Cell cycle-specific glucocorticoid growth regulation of a human cell line (NHIK 3025)

It has been reported that the human cell line NHIK 3025 has a specific cytoplasmic glucocorticoid receptor. When these cells were exposed to glucocorticoids, the cell cycle time was prolonged. Cells, synchronized by mitotic selection, were subjected to the synthetic glucocorticoid dexamethasone throughout the cell cycle. Only cells exposed in the first half of G1 phase had a lengthened cell cycle time. Most of the prolongation was also located within the G1 phase. The dexamethasone growth inhibition was reversible and could be detected only in the cell cycle where the cells were exposed to the steroid. DNA-histograms of asynchronous cells were recorded by flowcytometry at various times after steroid exposure. These histograms also showed G1 phase sensitivity and G1 phase prolongation after exposure to dexamethasone. Our results thus indicate that these cells have a dexamethasone-sensitive restriction point in mid-G1 phase of the cell cycle.     

Ras is active throughout the cell cycle,but is able to induce cyclin D1 only during G2 phase

The control of cell cycle progression has been studied in asynchronous cultures using image analysis and time lapse techniques. This approach allows determination of the cycle phase and signaling properties of individual cells, and avoids the need for synchronization. In past studies this approach demonstrated that continuous cell cycle progression requires the induction of cyclin D1 levels by Ras, and that this induction takes place during G2 phase. These studies were designed to understand how Ras could induce cyclin D1 levels only during G2 phase. First, in studies with a Ras-specific promoter and cellular migration we find that endogenous Ras is active in all cell cycle phases of actively cycling NIH3T3 cells. This suggests that cyclin D1 induction during G2 phase is not the result of Ras activation specifically during this cell cycle period. To confirm this suggestion oncogenic Ras, which is expected to be active in all cell cycle phases, was microinjected into asynchronous cells. The injected protein induced cyclin D1 levels rapidly, but only in G2 phase cells. We conclude that in the continuously cycling cell the targets of Ras activity are controlled by cell cycle phase, and that this phenomenon is vital to cell cycle progression.     

Changes in L-ornithine decarboxylase activity during the cell cycle

The activity of L-ornithine decarboxylase (L-ornithine carboxy-lyase; EC 4.1.1.17), the enzyme that catalyzes the initial and rate-limiting step in polyamine biosynthesis, has been studied in Chinese hamster ovary fibroblasts synchronized by selective detachment of mitotic cells. At various times after plating the distribution of cells among the G1, S and G2+M phases of the cell cycle was calculated from DNA distributions obtained by high-speed flow cytometric analysis. At these same times determination of the cellular L-ornithine decarboxylase activity showed that polyamine (putrescine) synthesis was initiated in mid-G1, that the rate of synthesis was maximal prior to DNA synthesis, and that it decreased during the S phase. A second increase in enzyme activity occurred before mitosis.     

Expression of proliferation associated antigens in the cell cycle of synchronized mammalian cells.

Flow cytometric bivariate analysis was used to investigate the expression of PCNA, p120 and p145 during the cell cycle of a mammalian cell line (CHO-K1). Initially, aliquots of cells in exponential and plateau (G0) phase were analyzed for proliferation associated antigen expression. Expression of PCNA and p145 during G0 was markedly depressed (less than 12% positive) while 54% of the G0 cells stained positive for p120. The fluorescent intensity (mean channel fluorescence) of these G0 positive p120 cells, however, was only slightly above the mean channel fluorescence (MCF) of cells stained with a negative isotype control. In asynchronous cultures, all three antigens were expressed in greater than 70% of the cells, with PCNA staining being greater than 95%. Cells were then synchronized using mitotic selection (mitotic index of 97%) and antigen levels were measured as cells progressed synchronously through the cell cycle. From DNA analysis histograms, it appeared that the degree of synchrony was approximately 90% throughout the remainder of the cell cycle. The bivariate DNA/PCNA, DNA/p120, and DNA/p145 histograms for mitotic cells indicated that both p120 and p145 expression were elevated (percent positive and MCF) while PCNA levels were near controls (MCF). In early G1, all three markers were depressed (less than 12% positive); however PCNA levels rose precipitously in mid-G1 (greater than 50% positive). In late G1 to early S, p145 levels increased concomitantly with increases in p120. All three antigens were elevated throughout S phase and began to decline as cells moved from G2/M to G1 of the next cell cycle with p145 expression decreasing first. This report indicates that all three proliferation associated antigens studied are differentially expressed in the cell cycle and therefore may be useful in detecting and assessing the proliferation state.     

