Glutamine synthetase regulation by dexamethasone, RU486, and compound A in astrocytes derived from aged mouse cerebral hemispheres is mediated via glucocorticoid receptor

Glucocorticoids (GCs) regulate astrocyte function, while glutamine synthetase (GS), an enzyme highly expressed in astrocytes, is one of the most remarkable GCs-induced genes. GCs mediate their effects through their cognate glucocorticoid receptor (GR alpha and GR beta isoforms); however, the mechanism via which these isoforms regulate GS activity in astrocytes remains unknown. We used dexamethasone (DEX), a classical GR alpha/GR beta agonist, RU486, which is a specific GR beta ligand, and Compound A, a known “dissociated” ligand, to delineate the mechanism via which GR modulates GS activity. Aged Mouse Cerebral Hemisphere astrocytes were treated with DEX (1 mu M), RU486 (1 nM-1 mu M) or compound A (10 mu M), alone or in combination with DEX. GS activity and expression, GR isoforms (mRNA and protein levels), and GR alpha subcellular trafficking were measured. DEX increased GS activity in parallel with GR alpha nuclear translocation. RU486 increased GS activity in absence of GR alpha nuclear translocation implicating thus a role of GR beta-mediated mechanism compound A had no effect on GS activity implicating a GR alpha-GRE-mediated mechanism. None of the compounds affected whole-cell GR alpha protein content. DEX reduced GR alpha and GR beta mRNA levels, while RU486 increased GR beta gene expression. We provide evidence that GS activity, in astrocytes, is regulated via GR alpha- and GR beta-mediated pathways with important implications in pathological conditions in which astrocytes are involved.