The cell cycle and the control of cellular reproduction

Immature Follicular Origins and Disrupted Oocyte Growth Pathways Contribute to Decreased Gamete Quality During Reproductive Juvenescence in Mice

Egg quality dictates fertility outcomes, and although there is a well-documented decline with advanced reproductive age, how it changes during puberty is less understood. Such knowledge is critical, since advances in Assisted Reproductive Technologies are enabling pre- and peri-pubertal patients to preserve fertility in the medical setting. Therefore, we investigated egg quality parameters in a mouse model of the pubertal transition or juvenescence (postnatal day; PND 11-40). Animal weight, vaginal opening, serum inhibin B levels, oocyte yield, oocyte diameter, and zona pellucida thickness increased with age. After PND 15, there was an age-associated ability of oocytes to resume meiosis and reach metaphase of meiosis II (MII) following in vitro maturation (IVM). However, eggs from the younger cohort (PND 16-20) had significantly more chromosome configuration abnormalities relative to the older cohorts and many were at telophase I instead of MII, indicative of a cell cycle delay. Oocytes from the youngest mouse cohorts originated from the smallest antral follicles with the fewest cumulus layers per oocyte, suggesting a more developmentally immature state. RNA Seq analysis of oocytes from mice at distinct ages revealed that the genes involved in cellular growth signaling pathways (PI3K, mTOR, and Hippo) were consistently repressed with meiotic competence, whereas genes involved in cellular communication were upregulated in oocytes with age. Taken together, these data demonstrate that gametes harvested during the pubertal transition have low meiotic maturation potential and derive from immature follicular origins.

Cell cycle synchronization by nutrient modulation

Living cells respond to changing environments by regulating their genes and activities. In unicellular organisms such as yeasts, the cell division cycle is coupled to the nutrient availability. However, it is unclear how tight this coupling is and how the intrinsic time scales of the different cell cycle processes respond to varying nutrient conditions. Here we study the cell cycle behavior of the budding yeast Saccharomyces cerevisiae in response to periodically modulated nutrient availability, using a microfluidic platform which allows for longtime cultivation, programmed medium switching, and automated time-lapse image acquisition. We observe that the division cycle of the yeast cells can follow a periodically modulated medium so that the whole population can be driven into synchrony. When the period of the nutrient modulation is optimized, as many as 80% of the cells in a population are continuously synchronized. The degree of synchronization as a function of the nutrient modulation period can be qualitatively captured by a stochastic phenomenological model. Our work may shed light on the coupling between the cell growth and cell division as well as provide a nontoxic and non-invasive method to continuously synchronize the cell cycle.

Synchrony in reaction-diffusion models of morphogenesis: applications to curvature-dependent proliferation and zero-diffusion front waves

The paper presents the classical age-dependent approach of the morphogenesis in the framework of the von Foerster equation, in which we introduce a new constraint and study a new feature: (i) the new constraint concerns cell proliferation along the contour lines of the cell density, depending on the local curvature such as it favours the amplification of the concavities (like in the gastrulation process) and (ii) the new feature consists of considering, on the cell density surface, a remarkable line (the null mean Gaussian curvature line), on which the normal diffusion vanishes, favouring local coexistence of diffusing morphogens, metabolites or cells, and hence the auto-assemblages of these entities. Two applications to biological multi-agents systems are studied, gastrulation and feather morphogenesis.

Lipid Biosynthesis and its Coordination with Cell Cycle Progression

The activation of cell cycle regulators at the G1/S boundary has been linked to the cellular protein synthesis rate. It is conceivable that regulatory mechanisms are required to allow cells to coordinate the synthesis of other macromolecules with cell cycle progression. The availability of highly synchronized cells and flow cytometric methods facilitates investigation of the dynamics of lipid synthesis in the entire cell cycle of the heterotrophic dinoflagellate Crypthecodinium cohnii. Flow cytograms of Nile red-stained cells revealed a stepwise increase in the polar lipid content and a continuous increase in neutral lipid content in the dinoflagellate cell cycle. A cell cycle delay at early G1, but not G2/M, was observed upon inhibition of lipid synthesis. However, lipid synthesis continued during cell cycle arrest at the G1/S transition. A cell cycle delay was not observed when inhibitors of cellulose synthesis and fatty acid synthesis were added after the late G1 phase of the cell cycle. This implicates a commitment point that monitors the synthesis of fatty acids at the late G1 phase of the dinoflagellate cell cycle. Reduction of the glucose concentration in the medium down-regulated the G1 cell size with a concomitant forward shift of the commitment point. Inhibition of lipid synthesis up-regulated cellulose synthesis and resulted in an increase in cellulosic contents, while an inhibition of cellulose synthesis had no effects on lipid synthesis. Fatty acid synthesis and cellulose synthesis are apparently coupled to the cell cycle via independent pathways.

How cells coordinate growth and division

Size is a fundamental attribute impacting cellular design, fitness, and function. Size homeostasis requires a doubling of cell mass with each division. In yeast, division is delayed until a critical size has been achieved. In metazoans, cell cycles can be actively coupled to growth, but in certain cell types extracellular signals may independently induce growth and division. Despite a long history of study, the fascinating mechanisms that control cell size have resisted molecular genetic insight. Recently, genetic screens in Drosophila and functional genomics approaches in yeast have macheted into the thicket of cell size control.

