P21 v-ras inhibits induction of c-myc and c-fos expression by platelet-derived growth factor

Oncogenic ras mediates apoptosis in response to protein kinase C inhibition through the generation of reactive oxygen species

Ras is a well established modulator of apoptosis. Suppression of protein kinase C (PKC) activity can selectively induce apoptosis in cells expressing a constitutively activated Ras protein. We wished to determine whether reactive oxygen species serve as an effector of Ras-mediated apoptosis. Ras-transformed NIH/3T3 cells contained higher basal levels of intracellular H(2)O(2) compared with normal NIH/3T3 cells, and PKC inhibition up-regulated ROS to 5-fold greater levels in Ras-transformed cells than in normal cells. Treatment with N-acetyl-l-cysteine reduced both the basal and inducible levels of intracellular H(2)O(2) in NIH/3T3-Ras cells and antagonized the induction of apoptosis by PKC inhibition. Culturing NIH/3T3-Ras cells in low oxygen conditions, which prevents ROS generation, also inhibited the apoptotic response to PKC inhibition. These results suggest that reactive oxygen species are necessary as downstream effectors of the Ras-mediated apoptotic response to PKC inhibition. However, the generation of ROS alone is not sufficient to induce apoptosis in Ras-transformed cells because inhibition of cell cycle progression prevented the induction of apoptosis in NIH/3T3-Ras cells without inhibiting the generation of intracellular H(2)O(2) observed after PKC inhibition. These findings suggest that continued cell cycle progression of Ras-transformed cells during PKC inhibition is also necessary for the induction of apoptosis.

Attenuated expression of the serum responsive T1 gene in ras transformed fibroblasts due to the inhibition of c-fos gene activity

The T1 gene encodes a protein, which shares homology with the IL-1 receptors. In fibroblasts, T1 is induced by growth factors and in response to the onset of oncogene expression. The c-fos gene is transiently activated in these situations and was shown to be the major mediator of T1 gene induction. In contrast, the sustained expression of a ras oncogene in NIH3T3 cells resulted in the downregulation of basal T1 gene activity and the attenuation of T1 gene induction in response to mitogenic signals. Likewise, the immediate early genes encoding c-Fos, FosB, and Fra-2 are repressed in these cells. T1 gene repression could be overcome by the forced expression of c-fos in ras transformed fibroblasts. Thus, the lack of c-fos gene expression is the likely cause for ras mediated T1 gene repression. Fra-1, in contrast to the other three members of the Fos family, is permanently synthesized in high amounts in ras transformed NIH3T3 fibroblasts. We show that AP-1, which is abundant in these cells throughout the whole cell cycle, consists predominantly of Fra-1/c-Jun and Fra1/JunD heterodimers. We provide evidence that Fra1/c-Jun heterodimers are responsible for the repression of c-fos gene induction following serum stimulation. The introduction of a dominant negative version of c-Jun into ras transformed fibroblasts was able to rescue c-fos gene induction in response to serum stimulation, further demonstrating that AP-1 is indeed involved in c-fos gene repression. We conclude that oncogenic ras mediates the activation of the fra-1 gene which results in elevated AP-1 activity throughout the cell cycle. Fra-1 containing AP-1 complexes repress the c-fos and possibly other immediate early genes thereby preventing the induction of certain delayed early genes such as the T1 gene in response to mitogenic stimulation.

PKR, apoptosis and cancer

The double-stranded (ds) RNA-regulated serine/threonine protein kinase, PKR, is an interferon-inducible enzyme of widespread occurrence in mammalian cells. PKR is activated by dsRNA via a mechanism involving autophosphorylation. Once activated, the enzyme phosphorylates the alpha-subunit of protein synthesis initiation factor eIF2, thereby inhibiting translation. Accumulating data suggest that PKR has additional substrates, and that the kinase may also regulate gene transcription and signal transduction pathways. Although PKR plays an important role in mediating the antiviral effects of interferons, PKR is also implicated in regulating cell proliferation in uninfected cells and may have a tumor suppressor function under normal conditions. Studies of human malignancies and tumor cell lines suggest that, in general, patients bearing tumors with a higher PKR content have a more favorable prognosis. However, in human breast carcinoma cells, dysregulation of PKR may be associated with the establishment or maintenance of the transformed state.

