c-myc regulation during retinoic acid-induced differentiation of F9 cells is posttranscriptional and associated with growth arrest

Regulation of TFIIIB during F9 cell differentiation

BACKGROUND

Differentiation of F9 embryonal carcinoma (EC) cells into parietal endoderm (PE) provides a tractable model system for studying molecular events during early and inaccessible stages of murine development. PE formation is accompanied by extensive changes in gene expression both in vivo and in culture. One of the most dramatic is the ~10-fold decrease in transcriptional output by RNA polymerase (pol) III. This has been attributed to changes in activity of TFIIIB, a factor that is necessary and sufficient to recruit pol III to promoters. The goal of this study was to identify molecular changes that can account for the low activity of TFIIIB following F9 cell differentiation.

RESULTS

Three essential subunits of TFIIIB decrease in abundance as F9 cells differentiate; these are Brf1 and Bdp1, which are pol III-specific, and TBP, which is also used by pols I and II. The decreased levels of Brf1 and Bdp1 proteins can be explained by reduced expression of the corresponding mRNAs. However, this is not the case for TBP, which is regulated post-transcriptionally. In proliferating cells, pol III transcription is stimulated by the proto-oncogene product c-Myc and the mitogen-activated protein kinase Erk, both of which bind to TFIIIB. However, c-Myc levels fall during differentiation and Erk becomes inactive through dephosphorylation. The diminished abundance of TFIIIB is therefore likely to be compounded by changes to these positive regulators that are required for its full activity. In addition, PE cells have elevated levels of the retinoblastoma protein RB, which is known to bind and repress TFIIIB.

CONCLUSION

The low activity of TFIIIB in PE can be attributed to a combination of changes, any one of which could be sufficient to inhibit pol III transcription. Declining levels of essential TFIIIB subunits and of activators that are required for maximal TFIIIB activity are accompanied by an increase in a potent repressor of TFIIIB. These events provide fail-safe guarantees to ensure that pol III output is appropriate to the diminished metabolic requirements of terminally differentiated cells.

Evidence of extra-telomeric effects of hTERT and its regulation involving a feedback loop

The human telomerase reverse transcriptase (hTERT) is the catalytic subunit of the enzyme telomerase which is responsible for telomeric maintenance and extension. Using RNA interference to knock down hTERT mRNA expression, we provide evidence that hTERT exerts extra-telomeric effects on the cell cycle and on its own regulatory proteins, specifically: p53 and p21. We tested our hypothesis that hTERT regulates its own expression through effects on upstream regulatory genes using transformed human embryonic kidney (HEK 293) cells, p53 and p16(INK4a) null human ovarian cancer SKOV-3 cells, and p53-null MDA-MB-157 human mammary cancer cells. In HEK 293 cells, hTERT knockdown resulted in elevated p53 and p21 transcription and a decrease in cellular proliferation. Similar results were observed in the MDA-MB-157 cell line where p21 was upregulated, correlating with cell growth inhibition. In contrast, we observed a decrease in expression of p21 in SKOV-3 cells with hTERT knockdown and cell growth appeared to be unaffected. These findings suggest that hTERT may be involved in a feedback loop system, thereby playing a role in its own regulation.

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.

Coding elements in exons 2 and 3 target c-myc mRNA downregulation during myogenic differentiation

Downregulation in expression of the c-myc proto-oncogene is an early molecular event in differentiation of murine C2C12 myoblasts into multinucleated myotubes. During differentiation, levels of c-myc mRNA decrease 3- to 10-fold despite a lack of change in its transcription rate. To identify cis-acting elements that target c-myc mRNA for downregulation during myogenesis, we stably transfected C2C12 cells with mutant myc genes or chimeric genes in which various myc sequences were fused to the human beta-globin gene or to the bacterial chloramphenicol acetyltransferase (CAT) gene. Deletion of coding sequences from myc exon 2 or exon 3 abolished downregulation of myc mRNA during myogenic differentiation, while deletion of introns or sequences in the 5' or 3' untranslated regions (UTRs) did not, demonstrating that coding elements in both exons 2 and 3 are necessary for myc mRNA downregulation. Fusion of coding sequences from either myc exon 2 or 3 to beta-globin mRNA conferred downregulation onto the chimeric mRNA, while fusion of myc 3' UTR sequences or coding sequences from CAT or ribosomal protein L32 did not, demonstrating that coding elements in myc exons 2 and 3 specifically confer downregulation. These results present the apparent paradox that coding elements in either myc exon 2 or myc exon 3 are sufficient to confer downregulation onto beta-globin mRNA, but neither element alone was sufficient for myc mRNA downregulation, suggesting that some feature of beta-globin mRNA may potentiate the regulatory properties of myc exons 2 and 3. A similar regulatory function is not shared by all mRNAs because fusion of either myc exon 2 or myc exon 3 to CAT mRNA did not confer downregulation onto the chimeric mRNA, but fusion of the two elements together did. We conclude from these results that two myc regulatory elements, one exon 2 and one in exon 3, are required for myc mRNA downregulation. Finally, using a highly sensitive and specific PCR-based assay for comparing mRNA levels, we demonstrated that the downregulation mediated by myc exons 2 and 3 results in a decrease in cytoplasmic mRNA levels, but not nuclear mRNA levels, indicating that regulation is a postnuclear event.

