Steps in the neoplastic transformation of hamster embryo cells by polyoma virus

Interactions of polyoma and mouse DNAs. I. Lytic infection of bromodeoxyuridine-prelabeled mouse embryo cells

On CsCl isopycnic centrifugation of the DNA extracted from secondary mouse embryo (ME) cultures grown in the presence of 5-bromodeoxyuridine (BUdR) and 5-fluorodeoxyuridine (FUdR) for 40 h, 10 to 25% of the DNA was found to be unsubstituted, 70 to 80% was bromouracil-hybrid DNA, and 5 to 10% was heavy DNA. These results together with cell number determinations, autoradiography, and Feulgen microspectrophotometry revealed three types of cells in these cultures: (i) 60 to 80% of the cells replicated their DNA once, divided, and then stopped mitotic activity, (ii) 5 to 10% were going through a second round of DNA replication; whereas (iii) 10 to 30% did not replicate DNA during the BUdR-FUdR exposure. After the transfer of these cultures to normal medium (without BUdR-FUdR), up to 20% of the cells resumed DNA synthesis asynchronously within 60 h, but no increase in cell number was observed. BUdR-FUdR-treated cultures, which were infected with polyoma virus in the absence of the thymidine analogues, supported a lytic infection to the same extent as did untreated ME cultures. This was concluded from the similar number of cells, which were induced to synthesize DNA, from the similar replication rate of the viral DNA, from the similar number of cells containing polyoma capsid proteins, and from the similar yields of progeny virus determined by hemagglutination and plaque formation. Thus, BUdR-prelabeled ME cultures are suitable for the investigation of interactions of the polyoma and mouse genomes during the lytic infection.

The distribution and asymmetry of mammalian cell surface saccharides utilizing ferritin-conjugated plant agglutinins as specific saccharide stains

The preparation, properties, and some applications of ferritin conjugates of two plant agglutinins, concanavalin A and Ricinus communis agglutinin, are reported. These conjugates serve as specific electron-dense stains for cell- and membrane-bound saccharide residues of the alpha-D-mannopyranosyl and beta-D-galactopyranosyl configurations, respectively, and as examples of a wide range of ferritin-plant agglutinin conjugates useful as high resolution saccharide stains. By using a technique for preparing flattened membrane specimens, it was found with a variety of mammalian cell plasma membranes (lymphocyte, lymphoma, and myeloma and normal, spontaneously and virally transformed fibroblasts) that the ferritin conjugates were localized exclusively to the exterior face of the membrane, with essentially none found on the cytoplasmic face. On the exterior face the topographical distribution of ferritin conjugates appeared to be random. The asymmetrical distribution of saccharide residues to the outer membrane face can be explained by an "assembly line" process whereby new plasma membrane is made from intracellular precursor membranes. It also suggests that the saccharide-containing components of the plasma membrane do not rotate at any appreciable rate from one membrane surface to the other.

Biosynthesis of immunoglobulin A (IgA) and immunoglobulin M (IgM). Requirement for J chain and a disulphide-exchanging enzyme for polymerization

Mouse myeloma cells secreting 19S IgM (immunoglobulin M) (MOPC 104E and TEPC 183) or monomer and polymer IgA (immunoglobulin A) (MOPC 315) were incubated with radioactive leucine and the intracellular and secreted immunoglobulins and immunoglobulin subunits were prepared by preparative sucrose-density-gradient centrifugation. Samples were reduced in the presence or absence of isolated J chain, passed over Sephadex G-25 and then incubated at 37 degrees C for 30min with or without a source of disulphide-interchange enzyme. The extent of reassembly of reduced subunits was then evaluated by electrophoresis in polyacrylamide gels. Provided that J chain and the disulphide-interchange enzyme were supplied, both IgM and IgA could be assembled from their respective subunits, obtained by reductive cleavage of polymeric forms. Under similar conditions, assembly of polymeric forms from intracellular or secreted 7S monomer subunits also occurred. Under these conditions polymerization was total, there being no residue of the monomeric form. Reassembly did not occur in the absence of either J chain or the enzyme. All of the J chain released from IgM by reductive cleavage was incorporated back into the reassembled polymer. The J chain is therefore likely to be an essential structural requirement for polymeric immunoglobulins. A variety of controls ruled out non-specific interactions, and further suggested that the amino acid sequence of polypeptide chains determines the specificity of polymerization. The fact that intracellular IgA and IgM monomer subunits known to be deficient in galactose and fucose can be completely polymerized suggests that the addition of carbohydrate does not control polymerization.

