Induction of prolactin synthesis in rat pituitary tumor cells by 5-bromodeoxyuridine.

The clonal strain of pituitary tumor cells GH₁2C₁ does not produce detectable amounts of prolactin (<5 ng/mg cell protein per 24 hr), although it does synthesize growth hormone. When GH₁2C₁ cells were grown in the presence of 5-bromodeoxyuridine (BrdU, 3 μg/ml), the cells did produce prolactin as determined by quantitative microcomplement fixation and incorporation of ³H-leucine into ³H-prolactin. BGH₁2C₁ and F₁BGH₁2C₁, two BrdU-resistant (r) substrains derived from GH₁2C₁ which grow in the presence of 30 μg/ml BrdU, also synthesized prolactin (100–500 ng/mg cell protein per 24 hr). Growth of BrdU^r strains was not dependent upon on the presence of the drug in the medium; however, the continued production of prolactin by F₁BGH₁2C₁ cells was dependent upon the presence of BrdU. Growth hormone production in both BrdU^s and BrdU^r strains was not affected by BrdU. Resistance of F₁BGH₁2C₁ cells to BrdU was not due to a defect in BrdU uptake. Thymidine inhibited the incorporation of ³H-BrdU into DNA in both sensitive and resistant strains, and also reduced BrdU-induced prolactin synthesis in F₁BGH₁2C₁. We postulate that induction of prolactin synthesis by BrdU in GH₁2C₁ and F₁BGH₁2C₁ cells is mediated by the incorporation of the drug into cellular DNA. Furthermore, the lack of measurable prolactin synthesis by the parent strain GH₁2C₁ is not due to deletion of the gene for prolactin, but is probably the result of regulatory mechanisms which do not permit expression of this gene.     

Regulated expression of the prolactin gene in rat pituitary tumor cells

Prolactin (PRL) gene expression in three strains of GH cells (rat pituitary tumor cells) has been quantitated by measurement of: (a) intracellular and extracellular PRL, (b) cytoplasmic translatable PRL-specific mRNA (mRNAPRL), and (c) molecular hybridization of cytoplasmic poly(A) RNA to cDNAPRL (DNA complementary to mRNAPRL). Three GH cell lines utilized in this investigation were a PRL-producing (PRL+) strain, GH4C1, a PRL nonproducing 5-bromo-deoxyuridine resistnat (PRL- BrdUrdr) strain, F1BGH12C1, and a new strain, 928-9b, derived by fusion of PRL+ cells with a nuclear monolayer of the PRL-, BrdUrdr GH cell strain. PRL production is a characteristic of 928-9b cells, but the level of PRL production (2-4 micrograms/mg protein/24 h) is much lower than that of the PRL+ strain, GH4C1 (15-25 micrograms/mg protein/24 h). Levels of cytoplasmic translatable mRNAPRL and cytoplasmic PRL-RNA sequences quantitated with a cDNAPRL probe were also much lower in 928-9b as compared to the PRL+ parent. PRL-RNA sequences could not be detected in the PRL- strain. Thyrotopin-releasing hormone (TRH) stimulates PRL synthesis about threefold and inhibit a growth hormone (GH) synthesis 72% in the PRL+ strain. TRH has no effect on the synthesis of either PRL or GH in the 928-9b strain, although TRH receptors could be detected in these cells. Stimulation of PRL synthesis in the PRL+ strain by TRH could be correlated with increases in levels of cytoplasmic translatable mRNAPRL and increases in cytoplasmic PRL-RNA sequences. These results demonstrate that the graded expression of the PRL gene at the basal level, and in response to TRH, is caused by the regulated production of specific mRNA, i.e., mRNAPRL in these three GH cell strains.     

Intracellular site of prolactin synthesis in rat pituitary cells in culture

Free and membrane-bound polyribosomes were isolated from control and thyrotropin releasing hormone-treated GH₃ cells. The two polysome fractions were used to direct {³H}leucine incorporation into prolactin in both heterologous and homologous cell-free protein-synthesizing systems. Prolactin was measured by immunoprecipitation and SDS-disc gel electrophoresis of the reaction products. Only membrane-bound polysomes directed incorporation of {³H}leucine into labeled prolactin. In additon, intact cells were pulselabeled with {³H}leucine, free and membrane-bound polysomes were isolated, and newly synthesized prolactin associated with each polysome fraction was measured. In control cells, {³H}prolactin represented about 0.4 and 4.2% of total acid-insoluble radioactivity in free and membrane-bound polysomes, respectively; whereas, in thyrotropin releasing hormone-treated cells, these values were about 1 and 20%, respectively. Added {³H}prolactin did not associate nonspecifically with membrane-bound polysomes. We conclude that prolactin is synthesized predominantly on membrane-bound polysomes in GH₃ cells.     