Identification of sequences in c-myc mRNA that regulate its steady-state levels

The level of cellular myc proto-oncogene expression is rapidly regulated in response to environmental signals and influences cell proliferation and differentiation. Regulation is dependent on the fast turnover of c-myc mRNA, which enables cells to rapidly alter c-myc mRNA levels. Efforts to identify elements in myc mRNA responsible for its instability have used a variety of approaches, all of which require manipulations that perturb normal cell metabolism. These various approaches have implicated different regions of the mRNA and have led to a lack of consensus over which regions actually dictate rapid turnover and low steady-state levels of c-myc mRNA. To identify these regions by an approach that does not perturb cell metabolism acutely and that directly assesses the effect of a c-myc mRNA region on the steady-state levels of c-myc mRNA, we developed an assay using reverse transcription and PCR to compare the steady-state levels of human myc mRNAs transcribed from two similarly constructed myc genes transiently cotransfected into proliferating C2C12 myoblasts. Deletion mutations were introduced into myc genes, and the levels of their mRNAs were compared with that of a near-normal, reference myc mRNA. Deletion of most of the myc 3' untranslated region (UTR) raised myc mRNA levels, while deletion of sequences in the myc 5' UTR (most of exon 1), exon 2, or the protein-coding region of exon 3 did not, thus demonstrating that the 3' UTR is responsible for keeping myc mRNA levels low. Using a similar reverse transcription-PCR assay for comparing the steady-state levels of two beta-globin-myc fusion mRNAs, we showed that fusion of the myc 3' UTR lowers globin mRNA levels by destabilizing beta-globin mRNA. Surprisingly, fusion of the protein-coding region of myc exon 3 also lowered globin mRNA steady-state levels. Investigating the possibility that exon 3 coding sequences may play some other role in regulating c-myc mRNA turnover, we demonstrated that these sequences, but not myc 3' UTR sequences, are necessary for the normal posttranscriptional downregulation of c-myc mRNA during myoblast differentiation. We conclude that, while two elements within c-myc mRNA can act as instability determinants in a heterologous context, only the instability element in the 3' UTR regulates its steady-state levels in proliferating C2C12 cells.

Altered distribution of the nuclear receptor RAR beta accompanies proliferation and differentiation changes caused by retinoic acid in Caco-2 cells

All epithelial cells require retinoic acid for growth, maintenance, and differentiation. Although the epithelial cells that line the gastrointestinal tract are exposed to extreme retinoid concentration fluctuations in luminal fluid, whether proliferation and differentiation in these cells are significantly affected is not known. We have investigated this question using Caco-2 cells as a model because, although they are derived from a colon adenocarcinoma, they differentiate spontaneously in a manner similar to enterocytes in the small intestine. We found that retinoic acid caused maximum inhibition of cell growth and ornithine decarboxylase activity during the proliferative period. Retinoic acid increased brush border enzyme activities only in differentiating cells but stimulated transglutaminase activity in cells at all stages. In untreated proliferating cells, we found an early peak of transglutaminase activity that has not been reported before. Retinoic acid in intestinal cells acts through its nuclear receptor, RAR beta. The nuclear distribution of this receptor has not been demonstrated. In this study, we show that RAR beta responds to increasing concentrations of retinoic acid with a shift to the nuclear membrane in undifferentiated cells and progressive aggregation, diffusion, and loss in differentiated cells. We conclude that retinoic acid can inhibit proliferation and stimulate differentiation in Caco-2 cells depending on concentration and cell stage, and that these effects are accompanied by changes in distribution, as well as by the loss of RAR beta.

Changes of proliferative activity and phenotypes in spontaneous differentiation of a colon cancer cell line

We examined the alterations of proliferative activity and c-myc expression of a colon cancer cell line (Caco-2) during its spontaneous differentiation. Caco-2 cells were cultured in various types of media and the degree of differentiation was monitored in terms of dome formation in cell monolayers and expression of alkaline phosphatase (ALP) activity. In Caco-2 cells cultured with Eagle's minimum essential medium (EMEM) containing 10% fetal calf serum (FCS), dome formation was demonstrated and ALP activity was markedly increased after the cells reached confluence. Five-fold reduction of c-myc mRNA and a marked decrease in S-phase cells were observed in the differentiated cells. These changes were not induced in FCS-free EMEM. The addition of insulin and transferrin to FCS-free EMEM did not induce cell differentiation or reduction of c-myc mRNA expression. When Caco-2 cells were cultured with three different serum-free media, the induction of dome formation and the increase of ALP activity were observed to varying degrees. Expression of c-myc mRNA in the cells cultured with one serum-free medium decreased to a level similar to that in fully differentiated cells cultured with EMEM containing 10% FCS. These results suggest that a spontaneous switch from a proliferative state with high c-myc expression to differentiated phenotype occurs after cells reach confluence and depends on the culture conditions.

A negative regulatory element in the human papillomavirus type 16 genome acts at the level of late mRNA stability

A negative regulatory element present in the human papillomavirus type 16 genome has been characterized. Deletion analysis has localized the 5' end of the element to the late region of the genome at the extreme 3' end of the coding region of the L1 open reading frame, around the L1 stop codon, with the element extending into the L1 3' untranslated region. For the cell lines used, the element's function was independent of cell type, tissue, or species of origin, unlike papillomavirus infection, which is very dependent on such factors. By using an mRNA decay assay, we have determined that polyadenylated RNA containing the element is much less stable than polyadenylated RNA lacking the element. This indicates that the element acts as an mRNA instability element. The significance of A-rich, GU-rich, and AUG-rich sequences for the functioning of this human papillomavirus type 16 instability element is discussed.

