Synchronization of Rous sarcoma virus production in chick embryo cells

Cyclic AMP-mediated growth arrest is associated with increased expression of human T cell leukemia virus type I structural and transforming genes

The effect of increased intracellular cyclic AMP levels on gene expression of the human T cell leukemia virus type I (HTLV-I) provirus was examined. Induction of infected cells to produce elevated levels of cyclic AMP was associated with specific increases in viral surface antigen expression, protein synthesis, p24 release into the supernatant, and RNA levels. The patterns of HTLV-I proviral gene expression observed support results from transfection experiments regarding the function of Tax, Rex, and cyclic AMP in HTLV-I gene regulation. As evidenced by thymidine incorporation, treatment of the infected cells to produce cyclic AMP caused reversible growth arrest. The data indicate that HTLV-I RNA and protein synthesis can proceed at an elevated level in the absence of cell growth. Sustained increases in the intracellular level of cyclic AMP may represent a method for enriching cell cultures in HTLV-I proteins.

Serum and v-src increase the level of a CCAAT-binding factor required for transcription from a retroviral long terminal repeat

Transcription from the long terminal repeat (LTR) of Rous sarcoma virus (RSV) in rat 3Y1 fibroblasts was dependent on the presence of serum. Within 1 hr after addition of serum to a serum-deprived culture, there was a fivefold increase in the level of transcripts initiated at the LTR. This stimulation did not require synthesis of new proteins. The induction of transcription by serum was mostly dependent on two CCAAT boxes in the LTR. Within 1 hr after addition of serum, there was also an increase in the level of a nuclear protein that bound to the two CCAAT boxes, even in the presence of cycloheximide. This serum-induced CCAAT factor also bound CCAAT sequences from other promoters, for example, those of human heat shock protein 70, human c-Ha-ras, and human histone 1, but not to the adenovirus origin of replication or the SV40 enhancer core sequence, suggesting that it was related to CP1 or CP2. Expression from the RSV LTR was not dependent on serum in v-src-transformed cells. Using temperature-sensitive v-src, it was shown that the tyrosine kinase activity of the oncogene increased the amount of CCAAT factor that was present in the nucleus. These findings demonstrate that a basal transcription factor, the CCAAT-binding factor, could be a second messenger for transducing a primary signal from serum to the cellular transcriptional apparatus. This also suggests a pathway by which a tyrosine kinase oncogene could influence the transcription of several genes in the nucleus.

The pathophysiology of murine retrovirus-induced leukemias

Murine leukemia viruses (MuLVs) are retroviruses which induce a broad spectrum of hematopoietic malignancies. In contrast to the acutely transforming retroviruses, MuLVs do not contain transduced cellular genes, or oncogenes. Nonetheless, MuLVs can cause leukemias quickly (4 to 6 weeks) and efficiently (up to 100% incidence) in susceptible strains of mice. The molecular basis of MuLV-induced leukemia is not clear. However, the contribution of individual viral genes to leukemogenesis can be assayed by creating novel viruses in vitro using recombinant DNA techniques. These genetically engineered viruses are tested in vivo for their ability to cause leukemia. Leukemogenic MuLVs possess genetic sequences which are not found in nonleukemogenic viruses. These sequences control the histologic type, incidence, and latency of disease induced by individual MuL Vs.

Regulation of Moloney murine leukemia virus replication in chronically infected cells arrested at the G0/G1 phase

The replication of Moloney murine leukemia virus (MMuLV) in chronically infected mouse cells arrested at the G0/G1 phase of the cell cycle by different procedures was investigated. MMuLV production was inhibited in glutamine- and isoleucine (Gln-Ile)-deprived G0/G1 cells. In contrast, butyric acid treatment, which efficiently arrested the cells at the G0/G1 phase of the cell cycle, did not inhibit MMuLV production. Furthermore, the inhibition of MMuLV production caused by either Gln-Ile deprivation or by interferon (IFN) treatment was overcome by butyric acid treatment. Thus, the replication of MMuLV could be dissociated from cell proliferation. The inhibition of MMuLV production in Gln-Ile-deprived cell cultures was compared to the inhibitory effect of IFN, which is known to affect budding and release of the virus. Rates of MMuLV protein synthesis were not affected in both the IFN-treated and Gln-Ile-deprived cells. However, processing of the viral polyprotein Pre65gag into p30 was blocked in the Gln-Ile-deprived cells. Furthermore, whereas in IFN-treated cells, MMuLV accumulated on the cell surface and could be released upon treatment with trypsin, in Gln-Ile-deprived cells, no virions were released by such treatment. These results indicate that in cells arrested by Gln-Ile deprivation, MMuLV is inhibited at a posttranslation step. This step appears to precede the anti-MMuLV block induced by IFN.

