Effects of inhibition of RNA or protein synthesis on CHO cell cycle progression

Chinese hamster ovary (CHO) cells, synchronized by selective detachment at mitosis, were treated with various concentrations of actinomycin D (AMD) or cycloheximide (CHX) either immediately, or 1, 2, or 3 hr after mitosis. Since the minimum duration of G₁ phase in these cultures was 3.4 hr, the addition of RNA or protein synthesis inhibitors took place at the beginning, first third, second third, or end (G₁–S boundary) of G₁ phase. The kinetics of exit from G₁ phase, the rate and extent of traverse of S phase, and the reaccumulation of RNA were estimated under each set of growth conditions by flow cytometry of acridine orange-stained cells. A mathematical model was constructed to describe the trajectories of the cell populations with respect to their increase in RNA and DNA content in the absence or presence of the inhibitor. The chronologic synchrony imposed on the CHO cell population began to decay within 3 hr, resulting in stochastic entrance of cells into S phase in the absence of inhibitor. Addition of AMD or CHX at 0, 1, 2, or 3 hr after mitosis, regardless of the inhibitor concentration, did not provide evidence of a critical restriction point in G₁ beyond which cells were committed to enter S phase and were no longer sensitive to moderate suppression of RNA or protein synthesis. The observed kinetics of cell entrance into and traverse of S phase were consistent with an inherently heterogenous response to serum stimulation occurring at or just after cell division.     

THE SYNTHESIS OF ACIDIC CHROMOSOMAL PROTEINS DURING THE CELL CYCLE OF HELA S-3 CELLS : I. The Accelerated Accumulation of Acidic Residual Nuclear Protein before the Initiation of DNA Replication

The synthesis and accumulation of acidic proteins in the tightly bound residual nuclear fraction goes on throughout the cell cycle of continuously dividing populations of HeLa S-3 cells; however, during late G₁ there is an increased rate of synthesis and accumulation of these proteins which precedes the onset of DNA synthesis. Unlike that of the histones, whose synthesis is tightly coupled to DNA replication, the synthesis of acidic residual nuclear proteins is insensitive to inhibitors of DNA synthesis. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of acidic residual nuclear proteins shows different profiles during the G₁, S, and G₂ phases of the cell cycle. These results suggest that, in contrast to histones whose synthesis appears to be highly regulated, the acidic residual proteins may have a regulatory function in the control of cell proliferation in continuously dividing mammalian cells.     

CELL CYCLE TIME DETERMINATIONS BASED ON LIQUID SCINTILLATION COUNTING OF 3H-TdR LABELED MITOTIC CELLS

Following a 10 min pulse labeling with ³H-TdR, flasks of asynchronous monolayer cultures of Chinese hamster ovary cells were subjected to mitotic selection at 2 hr intervals. The mitotic index of the selected populations was always greater than 90%. Counts per min per cell obtained by liquid scintillation counting were plotted versus time after the pulse label. Comparisons were made between cycle times obtained by the mitotic-scintillation counting method and by the standard per cent labeled mitosis technique. The resulting curves were used for calculations of the cell cycle times and the lengths of G₁, S, G₂ and M phases of the cell cycle. There was less than 2% difference in the cell cycle times obtained using the scintillation method as compared to times calculated from autoradiographic data obtained from individual petri dishes. The mitotic-scintillation counting technique is simple, accurate and rapid and allows the calculation of the cell kinetics parameters within 1 hr of the end of the experiment.     

Replacement perfusion of cultured eucaryotic cells: A method for the accurate measurement of the rates of growth,protein synthesis,and protein turnover

