Effect of the irradiated microenvironment on the expression and retrotransposition of intracisternal type A particles in hematopoietic cells.

We have previously demonstrated that the frequency of transformation of the factor-dependent hematopoietic cell line FDCP-1JL26 was dramatically increased when cells were cocultured with the irradiated bone marrow cell line D2XRII. In many of our factor-independent subclonal cell lines that we examined, transformation to factor independence appeared to be due to the retrotransposition of intracisternal type A particles (IAP) into the growth factor genes that are normally required for survival and growth of FDCP-1JL26 cells. To determine the role of the irradiated microenvironment in the evolution of factor-independent cells, we have examined the expression and retrotransposition of IAPs after exposure to the irradiated bone marrow stromal cell line D2XRII. Differential display and Northern blot analysis demonstrated that IAPs were overexpressed in a nonautocrine factor-independent subclonal cell line, FI7CL2. The frequency of retrotransposition was determined by the introduction of the IAP-neo(RT) plasmid into FDCP-1JL26 cells. The IAP-neo(RT) contains a neomycin resistance gene (neo) that only becomes active after retrotransposition, and thus the frequency of retrotransposition in FDCP-1JL26 cells was quantified by determining the frequency of neo-resistant cells.No significant increases in the expression of IAPs were observed after the cells were exposed to the irradiated stromal cells. This observation is in agreement with the observation that no increase in the frequency of retrotransposition could be detected. These results suggest that the irradiated bone marrow may have a passive role in the selection of factor-independent cells. During cocultivation, bone marrow stromal cells may provide a factor(s) to hematopoietic cells that allow it to survive in medium lacking IL-3. At random, a retrotransposition may occur that provides a selective advantage to the hematopoietic cells. In the absence of the irradiated stromal cells, the hematopoietic cells are perhaps more likely to die and therefore are not available for a random retrotransposition event to occur. This model is to be distinguished from an active role in which the irradiated microenvironment would synthesize or activate a factor(s) that promotes retrotransposition.

Activation of the nitric oxide synthase 2 pathway in the response of bone marrow stromal cells to high doses of ionizing radiation.

Reverse transcription-polymerase chain reaction and immunofluorescence analysis of D2XRII murine bone marrow stromal cells showed that gamma irradiation with doses of 2-50 Gy from (137)Cs stimulated expression of nitric oxide synthase 2 (Nos2, also known as iNos). The activation of Nos2 was accompanied by an increase in the fluorescence of 4,5-diaminofluorescein diacetate, a nitric oxide trap, and accumulation of 3-nitrotyrosine within cellular proteins in a dose-dependent manner. These effects were inhibited by actinomycin D and by N-[3-(aminomethyl)benzyl]acetamidine dihydrochloride, a specific inhibitor of Nos2. The induction of Nos2 expression and Nos2-dependent release of nitric oxide in D2XRII cells was observed within 24 h after irradiation and was similar in magnitude to that observed in cultures incubated with Il1b and Tnf. We conducted (1) confocal fluorescence imaging of 3-nitrotyrosine in bone marrow cells of irradiated C57BL/6J mice and (2) 3-nitrotyrosine fluorescence imaging of FDC-P1JL26 hematopoietic cells that were cocultured with previously irradiated D2XRII bone marrow stromal cells. Exposure to ionizing radiation increased the production of 3-nitrotyrosine in irradiated bone marrow cells in vivo and in nonirradiated FDC-P1JL26 cells cocultured with irradiated D2XRII cells for 1 or 4 h. We suggest that nitrative/oxidative stress to the transplanted multilineage hematopoietic cells due to exposure to nitric oxide released by host bone marrow stromal cells may contribute to the genotoxic events associated with malignant alterations in bone marrow tissue of transplant recipients who are prepared for engraftment by total-body irradiation.

The role of intracisternal A-type particles in the evolution of factor-independent murine hematopoietic cell lines.

Cocultivation of a clonal factor-dependent hematopoietic cell line (FDC-P1JL26) with an irradiated bone marrow stromal cell line (D2XRII) significantly increased the frequency of isolation of factor-independent subclones. Eight out of nine factor-independent subclonal lines showed expression of IL-3, GM-CSF or both cytokine mRNAs by reverse-transcription polymerase chain reaction (RT-PCR) and seven of these expressed biologically active GM-CSF or IL-3. In three cell lines that synthesized biologically active IL-3 (FIJ1, FIJ4D and FIJ10D) insertion of an IAP sequence into the IL-3 gene was detected by PCR analysis and the insertions were confirmed by DNA sequence analysis of PCR or RT-PCR fragments. In the four cell lines in which no IL-3 expression was detected no IAP insertions were detected. Rearrangements of the GM-CSF gene were detected in three factor-independent cell lines and an insertion of an IAP into the GM-CSF gene was confirmed by DNA sequence analysis of PCR fragments. In contrast to results with IL-3, insertion of an IAP into the GM-CSF gene did not correlate with GM-CSF expression. In one cell line that contained an IAP insertion into the GM-CSF gene, no GM-CSF was detected by biological assay nor by RT-PCR. Retrotransposition of IAPs may be responsible for the emergence of factor-independent cells in our cocultivation system and other IAP insertions may prove to be responsible for the factor-independent phenotype seen in the non-autocrine factor-independent cell line, FI7CL2.

