CYCLOSPORINE-A POTENTIATES THE DEXAMETHASONE-INDUCED MOUSE MAMMARY-TUMOR VIRUS-CHLORAMPHENICOL ACETYLTRANSFERASE ACTIVITY IN LMCAT CELLS - A POSSIBLE ROLE FOR DIFFERENT HEAT-SHOCK PROTEIN-BINDING IMMUNOPHILINSIN GLUCOCORTICOSTEROID RECEPTOR-MEDIATED GENE-EXPRESSION

As previously observed for FK506, we report here that cyclosporin A (C sA) treatment of mouse fibroblast cells stably transfected with the mo use mammary tumor virus-chloramphenicol acetyltransferase (MMTV-CAT) r eporter plasmid (LMCAT cells) results in potentiation of dexamethasone (Dex)-induced CAT gene expression, Potentiation by CsA is observed in cells treated with 10-100 nM Dex but not in cells treated with 1 mu M Dex, a concentration of hormone which results in maximum CAT activity , At 10 nM Dex, 1-5 mu M CsA provokes an approximate to 50 fold increa se in CAT gene transcription, compared with transcription induced by D ex alone, No induction of CAT gene expression is observed in cells tre ated with CsA or FK506 in the absence of Dex, The antisteroid RU 486 a bolishes effects obtained in the presence of Dex, Using a series of Cs A, as well as FK506, analogs, including some devoid of calcineurin pho sphatase inhibition activity, we conclude that the potentiation effect s of these drugs on Dex-induced CAT gene expression in LMCAT cells do not occur through a calcineurin-mediated pathway. Western-blotting exp eriments following immunoprecipitation of glucocorticosteroid receptor (GR) complexes resulted in coprecipitation of GR, heat shock protein hsp90 and two immunophilins: the FK506-binding protein FKBP59 and the CsA-binding protein cyclophilin 40 (CYP40). Two separate immunophilin- hsp90 complexes are present in LMCAT cells: one containing CYP40-hsp90 , the other FKBP59-hsp90. Thus, both FKBP59 and CYP40 can be classifie d as hsp-binding immunophilins, and their possible involvement as targ ets of immunosuppressants potentiating the GR-mediated transcriptional activity is discussed.