The ras gene. Transformer and transducer

NOGOB receptor-mediated RAS signaling pathway is a target for suppressing proliferating hemangioma

Infantile hemangioma is a vascular tumor characterized by the rapid growth of disorganized blood vessels followed by slow spontaneous involution. The underlying molecular mechanisms that regulate hemangioma proliferation and involution still are not well elucidated. Our previous studies reported that NOGOB receptor (NGBR), a transmembrane protein, is required for the translocation of prenylated RAS from the cytosol to the plasma membrane and promotes RAS activation. Here, we show that NGBR was highly expressed in the proliferating phase of infantile hemangioma, but its expression decreased in the involuting phase, suggesting that NGBR may have been involved in regulating the growth of proliferating hemangioma. Moreover, we demonstrate that NGBR knockdown in hemangioma stem cells (HemSCs) attenuated growth factor-stimulated RAS activation and diminished the migration and proliferation of HemSCs, which is consistent with the effects of RAS knockdown in HemSCs. In vivo differentiation assay further shows that NGBR knockdown inhibited blood vessel formation and adipocyte differentiation of HemSCs in immunodeficient mice. Our data suggest that NGBR served as a RAS modulator in controlling the growth and differentiation of HemSCs.

Metabolic control of glucose degradation in yeast and tumor cells

Regulation of glucose degradation in both yeasts and tumor cells is very similar in many respects. In both cases it leads to excretion of intermediary metabolites (e.g., ethanol, lactate) in those cell types where uptake of glucose is unrestricted (Saccharomyces cerevisiae, Bowes melanoma cells). The similarities between glucose metabolism observed in yeast and tumor cells is explained by the fact that cell transformation of animal cells leads to inadequate expression of (proto-)oncogenes, which force the cell to enter the cell cycle. These events are accompanied by alterations at the signal transduction level, a marked increase of glucose transporter synthesis, enhancement of glycolytic key enzyme activities, and slightly reduced respiration of the tumor cell. In relation to homologous glucose degradation found in yeast and tumor cells there exist strong similarities on the level of cell division cycle genes, signal transduction and regulation of glycolytic key enzymes. It has been demonstrated that ethanol and lactate excretion in yeast and tumor cells, respectively, result from an overflow reaction at the point of pyruvate that is due to a carbon flux exceeding the capacity of oxidative breakdown. Therefore, the respiratory capacity of a cell determines the amount of glycolytic breakdown products if ample glucose is available. This restricted flux is also referred to as the respiratory bottleneck. The expression "catabolite repression", which is often used in textbooks to explain ethanol and acid excretion, should be abandoned, unless specific mechanisms can be demonstrated. Furthermore, it was shown that maximum respiration and growth rates are only obtained under optimum culture conditions, where the carbon source is limiting.

Analysis of prostatic fluid: evidence for the presence of a prospective marker for prostatic cancer

In an endeavor to identify marker(s) for prostatic cancer, proteins in prostatic fluids were analyzed by two-dimensional (2-D) gel electrophoresis. The fluids were obtained from five males who had no prostate lesions and five patients each with benign prostatic hyperplasia (BPH) and prostatic carcinoma (PCA). The specimens were collected directly over a mixture of protease inhibitors and centrifuged, and the supernatants were lyophilized and solubilized in sodium dodecyl sulfate mix. Identical amounts of proteins were pooled according to donors' prostate disease and the resulting samples were subjected to 2-D gel analysis employing the ISO-DALT system. The electrophoretograms were developed by silver or double stain. The samples of each group exhibited distinctive profiles with the exception of similar relative positions of major protein spots. A predominant protein occurring as several charge variants was consistently present in prostatic fluids of patients with PCA. This protein appeared to be a previously unknown constituent that we have called protein D (molecular weight approximately 22 kDa and isoelectric point approximately 4), and was undetectable in the fluids of "normal" men and patients with BPH. An analysis of pooled, unprocessed urine from PCA patients revealed that perhaps this protein is excreted in urine in very low quantities. These results strongly suggest that the potential of this protein as a marker for prostatic cancer should be further explored.

