Animal cells reversibly permeable to small molecules

The potential of nucleotide analogs as inhibitors of retroviruses and tumors

The biologically active form of most purine or pyrimidine analogs is the nucleoside 5'-mono, di- or triphosphate. The nucleoside form is most often administered because of the ease with which it penetrates cells by facilitated transport. However, many nucleoside derivatives fail to exhibit significant antiviral or antitumor activity because they are not phosphorylated by cellular enzymes to the active nucleotide form. In this review, the potential use of suitable nucleotide analogs as selective inhibitors of ribonucleotide reductase and viral reverse transcriptase is considered. Masked nucleotides such as phosphoramidates or methyl phosphates could be employed to allow transport across cellular membranes. Furthermore, phosphonocarboxamide, phosphonoformate or sulfamidophosphoramidate may mimic nucleotide di- and triphosphates. Tumor cells and virally infected cells are often more permeable to nucleotides and their analogs than normal cells, which could provide a therapeutic advantage. There could be considerable therapeutic potential for nucleotide analogs that can penetrate the tumor cell membranes and that are resistant to enzymatic hydrolysis and are non-incorporable into DNA or RNA.

Preapoptotic chromatin changes induced by ultraviolet B irradiation in human erythroleukemia K562 cells

Exponentially growing human erythroleukemia K562 cells were permeabilized and the dose dependent decrease of DNA synthesis rate was measured after ultraviolet (UV B, 290 nm) irradiation. Cells were able to overcome 2 and 5 J/m2 UV doses, partial recovery was observed at 15 J/m2, while at high (25 J/m2) UV dose replicative DNA synthesis remained suppressed. K562 cells were subjected to synchronization prior to and after UV irradiation (24 J/m2) and 18 fractions were collected by centrifugal elutriation. Cell cycle analysis by flow cytometry did not show early apoptotic cells after UV irradiation. The gradual increase in DNA content typical for non-irradiated cells was contrasted by an early S phase block between 2.2 and 2.4 C-values after UV irradiation. Cell cycle dependent chromatin changes after ultraviolet irradiation were seen as a fine fibrillary network covering the mainly fibrous chromatin structures and incompletely folded primitive chromosomes. Based on observations after UV irradiation and on earlier results with cadmium treatment and gamma irradiation, we confirm that typical chromatin changes characteristic to genotoxic agents can be recognized and classified.

Permeabilization of the plasma membrane of L1210 mouse leukemia cells using lithotripter shock waves

Permeabilization of L1210 cells by lithotripter shock waves in vitro was monitored by evaluating the accumulation of fluorescein-labeled dextrans with a relative molecular mass ranging from 3,900-2,000,000. Incubation with labeled dextran alone caused a dose- and time-dependent increase in cellular fluorescence as determined by flow cytometry, with a vesicular distribution pattern in the cells consistent with endocytotic uptake. Shock wave exposure prior to incubation with labeled dextran revealed similar fluorescence intensities compared to incubation with labeled dextran alone. When cells were exposed to shock waves in the presence of labeled dextran, mean cellular fluorescence was further increased, indicating additional internalization of the probe. Confocal laser scanning microscopy confirmed intracellular fluorescence of labeled dextran with a diffuse distribution pattern. Fluorescence-activated cell sorting with subsequent determination of proliferation revealed that permeabilized cells were viable and able to proliferate. Permeabilization of the membrane of L1210 cells by shock waves in vitro allowed loading of dextrans with a relative molecular mass up to 2,000,000. Permeabilization of tumor cells by shock waves provides a useful tool for introducing molecules into cells which might be of interest for drug targeting in tumor therapy in vivo.

Permeabilization of ultraviolet-irradiated Chinese hamster cells with polyethylene glycol and introduction of ultraviolet endonuclease from Micrococcus luteus

Chinese hamster V-79 cells were made permeable by treatment with polyethylene glycol and then incubated with a Micrococcus luteus extract containing ultraviolet-specific endonuclease activity. This treatment introduced nicks in irradiated, but not in unirradiated, deoxyribonucleic acid. The nicks remained open for at least 3 h; there was no loss of endonuclease-sensitive sites, and no excision of dimers as measured by chromatography was detected. In addition, there was no increase in ultraviolet resistance in treated cells. This suggests that the absence of a significant amount of excision repair in rodent cells is due to the lack of both incision and excision capacity.

