E1A represses wild-type and F9-selected polyomavirus DNA replication by a mechanism not requiring depression of large tumor antigen transcription

Abstract

Polyomavirus (Py) DNA replication may be regulated to a low-level replication state in specific target cells in mice as well as in certain undifferentiated murine cell lines, such as embryocarcinoma (EC) cells. To investigate possible mechanisms by which such control may occur, we have examined the effects of ElA on Py DNA replication. Adenovirus ElA proteins repress transcriptional activation of various enhancers, including those of Py, and can stimulate DNA replication in quiescent cells, but ElA effects on Py DNA replication were unknown. We found that constitutive ElA expression in NIH 3T3 cells depressed Py DNA replication very strongly. Two F9 EC cell-selected Py enhancer variants, PyF441 and PyFlOl, were also examined because undifferentiated EC cells are hypothesized to have an ElA-like activity responsible for the Py restriction, and these variants activate Py DNA replication in cis in undifferentiated F9 cells. Both variants were repressed by ElA, indicating that EIA activity in 3T3 cells is not equivalent to undifferentiated F9 cell ElA-like activity. We also examined transient inducible ElA expression in cells supplying Py large tumor antigen (T-Ag). Py DNA replication was again repressed, and the inhibition increased with ElA induction. Analysis of TAg mRNA levels indicated that ElA repression of Py DNA replication was not an indirect result of depression of TAg transcription. This suggests that E1A may repress Py DNA replication by a more direct mechanism, possibly by blocking enhancer activation of DNA replication in a manner uncoupled with enhancer transcriptional control.

The evolution of small DNA viruses of eukaryotes: Past and present considerations

“…More recently, in vivo results with mouse polyomavirus also support the view that virus replication is typically associated with differentiation of infected tubular epithelial cells, which can occur either at birth or following various types of tissue damage (59). A similar episomal maintenance of py or SV40 genomes in various undifferentiated cell types has also been reported (70)(71)(72)(73)(74). With the parvoviruses, it had previously been thought that virus replication requires cycling cells, but here too it has more recently been reported that differentiation of infected host cells will facilitate vegetative replication (43).…”

Sequences flanking the pentanucleotide T-antigen binding sites in the polyomavirus core origin help determine selectivity of DNA replication

“…This cell type/species selectivity for replication may have several causes. One might be that the viral enhancer functions only in certain cell types and thereby restricts replication (30,91,92). Another cause might be incompatibility between the viral T antigen and the replication proteins of the heterologous host (1,9,40,74,75,93).…”

AP1 enhances polyomavirus DNA replication by promoting T-antigen-mediated unwinding of DNA

“…Alternatively, two or more copies of the PEA1 site can substitute for the viral enhancer to activate DNA replication (20,33,41,51,55). Additional evidence for the involvement of Fos and Jun in viral DNA replication is provided by the observations that ectopic expression of Fos and Jun (or v-Jun) in murine F9 embryonal carcinoma cells (which contain low levels of these proteins) stimulates viral DNA replication and that ectopic expression of the adenovirus E1A protein (which blocks binding of AP1 to DNA) in mouse fibroblasts represses viral DNA replication (13,22,39,44,60).…”