Biomedical subjects

J León

Publications and source records attributed to J León.

At least 37 records · Page 2Linked to original sources

PubMed2000

Structure-related inhibition of calmodulin-dependent neuronal nitric-oxide synthase activity by melatonin and synthetic kynurenines.

We recently described that melatonin and some kynurenines modulate the N-methyl-D-aspartate-dependent excitatory response in rat striatal neurons, an effect that could be related to their inhibition of nNOS. In this report, we studied the effect of melatonin and these kynurenines on nNOS activity in both rat striatal homogenate and purified rat brain nNOS. In homogenates of rat striatum, melatonin inhibits nNOS activity, whereas synthetic kynurenines act in a structure-related manner. Kynurenines carrying an NH(2) group in their benzenic ring (NH(2)-kynurenines) inhibit nNOS activity more strongly than melatonin itself. However, kynurenines lacking the NH(2) group or with this group blocked do not affect enzyme activity. Kinetic analysis shows that melatonin and NH(2)-kynurenines behave as noncompetitive inhibitors of nNOS. Using purified rat brain nNOS, we show that the inhibitory effect of melatonin and NH(2)-kynurenines on the enzyme activity diminishes with increasing amounts of calmodulin in the incubation medium. However, changes in other nNOS cofactors such as FAD or H(4)-biopterin, do not modify the drugs' response. These data suggest that calmodulin may be involved in the nNOS inhibition by these compounds. Studies with urea-polyacrylamide gel electrophoresis further support an interaction between melatonin and NH(2)-kynurenines, but not kynurenines lacking the NH(2) group, with Ca(2+)-calmodulin yielding Ca(2+)-calmodulin-drug complexes that prevent nNOS activation. The results show that calmodulin is a target involved in the intracellular effects of melatonin and some melatonin-related kynurenines that may account, at least in part, for the neuroprotective properties of these compounds.

Animals↗

PubMed1999

Susceptibility of HIV-1-TAT transfected cells to undergo apoptosis. Biochemical mechanisms.

The effects of HIV-1 Tat protein on mitochondria membrane permeability and apoptosis were analysed in lymphoid cells. In this report we show that stable-transfected HIV-Tat cells are primed to undergo apoptosis upon serum withdrawal. This effect was observed in both the Jhan T cell line and the K562 cells, the latter expressing the bcr-abl chimeric gene, which confers resistance to apoptosis induced by different stimuli. Using a cytofluorimetric approach we have determined that serum withdrawal induces a disruption of the transmembrane mitochondrial potential (Deltapsim) followed by an increase of reactive oxygen species (ROS) and the subsequent DNA nuclear loss in K562-Tat cells but not in the K562-pcDNA cell line. These pre-apoptotic events were associated with the cleavage of the caspase-3, while the expression of Bcl-2, Bcl-XL and Bax proteins was not affected by the presence of Tat. Regardless of the steady state of the Bax protein, we found that in both K562 and K562-Tat cells, this protein is located in the nucleus, but after serum withdrawal its localization was mainly in the cytoplasm. The activity of caspase-3 detected in K562-Tat cells after serum withdrawal paralleled with the mitochondria permeability transition. Nevertheless, in Jhan-Tat cells the inhibition of this caspase with the specific inhibitor, z-DEVD-cmk, did not affect the disruption of the mitochondria potential induced by serum withdrawal. Interestingly, we found that HIV-Tat protein accumulates at the mitochondria in the K562-Tat cells cultured under low serum conditions, and this mitochondrial localization correlated with the Deltapsim disruption detected in these cells. In addition, HIV-1 Tat protein synergies with protoporphyrin IX (PPIX), a ligand of the mitochondrial benzodiazepine receptor, in the induction of apoptosis in both Jhan and K562 cells. Thus, HIV-1 Tat protein may induce apoptosis by a mechanism that involves mitochondrial PT and may contribute to the lymphocyte depletion seen in AIDS patients.

Apoptosis↗

PubMed1999

Apoptosis and mitotic arrest are two independent effects of the protein phosphatases inhibitor okadaic acid in K562 leukemia cells.

