Characterization of ts13 cells a temperature-sensitive mutant of the G1 phase of the cell cycle

ts 13 cells are a temperature-sensitive (ts) mutant of BHK cells that are known to arrest in G1 when shifted to the nonpermissive temperature. We have determined the entry into S of ts13 cells in five different growth conditions, namely: 1) quiescent, sparse cultures stimulated to proliferate by serum. 2) Quiescent, dense cultures stimulated by serum. 3) Quiescent, sparse cultures stimulated by trypsinization and replating. 4) Quiescent, dense cultures stimulated by trypsinization and replating. 5) Mitotic cells collected by mitotic detachment. For each different growth condition we have also determined the execution point of the mutant function, i.e. the time at which a shift-up to the nonpermissive temperature no longer prevents the entry of cells into S. The median time of entry into S and the execution point varied in different growth conditions, but the distance between the median execution point and the median time of entry into S was remarkably constant, i.e. 3.2 hr. In addition we have fused ts 13 cells cells with chick erythrocytes and studied the ability of ts13 cells in heterokaryon formation to induce DNA synthesis in chick nuclei. Although ts13 cells can induce DNA synthesis in chick nuclei at the permissive temperature, they fail to do so when fused and stimulated at the nonpermissive temperature of 39.5 degrees C.     

Expression of histone genes in a G1-specific temperature-sensitive mutant of the cell cycle

The expression of genes coding for the four core histones (H2A, H2B, H3, and H4) was studied in tsAF8 cells. These baby hamster kidney-derived cells are a temperature-sensitive (ts) mutant of the cell cycle that arrest in G1 at the restrictive temperature. When serum-deprived tsAF8 cells are stimulated with serum, they enter the S phase at the permissive temperature of 34 degrees C, but are blocked in G1 at the nonpermissive temperature of 39.6 degrees C. Northern blot analysis using cloned human histone DNA probes detected only very low levels of histone RNA either in quiescent tsAF8 cells or in cells serum stimulated at the nonpermissive temperature for 24 h. Cellular levels of histone RNA were markedly increased in cells serum stimulated at 34 degrees C for 24 h. Temperature shift-up experiments after serum stimulation of quiescent populations showed that the amount of histone RNA was related to the number of cells that entered the S phase. Those cells that synthesized histone RNA and entered the S phase were capable of dividing. This is the first demonstration in a mammalian G1-specific ts mutant that the expression of H2A, H2B, H3, and H4 histone genes depends on the entry of cells into the S phase of the cell cycle.     

Cell cycle perturbations in normal and transformed fibroblasts caused by detachment from the substratum

Exponentially growing, anchorage-dependent fibroblasts were impeded in their progress through the cell cycle as a result of brief trypsinization from the substratum followed by replating. Untransformed mouse (3T3, clone A31), hamster (CHEF/18-1) and human (FS2) fibroblasts were partially inhibited from entering the DNA synthetic (S) phase of the cell cycle for 8 or 12 hours after detachment, even though the cells reattached within an hour of replating and attained a spread morphology 5 or 8 hours later. The decline in the proportion of cells in S phase was accompanied accumulation of cells in G1 as measured by autoradiography and flow microfluorimetry. Cells removed from the substratum by EDTA alone showed identical disturbances of exponential growth. These cell cycle perturbations could be a result of the detachment per se, as opposed to the rounded morphology. Synchronized A31 cells, exposed to colcemid or cytochalasin B for two hours, were not delayed in their entry into S, whereas trypsinization delayed S phase entry by 4 to 5 hours. These drugs disrupt the cytoskeleton without causing detachment. Isotope incorporation experiments revealed no decreases in the rates of protein or RNA synthesis following replating. However, exponentially growing A31 cells, treated for 2 hours with an inhibitor of protein synthesis behaved similarly to those briefly detached from their substratum: 7 hours after treatment, there were fewer cells in S and more cells in G1 relative to untreated cells. Brief treatment with an inhibitor of hn-RNA synthesis did not alter the cell cycle distribution of these fibroblasts. Three tumorogenic A31 derivatives were less affected by brief detachment from the substratum than were the untransformed cells. The derivative exhibiting the least in vitro growth control (an SV-40 transformant) showed the least sensitivity to trypsinization, while that derivative having the most in vitro growth control (a Moloney sarcoma virus transformant) was most sensitive. A chemically [benzo(a)pyrene] transformed derivative gave intermediate results with respect to both growth control and sensitivity to detachment from the substratum. The results suggest that as yet unidentified protein(s) necessary for the normal transit through G1 may be quite sensitive to the presence of an anchoring substratum.     

