Modulation of the spontaneous G2 phase blockage in Fanconi anemia cells by caffeine: differences from cells arrested by X-irradiation

The effect of caffeine on the endogenous G2 phase cell cycle blockage of Fanconi anemia (FA) cells was compared with the effect of caffeine on the G2 phase blockage induced in control cells by X-irradiation. The G2 phase accumulations in FA cells could be completely resolved by exposure to 1.5 mM caffeine. This was also observed in three brothers with endogenous G2 phase blockage due to unusual BrdU sensitivity. In contrast, G2 phase blockage induced by X-irradiation was only partially resolved by exposure to caffeine. The rescued G2 phase cells from FA patients were arrested within the following G1 phase compartments. This was not seen in X-irradiated cells from control donors. These results point towards a different nature and/or repair mechanism of the endogenous G2 phase lesion in FA cells compared to that induced by X-irradiation in control cells.

UV guided dendritic growth patterns and the networking of melanocytes

Whole skin organ cultures of vitiliginous skin show that the marginal melanocytes are highly sensitive to a pulse of UV exposure (210-380 nm) during the G2 phase of the cell cycle, as seen by prominent dendricity. Melanocytes are highly dendritic in the epidermis overlying rapidly growing tumors, as well as within proliferative lesions such as basal cell carcinomas and aggressive seborrheic keratosis. In the organ cultures the dendrites extend towards the source of UV, i.e. the surface, while the main body lies along the basement membrane. The epidermal melanocytes overlying tumors lie almost vertically, dendrites aligned towards the underlying tumor on one side and the surface on the other. Within tumors dendritic elongation is guided by mitotic and PCNA positive (S-phase) tumor cells, which are a source of ultraweak UV emissions in the range of 210-330 nm. These observations indicate that ultraweak biophoton emissions from neighbouring cells can simulate environmental cues and contribute to the plasticity of networks such as the melanocytes or the visual pathways.

Induction of tetraploid DNA content by simian virus 40 is dependent on T-antigen function in the G2 phase of the cell cycle

Previous experiments with the simian virus 40 mutant tsA357R-K (tsA30) demonstrated a T-antigen function that is required for production of cells with a greater-than-G2-phase DNA content. In this study, temperature shift experiments indicated that the temperature-sensitive function of tsA357R-K, which is necessary for entry into the greater-than-G2 phase, is not required in G1 or S but must be supplied in the G2 phase.

Competition between chromatin and transcription complex assembly regulates gene expression during early development

Xenopus early development is characterized by a generalized absence of transcription, which resumes at the midblastula transition (MBT). We analyzed this regulation using a plasmid containing the c-myc promoter that is under the same developmental control when injected into fertilized eggs. We find that the repression of transcription can be relieved simply by preincubating the reporter plasmid with TATA binding protein (TBP). However, the repression of gene activity normally occurring before the MBT soon becomes dominant over this activation independent of cell cycle phases. This inactivation correlates with chromatin assembly, and titration of chromatin components not only relieves repression of TBP-dependent transcription but also permits the establishment of stable transcription during early development. Our data suggest that the large excess of histones represses gene activity during early development through a dynamic competition between chromatin assembly and transcription complex assembly.

Enhanced repair endonuclease activities from radiation-arrested G2 phase mammalian cells

HeLa cells arrested in G2 phase 22 h after receiving 11.5 Gy gamma-radiation contained 3.6-fold more EDTA-resistant DNA repair endonuclease activity than unirradiated cells. Enzyme activity was determined by measuring the release of fragments from an irradiated repetitive alpha DNA substrate or from synthetic substrates containing a single modified base, 8-oxoguanine (8-oxo-G), a major radiation product. It appeared that the radiation-induced enhanced repair activity in some cells might be a feature of radiation-induced G2 arrest. Indeed, unirradiated G2 HeLa cells that had been synchronized by double thymidine block contained 3-7-fold more endonuclease activity than G1 or S-phase cells. Similarly, two of four other cell lines tested exhibited elevated repair endonuclease activity in G2. However, all six cell lines tested exhibited radiation-enhanced repair endonuclease activity. Therefore, the underlying mechanism for radiation enhancement of enzyme activity remains to be clarified and does not seem to be completely accounted for as a consequence of G2 arrest. The results showed different substrate specificities among cell lines as well as differences during the cell cycle of individual cell lines. Repair endonuclease activity from all cell lines which we have tested were associated with 60-70 kDa proteins from Superose 12 columns. Since reports from other laboratories have described several different DNA repair activities in 50-70 kDa Superose 12 fractions, it seems possible that the DNA repair enzymes may be associated in a repairosome structure.

