, ERK1, was observed in wild type but not in mutant middle-T-expressing cells. Wortmannin, an inhibitor of PI 3-kinase, prevented MAP kinase activation and nuclear localization in middle-T-transformed cells. PI 3-kinase activity was also required for activation of the MAP kinase pathway in normal serum-stimulated cells, generalizing the concept that signaling through MAP kinases requires not only Ras-but also PI 3-kinase-mediated signals.

Mitogenic signals and transforming potential of Nyk, a newly identified neural cell adhesion molecule-related receptor tyrosine kinase

Nyk/Mer is a recently identified receptor tyrosine kinase with neural cell adhesion molecule-like structure (two immunoglobulin G-like domains and two fibronectin III-like domains) in its extracellular region and belongs to the Ufo/Axl family of receptors. The ligand for Nyk/Mer is presently unknown, as are the signal transduction pathways mediated by this receptor. We constructed and expressed a chimeric receptor (Fms-Nyk) composed of the extracellular domain of the human colony-stimulating factor 1 receptor (Fms) and the transmembrane and cytoplasmic domains of human Nyk/Mer in NIH 3T3 fibroblasts in order to investigate the mitogenic signaling and biochemical properties of Nyk/Mer. Colony-stimulating factor 1 stimulation of the Fms-Nyk chimeric receptor in transfected NIH 3T3 fibroblasts leads to a transformed phenotype and generates a proliferative response in the absence of other growth factors. We show that phospholipase C gamma, phosphatidylinositol 3-kinase/p70 S6 kinase, Shc, Grb2, Raf-1, and mitogen-activated protein kinase are downstream components of the Nyk/Mer signal transduction pathways. In addition, Nyk/Mer weakly activates p90rsk, while stress-activated protein kinase, Ras GTPase-activating protein (GAP), and GAP-associated p62 and p190 proteins are not activated or tyrosine phosphorylated by Nyk/Mer. An analysis comparing the Nyk/Mer signal cascade with that of the epidermal growth factor receptor indicates substrate preferences by these two receptors. Our results provide a detailed description of the Nyk/Mer signaling pathways. Given the structural similarity between the Ufo/Axl family receptors, some of the information may also be applied to other members of this receptor tyrosine kinase family.

Disruption of oncogenic K-Ras4B processing and signaling by a potent geranylgeranyltransferase I inhibitor

Prenylation of the carboxyl-terminal CAAX (C, cysteine; A, aliphatic acid; and X, any amino acid) of Ras is required for its biological activity. We have designed a CAAX peptidomimetic, GGTI-287, which is 10 times more potent toward inhibiting geranylgeranyltransferase I (GGTase I) in vitro (IC50 = 5 nM) than our previously reported farnesyltransferase inhibitor, FTI-276. In whole cells, the methyl ester derivative of GGTI-287, GGTI-286, was 25-fold more potent (IC50 = 2 microM) than the corresponding methyl ester of FTI-276, FTI-277, toward inhibiting the processing of the geranylgeranylated protein Rap1A. Furthermore, GGTI-286 is highly selective for geranylgeranylation over farnesylation since it inhibited the processing of farnesylated H-Ras only at much higher concentrations (IC50 > 30 microM). While the processing of H-Ras was very sensitive to inhibition by FTI-277 (IC50 = 100 nM), that of K-Ras4B was highly resistant (IC50 = 10 microM). In contrast, we found the processing of K-Ras4B to be much more sensitive to GGTI-286 (IC50 = 2 microM). Furthermore, oncogenic K-Ras4B stimulation inhibited potently by GGTI-286 (IC50 = 1 microM) but weakly by FTI-277 (IC50 = 30 microM). Significant inhibition of oncogenic K-Ras4B stimulation of MAP kinase by GGTI-286 occurred at concentrations (1-3 microM) that did not inhibit oncogenic H-Ras stimulation of MAP kinase. The data presented in this study provide the first demonstration of selective disruption of oncogenic K-Ras4B processing and signaling by a CAAX peptidomimetic. The higher sensitivity of K-Ras4B toward a GGTase I inhibitor has a tremendous impact on future research directions targeting Ras in anticancer therapy.