A temperature-sensitive cell cycle mutant of the BHK cell line

A temperature-sensitive growth mutant derived from the BHK 21 cell Line, ts AF8, was found to have greatly reduced DNA synthesis at the nonpermissive temperature. This reduction is mainly due to a decrease in the frequency of cells synthesizing DNA. Upon shift up, ts AF8 becomes blocked in the G1 phase of the cell cycle. The cells acquire elevated cAMP levels and a unimodal distribution of DNA content, equivalent to that of G1 cells at the permissive temperature, Ts AF8 cells blocked at the G1/S boundary with hydroxyurea will enter S when shifted to the nonpermissive temperature. On the other hand, ts AF8 cells arrested m G1 by serum deprivation and shifted to the nonpermissive temperature at the moment of serum addition do not enter S, while those synchronized by isoleucine deprivation and shifted at the time of isoleucine addition will enter S. These data suggest that the cycle arrest point of the ts AF8 mutation is located in G1 between the blocks induced by serum starvation and isoleucine deprivation. The reduction in DNA synthesis caused by the ts AF8 mutation is not reversed by infection or transformation with Polyoma virus. Mitochondrial DNA continues to be synthesized at wild-type levels at the nonpermissive temperature.     

Changes in template activity and structure of nuclei from WI-38 cells in the prereplicative phase.

Quiescent confluent monolayers of WI-38 fibroblasts were stimulated to proliferate by either adding 10% fetal calf serum or by trypsinization and replating at lower density. The length of the prereplicative phase was 12 hr after serum stimulation and 18 hr after trypsinization and replating at lower density. Nuclei were isolated from WI-38 cells at different time intervals after either type of stimulation and their template activity, circular dichroism spectra, and ability to bind ethidium bromide were investigated. All these parameters were similarly increased after either type of stimulation. However, these changes, like the onset of DNA synthesis, were delayed 6 hr in cells trypsinized and replated at lower density. While there were no detectable changes in nuclear protein content after serum stimulation, at least 40% of nuclear protein, mostly nonhistone chromosomal proteins, were lost after trypsinization. The amount of nuclear proteins returned to prestimulation levels only 6-8 hr after replating. These data seem to suggest that nonhistone chromosomal proteins lost by trypsinization are essential for the entrance of WI-38 cells into the "prereplicative phase".     

Failure of reactivation of chick erythrocytes after fusion with temperature-sensitive mutants of mammalian cells arrested in G1

Two temperature-sensitive (ts) mutants of mammalian cell lines (AF8 and cs4D3) that arrest in G1 at the nonpermissive temperature were fused with chick erythrocytes and the induction of DNA synthesis was studied in the resulting heterokaryons. While both AF8 and cs4D3 could induce DNA synthesis in chick nuclei at the permissive temperature, they both failed to do so when arrested in G1 at the nonpermissive temperature. When S phase AF8 cells were fused with chick erythrocytes, chick nuclei were reactivated even if the heterokaryons were incubated at the temperature nonpermissive for AF8. A third ts mutant, ts111, that is blocked in cytokinesis but continues to synthesize DNA, reactivated chick nuclei at both permissive and nonpermissive temperature. It is concluded that chick erythrocyte reactivation depends on the presence of S phase-specific factors.     