MPF components and meiotic competence in growing pig oocytes

Growing pig oocytes (< or = 90 microns in diameter) are unable to resume meiosis in vitro. The objective of our present experiments has been to identify the reasons for meiotic competence in these cells. By comparing histone H1 kinase activity in growing and fully grown oocytes we demonstrate that incompetence is associated with an inability to activate H1 kinase in growing oocytes. Immunoblotting was used to determine whether this kinase inactivity resulted from a lack of either p34cdc2 protein or B-type cyclin. The results established that each of these cell cycle molecules are present in comparable amounts in both growing and fully grown oocytes. In the third series of studies experiments were carried out in an attempt to induce p34cdc2 activation during growth. Treatment with okadaic acid, an inhibitor of phosphatase 1 and 2A known to stimulate and accelerate the transition into M-phase of the meiotic cycle in a number of different species, was able to induce p34cdc2 kinase activity and facilitated the G2- to M-phase in growing oocytes. We conclude that although growing oocytes in pigs have sufficient key cell cycle components for the G2 to M transition, they remain incapable of converting these components to active maturation-promoting factor (MPF) until growth is virtually completed. Inhibition of phosphatase 1 or 2A induces the formation of active MPF during growth by an as yet unidentified pathway.

Simian virus 40 prevents activation of M-phase-promoting factor during lytic infection

Simian virus 40 (SV40) infection stimulates confluent cultures of monkey kidney cells into successive rounds of cellular DNA synthesis without intervening mitosis. As an initial step in defining the mechanisms responsible for viral inhibition of mitosis, M-phase-promoting factor (MPF) was examined in SV40-infected CV-1 cells passing from G2 phase into a second S phase. MPF is a serine-threonine protein kinase that is essential for mitosis in eukaryotic cells. In SV40-infected cells exiting G2 phase, there was a reduced amount of MPF-associated H1 kinase activity relative to that of uninfected cells passing through mitosis. Both subunits of MPF, cyclin B and the p34cdc2 catalytic subunit, were present and in a complex in infected cells. In uninfected cultures, passage through mitosis was associated with the dephosphorylation of the p34cdc2 subunit, which is characteristic of MPF activation. In contrast, the p34cdc2 subunit remained in the tyrosine-phosphorylated, inactive form in SV40-infected cells passing from G2 phase into a second S phase. These results suggest that although the MPF complex is assembled and modified normally, SV40 interferes with pathways leading to MPF activation.

Radiation-induced chromosomal aberrations in mouse 10T1/2 cells: dependence on the cell-cycle stage at the time of irradiation

Cell-cycle stage radiosensitivity for the induction of chromosome aberrations has been investigated in C3H 10T1/2 cells. Exponentially growing cells were irradiated with 3 Gy X-rays (80 kVp) or 0.6 Gy alpha-particles (LET = 101 keV/micron). The two doses produce the same survival level (37%) in the asynchronous population. Cells were harvested at four different times following irradiation and cell-cycle phase at the time of irradiation was assessed by using the differential replication staining technique. The frequency of chromosome aberrations produced in a given stage of the cell cycle was not constant as a function of the sampling time, but this could not be simply related to the existence of subphases exhibiting different radiosensitivity, because of cell-cycle perturbation introduced by radiation. X-radiation induced more exchanges than deletions, whereas a predominance of isochromatid deletions was observed after alpha-irradiation. This can be interpreted on the basis of the different patterns of energy deposition of densely- and sparsely-ionizing radiation. Both X- and alpha-rays produced a significant increase in the frequency of Robertsonian translocations when cells were exposed in G1 or S phase, but not in G2 phase.