Staphylococcal alpha toxin--recent advances

The elucidation of the amino acid sequence of alpha toxin in 1984 has greatly promoted our understanding of the basic biochemistry and interaction of this toxin with membranes. These aspects are discussed and the concept of alpha toxin as a channel forming protein is critically evaluated. The lethal action of alpha toxin has not yet been clarified, but the previously postulated action as a neurotoxin is not supported by recent observations.

Introduction and metabolism of pentose and hexose phosphates in permeabilized Morris hepatoma 5123TC cells

Metabolism of arabinose 5-P, ribose 5-P and glucose 6-P in permeabilized and resealed Morris hepatoma 5123TC cells was investigated by measuring the contribution of these compounds to nucleic acid biosynthesis. The level of [14C]-arabinose (non-phosphorylated) incorporation into nucleic acids was slight, presumably due to the low activity of the transport system or the absence or low activity of a specific 'kinase' enzyme. The permeabilizing procedure involved the brief treatment of Morris hepatoma 5123TC cells with lysolecithin and resulted in a cell population which was permeable to charged compounds i.e. sugar phosphates and nucleotides, that otherwise could not cross the plasma membrane. The permeabilized (and resealed cells) retained normal cellular morphology and intactness of specific organelles as judged by the maintenance of functional properties. Following permeabilization, these cells resealed when transferred back to normal growth medium, and continued to divide and increase at the same rates as control non-permeabilized cell cultures. The permeabilized cells incorporated deoxyribonucleotides ([methyl -3H]-TTP) into DNA at a linear rate of 0.047 nmol per 10(7) cells min-1, representing 90-100 per cent of the DNA synthesis rate in vivo. The permeabilization technique, when coupled with procedures to establish cell synchrony, permitted the comparative estimate of the contributions of [14C]-labelled arabinose 5-P, ribose 5-P and glucose 6-P to RNA, DNA, amino acids, CO2, lactate and sugar mono- and bisphosphates. The percentage of [14C]-isotope incorporated into total nucleic acids by these three labelled sugar phosphates were 2.3, 4.9 and 6.3 respectively. Possible reasons for the lower incorporation of 14C from arabinose 5-P are given. The results are consistent with the proposal that arabinose 5-P, an intermediate of the L-type pentose pathway activity of 5123TC cells, was incorporated into nucleic acids by its interconversion with ribulose 5-P and ribose 5-P and thus into PRPP. This study represents the first report of sugar phosphate as opposed to free sugar metabolism by tumour cells in culture.

Organellar DNA synthesis in permeabilized soybean cells

Cultured cells of Glycine max (L.) Merr. v. Corsoy were permeabilized by treatment with L-α-lysophosphatidylcholine (LPC). The permeabilized cells were capable of uptake and incorporation of deoxynucleoside triphosphates into DNA. Incorporation of exogenous nucleotides into DNA was linear for at least 90 minutes and the initial rate of incorporation approached 50% of the theoretical in vivo rate of DNA synthesis. However, DNA synthesis in the permeabilized cells was unaffected by the potent DNA polymerase α inhibitor, aphidicolin. Analysis of newly synthesized DNA by molecular hybridization revealed that only organellar DNA was synthesized by the permeabilized cells. The LPC treated cells were also permeable to a protein as large as DNase I. The permeabilized cells were capable of RNA and protein synthesis as indicated by incorporation of radiolabeled UTP and leucine, respectively, into acid-precipitable material.

