Potentiation of Progesterone Receptor-Mediated Transcription by the Immunosuppressant FK506

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Abstract
The nontransformed steroid receptors contain several non-steroid binding proteins, such as hsp90, hsp70, and p59. Recently, we and others have shown that p59 (FKBP59) is an immunophilin which binds two potent immunosuppressants, FK506 and rapamycin. This raises the possibility that 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 response element/glucocorticoid response element (PRE/GRE) upstream of the CYCl promoter which are linked to the lacZ gene of Escherichia coli. We found that FK506 potentiated the ability of progesterone in activating transcription. To gain insight into the mechanism of FK506's regulation of PR action, we questioned whether calcineurin is involved, because it has been shown that FK506 is a specific inhibitor of calcineurin, a Ca2+- and calmodulin-regulated phosphatase, through the formation of an FKBPl2-FK506-calcineurin~almodulin complex. We found that 15-0-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 antagonist, mimicked FK506's action. Furthermore, immunoblot analysis showed that both FK506 and calmidazolium potentiated the effect of progesterone in decreasing the mobility of hPR-B upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). This suggests that FK506 and calmidazolium may cooperate with progesterone in increasing the level of hPR-B phosphorylation. Taken as a whole, we conclude that FK506 potentiates the transcriptional activity of PR, perhaps by protecting it from dephosphorylation. Thus, we suggest that calcineurin may play an important role in regulating PR function. Finally, our data eliminate FKBP12 as the mediator for the FK506 action observed in this study, because FK506 potentiation of progesterone receptor transactivation was preserved in FKBPl2-deficient yeast mutant cells.

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