Regulated expression of the small nuclear ribonucleoprotein particle protein SmN in embryonic stem cell differentiation

The SmN protein is a component of small nuclear ribonucleoprotein particles and is closely related to the ubiquitous SmB and B' splicing proteins. It is expressed in a limited range of tissues and cell types, including several undifferentiated embryonal carcinoma cell lines and undifferentiated embryonic stem cells. The protein declines to undetectable levels when embryonal carcinoma or embryonic stem cells are induced to differentiate, producing primitive endoderm or parietal endoderm or yielding embryonal bodies. This decline is due to a corresponding decrease in the level of the SmN mRNA. The potential role of SmN in the regulation of alternative splicing in embryonic cell lines and early embryos is discussed.

Regulated expression of the small nuclear ribonucleoprotein particle protein SmN in embryonic stem cell differentiation

The SmN protein is a component of small nuclear ribonucleoprotein particles and is closely related to the ubiquitous SmB and B' splicing proteins. It is expressed in a limited range of tissues and cell types, including several undifferentiated embryonal carcinoma cell lines and undifferentiated embryonic stem cells. The protein declines to undetectable levels when embryonal carcinoma or embryonic stem cells are induced to differentiate, producing primitive endoderm or parietal endoderm or yielding embryonal bodies. This decline is due to a corresponding decrease in the level of the SmN mRNA. The potential role of SmN in the regulation of alternative splicing in embryonic cell lines and early embryos is discussed.

E1a-dependent expression of adenovirus genes in OTF963 embryonal carcinoma cells: role of E1a-induced differentiation

Some undifferentiated F9 embryonal carcinoma cells allow adenovirus genes to be expressed independently of the E1a oncogene normally required for their activation; this has been attributed to a cellular equivalent of E1a in F9 cells. However, transcription of all early genes was low in undifferentiated OTF963 embryonic carcinoma cells during the first 48 hr after infection with adenovirus type 5 (Ad5). Transcription then increased to about the level seen 16 hr after infection of cells induced to differentiate by retinoic acid (RA) (referred to as RA-dF9 cells), but this increase did not occur in cells infected by the E1a deletion mutant dl312. Addition of E1a in trans, or of RA, had no immediate effect on viral transcription in OTF963 cells, but viral transcription increased about 48 hr after these additions. Ad5 induced transcription of several differentiation-specific genes in OTF963 cells with about the same kinetics as their induction by RA. These genes were superinduced in RA-dF9 cells by cAMP or infection by adenovirus. We suggest the small amount of E1a produced early in infection of OTF963 cells activates cellular genes, some of which are differentiation specific and required for efficient transcription of viral genes, so that E1a both induces and is induced by differentiation. The simple hypothesis of a cellular equivalent to E1a does not adequately explain the complex interactions between viral and cellular genes in OTF963 embryonic carcinoma cells.

Defective expression of adenovirus genes during early infection of undifferentiated OTF963 embryonal carcinoma cells

Adenovirus infection was compared in F9 (OTF963) cells and cells induced to differentiate with retinoic acid, in order to study expression of early genes under the control of the reported "E1a-like factor" in F9 cells. However, not only was transcription of the viral E1a gene defective in undifferentiated cells but expression of all the other early genes was found to be reduced in OTF963 cells in comparison to differentiated cells. The defect in early gene expression was detected at the level of transcriptional initiation during the first 48 h of infection and resulted in similarly low levels of viral cytoplasmic mRNA and viral protein synthesis. Viral DNA replication was delayed and reduced. After 48 h of infection, the defect in transcription in OTF963 cells of E1a and other early genes was relieved, so that by 72 h postinfection the level of transcription was similar to that 16 h after infection of differentiated cells. At no time did adenovirus early gene expression occur independently of viral E1a. These results suggest limits to the generality and explanatory power of the hypothesis that F9 embryonal carcinoma cells contain an E1a-like factor.

c-myc, c-fos, and c-jun regulation in the regenerating livers of normal and H-2K/c-myc transgenic mice

We investigated the mechanisms of regulation of c-myc, c-fos, and c-jun at the early stages of liver regeneration in mice. We show that the transient increase in steady-state levels of c-myc mRNA at the start of liver regeneration is most probably regulated by posttranscriptional mechanisms. Although there was a marked increase in c-myc transcriptional initiation shortly after partial hepatectomy, a block in elongation prevented the completion of most transcripts. To gain further information on the mechanism of regulation of c-myc expression during liver regeneration, we used transgenic mice harboring the human c-myc gene driven by the H-2K promoter. In these animals, the murine c-myc responded to the growth stimulus generated by partial hepatectomy, whereas the expression of the transgene was constitutive and did not change in the regenerating liver. However, the mRNA from both genes increased markedly after cycloheximide injection, suggesting that the regulation of c-myc mRNA abundance in the regenerating liver differs from that occurring after protein synthesis inhibition. Furthermore, we show that in normal mice c-fos and c-jun mRNA levels and transcriptional rates increase within 30 min after partial hepatectomy. c-fos transcriptional elongation was restricted in nongrowing liver, but the block was partially relieved in the regenerating liver. Nevertheless, for both c-fos and c-jun, changes in steady-state mRNA detected after partial hepatectomy were much greater than the transcriptional increase. In the regenerating liver of H-2K/c-myc mice, c-fos and c-jun expression was diminished, whereas mouse c-myc expression was enhanced in comparison with that in nontransgenic animals.

c-myc, c-fos, and c-jun regulation in the regenerating livers of normal and H-2K/c-myc transgenic mice

