FMS-like tyrosine kinase 3 interacts with the glucocorticoid receptor complex and affects glucocorticoid dependent signaling.

The glucocorticoid receptor (GR) forms part of a multiprotein complex consisting of chaperones and proteins active in glucocorticoid signaling and other pathways. By immunoaffinity purification of GR, followed by Edman sequencing and Western blotting, we identified the FMS-like tyrosine kinase 3 (Flt3) as a GR-interacting protein in rat liver and hepatoma cells. Flt3 interacts with both non-liganded and liganded GR. The DNA-binding domain of GR is sufficient for Flt3 interaction as shown by GST-pull down experiments. Studies of the effects of Flt3 and its ligand FL in glucocorticoid-driven reporter-gene assays in Cos7 cells, show that co-transfection with Flt3 and FL potentiates glucocorticoid effects. Treatment with FL had no effect on GR location and Dex induced translocation of GR was unaffected by FL. In summary, GR and Flt3 interact, affecting GR signaling. This novel cross-talk between GR and a hematopoietic growth factor might also imply glucocorticoid effects on Flt3-mediated signaling.

Proteomic identification of glucocorticoid receptor interacting proteins.

The glucocorticoid receptor (GR) acts as a ligand dependent transcription factor but can also cross talk with other signaling pathways via protein-protein interactions. In this paper we describe methods to study novel cytosolic GR interacting proteins, using mAb based immunoaffinity chromatography of GR from rat liver cytosol. Co-purifying proteins were identified by 2-DE in combination with MALDI-TOF-MS. Non-liganded/non-activated and in vitro liganded/activated GR, respectively, co-purifies with specific sets of proteins. Of these 34 were conclusively identified, seven have previously been reported to be part of the GR-complex, revealing 27 new possible interacting candidates for the GR-complex. Of the novel GR interacting proteins the major vault protein, TATA binding interacting protein 49a and glycoprotein PP63 were of special interest. Furthermore, using 2-D DIGE we show that the set of proteins interacting with non-liganded GR is distinctly different in protein amount compared to the proteins found with liganded/activated GR. This suggests the presence of different GR complexes in the cell, which was further substantiated by the finding of several separate GR native protein complexes, "GR-receptosomes", using blue native gel electrophoresis. Our findings suggest the existence of several new mechanisms for GR signaling and regulation.

Cytosolic glucocorticoid receptor interaction with nuclear factor-kappa B proteins in rat liver cells.

The glucocorticoid receptor (GR) acts as an anti-inflammatory factor. To a large extent, this activity is exerted by the interference of pro-inflammatory nuclear factor kappa B (NF-kappa B) activity. In their respective inactive forms, both GR and NF-kappa B reside in the cytoplasm and translocate to the nucleus on relevant stimulation. Previously, p65, a component of the NF-kappa B complex, and GR have been shown to interact physically in vitro, and the interaction is assumed to take place in the nucleus of cells [McKay and Cidlowski (1999) Endocrine Rev. 20, 435-459]. We have studied the interaction between GR and NF-kappa B using in vivo -like conditions. Using immunoaffinity chromatography or immunoprecipitation, combined with Western blotting, we observed that, with endogenous protein levels in cytosolic extracts of rat liver and of H4-II-E-C3 hepatoma cells and in contrast with the current belief, p65, p50 and inhibitory kappa B alpha complex interact with GR, even in the absence of glucocorticoid or an inflammatory signal. The interaction between non-liganded/non-activated GR and p65/p50 has also been verified by both p65 and p50 co-immunoprecipitations. Intracellular localization studies, using Western blotting, revealed that glucocorticoids can decrease tumour necrosis factor alpha (TNFalpha)-induced nuclear entry of p65, whereas glucocorticoid-induced GR translocation was much less affected by TNFalpha. We were also able to demonstrate a nuclear interaction of GR and p65 and p50 using in vivo -like protein concentrations. Furthermore, nuclear GR interaction with heat-shock protein 90 was enhanced distinctly by TNFalpha treatment. In conclusion, our studies suggest a strong interconnectivity between the NF-kappa B and GR-signalling pathways where also, somewhat unexpectedly, a physical interaction in the cytosol constitutes an integral part of GR-NF-kappa B cross-talk.