Distribution of 5-Bromodeoxyuridine in the DNA of Rat Embryo Cells

Rat embryo cells were treated with (3)H-labeled 5-bromodeoxyuridine (BrdU) in order to determine the nature of substitution and distribution of the analog in DNA. At optimal oncornavirus-inducing doses of BrdU (10(-4) M), density gradient centrifugation and DNA.DNA reassociation experiments revealed extensive (>90%) and uniform base substitution in repetitive, intermediate, and unique DNA sequences. Suboptimal doses (10(-7) M) of [(3)H]BrdU resulted in less than 5% thymine replacement and nonrandom labeling of DNA. Treatment of cells with 10(-7) M [(3)H]dT resulted in relatively uniform labeling throughout all DNA sequences. At low concentrations, BrdU was incorporated predominantly within the repetitive and intermediate DNA of rat embryo cells. Moreover, the single-copy DNA sequences were lightly substituted and reassociated extensively only in the presence of excess unlabeled DNA. Such specificity of base substitution may be related to the unusually selective nature of BrdU effects on animal cells.     

Increase in nuclear thyroid and glucocorticoid receptors and growth hormone production during the deoxyribonucleic acid synthesis phase of the cell growth cycle

We have previously described a significant increase in cellular DNA and nuclear T3 receptor concentration in S phase cultures of GC cells, a rat pituitary cell line that produces GH. We have now measured GH production and some aspects of GH regulation in asynchronous cells and in cultures that were synchronized in early S phase by 25-h treatment with 2 mM thymidine (dT). Cellular DNA and both cellular and medium GH increased significantly at the end of dT treatment and for 3-6 h after removal of dT (S phase). Pulse-labeling experiments with [3H]leucine followed by specific immunoprecipitation of rat GH suggested that an S phase-associated 87% increase in GH could be attributed predominantly to a 101% increase in the GH synthesis rate. The relative GH synthesis rate (GH synthesis divided by total protein synthesis) increased from 0.85% to 1.70% during the S phase. Since cortisol (115 nM) augmented GH production in both asynchronous and S phase cultures, studies of glucocorticoid receptor concentration were also carried out. In comparison with asynchronous cultures, a significant increase in glucocorticoid receptor was identified in the nuclear fraction but not in the cytosol of cultures synchronized in the S phase. Studies with cells grown in media which were selectively depleted or repleted with T3 and/or cortisol and experiments with the glucocorticoid antagonist 17 alpha-methyltestosterone suggested that T3 was primarily responsible for augmented GH during the S phase and that cortisol modulated the amplitude of this response. Our studies suggest that increases in DNA, nuclear receptors for T3 and glucocorticoids, and GH synthesis occur in GC cell cultures synchronized in the S phase. We conclude that the GH production rate observed in asynchronous cultures may be an integrated value for all phases of the cell cycle, each of which may have a distinctive rate of hormone synthesis.     

Distribution of 5-bromodeoxyuridine and thymidine in the DNA of developing chick cartilage.

In order to study the mechanism of the irreversible effects of BrdUrd on the differentiation of limb bud mesenchyme to cartilage, the reannealing behavior of DNA obtained from such cells was examined. Cells incubated with [3H]thymidine ([3H]dThd) during days 1 and 2 of culture incorporated label into repetitive, moderately repetitive, and unique classes of DNA. In contrast, when 5-bromo-2'-[3H]deoxyuridine ([3H]Brd Urd) was added during the first 48 hr (in the presence of 32 muM BrdUrd), the label was preferentially incorporated into a late moderately repetitive region. Simultaneous incubation of unlabeled BrdUrd and [3H]dThd revealed a selective inhibition of [3H]dThd incorporation into moderately repetitive regions. Cultures incubated during days 3 and 4 with [3H]dThd incorporated label into all three classes of DNA; however, when [3H]dThd was present during days 3 and 4 in cultures previously incubated with BrdUrd during days 1 and 2, the [3H]dThd was incorporated preferentially in the late moderately repetitive region. The melting behavior of this reannealed DNA was identical with that of the reannealed 1-2 day [3H]BrdUrd-labeled, late moderately repetitive DNA. Turnover experiments revealed that whereas there was no loss of [3H]deoxycytidine or [3H]dThd, 37% of [3H]BrdUrd activity was lost from the DNA in 2 days after removal of the isotopes.     

