Extreme instability of myc mRNA in normal and transformed human cells

In vitro mRNA degradation system to study the virion host shutoff function of herpes simplex virus

The virion host shutoff (vhs) gene of herpes simplex virus encodes a virion polypeptide that induces degradation of host mRNAs at early times and rapid turnover of viral mRNAs throughout infection. To better investigate the vhs function, an in vitro mRNA degradation system was developed, consisting of cytoplasmic extracts from HeLa cells infected with wild-type herpes simplex virus type 1 or a mutant encoding a defective vhs polypeptide. Host and viral mRNAs were degraded rapidly in extracts from cells productively infected with wild-type herpes simplex virus type 1 but not in extracts from mock-infected cells or cells infected with the mutant vhs1. In contrast, 28S rRNA was stable in all three kinds of extract. Accelerated turnover of host mRNAs was also observed in extracts from cells infected with wild-type virus in the presence of dactinomycin, indicating that the activity was induced by a structural component of the infecting virions. The in vitro vhs activity was inactivated by heat or proteinase K digestion but was insensitive to brief treatment of the extracts with micrococcal nuclease. It was not inhibited by placental RNase inhibitor, it exhibited a strong dependence upon added Mg2+, it was active at concentrations of K+ up to 200 mM, and it did not require the components of an energy-generating system. In summary, the in vitro mRNA degradation system appears to accurately reproduce the vhs-mediated decay of host and viral mRNAs and should be useful for studies of the mechanism of vhs action.

Deprivation of a single amino acid induces protein synthesis-dependent increases in c-jun, c-myc, and ornithine decarboxylase mRNAs in Chinese hamster ovary cells

Genes of higher eucaryotic cells are considered to show only a limited response to nutritional stress. Here we show, however, that omission of a single essential amino acid from the medium caused a marked rise in the mRNA levels of c-myc, c-jun, junB and c-fos oncogenes and ornithine decarboxylase (ODC) in CHO cells. There was no general accumulation of mRNAs in amino acid-starved cells, since the gamma-actin, beta-tubulin, protein kinase C, RNA polymerase II, and glyceraldehyde-3-phosphate dehydrogenase mRNAs and the total poly(A)+ mRNA were not increased. The levels of c-myc, ODC, and c-jun mRNAs were elevated more by amino acid starvation than by inhibition of protein synthesis with cycloheximide, which is known to increase the levels of these mRNAs. Importantly, however, cycloheximide present during amino acid starvation reduced the rise in the levels of the mRNAs down to the level obtained with cycloheximide alone. This implies that protein synthesis is required for the accumulation of c-myc, ODC, and c-jun mRNAs in amino acid-deprived cells. The junB and c-fos mRNAs, instead, were increased to the same extent or less by amino acid starvation than by cycloheximide treatment. The accumulation of the c-myc mRNA in amino acid-starved cells was due to both stabilization of the mRNA and increase of its transcription. The rise in the c-jun mRNA level seemed to be caused merely by stabilization of the mRNA. Further, despite the inhibition of general protein synthesis, amino acid starvation led to an increase in the synthesis of c-myc polypeptide. The results suggest that mammalian cells have a specific mechanism for registering shortages of amino acids in order to make adjustments compatible with cellular growth.

Regulation of N-myc gene expression: use of an adenovirus vector to demonstrate posttranscriptional control

We present evidence that differences in the levels of N-myc mRNA among different cell types are the result of posttranscriptional control. First, we noted that while steady-state mouse N-myc mRNA could be detected only in fetal mouse brain, it was transcribed at an equivalent rate in adult brain, liver, spleen, and placenta and in fetal brain. Similarly, the human N-myc gene was transcribed at an equivalent rate in HeLa cells, which do not accumulate this RNA in the cytoplasm, and cell lines G401 (a Wilms tumor-derived cell line) and SKNMc (established from a primitive neuroepithelioma), which do express N-myc RNA. As expected, the N-myc promoter functioned at equivalent rates, as demonstrated by the level of a reporter gene, when introduced into these cell types by using a recombinant adenovirus vector. The suggestion that posttranscriptional mechanisms control the level of this RNA was supported by the observation that sequences in the N-myc third exon specifically decreased the level of E1A mRNA when these sequences were placed downstream of the E1A promoter in a recombinant adenovirus. Finally, we further localized these sequences to a 600-bp fragment of the third exon by introducing various subclones of this sequence downstream of the E1A promoter in both viral and plasmid vectors.

Poly(A) tail shortening is the translation-dependent step in c-myc mRNA degradation

The highly unstable c-myc mRNA has been shown to be stabilized in cells treated with protein synthesis inhibitors. We have studied this phenomenon in an effort to gain more insight into the degradation pathway of this mRNA. Our results indicate that the stabilization of c-myc mRNA in the absence of translation can be fully explained by the inhibition of translation-dependent poly(A) tail shortening. This view is based on the following observations. First, the normally rapid shortening of the c-myc poly(A) tail was slowed down by a translation block. Second, c-myc messengers which carry a short poly(A) tail, as a result of prolonged actinomycin D or 3'-deoxyadenosine treatment, were not stabilized by the inhibition of translation. We propose that c-myc mRNA degradation proceeds in at least two steps. The first step is the shortening of long poly(A) tails. This step requires ongoing translation and thus is responsible for the delay in mRNA degradation observed in the presence of protein synthesis inhibitors. The second step involves rapid degradation of the body of the mRNA, possibly preceded by the removal of the short remainder of the poly(A) tail. This last step is independent of translation.

