c-myc RNA degradation in growing and differentiating cells: possible alternate pathways

Transcripts of the proto-oncogene c-myc are composed of a rapidly degraded polyadenylated RNA species and an apparently much more stable, nonadenylated RNA species. In this report, the extended kinetics of c-myc RNA turnover have been examined in rapidly growing cells and in cells induced to differentiate. When transcription was blocked with actinomycin D in rapidly growing cells, poly(A)+ c-myc was rapidly degraded (t1/2 = 12 min). c-myc RNA lacking poly(A) initially remained at or near control levels; however, after 80 to 90 min it was degraded with kinetics similar to those of poly(A)+ c-myc RNA. These bizarre kinetics are due to the deadenylation of poly(A)+ c-myc RNA to form poly(A)- c-myc, thereby initially maintaining the poly(A)- c-myc RNA pool when transcription is blocked. In contrast to growing cells, cells induced to differentiate degraded both poly(A)+ and poly(A)- c-myc RNA rapidly. The rapid disappearance of both RNA species in differentiating cells suggests that a large proportion of the poly(A)+ c-myc RNA was directly degraded without first being converted to poly(A)- c-myc RNA. Others have shown that transcriptional elongation of the c-myc gene is rapidly blocked in differentiating cells. We therefore hypothesize that in differentiating cells a direct, rapid degradation of poly(A)+ c-myc RNA may act as a backup or fail-safe system to ensure that c-myc protein is not synthesized. This tandem system of c-myc turnoff may also make cells more refractory to mutations which activate constitutive c-myc expression.

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Defective c-myc and c-myb RNA turnover in acute myeloid leukemia cells

Blood ◽

10.1182/blood.v79.5.1319.bloodjournal7951319 ◽

1992 ◽

Vol 79 (5) ◽

pp. 1319-1326 ◽

Cited By ~ 1

Author(s):

MR Baer ◽

P Augustinos ◽

AJ Kinniburgh

Keyword(s):

Acute Myeloid Leukemia ◽

Posttranscriptional Regulation ◽

Myeloid Leukemia ◽

Actinomycin D ◽

The Other ◽

Rna Turnover ◽

Gm Csf ◽

Acute Myeloid Leukemia Cells ◽

Acute Myeloid ◽

In Cells

Dysregulated expression of the c-myc and c-myb protooncogenes has been implicated in the pathogenesis of acute myeloid leukemia (AML). To elucidate mechanisms of c-myc dysregulation in AML cells, we studied c- myc RNA turnover in peripheral blood blasts from eight patients using actinomycin D transcription blockade. Rapid c-myc RNA turnover was seen in cells from six patients, with half-lives of approximately 30 minutes, similar to those reported in normal myeloid cells, in HL-60 cells, and in other cell lines. c-myc RNA turnover was prolonged in cells of the other two patients, with half-lives of greater than 75 minutes. c-fos RNA turnover was rapid in blasts from all eight patients, with half-lives of approximately 15 minutes. Stabilization of GM-CSF transcripts was not observed. In contrast, c-myb RNA half-lives were greater than 75 minutes in cells of the two patients with prolonged c-myc RNA turnover, as compared to 30 minutes in cells of the other six patients. Enhanced stability of both c-myc and c-myb RNA species suggests that a defect exists in a trans-acting factor that destabilizes both of these normally labile RNAs. Incomplete correlation between c-myc RNA levels and half-lives indicates regulation of c-myc expression at the level of transcription or nuclear transport in addition to posttranscriptional regulation.

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Defective c-myc and c-myb RNA turnover in acute myeloid leukemia cells

Blood ◽

10.1182/blood.v79.5.1319.1319 ◽

1992 ◽

Vol 79 (5) ◽

pp. 1319-1326 ◽

Cited By ~ 19

Author(s):

MR Baer ◽

P Augustinos ◽

AJ Kinniburgh

Keyword(s):

