Vanadate increases glucocorticoid receptor-mediated gene expression: a novel mechanism for potentiation of a steroid receptor

Abstract

Transition metal oxyanions, such as molybdate, tungstate and vandadate, have been shown to prevent in vitro hormone-induced activation of the glucocorticoid receptor (GR) by blocking dissociation of the GR/heat shock protein heterocomplex. In this work, we report a novel effect of vanadate: in vivo potentiation of GR-mediated gene expression. In cells stably-transfected with complex (mouse mammary tumor virus (MMTV)) or minimal GR-regulated CAT reporters, treatment with 500 M vanadate caused CAT gene expression to dramatically increase, even at saturating concentrations of dexamethasone; while no such effect was seen in response to RU486 antagonist. Similar treatment with molybdate had no effect on GR activity, suggesting that the response to vanadate was not a general property of transition metal oxyanions. Treatment with vanadate after hormone-induced nuclear translocation of the GR also caused potentiation, demonstrating that vanadate was acting on a post-transformation event, perhaps by affecting the transactivation function of DNA-bound GR. Paradoxically, vanadate caused an apparent but temporary "loss" of GR protein immediately after treatment (as measured by loss of reactivity to BuGR2 antibody and of hormone-binding capacity) that returned to normal at approximately 8 h post-treatment, suggesting that potentiation of GR transactivation function (as measured by our CAT assays) was probably occurring during the later stages (8-24 h) of this assay. However, gel shift analyses revealed that vanadate could induce binding of the hormone-free GR to glucocorticoid response element (GRE)-containing oligonucleotides immediately after treatment. Thus, the rapid vanadate-induced "loss" of GR was not due to degradation of GR protein. Yet, vanadate in the absence of hormone had no effect on CAT reporter expression, demonstrating that this form of the GR still requires agonist for its enhanced transcriptional activity. As an indication of the potential mechanism of vanadate action, vanadate was found to dramatically stimulate the mitogen-activated protein kinases, ERK-1 and ERK-2. In addition, vanadate potentiation of GR reporter gene expression was completely blocked by the tyrosine kinase inhibitor herbimycin A. Taken as a whole, our results suggest that vanadate can have dramatic and complex effects on GR structure and function, resulting in hormone-free activation of GR DNA-binding function, as well as alterations to the BuGR2 epitope and hormone-binding domains-while at the same time stimulating tyrosine phosphorylation pathways controlling GR-mediated gene transcription.

Interaction of rat renal glucocorticoid receptor with Hsp90 and Hsp70 upon stress provoked by mercury

“…It was previously published that mercury led to a reduction of the level of hepatic GR immunoadsorbed by BuGR2 antibody, and this decline was ascribed to a conformational change of the receptor causing BuGR2 epitope masking (Brkljacib et al, 2004a, b). Similar results regarding vanadate-induced changes in GR conformation leading to a temporary loss of BuGR2 antibody reactivity and functional alterations in steroid-binding were reported by Calzi et al (2002). Considering the fact that the concentration of mercury determined by atomic absorption spectrometry was 30-fold higher in renal than in hepatic cytosol after i.p.…”

Molecular interactions between glucocorticoids and long-acting β2-agonists

“…Vanadate, a transition metal oxyanion similar to molybdate, has been shown to induce time-dependent and concentrationdependent increases in both ERK and nonreceptor tyrosine kinase activity that result in an increase in unliganded receptor GRE binding and markedly enhance transactivation in the presence of the ligand. 27 The cyclooxygenase-2 inhibitor nimesulide is also able to enhance ERK activity, GR phosphorylation, GR-GRE interactions, and transcriptional activity without affecting GR expression or nucleocytoplasmic shuttling. Many of the actions of nimesulide are thought to be mediated through these mechanisms.…”

Two Pioneer Transcription Factors, Krüppel-Like Transcription Factor 4 and Glucocorticoid Receptor, Cooperatively Transactivate the Bovine Herpesvirus 1 ICP0 Early Promoter and Stimulate Productive Infection

“…In general, nuclear hormone receptors, including GR, are activated by a ligand, in this case DEX (10). However, under certain circumstances, GR can be translocated from the cytoplasm to the nucleus and activate gene expression by ligand-independent mechanisms (35,36). In fact, the promoter that drives BRCA1 (breast cancer-associated gene 1) expression is positively regulated by GR via an unliganded mechanism (37).…”