The fractional rates of protein synthesis (k_s) and degradation (k_p) were studied in the myeloma cell line SP2/0-AG14 grown at different rates (k_g). Cells in spinner flask suspension cultures were maintained at constant cellular density for prolonged periods by replacement perfusion of labeling medium at a rate equivalent to the rate of growth. Total protein synthesis was calculated from the specific-radioactivity of labeled L-leucine in the precursor (medium) and cellular protein. Fractional synthesis rates determined by approach to equilibrium labeling were the same as those determined by equilibrium-pulse labeling kinetics and pulse-chase kinetics. The rate of protein degradation was determined from the established relationship k_g = k_s – k_p. Protein synthesis rates remained constant over a threefold range in the rate of cell growth. At relatively slow growth rates (k_g = 0.017/hr) turnover represented a major fraction of total synthesis (k_p = 0.032/hr = 0.65ks). At rapid growth rates (k_g = 0.058/hr) the value of k_p was less than 0.005/hr. No major difference was observed between the k_s determined for individual cellular proteins (separated by SDS-polyacrylamide (7.5%) gel electro-phoresis) from rapid- and slow-growing cultures. Thus, with an invariable k_s, any change in growth rate is due to an inverse change in the rate of turnover. Since turnover is the balance between synthesis and degradation and since synthesis is unchanging then changes in the growth rate of SP2/0-AG14 should be due to changes in the rate of protein degradation. Experiments were therefore performed to determine the origin of the degradative machinery, ie, cytosolic or lysosomal; autolysis of prelabeled cellular protein (in vitro) was observed only at acidic pH (4.2) and WUS totally inhibited by addition of lcupcptin (10 μM) and pepstatin (2 μM), the specific inhibitors of lysosomal cathepsins B (L) and D, respectively. Since growth rate appears to be regulated by the alterations in the rate of protein degradation and degradation (in vitro) in SP2/0-AG14 appearsto be lysosomal, then one should be able to alter the rate of cellular growth by interfering with rate of lysosomal proteolysis. Indeed, when the lysosomotropic amine NH ₄Cl (10 mM) is added to cells growing with a k_g of 0.018/hr ± 0.001 (k_s = 0.050/hr ± 0.002) the growth rate increased to 0.051/hr ± 0.002 without change in the rate of protein synthesis (k_s = 0.049/hr ± 0.003). It is suggested from our data that the cellular growth rate of SP2/0-AG14 is regulated by the lysosomal apparatus; whether this regulation is itself regulated by either a specific compartmentalization of the lysosomal proteinases and/or their substrates or by endogenous protease inhibitors, should prove to be an exciting area for future investigation.     

Changes in the phosphorylation of non-histone chromatin proteins during the cell cycle of HeLa S 3 cells

The phosphorylation of non-histone chromatin proteins in synchronized HeLa S₃ cells was studied in 5 phases of the cell cycle: mitosis, G₁, early and late S, and G₂. The rate of non-histone chromatin protein phosphorylation was found to be maximal during G₁ and G₂, somewhat decreased during S phase, and almost 90% depressed during mitosis. Analysis of the phosphorylated non-histone chromatin proteins by SDS-acrylamide gel electrophoresis showed a heterogeneous pattern of phosphorylation as measured by labeling with ³²P. Significant variations in the labeling pattern were seen during different stages of the cell cycle, and particular unique species appeared to be phosphorylated selectively during certain stages of the cycle.     

DNA synthesis and proliferation of human lymphocytes in vitro: III. Fate of cycling cells in aging cultures of phytohemagglutinin stimulated human lymphocytes

There are few data available on cell cycle events that occur when proliferation of normal cells in culture is curtailed due to “natural aging” of the culture conditions. Stathmokinetic and cytofluorometry studies were performed on PHA-stimulated human lymphocyte cultures for eight consecutive days. Cell proliferation peaked on day 5 and then gradually decreased. Percent labeled mitosis curves performed each day demonstrated that, for those cells which progressed to mitosis, the cell cycle time remained constant at 18 ± 1 hour throughout the entire period of culture. However when the fate of all cells pulse-labeled with ³H-thymidine (S phase cells) was followed daily, only 64 ± 5% of labeled cells reached mitosis on day 3 and <20% on day 6. When the growth fraction was estimated by standard methods (with the labeling index) and used to predict future cell counts in the culture, proliferation was greatly overestimated; but after correcting the growth fraction for labeled cells not reaching mitosis, proliferation was accurately predicted by a newly derived “dividing fraction.” Flow cytofluorometry confirmed accumulation of cells in S and G₂ + M phases, and mitotic indices ruled out accumulation in M phase. Assessment of non-viable cells with cytofluorometry demonstrated that death occurred in all phases of the cell cycle. We conclude that with increasing age of culture, an increased fraction of cycling PHA-stimulated lymphocytes fail to progress all the way to mitosis and are arrested in S or G₂ phases. These observations provide evidence against the existence of a specific “restriction point” in G₁ or at the G₁/S interface in aging proliferating human lymphocyte cultures, but it remains to be determined whether cells arrested in S or G₂ phases retain the capacity to complete the cell cycle in more favorable culture environments.     