Analyzing changes of chromatin-bound replication proteins occurring in response to and after release from a hypoxic block of replicon initiation in T24 cells

It was shown previously [Riedinger, H. J., van Betteraey-Nikoleit, M & Probst, H. (2002) Eur. J. Biochem.269, 2383-2393] that initiation of in vivo SV40 DNA replication is reversibly suppressed by hypoxia in a state where viral minichromosomes exhibit a nearly complete set of replication proteins. Reoxygenation triggers fast completion and post-translational modifications. Trying to reveal such fast changes of chromatin-bound replication proteins in the much more complex replication of the cellular genome itself, we developed a protocol to extend these studies using the human bladder carcinoma cell line T24, which was presynchronized in G1 by starvation. Concomitantly with stimulation of the cells by medium renewal, hypoxia was established. This treatment induced T24 cells to contain a large amount of replicons arrested in the 'hypoxic preinitiation state', ready to initiate replication as soon as normal pO2 was restored. Replicons in other stages of replicative activity were not detectable. Consequently the arrested replicons were rapidly released into synchronous initiation and succeeding elongation. Extraction of T24 nuclei with a Triton X-100 buffer yielded a fraction containing the cellular chromatin, including DNA-bound replication proteins, while unbound proteins were removed. The usefulness of this protocol was tested by the proliferation marker PCNA. We demonstrate here that this protein switches from the remainder cellular protein pool into the Triton-extracted nuclear fraction upon reoxygenation. Employing this protocol, analyses of chromatin-bound MCM2, MCM3, Cdc6 and cdk2 suggests that the 'classical' prereplication complex is already formed during hypoxia.

Combinatorial roles for pRB, p107, and p130 in E2F-mediated cell cycle control

Numerous studies have implicated the pRB family of nuclear proteins in the control of cell cycle progression. Although over-expression experiments have revealed that each of these proteins, pRB, p107, and p130, can induce a G(1) cell cycle arrest, mouse knockouts demonstrated distinct developmental requirements for these proteins, as well as partial functional redundancy between family members. To study the mechanism by which the closely related pRB family proteins contribute to cell cycle progression, we generated 3T3 fibroblasts derived from embryos that lack one or more of these proteins (pRB(-/-), p107(-/-), p130(-/-), pRB(-/-)/p107(-/-), pRB(-/-)/p130(-/-), and p107(-/-)/p130(-/-)). By comparing the growth and cell cycle characteristics of these cells, we have observed clear differences in the manner in which they transit through the G(1) and S phases as well as exit from the cell cycle. Deletion of Rb, or more than one of the family members, results in a shortening of G(1) and a lengthening of S phase, as well as a reduction in growth factor requirements. In addition, the individual cell lines showed differential regulation of a subset of E2F-dependent gene promoters, as well as differences in cell cycle-dependent kinase activity. Taken together, these observations suggest that the closely related pRB family proteins affect cell cycle progression through distinct biochemical mechanisms and that their coordinated action may contribute to their diverse functions in various physiological settings.

Mitotic regulation of TFIID: inhibition of activator-dependent transcription and changes in subcellular localization

Mitosis in higher eukaryotes is accompanied by a general inhibition of transcription. To begin to understand the mechanisms underlying this inhibition we have examined the behavior of the general transcription factor TFIID during mitosis. Immunocytochemistry and subcellular fractionation studies indicate that the majority of TFIID is displaced from the disassembling prophase nucleus to the mitotic cytoplasm around the time of nuclear envelope breakdown. However, a subpopulation of TFIID remains associated tightly with the condensed mitotic chromosomes. Metabolic labeling of mitotic cells revealed that several subunits of TFIID undergo mitosis-specific phosphorylation, but in spite of these changes, the TFIID complex remains intact. Functional analysis of purified TFIID from mitotic cells shows that phosphorylated forms are unable to direct activator-dependent transcription, but that this activity is restored upon dephosphorylation. These results demonstrate that TFIID regulation by phosphorylation is likely to have an important role in mitotic inhibition of RNA polymerase II transcription. In addition, they suggest a mechanism for regulating gene expression through the selective disruption of polymerase II promoter structures during mitosis.

Fas-activated serine/threonine kinase (FAST) phosphorylates TIA-1 during Fas-mediated apoptosis

We have identified a serine/threonine kinase that is rapidly activated during Fas-mediated apoptosis. Fas-activated serine/threonine kinase (FAST) is phosphorylated on serine and threonine residues in Jurkat cells. In response to Fas ligation, it is rapidly dephosphorylated and concomitantly activated to phosphorylate TIA-1, a nuclear RNA-binding protein that has been implicated as an effector of apoptosis. Phosphorylation of TIA-1 precedes the onset of DNA fragmentation, suggesting a role in signaling downstream events in the apoptotic program. Our results introduce Fast and TIA-1 as components of a molecular cascade involved in signaling Fas-mediated apoptosis.

The human CCG1 gene, essential for progression of the G1 phase, encodes a 210-kilodalton nuclear DNA-binding protein

The human CCG1 gene complements tsBN462, a temperature-sensitive G1 mutant of the BHK21 cell line. The previously cloned cDNA turned out to be a truncated form of the actual CCG1 cDNA. The newly cloned CCG1 cDNA was 6.0 kb and encoded a protein with a molecular mass of 210 kDa. Using an antibody to a predicted peptide from the CCG1 protein, a protein with a molecular mass of over 200 kDa was identified in human, monkey, and hamster cell lines. In the newly defined C-terminal region, an acidic domain was found. It contained four consensus target sequences for casein kinase II and was phosphorylated by this enzyme in vitro. However, this C-terminal region was not required to complement tsBN462 mutation since the region encoding the C-terminal part was frequently missing in complemented clones derived by DNA-mediated gene transfer. CCG1 contains a sequence similar to the putative DNA-binding domain of HMG1 in addition to the previously detected amino acid sequences common in nuclear proteins, such as a proline cluster and a nuclear translocation signal. Consistent with these predictions, CCG1 was present in nuclei, possessed DNA-binding activity, and was eluted with similar concentrations of salt, 0.3 to 0.4 M NaCl either from isolated nuclei or from a DNA-cellulose column.