Treatment of human myeloid leukemia K562 cells with the serine/threonine protein phosphatases inhibitor okadaic acid induced mitotic arrest followed by apoptosis in a synchronized manner. The effect was observed at drug concentrations that inhibited the protein phosphatase type 2A but not type 1. We investigated whether apoptosis was a consequence of the preceding mitosis arrest or was induced independently by okadaic acid. We found that (1) apoptosis, but not mitotic arrest, was inhibited in cells with constitutive expression of Bcl-2; (2) pretreatment of cells with the DNA synthesis inhibitor hydroxyurea blocked the mitotic arrest but not the apoptosis mediated by okadaic acid; (3) down-regulation of c-myc gene was associated with apoptosis, but not with mitotic arrest; and (4) inhibition of protein synthesis abrogated mitotic arrest, but not apoptosis. The results suggest that inhibition of protein phosphatase 2A by okadaic acid provokes mitotic arrest and apoptosis of leukemia cells by independent mechanisms.

Apoptosis↗

PubMed1999

Differential expression and phosphorylation of CTCF, a c-myc transcriptional regulator, during differentiation of human myeloid cells.

CTCF is a transcriptional repressor of the c-myc gene. Although CTCF has been characterized in some detail, there is very little information about the regulation of CTCF activity. Therefore we investigated CTCF expression and phosphorylation during induced differentiation of human myeloid leukemia cells. We found that: (i) both CTCF mRNA and protein are down-regulated during terminal differentiation in most cell lines tested; (ii) CTCF down-regulation is retarded and less pronounced than that of c-myc; (iii) CTCF protein is differentially phosphorylated and the phosphorylation profiles depend on the differentiation pathway. We concluded that CTCF expression and activity is controlled at transcriptional and post-transcriptional levels.

Blotting, Northern↗

PubMed1999

Antisense-mediated depletion of a potato lipoxygenase reduces wound induction of proteinase inhibitors and increases weight gain of insect pests.

De novo jasmonic acid (JA) synthesis is required for wound-induced expression of proteinase inhibitors and other defense genes in potato and tomato. The first step in JA biosynthesis involves lipoxygenase (LOX) introducing molecular oxygen at the C-13 position of linolenic acid. We previously have shown that, in potato, at least two gene families code for 13-LOX proteins. We have now produced transgenic potato plants devoid of one specific 13-LOX isoform (LOX-H3) through antisense-mediated depletion of its mRNA. LOX-H3 depletion largely abolishes accumulation of proteinase inhibitors on wounding, indicating that this specific LOX plays an instrumental role in the regulation of wound-induced gene expression. As a consequence, weight gain of Colorado potato beetles fed on antisense plants is significantly larger than those fed on wild-type plants. The poorer performance of LOX-H3-deficient plants toward herbivory is more evident with a polyphagous insect; larvae of beet armyworm reared on the antisense lines have up to 57% higher weight than those fed on nontransformed plants. LOX-H3 thus appears to regulate gene activation in response to pest attack, and this inducible response is likely to be a major determinant for reducing performance of nonspecialized herbivores. However, the regulatory role of LOX-H3 is not caused by its involvement in the wound-induced increase of JA, as wild-type and LOX-H3 deficient plants have similar jasmonate levels after wounding. LOX-H3-deficient plants have higher tuber yields. The apparent effect of suppressing the inducible defensive response on plant vigor suggests that it may pose a penalty in plant fitness under nonstress situations.

Agrobacterium tumefaciens↗

PubMed1999

Thymic carcinoid and parathyroid hyperplasia detection with 99mTc-MIBI men type 1.

We report a case of a 35-year-old male, with a history of diarrhea, renal lithiasis with frequent expulsions of calculus and hypercalcemia during the last 2 years. The patient was studied and diagnosed with a multiple endocrine neoplasia type I (MEN I), familiar (mother with MEN I). A scintigraphic study with 99mTc-MIBI was performed in order to localize hyperfunctioning parathyroid glands because of biochemical diagnosis of primary hyperparathyroidism. Double phase 99mTc-MIBI scan detected one hyperfunctioning parathyroid gland and a large anterior mediastinal mass. Subsequent, plain radiograph and CT of the chest showed a soft-tissue mass in that localization. Punch biopsy of the lesion guided by CT revealed malignant cells of neuroendocrine tumor. The tumor was removed and histologically confirmed as a carcinoid within a thymus in a MEN type I syndrome. MEN I patients can benefit from the examination with this agent which can potentially localize not only parathyroid endocrine pathology but also unknown associated tumors.

Adult↗

PubMed1999

Cross-talk between wound signalling pathways determines local versus systemic gene expression in Arabidopsis thaliana.