Allocation and Turnover of Photosynthetically Assimilated CO(2) in Leaves of Glycine max L. Clark

The allocation and turnover of photosynthetically assimilated ¹⁴CO₂ in lipid and protein fractions of soybean (Glycine max L. Clark) leaves and stem materials was measured. In whole plant labeling experiments, allocation of photosynthate from a pulse of ¹⁴CO₂ into polymeric compounds was: 25% to proteins in 4 days, 20% to metabolically inert cell wall products in 1 to 2 days, 10% to lipids in 4 days, and 4% to starch in 1 day. The amount of ¹⁴C labeled photosynthate that an actively growing leaf (leaf 4) used for its own lipid synthesis immediately following pulse labeling was about 25%. The ¹⁴C of labeled proteins turned over with half-lives of 3.8, 3.3, and 4.1 days in leaves 1, 2, and 3, respectively; and turnover of ¹⁴C in total shoot protein proceeded with a half-life of 5.2 days. Three kinetic ¹⁴C turnover patterns were observed in lipids: a rapid turnover fraction (within a day), an intermediate fraction (half-life about 5 days), and a slow turnover fraction. These results are discussed in terms of previously published accounts of translocation, carbon budgets, carbon use, and turnover in starch, lipid, protein, and cell wall materials of various plants including soybeans.     

Reentry into the cell cycle of differentiated skeletal myocytes

The activation of muscle-specific myosin synthesis and its relationship to withdrawal from the cell cycle have been examined in cycle-synchronized myoblasts under growth-restrictive, fusion-impermissive (low Ca²⁺) culture conditions. Under these conditions, embryonic quail skeletal myoblasts, collected in mitosis by mechanical shake-off, complete one normal cycle and arrest in G₁. The presence of skeletal muscle myosin is first detected, by indirect immunofluorescence, 8 hr into this protracted G₁. Within the next 10–11 hr the percentage myosin positive (Myo+) cells increases with good synchrony, reaching approximately 95%. Refeeding with a proliferation-stimulating, low Ca²⁺ medium when approximately 50% of the cells are Myo+ induces reentry into S. Applying a 15-min pulse with [³H]TdR immediately preceding fixation at regular intervals following refeeding, cells can be detected which are Myo+ and whose nuclei have incorporated [³H]TdR. The numbers of such doubly labeled cells are small but consistent with the fraction of cells in S (by time-lapse analysis) at the postfeeding times sampled. These cinematographic studies also indicate that progression to mitosis following stimulation occurs slowly and asynchronously. The kinetics of progression of the stimulated cells suggest that they reenter S from a different compartment in G₁ than do log-phase myoblasts. We conclude that in fusion-blocked quail myocytes irreversible withdrawal from the cell cycle is neither an obligate precondition for, nor an immediate consequence of the activation of the muscle-specific contractile gene set.     

ANALYSIS OF THE GROWTH KINETICS OF A HUMAN LYMPHOMA CELL LINE

The growth kinetics of an established human lymphoma cell line were analyzed by a variety of techniques utilizing various cell inocula (5 x 10⁴ - 5 x 10⁵ cells) dispensed into 60 mm diameter dishes. Techniques included pulse-labeled mitosis (PLM), continuous labeling with ³H-TdR, time-lapse photography (TLP), cell counts by electronic particle counter, and DNA histography obtained by pulse cytophotometry (PCP). There were no significant differences among values determined for any kinetic parameters as a function of cell concentration. the average doubling time of exponentially growing cells, regardless of cell inoculum, was 44.1 hr. the generation time determined by PLM was 31.1 hr with a SD of 4.7 hr. Transit times for each stage were: T_G1= 10.6 hr, T_s= 9.9 hr, T_G2= 9.9 hr, and T_m= 0.7 hr. Repeated experiments using continuous labeling with ³H-TdR demonstrated a T_G2 of 6.3 hr. the longer value determined by PLM is possibly due to the technical manipulations of this procedure which may delay pulse-labeled cells from resuming cell cycle transit. Hence, values for cell cycle stages were recalculated to give T_G1= 14.1 hr, T_s= 9.9 hr, T_G2 = 6.3 hr, and T_m= 0.7 hr. These results were used to compute the size of each cell cycle stage compartment pool and corresponded very closely to values defined directly by PCP. TLP analysis considered only cells that produced colonies of at least thirty-two cells. Generation times ranged from 8 to 89 hr and showed a positive skewness. the average value measured for 330 divisions was 34.5 hr with a SD of 13.2 hr. Thus, the variance predicted by curve fitting of the PLM data did not correlate with that defined by time-lapse photography nor did it encompass the range in generation times observed directly by TLP. There was a positive correlation between sister-sister cell generation times (+0.66) but no relation was noted for mother-daughter values.     

