Cell transformation by c-fos requires an extended period of expression and is independent of the cell cycle

Immediate Early Gene c-fos in the Brain: Focus on Glial Cells

The c-fos gene was first described as a proto-oncogene responsible for the induction of bone tumors. A few decades ago, activation of the protein product c-fos was reported in the brain after seizures and other noxious stimuli. Since then, multiple studies have used c-fos as a brain activity marker. Although it has been attributed to neurons, growing evidence demonstrates that c-fos expression in the brain may also include glial cells. In this review, we collect data showing that glial cells also express this proto-oncogene. We present evidence demonstrating that at least astrocytes, oligodendrocytes, and microglia express this immediate early gene (IEG). Unlike neurons, whose expression changes used to be associated with depolarization, glial cells seem to express the c-fos proto-oncogene under the influence of proliferation, differentiation, growth, inflammation, repair, damage, plasticity, and other conditions. The collected evidence provides a complementary view of c-fos as an activity marker and urges the introduction of the glial cell perspective into brain activity studies. This glial cell view may provide additional information related to the brain microenvironment that is difficult to obtain from the isolated neuron paradigm. Thus, it is highly recommended that detection techniques are improved in order to better differentiate the phenotypes expressing c-fos in the brain and to elucidate the specific roles of c-fos expression in glial cells.

Crk and CrkL are required for cell transformation by v-fos and v-ras

Crk and CrkL are SH2- and SH3-containing cytosolic adaptor proteins that can induce anchorage-independent growth of fibroblasts. Crk and CrkL play key roles in maintaining cytoskeletal integrity, cell motility and migration. We investigated the role of these two proteins in oncogenic transformation induced by v-fos and v-ras oncogenes using cell lines and fibroblasts carrying conditional alleles of Crk or CrkL. Transformation was assessed by cell morphology, saturation density and anchorage-independent growth in soft agar. We found that cell lines expressing v-fos or v-ras in the absence of Crk or CrkL displayed no evident morphological alterations and reduced anchorage-independent growth compared to those retaining Crk and CrkL. Similarly, overexpression of v-fos in mouse embryonic fibroblasts conferred a growth advantage and induced morphological changes, both of which were abrogated in the absence of either Crk or CrkL. In contrast, Crk, but not CrkL, contributed to v-ras-induced transformation of embryonic fibroblasts. These results suggest that both Crk and CrkL are required for the acquisition of cellular transformation by v-fos, whereas Crk plays a more prominent role than CrkL in v-ras-induced transformation.

JunB contributes to Id2 repression and the epithelial-mesenchymal transition in response to transforming growth factor-β

JunB helps set in motion the transcriptional program necessary for the epithelial–mesenchymal transition and tissue fibrosis in response to TGF-β.

The process of epithelial–mesenchymal transition (EMT) in response to transforming growth factor–β (TGF-β) contributes to tissue fibrosis, wound healing, and cancer via a mechanism that is not fully understood. This study identifies a critical role of JunB in the EMT and profibrotic responses to TGF-β. Depletion of JunB by small interfering ribonucleic acid abrogates TGF-β–induced disruption of cell–cell junctions, formation of actin fibers, focal adhesions, and expression of fibrotic proteins. JunB contributes to Smad-mediated repression of inhibitor of differentiation 2 through interaction with transcription repressor activating transcription factor 3. Importantly, JunB mediates the TGF-β induction of profibrotic response factors, fibronectin, fibulin-2, tropomyosin (Tpm1), and integrin-β3, which play critical roles in matrix deposition, cell–matrix adhesion, and actin stress fibers. In summary, JunB provides important input in setting the transcriptional program of the EMT and profibrotic responses to TGF-β. Thus, JunB represents an important target in diseases associated with EMT, including cancer and fibrosis.

c-Fos accelerates hepatocyte conversion to a fibroblastoid phenotype through ERK-mediated upregulation of paxillin-Serine178 phosphorylation

Transforming growth factor beta (TGF-beta) exerts an important role in the late steps of carcinogenesis by cooperating with Ras to induce cell motility and tumor invasion. The transcription complex AP-1 has been implicated in the regulation of genes involved in motility and invasion, by mechanisms not yet delineated. We utilized a model of immortalized human hepatocytes (IHH) overexpressing c-Fos (IHH-Fos) or not (IHH-C) to investigate the role of c-Fos on cell motility in response to a prolonged treatment with TGF-beta, EGF or a combination of both. Cotreatment with EGF and TGF-beta, but neither cytokine alone, induced the conversion of hepatocytes to a fibroblastoid phenotype and increased their motility in Boyden chambers. EGF/TGF-beta cotreatment induced a higher effect on ERK phosphorylation compared to TGF-beta treatment alone. It also induced an increase in total and phosphorylated Ser(178) paxillin, a protein previously implicated in cell motility. This response was inhibited by two specific MEK inhibitors, indicating the involvement of the ERK pathway in paxillin activation. Overexpression of c-Fos correlated with increased cell scattering and motility, higher levels of ERK activation and phospho Ser(178) paxillin, increased levels of EGF receptor (EGF-R) mRNA and higher EGF-R phosphorylation levels following EGF/TGF-beta cotreatment. Conversely, siRNA-mediated invalidation of c-Fos delayed the appearance of fibroblastoid cells, decreased EGF-R mRNA and downregulated ERK and Ser(178) paxillin phosphorylations, indicating that c-Fos activates hepatocyte motility through an EGF-R/ERK/paxillin pathway. Since c-Fos is frequently overexpressed in hepatocarcinomas, this newly identified mechanism might be involved in the progression of hepatic tumors in vivo.

c-Fos overexpression increases the proliferation of human hepatocytes by stabilizing nuclear Cyclin D1

AIM: To investigate the effect of stable c-Fos overexpression on immortalized human hepatocyte (IHH) proliferation.

METHODS: IHHs stably transfected with c-Fos (IHH-Fos) or an empty vector (IHH-C) were grown in medium supplemented with 1% serum or stimulated with 10% serum. Cell proliferation was assessed by cell counts, 3H-thymidine uptake and flow cytometry analyses. The levels of cell cycle regulatory proteins (Cyclin D1, E, A) cyclin dependent kinases (cdk) cdk2, cdk4, cdk6, and their inhibitors p15, p16, p21, p27, total and phosphorylated GSK-3β and epidermal growth factor receptor (EGF-R) were assayed by Western blotting. Analysis of Cyclin D1 mRNA levels was performed by reverse transcription-polymerase chain reaction and real-time polymerase chain reaction (PCR) analysis. Stability of Cyclin D1 was studied by cycloheximide blockade experiments.

RESULTS: Stable c-Fos overexpression increased cell proliferation under low serum conditions and resulted in a two-fold increase in [³H]-thymidine incorporation following serum addition. Cell cycle analysis by flow cytometry showed that c-Fos accelerated the cell cycle kinetics. Following serum stimulation, Cyclin D1 was more abundantly expressed in c-Fos overexpressing cells. Cyclin D1 accumulation did not result from increased transcriptional activation, but from nuclear stabilization. Overexpression of c-Fos correlated with higher nuclear levels of inactive phosphorylated GSK-3β, a kinase involved in Cyclin D1 degradation and higher levels of EGF-R mRNA, and EGF-R protein compared to IHH-C both in serum starved, and in serum stimulated cells. Abrogation of EGF-R signalling in IHH-Fos by treatment with AG1478, a specific EGF-R tyrosine kinase inhibitor, prevented the phosphorylation of GSK-3β induced by serum stimulation and decreased Cyclin D1 stability in the nucleus.

CONCLUSION: Our results clearly indicate a positive role for c-Fos in cell cycle regulation in hepatocytes. Importantly, we delineate a new mechanism by which c-Fos could contribute to hepatocarcinogenesis through stabilization of Cyclin D1 within the nucleus, evoking a new feature to c-Fos implication in hepatocellular carcinoma.

