Growth-stimulated Cells Research Articles

Evidence for transcriptional and post-transcriptional control of the cellular thymidine kinase gene.

We have studied the cell cycle-regulated expression of the thymidine kinase (TK) gene in mammalian tissue culture cells. TK mRNA and enzyme levels are low in resting, G0-phase cells, but increase dramatically (10- to 20-fold) during the S phase in both serum-stimulated and simian virus 40-infected cells. To determine whether an increase in the rate of TK gene transcription is responsible for this induction, nuclear run-on transcription assays were performed at various times after serum stimulation or simian virus 40 infection of growth-arrested simian CV1 cells. When assays were performed at 12-h intervals, a small (two- to threefold) but reproducible increase in TK transcription was detected during the S phase. When time points were chosen to span the G1-S interface a larger (six- to sevenfold) increase in transcriptional activity was observed in serum-stimulated cells but not in simian virus 40-infected cells. The large increase in TK mRNA levels and the relatively small increase in transcription rates in growth-stimulated cells suggest that TK gene expression is controlled at both a transcriptional and post-transcriptional level during the mammalian cell cycle. To identify the DNA sequences required for cell cycle-regulated expression, several TK cDNA clones were transfected into Rat-3 TK- cells, and their expression was examined in resting and serum-stimulated cultures. These experiments indicated that the body of the TK cDNA is sufficient to insure cell cycle-regulated expression regardless of the promoter or polyadenylation signal used.

Evidence for transcriptional and post-transcriptional control of the cellular thymidine kinase gene.

We have studied the cell cycle-regulated expression of the thymidine kinase (TK) gene in mammalian tissue culture cells. TK mRNA and enzyme levels are low in resting, G0-phase cells, but increase dramatically (10- to 20-fold) during the S phase in both serum-stimulated and simian virus 40-infected cells. To determine whether an increase in the rate of TK gene transcription is responsible for this induction, nuclear run-on transcription assays were performed at various times after serum stimulation or simian virus 40 infection of growth-arrested simian CV1 cells. When assays were performed at 12-h intervals, a small (two- to threefold) but reproducible increase in TK transcription was detected during the S phase. When time points were chosen to span the G1-S interface a larger (six- to sevenfold) increase in transcriptional activity was observed in serum-stimulated cells but not in simian virus 40-infected cells. The large increase in TK mRNA levels and the relatively small increase in transcription rates in growth-stimulated cells suggest that TK gene expression is controlled at both a transcriptional and post-transcriptional level during the mammalian cell cycle. To identify the DNA sequences required for cell cycle-regulated expression, several TK cDNA clones were transfected into Rat-3 TK- cells, and their expression was examined in resting and serum-stimulated cultures. These experiments indicated that the body of the TK cDNA is sufficient to insure cell cycle-regulated expression regardless of the promoter or polyadenylation signal used.

Cell-cycle-directed regulation of thymidylate synthase messenger RNA in human diploid fibroblasts stimulated to proliferate

Human diploid fibroblasts were synchronized in the resting phase by incubation in medium containing a low level of serum and then stimulated to proliferate by adding a high concentration of serum. DNA replication started 12 hours after addition of serum, and reached a maximum after 24 hours. Thymidylate synthase activity was very low in resting cells, but began to increase 12 hours after growth stimulation and thereafter continued to increase. Thymidylate synthase mRNA in the growing cells was compared with that in resting cells, using cloned human thymidylate synthase cDNA as a probe. Results showed that the mRNA content as a percentage of total RNA began to increase six hours after stimulation, reaching a level about 14 times that in unstimulated cells after 24 hours. However, the mRNA content relative to poly(A) + RNA had increased two- to fourfold by 24 hours after growth stimulation. Transcription of the thymidylate synthase gene, determined by hybridizing labelled nascent transcripts obtained in isolated nuclei to immobilized human thymidylate synthase cDNA, was similar in the nuclei of resting and of growth-stimulated cells. These results show that the increase in thymidylate synthase mRNA in growth-stimulated cells is caused by an increase in post-transcriptional events.

Regulation of thymidylate synthase enzyme synthesis in 5-fluorodeoxyuridine-resistant mouse fibroblasts during the transition from the resting to growing state.

Thymidylate synthase (TS) activity is very low in resting mouse 3T6 fibroblasts but increases sharply in growth-stimulated cells at about the same time the cells enter S phase. To study the mechanism responsible for the increase in TS level, we isolated a 5-fluorodeoxyuridine (5-FdUrd)-resistant cell line (LU3-7) that overproduces TS and its mRNA about 50-100-fold. In this paper we show that the LU3-7 cells were able to rest in the G0 state of the cell cycle when maintained in medium containing 0.5% serum. When the serum concentration was increased to 10%, the resting cells reentered the cell cycle and began DNA replication about 12 hr later. TS activity remained at the resting level until DNA replication began, then increased at later times. The increase was not affected when the cells were stimulated in the presence of DNA synthesis inhibitors. The rate of synthesis of TS (as determined in a pulse-labeling experiment) remained at the resting level for the first 10 hr following stimulation, then increased 8-9-fold by 25 hr following serum stimulation. The half-life of TS in growing LU3-7 cells was measured in a pulse-chase experiment and found to be greater than 24 hr. Therefore the increase in TS activity was primarily due to an increase in the rate of synthesis of the enzyme. Since TS gene expression appears to be regulated in a similar manner in LU3-7 cells and in the parental 3T6 cells, the LU3-7 cells should be a good model system for detailed analysis of the mechanism for regulating TS gene expression in mammalian cells.

Effect of sodium butyrate on induction of ornithine decarboxylase activity in phytohemagglutinin-stimulated lymphocytes

Effect of sodium butyrate on DNA synthesis and the induction of ornithine decarboxylase (EC 4.1.1.17), a rate-limiting enzyme of polyamine biosynthesis, was studied in phytohemagglutinin(PHA)-stimulated bovine lymphocytes. Millimolar concentrations of butyrate completely inhibited the incorporation of [ 3H] thymidine into the acid-insoluble fraction and reversibly suppressed the induction of ornithine decarboxylase. Other shortchain fatty acids were much less active than butyrate. These results suggest that the suppression of ornithine decarboxylase activity may be one of the reasons for the inhibition of DNA synthesis with butyrate in bovine lymphocytes, because our previous experimental results have shown that the induction of ornithine decarboxylase closely correlates with the DNA synthesis in growth-stimulated cells.