Phosphorylation of eIF4GII and 4E-BP1 in response to nocodazole treatment: A reappraisal of translation initiation during mitosis

Translation mechanisms at different stages of the cell cycle have been studied for many years, resulting in the dogma that translation rates are slowed during mitosis, with cap-independent translation mechanisms favored to give expression of key regulatory proteins. However, such cell culture studies involve synchronization using harsh methods, which may in themselves stress cells and affect protein synthesis rates. One such commonly used chemical is the microtubule de-polymerization agent, nocodazole, which arrests cells in mitosis and has been used to demonstrate that translation rates are strongly reduced (down to 30% of that of asynchronous cells). Using synchronized HeLa cells released from a double thymidine block (G₁/S boundary) or the Cdk1 inhibitor, RO3306 (G₂/M boundary), we have systematically re-addressed this dogma. Using FACS analysis and pulse labeling of proteins with labeled methionine, we now show that translation rates do not slow as cells enter mitosis. This study is complemented by studies employing confocal microscopy, which show enrichment of translation initiation factors at the microtubule organizing centers, mitotic spindle, and midbody structure during the final steps of cytokinesis, suggesting that translation is maintained during mitosis. Furthermore, we show that inhibition of translation in response to extended times of exposure to nocodazole reflects increased eIF2α phosphorylation, disaggregation of polysomes, and hyperphosphorylation of selected initiation factors, including novel Cdk1-dependent N-terminal phosphorylation of eIF4GII. Our work suggests that effects on translation in nocodazole-arrested cells might be related to those of the treatment used to synchronize cells rather than cell cycle status.     

EFFECTS OF IRRADIATION ON THE GENERATIVE CYCLE OF THE ESTROGEN STIMULATED VAGINAL EPITHELIUM

The effects of irradiation (300, 500 and 1500 rads) on mitosis and DNA synthesis in the estrogen primed vaginal epithelium have been studied. Dose-effect relations and the time sequence of effects on the two processes were investigated. The technique of tritiated thymidine labeling of DNA with autoradiography was used, in conjunction with the mitotic count, to study alterations in the generative cycle. Prior to irradiation, ovariectomized female rats were treated daily with diethylstilbestrol for a period of 2 weeks to create a steady state in the vaginal cell population. It was observed that:

• 1 Within 1 hr following ionizing radiation, mitotic figures disappear from the population and reappear at a time that is dose dependent. Those cells that have begun mitosis at the time of irradiation were able to complete that phase but no cells which were in G₂ were able to begin mitosis. Therefore, a G₂ block occurs within 1 hr post-irradiation.
• 2 Radiation reduces the rate of DNA synthesis thus prolonging the S phase. There is no evidence of a radiation-induced G₁ to S block in this system.

Based on these observations, it was further hypothesized that:

• 1 Cells in G₁ at the time of irradiation are relatively insensitive and continue to progress through the generative cycle at a rate primarily determined by the level of estrogen stimulation.
• 2 Radiation may interfere with the estrogen priming mechanism in this hormonedependent system thereby reducing the effective level of estrogen stimulation. This is seen in the behavior of cells which were in G₁ at the time of irradiation. The extent of the blockage of estrogen increases with radiation dose and after 1500 rads, estrogen stimulation is essentially at castrate level.

