Inhibition of dexamethasone binding to human glucocorticoid receptor by New World primate cell extracts

To determine if New World primates express an inhibitor that influences glucocorticoid receptor (GR) binding characteristics, we examined [³H] dexamethasone binding in cytosol prepared from B95-8 lymphoid cells, derived from the cotton top tamarin (Saguinus oedipus), in combination with cytosol prepared from human or rat tissues. B95-8 cytosol inhibited specific binding of [³H]dexamethasone (P < 0.01) when mixed with cytosol prepared from either a human lymphoid cell line (HL) or rat thymus. The inhibitory activity was heat labile and trypsin sensitive. Peak inhibitory activity was found in the 150-200 kd fractions after Sephacryl G-200 ultrafiltration. Scatchard analysis of [³H]dexamethasone binding using mixed cytosol showed a diminished GR apparent binding affinity when compared to HL cytosol. Kinetic studies using mixed cytosol indicated that B95-8 cytosol did not affect the apparent dissociation rate of [³H] dexamethasone. These data demonstrate that B95-8 cells contain a competitive inhibitor that prevents binding of dexamethasone to its cognate receptor.