Neuregulin 1-Beta cytoprotective role in AML 12 mouse hepatocytes exposed to pentachlorophenol

Neuregulins are a family of growth factor domain proteins that are structurally related to the epidermal growth factor. Accumulating evidence has shown that neuregulins have cyto- and neuroprotective properties in various cell types. In particular, the neuregulin-1 Beta (NRG1-Beta) isoform is well documented for its antiinflammatory properties in rat brain after acute stroke episodes. Pentachlorophenol (PCP) is an organochlorine compound that has been widely used as a biocide in several industrial, agricultural, and domestic applications. Previous investigations from our laboratory have demonstrated that PCP exerts both cytotoxic and mitogenic effects in human liver carcinoma (HepG2) cells, primary catfish hepatocytes and AML 12 mouse hepatocytes. We have also shown that in HepG2 cells, PCP has the ability to induce stress genes that may play a role in the molecular events leading to toxicity and tumorigenesis. In the present study, we hypothesize that NRG1-Beta will exert its cytoprotective effects in PCP-treated AML 12 mouse hepatocytes by its ability to suppress the toxic effects of PCP. To test this hypothesis, we performed the MTT-cell respiration assay to assess cell viability, and Western-blot analysis to assess stress-related proteins as a consequence of PCP exposure. Data obtained from 48 h-viability studies demonstrated a biphasic response; showing a dose-dependent increase in cell viability within the range of 0 to 3.87 microg/mL, and a gradual decrease within the concentration range of 7.75 to 31.0 microg/mL in concomitant treatments of NRG1-Beta+PCP and PCP. Cell viability percentages indicated that NRG1-Beta+PCPtreated cells were not significantly impaired, while PCP-treated cells were appreciably affected; suggesting that NRG1-Beta has the ability to suppress the toxic effects of PCP. Western Blot analysis demonstrated the potential of PCP to induce oxidative stress and inflammatory response (c-fos), growth arrest and DNA damage (GADD153), proteotoxic effects (HSP70), cell cycle arrest as consequence of DNA damage (p53), mitogenic response (cyclin- D1), and apoptosis (caspase-3). NRG1-Beta exposure attenuated stress-related protein expression in PCP-treated AML 12 mouse hepatocytes. Here we provide clear evidence that NRG1-Beta exerts cytoprotective effects in AML 12 mouse hepatocytes exposed to PCP.

Transcription factors control invasion: AP-1 the first among equals

Metastasis, the aggressive spread of a malignant tumor to distant organs, is a major cause of death in cancer patients. Despite this critical role in cancer outcomes, the molecular mechanisms that control this process are just beginning to be understood. Metastasis is largely dependent upon the ability of tumor cells to invade the barrier formed by the basement membrane and to migrate through neighboring tissues. This review will summarize the evidence that tumor cell invasion is the result of oncogene-mediated signal transduction pathways that control the expression of a specific set of genes that together mediate tumor cell invasion. We focus on the role of the transcription factor AP-1 to both induce the expression of genes that function as invasion effectors and repress other genes that function as invasion suppressors. This identifies AP-1 as a critical regulator of a complex program of gene expression that defines the invasive phenotype.

Invasion is a genetic program regulated by transcription factors

The invasive and metastatic behaviour of tumours impacts crucially on the clinical management of cancer. Accordingly, it is important to understand the regulation of tumour cell invasiveness. Genetic analysis of worms, Drosophila and mice has provided evidence that invasion is a genetic pathway regulated by transcription factors that are often implicated in tumour cell invasion. Recent evidence has revealed much concerning the role of one particular transcription factor, AP1, which is involved in the regulation of a multigenic invasion program in which upregulated and downregulated genes function as invasion effectors and suppressors, respectively. Differentially expressed genes cooperatively enhance pseudopod elongation during the mesenchymal mode of invasion by altering the function, localisation and activity of non-differentially expressed proteins.

Redox regulation of cell-cycle re-entry: cyclin D1 as a primary target for the mitogenic effects of reactive oxygen and nitrogen species

Reactive oxygen and nitrogen species inhibit or promote cell proliferation by modulating the cell signaling pathways that dictate decisions between cell survival, proliferation, and death. In the growth factor-dependent pathways that regulate mitogenesis, numerous positive and negative effectors of signaling are influenced by physiological fluctuations of oxidants, including receptor tyrosine kinases, small GTPases, mitogen-activated protein kinases, protein phosphatases, and transcription factors. The same mitogenic pathways that are sensitive to oxidant levels also directly regulate the expression of cyclin D1, a labile factor required for progression through the G1 phase on the cell cycle. Because the transition from G0 to G1 is the only phase of the cell cycle that is not regulated by cyclin-dependent kinases, but rather by redox-dependent signaling pathways, expression of cyclin D1 represents a primary regulatory node for the dose-dependent effects of oxidants on the induction of cell growth. We suggest that expression of cyclin D1 represents a useful marker for assessing the integration of proliferative and growth inhibitory effects of oxidants on the redox-dependent signaling events that control reentry into the cell cycle.

T-kininogen can either induce or inhibit proliferation in Balb/c 3T3 fibroblasts, depending on the route of administration

T-kininogen (T-KG) is a precursor of T-kinin, the most abundant kinin in rat serum, and also acts as a strong and specific cysteine proteinase inhibitor. Its expression is strongly induced during aging in rats, and expression of T-KG in Balb/c 3T3 fibroblasts results in inhibition of cell proliferation. However, T-KG is a serum protein produced primarily in the liver, and thus, most cells are only exposed to the protein from the outside. To test the effect of T-KG on fibroblasts exposed to exogenous T-KG, we purified the protein from the serum of K-kininogen-deficient Katholiek rats. In contrast to the results obtained by transfection, exposure of Balb/c 3T3 fibroblasts to exogenously added T-KG leads to a dose-dependent increase in [3H]-thymidine incorporation. This response does not require kinin receptors, but it is clearly mediated by activation of the ERK pathway. As a control, we repeated the transfection experiments, using a different promoter. The results are consistent with our published data showing that, under these circumstances, T-KG inhibits cell proliferation. We conclude that T-KG exerts opposite effects on fibroblast proliferation, depending exclusively on the way that it is administered to the cells (transfection versus exogenous addition).

PPARgamma ligand attenuates PDGF-induced mesangial cell proliferation: role of MAP kinase

BACKGROUND

Mesangial proliferation is a key feature in the pathogenesis of a number of renal diseases and can be experimentally induced by the mitogen platelet-derived growth factor (PDGF). Mitogen-activated protein kinase (MAPK) signaling plays a key role in mesangial cell proliferation. In the present study we examined whether peroxisome proliferator-activated receptor gamma (PPARgamma) activators/ligands, thiazolidinediones such as ciglitazone, troglitazone, and rosiglitazone, can inhibit cell proliferation by modulating individual steps in the MAPK pathway.

METHODS

Mouse mesangial cells were made quiescent and proliferation was measured following the application of PDGF. Using ciglitazone as the model compound, the mechanism of the antiproliferative effect of PPARgamma activators on MAPK and specific cell cycle regulatory proteins were examined by Western blot analysis and transfection studies.

RESULTS

Ciglitazone inhibited PDGF-induced mesangial cell proliferation in a dose-dependent manner (1 to 20 micromol/L). The inhibitory effect was blocked by a peroxisome proliferator-activated receptor element (PPRE) decoy oligonucleotide, indicating that the observed effect of ciglitazone was via PPARgamma activation. Ciglitazone (1 to 20 micromol/L) did not affect extracellular signal-regulated protein kinase (ERK) activation but inhibited the activation of serum response element (SRE) by 85 +/- 6% (P < 0.01). This effect was associated with a reduction in c-fos expression (80 +/- 9%, P < 0.01). Ciglitazone (1, 10, and 20 micromol/L) also inhibited cyclin D1 expression by 37 +/- 8%, 79 +/- 15%, and 87 +/- 12%, respectively (P < 0.001 to 0.001), and p21 expression by 45 +/- 6% (P < 0.01), 61 +/- 10% (P < 0.001), and 72 +/- 8% (P < 0.001), respectively. Ciglitazone inhibited PDGF-mediated up-regulation of p27. In addition, the antiproliferative effect of ciglitazone was potentiated by PD98059, a mitogen-activated protein (MAP) kinase kinase (MEK) inhibitor that acts at a step upstream from ERK.

CONCLUSION

These data indicate that PPARgamma activation may inhibit mesangial growth directly by affecting MAPK and cell cycle regulatory proteins. Furthermore, a MAP kinase inhibitor can potentiate the antiproliferative effect.

Dual mechanisms for lysophosphatidic acid stimulation of human ovarian carcinoma cells

BACKGROUND

Lysophosphatidic acid (LPA), at concentrations present in ascitic fluid, indirectly stimulates the growth of malignant ovarian tumors by increasing the expression of vascular endothelial growth factor (VEGF) in ovarian cancer cells. We investigated whether LPA could also directly promote ovarian tumor growth by increasing the level of cyclin D1, a key G1-phase checkpoint regulator, which thereby increases cell proliferation.