Acute Myeloid Leukemia ◽

Posttranscriptional Regulation ◽

Myeloid Leukemia ◽

Actinomycin D ◽

The Other ◽

Rna Turnover ◽

Gm Csf ◽

Acute Myeloid Leukemia Cells ◽

Acute Myeloid ◽

In Cells

Abstract Dysregulated expression of the c-myc and c-myb protooncogenes has been implicated in the pathogenesis of acute myeloid leukemia (AML). To elucidate mechanisms of c-myc dysregulation in AML cells, we studied c- myc RNA turnover in peripheral blood blasts from eight patients using actinomycin D transcription blockade. Rapid c-myc RNA turnover was seen in cells from six patients, with half-lives of approximately 30 minutes, similar to those reported in normal myeloid cells, in HL-60 cells, and in other cell lines. c-myc RNA turnover was prolonged in cells of the other two patients, with half-lives of greater than 75 minutes. c-fos RNA turnover was rapid in blasts from all eight patients, with half-lives of approximately 15 minutes. Stabilization of GM-CSF transcripts was not observed. In contrast, c-myb RNA half-lives were greater than 75 minutes in cells of the two patients with prolonged c-myc RNA turnover, as compared to 30 minutes in cells of the other six patients. Enhanced stability of both c-myc and c-myb RNA species suggests that a defect exists in a trans-acting factor that destabilizes both of these normally labile RNAs. Incomplete correlation between c-myc RNA levels and half-lives indicates regulation of c-myc expression at the level of transcription or nuclear transport in addition to posttranscriptional regulation.

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Early meiotic transcripts are highly unstable in Saccharomyces cerevisiae.

Molecular and Cellular Biology ◽

10.1128/mcb.12.9.3948 ◽

1992 ◽

Vol 12 (9) ◽

pp. 3948-3958 ◽

Cited By ~ 23

Author(s):

R T Surosky ◽

R E Esposito

Keyword(s):

Saccharomyces Cerevisiae ◽

Vegetative Growth ◽

Degradation Mechanism ◽

Rna Degradation ◽

Specific Gene ◽

Rna Turnover ◽

Coding Region ◽

Mrna Accumulation ◽

Rapid Degradation ◽

Transcriptional Induction

Meiosis in Saccharomyces cerevisiae requires the induction of a large number of genes whose mRNAs accumulate at specific times during meiotic development. This study addresses the role of mRNA stability in the regulation of meiosis-specific gene expression. Evidence is provided below demonstrating that the levels of meiotic mRNAs are exquisitely regulated by both transcriptional control and RNA turnover. The data show that (i) early meiotic transcripts are extremely unstable when expressed during either vegetative growth or sporulation, and (ii) transcriptional induction, rather than RNA turnover, is the predominant mechanism responsible for meiosis-specific transcript accumulation. When genes encoding the early meiotic mRNAs are fused to other promoters and expressed during vegetative growth, their mRNA half-lives, of under 3 min, are among the shortest known in S. cerevisiae. Since these mRNAs are only twofold more stable when expressed during sporulation, we conclude that developmental regulation of mRNA turnover can be eliminated as a major contributor to meiosis-specific mRNA accumulation. The rapid degradation of the early mRNAs at all stages of the yeast life cycle, however, suggests that a specific RNA degradation system operates to maintain very low basal levels of these transcripts during vegetative growth and after their transient transcriptional induction in meiosis. Studies to identify specific cis-acting elements required for the rapid degradation of early meiotic transcripts support this idea. A series of deletion derivatives of one early meiosis-specific gene, SPO13, indicate that its mRNA contains determinants, located within the coding region, which contribute to the high instability of this transcript. Translation is another component of the degradation mechanism since frameshift and nonsense mutations within the SPO13 mRNA stabilize the transcript.

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Early meiotic transcripts are highly unstable in Saccharomyces cerevisiae

Molecular and Cellular Biology ◽

10.1128/mcb.12.9.3948-3958.1992 ◽

1992 ◽

Vol 12 (9) ◽

pp. 3948-3958

Author(s):

R T Surosky ◽

R E Esposito

Keyword(s):

Saccharomyces Cerevisiae ◽

Vegetative Growth ◽

Degradation Mechanism ◽

Rna Degradation ◽

Specific Gene ◽

Rna Turnover ◽

Coding Region ◽

Mrna Accumulation ◽

Rapid Degradation ◽

Transcriptional Induction

Meiosis in Saccharomyces cerevisiae requires the induction of a large number of genes whose mRNAs accumulate at specific times during meiotic development. This study addresses the role of mRNA stability in the regulation of meiosis-specific gene expression. Evidence is provided below demonstrating that the levels of meiotic mRNAs are exquisitely regulated by both transcriptional control and RNA turnover. The data show that (i) early meiotic transcripts are extremely unstable when expressed during either vegetative growth or sporulation, and (ii) transcriptional induction, rather than RNA turnover, is the predominant mechanism responsible for meiosis-specific transcript accumulation. When genes encoding the early meiotic mRNAs are fused to other promoters and expressed during vegetative growth, their mRNA half-lives, of under 3 min, are among the shortest known in S. cerevisiae. Since these mRNAs are only twofold more stable when expressed during sporulation, we conclude that developmental regulation of mRNA turnover can be eliminated as a major contributor to meiosis-specific mRNA accumulation. The rapid degradation of the early mRNAs at all stages of the yeast life cycle, however, suggests that a specific RNA degradation system operates to maintain very low basal levels of these transcripts during vegetative growth and after their transient transcriptional induction in meiosis. Studies to identify specific cis-acting elements required for the rapid degradation of early meiotic transcripts support this idea. A series of deletion derivatives of one early meiosis-specific gene, SPO13, indicate that its mRNA contains determinants, located within the coding region, which contribute to the high instability of this transcript. Translation is another component of the degradation mechanism since frameshift and nonsense mutations within the SPO13 mRNA stabilize the transcript.

