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-Subunit of Human Chorionic Gonadotropin by the Embryonic Transcription Factor Oct-3/4
Departments of Biological Sciences (L.L.) and Animal Sciences and Biochemistry (D.L., M.V., R.M.R.), University of Missouri, Columbia, Missouri 65211
CG is required for maintenance of the corpus
luteum during pregnancy in higher primates. As CG is a heterodimeric
molecule, some form of coordinated control must be maintained over the
transcription of its two subunit genes. We recently found that
expression of human CG ß-subunit (hCGß) in JAr human
choriocarcinoma cells was almost completely silenced by the embryonic
transcription factor Oct-3/4, which bound to a unique ACAATAATCA
octameric sequence in the hCGß gene promoter. Here we report that
Oct-3/4 is also a potent inhibitor of hCG 
-subunit (hCG)
expression in JAr cells. Oct-3/4 reduced human GH reporter expression
from the -170 hCG promoter in either the presence or absence of
cAMP by about 70% in transient cotransfection assays, but had no
effect on expression from either the -148 hCG or the -99 hCG
promoter. Unexpectedly, no Oct-3/4-binding site was identified within
the -170 to -148 region of the hCG promoter, although one was
found around position -115 by both methylation interference
footprinting and electrophoretic mobility shift assays. Site-directed
mutagenesis of this binding site destroyed the affinity of the promoter
for Oct-3/4, but did not affect repression of the promoter. Therefore,
inhibition of hCG gene transcription by Oct-3/4 appears not to
involve direct binding of this factor to the site responsible for
silencing. When stably transfected into JAr cells, Oct-3/4 reduced the
amounts of both endogenous hCG mRNA and protein by 70–80%. Oct-3/4
is therefore capable of silencing both hCG and hCGß gene
expression. We suggest that as the trophoblast begins to form,
reduction of Oct-3/4 expression permits the coordinated onset of
transcription from the hCG and hCGß genes.
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