Deprivation of a single amino acid induces protein synthesis-dependent increases in c-jun, c-myc, and ornithine decarboxylase mRNAs in Chinese hamster ovary cells

Genes of higher eucaryotic cells are considered to show only a limited response to nutritional stress. Here we show, however, that omission of a single essential amino acid from the medium caused a marked rise in the mRNA levels of c-myc, c-jun, junB and c-fos oncogenes and ornithine decarboxylase (ODC) in CHO cells. There was no general accumulation of mRNAs in amino acid-starved cells, since the gamma-actin, beta-tubulin, protein kinase C, RNA polymerase II, and glyceraldehyde-3-phosphate dehydrogenase mRNAs and the total poly(A)+ mRNA were not increased. The levels of c-myc, ODC, and c-jun mRNAs were elevated more by amino acid starvation than by inhibition of protein synthesis with cycloheximide, which is known to increase the levels of these mRNAs. Importantly, however, cycloheximide present during amino acid starvation reduced the rise in the levels of the mRNAs down to the level obtained with cycloheximide alone. This implies that protein synthesis is required for the accumulation of c-myc, ODC, and c-jun mRNAs in amino acid-deprived cells. The junB and c-fos mRNAs, instead, were increased to the same extent or less by amino acid starvation than by cycloheximide treatment. The accumulation of the c-myc mRNA in amino acid-starved cells was due to both stabilization of the mRNA and increase of its transcription. The rise in the c-jun mRNA level seemed to be caused merely by stabilization of the mRNA. Further, despite the inhibition of general protein synthesis, amino acid starvation led to an increase in the synthesis of c-myc polypeptide. The results suggest that mammalian cells have a specific mechanism for registering shortages of amino acids in order to make adjustments compatible with cellular growth.

Changes in the stability of specific mRNA species in response to growth stage in Bacillus subtilis

In this study we compared the cellular concentrations and stability of the mRNA transcribed from the aprE (subtilisin) gene (a gene preferentially expressed in stationary growth phase) with those of a vegetative mRNA, succinate dehydrogenase (SDH) mRNA. The subtilisin transcript was shown to be at least 3 times more stable in early stationary phase than it is 2 hr further into stationary phase. When cells were shifted from maximum expression of the subtilisin transcript in stationary phase to physiological conditions, which allowed for the resumption of vegetative growth, the cellular concentration of the subtilisin mRNA decreased rapidly. We conclude that mRNA degradation is one of the means by which the cellular concentrations of the SDH and subtilisin transcripts are adjusted in response to growth stage.

Regulation of the relative abundances of c-myc mRNAs in human promyelocytic HL60 cells

Transcription of the c-myc gene is initiated mainly from two promoters, P1 and P2. By S1 nuclease analysis we found that there is 8 times more P2- than P1-initiated RNA in total RNA from HL60 cells. The half-lives of P1- and P2-initiated transcripts are 26 and 18 min, respectively, so the difference in the relative abundance of the mRNAs is not due to differences in their stabilities. The relative rates of transcription from the P1 and P2 promoters, estimated by in vitro nuclear run-on analysis, were found to differ by about 10-fold, sufficient to account for the difference in the steady-state levels of the two mRNAs. The abundance of c-myc mRNA changes dramatically during differentiation of HL60 cells. Dimethyl sulphoxide causes a very rapid reduction in total c-myc mRNA, while with phorbol ester a transient increase occurs followed by a more gradual decline. At no time during these dramatic alterations were significant changes detected in the relative abundance of P1- and P2-initiated mRNAs, or in their stabilities.

Cell cycle and gene expression in the insulin producing pancreatic cell line beta TC1

The pancreatic cell line beta TC1, established from insulinomas of transgenic mice carrying a hybrid insulin-promoted large T antigen gene, has retained several characteristics of normal cells, including the insulin content and inducibility of insulin secreting by glucose. We show here that the growth of beta TC1 cells is arrested in low serum-concentration medium. Cells exposed for three days to 0.25% fetal calf serum ceased to incorporate [3H]thymidine but were still able to resume the cell division cycle upon addition of serum. In this cell line, we have determined by cytofluorometry the cell cycle kinetic parameters to be of 21 h, 10 h 30 min and 12 h for the G1, S and G2/M phases, respectively. Quiescent beta TC1 cells constitutively expressed the protooncogene c-jun that codes for the transcriptional factor AP1, as well as cdc2, another cell cycle-related gene. A large transient increase in the expression of the c-fos gene was obtained rapidly, 30 min after addition of serum and a similar increase in c-jun expression after one hour. Expression of the cdc2 gene was also enhanced to a lesser extent. The same effects were also observed in the presence of cycloheximide, thus proving that the expression of these three genes is directly stimulated by serum growth factors. Consequently, quiescent beta TC1 cells provide a good model for studying the short- and long-term effects of growth factors on Beta-cell proliferation.

Induction of a chicken small heat shock (stress) protein: evidence of multilevel posttranscriptional regulation

A novel form of regulation of expression of a vertebrate heat shock gene is described. A cDNA clone encoding human Hsp27 was shown to specifically recognize chicken Hsp23 RNA by Northern (RNA) blot analysis and hybrid-select translation. This probe was then used to measure chicken hsp23 gene activity in control and heat-stressed cells. The hsp23 gene(s) was transcriptionally active in non-heat-stressed cells, and its rate of transcription did not increase significantly upon heat shock. Cytoplasmic Hsp23 mRNA, which was metabolically very stable in nonstressed cells, underwent a fourfold increase in amount after a 1-h heat shock, resulting in a twofold increase in Hsp23 mRNA in polysomes. Hsp23 mRNA was relatively abundant and translationally active even in non-heat-shocked cells. Taken together, these data implicated posttranscriptional nuclear events as an important control point for induction of Hsp23 RNA transcripts. The protein half-life of Hsp23 increased from approximately 2 h in control cultures to 13 h in heat-shocked cells, revealing a second major control point. Hsp23 which was synthesized prior to heat shock also increased in stability and contributed to the overall accumulation of Hsp23 in heat-shocked cells. Cycloheximide had no effect on this change in Hsp23 half-life, while dactinomycin blocked the stabilization of Hsp23, suggesting a need for newly synthesized RNA. These data indicated that stabilization of Hsp23 protein and posttranscriptional nuclear events resulting in increased production of Hsp23 mRNA were primarily responsible for a 13-fold increase in the accumulation of newly synthesized Hsp23 after 1 h of heat shock. The regulation of the hsp23 gene is discussed in comparison with several other posttranscriptionally regulated genes, including the proto-oncogene c-fos, the developmentally regulated chicken delta-crystallin gene, and regulation of cellular gene expression by the proto-oncogene c-myc.