Stromal cell involvement in leukemogenesis and carcinogenesis.

There is increasing evidence that the supportive cells (stromal cells) in nearly all organs containing cellular self-renewal systems are involved in carcinogenesis. One body of evidence specific to irradiation leukemogenesis documents the role of irradiated murine stromal cells in the cell biologic changes associated with evolution of leukemia in cocultivated, nonirradiated stem cells. Stem cell phenotypic changes that have been documented include upregulation of cell surface c-fms, downregulation of growth requirement for obligatory growth factors, and the appearance of novel transcripts detected by differential display. A second body of evidence documents the potential role of stromal cells functioning as biologic tumor promoters through their release of reactive oxygen species (ROS), and production of altered adhesion molecules or growth factors during the chronic response to chemical or physical carcinogens. These molecular biologic mechanisms, potentially operative in stromal cells, can block apoptosis and induce DNA strand breaks in closely associated self-renewing stem cells. In an in vivo model of irradiation effects on lung stromal cells, we have irradiated the lungs of control C57BL/6J mice or other mice with orthotopic Lewis lung tumors and shown that TGF-beta release is increased following irradiation. The TGF-beta increase by irradiation may specifically be inhibited by administering an inhalation plasmid liposome mixture containing a transgene for human manganese superoxide dismutase prior to irradiation. An appreciation of the role of stromal cells in leukemogenesis and carcinogenesis may also be very relevant to the design of new therapeutic strategies for treatment of cancer, particularly since current strategies focus on eradication of stem cell transformants and do not rigorously address the persistence of surviving stromal cells.

Role of bone marrow stromal cells in irradiation leukemogenesis.

The role of bone marrow stromal cells of the hematopoietic microenvironment in ionizing-irradiation leukemogenesis is a focus of current investigation. Evidence from recent in vitro and in vivo experiments suggests that damage by slowly proliferating cells of the hematopoietic microenvironment contributes to the sustained survival of irradiation-damaged hematopoietic progenitor cells/stem cells and can contribute to the selection and proliferation of a malignant clone. The molecular mechanism of the interaction of irradiated stromal cells with attached hematopoietic cells has been difficult to evaluate. Irradiated bone marrow stromal cell line D2XRII demonstrated altered patterns of fibronectin splicing and increased expression of several transcriptional splice variants of macrophage-colony-stimulating factor. Differential display has revealed specific radiation-induced gene transcripts which persist after irradiation of stromal cells in vitro or in vivo. In recent experiments, we demonstrated that irradiation of mouse bone marrow stromal cell line D2XRII induces release of significant levels of transforming growth factor (TGF)-beta into the tissue culture medium despite the lack of a detectable increase in TGF-beta mRNA. Since TGF-beta is known to induce reactive oxygen species (ROS), we tested how a target hematopoietic cell line, responsive to ROS by up-regulation of a transgene for an antioxidant protein, responded to cocultivation with irradiated bone marrow stromal cells. Bone marrow stromal cell line GPIa/GBL, derived from long-term bone marrow culture of a C57BL/6J-GPIa mouse, was irradiated in vitro and then cocultured with the interleukin (IL)-3-dependent hematopoietic progenitor cell line 32D cl 3, or with each of several subclonal lines expressing a transgene for human manganese superoxide dismutase (MnSOD). Cobblestone island formation, as a measure of adherence and proliferation by 32D-MnSOD clones in the presence or absence of IL-3, was increased with irradiated compared to control GPIa cells. Furthermore, using a fluorescent dye which detects ROS, hematopoietic cells cocultivated with irradiated stromal cells demonstrated higher levels of intracellular ROS than cells cocultivated and forming cobblestone islands on nonirradiated stromal cells. Since ROS are known to induce mutations in hot spots in the p53 gene, it appears worthwhile to investigate a potential mechanism for irradiated stromal cell induction of hematopoietic stem cell transformation through ROS-induced mutations. The present cell culture and molecular biology techniques provide new methods to analyze the effects of irradiated stromal cells on closely attached hematopoietic stem cells during irradiation leukemogenesis.

The single base pair substitution responsible for the Syn phenotype of herpes simplex virus type 1, strain MP.