     

ANALYSIS OF THE PROLIFERATION KINETICS OF BURKITT'S LYMPHOMA CELLS

The in vitro proliferation kinetics of a cell line derived from a patient with American Burkitt's lymphoma were investigated at three different growth phases: lag (day 1), exponential (day 3) and plateau (day 5). The growth curve, labeling and mitotic indices, percentage labeled mitosis (PLM) curves and DNA content distributions were determined. The data obtained have been analysed by the previously developed discrete-time kinetic (DTK) model by which a time course of DNA distributions during a 10-day growth period was characterized in terms of other cell kinetic parameters. The mean cell cycle times, initially estimated from PLM curves on days 1, 3 and 5, were further analysed by the DTK model of DNA distributions and subsequently the mean cell cycle times with respect to DNA distributions during the entire growth period were determined. The doubling times were 39·6, 31·2 and 67·2 hr, respectively, at days 1, 3 and 5. The mean cell cycle time increased from 23·0 to 37·7 hr from day 3 to day 5 mainly due to an elongation of the G₁ and G₂ phases. A slight increase in the cell loss rate from 0·0077 to 0·0081 fraction/hr was accompanied by a decrease in the cell production rate from 0·0299 to 0·0184 fraction/hr. This calculated cell loss rate correlated significantly with the number of dead cells determined by trypan blue exclusion. Analysis of the number of dead cells in relation to the cell cycle stage revealed that a majority of cell death occurred in G₁ (r= 0·908; P < 0·0001). There was a good correlation between the in vitro proliferation kinetics at plateau phase of this Burkitt's lymphoma derived cell line and the in vivo proliferation kinetics of African Burkitt's lymphoma (Iversen et al., 1974), suggesting the potential utility of information obtained by in vitro kinetic studies.     

Autoradiographic studies on the incorporation of3H-amino acids into chromosomes during the mitotic cell cycle ofHaplopappus gracilis

The synthesis of chromosomal proteins and the incorporation of labelled proteins into chromosomes in the mitotic cell cycle ofHaplopappus gracilis, 2n=4, were traced autoradiographically with³H-arginine,³H-lysine, and³H-tryptophane. The duration of the mitotic cell cycle in the root tip cells was determined by³H-thymidine autoradiography and was measured to be 13.0 hr (G₁ 1.3 hr, S 6.5 hr, G₂ 3.8 hr and M 1.4 hr).³H-arginine labelled proteins which were synthesized at S and G₂ were found to be incorporated into chromosomes to a greater extent than proteins which were synthesized either at G₁, at the transition phase from late S to early G₂, or at the mitotic phase. Such varied incorporation was also found in³H-lysine labelled proteins, but not in³H-tryptophane labelled proteins. These findings indicate that the chromosomal proteins are synthesized mainly at S and G₂. Some of the³H-arginine labelled proteins which were synthesized during the first mitotic cell cycle, were found to be incorporated into the chromosomes of the second mitotic cell cycle. The incorporation of the proteins synthesized at one stage of the mitotic cell cycle was found to occur locally in some regions of the chromosomes, while the pattern of incorporation was observed to be similar between euchromatic and heterochromatic regions.     

Protein synthesis and transport in the regenerating goldfish visual system

The nature of the proteins synthesized in the goldfish retina and axonally transported to the tectum during optic nerve regeneration has been examined. Electrophoretic analysis of labeled soluble retinal proteins by fluorography verified our previous observation of a greatly enhanced synthesis of the microtubule subunits. In addition, labeling of a tubulin-like protein in the retinal particulate fraction was also increased during regeneration. Like soluble tubulin, the particulate material had an apparent MW of 53–55K and could be tyrosylated in the presence of cycloheximide and [³H]tyrosine. Comparison of post-crush and normal retinal proteins by two-dimensional gel electrophoresis also revealed a marked enhancement in the labeling of two acidic 68–70K proteins. Analysis of proteins slowly transported to the optic tectum revealed changes following nerve crush similar to those observed in the retina, with enhanced labeling of both soluble and particulate tubulin and of 68–70K polypeptides. The most striking change in the profile of rapidly transported protein was the appearance of a labeled 45K protein which was barely detectable in control fish.     