Mutagenic mechanism of 5-bromodeoxyuridine in Chinese hamster cells

Resistance to 6-thioguanine was induced by 5-bromodeoxyuridine (BUdR) in synchronous Chinese hamster cells. The yield of mutant colonies was not proportional to the amount of BUdR incorporated into DNA; thus mutants were not due to mispairing of BUdR with guanine during replication. Few mutants were induced until BUdR concentrations exceeded that of the intracellular thymidine triphosphate pool and mutant yield was depressed by addition of thymidine to the medium. These data suggest that BUdR exerts an allosteric effect on the DNA synthesizing system which renders it more error prone.     

Hormonal regulation of prolactin storage in a clonal strain of rat pituitary tumor cells

GH4C1 cells (GH cells) are a clonal strain of rat pituitary tumor cells which secrete prolactin. GH cells have been used to study hormone secretion, but they store relatively little prolactin compared to normal prolactin-secreting cells. They are not suitable, therefore, for studying some aspects of pituitary function. We have found that the amount of prolactin GH cells store can be regulated. When GH cells were plated at 10(6) cells/well and treated for six days with 180 nM insulin or 1 nM estradiol, there was a 60 percent increase in prolactin storage compared to control cells. Insulin and estradiol in combination acted synergistically to cause a 190 percent increase in prolactin storage. In contrast, they were additive in increasing extracellular prolactin; there was a 40 percent increase in extracellular prolactin after insulin, a 20 percent increase after estradiol, and a 50 percent increase after insulin plus estradiol. The increases in prolactin storage were always greater than the increases in extracellular prolactin. The increases in prolactin storage were dose-dependent and reached maximal levels after four days of treatment with 180 nM insulin plus 1 nM estradiol. Reducing the plating density to 10(3) cells/well increased the response to insulin and estradiol to nineteenfold. Epidermal growth factor (10 nM) acted synergistically with estradiol and insulin in combination to increase prolactin storage 27-fold. The insulin- and estradiol-induced increase in extracellular prolactin was caused by a specific increase in the rate of prolactin synthesis. The fractional increase in prolactin storage above the increase in prolactin production could not be explained by an increase in prolactin synthesis, an increase in intracellular transit time, or a change in the cell-cycle distribution of the population. Hormone storage can, therefore, be regulated independently from other processes which control hormone production. The prolactin stored in response to insulin and estradiol was releasable by potassium depolarization. Following depletion of intracellular prolactin by depolarization, the cells retained their increased capacity for prolactin storage. The ability to increase prolactin storage will make GH cells a more useful system in which to study pituitary function.     

Novel prolactin related mRNAs in rat pituitary cells

From cytoplasm of rat pituitary GH4C1 tumour cells, anti prolactin anti-serum precipitates a polypeptide with apparent molecular weight of 75.000 in addition to prolactin. In vitro translation of size fractionated RNA shows that a 82.000 molecular weight PRL-like polypeptide is encoded by a mRNA larger than the 1 kb prolactin mRNA. Northern blot analysis shows that a rat prolactin cDNA probe hybridize to a 3.2 kb RNA and a 1.5 kb RNA in addition to the 1 kb PRL mRNA. The 82.000 molecular weight translation product and the 3.2 kb mRNA is also detected in rat anterior pituitary cytoplasm. We conclude that at least one high molecular weight mRNA which code for a prolactin-like polypeptide, is present in normal rat anterior pituitary gland and in GH4C1 cells.     

Glucocorticoid induction of growth hormone synthesis in a strain of rat pituitary cells

Conditions were determined for measuring growth hormone synthesis by a clonal strain of rat pituitary cells grown in suspension culture. Incubation of the cells with [3H]leucine in either continuous labeling or pulse-chase experiments showed that secretion of newly synthesized growth hormone commences only after a lag of about 15 min. The pulse-chase experiments also demonstrated that there is no detectable degradation by the cells of growth hormone. Thus growth hormone synthesis could be measured, in the absence of complications arising either from secretion or degradation of growth hormone, by incubating the cells with [3H]leucine for 10 min. Exposure of cells grown under the usual culture conditions to dexamethasone (a synthetic glucocorticoid) led to an average stimulation of specific growth hormone synthesis (growth hormone synthesis/total cytoplasmic protein synthesis) of only 2.6-fold. However, two other growth conditions were found in which dexamethasone routinely yielded a 5- to 15-fold stimulation of specific growth hormone synthesis. One of these conditions, involving substitution of 10% fetal calf serum for the normal serum supplement, was employed in subsequent experiments. A stimulation of specific growth hormone synthesis could be observed at 10(-9) M dexamethasone, and the maximum stimulation was observed at dexamethasone concentrations of about 10(-8) to 10(-7) M. There was a lag of about 6 h before a stimulation by dexamethasone of specific growth hormone synthesis was detected. Thereafter, the stimulation increased in a nearly linear fashion until maximum stimulation was reached at about 48 h.     