Nucleotide sequences were determined for portions of the genomes of the syncytial (Syn) mutant of herpes simplex virus type 1, strain MP, and the related wild-type strain mP. Comparisons of the nucleotide sequences showed only 1 bp difference between the DNAs of strains MP and mP in the region to which the Syn mutation of MP had previously been mapped. This base pair substitution in MP (at map coordinate 0.737) eliminates a ThaI restriction endonuclease recognition site that is present in mP DNA. Analyses of MP X mP recombinant viruses showed that presence of the ThaI site correlates with the Syn+ phenotype and absence of the ThaI site correlates with the Syn phenotype as predicted. We conclude that the base pair substitution at map coordinate 0.737 is responsible for the Syn phenotype of MP. This mutation could alter translation in four of the six reading frames, causing amino acid substitutions. From only one of these reading frames is a product likely to be expressed. The 338-amino acid polypeptide that could be expressed has features characteristic of membrane-associated proteins, including hydrophobic domains, potential sites for the attachment of N-linked carbohydrate, and a potential cleavable signal sequence.

Enhanced rate of conversion or recombination of markers within a region of unique sequence in the herpes simplex virus genome.

Insertion mutants of herpes simplex virus type 1, containing a second copy of the sequences of BamHI fragment L (map coordinates 0.706 to 0.744) inserted in inverted orientation into the thymidine kinase gene (at map coordinate 0.315), have been further characterized. We reported previously that, as a result of intramolecular or intermolecular recombination between copies of the BamHI-L sequence at the normal locus and inserted locus, a high proportion of progeny genomes exhibited either inversions of the unique sequence flanked by these inverted repeats or other rearrangements. Now we report that a genetic marker (syn-1 or syn-1+) originally present only in the inserted copy of BamHI fragment L appears in progeny at both the normal and inserted loci, and vice versa, at high frequency. Because these phenomena have not been observed with other insertion mutants containing duplications of other sequences from unique regions of the genome, we conclude that BamHI fragment L contains an element that enhances the rate of homologous recombination in adjacent sequences, resulting in genome rearrangements and gene conversion-like events.

Novel rearrangements of herpes simplex virus DNA sequences resulting from duplication of a sequence within the unique region of the L component.

We constructed insertion mutants of herpes simplex virus type 1 that contained a duplication of DNA sequences from the BamHI-L fragment (map units 0.706 to 0.744), which is located in the unique region of the L component (UL) of the herpes simplex virus type 1 genome. The second copy of the BamHI-L sequence was inserted in inverted orientation into the viral thymidine kinase gene (map units 0.30 to 0.32), also located within UL. A significant fraction of the progeny produced by these insertion mutants had genomes with rearranged DNA sequences, presumably resulting from intramolecular or intermolecular recombination between the BamHI-L sequences at the two different genomic locations. The rearranged genomes either had an inversion of the DNA sequence flanked by the duplication or were recombinant molecules in which different regions of the genome had been duplicated and deleted. Genomic rearrangements similar to those described here have been reported previously but only for herpes simplex virus insertion mutants containing an extra copy of the repetitive a sequence. Such rearrangements have not been reported for insertion mutants that contain duplications of herpes simplex virus DNA sequences from largely unique regions of the genome. The implications of these results are discussed.

Fine mapping of mutations in the fusion-inducing MP strain of herpes simplex virus type 1.

Previous studies [W. T. Ruyechan, L. S. Morse, D. M. Knipe, and B. Roizman (1979) J. Virol. 29, 677-697] have shown that multiple mutations are responsible for the mutant phenotypes of herpes simplex virus type 1, strain MP, and have indicated that these mutations may be located on the genome between map coordinates 0.70 and 0.83. Strain MP produces large syncytial (Syn) plaques on many cell types and does not express glycoprotein C (gC-), whereas a sibling strain mP produces wild-type, small, nonsyncytial (Syn+) plaques and is gC+. Cloned DNA fragments from strains MP and mP (and strain F) were used in marker transfer and marker rescue experiments to map more precisely the mutations in MP. It was found that a 680-bp fragment from MP DNA (map coordinates 0.735 to 0.740) could transfer a Syn marker to mP and that, conversely, an overlapping fragment from mP DNA (map coordinates 0.728 to 0.744) could rescue the Syn mutation of MP. Recombinant viruses obtained in these experiments differed from the donor of the cloned DNA fragment in plaque size, however, indicating that mutation(s) at other regions of the MP genome cause enlarged plaques, in which the infected cells are less rounded than in wild-type plaques. A fragment of MP DNA from map coordinates 0.60 to 0.64 transferred a mutation causing the gC- phenotype to strain mP, and a fragment of F DNA from map coordinates 0.62 to 0.64 rescued the gC- mutation of MP. These results, coupled with data published by Frink et al. [(1983) J. Virol. 45, 643-467], indicate that the mutation responsible for the gC- phenotype of MP may be in the structural gene for gC.

