No abstract

No abstract

We tested the ability of sequences in the long terminal repeat (LTR) of a mink cell focus-forming (MCF) murine leukemia virus to function as an enhancer in a cell-type-specific manner. In a stable transformation assay, the MCF or Akv LTR and the simian virus 40 enhancer had similar activities in murine fibroblasts. In contrast, the MCF LTR had a significantly greater activity in murine T lymphoid cells than did either the simian virus 40 enhancer or the Akv LTR.One of the regulatory signals for transcription of eucaryotic genes is cis-acting enhancer elements that can augment the transcriptional activity of different genes (for review, see references 14 and 18). Enhancer elements were first identified in the simian virus 40 (SV40) genome but have subsequently been found in other viruses, including retroviruses (17,23,24), and have recently been identified for cellular genes (1,13,29). Unique characteristics of these enhancer elements include their ability to function in an orientationindependent manner and over relatively long distances (2,12,25). Enhancer activity can also occur in a tissue-or species-specific manner (10, 21). Enhancers have been identified in the U3 region of the long terminal repeats (LTRs) of retroviruses (17,19,20). More recently, the LTRs of murine leukemia viruses (MLVs) have been implicated in the specific type of neoplasia caused by these viruses (5, 9, 22, 28), and it is possible that the sequences resembling enhancers are responsible for this specificity (4).We have examined the LTR of a mink cell focus-forming (MCF) MLV ) to study the role that these terminal repeated sequences play in the specificity of tumor formation by MCF viruses. Lymphomagenic MCF MLVs have been implicated in the development of thymic lymphomas in strain AKR mice because of their ability to accelerate the onset of tumorigenesis (6,27 (Fig. 1, in brackets) in the U3 region. From a comparison of the nucleotide sequence of the LTRs of MCF-13 and Akv (Fig. 1), it seems clear that the greatest differences are present in the U3 region. This is a region in the LTR where duplicated sequences resembling enhancer elements have been identified for other retroviruses, and these sequences also appear to be present in the case of MCF-13 and Akv (Fig. 1). We wished to determine whether the repeated sequences for MCF-13 and Akv could also * Corresponding author. function as enhancer elements and whether this activity could occur in a cell-type-specific manner. Tissue-specific differences in the activities of these enhancerlike elements could help to account for the different pathogenic properties of these two viruses.To test this idea, we compared the LTRs of MCF-13 and Akv, along with the prototypical SV40 enhancer, in plasmid constructs containing each enhancer and the selectable gene conferring neomycin resistance (Neor) in murine cells in culture. A clone generated by Southern and Berg (33) which contains the SV40 origin of replication was used to test the SV40 enhancer (Fig. 2). The SV40 sequences that are present in thi...

Induction of apoptosis by different types of pathogenic retroviruses is an important step in disease devel

No abstract

No abstract

We have identified nucleotide sequences that regulate transcription in both a cell-type-specific and general manner in the long terminal repeat of the MCF13 murine leukemia virus. Besides the enhancer element, we have observed that the region between the enhancer and promoter (DEN) has a profound effect on transcription in different cell types. This effect, however, was dependent on the copy number of enhancer repeats and was detectable in the presence of a single repeat. When two enhancer repeats were present, the effect of DEN on transcription was abrogated except in T cells. DEN also makes a significant contribution to the leukemogenic property of the MCF13 retrovirus. Its deletion from the MCF13 virus dramatically reduced the incidence of thymic lymphoma and increased the latency of disease in comparison with the wild-type virus. This effect was most marked when one rather than two enhancer repeats was present in the mutant viruses. We also observed that the removal of one repeat alone remarkably reduced leukemogenicity by the MCF13 virus. A newly identified protein-binding site (MLPal) located within DEN affects transcription only in T cells, and its deletion attenuates the ability of an MCF13 virus with a single enhancer repeat to induce thymic lymphoma. This observation suggests that the MLPal protein-binding site contributes to the effect of the DEN region on T-cell-specific transcription and viral leukemogenicity. This study identifies the importance of nonenhancer sequences in the long terminal repeat for the oncogenesis of the MCF13 retrovirus.

The soy isoflavone genistein has been identified as having antiproliferative and apoptotic effects on various malignant cell types derived from solid tumors. Because little information regarding the effect of genistein on hematopoietic malignancies is available, we undertook this study of T-cell lymphomas. We tested the effect of genistein on murine T-cell lines derived from thymic lymphomas induced by an oncogenic murine leukemia virus. When T lymphoma cells were treated with genistein concentrations of 15 microM and greater, it was observed that the percentage of viable cells was significantly reduced in a dose- and time-dependent manner. The observed cell killing was found to be the result of apoptosis as detected by flow cytometric analysis of cells stained with annexin V and propidium iodide and assays for caspase-3 activation and DNA fragmentation. Cell staining with the mitochondrial specific dye JC-1 and detection of caspase-9 activation revealed that genistein produced mitochondrial depolarization as an early step in the induction of apoptosis. Bongkrekic acid inhibition of mitochondrial depolarization identified the mitochondria permeability transition pore (PTP) as a potential target of genistein activity. These results indicate that the induction of apoptosis by pharmacological concentrations of genistein in T lymphoma cells occurs via mitochondrial damage with the involvement of the PTP.