Polyamine metabolism in enucleated mouse L-cells

The distribution of polyamines between the nucleus and the cytoplasm, and the role of the nucleus in polyamine metabolism, have been studied using cells enucleated with cytochalasin B. Spermidine and spermine were found in the nuclear and the cytoplasmic fractions of L929 cells; their concentration was 3-fold higher in the former fraction. Ornithine decarboxylase activity was only found in the cytoplasm, and this activity could be stimulated in enucleated cells by the addition of fresh medium. These cells synthesized putrescine actively, but the putrescine made was not converted to spermidine, and accumulated to relatively high concentrations. Similarly, methionine did not act as a precursor to spermidine in enucleated cells, in contrast to whole cells, although it was incorporated into cell protein. Spermidine synthesis, unlike putrescine synthesis, appears to be completely dependent on a nuclear component.     

Glucocorticoid receptor phosphorylation in mouse L-cells

This paper summarizes our observations on the phosphorylation state of untransformed and transformed glucocorticoid receptors isolated from 32P-labeled L-cells. The 300-350-kDa 9S untransformed murine glucocorticoid receptor complex is composed of a 100-kDa steroid-binding phosphoprotein and one or possibly two units of the 90-kDa heat shock protein (hsp90), which is also a phosphoprotein. Transformation of this complex to the 4S DNA-binding state is accompanied by dissociation of hsp90. When receptors in cytosol are transformed by heating at 25 degrees C, there is no gross change in the degree of phosphorylation of the steroid-binding protein. Both receptors that are bound to DNA after transformation under cell-free conditions and receptors that are located in the nucleus of cells incubated at 37 degrees C in the presence of glucocorticoid are labeled with 32P. The results of experiments in which the 32P-labeled receptor was submitted to limited proteolysis suggest that the 16-kDa DNA-binding domain is phosphorylated and that the 28-kDa steroid-binding domain is not.     

Large scale transfection of mouse L-cells by electropermeabilization

Mouse L-cells were transfected by electropermeabilization using the selectable plasmid pSV2-neo which confers resistance to G-418 (Geneticin). The DNA concentration used was 1 microgram/ml, the field strength was 10 kV/cm, the duration of the pulse was 5 microseconds. Transfection yield was optimal at a temperature of 4 degrees C when using a time in between consecutive pulses of 1 minute compared to shorter (of the order of seconds) or longer (3 minutes) time intervals. A more detailed study of the relationship between the number of pulses applied (up to 10) and transfection yield showed it to be almost linear in this range at 4 degrees C. The yield of transfectants in response to 10 pulses was up to 1000 per 10(6) cells (using 3.3 pg DNA per cell). The influence of the growth phase of the cells on the transfection yield and/or the subpopulation of the mouse L-cell line used was shown. Furthermore the clone yield depended on the DNA per cell ratio within a very small range.     

Effects of hydroxyurea on DNA synthesis in mouse L-cells

The effect of the anti-metabolite hydroxyurea on DNA synthesis in mouse L-cells has been examined. It was shown previously that when DNA synthesis was diminished to very low levels by treatment with the drug there was preferential incorporation of added [³H]dThd into low molecular weight fragments (Martin, R.F., Radford, I. and Pardee, M. (1977) Biochem. Biophys. Res. Commun. 74, 9–15). On the basis of several criteria it is concluded here that these fragments are a product of semi-conservative nuclear DNA replication. The preferential labelling of DNA fragments, but not their size, is shown to be dependent on the hydroxyurea concentration used. These DNA fragments are also shown, by comparison with normal DNA replication intermediates, to comprise a heterogeneous population of ‘larger-than-normal’ fragments. Different models to account for these findings are considered and it is concluded that the results are compatible with a loss of coordination of DNA synthesis following drug treatment.     

Age-dependent characteristics of immune cytolysis of mouse cells with cytogenetic changes induced by streptolysin O

It has been determined that streptolysin-O induces cytogenetic alterations in the cell culture of murine kidneys in all the mice examined, the most pronounced alterations being noted in the cells of newborn and, especially, old mice. The injection of syngeneic lymphocytes from newborn and sexually mature mice of the middle age group has led to a considerable decrease in the number of cells with altered chromosome number in cultures. The lymphocytes from old mice have no antimutagenic activity.     

