Synthesis of RNA and protein required for the mitosis of mammalian cells

Final report on the safety assessment of EDTA, calcium disodium EDTA, diammonium EDTA, dipotassium EDTA, disodium EDTA, TEA-EDTA, tetrasodium EDTA, tripotassium EDTA, trisodium EDTA, HEDTA, and trisodium HEDTA

EDTA (ethylenediamine tetraacetic acid) and its salts are substituted diamines. HEDTA (hydroxyethyl ethylenediamine triacetic acid) and its trisodium salt are substituted amines. These ingredients function as chelating agents in cosmetic formulations. The typical concentration of use of EDTA is less than 2%, with the other salts in current use at even lower concentrations. The lowest dose reported to cause a toxic effect in animals was 750 mg/kg/day. These chelating agents are cytotoxic and weakly genotoxic, but not carcinogenic. Oral exposures to EDTA produced adverse reproductive and developmental effects in animals. Clinical tests reported no absorption of an EDTA salt through the skin. These ingredients are likely, however, to affect the passage of other chemicals into the skin because they will chelate calcium. Exposure to EDTA in most cosmetic formulations, therefore, would produce systemic exposure levels well below those seen to be toxic in oral dosing studies. Exposure to EDTA in cosmetic formulations that may be inhaled, however, was a concern. An exposure assessment done using conservative assumptions predicted that the maximum EDTA dose via inhalation of an aerosolized cosmetic formulation is below that shown to produce reproductive or developmental toxicity. Because of the potential to increase the penetration of other chemicals, formulators should continue to be aware of this when combining these ingredients with ingredients that previously have been determined to be safe, primarily because they were not significantly absorbed. Based on the available data, the Cosmetic Ingredient Review Expert Panel found that these ingredients are safe as used in cosmetic formulations.

Unexpected cytokinetic effects induced by puromycin include a G2-arrest, a metaphase-mitotic-arrest, and apoptosis

The potent effects of low doses of PM on the cell cycle have to date been obscured by the conventional usage of this drug at high concentrations (5-50 micrograms/ml) to inhibit protein synthesis. In this in vitro study undertaken in a variety of malignant and non-malignant human and murine cell types, we found that low doses of PM (0.1-0.5 microgram/ml) disrupted significant phase-to-phase cell cycle transitions, causing a G2-arrest, a metaphase-mitotic-arrest, and apoptosis. In HL-60 cells these observations were elicited by PM concentrations starting at 0.1 microgram/ml, and were more pronounced at slightly higher PM concentrations, including that (0.5 microgram/ml) which inhibited [14C]leucine incorporation by approximately 20% after one hour, and by approximately 50% after 24 h. A concentration of CHX (0.25 microgram/ml) which was equivalent to 0.5 microgram/ml of PM, both in terms of molarity (0.9 microM) and degree of inhibition of [14C]leucine incorporation, failed to induce similar changes to those induced by PM. This suggests that at these particular concentrations the PM-induced changes were likely to have been related to the different mechanisms of protein synthesis inhibition exerted by these two 'classical' translation inhibitors. PM but not CHX generates nascent peptidyl-PM complexes (PMPs), and we therefore propose that the subsequent intracellular effects exerted by the PMPs may account, in part, for the differential cytokinetic effects elicited by these drugs. The role of PM is currently being evaluated in vivo as a low-dose component of a multidrug chemotherapeutic regimen in which its cell cycle-specific effects could potentially be synergistic with other agents.

The requirements for protein synthesis and degradation, and the control of destruction of cyclins A and B in the meiotic and mitotic cell cycles of the clam embryo

Fertilization of clam oocytes initiates a series of cell divisions, of which the first three--meiosis I, meiosis II, and the first mitotic division--are highly synchronous. After fertilization, protein synthesis is required for the successful completion of every division except meiosis I. When protein synthesis is inhibited, entry into meiosis I and the maintenance of M phase for the normal duration of meiosis occur normally, but the chromosomes fail to interact correctly with the spindle in meiosis II metaphase. By contrast, inhibition of protein synthesis immediately after completion of meiosis or mitosis stops cells entering the next mitosis. We describe the behavior of cyclins A and B in relation to these "points of no return." The cyclins are synthesized continuously and are rapidly destroyed shortly before the metaphase-anaphase transition of the mitotic cell cycles, with cyclin A being degraded in advance of cyclin B. Cyclin destruction normally occurs during a 5-min window in mitosis, but in the monopolar mitosis that occurs after parthenogenetic activation of clam oocytes, or when colchicine is added to fertilized eggs about to enter first mitosis, the destruction of cyclin B is strongly delayed, whereas proteolysis of cyclin A is maintained in an activated state for the duration of metaphase arrest. Under either of these abnormal conditions, inhibition of protein synthesis causes a premature return to interphase that correlates with the time when cyclin B disappears.

