Proviral Activation Research Articles

Tumor model-specific proviral insertional mutagenesis of the Fos/Jdp2/Batf locus

Retroviral activation of the AP-1/ATF super family member Jdp2 was recently reported to be a common event in M-MLV-induced T cell lymphoma in p27-null C57x129 mice as compared to wild type-inoculated mice but has not been found important in other models. On the basis of retroviral tag retrieval from 1190 individual Akv- and SL3-3-induced lymphomas, we here report that insertional mutagenesis into the 250-kb Fos/Jdp2/Batf locus is associated with SL3-3 MLV-induced T but not Akv-induced B cell lymphomas of NMRI and SWR mice. Integration pattern and clonality analyses suggest that Jdp2 participates in SL3-3-induced tumorigenesis distinctly as compared to the M-MLV setting. Northern blot analysis showed Jdp2 to be alternatively spliced in various normal tissues as well as MLV-induced lymphomas. Interestingly, in some tumors, proviral insertion seems to activate different mRNA sub-species. Whereas elevated mRNA levels of the Fos gene could not be correlated with provirus presence, in one case, Northern blot analysis as well as quantitative real-time PCR indicated proviral activation of the AP-1 super family member Batf, a gene not previously reported to be a target of insertional mutagenesis. A novel integration cluster between Jdp2 and Batf apparently did not influence the expression level of either gene, underscoring the importance of addressing expression effects to identify target genes of insertion. Altogether, such distinct insertion patterns point to different mechanism of activation of specific proto-oncogenes and are consequently of importance for the understanding of proviral activation mechanisms as well as the specific role of individual oncogenes in tumor development.

Protein phosphatase 2A enhances activation of human immunodeficiency virus type 1 by phorbol myristate acetate.

The viral replication rate in patients infected with human immunodeficiency virus type 1 (HIV-1) is controlled in part by regulation of the transcription of viral genes. The rate of transcription is determined by a complex interplay between cellular and viral proteins and the promoter elements found in the long terminal repeats. Protein phosphatase 2A (PP2A) is a phosphoprotein that plays important roles in the regulation of signal transduction and cell growth. In this report, we demonstrate that overexpression of the catalytic subunit of protein phosphatase 2A (PP2Ac) increases the basal activity of the HIV-1 promoter and, especially, enhances the promoter's response to the protein kinase C (PKC) activator 12-O-tetradecanoyl phorbol-13-acetate (PMA). Additionally, ectopic PP2Ac enhances activation of HIV-1 provirus by PMA. Okadaic acid, a potent inhibitor of PP2A, markedly reduces both HIV-1 enhancer and proviral activation. Fostriecin, a PP2A inhibitor which has been used as an antineoplastic agent in clinical trials, is also able to inhibit PMA-stimulated HIV-1 proviral activation. These observations demonstrate a role for the important cellular phosphatase PP2A in HIV-1 transcription and replication and also suggest that PKC can potentiate the activity of PP2A. PP2A is a potential target for therapeutic intervention in patients infected with HIV-1.

Activation of the Rap1 Guanine Nucleotide Exchange Gene,CalDAG-GEF I, in BXH-2 Murine Myeloid Leukemia

Here we report the recurrent proviral activation of the Rap1-specific guanine nucleotide exchange factor CalDAG-GEF I (Kawasaki, H., Springett, G. M., Toki, S., Canales, J. J., Harlan, P., Blumenstiel, J. P., Chen, E. J., Bany, I. A., Mochizuki, N., Ashbacher, A., Matsuda, M., Housman, D. E., and Graybiel, A. M. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 13278-13283; Correction (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 318) gene in BXH-2 acute myeloid leukemia. We also show that CalDAG-GEF I encodes two protein isoforms, a full-length isoform (CalDAG-GEF Ia) and a C-terminally truncated isoform (CalDAG-GEF Ib). Expression of the full-length CalDAG-GEF Ia isoform in Rat2 fibroblasts enhances growth in low serum, whereas expression in Swiss 3T3 cells causes morphological transformation and increased saturation density. In FDCP1 myeloid cells, CalDAG-GEF Ia expression increases growth and saturation density in the presence of the diacylglycerol analogs phorbol 12-myristate 13-acetate (PMA), which activates CalDAG-GEF Ia exchange activity. Likewise, in 32Dcl3 myeloblast cells, CalDAG-GEF Ia expression increases cell adherence to fibronectin in response to PMA and calcium ionophore and allows higher saturation densities and prolonged growth on fibronectin-coated plates. These effects were correlated with increased Rap1, but not Ras, protein activation following PMA and calcium ionophore treatment. Our results suggest that Rap1-GTP delivers signals that favor progression through the cell cycle and morphological transformation. The identification of CalDAG-GEF I as a proto-oncogene in BXH-2 acute myeloid leukemia is the first evidence implicating Rap1 signaling in myeloid leukemia.