Nephron segment and calcium as determinants of anoxic cell death in renal cultures

Proximal tubules of the S1, S2 and S3 segments, medullary thick ascending limbs of Henle's loop (MAL) and cortical collecting tubules (CCT) were individually microdissected from rabbit kidneys and cultured for seven days in hormonally defined media. Anoxia was induced by incubation of cultures in normal medium for 45 min at 25 degrees C in an atmosphere of nitrogen (N2), and cell death was measured by nigrosine dye uptake. Immediately after anoxia, cell death was highest in S3 and MAL segments greater than S2 greater than S1 = CCT. The combined effects of anoxia and substrate (glucose, vitamins, amino acid) omission determined after incubation of cultures in phosphate buffered saline containing Ca2+ and Mg2+ (PBS) for 45 min in N2 also showed differential killing dependent on segment of origin: MAL greater than S3 greater than S2 CCT greater than S1. The effects of in vitro "reflow" were tested by returning cells to their normal oxygenated culture media at 37 degrees C. After the 45 min of anoxia and four to six hr of reflow in normal calcium-containing media, all cells from each segment were dead. Reflow in media lacking calcium for two hr immediately after anoxia then followed by return to normal calcium-containing media was associated with the survival of a significant percentage of cells for 48 hr: S1 (35.3 +/- 2.0%), S2 (30.0 +/- 2.0%), S3 (46.2 +/- 3.0%), MAL (38.7 +/- 3.0%), CCT (28.2 +/- 2.0%).(ABSTRACT TRUNCATED AT 250 WORDS)

Permeability change in transformed mouse fibroblasts caused by ionophores, and its relationship to membrane permeabilization by exogenous ATP

Electrogenic ionophores have been found to induce membrane permeabilization in Swiss mouse 3T3 cells that had undergone spontaneous transformation (3T6 cells). Cells attached to plastic dishes were loaded with [3H] uridine, and then the medium was replaced by buffered salt solution at pH 7.8. The enhancement of membrane permeability was assayed by following the efflux of uridine nucleotides, normally impermeant substances. Titration with electrogenic ionophores, such as carbonylcyanide m-chlorophenylhydrazone (CCCP), SF-6847 and gramicidin D, markedly increased the membrane permeability within a very narrow range of ionophore concentration. Non-electrogenic ionophores, such as monensin and nigericin, did not affect membrane permeability. Measurements of the distribution of the lipophilic cation tetraphenylphosphonium (TPP+) between the cells and their environment implied that the remarkable increase in permeability took place within a narrow range of membrane potential (delta psi). The data could be explained by a delta psi threshold value, under which aqueous channels are opened in the plasma membrane. The effects exerted by electrogenic ionophores on the plasma membrane were found to be similar to those induced by exogenous ATP. In both cases rapid efflux of K+, influx of Na+ and reduction of delta psi preceded membrane permeabilization to low molecular weight, charged molecules, such as nucleotides. It is suggested that dissipation of delta psi induces conformational alterations in membranal components, and/or topological changes, such as aggregation of protein molecules, to form membranal aqueous channels. Electrogenic ionophores permeabilize both normal (3T3) and transformed (3T6) mouse fibroblasts, whereas ATP effects are specific for transformed cells. Thus, it is postulated that ATP acts via specific sites on the surface of transformed cells.

Nick-translation of metaphase chromosomes: in vitro labeling of nuclease-hypersensitive regions in chromosomes

Chinese hamster metaphase chromosomes were labeled by nick-translation, which involved pretreatment of metaphase chromosomes with low levels of DNase I followed by incubation with DNA polymerase I and radioactively labeled nucleotides. The labeled DNA was located on nuclease-hypersensitive regions of the chromosomes, as suggested by the following observations. (i) The labeled DNA was hypersensitive to the subsequent DNase I digestion. (ii) The labeled DNA contained no nucleosomes. DNA reassociation kinetic analysis suggested that the labeled DNA was enriched in repetitive DNA sequences. Base composition analyses showed that the labeled DNA was highly enriched in guanine and adenine residues, suggesting that the nick-translation reaction was asymmetrical and the strand enriched in purine was preferentially translated. Autoradiographic analysis revealed that the label was distributed on every chromosome, but there was a lower grain density on the Y chromosome, which is heterochromatic and exhibits a relatively low level of gene activity. The locations of silver grains on the Y chromosomes were generally consistent with that revealed by the in situ hybridization using [3H]cDNA synthesized from the total Chinese hamster messenger RNA. These observations suggest that a specific subset of genomic DNA on active chromatin is the preferred site of the nick-translation.