An acute release of Ca2+ from sequestered intracellular pools is not the primary transduction mechanism causing the initial burst of PRL and TSH secretion induced by TRH in normal rat pituitary cells

With 1.5 mM [Ca2+]e, 10 nM TRH induced a prompt high-amplitude burst of hormone secretion and an initial high-amplitude [Ca2+]i burst (first phase) followed by a sustained low-amplitude [Ca2+]i increment (second phase) in both tumor-derived GH4C1 and normal adenohypophyseal (AP) cells. With less than 2 microM [Ca2+]e, in both cell types the TRH-induced first phase rise in [Ca2+]i was suppressed 30% while the second phase rise was completely abolished; however, hormone secretion was inhibited only 20-30% in GH4C1 but greater than 80% in AP cells. Thapsigargin induced a first-phase rise in [Ca2+]i in AP cells equal to that induced by 10 nM TRH but only 20% as much first-phase hormone secretion. Blocking Ca2+ channels with nifedipine inhibited TRH-induced secretion in AP cells significantly more than in GH4C1 cells. Our data indicate that the TRH-induced first-phase spike in [Ca2+]i from intracellular Ca2+ stores may play a major transduction role in hormone secretion in GH4C1 cells but not in normal AP cells. Transduction mechanisms coupled to Ca2+ influx through Ca2+ channels in the plasmalemma are apparently a much more important component of TRH-induced secretion in normal than in tumor-derived pituitary cells.

TCDD causes stimulation of c-ras expression in the hepatic plasma membranes in vivo and in vitro

A series of in vivo and in vitro experiments were conducted to determine the effects of 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD) administered on the expression of c-ras. Differences in c-ras expression between control and TCDD treated groups were determined by immunoassay of p21ras protein, or indirectly measured by the specific binding of 3H-GTP to hepatic plasma membrane preparations. Intraperitoneal injection of sublethal doses of TCDD significantly elevated (P less than 0.05, Student t test) levels of hepatic p21ras protein in Sprague-Dawley rats and TCDD sensitive C57BL/6J mice. Such an increase occurred at an early stage of poisoning in the C57BL/6J mice. The earliest increase was detectable 6 hr after dosing, and the difference became statistically significant by 12 and 24 hr after dosing. In contrast, TCDD tolerant DBA/2J mice had only a marginal increase in hepatic p21ras protein which did not become statistically significant even at 24 hr host-dosing. TCDD evoked increases in hepatic p21ras protein of C57BL/6J mice were accompanied by the increase in the specific binding of GTP to hepatic plasma membranes. Column chromatography of solubilized rat hepatic membrane proteins on sephadex G-50 showed TCDD administration increased levels of a 3H-GTP binding protein with MW of approximately 21 Kd. 3H-GTP binding in total hepatic membranes was also elevated (P less than 0.05, Fisher PLSD multiple comparison test) 6 hr and 24 hr after dosing of C57BL/6J mice, but as expected the effect of TCDD was not as conspicuous as that found in the plasma membrane. TCDD treatment increased levels of a 21 Kd protein found in the in vitro translation products of RNA purified from guinea pig liver. This protein was identified as a c-ras protein based upon its ability to bind GTP, precipitation by a polyclonal antibody against the rasHa and Ki proteins and subsequent SDS-PAGE which showed a single protein band of approximately 21 Kd.

Unitary model of cell activation, growth control, cancer and other diseases: 1. Activated oxygen species and arachidonic acid modulation of solute permeabilities, internal Ca, Na and AOS levels and DNA transcription and synthesis

A comprehensive model of cellular activation and proliferation is developed. The model has arachidonic acid (ARA) produced mainly from PLA2 on both sides of the membrane, and superoxide and other activated oxygen species (AOS) formed from O2 by electrons passing out through membrane NANPH and NADH oxidases, as the immediate stimulants of solute permeability. Both ARA and AOS interact with the various solute channel proteins especially their external thiols and disulfides, to increase influx of metabolic substrates, Na, Ca and O2. PLA2 and NADPH oxidase are turned on by growth factors at their receptors acting through tyrosine kinase phosphorylations of messenger proteins GP and ras p-21, stimulated proteases, and by Ca-calmodulin. The adenylate cyclase system has opposite, deactivating character as it increases efflux of Ca and desensitizes growth factor receptors by phosphorylation to shut down the increased solute permeability. Most cancer types are due to carcinogen binding to cell membrane channel and mitochondrial sites for increased solute influx with excessive AOS production inside the cell from mitochondria and other vesicles. High Ca, Na and AOS stimulate proliferation with extra high levels causing transformation to the autogenic, more embryonic-type cancer cell.