METHODS

Expression of cyclin D1 and LPA receptors (EDG4 and EDG7) was determined in six ovarian cancer cell lines (including OVCAR-3 cells) and immortalized ovarian surface epithelial cells (IOSE-29). Cyclin D1 promoter activity was measured in LPA-treated OVCAR-3 cells cotransfected with cyclin D1 promoter-driven luciferase constructs and cDNA expression plasmids for IkappaBalphaM (a nuclear factor kappaB [NFkappaB] super-repressor).

RESULTS

Four of six cancer cell lines, including OVCAR-3, overexpressed cyclin D1 protein relative to levels in IOSE-29 cells. LPA treatment increased cyclin D1 protein in a dose- and time-dependent manner in OVCAR-3 cells but not in IOSE-29 cells. LPA stimulated cyclin D1 promoter activity (3.0-fold, 95% confidence interval [CI] = 2.7-fold to 3.3-fold). Mutation of the NFkappaB-binding site in the cyclin D1 promoter to block NFkappaB binding and expression of IkappaBalphaM, which binds NFkappaB and inhibits its binding to the promoter, markedly diminished LPA stimulation of cyclin D1 promoter activity (activity stimulated only 1.4-fold, 95% CI = 1.1-fold to 1.7-fold, and 0.7-fold, 95% CI = 0.6-fold to 0.8-fold, respectively). EDG4 was overexpressed in all cancer cell lines studied relative to that in IOSE-29 cells, but EDG7 was overexpressed in only two lines.

CONCLUSIONS

Dual mechanisms are probably involved in LPA stimulation of ovarian tumor growth in vivo. In addition to the previously characterized indirect mechanism that increases angiogenesis via VEGF, LPA may directly increase the level of cyclin D1 in ovarian cancer cells, increasing their proliferation.

Aberrant ubiquitin-mediated proteolysis of cell cycle regulatory proteins and oncogenesis

The ubiquitin pathway plays a central role in the regulation of cell growth and cell proliferation by controlling the abundance of key cell cycle proteins. Increasing evidence indicates that unscheduled proteolysis of many cell cycle regulators contributes significantly to tumorigenesis and is indeed found in many types of human cancers. Aberrant proteolysis with oncogenic potential is elicited by two major mechanisms: defective degradation of positive cell cycle regulators (i.e., proto-oncoproteins) and enhanced degradation of negative cell cycle regulators (i.e., tumor suppressor proteins). In many cases, increased protein stability is a result of mutations in the substrate that prevent the recognition of the protein by the ubiquitin-mediated degradation machinery. Alternatively, the specific recognition proteins mediating ubiquitination (ubiquitin ligases) are not expressed or harbor mutations rendering them inactive. In contrast, the overexpression of a ubiquitin ligase may result in the enhanced degradation of a negative cell cycle regulator. This chapter aims to review the involvement of the ubiquitin pathway in the scheduled destruction of some important cell cycle regulators and to discuss the implications of their aberrant degradation for the development of cancer.

Epigenetic Theories of Cancer Initiation

I argue that carcinogenic insults injure many cells rather than mutate a few. This results from evidence that such insults convert too many cells to a precancerous state and that too many of the converted cells then revert to plausibly involve mutation and its repair; from evidence that the delays between such insults and chemically demonstrable mutations are long enough to easily allow nonmutational mechanisms to work; from evidence that even ionizing radiation first acts on the cytoplasm and mainly affects cells unhit by it; from the fact that such insults induce proto-oncogene expression far too quickly to do so by mutation; and from the fact that fusions of various cells and cell parts show that the tumorous or nontumorous nature of the product depends on its cytoplasmic rather than its nuclear component. I further argue that reduced DNA methylation, modifications of the histone code, and tissue disorganization are the three main mechanisms of epigenetic cancer initiation. Hypomethylation would result from DNA excision repair. Moreover, a methyl-deficient diet is carcinogenic and demethylation is also known to be carcinogenic via the histone code. Finally, I strongly argue for tissue disorganization as a mechanism of cancer initiation. This results from evidence that skin carcinogens disrupt the dermal/epidermal connection and from the fact that tumorigens swiftly disrupt gap junctions, as well as from evidence that such disruption is tumorigenic.

Multiple degradation pathways for Fos family proteins

c-Fos protooncoprotein is a short-lived transcription factor with oncogenic potential. It is massively degraded by the proteasome in vivo under various experimental conditions. Those include consititutive expression in exponentially growing cells and transient induction in cells undergoing the G0/G1 phase transition upon stimulation by serum. Though there is evidence that c-Fos can be ubiquitinylated in vitro, the unambigous demonstration that prior ubiquitinylation is necessary for degradation by the proteasome in vivo is still lacking. c-Jun, one of the main dimerization partners of c-Fos within the AP-1 transcription complex, is also an unstable protein. Its degradation is clearly proteasome dependent. However, several lines of evidence indicate that the mechanisms by which it addresses the proteasome are different from those operating on c-Fos. Moreover, genetic analysis has indicated that c-Fos is addressed to the proteasome via pathways that differ depending on the conditions of expression. c-Fos has been transduced by two murine osteosarcomatogenic retroviruses in mutated forms, which are more stable and more oncogenic. The stabilization is not simply accounted for by simple deletion of one of the main c-Fos destabilizers but, rather, by a complex balance between opposing destabilizing and stabilizing mutations. However, although viral Fos proteins have acquired full resistance to proteasomal degradation, stabilization is limited because the mutations they have accumulated, during or after c-fos gene transduction, confer sensitivity to an unidentified proteolytic system(s). This observation is consistent with the idea that fos-expressing viruses have evolved expression machineries to ensure controlled protein levels in order to maintain an optimal balance between prooncogenic and proapoptotic activities of v-Fos proteins.

Evasion from proteasomal degradation by mutated Fos proteins expressed from FBJ-MSV and FBR-MSV osteosarcomatogenic retroviruses

c-Fos proto-oncoprotein is highly unstable, which is crucial for rapid gene expression shut-off and control of its intrinsic oncogenic potential. It is massively degraded by the proteasome in vivo in various situations. Although there is evidence that c-Fos can be ubiquitinylated in vitro, the unambiguous demonstration that ubiquitinylation is necessary for recognition and subsequent hydrolysis by the proteasome in vivo is still lacking. Moreover, genetic analysis have also indicated that c-Fos can be addressed to the proteasome via different mechanisms depending on the conditions studied. c-Fos has been transduced by two murine osteosarcomatogenic retroviruses under mutated forms which are more stable and more oncogenic. The stabilization is not simply accounted for by simple deletion of a C-terminal c-Fos destabilizer but, rather, by a complex balance between opposing destabilizing and stabilizing mutations. Though mutations in viral Fos proteins confer full resistance to proteasomal degradation, stabilization is limited because mutations also entail sensitivity to (an) unidentified proteolytic system(s). This observation is consistent with the idea that Fos-expressing viruses have evolved gene expression controls that avoid high protein accumulation-linked apoptosis.

Identification of a C-terminal tripeptide motif involved in the control of rapid proteasomal degradation of c-Fos proto-oncoprotein during the G(0)-to-S phase transition

c-Fos proto-oncoprotein is rapidly and transiently expressed in cells undergoing the G(0)-to-S phase transition in response to stimulation for growth by serum. Under these conditions, the rapid decay of the protein occurring after induction is accounted for by efficient recognition and degradation by the proteasome. PEST motifs are sequences rich in Pro, Glu, Asp, Ser and Thr which have been proposed to constitute protein instability determinants. c-Fos contains three such motifs, one of which comprises the C-terminal 20 amino acids and has already been proposed to be the major determinant of c-Fos instability. Using site-directed mutagenesis and an expression system reproducing c-fos gene transient expression in transfected cells, we have analysed the turnover of c-Fos mutants deleted of the various PEST sequences in synchronized mouse embryo fibroblasts. Our data showed no role for the two internal PEST motifs in c-Fos instability. However, deletion of the C-terminal PEST region led to only a twofold stabilization of the protein. Taken together, these data indicate that c-Fos instability during the G0-to-S phase transition is governed by a major non-PEST destabilizer and a C-terminal degradation-accelerating element. Further dissection of c-Fos C-terminal region showed that the degradation-accelerating effect is not contributed by the whole PEST sequence but by a short PTL tripeptide which cannot be considered as a PEST motif and which can act in the absence of any PEST environment. Interestingly, the PTL motif is conserved in other members of the fos multigene family. Nevertheless, its contribution to protein instability is restricted to c-Fos suggesting that the mechanisms whereby the various Fos proteins are broken down are, at least partially, different. MAP kinases-mediated phosphorylation of two serines close to PTL, which are both phosphorylated all over the G(0)-to-S phase transition, have been proposed by others to stabilize c-Fos protein significantly. We, however, showed that the PTL motif does not exert its effect by counteracting a stabilizing effect of these phosphorylations under our experimental conditions.

