Glucocorticoid regulation of transcription at an amplified, episomal promoter.

Abstract

The mouse mammary tumor virus long terminal repeat (MMTV LTR) has been introduced into cultured murine cells, using the 69% transforming fragment of bovine papilloma virus type 1 (BPV). Transformed cells contain up to 200 copies of the chimeric molecules per diploid genome. The restriction endonuclease map of the acquired recombinants, as well as the physical structure of the DNA, indicates that the LTR-BPV molecules present in these cells occur exclusively as unintegrated, extrachromosomal episome. When a 72-base pair direct repeat "enhancer" element (derived from the Harvey sarcoma retrovirus) was included in the MMTV LTR-BPV chimeric plasmids, DNA acquired through transfection, with a single exception, was integrated or rearranged or both. The transcriptional potential of the episomal MMTV promoter present in these cells was tested in two ways. First, steady-state levels of MMTV-initiated RNA were measured by quantitative Si mapping. Second, the relative number of transcription complexes initiated in vivo was determined by using a subnuclear fraction highly enriched for MMTV-BPV minichromosomes in an in vitro transcription extension assay. Both approaches showed that the MMTV LTR present in the episomal state was capable of supporting glucocorticoid hormone-regulated transcription. We have therefore demonstrated the hormone response for the first time in a totally defined primary sequence environment. Significant differences both in the basal level of MMTV-initiated transcription and in the extent of glucocorticoid induction were observed in individual cell lines with similar episomal copy numbers. These phenotypic variations suggest that epigenetic structure is an important component of the mechanism of regulation.The regulation of mouse mammary tumor virus (MMTV) expression by glucocorticoids has attracted considerable interest as a model for hormone action (13, 37). The early availability of homogeneous cDNA probes from purified virion preparations led to the demonstration that an increase in newly synthesized virus-specific RNA can be detected very quickly after the addition of hormone to MMTV-producing tumor cells in culture (40, 50), suggesting that RNA levels increase as a direct result of a stimulation in the rate of transcription. The subsequent finding that hormone regulation of MMTV expression is maintained in nonmurine cells chronically infected with MMTV (38) indicated that the virus either encodes the sequences necessary for hormone responsiveness or preferentially integrates in regions of cellular DNA containing such regulatory signals.More recently, the long terminal repeat (LTR) of the provirus has been fused to coding sequences that provide a selective function in mammalian cells, and the resulting chimeras have been introduced into cultured cells by calcium phosphate-mediated transfection. These investigations implied that LTR sequences alone are sufficient to establish hormone regulation of the selectable gene product (17,24). Chimeric molecules are assimilated in these transfection exper...

Nucleoprotein Structure Influences the Response of the Mouse Mammary Tumor Virus Promoter to Activation of the Cyclic AMP Signalling Pathway

“…Cell lines 904.13 and 1470.2 were both derived from C127 mouse mammary adenocarcinoma cells and contain stably replicating copies of bovine papillomavirus (BPV)-MMTV LTR fusions. Cell line 904.13 contains a BPV-MMTV ras fusion and has been described previously (50). Cell line 1470.2 contains a BPV-MMTV chloramphenicol acetyltransferase (CAT) fusion derived from plasmid pM25 (3).…”

In vitro methylation of bovine papillomavirus alters its ability to transform mouse cells.

“…BPV has been used successfully as a vector for the extrachromosomal maintainance of several different genes in mouse fibroblast cells, including the rat preproinsulin gene (37), the phosphotransferase neomycin resistance gene of TnS (22), the human metallothionein gene (16), the human ,B-globin gene (5), the human beta interferon gene (31,51), and other genes (32,48). Its use as a vector system has numerous advantages, including the fact that the inserted gene of interest is maintained extrachromosomally, thereby eliminating position effects due to the position of integration of a transferred sequence.…”

The bovine papillomavirus genome and its uses as a eukaryotic vector

“…Furthermore, if bacterial sequences are linked to the 69 % region then this outcome is even more likely [61,84]. Removal of these bacterial sequences prior to transfection tends to restore the episomality of the vector and leads to less rearrangement provided that the linear plasmid is recircularized before transfection [76,80,81]. The lost enhancer function could to a certain extent, be restored by 'stimulatory segments' [51,53,63] and vectors containing these sequences tended to remain episomal [62,63,65].…”

“…It is possible that this loss of regulation results from a changed chromosome environment around the metallothionein.gene [77][78][79]. However, in two other examples in which giucocorticoid-responsive promoters were used with BPV vectors, induction by dexamethasone was retained [80,81].…”