Involvement of fos in spontaneous and ultraviolet light-induced genetic changes

Transient overexpression of ras, mos, or fos transcribed from various inducible promoters in NIH 3T3 cells causes significant increases in the frequency of chromosomal aberrations and, as shown for fos, in gene mutations. Under the experimental conditions of exponential growth and full serum supply, overexpression of the oncogenes does not increase the proliferation rate of cells. The generation of ras- and mos-induced chromosomal aberrations was suppressed in cells that had been deprived of fos protein by antisense c-fos oligodeoxynucleotides. The induction of chromosomal aberrations by ultraviolet irradiation is also suppressed by antisense c-fos oligodeoxynucleotides. The data suggest that fos protein alone, or a transcription factor that contains fos protein as a subunit, activates or induces the synthesis of one or several mutator functions. Oncogene-driven mutagenesis could account for the accumulation of additional mutations after the activation of an oncogene, which may furnish a mechanistic basis for tumor promotion and tumor progression.

Growth advantage by overexpression of normal Harvey ras proto-oncogene in cultured rat epidermal keratinocytes

The ras proto-oncogene is frequently amplified and overexpressed in the hyperproliferative conditions of epidermal keratinocytes. To investigate the effects of its overexpression on the growth of keratinocytes in a model system, we constructed expression vectors for normal human Ha-ras and introduced them into FRSK cells, a fetal rat epidermal keratinocyte cell line. Several clones containing the transfected Ha-ras were isolated, and two of them overexpressed this gene. In these clones DNA synthesis and cell growth were greater than in other clones expressing this gene at low levels. Thus we suggest that overexpression of normal ras gene may provide growth advantage to epidermal keratinocytes.

Oncogene N-ras mediates selective inhibition of c-fos induction by nerve growth factor and basic fibroblast growth factor in a PC12 cell line

A cell line was generated from U7 cells (a subline of PC12 rat pheochromocytoma cells) that contains a stably integrated transforming mouse N-ras (Lys-61) gene under the control of the long terminal repeat from mouse mammary tumor virus. Such cells, designated UR61, undergo neuronal differentiation upon exposure to nanomolar concentrations of dexamethasone, as a consequence of expression of the activated N-ras gene (I. Guerrero, A. Pellicer, and D.E. Burstein, Biochem, Biophys. Res. Commun. 150:1185-1192, 1988). Exposure of UR61 cells to either nerve growth factor (NGF) or basic fibroblast growth factor (bFGF) results in a marked induction of c-fos RNA, with kinetics paralleling those of NGF- or bFGF-induced expression of c-fos RNA in PC12 cells. Dexamethasone-induced expression of activated N-ras p21 results in blocking of c-fos RNA induction by NGF or bFGF in a time-dependent manner. Activated N-ras p21-mediated inhibition of c-fos RNA induction in UR61 cells is selective for NGF and bFGF and is not due to selective degradation of c-fos RNA. Normal and transforming N-ras can trans activate the chloramphenicol acetyltransferase gene linked to mouse c-fos regulatory sequences when transient expression assays are performed. Our observations suggest that N-ras p21 selectively interacts with pathways involved in induction of c-fos expression which initiate at the receptors for NGF and bFGF.

Ras gene product expression in blood and marrow smears of patients with acute leukemia: importance of fixation

Activation of ras protooncogenes by any of several possible mutations in codons 12, 13 or 61 has been demonstrated in a variety of human malignancies, including acute non-lymphoblastic leukemia (ANLL). In situ staining for the ras gene product, p21, has been demonstrated in carcinomas of several sites. High levels of p21 expression have been associated with histologic anaplasia in prostate cancer and regional lymph node metastasis in breast cancer. We examined 16 marrow aspirates and blood smears from patients with acute leukemia, predominantly ANLL, and eight controls. Marrow aspirates or blood were smeared on glass slides and fixed immediately in 10% buffered formalin. p21 was examined with avidin-biotin linked immunoperoxidase visualization. Particular attention must be paid to antibody selection and fixation protocol to demonstrate p21, owing to its rapid degradation ex vivo. Three of 16 patients exhibited occasional high p21 expression primarily in leukemic blasts, but in no case were more than 10% of blast cells positive. Normal reticuloendothelial and myeloid cells occasionally exhibited mild to moderately heavy staining, but megakaryocytes, erythroid precursors, lymphocytes and plasma cells were consistently negative. Most patients, 5 normal volunteers and 3 patients with non-malignant disease, exhibited no reactivity, or only a faint blush. These data suggest that while point mutation and concomitant activation of c-N-ras occurs regularly in ANLL, high levels of ras p21 expression are rarely found with this technique.

