Polyoma virus and cyclic AMP-mediated control of dihydrofolate reductase mRNA abundance in methotrexate-resistant mouse fibroblasts

Activation of CREB/ATF sites by polyomavirus large T antigen

Polyomavirus large T antigen (LT) has a direct role in viral replication and a profound effect on cell phenotype. It promotes cell cycle progression, immortalizes primary cells, blocks differentiation, and causes apoptosis. While much of large T function is related to its effects on tumor suppressors of the retinoblastoma susceptibility (Rb) gene family, we have previously shown that activation of the cyclin A promoter can occur through a non-Rb-dependent mechanism. Here we show that activation occurs via an ATF/CREB site. Investigation of the mechanism indicates that large T can synergize with CREB family members to activate transcription. Experiments with Gal4-CREB constructs show that synergy is independent of CREB phosphorylation by protein kinase A. Examination of synergy with Gal4-CREB deletion constructs indicates that large T acts on the constitutive activation domain of CREB. Large T can bind to CREB in vivo. Genetic analysis shows that the DNA-binding domain (residues 264 to 420) is sufficient to activate transcription when it is localized to the nucleus. Further analysis of the DNA-binding domain shows that while site-specific DNA binding is not required, non-site-specific DNA binding is important for the activation. Thus, CREB binding and DNA binding are both important for large T activation of CREB/ATF sites. In contrast to previous models where large T transactivation occurred indirectly, these results also suggest that large T can act directly at promoters to activate transcription.

Genetic analysis of polyomavirus large T nuclear localization: nuclear localization is required for productive association with pRb family members

Polyomavirus large T antigen (LT) is a multifunctional nuclear protein. LT has two nuclear localization signals (NLS2), one spanning residues 189 to 195 (NLS1) and another spanning residues 280 to 286 (NLS2). Site-directed mutagenesis showed that each signal contains at least two critical residues. The possibility of connections between NLSs and adjacent phosphorylations has attracted much attention. Cytoplasmic LT (CyT) mutants were underphosphorylated, particularly at sites adjacent to NLS2. However, since a nuclear LT bearing an inactivated NLS2 was phosphorylated normally at adjacent sites, the signal was not directly required for phosphorylation. Conversely, LT could be translocated to the nucleus via NLS2 even when the adjacent phosphorylation sites were deleted. CyT was examined to probe the importance of LT localization. CyT was unable to perform LT functions related to interactions with retinoblastoma susceptibility gene (pRb) family members. Hence, CyT was unable to immortalize primary cells or to transactivate an E2F-responsive promoter. Consistent with these findings, CyT, though capable of binding pRb in vitro, did not cause relocalization of pRb in cells. Assays of transactivation of the simian virus 40 late promoter and of the human c-fos promoter showed that defects of CyT were not limited to functions dependent on pRb interactions.

Specific binding of human dihydrofolate reductase protein to dihydrofolate reductase messenger RNA in vitro

Dihydrofolate reductase (DHFR) is a critical enzyme in de novo purine and thymidylate biosynthesis. An RNA gel mobility shift assay was used to demonstrate a specific interaction between human recombinant DHFR protein and its corresponding DHFR mRNA. Incubation of DHFR protein with either its substrates, dihydrofolate or NADPH, or with an inhibitor, methotrexate, repressed its ability to interact with DHFR mRNA. An in vitro rabbit reticulocyte lysate translation system was used to show that the addition of exogenous human recombinant DHFR protein to in vitro translation reactions specifically inhibited DHFR mRNA translation. These studies suggest that the direct interaction between DHFR protein and its mRNA may be a mechanism for regulation of DHFR synthesis.

Effects of methotrexate on astrocytes in primary culture: light and electron microscopic studies

Recent studies on the animal model suggest that astrocytes may be a primary target for methotrexate (MTX) toxicity. To establish whether the astroglial alterations are due to a direct toxic effect of the drug, we studied the morphologic alterations, mitotic index, viability and growth rate of astrocytes in primary culture after exposure to varying concentrations of MTX in the absence or presence of dibutyryl cyclic AMP (dBcAMP). Dense bodies and cellular debris were noted by light and electron microscopy, and became more prominent with increasing doses and greater frequency of treatment. Degenerating cells and areas of necrosis were seen at higher concentrations. These changes became less conspicuous when MTX was given concurrently with dBcAMP. Large reactive-like astrocytes were also seen after MTX administration both in the absence or presence of dBcAMP. Mitotic rate inhibition was noted at all concentrations but was not dose-related. Cell viability was reduced and remained low up to 48 h after withdrawal of MTX and correlated well with drug concentration, although growth rate did not vary significantly from the control. Our findings show that pure populations of astrocytes can be adversely affected by MTX especially in the absence of bBcAMP, while also causing reactive-like changes in some cells. This report provides further evidence that astrocytes may be a primary target for MTX toxicity and suggests that the gliosis seen in MTX encephalopathy may in part be related to MTX-induced astrocytic injury.