We investigated the mechanisms of regulation of c-myc, c-fos, and c-jun at the early stages of liver regeneration in mice. We show that the transient increase in steady-state levels of c-myc mRNA at the start of liver regeneration is most probably regulated by posttranscriptional mechanisms. Although there was a marked increase in c-myc transcriptional initiation shortly after partial hepatectomy, a block in elongation prevented the completion of most transcripts. To gain further information on the mechanism of regulation of c-myc expression during liver regeneration, we used transgenic mice harboring the human c-myc gene driven by the H-2K promoter. In these animals, the murine c-myc responded to the growth stimulus generated by partial hepatectomy, whereas the expression of the transgene was constitutive and did not change in the regenerating liver. However, the mRNA from both genes increased markedly after cycloheximide injection, suggesting that the regulation of c-myc mRNA abundance in the regenerating liver differs from that occurring after protein synthesis inhibition. Furthermore, we show that in normal mice c-fos and c-jun mRNA levels and transcriptional rates increase within 30 min after partial hepatectomy. c-fos transcriptional elongation was restricted in nongrowing liver, but the block was partially relieved in the regenerating liver. Nevertheless, for both c-fos and c-jun, changes in steady-state mRNA detected after partial hepatectomy were much greater than the transcriptional increase. In the regenerating liver of H-2K/c-myc mice, c-fos and c-jun expression was diminished, whereas mouse c-myc expression was enhanced in comparison with that in nontransgenic animals.

Post-transcriptional control of c-myc proto-oncogene expression by glucocorticoid hormones in human T lymphoblastic leukemic cells

We have studied the regulation of the human c-myc proto-oncogene by glucocorticoid hormones in T lymphoblastic leukemic cells. A significant decrease (50%) of the steady state levels of c-myc mRNA was observed as early as 3 h after dexamethasone treatment of CEM-1.3 human lymphoma cells, reaching less than 5% values, with respect to untreated cells, 24 h after hormone administration. Nuclear run-on experiments showed no modifications of the transcriptional rate from the first exon. However, a slight decrease (15%) of the transcript elongation from the first exon/first intron boundary was observed in the dexamethasone-treated cells. Using actinomycin D to block gene transcription, we have observed a significant increase in the rate of c-myc RNA specific decay after dexamethasone treatment. Furthermore, cycloheximide was able to overcome completely the dexamethasone-induced down-regulation of the c-myc gene expression. Our data suggest that dexamethasone is able to inhibit human c-myc gene expression primarily at the post-transcriptional level, through the synthesis of hormone-induced regulatory protein(s) controlling c-myc transcript stability.

Rapid and transient decrease of N-myc expression in retinoic acid-induced differentiation of OTF9 teratocarcinoma stem cells

The level of expression of N-myc in mouse teratocarcinoma stem cells is very high. Previous studies have shown that N-myc expression significantly decreases when the stem cells are subjected to long-term induction for differentiation by retinoic acid (RA). We found that in a stem cell line, OTF9, a steep yet transient decrease of N-myc expression takes place much earlier, immediately after induction by RA. To examine whether this decrease is responsible for differentiation, we constructed a gene, miwNmyc, to express N-myc cDNA constitutively and transformed OTF9 cells with this gene construct. Transformants under the constitutive expression of miwNmyc differentiated normally, as judged by morphological changes and by modulation of c-myc, Hox1.1, and laminin B1 expression. Therefore, transient decrease of N-myc expression may be the consequence of RA-induced differentiation, even though it occurs very early in the process. Alternatively, in addition to N-myc decrease, there may be redundant mechanisms which lead to OTF9 differentiation after induction by RA, so that suppression of N-myc decrease is bypassed by at least one other mechanism.

Rapid and transient decrease of N-myc expression in retinoic acid-induced differentiation of OTF9 teratocarcinoma stem cells

The level of expression of N-myc in mouse teratocarcinoma stem cells is very high. Previous studies have shown that N-myc expression significantly decreases when the stem cells are subjected to long-term induction for differentiation by retinoic acid (RA). We found that in a stem cell line, OTF9, a steep yet transient decrease of N-myc expression takes place much earlier, immediately after induction by RA. To examine whether this decrease is responsible for differentiation, we constructed a gene, miwNmyc, to express N-myc cDNA constitutively and transformed OTF9 cells with this gene construct. Transformants under the constitutive expression of miwNmyc differentiated normally, as judged by morphological changes and by modulation of c-myc, Hox1.1, and laminin B1 expression. Therefore, transient decrease of N-myc expression may be the consequence of RA-induced differentiation, even though it occurs very early in the process. Alternatively, in addition to N-myc decrease, there may be redundant mechanisms which lead to OTF9 differentiation after induction by RA, so that suppression of N-myc decrease is bypassed by at least one other mechanism.

Expression of REX-1, a gene containing zinc finger motifs, is rapidly reduced by retinoic acid in F9 teratocarcinoma cells

In the presence of retinoic acid (RA), cultured F9 murine teratocarcinoma stem cells differentiate into nontumorigenic cells resembling the extraembryonic endoderm of the early mouse embryo. By differential hybridization screening of an F9 cell cDNA library, we isolated a 1,745-nucleotide cDNA for a gene, REX-1 (for reduced expression), whose steady-state mRNA level began to decline in F9 cells in monolayer culture within 12 h after the addition of RA. By 48 to 96 h after RA treatment of F9 cells in monolayer culture, the REX-1 steady-state mRNA level was more than sevenfold lower than the level in undifferentiated F9 stem cells. The REX-1 mRNA decrease did not result from the reduction in cell growth rate associated with the differentiation process, since the REX-1 mRNA level did not decline in F9 cells that were partially growth arrested after 48 h of isoleucine deprivation. The RA-associated REX-1 mRNA decrease resulted primarily from a reduction in the transcription rate of the REX-1 gene in the presence of RA. In contrast to results in F9 cells, we have been unable thus far to detect REX-1 mRNA in day 7.5 to 12.5 mouse embryo RNA samples or in the P19 teratocarcinoma stem cell line. The putative REX-1 protein identified by DNA sequence analysis contains four repeats of the zinc finger nucleic acid-binding motif and a potential acidic activator domain, suggesting that REX-1 encodes a regulatory protein. The REX-1 gene is not identical to the previously reported murine genes that encode zinc finger-containing proteins.