Gamma-Aminobutyric Acid Binding to Receptor Sites in the Rat Central Nervous System

[(3)H]Gamma-aminobutyric acid (GABA) binds to synaptic membrane fractions of rat brain in a selective fashion representing an interaction with postsynaptic GABA receptors. Inhibition of [(3)H]GABA binding by a variety of amino acids closely parallels their ability to mimic the synaptic inhibitory actions of GABA and does not correlate with their relative affinity for the presynaptic synaptosomal GABA uptake system. [(3)H]GABA binding is saturable with an affinity constant of about 0.1 muM. The GABA antagonist bicuculline inhibits [(3)H]GABA binding with half maximal effects at 5 muM, whereas it requires a concentration of 0.5 mM to reduce synaptosomal GABA uptake by 50%. In subcellular fractionation experiments [(3)H]GABA binding is most enriched in crude synaptic membranes. [(3)H]GABA binding is greatest in the cerebellum, least in the spinal cord and medulla oblongatapons, with intermediate values in the thalamus, hippocampus, hypothalamus, cerebral cortex, midbrain, and corpus striatum.     

Cycle-related toxicity and transformation in 10T1/2 cells treated with N-methyl-N''-nitro-N-nitrosoguanidine.

Exposure of C3H 10T1/2 Cl 8 cells, synchronized by release from confluence-induced arest of proliferation, to different concentrations of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) for 30 min at various points during the cell cycle causes dose-dependent toxicity (decrease in relative colony-forming efficiency or "survival") that increases linearly during the first G1 phase, reaches a maximum in early to middle S phase, and decreases during late S. In the course of the second S phase, toxicity again becomes maximal. The transformation rate (type III foci) increases and decreases with a similar pattern, increasing during the first G1 phase to a maximum during early S phase, subsequently decreasing, and then increasing again during the second S phase. Although periods of maximal toxicity and transformation roughly coincide with some portion of the S phase, the mechanisms underlying these phenomena appear to differ for the following reasons: (a) toxicity is linearly related to dose of MNNG, whereas the latter is linearly related to the logarithm of transformation rate, and (b) the ratio between toxicity and transformation varies with the cycle phase and the dose of MNNG.     

Reassessment of histone gene expression during cell cycle in human cells by using homologous H4 histone cDNA.

The representation of H4 histone mRNA sequences in RNAs isolated from G1 and S phase HeLa cells was assessed by use of a homologous H4 histone cDNA. S phase cells were obtained by double thymidine block, and G1 cells were obtained by double thymidine block or mitotic selective detachment. Nuclear and cytoplasmic RNAs from S phase cells hybridized with H4 histone cDNA as did nuclear and cytoplasmic RNAs from G1 cells synchronized by double thymidine block. In contrast, significant levels of hybridization were not observed between H4 histone cDNA and nuclear, polysomal, or postpolysomal cytoplasmic RNAs of G1 cells synchronized by mitotic selective detachment. Double thymidine block yields a G1 cell population containing 20-25% S phase cells whereas the G1 population obtained by mitotic detachment contains less than 0.1% S phase cells. The ability of H4 histone cDNA to hybridize with the RNAs from G1 cells obtained after release from double thymidine block can therefore be explained by the presence of S phase cells in such a G1 population--an artifact of the synchronization procedure. We interpret these results to be consistent with the presence of H4 histone mRNA sequences during the S but not G1 phase of the cell cycle in continuously dividing HeLa S3 cells.     

Replication of alpha and beta globin DNA sequences occurs during early S phase in murine erythroleukemia cells.

Murine erythroleukemia cells (MELC) can be induced to express the characteristics of erythroid differentiation by a variety of agents. Previous studies indicate that an action of inducer, occurring during early S phase, may be critical to the expression of differentiated characteristics such as initiation of accumulation of newly synthesized alpha and beta globin mRNAs. In this investigation, the time of replication of globin genes in MELC was studied. DNA was isolated from synchronous populations of cells obtained by centrifugal elutriation. Newly replicated DNA sequences were prepared from synchronized cells cultured for 1 1/2 hr with 5-bromodeoxyuridine; bromodeoxyuridine-containing DNA was isolated by CsCl gradient centrifugation. By employing cloned probes for hybridization to newly synthesized DNA, it was found that alpha and beta globin gene sequences are replicated early in S phase, while ribosomal RNA gene sequences are replicated to about the same extent in early, middle, and late S phases.     

The Binding Characteristics and Number of β-Adrenergic Receptors on the Turkey Erythrocyte

Turkey erythrocyte ghosts (empty membranes) possess a class of receptors that can bind both L-[(3)H]isoproterenol and DL-[(3)H]propranolol. The binding of [(3)H]isoproterenol to these receptors occurs with a dissociation constant of 0.15 muM and can be fully inhibited by 1 muM propranolol. The binding of [(3)H]propranolol occurs with a dissociation constant of 2.5 nM and can be fully inhibited by 0.2 mM DL-isoproterenol. Ligand binding is sensitive to sonication, boiling, and 8 M urea. The cells possess 500 to 1000 beta-adrenergic receptors per cell. Binding of propranolol to the beta-receptor was found to be stereospecific for the L stereoisomer. If one assumed a 1:1 relationship between beta-adrenergic receptors and adenylate cyclase, the turnover number of this adenylate cyclase would be close to 100 min(-1).     