Cytoplasmic regulation of two G1-specific temperature-sensitive functions

tsAF8 and ts13 cells are temperature-sensitive (ts) mutants of BHK cells that specifically arrest, at nonpermissive temperature, in the G1 phase of the cell cycle. These two mutants can complement each other. Both cell lines can be made quiescent by serum deprivation (G0). When subsequently stimulated by serum, they can enter S phase at 34 degrees C but not at 39.5 degrees-40.6 degrees C. We have used these mutants to determine whether the nucleus is needed during the G0 leads to S transition for the expression of the G1 ts functions. For this purpose, we fused cytoplasts of G0-tsAF8 with whole ts13 cells in G0, and cytoplasts of G0-ts13 with whole tsAF8 cells in G0. Serum stimulation at the nonpermissive temperature induced DNA synthesis in both types of such fusion products. No DNA synthesis was induced by serum stimulation at the nonpermissive temperature in fusion products constructed between either G0-tsAF8 cytoplasts and whole G0-tsAF8 cells or G0-ts13 cytoplasts and whole G0-ts13 cells. These results demonstrate that the information for these two ts functions, which are required for entry of serum-stimulated cells into the S phase, are already present in the cytoplasm of G0 cells--that is, before serum stimulation commits them to the transition from the nonproliferating to the proliferating state.     

Analysis of the growth factor requirements for stimulation of WI-38 cells after extended periods of density-dependent growth arrest

When cultures of WI-38 human diploid fibroblasts reach high cell densities, they cease to proliferate and enter a viable state of quiescence. WI-38 cells can remain in this quiescent state for long periods of time; however, the longer the cells remain growth arrested, the more time they require to leave G0, progress through G1, and enter S after stimulation with fresh serum. The experiments presented here compare the response of long-term quiescent WI-38 cells (stimulated 26 days after plating) and short-term quiescent WI-38 cells (stimulated 12 days after plating) to treatment with a variety of individual purified growth factors instead of whole serum. Our results show that the qualitative and quantitative growth factor requirements necessary to stimulate G1 progression and entry into S were the same for both short- and long-term quiescent WI-38 cells, in that the same defined medium (supplemented with epidermal growth factor [EGF], recombinant human insulin-like growth factor 1 [IGF-1], and dexamethasone [DEX]) stimulated both populations of cells to proliferate with the same kinetics and to the same extent as serum. However, the long-term quiescent WI-38 cells were found to exhibit a difference in the time during which either serum or these individual growth factors were required to be present during the prereplicative period. We believe that this difference may be the cause of the prolongation of the prereplicative phase after stimulation of long-term density-arrested WI-38 cells.     

INHIBITION OF CELL GROWTH IN THE G1 PHASE BY ADENOSINE 3'',5''-CYCLIC MONOPHOSPHATE

Incorporation of tritiated thymidine into acid-precipitable material was used to measure the rate of DNA synthesis in secondary cultures of human diploid fibroblasts. Confluent cultures of human diploid fibroblasts, which are synchronized in the G₁ phase due to contact inhibition, were released from growth inhibition either by the addition of fresh medium to the cultures or by trypsinization and replating at nonconfluent densities. Either treatment resulted in a synchronous wave of DNA synthesis beginning 10–15 h after treatment and peaking at 20–25 h. In confluent cultures stimulated by fresh medium, either the addition of 0.25 mM N⁶, O²-dibutyryl-adenosine 3',5'-cyclic monophosphate (db-cAMP) to the medium in the interval 4–8 h after stimulation or the replacement of the fresh medium in that same 4 h interval with the depleted medium present on the cells for the 2 day period before stimulation delayed the synchronous onset of DNA synthesis in the cultures by about 4 h. In nonconfluent cultures freshly seeded from trypsinized confluent cultures, this same depleted medium obtained after a 2 day incubation of fresh medium on confluent cultures is shown to support the progress of the cells into S phase; however, the addition of 0.25 mM db-cAMP to the medium 3½ h after replating still partially prevented the initiation of DNA synthesis in the cultures. The results are discussed in terms of the role of serum and cAMP in the control of cell growth in fibroblast cultures.     

Transcriptional activity and chromatin structural changes in a temperature-sensitive mutant of BHK cells blocked in early G1.