Ectopic Expression of the Erythropoietin Receptor in a Murine Interleukin-6–Dependent Plasmacytoma Cell Line (TEPC-2027) Confers Proliferative Responsiveness to Erythropoietin

To compare the signal transduction pathways used by erythropoietin (Epo) and interleukin-6 (IL-6), the cDNA for the murine Epo receptor (Epo-R) was introduced into an IL-6–responsive plasmacytoma cell line (TEPC-2027) by retrovirally mediated gene transfer. G418-resistant clones were amplified in IL-6 and studied for their ability to grow and differentiate in response to Epo. Epo-R synthesized from the viral gene showed the same affinity for Epo as did the receptor on erythroid cells; however, the numbers of Epo receptors expressed on the cell membrane varied among clones. After a delay of 3 to 5 days in the presence of Epo, all the clones studied proliferated as well in response to Epo as in response to IL-6. In response to IL-6, Stat3 was activated and JunB mRNA was accumulated, whereas in response to Epo, Jak2 and Stat5 were activated and JunB mRNA was not accumulated in Epo-R–expressing TEPC (Epo-R/TEPC) cells. These results suggest that Epo and IL-6 transduced their proliferative signals through different pathways. Further studies showed that, in Epo-R/TEPC cells, Epo neither induces the synthesis of erythroid-specific mRNA nor modifies the synthesis of γ1 Ig heavy chain, suggesting that ectopic expression of the Epo-R in plasmacytoma cells does not modify their differentiative potential. The data show that Epo induces a proliferative response without differentiation providing a new cellular model for evaluating molecular events specific for proliferation.

Sodium butyrate inhibits platelet-derived growth factor-induced proliferation of vascular smooth muscle cells

Sodium butyrate (SB), a naturally occurring short-chain fatty acid, was investigated for its therapeutic value as an antiproliferative agent for vascular smooth muscle cells (SMCs). At 5-mmol/L concentration, SB had no significant effect on rat SMC proliferation. However, at the same concentration, SB inhibited platelet-derived growth factor (PDGF)-AA-, -AB-, and -BB-induced proliferation of SMCs. Exposure of SMCs to PDGF-BB resulted in activation of receptor intrinsic tyrosine kinase activity and autophosphorylation of beta-PDGF-receptor (beta-PDGFR). The activated beta-PDGFR physically associated and phosphorylated signaling molecules such as ras-GTPase activating protein (GAP) and phospholipase C gamma (PLC gamma). SB, in the absence of PDGF-BB, caused neither beta-PDGFR tyrosine phosphorylation nor phosphorylation and association of GAP and PLC gamma with beta-PDGFR. PDGF-BB-enhanced activation of receptor intrinsic tyrosine kinase activity and autophosphorylation of tyrosine residues of beta-PDGFR were unaffected by SB irrespective of whether SMCs were preincubated with SB before exposure to PDGF-BB plus SB or incubated concomitantly with PDGF-BB plus SB. Likewise, phosphorylation and association of GAP and PLC gamma with PDGF-BB-activated beta-PDGFR were unaffected. In addition, SB did not block PDGF-BB-stimulated, PLC gamma-mediated production of inositol triphosphate. Similarly, PDGF-BB-induced beta-PDGFR degradation was unaffected when SMCs were exposed to PDGF-BB plus SB, and SB by itself had no influence on beta-PDGFR degradation. Unlike beta-PDGFR kinase activity, mitogen-activated protein kinase (MAP-kinase) activity was stimulated by SB by about 2.7-fold. Exposure of SMCs to PDGF-BB caused an approximately 11.4-fold increase in MAP-kinase activity and this increase in activity was not significantly affected when cells were coincubated with PDGF-BB and SB (10.3-fold). However, pretreatment of SMCs with SB for 30 minutes and subsequent incubation in PDGF-BB plus SB abolished most of the PDGF-BB-induced MAP-kinase activity (4.6-fold). Transcription of growth response genes such as c-fos, c-jun, and c-myc were induced by PDGF-BB, and their induction was suppressed, particularly c-myc, by incubating SMCs with PDGF-BB plus SB. Similarly, preincubation of cells with SB for 30 minutes and subsequent incubation in PDGF-BB plus SB diminished PDGF-BB-induced transcription of c-fos, c-jun, and c-myc. However, SB by itself had no significant effect on c-fos, c-jun, and c-myc transcription.(ABSTRACT TRUNCATED AT 400 WORDS)