Cell cycle specific fluctuations in adenosine 3'':5''-cyclic monophosphate and polyamines of Chinese hamster cells.

Chinese hamster V79 cells were synchronized by mitotic selection, which resulted in approximately 95% synchrony. The adenosine 3':5'-cyclic monophosphate level was elevated within 3 hr (G1 phase) and reached a level 2-fold higher than in early G1 within 6 hr (early S phase). An increase in ornithine decarboxylase activity (6-ornithine carboxy-lyase, EC 4.1.1.17), the initial enzyme in the polyamine biosynthetic pathway, was detected within 4 hr and was maximal at 8 hr. Since about 20% of the cells were labeled with [3-H]thymidine at 4 hr, ornithine decarboxylase exhibits cell-cycle specific activity starting in late G1 and continuing through middle S phase. The activity of S-adenosylmethionine decarboxylase (S-adenosyl-L-methionine carboxylase, EC 4.1.1.50) increased within 5 hr, i.e., early S phase. It is suggested on the basis of these data and other studies discussed herein that the increase in ornithine decarboxylase activity, which parallels closely the elevation in cyclic AMP, is an example of adenosine 3':5'-cyclic monophosphate-mediated protein synthesis.     

Tetraethylammonium: Voltage-dependent action on endplate conductance and inhibition of ligand binding to postsynaptic proteins

Tetraethylammonium (Et(4)N(+)) ions depressed the amplitude and accelerated the decay rate of spontaneously occurring and nerve-evoked endplate currents (EPCs) in frog sartorius muscle. The relationship between peak EPC amplitude and membrane potential became nonlinear in the presence of 100 muM Et(4)N(+), and with drug concentrations of 250 muM or greater the current-voltage relationship exhibited negative conductance in the hyperpolarized region. Et(4)N(+) modified the exponential dependence of the EPC decay on membrane potential such that the decays between -150 and -50 mV were abbreviated and voltage independent but remained near control levels at more positive membrane potentials. The minimal effective concentration of Et(4)N(+) for altering the EPC time course was 10, and maximal effects were attained with 100 muM. Little additional shortening in the EPC decay phase was detected on raising the drug concentration to 1000 muM. Acetylcholine noise analysis revealed a voltage-dependent reduction in the mean channel open time, which was comparable in magnitude to the shortening in the EPC decay, and a depression of single-channel conductance. In concomitant biochemical studies, Et(4)N(+) was found to inhibit the binding of both [(3)H]acetylcholine and [(3)H]perhydrohistrionicotoxin to receptor-rich membranes from the electric organ of Torpedo ocellata with K(i) values of 200 muM and 280 muM, respectively. These results suggest that Et(4)N(+) interacts with both the acetylcholine receptor and its associated ionic channel. The voltage-dependent actions of Et(4)N(+) are attributed to blockade of the ionic channel in closed as well as open conformation.     

Farnesol is utilized for isoprenoid biosynthesis in plant cells via farnesyl pyrophosphate formed by successive monophosphorylation reactions

The ability of Nicotiana tabacum cell cultures to utilize farnesol (F-OH) for sterol and sesquiterpene biosynthesis was investigated. [(3)H]F-OH was readily incorporated into sterols by rapidly growing cell cultures. However, the incorporation rate into sterols was reduced by greater than 70% in elicitor-treated cell cultures whereas a substantial proportion of the radioactivity was redirected into capsidiol, an extracellular sesquiterpene phytoalexin. The incorporation of [(3)H]F-OH into sterols was inhibited by squalestatin 1, suggesting that [(3)H]F-OH was incorporated via farnesyl pyrophosphate (F-P-P). Consistent with this possibility, N. tabacum proteins were metabolically labeled with [(3)H]F-OH or [(3)H]geranylgeraniol ([(3)H]GG-OH). Kinase activities converting F-OH to farnesyl monophosphate (F-P) and, subsequently, F-P-P were demonstrated directly by in vitro enzymatic studies. [(3)H]F-P and [(3)H]F-P-P were synthesized when exogenous [(3)H]F-OH was incubated with microsomal fractions and CTP. The kinetics of formation suggested a precursor-product relationship between [(3)H]F-P and [(3)H]F-P-P. In agreement with this kinetic pattern of labeling, [(32)P]F-P and [(32)P]F-P-P were synthesized when microsomal fractions were incubated with F-OH and F-P, respectively, with [gamma-(32)P]CTP serving as the phosphoryl donor. Under similar conditions, the microsomal fractions catalyzed the enzymatic conversion of [(3)H]GG-OH to [(3)H]geranylgeranyl monophosphate and [(3)H]geranylgeranyl pyrophosphate ([(3)H]GG-P-P) in CTP-dependent reactions. A novel biosynthetic mechanism involving two successive monophosphorylation reactions was supported by the observation that [(3)H]CTP was formed when microsomes were incubated with [(3)H]CDP and either F-P-P or GG-P-P, but not F-P. These results document the presence of at least two CTP-mediated kinases that provide a mechanism for the utilization of F-OH and GG-OH for the biosynthesis of isoprenoid lipids and protein isoprenylation.     

