Preferential inhibition by 5-bromodeoxyuridine of the synthesis of tyrosine aminotransferase in hepatoma cell cultures

Expression and amplification of cloned rat liver tyrosine aminotransferase in nonhepatic cells

A full-length cDNA for the rat liver enzyme tyrosine aminotransferase has been used to construct mammalian expression vectors by recombinant DNA techniques. These vectors, which have employed either a simian virus 40 or a Rous sarcoma virus promoter, were transfected into a variety of nonhepatic mammalian cell lines in culture. Transient expression of tyrosine aminotransferase was readily observed after transfection into monkey COS cells and mouse L cells. Stable clones that express cloned tyrosine aminotransferase have been isolated from mouse L cells, hamster Wg1a fibroblasts, and Chinese hamster ovary (CHO) cells. A vector capable of expressing both tyrosine aminotransferase and dihydrofolate reductase was stimulated to undergo amplification by treatment with methotrexate in a CHO cell line deficient in the latter enzyme. Levels of tyrosine aminotransferase as much as 50-fold higher than typically seen in glucocorticoid-induced hepatoma cells were achieved in some CHO clones by this technique. The tyrosine aminotransferase produced at these highly amplified levels appeared structurally normal and had no major harmful effects on the cells.

Anomalous neural differentiation induced by 5-bromo-2'-deoxyuridine during organogenesis in the rat

The influence of 5-bromo-2'-deoxyuridine (BrdU) on rat embryo development and neurogenesis was investigated using a rat conceptus culture system during organogenesis (pregnancy days 10-13). The embryos and visceral yolk sacs of conceptuses cultured with BrdU were examined for overall growth, morphological anomalies, incorporation of radiolabeled BrdU into DNA, and neurotransmitter enzyme activities in embryos. In addition, neural tubes from cultured whole embryos were isolated and mechanically dissociated into fragments and cultured again to assess neural cell differentiation into neuron-like cells. BrdU was found to incorporate differentially into embryonic and visceral yolk sac DNA with simultaneous stage-specific retardation and anomalous organogenesis in proportion to the increasing concentrations used. Neural tube differentiation of cultured embryos was markedly altered, and there were morphologically distinct neural anomalies. The neurite outgrowth from neuroblast cells (type 1) of explanted spinal neural tube fragments from BrdU-treated embryos was markedly reduced in length and number compared to those from similar areas of embryos grown without BrdU. In contrast, BrdU at the same doses did not affect differentiation of a number of neural tissue-related enzymes. These results indicate that BrdU incorporation into DNA of primordial embryonic cells significantly affects neurogenesis and differentiation of neurites from neuroblasts, which is a specific neural cytodifferentiation characteristic of neuronal cells.

Mutational and pseudomutational effects of 5-bromodeoxyuridine in human lymphoblasts

We have studied the effects of 5-bromodeoxyuridine (BrdUrd) at two genetic loci in diploid human lymphoblast cells. In thymidine kinase heterozygotes (tk +/-), a 2-h dose of BrdUrd induced a transient, non-heritable resistance to the thymidine analogue, trifluorothymidine (F3TdR). We have called this phenomenon pseudomutation and have shown that affected cells acquire the ability to survive in the presence of F3TdR and then, after degradation of F3TdR in the medium, return to an apparently normal wild-type state. Our data suggest that BrdUrd incorporation into DNA as a thymidine analogue is responsible for the effect, which we interpret as a temporary loss of thymidine kinase activity. This effect is not seen in tk +/+ homozygotes. In contrast, at the hypoxanthine-guanine phosphoribosyl transferase locus in tk +/- heterozygotes, BrdUrd did not induce a permanent, heritable resistance to 6-thioguanine (gene locus mutation). We detected such mutations only in the tk +/+ homozygote and only at external BrdUrd concentrations considerably higher than those which saturate the uptake of BrdUrd into DNA as a thymidine analogue. We postulate that the reduced TK enzyme levels (30%) in the heterozygote prevent the build-up of a sufficiently high intracellular BrdUrd triphosphate pool to promote the misincorporations as deoxycytidine triphosphate which may be responsible for gene locus mutation.