A small GTP-binding protein, Rho, associates with the platelet-derived growth factor type-beta receptor upon ligand binding

Ligand binding to the platelet-derived growth factor (PDGF) receptor initiates a complex and diverging cascade of signaling pathways. GTP-binding proteins with intrinsic GTPase activity (G-proteins) frequently link cell surface receptors to intracellular signaling pathways, but no close associations of the PDGF receptor and any small G-proteins, nor any such associations activated by ligand binding to the receptor have been previously reported. We demonstrate that a small GTP-binding protein binds specifically to the murine and human PDGF type-beta receptor. In response to PDGF-BB stimulation, there is an increase in the amount of labeled small G-protein associated with the PDGF type-beta receptor. The GTP-binding protein did not undergo ligand-induced association with a mutant receptor protein that was unable to bind ATP. Proteolytic cleavage analysis, together with two-dimensional separation techniques, identified the small G-protein specifically associating with the PDGF type-beta receptor after ligand binding as a member of the Rho family. This was confirmed by demonstration that the small G-protein coimmunoprecipitated by the anti-PDGF receptor antibody was a substrate for the ADP-ribosyltransferase C3 exoenzyme. Thus, the PDGF type-beta receptor may form a complex with one or more small G-proteins upon binding PDGF-BB, and the Rho small G-protein is likely to be an important component of the proteins making up the multimeric signaling complex of the PDGF type-beta receptor.

PKR: a new name and new roles

The double-stranded RNA (dsRNA)-activated protein kinase, now called PKR, was first discovered by virtue of its ability to phosphorylate translation initiation factor eIF-2 and inhibit its activity. Recent studies have shown that expression of inactive mutants of PKR in cultured cells causes them to acquire characteristics typical of transformed cells. These and other findings indicate that PKR plays a role in the normal control of cell growth and differentiation. It seems likely that, in addition to eIF-2, PKR has other substrates including the protein I-kappa B, which regulates the transcription of certain genes. Indeed, it now seems likely that PKR mediates the regulation of selected genes by dsRNA.

Reduction in platelet-derived growth factor receptor mRNA in v-src-transformed fibroblasts

The status of the platelet-derived growth factor (PDGF) receptor in normal rat kidney (NRK) fibroblasts and in NRK fibroblasts transformed by the v-src oncogene or the polyoma middle T (pmt) antigen has been compared. v-src-NRK cells have 7-fold fewer surface binding sites for PDGF than NRK cells, but the affinity of the residual receptors for PDGF is reduced only 2-fold. Levels of the PDGF receptor measured by Western blotting or in an autophosphorylation assay in vitro are 8- and 4-fold lower respectively in v-src-NRK cells than in NRK cells. No PDGF-induced phosphorylation of the PDGF receptor is apparent after 32P-labelling of intact v-src-NRK cells, implying that the reduction in PDGF receptor levels is not a consequence of production of autocrine PDGF. A 10-fold reduction in the amount of mRNA for the PDGF receptor is also observed in v-src-NRK cells. No decrease in PDGF receptor protein or mRNA levels is observed in pmt-NRK cells. We conclude that levels of the PDGF receptor in v-src-transformed NRK fibroblasts are modulated by reduction in the level of PDGF receptor mRNA.

The effects of N-ras oncogene expression on PDGF-BB stimulated responses in cultured mouse myoblasts