Effects of transforming growth factor-beta 1 on human arterial smooth muscle cells in vitro

Control of the thickness of the arterial wall is critical, as excessive overgrowth of constituent smooth muscle cells (SMCs) may interfere with blood flow. Effects on SMCs in vitro of several growth factors that are present in blood and/or that are produced endogenously in the arterial wall under certain conditions suggest that influences of endocrine, paracrine, and autocrine nature from stimulating and inhibiting factors may control the smooth muscle tissue mass in the artery. This possibility was explored further by investigating the degree of myodifferentiation in terms of the presence of differentiation-specific filamentous alpha-smooth muscle actin and growth, as measured by the synthesis of DNA and cell number, of SMCs as influenced by their exposure to the mitogens, platelet-derived growth factor and epidermal growth factor, and the bifunctional growth factor, transforming growth factor-beta 1 (TGF-beta 1). Exposure to TGF-beta 1 markedly enhanced differentiation-specific filamentous alpha-smooth muscle actin. This effect did not require arrest of growth, which speaks against a direct causal relation between loss of myodifferentiation (modulation) and multiplication. When quiescent cultures were exposed to TGF-beta 1, alpha-smooth muscle actin was further increased, indicating a more specific differentiation-promoting effect by TGF-beta 1 than mere inhibition of growth. Exposure to TGF-beta 1 also increased spreading, which occurred in parallel with increased filamentous alpha-smooth muscle actin and appearance of stress fibers. Exposure to platelet-derived growth factor under serum-free conditions and to epidermal growth factor in cultures exposed to serum markedly decreased the number of alpha-actin-positive SMCs, indicating a dedifferentiating effect by these mitogens. Exposure of SMCs to TGF-beta 1 under serum-free conditions had pronounced effects on growth, with a concentration-dependent inhibition of platelet-derived growth factor-induced DNA synthesis and cell multiplication. The basal synthesis of DNA in the absence of added growth factors was also greatly inhibited. With serum-free cultures, some loss of cells occurred even with very low concentrations of TGF-beta 1 (5 pg/ml), against which platelet-derived growth factor or a dense cultural state had a protective effect. Enhancement of cell multiplication was not detected for cultivated human SMCs exposed to TGF-beta 1, irrespective of culture density, in contrast to that reported for dense cultures of rat SMCs. TGF-beta 1 is present in and may be released from platelets in situations that promote platelet adherence such as endothelial injury; TGF-beta 1 may also be released from activated macrophages and T lymphocytes either during an immune reaction or inflammation or from the endothelium.(ABSTRACT TRUNCATED AT 400 WORDS)

Differentiation of HL-60 myeloid leukaemia cells is associated with a transient block in the G2 phase of the cell cycle

The human promyelocytic leukaemia cell line HL-60 can be induced to differentiate towards mature granulocytes by treatment with dibutyryl cyclic adenosine-3',5'-monophosphate (dbcAMP). Differentiation begins within 16-24 h of treatment and is associated with a time- and dose-dependent accumulation of cells in the G0/G1 phase of the cell cycle with a concomitant decrease in the number of cells in the S and G2 + M phases. Using acridine orange staining, we found that the RNA content of the cells also decreased following differentiation. Stathmokinetic analysis of HL-60 cell populations following dbcAMP treatment showed no effect on the total number of cells in the G0/G1 or S phases, or the rate of progression of cells through these cell cycle compartments. In contrast, dbcAMP was found to induce a transient arrest of the cells in the G2 phase. We also found that differentiation induced by dbcAMP did not require progression of the cells through the cell cycle. Cells arrested in either G1/S by hydroxyurea or G2 + M by colcemid eventually expressed markers of mature granulocytes. These results demonstrate that dbcAMP modulates cell cycle progression. However, these cell cycle changes alone are insufficient to induce granulocytic differentiation of HL-60 cells.

The human CCG1 gene, essential for progression of the G1 phase, encodes a 210-kilodalton nuclear DNA-binding protein

The human CCG1 gene complements tsBN462, a temperature-sensitive G1 mutant of the BHK21 cell line. The previously cloned cDNA turned out to be a truncated form of the actual CCG1 cDNA. The newly cloned CCG1 cDNA was 6.0 kb and encoded a protein with a molecular mass of 210 kDa. Using an antibody to a predicted peptide from the CCG1 protein, a protein with a molecular mass of over 200 kDa was identified in human, monkey, and hamster cell lines. In the newly defined C-terminal region, an acidic domain was found. It contained four consensus target sequences for casein kinase II and was phosphorylated by this enzyme in vitro. However, this C-terminal region was not required to complement tsBN462 mutation since the region encoding the C-terminal part was frequently missing in complemented clones derived by DNA-mediated gene transfer. CCG1 contains a sequence similar to the putative DNA-binding domain of HMG1 in addition to the previously detected amino acid sequences common in nuclear proteins, such as a proline cluster and a nuclear translocation signal. Consistent with these predictions, CCG1 was present in nuclei, possessed DNA-binding activity, and was eluted with similar concentrations of salt, 0.3 to 0.4 M NaCl either from isolated nuclei or from a DNA-cellulose column.

