The cytoplasmic appearance of three functions expressed during the G0 leds to G1 leads to S transition is nucleus-dependent

Two zinc-dependent steps during G1 to S phase transition

The influence of zinc (Zn) availability on thymidine kinase mRNA concentration has been investigated in cells in which production of the mRNA was regulated by either truncated thymidine kinase promoters or by the SV40 early promoter. Thymidine kinase mRNA concentrations were decreased by low Zn availability even when the promoter was truncated to 80 bp but not when it was replaced by the SV40 promoter. However, thymidine incorporation by the SV40 cells was still sensitive to lack of Zn, suggesting a second Zn-sensitive process involved in commitment to S phase. The increase in histone H3 mRNA production prior to S phase was not inhibited by lack of Zn leading to a preferential increase in this mRNA in exponentially growing cells deprived of Zn.

Expression of growth-regulated genes in normal and SV40 transformed hamster fibroblasts

Transformation by the oncogenic virus SV40 has been shown to alter the expression of cellular genes at the level of RNA abundance. Many of these genes have yet to be identified. We have determined, by Northern blot analysis, the abundance levels of several growth-regulated genes in SV40-transformed cell lines to determine if their expression is altered and correlates with the ability of SV40 transformed cells to grow in low serum containing media. The mRNA abundance levels of the G1-specific genes 2A9/calcyclin, 2F1/translocase, and 4F1/vimentin were determined in the parental hamster fibroblast cell line, tk-ts13, and in two SV40 transformants, HR5 and HR8 cells, grown in medium containing 10% calf serum (normal medium) and in HR5 and HR8 cells adapted to passage in medium containing low serum. A spontaneous transformant of the parental line capable of growth in low serum in the absence of SV40 transformation (tk-ts13/1%), was also included in these studies. The low serum adapted SV40-transformed cells and the spontaneous tk-ts13 transformed cells grew more vigorously than their nonadapted counterparts in medium containing low serum. The low serum adapted cells also grew to higher saturation densities in low serum and to densities comparable to those in high serum, whereas the nonadapted cells grew to low saturation densities in low serum, but not as low as the untransformed parental.(ABSTRACT TRUNCATED AT 250 WORDS)

CAD gene expression in serum-starved and serum-stimulated hamster cells

The enzymes in the pathway for de novo pyrimidine biosynthesis, including those associated with the tri-functional CAD protein, show a marked increase in activity in rapidly growing cells and tissues. To learn more about the relationship of this pathway to cellular proliferation, we have studied changes in levels of CAD RNA, rates of CAD protein synthesis, and levels of aspartate transcarbamylase activity in Syrian hamster ts13 cells in response to serum starvation and serum stimulation. The steady-state level of CAD RNA and the synthetic rate of CAD protein decrease by 12- to 15-fold following 24 hr of serum starvation, as compared to exponentially growing cells. Upon serum stimulation of quiescent cells, steady-state CAD RNA levels increase substantially (13-fold), peaking during mid to late G1. Parallel increases occur in the synthesis of new CAD protein and in aspartate transcarbamylase activity. At the same time, the rate of CAD transcription increases only about twofold. These findings indicate that regulation of CAD expression in this system is primarily at the post-transcriptional level. This is in contrast to the transcriptional regulation of CAD previously reported in terminally differentiating HL60 cells (Rao et al., Mol. Cell. Biol. 7, 1961-1966, 1987). While both systems indicate that CAD gene expression is dependent on cell growth, there apparently are alternative mechanisms that can produce the same effect. Evidence is also presented that indicates that the accumulation of CAD transcripts during serum stimulation requires the synthesis of new proteins.

Transforming potential of deletion mutants of the SV40 T antigen coding gene in Syrian hamster cells

Thymidine kinase-deficient syrian hamster cells were cotransfected with recombinant plasmids containing the thymidine kinase (TK) gene of Herpes Simplex Virus Type 1, and either intact or partially deleted SV40 T antigen-coding genes. The transformants were selected by their ability to grow in gHAT medium. After selection and cloning, the TK-positive transformants that also expressed T antigen were tested for the extent of their transformation with respect to a number of characteristics, which included saturation density, ability to grow in soft agar, resistance to butyrate and to dibutyryl-cAMP, and plating efficiency. The combined results of these various tests indicate that cells containing partially deleted SV40 T antigen-coding genes are less transformed than cells containing an intact SV40 T antigen-coding gene. However, the amounts of T antigen are lower in cells transformed by deletion mutants than in cells transformed by wild-type T antigen-coding gene. Our data indicate that both the quantity and the quality of T antigen may be important in determining the degree of transformation in Syrian hamster cells.

Changes in calmodulin and its mRNA accompany reentry of quiescent (G0) cells into the cell cycle

Release of CHO-K1 cells from plateau or stationary phase and reentry into the cell cycle is specifically and reversibly blocked at two distinct sites by the anticalmodulin drug W13. The first block occurs early during release while the cells are still at G0/G1, whereas the second occurs later in reentry during early S phase. As determined by radioimmunoassay, calmodulin levels undergo changes at three distinct steps in plateau-phase entry and release. First, the entry of exponentially growing cells into plateau phase is accompanied by an increase in the calmodulin level. The second change is a reduction in the calmodulin content of cells within the first hour following release from plateau phase. The third change is the subsequent increase in calmodulin levels, which precedes entry of the cells into S phase. Analysis of calmodulin mRNA levels by dot-blot hybridization demonstrates that the changes in calmodulin protein are preceded by changes in calmodulin mRNA. Furthermore, whereas a decrease in CaM mRNA is observed within the first hour following plateau release, no such decrease is observed for beta-actin mRNA, suggesting that this decrease may be selective for calmodulin. This selectivity is further substantiated by the fact that identical changes in calmodulin and calmodulin mRNA are observed in cells released from plateau by two different techniques. Taken together, these data suggest that calmodulin may play an important role in the reentry of cells into the cell cycle.