Murine erythroleukemia cell variants: isolation of cells that have amplified the dihydrofolate reductase gene and retained the ability to be induced to differentiate

A series of murine erythroleukemia cell (MELC) variants was generated by selection for the ability to grow in increasing concentrations of the folate antagonist methotrexate (MTX). Growth of the parental MELC strain DS-19 was completely inhibited by 0.1 microM MTX. We isolated cells able to grow in 5, 40, 200, 400, and 800 microM MTX. Growth rates and yields were essentially the same in the presence or absence of the selective dose of MTX for all variants. MTX resistance was not the result of a transport defect. Dihydrofolate reductase (DHFR) from our variants and DS-19 was inhibited to the same extent by MTX. Variants had increased dihydrofolate reductase activities. The specific activity of DHFR was proportional to the selective concentration of MTX employed to isolate a given variant. DNA dot blotting established that the cloned variant (MR400-3) had a 160-fold increase in DHFR gene copy number relative to the parental strain (DS-19). Hybridization studies performed in situ established the presence of amplified DHFR genes on the chromosomes of the MTX-resistant but not the MTX-sensitive (parental) cells. Quantitation of DHFR mRNA by cytoplasmic dot blotting established that the amplified DHFR gene expression was proportional to gene copy number. Thus, MTX resistance was due to amplification of the DHFR gene. The variants retained the ability to be induced to differentiate in response to dimethyl sulfoxide and hexamethylenebis(acetamide) as evaluated by the criteria of globin mRNA accumulation, hemoglobin accumulation, cell volume decreases, and terminal cell division.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular cloning of a cDNA for Chinese hamster ovary asparagine synthetase

In previous reports we have described the isolation and characterization of a number of Chinese hamster ovary (CHO) cell mutants resistant to the amino acid analogue albizziin (Alb). Multistep mutants were derived which showed a high degree of drug resistance and expressed increased levels of asparagine synthetase (AS) levels up to 300-fold over that of the parental cell line. Karyotypic analysis of these mutants revealed homogeneously staining regions (HSRs) usually indicative of gene amplification. In the present work, we provide further proof for gene amplification by showing that the mutants greatly overproduce functional AS mRNA, as evidenced by in vitro translation of purified mRNA and immunoprecipitation of AS. By using these overproducing mutants as sources of mRNA coupled with velocity centrifugation, we have been able to greatly enrich for AS sequences in our mRNA preparations to the point where they represent 1-5% of the total message. This facilitated cloning and selection of the cDNA sequences complementary to the gene. Utilizing these cloned cDNAs, we have demonstrated a correlation between gene copy number and enzyme expression in the parent and Alb-resistant mutants, thus providing direct evidence that drug resistance is due to gene amplification.

The intracellular content of dihydrofolate reductase: possibilities for control and implications for chemotherapy

Intracellular levels of DHFR can be modulated by mechanisms other than gene amplification. We found that MTX itself has an effect and the important features of this mechanism are as follows: (a) Sub-saturating doses of MTX induce intracellular DHFR activity by increasing DHFR synthesis; (b) The time-dependent effect seems quite specific for DHFR and is reversible (7); (c) Elevated DHFR synthesis is accompanied by disproportionate increases in DHFR mRNA; (d) The time scale for maximum induction is appreciably longer than the cell generation time. We suggest that part of the control involved is translational and we postulate that DHFR may regulate its own biosynthesis through feedback mechanisms. It is conceivable that the induction phenomenon could affect the clinical efficacy of MTX-therapy in some instances.