A new method to discriminate G1, S, G2, M, and G1 postmitotic cells

A new flow cytometric method combining light scattering measurements, detection of bromodeoxyuridine (BrdU) incorporation via fluorescent antibody, and quantitation of cellular DNA content by propidium iodide (PI) allows identification of additional compartments in the cell cycle. Thus, while cell staining with BrdU-antibodies and PI reveals the G1, S, and G2 + M phases of the cell cycle, differences in light scattering allow separation of G2 phase cells from M phase cells and subdivision of G1 phase into two compartments, i.e., G1A representing postmitotic cells which mature to G1B cells ready to initiate DNA synthesis. The method involves fixation of cells in 70% ethanol, extraction of histones with HC1, and thermal denaturation of DNA. This treatment appears to enhance the differences in chromatin structure of cells in the various phases of the cell cycle to the extent that cells could be separated on the basis of the 90 degrees scatter. Mitotic cells show much lower scatter than G2 phase cells, and G1 postmitotic cells (G1A) show lower scatter than G1 cells about to enter the S phase (G1B). Light scattering is correlated with chromatin condensation, as judged by microscopic evaluation of cells sorted on the basis of light scatter. The method has the advantage over the parental BrdU/DNA bivariate analysis in allowing the G2 and M phases of the cell cycle to be separated and the G1 phase to be analyzed in more detail. The method may also allow separation of unlabeled S phase cells from mitotic cells and distinguish between labeled and unlabeled mitotic cells.     

Mode of estrogen action on cell proliferation in CAMA-1 cells: II. Sensitivity of G1 phase population

The mammary cancer cell line CAMA-1 synchronized at the G1/S boundary by thymidine block or at the G1/M boundary by nocodazole was used to evaluate 1) the sensitivity of a specific cell cycle phase or phases to 17 beta-estradiol (E2), 2) the effect of E2 on cell cycle kinetics, and 3) the resultant E2 effect on cell proliferation. In synchronized G1/S cells, E2-induced 3H-thymidine uptake, which indicated a newly formed S population, was observed only when E2 was added during, but not after, thymidine synchronization. Synchronized G2/M cells, enriched by Percoll gradient centrifugation to approximately 90% mitotic cells, responded to E2 added immediately following selection; the total E2-treated population traversed the cycle faster and reached S phase approximately 4 hr earlier than cells not exposed to E2. When E2 was added during the last hour of synchronization (ie, at late G2 or G2/M), or for 1 hr during mitotic cell enrichment, a mixed response occurred: a small portion had an accelerated G1 exit, while the majority of cells behaved the same as controls not incubated with E2. When E2 addition was delayed until 2 hr, 7 hr, or 12 hr following cell selection, to allow many early G1 phase cells to miss E2 exposure, the response to E2 was again mixed. When E2 was added during the 16 hr of nocodazole synchronization, when cells were largely at S or possibly at early G2, it inhibited entry into S phase. The E2-induced increase or decrease of S phase cells in the nocodazole experiments also showed corresponding changes in mitotic index and cell number. These results showed that the early G1 phase and possibly the G2/M phase are sensitive to E2 stimulation, late G1, G1/S, or G2 are refractory; the E2 stimualtion of cell proliferation is due primarily to an increased proportion of G1 cells that traverse the cell cycle and a shortened G1 period, E2 does not facilitate faster cell division; and estrogen-induced cell proliferation or G1/S transition occurs only when very early G1 phase cells are exposed to estrogen. These results are consistent with the constant transition probability hypothesis, that is, E2 alters the probability of cells entering into DNA synthesis without significantly affecting the duration of other cell cycle phases. Results from this study provide new information for further studies aimed at elucidating E2-modulated G1 events related to tumor growth.     

DNA repair in UV-irradiated heteroploid cells at different phases of the cell cycle

The variation of DNA repair activity during the cell cycle was studied by analysing the UV-stimulated DNA synthesis in cells synchronized in mitosis. This activity was detected both by autoradiography and by directly measuring the incorporation of tritiated thymidine in cells irradiated and incubated in the presence of hydroxyurea. Cells in all phases were found to be able to perform repair. However the activity appeared to be considerably lower in mitotic cells than in cell in other phases. Increasing values of repair capacity were observed in G1 cells, in mixed G2, S and M cells and in asynchronous cells. The relationship between these findings and data on survival rates in the same synchronized cells is discussed.     