Evidence for differences in the mechanism of cell cycle arrest between senescent and serum-deprived human fibroblasts: heterokaryon and metabolic inhibitor studies

It has previously been shown that serum-deprived, early passage quiescent human diploid fibroblastlike (HDFL) cells are able to inhibit cycling cells from entry into DNA synthesis upon cell fusion. We have found that the degree of inhibition of DNA synthesis in the heterokaryon correlates with the duration of serum deprivation, which is consistent with the suggestion that serum-deprived cells may enter progressively deeper stages of G0 as they increase their time in quiescence. In contrast to fusions with senescent cells, in heterokaryons between serum-deprived early passage and cycling young cells transient inhibition of protein synthesis with cycloheximide or inhibition of RNA synthesis with 5-6-dichloro-1-beta-D-ribofuranosyl benzimidazole (DRB) did not stimulate nuclear [3H]-thymidine incorporation. These results suggest that differences may exist in the mechanisms responsible for inhibiting cell cycle progression in senescent vs early passage quiescent HDFL cells.

The role of adenovirus early region 1A in the regulation of early regions 2A and 1B expression

The role of the adenovirus early region 1A (E1A) in the regulation of expression of early regions 2A (E2A) and 1B (E1B) has been investigated by microinjection of cloned segments of viral DNA into cultured cells. Plasmids carrying adenoviral sequences that code for the DNA binding protein (DBP) associated with either the early or the late E2A promoter (at map coordinates 75 and 72, respectively), or both promoters, were injected into cell nuclei in the presence or absence of a plasmid DNA containing early region 1A. The injected cells were scored for the expression of the DNA binding protein by indirect immunofluorescence using specific antisera. These studies suggest that the product(s) encoded by early region 1A differentially control the expression of the DNA binding protein. DBP produced from the early promoter is stimulated by addition of the E1A region. By contrast E1A gene product(s) inhibit the synthesis of the DBP species transcribed from the late promoter. It has been shown previously by a variety of studies that E1A regulates E1B expression. However, experiments in which E1B DNA was microinjected together with E1A and E2A suggest that the DNA binding protein may have a stimulatory effect on the synthesis of E1B products.

Induction of cellular deoxyribonucleic acid synthesis in butyrate-treated cells by simian virus 40 deoxyribonucleic acid

Sodium butyrate (3 mM) inhibited the entry into the S phase of quiescent 3T3 cells stimulated by serum, but had no effect on the accumulation of cellular ribonucleic acid. Simian virus 40 infection or manual microinjection of cloned fragments from the simian virus 40 A gene caused quiescent 3T3 cells to enter the S phase even in the presence of butyrate. NGI cells, a line of 3T3 cells transformed by simian virus 40, grew vigorously in 3 mM butyrate. Homokaryons were formed between G1 and S-phase 3T3 cells, Butyrate inhibited the induction of deoxyribonucleic acid synthesis that usually occurs in B1 nuclei when G1 cells are fused with S-phase cells. However, when G1 3T3 cells were fused with exponentially growing NGI cells, the 3T3 nuclei were induced to enter deoxyribonucleic acid synthesis. In tsAF8 cells, a ribonucleic acid polymerase II mutant that stops in the G1 phase of the cell cycle, no temporal sequence was demonstrated between the butyrate block and the temperature-sensitive block. These results confirm previous reports that certain virally coded proteins can induce cell deoxyribonucleic acid synthesis in the absence of cellular functions that are required by serum-stimulated cells. Our interpretation of these data is that butyrate inhibited cell growth by inhibiting the expression of genes required for the G0 leads to G1 leads to S transition and that the product of the simian virus 40 A gene overrode this inhibition by providing all of the necessary functions for the entry into the S phase.

Induction of cellular deoxyribonucleic acid synthesis in butyrate-treated cells by simian virus 40 deoxyribonucleic acid

Sodium butyrate (3 mM) inhibited the entry into the S phase of quiescent 3T3 cells stimulated by serum, but had no effect on the accumulation of cellular ribonucleic acid. Simian virus 40 infection or manual microinjection of cloned fragments from the simian virus 40 A gene caused quiescent 3T3 cells to enter the S phase even in the presence of butyrate. NGI cells, a line of 3T3 cells transformed by simian virus 40, grew vigorously in 3 mM butyrate. Homokaryons were formed between G1 and S-phase 3T3 cells, Butyrate inhibited the induction of deoxyribonucleic acid synthesis that usually occurs in B1 nuclei when G1 cells are fused with S-phase cells. However, when G1 3T3 cells were fused with exponentially growing NGI cells, the 3T3 nuclei were induced to enter deoxyribonucleic acid synthesis. In tsAF8 cells, a ribonucleic acid polymerase II mutant that stops in the G1 phase of the cell cycle, no temporal sequence was demonstrated between the butyrate block and the temperature-sensitive block. These results confirm previous reports that certain virally coded proteins can induce cell deoxyribonucleic acid synthesis in the absence of cellular functions that are required by serum-stimulated cells. Our interpretation of these data is that butyrate inhibited cell growth by inhibiting the expression of genes required for the G0 leads to G1 leads to S transition and that the product of the simian virus 40 A gene overrode this inhibition by providing all of the necessary functions for the entry into the S phase.