Dominant negative c-jun inhibits activation of the cyclin D1 and cyclin E kinase complexes

The AP-1 transcription factor is activated by oncogenic signal transduction cascades and its function is critical for both mitogenesis and carcinogenesis. To define the role of AP-1 in the context of a human fibrosarcoma cell line, HT1080, we expressed a dominant negative c-jun mutant fused to the green fluorescent protein in an ecdysone-inducible system. We demonstrated that high levels of this mutant, GFP-TAM67, inhibit AP-1 activity and arrest cells predominantly in the G1 phase of the cell cycle. This arrest is reversible and occurs only above a threshold concentration; low to moderate levels of GFP-TAM67 are insufficient for growth arrest. Contrary to expectations based on the literature, GFP-TAM67 does not inhibit expression of cyclin D1, cyclin E, or their respective cyclin-dependent kinases. However, pRB is hypophosphorylated in GFP-TAM67-arrested cells and the activity of both the cyclin D1:cdk and the cyclin E:cdk complexes are impaired. Both of these complexes show an increased association with p21(CIP1/WAF1), concomitantly with induction of the p21 mRNA by GFP-TAM67. These results suggest a novel function of AP-1 in the activation of the G1 cyclin:cdk complexes in human tumor cells by regulating the expression of the p21(CIP1/WAF1) gene.

Down-regulation of particulate protein kinase Cepsilon and up-regulation of nuclear activator protein-1 DNA binding in liver following in vivo exposure of B6C3F1 male mice to heptachlor epoxide

The effects of in vivo administration of the cyclodiene tumor promoter heptachlor epoxide on mouse liver protein kinase C were studied in male B6C3F1 mice by protein kinase C activity assays and Western blotting under conditions known to increase the incidence of hepatocellular carcinoma because protein kinase C is thought to be critical in phorbol ester-induced tumor promotion. Under these test conditions, 20 ppm dietary heptachlor epoxide for 1-20 days increased cytosolic and decreased particulate total protein kinase C activities, while 10 ppm had no effect. Further, total cytosolic and particulate protein kinase C activities were decreased within 1 hour by 10 mg/kg intraperitoneal (i.p.) heptachlor epoxide. Western blotting showed that conventional protein kinase Calpha and beta isoforms were unaffected by heptachlor epoxide. Particulate novel protein kinase Cepsilon, however, was selectively down-regulated by 1, 10, and 20 ppm dietary heptachlor epoxide, whereas the cytosolic isoform was decreased by 1 and 10 ppm heptachlor epoxide for 10 days. The high-dose treatment for 24 hours also decreased particulate novel protein kinase Cepsilon but increased the cytosolic titer. These results demonstrate that this isoform is unique in its sensitivity to heptachlor epoxide. Activator protein-1 DNA binding, a critical factor in tumor promotion, was substantially increased at 3 and 6 hours with 3.7 mg/kg (i.p.) heptachlor epoxide and at 3 and 10 days with 20 ppm dietary heptachlor epoxide. The effects of heptachlor epoxide on protein kinase C and activator protein-1 are similar to those caused by phorbol ester treatments and correlate well to heptachlor levels found to induce tumors in mice. However, heptachlor epoxide did not initially activate protein kinase C with in vivo treatments or with in vitro treatments of a plasma membrane fraction aimed at demonstrating direct activation, as has been shown for phorbol esters. The ability of heptachlor epoxide to down-regulate particulate novel protein kinase Cepsilon correlates to dosages used in in vivo tumor promotion studies. However, this may represent a negative feedback response rather than a causative effect.

AP-1 in mouse development and tumorigenesis

Genetically modified mice have provided important insights into the biological functions of the dimeric transcription factor complex AP-1. Extensive analyses of mice and cells with genetically modified Fos or Jun proteins provide novel insights into the physiological functions of AP-1 proteins. Using knock-out strategies it was found that some components, such as c-Fos, FosB and JunD are dispensable, whereas others, like c-Jun, JunB and Fra-1 are essential in embryonic development and/or in the adult organism. Besides the specific roles of AP-1 proteins in developmental processes, we are beginning to obtain a better molecular understanding of the cell-context dependent function of AP-1 in cell proliferation and apoptosis, in bone biology as well as in multistep tumorigenesis.

Degradation of cellular and viral Fos proteins

c-Fos proto-oncoprotein is a short-lived transcription factor with oncogenic potential. We have shown that it is massively degraded by the proteasome in vivo under various experimental conditions. Other proteolytic systems including lysosomes and calpains, might, however, also marginally operate on it. Although there is evidence that c-Fos can be ubiquitinylated in vitro, the unambiguous demonstration that ubiquitinylation is necessary for its addressing to the proteasome in vivo is still lacking. c-Jun, one of the main dimerization partners of c-Fos within the AP-1 transcription complex, is also an unstable protein. Its degradation is clearly proteasome- and ubiquitin-dependent in vivo. Interestingly, several lines of evidence indicate that the addressing of c-Fos and c-Jun to the proteasome is, at least in part, governed by different mechanisms. c-Fos has been transduced by two murine osteosarcomatogenic retroviruses under mutated forms which are more stable and more oncogenic. The stabilization is not simply accounted for by simple deletion of c-Fos main destabilizer but, rather, by a complex balance between opposing destabilizing and stabilizing mutations. Though mutations in viral Fos proteins confer full resistance to proteasomal degradation, stabilization is limited because mutations also entail sensitivity to an unidentified proteolytic system. This observation is consistent with the idea that Fos-expressing viruses have evolved to ensure control protein levels to avoid high protein accumulation-linked apoptosis. In conclusion, the unveiling of the complex mechanism network responsible for the degradation of AP-1 family members is still at its beginning and a number of issues regarding the regulation of this process and the addressing to the proteasome are still unresolved.

Cellular and viral Fos proteins are degraded by different proteolytic systems

c-Fos proto-oncoprotein is a short-lived transcription factor degraded by the proteasome in vivo. Its mutated forms expressed by the mouse osteosarcomatogenic retroviruses, FBJ-MSV and FBR-MSV, are stabilized two- and threefold, respectively. To elucidate the mechanisms underlying v-Fos(FBJ) and v-Fos(FBR) protein stabilization, we conducted a genetic analysis in which the half-lives and the sensitivities to various cell-permeable protease inhibitors of a variety of cellular and viral protein mutants were measured. Our data showed that the decreased degradation of v-Fos(FBJ) and v-Fos(FBR) is not simply explained by the deletion of a c-Fos destabilizing C-terminal domain. Rather, it involves a complex balance between opposing destabilizing and stabilizing mutations which are distinct and which include virally-introduced peptide motifs in both cases. The mutations in viral Fos proteins conferred both total insensitivity to proteasomal degradation and sensitivity to another proteolytic system not naturally operating on c-Fos, explaining the limited stabilization of the two proteins. This observation is consistent with the idea that FBR-MSV and FBJ-MSV expression machineries have evolved to ensure controlled protein levels. Importantly, our data illustrate that the degradation of unstable proteins does not necessarily involve the proteasome and provide support to the notion that highly related proteins can be broken down by different proteolytic systems in living cells.

