Expression of c-fos antisense RNA inhibits the differentiation of F9 cells to parietal endoderm

Towards Gene-Inhibition Therapy: A Review of Progress and Prospects in the Field of Antiviral Antisense Nucleic Acids and Ribozymes

Antisense RNA and its derivatives may provide the basis for highly selective gene inhibition therapies of virus infections. In this review, I concentrate on advances made in the study of antisense RNA and ribozymes during the last five years and their implications for the development of such therapies. It appears that antisense RNAs synthesized at realistic levels within the cell can be much more effective inhibitors than originally supposed. Looking at those experiments that enable comparisons to be made, it seems that inhibitory antisense RNAs are not those that are complementary to particular sites within mRNAs but those that are able to make stable duplexes with their targets, perhaps by virtue of their secondary structure and length. The inclusion of ribozyme sequences within antisense RNAs confers RNA-cleaving activity upon them in vitro and possibly in cells, thereby offering the possibility of markedly increasing their therapeutic potential. The varieties of natural ribozyme and their adaptation as artificial catalysts are reviewed. The implications of these developments for antiviral therapy are discussed.

Regulation of Ras-MAPK pathway mitogenic activity by restricting nuclear entry of activated MAPK in endoderm differentiation of embryonic carcinoma and stem cells

In response to retinoic acid, embryonic stem and carcinoma cells undergo differentiation to embryonic primitive endoderm cells, accompanied by a reduction in cell proliferation. Differentiation does not reduce the activation of cellular MAPK/Erk, but does uncouple mitogen-activated protein kinase (MAPK) activation from phosphorylation/activation of Elk-1 and results in inhibition of c-Fos expression, whereas phosphorylation of the cytoplasmic substrate p90RSK remains unaltered. Cell fractionation and confocal immunofluorescence microscopy demonstrated that activated MAPK is restricted to the cytoplasmic compartment after differentiation. An intact actin and microtubule cytoskeleton appears to be required for the restriction of MAPK nuclear entry induced by retinoic acid treatment because the cytoskeletal disrupting agents nocodazole, colchicine, and cytochalasin D are able to revert the suppression of c-Fos expression. Thus, suppression of cell proliferation after retinoic acid–induced endoderm differentiation of embryonic stem and carcinoma cells is achieved by restricting nuclear entry of activated MAPK, and an intact cytoskeleton is required for the restraint.

c-Fos activated phospholipid synthesis is required for neurite elongation in differentiating PC12 cells

We have previously shown that c-Fos activates phospholipid synthesis through a mechanism independent of its genomic AP-1 activity. Herein, using PC12 cells induced to differentiate by nerve growth factor, the genomic effect of c-Fos in initiating neurite outgrowth is shown as distinct from its nongenomic effect of activating phospholipid synthesis and sustaining neurite elongation. Blocking c-Fos expression inhibited differentiation, phospholipid synthesis activation, and neuritogenesis. In cells primed to grow, blocking c-Fos expression determined neurite retraction. However, transfected cells expressing c-Fos or c-Fos deletion mutants with capacity to activate phospholipid synthesis sustain neurite outgrowth and elongation in the absence of nerve growth factor. Results disclose a dual function of c-Fos: it first releases the genomic program for differentiation and then associates to the endoplasmic reticulum and activates phospholipid synthesis. Because phospholipids are key membrane components, we hypothesize this latter phenomenon as crucial to support membrane genesis demands required for cell growth and neurite elongation.

Disassociation of MAPK activation and c-Fos expression in F9 embryonic carcinoma cells following retinoic acid-induced endoderm differentiation