CYTOKINETIC STUDIES OF THE REGENERATIVE PHASE IN THE JB-1 ASCITES TUMOUR

The cell kinetics of recurrent growth of the murine JB-1 ascites tumour have been investigated 0 hr and 24 hr after aspiration of the main part of the tumour in the plateau phase of growth. The experimental data: growth curve, percentage of labelled mitoses curve and continuous labelling curves combined with cytophotometric determination of single-cell DNA content were analysed using two alternative mathematical models for the cell kinetics. Investigations 24 hr after aspiration showed that the doubling time had decreased to 70 hr as compared with 240 hr in the plateau tumour. This was due to a release of non-proliferating cells into the cell cycle, resulting in an increase in the growth fraction from 44% to 72%. The decrease in the doubling time was also due to a shortening of the mean cell cycle time from 41 to 20.5 hr. The analysis rendered it likely that the aspiration caused a shift in the mode of cell loss from an age-specific elimination of old non-cycling cells with post-mitotic DNA content in the plateau tumour to an elimination of younger cells immediately after mitosis. Investigations from 0 to 10 hr after aspiration verified the release of non-proliferating cells with both G₁ and G₂ DNA content into the cell cycle. The release was initiated from 3 to 6 hr after aspiration. 24 hr after aspiration the experimental data did not indicate any further transition.     

Purine nucleoside phosphorylase synthesis and turnover in human lymphoid cells

We have studied the turnover and synthesis of purine nucleoside phosphorylase by using a polyclonal rabbit antiserum to this protein. The turnover of purine nucleoside phosphorylase was studied in the B lymphoblast cell, WI-L2, by specific immunoprecipitation of [3H]leucine-labeled proteins. The half-lives for total protein and purine nucleoside phosphorylase were 14.5 and 14.1 hr, respectively. For cells cultured in the presence of inosine the half-life of purine nucleoside phosphorylase was reduced to 11.2 hr. The synthesis of purine nucleoside phosphorylase was analyzed during phytohemagglutinin-stimulated T cell transformation by pulse labeling cells with [35S]methionine. Purine nucleoside phosphorylase synthesis increased greater than 10-fold during the first 12 hr of transformation and continued to a maximum of 30-fold. The relative rate of purine nucleoside phosphorylase labeled to total proteins was 0.04% in unstimulated T cells and increased to 0.18% 12 hr after stimulation. These studies identify some preferential synthesis of purine nucleoside phosphorylase during the early stages of T cell transformation.     

Disruption of CENP antigen function perturbs dynein anchoring to the mitotic kinetochore

Injection of purified autoantibodies against human centromeric proteins into HeLa cells during interphase disrupts the organization of the kinetochore and interferes with chromosomal movements during the subsequent mitosis even though the chromosomes retain the ability to bind microtubules. We have investigated the hypothesis that this phenotype arises from effects on cytoplasmic dynein, the microtubule motor protein. In previous experiments we found that introduction of anticentromere antibodies into cell nuclei during the G₁- or S-phases causes a prometaphase-like arrest, while injections during G₂-phase cause a metaphase arrest. We show here that, in both cases, the level of detectable cytoplasmic dynein at kinetochores is significantly decreased. In contrast, when injected cells were permitted to enter mitosis in the absence of microtubules (conditions where trilaminar kinetochores could be detected by electron microscopy), the intensity of dynein labeling on the kinetochores was identical to that seen in uninjected control cells exposed to colcemid. Therefore, the loss of dynein label on mitotic kinetochores was correlated both with the injection of anticentromere antibodies and with the presence of intact spindle microtubules. We suggest that the injection of anticentromere antibodies somehow weakens the association of dynein with the kinetochore, so that when microtubules are present, these motor molecules are pulled away from the kinetochores as they generate force. This model offers an explanation for the failure of chromosomes of injected cells to move normally in mitosis even though they have attached microtubules.     