Evidence for a coupling of prolactin secretion and synthesis by rat pituitary cells in culture

The relationship between the release and the synthesis of prolactin by rat pituitary cells in culture was studied using a microtubule-disrupting drug, vinblastine. (1) Prolactin secretion was inhibited by vinblastine in short-term incubations. Vinblastine did not act via the dopamine pathway, since a potent anti-dopaminergic drug, fluphenazine, was unable to reverse the inhibiting action of the antimicrotubular agent. (2) Continuous treatment by vinblastine induced a progressive decrease of the rate of incorporation of [3H]leucine in prolactin. The half-inhibition time was about 2 days. This inhibition of prolactin synthesis was selective, since total protein synthesis remained unaffected. (3) Measurements of radioimmunoassayable prolactin showed that the inhibition of hormone release by vinblastine led to a transient increase of the intracellular content of prolactin. The phase of over-accumulation was followed by a progressive reduction of the total (cell + medium) prolactin. This result is in agreement with the observed inhibition of de novo synthesis of prolactin and indicates that a degradation process takes place in pituitary cells in culture. In conclusion, the use of vinblastine allows us to demonstrate that the rate of prolactin synthesis is dependent upon the secretory status of the cell.     

Evidence for a coupling of prolactin secretion and synthesis by rat pituitary cells in culture

The relationship between the release and the synthesis of prolactin by rat pituitary cells in culture was studied using a microtubule-disrupting drug, vinblastine. (1) Prolactin secretion was inhibited by vinblastine in short-term incubations. Vinblastine did not act via the dopamine pathway, since a potent anti-dopaminergic drug, fluphenazine, was unable to reverse the inhibiting action of the antimicrotubular agent. (2) Continuous treatment by vinblastine induced a progressive decrease of the rate of incorporation of [³H]leucine in prolactin. The half-inhibition time was about 2 days. This inhibition of prolactin synthesis was selective, since total protein synthesis remained unaffected. (3) Measurements of radioimmunoassayable prolactin showed that the inhibition of hormone release by vinblastine led to a transient increase of the intracellular content of prolactin. The phase of over-accumulation was followed by a progressive reduction of the total (cell + medium) prolactin. This result is in agreement with the observed inhibition of de novo synthesis of prolactin and indicates that a degradation process takes place in pituitary cells in culture. In conclusion, the use of vinblastine allows us to demonstrate that the rate of prolactin synthesis is dependent upon the secretory status of the cell.     

Basal and thyroliberin-stimulated prolactin synthesis in single-cell cultures and in populations of rat pituitary cells.

1. Newly synthesized prolactin was obtained from cultures of rat pituitary tumour cells (GH4C1 cells) after incubation with [35S]methionine. 2. Radioactive synthesized and secreted prolactin was quantified by an immunoprecipitation method by using disc-gel electrophoresis of the dissolved immunoprecipitate in the presence of sodium dodecyl sulphate. By using a microanalytical modification, hormone synthesis and secretion could also be studied in single-cell cultures. This technique was combined with a cytoimmunofluorescence method in which rhodamine-conjugated antibodies were used for studying intracellular prolactin. 3. The presence of radioactive synthesized and secreted prolactin was demonstrated in nine out of 13 single-cell cultures. Cell cultures containing 10 cells or more and clonal populations originating from one cell always secreted radioactive prolactin. 4. Thyroliberin treatment (2 muM) for 24h increased the extracellular accumulation of radioactive prolactin in five out of seven single-cell cultures and always in populations of cells. 5. The number of cells showing prolactin specific fluorescence increased from 20 to 50% and the intensity of this fluorescence became greater after thyroliberin treatment. 6. Studies of [35S]prolactin secretion from single cells and immunochemical detection of intracellular prolactin showed that some cells in an unsynchronized population did not secret radioactive prolactin or show prolactin specific fluorescence. 7. The quantitative effect of thyroliberin as studied in single-cell cultures suggested that the main if not the only effect was to increase prolactin synthesis in cells already producing hormone.     

5-Azacytidine inhibits the induction of transient TK-deficient cells by 5-bromodeoxyuridine. A novel hypothesis for the facilitation of hypermethylation by 5-bromodeoxyuridine.