Significance of macrophage chemoattractant protein-1 in macrophage recruitment, angiogenesis, and survival in human breast cancer

Tumor cells stimulate the formation of stroma that secretes various mediators pivotal for tumor growth, including growth factors, cytokines, and proteases. However, little is known about the local regulation of these soluble mediators in the human tumor microenvironment. In this study, the local expression of cytokines, chemokines, and angiogenic factors was investigated in primary breast cancer tissue. The concentrations of interleukin (IL)-1, IL-4, IL-6, IL-10, IL-12, tumor necrosis factor (TNF)-alpha, IFN-gamma, IL-8, macrophage chemoattractant protein (MCP)-1, epithelial-neutrophil activating peptide-78, vascular endothelial growth factor, and thymidine phosphorylase (TP) were measured in 151 primary breast cancer extracts by ELISA. Tumor-associated macrophages (TAMs) were also examined by immunohistochemistry with anti-CD68 antibodies. The correlation between soluble mediators and the relationship between TAM count and soluble mediators were evaluated. MCP-1 concentration was correlated significantly with the level of vascular endothelial growth factor, TP, TNF-alpha, and IL-8, which are potent angiogenic factors. IL-4 concentration was correlated significantly with IL-8 and IL-10. On the other hand, an inverse association was observed between TP and IL-12. The level of MCP-1 was associated significantly with TAM accumulation. In the immunohistochemical analysis, MCP-1 expression was observed in both infiltrating macrophages and tumor cells. Prognostic analysis revealed that high expression of MCP-1, as well as of VEGF, was a significant indicator of early relapse. These findings indicate that interaction between the immune network system and angiogenesis is important for progression of human breast cancer, and that MCP-1 may play an important role in the regulation of angiogenesis and the immune system.

Regulation of a multigenic invasion programme by the transcription factor, AP-1: re-expression of a down-regulated gene, TSC-36, inhibits invasion

The transcription factor AP-1 (activator protein-1) is required for transformation by many oncogenes, which function upstream of it in the growth factor-ras signal transduction pathway. Previously, we proposed that one role of AP-1 in transformation is to regulate the expression of a multigenic invasion programme. As a test of this proposal we sought to identify AP-1 regulated genes based upon their differential expression in 208F rat fibroblasts transformed by FBR-v-fos (FBR), and to determine if they functioned in the invasion programme. Subtracted cDNA libraries specific for up- or down-regulated genes in FBRs compared to 208Fs were constructed and analysed. Northern analysis revealed that the cDNAs in both libraries represented differentially expressed genes. Nucleic acid sequence analysis of randomly selected cDNA clones from each library coupled with searches of nucleic acid and amino acid sequence databases determined that many of the cDNAs represented proteins that function in various aspects of the invasion process. Functional analysis of one the down-regulated genes, TSC-36/follistatin-related protein (TSC-36/Frp), which has not previously been associated with invasion, demonstrated that its expression in FBRs inhibited in vitro invasion. These results support the proposal that AP-1 in transformed cells regulates a multigenic invasion programme.

Regulation of FOS by different compartmental stresses induced by low levels of ionizing radiation

We irradiated different cellular compartments and measured changes in expression of the FOS gene at the mRNA and protein levels. [(3)H]Thymidine and tritiated water were used to irradiate the nucleus and the whole cell, respectively. (125)I-Concanavalin A binding was used to irradiate the cell membrane differentially. Changes in FOS mRNA and protein levels were measured using semi-quantitative RT-PCR and SDS-PAGE Western blotting, respectively. Irradiation of the nucleus or the whole cell at a dose rate of 0.075 Gy/h caused no change in the level of FOS mRNA expression, but modestly (1.5-fold) induced FOS protein after 0.5 h. Irradiation of the nucleus at a dose rate of 0.43 Gy/h induced FOS mRNA by 1.5-fold after 0.5 h, but there was no significant effect after whole-cell irradiation. FOS protein was transiently induced 2.5-fold above control levels 0.5 h after a 0. 43-Gy/h exposure of the nucleus or the whole cell. Irradiation of the cell membrane at a dose rate of 1.8 Gy/h for up to 2 h caused no change in the levels of expression of FOS mRNA or protein, but a dose rate of 6.8 Gy/h transiently increased the level of FOS mRNA 3-fold after 0.5 h. These data demonstrate the complexity of the cellular response to radiation-induced damage at low doses. The lack of quantitative agreement between the transcript and protein levels for FOS suggests a role for post-transcriptional regulation.

Transcriptional regulation of cell invasion: AP-1 regulation of a multigenic invasion programme

The focus of this review will be on the regulation of the multigenic invasion programme by activator protein-1 (AP-1). Investigation of AP-1-regulated gene expression in transformed cells can be used to identify the genes in the multigenic invasion programme and to validate them as targets for diagnosis or therapy.

Control of cell cycle gene expression in bone development and during c-Fos-induced osteosarcoma formation

We have used c-Fos transgenic mice which develop osteosarcomas to determine the expression patterns of cyclins, cyclin-dependent kinases (CDKs), and cyclin-dependent kinase inhibitors (CKIs) in different bone cell populations in order to define the potential mechanisms of c-Fos transformation. Immunohistochemical analysis in embryonic and early postnatal bone demonstrated that cyclin E and its kinase partner CDK2 were expressed specifically in bone-forming osteoblasts. Cyclin D1 expression was absent despite high levels of CDK4 and CDK6, and the CKI p27 was expressed in chondrocytes, osteoclasts, and at lower levels in osteoblasts. Following activation of the c-fos transgene in vivo and before overt tumor formation, cyclin D1 expression increased dramatically and was colocalized with exogenous c-Fos protein specifically in osteoblasts and chondrocytes, but not in osteoclasts. Prolonged activation of c-Fos resulted in osteosarcoma formation wherein the levels of cyclin D1, cyclin E, and CDKs 2, 4, and 6 were high in a wide spectrum of malignant cell types, especially in transformed osteoblasts. The CKI p27 was expressed at very high levels in bone-resorbing osteoclasts, and to a lesser extent in chondrocytes and osteoblasts. These in vivo observations suggest that cyclin D1 may be a target for c-Fos action and that elevation of cyclin D1 in osteoblasts which already express cyclin E/CDK2 and the cyclin D1 partners CDKs-4 and 6, may predispose cells to uncontrolled cell growth leading to osteosarcoma development. This study implicates altered cell cycle control as a potential mechanism through which c-Fos causes osteoblast transformation and bone tumor formation.

Exposure of nerve growth factor-treated PC12 rat pheochromocytoma cells to a modulated radiofrequency field at 836.55 MHz: effects on c-jun and c-fos expression

Rat PC12 pheochromocytoma cells have been treated with nerve growth factor and then exposed to athermal levels of a packet-modulated radiofrequency field at 836.55 MHz. This signal was produced by a prototype time-domain multiple-access (TDMA) transmitter that conforms to the North American digital cellular telephone standard. Three slot average power densities were used: 0.09, 0.9, and 9 mW/cm2. Exposures were for 20, 40, and 60 min and included an intermittent exposure regimen (20 min on/20 min off), resulting in total incubation times of 20, 60, and 100 min, respectively. Concurrent controls were sham exposed. After extracting total cellular RNA, Northern blot analysis was used to assess the expression of the immediate early genes, c-fos and c-jun, in all cell populations. No change in c-fos transcript levels were detected after 20 min exposure at each field intensity (20 min was the only time period at which c-fos message could be detected consistently). Transcript levels for c-jun were altered only after 20 min exposure to 9 mW/cm2 (average 38% decrease).

Conditional cell transformation by doxycycline-controlled expression of the ASV17 v-jun allele

To investigate the molecular basis of oncogenesis induced by the v-jun oncogene of avian sarcoma virus 17 (ASV17), we developed a conditional cell transformation system in which transcription of the ASV17 v-jun allele is controlled by a doxycycline-sensitive transactivator (tTA) or a reverse (doxycycline-dependent) transactivator (rtTA), respectively. Permanent cell lines of quail embryo fibroblasts conditionally transformed by a doxycycline-controlled v-jun allele revert to the normal phenotype within 3 days and lose their ability to grow in soft agar, strictly dependent on the addition or removal of the drug, respectively. The reverted cells are rapidly retransformed on conditional activation of v-jun. While full-level synthesis of v-jun mRNA and v-Jun protein in these cells is established within 2 and 14 h, respectively, after switching to the permissive conditions, the first morphological alterations are observed after 24 h, and as early as 2 days later the morphology has changed entirely from flat cells resembling normal fibroblasts to spindle-shaped fusiform cells showing a typical jun-transformed phenotype. Kinetic expression analysis revealed that transcriptional activation of the direct jun target gene BKJ precisely coincides with the establishment of full-level v-Jun protein synthesis. Furthermore, we have analyzed the expression of a novel candidate v-jun target gene, termed JAC, which shows no sequence homology to known genes. Similar to BKJ, JAC is specifically activated in jun-transformed fibroblasts, and induction of JAC is tightly linked to the conditional expression of oncogenic v-Jun. These results demonstrate the high stringency of the doxycycline-controlled v-jun expression system, and they also indicate that expression of v-jun in these cells is indispensable for enhanced proliferation, cell transformation, and the induction of specific expression patterns of downstream target genes.

