SILENCING OF THE GENE FOR THE ALPHA-SUBUNIT OF HUMAN CHORIONIC-GONADOTROPIN BY THE EMBRYONIC TRANSCRIPTION FACTOR OCT-3 4/

CG is required for maintenance of the corpus luteum during pregnancy i n higher primates. As CG is a heterodimeric molecule, some form of coo rdinated control must be maintained over the transcription of its two subunit genes. We recently found that expression of human CG beta-subu nit (hCG beta) in JAr human choriocarcinoma cells was almost completel y silenced by the embryonic transcription factor Oct-3/4, which bound to a unique ACAATAATCA octameric sequence in the hCG beta gene promote r. Here we report that Oct-3/4 is also a potent inhibitor of HCG alpha -subunit (hCG alpha) expression in JAr cells. Oct-3/4 reduced human GH reporter expression from the -170 hCG alpha promoter in either the pr esence or absence of cAMP by about 70% in transient cotransfection ass ays, but had no effect on expression from either the -148 hCG alpha or the -99 hCG alpha promoter. Unexpectedly, no Oct-3/4-binding site was identified within the -170 to -148 region of the hCG alpha promoter, although one was found around position -115 by both methylation interf erence footprinting and electrophoretic mobility shift assays. Site-di rected mutagenesis of this binding site destroyed the affinity of the promoter for Oct-3/4, but did not affect repression of the promoter. T herefore, inhibition of hCG alpha gene transcription by Oct-3/4 appear s not to involve direct binding of this factor to the site responsible for silencing. When stably transfected into JAr cells, Oct-3/4 reduce d the amounts of both endogenous hCG alpha mRNA and protein by 70-80%. Oct-3/4 is therefore capable of silencing both hCG alpha and hCG beta gene expression. We suggest that as the trophoblast begins to form, r eduction of Oct-3/4 expression permits the coordinated onset of transc ription from the hCG alpha and hCG beta genes.