Isolation of the origin of replication associated with the amplified Chinese hamster dihydrofolate reductase domain.

Autoradiography of restriction digests of DNA labeled in early S phase indicates that replication of the amplified dihydrofolate reductase (DHFR) domain of methotrexate-resistant CHOC 400 cells initiates within a 6.1-kilobase pair (kb) EcoRI-doublet located on the 3' side of the DHFR gene. To localize the DHFR origin fragment, synchronized CHOC 400 cells were either pulse labeled with [3H]thymidine in vivo or permeabilized and incubated with [32P]dATP under conditions that support limited chromosomal DNA replication. The temporal order of replication of amplified fragments was determined by hybridization of the in vivo or in vitro replication products to cloned fragments spanning the earliest-replicating portion of the DHFR domain. At the G1/S boundary, the labeled products derived from the replication of amplified sequences, either in whole or permeabilized cells, are distributed about an amplified 4.3-kb Xba I fragment that maps 14 kb downstream from the DHFR gene. As cells progress through the S phase, bidirectional replication away from this site is observed. These studies indicate that the 4.3-kb Xba I fragment contains the origin of replication associated with the amplified DHFR domain.     

Binding of 25-hydroxycholesterol and cholesterol to different cytoplasmic proteins

Studies were carried out to determine whether or not oxygenated derivatives of cholesterol (e.g., 25-hydroxycholesterol) that specifically suppress the activity of 3-hydroxy-3-methylglutaryl-CoA reductase [mevalonate:NADP(+) oxidoreductase (CoA-acylating), EC 1.1.1.34], bind to a soluble component of the cytoplasm different from that which binds the nonsuppressor, cholesterol. Density gradient fractionation of the cytosolic fraction isolated from L cell cultures that had been incubated with low concentrations of 25-hydroxy[26,27-(3)H]cholesterol or [1,2-(3)H]cholesterol provided evidence for the existence of at least two different sterol-binding proteins. Bound cholesterol sedimented in a sucrose density gradient as two or more broad bands with coefficients of approximately 9 S and 21 S. Two relatively narrow bands of bound 25-hydroxycholesterol had sedimentation coefficients of 5 S and 8 S. Preincubation of the cells with a relatively high concentration of unlabeled 25-hydroxycholesterol altered the banding pattern of the 25-hydroxy[(3)H]cholesterol taken up during a subsequent incubation period by decreasing the size of the major (8S) band. Under these conditions, cholesterol did not affect the banding pattern of 25-hydroxy[(3)H]cholesterol. The density gradient banding pattern of bound [(3)H]cholesterol was only slightly affected by preincubating the cells with unlabeled cholesterol or 25-hydroxycholesterol. Both sterols appeared to be bound to proteins because the bound sterols were eliminated from cytosol that had been heated at 100 degrees , and their sedimentation coefficients were altered by proteolysis.     

Action of lycurim and pyrazofurin on hepatoma 3924A cells in culture

The purpose of this study was to test for a possible synergism in the antitumor action of the antimetabolite pyrazofurin (PF) and the alkylating agent lycurim (LY) [1,4-di(2'-mesyloxy-ethylamino)-1,4-dideoxy-m-erythritol dimethylsulfonate] on proliferating and resting rat hepatoma 3924A cells in tissue culture. Earlier studies showed that PF killed hepatoma cells only in the logarithmic phase of growth as determined by colony-forming ability. Now we report that LY killed hepatoma cells in both the exponential and plateau phases. Treatment of cultures in each phase for 1 or 2 hrs resulted in threshold exponential type dose-response curves, yielding Do values of 3 and 1 microM, respectively. LY is a cycle-specific drug, although fluctuations in cell age-dependent survival were also demonstrated. In synchronized cultures, cells were most sensitive to 0.1 microM LY in early G1 phase, but in higher concentration (5 microM), LY killed the cells during the entire cell cycle, especially in G1, early and mid S phases, and M phase. Lycurim and PF given together in their IC50 concentrations (0.016 and 0.085 microM, respectively) yielded synergistic cell kill in 7-day treatment of proliferating cultures. Two-hour treatment of exponentially growing cells with LY and PF at their IC50 concentrations (1 and 20 microM, respectively) also yielded synergistic killing action, but in plateau-phase cells only summation was observed. Synergistic activity of LY and PF in hepatoma 3924A cells was dependent on drug concentration and incubation time.     