Ras CAAX peptidomimetic FTI-277 selectively blocks oncogenic Ras signaling by inducing cytoplasmic accumulation of inactive Ras-Raf complexes

Ras-induced malignant transformation requires Ras farnesylation, a lipid posttranslational modification catalyzed by farnesyltransferase (FTase). Inhibitors of this enzyme have been shown to block Ras-dependent transformation, but the mechanism by which this occurs remains largely unknown. We have designed FTI-276, a peptide mimetic of the COOH-terminal Cys-Val-Ile-Met of K-Ras4B that inhibited potently FTase in vitro (IC50 = 500 pM) and was highly selective for FTase over geranylgeranyltransferase I (GGTase I) (IC50 = 50 nM). FTI-277, the methyl ester derivative of FTI-276, was extremely potent (IC50 = 100 nM) at inhibiting H-Ras, but not the geranylgeranylated Rap1A processing in whole cells. Treatment of H-Ras oncogene-transformed NIH 3T3 cells with FTI-277 blocked recruitment to the plasma membrane and subsequent activation of the serine/threonine kinase c-Raf-1 in cells transformed by farnesylated Ras (H-RasF), but not geranylgeranylated, Ras (H-RasGG). FTI-277 induced accumulation of cytoplasmic non-farnesylated H-Ras that was able to bind Raf and form cytoplasmic Ras/Raf complexes in which Raf kinase was not activated. Furthermore, FTI-277 blocked constitutive activation of mitogen-activated protein kinase (MAPK) in H-RasF, but not H-RasGG, or Raf-transformed cells. FTI-277 also inhibited oncogenic K-Ras4B processing and constitutive activation of MAPK, but the concentrations required were 100-fold higher than those needed for H-Ras inhibition. The results demonstrate that FTI-277 blocks Ras oncogenic signaling by accumulating inactive Ras/Raf complexes in the cytoplasm, hence preventing constitutive activation of the MAPK 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.

Neurofibromin-deficient fibroblasts fail to form perineurium in vitro

To identify cell type(s) that might contribute to nerve sheath tumors (neurofibromas) in patients with neurofibromatosis type 1, we generated cell cultures containing neurons. Schwann cells and fibroblasts from transgenic mouse embryos in which the type 1 neurofibromatosis gene was disrupted by homologous recombination (Brannan et al. (1994) Genes Development, 8,1019-1029). Normal fascicle formation by perineurial cells failed to occur in the absence of neurofibromin. Fascicles were reduced in number and showed abnormal morphology when normal neurons and Schwann cells were cultured up to 37 days with fibroblasts lacking neurofibromin. Proliferation was increased in a majority of fibroblast cell strains analyzed from embryos lacking neurofibromin. These observations suggest that mutations in the neurofibromatosis type I gene affect fibroblast behavior that might contribute to neurofibroma formation in patients with neurofibromatosis type 1.

Divergent mechanisms for homologous desensitization of p21ras by insulin and growth factors

Previous work suggested that desensitization of p21ras in response to growth factors such as epidermal growth factor (EGF) results from receptor down-regulation. Here we show that p21ras is desensitized by insulin in 3T3-L1 adipocytes in the continued presence of activated insulin receptors, while loss of epidermal growth factor and platelet-derived growth factor (PDGF) receptors in response to their ligands correlates with p21ras desensitization. Furthermore, elevated amounts of Grb2/Shc complexes persisted throughout p21ras desensitization by insulin. However, immunoblotting of anti-Son-of-sevenless (Sos) 1 and 2 immunoprecipitates with anti-Grb2 antisera revealed that p21ras desensitization in response to insulin and PDGF, but not EGF, is associated with a marked decrease in cellular complexes containing Sos and Grb2 proteins. Nonetheless, the desensitization of p21ras in response to these stimuli was homologous, in that each peptide could reactivate [32P]GTP loading of p21ras after desensitization by any of the others. Taken together, these data indicate that insulin, EGF, and PDGF all cause disassembly of Sos proteins from signaling complexes during p21ras desensitization, but at least two mechanisms are involved. Insulin elicits dissociation of Sos from Grb2 SH3 domains, whereas EGF signaling is reversed by receptor down-regulation and Shc dephosphorylation, releasing Grb2 SH2 domains. PDGF action triggers both mechanisms of Grb2 disassembly, which probably operate in concert with GAP to attenuate p21ras signaling.