The role of the ras oncogene in the signalling pathway triggered by platelet-derived growth factor BB (PDGF-BB) has been investigated in a cell line which normally differentiates into myotubes. Following the activation of the N-ras oncogene, however, the cells proliferate and form foci. PDGF-BB stimulated the phosphorylation of tyrosine in several cellular proteins of molecular weight 185, 160, 94, 54, 44, 42 kDa and furthermore Ca2+ was released from internal stores. Activation of the N-ras gene by treatment of cells with dexamethasone (DEX) inhibited these responses to PDGF-BB. On the other hand, both ras-induced and -non induced cells responded to bradykinin (BK), foetal calf serum (FCS) and ionomycin (ION) by releasing Ca2+ from intracellular stores. The inhibition of the response to PDGF-BB in ras-activated cells has been further investigated. The binding of [125I]-PDGF-BB to its receptors was low and western blotting showed a low level of PDGF-BB receptor protein. This was in marked contrast to the receptor number seen in cells grown in growth medium or fusion promoting medium. These results indicate that cells transformed with the N-ras oncogene fail to respond to platelet-derived growth factor and exhibit a very low level of PDGF receptors. This suggests a role for the ras oncogene in the earliest steps of the signalling pathway.

Platelet-derived growth factor signal transduction through the interferon-inducible kinase PKR. Immediate early gene induction

The interferon-inducible, double-stranded RNA (dsRNA)-dependent eukaryotic initiation factor-2 alpha kinase PKR has primarily been characterized as a component of the interferon-mediated cellular antiviral response. Several lines of evidence now exist that suggest that PKR plays a role in the regulation of growth in uninfected cells. The most direct examples are the finding of an oncogenic variant of PKR and the effects of activators and inhibitors of PKR phosphorylation on the expression of platelet-derived growth factor (PDGF)-inducible genes. Previous reports have shown that 1) dsRNA, a direct activator of PKR, induces the genes c-myc, c-fos, and JE; 2) 2-aminopurine, a chemical inhibitor of PKR, blocks the induction of these genes by serum; and 3) activated p21ras induces a cellular inhibitor of PKR. We report here that activation of PKR was correlated with the induction of the immediate early genes c-fos, c-myc, and JE by PDGF in the following situations: 1) PDGF induction of these genes, also inducible by dsRNA, was blocked by two inhibitors of PKR activation: 2-aminopurine and v-ras; 2) PDGF induction of another immediate early gene, egr-1, which could not be induced by dsRNA, was not blocked by 2-aminopurine or v-ras; 3) agents that reverse v-ras inhibition of PKR activation also reversed the v-ras block of PDGF induction of c-myc, c-fos, and JE; 4) down-regulation of PKR protein levels by antisense inhibition of translation blocked the induction of c-myc, c-fos, and JE by PDGF, but had no effect on egr-1 induction; and finally, 5) PKR was autophosphorylated in vivo in response to PDGF. These results provide direct evidence that PKR activation functions as a second messenger in a growth factor signal transduction pathway. Thus, PKR may serve as a common mediator of growth-promoting and growth inhibitory signals.

c-Ha-rasEJ transfection of rat aortic smooth muscle cells induces epidermal growth factor responsiveness and characteristics of a malignant phenotype

Although the role of several protooncogenes, including sis, myc, and myb in the regulation of growth and differentiation of vascular cells has been examined in some detail, limited information is available on the contribution of ras genes to these processes. In the present studies the influence of oncogenic ras transfection on the phenotypic expression of rat aortic smooth muscle cells (SMCs) was examined. Cultured rat aortic SMCs during early passage (P4) were transfected by lipofection with c-Ha-rasEJ in a pSV2 neo vector or with pSV2 neo vector alone. Stable transfectants were selected in G418 over a 6-week period. Oncogene-transfected cells (ras-LF-1) exhibited differences in morphology and growth pattern relative to vector controls (neo-LF-1), or naive SMCs, including the development of prominent processes and the appearance of focal cellular arrangements giving rise to latticelike structures. Southern analysis revealed multiple integration of oncogenic ras in ras LF-1 cells. Transfection of c-Ha-rasEJ was associated with a twofold increase in p21 levels relative to pSV2 vector controls demonstrating that exogenous ras was expressed in these cells. Overexpression of ras p21 afforded SMCs a lower serum requirement for growth compared to vector controls, anchorage independent growth on soft agar, and acquisition of epidermal growth factor (EGF) responsiveness. Stimulation of serum-deprived SMCs with 5% fetal bovine serum (FBS) increased steady-state levels of c-Ha-ras mRNA in both ras-LF-1 and neo-LF-1 but ras induction was more pronounced in ras-transfected cells. alpha-smooth muscle (SM) actin gene expression was markedly reduced in ras-transfected cells relative to vector controls. These results show that transfection of c-Ha-rasEJ into aortic SMCs induces an altered phenotypic state characterized by alterations in growth factor-related signal transduction and tumorigenic potential.