Increased levels of dihydrofolate reductase mRNA can be measured in normal, growth-stimulated mouse fibroblasts

Levels of mRNA for the enzyme dihydrofolate reductase (EC 1.5.1.3) were determined in growth-stimulated 3T6 cells which contained wild-type dosage of the gene coding for this enzyme. As in the case of methotrexate-resistant cells having highly amplified levels of genes for dihydrofolate reductase, an increase in dihydrofolate reductase mRNA by a factor of 2-4 can be determined when cells enter the S phase. This increase is inhibited by sodium butyrate (which inhibits growth-stimulated 3T6 cells in mid G1 phase) but not by hydroxyurea (which inhibits in early S phase). We conclude that with the available methods it is possible to study the regulation of S phase-specific enzymes after growth stimulation at the level of the mRNA, even if gene amplification is not possible or desirable.

A preliminary model of double-minute-mediated gene amplification

A mathematical model of double minute (dm) population dynamics has been developed based upon current concepts of the saltatory replication, random partitioning, nuclear exclusion and loss, and cellular growth inhibition of these extrachromosomal elements. A highly accurate approximate analytical solution has been obtained for the dm frequency distribution at steady state and preliminary analysis of transient states has been performed. The steady state solution has been fit to experimental frequency data of the SW527N carcinoma line, the excellent goodness of fit (X2 = 2.6, d.f. = 29) providing preliminary evidence for the consistency of this set of mechanisms. Two special cases are examined in which extrareplicative dms are produced on both the chromosome and existing dms at equal rates or on the chromosome alone. The model predicts that the population--average rate of extrareplicative dm production is 0.039 +/- S.E. 0.009 dms/hr/cell in the first case and is tenfold higher than when such replication occurs on the chromosome alone (0.0043 +/- S.E. 0.0004 dms/hr/cell). Allowable ranges of the extent of dm-related growth inhibition and dm loss are determined for the SW527N cell line. It is found that dm-related growth inhibition can be nearly as high as that observed for the S180 sarcoma lines (on the order of 0.5% per dm lengthening of the doubling time) or as low as zero.

Expression of a methotrexate-resistant dihydrofolate reductase gene by transformed hematopoietic cells of mice

DNA-mediated gene transfer was used to introduce DNA from a methotrexate-resistant mouse fibroblast cell line into mouse bone marrow cells. This cell line contained a methotrexate-resistant dihydrofolate reductase, active at 10(-4) M methotrexate, which was electrophoretically separable from the wild-type mouse enzyme. Transformed hematopoietic cells were returned to irradiated mice and selected in vivo by methotrexate administration. Some recipients of transformed marrow cells expressed the electrophoretically distinct, methotrexate-resistant dihydrofolate reductase in hematopoietic cells. These observations suggest that successful transformation of marrow stem cells to methotrexate resistance is accomplished by insertion of a dihydrofolate reductase gene coding for a mutant enzyme that is highly resistant to methotrexate.

Chromosome-mediated transfer and amplification of an altered mouse dihydrofolate reductase gene

We have conferred methotrexate resistance on mouse 3T6 fibroblasts by chromosome-mediated transfer of an altered dihydrofolate reductase gene encoding a highly methotrexate-insensitive enzyme. The methotrexate-resistant 3T6 cell line from which the chromosomes were prepared contains multiple copies of the altered dihydrofolate reductase gene, all of which appear to reside on double-minute chromosomes. Transformants selected at 0.2 microM methotrexate contain 10-20 times more of the transferred altered gene than of the resident normal gene. The altered genes are associated with double-minute chromosomes and are permanently lost following growth of the transformants in the absence of methotrexate. Growth of the transformants in increasing concentrations of methotrexate leads to the emergence of cells which have accumulated double-minute chromosomes and which have amplified only the transferred dihydrofolate reductase gene.

Regulation of dihydrofolate reductase gene transcription in methotrexate-resistant mouse fibroblasts