Evidence for a bifurcation of the mitogenic signaling pathway activated by Ras and phosphatidylcholine-hydrolyzing phospholipase C

NIH 3T3 cells stably transfected with the gene encoding phosphatidylcholine-hydrolyzing phospholipase C (PC-PLC) from Bacillus cereus display a chronic elevation of intracellular diacylglycerol levels and a transformed phenotype. We have used such PC-PLC-transformed cells to evaluate the roles of the cytoplasmic serine/threonine kinases Raf-1, zeta protein kinase C (zeta PKC) and protein kinase A (PKA) in oncogenesis and mitogenic signal transduction elicited by phosphatidylcholine hydrolysis. We demonstrate here that stable expression of dominant negative mutants of both zeta PKC and Raf-1 lead to reversion of PC-PLC-transformed cells. Interestingly, expression of kinase defective zeta PKC also reverted NIH 3T3 cells transformed by the v-Ha-ras oncogene. Activation of PKA in response to elevation of cAMP levels also lead to reversion of PC-PLC-induced transformation, implicating PKA as a negative regulator acting downstream of PC-PLC. On the other hand, inhibition or depletion of phorbol ester responsive PKCs attenuated but did not block the ability of PC-PLC-transformed cells to induce DNA synthesis in the absence of growth factors. These results clearly implicate both Raf-1 and zeta PKC as necessary downstream components for transduction of the mitogenic/oncogenic signal generated by PLC-mediated hydrolysis of phosphatidylcholine and suggest, together with other recent evidence, a bifurcation in the signaling pathway downstream of PC-PLC.

Protein kinase C-and ras-dependent activation of c-jun gene by human papillomavirus type 11 E5a oncoprotein

E5a of HPV-11 is a transforming oncogene. Previously, we have shown that E5a constitutively activates the expression of protooncogene c-jun by transcriptional regulation through the AP-1 binding site in the c-jun promoter. In the present study, we used two different types of cells: the E5a transfected NIH 3T3 cells and human epidermal keratinocytes, and selectively inhibited different signal transduction pathways to investigate effects of E5a on c-jun expression. We find that protein kinase C and ras-dependent pathways are important for the c-jun induction by E5a, but not the cAMP-dependent pathway.

Differential effects on cAMP on the MAP kinase cascade: evidence for a cAMP-insensitive step that can bypass Raf-1

Because cAMP exerts opposite effects on cell proliferation in different cell types, we undertook to study its effect on the mitogen-activated protein kinase (MAPK) pathway in three cell lines (Rat-1, Swiss-3T3, and COS-7) chosen for their different mitogenic responses to cAMP. We measured the effect of cAMP on MAPK, MEK, and Raf-1 activities after stimulation by agonists acting through a tyrosine kinase receptor (epidermal growth factor) or a G protein-coupled receptor (lysophosphatidic acid). In Rat-1 cells we found that cAMP strongly inhibited all three activities (MAPK, MEK, and Raf-1), in good agreement with its effect on cell proliferation in these cells. In Swiss-3T3 and COS-7 cells, on the contrary, cAMP did not inhibit epidermal growth factor- and lysophosphatidic acid-induced stimulation of MAPK and MEK activities, and even stimulated MAPK activity slightly on its own. Again these results are in good agreement with the proliferative effect of cAMP in Swiss-3T3 cells. Raf-1 activity on the hand, was inhibited by cAMP in Swiss-3T3 and COS-7 as it was in Rat-1 cells. This result indicates that signaling pathways in Swiss-3T3 and COS-7 cells can activate MEK and MAPK in a Raf-1-independent and cAMP-insensitive manner. Our results add to growing evidence for the existence of Ras- and/or Raf-1-independent pathways leading to MEK and MAPK activation.