Retrovirus gene expression during the cell cycle. I. Virus production, synthesis, and expression of viral proteins in Rauscher murine leukemia virus-infected mouse cells

Synchronized mouse cells (JLS-V9) chronically infected with Rauscher murine leukemia virus were used to study virus production, the synthesis of gag and env precursor proteins, and the expression of env protein on the cell surface during the cell cycle. The amount of virus released into the medium by synchronized cells during a 30-min interval was determined by using the XC plaque assay and by measuring reverse transcriptase activity. The results show that virus production occurs during mitosis. Labeling of the cell surface of synchronized cells with 125I or with fluorescein-conjugated antiserum shows that the amount of gp 70env on the cell surface parallels cellular growth. Therefore, the cell cycle-dependent release of virus is not accompanied by similar variations in the amount of viral envelope protein on the cell surface. Immunoprecipitation of cells labeled with [35S]methionine, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, was used to measure viral protein synthesis during the cell cycle. The rate of synthesis of gag precursor proteins show three maximums corresponding to the G1, middle S, and late S to G2 phases of the cell cycle. The rate of synthesis of env precursor proteins does not change, suggesting that in these cells the synthesis of these two gene products is controlled separately.

Relationship of Friend murine leukemia virus production to growth and hemoglobin synthesis in cultured erythroleukemia cells

The factors that control oncornavirus formation were analyzed in Friend leukemia cells that undergo hematopoiesis when treated with dimethyl sulfoxide. Suspension cultures of Ostertag FSD-1 cell line were found to enter a G or resting state at the end of their proliferative phase and to simultaneously cease producing helper and dependent components of Friend virus. Whereas the decline in virus production is at least 100-fold, rates of cellular RNA and protein synthesis are only slightly lower in resting than in growing cells. Both resting and growing cells contain similarly large concentrations of the viral proteins P(30) and P(12). Dimethyl sulfoxide induces hemoglobin synthesis in growing cells, but its effects on virus production appear to be indirect results of its action to inhibit cell growth and thus to delay entry of cells into the G resting state. Furthermore, variant cell lines were obtained with differing abilities to synthesize virus or hemoglobin. Some lines no longer produce infectious virus, although they all harbor murine leukemia virus genes which are expressed to varying extents. The major internal protein of these oncornaviruses, P(30), is synthesized in large amounts by all of the cell lines. These results suggest that Friend virus production is not coinduced with erythroid differentiation, as had been proposed, but rather is controlled by a cellular growth cycle.

Effect of Fv-1 gene product on synthesis of N-tropic and B-tropic murine leukemia viral RNA

The production of virus and the synthesis of virus-specific RNA has been studied in Fv-1n/n (NIH/3T3, SIM) and Fv-1b/b (BALB/3T3, SIM-R) cell lines after infection with N- or B-tropic MuLV. It was found that virus production, measured by reverse transcriptase activity in the medium, was 70-100 fold lower in cells resistant at the Fv-1 locus than in permissive cells. The virus-specific RNA, detected by hybridization. In RNA excess with complementary DNA, was reduced by approximately 70-100 fold in cytoplasm of resistant cells compared to permissive cells. A reduction of the same magnitude was observed in the levels of virus-specific RNA extracted from nuclei of resistant cells. Our data therefore show that virus-specific RNA levels are reduced in cells nonpermissive at the Fv-1 locus, suggesting that restriction of the Fv-1 gene product occurs at the level of transcription of the viral genome or at a pre-integration step, or, alternatively, that the RNA transcripts are rapidly degraded after their synthesis.

Requirement for cell division for initiation of transcription of Rous sarcoma virus RNA

Stationary chicken embryo fibroblasts exposed to Rous sarcoma virus (RSV) did not synthesize virus RNA until after division of the infected cells. Initiation of RNA transcription after cell division resulted in transcription of the entire viral genome and was followed shortly thereafter by appearance of progeny virus. Addition of colchicine to RSV-infected stationary chicken embryo fibroblasts resulted in inhibition of cell division and a 24-h delay in the appearance of virus RNA. RSV-infected chicken embryo fibroblasts made stationary 10 to 15 cell generations after infection continued to produce an amount of progeny virus similar to that produced in dividing RSV-infected cells. Teh virus produced from cells made stationary after virus production had begun contained RNA synthesized in stationary cells. Therefore, division of the RSV-infected chicken embryo fibroblast is required for the initiation of transcription of virus RNA, but it is not required for the maintenance of transcription of virus RNA.

Replication process of the parvovirus H-1. I. Kinetics in a parasynchronous cell system

Parasynchronous cultures of hamster embryo cells were used to study some of the events in the replication process of the parvovirus H-1. Synthesis of viral DNA, viral hemagglutinating antigen, and infectious virus were examined. It was found that initiation of DNA synthesis, on which subsequent viral hemagglutinin synthesis was dependent, occurred at a specific time in late S-phase. Production of H-1 viral protein was shown to be sensitive to inhibition by alpha-amanitin. The implications of these findings are discussed.