Overexpression of the human glucocorticoid receptor α and β isoforms inhibits AP-1 and NF-κB activities hormone independently

The human glucocorticoid receptor isoforms GRα and GRβ are generated by alternative splicing. Upon hormone binding, GRα regulates positively or negatively transcription. In particular, it represses numerous genes encoding pro-inflammatory mediators by inhibiting the transcription factors activator protein (AP)-1 and nuclear factor (NF)-κB. The observation that GRβ, which does not bind the hormone, may act as a dominant negative receptor is subject to controversy. Because GRβ must be more abundant than GRα to act as such, we evaluated the relative amounts of GRα and GRβ in COS-1, A549 and HeLa cells using a monoclonal antibody that recognises the two isoforms equally well on western blots. Messenger RNA levels of GRα and GRβ were compared by reverse transcriptase polymerase chain reaction analysis. To gain insight into the possible function of GRβ, we examined the ability of overexpressed GRβ to alter transcription of glucocorticoid, AP-1 and NF-κB inducible reporter genes using transient transfection in COS-1 and A549 cells. Subcellular localisation of GRβ was determined in A549 cells by immunofluoresence microscopy. Data indicate that GRα is the predominant endogenous isoform in A549 and HeLa cells. GRβ became the major form after transfection with the corresponding expression vector and translocated into cell nuclei even in the absence of hormone. Overexpression of GRβ inhibited glucocorticoid-induced transcription markedly in COS-1 cells but weakly in A549 cells. We found that GRβ did not act as a dominant negative modulator of GRα for repression of AP-1 and NF-κB activities. In fact, both GRβ and GRα inhibited hormone-independently these activities by 25–60%. This property was not shared by the closely related mineralocorticoid receptor. Our results suggest that overexpression of either GRα or GRβ may have an anti-inflammatory effect.