Squirrel monkey immunophilin FKBP51 is a potent inhibitor of glucocorticoid receptor binding

Squirrel monkeys have high circulating cortisol to compensate for expressio n of low-affinity glucocorticoid receptors (GRs). We have demonstrated that the FK506-binding immunophilin FKBP51 is elevated in squirrel monkey lymph ocytes (SML) and, in preliminary studies, have shown that squirrel monkey F KBP51 is inhibitory to GR binding. In this report, we have demonstrated tha t elevated FKBP51 is the unequivocal cause of glucocorticoid resistance in SML in the following ways: 1) FK506 increased GR binding in cytosol from SM L in a concentration-dependent manner, an effect reproduced by rapamycin bu t not cyclosporin A. The apparent K-d (6.1 nM) and rank-order of steroid di splacement of [H-3]dexamethasone binding in FK506-treated SML cytosol are c haracteristic of high-affinity GR binding. 2) cytosol from COS-7 cells expr essing squirrel monkey FKBP51 inhibited GR binding in cytosol from human ly mphocytes by 74%. Cytosol from COS-7 cells expressing human FKBP51 inhibite d GR binding by 23%. 3) expression of squirrel monkey FKBP51 increased the median effective concentration (EC,,) for dexamethasone in GR transactivati on studies in COS-7 cells by approximately 17-fold, compared with the EC,, in control cells. The expression of human FKBP51 increased the EC,, for dex amethasone in COS-7 cells by less than 3-fold, compared with control. Squir rel monkey FKBP51 shares 94% overall amino acid homology with human FKBP51, with 92% and 99% homology with human FKBP51 in the peptidyl-prolyl isomera se and the tetratricopeptide repeat domains, respectively. Amino acid diffe rences in the more variable N- or C-terminal regions or in regions which jo in the highly homologous functional domains may be responsible for its more potent inhibitory activity.