Gene-specific characterization of human histone H2B by electron capture dissociation

The basis set of protein forms expressed by human cells from the H2B gene family was determined by Top Down Mass Spectrometry. Using Electron Capture Dissociation for MS/MS of H2B isoforms, direct evidence for the expression of unmodified H2B.Q, H2B.A, H2B.K/T, H2B.J, H2B.E, H2B.B, H2B.F, and monoacetylated H2B.A was obtained from asynchronous HeLa cells. H2B.A was the most abundant form, with the overall expression profile not changing significantly in cells arrested in mitosis by colchicine or during mid-S, mid-G2, G2/M, and mid-G1 phases of the cell cycle. Modest hyperacetylation of H2B family members was observed after sodium butyrate treatment.     

The dependence of the cytokinetic effects of alkylating antitumor preparations on the phase of the hepatocyte cell cycle in regenerating liver]

Effects of alkylating antitumor drugs on resting (G0 phase of cell cycle) and proliferating (G1, S, G2 and M phases) hepatocytes were studied in regenerating mouse liver. Cell cycle kinetics (fraction of labeled mitoses, labeling and mitotic indices) were determined by 3H-thymidine autoradiography. Dipin and fotrin as a DNA-damaging agents attack mainly resting (G0) and proliferating (G1) cells. Effect of the damage results in the inhibition of DNA synthesis and G2 phase arrest in the following mitotic cycle. An alkylating drug phopurin as well as ara-C both suppress the mitotic progression in proliferating hepatocytes and do not influence the resting cells.     

Fluctuation of trypsin-like proteinase activity in the cell cycle of synchronized and growing HeLa cells, and the effect of GMCHA-OPhBut

HeLa cells synchronized by double-thymidine block were grown in Eagle's minimum essential medium supplemented with 10% calf serum, and the fluctuation of trypsin-like protease activity in the cell cycle was examined. Seven distinct activity peaks were observed in one cell cycle at a cell density of 2%: two peaks in S phase, one peak at the S/G2 boundary, one peak in early M phase and one at the M/G1 boundary, and two peaks in G1 phase. HeLa cells synchronized by a mitotic detachment technique also showed similar results at cell density of 4.8%. The appearance of trypsin-like proteinase activity in the cell cycle was markedly affected by cell density, and no definite peak was observed above 8%. trans-Guanidinomethylcyclohexanecarboxylic and 4-tert-butylphenyl ester (GMCHA-OPhBut), a specific inhibitor for trypsin and a strong inhibitor of HeLa cell growth, had no effect on the various events in the first S, G2 and M phases, such as the incorporation of [methyl-3H]thymidine into DNA, the increase in the cell concentration, and the appearance of trypsin-like proteinase activity, whereas it retarded the onset of the second S phase and the various events in the second S, G2 and M phases for 3 h. In particular, it induced the appearance of a new proteinase peak at the G1/S boundary.     

Cell population kinetics of 1,2-dimethylhydrazine-induced colonic neoplasms and their adjacent colonic mucosa in the mouse.

The parameters of cell population kinetics of symmetrical 1,2-dimethylhydrazine-induced colonic neoplasms and their adjacent colonic mucosa in the mouse were analyzed using the fraction labeled-mitoses curve method and compared with those of three groups of epithelial cells in the crypt of the descending colon of normal mouse. The analysis of three groups of epithelial cells in the crypt of normal mouse indicates that differentiation of epithelial cells was associated not only with a smaller proliferative pool of cells but also with a shortening of the duration of G2 phase and a prolongation of mitotic time. Other parameters of cell cycle did not change significantly. The mean cell cycle time of neoplastic cells in chemically induced colonic neoplasms was similar to that of epithelial cells in normal colon, but the variance was much greater in neoplastic cells. In neoplastic cells, the proliferative pool was greater, the G1 phase prlonged, and the S phase and the mitotic time became shorter as compared to epithelial cells in normal colon. The duration of G2 phase of neoplastic cells fell between the values of presumptive stem cells and differentiating cells in normal colon, compatible with the hypothesis that neoplastic cells are transformed stem cells defective in cellular differentiation. In the colonic mucosa immediately adjacent to neoplasms, the fraction-labeled-mitoses curve showed a flat second wave, indicating that the group of cells initially labeled by the pulse became a mixture of cells, some continuing the proliferative cycle normally, some going out of cycle, some slowing down in their passage from S through G2 to M, and some being arrested in mitotic phase. Such heterogeneous behavior of cells may be closely related to expansion of neoplasms. With some assumptions, however, cell cycle parameters of those normally cycling cells were estimated: the cell cycle time and the duration of G1 phase and mitotic phase were prolonged as compared to neoplastic cells and epithelial cells of normal colon.     