Oncogene N-ras mediates selective inhibition of c-fos induction by nerve growth factor and basic fibroblast growth factor in a PC12 cell line

A cell line was generated from U7 cells (a subline of PC12 rat pheochromocytoma cells) that contains a stably integrated transforming mouse N-ras (Lys-61) gene under the control of the long terminal repeat from mouse mammary tumor virus. Such cells, designated UR61, undergo neuronal differentiation upon exposure to nanomolar concentrations of dexamethasone, as a consequence of expression of the activated N-ras gene (I. Guerrero, A. Pellicer, and D.E. Burstein, Biochem, Biophys. Res. Commun. 150:1185-1192, 1988). Exposure of UR61 cells to either nerve growth factor (NGF) or basic fibroblast growth factor (bFGF) results in a marked induction of c-fos RNA, with kinetics paralleling those of NGF- or bFGF-induced expression of c-fos RNA in PC12 cells. Dexamethasone-induced expression of activated N-ras p21 results in blocking of c-fos RNA induction by NGF or bFGF in a time-dependent manner. Activated N-ras p21-mediated inhibition of c-fos RNA induction in UR61 cells is selective for NGF and bFGF and is not due to selective degradation of c-fos RNA. Normal and transforming N-ras can trans activate the chloramphenicol acetyltransferase gene linked to mouse c-fos regulatory sequences when transient expression assays are performed. Our observations suggest that N-ras p21 selectively interacts with pathways involved in induction of c-fos expression which initiate at the receptors for NGF and bFGF.

Transfection of activated ras into an excitable cell line (AtT-20) alters tetrodotoxin sensitivity of voltage-dependent sodium current

The sensitivity of voltage-dependent sodium current to the sodium channel blocker tetrodotoxin (TTX) is altered by transfection of a c-Ha-ras oncogene into an excitable cell line. Control AtT-20 cells, a cell line derived from a mouse anterior pituitary tumor, were found to express both a TTX-sensitive and a TTX-resistant sodium current. AtT-20 cells transfected with the c-Ha-ras gene expressed only a TTX-sensitive current. Properties of TTX-sensitive and -resistant currents were also examined. No differences in voltage dependence of activation or inactivation between the TTX-sensitive and -resistant currents were observed. The rate of inactivation of the TTX-resistant current in control cells was slower, than that of the TTX-sensitive current in either control or ras-transfected AtT-20 cells.

Infrequent ras oncogene point mutations in renal cell carcinoma

The role of ras oncogenes in the pathogenesis of renal cell carcinoma is unclear. We have previously shown that insertion of a mutated ras oncogene into cultured human proximal tubular cells, the normal counterpart of renal cell carcinomas, initiates a series of transformation events which results in cells possessing a renal cancer phenotype. These data suggested a role for mutated ras genes in the initiation and maintenance of this disease. Therefore, to assess the involvement of ras genes in renal carcinogenesis, 51 primary and metastatic renal carcinomas, including three oncocytomas, were analyzed for point mutations in codons 12, 13 and 61 of the Ha-ras, Ki-ras and N-ras proto-oncogenes using polymerase-catalyzed chain reaction methodology. A mutated Ha-ras gene was found in one renal cancer metastatic to lung for an overall incidence of 2%. These data indicate that ras oncogenes, activated by point mutations, do not play a major role in the initiation, maintenance or metastases of renal carcinomas.