Expression of REX-1, a gene containing zinc finger motifs, is rapidly reduced by retinoic acid in F9 teratocarcinoma cells

In the presence of retinoic acid (RA), cultured F9 murine teratocarcinoma stem cells differentiate into nontumorigenic cells resembling the extraembryonic endoderm of the early mouse embryo. By differential hybridization screening of an F9 cell cDNA library, we isolated a 1,745-nucleotide cDNA for a gene, REX-1 (for reduced expression), whose steady-state mRNA level began to decline in F9 cells in monolayer culture within 12 h after the addition of RA. By 48 to 96 h after RA treatment of F9 cells in monolayer culture, the REX-1 steady-state mRNA level was more than sevenfold lower than the level in undifferentiated F9 stem cells. The REX-1 mRNA decrease did not result from the reduction in cell growth rate associated with the differentiation process, since the REX-1 mRNA level did not decline in F9 cells that were partially growth arrested after 48 h of isoleucine deprivation. The RA-associated REX-1 mRNA decrease resulted primarily from a reduction in the transcription rate of the REX-1 gene in the presence of RA. In contrast to results in F9 cells, we have been unable thus far to detect REX-1 mRNA in day 7.5 to 12.5 mouse embryo RNA samples or in the P19 teratocarcinoma stem cell line. The putative REX-1 protein identified by DNA sequence analysis contains four repeats of the zinc finger nucleic acid-binding motif and a potential acidic activator domain, suggesting that REX-1 encodes a regulatory protein. The REX-1 gene is not identical to the previously reported murine genes that encode zinc finger-containing proteins.

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

The viral oncogene v-ras inhibited the platelet-derived growth factor (PDGF)-induced upregulation of c-myc and c-fos proto-oncogene expression in fibroblast monolayers. These v-ras-containing cells proliferated in the absence of c-myc induction and no longer required PDGF to support growth. Fibroblasts expressing v-ras continued to express the same number of functional PDGF receptors on their surface as uninfected cells, yet the usual induction of transcription of the genes c-myc, c-fos, and JE in response to PDGF stimulation did not occur in the presence of newly introduced v-ras or chronic v-ras gene expression, and synthesis of c-myc protein did not occur. This inhibitory effect on growth factor-mediated induction of cellular proto-oncogenes was specific for PDGF in that induction of the c-myc and c-fos genes by certain other factors was not impaired.

Multiple mechanisms for transcriptional regulation of the myc gene family in small-cell lung cancer

The molecular mechanisms reported to regulate the expression of myc family genes are multiple and complex and include gene amplification, transcriptional activation, transcriptional attenuation, and mRNA stability. We have investigated which of these mechanisms are responsible for the extreme variation in myc gene family mRNA levels observed in human small-cell lung cancer cell lines. In addition to gene amplification, a block to nascent mRNA chain elongation, causing attenuation of transcription, is an important regulatory mechanism controlling the steady-state levels of c-myc and L-myc mRNA. The loss of transcriptional attenuation is correlated with overexpression of these two genes in cell lines which do not show gene amplification. Expression of c-myc mRNA appears to be dependent on promoter activity and attenuator function. In contrast, regulation of expression of the N-myc gene does not involve transcriptional attenuation; steady-state mRNA levels are correlated with promoter activity as well as gene amplification. We conclude that transcriptional regulation of each member of the myc gene family is accomplished by a different assortment of complex mechanisms, including gene copy number, promoter activation, and transcriptional attenuation. Interference at multiple points in this complex regulatory process appears to be an important mechanism by which small-cell lung cancer and other human tumors evade growth control.

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

The viral oncogene v-ras inhibited the platelet-derived growth factor (PDGF)-induced upregulation of c-myc and c-fos proto-oncogene expression in fibroblast monolayers. These v-ras-containing cells proliferated in the absence of c-myc induction and no longer required PDGF to support growth. Fibroblasts expressing v-ras continued to express the same number of functional PDGF receptors on their surface as uninfected cells, yet the usual induction of transcription of the genes c-myc, c-fos, and JE in response to PDGF stimulation did not occur in the presence of newly introduced v-ras or chronic v-ras gene expression, and synthesis of c-myc protein did not occur. This inhibitory effect on growth factor-mediated induction of cellular proto-oncogenes was specific for PDGF in that induction of the c-myc and c-fos genes by certain other factors was not impaired.

Antisense Myc sequences induce differentiation of F9 cells

Down-regulation of Myc expression is the earliest documented change in gene expression in retinoic acid-induced differentiation of murine F9 teratocarcinoma cells. F9 cells transfected with plasmids expressing antisense Myc sequences under control of the simian virus 40 (SV40) early promoter exhibit a decrease in Myc protein. The result of this decrease is the spontaneous differentiation into cells that resemble retinoic acid-treated F9 cells as judged by plasminogen activator assays. In contrast, when F9 cells are transfected with a plasmid expressing Myc under control of the SV40 early promoter, resulting cell clones are resistant to differentiation by retinoic acid as shown by the lack of induction of plasminogen activator. These results suggest that down-regulation of Myc is sufficient and necessary for F9 cell differentiation.