Plants react to mechanical damage by activating a set of genes, the products of which are thought to serve defensive functions. In solanaceous plants, cell wall-derived oligosaccharides and the plant hormones jasmonic acid and ethylene participate in the signalling network for wound-induced expression of proteinase inhibitors and other defence-related genes, both in the locally damaged and in the systemic non-damaged leaves. Here we show that in Arabidopsis thaliana, these signalling components interact in novel ways to activate distinct responses. In damaged tissues, oligosaccharides induce the expression of a specific set of wound-responsive genes while repressing jasmonic acid-responsive genes that are activated in the systemic tissues. The oligosaccharide-mediated repression of the jasmonic acid-dependent signalling pathway is exerted through the production and perception of ethylene in the locally damaged tissue. This cross-talk between separate wound signalling pathways thus allows the set up of different responses in the damaged and the systemic tissues of plants reacting to injury.

Journal Article↗

PubMed1999

Antisense-mediated depletion of potato leaf omega3 fatty acid desaturase lowers linolenic acid content and reduces gene activation in response to wounding.

Fatty acid omega3 desaturases act on membrane lipids to catalyse the formation of trienoic fatty acids, the most abundant in plant tissues being alpha-linolenic acid. This fatty acid is a precursor of jasmonic acid, a plant growth regulator involved in the control of wound-induced gene activation in plants and in the induction of tuberization in potato. We isolated a potato omega3 desaturase cDNA, possibly encoding a plastidial isoform, and used it to investigate its expression pattern throughout plant development and in response to wounding. Plastidial omega3 desaturase gene transcripts accumulate rapidly upon wounding, preceding the jasmonate-dependent induction of the wound-responsive proteinase inhibitor II gene. We generated transgenic potato plants constitutively expressing an antisense RNA to this plastidial omega3 desaturase. Selected transgenic lines in which the cognate omega3 desaturase mRNA is largely depleted show a marked reduction, of up to 60%, in trienoic acids in leaves and tubers. In these lines, a corresponding reduction in jasmonate content and proteinase inhibitor II expression is observed upon wounding. Our results indicate that a reduction in omega3 desaturase mRNA levels compromises the wound-induced activation of proteinase inhibitor II, suggesting that wound-induced synthesis of linolenic acid is required for jasmonic acid production. The antisense-mediated depletion of fatty acid omega3 desaturases is a viable alternative for reducing trienoic fatty acid content in plant species in which a mutant screening approach is not applicable.

Amino Acid Sequence↗

PubMed1999

Regulation of c-Myc and Max in megakaryocytic and monocytic-macrophagic differentiation of K562 cells induced by protein kinase C modifiers: c-Myc is down-regulated but does not inhibit differentiation.

We have studied the regulation and role of c-Myc and Max in the differentiation pathways induced in K562 cells by 12-O-tetradecanoyl phorbol-13 acetate (TPA) and staurosporine, an activator and inhibitor, respectively, of protein kinase C (PKC). We found that staurosporine induced megakaryocytic differentiation, as revealed by the cellular ultrastructure, platelet formation, and DNA endoreduplication. In contrast, TPA induced a differentiated phenotype that more closely resembled that of the monocyte-macrophage lineage. c-myc expression was down-regulated in K562 differentiated by both TPA and staurosporine, whereas max expression did not change in either case. Although PKC enzymatic activity was low in cells terminally differentiated with TPA and staurosporine, inhibition of PKC activity by itself did not induce c-myc down-regulation. We conclude that the c-myc gene is switched off as a consequence of the differentiation process triggered by these drugs in a manner independent from PKC. Ectopic overexpression of c-Myc in K562 cells did not affect the monocytic-macrophagic and megakaryocytic differentiation, indicating that c-Myc suppression is not required for these processes in K562. Similarly, both differentiation pathways were not affected by Max overexpression or by concomitant overexpression of c-Myc and Max. This result is in contrast with the inhibition of erythroid differentiation of K562 exerted by c-Myc, suggesting divergent roles for c-Myc/Max, depending on the differentiation pathway.

Basic Helix-Loop-Helix Leucine Zipper Transcriptio↗

PubMed1998

Spi-1/PU.1 proto-oncogene induces opposite effects on monocytic and erythroid differentiation of K562 cells.