Identification of a protein associated with p21ras by chemical crosslinking

The products of the ras oncogenes (p21ras) are ubiquitous membrane-associated proteins that bind guanine nucleotides and possess an intrinsic GTPase activity. Because of their functional homologies with regulatory guanine nucleotide-binding proteins, they are thought to be involved in the control of cellular proliferation as transducers of incoming growth signals. In an effort to identify proteins interacting with p21ras, we have used in vivo crosslinking techniques on Rat-1 fibroblasts and derived cell lines overexpressing p21ras and immunoprecipitation with polyclonal anti-p21ras antibodies. Under those conditions, using the homobifunctional crosslinker dithiobis(succinimidyl propionate), a protein of Mr 60,000 (p60) is found to be associated with p21ras, and this association is enhanced by the treatment of quiescent cells with serum. Upon sedimentation of detergent extracts from crosslinked cells on sucrose gradients, a p21-p60 complex could be demonstrated with a Mr of 200,000-300,000, p60 does not appear to be related to pp60src nor to the cytosolic GTPase activating protein that interacts with p21ras to enhance its GTPase activity. The amount of p60 seems to be limiting relative to p21ras in fibroblasts, since similar levels of p60 are immunoprecipitated from Rat-1 cells and transfectants overexpressing Ha-, Ki-, and N-ras p21s; the same protein is also found to associate with p21ras in numerous mammalian cell lines. The relevance of this component to the role of ras proteins in signal transduction is discussed.

Cyclic AMP can partially restore platelet-derived growth factor-stimulated prostaglandin E2 biosynthesis, and calcium mobilization in EJ-ras-transformed NIH-3T3 cells

NIH-3T3 cells transformed by the EJ-ras oncogene display reduced platelet-derived growth factor (PDGF)-stimulated phospholipase C activity as measured by inositol 1,4,5-triphosphate (IP3) synthesis and Ca2+ mobilization. The lack of PDGF-stimulated Ca2+ mobilization in EJ-ras transformed cells is not due to a loss of IP3 sensitivity, because microinjected IP3 elevates intracellular Ca2+. Treatment of EJ-ras transformed cells with cholera toxin or 8-bromo-cyclic AMP, but not pertussis toxin or the beta-subunit of cholera toxin, results in a slight recovery of PDGF-stimulated IP3 synthesis, a marked increase in intracellular Ca2+ mobilization, and an almost complete recovery of prostaglandin E2 biosynthesis. These data suggest that EJ p21-mediated inhibition of PDGF-stimulated intracellular events can be partially and transiently reversed by cyclic AMP.

Transfection of insulin-producing cells with a transforming c-Ha-ras oncogene stimulates phospholipase C activity

Pancreatic islet beta-cells and insulin-producing RINm5F cells were electroporated in the presence of the c-Ha-ras oncogene, to assess the possible involvement of the encoded product in coupling extracellular receptors to phospholipase C. After two days the c-Ha-ras-transfected cells increased their expression of c-Ha-ras mRNA. These cells were also found to contain more [3H]InsP3, suggesting an increased basal (non-ligand-activated) phospholipase C activity. In addition, the transfected cells were unable to respond to ligand (bombesin) activation of phospholipase C. The ras-transfected insulin-producing cells showed enhanced phosphorylation of a 200 kDa substrate crossreacting with an antibody to an 80 kDa protein kinase C substrate. The phorbol ester 12-O-tetradecanoyl 13-acetate and bombesin also induced phosphorylation of the 200 kDa substrate. All of these changes occurred without changes in the rates of [3H]thymidine incorporation. The results suggest that the mutated c-Ha-ras oncogene directly or indirectly stimulates the basal phospholipase C activity of these cells.

Nerve growth factor potentiates the hormone-stimulated intracellular accumulation of inositol phosphates and Ca2+ in rat PC12 pheochromocytoma cells: comparison with the effect of epidermal growth factor

The effects of nerve growth factor (NGF) and epidermal growth factor (EGF) on the intracellular accumulation of inositol phosphates and on cytosolic free Ca2+ concentrations were studied in rat PC12 pheochromocytoma cells. Both NGF and EGF potentiate in these cells the increase in the accumulation of inositol phosphates that is elicited by bradykinin and carbachol. A corresponding potentiation was also found for the agonist-induced increase of cytosolic Ca2+ concentrations. The effect of NGF, but not that of EGF, is abolished when the cells are preincubated with 5'-deoxy-5'-methylthioadenosine, an inhibitor of S-adenosylhomocysteine hydrolase. These results suggest that an increased response to hormones, which act via phosphoinositide-derived second messengers, may be important in the mechanism of action of NGF and EGF.