Retinoic acid induces cell differentiation and suppresses cell growth in a wide spectrum of cell lines, and down-regulation of activator protein-1 activity by retinoic acid contributes to these effects. In embryonic stem cell-like F9 teratocarcinoma cells, which are widely used to study retinoic acid actions on gene regulation and early embryonic differentiation, retinoic acid treatment for 4 days resulted in suppression of cell growth and differentiation into primitive and then visceral endoderm-like cells, accompanied by a suppression of serum-induced c-Fos expression. The MAPK (ERK) pathway was involved in mitogenic signaling in F9 cells stimulated with serum. Surprisingly, although c-Fos expression was reduced, the MAPK activity was not decreased by retinoic acid treatment. We found that retinoic acid treatment inhibited the phosphorylation of Elk-1, a target of activated MAPK required for c-Fos transcription. In F9 cells, the MAPK/MEK inhibitor PD98059 suppressed Elk-1 phosphorylation and c-Fos expression, indicating that MAPK activity is required for Elk-1 phosphorylation/activation. Phosphoprotein phosphatase 2B (calcineurin), the major phosphatase for activated Elk-1, is not the target in the disassociation of MAPK activation and c-Fos expression since its inhibition by cyclosporin A or activation by ionomycin had no significant effects on serum-stimulated c-Fos expression and Elk-1 phosphorylation. Thus, we conclude that retinoic acid treatment to induce F9 cell differentiation uncouples Ras/MAPK activation from c-Fos expression by reduction of Elk-1 phosphorylation through a mechanism not involving the activation of phosphoprotein phosphatase 2B.

Induction of differentiation in F9 cells and activation of peroxisome proliferator-activated receptor delta by valproic acid and its teratogenic derivatives

The antiepileptic drug valproic acid (VPA) is teratogenic, because it induces birth defects in some children of mothers treated for epilepsy. Cellular and molecular actions associated with teratogenicity were identified by testing differentiation of F9 embryocarcinoma cells. VPA altered cell morphology and delayed proliferation. Specific differentiation markers (e.g., c-fos and keratin 18 mRNA and particularly the activating protein-2 transcription factor protein) were induced. This pattern differs from the pattern induced by other teratogens or F9 cell-differentiating agents. Induction of differentiation correlated with teratogenicity because teratogenic derivatives of VPA, such as (S)-4-yn-VPA, induced differentiation, whereas closely related nonteratogenic compounds, such as (R)-4-yn-VPA, 2-en-VPA, and 4-methyl-VPA, did not. In the cellular signaling network, the peroxisome proliferator-activated receptor delta (PPARdelta) was activated selectively by VPA and teratogenic derivatives. Depletion of PPARdelta by antisense RNA expression precluded the response of F9 cells to VPA. In conclusion, our data show that VPA and its teratogenic derivatives induce a specific type of F9 cell differentiation and that PPARdelta is a limiting factor in the control of differentiation.

Antisense targeting of c-fos transcripts inhibits serum- and TGF-beta 1-stimulated PAI-1 gene expression and directed motility in renal epithelial cells

Plasminogen activator inhibitor type-1 (PAI-1), the major regulator of pericellular plasmin generation, and the c-FOS transcription factor are expressed by migrating cells in response to monolayer wounding. Induced c-fos and PAI-1 transcripts were evident within 30 min and 2 h, respectively, of scrape injury to confluent, growth-arrested, cultures of NRK epithelial cells. Since c-FOS/AP-1 DNA-binding activity modulates both basal and inducible modes of PAI-1 gene control, and AP-1 motif binding factors were present in quiescent as well as stimulated NRK cells, a model of directionally regulated cell movement (migration into scrape-denuded "wounds") was used to assess the consequences of c-fos transcript targeting on PAI-1 expression and cell motility. This in vitro model of epithelial injury closely approximated in vivo wound repair with regard to the spatial and temporal emergence of cohorts of cells involved in migration, proliferation, and PAI-1 expression. Stable cell lines (NRKsof) were generated by transfection of parental NRK cells with a c-fos antisense expression vector. Serum-inducible c-fos transcripts and PAI-1 protein levels were significantly attenuated in NRKsof transfectants relative to parental controls or cells transfected with a neo(R) vector without the sof insert. NRKsof cells had a markedly impaired ability to repair scrape-generated monolayer wounds under basal, serum-stimulated, or TGF-beta 1-supplemented culture conditions. Since injury closure and PAI-1 induction were attenuated in c-fos antisense cells, it was important to clarify the role of specific AP-1 sites in serum-mediated PAI-1 transcription. PAI-1 "promoter"-driven CAT reporter expression was assessed within the real time of serum-stimulated PAI-1 induction. A segment of the PAI-1 promoter corresponding to nucleotides -533 to -764 upstream of the transcription start site functioned as a prominent serum-responsive region (SSR). The 9-fold increase in CAT mRNA levels attained with the -533 to -764 bp PAI-1 SRR ligated to a minimal PAI-1 promoter (i.e., 162 bp of 5' flanking sequence containing the basal transcription complex) closely approximated the serum-induced transcriptional activity of a fully responsive (1,230 bp) PAI-1 promoter construct as well as the endogenous PAI-1 gene. AP-1-like, CTF/NF-1-like, and AP-2 sites were identified in the SRR. The SRR AP-1 motif was homologous to the sequence TGACACA that mapped between nucleotides -740 and -703 in the human PAI-1 gene, a region essential for growth factor-inducible PAI-1 transcription. While the functionality of this AP-1 site in wound-regulated PAI-1 synthesis remains to be determined, antisense c-fos transcripts effectively attenuated PAI-1 induction and basal as well as growth factor-stimulated cell locomotion, indicating that expression of both the PAI-1 and c-fos genes is necessary for wound-initiated NRK cell migration.