The cell cycle dependence of protein synthesis during Xenopus laevis development

The regulation of early embryonic development in the amphibian Xenopus laevis depends largely upon translational and post-translational regulatory mechanisms to direct the complex cytodifferentiations that take place during early cleavage and blastula formation. The cell cycle dependence of protein synthesis was examined in developing Xenopus embryos as well as in cycling cell-free lysates from Xenopus eggs. In both cases M-phase and the activation of the M-phase kinase were found to be correlated with an inhibition of translation. Translation in both the rough endoplasmic reticulum and cytosolic-free ribosomes were affected by this inhibition. Since elongation was found to be unaffected by M-phase, shifts in the polysome profiles during M-phase indicated that the inhibition affected initiation processes. The activity of the M-phase kinase may inhibit initiation through the modification of initiation factors or some other component during this process. The cell cycle dependence of translation may affect developmental mechanisms controlled by the titration of regulatory proteins.

The cell cycle dependence of the secretory pathway in developing Xenopus laevis

The cell cycle during the cleavage period of the amphibian Xenopus laevis is about 30 min long and oscillates between equal periods of mitosis and interphase. At the midblastula transition (MBT) the length of interphase begins to elongate and brings about corresponding changes in the activities of cell cycle-dependent processes. In this study protein secretion and Golgi processing during embryonic Xenopus development were examined. The elongation of interphase, either during normal development or experimentally induced, resulted in an increase in the secretion of both endogenous and exogenous proteins. Secretion was found to increase linearly with the increase in interphase length, indicating that the rate of secretion was constant and was regulated by the length of interphase. M-phase arrest in embryos and oocytes produced an inhibition of protein secretion that was reversible if the cell cycle was returned to interphase. This M-phase block of the secretory pathway was found to take place between the trans Golgi compartment and the plasma membrane. The developmental increase in the function of this pathway after the MBT may affect the expression of surface and secreted proteins important for the cell-cell interactions necessary for subsequent development through gastrulation.

Translation of cyclin mRNA is necessary for extracts of activated xenopus eggs to enter mitosis

The cyclins are a family of proteins encoded by maternal mRNA. Cyclin polypeptides accumulate during interphase and are destroyed during mitosis at about the time of entry into anaphase. We show here that Xenopus oocytes contain mRNAs encoding two cyclins that are major translation products in a cell-free extract from activated eggs. Cutting these mRNAs with antisense oligonucleotides and endogenous RNAase H blocks entry into mitosis in a cell-free egg extract. The extracts can enter mitosis if either of the cyclin mRNAs is left intact. We conclude that the synthesis of these cyclins is necessary for mitotic cell cycles in cleaving Xenopus embryos.

Periodic mitotic events induced in the absence of DNA replication

We have discovered and report here a means of separating a mitotic "subcycle" from the G1- and S-phase events of the mammalian cell cycle. Time-lapse videomicroscopy of Syrian hamster fibroblast (BHK) cells revealed that caffeine could induce multiple entries into mitosis while cells were blocked in DNA synthesis. As with normal mitoses, the abundance of mitosis-specific phosphoproteins was coupled with the condensation of chromatin. The BHK temperature-sensitive mutant tsBN2 also completed multiple entries into mitosis while arrested during DNA replication and raised to the restrictive temperature. Periodic mitotic events occurred even when BHK cells were exposed to low concentrations of serum or cycloheximide, conditions that prevent the cycling of BHK cells by blocking their entry into S phase. These results suggest that an oscillation governing the activation and inactivation of mitotic factors can be generated in mammalian cells and uncoupled from the G1 and DNA replication events of the normal cell cycle. This system will be useful for examining the molecular nature of mitotic factors.

The clam embryo protein cyclin A induces entry into M phase and the resumption of meiosis in Xenopus oocytes

Fertilized clam embryos synthesize several new cell-cycle-related proteins. The cloned cDNA and derived amino acid sequences of one of these, cyclin A, are presented here. Immunoblots with an anti-cyclin A antibody reveal that cyclin A is undetectable in oocytes, appears within 15 min of fertilization, and is destroyed near the end of each meiosis and mitosis. We directly tested the ability of cyclin A to induce M phase by injecting SP6 cyclin A mRNA into Xenopus oocytes, which are arrested at the G2/M border of first meiosis. The injected mRNA was translated, with the result that the Xenopus oocytes entered meiosis. These findings indicate that the rise in cyclin A plays a direct and natural role in driving cells into M phase.