Overexpression of Frat1 in transgenic mice leads to glomerulosclerosis and nephrotic syndrome, and provides direct evidence for the involvement of Frat1 in lymphoma progression.

The proto-oncogene Frat1 was originally identified as a common site of proviral insertion in transplanted tumors of Moloney murine leukemia virus (M-MuLV)-infected Emu-Pim1 transgenic mice. Contrary to most common insertion sites implicated in mouse T cell lymphomagenesis, retroviral insertional mutagenesis of Frat1 constitutes a relatively late event in M-MuLV-induced tumor development, suggesting that proviral activation of Frat1 contributes to progression of T cell lymphomas rather than their genesis. To substantiate this notion we have generated transgenic mice that overexpress Frat1 in various organs, including lymphoid tissues. Frat1 transgenic mice develop focal glomerulosclerosis and a nephrotic syndrome, but they do not exhibit an increased incidence of spontaneous lymphomas. Conversely, these mice are highly susceptible to M-MuLV-induced lymphomagenesis, and Frat1/Pim1 bitransgenic animals develop lymphomas with increased frequency compared to Pim1 transgenic littermates. These data support a role for Frat1 in tumor progression.

Methotrexate selectable retroviral vectors for Gaucher disease.

To develop a gene therapy protocol suitable for the treatment of a benign disease such as Gaucher disease, we have developed two bicistronic vectors that allow transduced cells to be selected for with methotrexate (MTX). The two vectors differ in the presence or absence of a mutant polyoma enhancer (delta Mo + PyF101) replacing the wild-type retroviral enhancer in the LTR. Infection of human TF-1 and K562 cells, Gaucher type II fibroblasts and murine hemopoietic bone marrow cells conferred MTX resistance and glucocerebrosidase (GC) expression. Upon increasing MTX concentrations, the number of proviral copies and GC activity increased, demonstrating in vitro selection of retrovirus-transduced cells. At high MTX selection pressure, up to 140 microM for infected Gaucher type II fibroblasts, no endogenous wild-type DHFR amplification could be detected, indicating that both retroviral constructs provide sufficient DHFR protein levels. Upon transduction, murine bone marrow cells were protected against otherwise lethal MTX concentrations (range 1-5 microM MTX). Flow cytometry specific for human GC (hGC) demonstrated that in vitro selection resulted in increased percentages of hGC-positive murine cells. In conclusion, the generated bicistronic vectors are ideally suited to investigate whether an in vivo selection approach for retrovirus-transduced cells is feasible. Such a strategy might abolish the need for a high initial transduction efficiency and might result in a gene therapy protocol devoid of the undesirable need for marrow ablative treatment of the recipient.

A full-length Notch1 allele is dispensable for transformation associated with a provirally activated truncated Notch1 allele in Moloney MuLV-infected MMTV(D)/myc transgenic mice.

The Notch1 gene was previously found to be targetted by provirus insertion in a high proportion of T-cell lymphomas arising in Moloney MuLV-inoculated MMTV(D)/myc transgenic mice. Proviral activation of Notch1 was associated with overexpression of truncated Notch1, deleted of the sequences coding for the extracellular domain. The high levels of truncated Notch1 RNA and proteins in these tumors are thought to be involved in the oncogenic transformation. However, in addition to these truncated RNA and proteins, high level expression of full-length Notch1 RNA and proteins was also observed in several tumors, suggesting that they could also contribute to the transformation process. To test this hypothesis, we used a genetic approach and studied MMTV(D)/myc transgenic mice in which one of the Notch1 alleles was mutated by targeted mutagenesis (Notch1+/- mice). Heterozygote (Notch1+/-) and wild-type (Notch1+/+) transgenic mice were inoculated with Moloney MuLV and the frequency of Notch1 rearrangements was compared between both groups. Notch1 was rearranged at similar frequencies in both groups, indicating that the full-length Notch1 allele is dispensable in tumors harboring an activated Notch1 allele.