A model to account for the effects of oncogenes, TPA, and retinoic acid on the regulation of genes involved in metastasis

We have postulated that signals from the microenvironment can induce shifts in tumor cell phenotypes and that microenvironmental factors are therefore important for cancer metastasis. In this article we expand on this hypothesis and propose a model to explain (a) how extracellular signals can lead to changes in tumor phenotypes, and (b) how cytoplasmic oncogenes, which influence signal transducing pathways as well as nuclear oncogenes regulating gene expression via DNA binding transacting factors, might affect metastatic competence.

Role of proto-oncogenes in myocardial hypertrophy

A question of major clinical significance in cardiology is the nature of the signals that initiate and maintain the various types of myocardial hypertrophy, either in response to hemodynamic loading or in the absence of altered load. This review suggests that the proto-oncogene model, a concept derived from the study of cancer, can be very useful in identifying these signals. The proto-oncogene model conceives of cell growth regulation in terms of a limited number of classes of critical regulatory proteins: growth factors, growth factor receptors, intracellular transducing proteins and ribonucleic acid (RNA) transcription factors. Growth of all cells has dissociable components: hypertrophy (growth in size), deoxyribonucleic acid synthesis, mitosis and cytokinesis. Hypertrophy may be the end result of activation of RNA transcription. The various types of hypertrophy could reflect transcription of specific myocyte genes in response to different growth factors. At least 1 member of each functional class of proto-oncogenes has been detected in the myocardium or myocytes, or both. The alpha 1-adrenergic receptor has been shown to be a growth factor receptor and to regulate RNA transcription. Continued work on proto-oncogenes in myocytes may open the way to manipulate the growth of these cells.

Cotransfection of granulosa cells with simian virus 40 and Ha-RAS oncogene generates stable lines capable of induced steroidogenesis

Cellular and viral oncogenes are usually defined on the basis of their ability to elicit neoplastic transformation. However, oncogene activity has also been implicated in the control of differentiation. We have tested whether transfection of primary cultured granulosa cells with various oncogenes can yield cell lines that maintain their differentiated properties. Primary granulosa cells were prepared from diethylstilbestrol-treated immature female rats and transfected with simian virus 40 (SV40) DNA or with SV40 plus activated human Ha-RAS oncogene. Transfection with SV40 plus Ha-RAS yielded cell lines that lost response to gonadotropins but, after 48 hr of stimulation with isoproterenol, cholera toxin, forskolin, or 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP), produced progesterone at levels comparable to those of differentiated primary cells. In contrast, cells transformed only by SV40 lost their ability to produce progesterone. Whereas in primary cell cultures progesterone production was already evident after a 3-hr incubation with 1 mM 8-Br-cAMP, in cotransfected cells progesterone production became evident only after 12 hr. All cotransformed cell lines produced SV40 large tumor antigen as well as human RAS p21 protein. The expression of the expected oncogenes in the various cell lines was confirmed by mRNA analysis. These results suggest that the expression of an activated RAS oncogene in granulosa cells can play a role in preserving inducible steroidogenesis.

The mode of action of nerve growth factor in PC12 cells

This review deals with the mechanism of nerve growth factor action. In view of the many and diversified effects of this growth factor, and since it could utilize different mechanism(s) in distinct types of cells, we have confined our analysis to the best characterized and more extensively studied target, the clonal cell line PC12. When exposed to NGF in vitro, these neoplastic cells recapitulate the last major steps of neuronal differentiation, i.e., the commitment to become a neuron and the acquisition of the neuronal phenotype. This is characterized by electrically excitable neurites, a display of a highly organized cytoskeleton, and the specific chemical and molecular neuronal properties. These effects are elicited upon the interaction of NGF with a receptor whose gene has been cloned and whose kinetic properties are now relatively well characterized. It is not yet clear, on the contrary, if and which of the several potential second messengers (cAMP, Ca, or phosphoinositides) that undergo marked fluctuations following NGF binding, transduce and amplify the NGF message. Among both the early and late effects of NGF is the modulation of expression of several genes. Some of the products of these genes are mainly restricted to nerve cells and others appear to play a crucial role in regulating the proper assembly of cytoskeletal elements. It is hypothesized that this complex array of chemical, molecular, and ultrastructural changes is triggered by NGF, not through activation of a single pathway, but more likely via combinatorial processes whereby several intracellular signals interplay before the irreversible commitment of becoming a neuron is undertaken.