Multiple mechanisms for transcriptional regulation of the myc gene family in small-cell lung cancer

The molecular mechanisms reported to regulate the expression of myc family genes are multiple and complex and include gene amplification, transcriptional activation, transcriptional attenuation, and mRNA stability. We have investigated which of these mechanisms are responsible for the extreme variation in myc gene family mRNA levels observed in human small-cell lung cancer cell lines. In addition to gene amplification, a block to nascent mRNA chain elongation, causing attenuation of transcription, is an important regulatory mechanism controlling the steady-state levels of c-myc and L-myc mRNA. The loss of transcriptional attenuation is correlated with overexpression of these two genes in cell lines which do not show gene amplification. Expression of c-myc mRNA appears to be dependent on promoter activity and attenuator function. In contrast, regulation of expression of the N-myc gene does not involve transcriptional attenuation; steady-state mRNA levels are correlated with promoter activity as well as gene amplification. We conclude that transcriptional regulation of each member of the myc gene family is accomplished by a different assortment of complex mechanisms, including gene copy number, promoter activation, and transcriptional attenuation. Interference at multiple points in this complex regulatory process appears to be an important mechanism by which small-cell lung cancer and other human tumors evade growth control.

Differential expression of the homeobox gene Hox-1.3 in F9 embryonal carcinoma cells

The Hox-1.3 gene is located on mouse chromosome 6 and has been previously shown to be expressed in mouse embryos and adults. In this study, we have examined the steady-state levels of the Hox-1.3 transcripts in undifferentiated and differentiated F9 embryonal carcinoma cells. We find that there is a rapid increase of Hox-1.3 transcripts after differentiation induction of F9 cells. The level of the major 1.85-kilobase (kb) transcript peaks at 16-24 hr after differentiation induction of F9 cells. By using primer extension techniques the 5' ends of the major 1.85-kb transcript have been mapped to two sites in induced F9 cells. Cellular fractionation of RNA and transfer blot gel analysis has localized one minor transcript to the nucleus, whereas the major transcript and two additional minor transcripts appear in the nucleus and the cytoplasm of induced F9 cells. The results of nuclear run-off experiments with uninduced and induced F9 cell nuclei indicate that there is a substantial increase in the rate of Hox-1.3 transcription upon induction of F9 cells with retinoic acid.

Poly(A) shortening and degradation of the 3' A+U-rich sequences of human c-myc mRNA in a cell-free system

The early steps in the degradation of human c-myc mRNA were investigated, using a previously described cell-free mRNA decay system. The first detectable step was poly(A) shortening, which generated a pool of oligoadenylated mRNA molecules. In contrast, the poly(A) of a stable mRNA, gamma globin, was not excised, even after prolonged incubation. The second step, degradation of oligoadenylated c-myc mRNA, generated decay products whose 3' termini were located within the A+U-rich portion of the 3' untranslated region. These products disappeared soon after they were formed, consistent with rapid degradation of the 3' region. In contrast, the 5' region, corresponding approximately to c-myc exon 1, was stable in vitro. The data indicate a sequential degradation pathway in which 3' region cleavages occur only after most or all of the poly(A) is removed. To account for rapid deadenylation, we suggest that the c-myc poly(A)-poly(A)-binding protein complex is readily dissociated, generating a protein-depleted poly(A) tract that is no longer resistant to nucleases.

Two distinct mechanisms of transcriptional control operate on c-myc during differentiation of HL60 cells

We examined the mechanisms that control the downregulation of the c-myc mRNA during differentiation of HL60 cells. On treatment with dimethyl sulfoxide, HL60 cells downmodulated their steady-state c-myc message levels, ceased to proliferate, and underwent terminal differentiation. In nuclear run-on assays in which distinct segments of the c-myc gene were used as probes, an increased blocking to elongation of nascent c-myc transcripts was shown during the early phase of differentiation. During a later phase, however, a loss of transcriptional initiation was observed. This loss of promoter activity correlated well with dramatic changes in the chromatin structure of the c-myc gene, as determined by DNase I-hypersensitive site analysis. In particular, two hypersensitive sites near the two major c-myc promoters disappeared at the time that promotion abated. The newly described, later-acting negative transcriptional control of c-myc also correlated temporally with the inability to reverse the downregulation of the c-myc message quickly on withdrawal of the differentiating agent. Therefore, a terminal step during differentiation may be linked to the later-acting mode of transcriptional regulation of c-myc. The evidence presented in this report has implications for tumorigenesis in Burkitt lymphomas, in which the germ line, nontranslocated c-myc allele is transcriptionally silent.

Cell cycle expression of RNA duplex unwindase activity in mammalian cells

An RNA duplex unwindase activity has been found by using an in vitro assay with various types of mammalian, somatic cells, including HeLa, mouse plasmacytoma, and Burkitt lymphoma. The unwindase activity is very low in mouse fibroblast 3T3 cells arrested into quiescence, but increases when the cells are released into renewed growth by serum. In addition, a gel retardation assay proved to be specific and sensitive for detection of RNA duplex-unwindase complexes.

Rapid cytoplasmic turnover of c-myc mRNA: requirement of the 3' untranslated sequences

Expression of the c-myc gene can be controlled by transcriptional or posttranscriptional mechanisms (or both), depending on the cell type and the growth conditions. An important mechanism of posttranscriptional regulation is modulation of cytoplasmic c-myc mRNA stability; normal human and murine c-myc mRNAs have cytoplasmic half-lives of 30 min or less. To elucidate the c-myc sequences which impart this unusually high rate of cytoplasmic transcript turnover, we have constructed various deletion and hybrid c-myc genes and analyzed the cytoplasmic stability of the mRNAs produced from them in stably transfected murine fibroblasts. The results indicate that sequences contained within the 5' and 3' ends of the c-myc transcript can affect cytoplasmic stability. Specifically, the 3' untranslated sequences of c-myc exon 3 are required for, but do not ensure, a high rate of transcript turnover in the cytoplasm. Exon 2 coding sequences do not seem to be involved, and exon 1 sequences at the 5' end of the transcript have only a small effect on cytoplasmic transcript stability. The sequences that are primarily responsible for the short c-myc RNA half-life were localized to a region of 140 bases in the 3' untranslated region.

