Abstract
To better understand molecular mechanisms by which the trichothecene vomitoxin (VT) superinduces cytokine gene expression, we studied the posttranscriptional effects of this mycotoxin on interleukin-2 (IL-2) gene expression in murine EL-4 thymoma cells stimulated with phorbol 12-myristate 13-acetate and ionomycin (PMA + ION). Northern analysis revealed that doses of 50 to 500 ng/ml VT superinduced IL-2 mRNA expression in a dose- and time-dependent manner in a synchronous model where VT was added at onset of PMA + ION stimulation. In accordance with the mRNA levels, IL-2 production was significantly elevated in the presence of 50 to 250 ng/ml VT. Superinduction of IL-2 mRNA was also observed in a delayed synchronous model (VT added 20 hr after PMA + ION stimulation) and an asynchronous model (VT added 20 hr after PMA + ION stimulation and removal). To assess the effects of VT (500 ng/ml) on IL-2 mRNA half-life, three transcriptional inhibitors were used in the delayed synchronous model. Actinomycin D (ActD) had a pronounced stabilizing effect on IL-2 mRNA but not on mRNA for the housekeeping gene GAPDH. VT did not affect IL-2 mRNA levels in ActD-treated cells. Although 5,6-dichloro-β- d-ribofuranosyl-benzimidazole (DRB) also had a stabilizing effect on IL-2 mRNA, IL-2 mRNA half-life t 1/2in VT-treated cells was three times that of control. In contrast, inclusion of cyclosporin A (CsA) into the cultures specifically arrested IL-2 transcription in EL-4 cells without any stabilizing effect. VT exposure in the presence of CsA markedly prolonged the half-life of IL-2 mRNA in a dose-dependent manner. The t 1/2for IL-2 mRNA in the control culture was 2.1 hr, whereas t 1/2was 3.1, 3.4, 4.2, and 10.5 hr in cultures containing 50, 100, 250, and 500 ng/ml VT, respectively. These results suggest that VT can superinduce IL-2 at both the mRNA and the protein level and that this superinduction can be explained, in part, by posttranscriptional mechanisms such as enhanced mRNA stability.
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