CHARACTERISTICS OF THE ISOPRENALINE STIMULATED PROLIFERATIVE RESPONSE OF RAT SUBMAXILLARY GLAND

A simple stochastic model has been developed to determine the cell cycle kinetics of the isoprenaline stimulated proliferative response in rat acinar cells. The response was measured experimentally, using ³H-TdR labelling of interphase cells and cumulative collections of mitotic cells with vincristine. The rise and fall of the fraction of labelled interphase cells and of metaphase cells is expressed by the product of the proliferative fraction and a difference of probability distributions. The probability statements of the model were formulated and then compared by an iterative fitting procedure to experimental data to obtain estimates of the model parameters. The model when fitted to the combined fraction labelled interphase (FLIW) and fraction metaphase (FMW,) waves gave a mean G_is transit time of 21-2 hr, mean G_is+ S transit time of 270 hr, and mean G_is+ S + G₂ transit time of 35-8 hr for a single injection of isoprenaline, where G_is is the initiation to S phase time. When successive injections of isoprenaline were given at intervals of 24 and 28 hr the corresponding values after the third injection were 12-4 hr, 20-8 hr and 25-7 hr respectively. The variance of the G_is phase dropped from 18-1 to 1–3 while the other variances remained unchanged. The estimated proliferative fraction was 0–24 after a single injection of isoprenaline, and 0–31 after three injections of the drug. Independently determined values of the proliferative fraction, obtained from repeated ³H-TdR injections, were 0–21 and 0–36 respectively.     

STUDIES ON THE DIVISION CYCLE OF MAMMALIAN CELLS : V. Modifications of Time Parameters by Different Steady-State Culture Conditions

In cultures of murine neoplastic mast cells, the duration of different phases of the division cycle (G₁, S, G₂, and mitosis [M]) was determined under optimal and several well-defined suboptimal growth conditions. Two methods of evaluation were applied to the same culture system: first, the relative number of G₁, S, G₂, and M cells was determined by pulse labeling of samples with thymidine-³H and subsequent radioautography in conjunction with a microfluorometric technique permitting rapid measurements of cellular DNA content; second, after pulse labeling with thymidine-³H, the variations with time of the mitotic labeling index were analyzed. Suboptimal culture conditions were obtained by reducing the concentration of single essential medium components (leucine, glucose, or serum) or by the addition of specific metabolic inhibitors (actinomycin D, amethopterin). Growth-limiting culture conditions resulted in increased generation times. Even under control conditions, the cell number doubling time exceeded the generation time, and this difference was more pronounced in suboptimal media. Under most of the suboptimal conditions tested, the increase in generation time was attributable primarily to an extended duration of the G₁ phase. Under certain growth-limiting conditions, however, other phases were also prolonged. In addition, the variabilities of the generation time and of certain cell cycle phases were increased under suboptimal culture conditions. Results obtained by the two methods of evaluation were, in general, in good agreement with each other. Some differences were, however, observed and interpreted in terms of cell death and/or asymmetric frequency distributions of cell cycle parameters.     

Temporal synthesis and intranuclear accumulation of the nuclear acidic proteins during periods of chromatin reactivation in Physarum polycephalum

In the lower eukaryote Physarum polycephalum, depending upon the existing cell state (i.e., actively growing plasmodia or metabolically quiescent cysts), there is in the complement of acidic chromatin proteins certain “proliferation” or “nonproliferation” associated proteins. Nonproliferative microplasmodia can be induced to undergo a 12 hr period of physiological and metabolic reorganization resulting in mitosis, DNA synthesis, and the reestablishment of active synchronous growth. During the 12 hr period of chromatin reactivation the specific acidic proteins associated with inactive chromatin and nonproliferative cell states decrease in intranuclear concentration in a continuous and linear fashion. The specific proteins associated with metabolically active chromatin and proliferative cell states are synthesized preferentially at different times during the 12 hr transition period. While several of the proliferation-associated proteins increase continuously in intranuclear concentration during the reactivation period others show maximum increases in their intranuclear concentration during the 2 hr period just preceding mitosis and DNA synthesis. The changes which develop in the acidic chromatin and nucleolar proteins during the period of chromatin reactivation occur independent of and prior to DNA synthesis and mitosis. Incorporation studies using [¹⁴C]-glutamic acid have provided additional evidence for periods of pooled protein and delayed intranuclear binding. The relative specific activities of individual protein bands determined through gel radiography show temporal differences independent of intranuclear protein concentration.It has been estimated that the proteins which are synthesized and accumulate in the nucleus during periods of chromatin reactivation are present in intranuclear concentrations between 80,000 and 1,000,000 copies per nucleus.