We have used a methotrexate-resistant mouse 3T6 cell line (M50L3) that overproduces dihydrofolate reductase (DHFR) and its mRNA by a factor of about 300 to study the regulation of DHFR hnRNA synthesis. We have previously shown that when resting (G0) M50L3 cells are serum stimulated to reenter the cell cycle, the amount and rate of synthesis of DHFR and the content of DHFR mRNA all begin to increase as the cells enter the S phase of the cell cycle. The increase in DHFR mRNA content is due to an increase in the rate of mRNA production. In the presnt study, we have used the technique of DNA-excess filter hybridization to determine the rate of synthesis of DHFR hnRNA relative to total hnRNA at various times following serum stimulation. We found that the relative rate of DHFR hnRNA synthesis began to increase at about the same time (6 hours), and increased to about the same extent (three to fourfold by 15 hours following stimulation) as we observed previously for DHFR mRNA production. This suggests that the increase in DHFR mRNA production (and consequently DHFR gene expression) is controlled primarily, if not exclusively, at the level of transcription. We also studied the effect of addition of high concentrations of dibutyryl cAMP and theophylline on DHFR gene transcription. We found that addition of these drugs at the time of stimulation completely blocked the increase in DHFR hnRNA synthesis as well as entry into S phase. Addition of the drugs at either 13 or 20 hours following stimulation led to a rapid decrease in DHFR hnRNA synthesis. The drugs were found to have little effect on the ability of the cells to complete S phase when they were added at 13 hours following stimulation. Our results suggest that high intracellular concentrations of cAMP may affect DHFR gene expression not only by preventing the progression of cells through the G1 phase of the cell cycle but also by affecting the rate of DHFR gene transcription in a more direct manner.

Transforming genes and gene products of polyoma and SV40

The small DNA-containing viruses, SV40 and polyoma, transform cells in vitro and induce tumors in vivo. For both viruses two genes required for transformation have been found. The genes required for transformation are also involved in productive infection. Although the two viruses are similar in their effects on cells, the organization of the transforming genes and gene products is different. The purpose of this review is to compare what is known about the biology and the biochemistry of the early regions of the two viruses. The genetic and biochemical studies defining the sequences important for transformation will be reviewed. Then, the products of the transforming genes, called T antigens, will be discussed in detail. There is a substantial body of descriptive information on those products, and studies on the function of the T antigens have also begun.

Unstable amplification of an altered dihydrofolate reductase gene associated with double-minute chromosomes

We have studied a line of 3T6 mouse fibroblasts grown in progressively increasing concentrations of methotrexate. Initially, drug resistance results from amplification of the gene encoding the normal dihydrofolate reductase. Growth of these methotrexate-resistant populations at higher methotrexate concentrations results in the emergence of cells expressing high levels of dihydrofolate reductase with a reduced methotrexate affinity. Using the fluorescence-activated cell sorter, we demonstrate that the variant gene is not present in the population of cells resistant to lower levels of methotrexate, and hence we postulate that the mutational event occurred in cells already containing multiple normal dihydrofolate reductase genes. Growth of the variant cells in the absence of selection is associated with the permanent loss of the altered genes and the disappearance of double-minute chromosomes, on which these genes reside. The pattern of accumulation and loss of double-minute chromosomes is reproduced following transformation of methotrexate-sensitive cells with the altered genes. Our results are consistent with autonomous replication of double-minute chromosomes and a selective advantage of cells with the smallest number of extrachromosomal elements necessary for survival at a given methotrexate concentration.

The effect of polyoma virus, serum factors, and dibutyryl cyclic AMP on dihydrofolate reductase synthesis, and the entry of quiescent cells into S phase

Three procedures were used to induced dihydrofolate reductase synthesis in quiescent cultures of methotrexate resistant mouse fibroblasts: 1) lytic infection with polyoma virus, 2) growth stimulation by replating cells at lower density in fresh cell culture medium, and 3) the addition of fresh medium to confluent cells. Following polyoma infection, an increase in the percentage of S-phase cells began at approximately 20 hours; dihydrofolate reductase synthesis also increased following a lag of 20 hours or more, and continued to increase throughout the late phase of lytic infection, reaching values nearly fivefold greater than that originally present in the quiescent cells. When quiescent cells received fresh medium (with or without replating), the percentage of cells in S phage began to increase by 10 hours and was accompanied by an increase in dihydrofolate reductase synthesis which reached a maximum by approximately 25 hours. These observations show that the initial entry of cells into S phase following mitogenic stimulation is associated with an induction of dihydrofolate reductase synthesis. Dibutyryl cyclic AMP blocked the stimulation of dihydrofolate reductase synthesis and the increase in the percentage of S-phase cells that resulted from the addition of fresh medium to confluent cells. When dibutyryl cyclic AMP was added at various times following the addition of fresh medium, the block in the induction of dihydrofolate reductase synthesis was correlated with a corresponding block in the increase in S-phase cells. These results suggest that dibutyryl cyclic AMP blocks cells at a point in G1 prior to either the induction of dihydrofolate reductase synthesis of the beginning of S phase. The relationship between the control of dihydrofolate reductase synthesis and entry into S phase suggests some form of coordinate control over these two parameters.