AF8, a temperature-sensitive mutant of BHK $\text{21}\text{13}$ cells, has been shown to arrest at the non-permissive temperature in the G1 phase of the cell cycle. When AF8 cells are released from density-dependent arrest of growth by trypsinization and replating at lower density, 60–80% of the cells enter DNA synthesis and divide at the permissive temperature (33 °C), while only 20% enter DNA synthesis at the non-permissive temperature (39.5 °C). The temperature-sensitive block has been localized 4 to 8 h before the onset of DNA synthesis which begins at 12 h after stimulation. Two biochemical events of the prereplicative phase have been temporally related to this temperature-sensitive block. RNA synthesis as measured in isolated nuclei increases initially at both temperatures, then levels off and declines to control levels at 39.5 °C while continuing to increase at 33 °C. Parallel changes are found in circular dichroism spectra and ethidium bromide binding capacity of isolated chromatin. The results suggest that these biochemical changes are involved in the regulation of the prereplicative phase and the subsequent entrance of cells into DNA synthesis.     

Identification of adenovirus 2 early genes required for induction of cellular DNA synthesis in resting hamster cells.

tsAF8 cells are temperature-sensitive (ts) mutants of BHK-21 cells that arrest at the nonpermissive temperature in the G1 phase of the cell cycle. When made quiescent by serum restriction, they can be stimulated to enter the S phase by 10% serum at 34 degrees C, but not at 40.6 degrees C. Infection by adenovirus type 2 or type 5 stimulates cellular DNA synthesis in tsAF8 cells at both 34 and 40.6 degrees C. Infection of these cells with deletion Ad5dl312, Ad5dl313, Ad2 delta p305, and Ad2+D1) and temperature-sensitive (H5ts125, H5ts36) mutants of adenovirus indicates that the expression of both early regions 1A and 2 is needed to induce quiescent tsAF8 cells to enter the S phase at the permissive temperature. This finding has been confirmed by microinjection of selected adenovirus DNA fragments into the nucleus of tsAF8 cells. In addition, we have shown that additional viral functions encoded by early regions 1B and 5 are required for the induction of cellular DNA synthesis at the nonpermissive temperature.     

Expression of thymidine kinase and dihydrofolate reductase genes in mammalian ts mutants of the cell cycle

Thymidine kinase and dihydrofolate reductase mRNA levels and enzyme activities were determined in two temperature-sensitive cell lines, tsAF8 and ts13, that growth arrest in the G1 phase of the cell cycle at the restrictive temperature. The levels of thymidine kinase mRNA and enzyme activity increased markedly in both cell lines serum stimulated from quiescence at the permissive temperature. At the nonpermissive temperature, the levels of thymidine kinase mRNA and enzyme activity remain at the low levels of quiescent G0 cells. The levels of dihydrofolate reductase mRNA as well as the enzyme activity also increase when both cell lines are serum stimulated at the permissive temperature. When ts13 cells are serum stimulated at the nonpermissive temperature dihydrofolate reductase enzyme activity declines rapidly and dihydrofolate reductase mRNA is below detectable levels. On the contrary, when tsAF8 cells are serum stimulated at the nonpermissive temperature dihydrofolate reductase enzyme activity increases and mRNA levels are detectable slightly above G0 levels, even though the cells are blocked in the G1 phase. Studies with 2 other cDNA clones (one with an insert whose expression is cell cycle dependent and the other with an insert whose expression is not cell cycle dependent) indicate that the results are not due to aspecific toxicity or the effect of temperature. We conclude that the expression of different genes is affected differently by the ts block in G1, even when these genes are all growth-related.     

