Rapamycin destabilizes interleukin-3 mRNA in autocrine tumor cells by a mechanism requiring an intact 3' untranslated region

We analyzed the effect of rapamycin on autocrine mast cell tumor lines with abnormally stable interleukin-3 (IL-3) transcripts due to a defect in mRNA degradation. Rapamycin inhibited IL-3 mRNA expression specifically, while transcripts of IL-4 and IL-6 were not affected. As indicated by the use of the transcriptional inhibitor actinomycin D or by reporter constructs, inhibition was posttranscriptional and resulted from destabilization of the mRNA. Transcripts from transgenes lacking the AU-rich 3' untranslated region were refractory to drug-induced degradation, suggesting that these 3' sequences contain the target of the rapamycin effect. Rapamycin did not promote IL-3 mRNA degradation in cells of a tumor variant lacking expression of FKBP12, the binding protein of rapamycin. Experiments with wortmannin indicated that rapamycin does not act via p70S6 kinase. FK-506, another ligand of FKBP12 affecting the phosphatase calcineurin, did not antagonize but shared the effect of rapamycin. Our data fit a model whereby both FKBP12 and calcineurin target an unknown regulator of IL-3 mRNA turnover.