Differential sensitivity to the induction of apoptosis by cisplatin in proliferating and quiescent immature rat thymocytes is independent of the levels of drug accumulation and DNA adduct formation

Immature rat thymocytes readily undergo apoptosis following exposure to many different stimuli, including agents which cause DNA damage, such as the topoisomerase II inhibitor etoposide and irradiation. We have shown previously that cells isolated from the immature rat thymus are resistant to the induction of apoptosis by the DNA-damaging agent cis-diamminedichloroplatinum(II) (cisplatin) (D. L. Evans and C. Dive, Cancer Res., 53:2133-2139, 1993). More than 85% of these thymocytes are quiescent. Here, we demonstrate that following purification of the minority subpopulation of thymocytes that are proliferating, a 2-h exposure to 50 microM cisplatin resulted in rapid apoptosis with 66% apoptotic cells by 12 h. In contrast, purified, nonproliferating thymocytes treated with cisplatin exhibited control levels of apoptosis at 12 h. Both proliferating and nonproliferating thymocytes rapidly underwent apoptosis following continuous exposure to methylprednisolone (10 microM) and etoposide (10 microM). The discrepancy in the levels of apoptosis seen in proliferating and quiescent thymocytes in response to cisplatin could not be attributed to changes in total cellular levels of cisplatin or to the number of DNA-platinum adducts which were determined, respectively, by atomic absorption spectrometry and competitive enzyme-linked immunoadsorbent assay. These results imply that in contrast to engagement of thymocyte apoptosis by methylprednisolone and etoposide, where apoptosis was proliferation independent, cisplatin-induced apoptosis depends on the presence of cells in S and G2-M phases of the cell cycle. Moreover, comparison of etoposide and cisplatin responses in thymocytes suggests that DNA damage per se may not be sufficient to induce apoptosis and that the type of DNA damage is important in this regard.

A dual block to cell cycle progression in HL60 cells exposed to analogues of vitamin D3

The physiologically active form of vitamin D3, 1,25-dihydroxy-vitamin D3 (1,25(OH)2D3), induces differentiation of several types of myeloid leukaemia cells. The acquisition of monocyte-like phenotype is accompanied by slower progression through the cell cycle, and G1 block has been reported to be the basis of this effect. It is shown here that human promyelocytic leukaemia HL60 cells treated with analogues of vitamin D3 which are potent inducers of monocytic differentiation have an additional cell cycle block. Exposure to 10(-7) M 1,25(OH)2D3 or 1,25-(OH)2-16-ene-D3 resulted in monocytic differentiation and the expected G1 block evident at approximately 48 h in a rapidly differentiating variant of HL60 cells (HL60-G), and at 96 h in the more slowly differentiating HL60-240 cells. In addition, a G2+M block was noted at approximately 72 h in HL60-G and HL60-240 cells. Exposure to vitamin D3 analogues also markedly increased the number of dikaryons, suggesting that cytokinesis was impaired more than karyokinesis. Treatment with a third analogue 25-hydroxy-16,23-diene-D3 produced little differentiation and had minimal effects on the cell cycle parameters. These findings indicate that vitamin D3 analogues regulate cell proliferation by control of the transition of G1 and G2+M phases, reminiscent of the cdc2/CDK2 type of cell cycle control.

Association of deficient DNA repair during G2 phase with progression from benign to malignant state in a line of human skin keratinocytes transfected with ras oncogene

Human skin keratinocytes after malignant neoplastic transformation by infection with Kirsten murine sarcoma virus (KiMSV) or transfection with pSV2 ras (containing an activated c-Ha-ras oncogene) showed a DNA repair deficiency(ies). The repair deficiency was manifest as an abnormally high frequency of chromatid breaks and gaps persisting after X-ray-induced DNA damage inflicted during the G2 phase of the cell cycle. Non-tumorigenic control cells at that time were clearly repair-efficient. By analyzing benign and malignant tumorigenic HaCaT-ras clones, we could exclude ras p21 oncoprotein expression as the causal mechanism for repair deficiency, since both clone types expressed similar levels of the mutated protein and only the malignant tumorigenic cells showed repair deficiency. The results suggest that mutated p21 ras provided the human keratinocytes with a growth advantage in vivo (benign tumor growth), but acquisition of repair deficiency is required for progression from benign to malignant state.