Relationship between chromatin structure and replication in mouse L-cells

Mouse L-cells treated with cytosine arabinoside, hydroxyurea, fluorodeoxyuridine, methotrexate, or mitomycin C rapidly cease DNA synthesis and stop dividing. Such inhibition of DNA replication is followed by interruption of formation of lysine- and arginine-containing proteins, including chromatin-bound histones, and by a major reorganization of the heterochromatin of the central nucleoplasm, manifest as disaggregation of large clumps of this condensed chromatin. Morphometric analysis revealed both cell and nuclear enlargement in cells treated with such antimetabolites of DNA replication. These observations are in contrast to those made with WT-4 cells starved of isoleucine or treated with cycloheximide. Isoleucine depletion was associated with inhibition of DNA synthesis and continued increase of cell and nuclear volume, but not with massive disaggregation of heterochromatin. Cycloheximide produced inhibition of DNA synthesis and protoplasmic growth, and also prevented structural reorganization of chromatin. A model is presented which suggests that initiation of chromatin replication is associated with a process, dependent upon de novo protein synthesis, which results in chromatin disaggregation. This can be revealed by inhibition of the correct replication of chromatin DNA and chromatin protein.     

Accumulation of short DNA fragments in hydroxyurea treated mouse L-cells

Treatment of L-cells with hydroxyurea markedly inhibits the incorporation of [³H]thymidine into DNA. The ³H incorporation that persists during hydroxyurea inhibition is largely into 7S DNA chains. The labelled fragments can be chased into higher MW DNA, suggesting that they are intermediates in the replication process. This interpretation concurs with that of earlier reports which describe a similar effect of hydroxyurea on the replication of viral DNA.     

Polyadenylated RNA complementary to repetitive DNA in mouse L-cells.

Complementary DNA, synthesized with L-cell polyadenylated RNA as template, renatured with total L-cell DNA to about 70%. About 30% complementary to unique sequence DNA and another 10 and 30% corresponded to sequences about 20- and 500-fold repetitive. Complementary DNA was fractionated after partial hybridization with total polyadenylated RNA to obtain preparations enriched or impoverished in complements of the most frequent polyadenylated RNA. Renaturation of these complementary DNA fractions with L-cell DNA revealed that most frequent RNAs are transcribed from repetitive DNA sequences, Complementary DNA, density labeled with bromodeoxyuridine, was fractionated by renaturation with L-cell DNA to yield fractions enriched in repetitive and unique sequence DNA. The denisty labeled complementary DNA was purified by equilibrium centrifiguation in an alkaline Cs2SO4 gradient. The complementary DNA representing mainly repetitive DNA sequences hybridized preferentially to frequent polyadenylated RNA.     

Characterization of RNA molecules restricted to the nucleus in mouse L-cells

RNA molecules which are restricted to the nucleus in mouse L-cells were characterized by the technique of RNA/DNA hybridization. Competition of cytoplasmic RNA with labeled nuclear RNA of various sizes revealed that the RNA restricted to the cell nucleus is heterogeneous in size. Competition for sites on fractions of mouse DNA of various base compositions indicated that this unstable RNA is also heterogeneous in base composition. Fractionation of nuclei into three subfractions failed to separate the uniquely nuclear RNA from the precursors of cytoplasmic RNA. The significance of the selective transport of RNA from the nucleus to the cytoplasm and its importance in the control of gene activity in eucaryotic cells is discussed.     

NH2-terminal processing of actin in mouse L-cells in vivo

When Dictyostellium discoideum actin is synthesized in vitro, it is made as a 43,000-dalton polypeptide with an NH2-terminal N-acetylmethionine. The acetylmethionine is then cleaved post-translationally, and the new NH2-terminal aspartic acid is acetylated to give the mature form of actin. Inhibition of methionine acetylation prevents methionine cleavage (Redman, K., and Rubenstein, P. (1981) J. Biol. Chem. 256, 13226-13229). In this paper, we describe the results of experiments designed to discover whether this novel actin processing pathway is peculiar to the rabbit reticulocyte lysate system or whether it is utilized by mammalian cells in vivo as well. We show that in mouse L-929 cells, actin is made as a 43,000-dalton protein with an NH2-terminal N-acylmethionine residue. Experiments using thin layer chromatography and digestion of the acylmethionine residue with hog kidney acylase I demonstrate that the acyl group is an acetyl residue. Pulse-chase experiments show that over the course of 1 h, this precursor is transformed first to an actin with a free NH2-terminal aspartic acid and is subsequently converted to mature L-cell actin with an acetylaspartic acid NH2 terminus. The half-life of the initial actin precursor in the cell appears to be approximately 12-15 min. These studies demonstrate the existence of this novel actin processing pathway in vivo and suggest that it is used for those actins where, in the gene, the initiator methionine codon directly precedes the codon for aspartic or glutamic acids, the residues normally found at the actin NH2 terminus.     