Pertubation of B and T cell development and predisposition to lymphomagenesis in Emu Bmi1 transgenic mice require the Bmi1 RING finger.

Proviral activation of the Bmi1 gene has implicated Bmi1 as a collaborator of c-Myc in lymphomagenesis. To determine the effect of Bmi1 overexpression on hematopoiesis and lymphomagenesis transgenic mice were generated that overexpress different forms of the Bmi1 protein in their lymphoid compartment. Emu Bmi1 transgenic mice, overexpressing the wild type Bmi1 protein showed a perturbed lymphoid development and were highly susceptible to B and T cell lymphomagenesis. Mutational analysis of the Bmi1 protein demonstrated that the conserved N-terminal RING finger and central part of Bmi1 are essential for its oncogenic potential whereas the C-terminal Pro-Ser rich region is not required. We have used provirus tagging in the Emu Bmi1 mice to identify genes that cooperate with Bmi1 in lymphomagenesis. MoMLV infection in Emu Bmi1 transgenic mice accelerated lymphoma development. Proviral activation of the Pim and Myc genes but not the Gfi1 gene were frequently observed in these tumors. These results demonstrate that Bmi1 is a potent oncogene and suggest that it plays an important role in early lymphoid development.

MoMuLV proviral integrations identified by Sup-F selection in tumors from infected myc/pim bitransgenic mice correlate with activation of the gfi-1 gene.

Infecting mice with a mutant Moloney murine leukemia virus which contains the bacterial suppressor tRNA supF in its LTR allows rapid cloning of proviral integration sites from genomic tumour DNA. In a previous study Emu pim-1/Emu L-myc bitransgenic mice had been inoculated neonatally with MoMuLV supF virus. The retroviral infection led to acceleration of lymphomagenesis indicating the proviral activation of further oncogenes cooperating with myc and pim-1 in tumour development. Using a functional supF screen for analysis of genomic mouse tumour DNA libraries which had been constructed in the phage vector EMBL3A, a common proviral integration site on mouse chromosome 5 was cloned and found to be identical to the proviral integration site evi-5 which has recently been identified in an AKXD T-cell lymphoma and which is located 18 kb upstream of the gfi-1 gene. Tumours bearing evi-5 integrations showed an enhanced gfi-1 expression level suggesting that gfi-1 is the target gene for insertions at the evi-5 locus. Together with three other previously described Moloney integration clusters all responsible for enhanced gfi-1 expression the number of tumours from infected double transgenic Emu L-myc/Emu pim-1 transgenic mice with retrovirally activated gfi-1 added up to 53% underscoring the role of GFI-1 as an effective collaborator for MYC and PIM-1 in the process of lymphomagenesis.

Cooperative activation of Hoxa and Pbx1-related genes in murine myeloid leukaemias

Retroviruses induce myeloid leukaemia in BXH-2 mice by the insertional mutation of cellular proto-oncogenes or tumour suppressor genes. Disease genes can thus be identified by proviral tagging through the identification of common viral integration sites in BXH-2 leukaemia. Here, we describe a new approach for proviral tagging that greatly facilitates the identification of BXH-2 leukaemia genes. Using this approach, we identify three genes whose expression is activated by proviral integration in BXH-2 leukaemias; Hoxa7, Hoxa9, and a Pbx1-related homeobox gene, Meis1. Proviral activation of Hoxa7 or Hoxa9 is strongly correlated with proviral activation of Meis1 implying that Hoxa7 and Hoxa9 cooperate with Meis1 in leukaemia formation. These studies provide the first genetic evidence that Pbx1-related genes cooperate with Hox genes in leukaemia formation and identify a number of new murine myeloid leukaemia genes.

Tissue- and stage-specific activation of an endogenous provirus after transcription through its integration site in the opposite orientation.

Endogenous proviruses of the Moloney murine leukemia retrovirus (Mo-MuLV) are transcriptionally blocked in early embryos and in general remain silent even when the tissues have become permissive to the expression of newly integrated copies. Eventually, activation in presumably very few cells initiates rapid superinfection leading to viremia and leukemia, but the processes leading to provirus activation are unknown. Differences in the onset and development of viremia between several mouse strains carrying an endogenous Mo-MuLV (Mov lines) are attributed to a chromosomal position effect, but neither cell type nor stage of provirus activation is known for any strain. We have now monitored the appearance of viral transcripts and particles in the Mov13 strain, which carries the Mo-MuLV provirus in inverted orientation in the first intron of a collagen gene (Col1a1) with well-characterized transcriptional activity. We report obligatory tissue- and stage-specific virus activation in osteoblasts and odontoblasts. The significance of this activation pattern is indicated by the fact that of the great variety of cells expressing the wild-type collagen gene, only these two cell types can also transcribe the mutant allele including its viral insert. We propose that this transcription of the proviral genome, albeit in the opposite direction, leads to the activation of the viral promoter.