Induction of a chicken small heat shock (stress) protein: evidence of multilevel posttranscriptional regulation

A novel form of regulation of expression of a vertebrate heat shock gene is described. A cDNA clone encoding human Hsp27 was shown to specifically recognize chicken Hsp23 RNA by Northern (RNA) blot analysis and hybrid-select translation. This probe was then used to measure chicken hsp23 gene activity in control and heat-stressed cells. The hsp23 gene(s) was transcriptionally active in non-heat-stressed cells, and its rate of transcription did not increase significantly upon heat shock. Cytoplasmic Hsp23 mRNA, which was metabolically very stable in nonstressed cells, underwent a fourfold increase in amount after a 1-h heat shock, resulting in a twofold increase in Hsp23 mRNA in polysomes. Hsp23 mRNA was relatively abundant and translationally active even in non-heat-shocked cells. Taken together, these data implicated posttranscriptional nuclear events as an important control point for induction of Hsp23 RNA transcripts. The protein half-life of Hsp23 increased from approximately 2 h in control cultures to 13 h in heat-shocked cells, revealing a second major control point. Hsp23 which was synthesized prior to heat shock also increased in stability and contributed to the overall accumulation of Hsp23 in heat-shocked cells. Cycloheximide had no effect on this change in Hsp23 half-life, while dactinomycin blocked the stabilization of Hsp23, suggesting a need for newly synthesized RNA. These data indicated that stabilization of Hsp23 protein and posttranscriptional nuclear events resulting in increased production of Hsp23 mRNA were primarily responsible for a 13-fold increase in the accumulation of newly synthesized Hsp23 after 1 h of heat shock. The regulation of the hsp23 gene is discussed in comparison with several other posttranscriptionally regulated genes, including the proto-oncogene c-fos, the developmentally regulated chicken delta-crystallin gene, and regulation of cellular gene expression by the proto-oncogene c-myc.

Stability of maternal mRNA in Xenopus embryos: role of transcription and translation

The first 12 cell divisions of Xenopus laevis embryos do not require gene transcription. This means that the regulation of gene expression during this period is controlled at post transcriptional levels and makes Xenopus early development a potentially interesting biological system with which to study the mechanisms involved. We describe here the stability characteristics of several maternal Xenopus mRNAs which are deadenylated soon after fertilisation (J. Paris and M. Philippe, Dev. Biol., in press). We show that these mRNAs were only degraded in the embryo after the midblastula transition (MBT), when gene transcription was initiated. The kinetics with which the deadenylated maternal mRNAs decreased in the post-MBT embryos showed sequence specificity. The degradation of these mRNAs after the MBT was inhibited by cycloheximide but was not affected by dactinomycin. Therefore, the destabilization of these mRNAs does not appear to be initiated by new embryonic gene transcripts. Sequence comparisons of the 3' untranslated region of these mRNAs identified several motifs which may be involved in the posttranscriptional control of these gene products.

Lipopolysaccharide and dexamethasone induce mouse mammary tumor proviral gene expression and differentiation in B lymphocytes through distinct regulatory pathways

Endogenous mouse mammary tumor virus (MMTV) proviral transcripts are up regulated during the normal course of B-lymphocyte differentiation. We report here that the regulatory mechanisms which lead to increased levels of MMTV transcripts in differentiating, lipopolysaccharide (LPS)-stimulated normal B cells and in the inducible B-cell lymphoma line CH12 are at least partially distinct from those controlling increases in immunoglobulin and J-chain gene expression. In studies designed to characterize the stimulatory pathways leading to MMTV expression in CH12 cells, we found that stimulation with either LPS or dexamethasone (Dex), a transcriptional activator of MMTV genes, induced not only MMTV expression but also differentiation to antibody secretion. Only Dex-induced and not LPS-induced MMTV expression and differentiation were inhibited by the glucocorticoid antagonist RU486, demonstrating that Dex and LPS stimulate B cells by distinct molecular pathways. Therefore, in B cells, MMTV expression can be regulated via either the conventional hormone receptor-dependent pathway or a hormone receptor-independent pathway. Furthermore, these results suggest that steroid stimulation of B cells can lead to alterations in the expression of other results suggest that steroid stimulation of B cells can lead to alterations in the expression of other steroid-responsive genes that can become involved in the process of B-cell differentiation.

Lipopolysaccharide and dexamethasone induce mouse mammary tumor proviral gene expression and differentiation in B lymphocytes through distinct regulatory pathways

Endogenous mouse mammary tumor virus (MMTV) proviral transcripts are up regulated during the normal course of B-lymphocyte differentiation. We report here that the regulatory mechanisms which lead to increased levels of MMTV transcripts in differentiating, lipopolysaccharide (LPS)-stimulated normal B cells and in the inducible B-cell lymphoma line CH12 are at least partially distinct from those controlling increases in immunoglobulin and J-chain gene expression. In studies designed to characterize the stimulatory pathways leading to MMTV expression in CH12 cells, we found that stimulation with either LPS or dexamethasone (Dex), a transcriptional activator of MMTV genes, induced not only MMTV expression but also differentiation to antibody secretion. Only Dex-induced and not LPS-induced MMTV expression and differentiation were inhibited by the glucocorticoid antagonist RU486, demonstrating that Dex and LPS stimulate B cells by distinct molecular pathways. Therefore, in B cells, MMTV expression can be regulated via either the conventional hormone receptor-dependent pathway or a hormone receptor-independent pathway. Furthermore, these results suggest that steroid stimulation of B cells can lead to alterations in the expression of other results suggest that steroid stimulation of B cells can lead to alterations in the expression of other steroid-responsive genes that can become involved in the process of B-cell differentiation.