Spi-1/PU.1 is a hematopoietic transcription factor of the Ets family. To analyze the effects of ectopic expression of spi-1 on the proliferation/differentiation of human myeloid leukemia cells, K562 cells were stably transfected with a spi-1 expression vector. The transfected cell lines expressed elevated levels of spi-1 mRNA and protein and high Spi-1-DNA binding activity. The spi-1 transfected cells showed reduced growth rates and reduced clonogenic cell growth. When the erythroid and monocytic differentiation markers were analyzed, spi-1 overexpression resulted in opposite effects: erythroid differentiation was significantly inhibited in spi-1 transfectants, while spi-1 overexpression increased the monocytic differentiation of cells. These results indicate a differential role of Spi-1 on the differentiation of human myeloid leukemia cells.

Antigens, CD↗

PubMed1998

Rho-regulated signals induce apoptosis in vitro and in vivo by a p53-independent, but Bcl2 dependent pathway.

Rho proteins are a branch of GTPases that belongs to the Ras superfamily which are critical elements of signal transduction pathways leading to a variety of cellular responses. This family of small GTPases has been involved in diverse biological functions such as cytoskeleton organization, cell growth and transformation, cell motility, migration, metastasis, and responses to stress. We report that several human Rho proteins including Rho A, Rho C and Rac 1, are capable of inducing apoptosis in different cell systems like murine NIH3T3 fibroblasts and the human erythroleukemia K562 cell line. Since K562 cells are devoid of p53, apoptosis induced by Rho in this system is independent of p53. Rho-dependent apoptosis is mediated by the generation of ceramides, and it is drastically inhibited by ectopic expression of Bcl2, both under in vitro and in vivo conditions. Furthermore, the human oncogenes vav and ost that have been shown to function as guanine exchange factors for Rho proteins, were also able to induce apoptosis under similar conditions. Finally, we also report that the levels of endogenous Rho proteins are increased when U937 myeloid leukemia cells are exposed to apoptosis-inducing conditions such as TNF alpha treatment. Furthermore, TNF alpha-induced apoptosis in these cells is inhibited by expression of a dominant negative mutant of Rac 1 but it is not affected by a similar mutant of Rho A. These results suggest that Rho proteins play an important role in the physiological regulation of the apoptotic response to stress-inducing agents.

3T3 Cells↗

PubMed1998

Modulation of rat striatal glutamatergic response in search for new neuroprotective agents: evaluation of melatonin and some kynurenine derivatives.

Melatonin attenuates the excitatory response of striatal neurons to sensorimotor cortex (SMCx) stimulation, which may be the basis for its neuroprotective role. Searching for new compounds with melatonin-like properties, the effects of several kynurenine derivatives in the response of the rat striatum to SMCx stimulation were studied using electrophysiological and microiontophoretical techniques. Melatonin iontophoresis (-100 nA) significantly attenuated the striatal excitatory response in 89.4% of the recorded neurons, showing excitatory properties in the other 10.6%. Compound A [2-acetamide-4-(3-methoxyphenyl)-4-oxobutyric acid] (-100 nA) displayed similar attenuating effects (86.7% of neurons inhibited vs. 13.3% excited). Compound B [2-acetamide-4-(2-amine-5-methoxyphenyl)-4-oxobutyric acid] (-100 nA) was more potent than melatonin itself to attenuate the excitatory response in 100% of the recorded neurons. Compound C [2-butyramide-4-(3-methoxyphenyl)-4-oxobutyric acid] (-100 nA) significantly increased the excitatory response in 84.2% of the recorded neurons, showing attenuating effects on the other 15.8% of the neurons. Interestingly, compound C iontophoresis excited the neurons in which melatonin had attenuating properties, whereas it inhibited the neurons showing excitatory responses to melatonin. These data suggest melatonin inverse agonist properties for compound C. Also, the effects of compounds B and C appeared immediately after they were iontophoretized, whereas both melatonin and compound A onset latencies were longer (2-4 min). The lack of latency shown by these melatonin analogs points to the possibility that melatonin itself was metabolized before producing its effects on striatal neurons. The results show a family of structurally-related melatonin analogs that may open new perspectives in search for new neuroprotective agents, including its clinical potentiality.

Animals↗

PubMed1998

Jasmonic acid-dependent and -independent wound signal transduction pathways are differentially regulated by Ca2+/calmodulin in Arabidopsis thaliana.