Mediation of suppression of c-fos transcription in rasT24-transformed rat cells by a cis-acting repressor element

Prolonged expression of activated ras mutants resulted in both neoplastic transformation and suppression of serum-induced c-fos expression in Rat1 fibroblasts. Expression of other serum-inducible genes, including c-jun and beta-actin, was not suppressed in ras-transformed Rat1 cells, indicating that these effects are specific for c-fos and that growth-factor signal transduction pathways remain essentially intact. Run-on transcription studies indicated that c-fos transcription was blocked at the level of initiation in these cells. Transient transfection studies using 360 bp from the wild-type c-fos promoter as well as a series of mutated c-fos promoter fragments linked to the chloramphenicol acetyltransferase gene indicated that repression of c-fos was mediated by approximately 49 bp immediately upstream of the dyad symmetry element (DSE). Deletion of this region, referred to as the upstream repressor region (URR), restored serum inducibility to the c-fos promoter in ras-transformed cells. In contrast, suppression of c-fos transcription was not affected by either deletion of 240 bp between the DSE and the TATA element or by base-substitution mutations that inactive the ternary complex factor and fos-AP-1-like binding sites. In addition, in vitro competition studies indicated that ras-transformed cells express one or more repressor factors that interact with as-yet-unidentified elements within the c-fos promoter (possibly the URR) and block serum induction of c-fos. These findings suggest that prolonged expression of activated ras results in the activation of one or more as-yet-unidentified proteins that suppress transcription of the c-fos gene by interacting with the URR.

Proteins that interact with PKR

The in vitro activities of recombinant gene products of the vaccinia virus E3L and K3L genes have been compared. These proteins are both potent inhibitors of the dsRNA activated protein kinase (PKR) as assayed in cell-free translation systems or with purified PKR. The two gene products function at similar molar concentrations. Both proteins are expressed early in vaccinia virus infection suggesting that vaccinia virus maintains redundant mechanisms for the down regulation of PKR. The K3L gene product can be shown to be associated with PKR in vaccinia virus infected cells. The activities of the vaccinia virus PKR inhibitors are compared with other viral protein inhibitors of PKR. A variety of cellular proteins have also been identified by their ability to inhibit PKR activity or to prevent PKR activation. These cellular PKR interacting proteins have been uncovered from the studies of viral strategies to prevent PKR activation, as well as from studies looking at the effects of growth control, growth factors or oncogene expression on PKR activity. A picture emerges of PKR fulfilling a complex regulatory role in cell function with the regulation of its activity as part of a complex cascade interfacing with the signal transduction/cell cycle control machinery.

High-resolution analyses of two different classes of tumor cells in situ tagged with alternative histochemical marker genes

To evaluate interactions of two different tumor cell classes during the establishment of micrometastases at the single-cell level, two different BALB/c 3T3 tumor cell derivatives were established that harbor different histochemical marker genes: bacterial lacZ in a EJ-Harvey ras transformant (abbreviated LZEJ cells) and human placental alkaline phosphatase (ALP) gene in a human c-sis transformant (APSI cells). Several different histochemical staining methods were evaluated, using the distinctiveness of lacZ and ALP gene activities, for identification of these cell classes singly or together in the lung after their intravenous injection into nude mice. LZEJ and APSI cells could readily be distinguished from each other after co-injection by using specific and sequential staining protocols of whole organs or sections; staining of host organ cells was minimized. Co-injection of the two tumor cell classes resulted in similar numbers of homogeneous microfoci in lungs of LZEJ or APSI cells within minutes after injection that persisted for several hours before clearance of most of them. Furthermore, a significant percentage of foci could be identified containing both classes of tumor cells on whole-organ or section evaluations; these cohabiting foci resisted clearance from lungs. Therefore, use of two different histochemical marker genes to tag different classes of tumor cells provides a powerful approach for determining their in situ co-localization, cooperation, or interference with the establishment and development of micrometastases, as well as an opportunity to evaluate gene regulation in situ at the single-cell level.