Premature chromosome condensation is induced by a point mutation in the hamster RCC1 gene

At the nonpermissive temperature, premature chromosome condensation (PCC) occurs in tsBN2 cells derived from the BHK cell line, which can be converted to the Ts+ phenotype by the human RCC1 gene. To prove that the RCC1 gene is the mutant gene in tsBN2 cells, which have RCC1 mRNA and protein of the same sizes as those of BHK cells, RCC1 cDNAs were isolated from BHK and tsBN2 cells and sequenced to search for mutations. The hamster (BHK) RCC1 cDNA encodes a protein of 421 amino acids homologous to the human RCC1 protein. In a comparison of the base sequences of BHK and BN2 RCC1 cDNAs, a single base change, cytosine to thymine (serine to phenylalanine), was found in the 256th codon of BN2 RCC1 cDNA. The same transition was verified in the RCC1 genomic DNA by the polymerase chain reaction method. BHK RCC1 cDNA, but not tsBN2 RCC1 cDNA, complemented the tsBN2 mutation, although both have the same amino acid sequence except for one amino acid at the 256th codon. This amino acid change, serine to phenylalanine, was estimated to cause a profound structural change in the RCC1 protein.

Premature chromosome condensation is induced by a point mutation in the hamster RCC1 gene

At the nonpermissive temperature, premature chromosome condensation (PCC) occurs in tsBN2 cells derived from the BHK cell line, which can be converted to the Ts+ phenotype by the human RCC1 gene. To prove that the RCC1 gene is the mutant gene in tsBN2 cells, which have RCC1 mRNA and protein of the same sizes as those of BHK cells, RCC1 cDNAs were isolated from BHK and tsBN2 cells and sequenced to search for mutations. The hamster (BHK) RCC1 cDNA encodes a protein of 421 amino acids homologous to the human RCC1 protein. In a comparison of the base sequences of BHK and BN2 RCC1 cDNAs, a single base change, cytosine to thymine (serine to phenylalanine), was found in the 256th codon of BN2 RCC1 cDNA. The same transition was verified in the RCC1 genomic DNA by the polymerase chain reaction method. BHK RCC1 cDNA, but not tsBN2 RCC1 cDNA, complemented the tsBN2 mutation, although both have the same amino acid sequence except for one amino acid at the 256th codon. This amino acid change, serine to phenylalanine, was estimated to cause a profound structural change in the RCC1 protein.

Transforming growth factor-beta and its receptor are differentially regulated in human embryonal carcinoma cells

The human embryonal carcinoma cell lines Tera-2 clone 13 and NTera-2 clone D1 can be induced by retinoic acid to differentiate in vitro into neuroectodermal derivatives. The undifferentiated cells are rapidly proliferating and tumorigenic, whereas retinoic-acid-treated cells possess a decreased growth rate, lose their transformed phenotype and show a finite lifespan. Differentiation is accompanied by a marked increase in the levels of mRNA for TGF-beta 1 and TGF-beta 2 and the production of TGF-beta activity. Just like murine embryonal carcinoma cells the growth of Tera-2 clone 13 cells is not affected by the addition of either TGF-beta 1 or TGF-beta 2 to the culture medium. In contrast to published data on murine embryonal carcinoma cells, Tera-2 clone 13 and NTera-2 clone D1 cells bind TGF-beta 1 with high affinity, which is due to the presence of type-III TGF-beta receptors. Furthermore, and again in contrast to murine embryonal carcinoma cells, treatment of the human embryonal carcinoma cells with retinoic acid causes a nearly complete loss of TGF-beta 1 binding sites. These results are discussed in the light of similarities and differences in the regulation of growth and differentiation of human and murine embryonal carcinoma cell lines.

Centriole ciliation and cell cycle variability during G1 phase of BALB/c 3T3 cells

Although variability in the duration of the cell cycle is thought to reflect growth-regulatory processes that control cell cycle progression, the precise timing of the variable period within the G1 phase of the cell cycle has not been defined. In particular, the timing of cell cycle variability in relation to the cell's commitment (R point) to the initiation of DNA synthesis remains controversial. In order to investigate cell cycle variability, indirect immunofluorescence was used to measure the formation of the primary cilium as a possible marker of G1 events in both stimulated quiescent and exponentially growing cells. The primary cilium, an internal "9 + 0" nonmotile structure formed by one of the interphase centrioles, was first detected in postmitotic BALB/c 3T3 cells 5 hr before the initiation of DNA synthesis, an interval similar to that for the reassembly of the primary cilium in serum-stimulated quiescent fibroblasts. This similarity in the timing of ciliation suggests that serum-stimulated quiescent cells reenter the cell cycle in early G1 and recapitulate much of G1. Moreover, the rate of cilia formation in both postmitotic and serum-stimulated quiescent cells was identical to the rate of DNA synthesis initiation. Thus, cell cycle variability occurs before ciliation in both stimulated quiescent and exponentially growing cells. Furthermore, since ciliation also precedes the R point, variability in the centriole cycle occurs before the R point and thus may reflect processes controlling the cell's commitment to the initiation of DNA synthesis.

