The immunosuppressive drugs cyclosporin A and FK506 inhibit calcineurin phosphatase activity and gene transcription mediated through the cAMP-responsive element in a nonimmune cell line.

Cyclosporin A and the macrolide tacrolimus (FK506) are powerful immunosuppressive drugs that in T cells inhibit the calcium/calmodulin-dependent phosphatase calcineurin thereby preventing the activation of T-cell-specific transcription factors, such as NF-AT, involved in lymphokine gene expression. While this may explain, at least in part, the mechanism of cyclosporin A/FK506 immunosuppression, additional mechanisms have to be invoked in order to explain the pharmacological properties and toxic effects of these drugs, such as nephrotoxicity and neurotoxicity. We have studied the effects of cyclosporin A and FK506 on calcineurin phosphatase activity and gene transcription mediated by the cAMP-responsive element (CRE), a binding site of the ubiquitous transcription factor CREB. A reporter gene was placed under the transcriptional control of the CRE of the rat glucagon gene and transiently transfected into the glucagon-expressing cell line alpha TC2. Cyclosporin A and FK506 inhibited depolarization-induced gene transcription in a concentration-dependent manner (IC50 of about 1 nM and 30 nM for FK506 and cyclosporin A, respectively). Both cyclosporin A and FK506 inhibited calcineurin phosphatase activity at drug concentrations that inhibited gene transcription. The FK506 analogue rapamycin had no effect on calcineurin activity and gene transcription, but excess concentrations of rapamycin prevented the effects of FK506 on both calcineurin activity and gene transcription. These results support the notion that the interaction of drug-immunophilin complexes with calcineurin may be the molecular basis of cyclosporin A/FK506-induced inhibition of CREB/CRE-mediated gene transcription. The ability to interfere with CREB/CRE-mediated gene transcription represents a novel mechanism of cyclosporin A/FK506 action which may underlie pharmacological effects and toxic manifestations of these potent immunuosuppressive drugs.