Mutual transrepression of Fos and the glucocorticoid receptor: involvement of a functional domain in Fos which is absent in FosB.

In this study, we show that Fos protein can repress transactivation by the glucocorticoid receptor (GR). In addition, we demonstrate that GR is capable of inhibiting, in a hormone-dependent fashion, Fos-mediated transactivation of AP-1 dependent transcription. Moreover, repression of the serum response element by Fos is abolished by the GR in the presence of hormone. Transrepression of glucocorticoid mediated induction involves a region of Fos, located between amino acids 40 and 111, to which no function has been previously assigned, and which is poorly conserved among Fos, FosB and Fra-1. In agreement with this finding, FosB is not capable of transrepressing GR activation of transcription, representing the first functional difference between Fos and FosB. We have mapped the domain of the GR which is required for repression of AP-1 dependent transcription, to the region of central DNA binding domain. Our results suggest that Fos and the GR may form transcriptionally inactive complexes and point to a regulatory interrelationship between different signal transduction pathways.