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RNA SYNTHESIS AND TURNOVER DURING DENSITY-INHIBITED GROWTH AND ENCYSTMENT OF ACANTHAMOEBA CASTELLANII

The Journal of Cell Biology ◽

10.1083/jcb.57.2.525 ◽

1973 ◽

Vol 57 (2) ◽

pp. 525-537 ◽

Cited By ~ 24

Author(s):

A. R. Stevens ◽

P. F. Pachler

Keyword(s):

Stationary Phase ◽

Acanthamoeba Castellanii ◽

Rna Turnover ◽

Uridine Incorporation ◽

Rna Content ◽

Cell Densities ◽

Reduced Rate ◽

Kinetics Of ◽

Growth Deceleration ◽

In Cells

Alterations in transcription that precede and accompany encystment (E) of suspension grown A. castellanii have been investigated. Comparative studies were performed on cells undergoing spontaneous E in high density stationary phase cultures or after experimental induction of E at low cell densities by deprivation of nutrients in exponential growth. Onset of growth deceleration at high cell densities was accompanied by an increase in the cellular RNA. The maximum RNA content occurred in cells at stationary phase and subsequently declined with the appearance of cysts in the cultures. On the contrary, the RNA content in cells whose growth was immediately terminated by experimental E induction remained at a constant exponential level through 5 h postinduction and then began to decline shortly before the appearance of cysts. The mature cyst formed in stationary phase cultures and after experimental E induction contained an equivalent amount of RNA (∼50% of the exponential value). Comparison of the kinetics of [3H]uridine incorporation demonstrated that there was an abrupt reduction in the rate of uridine incorporation into RNA with onset of growth deceleration or after growth termination in experimental E induction. The reduced incorporation of uridine into RNA could not be attributed to to a reduced uptake of the isotope by the cells or an altered capacity of the cells to phosphorylate uridine. Uridine continued to be incorporated into RNA at a reduced rate in cells throughout growth deceleration, in stationary phase, and up to 12 h postexperimental induction. Considered together, these results indicate that a buildup in RNA is not necessary for induction of encystment in acanthamoeba. The accumulated RNA in stationary phase cells appears to be due to the greater reduction in the growth rate than in transcription and the absence of RNA turnover in cells during growth deceleration. Initiation of RNA turnover appears to accompany growth termination and induction of E. The results further demonstrate that the regulation of the rate of transcription is closely coordinated with the control of growth and encystment in acanthamoeba.

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Effect of Some Metabolic Inhibitors on Vinblastine-Induced Ribosomal-Granular Material Complexes

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100066796 ◽

1971 ◽

Vol 29 ◽

pp. 548-549

Author(s):

Awtar Krishan ◽

Dora Hsu

Keyword(s):

Granular Material ◽

Rna Synthesis ◽

Culture Medium ◽

Actinomycin D ◽

Metabolic Inhibitors ◽

Protein And Rna Synthesis ◽

Apparent Reduction ◽

Plant Alkaloids ◽

In Cells

Cells exposed to antitumor plant alkaloids, vinblastine and vincristine sulfate have large proteinacious crystals and complexes of ribosomes, helical polyribosomes and electron-dense granular material (ribosomal complexes) in their cytoplasm, Binding of H3-colchicine by the in vivo crystals shows that they contain microtubular proteins. Association of ribosomal complexes with the crystals suggests that these structures may be interrelated.In the present study cultured human leukemic lymphoblasts (CCRF-CEM), were incubated with protein and RNA-synthesis inhibitors, p. fluorophenylalanine, puromycin, cycloheximide or actinomycin-D before the addition of crystal-inducing doses of vinblastine to the culture medium. None of these compounds could completely prevent the formation of the ribosomal complexes or the crystals. However, in cells pre-incubated with puromycin, cycloheximide, or actinomycin-D, a reduction in the number and size of the ribosomal complexes was seen. Large helical polyribosomes were absent in the ribosomal complexes of cells treated with puromycin, while in cells exposed to cycloheximide, there was an apparent reduction in the number of ribosomes associated with the ribosomal complexes (Fig. 2).

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