Mechanism of protection by verapamil and nifedipine from anoxic injury in isolated cardiac myocytes

This study was designed to determine whether slow Ca2+ channel blocking agents exert a direct protective effect on the anoxic myocardial cell and, if so, what the mechanism of protection is. Isolated Ca2+-tolerant rat cardiac myocytes were incubated under aerobic or anaerobic conditions, with or without verapamil or nifedipine, in the resting and contractile state. Protection against cell injury was assessed by preservation of rod-shaped morphology, cellular ATP levels, intracellular ionic composition, and lactate dehydrogenase release. Resting myocytes incubated anaerobically lost their rod-shaped appearance, accumulated Na+ and lost K+, and suffered a significant loss of cellular ATP. The release of lactate dehydrogenase into the medium was increased twofold, indicating significant membrane injury. Verapamil (1 microM) or nifedipine (1 microM) did not afford any protection against anoxic injury as measured by these parameters. Furthermore, on reoxygenation, anoxic verapamil- and nifedipine-treated myocytes had significantly higher cellular Ca2+ levels than control aerobic cells. When anoxic myocytes were paced at a rate of 300/min for 10 min, there were marked decreases in the number of rod-shaped cells and cellular ATP levels, whereas identically paced aerobic cells sustained no significant injury. Verapamil (1 microM) or nifedipine (1 microM) protected cells paced at 300/min from anoxic injury, but the cells were unable to sustain contraction rates at the frequency of the imposed pacing.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of DNA polymerase inhibitors on replicative and repair DNA synthesis in ultraviolet-irradiated HeLa cells

Aphidicolin specifically inhibits eukaryotic DNA polymerase alpha, while 2',3'-dideoxythymidine 5'-triphosphate (d2TTP) inhibits DNA polymerase beta and gamma but not alpha. 1-beta-D-Arabinofuranosylcytosine 5'-triphosphate (araCTP) inhibits both DNA polymerase alpha and beta although to a different extent. Here we measured the effects of these inhibitors on repair DNA synthesis of U.V.-irradiated HeLa cells by two different methods. Firstly, aphidicolin, 1-beta-D-arabinofuranosylcytosine (araC, a precursor of araCTP) and 2',3'-dideoxythimidine (d2Thd, a precursor of d2TTP) were added directly to the culture medium. In this case, aphidicolin and araC strongly inhibited replicative DNA synthesis of HeLa cells, and they also inhibited repair synthesis after U.V.-irradiation but to a much lesser extent. In contrast, high concentrations of d2Thd inhibited repair DNA synthesis to a higher extent than replicative DNA synthesis. Secondly, the active form of inhibitor, d2TTP, was microinjection directly into cytoplasm or nuclei or U.V.-irradiated HeLa cells. Microinjection of d2TTP effectively inhibited repair synthesis. The microinjection of d2TTP, into either cytoplasm or nucleus, strongly inhibited replicative synthesis. These results might indicate that multiple DNA polymerases are involved in repair synthesis as well as in replicative synthesis.

Effects of extracellular calcium removal and anoxia on isolated rat myocytes

Calcium-tolerant myocytes were isolated from adult rat ventricles by successive perfusion and incubation with buffer containing collagenase and hyaluronidase. Greater than 70% of the cells excluded trypan blue, maintained normal morphology, and contracted in response to an externally applied electric field. We have characterized metabolic defects present in isolated calcium-tolerance myocytes when exposed to low concentrations of extracellular calcium under aerobic and anaerobic conditions. In control cells exposed to 1.25 mM Ca2+, the following metabolic parameters were measured (in mumol/g protein): adenosine triphosphate (ATP) 28.8 +/- 3.3, creatine phosphate (CrP) 49.1 +/- 7.5, intracellular Na+ 37.7 +/- 8.1, intracellular K+ 352.9 +/- 49.3, cellular Ca2+ 12.3 +/- 1.8, as well as rate of protein synthesis 0.34 +/- 0.03 mumol . g protein-1 . h-1. In aerobic cells incubated in medium without added Ca2+, the corresponding values (in mumol/g protein) were ATP 27.9 +/- 4.4, CrP 25.3 +/- 4.3, intracellular Na+ 130.9 +/- 23.1, intracellular K+ 217.2 +/- 32.0, cellular Ca2+ 3.9 +/- 1.0, and rate of protein synthesis 0.09 +/- 0.02 mumol . g protein-1 . h-1. These data indicated major metabolic aberrations in myocytes exposed to medium low in Ca2+ (less than 10 microM). Metabolic depression was most severe in cells incubated in the absence of both Ca2+ and O2. It is postulated that Ca2+ removal resulted in an increase in Na+ and K+ permeability, causing a net gain of intracellular Na+ and loss of intracellular K+. These ionic shifts might stimulate the activity of membrane-associated Na+-K+-ATPase, accounting for lower levels of CrP.