Stability of maternal mRNA in Xenopus embryos: role of transcription and translation

The first 12 cell divisions of Xenopus laevis embryos do not require gene transcription. This means that the regulation of gene expression during this period is controlled at post transcriptional levels and makes Xenopus early development a potentially interesting biological system with which to study the mechanisms involved. We describe here the stability characteristics of several maternal Xenopus mRNAs which are deadenylated soon after fertilisation (J. Paris and M. Philippe, Dev. Biol., in press). We show that these mRNAs were only degraded in the embryo after the midblastula transition (MBT), when gene transcription was initiated. The kinetics with which the deadenylated maternal mRNAs decreased in the post-MBT embryos showed sequence specificity. The degradation of these mRNAs after the MBT was inhibited by cycloheximide but was not affected by dactinomycin. Therefore, the destabilization of these mRNAs does not appear to be initiated by new embryonic gene transcripts. Sequence comparisons of the 3' untranslated region of these mRNAs identified several motifs which may be involved in the posttranscriptional control of these gene products.

Expression of the avian gag-myc oncogene in Saccharomyces cerevisiae

The gag-myc oncogenic sequence of the avian retrovirus MC29 was first inserted in a multicopy expression vector allowing its expression in Saccharomyces cerevisiae. The oncogene transcripts were detected in yeast by Northern blot hybridization and gag-myc proteins were revealed by immunoprecipitation. On solid medium, the average size of gag-myc transformant colonies was smaller than control. In liquid cultures, the gag-myc strains had a doubling time of 4.7 h compared with 3.1 h in the controls. In one of the recipient strains, and after an initial transient period of 5 days, the gag-myc transformants became physiologically indistinguishable from control. In another recipient strain, the slow-growth phenotype is permanent. Plasmid instability is increased in gag-myc transformants. When a single copy of the gag-myc gene was inserted in a yeast chromosome, no phenotype was observed, showing that slow growth is the consequence of plasmid loss.

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.

Modulation by interferons of the expression of monocyte complement genes

Interferons-alpha, -beta and -gamma (IFNs-alpha, -beta and -gamma) stimulated the synthesis of the second complement component (C2), Factor B (B) and C1 inhibitor (C1-inh) by human monocytes in vitro. The degree of increase of the secretion rates of C2, B and C1-inh was dose-dependent and proportional to increases in the abundances of their respective mRNAs. IFN-gamma was the most effective at stimulating monocyte C1-inh synthesis, whereas IFN-alpha and IFN-beta were marginally more effective at stimulating monocyte C2 and B synthesis. Kinetic studies showed that the effect of the IFNs was rapid, with maximum stimulation occurring within 1-2 h for all three proteins. After the removal of IFNs from cultures the C1-inh mRNA abundance remained elevated for over 24 h in IFN-gamma-treated monocytes but returned to control levels within 8 h in IFN-alpha-treated and IFN-beta-treated monocytes. The abundances of C2 mRNA and B mRNA also returned to basal values within 8 h after removal of any of the three cytokines from the cultures. Both IFN-alpha and IFN-beta acted synergistically with IFN-gamma to stimulate synthesis of C1-inh and B. This synergistic effect only occurred when the cytokines were present in the cultures simultaneously. The effects of IFN-gamma plus IFN-alpha or IFN-beta on C2 synthesis appeared to be additive rather than synergistic. IFN-gamma inhibited synthesis of C3 by monocytes, but IFN-alpha and IFN-beta had no effect on the synthesis of this protein. Furthermore, none of the three cytokines had any effect on the expression of actin mRNA in monocytes.

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.

The human Pim-1 gene is selectively transcribed in different hemato-lymphoid cell lines in spite of a G + C-rich housekeeping promoter

The expression of the Pim-1 proto-oncogene was studied by using the K562, Daudi, and Jurkat cell lines. In K562, Pim-1 mRNA levels were more than 20-fold higher than in Daudi and 50-fold higher than in Jurkat. Nuclear run-on assay data correlated directly with the steady-state mRNA levels, suggesting that the rate of transcription was responsible for the selective expression of this gene. Furthermore, the half-life of Pim-1 mRNA was shown to be 47 min in K562, 71 min in Daudi, and 35 min in Jurkat. This indicated that selective Pim-1 mRNA expression did not depend on posttranscriptional regulation. Therefore, 1.7 kilobases of the Pim-1 promoter was sequenced and studied in detail. The sequence showed that the region from nucleotide -1 to -873 was G + C rich (71%). Study of promoter deletions defined two major functional regions, a proximal element (nucleotide -104 to -1) and a distal element (nucleotide -427 to -336). DNase I protection assays identified binding sites for the Sp1 and AP2 proteins in these elements. A possible new transcription factor binds at position -348 in the distal element. In our study of the 1.7-kilobase Pim-1 promoter, we found no differences between K562 and Jurkat that could explain large differences in transcription. Therefore, the Pim-1 promoter appears to function constitutively, and we conclude that distant elements must regulate the tissue-selective expression of this gene. Although the Pim-1 gene has a G + C-rich housekeeping promoter, expression is carefully regulated at the level of transcription.

The human Pim-1 gene is selectively transcribed in different hemato-lymphoid cell lines in spite of a G + C-rich housekeeping promoter

The expression of the Pim-1 proto-oncogene was studied by using the K562, Daudi, and Jurkat cell lines. In K562, Pim-1 mRNA levels were more than 20-fold higher than in Daudi and 50-fold higher than in Jurkat. Nuclear run-on assay data correlated directly with the steady-state mRNA levels, suggesting that the rate of transcription was responsible for the selective expression of this gene. Furthermore, the half-life of Pim-1 mRNA was shown to be 47 min in K562, 71 min in Daudi, and 35 min in Jurkat. This indicated that selective Pim-1 mRNA expression did not depend on posttranscriptional regulation. Therefore, 1.7 kilobases of the Pim-1 promoter was sequenced and studied in detail. The sequence showed that the region from nucleotide -1 to -873 was G + C rich (71%). Study of promoter deletions defined two major functional regions, a proximal element (nucleotide -104 to -1) and a distal element (nucleotide -427 to -336). DNase I protection assays identified binding sites for the Sp1 and AP2 proteins in these elements. A possible new transcription factor binds at position -348 in the distal element. In our study of the 1.7-kilobase Pim-1 promoter, we found no differences between K562 and Jurkat that could explain large differences in transcription. Therefore, the Pim-1 promoter appears to function constitutively, and we conclude that distant elements must regulate the tissue-selective expression of this gene. Although the Pim-1 gene has a G + C-rich housekeeping promoter, expression is carefully regulated at the level of transcription.