Fluorine-NMR studies of chimpanzee hemoglobin

It has been demonstrated that 4-fluorophenylalanine, a known inhibitor of protein synthesis, becomes incorporated into hemoglobin when present in the diet of a chimpanzee. 19F-NMR spectra of various forms of this protein show well-resolved lines, each line presumably corresponding to a unique phenylalanine/fluorophenylalanine position of the primary sequence. Fluorine chemical shifts and, by implication, tertiary structures vary with the oxidation state and ligand.

Studies on cell division in mammalian cells: VI. A temperature-sensitive mutant blocked in both G1 and G2 phases of the cell cycle

A temperature-sensitive mammalian cell cycle mutant with blocks in G1 and G2 phases of the cell cycle has been isolated in culture. When shifted from the permissive temperature of 33 degrees C to the nonpermissive temperature of 39 degrees C, the fraction of cells initiating DNA synthesis as well as the fraction of cells entering mitosis decreased rapidly. Combined cytophotometric and autoradiographic analysis on the cells at 39 degrees C showed that G1 cells, with the exception of those in late G1, were arrested in that phase. Cells is S phase at the time of temperature shift, together with the late g1 cells which subsequently entered S, continued through S into G2, but were blocked in that phase of the cell cycle and unable to initiate mitosis. Those cells already in mitosis completed cell division at 39 degrees C. The G1 block point of ts-550C was found to be located after the serum starvation and isoleucine deprivation arrest points, approximately 3 h before initiation of DNA synthesis.

The control of growth of mouse mastocytoma cells by N6,O2'-dibutyryladenosine cyclic 3',5'-monophosphate

Addition of N6,O2'-Dibutyryladenosine cyclic 3',5'-monophosphate (DB cyclic AMP) plus theophylline or transfer to medium containing 0.2% serum slowed the growth of cultured mouse mastocytoma cells and eventually arrested their growth in G1 phase. Examination of the properties of cells arrested by either procedure suggested that the drugs arrested cells in G1 phase 1.5-2 h after the point of low serum arrest. Cycloheximide prevented the recovery of cell growth after low serum or drug-induced arrest demonstrating that protein synthesis was necessary to pass either growth restriction point. Cordycepin also prevented drug-arrested cells from progressing into cycle indicating a requirement for RNA synthesis to overcome the drug-induced growth arrest. Evidence is also presented that DB cyclic AMP prevented the cells receiving a pulse of calcium necessary to proceed past the DB cyclic AMP-sensitive growth restriction point. It is suggested that high cyclic AMP levels prevent mastocytoma cells from receiving a surge of calcium in G1 phase that is necessary if the cells are to proceed to S phase and eventually divide.

The molecular basis of drug-induced G2 arrest in mammalian cells

The purpose of this review was to focus mainly on the molecular events related to the progression of cells through the G2 period to examine the cause for G2-arrest in mammalian cells after exposure to various anticancer drugs. With few exceptions, most of the eukaryotic cells exhibit a G2 period in their life cycles. The G2 period, which separates S phase from mitosis, represents the time necessary for the synthesis of the various components related to the condensation of chromosomes, assembly of the mitotic spindle, and cytokinesis. Continued synthesis of RNA and protein is necessary for the successful completion of G2 and the initiation of mitosis. Inhibition of RNA and protein synthesis, replacement of phenylalanine by its analog paraversible G2 arrest in cultured cells. Exposure of cells to certain antineoplastic drugs also blocks cells preferentially in G2. This irreversible drug-induced G2 arrest is associated with extensive chromosome damage. The G2-arrested cells were found to be deficient in certain proteins that may be specific for the G2-mitotic transition. These mitotic or chromosome condensation factors synthesized during the G2 period, reach their maximum levels at mitosis. A preliminary characterization of the chromosome condensation factor revealed that it is a heat labile, Ca2+-sensitive, nondialyzable protein with a sedimentation value of 4-5S.

