Regulation of interferon‐γ mRNA in a cytolytic T cell clone: Ca2+‐induced transcription followed by mRNA stabilization through activation of protein kinase C or increase in cAMP

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Abstract
Activation pathways inducing the expression of the interferon (IFN)‐γ gene in a cytotoxic T lymphocyte (CTL) clone were studied for their effects on transcription and on mRNA stability. IFN‐γ 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‐γ 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‐γ 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‐γ gene. When transcription of the IFN‐γ 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‐γ 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‐γ mRNA, which led to an increase in secreted IFN‐γ. 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‐γ expression in activated lymphocytes, and (ii) specific stabilization of IFN‐γ mRNA can be mediated by activation of two different protein kinases involved in T cell activation.