Elongation and maturation of c-myc RNA is inhibited by differentiation inducing agents in HL60 cells

Maturation of c-myc mRNA proceeds in a given order in HL60 cells. It starts with splicing of intron 2, continues with splicing of intron 1 and ends with 3' cleavage and polyadenylation of the primary transcript. This process is inhibited, when HL60 cells were induced to terminal differentiation by dimethylsulfoxide (DMSO). DMSO interferes specifically with maturation of c-myc but not c-abl RNA in HL60 cells. Simultaneously, DMSO induces a block to RNA elongation at the boundary of c-myc exon 1 and intron 1 in HL60 cells. Participation of the same factor(s) in the regulation of c-myc RNA elongation and splicing is supposed.

Regulation of mRNA abundance in activated T lymphocytes: identification of mRNA species affected by the inhibition of protein synthesis

Inhibition of protein synthesis has often been observed to increase the concentration of mRNAs that encode proteins associated with the regulation of cell division. As two-dimensional gel electrophoresis permits the simultaneous monitoring of individual elements in large populations of gene products, we have used this technique to assess the effect of cycloheximide treatment on the mRNA complement of activated mouse T cells in an objective fashion. Two-dimensional gels of proteins generated by cell-free translation of mRNA from T-cell blasts display about 400 spots; only 5 of these are reproducibly enhanced by cycloheximide treatment and about 4 are diminished. The cDNA cloning vector lambda jac allows analysis of large arrays of molecular clones by cell-free expression, and we have used it in a sibling selection scheme to isolate a clone of one of the prominently induced mRNA species, which we refer to as chx1. chx1 mRNA concentration is increased by cycloheximide treatment of activated B cells, as well as T cells, and it is rapidly and transiently induced, in a cycloheximide-enhanced manner, upon serum stimulation of resting 3T3 fibroblastoid cells. The chx1 protein is hydrophilic, is slightly basic, and has patches of homology with the Jun-D gene product. The chx1 gene is remarkable in its lack of detectable introns and of strong bias against CpG dinucleotides.

Fibroblast growth factor-dependent mitogenic signal transduction pathway in chemically transformed mouse fibroblasts is similar to but distinct from that initiated by phorbol esters

Treatment of the quiescent, chemically transformed Balb/c mouse 3T3 cells (BP-A31) with fibroblast growth factor (FGF) leads to reinitiation of the cell division cycle in a large proportion of the cells. The characteristics of the mitogenic action of FGF closely resemble those of phorbol esters (activators of protein kinases type C) and differ from those of insulin (mediated by insulin-like growth factor 1 receptors). In particular, the effects of FGF as well as of phorbol-2-myristate-13-acetate (PMA), unlike the effects of insulin, are prevented by a low concentration (7.5 nM) of staurosporin (an efficient inhibitor of protein kinase C) as well as by 3-isobutyl-1-methyl xanthin (IBMX). Both FGF and PMA are good inducers of the accumulation of c-fos and c-jun mRNAs, whereas insulin has little effect. However, FGF was fully active (both as a mitogen and as inducer of c-fos mRNA accumulation) also in cells where the protein kinase C-mediated pathway had been downregulated by a long exposure to phorbol dibutyrate. We propose that the mitogenic effect of FGF does not require activation of protein kinase C, but that the subsequent events in the transduction pathways initiated by FGF and PMA, respectively, are (in part) coincident.

Serum-inducible expression of transfected human c-myc genes

Activation of the c-myc proto-oncogene is implicated in the initiation or progression of many vertebrate cancers. In nontransformed cells, the expression of c-myc is induced by growth factors. Studies have indicated that such induction is effected by multiple mechanisms. To study regulation of c-myc expression, a transfection system has been developed in which introduced c-myc genes exhibit serum-responsive activity. The responsiveness assayed is not mediated by increased transcription initiation. Rather, it is effected at a point(s) between transcription and stabilization of the RNA.

Estradiol and estrogen receptor-dependent stabilization of a minivitellogenin mRNA lacking 5,100 nucleotides of coding sequence

We have developed a transfection assay to investigate the estrogen-mediated stabilization of cytoplasmic vitellogenin mRNA. A minivitellogenin (MV5) gene containing the 5' and 3' untranslated and coding regions but lacking 5,075 nucleotides of internal coding sequence was constructed. Cotransfection of the MV5 plasmid and a Xenopus estrogen receptor expression plasmid into Xenopus liver tissue culture cells yielded a 529-nucleotide MV5 mRNA, which was specifically stabilized by estrogen. MV5 mRNA exhibited the increased stability indicative of positive regulation when the estradiol-estrogen receptor complex was present and was not destabilized by unliganded estrogen receptor. Transfected estrogen receptor, estradiol, and 529 nucleotides of the 5,604-nucleotide vitellogenin B1 mRNA were sufficient for stabilization.

Estradiol and estrogen receptor-dependent stabilization of a minivitellogenin mRNA lacking 5,100 nucleotides of coding sequence

We have developed a transfection assay to investigate the estrogen-mediated stabilization of cytoplasmic vitellogenin mRNA. A minivitellogenin (MV5) gene containing the 5' and 3' untranslated and coding regions but lacking 5,075 nucleotides of internal coding sequence was constructed. Cotransfection of the MV5 plasmid and a Xenopus estrogen receptor expression plasmid into Xenopus liver tissue culture cells yielded a 529-nucleotide MV5 mRNA, which was specifically stabilized by estrogen. MV5 mRNA exhibited the increased stability indicative of positive regulation when the estradiol-estrogen receptor complex was present and was not destabilized by unliganded estrogen receptor. Transfected estrogen receptor, estradiol, and 529 nucleotides of the 5,604-nucleotide vitellogenin B1 mRNA were sufficient for stabilization.