Effect of 5-bromodeoxyuridine on the appearance of the liver isoenzyme of pyruvate kinase

Hepatocyte cultures derived from 15-day foetal rats produce the liver form of pyruvate kinase (EC 2.7.1.40) only after 3 days of culture. The appearance of the liver form of the enzyme can be blocked by the addition of 5-bromodeoxyuridine on day 2 of culture, but not by addition on day 3 of culture. The reversibility of the action of 5-bromodeoxyuridine was shown when the inhibitor was added on day 2 and removed on day 4. By day 6 of culture the liver form of pyruvate kinase was detectable. The specificity of the action of 5-bromodeoxyuridine was monitored by following changes in the closely related embryonic form of the enzyme as a control. This was unaltered by the inhibitor.

Unequal forms of 140-110 kD glycoproteins in B16 melanoma cells with differing detachment properties and metastatic behavior: influence of bromodeoxyuridine

Growth of B16 melanoma metastatic variants with 2.5 micrograms/ml of bromodeoxyuridine (BrdU) decreases cell detachment from the substratum, as measured by resistance to release by trypsin-EDTA treatment. Using an antiserum to intact melanoma cells and metabolic labelling of melanoma metastatic variants with 3H-glucosamine and subsequent electrophoretic analysis, we are now able to show that: (1) differential solubilization and immune precipitation permit the identification of hydrophilic glycoproteins of about 140 kD and 110 kD in melanoma cells with low colonizing ability; (2) the effects of BrdU on B16 melanoma appear to be exerted differentially on cells with differing metastatic behavior, since only poorly invasive melanoma cells show a stimulating effect of BrdU on the expression of the hydrophilic 140 kD glycoproteins; (3) cells with increased lung colonizing ability reveal hydrophobic 140 and 110 kD glycoprotein species with increased susceptibility to mild protease treatment, as compared with the corresponding components from poorly invasive cells. The possible relationship of the 140-110 kD glycoproteins to B16 melanoma biological behavior and cell-substratum interactions is suggested by the fact that such components undergo significant changes in cells with differing invasive behavior and detachment properties.

Factors affecting the CEA secretion of human adenocarcinoma cell lines into the spent medium

CEA producing cell lines were established from human gastric cancer (HGC-Y1), pancreatic cancer (HPC-Y9) and lung cancer (HLC-Y1). The culture medium was used RPMI-1640 supplemented with 10% fetal bovine serum. The secretion of carcinoembryonic antigen (CEA) into the spent medium from these cultured cell lines was modified by several factors, such as theophylline, cyclic AMP (cAMP), dibutyryl cyclic AMP (dbcAMP), Bromodeoxyuridine (BrdUrd), dimethyl sulfoxide (DMSO), Prostaglandin E2 (PGE2) and human interferon (INF). CEA secretion was enhanced by theophylline, cAMP, PGE2 and INF. Theophylline had an optimal dose to maximally enhance CEA secretion. cAMP and INF apparently enhanced CEA secretion dose dependently. PGE2 appeared to enhance CEA secretion, although cell growth was markedly suppressed dose dependently, dbcAMP, DMSO and BrdUrd did not affect CEA secretion. Here, the kinetics of CEA secretion was discussed.

Regulation of the synthesis of tyrosine aminotransferase: the relationship to mRNATAT

The activity of the hepatic enzyme tyrosine aminotransferase (TAT) is the sum of many diverse regulatory factors. These include the developmental stage of the animal, the hormonal and nutritional environment of the animal (or tissue culture cell), other extrinsic and intrinsic regulatory cycles and factors (including cytoplasmic substances), and chromatin structure. Although TAT is subject to a number of post-translational modifications, alterations in catalytic activity always parallel changes in enzyme amount. In a few instances this is due to a selective change in TAT degradation, but most are due to changes in the rate of aminotransferase synthesis. Recent studies have shown that TAT synthesis is generally directly correlated with the activity, and presumably amount, of the mRNA that codes for tyrosine aminotransferase.