Potential and limitations of cultivated fibroblasts in the study of senescence in animals. A review on the murine skin fibroblasts system

Senescence is the last period of the life span, leading to death. It happens in all animals, with the exception of a few didermic species (Hydras) having a stock of embryonic cells and being immortal. The causes of animal senescence are badly known. They depend both on genetic characters (maximum life span of a species) and on medium factors (mean expectation of life of the animals of a species). Animal senescence could depend on cell aging: (1) by senescence and death of the differentiated cells, (2) by modified proliferation of the stem cells of differentiated tissues, (3) by alterations in the extracellular matrices, (4) by interactions between factors (1) (2) and (3) in each tissue, and (5) by interactions between the several tissues of an organism. This complexity badly impedes the experimental study of animal senescence. Normal mammal cells are aging when they are cultivated (in vitro aging). Present literature upon in vitro aging of cultivated human fibroblasts consists essentially of papers devoted to proliferation and differentiation characteristics and not to cell senescence. Murine skin fibroblasts have been studied in our laboratory, using different systems: (1) primary cultures isolated from peeled skins of mouse embryos, (2) mouse derms analysed in the animals, (3) cultivated explants of skins, (4) serial sub-cultures of fibroblasts isolated from these explants, (5) cells cultivated comparably on plane substrates (glass, plastic, collagen films) and on three-dimensional matrices (collagen fibres). In primary cultures (system 1) all the cell generations have been analysed, including the last one until death of the culture. We have shown that many characters are varying with cell generation. All the observed variations were: progressive, non-linear and correlated (intracellular feedbacks). We come to the conclusion that the main effects of cell mitotic age are (1) to depress the plasticity of the chromatin, (2) to change the organization of the cytoplasmic filaments, (3) to change the organization of the extracellular matrix. The collagen fibres are also acting upon nucleus and filaments either in the animals or in the cultures. The phenotype of a fibroblastic cell is thus both age- and environment-dependent. Overall data on in vitro cell aging point to the hypothesis that senescent cells are phenotypic variants and not mutant cells. Aging cell cultures are remarkably useful to the studies on cell proliferation decrease and cell cycle lengthening shown by the stem cells in animal tissues. We propose the hypothesis that the fibroblasts of the vertebrates would be homologous to the pluripotent mesenchyme cells of their embryos.

Sequential change of DNA synthesis in cultured aortic smooth muscle cells stimulated by hyperlipidemic serum

Smooth muscle cells from monkey aorta quiescent in 5% calf serum have been shown to be stimulated to renewed proliferation by hyperlipidemic serum or LDL from such serum. This proliferative response evidently is not dependent on platelet-derived growth factor present in our system in large quantities. The least exposure time required for reaction between the mitogen and the cells in order to initiate maximal DNA synthesis by this mechanism was studied using autoradiography. Stationary primary cultures and subcultures from monkey aortic media required at least 4 and 8 hr of contact with hyperlipidemic serum or LDL so that a significant number of cells reentered the mitotic cycle. Compared to the primary culture system, subcultures needed a slightly longer time of contact with serum to initiate DNA synthesis. Since there was no significant difference in labeling index between the primary cultures stimulated by serum for 8 and 48 hr and the subcultures exposed between 6 and 48 hr, it is concluded that a relatively brief stimulation commits the majority of responsive cells to reenter the cycle and initiate DNA synthesis.

Isolation and characterization of the active cDNA of the human cell cycle gene (RCC1) involved in the regulation of onset of chromosome condensation

The human RCC1 gene was cloned after DNA-mediated gene transfer into the tsBN2 cell line, which shows premature chromosome condensation at nonpermissive temperatures (39.5-40 degrees C). This gene codes for a 2.5-kb poly(A)+ RNA that is well conserved in hamsters and humans. We isolated 15 cDNA clones from the Okayama-Berg human cDNA library, and found two that can complement the tsBN2 mutation with an efficiency comparable to that of the genomic DNA clone. The base sequences of these two active cDNA clones differ at the 5' proximal end, yet both have a common open reading frame, encoding a protein of 421 amino acids with a calculated molecular weight of 44,847 and with seven homologous repeated domains of about 60 amino acids. This human RCC1 gene was located to human chromosome 1 using sorted chromosomal fractions.

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.

Cell cycle regulation of a mouse histone H4 gene requires the H4 promoter

The mouse histone H4 gene, when stably transformed into L cells on the PSV2gpt shuttle vector, is cell cycle regulated in parallel with the endogenous H4 genes. This was determined in exponentially growing pools of transformants fractionated into cell cycle-specific stages by centrifugal elutriation, a method for purifying cells at each stage of the cell cycle without the use of treatments that arrest growth. Linker additions in the 5' noncoding region of the H4 RNA or in the coding region of the gene did not affect the cell cycle-regulated expression of the modified H4 gene even though the overall level of expression was altered. However, replacing the H4 promoter with the human alpha-2 globin promoter, so that the histone transcript produced by the chimeric gene remains essentially unchanged, resulted in the constitutive expression of H4 mRNA during all phases of the cell cycle with no net increase in H4 mRNA levels during the G1-to-S transition. From these results we conclude that all the information necessary for the cell cycle-regulated expression of the H4 gene is contained in the 5.2-kilobase subclone used in these studies with 228 nucleotides of 5'-flanking DNA and that the increase in H4 mRNA during the G1-to-S transition in the cell cycle is mediated by the H4 promoter and not by the increased stability of the H4 RNA.

Isolation of a human X chromosome-linked gene essential for progression from G1 to S phase of the cell cycle

The tsBN462 cell line, a temperature-sensitive (ts) mutant isolated from the hamster cell line, BHK21/13, cannot progress into S phase at 39.5 degrees C, following the release from isoleucine deprivation. The mutant cells were transfected with high molecular weight (HMW) DNA from human KB cells, and several human DNA bands were found to be conserved through three cycles of ts+ transformation. Conserved human DNA was isolated from the cosmid library of the secondary ts+ transformant (K-1-1), using 32P-labelled total human DNA as a probe. The isolated human DNA covers about 70 kb of human DNA flanked with hamster DNA, and originates from the human X chromosome. The middle part (56 kb) of the isolated human DNA was conserved through the primary, secondary and tertiary ts+ transformation, without gross rearrangement.