Kinetics of MPF and histone H1 kinase activity differ during the G2- to M-phase transition in mouse oocytes

Maturation promoting factor (MPF) is universally recognized as the biological entity responsible for driving the cell cycle from G2- to M-phase. Histone H1 kinase activity is widely accepted as a biochemical indicator of p34cdc2 protein kinase complex activity and therefore MPF activity. In this paper we present results which indicate that during the G2- to M-phase transition in mouse oocytes the dynamic of p34cdc2 related histone H1 kinase activity differs markedly from the biological activity of MPF as measured by classical cell fusion procedures. MPF is activated just before germinal vesicle breakdown (GVBD) whereas histone H1 kinase is activated 5-7 h later coincident with the formation of the definitive first metaphase plate. The biological activity of MPF is merely reduced to about 50% of control levels by a short period of protein synthesis inhibition (1-2 h) and completely suppressed after a prolonged period of inhibition (4-5 h). By contrast, inhibition of protein synthesis in mouse oocytes results in a rapid and complete suppression of histone H1 kinase activity. Therefore, biological MPF and histone H1 kinase activity should not be used in an interchangeable manner during the G2- to M-phase transition in mouse oocytes.

Rapid method for cell cycle analysis in a predatory marine dinoflagellate

Oxyrrhis marina (Dujardin) is a predatory marine dinoflagellate that feeds phagocytically on live phytoplanktonic "prey" cells from the surrounding environment. A rapid method was developed to separate the cell cycle characteristics of these predators from their prey cells in order to study the cell cycle dynamics of this organism. Nuclei from Oxyrrhis were isolated in low salt buffer (PBS) using detergent and mechanical agitation and the DNA stained with Hoechst 33258 in a one step procedure. The method permitted the isolation of nuclei from the Oxyrrhis cells with > 95% efficiency. Discrimination between prey cell nuclei and those of Oxyrrhis was achieved during flow cytometric analysis which yielded routinely G1 CVs of 3-6% for exponentially growing cell populations and 2-3% for stationary phase cells. The method was used to demonstrate the changes in cell cycle dynamics during the exponential and stationary phases of growth. Results indicated that in contrast to most mammalian and phytoplankton cell types Oxyrrhis spent the major portion (ca. 50%) of its cell cycle in G2 + M when actively dividing. Analysis of stationary phase populations also suggests that specific cell cycle control (or restriction) points were present in both G1 and G2 in this species.

Petunia p34cdc2 protein kinase activity in G2/M cells obtained with a reversible cell cycle inhibitor, mimosine

Protoplasts isolated from petunia leaf mesophyll are non-cycling cells mostly with 2C content. Cells regenerating from protoplast culture enter mitosis after 48 h. This experimental model is used to relate p34cdc2 kinase activity to cell cycle phase. Our results show that the histone H1 phosphorylation, and hence p34cdc2 kinase activity, peaks with G2+early M cell cycle phase. However, a trace kinase activity was already present when most cells were entering S phase. To obtain a maximum of cells in G1+S phases, the protoplast culture was treated with the rare amino acid, mimosine. Mimosine blocked plant cells derived from protoplast culture both at G1 and in early and mid S phase. Despite the increased G1+S level, p34cdc2 kinase activity did not increase. This suggests that the trace activity appearing when the majority of cells are entering S does not correspond to any putative p34cdc2 activation at G1/S transition but to the activation of the minor 4C population initially present in the leaf: the hypothesis remains that p34cdc2 kinase activity is solely related to G2+M phase in petunia.