Suppression of post-ischemic-induced fos protein expression by an antisense oligonucleotide to c-fos mRNA leads to increased tissue damage

Activation of c-fos, an immediate early gene, and the subsequent upregulation of Fos protein expression occur following neural injury, including focal cerebral ischemia (fci). Fos and Jun form a heterodimer known as activator protein 1, which regulates the expression of many late effector genes. To study the downstream effects of c-fos expression following ischemia, we suppressed the translation of c-fos by administering an antisense oligonucleotide (AO) to c-fos mRNA. Eighteen hours prior to fci, male, Long Evans (LE) rats received intraventricular injections of AO, mismatched AO (MS) or artificial cerebrospinal fluid (aCSF). Fci was induced by permanent right middle cerebral artery occlusion. At 24-h post-occlusion, neurological function was assessed, and the animals were sacrificed. The brains were removed and stained with triphenyltetrazolium chloride for infarct volume determination. Fos immunohistochemistry was performed in separate animals to determine the effects of treatment on Fos expression number of Fos positive cells. AO administration reduced the number of cells with fci-induced Fos expression by approximately 75%. No differences in neurological scores existed between any of the groups. AO-treated LE developed larger infarcts (40.1+/-1.0%, mean+/-S.D., p<0.001) than MS- or aCSF-treated controls (34.3+/-1.0%, 34.6+/-1.0%, respectively). These results suggest that c-fos activation and subsequent Fos protein expression exerts a neuroprotective effect, which is likely via upregulation of neurotrophins, following focal cerebral ischemia. This response, among others, may contribute to brain adaptation to injury that underlies functional recovery after stroke.

Suppression of postischemic hippocampal nerve growth factor expression by a c-fos antisense oligodeoxynucleotide

We examined the uptake and distribution of an antisense phosphorothioated oligodeoxynucleotide (s-ODN) to c-fos, rncfosr115, infused into the left cerebral ventricle of male Long-Evans rats and the effect of this s-ODN on subsequent Fos, NGF, neurotrophin-3 (NT-3), and actin expression. To establish the uptake and turnover of s-ODN in the brain, we studied the copurification of the immunoreactivity of biotin with biotinylated s-ODN that was recovered from different regions of the brain. A time-dependent diffusion and the localization of s-ODN were further demonstrated by labeling the 3'-OH terminus of s-ODN in situ with digoxigenin-dUTP using terminal transferase and detection using anti-digoxigenin IgG-FITC. Cellular uptake of the s-ODN was evident in both the hippocampal and cortical regions, consistent with a gradient originating at the ventricular surface. Degradation of the s-ODN was observed beginning 48 hr after delivery. The effectiveness of c-fos antisense s-ODN was demonstrated by its suppression of postischemic Fos expression, which was accompanied by an inhibition of ischemia-induced NGF mRNA expression in the dentate gyrus. Infusion of saline, the sense s-ODN, or a mismatch antisense s-ODN did not suppress Fos expression. That this effect of c-fos antisense s-ODN was specific to NGF was demonstrated by its lack of effect on the postischemic expression of the NT-3 and beta-actin genes. Our results demonstrate that c-fos antisense s-ODN blocks selected downstream events and support the contention that postischemic Fos regulates the subsequent expression of the NGF gene and that Fos expression may have a functional component in neuroregeneration after focal cerebral ischemia-reperfusion.