A c-fos/Estrogen receptor fusion protein promotes cell cycle progression and proliferation of human cancer cell lines

c-fos is the prototypic member of a family of transcription factors that regulate many cellular processes, including proliferation. c-fos heterodimerizes with jun family members to form the AP-1 transcription factor complex which binds specific DNA recognition elements in the promoters of many genes. Following rapid induction in response to serum or growth factors, c-fos regulates expression of downstream target genes involved in cellular proliferation. Although much work has focused on activation of cell cycle regulatory genes by c-fos, less is known about negative regulation of gene expression by this transcription factor. The cyclin-dependent kinase (cdk) inhibitor p21(Cip1/WAF1) is a negative regulator of cdk activity, thereby impeding cell cycle progression. By sequence analysis, we identified a putative AP-1 element in the p21(Cip1/WAF1) promoter. To investigate how this site regulated p21(Cip1/WAF1) expression and mitigate external effects on c-fos expression, we used a c-fos/estrogen receptor (c-fosER) fusion construct in which this transcription factor is conditionally activated by estradiol. In the presence of estradiol, c-fosER downregulated p21(Cip1/WAF1) promoter activity. This inhibition was dependent on the putative AP-1 site. Activation of c-fosER induced cell cycle progression and proliferation in a manner similar to serum stimulation. We concluded that activation of c-fosER mediated transcriptional inhibition of p21(Cip1/WAF1) through a previously uncharacterized AP-1 site, revealing an important role for c-fos in negative control of cell cycle regulatory genes.

Influence of the proto-oncogene c-fos on cisplatin sensitivity

Cisplatin resistance has been associated with overexpression of the c-fos gene in a human ovarian carcinoma cell line. To determine whether the correlation between c-fos overexpression and cisplatin resistance was limited to this cell line or was a more generalized phenomenon, we investigated cisplatin sensitivity in rat fibroblast cells that overexpressed the c-fos gene. The cisplatin Ic50 values for two different c-fos transfectants, CMVc-fos and L1-3c-fos, were 7.6 +/- 0.8 and 5.6 +/- 1.0 microM, respectively, whereas the cisplatin Ic50 value for the parental line, 208F, was 2.4 +/- 0.1 microM. This represented a 3.2- and 2.3-fold resistance to cisplatin for CMVc-fos and L1-3c-fos cells, respectively. The correlation between c-fos expression and cisplatin resistance also was examined in a human ovarian carcinoma cell line, 2008, and its cisplatin-resistant variant, C13*. Expression of c-fos was elevated slightly at both the mRNA and protein levels in the C13* cells compared with 2008 cells, and c-Fos protein levels were induced in C13* cells following cisplatin treatment. In addition, it was observed that C13* cells were significantly more sensitive than 2008 cells to a c-fos antisense oligonucleotide. The Ic50 values for the c-fos antisense oligonucleotide were 19.9 +/- 5.0 pmol for C13* cells and 58.1 +/- 6.0 pmol for 2008 cells (P = 0.0012). Furthermore, combinations of c-fos antisense and cisplatin reduced the amount of cisplatin required to kill 50% of the C13* cells, although the interaction was not synergistic. These results suggest that expression of the c-fos gene can influence cisplatin sensitivity, and that c-fos antisense oligonucleotide based therapy may be effective at killing parental and cisplatin-resistant ovarian carcinoma cells, either alone or in combination with cisplatin.

Inhibition of chondrocyte differentiation in vitro by constitutive and inducible overexpression of the c-fos proto-oncogene

We have investigated the role of c-Fos in chondrocyte differentiation in vitro using both constitutive and inducible overexpression approaches in ATDC5 chondrogenic cells, which undergo a well-defined sequence of differentiation from chondroprogenitors to fully differentiated hypertrophic chondrocytes. Initially, we constitutively overexpressed exogenous c-fos in ATDC5 cells. Several stable clones expressing high levels of exogenous c-fos were isolated and those also expressing the cartilage marker type II collagen showed a marked decrease in cartilage nodule formation. To investigate further whether c-Fos directly regulates cartilage differentiation independently of potential clonal variation, we generated additional clones in which exogenous c-fos expression was tightly controlled by a tetracycline-regulatable promoter. Two clones, DT7.1 and DT12.4 were capable of nodule formation in the absence of c-fos. However, upon induction of exogenous c-fos, differentiation was markedly reduced in DT7.1 cells and was virtually abolished in clone DT12.4. Pulse experiments indicated that induction of c-fos only at early stages of proliferation/differentiation inhibited nodule formation, and limiting dilution studies suggested that overexpression of c-fos decreased the frequency of chondroprogenitor cells within the clonal population. Interestingly, rates of proliferation and apoptosis were unaffected by c-fos overexpression under standard conditions, suggesting that these processes do not contribute to the observed inhibition of differentiation. Finally, gene expression analyses demonstrated that the expression of the cartilage markers type II collagen and PTH/PTHrP receptor were down-regulated in the presence of exogenous c-Fos and correlated well with the differentiation status. Moreover, induction of c-fos resulted in the concomitant increase in the expression of fra-1 and c-jun, further highlighting the importance of AP-1 transcription factors in chondrocyte differentiation. These data demonstrate that c-fos overexpression directly inhibits chondrocyte differentiation in vitro, and therefore these cell lines provide very useful tools for identifying novel c-Fos-responsive genes that regulate the differentiation and activity of chondrocytes.

Neuronal interleukin-16 (NIL-16): a dual function PDZ domain protein

Interleukin (IL)-16 is a proinflammatory cytokine that has attracted widespread attention because of its ability to block HIV replication. We describe the identification and characterization of a large neuronal IL-16 precursor, NIL-16. The N-terminal half of NIL-16 constitutes a novel PDZ domain protein sequence, whereas the C terminus is identical with splenocyte-derived mouse pro-IL-16. IL-16 has been characterized only in the immune system, and the identification of NIL-16 marks a previously unsuspected connection between the immune and the nervous systems. NIL-16 is a cytosolic protein that is detected only in neurons of the cerebellum and the hippocampus. The N-terminal portion of NIL-16 interacts selectively with a variety of neuronal ion channels, which is similar to the function of many other PDZ domain proteins that serve as intracellular scaffolding proteins. Among the NIL-16-interacting proteins is the class C alpha1 subunit of a mouse brain calcium channel (mbC alpha1). The C terminus of NIL-16 can be processed by caspase-3, resulting in the release of secreted IL-16. Furthermore, in cultured cerebellar granule neurons undergoing apoptosis, NIL-16 proteolysis parallels caspase-3 activation. Cerebellar granule neurons express the IL-16 receptor CD4. Exposure of these cells to IL-16 induces expression of the immediate-early gene, c-fos, via a signaling pathway that involves tyrosine phosphorylation. This suggests that IL-16 provides an autocrine function in the brain. Therefore, we hypothesize that NIL-16 is a dual function protein in the nervous system that serves as a secreted signaling molecule as well as a scaffolding protein.

Functional analysis of newly discovered growth control genes: experimental approaches

A large number of DNA sequences corresponding to human and animal transcripts have been filed in data banks, as cDNAs or ESTs (expression sequence tags). However, the actual function of their corresponding gene products is still largely unknown. Several of these genes may play a role in regulation of important biological processes such as cell division, differentiation, malignant transformation and oncogenesis. Elucidation of gene function is based on 2 main approaches, namely, overexpression and expression interference, which respectively mimick or suppress a given phenotype. The currently available tools and experimental approaches to gene functional analysis and the most recent advances in mass cDNA screening by functional analysis are discussed.

