Transformation by murine sarcoma virus: fixation (deoxyribonucleic acid synthesis) and development

Enhancement of endogenous xenotropic murine retrovirus expression by tuftsin

The exposure of a high-passage clone of Kirsten sarcoma virus transformed Balb/c (K-Balb) mouse cells to tuftsin (Thr-Lys-Pro-Arg) enhanced the expression of endogenous xenotropic retrovirus. The tetrapeptide increased the expression of virus that was infectious for rat, but not mouse, cells in a concentration-dependent fashion (0.001-1000 micrograms/ml). Increased virus expression could be achieved during short-term incubations (3-4 hr), with maximum enhancement occurring over longer time periods (16-18 hr). The enhancement of virus expression by tuftsin was proportional to the spontaneous release of virus. The infectivity of the enhanced virus was neutralized by goat anti-RLV gp 70 serum. Actinomycin D inhibited the induction of virus, suggesting that enhanced expression required de novo RNA synthesis. Tuftsin stimulated DNA, RNA, and protein synthesis in K-Balb cells during 16-hr incubations. Increased cellular proliferation was also seen at various time periods. The effects observed using K-Balb cells offer an opportunity to study the modulation of gene expression by tuftsin in a fibroblast culture system.

Attempts to induce tubular deformations and subsequent mixed tumours from organotypic cultures of mouse renal mesenchyma

With the aim of expanding knowledge on the pathogenesis of nephroblastomata, an embryological organ culture system (Grobstein, 1956) was tested for its applicability to in vitro carcinogenesis experiments by using murine sarcoma virus (MSV-M) and 3-methylcholanthrene (MCA). Treatment of CBA/H-T6 mouse metanephrogenic mesenchyma with MSV-M at the pretubular stage neither disturbed the glomerulogenesis nor induced rapid malignant transformation. Treatment of the same tissues with MCA considerably inhibited the glomerulogenesis but failed to also reduce rapid malignant transformation. However, one MSV-M, one MCA and two untreated cultures showed malignant transformation after prolonged survival in vitro and produced different histological types of tumours upon transplantation into newborn CBA/H-T6 mice.

Rescue of murine sarcoma virus from a sarcoma-positive leukemia-negative cell line: requirement for replicating leukemia virus

The nature of murine sarcoma virus (MSV) "defectiveness" was investigated by employing an MSV-transformed mouse 3T3 cell line which releases noninfectious virus-like particles. Rescue kinetics of MSV, observed after murine leukemia virus (MuLV) superinfection of these "sarcoma-positive leukemia-negative (S + L -)" mouse 3T3 cells, consisted of a 9- to 12-hr eclipse period followed by simultaneous release of both MSV and MuLV with no evidence for release of infectious MSV prior to the production of progeny MuLV. Addition of thymidine to the growth medium of MuLV-superinfected S + L - cells at a concentration suppressing deoxyribonucleic acid synthesis inhibited the replication of MuLV and the rescue of MSV. MSV production closely paralleled MuLV replication under a variety of experimental conditions. These results suggest that replication of MuLV is required for the rescue of infectious MSV from S + L - cells and that one (or more) factor, produced late in the MuLV replicative cycle, is utilized by both viruses during virion assembly. During the course of these experiments, virus stocks were recovered which contained infectious MSV in apparent excess over MuLV. These stocks were used for generating new S + L - cell lines by simple end point dilution procedures.

Evidence for a DNA replicative genome for RNA-containing tumor viruses

The requirement for DNA synthesis in the reproduction of RNA tumor viruses was examined using 5-bromodeoxyuridine (BrdU). Addition of BrdU for the first 12 hr after exposure of cells to Rous sarcoma virus resulted in a large decrease in infectious progeny. However, incorporation of [(3)H]uridine into the viral RNA of virions was unaffected. Also, the numbers of virions visualized by electron microscopy were similar in BrdU-treated and untreated samples. Viral group-specific antigen was synthesized after BrdU treatment, and BrdU had little effect on transformation of cells by Rous sarcoma virus. The results support the hypothesis that the replicative genome of RNA-containing tumor viruses contains DNA.

Mechanism of carcinogenesis by RNA tumor viruses. I. An RNA-dependent DNA polymerase in murine sarcoma viruses

A highly active and stable DNA polymerase was found in purified preparations of two murine sarcoma viruses. Enzyme activity is not detected in most virus preparations unless they are treated with low concentrations of a nonionic detergent such as Nonidet P-40. The incorporation of labeled thymidine triphosphate requires all four deoxyribonucleoside triphosphates and either Mg(2+) or Mn(2+). Enzyme activity is proportional to virus concentration and is linear with time up to 90 min. That the template is RNA is suggested by the reduction in polymerase activity upon treatment of murine sarcoma virus with RNase, and by the absence of detectable amounts of DNA in the virus. That the product is DNA is shown by the incorporation of all four deoxyribo-nucleoside triphosphates into an acid-insoluble product which is stable in alkali, is destroyed by DNase, sediments in alkaline sucrose gradients with a sedimentation coefficient of 7 S, and bands in isopycnic CsCl gradients with a mean buoyant density of 1.700.

On the role of DNA synthesis in avian tumor virus infection

Preinfection of chick embryo fibroblasts with an avian tumor virus does not eliminate the requirement of early DNA synthesis for other avian tumor viruses superinfecting the same cells several days after the primary inoculum. The function served by the early DNA in avian tumor virus infection either fails to persist into late infection or is genetically specific for each viral type.

Interferon production by human leukocytes in vitro: some biological characteristics

Interferon was optimally produced in human peripheral leukocyte cultures incubated for approximately 19 hr in the presence of Sendai virus at a multiplicity of 10 to 50 EID(50)/cell. For determining whether deoxyribonucleic acid (DNA) synthesis per se was essential for interferon production, 1-beta-D-arabinofuranosylcytosine (Ara-C), a potent DNA inhibitor was studied for its effect on interferon production in leukocytic and bone marrow cell cultures. These cells showed no impaired capacity to produce interferon when treated with 15 mug of Ara-C per ml. Interferon yields were also determined in leukocyte cultures treated with actinomycin D (0.1 mug/ml) and puromycin (10 mug/ml) at various times before and after virus inoculation. The data suggested that sequential transcriptive and translational events were required for the de novo synthesis of interferon by the infected leukocytes, in a manner similar to other known virus-induced interferon-producing systems. The synthesis of macromolecules and the effects of antimetabolites in leukocytes and bone marrow cell cultures were followed by measuring the incorporation of thymidine-2-(14)C, uridine-5-(3)H, and L-phenylalanine-1-(14)C. The effect of 0.1 mug of actinomycin per ml on the capacity of leukemic leukocytes to produce interferon was also studied. Preliminary data showed that, in contrast to nonleukemic leukocytes, interferon production by leukemic leukocytes was only partially inhibited by actinomycin.

Mouse leukemia virus: "spontaneous" release by mouse embryo cells after long-term in vitro cultivation

BALB/c mouse embryo cells maintained in tissue culture on a schedule of rapid transfer at high cell density develop into tumorigenic lines that have lost contact inhibition of cell division. After several months in culture, certain of these lines begin to release mouse leukemia virus. This virus has properties indistinguishable from those of virus found in adult BALB/c mice. The evidence presented here demonstrates that the murine leukemia virus genome must be present in the original embryo cultures. The possibility that the genetic information for making murine leukemia virus is present in a repressed form in every mouse embryo cell is discussed.