Correlation between cell cycle duration and RNA content

The metachromatic fluorochrome acridine orange was used to differentially stain DNA and RNA in Chinese hamster ovary (CHO) cells and in mitogen-stimulated human lymphocytes during their progression through the cell cycle. Green and red fluorescence of individual cells, representing cellular DNA and RNA, respectively, was measured by flow cytometry. CHO cells were synchronized by selective detachment at mitosis. Their rate of progression through G1 and subsequently through S phase correlated with the content of stainable RNA. The mean duration of the G1 phase was 5.2 hours for cells with high RNA content (highest 25 percentile population) and 8.1 hours for cells with low RNA (lowest 25 percentile). The duration of S phase was 5.9 and 7.5 hours for high- and low-RNA, 25 percentile subpopulations, respectively. Lymphocytes synchronized at the G1/S boundary by hydroxyurea or 5-fluorodeoxyuridine showed extremely high intercellular variation with respect to content of stainable RNA. After release from the block they traversed S phase at rates linearly proportional to the content of stainable RNA. The duration of S phase was five hours for cells with high RNA-, six to nine hours for cells with moderate RNA- and up to 27 hours for cells with minimal RNA-content. The data suggest that the rate of progression through the cell cycle of individual cells within a population may be correlated with the number of ribosomes per cell.

A serum factor requirement for the passage of cultured Vero cells through G2

When Vero cells, a line derived from and African Green Monkey kidney, are grown under conditions where the saturation density is limited by serum, they deplete the growth medium of a factor necessary for cell division. The factor is a component of serum. When Vero cells are plated at low density (2 X 10(4)/cm2) in this depleted growth medium (after dialysis against serum-free Dulbecco's Modified Eagle's Medium) they initiate an unbalanced program of growth. Protein synthesis proceeds at the same rate as parallel cells in fresh serum, and and the cells accumulate protein as a function of time. DNA synthesis is also initiated in these cells, and the amount of DNA per cell increases for the next four days plating. However the cells quickly stop dividing. Measurements of DNA per cell using microspectrofluorometry show that the cells are accumulating in the late S and G2 period during this time. Thus we conclude that these cells cannot pass through a transition point in G2. When fresh serum is added to cells after three days in depleted growth medium, they divide before they begin to synthesize DNA. This further confirms that they are in late S and G2. Cell division is promoted in Vero cells in depleted growth medium by bovine fetuin, and to a lesser extent by bovine albumin. Cell division is not promoted by insulin, hydrocortisone, dexamethasone, linolenic acid, calcium, and typsin inhibitor form ovomucoid. From these data we conclude that transit through G2 requires the prescence of an extracellular factor.

Localization in the cell cycle of the antimitotic action of the pyrrolizidine alkaloid, lasiocarpine and of its metabolite, dehydroheliotridine

The antimitotic action of the pyrrolizidine alkaloid lasiocarpine on rat liver parenchyma was investigated using as the experimental model the wave of mitosis produced in liver by a single dose of thioacetamide. A single low dose of lasiocarpine administered two weeks before the thioacetamide, almost completely inhibited the mitotic wave without inhibiting to the same extent the preceding wave of DNA synthesis. By the use of selective inhibitors and radioisotope labelling, the location of the mitotic block was found to be either in the latter half of the DNA synthetic phase, S, or early in G2, the post-synthetic phase. The mitotic wave was similarly inhibited by pretreatment of the rats with a single injection of dehydroheliotridine, a pyrrolic metabolite of heliotridine-based pyrrolizidine alkaloids.

Vaccinia virus infection of synchronized pig kidney cells

Vaccinia virus replication was studied in pig kidney (PK-15) cells synchronized by excess thymidine treatment. It was found that virus replication with concomitant inhibition of mitosis can occur at any period in the life cycle of the cell except for the narrow span of time from late prophase through telophase. Cells infected at this time continue to divide, and vaccinia does not replicate until cell division is complete.

Studies on the maintenance of oral development in Tetrahymena pyriformis GL-C. II. The relationship of protein synthesis to cell division and oral organelle development

The effects of puromycin on synchronized Tetrahymena pyriformis were investigated at two different concentrations, 43 microg per ml and 430 microg per ml. The rate of incorporation of histidine-(14)C into hot TCA-insoluble material was reduced by 30% at the low concentration and by 80-90% at the high concentration. The rate of oxygen uptake was lowered by only 10-20% at both concentrations. Cell division was prevented at both concentrations, if the drug was added prior to a "transition point" at about 45 min after the end of the synchronizing treatment. Development of "anarchic field" oral primordia was arrested, while primordia in early stages of membranelle differentiation were resorbed. Resorption began shortly after addition of the drug, and proceeded most rapidly at the lower concentration. If the drug was added after the "transition point," cell division and oral primordium formation were completed with only slight delay at the low concentration, and with considerable delay (in some cases complete arrest) at the high concentration. The results thus indicate that protein synthesis is involved in the later as well as the earlier stages of development; what specially characterizes the earlier stages, prior to the "transition point," is a dramatic response to partial inhibition of protein synthesis. It is suggested that this response involves the activation or release of a latent intracellular degradative system which is specific for developing structures.