Proviral activation of the c-myb proto-oncogene is detectable in preleukemic mice infected neonatally with Moloney murine leukemia virus but not in resulting end stage T lymphomas.

Moloney murine leukemia virus induces myeloid leukemia when inoculated intravenously into pristane-primed adult BALB/c mice. One hundred percent of these tumors show insertional activation of the c-myb proto-oncogene, and reverse transcriptase PCR assays have shown that the c-myb activation could be detected soon after infection. We tested BALB/c and NIH Swiss mice that had been inoculated as newborns with Moloney murine leukemia virus, under which conditions they develop T lymphomas exclusively. Reverse transcriptase-PCR assays indicated that c-myb activations were detectable soon after neonatal infection. However, none of the resulting T lymphomas contained c-myb activations. The implications of these results to the timing of proto-oncogene activations in leukemogenesis and the specificity of proto-oncogene activations for different diseases are discussed.

Cellular Latency in HIV-1 Infection

There are multiple levels of transcriptional regulation that a cell can bring to bear on either native cellular genes or integrated proviral genes. One such level of regulation is inducible sequence-specific transcriptional factors, such as NF-kappa B, which have been extensively investigated with respect to the HIV-1 LTR. This level of control can be modified by organization of the integrated HIV-1 provirus into higher order chromatin structures such as nucleosomes and chromatin domains. The overall level of proviral transcriptional activity within any given cell depends on the relative contribution of each of these levels of control.

Tumor Necrosis Factor-Dependent Production of Human Immunodeficiency Virus 1 in Chronically Infected HL-60 Cells

AbstractTumor necrosis factor (TNF) may play a central role in pro-viral activation and release from latency in cells infected with the human immunodeficiency virus (HIV). We studied viral production and its relation to TNF in a HL-60 cell line (J22-HL-60) infected with a monocytotropic strain of HIV-1jr-fl. Viral production was stimulated to similar levels by TNF, 12-0-tetradecanoyl-phorbol-13-acetate (TPA), and 1,25-dihydroxyvitamin D3 (1,25[OH]2D3). Production of the virus was not suppressed by 3’-azido-3’-deoxythymi-dine (AZT), indicating that viral production was not caused by superinfection. Low concentrations of TNF (0.1 ng/mL) induced viral production with a short lag period of 8 hours, and this proviral activation was specifically suppressed by anti-TNF antibodies. However, induction of virus production by 1,25(OH)2D3 showed an extended lag period of 2 to 3 days. The effect of 1,25(OH)2D3 on virus production was also blocked by anti-TNF antibodies, which suggests the direct participation of TNF in this process. TNF accumulated in the culture supernatant of cells stimulated with 1,25(OH)2D3 with a kinetics consistent with its involvement in the action of 1,25(OH)2D3 on viral production. The J22-HL-60 cell line produced low levels of virus when cultured in the absence of an external stimulus; however, this basal viral production was suppressed greater than 80% in the presence of anti-TNF antibodies. Corresponding low levels of TNF were detected in the culture supernatants. Viral production decreased slowly with increasing passage of the cells, and no virus was detected in the supernatants of cells maintained in culture for several months. Concomitantly, TNF was no longer detected in the supernatant of these cells, which suggests that endogenous autocrine production of TNF drives viral production in the unstimulated cells. However, viral production was stimulated in these cells by low concentrations (0.1 ng/mL) of added TNF. These results argue for a central role for TNF in HIV proviral activation in chronically infected myeloid cells.