Glutamine synthetase induction by glucocorticoids in the glucocorticoid-sensitive human leukemic cell line CEM-C7

Treatment of CEM-C7 cells with glucocorticoids produces a 2.5-fold increase in the activity of the enzyme glutamine synthetase (GS). This increase is specific for steroids with glucocorticoid activity adn occurs over a range of steroid concentrations consistent with a receptor-mediated mechanism. Half-maximal and maximal inductions by dexamethasone (dex) occur at 2 X 10(-8) M and 2 X 10(-7) M dex, respectively, concentrations approximately equal to those necessary to produce half and full occupancy of glucocorticoid receptors. GS activity began to increase 1 hour after dex treatment and was complete by 12 hours. This is well before any of the growth inhibitory or cytolytic effects of dex on this cell line occur. This increase was dependent on the presence of glucocorticoid receptors and required both RNA and protein synthesis. Removal of dex following stimulation to maximal levels resulted in a decrease of GS activity to preinduced levels with a half-time of 5 hours. Glutamine deprivation of cells resulted in increased GS activity. However, even in the total absence of glutamine, dex treatment elicited a 2.0-2.5-fold increase in GS activity, ruling out inhibition of glutamine uptake as a mechanism for the dex-induced increase. Experiments with 5'-bromodeoxyuridine (BrdU) demonstrated that GS elevation was sensitive to BrdU substitution of DNA, while dex-induced growth inhibition was not. Therefore GS elevation and growth inhibition in this cell line appear to be independently expressed steroid responses.

Antiviral action of 5-bromo-2'-deoxyuridine and polyoma virus-specific RNA synthesis

The influence of 5-bromo-2'-deoxyuridine on the synthesis of polyoma virus-specific RNA in mouse embryo cells at a late phase of infection was assayed by molecular hybridization. The effect of the analogue on virus yields in terms of infectivity (p.f.u.) and haemagglutination activity (h.a.u.) was also quantitated. 12.68-126.80 micrograms/ml bromodeoxyuridine inhibited viral RNA synthesis to 15-10%, h.a.u. titers of virus yield to 50-10%, p.f.u. titers of virus yield to 0.10-0.01%, and p.f.u./h.a.u. ratios to 1.0-0.1% of the respective controls. The inhibition of viral RNA synthesis was reversed by thymidine. Since i) bromodeoxyuridine inhibited viral RNA synthesis and h.a.u. titers only partially and to a different extent, and ii) it reduced p.f.u. titers and p.f.u./h.a.u. ratios by several orders, it is likely that viral RNA synthesis represents only one of the sites of the antiviral action of the analogue.

The binding of androgen receptor to DNA and RNA

The androgen receptor from mouse kidney cytosol has been studied for its nucleic acid binding properties by DNA-cellulose centrifugation assay. The receptor appears to bind to RNA (mRNA, tRNA, rRNA) as well as to DNA. Salt and heat activation of the androgen receptor enhances both DNA and RNA binding. The receptor binds slightly better to denatured DNA than to native DNA. The androgen receptor binds about 2-fold tighter to poly(dG-dC) than to poly (dA-dT). The interaction of the receptor with DNA is not greatly affected by the BrdUrd substitution. The observation that androgen receptor shows a significant affinity to RNA may imply that androgen receptor-RNA interaction could play a role in gene regulation.

5-Bromodeoxyuridine specifically inhibits the synthesis of estrogen-induced proteins in MCF7 cells

In the MCF7 human breast cancer cell line, estrogens induce a secreted glycoprotein with an Mr of 46 000 (46 000-Mr protein) and the progesterone receptor; they also increase to a lesser extent the secretion of [35S]-methionine-labelled proteins. When the cells are grown in the presence of 5-bromo-2'-deoxyuridine (BrdUrd, 5 microgram/ml for 4 days) these estrogen-induced responses are substantially inhibited while other proteins not regulated by estrogens and cell growth are not affected by BrdUrd. This effect of BrdUrd is not secondary to a decrease in estrogen receptor levels and appears to require incorporation of BrdUrd into DNA for the following reasons. First, there is a lag before any effect of BrdUrd is seen which is similar to the doubling time of the cells. Second, the effect of BrdUrd is not seen when cells are cultured in medium containing BrdUrd and 1-beta-D-arabinofuranosylcytosine, a DNA synthesis inhibitor, or excess thymidine, which blocks the incorporation of BrdUrd into DNA. Finally 2'-deoxycytidine, which reverses the effects of non-incorporated BrdUrd, is without effect on the inhibition of the estrogen-induced 46 000-Mr protein. We conclude that BrdUrd selectively prevents the effects of estrogens in MCF7 cells and that the mechanism of this anti-estrogenic effect of BrdUrd probably requires its incorporation into DNA and occurs beyond the nuclear translocation step of the estrogen receptor. Alterations in the recognition of chromatin effector sites by the estrogen-receptor complex could be involved in preventing hormone action.