Characterization of isolated rat-liver cells made permeable with filipin

When isolated rat-liver cells were incubated for 1 min at 37 degrees C with filipin at a concentration of 50 microM, the plasma membrane became permeable to sucrose, inulin, glycerol 3-phosphate and other low-molecular-weight compounds. Upon removal of the filipin and subsequent incubation of the cells at 37 degrees C there was a gradual leakage of lactate dehydrogenase from the cells. However, the leakage of lactate dehydrogenase could be prevented for about 10 min by including glutathione and ATP in the incubation medium. The filipin-treated cells were able to metabolize phosphorylated sugars. The conversion of fructose 1,6-bisphosphate to fructose 6-phosphate, glucose-6-phosphate and glucose was inhibited by AMP but not by high concentrations of fructose 1,6-bisphosphate. The results indicate that filipin-treated cells can be used to study the kinetic parameters of enzymes in their macromolecular environment in situ.

Permeabilization of ultraviolet-irradiated Chinese hamster cells with polyethylene glycol and introduction of ultraviolet endonuclease from Micrococcus luteus

Chinese hamster V-79 cells were made permeable by treatment with polyethylene glycol and then incubated with a Micrococcus luteus extract containing ultraviolet-specific endonuclease activity. This treatment introduced nicks in irradiated, but not in unirradiated, deoxyribonucleic acid. The nicks remained open for at least 3 h; there was no loss of endonuclease-sensitive sites, and no excision of dimers as measured by chromatography was detected. In addition, there was no increase in ultraviolet resistance in treated cells. This suggests that the absence of a significant amount of excision repair in rodent cells is due to the lack of both incision and excision capacity.

Permeabilization of animal cells for kinetic studies of intracellular enzymes: in situ behavior of the glycolytic enzymes of erythrocytes

Intracellular enzymes in erythrocytes can be made accessible for in situ kinetic studies by treating the cells with bifunctional reagents to crosslink proteins, thus creating a network that allows subsequent permeabilization by delipidation without escape of intracellular proteins. Dimethyl suberimidate, dimethyl 3,3'-dithiobispropionimidate, and toluene-2,4-diisocyanate have been used successfully as crosslinking reagents, and digitonin has been used for delipidation. In a systematic study of the in situ behavior of the 11 glycolytic enzymes of rat erythrocytes, it was observed that Km and Vmax values for the majority of the enzymes are essentially the same in situ as in vitro. Lactate dehydrogenase (L-lactate:NAD+ oxidoreductase, EC 1.1.1.27) is inhibited by excess of pyruvate as much in situ as in vitro. Hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) was allosterically inhibited by glucose 6-phosphate nearly as much in situ as in vitro but was not affected by 2,3-biphosphoglycerate. The allosteric properties of 6-phosphofructokinase (ATP:D-fructose 6-phosphate 1-phosphotransferase, EC 2.7.1.11), glyceraldehyde-phosphate dehydrogenase [D-glyceraldehyde-3-phosphate:NAD+ oxidoreductase (phosphorylating), EC 1.2.1.12], and pyruvate kinase (ATP: pyruvate 2-O-phosphotransferase, EC 2.7.1.40) in situ were qualitatively similar to those observed in vitro, but some important quantitative differences were noticed. Particularly striking was the much greater activity of phosphofructokinase in situ compared to that in vitro at physiological concentrations of effector metabolites.