The induction of CD10 on a pre-B leukemia cell line occurs with progression of the disease in scid mice

We have previously demonstrated the engraftment and dissemination of human pre-B acute lymphoblastic leukemia (ALL) cells into scid mice. In the current study, the temporal pattern of infiltration of a CD10- pre-B leukemia line (G2) in various murine tissues and the progression of the disease in the whole animal were monitored by quantifying human CD44 mRNA expression by the polymerase chain reaction (PCR). Irradiated scid mice were injected intravenously with 10(6) G2 cells and killed 3 days to 10 weeks later. After 2 weeks, leukemic cells were found mostly in bone marrow, but also in lung. At 6 to 7 weeks, spleen and lung contained 30% human RNA, while peripheral blood, liver, and kidney contained 2-3%. Infiltration to brain and thymus was observed at 8-9 weeks. In terms of the whole animal, spleen and liver were the major sites of tumor burden. The induction of CD10 expression was previously observed in transplanted CD10- G2 leukemic cells recovered from scid thymus at 10-12 weeks, which corresponds to the terminal stage of disease. In this study, the CD10 expression on the leukemic cells was monitored at earlier time points by flow cytometry and quantitative PCR. Induction of CD10 was first observed in bone marrow, spleen, peripheral blood, and liver at 6-7 weeks (10-fold), at the time of the onset of dissemination of the leukemia. Despite the presence of 30% human RNA in lung at 6-7 weeks, CD10 induction was not significant in that site before 10 weeks. Increased levels of CD10 were seen in all tissues between 8 and 10 weeks; the highest levels were observed in leukemic cells proliferating in thymus (113-fold) and in those found in circulation. These findings suggest that initial induction of CD10 occurs in hematopoietic tissues at the time of rapid proliferation of the leukemic cells and their infiltration of several tissues. At later time points, the increase in CD10 expression is seen on the leukemic cells found in all peripheral organs suggesting an association with disease progression.

[DNA flow cytometry of malignant melanoma of the choroid]

Thirty-five choroidal melanomas were analyzed by flow cytometry. DNA euploidy, aneuploidy and polyploidy were found in approximately 54, approximately 46, and approximately 20% of the tumors, respectively. No correlation between histological type and ploidy was found. The synthesis phase was irregular in approximately 25 and approximately 38% of the spindle and the mixed cell melanomas, respectively. The G2M peak was broadened in approximately 50% and in approximately 23% of the spindle and the mixed cell melanomas. About 20% of the malignant melanomas were highly proliferative. In 25% of the melanomas chemotherapy seemed to be useful.

Mechanisms that help the yeast cell cycle clock tick: G2 cyclins transcriptionally activate G2 cyclins and repress G1 cyclins

In budding yeast, G1 cyclins such as CLN1 and CLN2 are expressed in G1 and S phases, while mitotic cyclins such as CLB1 and CLB2 are expressed in G2 and M phases. We find that the CLBs play a central role in the transition from CLNs to CLBs: the CLBs stimulate their own expression while repressing that of CLNs. This negative regulation of CLNs may occur via the transcription factor SWI4, because CLBs are necessary for G2 repression of SCB-regulated genes like CLN1 and CLN2 but not for repression of MCB-regulated genes like DNA polymerase and CLB5. Furthermore, SW14 associates with CLB2 protein and is a substrate for the CLB2-associated CDC28 kinase in vitro.

A product of phosphatidylinositol-3 kinase is elevated in dividing HT29 colonic epithelial cells

Phosphatidylinositol 3 kinase (PI-3 kinase) activity has been linked to cell proliferation and growth regulation. Therefore, we studied changes in phosphoinositide metabolism during the cell cycle of HT-29 cells, a colonic epithelial cell line. HT29 cells were treated with the microtubule disrupter, nocodozole, separated into mitotic and quiescent populations and their phospholipid composition was analyzed. Radiolabelled phospholipids from cells labelled with 32PO4 or [3H]myoinositol were analyzed by TLC and/or deacylated and analyzed by HPLC. In all cases, levels of phosphatidylinositol 3 phosphate from mitotic phase cells were double that in resting populations. Therefore, levels of a product of PI-3 kinase are elevated and may play a role in cell division.