[Cell division and cell cycle]

The paper gives a short review of biochemical and genetic analyses of the eukaryotic cell cycle and cell division. Emphasis is placed on the interrelationship of macromolecular syntheses during chromosome replication, the possible involvement of protein phosphorylation in chromosome condensation, the function of contractile proteins in mitosis and cytokinesis and on mechanisms which trigger cell proliferation.

Insertion of new genes into bone marrow cells of mice

Drug resistance genes such as those coding for a methotrexate-resistant dihydrofolate reductase (DHFR) or the thymidine kinase from herpes simplex virus can be used to confer a proliferative advantage on bone marrow cells of mice. As a result of this proliferative advantage, transformed cells become the predominant population in the bone marrow. Efficient gene expression was obtained for both the thymidine kinase and DHFR genes inserted into mouse bone marrow. Such gene insertion techniques may ultimately lead to the cure of life-threatening globinopathies such as sickle cell disease or the beta thalassemias. They may also be useful in reducing the hematopoietic toxicity of anticancer drugs.

Relative increase in polysomal mRNA for R1 cAMP-binding protein in neuroblastoma cells treated with 1,N6-dibutyryl-adenosine 3',-5'-phosphate

Polysomal RNAs were isolated from control neuroblastoma cells and those treated with 1,N6-dibutyrl-adenosine 3',5'-phosphate (Bt2cAMP) and translated in wheat germ lysates. Comparison of proteins synthesized in vitro on two-dimensional gel electrophoretograms showed that there was a specific induction in the synthesis of a protein, Mr 48000, by the polysomal RNAs from Bt2cAMP-treated cells. This protein was identified as the R1 cAMP-binding protein by its coelectrophoresis with unlabelled binding protein and by its specific retention on 8-(6-aminohexylamino)-adenosine 3',5'-phosphate linked to Sepharose. Quantification of the proteins synthesized in vitro with subsaturating inputs of polysomal RNAs showed that there was a 1.4--1.7-fold increase in the synthesis of the R1 cAMP-binding protein by polysomal RNAs isolated from Bt2cAMP-treated cells. There was a similar increase when purified polyadenylated mRNA populations were compared. showing there was no change in the ratio of adenylated to nonadenylated mRNAs in the induced mRNA population. There was no corresponding increase in the synthesis of the R2 cAMP-binding protein although the relative synthesis of several other proteins was also increased and the synthesis of actin and the alpha and beta-tubulin subunits was decreased. The increased levels of the R1 cAMP-binding protein found in Bt2cAMP-treated neuroblastoma cells are therefore partly caused by a specific accumulation of its mRNA on polysomes. The mRNA content of the cytoplasmic messenger ribonucleoprotein (mRNP) population of control cells was insufficient to account for this increase by a translocation of R1 mRNA from the mRNP to the polysome fraction in Bt2cAMP-treated cells. The increase in polysomal R1 mRNA is therefore caused by its increased transcription of post-transcriptional processing or its decreased rate of degradation in Bt2cAMP-treated cells. Although the R1 and R2 binding proteins have identical molecular weights and similar pI values, the specific induction of the mRNA for R1 cAMP-binding protein and the differential distribution of the R1 and R2 mRNAs between the polysomal and messenger ribonucleoprotein compartments show that these two cAMP-binding proteins are encoded by different mRNA populations.

Structure and genomic organization of the mouse dihydrofolate reductase gene

The genomic organization of the mouse dihydrofolate reductase gene has been determined by hybridization of specific cDNA sequences to restriction endonuclease-generated fragments of DNA from methotrexate-resistant S-180 cells. The dihydrofolate reductase gene contains a minimum of five intervening sequences (one in the 5' untranslated region and four in the protein-coding region) and spans a minimum of 42 kilobase pairs on the genome. Genomic sequences at the junction of the intervening sequence and mRNA-coding sequence and at the polyadenylation site have been determined. A similar organization is found in independently isolated methotrexate-resistant cell lines, in the parental sensitive cell line and in several inbred mouse strains, indicating that this organization represents that of the natural gene.