Association of c-fos mRNA expression and excitotoxicity in primary cultures of mouse neocortical and cerebellar neurons

The effect of excitatory amino acids (EAAs) on c-fos mRNA expression was studied in primary cultures of mouse cerebellar granule cells and in neocortical neurons after 2 and 7 days in vitro (div). In cultured granule cells at 2 and 7 div, and in cortical neurons at 2 div, exposure to low levels (< or = 10 microM) of a variety of EAAs (viz. glutamate [Glu], S-sulpho-L-cysteine [SC], N-methyl-D-aspartate [NMDA], alpha-amino-3-hydroxy-5-methyl-4-isoxazole [AMPA], and kainate [KA]) resulted in a transient increase in the level of c-fos mRNA which peaked at 30 min but returned to a basal level by 120 min. However, exposure of granule cells (7 div) to high levels (250 microM) of Glu, NMDA, KA, SC and of cortical neurons (7 div) to high levels (250 microM) of Glu, NMDA, KA, SC, or AMPA and to low levels (< or = 10 microM) of Glu and AMPA resulted in a delay in c-fos mRNA induction but a subsequent, progressive increase that was sustained for at least 240 min. Furthermore, this effect was accompanied by a dose-related increase in the release of the cytosolic enzyme, lactate dehydrogenase, used as an indicator of excitotoxicity. A ratio (Q240/30) for the steady-state levels of c-fos mRNA after 30 min and 240 min of exposure to EAAs was determined which showed that Q240/30 >2 correlated reproducibly with excitotoxic cell death, whereas a ratio of < or = 1 correlated with a nonexcitotoxic event. In both cell types at 7 div, coadministration of the selective NMDA receptor antagonist, DL(+/-)-2-amino-5-phosphonopentanoic acid (APV) with cytotoxic levels of Glu 1) protected against EAA-induced neurotoxicity and 2) exhibited a transient c-fos mRNA expression (Q240/30 values approximately 1). In contrast, the AMPA/KA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), provided no protection against excitotoxicity and had no significant effect on the Glu-induced delay in c-fos mRNA expression. These results suggest that the Q240/30 c-fos mRNA ratio may 1) be used as a predictive index for excitotoxic neuronal death, 2) provide information on the identity of the receptor subtype mediating excitotoxicity in different brain cell types, and 3) aid in establishing the role of excitotoxicity during the development of neurons in vitro.

Brain substrates activated by electroacupuncture (EA) of different frequencies (II): Role of Fos/Jun proteins in EA-induced transcription of preproenkephalin and preprodynorphin genes

Antisense oligodeoxynucleotides (ODNs) of c-fos and/or c-jun were used in this study to investigate the role of Fos and Jun proteins in electroacupuncture (EA)-induced transcription of the opioid genes, preproenkephalin (PPE), preprodynorphin (PPD) and proopiomelanocortin (POMC). As the results showed, EA-induced Fos and fun expression was blocked efficiently and specifically by e-fos and c-jun antisense ODNs, respectively. This treatment significantly prevented EA-induced PPD, but not PPE, mRNA expression. These results suggest that Fos and Jun proteins are involved in PPD rather than PPE gene transcription activated by EA stimulation.

Suppression of ischemia-induced fos expression and AP-1 activity by an antisense oligodeoxynucleotide to c-fos mRNA

Activation of c-fos, an immediate early gene, and the subsequent expression of the Fos protein have been noted following focal cerebral ischemia. Fos and Jun form a heterodimer as activator protein 1 (AP-1), which transregulates the expression of several genes. To study the postischemic events related to c-fos expression, we suppressed the expression of c-fos by intraventricular infusion of an antisense oligodeoxynucleotide (anti-rncfosr115) of c-fos mRNA. The effectiveness of anti-rncfosr115 was confirmed first by its capability to block in vitro c-fos mRNA translation. In vivo, after intraventricular infusion of 32P-labeled anti-rncfosr115, the oligodeoxynucleotide was internalized within 6 hours and detectable also in the nucleic acids fraction up to 41 hours. Treatment of the recovered nucleic acids with RNase H separated the labeled oligodeoxynucleotide from the nucleic acid fraction, indicating an association of the antisense oligodeoxynucleotide and cellular RNA after uptake. When focal cerebral ischemia was induced 16 hours after the infusion of anti-rncfosr115, the postischemic increase in Fos expression and AP-1 binding activity were suppressed. Specificity of the effect of anti-rncfosr115 was suggested by its failure to suppress the DNA binding activity of nuclear cyclic AMP response elements. These results support the hypothesis that increased AP-1 binding activity following focal cerebral ischemia is dependent on Fos expression and can be inhibited in vivo by antisense c-fos oligodeoxynucleotides.