Nerve-muscle interaction in vitro. Role of acetylcholine

Nerve and muscle cells from clonal lines interact in vitro, resulting in the association on the muscle surface of an area of increased acetylcholine sensitivity with a site of nerve-muscle contact. This localization of acetylcholine sensitivity on the muscle cell to a site of contact between nerve and muscle was found to occur when acetylcholine receptors on the muscle had been blocked with alpha-neurotoxin. Localization was also found to occur when the nerve cell had been prevented from releasing acetylcholine. It is concluded that neither the presence of active acetylcholine receptors on the muscle, nor the release of acetylcholine from the nerve, was required for the events leading to the localization of acetylcholine sensitivity in vitro.

Cell contact-dependent increase in membrane D-galactopyranosyl-like residues on normal, but not virus- or spontaneously-transformed, murine fibroblasts

Ricinus communis agglutinin (a lectin specific for beta-D-galactopyranosyl-like oligosaccharide residues) was used to investigate differences in lectin-binding sites on normal 3T3 and transformed SV3T3 and 3T12 cells grown to various cell densities in culture. The agglutinability of SV3T3 cells by R. communis agglutinin decreased when cells were grown to confluency or to a point where the cells were in contact. The agglutinability of 3T3 cells also decreased at confluency, but this result was variable and not as dramatic as with SV3T3 cells. Saturation-binding experiments performed at 4 degrees with (125)I-labeled R. communis agglutinin were used to monitor the number of lectin-binding sites on cells grown to different densities. Transformed cells (SV3T3 and 3T12) did not show cell-culture density-dependent changes in R. communis sites; sparse, touching, and confluent transformed cells had equivalent numbers of sites. However, touching (or confluent) 3T3 cells possessed 2.5-times the number of R. communis sites per cell compared to growing, sparsely populated 3T3 cells. When growing sparse 3T3 cells are compared to growing SV3T3 or 3T12 cells, there are 5-times R. communis sites per unit surface area on the transformed cells. The change in R. communis sites during cell growth in culture is discussed in relation to membrane fluidity and topography of plasma membrane oligosaccharides and glycosyl transferases, which can modify cell surfaces at contact.

Escape from isoantiserum inhibition of lymphocyte-mediated cytotoxicity

Isoantiserum (IS) inhibition of lymphocyte-mediated cytotoxicity (LMC) was studied using an in vitro (51)Cr release assay system. In the early phase of incubation, LMC was competitively inhibited by IS. However, as the incubation continued, LMC irreversibly overcame IS inhibition (the "escape" phenomenon). Addition of fresh antiserum did not alter the course of the escape. Low-temperature incubation of isoantibody-coated target cells delayed the onset of the escape. We have excluded the possibility that the escape phenomenon is induced by complement or by LMC mediated by antigen-antibody complex. It is hypothesized that antibody directed toward an actively metabolizing target cell induces an alteration in the cell membrane that alters further interaction with the antibody. However, sensitivity to lymphocyte cytotoxicity is maintained.

Appearance of acetylcholine receptors during differentiation of a myogenic cell line

Acquisition of acetylcholine receptors during differentiation of a clonal myoblast cell line was monitored with a neurotoxin isolated from venom of the Indian Cobra Naja naja. Toxin bound specifically and reversibly to acetylcholine receptors of the differentiated cells. Specificity of the binding reaction was assayed by measurement of the ability of various cholinergic agonists and antagonists to compete with neurotoxin for its binding site. The rate of toxin binding paralleled the rate of inactivation of functional acetylcholine receptors, as measured by iontophoretic application of acetylcholine. Bound toxin was released from the cells with a half-life of about 7 hr. This release was not associated with a decrease in the total number of toxin-binding sites. A slow hyperpolarizing response to acetylcholine seen in myoblasts was insensitive to toxin; the appearance of toxin-binding sites parallels the appearance of fused fibers during differentiation of the muscle cells in tissue culture.