Glucocorticoid hormones reduce the expression of major histocompatibility class I antigens on human epithelial cells

Expression of a critical level of major histocompatibility complex (MHC) class I antigens on epithelial cells is a prerequisite for the action of specific cytolytic immune response cells. Glucocorticoid hormones have strong immunosuppressive effects. Therefore, we investigated the influence of the synthetic glucocorticoid dexamethasone on the expression level of MHC class I antigens on human epithelial cell lines. Long-term treatment with dexamethasone leads to reduced MHC class I surface antigen expression and to decreased total membrane-bound MHC class I protein. The steady-state mRNA level is significantly decreased and the transcription rate of MHC class I genes is reduced.

Modifications in molecular mechanisms associated with control of cell cycle regulated human histone gene expression during differentiation

Histone proteins are preferentially synthesized during the S-phase of the cell cycle, and the temporal and functional coupling of histone gene expression with DNA replication is mediated at both the transcriptional and posttranscriptional levels. The genes are transcribed throughout the cell cycle, and a 3-5-fold enhancement in the rate of transcription occurs during the first 2 h following initiation of DNA synthesis. Control of histone mRNA stability also accounts for some of the 20-100fold increase in cellular histone mRNA levels during S-phase and for the rapid and selective degradation of the mRNAs at the natural completion of DNA replication or when DNA synthesis is inhibited. Two segments of the proximal promoter, designated Sites I and II, influence the specificity and rate of histone gene transcription. Occupancy of Sites I and II during all periods of the cell cycle by three transacting factors (HiNF-A, HiNF-C, and HiNF-D) suggests that these protein-DNA interactions are responsible for the constitutive transcription of histone genes. Binding of HiNF-D in Site II is selectively lost, whereas occupancy of Site I by HiNF-A and -C persists when histone gene transcription is down regulated when cells terminally differentiate. These results are consistent with a primary role for interactions of HiNF-D with a proximal promoter element in rendering cell growth regulated human histone genes transcribable in proliferating cells.

Germ line c-myc is not down-regulated by loss or exclusion of activating factors in myc-induced macrophage tumors

As in tumors with c-myc chromosomal translocations, c-myc retrovirus-induced monocyte tumors constitutively express an activated form of c-myc (the proviral gene), whereas the normal endogenous c-myc genes are transcriptionally silent. Treatment of these retrovirus-induced tumor cells with a number of bioactive chemicals and growth factors that are known to induce c-myc expression in cells of the monocyte lineage failed to induce the endogenous c-myc gene. In contrast, the same treatments induced the c-fos gene in both tumors and a control macrophage line. To investigate c-myc suppression further, a normal copy of the human c-myc gene was introduced into tumor and control cell lines by using a retrovirus with self-inactivating long terminal repeats. This transduced normal gene was expressed at equivalent levels in all cells, regardless of the state of endogenous c-myc gene expression, and was strongly induced by agents that induce the normal gene in the control cells. These results indicate that the signal transduction pathways that normally activate the c-myc gene are functional in myc-induced tumor cells and suggest that endogenous c-myc is actively suppressed. An examination of the c-myc locus itself showed that the lack of transcriptional activity correlated with the absence of several prominent DNase I-hypersensitive sites in the 5'-flanking region of the gene but without loss of general DNase sensitivity. Furthermore, analysis of 22 methylation-sensitive restriction enzyme sites in the 5'-flanking region, first exon, and first intron indicated that the silent c-myc genes remained in the same unmethylated state as did actively expressed genes. Thus, c-myc suppression does not appear to result from the most frequently described mechanisms of gene inactivation.

Serum-inducible expression of transfected human c-myc genes

Activation of the c-myc proto-oncogene is implicated in the initiation or progression of many vertebrate cancers. In nontransformed cells, the expression of c-myc is induced by growth factors. Studies have indicated that such induction is effected by multiple mechanisms. To study regulation of c-myc expression, a transfection system has been developed in which introduced c-myc genes exhibit serum-responsive activity. The responsiveness assayed is not mediated by increased transcription initiation. Rather, it is effected at a point(s) between transcription and stabilization of the RNA.

Dissociation of cellular proliferation and c-myc expression by buttercup extract

Buttercup extract (BE), an extract of the buttercup plant (Zanthoriza simplicissima), inhibits RNA and DNA synthesis by HL-60 promyelocytic leukemia cells. Exposure of these cells to 3% BE for 48 hours results in dramatic inhibition of RNA synthesis without loss of cell viability. The effect of BE is partially reversible over 12-24 hours with the level of RNA synthesis returning nearly to control levels during this time period. DNA synthesis is also reversibly inhibited by exposure to BE. Despite the inhibition of RNA synthesis in HL-60 cells, there is no decrease in the level of c-myc mRNA, even at high BE concentrations. The level of gene-specific mRNA for the c-Ha-ras, c-fms, and c-mos genes in these cells also remained constant during exposure to BE. Ribosomal RNA is not degraded during 24 hours of BE treatment in vitro, suggesting that BE does not maintain the relative mRNA level for these genes by selective degradation of other RNA species. The inhibition of RNA and DNA synthesis by BE without a corresponding alteration in the level of expression of the c-myc gene suggests that this agent dissociates c-myc expression and cellular proliferation in these cells.