This paper examines the mechanism by which 5-bromodeoxyuridine (BrdUrd) induces a high frequency of transient trifluorothymidine (F3TdR)-resistant variants in the TK6 human lymphoblast cell line (a TK +/- heterozygote). This phenomenon has previously been termed 'pseudomutation' (Liber et al., 1985). We now report that 5-azacytidine (5-AzaC), an inhibitor of DNA methylation, reverses BrdUrd-induced pseudomutation in a dose-dependent manner. The inhibition by 5-AzaC is highly specific and does not appear to involve nucleotide pool perturbations. 5-AzaC inhibits the pseudomutagenic effect (transient trifluorothymidine resistance in a thymidine kinase heterozygote), but not the stable mutagenic effect (stable 6-thioguanine resistance or trifluorothymidine resistance in a hypoxanthine-guanine phosphoribosyltransferase-proficient cell) induced by BrdUrd. 5-AzaC did not affect the induction nor expression of mutation induced by several other chemical mutagens at either the tk or hgprt loci. Inhibition of pseudomutation by 5-AzaC did not appear to be caused by a number of potential confounding factors. Although significant changes in the levels of DNA methylation were detected by HPLC analysis in BrdUrd-treated cells, the dose response for inhibition of pseudomutation by 5-AzaC was correlated with a significant decrease in 5-methylcytidine levels. These results and additional data in the literature have led us to postulate a novel mechanism in which the substitution of BrdUrd in a TpG dinucleotide(s) may serve as a substrate for non-heritable methylation and hence transiently inactivate tk gene expression.     

The inhibition of prolactin synthesis in GH3 rat pituitary tumor cells by monohydroxytamoxifen is associated with changes in the properties of the estrogen receptor

E2 (1 nM) stimulated the synthesis of PRL in GH3 cells. OH TAM (100 nM) did not affect basal PRL synthesis, but completely inhibited the increase produced by 1 nM E2. [3H]E2 and [3H]OH TAM both bound to the cytosolic 8S ER and these were split into 4S subunits on sucrose gradients containing 0.4 M KCl. By comparison, ER complexes extracted from nuclei of GH3 cells cultured in media containing [3H]E2 or [3H]OH TAM both sedimented at 5S on sucrose gradients containing 0.4 M KCl. Both 4S and 5S ER complexes were recognized by the monoclonal antibody D547 which increased their sedimentation coefficients to 8-9S. In contrast, a polyclonal antibody raised to calf uterine ER in the goat, interacted with the cytosolic ER so that the binding of [3H]E2 was inhibited but the binding of [3]OH TAM was only slightly reduced. A molecular model is proposed to describe the binding of E2 and OH TAM to the ER that might contribute to an understanding of estrogen and antiestrogen action.     

Localization of microfilaments in prolactin cells of the rat anterior pituitary gland

Summary Prolactin cells from anterior pituitary glands of normal non-lactating female rats, and lactating animals, some of which were separated from their pups for 48 hours, were examined ultrastructurally for the presence of microfilaments. Microfilaments were found in specific intracellular locations in all cells examined. They were in association with the nuclear envelope, the Golgi complex, the endoplasmic reticulum, small vesicles of the endoplasmic reticulum, and secretory granules. The possible role of microfilaments in the movement of intracellular organelles is considered.This investigation was supported by the National Institutes of Health grants AM 12583 and TW 02023.The authors wish to express their gratitude to Mr. M. G. Williams and Miss Pauline Cisneros for their excellent technical assistance.     

Synergistic induction of the senescence-associated genes by 5-bromodeoxyuridine and AT-binding ligands in HeLa cells

5-Bromodeoxyuridine induces a senescence-like phenomenon in mammalian cells. This effect was dramatically potentiated by AT-binding ligands such as distamycin A, netropsin, and Hoechst 33258. The genes most remarkably affected by these ligands include the widely used senescence-associated genes and were located on or nearby Giemsa-dark bands of human chromosomes. We hypothesize that AT-rich scaffold/nuclear matrix attachment region sequences are involved in this phenomenon. In fact, upon substitution of thymine with 5-bromouracil, a rat S/MAR sequence reduced its degree of bending and became insensitive to cancellation of the bending by distamycin A. The S/MAR sequence containing 5-bromouracil also bound more tightly to nuclear scaffold proteins in vitro and this binding was not inhibited by distamycin A. Under the same conditions, the S/MAR sequence containing thymine easily dissociated from the nuclear scaffold proteins. Taken together, the synergistic induction of the genes may be explained not only by opening of condensed chromatin by distamycin A but also by increase in the binding of 5-bromouracil-containing S/MAR sequences to the nuclear scaffolds.