Absence of PDGF-induced, PKC-independent c-fos expression in a chemically transformed C3H/10T1/2 cell clone

The effect of platelet-derived growth factor (PDGF) on c-fos mRNA transcription was studied in the immortalized mouse embryo fibroblast C3H/10T1/2 Cl 8 (10T1/2) cells and the chemically transformed, tumorigenic subclone C3H/10T1/2 Cl 16 (Cl 16). In the 10T1/2 cells as well as the Cl 16 subclone, the dose-dependent PDGF stimulation of c-fos mRNA synthesis was similar in both logarithmically growing and confluent cultures. c-fos mRNA was induced severalfold by 12-O-tetradecanoylphorbol-13-acetate (TPA) in both 10T1/2 and Cl 16. Down-regulation of protein kinase C (PKC) activity by TPA pretreatment inhibited PDGF-stimulated c-fos mRNA expression in Cl 16 cells but did not affect this induction in the 10T1/2 cells. This inhibition was not a general phenomenon of 3-methylcholanthrene-mediated transformation of 10T1/2 cells since experiments with another transformed 10T1/2 cell clone, C3H/10T1/2 TPA 482, gave qualitatively the same results as the 10T1/2 cells. Receptor binding experiments showed that the nontransformed and transformed cells had a comparable number of PDGF receptors, 1.3 x 10(5) and 0.7 x 10(5) receptors per cell, respectively. Furthermore, cAMP-induced c-fos expression induced by forskolin is formerly shown to be independent of PKC down-regulation. In our experiments, forskolin induced c-fos expression in both clones. However, PKC down-regulation inhibited the forskolin-induced c-fos expression in Cl 16 cells. This apparently demonstrates cross talk between PKC and PKA in the c-fos induction pathway. The present results provide evidence for an impaired mechanism for activating c-fos expression through PKC-independent, PDGF-induced signal transduction in the chemically transformed Cl 16 fibroblasts compared to that in nontransformed 10T1/2 cells.

Immediate early gene responses of NIH 3T3 fibroblasts and NMuMG epithelial cells to TGF beta-1

Transforming growth factor beta has a wide range of physiological effects on cell growth and metabolism. We have previously reported on the rapid induction of jun transcription factors in TGF beta-treated cells. Here we show that the early genomic response to TGF beta-1 includes activation of a broad spectrum of serum-inducible genes both in NIH 3T3 fibroblasts and in NMuMG epithelial cells, which are growth-stimulated and growth-inhibited by TGF beta, respectively. Of particular interest is the presence of a putative nuclear DNA-binding receptor (N10) and zinc finger transcription factors (Krox 20 and Krox 24) among the TGF beta-induced genes. In addition to the stimulatory effects of TGF beta, expression of a few genes including c-myc is decreased in both types of cells. In cells transformed by neu or ras oncogenes the immediate early mRNA responses to TGF beta are deregulated. Our results suggest that certain transcription factors are required for both positive and negative regulation of cell proliferation by TGF beta, and that their relative concentrations may determine the subsequent cellular responses.

Elevation of glucose transporter, c-myc, and transin RNA levels by Ha-rasT24 is independent of its effect on the cell cycle

Elevation of the steady-state mRNA levels of glucose transporter and c-myc are among the earliest changes in gene expression observed after Ha-rasT24 stimulation of Rat-1 fibroblasts to enter the cell cycle. Since the expression of these genes may be the result of either increased cell proliferation or a specific response to rasT24, we evaluated the expression of glucose transporter and c-myc and their induction during the cell cycle in both parental Rat-1 cells and cell lines bearing a metallothionein rasT24 fusion gene (MTrasT24). We showed that, although levels of glucose transporter and c-myc mRNAs in Rat-1 cells underwent a transient increase within hours of the addition of serum, epidermal growth factor, or 12-O-tetradecanoylphorbol-13-acetate to quiescent (G0) cells, the levels of glucose transporter and c-myc mRNA otherwise remained constant throughout the normal cell cycle. In cells carrying MTrasT24 (MR5 cells), induction of rasT24 expression by ZnSO4 led to a rapid induction of glucose transporter and c-myc mRNA expression in both quiescent (density-arrested) and G1/S-synchronized (aphidicolin-blocked) cells. These increases exceeded the constitutive levels expressed in rapidly proliferating Rat-1 cells, indicating that the ras oncogene has an effect on these genes that is independent of growth status. In addition, the transin gene, which is not expressed in proliferating Rat-1 cells in the continuous presence of serum growth factors, was also induced after increased expression of the mutant ras gene. These results suggest that the induction of glucose transporter, c-myc, and transin is the direct result of rasT24-mediated alterations in cellular gene expression and is distinct from normal cell-cycle events.

