Glucocorticoid receptor interaction with 14-3-3 and Raf-1, a proposed mechanism for cross-talk of two signal transduction pathways

The glucocorticoid receptor (GR) functions as a ligand-dependant transcript ion factor. In the present study we describe a specific immunoaffinity chro matography purification of GR from liver cytosol from adrenalectomized rats that may be used to identify hitherto unknown cytosolic GR interacting pro teins. me have identified the ubiquitously expressed 14-3-3 as well as Raf- 1, a downstream effector of Ras, as GR co-purifying proteins. In our semi-q uantitative analysis liganded/activated GR showed the strongest interaction with 14-3-3 and Raf-l, but 14-3-3 was also found to co-purify with GR in a nonliganded/nonactivated state. By extensive salt washes we were also able to demonstrate that the glucocorticoid induced interaction between GR, 14- 3-3, and Raf-1, respectively, is remarkably stable and withstood 2.4 M salt . The interaction between GR and 14-3-3 was also verified by 14-3-3 co-immu noprecipitation studies. Our observations that GR and Raf-1 are found withi n the same protein complex ("receptosome") in the cytoplasm of rat liver ce lls could provide a mechanistic explanation for glucocorticoid effects on t he Raf-1-Ras signaling pathway.