Cytosolic glucocorticoid receptor interaction with nuclear factor-κB 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 kappaB (NF-kappaB) activity. In their respective inactive forms, both GR and NF-kappaB reside in the cytoplasm and translocate to the nucleus on relevant stimulation. Previously, p65, a component of the NF-kappaB 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-kappaB 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 kappaBalpha 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-kappaB and GR-signalling pathways where also, somewhat unexpectedly, a physical interaction in the cytosol constitutes an integral part of GR-NF-kappaB cross-talk.