Multienzyme complex for metabolic channeling in mammalian DNA replication

In the DNA-synthesizing phase (S phase) of CHEF/18 Chinese hamster embryo fibroblast cells, six enzymes associated with DNA metabolism, including DNA polymerase (deoxynucleoside triphosphate:DNA deoxynucleotidyl-transferase, EC 2.7.7.7), were largely localized in the nuclear region (karyoplasts). By contrast, in quiescent and G1 phase cells these enzymatic activites were mainly absent from the nucleus and were recovered in the cytoplasmic portion (cytoplasts). These nuclear (but not cytoplasmic) enzymatic activities cosedimented rapidly on sucrose density gradients. Further, the rapidly sedimenting enzyme activities were unique to cells in S phase. An organized supramolecular structure that allows channeling of metabolites into DNA was demonstrated by kinetics of nucleotide incorporation. "Permeabilized" cells selectively channeled incorporation of ribonucleoside diphosphates into DNA in preference to deoxyribonucleoside triphosphates. Deoxyribonucleoside triphosphate incorporation occurred when ribonucleoside-diphosphate reductase (2'-deoxyribonucleoside-diphosphate: oxidized-thioredoxin 2'-oxidoreductase, EC 1.17.4.1) activity was abolished by hydroxyurea. Our interpretation is that during DNA replication, the nucleus contains a complex of DNA precursor-synthesizing enzymes juxtaposed with the "replication apparatus" comprising DNA polymerase, other enzymes, and structural proteins. Functional integrity of this structure is impaired when one of its essential components is inactivated. We propose the name "replitase" for this multienzyme complex for DNA replication and suggest that it incorporates precursors rapidly and efficiently. Possibly its assembly signals the initiation of the S phase of the cell cycle.

Nature of virally mediated changes in membrane permeability to small molecules

1. The changes in membrane permeability to small molecules caused by Sendai virus [Pasternak & Micklem (1973) J. Membr. Biol. 14, 293-303] have been further characterized. The uptake of substances that are concentrated within cells is inhibited. Choline and 2-deoxyglucose, which become phosphorylated, and aminoisobutyrate and glycine, which are driven by a Na+-linked mechanism, are examples. The uptake of each compound under conditons where its diffusion across the plasma membrane is rate-limiting is stimulated by virus. Choline, 2-deoxyglucose and amino acids at high concentration, amino acids in Na+-free medium, and most substances at low temperature, are examples. It is concluded that virally mediated decrease of uptake is due to one of two causes. Substances that are accumulated by phosphorylation are not retained because of leakage of the phosphorylated metabolites out of cells. Substances that are accumulated by linkage to a Na+ gradient are no longer accumulated because of collapse of the gradient resulting from an increased permeability to Nat 2. Increased permeability to K+ and Na+ results in (a) membrane depolarization and (b) cell swelling. The latter event leads to haemolysis (for erythrocytes) and can lead to giant-cell (polykaryon) formation (for several cell types). 3. Recovery of cells can be temporarily achieved by the addition of Ca2+; permanent recovery requires incubation for some hours at 37 degrees C. 4. The possible significance of virally mediated permeability changes, with regard to clinical situations and to cell biology, is discussed.

The use of antibody and complement to gain access to the interior of presynaptic nerve terminals

Treatment of synaptosomes with sera containing antibodies (Ab) directed against synaptosomal membranes and complement (C) alters the plasma membrane so that it becomes selectively permeable to small molecules and ions but not to proteins. When synaptosomes are incubated with Ab and C, a rapid release of intracellular K occurs. This release does not occur after treatment with antiserum alone, or with normal serum + C. Ab + C treatment releases approximately the same amount of K as does detergent treatment or hypotonic lysis, two procedures that extensively disrupt the plasma membrane. The selectivity of the complement-induced lesion is consistent with the equivalent pore radius determined in other systems (Michaels and Mayer 1978; Sears et al. 1964). The lesions are large enough to allow the rapid permeation of small ions, but too small to permit the escape of the soluble cytoplasmic enzyme, lactate dehydrogenase. In addition, electron microscopic studies indicate that Ab + C treatment does not lead to gross morphological disruption of the synaptosomes. Ab + C treated synaptosomes are also permeable to calcium and ATP, as demonstrated by the stimulation of Ca sequestration into endoplasmic reticulum when 45Ca and ATP are added to the incubation medium.