Constitutively activated neu oncoprotein tyrosine kinase interferes with growth factor-induced signals for gene activation

The neu receptor oncoprotein tyrosine kinase, capable of transforming cultured fibroblasts and causing mammary carcinomas in transgenic mice, carries a point mutation in its transmembrane domain and shows a constitutive tyrosine kinase activity. We analyzed the neu tyrosine kinase and its substrates in transfected NIH 3T3 fibroblasts by phosphotyrosine immunoblotting. Tyrosine phosphorylated proteins were similar but not identical in epidermal growth factor (EGF)-stimulated cells expressing the human EGF receptor (EGFR) or a chimeric EGFR/neu receptor but differed from phosphotyrosyl proteins constitutively expressed in neu oncogene-transformed cells. The neu oncoprotein in the latter cells was phosphorylated in tyrosine in a ligand-independent manner and had a shortened half-life in comparison with the normal neu protein. Tumor promoter pretreatment inhibited ligand-induced receptor tyrosine phosphorylation and decreased tyrosine phosphorylated neu oncoprotein. Prolonged pretreatment with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) also prevented the induction of immediate early growth factor-regulated genes in response to neu activation. Expression of the neu oncogene but not the protooncogene in NIH 3T3 cells was associated with enhanced levels of the jun and fos oncoproteins and loss of serum growth factor induction of immediate early mRNA responses. The constitutively activated neu oncoprotein tyrosine kinase thus deregulates cellular genomic responses to growth factors.

PDGF beta-receptor stimulates tyrosine phosphorylation of GAP and association of GAP with a signaling complex

Platelet-derived growth factor (PDGF) stimulated the tyrosine phosphorylation of the GTPase activating protein (GAP) in 3T3 cells and in CHO cells expressing wild-type PDGF receptors, but not in several CHO cell lines expressing mutant receptors defective in transmitting mitogenic signals. Following PDGF treatment of cells, GAP physically associated with the PDGF receptor and with Raf-1, phospholipase c-gamma, and PI-3 kinase, suggesting that PDGF induced the formation of complexes of signaling molecules. The association of GAP with the PDGF receptor and the phosphorylation of GAP with the PDGF receptor and the phosphorylation of GAP were reconstituted in vitro using purified protein and in insect cells expressing murine PDGF receptor and human GAP. However, in cells transformed by activated c-Ha-ras, which are defective in certain responses to PDGF, GAP failed to associate with the PDGF receptor or increase its phosphotyrosine content in response to PDGF. The association of GAP with ligand-activated PDGF receptors may directly link PDGF and ras signaling pathways.

Molecular oncogenetics of metastasis

Metastasis is a complex non-stochastic process that is most likely the result of genetic and epigenetic interactions of a wide variety of genes. The search for a single gene which can encompass such a pleiotropic response as to account for the observed phenotypic characteristics of metastatic tumour populations has been unsuccessful. Particular studies involving gene transfection, subtractive hybridisation and cell fusion are beginning to identify specific genes which contribute to metastasis in some cell types. However, such analyses are complicated by the inherent genetic instability and phenotypic heterogeneity present in tumour populations. A more detailed understanding of the metastatic process may require an abandoning of current generalised approaches to metastasis in favour of concentrating on key components of the metastatic cascade such as adhesion and invasion.