Tumor necrosis factor-dependent production of human immunodeficiency virus 1 in chronically infected HL-60 cells

Tumor necrosis factor (TNF) may play a central role in proviral activation and release from latency in cells infected with the human immunodeficiency virus (HIV). We studied viral production and its relation to TNF in a HL-60 cell line (J22-HL-60) infected with a monocytotropic strain of HIV-1JR-FL. Viral production was stimulated to similar levels by TNF, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), and 1,25-dihydroxyvitamin D3 (1,25[OH]2D3). Production of the virus was not suppressed by 3′-azido-3′-deoxythymidine (AZT), indicating that viral production was not caused by superinfection. Low concentrations of TNF (0.1 ng/mL) induced viral production with a short lag period of 8 hours, and this proviral activation was specifically suppressed by anti- TNF antibodies. However, induction of virus production by 1,25(OH)2D3 showed an extended lag period of 2 to 3 days. The effect of 1,25(OH)2D3 on virus production was also blocked by anti-TNF antibodies, which suggests the direct participation of TNF in this process. TNF accumulated in the culture supernatant of cells stimulated with 1,25(OH)2D3 with a kinetics consistent with its involvement in the action of 1,25(OH)2D3 on viral production. The J22-HL-60 cell line produced low levels of virus when cultured in the absence of an external stimulus; however, this basal viral production was suppressed greater than 80% in the presence of anti-TNF antibodies. Corresponding low levels of TNF were detected in the culture supernatants. Viral production decreased slowly with increasing passage of the cells, and no virus was detected in the supernatants of cells maintained in culture for several months. Concomitantly, TNF was no longer detected in the supernatant of these cells, which suggests that endogenous autocrine production of TNF drives viral production in the unstimulated cells. However, viral production was stimulated in these cells by low concentrations (0.1 ng/mL) of added TNF. These results argue for a central role for TNF in HIV proviral activation in chronically infected myeloid cells.

Mouse mammary-tumor virus activates Fgf-3/Int-2 less frequently in tumors from virgin than from parous mice.

Tumorigenesis by mouse mammary-tumor virus (MMTV) involves proviral disruption and transcriptional activation of a number of cellular oncogenes, generically termed Int. The frequencies with which different Int genes are activated in different mouse strains can be quite variable, and previous surveys have suggested that insertions at Int-2/Fgf-3 occur primarily in strains that develop pregnancy-dependent mammary tumors. To address this issue, we have determined the relative contributions of 5 known Int genes (Wnt-1, Wnt-3, Fgf-3, Fgf-4 and Int-3) in mammary tumors from virgin BR6 and multiparous BR6, BALB/cfBR6 and RIII mice. Whereas Fgf-3 was implicated in 66%, 80% and 92% of the tumors from the respective parous animals, only 20% of the tumors from virgin mice expressed Fgf-3. This reduced involvement of Fgf-3 was compensated by proviral insertions in Fgf-4, Int-3 and Wnt-3, but the frequency of Wnt-1 activation was relatively constant. These data strengthen the link between Fgf-3 and a pregnancy-dependent phenotype and suggest that, in the strains analyzed, the frequency of Int-gene activation was influenced more by the hormonal status than by the genetic background.

Expression of functional β-platelet-derived growth factor receptors on hematopoietic cell lines

The β-type receptor of platelet-derived growth factor (βPDGFR) is a class III transmembrane receptor with tyrosine kinase activity. The βPDGFR gene is located on mouse chromosome 18 close to the c- fms gene which codes for the colony stimulating factor-1 receptor (CSF-1R). We previously reported that in a high percentage of myeloblastic leukemias induced by the Friend helper murine leukemia virus (F-MuLV), proviruses were integrated in the first intron of the c- fms gene leading to an enhanced expression of c- fms mRNA. Since activation by proviral insertion can act at long distance, we studied βPDGF receptor gene expression in murine myeloblastic leukemias. This gene was found to be frequently expressed but the level of βPDGF receptor mRNA was weak and not related to proviral activation. High affinity binding sites were expressed on myeloblastic cells and ligand binding induced cell proliferation. To determine whether βPDGFR expression is a common feature in hematopoietic cells, we tested cell lines belonging to other hematopoietic lineages. We found that multipotent stem and mast cell lines also expressed the βPDGF receptor gene. This suggests that PDGF, known as a mitogen for connective tissue cells, could also play a role in normal hematopoiesis.

Capture of a cellular transcriptional unit by a retrovirus: mode of provirus activation in embryonal carcinoma cells.

