Direct interaction between estrogen receptor α and NF-κB in the nucleus of living cells

Inhibition of NF-κB transcriptional activity by steroid receptors is the basis for the antiinflammatory actions of steroid hormones and the molecular mechanism underlying this cross-talk is thought to involve direct protein–protein interactions. In this study, we show that estrogen receptor (ER)α and NF-κB interact in vivo by using fluorescence resonance energy transfer (FRET) and co-immunoprecipitation. U2-OS cells were used to study direct interactions between fluorescent fusion proteins of ERα and the NF-κB subunits p50 and p65. Interactions were observed only in the nucleus and maximal FRET signal was detected when ERα is co-expressed with both NF-κB subunits and cells were stimulated with estrogen. This is in agreement with the induction of nuclear co-localization of the proteins under this condition. Moreover, in a U2-OS clone stably expressing ERα, interaction with NF-κB was confirmed. A p65 deletion mutant lacking the Rel homology domain was strongly impaired in its interaction with ERα showing the importance of this domain. Taken together, these findings provide a strong basis for the direct protein–protein interaction model for cross-talk between ERα and NF-κB.