We have used wound- and jasmonic acid (JA)-responsive genes as molecular markers to elucidate the pathway(s) of wound signal transduction in Arabidopsis thaliana. The JA-responsive (JR) genes JR1, JR2, and JR3 are strongly induced by wounding and by JA, while the wound-responsive (WR) genes WR3 and acyl CoA oxidase (ACO) are induced by wounding only. Accumulation of JR transcripts upon wounding was blocked by indomethacin. However, indomethacin did not affect either induction of these genes by JA or wound-induced expression of WR genes, suggesting that JA synthesis is only needed for wound-dependent induction of JR genes, and also that separate JA-dependent and -independent wound signal transduction pathways exist in Arabidopsis. The two pathways are differentially regulated by Ca2+ and calmodulin. Mobilization of intracellular Ca2+ pools blocked induction of JR genes by both wounding and JA, but not the induction of WR genes by wounding, but this effect could not be reproduced by increasing intracellular Ca2+ levels using ionophores. In addition, calmodulin antagonists blocked the expression of JR genes and up-regulated WR gene expression. Ca2+ and calmodulin seem to act downstream of both JA and the COI1 gene in the JA-dependent pathway, and downstream of reversible phosphorylation events that differentially regulate JA-dependent and JA-independent wound signal transduction pathways.

Arabidopsis↗

PubMed1998

Modification of nitric oxide synthase activity and neuronal response in rat striatum by melatonin and kynurenine derivatives.

Tryptophan is mainly metabolized in the brain through methoxyindole and kynurenine pathways. The methoxyindole pathway produces (among other compounds) melatonin, which displays inhibitory effects on human and animal central nervous systems, including a significant attenuation of excitatory, glutamate-mediated responses. The kynurenine pathway produces kynurenines that interact with brain glutamate-mediated responses. Nitric oxide (NO) increases glutamate release, and melatonin and kynurenines may act via modification of NO synthesis. In the present study, the effects of melatonin and four synthetic kynurenines were studied on the activity of rat striatal nitric oxide synthase (NOS) and on the response of rat striatal neurons to sensorimotor cortex (SMCx) stimulation, a glutamate-mediated response. Melatonin inhibited both NOS activity and the striatal glutamate response, and these effects were dose-related. Compound A (2-acetamide-4-(3-methoxyphenyl)-4-oxobutyric acid) did not inhibit NOS activity but inhibited the striatal response similarly to melatonin. Compound B (2-acetamide-4-(2-amino-5-methoxyphenyl)-4-oxobutyric acid) was more potent than melatonin in inhibiting both NOS activity and the striatal response. Compound C (2-butyramide-4-(3-methoxyphenyl)-4-oxobutyric acid) did not change NOS activity, but increased the striatal response. Compound D (2-butyramide-4-(2-amino-5-methoxyphenyl)-4-oxobutyric acid) showed potent inhibitory effects on both NOS activity and striatal glutamate-mediated response. A structure-related effect of the kynurenine derivatives was observed, and those with an amino group in position 2 of the benzenic ring had more potent effects than melatonin itself in inhibiting striatal NOS activity and the response of striatal neurons to SMCx.

Animals↗

PubMed1998

Reversible protein phosphorylation regulates jasmonic acid-dependent and -independent wound signal transduction pathways in Arabidopsis thaliana.

Plants responses to mechanical injury are complex and include the induced expression of defence-related genes. The phytohormone JA has been reported to mediate some of these responses. To elucidate further the signal transduction processes involved, the action of specific agonists and antagonists of known signalling effectors on the response of Arabidopsis thaliana plantlets to JA and wounding was investigated. The identification and characterization of a reversible protein phosphorylation step in a transduction pathway leading to JA-induced gene transcription is reported. This phosphorylation event involved the opposing activities of a staurosporine-sensitive protein kinase, negatively regulating the pathway, and a protein phosphatase, most probably of type 2 A, which activated JA-responsive gene expression. JA activation via this pathway was blocked in the A. thaliana JA-insensitive mutants jin1, jin4 and coi1, and by exogenous application of cycloheximide or auxins. Wound-induced activation of JA-responsive genes was also regulated by this protein phosphorylation step. An alternative wound signalling pathway, independent of JA, was also identified, leading to the transcriptional activation of a different set of genes. This JA-independent pathway was also regulated by a protein phosphorylation switch, in which the protein kinase positively regulated the pathway while the protein phosphatase negatively regulated it. Moreover, a labile protein apparently repressed the expression of these genes. One of the genes analysed, JR3, had a complex pattern of expression, possibly because it was regulated via both of the wound signalling pathways identified. According to the function of an homologous gene, JR3 may be involved in feedback inhibition of the JA response.