The effect of polyoma virus,serum factors,and dibutyryl cyclic AMP on dihydrofolate reductase synthesis,and the entry of quiescent cells into S phase

Three procedures were used to induce dihydrofolate reductase synthesis in quiescent cultures of methotrexate resistant mouse fibroblasts: (1) lytic infection with polyoma virus, (2) growth stimulation by replating cells at lower density in fresh cell culture medium, and (3) the addition of fresh medium to confluent cells. Following polyoma infection, an increase in the percentage of S-phase cells began at approximately 20 hours; dihydrofolate reductase synthesis also increased following a lag of 20 hours or more, and continued to increase throughout the late phase of lytic infection, reaching values nearly fivefold greater than that originally present in the quiescent cells. When quiescent cells received fresh medium (with or without replating), the percentage of cells in S phase began to increase by 10 hours and was accompanied by an increase in dihydrofolate reductase synthesis which reached a maximum by approximately 25 hours. These observations show that the initial entry of cells into S phase following mitogenic stimulation is associated with an induction of dihydrofolate reductase synthesis. Dibutyryl cyclic AMP blocked the stimulation of dihydrofolate reductase synthesis and the increase in the percentage of S-phase cells that resulted from the addition of fresh medium to confluent cells. When dibutyryl cyclic AMP was added at various times following the addition of fresh medium, the block in the induction of dihydrofolate reductase synthesis was correlated with a corresponding block in the increase in S-phase cells. These results suggest that dibutyryl cyclic AMP blocks cells at a point in Gl prior to either the induction of dihydrofolate reductase synthesis or the beginning of S phase. The relationship between the control of dihydrofolate reductase synthesis and entry into S phase suggests some form of coordinate control over these two parameters.     

DNA-negative temperature-sensitive mutants of herpes simplex virus type 1: patterns of viral DNA synthesis after temperature shift-up.

Temperature-sensitive mutants of herpes simplex virus type 1 belonging to four DNA- complementation groups exhibited two distinct patterns of viral DNA synthesis after shift-up to the nonpermissive temperature. In cultures infected with mutants belonging to complementation groups A, C, and D, little or no viral DNA was synthesized after shift-up. In cultures infected with a mutant in complementation group B, nearly normal amounts of viral DNA were synthesized after shift-up.     

Novel acriflavin resistance genes, acrC and acrD, in Escherichia coli K-12.

Acriflavine-resistant mutants were isolated from an acriflavine-sensitive (acrA) strain of Escherichia coli K-12 and then tested for temperature sensitivity of cell division. Genetic analysis characterized two new genetic loci, acrC and acrD. The former was mapped between tonA and proA, and the latter between the origin of genetic transfer of HfrH and serB. acrC and acrD mutants could divide but did not initiate a new round of deoxyribonucleic acid (DNA) replication at 43 degrees C. DNA synthesis of the acrC mutant cells ceased after a period of time following temperature shift-up, and thereafter DNA degradation occurred. However, cell mass continued to increase for a long time at the nonpermissive temperature. On the other hand, DNA synthesis of the acrD mutant cells ceased soon after the shift-up, and the cell mass did not appreciably increase during the prolonged incubation.     

The ts41 mutation in Chinese hamster cells leads to successive S phases in the absence of intervening G2, M, and G1.

The ts41 mutation of Chinese hamster cells was first isolated and characterized by Hirschberg and Marcus (1982) who showed that at nonpermissive temperature, cells accumulate up to 16C equivalents of DNA. Here we show that the mutation is recessive and at nonpermissive temperature, cells replicate their genome normally, but instead of going on into G2, M, and G1, they pass directly into a second S phase. Entry into a second S phase does not require serum nor is it inhibited by G2 checkpoints or mitotic inhibitors. Temperature-shift experiments suggest that the ts41 gene product participates in two functions in the cell cycle: entry into mitosis and inhibition of entry into S phase. The ts41 mutation seems to define a class of cell cycle mutant that couples the sequential events of DNA replication and mitosis.     

