Mj. O'Connor et al., The differentiation-specific factor CDP/Cut represses transcription and replication of human papillomaviruses through a conserved silencing element, J VIROLOGY, 74(1), 2000, pp. 401-410
The life cycles of human papillomaviruses (HPVs) are intimately linked to t
he differentiation program of infected stratified epithelia, with both vira
l gene expression and replication being maintained at low levels in undiffe
rentiated basal cells and increased upon host cell differentiation. We rece
ntly identified, in HPV-16, a negative regulatory element between the epith
elial-cell-specific enhancer and the E6 promoter that is capable of silenci
ng E6 promoter activity, and we termed this element a papillomavirus silenc
ing motif (PSM) and the unknown cellular factor that bound to it PSM bindin
g protein (PSM-BP). Here we show that the homologous genomic segments of si
x other distantly related genital HPV types contain a PSM that binds PSM-BP
and is capable of repressing transcription. Conservation of the PSM sugges
ts that it is indispensable for the HPV life cycle. Purification, electroph
oretic mobility shift assay experiments, and the use of specific antibodies
proved that the cellular factor PSM-BP is identical to a previously descri
bed transcriptional repressor, the CCAAT displacement protein (CDP), also r
eferred to as the human Cut protein (Cut). CDP/Cut repression of HPV-16 may
stem from the modification of specifically positioned nucleosomes, as sugg
ested by transcriptional stimulation under the influence of the histone dea
cetylase inhibitor trichostatin A. CDP/Cut is an important developmental re
gulator in several different tissues. It was recently shown that CDP/Cut is
expressed in basal epithelial cells but not in differentiated primary kera
tinocytes. This suggests the possibility that repression by PSM couples HPV
transcription to the stratification of epithelia. In each of the studied H
PV types, the two CDP/Cut binding sites of PSM overlap with the known or pr
esumed binding sites of the replication initiator protein El. Transfection
of CDP/Cut expression vectors into cells that support HPV-16 or HPV-31 repl
ication leads to the elimination of viral episomes. Similarly, two PSM-like
motifs overlapping the El binding site of bovine papillomavirus type 1 bin
d CDP/Cut, and CDP/Cut overexpression reduces the copy number of episomally
replicating BPV-1 genomes in mouse fibroblasts. CDP/Cut appears to be a ma
ster regulator of HPV transcription and replication during epithelial diffe
rentiation, and PSMs are important cis-responsive targets of this repressor
.