Physiologic and Pathologic Consequences of the Interaction of Glucocorticoid Receptor with p53

B. Wasylyk

Institut de Génétique et de Biologie Moléculaire et Cellulaire, France.

The tumour supressor p53 and the glucocorticoid receptor (GR) respond to different types of stress. We found that dexamethasone-activated endogenous and exogenous GR inhibit p53 dependent functions, including trans-activation, up- (Bax and p21WAF1/CIP1) and down- (Bcl2) regulation of endogenous genes, cell cycle arrest and apoptosis. GR forms a complex with p53 in vivo, resulting in cytoplasmic sequestration of both p53 and GR.

In neuroblastoma (NB) cells, cytoplasmic retention and inactivation of wild-type p53 involves GR. p53 and GR form a complex that is dissociated by GR antagonists, resulting in accumulation of p53 in the nucleus, activation of p53 responsive genes, growth arrest and apoptosis. These results suggest that molecules that efficiently disrupt GR-p53 interactions would have a therapeutic potential for the treatment of neuroblastoma and perhaps other diseases in which p53 is sequestered by GR.

Using normal endothelial cells (HUVEC), we studied the mechanism of the mutual inhibition of GR and p53 in response to hypoxia, a physiological stress, and mitomycin C that damages DNA. Dexamethasone (Dex) stimulates the degradation of endogenous GR and p53 by the proteasome pathway in HUVEC under hypoxia and mitomycin C treatments, and also in hepatoma cells (HepG2) under normoxia. Dex inhibits the functions of p53 (apoptosis, Bax and p21WAF1/CIP1 expression) and GR (PEPCK and G-6-Pase expression). Endogenous p53 and GR form a ligand dependent trimeric complex with Hdm2 in the cytoplasm. Disruption of the p53-HDM2 interaction prevents Dex induced ubiquitylation of GR and p53. The ubiquitylation of GR requires p53, the interaction of p53 with Hdm2, and Hdm2’s E3 ligase activity. These results provide a mechanistic basis for GR and p53 acting as opposing forces in the decision between cell death and survival (Fig. [1]).

Fig. 1

p53 stimulates hematopoietic cell maturation and antagonises GR activity in hypoxia, suggesting that it may inhibit stress erythropoiesis. We report that mouse foetal liver ebls that lack p53 proliferate better than wild type cells in the presence of the GR agonist dexamethasone. An important mediator of GR induced ebl self-renewal, the c-myb gene, is induced to higher levels in p53-/- ebls by dexamethasone. The stress response to anaemia is faster in the spleens of p53-/- mice, as shown by the higher levels of CFU-Es (colony forming units-erythroid) and the increase in the CD34/c-Kit double positive population. Our results show that p53 antagonises GR mediated ebl expansion and demonstrate for the first time that GR-p53 cross-talk is important in a physiological process in vivo, stress erythropoiesis.