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Mol. Cell. Biol., 02 1995, 1034-1048, Vol 15, No. 2
E Ben-Shushan, H Sharir, E Pikarsky and Y Bergman
The Oct-3/4 transcription factor is a member of the POU family of
transcription factors and, as such, probably plays a crucial role in
mammalian embryogenesis and differentiation. It is expressed in the
earliest stages of embryogenesis and repressed in subsequent stages.
Similarly, Oct-3/4 is expressed in embryonal carcinoma (EC) cells and is
repressed in retinoic acid (RA)-differentiated EC cells. Previously we have
shown that the Oct-3/4 promoter harbors an RA-responsive element, RAREoct,
which functions in EC cells as a binding site for positive regulators of
transcription and in RA-differentiated EC cells as a binding site for
positive regulators of transcription and in RA- differentiated EC cells as
a binding site for negative regulators. Our present results demonstrate
that in P19 and RA-treated P19 cells, the orphan receptors ARP-1/COUP-TFII
and EAR-3/COUP-TFI repress Oct-3/4 promoter activity through the RAREoct
site in a dose-dependent manner. While the N-terminal region of the
ARP-1/COUP-TFII receptor is dispensable for this repression, the C-terminal
domain harbors the silencing region. Interestingly, three different RA
receptor:retinoid X receptor (RAR:RXR) heterodimers, RAR alpha:RXR alpha,
RAR beta:RXR alpha, and RAR beta:RXR beta, specifically bind and activate
Oct-3/4 promoter through the RAREoct site in a ligand-dependent manner. We
have shown that antagonism between ARP-1/COUP-TFII or EAR-3/COUP-TFI and
the RAR:RXR heterodimers and their intracellular balance modulate Oct-3/4
expression. Oct-3/4 transcriptional repression by the orphan receptors can
be overcome by increasing amounts of RAR:RXR heterodimers. Conversely,
activation of Oct-3/4 promoter by RAR:RXR heterodimers was completely
abolished by EAR-3/COUP-TFI and by ARP-1/COUP-TFII. The orphan receptors
bind the RAREoct site with a much higher affinity than the RAR:RXR
heterodimers. This high binding affinity provides ARP- 1/COUP-TFII and
EAR-3/COUP-TFI with the ability to compete with and even displace RAR:RXR
from the RAREoct site and subsequently to actively silence the Oct-3/4
promoter. We have shown that RA treatment of EC cells results in
up-regulation of ARP-1/COUP-TFII and EAR-3/COUP- TFI expression. Most
interestingly, in RA-treated EC cells, the kinetics of Oct-3/4 repression
inversely correlates with the kinetics of ARP-1/COUP-TFII and
EAR-3/COUP-TFI activation. These findings are in accordance with the
suggestion that these orphan receptors participate in controlling a network
of transcription factors, among which Oct-3/4 is included, which may
establish the pattern of normal gene expression during development.
Copyright © 1995, American Society for Microbiology
A dynamic balance between ARP-1/COUP-TFII, EAR-3/COUP-TFI, and retinoic acid receptor:retinoid X receptor heterodimers regulates Oct-3/4 expression in embryonal carcinoma cells
Hubert H. Humphrey Center for Experimental Medicine and Cancer Research, Hebrew University-Hadassah Medical School, Jerusalem, Israel.
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