Overexpression of the FK506-binding immunophilin FKBP51 is the common cause of glucocorticoid resistance in three new world primates

Many New World primates have high circulating levels of cortisol to compens ate for the expression of glucocorticoid receptors (GRs) with low activity. Recent work in squirrel monkeys has suggested that this may be due to eith er the expression of GRs that are transcriptionally incompetent or the expr ession of an FK506-binding immunophilin that inhibits GR binding. The goal of this study was to resolve this controversy by determining the molecular basis of glucocorticoid resistance not only in species of squirrel monkeys but also in other glucocorticoid-resistant New World primates. First, the t ranscriptional activity of the GR from the Bolivian squirrel monkey was com pared to that of the human GR. Incubation of COS-7 cells transfected with t he squirrel monkey GR with 10 nM dexamethasone resulted in a robust stimula tion of MMTV-luciferase activity (up to 260-fold), which was similar in mag nitude to that achieved with the human GR. Second, the effect of FK506 on G R binding was determined in cytosol from cells from two species of squirrel monkeys as well as glucocorticoid-resistant cotton-top tamarins and owl mo nkeys. Incubation with 10 muM FK506 increased GR binding by at least 4-fold in cytosol from cells of each of the New World primates but had no effect on GR binding in cytosol from human WI-38 VA13 cells. Third, Western blots showed elevated expression of FKBP51 in New World primate cells and liver s amples from two squirrel monkey species, On the other hand, the levels of F KBP52 were significantly lower in cells and liver from New World primates. The sequences of FKBP51 from the cotton-top tamarin, owl monkey and squirre l monkey are closely related and share differences from the human, rhesus m onkey, mouse, and lemur FKBP51 sequences in the same 18 positions. Fourth, the relative activities of FKBP51 from the cotton-top tamarin, owl monkey a nd squirrel monkey were determined in cytosol mixing and GR transactivation studies and showed that FKBP51 from each of these primates was a potent in hibitor of GR activity. These results indicate that the elevated expression of FKBP51 contributes to glucocorticoid resistance in three New World prim ate genera. (C) 2001 Academic Press.