Estrogen receptor (ER) beta is the predominant ER in granulosa cells of the ovary.
ERbeta is expressed at high levels in
granulosa cell tumors (GCT) and in the human GCT-derived cell lines, COV434 and KGN. To gain insight into
ERbeta function in granulosa cells and in GCT, we have used the COV434 and KGN cell lines. Although the cells bind
estradiol (E2), transcriptional activation of a transfected
estrogen-responsive reporter vector construct (ERE2-luc) by E2 was not observed. Transactivation was also not observed with cotransfected
ERalpha or beta. This transcriptional resistance is specific to
steroid receptor transactivation; reporter plasmids that are activated by the
transcription factors activator protein 1 (AP-1) and
nuclear factor kappaB (
NF-kappaB) demonstrate both constitutive and inducible transactivation.
AP-1 and
NF-kappaB are known to cause transrepression of both
ERalpha- and
glucocorticoid receptor-mediated transcription. We therefore examined the possibility that activation of these pathways was responsible for the lack of a response to
estrogen by using inhibitors of
AP-1 or
NF-kappaB. The
AP-1 inhibitors alone had no effect, whereas inhibition of
NF-kappaB signaling allowed a 3- to 4-fold E2-mediated induction of ERE2-luc. This response was both
ligand and ER dependent. Repression of
ERbeta signaling by
NF-kappaB has not previously been reported. Recent evidence suggests that
ERbeta may function to promote differentiation. The inhibition of
ERbeta in combination with the antiapoptotic properties of
NF-kappaB may therefore contribute to pathogenesis of GCT.