Pleiotropic effects of a null mutation in the c-fos proto-oncogene

The c-fos proto-oncogene has been implicated as a central regulatory component of the nuclear response to mitogens and other extracellular stimuli. Embryonic stem cells targeted at the c-fos locus have been used to generate chimeric mice that have transmitted the mutated allele through the germline. Homozygous mutants show reduced placental and fetal weights and significant loss of viability at birth. Approximately 40% of the homozygous mutants survive and grow at normal rates until severe osteopetrosis, characterized by foreshortening of the long bones, ossification of the marrow space, and absence of tooth eruption, begins to develop at approximately 11 days. Among other abnormalities, these mice show delayed or absent gametogenesis, lymphopenia, and altered behavior. Despite these defects, many live as long as their wild-type or heterozygous littermates (currently 7 months). These data indicate that c-fos is not required for the growth of most cell types but is involved in the development and function of several distinct tissues.

Antisense promoter mapping. Inhibitory methods of transcriptional analysis

We have employed antisense methods to study the transcriptional functions of c-fos protein (Fos). Clones expressing inducible anti-fos RNA have been employed to inhibit c-fos expression, resulting in activation of c-fos transcription by inhibiting its normal repressor function. The sites of negative regulation by Fos have been mapped using this antisense mapping method which demonstrates that the serum response element represents the major site of repression by endogenous c-fos protein. A similar strategy (antisense cloning) has been employed to clone four target genes that are Fos dependent. These cDNAs encode mRNAs that are rapidly induced by serum (although this induction is blocked by cycloheximide) but are blocked by induction of anti-fos RNA. These inhibitory methods of studying transcription factor function are extremely useful for transcription factors (like Fos) that require cooperation with other factors to modulate gene transcription.

Long-term expression of the c-fos protein during the in vitro differentiation of cerebellar granule cells induced by potassium or NMDA

Levels of the c-fos protein were assayed in mouse cerebellar granule cells during their in vitro development under different culture conditions. When grown in media favoring both their survival and differentiation, i.e. in the presence of 30 mM K+ or 12.5 mM K+ plus 100 microM N-methyl-D-aspartate (NMDA), the c-fos protein becomes detectable in the nucleus of granule cells on and after 6 days and persists to high levels until the culture begins to decline. The protein c-fos appears therefore after the critical period described for the survival effect of K+ depolarization or NMDA receptor stimulation which corresponds to days 2-5 after plating. The c-fos protein remains however scarcely detectable or undetectable throughout the life-span of cells cultured under conditions providing poor survival and differentiation, i.e. in the presence of low K+ (5 or 12.5 mM) alone or when the effect of NMDA is blocked by the NMDA receptor antagonist MK-801. Interestingly, in cortical and striatal neurons, the survival and differentiation of which being not affected by depolarizing media, no c-fos protein is detected whatever the culture conditions tested at least during the first 18 days in vitro. This suggests that long-term expression of the c-fos gene might be related to some aspect of the late in vitro differentiation process of cerebellar granule cells.

The transcription factor, Egr-1, is rapidly modulated in response to retinoic acid in P19 embryonal carcinoma cells

The pluripotent murine embryonal carcinoma cell line, P19, differentiates along at least three main pathways under the inductive influence of retinoic acid (RA). The events most critical to the establishment of a particular differentiation pathway must occur early since P19 cells are committed to differentiation pathways after 30 min of exposure to RA (M. W. McBurney, personal communication and our unpublished results). We have, therefore, looked for genes that are induced (or repressed) within 30 min of RA addition and find that Egr-1 is one of these genes. Egr-1 is a transcription factor of the zinc-finger class and is known to transactivate genes after binding to specific oligonucleotide sequences. We describe here the extremely rapid and transient increase of Egr-1 transcript and protein levels in P19 cells after RA addition. Stable induction of Egr-1 transcripts occurred in the presence of protein synthesis inhibitors. Simultaneous addition of RA and cycloheximide did not result in an additive effect. The mechanism of induction with either drug appears to involve relief of a block to transcriptional elongation. The response was more rapid at high RA concentrations and this suggests that the Egr-1 transcription factor could play a role in initiation of differentiation pathways of P19 EC cells.