Characterization of the effects of butyric acid on cell proliferation, cell cycle distribution and apoptosis

We examined concentration-dependent changes in cell cycle distribution and cell cycle-related proteins induced by butyric acid. Butyric acid enhanced or suppressed the proliferation of Jurkat human T lymphocytes depending on concentration. A low concentration of butyric acid induced a massive increase in the number of cells in S and G2/M phases, whereas a high concentration significantly increased the accumulation of cells in G2/M phase, suppressed the accumulation of cells in G0/G1 and S phases, and induced apoptosis that cell cycle-related protein expression in Jurkat cells treated with high levels of butyric acid caused a marked decrease in cyclin A, cyclin E, cyclin-dependent kinase 2 (CDK2), CDK4 and CDK6 protein levels in G0/G1 and S phases, with apoptosis induction, and a decrease in cyclin B, Cdc25c and p27KIP1 protein levels, as well as an increase in p21CIP1/WAF1 protein level, in the G2/M phase. Taken together, our results indicate that butyric acid has bimodal effects on cell proliferation and survival. The inhibition of cell growth followed by the increase in apoptosis induced by high levels of butyric acid were related to an increase in cell death in G0/G1 and S phases, as well as G2/M arrest of cells. Finally, these results were further substantiated by the expression profile of butyric acid-treated Jurkat cells obtained by means of cDNA array.     

Initiation of DNA synthesis in the prematurely condensed chromosomes of G1 cells

The objective of this study was to investigate whether G1 cells could enter S phase after premature chromosome condensation resulting from fusion with mitotic cells. HeLa cell synchronized in early G1, mid-G1, late G1, and G2 and human diploid fibroblasts synchronized in G0 and G1 phases were separately fused by use of UV-inactivated Sendai virus with mitotic HeLa cells. After cell fusion and premature chromosome condensation, the fused cells were incubated in culture medium containing Colcemid (0.05 micrograms/ml) and [3H]thymidine ([3H]ThdR) (0.5 microCi/ml; sp act, 6.7 Ci/mM). At 0, 2, 4, and 6 h after fusion, cell samples were taken to determine the initation of DNA synthesis in the prematurely condensed chromosomes (PCC) on the basis of their morphology and labeling index. The results of this study indicate that PCC from G0, G1, and G2 cells reach the maximum degree of compaction or condensation at 2 h after PCC induction. In addition, the G1-PCC from normal and transformed cells initiated DNA synthesis, as indicated by their "pulverized" appearance and incorporation of [3H]ThdR. Further, the initiation of DNA synthesis in G1-PCC occurred significantly earlier than in the mononucleate G1 cells. Neither pulverization nor incorporation of label was observed in the PCC of G0 and G2 cells. These findings suggest that chromosome decondensation, although not controlling the timing of a cell's entry into S phase, is an important step for the initiation of DNA synthesis. These data also suggest that the entry of a S phase may be regulated by cell cycle phase-specific changes in the permeability of the nuclear envelope to the inducers of DNA synthesis present in the cytoplasm.     

DNA topoisomerase I and II activities during cell proliferation and the cell cycle in cultured mouse embryo fibroblast (C3H 10T1/2) cells

We have used C3H 10T1/2 cells to examine the regulation of topoisomerase activities during cell proliferation and the cell cycle. The specific activity of topoisomerase I was about 4-fold greater in proliferating (log phase) cells than in non-proliferating (confluent) cells. In synchronized cells, the bulk of the increased activity occurred during or just prior to S phase, depending upon the method of synchronization. A smaller increase in activity also occurred during G1 phase. The increase in activity during S phase was not altered by a hydroxyurea block at the G1/S phase boundary indicating that it is not directly coupled to DNA synthesis and is not the result of topoisomerase I gene dosage. The increase was inhibited by blocking cells at mid-G1 phase using isoleucine deprivation. Thus, the increase in activity during S phase is dependent on events occurring during mid- to late G1 phase. In contrast to the changes in topoisomerase I levels, the specific activity of topoisomerase II showed no detectable difference in proliferating vs non-proliferating cells. In addition, no detectable difference in topoisomerase II specific activity was seen in G1, S and M phases of the cell cycle. The differences in the activity profiles of the topoisomerases I and II during the cell cycle suggest that the two activities are regulated independently and may be required for different functions.     