Reduced induction of c-fos but not of c-myc expressions in a nontumorigenic revertant R1 of Ej-ras-transformed NIH/3T3 cells treated with 12-O-tetradecanoylphorbol-13-acetate (TPA)

It has been reported that both c-fos and c-myc mRNAs are induced in NIH/3T3 cells after 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment. We have studied the effect of TPA on the expression of c-fos and c-myc in EJ-ras-transformed NIH/3T3 and its nontumorigenic flat revertant R1 cells. Although TPA treatment induces c-myc mRNA, as in the case of NIH/3T3 cells, the induced level of c-fos mRNA is greatly reduced not only in slow-growing EJ-ras-transformed NIH/3T3 but also in quiescent R1 cells. In addition, serum-induced c-fos expression is also reduced in EJ-ras-transformed NIH/3T3 and R1 cells. These observations suggest that the pathway from TPA to c-fos gene is different from that to c-myc gene and that the former pathway is down-regulated in association not with the transformed phenotype, but with EJ-ras expression, and it is possible that this reduced induction of c-fos is not specific to TPA.

Abolishment of c-fos inducibility in ras-transformed mouse osteoblast cell lines

Proto-oncogene c-fos is induced by many types of cellular stimuli, such as 12-O-tetradecanoylphorbol-13-acetate (TPA), epidermal growth factor (EGF), serum (fetal bovine), calcium ionophore A23187, and dibutyryl cAMP (But2cAMP). In this study, c-fos induction was abolished in ras-transformed mouse osteoblast cells (MC3T3). Transformants of MC3T3 were isolated after transfection with Ki or Ha murine sarcoma virus DNA. All Ki- or Ha-ras transformed MC3T3 clones examined showed exceedingly low levels of c-fos induction by all inducers, as determined by the change in amounts of c-fos mRNA or its product. Induction of other TPA-responsive genes, such as metallothionein, was not altered in some ras-transformed cells; c-myc and c-jun expression was constitutively high in all the ras-transformed clones. Nuclear extracts and gel shift assay showed that the binding activity to c-fos enhancer element (serum response element) was altered in ras-transformed cells. These results indicate that transformation with ras oncogene induces modification of c-fos enhancer binding factors and that this modification is one cause for the decrease in c-fos induction.

Oncogenes, protooncogenes, and signal transduction: toward a unified theory?

This paper has reviewed, in a broad sense, the potential involvement of the oncogenes and their progenitors, the protooncogenes, in signal transduction pathways. The membrane-associated oncogene products appear to be connected with the generation and/or regulation of secondary messengers, particularly those associated with Ca2+/phospholipid-dependent activation of the serine/threonine kinase protein kinase C. Activation of transmembrane receptors, either through binding their native ligand or through point mutations that lead to constitutive expression, results in the expression of their intrinsic tyrosine-specific protein kinases. In PDGF-stimulated cells, this results in the increased turnover of phosphatidylinositols and the subsequent release of IP3 (Habenicht et al., 1981; Berridge et al., 1984). This coincides with activation of a PI kinase activity (Kaplan et al., 1987). Likewise, the fms product, which is the receptor for CSF-1, induces a guanine nucleotide-dependent activation of phospholipase C (Jackowski et al., 1986). Receptor functions are potentially regulated through differential binding of ligands (as proposed with PDGF), through interactions with other receptors, and through the "feedback" regulation mediated by protein kinase C. PDGF stimulation leads to modulation of the EGF receptor through protein kinase C (Bowen-Pope et al., 1983; Collins et al., 1983; Davis and Czech, 1985). Similarly, the neu product becomes phosphorylated on tyrosine residues following treatment of cells with EGF, although the neu protein does not bind EGF itself (King et al., 1988; Stern and Kamps, 1988). The tyrosine kinases of the src family are not receptors themselves, although they may mediate specific receptor-generated signals. The clck product is physically and functionally associated with the T-cell receptors CD4 and CD8, and becomes active upon specific stimulation of cells expressing those markers (Veillette et al., 1988a,b). The precise physiological role of the src family products has not been established, but their kinase activity is intrinsic to that function. The v- and c-src products are hyperphosphorylated during mitosis (Chackalaparampil and Shalloway, 1988), which correlates with periods of reduced cell-to-cell adhesion and communication (Warren and Nelson, 1987; Azarnia et al., 1988). Furthermore, pp60c-src is associated with a PI kinase activity when complexed with MTAg of polyoma virus, suggesting a function in stimulating increased turnover of the phosphatidylinositols (Heber and Courtneidge, 1987; Kaplan et al., 1987).(ABSTRACT TRUNCATED AT 400 WORDS)