Overexpression of uvomorulin in a compaction-negative F9 mutant cell line

The mutant F9 cell line F9att-5.51 synthesizes reduced amounts of uvomorulin (UM) protein and we hypothesized earlier (Adamson, Baribault, and Kemler, Dev. Biol. (1990), 138, 338) that this may account for its inability to compact into tightly aggregated balls of cells. Subsequently, when 5.51 cells are treated with retinoic acid to stimulate their differentiation, they are unable to form embryoid bodies as do wild-type cells which form an outer epithelial layer of visceral endoderm cells. We have now examined the possibility that the UM protein made in the mutant line is defective, but find that it is normal in structure and stability. The gene coding for UM appears to be normal as does the mRNA which is synthesized at a normal rate but is severely reduced in steady-state measurements of mutant cells. A rescue experiment was performed by increasing levels of UM in mutant cells by means of transfection with a UM expression vector. The resulting cells expressed abundant UM mRNA and protein but were still unable to form compacted aggregates and did not differentiate into embryoid bodies. Interestingly, the stability of endogenous UM mRNA was improved in the presence of exogenous UM; therefore, a positive feedback mechanism contributes to low mRNA levels in mutant cells. The accumulated data suggest that UM in 5.51 cells is unable to mount a compaction activity because a distal connecting link in the multicomponent process initiated by UM is missing or or aberrant. The missing component is likely to connect UM to actin and the cytoskeleton of the cell.

Developmental regulation of gap junction gene expression during mouse embryonic development

The expression of products from three different gap junction genes (alpha 1, beta 1 and beta 2) was studied in pre- and postimplantation mouse embryos, during organogenesis, during differentiation of F9 teratocarcinoma cells, and in cultured embryonic stem (ES) cells. In this analysis, the following results were obtained. 1) Pre- and postimplantation mouse embryos. The alpha 1 transcript was the earliest gap junction RNA detected (in the 4 cell stage embryo) and its abundance increased significantly throughout subsequent development. 2) Organogenesis. Evidence was obtained for developmental expression of these three different gap junction genes during early embryogenesis and throughout the late stages of organogenesis. The expression patterns for these genes may be related to differences in gap junctional communication requirements for fetal organ development versus neonatal and adult organ function, or the utilization of different genes by different cell types during organogenesis. 3) During the differentiation of F9 cells in culture, expression of these three genes was modulated. Thus, this is the first evidence for modulation of gap junction gene expression during the differentiation of a single cell type in culture. 4) In an ES cell culture line, alpha 1 was the only gap junction gene product detected. This is consistent with the findings of alpha 1 expression in the embryonic inner cell mass region and in undifferentiated teratocarcinoma cells.

The rodent B2 sequence can affect expression when present in the transcribed region of a reporter gene

The mouse B2 element is a moderately repetitive nt sequence of 180 bp transcribed by RNA polymerase III (Pol III) at high levels in embryonic and transformed cells. The B2 sequence is present in either orientation within the noncoding regions of a number of genes transcribed by RNA polymerase II (Pol II). We sought to determine if the small B2 transcripts generated by Pol III are natural antisense RNA molecules which might hybridize to complementary sequences present within Pol II transcripts. Chimaeric reporter genes encoding Escherichia coli gpt were constructed containing a B2 repeat in either orientation within the 5'- or 3'-untranslated regions. These constructs were transfected into embryonal carcinoma (EC) cells and expression of the reporter gene was analysed in EC cells and retinoic acid-treated EC cells, which contain high and low levels of small B2 RNAs, respectively. Although the B2 sequences affected expression of the reporter gene, these effects did not appear to be due to hybridization of the small B2 RNA to the reporter transcripts. The presence of B2 sequences near a Pol II-transcribed gene can alter expression of that gene in a position- and orientation-dependent manner, suggesting these repetitive elements may be cis-acting regulators of gene expression.

Antisense RNA production in transgenic mice

There are many reports of antisense inhibition of gene expression in cultured cells. We have generated four strains of transgenic mice expressing antisense hypoxanthine guanine phosphoribosyltransferase (HPRT) RNA in brain, or heart and liver, or all three organs. In the brain of one strain, the level of antisense RNA in the different brain regions roughly correlates with the degree of inhibition of the native HPRT mRNA in those same regions. Despite this decrease of up to 60% of endogenous HPRT mRNA, no reproducible reduction in HPRT activity has been observed. Possible reasons for the differences between the effectiveness of antisense inhibition in cultured cells and transgenic animals are discussed.