Shortening of life span caused by administration of 5-bromodeoxyuridine to neonatal rats

Newborn rats and adult animals in which liver cell replication had been induced by partial hepatectomy were injected with 5-bromodeoxyuridine (BUdR), and were kept without further treatment for the remainder of their life span. No evidence for carcinogenicity of BUdR was obtained, but a dose-dependent shortening of the life span was observed after administration to neonatal animals.

Tyrosine aminotransferase sensitivity to bromodeoxyuridine during restricted intervals of S phase in hepatoma cells

Synchronized hepatoma tissue culture (HTC) cells, accumulated at the G1/S boundary with aminopterin, were released into S phase with either thymidine or 5-bromodeoxyuridine (BUdR). Tyrosine aminotransferase (TAT) activity was found to be unaffected by BUdR over the initial 3 h of S phase, but then to rapidly decline to a new basal level of 40% of control by 9 h. There was no corresponding response in the activities of alcohol dehydrogenase, malate dehydrogenase, acid phosphatase, and alkaline phosphatase, or in the rate of protein and RNA synthesis. If BUdR incorporation was restricted to limited periods of S phase, TAT was found to be maximally suppressed by incorporation into the initial 40% of the DNA. Incorporation of the analogue into the latter 60% of DNA synthesized during S phase had no effect on TAT. This is the first report that the effect of BUdR on TAT in HTC cells is associated with incorporation of the analog into DNA synthesized during a specific interval of S phase.

Inhibition of Friend erythroleukemic cell differentiation by bromodeoxyuridine: correlation with the amount of bromodeoxyuridine in DNA

These studies were undertaken to examine the relationship between the inhibition by 5-bromodeoxyuridine (BrdU) of erythroid differentiation in Friend erythroleukemia cells and the incorporation of BrdU into DNA. Experiments were carried out in which the incorporation of BrdU into DNA and the concentration of BrdU to which the cells were exposed were varied independently of each other. In addition, the ability of deoxycytidine (dC) to reverse the effects of BrdU on hemoglobin production and to reduce the amount of BrdU in DNA was analyzed. Under all the conditions tested, the effects of BrdU were correlated with the amount of BrdU incorporated into nuclear DNA. These results differ from those of recent studies on the inhibition of pigmentation and the induction of mutations by BrdU in Syrian hamster melanoma cells. The results suggest that BrdU may be producing its biological effects by a variety of different mechanisms.

The requirement of DNA synthesis for the induction of alkaline phosphatase by bromodeoxyuridine in a derivative of the HeLa cell line

Non-lethal concentrations of bromodeoxyuridine induce a 2- to 5-fold increase in the specific activity of alkaline phosphatase in a HeLa subclone, S3G. Experiments employing 10-hour pulses of BRdU showed that 48 hours were required before induction commenced, and that maximal induction was attained by 96 hours. Under conditions in which DNA synthesis was prevented with hydroxyurea induction did not occur. Upon removal of hydroxyurea both DNA synthesis and induction were rapidly reestablished. Furthermore, experiments employing radiolabelled BRdU demonstrated that the kinetics of the induction process paralleled the incorporation of the analogue into cellular DNA. These results indicate that DNA synthesis, or some process intimately linked to DNA synthesis, is required for the induction of alkaline phosphatase, and suggest that the mode of the induction may be through the incorporation of the analogue into cellular DNA.