Time of replication of genes responsible for a temperature-sensitive function in a cell cycle-specific ts mutant from a hamster cell line

A method involving short pulses of 5-bromodeoxyuridine (brUdRib) followed by irraidation with 313 nm light was used to locate the time of replication of certain genes during the cell cycle of two cell lines, AF8 and AL106. AF8, a temperature-sensitive mutant of BHK21/13 cells, grows at 33°C but not at 39.5°C. AL106, a hybrid clone of tsAF8 and SV-40 transformed Lesch-Nyhan fibroblasts (LNSV), which retains all hamster chromosomes and one human chromosome (No. 3), has the ability to grow at 39.5°C. AF8 and AL106 cells synchronized at the G₁-S boundary were released from their block and pulsed with brUdRib for 2-hour periods during the S phase. The cells were subsequently irradiated with 313 nm light. Colony-forming efficiency and revertants frequency were studied. Incorporation of brUdRib during the early S phase (0–4 hours from the begining of S), decreased the colony-forming efficiency of AL106 cells both at 33°C and 39.5°C, and also of AF8 cells at 33°C. No AF8 colonies grew at the nonpermissive temperature regardless of the treatment. Thus the time of replication of genes responsible for colony-forming ability was the same in tsAF8 at the permissive temperature and in AL106 at both temperatures. The time of replication of the genes responsible for the ts function in AF8 cells was located by determining the revertants frequency in synchronized AF8 cells pulsed with brUdRib and irradiated during 1- to 2-hour periods of the S phase. Back-mutants were scored by counting the number of clones capable of growing at 39.5°C (nonpermissive for AF8 cells). The highest frequency of induced back-mutations occurred in synchronized AF8 cells pulsed with brUdRib (and irradiated) between two to four hours from the begining of the S phase. Exposure to brUdRib during other periods of the S phase or during G₁ had no effect on the reversion rate. This method can be used to locate the time of replication (in S) of ts genes in other temperature-sensitive mutants or of other specific genes in other conditional mutants.     

Conditionally lethal mutations in Chinese hamster cells. I. Isolation of a temperature-sensitive line and its investigation by cell cycle studies

The isolation of a temperature sensitive cell line from the Chinese hamster line CCL39 of the American Type Culture Collection is described. At the nonpermissive temperature (39°C) the cells become attached to the surface of tissue culture dishes, but no microscopically observable colonies are formed upon prolonged incubation. Exposure to the high temperature for more than 24 hours leads to an almost complete loss in viability. A karyotypic analysis showed that this new line has lost one of the medium-sized metacentric chromosomes, although no proof is available so far to show that this loss is not simply coincidental. In nonsynchronized cultures transferred to 39°C DNA synthesis stops first, RNA synthesis shortly thereafter, while protein synthesis (turnover) continues for a longer time. After such a shift the cell number increases by less than 15% as measured with the Coulter counter. Studies with synchronized cultures give the following results: (1) one round of DNA synthesis can occur at 39°C when the cells are released from serum starvation or a hydroxyurea block, or when mitotic cells are placed at 39°C; (2) the entry of cells into metaphase of mitosis at 39°C is almost normal when the preceding time interval at 39°C is only eight hours (release of cells from G₁/S boundary), but considerably reduced when the cells spend an additional 12 to 15 hours at 39°C in G₁ (release from serum starvation). Infection by SV40 virus temporarily induces DNA synthesis after it has come to a stop at the nonpermissive temperature, but cells permanently transformed by SV40 still exhibit the temperature-sensitive phenotype.     

The role of a cell surface inhibitor in early signal transduction associated with the regulation of cell division and differentiation

Serum stimulation of quiescent human fibroblast cultures resulted in a hyperphosphorylation of the nuclear retinoblastoma gene susceptibility product (RB). However, serum stimulation in the presence of 9 x 10(-8) M of a purified bovine sialoglycopeptide (SGP) cell surface inhibitor abrogated the hyperphosphorylation of the RB protein and the subsequent progression of cells through the mitotic cycle. The experimental results suggest that the SGP mediated its cell cycle arrest at a site in the cell cycle that was at the time of RB phosphorylation or somewhat upstream of the modification of this regulatory protein of cell division. Both cells serum-deprived and serum stimulated in the presence of the SGP displayed only a hypophosphorylated RB protein, consistent with the SGP-mediated cell cycle arrest point being near the G1/S interface.