Poly(A) shortening and degradation of the 3' A+U-rich sequences of human c-myc mRNA in a cell-free system

The early steps in the degradation of human c-myc mRNA were investigated, using a previously described cell-free mRNA decay system. The first detectable step was poly(A) shortening, which generated a pool of oligoadenylated mRNA molecules. In contrast, the poly(A) of a stable mRNA, gamma globin, was not excised, even after prolonged incubation. The second step, degradation of oligoadenylated c-myc mRNA, generated decay products whose 3' termini were located within the A+U-rich portion of the 3' untranslated region. These products disappeared soon after they were formed, consistent with rapid degradation of the 3' region. In contrast, the 5' region, corresponding approximately to c-myc exon 1, was stable in vitro. The data indicate a sequential degradation pathway in which 3' region cleavages occur only after most or all of the poly(A) is removed. To account for rapid deadenylation, we suggest that the c-myc poly(A)-poly(A)-binding protein complex is readily dissociated, generating a protein-depleted poly(A) tract that is no longer resistant to nucleases.

Two distinct mechanisms of transcriptional control operate on c-myc during differentiation of HL60 cells

We examined the mechanisms that control the downregulation of the c-myc mRNA during differentiation of HL60 cells. On treatment with dimethyl sulfoxide, HL60 cells downmodulated their steady-state c-myc message levels, ceased to proliferate, and underwent terminal differentiation. In nuclear run-on assays in which distinct segments of the c-myc gene were used as probes, an increased blocking to elongation of nascent c-myc transcripts was shown during the early phase of differentiation. During a later phase, however, a loss of transcriptional initiation was observed. This loss of promoter activity correlated well with dramatic changes in the chromatin structure of the c-myc gene, as determined by DNase I-hypersensitive site analysis. In particular, two hypersensitive sites near the two major c-myc promoters disappeared at the time that promotion abated. The newly described, later-acting negative transcriptional control of c-myc also correlated temporally with the inability to reverse the downregulation of the c-myc message quickly on withdrawal of the differentiating agent. Therefore, a terminal step during differentiation may be linked to the later-acting mode of transcriptional regulation of c-myc. The evidence presented in this report has implications for tumorigenesis in Burkitt lymphomas, in which the germ line, nontranslocated c-myc allele is transcriptionally silent.

Cell cycle expression of RNA duplex unwindase activity in mammalian cells

An RNA duplex unwindase activity has been found by using an in vitro assay with various types of mammalian, somatic cells, including HeLa, mouse plasmacytoma, and Burkitt lymphoma. The unwindase activity is very low in mouse fibroblast 3T3 cells arrested into quiescence, but increases when the cells are released into renewed growth by serum. In addition, a gel retardation assay proved to be specific and sensitive for detection of RNA duplex-unwindase complexes.

Rapid cytoplasmic turnover of c-myc mRNA: requirement of the 3' untranslated sequences

Expression of the c-myc gene can be controlled by transcriptional or posttranscriptional mechanisms (or both), depending on the cell type and the growth conditions. An important mechanism of posttranscriptional regulation is modulation of cytoplasmic c-myc mRNA stability; normal human and murine c-myc mRNAs have cytoplasmic half-lives of 30 min or less. To elucidate the c-myc sequences which impart this unusually high rate of cytoplasmic transcript turnover, we have constructed various deletion and hybrid c-myc genes and analyzed the cytoplasmic stability of the mRNAs produced from them in stably transfected murine fibroblasts. The results indicate that sequences contained within the 5' and 3' ends of the c-myc transcript can affect cytoplasmic stability. Specifically, the 3' untranslated sequences of c-myc exon 3 are required for, but do not ensure, a high rate of transcript turnover in the cytoplasm. Exon 2 coding sequences do not seem to be involved, and exon 1 sequences at the 5' end of the transcript have only a small effect on cytoplasmic transcript stability. The sequences that are primarily responsible for the short c-myc RNA half-life were localized to a region of 140 bases in the 3' untranslated region.

Definition of regions in human c-myc that are involved in transformation and nuclear localization

To study the relationship between the primary structure of the c-myc protein and some of its functional properties, we made in-frame insertion and deletion mutants of the normal human c-myc coding domain that was expressed from a retroviral promoter-enhancer. We assessed the effects of these mutations on the ability of c-myc protein to cotransform normal rat embryo cells with a mutant ras gene, induce foci in a Rat-1-derived cell line (Rat-1a), and localize in nuclei. Using the cotransformation assay, we found two regions of the protein (amino acids 105 to 143 and 321 to 439) where integrity was critical: one region (amino acids 1 to 104) that tolerated insertion and small deletion mutations, but not large deletions, and another region (amino acids 144) to 320) that was largely dispensable. Comparison with regions that were important for transformation of Rat-1a cells revealed that some are essential for both activities, but others are important for only one or the other, suggesting that the two assays require different properties of the c-myc protein. Deletion of each of three regions of the c-myc protein (amino acids 106 to 143, 320 to 368, and 370 to 412) resulted in partial cytoplasmic localization, as determined by immunofluorescence or immunoprecipitation following subcellular fractionation. Some abnormally located proteins retained transforming activity; most proteins lacking transforming activity appeared to be normally located.

