Endocrine Abstracts

Glucocorticoids (Gc) act through the glucocorticoid receptor (GR) to regulate gene transcription, but also initiate a number of rapid non-genomic effects. The inactive GR is found complexed with hsp90 and immunophilins. Several other signalling molecules including c-src and raf-1 also associate with hsp90. Evidence suggests that Gc treatment results in the rapid activation and dissociation of GR and c-src from these complexes. We speculate that GR-mediated activation of one or more proteins at the hsp90 level or at the membrane may initiate the non-genomic effects of Gc.

A549 cells treated with 100 nM Dexamethasone, displayed no detectable change in activation of p42, p44, or p38, but Gc treatment promoted rapid and transient phosphorylation of PKB. This was abolished following pretreatment with LY249002, wortmannin and RU486 suggesting that PKB activation was GR and PI3-Kinase dependent. We propose that c-src activation by Gc may link GR and PI3-Kinase. As the caveolar protein, caveolin, is implicated in regulating both PI3K/PKB and c-src pathways, we have investigated whether caveolin may be important in facilitating these non-genomic effects. Co-immunoprecipitation studies demonstrated an interaction between GR, c-src, and caveolin and density-gradient centrifugation demonstrated a co-localisation of GR and c-src to caveolin containing membrane fractions. These data suggest that GR may associate with the plasma membrane, and that caveolin may support the interaction between GR and c-src. Interestingly, Gc treatment induced a time dependent increase in the phosphorylation of caveolin and also IRS-1. This phosphorylation was disrupted following expression of a dominant negative caveolin molecule, and the use of the src-kinase inhibitor PP2. The fact that both approaches also abolished the Gc dependent phosphorylation of PKB suggests a functional role for both c-src and caveolin in Gc actions.

We have identified important functional interactions between GR, caveolin and c-src which leads to activation of PKB.