Protein synthesis in transformed 3T3 cells permeabilized by exogenous ATP

Exogenous ATP has been shown to cause a rapid and reversible increase in permeability in transformed 3T3 cells (3T6 and SV3T3) but not in untransformed 3T3 cells. The cells remain viable, but lose intracellular acid-soluble pools. Treatment of transformed cells with ATP greatly reduces incorporation of 14C-leucine into protein, which is restored by the incubation of the cells with Dulbecco's modified Eagle's medium or by the external additions of certain ions and energy sources. tRNA is not required for the restoration of protein synthesis. In the permeabilized cells the energy for protein synthesis can be provided by glycolysis, oxidative phosphorylation, or direct addition of ATP. These studies demonstrate the usefulness of this method for studying the control of metabolism and macromolecular synthesis in monolayer cultures of transformed mammalian cells.

Studies on the penetration of mammalian cells by deoxyribonucleoside-5'-phosphates

We have tested the ability of [5'-32P]-deoxyribonucleoside monophosphates (dNMPs) to penetrate living mouse fibroblast L cells and human HeLa cells. Under the conditions of our experiments, small numbers of apparently intact dNMP molecules appeared to penetrate into the interior of L cells and be incorporated into DNA. This incorporation was not due to mycoplasma contamination nor to extracellular hydrolysis of the dNMPs followed by resynthesis inside the cell. Under these same conditions, penetration of HeLa cells by intact dNMPs did not occur to a significant extent. However, HeLa cells were capable of hydrolyzing extracellular dNMPs to Pi and deoxyribonucleosides at a much faster rate than L cells. These experiments provide a starting point for attempts to specifically label the DNA in intact, living eukaryotic cells with [32P]-dNMPs.

Replication of mammalian DNA in vitro. Evidence for initiation from fiber autoradiography

We have used fiber autoradiography to examine the DNA product made in vitro in a lysed cell system. CHO cells were treated with 0.01% Brij-58 and the lysates were incubated at 30 degrees C in a complete reaction mixture for in vitro DNA synthesis with [3H]thymidine triphosphate ([3H]TTP) as the radioactive tracer. Fiber autoradiograms prepared from the DNA showed that it was synthesized on tandemly arranged replication units that were of average size of 20 micrometers, very similar to the size of units found in vivo. The rate of replication fork movement was 25--50% of the in vivo rate. More than 80% of forks stopped functioning by 15 min, and 95% stopped by 60 min. This suggests that synthesis is halted by premature terminations. Evidence for new initiations was provided by replication units with labeled origins in DNA synthesized in an in vitro reaction in which radioactivity was omitted for the first 10 min of incubation. This, plus the observations that the distance between initiation points (replication unit size) is not increased and that premature termination accounts largely for the cessation of synthesis, suggest that significant initiation takes place in this in vitro replication system.

Increased nuclease activity in cells treated with pppA2'p5'A2'p5' A

A series of 2'-5'-linked oligo(adenylic acid) triphosphate (2'-5' A) inhibitors of protein synthesis were described recently. These inhibitors are synthesized from ATP by an enzyme activated in interferon-treated cell extracts or rabbit reticulocyte lysates by double-stranded RNA. We show here that 2'-5' A is a potent inhibitor of protein synthesis in intact cells of different origin (human, monkey, hamster, and mouse). At a concentration of 10 nM (in AMP equivalents), protein synthesis is inhibited by 50-85%. There is also a secondary effect on the total RNA synthesis which becomes evident several hours after inhibition of protein synthesis. All of these effects, however, are transient and, after a recovery period, both RNA and protein synthesis resume rates comparable to the appropriate controls. A nuclease activity is detected in cells after treatment with 2'-5'A. The total polyadenylylated RNA is much reduced in comparison to that from untreated cells, and electrophoretic analysis in polyacrylamide slab gels provides evidence for its degradation. Similarly, there is an apparent degradation of ribosomal RNA. Consistent with these results, extracts from cells that had been treated with 2'-5'A manifest an enhanced nuclease activity in vitro on incubation with exogenous RNA. Here, we propose that, as in cell-free systems, the mechanism of action of 2'-5'A in intact cells involves activation of a nuclease. This activation is transient, but the nuclease remains sensitive to further activation by the inhibitor.