G-proteins, the inositol lipid signalling pathway, and secretion

The formation of the second messenger cyclic AMP (cAMP) is known to be coupled to its receptor via a guanine nucleotide regulatory protein, GS. Ca2+-mobilizing receptors stimulate the hydrolysis of phosphatidylinositol bisphosphate (PtdIns(4,5)P2), which generates two intracellular signals Ins(1,4,5)P3 and diacylglycerol. We review the evidence that this signalling system is also composed of three types of proteins: receptor, G-protein and effector. The G-protein that couples to the effector, polyphosphoinositide phosphodiesterase (PPI-PDE), is a novel G-protein, GP, which is a substrate for pertussis toxin in some cells (e.g. neutrophils and platelets) but not others (e.g. pancreatic acinar cells and GH3 cells). This implies that GP is not a single G-protein but encompasses a family of proteins that can activate PPI-PDE. We have also identified a role for another G-protein, GE, which is involved in the secretory process in mast cells and neutrophils. In this case, neither the receptor nor effector has been identified and the main evidence for proposing this second G-protein is based on the ability of guanine nucleotide analogues (e.g. GTP gamma S) to stimulate secretion independently of PPI-PDE activation.

2,3,7,8-Tetrachlorodibenzo-p-dioxin causes increases in expression of c-erb-A and levels of protein-tyrosine kinases in selected tissues of responsive mouse strains

2,3,7,8,-Tetrachlorodibenzo-p-dioxin (TCDD) administered in vivo causes drastic reduction in the weight of the mouse thymus at low doses (e.g., 30 micrograms/kg single i.p. injection), the reduction becoming statistically significant after 2 days. To understand the cause for such thymic involution TCDD-evoked changes in various biochemical parameters in this tissue were examined. The most noticeable change was observed in the increased activity of specific protein-tyrosine kinases and protein kinase C and an increased level of p21ras-associated binding of [3H]GTP. Since no significant change was observed with cAMP-stimulated protein kinases and cAMP levels, the above changes appear to be a selective effect on these special classes of proteins. As a result of a time sequence study it has become apparent that the rise in protein-tyrosine kinase activities becomes significant within 24 hr, whereas the rise in protein kinase C does not become significant until 48 hr. Among protein-tyrosine kinases, pp60c-src and probably pp561skT were found to be significantly elevated by TCDD treatment. In view of similarities between TCDD and thyroid hormones in causing thymic involution, the levels of c-erb-A expression were assessed in the liver by using avian 32P-labeled v-erb-A probe and RNA transfer blot hybridization technique. The results clearly indicate that TCDD has the property to elevate levels of mRNA bearing homology to v-erb-A. Such changes in c-erb-A expression and protein-tyrosine kinase occurred only in TCDD-susceptible (responsive) strains but not in tolerant (nonresponsive) strains of mice at the dose tested. Based on such observations a hypothesis has been proposed that TCDD owes its potency to its ability to stimulate the expression of one of a family of DNAs bearing homology to v-erb-A and that one of the major consequences of such an action is stimulation of various tyrosine kinases.

NIH-3T3 cells transformed by the EJ-ras oncogene exhibit reduced platelet-derived growth factor-mediated Ca2+ mobilization

