Cyclosporin A is a differential inhibitor of eukaryotic RNA polymerases

In order to investigate the molecular mechanism of biological action of the drug cyclosporin A (CsA), we have studied its effect on eukaryotic RNA polymerase activities in two systems: (a) in vitro transcription; (b) injection into frog oocytes. The electron microscopic transcription R-loop method [1] has been used to assay for specific transcription of an immunoglobulin gene clone in HeLa cell lysates. In this system, CsA inhibits efficiently the transcription of immunoglobulin genes. Dilution experiments show that RNA polymerase II activity is inhibited by nanomolar concentrations (i.e. amounts comparable to the ones needed for the inhibition of lymphocyte stimulation), whereas polymerase III is inhibited at micromolar concentrations. In vitro transcription with E. coli RNA polymerase is not inhibited. Parallel experiments with the mushroom toxin alpha-amanitin have shown that the eukaryotic RNA polymerase II displays similar sensitivity towards the two cyclic peptides CsA and alpha-amanitin in vitro. When CsA is injected into frog oocytes, together with a cloned Xenopus laevis U1 small nuclear RNA gene, transcription of this gene by endogenous RNA polymerase II is inhibited by 60%. RNA polymerase III and RNA polymerase I transcription is not inhibited in the oocyte. Various possibilities for the selective action of CsA on stimulated lymphocytes are discussed.