Regulation of interferon-gamma mRNA in a cytolytic T cell clone: Ca(2+)-induced transcription followed by mRNA stabilization through activation of protein kinase C or increase in cAMP.

Activation pathways inducing the expression of the interferon (IFN)-gamma gene in a cytotoxic T lymphocyte (CTL) clone were studied for their effects on transcription and on mRNA stability. IFN-gamma was secreted by the CTL clone in response to the Ca2+ ionophore ionomycin when used in conjunction with either protein kinase C (PKC)-activating phorbol 12-myristate 13-acetate (PMA) or with agents increasing cAMP, including prostaglandin E2. We describe that ionomycin induced IFN-gamma gene transcription, which was totally inhibited in the presence of cyclosporin A (CSA), an immunosuppressant forming a calcineurin-inhibiting complex with cyclophilin. Ionomycin did not, however, permit accumulation of IFN-gamma mRNA. Activation of PKC by PMA or of cAMP-dependent protein kinase through increase in cAMP had no transcription-inducing effect, either alone or in conjunction with ionomycin, as measured in run on assays of the IFN-gamma gene. When transcription of the IFN-gamma gene, initiated in the presence of ionomycin and an agent increasing intracellular cAMP, was inhibited by CSA in the absence of PKC or cAMP-dependent protein kinase activation, the IFN-gamma mRNA was rapidly degraded (half-life = 30 min). When either PKC was activated or intracellular cAMP was increased at the time of inhibition with CSA, a stabilizing effect was observed on IFN-gamma mRNA, which led to an increase in secreted IFN-gamma. These effects were selective, they did not affect the rate of transcription of the actin gene, nor the accumulation of actin mRNA. These results show that (i) post-transcriptional events can be critical for IFN-gamma expression in activated lymphocytes, and (ii) specific stabilization of IFN-gamma mRNA can be mediated by activation of two different protein kinases involved in T cell activation.