Human papillomavirus type 16 E7 associates with a histone H1 kinase and with p107 through sequences necessary for transformation

The transforming function of human papillomavirus type 16 (HPV16) E7 has been shown to depend on activities additional to the ability to bind RB. In this paper we describe two further properties of E7 which may also contribute to transformation, an association with a histone H1 kinase at the G2/M phase of the cell cycle and an ability to bind the RB-related protein p107. The region of E7 identified previously as important for RB binding was found to be involved in the association with the kinase and complex formation with p107, although analysis of E7 point mutants within this region revealed a difference in the precise sequence requirement for RB and p107 binding. Association with the kinase activity correlated with the ability to bind RB, but the restriction of the kinase association to the G2/M phase of the cell cycle implies that this activity might not be directly mediated by RB binding. Since kinase-binding-deficient E7 mutants are also transformation defective, this may represent an independent function of E7 which plays a role in the G2/M phase of the cell cycle.

The Cdk-associated protein Cks1 functions both in G1 and G2 in Saccharomyces cerevisiae

The CKS1 gene of Saccharomyces cerevisiae encodes a small essential protein shown to interact genetically and physically with the Cdc28 protein kinase. To investigate the specific functions of the CKS1 gene product, conditional temperature-sensitive mutant alleles were generated. The mutations were found to impair the ability of cells to undergo both the G1/S-phase and G2/M-phase transitions of the cell cycle, as well as the ability to bud. Mutants were not defective, however, in their ability to activate Cdc28 kinase as assayed in vitro on the substrate histone H1. It is likely, therefore, that Cks1 mediates a more specialized function of the Cdc28 kinase such as its ability to form specific multimeric complexes or to localize properly in cellular compartments.

Cell cycle dependence for the induction of 6-thioguanine-resistant mutations: G2/M stage is distinctively sensitive to 252Cf neutrons but not to 60Co gamma-rays

Cell cycle dependence for the induction of 6-thioguanine-resistant mutation was investigated using synchronized mouse L5178Y cells after exposure to 4 Gy to 60Co gamma-rays or 1 Gy of 252Cf radiation (fission neutrons). Maximal mutation frequency was observed immediately after release from colcemid block (G2/M phase) after 252Cf radiation, whereas it was 1 h after the release (G1 phase) after 60Co gamma-rays. When the mutation frequency was plotted against the surviving fraction, a general correlation was observed between the two parameters except for G2/M and G1 phases which were more mutable per lethal event for 252Cf and 60Co radiations, respectively.

Synergistic action of Drosophila cyclins A and B during the G2-M transition

A variety of different cyclin proteins have been identified in higher eukaryotes. In the case of cyclin B, functional analyses have clearly demonstrated an important role in the control of entry into mitosis. The function of cyclin A is more complex. It appears to function in the control of both S- and M-phase. The results of our genetic analyses in Drosophila demonstrate that cyclin A has a mitotic function and that it acts synergistically with cyclin B during the G2-M transition. In double mutant embryos that express neither cyclin A nor cyclin B zygotically, cell cycle progression is blocked just before the exhaustion of the maternally contributed cyclin A and B stores. BrdU-labeling experiments indicate that cell cycle progression is blocked in G2 before entry into the fifteenth round of mitosis. Expression of either cyclin A or B from heat-inducible transgenes is sufficient to overcome this cell cycle block. This block is also not observed in single mutant embryos deficient for either cyclin A or B. In cyclin B deficient embryos, cell cycle progression continues after the apparent exhaustion of the maternal contribution, suggesting that cyclin B might not be essential for mitosis. However, mitotic spindles are clearly abnormal and progression through mitosis is delayed in these cyclin B deficient embryos.