Sustained MAP kinase activation is required for the expression of cyclin D1, p21Cip1 and a subset of AP-1 proteins in CCL39 cells

In CCL39 cells thrombin is a potent growth factor which requires sustained activation of mitogen activated protein kinases (MAPKs) to promote DNA synthesis. Some of the effects of thrombin can be mimicked by peptides based on the new amino terminus of the cleaved receptor; however, these thrombin receptor peptides (TRPs) fail to induce sustained activation of MAPK or DNA synthesis. We have used thrombin, TRP-7 and other agonists which elicit sustained or transient MAPK activation to identify immediate-early and delayed-early genes which are only expressed under conditions of sustained MAPK activation focusing on cyclin D1, p21CiP1 and the AP-1 transcription factor. Of the stimuli tested only FBS and thrombin were able to stimulate a sustained activation of MAPK, expression of cyclin D1, p21Cip1 and cell cycle re-entry. The expression of cyclin D1 was strongly, though not completely, inhibited by the MEK1 inhibitor PD098059. Thrombin stimulated a rapid but transient accumulation of c-Fos whereas the expression of Fra-1, Fra-2, c-Jun and JunB was sustained throughout the G1 phase of the cell cycle. We focussed our analysis on c-Fos (typical of AP-1 genes which are expressed rapidly and transiently) and Fra-1 and JunB (typical of AP-1 genes expressed after a delay but in a sustained manner). The expression of c-Fos, Fra-1 and JunB was dependent upon the activation of MAPK since these responses were inhibited by PD098059. However, a comparison of responses to FBS, thrombin, TRPs, LPA and EGF revealed that Fra-1 and JunB expression required sustained activation of MAPK whereas c-Fos expression was strongly induced even by non-mitogenic stimuli which elicited only transient MAPK activation. The expression of c-Fos (in response to thrombin, TRP or LPA) or Fra-1, JunB and cyclin D1 (thrombin only) was also inhibited by pertussis toxin. This suggests that both early and late AP-1 gene expression is regulated by the same Gi-mediated, MEK-dependent MAPK signalling pathway but that expression of late AP-1 genes and cyclin D1 requires that this pathway be persistently activated. The results suggest that the duration of receptor signalling and therefore MAPK activation is a key determinant of qualitative changes in gene expression during cell cycle re-entry.

Proliferative activity and tumor angiogenesis is closely correlated to stromal cellularity of fibroadenoma: proposal fibroadenoma, cellular variant

Fibroadenoma (FA) is the most common benign tumor of the breast in adult women. Some FA have a highly cellular stroma, making it difficult to differentiate from phyllodes tumors (PT). Forty-three FA were grouped into: (i) 27 conventional type (FACT) median stromal cellularity (SC) of highest cellular area (HCA), < or = 125 cells/1 high-power field (HPF); and (ii) 16 cellular variant (FACV) median SC of HCA, > 125 cells/1 HPF. These were studied for the proliferative activity of their stromal cells. Expression of c-fos, p53, basic fibroblast growth factor (bFGF), fibroblast growth factor receptor (FGFR), and vascular endothelial growth factor (VEGF) in the stromal cells were examined in the FA and 12 PT to determine whether it is possible to separate FACV from FACT. The proliferative activity of stromal cells was evaluated by the labeling index (LI) of proliferating cell nuclear antigen (PCNA). Conventional type fibroadenoma stromal cells had the lowest frequency of c-fos, p53, bFGF, FGFR and VEGF protein expression; PT stromal cells had the highest frequency of expression; and FACV stromal cells had an intermediate frequency of expression. Multivariate analysis demonstrated that bFGF and FGFR expression are significantly correlated with SC of FA. Separation of FACV from FACT by SC seems appropriate in revealing the phenotypic and biological differences of FA. The SC of FA seems to be regulated by bFGF and FGFR expression.

The G10BP-1 gene encoding a GC box binding protein, is a target of Myc and Jun/Fos

BACKGROUND

G10BP, a serum-inducible factor, represses the transcription of the fibronectin gene through binding to the G-rich sequences in the promoter excluding Sp1 from binding to these sequences.

RESULTS

The 5' flanking sequence of the G10BP-1 gene was isolated by polymerase chain reaction of the adaptor-ligated genomic DNA library using the adaptor primer and the G10BP-1 cDNA primer. The elements required for activation of the G10BP-1 promoter following serum stimulation were analysed by transfection of quiescent rat 3Y1 cells with G10BP-1 promoter-luciferase cDNA constructs containing 5' sequential deletions or base substitutions. The results showed that the promoter was activated by Myc and Jun through the E box and AP1 sites. The formation of DNA-protein complexes with 32P-labelled oligonucleotides containing the E box or AP1 site with cell extracts prepared during G1 progression was correlated with the promoter activation and greatly reduced by immunodepletion of Myc or c-Jun from the extracts.

CONCLUSION

These results indicate that the G10BP-1 gene is a target of Myc and Jun/Fos and that these factors repress the fibronectin gene expression through induction of G10BP-1 during G1-to-S phase progression.

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.

Mitochondrial membrane potential regulation is independent of c-fosexpression

Tumour cells contain mitochondria with elevated membrane potentials compared with normal cells, and thus this feature provides a selective target for destroying tumour cells. To improve mitochondrial-based therapies, a better understanding of the factors involved in regulating mitochondria are required. Since v-fos overexpression has been shown to elevate mitochondrial membrane potentials in rat fibroblasts, we investigated whether the human homologue, c-fos, was also capable of regulating the mitochondrial membrane potential in cells. Rat fibroblasts transfected with the c-fos gene did not accumulate more rhodamine 123 (Rh123) nor did they retain this Rh123 for extended periods of time compared with their parental line. Moreover, there was no difference in survival following dequalinium chloride (Deca) treatment between transfectants and controls. Similarly, reduction of c-fos expression in rat fibroblasts did not significantly alter their mitochondrial membrane potential. In addition, human ovarian carcinoma cells, which overexpress the c-fos gene, did not accumulate more Rh123 nor were they hypersensitive to Deca compared with their parental line. In another human ovarian carcinoma cell line, selection of variants with lower mitochondrial membrane potential did not alter c-fos mRNA or protein levels. These data suggest that alterations in c-fos expression do not regulate the magnitude of the mitochondrial membrane potential.Key words: c-fos, mitochondria, membrane potential, rhodamine 123 (Rh123), lipophilic cations.

Prolonged expression of c-fos suppresses cell cycle entry of dormant hematopoietic stem cells

The proto-oncogene c-fos was transiently upregulated in primitive hematopoietic stem (Lin-Sca-1(+)) cells stimulated with stem cell factor, interleukin-3 (IL-3), and IL-6. To investigate a role of the c-fos in hematopoietic stem cells, we used bone marrow (BM) cells from transgenic mice carrying the c-fos gene under the control of the interferon-alpha/beta-inducible Mx-promoter (Mx-c-fos), and fetal liver cells from c-fos-deficient mice. Prolonged expression of the c-fos in Lin-Sca-1(+) BM cells inhibited factor-dependent colony formation and hematopoiesis on a stromal cell layer by keeping them at G0/G1 phase of the cell cycle. These Lin-Sca-1(+) BM cells on a stromal layer entered into the cell cycle whenever exogenous c-fos was downregulated. However, ectopic c-fos did not perturb colony formation by Lin-Sca-1(+) BM cells after they entered the cell cycle. Furthermore, endogenous c-fos is not essential to cell cycle progression of hematopoietic stem cells because the factor-dependent and the stroma-dependent hematopoiesis by Lin-Sca-1(+) fetal liver cells from c-fos-deficient mice was not impaired. These results suggest that the c-fos induced in primitive hematopoietic stem cells negatively controls cell cycle progression and maintains them in a dormant state.

Prolonged Expression of c-fos Suppresses Cell Cycle Entry of Dormant Hematopoietic Stem Cells

The proto-oncogene c-fos was transiently upregulated in primitive hematopoietic stem (Lin−Sca-1+) cells stimulated with stem cell factor, interleukin-3 (IL-3), and IL-6. To investigate a role of the c-fos in hematopoietic stem cells, we used bone marrow (BM) cells from transgenic mice carrying the c-fos gene under the control of the interferon-/β–inducible Mx-promoter (Mx–c-fos), and fetal liver cells from c-fos–deficient mice. Prolonged expression of the c-fos in Lin−Sca-1+ BM cells inhibited factor-dependent colony formation and hematopoiesis on a stromal cell layer by keeping them at G0/G1 phase of the cell cycle. These Lin−Sca-1+ BM cells on a stromal layer entered into the cell cycle whenever exogenous c-fos was downregulated. However, ectopic c-fos did not perturb colony formation by Lin−Sca-1+ BM cells after they entered the cell cycle. Furthermore, endogenous c-fos is not essential to cell cycle progression of hematopoietic stem cells because the factor-dependent and the stroma-dependent hematopoiesis by Lin−Sca-1+ fetal liver cells from c-fos–deficient mice was not impaired. These results suggest that the c-fos induced in primitive hematopoietic stem cells negatively controls cell cycle progression and maintains them in a dormant state.