Proteins of BrdU-dependent hamster cell lines as characterized by two-dimensional gel electrophoresis

Cell lines which exhibit the BrdU-dependent phenotype (B4 and HAB) were studied with respect to BrdU-induced alterations in genetic expression by two-dimensional gel electrophoresis. A comparison of the proteins from the HAB cells, in which the DNA is 100% substituted by BrdU, to those of the unsubstituted parent line (3460) showed 55 protein alterations; the synthesis of 15 increased while that of the other 40 decreased. When 3460 cells were grown in BrdU such that their DNA was greater than 50% substituted, 27 protein changes could be detected; of these, the synthesis of 10 increased while that of 17 decreased. A comparison of all these changes in the various cell lines showed six which were common to the BrdU-substituted cell lines. The proteins from another Syrian hamster cell line, BHK-21 (C-13) and those of HAB cells grown in thymidine or BrdC were also examined on two-dimensional gels. Although BrdU has a dramatic effect on many cellular functions, relatively few changes in the pattern of protein synthesis could be detected in these cell lines, perhaps reflecting the specialized action of this analogue on particular cellular functions.

Dexamethasone induces irreversible G1 arrest and death of a human lymphoid cell line

Growth of a human leukemic T-cell line (CEM C7) in 10(-6) M dexamethasone results in inhibition of growth and rapid loss of cell viability after a delay of approximately 18 to 24 hours. Analysis of dexamethasone-treated cells by flow-microfluorometry showed that they were arrested in the G1 phase of the cell cycle. Loss of cell viability began at the same time as G1 accumulation was first detectable, and 20% of all cells were found to be blocked in G1 at this time suggesting that loss of viability and G1 arrest were coincident events. Half-maximal and maximal effects on both viability and G1 arrest after 48 hours in steroid were nearly identical with respect to steroid concentration and corresponded to half-maximal and full occupancy of glucocorticoid specific receptor by hormone, consistent with a glucocorticoid receptor mediated mechanism for both phenomena. Most non-viable cells were arrested in G1, and accumulation of cells in G1 was irreversible; removal of steroid in the presence of colcemid did not result in a decreased fraction of G1 cells. Furthermore, dexamethasone treatment did not protect cells against the effects of 33258 Hoechst-amplified killing of bromodeoxyuridine substituted cells exposed to light. These results show that dexamethasone arrests these leukemic cells in G1 and strongly suggest that dexamethasone-treated cells are killed upon entry into G1.

Glucocorticoid induction of tyrosine aminotransferase in cultured cells

For over a decade, tyrosine aminotransferase induction in tissue culture cells has been a useful model system in which to study glucocorticosteroid action. In the 1960s, the establishment in culture of rat hepatomas expressing the inducible enzyme, already known to be induced in liver in vivo, provoked a wide-ranging series of experiments. The data from these experiments have provided considerable information regarding the mechanism of action of steroids. These include the fundamental facts that the steroids act directly on the induced cell in unmetablized form, that removal of steroid results in deinduction, that induction does not require DNA synthesis or massive changes in RNA synthesis, and that cytoplasmic receptor occupancy by active steroids correlates closely with the steroids' ability to affect inductions. Studies in tissue culture cells have led to the analysis of transcriptional and posttranscriptional models attempting to explain enzyme induction. The effects on enzyme induction of nonsteroid hormones and other factors have been studied through the use of tissue culture cells. Finally, cells and clones of cell variants are being used to study enzyme induction, through biochemical analysis and cell genetic approaches, including somatic cell hybridization.

RNA metabolism in M1 adrenergic neuroblastoma cells

RNA metabolism of M1 adrenergic neuroblastoma cells was investigated in logarithmic proliferating, stationary and differentiated states. [3h] uridine labelling experiments showed that in the stationary phase (10 days of culture) the cells incorporated less radioactive precursors into RNA than in the logarithmic phase (3 days of culture). Cells differentiated by bromodeoxyuridine treatment were compared with proliferating cells. At short labelling time, the BrdU-treated cells incorporated more [3H] uridine in RNA than the controls. But when morphological differentiation became irreversible, the treated cells incorporated less [3H] uridine into RNA. Our results show that morphological differentiation by BrdU-treatment of neuroblastoma M1 cells is not accompanied by striking changes in RNA metabolism.