NIH-3T3 cells transformed by the EJ-ras oncogene synthesize only 10-15% as much inositol 1,4,5-trisphosphate (InsP3) as control cells after stimulation with platelet-derived growth factor (PDGF). This is despite the fact that the basal (unstimulated) levels of InsP3 synthesized in control and EJ-ras-transformed cells are not significantly different. Using the fluorescent indicator fura-2 and digital-imaging techniques, we have visualized and quantified changes in intracellular Ca2+ concentrations in control and EJ-ras-transformed NIH-3T3 cells in response to PDGF. Within 3 min after exposure of control cells to PDGF, intracellular Ca2+ levels are increased 3- to 9-fold, paralleling the increase in InsP3. In contrast, the majority (greater than 90%) of the EJ-ras-transformed cells show no increase in Ca2+ levels after PDGF exposure and the few that did respond exhibited only a small transient increase. Pronounced differences in the intracellular localization of Ca2+ increases in control and the responding EJ-ras-transformed cells were also observed. Despite the inhibition of InsP3 synthesis and subsequent Ca2+ mobilization, the EJ-ras-transformed cells respond mitogenically to PDGF. These data do not support the hypothesis that the EJ-ras gene product (p21) stimulates a phosphatidylinositol 4,5-bisphosphate-specific phospholipase C in NIH-3T3 cells; instead they suggest that the EJ-ras p21 may uncouple the PDGF receptor from phospholipase C resulting in inhibition of PDGF-stimulated activity of phospholipase C, InsP3 synthesis, and Ca2+ mobilization.

A Harvey-ras responsive transcription element is also responsive to a tumour-promoter and to serum

The ras oncogenes are implicated in the onset of some human tumours, and in cellular proliferation and terminal differentiation. The ras proteins are plasma membrane bound transducers of signals between the outside of the cell and unknown targets in the cell. Identifying these targets and understanding how they are regulated will have a major impact on our understanding of the molecular basis of transformation. We have already shown that c-Ha-ras and the tumor promoter TPA (12-o-tetradecanoyl phorbol-13-acetate) can activate a transcriptional enhancer. We now report the identification of a short sequence in the polyoma virus (Py) enhancer which mediates Ha-ras activation, and show that this sequence (ras responsive element, RRE) also mediates activation by TPA and serum. This responsive element is a specific binding-site for the mouse transcription factor PEA1 (ref. 4 and below) and for the jun oncogene (ref. 5 and M. Karin, personal communication). These results are in keeping with a role for ras protein in signal transduction from outside the cell to a transcription factor in the nucleus, through protein kinase C. The striking similarity between RRE and DNA sequences present in the promoter regions of a number of transformation-related genes suggests that deregulated activation of RRE is a critical event in transformation.

Binding of ras p21 to bands 4.2 and 6 of human erythrocyte membranes

The direct binding protein(s) of ras p21 was (were) investigated in inside-out vesicles of human erythrocyte ghosts using the pure v-Kirsten (Ki)-ras p21 synthesized in E. coli. The bound ras p21 was detected immunochemically using an anti-v-Ki-ras p21 monoclonal antibody, ras p21 bound to vesicles. Prior digestion of the vesicles with trypsin reduced this binding significantly. When ras p21 was laid over vesicle proteins immobilized on a nitrocellulose sheet by transfer from the gel of SDS-polyacrylamide gel electrophoresis, ras p21 bound to bands 4.2 and 6. ras p21 binding to these proteins was reduced by prior incubation of ras p21 with the purified band 4.2 or 6 protein. These results indicate that v-Ki-ras p21 can bind directly to bands 4.2 and 6 of human erythrocyte membranes as far as tested in an in vitro cell-free system.

Modified responsiveness of v-Ha-ras-transfected rat fibroblasts to growth factors and a tumor promoter

The correlation of the phenotypic changes of v-Ha-ras transfected cells with the expression of p21ras and the modified responses to growth factors and a tumor promoter were examined. Transfection of the v-Ha-ras gene together with the neomycin-resistance gene into 208F rat fibroblasts yielded transformed clones characterized by morphological changes, anchorage-independent growth, and tumorigenicity in nude mice. The degrees of these biological alterations were parallel with the expression of mRNA and protein of the ras gene. In ras-transformed cells, anchorage-independent growth was stimulated by epidermal growth factor (EGF), insulin, bombesin, and fibroblast growth factor, whereas in the parental 208F cells, anchorage-independent growth was observed only in the presence of EGF, and there were many fewer EGF-induced colonies than those in the ras-transformed clones. A tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA) also augmented anchorage-independent growth of ras-transformed cells and induced morphological changes in monolayer cultures without altering the expression of the ras gene or phosphorylation of the p21ras protein. Retinoic acid inhibited the TPA-induced anchorage-independent growth. These results showed a good correlation of the expression of p21ras with the phenotypic changes and the increased sensitivity of the p21ras-expressing cells to the stimulation of growth factors and tumor promoter.