Overexpression of the Human Glucocorticoid Receptor Alpha and Beta Isoforms Inhibits AP-1 and NF-kappaB Activities Hormone Independently

The human glucocorticoid receptor isoforms GRalpha and GRbeta are generated by alternative splicing. Upon hormone binding, GRalpha 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)-kappaB. The observation that GRbeta, which does not bind the hormone, may act as a dominant negative receptor is subject to controversy. Because GRbeta must be more abundant than GRalpha to act as such, we evaluated the relative amounts of GRalpha and GRbeta 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 GRalpha and GRbeta were compared by reverse transcriptase polymerase chain reaction analysis. To gain insight into the possible function of GRbeta, we examined the ability of overexpressed GRbeta to alter transcription of glucocorticoid, AP-1 and NF-kappaB inducible reporter genes using transient transfection in COS-1 and A549 cells. Subcellular localisation of GRbeta was determined in A549 cells by immunofluoresence microscopy. Data indicate that GRalpha is the predominant endogenous isoform in A549 and HeLa cells. GRbeta became the major form after transfection with the corresponding expression vector and translocated into cell nuclei even in the absence of hormone. Overexpression of GRbeta inhibited glucocorticoid-induced transcription markedly in COS-1 cells but weakly in A549 cells. We found that GRbeta did not act as a dominant negative modulator of GRalpha for repression of AP-1 and NF-kappaB activities. In fact, both GRbeta and GRalpha 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 GRalpha or GRbeta may have an anti-inflammatory effect.