Calcium and A23187-induced cytolysis of mouse thymocytes

The cytotoxic effects of ionophore A₂₃₁₈₇ were studied in parallel with its action on calcium uptake in isolated mouse thymocytes. Under conditions where the cells were preincubated in a calcium-containing medium prior to ionophore treatment a close relationship could be observed between the extent of cell lysis and the stimulation of calcium uptake in the presence of A₂₃₁₈₇. In addition, increasing concentrations of calcium ions in the incubation medium lead to a pronounced decrease of cell viability and to a stimulation of calcium uptake suggesting that calcium is critical for cell survival.     

DNA-binding proteins in the cytoplasm of vaccinia virus-infected mouse L-cells.

Mouse L cell fibroblasts were infected with vaccinia virus and labeled 2 to 3 h postinfection with [35S]methionine. Labeled proteins were fractionated on native and denatured DNA-cellulose columns and then analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Twenty-four 90,000 to 12,500, were detected. VDP-12A (molecular weight, 29,750) had affinity for denatured but not native DNA, and its synthesis was dependent on viral DNA replication. VDP-20 (molecular weight, 41,000) bound very tightly to native and denatured DNA and was displaced only after boiling the protein-DNA-cellulose matrix in 1% sodium dodecyl sulfate. VDP-8,-11,-12,-13, -and-14 behaved electrophoretically like the polypeptide species previously shown to be present in DNA-protein complexes prepared from infected cells. The molecular weights of VDP-10 (50,000), VDP-11 (36,000), and VDP-8 (67,000) were similar to the polypeptide subunits of polyadenylate polymerase and phosphohydrolase I, enzymes purified from virions which have also been shown to have affinity for DNA.     

Synthesis of two non-phosphorylated proteins induced in mouse L-cells by homologous interferon

We have shown that two proteins P1 and P2 of M_r 43000 and 40800 are always detected by two-dimensional gel electrophoresis of interferon-treated mouse L-929 cell extract. These two proteins have an isoelectric point of pH 4.6 and pH 4.7 respectively. If Pl is detectable in small amount in the control gels, P2 is completely absent. Actinomycin D added at the same time as interferon, prevents both P1 and P2 synthesis, but enhances their production when added between 4 to 6 h after interferon. Using molecular weight and isoelectric point as criteria, we have tried to compare P1 and P2 to enzymes induced by interferon. With double-labelled two-dimensional gels by |³⁵S| methionine and |γ³²P| ATP, we have shown that neither P1 nor P2 is phosphorylated. This experimental procedure has allowed us to obtain new data on substrates phosphorylared by interferon induced protein kinase.     

Insulin catabolism by transformed, cultivated mouse fibroblasts (L-cells)]

Cultivated, transformed mouse fibroblasts (L-cells) are endowed with enzyme systems for insulin breakdown that are comparable to those of many other types of non-dedifferentiated insulin target cells (time and substrate dependence, Km value). It cannot yet be decided, however, which of the potential enzyme systems (thiolproteindisulfide-oxidoreductase (TPO) [E.C. 1.8.4.2] or proteinase) is prevalent in insulin breakdown in intact L-cells. Despite the lack of typical target symptoms for insulin, the cultivated L-cell could provide an appropriate experimental model for the insulin metabolism.     

Isolation of rapidly labeled RNA from mouse L-cells using cupric sulfate.

Cultured mouse L-cells, pulse labeled for 5 min with 3H-uridine, were gently suspended in 0.5mM cupric sulfate and 0.5% sodium dodecyl sulfate at 4 degrees C and treated with cold phenol. Only the RNA, containing less than 1% DNA, was extracted by this procedure. The rapidly labeled ribosomal RNA precursors (44S, 34S) and cytoplasmic 8S RNA showed specific activities higher than that of tRNA and were present in the RNA fraction insoluble in 2M NaCl.