The expression of murine leukemia provirus in embryonal carcinoma (EC) cells is blocked by a mechanism still incompletely understood. The blockage is not overcome by deleting a large portion of the enhancer region (in U3) in recombinant retroviruses (M-MuLVneo delta Enh). This confirms the presence of negative elements outside the viral 82-bp repeats. However, a few sites in the genomes of EC cells permit M-MuLVneo delta Enh proviral expression. One such site, identified in PCC4, PCC3, and LT, was studied. The complete analysis of the mechanism of activation by Northern (RNA) blotting, cloning, and sequencing of partial cDNA copies of the viral transcript and of the site of integration establishes that viral transcripts are initiated from an upstream host-cell promoter and are spliced from a host donor to a cryptic viral acceptor at position 542 in the Moloney murine leukemia virus (M-MuLV) genome. In consequence, the mature transcripts are host cell-virus fusion transcripts from which M-MuLV sequences, including the cis-active negative elements of the 5' long terminal repeat-containing region, are absent. The provirus integrates apparently randomly into any of the three most proximal introns of the transcriptional unit. The host cell promoter contains a TATA box and 14 potential SpI binding sites included in a 1.0-kb GC-rich island. These elements promote gene expression of recombinant vectors in EC and differentiated cells. The mechanism described points to a mechanism by which retroviruses can be transcribed from upstream nonviral elements and can acquire host genes by 5' annexation of exons.

An enhancer variant of Moloney murine leukemia virus defective in leukemogenesis does not generate detectable mink cell focus-inducing virus in vivo.

Moloney murine leukemia virus (Mo-MuLV) induces T-cell lymphoma when inoculated into neonatal mice. This is a multistep process. Early events observed in infected mice include generalized hematopoietic hyperplasia in the spleen and appearance of mink cell focus-inducing (MCF) recombinants; end-stage tumors are characterized by insertional proviral activation of protooncogenes. We previously showed that an Mo-MuLV enhancer variant, Mo+PyF101 Mo-MuLV, has greatly reduced leukemogenicity and is deficient in induction of preleukemic hyperplasia. In this report, we have examined Mo+PyF101 Mo-MuLV-inoculated mice for the presence of MCF recombinants. In contrast to wild-type Mo-MuLV-inoculated mice, Mo+PyF101 Mo-MuLV-inoculated mice did not generate detectable MCF recombinants. This failure was at least partly due to an inability of the MCF virus to propagate in vivo, since a molecularly cloned infectious Mo+PyF101 MCF virus did not replicate, even when inoculated as a Mo+PyF101 Mo-MuLV pseudotype. These results show that the leukemogenic defect of Mo+PyF101 Mo-MuLV is associated with its inability to generate MCF recombinants capable of replication in vivo. This, in turn, is consistent with the view that MCF recombinants play a significant role in Mo-MuLV-induced disease and, in particular, may play a role early in the disease process.

Tumour‐induced host stromal‐cell transformation: Induction of mouse spindle‐cell fibrosarcoma not mediated by gene transfer

Tumour-induced host-cell transformation has been addressed by examining human tumours in situ and following xenograft to nude mice. We have found evidence for the transformation of host stromal fibroblasts both in vivo and following the introduction of the tumours to in vitro culture. The in vitro culture of one such xenograft--derived from a human prostatic adenocarcinoma--resulted in the outgrowth of a transformed aneuploid mouse cell line. This transformed line was tumourigenic both in BALB/c nu/nu (nude) mice and in heterozygous nu/+mice, with the morphology of a spindle-cell sarcoma. The cell line did not express human isozymes or human histocompatibility antigens, nor were human chromosomes present. Moreover, human DNA sequences were not detected by human Alu repeat sequence element probing in the transformed cell line grown either in vitro or in vivo. The line contained retroviral long terminal repeat sequences but there was no evidence of proviral activation. These findings indicate that tumour cells may cause transformation of neighbouring stromal cells; that this transformation may proceed in the absence of DNA transfer or activation of endogenous proviruses; and that the means of this observed transformation may involve humoral factors elaborated by the tumour cells.

Expression from the HIV-LTR Is Stimulated by Glucocorticoids and Pregnancy

Gene regulation in several retroviral systems is subject to steroid hormone control. Clinical studies have shown that the human immunodeficiency virus (HIV) core protein p24 is more readily detectable in infected women during pregnancy. Here we show that expression of the HIV long terminal repeat (LTR) is induced by glucocorticoids in tissue culture cells and by pregnancy in placental and uterine tissue of transgenic mice. We suggest that hormonal stimulation could influence proviral activation and that placental expression of the HIV-LTR could contribute to the high perinatal transmission rate of HIV.