Strand orientation of simian virus 40 transcription in productively infected cells

Strand orientation of transcription in BSC-1 cells infected by simian virus 40 (SV40) was investigated by annealing RNAs extracted from infected cells with asymmetric complementary RNA from SV40 DNA synthesized in vitro by RNA polymerase from Escherichia coli. The results suggest that the early viral RNA sequences (those made before replication of viral DNA) and the late RNA sequences (those synthesized concurrently with replication of viral DNA) are transcribed from opposite strands of the SV40 DNA. The RNA synthesized in vitro is of the same polarity as the early in vivo RNA and is complementary to the late in vivo RNA. Knowledge of strand selection in lytic infection is a step towards understanding the regulation of transcription of this viral DNA.

Development of chicken lymphoid system. I. Synthesis and secretion of immunoglobulins by chicken lymphoid cells

Synthesis and secretion of Ig by chicken lymphoid cells was studied. Both spleen and bursa cells synthesize and secrete IgM and IgG whereas Ig was not detected in thymus cells. In contrast to the spleen cells which synthesize H and L chains in balanced quantities, the bursa cells synthesize and secrete free L chains. In addition to the lymphoid cells which secrete IgM or IgG, the bursa appears to contain a cell population which synthesizes nonsecretory Ig. The structure of this Ig was studied by specific serological precipitation and by SDS-acrylamide gel electrophoresis. The H chains of this nonsecretory Ig are serologically related to micro-chains and exhibit a smaller molecular weight (i.e., approximately 50,000) in SDS-acrylamide gel electrophoresis than H chains of IgG and IgM synthesized by the spleen cells (i.e., approximately 70,000).

Two kinds of antigen suppression in tumor cells revealed by cell fusion

The Ehrlich ascites tumor, which has been subjected to prolonged selection for growth in allogeneic hosts, possesses powerful mechanisms for the suppression of antigens normally found on the cell surface. It has previously been shown (1, 2) that when cells in which surface antigens are fully expressed are fused with Ehrlich cells, the suppressive mechanisms of the latter continue to operate and the new surface antigens introduced into the hybrid cell by the other partner are also suppressed. In the present paper we describe the properties of hybrids in which one parent cell was the TA3 ascites carcinoma. There are two sublines of this carcinoma which originally arose as a spontaneous mammary carcinoma in a strain A mouse. The TA3/St line has a high concentration of H-2(a) antigens and shows a strain-specific transplantation behavior; the TA3/Ha subline has a drastically reduced antigen concentration and readily transgresses histoincompatibility barriers. The immunoresistant TA3/Ha subline arose spontaneously without having been subjected to any known immunological selection pressure. Hybridization of TA3/Ha cells with normal diploid ACA fibroblasts reestablished full expression of H-2(a) antigens in nine independently derived hybrid clones. Full reestablishment of both D- and K-end components of the H-2(a) complex could be demonstrated. In some hybrid clones the concentration of H-2(a) antigens was found to be comparable to that seen in (A x ACA)F(1) fibroblasts, whereas in others a higher concentration was observed, even exceeding, in some cases, the levels found in the TA3/St line. The H-2(f) complex, contributed by the ACA parent cell, was fully expressed in eight of the nine hybrid clones studied. Antigen suppression thus behaves as a recessive character in the TA3/Ha hybrids, whereas in the Ehrlich hybrids antigen suppression is dominant.

Insulin receptors in human circulating cells and fibroblasts

Human lymphocytes obtained from fasted adult subjects and cultured human tumor lymphocytes were investigated for specific insulin receptors. By use of monoiodoinsulin, specific insulin binding sites were demonstrated in peripheral human lymphocytes, cultured human lymphocytes, and in other types of human circulating cells. Insulins and insulin derivatives that varied in their potency to stimulate glucose oxidation in the fat cell and to inhibit binding of [(125)I]insulin to purified plasma membranes, varied in an analogous fashion in their ability to inhibit the binding of labeled insulin to human lymphocytes. Hormones that had no effect on the binding of insulin to fat cells or liver membranes also had no effect on the binding of insulin to lymphocytes. Binding was time and temperature dependent; dissociation of [(125)I]insulin was rapid upon addition of 10 muM insulin. These findings afford a direct approach to the study of endocrine disorders in man.