Oncogenes, growth, and the cell cycle: an overview

In spite of the complexity of the network of regulatory factors which control the balance between the cell cycle and quiescence, a picture is emerging, if only in outline. Several dozens of protooncogenes participate in growth signal transduction and integration, and, when expressed inappropriately, generate growth signals that may override other cellular controls. Some of these controls are provided by the negatively regulating growth factors, and when these are lost (e.g. by chromosomal deletion), or inactivated (e.g. by binding to an inactive analogue or a DNA viral oncoprotein), cell cycle activity is favoured over quiescence. Embryonic tissues are rapidly growing, so their cells are actively cycling and expression of proto-oncogenes is usually observed (Schuuring et al., 1989). As embryonic and stem cells in adult tissues mature, expression of the active proto-oncogenes is generally lost, but other proto-oncogenes may now be expressed (e.g. Muller et al., 1982). These changes in proto-oncogene expression are not achieved by modulation of transcriptional rates alone; transcriptional attenuation, message processing and stability, and post-translational protein modifications are all known to be important for the regulation of proto-oncogene expression during the transition from growth to the differentiated state. When quiescent cells re-enter the cell cycle approximately 60 genes become up-regulated, including proto-oncogene c-fos, the jun family, and c-myc (Zipfel et al., 1989). Evidence is strong that fos and jun proteins are transcriptional regulators. Terminal differentiation, on the other hand, is sometimes accompanied by the up-regulation of the ras gene family, as well as of several other proto-oncogenes. Proto-oncogene function is essential to the cell cycle traverse, but the genes involved are different in various cell types, and the precise order of oncogene expression may not turn out to be important. This is because cell cycle traverse appears to be more dependent on a critical threshold of growth signals propagated by parallel pathways, rather than on a strict order of predetermined steps. The participation of proto-oncogenes in growth signal transduction offers opportunities for errors, and abnormal growth may result from aberrant oncogene products generating a persistent or excessive growth signal, which shifts the balance of input to the integrating genes from quiescence to an active cell cycle. Thus, cancer may result from an entirely normal processing of growth signals that are abnormal.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence that IFN-gamma does not affect MHC class II gene expression at the post-transcriptional level in a mouse macrophage cell line

Mouse class II major histocompatibility complex genes have been shown to be regulated at the level of transcription for both tissue-specific and inducible expression. In particular, IFN-gamma induction of the class II genes has been shown to occur at the transcriptional level, although the role that additional post-transcriptional mechanisms of regulation may play in this induction is not known. To evaluate IFN-gamma effects on transcriptional and post-transcriptional events of class II gene expression, we examined the rate of decline of class II transcription, steady-state mRNA, and cell surface protein following the removal of IFN-gamma from maximally stimulated WEHI-3 cells (an IFN-gamma inducible, myelomonocytic cell line). We determined that transcription of class II genes almost completely returned to baseline levels eight hours after removal of IFN-gamma. However, the steady-state level of class II mRNA's required 4 days, and membrane Ia expression required 5 days to return to baseline levels. This decay was linear and allowed us to determine a half-life value of 16-20 h for class II transcripts. These data demonstrate that, following removal of IFN-gamma from fully stimulated cells, transcription of the class II genes declined rapidly, but mRNA was quite long-lived. We also assessed the class II mRNA stability in unstimulated WEHI-3 cells and the B-cell lymphoma. A20/2J, by actinomycin D treatment and northern blot analysis. In agreement with the IFN-gamma washout experiments, transcripts from all four class II genes were quite long-lived in these cell types, with a half-life greater than ten hours. These data support the concept that IFN-gamma acts primarily at the level of class II transcription and argues against IFN-gamma playing a major role in post-transcriptional modulation of class II expression.

Cellular remodeling of HCO3(-)-secreting cells in rabbit renal collecting duct in response to an acidic environment

The renal cortical collecting duct (CCD) consists of principal and intercalated cells. Two forms of intercalated cells, those cells involved in H+/HCO3- transport, have recently been described. H+-secreting cells are capable of apical endocytosis and have H+ATPase on the apical membrane and a basolateral Cl-/HCO3- exchanger. HCO3(-)-secreting cells bind peanut agglutinin (PNA) to apical membrane receptors and have diffuse or basolateral distribution of H+ATPase; their Cl-/HCO3- exchanger is on the apical membrane. We found that 20 h after acid feeding of rabbits, there was a fourfold increase in number of cells showing apical endocytosis and a numerically similar reduction of cells binding PNA. Incubation of CCDs at pH 7.1 for 3-5 h in vitro led to similar, albeit less pronounced, changes. Evidence to suggest internalization and degradation of the PNA binding sites included a reduction in apical binding of PNA, decrease in pH in the environment of PNA binding, and incorporation of electron-dense PNA into cytoplasmic vesicles. Such remodeling was dependent on protein synthesis. There was also functional evidence for loss of apical Cl-/HCO3- exchange on PNA-labeled cells. Finally, net HCO3- flux converted from secretion to absorption after incubation at low pH. Thus, exposure of CCDs to low pH stimulates the removal/inactivation of apical Cl-/HCO3- exchangers and the internalization of other apical membrane components. Remodeling of PNA-labeled cells may mediate the change in polarity of HCO3- flux observed in response to acid treatment.

Analysis of polyadenylation site usage of the c-myc oncogene

The c-myc gene contains 2 well conserved polyadenylation (pA) sites. In all human and rat cell lines from various differentiation stages and tissue types the amount of mRNA terminating at the second pA site is 6-fold higher than the amount ending at the upstream site. This is not due to a difference in stability of the two mRNA types and therefore must be due to preferential usage of the downstream site. The usage of the pA sites is not altered during growth factor induction of quiescent cells. We have not been able to detect differences in behavior between mRNAs ending at either pA site. Both types of mRNA are induced upon treatment of cells with cycloheximide. Furthermore, we have shown that the poly(A) tail of c-myc mRNA is lost during degradation of the messenger, as was described previously for c-myc mRNA in an in vitro system. The time required for the loss of the poly(A) tail is similar to the half-life of c-myc mRNA.