Arabidopsis↗

PubMed1998

Melatonin interaction with magnesium and zinc in the response of the striatum to sensorimotor cortical stimulation in the rat.

The sensorimotor cortex (SMCx) sends numerous projections to the striatum. These projections are excitatory and glutamate mediated. Glutamatergic receptors, specifically those of NMDA type-receptors, are closely related to excitotoxicity. Thus, in some circumstances, an excess of Ca2+ influx through NMDA channels alters neuronal metabolism and may become lethal for the cell. Two other divalent cations, Mg2+ and Zn2+, have inhibitory effects on NMDA receptors. Magnesium ions exert a voltage-dependent block of the NMDA calcium channel, whereas zinc ions exert a voltage-independent NMDA block. In the present work, the effects of iontophoresis of Mg2+ and Zn2+ on the striatal response to SMCx stimulation were studied. Moreover melatonin, an indoleamine with anticonvulsant properties and inhibitory effects on the NMDA receptor, was also iontophorized alone or in combination with Mg2+ and Zn2+. Single pulse electrical stimulation of SMCx produced an excitatory response in the striatum. Iontophoresis of melatonin, Mg2+ and Zn2+ produced a potent attenuation of the excitatory response of the striatum to SMCx stimulation, although the latency of the effect of melatonin was longer than those of Mg2+ and Zn2+. When these cations were simultaneously ejected with melatonin, additive inhibitory effects were recorded. These observations suggest that the inhibitory effects produced by Mg2+ and Zn2+ and melatonin are produced via different processes, and thus the inhibitory role of melatonin on the NMDA receptor activity is exclusive of a direct action on the NMDA calcium channel.

Animals↗

PubMed1997

Interferon induces up-regulation of Spi-1/PU.1 in human leukemia K562 cells.

The human K562 cell line is derived from a chronic myelogenous leukemia in blastic crisis. Treatment of K562 cells with interferons alpha, beta or gamma resulted in inhibition of cell proliferation. Spi-1/PU.1 is a transcription factor of the Ets family which is required for normal hematopoyesis. We have found that spi-1 mRNA and protein as well as Spi-1-DNA binding activity increase after exposure of K562 cells to interferons. The increase in spi-1 expression ranged from 4- to 8-fold with the different interferons. K562 cells can be differentiated in vitro towards erythroid cells or monocyte-macrophage cells. Interestingly, the regulation of spi-1 by interferon-alpha depended on the differentiated phenotype of K562 cells: interferon-alpha failed to induce spi-1 in erythroid differentiated cells, whereas it induced spi-1 in monocyte-macrophage differentiated cells. The results suggest a role for Spi-1 in the cytostatic response to interferons.

Animals↗

PubMed1997

Positive autoregulation of ras genes expression in fibroblasts.

We have studied the effect of ectopic overexpression of a ras gene on the expression of the other two members of the ras gene family. We obtained NIH3T3 cell lines stably transfected with inducible H-ras and N-ras oncogenes. The expression of these genes is driven by a glucocorticoid-responsive promoter and the addition of dexamethasone resulted in a dramatic induction (10-20-fold) of H- or N-ras mRNA, peaking 4 h after hormone addition. The induction of the expression of ras oncogenes resulted in a transformed phenotype. In quiescent NIH3T3 cells transfected with inducible H-ras oncogenes, the induction of H-Ras was followed 12 h later by a 3-fold increase in the mRNA expression of endogenous K-ras and N-ras. Similarly, in NIH3T3 transfected with inducible N-ras oncogene, the induction of N-ras was followed by an increase in the expression of endogenous K- and H-ras genes. Interestingly, the effect was not limited to the mutated N-ras, as a similar result was obtained in cells transfected with N-ras proto-oncogene. The induction of ras genes expression was not linked to cell cycle progression as it was reproduced in cells arrested in S-phase by pretreatment with hydroxyurea. These results suggest the presence of a positive cross-regulation in the expression among the members of the Ras family. This effect could play a role in Ras-mediated carcinogenesis.

3T3 Cells↗