Biosynthesis of the carbohydrate portion of immunoglobulins. Incorporation of radioactive fucose into immunoglobulin G1 synthesized and secreted by mouse plasma-cell tumour MOPC 21

Incorporation of radioactive fucose into the immunoglobulin G1 myeloma protein secreted by mouse plasma-cell tumour MOPC 21 is stereospecific for the l-isomer. Heavy chains of the secreted form of the myeloma protein carry 90% of the label in fucose residues of their carbohydrate moieties. A small but significant amount of the intracellular immunoglobulin G1 of the mouse plasma-cell tumour MOPC 21 appears to be labelled. Serum in the incubation medium supplies low-molecular-weight diffusible substances necessary to maintain continuous secretion of fucose-labelled myeloma protein beyond 2-3h, and of leucine-labelled myeloma protein beyond 6-8h. In medium containing extensively dialysed serum the secretion of leucine- and fucose-labelled myeloma protein can be restored by the addition of 250mum-d-mannose, 250mum-d-galactose and 250mum-glucosamine. Synthesis and secretion appear to be facilitated in the presence of these sugars, although secretion of myeloma protein devoid of terminal fucose residues is possible for a limited time-period.

Biosynthesis of the carbohydrate portion of immunoglobulins. Kinetics of synthesis and secretion of [3H] leucine-, [3H] galactose- and [3H] mannose-labelled myeloma protein by two plasma-cell tumours

The kinetics of incorporation of leucine, galactose and mannose into intracellular and secreted myeloma protein, MOPC 21 IgG(1) and MOPC 46 kappa-type light chain, by cell suspensions of two myeloma plasma-cell tumours, MOPC 21 and MOPC 46, were similar. Radioactive galactose was incorporated to over 90% into galactose residues of intracellular and secreted protein, mannose to over 90% into glucosamine and mannose residues of intracellular protein and to over 90% into glucosamine, mannose and fucose residues of secreted protein, but not into galactose residues. The results show that specific residues in the carbohydrate portion of myeloma proteins can be labelled by specific radioactive monosaccharides, and suggest that fucose residues are added, while myeloma protein is in its final stage of secretion from the plasma cell. The kinetics of incorporation indicate at least three sequential precursor-product relationships between different intracellular forms and the secreted form of myeloma protein.

Antigen-specific synergism in the immune response of irradiated mice given marrow cells and peritoneal cavity cells or extracts

A synergistic immune response to foreign erythrocytes may be induced in heavily irradiated mice injected with a mixture of isologous cells obtained from marrow and from the peritoneal cavity. Under appropriate conditions, homologous or heterologous peritoneal cavity cells, heat-killed cells, or cellfree extracts made from such cells are also effective. The activity of the peritoneal cavity cells or extracts is antigen-specific, in the sense that cells or extracts obtained from animals previously immunized with the test antigen produce much stronger synergistic effects than do cells from animals immunized with some other antigen; however, the peritoneal cavity cells or extracts are not immunogenic when tested in primed animals. The marrow cells, demonstrated to contain precursors of the antibody-forming cells produced during this synergistic immune response, also show a form of antigen-specificity.

Leukemia viruses associated with mouse myeloma cells

Myeloma cells derived from BALB/c and C3H mice show evidence of infection by a murine leumemia virus. The immunoglobulin-producing myelomas secrete an RNA-containing virus with a density of 1.20 to 1.22 gm/cm(3). RNA with a sedimentation coefficient of 74 S in 0.1 M sodium sodium chloride has been isolated from secreted virus particles and has a base composition similar to that found for other murine leukemia virus RNA. An intracellular virus particle has been partially purified and has a density of 1.29 to 1.32 gm/cm(3). Both extracellular and intracellular virus particles contain the leukemia virus group-specific antigen.

In vitro differentiation of a mouse neuroblastoma

Mouse tumor C1300 has been established in tissue culture. The cells have a round cell morphology in both the subcutaneous tumor and in suspension culture. However, when given a surface on which to attach, they send out processes up to 3 mm in length and assume the morphology of mature neurons. The attached cells are stained by the Bodian silver procedure for neurons, whereas the cells grown in suspension are not. Electron microscopy reveals that the attached cells contain neurofilaments, neurotubules, and densecore vesicles indicative of nerve fibers. Both free-floating and attached cells have tyrosine hydroxylase activity characteristic of sympathetic nervous tissue. Apparently cell attachment can induce morphological differentiation from an anaplastic round cell to a cell which has many properties of a mature neuron.

Histopathology of sarcomas induced in hamsters by clones of in-vitro SV40-transformed homologous heart cells. Attempts to correlate morphologic features and degree ofoncogenic potential

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