POTENTIATION OF PROGESTERONE RECEPTOR-MEDIATED TRANSCRIPTION BY THE IMMUNOSUPPRESSANT FK506

The nontransformed steroid receptors contain several non-steroid bindi ng proteins, such as hsp90, hsp70, and p59. Recently, we and others ha ve shown that p59 (FKBP59) is an immunophilin which binds two potent i mmunosuppressants, FK506 and rapamycin. This raises the possibility th at FK506 or rapamycin may modify the function of steroid receptors. To develop this line of inquiry, we chose a yeast model system in which the human progesterone receptor form B (hPR-B) was cotransformed with a reporter gene. The reporter contains two copies of a progesterone re sponse element/glucocorticoid response element (PRE/GRE) upstream of t he CYC1 promoter which are linked to the lacZ gene of Escherichia coil . We found that FK506 potentiated the ability of progesterone in activ ating transcription. To gain insight into the mechanism of FK506's reg ulation of PR action, we questioned whether calcineurin is involved, b ecause it has been shown that FK506 is aspecific inhibitor of calcineu rin, a Ca2+- and calmodulin-regulated phosphatase, through the formati on of an FKBP12-FK506-calcineurin-calmodulin complex. We found that 15 -O-desmethyl-FK520, an FK506 analogue which is an excellent ligand of FKBP12, but a poor inhibitor of calcineurin, failed to induce the same effect as FK506. We also found that calmidazolium, a calmodulin antag onist, mimicked FK506's action. Furthermore, immunoblot analysis showe d that both FK506 and calmidazolium potentiated the effect of progeste rone in decreasing the mobility of hPR-B upon sodium dodecyl sulfate-p olyacrylamide gel electrophoresis (SDS-PAGE). This suggests that FK506 and calmidazolium may cooperate with progesterone in increasing the l evel of hPR-B phosphorylation. Taken as a whole, we conclude that FK50 6 potentiates the transcriptional activity of PR, perhaps by protectin g it from dephosphorylation. Thus, we suggest that calcineurin may pla y an important role in regulating PR function. Finally, our data elimi nate FKBP12 as the mediator for the FK506 action observed in this stud y, because FK506 potentiation of progesterone receptor transactivation was preserved in FKBP12-deficient yeast mutant cells.