Cyclosporin A inhibits Ca2+-mediated upregulation of the DNA repair enzymeDNA polymerase beta in human peripheral blood mononuclear cells

Alterations in gene expression may represent an underlying cause of undesir ed side-effects mediated by the immunosuppressant cyclosporin A (CsA). We e mployed the method of differential display PCR to identify new genes whose expression is modulated by CsA. Human peripheral blood mononuclear cells (P BMCs), or subpopulations thereof, were simultaneously stimulated with the p horbol ester 4 beta-phorbol 12-myristate 13-acetate (PMA) and the calcium i onophore ionomycin, in the presence or absence of therapeutic concentration s of CsA. We identify the gene encoding the DNA repair enzyme DNA polymeras e beta (Pol beta) as a novel CsA-sensitive transcription unit. Our data sho w that transcription of pol beta mRNA is induced by Ca2+ and that CsA signi ficantly inhibits PMA/ionomycin- and ionomycin-mediated upregulation of bot h pol beta mRNA and Pol beta protein. The CsA-mediated inhibition of pol be ta upregulation is maintained for at least 21 h after gene activation and i s exerted via the phosphatase calcineurin. FK506, another immunosuppressant that targets calcineurin, also inhibits pol beta upregulation, while rapam ycin competes with FK506 action. This work identifies Ca2+ as an inducer of pol beta gene activity in primary blood cells. The demonstrated CsA sensit ivity of this process suggests a novel molecular mechanism that may contrib ute to the increased tumor incidence in patients receiving CsA treatment.