Cell cycle regulation of a mouse histone H4 gene requires the H4 promoter

The mouse histone H4 gene, when stably transformed into L cells on the PSV2gpt shuttle vector, is cell cycle regulated in parallel with the endogenous H4 genes. This was determined in exponentially growing pools of transformants fractionated into cell cycle-specific stages by centrifugal elutriation, a method for purifying cells at each stage of the cell cycle without the use of treatments that arrest growth. Linker additions in the 5' noncoding region of the H4 RNA or in the coding region of the gene did not affect the cell cycle-regulated expression of the modified H4 gene even though the overall level of expression was altered. However, replacing the H4 promoter with the human alpha-2 globin promoter, so that the histone transcript produced by the chimeric gene remains essentially unchanged, resulted in the constitutive expression of H4 mRNA during all phases of the cell cycle with no net increase in H4 mRNA levels during the G1-to-S transition. From these results we conclude that all the information necessary for the cell cycle-regulated expression of the H4 gene is contained in the 5.2-kilobase subclone used in these studies with 228 nucleotides of 5'-flanking DNA and that the increase in H4 mRNA during the G1-to-S transition in the cell cycle is mediated by the H4 promoter and not by the increased stability of the H4 RNA.

Molecular cloning of a human gene that regulates chromosome condensation and is essential for cell proliferation

The tsBN2 cell line, a temperature-sensitive (ts) mutant of baby hamster kidney cell line BHK21/13, seems to possess a mutation in the gene that controls initiation of chromosome condensation. At the nonpermissive temperature (39.5 degrees C), the chromatin of tsBN2 cells is prematurely condensed, and the cells die. Using tsBN2 cells as a recipient of DNA-mediated gene transfer, we investigated a human gene that is responsible for regulation of chromosome condensation and cell proliferation. We found that the human gene complementing the tsBN2 mutation resides in the area of the 40- to 50-kilobase HindIII fragment, derived from HeLa cells. Based on this finding, we initiated cloning of a human gene complementing the tsBN2 mutation. From lambda and cosmid libraries carrying partial digests of DNA from the secondary transformants, the 41.8-kilobase HindIII fragment containing the human DNA was isolated. The cloned human DNA was conserved in ts+ transformants through primary and secondary transfections. Two cosmid clones convert the ts- phenotype of tsBN2 cells to ts+ with more than 100 times a higher efficiency, compared with cases of transfection with total human DNA. Thus, the cloned DNA fragments contain an active human gene that complements the tsBN2 mutation.

The effect of inhibiting DNA replication in the one-cell mouse embryo

Inhibition of DNA replication in the 1-cell mouse embryo causes arrest in interphase. In such arrested cells the pattern of protein synthesis progresses as normal through the first cell cycle but does not show the characteristic changes that are associated with mitosis. Synthesis does occur of the 68/70 kd heat shock-like proteins, on the earliest detectable embryonic transcripts synthesised shortly after first cleavage in control embryos. However, the quantitative change in the pattern of protein synthesis, indicative of the major activation of the embryonic genome that normally occurs during G2 of the second cell cycle, is prevented.

Molecular cloning of a human gene that regulates chromosome condensation and is essential for cell proliferation

The tsBN2 cell line, a temperature-sensitive (ts) mutant of baby hamster kidney cell line BHK21/13, seems to possess a mutation in the gene that controls initiation of chromosome condensation. At the nonpermissive temperature (39.5 degrees C), the chromatin of tsBN2 cells is prematurely condensed, and the cells die. Using tsBN2 cells as a recipient of DNA-mediated gene transfer, we investigated a human gene that is responsible for regulation of chromosome condensation and cell proliferation. We found that the human gene complementing the tsBN2 mutation resides in the area of the 40- to 50-kilobase HindIII fragment, derived from HeLa cells. Based on this finding, we initiated cloning of a human gene complementing the tsBN2 mutation. From lambda and cosmid libraries carrying partial digests of DNA from the secondary transformants, the 41.8-kilobase HindIII fragment containing the human DNA was isolated. The cloned human DNA was conserved in ts+ transformants through primary and secondary transfections. Two cosmid clones convert the ts- phenotype of tsBN2 cells to ts+ with more than 100 times a higher efficiency, compared with cases of transfection with total human DNA. Thus, the cloned DNA fragments contain an active human gene that complements the tsBN2 mutation.

Colony stimulating factor-induced differentiation of murine M1 myeloid leukemia cells is permissive in early G1 phase

Granulocyte-colony stimulating factor (G-CSF) induces differentiation of M1 murine myeloid leukemia cells into mature granulocytes/macrophages and also causes accumulation of the cells in the G1 phase of the cell cycle. We examined, therefore, whether synchronization of M1 cells in the G1 phase could affect G-CSF-induced differentiation as quantitated by expression of Fc fragment receptors (FcR) and lysozyme activity. Cells were arrested in early G1 by density inhibition in the absence of serum and in late G1 by addition of aphidicolin. Cells synchronized in early G1, when stimulated with G-CSF, showed enhanced expression of FcR and lysozyme activity. Eighty percent of the cells expressed FcR 18 hr after addition of G-CSF while, in exponentially growing cells, this percentage was reached 72 hr after addition of G-CSF. Cells synchronized in late G1 did not show enhanced expression of differentiation markers. These results imply that with respect to G-CSF-induced differentiation, the G1 phase can be separated into an early permissive and a later nonpermissive stage.