The suppression of the synthesis of a nuclear protein in cells blocked in G2 phase: identification of NP-170 as topoisomerase II

Previous studies of a nuclear protein of molecular weight 170 kDa (NP-170) have shown it to have two interesting properties. First, NP-170 synthesis began in mid- to late S phase and became maximal in G2 phase. Second, the synthesis of NP-170 was suppressed in cells blocked in G2 phase following irradiation with 6.8 Gy (J. M. Holland et al., Radiat. Res. 122, 197-208, 1990). The molecular weight of NP-170 is the same as that of Topoisomerase II (Topo II), an enzyme involved in the alteration of DNA supercoiling status with a double-strand passing function. This study was undertaken to determine whether NP-170 could be Topo II. The results from the present study show that both the proteins have identical cell cycle synthesis patterns. The synthesis of both these proteins is suppressed following irradiation. NP-170 was found to be recognized by a Topo II antibody in both Western blots and immunoprecipitation. This study characterizes NP-170 as Topo II.

Proposed role of phosphatidic acid in the extracellular control of the transition from G2 phase to mitosis exerted by epidermal growth factor in A431 cells

Epidermal growth factor (EGF) has been shown to cause an inhibition of A431 cells in G2 phase within approximately 10 min, i.e., shortly before mitosis (Kinzel et al., Cancer Res., 50: 7932-7936, 1990). This system has been used to study the proposed role phospholipid metabolites, particularly phosphatidic acid (PA), may play (Kaszkin et al., Cancer Res., 51: 4328-4335, 1991) in the extracellular control of cells at the physiological restriction site in G2 phase. A431 cells responded to EGF with a dose-dependent formation of phosphatidic acid (PA) which correlated with the dose-dependent G2 delay as well as with their time courses. The G2 delay induced by EGF as well as PA mobilization were effected in conditioned medium or in fresh medium containing bovine serum albimun instead of serum, i.e., under the conditions necessary for precursor studies to be carried out. The major pathway of PA formation was probably via phospholipase C-mediated breakdown of phosphatidylinositol and diacylglycerol kinase: (a) the dose response of PA formation correlated with that of total inositol phosphate accumulation; (b) little diacylglycerol was found and then only at a high EGF concentration; (c) prelabeling with [1-14C]arachidonic acid resulting in a large specific labeling of phosphatidylinositol led to an EGF-induced, dose-dependent formation of radioactive arachidonyl-PA (correlated with that of total PA and inositol phosphate), but in the presence of a primary alcohol not to the formation of radioactive phosphatidylalcohol; (d) prelabeling with [1-14C]oleic acid led to the EGF-induced formation of labeled PA, which in the presence of a primary alcohol was only slightly reduced to the advantage of very low levels of labeled phosphatidyl alcohol, thus demonstrating that an EGF-effected activation of phospholipase D did occur but contributed little to the general PA level. An alternative mobilization of PA was attempted with the phorbolester 12-O-tetradecanoylphorbol-13-acetate (TPA), which was shown to activate phospholipase D in A431 cells and to elicit PA from a phospholipid pool which was not significantly labeled with radioactive arachidonic acid. The TPA-induced degree of PA formation and of the G2 delay correlated. Both phenomena were considerably larger with fresh medium containing 0.5% bovine serum albumin instead of serum than in conditioned medium.(ABSTRACT TRUNCATED AT 400 WORDS)

CENP-E is a putative kinetochore motor that accumulates just before mitosis

The mechanics of chromosome movement, mitotic spindle assembly and spindle elongation have long been central questions of cell biology. After attachment in prometaphase of a microtubule from one pole, duplicated chromosome pairs travel towards the pole in a rapid but discontinuous motion. This is followed by a slower congression towards the midplate as the chromosome pair orients with each kinetochore attached to the microtubules from the nearest pole. The pairs disjoin at anaphase and translocate to opposite poles and the interpolar distance increases. Here we identify CENP-E as a kinesin-like motor protein (M(r) 312,000) that accumulates in the G2 phase of the cell cycle. CENP-E associates with kinetochores during congression, relocates to the spindle midzone at anaphase, and is quantitatively discarded at the end of the cell division. CENP-E is likely to be one of the motors responsible for mammalian chromosome movement and/or spindle elongation.