Effects of BUdR on developmental functions of Dictyostelium discoideum

The development of Dictyostelium discoideum cells, as measured by spore yield, is somewhat more sensitive to the presence of BUdR during vegetative growth than is growth itself. Observations on the development of control and BUdR-grown cells, their protein labelling patterns and assays of 4 developmentally regulated proteins all reveal a consistent picture. BUdR appears to block spore formation by partially inhibiting several or many different earlier events during development. The relative sensitivity of development compared to growth to inhibition by the drug may be a consequence of the nature of the developmental process rather than of some unique specificity of the inhibitor.

Bromodeoxyuridine induction of deoxycytidine deaminase activity in a hamster cell line

The Syrian hamster cell line, RPMI 3460, was found to express barely detectable levels of the enzyme deoxycytidine deaminase. In contrast, the cell lines B4 and HAB, which are derived from 3460 cells and have approx. 60 and 100% bromodeoxyuridine substitution in DNA, respectively, show an approx. 50-fold higher enzyme activity. Deoxycytidine deaminase activity can be "induced" in 3460 cells by growth in 10(-5) M bromodeoxyuridine, as well as by the other halogenated pyrimidines, iododeoxyuridine and chlorodeoxy-uridine. The time required for maximal enzyme activity to accrue (approx. 8 days) suggests that new genetic expression is required for enhanced deoxycytidine deaminase activity and inhibition of induction in the presence of Ara. C shows that bromodeoxyuridine must be incorporated into DNA. In addition, the extent of enhanced deoxycytidine deaminase activity is directly related to the level of bromodeoxyuridine substitution in DNA. Another hamster cell line, BHK21/C13, which shows no detectable deoxycytidine deaminase activity, cannot be induced by bromodeoxyuridine. These results are discussed with respect to a mechanism by which bromodeoxyuridine may alter gene expression due to an altered binding of both positive and negative regulatory proteins to DNA.

Inhibition of early postimplantation development of cultured mouse embryos by bromodeoxyuridine

Mouse embryos were treated with 5-bromodeoxyuridine (BrdUrd) for 24 hours at various preimplantation stages to determine its effect on early postimplantation development. The inhibitory effect of BrdUrd (10(-7) to 10(-6) M) on trophoblast outgrowth and inner cell mass (ICM) development was least severe when embryos were treated at the 2-cell stage and was most severe when they were treated at the morula stage. When embryos were treated at the blastocyst stage, the inhibitory effect on trophoblast outgrowth decreased, but that on ICM development remained severe. The severity of inhibition, particularly that on ICM development, appeared to be related to decreased cell numbers in BrdUrd-treated embryos. However, increasing the cell number by aggregating two ICM's isolated from BrdUrd-treated blastocysts did not increase their chance of survival or of forming two primary germ layers. This indicates that the decrease in cell numbers alone is not the cause of the failure of BrdUrd-treated embryos to develop. The mechanism of BrdUrd inhibition was studied by adding thymidine or deoxycytidine during BrdUrd treatments. A 10-fold excess of thymidine completely protected embryos from the inhibitory effect of BrdUrd. A 10-fold excess of deoxycytidine was less effective. Autoradiography indicated that both thymidine and deoxycytidine protected embryos by interfering with the incorporation of BrdUrd into the DNA.

Induction of placental alkaline phosphatase in choriocarcinoma cells by 5-bromo-2'-deoxyuridine

Growth of choriocarcinoma cells in the presence of 5-bromo-2'-deoxyuridine (BrdUrd) results in a 30- to 40-fold increase in alkaline phosphatase activity. The effects of BrdUrd is specific for phosphatase with an alkaline pH optimum. The induction by BrdUrd is probably not due to the production of an altered enzyme, since the induced enzyme resembles the basal enzyme in thermal denaturation and kinetic properties. Enzyme induction can be prevented by thymidine but not by deoxycytidine or deoxyuridine. The induction of alkaline phosphatase appears to require incorporation of the BrdUrd into cellular DNA. The presence of BrdUrd in the growth medium is not necessary for alkaline phosphatase induction in proliferating cells containing BrdUrd-substituted genomes. However, enzyme induction and maintenance of the induced levels of alkaline phosphatase in nonproliferating cells containing BrdUrd-substituted DNA requires the presence of the analogues in the medium. The induction of alkaline phosphatase by BrdUrd in probably an indirect process.