Enhanced expression of c-fos is not obligatory for retinoic acid-induced F9 cell differentiation

c-fos was studied in F9 cells to determine whether changes in its expression are an early and/or obligatory event in retinoic acid-induced F9 cell differentiation. Induction of c-fos transcripts was not observed at times early or late during retinoic acid-promoted differentiation, but a decrease in c-myc mRNA was noted as early as 1 h after retinoic acid dosing. Induction of a rapid and transient change in c-fos expression in F9 cells was observed only in response to serum stimulation. Therefore, although expression of c-fos may be involved in the cellular growth and proliferation of F9 cells, as indicated by the response to serum, an increase in c-fos is not required for retinoic acid-induced differentiation.

Activation of an intron enhancer within the keratin 18 gene by expression of c-fos and c-jun in undifferentiated F9 embryonal carcinoma cells

The mouse forms of human keratins 18 and 8 (K18 and K8) are the first members of the large intermediate filament gene family to be expressed during embryogenesis. To identify potential regulatory elements of the human K18 gene, various recombinant constructions were expressed in cultured cells. An enhancer element was found in the first intron that functions on both the K18 and thymidine kinase promoters in differentiated cells. In F9 embryonal carcinoma cells, the level of expression was low in the presence or absence of the first intron. Cotransfection of F9 cells with K18 constructs that include the first intron and increasing amounts of an expression vector of c-jun results in a modest increase in the reporter gene expression. Cotransfection of the same construct with increasing amount of the mouse c-fos gene results in activation of the reporter gene by as much as 15-fold, with a near linear response to the amount of c-fos gene added. Site-specific mutagenesis of a putative AP-1 site within the intron abolishes trans-activation by c-fos in F9 cells. Furthermore, induction of c-fos in a derivative of F9 cells results in increased expression of the endogenous mouse form of K18. Cotransfection with c-jun or c-fos expression vectors had little effect on the expression of the K18 reporter construct in a parietal endodermal cell line already expressing the endogenous mouse gene. These results identify an enhancer within the first intron of K18 that may interact directly with c-jun and c-fos via a conserved AP-1-binding site. K18 expression in undifferentiated F9 cells may be limited by the low levels of c-jun and c-fos.

Cerebellar granule cell survival and maturation induced by K+ and NMDA correlate with c-fos proto-oncogene expression

Persistent depolarization with a high K+ concentration (30 mM) or sustained activation of N-methyl-D-aspartate (NMDA) receptors (12.5 mM K+ plus 100 microM NMDA) enhance both survival and maturation of mouse cerebellar granule neurons in vitro taking as criteria the amount of protein and DNA and the release of endogenous glutamate respectively. K+ and NMDA neurotrophic effects are associated with c-fos protein expression in the nucleus of these cells suggesting that c-fos protein could play a role in the survival and/or maturation of granule neurons.

Stepwise aggregation, compaction, and differentiation of uncompacted F9 cells

To study the relationship between compaction and differentiation in aggregates of F9 embryonal carcinoma cells, a subline was developed which grows mostly uncompacted in monolayer culture in medium containing a low concentration of calcium (about 0.05 mM). When these cells were trysinized and cultured in suspension in the same medium, they formed loose, open aggregates, which failed to differentiate into embryoid bodies after exposure to 10 nM retinoic acid, confirming the requirement of compaction for differentiation. If, after culture for 3 days, the uncompacted F9 aggregates were exposed to additional calcium (4 nM), all compacted within an hour. The number of days necessary for aggregates to acquire this ability to compact rapidly was reduced if the monolayer of cells from which the aggregates were derived had been exposed to additional calcium to cause compaction for several days prior to trypsinization and aggregation. Next, treatment of the compacted F9 aggregates with 10 nM retinoic acid was followed by differentiation into embryoid bodies. The number of days required for this was also reduced if the aggregates were formed from previously compacted cells, presumably because compaction of the aggregates occurred sooner. The acceleration in compaction and differentiation in aggregates formed from previously compacted cells suggests that some of the proteins important for compaction, which are synthesized in a monolayer of compacted cells, persist through trypsinization and are carried over from monolayer to aggregates. Alternatively, an inhibitor of compaction is decreased in the compacted monolayer.(ABSTRACT TRUNCATED AT 250 WORDS)