Definition of regions in human c-myc that are involved in transformation and nuclear localization

To study the relationship between the primary structure of the c-myc protein and some of its functional properties, we made in-frame insertion and deletion mutants of the normal human c-myc coding domain that was expressed from a retroviral promoter-enhancer. We assessed the effects of these mutations on the ability of c-myc protein to cotransform normal rat embryo cells with a mutant ras gene, induce foci in a Rat-1-derived cell line (Rat-1a), and localize in nuclei. Using the cotransformation assay, we found two regions of the protein (amino acids 105 to 143 and 321 to 439) where integrity was critical: one region (amino acids 1 to 104) that tolerated insertion and small deletion mutations, but not large deletions, and another region (amino acids 144) to 320) that was largely dispensable. Comparison with regions that were important for transformation of Rat-1a cells revealed that some are essential for both activities, but others are important for only one or the other, suggesting that the two assays require different properties of the c-myc protein. Deletion of each of three regions of the c-myc protein (amino acids 106 to 143, 320 to 368, and 370 to 412) resulted in partial cytoplasmic localization, as determined by immunofluorescence or immunoprecipitation following subcellular fractionation. Some abnormally located proteins retained transforming activity; most proteins lacking transforming activity appeared to be normally located.

Transcriptional and posttranscriptional control of c-myc gene expression in WEHI 231 cells

Incubation of WEHI 231 cells, derived from a murine B-cell lymphoma, with antisera directed against its surface immunoglobulin results in the inhibition of growth within 24 h. Previously, we demonstrated that this treatment selectively affects cytoplasmic levels of c-myc mRNA (J. E. McCormack, V. H. Pepe, R. B. Kent, M. Dean, A. Marshak-Rothstein, and G. E. Sonenshein, Proc. Natl. Acad. Sci. USA 81:5546-5550, 1984). An initial increase in the cytoplasmic mRNA level is followed by a precipitous drop. We now show that the early increase results from a dramatic increase in the rate of c-myc gene transcription, as well as from partial stabilization of the mRNA in the cytoplasm. The later decrease results from a shutdown in transcription of the c-myc gene and a return to the normal lability of the cytoplasmic c-myc mRNA. Treatment with phorbol ester, like treatment with anti-immunoglobulin sera, inhibited WEHI 231 cell growth and caused similar changes in cytoplasmic c-myc mRNA levels, which can also be related to alterations in c-myc gene transcription. These results indicate that the control of c-myc gene expression in B cells is effected through regulation at multiple levels.

Transcriptional and post-transcriptional regulation of c-myc expression during the differentiation of murine erythroleukemia Friend cells

c-myc RNA rapidly decreases to barely detectable levels in Friend erythroleukemia cells induced to differentiate upon the addition of dimethylsulfoxide. We show here that c-myc gene is down-regulated both at the transcriptional level presumably by a block in the elongation of primary transcripts and at the post-transcriptional level by an increase in the degradation of its mRNA.

Transcriptional and posttranscriptional control of c-myc gene expression in WEHI 231 cells

Incubation of WEHI 231 cells, derived from a murine B-cell lymphoma, with antisera directed against its surface immunoglobulin results in the inhibition of growth within 24 h. Previously, we demonstrated that this treatment selectively affects cytoplasmic levels of c-myc mRNA (J. E. McCormack, V. H. Pepe, R. B. Kent, M. Dean, A. Marshak-Rothstein, and G. E. Sonenshein, Proc. Natl. Acad. Sci. USA 81:5546-5550, 1984). An initial increase in the cytoplasmic mRNA level is followed by a precipitous drop. We now show that the early increase results from a dramatic increase in the rate of c-myc gene transcription, as well as from partial stabilization of the mRNA in the cytoplasm. The later decrease results from a shutdown in transcription of the c-myc gene and a return to the normal lability of the cytoplasmic c-myc mRNA. Treatment with phorbol ester, like treatment with anti-immunoglobulin sera, inhibited WEHI 231 cell growth and caused similar changes in cytoplasmic c-myc mRNA levels, which can also be related to alterations in c-myc gene transcription. These results indicate that the control of c-myc gene expression in B cells is effected through regulation at multiple levels.

Induction of c-fos and c-myc mRNA by epidermal growth factor or calcium ionophore is cAMP dependent

Phorbol esters activate protein kinase C and induce expression of the c-fos and c-myc protooncogenes in density-arrested BALB/c 3T3 (A31) cells; in contrast, epidermal growth factor (EGF) does not activate protein kinase C and is a poor inducer of c-fos and c-myc in these confluent cells. We show that, when A31 cells were subconfluent and made quiescent by serum deprivation, the phorbol ester phorbol 12-myristate 13-acetate induced c-fos and c-myc mRNA poorly, whereas EGF was a better inducer. Another platelet-derived growth factor-inducible gene, JE, did not show this differential regulation by phorbol 12-myristate 13-acetate and EGF. The ability of EGF to induce protooncogene mRNA was associated with elevated levels of intracellular cAMP. First, serum-deprived cells maintained cAMP at about 2-fold higher level than density-arrested cells. Second, induction was greatly enhanced by cholera toxin and 3-isobutyl-1-methylxanthine, which increased intracellular cAMP 3- to 10-fold. The calcium ionophore A23187 mimicked EGF in that it elevated c-fos and c-myc mRNA when administered with cholera toxin and isobutylmethylxanthine. Neither cholera toxin and isobutyl-methylxanthine nor A23187 appreciably induced these mRNAs when used alone. Our results suggest that c-fos and c-myc expression can be regulated by an EGF-directed pathway that utilizes calcium and cAMP as cooperating cytoplasmic messengers.