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.     

Functional interaction between the glucocorticoid receptor and GANP/MCM3AP

Glucocorticoids are widely used to treat inflammatory diseases but have a number of side effects that partly are connected to inhibition of cell proliferation. Glucocorticoids mediated their action by binding to the glucocorticoid receptor. In the present study, we have identified by two-hybrid screens the germinal center-associated protein (GANP) and MCM3-associated protein (MCM3AP), a splicing variant of GANP, as glucocorticoid receptor interacting proteins. GANP and MCM3AP can bind to the MCM3 protein involved in initiation of DNA replication. Glutathione-S-transferase-pull-down and co-immunoprecipitation assays showed that the C-terminal domain of GANP, encompassing MCM3AP, interacts with the ligand-binding domain of the glucocorticoid receptor. Characterization of the intracellular localization of GANP revealed that GANP is shuttling between the nucleus and the cytoplasm. Furthermore, we show that glucocorticoids are unable to inhibit DNA replication in HeLa cells overexpressing MCM3AP suggesting a role for both glucocorticoid receptor and GANP/MCM3AP in regulating cell proliferation.     

11beta-hydroxysteroid dehydrogenases,cell proliferation and malignancy

The enzymes 11β-hydroxysteroid dehydrogenase type 1 and 2 (11β-HSD1 and 2) have well-defined roles in the tissue-specific metabolism of glucocorticoids which underpin key endocrine mechanisms such as adipocyte differentiation (11β-HSD1) and mineralocorticoid action (11β-HSD2). However, in recent studies we have shown that the effects of 11β-HSD1 and 2 are not restricted to distinct tissue-specific hormonal functions. Studies of normal fetal and adult tissues, as well as their tumor equivalents, have shown a further dichotomy in 11β-HSD expression and activity. Specifically, most normal glucocorticoid receptor (GR)-rich tissues such as adipose tissue, bone, and pituitary cells express 11β-HSD1, whereas their fetal equivalents and tumors express 11β-HSD2. We have therefore postulated that the ability of 11β-HSD1 to generate cortisol acts as an autocrine anti-proliferative, pro-differentiation stimulus in normal adult tissues. In contrast, the cortisol-inactivating properties of 11β-HSD2 lead to pro-proliferative effects, particularly in tumors. This proposal is supported by experiments in vitro which have demonstrated divergent effects of 11β-HSD1 and 2 on cell proliferation. Current studies are aimed at (1) characterizing the underlying mechanisms for a ‘switch’ in 11β-HSD isozyme expression in tumors; (2) defining the molecular targets for glucocorticoids as regulators of cell proliferation; (3) evaluating the potential for targeting glucocorticoid metabolism as therapy for some cancers. These and other issues are discussed in the present review.