The diverse effects of 5'-bromodeoxyuridine on enzyme activities in cultured H35 hepatoma cells

Reuber (H35) hepatoma cells were grown in medium containing 10(-5)M bromodeoxyuridine (BrdU), which was incorporated into their DNA. Cell growth rate was unaffected by BrdU for the first two generations, after which it was reduced by about 50%. The effect of BrdU incorporation on the activities of several enzymes with rapid turnover rates was examined to test the hypothesis that the synthesis of such enzymes will be preferentially inhibited by BrdU. Tyrosine amino-transferase (TAT) activity decreased by 70% within two generations whereas thymidine kinase activity remained at control values. PEP carboxykinase activity was unchanged during the first generation in BrdU-containing medium but, during the second, its activity increased by at least 30%. Ornithine decarboxylase levels decreased by about 50% only after two generations in the presence of BrdU. There appeared to be no simple relationship between turnover rates and the effect of BrdU on enzyme activity. Incorporation of BrdU was found to inhibit the induction of both TAT and PEP carboxykinase by dexamethasone and to enhance the inhibition of cell growth by this steroid. These results are discussed with respect to possible mechanisms of gene expression and development in both normal and neoplastic cells.

Changes in type of collagen synthesized by chick fibroblasts in vitro in the presence of 5-bromodeoxyuridine

Chick embryo fibroblasts cease to synthesize their normal collagen product when grown in the presence of the thymidine analogue, 5-bromodeoxyuridine (BrdU). The drug causes an alteration of synthesis from the normal Type I collagen (alpha1 (I)2alpha2) to a mixture of Type I and Type I trimer (alpha1(I)3). While the significance of the synthesis of Type I trimer is unclear, it has been noted that chondrocytes synthesize this collagen type following in vitro senescence and in the presence of BrdU. Since BrdU may cause a switching in the temporal pattern of collagen biosynthesis in chondrocytes and in fibroblasts it is proposed that BrdU may alter the normal regulatory controls acquired by the cells during the course of their differentiation. The synthesis of type I trimer might provide a marker for such a break-down in a wide variety of cell types.

Inhibition and stimulation by 5-bromodeoxyuridine of erythropoiesis by chick blood island cells

Erythropoiesis in liquid cultures of cell populations resolved from chick blastodiscs at the primitive streak and head-fold stages was totally inhibited by 5-8 mug/ml of 5-bromodeoxyuridine. However, concentrations of 0.2 mug/ml of the nucleoside enhanced the number of erythroid cells formed.

Selective interaction of 5'-bromodeoxyuridine substituted DNA with different chromosomal proteins

Chromosomal proteins selectively interact with 5'-bromodeoxyuridine (BrdUrd) substituted DNA relative to unsubstituted DNA. The relative affinities of chromosomal proteins for BrdUrd-DNA and unsubstituted DNA were measured by both thermal chromatography on hydroxylapatite and selective retention on nitrocellulose filters. Certain chromosomal proteins have a high affinity for hydroxylapatite; thus, during thermal chromatography of chromatin, the single-stranded DNA component percolates across a bed of adsorbed proteins as it elutes. We have measured the relative affinities of Brd-Urd-DNA and normal DNA for chromosomal proteins by chromatographing appropriate mixtures on hydroxylapatite. The results show that, under these conditions, the histone components, rather than the nonhistone chromatin proteins, retard the BrdUrd-substituted DNA. In addition, the individual histones vary in the degree of their affinity for BrdUrd-DNA in the order H3 greater than H4 greater than H2A greater than H2B greater than H1. We have used the property that protein-DNA complexes have a preferential affinity for nitrocellulose filters over naked DNA to measure the selective binding of BrdUrd-DNA and unsubstituted DNA's to both histone and nonhistone chromosomal proteins at low temperatures. The histones selectively retained BrdUrd-DNA on filters in the order H4 greater than H2A greater than H3 greater than H2B greater than H1. Using this assay, the nonhistones displayed greater selectivity toward BrdUrd-DNA than the histone fraction. We interpret these results to mean BrdUrd-containing DNA has a specific affinity for certain chromosomal proteins with BrdUrd-DNA may be the basis for selective inhibition of cytodifferentiation by the thymidine analogue, BrdUrd.