Interaction of glucocorticoid receptor isoforms with transcription factors AP-1 and NF-kappaB: lack of effect of glucocorticoid receptor beta.

Glucocorticoids act through the glucocorticoid receptor (GR) to enhance or repress transcription of glucocorticoid responsive genes depending on the promoter context and cellular background. The human GR primary transcript is alternatively spliced resulting in hGR alpha and hGR beta isoforms. Transactivation and transrepression are mediated by hGR alpha and while it has been demonstrated that hGR beta, can act as a dominant negative inhibitor of hGR alpha mediated transactivation, its effects on transrepression are not known. To investigate hGR beta actions, we used GR-deficient COS-7 and HEK-293 cells. When hGR alpha (0.5 microg 10(6) cells(-1)) was transfected into COS-7 cells dexamethasone (150 nM) inhibited TNF alpha (80 U ml(-1)) effects on a NF-kappaB responsive reporter gene by 40%. There was no evidence of a dominant negative effect when hGR beta (1-10 microg) was co-transfected with hGR alpha up to ratios of 10:1. Similarly hGR beta had no effect on hGR alpha inhibition of a phorbol ester stimulated Ap-1-responsive reporter gene in COS-7 or HEK-293 cells. In comparison, an apparent dominant negative effect of hGR beta on hGR alpha-mediated transactivation was found to be attributable to non-specific transcriptional squelching in COS-7 cells. In summary, the potential for hGR beta, to act as a dominant negative inhibitor of hGR alpha-mediated transactivation remains controversial, but our data suggest that hGR beta, was unable to act as a dominant negative inhibitor of either hGR alpha-mediated transrepression or transactivation in these promoter and cell contexts.