Expression of herpes simplex virus glycoprotein C from a DNA fragment inserted into the thymidine kinase gene of this virus.

Previous reports have described mutants of herpes simplex virus type 1 that fail to produce or accumulate one of the major glycoproteins, glycoprotein C (gC). This defect is not lethal in cell culture, has been associated with the syncytial plaque morphology of some mutants, and may result from mutations that map to a region on the genome noncontiguous with the structural gene for gC. To investigate the conditions required for, and consequences of, gC expression in a specific genetic background, we have inserted a wild-type allele of the gC gene into the thymidine kinase gene (tk) of a gC- fusion-inducing viral mutant, strain MP. This was accomplished by identifying cloned viral DNA fragments homologous to gC mRNA, inserting the appropriate fragments into the viral tk cloned in pBR322, and then cotransfecting cells with the recombinant plasmids and DNA from strain MP, for selection of insertional TK- mutants. All TK- mutants containing insertions of appropriate sequences (in either orientation) into tk were found to express gC while maintaining the syncytial plaque morphology of strain MP. Elimination of the insertion from one of the TK- mutants was accompanied by loss of ability to produce gC. Our results permit more precise mapping of the DNA sequence encoding gC, to a subfragment of Sal I fragment R (map coordinates 0.620-0.640) and indicate also that promoter sequences for the gC gene may be located in this fragment. Moreover, we can conclude that the previously described regulatory mutation of strain MP does not prevent expression of gC from the DNA inserted into its gene tk and that the syncytial phenotype of MP cannot be due solely to absence of gC.

Nucleotide sequence analysis of in vivo recombinants between bacteriophage lambda DNA and pBR322.

The nucleotide sequences involved in the illegitimate recombination of four recombinants between bacteriophage lambda DNA and pBR322 in E. coli (lambda TA6, lambda KA3, lambda TA1R, and lambda KA7) were determined. Each resulted from recombination between regions of homology of 10 to 13 base pairs. The presence of a recA+ allele was found to stimulate recombination between lambda DNA and pBR322 approximately 10-fold. Lambda TA6, lambda KA3, and lambda KA7 were isolated in the presence of a recA+ allele and therefore, may have been generated by the recA recombination system. However, lambda TA1R was isolated in a recA mutant, and was presumably generated by a different recombination system. The possibility that it was generated by DNA gyrase is discussed. Two recombination events were required to form lambda KA7, which may indicate that it also was generated by DNA gyrase.

Identification of a second tetracycline-inducible polypeptide encoded by Tn10.

Three Tn10 polypeptides were detected by analyzing the proteins synthesized in ultraviolet light-irradiated Escherichia coli cells after infection with lambda::Tn10. One of these polypeptides was the previously identified 36,000-dalton TET polypeptide. The other two had approximate sizes of 25,000 and 13,000 daltons. The syntheses of both the TET polypeptide and the 25,000-dalton polypeptide were inducible by tetracycline in lambda-immune hosts. Similarly, the synthesis of the TET polypeptide was inducible in nonimmune hosts. However, the synthesis of the 25,000-dalton polypeptide was constitutive in nonimmune hosts. An amber mutation in a gene required for tetracycline resistance on lambda::Tn10 was isolated that eliminated the synthesis of the TET polypeptide in sup+ hosts but not the synthesis of the 25,000-dalton or the 13,000-dalton polypeptides. The expression of tetracycline resistance from wild-type Tn10 was found to be anomalous in E. coli strains carrying the amber suppressors supD, supE, and supF. In general, strains containing these nonsense suppressors were less resistant to tetracycline.

Recombination between bacteriophage lambda and plasmid pBR322 in Escherichia coli.

Recombinant lambda phages were isolated that resulted from recombination between the lambda genome and plasmid pBR322 in Escherichia coli, even though these deoxyribonucleic acids (DNAs) did not share extensive regions of homology. The characterization of these recombinant DNAs by heteroduplex analysis and restriction endonucleases is described. All but one of the recombinants appeared to have resulted from reciprocal recombination between a site on lambda DNA and a site on the plasmid. In general, there were two classes of recombinants. One class appeared to have resulted from recombination at the phage attachment site that probably resulted from lambda integration into secondary attachment sites on the plasmid. Seven different secondary attachment sites on pBR322 were found. The other class resulted from plasmid integration at other sites that were widely scattered on the lambda genome. For this second class of recombinants, more than one site on the plasmid could recombine with lambda DNA. Thus, the recombination did not appear to be site specific with respect to lambda or the plasmid. Possible mechanisms for generating these recombinants are discussed.