The Homopolyadenylate and Adjacent Nucleotides at the 3′-Terminus of 30-40S RNA Subunits in the Genome of Murine Sarcoma-Leukemia Virus

Adenosine is the major 3'OH-terminal nucleoside of the 60-70S RNA genome of the murine sarcoma-leukemia virus, its 30-40S RNA subunits, and the poly(A) segments derived by RNase treatment of both RNA species, as determined by periodate oxidation-[(3)H]-borohydride reduction. The binding 30-40S RNA to oligo(dT)-cellulose suggests that most viral RNA subunits contain poly(A). The molecular weight of poly(A) derived from viral RNA by digestion with RNase and purified by affinity chromatography is 64,000-68,000, as determined by gel electrophoresis. From the size of poly(A) and the poly(A) content of viral RNA (1.6%), it is estimated that there is about one poly(A) segment for each viral 30-40S RNA subunit. The results of 3'-termini labeling with [(3)H]borohydride, in vivo labeling with [(3)H]adenosine, and base composition of [(32)P]poly(A) indicate that a homopoly(A) segment is located at the 3'-end of a 30-40S RNA subunit. The homogeneous poly(A) segments isolated from RNase T1 digests of 60-70S [(32)P]RNA consist of one cytidylate, one uridylate, and about 190 adenylate residues, while those isolated from RNase A digests consist exclusively of adenylate residues. These results indicate that -G(C,U)A(190)A(OH) is the 3'-terminal nucleotide sequence of the viral 30-40S RNA subunits.     

Evidence for a DNA Replicative Genome for RNA-Containing Tumor Viruses

The requirement for DNA synthesis in the reproduction of RNA tumor viruses was examined using 5-bromodeoxyuridine (BrdU). Addition of BrdU for the first 12 hr after exposure of cells to Rous sarcoma virus resulted in a large decrease in infectious progeny. However, incorporation of [(3)H]uridine into the viral RNA of virions was unaffected. Also, the numbers of virions visualized by electron microscopy were similar in BrdU-treated and untreated samples. Viral group-specific antigen was synthesized after BrdU treatment, and BrdU had little effect on transformation of cells by Rous sarcoma virus. The results support the hypothesis that the replicative genome of RNA-containing tumor viruses contains DNA.     

Alteration of Nucleoside Transport of Chinese Hamster Cells by Dibutyryl Adenosine 3′:5′-Cyclic Monophosphate

Cultured Chinese hamster ovary cells showed no significant change in generation time or fraction in the S-phase in the presence of 1 mM N(6),O(2')-dibutyryl adenosine 3':5'-cyclic monophosphate. Growth continued for at least two generations after expression of the morphological transformation induced by this cyclic AMP analog. Despite identical growth rates, apparent rates of DNA and RNA synthesis (incorporation of [(3)H]-thymidine or [(3)H]uridine) were reduced up to 15-fold in log phase by 1 mM cyclic nucleotide. [(3)H]Deoxycytidine incorporation was much less sensitive to dibutyryl cyclic AMP. Uptake studies with [(3)H]thymidine demonstrated an inhibition of transport rate dependent on the concentration of dibutyryl cyclic AMP in the growth medium. The rate of thymidine uptake at 1 degrees was decreased 21-fold by 1 mM cyclic nucleotide; half-maximal inhibition occurred at 6 muM. At 37 degrees , the pool size of acid-soluble thymidylate was strongly reduced by 1 mM cyclic nucleotide, and synergistic reduction of the pool size was found with 0.5 mM aminophylline. Phosphorylation of the acid-soluble intracellular label was unaffected by dibutyryl cyclic AMP. Inhibition of thymidine uptake is attributed to an observed decrease in thymidine kinase activity caused by growth in 1 mM dibutyryl cyclic AMP, and possibly to a simultaneous alteration in membrane permeability. Kinase-facilitated uptake of other metabolites may be regulated in a similar fashion by cyclic AMP.