Glucocorticoid receptor properties and glucocorticoid regulation of glutamine synthetase activity in sensitive C6 and resistant C6H glial cells

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Abstract
The relationship between induction of glutamine synthetase activity by dexamethasone and binding of the steroid to cytosolic glucocorticoid receptors was examined in sensitive C6 and resistant C6H glial cell cultures.Glutamine synthetase activity increased 3–4‐fold when C6 cultures were exposed to 7.6 × 10−6 M dexamethasone. This inductive response was reversible, dose‐dependent (ED50 ∼ 2 × 10−8 M), required de novo protein and RNA synthesis, and was elicited only by glucocorticoid steroids. Progesterone, but not epicortisol, antagonized the dexamethasone‐induced enzyme increase. In contrast, only a slight inductive effect was observed in dexamethasone‐treated C6H cells.Competitive binding assays demonstrated that specific binding of [3H]‐dexa‐methasone to cytosolic receptors was also dose‐dependent. The ED50 was ∼ 10−8 M for both C6 and C6H cells. Scatchard analysis revealed that each C6 cell contained ∼10,800 receptor sites and that the equilibrium dissociation constant (Kd) was 4.5 × 10−9 M. Each C6H cell possessed ∼ 12,200 sites, and the Kd was 6.7 × 10−9 M. Unlabeled dexamethasone and cortisol (but not epicortisol) competed effectively with [3H]‐dexamethasone for binding to cytosolic receptor sites and nuclear sites of both cell types.These results suggest that induction of glutamine synthetase activity in dexamethasone‐treated C6 cells is a glucocorticoid‐directed response. Since C6H cells are refractory in this regard but contain functional cytosolic receptors which interact with cell nuclei, the basis for their resistance appears to involve some step beyond these cellular processes.