The 1985 Walter Hubert lecture. Malignant cell differentiation as a potential therapeutic approach

Most drugs available for cancer chemotherapy exert their effects through cytodestruction. Although significant advances have been attained with these cytotoxic agents in several malignant diseases, response is often accompanied by significant morbidity and many common malignant tumours respond poorly to existing cytotoxic therapy. Development of chemotherapeutic agents with non-cytodestructive actions appears desirable. Considerable evidence exists which indicates that (a) the malignant state is not irreversible and represents a disease of altered maturation, and (b) some experimental tumour systems can be induced by chemical agents to differentiate to mature end-stage cells with no proliferative potential. Thus, it is conceivable that therapeutic agents can be developed which convert cancer cells to benign forms. To study the phenomenon of blocked maturation, squamous carcinoma SqCC/Y1 cells were employed in culture. Using this system it was possible to demonstrate that physiological levels of retinoic acid and epidermal growth factor were capable of preventing the differentiation of these malignant keratinocytes into a mature tissue-like structure. The terminal differentiation caused by certain antineoplastic agents was investigated in HL-60 promyelocytic leukaemia cells to provide information on the mechanism by which chemotherapeutic agents induce cells to by-pass a maturation block. The anthracyclines aclacinomycin A and marcellomycin were potent inhibitors of N-glycosidically linked glycoprotein biosynthesis and transferrin receptor activity, and active inducers of maturation; temporal studies suggested that the biochemical effects were associated with the differentiation process. 6-Thioguanine produced cytotoxicity in parental cells by forming analog nucleotide. In hypoxanthine-guanine phosphoribosyltransferase negative HL-60 cells the 6-thiopurine initiated maturation; this action was due to the free base (and possibly the deoxyribonucleoside), a finding which separated termination of proliferation due to cytotoxicity from that caused by maturation.

Sequence specificity of cytosine methylation in the DNA of the Chinese hamster ovary (CHO-K1) cell line

We have determined the DNA renaturation kinetics for those DNA sequences of the Chinese hamster ovary (CHO-K1) cells in which enzymatic cytosine methylation occurred immediately after strand synthesis and for those in which methylation was delayed after strand synthesis. DNA sequences showing immediate or delayed methylation were found to be distributed throughout all repetition classes of the DNA of these cells, with a slight concentration of immediate methylation in moderately repetitive sequences and with delayed methylation being slightly over-represented in the highly repetitive fraction. However, DNA sequences showing both classes of methylation were represented equally in unique DNA sequences. We interpret these data to mean that the methylase acting near the replication forks (the 'immediate' methylase) is a relatively inefficient enzyme, missing some 20% of hemimethylated sites produced by DNA replication in these cells. We suggest that the methylase performing maintenance methylation at sites remote from the replication forks (the 'delayed' methylase) is simply a back-up enzyme for the first and that it has no true sequence specificity. The implications of this for the function(s) of DNA methylation in mammalian cells are discussed.

The role of the G1 period in the life cycle of eukaryotic cells

The objective of this study was to test the concept that the G1 period lacks any specific function in the life cycle of mammalian cells and hence could be drastically reduced without any effect on the generation time. HeLa cells were grown in medium containing an optimum dose (60 microM) of hydroxyurea at which the duration of S period was prolonged with little or no increase in generation time. At this concentration of hydroxyurea, we observed a maximum of 3 h (or 28.5%) reduction in the G1 period. We also studied the effects of synchronization in S phase by single and double thymidine blocks on cell size and its relationship to the duration of G1 in the subsequent cycle. By these treatments, we could reduce the G1 period by not more than 2 to 3 h. The reduction in G1 period was not directly proportional to the size (volume) of the G1 cells. These results suggest that G1 period has certain specific functions and cannot be eliminated by alterations in culture conditions.

Cell cycle regulation of mouse H3 histone mRNA metabolism

The mechanisms responsible for the periodic accumulation and decay of histone mRNA in the mammalian cell cycle were investigated in mouse erythroleukemia cells, using a cloned mouse H3 histone gene probe that hybridizes with most or all H3 transcripts. Exponentially growing cells were fractionated into cell cycle-specific stages by centrifugal elutriation, a method for purifying cells at each stage of the cycle without the use of treatments that arrest growth. Measurements of H3 histone mRNA content throughout the cell cycle show that the mRNA accumulates gradually during S phase, achieving its highest value in mid-S phase when DNA synthesis is maximal. The mRNA content then decreases as cells approach G2. These results demonstrate that the periodic synthesis of histones during S phase is due to changes in the steady-state level of histone mRNA. They are consistent with the conventional view in which histone synthesis is regulated coordinately with DNA synthesis in the cell cycle. The periodic accumulation and decay of H3 histone mRNA appear to be controlled primarily by changes in the rate of appearance of newly synthesized mRNA in the cytoplasm, determined by pulse-labeling whole cells with [3H]uridine. Measurements of H3 mRNA turnover by pulse-chase experiments with cells in S and G2 did not provide evidence for changes in the cytoplasmic stability of the mRNA during the period of its decay in late S and G2. Furthermore, transcription measurements carried out by brief pulse-labeling in vivo and by in vitro transcription in isolated nuclei indicate that the rate of H3 gene transcription changes to a much smaller extent than the steady-state levels of the mRNA or the appearance of newly synthesized mRNA in the cytoplasm. The results suggest that post-transcriptional processes make an important contribution to the periodic accumulation and decay of histone mRNA and that these processes may operate within the nucleus.