E1A + cHa-ras transformed rat embryo fibroblast cells are characterized by high and constitutive DNA binding activities of AP-1 dimers with significantly altered composition

Transcription factors of the AP-1/ATF family, including c-Fos, c-Jun, and ATF-2, play an important role in the regulation of cell proliferation and differentiation, and changes in their levels and/or activities may contribute to oncogenesis. We analyzed the alterations of AP-1/ATF transcription factors upon immortalization and transformation in a panel of cell lines derived from rat embryo fibroblast (REF) cells. The tumorigenic E1A + cHa-ras cells are characterized by high and constitutive DNA binding activities of AP-1, in contrast to nontransformed cells and the E1A cells. The expression of c-fos and c-jun genes was affected differently by the oncogenic transformation. By using antibodies to c-Jun and c-Fos proteins in electrophoretic mobility shift assays (EMSA), we showed that E1A + cHa-ras transformants did not contain c-Fos under any condition of cell cultivation and growth factor stimulation, whereas c-Jun was constitutively upregulated. In the absence of c-fos gene expression, c-Fos protein appears to be replaced by proteins of Fos family (Fra-1) and ATF family (ATF-2 and ATFa). To determine the possible mechanisms of c-fos downregulation in E1A + cHa-ras transformants we have obtained populations of geneticin-resistant clones containing integrated reporter construct -711fos-CAT and its mutants in serum-responsive element (SRE) and cAMP-responsive element (CRE). Data obtained show that the mutations within the SRE lead to a manifold activation of fos-CAT expression. This allows to suggest that c-fos downregulation in E1A + cHa-ras transformants is provided by a negative control mediated through the SRE regulatory region. The profound differences in regulation and composition of transcription factors of the AP-1 family probably play a pivotal role in the transformation of REF cells by E1A and cHa-ras oncogenes.

The transcription factor AP-1 is required for EGF-induced activation of rho-like GTPases, cytoskeletal rearrangements, motility, and in vitro invasion of A431 cells

Human squamous cell carcinomas (SCC) frequently express elevated levels of epidermal growth factor receptor (EGFR). EGFR overexpression in SCC-derived cell lines correlates with their ability to invade in an in vitro invasion assay in response to EGF, whereas benign epidermal cells, which express low levels of EGFR, do not invade. EGF-induced invasion of SCC-derived A431 cells is inhibited by sustained expression of the dominant negative mutant of c-Jun, TAM67, suggesting a role for the transcription factor AP-1 (activator protein-1) in regulating invasion. Significantly, we establish that sustained TAM67 expression inhibits growth factor-induced cell motility and the reorganization of the cytoskeleton and cell-shape changes essential for this process: TAM67 expression inhibits EGF-induced membrane ruffling, lamellipodia formation, cortical actin polymerization and cell rounding. Introduction of a dominant negative mutant of Rac and of the Rho inhibitor C3 transferase into A431 cells indicates that EGF-induced membrane ruffling and lamellipodia formation are regulated by Rac, whereas EGF-induced cortical actin polymerization and cell rounding are controlled by Rho. Constitutively activated mutants of Rac or Rho introduced into A431 or A431 cells expressing TAM67 (TA cells) induce equivalent actin cytoskeletal rearrangements, suggesting that the effector pathways downstream of Rac and Rho required for these responses are unimpaired by sustained TAM67 expression. However, EGF-induced translocation of Rac to the cell membrane, which is associated with its activation, is defective in TA cells. Our data establish a novel link between AP-1 activity and EGFR activation of Rac and Rho, which in turn mediate the actin cytoskeletal rearrangements required for cell motility and invasion.

Glucocorticoid administration alters nuclear transcription factors in fetal rat brain: implications for the use of antenatal steroids

A recent Consensus Conference endorsed antenatal steroid use in prematurity, but indicated the need for future work on molecular and cellular effects on the developing brain. In the current study, pregnant rats were given dexamethasone during late gestation, in doses spanning those recommended for use, and effects on nuclear transcription factors were evaluated. Within the first hour after a single dose of dexamethasone, and intensifying over 4 h, marked induction of brain c-fos was seen. With repeated administration, c-fos became suppressed in some brain regions, but remained elevated in others. Dexamethasone also elicited suppression of the AP-1 family of nuclear binding proteins, but with a slower time course than seen for c-fos induction. The magnitude of the effects of late gestational exposure to dexamethasone on these transcription factors was comparable to those seen when repeated doses were administered to midgestation embryos in the context of dysmorphogenesis. Similarly, the effects on brain c-fos expression were substantially greater than those in the liver, an archetypal glucocorticoid target tissue. These results indicate that even a single, low dose of glucocorticoids used in late gestation, can disrupt the transcription factors that regulate brain cell differentiation.

c-Fos degradation by the proteasome. An early, Bcl-2-regulated step in apoptosis

c-Fos is a transcription factor that promotes cell growth, differentiation, and transformation. We found that c-Fos was degraded when WEHI7.2 mouse lymphoma cells were induced to undergo apoptosis with the calcium ATPase inhibitor, thapsigargin, or the glucocorticoid hormone, dexamethasone. The degradation of c-Fos preceded caspase-3 activation and apoptotic nuclear chromatin condensation and was inhibited by the proteasome inhibitors MG132, N-acetyl-leucyl-leucyl-norleucinal, and lactacystin. Stable transfection of WEHI7.2 cells with a mutant form of c-Fos that was not degraded by the proteasome inhibited apoptosis. Also, overexpression of Bcl-2 in WEHI7.2 cells blocked c-Fos degradation and inhibited apoptosis. The results indicate that proteasome-mediated degradation of c-Fos is an early, Bcl-2-regulated step in apoptosis induction by thapsigargin and dexamethasone. These findings suggest that c-Fos may have a protective action that is eliminated by proteasome-mediated degradation and preserved by Bcl-2.

Upregulation of Jun and Fos family members and permanent JNK activity lead to constitutive AP-1 activation in Theileria-transformed leukocytes

Theileria parasitises bovine leukocytes and transforms them into proliferating, metastatic tumours, where the infection resembles a leukaemia-like disease. We have studied the signal transduction pathways leading to activation of the transcription factor AP-1 in different transformed leukocytes. Parasite infection leads to an up-regulation of all members of the Jun/Fos family of proteins and surprisingly, this occurs in the absence of any detectable ERK, or p38 MAP kinase activity. In the parasitised B-sarcoma TBL3, AP-1 induction occurs in the absence of any JNK activity. In contrast, in infected macrophage and B-cell lines, AP-1 transcriptional activity is strictly associated with the parasite-induced constitutive activation of JNK and subsequent c-Jun N-terminal phosphorylation. Thus, constant AP-1 transcriptional activity involves both an upregulation in the levels of Jun and Fos proteins and constitutive JNK activation.

Human FIGF: cloning, gene structure, and mapping to chromosome Xp22.1 between the PIGA and the GRPR genes

We report the identification, structural characterization, and mapping of the human FIGF gene. FIGF is the human homologue of mouse figf (c-fos-induced growth factor), a new member of the platelet-derived growth factor/vascular endothelial growth factor (PDGF/VEGF) family. It codes for a secreted factor with mitogenic and morphogenic activity on fibroblast cells. The predicted amino acid sequence of FIGF is 84% identical to that of the mouse protein, and it is highly conserved (up to 40%) in the dimerization domain with respect to the VEGF members of the family. The 2.5-kb mRNA of FIGF was detected in adult lung and heart tissues. The gene spans about 50 kb and is organized into seven exons and six introns. The FIGF promoter contains an optimal AP-1-binding site and lacks a canonical TATA box. Fluorescence in situ hybridization mapped FIGF to chromosomal region Xp22.1. The subsequent identification of YAC positive clones from this region allowed us to refine the map and localize FIGF centromeric to the phosphatidylinositol glycan complementation class A (PIGA) gene and telomeric to the gastrin-releasing peptide receptor (GRPR) gene. FIGF and PIGA genes lie next to each other in a head-to-tail orientation, with the FIGF polyadenylation signal about 12 kb from the PIGA transcriptional start site.