Alterations in the cell cycle characteristics of granulosa cells during the periovulatory period: evidence of ovarian and oviductal influences

Granulosa cells at different stages of differentiation were collected from ovarian follicles and oviducts during the periovulatory period, and their nuclear DNA content was monitored by flow cytometry to establish their cell cycle characteristics (G0 + G1, S, G2 + M). The proportion of cells in the three phases of the cell cycle varied in characteristics patterns depending upon the time they were collected, before or following ovulation. Granulosa (cumulus) cells recovered from ovulated oocytes were mitotically inactive as shown by the large proportion of cells with a 2C amount of DNA and the absence of cells in S phase. The proportion of granulosa cells in G2 + M decreased when recovery from the oviducts was delayed. In contrast, granulosa (cumulus and/or mural) cells recovered from preovulatory follicles prior to luteinizing hormone (LH) exposure contained a considerable population of cells undergoing DNA synthesis, and a decreased proportion of cells with a 2C DNA content. Our findings indicate that granulosa cells undergo dynamic and characteristics changes in all cell cycle phases during the periovulatory period, within follicular and oviductal environments. Intrafollicular events appear to play a major role in controlling DNA synthesis, proliferation, and related cell cycle events in the granulosa cells. Flow cytometric techniques provide objective and detailed information on the cell cycle characteristics of granulosa cell populations at different stages of differentiation. Elucidation of the mechanisms regulating cell cycle parameters of granulosa cells and their physiological significance thus seems feasible.     

Cell synchrony techniques. I. A comparison of methods

Selected cell synchrony techniques, as applied to asynchronous populations of Chinese hamster ovary (CHO) cells, have been compared. Aliquots from the same culture of exponentially growing cells were synchronized using mitotic selection, mitotic selection and hydroxyurea block, centrifugal elutriation, or an EPICS V cell sorter. Sorting of cells was achieved after staining cells with Hoechst 33258. After synchronization by the various methods the relative distribution of cells in G1, S, or G2 + M phases of the cell cycle was determined by flow cytometry. Fractions of synchronized cells obtained from each method were replated and allowed to progress through a second cell cycle. Mitotic selection gave rise to relatively pure and unperturbed early G1 phase cells. While cell synchrony rapidly dispersed with time, cells progressed through the cell cycle in 12 hr. Sorting with the EPICS V on the modal G1 peak yielded a relatively pure but heterogeneous G1 population (i.e. early to late G1). Again, synchrony dispersed with time, but cell-cycle progression required 14 hr. With centrifugal elutriation, several different cell populations synchronized throughout the cell cycle could be rapidly obtained with a purity comparable to mitotic selection and cell sorting. It was concluded that, either alone or in combination with blocking agents such as hydroxyurea, elutriation and mitotic selection were both excellent methods for synchronizing CHO cells. Cell sorting exhibited limitations in sample size and time required for synchronizing CHO cells. Its major advantage would be its ability to isolate cell populations unique with respect to selected cellular parameters.     

Haspin inhibition delays cell cycle progression through interphase in cancer cells

Haspin (Haploid Germ Cell-Specific Nuclear Protein Kinase) is a serine/threonine kinase pertinent to normal mitosis progression and mitotic phosphorylation of histone H3 at threonine 3 in mammalian cells. Different classes of small molecule inhibitors of haspin have been developed and utilized to investigate its mitotic functions. We report herein that applying haspin inhibitor CHR-6494 or 5-ITu at the G1/S boundary could delay mitotic entry in synchronized HeLa and U2OS cells, respectively, following an extended G2 or the S phase. Moreover, late application of haspin inhibitors at S/G2 boundary is sufficient to delay mitotic onset in both cell lines, thereby, indicating a direct effect of haspin on G2/M transition. A prolonged interphase duration is also observed with knockdown of haspin expression in synchronized and asynchronous cells. These results suggest that haspin can regulate cell cycle progression at multiple stages at both interphase and mitosis.