Regulation of c-myc mRNA stability in vitro by a labile destabilizer with an essential nucleic acid component

The turnover rates of some mRNAs vary by an order of magnitude or more when cells change their growth pattern or differentiate. To identify regulatory factors that might be responsible for this variability, we investigated how cytosolic fractions affect mRNA decay in an in vitro system. A 130,000 X g supernatant (S130) from the cytosol of exponentially growing erythroleukemia cells contains a destabilizer that accelerates the decay of polysome-bound c-myc mRNA by eightfold or more compared with reactions lacking S130. The destabilizer is deficient in or absent from the S130 of cycloheximide-treated cells, indicating that it is labile or is repressed when translation is blocked. It is not a generic RNase, because it does not affect the turnover of delta-globin, gamma-globin, or histone mRNA and does not destabilize a major portion of polysomal polyadenylated mRNA. The destabilizer accelerates the turnover of the c-myc mRNA 3' region, as well as subsequent 3'-to-5' degradation of the mRNA body. It is inactivated in vitro by mild heating and by micrococcal nuclease, suggesting that it contains a nucleic acid component. c-myb mRNA is also destabilized in S130-supplemented in vitro reactions. These results imply that the stability of some mRNAs is regulated by cytosolic factors that are not associated with polysomes.

Germ line c-myc is not down-regulated by loss or exclusion of activating factors in myc-induced macrophage tumors

As in tumors with c-myc chromosomal translocations, c-myc retrovirus-induced monocyte tumors constitutively express an activated form of c-myc (the proviral gene), whereas the normal endogenous c-myc genes are transcriptionally silent. Treatment of these retrovirus-induced tumor cells with a number of bioactive chemicals and growth factors that are known to induce c-myc expression in cells of the monocyte lineage failed to induce the endogenous c-myc gene. In contrast, the same treatments induced the c-fos gene in both tumors and a control macrophage line. To investigate c-myc suppression further, a normal copy of the human c-myc gene was introduced into tumor and control cell lines by using a retrovirus with self-inactivating long terminal repeats. This transduced normal gene was expressed at equivalent levels in all cells, regardless of the state of endogenous c-myc gene expression, and was strongly induced by agents that induce the normal gene in the control cells. These results indicate that the signal transduction pathways that normally activate the c-myc gene are functional in myc-induced tumor cells and suggest that endogenous c-myc is actively suppressed. An examination of the c-myc locus itself showed that the lack of transcriptional activity correlated with the absence of several prominent DNase I-hypersensitive sites in the 5'-flanking region of the gene but without loss of general DNase sensitivity. Furthermore, analysis of 22 methylation-sensitive restriction enzyme sites in the 5'-flanking region, first exon, and first intron indicated that the silent c-myc genes remained in the same unmethylated state as did actively expressed genes. Thus, c-myc suppression does not appear to result from the most frequently described mechanisms of gene inactivation.

Enhanced messenger RNA stability and differentiation of HL 60 cells treated with 1,25-dihydroxyvitamin D3 and cordycepin

The effect of inhibitors of RNA synthesis on 1,25(OH)2 vitamin D3-induced monocytic differentiation was studied in a well-differentiating clone AB 47 of HL 60 cells. The concentrations of these inhibitors were chosen to permit the maintenance of cell viability for at least 48 hours, and resulted in 40-60% inhibition of total cellular RNA synthesis. No impairment of 1,25(OH)2 vitamin D3-induced monocytic differentiation was observed with all inhibitors tested, and the presence of cordycepin actually enhanced differentiation. The phenotypic evidence of monocytic differentiation correlated with the increased levels of mRNA for c-fos and c-fms measured by hybridization to appropriate nick-translated cDNA probes. In contrast, nuclear run-on experiments showed the expected inhibition of transcription of these genes by the compounds used. The data suggest that interference by external agents with transcription of genes essential for a differentiation program brings into play compensatory mechanisms which permit the program to continue. Thus, differentiation appears to have a high priority among various competing intracellular pathways in 1,25(OH)2 vitamin D3-treated HL 60 cells. Stabilization of messenger RNA levels evident in this study may therefore represent a general cellular mechanism for the correction of unwanted effects of xenobiotics on the cell.

Analysis of 3'-untranslated regions of seven c-myc genes reveals conserved elements prevalent in post-transcriptionally regulated genes

We have characterized the complete sequence of two c-myc cDNAs from the amphibian Xenopus laevis, and could thus compare the 3'-non-coding sequences of 7 myc cDNAs from 6 species spread over 350 million years of evolution. Although the size of these sequences is heterogeneous, we identified three completely conserved sequences of 10, 11 and 12 contiguous nucleotides. We observed that two of these elements may be contained in conserved stem-loop structures previously implicated in mRNA turnover. The length of these motifs, their existence in conserved predicted structures, and their presence in regulated eukaryote mRNA with a frequency greater than predicted by chance, suggest that they are functionally important.

Evidence that the familial adenomatous polyposis gene is involved in a subset of colon cancers with a complementable defect in c-myc regulation

Human colorectal carcinomas frequently express elevated levels of c-myc mRNA in the absence of a gross genetic change at the c-myc locus. To test the hypothesis that these tumors are defective in a gene function necessary for the regulation of c-myc expression, we fused an osteosarcoma cell line that exhibits normal c-myc regulation with two colon carcinoma cell lines that express deregulated levels of c-myc mRNA. The levels of c-myc transcripts in all of the hybrid clones examined were normal and were induced normally by a mitogenic stimulus. Since rates of c-myc mRNA turnover in the colon carcinoma cells were found to be comparable to those in normal cells, increased message stability cannot account for the increased steady-state levels of transcripts. Our findings suggest that loss of function of a trans-acting regulator is responsible for the deregulation of c-myc expression in a major fraction of colorectal carcinomas. Analysis of restriction fragment length polymorphisms in tumor/normal tissue pairs from patients with primary colorectal lesions indicated that deregulation of c-myc expression in the tumors is correlated with frequent loss of alleles of syntenic markers on chromosome 5q; allele loss on 5q could be detected in 9 of 19 tumors expressing deregulated levels of c-myc mRNA, but not in any of 8 tumors expressing normal levels of c-myc RNA. Chromosome 5q is the region known to contain the gene for familial adenomatous polyposis, an inherited predisposition to colon cancer. These findings, together with the earlier finding that the colonic distribution of tumors exhibiting deregulated c-myc expression is similar to that reported for familial polyposis, provide evidence that loss of function of the familial adenomatous polyposis gene is involved in a subset of colorectal cancers in which c-myc expression is deregulated.