Cell cycle regulation of mouse H3 histone mRNA metabolism

The mechanisms responsible for the periodic accumulation and decay of histone mRNA in the mammalian cell cycle were investigated in mouse erythroleukemia cells, using a cloned mouse H3 histone gene probe that hybridizes with most or all H3 transcripts. Exponentially growing cells were fractionated into cell cycle-specific stages by centrifugal elutriation, a method for purifying cells at each stage of the cycle without the use of treatments that arrest growth. Measurements of H3 histone mRNA content throughout the cell cycle show that the mRNA accumulates gradually during S phase, achieving its highest value in mid-S phase when DNA synthesis is maximal. The mRNA content then decreases as cells approach G2. These results demonstrate that the periodic synthesis of histones during S phase is due to changes in the steady-state level of histone mRNA. They are consistent with the conventional view in which histone synthesis is regulated coordinately with DNA synthesis in the cell cycle. The periodic accumulation and decay of H3 histone mRNA appear to be controlled primarily by changes in the rate of appearance of newly synthesized mRNA in the cytoplasm, determined by pulse-labeling whole cells with [3H]uridine. Measurements of H3 mRNA turnover by pulse-chase experiments with cells in S and G2 did not provide evidence for changes in the cytoplasmic stability of the mRNA during the period of its decay in late S and G2. Furthermore, transcription measurements carried out by brief pulse-labeling in vivo and by in vitro transcription in isolated nuclei indicate that the rate of H3 gene transcription changes to a much smaller extent than the steady-state levels of the mRNA or the appearance of newly synthesized mRNA in the cytoplasm. The results suggest that post-transcriptional processes make an important contribution to the periodic accumulation and decay of histone mRNA and that these processes may operate within the nucleus.

Centrifugal elutriation of human lymphoid cells: synchronization and separation of aneuploid cells into diploid and tetraploid populations

Both normal and leukemic human lymphoid cell lines were separated into populations corresponding to different positions in the cell cycle by centrifugal elutriation. Each population was analyzed for cell concentration, cell volume, [3H]thymidine incorporation, percentage S phase by autoradiography, and percent G1, S, and G2/M phases by flow cytometry. The smallest cells, collected at the lowest flow rate, were in G1 phase. Cells collected at increasing flow rates progressively increased in volume and represented distinct positions in the cell cycle transition from G1 phase, through S phase, and into G2/M phase. The purity of the G1 population varied according to cell load. One hundred percent of cells were recovered and cells collected in G1- and S-phase populations proliferated in culture with patterns characteristic of synchronized cells. An aneuploidy leukemia cell line, CEM, was separated into near-diploid and near-tetraploid populations by centrifugal elutriation. This method of cell separation provides large numbers of human lymphoid cells at different positions in the cell cycle for investigating the relationship between the cell cycle and various surface membrane and metabolic properties of cells. Aneuploid leukemia and lymphoma cells can be separated by centrifugal elutriation into populations which contain different numbers of chromosomes for comparisons of their biologic properties.

Kinetic analysis of entry into S phase in resting rat 3Y1 cells stimulated by serum. Effects of serum concentration and temperature

The kinetics of entry into S phase after stimulation of resting 3Y1 cells by serum was examined in relation to serum concentration, temperature and the time at which the serum was withdrawn or at which the temperature was shifted. The kinetics of entry into S phase could be represented not only by a lag phase followed by a negative exponential curve (fit 1), but also by a normal distribution of the reciprocals of the time required for cells to enter S phase (velocities) (fit 2). As the temperature was lowered below 37 degrees C, the exponential slope decreased and the lag period increased (fit 1), and both the mean velocity and its standard deviation decreased (fit 2). As the serum concentration decreased below 10%, the exponential slope decreased without change in the lag period (fit 1), and the mean velocity decreased with increase in the standard deviation (fit 2). The cells which did not enter S phase within 8 h on removal of serum, stopped or delayed entry into S phase. In this case the lag phase was not changed (11 h). When serum was removed just before the end of the lag phase, no effect was seen on the kinetic curve. When the temperature was shifted at any time, including after the lag phase, the characteristics of the kinetic curve (lag phase, synchrony) changed. These facts indicate that there is a serum-non-requiring, but temperature-dependent period before S phase. Most of the asynchrony in entry into S phase under conditions of low serum seems to be generated during the serum-requiring period presumably by the random transition to the state in which cells are committed to enter S phase or by the variability of reaction rates at unpredictable times due to undeterministic effects.

Inducers of DNA synthesis: levels higher in transformed cells than in normal cells

The objective of this study was to determine whether transformed cells have greater DNA synthesis-inducing ability (DSIA) than normal cells when fused with G1 phase cells. HeLa cells synchronized in G1 phase, prelabeled with large latex beads, were fused separately with (a) quiescent human diploid fibroblasts (HDF), (b) HDF partially synchronized in late G1, and random populations of (c) HeLa, (d) WI-38, (e) SV-40 transformed WI-38, (f) CHO, (g) chemically transformed mouse cells (AKR-MCA), and (h) T98G human glioblastoma cells (all prelabeled with small latex beads) using UV-inactivated Sendai virus. The fusion mixture was incubated with [3H] thymidine, sampled at regular intervals, and processed for radioautography. Among the heterodikaryons, the frequency of those with a labeled and an unlabeled nuclei (L/U) were scored as a function of time after fusion. The faster the induction of DNA synthesis in HeLa G1, the steeper the drop in the L/U class and hence the higher DSIA in the S phase cells. The DSIA, which is indicative of the intracellular levels of the inducers of DNA synthesis, was the highest in HeLa and virally transformed WI-38 cells and the lowest in normal human diploid fibroblasts (HDF) while those of chemically and spontaneously transformed cells are intermediate between these two extremes. Higher level of DNA synthesis inducers appears to be one of the pleotropic effects of transformation by DNA tumor viruses. These studies also revealed that initiation of DNA synthesis per se is regulated by the presence of inducers and not by inhibitors.